Which listed polymer exhibits the weakest intermolecular forces (and hence lowest room-temperature cohesive strength) compared to the others?

Introduction / Context:
Intermolecular forces such as hydrogen bonding, dipole interactions, and van der Waals forces determine the mechanical strength and thermal properties of polymers. Elastomers like polyisoprene rely on light crosslinking for elasticity and typically have weaker intermolecular attractions between chains compared to polar or crosslinked polymers.

Given Data / Assumptions:

• Nylon-66 has strong hydrogen bonding between amide groups → higher cohesive strength.
• Bakelite is a crosslinked thermoset → rigid network with strong covalent crosslinks.
• Polystyrene is a glassy polymer with rigid phenyl groups, offering significant stiffness at room temperature.
• Polyisoprene in the uncrosslinked or lightly crosslinked state displays low secondary bonding between chains.

Concept / Approach:
Weak secondary forces translate to lower modulus and lower tensile strength unless reinforced or crosslinked. Polyisoprene’s hydrocarbon backbone and lack of strong polar functionalities make it comparatively weak in intermolecular attractions, enabling rubber-like extensibility but lower unfilled strength.

Step-by-Step Solution:
Survey each polymer’s bonding and morphology.Identify the one lacking strong polar groups or network crosslinks.Select polyisoprene as having the weakest intermolecular forces among the list.

Verification / Alternative check:
Material property tables show high hydrogen-bonded strength for nylons and network rigidity for Bakelite; PS is stiff and glassy; unfilled elastomers are comparatively soft.

Why Other Options Are Wrong:
Nylon-66: strong H-bonding raises cohesion.Bakelite: crosslinked network is very rigid.Polystyrene: glassy with relatively high modulus at room temperature.Polycarbonate: tough engineering plastic with strong intermolecular interactions.

Common Pitfalls:
Equating elasticity with strength; elastomers stretch easily but do not necessarily have high cohesive strength without fillers/crosslinks.

Final Answer:
Polyisoprene (natural rubber type)