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Designing network topology with tunable dynamics and rheology behaviors is the key for vitrimers applications. In this study, PET vitrimers with varying crosslinking density and network homogeneity were created by using different M w of precursors and branching agents (catalysts). Our results show that the dynamic network formed by propane-type branching agent such as 1,3-bis[tris(hydroxymethyl) methylamino] propane (BIS-TRIS propane), is more uniform due to steric hindrance effect compared to 2,2-Bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol (BIS-TRIS). The thermal behavior of PET vitrimers was investigated by non-isothermal experiments, including glass transition and crystallization process. In particular, rheology behavior is sensitive to network structure and bond exchange process. The linear viscoelasticity (LVE) region of PET vitrimers is characterized by constructed pseudo-master curves that are guided by two distinct relaxation mechanisms: Rouse-type relaxation of strands and terminal relaxation of the network. Each relaxation mechanism is associated with a unique activation energy. The mechanical and processing properties of the PET vitrimers are reflected in their plateau modulus and characteristic relaxation time, which varies depending on the network topology. Based on the LVE analysis, we further investigated nonlinear rheology of PET vitrimers under extensional flow. Our results indicate that PET vitrimers exhibit significant strain hardening behavior, but display distinct stretching trends, corresponding to polymer melts' ductility. The ductility of materials decreases with a decrease in precursor's molecular weight (M w), but improved with enhanced network homogeneity. Therefore, the crosslinking density and network homogeneity of dynamic network can be selectively tailored using appropriate precursors and branching agents to meet various industrial applications. [Display omitted] • Tunable PET vitrimer dynamics via precursor and branching agent selection. • Steric hindrance from propane-type agents enhances network uniformity. • LVE characterized by unique relaxation mechanisms and activation energies. • Improved ductility via network homogeneity. [ABSTRACT FROM AUTHOR]

A study of polystyrene resins containing supercritical CO 2 was conducted to understand the thermodynamics of mixing to predict relevant viscoelastic properties for foaming applications. Three polystyrene resins with different molecular weight distribution were tested at three temperatures (170, 185 and 200 °C) and two pressures (6.89 MPa and 13.78 MPa) using a parallel plate rheometer equipped with a pressure cell. Effects of temperature, pressure and CO 2 concentration on viscoelastic properties were evaluated and isolated according to a time-temperature-pressure-concentration superposition (TTCS) to describe the thermodynamic path of mixing. Finally, elongational viscosities of the mixtures were estimated using the Phan-Thien-Tanner model to speculate about the polymer's foaming behavior at different operating conditions. The analysis proved that while the viscosities of the resins are very similar at atmospheric pressure, the differences between resins' viscosities were stronger at high temperatures and pressures. [ABSTRACT FROM AUTHOR]

In this work, a coarse-grained model of dynamic covalent polymers (DCP) based on dynamic covalent bonds (DCBs) is first constructed. Then, the dynamics of DCBs is characterized by analyzing the time correlation function where lifetime exhibits an Arrhenius dependence on temperature. Following it, the viscoelasticity of DCP is explored by tuning the number of DCBs and temperature. The storage modulus exhibits a significant decrease at the critical shear strain amplitude (γ 0) while the loss modulus reaches the maximum value. This is because of the orientation and stretching of chains as a function of the shear strain amplitude. The decrease magnitude in the storage modulus rises with increasing the number of DCBs. This can be understood well by characterizing the number, lifetime, bond length distribution of DCBs, and bond energy per DCBs at the critical γ 0. Furthermore, both the storage modulus and loss modulus decrease with increasing the temperature. This is because the number and lifetime of DCBs are reduced with increasing the temperature while the bond energy per DCBs is improved. Interestingly, the maximum loss modulus peak at the critical γ 0 gradually disappears with reducing the temperature. Finally, the self-healing process is quantitatively simulated by analyzing the self-healing degree. The relationship between the characteristic self-healing time and the self-healing temperature follows an Arrhenius behavior. In summary, this work presents a new understanding on the effect of DCBs on the viscoelasticity and self-healing property of DCP at the molecular scale. Viscoelasticity and self-healing property of dynamic covalent polymers: A molecular dynamics simulation. [Display omitted] • A coarse-grained model of dynamic covalent polymers (DCP) based on dynamic covalent bonds (DCBs) is constructed. • The dynamics of DCBs and chains are characterized where lifetime exhibits an Arrhenius dependence on temperature. • The viscoelasticity and self-healing property of DCP is explored by tuning the number of DCBs and temperature. [ABSTRACT FROM AUTHOR]

Extrusion-based 3D printing is a well-established material processing technique that has shown promise in employing a variety of polymers. Poly(lactic acid) is the most studied and widely used raw material in 3D printing, which can be modified by copolymerization with trimethylene carbonate (TMC), for tissue engineering applications. In this study, the aim was to explore the characterization of Poly(L-co-D,L-lactic acid-co-trimethylene carbonate) (PLDLA-TMC) at different ratios (60/40, 70/30, 80/20, and 90/10), focusing on their physicochemical, thermal, and rheological characterization, as well as their cytocompatibility. The results demonstrated shear-thinning behavior and highlighted the influence of TMC content on viscoelastic properties. The printing speed of 4 mm s−1 allowed the obtaining of scaffolds with rectangular pores and controlling of extrusion temperature promoted the formation of uniform filaments with high print quality. Additionally, the cytocompatibility assessment of the scaffolds revealed the terpolymer's potential for tissue engineering across a wide range of applications. [Display omitted] • Poly(L-co-D,L-lactic acid-co-trimethylene carbonate) has potential for 3D printing. • Changes in TMC content affect viscoelastic properties, tailoring material behavior. • The optimization of printing parameters led to scaffolds with high print quality. • The 3D printed scaffolds have suitable biocompatibility for tissue engineering. [ABSTRACT FROM AUTHOR]

Understanding the inelastic, rate-dependent mechanical response of biodegradable polymers is important for the design of load-bearing biodegradable structures with controlled deformation and failure response. In this study, we investigate the mechanical response of amorphous polylactic acid (PLA) in dry and wet conditions prior to the onset of degradation at body temperature. The presence of water decreases the glass transition temperature by 4.5 °C, the storage modulus by 21%, and the compressive and tensile yield strengths by about 10%, despite a small water uptake of 0.93 wt%. The tensile response of PLA is dominated by craze yielding, rather than shear plasticity, and is stable against necking despite pronounced strain softening and local strain heterogeneities measured by Digital Image Correlation (DIC). Further analysis of the DIC strain fields in dry and wet samples suggests a transition from pure craze yielding in dry samples to a coexistence of craze yielding and shear plasticity in wet samples. The mechanism shift between tension and compression behaviour of dry and wet PLA has implications for the design of load-bearing structures and for constitutive modelling. [Display omitted] • Rate-dependent mechanical properties of dry and wet amorphous PLA were characterised. • The plasticising effect of water is similar to a temperature rise of 4.5 °C. • DIC reveals strong strain heterogeneities in tension but no necking. • The tensile flow behaviour of dry PLA is dominated by craze-yielding. • Shear yielding and crazing likely co-exist in wet PLA. [ABSTRACT FROM AUTHOR]

Thermoplastic polyester ether elastomer (TPEE) vitrimers with different dynamic crosslinked networks were synthesized via zinc (II) acetylacetonate-catalyzed epoxy chemistry. The effects of bond exchange within networks, characterized by number of catalytic centers per crosslink (CC), and the dynamic crosslinked topology, characterized by number of entanglement and crosslinking points per crosslink (EC), on thermal, rheological, and foaming properties of TPEE vitrimer were compared. The results show that increasing EC is more favorable for lower crystallization temperature than improving bond exchange owing to the bad structural symmetry but almost constant crystallinity owing to the branches. TPEE vitrimer with a complex dynamic crosslinked topology exhibited slow relaxation with a stronger relaxing response and steeper strain hardening than that with more bond exchanges, indicating better viscoelasticity. Moreover, the good foaming behavior of TPEE vitrimers was ascribed to the dynamic crosslinked networks, which promoted heterogeneous nucleation to increase the cell density, and strain hardening good for cell growth as it suppressed cell coalescence and rupture. Furthermore, increasing CC facilitates bond exchange, promoting more uniform heterogeneous nucleation and cell size dispersity than those afforded by a complex dynamic crosslinked topology achieved by increasing EC. The improvement of foaming behavior for TPEE vitrimer is quite different from that of branched polymer by enhancing cell growth rather than cell density. [Display omitted] • TPEE vitrimers were successfully synthesized based on epoxy-acid chemistry. • Complex dynamic crosslinked topology resulted in good viscoelasticity. • Dynamic crosslinked networks contributed to good foaming behavior. • Complex dynamic crosslinked topology resulted in higher expansion ratio of TPEE vitrimer foams. • Bond exchanges led to a more uniform cell size dispersity of TPEE vitrimer foams. [ABSTRACT FROM AUTHOR]

Replacing carbon black and silica with organic fillers can reduce the weight of polymer nanocomposites. Soft-nanoparticles (SNPs), a type of organic filler, can be produced in large quantities through microemulsion polymerization. Their elasticities and diameters can be tuned for a wide range. This study aimed to investigate the linear viscoelasticity of athermal all-polystyrene nanocomposites with different molecular weights of matrix chains. Polymer nanocomposites filled with soft SNPs do not show this second plateau. But the second plateau is observed with semi-rigid SNPs if the weight percentage (φ) of SNPs is above a critical value φ c. Semi-rigid refers to the intermediate elasticity compared to phenol-capped silica nanoparticles and long-chain grafted silica nanoparticles/soft SNPs. In both cases of all-polystyrene nanocomposites, the entanglement plateau and higher frequency regions remain the same. This hints that polymer chains do not thread the network. Flory-Rehner theory further confirms negligible swelling of long polymer chains into dense crosslinking networks of SNPs. The modulus of this plateau scales to the 3.8 power of φ and the 0.26 power of M w. The critical weight percentage φ c for the onset of the second plateau corresponded with the compression of the polymer coil between two SNPs. As φ increases above φ c , polymer chains would be depleted from neighboring SNPs, resulting in this network of semi-rigid SNPs. Thus, we believe that the second plateau originated from this network. This study analyzed the second plateau and highlighted the variety of viscoelastic features associated with the elasticity of SNPs filled in polymer nanocomposites. [Display omitted] • The network of semi-rigid (without swelling) SNPs is formed by depleting polymers and leads to the second plateau. • The modulus of this plateau scales to the 3.8 power of the weight fraction of SNPs and the 0.26 power of Mw. • The onset of the second plateau corresponded with the squeeze of the polymer coil between neighboring SNPs. [ABSTRACT FROM AUTHOR]

This study applies a mechanical approach based on time-temperature equivalence to relate chain networks and properties of polyethylene-based materials (HDPE, PEXb, and especially vitrimers). We first characterize the chain network behavior from the microscopic to mesoscopic level using solvent extraction, DSC, DMTA, and rheology. The chain networks can be distinguished from the cross-linking degree, degree of crystallinity, and time-temperature-dependent linear viscoelasticity in solid state or above melting temperatures. As core studies, we then properly characterize the tensile mechanical behavior at large deformation. PEXb is stiffer than HDPE and PE-vitrimer at all tested physical states (from viscoelastic to flow states). Tensile mechanical behaviors between PE-vitrimer and HDPE are i) equivalent at the end of the glassy state (i.e., lower temperatures or high equivalent strain rates), and ii) distinct approaching flow (i.e., higher temperature or low equivalent strain rates). Therefore, an approach based on the time-temperature superposition extended to large deformation is suitable to link material properties to the chain networks. [Display omitted] • Structure-property relationships for polyethylene-based thermoplastic, thermoset, and vitrimer (PEs). • Successful application of the time-temperature equivalence to characterize mechanical behavior at large deformation. • Relation of the time-temperature-dependent mechanical behavior of PEs to their chain network organization. [ABSTRACT FROM AUTHOR]

This paper synergistically addressed the microstructure alteration and the mechanical behavior of rubber during thermo-oxidative aging process from both experimental and numerical modeling aspects. Through the FTIR spectra analysis, equilibrium swelling experiment, low-field NMR measurement and mechanical tests of styrene-butadiene rubber (SBR) samples, a macromolecular network alteration mechanism was introduced. The mechanism is related to the variation of the crosslinking network and dangling chain ends of SBR during aging. A finite deformation based hyper-viscoelastic constitutive model was then established to capture the mechanical behavior of SBR during aging. Based on the interpretation of the microscopic mechanism, the above-mentioned network alterations during aging were considered in the model. After the implementation by means of the finite element simulation software ABAQUS, the accuracy of the established model was checked via the comparison of the numerical results with the corresponding data obtained from mechanical tests. The work is helpful for estimating rubber mechanical performance during aging process. Image 10405 • A macromolecular alteration mechanism relating to crosslink network and dangling chain ends during SBR aging was proposed. • A hyper-viscoelastic constitutive model relating to the observed microscopic mechanism of SBR aging was established. • Helmholtz free energy was decomposed into the equilibrium and the non-equilibrium part, respectively. • The numerical model was implemented through the finite element method and the accuracy of the model was verified. [ABSTRACT FROM AUTHOR]

Abstract The shear viscosity of binary blends comprising polycarbonate (PC) and low-molecular-weight polystyrene (L-PS) was examined. It was found that the viscosity of PC significantly decreases by the addition of L-PS. Although the dynamic mechanical properties indicated that L-PS is immiscible with PC, the morphology observation of the extruded strand clarified that shear-induced phase-mixing occurs. Furthermore, the viscosity decrease was found to be pronounced in the high shear stress condition, which is much more obvious than that by the addition of low-molecular-weight PC. Consequently, the addition of L-PS improves the flowability at injection-molding greatly. Graphical abstract Image 10992 Highlights • A blend of polycarbonate and polystyrene shows flow-induced phase-mixing. • Addition of polystyrene greatly decreases the shear viscosity of polycarbonate. • Because of the viscosity drop, the blend shows high flowability at injection-molding. [ABSTRACT FROM AUTHOR]

Abstract The objective of the present work was to study the dependence of the foamability and the foaming window of a polystyrene/poly (methyl methacrylate) (PS60/PMMA40) blend on the structural evolution driven by phase separation. A compression molding machine was used to prepare the blend samples thermally annealing for various times. The phase contrast optical microscope (PCOM) and scanning electron microscopy (SEM) were used for phase morphological change study. The samples were foamed via a batch foaming process, and SEM was used for cell structure observation. The desorption curves of the blend samples were studied to show the dependence of the gas desorption diffusion coefficient on the annealing. It was found that the phase domain size was increased in a wide range with the annealing, and that the foaming behavior was significantly affected by the structural evolution for the blend with big phase domains and the saturation under high pressures. The gas desorption diffusion coefficient was increased for the interface with the phase domain, which resulted in the decreased expansion ratio after foaming, with varying reduction degrees for varying saturation pressures. Further, the suitable foaming temperature range was studied for the blend samples annealing for a short and a long period and saturated under each pressure. It was found that both the upper and the lower foaming temperatures were decreased with the saturation pressure, and the upper one was more vulnerable to the higher pressures for the blend samples after long-period annealing, leading to the much narrower suitable foaming temperature window. It was believed that the difference in viscoelasticity of the dispersed phase and the matrix was responsible for this phenomenon. Graphical abstract Image 1017 Highlights • Foaming behavior is affected by structural evolution via phase separation. • Phase domain size plays a significant role in affecting cell structure. • Upper foaming temperature is vulnerable to large domain and high pressure. • Foaming window is narrowed for combination of large domain and high pressure. • Viscoelasticity difference of two phases affects foaming behavior significantly. [ABSTRACT FROM AUTHOR]

Abstract Untethered octaisobutyl- polyhedral oligomeric silsesquioxane (POSS) molecularly dispersed into low-density polyethylene (LDPE) induced entanglement dilation and dynamics acceleration, reduced the melt viscosity and acted as nano-plugs thus reducing O 2 transmission. POSS nanoparticles were melt mixed with LDPE using a twin-screw extruder and adding up to 10 wt% POSS. High-resolution transmission electron microscopy (HRTEM) demonstrated excellent dispersion of POSS into LDPE, at a single POSS unit at low concentrations. The dynamics was probed by shear rheometry constructing master curves at T ref = 190 °C using the time-temperature superposition (TTS) principle. In the low concentration range (c POSS <5 wt%) the terminal (liquid-like) regime showed that POSS nanoparticles induced accelerated dynamics, decreasing the terminal relaxation time τ t. This translated into an anomalous, non-Einstein, reduction of viscosity. Increasing concentration to c∼10 wt% POSS formed clusters of ca. 10 units, the terminal relaxation time increased and the melt exhibited higher viscosity than the neat LDPE, as predicted for particulate filled suspensions. Therefore, at low concentrations the size of POSS (ca. 1.5 nm), smaller than entanglement mesh size of polyethylene (tube diameter d t = 3.28 nm), would drive chain disentanglement and be responsible for the dilution effect. At higher concentrations POSS aggregated and therefore increased the viscosity of the nanocomposite, a behavior akin to Einstein's prediction. Strikingly, the intercalation of POSS into the entangled molecular mesh reduced oxygen transmission, suggesting that POSS acted as nano-plugs by filling the voids in the entangled mesh. Graphical abstract Image 1 Highlights • Untethered POSS dispersed at a single unit in LDPE. • POSS induced chain disentanglement and viscosity reduction in LDPE. • POSS acted as nano-plug reducing O 2 transmission in LDPE. [ABSTRACT FROM AUTHOR]

Abstract Hydroentanglement is a versatile and important process used to form highly entangled nonwoven materials comprised of polymer fibers. It is a key element of polymer processing in the world, with the nonwovens market worth of tens of billions of the US dollars. No fundamental theoretical models of hydroentanglement were available so far, even though such modeling is required to facilitate the entirely empirical efforts to optimize the process. No model experiments on hydroentanglement aiming at its underlying physics were conducted either. These challenging problems are in focus in the present work. A model experiment is conducted and a quasi-one-dimensional model of individual polymer fibers is proposed and implemented for a number of fibers subjected to water jets impacting as a curtain normally to the nonwoven surface and undergoing filtration motion in the inter-fiber pores. The nonwoven is assumed to be located on a moving substrate with water suction through it. The model allows for a number of two-dimensional water jets and a number of suction ports in the substrate. The quasi-one-dimensional model allows for the three-dimensional motion of individual viscoelastic polymer fibers and the fiber-fiber interaction, which makes them non-self-intersecting. Then, the governing quasi-one-dimensional equations of fiber motion are discretized and solved numerically by using a discrete element method with mesh refinement of the Lagrangian mesh during the time marching. Using the developed code, several basic model problems were solved. Specifically, the behavior of several viscoelastic polymer fibers under the action of water jets with and without fiber-fiber interaction is described. Accordingly, a quantitative measure of the fiber-fiber entanglement is introduced based on the observed morphologies in the accompanying second part of this work. Graphical abstract Model of a single fiber in hydroentanglement with the corresponding simulation results versus SEM image. Image 1 Highlights • A theoretical model of hydroentanglement of polymer fibers was proposed. • An analytical solution of filtration flow inside the nonwovens mat was incorporated. • Numerical simulations of polymer fibers evolution with and without fiber-fiber and self-interaction were performed. • A model hydroentanglement experiment developed. [ABSTRACT FROM AUTHOR]

Abstract Annealing is widely utilized in industry to manipulate the mechanical properties of semi-crystalline polymers. In this work, poly(ether-ether-ketone) (PEEK) was annealed at different temperatures above its glass transition temperature (T g) to investigate its influences on the viscoelastic behavior of PEEK. A set of master curves on long-term creep and stress relaxation behaviors of PEEK were obtained via time-temperature superposition and showed that a higher annealing temperature leads to more retardation. The underlying molecular mechanism is related to a higher activation energy of β-relaxation (E β) associated with the molecular motions below T g. A physics-based Ngai's Coupling model was chosen for the analysis, suggesting a higher degree of restriction on molecular mobility due to annealing, which is mainly attributed to the increases in the density of the amorphous phase and the crystallinity. Usefulness of the Ngai's Coupling model for the prediction of long-term viscoelastic behavior of semi-crystalline polymers is discussed. Graphical abstract Image 1 Highlights • Ngai's Coupling model fits the creep behavior of semicrystalline PEEK. • E β responsible for molecular motions below T g is increased upon annealing. • Annealing causes increase in ρ amorphous phase and χ c to restrict molecular mobility. [ABSTRACT FROM AUTHOR]

Abstract A transform was proposed in the earlier work to convert the frequency domain storage modulus obtained from dynamic mechanical analysis (DMA) to elastic modulus over a wide range of temperatures and strain rates (Polymer, 2016, 101, 1–6) from testing of a single specimen. However, the method still required conducting temperature and frequency sweeps in DMA experiments. The present work is focused on developing an adaptive DMA experimental scheme for achieving further reduction in experimentation required to characterize a material using adaptive design of experiments (DoE) method. First, the surrogate model is established based on time-temperature superposition principle using the Particle Swarm Optimization. Then four sampling methods are used to fit the data on the surrogate model. The Sobol Quasi Monte Carlo (QMC) method converges faster and has better accuracy than other methods. Then, using integral relations of viscoelasticity, the surrogate model is transformed to time domain for obtaining a temperature dependent relaxation function from which the strain rate sensitive elastic modulus is extracted and validated with tensile test results. The error is found to be below 3.9% for 5% magnitude of noise and 4.4% for 10% magnitude of noise in the strain rate range 10−5 to 10−2 s−1 using Sobol QMC method. The close agreement indicates that the adaptive DMA scheme can significantly reduce the time and cost involved in materials characterization and eliminate the need for tensile tests for measuring material modulus. Graphical abstract Image Highlights • Storage modulus is transformed to elastic modulus over strain rates and temperatures. • The transform is validated with experimental results and found to match closely. • An adaptive design of experiments scheme is used for reduction in experiments. • A single specimen can yield elastic modulus over temperature and strain rate ranges. [ABSTRACT FROM AUTHOR]

Abstract Nanoparticles significantly reinforce rubber and enhance nonlinear mechanical behavior while the underlining mechanisms are not clear. Herein the effects of crosslinking, filler content and filler surface chemistry on the linear and nonlinear rheological responses of silica filled nitrile butadiene rubber are investigated. A time-concentration superposition principle is employed for constructing rheological master curves. While the strain amplification and dynamics retardation contribute to the high-frequency linear rheology of the compounds, the former is crucial for the vulcanizates with matrix of similar crosslinking degree. In both the compounds and vulcanizates, the Payne effect is initiated at a similar microscopic strain of the rubber phase. A weak overshoot of loss modulus appears in compounds containing hydrophilic silica and vulcanizates containing hydrophobic silica. It is suggested that the viscoelastic matrix is highly important for the rheological responses of the filled compounds and vulcanizates. Graphical abstract Image 1 Highlights • The stain amplification and dynamics retardation of uncured rubber contribute to high-frequency linear rheology of the compounds. • The stain amplification is crucial for linear rheology of vulcanizates with matrix of similar crosslinking degree. • In both the compounds and vulcanizates, the Payne effect is initiated at a similar microscopic strain of the rubber phase. • The strain amplification factor can be well explained by effective filler volume fraction. [ABSTRACT FROM AUTHOR]

Abstract Mechanical and viscoelastic properties of polybutadiene-based rubber materials are highly dependent on polybutadiene microstructure along with molar mass, dispersity, polymer architecture and styrene content for copolymers. In addition, the living character of anionic polymerization of butadiene allows the precise chain-end functionalization and synthesis of block copolymers. Knowing that mainly the nature of the counter-ion or solvents influence strongly the polybutadiene microstructure, a summary of all anionic or related strategies is proposed in this review to tune polybutadiene microstructures. The influence of alkali metals, alkaline earth metals and multi-metallic compounds as well as their environment is discussed. Graphical abstract Image Highlights • Anionic polymerization allows to tune polybutadiene microstructure. • Counter-ion and solvent influence polybutadiene microstructure. • High 1,4-cis or 1,4-trans content, up to 85–90%, can be reached. • 1,2 units can be varied from 8% to 88%. [ABSTRACT FROM AUTHOR]

Abstract Phase separation of trimethylolpropanetriacrylate/poly(styrene-co-methylmethacrylate) induced by UV-irradiation (photo-PIPS) has been studied. Three poly(St-co-MMA) with St:MMA monomer ratio of 25:75, 50:50 and 75:25 were synthesized with average molecular weight of Mw ≈ 5 × 104 and average Tg being ≈345 K, due to stereoirregularity in their structure. Difference in TMPTA/poly(St-co-MMA) blends compatibility caused different extents of photo-PIPS and final conversion status depended on the copolymer type and amount. Optical microscopy used to study time-evolution of photo-PIPS. SEM observation showed that, within studied range of copolymer content, droplet domains of copolymer-rich phase appeared in TMPTA-rich matrix through viscoelastic photo-PIPS and in copolymer-rich domains network-like structures of TMPTA were formed. Also some columns were created due to copolymer extrusion to the surface of the sample. Size of the copolymer-rich domains, their formation procedure and size of the columns depended on the copolymer type and amount. SEM Carbon-mapping confirmed copolymer extrusion to the surface. Based on AFM phase-mode images medium-, low- and high-modulus regions were detected and assigned to the TMPTA-rich matrix phase, copolymer-rich phase and TMPTA-rich phase of copolymer-rich domains, respectively. These corresponds to peaks of tan δ curves used to determine mentioned phases composition. Graphical abstract Image 1 Highlights • Use compatibility of thermoplastic/thermoset blends to create special morphologies. • Reduction of crosslinking kinetics to make phase separation reach the desired extent. • Clarification of viscoelastic phase separation in bulk and phase separated domains. • Use of thermoplastics to increase final conversion in photo-polymerized blend. [ABSTRACT FROM AUTHOR]

Abstract Shear-thinning hydrogels are often employed in direct-write 3D printing, however, the viscoelastic behaviors that define a printable hydrogel have not been fully established. Herein, we demonstrate a library of hydrogel inks based on the incorporation of water-soluble reactive meth(acrylate) monomers into F127-dimethacrylate hydrogels. This strategy afforded printed hydrogels with a broad range of chemical functionalities and mechanical properties. A systematic investigation was also performed to correlate the printability and mechanical properties to the viscoelastic properties of the hydrogel ink formulations. The materials with a high dynamic yield stress afforded extruded filaments that correlated well with the inner diameter of the printing nozzle. The static yield stress of the material was correlated to the extrusion pressure and print speed required for optimal printing. Thus, this study provides a guide for the future development of hydrogel inks for direct-write 3D printing along with a new set of functional hydrogel inks. Graphical abstract Image 1 Highlights • Library of hydrogel ink compositions for direct-write 3D printing. • Printability was correlated to the viscoelastic properties of the hydrogel inks. • Static and dynamic yield stress are important parameters for printability. [ABSTRACT FROM AUTHOR]

Dielectric polymers can achieve large, reliable deformation in response to an external electric field and have attracted significant interest as actuators and transducers. The electromechanical performance depends on the interaction between the visco-hyperelastic behaviour of materials and the Maxwell stress caused by the electric field. However, the mechanisms of complex structures, such as a surface-treated and filler-reinforced space system, are still not completely understood. A mechanical model for evaluating the surface effect and electromechanical performance is first proposed on the basis of visco-hyperelastic theory and statistical mechanics. Then, several factors affecting the geometry and material properties of complex structures are quantitatively investigated. The model will contribute to the development of dielectric polymers because its theoretical predictions are in agreement with existing experimental data. [ABSTRACT FROM AUTHOR]

The linear viscoelastic behavior of molten hybrid nanocomposites poly [(propylmethacryl-heptaisobutyl-POSS)-co-methyl methacrylate], denoted POSSn-MMA, was investigated; POSS content varied up to n  = 45 wt%. The nanocomposites were amorphous and ΔT g  ≈ −20 K. The melts exhibited linear viscoelastic behavior, and master curves were constructed using time-temperature superposition (TTS) at T ref  =  T g +50 K. The viscoelastic response covered terminal, rubber-like and transition regimes. Strikingly, POSS induced considerable fast dynamics with over three orders of magnitude reduction of terminal relaxation time τ t . Furthermore, POSS induced entanglement dilution where the rubbery modulus G e decreased three-fold (i.e., packing length p increased). The melt viscosity exhibited Newtonian and shear thinning regimes, and the zero-shear viscosity η ο decreased up to three orders of magnitude. The unusual viscosity reduction suggests that the viscoelastic response arises from (relatively weak) POSS-POSS associations and not chain entanglements as in linear polymers. Hence, shear would easily break the associations and rather low viscosity would be experienced. At Angstrom-scale POSS modified the structure of PMMA but did not crystallize, consistent with DSC results. At nm-scale POSS-POSS associations were detected by small-angle X-ray scattering, and the size of aggregates increased with POSS content. [ABSTRACT FROM AUTHOR]

Design and fabrication of organic and polymeric soft materials exhibiting solvent-free isothermal changes in mechanical properties triggered by photostimulation are challenging. Here, we report dramatic changes in viscoelasticity of coumarin end-functionalized poly(dimethyl siloxane) with a branched macromolecular architecture under a solvent-free isothermal condition upon alternating irradiation of UV at the wavelength of 365 and 254 nm (UV 365 and UV 254 ). Significantly, it is disclosed that storage modulus ( G′ ) was increased by a factor of more than 30000. Moreover, the subsequent UV 254 irradiation caused sharp decrease in G′ and it became comparable to loss modulus. Furthermore, alternating irradiation of UV 365 and UV 254 caused repetitive increase and decrease in G′ of the material. [ABSTRACT FROM AUTHOR]

Polyhedral oligomeric silsesquioxane copolymerized with butyl acrylate (BA), methyl methacrylate (MMA), and acrylic acid (AA) induced ca. twofold increase of fracture energy Γ and of Young's modulus E of cast films at only 1 wt% content. The copolymer of composition BA:MMA:AA:POSS 56:41:2:1 mol% was synthesized in situ by seeded semi-batch emulsion polymerization. Films cast from the corresponding emulsions were optically transparent and transmission electron microscopy (TEM) showed that POSS was predominantly dispersed at nearly single unit in the acrylic matrix. Shear rheology and time-temperature superposition (TTS) analysis demonstrated an entangled state with the absence of terminal regime, and POSS increased the segmental relaxation time τ e and the rubbery modulus G e , hence the free volume and packing length p decreased, relative to the neat copolymer. The efficient dispersion of POSS and its size smaller than the virtual tube diameter suggests that POSS is intercalating the molecular mesh. Therefore, the entanglement density increased and induced an increase of tensile mechanical modulus. The mesh intercalation by POSS means that it did not act as stress concentrator and consequently the critical fracture strain ε* and the fracture energy Γ also increased. These results evidenced the topological constraints imposed by the bulky POSS to hinder chains past each other thus inducing an increase of entanglement density and this manifested in simultaneous mechanical reinforcement and resistance to fracture. [Display omitted] • POSS simultaneously increased mechanical modulus and toughness of acrylic nanocomposite. • Well-dispersed POSS intercalated the macromolecular mesh. • The intercalation increased the entanglement density and hence induced mechanical reinforcement. • The intercalated POSS did not act as stress concentrator. [ABSTRACT FROM AUTHOR]

We investigate the properties of polymer nanocomposites consisting of narrow disperse spherical nanoparticles that are coated with narrow disperse polymers. These nanoparticles can be homogeneously dispersed in a polymer matrix. The structure of the polymer-coated nanoparticles was examined by using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), and was related to the viscoelastic properties of the nanocomposites. We observe a systematic retardation of the polymer relaxation dynamics with increasing particle loading. Nanoparticle aggregation leading to the formation of chain-like structures causes a strong reinforcement effect. [ABSTRACT FROM AUTHOR]

The ability to relax a macroscopically applied stress is often associated with molecular mobility, or the possibility for a molecule to move outside the confines of its current position, within the material of which the stress is applied. Here, a viscoelastic constitutive analysis is used to investigate the counter-intuitive experimental observation of “ mobility decrease with increased deformation through yield” [1] for a glass forming polymer during stress relaxation while under compressive and tensile loading conditions. The behavior of an epoxy thermoset is examined using an extensively validated, thermorheologically simple, material “clock” model, the Simplified Potential Energy Clock (SPEC) model.[2] This methodology allows for a comparison between the linear viscoelastic (LVE) limit and the true non-linear viscoelastic (NLVE) representation and enables exploration of a wide range of conditions that are not practical to investigate experimentally. The model predicts the behavior previously described as “ mobility decrease with increased deformation” in the LVE limit and at low strain rates for NLVE. Only when loading rates are sufficient to decrease the material shift factor by multiple orders of magnitude is the anticipated deformation induced mobility or “mobility increase with increased deformation” observed. While the model has not been “trained” for these behaviors, it also predicts that the normalized stress relaxation response is indistinguishable amongst strain levels in the “post-yield” region, as has been experimentally reported. At long time, which has not been examined experimentally, the model predicts that even the normalized relaxation curves that exhibit “mobility increase with increased deformation” “cross back over” and return to the LVE ordering. These findings demonstrate the ability of rheologically simple models to represent the counter-intuitive experimentally measured material response and present predictions at long time scales that could be tested experimentally. [ABSTRACT FROM AUTHOR]

Shape memory polymers (SMPs) are one of the polymeric smart materials which can undergo transition between two different shapes (temporary shape and permanent shape) induced by external stimuli, such as temperature, light etc. Increasing applications of SMPs have motivated a great progress in developing adequate constitutive models for this kind of material. In this work, we propose a new constitutive model of SMPs combining phase transition and viscoelasticity. The SMP materials are treated as a mixture of rubbery phase and glassy phase. Thus, it can physically relate the shape memory effect to glass transition. As the proposed model is also based on viscoelastic theory, it can successfully characterize the viscoelastic properties and predict the rate-dependent shape memory response of SMPs. By ignoring the viscosity of the material, the proposed model could be degenerated to a purely elastic phase transition model. To verify the model, we have simulated the shape memory behavior of polyurethane under different pre-strains and have compared the simulation results with the experimental results available in the literature. Good agreement between numerical calculations and experiments can be observed. [ABSTRACT FROM AUTHOR]

Stable and single droplet generation is essential in drop-on-demand (DOD) inkjet printing process to deposit homogeneous and flat film. However, polymer's rheological properties and viscoelasticity can influence filament break up and droplet formation, which may lead to the formation of satellite droplets. Controlling the Z constant (related to fluid viscosity, and surface tension) and Weissenberg (Wi) number can suppress the satellite droplets and get stable and single droplet. Here, mixed solvents (chloride benzene (CB) and high viscosity cyclohexylbenzene (ChB)), different molecular weight and concentration were employed to tune the polyfluorene (PFO) viscosity, surface tension and viscoelasticity. According to the droplet ink-jetting behavior of different CB/ChB ratio, PFO molecular weight and concentration, it's found that only when the speed of the secondary droplet is faster than that of the primary droplet, a single droplet could be formed. At last, we concluded that for PFO inks 9 ≤ Z ≤ 55, Re ≥ 3 and Wi = 0.5 were adequate judgement for a stable and single droplet formation and the two factors should be controlled simultaneously to ensure a stable and single droplet. [ABSTRACT FROM AUTHOR]

Soft, biphasic materials such as hydrogels are commonly used to mimic lubrication and confinement mechanics of biological tissue such as articular cartilage or the cornea. In-depth understanding of such mechanics is crucial for designing synthetic replacements for cartilage, contact-lens materials or soft coatings for medical devices. Using colloidal-probe atomic force microscopy (AFM), surfaces can be investigated at the nanoscale and information on the contact modulus, poro-viscoelastic properties and the permeability can be extracted. Yet, probing the surface of a soft material in a liquid environment is challenging, since the point of contact between a probe and sample surface during finite-rate indentation can be obscured by viscous squeeze-out effects of temporarily confined liquid. To address this issue, we have developed a 2-step indentation method that enables accurate alignment of finite-rate indentation curves with respect to the contact point of quasi-static indentation of soft matter in liquid. In this work, the issue and the method are illustrated by measurements on a commonly used poly(acrylamide) (PAAm) hydrogel. We have shown that liquid squeeze-out may cause non-negligible force offsets that can result in false contact-point determination during finite-rate indentation. The presented method allows accurate alignment of the indentation curves, enables one to accurately study the rate-dependent contact moduli and related stiffening effects, and thus greatly facilitates mechanical characterization of both biological as well as synthetic soft materials. [ABSTRACT FROM AUTHOR]

Engineered rubbery materials in practical applications usually contain nanoparticle and microscopic short fiber while no theories could describe the viscoelasticity of the compounds varying with these two kinds of fillers. We describe how the linear rheology of natural rubber compounds varies with the loading of precipitated silica, short nylon fiber, or both. We for the first time disclose a filler dimension-dependent hydrodynamic-to-non-hydrodynamic transition by applying the time-concentration superposition principle for accounting for the apparent liquid-to-solid transition related to filler composition and loading and the retarded dynamics of the matrix. A framework for simultaneous solving the reinforcement and dissipation of the multi-component compounds is suggested, providing a new perspective on understanding the filling effect for manufacturing high-performance rubber materials. [ABSTRACT FROM AUTHOR]

Controlling the dispersed-to-continuous transition (DCT) in immiscible polymer blends is still a challenging task due to multiple parameters dependency of DCT in the mixing process. In this study, the DCT of different polymer blends was investigated through controlling the viscoelastic asymmetry of polymers. The results demonstrate that the DCT is well correlated with the deformation of the dispersed domains and the ability to raise the aspect ratio of the dispersed domains. Both the viscosity ratio and the elasticity ratio are important to determine the aspect ratio of the dispersed domains. A diagram was suggested to control the DCT concentration by viscoelastic asymmetry, i.e., viscosity ratio and elasticity ratio. It is proved that DCT concentration as low as about 10% can be achieved for Polycarbonate/Poly (lactic acid) (PC/PLA) blends at the suitable viscoelastic asymmetry. [ABSTRACT FROM AUTHOR]

Impact mitigation in layered polymeric structures is studied to quantify the importance of impact impedance mismatch and energy dissipation modes on the overall transmission of a pressure wave. Internal reflection within the laminate structure is enhanced by optimizing the impedance between the laminate layers creating a virtual echo chamber for the impact wave. The high internal reflection potential of the structure enhances the residence of the energy within an energy dissipating viscoelastic layer. All the laminates of this research are composed of commonly available polymeric materials. A novel approach to measurement employs a Newton's cradle to correctly and accurately account for all input and output energies. As much as 90% of the theoretical transferrable energy can be dissipated by this mechanism. Statistical analysis of transmission and dissipation data shows that dissipative modes are of primary importance for mitigating impact forces, while other factors are of secondary importance. The most significant model is obtained when both dissipation and impedance are accounted for. The effect of increasing internal reflection within the dissipative structure results in the most significant variable for energy dissipation and transmission being the dissipative potential of a viscoelastic layer as determined from its tan-δ envelope. A dynamic finite element analysis technique has also been developed that models the time evolution of energy within the laminate structure following an idealized impact, and verifies the experimental results. [ABSTRACT FROM AUTHOR]

Microrheology based on diffusing-wave spectroscopy, dynamic light scattering (DLS), and macrorheology were performed on solutions of polyacrylamide-derivative associating polymer containing imidazole groups. By adding Ni ion, the physical gel is formed by metal-ligand bond. The physical gel point, where the percolation of a transient network is supposed to occur, was microrheologically determined. In the DLS measurement showing ergodicity at all the conditions studied, we observed three modes in the autocorrelation functions: fast, intermediate, and slow modes. The fast mode is the collective diffusion (gel mode), while the slow mode is attributed to the diffusion of clusters. For the intermediate mode, with increase in Ni ion concentration, the transition from diffusive mode to relaxation mode was observed near the microrheologically determined physical gel point. In postgel regime, intermediate mode can be attributed to the macroscopic relaxation since the activation energies are comparable with the relaxation energy estimated by macrorheology. [ABSTRACT FROM AUTHOR]

Phase separation kinetics of dynamically asymmetric mixtures poly(ethylene oxide)/ionic liquid (PEO/[EMIM][BF 4 ]) was investigated through optical microscopy and rheological measurements. The distinct phase-separating morphologies including network structure at lower PEO concentrations of 10∼30 wt % (P10, P30) driven by viscoelastic phase separation (VPS), co-continuous structure at the moderate concentrations (P40, P50) governed by spinodal decomposition (SD) and droplets-matrix at higher concentrations (P60, P70) driven by either SD or nucleation and growth (NG) are observed under different temperatures. Evident deviation of the storage modulus G ′ and gel-like behavior on loss tangent tanδ at low frequencies can be seen upon entering into the two-phase region dependent on different morphologies. During time sweeps, the initial increase of G ′ can be ascribed to the growth of concentration fluctuation, and the subsequent decrease possibly due to the phase domain coarsening, breaking up or coalescence. The interfacial tension σ at the late stage of phase separation with droplet-matrix morphologies is estimated by using the Gramespacher-Meissner (G-M) model and Palierne model. [ABSTRACT FROM AUTHOR]

Although 3D printing has the potential to transform manufacturing processes, the strength of printed parts often does not rival that of traditionally-manufactured parts. The fused-filament fabrication method involves melting a thermoplastic, followed by layer-by-layer extrusion of the molten viscoelastic material to fabricate a three-dimensional object. The strength of the welds between layers is controlled by interdiffusion and entanglement of the melt across the interface. However, diffusion slows down as the printed layer cools towards the glass transition temperature. Diffusion is also affected by high shear rates in the nozzle, which significantly deform and disentangle the polymer microstructure prior to welding. In this paper, we model non-isothermal polymer relaxation, entanglement recovery, and diffusion processes that occur post-extrusion to investigate the effects that typical printing conditions and amorphous (non-crystalline) polymer rheology have on the ultimate weld structure. Although we find the weld thickness to be of the order of the polymer size, the structure of the weld is anisotropic and relatively disentangled; reduced mechanical strength at the weld is attributed to this lower degree of entanglement. [ABSTRACT FROM AUTHOR]

A study of flexoelectric behavior of polyvinylfluoride, PVDF-based materials, was carried out considering the design in macroscopic terms. The materials are prepared using solution blow spinning (SBS) manufacturing multilayers of PVDF filled with multi-walled carbon nanotubes (MWCNT) with good control of concentration. Flexoelectric behavior of the materials is studied with a testing machine of our own design to obtain output voltage signals under cyclic mechanical perturbation as a function of perturbation rate. Mechanical behavior of the materials is described as a function of their porosity and MWCNT concentration. It is demonstrated that flexoelectricity does not only depend on the strain gradient but also on the time of mechanical response of the materials or their viscoelastic properties. This work offers a new insight on the study of flexoelectric behavior of polymer composite materials and how designing PVDF-based materials with high flexoelectric yields and, therefore, with high potentiality to be used as easy-to-handle sensors. [Display omitted] • SBS allows preparing PVDF-based composites with complex macroscopic designs. • An easy method to study flexoelectricity of polymer-based systems is developed. • Flexoelectricity do not only depend on the strain gradient but also on the viscoelastic properties. • Flexoelectric yield in polymer composites depends on flexoelectric coefficient, macroscopic design and viscoelasticity. [ABSTRACT FROM AUTHOR]

Polymeric surfactant is an attractive alternative to conventional surfactant-polymer system for application in enhanced oil recovery, as it plays dual role of reduction of interfacial tension (IFT) and improvement of mobility ratio. The present paper deals with the synthesis and rheological characterization of a novel polymeric surfactant derived from Jatropha oil. Molecular aggregation of polymer in aqueous solution has been confirmed by DLS studies, which increases with increase in concentration. The viscosity results indicate the good shear stability of system at higher shear rate as well as higher temperature for application in conventional oil reservoir. The polymeric solutions show peculiar behaviour with shear thinning nature at low shear rate and shear thickening nature at higher shear rate because of its unique molecular properties. The viscoelastic properties of the solutions were investigated and it has been found that storage modulus (G′) and loss modulus (G″) follow the Maxwellian fluid behavior. Specific frequency (SF) indicated by the crossing point between G′ and G″ on the viscoelastic curve represents the point of transition between elastic and viscous phases of the polymeric surfactant system. The effect of silica nanoparticle on polymer viscosity was also studied and it has been found that introduction of silica nanoparticle dramatically increases the polymer viscosity and stability of the solution with respect to temperature. [ABSTRACT FROM AUTHOR]

Experimental characterization and theoretical modeling of the viscoelastic behaviors of two epoxy compounds are aimed in the present work. Through bending and torsion dynamic analysis, long-term master curves are obtained for the materials moduli over more than 30 frequency and time decades, and the relevant shift factor over a wide temperature range. The uniqueness of the shift factor for the independent viscoelastic properties is shown. The behavior of the shift factor is categorized in regions below and above the glass transition temperature and expressed by different models. Substantial time dependency of the long-term relaxation bulk modulus and the Poisson's ratio are obtained through incremental relations using the measured relaxation moduli. The characterized viscoelastic behavior of the molding compound is verified by means of a creep test performed at isothermal and non-isothermal conditions. It is further shown that when the adhesive compound is cured between two members as in practice, very low glass transition temperature and mechanical properties are expected due to the more residual diluent. [ABSTRACT FROM AUTHOR]

In this letter, both the viscoelastic and electrical properties of poly(methyl methacrylate)/carbon black (PMMA/CB) nanocomposites with different CB concentrations prior to and after annealing were investigated. The linear steady-state recoverable compliance of pure PMMA is independent of annealing as it reflects only polymer-polymer interactions. While the conductivity of PMMA/CB nanocomposites increases gradually with annealing, and the viscoelastic properties show different regions of response depending on the CB concentration. At CB contents below the percolation threshold the recoverable compliances increase after annealing, whereas the reverse trend is found at CB contents above the percolation threshold. This behavior is related to the interplay between network formation and particle-particle interactions. [ABSTRACT FROM AUTHOR]

Tissue engineering requires scaffolding techniques based on non-toxic processes that permits the fabrication of constructs with tailored properties. Here, a two-step methodology based on the gelation and precipitation of the poly(lactic) acid/ethyl lactate organogel system is presented. With this technique nanofibrous matrices that resemble natural extracellular matrix can be easily obtained, while allowing control over the mechanical properties of the device. Gelation temperature and the dynamics of the gelation of the organogel system are characterized, and the final mechanical and viscoelastic properties, as well as porosity, as function of the initial polymer concentration are described. We show that gelation temperature of the system is concentration independent and below 44.5 °C, which permits gelation at room temperature. Furthermore, mechanical properties are found in the range of the soft organic tissues, and the obtained micro-network architecture gives place to a flexible structure. Such structure presents tuneable elastic modulus and viscoelastic properties as function of nanofibers density. Moreover, centimetre-long tubular scaffolds with the diameter of medium-caliber blood vessels were produced. The fibrous nano-architecture mimics the native extracellular matrix fibres diameter and morphology was proven to be suitable to support endothelialization of the lumen of the tube. Thus, this strategy, based on biocompatible green compound might be promising for the fabrication of large 3D scaffolds for tissue engineering applications. [ABSTRACT FROM AUTHOR]

The sol-gel transition of silica-polyacrylamide hydrogel nanocomposites is studied when approaching the gel point by independently varying the nanoparticle (NP) and chemical crosslinker concentrations. Time-concentration-superposition (TCS) principles are applied to ascertain the effect of NPs on dynamics at the gel point. Doping dilute and concentrated polyacrylamide (PA) solutions with silica NPs indicates that these particles do not crosslink PA; neither do they form percolating networks. Nevertheless, silica NPs interact with PA to influence the storage and loss moduli to a much greater degree than expected for passive fillers. Interestingly, when silica NPs are embedded in very weakly chemically crosslinked PA (with a bis crosslinker concentration that would otherwise not form PA hydrogels), they significantly enhance the effective degree of crosslinking, forming viscoelastic solids. From TCS analysis of dynamics at the gel point, the bond probability is found to scale with the NP volume fraction as p ∼ ϕ 1.5 , whereas chemical crosslinking furnishes a bond probability that is linear in the crosslinker concentration. [ABSTRACT FROM AUTHOR]

We present an approach to quantify different contributions to dissipation in elastomer nanoparticle composites based on strain sweeps. A modified Kraus equation is successfully used to approximate the loss modulus depending on strain amplitude G ″ γ measured at different temperatures. For natural rubber composites containing > 10 vol% carbon black or > 3 vol% carbon nanotubes two different contributions to dissipation due to (i) breaking and (ii) deformation of glassy rubber bridges in the filler network are identified. Filler fraction and temperature-dependent trends support physical pictures considering that glassy rubber bridges are responsible for the visco-elasticity of the filler network. Constant extra contributions to dissipation are associated with the bulk-like fraction of the elastomer matrix or filler network independent effects. The achieved understanding should be very important for the optimization of elastomer nanoparticle composites for special applications like tire treads. [ABSTRACT FROM AUTHOR]

In this study, the potential of amphiphilic diblock copolymers on the example of polystyrene- b -poly(4-vinylpyridine) (PS- b -P4VP) and poly(4-methylstyrene)- b -poly(4-vinylpyridine) (P4mS- b -P4VP) for producing open-celled foams was evaluated. The porous foam structure was realised by employing carbon dioxide and water as environmental-friendly blowing agents. Rheological measurements in shear and elongation revealed a similar strain-softening behaviour of the diblock copolymers in the melt state. The transient shear and elongational viscosity of the P4mS homopolymer agreed with the linear viscoelastic prediction. On the contrary, the employed PS homopolymer showed strain-hardening. The rheological behaviour of the PS and P4mS homopolymers is consistent with their foaming ability. The PS homopolymer led to a homogeneous, closed cell structure, while the P4mS homopolymer did not foam at all. Because of strain-softening in melt elongation, the PS- b -P4VP diblock copolymer generated homogeneous, open-celled foams throughout the whole sample. In contrast, the P4mS- b -P4VP diblock copolymer generated partially open-celled structures in coexistence with compact areas. In this work, it was demonstrated that the combination of carbon dioxide and water led to open-celled diblock copolymer foams even if the major component of the block copolymer generates homogeneous closed-celled foams or is not foamable, respectively. [ABSTRACT FROM AUTHOR]

In this work, specific experiments on an isotactic polypropylene are carried out, aiming to investigate the flow induced crystallization and the final morphology. The viscoelastic nature of the polymer is described by a non-linear Maxwell model applied to the conformation tensor. Shear stress evolutions, recorded during step shear isothermal experiments, are satisfactory described considering the molecular stretch, i.e. the difference between the two main eigenvalues of the conformation tensor. In the general model, the effect of temperature, pressure, and crystallinity are taken into account. Furthermore, a modeling framework is proposed to describe flow-induced crystallization of isotactic polypropylene. The spherulitic growth rate is analyzed on the basis of a flow dependent equilibrium melting temperature, using the molecular stretch as the key parameter. A phenomenological correlation of the nucleation rate with growth rate is observed. By combining the morphological models, both for nucleation and growth rate, for flow induced crystallization is possible to explain the effect of shear rate and shearing times in different experimental results, and potentially in the simulation of polymer processing. [ABSTRACT FROM AUTHOR]

In this work the three-dimensional architecture and properties of solution-blown laydown formed on a rotating drum are studied using the system of quasi-one-dimensional equations of the dynamics of free liquid polymer viscoelastic jets moving, evaporating and solidifying, while being driven by a surrounding high-speed air jet. Solution blowing of multiple polymer jets simultaneously issued from a nosepiece and collected on a rotating drum is modelled numerically. The numerical results on the volumetric porosity of nonwoven laydown are compared with the experimental data of the present work. The numerical predictions are in good agreement with the experimental data and elucidate the effect of the angular drum velocity on the mass and angular fiber distribution, as well as the volumetric porosity and permeability of the solution-blown nonwovens. It was found that instead of doing any upstream modification of the solution blowing process, the easiest way to control the laydown structure (the mass and angular fiber distribution, as well as the volumetric porosity and permeability) is to vary the angular velocity of the collecting drum. [ABSTRACT FROM AUTHOR]

We developed a novel Hierarchical Multi-mode Molecular Stress Function (HMMSF) model for polydisperse polymer melts, which implements the basic ideas of (i) hierarchical relaxation, (ii) dynamic dilution, and (iii) interchain tube pressure. Here, the capability of this approach is demonstrated by comparison of predictions of the HMMSF model with uniaxial extensional viscosity data of sixteen different grades of high and low density polyethylene melts, as well as two different polystyrene melts with defined topology. The modelling is solely based on the linear-viscoelastic relaxation modulus with only one non-linear material parameter, the dilution modulus. [ABSTRACT FROM AUTHOR]

A series of biodegradable nanocomposites were prepared by dispersing graphene nanoplatelets (GNPs) in poly (butylene adipate-co-terephthalate) (PBAT). Variations of dynamic and steady shear rheology of the nanocomposites with GNP loading and temperature were investigated via oscillatory measurements. Viscoelastic properties of the matrix exhibited significant enhancement with GNP addition. Solid-like flow behaviour was observed for highly filled samples while pure PBAT and nanocomposites with low GNP loadings showed liquid-like behaviour. Interestingly, the liquid-solid transition was found to occur at lower GNP concentrations with increasing temperature; dropping from 11.5 wt% at 160 °C to 7 wt% at 220 °C. Furthermore, in contrast to ideal melts, viscoelastic properties of some of the nanocomposites increased with increasing temperature. These observations suggest that the percolation did not originate only from network formation between the platelets but may be related to a combined GNP-PBAT gelling network, which enhanced with increasing temperature, leading to a more solid-like response at elevated temperatures. Variation of shear viscosity of nanocomposites with temperature also showed that increasing GNP loading reduces the temperature sensitivity of viscosity, leading to an increase in relative viscosity of nanocomposites with increasing temperature. [ABSTRACT FROM AUTHOR]

Recent interest in understanding the effect of strain rate on mechanical properties has motivated this study to develop a correlation between frequency domain dynamic mechanical analysis (DMA) results and elastic modulus values that are obtained from a separate set of elaborate tensile tests conducted over a wide range of strain rates. Using the time-temperature superposition principle and the integral relations of viscoelasticity, the DMA results are converted into a time-domain relaxation function in order to predict the strain-rate dependent modulus. The transformation technique is validated with experimental results for high density polyethylene (HDPE) resin and is found to be accurate over a wide range of strain rates. Cross correlation between DMA results and tensile test results over a wide range of strain rates can help in substantially reducing the requirement for tests that are needed to characterize the material behavior with respect to strain rates, temperature and loading frequency. [ABSTRACT FROM AUTHOR]

Based on nonlinear viscoelastic theory and coupled thermo-mechanical approach, heat build-up analysis of a rubber specimen under cyclic loading was performed through finite element analysis. The determination of the rubber material properties attracted particular attention. The thermo-mechanical coupling approach can be divided into three major parts: deformation, dissipation, and thermal modules. In the deformation module, uniaxial, planar, and equibiaxial tensile tests were used to determine the hyperelastic constitutive equation. In the dissipation module, an analytical method for calculating the energy dissipation rate was established. The dynamic properties were updated as a function of the strain and temperature based on the modified Kraus model. Creep effect and dynamic property softening effect on the viscoelastic properties were firstly considered. In the thermal module, the dependence of thermal parameters on temperature was established. A highly sophisticated equipment which can measure the temperature rise curves both on the surface and at the heart of the rubber specimen was used to verify the calculated results. The comparison between numerical results and experimental data shows that the proposed analysis method provides a satisfactory way to predict heat build-up for rubber compounds. [ABSTRACT FROM AUTHOR]

Hybrid particles of multiwall carbon nanotubes (CNTs) grafted onto silica gel were shown to provide a low viscosity processing route to forming electrically-conductive epoxy resin composites. Rheological studies showed the viscoelastic shear moduli of epoxy resin suspensions containing these particles (named SG6_3) remained within the same order of magnitude as that of the neat resin at ≤5 wt% loading (containing up to 1.65 wt% of grafted CNTs). Whereas, the addition of 1.65 wt% of discrete (non-grafted) CNTs into the same resin increased shear moduli by up to seven orders of magnitude. The complex viscosities of suspensions containing ≤5 wt% of SG6_3 remained essentially unchanged, whereas an increase of up to five orders of magnitude resulted upon the addition of 1.65 wt% of non–grafted CNTs. Whilst rheological studies showed SG6_3 did not form a percolated network at levels ≤5 wt%, addition of only 2 wt% (containing 0.66 wt% CNT) was found to form an electrically–conductive percolated network in an epoxy resin composite with conductivity ∼ 10 −4 S/m. [ABSTRACT FROM AUTHOR]

The connection between the structures and the linear viscoelasticity was revealed in this work for polymer nanocomposites containing agglomerated particles. A new image analysis method using the greyscale histogram of TEM images was suggested to determine the local volume fraction of nanoparticles inside an agglomerate, which represents the degree of agglomeration. It is found that the local volume fraction of nanoparticles is strongly related to the particle-polymer interaction. Such result was justified by a new rheological analysis based on the recently suggested modulus decomposition in the linear viscoelastic properties of polymer nanocomposites, from which the combined shift factor due to the hydrodynamic effect was used to quantify the local volume fraction of nanoparticles. Moreover, a two phase model was suggested to describe the linear viscoelasticity of such polymer nanocomposites. The success of this model in different polymer nanocomposites implies the rationality to regard the isolated agglomerate as a composite droplet in the polymer matrix. [ABSTRACT FROM AUTHOR]