Chapter 15. Polymers. Characteristics, Applications and Processing

Stress-Strain Behavior
The description of stress-strain behavior is similar to that of metals, but a very important consideration for polymers is that the mechanical properties depend on the strain rate, temperature, and environmental conditions.
The stress-strain behavior can be brittle, plastic and highly elastic (elastomeric or rubber-like), . 1. Tensile modulus (modulus) and tensile strengths are orders of magnitude smaller than those of metals, but elongation can be up to 1000 % in some cases. The tensile strength is defined at the fracture point and can be lower than the yield strength.
Mechanical properties change dramatically with temperature, going from glass-like brittle behavior at low temperatures (like in the liquid-nitrogen demonstration) to a rubber-like behavior at high temperatures.
In general, decreasing the strain rate has the same influence on the strain-strength characteristics as increasing the temperature: the material becomes softer and more ductile.

Deformation of Semicrystalline Polymers
Many semicrystalline polymers have the spherulitic structure and deform in the following steps :
· elongation of amorphous tie chains
· tilting of lamellar chain folds towards the tensile direction
· separation of crystalline block segments
· orientation of segments and tie chains in the tensile direction
The macroscopic deformation involves an upper and lower yield point and necking. Unlike the case of metals, the neck gets stronger since the deformation aligns the chains so increasing the tensile stress leads to the growth of the neck..

Factors that Influence the Mechanical Properties of Polymers
The tensile modulus decreases with increasing temperature or diminishing strain rate.
Obstacles to the steps mentioned in 16.4 strengthen the polymer. Examples are cross-linking (aligned chains have more van der Waals inter-chain bonds) and a large mass (longer molecules have more inter-chain bonds). Crystallinity increases strength as the secondary bonding is enhanced when the molecular chains are closely packed and parallel. Pre-deformation by drawing, analogous to strain hardening in metals, increases strength by orienting the molecular chains. For undrawn polymers, heating increases the tensile modulus and yield strength, and reduces the ductility - opposite of what happens in metals.

Crystallization, Melting, and Glass Transition Phenomena
Crystallization rates are governed by the same type of S-curves we saw in the case of metals. Nucleation becomes slower at higher temperatures.
The melting behavior of semicrystalline polymers is intermediate between that of crystalline materials (sharp density change at a melting temperature) and that of a pure amorphous material (slight change in slope of density at the glass-transition temperature). The glass transition temperature is between 0.5 and 0.8 of the melting temperature.
The melting temperature increases with the rate of heating, thickness of the lamellae, and depends on the temperature at which the polymer was crystallized.
Melting involves breaking of the inter-chain bonds, so the glass and melting temperatures depend on:
· chain stiffness (e.g., single vs. double bonds)
· size, shape of side groups
· size of molecule
· side branches, defects
· cross-linking
Rigid chains have higher melting temperatures.

Thermoplastic and Thermosetting Polymers
Thermoplastic polymers (thermoplasts) soften reversibly when heated (harden when cooled back)
Thermosetting polymers (thermosets) harden permanently when heated, as cross-linking hinder bending and rotations. Thermosets are harder, more dimensionally stable, and more brittle than thermoplasts.

Viscoelasticity
At low temperatures, amorphous polymers deform elastically, like glass, at small elongation. At high temperatures the behavior is viscous, like liquids. At intermediate temperatures, the behavior, like a rubbery solid, is termed viscoelastic.
Viscoelasticity is characterized by the viscoelastic relaxation modulus
Er = s(t)/e0.
If the material is strained to a value e0.it is found that the stress needs to be reduced with time to maintain this constant value of strain (see figs. 16.11 and 16.12).
In viscoelastic creep, the stress is kept constant at s0 and the change of deformation with time e(t) is measured. The time-dependent creep modulus is given by
Ec = s0/e(t).

Deformation and Elastomers
Elastomers can be deformed to very large strains and the spring back elastically to the original length, a behavior first observed in natural rubber. Elastic elongation is due to uncoiling, untwisting and straightening of chains in the stress direction.
To be elastomeric, the polymer needs to meet several criteria:
· must not crystallize easily
· have relatively free chain rotations
· delayed plastic deformation by cross-linking (achieved by vulcanization).
· be above the glass transition temperature

Fracture of Polymers
As other mechanical properties, the fracture strength of polymers is much lower than that of metals. Fracture also starts with cracks at flaws, scratches, etc. Fracture involves breaking of covalent bonds in the chains. Thermoplasts can have both brittle and ductile fracture behaviors. Glassy thermosets have brittle fracture at low temperatures and ductile fracture at high temperatures.
Glassy thremoplasts often suffer grazing before brittle fracture. Crazes are associated with regions of highly localized yielding which leads to the formation of interconnected microvoids . Crazing absorbs energy thus increasing the fracture strength of the polymer.

Miscellaneous Characteristics
Polymers are brittle at low temperatures and have low impact strengths (Izod or Charpy tests), and a brittle to ductile transition over a narrow temperature range.
Fatigue is similar to the case of metals but at reduced loads and is more sensitive to frequency due to heating which leads to softening.

Polymerization
Polymerization is the synthesis of high polymers from raw materials like oil or coal. It may occur by:
· addition (chain-reaction) polymerization, where monomer units are attached one at a time
· condensation polymerization, by stepwise intermolecular chemical reactions that produce the mer units.

Elastomers
In vulcanization, crosslinking of the elastomeric polymer is achieved by an irreversible chemical reaction usually at high temperatures (hence ‘vulcan’), and usually involving the addition of sulfur compounds. The S atoms are the ones that form the bridge cross-links. Elastomers are thermosetting due to the cross-linking.
Rubbers become harder and extend less with increasing sulfur content. For automobile applications, synthetic rubbers are strengthened by adding carbon black.
In silicone rubbers, the backbone C atoms are replaced by a chain of alternating silicon and oxygen atoms. These elastomers are also cross-linked and are stable to higher temperatures than C-based elastomers.