Probe dynamics in semidilute polymer solutions and gels

We investigate the dynamics of fluorescently labeled linear macromolecules and spherical particles being enclosed in semidilute polymer matrixes. The experiments were designed such that the transition from a semidilute solution to a permanent network could be covered. This was achieved by employing a matrix polymer, polyacrylamide, carrying pendent dimethylmaleimide groups. Stepwise irradiation of such samples in presence of a triplet sensitizer causes successive dimerization of the maleimides leading to progressive crosslinking. Mobilities of tracers enclosed in these systems were estimated by fluorescence recovery after photobleaching. Parameters varied were the concentration of matrix polymer (20-80 g.L-1) as well as the molar masses (200,000-1,300,000 g.mol(-1)) and particle radii (17 and 36 nm) of the labeled probes. The results show notable differences between the behavior of linear and spherical tracers: while the mobility of flexible linear chains remains nearly unaffected by the transition from a semidilute polymer solution into a chemically crosslinked network, rigid spheres get completely immobilized when the degree of crosslinking exceeds a certain threshold.