Superstructural hierarchy developed in coupled high shear/high thermal gradient conditions of injection molding in nylon 6 nanocomposites

In cast film extrusion, the polymer melt is extruded through a slit die, slightly stretched in air and then cooled on a chill-roll. An important part of the present work was devoted to the development and application of a thermomechanical'model of stretching in air and of a purely thermal model of cooling on the roll, able to take into account the polymer crystallization according to two different methods. The structures and morphologies of three series of extruded polypropylene films were investigated using various experimental techniques. These experimental results combined with theoretical predictions obtained from the model allow us to discuss the respective cffects of stretching in air and cooling on the roll, and to underline the importance of some key parameters. The series 1-f i lms make it possible to describe the influence of stretching in air and nucleation at the roll surface (transcrystallinity), and to show the general effects of the cooling conditions (cooling rates and thermal gradients) on the microstructure. In the scries 2 films, the role of the roll temperature TR is principally investigated. Eventually, in the third series, the influence of both the extrusion temperature T 0 and the roll temperature TR is analyzed. An important conclusion is that according to the values of To and TR , it is possible to obtain either a symmetrical or a dissymmetrical microstructure within the films.

Cast film extrusion of polypropylene. thermomechanical and physical aspects

Crystallographic Data and Melting Points for Various Polymers

Literature relating to the fundamental nature of the fibrous state is reviewed with the goal of illustrating evidence for the existence of a possibly fundamental morphological unit, the elemental microfiber or fibril. Following a short summary of the rather well documented situation in natural fibers, the problem of the microfiber in synthetic fibers is probed in terms of phenomena in solution and bulk polymeric systems. The reviewed literature indicates the existence of unique, elemental microfibers in synthetic fibers is largely tenuous; however, there is wide evidence of the existence of complex elongated structures axially aligned within macrofibers which future work may resolve into a central microfiber. The structural complexity appears to simplify for transformations occurring in high shear fields. This review covers articles through 1971, and it is notable that in 1971 approximately 100 articles of high relevance appeared in major journals.

Microfibers within fibers: A review

Light scattering by liquid crystals.

Features of the spherulites arising in uniaxially drawn streams of melted isotactic polypropylene

Isothermal crystallization in solutions of isotactic polypropylene

Polarization optical method of studying structural transitions in polymeric solutions and films

The effect of stretching on the process of structure formation in fiber spinning and film extrusion and in isothermal crystallization of crosslinked polyethylene has been investigated. It has been shown that the stretching forces (jet stretching) in these systems cause the formation of spherulites flattened with respect to the stretching direction. As a rule, the spherulite radius decreases while the degree of flattening increases with the jet stretching. Thermodynamic analysis of the kinetics and morphology of the spherulite growth during stretching enabled us to interpret these effects and to establish relationships connecting the degree of flattening of the spherulites with the characteristics of molecular orientation in the melt. Taking into account the changes in the conformational entropy of the deformed macromolecule (or the network) in the course of nucleation and crystal the deformed macromolecule (or the network) in the course of nucleation and crystal growth it is possible to explain not only the effect of the spherulite flattening but also the formation of tilted crystals and other peculiarities of crystallization taking place with the occurrence of molecular orientation. A general picture is given of the effect of a mechanical field on the morphology of the crystallizing polymers at various degrees of stretching.

Formation of supermolecular structures of semicrystalline polymers under molecular orientation

A relatively simple device is described for studies of phase transitions in polymer solutions and films by means of low-angle scattering of polarized light. The device differs from that described by Stein. It functions as a horizontal polarizing microscope, as a photodiffractometer, and as a photometer. A study was carried out of the structure formation in concentrated solutions of poly-γ-benzyl-L-glutamate with Mw = 105 in dioxane. At room temperature and in the absence of external fields a successive development of several levels of supermolecular order is observed. The structure formation starts with the spherulitic phase characterized by a cross-shaped diffraction pattern. During their progressive growth the spherulites fill the volume of the solution and finally begin to collapse. This changes the scattering pattern to a circular one. The mechanical instability of this phase and the thermodynamic advantage of a parallel packing of rodlike particles leads to the appearance of an orientational order in the system with a rhombic-shaped scattering pattern. The structure formation is completed with the development of a highly ordered liquid crystalline (cholesteric) phase characterized by a specific pattern similar to the x-ray diffraction patterns of oriented crystalline polymers. A detailed interpretation of rhombic scattering pattern is presented based on an analysis of polarized light scattering from a system of noncoherent scattering elements with the shape of a rectangular parallelepiped. Expressions are derived connecting the shape and the size of the scattering pattern with the dimensions of a single scattering element, and with the optical properties of this element and of the supporting medium. The spherulitic growth habit in isotactic polypropylene solutions when subjected to cooling also has been studied. At temperatures below 100°C. the spherulites appear and the rate of their growth is dependent on the thermal history of the solution. Kinetic curves are obtained showing the existence of a “kinetic memory” in the system which is manifested by a pronounced acceleration of growth of the structure when the fusioncrystallization cycles are repeated.

T.I. Volkov

Papers

Features of the spherulites arising in uniaxially drawn streams of melted isotactic polypropylene

Isothermal crystallization in solutions of isotactic polypropylene

Polarization optical method of studying structural transitions in polymeric solutions and films

Formation of supermolecular structures of semicrystalline polymers under molecular orientation

A study of the formation of supermolecular order in polymeric solutions by means of low‐angle scattering of polarized light