Poly , normal stress difference

Fig. 2.1. First normal stress difference (pu—p22) as a function of shear rate q and doubled storage modulus 2 G as a function of angular frequency for a poly-dimethyl siloxane (M = 536,000) at a measurement temperature of 20° C. (o) (A n/C) cos 2y,...

Figure 3.34 Shear stress (open symbols) and first normal stress difference (closed symbols) as functions of shear rate for two solutions of very-high-molecular-weight poly--4nethylmethacrylate (M = 23.8 x 10 )...

Figure 13.32 Primary normal stress difference as a function of the shear rate and molecular weight for 3% (mass) poly(oxyethylene) solutions in water and glycerine, (From Ref. 51.)...

Thermotropic LCPs have high melt elasticity, but exhibit little extrudate swell. The latter has been attributed to a yield stress and to long relaxation times (60). The relaxation times for LCPs are normally much longer than for conventional polymers. Anomalous behavior such as negative first normal stress differences, shear-thickening behavior and time-dependent effects have also been observed in the. rheology of LCPs (56). Several of these phenomena are discussed for poly(benzylglutamate) solutions in the chapter by Moldenaers et al. 

Normal stress measurements for some MLC nematics was reported to be consistent with that of a second-order fluid, that the low frequency limit of G /co equaled the low shear limit of N /(dy/dty [36]. Coleman and Markowitz demonstrated that for a second-order fluid in slow Couette flow, the viscoelastic contribution to the normal thrust must have a sign opposite to the inertial contribution on thermodynamic grounds [37]. A textbook by Walters stated that the measurements of first normal stress difference have invariably led to a positive quantity except for one case which was later found to be in error [38]. Adams and Lodge reported the possible observation of a negative value for Nj for solutions of poly isobutylene + decalin [39]. This result was obtained by a combination of obtained from radial... 

The primary normal stress difference also experiences an overshoot during the establishment of steady flow following onset of a constant shear rate. An example is included in Fig. 17-33 the normal stress maximum appears considerably later than the shear stress maximum, as found also in earlier measurements on a solution of polyisobutylene by Huppler and collaborators. In relaxation following cessation of steady-state flow, the primary normal stress difference falls more slowly than the shear stress, as seen in Fig. 2-13 and also the measurements on poly(a-methyl styrene) by Nagasawa and collaborators when normalized by the steady-flow value, the primary normal stress difference appears to fall somewhat more rapidly with increasing 7. 

Figure 6.41 Normal stress differences for a solution of poly(Y-benzyl-l-glutamate). Schematic curve. Drawn after data from Magda et al. (1991).

Isono and Nagasawa report shear thinning and normal stress differences from a cone-and-plate rheogoniometer applied to solutions of poly-a-methylstyrenes in good and Theta solvents(36). These solutions were all entangled in the viscometric sense Je c - Figure 13.24 shows r] K) has a simple exponential decay for k as... 

Very few investigators have reported on strain scaling with first normal stress difference N (t, y) for lyotropic LCPs after reversal in flow direction. Chow et al. (1992) reported variations of N t,y) with yt for lyotropic solutions of poly(p-phenylenebenzobisthiazole) (PBZT) after reversal in flow direction, indicating that for this system (t, y) does not follow strain scaling after flow reversal. Hongladarom... 

Several attempts have been made to superimpose creep and stress-relaxation data obtained at different temperatures on styrcne-butadiene-styrene block polymers. Shen and Kaelble (258) found that Williams-Landel-Ferry (WLF) (27) shift factors held around each of the glass transition temperatures of the polystyrene and the poly butadiene, but at intermediate temperatures a different type of shift factor had to be used to make a master curve. However, on very similar block polymers, Lim et ai. (25 )) found that a WLF shift factor held only below 15°C in the region between the glass transitions, and at higher temperatures an Arrhenius type of shift factor held. The reason for this difference in the shift factors is not known. Master curves have been made from creep and stress-relaxation data on partially miscible graft polymers of poly(ethyl acrylate) and poly(mcthyl methacrylate) (260). WLF shift factors held approximately, but the master curves covered 20 to 25 decades of time rather than the 10 to 15 decades for normal one-phase polymers. 

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