poly(vinyl acetate) copolymer in aqueous solution

Eur. Polym. J. Vol. 24, No. 6, pp. 505-509, 1988

0014-3057/88 $3.004-0.00 Copyright © 1988PergamonPress plc

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PHOTOCROSSLINKING OF PARTIALLY CINNAMOYLATED POLY(VINYL ALCOHOL)/POLY(VINYL ACETATE) COPOLYMER IN AQUEOUS SOLUTION HEIKKI TENHU, JUTTA NUORTILA-JOKINENand FRANCISKASUNDHOLM Department of Wood and Polymer Chemistry, University of Helsinki, Meritullinkatu 1 A, 00170 Helsinki, Finland (Received 11 September 1987)

Abstract--A photocrosslinkable water-soluble polymer has been prepared by partial esterification of poly(vinyl acetate-co-vinyl alcohol) with cinnamic acid. The photochemical [2 + 2] cycloaddition reaction has been used as a chemical probe for the investigation of different conformational states of the polymer chain in aqueous solution. At around 20°, the polymer adopts a compressed conformation with the hydrophobic side-groups turned out from the coil, this conformation leading to intercatenary crosslinki~ig during irradiation. At temperatures lower or higher than 20°, the polymer coil expands; the intercatenary reaction is no longer dominant. The dynamics of the polymer in solution, as well as the interaction between water molecules and the polymer, have been studied with nitroxide sonds and calorimetrically. Crosslinking is shown to prevent the conformational change detected in the parent polymer around 20°; crosslinking also affects the water-polymer interaction.

INTRODUCTION Photodimerization of cinnamic acid, as well as photocrosslinking of its polymeric derivatives, has been extensively studied; these polymers are known as the first negative photoresists [1, 2]. Irradiation of cinnamic esters of polymers in the solid state may be supposed to cause mainly intercatenary crosslinking due to overlap of the polymer coils. In solution, however, inter- and intracatenary reactions compete. Shindo et al. [3] have studied the change in polymer coil dimension during the [ 2 + 2 ] cycloaddition reaction in tetrahydrofuran solution. Using poly(2-cinnamoyloxyethyl methacrylate) and copolymers of 2-cinnamoyloxyethyl methacrylate and methyl methacrylate, they showed that in very dilute solution the polymer coil contracts due to predominant intracatenary photodimerization. Increasing the polymer concentration was shown to lead to an increase in molar mass due to competing intra- and intercatenary reactions. Several new photocrosslinkable polymers based on poly(vinyl alcohol) (PVAL) have been synthesized recently, to increase the photosensitivty with or without a sensitizer. Some PVAL derivatives have been compared by Ichimura and Watanabe, who suggested that the aggregation of photofunctional groups is an important factor affecting the sensitivity of the polymers [4]. Poly(vinyl cinnamate) was used as a standard in this comparison. We have chosen to prepare partially cinnamoylated PVAL mainly because the cycloaddition reaction seemed to serve as an interesting method for studying conformational states of the polymer in water. Poly (vinyl acetate-co-vinyl alcohol) (PVAA) containing 10-20% residual acetate groups is readily soluble in 505

water at room temperature, the copolymer with 20% acetate groups having a solubility maximum at 20 ° [5]. Eagland et al. [6, 7] have concluded that in dilute aqueous solutions the polymer undergoes a change in conformation as a function of temperature, particularly in the region of 20°. Thus a thermally forbidden photocycloaddition reaction performed at different temperatures should give information about the conformation of the polymer as a whole, as well as about the association of the hydrophobic side-groups. We have investigated not only the rate of reaction but also the dynamics of the polymer solutions with nitroxide free radicals before and after irradiation. Furthermore, we were interested to see whether crosslinking affects the interaction between water molecules and the polymer. This has been investigated by DSC, studying the thermal behaviour of water associated with the polymer.

EXPERIMENTAL

The polymer used was PVAL with 88% degree of hydrolysis, and molar mass 125,000 (BDH Chemicals). Esterification was performed in pyridine by adding 3.5 g of trans-cinnamoyl chloride (Aldrich) to 50 g of polymer. After 12 hr the polymer was precipitated with methanol. After two reprecipitations from aqueous solution, the product was dialysed for several days against distilled water and then dried. The intrinsic viscositieswere 90, 35 and 85 ( _+5) cm3/g at 10, 20 and 50°, respectively. The cinnamoyl content was determined from the u.v. spectrum of the product by the absorption maximum at 275 nm [8] with cinnamic acid as a standard. The product was estimated to contain 1.5 cinnamoyl groups per 100 constitutional repeating units. The cycloaddition reaction was followed by the same method.

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HEIKKI TENHU et al.

The irradiation procedure was as follows. A 200 W low pressure Hg lamp, in a Pyrex tube with circulating thermostated water, was immersed in deaerated 5% (w/v) aqueous polymer solution. The whole reaction system was kept in a thermostated water bath. The reaction (usually 90 min) was conducted at three temperatures, viz. 10, 20 and 50°. Samples from each solution were taken at 15-min intervals. Ultraviolet spectra were taken for each sample, and the viscosities were measured at 20 ° with an Ubbelohde viscometer, The 5% polymer solutions were investigated with two nitroxide sonds, viz. 2,2,6,6-tetramethylpiperidine-l-oxyl

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(TEMPO) and 2,2,6,6-tetramethyl-4-piperidinol-l-oxyl (TEMPOL), dissolved in the samples (~
RESULTS

AND

DISCUSSION

By comparing the intrinsic viscosity data at different temperatures, we conclude that the polymer coil is contracted at 20 ° . The change in conformation detected at infinite dilution is supposed to occur also in more concentrated solutions, although it is difficult to verify with a flow viscometer due to the high shear [9]. As shown in Fig. 1, the viscosity of a 5% solution of the polymer gives a straight line as a function of temperature. Ten-fold dilution of this solution reveals discontinuities in the viscosity vs temperature curve. We used the 5% solution in the crosslinking reactions because of the reasonable reproducibility of the viscosity data. The relative amount of the reactive double bonds measured by u.v. spectroscopy is reported in Fig. 2 for three samples irradiated at 10, 20 and 50 °, respectively. It can be seen that the reaction is slowest at 20 °, and is second order in character. At 50 ° the cycloaddition reaction is fast: over 40% of the double bonds react during the first 15 min, after which the reaction continues slowly. The shape of the curve obtained at 10 ° is intermediate; however, 40% of the double bonds react during 45 min. It is interesting to note that, in the study of cinnamoylated polymer in tetrahydrofuran solution by Shindo et al. [3], the

1 30

I

I

50

I

I

70

T (*C)

Fig. 1. The inherent viscosity of a 5% ( 0 ) and a 0.5% (O) aqueous solution of the copolymer as a function of temperature. change in concentration of cinnamate groups due to the cyclodimer formation could be expressed as a straight line; - d [ C ] / d t was independent of polymer concentration. Following the arguments of these authors, we suppose that at 50 ° the reaction is mainly intracatenary, the quantum yield changing during the reaction. At 20 ° intra- and intercatenary reactions compete. The data in Fig. 2 should be compared with the inherent viscosities presented in Fig. 3. The viscosity gradually increases only in the sample irradiated at 20 °. Since the viscosity remains practicall3~ constant in the other two samples, it is concluded that, in addition to the dominating intracatenary crosslinking reaction, there is also some intercatenary crosslinking at 10 and 50 °. If the reaction was exclusively intracatenary, some decrease in the viscosity should be observed. At 20 ° the intercatenary reaction becomes dominant; this is in accordance with the second order kinetics observed. 1.0

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30

I

i

60

I

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90 TIME (rain)

Fig. 2. The relative amount of the reactive double bonds as a function of irradiation time. Irradiation temperature: lO° (O), 20 ° (O) and 50° (A).

Photocrosslinking of PVAA

where m, is the nitrogen nucleus spin quantum number. A, B and C contain terms arising from the hyperfine interaction tensor A and the spectroscopic g tensor. They are dependent on the correlation time. Terms B and C can be experimentally obtained from the peak heights of the three hyperfine lines and the line width of the central line in the observed spectra. This method is based on the assumptions of axial symmetry of the radical and of isotropic motion of the probe. Due to these assumptions, the calculated correlation times are only estimates which, however, may well be used to compare the samples. The anisotropy of motion has been evaluated by the simple method developed by Wasserman [16], using a parameter E:

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70 I

I

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507

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Fig. 3. The inherent viscosity of the samples at 20° as a function of irradiation time. The symbols refer to irradiation temperature and are the same as in Fig. 2. At first sight, these results seem rather confusing, because it would be logical to expect the crosslinking to occur intercatenarily when the polymer coil is expanded. Thus we conclude that at 20 ° the hydrophobic side-groups (cinnamate and acetate groups) are, at least to some extent, turned out from the contracted polymer coil into the surrounding water. In this conformation, the reactive groups are randomly distributed in the polymer solution and the crosslinking proceeds slowly. At lower or higher temperature, the polymer coil expands and the hydrophobic groups turn towards the inside of it. The associated cinnamate groups inside the polymer react rapidly; after this initial stage, the reaction continues very slowly. Notice that in Fig. 3 the viscosities of the samples irradiated at 10 or 50 ° decrease slightly at the beginning of the irradiation; this could be an indication of an intracatenary reaction as a first stage of crosslinking. More detailed information of the system under study has been obtained with two nitroxide sonds with different polarities, TEMPO and TEMPOL. It is well-known that the shape of the ESR spectrum of a nitroxide free radical varies as a function of the rotational motion of the radical, and is thus affected by the rate and possible anisotropy of the diffusional motion of the probe [10]. Some recent examples of the application of the spin probe method to aqueous systems include studies of water adsorbed on silica surfaces [11, 12] and a study of micellar solutions [13]. Watanabe et al. [14] studied aqueous PVAL gels with different crosslink densities using five nitroxide sonds. We have used nitroxides in a study of aqueous solutions and gels of poly(N-vinyl-2-pyrrolidone), PVP [15]. The spectra obtained from the liquid samples were typical motionally narrowed spectra, the correlation times for the rotational motion of the sonds being in the range 10-aLl0 -9 sec. The correlation times were calculated in the usual way [10] based on the expression for the line width, 1/T2:

1/T:= A + B m l + C m ~

(2)

E = (B + C)/(B - C).

I

90 TIME (rain)

(1)

B and C have here the same meaning as in expression (1), and parameter E can thus be calculated from the intensities of the peaks in the experimental spectrum. Recently, Hamada et al. have successfully utilised this parameter in the analysis of the mobility of spin probes in nylon films [17, 18]. Correlation times of the two sonds in a 5% aqueous solution of the polymer before irradiation are presented in Fig. 4. The correlation time for TEMPOL varies linearly with temperature, whereas the less polar TEMPO clearly shows a minimum around 20 ° . The data indicate faster motion for TEMPO at 20 ° than at other temperatures. This is in accordance with the model suggested above: the less polar sond evidently tends to associate with the hydrophobic side-groups of the polymer and thus its diffusion is faster when these groups are turned outwards from the polymer coil. Correlation times of the sonds in irradiated samples are shown in Figs 5 and 6. In the sample irradiated at 20 ° , the conformation change occurs above 30 ° probably due to restriction by intercatenary crosslinks. Samples irradiated at 10 and 50 ° closely resemble each other. The conformation change is absent due to extensive cycloaddition reaction which has fixed the hydrophobic cinnamoyl groups into the interior of the coil.

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Fig. 4. The correlation time, ~R, for TEMPO (O) and TEMPOL (O) in 5% aqueous polymer solution as a function of temperature.

508

HEXKKI

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Fig. 5. Trt for TEMPO (O) and TEMPOL ( 0 ) as a function of temperature in a solution irradiated 60 rain at 20°. The values obtained for the parameter E are in the range - 0 . 6 to 0 for TEMPO and 0.6 to 0.2 for TEMPOL. This indicates that the motion of both radicals is anisotropic. We define the axis system of the nitroxide molecule with the z-axis along the 2pn-orbital of nitrogen, the x-axis along the N - - O bond and the y-axis perpendicular to these. It seems clear that TEMPO rotates anisotropically around the x-axis, and TEMPOL most probably around the y-axis. We conclude that TEMPOL is attached to the hydrophilic polymer segment with a hydrogen bond, and reflects the motion of this segment. This is the reason for the Arrhenius-type temperature dependence of the rotational motion of TEMPOL, seen in Figs 4-6. The activation energy E~ for the rotational motion calculated for TEMPOL from the slope of the straight line zR vs 1/T in the temperature range 0--50 ° is 11.3 kJ/mol in the sample before irradiation. Ea is 11.0, 26.6 and l l.4kJ/mol in the samples irradiated at 10, 20 and 50 °, respectively. Only the

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Fig. 6. Tg for TEMPO (closed symbols) and TEMPOL (open symbols) as a function of temperature in the solutions irradiated 60 min at 10° (0, ©) or 50° (A, A).

TENHU

et

al.

intercatenary crosslinking occurring at 20 ° slows down the motion of the hydrophilic polymer segments. We take this as further evidence for the model suggested above; cycloaddition reaction between the groups which are already associated inside the polymer coil leaves the hydrophilic part of the macromolecule unaffected. There are some difficulties in the analysis of the ESR spectra, probably due to inhomogeneous line broadening effects discussed by Watanabe et al. [14]. As is well-known, a decrease in solvent polarity results in a decrease in the nitrogen coupling constant. However, the electronic distribution of the nitroxide molecule is differently affected by different solvent parameters, and it is correlated with the conformation of the radicals. Recently, Ottaviani has discussed this matter in a profound analysis of the spectra of two neutral nitroxides, in ethanol and pyridine respectively [19]. There are some deviations from linearity for TEMPOL in Figs 4-6; and the situation is even more complex with TEMPO. Changes of the micro-environment of the latter nitroxide are detected as small changes in nitrogen isotropic coupling constant with varying temperature. Due to the complexity of the system under study, we have considered it sufficient at this stage to compare the samples with the rather qualitative methods above. Finally we have investigated, caiorimetrically, the changes in water-polymer interaction, produced by crosslinking. It is well established that, in an aqueous polymer sample, there is some of the water which is not capable of crystallizing even at temperatures far below the freezing temperature of pure water. In previous studies of the effect of crosslinking on water-polymer interactions, we concluded that the amount of non-freezable water associated with the polymer decreases, if the crosslinking reaction leads to more compact structures than in the original material [20, 21]. Hatakeyama et al. have studied gels of PVAL obtained either by ?-irradiation [22] or by freezing aqueous PVAL solutions [23]. They have showed that the frozen water in the gel melts in two fractions; these are called bound (or restrained) and free water, respectively. Furthermore, in these studies the amount of the non-freezable water was estimated to be about 0.5 g/g polymer in the case of irradiated gel, and over 2 g/g in the gel obtained by freezing the solution. It is noteworthy that, in the case of PVAL gel obtained by ~,-irradiation, Hatekeyama et al. concluded that the amount of non-freezable water is almost independent of crosslink density. However, the authors have used gels swollen in equilibrium with water, and the amount of water in strong interaction with the polymer is thus expressed not only as a function of crosslink density but also as a function of concentration. Most remarkable variations were reported in the ratio between free and bound (restrained) water. In this study the melting thermogram of water in the copolymer was found to be complex showing three distinct melting peaks. Thermograms of two samples containing 42 wt% polymer are shown in Fig. 7. Thermograms obtained of the samples crosslinked at 20 and 50 ° are exactly similar to that of the linear polymer. The peak appearing at the highest

Photocrosslinking of PVAA

509

with decreasing temperature; below 7° the solution viscosity is reported to be lower than that of pure water. Summarizing, we conclude that in an aqueous solution the partially esterified PVAA undergoes a change in conformation as a function of temperature, very similar to that reported for the parent polymer [6,7]. Crosslinking by photocycloaddition either moves this change to higher temperature or prevents it totally, depending on whether the reaction has occurred inter- or intracatenarily. REFERENCES

-2b -1'0 T (*C) Fig. 7. Melting endotherms of water in a polymer sample before irradiation (left) and after 60 min irradiation at 10° (ri~t).

temperature (about 0°) may be defined as "free" water. The amount of non-freezable water was 0.62 g/g polymer in samples with a polymer content of 20 wt%. Increasing the amount of polymer to 42 wt% revealed some small differences between the samples, the amount of non-freezable water then being 0.54, 0.47, 0.45 and 0.50g/g in the linear polymer and polymers crosslinked at 10, 20 and 50°, respectively. The intercatenarily crosslinked sample has the lowest content of non-freezable water. However the cycloaddition reaction does not change the micro-environment inside the pores of the polymer enough to affect dramatically the formation of ice crystals. Some expected differences exist between the samples both in the ratio of "free" to "bound" water as well as in the amount of non-freezable water, although they are probably too small to allow any definite conclusions. The melting thermograms show that the sample crosslinked at 10° differs from the polymers crosslinked at higher temperatures: the ratio of "free" to "bound" water is clearly different. This gives a reason to suppose that the association of the hydrophobic groups is not the only factor affecting the conformation of the copolymer and its crosslinked derivatives. PVAL chains are known to form hydrogen bonds between the hydroxyl groups; this is the reason for the low water solubility of pure PVAL. Lowering the temperature of the solution leads to an increase in the amount of hydrogen bonds; this is evidently the reason for the observed difference. The sample crosslinked at 10° contains more hydroxyl groups accessible for interaction with water molecules, than do the samples crosslinked at higher temperatures. This matter clearly needs further investigation. However, it has been found [7] that the properties of an aqueous PVAA solution change

1. B. Duncalf and A. S. Dunn. Polyvinyl Alcohol-Properties and Applications (Edited by C. A. Finch), p. 461. Wiley, London (1973). 2. V. Ramamurthy and K. Venkatesan. Chem. Rev. 87, 433 (1987). 3. Y. Shindo, T. Sugimura, K. Horie and I. Mita. Eur. Polym. J. 22, 859 (1986). 4. K. Ichimura and S. Watanabe. J. Polym. Sci. Chem. 20, 1419 (1982). 5. K. Toyoshima. Polyvinyl Alcohol--Properties and Applications (Edited by C. A. Finch), p. 17. Wiley, London (1973). 6. D. Eagland and N. J. Crowther. Faraday Syrup. chem. Soc. 17, 141 (1982), 7. F. F. Vercauteren, W. A. B. Donners, R. Smith, N. J. Crowther and D. Eagland. Eur. Polym. J. 23, 711 (1987). 8. Y. Shindo, K. Horie and I. Mita. Chem. Lett. 639 (1983). 9. D. Eagland, G. C. Wardlaw and I. Thorn. Colloid Polym. Sci. 256, 1073 (1978). 10. P. L. Nordio. Spin Labeling--Theory and Applications, Vol. 1 (Edited by L. J. Berliner), p. 5. Academic Press, New York (1976). 11. G. Martini, M. F. Ottaviani and M. Romanelli. J. Colloid Interface Sci. 94, 105 (1983). 12. M. Romanelli, M. F. Ottaviani and G. Martini. J. Colloid Interface Sci. 96, 373 (1983). 13. M. F. Ottaviani, P. Baglioni and G. Martini. J. phys. Chem. 87, 3146 (1983). 14. T. Watanabe, T. Yahagi and S. Fujiwara. J. Am. chem. Soc. 102, 5187 (1980). 15. F. Sundholm and H. Tenhu. Acta polytech, scan. 178, 85 (1987). 16. A. M. Wasserman and A. L. Kovarski. Spin Probes and Sonds in the Physical Chemistry of Polymers. Nauka, Moscow (1986). In Russian. 17. K. Hamada, T. Iijima and R. McGregor. ~,lacromolecules 19, 1443 (1986). 18. K. Hamada, T. lijima and R. McGregor. Macromolecules 20, 215 (1987). 19. M. F. Ottaviani. J. phys. Chem. 91, 779 (1987). 20. H. Tenhu and F. Sundholm. Eur. Polym. J. 22, 629 (1986). 21. H. Tenhu, O. Rimpinen and F. Sundholm. Biological and Synthetic Polymer Networks (Edited by O. Kramer). Elsevier, New York (in press). 22. T. Hatakeyama, A. Yamauchi and H. Hatakeyama. Eur. Polym. J. 20, 61 (1984). 23. T. Hatakeyama, A. Yamauchi and H. Hatakeyama. Eur. Polym. J. 23, 361 (1987).