Reorientational motion of a cross‐link junction in a poly(dimethylsiloxane) network measured by time‐resolved fluorescence depolarization

The role of ultrafast structural dynamics with physical and chemical changes in polydimethylsiloxane thin films by two-dimensional IR spectroscopy

Fourier transform infrared (FTIR) and two-dimensional IR (2D-IR) spectroscopies were applied to polydimethylsiloxane (PDMS) cross-linked elastomer films. The vibrational probe for the systems studied was a silicon hydride mode that was covalently bound to the polymer chains. The structure and dynamics reported by this mode were measured in response to a wide range of chemical and physical perturbations, including elevated curing temperature, increased curing agent concentration, mechanical compression, and cooling to near the glass transition temperature. The FTIR spectra were found to be relatively insensitive to all of these perturbations, and 2D-IR spectroscopy revealed that this was due to the overwhelming influence of heterogeneity on the spectral line shape. Surprisingly, the deconvoluted spectral line shapes showed that there were only slight differences in the heterogeneous and homogeneous dynamics even with the drastic macroscopic changes occurring in different systems. In the context of modeling polymer behavior, the results confirm that dynamics on the ultrafast time scale need not be included to properly model PDMS elasticity.

Molecular Motion of the Junction Points in Model Networks Prepared by Acyclic Triene Metathesis

The junction dynamics in a selectively deuterated model polymer network containing junctions on every 21st chain carbon is studied by solid state (2) H echo NMR. Polymer networks are prepared via acyclic triene metathesis of deuteron-labeled symmetric trienes with deuteron probes precisely placed at the alpha carbon relative to the junction point. The effect of decreasing the cross-link density on the junction dynamics is studied by introduction of polybutadiene chains in-between junctions. The networks are characterized by swelling, gel content, and solid state (1) H MAS NMR. Line shape analysis of the (2) H quadrupolar echo spectra reveals that the degree of motion anisotropy and the distribution of motion correlation times depend on the cross-link density and structural heterogeneity of the polymer networks. A detailed model of the junction dynamics at different temperatures is proposed and explained in terms of the intermolecular cooperativity in densely-packed systems.

Fractional Generalizations of Maxwell and Kelvin-Voigt Models for Biopolymer Characterization

The paper proposes a fractional generalization of the Maxwell and Kelvin-Voigt rheological models for a description of dynamic behavior of biopolymer materials. It was found that the rheological models of Maxwell-type do not work in the case of modeling of viscoelastic solids, and the model which significantly better describes the nature of changes in rheological properties of such media is the modified fractional Kelvin-Voigt model with two built-in springpots (MFKVM2). The proposed model was used to describe the experimental data from the oscillatory and creep tests of 3% (w/v) kuzu starch pastes, and to determine the values of their rheological parameters as a function of pasting time. These parameters provide a lot of additional information about structure and viscoelastic properties of the medium in comparison to the classical analysis of dynamic curves G’ and G” and shear creep compliance J(t). It allowed for a comprehensive description of a wide range of properties of kuzu starch pastes, depending on the conditions of pasting process.