Synthesis, bulk characterization, and surface characterization of p-hydroxystyrene/styrene copolymers

Synthesis, Bulk Characterization, and Surface Characterization of

p-Hydroxystyrene/Styrene Copolymers

DEBESH MAJUMDAR and BUDDY D. RATNER,* Department of Chemical Engineering7 BF-10, University of Washington, Seattle,

Washington 981 95

Synopsis

Styrene/p-hydroxystyrene copolymers were prepared from copolymers of styrene and p-benzyloxystyrene by cleavage of the ether bond. Nuclear magnetic resonance spectroscopy was used to characterize the polymers before and after the ether cleavage reaction. Reactivity ratios were calculated by applying the method of least squares to data generated from the Fineman-Ross equation ( r , = 0.37, r . = 0.28). The surface chemistry of centrifugally cast films of the homopoly- mers and the copolymers was studied by using electron spectroscopy for chemical analysis (ESCA). The Zisman contact angle method was used to determine the critical surface tension in air for each polymeric surface. Water contents of hydrated films were determined gravimetrically. The polar character of the surface was shown to increase to a small degree with an increase in the p-hydroxystyrene component. Polymers with high p-hydroxystyrene contents did not exhibit pronounced hydrogel character.

INTRODUCTION

Polymeric materials have had a long history of medical and surgical appli- cation. In an effort to understand how one physicochemical property, the surface polar/apolar character, affects the biological response to synthetic polymers, we devised a model system consisting of polymers and copolymers of styrene (S) and p-hydroxystyrene (HS).

The rationale for the development of this model system for biological studies evolved from ow experience with other polymeric systems having both hydrophilic and hydrophobic character. In earlier studies, poly(2-hydroxy- ethyl methacrylate/ethyl methacrylate) and poly(acrylamide/N, N-dibu- tylacrylamide) copolymer systems were prepared and examined.'s2 Both of these systems varied in a graded predictable fashion from a hydrophilic extreme to a hydrophobic extreme. For these polymer series, as the surface polar character increased, both the equilibrium water content and the surface compliance increased. Significant water content and high compliance are characteristics associated with hydrogels. In using such polymer surfaces to study biological interactions, interpretation of the experiments was com- plicated by multiple variables (i.e., was the parameter under study surface polarity, surface compliance or water content?). Therefore, the HS/S copolymer series was developed with the expectation of observing a graded in-

*To whom correspondence should be addressed.

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 26, 1991-2002 (1988) 0 1988 John Wiley & Sons, Inc. CCC 0360-6376/88/081991-12$04.00

1992 MAJUMDAR AND RATNER

crease in the surface polar character without the concurrent development of hydrogel character as the p-hydroxystyrene component is increased.

The synthesis of high molecular weight poly(p-hydroxystyrene) from p- acetoxystyrene and also from p-benzyloxystyrene was reported by F e r r ~ t i . ~ ? ~ The polymerization of HS has produced low molecular weight p ~ l y m e r s . ~ ~ ~ . ~ Moreover, the free radical polymerization of HS has often produced polymers whose structures are not fully polyvinylic.' Catala and Brossas have reported the synthesis of a hydroxystyrene/styrene copolymer from polystyrene; however, the introduction of the hydroxyl group was not specific (i.e., meta : para = 2 : 1).

In this paper, we describe the copolymerization and determine the reactivity ratio of styrene and p-benzyloxystyrene. The p-benzyloxystyrene/styrene copolymers were converted to HS/S copolymers by reaction with hydrobromic acid. The reaction method used is shown in Scheme 1.

We examined the copolymers by using nuclear magnetic resonance (NMR) spectroscopy before and after the ether cleavage reaction. We also used electron spectroscopy for chemical analysis (ESCA) and contact angle measurements tQ characterize the surface chemistry and surface energy of these copolymers. Investigations of the interactions of these polymers with living cells have been described else~here.~

Styrene ? 0

{H 2

p- Benz ylox yst y rcne Pol y(styrene/p- benzyloxyttyrene)

O H

Pol y(styrene/p- hydroxystyrene)

Scheme 1. Steps used to obtain poly(styrene/p-hydroxystyrene).

p-HYDROXYSTYRENE/SmRENE COPOLYMERS 1993

EXPERIMENTAL

Monomer Preparation

To prepare the p-benzyloxystyrene required for the copolymerization with styrene, the synthesis steps shown in Scheme 2 were performed.

Synthesis of p-Benzybxyacetopheenone

To synthesize p-benzyloxyacetophenone, the method of Sulzbacher was utilized with some modifications.1° A mixture of 136 g (1 mol) p-hydroxy- acetophenone, 121.5 mL (1.1 mol) benzyl chloride, 308 g (2.2 mol) anhydrous potassium carbonate, 50 mg sodium iodide, and lo00 mL 2-butanone was refluxed for 20 h. The reaction mixture was cooled and poured into 2000 mL cold water. The resulting mixture was extracted three times with 400 mL portions of ether. The combined ether extract was washed three times with 150 mL portions of 10% aqueous sodium hydroxide solution, washed twice with water, dried, and evaporated under vacuum to yield an oily residue which crystallized spontaneously. After trituration with petroleum ether (bp 35-6OoC), the p-benzyloxyacetophenone was recrystallized from isopropyl alcohol. It melted at 97-98°C (Sulzbacher reported 96-97"Clo), and the yield was 195 g (86%). The 'H-NMR spectrum (CDC1, solvent) exhibited peaks at 62.56 (s, 3, C o g , ) , 5.17 (s, 2, OCH,), 7.06 (d, 2, 0-Ar), - 7.46 (m, 5, aromatic) and 7.66 (d, 2, CO- Ar). -

Synthesis of p-Bemyhxystyrene

p-Benzyloxystyrene was synthesized according to the method of Ferruti with the following modification: a mixture of 22.8 g (0.1 mol) p-benzyloxy-d- methylbenzyl alcohol, which was obtained by the reduction of p-benzyloxyacetophenone with lithium aluminum hydride, and 0.6 g powdered KHSO, was heated over 0.1 g powdered copper or copper sulfate. p-Benzyloxystyrene was distilled at 75°C at 5 X torr. The high vacuum and the presence of the copper salt inhibited thermal polymerization. The product was crystallized twice from methanol [16 g (76%) yield and mp 69-70"Cl. The 'H-NMR

p 3 ? 4 3 c-0 H-C-OH

I

C H 2

6 p - h ydroxyscetop henone $2 $2

p- benzyloxyacetophenone p- benzyloxy-d- p- benzyloxystyrene methyl benzyl alcohol

Scheme 2. Steps used to synthesize p-benzyloxystyrene.


spectrum (CDC1,) exhibited peaks at 65.05 (s, 2, OCH -2 ) 7 5-6.9 (m, 3, vinylic), 7.05 (m, 2, 0- - Ar), 7.46 (m, 7, aromatic).

Copolymerization

Copolymerization of p-Benzyloxystyrene and Styrene

A polymerization tube was charged with p-benzyloxystyrene (3.15 g, 15 mmol), styrene (0.502 g, 5 mmol), 2,2'-azobisisobutyronitrile (AIBN; 0.26 g, 8 mol %, recrystallized from methanol) and toluene (8 mL), and flushed with nitrogen. Then the tube was degassed by three freeze-thaw cycles, sealed at 1 x torr, and placed in a constant temperature bath at 60°C. After 24 h the tube was opened, the contents were diluted with 50 mL of benzene, and the polymer was precipitated by pouring the mixture slowly into cold, stirred methanol. The white, powdery polymer was collected by filtration, washed several times with cold methanol, and dried under vacuum. The yield was 3.17 g (86%) p-benzyloxystyrene/styrene copolymer. The H-NMR spectrum (CDC1,) edibited peaks at 60.9-2.4 (m, 6, - CH,CH -2 ) ' 5.0 (s, 2, 0%) and 6.6-7.7 (m, Ar-H). According to the 'H-NMR spectrum, the copolymer consisted of c x 2 3 mol % styrene and ca. 77 mol % p-benzyloxystyrene, based upon the relative integrated signal intensities of the benzyl ether protons and aromatic atoms.

For the determination of reactivity ratios, copolymerizations were performed by heating the reaction mixture for 1 h at 60°C (to less than 10% conversion). The relative ratios of the monomers in the copolymers were determined by examination of the 'H-NMR spectra.

1

Conversion of p-Benzyloxystyrene / Styrene Copolymers to p-Hydroxystyrene / Styrene Copolymers

A solution of 1 g p-benzyloxystyrene/styrene copolymer (from ' H-NMR: 23% styrene, 77% p-benzyloxystyrene) in 100 mL dry toluene was cooled to - 5°C and slowly saturated with anhydrous, bromine-free hydrobromic acid. The reaction flask was then tightly stoppered and allowed to warm up to room temperature overnight. The resulting cloudy suspension was poured into cold, stirred hexane. The product precipitated out immediately and was collected by filtration, washed several times with hexane, and dried. The crude p-hydroxystyrene/styrene copolymer was dissolved in a 10% aqueous sodium hydroxide solution, reprecipitated with 10% hydrochloric acid, washed several times with water, and dried at 40°C and 1 X torr. The yield was 0.59 g (97%). The 'H-NMR spectrum (CHC1,) exhibited peaks at 63.3 (broad, 1, OH)

For thisgraded series of copolymers, the purification procedure used dif- fered according to the copolymer ratio. Copolymers containing 50% or more p-hydroxystyrene were purified by dissolving them in an aqueous sodium hydroxide solution and precipitating them from hydrochloric acid. The copolymers containing less than 50% p-hydroxystyrene were precipitated twice by dissolving them in methyl ethyl ketone, pouring the solution into hexane, and washing the product several times with water.

and 6-7.5 (Ar-H).

p-HY DROXY STYRENE/STY RENE COPOLYMERS 1995

Physical Measurements

The 'H-NMR spectra were obtained on a Varian model EM360 spectropho- tometer. Melting points were measured on a Hoover Uni-melt capillary melting point apparatus. Molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC) (Waters model 201; lo3 A, lo4 A, lo5 A p-Styragel columns; RI detector). Electron spectroscopy for chemical analysis (ESCA) spectra of spin-cast polymer surfaces on glass were taken by using a Hewlett-Packard 5950B spectrometer and a Surface Science Laboratories SSX-100 spectrometer. Both operate with an aluminum anode, a monochromatized x-ray source, and a low energy electron floodgun to neutralize charging. The water content of each polymer was measured gravimetrically, following the procedure of Sasaki et al.'

Surface energies of spin-cast samples were determined by using the Zisman method." Each contact angle value used in the Zisman calculation represents an average of twelve measurements (measuring two sides of three advancing angles of two drops on each surface). Two critical surface tension values were obtained for each type of polymer by examining two samples of each polymer on different days.

RESULTS AND DISCUSSION

p-Benzyloxystyrene / Styrene Copolymers

The yield of the p-benzyloxystyrene/styrene copolymers ranged from 80-98%. For determining the reactivity ratios, however, the copolymerization was carried out for a period of 1 h at 60°C to less than 10% conversion. The monomer mixtures and the copolymer compositions are given in Table I, and the relationship between the monomer and copolymer compositions is illustrated in Figure l.

The reactivity ratios were determined using the Fineman and Ross method.', The linear least squares fit of the data generated by the Fineman-Ross equation is plotted for this system in Figure 2. The slope and the intercept of the least squares line are the reactivity ratios: rl = 0.37, r, = 0.28.

TABLE I Monomer and Copolymer Compositions:

p-Benzyloxystyrene (Ml) and Styrene (M2y

M1 M 2 ml m2 f F

0.800 0.200 0.714 0.286 2.50 4 0.750 0.250 0.638 0.362 1.76 3 0.666 0.333 0.574 0.426 1.35 2 0.500 0.500 0.606 0.394 1.54 1 0.333 0.666 0.414 0.586 0.71 0.5 0.250 0.750 0.354 0.646 0.55 0.33 0.200 0.800 0.319 0.681 0.47 0.25

"M1 = mole fraction of p-benzyloxystyrene in monomer feed, M2 = mole fraction of styrene in monomer feed, m l = mole fraction of p-benzyloxystyrene in the copolymer, m2 = mole fraction of styrene in the copolymer, f = ml/m2, F = Ml/M2.


/ /

/ .- s +

/ 0

Lt" off I I I I I I 1 I I 0 0.2 0.4 0.6 0.8 1.0

Fraction of Styrene in Monomer Mixture

Fig. 1. Relationship between the fraction of styrene in the monomer mixture and the fraction of styrene units in the copolymer.

n 2.5

-0.5 I I I I I I I

0 I 2 3 4 5 6 , F 2/f

Fig. 2. Fineman-Ross plot for the p-benzyloxystyrene/styrene copolymer system.

p-Hydroxystyrene / Styrene Copolymers

For the conversion of the p-benzyloxystyrene/styrene copolymers to p- hydroxystyrene/styrene copolymers, a dilute solution of the uncleaved copolymer was treated with hydrobromic acid. The conversions were almost quantitative. Examination of the 'H-NMR spectra for each conversion of the copolymer series revealed the disappearance of the benzyloxy proton peak (65.0) and the appearance of the hydroxyl proton peak (63.3). The homopoly- mer spectra shown in Figure 3 illustrate the definite changes in structure that occurred during the cleavage reaction.

p-HYDROXYSTYRENE/STYRENE COPOLYMERS 1997

I H' NMR - p-hydroxystyrene I - O H

8 7 6 5 4 3 2 1 (

(b)

6 (ppm)

Fig. 3. (a) 'H-NMR spectrum of p-benzyloxystyrene (before cleavage) and (b) 'H-NMR spectrum of p-hydroxystrene (after cleavage).

The surface chee t ry of the p-hydroxystyrene/stryene copolymers in the uppermost 50-100 A is suggested by the ESCA data shown in Table 11. To obtain atomic compositions, the total area under the photoemission peak was normalized by the number of scans and corrected with photoemiasion efficiency sensitivity factors appropriate for the ESCA instrument. The ratios of the compositions of the two elements present in the surface (C/O) are given in Table 11. The main Cls curve envelope was resolved by using the hydrocarbon peak (285.0 eV) as the reference and assigning the -C-O- species to 286.5 eV. The rationale for this curve fitting procedure has been ~resented.'~ The Cls


TABLE I1 Hydroxystyrene/Styrene Copolymer ESCA Data'

Monomer Ratio ClS

Styrene p-Benzyloxystyrene cshakeup/cls -C-O- CH c/o 100 0 80 20 75 25 50 50 25 75 20 80 0 100

Theoretical values (based on HS/S copolymer)

100 0 0 100

0.065 1.5 0.062 4.5 0.049 7.4 0.056 7.9 0.054 12 0.051 12 0.039 14

0 14

98.5 109 95.5 54 93 25 92 16 88 10 88 9.1 86.5 10

100 - 86 8

'Values for 0,20, 80 and 100% p-benzyloxystyrene are averages of 2-4 results, whereas values for 25, 50, and 75% represent single ESCA measurements.

shakeup was calculated separately from the main Cls envelope, and is ex- pressed as a percentage of the total composition in Table 11. The resolved ESCA Cls spectrum of poly( p-hydroxystryene) is shown in Figure 4.

Figures 5-8 are plotted as a function of the mole fraction of p-hydroxystyrene in the bulk copolymer, assuming 100% cleavage. This assumption is valid because, as mentioned earlier, the conversions were almost quantitative. As the percentage of p-hydroxystyrene in the bulk copolymer increases, the O/C ratio increases and the percentage of -C-0- species increases. These trends are illustrated in Figures 5 and 6. As shown in Figure 5, the comparison of the bulk copolymer O/C ratios (calculated from NMR data) with the ESCA surface O/C ratios indicates that the bulk and surface compositions are very similar in this series of polymers. In Figure 7, the ratio of the area of the Cls shakeup to the area of the main Cls envelope decreases as a function of

ESCA C,. - POLY (D-HYDROXYSTYRENE!

"Shokeup"

I I I I 1 I I 1

293 289 285 281 - Binding Energy (eV)

Fig. 4. ESCA Cls spectrum of poly(p-hydroxystyrene).


d

0.10-

0.05 -

I Surface 4 Bulk

0

I 1 I I

0.0 0.2 0.4 0.6 0.8 1 .o Mole fraction p-hydroxystyrene

Fig. 5. The ESCA O/C ratio in the HS/S copolymer surfaces increases as the fraction of p-hydroxystyrene in the bulk copolymer increases. A s shown, these surface results closely follow the O/C line calculated from NMR bulk data.

increasing p-hydroxystyrene in the copolymer. Therefore, the surface results in these figures are consistent with the expectations for this system and with the NMR results describing the cleaved copolymer.

-4 rigorous interpretation of shake-up satellite intensities for aromatic polymers cannot be made. However, these satellite peaks have been the subject of much recent interest and these intensities have been studied empirically with regard to copolymer^.'^-^^ In this work, a decrease in shake-up intensity is associated with increased p-hydroxystyrene content.

The weight-average molecular weights of the copolymers determined by using GPC are given in Table 111. For all ratios except the 1.54 copolymer

0.20 ,

0.1 5

0.10

0.05

0.0

I


Fig. 6. The ESCA ratio of the C-0 species to the C-C species in the HS/S copolymer surface increases as the fraction of p-hydroxystyrene in the bulk copolymer increases.

2000

0.04 - 0.03 - 0.02 - 0.01

. n 3 Q) x a C UJ

I

I I I I

0

MAJUMDAR AND RATNER

Fig. 7. The ratio of the Cls shakeup peak to the main Cls envelope of the HS/S copolymer surface decreases as a function of the increasing fraction of p-hydroxystyrene in the bulk copolymer.

ratio, the apparent molecular weight of the p-hydroxystyrene/styrene copolymer is greater than that of the p-benzyloxystyrene/styrene copolymer. This increased calculated molecular weight implies a decrease in elution time, which was probably caused by either an increase in the mean square radius of gyration of the molecules in solution or a reduced extent of interaction between the polymer molecules and the hydrophobic, crosslinked, macropor- ous chromatography column. Either factor could lead to an easier passage through the solvated gel. Some polymer chain backbone degradation may have occurred during the cleavage step for the 1.54 ratio copolymer.

50

40

30

1

f

t A Setone Settwo

20 I I I I I i


Fig. 8. The critical surface tension of the HS/S copolymer increases as the mole fraction of p-hydroxystyrene in the bulk copolymer increases.


TABLE 111 Molecular Weights of p-Benzyloxystyrene/Styrene and

p-Hydroxystyrene/Styrene Copolymers

Copolymer ratio Mw x 10-6 ~w x 10-~ (BS/S) (=/s) (HS/S)

2.50 1.76 1.54 0.55 0.47

4.13 4.33 2.73 1.85 4.71

63.5 54.9 2.62 3.05

12.6

TABLE IV Critical Surface Tensions for the HS/S Copolymer System

Polystyrene 27.6 f 0.2 30.0 f 1.1 p-Hydroxystyrene/styrene (1 : 1) 40.9 0.8 p-Hydroxystyrene/styrene (3 : 1) 42.6 f 1.4 43.5 f 5.6 p-Hydroxystyrene/styrene (4 : 1) 43.3 f 1.9 42.3 f 6.3 Poly( p-hydroxystyrene) 44.2 f 3.4 43.4 f 3.1

"Errors represent the range in the extrapolated surface tension values calculated from the error in the contact angle measurements.

As shown in Figure 8 and Table IV, the critical surface tension increases slightly with an increase in the percentage of p-hydroxystyrene. A higher critical surface energy indicates that the surface is more polar and more wettable. In Figure 8, the critical surface tension values represent results from analyses performed at two separate times on two sets of these films.

The water content of the polymer films given in Table V increases with an increase in the percentage of p-hydroxystyrene. The increased number of hydroxyl groups should result in increased water retention. However, the highest observed water content (14.7%) is not large enough to produce significant hydrogel character (i.e., high compliance) in the polymer. This is a

TABLE V Water Content of the HS/S Copolymers

Dry weight Wet weight Polymers (mole ratio) (9) (g) Percent water

Polystyrene Copolymer of

1 . w 9 1.0820 1.4 1.0249 1.0711 4.3

p-hydroxystyrene/styrene (1 : 3)




Copolymer of 1.0345 1.1177 7.4

Copolymer of 0.9861 1.0940 9.8

Copolymer of 0.6744 0.7701 12.4

Poly( p-hydroxystyrene) 0.3570 0.4188 14.7


reflection of the limited molecular mobility in these polymers related to steric and hydrophobic interactions of the bulky phenyl rings. The advancing contact angles measured for use in the Zisman method have increased validity on rigid polymers that might not absorb significant amounts of liquid. I t should be kept in mind that advancing contact angles are more sensitive to the dispersion force component of the surface energy than receding contact angle measurements. If receding contact angles had been measured, a larger polar contribution to the surface energy would have been noted.

CONCLUSIONS

p-Hydroxystyrene/styrene copolymers were prepared from the p-benzyloxystyrene/styrene copolymers by cleavage of the ether bond. 'H-NMR spectra showed the absence of the benzyloxy peak and the presence of the hydroxyl peak in the cleaved compounds.

ESCA data indicated an increasing O/C ratio, an increasing percentage of -C-0- species, and a decreasing (Cls shakeup/primary Cls) ratio with an increase in the bulk copolymer fraction of p-hydroxystyrene. Therefore, for this class of polymers, the surface structure closely, although not exactly, reflects the bulk structure. The critical surface tension and the water content increased with an increasing percentage of the bulk p-hydroxystyrene component. All of these results suggest a graded increase in surface polar character in this series of copolymers. However, based on the water content results, no significant hydrogel character developed with this polar character.

Support for this work was received from N.I.H. grants HL22163, HL19419 and RR01296. Valuable suggestions with regard to this work were received from A. S. Hoffman. Editorial assistance from N. B. Mateo is also gratefully acknowledged.

References 1. T. Sasaki, B. D. Ratner and A. S. Hoffman, in ACS Symposium Series No. 31, J. D.

2. B. D. Ratner and A. S. Hoffman, in Synthetic Biomedical Polymers, M. Szycher and W. J.

3. F. Danusso, P. Ferruti, and C. Gazzamiga Marabelli, Chim. Znd., 47, 493 (1965). 4. P. Ferruti and A. Fere, J. Polym. Sci. Polym. Chem. Ed., 9, 3671 (1971). 5. R. C. Sovish, J. Org. Chem., 24, 1345 (1959). 6. M. Kato, J. Polym. Sci. Polym. Chem. Ed., 7, 2175 (1969). 7. M. Kato, J. Polym. Sci. Polym. Chem. Ed., 7 , 2405 (1969). 8. J. M. Catala and J. Brossas, Polym. Bull., 2, 137 (1980). 9. A. J. Lentz, T. A. Horbett, L. Hsu, and B. D. Ratner, J. Biomed. Mat. Res., 19(9), 1101

Andrade, Ed., American Chemical Society, Washington, D. C., 1976, p. 283.

Robinson, Eds., Technomic, Westport, CT, 1980, p. 133.

(1985). 10. M. Sulzbacher, J. Am. Chem. Soc., 72, 5346 (1950). 11. W. A. Zisman, in ACS Advances in Chemistry Series No. 43, F. Fowkes, Ed., American

12. M. Fineman and L. D. Ross, J. Polym. Sci., 5, 259 (1950). 13. D. T. Clark and H. R. Thomas, J. Polym. Sci., Polym. Chem. Ed., 14, 1671 (1976). 14. J. J. O'Malley, H. R. Thomas and G. M. Lee, Macromolecules, 12, 996 (1979). 15. L. C. Lopez, D. W. Dwight and M. B. Polk, Surf. Znterf. Anal., 9, 405 (1986). 16. J. A. Gardella, S. A. Ferguson and R. L. Chin, Appl. Spectrosc., 40(2), 224 (1986).

Chemical Society, Washington, D.C., 1964, p. 1.

Received December 16, 1985 Accepted October 15, 1987

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Synthesis, bulk characterization, and surface characterization of p-hydroxystyrene/styrene copolymers