Extensional properties of hydroxypropyl ether guar gum solutions.

The extensional properties of 2-hydroxypropyl ether guar gum solutions were investigated using a capillary breakup extensional rheometer (CaBER). Optimization of the geometric parameters of this device allowed for the measurement of the characteristic relaxation times and the apparent extensional viscosities of a series of dilute to semidilute guar gum solutions. The measured relaxation times were compared with predicted Zimm relaxation times, assuming that the hydrophobically modified guar was in a good solvent. Good agreement was found at low concentrations (0.01 wt % approximately 0.17 c*, where c* is the polymer overlap concentration), and this technique allowed for relaxation times on the order of 1 ms to be measured for solutions with shear viscosities of 2 mPa.s. Both the shear and (apparent) steady-state extensional viscosities of this set of industrially relevant fluids exhibited two regions of dependency on polymer concentration: linear up to concentrations of 0.2 wt % ( c/ c* approximately 3) and power law thereafter, where interchain interactions became significant. The extracted relaxation times followed the same trend (i.e., having a near linear dependency on concentration up to 0.2 wt % and a power-law dependency on concentration up to 9 c*). The results indicate that the transition from dilute to semidilute behavior occurs at a nominal concentration of approximately 3 c* instead of c*. The results presented suggest that interchain interactions for this modified guar are weak overall, and the solutions investigated are absent of entanglements over the whole range of frequencies and concentrations explored ((0.17-9) c*).