Meng Zhiyong, Cho Jae Kyu, Breedveld Victor, Lyon L Andrew
School of Chemistry and Biochemistry, Parker H. Petit Institute for Bioengineering & Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332, USA.
J Phys Chem B. 2009 Apr 9;113(14):4590-9. doi: 10.1021/jp8104979.
Quantitative microscopy measurements have been made on poly(N-isopropylacrylamide-co-acrylic acid) (pNIPAm-AAc) microgel dispersions as a function of time, temperature, pH, and volume fraction. These studies reveal an extreme degree of complexity in the physical aging and phase behavior of the dispersions; this complexity arises from a convolution of the system energetics at the colloidal, polymer-chain, and molecular scales. Superficially, these dispersions display the classic colloidal phases observed for spherical particles (i.e., gas, fluid, crystal, and glass). However, unlike simple repulsive hard spheres, pNIPAm-AAc dispersions are observed to evolve from a diffusive, fluidlike state immediately after being introduced into rectangular capillary tubes, to very slow crystalline or glassy phases after days or weeks of aging. In addition to this structural evolution, the free volume accessible to the microgels in crystalline or glassy phases (i.e., the cage size) decreases with time, indicating that the physical aging process does not end following assembly, but instead continues to evolve as the dispersion slowly proceeds to an equilibrium state. The temperature dependence of pNIPAm-AAc microgel swelling and how it influences the colloidal assembly was evaluated during the aging process as well. These thermal melting experiments revealed an enhancement in the thermal stability (i.e., a decrease in the influence of temperature on the phase behavior) of the assemblies during the aging process that we associate with an evolution of attractive interparticle interactions during aging. These attractive interactions dictate the time scale for assembly (aging), the final phase adopted by the dispersion, the dynamics of the final state, and the ultimate thermal stability. The culmination of these studies is the pseudoequilibrium phase behavior of pNIPAm-AAc microgel dispersions, which we present as a function of pH and volume fraction following approximately 1 month of aging. This diagram reveals highly complex dispersion characteristics that appear to be intrinsically tied to the degree of AAc protonation. In general, we find that, at pH < pK(a), the final dispersions behave in a manner that can be associated with attractive interparticle interactions, whereas at pH > pK(a), repulsive interactions appear to be dominant. These results are discussed in the context of the slow evolution of microgel swelling and attractive interaction potentials arising from reorganization and association of polymer chains via multiple weak hydrogen-bonding interactions.
已对聚（N-异丙基丙烯酰胺-共-丙烯酸）（pNIPAm-AAc）微凝胶分散体进行了定量显微镜测量，测量结果作为时间、温度、pH值和体积分数的函数。这些研究揭示了分散体在物理老化和相行为方面的极端复杂性；这种复杂性源于胶体、聚合物链和分子尺度上系统能量学的卷积。从表面上看，这些分散体呈现出球形颗粒所观察到的经典胶体相（即气体、流体、晶体和玻璃）。然而，与简单的排斥性硬球不同，pNIPAm-AAc分散体在被引入矩形毛细管后立即从扩散的、流体状状态演变为老化数天或数周后的非常缓慢的结晶或玻璃态。除了这种结构演变外，结晶或玻璃态相中微凝胶可利用的自由体积（即笼尺寸）随时间减小，这表明物理老化过程在组装后并未结束，而是随着分散体缓慢达到平衡状态而继续演变。在老化过程中，还评估了pNIPAm-AAc微凝胶溶胀的温度依赖性及其对胶体组装的影响。这些热熔融实验表明，在老化过程中，组装体的热稳定性有所增强（即温度对相行为的影响减小），我们将其与老化过程中颗粒间吸引相互作用的演变联系起来。这些吸引相互作用决定了组装（老化）的时间尺度、分散体最终采用的相、最终状态的动力学以及最终的热稳定性。这些研究的 culmination 是pNIPAm-AAc微凝胶分散体的准平衡相行为；我们在老化约1个月后将其作为pH值和体积分数的函数呈现出来。该图揭示了高度复杂的分散特性，这些特性似乎与AAc质子化程度内在相关。一般来说，我们发现，在pH < pK(a)时，最终分散体的行为方式可与颗粒间吸引相互作用相关联，而在pH > pK(a)时，排斥相互作用似乎占主导地位。在通过多个弱氢键相互作用进行聚合物链的重组和缔合而导致微凝胶溶胀和吸引相互作用势缓慢演变的背景下，对这些结果进行了讨论。 （注：“culmination”原词有误，推测可能是“culmination”，这里翻译为“高潮、顶点、最终成果”等意思，此处结合语境译为“最终成果”）
