Blends Of Linear Chains Research Articles

Rheology of Crystallizing LLDPE.

Polymer crystallization occurs in many plastic manufacturing processes, from injection molding to film blowing. Linear low-density polyethylene (LLDPE) is one of the most commonly processed polymers, wherein the type and extent of short-chain branching (SCB) may be varied to influence crystallization. In this work, we report simultaneous measurements of the rheology and Raman spectra, using a Rheo-Raman microscope, for two industrial-grade LLDPEs undergoing crystallization. These polymers are characterized by broad polydispersity, SCB and the presence of polymer chain entanglements. The rheological behavior of these entangled LLDPE melts is modeled as a function of crystallinity using a slip-link model. The partially crystallized melt is represented by a blend of linear chains with either free or crosslinked ends, wherein the crosslinks represent attachment to growing crystallites, and a modulus shift factor that increases with degree of crystallinity. In contrast to our previous application of the slip-link model to isotactic polypropylene (iPP), in which the introduction of only bridging segments with crosslinks at both ends was sufficient to describe the available data, for these LLDPEs we find it necessary to introduce dangling segments, with crosslinks at only one end. The model captures quantitatively the evolution of viscosity and elasticity with crystallization over the whole range of frequencies in the linear regime for two LLDPE grades.

A slip-link model for rheology of entangled polymer melts with crystallization

Many industrial polymer melts are entangled and undergo crystallization during processing. Several recent studies have reported experimental data for the rheology of crystallizing entangled polymers. Meanwhile, over the past few years, slip-link models have been demonstrated to describe the rheology of entangled melts under a variety of nonlinear deformation conditions. In this work, we present a modification of the slip-link model to describe the rheology of an entangled melt undergoing crystallization. The partially crystallized melt is represented by a blend of linear chains with free ends and cross-linked, bridgelike chains with fixed ends that resemble the tie molecules between developing crystallites. Two new parameters are introduced: the fraction of cross-linked chains and a modulus shift factor, both of which are functions of the degree of crystallinity. The model captures the evolution of viscosity and elasticity simultaneously over the whole range of available frequencies in the linear regime. The model is validated using experimental datasets for isotactic polypropylene.Many industrial polymer melts are entangled and undergo crystallization during processing. Several recent studies have reported experimental data for the rheology of crystallizing entangled polymers. Meanwhile, over the past few years, slip-link models have been demonstrated to describe the rheology of entangled melts under a variety of nonlinear deformation conditions. In this work, we present a modification of the slip-link model to describe the rheology of an entangled melt undergoing crystallization. The partially crystallized melt is represented by a blend of linear chains with free ends and cross-linked, bridgelike chains with fixed ends that resemble the tie molecules between developing crystallites. Two new parameters are introduced: the fraction of cross-linked chains and a modulus shift factor, both of which are functions of the degree of crystallinity. The model captures the evolution of viscosity and elasticity simultaneously over the whole range of available frequencies in the linear regime. ...

The Effect of Molecular Architecture on the Phase Diagram and Mobility of a Polymer Blend Exhibiting a Lower Critical Solution Temperature: Cycles Mixed with Linear Chains.

AbstractWe examine the role of molecular architecture on the phase diagram of the PS/PVME (poly[styrenel /poly[vinyl methyl ether]) blend, a mixture which in previous studies with linear chains exhibited a lower critical solution temperature, (LCST) i.e. it phase separated on heating. In this investigation, two blends with components exceeding the critical molecular weight for entanglement were compared: one consisting of linear PS and PVME and a second with cyclic PS and linear PVME. Cloud point experiments over a broad composition range reveal that the blend containing cyclic PS undergoes phase separation at temperatures 7-8 °C higher than the analogous linear blend. In other words, the mixture of cycles and linear chains is more thermodynamically stable than the mixture of two linear chains.The LCST nature of the system facilitates examining chain mobility by considering the phase separation kinetics. Time-resolved light scattering studies of blends near their critical compositions tracked the spinodal decomposition following a rapid temperature jump from the one-phase to the two-phase region. An analysis of the scattering intensity growth ultimately led to mutual diffusion coefficients whose temperature dependence confirmed the observed cloud points. An approximation of the second derivative of the free energy function based on SANS studies of the linear PS/PVME blend allowed us to estimate mutual mobilities. The values determined for the cycle-containing blend were considerably lower than those for the blend of linear chains at these molecular weights.