Dynamics of rodlike macromolecules in nondilute solutions: poly(n-alkyl isocyanates)

Abstract

Theories of caging of rigid rodlike molecules of length L and diameter d in semidilute solution are extended to higher concentrations. Packing considerations indicate that ordering in concentrated rigid rod systems must occur below CL2d = 2.26, where C is the number concentration of rods. In addition, the concentration at which the mean square deflection of a semiflexible polymer chain is equal to the cage size is calculated in terms of the ratio of the persistence length to the contour length. These and other considerations are used to interpret a series of dynamic light scattering experiments on solutions of low molecular weight samples of poly(n-butyl isocyanate), poly(n-hexyl isocyanate), and poly(n-octyl isocyanate). Samples of these materials were fractionated by a recrystallization procedure that was developed to minimize complications due to the presence of water. The polymer fractions selected for study were characterized by capillary viscometry, differential refractometry, infrared and ultraviolet spectroscopy, vapor pressure measurements, q d total intensity light scattering. The dynamic light scattering experiments in the semidilute region, as analyzed by the programs DISCRETE and CONTIN, usually give multimodal time correlation functions that are qualitatively interpreted in terms of caging theories and rotational-translational coupling. This coupling grows in importance in a fashion consistent with caging theories as the concentration is raised or the molecular diameter is increased. In the high CL2d region the solutions studied form either a mesophase or a gel depending on the method of solution preparation and the molecular mobility (which depends in turn on other parameters such as the axial ratio and the flexibility). Present address: Tennessee Eastman Company, P.O. Box 1972, Kingsport, Tennessee 37662.confluence of two binodal regions) and unstable elliptic types. Sector vertices represent unique systems where the effect of unequal chain lengths is just balanced by interactions to produce a constant critical temperature, independent of composition. Conditions for multiple occurrence of HODCP's and relations for orientation of spinodals a t HODCP's are derived. Special attention is paid to systems located on hyperbolic boundaries between elliptic and hyperbolic regions where HODCP's overlap with heterogeneous double critical points.A criterion is also given for improving the solvent power in terms of interaction parameter perturbations.

Light scattering analysis of fibril growth from the amino-terminal fragment beta(1–28) of beta-amyloid peptide

“…Nonrandom orientation of fibrils may occur simply due to hard core interactions at high concentrations. Keep and Pecora (1988) showed that a nonrandom orientation of rods must occur at NL2d > 2.26, and may occur at lower values. If fibril length and/or fibril density is greater at the higher ionic strength, then hard-core interactions alone could account for the difference in fibril orientation.…”

“…The most likely explanations for the anomalously low diffusivities are the formation of loose fibril aggregates and the onset of caging, in which the rotational and sideways translational diffusion are reduced due to the effect of interactions with neighboring rods. The effect of caging is estimated to become measurable at values of NL3 > 35-50 (Keep and Pecora, 1988), where N is the number density of rods. For fibrils with lengths of -1000 nm, and molecular weights of -107, the corresponding concentration of fibrils at which caging might be observed is -0.5-0.8 mg/ml, compared to influence dynamic properties such as diffusion without altering static properties such as radius of gyration.…”

Dynamics of Hairy-Rod Polymers: Semidilute Regime

“…11,12 The results from the relevant studies are not fully conclusive concerning three aspects: (i) the mechanisms for relaxing concentration fluctuations in solution, especially in the concentrated region, where a coupling of translational to rotational diffusion is expected; (ii) the reorientational relaxation dynamics; (iii) the dependence of the self-diffusion on molecular parameters and concentration. On the other hand, investigations of the orientational dynamics, which are of primary interest in such anisotropic molecules were conducted more by electric birefringence and less by PCS experiments, 7,11,[13][14][15] due to the small optical anisotropy of these systems, resulting in difficult, and sometimes ambiguous, measurements. Recent advances in macromolecular chemistry have led to the synthesis of well-defined poly(p-phenylenes), PPP, with aliphatic side chains, with varying molecular weights and much higher optical anisotropies compared to other molecules.…”

The Shear Response of Low Molecular Weight Poly(n-hexyl isocyanate) Wormlike Three-Arm Stars in Semidilute Solution

“…Additional insight into each mode is gained by decoupling the slow mode from the fast mode. ∆n′ 2 ) ∆n′ f 2 + ∆n′ s 2 + 2∆n′ f ∆n′ s cos[2( ′ s -′ f )] (12) tan(2 ′) ) ∆n′ s sin(2 ′ s ) + ∆n′ f sin(2 ′ f ) ∆n′ s cos(2 ′ s ) + ∆n′ f cos(2 ′ f ) (13) Shear rate, γ , dependence of the steady state dichroism orientation angle, ′′.…”

“…Traditional studies have been limited to the two extremes of flexibility: the fully flexible chain and the rigid rod. [11][12][13][14][15] A previous rheo-optical study of linear PHIC has shown that concentration scaling of the shear response of the refractive index tensor, n xy is dependent on the chain flexibility. [1][2][3][4][5] However, there are a great many systems that fall between these two stiffness regimes.…”