Abstract
Star-shaped porphyrin-PNIPAM(4) (PP) conjugates having four PNIPAM arms connected to a central tetraphenylporphyrin unit were synthesized using reversible addition-fragmentation chain-transfer polymerization. Temperature-induced phase-separation behavior of the conjugates was investigated, and the lower critical solution temperature (type II)-composition phase diagram was constructed using Flory-Huggins theory.
Interestingly, in contrast to PNIPAM homopolymers, the shorter PNIPAM arms of PP conjugates lead to a lower phase-separation temperature (T(p)). The concentration dependency of the size of the cooperative domain was also determined.
Below T(p), experimental data indicate that PP behaves as a 1D supramolecular polymer with a concentration-dependent length, while above T(p), PP globules adopt a larger spherical shape. Various temperature- pH reversible and irreversible interdependencies ("cross-effects") between phase separation and protonation were observed.
The PP conjugates represent a dual temperature-pH-responsive model system possessing various aggregated states, making them candidates for visual indicators, pH or temperature sensors, or singlet oxygen generators for biomedical applications.