Reversible gelation of poly(dimethylsiloxane) with ionic and hydrogen-bonding substituents

Poly(dimethylsiloxane) copolymers containing a small fraction of carboxylic acid or Zn-carboxylate groups were prepared and compared regarding reversi ble gelation by hydrogen-bonding and ion-pair interaction. The polymers wer e synthesized by condensation of a t-butylcarboxylate functionalized dichlo rosilane with an alpha,omega-dihydroxy-poly(dimethylsiloxane), followed by thermal cleavage of the ester bond. Neutralization of the resulting carboxy lic acid substituents was achieved by addition of Zn (acac)(2). Reversible crosslinking was investigated by step stress and oscillating shear experime nts. The carboxylic acid containing poly(dimethylsiloxane) became rubberlik e upon increasing the temperature and liquified again when it was brought b ack to room temperature. This observation has been explained tentatively by segregation of the carboxylic acid groups into polar domains at high tempe ratures [i.e., a behavior like it is observed for systems with a lower crit ical solution temperature (LCST)]. At ambient temperature, the carboxylic a cid groups undergo hydrogen bonding to the Si-O-Si backbone. Clustering of the carboxylic acid groups occurs only as these hydrogen bonds break upon r aising temperature. Moisture was found to have a strong influence on the re versal of the crosslinking. Addition of zinc acetylacetonate resulted in th e formation of an elastic network already at ambient conditions consistent with the concept of ionomers which undergo reversible gelation by formation of ion-pair multiplets and clusters in the hydrophobic polymer matrix in p articularly at low temperatures. At high temperature, both the carboxylic a cid and the carboxylate sample exhibited a rather similar viscoelastic beha vior consistent with a common structure where transient crosslinks are form ed by clusters of the carboxylic acid and the carboxylate groups. (C) 1999 John Wiley & Sons, Inc.