Crystallization and Enthalpy Relaxation of Physically Associating, End-Linked Polymer Networks: Telechelic Pyrene-Labeled Polydimethylsiloxane

The effects of physical crosslinking or gelation and molecular weight on the crystallization behavior of polydimethylsiloxane (PDMS) are determined by thermal analysis of telechelic, pyrene-end-labeled PDMS and its precursor, telechelic, amine-end-labeled PDMS. The pyrene-end-labeled PDMS forms physical crosslinks while the amine-end-labeled PDMS does not. The presence of physical crosslinks leads to a major reduction in PDMS crystallinity achieved during a 5 °C/min quench that is a striking function of molecular weight. In going from the amine-end-labeled PDMS to the pyrene-end-labeled PDMS, a 25 kg/mol sample exhibits a reduction in PDMS crystallinity from 59% to 30% while a 4-7 kg/mol sample exhibits a reduction in PDMS crystallinity by more than an order of magnitude, from 53% to 2 to 4%, with the latter value depending on the extent of physical crosslinking achieved by the pyrenyl units at the start of the cooling cycle. The PDMS melting temperature is a strong function of physical crosslinking, while the PDMS glass transition temperature is invariant with molecular weight and type of end unit. However, the extent of enthalpy relaxation or physical aging of the PDMS achieved during a brief quench is reduced in the presence of pyrenyl end units and thus physical crosslinking.