Joseph E. Spruiell

High speed melt spinning of poly(L-lactic acid) filaments

Poly(L-lactic acid) filaments were prepared by high speed melt spinning at take-up velocities up to 5000 m/min. The crystallinity, birefringence, tensile strength, Youngs modulus and yield strength all exhibit maxima at take-up velocities between 2000 and 3000 m/min. The boiling water shrinkage exhibits a minimum in this range. The maximum tensile strength of the as-spun filaments was 385 MPa and the maximum modulus was 6 GPa.

An experimental method for studying nonisothermal crystallization of polymers at very high cooling rates

A new technique based on light depolarizing microscopy was developed for studying non-isothermal crystallization of polymers at average cooling rates up to about 5000°C/min. The polymer is cooled down by a gaseous cooling medium supplied at a constant temperature. The temperature of polymer is measured by a thermocouple imbedded directly in the sample. A heat transfer analysis was used to establish appropriate sample geometry to assure that, under the applied cooling condition, the temperature distribution along the sample thickness can be neglected. A light-scattering effect, which occurs when crystallization is carried out under high cooling rates, was observed. This required the development of a method to correct the depolarized light intensity for the effect of light scattering. An appropriate correction method was developed based on both a theoretical and an experimental analysis of the light intensity measurement. This provided a means to measure the overall crystallization kinetics. Examples of such measurements for iPP, HDPE, and LDPE are presented. In addition to the overall crystallization kinetics, the developed technique includes a video camera and VCR system used for measurements of spherulite growth rates during crystallization under high cooling rates. Constant spherulite growth rates were observed for isotactic polypropylene crystallized under very non-isothermal conditions.

Isothermal melt- and cold-crystallization kinetics and subsequent melting behavior in syndiotactic polypropylene: a differential scanning calorimetry study

The isothermal melt- and cold-crystallization kinetics and subsequent melting behavior of syndiotactic polypropylene ( s-PP) were investigated using differential scanning calorimetry (DSC). The overall crystallization kinetics was determined by directly fitting the experimental data to the Avrami and Malkin macrokinetic models using a non-linear multi-variable regression program. When plotted as a function of crystallization temperature, the overall crystallization rate parameters for melt-crystallization process exhibited an unmistakable double bell-shaped curve, while those for cold-crystallization process showed the typical bell-shaped curve. Comparison of the overall crystallization rate parameters obtained for both melt- and cold-crystallization processes indicated that crystallization from the glassy state proceeded at a much greater rate than that from the melt state. Melting of samples isothermally crystallized at low and moderate crystallization temperatures exhibited multiple-melting phenomenon. Determination of the equilibrium melting temperature according to the “linear” and “non-linear” Hoffman‐Weeks extrapolative methods provided values of ca. 145 and 182 8C, respectively. q 2000 Elsevier Science Ltd. All rights reserved.

Crystalline memory effects in isothermal crystallization of syndiotactic polypropylene

Isothermal crystallization behavior after partial or complete melting of syndiotactic polypropylene was investigated by differential scanning calorimetry (DSC). On partial melting, the total concentration of predetermined nuclei was found to decrease with increasing fusion temperature and increasing time period that the sample spent at a specific fusion temperature. A significant effect of the rate of heating to the fusion temperature was also observed. On complete melting, the total concentration of predetermined nuclei was found to approach a constant value, which is the concentration of infusible heterogeneous nuclei (e.g., impurities, catalyst residues) present originally in the sample. At a specific fusion temperature, the concentration of predetermined athermal nuclei was found to decrease exponentially with the time period spent in the melt.

A WAXD/SAXS/DSC Study on the Melting Behavior of Ziegler-Natta and Metallocene Catalyzed Isotactic Polypropylene

A small- and wide-angle X-ray scattering study was performed on two metallocene catalyzed isotactic polypropylene (miPP) resins. The results were compared with two similar molecular weight Ziegler-Natta catalyzed isotactic polypropylene (zniPP) materials. Wide-angle X-ray diffraction (WAXD) results showed the existence of two crystalline structures in the metallocene samples, the α-monoclinic and γ-orthorhombic crystal structure, with increasing relative amounts of γ-orthorhombic phase as the lamellae thickness increased. Differential scanning calorimetry (DSC) scans exhibited a melting peak for each crystal structure. The metallocene resins had the same equilibrium melting temperature (186 ± 2 °C) as the high tacticity Ziegler-Natta (ZNHT) resin, whereas a second Ziegler-Natta resin had a lower equilibrium melting temperature (178 ± 2 °C). The equilibrium melting temperature for the γ-orthorhombic crystal structure in the metallocene resins was found to be 178 ± 4 °C. The results were explained by the distribution of defects within the miPP chains, generating higher fold surface free energies for the miPP resins.

Interpretation of the nonisothermal crystallization kinetics of polypropylene using a power law nucleation rate function

A nucleation rate function is proposed for use in analyzing the overall crystallization kinetics of polymers. This function allows for the possibility that the nucleation rate varies substantially during the crystallization. This feature is particularly useful in analyzing nonisothermal crystallization, but it can be used to analyze isothermal crystallization as well. The nucleation rate function was used in the derivation of a modified transformation kinetics equation of the Avrami type. The modified Avrami equation was found to be suitable for kinetics analysis for the data obtained from nonisothermal crystallization at rapid cooling rates. Kinetics parameters used to describe nonisothermal crystallization under rapid cooling rates are presented and discussed. These include crystallization induction time, plateau (crystallization) temperature, crystallization half-time, crystallization rate constant, Avrami index, and newly defined quantities called nucleation index, geometric index, and nucleation rate constant. The procedure used to obtain the nucleation rate constant and nucleation index for the nucleation rate function is described and illustrated by application to the analysis of the crystallization kinetics of polypropylene.

Microstructural stability of titanium-modified type 316 and type 321 stainless steel

Optical metallography, transmission electron microscopy, and X-ray diffraction from bulk extracted residues were used to investigate the microstructural stability in the temperature range 450°C to 950°C of a titanium-modified type 316 stainless steel and to compare this steel to a type 321 heat. The effect of cold deformation prior to aging was also investigated. Compared to standard type 316 stainless steel, the nucleation of M23C6 was delayed and its growth retarded in the titanium modified alloy due to early formation of TiC and Ti4C2S2 which reduced the carbon content in the matrix. Precipitation of the intermetallic σ and χ phases was faster in the titanium modified alloy. The type 321 material formed both M23C6 and the intermetallic phases less rapidly than either standard or titanium-modified type 316 steels. The relative tendencies toward intermetallic compound formation in various austentic stainless steels are discussed in terms of an “effective equivalent Cr content” remaining in the austenitic matrix after carbide precipitation. Cold work accelerated the precipitation rate of M23C6 and σ, but it suppressed χ formation due to preferential early σ formation. Early sigma formation was often associated with recrystallization of the cold worked matrix. Mechanisms accounting for this behavior are discussed.

Thermal Properties and Isothermal Crystallization of Syndiotactic Polypropylenes: Differential Scanning Calorimetry and Overall Crystallization Kinetics

Isothermal crystallization and subsequent melting behavior of five samples of syndiotactic polypropylene are presented. Crystallization studies were carried out in the temperature range of 60°C to 97.5°C using a differential scanning calorimeter (DSC). Subsequent DSC scans of isothermally crystallized samples exhibited double melting endotherms. The high melting peak was concluded to be the result of the melting of crystals formed by recrystallization during the reheating process. Overall crystallization kinetics was studied based on the traditional Avrami analysis. Analysis of crystallization times based on the modified growth rate theory suggested that, within the crystallization temperature range studied, the syndiotactic polypropylenes crystal- lize in regime III. Kinetic crystallizability parameters also were evaluated, and were found to be in the range of 0.41°C s 21 to 2.14°C s 21 .

Nonisothermal bulk crystallization studies of high density polyethylene using light depolarizing microscopy

The quiescent nonisothermal bulk crystallization kinetics of two high-den- sity polyethylene resins were investigated by a modified light-depolarizing microscopy (LDM) technique. The technique allows studies at average cooling rates up to 25007C/ min. The polymer was found to crystallize at a pseudo-isothermal temperature even at these very high cooling rates. The overall bulk crystallization rate increased rapidly as the cooling rate and supercooling increased. Crystallization kinetics was analyzed by Avrami analysis. Avrami exponents near 3 suggested spherical growth geometry and instantaneous nucleation at predetermined sites. Observation of spherulites by optical microscopy together with a number density of spherulites that changed little with increase in cooling rate or supercooling supported this model of crystallization behavior. Analysis of the half-time of crystallization based on the Lauritzen and Hoff- man secondary nucleation theory indicated that the regime II-III transition was found to occur at a degree of supercooling of approximately 227C. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 681-692, 1998

Application of the Avrami, Tobin, Malkin, and Simultaneous Avrami Macrokinetic Models to Isothermal Crystallization of Syndiotactic Polypropylenes

Various macrokinetic models (the Avrami, Tobin, Malkin, and simultaneous Avrami models) were applied to describe the primary crystallization of syndiotactic polypropylene (sPP) under isothermal conditions. Analysis of the experimental data was carried out using a direct-fitting method such that the experimental data were fitted directly to each macrokinetic model using a nonlinear multivariable regression program. Comparison of the kinetics parameters obtained from the program to those obtained from the traditional analytical procedure suggested that applicability and reliability of the direct-fitting method are satisfactory. Prediction of the time-dependent relative evolution of crystallinity at other crystallization temperatures was demonstrated based on the bulk kinetics parameters obtained from the analysis.