Glass (Tg) and Stimuli-Responsive (Tsr) Transitions in Random Copolymers
Originally published in Macromolecules, 2010, 43 (12), pp 5330–5337
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In an effort to elucidate the origin of stimuli-responsive (TSR) transitions and correlate them to the glass transition temperature (Tg), poly(N-acryloyl-N′-propylpiperazine-co-2-ethoxyethyl methacrylate) (p(AcrNPP/EEMA)), poly(N-vinylcaprolactam-co-n-butyl acrylate) (p(VCl/nBA)), poly(N-isopropyl methacrylamide-co-n-butyl acrylate) (p(NIPMAm/nBA)), and poly(2-(N,N′-dimethylamino)ethyl methacrylate-co-n-butyl acrylate) (p(DMAEMA/nBA)) colloidal dispersions were synthesized, which upon coalescence form solid films. These studies showed that molecular rearrangements responsible for the TSR transitions are attributed to the backbone buckling and collapse of stimuli-responsive components. Based on empirical data, the relationship between Tg and TSR was established: log(V1/V2) = (P1(TSR − Tg))/(P2 + (TSR − Tg)), where the V1 and V2 are the copolymer total volumes below and above the TSR, respectively, Tg is the glass transition temperature of the copolymer, and P1 and P2 are the fraction of the free volume (ffree) at Tg (P1) and (Tg,midpoint − TSR)50/50) for each random copolymer (P2), respectively. This relationship can be utilized to predict the total volume changes as a function of TSR − Tg for different copolymer compositions. To our best knowledge, this is the first study that provides the relationship between the TSR, Tg, free volume, chain mobility, and dimensional changes in stimuli-responsive random copolymer networks.