MOLECULAR-STRUCTURES IN ISOTACTIC POLYSTYRENE THERMOREVERSIBLE GELS

The chain helical structure, the chain-solvent arrangement, and the chain trajectory are investigated in isotactic polystyrene (iPS) thermoreversible gels by three techniques: neutron diffraction, swelling experiments, and small-angle neutron scattering. From the neutron diffraction experiments on gels prepared from various solvents, it is again concluded that the 121 helical form is absent in the nascent state (undried and unstretched). As already proposed in a previous paper, a solvated near-31 helix seems to be a better candidate to account for the results (ladderlike model). The swelling experiments support this model by indicating that the gel behaves as a polymer-solvent compound whose stoichiometry is defined at Cγ ≈ 0.3 in cis-decalin and Cγ ≈ 0.38 in irans-decalin, values in good agreement with those found previously by calorimetry and solvent crystallization. The small-angle neutron scattering on iPS/cis-decalin gels reveals a most unusual behavior in that the chains become stiffer and stiffer up to C = 0.3 (increase of the chain radius of gyration and q-1 behavior consistent with rodlike portions larger than 300 Å) then lose their stiffness (decrease of the chain radius of gyration and loss of the q-1 behavior). Such behavior is in accord with the temperature-concentration phase diagram established earlier. All the results are discussed in the light of various molecular models. © 1990, American Chemical Society. All rights reserved.