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The nitroxide-mediated polymerization (NMP) of isoprene (I) initiated by the poly(ethylene-stat-butylene)-(SG1)2 dialkoxyamine (PEB-(SG1)2) at 115 °C in 50 vol% pyridine led to cis-1,4-rich poly(isoprene)s (PIs), exhibiting Mn,GPC = 52–59 K (PI equivalents) and Ð = 1.46–1.59. S-I-S triblock copolymers (S = styrene, Mn,GPC = 95–109 K, Ð = 2.11–2.29, FS = 0.30–0.49) were synthesized via subsequent S chain-extensions in bulk at 115 °C from these PI-(SG1)2 macro-initiators. A micro-phase separation, exhibiting possibly a cylindrical or lamellar morphology, was observed by atomic force microscopy for an S-I-S having FS = 0.30. Tensile strength at break σB = 2.8–4.1 MPa and elongation at break eB ~ 375–450% were measured for S-I-S samples with FS = 0.30–0.38. The substitution of the PS outer segments by more rigid segments rich in isobornyl methacrylate (IBOMA) units allowed to improve the stress–strain properties, with σB = 11.4 ± 0.6 MPa and eB = 1356 ± 214%. This IBOMA-I-IBOMA type copolymer also displayed an upper service temperature greater than 150 °C.

β-Myrcene (My), a natural 1,3-diene, and isobornyl methacrylate (IBOMA), from partially bio-based raw materials sources, were copolymerized by nitroxide-mediated polymerization (NMP) in bulk using the SG1-based BlocBuilder™ alkoxyamine functionalized with an N-succinimidyl ester group, NHS-BlocBuilder, at T = 100 °C with initial IBOMA molar feed compositions fIBOMA,0 = 0.10–0.90. Copolymer reactivity ratios were rMy = 1.90–2.16 and rIBOMA = 0.02–0.07 using Fineman–Ross, Kelen–Tudos and non-linear least-squares fitting to the Mayo–Lewis terminal model and indicated the possibility of gradient My/IBOMA copolymers. A linear increase in molecular weight versus conversion and a low dispersity (Đ ≤ 1.41) were exhibited by My/IBOMA copolymerization with fIBOMA,0 ≤ 0.80. My-rich and IBOMA-rich copolymers were shown to have a high degree of chain-end fidelity by performing subsequent chain-extensions with IBOMA and/or My, and by 31P NMR analysis. The preparation by NMP of My/IBOMA thermoplastic elastomers (TPEs), mostly bio-sourced, was then attempted. IBOMA-My-IBOMA triblock copolymers containing a minor fraction of My or styrene (S) units in the outer hard segments (Mn = 51–95 kg mol−1, Đ = 1.91–2.23 and FIBOMA = 0.28–0.36) were synthesized using SG1-terminated poly(ethylene-stat-butylene) dialkoxyamine. The micro-phase separation was suggested by the detection of two distinct Tgs at about −60 °C and +180 °C and confirmed by atomic force microscopy (AFM). A plastic stress–strain behavior (stress at break σB = 3.90 ± 0.22 MPa, elongation at break eB = 490 ± 31%) associated to an upper service temperature of about 140 °C were also highlighted for these triblock polymers.

This contribution addresses the challenging investigation of the chemical mechanisms occurring at the carbon fiber-matrix interface of carbon fiber-reinforced polymer (CFRP) composite materials cured out-of-autoclave. A high energy radiation curing method (electron-beam) was used to initiate free radical polymerization of acrylate-based matrices. Comparison with state-of-the-art thermally cured composites reveals the lower transverse mechanical properties of the radiation-cured CFRP composites. To improve the interfacial layer, several points related to the polymerization at the interphase have been investigated. Notably, the influence on the polymerization of acrylate matrices of the chemical functional groups present at the carbon fiber surface and its graphite-like properties are discussed using representative molecules. It is shown that these additives exhibit strong inhibiting effects, whereas thiol groups efficiently sensitize the initiation mechanisms and undergo transfer reactions. Moreover, it is discussed that graphite-like molecules affect the matrix polymerization and the mechanical properties of the composites. An adapted solution is proposed.

This paper concerns engineered composites integrating metallic particles to enhance thermal and electrical properties. However, these properties are strongly dependent on the forming process itself that determines the particle distribution and orientation. At the same time, the resulting enhanced thermal properties affect the reinforced resin viscosity whose flow is involved in the intimate contact evolution. Thus, a subtle and intricate coupling appears, and the process cannot be defined by ignoring it. In this paper, we analyze the effects of particle concentration and orientation on the process and processability. For this purpose, three main models are combined: (i) a multi-scale surface representation and its evolution, by using an appropriate numerical model, (ii) flow-induced orientation, and (iii) the impact of the orientation state on the homogenized thermal conductivity.

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Nitroxide-mediated polymerization (NMP) of β-myrcene (My) at 120 °C in bulk using unimolecular SG1-based succinimidyl ester-functionalized BlocBuilder (BB) alkoxyamine resulted in low dispersity (Đ = 1.1–1.4) poly(myrcene)s P(My)s with high SG1 chain-end fidelity. The polymerizations also showed the number-average molecular weights (Mn) increased almost linearly with conversion. SG1-terminated P(My) macroinitiators were cleanly chain-extended with styrene (S) and the S-rich P(My-b-S) diblock copolymers exhibited two distinct glass transition temperatures (Tgs), indicative of microphase separation. P(My-b-S) diblocks showed brittle stress–strain behavior, plausibly due to relatively low Mn. My/S mixtures with initial S molar feed compositions fS,0 = 0.10–0.94 were also statistically copolymerized (Mn = 8.2–19.8 kg mol–1, Đ ≤ 1.37, and monomodal distributions). Copolymer reactivity ratios were rS = 0.25 ± 0.04/0.34 ± 0.19 and rMy = 1.88 ± 0.12/2.19 ± 0.07 using Fineman–Ross and Kelen–Tudos methods. The stat...

The lower transverse mechanical properties of radiation-cured acrylate-based composites reinforced with carbon-fibre with respect to the thermosettable analogues was investigated from the viewpoint of chemical interactions at the interface between the matrix and the carbon material. XPS analysis of representative commercial carbon fibres revealed the presence of a significant amount of chemical functions potentially exerting an adverse effect on the initiation and propagation of the free radical polymerization initiated under high energy radiation. The EB-induced polymerization of n -butyl acrylate as a simple model monomer was conducted in the presence of various aromatic additives exhibiting a strong inhibiting effect, whereas thiols efficiently sensitize the initiation mechanism and undergo transfer reactions. A method based on the surface modification of sized fibres by thiomalic acid is proposed for overcoming the localized inhibition phenomenon and for improving the mechanical properties of the resulting acrylate-based composites.

The crosslinking polymerization of multifunctional monomers is known to yield brittle matrices, therefore limiting the development of this technique for the production of high performance composite materials. Among the various possible causes of the brittleness, the spontaneous formation of nanoheterogeneities during radiation-initiated polymerization is supported by atomic force microscopy imaging and by calorimetric analyses. The controlled polymerization-induced phase separation of nanosized clusters of polyethersulfone was evaluated as a means for alleviating the inherent tendency of the diacrylate materials to fragile failure. Various homogeneous formulations including the aromatic diacrylate monomers, and polyethersulfone together with a compatible reactive diluent were prepared and polymerized by electron beam irradiation. The resulting toughened materials show optimized critical stress intensity factor ( K Ic ) over 2 MPa m 0.5 , whereas the K Ic value is about 1 MPa m 0.5 for the unmodified reference resin.

Cross-linking polymerization initiated by high-energy radiation is a very attractive technique for the fabrication of high-performance composite materials. The method offers many advantages compared to conventional energy- and time-consuming thermal curing processes. Free radical and cationic poly-addition chemistries have been investigated in some details by various research groups along the previous years. A high degree of control over curing kinetics and material properties can be exerted by adjusting the composition of matrix precursors as well as by acting on process parameters. However, the comparison with state-of-the-art thermally cured composites revealed the lower transverse mechanical properties of radiation-cured composites and the higher brittleness of the radiation-cured matrix. Improving fiber–matrix adhesion and upgrading polymer network toughness are thus two major challenges in this area. We have investigated several points related to these issues, and particularly the reduction of the matrix shrinkage on curing, the wettability of carbon fibers, the design of fiber–matrix interface and the use of thermoplastic toughening agents. Significant improvements were achieved on transverse strain at break by applying original surface treatments on the fibers so as to induce covalent coupling with the matrix. A drastic enhancement of the K IC value exceeding 2 MPa m 1/2 was also obtained for acrylate-based matrices toughened with high T g thermoplastics.

The cationic polymerization of diglycidylether of bisphenol A (DGEBA) initiated in the presence of a diaryliodonium salt (DAIS) by electron beam irradiation has been studied by FTIR spectroscopy and by dynamic mechanical thermal analysis (DMA). The obtained results show the gradual increase of the temperature for the network thermo-mechanical transition ( T α , associated with the glass transition temperature T g ) over a broad range of conversion ( π ) and reveal a peculiar behavior at high conversion. In this domain ( π >0.90), the material's T g is shown to decrease when conversion approaches unity. Moreover, the post-irradiation thermal treatment of the materials, that generally yields effective post-polymerization, appears to induce a decrease of T g , with an amplitude correlated with the amount of DAIS in the formulation. Owing to the particular nature of the propagating centers in cationic polymerization, the thermal relaxation of ionic clusters trapped in the glassy matrix can be reasonably invoked as a possible cause for this behavior.

Adhesion between an electron-beam-cured Diglycidyl Ether of Bisphenol A (DGEBA) epoxy matrix and AS4 carbon fibers has been evaluated with the microindentation test method and compared with similar thermally cured materials. The results indicate that the absence of amine compounds and of high temperature treatment associated with thermally cured epoxy matrices are detrimental to fiber-matrix adhesion in electron-beam-cured epoxy matrices when measured by the microindentation test. Electron beam processing was not found responsible for any adsorption and/or deactivation of the irradiated carbon fiber surface as determined by surface analysis with X-ray Photoelectron Spectroscopy (XPS). Moreover, the relationship between electron-beam processing conditions (namely, dose and dose increment) with the resulting matrix properties and the adhesion to carbon fiber have revealed a strong dependency of fiber-matrix adhesion on the bulk matrix properties independent of the electron beam processing history. Undercure...

The electron beam (EB) induced polymerization of a typical epoxy acrylate (EP-AC) formulation designed for high performance fibre-reinforced composites has been investigated in order to quantify the influence of various processing parameters on polymerization kinetics. Crosslinking polymerization was first conducted on thin EP-AC films coated on a NaCl plate and irradiated in nitrogen with incremental EB-dose delivered by a 175 kV laboratory processor. In such conditions and with the particular geometry of the samples, thermal effects are small, inhibition by air is avoided and post-polymerization is reduced. The actual conversion vs. dose profiles can be reconstructed from discontinuous transmission FTIR measurements following each incremental dose application. Two limiting kinetic regimes were shown to be dependent on the square root of the dose rate (Ḋ) in the initial stage, and proportional to Ḋ in the final stage, as an expectable consequence of the change in chain termination mechanism. A second series of polymerizations was conducted with the reactive layer covered by a PET film that isolates the polymerizable medium from ambient air. The influence of the dose rate and of the temperature was examined under these conditions which allow some post-polymerization to take place. The activation energy determined from the Arrhenius plot drawn from the data recorded during the first regime was about 4 kJ mol–1. The polymerization that proceeds in the third kinetic regime was not significantly dependent on temperature, as expected for a process requiring segmental mobility that takes place in the glassy state. This kinetic study provides a useful set of information for developing a model based on the change of termination mechanism in a medium that is gradually solidifying upon simultaneous network densification and monomer consumption.

The electron beam induced curing of a typical epoxy acrylate formulation designed for high performance fiber reinforced composites has been investigated in order to quantify the influence of the beam parameters on polymerization kinetics. Experimental results illustrating the effects of dose rate on curing kinetics observed for a pulsed irradiation of the epoxy acrylate resin are detailed and compared to those of a continuous beam. Variations of the beam frequency in the case of pulsed irradiation do not lead to the same dose rate dependence of the polymerization rate as do current variations in continuous irradiation, or changes in the distance between gun and sample during pulsed irradiation. Simulations of the free radical concentration profiles using a reasonable selection of values for the rate constants provide the basis of tentative explanations and contribute to having a good control over the industrial process.

The thermal effects taking place during the electron beam-induced polymerization of acrylate type formulations were numerically simulated on the basis of the general heat equation applied to a one-dimensional system, The nature, the dimensions and the environment of the polymerizing medium were defined for representing the actual conditions of kinetic experiments performed with a 175 kV laboratory accelerator and FTIR monitoring. The modeled system was constituted of a polymerizable composition coated onto a NaCl plate, initially at 20°C in gaseous nitrogen at the same constant temperature, with or without a PET film covering the reactive layer, Polymerization profiles describing the progress of the reaction as a function of dose were modeled on a phenomenological basis from actual data obtained by discontinuous FTIR monitoring of typical epoxy acrylate or polyurethane acrylate compositions. The influence of the reactive layer thickness (10/100 μm), dose rate (10-110 kGy.s -1 ), maximum polymerization heat (200-400 J.g -1 ) on the temperature-time variations was examined for continuous irradiation. In spite of the relatively small thickness of the reactive layer, significant temperature rise is simulated when heat production is large and fast compared to energy dissipation at the reactive layer boundaries. The obtained data substantiate the fact that upon fractionated EB-treatment with small dose increments (down to 0.6 kGy per pass) at low dose rate (down to 10 kGy.s -1 ) the heat release can be considered weak and without noticeable influence on the conversion data processed for a detailed kinetic analysis. For example, a maximal temperature rise of 6°C was calculated for a fractionated irradiation of 2 kGy increments at 19 kGy.s -1 applied to a polymerizable formulation releasing a maximum enthalpy of 300 J.g -1 .

The electron-beam induced polymerization of some representative formulations including acrylate functional oligomers and diluents has been investigated by means of FTIR spectroscopy applied to films that were cured under a nitrogen flow. In order to gain a deeper insight into the reactivity of the polymerizable systems, the conversion–dose relationship was examined with emphasis on the following points : depth cure profile of the films, and the additivity of effects of incremental radiation doses on monomer conversion. It was shown to be possible to reproduce the actual polymerization profile from discontinuous measurements. This remarkable result is tentatively explained by the geometry of the samples causing limited thermal effects and by the minor influence of possible inhibition and post-polymerization that could influence each of the incremental transformations compared to a single large dose treatment. This method provides a fine tool for revealing differences in kinetic behavior between polymerizable mixtures of various compositions.

Maleic anhydride plasma polymer was deposited at the surface of carbon fibers and functionalized with vinyl and thiol groups to improve its adhesion strength with an acrylate matrix cured by an electron beam. A characterization of the fiber surface properties was done before and after coating (topography, surface chemistry, and surface energy). Sharp improvements of the interfacial shear strength (+ 120%), measured by a micromechanical test derived from the pull-out test, were obtained and, to the best of our knowledge, never reported before. The values were close to the ones obtained with a thermal cure. The comparison of this approach with other types of surface treatments (oxidation, grafting of coupling agents) enabled the establishment of a general strategy for the improvement of the interfacial adhesion in carbon fiber composites cured by an electron beam and potentially the improvement of their mechanical properties. This strategy is based on a high surface density of functionalities that are generating covalent bonding during the polymerization of the matrix and on the insertion of a polymer layer strongly attached to the fiber surface and acting as a buffer between the fiber surface and the matrix to counteract the generation of stress in the interphase.

International audience; Recent advances in the radiation-induced polymerization of epoxy-based resins: reactivity, network structure and material performances

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Glassy network epoxies have widespread applications as matrices for advanced composites As an alternative to thermal curing, electron beam (ebeam) processing has been demonstrated as a very powerful tool for achieving fast and efficient curing [1,2]. E-beam curing of epoxy based resins using suitable onium salts as initiators proceeds via a cationic mechanism. The final polymer network properties are highly dependent on processing conditions and curing kinetics. It was demonstrated that under certain processing conditions, heterogeneities can appear in the material, due to the agglomeration of initiator residue. Initially the initiator is miscible with the resin; however, reaction-induced phase separation occurs as the increasing molecular weight of the polymer reduces the solubility of the initiator.

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