INTERFACIAL COIL-TO-GLOBULE TRANSITIONS - THE EFFECTS OF MOLECULAR-WEIGHT

The coil-to-globule transitions of fractionated poly(N-isopropylacryla mide) chains attached to electrosterically stabilized polystyrene late x particles were studied for molecular weights ranging from 3 x 10(5) to 2 x 10(6). The shapes of the transitions proved to be a sensitive f unction of the polymer molecular weight: the component of the transiti on that occurred under better than B-solvency conditions increased sig nificantly (from 30 to 80%) as the molecular weight was decreased by a n order of magnitude; concomitantly, the component that occurred under worse than theta-solvency conditions was substantially decreased. Stu dies carried out with C-13 NMR supported the hypothesis that these two types of transitions were phenomenologically different. The transitio ns in worse than theta-solvents can be attributed to the strong attrac tive binary interactions that characterize worse than theta-solvents, whereas the transitions in better than theta-solvents arose from the w eaker attractive many-body interactions, i.e. n-clustering. Simple sca ling law considerations imply that the component of the coil-to-globul e transitions arising from ii-clusters should decrease with increasing molecular weight of the interfacial polymer, in conformity with the e xperimental data.