polymer structures and properties

Polymer Structures and Properties

Dr. Ying-Chieh Yen 嚴英傑Ref: INTRODUCTION TO POLYMERS 2nd edition, R. J. Young and P. A. Lovell.

Polymer Structures

Basic Definitions and Nomenclature

A polymer is a substance composed of molecules which have long sequences of one or more species of atoms or groups of atoms linked to each other by covalent bonds.

The words, polymers and macromolecules are used interchangeably, the latter strictly defines molecules of which the former is composed.

Macromolecules are formed by linking together monomer molecules through chemical reactions, the process by which this is achieved being known as polymerization.

Basic Definitions and Nomenclature

Molecular Mass and Polydispersity Number average molecular mass ( ):

Weight average molecular mass ( ):

The ratio must be greater than unity for a polydisperse polymer and is known as the polydispersity or heterogeneity index.

A perfectly monodisperse polymer would have polydispersity = 1.00.

nM i

iiN

MN

wM

ii

iiMN

MN 2

n

w

MM

Classification of Polymers

Classification of Polymers Thermoplastics: It often referred to just as plastics (linear

or branched polymers) which can be melted upon the application of heat.

Crystalline: Those which do

crystallize invariably do not form perfectly crystalline materials but instead are semi-crystalline with both crystalline and amorphous regions. (Tm)

Amorphous: Many thermoplastics are completely amorphous and incapable of crystallization. (Tg) At the temperature, thermoplastics transform abruptly from the glass state (hard) to the rubbery state (soft).

Classification of PolymersElastomers are rubbery polymers (i.e. rubbery netw

orks) which can be stretched easily to high extensions (e.g. 3x to 10x their original dimensions) and which rapidly recover their original dimensions when the applied stress is released.

Thermosets normally are rigid materials and are network polymers in which chain motion is greatly restricted by a high degree of crosslinking.

Skeletal StructuresLinear structure: A chain with two ends.

Non-linear structures: Branched structure: Side chains, or branches, of

significant length bonded to the main chain at branch points (junction points).

Skeletal StructuresNon-linear structures: Network structure (crosslinked): Polymers have thre

e-dimensional structures in which each chain is connected to all others by a sequence of junction points and other chains.

HomopolymersThe formal definition of a homopolymer is a polyme

r derived from one species of monomer.

However, it often is used more broadly to describe polymers whose structure can be represented by multiple repetition of a single type of repeat unit.

Some Common Homopolymers

Some Common Homopolymers

Some Common Homopolymers

Tacticity

• For polymers prepared from monomers of the general structure CH2=CXY, where X and Y are two different substituent groups, there are two distinct configurational arrangements of the repeat unit.

Tacticity

• Isotactic:

• Syndiotactic:

• Atactic:

CopolymersThe formal definition of a copolymer is a polymer

derived from more than one species of monomer.

However, it often is used more broadly to describe polymers whose molecules contain two or more different types of repeat unit.

CopolymersStatistical copolymers: The sequential distribution

of the repeat unit obeys the statistical laws. (Markovian)

Random copolymers: A special type of statistical copolymer in which the distribution of repeat units is truly random. (Older textbooks and scientific papers often use the term random copolymer to describe both random and non-random statistical copolymers.)

CopolymersAlternating copolymers: Only two different types

of repeat units are arranged alternately along the polymer chain.

Block copolymers: Linear copolymers with repeat units existing only in long sequences or blocks.

CopolymersGraft copolymers: Branched polymers with the

branches having different chemical structure to that of the main chain.

Polymer Properties

The Glass TransitionIf the melt of a non-crystallizable polymer is cooled

it becomes more viscous and flows less readily. If the temperature is reduced low enough it becomes rubbery and then as the temperature is reduced further it becomes a relatively hard and elastic polymer glass.

The temperature at which the polymer undergoes the transformation from a rubber to a glass is known as the glass transition temperature, Tg.

The Glass TransitionThere is a dramatic change in the properties of a pol

ymer at glass transition temperature. For example, there is a sharp increase in the stiffness of an amorphous polymer when its temperature is reduced below Tg.

There are also abrupt changes in other physical properties such as heat capacity and thermal expansion coefficient.

There have been attempts to analyse the glass transition from a thermodynamic viewpoint.

The Glass Transition In the first-order transition there is an abrupt change i

n a fundamental thermodynamic property such as enthalpy, H or volume, V, whereas in a second-order transition only the first derivative of such properties changes.

This means that during a first-order transition, such as melting, H and V will change abruptly whereas for a second-order transition changes will only be detected in properties such as heat capacity, Cp or volume thermal expansion coefficient, α which are definded as:

The Glass TransitionAs both of these parameters are found to change abr

uptly at the glass transition temperature it would appear that it may be possible to consider the glass transition as a second-order thermodynamic transition.

At the glass transition, the molecules which are effectively frozen in position in the polymer glass become free to rotate and translate and so it is not surprising that the value of the Tg will depend upon the physical and chemical structure of the polymer molecules.

The Glass TransitionThe most important factor is chain flexibility which

is governed by the nature of the chemical groups which constitute the main chain.

The Glass Transition

In vinyl polymers of the type (-CH2-CHX-)n the nature of the side group (bulky and polar groups) has a profound effect upon Tg as can be seen in the table.

CrystallizationCrystallization is the process whereby an ordered

structure is produced from a disordered phase, usually a melt or dilute solution, and melting can be thought of as being essentially the opposite of this process.

Features: (a) Polymer crystals are usually thin and lamellar

when crystallized from both dilute solution and the melt.

Crystallization

(b) The lamellar thickness is related to the crystallization temperature.

(c) Chain folding is known to occur during crystallization.

(d) The growth rates of polymer crystals are found to be highly dependent upon the crystallization temperature and molar mass of the polymer.

Thermal Degradation Temperature

The thermal degradation temperature was determined from the changes in weight in relation to change in temperature using thermogravimetric analysis (TGA).

Surface Free Energy

Surface free energy from work :The reversible work required to create a unit surface area is related to the surface free energy of the material.

1

111W

11 12W

high surface free energy ↔ strong cohesion

surface tension of liquids corresponds to surfaceenergy of solids

Units: mJ/m2 = mN/m

The Theory of Surface Free EnergyThree-Liquid Acid-Base Method

γS : apolar component, accounting for Liftshitz-van der Waals type interactions

γS : polar component, accounting for acid-base or donor- acceptor type interactions

γS + : Lewis-acid component, electron acceptor γS

- : Lewis-base component, electron donor

LW

AB

LW AB AB 2

LW 2

LW LW + +L1 1 S L1 S L1 S L1

LW LW + +L2 2 S L2 S L2 S L2

LW LW + +L3 3 S L3 S L3 S L3

(1 cos ) 2( )

(1 cos ) 2( )

(1 cos ) 2( )

Fluoropolymers and Silicones

C C

F

F

F

Fn

poly(tetrafluoroethylene)

PTFE 221mJ / m

The low intermolecular forces present in fluorinated polymers have been recognized to account for the relatively low surface free energy.

Si O

CH3

CH3n poly(dimethylsiloxane) PDMS 219.9mJ / m

C C

H

Hn

H

Hpolyethylene PE 235.7mJ / m

Mechanical Properties

• Stress: In consideration of the mechanical properties of polymers we are mainly to interested in effect of applying surface forces such as stress or pressure to the material.

Mechanical Properties

• Strain: When forces are applied to a material the atoms change position in response to the force and this change is known as strain.

• Young’s modulus: E of a material which for simple uniaxial extension or compression is given by E = stress/strain.

Mechanical Properties

• Viscoelasticity: A distinctive feature of the mechanical behavior of polymers is the way in which their response to an applied stress or strain depends upon the rate or time period of loading.

The behavior of most polymers can be though of as being somewhere between that of elastic solids and liquids.

Mechanical Properties

At low temperatures and high rates of strain they display elastic behavior whereas at high temperatures and low rates of strain they behave in a viscous manner, flowing like a liquid.

Polymers are therefore termed viscoelastic as they display aspects of both viscous and elastic types of behavior.

Mechanical Properties• General time-dependent behavior:

Creep experiment

Relaxation experiment

Mechanical Properties

Stress-Strain Curve

Neck occurs!!

Brittle!!

Ductile!!

The EndThe EndThanks for Your AttentionThanks for Your Attention