Developments in dynamic testing of rubber compounds: assessment of non-linear effects

Abstract In the present work, a test method to characterize the dynamic behaviour of rubber compounds by electrodynamic shaker (ES) in the frequency range of 10–1000 Hz was developed. Data of dynamic moduli of two different rubber compounds were determined through the analysis of the transmissibility of a suitably designed test system. The results were compared with those of dynamic moduli master curves obtained through frequency–temperature reduction of data measured by a commercial dynamic mechanical thermal analyser (DMTA), by scanning temperature at various frequencies in the range 0.3–30 Hz. Very good agreement of the data obtained by the two different aproaches were found, in spite of the different range of frequency explored by the two instruments, ES and DMTA, respectively. For one of the material examined, non-linear effects at low strain amplitudes were investigated by the two experimental methods considered

High-rate J-testing of toughened polypropylene

2nonenoneF. MARTINATTI; T. RICCO'F., Martinatti; Ricco', Theoni

On the determination of the point of fracture initiation by the load separation criterion in J-testing of ductile polymers

International audienc

The two-way shape memory behaviour of crosslinked poly(ϵ-caprolactone) systems with largely varied network density

The two-way shape memory behaviour of semicrystalline networks was investigated on systems based on poly(epsilon-caprolactone) featuring significantly different network architecture. Crosslinked poly(epsilon-caprolactone)s were prepared by thermal curing from methacrylic end-capped linear chains having various methacrylation degrees. By conveniently reducing the methacrylation degree, the crosslink density of cured materials was varied over a range of one order of magnitude, leading to comparable changes in the material compliance in the rubbery region, but only to moderate variations in melting and crystallization temperatures (T-m and T-c) and in the crystallinity content. When subjected to constant non-zero stress and to cooling-heating cycles from above T-m to below T-c, the materials undergo a reversible two-way elongation-contraction effect, whose extent depends on material structure and applied stress. The structural changes in the crystalline phase accompanying the cooling-induced elongation were studied through differential scanning calorimetry and X-ray diffraction analyses. The elongation process involves different contributions of entropy- and crystallization-driven processes, whose amounts were investigated as a function of the loading conditions and the molecular architecture. The role of the network density towards a controlled two-way response is evidenced, showing that a proper value of the crosslink density has to be identified to maximize the two-way elongation capabilities

Delamination of organically modified montmorillonite for reducing the filler networking with carbon black in poly(1,4-cis-isoprene) based nanocomposites

This work presents the correlation between the organization of an organically modified montmorillonite (OMt) and the dynamic-mechanical properties of polymer nanocomposites containing a hybrid filler system, based on carbon black (CB) and OMt. Clay polymer nanocomposites (CPN) were prepared by melt blending synthetic poly(1,4-cis-isoprene) (PI), carbon black (CB) and OMt. Two types of OMt were used: with ammonium cations intercalated between the clay mineral layers (I-OMt) and delaminated (D-OMt). The latter was obtained via ball milling of I-OMt and presented a degree of delamination higher than 95%, as revealed by X-ray diffraction (XRD) analysis. The structure of CPN was studied with XRD and transmission electron microscopy (TEM) analyses. The mechanical behavior of uncrosslinked masterbatches and of nanocomposites crosslinked with sulfur based systems was assessed with dynamic shear experiments, by measuring the time needed to recover the initial modulus after a large strain perturbation and by obtaining master curves for the dependence of the storage modulus on frequency. I-OMt is shown to promote the filler networking phenomenon more easily than D-OMt: the storage modulus has a stronger reduction with the strain amplitude and a longer rubbery plateau is featured on the frequency scale. Modulus recovery measurements revealed that the structure of CPN can be restored after the application of large strain amplitudes. The extent of OMt delamination is presented as a key feature to control the dynamic-mechanical properties of CPN containing a filler, such as CB, suitable to establish an intimate interaction with OMt. The partial substitution of CB with D-OMt allowed the preparation of crosslinked nanocomposites with lower Payne effect

Two-way reversible shape memory behaviour of crosslinked poly(ε-caprolactone)

Polymers capable of reversible “two-way” shape memory behaviour are of great interest for applications where reversible actuation is demanded, and semicrystalline crosslinked systems have been indicated as an interesting solution towards this end. In this work we have explored the two-way shape memory response of semicrystalline poly(ε-caprolactone)-based polymer networks, prepared with various macromolecular architectures starting from linear, three- and four-arm star poly(ε-caprolactone) func- tionalized with methacrylate end-groups. All the materials have revealed two-way shape memory capabilities. The effect arises from an elongation process that takes place when the material is cooled under an applied load below the crystallization temperature, and that is completely reversed when heated again above melting temperature, in a manner that strongly depends on the applied load and on the material crosslink density. Two-dimensional XRD analysis, carried out on elongated specimens, shows that the elongation on cooling is accompanied by a change in the crystallinity orientation along the direction of stretch

Chemical and thermomechanical tailoring of shape memory effect in poly(e-caprolactone)-based systems

The thermally activated shape memory response of polymeric materials results from a combination of the material molecular architecture with the thermal/deformational history, or 'programming'. In this work, we investigate the shape memory response of systems based on poly(ε-caprolactone) (PCL) so as to explore the adoption of proper chemical and thermomechanical tailoring routes. Cross-linked semicrystalline PCL-based materials are prepared by different molecular architectures starting from linear, three- and four-arms star PCL functionalized with methacrylate end groups, allowing to tune the melting temperature, T m, ranging between 36 and 55 C. The materials' ability to display the shape memory is investigated by the application of proper thermomechanical cycles on specimens deformed at two different temperatures (23 and 65 C, i.e. below and above the T m, respectively). The shape memory response is studied under dynamic thermal conditions in thermally activated recovery tests, to identify the typical transformation temperatures, and under isothermal conditions at given recovery temperatures, to monitor shape recovery as a function of time. All the specimens are capable of full recovery on specific thermal ranges influenced by both melting and deformation temperatures. Specimens deformed above T m are able to recover the whole deformation in a very narrow temperature region close to T m, while those deformed at room temperature display broader recovery processes, those onset at about 30 C. Isothermal tests reveal that when the deformed material is subjected to a constant recovery temperature, the amount of recovered strain and the time required strongly depend on the particular combination of melting temperature, deformation temperature and recovery temperature

Delaminated and intercalated organically modified montmorillonite in poly(1,4-cis-isoprene) matrix. Indications of counterintuitive dynamic-mechanical behavior

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. © 2014 Elsevier B.V

The Role of CNTs in Promoting Hybrid Filler Networking and Synergism with Carbon Black in the Mechanical Behavior of Filled Polyisoprene

The filler networking process promoted by multiwalled CNTs is studied in neat and CB-filled poly(1,4-cis-isoprene) matrices. TEM analysis, tensile, dynamic-mechanical, and electrical measurements reveal that the CNTs form a filler network at low concentration in neat PI and a continuous hybrid filler network at a lower CNT concentration in the presence of CB, with a remarkable increase of the nonlinear dynamic- mechanical behavior of the nanocomposites at low deformation. A synergistic effect between CB and CNTs is demonstrated. The addition of CNTs to the CB-filled PI matrix leads to initial modulus values much larger than those calcu- lated by simple addition of the two initial moduli of the composites containing only CB and only CNTs, respectively