Study on thermal expansion in injection‐molded isotactic polypropylene and thermoplastic elastomer blends

Abstract

AbstractThe linear thermal expansion coefficients (CLTEs) along flow direction (FD) for the injection‐molded blends composed of isotactic polypropylene (iPP) and various ethylenic thermoplastic elastomers (TPEs) were investigated using a thermo‐mechanical analyzer. The iPP/TPE blends with higher comonomer contents in the TPE showed extremely low CLTE. TEM observation revealed that the array of the TPE whose MFR was adjusted to be higher than the iPP matrix was in lamella‐like sheet stacked normal to normal direction (ND) with being elongated along both FD and transverse‐to‐flow direction. At higher magnification of TEM, the iPP lamellae in the blend with higher comonomer contents in the TPE deeply penetrated into the TPE phase as a consequence of the faster iPP crystallization before the completion of the phase‐separation. Hence, the location of the iPP amorphous chains would change depending on the comonomer contents in the TPE; in the case of the iPP/TPE blend with higher comonomer contents, large amount of the iPP amorphous chains would be trapped inside the TPE phase because of incomplete phase‐separation arrested by faster crystallization. Therefore, the extremely low CLTE for the iPP/TPE blend with higher comonomer contents was accounted for by the simultaneous suppression of the thermal expansions from both the TPE phase and the iPP amorphous chains trapped inside the TPE by rigid iPP crystalline lamellae connecting in parallel with the TPE phase. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008