Equilibrium and nonequilibrium molecular dynamics (MD) simulations have been performed in both isochoric-isothermal (NVT) and isobaric-isothermal (NPT) ensemble systems. Under steady state shearing conditions, thermodynamic states and rheological properties of liquid n-hexadecane molecules have been studied. Between equilibrium and nonequilibrium states, it is important to understand how shear rates (gamma) affect the thermodynamic state variables of temperature, pressure, and density. At lower shear rates of gamma<1 x 10(11) s(-1), the relationships between the thermodynamic variables at nonequilibrium states closely approximate those at equilibrium states, namely, the liquid is very near its Newtonian fluid regime. Conversely, at extreme shear rates of gamma>1 x 10(11) s(-1), specific behavior of shear dilatancy is observed in the variations of nonequilibrium thermodynamic states. Significantly, by analyzing the effects of changes in temperature, pressure, and density on shear flow system, we report a variety of rheological properties including the shear thinning relationship between viscosity and shear rate, zero-shear-rate viscosity, rotational relaxation time, and critical shear rate. In addition, the flow activation energy and the pressure-viscosity coefficient determined through Arrhenius and Barus equations acceptably agree with the related experimental and MD simulation results.
SYNOPSISThe melt viscosity of polycarbonate/acrylonitrile-butadiene-styrene ( PC / ABS ) blends relative to PC is significantly lower, even lower than that of pure ABS in some compositions. Annealing above the TB of PC coalesces and coarsens phase structure in core and skin regions. Increase in the molecular weight of PC in PC /ABS blends results in low-temperature fracture toughness improvement but suffers from the disadvantage of higher melt viscosity. The selection of PC in PC/ABS blends must be a compromise between the toughness advantages of higher PC molecular weight and the disadvantages of higher melt viscosity.
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.
The B‐ala/AIBN PBZ system has a high extent of ring‐opening of oxazine because phenol‐containing oligomers are formed at the early stage of the curing process. As a result, the B‐ala/AIBN PBZ system possesses a relatively stronger intramolecular hydrogen bonding and lower surface energy than the pure B‐ala system at low temperature curing. In this context, poly(4‐vinyl pyridine), poly(4‐vinyl phenol) thin films and polycarbonate substrates, which lack liquid resistance, possess low surface free energy after modification with B‐ala/AIBN = 5/1 PBZ.magnified image
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