LecturePolymerPhysics.pptx

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,#,1,Lecture at ECUST:Polymer Physics,Shanfeng Wang,Ph.D.,Assistant Professor,The University of Tennessee,Knoxville,Department of Materials Science&Engineering,2,Professor Mark Foster and Family(,Univ.of Akron Polymer Science Department,)with all their oil-based polymer belongings!-,National Geographic Magazine,June 2004,How Important are Polymers in Our Life?,Photographer:Sarah Leen,3,Questions,What makes polymers so“different”?,Whats the difference between long chain polymer and short chain polymer?,How do polymer chains move?,Intrinsic chemical structure:intermolecular interaction,molecular weight,nature of chain packing.,(1)How high must the molecular weight be before the molecule displays“polymer properties”?,(2)Different polymer types-different critical molecular weight.,The issue of observation time;the issue of experimental temperature;their correlation.,4,A Few Important Issues in Polymer Science,Glass transition temperature(T,g,);,Viscoelasticity;,Time-temperature superposition;,Models for describing polymer dynamics when polymer chain is at different lengthsLength scale,Time scale,Intrinsic chemical structure;,Molecular weight;,Packing;Motion.,Temperature,Length Scale and Time Scale,5,“,Scaling concepts in Polymer Physics,”by P.G.de Gennes.,“,Rheology:Principles,Measurements,and Applications,”by Christopher W.Macosko.,“Various animals attempting,to follow a scaling law”,“Even the mountains flowed,before the Lord”,From the song of Deborah after her,Victory over the Philistines,Judges 5:5,6,Materials Design Strategy:Length Scale Really Matters,Fabrication methods and product design,Mechanical properties,biodegradability,etc,Kuhn length,Packing length,Entanglement length,Radius of gyration,Chain rigidity,Crystallization,Phase separation,Chain packing,Self-assembled,structure,etc,A,o,nm,m,m,mm,7,Polymer Physics,Condensed State Physics,Soft Matter(or Complex Fluid),Physics,Structure&Morphology,:,Crystalline,liquid-crystalline,or amorphous,Polymer Dynamics&,Rheology,Physical Properties,Thermal,Rheological,Mechanical,8,Organization of Atoms and Molecules,Polymer“Solid“State,Semi-Crystalline,Amorphous,Glassy,Rubbery,9,Conformations of Polymer Chains,Ordered,Disordered,The amorphous state is disordered.,helix,Planar,Zigzag,Liquid crystal structures,cholesteric,smectic A,smectic C,discotic,nematic,10,Glassy State,State of a polymer depends on the,temperature,and on the,time,allotted to the experiment.,At low temperatures all amorphous polymers become,stiff and glassy,.,The glassy state corresponds to an,absence of long range motion of chains,.,If we increase the temperature,a glassy polymer will soften and become rubberythe temperature where this transition occurs is call the glass transition temperature,T,g,.,T,g,is the most important characteristic temperature of amorphous polymers.,11,Measurement of T,g,by DSC,Example of differential scanning calorimetry(,DSC,)trace of poly(ethylene terephthalate-,stat,-,p,-oxbenzoate),quenched,reheated,and cooled at 0.5 K/min through the glass transition,and reheated for measurement at 10 K/min.T,g,is taken at the temperature at which half the increase in heat capacity has occurred.The width of the glass transition is indicated by,D,T.Note that,D,H,c,and,D,H,f,are equal in magnitude but opposite in sign.,12,Effect of Molecular Weight on T,g,As molecular weight increases,the density of chain ends decreases,causing a decrease in free volume.,Effect can be expressed with the following equation,where,K,is a constant depending on the polymer type,and,a,R,and,a,G,represented the cubic(volume)expansion coefficients in the rubbery and glassy states,respectively,.,Polystyrene,T,g,(,o,C),log M,n,13,Dependence of T,g,on Chemical Structure,Factors that increase energy required for molecular motion increase T,g,and factors that decrease this energy decrease T,g,;,Nature of intermolecular bonding;,Chain flexibility:,segmental mobility is highly dependent on chain stiffness;,Side chain substitution:,(1),Short bulky groups increase T,g,(stiffen chain by hindering rotation);,(2),Long linear flexible pendant groups lower T,g,(internal diluents,hinder packing,increase free volume);,(3),Non-symmetrical or symmetrical di-substitution.,14,Examples,CH,2,CH,n,COOCH,3,CH,2,CH,n,COOH,CH,2,CH,n,CO,2,-,K,+,CH,2,CH,n,CO,2,-,Ca,+,T,g,(,o,C),O-C-(CH,2,),4,-C-OCH,2,CH,2,n,O,O,O-C C-OCH,2,CH,2,n,O,O,6,106,280,500,500,T,g,=-63,o,C,T,g,=69,o,C,T,g,=69,o,C,T,g,=69,o,C,T,g,=-63,o,C,T,g,=69,o,C,Dipole-dipole,Hydrogen,bonding,ionic,Stronger,ionic,15,Effect of Side Chain Length on T,g,of Polymethacrylates,16,Summary of Factors That Affect T,g,Increase T,g,Intermolecular forces,High Cohesive Energy Density,Intrachain steric hindrance,Bulky,stiff side groups,Decrease T,g,In-chain groups promoting flexibility,(double-bonds and ether linkages),Flexible side groups,Symmetrical substitution,17,Plasticizers,Plasticizers are low molecular weight diluents.They are used to increase free volume and decrease T,g,.,Plasticizers:organic,high boiling point,weakly polar.,T,g,:-150 to 50,o,C.,Aromatic diesters,phosphates.,Example:PVC(polyvinyl chloride)T,g,=87,o,C.Plasticizer lowers T,g,and broadens transition region between glassy and rubbery region.,18,Effect of Copolymerization or Blending on T,g,The T,g,of a blend or a random copolymer can be estimated from the following equation where,D,C,p,refers to the change in heat capacity at the glass transition and M,i,is the mass fraction of component i;,This relation is often simplified to the,Fox equation,:,Both of these equations also apply to plasticizers which are small molecules added to polymers to reduce T,g,.,19,Glass Transition Temperature of Polymer Blends,20,Five Regions of Viscoelastic Behavior,Solid line:,amorphous polymer;,Dashed line:,semi-crystalline polymer;,Dotted line:,crosslinked amorphous polymer.,Glassy region,Glassy transition region,Rubbery plateau region,Rubbery flow region,Liquid flow region,21,Regions of Viscoelastic Behavior,Difference is in the range of,molecular motions,that are possible.,Glass region,:only vibrational or short range rotational motions(1 to 4 chain atoms involved).,Glass transition region,:onset of long range coordinated motions(10-50 chain atoms involved).,Rubbery plateau,:rapid coordinated motions of much of the chain:,(1)for semi-crystalline and crosslinked polymers the chain segments between crystallites or crosslinks move in a coordinated manner;,(2)for linear amorphous polymers the chain segments between entanglements can move.,Flow region,:only exists in the absence of crosslinks,entire chain can move in a coordinated fashion(reptation).,22,Effect of Molecular Weight on Length of Plateau,PB:Polybutadiene,The width of the plateau,is governed primarily by,the,molecular weight,.,The higher the molecular,weight,the longer is the,plateau.,When the molecular weight,is lower than No.4,it is,possible for the plateau to,disappear due to the lack,of“,entanglement,”.,23,Time-Temperature Superposition,Time-temperature equivalence,implies that the viscoelastic behavior can,be related to that at another temperature by a change in the time-scale only!,24,Amorphous Polymers,Temperature,Deformation,T,g,T,f,Glassy region,Glassy transition region,Rubbery plateau region,Rubbery flow region,Liquid flow region,Glassy region,Glassy transition region,Rubbery plateau region,Rubbery flow region,Liquid flow region,Different Polymer Species(Rubber or Plastics),25,Temperature(,o,C),Deformation,1,2,3,-100,0,100,Crystalline Polymers,26,Crystallinity:1,2 3 4;,Molecular weight:2 1,3,4,Deformation,Temperature,1,2,3,4,T,g,T,m,T,f,Crystallites,can be,considered as,fillers,to,increase,the elastic modulus and therefore to,decrease,the deformation when the same stress is applied.,Effect of Crosslinking Density,27,Deformation,Temperature,1,2,3,4,Crosslinking density:1 2 3 2 3,Plasticizers are to,decrease T,g,.,Plasticizers are low molecular weight diluents.They are used to increase free volume and decrease T,g,.,Plasticizers:organic,high boiling point,weakly polar.,T,g,:-150 to 50,o,C.,Aromatic diesters,phosphates.,Example:PVC(polyvinyl chloride)T,g,=87,o,C.Plasticizer lowers T,g,and broadens transition region between glassy and rubbery region.,Molecular Weight,29,Deformation,Temperature,1,2,3,4,5,6,7,Molecular weight:1 2 3 4 5 6 1.0,Slope 3.0,M,c,=2-3,M,e,Solid:experimental data;,Dotted:Theoretical predictions.,31,Polymer Dynamics and Rheology,(1)Unentangled Systems,a.Rouse theory,-“free-draining”,-bead and spring,-pure unentangled melt,DN,-1,t,N,2,h,N,(2)Entangled Systems,Reptation theory:Tube,DN,-2,t,h,N,3,Modification:Free volume theory(or iso-friction correction),Modification:Contour length fluctuation,Constraint Release.,What is Rheology Anyway?,Rhe-:,rei,Greek for“flow”;,-ology:“study of”.,“,What is Rheology Anyway?,”Faith A.Morrison.,The Industrial Physicist,10(2)29-31,April/May 2004,32,Rheology=the study of deformation and flow,“,The Deborah Number,”M.Reiner.,Physics Today,62,January 1964,Society of Rheology,Established in,1929,by,E.C.Bingham,panta rei,Everything flows,“panta rei”,33,Polymeric Materials Are Viscoelastic,Exhibit both viscous and elastic behavior,Elastic,-characteristic of solids,-react like a spring,stress immediately deforms to strain,-recovers original dimensions when stress removed,-strain is proportional to the applied stress,but independent of the rate of deformation,Viscous,-characteristic of liquids,-instant response to stress is flow,-tend to irreversible deformation,-stress is proportional to the rate of strain.,Classic elastic and viscous laws:,Hookean Law,：,Newtonian Law,：,Simplification:homogenous,isotropic,34,E:Elastic modulus;,G:Shear modulus.,h,:viscosity.,Hookean solid,Newtonian Liquid,35,“,Rheology:Principles,Measurements,and Applications,”,by Christopher W.Macosko.,At short times Silly Putty behaves like a,Hookean solid,but at long times it behaves more like a,Newtonian liquid,.,This behavior can be nicely modeled by a modulus that relaxes with time,G(t).,36,“,Rheology:Principles,Measurements,and Applications,”,by Christopher W.Macosko.,Mayonnaise spreads easily,at the high shear stress of the,moving knife,but left alone it will,sit in a lump on a piece of bread.,Hookean solid,Newtonian Liquid,Shear Thinning,Shear Thinning,37,Viscosity(,h,Pa.s,),Shear rate(,g,1/s,),tg1,.,.,tg1,.,.,Rod Climbing(Weissenberg Effect),38,Normal force,Rod Climbing,is not a subtle effect,as demonstrated on the cover by Ph.D.student Sylvana Garcia-Rodrigues from Columbia.Ms.Garcia-Rodrigues is studying rheology in the Mechanical Engineering Department at U.of Wisconsin-Madison,USA.,The Apparatus shown was created by,UW Madison Professors Emeriti John L.Schrag and Arthur S.Lodge.The fluid shown is a 2%aqueous polyacrylamide solution,and the rotational speed is nominally 0.5 Hz.Photo by Carlos Arango Sabogal(2006),Rod Climbing,39,(,N,):For the Newtonian fluids the surface near the rod is slightly,depressed,and acts as a sensitive manometer for the,smaller pressure near the rod,generated by centrifugal force.,(N),(P),(,P,):The Polymeric fluids exhibit an extra tension along the streamlines,that is,in the“,”direction.In terms of chemical structure,this extra tension arises from the,stretching,and,alignment,of the polymer molecules along the streamlines.The thermal motions make the polymer molecules act as small“rubber bands”wanting to snap back.,Phenomenological Interpretation,Application of Weissenberg Effect,41,Feed the polymer,Rotate,and mix,Extrude,the polymer,out from,the exit,Weissenberg effect occurs here,Die Swell(Barus Effect),42,“,Understanding Rheology,”by Faith A.Morrison.,d,j,/D1,Elastic recovery,As the liquid polymer exits,A die,the diameter of the liquid,Increases by up to an order,of magnitude.,It is caused by the relaxation,of extended polymer coils as,the stress in a polymeric liquid,reduces from,the high,flow-producing stresses,present within the die to,the low stresses,associated with the extruded stream moving through,ambient air.,Capillary Flow Instabilities,43,“,Understanding Rheology,”by Faith A.Morrison.,Shapes like shark-skin,bamboo,etc.,Shear:edge fracture(meniscus instability)when shear stress is 10,4,Pa.,Secondary Flow,44,Secondary flow,effects may occur when measuring low-viscosity fluids at high shear rates and can lead to turbulences.However,the advantage of secondary flow is to mix the polymers better in processing.,Ductless Siphon:Extensional Thickening45,“,Rheology:Principles,Measurements,and Applications,”,by Christopher W.Macosko.,Some,non-Newtonian fluids,can be stretched out of for short,times like,a solid,.,When a siphon is slowly,removed from a,Newtonian fluid,it breaks very close to the surface,but some,complex fluids,can be,pulled up many centimeters,above the free surface.,Polymeric and other complex,liquids can show,much higher,viscosities in extension than in,shear.,Applications:Fiber spinning and film forming,Flow-induced Change,Flow-induced crystallization,.,Associating polymer solutions.Micelles,.,Shear mixing and de-mixing.,46,Shear-induced mixing is interpreted as being a disruption of slightly entangled clusters under the influence of the shear energy.,Newtonian Fluids,47,Newtonian fluids,show the relationship between shear stress and shear rate and the fluids viscosity at a varying shear rate.Typical Newtonian fluids include water and thin motor oils.,So,at a given temperature the viscosity of a Newtonian fluid remains constant regardless of which viscometer model,spindle or speed is used to measure it.,The behavior of Newtonian liquids in experiments conducted at constant temperature and pressure has the following features:the only stress generated in simple shear flow is the shear stress,the two normal stress differences are zero the shear viscosity doesnt vary with shear rate;the viscosity is constant with respect to the time of shearing and the stress in liquid falls to zero immediately the shearing is stopped;the viscosities measured in different types of deformation are always in simple proportion to one another.,A liquid showing any deviation from the above features is non-Newtonian.,www.sju.edu/phabdas/physics/rheo.html,Non-Newtonian Fluids:Pseudoplastic,48,The,viscosity,of non-Newtonian fluids,changes,as the,shear rate is varied,.Thus,the parameters of viscometer model,spindle and rotational speed all have an effect on the measured viscosity.This measured viscosity is called apparent viscosity and is accurate when explicit experimental parameters are adhered to.There are several types of non-Newtonian flow behavior,characterized by the way a fluids viscosity changes in response to variations in shear rate.,www.sju.edu/phabdas/physics/rheo.html,Pseudoplastic:,Fluid displays a decreasing viscosity with an increasing shear rate,some examples include paints and emulsions.This type