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We carried out a large set of tests to establish a correlation between the molecular (network) structure (influenced by molecular weight, molecular weight distribution, and melt predeformation) and mechanical responses of several glassy polymers to uniaxial compression at different temperatures and different compression speeds. The experimental results show that to have ductile responses there must be an adequate chain network, afforded by the interchain uncrossability among sufficiently long chains. Specifically, polystyrene (PS) and poly(methyl methacrylate) of sufficiently low molar mass do not have chain network and are found to be very brittle. Binary PS mixtures are brittle at room temperature when the volume fraction of the high‐molecular‐weight component is sufficiently low (e.g., at and below 27.5%). Moreover, sufficiently melt‐stretched PS mixtures show brittle fracture when compressed along the same direction, along which melt stretching was made. All the experimental findings confirm that a robust chain network is also a prerequisite for yielding and ductile cold compression of polymer glasses, as is for extension. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 758–770 [ABSTRACT FROM AUTHOR]

In comparison to the behavior of linear block copolymers, much less is known about the structure and properties of highly branched polymeric materials. Motivated by this, the solution properties of a series of 26- and 40-arm polystyrene-poly(2-vinylpyridine) (PS-PVP) star diblock copolymers of different weight-average molecular weight (Mw) and styrene to 2-vinylpyridine (S/V) ratios are studied. These stars are investigated in tetrahydrofuran (THF), a thermodynamically good solvent for both blocks, and in toluene, a solvent that is selective for the inner PS blocks. It is found that in both THF and toluene, the 26- and 40-arm stars remain dispersed as unimolecular star block copolymers across the concentration range studied, 0.001< c < 10.0 mg/mL. The hydrodynamic radius, Rh, increases with PS Mw and number of arms, with the stars of highest Mw and number of arms having the largest Rh. The characteristic ρ ratio, Rg/Rh, is -1.0, suggesting that these stars do not behave as homogeneous hard spheres, but rather exhibit Gaussian soft sphere characteristics. TEM images indicate that these stars adopt an unusual asymmetric structure in toluene due to intramicellar microphase segregation of the arms: The PVP blocks collapse and aggregate within the unimolecular structure while the PS blocks stretch and shield the aggregated PVP domain. Despite the strong tendency of PVP to drive aggregation in toluene, repulsive steric interactions between solvated PS blocks and this rearrangement prevent PVP domains of selectively solvated stars from assembling into multimolecular aggregates. This morphological behavior is consistent with inferences drawn from analyses of the concentration dependence of the z-average diffusion coefficient, which suggests that frictional interactions are stronger than star-star interactions. In total, these results shed new light on how topologically complex, amphiphilic block copolymers organize in solution. [ABSTRACT FROM AUTHOR]

The poly(C60-co-styrene)s prepared by radical copolymerizations of C60 and styrene in bulk were characterized by multiple-detector size-exclusion chromatographs consisting of a refractometer, a differential viscometer, and/or a right-angle laser-light scattering photometer. The multidetector systems enabled the determinations of reliable, absolute molecular weights of the copolymers. The plots of intrinsic viscosity vs. molecular weight and radius of gyration vs. molecular weight offered valuable information about the molecular architectures of the copolymers. The slopes of the plots reveal that the structure of the copolymer changes with its C60 content: the copolymer with a low C60 content of 0.58 wt% possesses a linear structure, whereas its conger with a high C60 content of 1.14 wt% possesses a branched structure. [Copyright &y& Elsevier]

Poly(acrylic acid-co-itaconic acid) copolymers containing pendent methacrylates were synthesized and used to formulate redox-initiated in situ cured glass-ionomer cements (GICs) by mixing with reactive glass fillers (Fuji II LC). Various formulations for the redox initiator were studied, and flexural strength (FS) was used as a screening tool for optimization. Effects of molecular weight (MW), grafting ratio, comonomer, polymer content in the liquid composition, powder/liquid (P/L) ratio, and aging on FS were investigated. The results show that the in situ cured GICs demonstrated higher FS (89.6–123.2 MPa), as compared to commercial Fuji II LC GIC (57.1 MPa). The optimal concentrations for redox initiators were found to be 0.15% (by weight) for K2S2O8 and 0.2% for ascorbic acid (or 0.6% for microencapsulated ascorbic acid), respectively. Effects of MW, grafting ratio, P/L ratio and polymer content in the liquid formulation were significant. During aging, the cement showed an increase in strength over 24 h and then no change for time periods up to six months. SEM analysis supports the strength data associated with the formulations. The exotherm and setting time suggest that novel redox-initiated resin-modified GICs hold promise as biocompatible and workable cement for orthopedic applications. [Copyright &y& Elsevier]

Six methacrylate or acrylate derivatives of natural amino acids were synthesized and characterized. Based upon these monomers, six terpolymers [poly(acrylic) acid-co-itaconic acid-co-amino acid] were prepared and characterized. The synthesized polymers were used to formulate glass-ionomer cements (GICs) using Fuji II glass filler. The effects of the molecular weight (MW) and powder/liquid (P/L) ratio were evaluated. Scanning electron microscopy (SEM) was used to examine the fracture surfaces of the selected cement specimens. Results show that all the amino acid modified GICs exhibited higher compressive strengths (CS, 193–236 MPa) and much higher flexural strengths (FS, 55–71 MPa) as compared to commercial Fuji II GIC (191 in CS and 16 in FS). Both MW and P/L ratio affected the strength of the formed cement. It was important to find the optimal MW and P/L ratio to obtain the highest FS. In this study, optimized MW (number average) of the polyacids and P/L ratio were around 50,000 and 2.7/1, respectively. The microstructures of the fracture surfaces helped to explain the strength differences among the materials tested in the study. SEM analysis suggests that more integrated microstructures and fewer defects can lead to higher FS. [Copyright &y& Elsevier]

The properties of the interfacial layer between the polymer matrix and nanoparticles largely determine the macroscopic properties of polymer nanocomposites (PNCs). Although the static thickness of the interfacial layer was found to increase with the molecular weight (MW), the influence of MW on segmental relaxation and the glass transition in this layer remains to be explored. In this Letter, we show an unexpected MW dependence of the interfacial properties in PNC with attractive polymer-nanoparticle interactions: the thickness of the interfacial layer with hindered segmental relaxation decreases as MW increases, in sharp contrast to theoretical predictions. Further analyses reveal a reduction in mass density of the interfacial layer with increasing MW, which can elucidate these unexpected dynamic effects. Our observations call for a significant revision of the current understandings of PNCs and suggest interesting ways to tailor their properties. [ABSTRACT FROM AUTHOR]