Effects of the crystallization temperature on the crystal structure and its melting behavior of poly (l-lactic acid) (PLLA) have been investigated by means of wide-angle (WAXS) and small-angle (SAXS) X-ray scattering, optical microscopy, and differential scanning calorimetory (DSC). PLLA was found to crystallize as the α form when the crystallization temperature Tc was higher than 120 °C, while significant change in lattice parameters was seen for Tc's below 120 °C. The ratio of the a- and b-axis lengths begins to decrease with Tc below 120 °C and is 31/2 below 90 °C, which suggests a new crystalline form with hexagonal packing, namely, the α‘ form. The possible reason for α‘ formation is discussed. High-temperature WAXS and SAXS measurements showed that α‘ crystal transforms into ordered a form during heating. The transition takes place at 150 °C without a decrease in scattering intensity and without heating rate dependence. The mechanism for the transition is discussed.

Crystallization and Melting Behavior of Poly (l-lactic Acid)

The crystalline structure, spherulitic crystallization and melting behaviour of isotactic polypropylene (iPP) is comprehensively reviewed, illustrated and discussed. It is demonstrated and summarized how the nucleation, development and growth of the different spherulites depend on the crystallization and melting conditions both in quiescent and sheared melt. Based on the results the microtexture development during real processing conditions is elucidated for selected examples. Several subjects of intense debate on the above topics are clarified using evidence from polarized optical micrography and differential scanning calorimetry.

Supermolecular structure of isotactic polypropylene

Polymorphism in isotactic polypropylene

Crystallization processes in quiescent and moving polymer melts under heat transfer conditions

Mesophases in polyethylene, polypropylene, and poly(1-butene)

To account for the observed diffraction pattern of a commercial specimen of oriented fibers of isotactic polypropylene in the a modification, the crystal structure is described as belonging to the space group P 21/c but having a certain disorder in the distribution of “up” and “down” chain direction. As a result of this disorder, reflections having the (h + k) sum odd are weaker than in the ideal P 21/c space group, and the structure thus resembles partially the situation in the C 2/c space group where no such order is required or possible. In the specimen studied in this work, best agreement between theory and experiment (R = 0.158) was achieved if it was assumed that in about 25 % of matter the chains were placed in the wrong direction.

Crystal structure of isotactic polypropylene

The unit cell of poly(tetramethylene terephthalate) is triclinic with parameters a = 5.96 A, b = 4.83 A, c (fiber axis) = 11.62 A, α = 115.2. β = 99.9, and γ = 111.3°; space group P1, calculated crystalline density 1.41 g/cc. The plane of the benzene ring is found to be inclined by about 15° from the fiber axis, contributing to a shortening of the fiber period as compared to the period expected on the basis of analogy with other members of the terephthalate ester series. The remaining shortening of the fiber period occurs in the O°CH2°CH2segment of the chain. No abnormally short distances among neighboring chain atoms were observed. A typical texture pattern was found in specimens of this polymer that were cold rolled and subsequently annealed. In this texture the c axis of the unit cell is highly oriented in the rolling direction; the a and b axes of the unit cell are oriented preferentially so that the terephthalate residue lies as close as possible to the plane of rolling.

The crystal structure of poly(tetramethylene terephthalate)

Morphology of injection‐molded polypropylene

Injection-molded specimens of polypropylene were examined by x-ray diffraction for texture and orientation patterns as a function of the distance from the surface and of molding conditions. At least three distinct layers in the surface region whose texture patterns differ and an isotropic core of the moldings have been identified. These layers show preferred orientations; the axes of symmetry of the oriented textures lie in a plane passing through the direction of flow and perpendicular to the mold surface, but are inclined to the mold surface at a small angle. Some of these layers may be missing in conditions of either very high or very low shear during the injection-molding flow. A tentative explanation of the formation of the texture patterns is proposed.

Zdenek Mencik

Papers

Crystal structure of isotactic polypropylene

The crystal structure of poly(tetramethylene terephthalate)

Morphology of injection‐molded polypropylene

Texture of injection‐molded polypropylene

Crystal structure of polychlorotrifluoroethylene