NETWORK STRUCTURES OF RADIATION-CROSS-LINKED STAR POLYMER GELS

The network structure of radiation-cross-linked solutions of star mole cules is analyzed theoretically and experimentally. We synthesize gels via electron beam irradiation of aqueous solutions of high-functional ity (f > 30) star PEO in the concentration regime where the molecules exhibit extensive overlap. The expected network properties of the resu lting gels are systematically varied by using PEO stars with different arm lengths and functionalities and by adjusting the irradiation dose and initial polymer concentration. The cross-link density in these ge ls is analyzed three ways: (1) elastic modulus, (2) swelling equilibri um, and (3) comparison to the cross-link density of linear gels prepar ed in a comparable fashion. In order to predict the network structure of cross-linked star molecules from swelling, Flory's equilibrium swel ling theory is modified to include a bimodal distribution of cross-lin k functionalities. The modified equation fits the trends in swelling b ehavior of the series of radiation-cross-linked star PEO gels. The app arent cross-link density of radiation-cross-linked star PEO gels obtai ned by physical property measurement-swelling and elastic modulus-diff er from the cross-link density expected based on comparison to linear gels cross-linked under identical conditions. Cross-link densities bas ed on the elastic modulus are lower than the expected values, suggesti ng that a significant fraction of elastically ineffective cross-links form between star af ms within the same molecule.