Temperature-Induced Self-Assembly of Triple-Responsive Triblock Copolymers in Aqueous Solutions

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• 作者：Jan Weiss ; Andre虂 Laschewsky
• 刊名：Langmuir
• 出版年：2011
• 出版时间：April 19, 2011
• 年：2011
• 卷：27
• 期：8
• 页码：4465-4473
• 全文大小：994K
• 年卷期：v.27,no.8(April 19, 2011)
• ISSN：1520-5827

文摘

A series of triple-thermoresponsive triblock copolymers from poly(N-n-propylacrylamide) (PNPAM, A), poly(methoxydiethylene glycol acrylate) (PMDEGA, B), and poly(N-ethylacrylamide) (PNEAM, C) was synthesized by sequential reversible addition鈭抐ragmentation chain transfer polymerizations. Polymers of differing block sequences, ABC, BAC, and ACB, with increasing phase transition temperatures in the order A < B < C were prepared. Their aggregation behavior in dilute aqueous solution was investigated using dynamic light scattering, turbidimetry, and NMR spectroscopy. The self-organization of such polymers was found to dependent strongly on the block sequence. While polymers with a terminal low-LCST (lower critical solution temperature) block undergo aggregation above the first phase transition temperature at 20鈭?5 掳C, triblock copolymers with the low-LCST block in the middle show aggregation only above the second phase transition. The collapse of the middle block is not sufficient to induce aggregation but produces instead stable, unimolecular micelles with a collapsed middle block, as supported by NMR and fluorescence probe data. Continued heating of all copolymers led to two additional thermal transitions at 40鈭?5 and 70鈭?0 掳C, which could be correlated to the phase transitions of the B and C blocks, respectively. All polymers show a high tendency for cluster formation, once aggregation is induced. The carrier abilities of the triple responsive triblock copolymers for hydrophobic agents were probed with the solvatochromic fluorescence dye Nile Red. With passing through the first thermal transition, the block copolymers are capable of solubilizing Nile Red. In the case of block copolymers with sequences ABC or ACB, which bear the low-LCST block at one terminus, notable amounts of dye are solubilized already at this stage. In contrast, the hydrophobic probe is much less efficiently incorporated by the BAC triblock copolymer, which forms unimolecular micelles. Only after the collapse of the B block, when reaching the second phase transition at about 45 掳C, does aggregation occur and solubilization becomes efficient. In the case of ABC and ACB polymers, the hydrophobic probe seems to partition between the originally collapsed A chains and the additional hydrophobic chains formed after the collapse of the less hydrophobic B block.