Crystallization of PDMS: The effect of physical and chemical crosslinks

Calorimetric studies of the crystallization behavior of physically and chemically crosslinked semicrystalline polymer, polydimethylsiloxane (PDMS) are presented. Physical crosslinks are introduced either via entanglements in high-molecular-weight PDMS, or by anchoring chain ends to rigid polyethylene oxide (PEO) endblocks in a PEO-b-PDMS-b-PEO triblock copolymer. Chemical end-linking of di-vinyl PDMS chains results in the formation of a crosslinked network. Comparison of the thermograms obtained for each of these systems at constant cooling/heating rates with their noncrosslinked analogues indicates that, contrary to conventional wisdom, the different types of crosslinks result in an increased crystallization tendency. We suggest that this effect is a manifestation of the enhancement of local ordering together with reduced dynamics as compared to the non-crosslinked melt.