ORGANIC AND PHYSICAL CHEMISTRY OF POLYMERS phần 3 ppsx

POLYMER CHAINS WITH REGULAR CONFORMATIONS 117
Guanine
(
G
)
NH
N
N
O
NH
2
C
y
tosine
(
C
)
HN
NO
NH
2
Chain moieties comprising these saccharide cycles and the bases are called nucle-
osides (adenosine, guanosine, cytidine, and thymine in the case of DNA), and the
phosphoric esters of these nucleosides are the nucleotides (adenylic acid, guanylic
acid, cytidylic acid, and thymidylic acid).
Hence, DNA can be regarded as a statistical copolymer between four different
comonomers (hence it is called quaterpolymer):
Adenine-thymine
N
N
N

N
Sugar
N
N
O
O
CH
3
H
Sugar
Guanine-cytosine
N
N
N
O
N
H
H
H
Sugar
N
N
O
NH
H
Sugar
NH
H
Unlike ribonucleic acids, which are single-strand polymers, DNA form double-
strand helices, a sort of twisted ladder, consisting of two complementary chains.

This complementarity occurs through intermolecular hydrogen bonding between
two pairs of bases, between the adenosine of one strand and the thymidine of
another, and likewise between cytidine and guanosine.
This double helix was identified by Crick and Watson in 1954; the rungs
of the twisted ladder correspond to the pair of bases. DNA is always formed
by replication/duplication upon separation of the double strands; the intermedi-
ate single strands are the matrices for the generation of new DNA chains. After
replication, each double helix includes one old strand and a new one. Three
successive nucleotides of DNA provide the code for one amino acid, and the
genetic code is determined by the sequence of these triplets.
Each nucleus in a living cell contains long, thread-like structures called chro-
mosomes, which carry bits of genes. Both chromosomes and genes are made of
DNA, which is often called the blueprint for life; every living cell contains indeed
a copy of the blueprint.
Figure 5.15 shows the complexity of such a structure, underscoring the progress
made by biology to unveil it.
118 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
C
G
A
T
A
T
T
G
G
C
A
C
Figure 5.15. Representation of the DNA double-helix organization showing A-T and C-G

links.
5.3. CHAIN PACKING
5.3.1. Assembly of Random Coils
Taken individually, random coils commonly exhibit a Gaussian distribution of their
constitutive units if one considers the distance of the latter to the center of mass of
the macromolecule under consideration. The chains constituting a sample add their
distributions and give an apparently homogeneous material down to the nanometer
level. Figure 5.16 schematizes the situation of an assembly of chains of different
size, showing the interpenetration of random coils in the condensed state. Such an
interpenetration leads to interchain entanglements and enhances the cohesion of the
corresponding material.
For reasons that are related to the rigidity of constitutive units, atoms in certain
polymers do not occupy entirely the space available to them in spite of the apparent
homogeneity of the latter as illustrated by the horizontal straight line of the P =f (r)
diagram (line corresponding to the addition of the probabilities of presence of
monomer units belonging to different chains). To account for this unfilled space,
CHAIN PACKING 119
P
r
Figure 5.16. Diagram showing the variation of the probability P of presence of monomeric units
belonging to an assembly of polymeric chains as a function of their distance R to a reference
point.
the concept of free volume was introduced. The free volume (see Chapter 11) plays
an important role with respect to thermomechanical properties (glass transition) and
transfer properties (permeability, etc.).
5.3.2. Packing of Sequences of Regular Chains
Due to the possible existence of defects in the molecular structure of monomeric
units and in their placement, an assembly of regular chains can be described only
for short sequences whose length is closely related to the extent of their regularity.
In that respect, only linear and stereoregular sequences can be taken into account

since branching points, junction points in networks, chain ends, and configurational
irregularities are structural defects that oppose the regular chain packing in their
totality. Given the difficulty for chains to organize on a large scale due to the
macromolecular state, only assemblies made up of a limited number of constitutive
units will be described.
Three categories of assemblies can be arbitrarily distinguished whose geometry
is determined by the molecular structure of the constitutive unit, the relative size and
bulkiness of side groups, and the conformation of the isolated chain. This geometry
is governed by the tendency of these assemblies to minimize their potential energy
and maximize their molecular interactions (intra- and interchain).
The first category of assemblies is that of chains which exhibit a cylindrical
overall shape and can be viewed as screws with small “threads.” For the maximum
development of molecular interactions, the chains tend to minimize the distance
between them (which corresponds to the maximum density), and it is the hexagonal
packing which complies best with this criterion as shown in Figure 5.17.
A typical example of such a packing is that of polytetrafluoroethylene which
crystallizes in a hexagonal system with a =0.554 nm and b =1.680 nm and whose
regular conformation was previously described (see page 111).
120 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
Figure 5.17. Hexagonal packing of chains similar to cylinders.
The second group is that of chains with helical conformation, which are different
from cylinders: their “threads” are indeed more prominent, and the number of
constitutive units per helix turn is fractional. The chain packing takes a tetragonal
symmetry with an interpenetration of the “threads” of a left-handed helix with
those of the four right-handed helices which surround it and vice versa. Figure 5.18
represents an arrangement of such chains in which the size of the thread is measured
by the ratio R/r. It also shows the way chains assemble and the relative direction
of the interpenetrated helices. In this group, isotactic poly(4-methylpent-1-ene) is
found whose regular conformation is a 7
2

helix. This means that the period of
identity along the fiber axis of the chain is 7 repetitive units regularly placed on 2
helix turns; the parameters of the corresponding tetragonal cell are: a =b =1.86 nm
and c =13.7 nm.
CH
2
CH
CH
2
CH
CH
3
CH
3
n
The third group includes chains similar to the preceding ones, whose number
of constitutive units per helix turn is a nonfractional number. In this case, the
symmetry of the assembly reflects that of the individual chain: ternary symme-
try for an assembly of chains with a ternary symmetry, and so on. Figures 5.19
and 5.20 show such an assembly with ternary and quaternary symmetries, respec-
tively.
It is worth stressing that the criteria that distinguish the above groups tolerate a
number of exceptions which can be found even for an usual polymer with a simple
structure.
CHAIN PACKING 121
Figure 5.18. Diagram of the packing of helical chains exhibiting a tetragonal symmetry.
Figure 5.19. Packing of ternary symmetry chains: isotactic polybut-1-ene (conformation 3
1
).
Figure 5.20. Packing of quaternary symmetry chains: isotactic polyacetaldehyde (confor-

mation 4
1
).
122 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
5.4. MORPHOLOGY OF MACROMOLECULAR SYSTEMS
The term morphology corresponds to the structure taken by polymers at the micro-
scopic level. The morphology of a macromolecular system is primarily determined
by the molecular regularity (placement of the constitutive units and configurational
regularity) and by the treatment undergone by the sample prior to or being in the
solid state. All situations can exist between the amorphous state corresponding to
the maximum entropy for a macromolecular system and the single crystal whose
only imperfections are due to chain folding and the molecular irregularity of their
ends. The various situations will be successively examined.
5.4.1. Homogeneous Amorphous State
The amorphous state can be depicted as a multitude of random coils being thor-
oughly entangled. At the microscopic level, this brings about an apparent homo-
geneity, which is responsible, in particular, for the transparency of these systems
to the visible light; such polymers are often called organic glasses.
The amorphous state results from the impossibility of chains to crystallize due
to the existence of defects at the molecular level or difficulty for the chains to
disentangle when cooling from the molten state. In the latter case, a fast cooling
quenches the disordered molten state.
Poly(methyl methacrylate) (PMMA) and polystyrene (PS) obtained by free
radical polymerization are amorphous due to their atacticity. Poly(ethylene tereph-
thalate) (PET) is also amorphous when quenched from the molten state but is
potentially crystallizable; the rigidity of the chains prevents them from disentan-
gling rapidly enough so that they remain as in the molten state—that is, in a
disordered state.
5.4.2. Extended Chain Polymers
Due to the molecular agitation at the time of the transition, chains can hardly crystal-

lize in an extended form and without folding. However, chains that are highly rigid
such as aromatic polyamides—for example, poly(p-phenyleneterephthalamide)
NN
HH
OO
n
with their rigid phenylene moieties and interchain hydrogen bonds between amide
functional groups (-CO-NH-)—crystallize almost unfolded. Application of an exter-
nal stress can also prevent chain folding. For instance, poly(oxymethylene) (POM)
[-(CH
2
-O)
n
-] obtained by solid-state radiation polymerization of cyclic trioxane
(CH
2
-O)
3
form extended crystalline chains. In this case, the monomer is polymer-
ized in its crystal form which affords directly stretched chains; the length of the
MORPHOLOGY OF MACROMOLECULAR SYSTEMS 123
Figure 5.21. Electron micrography of a fracture of a PE sample revealing zones comprising
extended chains.
extended crystalline part corresponds to the molar mass of the chain (chains in
total extension).
Extended chains whose folding occurs only beyond ∼100 nm are also consid-
ered. Such length corresponds to degrees of polymerization (for common vinyl
polymers) higher than 500. Such structures are observed in “nascent” polytetraflu-
oroethylene (PTFE), which forms partially extended highly rigid helical chains. In
another example, when polyethylene is crystallized under strong pressure (about

100 MPa), it can also give rise to extended chains of the type shown in Figure 5.21.
Chains extended under stressed conditions do not exhibit the same attractive-
ness applicationwise as those oriented monodimensionally (fibers and films) or
two-dimensionally (films) stretched. Section 5.5 will be devoted to the description
of orientated polymers.
5.4.3. Single Crystals
Upon cooling slowly dilute solutions of a polymer of great molecular regularity,
it is possible to obtain single crystals with a morphology close to that of simple
molecules as shown in Figure 5.22a (electron diffraction).
These single crystals exhibit the most regular arrangement possibly formed in a
polymer; they form lamellae (Figure 5.23) whose thickness (a few tens of nanome-
ters) is determined by the nature of the polymer and the thermodynamic conditions
of crystallization.
These lamellae can also pile up by means of screw dislocations (see Figure 5.28)
and afford more complex structures such as those shown in Figure 5.22b. Their
dimensions (about a few tens of micrometers) are such that optical or electron
microscopies are essential techniques to visualize them. By analysis of the elec-
tron diffraction patterns of these single crystals, it could be established that they
124 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
(a) (b)
Figure 5.22. Electron diffraction pattern (a) and transmission electron micrography (b) of a
single crystal of polyethylene of pyramidal shape. [Courtesy of J. C. Wittmann, ICS, CNRS
Strasbourg (France).]
Figure 5.23. Transmission electron micrography of monolamellar single crystals of polyethy-
lene.
comprise regularly folded linear chains, as schematically shown in Figure 5.24, and
that the chain axes are perpendicular to the surface of the lamellae. The existence
of such folding is proved by the fact that the thickness of the lamellae is generally
much lower than the length of totally stretched chains. Due to this folding, irregu-
larities related to the dispersion in the molar masses of the chains forming a same

single crystal can be somehow compensated.
From a thermodynamic point of view, the regular folding of the chains is the
result of a compromise between the increase of the free energy of the system related
to torsional and longitudinal oscillations of extended chains under the effect of
molecular agitation and the tendency of the crystal to exhibit a minimum surface
free energy. The existence of such a compromise indicates that the dimension of the
extended segments (thickness of the lamellae) is likely affected by the temperature
of crystallization.
This is what is actually observed experimentally. It is even possible to aug-
ment the thickness of a preexisting single crystal by means of a thermal treatment
MORPHOLOGY OF MACROMOLECULAR SYSTEMS 125
Face 110
Figure 5.24. Schematic representation of an ideal polyethylene single crystal resulting from
the folding of chains planar zigzag conformation.
Figure 5.25. Polyethylene single crystal ‘‘reconditioned’’ in a different thermodynamic envi-
ronment from that of the initial crystallization and resulting in the formation of ‘‘holes.’’
(annealing); as the other dimensions remain constant, “holes” appear in the crystal
to compensate the increase of the lamellae thickness (Figure 5.25).
5.4.4. Semi-crystalline State
It corresponds to a state intermediate between the amorphous state and a strongly
ordered one such as that of a single crystal. All polymers that exhibit a sufficiently
high molecular regularity to generate crystalline zones organize in a semicrys-
talline state when subjected to favorable thermal and kinetic conditions (see Section
12.3). Before describing various morphologies referring to this physical state, it is
126 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
necessary to define the degree of crystallinity (X) of semicrystalline polymers,
which is simply the proportion of crystalline matter; depending on whether this
proportion is expressed in mass or in volume, slightly different values will result.
The degree of crystallinity (X
v

) in volume is defined as
X
v
= V
c
/(V
a
+V
c
) = V
c
/V
a relation in which V
a
,V
c
,andV denote the respective volumes of the amorphous
and crystalline phases and the total volume phases of the sample studied.
Inthesameway,thedegree of crystallinity (X
m
) in mass can be defined as
X
m
= m
c
/(m
a
+m
c
) = m

c
/m
a relation in which m
a
,m
c
,andm denote the mass of the amorphous and crystalline
phases and the total mass phases, respectively.
If
V
c
, V,ρ
c
, and ρ are the bulk volumes and the densities of the crystalline
phase and of the entire sample, one can write
X
v
=
V
c
m
c
Vm
=
V
a
V
X
m
=

ρ
ρ
c
X
m
For the majority of polymers, both mass and volume degrees of crystallinity are
not very different; both are thus used indifferently, depending upon the method
utilized to carry out the measurement.
It is worth emphasizing that the physical, chemical, mechanical, and so on, prop-
erties of amorphous and crystalline phases are very different. In most cases, there
is a proportional additivity of the specific properties. If
P
a
, P
c
, and P represent,
respectively, the specific property of the amorphous phases, the crystalline phases,
and all the phases, one can write
P = XP
c
+(1 −X)P
a
and this relation is used for the measurement of the degree of crystallinity (see
Section 6.5).
Depending upon the degree of crystallinity of a polymer, regardless of whether
it is low or high, two different types of morphology for semicrystalline systems
can be distinguished.
For low degrees of crystallinity, the morphology can be described by the fringed
micelle model, with small-size crystallites being dispersed in an amorphous poly-
mer matrix. The size and the degree of perfection of these crystallites are closely

related to the length of the regular—and thus crystallizable—sequences in the
chains constituting the sample. Figure 5.26 schematically represents a polymer
exhibiting such a morphology. In this representation the crystallites result from
the packing of more or less long sequences belonging to different chains. In addi-
tion, the same chain can be involved in the formation of several crystallites; it
MORPHOLOGY OF MACROMOLECULAR SYSTEMS 127
Figure 5.26. Diagram showing the morphology of a semicrystalline polymer with a low degree
of crystallinity (crystallites are ringed).
is thus impossible to physically separate crystalline domains from the amorphous
phase. Crystallites are zones of high density of cohesive energy; they play the role
of physical cross-links and, even in small proportion, can significantly affect the
mechanical properties of polymers in the solid state. For instance, the difficulties
encountered in the processing of PVC are of rheological origin and due to the
crystallization of short syndiotactic sequences.
For high degrees of crystallinity, the crystalline zones give rise to an organi-
zation of higher order. They represent the majority of the sample and self-organize
in lamellae made of folded chains as described in the case of monocrystals. Both
impurities present in the medium and noncrystallizable sequences or sequences that
could not crystallize due to kinetic reasons are rejected into interlamellar zones and
form the amorphous phase (Figure 5.27). Because the matter in such amorphous
zones is less cohesive than that in the crystalline layers, its proportion and its
thermomechanical characteristics can considerably affect the overall mechanical
properties of the whole sample.
Figure 5.27. Diagram showing the detail of the lamellar structure of a spherulite in a polymer
with a high degree of crystallinity.
128 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
Figure 5.28. Representation of a screw dislocation: the arrow indicates the direction of growth
of the dislocation.
The examination of this structure shows that a same chain can be involved in
the formation of different lamellae, thus bringing about the cohesion between the

various layers. The three-dimensional filling of space by crystallized matter occurs
by means of dislocations (Figure 5.28) and various lamellae stuctures resulting
from crystallographic defects.
At the microscopic level a structure with an apparent spherical symmetry is
obtained, which is referred to as spherulite. Figure 5.29 explains how the growth
of such lamellae (direction of the arrows) and the space filling by the crystallized
matter in the perpendicular direction to the orientation of the chains occur.
The electron microscopy image of Figure 5.30 clearly shows that the crystal-
lization starts from a central nucleus and grows through the formation of layers
whose orientation corresponds to the representation of the Figure 5.29.
In addition, the examination of spherulites in polarized light reveals textures
which can be related to the orientation of lamellae and thus to that of the chains.
Figure 5.29. Representation of the directions of the growth of the lamellae starting from a
microcrystalline nucleus (crystallization from the molten state).
MORPHOLOGY OF MACROMOLECULAR SYSTEMS 129
Figure 5.30. Electron micrography of a polyethylene spherulite at the beginning of growth.
[Courtesy of B. Lotz, ICS-CNRS, Strasbourg (France).]
Figure 5.31. Microscopic texture of a polyethylene spherulite with radial lamellae. Observation
between crossed nicols. [Courtesy of B. Lotz, ICS-CNRS, Strasbourg (France).]
Figure 5.31 is characteristic of a highly crystalline structure in which the lamel-
lae are oriented along the radius of the spherulite; in Figure 5.32, the existence of
concentric extinction lines in the texture shows that the lamellae are twisted as
schematically represented in Figure 5.33.
Crystallization from the molten state is an important phenomenon whose mech-
anism, thermodynamic aspects and kinetics will be described in Chapter 12.
130 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
Figure 5.32. Microscopic texture of a spherulite of poly(ethylene adipate) with twisted lamellae.
Observation between crossed nicols. [Courtesy of B. Lotz, ICS-CNRS, Strasbourg (France).]
a
b

c
Figure 5.33. Schematical representation of a twisted lamella corresponding to a ringed texture
as shown in Figure 5.32.
5.4.5. Morphology of Phase-Separated Polymer Systems
Even if the heterogeneity of their structure is well established, semicrystalline
homopolymers are generally not included in the category of heterogeneous systems.
This term is reserved to polymers subject to a clear phase separation in which the
nonmiscibility is due to the presence of different molecular structures. The thermo-
dynamic aspect of the nonmiscibility of polymers is discussed in Section 4.4, where
it is shown that this phenomenon is practically general, with the only exception
of polymers with strong specific molecular interactions. Upon mixing two non-
miscible polymers molecular interactions develop within each phase but interphase
interactions remain weak. It results in poor mechanical characteristics for the cor-
responding materials. The electron micrography of Figure 5.34 clearly shows the
lack of interphase cohesion between two nonmiscible homopolymers.
The situation is different when a covalent link can be established between the
phases. In block copolymers the various blocks are also nonmiscible, but the dyad
linking the two blocks ensures the cohesion of the system and the phase dispersion.
MORPHOLOGY OF MACROMOLECULAR SYSTEMS 131
Figure 5.34. Scanning electron micrography of a polystyrene–polybutadiene blend in absence
of a compatibilizing agent. [Courtesy of BASF Cy (Ludwigshaffen Germany).]
5.4.5.1. Morphology of Block Copolymers—‘‘Self-Organized Polymers’’.
Two situations have to be considered in the case of block copolymers, depending
upon their situation with respect to the limit of segregation. If the system is far from
this limit, the segregation is clear and the morphology is determined only by the
molecular composition of the block copolymer. In the second case, an increase (in
absolute value) of the entropy of mixing with the temperature can result in a certain
compatibility at high temperature (see Section 4.4). For the sake of simplicity, only
systems that are far from the limit of segregation will be described.
When block copolymers exhibit a relatively well-defined molecular structure

(uniform composition and molar mass), four types of morphologies are observed,
depending upon their composition. Such systems are referred to as self-organized
polymers.
For a poly(A-b-B) copolymer whose mass ratio [A]/[B] (or [B]/[A]) is lower
than approximately 20%, the minority phase forms spherical domains that are regu-
larly dispersed in the matrix based on the majority block. The system self-organizes
in a centered cubic symmetry (Figures 5.35a and 5.35e). With the increase of the
proportion of the minority phase and for compositions [A]/[B] ranging between
20% and 35%, the spheres self-assemble into cylinders exhibiting a hexagonal
symmetry (Figures 5.35b and 5.35d). In the case of block copolymers with a bal-
anced composition ([A]/[B] from 40% to 60%), the cylinders self-assemble in
lamellae whose thickness is determined by the composition (Figure 5.35c). For
intermediate compositions [A]/[B] (or [B]/[A]) ranging from 35% to 40%, struc-
tures such as bicontinuous phases are observed. These biphasic morphologies (not
represented in the Figure 5.35) are intermediate between cylinder ones and lamel-
lae ones. The minor phase can form a sort of “network” (or two interpenetrated
132 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
(a) (b) (c) (d) (e)
< 15% 15–35% 35–65% 65–85% > 85%
0.5mm
Figure 5.35. Morphologies and micrographies of poly (styrene-b-butadiene) organized poly-
mers: (a, e) Spheres; (b, d) cylinders; (c) lamellae. (Percentages are those of styrene).
“networks”) regularly distributed in the matrix constituted by the major phase. Star
block copolymers also give rise to this type of structure.
Morphologies of self-organized copolymers are observed not only in diblock
copolymers but also in copolymers with a higher number of blocks. However, as
the number of blocks increases and their individual length decreases (segmented
polymers), it is more difficult to obtain a clear phase separation.
Triblock copolymers comprising a central block of polybutadiene (BR) and two
external blocks of polystyrene (PS) are commonly used as thermoplastic elas-

tomers.
Their composition induces the formation of a morphology in which polystyrene
spheres are distributed with a cubic symmetry in an elastomeric matrix of polybu-
tadiene. At the service temperature the spherical polystyrene nodules are in the
glassy state, whereas the polybutadiene chains connecting them are in the elas-
tomeric state. Figure 5.36 shows the rigid nodules of PS in their role of physical
cross-links, which are responsible for the reversibility of the deformations under-
gone by the sample.
When the temperature of the system is increased beyond the glass transition
temperature of polystyrene, the material becomes plastic and can be processed as
a viscous liquid. Upon lowering the temperature, the PS nodules become glassy
again and the elastomeric character of the material is restored.
5.4.5.2. ‘‘Compatibilization’’ of Polymer Mixtures—High Impact Poly-
mers. In block copolymers that are ill-defined (heterogeneity in composition, dis-
persity of molar masses, etc.) phase separation occurs with no defined order. This
is also observed in graft copolymers and polymer blends that are “compatibilized”
by means of a block (or graft) copolymer consisting of the same monomeric units
as those of the homopolymers to be mixed.
MORPHOLOGY OF MACROMOLECULAR SYSTEMS 133
Figure 5.36. Diagram of the morphology of relaxed S-B-S thermoplastic elastomers revealing
the physical cross-linking of the system by the glassy nodules of polystyrene.
Block or graft copolymers used in the latter case act as compatibilizers (surfac-
tants) of the polymer blend so as to “emulsify” the two homopolymers (Figure 5.38)
and thus improve its mechanical characteristics.
Remark. It is important to distinguish between miscibility and compatibil-
ity:themiscibility is a thermodynamic characteristic, whereas the compati-
bility is a phenomenon affecting a service property.
Upon thoroughly blending two homopolymers, such compatibilizers can some-
times be generated. Indeed, such a mechanical mixing can cause the homolytic
breaking of σ bonds and generate macromolecular free radicals that can give rise

to block copolymers by random recombination.
A particularly interesting case of polymer systems with heterogeneous mor-
phology is that of polymers with high-impact strength. These polymers exhibit a
strongly improved impact resistance as compared to that of common polymers as
a result of the dispersion of micron-size nodules of elastomers in a rigid phase.
High-impact polystyrene (HIPS) and ABS (acrylonitrile, butadiene, and styrene
copolymers) are the best-known examples.
HIPS is obtained by free radical polymerization of styrene in the presence of
polybutadiene. The labile character of the allylic hydrogen atom of polybutadiene
favors radical transfer from the growing polystyrene chains (see section 8.5.6.4),
which generates graft copolymers along with homopolystyrene:
134 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
R + Styrene Polystyrene
R +
CH
2
-CH=CH-CH
2
Polybutadiene
+
Styrene
CH-CH=CH-CH
2
CH-CH=CH-CH
2
Polystyrene
RH +
CH-CH=CH-CH
2
How does the morphology of the resulting polymer material occur and build

up?
Polybutadiene is first solubilized in styrene, which plays the role of a sol-
vent. Upon polymerizing, the polystyrene formed is still in a minor proportion—
compared to the polybutadiene present—and is thus dispersed as nodules in the
polybutadiene solution. As the yield of styrene increases, the polystyrene phase
becomes predominant, provoking an inversion of phases and in turn the dispersion
of the polybutadiene phase in the PS matrix. Figure 5.37 shows the various steps
of such a process.
The covalent links between the elastomeric nodules and the matrix are responsi-
ble for the cohesion of the whole material and the dispersed soft phase for stopping
the propagation of cracks resulting from an impact (Figure 5.38).
(a) 4 %
(b) 22 %
(c) 47 %
(d) 77 %
(e)
Figure 5.37. Scanning electron micrographies carried out at various steps of the formation of
a HIPS. (a–d) The given percentages correspond to the yield in styrene. (e) Final state; dark
parts consist of polybutadiene phase. [Courtesy of BASF Cy (Ludwigshaffen Germany).]
ORIENTED POLYMERS 135
Figure 5.38. Scanning electron micrography of a high-impact polystyrene showing the cohe-
sion between the polybutadiene nodules and the polystyrene matrix. [Courtesy of BASF Cy
(Ludwigshaffen Germany).]
Terpolymers referred to as ABS are obtained in the same way as HIPS, a
styrene/acrylonitrile mixture replacing styrene. The copolymerization of acryloni-
trile with styrene gives rise to a material with better cohesive properties.
It is worth mentioning that in heterogeneous multiphase systems, each phase
retains its characteristics. The best method to check the presence of a phase sepa-
ration in a system consists of the observation of two glass transition temperatures.
Techniques of microscopy are also widely used to characterize heterogeneous

systems (Figures 5.34 and 5.38) because they afford extremely precise infor-
mation with respect to the phase dispersion and the structure of the interphase
zones.
5.5. ORIENTED POLYMERS
5.5.1. Intrinsic and Shape Anisotropy of Polymers
A system consisting of oriented molecules generates anisotropic properties. This
anisotropy can take various facets—for example, an anisotropy of the refractive
index (i.e., birefringence).
The anisotropy of a material depends on the degree of orientation of its con-
stitutive molecules and on the molecular anisotropy of the latter. Both a shape
anisotropy and the molecule intrinsic anisotropy contribute to this molecular
anisotropy.
The shape anisotropy results from the molecular asymmetry: when placed in an
external electric field, an object of refractive index ˜n
0
modifies it in a nonisotropic
136 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
way due to an asymmetrical polarization of its charges. This shape anisotropy,
which is proportional to (˜n −˜n
0
)
2
, can be only positive. Due to their asymme-
try, macromolecular chains are characterized by a shape anisotropy that can only
increase with the deformation applied.
As for the intrinsic anisotropy of molecules, it depends on their chemical struc-
ture and more particularly on their polarizability. Double bonds and more particu-
larly conjugated ones—mainly in aromatic cycles—contribute to the anisotropy of
molecules. For a macromolecule to exhibit an intrinsic anisotropy, it has to consist
of anisotropic groups organized in a very regular manner along the chain. Indeed, an

assembly of anisotropic molecules, which would be randomly oriented, would gen-
erate a macroscopically isotropic system. Polymers forming double helixes (DNA)
or which are rigid [poly(benzyl glutamate)] are among the best-known examples of
polymers with strong intrinsic anisotropy. Macromolecular coils exhibit an intrin-
sic anisotropy that is less pronounced than that of highly organized polymers; it
depends essentially on differences in the polarizabilities of the backbone and of the
side-chain substituents. This difference is minimum in the case of the poly(methyl
methacrylate) whose main chain anisotropy is almost completely counterbalanced
by that of the side groups.
In the case of polystyrene, the contribution of the aromatic moieties perpendicu-
lar to the chain results in a negative intrinsic anisotropy of its segments. As observed
for the shape anisotropy, the intrinsic anisotropy of macromolecules is also expected
to strongly increase upon orientation of each of their anisotropic segments. In the
solid state, this orientation can be obtained by a mechanical solicitation (stretch-
ing); in the liquid state, it can be obtained by (a) an elongational flow (extrusion,
spinning) or (b) an electric (Kerr effect) or magnetic effect (Colton–Sheep effect)
when the chemical structure of the macromolecules is appropriate.
All macromolecules are not prone to undergo an orientation by one of the
means previously mentioned. The structural criteria determining the “orientability”
of polymer chains are almost the same as those required for manufacturing fibers.
Highly symmetrical and stereoregular chains that possess groups with a strong
energy of interaction [poly(vinyl chloride) (PVC), polyacrylonitrile (PAN), etc.]
are the best suited to retain an orientation upon drawing or in an elongational flow.
5.5.2. Orientation of Polymers
5.5.2.1. Uniaxial Drawing in a Solid State. Upon drawing, initially disordered
chains in an amorphous polymer undergo a phenomenon of orientation. The fact
that macromolecular coils are oriented upon application of an elongational stress
is materialized by the phenomenon of necking (point B of Figure 5.39); in the
subsequent step (B-C), the stretched chains self-organize in fibrillae, which reduces
the intermolecular distances and contributes to reinforce the interactions between

chains and the density of the cohesive energy of the system. PVC is an example
of an amorphous polymer whose mechanical properties can be improved to give
textile fibers after undergoing such a drawing.
Drawing also orientates the crystalline zones (when they exist) inducing the
transformation of spherulites into lamellar structures (see Figure 5.40). However,
ORIENTED POLYMERS 137
B
A
C
Figure 5.39. Schematic presentation of the necking phenomenon (B) obtained by the drawing
of an amorphous polymer sample.
Figure 5.40. Deformation under stretching of spherulites in a semicrystalline polymer.
more than the latter phenomenon, it is the orientation of the amorphous parts which
contributes to the increase of the Young modulus of such a stretched material in
the direction of drawing and to the increase of its fracture strength. Indeed, an
initially stretched sample that is subjected to a new drawing test exhibits a higher
Young modulus in the direction of stretching and a lower one in the perpendicular
direction. Such a drawing should be imperatively carried out at a temperature close
to, but lower than, the melting point (T
m
) of the crystalline zones to favor the
chain rearrangement and prevent their immediate relaxation as soon as the stress is
suppressed. When treated under such conditions, semicrystalline polymers such as
aromatic polyamides can exhibit a Young modulus equal to 130 GPa and a fracture
strength of 2.8 GPa.
5.5.2.2. Orientation by Elongational Flow. Crystallizable polymers can
undergo an orientation of their chains in dilute solutions. When subjected to an
elongational flow, these chains are forced to be oriented in the parallel direction and
thus form fibrillae; this phenomenon is referred to as fibrous crystallization.Inside
these fibrous parts which behave as nuclei for the epitaxial growth of remaining

macromolecules, shear stresses are low. This growth results in crystalline lamellae
that are perpendicular to the fiber axis. As for the axis of the chains folded in these
lamellae, it is parallel to that of the fibers: one speaks of “shish-kebab” (skewer
structures) as shown in Figure 5.41.
Dilute solutions of either polyethylene in toluene or isotactic polypropene in
chloronaphthalene afford such “shish-kebab” structures when crystallized in an
elongational flow. In the case of polyethylene of high molar mass spun from a vis-
cous solution, fibers of 3-GPa fracture strength and of 90-GPa Young modulus could
be obtained. These are very high values if one considers that only London-type
molecular interactions are responsible for the cohesion of the material.
138 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
(a)
(
b
)
fibrillar zone lamellar zone
Figure 5.41. (a) Representation and (b) scanning electron microscopy of a ‘‘shish-kebab’’
structure obtained in an elongating flow.
Whatever the method used to obtain the orientation of the macromolecular sys-
tems, an orientation function (referred to as “Hermans function” and indicated by
F
her
) can be defined to characterize the chain alignment with the reference direction
(in general the fiber axis in the case of an uniaxial drawing):
F
her
=
1
2
(3cos

2
−1)
where  is the angle between the direction of drawing and that of the chains axis.
If all the chains are completely oriented, then  =0andF
her
=1. The orientation
function is equal to 0.5 for a perpendicular orientation of the chains and 0 for a
random orientation.
5.5.2.3. Effect of Biaxial Drawing. Such a biaxial drawing is observed in
film-forming processes. It can be carried out on either amorphous or semicrys-
talline polymers. The two-dimensional orientation can be described using the angles

X
and 
Y
between the chain direction and those (X and Y) of drawing (gen-
erally orthogonal). The Hermans functions relative to each reference direction are
given by
F
herX
= 2cos
2

X
+cos
2

Y
−1
F

herY
= 2cos
2

Y
+cos
2

X
−1
LIQUID CRYSTALLINE POLYMERS 139
5.6. LIQUID CRYSTALLINE POLYMERS
5.6.1. Molecular Liquid Crystals
Between the crystalline state (characterized by a long-range three-dimensional
order), and the amorphous isotropic state, there is an intermediate state of matter
referred to as liquid crystal. It is specific to certain molecules, which simultane-
ously exhibit order like crystals and flow like fluids. Reinitzer, who observed that
cholesteric esters form opaque liquids that become transparent upon raising the tem-
perature, is considered as the precursor of this field. In addition to the term liquid
crystal, mesomorphic or mesophase can also be used (from Greek mesos meaning
“median”) to name this intermediate state between an isotropic liquid state and
the three-dimensional crystalline order as first proposed by Friedel. Molecules that
adopt a preferential orientation and result in an anisotropy are called mesogens.
Liquid crystalline molecules are classified in two families referred to as ther-
motropic and lyotropic, respectively. In thermotropic liquid crystals, the formation
of mesophases is temperature-dependent; as for lyotropic liquid crystals, they
necessitate the use of a solvent for forming mesophases. Liquid crystals are also
sensitive to other stimuli such as magnetic or electric fields, pressure, and so on.
Molecules that are prone to generate mesophases exhibit either “rod-like” or
“disk-like” rigid structures:

XZZ
"Rod"
"Disk"
Mesophases are characterized by a long-range orientational order which results
from the longitudinal alignment of the mesogenic groups along a directing axis.
Two categories of mesophases can be distinguished, depending upon the dimen-
sions and the degree of order that is attained (Figure 5.42). When mesogens are
organized in two-dimensional layers of regular size, the corresponding mesophases
n
n
(
a
)
smectic A
(c) nematic
n
(b) smectic C
Figure 5.42. Representation of the organization of mesogens in smectic phases (S
a
,S
c
)and
nematic one (N).
140 CONFORMATIONAL STRUCTURES AND MORPHOLOGIES
are called smectic (S). As lateral forces between the molecules of a mesophase
are quite larger than those between the layers, the fluid character results from the
relative slipping of the layers.
At least eight different smectic phases are known; they can be distinguished from
one another by adding alphabetical index to the letter S. The phase corresponding
to the highest degree of order and to a hexagonal ordering of mesogens within

a layer is referred to as smectic B (S
B
). In addition to S
B
phases, which exhibit
a three-dimensional arrangement of the mesogen groups, smectic mesophases E,
G, and H are also characterized by a similar degree of order, but in S
G
and S
H
the direction of alignment of mesogens is tilted with respect to the axis perpen-
dicular to the layer plane. S
A
mesophases are the least-ordered smectic structures
that are characterized by a random lateral distribution of mesogens even if their
longitudinal axis is perpendicular to the layer plane. S
C
mesophases possess the
same characteristics as S
A
ones, but in the latter case the mesogens are tilted by
an angle θ with respect to the axis perpendicular to the layer plane. Mesophases
S
F
and S
I
correspond to an intermediate degree of order between S
B
and S
A

mesophases.
Nematic mesophases (N) are less ordered than smectic ones because they exhibit
only a monodimensional order. In this case, even if the orientation order is retained
with respect to a directing axis—which can thus be regarded as the main directional
axis of the molecule—the centers of mass are not necessarily within a layer but can
be distributed in a random way. Within these domains, whose size is in general in
the micron range, the average degree of alignment with respect to a preferential axis
is described by the Hermans orientation factor (F
her
) (order parameter). The closer
this factor to 1, the higher the degree of order of the phase. Nematic mesophases
are more fluid than their smectic homologs.
The family of cholesteric mesophases is also part of the family of nematic
mesophases (N*) (Figure 5.43). Only mesogens carrying a chiral center (denoted
by the presence of * next to the letter N) and ordering themselves in nematic phases
can generate such phases. The presence of this chiral center forces mesogen groups
to adopt a screw-type structure corresponding to a helical variation of the nematic
directing axis. Smectic structures S
C
and S
A
can also afford chiral phases insofar
as the mesogenic group carries a chiral center. Chiral smectic phases C (S
C
∗
)are
known for their ferro- and piezoelectric properties.
Z
P
Figure 5.43. Representation of a chiral nematic phase known as ‘‘cholesteric.’’

LIQUID CRYSTALLINE POLYMERS 141
Finally, it is worth mentioning that a molecule can undergo several transitions
and experience successively the highly ordered state of a crystal (the nematic and
then smectic) and finally go to the isotropic states upon raising the temperature
(S
B
→S
C
→S
A
→N →I).
5.6.2. Liquid Crystalline (Mesomorphic) Polymers (LCP)
The association of simple molecular groups exhibiting mesomorphic properties with
polymers was considered soon after Flory predicted in 1956 that concentrated solu-
tions of “rod”-type rigid polymers could form ordered structures. Investigations of
the behavior of polymers with helical conformation such as those of poly(methyl
and/or benzyl glutamate) type showed that they self-align in a given direction,
thus corroborating this prediction. By associating polymers and mesogenic groups
within the same structure, one can design materials exhibiting simultaneously the
anisotropic characteristics of liquid crystals and the thermoplastic behavior of cer-
tain liquids. These mesogenic groups can be incorporated either in the main chain
or as side chains—that is, laterally grafted onto the backbone (Figure 5.44).
(b)
(c) (d)
(a)
Figure 5.44. Representation of liquid crystalline polymers with main-chain (a) and side-chain
(b, c) mesogenic groups and combination of both types of chains (d).
Certain liquid crystalline polymers comprise both main-chain and side-chain meso-
genic groups. Polymers that include mesogenic groups in the main chain are
generally obtained by step-growth polymerization (see Chapter 8). Depending upon

the nature and the size of the links which connect the mesogenic groups together,
main-chain mesomorphic polymers form either very rigid or semiflexible struc-
tures.
Polymers carrying side-chain mesogenic groups can be prepared in various ways:
•
By chemical modification of a flexible polymeric backbone as in the case of
polysiloxanes
•
By chain polymerization of a vinyl or related monomer carrying a mesogenic
group
•
By step-growth polymerization of mesomorphic monomers.
In the latter case, the polymeric backbone and the mesogens are separated by a
bivalent flexible molecular group called spacer.