Thermoreversible gelation of solutions of isotactic poly(methyl methacrylate) in 2-butanone

The mechanism of thermoreversible gelation in solutions of isotactic poly(m ethyl methacrylate) (iPMMA) was investigated through the study of the tempe rature-concentration relations, the melting behavior of the solvent and the gel, the change in molecular conformation by FTIR spectroscopy and structu ral analysis by wide angle X-ray scattering. Such a gelation proceeds very fast below -20 degrees C while crystallisation from solution necessitates a n induction period of several weeks, even at high degrees of undercooling. The molecular conformation is different from the one in crystalline samples . The mechanism consists of different steps: a coil to (double) helix trans ition followed by an intermolecular association that finally leads to a mac roscopic crystal lattice. The data clearly illustrate that such a crystalli ne state is reached by a mechanism that is different from the well-known fo lded chain crystallisation. (C) 1999 Elsevier Science Ltd. All rights reser ved.