1,721,042 research outputs found
Manufacturing of Polypropylene Laminates and Related Structural Reorganization In the Crystalline Phase
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New x-ray crystallographic data on crystalline polymers of methyl sorbate. Conformational energy calculations for different configurational hypotheses
No full textConformational calculations have been performed on chains of poly(methyl sorbate) having different configurations; the results of these calculations have been compared with NMR and X-ray data obtained for two kinds of poly(methyl sorbate) recently synthesized. The polymer having a prevailingly erythro configuration has a chain axis and a conformation of minimum energy in accordance with that reported in a previous paper. The polymer having a prevailingly threo configuration has a chain axis compatible with a regular conformation of low energy having two constitutional repeating units and in accordance with the distribution of intensities in the X-ray spectra. The two ideal conformations found are compatible with the presence of configurational defects
Conformational analysis of syndiotactic polymer chains in the crystalline state: Polypropylene and 1,2-poly(1,3-butadiene)
No full textComments On the Possibility That the Mesomorphic Form of Isotactic Polypropylene Is Composed of Small Crystals of the Beta-crystalline Form
No full textCrystal-Structure of the Alpha-Form of Syndiotactic Polystyrene
No full textModels for chain packing for ordered and disordered modifications of the alpha-form of syndiotactic polystyrene are suggested, through quantitative comparisons between X-ray diffraction intensities and calculated structure factors. According to the reported analysis, zig-zag planar chains are packed in the hexagonal unit cell proposed by Greis et al., but with different orientation and relative height of the phenyl groups. In particular, the phenyl groups in both ordered and disordered modifications are packed according to rhombohedral symmetry. The disorder in the alpha' modifications correspond to the statistical occurrence o two different, nearly isosteric, orientations of triplets of chains at well defined locations of a threedimensional lattice, which generate a rhombohedral symmetry for the whole unit cell
Evaluation of the Orientation Coefficient For the C-axis In Poly(ethylene-terephthalate) Fibers
No full textThe literature methods for the determination of the mean of the crystallite orientation distribution for the c axis, that is of the orientation coefficient f(c), for poly(ethylene terephthalate) (PET), based on the azimuthal scan of the (() over bar 105) reflection (which can be applied also to samples with ''tilted orientation''), is proposed. This method implies as a first step the determination of the tilt angle, for which the complete fiber pattern is required. A possible simplifying assumption, which allows use of the sole azimuthal (($) over bar 105) profile and makes the method also applicable to poorly oriented samples (for which the determination of the tilt angle is not easy), is also discussed. (C) 1995 John Wiley and Sons, Inc
Mesomorphic form (beta) of nylon 6
No full textThe structure of poly (epsilon-caprolactam) (nylon 6) in the beta mesomorphic form is here examined. The comparison of the diffraction intensity, calculated on modeled structures, with the experimental profiles, collected by us through an automatic diffractometer, is presented. This analysis has put into evidence the following limiting structural features. (a) The beta form of nylon 6 is made of small mesomorphic aggregates of chains (where the matter scatters coherently) with axes arranged in a hexagonal lattice (a = b = 0.48 nm; gamma = 120 degrees). (b) The chains have disordered conformations (and do not have a definite chirality, as it is the case for the 2(1) helices in the alpha and gamma forms) with the -CH2- chains close to nearly all-trans (antiperiplanar) and the two dihedral angles adjacent to the amide bond, +/-120 degrees) to antiperiplanar (180 degrees). This notwithstanding, the chains are straight and extended. As a result, the mean chain periodicity is close to 0.835 nm. (c) The H-bonds are formed along lines in the [100], [010], and [1(1) over bar0$] directions; they force neighboring chains within the small mesomorphic aggregates to adjust their conformation in such a way that nearly 100% of hydrogen bonds are always formed, in agreement with the IR data. In the case of the alpha and gamma more ordered forms of nylon 6, such lines are all in a unique direction, leading to hydrogen-bonded sheets (parallel to the chain axis) of enantiomorphous anticlined, alternately up and down chains (in the alpha form) or of isomorphous isoclined chains, (in the gamma form), (d) As a consequence, since the amide groups lie all at nearly the same height along z, in the beta form of nylon 6 the lines of hydrogen bonds lie in layers perpendicular to the chain asis and have the same direction within each layer. However, consecutive layers along z may have the lines of H-bonds which are not parallel (e.g., occasionally rotated by +120 or -120 degrees instead of the ''normal'' 180 degrees). (e) In the beta form of nylon 6, disorder arises also from the random substitution of up and down chains in the lattice positions
X-ray-analysis On Unoriented and Oriented Samples of the Quenched Form of Isotactic Polypropylene
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