Diffusion of flexible and semirigid polymers confined to the pore spaces in porous glass

The dynamics of polymer chains in solution within the pore spaces of controlled pore glasses has been studied experimentally by using dynamic light scattering and theoretically by computer simulation. For flexible chains, we have explored the dependence of diffusion on the degree of confinement (i.e. on molecular weight or on R_h/R_p where R_h is the polymer hydrodynamic radius in free solution and R_p is the glass pore radius), molecular architecture, and time or distance scale of the diffusion measurement. Flexible chain diffusion over macroscopic distances may be understood in terms of hydrodynamic interactions with the pore wall at low and intermediate confinement, and in terms of an entropic barrier model at strong confinement. At intermediate scattering wavevector (qR_p ≅ 1) the apparent diffusion coefficient is time-dependent, and exhibits a crossover from the free-solution value at short times to a smaller macroscopic (hindered) diffusion coefficient value at later times. Semirigid chains experience much stronger hindrance than flexible chains. We have developed a model of the dynamics based on reptation theory. The mean squared displacement of a sufficiently long chain is expected to display a power law time dependence; the exponent depends on the time, changing from approximately 1/4 at short times to 1 at long times. Diffusion measurements of a series of poly(hexylisocyanate) polymers in porous silica display a definite crossover to much slower hindered diffusion at higher molecular weight.