The effect of polydispersity on surface segregation of a lower molecular weight polymer component in a higher molecular weight linear polymer melt host is investigated theoretically. We show that the integrated surface excess z(M) of a polymer component of molecular weight M satisfies a simple relation z(M)=2U(e)(M/M-w-1)phi(M), where M-w is the weight averaged molecular weight, phi(M) is the polymer volume fraction, and U-e is the attraction of polymer chain ends to the surface. U-e is principally of entropic origin, but also reflects any energetic preference of chain ends to the surface. We further show that the surface tension gamma(M) of a polydisperse melt of high molar mass components depends on the number average degree of polymerization M-n as, gamma(M)=gamma(infinity)+2U(e)rho bRT/M-n. The parameter gamma(infinity) is the asymptotic surface tension of an infinitely long polymer of the same chemistry, rho(b) is the bulk density of the polymer, R is the universal gas constant, and T is the temperature. The predicted gamma(M) compare favorably with surface tension values obtained from self-consistent field theory simulations that include equation of state effects, which account for changes in polymer density with molecular weight. We also compare the predicted surface tension with available experimental data. (c) 2007 American Institute of Physics.
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