MOLECULAR TAILORING OF THERMOREVERSIBLE COPOLYMER GELS - SOME NEW MECHANISTIC INSIGHTS

We earlier reported the role of hydrophobic and hydrogen bonding inter actions on the transition temperatures of thermoreversible copolymer g els. We show here that the chemical structure of;the hydrophobe and it s,concentration determine the transition temperatures [lower critical solution temperature (LCST)] and the heat of transition of new hydroph obically modified poly(N-isopropyl acrylamide) [PNIPAm] copolymer gels . The gels, prepared by copolymerizing NIPAm monomer with hydrophobic comonomers containing increasing lengths of alkyl side groups and a te rminal carboxyl acid group, showed lower LCST and lower heat of-transi tion when compared to pure PNIPAm gel. The experimental results were a lso compared with theoretical calculations based on a lattice-fluid-hy drogen-bond [LFHB] model. We show experimentally and theoretically tha t a linear correlation exists between the transition temperature and l ength of the hydrophobic alkyl side group. Also, in apparent contradic tion to previous work, we found a reduction in the heat of transition with increasing hydrophobicity. We propose that the presence of the te rminal carboxyl acid group on the hydrophobic side chain of the comono mer prevents the association of water molecules around the hydrophobe, thereby causing a reduction in the heat of transition; The LFHB model :supports this argument. (C) 1998 American Institute of Physics.