John M. Pochan

Different approach for the correlation of the Tg of mixed amorphous systems

Abstract An empirical equation relating the glass transition (measured dynamically) of mixed systems to their pure components Tgs can be fitted by the following relationship: In T g =m 1 In T g1 + m 2 In T g2 where m is either volume or wt % of each component. The thermodynamic implications of the logarithmic relationship have been considered and it is shown that other relationships governing the Tg of mixed systems (Fox equation etc.) can be derived using certain thermodynamic constraints. A general power law dependence of log Tg on the apparent activation energy of Tg at a given frequency is also ascertained from experimental data and it is shown that this power law relationship can be obtained from the WLF approach to the glass transition. From this, a relationship concerning the apparent activation energies of Tg (at a fixed frequency) of a mixed system as a function of the pure component activation energies is derived.

Mechanism of Shear‐Induced Structural Changes in Liquid Crystals‐Cholesteric Mixtures

The reflection spectrum of a mixed nematic‐cholesteric liquid crystal has been obtained as a function of mechanical shear. Data are explained in terms of a layered structure consisting of tilted and untilted helical Grandjean texture and a dynamic focal‐conic texture for shear rates less than 103 sec−1. The correlated helical structure is shown to tilt under applied shear with slight distortion. At shear rates greater than 103 sec−1, homeotropic texture is generated. The results of this study are consistent with a theoretical model prepared earlier and suggest that the molecular forces maintaining the integrity of the helical structure in cholesteric systems in the Grandjean texture are significantly weakened by incorporation of nematic liquid crystalline materials.

Dielectric spectroscopy of a-Se and some a-Se--As alloys

Dielectric activity is observed in a‐Se and in some a‐Se : As alloy films in the neighborhood of their respective glass transition temperatures. When measurements are made at fixed frequency in the temperature scan mode, enthalpy relaxation and volume hysteresis effects are readily identified. Relaxation behavior conforms to the phenomenological WLF equations and for a:Se C1=10.4 and C2=20.5 K. The free volume fraction is 0.042. Arsenic addition to a‐Se significantly broadens the relaxation distribution function and increases the relaxation oscillator strength. The effective incremental dipolar contribution per As atom is 0.6 D despite the very small Pauling ionicity of the Se : As bond. The latter is consistent with the formation of a polar pyramidal complex at each incorporated As site. Sub‐Tg relaxation processes are not observed in a‐Se. Dielectric activity in homopolar a‐Se could reflect contributions from polar chemical impurities, deformation induced dipoles, or dipoles associated with native charg...

Normal Stress Effects in Cholesteric Mesophases

Normal forces have been found in rheological studies on cholesteryl chloride‐cholesteryl oleyl carbonate liquid crystal mixtures over the composition range 0–27 wt.% cholesteryl chloride. These normal forces appear to be associated with the homeotropic texture and are evident in the transition region for the shear induced focal conic to homeotropic transition. The Kotaka model for normal stresses arising from hydrodynamic interactions of rotating rods has been applied. The results are consistent with a model of the homeotropic texture of the cholesteric mesophase comprised of rodlike helical arrays of molecules rotating in an applied shear field. The calculated rod lengths are less than the characteristic pitch of the material.

Space-charge effects of semiconductive coatings on triboelectric charge exchange

Abstract Triboelectric charge exchange studies have revealed a previously unreported “semiconductive” effect. A semiconductive film on a metal substrate acquires the opposite sign of charge when contacted with another body than that expected on the basis of energy level considerations for the two contacting materials. The effect is explained in terms of a space-charge region (depletion or accumulation layer) caused by charge exchange of the semiconductor film with the substance onto which it is coated. It is estimated from our measurements that the space-charge depth for poly(N-vinylcarbazole) is between 3 and 8 μm.

Effects of oxidation on I2 doping of trans-polyacetylene as studied via e.s.r. and conductivity measurements

Oxidation of trans-polyacetylene lowers the activation energy for conduction from 16 kcal mol−1 to 11 kcal mol−1 for oxidation levels of (C2H2On)x, n = 0.144, 0.207 and 0.550. Maximum conductivity levels for 0 < x < 0.144 doped with I2 were almost identical. At long I2 doping times all samples exhibited some loss of conductivity. E.s.r. experiments indicate that: (1) linewidth is directly proportional to oxygen content, and (2) iodine doping is not diffusion controlled. All data are consistent with a molecular picture of PAC consisting of highly reactive localized spin states that are selectively oxidized, and less reactive delocalized states that affect conductivity more strongly via doping.

The Effect of Cooling Rate and Magnetic Field on the NMR Relaxation and Thermal Behavior of [N- (p-methoxybenzylidene) -p-n- butylaniline] (MBBA)

Abstract NMR relaxation time measurements and the dsc thermal behavior of the solid state of MBBA have been examined as a function of cooling rate and aligning magnetic field. The results indicate a complicated solid state morphology as a result of these effects. Aside from the normal crystal structure, anisotropies are introduced by the aligned magnetic field and the liquidcrystalline structure that is retained on quenching to the amorphous metastable state. These anisotropies are reflected in the occurance of additional endotherms in the thermal data, and by separate relaxations for methyl reorientation in the normal crystal and the metastable amorphous material in the nmr data. The activation energies for methyl reorientation are determined.

The mechanism of shear induced structural changes in liquid crystals—cholesteric‐polymer solutions

The reflectance spectra and rheological properties for a cholesteric liquid crystal‐polyisobutylene solution, have been obtained as a function of mechanical shear. The optical data are explained in terms of a layered structure consisting of tilted and untilted helical Grandjean texture and a dynamic focal‐conic texture. At low shear rates < 1.72 sec−1, this system behaves optically and rheologically as a normal mixed cholesteric system. At greater shear rates, the structural reinforcing properties of the polyisobutylene cause optical and rheological behavior different from that observed in mixed cholesteric and cholesteric‐nematic mixtures. The most striking effect of the polyisobutylene is its facility to disrupt the shear induced focal‐conic component, causing relaxation to the Grandjean texture in a way that is similar to raising the temperature or releasing the surface constraints on the system.

Dielectric relaxation studies in n‐butyl‐4,5,7‐trinitrofluorenone‐2‐carboxylate/LexanR polycarbonate solid solutions

The α and combined low‐temperature dielectric relaxations of n‐butyl‐4,5,7‐trinitrofluorenone‐2‐carboxylate (BuTNF)/LexanR polycarbonate solid solutions have been studied. It has been shown that only in the specific case of the β relaxations in the components does a correlated motion exist. For the α (Tg) relaxations, the relaxation modes are shown to coexist and are dependent upon composition. The intensity of the dielectric Tg relaxation is dominated by BuTNF. The sub‐Tg relaxations of the system are different. At intermediate compositions, a correlated β relaxation of the polymer–small‐molecule combination is observed. With the exception of the correlated β relaxation, the individual intensities appear to be relative functions of the molar concentrations. This study indicates that BuTNF acts as a plasticizer for the α and γ relaxations of polycarbonates and interacts directly with the phenyl group of the polycarbonate producing a correlated motion. This interaction affects the potential field causing t...

Kinetics of oxygen doping and oxidation of poly(1,6-heptadiyne), a conducting polyene

Abstract The oxidative stability of poly(1,6-heptadiyne) (a free-standing dopable polyene) has been studied via conductivity measurements. A kinetic model of degradation has been applied to the results and it has been shown that the degradation is first order with an activation energy of ∼ 13 kcal mol 1 . It is also shown that solvent wash during polymerization affects degradation rate but not activation energy. Simultaneous oxygen uptake-conductivity experiments are also described. The effect of light on degradation rates is presented. Results of these experiments are compared with earlier work on polyacetylene.