For each of the following pairs of polymers, do ( the following: (1) State whether it is possible to decide whether one polymer has a higher tensile strength than the other; (2) if this is possible, note which has the higher tensile strength and cite the reason(s) for your choice; and (3) if it is not possible to decide, state why. (a) Linear and isotactic poly(vinyl chloride) with a weight-average molecular weight of \(100,000 \mathrm{~g} /\) mol; branched and atactic poly(vinyl chloride) having a weight-average molecular weight of \(75,000 \mathrm{~g} / \mathrm{mol}\) (b) Graft acrylonitrile-butadiene copolymer with \(10 \%\) of possible sites crosslinked; alternating acrylonitrile-butadiene copolymer with \(5 \%\) of possible sites crosslinked (c) Network polyester; lightly branched polytetrafluoroethylene

Step by step solution

01

Pair (a) - Linear isotactic PVC vs branched atactic PVC

First, let's compare linear isotactic poly(vinyl chloride) (PVC) with a weight-average molecular weight of \(100,000 \thinspace\text{g/mol}\) to branched atactic PVC with a weight-average molecular weight of \(75,000 \thinspace\text{g/mol}\). (1) It is possible to decide which polymer has higher tensile strength based on their structure and molecular weight. (2) Linear isotactic PVC has higher tensile strength. The reasons for this are: - Higher molecular weight: A higher molecular weight correlates with stronger intermolecular forces, which results in higher tensile strength. - Linear structure & isotacticity: Linear and isotactic polymers can pack more tightly together, enhancing the chances of forming strong secondary bonds, leading to a higher tensile strength compared to branched and atactic polymers.

02

Pair (b) - Graft vs alternating acrylonitrile-butadiene copolymers

Now, let's compare graft acrylonitrile-butadiene copolymer with \(10\%\) of possible sites crosslinked to alternating acrylonitrile-butadiene copolymer with \(5\%\) of possible sites crosslinked. (1) It is possible to decide which polymer has higher tensile strength based on their structure and crosslinking percentage. (2) Graft acrylonitrile-butadiene copolymer has higher tensile strength. The reasons for this are: - Higher crosslinking percentage: The graft copolymer has more crosslinks, which leads to a more interconnected network and better load distribution, resulting in higher tensile strength compared to the alternating copolymer with fewer crosslinks.

03

Pair (c) - Network polyester vs lightly branched polytetrafluoroethylene

Lastly, let's compare network polyester to lightly branched polytetrafluoroethylene (PTFE). (1) It is not possible to decide which polymer has higher tensile strength based on the provided information. (3) Without more specific details (e.g., molecular weights, branching density, or the presence of crosslinks), we cannot accurately compare the tensile strengths of these two polymers.

Unlock Step-by-Step Solutions & Ace Your Exams!

• Full Textbook Solutions

  Get detailed explanations and key concepts

• Unlimited Al creation

  Al flashcards, explanations, exams and more...

• Ads-free access

  To over 500 millions flashcards

• Money-back guarantee

  We refund you if you fail your exam.

Start your free trial

Over 30 million students worldwide already upgrade their learning with Vaia!

Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Molecular Weight Effect on Polymers

When considering how molecular weight impacts polymer properties, it's essential to understand that polymers are made of long chains of repeating units, known as monomers. The length of these chains is defined by the molecular weight. As the molecular weight of a polymer increases, so does the number of monomers in a chain.

This affects the tensile strength of a polymer in a couple of ways:

• Enhanced Entanglement: Longer chains are more likely to entangle with each other, creating a network that can better resist applied forces.
• Increased Van der Waals Forces: With more atoms in each chain, the intermolecular forces, particularly van der Waals forces, also increase. This leads to a stronger cohesion between chains.

However, the effect of molecular weight on tensile strength isn't endless. Beyond a certain point, known as the 'entanglement molecular weight', additional increases in molecular weight have a much smaller effect on the mechanical properties.

Polymer Structure and Mechanical Properties

The structure of a polymer greatly determines its mechanical properties, including tensile strength, elasticity, and plasticity.

Polymers can be categorized based on their structure into three types:

• Linear Polymers: These have a straight chain structure, allowing them to pack closely together, resulting in denser materials with higher tensile strengths.
• Branched Polymers: Branches in the polymer chains prevent them from packing tightly, which typically leads to lower tensile strength due to the reduced effective area for intermolecular forces to act.
• Crosslinked Polymers: When polymer chains are linked together in a network, it drastically increases the material's resistance to deformation and enhances its tensile strength.

Another structural factor is tacticity, or the regularity of the side groups along the chain. Isotactic polymers, with orderly side groups, tend to be stronger than atactic polymers with random side group placement.

Crosslinking in Polymers

Crosslinking refers to the process of chemically joining two or more polymer chains together. It can be viewed as the polymer equivalent of building a network of bridges between chains.

The degree of crosslinking has a profound effect on polymer strength:

• Rubber Elasticity: Limited crosslinking introduces elasticity as seen in common rubbers. Without crosslinks, the polymer would simply flow apart when stress is applied.
• Tensile Strength: As crosslinking increases, the polymer becomes tougher and more resistant to being pulled apart. This leads to higher tensile strength because the network can distribute an applied load more evenly throughout its structure.

Notably, too much crosslinking can make a polymer too rigid, leading to brittleness under certain conditions. Thus, the optimal crosslinking level depends on the intended application of the polymer and the balance of properties required for its performance.