Conformation of comb‐like liquid crystal polymers in isotropic solution probed by small‐angle neutron scattering

Abstract

The conformational characteristics of a comb‐like side‐chain liquid crystal polysiloxane (SCLCP), dissolved in deuterated chloroform, were evaluated by small‐angle neutron scattering (SANS) measurements over a wide q range. SANS studies were carried out on specimens with constant backbone length (DP = 198) and variable spacer length (n = 3, 5, and 11), and with constant spacer length (n = 5) and variable DP (45, 72, 127, and 198). The form factor P(q) at high q was analyzed using the wormlike chain model with finite cross‐sectional thickness (Rc) and taking into account the molecular weight polydispersity. The analysis generated values of persistence length in the range lp = 28–32 Å, considerably larger than that of the unsubstituted polysiloxane chain (lp = 5.8 Å), with contour lengths per monomer comparable to the fully‐extended polysiloxane backbone (lm = 2.9 Å). This indicates a relatively rigid SCLCP chain due to the influence of the densely attached mesogenic groups. The SCLCP with n = 11 is more flexible (lp = 28 Å) than those with n = 3 and n = 5 (lp = 32 Å). The cross‐sectional thickness increases with spacer length, Rc ∼ n0.21±0.02 (3 ≤ n ≤ 11), and the contour length per monomer decreases with increasing spacer length, lm ∼ n−0.35±0.01. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2412–2424, 2006

Bridging Simulation Length Scales of Bottlebrush Polymers Using a Wormlike Cylinder Model

“…300. This is within the range of Kuhn lengths reported in the experiments. ,− This connection between Kuhn length λ –1 and N sc is plotted in Figure a as solid points and lines. Comparing the data for each N sc on increasing grafting density, we match the intuitive expectations that denser grafting should lead to higher stiffness.…”

“…While ostensibly describing the same molecular conformations, seemingly conflicting observations emerge. For example, simulation and scaling theory generally agrees that scaling exponents are similar to that of a flexible chain for synthetically relevant cases. ,, This is curiously distinct from the results that the Kuhn length of a bottlebrush can be as much as 40–100 times larger than that of a flexible backbone, as measured in an experiment assuming a wormlike geometry. ,− In contrast to a flexible chain, this suggests that bottlebrushes possess significant rigidity, consistent with experimental evidence of lyotropic ordering of bottlebrushes in nondilute solutions. − This prediction is also consistent with some simulation and theory for the bottlebrush persistence length.…”

Theory of Side-Chain Liquid Crystal Polymers: Bulk Behavior and Chain Conformation

“…Work by Kempe et al 13 shows that the SANS data of the end-on SCLCPs can be well fitted by eq in both the parallel and the perpendicular directions. Other groups 45,46 have made similar findings, although they point out that eq is a good approximation only on the large length scale (i.e., low q region when AER g 2 aeq 2 < 1).…”

Materials Design of Highly Branched Bottlebrush Polymers at the Intersection of Modeling, Synthesis, Processing, and Characterization

“…A large set of early experimental observations have shown that the stiffness of bottlebrush polymer molecules, as reflected in the Kuhn length λ –1 or persistence length l p , is several times more than that of the bare backbone. ,,− Early work by Wintermantel ,, found the ratio λ –1 / D to be 10 or more, consistent with lyotropic ordering in nondilute solutions. , Concomitantly, Nemoto et al reported λ –1 = 90 nm for poly­(methyl methacrylate)- g -poly­(styrene) bottlebrushes in benzene, which is much higher than λ –1 = 3.2 nm for bare poly­(methyl methacrylate); Gerle et al reported λ –1 = 120 nm for poly­(methyl methacrylate)- g -poly­(methyl methacrylate), and Lecommandoux et al reported l p = 11 nm for poly­(chlorovinyl ether)- g -poly­(styrene), in contrast to l p = 1.2 nm for the bare backbone.…”