ESR spin-label study of poly(styrene-co-methacrylic acid)/poly(epsilon-caprolactone) semi-interpenetrating polymer networks with controlled hydrogen-bond interactions

The morphology and miscibility of semi-interpenetrating polymer networks (semi-IPN) prepared with poly(styrene-co-methacrylic acid) [P(S-co-MAA)] of different carboxylic acid contents and poly(epsilon-caprolactone) (PCL) have been studied by ESR spin-label method. The ESR spectra of spin-labeled PCL showed one motional component at any specific temperature. It indicated that the spin-labeled molecules were located in one type of environment. The coexistence of two motional components in the ESR spectra of all semi-IPN samples was observed over a certain temperature range. This phenomenon suggested that the semi-IPNs were not compatible systems; they contained two microphases, a PCL-rich microdomain and a P(S-co-MAA)-rich microdomain. The miscibility could be improved by increasing the carboxylic acid content, which could enhance the hydrogen-bonding interactions between the ester groups of PCL and carboxylic acid groups in P(S-co-MAA). It was also found that the intracomponent cross-linking of the semi-IPNs was not in favor of the miscibility. The microphase separation occurred in all semi-IPNs, even in the samples having strong hydrogen-bonding interactions. With increasing cross-linking density, the microphase separation became more remarkable.