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The EPR investigation of the sulphur-only vulcanization of polybutadiene has afforded evidence of free radical activity and proposals for the identification of the paramagnetic intermediates are forwarded. Some of the species (polysulphonyl and sulphuranyl radicals) are thought to be direct evidence of a free radical component in the vulcanization mechanism. Indirect evidence of a vulcanization free radical mechanism comes also from other species, like stable tertiary carbon radicals and delocalized thiyl radicals which are characterised by absence of α and β hydrogen atoms. This structural feature is reckoned with sulphur induced dehydrogenation reactions which are known to be dominant at higher temperatures but are suggested to start to be active, as minor mechanism, already at the vulcanization temperatures. As a necessary support for the present and future vulcanization studies based on the EPR methodology, part of the work is devoted to a survey of all the possible radical intermediates and their properties which have been investigated with the help of DFT M.O. computations and specific ancillary EPR experiments to implement the available literature information. Amongst the results obtained is the observation that the g tensor of the polysulphanyl radicals RS n ( n ⩾ 2) is invariant with respect to he number of sulphur atoms in the polysulphane chains; however, in conditions of high sulphur atoms concentration (as in molten sulphur), a significant decrease of g av is predicted to occur as a consequence of through-space spin exchange phenomena. Other results concern the free radical vulcanization chemistry of sulphur whose attitude to act as scavenger of polybutadiene allyl radicals has been assessed. The vulcanization carried out in presence of t-butyl benzothiazole sulphenamide and Zn stearate has afforded very similar EPR spectra together with a moderately faster radicals forming kinetics; this result is considered proof that no change of the vulcanization free radical component into polar or concerted mechanisms is induced by the catalyst and the promoter

Montmorillonite with dimethyl di(hydrogenatedtalloyl) ammonium as the compensating cation was added to a silica based elastomeric composite and the hybrid filler system led to reduction of filler networking phenomenon, better stability of elastic modulus with temperature, enhancement of stresses at all elongations, improvement of ultimate properties. This composite was based on a blend of natural rubber, poly(1,4-cis-isoprene) and poly(styrene-co-butadiene) from anionic polymerization and contained 70 parts per hundred rubber (phr) of silica. The organically modified clay (OC) was below the threshold required to establish an hybrid OC-silica filler network. Such threshold (about 7 phr) was estimated by preparing silica based nanocomposites containing various amounts of OC and determining shear storage and loss moduli as a function of strain amplitude. This work demonstrates that exfoliated OC favour lower dissipation of energy of silica based elastomeric composites under dynamic mechanical stresses and paves the way for further large scale applications. (C) 2016 Published by Elsevier B.V.

Layered double hydroxides (LDHs) containing Mg2 + and Al3 + as divalent and trivalent cations, respectively, have been synthesized by a procedure based on hydrolysis of a mixture of metal alcoholates, and have been characterized mainly by Wide Angle X-ray diffraction (WAXD) and Fourier Transform Infrared (FTIR) measurements. The used procedure is suitable to prepare LDHs with Al molar fractions, xAl = Al/(Al + Mg), as low as 0.1. The same LDH preparation procedure, for xAl

Summary: The γ-ray induced “grafting” of polybutadiene olygomers onto precipitated silica and the radiation induced crosslinking of the blends prepared with the modified silica in SBR matrices have been investigated in the dose range up to 290 kGy. Enhanced mechanical properties have been detected for the SBR blends containing the modified silica after γ-ray induced crosslinking. The comparison with the analogue mixtures containing pristine silica, supported by EPR measurements, suggests that the PB coating is effective in enhancing the mechanism of formation of filler-rubber chemical links.

Ionizing radiations from a 60-Co source have been employed for inducing radical reactions finally leading to the grafting of unsaturated olygomers and vinyl monomers onto the surface and micropores of precipitated silica Zeosil 116 5 (150- 170 m 2 /g). The target was to obtain modified silica with enhanced polar compatibility with respect to polybutadiene and styrene-butadiene copolymers rubber matrices and suited to favour the formation of chemically bound rubber in a vulcanization process. The latter property was expected to follow from the characteristics of free radical reactivity arising from the unsaturations present in the organic coating. Three classes of modified silica have been prepared: A)silica coated with polybutadiene olygomers with M n in the range 1000-5000 and having a different relative content of 1,4-cis, 1 , 4 -trans and vinyl double bonds [1,2] ; B)Silica coated with polybutadiene olygomers further modified by subsequent grafting of styrene monomer. The type A samples were obtained by γ irradiation under vacuum of silica samples preimpregnated with the oligomers in the dose range up to 30-200 kGy. Type B samples were obtained via three different steps as outlined in the scheme below: SiO 2 (PB) γ vacuum→/298K SiO 2 -PB γ air→/298K SiO 2 -PB-OOH Fe 2+ →/Sty SiO 2 -PB-O-PS The characterization of the modified silica was carried on by elemental and thermogravimetric analysis, FTIR (diffuse reflectance), Raman spectroscopy, inverse gas chromatography (IGC), CP/MAS 29-Si NMR, granulometry and TEM microscopy.

Montmorillonite (Mt) and Mt organically modified (OMt) with dimethyl ditalloyl ammonium cations were delaminated through ball milling. X-ray diffraction (XRD) analysis allowed the observation a more efficient delamination for OMt than for Mt. In fact, 90% delamination was obtained for OMt, without appreciable variations of the in-plane Mt order. About 60% delamination was obtained for pristine Mt, in the presence of a substantial reduction (up to 40%) of crystalline order in the structural planes. A reflection due to the rotator order of talloyl chains was found both in pristine and in extensively delaminated OMt samples. Polymer nanocomposites based on poly(1,4-cis-isoprene) were prepared with two types of OMt: OMt, unmilled, with the ammonium cation intercalated in the interlayer space (I-OMt) and 90% delaminated OMt (D-OMt). XRD analysis of clay polymer nanocomposites (CPNs) revealed the unaltered crystalline order of I-OMt and a nearly complete maintenance of the delaminated structure of D-OMt. Dynamic-mechanical characterization of CPN gave counterintuitive results. I-OMt revealed a higher tendency to give rise to the filler networking phenomenon, in samples crosslinked with a sulfur based system. Moreover, I-OMt gave rise to a more pronounced extension of the rubbery plateau at low frequencies, in uncrosslinked masterbatches. This behavior is attributed to the higher volume fraction of I-OMt, as the intercalated alkylammoniums contribute to the I-OMt volume, whereas they essentially act as compatibilizers in D-OMt. Delamination of OMt is thus presented as a powerful tool to reduce the dissipation of energy in dynamic-mechanical applications of polymer melts and elastomers.

An innovative organoclay containing polybutadiene chains was prepared in order to make rubber nanocomposites based on natural rubber or styrene–butadiene copolymer. It was obtained by reacting a maleinised polybutadiene oligomer with the CH 2 CH 2 OH moieties present in the ammonium cation of a commercially available organoclay (Cloisite 30B). Elastomeric nanocomposites were also prepared with a reference organoclay, formed by intercalation during mixing with dialkyl dimethylammonium chloride. Sulphur cured nanocomposites were characterised in terms of curing kinetics, morphology, tensile and dynamic-mechanical properties; chemical interactions between the modifier of the new organoclay and the matrices can be inferred by experimental evidences.

Nanocomposites were prepared based on diene rubbers and layered clays modified with an alkyl quaternary ammonium cation (organoclay). A diene rubber, either polybutadiene, or synthetic or naturally occurring polyisoprene or styrene-butadiene copolymer, was melt blended with either a preformed organoclay or with a mixture of pristine clay and ammonium cation. Besides isolated lamellae, nanocomposites showed the presence of crystalline organoclays with intercalated organic layers made only by low molecular mass substances, essentially the ammonium cation, and no evidences for the intercalation of polymer chains were observed. Dynamic-mechanical properties of sulphur cured compounds with carbon black as the main filler and a minor amount of organoclay were investigated. The organoclay was found to bring about a reduction of Mooney viscosity, an increase of storage modulus at low temperature as well as an increase of thermoplasticity.

Apolar-rubber/clay composites obtained by rubber compounding with pristine and organo-modified montmorillonite (MMT) have been characterized by X-ray diffraction measurements. By reducing the clay content in the rubber, the (00l) reflections progressively reduce their intensity, with respect to (hk0) reflections, while their spacing remains unaltered. These results indicate the occurrence of a progressive reduction of the number of the packed clay layers without any polymer intercalation. This delamination process, different from the usual intercalation−exfoliation process, is more efficient for pristine rather than for organo-modified MMT. In particular, pristine MMT, as observed for contents lower than 10% by weight, can be fully delaminated by mixing with natural rubber. Moreover, MMT delaminated by compounding with natural rubber can be aggregated by aging in the presence of suitable organic modifiers (OM), leading to MMT/OM intercalate structures with basal spacing nearly double with respect to the u...