Thermo-Responsive Behavior of Block and Random Copolymers of N-Isopropylamide/N,N-Dimethylacrylamide Synthesized via Reversible AdditionFragmentation Chain Transfer Polymerization

Block and random copolymers of N-isopropylacrylamide (NIPAM) and N,N-dimethylacrylamide (DMA) with similar compositions and molecular weights were prepared by the reversible addition-fragmentation chain transfer (RAFT) polymerization with a trithiocarbonate (2-dodecylsulfanylthiocarbonyl- sulfanyl-2-methyl propionic acid) as the RAFT reagent. The aggregation behavior and morphology of block copolymers P(NIPAM-b-DMA) and random copolymers P(NIPAM-co-DMA) was investigated by dynamic light scattering (DLS) and atomic force microscopy (AFM). Although they have similar compositions and molecular weight, their solution properties and morphology differ greatly due to their different chemical sequence. The P(NIPAM-co-DMA) exhibits much higher lower critic solution temperature (LCST) at 46C than that of P(NIPAM-b-DMA) block copolymers (36C). The AFM results showed that P(NIPAM-b-DMA) molecules tend to coil and aggregate to form elliptical particles with average diameter 20 nm below the LCST (25C), while P(NIPAM-co-DMA) molecules form network configuration on SiO2 substrate. Above their LCST (50C), the strong hydrophobic aggregating transition takes place between the P(NIPAM-b-PDMA) segments, while the P(NIPAM-co-DMA) segments only show some evidence of entangling. These indicate that the distribution of DMA in the DMA/NIPAM copolymer chains can strongly interfere the hydrophobic assembling of NIPAM segments.