Control Over ABA-type Triblock Copolymer Latex Morpholopgy In RAFT Miniemulsion Polymerization And Mechanical Properties Of The Latex Films

Compared with anionic polymerization,"living" free radical emulsion polymerization has outstanding advantages in terms of the preparation of block copolymer.Recently,this technique has been employed to prepare a variety of ABA-type block copolymer thermoplastic elastomer.The mechanical properties of these copolymers show the typical features of thermoplastic elastomer.However,these studies exclusively focused on their solvent films instead of the latex film.ABA-type triblock copolymers consisting of outer polystyrene blocks with 30 K g/mol molecular weight and inner polyacrylate blocks were synthesized in two steps through chain extension from symmetric RAFT agents.A variety of novel particle microstructures were realized by regulating either block ratios or particle surface attraction for each blocks.The nanostructured latex were also used for casting latex films.The morphologies and mechanical properties of latex films were investigated.The results are as following:1.The structure of the ABA-type triblock copolymers can be accurately controlled by either macroRAFT or ATTCA.The monomer conversions of each stage exceded 90% during these two-stage polymerizations.The number average molecular weight of the products grew linearly with monomer conversion,indicating the “living” characteristics of polymerizations.However,the PDIs of the final products were as high as 3.5.2.A variety of novel particle microstructures are realized by regulating either block ratios or particle surface attraction for each blocks:?.As macroRAFT agent was employed,the produced PSt-b-PnBA-b-PSt latex particles showed “core-shell” structure when the the central blocks consisted pure PnBA,while the shell thickness decreased with the length of central block.?.As macroRAFT agent was employed,with the incorporation of ethyl acrylate(EA),the P(nBA-co-EA)block in the latex particles are more likely to migrate to the latex particle surface,due to the increased affinity of P(nBA-co-EA)towards particle surfaces.?.As macroRAFT agent was replaced by the small RAFT agent ATTCA,the P(nBA-co-EA)block is even more pone to migrate to the latex particle surface,due to the absence of hydrophilic PDMAA chain segments at the end of PSt blocks.It occurred as a result that the PSt blocks migrated into the center of the particles.3.The film formation performace of latex particles depended on either the paticle microstructures or copolymer compositions:?.In the case of PSt-b-PnBA-b-PSt “core-shell” latex,the glass transition temperature(Tg)the PSt shell decreased significantly with the decrease in its thickness.As PnBA chain length increased to 160 K g/mol,the shell thickness decreased to around 10 nm and has a Tg of 0 OC.Therefore,the latex attained a continuous film at room temperature.?.Among the block copolymers having a chain structure of 30K-90K-30 K,the P(nBA-co-EA)block is more concentrated at the particle surface than did PnBA.Therefore,PSt-b-P(nBA-co-EA)-b-PSt has a higher film formation capcity than did the PSt-b-PnBA-b-PSt latex.4.The increased annealing temperature was helpful for the film microphase structure approaching to the thermodynamic equilibrium structure.However,the current investigation indicated that film structure never attained the thermodynamic equilibrium one even after 48 h annealing at 160 OC.5.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 the scarce of bridging configuration of central block connecting different glassy domains.