Problem 93 - Chemistry: The Central Science in SI Units, Global Edition

Question

If you want to make a polymer for plastic wrap, should you strive to make a polymer that has a high or low degree of crystallinity?

Step-by-Step Solution

Answer

A polymer with a lower degree of crystallinity is more suitable for making plastic wrap, as it provides better flexibility, transparency, and ease of processing, which are essential for the functionality of plastic wrap.

Step 1: Define crystallinity

Crystallinity is the degree to which the polymer chains are organized in a regular and ordered structure. Some polymers exhibit crystalline regions, whereas others have an amorphous, disordered structure.

Step 2: Understand the properties of high crystallinity polymers

Polymers with a high degree of crystallinity exhibit better mechanical strength, have higher melting points, and are more resistant to solvents and chemicals. However, they can be more brittle and less transparent.

Step 3: Understand the properties of low crystallinity polymers

Polymers with a low degree of crystallinity, or with more amorphous regions, exhibit better clarity and transparency, are more flexible, and have a lower melting point. They may also have better resistance to cracking and are easier to process.

Step 4: Evaluate the desired properties for plastic wrap

Plastic wrap is typically used to cover and protect food items, so it needs to be flexible, transparent, and capable of sealing. It also needs to adhere to a variety of surfaces to be effective. It should be able to be easily processed and manufactured economically.

Step 5: Determine if high or low crystallinity is preferred for plastic wrap

Based on the desired properties of plastic wrap, a polymer with a lower degree of crystallinity is more suitable for this application. Low crystallinity polymers would provide better flexibility, transparency, and the ability to be processed easily, which are all essential for the functionality of plastic wrap.

Key Concepts

Crystalline Polymers

Crystalline polymers are fascinating materials due to their structured arrangement. In these polymers, the molecular chains are aligned in an orderly fashion, forming highly organized regions. This orderly arrangement gives crystalline polymers unique properties:

• Increased mechanical strength: The orderly structure contributes to enhanced rigidity and toughness.
• Higher melting points: The strong interactions within the crystalline regions require more energy to break, translating to a higher melting temperature.
• Resistance to solvents and chemicals: The tight packing of chains makes it harder for chemicals to penetrate the material.

Despite these benefits, crystalline polymers can also have downsides like brittleness and reduced transparency. They may not be the best choice when flexibility and clarity are crucial, such as in applications like plastic wrap.

Amorphous Polymers

Unlike crystalline polymers, amorphous polymers showcase a disordered arrangement of molecular chains. This random structure allows these polymers to display:+

• Excellent clarity and transparency: The lack of order makes them less opaque, ideal for applications where transparency is needed.
• Increased flexibility: With fewer rigid structures, these materials can bend and stretch more easily.
• Lower melting points: Less energy is required to disrupt their structure, so they tend to melt at lower temperatures.

Amorphous polymers offer additional benefits, such as improved resistance to cracking under stress and ease of processing. These are crucial qualities in products that demand frequent handling, like plastic wrap used for food protection.

Plastic Wrap Properties

Plastic wrap is a common household material meant for covering food to keep it fresh. It has specific property demands to perform its function effectively.
When used in the kitchen or industrially, plastic wrap must exhibit:

• High flexibility: To easily adhere to and conform around various shapes of food containers.
• Transparency: So the user can see the contents clearly without removing the wrap.
• Easy processability: To allow for economic manufacturing and tailoring to different sizes and uses.

Given these requirements, materials with lower crystallinity are typically preferred for making plastic wraps. They offer the needed balance of flexibility and clarity, enabling them to stretch across and cling to surfaces while staying see-through. This adaptability is vital for efficiently covering and sealing food items.