Unusual Nonlinearity and Nonexponentiality in Glass-Forming Polymers Governed by Ionic Interactions.

The glass transition is dynamically heterogeneous with non-Arrhenius, nonexponential, and nonlinear features, and those parameters are mutually correlated in most cases. This study systematically investigates these interrelated features through thermodynamic analysis of polyvinylpyrrolidone (PVP)/salt complexes, employing enthalpy relaxation parameters, specifically the heat capacity jump (Δ) at and enthalpy hysteresis (Δ). Notably, the introduction of ionic interactions induces simultaneous reductions in dynamic fragility (), nonexponential parameter (β), and nonlinear parameter (), thereby disrupting their previously established empirical correlations.

Material properties and progress in modification of hydrogel-based self-expandable poly(methyl methacrylate) bone cement.

Although traditional poly(methyl methacrylate) (PMMA) bone cements have been widely used in clinical practice, they are beset with many inherent drawbacks such as high polymerization heat, weak osteoinductive and osteoconductive ability, low bioactivity, volumetric shrinkage upon solidification, , which significantly limit its clinical application prospects. Chemical or physical modification of PMMA matrix to regulate its material properties has become a research hotspot in the field of bone tissue repair. Self-expandable PMMA bone cement (SBC) is a novel bone cement developed by copolymerizing PMMA matrix with hydrophilic monomers such as acrylic acid, hydroxyethyl acrylate, They are of high bioactivity and adjustable mechanical properties, along with excellent volumetric swelling capability due to the spontaneous water absorption in body fluids.

Experimental evidences for the correlation between chair-to-chair conversion and dynamic fragility in poly(cyclohexyl methacrylate)/hindered phenol blends.

Polymers with cyclic or ring topologies are typically endowed with unusual molecular dynamics considerably different from their linear or branched counterparts. Here, the primary and secondary dynamics of neat poly(cyclohexyl methacrylate) (PCHMA) and its blends with hindered phenols were systematically investigated by a combination of dielectric and enthalpy relaxation. Notably, neat PCHMA owns a glass transition temperature Tg ∼ 299 K, while the segmental fragility m can be as low as about 42.

Effect of intermolecular hydrogen bonding strength on the dynamic fragility of amorphous polyamides.

Small-molecular-induced intermolecular hydrogen bonding (inter-HB) interactions were reported to increase the glass transition temperature () while decrease the dynamic fragility () of polymers. Herein, enthalpy relaxation parameters heat capacity jump (Δ) at and enthalpy hysteresis (Δ) were investigated to help clarify the effect of macromolecular-induced inter-HB on and using amorphous polyamides as model polymers. The inter-HB strength was weakened by random copolymerization with varied chain rigidity, but was enhanced by decreasing steric hindrance.

Mediating the slow dynamics of polyacrylates by small molecule-bridged hydrogen bonds.

This report studied the changes in the slow dynamics of polyacrylate by adding a hindered phenol (CA) capable of forming three intermolecular hydrogen bonds (inter-HBs) per molecule with the polymer chain. The CA molecule apparently diminishes the slow modes (with lower peak temperatures and peak heights) of the polyacrylate, although it has a higher glass transition temperature () than the acrylic matrix, and the rigid CA-bridged HB network significantly amplifies the α-relaxation near (with higher peak temperatures and peak heights). Consequently, the mixtures exhibit a diminishing slow mode that gradually merges with the prominent -peak with increasing CA loadings.

Tuning the dynamic fragility of acrylic polymers by small molecules: the interplay of molecular structures.

This report studied changes in the dynamic fragility (m) of poly(butyl methacrylate) (PBMA) by introducing guest hindered phenols capable of forming two or three intermolecular hydrogen bonds (inter-HBs) per molecule with the host polymer. The small molecules effectively decrease the m value, even if they apparently increase the glass transition temperature (T) of mixtures. The reduction in m was confirmed by enthalpy relaxation in two aspects: adding the guest molecule leads to a stronger cooling rate dependence of the limiting fictive temperature together with an apparent increase in aging rate of PBMA hybrids at low concentrations.