Viscoelastic measurements have been carried out for both liquid crystalline and isotropic solutions of a helical synthetic polypeptide PBG (an equimolar mixture of L and D enantiomers of poly-γ-benzyl-glutamate) in
m-cresol to study the viscoelastic properties of lyotropic liquid crystal of rodlike macromolecules. Dynamic viscoelastic functions, shear storage modulus
G′ and shear loss modulus
G″, were determined over angular frequency(ω) range of 0.005-3 (rad·sec
-1) by oscillatory shear between cone and plate. The contour of stress-strain Lissajous' figure obtained for liquid crystalline solution and isotropic solution is almost elliptical within angular frequency range described above and it was not changed by increasing strain amplitude at least up to 80% and by repeated application of sinsoidal strain, which means linear viscoelastic response and no rheotropic behavior in the present system. The results of
G′ and
G″ revealed a distinct difference between liquid crystalline solution and isotropic solution. In the case of isotropic solution,
G′ and
G″ are proportional to ω
2 and ω, respectively and independent of strain amplitude. On the other hand, in the case of liquid crystalline solution the dependence of
G′ and
G″ on ω becomes weaker with decreasing angular frequency in the lower angular frequency region, which implies the existence of relaxation mechanism attributed to polydomain texture. The trend is more remarkable in
G′ than in
G″.
G′ and
G″ are slightly, but not negligibly, affected by the amount of strain amplitude. For liquid crystalline solution
G′ (
G″) at different temperatures can be superposed into one composite curve only when the curves are shifted both horizontally and vertically.
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