Effect of the Strain Rate and Drawing Temperature on the Mechanical Behavior of EVOH and EVOH Composites

Abstract

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Porous Membranes Based on Polypropylene-Ethylene Copolymers. Influence of Temperature on Extrusion, Annealing and Uniaxial Strain Stages

“…Endotherm curves for blends with the random copolymer were less pronounced and wider as the content of the mixture was increased, in comparison with the curves related to the blends with the heterophasic polypropylene. This behaviour is attributed to a broad range of crystals with different lamellar thickness [31]. As the random content in the blend increases (H-20R), a decrease in the melting point is also observed.…”

“…Shoulder at lower temperatures related to the annealing process [14,15,16,17,18] and also associated with the melting of different crystal populations disappears progressively as the annealing temperature increases. It seems that the increment on molecular mobility at higher temperatures promotes the formation of a more dense and stable crystalline form [31]. Membrane thermograms display similar endothermic curves for annealing temperatures above 150 °C.…”

“…After yielding, the tensile curves presented an increase in the nominal stress with the beginning of the strain hardening. The stress applied involved a pre-orientation of amorphous tie chains prior to crystal chain unfolding, which can be related to the first yield point [31]. Annealing the samples at 150 and 160 °C increased the yield stress and elastic modulus due to an increment of sample crystallinity.…”

“…These results are in accordance with the endothermic curves reported in Figure 4 and Figure 5, where for samples annealed above this temperature the shoulder on the left side of the main melting peak, associated with the secondary new crystalline region disappears to produce a more dense and stable crystalline structure. The pronounced strain hardening after yielding can be promoted by homogeneous crystal slip that makes the plastic deformation more difficult [18,31]. For this reason, when an optimum percentage near to the strain of 35% is achieved [10,24], the second uniaxial stretching step at high temperatures begins and thermal effects counteracting the strain hardening.…”

Multifunctional hydrolyzed EVA membranes with tunable microstructure and water barrier properties

“…where ΔH m is the experimental enthalpy of melting and ΔH 0 m ¼ 157.8 J/g is the poly(vinyl alcohol) (PVA) equilibrium enthalpy of melting (theoretical value of the polymer supposed to be 100% crystalline) [39,40]. It is shown that the ethylene segments in the EVOH copolymers with 40-80 mol% of the vinyl alcohol groups crystallize into a monoclinic lattice similar to that of PVA regardless of the ethylene sequences, located as punctual defects in the main crystalline lattice [40].…”