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1、Chapter VI The mechanical properties of polymers,Section I OverviewSection II High elasticitySection III ViscoelasticitySection IV Ultimate mechanical behavior(yield,destruction and Strength),Section I Overview,1-1 Classification of the mechanical properties 1-2 The basic physical quantities charact
2、erizing the mechanical properties 1-3 Characteristics of the mechanical properties of polymers,1-1 Classification of the mechanical properties,The mechanical properties are the base of excellent physical properties of polymer.E.g.:one polymer has good friction and wear(磨耗)performance,but with poor m
3、echanical properties,very crisp,can not be used for anti-friction(减摩)material.E.g.:the polymer for wire insulation,also requires certain mechanical properties:strength and toughness.If it is folded a few times,it will be broken.Then even if the electrical insulation is good,but can not be used as wi
4、re insulation.,Common terms:Mechanical behavior:refers deformation responsed to an external force applied to the material.Deformation properties:mechanical behavior under non-extreme conditions.Fracture properties(断裂性能):mechanical behavior under extreme conditions.Elasticity:For an ideal elastic bod
5、y,its elastic deformation can be expressed by Hookes law,namely:stress(应力)is directly proportional to strain(应变)and the strain is time-independent.,Viscosity:in the external force,the displacement occurs between the molecules,deformation of the ideal flow of viscous fluid can be described by Newtons
6、 law:stress is directly proportional to the strain rate.General Elasticity(普弹性):in large stress,only produced small,linear reversible deformation,it is caused by the change of the bond lengths and bond angles.Related to change of internal energy in materials:internal energy increases when the deform
7、ation,deformation recovery,release energy,work to outside(glassy,crystalline,polymers,metals,ceramics have such a performance).General elasticity also known as energy elasticity.,High elasticity:a great reversible deformation can occur under a small stress,caused by internal conformational entropy c
8、hange,so also known as entropy elasticity(rubber with high elasticity).Static mechanical properties:the mechanical behavior of constant stress or constant strain.Dynamic mechanical properties:viscoelastic behavior of the object in the alternating stress.Stress relaxation:in the case of constant stra
9、in,stress change with time.,Creep(蠕变):Under the constant stress,the deformation of an object changes over timeStrength:The stress materials can withstand Toughness(韧性):The absorbed energy when materials fracture,1-2 The basic physical quantities characterizing the mechanical properties,1-3 Character
10、istics of the mechanical properties of polymers,1Polymer material have the widest mechanical properties within all the known material,including liquids,soft rubber to hard solids,a variety of polymers responses varied widely for mechanical stress,for example:,PS products is very brittle,broken on a
11、knock(brittle)Nylon products are tough and difficult to deformation,are not easy to break(toughness)Slightly cross-linked rubber stretching,it can stretch several times,releasing force can almost restitution(elasticity)After deformation,clay(胶泥)remains completely new shape(viscosity)These diversitie
12、s of polymer mechanical properties provide a broad choice as different applications.,2 Most outstanding charcteristic of mechanical properties of polymers is flexibility and high viscoelasticity.,The high elasticity of polymer:due to a great molecular weight,polymer chains have many different confor
13、mations,and the changes of conformation lead to polymer chains have their unique flexibility.Chain flexibility is the essence of polymer high elasticity.The difference from the general elasticity is the conformation change:when deformation,the conformation entropy decreases,increases during recovery
14、.Internal energy is not the main role in the high elastic deformation(but it is a major cause of general elastic deformation).,(2)The visco-elastic polymer:refers to that the polymer material not only has the general characteristics of elastic materials,but also has some characteristics of viscous f
15、luid.Elasticity and viscosity are showing simultaneously,which is particular characteristic of polymer materials.The visco-elastic of polymer is shown in its outstanding mechanical relaxation phenomenon.In the study of its mechanical properties,the relationship among the stress,strain and time must
16、be considered.Temperature is also very important factor on mechanical properties.,Describe the viscoelastic mechanical behavior of polymer materials must consider four parameters simultaneously-the stress,strain,time and temperature.The mechanical properties of polymer materials on time and temperat
17、ure dependence are the focus to understand its mechanical properties,and also must pay great attention for testing and using.,Section II High elasticity,2-1 Characteristics of high elasticity2-2 Thermodynamic analysis of high elasticity in the equilibrium2-3 The use temperature of rubber,2-1 Charact
18、eristics of high elasticity,High-elastic state is a mechanical state unique to polymer based on segment motion,under certain conditions through the polymer glass transition,to achieve it.The polymer in the high-elastic state exhibits unique mechanical properties-high elasticity.This is a very excell
19、ent performance in a polymer.,Rubber is highly elastic material,high flexibility characterized by:large elastic deformation,up to 1000%,while the elastic deformation of metallic materials does not exceed 1%elastic modulus is small,but it increases with the absolute temperature increase;and elastic m
20、odulus of metallic materials reach,and it decreases with the absolute temperature increase.,In the fast tension(adiabatic(绝热)process),the polymer temperature rise;and metal temperature decreases.If you stretch the rubber sheet,stick it on lips or cheeks,you will feel heat when the rubber elongation.
21、absorbs heat when retraction.Deformation depends on time,the rubbers compression or tension by the external force(constant stress),deformation always evolve over time,and finally reaches the maximum deformation,the phenomenon called creep(蠕变).,Reason:Because the rubber is long-chain molecules,the mo
22、lecular movement must overcome intermolecular forces and internal friction.High-elastic deformation is originated from molecular chain movement.It may take a few minutes,hours or even years for the molecular chain from one equilibrium state to another equilibrium state that adapts the external force
23、.Under normal circumstances,that is saying the deformation always occurs delaying to the force,so the rubber deformation needs time.,2-2 Thermodynamic analysis of high elasticity in the equilibrium,High-elastic deformation can be divided into equilibrium deformation(reversible)and non-equilibrium de
24、formation(irreversible).Assuming high-elastic deformation of rubber occurs when stretching,when remove the external forces,it can be fully reinstated,and that is to say,the deformation is reversible,so the first law of thermodynamics and the second law can be used to analyze.,Physical meaning:extern
25、al force in the rubber,with the rubber stretching,on the one hand,the internal energy change,on the other hand,the rubber entropy changes.Or:the rubber tension is due to the internal energy and entropy change when deformation.,This is the thermodynamic equation of rubber.In experiments,is the vertic
26、al axis,T is the abscissa axis,mapping:,Intercept;slope.发现各直线外推到 时均通过原点，即截距为0,77%,33%,11%,4%,固定拉伸时的张力温度曲线,So：So when the rubber is tensioned,the internal energy almost does not change,but cause the entropy changes.That is to say,under the external force,the rubber molecular chains change from twiste
27、d disordered state to the straight orderly state.Entropy changes from large to small,order to disorder,final state is an unstable system.After the external force is removed,it will spontaneously revert to the initial state,rubber from stretched state to the original state is the entropy increasing p
28、rocess(spontaneous process),also explains why the high-elastic deformation is reversible.,As in tension,the entropy decrease,is negative,so should also be negative,indicating why release heat in the stretching process.As the ideal elastomer tension only cause entropy change or only the entropy chang
29、e contributes to the ideal elastomer flexibility,also known as entropy elasticity(熵弹性).,2-3 The use temperature of rubber,Higher than a certain temperature,the rubber loses its elasticity due to aging;less than a certain temperature,the rubber loses its elasticity as the glass transition.How to impr
30、ove the heat resistance and cold resistance of rubber?It is very important to expand its temperature range.,I Improve aging resistance,improved heat resistance Vulcanized(硫化)rubber has cross-linked network structure,unless the molecular chains breaks or cross linked chains destruction,it would not f
31、low,heat resistance of the vulcanized rubber appears good.However,in fact,vulcanized rubber at 120 has beendifficult to maintain their physical and mechanicalproperties,170 180 have already lost value,why?The main chain of rubber contains a large number ofdouble bonds,susceptible to oxygen damage an
32、dpyrolysis,the-methylene hydrogen next to the doublebond is easily oxidized and degraded or crosslinked.,In order to improve its aging resistent properties that we take1 Change the main chain structure of rubber(1)The main chain does not contain double bonds(2)The main chain has less butyl rubber wi
33、th double bonds(isobutylene and isoprene),(3)Polysulfide rubber with S atoms on main chain(4)polyether rubber containing O atom on the main chain(5)dimethyl silicone rubber,the main chain are non-carbon atoms,2.Change the structure of substituentsThe rubber with electron donor substituents easily ox
34、idized:natural rubber,styrene butadiene rubber.The rubber with electron withdrawing substituents is not easy oxidized:chloroprene rubber(氯丁橡胶),fluorine rubber,3 Change the structure of cross-linked chainsPrinciples:cross-linked chains with less sulfur have great bond energy,good heat resistance.If t
35、he crosslink bond is CC or CO,bond energy is greater,and heat resistance is better.(The Chloroprene rubber vulcanized with ZnO,cross-linked chains is the C-O-C-,natural rubber cross-linked by peroxide or radiation,cross-linked chains the-C-C-),2 Lower to avoid crystallization and improve cold resist
36、ance The reason of lack of cold resistance is due to glass transition or crystallization at low temperatures,which leads to the rubber brittle and harden,loss of elasticity.The reason of polymer glass transition is the molecules close to each other,strengthening the intermolecular forces,so that the
37、 movement of chain segments are frozen.,any measures to increase the activity of molecular chain,weaken the interaction between molecules will cause decline any measures to reduce crystallization ability and crystallization rate of polymer will increase the flexibility of the polymer,increase resist
38、ance to cold(because crystallization is regular arrangement of polymer chains or chains segments.It will greatly increase the interaction force between molecules,increase the polymer strength and decrease elasticity),1 plasticizers:the weakening of intermolecular forces 如氯丁胶-45，加葵二酸二丁酯（-80）可使其的-62；如
39、用磷酸三甲酚酯（-64）可使其-57。Plasticization effect not only relates to the plasticizer structure,but also relates to itself.The Tg of plasticizer lower,the of plasticized polymer is also lower.,Attention to the side effects of plasticizers It increases the molecular chains mobility,and also create the conditi
40、ons for forming crystalline structure,so using the plasticizer to reduce,we must also consider the possibility of crystal formation.,2.The copolymerization Polystyrene has a large side base,so rotation is difficult in the main chain,more rigid,higher than room temperature,but the copolymer of styren
41、e-butadiene rubber is-53 Polyacrylonitrile have polarity,so rotation difficult in the main chain,more rigid,higher than room temperature,using butadiene and acrylonitrile copolymer,the NBR(丁睛橡胶)to-42,3.Reduce the ability of polymer crystallization Linear polyethylene molecular chain is very flexible
42、,very low,but because of high regularity,so crystallization.Polyethylene can not use as the rubber.The introduction of smaller non-polar methyl substituents to disrupt the regularity of molecular chain of polyethylene,then destruct their crystallinity,which is ethylene and propylene copolymer rubber
43、=-60.,Destroying the regularity of the chain to reduce the ability of polymer crystallization,improving flexibility,but the side effects are detrimental to strength.,Section three Viscoelasticity,3-1 relaxation phenomenon3-2 Creep3-3 Stress relaxation3-4 Hysteresis3-5 Mechanical loss 3-6 Method of m
44、easuring viscoelasticity 3-7 Viscoelastic model3-8 Relationship of viscoelasticity and time,temperature(time-temperature equivalence)3-9 Boltzmann principle of superposition(迭加),3-1 Polymer mechanical relaxation phenomenon,Mechanical relaxation-the mechanical properties of polymers change with time
45、The most basic are:CreepStress relaxation Hysteresis(滞后)Mechanical loss,ideal elastic body subject to external forces,the balance deformation achieved instantaneous,strain is proportional to the stress,deformation is time-independent ideal viscous body subject to external forces,the deformation is l
46、inear development over time,the strain rate is proportional to the stress polymer deformation relate to time,this relationship between the ideal elastomer and the ideal viscous body,which means that both strain and strain rate are relate to the stress,therefore polymers are often called as viscoelas
47、ticity materials.,3-2 Creep,Creep:at a certain temperature and constant external force(tension,pressure,torsion(扭力),etc.),the material deformation increases with time gradually increasing.Creep process includes the following three deformations:General elastic deformation,high-elastic deformation,vis
48、cous flow,General elastic deformation Polymer materials under external force,bond lengths and angles within the molecular chain immediate change,deformation is small,after the removing of external forces,general elastic deformation immediately fully restored,independent of time.,Schematic diagram,Hi
49、gh-elastic deformationA process of molecular chain through segment movement has gradually extended,the deformation is much greater than the general elastic deformation,deformation is exponential relationship with time.When remove external force,the high-elastic deformation gradually restored.,Schema
50、tic diagram,Viscous flow Intermolecular non-cross-linked linear polymer,it will produce a relative slip between the molecules,it has linear relationship with time,after the removing of external forces,viscous deformation can not be restored,is the irreversible deformation.,Schematic diagram,Polymer
