Preparation of a new thermo-sensitive material by preirradiation grafting

Radiat. Phys. Chem. Vol.48, No. 4, pp. 525-527, 1996 Copyright© 1996ElsevierScienceLtd 0969-806X(95)00473-4 Printed in Great Britain.All fights reserved 0969-806X/96 $15.00+ 0.00

Pergamon

PREPARATION OF A NEW THERMO-SENSITIVE MATERIAL BY PREIRRADIATION GRAFTING M I N G H O N G WU, CHEN JIE, Z H O N G L I D I N G and ZUE-TEH M A Shanghai Applied Radiation Institute, Shanghai University of Science and Technology, Shanghai 201800, P.R. China (Received 20 October 1995; accepted 30 November 1995)

Abstract--In this paper, a new thermo-sensitive material that NIPAAm was grafted onto EVA by preirradiation grafting was obtained, and the effects of reaction condition on grafting reaction were discussed. The experimental results showed that the swelling of the grafted EVA was reversible and the lower critical solution temperature (LCST) of the surface was 32°C. Compared with the poly-NIPAAm gel, the grafted EVA with high mechanical strength behaves with similar thermo-sensitivity and the more swift response to the change of temperature. When acrylic or acrylamide was added to NIPAAm, the LCST of the grafted surface shifts higher with the increase of the grafted component hydrophilicity. Copyright © 1996 Elsevier Science Ltd

INTRODUCTION

Thermo-sensitive hydrogel, one kind of external stimuli response hydrogel, collapses at elevating temperature through the LCST. The large change of volume happens within a quite narrow temperature range (Tanaka, 1978; Hirotsu et al., 1987; Matsuo and Tanaka, 1988; Havasky et aL, 1985; Ulbrick and Kopeck, 1979). The gel swiftly absorbing and releases water in a way of switching on-off with the change of temperature. So this kind of material could be highly useful in many fields such as drug delivery (Hoffman et al., 1986; Bae, 1987), extraction (Freitas and Cussler, 1987), catalytic reaction rate control (Hoffman et al., 1986) and enzyme activity control (Dong and Hoffman, 1986). The most important factors of thermo-sensitive hydrogel are phase transition temperature, phase transition rate and the mechanical strength. Crosslinked poly-NIPAAm with a LCST about 32°C has attracted much attention for its representative thermo-sensitivity, but in swelling state, the mechanical strength is weak. To improve the mechanical strength, copolymerization and the interpenetrating polymer networks (IPN) have been tried (Hoffman et al., 1986; Bae et al., 1987; Mukae, 1990). The experimental showed the thermo-sensitivity is depended on the surface of hydrogel, the LCST may be kept at the same temperature as that of polyNIPAAm, so we expect to get a new material with high mechanical strength and thermo-sensitive surface through the grafting of N I P A A m onto a trunk polymer. In experiments, we prepared the material that NIPAAm was grafted on EVA. The grafted EVA with high mechanical strength performed with similar thermo-sensitivity to the poly-NIPAAm gel.

The effects on grafting and thermo-sensitive property are discussed. EXPERIMENTAL

Trunk polymer, EVA slices in size of 8 x 20 x 1 mm were carried out at a dose rate of 100 Gy/s and a dose of 100 kGy with EB of 0.9 MeV energy from a Van de Graft accelerator at room temperature in the presence of air. The irradiated slices and solution of NIPAAm with concentration of 20 wt% in deionized water were added to test tube, then atmosphere was pumped out of tube and nitrogen added to it. Then the test tube was placed in a constant temperature water pool which had already reached the reaction temperature for desired hours, and the grafting reaction was achieved. The grafting yield and the swelling ratio were defined as: G = ( W o . - Wo)/Wo × 100% R = (Wi -

Wo,)/Wo, x

100%

where Wo--the weight of the slice after grafting; Wo,--the weight of the slice after grating; Wi--the weight of the slice after equilibration in water; G - the grafting yield; R - - t h e swelling ratio.

RESULTS AND DISCUSSION

The influence o f reaction condition on the grafting reaction

In grafting reaction, we discussed the influence of reaction temperature, reaction time, monomer coneentration, preirradiation dose and dose rate on the

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Fig. 1. The influence of temperature on swelling ratio of EVA and PE sheet grafted by NIPAAm. grafting reaction. The results show: common characteristics of preirradiation grafting reaction are reflected in NIPAAm-EVA system, for example, the grafting yield increases with the increase of reaction temperature, reaction time and monomer concentration. When the reaction conditions were selected as: radiation dose rate 280kGy/s, dose 100 kGy, monomer concentration 20 wt%, reaction temperature 90°C, after 5 h grafting reaction, the grafting yield was 79%.

The selection of trunk polymer Figure 1 shows the swelling-deswelling curve of the surface of EVA and PE after grafted with NIPAAm. It is clear from Fig. 1 that, with the increasing temperature, the swelling ratio of the grafted EVA and PE abruptly changes around 32°C, it does not change with the different grafting trunk polymer. The swelling ratio of the grafted EVA is much higher than that of grafted PE. The volume of grafted EVA undergoes a much larger change within 2-3°C narrow temperature range through the LCST. The difference of the swelling ratio between the grafted EVA and PE is connected with the grafting yield on the trunk polymer, the grafting yield is

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Temp. (DSC) Fig. 3. DSC thermogram for the surface layer EVA-g-(AAm-NIPAAm) equilibrated in deionized water at 15°C (100 NIPAAm). changed with the trunk: (1) the ventilation of EVA is much better than that of PE. Oxygen diffuses more easily in preirradiation of the EVA slices, and it is favorable to the formation of peroxide. (2) On the other hand, the water pool temperature of grafting reaction is 90°C, it is close to the softening temperature (Tin) of EVA and far from the Tm of PE. So it is convenient for the diffusion and absorption of the grafting monomer to EVA. Therefore, the grafting yield of NIPAAm on EVA is much higher than that on PE, the grafting yield of EVA can reach 79% while the grafting yield of PE is only 38%. So the grafted EVA with NIPAAm shows the better thermo-sensitivity.

Influence of grafting yield on thermo-sensitivity The relationship between the swelling ratio and grafting yield is shown in Fig. 2, it's clear from the change of swelling as a function of temperature that LCST of the surface grafted with NIPAAm does not change with the variation of grafting yield. The swelling ratio increases with increase in grafting yield. As the grafting increases, the surface of the trunk polymer becomes more and more hydrophilic below LCST, so the swelling ratio increases. As to above LCST, the shrinked grafting copolymer contains water, though much less than it does below LCST. The more graft, the more water is contained. 2.00 --

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Temp. (DSC) Fig. 5. DSC thermogram for the surface layer EVA-g-(AAm-NIPAAm) equilibrated in deionized water at 15°C (85 NIPAAm, 15 AAm).

Influence o f component o f the grafting monomer on thermo -sensitivity Figures 3-5 are the DSC curves of the grafted surface with phase separation around 30°C. They show the effect of hydrophilic monomer AAm on LCST and the swelling behavior of the surface cografted with NIPAAm-AAm. With the increase of AAm component from 0, 5 to 15%, correspondingly, the temperature of the phase change starting point (ONSET) that stands for LCST enhancement increases from 30.63, 32.59 and 33.25°C. In the meantime, the heat of phase separation stands for the ratio of swelling decrease from 0.65, 0.29 and 0.03 cal/g. So when hydrophilic monomer AAm was added to NIPAAm grafting solution, the LCST of grafted surface shifted to higher temperature and the swelling ratio of grafted surface decreased with the increase of AAm component.

Swelling and deswelling kinetics The swelling-deswelling kinetics of EVA grafted with NIPAAm is shown in Fig. 6. It indicates that the swelling and deswelling of the grafted EVA were reversible. After several reversible swelling-

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deswelling change, both of the equilibration swelling ratio of swelling state and deswelling state basically keep constant. The response to temperature change is very quick. An abrupt decrease of swelling ratio is observed when the temperature is modulated from 20 to 40°C. It takes only 15 min to squeeze out 80% of water. The reswelling is relatively slow and needed 25 min to reach 80% of full swell. Poly-NIPAAm gel took about 1 h to extrude 70% water it contained in response to temperature from 25 to 40°C. In reswelling, 2 h are needed to absorb 80% of water. Compared with poly-NIPAAm, the EVA surface grafted with NIPAAm is more sensitive to temperature. When the temperature raising through the LCST, the outer layer of poly-NIPAAm is the first area to be affected, a skin is formed, which retards the flux of water out. The decreased rate of swelling ratio is slow. In contrast, the NIPAAm grafted on the EVA surface is quite thin and loose, no skin is formed as the temperature increase passes LCST. The water in the grafting polymer is very easy to pass through when it shrinks. Therefore, EVA-g-NIPAAm would be useful for immobilization of enzymes to have their activities thermally controllable.

CONCLUSIONS (1) The grafting reaction on EVA with NIPAAm has common characteristics of preirradiation grafting reaction. (2) EVA surface grafted with NIPAAm presents typical thermo-sensitivity similar to that of P-NIPAAm gel. The volume abruptly changes within 2-3°C around its LCST of 32°C. (3) The grafting on EVA with NIPAAm-AAm mixture, the LCST of the grafted shifts to higher temperature, and the swelling ratio diminishes. (4) The response of EVA surface grafted NIPAAm to the change of temperature is more swift than that of poly-NIPAAm.

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