- An increase in the degree of crystallinity in polymers significantly influences their physical properties.
- Crystallinity refers to the extent to which polymer chains are ordered in a regular, repeating pattern.
- When the degree of crystallinity increases, more polymer chains are tightly packed together, forming well-organized crystalline regions.

This change leads to several notable effects:
1. Increased stiffness: The ordered arrangement of polymer chains restricts their mobility, making the material more rigid and less flexible.
2. Higher melting point: Crystalline regions require more thermal energy to break the intermolecular forces holding the chains together, resulting in an elevated melting temperature.
3. Additionally, increased crystallinity typically leads to higher density and improved mechanical strength.

- In contrast, amorphous regions in polymers are less ordered and contribute to flexibility and lower melting points.

Therefore, the correct answer is “Increased stiffness and higher melting point”, as these are the direct consequences of increasing the degree of crystallinity in polymers.

Reference: Introduction to Polymers, 3rd Edition, Chapter 4: Polymer Structure and Properties, Pages 120-125