Bromberg Lev, Temchenko Marina, Moeser Geoffrey D, Hatton T Alan
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Langmuir. 2004 Jul 6;20(14):5683-92. doi: 10.1021/la0359530.
The temperature-induced structural changes and thermodynamics of ionic microgels based on poly(acrylic acid) (PAA) networks bonded with poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) (Pluronic) copolymers have been studied by small-angle neutron scattering (SANS), ultra-small-angle neutron scattering (USANS), differential scanning calorimetry (DSC), and equilibrium swelling techniques. Aggregation within microgels based on PAA and either the hydrophobic Pluronic L92 (average composition, EO8PO52EO8; PPO content, 80%) or the hydrophilic Pluronic F127 (average composition, EO99PO67EO99; PPO content, 30%) was studied and compared to that in the solutions of the parent Pluronic. The neutron scattering results indicate the formation of micelle-like aggregates within the F127-based microgel particles, while the L92-based microgels formed fractal structures of dense nanoparticles. The microgels exhibit thermodynamically favorable volume phase transitions within certain temperature ranges due to reversible aggregation of the PPO chains, which occurs because of hydrophobic associations. The values of the apparent standard enthalpy of aggregation in the microgel suspensions indicate aggregation of hydrophobic clusters that are more hydrophobic than the un-cross-linked PPO chains in the Pluronic. Differences in the PPO content in Pluronics L92 and F127 result in a higher hydrophobicity of the resulting L92-PAA-EGDMAmicrogels and a larger presence of hydrophobic, densely cross-linked clusters that aggregate into supramolecular structures rather than micelle-like aggregates such as those formed in the F127-PAA-EGDMA microgels.
基于与聚环氧乙烷-嵌段-聚环氧丙烷-嵌段-聚环氧乙烷（PEO-PPO-PEO）（普朗尼克）共聚物键合的聚丙烯酸（PAA）网络的离子微凝胶的温度诱导结构变化和热力学，已通过小角中子散射（SANS）、超小角中子散射（USANS）、差示扫描量热法（DSC）和平衡溶胀技术进行了研究。研究了基于PAA以及疏水的普朗尼克L92（平均组成，EO8PO52EO8；PPO含量，80%）或亲水的普朗尼克F127（平均组成，EO99PO67EO99；PPO含量，30%）的微凝胶中的聚集情况，并与母体普朗尼克溶液中的聚集情况进行了比较。中子散射结果表明，基于F127的微凝胶颗粒内形成了胶束状聚集体，而基于L92的微凝胶形成了致密纳米颗粒的分形结构。由于PPO链的可逆聚集，微凝胶在特定温度范围内表现出热力学上有利的体积相变，这种聚集是由疏水缔合引起的。微凝胶悬浮液中表观标准聚集焓的值表明，疏水簇的聚集比普朗尼克中未交联的PPO链更疏水。普朗尼克L92和F127中PPO含量的差异导致所得L92-PAA-EGDMA微凝胶具有更高的疏水性，并且存在更多疏水的、密集交联的簇，这些簇聚集成超分子结构，而不是像F127-PAA-EGDMA微凝胶中形成的胶束状聚集体。
