Glass (T-g) and Stimuli-Responsive (T-SR) Transitions in Random Copolymers
In an effort to elucidate the origin of stimuli-responsive (T-SR) transitions and correlate them to the glass transition temperature (T-g), poly(N-acryloyl-N'-propylpiperazine-co-2-ethoxyethyl methacrylate) (p(Acr-NPP/EEMA)), poly(N-vinylcaprolactam-co-n-butyl acrylatc) (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 T-SR transitions are attributed to the backbone buckling and collapse of stimuli-responsive components. Based on empirical data, the relationship between T-g and T-SR was established: log(V-1/V-2) = (P-1(T-SR - T-g))/(P-2 + (T-SR - T-g)), where the V-1 and V-2 are the copolymer total volumes below and above the T-SR, respectively, Tg is the glass transition temperature of the copolymer, and P-1 and P-2 are the fraction of the free volume (f(free)) at T-g (P-1) and (T-g.midpoint - T-SR)(50/50)) for each random copolymer (P-2), respectively. This relationship can be utilized to predict the total volume changes as a function of T-SR - T-g for different copolymer compositions. To our best knowledge, this is the first study that provides the relationship between the T-SR, T-g, free volume, chain mobility, and dimensional changes in stimuli-responsive random copolymer networks.