Reexamination of slow relaxation of polymer chains in sol–gel transition

Using [2+2] photocycloaddition of coumarin groups attached to the PMMA chain backbone, we can gradually transfer a semidilute copolymer solution to a uniform speckle-free gel. We reexamined the effects of concentration, crosslinking density, solvent quality on the controversial scaling exponent (αs) between the linewidth (Γs) of the slow mode and the scattering vector (q) during the sol–gel transition. Our results suggest that the slow mode is attributed to large thermally agitated density fluctuation and revealed that the variation of αs is due to the relative length of 1/q to the correlation length (ξ) of the density fluctuation; namely, if ξ>1/q, αs→3, while when ξ<1/q, αs→2.