THEORY OF GEL-ELECTROPHORESIS IN HIGH FIELDS - TRANSIENT REGIMES AT THE FIELD ONSET

We develop a model for the reorientation mechanism of long-chain elect rolytes that are initially at rest in a gel and suddenly submitted to an electric field. Two different behaviors occur. For molecules smalle r than a critical size N-c, the reorientation takes place through the extremities. The tube hypothesis of the biased reptation model remains valid. For larger molecules the tube hypothesis breaks down; the moti on of the molecules involves lateral loops. N-c is a function of the e lectric field: N-c(E) proportional to exp(E(O)/E). It is a measurable quantity. When the field is switched on, the orientation of a molecule on the scale of the Kuhn segment increases. If the molecular size is smaller than N-c, the growth of the orientation is regular and charact erized by a single overshoot time tau(ov), proportional to molecular s ize. The growth rate is roughly inversely proportional to the size. At time tau(ov) the orientation is maximum. For molecules larger than N- c, the growth of the orientation occurs in two stages. The duration ta u(c) of the first stage is size independent. It depends only on the el ectric field: tau(c)(E) proportional to E(-1) exp(E(O)/E). During the first stage, the growth is size independent. The second stage lasts un til a time tau(ov) that is proportional to the size, as for small mole cules. During the second stage, the growth rate is roughly inversely p roportional to the size. Some of these predictions have been observed experimentally. More systematic investigations will be welcome.