studies on grafting of methacrylic acid on to poly(vinyl chloride) films

British Polymer Journal 22 (1990) 89-95

Studies on Grafting of Methacrylic Acid on to Poly(viny1 chloride) Films

Jagbir Singh, Alok R . Ray & Harpal Singh*

Centre for Biomedical Engineering, Indian Institute of Technology, Delhi, New Delhi- 110016, India

(Received 27 September 1988; revised version received 16 January 1989; accepted 3 February 1989)

Abstract: The characterization of poly(viny1 chloride-g-methacrylic acid) (PVC-g- MAA) films, prepared by the simultaneous-irradiation technique, was undertaken through swelling measurements, attenuated total reflection infrared spectroscopy (ATRIR), scanning electron microscopy (SEM), electron spectroscopy for chemical analysis (ESCA) and thermogravimetric analysis (TGA). The per cent swelling of grafted PVC in various media increased with the increase in the graft level. A higher per cent swelling was observed in 0 . 5 ~ NaOH aqueous solution relative to that in distilled water. It is concluded from the ATRIR and ESCA studies that the concentration ratio of oxygen to chlorine atoms on the surface of grafted PVC increased compared with unmodified PVC, indicating grafting of methacrylic acid on to the surface. It was also found that the thermal properties of PVC improved after grafting with methacrylic acid.

Keywords: poly(viny1 chloride), methacrylic acid, swelling, ATRIR, SEM, ESCA, TGA.

1 INTRODUCTION analysis. Deb and Sankholkar' carried out the grafting of methylmethacrylate on to poly(viny1

Radiation-induced graft copolymerization offers a chloride) and used infrared spectroscopy and versatile method of changing the surface properties swelling measurements to characterize the graft of polymers. ' v 2 Poly(viny1 chloride) has been surface copolymers. Chapiro and Gouldoubendi6 studied grafted with methacrylic acid (MAA) to improve the the swelling behaviour of PVC-g-MAA films. surface hydrophilicity and to provide carboxylic Electron spectroscopy for chemical analysis (ESCA) groups for the immobilization of drugs. A graft characterization of PVC surfaces grafted with copolymer results from the chemical linkage of heparin complexing poly(amido-amine) chains has small macromolecular chains on to the preformed been done by Ferruti and coworker^.^ The present polymer. The interaction of the grafted moiety with study is aimed at the evaluation of changes induced the polymer backbone, the nature of the bonding in poly(viny1 chloride) as a result of graft copoly- and the distribution of grafted polymer on the trunk merization of methacrylic acid and the characteriza- polymer matrix strongly influence the properties of tion of the grafted PVC by various techniques. the graft c~polymer .~ It will be useful to determine the composition and structural changes in the 2 EXPERIMENTAL grafted polymer and to correlate these with the polymer properties. Busscher et aL4 have studied the surface characteristics of poly(viny1 chloride)/poly- (methylmethacrylate) (PVC/PMMA) blends using contact angle and electron scanning for chemical * To whom all correspondence should be addressed.

2.1 Materials

Food grade poly(viny1 chloride) (67ER092) was obtained from Indian Petrochemicals Ltd, Baroda, India. PVC films with and without plasticizer

89 British Polymer Journal 0007-1641/89/$03-50 0 1989 Society of Chemical Industry. Printed in Great Britain

90 Jagbir Singh, Alok R. Ray, Harpal Singh

(dioctylphthalate, DOP), were cast on glass plates from a 6% total solution of either PVC resin or PVC resin and DOP in cyclohexanone and toluene (60/40, v/v). PVC films with 30% DOP content were thus prepared for grafting studies. All the films were dried in a vacuum oven at 60°C and stored in a desiccator over anhydrous calcium chloride. Most of the experiments were carried out with PVC films containing 30% DOP, unless otherwise specified. Methacrylic acid obtained from E. Merck was distilled at 8O0C/lO mm Hg before use. Analytical grade methanol and laboratory grade cyclohexanone and toluene supplied by Glaxo, India, were used without further purification. The effects of different parameters on grafting of methacrylic acid on PVC have been reported in a previous paper.*

2.2 Methods

2.2. I Grafting procedure. Graft copolymerization was carried out in standard joint Corning tubes of 12.0 x 3.0 cm size under a nitrogen atmosphere. In a typical procedure, a weighed amount (-0.2g) of PVC film (6.5 cm x 3.0 cm x - 0.006 cm) was kept in a glass ampoule in an equivolume mixture of methanol-water for 24h. MAA monomer was added just before the exposure of samples and tubes were placed in the radiation chamber for the desired period. The experiments were carried out under the following conditions: Dose rate 56 rad/s, total dose 025 Mrad, monomer concentration 2.36 mol/litre, total volume of solvent 14 ml, methanol-water (1 : l), and DOP content of PVC films 30%. After irradiation, the films were removed from the bulk external polymer and thoroughly washed with hot water and soaked in water at 50°C for several hours to remove adhering homopolymer. Further extraction of the homopolymer was done by Soxhlet extraction in methanol for 4 h and then films were washed with water and dried at 60°C at reduced pressure and weighed. Grafting was calculated as follows and expressed in mg/cm2:

w, - wo Grafting= , A0

where W,=weight of the grafted sample, W,= weight of the original sample, and A , = area of the original sample.

The PVC films used in these experiments were prepared by a solution casting method. The films were not of uniform thickness. Therefore, where grafting occurred only on the surface, the degree of graft expressed as a percentage appears to vary with thickness of the substrate. We observed that the weight gain (grafting) on films of the same area with different thicknesses was almost the same. If these graft add-on results are expressed in mg/cm2, the

values are almost the same for both the films, but when graft add on results are expressed in weight percent grafting, the values are quite different although the total weight gain is almost the same. To avoid the variation we preferred to express our grafting results in mg/cm2, as also adopted by other

2.2.2 Swelling measurements. The grafted samples were dried and weighed. The swelling percent was investigated in distilled water, 0 . 5 ~ sodium hydroxide and Ringer phosphate buffer solution. Ringer phosphate buffer solution was prepared as described by Sidney and Lester.' ' The weight of the swollen samples was taken at various time intervals after keeping the samples between blotting paper for 30 s. The percent swelling was calculated from the expression

x 100 ws - Wd w* Percent swelling =

where W, is the weight of swollen film and W, is the weight of dry film. The assessment of swelling was carried out in triplicate and there was reproduci- bility in percent swelling with slight variation. There was no loss of plasticizer during all stages of assessment. Read et aLi2 and Darby and Searsi3 have reported that maximum loss of DOP from PVC films after extraction for 24h in water at 50°C is 0.0 1 Yo.

2.2.3 Attenuated total reflection infrared spectroscopy. Attenuated total reflection infrared analysis (ATRIR) of the grafted and ungrafted samples was carried out using a Perkin Elmer 683 spectro- photometer. The reflection element consists of KRS- 5 prisms giving 13 reflections at 45".

2.2.4 Scanning electron microscopy. Approximate sections of 6 mm x 6 mm were cut from the grafted and ungrafted films. These sections were mounted on to specimen holders. A silver coating, approximately 200A thick, was deposited on the surface in a vacuum evaporator. Micrographs of the film surface were obtained with a scanning electron microscope (SEM) model Stereoscan S4, Cambridge Scientific Instruments Ltd, UK. 2.2.5 Electron scanning for chemical analysis. The grafted poly(methacry1ic acid) branches were con- verted into calcium salt by immersing the films in an aqueous solution of 1 ?A calcium acetate at 50°C for 16 h. Approximate sections of 6 mm x 6 mm were cut from grafted and ungrafted films. These sections were mounted on sample holders. Spectra were recorded with an ESCA PHI Model 1800, using Mg K radiation (1253.6 eV) at 400 W. The resolution was 1.0 eV. The pressure in the sample chamber was approximately torr.

BRITISH POLYMER JOURNAL VOL.22, NO.2.1990

Grafting of methacrylic acid on to poly(viny1 chloride) jilms 91

2.2.6 Thermogravimetric analysis. Thermogravi- metric analysis (TGA) was carried out on a Stanton Redcroft TG-780 thermobalance in a nitrogen atmosphere, from ambient temperature to about 550°C at a heating rate of lO"C/min.

3 RESULTS AND DISCUSSION

3.1 Swelling measurements

Figure 1 shows the results of equilibrium swelling in different media of samples with various levels of grafting. It is observed that equilibrium swelling per cent for grafted samples in 0 . 5 ~ NaOH is greater than that in distilled water and Ringer phosphate buffer solution. The higher percent swelling in 0 . 5 ~ NaOH is due to the formation of the sodium salt of carboxylic acid or it may be due to the higher density of aqueous NaOH solution compared with water or buffer solution. It is also clear from Fig. 1 that per cent equjlibrium swelljng increases in all systems with increase in graft level. Figure 2 shows the per cent swelling behaviour of various samples at different time intervals. It is observed that equilibrium swelling is attained in about 8 h and subsequently remains constant with slight variation. The dimension of the film changes after swelling which can be observed by the naked eye.

3.2 Attenuated total reflection infrared spectroscopy

In order to confirm the presence of poly(methacry1ic acid) on the PVC surface, ATRIR has been carried out. Attenuated total reflection infrared spectra of unmodified PVC film shows strong peaks at 2930 and 2960cm-' due to CH, stretching, 1440 and 1480cm-' due to CH, bending and a weak peak at 1710cm-' due to C=O stretching. In the case of MAA-grafted PVC film, additional peaks at 3 4 0 s

= t p 80 0 .- -

/ a .- ! I

Grofting mg/crn2

Fig. 1. Effect of graft level on equilibrium swelling of PVC-g- MAA films in different media: (0) water; (A) Ringer phosphate;

(0) 0 . 5 ~ NaOH.

2 5 t

20 I

Time ( h )

Fig. 2. Effect of graft level of PVC-g-MAA films on per cent swelling in distilled water. Graft level (mgjcm'): (0) 059; (0)

1.08; (A) 1.63; (0) 2.63.

3500cm-' due to OH stretching and a strong peak at 1700cm-' due to C=O stretching are also observed. The presence of plasticizer in PVC definitely shows a peak at about 1700cm- ', but not at 3400cm-' as there is no acid group in the plasticizer. The peak at 3400-3500cm-' in grafted PVC is due to the OH group of the acid. The doublet at 63&690cm-' due to the C-CI stretching is prominent in PVC whereas it diminishes in grafted PVC. The presence of these two additional peaks in the ATRIR spectra of modified PVC samples indicates the presence of acidic groups. This is due to the grafted methacrylic acid on the surface of the PVC film.

3.3 Scanning electron microscopy

Figure 3 shows a scanning electron micrograph of the surface of unmodified PVC film at x500

Fig. 3. Scanning electron micrograph of PVC film at magnification of x 500.

BRITISH POLYMER JOURNAL VOL. 22, NO. 2,1990

92 Jagbir Singh, Alok R. Ray, Harpal Singh

(a) (b) Fig. 4. Scanning electron micrographs of grafted PVC. Graft level (a) 0,59mg/cm2 ( x 500); (b) 1.08mg/cm2 ( x 2200).

(4 (4 Fig. 5. Scanning electron micrographs of grafted PVC at magnification x 2200 at graft level (mg/cm2): (a) 1.08; (b) 1.63; (c) 2.63; (d) 3.1.


Crafting of methacrylic acid on to polv(oiny1 chloride) films 93

magnification. It is clear from the micrograph that the surface of the unmodified PVC film is almost smooth. Figures 4 and 5 illustrate the results of SEM studies of the surfaces of the grafted films with various degrees of grafting at different magnifica- tions. Scattered small particles are observed on the surface of MAA-grafted PVC at a graft level of 0.59 mg/cm2 (Fig. 4(a)). These particles increase in size and number with the increase in the degree of grafting until they fuse together to form larger particles (Figs 4 and 5). The particle size may increase with the increase in the chain length of poly(methacry1ic acid). It may be concluded from the above observation that the majority of the grafting occurs on the surface and the particles may be considered to consist of only poly(methacry1ic acid).14 Since the PVC does not swell in the grafting solvent medium and monomer, grafting may be restricted to the surface of the PVC film.

3.4 Electron spectroscopy for chemical analysis

The ESCA spectra of the unmodified PVC film and of the calcium salt of the MAA-grafted film are shown in Fig. 6. The scanning range was 0-660 eV in binding energy of electrons. The peaks at about 200, 290 and 530eV, assigned to Cl,,, C,, and Ols, respectively, are clearly observed in the spectrum of the unmodified plasticized PVC film (Fig. 6(a)). Grafted PVC film shows an additional peak at 350 assigned to Ca,, due to the presence of the calcium salt of methacrylic acid (Fig. 6(b)). Further, the peak of O,, becomes much stronger due to the presence of acidic groups after grafting. It is also noted that the

c ( is 1

c 1tS 1

( a )

I I I I r ?

660 550 440 330 220 110 0 BINDING ENERGY (EV)

Fig. 6. ESCA spectra of (a) ungrafted PVC film and (b) MAA- grafted PVC film (graft level 1.63 mg/cm2).

TABLE 1. Variation of surface composition of PVC and grafted PVC

Sample Atomic composition (%)

C 0 CI Ca

PVC 71.0 9.7 19.3 PVC-9-MAA" 71.2 11.2 9.3 8.3

a Calcium salt of grafted PVC.

intensity of the peak at 210 assigned to CI2, reduces drastically. The atomic compositions of the PVC and grafted PVC are given in Table 1. The per cent of oxygen increases and chlorine decreases after grafting. This reduction of chlorine might be because when PVC is exposed to radiation during grafting, some of the chlorine atoms are removed and grafting occurs on these sites. The elemental ratios of PVC and grafted PVC determined from ESCA are given in Table 2. A significant decrease in Cl/C ratio is

TABLE 2. ESCA elemental ratios for PVC and grafted PVC

Sample Elemental ratios

PVC 0.14 0.27 0 0.50 PVC-g-MAA 0.16 0.13 0.12 1.20

observed, whereas O/C and O/C1 increase after grafting. To provide further detail, the individual peaks were scanned near 210, 290, 350 and 530eV. The results are shown in Fig. 7. A comparison was made between the unmodified PVC and MAA- grafted PVA (graft level 1-63 mg/cm2). The C,, peak at 286.3 eV becomes broader by 1.75 eV in full width at half maximum for grafted PVC at 284-8eV and shows a shoulder at 290 eV. This non-uniformity of the C,, peak might be due to the different types of carbon atom introduced in the PVC backbone after grafting of methacrylic acid. These results are in accordance with the results of other workers.14-16

3.5 Thermogravimetric analysis

The primary thermograms obtained from TGA were expressed in terms of (i) the initial decomposition temperature (IDT), (ii) the integral procedural decomposition temperature (IPDT), and (iii) the 50% decomposition temperature (Ts0). The IDT is the temperature at which the actual decomposition starts. The IPDT, as proposed by Doyle,17 was obtained from summation of the whole shape of the


Jagbir Singh, AIok R . Ray, Harpnl Singh

CLS 286.3

I , I J 290 288' 286 284 282

BINDING ENERGY, EV

01s 534.0

538 536 534 532 530

BINDING ENERGY, EV

CLZP 20i.4 a

206 204 202 200 I98 BINDING ENERGY, EV

c is 284.8

I I I I I I I

292 290 288 286 284 282 280 BINDING ENERGY, EV

01s

I I I L I I

538 536 534 532 530 528 BINDING ENERGY, EV

CaZD

BINDING ENERGY, EV

Fig. 7. Core level spectra of (a) ungrafted PVC and (b) calcium salt of MAA-grafted PVC films.

100

90

80

70 z - 60 QA 5 0

5 4 0

30

20

10

n

cc

I I I I I I I

IS0 200 250 300 3SO 400 450 700 550

Tempernlure 1°C )

Fig. 8. Thermograms of ungrafted PVC and grafted PVC. (@) PVC; (A) PVC-g-MAA (graft level 1.08 mg/cm2); (0) PVC-g-

MAA (1.63mg/cm2); (0) PVC-g-MAA (2-63mg/cm2)).

normalized data curve. IPDT is a comprehensive index of the thermal stability which places all the materials on a common basis and provides the most valid comparison of different polymeric materials. IPDT is valid whether the decomposition occurs in a single step or in several consecutive steps. According to Doyle, IPDT is defined as follows:

IPDT = A*( T, - Ti) + Ti where A* is the ratio of the area of the thermogram and total area covered by the curve, Tf is the final decomposition temperature and T, is the initial decomposition temperature. T5,, is the temperature at which the polymer loses 50% of its weight.

Thermograms obtained by plotting the per cent residual weight against temperature for plasticized PVC and PVC-g-MAA are given in Fig. 8. The temperatures corresponding to IDT, T, , and IPDT were calculated from the thermograms and are given in Table 3. The IDT values of all the graft copolymers are higher than that of unmodified plasticized PVC. IDT values increase with increase in the graft level in modified samples. Similarly T,, increases from 300°C to 337.5-387-5"C on grafting of MAA on to PVC. A significant increase in IPDT was observed in all the graft copolymers. The IPDT values increase with increase in graft level. The rate of decomposition up to 350°C is very high for PVC.

TABLE 3. Effect of methacrylic acid grafting on thermal characteristics of poly(viny1 chloride)

Sample Graft IDT IPDT T, level ("C) ( " C ) ("C)

(mg/cm2)

PVC Nil 190 350 300 PVC -9 - M A A 1 908 202 372 337 PVC-g-MAA 1.63 210 380 365 PVC-g - MAA 2.63 217 390 387


Grafting of methacrylic acid on to poly(uiny1 chloride) films 95

This may be due to the fast dehydrochlorination of PVC up to this temperature. The decomposition rate up to 350°C is relatively low for graft copolymer samples. This may be due to the labile chlorine sites, which are responsible for initiating the dehydrochlorination of PVC, being removed by grafting which mainly takes place at such labile chlorines available in the unmodified PVC. It may be concluded that thermal stability of PVC is improved by grafting. Similar results have been reported by other^.'^.'^

REFERENCES

1 Bruck, S. D., J . Biomed. Muter. Res., 7 (1973) 387. 2 Hoffman, A. S., Radial. Phys. Chem., 9 (1977) 207. 3 Mukherjee, A. K. & Gupta, B. D., J. Macromol. Sri. Chem., 19(7)

4 Busscher, H. J., Hoogsteen, W., Dijkema, L., Swatsky, G. A,, van Pelf, (1983) 1069.

A. W. J., de Jong, H. P., Challa, G. & Arends, J., Polymer Commun., 26 (1985) 252.

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6 Chapiro, A. & Gouldoubendi, R., Eur. Polym. J., 11 (1975) 241. 7 Ferruti, P., Barbucci, R., Danzo, N., Torrisi, A., Puglisi, 0..

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17 Doyle, C. D., Anal. Chem., 33 (1961) 77. 18 Sharma, K. K., PhD thesis, IIT Delhi, 1979. 19 Kennedy, J. P. & Nakao, M., J. Appl. Polym. Sri., Appl. Polym. Svmp.,

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studies on grafting of methacrylic acid on to poly(vinyl chloride) films

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