Dynamics of the swelling or collapse of a homopolymer

We study the dynamics of a polymer when it is quenched from a $\theta$ solvent into a good or bad solvent by means of a Langevin equation. The variation of the radius of gyration is studied as a function of time. For the first stage of collapse or swelling, the characteristic time-scale is found to be independent of the number of monomers. Other scaling laws are derived for the diffusion regime at larger times. Although the present model is solved only for homopolymers and doesn't include hydrodynamic interactions, these results may be a first step towards the understanding of the early stages of protein folding.