Journal of Applied Spectroscopy, Vol. 79, No. 4, September, 2012 (Russian Original Vol. 79, No. 4, July–August, 2012)
DETERMINATION OF THE DEGREE OF CRYSTALLINITY OF POLY(VINYL ALCOHOL) BY FTIR SPECTROSCOPY O. N. Tretinnikov* and S. A. Zagorskaya
UDC 541.64:543.422.4
Quantitative correlations between the intensity of the crystalline band at 1144 cm–1 (A1144) normalized to the intensity of the C–O stretching band at 1094 cm–1 (A1094) in IR spectra of PVA films and the degree of crystallinity (α) of these films measured by an x-ray diffraction method were investigated. It was found that α and A1144/A1094 were related by the linear dependence α (%) = a + b(A1144/A1094) with correlation coefficient 0.999 for α values in the range 18–60%. Keywords: FTIR spectroscopy, poly(vinyl alcohol), degree of crystallinity, x-ray diffraction. Introduction. Poly(vinyl alcohol) (PVA) is a water-soluble synthetic polymer that is known for its resistance to organic solvents, thermal stability, low gas permeability, good adhesive properties, biocompatibility, and biodegradability [1]. Owing to these properties, it has a broad spectrum of practical applications including optical and biomedical materials; sorbents; ultrafiltration, gas-separation, and ion-exchange membranes; and protective and binding coatings. PVA is a partially crystalline polymer. The crystalline phase consists of plate-like single crystals (lamellae) 10–20 nm thick that are bound to each other by disordered polymer chains and form an amorphous polymer phase. The content of the crystalline phase in PVA (degree of crystallinity) is an important structural characteristic that determines many macroscopic properties of the polymer. Its mechanical properties improve, the water-resistance increases, and the permeability to gases and liquids decreases as the degree of PVA crystallinity increases. Therefore, it is important to obtain quantitative data on the PVA crystallinity in materials based on it in order to understand, predict, and control the operating characteristics of such materials. The degree of crystallinity of polymers (α) can be determined by x-ray diffraction, IR and NMR spectroscopy, calorimetry, and density measurements [2]. X-ray diffraction is the most direct and reliable method. However, it requires expensive equipment and significant time expenditures to take measurements so that it cannot be used in routine investigations. The density of the amorphous and crystalline phases must be known in order to determine α from density measurements. However, their true values for PVA are unknown because completely amorphous or completely crystalline samples of this polymer cannot be produced. Calorimetric measurements give the crystallinity of the sample o near its melting point (210–220 C for PVA). Therefore, the resulting α values correspond not to the initial state of the studied sample but to its condition resulting from the thermal action on the sample during recording of the thermograms. NMR spectroscopy determines α values from the content of protons with decreased mobility. However, it should be kept in mind when evaluating the reliability of the results that the mobility of protons can be restricted not only by crystallinity but also as the result of forming disordered aggregates with strong intermolecular bonds. IR-Fourier spectroscopy is a highly sensitive and useful method for analyzing the stereochemical and conformational structures of polymer chains, the molecular interactions between them, and their orientational and crystalline ordering [3]. As a rule, IR spectra of partially crystalline polymers contain crystallinity bands, i.e., absorption bands due to molecular vibrations in polymer crystalline regions. However, the extinction coefficients of these absorption bands cannot be determined from IR spectra if independent data on the degree of crystallinity are unavailable. Therefore, IR spectroscopy cannot be a direct method for determining α. The quantitative correlation between the intensity of a crystallinity band (normalized to the intensity of a reference band in order to exclude the effect of sample thick∗
To whom correspondence should be addressed.
B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68 Nezavisimost’ Ave., Minsk, 220072, Belarus; e-mail:
[email protected]. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 79, No. 4, pp. 538–543, July–August, 2012. Original article submitted February 10, 2012. 0021-9037/12/7904-0521 ©2012 Springer Science+Business Media New York
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ness) and α values obtained by an independent method must be known in order to determine the degree of polymer crystallinity from its IR spectrum. We know of only two studies [4, 5] addressing the establishment of this relation–1 ship for PVA. It was reported [4] that the ratio of intensities of the crystallinity band at 1144 cm and the band at 854 (A1144/A854) in attenuated total reflection Fourier-transform IR (ATR-FTIR) spectra of PVA films was related to α by the expression α = a(A1144/A854) + b.* However, the values of constants a and b were not reported [4]. Therefore, the results cannot be verified or used. An expression relating the degree of crystallinity of PVA films to the ratio –1 of band intensities at 1144 and 1096 cm (A1144/A1096) in transmission IR spectra of these films was α = 0.820(A1144/A1096) [5]. Unfortunately, this expression also cannot be verified or used in practice because the procedure for measuring the baselines for determining the intensities of the used analytical bands was not indicated. Furthermore, α values in both studies [4, 5] were determined from polymer density measurements. This method is not sufficiently accurate because of the aforementioned deficiencies. The goal of our work was to develop a method for determining the degree of PVA crystallinity using FTIR spectroscopy that was based on quantitative correlations between the relative intensity of a PVA crystallinity band in IR spectra of specially prepared PVA films with various degrees of crystallinity and the values of this characteristic measured using x-ray diffraction. Experimental. PVA of grade 16/1 (GOST 10779-78) was used. The polymer had degree of hydrolysis 98.7%, average degree of polymerization 1400, and sodium acetate content ≤0.6 mass%. PVA film of thickness 4–6 μm was prepared from a 4% aqueous solution of the polymer by pouring onto the bottom of polystyrene Petri dishes (Cell Culture Dish, Corning) with subsequent drying at room temperature for 1–3 d, after which the films were separated from the dish. Films with increased crystallinity were prepared by annealing films formed at room temperature for 1 o h at 90, 120, 135, and 150 C. Films with decreased crystallinity were prepared by adding LiCl to them. For this, a 4% PVA solution containing 0.1 M LiCl was prepared. Composite PVA–LiCl films were prepared from it analogously to the pure PVA films. IR spectra of films were recorded on a Nicolet Nexus 670 FT-IR Fourier-spectrometer with resolution –1 2.0 cm−1 by averaging 64 scans in the range 4000–400 cm . Spectra were analyzed and processed mathematically using the Omnic 7.3 software. X-ray studies were carried out on an HZG 4A diffractometer (Carl Zeiss, Jena) at poo tential 40 kV and current 30 mA using Ni-filtered CuKα-radiation with point-by-point detection in 0.1 steps. The complicated diffraction scattering curve was deconvoluted into fundamental Lorentz-shaped bands using the Origin 6.1 software. The details of the deconvolution procedure are given below. The degree of crystallinity was calculated using the formula α = Ic/(Ic + Ia), where Ic is the total intensity of all scattering bands of the polymer crystalline phase and Ia, the intensity of the amorphous-phase scattering band. Results and Discussion. IR spectroscopy has been applied many times to the study of PVA structure. Early studies were aimed at finding the absorption bands due to amorphous and crystalline polymer regions and stereochemically ordered parts of the polymer chains. A detailed bibliography of these studies was published [6]. Later work [4, 5, 7–11] was dedicated to studies of the effect of bound water on PVA crystallinity during its hydration and dehydration and the change of crystallinity upon adding nano-sized fillers and inorganic salts to the polymer composition. With two exceptions [4, 5], the degree of crystallinity itself and its relative changes from sample to sample were not determined from IR spectra. The crystallinity in all instances was estimated from the intensity of the absorption band at –1 1144 cm . Some researchers assigned it to ν(C–O) stretching vibrations in C–OH groups of the crystalline polymer phase; others, to ν(C–C) stretching vibrations of the carbon framework of the polymer chain in the crystalline phase. Obviously, the issue of assigning the vibration type is not critical with respect to using this band to determine α. Figure 1 shows IR spectra of a PVA film poured from an aqueous solution and dried at room temperature o and of the same film after crystallization by annealing at 150 C for 1 h. The observed large decreases of intensity for –1 bands with maxima at about 3340, 1660, and 640 cm that resulted from annealing the film were caused by removal from it of absorbed water [8]. With respect to spectral changes due to increased PVA crystallinity, they appeared most –1 strongly and clearly as an intensity increase of the absorption band at 1144 cm that was located on the high-fre*
The positions of the PVA crystallinity absorption maximum given in the literature vary in the range 1141–1145 cm–1. By definition (regardless of the literature source), we will use the value 1144 cm–1 that corresponds to the frequency obtained in our experiments.
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Fig. 1. IR spectra of poly(vinyl alcohol) film poured from aqueous solution and dried at room temperature (1) and the same film crystallized by annealing at 150oC for 1 h (2); spectra at an increased scale in the range 1200–930 cm−1 are shifted along the ordinate for clarity. *
–1
quency tail of the broad band of ν(C–O) stretching vibrations with a maximum at 1094 ± 2 cm . This band was the –1 most suitable as a reference band because it was the least removed from the band at 1144 cm . This point was important for analyzing samples by ATR-FTIR where the error in determining the ratio of band intensities that was due to non-ideal contact of the reflecting element with the sample increased quickly with increasing spectral gap between –1 the bands [12]. Figure 1 shows the range 1200–930 cm in which the analytical absorption bands used in the proposed method for determining α were situated. The procedure for constructing the baseline relative to which the peak intensities of the analytical bands (A1144 and A1094) were measured is also shown. The baseline passed through minima in the absorption spectrum that were located at the edges of the examined spectral range. Figure 2 shows the IR spectrum of a PVA film obtained from an aqueous polymer solution with added LiCl. –1 The lack of the peak at 1144 cm was interesting. Instead of it, only a weak shoulder on the high-frequency tail of the examined curve was observed. This indicated that the film had a low degree of crystallinity. We explained this as the inhibition of PVA crystallization by Li ions. It was interesting that adding halides of other alkali metals (K, Na, Cs) led to a substantial increase of crystallinity for this polymer [11]. The availability of a PVA sample with low crystallinity could expand the scope of applicability of the developed method for determining α to the range of small α values. Because the crystallinity band maximum in the measured spectrum was not observed in this instance, appropriate mathematical methods for processing the spectra were used to determine accurately its frequency. Figure 2 shows the spectrum of the PVA–LiCl film with the resolution enhanced mathematically using Fourier deconvolution. It can be seen that the crystallinity band maximum in this instance is practically the same as its position in the spec–1 trum of pure PVA film, 1143.8 and 1144.0 cm , respectively (Fig. 1). The intensity of the spectrum at the frequency equal to that of the crystallinity band maximum that was determined by Fourier deconvolution or any other method for separating hidden bands in a complicated spectrum was taken as the intensity of the crystallinity band that was not resolved in the experimental spectrum in the proposed method for determining α. The degree of crystallinity was determined by x-ray diffraction using the same PVA films that were used to record IR spectra. The shape of the scattering curve of the amorphous phase must be known in order to measure accurately by this method the degree of crystallinity. It was determined by analyzing the scattering curve of the PVA film with added LiCl that had, as noted above, low crystallinity. Figure 3a shows the diffraction pattern of this film and the result of deconvoluting the scattering curve into fundamental Lorentz-shaped components. The positions of the *
The position of the ν(C–O) band maximum varied in the range 1092–1096 cm–1 depending on the degree of PVA crystallinity. In all instances in the text (for clarity), we call it the band at 1094 cm–1.
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Fig. 2. IR spectrum of poly(vinyl alcohol) film prepared from an aqueous solution of the polymer with added LiCl (1) and its Fourier deconvolution (2).
Fig. 3. Diffraction patterns of poly(vinyl alcohol) film prepared from an aqueous solution with added LiCl (a) and prepared from an aqueous solution and annealed at 150oC (b). crystalline peak maxima in the diffraction pattern of highly crystalline film (Fig. 3b) were used as the starting posio tions for separation into components. The half-widths were set at 2 . The starting position and half-width of the amoro o phous band were 20 and 10 . The iteration was at first carried out only for band intensities at these fixed values of position and half-width. Then, the limitation on the position and half-width of all bands was removed, after which the final result was obtained from the last iteration. Deconvolution into components of diffraction patterns of other samples (with higher crystallinity) was carried out analogously with the exception that the half-width of the amorphous o band during the whole iteration process was constant and equal to its value for the low crystalline sample (19 ). The o resulting positions of scattering maxima of the polymer crystalline phase (2θ = 11.2, 16.3, 19.7, and 22.8 ) agreed well with the literature values [13]. Table 1 gives the ratio of intensities of the crystallinity and reference bands (A1144/A1094) in IR spectra of the studied PVA films and the degrees of crystallinity of these films that were obtained using x-ray diffraction. It can be seen that the lower limit of α could be extended to 17.8% by using the PVA film with added LiCl. The maximum α value was 60.2% for the PVA film annealed at 150oC. It is noteworthy that an attempt to produce a sample with o o even higher crystallinity by annealing a film at 165 C was unsuccessful. Annealing at 165 C gave lower crystallinity o than annealing at 150 C and increased the ratio A1144/A1094. It is well known [14] that PVA begins to be destroyed at o temperatures 150 C. As a result, hydroxyls are lost and polyene and other chemical structures are formed. The reason for the reduced α and simultaneous increased A1144/A1094 was most likely chemical destruction of the polymer. There524
Fig. 4. Calibration curve for determining degree of crystallinity of poly(vinyl alcohol) from its IR spectrum. TABLE 1. Values A1144/A1094 and Degree of Crystallinity α of Poly(vinyl alcohol) Films Prepared Under Various Conditions Solvent
Anneal temperature, oC
A1144 ⁄ A1094
α, %
H2O + LiCl H2O H2O H2O H2O H2O
– – 90 120 135 150
0.348 0.534 0.575 0.663 0.718 0.819
17.8 34.8 38.8 46.2 50.8 60.2
fore, IR spectroscopy should not be used to estimate the crystallinity of PVA samples annealed at temperatures exceeding its threshold of thermal stability. Figure 4 shows a calibration curve for determining the degree of PVA crystallinity from its IR spectrum that was produced based on the data in Table 1. It obeyed the linear function
α(%) = −13.1 + 89.5(A1144 ⁄ A1094) .
(1)
The correlation coefficient between the measured degree of crystallinity and that predicted by Eq. (1) was 0.999. The absolute error of determining α using Eq. (1) was ±0.3%. –1 Conclusion. Quantitative correlations between the intensity of the crystallinity band at 1144 cm (A1144) nor–1 malized to the intensity of the C–O stretching band at 1094 cm (A1094) in IR spectra of PVA films and the degree of crystallinity (α) of these films that was measured by x-ray diffraction were studied. It was found that α and A1144/A1094 were related by the linear function α(%) = a + b(A1144/A1094), where a = –13.1 and b = 89.5. The absolute error of determining α of PVA using IR spectroscopy and this expression was ±0.3% for α values in the range 18– 60%. IR spectroscopy cannot be used to determine α values of PVA annealed at temperatures exceeding its threshold of thermal stability.
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