Effects of temperature and swelling on chain dynamics during the sol-gel transition

The UV-irradiation-induced [2 + 2] photocycloaddition of 7-methacryloyloxy-4-methylcoumarin attached to PMMA can transfer a semidilute solution to a homogeneous and speckle-free chemical gel. It allows us to study the effect of temperature on the chain dynamics at each stage of the sol-gel transition by laser light scattering. The fast and slow relaxation modes in semidilute solution are preserved in the resultant gel. For the fast mode, the characteristic decay time <tau(c)>(f) and its related scattering intensity <I>(fast) are nearly temperature-independent because it is related to the motions of the subchains ("blobs") between two cross-linked points. In contrast, <tau(c)>(s) and <I>(slow) increase as the temperature decreases from 40 to 10 degreesC, which cannot be attributed to the chain conformation because chloroform remains a good solvent in this temperature range. The increase of <I>(slow) is because the lights scattered from the segments of different chains are more correlated at lower temperatures (higher viscosity). This study further supports that the slow mode is due to the density fluctuation. Moreover, the scattering vector (q) dependence of <tau(c)>(f) and <tau(c)>(s) shows that as the temperature decreases, the scaling exponent alpha(f) in 1/<tau(c)>(f) similar to q(alphaf) increases from 2.0 to 2.4, but alpha(s) in 1/<tau(c)>(s) similar to q(alphas) decreases from 3.0 to 2.2, which can be related to the decrease of static correlation length. The swelling of the resultant gel shows a similar effect as the increase of the temperature.