Cononsolvency-Induced Micellization of Pyrene End-Labeled Diblock Copolymers of N-Isopropylacrylamide and Oligo(ethylene glycol) Methyl Ether Methacrylate

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• 作者：Jingyi Rao ; Jian Xu ; Shizhong Luo ; Shiyong Liu
• 刊名：Langmuir
• 出版年：2007
• 出版时间：November 6, 2007
• 年：2007
• 卷：23
• 期：23
• 页码：11857 - 11865
• 全文大小：238K
• 年卷期：v.23,no.23(November 6, 2007)
• ISSN：1520-5827

文摘

Pyrene end-labeled double hydrophilic diblock copolymers, poly(N-isopropylacrylamide)-b-poly(oligo(ethyleneglycol) methyl ether methacrylate) (Py-PNIPAM-b-POEGMA), were synthesized via consecutive reversible addition-fragmentation chain transfer polymerization using a pyrene-containing dithioester as the chain transfer agent. Thesediblock copolymers molecularly dissolve in pure methanol and water, but form well-defined and nearly monodispersePNIPAM-core micelles in an appropriate mixture of them due to the cononsolvency of PNIPAM block. ¹H NMR,laser light scattering, fluorescence spectroscopy, and transmission electron microscopy were employed to characterizethe cononsolvency-induced PNIPAM-core micelles. When the volume fraction of water, _water, in the methanol/watermixture is in the range of 0.5-0.8, the sizes of micelles are in the range of 20-30 nm in radius for Py-PNIPAM₅₀-b-POEGMA₁₈. At _water = 0.5, the formed micelles possess the highest overall micelle density and the largest molarmass. The effects of varying the block lengths of Py-PNIPAM-b-POEGMA diblock copolymers on the structuralparameters of PNIPAM-core micelles have also been explored. Although we can observe the immediate appearanceof bluish tinge upon mixing the diblock copolymer solution in methanol with equal volume of water (_water = 0.5),which is characteristic of the formation of micellar aggregates, the whole micellization process apparently takes arelatively long time to complete, as revealed by monitoring the time dependence of fluorescence emission spectra.The excimer/monomer fluorescence intensity ratios, I_E/I_M, continuously decrease with time and then reach a plateauvalue after ~20 min. The decrease of I_E/I_M after the initial formation of pseudo-equilibrium micelles should be ascribedto the structural rearrangement and further packing of PNIPAM segments within the micelle core, restricting themobility of pyrene end groups and decreasing the probability of contact between them. Compared to the conventionalcosolvent approach employed for the micellization of block copolymers in selective solvents, the reported cononsolvency-induced unimer-micelle-unimer transition of Py-PNIPAM-b-POEGMA in methanol/water mixtures has beenunprecedented.