Molecular Weight-Dependent Behavior of the Twist Distortion in a Nematic Monodomain Containing a Main-Chain Liquid Crystal Polymer

Abstract

The twist viscoelastic coefficients of dilute nematic solutions in 4'-(pentyloxy)-4-cyanobiphenyl (50CB) of a main-chain liquid crystal polymer, TPB-10, which has a mesogenic group, l-(4-hydroxy-4/biphenylyl)-2-(4-hydroxyphenyl)butane, separated by flexible decamethylene spacers, were determined over a wide range of molecular weight using electric field-dependent dynamic tight scattering. A Mark-Houwink-Sakurada relationship [??] = KMya was found for the intrinsic twist viscosity, with different exponents for oligomeric and polymeric chains. The oligomer behavior is interpreted as comparable to that of a freedraining rigid rod, while the polymer chains appear to approach the free-draining random coil limit. By invoking the wormtike chain model, the chain persistence length is determined. In addition, the configurational anisotropies of the polymers can be estimated. The apparent axial ratio of the chain decreases with molecular weight and then becomes constant. The results are thus consistent with the theoretical prediction for the crossover from rigid rod to a biased random coil. Finally, a model calculation for the dimer, trimer, and tetramer leads to an estimate of the probability of a hairpin turn per repeat unit.

Electrorheological Behavior of Main-Chain Liquid Crystal Polymers Dissolved in Nematic Solvents

“…For a chain in an anisotropic solvent, the increment of η c can be expressed by 4 where C is polymer concentration, k is the Boltzmann constant, T is absolute temperature, R ∥ and R ⊥ are the rms end-to-end distances parallel and perpendicular to the director, respectively, and τ R is the configurational relaxation time, which can be expressed in terms of λ ∥ and λ ⊥ , the frictional coefficients parallel and perpendicular to the director: When R ∥ ≫ R ⊥ , eq 3 can be simplified to For a Gaussian chain, R ∥ 2 ≈ M , and assuming free draining behavior, λ ⊥ ≈ M , whence eq 5 predicts an exponent of 1, in agreement with the experimental result. This result is further consistent with our earlier dynamic light scattering measurements of the intrinsic twist viscosity, [γ 1 ], which were obtained at 52 °C:

5 Dependence of the intrinsic Miesowicz viscosity [η c ] on chain contour length of TPB10 in 5OCB at 52 °C.

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Twist Viscosity of Side-Chain Liquid-Crystalline Polysiloxanes in a Nematic Solvent

“…In EFDLS measurements, the depolarized intensity correlation functions of each specimen were obtained by photon correlation analysis with each level of AC electric field applied by a Hewlett-Packard audio frequency generator model 200CDR at 6000 Hz. Further details of the experimental method can be found elsewhere. ,, The concentration dependence of γ 1 was found to be a linear function of concentration up to 0.03 g/mL in agreement with previous studies. , Therefore, measurements to compare the effect of LCP structure on twist viscosity increment were made on 0.03 g/mL solutions.…”

“…Several groups have investigated experimentally the viscometric properties of dilute solutions of an LCP in a nematic solvent. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] Our own studies 4,7,[9][10][11][12][13][14][15] have clearly demonstrated that the change in various viscosity coefficients of the nematic solvent depends not only on the hydrodynamic volume and molecular weight of the dissolved LCP but also on its molecular architecture (e.g., side-chain vs mainchain). In this paper, we focus on our observation [13][14][15] that a side-chain liquid crystalline polymer (SCLCP), dissolved in a flow-aligning nematic solvent such as 5CB (R 2 R 3 > 0), alters the rheology to director-tumbling character (R 2 R 3 < 0); i.e., the director is continuously rotated around the vorticity axis by the hydrodynamic torques.…”

Newtonian Viscosity of Dilute, Semidilute, and Concentrated Polymer Solutions

“…The results show that with increasing molecular weight, τ R increases dramatically, τ R ∼ L b , with b = 1.99 at T = 52 • C, decreasing slightly to b = 1.94 at T = 52 • C, whereas R || /R ⊥ shows no clear trend with molar mass. Based on earlier studies of the twist viscosity of TPBn polymers [Chen and Jamieson, 1994], which encompassed oligomers to high polymers and which indicate that we are in the long polymer limit for TPB10, when L ≥ 62.99 nm, we have tentatively interpreted [Chiang et al, 1997b[Chiang et al, , 2000] the strong molecular-weight dependence of τ R to indicate that TPB10 behaves as a free-draining Gaussian coil. Specifically, from the Brochard model [Brochard, 1979],…”