Plastic behavior of monoclinic polypropylene under hydrostatic pressure in compressive testing

Abstract

The plastic deformation of monoclinic isotactic polypropylene is studied in compressive testing with particular attention to the effect of hydrostatic pressure up to 300 MPa, in the temperature range 20 -60°C. The coefficients of the Coulomb criterion are fairly consistent with those assessed from the comparison of the tensile and compressive yield stresses at atmospheric pressure, in the same temperature range. The high friction coefficient of polypropylene is ascribed to a strain-induced order-todisorder transition from the monoclinic to the smectic form accompanied by an increase of specific volume. This local and transient phase change is assumed to result from the mobile conformational chain defects that govern the elementary mechanism of plasticity in the crystalline phase of polypropylene consisting of 3/1 helix chains. A comparative discussion is made with polyethylene that displays a much lower friction coefficient. The dislocation-based approach that is proposed in relation to the viscoelastic relaxation processes provides a new insight into the molecular grounds of the elementary mechanism of plasticity of polypropylene.

A Model for the Elastoplastic Behavior of Isotactic Poly(propylene) Below the Yield Point

“…Plasticity and yield of iPP have been the focus of attention in the past decade, see, e.g., ref. [1][2][3][4][5][6][7][8][9][10] This may be explained by numerous applications of poly(propylene) in industry (ranged from oriented films for packaging to nonwoven fabrics and reinforcing fibres).…”

“…The latter case, however, is excluded from the consideration in this work, because it requires the mathematical apparatus of finite deformations to be used (at e > e y ¼ 0.13, Equation (1), (2) and (9) become inapplicable). Equation (7), (8) and (14) …”

The effect of annealing on the elastoplastic response of isotactic polypropylene

“…An important advantage of the stress-strain relations (1), (2), (4) to (6), (15) and (19) is that 3 constants, E 0 , a and ε, are found by fitting experimental data for the loading path of a stress-strain curve, whereas the other two parameters, K and κ, are determined by matching observations for the unloading path.…”

Dislocation approach to the plastic deformation of semicrystalline polymers: Kinetic aspects for polyethylene and polypropylene*

“…An attempt can be made to approach the plastic deformation kinetics of isotactic PP in a way similar to that developed for PE. The plastic behavior of PP in its stable monoclinic crystalline form in terms of crystal slip and dislocation activation has been specifically addressed by several authors 14, 15, 29, 54–56. Experimental and theoretical investigations of the molecular mobility in the crystal have shown that the basic motion of the chain stems with a 3/1 helix conformation is a threefold rotation about the stem axis accompanied by a c /3 translation parallel to the stem axis 46, 57.…”

“…As for PE, the PP stems cannot rotate and translate as a rigid entity but should rather involve the migration of a conformational defect 41. However, only hypotheses have been put forward regarding the nature of the mobile defect 56…”