Polymer Degradation and Stability SO (1995) 125-130

0 1995 Elsevier Science Limited

0141-3910(95)00145-x Printed in Northern Ireland. All rights reserved

0141-3910/95/$09.50

X-ray induced degradation of regenerated cellulose membrane films

Yoshio Kawano” & Amadeu J. M. Logarezzib “Institute de Quimica, Universidade de SZo Paula, Caixa Postal 26077, CEP 05599.970, SZo Paulo, Brazil

‘Departamento de Engenharia de Materiais, Universidade Federal de SZo Carlos, Caixa Postal 676, CEP 13565.905, SGo Carlos, SP, Brazil

(Received 1 June 1995; accepted 20 June 1995)

Polymeric membranes are often exposed to degrading conditions that may compromise their performance before or during their application. In particu- lar, membranes used in hemodialysis require sterilization by irradiation with high energy radiation.

The X-ray induced degradation of regenerated cellulose membranes (Cup- rophan film) has been investigated by infrared, ultraviolet/visible and electron spin resonance spectroscopies. The infrared spectra show a change in the intensity of several bands associated with the glucose ring. A new band appears at 1724 cm-‘, whose intensity increases with X-ray exposure time.

The UV/visible spectra show bands at 200 and 260 nm, whose intensity increases with X-ray irradiation time. The ESR spectrum of Cuprophan membranes is a seven-line spectrum composed of the overlapping of at least two triplet and one doublet signal, indicating the presence of several types of free radicals in the irradiated membranes.

1 INTRODUCTION

Hemodialysis has been widely used for the treatment of patients with renal failure. Several polymeric membranes have been used as materials for hemodialysis and these demand sterilization processes before utilization. The methods currently used for sterilization of dialysers include autoclaving, treatment with gaseous ethylene oxide and gamma irradiati0n.l Gaseous ethylene oxide requires special precau- tions in controlling the biocompatibility, while autoclaving and high energy irradiation carry the risk of membrane degradation, which might cause important and undesired changes in membrane properties, such as dimensional stability, mechanical resistance and hydraulic and diffusive permeabi1ity.l

The effects of high energy radiation on polymers have been extensively studied, and several studies on high energy degradation of cellulose and its derivatives have been published,2” Cuprophan is widely used for dialysis membranes and is sterilized by high energy radiation. Cuprophan membranes are

semicrystalline regenerated cellulose with ap- proximately 65 % of crystallinity.7

A recent study of the degradation of tubular dialysis membranes of regenerated cellulose by gamma irradiation4 concluded that several properties are altered by gamma irradiation; the tensile strength is significantly reduced and the pore radius diminishes, decreasing the per- meability to molecules of low molecular weight. The effect induced by gamma radiation differs depending on whether the irradiation is carried out under dry or wet conditions.

The present study investigates the structural changes in Cuprophan membranes induced by X-ray radiation, monitored by infrared (IR), UV/visible, electron spin resonance (ESR), X-ray diffraction and thermal gravimetric analysis.

2 EXPERIMENTAL

Commercial Cuprophan regenerated cellulose membranes (11 pm thick), from EnkaGranztaff,

125

126 Y. Kawano, A

were used. The films were irradiated at room temperature in vacua with X-ray beams, using a Philips X-ray fluorescence spectrometer (PW 1410) equipped with a tungsten tube, operating at under 40 kV and 20 mA, as previously described.” The sample support was rotated during the irradiation to insure uniform irradia- tion on all surfaces. Under these conditions, the rate of energy dose transferred to the sample was 160 kGy/h. The samples were irradiated for various times up to 16 h. Some experiments were carried out with membranes of different water contents (dry, with about 3%; normal, with 10%: and wet, with -45% of moisture, in mass percentage).

Infrared (IR) absorption spectra of the Cuprophan films were measured on a Bomem DA3.02 FT-IR spectrophotometer, in the range 400-4000 cm- ’ .

UV/visible spectra were obtained using a HP 8451, diode-array spectrophotometer in the range of 190-500 nm.

X-ray diffractograms were obtained on a Philips X-ray generator (PW 1410) and a goniometer (PW 1370) using Cu(Ka) monoch- romatic radiation (0.1541 nm).

Electron spin resonance (ESR) spectra were measured on a Bruker ER 200D-SRC spectro- meter with the samples fixed in a flat cell.

The thermal gravimetric measurements were obtained on a Perkin-Elmer TGA-7 thermo-

4900 3000 2000 1000 c”-1

Fig. 1. IR spectra of Cuprophan films: (A) unirradiated, (B) irradiated for 16 h.

I. M. Logarezzi

gravimetric system, using samples weighing 1 mg and a heating rate of lO”C/min.

3 RESULTS AND DISCUSSION

3.1 Infrared

Figure 1 shows the IR spectra of normal Cuprophan films in the range 400-4000 cm-’ before and after 16 h of X-ray irradiation. On X-ray irradiation there is a decrease in the intensities of some bands and an increase in others. The broad band at ca 3420 cm-’ is assigned to the O-H stretching vibration in the cellulose II structure” and the broad band at 1000-1200 cm-’ is a superposition of several bands that are assigned to ring stretching, C-O stretching and C-C stretching vibrations.Y-” The former band exhibits a significant intensity decrease with irradiation time, indicating the loss of the OH group; the latter band also shows an intensity decrease upon irradiation, indicating ring opening in the irradiated membranes. Shabaka et al.,” studying IR spectra of y- irradiated cotton linters, observed a decrease in intensity of the OH stretching band at 3350 cm ‘. A sharp and weak band at 894 cm-’ is assigned to glucosidic linkage stretching.” This band exhibits

Fig. 2. IR spectra of Cuprophan films after various irradiation times: (A) 0 h, (B) 1 h, (C) 2 h, (D) 4 h, (E), 8 h,

and (F) 16 h.

X-ray induced degradation of cellulose films 127

an intensity decrease with increasing exposition time, indicating scission of glucosidic linkages (cl-o-cd’).

Figure 2 shows the IR spectra in the region of 1200-3100 cm-l of normal Cuprophan irradiated for various times. The bands at 2898 and 1367 cm-‘, assigned to C-H stretching and C-H in-plane deformation vibrations, respectively, exhibit an intensity decrease with increasing exposure time. The bands at 2936 and 1415 cm-l, assigned to CH, anti-symmetric stretching and CH, bending vibrations” exhibit very little intensity decrease with increasing irradiation time. The band at 1331 cm-l, assigned to O-H in-plane bending vibration, also shows an intensity decrease with irradiation time. On the other hand, the intensity of the band at 1311 cm-l, assigned to CH, wagging vibration, changes very little with irradiation time. The appearance of a new band at 1724 cm-‘, whose intensity increases with increasing exposure time, may be assigned to the carbonyl stretching (C&O) vibration. The formation of carbonyl groups in the course of cellulose degradation by y-radiation was observed by high resolution solid state 13C nuclear magnetic resonance (NMR).’ The weak band at 1642 cm-’ is due to membrane-adsorbed water (around 10% in mass), and its intensity changes very little with increasing irradiation time.

Figure 3 shows the relationship between the intensities of the bands at 894, 1367, 1415, 1724 and 2898 cm-’ and the irradiation time. Clearly, the intensity decrease or increase is not linear with irradiation time. The curves of the 2898 and 1367 cm-l bands show a similar behavior and

0 5 10 l5 h

Fig. 3. Intensity changes versus irradiation time for several IR bands.

indicate a loss of C-H bonds in the Cuprophan structure, which are related to ring opening effects. The 1415 cm-l absorption shows a small intensity decrease with exposure time, compared to the intensity of the C-H vibration band, indicating a reduced loss of CH, group in the cellulose structure with irradiation time. The 1724 cm-’ band shows a strong intensity increase, which may result from ring opening, the scission of glucosidic linkages and the free radical at C, (the C outside the ring, according to the conventional numbering system for cellulose) with irradiation time.

3.2 Ultraviolet/visible

The UV/visible spectra of normal Cuprophan membranes (11 pm thickness) before and after irradiation are shown in Fig. 4. Unirradiated Cuprophan films absorb only below 200 nm. Films irradiated for 1, 2, 4, 8 and 16 h show absorption bands at approximately 200 and 260 nm. The absorption intensity increases with exposure time and a small red-shift of the absorption band at around 200 nm is observed with increasing exposure time. The absorption band at 260 nm can be assigned to the carbonyl group.’ The assignment of the absorption band at 200 nm is uncertain, and could be related to the absorption of free radical species, since its intensity decreases with aging time. The UV absorption profiles of irradiated and unirradiated Cuprophan films are similar to those reported for

3

100 2 so 300 310 100 45o nm

Fig. 4. UV/visible absorption spectra of Cuprophan films. Irradiation times: (A) 0 h, (B) 1 h, (C) 2 h, (C’) 2 h after 40 days, (D) 4 h, (E) 8 h, (E’) 8 h after 40 days, and (F) 16 h.

128 Y. Kawano, A. J. M. Logarezzi

unirradiated and y-irradiated dry regenerated cellulose membranes in Ref. 4.

Fig. 4 also shows the spectra (C* and E*) of membranes irradiated for 2 and 8 h and recorded 40 days after irradiation. A comparison of the corresponding spectra shows clearly that ir- radiated membranes exhibit a small aging effect.

Fig. 5 shows the relationship between the intensity of absorbance of the band at 260 nm and irradiation time. The intensity increase is clearly not linear and is similar to the curve observed in Fig. 3, for the IR band at 1724 cm ‘, assigned to the carbonyl stretching vibration.

3.3 X-Ray diffraction

Figure 6 shows X-ray diffractometer scans of unirradiated normal Cuprophan membranes. Unirradiated membranes show diffraction peaks at 12.2, 20.0 and 21.7“ corresponding to the (020) (110) and (110) pl anes, respectively, of cellulose II. The intensities of these peaks decrease very little after 4 h of irradiation, but after 8 h the membrane becomes undulated and difficult to handle in the X-ray diffractometer support. The degree of crystallinity clearly decreases with increasing dose, but it is very difficult to obtain reproducible data by the X-ray technique because of film sampling. High resolution solid-state 13C NMR on -y-irradiated cellulose showed that the degree of crystallinity decreased in a manner dependent on the dose of y-radiation.’

3.4 Thermogravimetry

Figure 7 shows the thermogravimetric (TG) curves of normal Cuprophan membranes before

Fig. 5.

0 s 10 IS h

UV absorbance changes vs irradiation time for the band at 260 nm.

I 20

Fig. 6. X-ray diffractogram of unirradiated Cuprophan films.

and after irradiation for 8 h. The TG curves exhibit a very small weight loss (less than 1%) up to 260°C (first step), the decomposition then proceeds rapidly, resulting in a significant weight loss up to 550°C. The TG curves of irradiated (8 h) membranes change very little compared to the unirradiated curve.

3.5 Electron spin resonance

Figure 8 shows the ESR spectra of dry Cuprophan membranes irradiated for 1, 2, 4 and 8 h. Line shapes and intensities of ESR signals are dependent on the degree of crystallinity, lattice type and molecular arrangement in the cellulose matrix.” The ESR signals of solid samples show a large peak width. The ESR spectral profile is very complex, indicating the formation of several free radical species induced

Fig. 7. Thermogravimetric curves of Cuprophan films: (-) unirradiated and (-) irradiated for 8 h.

X-ray induced degradation of cellulose jilms 129

irradiation and then decreases. The intensity decrease is more pronounced for the signal of doublet (III) compared to that of the triplet signals (I) and (II).

1 A B C D I

Fig. 8. ESR spectra of dry Cuprophan films. Irradiation times: (A) 1 h, (B) 2 h, (C) 4 h and (D) 8 h.

The ESR spectral profiles for wet membranes are different from those of dry membranes. The main change occurs in the relative intensity of the signal corresponding to the doublet (III), which is stronger in wet films than in dry films for the same irradiation time (Fig. 8 (A)-(C)).

by X-ray irradiation. The unirradiated Cup- rophan membrane shows no ESR signal. The ESR pattern is formed by the superposition of at least two triplet and one doublet signal, whose intensities increase up to 4 h of irradiation and decrease thereafter.

The ESR spectral profile of the irradiated membranes is very complex, and tentative signal assignments are largely based on the results of an extensive study of radical-induced effects on cellulose, cellobiose and disaccharides.5,6

The free radical corresponding to doublet (III) is more susceptible to radiation at high than at low water contents. The excess water content is localized preferentially in amorphous regions. It is known that the amorphous region of cellulose increases during water sorption.‘* Hence, most of the free radical of type III generated by radiation belongs to the amorphous region. It is known that the physico-chemical properties of cellulose are mainly dependent on the amorphous region.13

The triplet (I) with a hyperfine splitting constant of 2.7 mT may be assigned to a free radical at the C-2 or C-3 positions, using the conventional numbering system for cellulose. The triplet (II) with a hyperfine splitting constant of 1.6 mT may be assigned to the free radical formed at C-6 through the abstraction of a H atom. The doublet (III) with a hypertine splitting constant of 2.3 mT may be assigned to the free radical formed by the abstraction of the H atom at C-l, which induces cleavage of the glucosidic linkage.

Figure 10 shows the ESR spectra of wet Cuprophan membranes obtained 15 min, 40 min, 3 h and 24 h after irradiation for 4 h. The relative intensities of all signals decrease with time after irradiation. The decrease in intensity of doublet (III) is faster than that of triplets (I) and (II), indicating that the lifetime of doublet (III) is shorter than those of triplets (I) and (II). This means that it is located in a region with high mobility and for high diffusivity. Usually a long lifetime is characteristic of free radicals located in crystalline region.

The occurrence of a doublet typically indicates spin-coupling with one proton and suggests that the free radical arises from the ejection of a hydrogen atom from C-l.

Figure 9 shows the ESR spectra of wet ( - 45% water content) Cuprophan membranes irradiated for 1, 2, and 4 h. The relative intensity of the signals increases with irradiation time up to 2 h of

Crystalline regions of regenerated cellulose contain large numbers of more stable free radicals than amorphous regions.

The water content and the water-cellulose interactions in irradiated membranes change the ESR spectral profile, indicating a change in the mechanism of degradation of membranes by high energy radiation.

A recent study of cellulose degradation by

Fig. 9. ESR spectra of wet Cuprophan films. Irradiation times. (A) 1 h, (B) 2 h and (C) 4 h.

Fig. 10. ESR spectra of wet Cuprophan films irradiated for 4 h: (A) 15 min after irradiation, (B) 40 min after irradiation, (C) 3 h after irradiation, and (D) 24 h after

irradiation.

130 Y. Kawano. A. J. M. Logarezzi

y-radiation showed that the degree of crystalli- nity decreases with increasing irradiation dose above 1 MGY.~

Ultrafiltration experiments showed that mem- branes becomes altered by exposure to high energy radiation. The hydraulic permeability (L,) increases significantly after 400 kGy exposure, according to results obtained by Takesawa et al. ’ Wet Cuprophan membranes (45% water content) showed greater degradation, but it was not possible to measure L, for these samples. It is known that for regenerated cellulose films, water swelling decreases the degree of crystallinity.14

4 CONCLUSIONS

Prolonged exposure to X-ray radiation causes both physical and chemical changes in Cup- rophan membrane films. The main physical change is a loss of crystallinity. The irradiated material acquires a distinct light yellow colora- tion that increases with exposure time, and at long exposure times the films become brittle.

The main chemical changes observed are the appearance of carbonyl groups in the membrane structure, scission of the glucosidic bond and the loss of OH groups. The irradiated Cuprophan film shows the presence of several types of free radical.

ACKNOWLEDGEMENTS The authors thank Professor Dr Frank H. Quina

for helpful discussions. AJML thanks CAPES/PICD for a fellowship grant and YK thanks CNPq for a research fellowship and FAPESP and BID/USP for financial support.

REFERENCES

I.

2.

3 _ .

4.

5.

6.

7.

8.

9.

10. 11.

12.

13.

14.

Takesawa, S., Ohmi, S., Konno, Y., Sekiguchi, M., Shitaokoshi, S., Takahashi, T., Hidai, H. & Sakai, K., Nephrol. Dial. Transplant., 1 (1987) 254. Spevacek, J. & Kafka, D., Makromol. Chem., Rapid Commun., 12 (1991) 359. Shabaka, A. A., El-Agramy, A. M. & Nada, A. M. A., Polym. Deg. Stab., 31 (1991) 327. Takesawa, S., Satoh, S., Hidai, H., Sekiguchi, M. & Sakai, K., ASAZO, 10 (1987) 584. Phillips, G. 0. & Arthur J. C., Jr, in Cellulose Chemistry and its Applications, ed. T.P. Nevell & S.H. Zeronian, p. 290. Ellis Horwood, Chichester, 1985. Dilli, S., Ernst, I. T. & Garnett, J. L., Aust. J. Chem., 20 (1967) 911. Kulshreshtha, A. K. & Dweltz, N. E., J. Polym. Sci.; Polym. Lett. Ed., 16 (1978) 277. dos Santos, L. G. C. & Kawano, Y., Polym. Deg. Stab., 44 (1994) 27. Hatakeyama, H., Nagasaki, C. & Yurugi, T., Carbohydrate Res., 48 (1976) 148. Michell, A. J., Carbohydrate Res., 197 (1990) 53. Reddy, S. S., Bhaduri, S. K. & Sen, S. K., J. Appl. Polym. Sci., 41 (1990) 329. Hatakeyama, T., Ikeda, Y. & Hatakeyama, H., Makromol. Chem., l&3 (1987) 1875. Hatakeyama, T., in Cellulose Structural and Functional Aspects, ed. J. F. Kennedy, G. 0. Phillips & P. A. Williams, p. 45. Ellis Horwood, Chichester, 1989. Larena, A., Peha, M. C. & Pinto, G., J. Mater. Sci. Lett. 12 (1993) 1745.