Jeffrey R. Gillmor

Linear viscoelasticity of side chain liquid crystal polymer

Abstract Small amplitude oscillatory shear has been used to study thermotropic liquid-crystalline polymers that have mesogenic groups pendant to flexible backbones. The polymers studied form nematic and smectic glasses, enabling viscoelastic response to be studied over a wide range of frequencies using time-temperature superposition. In contrast to main chain liquid-crystalline polymers, the nematic side chain polymers exhibit linear viscoelastic response over a wide range of strain amplitudes that is independent of thermal and shear histories. Viscoelastic response is very sensitive to smectic-nematic and smectic-isotropic transitions, but insensitive to the nematic-isotropic transition, as time-temperature superposition applies across this transition. We compare viscoelastic data with diffusion data by calculating the time τ that it takes a polymer to diffuse a distance equal to its coil size R (τ=R2/D). At frequencies lower than 1/τ side chain polymers in their nematic show the terminal response charac...

Viscoelastic properties of a model main‐chain liquid crystalline polyether

We report oscillatory shear and steady‐shear measurements on a model main‐chain thermotropic polyether in its nematic and isotropic phases, and in the nematic–isotropic biphase. The nematic phase is viscoelastic on all time scales measured. Since these time scales are longer than the molecular time scale of diffusion, we conclude that the nematic viscoelasticity is controlled by the large‐scale defect structure. The isotropic phase approaches the response of a viscoelastic liquid, but there are much longer relaxation times present in the isotropic phase than would be expected for a flexible polymer. Viscoelasticity of the nematic and isotropic phases show very weak temperature dependences, but the viscoelastic response of the biphase is extremely temperature sensitive due to the first‐order phase transition. The Cox–Merz rule was tested and found to apply in the isotropic phase and fail in the nematic unless the sample is sheared just prior to the dynamic analysis.

Using Chemistry in Compositional Analysis by Thermogravimetry

Many of the compositional analysis applications involving thermogravimetry have focused on the determination of concentrations of one or two additives to a polymeric matrix. Paramount to the utility of such a thermogravimetric approach for filled polymers is that the matrix pyrolyzes repeatably, if not completely. Equally important is the understanding of the components constituting the high temperature residue (ash) and how they are derived. Two examples will be addressed in which controlling or understanding the chemistry of the experiment highlights the significance of the above-mentioned factors in achieving useful compositional analyses. In the first example, an ammoniated atmosphere is employed to achieve repeatable pyrolysis of silicone rubber to measure the carbon black filler content. The second example emphasizes the understanding of decomposition chemistry to distinguish two inorganic fillers retained as ash. In this example the evolution of CO 2 from CaCO 3 is used to identify the calcium oxide portion of the ash in a polystyrene + carbon black + silicone oil + calcium carbonate + titanium dioxide system.