Françoise Lauprêtre

Investigation of the relationships between the chain organization and rheological properties of atactic poly(vinyl alcohol) hydrogels

Abstract The structure and rheological behavior of atactic poly(vinyl alcohol) (a-PVA) hydrogels prepared by freeze/thaw cycles were investigated as a function of polymer concentration and number of freeze/thaw cycles. The presence of phases with different mobilities was observed using 13 C CP/MAS and DP/MAS NMR experiments. The degree of crystallinity of the a-PVA-rich phase was determined by 1 H NMR free decay experiments. Measurements of the shear storage and loss modulus were performed at a fixed frequency of 1 Hz and a strain value of 0.1%, i.e. under conditions where the deformation imposed on the gel structure is entirely reversible. Results thus obtained showed that an increase in the number of freeze/thaw cycles induces an increase in the degree of crystallinity in the polymer-rich phase together with an increase in the storage modulus. The a-PVA hydrogels became more fragile as the number of freeze/thaw cycles was increased. Moreover, both the percentage of protons in a rigid environment measured by 1 H NMR and the storage modulus values tended to a limiting value after six freeze/thaw cycles. These results show that the first five or six freeze/thaw cycles play a very important role in determining the hydrogel structure and rheological properties. A more detailed comparison of NMR and rheological data led to the conclusion that the storage modulus is mainly controlled by the a-PVA crystallinity while the hydrogen bond interactions have a much smaller contribution.

Soluble supramolecular polymers based on urea compounds

N,N′-Dialkylureas, which form intermolecular hydrogen bonds, are interesting precursors in the field of supramolecular chemistry. Particularly, N,N′-di(2-ethylhexyl)urea, which is soluble in nonpolar solvents, was shown by viscosimetry and FTIR spectroscopy to be associated in heptane at concentrations higher than 10 g l−1. The interactions involved are strongly reinforced by the cooperative association of two urea groups in the case of bis-ureas prepared from 2,4-toluene diisocyanate.

Secondary relaxation processes in polyethylene terephthalate-additive blends : 1. N.m.r. investigation

High-resolution carbon-13 and deuterium n.m.r. experiments have been used to examine the effects that low-molecular weight additives have on the secondary relaxation processes in polyethylene terephthalate (PET). These experiments have revealed that both the phenyl rings and the carbonyl groups in PET undergo small angle oscillations at temperatures below the glass transition temperature. It would, however, appear from the deuterium n.m.r. experiments that the phenyl rings also undergo rapid 180 flips, which appear to be closely related to the relaxation peak that is observed in dynamic mechanical experiments. Extremely little motion has, however, been observed in the ethylene glycol units and it has therefore been concluded that this group does not contribute to the secondary relaxation processes. Finally it has been observed that by blending low-molecular weight additives into the polymer it is possible to significantly reduce the number of the phenyl ring flips in the polymer and thereby suppress the relaxation peak.

Polymers with mesogenic elements in the main chain: a nematic aromatic copolyester

Abstract Properties of a copolyester based on methylhydroquinone, pyrocatechol and terephthalic acid have been described. Light scattering data for the copolyester in chloroform and dichloroacetic acid are well represented by M w =220 000 while the intrinsic viscosity is 0.22 dl g−1, which implies a compact arrangement of the molecule in dilute solution. Differential scanning calorimetry (d.s.c.) traces show that the copolyester is essentially non-crystalline in character. Polarizing microscopy (texture observations, miscibility tests) provides definite proof as to the nematic structure of the melt. Well aligned samples can be produced either by subjecting nematic melt to a magnetic field (Hc = 0.6 T) or by suitable treatments of the slides between which the melt is observed. The conoscopic interference pattern corresponds to that of a uniaxial system showing an optically positive character. A structural model is proposed to account for the X-ray patterns obtained from oriented samples.

N.m.r. and d.s.c. investigations of the miscibility of poly(methyl methacrylate)/ poly(ethylene oxide) blends

Abstract 1H spin-lattice relaxation times in the rotating frame and free induction decays were measured on poly(methyl methacrylate)/poly(ethylene oxide) blends over the whole composition range. In these systems, in which one component is an amorphous polymer and the other one can crystallize, experiments combining both 1H and 13C n.m.r. spectroscopies allowed the determination of the composition of each phase. Results thus obtained led to the conclusion that phase separation occurs during the freeze-drying step. Therefore, poly(ethylene oxide) crystallization takes place in a poly(ethylene oxide) rich phase.

Synthesis and mesomorphic properties of a new thermotropic liquid-crystalline ‘backbone’ copolyester

Details of the thermal behaviour of the following liquid-crystalline ‘backbone’ copolyester are reported: — [OCC6O]0.6+[OOCC6H4OCH2CH2OC2H4COOCH2CH2]0.4? Between 218° and 337°C, observations using a polarizing microscope indicate the formation of a birefringent melt that exhibits a Schlieren texture. From the observation of disclinations of strength ± 12, the mesophase can be identified as a nematic phase. The isobaric phase diagram of mixtures of copolyester and N,N′-di-(ρ-methoxy benzylidene)-α,α′-bi-ρ-toluidine and X-ray diffraction patterns provide definite proof as to the nematic structure of the birefringent melt. Addition of a chiral compound to the copolyester in the nematic state causes the formation of the typical textures of cholesterics. The temperature dependence of the order parameter has been determined from FTi.r. polarized spectra. At high temperature S is found to be 0.3. Such a result confirms the recent theory of Ronca and Yoon: the isotropic-nematic transition of semiflexible polymers does not entail a very high degree of order in the resultant nematic phase.

High-resolution solid-state 13C nuclear magnetic resonance investigation of local motions in model epoxy resins

The local dynamics of some ‘model’ epoxy networks based on various diepoxides and diamines was investigated in the bulk state using high-resolution solid-state 13C nuclear magnetic resonance. The strength of the 13C-1H dipolar coupling was determined from the rises of 13C magnetization in cross-polarization experiments using very short contact times. A restricted motional averaging of the 13C-1H dipolar coupling was observed below the glass transition temperature for the hydroxypropyl ether groups and methylene carbons adjacent to the crosslinks, indicating the existence of local motions of these units in the tens of kilohertz range. The extent and temperature dependence of the motional averaging depend on the chemical structure of the epoxy network considered. Comparison with results derived from dynamic mechanical experiments leads to the conclusion that the motions of the hydroxypropyl ether groups are likely to participate in the phenomena responsible for the β secondary relaxation. Moreover, the relatively high mobility of the central methylene units in the aliphatic sequences of the diglycidyl ether of the butanediol-based network, and of the hexamethylenediamine-based network, can be associated with the γ relaxation of epoxy networks.

Molecular motion of polyethylene oxide in dilute solutions studied by electron spin resonance and nuclear magnetic relaxation

The isotropic rotational model appears a satisfactory approximation for describing both e.s.r. and 1H and 13C n.m.r. relaxation measurements of PEO in dilute solution, indicating the highly flexible nature of this polymer. Correlation times determined by e.s.r. are proportional to the solvent viscosity as predicted by the Stokes–Einstein law. In contrast, the dependence of correlation times obtained by n.m.r. techniques as a function of solvent viscosity must be described in terms of the Magee diffusion model. These different modes of behaviour are expected to arise from a somewhat larger free volume available for the terminal segment than for central segments of the macromolecule.

Comparison of the local chain dynamics in a series of bulk polymers at temperatures well above the glass transition temperature

Abstract 13 C n.m.r. spin-lattice relaxation times and excimer fluorescence data obtained on a series of bulk polymers at temperatures well above the glass transition temperature have been analysed in terms of local dynamics. Results thus obtained have shown that the intramolecular conformational changes of the excimer-forming probes are mostly governed by the segmental motions of the polymer chains involved in the glass transition phenomenon. Moreover, the frequency of these intramolecular motions is controlled by the monomeric friction coefficient of the surrounding matrix. Simultaneous analysis of 13 C n.m.r. spin-lattice relaxation times and excimer fluorescence data has led to an estimation of the displacement associated with the segmental motion of the chain. The jump amplitude is determined by the precise chemical nature of the polymer chain. In addition, 13 C n.m.r. data have shown that, in all the polymers investigated, libration of the C-H vector is superposed on the conformational reorientation. The libration amplitude depends on the chemical structure of the polymer and, for a given polymer, on the steric hindrance at the site of each carbon considered.

Conformation of poly(glutamic acid) and of poly(aspartic acid) in the solid state. X-ray diffraction, infra-red and 13C cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopic study

Abstract In order to assess the manifestation of possible conformations of poly(amino acids) in 13C cross-polarization/magic angle spinning nuclear magnetic resonance (CP/MAS n.m.r.) spectra, the conformational structures of poly(α- l -glutamic acid) (poly(Glu)), of poly(α-L-aspartic acid) (poly(Asp)) and of irregular poly(α,β,- dl -aspartic acid) in the solid state were studied by X-ray diffraction and infra-red spectroscopy and compared with the results of 13C CP/MAS n.m.r. spectroscopy. None of the methods alone can fully characterize the conformation; by a combination of the results it could be determined that poly(Glu) and poly(Asp) in the free acid form exhibit both α-helical and β-structure, whereas the random-coil structure predominates in their sodium salts. Both the free acid and the sodium salt of poly(α,β- dl -Asp) exist predominantly in the random-coil form. It turns out that the sodium salts are especially suitable for determining the n.m.r. characteristics of the random-coil conformation in the solid state. The dynamic behaviour studied by the spin-lock cross-polarization experiment indicates that the studied poly(amino acids) in the free acid form are much more rigid than, for example, the poly(alkyl methacrylates).