Lecture . 2: material physics / 2nd class/ polymer branch
Structure of materials:
Introduction:
Firstly, there are five main types of materials , these materials are:
1- metallic
2- ceramic
3- polymer
crystal
4- liquid
5- fibers
These materials different each other by its structure, this structure lead to
each properties of each one. Then before we discus each one of these
materials, must be known what's mean by structure of materials?
The Concept of Structure
The structure of a material may be divided into four levels: atomic
structure, atomic arrangement, microstructure, and macrostructure.
Although the main thrust of [the materials engineer] is to understand and
control the microstructure and macro-structure of various materials, [you]
must first understand the atomic and crystal structures.
Atomic structure influences how the atoms are bonded together, which in
turn helps one to categorize materials as metals, ceramics, and polymers
and permits us to draw some general conclusions concerning the
mechanical properties and physical behavior of these three classes of
materials.
There are many methods that the atoms are bonded together, this lead to
three main types of materials depend on this method, as following:
1- Crystal structure such as ceramic and metallic materials in solid
state
2- Simi crystalline materials such as some types of polymer
3- Amorphous materials such as among of polymer types and glassy
materials.
1- Crystal structure:
In mineralogy and crystallography, a crystal structure is a unique
arrangement of atoms or molecules in a crystalline liquid or solid.[1] A
crystal structure describes a highly ordered structure, occurring due to the
intrinsic nature of molecules to form symmetric patterns. A crystal
structure can be thought of as an infinitely repeating array of 3D 'boxes',
known as unit cells. The unit cell is calculated from the simplest possible
representation of molecules, known as the asymmetric unit. The
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Lecture . 2: material physics / 2nd class/ polymer branch
asymmetric unit is translated to the unit cell through symmetry
operations, and the resultant crystal lattice is constructed through
repetition of the unit cell infinitely in 3-dimensions. Patterns are located
upon the points of a lattice, which is an array of points repeating
periodically in three dimensions. The lengths of the edges of a unit cell
and the angles between them are called the lattice parameters. The
symmetry properties of the crystal are embodied in its space group.[1]
A crystal's structure and symmetry play a role in determining many of its
physical properties, such as cleavage, electronic band structure, and
optical transparency.
Unit cell: The crystal structure of a material (the arrangement of atoms
within a given type of crystal) can be described in terms of its unit cell.
The unit cell is a small box containing one or more atoms arranged in 3dimensions. The unit cells stacked in three-dimensional space describe
the bulk arrangement of atoms of the crystal. The unit cell is represented
in terms of its lattice parameters, which are the lengths of the cell edges
(a,b and c) and the angles between them (alpha, beta and gamma),
while the positions of the atoms inside the unit cell are described by the
set of atomic positions (xi , yi , zi) measured from a lattice point.
Commonly, atomic positions are represented in terms of fractional
coordinates, relative to the unit cell lengths
Miller indices
Vectors and atomic planes in a crystal lattice can be described by a threevalue Miller index notation (ℓmn). The ℓ, m, and n directional indices are
separated by 90°, and are thus orthogonal.
By definition, (ℓmn) denotes a plane that intercepts the three points a1/ℓ,
a2/m, and a3/n, or some multiple thereof. That is, the Miller indices are
proportional to the inverses of the intercepts of the plane with the unit cell
(in the basis of the lattice vectors).
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Lecture . 2: material physics / 2nd class/ polymer branch
Planes with different Miller indices in cubic crystals
Planes and directions
The crystallographic directions are geometric lines linking nodes (atoms,
ions or molecules) of a crystal. Likewise, the crystallographic planes are
geometric planes linking nodes. Some directions and planes have a higher
density of nodes. These high density planes have an influence on the
behavior of the crystal as follows:[1]

Optical properties: Refractive index is directly related to density
(or periodic density fluctuations).

Adsorption and reactivity: Physical adsorption and chemical
reactions occur at or near surface atoms or molecules. These
phenomena are thus sensitive to the density of nodes.

Surface tension: The condensation of a material means that the
atoms, ions or molecules are more stable if they are surrounded
by other similar species. The surface tension of an interface thus
varies according to the density on the surface.
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Lecture . 2: material physics / 2nd class/ polymer branch
Dense crystallographic planes

Microstructural defects: Pores and crystallites tend to have
straight grain boundaries following higher density planes.

Cleavage: This typically occurs preferentially parallel to higher
density planes.

Plastic deformation: Dislocation glide occurs preferentially parallel
to higher density planes. The perturbation carried by the
dislocation (Burgers vector) is along a dense direction. The shift of
one node in a more dense direction requires a lesser distortion of
the crystal lattice
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Lecture . 2: material physics / 2nd class/ polymer branch
Increasing of symmetry
Cubic
Hexagonal
Tetragonal
Trigonal
Orthorhombic
Monoclinic
Triclinic
Non-crystalline materials or "glassy", "amorphous"
"amorphous solid" is considered to be the overarching concept, and
glass the more special case: A glass is an amorphous solid that exhibits a
glass transition.[1] Polymers are often amorphous. Other types of
amorphous solids include gels, thin films, and nanostructured materials.
A polymer [ (poly-, "many" + -mer, "parts") is a large molecule, or
macromolecule, composed of many repeated subunits. Because of their
broad range of properties,[4] both synthetic and natural polymers play an
essential and ubiquitous role in everyday life.[5] Polymers range from
familiar synthetic plastics such as polystyrene to natural biopolymers
such as DNA and proteins that are fundamental to biological structure
and function. Polymers, both natural and synthetic, are created via
polymerization of many small molecules, known as monomers. Their
consequently large molecular mass relative to small molecule compounds
produces unique physical properties, including toughness, viscoelasticity,
and a tendency to form glasses and semicrystalline structures rather than
crystals.
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Lecture . 2: material physics / 2nd class/ polymer branch
Polymer Structure
Although the fundamental property of bulk polymers is the degree of
polymerization, the physical structure of the chain is also an important
factor that determines the macroscopic properties.
The terms configuration and conformation are used to describe the
geometric structure of a polymer and are often confused. Configuration
refers to the order that is determined by chemical bonds. The
configuration of a polymer cannot be altered unless chemical bonds are
broken and reformed. Conformation refers to order that arises from the
rotation of molecules about the single bonds. These two structures are
studied below.
Configuration
The two types of polymer configurations are cis and trans. These
structures can not be changed by physical means (e.g. rotation). The cis
configuration arises when substituent groups are on the same side of a
carbon-carbon double bond. Trans refers to the substituents on opposite
sides of the double bond.
Stereoregularity is the term used to describe the configuration of polymer
chains. Three distinct structures can be obtained. Isotactic is an
arrangement where all substituents are on the same side of the polymer
chain. A syndiotactic polymer chain is composed of alternating groups
and atactic is a random combination of the groups. The following
diagram shows two of the three stereoisomers of polymer chain.
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Lecture . 2: material physics / 2nd class/ polymer branch
Isotactic
Syndiotactic
Conformation
If two atoms are joined by a single bond then rotation about that bond is
possible since, unlike a double bond, it does not require breaking the
bond.
The ability of an atom to rotate this way relative to the atoms which it
joins is known as an adjustment of the torsional angle. If the two atoms
have other atoms or groups attached to them then configurations which
vary in torsional angle are known as conformations. Since different
conformations represent varying distances between the atoms or groups
rotating about the bond, and these distances determine the amount and
type of interaction between adjacent atoms or groups, different
conformation may represent different potential energies of the molecule.
There several possible generalized conformations: Anti (Trans), Eclipsed
(Cis), and Gauche (+ or -). The following animation illustrates the
differences between them.
Conformation Lattice Simulation
Like the polymer growth simulation, the conformation lattice simulation
takes a statistical approach to the study of polymers. Probabilities of the
different conformations are assigned which produces a polymer chain
with many possible shapes. Click the icon to enter the virtual laboratory.
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Lecture . 2: material physics / 2nd class/ polymer branch
Other Chain Structures
The geometric arrangement of the bonds is not the only way the structure
of a polymer can vary. A branched polymer is formed when there are
"side chains" attached to a main chain. A simple example of a branched
polymer is shown in the following diagram.
There are, however, many ways a branched polymer can be arranged.
One of these types is called "star-branching". Star branching results
when a polymerization starts with a single monomer and has branches
radially outward from this point. Polymers with a high degree of
branching are called dendrimers Often in these molecules, branches
themselves have branches. This tends to give the molecule an overall
spherical shape in three dimensions.
A separate kind of chain structure arises when more that one type of
monomer is involved in the synthesis reaction. These polymers that
incorporate more than one kind of monomer into their chain are called
copolymers. There are three important types of copolymers. A random
copolymer contains a random arrangement of the multiple monomers. A
block copolymer contains blocks of monomers of the same type. Finally,
a graft copolymer contains a main chain polymer consisting of one type
of monomer with branches made up of other monomers. The following
diagram displays the different types of copolymers.
Block Copolymer
Graft Copolymer
Random Copolymer
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Lecture . 2: material physics / 2nd class/ polymer branch
An example of a common copolymer is Nylon. Nylon is an alternating
copolymer with 2 monomers, a 6 carbon diacid and a 6 carbon diamine.
The following picture shows one monomer of the diacid combined with
one monomer of the diamine:
Cross-Linking
In addition to the bonds which hold monomers together in a polymer
chain, many polymers form bonds between neighboring chains. These
bonds can be formed directly between the neighboring chains, or two
chains may bond to a third common molecule. Though not as strong or
rigid as the bonds within the chain, these cross-links have an important
effect on the polymer. Polymers with a high enough degree of crosslinking have "memory." When the polymer is stretched, the cross-links
prevent the individual chains from sliding past each other. The chains
may straighten out, but once the stress is removed they return to their
original position and the object returns to its original shape.
One example of cross-linking is vulcanization . In vulcanization, a series
of cross-links are introduced into an elastomer to give it strength. This
technique is commonly used to strengthen rubber.
Classes of Polymers
Polymer science is a broad field that includes many types of materials
which incorporate long chain structure of many repeat units as discussed
above. The two major polymer classes are described here.
Elastomers,or rubbery materials, have a loose cross-linked structure. This
type of chain structure causes elastomers to possess memory. Typically,
about 1 in 100 molecules are cross-linked on average. When the average
number of cross-links rises to about 1 in 30 the material becomes more
rigid and brittle. Natural and synthetic rubbers are both common
examples of elastomers. Plastics are polymers which, under appropriate
conditions of temperature and pressure, can be molded or shaped (such as
blowing to form a film). In contrast to elastomers, plastics have a greater
stiffness and lack reversible elasticity. All plastics are polymers but not
all polymers are plastics. Cellulose is an example of a polymeric material
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Lecture . 2: material physics / 2nd class/ polymer branch
which must be substantially modified before processing with the usual
methods used for plastics. Some plastics, such as nylon and cellulose
acetate, are formed into fibers (which are regarded by some as a separate
class of polymers in spite of a considerable overlap with plastics). As we
shall see in the section on liquid crystals, some of the main chain polymer
liquid crystals also are the constituents of important fibers. Every day
plastics such as polyethylene and poly(vinyl chloride) have replaced
traditional materials like paper and copper for a wide variety of
applications. The section on Polymer Applications will go into greater
detail about the special properties of the many types of polymers.
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