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.
Isothermal step thickening in a long-spaced aliphatic polyester
Abstract Discontinuous lamellar thickening has been found at the isothermal crystallization temperatures of a long-spaced aliphatic polyester (PE-48,48) synthesized via polycondensation of the diol and diacid. The isothermal step thickening is characterized by a discrete increase in melting temperature with increasing crystallization time and an increase in heat of fusion of the high melting peak at the expense of the low melting peak. The increase in crystal thickness, quantified by SAXS, is consistent with a straightening of the chains at the basal surfaces of the lamellae to complete a full repeat of the crystalline layer. The increase in thickness and decrease in surface energy are associated with the sudden increase in crystal melting. The isothermal thickening follows a logarithmic dependence of time with a rate that increases with temperature. The thickening process is thermally activated and follows Arrhenius behavior with an activation energy of 238 ± 4 kJ/mol.
NMR Experiments and Molecular Dynamics Simulations of the Segmental Dynamics of Polystyrene
We have performed NMR spin−lattice relaxation experiments and molecular dynamics (MD) computer simulations on atactic polystyrene (a-PS). The segmental correlation times of three different molecular weight a-PS (Mn = 1600, 2100, 10 900 g/mol) were extracted from NMR by measuring the 2H spin−lattice relaxation times (T1) over a broad temperature range (390−510 K). MD simulations of an a-PS melt of molecular weight 2200 g/mol were carried out at 475, 500, and 535 K. Comparisons between experiments and simulations show that the MD simulations reproduce both the shape of the P2(t) orientation autocorrelation function and its temperature dependence, while the simulated segmental correlation times are slower than experimental results by a factor of 1.8. If the simulations are rescaled by this factor, they reproduce both the experimental T1 values and the slight difference in dynamics between the backbone and side group of PS.