Lec.1………………………………………………………………………………….Introduction to polymer morphology
Introduction to Polymer Morphology
Morphology is the science of form and structure. As applied to polymers,
morphology involves the study of the arrangement of polymer molecules into
crystalline and amorphous regions.
Molecular shape and the way molecules are arranged in a solid are important
factors in determining the properties of polymers.
Lec.1………………………………………………………………………………….Introduction to polymer morphology
Low molar mass organic molecules and polymeric materials are often found
as solids and their physical properties are a consequence of the way in which
the molecules are organized: their morphology.
The morphology is a result of specific molecular interactions which control
the processes involved in the individual molecules packing together to form a
solid phase. Depending on the extent of the molecular organization, a
crystalline solid, liquid crystals or amorphous solid may be formed.
Lec.1………………………………………………………………………………….Introduction to polymer morphology
Importance of studying the morphology:
1- It provides the link between the molecular structure and the bulk properties.
Example: Barrier properties
2- There is a relationship between morphology and processing conditions
Example: Flexural Modulus
Lec.1………………………………………………………………………………….Introduction to polymer morphology
3- Crystalline polymers show ordering at a variety of dimensional levels, from
interatomic spacings to macroscopic measures. The study of polymer
morphology is primarily concerned to elucidate this organization.
Lec.1………………………………………………………………………………….Introduction to polymer morphology
4- Furthermore, the morphology is a record of the past history of a sample
which, with sufficient understanding, may be read to disclose not only
crystallization, annealing or deformation treatments to which it has been
subjected but can also
5-Provide an indication of certain intrinsic properties, such as the molecular
mass range within a specimen or the nature and extent of molecular branching.
History of polymer morphology:
►Historically, the first detailed knowledge of polymer morphology
obtained was that of the crystal structures, i.e. chain packing using the
techniques of X-ray crystallography. In addition to the sharp, crystalline
reflexions used for the structural analyses, polymer samples generally show
diffuse, liquid-like diffraction indicative of more disordered molecular
arrangements (amorphous regions).
► For many years the fringed-micelle model held sway. A sketch of a
possible arrangement is shown in Fig. I .2.
Fig. 1.2. The fringed-micelle model of polymeric texture
An individual molecule would be likely to pass through different regions
of order and disorder.
The fraction of the material supposed to be fully-crystalline is known as the
(degree of) crysrallinity of a sample and is a widely used parameter in
comparisons of similar textures and provided numerical values.
►The textural scale of the fringed-micelle model was believed, primarily
on the basis of crystallite sizes estimated from the widths of X-ray diffraction
rings, to be on the scale of a few tens of nm. Such dimensions were not then
Lec.1………………………………………………………………………………….Introduction to polymer morphology
observable at a time when the first electron microscopes were just being
developed. Not until 1945 was it appreciated, first for PE and subsequently
for other polymers, that there was additional ordering on the scale of several
μm. This is due to the prevalent crystallization of high polymers as
spherulites, literally little spheres, a mode previously found mostly in viscous
minerals.
► The study of spherulites with the polarizing optical microscope then
became the second major area of polymer morphology to be investigated.
► At this stage the link of morphology with properties becomes particularly
evident. On one hand spherulites as optical inhomogeneities on a scale of μm
scatter light strongly and are responsible for the cloudiness of, for example,
PE film which may be a disadvantage for a packaging material, on the other
spherulites are associated with, and believed to be due to, the segregation of
different molecular species in a sample. For example shorter molecules are
likely to predominate in inter-spherulitic boundaries and give these regions
Lec.1………………………………………………………………………………….Introduction to polymer morphology
different mechanical properties leading in certain circumstances to
preferential fracture between spherulites.
►When such behaviour results it becomes particularly important to
understand what the morphological texture is, how it formed, how it can be
controlled and, if possible, modified to give improved properties.
►Matters became a good deal clearer when the introduction of the first
generation of modern electron microscopes, coinciding with the synthesis of
highly linear and stereoregular polymers following Ziegler and Nat ta, led to
the discovery of individual polymer crystals grown from very dilute
solutions. This similarity suggested that molecules - typically 5-10 μm long –
lay in the crystals with their lengths across the thin dimension (12 nm) of the
lamellae. The inescapable conclusion was that the chains must fold back
on themselves repetitively at each crystal surface alternately, a phenomenon
now known to be widespread and called chainfolding (Fig. 1.4).
Fig. 1.4. A chainfolded conformation (schematic).
Lec.1………………………………………………………………………………….Introduction to polymer morphology
►These twin discoveries of polymer lamellar crystals and chainfolding lie
at the heart of modern understanding of polymeric morphologies.
►So far as bulk, melt-crystallized polymers are concerned, evidence for their
containing lamellae has been much harder to obtain than for solution-grown
specimens but it has always been clear that folding would be neither so
complete nor so regular as from solution.
Nevertheless, evidence for lamellae has accumulated mostly by comparison
with the similar behaviour of solution-grown crystals using a variety of
techniques such as small-angle X-ray scattering, thermal measurements
and molecular mass measurements of chemically degraded materials.
What has not been possible, until very recently, is to observe representative
lamellae microscopically in melt-crystallized polymer.
The introduction of new techniques of specimen preparation has now made
this possible and there is little doubt that it will lead to much more firmly
Lec.1………………………………………………………………………………….Introduction to polymer morphology
based understanding of the organization and crystallization of polymers
grown from the melt.
At the same time the new tool of small-angle neutron scattering has begun
to give information on the conformations of individual molecules in both
molten and melt-crystallized polymers.
Polymer Structure
Configuration vs. Conformation:
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.
Lec.1………………………………………………………………………………….Introduction to polymer morphology
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.
Lec.1………………………………………………………………………………….Introduction to polymer morphology
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.
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.
In the case of the n-alkanes, the bond lengths for the C–C and C–H bonds
are, respectively, 1.5 and 1.10 A ˚ and the C–C–C bond angle is 112o .The H–
C–H bond angle has been found to be 109.
The conformational changes can be described by a potential energy diagram
(Figure 1.5).
Lec.1………………………………………………………………………………….Introduction to polymer morphology
Figure 1.5 Potential energy curve obtained for n-butane for rotation about
the 2,3 central bond of the molecule.
A higher energy gauche state exists in which the interaction between
neighbouring atoms is greater than in the trans conformation.
At any temperature above absolute zero there will be a finite population of
the higher energy gauche state dictated by the Boltzmann distribution:
where ∆E is the energy difference between the gauche and trans states, g 1 and
g2 are, respectively, the degeneracy of the trans and gauche states at the
temperature T and R is the gas constant.
The trans conformation is the lowest energy state and is able to nucleate
crystal growth.
Abe et al. have shown that the potential energy profile can be reproduced by
selecting a barrier to the interchange between the trans and gauche forms of
12 540 J mol_1 and the energy difference between the two conformations has
a value of 2090 J mol_1. The energy and barrier to rotation are a result of
Lec.1………………………………………………………………………………….Introduction to polymer morphology
nonbonding repulsive and attractive interactions between the hydrogen–
hydrogen and hydrogen–carbon atoms on neighbouring carbon atoms.
The conformation of n-butane, the simplest n-alkane, is described by three
rotation angles (Figure 1.6).
Lec.1………………………………………………………………………………….Introduction to polymer morphology
Lec.1………………………………………………………………………………….Introduction to polymer morphology
Poly(methylene) Chains
The values of the energies that are usually used to describe the curve are 2.1
kJ mol_1 for the energy difference and 8.4 kJmol_1 for the eclipsed state. Flory
and others have shown that this simple approach can be applied successfully
to many other chains.
The basis of the so-called Rotational Isomeric States Model (RISM) used
extensively for the prediction of the physical properties of polymers is thus
based on simple additive effects of nonbonding interactions between the
atoms attached to the backbone carbon atoms.