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Thermodynamic and spectroscopic analysis of the conformational transition of poly(vinyl alcohol) by temperature-dependent FTIR
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 37–42
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Thermodynamic and spectroscopic analysis of the conformational transition of poly(vinyl alcohol) by temperature-dependent FTIR Shan Han a, Ye-Mei Luan b, Shu-Feng Pang a,⇑, Yun-Hong Zhang a,⇑ a b
Institute of Chemical Physics, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China College of Textile, Hebei University of Science and Technology, Shijiazhuang 050018, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
FTIR technique was used to study the
trans/gauche translation. The structural information was
gained by analyzing the differential spectra. As the increase of temperature, PVA film took the conformational transition from trans to gauche.
a r t i c l e
i n f o
Article history: Received 26 June 2014 Received in revised form 18 November 2014 Accepted 25 November 2014 Available online 18 December 2014 Keywords: Poly(vinyl alcohol) Conformational changes FTIR spectroscopy Temperature-dependent TGA
a b s t r a c t The conformational change of poly(vinyl alcohol) has been studied by Fourier transform infrared spectroscopy at various temperatures in the 4000–400 cm1 region. The molecular motion and the trans/ gauche content are sensitive to the CAH, CAC stretching modes. FTIR spectra show that the I2920/I2849 decreases from 1.84 to 1.0 with increasing temperature, companying the decrease in I1047/I1095 from 0.78 to 0.58, implying the conformational transition from trans to gauche in alkyl chain. Based on the van’t Hoff relation, the enthalpies and entropies have been calculated in different temperatures, which are 4.61 kJ mol1 and 15.23 J mol1 K1, respectively, in the region of 80–140 °C. From the C@O stretching mode and OAH band, it can be concluded that the intermolecular hydrogen bonds decrease owing to elevating temperature, which leads to more gauche conformers. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Conformations of polymers have attracted many scientists because many macroscopic properties and industrial applications were decided by chain conformations. In general, two main conformations of trans and gauche will be translated each other dependent upon variable kinds of molecular interactions and external environment. When the interactions between the chains and surrounding compounds are stronger, the population of trans conformer is more. ⇑ Corresponding authors. Tel.: +86 10 68913596; fax: +86 10 68912652. E-mail addresses: [email protected] (S.-F. Pang), [email protected] (Y.-H. Zhang). http://dx.doi.org/10.1016/j.saa.2014.11.100 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Wu et al. [1] have utilized temperature-dependent Raman spectroscopy to observe the conformational changes in novel thermotropic liquid crystalline polymer. The results show that intermolecular interactions decrease as the temperature increases, which leads to the increased rotational and vibrational freedom, so that more gauche conformations exist in the alkyl chains. The temperaturedependent ATR-FTIR spectra of poly(N-isopropylacrylamide) (PNiPA) aqueous solution have also revealed that the populations of the gauche conformation in the polymer chain decrease when PNiPA changes into the globule state [2]. Poly (vinyl alcohol) (PVA) film has been studied systematically in the past few years because of its many desirable characteristics
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S. Han et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 37–42
specifically for various industrial [3,4] and pharmaceutical applications [5–10]. Many PVA applications including polymer plastic alloy, textile industry are always related to the heating process, so the advanced research and analysis of micro-structure of polymer as a function of temperature are very important to such applications. Horii et al. [11–13] have studied the chain conformation and hydrogen bonds in frozen PVA solutions and found it depends upon the solvents and annealing process through the analysis of CAH lines by cross polarization/magic angle spinning 13C nuclear magnetic resonance (CP/MAS 13C NMR) spectroscopy. Eagland et al. [14] have shown that there are trans/gauche conformational changes in PVA with the temperature in the temperature range of 2–30 °C. Though stabilization of structure is closely associated with temperature, little information on the conformational transition of PVA from room temperature to high temperature is found. Infrared spectroscopy is well known to be a powerful tool to directly obtain molecular information about the conformational change including the molecular motion (chain coupling, twisting and bending), the relative content of trans and gauche conformers, and the specific interactions [15,16]. Thus in this paper, we analyze the conformational transition of PVA film from 20 to 180 °C by FTIR technique. Based on the spectral signals, we also calculate the enthalpy and entropy values in different temperature ranges by van’t Hoff equation.
which is obtained by subtracting the IR spectrum of PVA film at 20 °C. Thermo-gravimetric analysis Thermo-gravimetric analysis (TGA) was performed on a TGAQ 5000 thermo-gravimetric analyzer from 25 to 180 °C at a heating rate of 8 °C/min under a nitrogen atmosphere (flow rate: 20 cm3 min1). The sample weight was about 6 mg. Results and discussion FTIR-ATR spectral features Some representative FTIR spectra of PVA film are shown in Fig. 2 and the assignments are listed in Table 1. It can be found that four main bands at 3700–3000, 3000–2750, 1790–1660 and 1150– 950 cm1 are attributed to OAH stretching band, the CAH stretching mode, C@O stretching mode, and CAC stretching mode, respectively. Usually the relative intensities of CAH and CAC stretching modes are sensitive to conformational changes of chain in PVA film, While the peak positions of OAH and C@O group stretching mode are variable owing to different hydrogen bonds. They are correlated to vibrational freedom of chain, leading to conformational transition. So the four peaks will be analyzed in more detail in following sections.
Experimental section The CAH stretching region Materials The PVA film, was purchased from Beijing Chemical Reagents Company and used without any further purification. The degree of polymerization (DP) is 1700–1800, the alcoholysis degree of PVA is 88%, polymer weight is over 5,000,000 and the thickness of film is about 100 lm. Because PVA is prepared by partial hydrolysis of poly(vinyl acetate) (PVAc), therefore there are a certain amount of ester groups in PVA chain. It means, PVA always is a copolymer of PVA and PVAc with different ratios. The chemical structure of PVA is shown in Fig. 1.
FTIR spectroscopy The FTIR spectra were recorded on a Nicolet MAGNA-IR 560 Fourier transform infrared (FT-IR) spectrometer equipped with a DTGS detector. A total accumulation of 24 scans and a resolution of 4 cm1 were adequate to obtain a high signal-to-noise spectrum in range of 4000–400 cm1. For the measurement of infrared spectra at elevated temperature, a thin PVA film was placed into a heating cell composed of two KBr windows. The temperature was controlled to rise at a distance of 10 °C with a thermocouple, and a digital temperature sensor was used to probe the temperature of the PVA film. When the temperature arrives to the preset value, 10 min was allowed to reach the thermal equilibrium. The white PVA film gradually becomes yellow with the increase of temperature. This experiment was repeated for more than three times to ensure reproducibility. The differential spectra were used to discuss the effect of temperature on conformational change of PVA,
Fig. 1. The chemical structure of PVA.
The changes of CAH stretching region of the alkyl chain have been extensively used to determine the conformation of hydrocarbon chains in various phases [17–19]. A small change in the CAH stretching mode can reflect the difference in alkyl chain. In general, a higher-frequency shift, reduction in intensity or band-width increase of the CH2 stretching bands indicates the more gauche conformers in the hydrocarbon chain [20]. However, this phenomenon is not very clear in our spectra. In order to gain the explicit information on chain conformers, we analyzed the differential spectra shown in Fig. 3a. It clearly illustrates four impressive negative peaks at 2954, 2920, 2881 and 2849 cm1. The peaks at 2881 and 2954 cm1 were assigned to the symmetric CH3 (ms-CH3) and asymmetric CH3 stretching mode (mas-CH3), which are caused by the poly(vinyl acetate) part, and the peaks at 2849 and 2920 cm1 correspond to the symmetric (ms-CH2) and asymmetric CH2 stretching mode (mas-CH2), respectively [20–23]. Rousseau described the utility of the relative
Fig. 2. FTIR spectra of PVA films in the 4000–400 cm1 region at 20, 90, 150 and 180 °C.
S. Han et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 37–42 Table 1 Assignments of FTIR spectra for PVA. Wavenumber (cm1)
Assignments
3521 3222 2954 2920 2881 2849 1735 1640 1434 1328 1245 1095 1047 1024 945 849
m(OAH) week hydrogen bonda m(OAH) strong hydrogen bonda mas(CH3) a mas(CH2) a ms(CH3) a ms(CH2) a m(C@O) ester groupa
a
m, the stretching mode.
b
d, the bending mode. p, the rocking mode. x, the wagging mode.
c d
d(OAH)b d(CH)3b d(CH)2b m(CAOAC) ester groupa m(CACAG)a m(CACAT)a m(CACACAO) main chaina p(CH)c x(CH)d
39
chain conformations change, the mas(CH2) and ms(CH2) bands will shift due to the coupling effect. In present PVA chain, the coupling of CAH stretching mode would present similar behavior as methylene chain. So in this paper the intensity ratio of asymmetric and symmetric CH2 stretching modes (I2920/I2849) in the temperature variable IR spectra is applied here to investigate the conformational change in PVA film. Fig. 3b gives the intensity ratio of the band 2920 and 2849 cm1 (I2920/I2849) as a function of temperature. In this paper, it can be easily found that the I2920/I2849 decreases with increasing temperature and there are two obvious inflection points at 80 and 140 °C, so the curve can be divided into three regions via 80 and 140 °C as boundaries. In region I (30–80 °C), the I2920/I2849 decreases quickly from 1.84 at 30 °C to 1.55 at 80 °C. And then, when the temperature enters into region II (80–140 °C), the I2920/I2849 changes more steeply from 1.55 at 80 °C to 1.0 at 140 °C. As the temperature rises to region III (140–180 °C), the I2920/I2849 is insensitive to the temperature. According to the earlier analysis [23–26], the ratio of I2920/I2849 can characterize the degree of order of aliphatic chain. The smaller value implies the more gauche conformation and more disordered of aliphatic chain. So the present decrease in I2920/I2849 implies the increased gauche conformation in PVA chain with temperature. In the region II (80–140 °C), the trans conformations transform into gauche conformations most rapidly, which leads to phase transition of aliphatic chain. Above 140 °C, the chain conformations almost keep constant. Analysis of CAC stretching mode and thermodynamic estimation The chain conformation of PVA film not only can be characterized by the ratio of I2920/I2849, not only by the intensity ratio of asymmetric and symmetric CAC stretching modes Isym[m(CAC)]/ Iasy[m(CAC)]. Furthermore, CAC stretching modes can provide a measurement of PVA aliphatic chain intermolecular mobility [27,28]. The bands around 1047 and 1095 cm1 in Fig. 4a arise from symmetric and antisymmetric CAC stretching modes, respectively. The relative numbers of trans and gauche conformers in skeletal chains are indicated by the intensity ratio of m(C–C–T) (1047 cm1) to m(C–C–G) (1095 cm1), which is shown in Fig. 4b. Compared to Fig. 3b, I1047/I1095 has a similar trend and can also be divided into three regions. In region I, the ratio decreased gradually from 0.78 to 0.74, and then quickly dropped to 0.60 at 140 °C. Above 140 °C, it reduced slowly to 0.58 at 180 °C. It means that the more gauche conformers were formed in the alkyl chain, which is consistent with the above results from CH2 stretching mode. The slow changes of trans/gauche ratio in the region III implying that there are almost no more gauche conformers and the backbone alkyl chain properties keep stable. The thermodynamic behavior of conformational transition between trans and gauche could be determined by applying the van’t Hoff equation [1,29,30].
Fig. 3. (a) Differential spectra of the PVA film in the 3000–2750 cm1 region in the temperature of 30–180 °C by subtracting the spectrum at 20 °C. (b) I2920/I2849 as a function of the temperature.
ln K t=g ¼
K t=g ¼ intensity of CAH stretching mode to probe the conformation, environment and dynamics of hydrocarbon chains in triglycerides [29]. The asymmetric (mas(CH2)) and symmetric (ms(CH2)) bands are originally from the result of out phase and in phase coupling of the two CAH stretching modes in the CH2 group. For the methylene chain, there are a lot of CH2 units, which will be further coupling for the mas(CH2) and ms(CH2) modes. The coupling effect intrinsically determines the position of the mas(CH2) and ms(CH2) bands. When the
DH 1 DS þ R T R
½trans I1047 ¼ ½gauche I1095
ð1Þ
ð2Þ
where equilibrium constant Kt/g represents the relative trans/ gauche content. In this relationship, it is assumed that the enthalpy and entropy changes are independent of temperature. The natural logarithm of K as a function of the reciprocal of absolute temperature is shown in Fig. 5. This curve also can be divided into three parts and can be analyzed by liner fitting to obtain the enthalpy and entropy change of each part. The corresponding DH and DS are DH1 = 4.91 kJ mol1, DS1 = 0.87 J mol1 K1 in region I,
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understand conformational transition. The FTIR spectra in the region of 1780–1680 cm1 contain some information about the geometry of the carbonyl groups in the ester [32,33]. Fig. 6a shows differential spectra of the carbonyl groups. There was a positive band around 1745 cm1 and a negative band around 1705 cm1. The band at 1745 cm1 can be attributed to free C@O groups without hydrogen bonded to hydroxyl groups, while the band at 1705 cm1 is assigned to the hydrogen bonded C@O groups. The stronger bands at 1745 cm1 and weaker bands at 1705 cm1 in Fig. 6a show that the hydrogen bonds between chains became weaker. It is well known that the conformational change arises from intermolecular interactions. When the intermolecular interaction between chains became stronger, the chains were restrained and trans conformers became more. Herein, the intermolecular interactions became weaker with elevating temperature, which lead to more gauche conformers. In fact, Ellen et al. [31] have directly assigned the band at 1745 cm1 to gauche conformer while the band at 1705 cm1 represents the trans conformer. Thus the two bands of carbonyl groups at 1745 and 1705 cm1 can also be used to investigate the conformational transition process [34,35]. Fig. 6b shows the intensities of the band at 1745 cm1 and the band at 1705 cm1 are dependent upon temperature. With the temperature increasing from 30 to 140 °C, the intensity of the free C@O increases sharply from 0.034 to 0.173, while the intensity of hydrogen bonded C@O decreases from 0.03 to 0.121. After 150 °C, the intensity of both of the two bands show opposite change trends. The changes indicate that carbonyl groups are sensitive to a conformational transition and the hydrogen bonds decrease with the temperature, which results in more gauche conformers in the poly chains. Fig. 4. (a) Spectra of the PVA film in the 1186–962 cm1 region in the temperature of 20–180 °C. (b) I1047/I1095 as a function of the temperature.
Fig. 5. A van’t Hoff plot for PVA film and the corresponding thermodynamic parameters in the conformational transition.
DH2 = 14.17 kJ mol1, DS2 = 4.21 J mol1 K1 in region II and DH3 = 8.77 kJ mol1, DS3 = 1.88 J mol1 K1 in region III, respectively. The different thermodynamically functional values indicate that there is an intermediate state between the regions I and III. Analysis of ester carbonyl groups As mentioned above, there are a certain amount of ester groups in PVA chains because of the partial hydrolysis in the process of PVA preparation. Usually the frequency of carbonyl groups is not sensitive to rotation about the CAO bond but CAC bond [31]. Thus, analysis of the frequency of carbonyl group is necessary for us to
Fig. 6. (a) Differential spectra of the PVA film in the 1780–1670 cm1 region in the temperature of 30–180 °C by subtracting the spectrum at 20 °C. (b) The intensities of the band at 1745 and 1705 cm1 as a function of the temperature.
S. Han et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 37–42
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Hydroxyl group in the PVA film Every repeating unit of PVA contains one hydroxyl group that can be easily formed intramolecular or intermolecular hydrogen bond with itself, carbonyl group or water molecule [36]. The frequency of the OAH stretching mode is a vital indicator of the presence and the strength of hydrogen bond. Thus the OAH stretching mode can be used to analyze the intermolecular interactions. Fig. 7a shows differential spectra of the PVA film in the 3700– 2980 cm1 region obtained in the temperature region of 30– 180 °C. There are two bands at 3521 and 3222 cm1, which are attributed to the week hydrogen bond (interaction between carbonyl group and water molecular etc.) and strong hydrogen bond (interaction between water and water molecular, hydroxyl group of PVA and water molecular etc.), respectively. It can be seen that the intensities of the two bands change with the temperature, which means that the strength of the strong hydrogen bond depends on the temperature. The intensity of the weak and strong hydrogen bond as a function of temperature is shown in Fig. 7b. It is obvious that the peak intensity of the strong hydrogen bond decreased continuously with increase of temperature from 30 to 180 °C, while that of weak hydrogen bond decreased sharply until 70 °C, and then gradually increased as the temperature increased from 70 to 180 °C. In general, the changeable trend of strong and weak hydrogen bonds should be contrary. Thus, the same change trends of the strong hydrogen bond and weak hydrogen bond below 70 °C should be related to the existence of free water. Above 70 °C, the hydrogen bonds became weak as the temperature increased, which lead to more chain mobility.
Fig. 8. TGA curve of PVA film.
In order to make clear the reason why the strong and weak hydrogen bonds change at the same trends below 70 °C, TGA analysis has been employed (shown in Fig. 8). There are two distinct stages in the TG profile. The weight loss is going on till 75 °C, which is consistent with the trend of the weak hydrogen bonds below 70 °C. It shows that the above-mentioned decrease in intensity of the peaks at 1705 cm1 is caused by the evaporation of the moisture in PVA film. At the temperature above 75 °C, the sample mass keeps almost constant. In this stage, rather than being associated with dehydration, the variation about OAH stretching mode was linked to molecular structure changes. The free water almost completely evaporated, the rest existed as bound water. The strength of strong hydrogen bond weakened as the temperature increased from 70 to 180 °C, which suggested the decrease of number of strong hydrogen bond. Meanwhile, a certain number of weak hydrogen bonds were formed owing to the destruction of strong hydrogen bonds. From the TGA curve, it can be learned that PVA does not decomposed during 70–180 °C. These data suggest that significant changes in conformational order occur as 70–180 °C, and also some transition in intermolecular and intramolecular structures of the PVA film happened. Comparison found that increase of the temperature leads to less strong hydrogen bonds and more disordered alkyl chain. For PVA film, increase of temperature results in weakening the hydrogen bonds and disordering alkyl chain. The order degree of chain is dependent on the interchain interaction. From the structure of PVA macromolecules, the interaction between the skeleton chains bearing hydroxyl groups is mainly decided by the interchain hydrogen bonds. The weaker hydrogen bonds between chains suggest the weaker interchain interaction, which leads to more disordered chain and more gauche conformation.
Conclusion
Fig. 7. (a) Differential spectra of the PVA film in the 3700–2980 cm1 region in the temperature of 30–180 °C by subtracting the spectrum at 20 °C. (b) The intensities of the band at 3521 and 3222 cm1 as a function of the temperature.
In the present study, we used temperature-dependent FTIR technique to demonstrate the conformational transition between trans and gauche in PVA film. The I2920/I2848 and I1047/I109 both show that the intermolecular interactions became weaker and more gauche conformer are formed in the alkyl chains as the temperature increased. From the intensity ratio of the peaks at 1047 and 1095 cm1 (I1047/I1095), the values of enthalpy and entropy change for the transition from trans to gauche conformer have been obtained from the van’t Hoff relation. Analysis of the C@O stretching modes and OAH bands suggests that the intermolecular hydrogen bonds became weaker as the temperature is rising, which should be responsible for trans to gauche transition. In the whole studied temperature range, significant changes in conformational order occur during 70–140 °C.
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Acknowledgements The authors would like to thanks Damin Li for associate work. And we gratefully appreciate financial support from the National Natural Science Foundation of China (41175119, 20933001 and 21373026), and the 111 Project (B07012). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
H. Tang, S. Sun, J. Wu, P. Wu, X. Wan, Polymer 51 (2010) 5482. T.T.Y. Katsumoto, H. Sato, Y. Ozaki, J. Phys. Chem. A 106 (2002) 3429. K. Aoi, H. Aoi, M. Okada, Macromol. Chem. Phys. 203 (2002) 1018. Q.Y. Soundararajah, B.S.B. Karunaratne, R.M.G. Rajapakse, J. Compos. Mater. 44 (2010) 303. A. Kaboorani, B. Riedl, Compos. Part A Appl. S. 42 (2011) 1031. X.H. Qin, S.Y. Wang, J. Appl. Polym. Sci. 109 (2008) 951. A.B.R. Moreira, V.H. Perez, G.M. Zanin, H.F. De Castro, Energy Fuels 21 (2007) 3689. N. De Prisco, B. Immirzi, M. Malinconico, P. Mormile, L. Petti, G. Gatta, J. Appl. Polym. Sci. 86 (2002) 622. M. Vieira, C.R. Tavares, R. Bergamasco, J.C.C. Petrus, J. Membrane, Science 194 (2001) 273. T. Yasukawa, H. Kimura, Y. Tabata, Y. Ogura, Adv. Drug Deliver. Rev. 52 (2001) 25. K. Masuda, F. Horii, Macromolecules 31 (1998) 5810. H. Yang, S.H. Hu, F. Horii, R. Endo, T. Hayash, T. Hayashi, Polymer 47 (2006) 1995. K. Masuda, H. Kaji, F. Horii, J. Polym. Sci. Poly. Phys. 38 (2000) 1.
[14] D. Eagland, D.J. Maitland, N.J. Crowther, W.L. Titterton, Polymer 35 (1994) 3389. [15] E. Meaurio, E. Zuza, N. Lopez-Rodriguez, J.R. Sarasua, J. Phys. Chem. B 110 (2006) 5790. [16] Y.C. Cheng, X. Wang, J.B. Cheng, L. Sun, W.Q. Xu, B. Zhao, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 61 (2005) 905. [17] R.G. Snyder, H.L. Strauss, C.A. Elliger, J. Phys. Chem. 86 (1982) 5145. [18] L.C. Hector, H.M. Henry, G.C. David, J. Chem. Phys. 77 (1982) 2825. [19] R.A. Macphail, H.L. Strauss, R.G. Snyder, C.A. Elliger, J. Phys. Chem. 88 (1984) 334. [20] L.C. Hector, H.M. Henry, Biochim. Biophys. Acta 779 (1984) 381. [21] D.J. Moore, M.E. Rerek, R. Mendelsohn, J. Phys. Chem. B 101 (1997) 8933. [22] E.A. Disalvo, A.M. Bouchet, M.A. Frias, Biochim. Biophys. Acta 1828 (2013) 1683. [23] N.V. Venkataraman, S. Vasudevan, J. Phys. Chem. B 105 (2001) 1805. [24] G.B. Kenneth, J. Phys. Chem. 91 (1987) 3436. [25] S. Bresson, M. El Marssi, B. Khelifa, Vib. Spectrosc. 40 (2006) 263. [26] S. Bresson, A. El Marssi, B. Khelifa, Chem. Phys. Lipids 134 (2005) 119. [27] Y. Maeda, T. Nakamura, I. Ikeda, Macromolecules 34 (2001) 1391. [28] R.W. Wang, J. Guo, G. Baran, S.L. Wunder, Langmuir 16 (2000) 568. [29] D.E. Silva, D. Rousseau, Phys. Chem. Chem. Phys. 10 (2008) 4606. [30] M.J. Zheng, W.M. Du, Vib. Spectrosc. 40 (2006) 219. [31] B.B. Ellen, G.B. Kenneth, B.P. Willis, J. Am. Chem. Soc. 102 (1980) 5486. [32] A. Lejardi, A. Etxeberria, E. Meaurio, J.R. Sarasua, Polymer 53 (2012) 50. [33] L. Senak, Z.M. Ju, N. Noy, R. Callender, D. Manor, Biospectroscopy 3 (1997) 131. [34] S. Bresson, M. El Marssi, B. Khelifa, Vib. Spectrosc. 34 (2004) 231. [35] S. Bresson, D. Rousseau, S. Ghosh, M. El Marssi, V. Faivre, Eur. J. Lipid Sci. Technol. 113 (2011) 992. [36] M. Watase, K. Nishinali, Polym. J. 21 (1989) 597.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Thermodynamic and spectroscopic analysis of the conformational transition of poly(vinyl alcohol) by temperature-dependent FTIR Shan Han a, Ye-Mei Luan b, Shu-Feng Pang a,⇑, Yun-Hong Zhang a,⇑ a b
Institute of Chemical Physics, School of Chemistry, Beijing Institute of Technology, Beijing 100081, China College of Textile, Hebei University of Science and Technology, Shijiazhuang 050018, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
FTIR technique was used to study the
trans/gauche translation. The structural information was
gained by analyzing the differential spectra. As the increase of temperature, PVA film took the conformational transition from trans to gauche.
a r t i c l e
i n f o
Article history: Received 26 June 2014 Received in revised form 18 November 2014 Accepted 25 November 2014 Available online 18 December 2014 Keywords: Poly(vinyl alcohol) Conformational changes FTIR spectroscopy Temperature-dependent TGA
a b s t r a c t The conformational change of poly(vinyl alcohol) has been studied by Fourier transform infrared spectroscopy at various temperatures in the 4000–400 cm1 region. The molecular motion and the trans/ gauche content are sensitive to the CAH, CAC stretching modes. FTIR spectra show that the I2920/I2849 decreases from 1.84 to 1.0 with increasing temperature, companying the decrease in I1047/I1095 from 0.78 to 0.58, implying the conformational transition from trans to gauche in alkyl chain. Based on the van’t Hoff relation, the enthalpies and entropies have been calculated in different temperatures, which are 4.61 kJ mol1 and 15.23 J mol1 K1, respectively, in the region of 80–140 °C. From the C@O stretching mode and OAH band, it can be concluded that the intermolecular hydrogen bonds decrease owing to elevating temperature, which leads to more gauche conformers. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Conformations of polymers have attracted many scientists because many macroscopic properties and industrial applications were decided by chain conformations. In general, two main conformations of trans and gauche will be translated each other dependent upon variable kinds of molecular interactions and external environment. When the interactions between the chains and surrounding compounds are stronger, the population of trans conformer is more. ⇑ Corresponding authors. Tel.: +86 10 68913596; fax: +86 10 68912652. E-mail addresses: [email protected] (S.-F. Pang), [email protected] (Y.-H. Zhang). http://dx.doi.org/10.1016/j.saa.2014.11.100 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Wu et al. [1] have utilized temperature-dependent Raman spectroscopy to observe the conformational changes in novel thermotropic liquid crystalline polymer. The results show that intermolecular interactions decrease as the temperature increases, which leads to the increased rotational and vibrational freedom, so that more gauche conformations exist in the alkyl chains. The temperaturedependent ATR-FTIR spectra of poly(N-isopropylacrylamide) (PNiPA) aqueous solution have also revealed that the populations of the gauche conformation in the polymer chain decrease when PNiPA changes into the globule state [2]. Poly (vinyl alcohol) (PVA) film has been studied systematically in the past few years because of its many desirable characteristics
38
S. Han et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 37–42
specifically for various industrial [3,4] and pharmaceutical applications [5–10]. Many PVA applications including polymer plastic alloy, textile industry are always related to the heating process, so the advanced research and analysis of micro-structure of polymer as a function of temperature are very important to such applications. Horii et al. [11–13] have studied the chain conformation and hydrogen bonds in frozen PVA solutions and found it depends upon the solvents and annealing process through the analysis of CAH lines by cross polarization/magic angle spinning 13C nuclear magnetic resonance (CP/MAS 13C NMR) spectroscopy. Eagland et al. [14] have shown that there are trans/gauche conformational changes in PVA with the temperature in the temperature range of 2–30 °C. Though stabilization of structure is closely associated with temperature, little information on the conformational transition of PVA from room temperature to high temperature is found. Infrared spectroscopy is well known to be a powerful tool to directly obtain molecular information about the conformational change including the molecular motion (chain coupling, twisting and bending), the relative content of trans and gauche conformers, and the specific interactions [15,16]. Thus in this paper, we analyze the conformational transition of PVA film from 20 to 180 °C by FTIR technique. Based on the spectral signals, we also calculate the enthalpy and entropy values in different temperature ranges by van’t Hoff equation.
which is obtained by subtracting the IR spectrum of PVA film at 20 °C. Thermo-gravimetric analysis Thermo-gravimetric analysis (TGA) was performed on a TGAQ 5000 thermo-gravimetric analyzer from 25 to 180 °C at a heating rate of 8 °C/min under a nitrogen atmosphere (flow rate: 20 cm3 min1). The sample weight was about 6 mg. Results and discussion FTIR-ATR spectral features Some representative FTIR spectra of PVA film are shown in Fig. 2 and the assignments are listed in Table 1. It can be found that four main bands at 3700–3000, 3000–2750, 1790–1660 and 1150– 950 cm1 are attributed to OAH stretching band, the CAH stretching mode, C@O stretching mode, and CAC stretching mode, respectively. Usually the relative intensities of CAH and CAC stretching modes are sensitive to conformational changes of chain in PVA film, While the peak positions of OAH and C@O group stretching mode are variable owing to different hydrogen bonds. They are correlated to vibrational freedom of chain, leading to conformational transition. So the four peaks will be analyzed in more detail in following sections.
Experimental section The CAH stretching region Materials The PVA film, was purchased from Beijing Chemical Reagents Company and used without any further purification. The degree of polymerization (DP) is 1700–1800, the alcoholysis degree of PVA is 88%, polymer weight is over 5,000,000 and the thickness of film is about 100 lm. Because PVA is prepared by partial hydrolysis of poly(vinyl acetate) (PVAc), therefore there are a certain amount of ester groups in PVA chain. It means, PVA always is a copolymer of PVA and PVAc with different ratios. The chemical structure of PVA is shown in Fig. 1.
FTIR spectroscopy The FTIR spectra were recorded on a Nicolet MAGNA-IR 560 Fourier transform infrared (FT-IR) spectrometer equipped with a DTGS detector. A total accumulation of 24 scans and a resolution of 4 cm1 were adequate to obtain a high signal-to-noise spectrum in range of 4000–400 cm1. For the measurement of infrared spectra at elevated temperature, a thin PVA film was placed into a heating cell composed of two KBr windows. The temperature was controlled to rise at a distance of 10 °C with a thermocouple, and a digital temperature sensor was used to probe the temperature of the PVA film. When the temperature arrives to the preset value, 10 min was allowed to reach the thermal equilibrium. The white PVA film gradually becomes yellow with the increase of temperature. This experiment was repeated for more than three times to ensure reproducibility. The differential spectra were used to discuss the effect of temperature on conformational change of PVA,
Fig. 1. The chemical structure of PVA.
The changes of CAH stretching region of the alkyl chain have been extensively used to determine the conformation of hydrocarbon chains in various phases [17–19]. A small change in the CAH stretching mode can reflect the difference in alkyl chain. In general, a higher-frequency shift, reduction in intensity or band-width increase of the CH2 stretching bands indicates the more gauche conformers in the hydrocarbon chain [20]. However, this phenomenon is not very clear in our spectra. In order to gain the explicit information on chain conformers, we analyzed the differential spectra shown in Fig. 3a. It clearly illustrates four impressive negative peaks at 2954, 2920, 2881 and 2849 cm1. The peaks at 2881 and 2954 cm1 were assigned to the symmetric CH3 (ms-CH3) and asymmetric CH3 stretching mode (mas-CH3), which are caused by the poly(vinyl acetate) part, and the peaks at 2849 and 2920 cm1 correspond to the symmetric (ms-CH2) and asymmetric CH2 stretching mode (mas-CH2), respectively [20–23]. Rousseau described the utility of the relative
Fig. 2. FTIR spectra of PVA films in the 4000–400 cm1 region at 20, 90, 150 and 180 °C.
S. Han et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 37–42 Table 1 Assignments of FTIR spectra for PVA. Wavenumber (cm1)
Assignments
3521 3222 2954 2920 2881 2849 1735 1640 1434 1328 1245 1095 1047 1024 945 849
m(OAH) week hydrogen bonda m(OAH) strong hydrogen bonda mas(CH3) a mas(CH2) a ms(CH3) a ms(CH2) a m(C@O) ester groupa
a
m, the stretching mode.
b
d, the bending mode. p, the rocking mode. x, the wagging mode.
c d
d(OAH)b d(CH)3b d(CH)2b m(CAOAC) ester groupa m(CACAG)a m(CACAT)a m(CACACAO) main chaina p(CH)c x(CH)d
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chain conformations change, the mas(CH2) and ms(CH2) bands will shift due to the coupling effect. In present PVA chain, the coupling of CAH stretching mode would present similar behavior as methylene chain. So in this paper the intensity ratio of asymmetric and symmetric CH2 stretching modes (I2920/I2849) in the temperature variable IR spectra is applied here to investigate the conformational change in PVA film. Fig. 3b gives the intensity ratio of the band 2920 and 2849 cm1 (I2920/I2849) as a function of temperature. In this paper, it can be easily found that the I2920/I2849 decreases with increasing temperature and there are two obvious inflection points at 80 and 140 °C, so the curve can be divided into three regions via 80 and 140 °C as boundaries. In region I (30–80 °C), the I2920/I2849 decreases quickly from 1.84 at 30 °C to 1.55 at 80 °C. And then, when the temperature enters into region II (80–140 °C), the I2920/I2849 changes more steeply from 1.55 at 80 °C to 1.0 at 140 °C. As the temperature rises to region III (140–180 °C), the I2920/I2849 is insensitive to the temperature. According to the earlier analysis [23–26], the ratio of I2920/I2849 can characterize the degree of order of aliphatic chain. The smaller value implies the more gauche conformation and more disordered of aliphatic chain. So the present decrease in I2920/I2849 implies the increased gauche conformation in PVA chain with temperature. In the region II (80–140 °C), the trans conformations transform into gauche conformations most rapidly, which leads to phase transition of aliphatic chain. Above 140 °C, the chain conformations almost keep constant. Analysis of CAC stretching mode and thermodynamic estimation The chain conformation of PVA film not only can be characterized by the ratio of I2920/I2849, not only by the intensity ratio of asymmetric and symmetric CAC stretching modes Isym[m(CAC)]/ Iasy[m(CAC)]. Furthermore, CAC stretching modes can provide a measurement of PVA aliphatic chain intermolecular mobility [27,28]. The bands around 1047 and 1095 cm1 in Fig. 4a arise from symmetric and antisymmetric CAC stretching modes, respectively. The relative numbers of trans and gauche conformers in skeletal chains are indicated by the intensity ratio of m(C–C–T) (1047 cm1) to m(C–C–G) (1095 cm1), which is shown in Fig. 4b. Compared to Fig. 3b, I1047/I1095 has a similar trend and can also be divided into three regions. In region I, the ratio decreased gradually from 0.78 to 0.74, and then quickly dropped to 0.60 at 140 °C. Above 140 °C, it reduced slowly to 0.58 at 180 °C. It means that the more gauche conformers were formed in the alkyl chain, which is consistent with the above results from CH2 stretching mode. The slow changes of trans/gauche ratio in the region III implying that there are almost no more gauche conformers and the backbone alkyl chain properties keep stable. The thermodynamic behavior of conformational transition between trans and gauche could be determined by applying the van’t Hoff equation [1,29,30].
Fig. 3. (a) Differential spectra of the PVA film in the 3000–2750 cm1 region in the temperature of 30–180 °C by subtracting the spectrum at 20 °C. (b) I2920/I2849 as a function of the temperature.
ln K t=g ¼
K t=g ¼ intensity of CAH stretching mode to probe the conformation, environment and dynamics of hydrocarbon chains in triglycerides [29]. The asymmetric (mas(CH2)) and symmetric (ms(CH2)) bands are originally from the result of out phase and in phase coupling of the two CAH stretching modes in the CH2 group. For the methylene chain, there are a lot of CH2 units, which will be further coupling for the mas(CH2) and ms(CH2) modes. The coupling effect intrinsically determines the position of the mas(CH2) and ms(CH2) bands. When the
DH 1 DS þ R T R
½trans I1047 ¼ ½gauche I1095
ð1Þ
ð2Þ
where equilibrium constant Kt/g represents the relative trans/ gauche content. In this relationship, it is assumed that the enthalpy and entropy changes are independent of temperature. The natural logarithm of K as a function of the reciprocal of absolute temperature is shown in Fig. 5. This curve also can be divided into three parts and can be analyzed by liner fitting to obtain the enthalpy and entropy change of each part. The corresponding DH and DS are DH1 = 4.91 kJ mol1, DS1 = 0.87 J mol1 K1 in region I,
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understand conformational transition. The FTIR spectra in the region of 1780–1680 cm1 contain some information about the geometry of the carbonyl groups in the ester [32,33]. Fig. 6a shows differential spectra of the carbonyl groups. There was a positive band around 1745 cm1 and a negative band around 1705 cm1. The band at 1745 cm1 can be attributed to free C@O groups without hydrogen bonded to hydroxyl groups, while the band at 1705 cm1 is assigned to the hydrogen bonded C@O groups. The stronger bands at 1745 cm1 and weaker bands at 1705 cm1 in Fig. 6a show that the hydrogen bonds between chains became weaker. It is well known that the conformational change arises from intermolecular interactions. When the intermolecular interaction between chains became stronger, the chains were restrained and trans conformers became more. Herein, the intermolecular interactions became weaker with elevating temperature, which lead to more gauche conformers. In fact, Ellen et al. [31] have directly assigned the band at 1745 cm1 to gauche conformer while the band at 1705 cm1 represents the trans conformer. Thus the two bands of carbonyl groups at 1745 and 1705 cm1 can also be used to investigate the conformational transition process [34,35]. Fig. 6b shows the intensities of the band at 1745 cm1 and the band at 1705 cm1 are dependent upon temperature. With the temperature increasing from 30 to 140 °C, the intensity of the free C@O increases sharply from 0.034 to 0.173, while the intensity of hydrogen bonded C@O decreases from 0.03 to 0.121. After 150 °C, the intensity of both of the two bands show opposite change trends. The changes indicate that carbonyl groups are sensitive to a conformational transition and the hydrogen bonds decrease with the temperature, which results in more gauche conformers in the poly chains. Fig. 4. (a) Spectra of the PVA film in the 1186–962 cm1 region in the temperature of 20–180 °C. (b) I1047/I1095 as a function of the temperature.
Fig. 5. A van’t Hoff plot for PVA film and the corresponding thermodynamic parameters in the conformational transition.
DH2 = 14.17 kJ mol1, DS2 = 4.21 J mol1 K1 in region II and DH3 = 8.77 kJ mol1, DS3 = 1.88 J mol1 K1 in region III, respectively. The different thermodynamically functional values indicate that there is an intermediate state between the regions I and III. Analysis of ester carbonyl groups As mentioned above, there are a certain amount of ester groups in PVA chains because of the partial hydrolysis in the process of PVA preparation. Usually the frequency of carbonyl groups is not sensitive to rotation about the CAO bond but CAC bond [31]. Thus, analysis of the frequency of carbonyl group is necessary for us to
Fig. 6. (a) Differential spectra of the PVA film in the 1780–1670 cm1 region in the temperature of 30–180 °C by subtracting the spectrum at 20 °C. (b) The intensities of the band at 1745 and 1705 cm1 as a function of the temperature.
S. Han et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 139 (2015) 37–42
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Hydroxyl group in the PVA film Every repeating unit of PVA contains one hydroxyl group that can be easily formed intramolecular or intermolecular hydrogen bond with itself, carbonyl group or water molecule [36]. The frequency of the OAH stretching mode is a vital indicator of the presence and the strength of hydrogen bond. Thus the OAH stretching mode can be used to analyze the intermolecular interactions. Fig. 7a shows differential spectra of the PVA film in the 3700– 2980 cm1 region obtained in the temperature region of 30– 180 °C. There are two bands at 3521 and 3222 cm1, which are attributed to the week hydrogen bond (interaction between carbonyl group and water molecular etc.) and strong hydrogen bond (interaction between water and water molecular, hydroxyl group of PVA and water molecular etc.), respectively. It can be seen that the intensities of the two bands change with the temperature, which means that the strength of the strong hydrogen bond depends on the temperature. The intensity of the weak and strong hydrogen bond as a function of temperature is shown in Fig. 7b. It is obvious that the peak intensity of the strong hydrogen bond decreased continuously with increase of temperature from 30 to 180 °C, while that of weak hydrogen bond decreased sharply until 70 °C, and then gradually increased as the temperature increased from 70 to 180 °C. In general, the changeable trend of strong and weak hydrogen bonds should be contrary. Thus, the same change trends of the strong hydrogen bond and weak hydrogen bond below 70 °C should be related to the existence of free water. Above 70 °C, the hydrogen bonds became weak as the temperature increased, which lead to more chain mobility.
Fig. 8. TGA curve of PVA film.
In order to make clear the reason why the strong and weak hydrogen bonds change at the same trends below 70 °C, TGA analysis has been employed (shown in Fig. 8). There are two distinct stages in the TG profile. The weight loss is going on till 75 °C, which is consistent with the trend of the weak hydrogen bonds below 70 °C. It shows that the above-mentioned decrease in intensity of the peaks at 1705 cm1 is caused by the evaporation of the moisture in PVA film. At the temperature above 75 °C, the sample mass keeps almost constant. In this stage, rather than being associated with dehydration, the variation about OAH stretching mode was linked to molecular structure changes. The free water almost completely evaporated, the rest existed as bound water. The strength of strong hydrogen bond weakened as the temperature increased from 70 to 180 °C, which suggested the decrease of number of strong hydrogen bond. Meanwhile, a certain number of weak hydrogen bonds were formed owing to the destruction of strong hydrogen bonds. From the TGA curve, it can be learned that PVA does not decomposed during 70–180 °C. These data suggest that significant changes in conformational order occur as 70–180 °C, and also some transition in intermolecular and intramolecular structures of the PVA film happened. Comparison found that increase of the temperature leads to less strong hydrogen bonds and more disordered alkyl chain. For PVA film, increase of temperature results in weakening the hydrogen bonds and disordering alkyl chain. The order degree of chain is dependent on the interchain interaction. From the structure of PVA macromolecules, the interaction between the skeleton chains bearing hydroxyl groups is mainly decided by the interchain hydrogen bonds. The weaker hydrogen bonds between chains suggest the weaker interchain interaction, which leads to more disordered chain and more gauche conformation.
Conclusion
Fig. 7. (a) Differential spectra of the PVA film in the 3700–2980 cm1 region in the temperature of 30–180 °C by subtracting the spectrum at 20 °C. (b) The intensities of the band at 3521 and 3222 cm1 as a function of the temperature.
In the present study, we used temperature-dependent FTIR technique to demonstrate the conformational transition between trans and gauche in PVA film. The I2920/I2848 and I1047/I109 both show that the intermolecular interactions became weaker and more gauche conformer are formed in the alkyl chains as the temperature increased. From the intensity ratio of the peaks at 1047 and 1095 cm1 (I1047/I1095), the values of enthalpy and entropy change for the transition from trans to gauche conformer have been obtained from the van’t Hoff relation. Analysis of the C@O stretching modes and OAH bands suggests that the intermolecular hydrogen bonds became weaker as the temperature is rising, which should be responsible for trans to gauche transition. In the whole studied temperature range, significant changes in conformational order occur during 70–140 °C.
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Acknowledgements The authors would like to thanks Damin Li for associate work. And we gratefully appreciate financial support from the National Natural Science Foundation of China (41175119, 20933001 and 21373026), and the 111 Project (B07012). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
H. Tang, S. Sun, J. Wu, P. Wu, X. Wan, Polymer 51 (2010) 5482. T.T.Y. Katsumoto, H. Sato, Y. Ozaki, J. Phys. Chem. A 106 (2002) 3429. K. Aoi, H. Aoi, M. Okada, Macromol. Chem. Phys. 203 (2002) 1018. Q.Y. Soundararajah, B.S.B. Karunaratne, R.M.G. Rajapakse, J. Compos. Mater. 44 (2010) 303. A. Kaboorani, B. Riedl, Compos. Part A Appl. S. 42 (2011) 1031. X.H. Qin, S.Y. Wang, J. Appl. Polym. Sci. 109 (2008) 951. A.B.R. Moreira, V.H. Perez, G.M. Zanin, H.F. De Castro, Energy Fuels 21 (2007) 3689. N. De Prisco, B. Immirzi, M. Malinconico, P. Mormile, L. Petti, G. Gatta, J. Appl. Polym. Sci. 86 (2002) 622. M. Vieira, C.R. Tavares, R. Bergamasco, J.C.C. Petrus, J. Membrane, Science 194 (2001) 273. T. Yasukawa, H. Kimura, Y. Tabata, Y. Ogura, Adv. Drug Deliver. Rev. 52 (2001) 25. K. Masuda, F. Horii, Macromolecules 31 (1998) 5810. H. Yang, S.H. Hu, F. Horii, R. Endo, T. Hayash, T. Hayashi, Polymer 47 (2006) 1995. K. Masuda, H. Kaji, F. Horii, J. Polym. Sci. Poly. Phys. 38 (2000) 1.
[14] D. Eagland, D.J. Maitland, N.J. Crowther, W.L. Titterton, Polymer 35 (1994) 3389. [15] E. Meaurio, E. Zuza, N. Lopez-Rodriguez, J.R. Sarasua, J. Phys. Chem. B 110 (2006) 5790. [16] Y.C. Cheng, X. Wang, J.B. Cheng, L. Sun, W.Q. Xu, B. Zhao, Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 61 (2005) 905. [17] R.G. Snyder, H.L. Strauss, C.A. Elliger, J. Phys. Chem. 86 (1982) 5145. [18] L.C. Hector, H.M. Henry, G.C. David, J. Chem. Phys. 77 (1982) 2825. [19] R.A. Macphail, H.L. Strauss, R.G. Snyder, C.A. Elliger, J. Phys. Chem. 88 (1984) 334. [20] L.C. Hector, H.M. Henry, Biochim. Biophys. Acta 779 (1984) 381. [21] D.J. Moore, M.E. Rerek, R. Mendelsohn, J. Phys. Chem. B 101 (1997) 8933. [22] E.A. Disalvo, A.M. Bouchet, M.A. Frias, Biochim. Biophys. Acta 1828 (2013) 1683. [23] N.V. Venkataraman, S. Vasudevan, J. Phys. Chem. B 105 (2001) 1805. [24] G.B. Kenneth, J. Phys. Chem. 91 (1987) 3436. [25] S. Bresson, M. El Marssi, B. Khelifa, Vib. Spectrosc. 40 (2006) 263. [26] S. Bresson, A. El Marssi, B. Khelifa, Chem. Phys. Lipids 134 (2005) 119. [27] Y. Maeda, T. Nakamura, I. Ikeda, Macromolecules 34 (2001) 1391. [28] R.W. Wang, J. Guo, G. Baran, S.L. Wunder, Langmuir 16 (2000) 568. [29] D.E. Silva, D. Rousseau, Phys. Chem. Chem. Phys. 10 (2008) 4606. [30] M.J. Zheng, W.M. Du, Vib. Spectrosc. 40 (2006) 219. [31] B.B. Ellen, G.B. Kenneth, B.P. Willis, J. Am. Chem. Soc. 102 (1980) 5486. [32] A. Lejardi, A. Etxeberria, E. Meaurio, J.R. Sarasua, Polymer 53 (2012) 50. [33] L. Senak, Z.M. Ju, N. Noy, R. Callender, D. Manor, Biospectroscopy 3 (1997) 131. [34] S. Bresson, M. El Marssi, B. Khelifa, Vib. Spectrosc. 34 (2004) 231. [35] S. Bresson, D. Rousseau, S. Ghosh, M. El Marssi, V. Faivre, Eur. J. Lipid Sci. Technol. 113 (2011) 992. [36] M. Watase, K. Nishinali, Polym. J. 21 (1989) 597.