Name three things, related to the polymer chains (excluding positioning of side
ID: 509544 • Letter: N
Question
Name three things, related to the polymer chains (excluding positioning of side groups) and their packing, that has a strong affect on the mechanical properties of a polymer in bulk form. Name two things, related to the external environment and applied loading, that has a strong affect the mechanical behaviour of a polymer in bulk form. The molecular weight of one polymer chain is M = 50, 400 g/mol and the degree of polymerization is 1, 200. Is the polymer PE (C_2 H_4)_n, PVC (C_2 ClH_3)_n, PP (C_3 H_6)_n or PTFE (C_2 F_4)_n? Justify your answer. Polymer A has a melting temperature of 200C and a glass transition temperature of 143C. Polymer B has no melting temperature but a glass transition temperature of 120C. What can you conclude about the degree of crystallinity of polymers A and B (at room temperature)? For a thermoplastic polymer that behaves in a visco-elastic manner: i) sketch the stress as a function of time if you apply a strain at time zero and hold it constant for a time delta t and then quickly reduced it to zero, ii) sketch the strain as a function of time if you apply a stress at time zero and hold it constant for a time delta t and then quickly reduce it to zero.Explanation / Answer
Answer for 5a:
Polymers are long chain molecules and their structure influence many of its properties including mechanical properties.
Mechanical properties of polymers depend how well the individual chains pack together and their ability to move relative to each other.
In a polymer when its chains pack in a regular fashion those areas of polymer is known as crystalline regions.
However when its chains are aligned in a random fashion those areas of polymer is known as amorphous regions.
Highly crystalline polymers have high percentage of crystalline regions. Such regions of polymers have very strong mechanical stregnth as the chains aligned are parallel and have strong attractive forces between them.
Whereas flexible polymers have high percentage of amorphous regions. In these polymers the chains are randomly aligned and slide over each other without disrupting the weaker intermolecular attractions between the chains.
Crystalline polymers (ex. High Density PolyEethylene (HDPE) are denser than amorphous polymers (ex. Low Density PolyEethylene (LDPE).
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Answer for 5b:
Polymers are usually described as viscoelastic materials, which emphasizes their intermediate position between viscous liquids and elastic solids.
An ideal linear elastic solid obeys Hooke’s law, i.e. stress is proportional to strain.
An ideal viscous liquid obeys Newton’s law, i.e. stress is proportional to the rate of change of strain.
At low temperatures or high frequencies of measurement, a polymer may behave like a glass with a Young’s modulus of 109 N/m2 to 1010 N/m2 and will break at strains greater than about 5%.
At high temperatures or low frequencies, the same polymer may display rubber-like behavior with a low modulus of 106 N/m2 to 107 N/m2 and extension larger than 100% without any permanent set.
At still higher temperatures, the polymer behaves like a highly viscous liquid.
Answer for 5c:
Molecular weight of the polymer = 50,400 g/mol
Degree of polymerization = 1200
Molecular weight of the monomer unit = molecular weight of the polymer / degree of polymerization
Molecular weight of the monomer unit = 50400/1200 = 42
Molecular weight of the monomer unit = 42 = C3H6 =
So the polymer is Poly Propylene PP (C3H6)n
Answer for 5d:
Polymer B is not crystalline at room temperature as it has no melting point and its amorphous unlike Polymer A which has clear melting temperature.
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