Preparationand Self-Assembly Of Block Copolymers Containing Temperature Sensitive And Degradable Chain Segments

Block copolymer has the merits of self-assembly. Usually, the thermodynamic incompatibility between molecular chains drives block copolymer forming rich micro-phase structures, which endow materials with specialproperties. It has wide application in thermoplastic elastomer, information storage, drug delivery and so on. Through molecular design, block copolymers containing temperature sensitive and degradable chain segments polylactide-b-poly?N-isopropylacrylamide? ?PLA-b-PNIPAM? and polylactide-b-poly?N-isopropylacrylamide?-b-poly?styrene? ?PLA-b-PNIPAM-b-PS? were synthesized through a combination of controlled ring opening polymerization ?ROP? and reversible addition fragmentation chain transfer ?RAFT? polymerization. The self-assembly behavior of block copolymer were studied in detail, which lay a certain foundation for the application of the intelligent membrane, drug controlled release or nano pattern materials. The main works of this paper are as follows:1. Preparation and characterization of PLA-b-PNIPAM diblock copolymers and its micelles?1? Firstly, hydroxy-group terminated PLA ?PLA-OH? was prepared by the ring opening polymerization ?ROP? of D,L-lactide using a stannous octanoate [Sn?Oct?2] catalyst and a benzyl alcohol initiator. Then, PLA-OH was coupled to a kind of RAFT agent 2-methyl-2-[?dodecylsulfanylthiocarbonyl? sulfanyl] propanoic acid ?TC?, which was acylchlorided by oxalyl chloride. This macro chain transfer agent ?PLA-TC? was then used to synthesize three narrow distribution diblock polymers (PLA₃₄₀₀-b-PNIPAM4500, PLA₃₄₀₀-b-PNIPAM5700 and PLA₃₄₀₀-b-PNIPAM8600) combined with N-isopropylacrylamide ?NIPAM? as monomers, AIBN as initiator, implemented by a RAFT process. The structure and molecular weight of block copolymers were characterized by IR,'H-NMR and GPC.?2? The self assembly of PLA-b-PNIPAM in water was relied on the hydrophobic PLA segment and hydrophilic PNIPAM segment, and the morphology and particle size of the micelles were characterized by TEM and laser particle size analyzer. The results show that block copolymers PLA-b-PNIPAM can be self-assembled into micelle in water, where PLA form as the core, and PNIPAM as the shell. The size of the micelle is related to the PNIPAM length. When the length of the PLA chain is certain, the larger the fraction of PNIPAM ?fPNIPAM? is, the bigger the size of the micelle.?3? The temperature sensitivity of PLA-b-PNIPAM micelle was studied by UV spectrophotometer and laser particle size analyzer. The results showedthat the low critical solution temperature ?LCST? and turbidity ?CP? of PLA-b-PNIPAM are mainly related to the concentration of copolymer, while the PNIPAM chain length has little influence on the results.For example, when the micelle concentration of PLA₃₄₀₀-b-PNIPAM8600 changed from 0.30 mg/mL to 0.04 mg/mL, the LCST of the block copolymer changed from 24 ? to 28.2 ?,and CP value from 29.5 ? to 30.4 ?. In addition, it was found that in the temperature variation range from 20 ? to 40 ?, the diameter of PLA34oo-b-PNIPAMg6oo micelles change from 304 nm to 280 nm, but the diameter of PLA₃₄₀₀-b-PNIPAM5700 micelles change from 239 nm to 267 nm. The completely opposite phenomenon of particle diameter can be explained as thatwhen the temperature increase above the LCST, PNIPAM chains collapse on the PLA core and reduce the particle size. In contrast, micelles self-assembled from PLA-b-PNIPAM with short PNIPAM chains tend to aggregate and increase the particles size, results from the loss of stabilization of PNIPAM chains in water.2. Preparation and characterization of PLA-A-PNIPAM-6-PS triblock copolymer and its films?1? Based on the two block copolymer PLA-b-PNIPAM preparation, we use PLA-b-PNIPAM-TC as the macromolecular chain transfer agent, AIBN as initiator, styrene ?St? as the monomer,to obtain narrow distribution triblock copolymers (PLA₃₄₀₀-b-PNIPAM3500-b-PS12900 and PLA₃₄₀₀-b-PNIPAM86oo-b-PS58800) by RAFT process. The structure and molecular weight of block copolymers were characterized by IR,1H-NMR and GPC.?2? To research the self-assembly behavior of block copolymer film under thermal field, PLA34oo-b-PNIPAM35oo-b-PS 12900 triblock copolymer films were prepared by spin coating of thechlorobenzene solution. We set the temperature to 110 ?,130 ?,170 ? and 200 ? and the film morphology was characterized by atomic force microscopy ?AFM?. The results show thatafter thermal annealing of 12 h, the thin film formed the smooth, dark and light, point distribution micro morphology. And the higher the temperature is, the stronger the move ability of molecular chain segment, which incur the increase of diameter of the "concave" dispersed phase ?D? ?262.2 nm?281.2 nm.331.2 nm?347.8 nm?and center distance?dc.c??192.9 nm,1084.5 nm? 1123.3 nm?1343.0 nm?. But higher temperature will enable the PLA chain degradation, the remaining PNIPAM-b-PS will "dissolved" by PS matrix, to further expand dc-c of "concave" dispersed phase.?3? To investigate the self-assembly behavior of block copolymer film under solvent field, PLA₃₄₀₀-b-PNIPAM8600-b-PS58800 triblock copolymer films were prepared by spin coating of thechlorobenzene solution. Different solvents field such as acetone, chlorobenzene and tetrahydrofuran ?THF? were used and the film morphology was characterized by AFM. The results show that it is more likely to get regular self assembling structure in THF solvent atmosphere after annealing 4 h at room temperature. With the increase of solvent annealing time, the diameter of the dispersed phase is larger and the center distance ofdispersed phase is larger, too. The effect of concentration of copolymer solution on the surface morphology of the thin films is important. It is interesting that if the concentration of copolymer was high, "concave" self-assembled morphologies can be obtained. In contrast, if the concentration was low, "ridgy" self-assembled morphologies can be gotten. For example, when the concentration of copolymer is 2.5 mg/ml, the thin film can be formed a relatively regular hexagonal packing surface morphology, and the diameter of "ridgy" is 30 nm, the center distance is 50 nm if the thin film was treated with THF solvent at room temperature for 4 h.?4?The PLA₃₄₀₀-b-PNIPAM86oo-b-PS58800 thin film was selectively etched off the PLA chain segment after self assembly at room temperature for 4 hin THF atmosphere, and the porous membrane with an aperture of about 800 nm was obtained.