Thermorheological complexity of poly(methyl methacrylate)/poly(vinylidene fluoride) miscible polymer blend: Terminal and segmental levels

Revealing the dynamic heterogeneity of PMMA/PVDF blends: from microscopic dynamics to macroscopic properties

An effort was made to demonstrate the dynamic heterogeneity of poly(methyl methacrylate) (PMMA)/poly(vinylidene fluoride) (PVDF) blends, where its composition dependence and the role of interphase were probed. Firstly, the composition dependence of thermorheological complexity of PMMA/PVDF blends in the melt was revealed. The molecular entanglement state involving intra- and interchain entanglements was found to govern the scenario of thermorheological complexity. Intriguingly, local heterogeneity was further demonstrated to exist in the melt-state blends with intermediate compositions, and its origin was depicted to be the interphase. The interphase, coupled with unfavourable interchain entanglements in those blends, could explain the reduced viscosity and speed-up relaxations, contributing to the overall thermorheological complexity. Besides, two experimental glass transition temperatures of blends were resolved in view of segment motions in the miscible phase and the crystal-amorphous interphase, and further assessed via the "self-concentration" concept. The presence of a crystal-amorphous interphase, likely leading to three distinct dynamics of segments in blends, was supposed to contribute to the dynamic heterogeneity in segment relaxations for PMMA/PVDF blends in the solid state. Lastly, effects of dynamic heterogeneity on dynamic mechanical properties were also evaluated.

Modeling Based Characterization of Thermorheological Properties of Polyurethane ESTANE™

Shape-Memory Polymers (SMPs) have the ability to be deformed and memorize this deformation until an external activation stimulus (e.g., heat) is applied. Therefore, they have attracted great interest in many areas, especially for applications where reconfigurable structures are required (e.g., Shape-Memory (SM) stents or micro air vehicles). Nevertheless, prior to technical application, the effective thermomechanical behavior of SMPs must be thoroughly understood. In the current contribution, an assessment of thermorheological properties of the commercially available polyurethane system ESTANE is presented. Thermorheological properties were investigated using Dynamic Mechanical Thermal Analysis (DMTA) and complementary uniaxial stress relaxation experiments. Upon material parameter optimization, a finite viscoelastic and incompressible material model was used to model experimentally observed viscoelastic properties.

Composition-dependent depression of the glass transition temperature of the rubber phase in a PE-SBR blend

A thermoplastic elastomeric blend was introduced based on high-density polyethylene and styrene-butadiene rubber (SBR). The morphology of the blend system was found to change from two-phase to co-continuous, depending on the polyethylene (PE) concentration. The thermo-mechanical measurements showed that, at the PE concentrations, which seemed to have a co-continuous morphology, the SBR inclusions were very fine. The fineness of the SBR domains resulted in a 20° depression of the glass transition temperature (Tg) of the SBR phase. This was attributed to the smooth interfaces at which the SBR chain sensed a higher free volume as compared with that in bulk SBR. A model correlating thin polymer thickness to the Tg of the confined polymer in the films was employed to correlate SBR domain size to the experimental Tg measured. The adjusting parameters A=129 and δ=1.45 were successfully used to fit the experimental data.

Surface indentation arrays for high-throughput analysis of viscoelastic material properties

Viscoelastic relaxation processes factor into polymer performance and stability throughout an application lifetime, controlled by the polymer network structure and dynamics which occur over a wide spectrum of time scales. In this work, we detail the design and operation of an independent array of surface indenters which can measure the creep response at multiple points on a polymer substrate. Samples with composition and temperature gradients are used to exhibit the ability to measure viscoelastic properties under unique conditions for each indentation. Methacrylate photopolymer systems are measured at different compositions and crosslink densities simultaneously within an indenter array to increase the measurement throughput, with a measured creep compliance ranging from 10(-9) Pa(-1) to 10(-5) Pa(-1). The application of temperature gradients allows for the viscoelastic measurements to be assembled onto a master curve using time-temperature superposition.