No assignee for this patent application, patent application serial number 842265, has been made.
Reporters obtained the following quote from the background information supplied by the inventors: "In polymeric crystals, polymer chains are arranged in a two-dimensional pattern. Due to statistical and mechanical requirements, a complete polymer chain cannot form a single straight stem, the straight stems being limited to a certain length depending on the crystallization temperature. As a result thereof, the stems fold and reenter into a lattice. This reentry can be adjacent to the previous stem or at a random lattice point. The perfectly ordered portion of a polymer is crystalline and the folded surface is amorphous. As such, polymers are semi-crystalline. The crystalline portion may occur either in isolation or as an aggregate with other similar crystals leading to the formation of mats or bundles or spherulites.
"The first step in the formation of spherulites, wherein a straight stem of a polymer chain called a nucleus forms from a random coil, is called nucleation. The rest of the process that includes lamellae growth and spherulite formation is cumulatively called crystal growth. In general, single crystals take the form of thin lamellae that are relatively large in two dimensions and bounded in the third dimension by the folds. Typically all the lamellae within one spherulite originate from a single point. As the spherulite grows, the lamellae get farther and farther apart. When the distance between two lamellae reaches a critical value, they tend to branch. Since the growth process is isotropic, the spherulites have a circular shape in two dimensions and a spherical shape in three dimensions for solidification in a uniform thermal field.
"A certain degree of crystallinity is often desired during injection molding or extrusion operations due to the higher thermal and mechanical stability associated therewith. If the crystallization rate is slow or uneven, the resultant product properties may have a wide variation in morphology, creating a potential for lines of imperfection that may lead to material failure and result in lower production capacity and reduced quality of the final product.
"Absorbable polymers are known to be generally slow crystallizing materials. As is well known to those skilled in the art, poly(L-lactic acid) (PLLA) belongs to the group of very slow crystallizable polyesters. High molecular weight PLLA crystallizes with even more difficulty, due to the reduced mobility of its highly entangled macromolecules. The crystallinity of different molecular weight PLLA polymers (18,000, 31,000, 156,000 and 425,000 g/mol) has been studied by calorimetric methods (see: Clinical Materials, 1991, 8(1-2), 111. As demonstrated by that study, during cooling from the melt (rate=-0.5.degree. C./min), only the lower molecular weight polymers were able to develop any measurable crystallinity.
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