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Time-resolved polymer deformation using polarized planar array infrared spectroscopy
Vibrational Spectroscopy 51 (2009) 34–38
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Time-resolved polymer deformation using polarized planar array infrared spectroscopy Baptiste Farbos, Damien Mauran, Christian Pellerin * Centre for Self-Assembled Chemical Structures (CSACS/CRMAA) and De´partement de chimie, Universite´ de Montre´al, Montre´al, QC, H3C 3J7 Canada
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 August 2008 Accepted 24 September 2008 Available online 4 October 2008
Polarized planar array infrared (PA-IR) spectroscopy is shown for the first time to be a powerful approach to study the mechanical deformation of polymers. A dual-beam PA-IR spectrometer was built to enable the simultaneous recording of parallel- and perpendicular-polarized spectra at the same sample spot. The technique provides orientation and structural information during and after fast irreversible deformations with a low-ms (or sub-ms) time resolution and a low data scatter. In proof-of-concept experiments, the possibilities of polarized PA-IR spectroscopy are illustrated by studying the deformation of poly(ethylene terephthalate) thin films. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Planar array infrared spectroscopy Time-resolved Polymer Orientation Conformation Poly(ethylene terephthalate)
1. Introduction The structure of a polymer, for instance its molecular orientation, degree of crystallinity and morphology, has a major impact on its physical properties. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and Raman scattering are amongst the most powerful approaches to study the orientation and structure of polymers [1]. While their static use is relatively common, performing time-resolved analyses during polymer processing has proven much more challenging, let alone at the high speeds relevant to industrial processes. The low quantum yield of the Raman inelastic scattering phenomenon generally precludes its use for such measurements. Time-resolved XRD studies with an excellent 50 ms resolution have been performed using a synchrotron source [2,3], but this is evidently unpractical for most applications. In contrast, time-resolved FT-IR spectra can be acquired using standard instrumentation and has therefore been widely used for dynamic studies of polymer deformation [4,5]. IR characterization of uniaxially oriented samples, such as oneway drawn films and fibers, requires the acquisition of two spectra
* Corresponding author at: De´partement de chimie, Universite´ de Montre´al, CP 6128 Succ. Centre-ville, Montre´al, QC, H3C 3J7 Canada. Tel.: +1 514 340 5762; fax: +1 514 340 5290. E-mail address: [email protected] (C. Pellerin). 0924-2031/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2008.09.010
polarized parallel (p) and perpendicular (s) with respect to the orientation direction. The dichroic difference provides information about molecular orientation, while details about the crystallinity, conformation, etc., can be obtained by calculating a structural absorbance spectrum. Early studies involved quenching samples below their glass transition temperature (Tg) in order to freeze the structure and allow static measurements [6–8]. Siesler recognized the limitations of this approach and built a rheo-optical spectrometer in which the polarizer was rapidly rotated by a pneumatic system, allowing both polarized spectra to be recorded in a matter of seconds [9]. The Siesler group used this method to follow in real time the slow deformation and relaxation of many polymers and copolymers [5,10]. Pe´zolet and coworkers have demonstrated that polarization modulation infrared linear dichroism (PM-IRLD) provides a much improved 400 ms time resolution, and used it to study various polymer systems [4,11–15]. We have very recently overcome a major limitation of PM-IRLD, the impossibility of obtaining the structural absorbance spectrum, in an approach called polarization modulation IR structural absorbance spectroscopy (PM-IRSAS) [16]. The best time resolution thus far has been obtained using an ultra-rapid-scanning FT-IR (URS-FT-IR) spectrometer developed by Manning Applied Technology and the Griffiths group [17]. The deformation of poly(ethylene terephthalate) (PET) above and below its Tg could be followed with time resolutions of 40 and 10 ms, respectively [18,19]. However, a serious drawback for deformation studies is that the technique records spectra in a
B. Farbos et al. / Vibrational Spectroscopy 51 (2009) 34–38
single polarization at a time, so that two perfectly repeatable samples are required to extract the orientation and structural information. Recently, Rabolt and coworkers have developed planar array infrared (PA-IR) spectroscopy, a technique that combines a spectrograph with a two-dimensional focal plane array (FPA) detector to enable true multichannel IR spectroscopy [20,21]. Pellerin et al. used it to follow the electric field-induced reorientation of a liquid crystal with an 8.7-ms time resolution [22]. It was demonstrated that the two polarized spectra could be recorded simultaneously by using different sections of the FPA without affecting the time resolution. In this paper, we demonstrate for the first time the potential of polarized PA-IR spectroscopy for the ultra-rapid dynamic study of polymer deformation and relaxation. It will be shown that orientation and structural information can be obtained with a ms or sub-ms time resolution while preserving an excellent noise level. The deformation of amorphous PET above Tg will be used as a model case. 2. Experimental 2.1. Samples and deformations Amorphous PET films with 7 mm thickness were prepared by blow molding at the National Research Council of Canada and cut into 20-mm long and 6-mm wide strips. Pyrotape (Aremco Products #546) was used to prevent slippage during the elongation. Deformations to draw ratios of 1.5 or 2 at a nominal speed of 3.21 cm/s (with acceleration and deceleration rates of 14.1 and 35.9 cm/s2, respectively) were performed at 90 8C (Tg + 12 8C) using a custom-built mechanical stretcher fitted with ZnSe windows to allow in situ measurements. A waveform generator (Agilent 33220A) was used to trigger simultaneously the FPA detector and the mechanical stretcher. 2.2. PA-IR spectrometer Fig. 1 shows a schematic layout of the new dual-polarization PA-IR spectrometer used in this work. The source was a silicon carbide element (Oriel 80030) powered at 12 V and 2 A using an Agilent E3634A power supply. The IR radiation was collected,
35
focused onto the sample in the stretcher, and re-collimated using three identical aluminum-coated off-axis parabolic (OAP) mirrors (Janos Technology) with a focal length of 15 cm. A beam stop was used before measurements to prevent sample heating by the IR source. The radiation was then separated into two beams using a gold-coated 908 prism mirror (Edmund Optics) and two OAP mirrors. A linear polarizer (KRS-5 or ZnSe) was inserted in each beam to polarize the radiation p and s with respect to the stretching direction. An identical arrangement of a 908 prism mirror with two other OAP mirrors was used to recombine the polarized beams. It was then focused onto the 300-mm wide entrance slit of a MicroHR imaging spectrograph (Horiba JobinYvon) equipped with a gold-coated plane reflection grating with 50 grooves/mm and a blazing wavelength of 6 mm. A 5.5-mm long-pass optical filter (Janos Technology) was inserted before the spectrograph to remove unwanted diffraction orders. Finally, a 128 128 mercury–cadmium–telluride (MCT) FPA detector (SBF161) with a NINOX compound lens was used to detect the radiation. The integration time was set to 123 ms and the frame rate to 1610 Hz, yielding a 621-ms acquisition time. This configuration allowed recording multiplex spectra over a spectral range from approximately 1630 to 1160 cm1, with an average optical resolution of 7–8 cm1. The signal-to-noise ratio could be improved at the expense of optical resolution by increasing the slit width, but it was not found to be necessary. Time-resolved PA-IR experiments were conducted by recording 6000 single frame spectral images for a total measurement time of 3.7 s. Background and dark background spectra were obtained by averaging 1000 frames with an open beam and with the beam blocked by a metal plate, respectively. All calculations were performed using custom-made programs in Matlab R2006a (MathWorks). The procedure described by Pelletier et al. was used to bin multiple pixel rows of the FPA in order to improve the signal-to-noise ratio in the spectra [21]. Since a PA-IR spectrometer is a dispersive instrument, a calibration was performed to convert the pixels to wavenumbers. The maxima of several PET bands in PA-IR and FT-IR spectra were determined using Matlab and Grams/ AI (ThermoFisher Scientific), respectively, and the Excel (Microsoft) solver tool was used to perform a least-square minimization using a third order polynomial function. A common practice in macroscopic polymer deformation studies is to average the results obtained from multiple samples
Fig. 1. Schematic layout of the planar array infrared (PA-IR) spectrometer with a dual-polarization configuration.
36
B. Farbos et al. / Vibrational Spectroscopy 51 (2009) 34–38
in order to reduce the data scatter and to average out sample-tosample variations. While the PET samples used in this work yielded highly reproducible results, it is important to emphasize that no sample averaging was performed since the objective was to illustrate the data quality that can be obtained in a single polarized PA-IR experiment. 3. Results and discussion Fig. 2 shows a series of p- and s-polarized spectra recorded at different draw ratios during the rapid deformation of a PET thin film to a draw ratio of 1.5, as well as after 3.3 s of relaxation. Each pair of polarized spectra was recorded simultaneously and in a single frame acquisition of 621 ms. In spite of this very short measurement time, the signal-to-noise ratio is sufficient to allow quantitative analysis at such time resolution. Because of the dualbeam configuration of the instrument, both polarized spectra originate from the same spot on the sample. This has the benefit of preventing potential artifacts that could arise in the case of nonperfectly homogeneous deformations. As expected, the two polarized spectra recorded at the earliest point of the deformation (l 1) are very similar since the initial sample is almost isotropic. A significant dichroism can be observed for several bands when the sample is drawn to l = 1.25. This is most obvious for the CH2 wagging deformation modes at 1340 and 1370 cm1 which are due to segments with glycol groups in trans and gauche conformation, respectively. Both show a positive dichroic difference, DA = Ap As, with values of 0.049 and 0.016, respectively, since their transition dipole moment is mostly parallel to the polymer chain. The bands at 1410 and 1505 cm1, attributed to benzene ring in-plane vibrations, also show parallel dichroism with DA = 0.039 and 0.024, respectively. For all bands, the dichroic difference reaches a maximum at l = 1.5, up to 0.100 and 0.027 for the 1340 and 1370 cm1 bands, respectively, and then decreases in the spectrum recorded 3.3 s after the end of the deformation because the polymer chains then relax toward the isotropic state. The band at 1410 cm1 is an interesting case since it has frequently been considered as insensitive to orientation and conformation. As such, it has often been used as an internal reference, in particular to take into account sample thickness variations in transmission experiments or the imperfect contact in attenuated total reflectance (ATR) measurements. We have recently shown using PM-IRLD that this band actually presents a small but non-negligible dichroism in cold drawn PET [18]. This observation is confirmed by the spectra of Fig. 2, in which a positive dichroic difference is clearly observed and is found to evolve with draw ratio as that of the other bands.
Fig. 2. p- and s-polarized infrared spectra of PET recorded with a 621-ms acquisition time before (l = 1), during (l = 1.25 and 1.5) and 3.3 s after a deformation at 90 8C.
Fig. 3. Evolution of the p-polarized, s-polarized and structural (A0) absorbances during and after a deformation to l = 1.5 for (a) the gauche (1370 cm1) and (b) the trans (1340 cm1) conformers. The full line shows the expected A0 if only sample thinning occurred during deformation.
The evolution of the p- and s-polarized absorbances for the gauche (1370 cm1) and trans (1340 cm1) conformers is shown in Fig. 3 as a function of time during the deformation and relaxation periods. The deformation lasted 438 ms (taking into account the acceleration and deceleration of the stretcher) and is indicated as negative times so that the relaxation time starts at 0 ms. The data were obtained by measuring band heights in spectra resulting from the co-addition of 10 frames of a single sample deformation. This yields an effective time resolution of 6.2 ms/spectrum, better than in any previous studies of irreversible polymer deformation by infrared spectroscopy. For the gauche conformers (Fig. 3a), both polarized absorbances decrease smoothly during deformation and then slightly increase during relaxation. A similar behavior is observed for the As of the trans conformers (Fig. 3b) but, in contrast, Ap increases during deformation and decreases during relaxation. These results are due to the combined effects of sample thinning, orientation and conformational changes. The effect of conformational changes can be more directly observed by calculating the structural absorbance of the sample as A0 = (Ap + 2As)/3. A0 corresponds to the absorbance that the sample would have if it was isotropic but had an otherwise identical structure. It can be observed in Fig. 3 that A0 decreases for both bands during stretching because of sample thinning. Indeed, assuming a deformation at constant volume with transverse isotropy, the absorbance of a band should decrease as the square root of the draw ratio during deformation, and then remain constant during relaxation. This expected behavior is shown as full lines and is not completely followed in practice. Rather, the A0 of the gauche band overshoots to lower values during deformation, followed by a gradual increase toward the expected absorbance during the relaxation. The opposite is observed for the trans band.
B. Farbos et al. / Vibrational Spectroscopy 51 (2009) 34–38
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Fig. 4. Evolution of the trans and gauche conformer fractions during and after a deformation to l = 1.5.
Fig. 5. Evolution of the orientation function, hP2i, of the trans and gauche conformers during and after a deformation to l = 1.5.
This behavior suggests that a reversible gauche-to-trans conformational change is taking place. To obtain more quantitative information, the gauche and trans fractions were calculated as Ftrans = A0.1340/(A0.1340 + kA0.1370) and Fgauche = 1 Ftrans. The factor k is the ratio of the absorption coefficients for the 1340 and 1370 cm1 bands and was previously estimated as 4.2 using the approach of Gue`vremont et al. [23]. Fig. 4 shows that Ftrans is 14.5% before stretching, in agreement with published data for amorphous PET [23,24]. The fraction of trans conformers increases during deformation at the expense of gauche conformers and reaches a maximum value of 17%. This conformational change is indeed reversible, with Ftrans rapidly decreasing back toward the initial value as soon as the deformation is stopped. Although the original value is not fully recovered yet after 3.3 s of relaxation, it is interesting to observe the very rapid initial recovery, with 50% of the excess trans conformers converting back to gauche within the first 300 ms of relaxation. This phenomenon would have been missed at lower time resolution. Clearly, the amplitude of the gauche-to-trans conversion is very small for a draw ratio of only 1.5. The low scatter in the results demonstrates the potential of polarized PA-IR spectroscopy to study such minute conformational changes with a time resolution of 6 ms or better. In addition to sample thinning and conformational changes, the evolution of molecular orientation could also be noted in Fig. 2 by observing the difference between Ap and As. The orientation function, hP2i, of the gauche and trans conformers was calculated as [4]:
amorphous PET at 90 8C [11,26]. This interesting phenomenon will be further investigated in the near future. The deformation of PET at a relatively low draw ratio of 1.5 generated small conformational changes and moderate orientation levels. In contrast, Fig. 6 shows that much more pronounced changes occur for a deformation to a draw ratio of 2. It can be observed that the orientation function of the trans conformers increases to a large value of 0.83, close to the maximum hP2i value of unity. Furthermore, in sharp contrast to Fig. 5, this orientation does not relax after the end of the deformation but it actually slightly increases as a function of time. Similarly, the fraction of trans conformers increases much more than in Fig. 3, eventually reaching a value of 40%. This conformational change appears to be irreversible over the time scale of a few seconds, in contrast with the observations at a draw ratio of 1.5. Analysis of the FT-IR spectrum of the quenched sample indicates that the increase in trans conformers fraction is mainly due to the formation of a mesophase rather than to a crystallization. Both these phases are constituted of trans glycol conformers but they can be distinguished by the position or shape of other bands sensitive to the conformation around the C–O bonds and phenyl rings [25]. A predominant mesophase was also observed in cold drawn PET samples [18]. Aggregates of segments in the mesophase could then act as effective physical cross-linking points across the sample to prevent the relaxation of orientation after deformation. The results obtained in this work clearly demonstrate the great potential of polarized PA-IR spectroscopy for the time-resolved characterization of polymer deformations. The data scatter in the previous figures is very good and depends on the number of frames co-added to obtain a given time resolution. The original spectral images were reprocessed by averaging anywhere from 1 to 300
hP 2 i ¼
Ap As 2 3 cos2 a 1 3A0
(1)
where a, the angle between the transition dipole moment and the chain axis, was assumed to be 188 for both bands [18,25]. Fig. 5 shows that the orientation increases more or less linearly during the deformation, goes through a maximum at l = 1.5, and then decreases as a function of time because of segmental relaxation of the polymer chains. As observed before [11,19], the orientation of the trans conformers is much larger than that of the gauche segments with maximum hP2i values of 0.35 and 0.08, respectively. Their relaxation kinetics are nevertheless very similar, as is that of the 1410 cm1 band. These results are in good agreement with those obtained by PM-IRLD and URS-FT-IR under similar experimental conditions [19]. Interestingly, a rapid initial decay of the orientation occurs on a timescale of a few hundreds of ms, similar to that noted in Fig. 4 for the trans-to-gauche conversion. Previous work using slower techniques and/or drawing rates had rather reported first relaxation times on the order of 7–19 s for
Fig. 6. Evolution of the trans segments orientation function and conformer fraction during and after a deformation to l = 2.
38
B. Farbos et al. / Vibrational Spectroscopy 51 (2009) 34–38
frames per spectrum. It was observed that the data scatter in the polarized absorbances, and therefore in the other calculations, decreases approximately as the square root of the number of averaged frames, as expected for a detector-noise limited instrument. It is thus possible to select a time resolution as good as 0.6 ms/spectrum, or to improve drastically the data quality by averaging more frames if a reduced time resolution is acceptable. It should be emphasized that even when averaging 300 frames, the effective time resolution is still better than 200 ms/spectrum. Polarized PA-IR spectroscopy provides a much better time resolution than PM-IRSAS, but the latter is convenient when studying slow deformations or long relaxation times since it provides a better sensitivity to very small dichroic differences and more easily manageable datasets. The two techniques are therefore complementary. On the other side, polarized PA-IR spectroscopy compares very favorably with URS-FT-IR spectroscopy for fast time-resolved studies. In particular, the noise level obtained in this work is significantly better than that in our previous URS-FT-IR study of the same PET sample [19]. PA-IR spectroscopy also provides a better time resolution and acquires both polarized spectra in a single experiment, which is a key element for polymer deformation studies. Its most significant drawback is that it only gives access to limited spectral window and resolution, rather than full spectra as in URS-FT-IR measurements. These parameters depend mainly on the size of the FPA detector, the grating selection, and the spectral region of interest. As illustrated here, even a comparatively narrow spectral range can often be adequate for obtaining the relevant information about the sample dynamics. 4. Conclusion The proof-of-concept results obtained in this work using PET films demonstrate that polarized PA-IR spectroscopy is a powerful tool for following fast polymer deformations and relaxations. The technique provides information about the orientation and structure of the sample in a single experiment with an excellent signal-to-noise ratio. It further offers a time resolution in the low or sub-ms regime, better than any other approach to date. It therefore has interesting potential for the scientific inquiry of polymer deformation and relaxation mechanisms as well as for the realtime monitoring of industrial polymer processing operations.
Acknowledgments The authors acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI). CP thanks K.C. Cole of the National Research Council of Canada for providing the PET samples. References [1] I.M. Ward, Structure and Properties of Oriented Polymers, 2nd ed., Chapman & Hall, London, 1997. [2] D.J. Blundell, D.H. MacKerron, W. Fuller, A. Mahendrasingam, C. Martin, R.J. Oldman, R.J. Rule, C. Riekel, Polymer 37 (1996) 3303. [3] A. Mahendrasingam, D.J. Blundell, C. Martin, W. Fuller, D.H. MacKerron, J.L. Harvie, R.J. Oldman, C. Riekel, Polymer 41 (2000) 7803. [4] T. Buffeteau, M. Pe´zolet, in: P.R. Griffiths, J.M. Chalmers (Eds.), Handbook of Vibrational Spectroscopy, John Wiley & Sons Inc., Chichester, 2001, pp. 693. [5] A.K. Kalkar, H.W. Siesler, I. Zebger, F. Pfeifer, A. Ameri, S. Michel, U. Hoffmann, in: P.R. Griffiths, J.M. Chalmers (Eds.), Handbook of Vibrational Spectroscopy, John Wiley & Sons Inc., Chichester, 2001, p. 2559. [6] J.P. Faivre, B. Jasse, L. Monnerie, Polymer 26 (1985) 879. [7] B. Jasse, J.L. Koenig, Journal of Polymer Science, Polymer Physics Edition 17 (1979) 799. [8] J.F. Tassin, A. Baschwitz, J.-Y. Moise, L. Monnerie, Macromolecules 23 (1990) 1879. [9] H.W. Siesler, Advances in Polymer Science 65 (1984) 1. [10] H.W. Siesler, I. Zebger, C. Kulinna, S. Okretic, S. Shilov, U. Hoffmann, in: G. Zerbi (Ed.), Modern Polymer Spectroscopy, Wiley–VCH, Weinheim, 1999, pp. 33. [11] C. Duchesne, X.H. Kong, J. Brisson, M. Pe´zolet, R.E. Prud’homme, Macromolecules 35 (2002) 8768. [12] T. Buffeteau, B. Desbat, M. Pe´zolet, J.M. Turlet, Journal de Chimie Physique 90 (1993) 1467. [13] F. Lapointe, M. Pe´zolet, J. Brisson, Polymer 48 (2007) 5626. [14] C. Pellerin, M.-E. Rousseau, R.E. Prud’homme, M. Pe´zolet, Applied Spectroscopy 56 (2002) 17. [15] Y.X. Wang, C. Pellerin, C.G. Bazuin, M. Pe´zolet, Macromolecules 38 (2005) 4377. [16] Y. Liang, D. Mauran, R.E. Prud’homme, C. Pellerin, Applied Spectroscopy 62 (2008) 941. [17] P.R. Griffiths, B.L. Hirsche, C.J. Manning, Vibrational Spectroscopy 19 (1999) 165. [18] C. Pellerin, M. Pe´zolet, P.R. Griffiths, Macromolecules 39 (2006) 6546. [19] C. Pellerin, R.E. Prud’homme, M. Pe´zolet, B.A. Weinstock, P.R. Griffiths, Macromolecules 36 (2003) 4838. [20] D.L. Elmore, M.W. Tsao, S. Frisk, D.B. Chase, J.F. Rabolt, Applied Spectroscopy 56 (2002) 145. [21] I. Pelletier, C. Pellerin, D.B. Chase, J.F. Rabolt, Applied Spectroscopy 59 (2005) 156. [22] C. Pellerin, C.M. Snively, D.B. Chase, J.F. Rabolt, Applied Spectroscopy 58 (2004) 639. [23] J. Gue`vremont, A. Ajji, K.C. Cole, M.M. Dumoulin, Polymer 36 (1995) 3385. [24] K.C. Cole, A. Ajji, E. Pellerin, Macromolecular Symposia 184 (2002) 1. [25] K.C. Cole, A. Ajji, E. Pellerin, Macromolecules 35 (2002) 770. [26] A.K. Oultache, X.H. Kong, C. Pellerin, J. Brisson, M. Pe´zolet, R.E. Prud’homme, Polymer 42 (2001) 9051.
Contents lists available at ScienceDirect
Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec
Time-resolved polymer deformation using polarized planar array infrared spectroscopy Baptiste Farbos, Damien Mauran, Christian Pellerin * Centre for Self-Assembled Chemical Structures (CSACS/CRMAA) and De´partement de chimie, Universite´ de Montre´al, Montre´al, QC, H3C 3J7 Canada
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 August 2008 Accepted 24 September 2008 Available online 4 October 2008
Polarized planar array infrared (PA-IR) spectroscopy is shown for the first time to be a powerful approach to study the mechanical deformation of polymers. A dual-beam PA-IR spectrometer was built to enable the simultaneous recording of parallel- and perpendicular-polarized spectra at the same sample spot. The technique provides orientation and structural information during and after fast irreversible deformations with a low-ms (or sub-ms) time resolution and a low data scatter. In proof-of-concept experiments, the possibilities of polarized PA-IR spectroscopy are illustrated by studying the deformation of poly(ethylene terephthalate) thin films. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Planar array infrared spectroscopy Time-resolved Polymer Orientation Conformation Poly(ethylene terephthalate)
1. Introduction The structure of a polymer, for instance its molecular orientation, degree of crystallinity and morphology, has a major impact on its physical properties. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and Raman scattering are amongst the most powerful approaches to study the orientation and structure of polymers [1]. While their static use is relatively common, performing time-resolved analyses during polymer processing has proven much more challenging, let alone at the high speeds relevant to industrial processes. The low quantum yield of the Raman inelastic scattering phenomenon generally precludes its use for such measurements. Time-resolved XRD studies with an excellent 50 ms resolution have been performed using a synchrotron source [2,3], but this is evidently unpractical for most applications. In contrast, time-resolved FT-IR spectra can be acquired using standard instrumentation and has therefore been widely used for dynamic studies of polymer deformation [4,5]. IR characterization of uniaxially oriented samples, such as oneway drawn films and fibers, requires the acquisition of two spectra
* Corresponding author at: De´partement de chimie, Universite´ de Montre´al, CP 6128 Succ. Centre-ville, Montre´al, QC, H3C 3J7 Canada. Tel.: +1 514 340 5762; fax: +1 514 340 5290. E-mail address: [email protected] (C. Pellerin). 0924-2031/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2008.09.010
polarized parallel (p) and perpendicular (s) with respect to the orientation direction. The dichroic difference provides information about molecular orientation, while details about the crystallinity, conformation, etc., can be obtained by calculating a structural absorbance spectrum. Early studies involved quenching samples below their glass transition temperature (Tg) in order to freeze the structure and allow static measurements [6–8]. Siesler recognized the limitations of this approach and built a rheo-optical spectrometer in which the polarizer was rapidly rotated by a pneumatic system, allowing both polarized spectra to be recorded in a matter of seconds [9]. The Siesler group used this method to follow in real time the slow deformation and relaxation of many polymers and copolymers [5,10]. Pe´zolet and coworkers have demonstrated that polarization modulation infrared linear dichroism (PM-IRLD) provides a much improved 400 ms time resolution, and used it to study various polymer systems [4,11–15]. We have very recently overcome a major limitation of PM-IRLD, the impossibility of obtaining the structural absorbance spectrum, in an approach called polarization modulation IR structural absorbance spectroscopy (PM-IRSAS) [16]. The best time resolution thus far has been obtained using an ultra-rapid-scanning FT-IR (URS-FT-IR) spectrometer developed by Manning Applied Technology and the Griffiths group [17]. The deformation of poly(ethylene terephthalate) (PET) above and below its Tg could be followed with time resolutions of 40 and 10 ms, respectively [18,19]. However, a serious drawback for deformation studies is that the technique records spectra in a
B. Farbos et al. / Vibrational Spectroscopy 51 (2009) 34–38
single polarization at a time, so that two perfectly repeatable samples are required to extract the orientation and structural information. Recently, Rabolt and coworkers have developed planar array infrared (PA-IR) spectroscopy, a technique that combines a spectrograph with a two-dimensional focal plane array (FPA) detector to enable true multichannel IR spectroscopy [20,21]. Pellerin et al. used it to follow the electric field-induced reorientation of a liquid crystal with an 8.7-ms time resolution [22]. It was demonstrated that the two polarized spectra could be recorded simultaneously by using different sections of the FPA without affecting the time resolution. In this paper, we demonstrate for the first time the potential of polarized PA-IR spectroscopy for the ultra-rapid dynamic study of polymer deformation and relaxation. It will be shown that orientation and structural information can be obtained with a ms or sub-ms time resolution while preserving an excellent noise level. The deformation of amorphous PET above Tg will be used as a model case. 2. Experimental 2.1. Samples and deformations Amorphous PET films with 7 mm thickness were prepared by blow molding at the National Research Council of Canada and cut into 20-mm long and 6-mm wide strips. Pyrotape (Aremco Products #546) was used to prevent slippage during the elongation. Deformations to draw ratios of 1.5 or 2 at a nominal speed of 3.21 cm/s (with acceleration and deceleration rates of 14.1 and 35.9 cm/s2, respectively) were performed at 90 8C (Tg + 12 8C) using a custom-built mechanical stretcher fitted with ZnSe windows to allow in situ measurements. A waveform generator (Agilent 33220A) was used to trigger simultaneously the FPA detector and the mechanical stretcher. 2.2. PA-IR spectrometer Fig. 1 shows a schematic layout of the new dual-polarization PA-IR spectrometer used in this work. The source was a silicon carbide element (Oriel 80030) powered at 12 V and 2 A using an Agilent E3634A power supply. The IR radiation was collected,
35
focused onto the sample in the stretcher, and re-collimated using three identical aluminum-coated off-axis parabolic (OAP) mirrors (Janos Technology) with a focal length of 15 cm. A beam stop was used before measurements to prevent sample heating by the IR source. The radiation was then separated into two beams using a gold-coated 908 prism mirror (Edmund Optics) and two OAP mirrors. A linear polarizer (KRS-5 or ZnSe) was inserted in each beam to polarize the radiation p and s with respect to the stretching direction. An identical arrangement of a 908 prism mirror with two other OAP mirrors was used to recombine the polarized beams. It was then focused onto the 300-mm wide entrance slit of a MicroHR imaging spectrograph (Horiba JobinYvon) equipped with a gold-coated plane reflection grating with 50 grooves/mm and a blazing wavelength of 6 mm. A 5.5-mm long-pass optical filter (Janos Technology) was inserted before the spectrograph to remove unwanted diffraction orders. Finally, a 128 128 mercury–cadmium–telluride (MCT) FPA detector (SBF161) with a NINOX compound lens was used to detect the radiation. The integration time was set to 123 ms and the frame rate to 1610 Hz, yielding a 621-ms acquisition time. This configuration allowed recording multiplex spectra over a spectral range from approximately 1630 to 1160 cm1, with an average optical resolution of 7–8 cm1. The signal-to-noise ratio could be improved at the expense of optical resolution by increasing the slit width, but it was not found to be necessary. Time-resolved PA-IR experiments were conducted by recording 6000 single frame spectral images for a total measurement time of 3.7 s. Background and dark background spectra were obtained by averaging 1000 frames with an open beam and with the beam blocked by a metal plate, respectively. All calculations were performed using custom-made programs in Matlab R2006a (MathWorks). The procedure described by Pelletier et al. was used to bin multiple pixel rows of the FPA in order to improve the signal-to-noise ratio in the spectra [21]. Since a PA-IR spectrometer is a dispersive instrument, a calibration was performed to convert the pixels to wavenumbers. The maxima of several PET bands in PA-IR and FT-IR spectra were determined using Matlab and Grams/ AI (ThermoFisher Scientific), respectively, and the Excel (Microsoft) solver tool was used to perform a least-square minimization using a third order polynomial function. A common practice in macroscopic polymer deformation studies is to average the results obtained from multiple samples
Fig. 1. Schematic layout of the planar array infrared (PA-IR) spectrometer with a dual-polarization configuration.
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in order to reduce the data scatter and to average out sample-tosample variations. While the PET samples used in this work yielded highly reproducible results, it is important to emphasize that no sample averaging was performed since the objective was to illustrate the data quality that can be obtained in a single polarized PA-IR experiment. 3. Results and discussion Fig. 2 shows a series of p- and s-polarized spectra recorded at different draw ratios during the rapid deformation of a PET thin film to a draw ratio of 1.5, as well as after 3.3 s of relaxation. Each pair of polarized spectra was recorded simultaneously and in a single frame acquisition of 621 ms. In spite of this very short measurement time, the signal-to-noise ratio is sufficient to allow quantitative analysis at such time resolution. Because of the dualbeam configuration of the instrument, both polarized spectra originate from the same spot on the sample. This has the benefit of preventing potential artifacts that could arise in the case of nonperfectly homogeneous deformations. As expected, the two polarized spectra recorded at the earliest point of the deformation (l 1) are very similar since the initial sample is almost isotropic. A significant dichroism can be observed for several bands when the sample is drawn to l = 1.25. This is most obvious for the CH2 wagging deformation modes at 1340 and 1370 cm1 which are due to segments with glycol groups in trans and gauche conformation, respectively. Both show a positive dichroic difference, DA = Ap As, with values of 0.049 and 0.016, respectively, since their transition dipole moment is mostly parallel to the polymer chain. The bands at 1410 and 1505 cm1, attributed to benzene ring in-plane vibrations, also show parallel dichroism with DA = 0.039 and 0.024, respectively. For all bands, the dichroic difference reaches a maximum at l = 1.5, up to 0.100 and 0.027 for the 1340 and 1370 cm1 bands, respectively, and then decreases in the spectrum recorded 3.3 s after the end of the deformation because the polymer chains then relax toward the isotropic state. The band at 1410 cm1 is an interesting case since it has frequently been considered as insensitive to orientation and conformation. As such, it has often been used as an internal reference, in particular to take into account sample thickness variations in transmission experiments or the imperfect contact in attenuated total reflectance (ATR) measurements. We have recently shown using PM-IRLD that this band actually presents a small but non-negligible dichroism in cold drawn PET [18]. This observation is confirmed by the spectra of Fig. 2, in which a positive dichroic difference is clearly observed and is found to evolve with draw ratio as that of the other bands.
Fig. 2. p- and s-polarized infrared spectra of PET recorded with a 621-ms acquisition time before (l = 1), during (l = 1.25 and 1.5) and 3.3 s after a deformation at 90 8C.
Fig. 3. Evolution of the p-polarized, s-polarized and structural (A0) absorbances during and after a deformation to l = 1.5 for (a) the gauche (1370 cm1) and (b) the trans (1340 cm1) conformers. The full line shows the expected A0 if only sample thinning occurred during deformation.
The evolution of the p- and s-polarized absorbances for the gauche (1370 cm1) and trans (1340 cm1) conformers is shown in Fig. 3 as a function of time during the deformation and relaxation periods. The deformation lasted 438 ms (taking into account the acceleration and deceleration of the stretcher) and is indicated as negative times so that the relaxation time starts at 0 ms. The data were obtained by measuring band heights in spectra resulting from the co-addition of 10 frames of a single sample deformation. This yields an effective time resolution of 6.2 ms/spectrum, better than in any previous studies of irreversible polymer deformation by infrared spectroscopy. For the gauche conformers (Fig. 3a), both polarized absorbances decrease smoothly during deformation and then slightly increase during relaxation. A similar behavior is observed for the As of the trans conformers (Fig. 3b) but, in contrast, Ap increases during deformation and decreases during relaxation. These results are due to the combined effects of sample thinning, orientation and conformational changes. The effect of conformational changes can be more directly observed by calculating the structural absorbance of the sample as A0 = (Ap + 2As)/3. A0 corresponds to the absorbance that the sample would have if it was isotropic but had an otherwise identical structure. It can be observed in Fig. 3 that A0 decreases for both bands during stretching because of sample thinning. Indeed, assuming a deformation at constant volume with transverse isotropy, the absorbance of a band should decrease as the square root of the draw ratio during deformation, and then remain constant during relaxation. This expected behavior is shown as full lines and is not completely followed in practice. Rather, the A0 of the gauche band overshoots to lower values during deformation, followed by a gradual increase toward the expected absorbance during the relaxation. The opposite is observed for the trans band.
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Fig. 4. Evolution of the trans and gauche conformer fractions during and after a deformation to l = 1.5.
Fig. 5. Evolution of the orientation function, hP2i, of the trans and gauche conformers during and after a deformation to l = 1.5.
This behavior suggests that a reversible gauche-to-trans conformational change is taking place. To obtain more quantitative information, the gauche and trans fractions were calculated as Ftrans = A0.1340/(A0.1340 + kA0.1370) and Fgauche = 1 Ftrans. The factor k is the ratio of the absorption coefficients for the 1340 and 1370 cm1 bands and was previously estimated as 4.2 using the approach of Gue`vremont et al. [23]. Fig. 4 shows that Ftrans is 14.5% before stretching, in agreement with published data for amorphous PET [23,24]. The fraction of trans conformers increases during deformation at the expense of gauche conformers and reaches a maximum value of 17%. This conformational change is indeed reversible, with Ftrans rapidly decreasing back toward the initial value as soon as the deformation is stopped. Although the original value is not fully recovered yet after 3.3 s of relaxation, it is interesting to observe the very rapid initial recovery, with 50% of the excess trans conformers converting back to gauche within the first 300 ms of relaxation. This phenomenon would have been missed at lower time resolution. Clearly, the amplitude of the gauche-to-trans conversion is very small for a draw ratio of only 1.5. The low scatter in the results demonstrates the potential of polarized PA-IR spectroscopy to study such minute conformational changes with a time resolution of 6 ms or better. In addition to sample thinning and conformational changes, the evolution of molecular orientation could also be noted in Fig. 2 by observing the difference between Ap and As. The orientation function, hP2i, of the gauche and trans conformers was calculated as [4]:
amorphous PET at 90 8C [11,26]. This interesting phenomenon will be further investigated in the near future. The deformation of PET at a relatively low draw ratio of 1.5 generated small conformational changes and moderate orientation levels. In contrast, Fig. 6 shows that much more pronounced changes occur for a deformation to a draw ratio of 2. It can be observed that the orientation function of the trans conformers increases to a large value of 0.83, close to the maximum hP2i value of unity. Furthermore, in sharp contrast to Fig. 5, this orientation does not relax after the end of the deformation but it actually slightly increases as a function of time. Similarly, the fraction of trans conformers increases much more than in Fig. 3, eventually reaching a value of 40%. This conformational change appears to be irreversible over the time scale of a few seconds, in contrast with the observations at a draw ratio of 1.5. Analysis of the FT-IR spectrum of the quenched sample indicates that the increase in trans conformers fraction is mainly due to the formation of a mesophase rather than to a crystallization. Both these phases are constituted of trans glycol conformers but they can be distinguished by the position or shape of other bands sensitive to the conformation around the C–O bonds and phenyl rings [25]. A predominant mesophase was also observed in cold drawn PET samples [18]. Aggregates of segments in the mesophase could then act as effective physical cross-linking points across the sample to prevent the relaxation of orientation after deformation. The results obtained in this work clearly demonstrate the great potential of polarized PA-IR spectroscopy for the time-resolved characterization of polymer deformations. The data scatter in the previous figures is very good and depends on the number of frames co-added to obtain a given time resolution. The original spectral images were reprocessed by averaging anywhere from 1 to 300
hP 2 i ¼
Ap As 2 3 cos2 a 1 3A0
(1)
where a, the angle between the transition dipole moment and the chain axis, was assumed to be 188 for both bands [18,25]. Fig. 5 shows that the orientation increases more or less linearly during the deformation, goes through a maximum at l = 1.5, and then decreases as a function of time because of segmental relaxation of the polymer chains. As observed before [11,19], the orientation of the trans conformers is much larger than that of the gauche segments with maximum hP2i values of 0.35 and 0.08, respectively. Their relaxation kinetics are nevertheless very similar, as is that of the 1410 cm1 band. These results are in good agreement with those obtained by PM-IRLD and URS-FT-IR under similar experimental conditions [19]. Interestingly, a rapid initial decay of the orientation occurs on a timescale of a few hundreds of ms, similar to that noted in Fig. 4 for the trans-to-gauche conversion. Previous work using slower techniques and/or drawing rates had rather reported first relaxation times on the order of 7–19 s for
Fig. 6. Evolution of the trans segments orientation function and conformer fraction during and after a deformation to l = 2.
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frames per spectrum. It was observed that the data scatter in the polarized absorbances, and therefore in the other calculations, decreases approximately as the square root of the number of averaged frames, as expected for a detector-noise limited instrument. It is thus possible to select a time resolution as good as 0.6 ms/spectrum, or to improve drastically the data quality by averaging more frames if a reduced time resolution is acceptable. It should be emphasized that even when averaging 300 frames, the effective time resolution is still better than 200 ms/spectrum. Polarized PA-IR spectroscopy provides a much better time resolution than PM-IRSAS, but the latter is convenient when studying slow deformations or long relaxation times since it provides a better sensitivity to very small dichroic differences and more easily manageable datasets. The two techniques are therefore complementary. On the other side, polarized PA-IR spectroscopy compares very favorably with URS-FT-IR spectroscopy for fast time-resolved studies. In particular, the noise level obtained in this work is significantly better than that in our previous URS-FT-IR study of the same PET sample [19]. PA-IR spectroscopy also provides a better time resolution and acquires both polarized spectra in a single experiment, which is a key element for polymer deformation studies. Its most significant drawback is that it only gives access to limited spectral window and resolution, rather than full spectra as in URS-FT-IR measurements. These parameters depend mainly on the size of the FPA detector, the grating selection, and the spectral region of interest. As illustrated here, even a comparatively narrow spectral range can often be adequate for obtaining the relevant information about the sample dynamics. 4. Conclusion The proof-of-concept results obtained in this work using PET films demonstrate that polarized PA-IR spectroscopy is a powerful tool for following fast polymer deformations and relaxations. The technique provides information about the orientation and structure of the sample in a single experiment with an excellent signal-to-noise ratio. It further offers a time resolution in the low or sub-ms regime, better than any other approach to date. It therefore has interesting potential for the scientific inquiry of polymer deformation and relaxation mechanisms as well as for the realtime monitoring of industrial polymer processing operations.
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