Effect of Urea

Figure 5.8 – Viscosity curves of a) 2.5 and b) 5% w/w mamaku polysaccharide with 0 („), 1 (z), 2 (S), 3 (T), 4 (‹) & 5M (W) urea at 20qC

The effects of increasing urea concentration in 2.5 and 5% w/w mamaku solutions are shown in Figure 5.8. For 2.5% w/w mamaku (Figure 5.8a), a large reduction in zero-shear viscosity was initially observed when the polysaccharide was dissolved in 1 and 2M urea concentration. However, the effects on zero-shear viscosity reduction plateaued above the concentration of 2M urea. Shear-thickening was completely lost at t3M urea. Similar effects were observed for 5% w/w mamaku (Figure 5.8b), but with the shear-thickening being lost at

t5M urea. Therefore the concentration of urea required to suppress shear-thickening was proportional to polysaccharide concentration.

Urea is a chaotropic agent which disrupts hydrogen bonds by competing for binding sites. These observations strongly suggest that again, hydrogen bonds were most likely responsible for the viscosity of the polymer solution before and during shear-thickening, although affecting the latter more significantly. The reduction of zero-shear viscosity with urea indicates that hydrogen bonds were also present in the quiescent state, either due to breakup of aggregates (Jin, et al., 2013) or intra- and intermolecular bonds. No further reduction in zero-shear viscosity was achieved with increasing urea concentration t2M (5% w/w mamaku), although shear-

thickening further diminished. Hence there is strong evidence to indicate that the hydrogen bonds responsible for shear-thickening were formed only under shear. This agrees with the hypothesis that under shear, the polysaccharide could stretch out and exposes associative groups, changing from intra- to intermolecular bonds which would cause shear-thickening. In mamaku it is postulated therefore, that urea binds to associative groups along the polysaccharide, preventing the ‘zipping’ effect between mamaku chains during shear as it has been shown in literature with other polymers (Kjøniksen, et al., 2005; Kjøniksen, et al., 2003; Lele & Mashelkar, 1998). It is also possible that urea could disrupt the hydrogen-bonded polymer-solvent structure (Hammes & Swann, 1967), which contributes to rheological properties of synthetic polymers such as polyethylene oxide (Dormidontova, 2002) and polyvinyl alcohol (Gao, Yang, He, & Yang, 2010). This would affect the entire network structure of the mamaku in solution as well as the shear-induced intermolecular associations.

Figure 5.9 – a) Dependence of peak viscosity (Kmax) („) and characteristic time scale (1/ࢽሶ࢓ࢇ࢞) (z) on urea concentration for 5% w/w mamaku; b) inverse of maximum shear rate normalised by mamaku

concentration (2.5 („), 5 ({), 7% w/w (S)) at 20qC

Similar to temperature, the peak viscosity during shear-thickening and the characteristic time or association lifetime (inverse of shear rate at peak viscosity) exhibited an exponential dependency on urea concentration (~e0.8Curea_{) (Figure 5.9a). Likewise, a master curve could be obtained by normalising 1/ߛሶ}

௠௔௫ with mamaku

concentration which has the same exponential dependency of ~e-0.8Curea _{(Figure 5.3b). Therefore similar}

conclusions could be drawn with urea as with temperature, addition of urea did not change the average number of association groups per chain but only modified the average lifetimes of the association, and therefore the onset of shear-thinning (network rupture) occurred at an approximately constant value of stress (Appendix B; Figure B3).

Large amplitude oscillatory shear and extensional viscosity experiments were also carried out on the mamaku gum in the presence of urea (Appendix B; Figure B4 & B5). Similar rheological responses were measured, with loss of strain hardening in LAOS and decrease in extensional relaxation time with increasing urea concentrations.

5.3.2.1

Time-Urea Concentration Superposition

Analogous to time-temperature superposition, the viscosity curves in the nonlinear region (Figure 5.10) and the frequency sweeps in the linear viscoelastic region (Figure 5.11) of 5% w/w mamaku with various urea concentrations were superimposed using shift factors. The same principle should apply, whereby superimposable curves indicate that all relaxation mechanisms of the material have the same urea concentration dependence, and it does not undergo microstructural changes or phase transitions upon addition of urea.

The shear-thinning prior to onset of shear-thickening was superimposable up to 3M urea, but began to deviate at 4 and 5M urea concentrations. The shear-thickening regions were not superimposable as the extent of shear-thickening (Kmax/Kcrit) clearly diminished with increasing urea concentration. The shift factors aC and bC

had exponential dependencies of -0.9 and 0.4 on urea concentration, respectively. The larger exponent value of -0.9 for aC as compared to -0.1 for aT showed that urea had a more pronounced effect on shear-thickening

(or hydrogen bonds) per unit concentration than per unit temperature does. Again, this confirmed the presence of additional intermolecular associations during shear-thickening, which had a different relaxation mode from simple chain relaxation.

Similar to time-temperature superposition, the linear viscoelastic behaviour based on frequency sweeps was superimposable (Figure 5.11). Therefore the presence of urea was not likely to have altered the structure or phase properties of the mamaku polysaccharide, and it behaves like a thermorheologically simple material in the linear region. Again, the exponential value of -0.4 for the horizontal shift factor in the linear region is approximately half of the value in the nonlinear region (-0.9), suggesting that twice as many associations would occur during shear-thickening as compared to the quiescent state. This supports the theory of shear-induced intra- to intermolecular association since every two associative groups involved in one intramolecular bond would then convert to two intermolecular bonds under shear i.e. pairwise association (Higgs & Ball, 1989).

Figure 5.10 – Reduced viscosity (Kr=bC˜K) vs. reduced shear rate (ࢽሶ࢘=aC˜ࢽሶ) of 5% w/w mamaku with 0 („), 1 (z), 2 (S), 3 (T), 4 (‹) and 5M (W) urea; inset: horizontal (aC; z) and vertical (bC; {) shift factors with

Figure 5.11 – Time-urea concentration superposition of frequency sweeps of 5% w/w mamaku with 0 („), 1 (z), 2 (S), 3 (T), 4 (‹) and 5M (W) urea by shifting along x-axis with shift factor, ac; inset: shift factor