Polymer Crystallinity: Understanding the Crystallinity of Polymers Quiz and Flashcards

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the main transition temperature found in amorphous polymers?

Thermal transition temperature

Glass transition temperature (correct)

Crystallization temperature

Melting temperature

What happens to the polymer at the glass transition temperature (Tg)?

Frozen backbone sequences begin to move (correct)

It becomes a liquid

It becomes opaque

It crystallizes

What is the effect of chain ends on the glass transition temperature (Tg) of a polymer?

It depends on the molecular weight of the polymer

It decreases the Tg (correct)

It increases the Tg

It has no effect on the Tg

What is the approximate relationship between Tg and Tm for crystalline polymers with unsymmetrical chains?

Tg ≈ 0.5Tm Signup and view all the answers

What is the effect of side groups on the polymer below the glass transition temperature (Tg)?

They can change position Signup and view all the answers

What happens to a thermoplastic polymer when it is cooled to a temperature below its melting temperature (Tm)?

It crystallizes Signup and view all the answers

What is the effect of copolymerization on the glass transition temperature (Tg) range?

It broadens the Tg range Signup and view all the answers

What is the characteristic of crystalline polymers?

They have a melting temperature Signup and view all the answers

What is the main reason why complete crystallization does not occur in polymers, unlike in metals?

The difficulty of aligning every portion of each chain of the polymer Signup and view all the answers

What is the term used to describe the unordered or structure-less material surrounding crystalline regions in polymers?

Amorphous Signup and view all the answers

What is the effect of stretching a film on the crystallinity of the material?

It orients the crystallites and increases the crystallinity Signup and view all the answers

What is a characteristic of non-crystalline, amorphous polymers?

They have no melting point Signup and view all the answers

What is the relationship between orientation and crystallinity in polymers?

Only polymers that are capable of crystallization can be oriented Signup and view all the answers

What is the effect of bulky side groups on the glass transition temperature of a polymer?

Raise Tg Signup and view all the answers

What is the effect of high shear during processing and rapid cooling on the crystallinity of polymers?

It inhibits the crystallinity Signup and view all the answers

What is the characteristic of the melting temperature of an ordered copolymer?

A single, sharp melting temperature Signup and view all the answers

What is the effect of impurities such as catalysts, fillers, and pigments on the crystallization rate of polymers?

It diminishes the crystallization rate Signup and view all the answers

What happens to a polymer when both Tm and Tg lie below room temperature?

It becomes a liquid Signup and view all the answers

What is the characteristic of unfilled plastics that is related to their crystallinity?

Transparency Signup and view all the answers

What is the effect of molecular symmetry on the glass transition temperature of a polymer?

Lowers Tg Signup and view all the answers

What is the typical range of glass transition temperatures for commercially important crystallizable polymers?

Below 25°C Signup and view all the answers

What is the effect of substituting alternate hydrogens in polyethylene with methyl groups on the glass transition temperature?

Increases Tg Signup and view all the answers

What is the characteristic of the transition behavior of ordered copolymers?

Shows a single, characteristic Tg and a single, sharp melting temperature Signup and view all the answers

What is the effect of bulky side groups on the melting temperature of a polymer?

Raise Tm Signup and view all the answers

Study Notes

Physical Transitions in Polymers

Polymers can range from cloudy to opaque depending on the degree of crystallinity.
Non-crystalline (amorphous) polymers have a glass transition temperature (Tg) and crystalline polymers have a crystalline melting temperature (Tm).

Glass Transition Temperature (Tg)

Tg is the main transition temperature found in amorphous polymers.
At Tg, frozen backbone sequences begin to move.
Tg is determined by the main chain architecture and its immediate surroundings.
Chain ends and low-MW plasticizers lower the Tg of a polymer, while a sufficiently large number of cross-links increase Tg.
Above Tg, a few carbon atoms in each chain can still move with relative freedom, but below Tg, nearly all carbon atoms become fixed.

Crystalline Melting Temperature (Tm)

Crystalline polymers have a melting transition at a temperature called Tm.
Tm and Tg are important parameters that define the upper and lower temperature limits for numerous applications.

Relationship between Tg and Tm

For unsymmetrical chains, Tg ≈ 0.5-0.7Tm.
For symmetrical chains, Tg ≈ 0.8-0.9Tm.

Crystallinity

The degree of crystallinity is an important factor affecting polymer properties.
Crystallinity in plastics consists of thousands of small “islands” of crystalline regions surrounded by unordered or amorphous material.
The crystalline areas are known as crystallites.
High MW, narrow MWD, and linearity in the polymer backbone can yield high crystallinity.
The crystallization rate is diminished by the presence of impurities such as catalysts, fillers, and pigments.
Stretching a film orients the crystallites and realigns other molecules or segments of molecules, causing the total crystallinity of the film to increase.

Effect of Crystallinity on Polymer Properties

Transparency of unfilled plastics is a function of crystallinity, with non-crystalline polymers having excellent transparency.
The physical properties of a thermoplastic polymer depend on the values of Tm and Tg relative to room temperature.
If both Tm and Tg lie below room temperature, the polymer is a liquid.
If room temperature lies between Tm and Tg, the polymer is either a very viscous supercooled liquid or a crystalline solid.
If both Tm and Tg are above room temperature, an amorphous polymer is glassy in nature, tending to be brittle.

Factors Affecting Tg

Bulky side groups decrease the mobility of the chain and raise Tg.
Molecular symmetry tends to lower Tg.
Tg values are low for flexible, linear polymers such as PEs, and relatively high for stiff chain polymers such as PET and PC.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.

Description

This quiz explores the complex structure of polymers, including the degree of crystallinity and its impact on polymer properties. It also discusses the challenges of aligning polymer chains and the resulting crystallinity in plastics.

Polymer Crystallinity and Structure

Choose a study mode