The State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Laboratory for Advanced Materials, Fudan University, Shanghai 200433, China.
Phys Chem Chem Phys. 2018 Aug 15;20(32):20849-20855. doi: 10.1039/c8cp04147d.
Most of the existing studies have focused on the development of the lower critical solution temperature (LCST) behavior of thermoresponsive polymers, whereas research on the upper critical solution temperature (UCST) behavior from a molecular point of view is extremely rare. In this work, a molecular level study of the UCST thermal transition of poly-N-acryloylglycinamide (PNAGA)-based polymers in water and in a water/methanol mixture (χm = 6.9 mol%) was performed. Turbidimetric measurements revealed the increased UCST and enlarged hysteresis of the thermal transition of the polymers with the addition of methanol. Fourier transform infrared (FTIR) spectral analysis indicated that the increased hydrophilicity among the C-H groups along the polymer chains upon heating induced the UCST-type transition of the polymers in water, while the hydrogen bonding transformation predominated the thermal transition in the mixture. Moreover, in combination with two-dimensional correlation spectroscopy (2Dcos), the structures with different types of inter/intramolecular hydrogen bonds formed among the NAGA units have been distinguished and the transformation of the amide groups in PNAGA-based polymers from C[double bond, length as m-dash]OD-N hydrogen bonds to C[double bond, length as m-dash]OD2O ones during the UCST-type transition upon heating was captured.
大多数现有研究都集中在热敏聚合物的下临界溶液温度（LCST）行为的发展上，而从分子角度研究上临界溶液温度（UCST）行为的研究极为罕见。在这项工作中，我们对基于聚 N-丙烯酰基甘氨酸酰胺（PNAGA）的聚合物在水中和水/甲醇混合物（χm = 6.9 mol%）中的 UCST 热转变进行了分子水平研究。浊度测量表明，随着甲醇的加入，聚合物的 UCST 和热转变滞后增大。傅里叶变换红外（FTIR）光谱分析表明，加热过程中聚合物链上 C-H 基团之间亲水性的增加导致聚合物在水中发生 UCST 型转变，而氢键转变则主导混合物中的热转变。此外，结合二维相关光谱（2Dcos），区分了 NAGA 单元之间形成的不同类型的分子内/分子间氢键结构，并捕捉到 PNAGA 基聚合物中的酰胺基团在加热时从 C[双键，长度为 m-dash]OD-N 氢键到 C[双键，长度为 m-dash]OD2O 氢键的转变在 UCST 型转变过程中。
