Influence of Composition on Ultraviolet Absorption Spectra Styrene-Alkyl Methacrylate and Styrene-Alkyl Acrylate Random Copolymers

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1 ANALYTICAL SCIENCES FEBRUARY 1990, VOL Influence of Composition on Ultraviolet Absorption Spectra Styrene-Alkyl Methacrylate and Styrene-Alkyl Acrylate Random Copolymers of Sadao MORI Department of Industrial Chemistry, Faculty of Engineering, Mie University, Tsu, Mie 514, Japan The ultraviolet (UV) absorption spectra of styrene-alkyl methacrylate and -alkyl acrylate copolymers in chloroform solution altered with changing the styrene content. The molar absorption coefficients at and nm were constant over the entire range of composition, while those at and nm decreased with decreasing the styrene content. Hypochromic effects at the latter two wavelengths were observed. Wavelengths of nm and nm can be used as the key bands for the determination of the copolymer composition by UV. The bathochromic shift for the UV spectra of the styrene-alkyl methacrylate copolymers and the hypsochromic shift for those of the styrene-alkyl acrylate copolymers were observed. The hypochromic effect was independent of the types of comonomers; the ester group of the comonomer did not have any influence on UV absorption for the phenyl group. However, a methyl group attached to an a-carbon atom of the methacrylate monomer shifted the peak maxima toward lower absorption energies and a hydrogen atom on an a-carbon atom of the acrylate monomer toward higher absorption energies. Keywords Ultraviolet absorption spectra, copolymer, styrene-alkyl methacrylate copolymer, hypochromic effect, bathochromic shift, hypsochromic shift copolymer, styrene-alkyl acrylate Among a number of methods for the analysis of a copolymer composition are ultraviolet (UV) spectroscopy, infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and elemental analysis. Since the determination of the composition of copolymers by UV or IR is dependent on the accuracy of the absorptivities associated with the components, which are not always coincident with those of the homopolymers, the absolute error in measurements by UV or IR analysis is usually unknown. Evans et al. determined the composition of styrenemethyl methacrylate copolymers, (S-MMA), by pyrolysis gas chromatography, NMR and carbon analysis, and observed that the results were in excellent agreement.' However, different compositional results have been reported using various methods, even for the same copolymer samples.2,3 Forrette and Rozek studied the compositional analysis of styrene-isobutylene copolymers by IR, NMR and carbon-hydrogen elemental analysis, and observed that appropriate corrections were required for the use of IR as an analytical technique.2 Gruber and Elias analyzed (S-MMA) copolymers by IR, UV, NMR, elemental analysis and the differential index of refraction method, and observed that the UV method gave a higher MMA content in copolymers.3 Deviations of the UV absorption spectra originated in the styrene units in styrene containing copolymers from those of polystyrene (PS). For example, deviations from the additivity of the responses of the individual styrene units have been reported in the literature: the hypochromic effect of (S-MMA) copolymers in solutions4-6 and the changes of molar absorption coefficients at 269 nm for styrene in styrene-centered triads of different comonomers.' These deviations have been attributed to a change in the length of the styrene sequences5'6,s-io and to a conformation of the molecules in solution.4 However, Gracia-Rubio' concluded that these deviations observed in the molar absorption coefficients of styrene copolymers can be attributed simply to experimental error. Styrene-acrylate, styrene-methacrylate and styreneacrylonitrile copolymers have UV absorption spectra which originate from the styrene units in the copolymers in the 250 to 280 nm region, where the comonomer units (acrylate, methacrylate and acrylonitrile units) are transparent. Thus, the UV absorption spectra around 250 and 260 nm for the copolymers have been widely used as key bands for composition analysis. Further, the chemical heterogeneity of the copolymers has been measured by size-exclusion chromatography (SEC)-a dual-detector system, such as UV and refractive-index detectors.11,22 It is therefore important to estimate the effects of the composition and sequence length of

2 28 ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6 styrene units of comonomer units in the copolymers on the molar absorption coefficients at peaks around 250 and 280 nm. In the present work, the UV absorption spectra for styrene-alkyl methacrylate and styrene-alkyl acrylate copolymers were systematically studied by changing the composition of the copolymers. Bathochromic and hypsochromic shifts with the composition have been observed and some bands have shown a hypochromic effect with decreasing the styrene content in the copolymers. polymerization, as reported previously.14 The styrene content in each copolymer was measured by UV spectroscopy. Sample copolymers were dissolved in chloroform. A calibration curve was constructed with PS by plotting the absorbance at 260 nm against the PS concentration. The composition of (S-MMA) copolymers having narrow CCD was also measured by proton NMR spectroscopy. The composition of the copolymers was calculated from the integral areas of the phenyl peak to the total proton peak as follows: Experimental styrene mol%= 100X8(phenyl peak area) 5(total proton peak area) (1) Apparatus A UV-VIS spectrophotometer (Model UVIDEC- 610C, Japan Spectroscopic Co.) was used for measurements of the UV absorption spectra of styrene-alkyl acrylate and styrene-alkyl methacrylate copolymers. This spectrophotometer is operated with the assistance of a microcomputer. Two thousand data points at a wavelength interval of 1 / 50 of 2 scale can be stored in memory. The operational variables used to measure the UV spectra were: band width (resolution), 0.1 nm; time constant, 8 s; scan speed, 4 nm/ min; A set, nm; A scale, 5 nm/ cm; chart speed, 2 mm/ min; noise sensitivity value, 0.01; and quartz cells, 1-cm path length. To estimate the peak wavelengths at the UV absorption maxima, fourth-derivative spectra were measured. The derivatization scheme was as follows: smoothing of the spectra was carried out using every data point; the smoothed spectra were magnified 5 times; differentiation of the spectra was made every two data points; smoothing of the differentiated spectra was made every data point; these magnification, differentiation and smoothing procedures were repeated four times in this order; fourth-derivative spectra were finally printed out. The NMR spectra of (S-MMA) copolymers were recorded on a JEOL 60 MHz spectrometer (Model C- 60H). Deuterated chloroform was used as the solvent with tetramethylsilane as an internal standard. Samples (S-MMA) copolymers of narrow chemical composition distribution (CCD) were the same as those prepared previously.13 Styrene copolymers with ethyl methacrylate (EMA), butyl methacrylate (BMA), methyl acrylate (MA), ethyl acrylate (EA) and butyl acrylate (BA) were prepared by solution polymerization in benzene at a low degree of conversion. After polymerization, the products were purified by dissolving them in chloroform, followed by pouring the solutions into methanol in order to precipitate them. This purification was repeated twice. For (S-EA) and (S- BA) copolymers, hexanol was used instead of methanol as a poor-solvent. High-conversion (S-MMA) copolymers having broad CCD were also prepared by bulk Size-exclusion chromatography For fractionation of the copolymers, SEC was carried out by using a high performance liquid chromatograph with SEC columns (Shodex A80M). The mobile phase was chloroform. The copolymers were dissolved in chloroform and three fractions were obtained for each copolymer. The compositions of the fractionated copolymers were measured by IR spectroscopy. Results and Discussion The UV absorption spectra of (S-MMA) copolymers, (S-MA) copolymers, PS and PMMA are presented in Figs. 1 and 2. Absorption peaks and shoulders can be seen at about 256, 259, 262, 265 and 269 nm. It is clearly observed that the shapes of the UV absorption spectra of the copolymers altered with changing the styrene content in the copolymers. With decreasing the styrene content in the copolymers, the peak shapes for the copolymers altered considerably as follows: peak c decreased its intensity compared to peak b and became a shoulder to peak b; peak d became clear; peak e decreased its intensity compared to peak d; peak d for (S-MA) copolymers became clearer than that for (S- MMA) copolymers. Generally speaking, PMMA is transparent above 250 nm. However, the foot of the UV absorption spectrum of PMMA appears in the region of the benzenoid absorption band. The UV spectrum of PMMA in the region 250 to 280 nm is shown in Fig. 1 F. The molar concentration of PMMA in Fig. 1 F was the same for that of PS in Fig. la and the absorbance of PMMA was less than 1% of that of PS. Specific and molar absorption coefficients at several wavelengths for PS and PMMA are listed in Table 1. The contribution of UV absorption for MMA units to the absorbance of (S-MMA) copolymers was 3% when the composition of the copolymer was 20 mol% styrene, and 9% when it was 10 mol% styrene. Therefore, the contribution of the UV absorption for MMA units in the copolymers was negligible, except for the case where the copolymers had a styrene content lower than

3 ANALYTICAL SCIENCES FEBRUARY 1990, VOL Fig. 1 UV absorption spectra of (S-MMA) copolymers, PS and PMMA. Spectra: (A), PS; (B)-(E), copolymers. Styrene mol%: (B), 85.5; (C), 66.3; (D), 48.7; (E), 15.2; (F), PMMA. Concentration, g/l: (A), 0.339; (B), 0.349; (C), 0.501; (D), 0.587; (E), 1.957; (F), mol%. To estimate the variation of the UV absorption spectra for the (S-MMA) copolymers quantitatively, the molar absorption coefficients at the peaks of the styrene units in the copolymers were calculated. The composition of the copolymers was determined by proton NMR. The molar absorption coefficients were obtained by determining the absorbance at the specified wavelength and the styrene molar concentration in the sample solution. The results are shown in Fig. 3 as a function of the styrene content. The molar absorption coefficients given in Fig. 3 were measured at wavelengths of UV peak maxima of PS. Although UV wavelengths at the peak maxima of the copolymers changed slightly with the composition (as discussed later), the molar absorption coefficients obtained at the peak maxima of the copolymers agreed with those at the wavelengths of the peak maxima of PS within the experimenal error. The contribution of the UV absorption of the MMA units in the copolymers was corrected for by subtracting the UV absorbance corresponding to the MMA units from the total absorbance of the copolymers by using Table 1. Fig. 3 Molar absorption coefficients of the styrene unit in (S-MMA) copolymers at several wavelengths as a function of the styrene content. Wavelength: (a), nm; (b), nm; (c), nm; (d), nm. The styrene content was measured by proton NMR. Table 1 Specific and molar PS and PMMA absorption coefficients for Fig. 2 UV absorption spectra of (S-MA) copolymers and PS. Spectra: (A), PS; (B) - (E), copolymers. Styrene mol%: (B), 65.7; (C), 51.3; (D), 35.6; (E), Concentration, g/1: (A), 0.339; (B), 0.381; (C), 0.607; (D), 0.627; (E), a, specific coefficient. absorption coefficient; e, molar absorption

4 30 ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6 The molar absorption coefficients of styrene units at and nm were constant over the entire range of the composition, while those at and nm decreased with decreasing the styrene content in the copolymers. Hypochromic effects at the latter two wavelengths were observed. Similar tendencies have been seen in copolymers of (S-MA), (S-EA), (S-BA), (S-EMA) and (S-BMA). Therefore, the selection of wavelengths which do not show hyperchromic or hypochromic effects with composition is very important for the determination of the copolymer composition. Wavelengths at and nm for the styrenealkyl acrylate and styrene-alkyl methacrylate copolymers can be used as the key bands for composition analysis. Although the hypochromic effects at and nm for (S-MMA) copolymers have been reported in the literature5'10, the hypochromism at nm was also observed (wavelengths in the literature were 259.5, and 269 nm). This observation is inconsistent with the results observed in this work. Though the reason is not clear yet, several factors in addition to experimental error' are probably involved, such as the differences of the instrumental conditions of UV spectrophotometers used for the UV measurements. The smallest value of the molar absorption coefficients for the styrene units in (S-MMA) copolymers was found for the alternating copolymer, where every styrene uint is flanked by MMA units on both sides.5 The UV spectra of the random copolymers might become similar to those for the alternating copolymers when the styrene content in the random copolymers is lower than 10%. The ratio of the molar absorption coefficients for a random copolymer having 20 mol% styrene and PS at nm5 was 0.690; in this work it was The latter value corresponds to a copolymer having 50 mol% styrene5, where the decrease in the molar absorption coefficient at nm was only 2.5% that of PS. Similarly, the average sequence length of styrene units in a copolymer having 20 mol% styrene in our work was about 2.5, according to data from the literature. 10 The observations by Gallow and Russo4 were different from those from the literature5"0, and from our results as well. They observed that there were three different parts of the plot of the molar absorption coefficients at nm and the styrene content in (S- MMA) random copolymers (wavelength from the literature was nm): part a (from 0 to 25 mol% styrene) and part c (from 80 to 100 mol% styrene), where the same plot as PS has been obtained, and part b (from 25 to 80 mol% styrene), where the molar absorption coefficients of the random copolymers were lower than that of PS. The maximum decrease in the molar absorption coefficient corresponded to a copolymer containing 50 mol% styrene. A correction for the contriburion of alkyl acrylate or alkyl methacrylate units to the UV absorbance at the key bands is required. The molar absorption coefficients cannot be directly applied to the correction, since the Fig. 4 Bathochromic shift with the MMA content of peak maxima for UV spectra of (S-MMA) copolymers. Wavelength at peak maxima for PS: (a), nm; (b), nm; (c), nm; (d), nm; (e), nm. The styrene content was measured by UV. concentration of the copolymer solutions was expressed as weight per volume, not moles per volume. The specific absorption coefficient is used instead and the styrene weight fraction in the copolymers, x is calculated by using A=asbxC+aMMAb(1-x)C (2) where A is the absorbance at a key band, as and amma are the specific absorption coefficients for PS and PMMA at the key band, respectively, b is the cell length, and C is the concentration of the copolymers in the solution. The unit for C is g/ l. The peak maxima of the UV spectra for (S-MMA) copolymers shifted toward higher wavelengths with increasing the MMA content. The bathochromic shift with the MMA content is shown in Fig. 4. The tendencies of the bathocromic shift for the UV spectra of (S-EMA) and (S-BMA) copolymers were the same for those of (S-MMA) copolymers, while the peak maxima of the UV spectra for (S-EA) and (S-BA) copolymers shifted toward lower wavelengths with the EA or BA content. The bathochromic shift for (S- EMA) and (S-BMA) copolymers and the hypsochromic shift for (S-EA) and (S-BA) copolymers are shown in Fig. 5. The styrene content in the copolymers was measured by UV. The peak maxima of the UV spectra for the copolymers were estimated by a fourth-order derivatization of the UV spectra. The hypsochromic shifts for (S-MA) copolymers were also observed, though the results are not shown in Fig. 5. Regression lines were obtained by a least-square method. The UV peak positions of the copolymers in almost

5 ANALYTICAL SCIENCES FEBRUARY 1990, VOL Table 2 Styrene content and the ratio nm and nm of absorbances at Fig. 5 The bathochromic shift for UV spectra of (S-EMA) and (S-BMA) copolymers and the hypsochromic shift for those of (S-EA) and (S-BA) copolymers. Wavelength at peak maxima for PS: (a), nm; (b), nm; (c), nm. Copolymer: (0) S-EA; (0) S-BA; ( ) S-EMA; (U) S-BMA; (X) peak wavelength for PS. a. Calculated from Fig. 3 by knowing composition of each fraction. all cases were regarded as being fixed, and were observed 5' 10 However, a hypochromism accompanied by a very small shift toward higher wavelengths was also reported.4 In this work, the hypochromic effect and the bathochromic and hypsochromic shifts with decreasing styrene content in the copolymers were observed systematically. The hypochromic effect is mainly due to electronic interactions between the ester group and the phenyl ring.4 This effect is independent of the kind of comonomers; that is to say, the alkyl group attached next to the ester group of the comonomers does not affect the UV absorption for the phenyl groups. However, a methyl group attached to an a-carbon atom of the comonomers (methacrylates) shifted the peak maxima toward lower absorption energies and a hydrogen atom on an a- carbon atom (acrylates) toward higher absorption energies. These results tell us that the changes of peak intensities or the shifts of wavelengths with a change in the composition are not entirely attributed to the experimental error but, rather, mostly to the various sequence lengths of the styrene unit, or to conformation changes in the copolymers. The copolymers used here were random and statistical, and the average sequence length of the styrene units in the copolymers was assumed to decrease statistically with decreasing the styrene content. The positions and molar absorption coefficients of the peaks are dependent on the content of comonomer units and the sequence length distribution of the copolymers. Therefore, UV absorption measurements may be used as one method for estimating the sequence length of styrene units in the copolymers. Garcia-Rubio et al. determined the average sequence length of the styrene units for styrene-acrylonitrile copolymers as a function of molecular size by SEC.15 Absorbances at wavelengths of 254 and 269 nm were used for calculating the average length of the styrene sequences. Although the proposed parameters of the relation between the absorbances and the average length of the styrene sequences for (S-MMA) copolymers were not determined in this work, the ratio of the absorbances at wavelengths of and nm for the fraction by SEC gives some information regarding the styrene sequence length as a function of the molecular size. The results are shown in Table 2. Fraction 1 was eluted first and fraction 3 last, i.e., fracrtion 1 has a higher molecular weight than does fraction 3. Copolymer (S-MMA) III was prepared by solution polymerization and is supposed to have a relatively narrow CCD. Copolymers H-3 and H-7 were prepared by bulk polymerization and are supposed to have broader CCDs. It can be assumed that if the ratio of absorbances obtained experimentally is near that calculated from Fig. 3 by knowing the composition of each fraction, then the randomness of the styrene units is estimated to be average; in other words, the average length of styrene sequences is statistical. Fraction 2 of (S-MMA) III copolymer represents this case. The ratio obtained for fraction 1 of (S-MMA) III was higher than the calculated value, while the ratio obtained for fraction 3 was lower than that calculated value. These results tell us that fractions at the higher molecular weight portions have a block nature, and those at the lower molecular weight portion have a smaller length of styrene sequences than does the average length considered from composition. Fractions of H-3 and H-7 copolymers have a block nature and differences among fractions were not observed. The author wishes to express his gratitude to Mr. M. Mouri and Miss K. Akachi for their technical assistance.

6 32 ANALYTICAL SCIENCES FEBRUARY 1990, VOL. 6 References 1. D. L. Evans, J. L. Weaver, A. K. Mukherji and C. L. Beatty, Anal. Chem., 50, 857 (1978). 2. J. E. Forrette and A. L. Rozek, J. Appl. Polym. Sci., 18, 2973 (1974). 3. U. Gruber and H. G. Elias, Makromol. Chem., 86, 168 (1965). 4. B. M. Gallo and S. Russo, J. Macromol. Sci. Chem., A8, 521 (1974). 5. B. Stutzel, T. Miyamoto and H. J. Cantow, Polymer J., 8, 247 (1976). 6. K. F. O'Driscoll, W. Wertz and A. Husar, J. Polym. Sci., Al, 2159 (1967). 7. L. H. Garcia-Rubio, J. Appl. Polym. Sci., 27, 2043 (1982). 8. R. J. Brussau and D. J. Stein, Angew. Makromol. Chem., 12, 59 (1970). 9. E. Gruber and W. Knell, Makromol. Chem., 179, 733 (1978). 10. Z. Wojtczak and M. Switata, Makromol. Chem., 187, 2427 (1986). 11. S. Mori, J. Liq. Chromatogr., 4, 1685 (1981). 12. S. Mori, J. Chromatogr., 411, 355 (1987). 13. S. Mori, Y. Uno and M. Suzuki, Anal Chem., 58, 303 (1986). 14. S. Mori, Anal. Chem., 60,1125 (1988). 15. L. H. Garcia-Rubio, J. F. MacGregor and A. E. Hamielec, "Polymer Characterization: Spectroscopic, Chromatographic, and Physical Instrumental Methods", Chap. 17, in ACS Advances in Chemistry Series, No. 203, American Chemical Society, Washington, D. C., (Received May 29, 1989) (Accepted August 11,1989)