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Chapter 14: Problem 22

Explain briefly why the tendency of a polymer to crystallize decreases with increasing molecular weight.

Short Answer

Expert verified
Answer: The tendency to crystallize decreases in polymers with increasing molecular weight because high molecular weight polymers have more entangled chains and reduced chain mobility. This makes it difficult for the chains to align themselves into ordered structures, leading to slower nucleation rates and limited crystal growth.

Step by step solution

01

Understanding Polymer Crystallization

Polymer crystallization is a process that occurs when a polymer cools down from its liquid or melt state. It forms ordered structures called crystals. The crystalline regions consist of polymer chains folded into a regular, repeating pattern. The degree of crystallinity depends on various factors, such as molecular weight, chemical structure, and cooling rate.
02

Effect of Molecular Weight on Crystallization

Molecular weight is a significant factor that influences the crystallization process. The molecular weight is defined as the mass of a single molecule of a material, typically expressed as g/mol. Higher molecular weight polymers are more entangled and have a more complex structure compared to lower molecular weight polymers. As a result, they face more difficulty in forming an organized structure like crystals.
03

Entanglements and Chain Mobility

High molecular weight polymers have more entanglements, which can hinder the polymer chains' movement and reduce their chain mobility. These entanglements create a larger radius of gyration, making it tougher for the chains to move and align themselves into an ordered arrangement that would lead to crystallization.
04

Nucleation and Crystal Growth

The crystallization process involves two main steps: nucleation and crystal growth. Nucleation is the formation of small, ordered regions called nuclei, which act as the starting point for crystal growth. Crystal growth occurs when more polymer chains attach themselves to the nuclei, increasing the size of the crystal. Higher molecular weight polymers have slower nucleation rates and limited crystal growth due to their entangled nature, resulting in a decreased tendency to crystallize.
05

Conclusion

The tendency of a polymer to crystallize decreases with increasing molecular weight because high molecular weight polymers have more entangled chains and reduced chain mobility. This entanglement and restricted movement make it difficult for the chains to align themselves into ordered structures, leading to slower nucleation rates and limited crystal growth. This ultimately results in a decreased tendency to crystallize with increasing molecular weight.

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Crosslinked copolymers consisting of 35 wt \(\%\) (- ethylene and 65 wt \% propylene may have elastic properties similar to those for natural rubber. For a copolymer of this composition, determine the fraction of both repeat unit types.

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