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.
Crystallization of Long-Spaced Precision Polyacetals I: Melting and Recrystallization of Rapidly Formed Crystallites
Rapidly melt-crystallized polyethylenes with acetal groups (−O–CH2–O−) precisely spaced by 12, 18, 19, or 23 methylene backbone carbons exhibit two reorganizations and three melting endotherms on heating at relatively low heating rates. Real-time wide-angle X-ray diffraction (WAXD) experiments further confirm that these transitions are associated with a reorganization to a different crystalline structure. The differential scanning calorimetry endotherms are associated with melting of the initially formed disordered (or hexagonal) phases, followed by melting of Form I and by melting of Form II crystals with increasing temperature. In parallel with these structural changes, the long spacing undergoes a discrete step increase at the polymorphic transitions. Upon the transition from disordered to Form I crystals, the core crystal thickness increases by one repeating unit, whereas the crystal thickness remains basically unchanged during the transition from Form I to Form II crystallites. The effect of even vs ...
Linear Low-Density Polyethylene Containing Precisely Placed Hexyl Branches
A structural investigation into model linear low-density polyethylene containing precise hexyl branches has been completed using metathesis chemistry. These models based on ethylene/1-octene (EO) copolymers are versions of industrially produced metallocene copolymers; however, they contain exact primary structures and constant methylene sequence lengths. Acyclic diene metathesis (ADMET) polymerization has been used to produce copolymers containing only hexyl branches on every 9th, 15th, or 21st carbon along the backbone of polyethylene. Thermal examination of these macromolecules has demonstrated the first narrow melting profile for EO copolymers with high 1-octene incorporation (111, 67, and 48 hexyl branches per 1000 carbons). Further, a new synthetic methodology has been developed to produce branched, pure diene functional monomers with the ability to produce any model LLDPE in good yields.