Diffusion of Dendritic Polymers Through Concentrated Polymer Solutions
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Diffusion of Dendritic Polymers Through Concentrated Polymer Solutions James L. Thomas†, Wei Chen†, Yu Cheng‡, and Robert K. Prud’homme‡ † Department of Chemical Engineering, Columbia University, NY, NY ‡ Department of Chemical Engineering, Princeton University, Princeton, NJ Abstract Diffusional dynamics of polymers can be very sensitive to polymer architecture. Polymers with novel (or time-varying) architectures could facilitate the release of therapeutic compounds from gels or concentrated polymer solutions with unusual or novel kinetic profiles. Towards this end, we are studying the behavior of model dendritic polymers, the poly(amidoamine)(PAMAM) dendrimers, in aqueous solutions and in concentrated solutions of a “matrix” polymer, poly(ethylene oxide)(PEO). Fluorescence measurements of the environmental polarity of the dendrimers provide evidence for pHinduced confomational changes in mid-sized (generation 6), but not in small (generation 2) dendrimers. In aqueous solution, dendrimer diffusion measurements reveal the fractallike growth of these molecules, but measurements in aqueous PEO solutions failed to detect any pH dependence of the diffusion coefficient. Specific chemical interactions between the PEO and the PAMAM molecules may dominate their dynamic behavior. Introduction Polymer gels and polymer solutions are attractive matrices for controlled release formulations of pharmacologically active compounds. Polymeric systems have been designed to respond to temperature, pH, or metabolite concentration with changes in their structure and dynamics.1-3 Anchoring an active compound to a polymer with a novel architecture, such as a dendritic architecture, could produce novel diffusion and release kinetics in such systems. Star-branched polymers have been extensively studied in polymer melts and gels,4,5 and studies have shown that the polymeric arms do indeed entangle with the matrix polymer. Arm retraction appears to be a limiting step in the diffusive process, and consequently the number of arms is a critical determinant of diffusion rate. In this work, we report on the behavior of poly(amidoamine) dendritic polymers, both in solution and in concentrated solutions of a neutral polymer, poly(ethylene oxide). Poly(amidoamine) dendrimers have been studied for use in a number of biomedical applications, including DNA transfection6-8 and oral controlled release.9 There is still considerable debate concerning the spatial organization and dynamics of dendritic polymers. Polymer growth occurs (in a so-called divergent synthesis) by branching at each endgroup, resulting in exponential growth in the number of ends with increasing “generation” (number of synthetic cycles.) This exponential growth could result in surface crowding and the comcomittant formation of molecular cavities. 10 Alternatively, the chain ends could fold inward, relieving surface crowding.11-13 Dendrimer conformation could play an important role in the transport of dendrimers through polymeric media, since it can likely affect the propensity to form e
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