•Macro-synthetic fibres can substitute transverse reinforcement in slender beams.•Macro-synthetic fibres provide great shear strength and ductility.•Diagonal tension MOF was less brittle and more ...predictable in PFRC than in RC beams.
Shear behaviour in reinforced concrete (RC) elements can improve with an adequate amount of fibres. Research has recently determined how fibres affect shear strength, but has barely focused on macro-synthetic fibre-reinforced concrete (PFRC). This paper presents the experimental results of 16 full-scale beams (eight RC, eight PFRC), 12 without transverse reinforcement. Polypropylene fibres (10 kg/m3) were included. Mode of failure (MOF) in shear and behaviour throughout the loading process were studied. The results obtained with fibres showed significantly improved shear strength in the RC beams with/without transverse reinforcement. A synergy between transverse reinforcement and fibres was observed in some cases.
The relaxation behavior of two lignins (Alcell organosolv, OSL, and hydroxypropyl modified Kraft, ML) and bio-based thermoplastic polyurethane (TPU) blends have been studied by Differential scanning ...calorimetry (DSC), Dynamic-mechanical analysis (DMA) and Dielectric relaxation spectroscopy (DRS). The effect of blending on the glass and local relaxation processes as a function of composition, frequency, and temperature has been assessed. The dielectric spectra were resolved into dipolar relaxations as well as conductive processes for differing blend compositions. Characteristic relaxation times, activation energies and dielectric relaxation strengths of lignin/xTPU blends were also investigated. It was found that blending suppresses the α-relaxation process of the blends compared to virgin TPU. On the other hand, while the position of the β-relaxation was not influenced by blending, a reduction of the activation energies, Ea, of this process was observed in the lignin/xTPU blends. Finally, changes are observed in the conductivity behavior of both blends, with conductivity processes more favorable for the OSL/xTPU blends.
Poly(vinylpyrrolidone-co-butyl acrylate) samples with different proportions of monomers were prepared as tractable and hydrophilic materials. An analysis of the intermolecular interactions between ...the polymer groups was carried out by FTIR. The dependence of the CO bands (lactam and pendant butyl ester) with the composition suggests a strong interaction between the lactam groups. They exert an important influence in the molecular mobility, which was studied by DSC and DRS. A single narrow glass transition temperature (T g) is observed in each random copolymer, consistent with a single phase of low compositional nanoheterogeneity. The dependence of the T g with the composition suggests significant interactions between polymer components. The dielectric spectra show γ, β, and α relaxations in increasing order of temperature, followed by conductive contributions. The apparent activation energies for secondary relaxations have similar values for all the samples. The γ-process is related to the local motions of the butyl units and the β-process is a Johari–Golstein secondary relaxation that is related to the local motions of the pyrrolidone group together with the motion of polymer backbone segments.
Nanocomposite materials obtained from natural rubber (NR) reinforced with different amounts of cellulose II (cell) nanoparticles (in the range of 0 to 30phr) are studied by dielectric spectroscopy ...(DS) in a broad temperature range (−150 to 150°C). For comparative purposes, the pure materials, NR and cell, are also investigated. An analysis of the cell content effect on the conductive properties of the nanocomposites was carried out. The dielectric spectra exhibit conductivity phenomena at low frequencies and high temperatures: Maxwell–Wagner–Sillars (MWS) and electrode polarization (EP) conductive processes were observed in the nanocomposite samples.
•Study of the conductivity behaviour of natural rubber–cellulose nanocomposites•Analysis of the time/frequency temperature correspondence for the ac conductivity•Cell nanoparticles preserve the inherent good insulating properties of natural rubber.
► Polycarbonate polyurethane films modified with expanded graphite (EG). ► Thermal and dynamical thermal analysis indicates changes upon addition of EG. ► Relaxation processes and conductive ...phenomena (MWS and EP) are discussed and related with the EG content.
Segmented polycarbonatediol polyurethane (PUPH) has been synthesized and modified with different amounts of graphite conductive filler (from 0 to 50wt%). Thermal and dynamical thermal analysis of the composites clearly indicates changes in the polyurethane relaxations upon addition of graphite. Broadband dielectric spectroscopy has been used to study the dielectric properties of the (PUPH) and one composite in the frequency range from 10−2 to 107Hz and in the temperature window of −140 to 170°C. Relaxation processes associated with different molecular motions and conductivity phenomena (Maxwell–Wagner–Sillars and electrode polarization) are discussed and related to the graphite content.
This work is focused on the conductivity study of viscoelastic liquids, taking as a model poly(2,3-dimethoxybenzyl methacrylate). Each isotherm, displaying the conductivity in the frequency domain, ...shows a plateau in the low frequency region, representing the dc conductivity. The covered frequency range by the plateau increases with the temperature. The frequency corresponding to the end of the plateau, ω c , marks the onset of the ac conductivity, which correspond in increasing order of frequency to Maxwell–Wagner–Sillars, glass–rubber transition and secondary relaxations. The contributions of the relaxation processes to the ac conductivity in the wholly frequencies range were analyzed. The time–temperature correspondence principle holds for the reduced ac conductivity. However, this principle does not hold for the components of the complex dielectric permittivity due, among other things, to the different temperature dependences of each dipolar relaxation processes. Analogueies and differences between the conductivity behavior of viscoelastic liquids and disordered inorganic solids are discussed.
Natural rubber (NR) isolated from Hevea Brasiliensis was investigated by differential scanning calorimetry, dielectric spectroscopy, and high-pressure dielectric spectroscopy. In the range of ...frequencies (5 × 10−2 to 3 × 106 Hz), temperatures (−120 to 120 °C), and pressures (0.1 to 240 MPa) studied, the dielectric spectra exhibit two overlapped α-processes but no subglass relaxations. Thermal measurements revealed the presence of some moisture in NR. To elucidate the influence of water, dielectric measurements were carried out in dry and wet NR samples. The origin of the two dielectrically active processes was discussed in terms of (i) the apparent activation volume, (ii) the pressure coefficient of the respective glass temperatures, and (iii) the values of the ratio of activation energies, at constant volume and pressure. The latter allowed extracting the relative contribution of thermal energy and volume for each dynamic process. On the basis of these results, the faster α-processes is assigned to the rigidified rubber backbone dynamics whereas the slower to fatty acids (such as stearic acid) that are linked to the rubber chain.
Nanocomposite materials obtained from natural rubber (NR) reinforced with different amounts of cellulose II nanoparticles (in the range of 0 to 30
phr) are studied by dielectric spectroscopy (DS). ...For comparative purposes the pure materials, NR and cellulose, are also investigated. The dielectric spectra of the nanocomposites exhibit: (a) two overlapped α-relaxations associated respectively with the dynamic glass transitions of NR (faster process) and of the lipid present in NR; (b) a β-relaxation associated with local chain dynamics of cellulose and (c) a relaxation process associated to the presence of traces of water in cellulose. The spectra exhibit conductivity phenomena at low frequencies and high temperatures. The samples were also studied in the dry state. An explanation is given concerning the cellulose effect on the dielectric properties of the dry and wet nanocomposites.
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