Conformation in solution of side-chain liquid crystal polymers as a function of the mesogen-graft amount
Introduction
In the past 15 years, the backbone conformation of side-chain liquid crystal polymers has been extensively studied, mainly using small-angle neutron scattering (SANS) after labeling the backbone by deuterium, [1], [2], [3], [4], [5] or X-ray diffraction [6] experiments. These studies showed an anisotropy of the backbone conformation in the oriented mesomorphic phases. This anisotropy is small in the nematic phases, and very large in the smectic phases where the backbone is essentially confined between the smectic layers. Another basic question posed by these systems concerns their persistence length lp. A study carried out in the isotropic and unaligned nematic phases shows that the value of lp is near that of a conventional flexible polymer [7]. This surprising result indicates that the flexibility of the chain is not modified neither by grafting the mesogenic groups every ∼2.5 Å on the chain, nor by the nematic interactions. All these studies were carried out in the melt state. In order to see if this result is still valid for isolated chains, we have performed SANS experiments in dilute solutions of side-chain polysiloxanes exhibiting different mesogen-graft amounts. The results obtained show that the polymeric backbone becomes stiffer when the mesogen-graft amount is increased.
Section snippets
Samples
The ‘side-end’ fixed liquid crystal polymers (LCP) investigated correspond to the chemical formula shown in Fig. 1. The precursor backbones are homohydrogenmethysiloxanes (from Aldrich) or copolymers formed by hydrogenmethylsiloxane and dimethylsiloxane units. The copolymers were synthesized, as described elsewhere [8], in such a way that they have a similar degree of polymerization. The statistical distribution of the two types of units was checked by 29Si NMR.
Two different mesogenic groups,
Results
The neutron scattering results have been analyzed by means of the Zimm approach based on the development of the scattered intensity proposed by Guinier for qRg<1, where Rg is the radius of gyration. For polydispersed systems, and under the condition that where is the overlap concentration, Zimm has obtained for the reciprocal of the scattered intensity [13]:in which Rz2 is the mean-square z-averaged radius of gyration (third moment of the distribution), Mw
Discussion
From Fig. 4 and from the results displayed in Table 2 it is clear that increasing the mesogen-graft amount from 0 to 100% entails an increase in the radius of gyration by a factor of nearly two. Correspondingly, the second virial coefficient decreases by approximately one order in magnitude. This means that the increase in the radius of gyration does not arise from an increase in solvent quality but is an intrinsic property of the chains.
Since the chains with the grafted mesogens cannot be
Concluding remarks
The experimental determination of Rz points towards an increase of the backbone's persistence length with increasing the mesogen-graft amount in the dilute state. Recent results reported by Fourmaux-Demanges et al. [7] in the isotropic and nematic melt-state have shown that the persistence length is about 1 nm for liquid crystal polymers made of poly(methylmethacrylate) with mesogen-graft amounts of 100%. Since the persistence length for the standard poly(methylmethacrylate) is about 0.7 nm [24],
Acknowledgements
It is a pleasure to thank L. Noirez for many stimulating discussions and helpful remarks about the analysis of the data. The experiments at BENSC in Berlin were supported by the European Commission under the Access to Research Infrastructures Action of the Human Potential Programme (contract: HPRI-CT-1999-00020).
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