Effect of Cross-Linking on Dynamics of Semidilute Copolymer Solutions: Poly(methyl methacrylate- c o -7-acryloyloxy-4-methylcoumarin) in Chloroform

7-Acryloyloxy-4-methylcoumarin (AMC) can undergo a [2 + 2] photodimerization when irradiated with an UV light of 310 nm. Its copolymerization with methyl methacrylate (MMA) resulted in a photo-cross-linkable copolymer P(MMA- co -AMC), which enabled us to study the effect of cross-linking on the dynamics of semidilute solutions of such a copolymer in a controllable fashion. For each semidilute copolymer solution studied before the photoreaction, dynamic laser light scattering showed a fast and a slow relaxation mode respectively with line widths Γ f and Γ s . They were scalable to the scattering vector q as Γ f ∼ q α f with α f ≈ 2.0 ± 0.1, which is the hallmark of cooperative diffusion relaxation, and Γ s ∼ q α s with α s ≈ 3.0 ± 0.2, which could be related to internal motions of large transient clusters made of the entangled chains. Two relaxation modes were also observed during the photoreaction. The photo-cross-linking of the chains in less concentrated solutions (<3 × 10 -2 wt %) only resulted in finite clusters, where α f and α s essentially remained, indicating that there was no change in solution dynamics. On the other hand, in more concentrated solutions (>5 × 10 -2 wt %), transparent and homogeneous gels were eventually formed after a long time irradiation. As the cross-linking proceeds, the fast relaxation contributes less and less. At the same time, α f increases, but α s decreases, revealing that the diffusion relaxation are gradually mixed with the internal motions and finally diminished in the gel state.