Bruno Jasse

Intramolecular excimer formation in polystyrene model molecules

Abstract Intramolecular excimer formation has been investigated for three polystyrene model molecules: 2,4-diphenylpentane, 2,4,6-triphenylheptane and 2,4,6,8-tetraphenylnonane. The results show that isotactic conformations are more favourable for excimer formation than syndiotactic. A high efficiency of excimer sampling is observed for the isotactic forms of the three and four-units polystyrene model molecules with respect to the two-unit model compound.

Fourier transform infra-red and birefringence studies of orientation in uniaxially-stretched poly(2,6-dimethyl 1,4-phenylene oxide) — atactic polystyrene blends

Abstract Infra-red measurements of the dichroic ratio of polystyrene and poly(2,6-dimethyl 1,4-phenylene oxide) absorption bands provide a valuable method for the determination of orientation of chains of both polymers during stretching of their compatible blends. Influence of blend composition on orientation of both polymer chains has been studied in the overall range of concentration. A similar behaviour is observed for both polymers. Orientation is concentration-dependent when the corresponding component is present in a large amount up to a limit concentration C1. In the present case C 1 ⋍ 20% of PPO for PS orientation and C 1 ⋍ 25% of PS for PPO orientation. Between these two values, orientation of both components is insensitive to the composition of the blend. The increase in orientation observed for both polymers in the blends when compared with the pure components is in good agreement with an increase in friction coefficient.

Molecular dynamics of polystyrene model molecules: Solvent effects on intramolecular excimer formation in 2,4-diphenylpentanes

Abstract Intramolecular excimer formation in 2,4-diphenylpentanes has been examined in a homologous series of alkanes, in ethanol and in mixtures of ethanol, ethylene glycol and glycerol. The ratio of the emission intensities of dimer and monomer (ID/IM) is not affected in low viscosity solvents but, above 4 cP, viscosity effects are discernible and a relationship of the form ID/IM = Aη−2 is obeyed. In methylene chloride, only the dl molecule exhibits a decrease of the efficiency of excimer sampling. The temperature dependence of ID/IM in isooctane and methylene chloride has been interpreted in terms of the activation energy of the excimer sampling.

Tensile drawing of poly(aryl ether ether ketone): 1. Birefringence, infra-red dichroism and shrinkage-stress measurements

Abstract Birefringence, infra-red dichroism and shrinkage-stress data of poly(aryl ether ether ketone) (PEEK) uniaxially drawn above the glass transition have been analysed in the framework of the theory of rubber elasticity. It is shown that PEEK chains having a molecular weight Mn ≈ 13000 only contain three entangelements per chain on average. The very low level of shrinkage stress of the samples having a draw ratio below the threshold of strain-induced crystallization is perfectly consistent with a weakly entangled network. The birefringence versus stress curve discloses an orientation-dominated regime at low strains and a deformation-dominated regime starting from the onset of the strain-induced crystallization. The length of the random link, L= 1.8 nm, is in very good agreement with the one that can be theoretically predicted for the stiff-chain polymer relatives, poly(ethylene terephthalate), polycarbonate and poly(phenylene oxide). A value for the intrinsic birefringence of the amorphous phase, Δn°a = 0.38±0.04, is also reported.

Molecular dynamics of polystyrene model molecules. Conformational energy and ultrasonic relaxation of 2,4,6-triphenylheptane

Conformational energies have been calculated for isotactic, syndiotactic and heterotactic 2,4,6-triphenylheptane, giving the most likely states and the conformational paths between them. The ultrasonic attenuation of these compounds has been investigated as a function of temperature and frequency. A single relaxation is observed for isotactic and syndiotactic isomers and has been assigned to isomerism between the helix tg+tg+(or g–tg–t) and the transition form g–ttg+ for the isotactic, and between tttt and ttg+g+(or g–g–tt) conformations for the syndiotactic isomer. Two relaxation processes were needed for the heterotactic isomer, each involving one part of the molecule; the rotational isomerism of the meso diad is between g–ttt (or tg+tt) and tttt conformations, while the racemic diad involves a relaxation between g–ttt and g–tg+g+ conformations.

Molecular dynamics of polystyrene model molecules: ultrasonic relaxation of 2,4-di(parabromophenyl)pentane

Abstract Ultrasonic attenuation of meso and racemic 2,4-di(parabromophenyl)pentanes, model compounds of poly(parabromostyrene), have been investigated as a function of temperature and frequency. The single relaxation observed for each isomer has been assigned to similar rotational isomerism as for the 2,4-diphenylpentanes, despite the large size of the bromine atoms. The substituent effect mainly appears in the lowering of the relaxation frequency.