Control over ABA-type triblock copolymer latex morphology in RAFT miniemulsion polymerization and mechanical properties of the latex films

Thermoplastic elastomers (TPEs) are of growing commercial importance in the worldwide non-tire rubber market. It is because of their unique thermomechanical properties that associate with the phase morphologies. Controlled/living radical (mini)emulsion polymerization holds good promise for synthesis of ABA-type triblock copolymer TPEs. Both the mechanical and morphological investigations have been conducted on the solvent films of this copolymer latex. However, neither the copolymer latex particle nor the latex film has been investigated in terms of morphologies or mechanical properties. This work reports the development of triblock copolymer latex particle morphologies through reversible addition-fragmentation chain transfer (RAFT)-mediated miniemulsion polymerization. ABA-type triblock copolymers consisting of outer polystyrene blocks and inner polyacrylate blocks are synthesized in two steps through chain extension from symmetric RAFT agents. A variety of novel particle microstructures are realized by regulating either block ratios or particle surface attraction for each blocks. The nanostructured latex are also used for casting latex films. It is observed that the film morphologies do not attain thermodynamic equilibrium even after thermal annealing. In comparison to the solvent films with similar hard block contents, the latex films have higher elastic modulus, but exhibit inferior mechanical responses in terms of elongation at break and ultimate strength. The possible reason is attributed to the suppressed interparticle self-assembly of copolymers during latex film formation and scarce of bridging configuration of central block connecting different glassy domains within the final films.