Host鈥揋uest Interactions in Poly(N-isopropylacrylamide) Hydrogel Seen by One- and Two-Dimensional 1H CRAMPS Solid-State NMR Spectroscopy

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• 作者：Attila Domj谩n ; Enik艖 Manek ; Erik Geissler ; Krisztina L谩szl贸
• 刊名：Macromolecules
• 出版年：2013
• 出版时间：April 23, 2013
• 年：2013
• 卷：46
• 期：8
• 页码：3118-3124
• 全文大小：350K
• 年卷期：v.46,no.8(April 23, 2013)
• ISSN：1520-5835

文摘

Host鈥揼uest interactions in poly(N-isopropylacrylamide) (PNIPA) hydrogels with pyrocatechol were investigated by ¹H single-pulse as well as one- and two-dimensional combined rotation and multiple-pulse spectroscopy (CRAMPS) ¹H solid-state NMR techniques. Differential scanning calorimetry (DSC), swelling measurements, and density functional theory (DFT) calculations were used as supplementary methods. The results are compared with the phenol鈥揚NIPA system to explore possible similarities or differences in the effects of the two phenolic guest molecules on the temperature-induced volume phase transition of the hydrogel. Both small molecules shift the lower critical solution temperature (LCST) of the hydrogel, but with phenol the transition is broader. This observation may reflect changes in the ¹H signal from residual water trapped in cavities in the hydrogel above the LCST. Although in the pyrocatechol-containing hydrogel only two signals are observed, the phenol-containing hydrogel displays numerous signals. In the case of phenol, the formation of water cavities is a more complex process, although reaching the equilibrium state requires a very long time (about 40 days). The CRAMPS protocol increases the resolution and makes it possible to perform two-dimensional correlation ¹H鈥?sup>1H experiments. The rate matrix calculation yields a host鈥揼uest distance that is 0.3 呛 shorter for pyrocatechol than for phenol. Independent calculations by DFT indicate that the host鈥揼uest distance is 0.2 呛 shorter for pyrocatechol. The satisfactory agreement between the results from different methods demonstrates that the power of this NMR method extends not only to crystalline but also to macroscopically amorphous systems possessing only local order.