Dale S. Pearson

Flow‐Induced Birefringence of Concentrated Polyisoprene Solutions

Experiments measuring the birefringence following the inception of a steady shearing flow were conducted on a series of concentrated polyisoprene solutions. The steady‐state shear stress was also measured in a mechanical rheometer. The data obtained from the two experiments confirmed that the birefringence and stress were linearly related (stress‐optical law) over a range of shear rates that extended far into the non‐Newtonian region. Interesting nonlinear effects were also observed during the transient response at high shear rates. Both the shear stress and the first normal stress pass through maximums before reaching their steady‐state values. A maximum in the normal stress is not predicted by the original Doi‐Edwards model but a modification of their theory which includes chain stretching is in qualitative agreement with our results.

Solution processing of a doped conducting polymer

Abstract The solution processing of doped conducting polymers is a fast and simple method by which conducting articles may be formed, although aggregation of the polymer upon doping may inhibit processing. Solutions of poly(3-octylthiophene) (P3OT) doped with FeCl 3 · 6H 2 O and NOPF 6 were found to exhibit varying degress of aggregation dependent upon the polymer concentration and doping level. However, we have found that coherent films with conductivities up to 1 S/cm may be cast directly from the doped solutions. The effect of doping level and other factors on the physical state of the doped solutions is discussed and related to the conductivity achieved in the solution-cast films.

Statistics of the entanglement of polymers: Unentangled loops and primitive paths

Attempts to apply the reptation and tube model to describe polymer disentanglement and associated relaxations have had wide success. However, questions have arisen in various applications as to the statistics of entanglement of a random walk with an obstacle net. Calculations are presented of a number of probabilities which characterize the degree to which a polymer, represented by a lattice walk, entangles with an array of obstacles. In particular, we have calculated the number of ways that such a walk can form unentangled closed loops of various types. If one reels in a general random walk from its ends, pulling out unentangled loops, one is left with the so‐called primitive path, which is taken to represent the path of the tube. The probability that an N step random walk has a K step primitive path has been determined. Asymptotic formulas for this probability are presented.

FLOW PATTERNS AND DISCLINATION-DENSITY MEASUREMENTS IN SHEARED NEMATIC LIQUID CRYSTALS. I: FLOW-ALIGNING 5CB

Abstract We report the first direct measurements of disclination density during shear flow of a nematic liquid crystal. Samples of 4,4′-n-pentylcyanobiphenyl, or 5CB, are initially aligned in the homeotropic configuration within a torsional shear cell using surface treatment with lecithin. For low shearing rates, the director rotates within the shearing plane, except near the axis of rotation where irregular time-dependent out-of-plane orientation patterns exist. Disclinations form above a threshold shear rate, with the first ‘thick’ and ‘thin’ disclinations appearing near the sample edge, rapidly multiplying, and subsequently filling the sample. We measure the steady-state disclination density, ρA, the length of disclinations per unit projection area. The dimensionless disclination density, ρA h, where h is the sample thickness, is found to depend on two dimensionless quantities: Er, the Erickson number, and [rtilde] = r/h, the scaled radial position. In the limit of large [rtilde]([rtilde]) 100), the de...

Flow patterns and disclination-density measurements in sheared nematic liquid crystals ii: Tumbling 8cb

Abstract We examine the behaviour of nematic 4,4′-n-octylcyanobiphenyl (8CB) subjected to steady torsional shear flow at a temperature where this nematic is ‘tumbling’, i.e. the ratio α3/α2 of Leslie viscosities is negative. If the disc rotation speed is gradually increased from zero ( < 10−6 rad s−2), the director eventually departs from the shear plane and becomes oriented radially. However, if the disc rotation speed is increased from rest at higher rates, twist wall defects are created sequentially at the sample meniscus and propagate inward with velocity ≈0.85mm s−1. Eventually, disclinations are nucleated. Disclination density measurements reveal that over similar ranges of values of the Ericksen number Er, samples of 8CB have roughly ten times the disclination density of the flow-aligning nematic, 5CB, reported in the preceding paper. Additionally, a transition from a power-law exponent of 0.5 to 1.0 for the dependence of dimensionless disclination density on Er is observed near Er = 2000, consiste...

Stress optical measurement of the third normal stress difference in polymer melts under oscillatory shear

Results are reported for the dynamic moduli,G′ andG″, measured mechanically, and the dynamic third normal stress difference, measured optically, of a series bidisperse linear polymer melts under oscillatory shear. Nearly monodisperse hydrogenated polyisoprenes of molecular weights 53000 and 370000 were used to prepare blends with a volume fraction of long polymer,ΦL, of 0.10, 0.20, 0.30, 0.50, and 0.75. The results demonstrate the applicability of birefringence measurements to solve the longstanding problem of measuring the third normal stress difference in oscillatory flow. The relationship between the third normal stress difference and the shear stress observed for these entangled polymer melts is in agreement with a widely predicted constitutive relationship: the relationship between the first normal stress difference and the shear stress is that of a simple fluid, and the second normal stress difference is proportional to the first. These results demonstrate the potential use of 1,3-birefringence to measure the third normal stress difference in oscillatory flow. Further, the general constitutive equation supported by the present results may be used to determine the dynamic moduli from the measured third normal stress difference in small amplitude oscillatory shear. Directions for future research, including the use of birefringence measurements to determineN2/N1 in oscillatory shear, are described.

Structural response of nematic liquid crystals to weak transient shear flows

Using conoscopy, we have investigated the dynamic director response of nematics to oscillatory shear and step shear strain. In oscillation, the ratio of the amplitude of the director rotation to the strain amplitude and the phase difference between the director rotation and the applied strain are measured as functions of frequency for both 5CB and 8CB. Comparison with the Leslie–Ericksen theory allows extraction of several important material parameter ratios. In particular, data in the high frequency limit yield λ, the material parameter which indicates flow‐aligning (λ≳1) or tumbling (λ<1) behavior. For 5CB at 32.5 °C, λ≊1.03, while 8CB at 34 °C gives λ≊0.42. An analysis is presented for director field relaxation following distortion induced by a step strain. Step strain experiments on 5CB and 8CB provide an independent measurement of viscoelastic properties. Aside from confirming the flow classification of these model nematics, these new experimental methods establish a framework for the study of director dynamics in polymeric nematic liquid crystals.

The origin of stress-oscillation damping during startup and reversal of torsional shearing of nematics

Using a controlled-temperature shear cell mounted on a polarizing microscope, we observe the behavior of nematic 4,4′-n-octyl-cyanobiphenyl (8CB) during start-up and reversal of shearing in a torsional parallel-plate geometry and correlate this behavior with rheological measurements. During the start-up, a sequence of birefringent rings, or “twist walls”, are observed that originate at the sample edge and propagate radially inward. Each twist wall is a thin region in which the director is twisted out of the plane of the velocity and velocity-gradient directions. The radial variation of in-plane orientation can be explained by the variation of strain in the parallel-plate device. A high Ericksen-number solution of the Leslie-Ericksen equations predicts a damped oscillatory shear stress response which agrees quantitatively with the measured stress oscillations out to an edge strain of around 50. The damping of the stress oscillations is due to the nonuniformity of strain in the parallel-plate geometry. On reversal of the flow, if the strain, γ, is smaller than about 500 units, the damping of stress oscillations is reversed; this correlates with an outward radial migration of twist walls. When γ > 500, disclinations nucleate and spoil the reversibility of stress damping.

Phase behavior and rheology of blends containing polycarbonate and a thermotropic polyester

The phase behavior and rheology of binary blends of polycarbonate (PC) and a liquid crystalline polymer (LCP) have been investigated. The thermotropic LCP employed was a semiflexible polyester synthesized by melt condensation of t-butylhydroquinone and 4,4′ dichloroformyl-α,ω-diphenoxyhexane. It shows a distinct nematic-to-isotropic transition in the pure state and in the blends. Results of DSC and optical microscopy indicate that the LCP is solubilized in the mixture for weight fractions of LCP less than about 0.05 and shows partial miscibility with PC over the rest of the composition range. The phase separation is considered to be driven by both isotropic and anisotropic interactions between constituent chains. Dynamic oscillatory measurements show that there is some interaction between the separate isotropic and anisotropic phases, with complex viscosities of the blends being intermediate between those of pure components and showing significant deviation from a logarithmic rule-of-mixtures.

Relaxation dynamics of selected polymer chain segments and comparison with theoretical models

Simultaneous measurement of infrared dichroism and birefringence is used to study selected polymer segment dynamics in isotopically labeled block copolymers. Two different polymers were studied: polybutadiene and poly (ethylene propylene). The first type consisted of a triblock with a short middle block labeled and a diblock with a short end block labeled, while the second type consisted of a triblock with three equal blocks and the end blocks labeled. Results of step strain experiments at −10°C for polybutadiene and at room temperature for poly(ethylene propylene) indicated that segments located at chain ends relax faster than segments located at chain centers. These experimental data were compared to the predictions of two molecular models: the bead-spring model of Rouse and the tube model of Doi and Edwards, and it was found that both models correctly predict the qualitative features of segmental relaxation. However, the tube-model predictions were closer to the experimental results. In addition, when the effects of orientational coupling interactions between segments in the melt were incorporated into this model, its predictions quantitatively agreed with the experimental results. The orientational coupling coefficient for poly(ethylene propylene) was 0.45 as measured from previous work, and for polybutadiene it was found to be 0.4.