Jean-Michel Guenet

Thermoreversible gelation of syndiotactic polystyrene in benzene

Abstract The temperature-concentration phase diagram of syndiotactic polystyrene (sPS)/benzene gels establishes the existence of two compounds, C1 and C2, the stoichiometries of which, expressed as benzene molecules/monomer, are 4 1 (C1) and 1 1 (C2). C1 is an incongruently-melting compound as it transforms into C2 at about the boiling point of benzene. In both compounds the chains take on a 21 helical form. Neutron diffraction investigations confirm the existence of these compounds but also disclose that compound C1 possesses a novel crystalline form. Neutron scattering experiments reveal that sPS chains can be described by worm-like statistics in the molten state while still retaining a conformation close to the 21 helix. It is suggested that helix stabilization occurs by means of benzene molecules intercalated between the phenyl groups as previously proposed for isotactic polystyrene. A molecular model is put forward. The neutron scattering curves obtained in the gel state, although smeared by strong interchain scattering, are consistent with the absence of chain folding. The link between the fibrillar morphology of the gel and helix stabilization is discussed.

On the definition of thermoreversible gels: the case of syndiotactic polystyrene

Abstract We consider the four criteria recently proposed by one of us (JMG) for defining a thermoreversible gel and use them with syndiotactic polystyrene/solvent systems. We show that some of these systems, considered to be gels, do not obey these criteria. In addition, a simple mechanical test is presented which allows a thermoreversible gel to be recognized.

Thermoreversible gelation of syndiotactic polystyrene in toluene and chloroform

Abstract Temperature-concentration phase diagrams have been established for syndiotactic polystyrene (sPS)/ toluene gels and sPS/chloroform gels. The occurrence of polymer-solvent compound formation is shown in both systems and confirmed by neutron diffraction investigations. In particular it is found that a compound with a stoichiometry close to 1 / 1 occurs in both types of gels. Infra-red analysis show that the polymer chains take on a 21 helical form in both series of gels. In sPS/toluene gels, neutron scattering experiments reveal that the chains possess a worm-like conformation in the molten state while still retaining a conformation close to the 21 helix. The results are discussed in the light of a polymer-solvent intercalation model already proposed for sPS/benzene gels.

Agarose chain conformation in the sol state by neutron scattering

We report on small-angle neutron scattering investigations into the agarose conformation in water. We show that at 70 degrees C in the sol state agarose chains are fairly rigid with a lower limiting value for the persistence length of about 9 nm and a mass per unit length of muL = 360 +/- 36 g/mol x nm. The value of the latter parameter is consistent with single-stranded helices as those proposed recently by Foord and Atkins. Such a high rigidity together with such a low linear mass leads one to wonder how would the chains manage to intertwine should they form double helices in the gel state.

Neutron scattering investigations on the effect of crystallization temperature and thermal treatment on the chain trajectory in bulk-crystallized isotactic polystyrene

Abstract Small-angle neutron scattering (SANS) investigations coupled with differential scanning calorimetry (d.s.c.) have been performed on isotactic polystyrene (IPS) samples crystallized in bulk by one of three thermal treatments: crystallization at high supercooling ( T c = 140°C); crystallization at low supercooling ( T c = 200°C); crystallization at T c = 140°C then annealing at T an = 180°C. The neutron scattering experiments show that the chain trajectory in the semcrystalline medium varies depending on the molecular weight of the protonated matrix and the crystallization process. The SANS data, interpreted in the light of the d.s.c. measurements and conformational models of the semicrystallized chain, point out in particular that the size of the regularly-folded crystalline sequences (along the 330 plane) increases with chain mobility in the originally amorphous melt. These results are quite consistent with those of previous studies of IPS using bulk-crystallized samples and solution-grown single crystals.

Rheological and Neutron Scattering Investigations of the Jelly State of Binuclear Copper Complexes in Cyclohexane

Binuclear copper(II) carboxylates are known to exhibit thermotropic mesophases. When dissolved into cyclohexane, viscoelastic jellies are obtained. Oscillatory shear experiments are performed and the Maxwell model is used as a basic description of the rheological behaviour. At high frequency, a transient elastic three-dimensional network is evidenced. Small-angle neutron scattering experiments indicate that the network is constituted by aggregates with a rodlike local structure with a diameter of ??=?17.5?? (Cu2-S8) and only one molecule lying in the cross-section. The aggregates are formed with stacked molecules which are linked by axial copper-oxygen coordination bondings. The jelly state is found to be reminiscent with the related discotic thermotropism.

Contributions of phase diagrams to the understanding of organized polymer-solvent systems

Abstract When organized phases are formed in polymer-solvent systems through first-order transition, knowledge of the temperature-concentration phase diagram brings valuable information about their nature (polymer-solvent compound, peritectic, etc.), and also about the mechanisms involved during the organization process. The construction of these phase diagrams usually obeys Gibbs phase rules, even though formation often occurs under non-equilibrium conditions. In particular, the existence of invariants (monotectic transformations, peritectic, etc.) can still be observed. Knowledge of the phase diagram is also of prime importance for understanding rheological properties, particularly for thermoreversible gels. Examples dealing with these issues are given in this paper.

Glass transition of ultra-thin films of modified PVC

Abstract Ultra-thin layers ( h =50–150xa0nm) of PVC, functionalised in solution by thiophenol (t-PVC) and 4-aminothiophenol (am-PVC), were prepared by spin-coating on silicon wafers. The glass transition temperature of these thin films was investigated by kinetic scan ellipsometry. The Tg of the am-PVC and t-PVC modified PVC is found to increase more strongly with the degree of modification in thin layer geometry than in the bulk. Also, Tg( h ) of t-PVC exhibits a larger increase in the confined state as compared to am-PVC. This result is unexpected since no specific interactions are assumed between thiophenyl group and the surface indicating that other driving forces for the alteration of Tg( h ) should be taken into account. In a second step, the supported PVC thin films are chemically cross-linked in solution by terephthaloyl chloride. The Tg( h ) of the polymer layer increases with the cross-linking time more severely at a higher degree of modification but independently of the thickness of the layer indicating that the diffusion process is not altered in thin layers. Above Tg, the thermal expansion coefficient α of the thin films is not modified after cross-linking, whereas, it was found to decrease strongly between 4 and 8% of modification in the uncured state.

Structure of colloidal particles in water-oil mixtures stabilized by polymeric emulsifiers: 2. Small angle neutron scattering investigation

Abstract Small-angle neutron coherent scattering techniques have been used to characterize the conformation of amphiphilic macromolecules in polymeric microemulsions. The radius of gyration, the molecular weight and the aggregation number of the macromolecules within the micelles have been measured using a dilution procedure in conjunction with an adjustment of the contrast factor between polymer and solvents. Additional information on the internal structure of the micelles has been obtained from the asymptotic behaviour of the scattering form factor P ( q ). From considerations of the respective affinities between the two copolymer sequences and the solvent mixture, plausible models are proposed for the micellar structure.

Conformation of isotactic polystyrene (IPS) in the bulk crystallized state as revealed by small-angle neutron scattering

Abstract Neutron scattering experiments have been performed on isotactic polystyrene (IPS) samples in the bulk crystallized state (Tcrystallization = 185°C). The determination of the conformation of tagged chains ranging from 2.5 × 105 to 7 × 105 has been undertaken on two different hydrogenated IPS matrices. A matrix of usual molecular weight (Mw = 4 × 105) leads to results which do not agree with Florys model. In this case, measurements on radius of gyration Rg show on the one hand an important increase of this parameter (∼40%) with increasing crystallinity for the highest molecular weight tagged chains and on the other hand a variation with molecular weight like M0.78. These results are interpreted with a schematic model involving a long crystalline sequence incorporated in the monocrystal along the 110 plane and two amorphous wings. Such an assumption is confirmed by the scattering behaviours in the intermediate range. On the other hand, by using an IPSH matrix of very high molecular weight (Mw = 1.75 × 106), and the same tagged chains previously considered, a very weak variation of Rg with increasing crystallinity is observed. This leads to consider in this case Florys conformation which is corroborated by data obtained in the intermediate range.