RAFT-derived siloxane-based amphiphilic triblock copolymers: Synthesis, characterization, and self-assembly

Amphiphilic triblock copolymers (ATBCs) were prepared using polydimethylsiloxane (PDMS) as a central block and poly(2-hydroxyethyl methacrylate) (polyHEMA) side-blocks via reversible addition-fragmentation chain transfer (RAFT) polymerization. Initially, PDMS macro-initiator was synthesized by the esterification reaction of dihydroxyalkyl-terminated PDMS and a RAFT agent i.e., 4-cyano-4-(phenylcarbonothioylthio) pentanoic acid in the presence of 2-(1H-benzotriazole-1-yl)-1,1,3,3-tetramethylaminium tetrafluoroborate (TBTU) as a high-performance coupling reagent. RAFT copolymerization between the PDMS macro-RAFT agent and HEMA monomers was initiated by azobisisobutyronitrile (AIBN) and terminal hydroxyl functional groups of polyHEMA sideblocks were end-capped using acetic anhydride at the final step. The performance of the coupling and polymerization reactions was supported by FTIR spectroscopy. Conversion of the coupling reaction, chemical structure, and number-average molecular weight of the PDMS macro-initiator were determined by HNMR spectroscopy. HNMR integration ratio of -CH3/-OCH2- signals in deuterated chloroform and methanol proved ATBCs self-assembly which was further supported by the presence of partially-ordered and lamellar morphologies according to the AFM and SAXS findings. The molecular weights determined by GPC, HNMR, and SAXS experiments were in relative agreement with the corresponding theoretical values. The appearance of glass transition for both PDMS and polyHEMA side-blocks at differed temperatures confirmed the copolymer structures and phase-separated morphologies of ATBCs. Contact angle and surface free energy findings supported a significant decrement in water contact angle (WCA) and increment in surface tension for ATBCs by increasing the polyHEMA side-blocks length. Also, the hydrophilicity promotion of silicone rubber surface was obtained in the presence of ATBCs as a hydrophilicity enhancer additive.