上海交通大学：《食品物性学 Physical Properties of Foods》教学资源（英文PPT课件）SJTU4

Suspensions Particles and Polymers

Suspensions Particles and Polymers

Polymer Rheology trees and plants-gum-arabic,cellulose ● seeds Guar gum,locust bean gum ● seaweed carageenan,alginates,agar ● fruit -pectins ● grains starches ● microbial xanthan gum ● animal -gelatin,keratin and petroleum -'acrylic acid,polyacrylamide,and many plastics. Kinds of Polymer Chains Long Rods Coils/Strings Branching

Polymer Rheology Kinds of Polymer Chains Long Rods Coils/Strings Branching

Polymer melts and solutions Viscosity of Polymer melts -molecular weight distribution degree of branching shear rate Temperature and Pressure Polymer solutions all of the above -concentration -solvent interactions

Polymer melts and solutions Viscosity of Polymer melts -molecular weight distribution - degree of branching - shear rate - Temperature and Pressure Polymer solutions - all of the above -concentration -solvent interactions

Polymer melts Critical Molecular weight Mwe of a polymer Mw Mwe;no ofmolten polymer is almost directly proportional to the weight average molecular weight Mww hw>hwe;ηoiS .4 7。=K[Mw,e]

Polymer melts Critical Molecular weight Mwe of a polymer Mw Mwe ; ho is ho = K[Mww ] 3.4

Mwe is the point where at which entanglemments become important.Below this value the chains may be considered as non-penetration spheres. Mw.depends on chain configuration and is usually on the order of tens of thousands. At higher shear complete entanglement rates chain entanglements begin gradual loss of entanglements to decrease and development of alignment shear thinning complete alignment- occurs. Shear rate,y(log scale)

Mwe is the point where at which entanglemments become important. Below this value the chains may be considered as non-penetration spheres. Mwe depends on chain configuration and is usually on the order of tens of thousands. At higher shear rates chain entanglements begin to decrease and shear thinning occurs

All the individual curves for a series of Mw's, temperatures,and shear rates can be incorprated into a master curve by plotting 7 7'Mo2 versus no PT where a is a constant near unity

where is a constant near unity versus into a master curve by plotting temperatures, and shear rates can be incorprated All the individual curvesfor a series of Mw's,   h  h h   o  o 

All else being equal,broadening the molecular-weight distribution generally results in a polymer melt becoming non-Newtonian at lower values of shear rate,so by the time we come to those shear rates relevant to extruding etc.,the polymer is easier to handle with a lower viscosity. A small degree of branching of the polymer chains generally decreases the viscosity of a melt with the same average molecular weight since the branched chain is more compact.However,if the branching is extensive,for a reasonably high molecular weight(~106)the viscosity at low shear rates may be much higher,given the difficulties for these complicated interpenetrating structures to move.Imagine the structure as a collection of stars,i.e.*.Nevertheless,in almost all cases branching results in a lower viscosity at high shear rates

Polymer Solutions Solution Viscosity depends on same factors as polymer melts Viscosity also effect by concentration and solvent interaction effects

Polymer Solutions Viscosity also ef ect by concentration and solvent interaction ef ects Solution Viscosity depends on same factors as polymer melts

103 20 10 10 3.5 10 2 1 0.35 10 103 10 10 103 10 Shear rate,y/s The flow curve for 3%w/w polystyrene in toluene, for different molecular weights(shown in millions)

103 4 10 32 1 10 0.5 103 103 10 103 103 Shear rate,y/s The flow curve for polystyrene in toluene, Mw 20M,for different concentrations of polymer (shown as wt.%)