Thermal Expansion of Polypropylene Part II: Depression of In‐Plane Linear Thermal Expansion of Polypropylene by Elastomer Incorporation and Elucidation of Its Mechanism Through Temperature‐Variable X‐ray Scattering

Abstract

AbstractIn this study, the in‐plane linear thermal expansion behaviors from −20 °C to 100 °C along the flow direction (FD) for injection molded isotactic homo polypropylene (hPP) and their elastomer blends are thoroughly investigated. The elastomers used in this study are two different poly(ethylene‐co‐octene)(EO) thermoplastic elastomers which have the same density but different melt flow rate (MFR), 5.0 and 30 (dg/10 min). Thus, the blends are designated as hPP‐EO5.0 having the EO with 5.0 of MFR and hPP‐EO30 having the EO with 30 of MFR. It is found that from −20 °C to 40 °C, the linear thermal expansion profiles for neat hPP, hPP‐EO5.0, and hPP‐EO30 overlapped, indicating that with or without the EO, their linear thermal expansions proceeded under the same mechanism. On the other hand, above 40 °C, their thermal expansion behaviors greatly changed; the linear thermal expansions of hPP‐EO5.0 and hPP‐EO30 are suppressed to be much lower than that of neat hPP. The morphological observation by a transmission electron microscope (TEM) reveals that the shapes of the EO domains in the blend systems are found to be elongated along the FD. Thus, it would be expected that the suppressed linear thermal expansions of the hPP‐EO blends are attributed to the suppression of the linear thermal expansions of the hPP amorphous layers in the lamellae due to the retraction of the elongated EO domains caused due to the instability of the interfacial tension at elevated temperatures. Yet, temperature variable small‐angle X‐ray scattering (SAXS) results reveal that the linear thermal expansions of the hPP amorphous layers in the EO blends, on the contrary, increase. One of the plausible mechanisms is proposed to elucidate seemingly contradictory phenomenon that the EO blends have their suppressed bulk linear thermal expansions while enhanced linear thermal expansions of the hPP amorphous layers. Finally, the suitable elastomer design for low coefficient of linear thermal expansion (CLTE) PP is proposed.