The structure of part of a polyester chain is shown - AQA - A-Level Chemistry - Question 22 - 2018 - Paper 3

The correct explanation is that permanent dipole-dipole forces and van der Waals' forces exist between polyester chains. The presence of these intermolecular forces indicates that while the polyester chains are held together, they are not as strong as covalent bonds and can be overcome at elevated temperatures, leading to softening of the material.

To elaborate, if we analyze the options:

• Option A: Hydrogen bonds and van der Waals' forces - While van der Waals' forces may be present, hydrogen bonds do not significantly contribute to the stability of polyester structures.
• Option B: Permanent dipole-dipole forces and van der Waals' forces - This is the correct choice as it highlights the stronger interactions in play.
• Option C: The carbon-carbon bonds in the chain are strong - This is misleading because the strength of covalent bonds does not explain why the overall material softens at high temperatures.
• Option D: The carbon-oxygen bonds in the chain are strong - While these bonds are strong, they are part of the polymer structure and do not relate to the softening at high temperatures.

Thus, the correct answer is option B.