Isothermal crystallization of isotactic polypropylene at high pressures (from 200 to 250 MPa) have been carried out using a high-pressure dilatometer leading to a pure γ-form, as revealed by WAXD. The melting temperatures at the same pressures have been determined to assess the pressure effect on the equilibrium melting temperature by the Hoffman-Weeks analysis. The crystallization times have been analyzed according to the Hoffman-Lauritzen theory and corrections introduced to take into account the pressure effects on both the equilibrium melting temperature and the diffusion process. This analysis showed a transition from crystallization regime II to regime III for an undercooling of 53.7 K. The lateral and fold surface free energies were evaluated. Furthermore, the morphology of the samples investigated by polarized light microscopy, density measurements, and SAXS depends only on the crystallization undercooling, as a result of the influence of pressure on the equilibrium melting temperature. Moreover, the lamellar thickness experimental data agree well with the theoretical calculation.

ABSTRACT:The orientation of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) films with different morphologies were studied by wide-angle X-ray diffraction. Different structures were obtained by thermally treating biaxially stretched PEN samples. Virgin and thermally treated (1 h at 240, 250, and 260°C) samples of PEN bioriented films were characterized by DSC to determine the glass-transition temperature and the crystallinity ratio. To define the orientation of crystallites in the 25 m thick bioriented samples, pole figures were recorded for various PEN samples, as a function of their position in the transverse drawing direction. The significant result is that there is a dominant crystal population, whose c-axis direction varies from ϩ45°at one sample edge to Ϫ45°at the other edge, the orientation at the center being parallel to the transverse direction. There is also a secondary population, which can be seen only near the center. DSC studies also showed that by increasing the annealing temperature the crystallinity ratio was increased and pole figures showed that the texture was modified, probably because of disorientation mainly from an annealing temperature of 260°C.

RGSUMJ?L'Ctude des domaines cristallins au moyen de la diffraction des rayons X de fibres CtirCes bien cristallisCes a montrC que les polytCrCphtalates d'Cthykneglycol (la), de butanediol-1,4 (lb), de pentanediol-1,5 (lc) et d'hexanediol-1,6 (Id) cristallisent dans le systkme triclinique. L'evolution des param2tres ne se fait pas rCgulierement en fonction de la longueur du motif monombre. La connaissance du rCseau cristallin et de la structure chimique nous a conduit A proposer des conformations permettant aux chaines de s'arranger dans le rCseau et de proposer ainsi des hypothhes de structure pour les polyesters CtudiCs.Ces rCsultats sont confirm& par l'examen des spectres IR des Cchantillons cristallins qui presentent des bandes CaractCristiques de conformation trans tres importantes pour les polyesters l a et Id alors que les polyesters l b et lc presentent des bandes de structure gauche importantes. L'Ctude du taux de cristallinitC par differentes mCthodes (rayons X, calorimCtrie differentielie, spectres IR et colonne 2 gradient de densite) confirme la valeur proposCe pour le volume de la maille du polyester Id. SUMMARY:The study of the crystalline regions by means of X-ray diffraction of highly crystalline, drawn fibres, showed that the polyterephthalates from ethylene glycol (la), 1,4-butanediol (lb), 1,s-pentanediol (lc) or from 1,6-hexanediol (Id) crystallised in the triclinic system. The change of parameters is not a regular function of the length of the monomeric unit. The knowledge of the crystalline network and the chemical structure leads us to propose that the conformations allow the chains to rearrange themselves in the network, and so to suggest possible structures for the polyesters studied. The results were confirmed by IR spectroscopy of the crystalline samples which give strong characteristic bands indicating trans conformation for the polyesters l a and Id whereas the polyesters l b and l c give strong bonds corresponding to a gauche

The crystallization kinetics of virgin and nucleated polyamide 66 was investigated in non‐isothermal and isothermal conditions, by use of DSC and high pressure dilatometry. In non‐isothermal conditions, at atmospheric pressure, the results were first analyzed using the Ozawa equation, leading to an Avrami exponent of 4 in the case of the virgin grade and 2.1 for the nucleated polyamide. However, experimental and calculated kinetics showed a difference in the ending part of the crystallization, likely due to a large secondary crystallization phenomenon. Thus, the relative crystallinities were normalized to extract the primary crystallization from the whole signal and analyzed using the Douillard/Kim equation. In isothermal conditions, at atmospheric pressure, the Avrami exponents were similar to the values obtained in non‐isothermal conditions for both virgin and nucleated grades. Furthermore, the crystallization under pressure was examined in terms of crystallization supercooling. In non‐isothermal conditions, the crystallization supercooling (ΔTc) increases when the cooling rate increases, but it is not affected by the pressure since the crystallization temperature increase is due to the equilibrium melting temperature rise. In isothermal conditions, the supercooling (ΔT) rises according to the increase of the pressure for the same crystallization temperature.