Polyethylene Film Acrylic Acid Grafted by Electron-Beam

Chapter 5

Polyethylene Film Acrylic Acid Grafted by Electron-Beam Preirradiation Method

Downloaded by NORTH CAROLINA STATE UNIV on September 4, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0480.ch005

Properties of Grafted Materials 1,2

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1

J. Harada , R. T. Chern , and V. T. Stannett 1

Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695-7905 Tsukuba Research Laboratories, Mitsubishi Paper Mills, Ltd., 46 Wadai, Tsukuba City, Ibaraki 300-42, Japan

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The properties of acrylic acid grafted high density polyethylene films prepared by the electron beam preirradiation method have been studied. At high extents of grafting, the products are essentially poly(acrylic acid) hydrogels which are crosslinked by polyethylene. The hydrogels have a rubberlike elasticity. Although the tensile strength decreased with the extent of grafting, the highest grafted material which contained only 0.6 wt% polyethylene (on a dry basis) still had a tensile strength of 4.0 MPa at an elongation of about 790% after the sample was equilibrated with water. The water absorption capacity was also affected by the preirradiation dose, with the 8 Mrad irradiated sample exhibiting the highest water uptake. Grafting of hydrophilic monomers throughout the whole thickness of non-porous hydrophobic polymer films is known to lead to hydrophilic membranes. These hydrophilic membranes have been considered as biomedical materials because of their excellent biocompatibility, high water permeability, and characteristics desirable for enzyme immobilization and controlled release of drugs (1). The polarized gels have also been studied for use as mechano-chemical materials (2, 3), including pH sensors, polymer actuators, and separation membranes. The grafting reaction, including the use of high energy radiation, of acrylic acid or methacrylic acid onto polyethylene has been studied by many researchers because of its simplicity, high grafting ratio, and many application possibilities. These applications include water absorbents (4-7), battery separators (8-16), ion exchange and ion trap materials (17-21), selective separation membranes (19, 22, 23), anti static materials, deodorant materials, enzyme-immobilization substrates (24,25), metal coatings (26), and so on. In this paper, we will present the properties of acrylic acid grafted polyethylene films, prepared by the electron beam preirradiation method at a comparatively high reaction temperature, high irradiation doses, and under redox reagent-free conditions (27, 28).

0097-6156/92/0480-0080$06.00Α) © 1992 American Chemical Society

In Polyelectrolyte Gels; Harland, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

5. HARADA ET AL.

Acrylic Acid Grafted Polyethylene Films

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Experimental


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High density polyethylene (HDPE) film (d = 0.963 g/cm , 95 mm thick, crystallinity=70%, Mitsubishi Chemical Product) was used. Acrylic acid (Aldrich, containing 200 ppm hydroquinone monomethylether as a stabilizer) was used without purification. Potassium hydroxide and sodium hydroxide were A.C.S. reagent grade (Aldrich). An electron curtain type electron beam accelerator (175Kv, Energy Sciences Inc.) was used. The preirradiation was conducted under nitrogen atmosphere and room temperature. The maximum irradiation dosage for a single pass was 16 Mrad. For doses larger than 16 Mrad, the sample was exposed several timesfromboth sides of the sample. According to the dose-depth profile at 175Kv (29), the irradiation dosage was expected to be almost the same (>95%) throughout the sample thickness. The gel fraction of irradiated polyethylene was determined by weighing the sample after 48 hours of extraction in hot toluene. Monomer solutions were aqueous and were bubbled with nitrogen in die Erlenmeyer flask for at least 30 minutes to degas the solution before the grafting reaction. The irradiated polyethylene samples were dropped into the reaction flask which was kept in an isothermal (75° C) water bath. Nitrogen bubbling was continued during the grafting reaction. After reaction, the grafted samples were taken out and washed by running hot water for one day, followed by vacuum drying for 48 hours at 55° C. By this washing method, the grafted polymer weight reached almost the same value (within 2 wt. %) as the complete extraction results (2 days water reflux and 2 days methyl alcohol extraction). Grafting ratio (%) was calculated according to the following equation, 100* (weight after grafting - original weight) / original weight Strictly speaking, the "grafting ratio" is actually the fractional unextractable poly(acrylic acid). A microtome was used to slice the grafted film and a scanning electron microscope (JEOL, JXA-840) was used to observe the surfaces and cross sections of the grafted films. The grafted samples were neutralized by a 1% potassium hydroxide solution to enhance the contrast between the grafted layer and the ungrafted layer. A Monsanto Tensometer 10 was used to measure the tensile strength of grafted samples. The sample width and length were 2 cm and 5 cm, respectively. The elongation rate was 300 mn/min. Standard pH buffer solutions (Fisher) were used to check the pH dependence of swelling of the grafted polyethylene samples. The degree of swelling was defined as 100 X (length after swelling / original length). Results and Discussion Asymptotic Grafting Ratio and Crosslinking Table I shows the asymptotic grafting ratio of polyethylene samples irradiated to different doses. The asymptotic grafting ratio can be fitted to the following equation: Asymptotic grafting ratio = Κ * [D]b where Κ and b are constants, [D] is the irradiation dose (Mrad). The b value for HDPE is about 0.5 between 1 Mrad to 16 Mrad. Above 32 Mrad the asymptotic grafting ratio was much higher than that calculated from this equation (27). It is not


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POLYELECTROLYTE GELS

clear why this was so. Presumably, a combination of reduced termination and chain transfer reactions or grafting to existing grafted poly(acrylic acid) has led to these results.


Table I. Asymptotic Grafting Ratio of Irradiated Polyethylene Samples Irradiation Dose (Mrad) 0 1 2 4 8 16 32 48 64 96

Asymptotic Grafting Ratio (%) 0 428 659 993 1240 1709 3645 8685 26086 40653

Size Swelling LILo (%)* 100 192 250 310 335 370 432 590 1000 1170

Gel Fraction** (%) 0 0 0 0 0 43 66 71 78 84

Lateral dimension of the sample at the end of the grafting reaction. Gel fraction of the irradiated but ungrafted HDPE. Above 16 Mrad, effective crosslinking happened and an insoluble fraction of polyethylene remained after extraction with toluene for 48 hrs. The unextractable gel fraction increased with preirradiation dose. Crosslinking was found to affect the grafting reaction rate, penetration of grafting layer and the mechanical properties of grafted materials; these results will be presented below. Water Absorption of Samples in Acid Form Acrylic acid grafting of polyethylene has been shown by us to proceedfromthe surface to the center of the film (27). Figure 1 shows the equilibrium water uptake of the un-neutralized samples as a function of the grafting ratio, expressed in terms of g water / 100 g poly(acrylic acid) grafted. Water uptake increased initially with grafting ratio and asymptoted to a constant value after 20% grafting. Clearly, the grafted layer can absorb a significant amount of water even if the grafting ratio is very small (2 was rapid. Moreover, there was a much smaller hysteresis between the quasi-equilibrium values of the deswelling and die reswelling cycles (lines 2 and 3). (Because of slight differences in the swelling history of the samples, the degree of swelling at the two extremes of die curves in Fig. 7 do not coincide). Figure 8 shows the pH sensitivity of the swelling-deswelling properties of a sample with a lower grafting ratio. The sample started to swell at pH = 7 but the rate of swelling was very slow. The quasi-equilibrium swelling reached only 135% at pH = 10 after 30 hours. Similar to the data in Figure 7, there is a large hysteresis between the swelling and the deswelling cycle, but a smaller hysteresis between the deswelling and reswelling cycles.



5. HARADA ET AL.


Figure 5.

Stress-strain relationship for grafted HDPE.

20

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re

-ffl— 4 Mrad

PH

15

* - 8 Mrad • φ - 16 Mrad •+• 32 Mrad

t is

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10

CD

CO

m

5 μ

ι l ι ι ι ι

ι ι ι ι I ι ι ι ι 1 ι

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500

1000

1500

Grafting, Figure 6.

2000

2500 3000

%

Relationship between tensile strength and grafting ratio for samp irradiated to various doses.


88


170


160 150

e *

W

140

1=5

SP c 130 •S si 1J

120

J

Swelling Deswelling Reswelling

110

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100 90

12

Figure 7.

pH dependence of the quasi-equilibrium swelling of highly grafted HDPE. Grafting ratio = 20,000%.

150

' 1

140 1 c ϋ

m Swelling —O— Deswelling — -• — Reswelling

60

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•

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/

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237% grafting • , , ,

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pH dependence of the quasi-equilibrium swelling of relatively lightly grafted HDPE. Grafting ratio = 237%.



5. HARADAETAL.


89

The large hysteresis between the first swelling cycle and the deswelling cycle may be attributed to changes in the poly(acrylic acid) structure. Before neutralization, the grafted poly(acrylic acid) chains might betightlycoiled because the grafting reaction was carried out inside polyethylene. This highly coiled structure may significantly retard swelling at a moderate pH. After neutralization, all the carboxyl groups have been dissociated, and the ionic repulsion greatly expanded the originally tight coils of poly(acrylic acid). Upon deswelling, the expanded chains contracted to a much looser coil structure. During the subsequent reswelling, the conformation of the grafted poly(acrylic acid) never experienced the same degree of reorganization, therefore a much smaller hysteresis was observed. Conclusions Poly(acrylic acid) grafted HDPE gel was obtained easily by the electron beam preirradiation method. The gel had a high tensile strength (>4.0 MPa) even if it contained more than 50 wt% of water. The preirradiation dose strongly affected the tensile strength of the grafted HDPE. Higher preirradiation dose yielded higher tensile strength at the same grafting ratio. The amount of water uptake by the grafted sample before neutralization of the acid by NaOH was insensitive to the radiation dose or the grafting ratio as long as the grafting was above 200%. After neutralization, the sample preirradiated to 8 Mrad absorbed about twice as much water as samples of higher doses. Crosslinking of polyethylene at higher doses might have contributed to this reduced water absorption capacity. The un-neutralized hydrogel exhibited swellingdeswelling hysteresis when equilibrated in buffered solutions. The hysteresis probably originated in the transformation of the conformation of the poly(acrylic acid) chains from atightlycoiled state to a loosely coiled state after the expansioncontraction cycle caused by the pH changes. This paper contains a great deal of interesting new data but with only tentative explanations. Some additional data and a more detailed discussion will be presented in Part III of this series. Acknowledgment We thank Energy Sciences, Inc. for providing access to electron beam accelerator. The microscopy analyses provided by the Analytical group of Mitsubishi Paper Mills, Ltd. Central Research Laboratories are also highly appreciated. Literature Cited (1) (2) (3) (4) (5)

Niemoller A; Ellinghorst,G. Angew. Makromol. Chem., 1987, 148, 1. Suzuki M.; Ushida, T.; Fujishige, S. Biorheology, 1986, 23, 274. Osada, Y.;Hasebe, M. Chem. Lett., 1986, 1285. Japan Patent 53-46199, 1978. Vitta, S. B.; Stahel, E. P.; Stannett, V. T. J. Macromol. Sci., A, 1985, 22, 579. (6) Vitta, S. B.; Stahel, E. P.; Stannett, V. T. J. Appl. Polym. Sci., 1986, 32, 5799 (7) Hegazy, E.-S. Α.; Mokhtar, S. M.; Osman, M. B. S.; Motafa, A. I.-K. B. Radiat. Phys. Chem., 1990, 36, 365. (8) Charlesby, Α.; Fydelor, P. J. Radiat. Phys. Chem., 1972, 4 107. (9) Guimon, C. Radiat. Phys. Chem., 1979, 14, 841. (10) Lawler, J. P.; Charlesby, A. Radiat. Phys. Chem., 1980, 15, 595. (11) Sidorova, L. P.; Aliev, A. D.; Zlobin, V. B.; Aliev, R. E.; Chalkh, A. E.; Kabanov, V. Ya. Radiat. Phys. Chem., 1986, 28 407.



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(12) Grushevskaya, L. N.; Aliev, R. E.; Kabanov, V.Ya. Radiat. Phys. Chem., 1990, 36, 475. (13) Ishigaki, I.; Sugo, T.; Senoo, K.; Takayama, T.; Machi, S.; Okamoto, J. ; Okada, T. Radiat. Phys. Chem., 1981, 18, 899. (14) Ishigaki, I.;Sugo, T.; Senoo, K.; Okada, T.; Okamoto, J.; Senoo, K. J. Appl. Polym. Sci., 1982, 27, 1033. (15) Ishigaki, I.; Sugo, T.; Takayama, T.; Okada, T.; Okamoto, J.; Machi, S. J. Appl. Polym.Sci.,1982, 27, 1043. (16) Tanso, S.; Yoshida, S.;Senoo, K. Yuasa-Jiho (Japanese), 1985, 59, 25. (17) Omichi, H.; Okamota, J. J. Appl. Polym. Sci., 1985, 30, 1277. (18) Okamoto, J.; Sugo, T.; Katakai, A.;Omichi, H. J. Appl. Polym. Sci., 1985, 30, 2967. (19) Furusaki, S.; Okamoto, J.; Sugo, T.; Saito, K. Chem. Eng., 1987, 521. (20) Saito, K.; Yamada, S.; Furusaki, S.; Sugo, T.; Okamoto, J. J. Membr. Sci., 1987, 34, 107. (21) Omichi, H.; Chundury, D.; Stannett, V. T. J. Appl. Polym.Sci.,1986, 32, 4827. (22) Okamoto, J. Membrane (Japanese), 1989, 24, 277. (23) Pimg, Ζ. H.; Nguyen, Q. T.; Clement and, R.; Neel, J. J. Membr.Sci.,1990, 48, 297. (24) Hoffman, A. S. IAEA-TECDOC-486, Viena, Austria, 1988, 25. (25) Kaetsu, I. IAEA-TECDOC-486, Viena, Austria, 1988, 153. (26) Guimon, C. Radiat. Phys. Chem., 1986, 14, 841. (27) Harada, J.; Chern, R. T.; Stannett, V. T. RadTech '90 in North America, 1990, 1, 493. (28) Harada, J.; Chern, R. T.; Stannett, V. T. In Radiation Effects onPolymer;ACS Symp. Series, in press 1991. (29) Nablo, S. V.; Frutiger, W. A. Radiat. Phys. Chem., 1981, 18, 1023. (30) Ratner, D. B.; Hoffman, A. S. In Synthetic Hydrogels for Biomedical Applications,Andrade J. D., Ed.; ACS Symp. Ser. 31, 1976, 1. (31) Stoy, V. A. J. Biomat. Appl., 1989, 3, 552. RECEIVED August 26, 1991


Polyethylene Film Acrylic Acid Grafted by Electron-Beam

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