Araceli Flores

Microhardness studies of chain-extended PE: III. Correlation with yield stress and elastic modulus

Abstract The correlation between microhardness, H, yield stress, Y, and Youngs modulus, E, has been explored on various chain-extended polyethylene (PE) samples and compared to chain-folded PE. Mechanical properties have been derived from tensile and compressive experiments. The tensile yield stress, Yt, is shown to correlate with hardness following H∼3Yt while H∼Et/10 (Et is Youngs modulus derived from tensile experiments). On the other hand, the compressive yield stress, Yc, is shown to correlate to H following the mechanical models of elastoplastic indentation. Hence, H/Yc increases with decreasing elastic strain tending towards H∼3Yc for a fully plastic deformation. The atmospheric pressure compression-moulded samples show the lowest H/Yc ratios as a consequence of the largest fraction of compliant phase. Moreover, the H/Ec ratio (Ec being the elastic modulus under compressive force) is shown to diminish with the decrease of Yc/Ec. Chain extended PE provides the lowest values for the H/Ec ratio from all the PE samples investigated.

Mechanical properties of poly(ethylene terephthalate) at the near surface from depth-sensing experiments

The viscoelastic-plastic properties of glassy poly(ethylene terephthalate) in the micron and submicron range have been investigated by means of load displacement analysis from depth-sensing experiments. Experimental data have been modelled using two methods: firstly assuming the elastic behaviour during initial unloading to be that of a cylindrical punch; secondly using a power-law relation. The creep behaviour under the indenter has been examined. Furthermore, the influence of the maximum penetration depth and loading and holding times on the hardness values are discussed. Hardness data from the depth-sensing and imaging methods are shown to be in good agreement. When using the depth-sensing method, hardness values are shown to be constant with decreasing penetration depth (0.5 μm ≤ h ≤ 9 μm), provided that a correction procedure to account for the indenter tip defect is applied. Youngs modulus derived from the compliance method has been studied and results are discussed in the light of the various testing conditions employed.

Polymorphism of isotactic polybutene-1 as revealed by microindentation hardness. Part II: correlations to microstructure

The influence of polymorphism on the micromechanical properties of isotactic polybutene-1 (iPBu-1) has been investigated by means of the microhardness technique. Hardness data, H, of form I (hexagonal) are shown to be notably larger than those of form II (tetragonal). The H values of both polymorphic forms are shown to depend on the molecular weight, Mw, and the crystallization temperature, Tc. The hardness behaviour with Mw and Tc has been correlated to the changes of degree of crystallinity and nanostructure as derived from small angle X-ray scattering. The hardness values for iPBu-1 infinitely thick crystals (Hc ), of forms I and II, have been calculated for the first time. Hc of form I is shown to be notably larger than that of form II. This result is a consequence of the denser packing of the hexagonal crystal modification, and accounts for the large difference in H values found for forms I and II. Finally, the variation of the mechanical b-parameter, proportional to the surface-free energy of the crystals, with molecular weight is discussed. q 2002 Elsevier Science Ltd. All rights reserved.

Microhardness studies of chain‐extended PE. I. Correlations to microstructure

The hardness-microstructure correlation of various polyethylene (PE) samples crystallized at high pressure from the melt (chain-extended), with different molecular weights, has been investigated and compared to melt crystallized samples at atmospheric pressure (chain-folded). The hardness, H, of melt crystallized PE is confirmed to increase linearly with the logarithm of the annealing time, t a , at a constant annealing temperature. The H increase with t a is discussed in terms of the crystallinity and crystalline lamellar thickness variation. Unusually high hardness values are obtained for samples crystallized or annealed at high pressure as a consequence of the resulting high degree of crystallinity and large crystalline lamellar thickness values. However, it is shown that the high surface free energy value of the chain-extended crystals considerably lowers the hardness values from that of an ideal infinitely thick PE crystal. Analysis of the crystal hardness and the melting temperature data of different polymeric materials emphasizes the close existing relationship between both quantities.

Creep behavior and elastic properties of annealed cold-drawn poly(ethylene terephthalate): The role of the smectic structure as a precursor of crystallization

The creep behavior and elastic properties of cold-drawn poly(ethylene terephthalate) (PET) films, annealed in the range 60–240 °C have been investigated by means of microindentation testing. Two indentation methods have been used. The imaging method has been employed to examine the viscoplastic properties of the polymer materials while the depth-sensing method was used for the determination of Young’s modulus values. The creep behavior (plastic flow) of cold-drawn PET is shown to be intimately correlated to the nanostructural changes occurring upon annealing. The observed decrease in the rate of creep, when the glassy material is annealed at 60 °C, has been associated with the emerging smectic structure, which confers to the material a higher mechanical performance. The elastic properties of the smectic phase are found to be comparable to those of the glassy state. Young’s modulus E values of the semicrystalline samples are discussed in light of the parallel model of crystalline and amorphous layers. E va...

Microhardness studies of chain-extended PE: II. Creep behaviour and temperature dependence

The variation of hardness with indentation time has been investigated for chain-extended polyethylene (PE), other PE samples crystallised under different conditions and paraffins. Hardness is shown to decrease with indentation time for all the samples investigated according to a power-law. Chain-extended PE, produced by high pressure crystallization or annealing, flows at the lowest rate under the indenter of all the PE samples considered. On the other hand, paraffins creep at the highest rate. Creep behaviour depends markedly on the crystal thickness of the material. The mechanical properties at long indentation times seem to be determined primarily by the deformation modes of the crystals. The temperature dependence of hardness and that of the creep behaviour has also been investigated. In chain-extended PE, the softening of the sample and the higher rate of creep with increasing temperature are discussed in terms of the thermal expansion of the unit cell.

Influence of filler structure on microhardness of carbon black–polymer composites

The microhardness, H, of carbon black–polycarbonate and carbon black–low-density polyethylene composites was investigated. Two types of microadditives with different average particle sizes were employed. It has been shown that the morphology of the polymeric matrix conspicuously influences the hardness dependence of the composites with volume concentration of filler, ϕ. The microhardness of the carbon black–polycarbonate composites shows a steplike behavior with respect to carbon black content, while the H value of the carbon black–low-density polyethylene composite linearly increases with increasing ϕ. The influence of filler structure on the microhardness of the carbon black–polymer composites is also discussed. Results favor the concept that a smaller carbon black particle size (smaller aggregate diameters and interaggregate distances) enhances the microhardness of the composites.

Numerical-experimental method for the identification of plastic properties of polymers from microhardness tests

A computational method in conjunction with indentation measurements is presented to determine the yield stress and the tangential modulus of different polymeric materials. A novel approach is developed to select representative points at which finite element calculations are performed. Yield stress and tangential modulus can be obtained by minimizing the deviation between the calculated and the experimental data. Hardness values are also derived using finite element calculations and compared to those obtained from the fitting to the loading curve of depth-sensing experiments.

The role of the amorphous phase in the re-crystallization process of cold-crystallized poly(ethylene terephthalate)

The process of re-crystallization in poly(ethylene terephthalate) is studied by means of X-ray diffraction (SAXS and WAXS) and dynamical mechanical thermal analysis. Samples cold-crystallized for 9h at the temperatures Tc = 100 fcir#circ;C and Tc = 160 fcir#circ;C, i.e. in the middle of the

EVIDENCE OF A TRANSCRYSTALLINE INTERPHASE IN FIBER PE HOMOCOMPOSITES AS REVEALED BY MICRODIFFRACTION EXPERIMENTS USING SYNCHROTRON RADIATION

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