Chain Collapse and Revival Thermodynamics of ... - ACS Publications

J. Phys. Chem. B 2010, 114, 9761–9770

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Chain Collapse and Revival Thermodynamics of Poly(N-isopropylacrylamide) Hydrogel Shengtong Sun, Jun Hu, Hui Tang, and Peiyi Wu* The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, and Laboratory of AdVanced Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: April 27, 2010; ReVised Manuscript ReceiVed: June 6, 2010

Two-dimensional correlation infrared spectroscopy (2DIR) and a newly developed perturbation correlation moving window (PCMW) technique were employed to study the precise chain collapse and revival thermodynamic mechanism of poly(N-isopropylacrylamide) (PNIPAM) hydrogel. Both Boltzmann fitting and PCMW had figured out the volume phase transition temperature for heating and cooling processes to be about 35 and 33.5 °C, respectively, close to the results obtained from DSC. Furthermore, determination of the isosbestic points for V(CH3) and V(CdO) overlaid spectra showed that the chain collapse of PNIPAM hydrogel took place along with some intermediate states or a completely continuous phase transition while the chain revival occurred with only conversion between two single states. Finally, 2Dcos discerned all the sequence of group motions of PNIPAM hydrogel, indicating that in the heating process, PNIPAM hydrogel occurred to collapse along the backbone before water molecules were expelled outside the network, while in the sequential cooling process, PNIPAM hydrogel had water molecules diffusing into the network first before the chain revival along the backbone occurred. 1. Introduction A hydrogel is a network of hydrophilic polymers which can swell in water and hold a large amount of water while maintaining the structure.1 According to different cross-linking methods (chemically or physically), hydrogels can undergo reversible volume phase transitions or sol-gel phase transitions upon environmental stimuli, such as temperature,2,3 pH,2,4,5 light,6 pressure,7 electric fields,8 solvent composition,9 etc. These environment-sensitive hydrogels are also called “intelligent” or “smart” hydrogels. Due to their excellent and attractive stimuliresponsive properties, smart hydrogels have gained diverse applications in controlled drug delivery,1,2 tissue engineering,10 artificial muscles,8,11 soft machines,12 bioseparation,13 etc. Among various synthetic thermosensitive hydrogels, poly(Nisopropylacrylamide) (PNIPAM) hydrogel is the most studied example with negative thermal response or a lower critical solution temperature (LCST, ∼32 °C).14 Below LCST, PNIPAM hydrogel absorbs a high amount of water and exists in a transparent swollen state. When the temperature increases to above LCST, PNIPAM hydrogel would undergo a drastic, discontinuous volume phase transition and exists in a collapsed state above LCST.15 Similar to the coil-to-globule phase transition of PNIPAM aqueous solution, the swelling and deswelling transition of PNIPAM hydrogel is also thermally reversible. The transition is generally considered to be the competitive result of the hydrophobic interaction of pendent isopropyl groups and backbones and the hydrogen bonding association between amide groups and water molecules.16 A large amount of research has been devoted to investigate the novel volume phase transition behavior of PNIPAM hydrogel. Plenty of work tried to improve the volume phase transition speed in synthesis by changing synthesis solvent,17,18 gelling method,19 polymerizing technique,20 and most commonly * To whom correspondence [email protected].

should

be

addressed.

E-mail:

copolymerizing with other monomers.6,21,22 Although the mechanism of coil-to-globule phase transition of PNIPAM aqueous solution have been studied by many researchers,16,23–29 related research about the volume phase transition of PNIPAM hydrogel is still limited. Several temperature jump experiments30–35 were used to investigate the kinetics of PNIPAM hydrogel showing that the shrinking relaxation time of gels changes discontinuously by 102-104 times and the collective diffusion constants for shrinking and swelling processes could also be determined. In addition, the phase transition behavior of PNIPAM hydrogel has been predicted by different theories, such as statistical thermodynamic theory,36 lattice-fluid hydrogen bond theory,37 and Flory-Huggins-Staverman theory.38 Investigations on the swollen and collapsed state in constrained systems by either surface tethered method39,40 or physical absorption41,42 were also helpful for our understanding the volume phase transition of PNIPAM hydrogel. As we know, spectroscopy, especially IR and Raman spectroscopy, is rather sensitive to morphology and conformational changes by reflecting subtle information at the molecular level. However, up to now, compared to those on PNIPAM aqueous solution, only a few spectroscopic studies40,43 have been reported on the volume phase transition behavior of PNIPAM hydrogel. Recently, Aser et al.44 ultilized UV resonance Raman spectroscopy to determine the molecular mechanism of PNIPAM’s hydrophobic collapse, indicating that the amide bonds of PNIPAM do not engage in the interamide hydrogen bonding in the collapsed state but are still hydrogen bonded to water molecules, and above LCST PNIPAM forms local hydrophobic pockets which significantly reduce the solvent exposure of its pendent amide groups. Nevertheless, a whole dynamic analysis involving both heating and cooling processes of the volume phase transition of PNIPAM hydrogel is still lacking, and a precise thermally induced evolving mechanism has not yet been clarified.

10.1021/jp103818c  2010 American Chemical Society Published on Web 07/13/2010

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SCHEME 1: Synthesis of PNIPAM Hydrogel

In this paper, we present our FT-IR study of the chain collapse and revival thermodynamics of PNIPAM hydrogel during heating and cooling processes, mainly by two-dimensional correlation spectroscopy (2Dcos) as well as a newly developed perturbation correlation moving window (PCMW) technique. 2Dcos is a mathematical method whose basic principles were first proposed by Noda in 1986.45 Up to the present, 2Dcos has been widely used to study spectral variations of different chemical species under various external perturbations (e.g., temperature, pressure, concentration, time, electromagnetic, etc).46 Due to the different response of different species to external variable, additional useful information about molecular motions or conformational changes can be extracted that cannot be obtained straight from conventional 1D spectra. PCMW is a newly developed technique, whose basic principles can date back to the conventional moving window proposed by Thomas,47 and later in 2006 Morita48 improved this technique to much wider applicability through introducing the perturbation variable into the correlation equation. Except for its original ability in determining transition points as the conventional moving window did, PCMW can additionally monitor complicated spectral variations along the perturbation direction. 2. Experimental Methods 2.1. Materials. N-Isopropylacrylamide (NIPAM) was purchased from Tokyo Kasei Kogyo Co. (Tokyo, Japan) and recrystallized from cyclohexane before use. Azobis(isobutyronitrile) (AIBN) and N,N′-methylenebisacrylamide (MBAA) were purchased from Aladdin Reagent Co. and AIBN was recrystallized from ethanol. D2O was purchased from Cambridge Isotope Laboratories Inc. (D-99.9%). DMF was vacuum distilled from calcium hydride before use. 2.2. Preparation of PNIPAM Hydrogel. A detailed procedure for the preparation of PNIPAM hydrogel by free radical polymerization is described elsewhere,17,18 and only the chemical structure is shown in Scheme 1. Here, the feed ratio is [NIPAM]: [BIS]:[AIBN] ) 1000:50:20 mol/L. The obtained swollen PNIPAM hydrogel through dialysis was freeze-dried to dry gel before use. 2.3. Investigation Methods. 2.3.1. FT-IR Spectroscopy. PNIPAM dry gel was swollen in D2O at 4 °C for a week to ensure complete deuteration of all the N-H protons and sufficient swelling of PNIPAM hydrogel. The sample of PNIPAM hydrogel for FT-IR measurements was prepared by being sealed between two CaF2 tablets. All time-resolved FTIR spectra at different temperatures were recorded on a Nicolet Nexus 470 spectrometer with a resolution of 4 cm-1, and 32 scans were available for an acceptable signal-to-noise ratio. Temperature-dependent spectra were collected between 28 and 40 °C with an increment of 0.5 °C. Raw spectra were baselinecorrected by the software Omnic, ver. 6.1a. 2.3.2. 2D Correlation Spectroscopy. . FT-IR spectra collected in the temperature range 28-40 °C with 0.5 °C interval were used to perform 2D correlation analysis. 2D correlation analysis

Figure 1. Temperature-dependent FT-IR spectra of PNIPAM hydrogel (D2O) during heating and cooling between 28 and 40 °C.

was carried out with the software 2D Shige, ver. 1.3 ( Shigeaki Morita, Kwansei-Gakuin University, Japan, 2004-2005), and was further plotted into the contour maps by Origin program, ver. 8.0. In the contour maps, warm colors (red and yellow) are defined as positive intensities, while cool colors (blue) as negative ones. 2.3.3. Perturbation Correlation MoWing Window. FT-IR spectra used for 2D correlation analysis were also used to perform a perturbation correlation moving window analysis. Primary data processing was carried out with the method Morita provided and further correlation calculation was performed with the same software 2D Shige, ver. 1.3 (Shigeaki Morita, KwanseiGakuin University, Japan, 2004-2005). Similarly, the final contour maps were plotted by the Origin program, ver. 8.0, with the same colors defined as the same significations as 2D correlation analysis. An appropriate window size (2m + 1 ) 11) was chosen to generate PCMW spectra with good quality. 3. Results and Discussion 3.1. Conventional IR Analysis. Figure 1 shows us the temperature-dependent FT-IR spectra of PNIPAM hydrogel during one heating and cooling cycle between 28 and 40 °C. It should be noted that we used D2O instead of H2O as the solvent here in order to eliminate the overlap of the δ(OsH) band of H2O around 1640 cm-1 with the V(CdO) of PNIPAM hydrogel as well as the broad V(OsH) band of H2O around 3300 cm-1 with the V(CsH) bands of PNIPAM hydrogel.16 As reported, the transition temperature of the polymer gel in D2O is 0.7 °C higher than that in H2O. However, the deuterium isotope effect does not cause obvious changes on the magnitude of hysteresis.49,50 Thus, it is a good choice to choose D2O for IR analysis in other similar aqueous systems.

Poly(N-isopropylacrylamide) Hydrogel

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Figure 2. Temperature-dependent frequency shifts of (a) Vas(CH3) and (b) Vas(CH2) as well as the integral area in the regions (c) 1675-1653 and (d) 1626-1580 cm-1 during heating and cooling, respectively. The solid lines represent Boltzmann fitting curves.

Examining carefully the spectral variations of the two investigated regions in Figure 1 (CsH stretching bands in 3300-2847 cm-1, and CdO stretching band or amide I in 1675-1580 cm-1), we can find that during heating all the C-H stretching bands shifted slightly to lower frequency, while CdO exhibited a binary spectral intensity change. During cooling the case is just opposite to that in the heating process. The changes of V(CsH) bands can be explained by a hydrophobic interaction of polymer with neighboring water molecules of the solution. The higher the number of water molecules surrounding CsH groups is, the higher the vibrational frequency is.26 The V(CdO) band can be roughly considered to the combination of two bands at 1626 and 1653 cm-1. These two bands can be assigned to CdO stretching vibrations in CdO · · · D2O and CdO · · · DsN hydrogen bonding, respectively.16,23,27 The changes of V(CsH) and V(CdO) bands reveal that the chain collapse of PNIPAM hydrogel during heating is accompanied by the dehydration of hydrophobic CsH groups, the disassociation of CdO · · · D2O hydrogen bonds, and the formation of CdO · · · DsN hydrogen bonds. The chain revival process during cooling had the inverse changes. Judging from primitive conventional IR variations, PNIPAM hydrogel has a large similarity to PNIPAM aqueous solution,16 indicating that the volume phase transition of PNIPAM hydrogel is closely related to the coilto-globule phase transition of PNIPAM aqueous solution. To quantitatively describe the two volume phase transition processes during heating and cooling, the temperature-dependent frequency shifts of Vas(CH3) and Vas(CH2) as well as the half integral area of two kinds of CdO have been plotted in Figure 2. For an accurate determination of the transition temperature, Boltzmann fitting (using Origin program) was employed

for all the points in Figure 2. The corresponding equation is as follows:

y)

A1 - A2 1 + e(x-xo)/dx

+ A2

where A1 is the minimum value of the function; A2 is the maximum value of the function; x0 is the value on the x axis corresponding to the inflection of the curve, which also equals the transition temperature; and dx is the domain where this value lies.51 The LCSTs resulting from Boltzmann fitting are approximate to that obtained from differential scanning calorimetry (DSC, the heat flow curve not shown). It should also be noticed that there are large differences of retrieval degree after the cooling process between CH3 and CH2 as well as between the two kinds of CdO. The better retrieval of CH3 than that of CH2 should arise from the higher degree of freedom of CH3 in pendent isopropyl groups than that of CH2 in cross-linked backbone. The CdO · · · DsN hydrogen bonding can also easily recover because the intermolecular hydrogen bonding among networks would be inclined to form nearby constrained by the confined 3D network structure, which would also be disrupted easily. However, once the CdO · · · D2O disassociated and water molecules was expelled outside the network, to reassociate them through hydrogen bonding would require that the water molecules diffuse into the network first, which may cost a period of time. The retrieval differences of hydration and dehydration of CsH groups as well as the association and disassociation of CdO related hydrogen bonds strongly reveal that unlike in PNIPAM aqueous solution the

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Figure 3. Determination of the isosbestic points of V(CH3) and V(CdO) overlaid spectra during heating and cooling. Three curves at 28, 34, and 40 °C are highlighted for good observance.

diffusion process of water molecules through networks has unnegligible effects on the volume phase transition behavior of PNIPAM hydrogel. Generally, an isosbestic point in overlaid spectra occurs only when one species is quantitatively converted to another single species.52 To clearly determine whether there existed isosbestic points in Vas(CH3) and V(CdO) overlaid spectra during heating and cooling in PNIPAM hydrogel, four enlarged spectra with three highlighted curves at 28, 34, and 40 °C are presented in Figure 3. Interestingly, only the cooling process exhibits two isosbestic points at 2979 and 1642 cm-1, respectively, while the heating process has no strict isosbestic points. The absence of isosbestic points in the heating process indicates that the chain collapse of PNIPAM hydrogel occurred with some intermediate states or a completely continuous phase transition, which may result from a holistic phase transiton due to the free diffusion of water molecules through networks starting from a sufficient swollen hydrogel. Similarly, the presence of isosbestic points in the cooling process indicates that the chain revival of PNIPAM hydrogel occurred with only conversion between two single states;that is, the hydration and dehydration states of CH3 as well as CdO · · · D2O and CdO · · · DsN hydrogen bonds. This may arise from local phase transition due to the less free diffusion of water molecules through networks starting from a hard collapsed state of PNIPAM hydrogel during cooling. Largely different from PNIPAM hydrogel, PNIPAM aqueous solution (20%) showed isosbestic points in both heating and cooling processes,16 and the existence of them strongly depends on the concentration, which will be discussed in another paper from our group. 3.2. Perturbation Correlation Moving Window. Figure 4 shows PCMW synchronous and asynchronous spectra generated from all the spectra during heating and cooling between 28 and

40 °C. PCMW synchronous spectra are very helpful in finding transition points, which have been listed together in Table 1. The LCSTs obtained from PCMW are approximately equal to those from Boltzmann fitting. However, PCMW is much easier to operate, and LCSTs in the whole spectral region can all be reflected in the contour maps. In addition to determining transition points, PCMW can also monitor the spectral variations along temperature perturbation combining the signs of synchronous and asynchronous spectra by the following rules: positive synchronous correlation represents spectral intensity increasing, while negative correlation represents decreasing; positive asynchronous correlation can be observed for a convex spectral intensity variation while negative correlation can be observed for a concave variation.48 On the basis of this point, we can primarily ascertain that during heating all the peaks in the regions 3030-2979 and 1642-1580 cm-1 exhibited an anti-S shaped intensity decrease while all the peaks in the regions 2979-2847 and 1675-1642 cm-1 exhibited an S shaped intensity increase. The case during cooling is just the opposite. Interestingly, we can also determine the existence of isosbestic points from PCMW asynchronous spectra. For example, the Vas(CH3) peaks in PCMW asynchronous spectra show a “fourleaf” pattern in both heating and cooling processes, whereas the pattern in the heating process without an isosbestic point has the center partially connected, while that in the cooling process with an isosbestic point has four separate leaves. The existence of isosbestic points of V(CdO) can be determined similarly. It is worth noting that PCMW can also determine the transition temperature regions by the peaks in asynchronous spectra which are all turning points of the S or anti-S shaped curves. The results have also been listed in Table 1. Thus we



Figure 4. PCMW synchronous and asynchronous spectra in the heating and cooling processes of PNIPAM hydrogel generated from all the spectra between 28 and 40 °C. Here, warm colors (red and yellow) are defined as positive intensities, while cool colors (blue) as negative ones.

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TABLE 1: LCSTs and Transition Temperature Regions Obtained from DSC, Boltzmann Fitting, and PCMW heating/°C method DSC frequency shift integration area PCMW

LCST Vas(CH3) Vas(CH2) CdO · · · DsN CdO · · · DsOsD CH3 (dehydrated), CdO · · · DsOsD CH3 (hydrated), CH2, CdO · · · DsN

can know that the volume phase transition of PNIPAM hydrogel mainly occurred at 33.0-36.5 °C during heating and 3235.5 °C during cooling. This served as an important basis for the segmental mode of the following 2Dcos analysis. 3.3. Two-Dimensional Correlation Analysis. On the basis of the phase transition evolving regions obtained from PCMW, we chose all the spectra between 32 and 37 °C to perform 2Dcos analysis. Figure 5 shows the 2D synchronous and asynchronous spectra in the heating and cooling processes of PNIPAM hydrogel. 2D synchronous spectra reflect simultaneous changes

34.1 35.1 35.3 35.1 35.1 35.5 35.0

transition region

33.5-36.5 33.0-36.0

cooling/°C LCST 31.9 33.8 33.9 33.2 33.7 33.5 34

transition region

32-35.5 32-35.5

between two given wavenumbers. The bands at 2970, 2927, 2871, and 1653 cm-1 all have positive cross-peaks, indicating that they had similar response of spectral intensities to temperature perturbation;that is, all increased during heating and all decreased during cooling determined from raw spectra. On the other hand, the bands at 2987 and 1626 cm-1 have spectral intensities that both decrease during heating and increase during cooling. 2D asynchronous spectra can significantly enhance the spectral resolution. In Figure 5, many subtle bands during

Figure 5. 2D synchronous and asynchronous spectra in the heating and cooling process of PNIPAM hydrogel generated from all the spectra between 32 and 37 °C. Here, warm colors (red and yellow) are defined as positive intensities, while cool colors (blue) as negative ones.



TABLE 2: Tentative Band Assignments of PNIPAM Hydrogel According to 2Dcos during Heating and Cooling, Respectively16,23,26,27,53 heating

cooling

freq/cm-1

assignment

freq/cm-1

2987 2974

Vas(hydrated CH3) Vas(dehydrating CH3)

2871 2850

2970

Vas(dehydrated CH3)

1655

2941

Vas(hydrated CH2)

1645

2933

Vas(hydrating CH2)

1632

2927

Vas(dehydrated CH2)

1618

2893

V(CH)

1610

freq/cm-1

assignment

freq/cm-1

assignment

Vs(CH3) Vs(CH2)

2989 2970

Vas(hydrated CH3) Vas(dehydrated CH3)

2854 1655

V(CdO · · · DsN) (pendent group) V(CdO · · · DsN) (cross-linker) V(CdO · · · D2O) (pendent group) V(CdO · · · D2O) (pendent group) V(CdO · · · D2O) (cross-linker)

2947

Vas(hydrated CH2)

1632

2929

Vas(dehydrated CH2)

1624

2908

V(CH)

1608

2877

Vs(hydrated CH3)

vs(CH2) V(CdO · · · DsN) (pendent group) V(CdO · · · D2O) (pendent group) V(CdO · · · D2O) (pendent group) V(CdO · · · D2O) (cross-linker)

2871

Vs(dehydrated CH3)

assignment

heating such as the bands at 2974, 2893 cm-1 attributed to Vas(dehydrating CH3 or CH3 in intermediate state) and V(CH) as well as five CdO splitting bands at 1655, 1645, 1632, 1618, and 1610 cm-1 have been indentified. For the convenience of discussion, all the bands found in asynchronous spectra and their corresponding assignments during heating and cooling respectively have been presented in Table 2. 3.3.1. The Sequence of Group Motions of PNIPAM Hydrogel in the Heating Process. Except for enhancing spectral resolution, 2D correlation spectroscopy can also discern the specific order taking place under external perturbation. The judging rule can be summarized as Noda’s rule;that is, if the cross-peaks (V1, V2, and assume V1 > V2) in synchronous and asynchronous spectra have the same sign, the change at V1 may occur prior to that of V2, and vice versa. A simplified method for determination of sequence order has never been described before.54 In short, multiplication was performed on the two signs of each cross-peak in synchronous and asynchronous spectra, the final results of which have been presented in Table 3. To each sign of cross-peaks in Table 3, according to Noda’s rule, if the sign is positive (+), the larger wavenumber or the bottom wavenumber will respond to external perturbation earlier than the smaller wavenumber or the left wavenumber. Similarly, if the sign is negative (-), the left wavenumber will respond earlier than the bottom one. If the sign is zero (or blank), we cannot make an exact judgment. Thus we can easily deduce the final specific order for the heating process of PNIPAM hydrogel (f means prior to or earlier than): 1610 cm-1 f 1645 cm-1 f 1618 cm-1 f 2941 cm-1 f 2893 cm-1 f 1632 cm-1 f 2933 cm-1 f 2974 cm-1 f 1655 cm-1 f 2970 cm-1 f 2927 cm-1 f 2871 cm-1 f 2987 cm-1 f 2850 cm-1. This sequence order can be interpreted at the following three aspects: (1) Considering separately C-H related vibrations, the sequence can be extracted as follows: 2941 cm-1 f 2893 cm-1 f 2933 cm-1 f 2974 cm-1 f 2970 cm-1 f 2927 cm-1 f 2871 cm-1 f 2987 cm-1 f 2850 cm-1;that is, Vas(hydrated CH2) f V(CH) f Vas(hydrating CH2) f Vas(dehydrating CH3) f Vas(dehydrated CH3) f Vas(dehydrated CH2) f Vs(CH3) f Vas(hydrated CH3) f Vs(CH2). Without considering the differences in stretching modes, the sequence can be described as CH2 f CH f CH3. This reveals that during heating the backbone of PNIPAM hydrogel had an earlier response than pendent isopropyl groups. On the other hand, if we consider only stretching modes, an interesting sequence can be found for C-H stretching vibrations that the asymmetric stretching vibration had an earlier response than the symmetric stretching

vibration, no matter for methyl or methylene groups. As previously reported, the direction of asymmetric stretching vibration is parallel to the polymer chain axis while that of symmetric stretching vibration is vertical to the polymer chain axis.54 Therefore, we can conclude that PNIPAM hydrogel had the chain collapsed along the backbone first before water molecules were expelled outside the network. (2) Considering separately CdO related vibrations, the sequence can be extracted as follows: 1610 cm-1 f 1645 cm-1 f 1618 cm-1 f 1632 cm-1 f 1655 cm-1;that is, CdO (crosslinker) f CdO (pendent group). This reveals for the second time that during heating the backbone of PNIPAM hydrogel had an earlier response than pendent groups. (3) Combining CsH and CdO related stretching vibrations, the whole sequence can be summarized as fpllows: CdO (crosslinker) f CdO (pendent group) f CH2 f CH f CH3. This sequence suggests to us exciting information that the driving force for chain collapse of PNIPAM hydrogel during heating was the conversion of amide hydrogen bonds from waterassociated ones to intermolecular ones. 3.3.2. The Sequence of Group Motions of PNIPAM Hydrogel in the Cooling Process. Similarly, the sequence of group motions of PNIPAM hydrogel in the cooling process can also be deduced as follows: 2854 cm-1 f 2908 cm-1 f 2877 cm-1 f 2929 cm-1 f 1655 cm-1 f 1608 cm-1 f 2970 cm-1 f 2871 cm-1 f 2989 cm-1 f 1632 cm-1 f 1624 cm-1 f 2947 cm-1. We adopted the same analysis method as that in the heating process. (1) For C-H related stretching vibrations, the sequence can be extracted as follows: 2854 cm-1 f 2908 cm-1 f 2877 cm-1 f 2929 cm-1 f 2970 cm-1 f 2871 cm-1 f 2989 cm-1 f 2947 cm-1;that is, Vs(CH2) f V(CH) f Vs(hydrated CH3)f Vas(dehydrated CH2) f Vas(dehydrated CH3) f Vs(dehydrated CH3) f Vas(hydrated CH3) f Vas(hydrated CH2). Without considering the differences in stretching modes, the sequence can be described as CH2 f CH f CH3, indicating that during cooling the backbone of PNIPAM hydrogel still had an earlier response than pendent isopropyl groups. If we consider only stretching modes, the case is opposite to the heating process that the symmetric stretching vibration had an earlier response than the asymmetric stretching vibration. Thus we can conclude that PNIPAM hydrogel had water molecules diffusing into the network first before the chain revival along the backbone occurred. (2) For CdO related vibrations, the sequence can be extracted as follows: 1655 cm-1 f 1608 cm-1 f 1632 cm-1 f 1624

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TABLE 3: The Final Results of Multiplication on the Signs of Each Cross-Peak in Synchronous and Asynchronous Spectra during Heating and Cooling, Respectively

cm-1;that is, CdO (pendent group) f CdO (cross-linker). This sequence can be interpreted that CdO in pendent groups formed hydrogen bonding with D2O first due to relatively higher freedom than CdO in cross-linker. (3) Combining CsH and CdO related stretching vibrations, the whole sequence can be summarized as follows: CH2 f CH f CH3 f CdO (side group) f CdO (cross-linker). This suggests that the driving force for chain revival of PNIPAM hydrogel during cooling was the diffusion of water into hard collapsed networks or the physical swelling action. On the basis of the above analysis during heating and cooling, we proposed the chain collapse and revival thermodynamic mehanism of PNIPAM hydrogel, as outlined in Figure 6. At lower temperature below LCST, PNIPAM hydrogel was swollen by water molecules through the hydration of aliphatic groups and the hydrogen bonding association with amide groups. As tempeature increased up to above LCST, PNIPAM hydrogel occurred to collapse along the backbone before water molecules were expelled outside the network, and this process was driven by the conversion from water-associated amide hydrogen bonds to intermolecular ones. In the sequential cooling process, PNIPAM hydrogel had water molecules diffusing into the network first before the chain revival along the backbone occurred, and

this process was driven by the physical diffusion or swelling action. Our proposed mechanism for volume phase transition of PNIPAM hydrogel discerned for the first time the specific order taking place between the physical diffusion of water molecules and hydrogen bonding association among amide groups and additionally identified the driving force of each process. These two steps during both heating and cooling may be related to the bimodal peaks in DSC curves,55,56 which need to be further confirmed. It should also be noted that we did not provide much discussion about the sequence of different CdO splitting bands. This is because CdO is much more sensitive to conformational changes than other groups due to its being almost the largest dipole moment of all chemical bonds, and most importantly, there are still divergences16,23,27,44 about the hydrogen bonding types of amide groups (including CdO and NsH) in PNIPAM hydrogel. In this paper, we adopted the common perception of the hydrogen bonds formed by CdO (CdO · · · H2O in swollen state and CdO · · · HsN in collapsed state). 4. Conclusion In this paper, we ultilized FT-IR spectrsocpy to in situ trace the thermally induced reversible volume phase transition



Figure 6. Proposed chain collapse and revival thermodynamic mechanism of PNIPAM hydrogel according to 2Dcos analysis. Dashed arrows represent the chain collapse and revival directions along the backbone of PNIPAM hydrogel networks.

of PNIPAM hydrogel using D2O as the solvent. A newly developed PCMW technique and 2Dcos were both employed to elucidate the chain collapse and revival mechanism of PNIPAM hydrogel. Boltzamann fitting on the frequency shifts of Vas(CH3) and Vas(CH2) as well as the half integral area of V(CdO · · · D2O) and v(CdO · · · DsN) bands figured out the phase transition tempetures to be about 35.1 °C during heating and 33.7 °C during cooling, approximate to the results obtained from DSC. PCMW generated similar results and further determined the phase transition regions for heating and cooling process to be 33.0-36.5 and 32-35.5 °C, respectively. We additionally determined the existence of isosbestic points for Vas(CH3) and V(CdO) overlaid spectra and found that in the heating process the chain collapse of PNIPAM hydrogel occurred with some intermediate states or a completely continuous phase transition while in the cooling process the chain revival occurred with only conversion between two single states. 2Dcos discerned all the sequence of group motions during heating and cooling. According to 2Dcos results, we proposed the chain collapse and revival thermodynamic mechanism. That is, at lower temperature below LCST, PNIPAM hydrogel was swollen by water molecules through the hydration of aliphatic groups and the hydrogen bonding association with amide groups. As temperature increased up to above LCST, PNIPAM hydrogel occurred to collapse along the backbone before water molecules were expelled outside the network, and this process was driven by the conversion from water-associated amide hydrogen bonds to intermolecular ones. In the sequential cooling process, PNIPAM hydrogel had water molecules diffusing into the network first before the chain revival along the backbone occurred, and this process was driven by the physical diffusion

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