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Lab 4: Polymers

July 1 2000

1)         During crystallization the chains become closely aligned over an appreciable distance.  This causes a sharp increase in density as the coiled intertwined chains are arranged into a more orderly, close-packed structure.  Crystallization causes the polymer to become more hard, brittle, and glasslike.  Figure 1 show the stress-strain graph for high density PE and low density PE.  The high density PE represents a polymer that has undergone crystallization.  High density PE has a higher yield point than low density PE, which indicates it is stronger.

The type of monomer that makes up the chains in the polymer influence the bonding between chains and the ability of the chains to rotate or slide past one another when a stress is applied.  Polyethylene is comprised of repeating units of ethylene (C2H4) which does form strong polar bonds between chains.  As a result sliding of chains is easy and the polymer has low strengths.

Plasticizers are low-weight -molecular chains that reduce the glasss transtion temperature (Tg) and provide internal lubrication between the chains of a polymer. This addition improves the ductility of the polymer.  PVC has a glass transition temperature above room temperature, plasticizers are added to it to reduce the Tg and make it more ductile and formable at room temperature.

2)         When an external force is applied to a thermoplastic, both elastic and plastic deformation occurs.  The mechanism by which it defoms depends on how the polymer chains move.

In elastic deformation, an applied stress causes the covalent bonds within the chain to stretch and distort, elongating the chain elastically.  However in plastic deformation the move, rotate and disentangle form one another.

In the elastic deformation of polymers, an applied stress causes the covalent bonds within the chain to stretch and distort, allowing the chains to elongate elastically.  And when the stress is removed the chains go back to their original position.  A polymer experiences plastic deformation when the applied force exceeds the yield strength.  Plastic deformation results as the chins rotate, slide, and disentangle.  Both elastic and plastic deformation is depended on temperature.  When a polymer is at is glass temperature it will exhibit a greater

The ability of a stress to cause chain slippage and plastic deformation is related to time and strain rate.  If a stress is applied slowly (low strain rate) the chain slide past one another easily.  If a stress is applied rapidly (high strain rate) sliding of the is limited.

3) The ability of a stress to cause chain slippage and plastic deformation is related to time or the strain rate.  If a stress is applied slowly (low strain rate) the chains slide easily past one another.  However, if the stress is applied rapidly (high strain rate) there isnít enough time for the chains to slide and disentangle from one another for the polymer to deform.  Figure @#$@#$ shows the stress-strain graph for the same polymer but with a different strain rate.  The polymer tested with a crosshead speed of 750 mm/min behaved like a solid material.  It had a higher yield point than the polymer tested with a crosshead speed of 75 mm/min.

 

The polymer tested with the lower cross head speed behaved more as a viscous liquid.  As a result it showed a lower yield strength and a lower modulus of elasticity, with only a small portion of the graph having a direct relationship between stress and strain.  

 

4)         In a thermoplastic polymer weak secondary bonds exist between polymer chains.  When a stress is applied, the weak bonding is overcome and the chains can move and rotate.  The point where the bonds are overcome is related to temperature.

At or above the melting point the bonding within the chains is weak, and a low force is required to cause the chains to move and deform.  At this temperature the polymer has virtually no elastic strain, and the modulus of elasticity is nearly zero.

At or below the glass transition temperature the polymer is crystallising and aligning the chains close together, making the bonds stronger.  At this temperature the polymer becomes more hard and brittle and gains strength and stiffness, but loses ductility.   Figure x show the stress-strain graph for two samples of low density PE placed under different temperatures.  The sample exposed to the cold temperature had a relatively higher yield point with very little strain, when compared to the PE at room temperature.  This shows that PE at a colder temperature is brittle and strong.  Generally as the temperature gets lower a polymer displays increased strength but reduced ductility.

 

In  a thermoplastic polymer, bonding within the chain is covalent, but the chains are also held together by weak van der wall forces. When the stress is applied tp the thermoplastic, the weak bonding between the chain is overcome and the chains can rotate and slide.  The ease at with chains slide is dependent on  the temperature.

At or above the melting temperature the bonding between the chains are weak.  An applied force causes the chains to slide past one another with little resistance.  At this state the polymer has virtually no elastic strin, and the modulus of elasticity is nearly zero.

Below the melting temperature the bonding between the chian is slightly stronger, and as a result the polymer behaves in a rubbery manner.

Below the glass transition temperature the polymer becomes slightly crystalline with the chains becoming closely aligned.  This causes an increse in density as the coild and intertwined chains are rearranged into a more ordely close-packed structure which is more hard and glasslike.  Below the glass transion temperatur the density and ant modulus of elasticity change giving the polymer good strength, stiffness, and creep resistance, but poor ductility.

5) In thermoplastics polymer chains are held together by physical attraction caused by van der wall forces. When a thermoplastic is melted the van der wall forces are very weak, but as the polymer is cooled and recrystallised, the chains become closer, and the van der wall forces get stronger. Thus the polymer retains its intiall characteristics.
Thermosettings are highly cross-linked polymer chains that form a three-dimensional network structure. The cross-linking of the polymers is the result of a chemical reaction between the chains at certain conditions such as heat and pressure. This reaction is not reversible; once formed the thermoset cannot by recycled.