A Study of the Kinetics of Polylactic Acid Crystallization by Image Processing
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A Study of the Kinetics of Polylactic Acid Crystallization by Image Processing F. Hernández-Sánchez, H. Carrillo-Escalante. Centro de Investigación Científica de Yucatán, A.C. Calle 43 No. 130, Col. Chuburná de Hidalgo, C.P. 97200, Mérida. Yucatán, México. Email: [email protected]
ABSTRACT The polylactic acid (PLLA) is commonly used in the biomedical application. The physical and mechanical properties of PLLA depend on the molecular weight, crystallinity, and synthesis. In this work, we study the PLLA crystallization kinetics. One of the main attractive of these polymers is that its degree of crystallinity can easily be modified, from amorphous state to a high crystallinity degree. In this study a new technique is applied to follow the crystallization kinetic. An optical microscope with cross polarizers, warming plates and video camera were used. A series of images were obtained under isothermal conditions to observe the growth spherulite. Since an image is composed by pixels and each pixel has a value from 0 to 255, which represents gray intensity, being zero for black and 255 are white. Then, the pixels average versus time was plotting and from the plots crystallization kinetic and secondary crystallization are easily obtained. INTRODUCTION Polylactic acid (PLLA) is mostly used in biomedical applications [1]. The physical and mechanical properties of PLLA depend on the molecular weight, the crystallinity and synthesis [2]. One of its main attractive is that the degree of crystallinity can be easily modified, from an amorphous state to a high degree of crystallinity. During the crystallization, the numbers of spherulites observed in thermal experiments depend on the isothermal conditions, and the lower the isotherm temperature is, the larger the number of spherulites is [3,4]. The temperature range, where isothermal crystallization occurs, is within supercooling. In this study, from the crystalline fraction, it is possible to determine the Avrami´s constants using software for the analysis digital photograph (images). During crystallization, the number of spherulites depends on the isothermal temperature [5]. The lower the isothermal temperature the larger is the number of spherulites. The temperature range where the isothermal crystallization takes place is the supercooling temperature [6]. A new technique is applied to follow the crystallization kinetic. An optical microscope with crossed polarizers, and equipped with warming plates, and a photographic camera were used. A series of images was obtained under isothermal conditions to observe the spherulites growth. Since the image is composed by pixels and each pixel has a value from 0 to 255, which represents gray intensity being zero a black and 255, white. Then the pixels average versus time was plotted and from this plot, the crystallization kinetics and the secondary crystallization are easily obtained. EXPERIMENTAL Materials and Methods. PLLA was synthesized by classical polycondensation procedures. The polymerization reactions were carried out as described elsewh
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