Calorimetric studies of PEO-b-PMMA and PEO-b-PiPMA diblock copolymers synthesized via atom transfer radical polymerization

Abstract

Poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO-b-PMMA) and poly(ethylene oxide)-b-poly(isopropyl methacrylate) (PEO-b-PiPMA) diblock copolymers of different block ratios have been synthesized using atom transfer radical polymerization with functionalized PEO monomethylether as macroinitiator. The phase separation between the constituent blocks of the copolymers has been investigated by differential scanning calorimetry (DSC). In PEO-b-PMMA, the constituent blocks are completely miscible irrespective of their molar mass and block ratios. This behaviour remains the same for PEO-b-PiPMA with ≤15% PEO fraction; phase separation could only be observed for PEO-b-PiPMA with higher PEO content. These observations are supported by results of atomic force microscopy studies of films of two copolymers with comparable molecular weight and PEO fraction (≥24% PEO); a very well developed lamellar morphology was only observed for PEO-b-PiPMA, while the block domains were randomly dispersed in PEO-b-PMMA. Interestingly, the phase separation behaviour in PEO-b-PMMA with >30% PEO fraction has been found to be strongly dependent on its processing and thermal history. On the contrary, phase separation in PEO-b-PiPMA BCPs with ≥24% PEO fraction has not been affected by its processing or thermal history. Results indicate that the use of a block-selective solvent for the precipitation of the diblock copolymer promotes the formation of microphase separated structures even for copolymers with miscible blocks. The findings are relevant for ongoing attempts to utilize the microphase separation of such polymers to obtain well-defined nanoporous membranes and other materials.