Revisit to phase diagram of poly(N-isopropylacrylamide) microgel suspensions by mechanical spectroscopy.
Revisit to phase diagram of poly(N-isopropylacrylamide) microgel suspensions by mechanical spectroscopy.
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DOI：
10.1063/1.4861426
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发表时间：
2014-01
期刊：
影响因子：
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通讯作者：
Huaguang Wang;Xuebang Wu;Zhen-Gang Zhu;Changsong Liu;Zexin Zhang
中科院分区：
文献类型：
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作者：
Huaguang Wang;Xuebang Wu;Zhen-Gang Zhu;Changsong Liu;Zexin Zhang
微凝胶是一种柔软的颗粒，可以变形和压缩，这将导致有趣的相行为在高填充分数。聚（N-异丙基丙烯酰胺）（PNIPAM）微凝胶具有33 °C的低临界溶解温度（LCST），作为模型胶体引起了相当大的兴趣，因为它们的体积和微凝胶之间的相互作用可以通过温度精确地调节。在这项工作中，PNIPAM微凝胶悬浮液的线性粘弹性性能进行了研究，使用机械光谱。一个特别的注意力集中在高浓度下的相行为。随着浓度的增加，该系统经历了一个排斥的玻璃-凝胶转变低于LCST，而随着温度升高，整个LCST，该系统经历了一个凝胶-有吸引力的玻璃化转变。基于粒子间的方向性相互作用，提出了微凝胶相变的机理。在中等浓度或去溶胀微凝胶中，相互作用是各向同性的，导致玻璃相，而在浓缩和变形的微凝胶中，相互作用是定向的，导致凝胶相。我们的研究结果丰富了目前对微凝胶体系相变的理解，并为胶体悬浮体的相图提供了新的思路。
Microgels are soft particles that can be deformed and compressed, which would induce intriguing phase behaviors at high packing fractions. Poly(N-isopropylacrylamide) (PNIPAM) microgels, with a lower critical solution temperature (LCST) of 33 °C, have attracted considerable interests as model colloids, since the volume of them and the interaction between the microgels can be tuned precisely by temperature. In this work, the linear viscoelastic properties of PNIPAM microgel suspensions have been investigated using mechanical spectroscopy. A particular attention is focused on the phase behaviors at high concentrations. With increasing concentration the system undergoes a repulsive glass-to-gel transition below the LCST, while, as temperature is raised across the LCST, the system undergoes a gel-to-attractive glass transition. A mechanism of these transitions for the microgels is proposed based on the directional interaction between the particles. In moderate concentration or de-swelling microgels the interaction is isotropic leading to the glass phase, while in concentrated and deformed microgels the interaction is directional leading to the gel phase. Our results enrich the current understanding of the phase transition in microgel systems and shed new light on the phase diagram of colloidal suspensions in general.