Infrared Spectroscopy and X-ray Diffraction Characterization of Dimorphic Crystalline Structures of Polyethylenes with Halogens Placed at Equal Distance along the Backbone.
Polyethylenes with halogens placed on each and every 21st, 15th, or ninth backbone carbon display crystallization patterns enabled by the size of the halogen and by changing crystallization kinetics. The different structures have been identified from X-ray patterns combined with a detailed analysis of the infrared spectra of series containing F, Cl, or Br atoms that were either fast or isothermally crystallized from the melt. Under both crystallization modes, all specimens develop layered crystallites that accommodate 5-9 repeating units along the chain's axis. The size of the halogen and intermolecular staggering to maximize packing symmetry are responsible for striking structural differences observed between the series and between the two modes of crystallization. While the small size of the F atom causes a small perturbation to the crystal lattice and the orthorhombic structure is maintained for all members of the series either fast or isothermally crystallized, each Cl or Br-containing system presents dimorphism. Under fast crystallization, Cl and Br containing samples adopt the all-trans conformation (planar Form I), while in slowly crystallized samples gauche conformers set for bonds of the backbone carbons adjacent to the carbon with the halogen due to a close intermolecular staggering of halogens (herringbone Form II). In both forms the methylene sequence between halogens maintains the all-trans conformation. The structural details are extracted from the analysis of the C-halogen stretching region of the IR spectra, and from adherence to the n-alkane behavior of CH2 rocking, CH2 wagging, and C-C stretching progression modes.
Structure formation in nanophase-separated systems with lamellar morphology: Comb-like vs. linear precision polymers
Abstract We report findings of a comparative study of a comb-like polymer, poly(1,4-phenylene-2,5-n-didecyloxy terephthalate) [PPDOT] and a linear chain precision polymer consisting of a methyl protected 2,6-diaminopyridine group after every 20th methylene unit of a linear saturated polyethylene chain [PDAPS20]. Both polymers exhibit lamellar morphologies related to a nanophase separation of the methylene sequences from ring-like sub-units as seen in X-ray diffraction data. Although their chain architecture is different, investigations on oriented samples of both PPDOT and PDAPS show that their lamellae orient in such a way that their surface normals tend to be perpendicular to the shear fields leading to different average chain orientations. A novel approach considering the volume occupied per CH2 unit in the alkyl nanodomain calculated on the basis of unit cell considerations is employed to understand the packing state of the alkyl segments. While modification B of PPDOT exhibits densely packed alkyl nanodomains, modification A of PPDOT and PDAPS20 show a relatively loose packing. At the end, we attempt to bring together these two classes of polymers exhibiting nanophase-separated morphologies by means of a classification scheme based on the structural arrangement of the sub-units within their respective domains, highlighting their similarities and differences.
Marie Curie's doctoral thesis: Prelude to a Nobel Prize
This paper is a critical account for the most significant body of Marie Sklodowska Curie's scientific work, presented in the hopes that she be remembered for more of the "right" reasons. It also offers some new insights into her methods and results.