Doctoral Dissertations

Date of Award

5-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Joseph E. Spruiell

Committee Members

P. J. Philips, J. F. Fellers, D. C. Bogue

Abstract

A new technique based on Light Depolarizing Microscopy (LDM) for the study of nonisothermal crystallization of polymers was developed in this research. The cooling rates applied in this technique are not constant, and the polymer is cooled down by a cooling medium with constant temperature. The cooling rates applied in this dissertation ranged up to 2500 °C/min and the technique has a potential ability to reach much higher cooling rates. The temperature measurement was introduced into the LDM technique by using a thermocouple directly embedded in the polymer. A second light sensor was also introduced in the LDM technique to follow the light scattering effect caused by the numerous nuclei created under high cooling rates. This information is necessary in order to correct for the light scattering effect The developed technique includes a VCR system used for the measurements of spherulite growth during crystallization under high cooling rates. Two light intensities, temperature, and spherulite growth can be simultaneously measured in the developed technique.

A heat transfer analysis for the considered sample assembly used in the developed technique was carried out to check the influence of temperature distribution along the direction perpendicular to the sample plane. It was found that for a sample thickness less than 150 μm and cooling rates up to 2500 °C per minute the temperature distribution in the sample can be neglected. This heat transfer analysis thus supplies a basis for the believe that the temperature measurement in the technique is well controlled. Based on the heat transfer analysis, a cooling rate function (CRF) has been suggested for quantitatively describing the cooling condition applied in the experiments.

An optical analysis for the developed system was also made to supply the theoretical base for the technique. A new equation and a new quantity to correct for the light scattering effect encountered in the experiment were proposed based on this analysis. The proposed equation and quantity were successfully used in correction of the light scattering effect and was verified by numerous experiments. The optical analysis includes a potential ability to handle measurements of crystallization under orientation.

A power law nucleation rate function was proposed to analyze the kinetic data obtained by the technique. Based on the proposed nucleation function, the Avrami equation was derived and the Avrami index and rate constant were found to have new meaning. According to the nucleation function, various kinetic equations for nonisothermal crystallization appearing in the literature were found to be different forms of the Avrami equation. According to this new analysis the Avrami index can be separated into geometric and nucleation indexes, and from the crystallization constant, a nucleation rate constant can be calculated. The nucleation index and nucleation rate constant are the two parameters which determine the power law nucleation rate function. The nucleation function can thus be determined from the Avrami index and crystallization rate constant obtained through the usual Avrami analysis for the experimental kinetic data. From the determined nucleation function, the nucleation density as a function of time can be computed. Also through the power law nucleation rate function, the theoretical base for application of the Avrami equation to the nonisothermal kinetic data obtained through the developed technique was established.

The developed technique and theories were applied to six isotactic polypropylenes. Both overall crystallization rates and growth rates were studied during nonisothermal crystallization at rapid cooling rates. The growth rates in isothermal crystallization were also examined for the six iPPs to help understand the data obtained by the developed technique and supply the bridge connecting the technique to the traditional kinetic studies.

Important experimental facts discovered by the technique include the following. The growth rate is constant with time during nonisothermal crystallization under rapid cooling rates. The growth rate measured by the technique is well connected to the data obtained under isothermal condition; i.e., the data obtained by both the new technique and by the established isothermal crystallization method composed one picture describing the crystallization kinetics by growth rate. These facts suggest that the crystallization kinetic data obtained from the experiments can be explained by a local isothermal crystallization mechanism. The growth rate study by the technique thus extended the temperature range of growth rate studies to a much lower and wider range important to the understanding of crystallization occurring in polymer processing. The differences of the growth rates for different materials were found to be small in the examined range of molecular weight and molecular weight distributions. For the small possible difference in growth rate, for materials with different molecular weight and polydispersities, at high crystallization temperature, materials with lower molecular weight or broader polydispersity exhibit a higher growth rate, but these trends were found to be reversed in the lower crystallization temperature range. A possible mechanism has been suggested to explain these phenomena. Another important fact is that the overall crystallization rate is higher for materials which can produce higher nucleation rates during crystallization. The reason for materials producing higher nucleation rate may be attributed to a higher molecular weight and/or broader molecular weight distributions, or simply caused by different levels of nucleating impurities existing in the materials.

Equations describing the temperature dependence of nucleation rate constant and crystallization induction time were suggested according the experimental results obtained.

The crystallization kinetics data obtained for the six polypropylenes were used in developing an improved model for the melt spinning process. Other improvements in the melt spinning model included replacing the Newtonian viscosity equation by a generalized power law equation based on the data obtained in shear experiments for nine iPPs with different molecular weight and polydispersity, and introducing the dependence of molecular weight and polydispersity into the model. The improved model has been tested for variation in molecular weight, polydispersity, and spinning conditions including spinning speeds, melt temperature, ambient temperature, and mass throughput. Among these, the results from variation of molecular weight and polydispersity are most difficult to obtain and understand, and these results have been carefully discussed through key parameters such as crystallization onset stress, onset temperature, crystallization rate constant, Avrami index, nucleation rate constant, final birefringence, final crystallinity, final orientation functions and so on. By these tests, the model was found to give satisfactory simulations of the melt spinning process.

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