Etude Phsicochimique des Copolymères Triséquencés Poly(γ-L-glutamate de benzyle)/Poly(oxyéthylène)/Poly(γ-L-glutamate de benzyle). Rǒle de la Polydispersiteé dans la Détermination de la Conformation des Molécules en Solution Diluée
✍ Scribed by Léonard Reibel; Gérard Spach; Claude Dufour
- Publisher
- Wiley (John Wiley & Sons)
- Year
- 1973
- Tongue
- English
- Weight
- 948 KB
- Volume
- 12
- Category
- Article
- ISSN
- 0006-3525
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✦ Synopsis
Abstract
We have prepared a series of three‐block copolymers containing the same central block of poly(ethylene oxide) (POE) and two side blocks poly(γ‐L‐glutamate) (PBLG) of variable lengths. These molecules could present an unusual conformational model when dissolved in solvents where PBLG exhibits a helical structure: a helical structure: a gaussian chain connected at each end to a rigid rod. We checked this hypothesis by light scattering, viscosimetry, and dielectric adsorption measurements.
In order to take into account the possibility that the orientations of the two rods may not be completely independent, we have introduced in the calculations the angle θ, formed by the two rigid segments. We have mainly considered three types of conformation which can be described by assimilating the lods of PBLG of two vectors having their origin at the juncture with the POE chain: depending whether cos θ (the average value of cos θ) is equals to −1,0, or +1 ( where the two vectors are antiparalles, oriented independently, or parallel, respectively). We have tried to establish a theoretical expression for the molecular size obtained by each method of investigation, as a function of cos θ. We corrected this value to account for the effect of the polydispersity on the molecular size and then compared the theoretical values thus calculated with the experimental results. The measurements of the radius of gyration, R, by light scattering does not permit the unambiguous determination of cos θ. As soon as the system is not perfectly monodisperse, the variation of R as a funciton of cos θ is not very large. The viscosity results are also difficult to interpret because the effect of the polydispersity on the viscosity cannot be established fo each conformational model. However, a clear answer can be obtained by the dielectric absorption technique for two reasons: the variation of the dielectric absorption as a function of cos θ is very important and the influence of the polydispersity on the measurements is weaker than for the two former methods. The results show that cos θ is equal to −1, demonstrating that the two vectors are antiparallel.
From the dielectric dispersion, one sees that the total length of the molecule is roughly equals to the sum of the lengths of the two rigid blocks. To explain this result we propose a new conformational model. Since it is known that in some solvents and mixtures of solvents the molecules of PBLG tend to associate, we assume that the two rigid rods are antiparallel and partly associated. The associated part of the molecule is assumed to be surrounded by the mixture of POE and solvent, which acts as a poor solvent (ef. Fig. 2.).