Crystallization of propylene–ethylene random copolymers

Abstract

Of the three melting peaks typical of a propylene-ethylene random copolymer (with 5.1 wt % ethylene) crystallized between 110 and 140°C, the two higher peaks result from primary and secondary isothermal crystallization, whereas the material crystallized on cooling gives the lowest peak. In contrast to polypropylene homopolymers, which show strong morphological changes developing from the center of a spherulite, copolymer specimens are uniformly crosshatched. The highest melting peak is related to an open crosshatched framework of primary lamellae, and the next lower peak is related to later forming subsidiary lamellae filling the intervening space. The origin and nature of these double peaks are discussed in terms of the fractional crystallization and the ensuing constraints placed on isothermal lamellar thickening as a result of the exclusion of the comonomer from the polypropylene lattice.

Evidence for isothermal lamellar thickening at and behind the growth front as polyethylene crystallizes from the melt

“…A different explanation must apply to the reduced thickening at the centres of a-polypropylene spherulites whose texture is quite open with two sets of nearly orthogonal lamellae [12]. Their fold surfaces do not touch but there is mutual contact on the growth surfaces, which will affect refolding and so delay thickening.…”

Fractionation and Characterisation of Propylene‐Ethylene Random Copolymers: Effect of the Comonomer on Crystallisation of Poly(propylene) in the γ‐Phase

“…There is much debate surrounding the cause of the multiple melting endotherms,3,30–34 although there is usually a definite cause depending on the type of material that is being used and the conditions of analysis. Multiple melting endotherms could be the result of different polymorphic forms, melting‐recrystallisation, segregation by tacticity or molecular weight, the melting of different regions in the crystalline structure such as radial and transverse lamellae, or even orientation effects 3,30,31,33,35. Fraction A12 (115 °C), being the fraction with the most prevalent multiple melting, was analysed at different heating rates to determine if the cause of the multiple melting was due to a melting‐recrystallisation effect.…”

“…Multiple melting endotherms could be the result of different polymorphic forms, meltingrecrystallisation, segregation by tacticity or molecular weight, the melting of different regions in the crystalline structure such as radial and transverse lamellae, or even orientation effects. [3,30,31,33,35] Fraction A12 (115 8C), being the fraction with the most prevalent multiple melting, was analysed at different heating rates to determine if the cause of the multiple melting was due to a meltingrecrystallisation effect. It should be noted at this point that all the samples exhibiting the multiple melting phenomenon have a single, narrow crystallisation peak.…”

Differential scanning calorimetry of copolymer of isotactic polypropylene backbone with grafted poly(ethylene‐co‐propylene) branches

“…At high T c , the radial lamellae grow before the tangential lamellae, and thus the radial lamellae have greater thickness 43. Therefore, after crystallization at high temperatures, upon heating, the thin tangential lamellae melt first, and then the thicker radial lamellae melt 45–47. Weng et al demonstrated that different lamellar thicknesses cause multiple melting in metallocene‐catalyzed PP and in propylene‐ethylene random copolymers 46, 48…”