Observation of the first-order transition in ultrafiltration of flexible linear polymer chains.

Using a special double-layer membrane to avoid interaction among flow fields generated by different pores, we have, for the first time, observed the predicted discontinuous first-order transition in ultrafiltration of flexible linear polymer chains. Namely, the chain could pass through a pore much smaller than its unperturbed radius only when the flow rate is higher than a certain value. When only one chain and one pore are considered in theory, such a threshold is surprisingly independent of both the chain length and the pore size. Our results reveal that for a membrane with many pores and at a microscopic flow rate () lower than the threshold, the inevitable blocking of some pores by longer nonstretched coiled chains increases in those unblocked pores because the macroscopic flow rate () is a constant. Long chains have two populations, coiled and stretched, in a real ultrafiltration experiment when is lower than the threshold.