Polymers and Polymerization Methods

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Questions and Answers

What is the process called by which monomers are transformed into polymers?

Molecular fusion

Chemical bonding

Polymer synthesis

Polymerization (correct)

Which of the following statements best explains why different polymers exhibit different properties?

They have varying chemical compositions and structures. (correct)

They are always made from the same chemical composition.

The amount of heat used in their fabrication does not affect their properties.

They consist of the same monomer units but are structured differently.

What polymer is used in the production of flexible and transparent materials like sandwich bags?

Polystyrene (PS)

Polyvinyl chloride (PVC)

High-density polyethylene (HDPE)

Low-density polyethylene (LDPE) (correct)

What is the term used to describe the number of repeating units in a polymer chain?

Degree of polymerization Signup and view all the answers

Which of the following pairs of polymers indicates a difference in physical properties due to their structural characteristics?

Low-density polyethylene (LDPE) and High-density polyethylene (HDPE) Signup and view all the answers

What characterizes the growth mechanism in step-growth polymerization?

Growth occurs throughout the matrix. Signup and view all the answers

What happens to the average molecular weight of polymers during step-growth polymerization as conversion increases?

It increases slowly and requires high extents of reaction. Signup and view all the answers

Which statement is true regarding the presence of monomers during polymerization?

Chain-growth polymerization depletes monomer rapidly after initiation. Signup and view all the answers

What key factor distinguishes the glass transition temperature (Tg) of amorphous polymers?

It defines the temperature at which polymers undergo phase changes from rubbery to glassy states. Signup and view all the answers

Which polymerization process requires an initiator?

Chain-growth polymerization. Signup and view all the answers

What role does the glassy state play in polymer properties?

Polymers are hard, brittle, and glassy in this state. Signup and view all the answers

How does the activation of polymer chains differ between the two types of polymerization?

Chains in step-growth remain active with no termination. Signup and view all the answers

What best describes the change in physical state of amorphous polymers at Tg?

They convert from soft and flexible to hard and brittle. Signup and view all the answers

What is the primary characteristic of polymers when heated with formaldehyde or hexamine?

They undergo extensive cross linking Signup and view all the answers

Which polymer is formed when the molar ratio of phenol to formaldehyde is less than 1 with a base catalyst?

Resole Signup and view all the answers

Which of the following properties does Bakelite NOT possess?

Flexibility and elasticity Signup and view all the answers

What is the result of further reaction during curing of resoles due to the presence of alcoholic groups?

Permanent cross linking Signup and view all the answers

What type of product is created when novolacs are cured with formaldehyde or hexamine?

An infusible and rigid thermosetting product Signup and view all the answers

What are resoles specifically formed from?

Mono, di, and trimethylol phenols Signup and view all the answers

What percentage of sulfur might be found in a battery case rubber?

30% Signup and view all the answers

What makes Bakelite resistant to certain chemical environments?

Cross-linked structure Signup and view all the answers

What is the main purpose of compounding rubber?

To impart specific properties for particular jobs Signup and view all the answers

In the context of Bakelite, which component contributes to its poor resistance to certain substances?

Inherent alkali sensitivity Signup and view all the answers

Which of the following compounds is significantly increased in battery case rubber compared to tyre rubber?

Sulfur Signup and view all the answers

In terms of percentage, how much sulfur does tyre rubber typically contain?

3% to 5% Signup and view all the answers

Which of the following statements is true regarding the composition of rubber?

Different forms of rubber can contain varying amounts of sulfur. Signup and view all the answers

Compounding of rubber primarily involves which of the following activities?

Mixing raw rubber with other materials Signup and view all the answers

What might be an unexpected outcome of improperly compounded rubber?

Decreased product performance Signup and view all the answers

Why is sulfur added to rubber during the compounding process?

To improve the heat resistance and strength Signup and view all the answers

Which of the following is NOT typically a reason for compounding rubber?

Reducing manufacturing costs Signup and view all the answers

The compounding process results in rubber that can be tailored for specific applications. What is one of the materials it is commonly mixed with?

Plasticizers Signup and view all the answers

What is the primary function of elastomers?

To undergo significant elongation under stress Signup and view all the answers

What is natural rubber primarily composed of?

A colloidal solution called latex Signup and view all the answers

What temperature range is suitable for using raw rubber?

10 to 60˚C Signup and view all the answers

Which process enhances the elasticity of rubber?

Vulcanization Signup and view all the answers

What is the average degree of polymerization (D.P.) of rubber?

5000 Signup and view all the answers

Which compound is the monomer used to form natural rubber?

Isoprene Signup and view all the answers

Who patented the process of vulcanizing rubber?

Charles Goodyear Signup and view all the answers

What is a significant drawback of raw rubber's physical properties?

Large water absorption capacity Signup and view all the answers

Study Notes

Polymers

Polymers are large molecules made up of repeating smaller units called monomers.
Polymers are formed by the process of polymerization.
Examples of polymers we encounter daily include sandwich bags, carpets, nylon stockings, milk cartons, and more.
Different polymers have different properties due to variations in their chemical compositions, structures, and fabrication processes.
LDPE (Low-Density Polyethylene) is more transparent, flexible, and waxy compared to HDPE (High-Density Polyethylene), which is more opaque, rigid, and non-waxy.

Step-Growth vs Chain-Growth Polymerization

Step-Growth Polymerization: Growth occurs throughout the matrix, the monomer is rapidly lost early in the reaction, and average molecular weight increases slowly.
Chain-Growth Polymerization: Growth happens by adding monomers at one end of the chain, some monomers remain even after long reaction times, different mechanisms operate at different stages, and the molar mass of the backbone chain increases rapidly at the beginning.

Glass Transition Temperature (Tg)

At low temperatures, amorphous polymers are stiff and glassy.
Upon warming, polymers soften in a specific temperature range known as the glass-rubber transition region.
Tg is the temperature when the amorphous phase of the polymer transitions between rubbery and glassy states, considered a key mechanical property.
Below Tg, an amorphous polymer is brittle, hard, and glassy. Above Tg, it becomes flexible, soft, and rubbery.

Bakelite

Bakelite is a thermosetting polymer formed by condensation of phenol (P) and formaldehyde (F).
Novolacs are prepolymers with a molar ratio (P/F) greater than one. They are soluble and fusible and do not undergo cross-linking on their own.
Resoles are formed with a molar ratio (P/F) less than one and a base catalyst. They contain alcoholic groups and are soluble and fusible, undergoing cross-linking upon curing.
Bakelite is rigid, hard, scratch-resistant, infusible, water-resistant, insoluble, and resistant to non-oxidizing acids, salts, and many organic solvents.
Uses: Electric insulators, molded articles, impregnating fabrics, woods, and paper, adhesives, paints and varnishes, and in water softening.

Elastomers

Elastomers are long-chain polymers that can be stretched several times under stress and regain their original shape when the stress is released.

Natural Rubber

Natural rubber is obtained from the latex of rubber trees.
Latex is a milky colloidal solution containing 25-45% rubber dispersed in water, with small amounts of protein and other materials.
Natural rubber is a polymer of isoprene (2-methyl-1,3-butadiene) with an average degree of polymerization (DP) around 5000.

Drawbacks of Raw Rubber

Plasticity: Soft at high temperatures and brittle at low temperatures, limiting its use to 10-60℃.
Weakness: Low tensile strength of 200 kg/cm2.
Water Absorption: High water absorption capacity.
Solvent Resistance: Non-resistant to non-polar solvents like vegetable and mineral oils.
Oxidizing Agent: Easily attacked by oxidizing agents.
Swelling and Degradation: Swells in organic solvents and degrades over time.
Durability: Poor durability.

Vulcanization

Vulcanization is a process that increases rubber's elasticity and reduces its plasticity by forming a cross-linked molecular network.
Vulcanization involves heating rubber with sulfur or other vulcanizing agents under pressure.
The amount of sulfur used influences the properties of the final rubber product.

Compounding of Rubber

Compounding involves mixing raw rubber with other substances to impart specific properties for specific applications.

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Description

Explore the fascinating world of polymers, focusing on their formation through polymerization processes. Understand the differences between step-growth and chain-growth polymerization, along with examples of daily-use polymers. Dive into the properties of various types of polyethylene and their applications.