The University of Edinburgh
Division of Engineering
Session 2001-2002
Materials Science and Engineering
Introduction to polymers
1. Polymeric materials basic definitions, structure, classification
1.1 Molecular structure
Polymers (or macromolecules) are very large molecules made up of smaller units,
called monomers or repeating units, covalently bonded together (Fig 1). This spe-
cific molecular structure (chainlike structure) of polymeric materials is responsi-
A
A A A A
A A A
A A
A
A A
A
A is a M onom er unit
represents a covalent bond
Figure 1: A polymer chain.
ble for their intriguing mechanical properties. Polymer architecture can vary. In
Fig 2 three possible molecule architectures are depicted.
A linear polymer consists of a long chain of monomers. A branched polymer
has branches covalently attached to the main chain. Cross-linked polymers have
monomers of one chain covalently bonded with monomers of another chain. Cross-
linking results in a three-dimensional network; the whole polymer is a giant macro-
molecule. Elastomers are loosely cross-linked networks while thermosets are
1
(a)
(b)
(c)
Figure 2: Types of molecular configuration: (a) Linear chain. (b) Branched molecule. (c) Cross-
linked network: molecules are linked through covalent bonds; the network extends over the whole
sample which is a giant macromolecule.
densely cross-linked networks.
Another classification of polymers is based on the chemical type of the monomers
(Fig 3): Homopolymers consist of monomers of the same type; copolymers have
different repeating units. Furthermore, depending on the arrangement of the types
of monomers in the polymer chain, we have the following classification:
In random copolymers two or more different repeating units are distributed
randomly
Alternating copolymers are made of alternating sequences of the different
monomers
In block copolymers long sequences of a monomer are followed by long
sequences of another monomer
Graft copolymers consist of a chain made from one type of monomers with
branches of another type.
2
-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A- (a)
-A-A-B-A-B-B-A-B-A-A-B-A-A-A-B-A-B-B-A-B-B-B-A-A-B-A-A- (b)
-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B-A- (c)
-A-A-A-A-A-A-A-A-A-B-B-B-B-B-B-B-B-B-B-B-B-A-A-A-A-A-A- (d)
|
B
|
B
|
B
|
-A-A-A-A-A-A-A-A-A-A-A-A- A-A-A-A-A-A-A-A-A-A-A-A-A-A- (e)
| |
B B
| |
B B
| |
B B
| |
B B
| |
B
|
B
|
Figure 3: (a) Homopolymer. (b) Random copolymer. (c) Alternating copolymer. (d) Block
copolymer. (e) Graft copolymer.
3
(a)
(b)
Figure 4: (a) Amorphous polymer (observe the entanglements among the polymer chains) and (b)
semicrystalline polymer.
1.2 Microstructure
Many properties of polymeric materials depend on the microscopic arrangement
of their molecules. Polymers can have an amorphous or semicrystalline (partially
crystalline) structure (Fig 4). Amorphous polymers lack order and are arranged
in a random manner, while semicrystalline polymers are partially organised in
orderly crystalline structures.
1.3 Thermal Behaviour
Thermosets, which are densely cross-linked in the form of a network, degrade
upon heating, while thermoplastics, which do not contain cross-links, melt upon
heating.
4
(b)
(a)
(c)
Figure 5: Examples of types of forces: (a) Tensile force. (b) Compressive force. (c) Shear force.
1.4 Composite materials
Polymers are often mixed with inorganic particles (usually in the form of fibers,
such as fiberglass) in order to modify and improve their mechanical properties.
Such materials are called composites.
2. Mechanical behaviour of polymeric materials
Whenever a force is exerted on a solid material, the material will deform in re-
sponse to the force. Depending on the particular orientation of the force with
respect to the material surface different types of forces can be identified. In Fig 5,
some common types of forces are depicted. For our subsequent analysis, tensile
(pulling) forces will be used. A mechanical test using tensile forces is called a
tensile test. Generally speaking, the basic concepts remain the same for all types
of forces.
2.1 Elastic behaviour
A material is elastic, if upon an applied force, its deformation is instantaneous
and constant, and upon the removal of the force, its recovery is instantaneous and
5
complete (i.e. the material will return to its original shape).
In order to make the analysis of a mechanical test independent of the materials
size, it is useful to define a quantity called stress Ã; it is defined as the force F
divided by the cross-sectional area A of the material [units N/m2 = Pa]. The de-
formation is quantified by the strain µ which is defined as the length change "l
divided by the initial length l0 and it is dimensionless. In case of a tensile test the
strain is often called elongation and is usually expressed as a percentage increase
in length compared to the initial length.
The stress-strain relationship is given by the following equation
à Eµ (1)
where à F A, µ "l l0 and E is the Young s modulus [units N/m2 =Pa].
In Fig 6 two stress-strain(elongation) curves are shown. The first one is linear
and reminds us of Hooke s law of mechanical springs (F kx). In this case,
Young s modulus is constant and mathematically it is the slope of the stress-strain
curve. The solid materials that exhibit such behaviour are called linear or hookean.
Practically, this behaviour is encountered in almost all materials (metals, ceram-
ics and polymeric materials) only at sufficiently low stresses and deformations
(e.g. 1 per cent). The second stress-strain(elongation) curve corresponds to the
elastic behaviour of a non-linear or non-hookean solid and it is characteristic of
elastomers. In elastomers the elastic behaviour holds for very large deformations
(several times the original sample length, e.g. 400 per cent) which can be attained
by relatively low stresses. It is clear that elastic behaviour is not always linear.
A material with high Young s modulus is called stiff, while a material with low
Young s modulus is called compliant.
2.2 Viscous behaviour and viscoelasticity
Fluids show a characteristic resistance to movement (flow), which is called vis-
cosity. Viscosity results in a frictional energy loss, which dissipates in the fluid as
heat. Polymeric materials behave both as viscous fluids and elastic solids. They
6
(a)
Strain
(b)
Strain
Figure 6: Elastic behaviour: (a) at low strains for all materials; (b) at low stresses (but large
elongations) for elastomers (e.g. rubber elasticity).
7
Stress
Stress
(a)
Tim e
(c) (d)
(b)
Tim e Tim e Tim e
Figure 7: Mechanical response (deformation) of a material subjected to a constant load for a finite
time interval (up to the dotted line). (a) Load application; (b) solid elastic behaviour (c) liquid
viscous flow behaviour (d) polymer viscoelastic behaviour.
are viscoelastic materials. The most important characteristic of viscoelastic mate-
rials is that their mechanical properties depend on time.
2.3 Creep
The deformation of a material over time due to the application of a constant load
is called creep (Fig 7).
A purely elastic material responds instantaneously to the load and the deformation
remains the same, in addition, it will recover its initial shape upon the removal of
the load. On the contrary, a viscous liquid will deform as long as the load contin-
ues to be applied. Upon the removal of the load, the fluid does not return to its
initial position. The response of a viscoelastic material is intermediate between
the solid and the liquid (see Fig 7d). Creep depends on the applied load, molecular
characteristics, microstructure and temperature.
We can use combinations of springs (linear elastic behaviour) and dashpots (linear
8
Strain
Strain
Strain
Spring
dashpot
Ã
Figure 8: Voigt element: Combination of a spring and a dashpot in parallel.
viscous behaviour) in order to quantify the mechanical behaviour of polymeric
materials. In Fig 8 the Voigt element is shown. This is a parallel combination of a
spring and a dashpot. The total stress à is distributed between the spring Ã1 and
the dashpot Ã2, so that
à Ã1 Ã2 (2)
However the strains of the spring and dashpot are the same, that is
µ µ1 µ2 (3)
We know that
Ã1 Eµ1 (4)
For a viscous liquid
dµ2
Ã2 · (5)
dt
where · is the viscosity. Thus,
dµ
à Eµ · (6)
dt
Solving the differential equation for constant stress (as in a creep test), we
obtain
Ã
µ t 1 exp Et · (7)
·
9
For the recovery (relaxation, Ã 0), we obtain
µ t µ0 exp Et · (8)
Eqns 7 and 8 agree qualitatively with the viscoelastic behaviour shown in Fig
7d. Although the Voigt element and other more complicated combinations of
springs and dashpots provide some useful insight into the viscoelastic properties
of polymers, they are inadequate in describing the creep behaviour of a real poly-
meric material. Many empirical equations have been proposed. One which applies
to some of the common engineering plastics, has the form
µ t KÃtn (9)
where n, K are constants for a given polymer and 0 n 1. In cases where n 0,
the material behaves in a purely elastic manner. Alternatively, at n 1thematerial
behaves as viscous fluid. The value of n obtained from creep data is therefore a
measure of the relative contributions of elastic and viscous deformation to the
creep process.
2.4 High-strain behaviour and failure
If a material is subjected to high-strain deformation, it deforms permanently
(plastic deformation) and ultimately fails. In fig. 9, we show a graph of stress-
strain behaviour over the entire strain range and the ultimate failure (rupture) for a
typical polymeric material subjected to a tensile test. For sufficiently low stresses
and strains, the polymeric material behaves as a linear elastic solid. The point
where the behaviour starts to be non-linear is called the proportional limit. The
local maximum in the stress-strain curve is called the yield point and indicates the
onset of plastic (i.e. permanent) deformation. The corresponding stress and elon-
gation are called yield strength and elongation at yield. Beyond the yield point the
material stretches out considerably and a neck is formed; this region is called
the plastic region. Further elongation leads to an abrupt increase in stress (strain
hardening) and the ultimate rupture of the material. At the rupture point the cor-
responding stress and strain are called the ultimate strength and the elongation at
10
C
B
A
Strain
Figure 9: Typical stress-strain curve of a polymeric material. A is the proportional point, B the
yield point and C the rupture (break) point.
break, respectively.
The stress-strain behaviour of a polymeric material depends on various parameters
such as molecular characteristics, microstructure, strain-rate and temperature.
3. Brief molecular interpretation of the mechanical behaviour of polymeric
materials
The observed elastic behaviour of solids at low stress-strains is due to the
stretching of their chemical bonds, which are inherently short-range. Par-
ticularly in polymers, although the above mechanism cannot be excluded,
the elastic behaviour is more complicated due to the chain-like structure
of the macromolecules. A polymer chain resists stretching because it re-
duces its entropy (we can also say that a polymer chain resists stretching
due to its thermal movement which is significantly hindered if stretched).
The associated restoring force is elastic and it is the underlying cause for
the mechanical behaviour of elastomers (e.g. rubber elasticity).
Viscosity is a measure of the friction and the associated energy dissipation
between molecules of fluids. Polymeric materials due to their macromolec-
ular (long-chain) structure are expected to have high viscosities.
11
Stress
Creep, flow and plastic deformation in polymeric materials results from the
irreversible slippage, decoupling and disentanglements of polymer chains
(or groups of chains in semicrystalline polymers).
Strain hardening results from the high orientation and alignment of polymer
chains at high strains.
References
Main
W. D. Callister Jr Materials Science and Engineering: An introduction Fifth
edition, Wiley, 2000
Supplementary
Christopher Hall, Polymer Materials: An Introduction for Technologists and
Scientists, Second edition, Macmillan Education, 1989
Christopher Hall, Polymers , in N. Jackson and R. K. Dhir (eds) Civil
engineering materials, Fifth edition, Macmillan 1996
P. C. Painter and M. Coleman Fundamentals of Polymer Science: An Intro-
ductory Text Second edition, Technomic, 1997.
A. B. Strong Plastics: Materials and Processing Second edition, Prentice-
Hall, 2000.
A. Meyers and K. K. Chawla Mechanical Behaviour of Materials, Prentice-
Hall, 1999
12
Vasileios Koutsos
Revised 13 May 2002
13
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