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Thermal analyses of poly(N-isopropylacrylamide) in aqueous solutions and in nanocomposite gels

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Abstract

The coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPA) prepared by free-radical redox polymerization in aqueous solutions and its nanocomposite (NC) gels were investigated by differential scanning calorimetery. The lower critical solution temperatures (LCST) of aqueous solutions of PNIPA of different molecular weights were not significantly affected by molecular weight (M w: 0.19 × 106−4.29 × 106 g × mol−1) or polymer concentration (1−10 wt%), although the enthalpy of transition increased with molecular weight, at M w (<1.2 × 106 g × mol−1). The glass-transition temperature of PNIPA in the dried state also remained constant (138 °C), regardless of molecular weight. On the other hand, the enthalpy of the coil-to-globule transition of PNIPA in NC gels consisting of a PNIPA/clay network decreased with increasing clay concentration (C clay), while the onset temperature (≡LCST) was almost constant, regardless of C clay. The PNIPA chains in NC gels could be classified into the following three types: P-1, which exhibits a normal LCST transition, similar to that of linear PNIPA; P-2, exhibiting restricted transition at higher temperatures as a result of interactions with the clay; and P-3, which does not undergo that transition because of stronger restrictions. It was found that the proportion of P-3 increases with increasing C clay. However, some P-1 and P-2 was still observed, even in NC gels with high C clay. That the transition to the hydrophobic globular state was restricted by interactions with the clay was confirmed by measurements on PNIPA after removal of the clay from NC gels.

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Notes

  1. There is some ambiguity about the molecular weight of PNIPA prepared in aqueous solution. In Otake’s paper [36], PNIPA was prepared using the following conditions: Free-radical redox polymerization at 0 °C, [NIPA] = 0.7 M, KPS = 0.15 mM, TEMED = 2 mL for 1 l H2O. Based on our results [34], the conditions used there should result in a partially self-crosslinked gel, the molecular weight of which cannot be precisely determined by GPC

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Acknowledgment

This work was supported by the Ministry of Education, Science, Sports and Culture of Japan (grant-in-aid 23350117).

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  1. Material Chemistry Laboratory, Kawamura Institute of Chemical Research, 631 Sakado, Sakura, Chiba, 285-0078, Japan

    Kazutoshi Haraguchi & Yingjia Xu

  2. College of Material Science and Engineering, Donghua University, Shanghai, 201620, China

    Yingjia Xu

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  1. Kazutoshi Haraguchi
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Correspondence to Kazutoshi Haraguchi.

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Haraguchi, K., Xu, Y. Thermal analyses of poly(N-isopropylacrylamide) in aqueous solutions and in nanocomposite gels. Colloid Polym Sci 290, 1627–1636 (2012). https://doi.org/10.1007/s00396-012-2694-y

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  • DOI: https://doi.org/10.1007/s00396-012-2694-y

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