Stable critical gels of a crystallizing copolymer of ethene and 1-butene

Crystallization of molten polymers involves a liquid-solid transition. In its first stage, the solidification process may be viewed as a physical gelation, which can be monitored with small-amplitude oscillatory shear experiments. A threshold crystallinity, X-cg, is required for the sample to reach the critical gel state (gel point). Samples of a metallocene random copolymer of ethene and II mol % 1-butene were quenched from the melt to a crystallization temperature T-X and then held at that temperature for isothermal crystallization. The objective was to find the highest value of T-X at which the polymer would crystallize only to the threshold crystallinity X-cg(infinity) and no further. The result would be a stable critical gel at a temperature T-X = T-cg(infinity). We start at large degrees of supercooling where the critical gel is only a transient state which is passed through as the sample solidifies to greater degrees of crystallinity. The gel time increases with temperature, obeying a power law, and can be extrapolated to infinity at 68.8 degrees C. The crystallinity at the gel point is about X-cg = 1%, almost independent of T. The rheological experiments are inherently difficult since the characteristic transition behavior, where tan delta is independent of frequency, occurs only in the terminal frequency region which is at very low frequencies for the sample of this study.