NMR Study of Gels of Isotactic Polystyrene and cis-or trans-Decalin

Samples consisting of a high molecular weieht (M_w= 1.4 x 10⁶) isotactic polystyrene (iPS) mixed, 25% by weight, with cis-or trans-decalin have been investigated by proton and ¹³C NMR. The most studied samples were gels formed by quenching the solutions to 253 K, although some data on gels formed at 296 K are also reported. Comparison is further made with samples crystallized at 347 K. Most of the NMR data were obtained on nonspinning samples, although magic angle spinning was attempted on one gel sample. Variable-temperature proton spectra covered the range from solvent freezing to gel melting temperatures. The principal motivation was to investigate the nature of the iPS-rich phase in the gels since previous studies have claimed evidence either for a crystal structure different from the usual 3₁ helix or for a polymer/solvent complex. The existence of a polymer/solvent complex (or a solvated crystal structure) was shown to be unlikely on the basis of the NMR findings of high solvent mobilities and widely disperse segment mobilities for iPS. NMR results on the solvent freezing behavior support earlier DSC findings, i.e., sizeable fractions of solvent molecules undergo a gradual immobilization as temperature is lowered compared with the rapid freezing behavior of the pure solvent. In the absence of a polymer/solvent complex, the alternate explanation offered for the gradual freezing is that uncondensed iPS chain segments partition space into a distribution of cavity sizes, which, in turn, impose on the solvent a size-dependent depression of solvent freezing points and enthalpies. Above the solvent freezing temperatures, proton spectra were analyzed in terms of a broad, glasslike and a narrow, liquidlike signal. As the temperature is raised from the solvent freezing point, the broad signal weakens and narrows slightly; correspondingly, the liquidlike signal grows. The thermal reversibility of the line shapes was investigated and established between the solvent freezing points and 296 K. This continuous change and the reversibility offer insight into the growth process of the rigid iPS phase. Whether the structure of the rigid PS segments in the gel is ordered or disordered remains a largely unanswered question. Evidence for disorder is a faster proton dipolar relaxation time and a reduced line width at 296 K compared to either the glassy state or the 3₁ helical crystalline state. Evidence for order is that the aliphatic and the unprotonated aromatic carbon resonances in the CP-MAS spectrum at 296 K have intermediate line widths and a unique chemical shift difference compared to the corresponding parameters in glassy (disordered) and crystalline (ordered) 3₁ helical preparations. Also, the energetics of condensation favor the ordered over the disordered structures. It is concluded that the rigid portion of the gel probably has more order than the glass, but more disorder than the 3₁ helical crystal. Finally, it is shown that the NMR behavior of the gel is very different from that of a preparation of solution-crystallized iPS.