Polyethylene glycol (PEG) is a polymer with the repeating unit –CH2CH2O–. A researcher prepares two PEG samples of identical molar mass: Sample X is synthesized with every tenth oxygen atom substituted by nitrogen to give –CH2CH2NH– units, while Sample Y is left unmodified. Both samples are liquids at room temperature.
The researcher measures the following macroscopic properties at 298 K.
Property
Sample X
Sample Y
Dynamic viscosity (mPa·s)
110
280
Density (g mL−1)
1.11
1.12
Respond to the prompts below.
(a) Propose the dominant intermolecular forces operating between the polymer chains in each sample. Identify any differences that arise from the nitrogen substitution.
(b) Provide a molecular-level explanation for the observed difference in viscosity between Sample X and Sample Y.
(c) The researcher wants to design a PEG derivative with even lower viscosity than Sample X while keeping the molar mass constant. Suggest a structural modification (other than reducing molar mass) and justify how it would achieve the desired effect in terms of intermolecular forces.
(d) Predict qualitatively how the glass transition temperature (Tg) of the modified polymer you proposed in part (c) would compare with that of Sample Y. Support your prediction with reasoning based on intermolecular interactions and chain mobility.