Liliane Bokobza

Determination of polybutadiene microstructures and styrene–butadiene copolymers composition by vibrational techniques combined with chemometric treatment

Abstract Vibrational spectroscopy techniques have been used to study the polymer composition using only one technique at a time. In this paper, mid- and near-infrared (NIR) as well as Raman spectroscopies are combined with partial least squares (PLS) in order to determine the cis -, trans - and vinyl-configurations of polybutadienes as well as the composition of styrene–butadiene copolymers. A critical comparison of these techniques for the microstructure analysis will also be presented not only in terms of quantitative results but also in the way the samples can be analyzed and the spectra interpreted. Each of the techniques gives comparable results in terms of the precision of the models developed (limited to the precision of the reference method), but we can also see that the NIR spectroscopy is more reproducible in the short term due to the high signal to noise ratio obtained on the spectra. On the other hand, Raman excited in the NIR (FT-Raman) is the only technique which is free of sample preparation and handling allowing to get very good results in the long term.

Investigation of the mobility of polybutadienes: 1. Excimer fluorescence technique

Abstract Intramolecular excimer formation of meso-2,4-di( N -carbazolyl)pentane dissolved in polybutadienes of molecular weight 500 and 1000 has been investigated. The probe dynamics, which follow the WLF equation, are not similar in the two matrices. The effect cannot be ascribed to the difference between the glass transition temperatures of the two polymers but to a variation of their microstructures.

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.

Reinforcement of elastomeric networks by fillers

Although numerous investigations have been devoted to the analysis of filled systems, the mechanisms of the reinforcement have not been completely clarified. The present paper attempts to recall, through new experimental data, some of the basic mechanisms of filler reinforcement. The basic processes giving rise to the increase in the elastic modulus, generally observed by a filler addition in an elastomeric medium, are discussed in the light of mechanical measurements and of recent results based on measurements of chain orientation.

Molecular orientation studies in polymer films by polarization modulation FTi.r. spectroscopy

Abstract Measurements of molecular orientation in stretched films of polydimethylsiloxane using infra-red linear dichroism have been improved by the introduction of polarization modulation of the incident electromagnetic field. This technique is able to measure small dichroic effects with high sensitivity, even for very small sample draw ratios.

Investigation of local molecular motions in bulk polymers through the emission properties of an intramolecular excimer-forming probe

Abstract Intramolecular excimer formation of meso -bis[1-(2-pyrenyl)ethyl] ether dissolved in different elastomers has been investigated. The intramolecular conformational change required for the excimer sampling process is shown to be controlled by the segmental motions of the polymer matrix involved in the glass transition phenomenon. The results show that, at a fixed frequency of the probe motion, the fractional free volume as well as the free volume per atomic group vary from one polymer to another.

The use of near-infra-red spectroscopy coupled to the polarization modulation technique to investigate molecular orientation in uniaxially stretched polymers

Abstract The purpose of this brief communication is to demonstrate how near-infra-red (n.i.r.) spectroscopy can be used to characterize molecular orientation in stretched polymers. Combined with the polarization modulation technique, n.i.r. provides a useful way of measuring small anisotropy with high precision in thick polymer films

Investigations on natural rubber filled with multiwall carbon nanotubes

Composites based on natural rubber (NR) and multiwall carbon nanotubes (MWNTs) were prepared by using different mixing processes. The state of dispersion was evaluated by transmission electron microscopy and the extent of reinforcement imparted to the elastomeric matrix was assessed by equilibrium stress–strain measurements. Dynamic mechanical analysis was performed in order to get information on the polymer–filler interface. Molecular investigations including infrared analysis under strain or Raman spectroscopy were carried out in order to point out possible alterations in the crystalline phase or interfacial interactions between the filler and the matrix.

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.

Relation between excimer formation in small probes and free-volume theory in polymer melts

Abstract Intramolecular excimer formation of meso-2,4-di( N -carbazolyl)pentane dissolved in different elastomers has been investigated. The intramolecular conformational change required for the excimer sampling process is shown to be controlled by the segmental motions of the polymer matrix involved in the glass transition phenomenon. The results show that, at a fixed frequency of the probe motion, the fractional free volume as well as the free volume per atomic group vary from one polymer to another.