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.
Local and collective motions in precise polyolefins with alkyl branches: a combination of 2H and 13C solid-state NMR spectroscopy.
Branching out: The mobility of linear polymers changes upon branching, which has a pronounced effect on processability and drawability. Regularly branched model polyolefins were studied by advanced solid-state NMR spectroscopy, and twist defects around the branches in the crystalline regions are identified. For lower branch content, the twisting motions are decoupled; for higher content, collective motion is found (see picture).
gem-Dimethyl Effects in the Thermal Behavior of Polyethylene
Polyethylene can be rendered completely amorphous via the insertion of regularly spaced gem-dimethyl defects along its backbone, where frequency of insertion is the deciding factor in achieving the totally amorphous state. Accurate placement of defects along polyethylene's backbone is achieved with step polymerization metathesis chemistry (the ADMET reaction) rather than using chain polymerization techniques. These gem-dimethyl polyethylene macromolecules, when compared with other ADMET ethylene-based model materials, demonstrate that steric bulk, frequency, and distribution of the defect along the polymer backbone are more important than stereoregularity (tacticity) when the defects are spaced nine carbon atoms or more apart. Comparisons are made between gem-dimethyl defects and methyl defects in a polyethylene backbone.