Question 1 and 2:
Using the information in the table, what can you tell about the phase transitions (changes of physical state) of the plastic types listed? How would you apply the information in the table to predict the mechanical properties of the given plastic types at room temperature?
From the table, all given polymers have glass and melting transition temperature, which split the behavior of polymers into five regions. At heat below Tg, the physical state of the polymers is glassy, tough and hard in texture. When temperature increases and near Tg, polymers become softer and more leathery. Between Tg and Tm, the polymer reaches a rubbery period, where it is easily bent and flexible. In this case, all above polymers are not degraded before melting so when the heat is closer to Tm, the polymer starts to melt and expressed as a rubbery flow which is reversible if a force is removed. Above Tm, the polymer remains as a melt or liquid whose viscosity would depend on molecular weight and on the observed temperature.
In addition, the table also shows the effect of density has the on melting point of polyethylene, the thinner the substances, the lower the melting point.
In particular, at room temperature, when mechanical forces are applied, polyethylene, polypropylene and polybut-1-ene are in “between Tg and Tm stage” and tend to be elastic, rubbery and able to return to their original shapes when the force is removed. Meanwhile, room temperature is around the glass transition point of polybut-2-ene, polymer chains starts to be softened, glassy and rubbery components coexist in the material at the same time.
For PTFE and PCTFE, room temperature is much lower than their glass transition points, so the polymer chains move slowly, which is indicated through their rigid and breakable behavior.
Compare the two figures below. The curves represent two different plastic materials at the same temperature, how would describe the mechanical properties of the materials at this temperature?
Above is stress-strain behavior of two different plastics at the same temperature.
The linear part is elastic region where stress and strain are proportional and the slope of linear part is Young’s modulus (Hooke’s Law: ? = E x ?). Young’s modulus is known as elastic modulus, a measure of force resistance of material. As shown above, the first material has higher slope, which leads to higher modulus, higher tensile strength but lower elongation at break, its behavior is stiff and rigid, and vice versa, the second sample has lower elastic modulus, lower tensile strength and higher elongation at break, it is soft and more flexible.
Question 4: The figure below describes the stress (MPa) – strain curves obtained experimentally for PMMA at various temperatures. What can be concluded from the figure related to thermomechanical properties of PMMA?
Similar to question 3, the slope of stress-strain curve is Young’s modulus.
As shown in the diagram, Young’s modulus of PMMA reduces along with the growth of temperature. This is because enhancing in temperature leads to the increase in kinetic energy of molecules. Molecular segments move easier between each other and raises the free volume of polymer matrix. Therefore, Young’s modulus, toughness and tensile strength of material decrease, material behavior transforms from rigid, tough and breakable to flexible, rubbery and soft.
In addition, when the temperature rises, the mobility of polymer chains increase, so more strain is applied causing the growth of elongation at break.
Question 5: Tell about viscoelasticity of polymers.
In general, viscosity features the flow resistance of polymer to long stress while elasticity is the property to return to its original shape when removing force, so viscoelasticity is the ability of polymer to be viscous and elastic when deformed.
Viscoelastic polymer is characterized by its stress-strain curve:
· Strain rate depends on time
· Creep stage: increase in strain with constant stress and increase in temperature.
· Stress relaxation stage: decrease in stress with constant strain
· Different from purely elastic material, energy loses in loading and unloading cycle
Degree of viscoelasticity depends on testing temperature, deforming rate, the degree of crystallinity, molecular mass and crosslinking structure.
Question 6: What is rheology? How it is important for polymer processing?
Originally, the term rheology comes from ‘rheos’ in Greek, meaning the river, flowing, streaming
Rheology is the science of the deformation and flow of materials, where the key term is viscosity. In particular, Viscosity presents the flow resistance of molecules to frictional forces, viscosity depends on molar mass, intermolecular forces, processing temperature, structure of molecules and substituents, the way and magnitude of applied stress
Rheology is important to polymer handling and processing because it prescribes the flow and deformation of polymer, Rheological properties affect the viscoelastic properties in solids, flow dynamics of melted polymers and resins as well as the flow information in melting and manufacturing composite.