Electrophoretic migration of submicron polystyrene latex spheres in solutions of linear polyacrylamide

The electrophoretic migration of highly cross-linked carboxylated polystyre ne latex spheres of 55, 140, and 215 nm radius (R) in solutions of linear p olyacrylamide of 0.4 x 10(6), 0.6 x 10(6), and 1 x 10(6) molecular weight, in the 0.1-1% concentration range, was studied by capillary zone electropho resis. The electric field strengths applied ranged from 40 to 530 V/cm. At the ionic strength used, these particles must be considered "large", exhibi ting kappa R greater than or equal to 13 where kappa(-1) is the thickness o f electric double layer. In the semidilute polymer concentration regime, th e radius of the particles severalfold exceeds the average mesh size, xi, in the polymer network. It was found that particle retardation (expressed as mu/mu(0) where mu and mu(0) are particle electrophoretic mobilities in poly mer solution and buffer alone) at a given polymer concentration decreases w ith both increasing particle size and electric field strength but increases with polymer molecular weight. The dependence of retardation on polymer co ncentration, c, follows a "stretched exponent", mu/mu(0) = exp(-alpha c''). The prefactor a and the exponent nu are particle radius and electric field strength dependent. The microviscosity of polymer solutions defined as mu( 0)/mu was well below values of zero shear viscosity measured viscometricall y even when no dependence of microviscosity on electric field strength was observed. These findings were interpreted in terms of (i) a local shearlike deformation of the polymer network upon particle passage, resulting in a p rogressive decrease of the network entanglement density at the particle loc ales with particle translational velocity and, thus, a decrease of network resistance to particle penetration; and (ii) a progressive polymer depletio n near the particle surface, with increasing particle radius at the scale o f R/xi, which facilitates electrophoretic migration of the microparticle in the polymer solutions.