Author
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Carriere, Craig |
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Submitted to: Journal of Polymer Science Part B: Polymer Physics
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 3/11/1998 Publication Date: N/A Citation: N/A Interpretive Summary: Starch is an industrially-important polymer which is used in numerous applications including paper coatings and foods. Despite its widespread use, many unanswered questions remain regarding the basic rheological properties of starch. In order to process starch effectively, an understanding of the flow behavior of starch-based systems needs to be developed. This work investigates the physical basis for the unexpected viscosity rise and the consequent stable, flow-induced structure formation of waxy maize starch in solutions. The increased viscosity was observed for waxy maize starch, but not for normal maize starch at the same concentration. The data indicate that the viscosity increase observed for waxy maize starch is caused by a flow-induced highly-entangled network. Prior to the region of increased viscosity, the flow behavior of waxy maize starch was that expected for a random coil. Normal maize starch was found to behave as a rigid or semi-flexible rod in solution. This work represents some of the first attempts to utilize physical models to distinguish the solution flow behavior of different starches. Technical Abstract: Small-amplitude oscillatory shear experiments were used to investigate the network structure formed by semidilute solutions of waxy maize (WM) and normal maize starches in a solution of 90/10 dimethyl sulfoxide/water during a shear-loop experiment. The solution viscoelastic properties of WM starch prior to the observed shear-thickening region could be described by a Rouse bead-spring model. The macromolecules behave as random coils with a longest relaxation time of 0.58 +/ 0.03 s. For WM starch, the observed shear-thickening region creates a stable, highly-entangled network which resembles the behavior found in solutions in the concentrated regime. The shear-thickening behavior of WM starch could be described using a transient network model. NM starch does not exhibit a shear-thickening regime. The semidilute solution viscoelastic properties of NM starch resemble that of a rigid cylinder, or a semiflexible rod. The experimental data were fitted using a rigid rod model with an end-over-end relaxation time constant of 0.12 # 0.01 s. The experimental data may either be interpreted as a rigid rod or as part of the spectrum of a semiflexible cylinder where the remaining relaxation times are outside the frequency regime of the rheometer. |
