M. Dosière

A dual detector single readout system for simultaneous small (SAXS) and wide-angle X-ray (WAXS) scattering

Abstract A data acquisition system is described which allows to simultaneously record X-ray scattering patterns in different angular regimes both with high spatial and temporal resolution. It consists of two linear detectors with delay-line readout connected in series. A few examples illustrate its application in the study of the polymorphism of lipid systems and their phase transitions as well as of synthetic polymers.

Crystal growth rate as a function of molecular weight in polyethylene crystallized from the melt: an evaluation of the kinetic theory of polymer crystallization

The aim of this paper is a critical evaluation of the classical kinetic theory of polymer crystallization. This theory, even the most recent version of Hoffman and Miller4 does not explain the kinetic data of Labaig2 on crystallization of polyethylene (PE) from the melt. We define various characteristic lengths which are significant in Hoffman and Millers approach4 and state that their values, as calculated from the theory, are not consistent with assumptions which are basic to the model. Lastly, we recall some conceptual shortages of the theory. We conclude that the occurrence of regime I is completely hypothetical and that the use of the log G versus1TΔT plot to determine the product σσe of the macroscopic interfacial free enthalpies of the lateral and of the fold surfaces is unjustified.

Static and quasielastic light scattering from solutions of poly(ethylene oxide) in methanol

Abstract Static and quasielastic light scattering measurements on dilute and semi-dilute solutions of monodisperse poly(ethylene oxide) in methanol have been carried out at 25°C. Four PEO fractions having weight average molecular weights equal to 31 500, 90 000, 230 000 and 904 000 and degrees of polydispersity lower than 1.1 were used. The angular dependence of Zimm plots shows no downturn at low angles. In addition, monomodal distribution curves were computed from the quasielastic measurements, confirming that PEO are molecularly dispersed in methanol at 25°C. Experimental scaling laws for the radius of gyration R G , the second virial coefficient A 2 and the hydrodynamic radius R H have been determined. The exponents characterizing these scaling laws confirm that methanol is a good solvent for this polymer. Finally, the overlap concentrations corresponding to the transition between the dilute and semi-dilute regimes were determined for the different PEO samples.

Characterization of molecular heterogeneities of LLDPE by multiple crystallization–dissolution steps

Abstract This article demonstrates that a fractionation technique, based on molecular segregation during the crystallization from the melt and dissolution of the crystals, allows a complete characterization of the molecular heterogeneity in the comonomer distribution of linear low density polyethylene. An ethylene–1-hexene copolymer with a bimodal short chain branching distribution has been fractionated by multiple step crystallizations followed by selective dissolutions in tetrachloroethylene. The relative amount of each component of the distribution in each fraction was determined from the melting curves of isothermally crystallized fractions.

Morphology and crystal structure of the poly(ethylene oxide)–hydroquinone molecular complex

The occurrence of a molecular complex between poly(ethylene oxide) (PEO) and p-dihydroxybenzene (hydroquinone) has been determined using different experimental techniques such as differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and Fourier transform infrared spectroscopy (FTIR). From DSC investigations, an ethylene oxide/hydroquinone molar ratio of 2/1 was deduced. During the heating, the molecular complex undergoes a peritectic reaction and spontaneously transforms into a liquid phase and crystalline hydroquinone (incongruent melting). A triclinic unit cell (a = 1.17 nm, b = 1.20 nm, c = 1.06 nm, α = 78°, β = 64°, γ = 115°), containing eight ethylene oxide (EO) monomers and four hydroquinone molecules, has been determined from the analysis of the X-ray diffraction fiber patterns of stretched and spherulitic films. The PEO chains adopt a helical conformation with four monomers per turn, which is very similar to the 72 helix of the pure polymer. A crystal structure is proposed on the basis of molecular packing considerations and X-ray diffraction intensities. It consists of a layered structure with an alternation of PEO and small molecules layers, both layers being stabilized by an array of hydrogen bonds. The morphology of PEO–HYD crystals was studied by small angle X-ray scattering and DSC. As previously shown for the PEO–resorcinol complex, PEO–HYD samples crystallize with a lamellar thickness corresponding to fully extended or integral folded chains. The relative proportion of lamellae with different thicknesses depends on the crystallization temperature and time. Finally, the observed morphologies are discussed in terms of intermolecular interactions and chain mobility.

Metastabilities of lamellar crystals of molecular complexes

Abstract The molar stoichiometry and the crystal structure of four molecular complexes between poly(ethylene oxide) (PEO) and p -dichlorobenzene (PDCL), resorcinol (RES), hydroquinone (HYD) and p -nitrophenol (PNP) are compared. Even with a polymer weight fraction of 0.5, these molecular compounds form lamellar crystals when crystallized from the melt. The morphology of the lamellar crystals of these four PEO molecular complexes has been investigated in the early stages and after prolonged times of crystallization from the melt by time-resolved small-angle X-ray scattering coupled with wide-angle X-ray diffraction and differential scanning calorimetry. Non-integral-folded chain (NIFC) crystals are observed in the early stage of crystallization of PEO–RES and PEO–PNP molecular complexes from the melt. NIFC crystals of PEO–RES transform into integral-folded chain (IFC) and extended chain (EC) lamellar crystals with time. NIFC, 1-IFC and EC crystals can coexist in a sample of PEO–RES molecular complex. NIFC crystals of PEO–PNP do not transform into IFC or EC crystals even after long crystallization times or after heating until they melt. Stacks of IFC and EC crystals are present in the early stage of crystallization of PEO–HYD complexes at all the range of crystallization temperatures studied. The relative proportion of EC crystals of PEO–HYD increases with the crystallization temperature. PEO–PDCL forms only EC crystals at high crystallization temperatures ( T c >50°C). The type and magnitude of the interactions between host and guest molecules account for the different lamellar morphologies of these four molecular complexes. PEO–RES forms banded and unbanded spherulites in the temperature range 20 to 55°C. Banded spherulites are metastable and transform into unbanded spherulites through a solid-phase transition.

Use of the Hough transform to determine the center of digitized X-ray diffraction patterns

Abstract The use of area detector of X-ray diffraction yields digitized information. The position of the incident beam on such recorded images is needed with a high precision for a careful determination of the Bragg spacings. An application of the Hough transform is presented to obtain the coordinates of the center of the X-ray pattern with a good accuracy and without any important intervention of the operator.

Orientation and nonlinear optical properties of 4-(N,N-dimethylamino)-3-acetamidonitrobenzene crystals on nanostructured poly(tetrafluoroethylene) substrates

The growth of 4-(N,N-dimethylamino)-3-acetamidonitrobenzene (DAN) crystals on nanostructured poly(tetrafluoroethylene) (PTFE) substrates has been investigated by means of x-ray, micro-FTIR (Fourier-transform infrared) and nonlinear optical ellipsometry techniques. Two types of structural arrangements were identified, depending on growth conditions. In both cases, the (001)DAN plane contacts the (100)PTFE substrate plane. The major difference between both types of samples lies in the degree of azimuthal orientation at the interface. For thin samples (thickness 1 μm) are characterized by a uniaxial orientation with an isotropic random distribution in the plane of the layers. The comparison between various techniques used in this study to characterize the obtained thin films showed a remarkable agreement. In this context, ...

New dynamical effects in spherulitic growth

Abstract Two unexpected phenomena have been observed during the study of the growth of α and β spherulites of a poly(ethylene oxide)-resorcinol complex: an overgrowth rate (denoted as G i ) of the fastest, namely the α-form, along the interface of the β-form, and a transformation of the β-form into the α-form, which propagates as a dynamic front at a constant rate G t . The evidence for these two phenomena is demonstrated, and possible explanations are discussed. Measurements of the two rates, G i and G t , are also reported.

ON THE EFFECT OF REACTION CONDITIONS ON MORPHOLOGY OF AROMATIC POLY(ETHER-KETONE)S, PEKK*

Aromatic poly(ether ketone ketone)s (PEKKs) of regular structure, containing 1,3- and 1,4-substituted phthalic acid units in a 1-to-1 ratio, were prepared by two methods: electrophilic and nucleophilic aromatic substitutions. Electrophilic preparations were performed as low-temperature precipitation polycondensations, and resulting polymers were obtained in the form of particles. Nucleophilic syntheses were carried out in N-cyclohexyl-pyrrolidone and diphenyl sulfone at 260°C and 290°C, respectively, in the presence of sodium and potassium carbonates. Differential scanning calorimetry (DSC) curves of the polyketones “as obtained” and “reprecipitated” revealed differences in their thermal behavior depending on the polymerization method, the synthetic route, and the reaction conditions used. The ability of samples to undergo molecular reorganization during heating, affected by polymerization conditions, is responsible for the diversity of DSC curves observed. A parallel X-ray study of the samples revealed small differences between diffractograms from the as-obtained materials, while they were similar for the reprecipitated ones. However, X-ray diffractograms of thermally treated samples showed further differences between the emerging crystalline structures that should be related to a different chain organization produced during the polymerization process. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.