Gel formation via physical cross-linking in the soluble conjugated polymer, poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], in solution by addition of alkalies

In poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) solution in 1,3-dichlorobenzene (DCB), an addition of nonsolvent alkane can lead to a formation of gel and a growth of the content of aggregates (with intersegmental pi-pi interaction) on standing without stirring. Such a gel then becomes immobile after standing for about 13 h but mobile again after a vigor agitation. The viscosity of MEH-PPV solution in DCB is found to decrease with the addition of nonane until a minimum is reached at 30 vol % and then increase rapidly with further addition. Thus, we propose that the gel formation resulted from an occurrence of interchain physical cross-linking via interaction between neighboring alkoxy side chains with nonane as a bridge. The generated aggregates are found to be of intrachain because MEH-PPV chains tend to adopt a coil conformation in the presence of nonane as revealed by an initial drop of solution viscosity upon addition of nonane. Additions of other alkanes are investigated also, but a complete gelation can happen only as the linear chain length of alkanes is more than nine carbons.