Effect of Stirring on the Gelation Behavior of High-Density Polyethylene Solutions

In an earlier work we reported that polyethylene solutions of ultrahigh molecular weight once stirred at temperatures above the dissolution temperature of normal crystals form gels on subsequent cooling, even in the quiescent state. Following this observation and the earlier recognition by Pennings of changes in the shear stress of a polyethylene solution during stirring at high temperatures, the present work was undertaken, fundamentally, to examine whether there existed a correlation between the shear stress and its different stages of development with stirring time at different stirring temperatures and concentration on the one hand and the gel formation on subsequent cooling in the quiescent state on the other. Indeed, clear-cut correlations were observed together with information on the associated relaxation times and on the strength and thermal stability of the gels. From this we could deduce the existence of a gel-forming association gradually building up across the fluid gap followed by its breaking up into gel particles which continue to impart gel-forming ability to the liquid when cooled after cessation of stirring. The underlying basic association itself, while temporary, is most likely to be of a crystalline nature and thus of relevance to flow-induced crystallization and shish kebab formation for which they are likely to be precursor stages. The detailed nature of the latter is to be explored. Our findings, however, should also be relevant to all conventional solution studies relying on chains being molecularly dispersed. We discuss this work in the light of similar observations on noncrystalline polymers made during the 1960s and some recent theoretical developments.