Al-Manasir Nodar, Zhu Kaizheng, Kjøniksen Anna-Lena, Knudsen Kenneth D, Karlsson Göran, Nyström Bo
Department of Chemistry, University of Oslo, Blindern, N-0315 Oslo, Norway.
J Phys Chem B. 2009 Aug 13;113(32):11115-23. doi: 10.1021/jp901121g.
Chemically cross-linked poly(N-isopropylacrylamide) (PNIPAM) microgels and PNIPAM with different amounts of acrylic acid groups (PNIPAM-co-PAA) were synthesized and the temperature-induced aggregation behaviors of aqueous suspensions of these microgels were investigated mainly with the aid of dynamic light scattering (DLS) and turbidimetry. The DLS results show that the particles at all conditions shrink at temperatures up to approximately the lower critical solution temperature (LCST), but the relative contraction effect is larger for the microgels without acid groups or for microgels with added anionic surfactant (SDS). A significant depression of the cloud point is found in suspensions of PNIPAM with very low concentrations of SDS. The compression of the microgels cannot be traced from the turbidity results, but rather the values of the turbidity increase in this temperature interval. This phenomenon is discussed in the framework of a theoretical model. At temperatures above LCST, the size of the microgels without attached charged groups in a very dilute suspension is unaffected by temperature, while the charged particles (pH 7 and 11) continue to collapse with increasing temperature over the entire domain. In this temperature range, low-charged particles of higher concentration and particles containing acrylic acid groups at low pH (pH 2) aggregate, and macroscopic phase separation is approached at higher temperatures. This study demonstrates how the stabilization of microgels can be affected by factors such as polymer concentration, addition of ionic surfactant to particles without charged acid groups, amount of charged groups in the polymer, and pH.
合成了化学交联的聚（N-异丙基丙烯酰胺）（PNIPAM）微凝胶以及具有不同丙烯酸基团含量的PNIPAM（PNIPAM-co-PAA），并主要借助动态光散射（DLS）和比浊法研究了这些微凝胶水悬浮液的温度诱导聚集行为。DLS结果表明，在高达约低临界溶液温度（LCST）的温度下，所有条件下的颗粒都会收缩，但对于无酸基团的微凝胶或添加了阴离子表面活性剂（SDS）的微凝胶，相对收缩效应更大。在含有极低浓度SDS的PNIPAM悬浮液中发现浊点明显降低。从浊度结果无法追踪微凝胶的压缩情况，相反，在此温度区间内浊度值会增加。在一个理论模型的框架内对这一现象进行了讨论。在高于LCST的温度下，非常稀的悬浮液中无附着电荷基团的微凝胶尺寸不受温度影响，而带电颗粒（pH值为7和11）在整个温度范围内会随着温度升高而持续塌陷。在此温度范围内，较高浓度的低电荷颗粒以及低pH值（pH 2）下含丙烯酸基团的颗粒会聚集，在较高温度下会接近宏观相分离。这项研究表明了微凝胶的稳定性如何受到诸如聚合物浓度、向无电荷酸基团颗粒中添加离子表面活性剂、聚合物中电荷基团数量以及pH值等因素的影响。
