Polymers. Representing Polymer Structures

Transcription

1 (Callister: Chapter 14, 15) Polymers Polymers are organic, chain molecules. They can vary from a few, to hundreds of thousands of atoms long. There are three classes of polymers that we will consider in turn: Thermoplastic: flexible linear chains Thermosetting: rigid, 3-D networks Elastomeric: linear, cross-linked chains ES 021 Polymers 1 Representing Polymer Structures Polymers can be represented by 3-D solid models 3-D space models 2-D models Complex structures are also often simplified. The benzene ring in polystyrene is represented by a hexagon with an inscribed circle. ES 021 Polymers 2

2 Polymer Molecules Before we discuss how the polymer chain molecules are formed, we need to cover some definitions: The ethylene monomer looks like: The polyethylene molecule looks like: Polyethylene is built up from repeat units or mers. Ethylene has an unsaturated bond (the double bond can be broken to form two single bonds. The functionality of a repeat unit is the number of sites at which new molecules can be attached. The functionality of ethylene is 2 ES 021 Polymers 3 Molecular Weight When polymers are fabricated, there will always be a distribution of chain lengths. The properties of polymers depend heavily on the molecule length. There are two ways to calculate the average molecular weight: 1. Number Average Molecular Weight An indication of the weight that is most common 2. Weight Average Molecular Weight An indication of the average weight of each chain in the sample. ES 021 Polymers 4

3 Molecular Weight Number Average Molecular Weight M n = xim i where: x i = number of chains in the i th weight range M i = the middle of the i th weight range Weight Average Molecular Weight M w = wi M i where: w i = weight fraction of chains in the i th range M i = the middle of the i th weight range ES 021 Polymers 5 Molecular Weight ES 021 Polymers 6

4 Degree of Polymerization The average length of a linear polymer is also expressed in terms of the degree of polymerization. n n M = m n n w = M m where m is the molecular weight of the mer (repeat unit) w From the last example for PVC, the repeat unit is: m = ( 3 1) + ( 2 12) + ( ) = ,150 23,200 nn = = 338 n w = = ES 021 Polymers 7 Molecular Shape The mechanical properties of a polymer are dictated in part by the shape of the chain. Although we often represent polymer chains as being straight, They rarely are. The carbon carbon bonds in simple polymers form angles of 109º ES 021 Polymers 8

5 Molecular Structure The mechanical properties are also governed by the structure of the polymer chains. They can be: Linear Branched Cross-linked Network (3D) ES 021 Polymers 9 Molecular Configurations Polymers that have more than one type of side atom or group can have a variety of configurations: Stereoisomerism: (Head to Tail) Isotactic: all on the same side Syndiotactic: alternating sides Atactic: random ES 021 Polymers 10

6 Molecular Configurations Geometrical Isomerism is possible in mer units that contain a double bond (e.g. isoprene) cis structure trans structure ES 021 Polymers 11 Copolymers Polymer chains do not have to be comprised of the same mer. Random It is often beneficial to have two different mers in the same chain Alternating The possible arrangements are called: Random copolymers Alternating copolymers Block copolymers Graft copolymers Block Graft ES 021 Polymers 12

7 Polymer Crystallinity Thermoplastic polymers go through a series of changes with changes in temperature. (Similar to ceramic glasses) In their solid form they can be semi-crystalline or amorphous (glassy). ES 021 Polymers 13 Thermoplastics Bonding along the backbone of the chain is covalent. In simple thermoplastic polymers, the chains are bound to each other by weaker Van der Waal s forces and mechanical entanglement. Therefore, the chains are relatively strong, but it is relatively easy to slide and rotate the chains over each other. ES 021 Polymers 14

8 Amorphous Thermoplastics Glassy Polymers (T < T g ) Below the Glass Transition Temperature, amorphous polymers are hard and brittle. Rubbery or Leathery Polymers (T g < T < T m ) Between Tg and Tm, when a stress is applied, the polymer deforms elastically and plastically at the same time. When the stress is removed the elastic deformation is recovered, but the polymer is permanently deformed because of the movement of the chains. Liquid Polymers (T > T m ) Bonds between chains are very weak Chains slide past each other with almost no force ES 021 Polymers 15 Polymer Crystallinity Similar to metals and ceramics, polymers can be made to exhibit some long-range order (crystallize). For polymers to crystallize, entire chains must be ordered. ES 021 Polymers 16

9 Crystalline Thermoplastics Linear polymers never completely crystallize. However, some polymers partially crystallize. Neighbouring chains become aligned and fold back on themselves to form thin plates. These plates are connected to each other by amorphous chains and often form spherulites. ES 021 Polymers 17 Crystalline Thermoplastics The ability of a polymer to crystallize is affected by: 1. Complexity of the chain: Crystallization is easiest for simple polymers (e.g. polyethylene) and harder for complex polymers (e.g. with large side groups, branches, etc.) 2. Cooling rate: Slow cooling allows more time for the chains to align 3. Annealing: Heating to just below the melting temperature can allow chains to align and form crystals 4. Degree of Polymerization: It is harder to crystallize longer chains 5. Deformation: Slow deformation between T g and T m can straighten the chains allowing them to get closer together. ES 021 Polymers 18

10 Deformation of Polymers Polymers can be: A. Elastic Brittle B. Elastic Plastic C. Highly Elastic ES 021 Polymers 19 Deformation of Amorphous Polymers Deformation of thermoplastic polymers is more complicated than in metals or ceramics It depends on the applied stress AND the strain rate ES 021 Polymers 20

11 Deformation of Amorphous Polymers Elastic deformation is the result of: stretching the covalent bonds of the chain and, the rotation of the curved chains On unloading: the strain due to stretching the chains is recovered immediately the strain due to the rotation of the chains takes longer (sometimes much longer) ES 021 Polymers 21 Deformation of Amorphous Thermoplastics Plastic deformation in polymers does not occur by the motion of dislocations Deformation is accomplished by the motion of polymer chains relative to each other If the polymer is not brittle, after yielding the chains begin to disentangle, straighten and slide past each other. Necking begins almost immediately after yield, BUT this is different than necking in metals. ES 021 Polymers 22

12 Necking in Amorphous Thermoplastics A neck occurs in amorphous thermoplastics as the polymer chains align. Instead of being weaker because of the smaller cross-sectional area, the neck is stronger because the chains are aligned and we are pulling on the now stronger van der Waal s bonds and the covalent bonds of the backbone ES 021 Polymers 23 Deformation of Semi-Crystalline Thermoplastics Elongation occurs first in the amorphous regions between crystalline lamellae. Crystalline lamellae begin to align in the direction of the applied stress and then break into smaller segments. ES 021 Polymers 24

13 Deformation of Semi-Crystalline Thermoplastics Under an applied stress, small voids can open up in bands perpendicular to the applied load. This is known as crazing. In a transparent materials, these voids can appear as an opaque band through the material. A craze is not a crack and can still support a stress. As the voids elongate, the ligaments between them are subjected to higher stresses and eventually fracture causing the craze to grow. Eventually, the crack grows to a size that causes rapid fracture. ES 021 Polymers 25 Time Dependent Deformation We ve already said that deformation is accomplished by the motion of polymer chains relative to each other. Sliding polymer chains past each other takes some time. If we apply the load slowly, the chains will slide easily If the load is applied quickly, the chains do not have time to slide and the polymer behaves in a brittle manner This is viscoelastic behaviour. At high temperatures or low strain rates More Ductility At low temperatures or high strain rates Less Ductility ES 021 Polymers 26

14 Time Dependent Deformation Creep and Stress-Relaxation are time dependent phenomena. Creep: change in length at constant load Stress-Relaxation: change in load at constant extension Creep Stress Relaxation Strain Stress Time Time ES 021 Polymers 27 Time Dependent Deformation The rate at which the stress decreases in the stress-relaxation process is given by: σ = σ exp 0 ( t ) λ where: λ is the relaxation time (a material property) σ 0 is the initial stress ES 021 Polymers 28

15 Stress-Relaxation Example A stress of 17 MPa is applied to a polymer serving as a fastener in a complex assembly. At a constant strain, the stress drops to 16.5 MPa after 100 hours. If the stress on the part must remain above 14.5 MPa in order for the part to function properly, determine the life of the assembly. σ = σ exp 0 ( t ) λ Find the relaxation time, λ lnσ = lnσ 0 t λ lnσ lnσ 0 = t λ t λ = lnσ lnσ λ = 0 100hrs = 3350hrs ln16.7 ln17 Time to reach σ = 14.5 MPa ( lnσ lnσ ) t = λ t = 3350 t = 532hours t 22.2 days ( ln14.5 ln17) ES 021 Polymers 29 0 Temperature Dependence of Deformation Increasing Temperature: Decreases Young s Modulus Decreases the Yield Strength Increases Ductility Why? van der Waal s bonds are weak and easy to melt Energy Greater separation / C kt ES 021 Polymers 30

16 Viscoelastic Behaviour For the load profile shown in a), the strain response can be: Elastic Viscous Viscoelastic ES 021 Polymers 31 Relaxation Modulus The Viscoelastic Relaxation Modulus is another way to characterize the stress relaxation behaviour of a polymer. It is a time-dependent Young s Modulus Remember that a stress relaxation test measures stress vs. time at a constant strain The relaxation modulus is calculated as: () t E r = σ () t ε Stress σ 0 σ 1 σ 2 With time, the apparent stiffness of the material changes σ 3 t 0 t 1 t 2 t 3 Time ES 021 Polymers 32

17 Controlling the Strength of Thermoplastics Plastic deformation in thermoplastics is due to the rotation and sliding of chains over each other. To increase the strength of a thermoplastic, we have to make it harder for the chains to move. There are essentially three ways that we can control this: 1. Alter the length of the chains 2. Change the strength of the bonds within the chains 3. Change the strength of the bonds between the chains ES 021 Polymers 33 Chain Length Controlling the Strength of Thermoplastics If the polymer chains are made longer (increased degree of polymerization), they become more tangled and harder to pull apart. The strength is increased. Material Low Density Polyethylene (LDPE) DoP 7,000 MW 200,000 Tensile Strength 20 MPa Elongation 800% High Density Polyethylene (HDPE) 18, , MPa 130% ES 021 Polymers 34

18 Controlling the Strength of Thermoplastics Forces Between Chains In addition to the molecular weight, the degree of crystallinity influences the strength of polymers Higher crystallinity means more chains are aligned in crystals Stronger van der Waal s forces. ES 021 Polymers 35 Controlling the Strength of Thermoplastics Forces Within Chains Chains formed from more complex monomers often have much stronger bonds and resist rotation and sliding Higher bond strength also means: Material Low Density Polyethylene (LDPE) Tensile Strength 20 MPa Elastic Modulus 270 MPa Repeat Unit Polyimide (PI) 113 MPa 2000 MPa Polyetheretherketone (PEEK) 68 MPa 3660 MPa ES 021 Polymers 36

19 Controlling the Strength of Thermoplastics Forces Between Chains Changing some or all of the side groups in the polymer chain can alter the bond strength between chains and make it physically harder for the chains to rotate, disentangle and slide over each other. Material Low Density Polyethylene (LDPE) Polystyrene (PS) Tensile Strength 20 MPa 53 MPa Elongation 800% 60% Repeat Unit Polyvinyl Chloride (PVC) 60 MPa 100% Polymethylmethacrylate (PMMA) 80 MPa 5% ES 021 Polymers 37 Controlling the Strength of Thermoplastics Forces Between Chains Polymer chains with many side branches can not pack as tightly together and, therefore, the bonds between chains are not as strong. ES 021 Polymers 38

20 Controlling the Strength of Thermoplastics Forces Between Chains Tacticity: For polymers with nonsymmetrical repeat units, the location of the non-symmetrical side group affects the properties Atactic: least regular, poor packing, low strength and stiffness Syndiotactic: Regular alternation of the side groups promotes close packing and crystallization. Isotactic: Also favours easy crystallization. Chains form helices that nest together. ES 021 Polymers 39 Elastomers Common elastomers are made from highly coiled, linear polymer chains. In their natural condition, elastomers behave in a similar manner to thermoplastics (viscoelastic) i.e. applying a force causes the chains to uncoil and stretch, but they also slide past each other causing permanent deformation. This can be prevented by cross-linking the polymer chains ES 021 Polymers 40

21 Elastomers The process of vulcanization uses sulfur atoms to form crosslinks between the chains As more sulfur is added, more cross-links are formed and the elastomer becomes more rigid Vulcanized rubber is not recyclable (into new rubber) All deformation in elastomers is recoverable ES 021 Polymers 41 Thermoplastic Elastomers Thermoplastic Elastomers are a special class of elastomers. They are a combination of hard and soft mers (a block copolymer) Typically styrene ends and butadiene or isoprene middles. On cooling, the hard styrene ends tend to condense into hard zones These hard zones prevent chain sliding, but still allow the elastomeric middles to uncoil. They do not employ covalent crosslinks to maintain their shape They are recyclable ES 021 Polymers 42

22 Thermosetting Polymers Thermosets are very highly cross-linked polymers that form a three-dimensional network. Chains are prevented from moving by the cross-links. Strength, stiffness, and hardness are high ductility and impact properties are low Examples: epoxies, phenolics They are often formed from one or more linear polymers. The application of heat / pressure or even just mixing the components begins the cross-linking reaction. Cross-linking is not reversible. Once formed, thermosets cannot be reformed. ES 021 Polymers 43 Addition Polymerization Addition polymerization is the mechanism of growing polymer chains by adding individual repeat units to the end. Polyethylene is made this way. An initiator is used to start the chain. Additional repeat units add on to increase the length of the chain. Growth is rapid at first and slows as polymerization nears completion because the diffusion distances for the remaining mers becomes longer. ES 021 Polymers 44

23 Addition Polymerization The growing chains can be terminated in two ways: Two growing chains connect (Combination) An active chain removes a hydrogen atom from a second chain (Disproportionation) ES 021 Polymers 45 Condensation Polymerization Linear molecules can also be formed by condensation polymerization. This is a step process involving at least two different monomers. In both of the examples below, both monomers are bifunctional and therefore the chains can grow in both directions ES 021 Polymers 46

24 Polymer Additives In addition to the basic mers, various other components can be added to polymers to alter their properties Fillers Plasticizers Stabilizers Colorants Flame Retardants ES 021 Polymers 47 Thermoplastics Forming Polymers ES 021 Polymers 48

25 Forming Polymers Thermosets Compression Molding Transfer Molding ES 021 Polymers 49