ABSTRACT
IV. CONCLUDING REMARKS This chapter bas c.xamined the influence of processing variables such as temperture. pressure, and shear rate and the influence of stmctural variables such as molecular weight. molecular weight distribution, and chain branching on the steady shear viscosity of melts of flexible chain polymers. Typical data were presented and explained, and it was shown bow viscosity master curves could be generated. Many polymer processors. however. do not have access to sophisticated viscometers. They generally characterize polymer melts through a parameter called the mel! flow index (MFI). This is the mass of polymer in grams extruded in I 0 minutes through a capillary of specified length and diameter by the application of pressure using a dead weight as specified by 1\STM D 1238. The MFL therefore, is a single-poim viscosity measurement at celatively low shear rate and temperature. Sbenoy el al. [66] have shown that temperature-independent viscosity master curves can be generated for a given polymer type by plotting the product of the shear viscosity and MFl against the rcuio of the shear rate to MFI. They have also provided master curves for several commodity thermoplastics 166], engineering plastics [67], and specialty polymers [68]. A knowledge of the MFI alone. in conjunction with these master curves and the WLF equation, provides the entire flow curve. (See also Ref. 69.)
We emphasize that we have no! considered transient effects in this chapter. These arc important in polymer processing operations where the time to reach steady state may be comparable to or even longer than the time scale of the processing operation being analyzed. These transient effects are examined in the chapter on liquid crystal polymer rheology, where the behavior of flexible chain polymers is compared and contrasted with the 11ow behavior of' rigid chain polymers. Finally we point out that in the processing of semicrystalline polymers,
Shear Viscosity of Melts of Flexible Chain Polymers 59
the rheological behavior. in a temperature range between the melting point and the glass transition temperature. is influenced by the presence of crystallinity in the melt. There is a great paucity of data under these conditions. and it is common to account for the presence of crystals in an empirical manner [701.
