Influence of Annealing on Chain Entanglement and Molecular Dynamics in Weak Dynamic Asymmetry Polymer Blends

Effect of Morphology/Structure on the Phase Behavior and Nonlinear Rheological Properties of NR/SBR Blends

The evolution of the morphology/structure and the nonlinear viscoelasticity of rubber blends under large amounts of strain are key scientific issues for the design and manufacture of rubber blends. The rheological responses of natural rubber/styrene-butadiene rubber (NR/SBR) blends are traced over a wide range of blend compositions to gain an insight into the effect of blend morphology on their nonlinear viscoelasticity. We also prepare NR + SBR physical blends without melt mixing to distinguish the contributions of composition and blend morphology to the viscoelastic response. The microscopic heterogeneous gel-like structure of NR/SBR blends may remarkably weaken their strain softening and improve their modulus hysteretic recovery under large strain, which may be attributed to the heterogeneous microscopic deformation for the NR and SBR phases. Furthermore, additional elastic contribution resulted from the increasing interfacial energy of domain deformation. This may provide some new insights into the effect of blend morphology on the Payne effect of rubber blends.

Influence of ionic liquids on the chain dynamics and enthalpy relaxation of poly(methyl methacrylate)

Imidazolium ionic liquids (ILs) with various alkyl chain lengths on the cations ([C_nmim]⁺, n = 2, 4 and 8) and different combined anions ([TFSI]^- and [PF₆]^-) were blended with poly(methyl methacrylate) (PMMA), and the effects of the IL structure on the chain dynamics of PMMA were experimentally investigated by rheology and DSC measurements combined with a simulation method. The results indicate that the interaction between PMMA and ILs becomes stronger as the alkyl chain length on the imidazolium ring increases or the anion changes from [PF₆]^- to [TFSI]^-. As a result, a higher critical entanglement concentration and a larger entanglement molecular weight of PMMA were found in [C₈mim][TFSI] due to the stiffer conformation. Molecular dynamics (MD) simulations further demonstrated stronger interactions between PMMA and ILs with longer cationic alkyl chain lengths or [TFSI]^- anions, which showed smaller Flory-Huggins interaction parameters and larger radii of gyration, R_g. However, the larger size of alkyl chains or [TFSI]^- anions produced a larger free volume in the system as evidenced by positron annihilation lifetime spectroscopy (PALS), which competed with the molecular interaction and dominated the segmental motion. Therefore, a lower T_g and accelerated segmental relaxation were observed. Compared to alkyl chain length, the effect of anions on the interactions between ILs and PMMA is more prominent.

Chain entanglement and molecular dynamics of solution-cast PMMA/SMA blend films affected by hydrogen bonding between casting solvents and polymer chains

The effects of intermolecular interaction between casting solvents and polymer chains on molecular entanglement and dynamics in solution-cast PMMA/SMA films were probed by rheological, dielectric and modulated DSC methods.

Surface dynamics of poly(methyl methacrylate) films affected by the concentration of casting solutions

The effect of the concentration of casting solutions on the surface dynamics of the corresponding spin-coated poly(methyl methacrylate) (PMMA) film was investigated by measuring the surface reorganization of fluorine tracer-labeled PMMA. The onset temperature of fluorinated PMMA chain end reorganization (T(onsetR)) was identified and is shown to depend on the PMMA concentration in the film-forming solution. It was found that the surface T(onsetR) and relaxation activation energy E(a) of the PMMA films prepared from 4.2 wt% PMMA cyclohexanone solution are 70 °C and 260 kJ mol(-1), respectively, which are higher than those of the PMMA films prepared from 0.8 wt% PMMA cyclohexanone solution (55 °C and 144 kJ mol(-1), respectively). The T(onsetR) and E(a) of PMMA films increased with increasing concentration of casting solutions within the range of 1.8 wt% to 4 wt%. The chain entanglement of PMMA chains is proposed to be the speculative origin for these observed depressed dynamics of poly(methyl methacrylate) chains on the films' surface prepared using casting solutions of various concentrations.