Some principles of rheology are applied to the study of the shear‐induced crystallization of molten polymers. A new technique is described for measuring crystallization kinetics during isothermal flow at constant shear rate in a parallel plate rheometer. The crystallization rate is characterized by the time elapsed from the start of shearing until the rise in melt viscosity due to crystallization. The measured‐viscosity and induction time for crystallization are shown to be independent of sample geometry. Kinetic data are presented for crystallization of three linear polyethylenes at shear rates of 0.03 – 30 sec−1. It is shown that shear flow has a strong accelerating effect on crystallization when the deformation rate exceeds a critical value. Comparison of results for the different polyethylenes reveals that higher molecular weight materials crystallize faster at a given shear rate and temperature. Finally, shear‐induced crystallization of propylene polymers is shown to be unaffected by the presence of either a carbon black additive or a heterogeneous nucleating agent. It is concluded that the hydrodynamic origin of the shear‐induced crystallization is elastic chain extension due to entanglement couplings between molecules. Furthermore, it is suggested that transient orientation effects during the startup of shear flow may have a dominant influence on the observed phenomena.
where the Ms value is independent of pH, i.e., a = 1. (12) This is because the result by Behrens et al. involves indefinite amino acid residues: Glx, residues 266, 268, 382, and 397; Asx, residues 267 and 269. However, the ionizable group contents (1.70 mmol/g for the total acidic groups and 1.51 mmol/g for the total basic groups), calculated by assuming that Glx and Asx are equal to Glu and Asp (acid form) or Gin and Asn (amide form), approximately agree with those shown in Table I.
The squeezed state approach of the semiclassical limit of the timedependent Schrödinger equation Many physical properties of polymer glasses change spontaneously during isothermal aging by a process commonly modeled as collapse offree volume. The model has not been verified rigorously because free volume cannot be unambiguously measured. In the present investigation, we tentatively identify the free-volume fraction with the fraction of empty sites in the equation of state ofSimha and Somcynsky. With this theory, volume recovery measurements can be analyzed to yield directly the time-dependent, free-volume fraction. Using this approach, recent volume measurements on poly(methyl methacrylate) are analyzed. The resulting free-volume fractions are then used in the Doolittle equation to predict the shift in stress relaxation curves at 23 ·C. These predicted shift factors agree with the experimental measurements of Cizmecioglu et al. In addition, it is shown that previous assumptions concerning temperature dependence of free volume are inconsistent with the theory.
Semicrystalline polymers gelled from thermally quenched semidilute solutions can, in some cases, be supercritically dried to produce nano‐structured foams of exceedingly high specific surface area. This article investigates the nano‐morphology of these semicrystalline foams. The common morphological feature that these systems display in small‐angle scattering can be described by uncorrelated lamellar platelets. The morphological details, which can be obtained using microscopy and small‐angle scattering, indicate that these low‐density systems occupy a morphological niche between polymeric crystallites from dilute solutions, and spherulitic crystals derived from concentrated solutions and melts. Because these crystalline morphologies occur in concentration ranges between dilute and concentrated, they may offer simple insight into the mechanisms available for distortion of ideal, dilute‐solution‐derived crystallites as polymer concentration is increased. Several mechanisms for the observed distortions are proposed. © 1996 John Wiley & Sons, Inc.
SynopsisThe morphology of a chemically crosslinked urethane elastomer is correlated with its time-dependent mechanical properties. Evaluation of this amorphous elastomer by electron microscopy and small-angle x-ray scattering reveals that incompatible chain segments cluster into separate microphases having a periodicity in electron density of about 90 A. This observed domain structure is similar to that seen previously in uncrosslinked, thermoplastic urethane elastomers. As in earlier studies on such linear systems, thermal pretreatment of the crosslinked elastomer causes a timedependent change in its room temperature modulus. However, the magnitude of this modulus change (about 2W) is generally less than observed previously with the linear systems. Another contrast with previous findings is that this time-dependent phenomenon is apparently not caused by thermally activated changes in microphase segregation. Rather, the observed time dependence in modulus is believed to be caused by molecular relaxation resulting in densification of amorphous packing within the hard-segment domains. The validity of this proposed mechanism is supported by differential scanning calorimetry experiments showing evidence of enthalpy relaxation during roomtemperature aging of the elastomer. This relaxation is qualitatively similar to that observed previously during sub-T, annealing of single-phase glassy polymers.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.