Mechanical Behavior of Polymers Chapter 15 Part 2 Properties of Polymers Wide range of behaviors Elastic-Brittle (Curve A) Thermosets and thermoplastics Elastic-Plastic (Curve B) Thermoplastics Extended elasticity (Curve C) Elastomers Compared to metals Low elastic modulus Low strength High ductility under the right conditions Deformation Mechanisms Bond stretching Elastic deformation All types of polymers Chain uncoiling Elastic and plastic deformation Thermoplastics and elastomers Chain slipping Plastic deformation Thermoplastics Mechanical Behavior of Thermoplastics Tensile deformation Elastic deformation followed by plastic deformation Yield point phenomenon and a drop in strength upon initiation of plastic deformation when a neck forms The necked region is stronger than the rest of the sample The neck spreads with further deformation 1
Plastic Deformation of a Semicrystalline Polymer Mechanical Behavior of Thermoplastics Elongation of amorphous regions Separation into crystalline block segments Effect of temperature Over a very small range of temperature there is a change from a brittle material to a highly ductile material Example: PMMA Below the glass transition temperature T g elastic solid Well above T g viscous or liquid like behavior Intermediate temperatures viscoelastic behavior Tilting of crystalline regions Orientation of crystalline and amorphous regions Types of Deformation Applied load Elastic response Viscoelastic response Viscous response Load Relaxation Modulus (during viscoelastic deformation) A sample is extended in a testing machine to a strain ε 0 The length of the sample is held constant Over a period of time the load decreases (relaxes) due to molecular rearrangement The relaxation modulus is defined as the ratio between the load and strain, as the load relaxes σ ( t) E r ( t) = ε E r changes with both time and temperature When the material is relatively stiff (elastic), there is little change in load with time, and E r remains constant with time With increasing viscoelastic behavior, E r changes with time 0 Time 2
Viscoelastic Behavior Low T Viscoelastic Behavior High T Schematic variation of E r with time (log-log scale) Variation of E r for amorphous polypropylene showing five different regions of viscoelastic behavior Curve A: Crystalline isotactic polystyrene Curve B: Lightly crosslinked atactic polystyrene may not melt Curve C: Amorphous polystyrene Crystallization of Polymers Glass Transition Upon cooling from the liquid state, a thermoplastic polymer may Partly Crystallize Freeze to a glassy (amorphous) state Extent of crystallization depends upon the temperature at which the process occurs Nucleation and growth phenomenon Complete crystallization is never achieved As a fraction of the max. crystallization at a given temperature, amount of crystallization follows the Avrami equation y = 1 exp( kt n ) During cooling from a liquid Crystallline and semicrystalline solids exhibit a sudden decrease in specific volume at the melting point During glass transition, the specific volume decreases continuously, while the relaxation modulus increases Below T g, the material behaves like an elastic brittle glass In a glass, the molecular arrangement is random or amorphous Glass Semicrystalline solid Crystalline solid 3
Glass Transition Temperature Increases with increasing molecular weight See next slide Increases with bulky side groups PP versus PE Increases with polar atoms PVC versus PE Increases with double bonds and rings on the backbone PC versus PE Increases with decreasing number of side branches HDPE versus LDPE Mechanical Properties of Polymers Strength and elastic modulus depend upon the extent of inter-chain secondary bonding Since inter-chain sliding is the primary mode of plastic deformation in thermoplastics, factors that prevent this from happening also increase strength and stiffness Linear Polymer Branched Polymer Cross-linked Polymer 4
Strengthening Mechanisms Internal Mechanisms Up to a point, strength and stiffness increase with molecular weight of chains A TS TS = M n Degree of crystallization Inter-chain bonding increases with crystallization Pendant groups Bulky side groups Prevent sliding of chains Polar atoms Provide additional bonding between chains Strengthening Mechanisms Modification of main chain O, N, S atoms on the main chain Phenylene ring Pre-deformation (or drawing) Causes orientation of chains parallel to tensile deformation direction Heat treatment Annealing leads to increased crystallization Increased strength and stiffness, decreased ductility (effect is opposite to that in metals) Addition of reinforcements Fracture of Polymers Like metals, fracture can be brittle, ductile or a combination Thermosets Always brittle because the covalent network does not allow plastic deformation. Increasing temperature lowers the fracture strength Thermoplastics Below T g fracture is brittle Above T g fracture is ductile or ductile + brittle At slow deformation rates fracture is ductile since there is time for chain realignment Brittle Fracture of Polymers An amorphous glassy polymer, such as PMMA or Polystyrene, has a very high surface energy required to fracture much higher than that would be expected from simply breaking the C-C bond Crazing In regions of high stress, ahead of a crack tip, the formation of voids and alignment of chains absorbs a lot of energy. This results in high impact strength, 5
Fatigue Behavior of Polymers Natural rubber: cis-polyisoprene Produced from latex of Havea Brasiliensis tree The chains are periodically cross linked The chains uncoil when a stress is applied When the stress is removed, the cross-links provide a reference for the chains to re-coil The result is extensive elastic deformation Elastomers cis-polyisoprene trans-polyisoprene Vulcanization (Chemical Crosslinking) Vulcanization Vulcanization: Heating rubber with sulfur and lead carbonate A chemical method for creating additional cross links between chains Restricts molecular movement by crosslinking of molecules. Increases strength and stiffness Two sulfur atoms are required to link adjacent the carbon atoms two mers on adjacent chains The double bonds between carbon atoms on the main chain are replaced with covalent bonds connecting cross-linking sulfur atoms 6
Copolymerized Elastomers Copolymers of elastomers with hard components results in a physical cross-linking between chains Polystyrene-Butadiene or SBR or Buna S Other Elastomers Polychloroprene (Neoprene) Silicone Rubbers 7