Effect of molecular weight on rate of enthalpy relaxation of poly (methyl methacrylate) and its oligomers

The enthalpy relaxation process of methyl methacrylate oligomers and poly (methyl methacrylate) was analyzed by use of the excess enthalpy (ΔH_t), which was obtained from a differential scanning calorimetric measurement. The apparent relaxation time (τ_½) was estimated by the assumption of the single relaxation time response. The T_g of oligomers increased with an increase of molecular weight, and approached a constant value above 1 × 10⁴. The apparent relaxation time showed a similar molecular-weight dependency as T_g, and approached a constant value above 1 × 10⁴. It was expected that the molecular motion which caused the enthalpy relaxation was based on the segmental motion of main chains.