Reaction-Induced Self-Assembly of Gel Structure: A New Insight into Chemical Gelation Process of N-Isopropylacrylamide as Studied by Two-Dimensional Infrared Correlation Spectroscopy

Abstract

We report a new insight into the chemical gelation process of poly­(N-isopropylacrylamide) (PNiPAAm) hydrogel elucidated on the basis of the in situ observations with time-resolved Fourier-transform infrared spectroscopy (FTIR), its principal component analysis (PCA), and two-dimensional correlation spectroscopy (2D COS) at preparation temperatures below and above the lower critical solution temperature of PNiPAAm aqueous solution. First, we spectroscopically identified for the first time the specific time-spans for the two-stage reaction process: the first-stage giving rise to linear and branched random copolymers of NiPAAm and cross-linker monomers and the second-stage giving rise to cross-linking into macroscopic network structure. Second, we conducted 2D COS for both the first-stage and second-stage reaction processes in order to resolve highly superposed spectra in the amide I and II regions into fundamental spectra comprising the superposed spectra. We elucidated the time-sequence order of the changes in the fundamental spectra occurring in situ during the gelation process. We proposed molecular interpretations of the sequence of the spectroscopic events, which would give deep insights into molecular mechanisms of the gelation process as detailed in the text.

Two-dimensional correlation spectroscopy in polymer study

“…Various polymer systems, such as block copolymers (Kim et al, 2006b; Jia et al, 2010; Jin et al, 2011), biodegradable polymers (Guo et al, 2011; Unger et al, 2011; Ando et al, 2012; Hoshina et al, 2012, 2014; Wang et al, 2014), conducting polymer (Hong et al, 2005; Grzeszczuk et al, 2013), liquid crystals (Tang et al, 2010; Cheng et al, 2011), polymer blends (Oh et al, 2009; Unger et al, 2009, 2011; Jelèić et al, 2010; Popescu and Vasile, 2010, 2011; Kuo and Liu, 2011), and polymer nanocomposites (Huang et al, 2009; Huang and Kuo, 2010; Peng et al, 2010; Ando et al, 2012; Qu et al, 2012), etc., are analyzed by 2DCOS. Detailed information of polymers for polymerization (Izawa et al, 2001; Hong et al, 2005; Huang and Kuo, 2010; Qu et al, 2012; Noda, 2014d; Seo et al, 2014), melting behavior (Unger et al, 2009; Peng et al, 2010; Popescu and Vasile, 2010), crystallization (Huang et al, 2009; Zheng et al, 2010; Guo et al, 2011; Unger et al, 2011; Ando et al, 2012; Hoshina et al, 2012, 2014; Chai et al, 2013; Wang et al, 2013, 2014), gelation (Wang and Wu, 2011; Su et al, 2012; Park et al, 2013), photoreaction (Lee et al, 2010, 2012), hydration (Lai and Wu, 2013), sorption/desorption processes (Musto et al, 2011; Lai and Wu, 2013; Galizia et al, 2014), and phase transition/separation (Kim et al, 2006b; Cheng et al, 2011; Kuo and Liu, 2011; Hou et al, 2014), etc., which are undergoing under the influence of applied external perturbation, are obtained by 2DCOS. 2DCOS probes with various analytical techniques, such as IR, near-IR (NIR), Raman, X-ray, UV-Vis, THz, NMR spectroscopies, and even chromatography, have been successfully applied in polymer studies.…”

“…The generalized 2DCOS can elucidate information in spectral variations, e.g., IR (Kim et al, 2006b; Cerdà-Costa et al, 2009; Huang et al, 2009; Unger et al, 2009, 2011; Del Río et al, 2010; Huang and Kuo, 2010; Jelèić et al, 2010; Jia et al, 2010; Lee et al, 2010, 2012; Peng et al, 2010; Popescu and Vasile, 2010, 2011; Zheng et al, 2010; Cheng et al, 2011; Jin et al, 2011; Kuo and Liu, 2011; Musto et al, 2011; Quaroni et al, 2011; Wang and Wu, 2011; Zhang et al, 2011; Ando et al, 2012; Qu et al, 2012; Su et al, 2012; Wu et al, 2012; Chai et al, 2013; Lai and Wu, 2013; Park et al, 2013; Shinzawa et al, 2013; Wang et al, 2013, 2014; Galizia et al, 2014; Hou et al, 2014; Noda, 2014c; Seo et al, 2014), Raman (Radice et al, 2010; Tang et al, 2010; Ma et al, 2011; Ji et al, 2012; Pazderka and Kopecký Jr, 2012; Brewster et al, 2013; Grzeszczuk et al, 2013; Noda, 2014d), terahertz (THz) (Hoshina et al, 2012, 2014), X-ray (Guo et al, 2011), UV-Vis (Hong et al, 2005; Jiang and Wu, 2008; Sikirzhytski et al, 2012; Zhong et al, 2012), NMR (Oh et al, 2009; Li et al, 2013), fluorescence (Hur et al, 2011; Zhang et al, 2013), and even chromatography (Izawa et al, 2001), under various external perturbations, such as thermal, electrical, optical, magnetic, and chemical perturbations (Noda, 1986, 1993; Hong et al, 2005; Kim et al, 2006b; Jiang and Wu, 2008; Cerdà-Costa et al, 2009; Huang et al, 2009; Oh et al, 2009; Unger et al, 2009; Del Río et al, 2010; Huang and Kuo, 2010; Jelèić et al, 2010; Jia et al, 2010; Lee et al, …”

“…The most popularly applied external perturbation in 2DCOS is temperature (Kim et al, 2006b; Unger et al, 2009, 2011; Jia et al, 2010; Peng et al, 2010; Popescu and Vasile, 2010; Tang et al, 2010; Zheng et al, 2010; Cheng et al, 2011; Wang and Wu, 2011; Pazderka and Kopecký Jr, 2012; Chai et al, 2013; Li et al, 2013; Wang et al, 2013, 2014; Hoshina et al, 2014; Hou et al, 2014; Seo et al, 2014). Applications of 2DCOS in investigations of intriguing properties of polymer system measured by different types of analytical probes has been substantially increased (Izawa et al, 2001; Hong et al, 2005; Kim et al, 2006b; Jiang and Wu, 2008; Huang et al, 2009; Oh et al, 2009; Unger et al, 2009, 2011; Del Río et al, 2010; Huang and Kuo, 2010; Jelèić et al, 2010; Jia et al, 2010; Lee et al, 2010, 2012; Peng et al, 2010; Popescu and Vasile, 2010, 2011; Radice et al, 2010; Tang et al, 2010; Zheng et al, 2010; Cheng et al, 2011; Guo et al, 2011; Jin et al, 2011; Kuo and Liu, 2011; Ma et al, 2011; Wang and Wu, 2011; Zhang et al, 2011; Ando et al, 2012; Hoshina et al, 2012, 2014; Qu et al, 2012; Su et al, 2012; Wu et al, 2012; Zhong et al, 2012; Chai et al, 2013; Grzeszczuk et al, 2013; Lai and Wu, 2013; Park et al, 2013; Shinzawa et al, 2013; Wang et al, 2013, 2014; Hou et al, 2014; Noda, 2014c,d; Seo et al, 2014). Generalized 2D correlation spectra has notable advantages: examination of inter- or intra-molecular interactions and determination of the sequential order of events, which is hardly depicted in conventional spectroscopy.…”

Studies on Chemical IR Images of Poly(hydroxybutyrate–co–hydroxyhexanoate)/Poly(ethylene glycol) Blends and Two-Dimensional Correlation Spectroscopy

“…As polymer systems are very complicated to investigate using only conventional spectroscopic analysis, many kinds of advanced analytical methods, such as the secondary derivative, chemometrics, and two-dimensional correlation spectroscopy (2D-COS) analysis have been used. Among them, 2D-COS is a very powerful and reliable analytical method for polymer study [26,27,28,29,30,31,32,33,34]. 2D-COS can sort out very important and interesting information in the system, which is sometimes scarcely detected in conventional 1D spectral analysis.…”

Polymer Adsorbed Bilayer Membranes Form Self-Healing Hydrogels with Tunable Superstructure

“…Some recent works have succeeded in developing hydrogels with macroscale super-structures. [24][25][26][27] An interesting example is the dodecyl glyceryl itaconate/acrylamide (DGI/AAm) system. [28][29][30][31] The gel, consisting of poly(dodecyl glyceryl itaconate) (PDGI) that form lamellar bilayers and chemically cross-linked polyacrylamide (PAAm), exhibits many unique functions that are substantially superior to conventional hydrogels, such as structure color, unidirectional swelling, high toughness, and self-healing.…”