Nucleation and Spherulitic Growth Rates in Thin Films With Embedded Thermocouples

Abstract

Fine bare wire thermocouples (12.5 and 25 μm) embedded in polymer films have been used to generate cooling curves in a series of studies, reported earlier, involving crystallization at cooling rates as high as 2000°/min and comparable to those used in film manufacture in industry. The rapid cooling method has resulted in the extension of basic studies of crystallization to supercoolings previously inaccessible using existing methods. The results have been very enlightening and have verified existing theories, but have also generated unique information and heretofore unobserved significant changes in behavior of several polymers. The most startling result is the conclusion that the growth face of a crystal is at a higher temperature than any temperature that can be measured, thus raising serious questions regarding the interpretation of experimental data. The current study investigates the effects of embedding thermocouples in thin films of linear polyethylene on the nucleation and growth rates of spherulites in close proximity. Row nucleation is observed along the surface of the thermocouple, with lamellae growing preferentially perpendicular to the thermocouple axis. When growth rates of spherulites are plotted against spatial distance from the thermocouple, a sigmoidal dependence is observed. The distance at which the growth rate becomes independent of proximity to the thermocouple, or in other words, the width of the `characteristic zone of influence' is found to be a function of thermocouple size and cooling rate. The most remarkable finding is that spherulites growing within this characteristic zone displayed asymmetry in growth rates with radial direction. Consequently, the spherulites exhibit ellipsoidal or irregular shapes and orientation depending on the applied cooling rate. Two independent, albeit simultaneously occurring phenomena are identified to explain the observed behavior: (i) induced mechanical stress, that affects the overall orientation of the spherulites with respect to the thermocouple axis, and (ii) thermal gradient introduced by the heat conduction effect of the thermocouple. However, the relative contribution of these two factors is a function of the applied cooling rate; the behavior being dominated by stress under rapid cooling conditions, whereas the thermal gradient governed the behavior in slow cooling.

Crystallization kinetics modeling of high density and linear low density polyethylene resins

“…As mentioned earlier, Patki and Phillips7, 8 have measured isothermal crystallization rates of a HDPE resin (0.954 g/cc, M w = 101,300 g/mol, predicted melt index = 2) at high supercoolings using an elaborate technique of modified light depolarization microscopy method. They showed, for the first time experimentally, the characteristic maximum in bulk crystallization and spherulitic growth rate of a HDPE resin.…”

“…The K o values for the HDPE resin at each cooling rate for these values of C 3 are tabulated in Table III. It can be seen from Table III that 6) and (7) for the HDPE Resin…”

“…Thus, experimentally obtained isothermal crystallization rates by such standard techniques needs to be extrapolated to lower temperatures (high supercoolings) 6. Recently, a pioneering experimental technique has been developed by Patki and Phillips for measuring isothermal crystallization rates of rapidly crystallizing polymers (e.g., high density polyethylene) at high supercoolings, enabling the measurement of the temperature at which the crystallization rate goes through a maximum as well as the maximum rate of crystallization 7, 8. However, such techniques are quite elaborate and time consuming to get the desired crystallization rate data at high supercoolings.…”

Flash DSC crystallization study of blown film grade bimodal high density polyethylene (HDPE) resins. Part 2. Non‐isothermal kinetics

“…The technique consists of using a thermocouple imbedded directly in the sample; this thermocouple measures the temperature of the polymer. This technique, although effective, is very time consuming and elaborate, compared with another innovative technique presented by Patki and Phillips in 2007 . The Patki and Phillips technique enables the measurement of the temperature at which the crystallization growth rate reaches a maximum.…”