Preparation And Characterization Of PLA-PNIPAM-PS Block Copolymer And Self-assembled Films

Block copolymers,which have great potential application on the nanometer field,especially in the field of smart materials with environmental responsiveness segments,can form different nano-sized microscopic phase separation structure through self-assembly due to the incompatibility between different blocks.In this paper,a linear triblock copolymer,polylactide-poly(N-isopropylacrylamide)-polystyrene(PLA-PNIPAM-PS),was prepared by ring-opening polymerization(ROP)of lactide and reversible addition-fragmentation chain transfer(RAFT)polymerization of N-isopropylacrylamide(NIPAM)and styrene(St).PLA-PNIPAM-PS films were formed by spin-coating and was annealed in solvent atmosphere to self-assemble.PS nanoporous membrane suspended with PNIPAM chain on the cell wall could be gotten when PLA block was etched.It could have a good basis of PLA-PNIPAM-PS on smart response materials application.The main contents of this paper are:1.Preparation and characterization of PLA-PNIPAM-PSFirstly,PLA-OH was obtained by ROP of lactide in toluene,when benzyl alcohol was used as initiator and stannous octoate as catalyst.After the carboxyl group of TC was modified into acyl chloride group,it reacted with the hydroxyl group of PLA-OH to synthesize PLA-TC for RAFT polymerization.Next,NIPAM reacted with PLA-TC under the induction of azodiisobutyronitrile(AIBN)to synthesize PLA-PNIPAM,then PLA-PNIPAM-PS was prepared similarly.PLA-PNIPAM-PS was characterized by 1H-NMR,IR and GPC.Effect of reaction time on the molecular weight and distribution of PLA-PNIPAM and PLA-PNIPAM-PS were studied.Results showed that molecular weight of PLA-PNIPAM showed a linear increase with increasing conversion rate for a certain extent,and molecular weight distribution maintained lower than 1.3,which agreed with the RAFT polymerization character.However,molecular weight distribution of PLA-PNIPAM-PS was wider than 1.4,due to the loss of RAFT polymerization activity of some PLA-PNIPAM segments.2.Self-assembly of PLA-PNIPAM-PS triblock copolymer filmsPLA-PNIPAM-PS films was prepared by chlorobenzene solution with different concentrations through spin-coating.After PLA-PNIPAM-PS films annealed in solvent vapor,surface morphology of the self-assembled films were observed by atomic force microscope(AFM).Effects of annealing solvent,ratio of blocks,blend of homopolymer PLA and PLA-PNIPAM-PS,annealing time and concentration of PLA-PNIPAM-PS solution on the self-assembly behavior were investigated in detail.By comparing the AFM images of films before and after the annealing,it can be found that the solvent annealing made the block copolymer form distinct disperse phase which was described as a concave surrounding circular bulges.If films were annealed by single solvent(tetrahydrofuran,methylene chloride),the formed concave disperse phase were sparse due to their weaker selectivity for the PLA chain,which formed the most densely structured for blend solvent of toluene/acetone(7:3,v/v).As PLA content in PLA-PNIPAM-PS decreases,area and size of the dispersed phase formed decreased because PS limits PLA movement which confirmed by blending PLA-PNIPAM-PS and PLA homopolymers.If annealing time was extended,PLA tends to aggregate more and more and form larger-sized phase on the film surface.Thickness of PLA-PNIPAM-PS films can be controlled by the concentration of PLA-PNIPAM-PS solution in the process of spin-coating.Good dispersed phase hardly be gotten if PLA-PNIPAM-PS film was too thin.With thickness of films increasing,dispersed phase size will increased slightly.After PLA segments of PLA-PNIPAM-PS films were etched,PS-PNIPAM nanoporous films could be gotten which confirmed by SEM.The hole size was not changed much compared films etched before and after which also confirmed that the dispersed phase is PLA block.