Plastic yielding of semicrystalline polymers affected by amorphous phase

It is shown that plastic yielding of semicrystalline polymers is greatly affected by the state of their amorphous phase. Stress reduction at yield point in crystalline polymers, whose amorphous phase is modified by swelling, has been observed and the mechanism explained. Swelling of amorphous phase with low molecular weight penetrant, which does not affect crystalline phase, causes deformation of lamellae stacks. However, there is no expansion of stacks along lamellae basal planes while stretched chains of amorphous phase generate uniaxial tension transmitted to lamellae surfaces. This additional tensile stress was determined by recording the stress buildup for samples with fixed ends during desorption of penetrant. It appears that measured yield stress plus the stress exerted by swollen amorphous phase amounts exactly to the stress required for plastic deformation of crystals, therefore the yielding is determined by the same crystal plasticity despite different external load. The phenomenon was observed in several systems including polypropylene, polyethylene and polyamide 6 with various penetrants. These observations and explanations seem to pertain to each semicrystalline polymer–penetrant systems.

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► Yield stress of semicrystalline polymers is influenced by the state of amorphous phase.
► Amorphous layers between lamellae become uniaxially swollen perpendicular to lamellae.
► Uniaxially stretched amorphous layers reduce the yield stress of crystalline lamellae.
► Yielding appears at lower load though at similar critical shear stress for crystals.
► The observations and explanations seem to pertain to each semicrystalline polymer.