Small-angle neutron scattering from polystyrene—DVB networks containing a delta fraction of deuterated polystyrene: Evidence for aggregation during polymerization

Homopolymer polystyrene-divinylbenzene, PS-DVB, networks were made by free radical chemistry u.v. photopolymerization techniques, and characterized by small-angle neutron scattering (SANS). The networks contained a delta fraction of chains labelled with deuterium. Two different series of samples were made: Method A in which the insertion was made centrally or at the end of the polymerization; Method B in which the delta fraction of deuterated primary chains was inserted at the beginning of the polymerization. Method B samples yielded a molecular weight of about 70,000 g/mole and a radius of gyration of about 120 Å, values in the range expected for single primary chains. Method A samples, however, gave molecular weights of 1–2 million and radii of gyration ranging from 350 to 400 Å. Intrinsic viscosity and gel permeation chromatography (g.p.c.) studies on parallel fractions yielded weight-average molecular weights near 72,000 g/mole for most of the polymerization. These results strongly suggest the existence of aggregation of primary chains synthesized in the same conversion period within the network. A new method of characterizing crosslinking between network chains is proposed, since these experiments count the probability of a chain crosslinking with a chain polymerized immediately before in time.