Bradley Melanie, Ramos Jose, Vincent Brian
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, UK.
Langmuir. 2005 Feb 15;21(4):1209-15. doi: 10.1021/la047966z.
The use of microgels for controlled uptake and release has been an area of active research for many years. In this work copolymer microgels of N-isopropylacrylamide (NIPAM) and acrylic acid (AAc), containing different concentrations of AAc and also cross-linking monomer, have been prepared and characterized. These microgels are responsive to pH and temperature. As well as monitoring the equilibrium response to changes in these variables, the rates of swelling/de-swelling of the microgel particles, on changing either the pH or the temperature, have also been investigated. It is shown that the rate of de-swelling of the microgel particles containing AAc is much faster than the rate of swelling, on changing the pH appropriately. This is explained in terms of the relative mobilities of the H(+) and Na(+) ions, in and out of the particles. It was observed that the microgels containing AAc, at pH 8, de-swelled relatively slowly on heating to 50 degrees C from 20 degrees C. This is attributed to the resistance to collapse associated with the large increase in counterion concentration inside the microgel particles. The swelling and de-swelling properties of these copolymer microgels have also been investigated in aqueous poly(ethylene oxide) (PEO) solutions, of different MW (2000-300 000). The corresponding absorbed amounts of PEO from solution onto the microgels have also been determined using a depletion method. The results, as a function of AAc content, cross-linker concentration, PEO MW, pH, and temperature, have been rationalized in terms of the ease and depth of penetration of the PEO chains into the various microgel particles and also the H-bonding associations between PEO and either the -COOH of the AAc moeities and/or the H of the amide groups (much weaker). Finally, the adsorption and desorption of the PEO molecules in to and out of the microgel particles have been shown to be extremely slow compared to normal diffusion time scales for polymer adsorption onto rigid surfaces.
多年来，微凝胶用于可控摄取和释放一直是一个活跃的研究领域。在这项工作中，制备并表征了含有不同浓度丙烯酸（AAc）以及交联单体的N-异丙基丙烯酰胺（NIPAM）和丙烯酸（AAc）的共聚物微凝胶。这些微凝胶对pH值和温度有响应。除了监测对这些变量变化的平衡响应外，还研究了微凝胶颗粒在改变pH值或温度时的溶胀/去溶胀速率。结果表明，在适当改变pH值时，含AAc的微凝胶颗粒的去溶胀速率比溶胀速率快得多。这是根据H(+)和Na(+)离子进出颗粒的相对迁移率来解释的。观察到，在pH值为8时，含AAc的微凝胶在从20℃加热到50℃时去溶胀相对较慢。这归因于与微凝胶颗粒内部抗衡离子浓度大幅增加相关的抗塌陷性。还研究了这些共聚物微凝胶在不同分子量（2000 - 300000）的聚环氧乙烷（PEO）水溶液中的溶胀和去溶胀特性。还使用耗竭法测定了从溶液到微凝胶上相应的PEO吸收量。根据PEO链进入各种微凝胶颗粒的难易程度和深度以及PEO与AAc基团的 -COOH和/或酰胺基团的H之间的氢键缔合（弱得多），对作为AAc含量、交联剂浓度、PEO分子量、pH值和温度函数的结果进行了合理说明。最后，与聚合物吸附到刚性表面的正常扩散时间尺度相比，PEO分子进出微凝胶颗粒的吸附和解吸极其缓慢。
