Cononsolvency of Water/Methanol Mixtures for PNIPAM and PS-b-PNIPAM: Pathway of Aggregate Formation Investigated Using Time-Resolved SANS
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- 作者：Konstantinos Kyriakos ; Martine Philipp ; Joseph Adelsberger ; Sebastian Jaksch ; Anatoly V. Berezkin ; Dersy M. Lugo ; Walter Richtering ; Isabelle Grillo ; Anna Miasnikova ; Andr茅 Laschewsky ; Peter M眉ller-Buschbaum ; Christine M. Papadakis
- 刊名：Macromolecules
- 出版年：2014
- 出版时间：October 14, 2014
- 年：2014
- 卷：47
- 期：19
- 页码：6867-6879
- 全文大小：656K
- ISSN：1520-5835
文摘
We investigate the cononsolvency effect of poly(N-isopropylacrylamide) (PNIPAM) in mixtures of water and methanol. Two systems are studied: micellar solutions of polystyrene-b-poly(N-isopropylacrylamide) (PS-b-PNIPAM) diblock copolymers and, as a reference, solutions of PNIPAM homopolymers, both at a concentration of 20 mg/mL in D2O. Using a stopped-flow instrument, fully deuterated methanol was rapidly added to these solutions at volume fractions between 10 and 20%. Time-resolved turbidimetry revealed aggregate formation within 10鈥?00 s. The structural changes on mesoscopic length scales were followed by time-resolved small-angle neutron scattering (TR-SANS) with a time resolution of 0.1 s. In both systems, the pathway of the aggregation depends on the content of deuterated methanol; however, it is fundamentally different for homopolymer and diblock copolymer solutions: In the former, very large aggregates (>150 nm) are formed within the dead time of the setup, and a concentration gradient appears at their surface in the late stages. In contrast, the growth of the aggregates in the latter system features different regimes, and the final aggregate size is 鈭?0 nm, thus much smaller than for the homopolymer. For the diblock copolymer, the time dependence of the aggregate radius can be described by two models: In the initial stage, the diffusion-limited coalescence model describes the data well; however, the resulting coalescence time is unreasonably high. In the late stage, a logarithmic coalescence model based on an energy barrier which is proportional to the aggregate radius is successfully applied.