Synthesis And Properties Of Well-defined Multiblock Copolymers Via RAFT Emulsion Polymerization

Advances in reversible deactivation radical polymerization techniques open new possibilities for synthesizing block copolymers with controllable block numbers(n),block types(k),and sequences of repeating chains for the past two decades.Compared with traditional AB diblock or ABA triblock copolymers,multiblock copolymers(mBCP)with increasing(k,n)and a significant expansion of possible molecular architectures offers an unparalleled platform for designing and producing multifunctional or high-performance polymer materials with less cost.It remains challenging for the synthetic techniques of mBCPs to achieve high k,n,degree of polymerization(DP),high livingness of mBCP and high synthesis efficiency at the same time.Also,the theoretical prediction of nanomorphology and properties of mBCPs becomes extremely difficult due to the complexity of the block architectures.To produce mBCPs with tailor-made properties requires systematical experimental research on the relationships between block architectures and properties.To address the above issues,in the current thesis,we developed reversible addition-fragmentation chain transfer(RAFT)radical polymerization emulsion polymerization into an efficient tool to synthesize well-defined mBCPs with high n,k and DP.The relationships between block architectures and properties of mBCPs were also investigated.The main conclusions and achievements are as follows:(1)It was shown that one-pot RAFT emulsion polymerization should be a highefficient new technique for synthesizing well-defined mBCPs.Taking styrene as a model monomer,a "dodecablock" PS300-(PS 100)11(PS for polystyrene)was synthesized via sequential adding monomers.The monomer conversion reached over 98%in just 2 hr for each block synthesis.The experimental molecular weights were in excellent agreement with theoretical predictions.PS300-(PS100)11 showed high livingness(>93 mol%)and narrow molecular weight distribution(PDI<1.50).Polystyrene-b-poly(tbutyl acrylate)-b-poly(methyl acrylate)-b-poly(t-butyl acrylate)-b-polystyrene(PSPtBA-PMA-PtBA-PS)pentablock copolymer with total molecular weight over 300 kg/mol was also synthesized.The PtBA chains could be hydrolyzed to hydrophilic PAA chains in mild conditions,broadening available molecular architectures of mBCPs.(2)The process of RAFT emulsion polymerization was optimized to synthesize polystyrene-b-poly(n-butyl acrylate)-b-polystyrene(PS-PnBA-PS,abbreviated to SBAS)triblock copolymer with narrow molecular weight distribution.Firstly,narrow molecular weight distribution of the first block PS(PDI～1.15)was achieved by optimizing polymerization temperature and initiator concentration so that a high particle activation/deactivation rate was remained.Secondly,the polymerization temperature was reduced to 40～50℃ to suppress the formation of long-chain branches generated during the polymerization of acrylates.The PDI of SBAS(15k-60k-15k)prepared by the optimized process was 1.39,much lower than that prepared in a traditional process(PDI>3).SBAS with narrower molecular weight distribution showed more ordered nanomorphology and better transparency,processability and mechanical properties.(3)mBCPs(SBA)n(n=1,2,3,4)with alternated hard and soft blocks were designed and synthesized.(SBA)4(DPps=186,DPpnBA=286,molecular weighht=195 kg/mol)showed relatively low PDI(1.86)and high livingness(90 mol%).The high molecular weight in(SBA)n ensured microphase separation even though PS/PnBA system is known for small χ value.The tensile properties increased significantly with increased block number.(4)A series of "hard-soft-hard"(SBAS)X(x=1,2,3,DPps=1 86)block copolymers with various block compositions(PS wt%=50%,33%,and 25%)were designed and synthesized.The relationship between block architectures and mechanical properties were investigated.The tensile strength and toughness of all(SBAS)2 were significantly enhanced compared with their SBAS counterparts while the modulus was mildly increased.The enhancement of mechanical properties increased as the decrease of PS composition.In the case of 25 wt%PS,the tensile strength,toughness and modulus of(SBAS)2 were enhanced by 173%,151%,and 30%,respectively.It suggested that(hard-soft-hard)2 pentablock copolymers could be a soft yet strong and tough thermoplastic elastomer.(5)The degree of microphase separation played an important role on mechanical properties of(SBAS)x.For weakly segregated(SBAS)1,the modulus was increased due to the presence of an interface layer.The shear deformation of the interface layer led to remarkable energy dissipation,resulting in high toughness.Yet the strength was decreased.However,in the case of(SB AS)2 pentablock copolymer,the unique bridging and knotted looping of PS mid-chains reinforced the PS domains,resulting in high tensile strength.As a result,weakly segregated(SBAS)2 showed high strength and toughness simultaneously.