No abstract

R e~u le 20 mars 1969Des polymeres a structure "en Ctoile" ont CtC prCparCs par copolymCrisation sCquencCe anionique de styrene et de divinylbenzene (DVB), la proportion de ce dernier monomere etant de l'ordre de quelques lnolCcules par extrCmitC vivante. Toutes les opCrations ont CtC effectuCes sous vide pousse, et le benzene a CtC choisi comnle solvant. Les Cchantillons obtenus ont CtC trouvCs quasinlent exempts de polystyrene linCaire et leur polydispersitC est tres faible. Le nombre de branches des Ctoiles dCpend de la concentration globale, de la proportion de DVB nlise en jeu et de la masse molCculaire du prCcurseur linCaire. Les resultats obtenus avec des isomeres para et mCta du DVB ne different pas de f a~o n significative, pour ce qui est de la structure des produits obtenus. L'etude spectroscopique de la rCaction fait apparaitre cependant des differences notables, dues au fait que les 2 doubles liaisons de l'isomere para different par leurs reactivitis, ce qui n'est pas le cas de l'isomere mCta.Star-shaped polystyrene samples have been synthesized by anionic block-copolymerization of styrene and divinylbenzene (DVB), the proportion of the latter monomer being of the order of a few molecules per living end. Vacuum technique was used and benzene was chosen as solvent. The samples were found almost free of linear polystyrene, and of very low polydispersity. The number of branches was found to be a function of the overall concentration, of the amount of DVB, and of the molecular weight of the linear precursor. The results obtained with the meta and the para isomers did not differ significantly, as far as the products themselves are concerned. However, spectroscopic investigations carried out during reaction showed that the reactivity of the 2 double bonds of the para isomer differ significantly, whereas this is not the case with the meta isomer.

A series of polydimethylsiloxanes and two-block copolymers with styrene of reasonably narrow molecular weight distribution has been prepared using the monomer hexamethylcyclotrisiloxane. The initiator was sec-butyllithium. Evidence was found for a low level of side reactions which results in the molecular weight distributions becoming broader at very high molecular weights. Similarly block copolymers with > 2 X 10s were contaminated with both homopolymers. Purification methods to remove the homopolymers are described. The solution properties of polydimethylsiloxane agreed in general with earlier studies which used fractions produced by polymerization of octamethylcyclotetrasiloxane. The intrinsic viscosities of the block copolymers are, unlike those of styrene-isoprene two-block copolymers, greater than the average of those of the homopolymers. This suggests greater segregation of the two types of segment. The optical properties of the block copolymers are rather unusual. Refractive index increments of the two types of segment are markedly different and in toluene opposite in sign. The overall refractive index increment can therefore be rather small and the Pe function under these conditions greater than unity at finite angles. Zimm plots of unusual form are then observed. These effects are explained in terms of the Benoit theory of light scattering from block copolymers.

The dispersion state of sodium-sulphonated polystyrene (NaPSS) star-branched polyelectrolytes was investigated in salt-free aqueous solutions, by use of the small-angle X-ray scattering technique. With respect to polystyrene (PS) star-branched polymers of identical functionality, the ordering phenomenon occurring in the neighborhood of the overlap concentration c * is reinforced and observed in a larger range of concentrations. Moreover, the degree of order is no longer maximum at c * and is improved as the concentration decreases. The dispersion state is then mainly controlled by the electrostatic interaction. A crystalline order should therefore be achieved with stars of lower functionality, provided the electrostatic interaction is added to the osmotic repulsion. On the other hand, unusual scattering patterns are measured for aqueous solutions of NaPSS star polyelectrolytes. Indeed, a diffuse scattering is revealed at high angles, in addition to the regular diffraction rings related to preferred interstar distances. It is similar to the broad scattering peak produced by semidilute solutions of NaPSS linear polyelectrolytes and associated to the electrostatic correlation hole within the isotropic model. In the dilute regime (c < c * ), it is just an intramolecular characteristic and represents the electrostatic repulsion between arms belonging to the same star. In the semidilute regime (c > c * ), it also reflects the electrostatic repulsion between arms of distinct stars. So, as the concentration increases, it is mainly caused by the interpenetration of NaPSS stars. Such an observation is in agreement with the composite structure earlier proposed by Daoud and Cotton for star semidilute solutions. For c > c * , NaPSS star aqueous solutions can therefore be pictured as effective stars immersed in a matrix formed by the overlap of the arm ends. With respect to the dilute regime, the effective stars are smaller; the higher the concentration the smaller the size. PACS. 61.25.Hq Macromolecular and polymer solutions -82.70.-y Disperse systems