Partitioning and tracer diffusion of polymers in porous material

The partitioning of solvated polymer molecules with a bimodal molecular weight distribution between a confined geometry such as a porous medium and an external free volume is investigated. It is found that, as the concentration of the high molar mass macromolecule exceeds the overlap concentration in the exterior, the low molar mass fraction is driven into the restricted pore channels, while the high molar mass fraction remains outside. It is noted that this segregation effect could in principle be used to fractionate polydisperse polymers in solution. A novel scheme for the fractionation of polydisperse polymers in solution by utilizing nano-scale porous materials is presented. The efficiency of this fractionation scheme is estimated and the protocols to be followed for an optimal separation are proposed. The tracer diffusion of probe polystyrene (PS) molecules in a matrix polymer solution confined in a porous medium is studied by using dynamic light scattering (DLS). In these experiments, a sample of controlled pore glass was equilibrated with a ternary solution consisting of PS, the matrix polymer (polytetrahydrofuran (PTHF) or poly(vinyl methyl ether) (PVME) at higher matrix concentration), and the common solvent 2-fluorotoluene (2FT); the matrix polymer and 2FT are isorefractive with the porous glass. The PS concentration was held low, whereas the concentration of matrix polymer exterior to the pore was systematically increased within the homogeneous regime. The tracer diffusion coefficient D$\sb{\rm p}$ of PS in the interior of the pore decreased more rapidly than did the tracer diffusion coefficient D$\sb{\rm f}$ in the exterior as matrix concentration exterior to the pore increased in the range below the overlap concentration; this is ascribed to the enhanced polymer-polymer interactions in the narrow pore channels. However, D$\sb{\rm p}$ showed a weaker dependence on matrix concentration than D$\sb{\rm f}$ as matrix concentration exceeded overlap concentration. This result is ascribed to the smaller topological constraint exerted by matrix molecules in the interior of the pore than in the exterior free solution because of the lower interior matrix concentration