Strong stretching of polymer chains caused by polymerization of their end groups

The elastic properties of hydrogels obtained by end-group radical polymerization of poly(ethylene oxide) dimethacrylates with M = 4000 and 12000 were studied. For gels prepared at high concentration of poly(ethylene oxide) dimethacrylate, the shear modulus G increases with increasing degree of swelling Q. The increase in G is observed even at small deviation of e from the initial value Q(o). This indicates that poly(ethylene oxide) (PEO) chains are considerably stretched and begin to exhibit non-Gaussian properties when incorporated into the network. It is supposed that stretching of the PEO chains is due to high functionality of the network junctions. In the case studied, the network junctions are represented by polymethacrylate chains, which most likely form globules in an aqueous medium. The PEO chains connecting network junctions are stretched due to topological restrictions similar to those occurring in starlike macromolecules or spherical polymer brushes with high grafting density. The stretching coefficient alpha(0) estimated by describing the experimental dependence G(Q) in terms of the inverse Langevin function increases from 1.6 to 3.2 when the concentration of chains varies from 12 to 21 wt %, and decreases with increasing molecular mass of the chains. The functionality of the network junctions, calculated from ore, falls with in the range from 20 to 300, which is consistent in the order of magnitude with the data available in the literature on the degree of polymerization of macromonomers.