Shear Induced Phase Boundary Shift in the Critical and Off-Critical Regions for a Polybutadiene/Polyisoprene Blend

Physical Gels of Atactic Poly(N-isopropylacrylamide) in Water: Rheological Properties and As-Derived Spinodal Temperature

Aqueous solutions of atactic poly(N-isopropylacrylamide) (a-PNIPAM) undergo complex phase transitions at 20–33 °C. In this temperature range, the a-PNIPAM solution exhibits a phase behavior of lower critical solution temperature at the binodal temperature (Tb) and physical gel formation at the gel temperature (Tgel). On slow heating of the one-phase solution containing linear a-PNIPAM chains, branched chains are gradually developed to proceed with the physical gelation before phase separation considering that Tgel < Tb. Thus, the phase separation temperature determined from the conventional approaches, either by turbidity to derive the Tb or by scattering to derive the spindal temperature (Ts) from the Ornstein–Zernike analysis, is strictly the transition temperature associated with the a-PNIPAM hydrogel (or highly branched chains newly developed at elevated temperatures), rather than the initial a-PNIPAM solution prepared. Herein, the spinodal temperatures of a-PNIPAM hydrogels (Ts,gel) of various concentrations were determined from rheological measurements at a heating rate of 0.2 °C/min. Analyses of the temperature dependence of loss modulus G″ and storage modulus G′ give rise to the Ts,gel, based on the Fredrickson–Larson–Ajji–Choplin mean field theory. In addition, the specific temperature (T1) above which the one-phase solution starts to dramatically form the aggregated structure (e.g., branched chains) was also derived from the onset temperature of G′ increase; this is because as solution temperature approaches the spinodal point, the concentration fluctuations become significant, which is manifested with the elastic response to enhance G′ at T > T1. Depending on the solution concentration, the measured Ts,gel is approximately 5–10 °C higher than the derived T1. On the other hand, Ts,gel is independent of solution concentration to be constant at 32.8 °C. A phase diagram of the a-PNIPAM/H2O mixture is thoroughly constructed together with the previous data of Tgel and Tb.

Microphase separation/crosslinking competition-based ternary microstructure evolution of poly(ether-b-amide)

The temperature dependence of the rheological properties of poly(ether-b-amide) (PEBA) segmented copolymer under oscillatory shear flow has been investigated. The magnitude of the dynamic storage modulus is affected by the physical microphase separation and irreversible crosslinking network, with the latter spontaneously forming between the polyamide segments and becoming the dominant factor in determining the microstructural evolution at temperatures well above the melting point of PEBA. From the rheological results, the initial temperature of the rheological properties dominated by the microphase separation and crosslinking (T cross) structures were determined, respectively. Based on the two obtained temperatures, the microstructure evolution upon the heating can be separated into the ternary microstructure domains: homogenous (temperature below ), microphase separation dominating (between and T cross), and crosslinking dominating domains (above T cross). When the PEBA is heated to above T cross, the content of crosslinking network increases with time and temperature, leading to an irreversible and non-negligible influence on the rheological, crystallization, and mechanical properties. A more pronounced strain-hardening phenomenon during the uniaxial stretching is observed for the sample with a higher content of crosslinking network.

Characterization on the phase separation behavior of styrene-butadiene rubber/polyisoprene/organoclay ternary blends under oscillatory shear

The present work investigated the influence of organoclay (organo-montmorillonite, OMMT) on the phase separation behavior and morphology evolution of solution polymerized styrene-butadiene rubber (SSBR)/low vinyl content polyisoprene (LPI) blends with rheological methodology. It was found that the incorporation of OMMT not only reduced the droplet size of the dispersion phase, slowed down the phase separation kinetics, also enlarged the processing miscibility window of the blends. The determination on the wetting parameters indicated that due to the oscillatory shear effect, the OMMT sheets might localize at the interface between the two phases and act as compatibilizer or rigid barrier to prevent domain coarsening, resulting in slow phase separation kinetics, small droplet size, and stable morphology. The analysis of rheological data by the Palierne model provided further confirmation that the addition of OMMT can decrease the interfacial tension and restrict the relaxation of melt droplets. Therefore, a vivid "sea-fish-net" model was proposed to describe the effect of OMMT on the phase separation behavior of SSBR/LPI blends, in which the OMMT sheets acted as the barrier (net) to slow down the domain coarsening/coalescence in phase separation process of SSBR/LPI blends.

Manipulating the kinetics and mechanism of phase separation in dynamically asymmetric LCST blends by nanoparticles

The addition of nanoparticles in dynamically asymmetric LCST blends is used to induce the preferred phase-separating morphology by tuning the dynamic asymmetry, and to control the kinetics of phase separation by slowing down (or even arresting) the domain growth.

Controlling the kinetics of viscoelastic phase separation through self-assembly of spherical nanoparticles or block copolymers

Viscoelastic phase separation (VPS) can produce a network structure of the minor phase, which needs to be stabilized for designing a heterogeneous structure with desired mechanical and electrical functions. In this work, we investigate the stabilization of the VPS-induced network structure in a dynamically asymmetric PS/PVME blend by incorporation of a SEBS-g-MA block copolymer or dimethyldichlorosilane modified nanosilica. The addition of SEBS-g-MA retards the volume shrinking process and slows down the kinetics of phase separation due to its localization at the PS/PVME interfaces. Consequently, in the later stage of VPS, phase inversion occurs at longer times with respect to the neat blend due to the decreased interfacial tension. In contrast, hydrophobic nanoparticles self-assemble in the bulk of PS-rich phase and restrain the dynamics of polymer chains enhancing the dynamic asymmetry of the system. The efficiency of nanoparticles in controlling the kinetics of phase separation is found to be superior compared to block copolymer-based compatibilizers indicating the significance of chain dynamics. Moreover, beyond a critical nanoparticle volume fraction, phase separation is pinned due to particle percolation within the PS-rich phase, yielding a kinetically trapped VPS-induced network structure.

Does dynamic vulcanization induce phase separation?

Immiscible and miscible blends of poly(vinylidene fluoride) (PVDF) and acrylic rubber (ACM) were subjected to dynamic vulcanization to investigate the effect of crosslinking on phase separation. As a result of different processability, mixing torque behavior of miscible and immiscible blends was significantly different from one another. Scanning electron microscopy (SEM) was used to investigate the morphology of the system. After dynamic vulcanization, submicron ACM droplets were observed in the samples near the binodal curve of the system under mixing conditions. Small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) analysis were used to investigate the effect of dynamic vulcanization on the lamellar structure of the system. It was shown that for samples near the boundary of phase separation, increasing the crosslink density led to a decrease in the lamellar long period (L) as a sign of increment of crosslink density induced phase decomposition. Effects of shear rate on the final morphology of the system were investigated by changing the mixing temperature and by comparing the results of dynamic vulcanization at one phase and two phase regions.