Effect of Molecular Weight on Enthalpy Relaxation in Syndiotactic Poly(Methyl‐Methacrylate)

The structural relaxation behaviour of narrow fractions (M w /M n  < 1.1) of syndiotactic poly(methyl methacrylate) with molecular masses ranging from 2,000 to 200,000 Daltons have been studied by DSC with two classical procedures, namely: the rate of cooling and the isothermal approaches. The apparent activation energy ( Δh *) of enthalpy relaxation was evaluated from the dependence of the glass transition temperature on the cooling rate while a comparison of the apparent relaxation rates was appraised from the enthalpy loss by annealing the different samples at the same level of undercooling (T a  = T g  − 10 °C). As expected, the increase of molecular weights gives rise to both a continuous increase of Δh * and a decrease of the apparent isothermal relaxation rate . More interestingly, both Δh * and the apparent isothermal relaxation rate showed abrupt changes around the syndiotactic PMMA entanglement mass ( M e ).