Jinlong Cheng

Structure-property analysis for gel-spun, ultrahigh molecular mass polyethylene fibers

Abstract The structures of four gel-spun, ultrahigh molecular mass polyethylene fibers have been studied with the techniques of full-pattern x-ray diffraction, small-angle x-ray scattering, powder x-ray diffraction, solid-state 13C nuclear magnetic resonance, differential scanning calorimetry, and optical microscopy. A high molecular mass polyethylene fiber is also studied for comparison. At room temperature these fibers show mainly the common orthorhombic crystals and a small amount of monoclinic crystals in addition to an intermediate, oriented phase and the amorphous phase. The structure parameters of the orthorhombic phase change slightly with fiber processing history. The chains of the intermediate phase have largely a trans-conformation and are oriented preferentially parallel to the fiber axis, but are disordered laterally. The mobility (correlation time) of the carbon atoms of the intermediate phase is higher than that of the crystalline phase by 2 orders of magnitude, but lower than that of the a...

Solid state NMR study on the conformation and mobility of n-octadecyl chains in a silane coupling agent attached to the surface of colloidal silica.

The surface of colloidal silica of 22 nm and 100 nm in diameter has been modified with a silane coupling agent, n-octadecyltriethoxysilane. The solid state 13C NMR spectrum shows surprisingly that the covalently bonded n-octadecyl chains on the silica surface adopt largely extended all-trans conformation. By increasing the temperature to 70 degrees C, the trans conformations observed at room temperature can be converted to a disordered state in which the trans and gauche conformers are under rapid exchange (mobile disorder). On cooling to ambient temperature, the trans conformation cannot be recovered and a distribution of chain conformations can be observed, indicating that the disorder has been frozen-in (rigid disorder). Subsequent treatment of the surface modified silica with a second reagent, such as trimethylsilychloride or ethanol reacts with the remaining surface hydroxy groups and causes the n-octadecyl chains to collapse to the mobile, disordered state. The 13C spin-lattice relaxation times for carbon atoms in the all-trans conformation are longer than those in the mobile, disordered chains, indicating that there is more restriction of motion in the trans octadecyl chains. Instead of crystallization of the octadecyl chains on the silica surface, other factors, such as static interactions, might be responsible for the existence of trans conformation in the octadecyl chains.

Variable-temperature study of a gel-spun ultra-high molecular-mass polyethylene fiber by solid state NMR

Solid-state13C nuclear magnetic resonance (13C NMR) methods have been applied to study a gel-spun ultra-high molecular-mass polyethylene fiber. The mass fractions of the crystalline (orthorhombic and monoclinic), intermediate, and amorphous phases have been determined at temperatures between 296 and 413 K. The mobility of the polymer chains in the crystalline and the intermediate phases have been measured within the same temperature range, 296–413 K. Discussions on the chain conformation and the rate of motion are carried out based on the13C chemical shift and spin-lattice relaxation time, respectively.

Condis Crystals of Small Molecule VI. The Differences Between Smectic and Condis Phases, Evaluated by a Solid State 13C NMR Study of N,N′-bis(4-n-octyloxybenzal)-1,4-phenylenediamine (OOBPD)

Abstract Carbon-13 NMR spectra have been obtained for N,N′-bis(4-n-octyloxybenzal)-1,4-phenylenediamine (OOBPD) in the condis and smectic phases. Sharp, well-resolved spectra were obtained without magic angle sample spinning in the smectic phases under the conditions of cross-polarization or single carbon pulse followed by high-power proton decoupling. The smectic I-to-smectic C transition involves a discontinuous change in 13C chemical shifts due to a large degree to orientational disordering. Similar behavior was found between the spectra of smectic C and the isotopic state. Order parameters for various smectic phases are calculated based on the observed differences in 13C chemical shift between the isotropic state and the smectic phase. The spectra were measured at different sample orientation with respect to the external magnetic field, and show that reorienting the smectic molecules in a magnetic field is facile. Larger difference can be found between the spectra obtained by cross polarization and th...

Studies of molecular motions in novel syndiotactic alternating ethylene-propylene crystalline copolymer by dynamic mechanical analysis and solid-state 13C n.m.r.

A syndiotactic alternating ethylene-propylene (SYN-ALT-EP) copolymer was synthesized. Two molecular relaxation processes (α and β relaxations) have been found below room temperature via dynamic mechanical analysis (d.m.a.): the α relaxation at −60°C and the β relaxation at −125°C. The apparent activation energy of the α relaxation is 285 kJ mol−1, and the activation energy of the β relaxation is 43 kJ mol−1 based on the Arrhenius equation. Molecular motion in SYN-ALT-EP copolymer was probed by solid-state 13C n.m.r. experiments. At temperatures above the glass transition temperature (Tg) there are two major molecular motions in this copolymer: a backbone motion (the rotational motion about single bonds) and a methyl side group rotation. The backbone motion is frozen below Tg, but there is still a methyl rotation. As the temperature is further decreased to about −175°C, well below the β-transition observed in d.m.a., the methyl side group rotation slows down, suggesting that the methyl rotation may be associated with the observed β relaxation process.