(a) What molecular features make a polymer flexible? (b) If you cross-link a polymer, is it more flexible or less flexible than it was before?

Polymers are long chain molecules made up of smaller repeating units called monomers. They can be synthesized from both natural and synthetic sources. The physical and chemical properties of polymers are determined by their molecular structure, composition, and the interactions between polymer chains.

A polymer's flexibility depends on its molecular features, such as chain length, intermolecular forces, and the presence of flexible bonds or side groups. Some key molecular features that make a polymer flexible are: 1. **Long Polymer Chains:** Longer chains have more freedom of movement, which contributes to the flexibility of the polymer. 2. **Weak Intermolecular Forces:** Polymers with weak intermolecular forces between the chains (such as van der Waals forces) tend to be more flexible. These weak forces allow the polymer chains to slide past one another more easily. 3. **Flexible Bonds and Side Groups:** Polymers with flexible bonds (such as single bonds) and side groups, which can freely rotate, lead to increased flexibility. For example, polyethylene, which is made up of carbon-carbon single bonds and hydrogen side groups, is a flexible polymer. To summarize, a polymer is considered flexible if it has long chains, weak intermolecular forces, and flexible bonds or side groups.

Cross-linking is a process where polymer chains are connected through covalent bonds, ionic interactions, or other strong forces, creating a network structure. The change in flexibility due to cross-linking depends on the extent and nature of the cross-links. When a polymer is cross-linked: 1. **Restricted Chain Movement:** The cross-links restrict the movement of the polymer chains, making it more difficult for the chains to slide past one another. 2. **Increased Rigidity and Strength:** As the number of cross-links increases, the rigidity and strength of the polymer also increase. So, if a polymer is cross-linked, it generally becomes less flexible than it was before. However, the extent to which the flexibility decreases depends on the number and strength of the cross-links. In conclusion, cross-linking a polymer typically results in a less flexible material compared to the uncross-linked polymer.