Gan D, Lyon L A
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
J Am Chem Soc. 2001 Aug 8;123(31):7511-7. doi: 10.1021/ja010609f.
Thermoresponsive, core--shell poly-N-isopropylacrylamide (p-NIPAm) nanoparticles (microgels) have been synthesized by seed and feed precipitation polymerization, and the influence of chemical differentiation between the core and shell polymers on the phase transition kinetics and thermodynamics has been examined. The results suggest that the core--shell architecture is a powerful one for the design of colloidal "smart gels" with tunable properties. To examine these materials, differential scanning calorimetry (DSC), (1)H NMR, and temperature-programmed photon correlation spectroscopy (TP-PCS) have been employed. These measurements show that the addition of small concentrations of a hydrophobic monomer (butyl methacrylate, BMA) into the particle shell produces large decreases in the rate of thermo-induced particle collapse. Conversely, these low levels of hydrophobic modification do not perturb the thermodynamics of the particle phase transition. When these results are examined in light of previous studies of macroscopic hydrogels, they suggest that the formation of a thin, stable skin layer at the particle exterior during the early stages of particle collapse is the rate limiting factor in particle deswelling. Finally, the hydrophobicity (BMA content) of the shell determines the magnitude of the hydrogel collapse rate, while the thickness of the BMA containing region does not impact the observed kinetics. Together, these results suggest that control over the kinetics of microgel deswelling events can be accomplished simply by modification of the particle periphery, and therefore do not require homogeneous modification of the entire polymer structure.
通过种子和进料沉淀聚合合成了热响应性核壳聚N-异丙基丙烯酰胺（p-NIPAm）纳米颗粒（微凝胶），并研究了核聚合物和壳聚合物之间的化学差异对相变动力学和热力学的影响。结果表明，核壳结构对于设计具有可调性质的胶体“智能凝胶”是一种有效的结构。为了研究这些材料，采用了差示扫描量热法（DSC）、核磁共振氢谱（¹H NMR）和程序升温光子相关光谱法（TP-PCS）。这些测量结果表明，在颗粒壳中加入低浓度的疏水单体（甲基丙烯酸丁酯，BMA）会使热诱导颗粒塌陷的速率大幅降低。相反，这些低水平的疏水改性不会干扰颗粒相变的热力学。当根据先前对宏观水凝胶的研究来审视这些结果时，它们表明在颗粒塌陷的早期阶段，颗粒外部形成薄而稳定的表层是颗粒去溶胀的速率限制因素。最后，壳的疏水性（BMA含量）决定了水凝胶塌陷速率的大小，而含BMA区域的厚度不影响观察到的动力学。总之，这些结果表明，只需对颗粒周边进行改性就能实现对微凝胶去溶胀事件动力学的控制，因此不需要对整个聚合物结构进行均匀改性。
