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Due to the sensitive relations between aerogel physical, thermal, and mechanical properties, improving the polyimide aerogels performance requires understanding of their structure to properties relations, along with introducing a robust method to tailor aerogels properties to achieve the optimum performance. Given this background, this work extensively studies the structure to properties relations of pyromellitic dianhydride (PMDA) and biphenyl-tetracarboxylic acid dianhydride (BPDA) backbone polyimide aerogels at varying hybrid diamine fractions. Based on the results, combination of rigid and flexible diamine monomers at varying fractions successfully tailored the properties of the hybrid diamine polyimide aerogels. Also, physical, thermal, and mechanical properties of the two aerogel backbones presented a tendency to the hybrid diamine monomers fraction. However, the opposite trends of the two backbones suggested that, rather than the diamine or dianhydride monomers individually, their network structure and bonding nature would define the aerogel behavior. Furthermore, this work shows that the shrinkage and morphology are the two main parameters, those describe the aerogels properties. Therefore, tailoring these two parameters can provide aerogels with optimum performance. The fabricated aerogels presented wide range of properties with the maximum observed open cell content of 96.6% and compression modulus of 16.9 MPa along with the effective thermal conductivity and density as low as 32.5 mW/mK and 0.056 g/cm3, respectively.

Strong low-density polymer aerogels have received intensive attention as thermal or sound insulator, filtration media and sensors, etc. However, dimensional instability has been regarded as one of issues that limits the wide application of polymer aerogels in a harsh temperature. In present work, polyimide aerogels were reinforced with attapulgite (AT) nanorods, one of natural fibrillar minerals, through the strong hydrogen-bonding interaction. In hybrid aerogels, AT nanorods act as the rigid skeleton that supporting the framework of aerogels. The resultant hybrid aerogels exhibit improved mechanical properties. At the 80% strain, the compressive stresses for the aerogels containing 10 wt% AT are 100% as high as those of the pure polyimide aerogels. Interestingly, modulus of hybrid aerogels increases as the density decreases. For example, the compression Young's and specific modulus increase by 100% and 105% for the hybrid ODA-based aerogel compared to the pristine sample, meanwhile, the density decreases by 10%. Furthermore, the AT nanorods show strong supporting effects in maintaining the structural integrity of aerogels, which produce a significant effect on reducing the shrinkage of hybrid aerogels at high temperatures and retaining their excellent thermal insulation performance. The detailed investigation reveals that the AT is an effective and low-cost additive for preparing high performance polymer nanocomposites aerogels with improved mechanical property and thermal dimensional stability.

In this work, we study the tensile deformation of chicken muscle fibers and the temperature dependence of the tensile deformation of non-twisted nylon 6 lines and twisted nylon 6 (artificial muscles). Both the non-twisted nylon 6 lines and twisted nylon 6 are annealed at different temperatures of 150, 175, 190 and 200 °C. The chicken muscle fibers under tensile loading exhibit four deformation stages with the tensile load being a linear function of the elongation in each stage. The largest Young's modulus (the slope of the load versus the elongation curve) occurs at stage III. For the tensile deformation of the annealed non-twisted nylon 6 lines, the tensile load is proportional to the elongation. For the tensile deformation of the twisted nylon 6 (artificial muscles), there exist three stages with the tensile loading being a linear function of the elongation in stages I and III. The Young's modulus calculated from the load-elongation curves decreases with the increase of the testing temperature. For the testing conditions used in this work, the tensile deformation eventually leads to the fracture of both the chicken muscle fibers and the annealed non-twisted nylon 6 lines. The fracture stress of the annealed non-twisted nylon 6 lines decreases with the increase of the testing temperature.

Mechanical properties of ultra-fine polymeric nanofibers are highly size-dependent and the mechanisms for causality of the size dependency is still not quite clear yet. In this work, we investigate the origins of this size dependency at the atomistic level. By using molecular dynamics (MD) framework, two fabrication methods are utilized in this study to prepare non-drawn and hot-drawn fibers, in order to investigate the two distinguished mechanisms for nanofiber size dependency. One is rooted in the effect of surface chains which is intrinsic to low dimensional materials and the other is the chain alignment induced by drawing during fiber fabrication process. Our results show that the size dependency is not chiefly attributed to the effect of surface polymeric chains, but rather strongly relates to the chain alignment in nanofiber's microstructure. The atomistic study shows that non-drawn nanofibers have a dense core covered by a less dense shell layer, but interestingly, our investigations reveal that the core-shell structure itself will not result in the remarkable increase of modulus and strength when diameter of the fiber drops down. By testing the hot-drawn fibers, we found the size dependency mainly originates from the chain alignment. When fiber is formed by hot-drawing, higher drawing ratio leads to thinner fiber and more aligned chains in the microstructure. A three-fold increase of modulus and strength is observed when the chain orientation parameter of the hot-drawn nanofiber increases from 0.1491 to 0.3375 while the diameter of the fiber drops from 19 to 10 nm. By examining the energy evolution associated with the bond lengths, bond angles and dihedral angles at the atomistic scale, our results show that the dihedral angles play a vital role in the size-effect. Our results also show the mechanical response of fibers are affected by changing the temperature, additionally, the orientation-induced crystallization is monitored on hot-drawn fibers at 300 K, which makes the fibers become stiffer but less ductile.

This study reports the effects of post-draw processing on the structural and functional properties of electrospun poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanofibers. Previous studies have independently demonstrated the potential of the electrospinning process or the post-drawing process to enhance the piezoelectric properties of PVDF nanofiber, and thin film structures respectively. However, post drawing electrospun nanofibers has been difficult to achieve. To overcome this limitation, a parallel automated track device was implemented to post-draw thousands of individual PVDF-HFP electrospun nanofibers per minute. Relationships were established showing that yield strength, Young's modulus, and piezoelectric output were enhanced with increasing draw ratio. The normalized voltage output of PVDF-HFP nanofibers post-drawn to a draw ratio of 3 increased by four-fold compared to undrawn control. We hypothesize that polymer chain and crystal alignment in the direction of the fiber axis, which were increased by post drawing, resulted in enhanced voltage output of PVDF-HFP nanofibers under mechanical stimulation.

Based on the strategy of dynamic metal-coordination for improving mechanical properties, a further ideal was taken to prepare the tough hydrogels with the combination of self-crosslinking monomer and metal-coordination complexes. Herein, a series of hydrogels of poly(acrylamide-co-acrylic acid-co- N-hydroxymethyl acrylamide) (P(AM-co-AAc-co-NMAM)) consisting the chemical crosslinking induced by NMAM units and the physical crosslinking derived from the coordination complexes of carboxyl-Fe3+ were prepared. The molecular structure was investigated with the ATR-FTIR, Raman and UV-vis spectra. These hydrogels with different water content of 57–93% possess good mechanical performances. The optimal hydrogels possess high tensile strength (8.56 MPa), prominent modulus (15.5 MPa), remarkable toughness (37.85 MJ/m3) and superb tearing energy (7062 J/m2). The tough hydrogels also display excellent self-recovery (95% toughness recovery within 50 min), pH-triggered healing, shape memory and plasticity abilities. These hydrogels having high strength and toughness may broaden range of potential applications in load-bearing soft actuators, flexible electronics, etc.

Light-activated self-healing (LSH) materials have received extensive attentions recently due to their environmentally friendly repairing methods and excellent healing performance. However, the current reported LSH materials have poor mechanical strength and elasticity, which limit their applications. Herein, a novel series of coumarin-based polyurethanes with enhanced mechanical properties and elasticity have been synthesized which possessed a dramatic breaking stress of 27 MPa with a high breaking strain of 890%. Besides, the energy dissipation efficiency in continuous stretch cycles of the as-prepared polyurethanes varied negligibly, which proves the as-prepared LSHs have excellent elasticity. Meanwhile, after cutoff, the polymers exhibited excellent self-healing performance by UV irradiation for 40 min and the best healing efficiency of strain, stress and Young's modulus were 84%, 92%, 94%, respectively. The prepared self-healing polyurethanes showed great potential applications in the fields of smart materials such as self-healing coatings, electrochemical sensors and devices, electronic skin, etc.

High orientation structure in die-drawing products ensures excellent tensile strength and stiffness along the drawing direction but extremely poor toughness and ductility. Poly (ethylene-co-octene) (POE) was used to toughen orientated polypropylene (PP) prepared by die-drawing. However, the enhancement of toughness is at the expense of stiffness of matrix. To efficiently reserve the stiffness of PP matrix, high density polyethylene (HDPE) as auxiliary modifier, together with network structure through crosslinking and highly orientated structure through die-drawing process were applied. Specially, two different crosslinking strategies namely one-step and two-step crosslinking were compared. The results revealed that the combination of HDPE, POE and crosslinking reaction could realize simultaneous toughening and reinforcement of die-drawing PP blends. The tensile strength, modulus and impact strength of two-step crosslinking PP/POE/HDPE were 88.0 MPa, 1.7 GPa and 42.9 kJ/m2 respectively after die-drawing, nearly three times in strength and even ten times higher in impact strength than raw PP.

As the most promising bio-derived and biodegradable polymers with tremendous application potential, the practical application of poly( l -lactide) (PLLA) has been severely restricted by its inherent brittleness. Unfortunately, blending with various rubbers can substantially enhance the toughness of PLLA, but usually causes a significant deterioration in mechanical strength and modulus. In this work, taking novel PLLA/core-shell rubber (CSR) blends as an example, we describe a facile and robust strategy to obtain good stiffness-toughness balance by promoting the networking of CSR particles within PLLA matrix with the aid of stereocomplex (SC) crystallites. To do this, small amounts (5–20 wt%) of poly( d -lactide) (PDLA) were incorporated into the blends through simple melt-blending. The results demonstrate that the CSR particles can be uniformly dispersed in PLLA matrix due to the good interfacial compatibility, but the stereocomplex (SC) crystallites formed between the incorporated PDLA and PLLA matrix chains induces the organization of the CSR particles into a network-like structure in the matrix. Compared with the well-dispersed structure, the network-like structure can endow the PLLA/CSR blends with dramatically enhanced impact toughness, without compromising their mechanical strength and modulus, indicating a marvelous balance between the stiffness and toughness. Furthermore, the elongation at break of the blends is also enhanced greatly. These interesting findings suggest that the formation of SC crystallites could provide a promising avenue for fabricating CSR particles toughened PLLA blends with balanced stiffness-toughness.

Solid-state drawing to high extension ratios of melt-extruded VGCF-iPP nanocomposite filaments results in remarkable increases in modulus and strength, which mask any contribution of nano-reinforcement or inter-particle confinement observed previously. Based on X-ray diffraction and thermal analyses it is shown that the drawing process involves crumpling of the original 'shish-kebab' structure, accompanied by lateral shrinkage of the 'kebabs', disappearance and reorientation of daughter lamellae and significant increases in the degree of crystallinity by solid-state recrystallization. The effects of the initial diameter of the as-extruded filaments and those of the nanofibers and the 'shish-kebab' structure are dominated by the drawing process, which produces a highly aligned fibrillar structure with superior mechanical properties. A cartoon model of the effect of solid-state drawing of extruded iPP and nanocomposite filaments on the transition from 'shish-kebab' to fibril morphology is presented.

To strengthen the interfacial adhesion between aramid fiber and rubber, inspired by mussel adhesion protein and mimicking adhesive molecule, a new environmentally friendly dipping system was developed. Specifically, water-soluble pyrogallic acid and polyethyleneimine are reacted to form a network structure, which is then mixed with rubber latex to prepare the dipping solution. We use blocked isocyanate and epoxy aqueous solution to activate the fiber, and then conduct the dipping treatment. The H pull-out force between modified aramid fiber and rubber reached 147 N, achieving the level of traditional resorcinol-formaldehyde-latex dipping system (154N), while avoiding the utilization of harmful resorcinol and formaldehyde. In addition, we employed atomic force microscope for the first time to quantitatively characterize the modulus of the transition layer between the fiber and rubber. Results from this research provide new insights into the interfacial design of aramid fiber/rubber composites and will facilitate the development of aramid fiber/rubber composites.

An appropriate processing protocol was proposed to improve the rubber elasticity of a thermoplastic polyester elastomer (TPEE). Rheology measurements at the processing temperature and thermal measurements determined the critical temperature at which small numbers of crystals exist. It was found from dynamic mechanical analyses, wide- and small-angle X-ray measurements, and electron microscope observations that cooling from the critical temperature markedly promoted the crystallization of hard segments. Moreover, tensile tests revealed that the modulus and yield stress increased. Most importantly, compared with films obtained by cooling from high temperatures, the residual strain at a fixed stress was low—i.e., the rubber elasticity was improved owing to its well-developed crystalline structure. These effects were accelerated by a more optimized process—i.e., subsequent heating at slightly below the critical temperature. The technique is noteworthy because a TPEE with high modulus generally shows poor elasticity.

From experimental size exclusion chromatography/infrared detection and differential scanning calorimetry data coupled with random walk statistics, the primary structure parameter (PSP3*) and the average number of crystals per chain (ANCC) were estimated and used to correlate microstructure effects of various high density polyethylenes to their measured natural draw ratio (NDR) and strain hardening modulus (SHM). Results suggest that while both the NDR and SHM reflect the extent of connectivity between crystalline lamella, the SHM reflects the strength of those bridging entanglements and tie molecules. Based on this view and supporting stress strain curves, different but related mechanisms for SHM were suggested for homopolymers versus copolymers. Moreover, a delineation between the effects of branch type was suggested. A “crystal tie mesh (CTM)” model that views the system to be a type of network formed by “crosslinking” multiple bridging and entangled tie chains through common crystalline lamella was proposed. In this semi-empirical model, rather than a rigorous development, basic scaling arguments using the PSP3* and ANCC parameters along with branch length factors (fB) were used. A linear correlation (R2 = 0.9615) was found between the SHM and the structural relationship, fB (PSP3*3/ANCC), for samples used in this study which included monomodal and bimodal resins made using various catalyst types.

Epoxy composites with excellent toughness and strength have been successfully fabricated by direct incorporation of graphite fluoride (GrF) without using any solvent or surfactant modification. Owing to the strong interfacial interactions, the glass transition temperature of GrF/epoxy composites exhibits a significant improvement as observed by dynamic mechanical analysis. Mechanical characterizations on GrF/epoxy composites suggest that the optimal increase of tensile strength and Young's modulus is achieved at a low filler concentration of 0.5 wt%, which is attributed to the favorable level of the GrF dispersion in the epoxy matrix. A maximum toughening effect is obtained at the concentration of 1 wt% GrF in the epoxy matrix with a 1.5-fold improvement in fracture toughness. The generation of crack pinning and crack deflection in epoxy due to the existence of GrF platelets is proposed to be the main toughening mechanism. Besides, microcracks resulted from pulling out, layer breakage and interfacial debonding of GrF platelets also contribute to the improvement in the fracture toughness of GrF/epoxy composites.

The rapidly increased thermal load as a result of the continuous down-scaling of electronic devices and increase in power densities requires faster and more effective heat dissipation. This work has been carried out to study the effect of inclusion of boron nitride nanosheets (BNNSs) and cellulose nanocrystals (CNCs) on the thermal and mechanical properties of a polyvinyl alcohol (PVA) matrix. Both BNNSs and CNCs improved the thermal conductivity, tensile strength and modulus of the polymer matrix at a low content (

The aromatic epoxy isomers triglycidyl p-aminophenol and triglycidyl m-aminophenol were cured with two aromatic diamine isomers 4,4′ diaminodiphenyl sulphone and 3,3′ diaminodiphenyl sulphone, creating four variations of epoxy resin. Dynamic and static mechanical analyses were used to understand the influence of chemical and network structure on the thermal, volumetric and mechanical properties of the epoxy resin. Fracture toughness increases are observed for networks containing meta substituted phenylene ring amines compared to the para equivalents, however no difference is noticed when the meta substituted phenylene ring epoxy is used. Use of meta substituted phenylene rings increases glassy modulus, yield stress, density and strain to failure. Correspondingly, decreases are seen in the glass transition temperature, intensity of the beta transition and the rubbery modulus. The results are entirely consistent with internal antiplasticisation caused by the presence of the meta substituted phenylene rings.

Strong and tough hydrogels, important for several materials and bio-applications, typically consist of nanocomposites and double networks. There is the need to create hydrogels that exhibit tunable mechanical properties on demand without compromising other functional properties. This work demonstrates the rapid thermal stiffening of a hydrophobic association hydrogel while maintaining its optical quality and chemical composition. Up to 100-fold increase in modulus of the methacrylate-based hydrogel was achieved by an increase in temperature from 25 °C to 50 °C. Based on various characterizations, we proposed that the thermal stiffening is related to the polymer conformational changes and the ensuing increase in inter-chain hydrophobic associations at the expense of intra-chain associations. The temperature above which thermal stiffening occurs can be tuned with the polymer content in the hydrogel. Furthermore, hydrogels containing a lower mole fraction of the hydrophobic groups exhibit unusual “gel-sol-gel” transitions with temperature increase.

We used atomic force microscopy (AFM) nanomechanical mapping to image the surface of a semicrystalline polymer, specifically polymorphic isotactic poly (1-butene) (iPB), to determine the mechanical properties of a surface layer, and compared this to the properties in the bulk. The measured Young's modulus of the surface layer as a function of annealing time at room temperature was found to be higher than that of the bulk, indicating a more rapid transformation from form II to I polymorphs and an enhanced movement of polymer chain segments at the surface relative to the bulk for this polymer. Plate-like crystals were found at the surface and, as the annealing temperature increased from 70 to 110 °C, evidence of a surface layer was found that increased in thickness from ∼30 to ∼130 nm, respectively. After removing this layer, the morphology found in the bulk was markedly different. We also used AFM nanothermal analysis to determine the local melting point (Tm) and found, that the melting point, Tm, of the crystals at the surface was higher than that of the bulk, in keeping with the modulus measurements. The nanomechanical and nanothermal properties and the morphology at the surface suggest an enhanced movement of polymer chain segments at the surface and therefore, an enhanced crystallization at the surface.

In order for super absorbent polymer (SAP) to be used in an application undergoing mechanical stress, it is necessary to regulate its shape and mechanical properties, which have been rarely studies. Here, spherical SAPs were prepared through inverse-suspension polymerization of partially neutralized acrylic acid monomers with ethylene glycol diacrylate (EGDA) crosslinkers. The surface region of SAP spheres was additionally crosslinked with ethylene glycol diglycidyl ether (EGDE) to improve the mechanical properties, producing a core-shell structure characterized by lightly crosslinked core and more densely crosslinked shell. Mechanical responses of the cross-linked SAP to global compression were obtained from experiments using a custom-made indentation device. The local mechanical properties of crosslinked surface were measured by atomic force microscopy. While the global responses of SAP were similar independent of surface crosslinking, the modulus of crosslinked surface increased as concentration of EGDE. In our experimental conditions, the surface modulus of surface crosslinked SAP was increased 2.7 folds, compared to that of SAP without surface crosslinking. The structure of SAP spheres with and without surface crosslinking was also visualized, and thickness of crosslinked surface was measured with fluorescence microscopy. We found that the thickness of crosslinked surface increased with increasing EGDE concentration but became saturated. Furthermore, a computational model was developed for core-shell SAP spheres using the measured mechanical properties, and was utilized to predict the thickness of crosslinked surface. Finally, we propose an analytical diffusion model that describes the diffusion and surface crosslinking reaction to elucidate the mechanism over which the mechanical and diffusion properties of SAP sphere are determined.

Effects of hydrogen bonding on the micro-structure development and mechanical properties of a bulk-polymerized Polyurea are systematically investigated from the perspective of temperatures. A two-stage temperature dependence of bidentate hydrogen bonding among hard domains, namely, slight dissociation from 85 °C to 165 °C and dramatic destruction above 165 °C, are revealed by Fourier transform infrared spectroscopy (FTIR). Morphology developments and mechanical properties also exhibit similar stages in consistent with hydrogen bonding evolution. At the first stage, even slight dissociation of hydrogen bonding can lead to the loss of long-range connectivity of hard domains, resulting in the decrease of "energy loss coefficient". At the second stage, dramatic destruction of hydrogen bonding above 165 °C facilitates the "coarsening" process, which undoubtedly poses the decline of physical cross-link density, leading to the steep decline of Young's modulus and rubbery plateau modulus in dynamic mechanical analysis (DMA). Multi-scale characterizations are employed during the investigation.

The evolution of crystalline domain orientation vs. mechanical properties of aramid fibers under mechanical loading was investigated for initial crystalline domain orientations between 16.7° and 9.7°. The latter resulted in a broad range of longitudinal moduli between 66 GPa and 119 GPa but tensile strengths in the narrow range of 3.5–4.0 GPa. Mechanical conditioning up to 90% of the fiber tensile strength increased the initial modulus converging to 100 GPa which corresponds to a stable crystalline domain orientation of 11.6°, while the unloading modulus at stresses near the tensile strength converged to ∼165 GPa which approaches the theoretical modulus of 220 GPa. On the contrary, the tensile strength remained unchanged with increasing crystalline domain orientation, and was shown to be independent of the fiber gauge length, thus implying that failure is not due to flaws obeying weakest link statistics. Instead, short gauge length tests (200 μm) showed failure initiation at the fiber skin followed by crack propagation at the skin-core interface, resulting in extrusion of the fiber core, which points to the skin-core interface as an important factor limiting the tensile strength of this class of fibers.

Thermoplastic polyurethane elastomers (PUEs) consisting of biodegradable poly (e-caprolactone) (PCL) and low toxic aliphatic diisocyanates are of great potential in biomedical applications, but the inferior mechanical strength and elastic recovery at large strain have limited their practical utilization. Inspired by the outstanding thermal and mechanical properties of stereocomplex (SC) PLA, a thermoplastic PUE that contains SC crystallites (SC-PU) was prepared via racemically blending the PLA stereoisomers-based polyurethanes. The formation of SC crystallites in SC-PU was confirmed by WAXD and ATR-FTIR techniques. The effects of stereocomplexation on its thermal, morphological and mechanical properties were investigated. The results show that the SC crystalline domains, acting as ‘additional reinforcing phases’, endow the improved performance of SC-PU in tensile strength, Young's modulus, elastic recovery, and other properties. This study may provide a new method to improve the inferior mechanical properties of biodegradable PUEs based on PCL and aliphatic diisocyanates, which can contribute to the expansion of their applications in widespread fields.

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. ISSN:0032-3861 ISSN:1873-2291

Polyacrylonitrile (PAN) nanofibers have long been the subject of studies as precursor for high performance materials such as carbon nanofibers. Here, we present the first effort to study the mechanical properties of PAN nanofibers fabricated via near field electrospinning, in which bending instability was completely suppressed. The method allowed for the collection of continuous single-strand PAN nanofibers with aspect ratios exceeding 106 on a spool (rotating target). By controlling the electrospinning parameters, such as solution concentration, voltage and distance, we demonstrated that the morphology and diameter of the PAN nanofiber were well controlled. We also realized that slight changes in the distance can significantly change the shape of the cross section of the fibers, from a circular cross section to oval shaped (ribbons), which was explained in terms of solvent residues in the jet and momentum transfer between the fibers and the target, as the fibers reach the target. Our studies on individual PAN nanofibers revealed a strong mechanical size effect, in which reducing the diameter of the nanofibers from ∼290 nm to ∼200 nm, led to a large increase in modulus to as high as ∼9 GPa, among the highest in as-electrospun PAN nanofibers. The strong size effect was attributed to a loss in chain alignment in electrospun nanofibers facilitated by the plasticizing effect of residual solvent. In comparison to conventional electrospinning, the NFES led to a significantly narrower diameter distribution. The demonstrated size-dependent morphology, mechanical properties of NFES PAN nanofibers provides a solid foundation for fabricating polymer nanofiber with regularly patterning and controllable properties through NFES.

Although many effects to design smart self-healing sustainable hydrogels with sensitivity to multi-environmental stimuli have been documented, a facile fabrication of stimuli-responsive hydrogels with improved mechanical properties remains challenging. Here, smart self-healing sustainable hydrogels containing soy protein polymer (SPP) and/or graphene nanoplatelets (GNP) were fabricated using “slime” chemistry. The composites hydrogels exhibited impressive self-healing behavior and high ductility under mechanical stimuli, pH-modulation, and thermo sensitivity. An unexpected synergistic reinforcing effect was achieved via combination of SPP and GNP (ternary composite); while the usage of mere SPP and GNP (binary composite) even weakened the gel modulus. Surprisingly improved resistance to gel flowability caused by external heat stimuli was also achieved in the same system. The reinforcing mechanism was attributed to the interactions between SPP and GNP to restore the crosslinking structures. This character also offered merits to the higher viscosity of ternary composite system and thereby resulted in improved heat resistance.

Thermoplastic polyurethane (TPU) has attracted significant attention in various fields owing to its elasticity, abrasion resistance, and processability. However, high-molecular-weight TPUs need to be synthesized to achieve the desired mechanical properties, which generally require the tedious optimization of the synthetic process. Herein, we report a post-crosslinking process by which different types of TPUs with different levels of mechanical properties can be obtained. TPUs containing azide functional groups on the side chain (ATPU) are synthesized with different ratio of azide repeating units to non-azide repeating units, and post-crosslinking is performed via a click reaction using different types (soft and hard) and amounts of crosslinkers (di-alkyne) to adjust the mechanical properties of low-molecular-weight ATPU. The introduction of a soft and hard crosslinker respectively increases the strain and initial modulus. The post-crosslinking method will allow the production of TPUs with different levels of mechanical properties (strain or modulus) using a single synthetic process.

High dielectric constant and low modulus of thermoplastic polyurethane elastomers (PUEs) are required for various applications in actuators, sensors and energy harvestings. In this work, we controlled the microphase separated morphology of PUEs by regulation of the content of hydroxyl-terminated four-armed polycaprolactone (4amPCL) chain extender to achieve J-shaped stress-strain curve with low modulus and high electromechanical properties. Our results showed that the hydrogen bonding between hard segments (HSs) and crystallization of soft segment (SS) are significantly impeded by long-chain branched structure when added lower molar fraction ( 20% of chain extenders) of 4amPCL as chain extender. The PUE with 29% of 4amPCL (PU-4amPCL-0.29) showed lower modulus (1.5 MPa), higher tensile strength (17.5 MPa) at 1048% of elongation, and 3.9% of actuation strain at 20 kV/mm, and higher contribution (48.1%) of Maxwell stress on actuation strain at 20 kV/mm. We found that the microphase separation of branched long-chains induced semi-interpenetration polymer network like structure of PUEs played an important role in exhibiting J-shaped stress-strain behavior. Our results can provide new insight on fabrication of high electromechanical performance of dielectric polyurethane elastomers.

Studies of polypropylene with different degrees of isotacticity have shown a way of the rational design of material with predetermined mechanical properties starting from the synthesis stage already – controlled introducement of stereodefects will allow the smooth adjustment of the Young's modulus and elasticity in the range from plastic to elastomer materials. It was also revealed that modern theoretical models of the elasticity can be successfully applied not only for the description of the mechanical behaviour of polymers, but also for better understanding of the mechanism of elasticity in them. While in the low crystalline materials deformation has Gaussian nature, in the materials of the intermediate crystallinity (30–40%) percolation takes place, and the cross-linking network becomes harder, manifesting the switch to the thermotropic behaviour of the material. Simultaneously the divide between cross- and slip-links becomes substantial, as an extensibility grows sharply.

Understanding the relationship between structure and dielectric property of dielectric elastomers (DEs) is very important in their practical applications. In this work, we fabricated polyurethane elastomers (PUEs) with high dielectric properties using hydroxyl-terminated butadiene-acrylonitrile copolymer (HTBN) as soft segment and hexamethylene diisocyanate (HDI) and butanediol (BDO) as hard segment (HS). We also investigated the effect of HS content on the structure, mechanical, dielectric and electromechanical properties of HTBN-based PUEs. The HTBN-based PUEs were achieved larger than 8.7 of dielectric constant and lower than 0.02 of dissipation factor at 1 KHz. The dielectric constant and dissipation factor of PUEs were slightly decreased with increase of HS segment, whereas the breakdown strength and Young's modulus of PUEs were significantly increased. The actuated area strain of PUEs also showed decreased with increase of HS content under the same electric field. Our results indicated that the adjusting of HD structure via HS content is one of effective ways to optimize the dielectric and mechanical properties of PUEs.

The mechanical properties and shape memory behavior of compounds formed from a commercial poly (ethylene- co -methacrylic acid) ionomer and zinc stearate (ZnSt) are described. For compounds containing less than 10 wt% ZnSt, the components were miscible. Although the components were only partially miscible above 10 wt% ZnSt, the compounds were compatible, as judged by their macroscopic homogeneity and the improvements of the mechanical properties. The addition of 30 wt% ZnSt produced ductile materials and improved the room temperature tensile modulus by ∼50% and the stress at yield by ∼30%. The strain at yield, however, decreased by ∼45%. Compounds with 40 or 50 wt% ZnSt improved the modulus of the ionomer by nearly 100%, but the compounds were relatively brittle and broke before yielding. Shape memory behavior was demonstrated with a compound containing 50 wt% ZnSt where nanophase-separated ionic aggregates acted as a permanent network and a phase-separated ZnSt crystalline phase was a reversible, temporary network. The shape memory behavior was repeatable over five consecutive shape memory cycles with shape fixity and shape recovery values of ∼92% and ∼97%, respectively.

This study demonstrates the postpreparation easy tuning of the Young's modulus of elastomers by using polyesters bearing thermal and photo cross-linkable groups. In the first cross-linking step, only thermoreactive groups form cross-links, resulting in a highly deformable soft elastomer. In the second step, photoreactive groups are cross-linked by UV irradiation, and controlling the UV irradiation time can tune the cross-link density, resulting in modulus tuning over two orders of magnitude at room temperature. The use of photo reaction enables precise modulus patterning using photomasks with a series of slits. In horizontally patterned samples, the dually cross-linked stiff section shows negligible deformation, whereas large elongation is observed selectively in the single cross-liked soft sections. In vertically patterned samples, interestingly, crack propagation and complete fracture are significantly suppressed, compared with the fully dually cross-linked sample. The present data can thus provide important insight for developing new methodologies for controlling the tensile and fracture behavior of elastomers.

In this work, a triple-shape memory polymer (SMP) was prepared by chemically crosslinking melting blending from ethylene-vinyl acetate copolymer (EVA) and poly(propylene carbonate) (PPC). The EVA/PPC blends combine the glass transition of PPC (~25.6 °C) and melting transition of EVA (~72.2 °C) to function as the shape memory mechanisms. The triple-shape memory effect (triple-SME) and its dependence on composite ratios and micro-phase morphologies were systematically investigated. The results show that EVA/PPC blends with different composite ratios and morphologies all exhibit excellent triple-SMEs, with shape fixing ratio over 90% and shape recovery ratio close to 90%. The key to realize such triple SME is the certain matching of the modulus of the two components. Specifically, the modulus of glassy PPC is much higher than the modulus of semi-crystalline EVA, whereas the modulus of viscoelastic PPC is lower than the modulus of semi-crystalline and elastic EVA. This work provides a comparatively simple and efficient method to design and prepare high-performance triple-SMP.

Fully cured epoxy resins are typically brittle materials but according to recent research, cured epoxy fibers exhibit a singular mechanical behavior, including yielding followed by large deformation, and very high strength, toughness, and modulus. These properties appear to intensify as the fiber diameter decreases. The microstructural origin of this unusual behavior has not been fully determined. Here we use confocal polarized Raman spectroscopy to monitor the apparent molecular reorientation induced by plastic deformation of epoxy fibers, both qualitatively and quantitatively. Based on these and previous X-ray diffraction measurements, a likely molecular explanation for the extreme mechanical behavior of micro-sized epoxy fibers is proposed.

Low efficiency in the fabrication of ultra-high molecular weight polyethylene (UHMWPE) with excellent mechanical properties has limited its wide application, owing to its ultra-high melt viscosity. Present processing methods including compression molding (CM) and high velocity compaction (HVC) can't be used for manufacturing UHMWPE products possessing outstanding mechanical strength, toughness and wear resistance simultaneously. A novel molding process of pulse vibration molding (PVM) first utilizes application of pulsed vibration pressure to UHMWPE powders pre-heated close to melting temperature. This method effectively promotes consolidation at particle interfaces at low molding temperatures, while melt flow in a specific direction during the PVM process accelerates a further fusion at particle interfaces, thus obtaining UHMWPE products with orientation induced by melt flow during the PVM process (i.e. OPVM). The crystallinity and lamellae thickness of PVM-UHMWPE molded at 170 °C for 30 min (PVM-170-30) were significantly greater than those of CM-UHMWPE prepared at 210 °C for 60 min (CM-210-60), consequently yield strength, Young's modulus, and wear resistance of PVM-170-30 were much higher. Inside OPVM-UHMWPE molded at 170 °C for 30 min (OPVM-170-30), orientation of molecular chains along the flow direction generated by melt flow resulted in high crystallinity and formation of numerous shish-kebab structures at particle interfaces. Compared with those of CM-210-60, yield strength, Young's modulus, elongation at break and work-to-failure of OPVM-170-30 increased from 21.5 ± 0.4 MPa, 292 ± 3 MPa, 505 ± 20% and 114 ± 3 kJ/m2 to 23.0 ± 0.2 MPa, 334 ± 8 MPa, 571 ± 17% and 132 ± 6 kJ/m2, and its wear rates and wear index decreased from 3.8 ± 0.3% and 184 ± 15 to 2.8 ± 0.2% and 142 ± 6, respectively. The outstanding performance of OPVM-170-30 was originated from high crystallinity and formation of self-reinforcing shish-kebab superstructures at particle interfaces. The present results suggested OPVM-170-30 has high potential in applications requiring high wear resistance, mechanical strength, and fracture toughness, such as artificial joint.

Owing to the structural resemblance of synthetic hydrogels to biological tissues, numerous self-healing wet materials have recently emerged, based on either chemical or physical motifs. With a plethora of novel synthetic hydrogels, a reliable characterisation strategy to assess self-healing is necessary. Herein, we therefore propose rheological criteria to distinguish self-healing hydrogels by comparing two types of poly (vinyl alcohol) hydrogels prepared via freezing/thawing (FX) or borax addition (BX). Non-self-healing FX and self-healing BX hydrogels exhibit incompatible features for three rheological characteristics: 1) Bingham behaviour without a lower Newtonian flow region (LNFR) (FX) versus pseudoplastic behaviour with a LNFR (BX), 2) infinite chain flow relaxation time (τf) versus finite τf on a reasonable time scale, and 3) drastic gel-sol transition with heating versus negligible transition. The plotting of modulus master curves confirms the terminal flow behaviour of the self-healing BX hydrogels, in contrast to the rubbery nature of the FX gels. The flowability of the self-healing hydrogels is ascribed to the pseudo-state of the gel network, due to the dynamic nature of the borate-diol bonds. Thus, the rheological criteria provide a sound theoretical background for self-healing material characterisations.

In this work, various shape memory polyacrylamide/gelatin (PAM-Gels) hydrogels with interpenetrating double networks were synthesized via a facile one-pot method. Taking advantage of the thermal reversible property of physical gelatin network and the constant of chemical crosslinking polyacrylamide network, the prepared PAM-Gels hydrogels showed shape memory function under warm water. Moreover, the shape memory, mechanical, drug release, and swelling properties of the prepared PAM-Gels hydrogels could be tuned by altering the gelatin component content. Notably, the PAM-Gel5 hydrogel containing 5% w/v of gelatin exhibited the most effective enhancements in physicochemical properties, and the highest shape fixity and shape recovery rate were 78.2% and 93.5%, respectively. Meanwhile, it could retain the integrity of its shape even with high water uptake. More importantly, the PAM-Gel5 hydrogel possessed well balanced mechanical properties (e.g. tensile strength was 0.57 MPa and elongation at break was 1556% in tensile test; compression stress was 0.06 MPa and Young's modulus was 0.38 MPa in compression test) among the as-prepared PAM-Gels hydrogels, due to its uniform pore size distribution and smaller pore size. Furthermore, the drug release experiment revealed that the PAM-Gel5 hydrogel showed sustained release characteristics, and the majority of loaded model drug could be released within 24 h under physiological conditions. Additionally, cytotoxicity test suggested the representative PAM-Gel5 hydrogel exhibited good in vitro cytocompatibility. This work provides a viable avenue for developing bio-compatible shape memory hydrogel with acceptable mechanical and drug release properties applied as wound dressing.

The sandwich-structured fluorinated graphene (FGN)/poly(vinylidene fluoride-hexafluoro- propylene) [P(VDF-HFP)] composites with various FGN constituents were fabricated via a layer-by-layer solution casting technique, and their dielectric relaxation phenomenons of the trilayered composites were studied in a systematic way by dielectric relaxation spectroscopy (DRS) at various temperatures (25°C–200 °C) in the range of 20 Hz to 5 M Hz. The variations in dielectric parameters ( e ′ and e " ) with frequency at each studied temperature show that e ′ and e " increase with the rise of temperature and e " significantly decreases with the addition of FGN. Moreover, the asymmetric shapes of electric modulus spectra (M″) and Cole-Cole plots exhibit the relaxation process of non-Debye, especially in trilayered FGN/P(VDF-HFP) composites. AC conductivity is irrelevant to frequency and temperature under low frequency-high temperature, but frequency sensitive under high frequency region. Furthermore, the two activation energies (Eτ and Eσ) gradually increased with the addition of FGN, showing that the mobility of charge carriers is suppressed, thus they are unable to overcome the relaxing and conducing barrier.

Oxidative stabilization is the critical step to maintain the fibrous morphology of pitch fiber and affects mechanical properties of resultant carbon fiber, but the detailed oxidation mechanism is still unclear because of the complex polycyclic aromatic hydrocarbons composition in pitch fiber. In here, pitch fiber was obtained from coal tar pitch followed by air-blowing and spinning. The chemical structure character of corresponding fibers before and after oxidative stabilization at different temperatures (260–320 °C) and durations (2–12 h) were analyzed. The results indicated that aromatic nucleus in spinnable pitch was gradually increased, while aliphatic chains were gradually shorten with prolongation of air-blowing time. The spinnable pitch obtained by air-blowing at 330 °C for 4 h showed excellent spinning performance, and the as-spun pitch fiber displayed a higher oxidative stabilization efficiency because the lower oxygen content was introduced and more C-O-C groups were formed in the resultant oxidative stabilized pitch fiber. The obtained pitch-based carbon fiber, after carbonizing at 1000 °C, showed excellent mechanical properties with tensile strength of 1068.7 ± 100.0 MPa and Young's modulus of 52.5 ± 9.8 GPa. This work provided a facile approach to improve and optimize the oxidative stabilization efficiency for pitch carbon fiber preparation.

Employing computer simulation to accurately and quantitatively predict the mechanical properties of polymeric materials is a long-standing challenge. Herein, we consider two methods to simulate the mechanical properties based on a fully atomistic model of crosslinked styrene-butadiene rubber (SBR) network via molecular dynamics simulation. In the first method (stepwise deformation and relaxation), by keeping the tensile velocity constant and regulating the simulation time that each deformation period takes, sufficient relaxation occurs in the consecutive deformation, which enables each deformation period followed by a corresponding relaxation period. The engineering stress-strain curve finally obtained by this stepwise deformation and relaxation method exhibits similar features of elastomers compared with the experimental results. The second method divides the whole deformation process into several discontinuous parts. Sufficient relaxation also occurs in several independent SBR systems at the specific strain and the relaxed stress plateau is indicated to represent the stress that SBR system exhibits at specific strain after allowing the system to relax. Compared with the traditional uniaxial tensile method (continuous deformation in constant strain rate) and the first method we adopt, the second method could also characterize the mechanical behavior of elastomers and improve the computational capability on the level of a large-scale system. Compared with the excessively large values obtained by traditional method, the elastic modulus, the modulus at 100 %, 300 % and 500 % of the full atomistic SBR are calculated as 38.5 MPa, 12.0 MPa, 25.1 MPa and 83.1 MPa, respectively, which exhibits more realistic mechanical behavior and merely exceeds around one order of magnitude than the experimental results. These methods are believed to serve as a good basis for further accurately simulating and predicting the mechanical properties of elastomers.

We found that the birefringence of miscible blends of dissimilar polycarbonates was lower than those of the neat component polymers though the sign of the birefringence was same. The modulus and birefringence of the blends decreased more gently in the glass transition region but more steeply in the rubbery plateau region than those of the neat component polymers. The modulus and birefringence relaxation curve fitting with the KWW and modified Rouse equations revealed wider distortion relaxation and orientation distributions in the blends than in the neat component polymers. The wide distortion relaxation might be attributed to the dynamic asymmetry of the blends. The orientation relaxed faster in the blend than in the component polymers, indicating accelerated orientation relaxation, which could be explained by assuming cooperative relaxation of the component polymers strongly affected by fast relaxation component of the low-Tg one due to the dynamic asymmetry with a large glass transition temperature difference between the component polymers. Owing to the accelerated relaxation, the elongated blends exhibited lower birefringence than the component polymers.

Chemical mixture (C-M) of low density polyethylene (LDPE) and electronic waste made of acrylonitrile butadiene styrene (ABS) have been prepared via one step reactive extrusion. The morphology of prepared crosslinked blends has been investigated. Single phase morphology has been found in C-M as opposed to two phase morphology in physical blend. The extent of crosslinking has been observed by determining gel content through solvent extraction. The structural changes have been confirmed by FTIR and solid state 13C NMR spectroscopy. The C-M have been tested using DSC and TGA and have found to be thermally stable. The chemical mixture has showed excellent mechanical properties like increment in Young's modulus, ultimate tensile strength. The flexural properties have also been investigated and C-M has found to have enhanced flexural modulus and flexural stress. For C-M to be used in practical heat exposure conditions heat distortion temperature tests have been performed and C-M has shown better properties than pure LDPE and physical blend.

Strong, tough, transparent, and highly heat-resistant nanocomposites made of nanodiamond (ND) and poly (methyl methacrylate) (PMMA) were prepared via a simple water-based process. In addition, their structure and properties were investigated. The ND was dispersed in PMMA, and the high optical transparency of the acrylic glass polymer persisted. The nanocomposites exhibited a remarkable increase in mechanical and thermal properties at low ND contents. Young's modulus, tensile strength, and toughness were substantially increased by the incorporation of ND. In particular, the toughness increased by 47% for the nanocomposite with 1% w/w ND loading. The thermal degradation temperature and glass transition temperature of the nanocomposite with 1% w/w ND loading were also 8 °C and 30 °C higher than those of PMMA, respectively. Young's moduli and the thermal conductivities of the nanocomposites were discussed using the Halpin–Tsai and Maxwell theoretical models, respectively; both exceeded the theoretical values, demonstrating that the excellent properties of ND were successfully imparted to the nanocomposites.

Ultra High Molecular Weight Polyethylene with a reduced number of entanglements can be stretched in the solid state both uni-or biaxially to produce highly oriented tapes and films. The chain orientation, in combination with the reduced number of chain ends, is responsible for the high tensile modulus and tensile strength of the drawn materials, and, as we report here, also for the high thermal conductivity achieved through lattice movements. A property such as thermal conductivity in an electrical insulator makes UHMWPE tapes and films of great applicative interest. In-plane laser-flash thermal analysis has been applied to measure the thermal diffusivity of samples of different molecular weights stretched both uni-and biaxially, and a strong correlation has been found between the drawing ratio and the resulting in-plane thermal conductivity. Values of at least 40 W/m K have been achieved for UHMWPE having M-w comprised between 2 and 10 million, while higher values of 65 W/m K are observed for the higher M-w samples having relatively lesser number of chain ends. Surprisingly the biaxially stretched samples also show in-plane conductivity, with the highest value reaching 18 W/m K, comparable to stainless steel. (C) 2017 The Authors. Published by Elsevier Ltd.

Evaluation of principal residual stress with X-Ray diffraction for metals has been widely practiced, but not for polymers due to a large fraction of amorphous phase, though it is very important for many engineering applications. Such a method is established for rigid semicrystalline polymer with rubber amorphous which, in the present work, are defined as the polymers with an amorphous rubber phase and a crystal matrix (rigid crystal network) providing its plastic modulus. An equal strain model between crystal and amorphous phases and the Young's modulus contributed by both crystal and amorphous phases, instead of moduli from crystal region, are justified for the stress evaluation for rigid semicrystalline polymers. The principal residual stresses obtained with our approach show a very good correlation with crystal orientation and the anisotropic mechanical properties of the polymer films studied. The established method can be widely used for rigid semicrystalline polymers with rubbery amorphous.

Parts prepared by the fused deposition modeling (FDM) 3D printing process suffer from poor interfacial adhesion between layers. This is due to poor diffusion of the very large and slow polymer chains across the inter-filament interface. To address this issue, we have developed the use of a bimodal blend of poly(lactide) (PLA) comprised of a series of synthesized low molecular weight PLA components (8.5 k, 50 k, and 100 k) added to a commercially available PLA (220 k). Tensile testing results indicate that when the LMW additive is of a sufficient length, the maximum stress and modulus in the part printed orthogonal to the print head (transverse) is significantly improved. More specifically, this behavior is observed where increased diffusion and increased entanglement of chains across adjacent layers occurs. The extent of crystallization at various stages of processing is also analyzed and indicates no correlation between the mechanical properties obtained and extent of crystallinity.

Tremendous effort is focused on improving epoxy toughness, but common approaches diminish processability and mechanical properties. This work presents hybrid additives which simultaneously enhance toughness, modulus, and other mechanical properties without reducing the network's processability. The hybrid herein was molecularly designed to contain “hard” nanoparticles and “soft” organic segments which both covalently bond and interact to form strong interfaces with epoxy networks. It is shown that a hyperbranched epoxy (HBE) reacted with a molecular silica, polyhedral oligomeric silsesquioxane (POSS), can provide the desired simultaneous reinforcement and toughness enhancements while maintaining processability. Epoxy network incorporation of the hybrid toughener led to a 220% increase in fracture toughness while also bringing about an increase in Young's modulus of over 20%. These significant improvements are described by the cured network's multi-scale morphologies and toughener dispersion states, and are related to strong interactions which occur between POSS's pendants and the HBE.