Nematic Coupling in Polybutadiene from MD Simulations

Negative and Positive Energetic Elasticity of Polydimethylsiloxane Gels

We investigated the thermoelasticity of polydimethylsiloxane (PDMS) gels containing six types of solvents with different solubilities. The contribution of energetic elasticity to the total stress (σ_E/σ) ranges from +0.20 to -0.20 depending on the solvent species. The σ_E/σ values are positive for the solvents with low molecular mass. By contrast, it is negative for oligodimethylsiloxane (ODMS) or PDMS solvents acting as athermal solvents, each of which has the same chemical structure as the network strands. The investigation using a PDMS rubber without a solvent and the PDMS gels with various ODMS contents reveal a crossover of the σ_E/σ value from positive to negative with increasing ODMS content. The pronounced dependence of σ_E/σ on the solvent species and the negative energetic elasticity specific to the high contents of ODMS and PDMS unveil previously unknown aspects of thermoelasticity of polymer gels. The orientation coupling between the segments of the free polymeric chains and network strands is one of the possible scenarios to explain the negative energetic elasticity specific to the ODMS and PDMS solvents, because it stabilizes the aligned state, reducing the elastic energy.

Frustrating Strain-Induced Crystallization of Natural Rubber with Biaxial Stretch

The supreme mechanical performance of natural rubber (NR) is commonly attributed to strain-induced crystallization (SIC). The SIC of NR during uniaxial stretch has been extensively investigated, whereas that under multiaxial deformation has been rarely reported, which is close to real service conditions (i.e., tire). In this work, the crystallization behavior of NR under biaxial stretch was studied with in situ synchrotron radiation wide-angle X-ray diffraction in combination with a custom-built biaxial stretch machine. It is observed that biaxial stretch frustrates the SIC of NR: within λ_x/λ_y < 1.6, where λ_x and λ_y are stretch ratios of two mutually perpendicular axes, no crystallization emerges even under large drawing ratio until sample fracture at ambient temperature. This finding challenges the common wisdom of the self-reinforcement mechanism of SIC in NR under multiaxial deformation in real service conditions. A theoretical SIC model is proposed, which can decouple the contributions of conformational entropy reduction ΔS_f and amorphous chain orientation f to final Gibbs free energy change (ΔG) during multiaxial deformation. This model quantitatively renders a reproduction of the crystallinity during the biaxial stretch, which is well consistent with experimental results and can be further generalized for flow-induced crystallization of semicrystalline polymers.