Problem 87 - Chemistry: The Central Science

Question

(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?

Step-by-Step Solution

Answer

(a) A polymer's flexibility is influenced by its molecular features such as long molecular chains, low degree of crystallinity, presence of side groups, and weak intermolecular forces. These features allow the polymer chains to move and bend more easily. (b) When a polymer is cross-linked, its flexibility decreases because covalent bonds formed between polymer chains restrict their movement, the degree of crystallinity often increases, and the chains are more effectively locked in place due to the three-dimensional network created by cross-linking. Therefore, a cross-linked polymer is generally less flexible than it was before.

Step 1: Understanding Polymers

Polymers are large molecules made up of smaller repeating units called monomers. They can be found in various materials, such as plastics, rubbers, and textiles. The physical properties of polymers are determined by their molecular structure, particularly the arrangement of their monomers and the strength of the bonds between them.

Step 2: Molecular Features that make a Polymer Flexible

There are several molecular features that can contribute to a polymer's flexibility: 1. **Long molecular chains**: Long polymer chains can give the material more freedom to move and bend around, making it more flexible. 2. **Low degree of crystallinity**: Polymers with a lower degree of crystallinity (meaning that their chains are more disordered and randomly coiled) tend to be more flexible, as there are fewer strong intermolecular forces that restrict their movement. 3. **Presence of side groups**: The presence of relatively large or bulky side groups on a polymer chain can lead to increased flexibility since these groups can act as spacers between the chains and facilitate their movement. 4. **Weak intermolecular forces**: In a flexible polymer, the intermolecular forces, such as van der Waals forces or hydrogen bonds, are relatively weak. This allows the polymer chains to slide past one another more easily, giving the material greater flexibility.

Step 3: Cross-linking and its Effect on Flexibility

Cross-linking is a process in which individual polymer chains are connected to each other by strong covalent bonds, thereby forming a three-dimensional network. The resulting cross-linked material usually has improved properties such as higher strength or increased thermal stability. When considering the question of whether a cross-linked polymer is more or less flexible than a non-cross-linked one, one must examine the effect of cross-linking on the molecular features mentioned earlier that contribute to polymer flexibility.

Step 4: Flexibility Comparison: Cross-linked vs Non-cross-linked Polymers

Cross-linking generally results in a decrease in flexibility, for the following reasons: 1. The covalent bonds formed between polymer chains during cross-linking restrict their movement, making the overall material less flexible. 2. Cross-linking often increases the degree of crystallinity in the polymer, reducing its flexibility. 3. Due to the three-dimensional network formed by cross-linking, the polymer chains are more effectively locked in place, making it harder for them to slide past one another. So, to answer part (b) of the exercise: If you cross-link a polymer, it is generally less flexible than it was before.