SYNOPSISMechanical properties of polycarbonates ( PCs ) and elastomer-modified polycarbonates with various molecular weights (MW) are investigated. Higher MW PCs show slightly lower density, yield stress, and modulus. The ductile-brittle transition temperature (DBTT) of the notched impact strength decreases with the increase of PC MW and with the increase of elastomer content. The elastomer-modified PC has higher impact strength than does the unmodified counterpart if the failure is in the brittle mode, but has lower impact strength if the failure is in the ductile mode. The critical strain energy release rate (G,) measured at -30°C decreases with the decrease of PC MW. The extrapolated zero fracture energy was found at M , = 6800 or MFR = 135. The G, of the elastomer-modified PC (MFR = 15, 5% elastomer) is about twice that of the unmodified one. The presence of elastomer in the PC matrix promotes the plane-strain localized shear yielding to greater extents and thus increases the impact strength and G, in a typically brittle fracture. Two separate modes, localized and mass shear yielding, work simultaneously in the elastomer-toughening mechanism. The plane-strain localized shear yielding dominates the toughening mechanism at lower temperatures and brittle failure, while the plane-stress mass shear yielding dominates at higher temperatures and ductile failure. For the elastomer-modified PC ( 10% elastomer), the estimated extension ratio of the yielding zone of the fractured surface is 2 for the ductile failure and 5 for the brittle crack. A criterion for shifting from brittle to ductile failure based on precrack critical plastic-zone volume is proposed.

SYNOPSISThe crystallization kinetics and morphology of glass fiber-reinforced polypropylene ( PP/ GF ) were investigated in this work. Both isothermal and nonisothermal crystallization behaviors of 9OPP/lOGF, SOPP/20GF, and 70PP/30GF were examined with a DSC instrument. It was found that the addition of glass fiber would increase the crystallization rate of PP and increase the content of @ spherulite, which was most likely formed at temperatures between 390 and 400 K. The morphology of spherulites of PP/GF composites were examined with SEM and a polarized microscope. All experimental observations conformed rather well with the theoretical approach, a dynamic crystallization model, proposed in our previous work. The size of a spherulites of PP would decrease at lower crystallization temperature, or at higher cooling rate, or by adding glass fiber in it.

SYNOPSISThe ductile-brittle transition behavior of polycarbonate and methylmethacrylate-butadiene-styrene (MBS) elastomer modified polycarbonate has been investigated in terms of notch radius and temperature. At -40°C and 21-mil notch radius, polycarbonate fractures in three possible modes, ductile ( 25% ) , semi-ductile (50% ) , and brittle (25% ) . This semiductile mode fracture has never been reported previously with brittle characterization, but to a greater extent in localized shear yielding on the fracture surface and intermediate toughness. A two-dimensional fracture mode diagram in terms of temperature and notch radius has been constructed to interpret the observed phenomena. This diagram can also predict the existence of other conditions under which the triplet fracture modes may also occur. Another unstable zone has also been identified where the fracture occurs in either ductile mode or brittle mode over a broad temperature range, instead of the narrow temperature range typically observed for polycarbonate. A model based on the excessive precrack strain just below yielding due to the greater notch radius is proposed to explain such observed semi-ductile mode fracture.

SYNOPSISTo investigate the correlation of viscosity-morphology-compatibility of polymer blends, polystyrene, polybutadiene, and styrene /butadiene triblock copolymer were selected as our materials. A capillary rheometer was used to measure the viscosity. All samples were blended by a two-roll mill, a Brabender, or by the solution casting method. The morphologies of extruded samples from the capillary were observed by SEM. The relation between the viscosity and the composition of the blend changes with shear rate. It was found that the better the compatability between the two phases of the blend the smaller is the disperse phase domain. The viscosity of polyblends is related to the compatability and composition of the two phase in this blend. In the present study, the rheological behavior of polyblends is explained from the viewpoints of morphology and compatability. 0 1993 John Wiley & Sons, Inc.

Blends of polybutadiene (BR) and styrene-butadiene-styrene triblock copolymer (SBS) have been prepared by a two-roll mill. The morphologies of extruded samples from a capillary rheometer were observed by scanning electron microscopy (SEM). It is found that PS phase is dispersed in the BR phase. The glass transition temperature (T g ) of the blend has been examined by using differential scanning calorimetry (DSC). From the T g behavior and the electron microscopy study, it is found that certain degree of miscibility between the polystyrene phase and the BR phase is observed. The rheological behavior of the blend has been investigated by a capillary rheometer. It is found that the viscosity of the blend increases with increased content of PS phase. The behavior is in accord with the expected behavior of filler effect. To predict the filler effect of PS phase on the BR-SBS blend, a modified model of Chen and Cheng is proposed to elucidate the rheological properties of the BR-SBS blends with different compositions. Chen and Cheng's micromechanical model derived in Part I of this series, which relates the macroscopic shear stress to the macroscopic shear rate of a rigid non-Newtonian suspension when the direct contribution of Brownian force is completely neglected. The agreement between the theoretical predictions and the experimental results is satisfactory.