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In situ FT-IR spectroscopic study on the conformational changes of isotactic polypropylene in the presence of supercritical CO2
European Polymer Journal 44 (2008) 2619–2624
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
In situ FT-IR spectroscopic study on the conformational changes of isotactic polypropylene in the presence of supercritical CO2 Bin Li, Lei Li, Ling Zhao *, Weikang Yuan State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e
i n f o
Article history: Received 4 May 2007 Received in revised form 12 April 2008 Accepted 29 May 2008 Available online 6 June 2008
Keywords: FT-IR Supercritical CO2 Isotactic polypropylene Conformational change
a b s t r a c t The conformational changes of isotactic polypropylene (iPP) under supercritical CO2 condition with different pressure and temperature have been carefully studied by in situ Fourier-transform infrared spectroscopy (FT-IR). Analysis of the corresponding spectra shows that the conformational ordering by supercritical CO2 results in the intensity enhancement of the regularity bands of iPP. Due to the high CO2 concentration and strong intermolecular interaction, iPP can reach an equilibrium state in a short time at high CO2 pressure. The equilibrium time increases with soaking temperature. After supercritical CO2 treatment, two mechanisms, the formation of short helix from amorphous phase and the extension of short helix into long one, happen simultaneously. The latter mechanism undergoes quickly at the beginning of induced conformational changes and then slows down, resulting in the slight increase of crystallinity. At the same time, the conformational ordering in amorphous phase happens continuously until a thermodynamic equilibrium. In summary, in the presence of supercritical CO2, the conformational ordering of iPP chains occurs exclusively in the amorphous region, with no impact on the crystal part. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, supercritical CO2 (Tc = 31.1 °C, Pc = 7.37 MPa) has become a potential solvent for various polymer processing [1–4] such as grafting, foaming, impregnation of additives, etc. Supercritical CO2 can swell and plasticize polymers, resulting in a depression on the melting temperature and glass transition temperature. The plasticization of amorphous phase increases the chain mobility and thus induces crystallization of polymer. The CO2 induced crystallization can significantly change polymer structure and morphology, which is crucial for the applications of supercritical CO2 in polymer processing. Recently, there are an increasing number of reports on the CO2 induced crystallization of different polymers. For examples, Handa et al. [5] studied the CO2 induced crystallization of tert-butyl-substituted poly(ether ether ketone) * Corresponding author. Tel.: +86 21 64253470; fax: +86 21 64253528. E-mail address: [email protected] (L. Zhao). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.05.030
(tBuPEEK). They found that the melting point of tBuPEEK crystal increases linearly with the CO2 pressure. The dissolution of CO2 in polymer primarily facilitates the nucleation and then speeds up crystal growth. A similar phenomenon was also observed by Liao et al. [6], who found that bisphenol A polycarbonate exhibits two melting endotherms after supercritical CO2 treatment. The lower melting peak moves toward high temperature with increasing soaking temperature, pressure and time. The CO2 induced crystallization is generally explained through the improvement of macromolecular mobility which decreases the energy barriers for the polymer crystallization. The specific intermolecular interaction between CO2 and polymer is responsible for the rearrangement and crystallization of polymer chains. Fourier-transform infrared spectroscopy (FT-IR) is a powerful tool to study such interaction under different conditions. For example, Kazarian et al. [7] studied the intermolecular interaction between CO2 and different polymers, including PMMA, PVAc, PVF, PS, etc. Zhu et al. [8–14] used the in situ FT-IR to monitor the
2620
B. Li et al. / European Polymer Journal 44 (2008) 2619–2624
melting and crystallization processes of isotactic polypropylene (iPP). Absorption bands of infrared spectra are very sensitive to the physical structure of semicrystalline polymers. There are two types of the sensitive infrared bands, the crystalline band and regularity band. The former is caused by the intermolecular forces in the crystal lattice where the polymer molecules pack together on a regular three-dimensional arrangement. The latter is caused by the intramolecular vibration coupling within a single chain. For iPP, most of the absorption bands with the wavenumber lower than 1400 cm1 belong to the regularity band. It is well established that the specific regularity bands of iPP are related to the different critical length ‘‘n” of isotactic sequences [8–18]. The minimum n values for different IR bands at 973, 998, 841, and 1220 cm1 are 5, 10, 12 and 14 monomer units in helical sequences, respectively. Since supercritical CO2 can only diffuse into the amorphous phase instead of the crystal lattice, the conformational change can be characterized through analyzing the intermolecular interaction between CO2 molecules and iPP chain. In this work, the conformational change of iPP in the presence of supercritical CO2 is investigated by in situ FTIR. The IR spectra of iPP are recorded with the time-resolved FT-IR during chain rearrangement of iPP in supercritical CO2. The intensity changes of iPP regularity bands are used to analyze the effect of temperature, pressure and time on CO2 induced regularity improvement. 2. Experimental 2.1. Materials and sample preparation A commercial isotactic polypropylene (iPP) was used in this study. It was provided by Shanghai Petrochemical Company, China, in a powdery form under the tradename Y1600. It was purified through refluxing in acetone for 24 h and dried in a vacuum oven at ambient temperature. Fifteen gram of the purified iPP powder was moulded into films of about 100 lm in thickness using a hot press at 200 °C and then put the sample into liquid nitrogen for
20 min. CO2 (purity: 99.9%) was purchased from Air Products Co., Shanghai. 2.2. Experimental setup The FI-TR apparatus used was of type Bruker Equinox55 equipped with a Harrick high-pressure demountable liquid cell. A schematic view of the optical cell is shown in Fig. 1. Optical access is given by two ZnSe windows for the absorption measurements with a path length of 0.5 cm. The heating is performed with two cartridge heaters disposed in the body of the cell in which the thermocouple is placed. The accuracy of the controlled temperature is about 0.5 °C. 2.3. IR data analysis FT-IR spectra were recorded at a resolution of 4.0 cm1 and a rate of 1 spectrum per 32 s. The IR intensities refer to the peak height. The scanned wavenumber range was 4000–400 cm1. Since the regularity bands were overlapped by the amorphous bands at 808, 1100 and 1167 cm1, the curve fitting procedures were adopted with the following function [19]:
x x0 2 f ðxÞ ¼ ð1 LÞH exp ð4 ln 2Þ þ L
x
H
4
xx0 2 x
þ1
It is a sum of a Lorentzian function and a Gaussian function, in which x0 is the peak position, H the height, x the width at half-height and L the Lorentzian component. Due to the experiment limitations, it requires 20– 30 min for the system pressure and temperature reaching the value that preliminarily set. No data is collected during that period of time. 2.4. XRD and DSC analysis The wide angle X-ray diffractiometry (WAXD) and differential scanning calorimetry (DSC) are used to investigate the crystal structure and crystallinity of iPP after it
Fig. 1. Schematic diagram of the high pressure in situ FT-IR apparatus: (1) high pressure IR cell; (2) equilibrium still; (3) thermostatic bath; (4) magnetic stirrer; (5) diaphragm type compressor; (6) filter and (7) stirrer.
B. Li et al. / European Polymer Journal 44 (2008) 2619–2624
is treated by supercritical CO2. They are of type Rigaku D/ max 2550 VB/VC X-Ray Diffractometer (Cu Ka Ni-filtered radiation) and NETZSCH 204 HP. The scan rate of the WAXD was 1° (h)/min. DSC curves were obtained by heating the iPP sample from 20 to 200 °C with a heating rate of 10 °C/min using nitrogen as the purge gas.
3. Results and discussion Since the iPP film was immersed into the supercritical CO2 in the high pressure IR cell, the collected spectra contain bands of iPP and CO2. Fig. 2 shows the spectra of pure CO2 and iPP immersed in CO2 under same temperature and pressure. Obviously, the four peaks at 841, 973, 998, and
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1220 cm1 related to the helical structure of iPP do not appear in the spectrum of pure CO2, therefore they can be used to investigate the conformation change of iPP during the CO2 induced regularity enhancement. The intensity increment of these regularity bands shows an ordering of iPP helix sequences in supercritical CO2. The spectra were firstly recorded at different CO2 pressure (6–16 MPa) under an isothermal condition. The IR intensities of the regularity bands are normalized. Fig. 3 shows the time dependence of 973 cm1 band under different CO2 pressure. It is clear that its intensity increases with soaking time and increasing CO2 pressure shortens the equilibrium time. A similar evolution can be observed for other regularity bands, such as 998, 841 and 1220 cm1 (Fig. S1 in supplementary material). The dissolution of
Fig. 2. IR spectrum of pure CO2 (dash line) and iPP immersed in CO2 (solid line) at 12 MPa and 60 °C.
Fig. 3. Time dependence of 973 cm1 band under different CO2 pressures.
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CO2 in the free volume among iPP chain favors the rearrangement and ordering of the helix structures. The higher the CO2 concentration is, the faster the ordering process is. Generally, the chain rearrangement and induce crystallization from a glassy state usually takes a very long time [20,21]. For iPP, the cold crystallization occurs at temperature higher than 80–100 °C [22–24]. In the present work, the rearrangement of polymer chain finishes within 2 h when CO2 pressure is up to 16 MPa, indicating a fast chain arrangement and induced crystallization process. On the contrary, the equilibrium state cannot be reached even after 16 h at 6 MPa CO2 pressure (lower than the critical pressure). To obtain further information during the CO2 induced conformational changes under different pressure, the ratios of long helix and short one are plotted with soaking
time. Fig. 4 shows the ratio of band intensities at 1220 and 973 cm1 with the soaking time at different CO2 pressure. At low pressure, the ratio increases in the beginning, indicating a significant growth of long helix chains. However, the ratio decreases in the later stage, illustrating the formation of a limited number of long helix. Increasing the CO2 pressure up to 16 MPa causes a dramatically decrease of the ratio, representing a quick increase on the amount short helix which are supposed to be generated from the random coil in amorphous region, or a very fast initial ratio increase which cannot be observed. In summary, when interacting with CO2 molecules, the number of iPP helix sequence is enhanced, resulting in an increase of the relevant regularity band intensities. The induced conformational changes include two mechanisms, the formation of short helix from amorphous phase and the extension of short
Fig. 4. Ratio of band intensities at 1220 and 973 cm1 with the soaking time under 60 °C and different CO2 pressure.
Fig. 5. Time dependence of 973 cm1 regularity bands under different soaking temperature.
B. Li et al. / European Polymer Journal 44 (2008) 2619–2624
helix into long one. The latter is difficult and can only reach a certain extent in short time, and the former is much easier especially under higher pressure with a large concentration of CO2 molecules in iPP matrix. In the meantime, the equilibrium time is shortened with increasing pressure, which is coincident with the previous analysis. According to Sato et al. [19], the solubility of CO2 in iPP increases with temperature, which means a large concentration of CO2 molecules exists in iPP matrix at high temperature. Meanwhile, the polymer chain can get more energy and mobility to rearrange its conformation to a favorable state. Fig. 5 shows the time dependence of 973 cm1 band under different soaking temperature. The increment of band intensity is difficult to reach equilibrium at high temperature, indicating a persistent ordering of the helix structure. A similar evolution can be observed for other regularity bands, such as 998, 841 and 1220 cm1 (Fig. S2 in supplementary material). Based on the results in Fig. 5, it can be concluded that the contents of both short and long helix sequence increase with soaking time under certain CO2 pressure and different
2623
temperature. Similarly, the ratios of band intensities at 1220 and 973 cm1 are plotted with soaking time. The results are shown in Fig. 6. The two mechanisms of induced conformational changes are competitive each other during the process. Initially, the quick formation of long helix leads to the increase of intensity ratio, and then the generation of short helix becomes dominant since only a limited number of long chains are formed in this stage. The higher the temperature is, the longer the equilibrium time for induced conformational changes is. At low temperature (e.g. 60 °C), the formation of short helix can reach equilibrium in a short time, and the sustainable formation of long helix makes the ratio increase slowly. The X-ray diffraction patterns of virgin iPP and iPP treated by CO2 under 100 °C and 16 MPa are showed in the supplementary maetrial (Fig. S3). No obvious change happens on iPP crystal form, which means the CO2 induced conformational changes only favors the ordering of helix sequence instead of changing the crystal form. Importantly, Fig. 7 shows that the crystallinity of iPP increases slightly after supercritical CO2 treatment, which means
Fig. 6. Ratio of band intensities at 1220 and 973 cm1 with the soaking time under 12 MPa CO2 pressure and different temperature.
Fig. 7. Crystallinity of iPP before and after CO2 treatment under 60 °C and different temperature.
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that the amount of iPP crystals induced by supercritical CO2 is low. Actually, Figs. 3–6 have demonstrated that the extension of long helix conformation can be completed in a short time at high CO2 pressure, indicating the slight increase of crystallinity. Consequently, the influence of supercritical CO2 on iPP conformational change can be described as follows: In the initial stage, some long helical conformations are formed, which are transformed to the crystal phase very quickly. However, such transformation is limited and the crystallinity of final sample increases only slightly. Due to the interaction between CO2 and iPP chains, the conformational ordering in amorphous phase happens continuously and more and more random coils are transformed to short helix until a thermodynamic equilibrium. The conformational ordering of iPP chains occurs exclusively in the amorphous region. 4. Conclusion Supercritical CO2 induced conformational changes of iPP is monitored through analyzing the variation of the regularity bands by in situ FT-IR. The effects of CO2 pressure and soaking temperature on chain rearrangement and crystallization behavior are investigated. The experimental results show that the conformational ordering of iPP chains occurs exclusively in the amorphous region. In the presence of supercritical CO2, two mechanisms, the formation of short helix from amorphous phase and the extension of short helix into long one, happen simultaneously. The latter mechanism undergoes quickly at the beginning of induced conformational changes and then slows down, resulting in the slight increase of crystal phase. In the meantime, the conformational ordering in amorphous phase happens continuously and more and more random coils are transformed to short helix until a thermodynamic equilibrium. High CO2 pressure and low soaking temperature favor the completion of equilibrium state in a short time. Acknowledgements This work was supported financially by National Natural Science Foundation of China (Grant Nos. 20490200, 20676031), and the Science and Technology Commission of Shanghai Municipality (Grant No. 0552nm039). Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/ j.eurpolymj.2008.05.030. References [1] Tomasko DL, Li HB, Liu DH, et al. A review of CO2 applications in the processing of polymers. Ind Eng Chem Res 2003;42:6431–56.
[2] Liu T, Hu GH, Tong GS, Zhao L, Cao GP, Yuan WK. Supercritical carbon dioxide assisted solid-state grafting process of maleic anhydride onto polypropylene. Ind Eng Chem Res 2005;44:4292–9. [3] Xu ZM, Jiang XL, Liu T, Hu GH, Zhao L, Zhu ZN, Yuan WK. Foaming of polypropylene with supercritical carbon dioxide. J Supercritical Fluids, in press. [4] Li B, Hu GH, Cao GP, Liu T, Zhao L, Yuan WK. Supercritical carbon dioxide-assisted dispersion of sodium benzoate in polypropylene and crystallization behavior of the resulting polypropylene. J Appl Polym Sci 2006;102:3212–20. [5] Handa YP, Zhang ZY, Roovers J. Compressed-gas-induced crystallization in tert-butyl poly(ether ether ketone). J Polym Sci: B: Polym Phys 2001;39:1505–12. [6] Liao X, Wang J, Li G, He JS. Effect of supercritical carbon dioxide on the crystallization and melting behavior of linear bisphenol A polycarbonate. J Polym Sci: B: Polym Phys 2004;42:280–5. [7] Kazarian SG, Vincent MF, Bright FV, Liotta CL, Eckert CA. Specific intermolecular interaction of carbon dioxide with polymers. J Am Chem Soc 1996;118:1729–36. [8] Zhu XY, Yan DY, Yao HX, Zhu PF. In situ FTIR spectroscopic study of the regularity bands and partial-order melts of isotactic polypropylene. Macromol Rapid Commun 2000;21:354–7. [9] Zhu XY, Yan DY, Fang YP. In situ FTIR spectroscopic study of the conformational change of isotactic polypropylene during the crystallization process. J Phys Chem B 2001;105:12461–3. [10] Zhu XY, Yan DY. In situ FTIR spectroscopic study on the melting process of isotactic polypropylene. Macromol Chem Phys 2001;202:1109–13. [11] Zhu XY, Li YJ, Yan DY, Zhu PF, Lu QH. Influence of the order of polymer melt on the crystallization behavior: I. Double melting endotherms of isotactic polypropylene. Colloid Polym Sci 2001;279:292–6. [12] Zhu XY, Fang YP, Yan DY. A possible explanation to the structure change of isotactic polypropylene occurring at about 135 °C. Polymer 2001;42:8595–8. [13] Zhu XY, Fang YP, Li YJ, Yan DY, Zhu PF, Lu QH. I In situ FTIR spectroscopic study of the conformational change of isotactic polypropylene during the heating process. Chem J Chin Univ 2001;22:1425–7. [14] He P, Zhu XY. In situ Fourier transform infrared spectroscopic study of the thermal degradation of isotactic poly(propylene). Appl Spectr 2005;59:33–8. [15] Miyamoto T, Inagaki H. Structure and steric isomerism of polypropylene. J Polym Sci 1969;A-2(7):963–81. [16] Kobayashi M, Akita K, Tadokoro H. Infrared spectra and regular sequence lengths in isotactic polymer chains. Makromol Chem 1968;118:324–42. [17] Kissin YV, Rishina LA. Regularity bands in the IR spectra of C3H6– C3D6 copolymers. Eur Polym J 1976;12:757–9. [18] Kissin YV. Orientation of isotactic polypropylene in crystalline and amorphous phases: IR method. J Polym Sci, Polym Edition 1983;21:2085–96. [19] Sato Y, Fujiwara K, Takikawa T, Sumarno, Takishima S, Masuoka H. Solubilities and diffusion coefficients of carbon dioxide and nitrogen in polypropylene, high-density polyethylene, and polystyrene under high pressures and temperatures. Fluid Phase Equilib 1999;162:261–76. [20] Grasruck M, Strobl G. Crystallization of s-polypropylene from the glassy state: indications for a multistage process. Macromolecules 2003;36:86–91. [21] Bu HS, Cao J, Xu SY, Zheng Z, Wunderlich B. Multimolecule glasses and crystals of isotactic polystyrene. J Macromol Sci Phys 1999;B38:251–62. [22] Wang ZG, Hsiao BS, Srinivas S, Brown GM, Tsou AH, Cheng SZD, Stein RS. Phase transformation in quenched mesomorphic isotactic polypropylene. Polymer 2001;42:7561–6. [23] Zhu XY, Yan DY, Fang YP, Chen L. On-line FT-IR spectroscopic study of the disorientation of uniaxially stretched isotactic polypropylene upon heating. Appl Spectr 2003;57:104–6. [24] Chen L, Wang YP, Zhu XY, Yan DY. Influence of the conformational order of glassy isotactic polypropylene on its crystallization and melting behaviour. Polym Int 2004;53:131–5.
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
In situ FT-IR spectroscopic study on the conformational changes of isotactic polypropylene in the presence of supercritical CO2 Bin Li, Lei Li, Ling Zhao *, Weikang Yuan State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e
i n f o
Article history: Received 4 May 2007 Received in revised form 12 April 2008 Accepted 29 May 2008 Available online 6 June 2008
Keywords: FT-IR Supercritical CO2 Isotactic polypropylene Conformational change
a b s t r a c t The conformational changes of isotactic polypropylene (iPP) under supercritical CO2 condition with different pressure and temperature have been carefully studied by in situ Fourier-transform infrared spectroscopy (FT-IR). Analysis of the corresponding spectra shows that the conformational ordering by supercritical CO2 results in the intensity enhancement of the regularity bands of iPP. Due to the high CO2 concentration and strong intermolecular interaction, iPP can reach an equilibrium state in a short time at high CO2 pressure. The equilibrium time increases with soaking temperature. After supercritical CO2 treatment, two mechanisms, the formation of short helix from amorphous phase and the extension of short helix into long one, happen simultaneously. The latter mechanism undergoes quickly at the beginning of induced conformational changes and then slows down, resulting in the slight increase of crystallinity. At the same time, the conformational ordering in amorphous phase happens continuously until a thermodynamic equilibrium. In summary, in the presence of supercritical CO2, the conformational ordering of iPP chains occurs exclusively in the amorphous region, with no impact on the crystal part. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, supercritical CO2 (Tc = 31.1 °C, Pc = 7.37 MPa) has become a potential solvent for various polymer processing [1–4] such as grafting, foaming, impregnation of additives, etc. Supercritical CO2 can swell and plasticize polymers, resulting in a depression on the melting temperature and glass transition temperature. The plasticization of amorphous phase increases the chain mobility and thus induces crystallization of polymer. The CO2 induced crystallization can significantly change polymer structure and morphology, which is crucial for the applications of supercritical CO2 in polymer processing. Recently, there are an increasing number of reports on the CO2 induced crystallization of different polymers. For examples, Handa et al. [5] studied the CO2 induced crystallization of tert-butyl-substituted poly(ether ether ketone) * Corresponding author. Tel.: +86 21 64253470; fax: +86 21 64253528. E-mail address: [email protected] (L. Zhao). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.05.030
(tBuPEEK). They found that the melting point of tBuPEEK crystal increases linearly with the CO2 pressure. The dissolution of CO2 in polymer primarily facilitates the nucleation and then speeds up crystal growth. A similar phenomenon was also observed by Liao et al. [6], who found that bisphenol A polycarbonate exhibits two melting endotherms after supercritical CO2 treatment. The lower melting peak moves toward high temperature with increasing soaking temperature, pressure and time. The CO2 induced crystallization is generally explained through the improvement of macromolecular mobility which decreases the energy barriers for the polymer crystallization. The specific intermolecular interaction between CO2 and polymer is responsible for the rearrangement and crystallization of polymer chains. Fourier-transform infrared spectroscopy (FT-IR) is a powerful tool to study such interaction under different conditions. For example, Kazarian et al. [7] studied the intermolecular interaction between CO2 and different polymers, including PMMA, PVAc, PVF, PS, etc. Zhu et al. [8–14] used the in situ FT-IR to monitor the
2620
B. Li et al. / European Polymer Journal 44 (2008) 2619–2624
melting and crystallization processes of isotactic polypropylene (iPP). Absorption bands of infrared spectra are very sensitive to the physical structure of semicrystalline polymers. There are two types of the sensitive infrared bands, the crystalline band and regularity band. The former is caused by the intermolecular forces in the crystal lattice where the polymer molecules pack together on a regular three-dimensional arrangement. The latter is caused by the intramolecular vibration coupling within a single chain. For iPP, most of the absorption bands with the wavenumber lower than 1400 cm1 belong to the regularity band. It is well established that the specific regularity bands of iPP are related to the different critical length ‘‘n” of isotactic sequences [8–18]. The minimum n values for different IR bands at 973, 998, 841, and 1220 cm1 are 5, 10, 12 and 14 monomer units in helical sequences, respectively. Since supercritical CO2 can only diffuse into the amorphous phase instead of the crystal lattice, the conformational change can be characterized through analyzing the intermolecular interaction between CO2 molecules and iPP chain. In this work, the conformational change of iPP in the presence of supercritical CO2 is investigated by in situ FTIR. The IR spectra of iPP are recorded with the time-resolved FT-IR during chain rearrangement of iPP in supercritical CO2. The intensity changes of iPP regularity bands are used to analyze the effect of temperature, pressure and time on CO2 induced regularity improvement. 2. Experimental 2.1. Materials and sample preparation A commercial isotactic polypropylene (iPP) was used in this study. It was provided by Shanghai Petrochemical Company, China, in a powdery form under the tradename Y1600. It was purified through refluxing in acetone for 24 h and dried in a vacuum oven at ambient temperature. Fifteen gram of the purified iPP powder was moulded into films of about 100 lm in thickness using a hot press at 200 °C and then put the sample into liquid nitrogen for
20 min. CO2 (purity: 99.9%) was purchased from Air Products Co., Shanghai. 2.2. Experimental setup The FI-TR apparatus used was of type Bruker Equinox55 equipped with a Harrick high-pressure demountable liquid cell. A schematic view of the optical cell is shown in Fig. 1. Optical access is given by two ZnSe windows for the absorption measurements with a path length of 0.5 cm. The heating is performed with two cartridge heaters disposed in the body of the cell in which the thermocouple is placed. The accuracy of the controlled temperature is about 0.5 °C. 2.3. IR data analysis FT-IR spectra were recorded at a resolution of 4.0 cm1 and a rate of 1 spectrum per 32 s. The IR intensities refer to the peak height. The scanned wavenumber range was 4000–400 cm1. Since the regularity bands were overlapped by the amorphous bands at 808, 1100 and 1167 cm1, the curve fitting procedures were adopted with the following function [19]:
x x0 2 f ðxÞ ¼ ð1 LÞH exp ð4 ln 2Þ þ L
x
H
4
xx0 2 x
þ1
It is a sum of a Lorentzian function and a Gaussian function, in which x0 is the peak position, H the height, x the width at half-height and L the Lorentzian component. Due to the experiment limitations, it requires 20– 30 min for the system pressure and temperature reaching the value that preliminarily set. No data is collected during that period of time. 2.4. XRD and DSC analysis The wide angle X-ray diffractiometry (WAXD) and differential scanning calorimetry (DSC) are used to investigate the crystal structure and crystallinity of iPP after it
Fig. 1. Schematic diagram of the high pressure in situ FT-IR apparatus: (1) high pressure IR cell; (2) equilibrium still; (3) thermostatic bath; (4) magnetic stirrer; (5) diaphragm type compressor; (6) filter and (7) stirrer.
B. Li et al. / European Polymer Journal 44 (2008) 2619–2624
is treated by supercritical CO2. They are of type Rigaku D/ max 2550 VB/VC X-Ray Diffractometer (Cu Ka Ni-filtered radiation) and NETZSCH 204 HP. The scan rate of the WAXD was 1° (h)/min. DSC curves were obtained by heating the iPP sample from 20 to 200 °C with a heating rate of 10 °C/min using nitrogen as the purge gas.
3. Results and discussion Since the iPP film was immersed into the supercritical CO2 in the high pressure IR cell, the collected spectra contain bands of iPP and CO2. Fig. 2 shows the spectra of pure CO2 and iPP immersed in CO2 under same temperature and pressure. Obviously, the four peaks at 841, 973, 998, and
2621
1220 cm1 related to the helical structure of iPP do not appear in the spectrum of pure CO2, therefore they can be used to investigate the conformation change of iPP during the CO2 induced regularity enhancement. The intensity increment of these regularity bands shows an ordering of iPP helix sequences in supercritical CO2. The spectra were firstly recorded at different CO2 pressure (6–16 MPa) under an isothermal condition. The IR intensities of the regularity bands are normalized. Fig. 3 shows the time dependence of 973 cm1 band under different CO2 pressure. It is clear that its intensity increases with soaking time and increasing CO2 pressure shortens the equilibrium time. A similar evolution can be observed for other regularity bands, such as 998, 841 and 1220 cm1 (Fig. S1 in supplementary material). The dissolution of
Fig. 2. IR spectrum of pure CO2 (dash line) and iPP immersed in CO2 (solid line) at 12 MPa and 60 °C.
Fig. 3. Time dependence of 973 cm1 band under different CO2 pressures.
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CO2 in the free volume among iPP chain favors the rearrangement and ordering of the helix structures. The higher the CO2 concentration is, the faster the ordering process is. Generally, the chain rearrangement and induce crystallization from a glassy state usually takes a very long time [20,21]. For iPP, the cold crystallization occurs at temperature higher than 80–100 °C [22–24]. In the present work, the rearrangement of polymer chain finishes within 2 h when CO2 pressure is up to 16 MPa, indicating a fast chain arrangement and induced crystallization process. On the contrary, the equilibrium state cannot be reached even after 16 h at 6 MPa CO2 pressure (lower than the critical pressure). To obtain further information during the CO2 induced conformational changes under different pressure, the ratios of long helix and short one are plotted with soaking
time. Fig. 4 shows the ratio of band intensities at 1220 and 973 cm1 with the soaking time at different CO2 pressure. At low pressure, the ratio increases in the beginning, indicating a significant growth of long helix chains. However, the ratio decreases in the later stage, illustrating the formation of a limited number of long helix. Increasing the CO2 pressure up to 16 MPa causes a dramatically decrease of the ratio, representing a quick increase on the amount short helix which are supposed to be generated from the random coil in amorphous region, or a very fast initial ratio increase which cannot be observed. In summary, when interacting with CO2 molecules, the number of iPP helix sequence is enhanced, resulting in an increase of the relevant regularity band intensities. The induced conformational changes include two mechanisms, the formation of short helix from amorphous phase and the extension of short
Fig. 4. Ratio of band intensities at 1220 and 973 cm1 with the soaking time under 60 °C and different CO2 pressure.
Fig. 5. Time dependence of 973 cm1 regularity bands under different soaking temperature.
B. Li et al. / European Polymer Journal 44 (2008) 2619–2624
helix into long one. The latter is difficult and can only reach a certain extent in short time, and the former is much easier especially under higher pressure with a large concentration of CO2 molecules in iPP matrix. In the meantime, the equilibrium time is shortened with increasing pressure, which is coincident with the previous analysis. According to Sato et al. [19], the solubility of CO2 in iPP increases with temperature, which means a large concentration of CO2 molecules exists in iPP matrix at high temperature. Meanwhile, the polymer chain can get more energy and mobility to rearrange its conformation to a favorable state. Fig. 5 shows the time dependence of 973 cm1 band under different soaking temperature. The increment of band intensity is difficult to reach equilibrium at high temperature, indicating a persistent ordering of the helix structure. A similar evolution can be observed for other regularity bands, such as 998, 841 and 1220 cm1 (Fig. S2 in supplementary material). Based on the results in Fig. 5, it can be concluded that the contents of both short and long helix sequence increase with soaking time under certain CO2 pressure and different
2623
temperature. Similarly, the ratios of band intensities at 1220 and 973 cm1 are plotted with soaking time. The results are shown in Fig. 6. The two mechanisms of induced conformational changes are competitive each other during the process. Initially, the quick formation of long helix leads to the increase of intensity ratio, and then the generation of short helix becomes dominant since only a limited number of long chains are formed in this stage. The higher the temperature is, the longer the equilibrium time for induced conformational changes is. At low temperature (e.g. 60 °C), the formation of short helix can reach equilibrium in a short time, and the sustainable formation of long helix makes the ratio increase slowly. The X-ray diffraction patterns of virgin iPP and iPP treated by CO2 under 100 °C and 16 MPa are showed in the supplementary maetrial (Fig. S3). No obvious change happens on iPP crystal form, which means the CO2 induced conformational changes only favors the ordering of helix sequence instead of changing the crystal form. Importantly, Fig. 7 shows that the crystallinity of iPP increases slightly after supercritical CO2 treatment, which means
Fig. 6. Ratio of band intensities at 1220 and 973 cm1 with the soaking time under 12 MPa CO2 pressure and different temperature.
Fig. 7. Crystallinity of iPP before and after CO2 treatment under 60 °C and different temperature.
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that the amount of iPP crystals induced by supercritical CO2 is low. Actually, Figs. 3–6 have demonstrated that the extension of long helix conformation can be completed in a short time at high CO2 pressure, indicating the slight increase of crystallinity. Consequently, the influence of supercritical CO2 on iPP conformational change can be described as follows: In the initial stage, some long helical conformations are formed, which are transformed to the crystal phase very quickly. However, such transformation is limited and the crystallinity of final sample increases only slightly. Due to the interaction between CO2 and iPP chains, the conformational ordering in amorphous phase happens continuously and more and more random coils are transformed to short helix until a thermodynamic equilibrium. The conformational ordering of iPP chains occurs exclusively in the amorphous region. 4. Conclusion Supercritical CO2 induced conformational changes of iPP is monitored through analyzing the variation of the regularity bands by in situ FT-IR. The effects of CO2 pressure and soaking temperature on chain rearrangement and crystallization behavior are investigated. The experimental results show that the conformational ordering of iPP chains occurs exclusively in the amorphous region. In the presence of supercritical CO2, two mechanisms, the formation of short helix from amorphous phase and the extension of short helix into long one, happen simultaneously. The latter mechanism undergoes quickly at the beginning of induced conformational changes and then slows down, resulting in the slight increase of crystal phase. In the meantime, the conformational ordering in amorphous phase happens continuously and more and more random coils are transformed to short helix until a thermodynamic equilibrium. High CO2 pressure and low soaking temperature favor the completion of equilibrium state in a short time. Acknowledgements This work was supported financially by National Natural Science Foundation of China (Grant Nos. 20490200, 20676031), and the Science and Technology Commission of Shanghai Municipality (Grant No. 0552nm039). Appendix A. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/ j.eurpolymj.2008.05.030. References [1] Tomasko DL, Li HB, Liu DH, et al. A review of CO2 applications in the processing of polymers. Ind Eng Chem Res 2003;42:6431–56.
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