Glass transition in poly(propylene glycol) networks

Difficulty in controlling and determining the structural parameters of polymer networks has hindered experimental studies on the glass transition in crosslinked polymers. A series of wellcharacterized networks of poly(propylene glycol) having narrow network chain‐length distributions and average molecular weight between crosslinks M c in the range of 425–3000 has been prepared. The glass transition temperatures T g of these networks were found to vary linearly with M , consistent with several theoretical treatments. Both the physical crosslinking and the incorporation of crosslinking agent into the system (a “copolymer” effect) are shown to be responsible for increase in T g upon crosslinking in this system. Varying the network chain‐length distribution without changing M c did not affect the T g of the system. The chemical nature of the crosslinking agent, however, does affect the T g of the network, particularly at high crosslink densities.