Polystyrene melt, extensional viscosity

For polymer melts where the low shear rate limiting viscosity value is r ), r 3t]0 (14). Examples of extensional viscosity growth, either to a steady t](i ) value or to a strainhardening-like mode, are shown in Fig. 3.6 for the linear nonbranched polystyrene (PS), a high density polyethylene (HDPE) that is only slightly branched with short branches, and a long chain-branched low density polyethylene (LDPE) (15). 

Figure 9.1 Variation of steady state extensional viscosity with extensional rate for filled polystyrene melts at 30vol% of various types of fillers as indicated. (Reprinted from Ref. 29 with kind permission from American Chemical Society, vyashington DC. USA)...

Figure 9.4 shows the extensional viscosity vs. time curves for unfilled polystyrene melt at different extensional rates. It was found [19] that the extensional viscosity may tend to become constant at very low deformaticm rates, but become unbounded at higher and higher deformation rates. With filler concentration at low loading levels of 5 and 10% of carbon black filler, it weis fotmd [19] that the plots resembled those in Figure 9.4. However, at higher filler concentrations, constant extensional viscosities were achieved with time and these values were found to decrease with increasing extensional rate as shown in Figures 9.5 ctnd 9.6 for 20 and 25 vol% carbon black loading.

FIgur 9.5 Variation of extensional viscosity with time at different extensional rates for 20vol% carbon black filled polystyrene melt at 170°C. (Reprinted from Ref. 19 with kind permission from Society of Plastics Engineers Inc., Connecticut, USA.)... 

A typical example of extensional flow is the flow at the entrance of a capillary die. Cogswell [83] has shown that the pressure losses through sudi dies can be used as a measure of the extensional viscomty. This method has not lined popularity because of the skepticism in acc ting the complex converging-flow patterns at the die entrance as representative of true extensional flow with constant extensional rate. Cogswell [84] did suggest later that the die should be lubricated to reduce the shear flow and the profile of the die wall should vary at all cross sections in such a way as to ensure constant extensional rate along the die axis. Such a rheometer has been known to be developed and used for extensional viscosity data of polystyrene melt [85]. 

Figure 11.13 Steady-state uniaxial extensional viscosity normalized by the zero shear viscosity versus Weissenberg number Wi = re for nearly monodisperse polystyrene melts with molecular weights of 200,000 (-i-) and 390,000 ( ), where r is roughly the reptation time.The line is the prediction of the Doi-Edwards theory (from Bach etal. [56]).

---