Surface-enhanced nucleation in immiscible polypropylene and polyethylene blends: The effect of polyethylene chain regularity

In blends where polyethylene (PE) is the minor component dispersed in an isotactic-polypropylene (PP) matrix, an enhancement of the crystallization rate of PE was found when the crystallization temperature of the matrix phase is increased by means of a self-nucleation protocol. Such an effect is interpreted as the result of the epitaxial nucleation of PE droplet domains at the interface with the PP matrix (Macromolecules, 2021, 54, 19, 9100–9112). This study extends the findings on immiscible blends of a PP matrix and various industrially produced PE grades as the dispersed phase. Eight different PEs with varying molecular architecture, and thus density and melting temperatures, were mixed with PP in a 20/80 PE/PP weight ratio. The possible existence of surface nucleation was probed by applying a self-nucleation thermal protocol to the PP matrix and recording the concomitant variation of PE crystallization temperature (Tc). Different behaviours are observed for various PEs. In particular, those with relatively high density display a clear increase in crystallization temperature, over 3.0 °C, when the Tc of the matrix is increased (i.e., as PP lamellar thickness increased), indicating an efficient surface nucleation mechanism. Metallocene-made LLDPE also shows a small increase, about 1.7 °C, while PEs with lower densities, below 927 kg/m3, metallocene PEs or LDPE display no meaningful change in Tc with matrix self-nucleation. It was demonstrated that a threshold value of chain regularity is required to trigger the surface nucleation of PE on PP. Only polymers with a density above approximately 920 kg/m3 and melting temperatures exceeding about 115 °C can efficiently nucleate onto the PP substrate. It is postulated that the presence of a high amount of branches or comonomers along the PE chains hinders the epitaxial matching between PE and PP.