Polymers Flashcards
What is a Polymer?
consist on macromolecules. Large molecular chains which the atoms are held together by covalent bonds (bonds between the different chains are much weaker)
Mention the 3 different types of polymers
Thermoplastic, Elastomers (rubbers) and Thermoset (duromers)
What is a crosslink and the difference between the polymers
Crosslink = crucial fix chains relative to each other and thus render it impossible to draw out a single molecular chain. Thermplastic = NO crosslinks (have just weak links) Elastomers = Small # of cross links. Thermoset/duromers = High # of crosslinks
Mentions some characteristics of Thermoplastic
- Molecules are not chemically joined together.
- Have secondary bonds/intermolecular (van der waals)
- With heat, the 2ndary bonds broken and molecules move.
- Can be heat-softened , melted and reshaped as many times.
Mention some characteristics of Elastomers and thermosets
Are always completly amorphous because the chemical bonds make a regular arrangement of the molecular chains impossible.
- chemically joined together by crosslinks. (rigid 3D network structure)
- cannot be melted by application of heat.
Glass transition temperature
AT certain temperature, the curve of the specific volume has a kink and the specific volume grows more strongly with the temperature than before.
- Thermal expansion
- “the bonds melt when the Tg is reached”
thermal movement of the molecular chains is less restricted by intermolecular bonds above the Tg. THE MATERIALS IS HIGHLY VISCOELASTIC
Mention some characteristics of Thermoplastic with the Tg
- Bonds melt at Tg
- Thermal movement is less restricted.
- Material become highly viscoelastic
- above Tg material doesnot behave as liquid.
- Molecules are entrangled and sliding in geometrically constrained.
- behave as liquid, molecules have to able to separate by large distance
Mention some characteristics of Elastomers and Thermosets with the Tg
- with localized sliding of the molecules being consequently easier.
- Due to covalent bonds between the molecules it is not possible to pull single molecules out of the network.
- never becomes liquid and always remain solid.
- Thermoset exhibit lower creep & stress relaxation than thermoplastic poymers.
Define Viscoelastic with creep and stress relaxation
- stress raised abruptly from 0 to a Stress.
- Polymer answer with a time-dependent strain
- Strain increase instantaneously to a value of Strain0. As in the case without time-dependent elastic behavior but the it further increases with time.
- Load remove after a time T0, strain decrease instantaneously by the time-independent strain0 and then reduces slowly to zero (relaxation)
Draw the graphs of Strain and Stress dependent of time related with Creep and stress relaxation
- The strain is kept constant.
- STress increase instantaneously, but then it decrease with time and approaches a constant value (stress relaxation)
Generalities of VISCOELASTIC
- mechanical properties depending on the variable TIME.
- if we have viscoelastic we have creep and relaxation of stress.
- thermoplastic take in mind the viscoelastic
- thermoset or elastomers neglect viscoelastic.
Define the 3 models of Viscoelastic
- Kevin Voigt (parallel)
- Maxwell Model (series)
- Generalized MAxwell model (mix)
Main characteristics of Kevin Voigt
- THE MODEL DESCRIBES PURELY VISCOELASTIC MATERIAL.
- PREDICT CREEP.
- elastic contribution of the deformation (without time dependace)
- plastic properties of a polymer also strongly depend on time
- polymers are viscoelastic
Main characteristics of MAxwell model
- PREDICT RELAXATION
- linear behaviour is increasing and not as a curve.
Main characteristics of Generalized Maxwell model
- Stifness and damping are parameters of temperature and material dependent.
- small temperatures (Tg decrease) elastic behaviour dominates assentially linear-elastic
- Temperature increase->viscoelastic
- # of springs and dampers as you need
What is a ISOCHRONOUS CURVE
- Is the retardation of experiments kept at constant stress after a fixed load time is measured.
- Deformation becomes larger the longer the loading time is.
- Viscoelastic effects occur at temperatures well below the Tg
Mention some ELASTIC PROPERTIES of the Thermoplastics
- Elastic behavior determined by intermolecular bonds.
- Secondary bonds melt.
- Due to geometrical constrain, elastic modulus is not Zero.
- applying load bonds stretch after load dispates.
- In E-T Log diagram -> STIFNESS strongly decrease at temperatures close to the Tg
- *In E-T Log diagram divided in Energy elastic and Entropy elastic
Draw the E-Tlog diagram for Thermoplastic
E-T diagram
What is de difference between Energy elastic and Entropy elastic region in the E-TLog diagram
ENERGY = Energy needed to displace atomos from equilibrium position. On unloading, the atoms return to their original position which gas the lowest energy. [covalent bonds not contribute significatly to the elastic properties]
ENTROPY = Load removed, no force on straightened molecules. No reason why they should return to their initial position. Deformation below Tg, it is not smaller energy of the initial configuration that drives to return to this form.
Mention about Semi-Crystaline Thermoplastic Elastic Properties
- Semicrystalline thermoplastics show a different behaviour*
- Elastic stifness is usually larger than that of amorphous
- Yound modulus decrease. Is smaller because only the amorphous regions become entropy-elastic.
- chain molecules extended over several crystalline and amorphous regions.
Properties of the Plastic behaviour of Thermplastics
- Chain molecules sliding past each other over large distances.
- Strongly depends on the temperature because of the obstacles.
- Lower than 80% of Tg (bonds between the molecules are so strong and the specific volume is so small that chain molecules cannot move by sliding)
Which are the 2 main plastic behaviour deformation mechanicms of Plastic behavior in Thermoplastics
- CRAZING
- SHEAR BANDS
How can it describe a CRAZING mechanism
- crazes initial surface defects
- plastic deformation start in those regions.
- stress state become triaxial and increase hydrostatic tension (cavities star to deform plastically)
- Crazed form
How can it describe a SHERA BANDS mechanism
-LArge and localised plastic deformation
-Compression mechanism
(if one shear bond cross other shera band will be fail)
-Non-symmetrical behavior between tension-compression
-Hydrostatic pressure increase , yield strength increase
-Start from Von Mises
**have deformation Above and CLOSE to Tg
