ABSTRACT
Properties of polymers depend on both the structure of their molecular subsystem and their non-equilibrium excessive free volume VNEF. The data of densitometry and NMR spectroscopy for unannealed epoxy films demonstrate the presence of three types of defects in highly crosslinked polymers: n-defects of approximately atomic size, their clusters, and microvoids of nanometric size. The polymeric chains involved in the structure of n-defects and their complexes are stressed. The polymeric chains in a vicinity of a microvoid are unstressed and exhibit increasing molecular mobility for T < Tg. A physical model for the formation of the VNEF elements is suggested. It involves (1) a stage of formation n-defects in chemical reaction, (2) stepwise formation of the complexes of n-defects, and (3) a stage of microvoids formation by separation of non-equilibrium (excess) free volume in an individual phase. The duration and mechanism of annealing are associated with the size and molecular mobility of the VNEF structural elements. The n-defects, as the smallest ones, are rapidly annealed, at least partially, during heating a sample up to experimental temperature. The annealing the clusters of n-defects and microvoids occurs much slower and includes a step of their disintegration to form mobile n-defects, which then relax by diffusion. At high temperatures (for T ≫ Tg), the annealing of n-defects is accompanied by decomposition of the polymer network, the polymer chains involved in the structure of n-defects and their clusters exhibiting the fastest thermal decomposition.
