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(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 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(V₁/V₂) = (P₁(T_SR − T_g))/(P₂ + (T_SR − T_g)), where the V₁ and V₂ are the copolymer total volumes below and above the T_SR, respectively, T_g is the glass transition temperature of the copolymer, and P₁ and P₂ are the fraction of the free volume (f_free) at T_g (P₁) and (T_g,midpoint − T_SR)_50/50) for each random copolymer (P₂), 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.