Polymers Flashcards
What are the three polymer types?
Thermoplastics, elastomers and thermosets.
What is the structure of a polymer?
A polymer consists of long molecules (i.e. macromolecules) which contain chains of atoms held together by covalent bonds. A polymer is produced by a process known as polymerisation in which many monomer molecules react together chemically to form the polymer.
How do you calculate the molecular weight of a polymer.
The molecular weight of the polymer is the number of monomers in the chain multiplied by the molecular weight of the monomer
What is the chain formation of a thermoplastic?
Thermoplastics have linear or branched chains
What is the chain formation of an elastomer?
Elastomers have a few cross-links and remain elastic.
What is the chain formation of a thermoset?
Thermosets have many crosslinks.
What is a crosslinked rubber elastic material.
Vulcanised
What is a semi-crystalline polymer?
A polymer that contains crystalline and amorphous regions.
What bonds exist between linear chains.
Weak van der Waals / Hydrogen bonds. These break easily on heating?
What is the glass transition?
As an amorphous solid is heated, a temperature is reached where there is an increase in the rate of change of specific volume.
What increases the melting point of a thermoplastic?
Increasing crystallinity, chain stiffness, molecular weight and bond strength.
You have developed a product and have decided to package this in a polymer wrap which is semi-crystalline. How would you expect the stiffness of this packaging to vary as it is strained by a customer when unpacking? Explain this with respect to what is happening with the polymer chains
The stiffness of a material is related to it’s Young’s modulus. Initially the Young’s modulus is low as the force required to maintain a certain strain is dictated mainly by the breaking of the weaker van der Waals forces and chain slippage. After high strains the Young’s modulus increases as the stiffness is now mainly dictated by the strength of the carbon-carbon covalent bonds once the chains have been stretched.
How are the origins of tensile properties different for polymers and metals?
In metals the tensile properties are largely defined by dislocation movements and how much resistance there is to this movement of dislocation. In polymers chain entanglement is the main mechanism which affects the tensile properties of a polymer.
Describe what is the glass transition temperature and the effect of the mechanical properties of a polymer.
The glass transition temperature is the temperature at which a polymer changes its mechanical properties from one being glassy to one which is more viscoelastic. The effect that a polymer above its glass transition temperature is much softer and more ductile than one in which the temperature is below the glass transition temperature and therefore much stiffer and therefore more brittle. This is mostly dictated by the influence of the van der Waals forces between the individual polymer chains.
Explain briefly why the tendency of a polymer to crystallize decreases with increasing molecular weight.
The tendency of a polymer to crystallize decreases with increasing molecular weight because as the chains become longer it is more difficult for all regions along adjacent chains to align so as to produce the ordered atomic array.
