Microdynamics Mechanism of Thermal-Induced Hydrogel Network

Article pubs.acs.org/JPCB

Microdynamics Mechanism of Thermal-Induced Hydrogel Network Destruction of Poly(vinyl alcohol) in D2O Studied by TwoDimensional Infrared Correlation Spectroscopy Leilei Peng, Tao Zhou, Yun Huang, Long Jiang,* and Yi Dan* The State Key Laboratory of Polymer Materials Engineering of China, Polymer Research Institute, Sichuan University, Chengdu 610065, China ABSTRACT: Microdynamics mechanism of thermal-induced hydrogel network destruction of poly(vinyl alcohol) (PVA) in D2O at heating (25−62 °C) was studied by in situ Fourier transform infrared (FTIR) spectroscopy combining with moving-window two-dimensional (MW2D) technique and two-dimensional (2D) correlation analysis. The temperature range of hydrogel destruction was determined within 34−52 °C by dynamic rheological test at first, and then also monitored by MW2D FTIR spectra. The motion of vs(−C− O−, microcrystals) was important in the entire hydrogel destruction process. The microdynamics mechanism of PVA molecular chains can be elaborated as follows: At 32 °C, the number of D2O molecules in the swollen amorphous remains unchanged. At 32−37 °C, more D2O molecules enter into the swollen amorphous region, and the groups of −C−O−, together with −CH2−, are partially hydrated. At 37 °C, the intramolecular or intermolecular hydrogen bonds of PVA are dissociated. The physical cross-linking points of hydrogel are broken due to the melting of PVA microcrystals. At 42 °C, the dissociated hydroxyls from PVA microcrystals rapidly integrate solid hydrogen bonds with D2O molecules. The groups of −C−O− and −CH− are completely hydrated by D2O simultaneously. At 45−55 °C, PVA molecules are surrounded by more D2O molecules. The partially hydrated −CH2− is completely hydrated, and all of the PVA molecules are fully dissolved in D2O.

1. INTRODUCTION Poly(vinyl alcohol) (PVA) is a highly hydrophilic polymer and was first synthesized by Haehnel and Herrmann in 1924.1 Since then, PVA hydrogel has been especially popular in the biomedical applications2 of drug delivery systems,3,4 stabilizing colloids, wound dressing,5 tissue scaffolds, and prosthetics6−8 because of their biocompatible, good mechanical, nontoxic, and hydrophilic properties.9 Due to the functionality of the PVA hydrogels mainly depending on their structure,10 many studies have been carried out to investigate the structural organization, in which PVA chains and solvent molecules are organized through the interaction of hydrogen bonds. The studies on structural features of PVA hydrogels, such as crystallinity degree,11−14 pH,15,16 tensile mechanical properties, and crystallline phase, confirmed that the structure change of hydrogel had a great influence on the properties. Chiessi et al.10 used the molecular dynamics simulation to study the PVA hydrogel with an atomic detail. Through the investigation of local polymer dynamics, they found that the PVA mobility was affected by the interaction between macromolecules and water. On the basis of previous studies,11−14 the analysis revealed that the structural organization of PVA hydrogel contains two separated phases constituted by polymer-rich and polymer-poor regions, in which the crystalline domains act as knots17−19 of the gel network. The gel−sol transition at heating can be © 2014 American Chemical Society

characterized by the endothermic melting of the crystallites, the exothermic solubilization, and solvation of PVA chains in water. Proton low-field NMR spectroscopy20 analysis showed that a primary crystalline polymer phase constitutes the main support of the network structure. Correspondingly, progressive melting of the main PVA crystals led to the destruction of the network gel and the formation of an isotropic PVA solution. The dynamics behavior of PVA solutions during the chemical gelation process was explored by Anna-Lena Kjøniksen and Bo.21 For all the systems in the pregel domain, the time correlation data proved the existence of two relaxation modes (“fast” and “slow” mode). It is now accepted that the structural model of PVA hydrogel includes a polymer-rich and a polymerpoor phase, accompanied by the crystallization region and swollen amorphous of PVA in the polymer-rich phase.22−24 The presence of these tie chains, which constitute a sort of swollen amorphous region, ensures the connectivity of the macroscopic network. The widespread investigations on the PVA hydrogels revealed that the structure of hydrogels influence the properties by various techniques,1,5,7−21 such as NMR, QENS, dielectric Received: June 2, 2014 Revised: July 15, 2014 Published: July 21, 2014 9496

dx.doi.org/10.1021/jp5054259 | J. Phys. Chem. B 2014, 118, 9496−9506

The Journal of Physical Chemistry B

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relaxation, DSC, SANS, SAXS,25 and light scattering. However, the knowledge on the microdynamics mechanism for the structure formation or destruction of PVA hydrogel on the molecular level still lacks now. Moreover, this microdynamics mechanism is significant for scientific investigation and commercial applications. It is worth noting that in situ infrared spectroscopy combining with two-dimensional (2D) correlation analysis is a widely used technique for the detection of the changes in conformation, interaction, and the motions of chemical groups in polymer phase transitions.26 Generalized 2D correlation spectroscopy was first proposed by Noda in 1993.27,28 It is a powerful tool for the analysis of the spectral intensity variation induced by different external perturbation such as time,29−31 concentration,32 temperature,33,34 pH,35 and so on. The 2D correlation FTIR spectra can easily capture the subtle information which cannot be gained in the 1D spectra via spreading the peaks over the second dimension. The techniques of moving-window 2D correlation spectroscopy, including moving-window two-dimensional (MW2D) and perturbation-correlation moving-window two-dimensional (PCMW2D), were proposed in recent years.36,37 These two methods can be directly applied to see correlation intensity along both spectral variables (e.g., wavenumber) and perturbation variables (e.g., temperature) axis. Therefore, the transition points and the transition ranges can be obtained from the correlation intensity along the perturbation variables’ direction. In the past decades, in situ FTIR combined with 2D correlation analysis was proven to be successful for the study of the macromolecules microdynamics in transitions. The applications of 2D correlation FTIR spectroscopy on the behavior of some typical water-soluble polymers was carried out by scientists, such as poly(vinyl methyl ether) (PVME),38 poly(N-isopropylacrylamide) (PNIPAM),39,40 and poly(Nisopropylacrylamide-co-acrylic acid) (PNIPAM-co-AA).41 More details of microdynamics of the phase separation around lower critical solution temperature (LCST) in these systems were provided. In these water-soluble polymers, when the temperature is above LCST, the outward diffusion of water molecules occurs due to the dehydration of hydrophobic groups, resulting in an apparent volume phase transition. However, PVA studied here are completely different from above polymers since the existence of a number of −OH on PVA chains. PVA aqueous solution appears to present an upper critical solution temperature (UCST). There is a formation of PVA hydrogel when the temperature is below UCST due to the generation of PVA microcrystals. The destruction of PVA hydrogel occurs, and it is converted into a homogeneous solution when above UCST owing to the microcrystals melting and hydration. In the present study, we attempt to study the microdynamics mechanism of thermal-induced hydrogel network destruction of PVA hydrogels in D2O. The temperature range of hydrogel destruction was determined by dynamic rheological test at first, and then the in situ FTIR spectroscopy combined with 2D correlation analysis and MW2D technique was employed. The microscopic behavior of hydrogel network destruction was successfully monitored. The network destruction of PVA hydrogel at heating mainly seems to arise from the hydrogen bonds dissociation in microcrystals (the melting of PVA microcrystals) and the complete hydration of −C−O− and −CH− groups by D2O. A proposed microdynamics mechanism is also provided in the present study. The mechanism revealed

is possibly helpful in understanding the network destruction process of a complex hydrogel like PVA.

2. EXPERIMENTAL SECTION 2.1. Materials. An atactic PVA (1799) was provided by Sinopec Sichuan Vinylon Works (Chongqing, China) with a number-average polymerization of 1700, and its degree of hydrolysis was 99%. Deuterated water (D2O, D-99.9%) was purchased from Cambridge Isotope Laboratories, Inc. 2.2. Sample Preparation of PVA/H2O Solution and PVA/D2O Solution. PVA was dissolved in deionized water at 90 °C under continuous stirring to prepare a PVA/H2O solution with concentrations of 15% (w/w). PVA/D2O solution (with polymer concentrations of 15% w/w) was also prepared via dissolving PVA in deuterated water at 90 °C under continuous stirring to avoid inhomogeneities and local gelation. The PVA/D2O solution was then sealed and stored at 70 °C for 7 days to ensure a sufficient proton exchange between D2O and the hydroxyl protons of PVA. After that, the sealed PVA/D2O solution was kept at 5 °C for 10 days to form the hydrogel. 2.3. Dynamic Rheological Experiments. The dynamic rheological properties of PVA/H2O solutions were characterized using a Rotational rheometer (Thermo Scientific HAAKE Viscotester 550). Parallel plate geometry with a diameter of 60 mm and a gap of 1 mm was used to measure dynamic viscoelastic parameters, such as the storage modulus (G′) and tan δ. The values of the strain amplitude were set as 10% to ensure that the measurement was carried out within a linear viscoelastic regime. The effect of temperature on the viscoelastic response of the solutions was studied by performing an isochronal temperature sweep experiment. The rheological experiment was carried out with 1.0 Hz, and the temperature was scanned from 29 to 53 °C at a speed of 2 °C/min. 2.4. Temperature-Dependent FTIR. The hydrogel of PVA/D2O was chosen for temperature-dependent FTIR measurement. The reason is that the FTIR absorption peaks of PVA in PVA/D2O hydrogel is not covered up in the region 3000−2800 cm−1, which is different from PVA/H2O. PVA/ D2O hydrogel was sealed between two small ZnSe windows (Φ13 mm) and was then placed into a homemade in situ pool (programmable heating and cooling device).The sample was heated from 25 to 62 °C with 1.0 °C/min. At the same time, the FTIR spectra were collected. There were 63 IR spectra collected at heating. FTIR spectrometer was Nicolet iS10 which was equipped with a deuterated triglycine sulfate (DTGS) detector. The FTIR spectral resolution was 4 cm−1 and total of 20 scans were coadded. During the measurement, the sample was also protected by a static high-purity nitrogen gas. 2.5. Two-Dimensional Correlation Analysis. MW2D correlation FTIR spectra, as well as generalized 2D correlation FTIR spectra, were processed and calculated by 2DCS 6.1, developed by Tao Zhou at Sichuan University. The linear baseline corrections were performed in the region of 2985− 2815 cm−1 and 1160−1080 cm−1 before analysis. The linear baseline correction can preserve the peak shape and intensity distribution from the original spectra to the maximum extent. To produce clear MW2D spectra, the window size was selected as 15 (2m + 1). In the 2D correlation FTIR spectra, the red areas represent positive correlation intensity, and the blue areas represent the negative. The readers can refer to refs 42−44 for the theory of MW2D. 9497



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3. RESULTS AND DISCUSSION 3.1. Analysis of Dynamic Rheological Data. Dynamic rheological test with temperature range of 29−53 °C was performed using a rotational rheometer for PVA/H2O hydrogel. This method is always used to determine the dynamic viscoelastic behaviors, as well as the rheological behavior for a polymer solution or a polymer gel. The storage modulus (G′) and tan δ of PVA/H2O hydrogel with temperature changing are illustrated in Figure 1. In this case

Figure 2. Typical photos of PVA/H2O hydrogel at different temperatures. PVA/H2O was placed in a small inverted glass bottle. PVA/H2O is a typical gel when the ambient temperature is below 32 °C. PVA/H2O is a polymer solution above 45 °C.

solution when the temperature is above 45 °C. The solution rapidly flows to the bottom of the inverted bottle under gravity. PVA hydrogel network destruction was characterized by dynamic rheological test, and this process can also be observed with the eyes. How does this process proceed from the microscopic point of view? This aroused our curiosity. So, a detail FTIR spectroscopic study is necessary for us to understand its microdynamics mechanism. 3.2. Temperature-Dependent FTIR Spectra Analysis of the Heating Process. The PVA/D2O hydrogel was used for temperature-dependent FTIR measurement due to the FTIR absorption peaks in the region of 3000−2800 cm−1 not being covered up by D2O. Figure 3 shows in situ FTIR spectra of

Figure 1. Storage modulus (G′), tan δ, and the complex viscosity of PVA/H2O hydrogel with temperature increasing, using dynamic rheological test with temperature range of 29−53 °C. The viscosity gradually decreases with the temperature increasing.

we can obviously observe that the curve of G′ for the 15 wt % PVA/H2O hydrogel with a heating rate of 2 °C/min shows the different rheological regions. The first region is characterized by a slightly decreasing of G′, showing a small temperature dependence. Subsequently, in the temperature range 35−50 °C, the G′ decreases rapidly with the temperature increasing. This describes the destruction of the physical network PVA/H2O hydrogel, together with a process of converting into a concentrated solution. The physical network is damaged significantly within 35−50 °C. However, tan δ is different. After the first increasing within 37−45 °C, it then decreases rapidly above 45 °C. The curve of tan δ shows an obvious bread-shaped peak in the range of 34−52 °C, representing a significant viscoelastic behavior. It also indicates the enhancement of PVA intramolecular friction caused by a further swelling. In the case of >46 °C, the physical network is completely destroyed, and the swollen PVA is dissolved in H2O. A significant reduction of the friction among PVA molecules is shown, and therefore tan δ decreases rapidly. The curve of tan δ indicates that the swelling process is first involved in, and then the dissolution process. A complex viscosity from the rheological tests is also illustrated in Figure 1. It can be observed that the viscosity gradually decreases with the temperature increasing from 29 to 53 °C. In the present study, the macroscopic state of PVA/H2O hydrogel can be directly observed by our eyes. Typical photos of PVA/H2O hydrogel at different temperatures are shown in Figure 2. PVA/H2O was placed in a small glass bottle, and please note the bottle was inverted. The PVA/H2O shows the typical behavior of gel when the ambient temperature is below 32 °C. The stable hydrogel of PVA/H2O which can overcome the gravity without flowing or slipping stays in the upper part of the inverted bottle. However, PVA/H2O is a typical polymer

Figure 3. In situ FTIR spectra of PVA/D2O hydrogel in the region 2985−2815 cm−1 and 1160−1080 cm−1 upon heating from 25 to 62 °C.

PVA/D2O hydrogel in the region 2985−2815 cm−1 and 1160− 1080 cm−1 at heating from 25 to 62 °C. The region of 2985− 2815 cm−1 is the stretching vibration of alkyl groups of PVA main chains, and that of 1160−1080 cm−1 is assigned to the −C−O− stretching vibration of PVA hydroxyls. So, the bands at 2985−2815 cm−1 and 1160−1080 cm−1 can be conveniently used to describe the motions of functional groups of both PVA 9498



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backbone and side groups. At the same time, the spectral intensities within these two spectral ranges are high enough. This gives a high signal-to-noise ratio, which will bring a more precise and robust result of 2D correlation analysis in the next section. In the 2985−2815 cm−1 region, the gradual decreasing of spectral intensity at 2950 cm−1 and the increasing at 2877 cm−1 are observed (25−62 °C). In the region of 1160−1080 cm−1, the spectral intensity around 1146 cm−1 decreases with the temperature increasing, whereas the bands around 1113 and 1097 cm−1 both enhance. These changes of the spectral intensity at the specific bands indicate the complex motions of PVA molecular chains in PVA/D2O hydrogel, which could not be figured out up to the present. The correct assignments of these bands play a key rule on solving the problem of the complex motions of PVA molecular chains. The band at 2950 cm−1 is attributed to the asymmetric stretching vibration of −CH2− of PVA main chains, which are partially hydrated (swollen) by D2O (see Scheme 1a). In pure

Figure 4. Second derivative of the in situ FTIR spectra of PVA/D2O hydrogel in the region of 1160−1080 cm−1. Three bands can be distinguished, including 1146 cm−1, 1113 cm−1, and 1097 cm−1.

Scheme 1. PVA Molecules in D2Oa

groups of the PVA microcrystals. In solid PVA, much literature reported that the bands within 1141−1146 cm−1 is the characteristic bands in PVA crystals.45−49 People found that the crystallinity of PVA was proportional to the intensity of the absorption peak within 1141−1146 cm−1. Although the band at 1146 cm−1 in the present study is not easy to distinguish compared to the solid PVA (this band is very sharp in solid PVA), this crystalline band can still be judged via its intensity decreasing at heating. The band centered at 1097 cm−1 is assigned to −C−O− stretching vibration in hydroxyl groups of the PVA amorphous, which was also reported by much literature.45,47,50 The position of 1113 cm−1 is between 1146 and 1097 cm−1. The assignment of this band has not been proposed by any literature. In solid PVA, it is generally considered 1146 cm−1 to be contributed from PVA crystals, and 1097 cm−1 contributed from the amorphous PVA. So, 1113 cm−1 is probably attributed to the transition region between crystals and amorphous. Similarly, in PVA/D2O hydrogel, 1146 cm−1 is attributed to the PVA microcrystals, and 1097 cm−1 is assigned to the fully hydrated amorphous PVA. Therefore, 1113 cm−1 is probably assigned to the partially hydrated PVA (see Scheme 1a). This assignment is also confirmed by 2D correlation analysis. Here, we attribute 1113 cm−1 to the −C− O− stretching vibration in hydroxyl groups of the partially hydrated PVA. The assignments mentioned above are all listed in Table 1.

a

Key: (a) PVA main chains partially hydrated (swollen) by D2O; (b) PVA main chains fully hydrated (dissolved) by D2O.

PVA (solid), the asymmetric stretching vibration of −CH2− is usually around 2940 cm−1.45 After PVA molecules are partially hydrated, this asymmetric stretching vibration moves toward a higher wavenumber. The bands centered at 2877 cm−1 are attributed to the −CH− stretching vibration of PVA, which is fully hydrated (dissolved) (see Scheme 1b). In the present study, the assignment of these bands, which is confirmed by 2D correlation infrared spectroscopy, had no report by the literature before. According to the second derivative (Figure 4) in the region of 1160−1080 cm−1, three bands can be distinguished, including 1146 cm−1, 1113 cm−1, and 1097 cm−1. The second derivative was calculated according to the Savitsky−Golay method, and the Omnic 8.1 software was used. The curve of the second derivative is helpful to determine the overlapping bands in the original infrared spectra. In Figure 4, the wavenumber of downward peaks corresponds to that of hidden bands in the original infrared spectra. The band at 1146 cm−1 is attributed to the −C−O− stretching vibration in hydroxyl

Table 1. FTIR Band Assignments Reported by the Literature and Inferred from 2D Correlation Analysis wavenumber (cm−1) 2950 2877 1146 1113 1097

9499

assignments

explanations

vas (−CH2−, phydrated) vs (−CH−, fhydrated) vs (−C−O−, microcrystals) vs (−C−O−, phydrated) vs (−C−O−, fhydrated)

−CH2− asymmetric stretching of PVA partially hydrated (swollen) by D2O −CH− stretching of fully hydrated (dissolved) PVA −C−O− stretching of PVA in microcrystals −C−O− stretching of PVA partially hydrated (swollen) by D2O −C−O− stretching of PVA fully hydrated (dissolved) by D2O



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The formation mechanism of PVA hydrogel has been studied by many scientists.4 PVA hydrogel with a high mechanical strength can usually be obtained via a common method having heating and cooling cycles.4,51 It is widely believed that the reason for PVA hydrogel formation is the generation of physical cross-linking points through slow crystallization. In the present study, the microcrystals (1146 cm−1) of PVA/D2O hydrogel are also observed via in situ IR spectra. It can be seen that PVA microcrystals gradually disappear, and then completely dissolved into the D2O solution when the temperature rises. The disappearance of PVA microcrystals means the destruction of physical cross-linking points, and PVA/D2O hydrogel is gradually converted to a homogeneous solution. This is also the direct reason why the viscosity decreases with temperature increasing in the rheological test showing in Figure 1. In addition, as illustrated in Figure 4, it can also be observed that the content of the partially hydrated −CH2− (2950 cm−1) gradually decreases, and that of the fully hydrated −CH− (2877 cm−1) increases, which also shows the process of PVA/ D2O hydrogel converting into a solution. Although some useful information about PVA/D2O hydrogel network destruction can be gained via in situ infrared spectroscopy, the key information on microdynamics mechanism in which we are most interested are still unclear. Next, MW2D correlation FTIR technique and 2D correlation FTIR analysis were employed to reveal the microdynamics mechanism of the network destruction process of PVA/D2O hydrogel. 3.3. Analysis of Two-Dimensional Correlation FTIR Spectroscopy. 3.3.1. MW2D FTIR Spectra. MW2D FTIR spectra of PVA/D2O hydrogel at heating were calculated and illustrated in Figure 5. MW2D technique can be conveniently used to determine the temperature range, as well as the transition point, of PVA/D2O hydrogel destruction. In Figure 5, a correlation peak at 2950 cm−1 and 45 °C is observed. Two correlation peaks at 1146 and 1097 cm−1 appear, and their corresponding temperatures are 37 and 42 °C, respectively. 2877 cm−1 has a broad response of the correlation intensity in a wide temperature range of 34−52 °C. According to the assignments listed in Table 1, the following information can be gained. During PVA/D2O network destruction process, the transition point of partially hydrated −CH2− (2950 cm−1) in PVA main chains is 45 °C. The transition point of fully hydrated −CH− (2877 cm−1) is within 34−52 °C. The melting temperature of −C−O− (1146 cm−1) in PVA microcrystals is 37 °C, and the temperature at 42 °C is the transition point of fully hydrated −C−O− (1097 cm−1). The sequential order of 2950 cm−1, 1146 cm−1, and 1097 cm−1 participating in hydrogel network destruction can be inferred from the temperature difference of their transition points. 1146 cm−1 (37 °C) → 1097 cm−1 (42 °C) → 2950 cm−1 (45 °C). The temperature point or range of 1113 cm−1 cannot be distinguished in Figure 5. This is probably due to its correlation peak covered by the correlation intensity of 1097 cm−1. The temperature range (34−52 °C) of PVA/D2O hydrogel destruction determined by MW2D is fully consistent with that of determined by the rheology test. Within this temperature range, PVA/D2O hydrogel is destroyed and converts into a homogeneous solution. 3.3.2. Generalized 2D Correlation Analysis. The in situ FTIR spectra within 34−52 °C were used to perform the generalized 2D correlation analysis. The 2D correlation FTIR spectra are shown in Figure 6, Figure 7, and Figure 8.

Figure 5. MW2D FTIR spectra of PVA/D2O hydrogel in the region 2985−2815 cm−1 and 1160−1080 cm−1 upon heating from 25 to 62 °C. The horizontal dashed lines correspond to the temperature points at 37 °C, 42 °C, and 45 °C, respectively.

Generalized 2D correlation FTIR spectra include both synchronous and asynchronous spectra. The sequential order of variation of the spectral intensity at specific wavenumber can be easily judged by the sign of the correlation peaks according to Noda’s rules. Noda’s rules are summarized as follows: (1) If Φ(v1, v2) > 0, Ψ(v1, v2) > 0 or Φ(v1, v2) < 0, Ψ(v1, v2) < 0, then the movement of v1 is before that of v2. (2) If Φ(v1, v2) > 0, Ψ(v1, v2) < 0 or Φ(v1, v2) < 0, Ψ(v1, v2) > 0, then the movement of v1 is after that of v2. (3) If Φ(v1, v2) > 0, Ψ(v1, v2) = 0 or Φ(v1, v2) < 0, Ψ(v1, v2) = 0, then the movements of v1 and v2 are simultaneous. 3.3.2.1. Sequential Order of Group Motion in the C−H Stretching Vibration Region. Figure 6 is the synchronous (left) and the asynchronous (right) 2D correlation FTIR spectra in the region 2985−2815 cm−1. Since Φ(2950 cm−1, 2877 cm−1) < 0 and Ψ(2950 cm−1, 2877 cm−1) > 0, the sequential order is 2877 cm−1 → 2950 cm−1 according to Noda’s rules (“ → ”means before). That is to say vs(−CH−, fhydrated) → vas(−CH2−, p-hydrated). This indicates that the motion of fully hydrated −CH− is before that of partially hydrated −CH2− during the PVA/D2O hydrogel destruction. 3.3.2.2. Sequential Order of Group Motion in the C−O Stretching Vibration Region. Figure 7 is the synchronous 9500



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Figure 6. Synchronous and asynchronous FTIR spectra of PVA/D2O hydrogel in the region 2985−2815 cm−1 (34−52 °C). Red and blue areas represent positive and negative correlation intensity, respectively.

Figure 7. Synchronous and asynchronous FTIR spectra of PVA/D2O hydrogel in the region 1160−1080 cm−1 (34−52 °C).

Figure 8. Synchronous and asynchronous FTIR spectra of PVA/D2O hydrogel in the region 2985−2815 cm−1 vs 1160−1080 cm−1 (34−52 °C).

Φ(1146 cm−1, 1097 cm−1) < 0, Ψ(1146 cm−1, 1097 cm−1) < 0; and Φ(1113 cm−1, 1097 cm−1) > 0, Ψ(1113 cm−1, 1097 cm−1) > 0. The sequential orders are 1146 cm−1 ← 1113 cm−1 (“ ← ”

(left) and the asynchronous (right) 2D correlation FTIR spectra in the region 1160−1080 cm−1. It can be observed that Φ(1146 cm−1, 1113 cm−1) < 0, Ψ(1146 cm−1, 1113 cm−1) > 0; 9501



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means after), 1146 cm−1 → 1097 cm−1, and 1113 cm−1 → 1097 cm−1. It can be inferred that 1113 cm−1 → 1146 cm−1 → 1097 cm−1, expressed as vs(−C−O−, p-hydrated) → vs(−C−O−, microcrystals) → vs(−C−O−, f-hydrated). This shows that the motion of partially hydrated −C−O− is before that of −C−O− in PVA microcrystals, and the motion of −C−O− in microcrystals is prior to that of fully hydrated −CH−. 3.3.2.3. C−H versus C−O Stretching Vibration Region. The synchronous (left) and the asynchronous (right) generalized 2D correlation FTIR spectra in the region 2985−2815 cm−1 versus 1160−1080 cm−1 are shown in Figure 8. The signs of cross peaks of C−H versus C−O stretching vibration are summarized in Table 2. According to Noda’s rules, the

the motion of vs(−C−O−,microcrystals) is particularly important in the entire hydrogel destruction process. The motion of vs(−C−O−, microcrystals) is actually the destruction process of these physical cross-linking points. Its real physical meaning is the melting of microcrystals caused by temperature. Literature reported that the microcrystals of PVA hydrogels are the folded chain lamellae,4 and mainly in intramolecular folding. The impetus for the lamellae formation is the regular hydrogen bonds (intramolecular or intermolecular), and therefore hydrogen bonds plays an important role in the formation of microcrystals. In PVA/D2O hydrogel, the groups of −C−O− can form hydrogen bonds through hydroxyls, and the hydrogen bonds can also be formed between −C−O− and D2O. As the temperature rises, the dissociation of the regular hydrogen bonds (intramolecular or intermolecular) in PVA occurs, which is characterized as the melting of microcrystals. The hydrogen bonds between −C−O− groups are gradually substituted with the hydrogen bonds between −C−O− and D2O. From the sequential order of groups concluded above, it can be inferred that hydrogel network destruction starts from −C−O−, and ends with −CH− and −CH2−. The key step of PVA/D2O hydrogel destruction is the process of intramolecular or intermolecular regular hydrogen bonds of PVA substituted and overcame by the hydrogen bonds between PVA and D2O. Some of the literature predicted this process,52 but this work first discovered this key step from the 2D correlation FTIR spectra. It is generally recognized that the two-phase model is suitable to describe the structure of PVA hydrogels.4,13 The basic meaning of a two-phase model is that a hydrogel contains a PVA-rich phase and PVA-poor phase. The PVA-rich phase is a swollen PVA, and the PVA-poor phase is a completely dissolved PVA, considered as a half-concentrated aqueous solution. In a two-phase model, the PVA-rich phase also includes a microcrystal phase and a swollen amorphous phase. A small amount of the microcrystal phase is dispersed in the swollen amorphous phase. The microcrystals phase is a physical cross-linking point for the PVA-rich phase, served as the “rivet” to maintain the stabilization of the PVA-rich phase. In the PVArich phase, each of microcrystals is surrounded by the swollen amorphous phase. The exchange of water molecules between the swollen amorphous phase and PVA-poor phase carries out constantly, and the dynamic equilibrium is maintained. The steps of PVA/D2O hydrogel destruction inferred from 2D correlation FTIR spectra is illustrated in Scheme 2.The green areas represent the PVA-rich phase, and the white areas represent the PVA-poor phase. The yellow rectangular areas with parallel black lines are on behalf of microcrystals in the PVA-rich phase. Double-headed arrows indicate the dynamic equilibrium of the exchange of D2O molecules. One-headed arrows indicate the direction of D2O molecules penetrating. At 32 °C, the dynamic equilibrium of the exchange of D2O between the swollen amorphous phase and the PVA-poor phase is maintained, and the PVA microcrystals are stable. Within 32−37 °C, more D2O molecules from the PVA-poor phase penetrate into the swollen amorphous in PVA-rich phase as the temperature rises. The dynamic equilibrium of the D2O exchange is broken, resulting in the increasing of swelling degree of swollen amorphous in PVA-rich phase. The PVA microcrystals are still stable. At 37 °C, the physical cross-linking points disappear, caused by the melting of PVA microcrystals. At temperatures above 37 °C (37−55 °C), the PVA-rich phase without physical cross-linking points is rapidly penetrated by

Table 2. Sequential Orders of the Bands of the C−H Stretching Vibration Region, the C−O Stretching Vibration Region, and Their Cross Regions Gained from Figures 6−8 cross correlation peak (cm−1) (2950, (1146, (1146, (1113, (2950, (2950, (2950, (2877, (2877, (2877,

2877) 1113) 1097) 1097) 1146) 1113) 1097) 1146) 1113) 1097)

sign in synchronous spectra

sign in asynchronous spectra

sequential order

− − − + + − − − + +

+ + − + − + + + − 0

2950←2877 1146←1113 1146→1097 1113→1097 2950←1146 2950←1113 2950←1097 2877←1146 2877←1113 2877=1097

1113 cm−1 → 1146 cm−1 → 1097 cm−1 = 2877 cm−1 → 2950 cm−1; vs(−C−O−, p-hydrated) → vs(−C−O−, microcrystals) → vs(−C− O−, f-hydrated) = vs(−CH−, f-hydrated) → vas(−CH2−, p-hydrated).

sequential orders are 2950 cm−1 ← 1146 cm−1, 2950 cm−1 ← 1113 cm−1, 2950 cm−1 ← 1097 cm−1, 2877 cm−1 ← 1146 cm−1, 2877 cm−1 ← 1113 cm−1, and 2877 cm−1 = 1097 cm−1. Please note that the motions of the fully hydrated −CH− and the fully hydrated −C−O− are simultaneous. The groups of −CH− (2877 cm−1) and the groups of −C−O− (1097 cm−1) are directly linked from the view of the chemical structure. In the generalized 2D correlation FTIR spectroscopy, the functional groups that move simultaneously are always chemically linked. Since the groups of −C−O− (1097 cm−1) are assigned to a fully hydrated PVA, the −CH− (2877 cm−1) is also attributed to a fully hydrated PVA. This is why we assign 2877 cm−1 to the fully hydrated −CH− of PVA in Table 1. According to the sequential orders of C−H versus C−O stretching vibration, it can be judged that the motions of all C− O are generally prior to that of alkyls in PVA during the hydrogel destruction. This indicates the hydroxyls of PVA are the attack points of D2O. According to the results concluded above, the sequential order of the whole process is 1113 cm−1 → 1146 cm−1 → 1097 cm−1 = 2877 cm−1 → 2950 cm−1, expressed as vs(−C−O−, phydrated) → vs(−C−O−, microcrystals) → vs(−C−O−, fhydrated) = vs (−CH−, f-hydrated) → vas(−CH2−, phydrated). The partially hydrated −C−O− moves first, and then the motion of −C−O−in microcrystals follows. After that, both the full hydrated −C−O− and −CH− moves simultaneously, and the partially hydrated −C−O− moves finally. Since the physical cross-linking points of PVA/D2O hydrogel are microcrystals, 9502



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Scheme 2. Steps of PVA/D2O Hydrogel Network Destruction Inferred from 2D Correlation FTIR Spectraa

a

The green areas represent the PVA-rich phase, and the white areas represent the PVA-poor phase. The yellow rectangular areas with parallel black lines are microcrystals in the PVA-rich phase. Double-headed arrows indicate the dynamic equilibrium of the exchange of D2O molecules. Oneheaded arrows indicate the direction of D2O molecules penetrating.

4. CONCLUSION

D2O molecules. PVA molecular chains of the PVA-rich phase are completely hydrated and dissolved by D2O. The PVA/D2O hydrogel network is completely destroyed and converted into a PVA/D2O concentrated solution. The 2D correlation FTIR spectroscopy reveals the microdynamics information on PVA molecular chains in PVA-rich phase. Here, Scheme 3 is used to further elaborate the microdynamics mechanism of PVA molecular chains during the hydrogel network destruction. When the temperature is at 32 °C, the number of D2O molecules in the swollen amorphous remains unchanged. Within 32−37 °C, more D2O molecules enter into the swollen amorphous region, and the groups of −C−O−, as well as −CH2−, are partially hydrated. At 37 °C, the intramolecular or intermolecular hydrogen bonds of PVA molecular chains are dissociated. The physical cross-linking points of the hydrogel are broken due to the melting of PVA microcrystals. At 42 °C, the dissociated hydroxyls from PVA microcrystals rapidly integrate solid hydrogen bonds with D2O molecules. The groups of −C−O− and −CH− are completely hydrated by D2O simultaneously. When the temperature increases to 45−55 °C further, PVA molecules are surrounded by more D2O molecules. The partially hydrated −CH2− is completely hydrated, and all of the PVA molecules are fully dissolved in D2O. The PVA/D2O hydrogel transforms into a PVA/D2O concentrated solution.

A FTIR spectroscopic study combining with MW2D technique and 2D correlation analysis was used to investigate the microdynamics mechanism of PVA/D2O hydrogel network destruction at heating. After the network destruction, PVA/ D2O hydrogel is converted to a homogeneous solution. PVA hydrogel network destruction was characterized by dynamic rheological test, and its temperature range was determined within 34−52 °C. This temperature range is fully consistent with that of confirmed by MW2D FTIR spectra for PVA/D2O hydrogel. Through MW2D, it was also found that the motion temperature of partially hydrated −CH2− is 45 °C. The motion temperature of fully hydrated −CH− is within 34−52 °C. The melting temperature of −C−O− in PVA microcrystals is 37 °C, and the temperature at 42 °C is the motion of fully hydrated −C−O−. From the 2D correlation analysis of C−H stretching vibration region, C−O stretching vibration region, and their cross regions, the sequential orders was gained as vs(−C−O−, p-hydrated) → vs(−C−O−, microcrystals) → vs(−C−O−, f-hydrated) = vs(−CH−, fhydrated) → vas(−CH2−, p-hydrated). The microdynamics mechanism of PVA molecular chains during the hydrogel network destruction can be elaborated as follows: (1) At 32 °C, the number of D2O molecules in the swollen amorphous remains unchanged. 9503



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Scheme 3. Microdynamics Mechanism of PVA Molecular Chains during the Hydrogel Network Destructiona

a At 32 °C, the number of D2O molecules in the swollen amorphous remains unchanged. At 32−37 °C, more D2O molecules enter into the swollen amorphous region, and the groups of −C−O−, together with −CH2−, are partially hydrated. At 37 °C, the intramolecular or intermolecular hydrogen bonds of PVA are dissociated. The physical crosslinking points of the hydrogel are broken due to the melting of PVA microcrystals. At 42 °C, the dissociated hydroxyls from PVA microcrystals rapidly integrate solid hydrogen bonds with D2O molecules. The groups of −C−O− and −CH− are completely hydrated by D2O simultaneously. At 45−55 °C, PVA molecules are surrounded by more D2O molecules. The partially hydrated −CH2− is completely hydrated, and all of the PVA molecules are fully dissolved in D2O.

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(2) 32−37 °C, more D2O molecules enter into the swollen amorphous region, and the groups of −C−O−, together with −CH2−, are partially hydrated. (3) At 37 °C, the intramolecular or intermolecular hydrogen bonds of PVA are dissociated. The physical cross-linking points of hydrogel are broken due to the melting of PVA microcrystals. (4) 42 °C, the dissociated hydroxyls from PVA microcrystals rapidly integrate solid hydrogen bonds with D2O molecules. The groups of −C−O− and −CH− are completely hydrated by D2O simultaneously. (5) 45−55 °C, PVA molecules are surrounded by more D2O molecules. The partially hydrated −CH2− is completely hydrated, and all of the PVA molecules are fully dissolved in D2O.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-28-85407286; Fax: +86-28-85402465; E-mail: [email protected] (L. Jiang). *Tel.: +86-28-85407286; Fax: +86-28-85402465; E-mail: [email protected] (Y. Dan). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (No. 51173116) for support of this research. 9504



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Microdynamics Mechanism of Thermal-Induced Hydrogel Network

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