ABSTRACT:To understand the reversible gelation and subsequent aging of hydrogels prepared by freeze/thaw processing of poly(vinyl alcohol) (PVOH) solutions, the microstructures of gels prepared by different freeze/thaw protocols and aged to varying extents are studied by cryogenic transmission electron microscopy, solid-state nuclear magnetic resonance, X-ray scattering, and differential scanning calorimetry (DSC). As discussed in the literature, gelation by the freeze/thaw process occurs as a homogeneous aqueous poly(vinyl alcohol) solution is cycled, perhaps multiple times, between temperatures above 0°C and well below 0°C. The current investigation has determined that a few percent of chain segments crystallize during the first cycle, organizing themselves into 3-8 nm primary crystallite junctions separated on an irregular mesh by an average spacing of ϳ 30 nm. Aging or imposition of additional freeze/thaw cycles augments the level of crystallinity and transforms the as-formed liquid-like microstructure, characterized in the electron microscope by rounded ϳ 30 nm pores, into a fibrillar network. Observation that the transformation occurs at fixed mesh spacing and approximately constant average crystallite size suggests the formation of secondary crystallites that do not affect network connectivity. Dendritic ice crystallization and possibly spinodal decomposition superimpose on this nanoscale structure a matrix of much larger pores.

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To understand the reversible gelation and subsequent aging of hydrogels prepared by freeze/thaw processing of poly(vinyl alcohol) (PVOH) solutions, the microstructures of gels prepared by different freeze/thaw protocols and aged to varying extents are studied by cryogenic transmission electron microscopy, solid‐state nuclear magnetic resonance, X‐ray scattering, and differential scanning calorimetry (DSC). As discussed in the literature, gelation by the freeze/thaw process occurs as a homogeneous aqueous poly(vinyl alcohol) solution is cycled, perhaps multiple times, between temperatures above 0 °C and well below 0 °C. The current investigation has determined that a few percent of chain segments crystallize during the first cycle, organizing themselves into 3–8 nm primary crystallite junctions separated on an irregular mesh by an average spacing of ∼ 30 nm. Aging or imposition of additional freeze/thaw cycles augments the level of crystallinity and transforms the as‐formed liquid‐like microstructure, characterized in the electron microscope by rounded ∼ 30 nm pores, into a fibrillar network. Observation that the transformation occurs at fixed mesh spacing and approximately constant average crystallite size suggests the formation of secondary crystallites that do not affect network connectivity. Dendritic ice crystallization and possibly spinodal decomposition superimpose on this nanoscale structure a matrix of much larger pores. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3438–3454, 1999

The mechanical behavior of compatible glassy polyblends based upon poly(2.6‐dimethyl‐ 1,4‐phe nylene oxide) (PPO) was investigated. In particular, the influence of composition, molecular weight, and molecular weight distribution upon the tensile modulus of the blend was assessed. Various possible correlations between the experimentally determined moduli and theory are considered. Included are correlations with density, packing density, composite theory, and lattice fluid theory. The modeling of the properties of mixtures via Simplex lattice design is also presented. Finally, attention is given to the development of compatibility criteria based upon tensile modulus and density measurements.

The roles of poly(lactic acid) chain conformation and configuration on the enthalpy relaxation kinetics of amorphous poly(lactic acid) were examined. Enthalpic relaxation data, which were scaled to the same supercooling from the initial fictive temperature, were taken for three types of the polymer containing various d-lactyl monomers (5.7%, 13.0%, 50%) to assess the effects of configurational defects. The kinetics data were very similar from sample to sample. The effects of configurational defects were assessed using the generalized Kohlrausch−Williams−Watts (KWW) equation solved by the Tool−Narayanaswamy−Moynihan (TNM) equation. The major effect of increasing the d-lactyl contents was to lower the PLA glass transition temperature, thereby accelerating the kinetics of enthalpic relaxation. Configurational defects showed no significant effect on the other KWW/TNM fit parameters (x, Δh, ln A). A slightly larger KWW β stretched exponential parameter is observed for greater (50% d) than for lower (5.7% d) amount of d-lactyl monomer, although these differences are just within the experimental error. Raman spectroscopy showed that conformation does not change appreciably during physical aging.

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High-quality polarized Raman spectra have been obtained for various poly(vinylidene fluoride) (PVDF) structures, crystalline and amorphous. The results encouraged us to revisit the Raman band assignment, especially within the conformational sensitive region (400−1100 cm-1) and to use the new understanding to characterize the amorphous region of PVDF. Vibrational bands have been assigned on the basis of polarization characteristics observed and the potential energy distribution (PED) calculated. The simulated results agree well with the experimental polarized Raman study. On the basis of the calculated PED, combined with the simulation of different conformational sequences (tttt, tttg, tgtg‘ tggg, gggg), spectroscopic features (band intensity at 648 cm-1 and the frequency change of the 856 cm-1 band) were associated with the distribution of rotational isomeric states. Two rotational isomeric state (RIS) models were analyzed and compared in the simulation of the amorphous state. On the basis of the spectroscopic features of experimental and simulated Raman spectra, it was concluded that the model which predicts higher gauche population more accurately describes the amorphous state. This analysis provides an opportunity to describe the amorphous state in a quantitative manner.