Understanding the phase transition of linear poly(N-isopropylacrylamide) gel under the heating and cooling processes
Graphical abstract
Introduction
Intelligent polymers, such as poly(N-isopropylacrylamide) (PNiPAAm), depend on external stimuli, including temperature, pH, solvents, magnetic fields, and other factors. These polymers have been intensively investigated due to their applications to the biotechnology and pharmaceutical industries [1], [2], [3], [4], [5], [6], [7], [8], [9]. PNiPAAm, a thermosensitive polymer, exhibits a coil to globule transition above the lower critical solution temperature (LCST). The hydrophilic and swollen PNiPAAm gel is dramatically changed to hydrophobic and shrunken gel above LCST [10]. Various experimental techniques, such as nuclear magnetic resonance (NMR) [8], X-ray [8], [11], [12], [13] and infrared (IR) spectroscopy [14], [15], [16] methods, have been used to better understand the PNiPAAm coil to globule transition. Sun et al. [14] reported PNiPAAm hydrogel chain collapse and revival thermodynamics in D2O during heating and cooling using perturbation-correlation moving-window (PCMW) two-dimensional (2D) correlation spectroscopy. Hashimoto et al. [17] investigated the IR spectra of dry linear PNiPAAm film and linear PNiPAAm in D2O with increasing temperature that the linear PNiPAAm amide and alkyl group dehydrations induced the amide I band to shift to a lower wavenumber and C–H stretching band to shift to a higher wavenumber via the coil–globule transition during heating. However, the temperature-induced phase transition mechanisms of aqueous linear PNiPAAm are not clearly understood.
2D correlation spectroscopy is a powerful analytical technique that can be used to interpret spectral data obtained under external perturbations, which can provide new molecular level insights for understanding systems of interest. Based on advantages of 2D correlation spectroscopy, such as the enhancement of the spectral resolution, establishment of unambiguous assignments and determination of the sequence of events of the spectral peaks [18], [19], [20], [21], 2D correlation analysis has been applied in various polymer studies [18], [19], [20], [21], [22], [23], [24], [25], [26]. We have spectroscopically identified for the first time the chemical gelation process at preparation temperatures below and above the LCST of PNiPAAm hydrogel by using the in-situ observations with time-resolved FTIR spectroscopy and its principal component analysis (PCA) and 2D correlation spectroscopy [25].
In this study, the phase transition of linear PNiPAAm gel under the heating and cooling processes were investigated at molecular levels. To achieve a deeper insight into the phase transition mechanism of linear PNiPAAm gel, PCA and 2D correlation spectroscopy were applied to the temperature-dependent ATR-FTIR spectra during the heating and cooling processes.
Section snippets
Experimental section
A linear PNiPAAm polymer was synthesized by free radical polymerization as described in the previous paper [17]. A 56 wt % linear PNiPAAm aqueous solution was prepared and used to measure phase transitions via 10-times reflectance ATR-FTIR (Ge crystal and incident angle of 45°) with heating controller. All temperature-dependent ATR-FTIR spectra were recorded with a spectral resolution of 4 cm−1 using a Nicolet 6700 spectrometer equipped with a liquid nitrogen-cooled MCT detector. A total of
Results and discussion
Fig. 1(a) and (b) show the temperature-dependent ATR-FTIR spectra of linear PNiPAAm gel during the heating and cooling processes, respectively, between 25 and 40 °C. Each spectrum was measured at a 1 °C interval. We focus on the C–H stretching region and the region for the CO stretching coupled N–H bending modes, to investigate the coil to globule transition. The spectral intensities of the C–H and CO stretching including N–H bending bands exhibit small increases until 32 °C, as shown in Fig. 1
Conclusion
The coil to globule transitions of linear PNiPAAm gel during heating and cooling processes were investigated using ATR-FTIR, PCA and 2D correlation spectroscopy. Based on the PCA results, intensity of bands in ATR-FTIR spectra of linear PNiPAAm changed dramatically from 33 to 34 °C during the heating process. While the intensities were gradually decreased during the cooling process. The 2D correlation analysis investigated the linear PNiPAAm phase transition during the heating and cooling
Acknowledgments
This study was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (No. NRF2015R1D1A3A01015885). The authors thank the Central Laboratory of Kangwon National University for the measurements of IR spectra.
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