2986 zyxwvutsrqpon Macromolecules zyxwvu 1986,19, zyxwvut 2986-2991 screening of hydrodynamic interactions and as such does not address the issue of molecular topology directly. A more quantitative interpretation of these results will be appropriate after several related studies have been conducted. In addition to the measurements of linear PS diffusion in this matrix referred to previou~ly,~~ mea- surements are in progress on linear and 3-arm-star PS in other PVME matrices. Light scattering studies are being undertaken to determine the PS coil dimensions as a function of matrix concentration, as is further rheological characterization of semidilute and concentrated PVME solutions. Nevertheless, it is evident from these data that reptation must be considered zyxwvut as a significant contribution to diffusion of linear polymers in thoroughly entangled solutions. However, it is equally clear that coil overlap is not a sufficient condition for reptation to be the dominant mechanism of diffusion. Acknowledgment. We thank Dr. L. J. Fetters for generously providing the branched polymer samples and Dr. G. D. J. Phillies for making available a preprint of his paper. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work. Registry No. PS, 9003-53-6; PVME, 9003-09-2. References and Notes (1) Tirrell, M. Rubber Chem. Technol. 1984, 57, 523. (2) de Gennes, P.-G. J. Chem. Phys. 1971,55, 572. (3) Phillies, G. D. J. Macromolecules 1986,19, 2367. (4) Daoud, M; de Gennes, P.-G. J. Polym. zyxwvuts Sci., Polym. Phys. Ed. 1979,17, 1971. (5) Klein, J. Macromolecules 1981,14, 460. (6) Graessley, W. W. Adu. Polym. Sci. 1982, 47, 67. (7) Fujita, H.; Einaga, Y. Polym. J (Tokyo) 1985,17, 1131. (8) Martin, J. E. Macromolecules 1986,19, 922. (9) Ogston, A. G.; Preston, B. N.; Wells, J. D. Proc. R. SOC. Lon- don, zyxwvu A 1973,333, 297. (10) Freed, K. F.; Edwards, S. F. J. Chem. Phys. 1974,61, 3626. (11) de Gennes, P.-G. Macromolecules 1976, 9, 594. (12) Cukier, R. I. Macromolecules 1984,17, 252. (13) Altenbereer. A. R.: Tirrell. M.: Dahler. J. S. J. Chem. Phvs. . , , . 1986, 84,-5122. (14) de Gennes. P.-G. J. Phvs. 1975.36. 1199. (15) Klein, J.; Fletcher, D.; Fetters, L. J. Nature (London) 1983, 304, 526. (16) Bartels, C. R.; Crist, B., Jr.; Fetters, L. J.; Graessley, W. W. Macromolecules 1986,19, 785. (17) Antonietti, M.; Sillescu, H. Macromolecules 1986, 19, 798. (18) von Meerwall, E.; Tomich, D. H.; Hadjichristidis, N.; Fetters, L. J. Macromolecules 1982,15, 1157. (19) von Meerwall, E.; Tomich, D. H.; Grigsby, J.; Pennisi, R. W.; Fetters, L. J.; Hadjichristidis, N. Macromolecules 1983, 16, 1715. (20) Lodge, T. P. Macromolecules 1983,16, 1393. (21) Hanley, B.; Balloge, S.; Tirrell, M. Chem. Eng. Commun. 1983, 24, 93, 1983. (22) Markland, P.; Lodge, T. P., submitted for publication in Polymer. (23) Hanley, B.; Tirrell, M.; Lodge, T. P. Polym. Bull. (Berlin) 1985, 14, 137. (24) Lodge, T. P.; Wheeler, L. M.; Hanley, B.; Tirrell, M. Polym. Bull (Berlin) 1986,15, 35. (25) Wheeler, L. M.; Lodge, T. P.; Hanley, B.; Tirrell, M., submit- ted for publication in Macromolecules. (26) Koppel, D. E. J. Chem. Phys. 1972,57, 4814. (27) Provencher, S. W.; Hendrix, J.; De Maeyer, L.; Paulussen, N. J. Chem. Phys. 1978,69,4273. (28) Bauer, B. J.; Hanley, B.; Muroga, Y., to be submitted. (29) Huber, K.; Burchard, W.; Fetters, L. J. Macromolecules 1984, 17, 541. (30) Ferry, J. D. Viscoelastic Properties zyxw of Polymers, 3rd ed.; Wi- ley-Interscience: New York, 1980. (31) Hashimoto, T., personal communication. Thermodynamic Interactions in Copolymeric Hydrogels Malcolm B. H u g h , * Mahmoud M. A.-M. Rehab, and Mat B. Zakaria' Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, England. Received July 31, 1986 ABSTRACT: Xerogels comprising n-butyl acrylate and N-vinyl-2-pyrrolidone (30/70 wt/wt) and varying small amounts of a hexafunctional cross-linking agent have been prepared by y-ray-initiated copolymerization to complete conversion and then swollen in water to equilibrium at several different temperatures (T) within the range 284-349 K. Photographic determination of the sample dimensions afforded the volume fractions of polymer (42) and water within the resultant hydrogels, and the effective cross-linking densities (u,) were obtained via stress-strain measurements. Values of G2 increased with increasing T and with increasing u,. The elastic moduli at 294 K lay within the range 0.21-1.07 MN m-2. Although the enthalpic components (xH) were negative, the large entropic components (xs) were positive and dominant in the overall increase with T of the polymer-water interaction parameter zyxwvu (x). The negative values for the enthalpy and entropy of dilution are compatible with structuring of water via enhanced hydrogen bonding and by hydrophobic interaction. Introduction In previous reports1t2 we have shown that by means of y-irradiation n-butyl acrylate (BA) can be copolymerized with N-vinylpyrrolidone (VP) to more than 99.9% con- version to yield a series of xerogels having a range of hy- drophilicity. These materials are insoluble in liquids that are capable of dissolving the corresponding low-conversion copolymers. Hence it is inferred that the y-irradiation not only effects initiation but also induces some cross-linking into the copolymers. Incorporation of a hexafunctional Present address: Jabatan Kimia, Universiti Kebangsaan Ma- laysia, Bangi, Selangor, Malaysia. 0024-9297/86/2219-2986$01.50/0 cross-linking agent, l,l,l-trimethylolpropane trimeth- acrylate (TPT),3 into the feed mixture enhances the cross-linking and also reduces the water content of the hydrogels produced by swelling the xerogels in water. The influence of composition, dose, and dose rate on the swelling characteristics4 of the copolymers and observations on the dissolved oxygen permeability coefficient5 in the hydrogels have been reported. In this paper an attempt is made to characterize some of these hydrogels more fully, with specific reference to (a) copolymer-water interaction and (b) the influence of temperature on the degree of swelling. In addition to those abbreviations already indicated, PEO, PTFE, PU, and PVAL are used to denote poly- 0 1986 American Chemical Society