Study On Mechanical Properties Of Poly(Butylene Terephthalate)/Polycarbonate Blends

The mechanical properties of poly(butylene terephthalate)/polycarbonate(PBT/PC)under different conditions were studied systematically,including naturally cooled,isothermally crystallized and amorphous blends.Crystalline PBT has a multi-scale structure that spans microscopic,mesoscopic,macroscopic scale,thus the structure and structure evolution of PBT/PC blends were investigated by wide angle X-ray scattering(WAXS),small angle X-ray scattering(SAXS),and so on.Main contents of this thesis are listed below:1.The presence of PC components contributes to the kinetic hindrance of PBT crystallization during the curing process in PBT/PC melt blends,resulting in a decrease in the crystallinity of the blends,thus the mechanical properties such as yield strength and Young's modulus decrease with increasing PC content.When PBT samples are stretched at various temperatures,intralamellar slip may occur in the transverse direction and some small,imperfect crystallite further melt under external force via chain-unfolding;when stretched at 120 ~?,the lamellae thicken and crystallinity increases with increasing strain due to the stress-induced melting and recrystallization mechanism.Stretching induced phase separation occurs during deformation at 70 ~? for all the PBT/PC samples,the macroscopic strain mainly comes from the longitudinal slip of the fibers.2.When PBT/PC melt blends are isothermally crystallized at 180 ~?,PBT has sufficient time for structure rearrangment and conformational ajustments,the relative crystallinity of PBT in PBT/PC blends is independent of the composition,while the apparent crystallinity decreases with increasing PC content.Thus the factor that affect the mechanical properties of PBT/PC blends is the apparent crystallinity,not the relative crystallinity.The yield stress at various temperatures can be described by Ree-Eyring equation for PBT isothermally crystallized at 180 ~? and 100 ~?.The yield process involves the cooperative movement of chain segments even in the glassy state of PBT.Stretching induced phase separation also occurs during deformation at70 ~? for the PBT/PC samples isothermally crystallized at 180 ~?;at large strains,the long period is observed to increase with increasing PC content mainly due to an increase in lamellar thickness.3.The mesophase structure of PBT is obtained when amorphous PBT/PC blends are stretched in the glassy state(30 ~?),the addition of PC will reduce the orientation of molecular chains during deformation,thus reducing the content of the mesophase.When stretched at 50 ~? for amorphous PBT/PC blends,the presence of PC hinders the mobility of chains,the crystallinity decreases with the addition of PC.