Micellization Kinetics of a Novel Multi-Responsive Double Hydrophilic Diblock Copolymer Studied by Stopped-Flow pH and Temperature Jump
✍ Scribed by Yanfeng Zhang; Tao Wu; Shiyong Liu
- Publisher
- John Wiley and Sons
- Year
- 2007
- Tongue
- English
- Weight
- 292 KB
- Volume
- 208
- Category
- Article
- ISSN
- 1022-1352
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✦ Synopsis
Abstract
Double hydrophilic diblock copolymer, poly(N‐isopropylacrylamide)‐block‐poly(2‐diethylamino ethyl methacrylate) (PNIPAM‐b‐PDEA), was synthesized via reversible addition‐fragmentation chain transfer (RAFT) polymerization. Containing the well‐known thermo‐responsive PNIPAM block and pH‐responsive PDEA block, this novel diblock copolymer exhibits intriguing “schizophrenic” micellization behavior in aqueous solution, forming PDEA‐core micelles at alkaline pH and room temperature, and PNIPAM‐core micelles at acidic pH and elevated temperatures. The kinetics of the pH‐ and thermo‐responsive micellization processes were studied in detail using a stopped‐flow apparatus equipped with a newly developed millisecond temperature jump (mT‐jump) accessory. Upon a pH jump from 4 to 12 at 25 °C, the early stages of relaxation curves monitoring the formation PDEA‐core micelles can be well‐fitted using a double‐exponential function, leading to two characteristic relaxation time constants, τ~1~ and τ~2~. As τ~2~ decreases with increasing polymer concentration, the slow process is thus expected to proceed via micelle fusion/fission mechanism, approaching the final equilibrium state. Upon a temperature jump from 20 to 45 °C at pH 4, the relaxation curves monitoring the formation PNIPAM‐core micelles can also be well‐fitted using a double‐exponential function. The fast process (τ~1~) is associated with the quick association of unimers into a large amount of small micelles and the formation of quasi‐equilibrium micelles. τ~2~ is almost independent of polymer concentration, suggesting that unimer insertion/expulsion is the main mechanism for the slow process. The protonated PDEA corona of quasi‐equilibrium micelles renders the micelle fusion/fission mechanism less favorable due to electrostatic repulsion.
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