Making a polyester
4 marks · NormalThe table shows the properties of two polyesters.
| Polyester | Monomers Used | Melting Point / C | Tensile Strength |
|---|---|---|---|
| A | ethane-1,2-diol + benzene-1,4-dicarboxylic acid | 258 | High |
| B | propane-1,3-diol + benzene-1,4-dicarboxylic acid | 140 | Medium |
State one similarity and one difference in the monomers used to make Polyesters A and B.
Using the data in the table, suggest a reason for the difference in melting point between Polyester A and Polyester B.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Both use benzene-1,4-dicarboxylic acid, but have different diols (ethane-1,2-diol vs. propane-1,3-diol).
(b) Polyester A has a higher melting point because its chains are more regular and can pack closely, due to the shorter ethane-1,2-diol monomer. The longer propane-1,3-diol in Polyester B makes its chains less regular, lowering its melting point.
Must show: Refers to diol differences, Links structure to property
Common mistakes: Not referencing table for answer; Ignoring type of diol used
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States a correct similarity in the monomers used (both use benzene-1,4-dicarboxylic acid).
B2 (a) · 1 mark · Gives a correct difference (different diols: ethane-1,2-diol and propane-1,3-diol).
B3 (b) · 1 mark · Links melting point to chain regularity/packing due to monomer structure.
B4 (b) · 1 mark · Correctly applies data to explain how a shorter diol gives closer packing and higher melting point.
Worked solution
a) Both polyesters use benzene-1,4-dicarboxylic acid as a monomer. The difference is that Polyester A uses ethane-1,2-diol and Polyester B uses propane-1,3-diol.
b) Polyester A has a higher melting point (258°C vs. 140°C), likely because its chains are more regular and can pack more closely due to shorter ethane-1,2-diol, increasing intermolecular forces.