Title:

Crystallisation kinetics of polymer solutions

During the past two decades, it has been established that the crystallisation process in polymers is a phase transformation phenomenon akin to that occurring in low molecular weight compounds, The experimental crystallisation isotherms have been shown to obey, with a reasonable degree of precisions the Avrami equations, is the weight fraction of material remaining uncrystallised at time to The time exponentsn; is usiaIly found to have an integral value in the range of 1 to 4 and K is the rate ant involving both nucleation and growth processes. This equations when derived theoretically is based on three main assumptions (1) The nucleation is random in space and constant with respect to time. (2) The growth rate is constant and a linear function of time (3) The density of the growing crystalline phase is constant throughout the whole process. The above equation requires that 'n' should be an integer and sigmoidal curves should be obtained on plotting 0 against log t. Two types of experimental techniques have been used. Dilatometric measurements enabled the overall crystallisation process to be studied while microscopic observations of the separate growth and nucleation procesmeo enable the rate constant to bo measured by an independent medhod. Most of the early experimental work seemod to indicate integral values of the time ezponentp 0109 and these values vie e, then, used to provide information about the detailed crystallisation mechanism. Recent and more accurate Tio. has led to fractional values of ono being found experimentally and only one experimontal diiatorneti study has been made of the crystallisation of polymer.diluent mixtures from concentrated or moderately concentrated solutions. The results of this study deviated considerably from the original Avrami equation. The object of the present work was to Main more data on the crystallisation of polymercidiluent systems using both dilatometry and microscopy to try and gain some insight into the mechanism of the process. The system selected for study van polyethylene oxidediethyl sebacate. The polymer was chosen bemuse it is known to form large sphorulitic structures rather easily which facilitates microscopic determination of growth and nucleation rates The pure polymer we studied first and at all temperaturos, the crystallisation followed the simple Avrami equation with a constant value of n 2.5 t. 001 throughout the whole process. 14bnintegral values necannot arise Zrom the Avra41 equation and as the first two basic asoumptions noted abovo have been tested onperimontally, doubt ryas cast on the experimental validity of the last. Various densitytime relationships were used to modify the theoretical rate equation but none gave a constant value of it equal to 2.5. The results on the polymerdiluent systems showed that the nucleation process was basically heterogeneous as for the pure polymery and the growth rate was linear pith respect to time. The dilatometric results gave values of'ntwhich were a function of the weight fraction of unchanged material over a large portion of the crystallisation process. The initial value of ncwas 2.5v as in the case of pure polymer but after remaining at this value for a certain time which depended on the concentration of polymer in the mixture gng fell in a reasonably linear manner to 1.3 0.10. A reasonable interpretation of these re uta is that the crystalline phase begins to grow as a structure similar to that forming in pure polymer but after a certain time the diluent is incorporated into the crystalline phase leading to a reduced value of Wu Once again.5any physically reasonable densitytime relationship failed to give a theoretical equation which fitted the enperimental resulta.
