J.L. Halary

Dynamic mechanical and 13C n.m.r. investigations of molecular motions involved in the β relaxation of epoxy networks based on DGEBA and aliphatic amines

Abstract As shown by a precise analysis of the dynamic mechanical data in terms of activation energies and entropies, the extent of the β transition strongly depends on the role played by the crosslinks. In a densely crosslinked network, localized motions at the spatial scale of an epoxy-amine repeat unit occur at low temperatures, whereas cooperative modes implying more than six units are responsible for the high temperature part of the relaxation. In the quasi-linear systems, only modes with a low spatial extent exist. Measurements of the strength of the 13 C- 1 H dipolar couplings, together with other n.m.r. techniques, have shown that the motions of the hydroxypropylether units take part in the β transition. The DGEBA ring flips also take part in the β transition. The comparison of dynamic mechanical and 13 C n.m.r. results tends to indicate that the sensitivity of the n.m.r. experiments allows it to probe the cooperative motions, which induce a reorientation of quite a large amount of 13 C- 1 H bonds, rather than the isolated ones, whose effect on the 13 C- 1 H dipolar coupling is rather weak. In addition, it is shown that the β transition is broader and more intense in networks prepared in the presence of secondary diamines than in systems with the same crosslink density obtained by using primary monoamines. This result is interpreted in terms of motions that have a higher intramolecular cooperativity than the cooperativity developed in networks incorporating pending hexamethylene units, and a lower spatial extent than the one observed in the densely crosslinked network.

Hydrogen bonding in PMMA-fluorinated polymer blends: FTi.r. investigations using ester model molecules

Abstract The sensitivity of the carbonyl stretching vibration of methyl acetate to interactions with the surroundings is re-examined through FTi.r. analyses and Fourier self deconvolution. The relevant conclusions permit a study of the mixtures of methyl acetate and methyl pivalate with various fluorinated polymers. Evidence is given for the existence of hydrogen bonding between poly(vinylidene fluoride) and these model compounds of poly(methylmethacrylate). Arguments are put forward in favour of a similar behaviour in poly(vinylidene fluoride)/poly(methylmethacrylate) blends.

Phase separation in polystyrene-poly(vinylmethylether) blends: a a fluorescence emission analysis

Abstract Fluorescence emission of labels appears to be a new technique for the investigation of the LCST behaviour of polystyrene-poly(vinylmethylether) (PS-PVME) blends. Indeed, heating ternary blends of PVME/PS/labelled PS results in sharp increases in the fluorescence intensity, which occur simultaneously with their phase separation. Specific interactions between the anthracenic units of the labelled PS and the ether functions of PVME, are responsible for the fluorescence quenching, which occurs in the compatible blends. Quenching drops as phase separation proceeds, because of the lowering of the probability for label-PVME interactions in the two-phase state. By relating the phase separation curves obtained in this way to those acquired by the classical light scattering method, it is shown that fluorescence experiments may allow determination of both spinodal and binodal curves, provided that heating rate is appropriate.

Cement-polymer and clay-polymer nano- and meso-composites: spotting the difference

Calcium silicate hydrate (C-S-H), the most important reaction product of Portland cement with water, is a layered lattice silicate which has much in common with smectite clays. Both materials may be synthesised (C-S-H) or found in nature (smectites) in the form of particles which are stacks of negatively charged nanometre-thick platelets separated by water molecules and charge-compensating cations. Smectite clays are well known for their ability to intercalate molecular species including polymers, to “swell” in a remarkable variety of solvents and to form nano-composites with polymers in which total delamination may eventually be obtained. In this work, we explore the possibility of intercalating cationic, anionic and neutral water-soluble polymers in C-S-H. Contrary to recent reports, no clear signs for intercalation of the macromolecules were observed. Nevertheless, the significant amount of polymer retained by the silicate suggests that the composite materials formed may be considered as meso-composites in which the individual solid units are not the individual C-S-H sheets but crystallites thereof. The localisation of electric charges and the strong Coulombic forces acting in cement hydrates are thought to be responsible for this difference.

Dynamic mechanical and 13C n.m.r. analyses of the effects of antiplasticization on the β secondary relaxation of aryl-aliphatic epoxy resins

Abstract The antiplasticization of epoxy networks based on diglycidylether of bisphenol A (DGEBA) and hexamethylene diamine was investigated by both dynamic mechanical analysis and high-resolution solid-state 13 C nuclear magnetic resonance (n.m.r.) spectroscopy. Particular attention was paid to the influence of the antiplasticizer on the β secondary transition. In the case of densely crosslinked networks, the relevant loss peak of the antiplasticized material is dramatically reduced compared with that of the pure resin. The cooperative modes responsible for the high-temperature part of the β relaxation in the pure networks do not show up in the antiplasticized systems. N.m.r. measurements clearly demonstrate that the loss peak reduction is accompanied by a strong decrease in the mobility of the CH 2 groups that are close to the nitrogen crosslinks. In agreement with these observations, the efficiency of the antiplasticizer is weaker in the loosely crosslinked networks where the cooperative modes of the β transition are reduced. In the quasi-linear systems, where the β motions tend to be restricted to isolated motions, the efficiency of the antiplasticizer almost vanishes. This molecular description of the antiplasticization mechanism, in terms of hindrance of the short-scale cooperative motions in the glassy state, can be viewed as a dynamic coupling between the polymer and antiplasticizer molecule.

Hysteretic heating of modified poly(methylmethacrylate)

The hysteretic thermal response of two polymers, polymethyl-methacrylate (PMMA_REF) and a tougher modified PMMA þ N-methyl glutarimide (PMMA_MOD) has been investigated under compressive cyclic loading at high stress levels. The modification increases the ability of PMMA_REF to undergo plastic deformation. This work characterizes the thermomechanical response of both polymers at high cyclic stress levels (of the order of sy), that were not previously investigated. The comparison clarifies the contribution of increased chain mobility on the hysteretic heating phenomenon, with emphasis on the nature of a significant exothermal peak that was previously observed in commercial polycarbonate (PC). In this work, both PMMA_REF and PMMA_MOD were subjected to compressive cyclic loading, and the temperature was continuously monitored until specimen failure. The experimental results show that, despite the higher toughness PMMA_MOD, the two polymers fail in a very similar fashion, both in terms of temperature rise and bulging failure mode, without the initial thermal peak that was observed in commercial PC. It thus seems that the unique thermomechanical response of PC is neither related to its plastic flow properties, nor to its ability to undergo high cyclic stresses. It is suggested that a cyclic stress induced/related exothermal phase transition might be responsible for the thermal peak in this material. q 2003 Elsevier Science Ltd. All rights reserved.

Plasticity of polystyrene -poly(2,6,dimethy1,1,4,phenylene oxide) blends

Abstract The temperature and strain-rate dependence of the plastic yield in compression of blends between polystyrene (PS) and poly(2,6,dimethyl 1,4,phenylene oxide) (PPO) are investigated. The salient of our study is thhat an amount as small as 2 wt% of PPO can significantly lower the yield stress and elastic modulus of pure PS. This result is interpreted as being due to the creation of favourable nucleation sites for the onset of plastic deformation. The plastic flow stress, however, is unaffected by this small addition. When plotted at a constant T - T g our results show a nearly constant yield stress from 2% up to ca. 40 wt% PPO. At this point the yield stress starts to decrease and the activation volume increases, implying a more co-operative character of the plastic deformation mechanisms. This transition corresponds also to a change from more brittle to more ductile behaviour in tensile experiments.

On the role of entanglements in the embrittlement of poly(methyl methacrylate) based random copolymers

Abstract Composition dependences of the glass transition temperature (Tg), the molecular weight between entanglements (Me) and the thin film deformation behaviour have been studied in two series of methyl methacrylate-based random copolymers. When the comonomer is styrene, increases in both Me and brittle behaviour accompany the decrease of Tg. When the comonomer is N-substituted maleimide, the same conclusions hold with increasing Tg. For both series of materials, a correlation is proposed between the number of skeletal bonds in an entanglement strand, Ne, and the quantity (Tg—Tcs), where Tcs is the lower temperature at which shear deformation is observed in thin films in competition with crazing.

Molecular analysis of the viscoelastic properties of epoxy networks as deduced from the study of model systems

Abstract Various model systems were built from a diepoxide and different amines (or mixtures of amines) in order to investigate the dependence of molecular motions on the cross-link density and network chain flexibility. Dynamic mechanical experiments were performed over the frequency range 0.01–85 Hz at temperatures covering the β-relaxation process and the glass transition region. The glass transition temperature, T g , markedly depends on both the cross-link density and chain flexibility. The frequency-temperature superposition principle (WLF equation) was used to determine the viscoelastic coefficients C 1 g and C 2 g . C 1 g , related to the free volume fraction available at T g , mostly depends on cross-link density, whereas the product C 1 g C 1 g , related to the free volume expansion coefficient, is a function of both the chain flexibility and the cross-link density. Motions responsible for the β-process begin to develop at the same temperature, whatever the cross-link density and chain flexibility may be. However, an increase in cross-link density is accompanied by an increase in amplitude and a broadening towards high temperatures of both damping, tan δ, and loss modulus, E ′. This effect is responsible for the decrease of the elastic modulus, E ′, at room temperature with increasing cross-link density.

Orientation and relaxation in uniaxially stretched poly(o-chlorostyrene)-polystyrene blends

Abstract Orientation and relaxation of polymer chains have been analysed in uniaxially stretched films of poly( o -chlorostyrene) (PoCS) and of compatible PoCS/polystyrene (PS) blends. The intrinsic birefringence of PoCS is derived from fluorescence polarization and birefringence orientation measurements. Further, using Fourier transform infra-red spectroscopy, we have determined the angle between the dipole moment vector of some infra-red vibrations of PoCS and the chain axis. The orientation of the benzene ring with respect to the chain axis in oriented PoCS samples is identical to that observed in PS. In PoCS/PS blends, infra-red and birefringence measurements show that chain orientation of both polymers is almost the same and independent of the presence of the second component, in agreement with mechanical relaxation measurements. This result is in contrast with previous data on PS/poly(phenylene oxide) and PS/poly(vinyl methyl ether) blends. It corroborates well the assumption that change in friction coefficients is responsible for the orientation behaviour of compatible blends in which strong enough polymer-polymer interactions develop.