chain collapse and revival thermodynamics of poly(n ... · reversible volume phase transitions or...

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 ofMacromolecular 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 correlationmoving window (PCMW) technique were employed to study the precise chain collapse and revivalthermodynamic mechanism of poly(N-isopropylacrylamide) (PNIPAM) hydrogel. Both Boltzmann fitting andPCMW had figured out the volume phase transition temperature for heating and cooling processes to beabout 35 and 33.5 °C, respectively, close to the results obtained from DSC. Furthermore, determination ofthe isosbestic points for V(CH3) and V(CdO) overlaid spectra showed that the chain collapse of PNIPAMhydrogel took place along with some intermediate states or a completely continuous phase transition whilethe chain revival occurred with only conversion between two single states. Finally, 2Dcos discerned all thesequence of group motions of PNIPAM hydrogel, indicating that in the heating process, PNIPAM hydrogeloccurred to collapse along the backbone before water molecules were expelled outside the network, while inthe sequential cooling process, PNIPAM hydrogel had water molecules diffusing into the network first beforethe chain revival along the backbone occurred.

1. Introduction

A hydrogel is a network of hydrophilic polymers which canswell in water and hold a large amount of water whilemaintaining the structure.1 According to different cross-linkingmethods (chemically or physically), hydrogels can undergoreversible volume phase transitions or sol-gel phase transitionsupon environmental stimuli, such as temperature,2,3 pH,2,4,5 light,6

pressure,7 electric fields,8 solvent composition,9 etc. Theseenvironment-sensitive hydrogels are also called “intelligent” or“smart” hydrogels. Due to their excellent and attractive stimuli-responsive properties, smart hydrogels have gained diverseapplications 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(N-

isopropylacrylamide) (PNIPAM) hydrogel is the most studiedexample with negative thermal response or a lower criticalsolution temperature (LCST, ∼32 °C).14 Below LCST, PNIPAMhydrogel absorbs a high amount of water and exists in atransparent swollen state. When the temperature increases toabove LCST, PNIPAM hydrogel would undergo a drastic,discontinuous volume phase transition and exists in a collapsedstate above LCST.15 Similar to the coil-to-globule phasetransition of PNIPAM aqueous solution, the swelling anddeswelling transition of PNIPAM hydrogel is also thermallyreversible. The transition is generally considered to be thecompetitive result of the hydrophobic interaction of pendentisopropyl groups and backbones and the hydrogen bondingassociation between amide groups and water molecules.16

A large amount of research has been devoted to investigatethe novel volume phase transition behavior of PNIPAMhydrogel. Plenty of work tried to improve the volume phasetransition speed in synthesis by changing synthesis solvent,17,18

gelling method,19 polymerizing technique,20 and most commonly

copolymerizing with other monomers.6,21,22 Although the mech-anism of coil-to-globule phase transition of PNIPAM aqueoussolution have been studied by many researchers,16,23–29 relatedresearch about the volume phase transition of PNIPAM hydrogelis still limited. Several temperature jump experiments30–35 wereused to investigate the kinetics of PNIPAM hydrogel showingthat the shrinking relaxation time of gels changes discontinu-ously by 102-104 times and the collective diffusion constantsfor shrinking and swelling processes could also be determined.In addition, the phase transition behavior of PNIPAM hydrogelhas been predicted by different theories, such as statisticalthermodynamic theory,36 lattice-fluid hydrogen bond theory,37

and Flory-Huggins-Staverman theory.38 Investigations on theswollen and collapsed state in constrained systems by eithersurface tethered method39,40 or physical absorption41,42 were alsohelpful for our understanding the volume phase transition ofPNIPAM hydrogel.

As we know, spectroscopy, especially IR and Raman spec-troscopy, is rather sensitive to morphology and conformationalchanges by reflecting subtle information at the molecular level.However, up to now, compared to those on PNIPAM aqueoussolution, only a few spectroscopic studies40,43 have been reportedon the volume phase transition behavior of PNIPAM hydrogel.Recently, Aser et al.44 ultilized UV resonance Raman spectros-copy to determine the molecular mechanism of PNIPAM’shydrophobic collapse, indicating that the amide bonds ofPNIPAM do not engage in the interamide hydrogen bonding inthe collapsed state but are still hydrogen bonded to watermolecules, and above LCST PNIPAM forms local hydrophobicpockets which significantly reduce the solvent exposure of itspendent amide groups. Nevertheless, a whole dynamic analysisinvolving both heating and cooling processes of the volumephase transition of PNIPAM hydrogel is still lacking, and aprecise thermally induced evolving mechanism has not yet beenclarified.

* To whom correspondence should be addressed. E-mail:[email protected].

J. Phys. Chem. B 2010, 114, 9761–9770 9761

10.1021/jp103818c 2010 American Chemical SocietyPublished on Web 07/13/2010

In this paper, we present our FT-IR study of the chain collapseand revival thermodynamics of PNIPAM hydrogel duringheating and cooling processes, mainly by two-dimensionalcorrelation spectroscopy (2Dcos) as well as a newly developedperturbation correlation moving window (PCMW) technique.

2Dcos is a mathematical method whose basic principles werefirst proposed by Noda in 1986.45 Up to the present, 2Dcos hasbeen widely used to study spectral variations of differentchemical species under various external perturbations (e.g.,temperature, pressure, concentration, time, electromagnetic,etc).46 Due to the different response of different species toexternal variable, additional useful information about molecularmotions or conformational changes can be extracted that cannotbe obtained straight from conventional 1D spectra. PCMW is anewly developed technique, whose basic principles can dateback to the conventional moving window proposed by Thomas,47

and later in 2006 Morita48 improved this technique to muchwider applicability through introducing the perturbation variableinto the correlation equation. Except for its original ability indetermining transition points as the conventional movingwindow did, PCMW can additionally monitor complicatedspectral variations along the perturbation direction.

2. Experimental Methods

2.1. Materials. N-Isopropylacrylamide (NIPAM) was pur-chased from Tokyo Kasei Kogyo Co. (Tokyo, Japan) andrecrystallized from cyclohexane before use. Azobis(isobuty-ronitrile) (AIBN) and N,N′-methylenebisacrylamide (MBAA)were purchased from Aladdin Reagent Co. and AIBN wasrecrystallized from ethanol. D2O was purchased from CambridgeIsotope Laboratories Inc. (D-99.9%). DMF was vacuum distilledfrom calcium hydride before use.

2.2. Preparation of PNIPAM Hydrogel. A detailed proce-dure for the preparation of PNIPAM hydrogel by free radicalpolymerization is described elsewhere,17,18 and only the chemicalstructure is shown in Scheme 1. Here, the feed ratio is [NIPAM]:[BIS]:[AIBN] ) 1000:50:20 mol/L. The obtained swollenPNIPAM hydrogel through dialysis was freeze-dried to dry gelbefore use.

2.3. Investigation Methods. 2.3.1. FT-IR Spectroscopy.PNIPAM dry gel was swollen in D2O at 4 °C for a week toensure complete deuteration of all the N-H protons andsufficient swelling of PNIPAM hydrogel. The sample ofPNIPAM hydrogel for FT-IR measurements was prepared bybeing sealed between two CaF2 tablets. All time-resolved FT-IR spectra at different temperatures were recorded on a NicoletNexus 470 spectrometer with a resolution of 4 cm-1, and 32scans were available for an acceptable signal-to-noise ratio.Temperature-dependent spectra were collected between 28 and40 °C with an increment of 0.5 °C. Raw spectra were baseline-corrected by the software Omnic, ver. 6.1a.

2.3.2. 2D Correlation Spectroscopy. . FT-IR spectra collectedin the temperature range 28-40 °C with 0.5 °C interval wereused to perform 2D correlation analysis. 2D correlation analysis

was carried out with the software 2D Shige, ver. 1.3 ( ShigeakiMorita, Kwansei-Gakuin University, Japan, 2004-2005), andwas 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) asnegative ones.

2.3.3. Perturbation Correlation MoWing Window. FT-IRspectra used for 2D correlation analysis were also used toperform a perturbation correlation moving window analysis.Primary data processing was carried out with the method Moritaprovided and further correlation calculation was performed withthe same software 2D Shige, ver. 1.3 (Shigeaki Morita, Kwansei-Gakuin University, Japan, 2004-2005). Similarly, the finalcontour maps were plotted by the Origin program, ver. 8.0, withthe same colors defined as the same significations as 2Dcorrelation 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 thetemperature-dependent FT-IR spectra of PNIPAM hydrogelduring one heating and cooling cycle between 28 and 40 °C. Itshould be noted that we used D2O instead of H2O as the solventhere in order to eliminate the overlap of the δ(OsH) band ofH2O around 1640 cm-1 with the V(CdO) of PNIPAM hydrogelas 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 °Chigher than that in H2O. However, the deuterium isotope effectdoesnotcauseobviouschangeson themagnitudeofhysteresis.49,50

Thus, it is a good choice to choose D2O for IR analysis in othersimilar aqueous systems.

SCHEME 1: Synthesis of PNIPAM Hydrogel

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

9762 J. Phys. Chem. B, Vol. 114, No. 30, 2010 Sun et al.

Examining carefully the spectral variations of the twoinvestigated regions in Figure 1 (CsH stretching bands in3300-2847 cm-1, and CdO stretching band or amide I in1675-1580 cm-1), we can find that during heating all theC-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 heatingprocess. The changes of V(CsH) bands can be explained bya hydrophobic interaction of polymer with neighboring watermolecules of the solution. The higher the number of watermolecules surrounding CsH groups is, the higher thevibrational frequency is.26 The V(CdO) band can be roughlyconsidered to the combination of two bands at 1626 and 1653cm-1. These two bands can be assigned to CdO stretchingvibrations in CdO · · ·D2O and CdO · · ·DsN hydrogenbonding, respectively.16,23,27 The changes of V(CsH) andV(CdO) bands reveal that the chain collapse of PNIPAMhydrogel during heating is accompanied by the dehydrationof hydrophobic CsH groups, the disassociation ofCdO · · ·D2Ohydrogenbonds,and theformationofCdO · · ·DsNhydrogen bonds. The chain revival process during coolinghad the inverse changes. Judging from primitive conventionalIR variations, PNIPAM hydrogel has a large similarity toPNIPAM aqueous solution,16 indicating that the volume phasetransition of PNIPAM hydrogel is closely related to the coil-to-globule phase transition of PNIPAM aqueous solution.

To quantitatively describe the two volume phase transitionprocesses during heating and cooling, the temperature-dependentfrequency shifts of Vas(CH3) and Vas(CH2) as well as the halfintegral area of two kinds of CdO have been plotted in Figure2. 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 asfollows:

where A1 is the minimum value of the function; A2 is themaximum value of the function; x0 is the value on the x axiscorresponding to the inflection of the curve, which also equalsthe transition temperature; and dx is the domain where this valuelies.51 The LCSTs resulting from Boltzmann fitting are ap-proximate to that obtained from differential scanning calorimetry(DSC, the heat flow curve not shown).

It should also be noticed that there are large differences ofretrieval degree after the cooling process between CH3 and CH2

as well as between the two kinds of CdO. The better retrievalof CH3 than that of CH2 should arise from the higher degree offreedom of CH3 in pendent isopropyl groups than that of CH2

in cross-linked backbone. The CdO · · ·DsN hydrogen bondingcan also easily recover because the intermolecular hydrogenbonding among networks would be inclined to form nearbyconstrained by the confined 3D network structure, which wouldalso be disrupted easily. However, once the CdO · · ·D2Odisassociated and water molecules was expelled outside thenetwork, to reassociate them through hydrogen bonding wouldrequire that the water molecules diffuse into the network first,which may cost a period of time. The retrieval differences ofhydration and dehydration of CsH groups as well as theassociation and disassociation of CdO related hydrogen bondsstrongly reveal that unlike in PNIPAM aqueous solution the

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

y )A1 - A2

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

Poly(N-isopropylacrylamide) Hydrogel J. Phys. Chem. B, Vol. 114, No. 30, 2010 9763

diffusion process of water molecules through networks hasunnegligible effects on the volume phase transition behaviorof PNIPAM hydrogel.

Generally, an isosbestic point in overlaid spectra occurs onlywhen one species is quantitatively converted to another singlespecies.52 To clearly determine whether there existed isosbesticpoints in Vas(CH3) and V(CdO) overlaid spectra during heatingand cooling in PNIPAM hydrogel, four enlarged spectra withthree highlighted curves at 28, 34, and 40 °C are presented inFigure 3. Interestingly, only the cooling process exhibits twoisosbestic points at 2979 and 1642 cm-1, respectively, whilethe heating process has no strict isosbestic points. The absenceof isosbestic points in the heating process indicates that the chaincollapse of PNIPAM hydrogel occurred with some intermediatestates or a completely continuous phase transition, which mayresult from a holistic phase transiton due to the free diffusionof water molecules through networks starting from a sufficientswollen hydrogel. Similarly, the presence of isosbestic pointsin the cooling process indicates that the chain revival ofPNIPAM hydrogel occurred with only conversion between twosingle states;that is, the hydration and dehydration states ofCH3 as well as CdO · · ·D2O and CdO · · ·DsN hydrogen bonds.This may arise from local phase transition due to the less freediffusion of water molecules through networks starting from ahard collapsed state of PNIPAM hydrogel during cooling.Largely different from PNIPAM hydrogel, PNIPAM aqueoussolution (20%) showed isosbestic points in both heating andcooling processes,16 and the existence of them strongly dependson the concentration, which will be discussed in another paperfrom our group.

3.2. Perturbation Correlation Moving Window. Figure 4shows PCMW synchronous and asynchronous spectra generatedfrom all the spectra during heating and cooling between 28 and

40 °C. PCMW synchronous spectra are very helpful in findingtransition points, which have been listed together in Table 1.The LCSTs obtained from PCMW are approximately equal tothose from Boltzmann fitting. However, PCMW is much easierto operate, and LCSTs in the whole spectral region can all bereflected in the contour maps.

In addition to determining transition points, PCMW can alsomonitor the spectral variations along temperature perturbationcombining the signs of synchronous and asynchronous spectraby the following rules: positive synchronous correlation repre-sents spectral intensity increasing, while negative correlationrepresents decreasing; positive asynchronous correlation can beobserved for a convex spectral intensity variation while negativecorrelation can be observed for a concave variation.48 On thebasis of this point, we can primarily ascertain that during heatingall the peaks in the regions 3030-2979 and 1642-1580 cm-1

exhibited an anti-S shaped intensity decrease while all the peaksin the regions 2979-2847 and 1675-1642 cm-1 exhibited anS shaped intensity increase. The case during cooling is just theopposite.

Interestingly, we can also determine the existence of isosbesticpoints from PCMW asynchronous spectra. For example, theVas(CH3) peaks in PCMW asynchronous spectra show a “four-leaf” pattern in both heating and cooling processes, whereasthe pattern in the heating process without an isosbestic pointhas the center partially connected, while that in the coolingprocess with an isosbestic point has four separate leaves. Theexistence of isosbestic points of V(CdO) can be determinedsimilarly.

It is worth noting that PCMW can also determine thetransition temperature regions by the peaks in asynchronousspectra which are all turning points of the S or anti-S shapedcurves. The results have also been listed in Table 1. Thus we

Figure 3. Determination of the isosbestic points of V(CH3) and V(CdO) overlaid spectra during heating and cooling. Three curves at 28, 34, and40 °C are highlighted for good observance.


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


can know that the volume phase transition of PNIPAM hydrogelmainly occurred at 33.0-36.5 °C during heating and 32-35.5 °C during cooling. This served as an important basis forthe segmental mode of the following 2Dcos analysis.

3.3. Two-Dimensional Correlation Analysis. On the basisof the phase transition evolving regions obtained from PCMW,we chose all the spectra between 32 and 37 °C to perform 2Dcosanalysis. Figure 5 shows the 2D synchronous and asynchronousspectra in the heating and cooling processes of PNIPAMhydrogel. 2D synchronous spectra reflect simultaneous changes

between two given wavenumbers. The bands at 2970, 2927,2871, and 1653 cm-1 all have positive cross-peaks, indicatingthat they had similar response of spectral intensities to temper-ature perturbation;that is, all increased during heating and alldecreased during cooling determined from raw spectra. On theother hand, the bands at 2987 and 1626 cm-1 have spectralintensities that both decrease during heating and increase duringcooling.

2D asynchronous spectra can significantly enhance thespectral resolution. In Figure 5, many subtle bands during

TABLE 1: LCSTs and Transition Temperature Regions Obtained from DSC, Boltzmann Fitting, and PCMW

heating/°C cooling/°C

method LCST transition region LCST transition region

DSC 34.1 31.9frequency shift Vas(CH3) 35.1 33.8

Vas(CH2) 35.3 33.9integration area CdO · · ·DsN 35.1 33.2

CdO · · ·DsOsD 35.1 33.7PCMW CH3 (dehydrated), CdO · · ·DsOsD 35.5 33.5-36.5 33.5 32-35.5

CH3 (hydrated), CH2, CdO · · ·DsN 35.0 33.0-36.0 34 32-35.5

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


heating such as the bands at 2974, 2893 cm-1 attributed toVas(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 ofdiscussion, all the bands found in asynchronous spectra and theircorresponding assignments during heating and cooling respec-tively have been presented in Table 2.

3.3.1. The Sequence of Group Motions of PNIPAM Hy-drogel in the Heating Process. Except for enhancing spectralresolution, 2D correlation spectroscopy can also discern thespecific order taking place under external perturbation. Thejudging rule can be summarized as Noda’s rule;that is, if thecross-peaks (V1, V2, and assume V1 > V2) in synchronous andasynchronous spectra have the same sign, the change at V1 mayoccur prior to that of V2, and vice versa. A simplified methodfor determination of sequence order has never been describedbefore.54 In short, multiplication was performed on the two signsof each cross-peak in synchronous and asynchronous spectra,the final results of which have been presented in Table 3. Toeach sign of cross-peaks in Table 3, according to Noda’s rule,if the sign is positive (+), the larger wavenumber or the bottomwavenumber will respond to external perturbation earlier thanthe smaller wavenumber or the left wavenumber. Similarly, ifthe sign is negative (-), the left wavenumber will respond earlierthan the bottom one. If the sign is zero (or blank), we cannotmake an exact judgment. Thus we can easily deduce the finalspecific order for the heating process of PNIPAM hydrogel (fmeans prior to or earlier than): 1610 cm-1 f 1645 cm-1 f1618 cm-1 f 2941 cm-1 f 2893 cm-1 f 1632 cm-1 f 2933cm-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-1f 2893 cm-1

f 2933 cm-1 f 2974 cm-1 f 2970 cm-1 f 2927 cm-1 f2871 cm-1 f 2987 cm-1 f 2850 cm-1;that is, Vas(hydratedCH2) f V(CH) f Vas(hydrating CH2) f Vas(dehydrating CH3)f Vas(dehydrated CH3) f Vas(dehydrated CH2) f Vs(CH3) fVas(hydrated CH3) f Vs(CH2). Without considering the differ-ences in stretching modes, the sequence can be described asCH2 f CH f CH3. This reveals that during heating thebackbone of PNIPAM hydrogel had an earlier response thanpendent isopropyl groups. On the other hand, if we consideronly stretching modes, an interesting sequence can be foundfor C-H stretching vibrations that the asymmetric stretchingvibration had an earlier response than the symmetric stretching

vibration, no matter for methyl or methylene groups. Aspreviously reported, the direction of asymmetric stretchingvibration is parallel to the polymer chain axis while that ofsymmetric stretching vibration is vertical to the polymer chainaxis.54 Therefore, we can conclude that PNIPAM hydrogel hadthe chain collapsed along the backbone first before watermolecules were expelled outside the network.

(2) Considering separately CdO related vibrations, thesequence can be extracted as follows: 1610 cm-1f 1645 cm-1

f 1618 cm-1f 1632 cm-1f 1655 cm-1;that is, CdO (cross-linker) f CdO (pendent group). This reveals for the secondtime that during heating the backbone of PNIPAM hydrogelhad an earlier response than pendent groups.

(3) Combining CsH and CdO related stretching vibrations,the whole sequence can be summarized as fpllows: CdO (cross-linker) f CdO (pendent group) f CH2 f CH f CH3. Thissequence suggests to us exciting information that the drivingforce for chain collapse of PNIPAM hydrogel during heatingwas the conversion of amide hydrogen bonds from water-associated ones to intermolecular ones.

3.3.2. The Sequence of Group Motions of PNIPAM Hy-drogel in the Cooling Process. Similarly, the sequence of groupmotions of PNIPAM hydrogel in the cooling process can alsobe deduced as follows: 2854 cm-1f 2908 cm-1f 2877 cm-1

f 2929 cm-1 f 1655 cm-1 f 1608 cm-1 f 2970 cm-1 f2871 cm-1 f 2989 cm-1 f 1632 cm-1 f 1624 cm-1 f 2947cm-1.

We adopted the same analysis method as that in the heatingprocess.

(1) For C-H related stretching vibrations, the sequence canbe extracted as follows: 2854 cm-1f 2908 cm-1f 2877 cm-1

f 2929 cm-1 f 2970 cm-1 f 2871 cm-1 f 2989 cm-1 f2947 cm-1;that is, Vs(CH2) f V(CH) f Vs(hydrated CH3)fVas(dehydrated CH2) f Vas(dehydrated CH3) f Vs(dehydratedCH3) f Vas(hydrated CH3) f Vas(hydrated CH2). Withoutconsidering the differences in stretching modes, the sequencecan be described as CH2 f CH f CH3, indicating that duringcooling the backbone of PNIPAM hydrogel still had an earlierresponse than pendent isopropyl groups. If we consider onlystretching modes, the case is opposite to the heating processthat the symmetric stretching vibration had an earlier responsethan the asymmetric stretching vibration. Thus we can concludethat PNIPAM hydrogel had water molecules diffusing into thenetwork first before the chain revival along the backboneoccurred.

(2) For CdO related vibrations, the sequence can be extractedas follows: 1655 cm-1 f 1608 cm-1 f 1632 cm-1 f 1624

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 assignment freq/cm-1 assignment freq/cm-1 assignment

2987 Vas(hydrated CH3) 2871 Vs(CH3) 2989 Vas(hydrated CH3) 2854 vs(CH2)2974 Vas(dehydrating CH3) 2850 Vs(CH2) 2970 Vas(dehydrated CH3) 1655 V(CdO · · ·DsN)

(pendent group)2970 Vas(dehydrated CH3) 1655 V(CdO · · ·DsN)

(pendent group)2947 Vas(hydrated CH2) 1632 V(CdO · · ·D2O)

(pendent group)2941 Vas(hydrated CH2) 1645 V(CdO · · ·DsN)

(cross-linker)2929 Vas(dehydrated CH2) 1624 V(CdO · · ·D2O)

(pendent group)2933 Vas(hydrating CH2) 1632 V(CdO · · ·D2O)

(pendent group)2908 V(CH) 1608 V(CdO · · ·D2O)

(cross-linker)2927 Vas(dehydrated CH2) 1618 V(CdO · · ·D2O)

(pendent group)2877 Vs(hydrated CH3)

2893 V(CH) 1610 V(CdO · · ·D2O)(cross-linker)

2871 Vs(dehydrated CH3)


cm-1;that is, CdO (pendent group) f CdO (cross-linker).This sequence can be interpreted that CdO in pendent groupsformed hydrogen bonding with D2O first due to relatively higherfreedom than CdO in cross-linker.

(3) Combining CsH and CdO related stretching vibrations,the whole sequence can be summarized as follows: CH2f CHf CH3 f CdO (side group) f CdO (cross-linker). Thissuggests that the driving force for chain revival of PNIPAMhydrogel during cooling was the diffusion of water into hardcollapsed networks or the physical swelling action.

On the basis of the above analysis during heating andcooling, we proposed the chain collapse and revival ther-modynamic mehanism of PNIPAM hydrogel, as outlined inFigure 6. At lower temperature below LCST, PNIPAMhydrogel was swollen by water molecules through thehydration of aliphatic groups and the hydrogen bondingassociation with amide groups. As tempeature increased upto above LCST, PNIPAM hydrogel occurred to collapse alongthe backbone before water molecules were expelled outsidethe network, and this process was driven by the conversionfrom water-associated amide hydrogen bonds to intermo-lecular ones. In the sequential cooling process, PNIPAMhydrogel had water molecules diffusing into the network firstbefore the chain revival along the backbone occurred, and

this process was driven by the physical diffusion or swellingaction. Our proposed mechanism for volume phase transitionof PNIPAM hydrogel discerned for the first time the specificorder taking place between the physical diffusion of watermolecules and hydrogen bonding association among amidegroups and additionally identified the driving force of eachprocess. These two steps during both heating and coolingmay be related to the bimodal peaks in DSC curves,55,56 whichneed to be further confirmed.

It should also be noted that we did not provide muchdiscussion about the sequence of different CdO splittingbands. This is because CdO is much more sensitive toconformational changes than other groups due to its beingalmost the largest dipole moment of all chemical bonds, andmost importantly, there are still divergences16,23,27,44 aboutthe hydrogen bonding types of amide groups (including CdOand NsH) in PNIPAM hydrogel. In this paper, we adoptedthe common perception of the hydrogen bonds formed byCdO (CdO · · ·H2O in swollen state and CdO · · ·HsN incollapsed state).

4. Conclusion

In this paper, we ultilized FT-IR spectrsocpy to in situtrace the thermally induced reversible volume phase transition

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


of PNIPAM hydrogel using D2O as the solvent. A newlydeveloped PCMW technique and 2Dcos were both employedto elucidate the chain collapse and revival mechanism ofPNIPAM hydrogel.

Boltzamann fitting on the frequency shifts of Vas(CH3) andVas(CH2) as well as the half integral area of V(CdO · · ·D2O) andv(CdO · · ·DsN) bands figured out the phase transition tempe-tures to be about 35.1 °C during heating and 33.7 °C duringcooling, approximate to the results obtained from DSC. PCMWgenerated similar results and further determined the phasetransition regions for heating and cooling process to be33.0-36.5 and 32-35.5 °C, respectively.

We additionally determined the existence of isosbesticpoints for Vas(CH3) and V(CdO) overlaid spectra and foundthat in the heating process the chain collapse of PNIPAMhydrogel occurred with some intermediate states or acompletely continuous phase transition while in the coolingprocess the chain revival occurred with only conversionbetween two single states.

2Dcos discerned all the sequence of group motions duringheating and cooling. According to 2Dcos results, we proposedthe chain collapse and revival thermodynamic mechanism. Thatis, at lower temperature below LCST, PNIPAM hydrogel wasswollen by water molecules through the hydration of aliphaticgroups and the hydrogen bonding association with amide groups.As temperature increased up to above LCST, PNIPAM hydrogeloccurred to collapse along the backbone before water moleculeswere expelled outside the network, and this process was drivenby the conversion from water-associated amide hydrogen bondsto intermolecular ones. In the sequential cooling process,PNIPAM hydrogel had water molecules diffusing into thenetwork first before the chain revival along the backboneoccurred, and this process was driven by the physical diffusion

or swelling action. Our proposed mechanism may be helpful tounderstand the volume phase transition nature of PNIPAMhydrogel.

Acknowledgment. We gratefully acknowledge the financialsupport of National Science Foundation of China (NSFC) (No.20934002, 20774022), the National Basic Research Programof China (2009CB930000).

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


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