Are high molecular weights and a high degree of crystallinity always desirable properties of a polymer? Explain.

High molecular weights in polymers refer to longer chains and a higher number of repeating units. This usually results in increased strength, stiffness, and toughness of the polymer. A high degree of crystallinity refers to the extent to which a polymer solidifies in a periodic, ordered structure. This property typically influences the mechanical, thermal, and chemical resistance properties of a polymer. Some polymers will have a range of crystallinity, depending on how they are processed or how their molecular structure is arranged.

In some applications, high molecular weights, and high degrees of crystallinity can be desirable properties for a polymer material. For example, in engineering applications such as aerospace or automotive, where structural integrity and durability are crucial, polymers with these properties may be ideal. They can provide enhanced strength and rigidity, resistance to chemical attack, and excellent thermal stability.

However, high molecular weights and high degrees of crystallinity may not always be desirable properties for a polymer. These properties can lead to increased brittleness, decreased elasticity, longer processing times, and difficulty in recycling. For applications that require a flexible or stretchy material, such as packaging film, medical applications, or in garments, a polymer with lower molecular weight and lower crystallinity might be favored. Additionally, in applications where ease of processing or recyclability is important, these properties might be less desirable.

To further understand the desirability of high molecular weights and high degrees of crystallinity, we can consider specific examples. For instance, high-density polyethylene (HDPE) is a widely used polymer with a relatively high molecular weight and high crystallinity. It is extensively employed in industrial applications, like pipes and containers, thanks to its excellent mechanical and chemical resistance. On the other hand, low-density polyethylene (LDPE) has a lower molecular weight and crystallinity and is used for applications requiring flexibility and ease of processing, such as plastic films and bags. In conclusion, high molecular weights and a high degree of crystallinity are not always desirable properties of a polymer. Their desirability depends on the specific requirements of the application, including mechanical, thermal, chemical, and processing characteristics. Therefore, it is essential to carefully consider the properties and the specific needs of each application before choosing the most suitable polymer material.