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The microstructure of physically assembled gels depends on mechanical loading and environmental stimuli such as temperature. Here, we report the real-time change in the structure of physically assembled triblock copolymer gels that consist of 10 wt% and 20 wt% of poly(styrene)-poly(isoprene)-poly(styrene) [PS-PI-PS] triblock copolymer in mineral oil (i) during the gelation process with decreasing temperature, (ii) subjected to large oscillatory deformation, and (iii) during the stress-relaxation process after the application of a step-strain. The presence of loosely bounded PS-aggregates at temperatures higher than the rheometrically determined gelation temperature (Tgel) captures the progressive gelation process spanning over a broad temperature range. However, the microstructure fully develops at temperatures suciently lower than Tgel, and the storage modulus (G0 ) also reaches a plateau at those temperatures. The microstructure orients in the stretching direction with the applied strain. In an oscillation strain cycle, such oriented structure has been observed at low-strain. But, at large-strain, the oriented structure splits, and only a fraction of midblock participates in load-bearing. This has been attributed to the endblock pullout from the aggregates, likely caused by the strain localization in the samples. Both microstructure recovery and time-dependent moduli during the stress-relaxation process after the application of a step-strain can be captured using a stretched-exponential model. However, the microstructure recovery time has been found to be two orders of magnitude slower than the stress-relaxation time at room temperature, indicating a complex nature of relaxation process involving midblock relaxation, endblock pullout and reassociation process. Due to their viscoelastic nature, these gels' mechanical responses are sensitive to strain, temperature, and rate of deformation. Therefore, insights into the microstructural information as a function of these parameters will assist these gels' real-life applications and design new gels with improved properties

Endblock associative ABA gels in midblock selective solvents are attractive due to their easily tunable mechanical properties. Here, we present the effects of A- and B-block lengths on the rheological properties and microstructure of ABA gels by considering three low and one high polymer concentrations. The triblock polymer considered is poly(methyl methacrylate)-poly(

The nonlinear rheological response to extensional flows in entangled polymers is related to the segmental chain stretching and to the chemical identity of the monomeric units. The latter has a strong effect on the drag coefficients, and therefore, quantification of molecular conformation changes in the subnanometer scale (at the monomer level) are crucial to fully understand nonlinear viscoelastic behavior in polymer melts. We report in situ time-resolved extensional rheo-small-angle neutron scattering (tEr-SANS) and wide-angle X-ray scattering (tEr-WAXS) during startup of uniaxial flow on a monodisperse polystyrene melt. Flow-induced segmental alignment was quantified with tEr-SANS, whereas local alignment of the backbone-backbone and phenyl-phenyl interactions were measured with tEr-WAXS. Linear relations between the three alignment factors and stress were observed at low stresses, which confirmed the validity of simple stress-SANS and stress-WAXS rules (SSR and SWR, respectively). Significant differences in SSR and SWR coefficients, as well as the stress values for failure of the two rules suggest very different correlations between global (at the segmental level) and local (at the monomer level) conformations with stress.

The field of 3D bioprinting has rapidly grown, yet the fundamental ability to manipulate material properties has been challenging with current bioink methods. Here, we change bioink properties using our PEG cross-linking (PEGX) bioink method with the objective of optimizing cell viability while retaining control of mechanical properties of the final bioprinted construct. First, we investigate cytocompatible, covalent cross-linking chemistries for bioink synthesis (e.g. Thiol Michael type addition and bioorthogonal inverse electron demand Diels-Alder reaction). We demonstrate these reactions are compatible with the bioink method, which results in high cell viability. The PEGX method is then exploited to optimize extruded cell viability by manipulating bioink gel robustness, characterized by mass flow rate. Below a critical point, cell viability linearly decreases with decreasing flow rates, but above this point, high viability is achieved. This work underscores the importance of building a foundational understanding of the relationships between extrudable bioink properties and cell health post-printing to more efficiently tune material properties for a variety of tissue and organ engineering applications. Finally, we also develop a post-printing, cell-friendly cross-linking strategy utilizing the same reactions used for synthesis. This secondary cross-linking leads to a range of mechanical properties relevant to soft tissue engineering as well as highly viable cell-laden gels stable for over one month in culture. STATEMENT OF SIGNIFICANCE: We demonstrate that a PEG crosslinking bioink method can be used with various cytocompatible, covalent cross-linking reactions: Thiol Michael type addition and tetrazine-norbornene click. The ability to vary bioink chemistry expands candidate polymers, and therefore can expedite development of new bioinks from unique polymers. We confirm post-printed cell viability and are the first to probe, in covalently cross-linked inks, how cell viability is impacted by different flow properties (mass flow rate). Finally, we also present PEG cross-linking as a new method of post-printing cross-linking that improves mechanical properties and stability while maintaining cell viability. By varying the cross-linking reaction, this method can be applicable to many types of polymers/inks for easy adoption by others investigating bioinks and hydrogels.

The microstructure of physically assembled gels depends on mechanical loading and environmental stimuli such as temperature. Here, we report the real-time change in the structure of physically assembled triblock copolymer gels that consist of 10 wt% and 20 wt% of poly(styrene)-poly(isoprene)-poly(styrene) [PS-PI-PS] triblock copolymer in mineral oil (i) during the gelation process with decreasing temperature, (ii) subjected to large oscillatory deformation, and (iii) during the stress-relaxation process after the application of a step-strain. The presence of loosely bounded PS-aggregates at temperatures higher than the rheometrically determined gelation temperature (Tgel) captures the progressive gelation process spanning over a broad temperature range. However, the microstructure fully develops at temperatures suciently lower than Tgel, and the storage modulus (G0 ) also reaches a plateau at those temperatures. The microstructure orients in the stretching direction with the applied strain. In an oscillation strain cycle, such oriented structure has been observed at low-strain. But, at large-strain, the oriented structure splits, and only a fraction of midblock participates in load-bearing. This has been attributed to the endblock pullout from the aggregates, likely caused by the strain localization in the samples. Both microstructure recovery and time-dependent moduli during the stress-relaxation process after the application of a step-strain can be captured using a stretched-exponential model. However, the microstructure recovery time has been found to be two orders of magnitude slower than the stress-relaxation time at room temperature, indicating a complex nature of relaxation process involving midblock relaxation, endblock pullout and reassociation process. Due to their viscoelastic nature, these gels' mechanical responses are sensitive to strain, temperature, and rate of deformation. Therefore, insights into the microstructural information as a function of these parameters will assist these gels' real-life applications and design new gels with improved properties

Correction for ‘Temperature- and strain-dependent transient microstructure and rheological responses of endblock-associated triblock gels of different block lengths in a midblock selective solvent’ by Rosa Maria Badani Prado et al., Soft Matter, 2022, 18, 7020–7034, https://doi.org/10.1039/D2SM00567K.

End-block association dynamics are known to influence deformation behavior in block copolymer systems. The ability to obtain precisely sequence controlled systems can help inform this deformation behavior, which is often influenced by dispersity in sequence and molar mass. Elastin-like polypeptides (ELPs) are a class of protein that consists of a sequence of five amino acids (XPGVG) that thermoresponsively aggregate in solution. These ELPs can be used as end-blocks in triblock fusion proteins with coiled-coil associating midblock domains to result in dual-associating, network-forming materials. By modifying the standard glycine-containing ELP sequence (XPGVG) to instead contain alanine in the third position of the repeat sequence (XPAVG), it is possible to improve the properties of the material in both shear and extension. In extension at 50 °C, the alanine-containing triblock (A10P4A10) and the glycine-containing triblock (G10P4G10) have similar Young’s moduli. However, while G10P4G10 yields and breaks w...

Dual-associative protein di- and triblock copolymers composed of sticker-decorated midblocks and micelle-forming elastin-like polypeptide (ELP) endblocks form shear-thinning, thermoresponsively reinforceable hydrogels that are potentially useful as injectable materials for a variety of applications. Here, the combination of rheological and in situ scattering measurements under shear on these dual-associative gels is employed in order to better understand how block architecture plays a role in controlling microscopic structural rearrangement and the resulting macroscopic mechanical responses. These gels, which form a disordered sphere phase due to endblock aggregation under quiescent conditions with the midblock domains physically crosslinked by protein associations, exhibit both viscoelastic and thixotropic signatures with relative magnitudes dependent upon gel concentration and block architecture. In situ SAXS measurements during flow indicate that these thixotropic responses correspond to the development of ordered domains following start-up of shear. For both architectures, the rate of alignment increases with increasing concentration. However, the rate of domain formation when increasing the temperature from 35 to 50 °C depends on the interplay between thermoresponsive toughening of the endblocks and softening of the coiled-coil domains such that rate of rearrangement decreases in the triblock while it increases in the diblock. Following a step-down in shear flow, structural rearrangement within the samples results in a thixotropic stress response. Upon cessation of flow, gel recovery is characterized by a concentration-dependent restoration of the micellar network over time, with two timescales observed that correspond to two different length scales of network relaxation.

We report in situ small-angle x-ray scattering studies of a prealigned cylindrically ordered styrene-ethylene butylene-styrene block copolymer melt subjected to extensional flow. Samples are prepared via lubricated planar extensional flow and tested using two initial conditions: “parallel” with cylindrical microdomains oriented along the extensional flow direction and “perpendicular” with domains aligned transverse to the flow direction. The experiments employ a counter-rotating drum extensional flow fixture housed in an oven designed for in situ synchrotron access. The impacts of initial condition, extension rate, and final Hencky strain on the melt structure are analyzed both during and following flow. Stress and flow kinematics are strongly influenced by the initial sample orientation. While parallel samples exhibit uniaxial deformation, perpendicular samples exhibit planar extensional kinematics, attributed to susceptibility to compression along the cylindrical microdomain axis due to a microscopic buckling instability. Scattering data reveal both anisotropic deformation of microdomain spacing and reorientation induced by the flow. Persistence of higher order reflections confirms hexagonal packing throughout the flow process, and strong alignment along the stretching direction is attained for both initial conditions. Flow-induced deformation of microdomain spacing and mechanical stress relax on similar time scales upon flow cessation, while negligible relaxation of orientation is observed.

3D bioprinting shows significant promise for creating complex tissue and organ mimics to solve transplant needs and to provide platforms for drug testing and studying tissue morphogenesis[1][2][3]. However, the lack of 3D printable and cell-compatible bioinks as well as the limited ability to tune bioink material properties are cited as significant inhibitors to the growth of bioprinting[4][5]. Pioneers in the field of tissue engineering and biomaterials have established and validated that changing materials properties such as stiffness[6], bioactive moieties[7][8], and degradation[7][9] significantly impacts cell behavior and tissue formation. Thus, developing novel, versatile and tunable bioink methods that will facilitate advanced material and construct design will have important implications in the field of bioprinting and biofabrication. Versatile bioink synthesis techniques, ones that can be used with many materials, will improve both printability of existing bioinks and most importantly, can add completely new biomaterials to the 3D bioprinting material palette. Furthermore, development of tunable bioink methods will provide additional means to customize mechanical, chemical, physical, and biological properties of printed structures towards creating compositionally and structurally complex structures and functional tissues beyond the rudimentary tissue structures presented thus far.

The thermoreversible self-assembly of poly(methyl methacrylate)-b-poly(tert-butyl methacrylate) (PMMA–PtBMA) diblock copolymers in 2-ethylhexanol has been studied in the transition regime between spherical and cylindrical morphologies. In these materials the PMMA block exhibits a strong temperature dependence of the solvent quality that leads to reversible micelle formation, and the PtBMA block is a versatile polymer that can be hydrolyzed for further use as a polyelectrolyte. Self-consistent field theory was used in combination with a variety of experimental techniques to develop a simple criterion for the location of the sphere/cylinder transition in solutions with concentrations above the micelle overlap threshold. It is shown that the effective volume fraction of PMMA core, accounting for solvent swelling of the micelle core, is equal to ≈0.27 at the sphere/cylinder transition. For the spherical domain morphologies, a transition between disorded micelles and micelles packed on a body-centered-cubic la...

A hexagonally ordered styrene–ethylene-co-butylene–styrene triblock copolymer melt is studied during and immediately following uniaxial extensional flow using small-angle X-ray scattering. Strips of polymer melt are stretched between counter-rotating drums inside an oven designed for in situ synchrotron studies. Previous study of this sample demonstrated deformation and reorientation of PS microdomains and identified a critical Hencky strain ∼0.2 at which the structure is dramatically disrupted. To further probe microstructural dynamics, we study relaxation behavior of structure and stress in samples stretched in the melt to various Hencky strains. Upon cessation of stretching below the critical strain, structural relaxation is strongly retarded; stretching to higher strain leads to more rapid relaxation. At all strains, microdomain orientation relaxation following flow is slower than relaxation of flow-induced deformation and mechanical stress. The extension rate dependence of the sample’s response is al...

Compared with conventional polyolefins, ultrahigh molecular weight polyethylene (UHMWPE) possesses outstanding impact strength and crack resistance that make it desirable for a wide variety of applications. Unfortunately, UHMWPE has an ultrahigh viscosity that renders common, continuous melt-state processes ineffective for making UHMWPE products. Attempts to overcome this problem by blending UHMWPE with lower molecular weight high-density polyethylene (HDPE) by melt processing have typically led to poorly dispersed blends due to the vast viscosity mismatch between blend components. Here, we present solid-state shear pulverization (SSSP) as a mild, continuous, and simple approach for achieving effective and intimate mixing in UHMWPE/HDPE blends. These SSSP blends are easily processed by post-SSSP melt extrusion; for an SSSP blend with 50 wt% UHMWPE, we observe more than a factor of 1000 increase in viscosity at a shear rate of 0.01 s−1 but less than a factor of 5 increase at 100 s−1, the latter being more typical of melt-processing operations. Using extensional rheology, we confirm the strain hardening behavior of SSSP blends. Shear rheology and crystallization data show that the mixing between UHMWPE and HDPE can be improved with subsequent passes of SSSP and single-screw extrusion. Finally, we show that blending via SSSP leads to dramatic improvements in impact strength: as compared to literature results, injection-molded sample bars made from SSSP blends with 30–50 wt% UHMWPE exhibit very high values of notched Izod impact strength, 660–770 J/m (the impact strength of neat HDPE was 170 J/m).

We report in situ small-angle X-ray scattering (SAXS) studies of a cylindrically ordered styrene-ethylene butylene-styrene (SEBS) triblock copolymer melt subjected to uniaxial extensional flow. The flow is applied by stretching strips of polymer melt using a counter-rotating drum extensional flow fixture housed in a custom oven designed to provide X-ray access. SAXS patterns show two distinct modes of structural response during extensional flow: deformation of the microscopic structure, and re-orientation of PS microdomains towards the flow direction. The d-spacings of the hexagonally ordered domains measured parallel and perpendicular to the flow direction deform affinely until Hencky strains of ∼0.2. Departures in extensional viscosity from linear viscoelastic predictions are observed at similar strain. The azimuthal dependence of the primary diffraction peak reveals a complex re-orientation process whereby PS microdomains rotate toward the stretching direction. At intermediate strains, a '4-point' diffraction pattern indicates the presence of two discrete populations of microdomain orientation, attributed to a buckling instability of microdomains initially oriented perpendicular to the stretching direction. Flow-induced deformation and orientation both relax upon cessation of flow, albeit at very different rates, suggesting that these two modes of structural response are largely decoupled.

Transient network hydrogels cross-linked through histidine-divalent cation coordination bonds were studied by conventional rheologic methods using histidine-modified star poly(ethylene glycol) (PEG) polymers. These materials were inspired by the mussel, which is thought to use histidine-metal coordination bonds to impart self-healing properties in the mussel byssal thread. Hydrogel viscoelastic mechanical properties were studied as a function of metal, pH, concentration, and ionic strength. The equilibrium metal-binding constants were determined by dilute solution potentiometric titration of monofunctional histidine-modified methoxy-PEG and were found to be consistent with binding constants of small molecule analogs previously studied. pH-dependent speciation curves were then calculated using the equilibrium constants determined by potentiometric titration, providing insight into the pH dependence of histidine-metal ion coordination and guiding the design of metal coordination hydrogels. Gel relaxation dynamics were found to be uncorrelated with the equilibrium constants measured, but were correlated to the expected coordination bond dissociation rate constants.

We report measurements of shear-induced particle orientation in highly exfoliated polystyrene/clay nanocomposites. Samples were prepared using an in situ polymerization technique, in which native clay is organically modified with a cationic surfactant that incoporporates a polymerizable vinylbenzyl moiety. Controlled radical polymerization was used during the synthesis to limit the molecular weight and polydispersity of the nanocomposite polymer matrix. Flow-induced orientation was measured in the flow-gradient plane of shear flow using synchrotron-based small-angle X-ray scattering. Despite the small rotational diffusivity expected for the clay particles, significant particle orientation was only observed at relatively high rates in steady shear or at high frequencies in large-amplitude oscillatory shear. Measurements of orientation upon flow cessation provided direct evidence of a structural relaxation process that was orders of magnitude faster than estimates of rotational Brownian motion. It is sugges...

The development of molecular orientation in thermotropic liquid crystalline polymers (TLCPs) during injection molding has been investigated using two-dimensional wide-angle X-ray scattering coordinated with numerical computations employing the Larson–Doi polydomain model. Orientation distributions were measured in “short shot” moldings to characterize structural evolution prior to completion of mold filling, in both thin and thick rectangular plaques. Distinct orientation patterns are observed near the filling front. In particular, strong extension at the melt front results in nearly transverse molecular alignment. Far away from the flow front shear competes with extension to produce complex spatial distributions of orientation. The relative influence of shear is stronger in the thin plaque, producing orientation along the filling direction. Exploiting an analogy between the Larson–Doi model and a fiber orientation model, we test the ability of process simulation tools to predict TLCP orientation distributions during molding. Substantial discrepancies between model predictions and experimental measurements are found near the flow front in partially filled short shots, attributed to the limits of the Hele–Shaw approximation used in the computations. Much of the flow front effect is however “washed out” by subsequent shear flow as mold filling progresses, leading to improved agreement between experiment and corresponding numerical predictions. POLYM. ENG. SCI.,, 2011. © 2011 Society of Plastics Engineers

X-ray scattering is used to measure particle orientation in concentrated multiwalled carbon nanotube (MWNT) suspensions under shear flow. MWNTs were dispersed in a Newtonian suspending fluid (uncured epoxy). The dispersions exhibit shear thinning, approaching the matrix viscosity at high shear rates. This is accompanied by progressive development of MWNT orientation along the flow direction with increasing shear rate. The impact of MWNT aspect ratio and concentration on steady-state orientation is explored. In one sample (2 wt. % dispersion of short MWNTs), orientation was measured in both the flow-gradient (1-2) and flow-vorticity (1-3) planes of shear flow to provide a more complete picture of the three-dimensional orientation state. Also in this sample, 1-3 plane measurements were conducted using both small- and wide-angle x-ray scattering (SAXS and WAXS). While the two methods produce qualitatively similar results, WAXS-derived measures of flow-induced anisotropy are consistently larger than SAXS data...

We report steady and transient measurements of particle orientation in a clay dispersion subjected to shear flow. An organically modified clay is dispersed in a Newtonian polymer matrix at a volume fraction of 0.02, using methods previously reported by Mobuchon et al. (Rheol Acta 46: 1045, 2007). In accord with prior studies, mechanical rheometry shows yield stress-like behavior in steady shear, while time dependent growth of modulus is observed following flow cessation. Measurements of flow-induced orientation in the flow-gradient plane of simple shear flow using small-angle and wide-angle X-ray scattering (SAXS and WAXS) are reported. Both SAXS and WAXS reveal increasing particle orientation as shear rate is increased. Partial relaxation of nanoparticle orientation upon flow cessation is well correlated with time-dependent changes in complex modulus. SAXS and WAXS data provide qualitatively similar results; however, some quantitative differences are attributed to differences in the length scales probed by these techniques.

We report in situ X-ray scattering measurements of shear-induced orientation in polymer–clay dispersions. Two different organically modified clays, montmorillonite and fluorohectorite, are dispersed in a low molecular weight, viscous polymer melt, facilitating studies at room temperature. Orientation measurements are performed in the flow-gradient plane, allowing characterization of both the average degree and direction of particle orientation during shear. In all cases, the orientation angle is finite, indicating systematic misalignment of the particle long axes relative to the flow direction. In concentrated fluorohectorite and montmorillonite dispersions, anisotropy and orientation angle are roughly independent of shear rate, and negligible relaxation is observed upon flow cessation. Conversely, a lower concentration montmorillonite sample exhibits orientation that is more responsive to shear flow, and partially relaxes upon flow cessation. In this sample, the orientation behavior is interpreted in light of rotational diffusion of the clay particles. This same sample exhibits oscillatory structural dynamics upon shear flow reversal, attributed to tumbling rotations of the disk-like clay particles in shear. Large-amplitude oscillatory shear is similarly demonstrated to be capable of inducing significant particle orientation; the degree of orientation is principally determined by the applied strain amplitude. Complementary measurements of rheological properties exhibit many characteristics commonly reported in polymer–clay nanocomposites. Based on the structural measurements reported here, the rheological phenomena are interpreted to arise from a combination of flow-induced particle orientation and rate- and time-dependent destruction or reformation of particle networks.

The effect of polymer concentration on mechanical response and micelle morphology of ABA and AB copolymers in B-selective solvents has been systematically studied. Micelle morphology was determined using a combination of small-angle X-ray scattering, shear, and birefringence while mechanical response at low and high strains was determined using indentation techniques. Self-consistent field theory calculations were used to determine micelle volume fraction profiles and to construct an equilibrium phase map. The transition from spherical to cylindrical micelles increases the triblock gel modulus and energy dissipation. Combining knowledge of gel relaxation time, which determines the rate at which the gel can equilibrate its micelle structure, with the equilibrium phase map allows estimation of the experimental temperatures and time scales over which kinetic trapping will arrest micelle structure evolution. Kinetic trapping enables cylindrical morphologies to be obtained at significantly lower polymer fractions than is possible in equilibrated systems.

Dispersions of multiwalled carbon nanotubes (MWNT) in polypropylene (PP) were prepared via conventional melt batch mixing and solid-state shear pulverization. The properties and structure of each system were assessed via linear viscoelasticity, electrical conductivity, PP crystallization kinetics, dynamic mechanical analysis, scanning electron microscopy, and small angle X-ray scattering. Increasing either the duration or the intensity of melt mixing leads to higher degrees of dispersion of MWNT in PP, although at the cost of substantial melt degradation of PP for long mixing times. Samples prepared by pulverization exhibit faster crystallization kinetics and higher mechanical stiffness than the melt blended samples, but in contrast show no measurable low frequency elastic plateau in melt rheology, and lower electrical conductivity than melt-mixed samples. X-ray scattering demonstrates that neither sample has uniform dispersion down to the single MWNT level. The results illustrate that subtle differences in the size and distribution of nanotube clusters lead to differences in the nanotube networks with strong impact on bulk properties. The results also highlight distinctions between conductive networks and load transfer networks and demonstrate that a complete and comparative picture of dispersion cannot be determined by simple indirect property measurements. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1426–1436, 2009

We report process simulations of molecular orientation of liquid crystalline polymers for isothermal channel flows in extrusion. The simulations use a “polydomain” model due to Larson and Doi (Larson and Doi, J. Rheol., 35, 539 (1991)), which is implemented by exploiting a nearly exact analogy with a fiber orientation model that is widely used for analysis of composites processing. Simulation results are compared to experimental data previously obtained using a customized X-ray capable extrusion die that allows a wide range of channel flow geometries to be explored. Competition between inhomogeneous shear and extension in these kinematically complex flows has a profound effect on the resulting molecular orientation distributions. We find that the Larson–Doi model successfully predicts most aspects of the experimental observations, demonstrating its utility for process modeling. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

The degree of microphase or nanophase segregation in gradient copolymers with compositions varying across the whole copolymer backbone is studied via low-amplitude oscillatory shear (LAOS) measurements and small-angle X-ray scattering (SAXS). Studies are done as a function of comonomer segregation strength, molecular weight (MW), gradient architecture and temperature. Controlled radical polymerization is used to synthesize strongly segregating styrene/4-acetoxystyrene (S/AS) and the more weakly segregating S/n-butyl acrylate (S/nBA) gradient copolymers. Results are compared to those from S/AS and S/nBA random and block copolymers. The higher MW S/AS gradient copolymer exhibits LAOS behavior similar to the highly microphase segregated S/AS block copolymer, while the lower MW S/AS gradient copolymer exhibits complex, nonterminal behavior indicative of a lower degree of microphase segregation. The S/nBA gradient copolymers demonstrate more liquidlike behavior, with the lower MW sample exhibiting near-Newtoni...

The flow behavior and phase structure of two side-chain liquid-crystalline polysiloxanes with the same mesogens but different spacers of either four (PSi4) or six (PSi6) methylene groups are studied in the nematic phase by 2 H NMR, polarization microscopy, and scattering of light, neutrons, and X-rays. PSi4 is flow-aligning, forming a monodomain with the director close to the flow direction, whereas PSi6 is non-flow-aligning showing a polydomain structure with a preferred director orientation corresponding to the log-rolling state. The flow behavior of PSi6 is related to the presence of strong smectic clusters. The director tilt in the smectic clusters of both polymers is reported.

We have investigated the shear flow behavior of a dynamically symmetric polymeric bicontinuous microemulsion using rheology and in situ small angle x-ray scattering. The microemulsion consists of a ternary blend of poly (ethylene-alt-propylene) (PEP), poly (butylene oxide) (PBO), and a PEP-PBO diblock copolymer. Steady shear experiments reveal an unusual shear thickening behavior at the onset of the non-Newtonian regime, which is consistent with the strain hardening and frequency thickening (at large strains) under oscillatory shear. Scattering experiments indicate development of anisotropy in the bicontinuous structure within the thickening regime. Subsequent shear thinning is observed at intermediate shear rates. Shear-induced bulk phase separation is detected at very high rates. This work complements previous studies on a dynamically extremely asymmetric bicontinuous microemulsion, and thereby establishes the universal rheological properties of polymeric microemulsions. Possible underlying molecular me...

Large amplitude oscillatory shear (LAOS) is frequently capable of generating macroscopic alignment from an initially random orientation distribution in ordered polymer fluids. Side-group liquid crystalline polymers are of special interest in that the flow field may couple differently to the polymer backbone and the mesogen ordering. We report combined rheological and in situ X-ray scattering investigations of LAOS-induced alignment in smectic side-group LCPs. Synchrotron X-ray scattering is used to study orientation development using a rotating disk shear cell in which orientation is tracked within the flow-vorticity (1−3) plane. In all cases, we find that shear promotes anisotropic orientation states in which the lamellar normal tends to align along the vorticity direction of the shear flow (“perpendicular” alignment). We examine the effects of shear strain amplitude and polymer backbone molecular weight on the ability of LAOS to induce alignment. Rheological measurements of the dynamic moduli reveal that large amplitude shearing in the smectic phase causes a notable decrease in the modulus. X-ray and rheological data demonstrate that increasing strain promotes higher degrees of orientation, while increasing molecular weight impedes development of smectic alignment.

Elastic recoil detection (ERD) was used to characterize the phase behavior of blends involving neutralized poly(styrene-ran-styrenesulfonate) ionomers (P(S-SSx)-M) and deuterated polystyrene (dPS). The lightly sulfonated ionomers (acid mole fraction x = 0.007) were neutralized with various cations (M): sodium (Na+), barium (Ba2+), and zinc (Zn2+). The dPS:P(S-SS0.007)-M blends have a higher upper critical solution temperature (UCST) than the dPS:P(S-SS0.007) blends, indicating that neutralizing the acid copolymer reduces blend miscibility. The UCST is higher when P(S-SS0.007) is neutralized (125%) with divalent cations, Ba2+ and Zn2+, rather than with a monovalent cation, Na+. In addition, as the level of neutralization increases from 25% to 125%, the miscibility in the dPS:P(S-SS0.007)-Zn blends decreases; this was not observed in the dPS:P(S-SS0.007)-Na blends. Complementary linear viscoelastic measurements were performed on a copolymer and ionomers with a higher acid content. Upon neutralization, the ...

The equilibrium dynamics of a bicontinuous microemulsion composed of polystyrene, polyisoprene, and poly(styrene-block-isoprene) was characterized using rheology and X-ray photon correlation spectr...

In situ X-ray scattering measurements of molecular orientation under shear are reported for two commercial thermotropic liquid crystalline polymers (TLCPs), Vectra A950® and Vectra B950®. Transient shear flow protocols (reversals, step changes, and flow cessation) are used to investigate the underlying director dynamics. Synchrotron X-ray scattering in conjunction with a high-speed area detector provides sufficient time resolution to limit the total time spent in the melt during testing, whereas a redesigned X-ray capable shear cell provides a more robust platform for working with TLCP melts at high temperatures. The transient orientation response upon flow inception or flow reversal does not provide definitive signatures of either tumbling or shear alignment. However, the observation of clear transient responses to step increases or step decreases in shear rate contrasts with expectations and experience with shear-aligning nematics and suggests that these polymers are of the tumbling class. Finally, these two polymers show opposite trends in orientation following flow cessation, which appears to correlate with the evolution of dynamic modulus during relaxation. Specifically, Vectra B shows an increase in orientation upon flow cessation, an observation that can only be rationalized by the assumption of tumbling dynamics in shear. Together with prior observations of commercial LCP melts in channel flows, these results suggest that this class of materials, as a rule, exhibits director tumbling.

The structure and relaxation behavior of thermoreversible gels made with poly(methyl methacrylate)−poly(n-butyl acrylate)−poly(methyl methacrylate) [PMMA−PnBA−PMMA] triblock copolymers in 2-ethylhexanol, a midblock selective solvent, were studied by small-angle X-ray scattering (SAXS) and rheology. Effects of endblock length, endblock fraction, and gel concentration on the gel properties were investigated. A dramatic decrease in SAXS intensity was observed over a 20 °C interval where the gel transitions smoothly from elastic to viscous behavior. SAXS patterns were fit with a Percus−Yevick disordered hard-sphere model from which aggregation number and average domain spacing were calculated. Aggregation number increases with increasing gel concentration and endblock length. Increasing the endblock length from 9K to 25K increases the relaxation time of a gel with a polymer volume fraction of 0.15 by a factor of 10^6. For a given triblock endblock fraction and molecular weight, the micelle aggregation number is strongly correlated to the gel relaxation time. Arrhenius behavior with an effective activation energy of ~550 kJ/mol was observed for all triblocks and concentrations. This very high effective energy barrier describes gels relaxation behavior over a 40 °C temperature range, where the relaxation times vary by a factor of 10^(10).

Research demonstrates that varying sensory input, including the characteristics of a bolus, changes swallow physiology. Altering the consistency of fluids is a common compensatory technique used in dysphagia management to facilitate change. However, it is not known what variations in viscosity can be perceived in the oral cavity or oropharynx or if age affects oral and oropharyngeal perceptions of fluid viscosity. This study aims to establish the ability of normal adults to perceive fluid viscosity in the oral cavity and oropharynx and to determine if, within this population, there are age-related changes in oral and oropharyngeal perceptions. Sensitivity was established by deriving the exponent for the psychophysical law for fluid viscosity in both the oral cavity and the oropharynx, using modulus-free magnitude estimation with Newtonian fluids of corn syrup and water. Sixty normal volunteers, aged 21-84 years, participated. Results indicate that the exponent for oral perception of fluid viscosity was 0.3298, while for oropharyngeal perception it was 0.3148. Viscosity perception deteriorates with increasing age. Men exhibited a more marked deterioration in sensitivity than women. This study contributes to the literature on oral and oropharyngeal perceptions and on aging. The results provide a basis for work with individuals with dysphagia.

Injection molding thermotropic liquid-crystalline polymers (TLCPs) usually results in the fabrication of molded articles that possess complex states of orientation that vary greatly as a function of thickness. 'Skin-core' morphologies are often observed in TLCP moldings. Given that both 'core' and 'skin' orientation states may often differ both in magnitude and direction, deconvolution of these complex orientation states requires a method to separately characterize molecular orientation in the surface region. A combination of two-dimensional wide-angle X-ray scattering (WAXS) in transmission and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to probe the molecular orientation in injection molded plaques fabricated from a 4,4'-dihydroxy-{alpha}-methylstilbene (DH{alpha}MS)-based thermotropic liquid crystalline copolyester. Partial electron yield (PEY) mode NEXAFS is a noninvasive ex situ characterization tool with exquisite surface sensitivity that samples to a depth of 2 nm. The effects of plaque geometry and injection molding processing conditions on surface orientation in the regions on- and off- axis to the centerline of injection molded plaques are presented and discussed. Quantitative comparisons are made between orientation parameters obtained by NEXAFS and those from 2D WAXS in transmission, which are dominated by the microstructure in the skin and core regions. Some qualitative comparisons are also made withmore » 2D WAXS results from the literature.« less

The blend miscibilities of deuterated polystyrene (dPS) and sulfonated poly(styrene-ran- styrenesulfonate) (P(S-SS)) are examined by using forward recoil spectrometry (FRES) to probe the intermixing of bilayer films. This method directly determined the equilibrium coexistence compositions for dPS:P(S-SSx) blends where the degree of sulfonation (x) ranged from 0.2 to 2.6 mol %. In the temperature range 150-190 °C, FRES profiles reveal full miscibility for x e 0.2 mol % and complete immiscibility for x g 2.6 mol %. Partial miscibility exists in dPS:P(S-SSx) blends with x ) 0.7, 1.0, and 1.2 mol %, where between 150 and 190 °C the coexisting compositions show upper critical solution temperature (UCST) phase behavior. Blend interaction parameters, � blend, are calculated using the Flory-Huggins theory and the coexisting compositions of the partially miscible bilayers. The copolymer blend theory estimates the styrene-styrenesulfonate segmental interaction parameter to be extraordinarily large, � S/SS g 25. While the applicability of mean-field approaches is limited in this profoundly incompatible system, recent theories about random copolymers have established criteria for "self- demixing" due to their inherent compositional variations. Our estimate of the monomer-monomer interaction parameter suggests the potential for demixing in P(S-SSx) random copolymers that possess even a narrow distribution of compositions.

Relationships among mechanical properties, degree of molecular orientation and molding conditions are investigated in injection molded plaques fabricated from a 4,4′-dihydroxy-α-methylstilbene (DHαMS)-based thermotropic liquid crystalline copolyester. Wide-angle X-ray scattering (WAXS) patterns reveal bimodal orientation states at most locations in the plaques. One population aligns roughly along the anticipated flow direction while a separate population is generated as a result of transverse stretching associated with diverging streamlines during mold filling. Micro-tensile bars are cut from the plaques both parallel and perpendicular to the filling direction to assess anisotropy in properties. Enhanced molecular orientation and properties ‘in-shear’ are observed for thinner plaques fabricated at relatively low mold temperatures and melt temperatures slightly above the nominal melting point of the polymer. Injection fill speed is not found to have a significant effect on anisotropy in tensile strength/stiffness. Mechanical properties such as tensile modulus and fracture stress are found to obey a ‘universal’ correlation with X-ray measurements of molecular orientation projected onto the axis of the testing specimens. These results suggest that even in the presence of complex, spatially heterogeneous orientation states, simple average measures of orientation can provide a robust means of anticipating macroscopic properties.

Studies of immiscible blend compatibilization often involve laborious microscopy methods to characterize changes in the particle size distribution with time in the melt. Here we explore a simple alternative approach based on Porod scattering from the two-phase structure. Although micron-sized particles in immiscible polymer blends are too large to be fully characterized by small-angle scattering, Porod scattering measurements of the interfacial area combined with knowledge of the blend volume fraction allows determination of an average particle diameter from a single scattering measurement. This technique is illustrated in experiments monitoring coarsening of particle size in polystyrene/poly(methyl methacrylate) blends prepared either by melt blending or solid-state shear pulverization.

The process of injection-molding net-shape parts from thermotropic liquid-crystalline polymers results in a skin-core macrostructure. The underlying orientation in the core and the skin may differ both in magnitude and direction. A combination of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and two-dimensional wide-angle X-ray scattering (2D WAXS) in transmission was used to characterize the orientation in injection-molded plaques fabricated from thermotropic liquid-crystalline copolyesters based on either 4, 4{prime}-dihydroxy-{alpha}-methylstilbene or 6-hydroxy-2-naphthoic acid/6-hydroxybenzoic acid. NEXAFS is presented as a noninvasive in situ means of determining surface layer orientation that samples to a depth of as little as 2 nm and does not require slicing or ultramicrotoming of the samples. The effects of various processing conditions on the surface orientation in the region of the centerline of square injection-molded plaques are presented and discussed. Comparisons are made between orientation parameters obtained by 2D WAXS in transmission, which is dominated by the microstructure in the core, and the NEXAFS technique.

In-situ X-ray scattering methods have been used to measure the average degree of molecular orientation in the commercial thermotropic copolyesteramide, Vectra B. Experiments were conducted in both homogeneous shear flow and in extrusion-fed channel flows that provided mixed shear/extensional deformations. In the channel flows, extension has a dramatic effect on the average orientation state in the vicinity of stagnation points or expansions/contractions in cross-sectional area. Of particular note, a temporary increase and subsequent decay in orientation observed in a 4:1 slit-contraction flow provides additional indirect evidence supporting the hypothesis that Vectra B exhibits director tumbling. This is consistent with results from other fully aromatic copolyesters but contrasts with findings in “model” thermotropes incorporating flexible spacers. Thus, it seems that the stiffer backbone of commercial main chain LCPs is the main feature which, apparently, leads to tumbling. Measurements of average molecular orientation in transient shear flows show some connections with the corresponding mechanical response, but fail to show the distinctive characteristics that have previously been associated with either tumbling or aligning in LCPs using similar procedures. These experiments might be adversely affected by the comparatively slow rate of data acquisition, which leads to lengthy experiments in which the sample is more prone to degradation.

Wide-angle X-ray scattering (WAXS) is used to probe the molecular orientation in steady isothermal complex channel flows (in situ) and in injection molded plaques (ex situ) of a new, low-cost aromatic copolyester based on the mesogen 4,4′-dihydroxy-α-methylstilbene (DHαMS). Complex orientation states arise from the competition of inhomogeneous mixed shear and extension in isothermal flows. Slit-contraction flows lead to a significant but temporary increase in the average degree of molecular orientation, suggesting that this polymer is of the ‘shear-tumbling’ type. Conversely, bimodal orientation states are observed in slit-expansion flows, where transverse extension leads to a strong reduction in the average degree of molecular orientation along the flow direction. Similar bimodal orientation states are observed in injection molded plaques, suggesting that these kinematic concepts translate rather directly to the more complex transient non-isothermal case of injection molding. Variations in orientation state induced by changes in plaque thickness may be rationalized by systematic changes in the relative importance of shear and extension. These results suggest a complementary perspective on ‘skin-core’ morphologies in liquid crystalline polymer moldings, and provide a clear conceptual link between more fundamental studies in isothermal flows and structure development during processing.

Semi-interpenetrating polymer networks (semi-IPNs), comprised of poly(N-isopropylacrylamide-co-acrylic acid) (p(NIPAAm-co-AAc)) hydrogels and linear p(AAc) chains, were synthesized, and the effects of the p(AAc) chains on semi-IPN rheology were examined. Oscillatory shear rheometry studies were performed and the rheological data were analyzed as a function of temperature, frequency, and p(AAc) chain amount (weight average molecular weight (Mw) 4.5 x 10(5) g/mol). At 22 degrees C, the semi-IPNs, as well as control p(NIPAAm-co-AAc) hydrogels, demonstrated rheological data that were representative of soft, loosely cross-linked solids. Furthermore, only the highest p(AAc) chain amount tested affected the rigidity of the p(NIPAAm-co-AAc)-based semi-IPNs, as compared to the p(NIPAAm-co-AAc) hydrogels. At 37 degrees C, the complex shear moduli (G*) demonstrated by the p(NIPAAm-co-AAc)-based semi-IPNs were significantly greater than G* exhibited by the p(NIPAAm-co-AAc) hydrogels, and the semi-IPN G* values significantly increased with increasing p(AAc) chain amount. These results can be used to develop p(NIPAAm)-based semi-IPNs with tailored mechanical properties that may function as scaffolds in tissue engineering initiatives.

Flow birefringence provides a means to map stress distributions in complex flows of polymer solutions and melts, which may be used as a basis for evaluating performance of constitutive models in viscoelastic flow simulations. Application of this technique to axisymmetric geometries allows consideration of flows with greater kinematic complexity than is possible in the planar flow fields more traditionally studied using birefringence. This paper reports an extension of this technique to a time-dependent, axially symmetric stagnation flow, in which fluid is periodically forced back and forth against the end of a cylinder with a hemispherical tip. The geometry allows independent variation of Deborah and Weissenberg numbers (through independent control of the amplitude and frequency of the motion), and also imposes periodically reversing kinematics, in which fluid in the stagnation region alternately experiences uniaxial and equibiaxial extension. This new flow can thus provide tests of viscoelastic fluid models under severe conditions. The experimental procedures are validated by experiments on a concentrated but low MW polystyrene solution, with essentially Newtonian rheology, to demonstrate the principles and capabilities of the experiment. We further present preliminary data on a shear thinning and viscoelastic solution of higher molecular weight polystyrene, to illustrate the effects of nonlinear viscoelasticity in the stagnation region.

Transient rheological measurements are reported for a model polymeric bicontinuous microemulsion. The sample consists of a ternary blend of poly(ethyl ethylene) (PEE) and poly(dimethyl siloxane) (PDMS) homopolymers and a symmetric PEE-PDMS diblock copolymer. Steady-flow rheological data, reported previously, show four regimes as a function of increasing shear rate. Newtonian behavior is observed in regime I, followed by shear thinning in regime II. Flow-induced phase separation is the hallmark of regime III. The microemulsion starts ejecting homopolymer-rich phases, and the shear stress is independent of shear rate. In regime IV, complete phase separation occurs and the sample behaves like an immiscible blend. Transient rheological data on flow inception reveal linear viscoelastic response in regime I, and development of a stress overshoot in regime II. In regime III, a strong stress overshoot is observed, followed by a “shoulder” and a slow decay to the steady, rate-independent value. The normal stress s...

Ternary polymer blends of immiscible homopolymers and the corresponding diblock copolymer are useful models for studying equilibrium and nonequilibrium behavior of self-assembled fluids. We report linear viscoelastic data for a polymeric bicontinuous microemulsion, experiments that are possible due to the comparatively high viscosities of the principal components. After subtracting a viscous background contribution from the pure constituents, the microemulsion exhibits “excess” viscoelastic behavior similar in character to that resulting from the Rouse model of polymer dynamics. The data are compared to the predictions of a time-dependent Landau−Ginzburg model developed by Patzold and Dawson, using structural parameters derived from neutron scattering as input. This model captures the essential characteristics of the viscoelastic behavior very well. However, using independent dynamic light scattering measurements of the Onsager coefficient for these blends, it appears that the model fails to predict eithe...

We have investigated the effects of shear flow on a polymeric bicontinuous microemulsion using neutron scattering, light scattering, optical microscopy, and rheology. The microemulsion consists of a ternary blend of poly(ethyl ethylene) (PEE), poly(dimethyl siloxane) (PDMS), and a PEE–PDMS diblock copolymer. At equilibrium, the microemulsion contains two percolating microphases, one PEE rich and the other PDMS rich, separated by a copolymer-laden interface; the characteristic length scale of this structure is 80 nm. Low strain amplitude oscillatory shear measurements reveal behavior similar to that of block copolymer lamellar phases just above the order–disorder transition. Steady shear experiments expose four distinct regimes of response as a function of the shear rate. At low shear rates (regime I) Newtonian behavior is observed, whereas at intermediate shear rates (regime II) development of anisotropy in the morphology leads to shear thinning. When the shear rate is further increased, there is an abrup...

We report measurements of molecular orientation in solid specimens of a main-chain thermotropic liquid crystalline polymer (LCP) that were quenched from mixed shear-extensional channel flows. The polymer under investigation is a random copolyether with mesogens separated by flexible hydrocarbon spacers. This polymer is known to exhibit ‘flow aligning’ dynamics under slow shear flow. Experiments were designed to preserve the molecular orientation state, representative of steady, isothermal channel flow in the solid samples, so that comparisons could be made against in situ channel flow measurements on other main chain thermotropes without flexible spacers, including a commercial fully aromatic copolyster. In the flow aligning material, little change in orientation was found in slit-contraction flows, and only modest drops in orientation were found in slit-expansion flow. This contrasts strongly with data on commercial LCPs, suggesting that these materials may be of the ‘tumbling’ type.

Quantitative measurements of molecular orientation and rheology are reported for various transient shear flows of a nematic semiflexible copolyether. Unlike the case of lyotropic liquid crystalline polymers (LCPs), whose structure and rheology in shear are dominated by director tumbling, this material exhibits flow aligning behavior. The observed behavior is quite similar to that seen in a copolyester that we have recently studied [Ugaz and Burghardt (1998)], suggesting that flow aligning dynamics may predominate in main-chain thermotropes that incorporate significant chain flexibility. Since the flow aligning regime has received little attention in previous attempts to model the rheology of textured, polydomain LCPs, we attempt to determine whether available models are capable of predicting the orientation and stress response of this class of LCP. We first examine the predictions of the polydomain Ericksen model, an adaptation of Ericksen’s transversely isotropic fluid model which accounts for the polydo...

We report studies on the average molecular orientation state in steady and transient shear flow of two lyotropic liquid crystalline polymers: poly(benzyl glutamate) [PBG] and hydroxypropylcellulose [HPC], both in m-cresol solution. An annular cone and plate X-ray shear cell is used to probe molecular orientation in the “1−2” plane, allowing simultaneous measurements of the degree of anisotropy and the average orientation angle relative to the flow direction. In steady shear flow, molecular orientation increases with shear rate in both materials. Comparisons with separate measurements in the “1−3” plane indicate that both materials exhibit a macroscopically biaxial orientation distribution function. The orientation angle is always small and exhibits a sign change from positive to negative values with increasing shear rate. In transient flows, anisotropy and orientation angle both exhibit damped oscillations that scale with shear strain. The Larson−Doi polydomain model is in qualitative agreement with data...

We report the first direct, quantitative measurements of the shear aligning properties of a main-chain thermotropic liquid crystalline polymer (LCP). We find that a model thermotrope with alternating mesogen and spacer structure is of the shear aligning type throughout its nematic range. The director rotates uniformly in the shear flow toward the Leslie alignment angle as probed by in situ flow conoscopy. The Leslie alignment angle becomes progressively closer to the flow direction as temperature decreases, corresponding to a decrease of the tumbling parameter λ with increasing order parameter S. Our measurements of λ(S) enable direct comparison with predictions from molecular models, which predict that shear alignment prevails in the limit of flexible nematic chains. This is in direct contrast to rodlike lyotropic LCPs for which director tumbling is the rule.

We explore the origins of shear stress overshoots upon flow inception of main-chain thermotropic liquid crystalline polymers from a random polydomain initial condition. A simple polydomain simulation strategy, based on Ericksen's transversely isotropic fluid model, is capable of predicting a shear stress overshoot arising from the reorientation dynamics of domains toward a steady state flow aligned state. When model parameters are estimated using elementary molecular theories for semiflexible main-chain thermotropes, both the relative magnitude of the shear stress overshoot and the steady state ratio of first normal stress difference to shear stress are determined solely by the “tumbling” parameter, λ. This implies a universal correlation between these two variables. We test this prediction against an extensive data set on semiflexible polyesters and find good qualitative agreement between model and experiment.