Miscibility and rigid amorphous phase in blends of polypropylene with poly(propylene‐co‐ethylene)

Abstract

Thermal and mechanical properties of binary blends of polypropylene (PP) with a statistical copolymer (85 wt% propylene and 15 wt% ethylene, abbreviated here as VM) have been investigated. Both components were found to be miscible in the melt in the whole range of compositions. No separate domains of VM have been detected by polarizing optical microscopy, and the size of spherulites is going down with increasing VM content. The crystallization temperature of PP as detected by DSC, is going down to 83°C, for PP/VM blends 10:90. This is another evidence for miscibility in the melt. Viscoelastic properties of the melt were measured. Regarding the solid state, the melting enthalpy is exactly proportional to the PP content in the blend. The DSC step height at the glass transition is exactly proportional to the VM content. These results are suggesting a three‐phase model (crystalline, rigid amorphous, and mobile amorphous phase), where the VM component is mainly located in the mobile amorphous regions, while the amorphous part of PP is in the rigid amorphous regions. Stress–strain curves show a systematic increase of elongation at break with increasing VM%, to be extended from 7.8% to more than 550%.

Compatibilization of polypropylene/high‐density polyethylene blends using poly(propylene‐co‐ethylene)

“…This trend may indicate that the crystallizing portion of PP, respectively HDPE, is not reduced significantly by the addition of VM, in the studied range of compositions. A similar trend has been observed in binary blends of PP with VM 43 …”

“…At 5% VM and 9% VM, the spherulites look more disturbed, and at 17% VM, spherulites are not visible. Such a breakdown of the spherulitic structure was already observed in a series of PP/VM blends in our previous paper 43 . It could be related to a change in the stress–strain curves, from brittle to ductile behavior.…”

“…Of course, these additives should be miscible with PP. Blends of PP and VM have been investigated in a previous paper 43 . VM is essentially an amorphous material.…”

“…In our previous paper, 43 the effect of VM on the crystallization behavior of PP was investigated. In the same way, the effect of adding VM to HDPE was investigated here.…”

“…Pure HDPE exhibits a strong exothermic melt crystallization peak at 118 C. Its crystallization enthalpy is around 233 J/g and corresponds to X C = 80% crystallinity if 293 J/g is adopted as fictive enthalpy of 100% crystalline PE. 44 From our previous paper, 43 we found that a weak melt crystallization peak of pure VM is detected at 13 C, its crystallization enthalpy is around 3 J/g and corresponds to X C = 1% crystallinity. In the blends, the melt crystallization peak of HDPE appears at higher temperatures ($3 C higher), and this shift is independent of the VM content.…”