Materials Sciences and Applicatio ns, 2011, 2, 30-41
doi:10.4236/msa.2011.21005 Published Online January 2011 (
Copyright © 2011 SciRes. MSA
Rheological Properties of Polymers: Structure and
Morphology of Molten Polymer Blends
Oluranti Sadiku-Agboola, Emmauel Rotimi Sadiku, Adesola Taoreed Adegbola, Olusesan Frank
Department of Mechanical Engineering, Faculty of Engineering and the Built Environment, Tshwane University of Technology,
Lynnwoodridge, South Africa.
Received October 6th, 2010; revised December 16th, 2010; accepted December 26th, 2010.
The article reviews a brief literature on the rheological properties of polymer melts and blends. Experimental results on
polymer blends are summarized. Technically, vital types of multi-phase polymers such as compounds and blends are
discussed. The importance of the rheological properties of polymer mixtures in the development of the phase structure
is discussed. And the importance of considering the stress and/or strain history of a material sample in a rheological
investigation is discussed. Finally, the outlook on the past, present and future developments in the field of polymer
rheology are given. The review concludes with a brief discussion on the opportunities and challenges in the field of
polymer blends and blend rheology.
Keywords: Polymer Melts, Polymer Blends, Miscible Polymer Blends, Immiscible Polymer Blends, Compatibilization,
Rheological Properties and Phase Structures
1. Introduction
Rheology is a branch of physics that deals with the de-
formation and flow of matter under stress. It is particu-
larly concerned with the properties of matter that deter-
mine its behaviour when a mechanical force is exerted on
it. Rheology is distinguished from fluid dynamics be-
cause it is concerned with the three traditional states of
matters rather than only liquid and gases. Rheological
properties have important implications in many and di-
verse applications. Often, an additive is used to impart
the desired flow behaviour. Among these, organoclay
products, formed by the reaction of organic cations with
smectite clays, are the most widely used additives for
solvent-based coatings. The often used cation, usually a
quaternary ammonium salt, influences the performance
of the resultant organoclay. Criteria to consider in the
choice of a cation are molecular size, compatibility with
the fluid in which the organoclay is to be used, stability
and reactivity. Applications of rheology are important in
many areas of industries involving metal, plastic, and
many other materials. The results from rheological inves-
tigations provide the mathematical description of the
viscoelasticity behaviour of matter. An understanding of
the rheology of a material is important in the processing
of composites, whether the task is designing an injection
molded part or determining the cure cycle for a prepregs.
For many years, rheology has been used as semi-
quantitative tools in polymer science and engineering.
The relationship between the structure and rheology of a
polymer is of practical interest for two reasons: firstly,
rheological properties are very sensitive to certain aspect
of structure and they are simpler to use than analytical
methods, such as nuclear magnetic resonance. Secondly,
it is the rheological properties that govern the flow be-
haviour of polymers when they are processed in the mol-
ten state. Considering the structures of polymers by
means of the size and shape of molecules and the distri-
bution of these characteristics among the molecules,
structure formation and controlled assembly are the focus
of joint simulations and various experiments. Neves et al.
[1] investigated the main rheological features of vaginal
hydrophilic polymer gels and elucidated the relationship
between these characteristics, gels composition and their
general influence in therapeutic/usage purpose. In their
studies, two vaginal gels were studied by the cone-and-
plate rheometry, at body temperature. Several parameters
(apparent viscosity, complex viscosity, storage modulus,
loss modulus, critical oscillatory stress, tan δ, thixotropy
Rheological Properties of Polymers: Structure and Morphology of Molten Polymer Blends31
and yield stress) were measured and/or calculated. They
found that the rheological behaviour of vaginal gels
strongly depended on the type of gelling agent used;
which potentially influences their spreading and retention
properties when administered in the vaginal canal. Small
variations in gels composition can result in substantial
changes in their features, namely: viscosity, yield stress
and thixotropy. Rheological properties of tested gels ap-
peared to have correlated strongly with their therapeu-
tic/usage purpose.
Early investigations of the solution properties of
polymers are primarily responsible for proving that the
high polymers are not association of colloids, but are
macromolecules held together by covalent bonds. Meth-
ods of measuring the molecular weight of polymers were
necessary to prove the truth (or otherwise) of this hy-
pothesis [2]. It is striking that the rheology of macro-
molecules and suspensions reflects their size, shape, and
interactions in a flowing field. For nearly a century, solu-
tion properties of polymers, such as their viscosities, col-
ligative properties, solubility and light scattering behav-
iour have been studied. Materials with totally new prop-
erty combination may be achieved by blending together,
two or more polymers. In many polymer-polymer alloys,
immiscibility has been observed by shearing an initially
(static) phase-separated blend. A theoretical prediction
shows that shear mixing occurs at low and high shear
rates, with a demixing region at the intermediate shear
rates [3]. While immiscible blend often follows predict-
able rheology pattern (since they exhibit single phase
behaviour), the rheology of phase-separated system is
much more complex. Its phase morphology is a major
factor, since it critically affects the size and shape
changes in shear flow, as observed by viscosity. The con-
tinuous phase of the phase-separated blend is determined
by two important factors, viz: the volume fraction and
the viscosity of the components. High volume fraction
and low viscosity favour phase continuity [3]
The effect of viscosity ratio on the morphology of im-
miscible polymer blend has been studied by several re-
searchers. To achieve fine phase morphology during the
processing of immiscible polymer blends, it is often re-
quired to add compatibilizers [4]. Mayu et al. [5] studied
the morphology of blends of PS/PMMA, PC/SAN24 and
PMMA/EVA and compared the morphologies with and
without modified organoclays Cloisite 20 A or Cloisite 6
A. In each case, they found a large reduction in domain
size and the localization of the clay platelets along the
interfaces of the components. Increase in miscibility was
accompanied in some cases, with the reduction of the
system from multiple values of the glass transition tem-
peratures to one. In addition, the modulus of all systems
increased significantly. They proposed a model where
the in-situ grafts were forming on the clay surfaces dur-
ing blending and the graft had to be localized at the in-
terfaces. This mechanism reflects the composition of the
blend and is rather fairly nonspecific [5]. Young and Kim
[6] investigated the effect of the amount of in-situ formed
graft copolymers on the blend morphology for immis-
cible polymer blends of poly (butylenes terephthalate)
(PBT) and polystyrene (PS) with various amount of poly
(styrene-ran-glycidy methacrylate) (PS-GMA) as in-situ
compatibilizer. This article surveys published literature
which were concerned with the rheology, polymer melts
and blends. Also included are some studies of blend of
different polymers. The aims are:
To highlight the importance of polymer melts and
To highlight the rheological properties of poly-
meric materials in the molten state, in order to gain fun-
damental understanding of the processability of such
To highlight the importance of considering the
stress or strain history of a material sample in rheological
To review the past, present and possible future de-
velopments in the field of polymer blend rheology.
To discuss the current research issues.
To highlight the opportunities and the challenges
2. Polymer Melts
In the last three decades, considerable progress has been
made in the rheology of polymer solutions and polymer
melts. The measurement of rheological properties of any
polymeric material in the molten state is crucial in order
to gain fundamental understanding of the processability
of that material. This is because rheological behaviours
are strongly influenced by the material structure and the
interfacial characteristics. The rheology of polymer melt
is crucially essential for two reasons. Firstly, it has
helped to resolve many polymer problems, such as wide
gauge variations in films, poor optical quality of sheet
and films, slow production rates, dimensional instability
and poor mechanical properties. Secondly, it has been
employed for the analysis of parameters, such as: mo-
lecular structures, branching type and extent of branching,
content, entanglements and crosslink density [2]. Since
polymer melts are viscoelastic, both viscosity and elas-
ticity must be measured. This information is easily ob-
tainable from viscoelastic spectrum. Data can be treated
in many different ways. Master curves can be produced
from where retardation and relaxation spectra can be
generated. To include the wide range of relaxation
times-from the largest molecules to the smallest subseg-
ments, it is necessary to make measurement over a wide
Copyright © 2011 SciRes. MSA
Rheological Properties of Polymers: Structure and Morphology of Molten Polymer Blends
range of temperature and frequency (or shear rates).
Nichetti and Manas-Zloczower [7] employed a simple
superposition model to define the relationship between
molecular weight distribution and shear viscosity for
linear polymeric systems. Gel permeation chromatogra-
phy data for molecular weight distributions were fitted
using statistical distribution functions. A simple superpo-
sition model was then employed to calculate the shear
viscosity for the systems investigated. The effect of
polydispersity on the shape of the flow curves was cal-
culated. The simplicity of the model makes feasible its
use in numerical simulations of complex geometries as
encountered in polymer processing equipment. Their
study also sheds some light on the relationship between
entanglement and disentanglement phenomena in poly-
meric systems. Soskey and Winter [8] measured stress
relaxation after rapid extensional strain, in order to obtain
the extensional relaxation modulus. Their research had
the objectives of developing the lubricated squeezing
technique for molten polymers, by applying the tech-
nique to two different polymers, and testing the “separa-
bility hypothesis.” The time-dependence of the relaxation
modulus was found to be the same in extension as in
shear, giving the relaxation modulus of linear viscoelas-
3. Polymer Blends and Melts
3.1. Polymer Blends
Recently, there has been pronounced interest in polymer
blends. The enhanced activities are related to the hope of
producing advanced high performance materials based
on well known products and the need for basic knowl-
edge on their phase behaviour, which in turn offers some
flexibility for the control of morphology during process-
ing. Polymer blends are mixtures of different homopoly-
mers, copolymers, and terpolymers. They can be homo-
geneous (miscible) or heterogeneous (multiphase). This
includes both crystalline and amorphous polymers.
Whether a mixture of two chemically dissimilar poly-
mers is miscible or not depends on the thermodynamics
of mixing. In order to understand what governs poly-
mer-polymer miscibility on a molecular scale, it can be
approached via the polymer solution theory. The unique
factor affecting the thermodynamics of polymer blends
compared with polymer solutions is the large molecular
weight of both components [2]. Polymer blending has
been identified as the most versatile and economical
route to producing new multi-phase polymeric materials
that are able to satisfy the complex demands of perform-
ance [9]. Over the past few decades, polymer blend has
grown tremendously in leaps and bonds. Infact, the de-
sign and development of these multi-phase polymer
blend materials are strongly dependent on two major
parameters; which are:
The control of interface bonding.
The control of morphology.
Tol et al. [10] prepared (PPE/PS)/PA6 and PS/PA6
blends by means of melt-extrusion. They were compati-
bilized using the reactive styrene-maleic anhydride co-
polymer with 2 wt% maleic anhydride (SMA2). The ef-
fect of compatibilization on the phase inversion and the
stability of the resulting co-continuous blend structures
were investigated using scanning electron microscopy,
dissolution and extraction experiments. Depending on the
blend composition, the onset of co-continuity shifted
towards the lower PA6 concentrations according to the
change in blend viscosity ratio. The unmodified co-con-
tinuous blends were not stable and did break-up into
droplet/matrix type of morphology upon annealing in the
melt. Although the stability of the threads during anneal-
ing improved upon compatibilization because of the re-
sulting lower interfacial tension, the decreased possibility
for recombination and coalescence during flow reduced
the co-continuous region for the compatibilized blends. It
was proposed that a dynamic equilibrium between
break-up and recombination phenomena after the initial
network formation is necessary to maintain the network
Processing of polymer materials has a large influence
on the resulting mechanical and optical properties of the
end product. For instance, dimensional stability in preci-
sion injection moulding or yield strength, Young’s
modulus and even tear strength of blown films are af-
fected by the viscoelastic properties of the polymer melt.
In turn, the rheological behaviour is related to specific
molecular structures and the molecular weight distribu-
tion [11]. In the manufacturing of polymeric objects,
most of the shaping is carried out in the molten state, as it
is an important part of the physical property development.
Melt processing involves interplay between fluid me-
chanics and heat transfer in rheologically complex liq-
uids, and taken as a whole, it is a relevant example of the
importance of coupled transport processes. The viscoe-
lastic character of polymer melts reflects the entangled
microstructure and plays an important role in property
development and in flow stability. Viscoelasticity has
little effect on the evolution of many processing flows,
however, where the mechanics are dominated by the
temperature and shear-rate dependence of the viscosity;
this statement is especially true of extrusion and some
mold filling, but it equally applies to some extensional
flows, even when the polymer is a relatively inelastic
polyester or nylon.
Many investigations have been done on polymer melt.
Seemann et al. [12] studied the stability conditions of
Copyright © 2011 SciRes. MSA
Rheological Properties of Polymers: Structure and Morphology of Molten Polymer Blends33
thin (3 to 300 nm) liquid polymer films on various sub-
strates. The key role was played by the effective interface
potential, φ of the system air/film/substrate, which de-
termined the de-wetting scenario, in case, if the film is
not stable. In their study, they described how to distin-
guish spinodal de-wetting scenario from heterogeneous
and homogeneous de-wetting by analysing the emerging
structures of the film surface; by e.g. Minkowski meas-
ures. They also included line tension studies of tiny
droplets, showing that the long-range part of φ does af-
fect the drop profile, but only very close to the three
phase boundary line. The dynamic properties of the films
are characterized via various experimental methods: the
form of the de-wetting front, for example, was recorded
by scanning probe microscopy that gave insight into the
boundary condition between the liquid and the substrate.
They further reported experiments probing the viscosity
and the glass transition temperature of nm-thick films
using e.g. ellipsometry. They found that even short-
chained polymer melts exhibit a significant reduction of
the glass transition temperature as the film thickness is
reduced below 100 nm. Daoulas et al. [13] presented an
atomistic modeling approach for simulating the interface
between a polymer melt and a crystalline solid substrate.
As a test case, a thin film of polyethylene (PE) melt con-
fined between a semi infinite graphite phase on the one
side and vacuum on the other, were considered. The
simulation was carried out in the isothermal-isobaric sta-
tistical ensemble with an efficient Monte Carlo (MC)
algorithm based on state of-the-art variable connectivity
moves. The atomistic simulations are conducted by de-
scribing the PE and PP chains with a united atom model,
which considers each methylene (CH2) and methyl (CH3)
group along the chain backbone as single interaction sites.
To calculate the potential energy of interaction between
polymer atoms and the semi-infinite graphite substrate,
capable of incorporating the exact crystallographic struc-
ture of graphite, the method designed by Steele was im-
plemented. The new approach has allowed the authors to
analyze the structural and conformational properties on
the length scale of just a few angstroms from both sur-
faces. Detailed results are presented for the local mass
density, structure and conformation of PE at the two in-
terfaces, obtained from simulations with model, strictly
monodisperse PE samples of molecular length up to
C-400. Additional structural features of the adsorbed
layer, such as the distribution of skeletal carbon atoms in
train, loop and tail conformations and their statistics, are
also analyzed in detail and compared with the predictions
of the lattice-based Scheutjens-Fleer self-consistent mean
field theory in the limit of zero solvent concentration
(melt case). Their atomistic simulation data demonstrated
a stronger dependence of these descriptors of adsorbed
layer structure on chain length than what was calculated
by the mesoscopic Scheutjens-Fleer lattice model. In a
second step, thoroughly equilibrated configurations of
the confined model PE melt films are subjected to de-
tailed molecular dynamics (MD) simulations in the iso-
thermal-isobaric statistical ensemble to analyze their dy-
namic behaviour. The MD simulations are carried out
with the rRESPA method of multiple-time-step algorithm
and have allowed them to monitor segmental and chain
center-of-mass mean-square displacements over time
scales on the order of a few hundreds of nanoseconds.
Foteinopoulou et al. [14] subjected a large number of well-
equilibrated atomistic configurations of linear, strictly
monodisperse polyethylene (PE) melts of molecular
length ranging from C-24 up to C-1000, obtained through
extensive Monte Carlo simulations based on chain con-
nectivity altering algorithms, to a detailed topological
analysis. Primitive paths are geometrically constructed
connecting the two ends of a polymer chain (which in all
cases are considered as fixed in space) under the con-
straint of no chain cross ability, such that the multiple
disconnected (coarse-grained) path has minimum contour
length. When applied to a given, dense polymer configu-
ration in 3-D space, the algorithm returns the primitive
path and the related number and positions of entangle-
ments (kinks) for all chains in the simulation box, thus
providing extremely useful information for the topologi-
cal structure (the primitive path network) hidden in bulk
PE. In particular, their analysis demonstrated that once a
characteristic chain length value (around C-200) is ex-
ceeded, the entanglement molecular length for PE at T
450 K reaches a plateau value, characteristic of the en-
tangled polymeric behaviour). They further validated
recent analytical predictions about the shape of the dis-
tribution for the number of strands in a chain at equilib-
rium. At the same time, they showed that the number of
entanglements obtained by assuming random walk devi-
ates significantly from these predictions, which they re-
garded as clear evidence that by directly counting the
entanglements and their distribution functions, as pro-
posed in their work, offers advantages for a quantitative
analysis of the statistical nature of entanglements in
polymeric systems.
3.2. Vital Types of Multi-Phase Polymers:
Compounds and Blends
Over the years, the fact that polymer cannot mix with
each other to form homogeneous mixture at segmental
scale has been the driving force for a large number of
studies on multiphase polymeric materials. It has been
known for a long time that the physical properties of
these phase-separated polymers; such as toughness,
flowability, transparency and weather resistance, to men-
Copyright © 2011 SciRes. MSA
Rheological Properties of Polymers: Structure and Morphology of Molten Polymer Blends
tion a few, are dictated by their morphology [15]. As a
consequence, a variety of chemical and physical methods
has been extensively exploited to control morphology of
immiscible polymer mixtures. Reactive polymers are
widely utilized for various practical purposes such as the
modification of surface tension between different phases
via which morphology and physical properties of multi-
phase polymeric material can be modified and controlled.
These techniques are termed reactive compatibility [15].
Types of polymer blends are quite varied and comprise
of many diverse combinations of polymeric materials for
both academic and industrial interests. The primary dif-
ferentiation of polymer blends involves their phase be-
haviour; specifically miscibility versus phase-separation.
The primary advantage of miscible versus phase-sepa-
rated polymer blends is the blend property profile (me-
chanical properties), which is generally intermediate be-
tween that of the unblended constituents. The technology
involves in polymer blends includes a multitude of
polymer alloy compositions including elastomer blends,
engineering polymer blends, impact modified polymer
blends, etc.
3.3. Miscible Polymer Blends
Miscible polymer blends are an expensive, easy-to-use
approach for investigating and refining polymer combi-
nations. It facilitates combinations that work and some
that don’t. Miscible polymer blends behave similarly to
what is expected of a single phase system. Their proper-
ties are a combination of properties of pure components.
The characteristics of components affecting the proper-
ties of miscible blends are their chemical structure and
molecular weight, their concentration and intermolecular
interactions. While miscible blend system is of consid-
erable scientific and practical interest, it should not be
concluded that miscibility is always preferred with re-
spect to properties.
3.4. Immiscible Polymer Blends
Mixing immiscible polymers in the liquid state can result
in various phase morphologies. These depend on the na-
ture and the relative amounts of the polymers used and
on the flow history of the sample. Immiscible polymers
behave as different materials at different flow fields. The
phase morphology is an important factor in the rheology
of immiscible polymer blends. Most binary polymer
blends are immiscible. To a large extend, the characteris-
tics of these immiscible polymer blends are determined
by the state of the interface between the components. The
interaction of two melts at the interface is primarily rep-
resentented by a quantity k, called the interfacial tension.
“Interfacial tension is the excess free energy caused by
the existence of an interface, arising from the unbalanced
molecular forces [16]. Generally, chemically different
polymers are immiscible and their blending leads to ma-
terials with weak interfacial adhesion and thus poor me-
chanical integrity. The conversion of immiscible blend to
useful polymeric products with the desired properties
requires some manipulations of the interface. One of the
classical methods to ensure adhesion between the phases
(reduction of the interfacial tension) is the use of a third
component, a compatibilizer, which is miscible in both
cases. According to Elias et al. [17], in recent years, new
concept of compatibilization using rigid nano-materials
like silica nano-particles has been proposed.
3.5. Compatibilization
Compatibilization is the process of modification of inter-
facial properties in immiscible polymer blends. This re-
sults in the reduction of the interfacial tension, stabiliza-
tion of the desired morphology and improved interaction
between phases of solid state. In order words, compatibi-
lization results in the formation of polymer alloys. Com-
patibilization is accomplished by physical or chemical
means. In the former case, the desired level of dispersion
is generated by physical means, then physically stabilised
(e.g., by quenching, retardation, cross-linking or co-cry-
stallization). In the later case, the morphology does not
only depend on the level of mechanical mixing, but to a
great extent, it is controlled by the compatibilizer [18].
Table 1 below, shows the different type of polymer
blends [3].
4. Experimental Results on Polymer Blends
Young and Kim [6] investigated the effect of the amount
of in-situ prepared graft copolymers on the blend mor-
Table 1. Types of polymer blends.
1. Engineering polymer blends
2. Impact modified polymer blends
3. Emulsion blends
4. Crystalline-crystalline polymer blends
5. Crystalline-amorphous polymer blends
6. Thermosetting polymer blends
7. Impact modified polymer blends
8. Molecular composites
9. Biodegradable polymer blends
10. Reactive compatibilized blends
11. Liquid-crystalline polymer blends
12. Polyolefin blends
13. Interpenetrating polymer networks
14. Polyelectrolyte complexes
15. Recycled polymer blends
16. Water soluble polymer blends
17. Polymer blend composites
18. Black copolymer-homopolymers blends
19. Core-shell polymer system
20. Elastomer blends
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Rheological Properties of Polymers: Structure and Morphology of Molten Polymer Blends35
phology of immiscible polymer systems of poly (buty
lenes terephthalate) (PBT) and polystyrene (PS) with-
various amount of poly (styrene-ran-glycidy methacry-
late) (PS-GMA) as in-situ compatibilizers. They used
two different blending methods to prepare the blends: A
melt blending (MB) and a solution blending followed by
an oscillatory shearing in the molten state (SOM). The
molecular weight of the in-situ PS-g-PBT copolymers
formed from the reaction between PS-GMA and PBT in
the blend was determined using high temperature gel
permeation chromatography (GPC). The concentration of
the in-situ formed graft copolymers of PS-g-PBT in the
blends [copolymer blend] prepared either by MB or
SOM was determined by solvent extraction and followed
by Fourier Transform Infrared Spectroscopy (FTIR)
analysis. On the basis of the GPC and FTIR results, they
concluded that the PS-g-PBT in the blend has between
1.3-2 PBT chains grafted onto a PS-GMA chain. From
the FTIR analysis and the morphology of the blend in-
vestigated by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM), they found that
the interfacial area density of the in-situ formed
PS-g-PBT is ~0.1, chains/nm2, for the blends prepared by
SOM, regardless of the amounts of PS-GMA added ini-
tially in the blend.
Block copolymers may be added as surface-active
compatibilizers in order to control the morphology of
blends of immiscible polymers. Velankar et al. [19] in-
vestigated the effects of such added compatibilizers on
the rheological properties of droplet-matrix blends are
experimentally. They studied the model blends composed
of polyisobutylene (PIB) droplets in a polydimethylsi-
loxane (PDMS) matrix, compatibilized with a diblock
copolymer of PIB and PDMS. The viscosity and the first
normal stress difference under steady shear conditions,
and complex moduli after cessation of shear are meas-
ured. They found that addition of the compatibilizer
slightly raises the magnitude of the terminal complex
viscosity of blends at all ratios of viscosity. With addi-
tion of the compatibilizer, the terminal relaxation time
was found to increase sharply at high viscosity ratio,
whereas the steady shear was found to increase at low
viscosity ratios. Lu and Isacsson [20] studied the
rheological properties of styrene-butadiene-styrene co-
polymer (SBS) modified bitumen. The modified binders
were prepared using a laboratory mixer. Five types of
bitumen from four sources were mixed with two SBS
polymers at different polymer contents. The modified
binders were characterized using dynamic mechanical
analysis over a wide range of temperatures and frequen-
cies. It was found that the addition of the SBS polymers
increased the binder electrical properties at high tem-
peratures and improved the binder flexibility at low tem-
peratures. The temperature susceptibility of bitumen was
also reduced by SBS modification. However this prop-
erty cannot be evaluated with a single-valued parameter.
The degree of modification with respect to the binder
rheology varied with temperature and frequency, and was
dependent on the bitumen source/grade and the polymer
concentration and structure.
William and Wagner [21] demonstrated a new method
that applies the Porod limit of small angle neutron scat-
tering to measure the interface morphology in concen-
trated, immiscible blends under simple shear. The effect
of viscoelasticity contrast on rheology and blend micro-
structure was probed for a model dispersion of liquid
crystalline polymer dispersed in a linear polymer matrix.
In comparison to the theories demonstrated by the com-
plex morphological evolution, is a consequence of the
unique rheology of liquid crystalline polymers. The re-
sults at low shear rates were used to predict the onset of
stable micro-fibrillation at higher shear rates through
consideration of the contrast in the first normal stress
differences. Kuo and Chang [22] prepared polymer
blends of poly (vinylphenol) (PVPh) with poly (vi-
nylpyrrolidone) (PVP) by solution casting from the N,
N-dimethylformamide (DMF) solution. Differential
scanning calorimeter (DSC), Fourier transforms infrared
spectroscopy (FTIR) and solid state Nuclear Magnetic
Resonance (NMR) were used to investigate the hydrogen
bonding and miscibility behaviour of the blend. The
PVPh/PVP blend system has a single glass transition
temperature over the entire composition range, indicating
that this blend is able to form a miscible phase due to the
formation of inter-hydrogen bonding between the hy-
droxyl of PVPh and the carbonyl of PVP. FTIR and
solid-state NMR were further employed to study the hy-
drogen-bonding interaction between the PVPh hydroxyl
group and the PVP carbonyl group at various composi-
tions. According to the Painter-Coleman association
model (PCAM), the inter-association constant for the
PVPh/PVP blend is significantly higher than the self-
association constant of PVPh, revealing that the tendency
towards hydrogen bonding of the PVPh and PVP domi-
nates the intra-hydrogen bonding of the PVPh in the
mixture. Kim et al. [23] examined block copolymers of
PC-b-PMMA (polycarbonate-b-polymethylmethacrylate)
and PC-b-SAN (polycarbonate-b-(styrene-c-acryloni- trile)),
as compatibilizers for blends of PC with SAN copolymer.
The average diameter of the dispersed particles was
measured with an image analyzer, and the interfacial
properties of the blends were analyzed with an imbedded
fiber retraction (IFR) technique. The average diameter of
dispersed particles and interfacial tension of the PC/SAN
blends reached a minimum value when the SAN co-
polymer contained about 24 wt% AN. Interfacial tension
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Rheological Properties of Polymers: Structure and Morphology of Molten Polymer Blends
and particle size were further reduced by adding com-
patibilizer to the PC/SAN blends. In reducing the average
diameter of the dispersed particles and interfacial tension
of PC/SAN blend, PC-b-PMMA was more effective than
PC-b-SAN as a compatibilizer. A direct proportionality
between the particle diameter and interfacial tension was
also observed. The interfacial properties of the PC/SAN
blends were optimized by adding a block copolymer and
using an SAN copolymer that had minimum interaction
energy with PC.
5. Importance of Rheological Properties in
the Development of the Phase Structure
Rheological properties of a two-phase system depend,
not only on the rheological behaviour of the components,
but also the size and size distribution and the shape of
discrete phase droplets dispersed in a continuous matrix
phase. Rheological properties of multiphase systems are
strongly influenced by the morphology, which depends
on the thermodynamic interactions between the con-
stituent polymers and flow history. Therefore, rheologi-
cal properties are essential in order to relate the mor-
phology of the phase-separated state to the processing of
multiphase systems [24]. Most models of the morpho-
logical changes of polymer blends assume that an aver-
age response (e.g. an average size drop is being broken
or average size drop coalescence) provides good repre-
sentation of the whole system. This assumption should be
reasonably correct for blends with narrow distribution of
drop size. However, there are reports, for instance, that
during the initial stage of blending in the twin screw ex-
truder, the domain sizes may differ by three orders of
magnitude. Here, the average size may not be valid. Ac-
cording to Utracki [18], a recent kinetic theory of the
structure development in the moderately concentrated
polymer blends, was proposed. The break-up coalescence
in a steady state shearing was considered, assuming a
temporal population balance model. Kroger and Hess [25]
studied a certain critical molecular weight that controls
rheological properties of the multi-bead finitely extensi-
ble nonlinear elastic (FENE) chain model of polymer
melt. The rheological crossover (where G = G) mani-
fested itself in a change of power law behaviour for the
viscous properties at a critical number of beads per chain,
Nc = 100 ± 10. Their finding confirmed a newly proposed
relationship between dimensionless critical weight,
characteristic length and the flexibility which they ob-
tained as a side result. Sivashinsky et al. [26] studied
shear stress growth and relaxation behaviour of two
well-characterized SBS block copolymers using a melt
elasticity tester. Effects of casting solvents on the transient
rheological properties and structure variation during flow
were determined via samples cast from THF/MEK and
cyclohexane. The temperature-dependence of the shear
stress observed at the yield point and the steady-state
viscosities provide an insight into the mechanism of
structure breakdown, induced by the applied flow. Shear
stress relaxation, upon cessation of flow, reveals complex
behaviour, depending on temperature, casting solvents
and duration of previous flow. Strain recovery data have
also been obtained, indicating strong dependence on the
composition of the copolymers. Dispersed phase mor-
phology development has been mainly studied in capillary
flow. To explain the fibrillation processes, not only the
viscosity ratio, but also the elasticity effects and the in-
terfacial properties had to be considered. In agreement
with the microrheology of Newtonian systems, an upper
bound for the viscosity ratio, λ has also been reported for
polymer blends-above certain value of λ, which can sig-
nificantly be larger than the Newtonian value of 3.8,
during which time, the dispersed phase could not be de-
formed. By contrast, lower bounds of λ were not estab-
lished for polymer blends [18].
6. The Importance of Considering the Stress
or Strain History of a Material Sample in
a Rheological Investigation
In order to measure a material’s rheology, five criteria
must be met: geometric boundary conditions, stress, train,
strain rate and mode of deformation. For a good com-
prehension of the rheological properties of materials, it is
essential to either measure the deformation resulting from
a given force or measure the force required to produce a
given deformation. As a measure of force, one can use the
stress which is defined as the ratio of applied force to the
cross sectional area on which the force acts. Deformation
can be described in term of strain or rate of strain. There
are two basic flows used to characterize polymer: shear
and shear-free flows. For these two types of flow, the
component of the stress and the rate of deformation tensor
take on a distinct form. Owing to its sensitive response to
changes of microstructures for heterogeneous polymers,
rheological measurement has been a preferred approach in
the characterization of the formation and evolution of
microstructures for multi-component or multi-phase
polymeric material systems. According to Qiang et al.
[27], some of the recent progresses made in the studies on
the correlations between the micro-structural change and
rheological response have been introduced as two aspects,
viz: relationships between viscoelastic behaviour and
microstructure of nano-composites and relationships be-
tween rheological behaviour and liquid-solid transition in
isothermal crystallization of polyolefins. By means of
rheological measurements, not only are some valuable and
pertinent information responsible for the evolution of
morphology and structure development, dealing with
Copyright © 2011 SciRes. MSA
Rheological Properties of Polymers: Structure and Morphology of Molten Polymer Blends37
these polymer systems can be obtained, but also the cor-
responding results are in favor of designing and preparing
novel polymer-based composites and functional materials.
Kohli et al. [28] assessed the effect of deformation his-
tory on the morphology and properties of liquid crystal-
line polymers (LCP) blended with polycarbonate resin.
The addition of an immiscible LCP phase was found to
improve the melt processability of the host thermoplastic
polymer. In addition, by employing a suitable deforma-
tion history, the LCP phase may be elongated and ori-
ented such that microfibrillar morphology can be retained
in the solid state. This has important ramifications for the
development of self-reinforcing polymer blends to com-
pete with conventional inorganic fiber reinforced poly-
mers. Shear flows are generally ineffective in developing
these morphologies, but flows that incorporate an exten-
sional region, such as the converging flow found at the
entrance to a capillary or die can produce an elongated
LCP phase. Bousmina et al. [29] assessed the phase seg-
regation in the poly (styrene-co-acrylonitrile)/poly (methyl
methacrylate) (SAN/PMMA) blend with lower critical
solution temperature (LCST) by linear viscoelastic
rheology, optical microscopy and inverse gas chroma-
tography (IGC) techniques for various blend composi-
tions. At low temperatures, the blends showed a classical
behaviour of homogeneous polymer melts, whereas in
the vicinity of phase segregation, a shoulder in the stor-
age modulus and in the linear relaxation modulus, G(t)
was observed. The width of such a low-frequency/
longer-time plateau and the terminal relaxation time were
found to increase with temperature. Such behaviour was
attributed to variable morphologies appearing at different
temperatures. The development of the morphology was
found to take place within a given interval of temperature
rather than at a single critical temperature. Optical mi-
croscopy and IGC analyses supported the peculiar be-
haviour observed, of such a blend. Time-temperature
superposition, origin of elasticity, and the Fredrickson
and Larson theory were discussed in light of the results
7. Developments in the Field of Polymer
Blend Rheology
7.1. Past, Present and Future Developments in
the Field of Polymer Blend Rheology
The technology of polymer blends has been one of the
major areas of research and development in polymer
science in the past three decades. The advantages of
polymer blends versus developing new polymeric struc-
tures have been well documented. The ability to combine
exiting polymers into new compositions with commer-
cializable properties offer the advantage of reduced re-
search and development expenses compared with the
development of new monomers and polymers to yield
similar property profile. Another specific advantage of
polymer blends versus new monomers/polymers compo-
sition is that the blends can often offer property profile
combination not easily obtained with new polymeric
structures. In rapidly emerging technological landscape,
polymer blend technology can quickly respond to devel-
oping needs much faster than the time consuming re-
search and development involved with new mono-
mer/polymer development. The technical response to
emerging needs is now primarily directed to polymer
blend technology in order to determine if such needs can
be met compared to the development of wholly new
polymeric compositions [3]. The understanding of poly-
mer blend technology to design the specific compositions
to meet application requirements is of primary impor-
tance. A number of useful analytical and characterization
methods have been developed for polymer blends, al-
lowing for an improved understanding of the nature of
their miscibility and phase behaviours.
While miscible polymer blends have attracted consid-
erable interest due to the thermodynamic implications
and commercial relevance, phase-separated blends have
had a prominent role in polymer blend technology. While
mechanical compatibility is assured in miscible blends,
phase-separated blends can often achieve property ad-
vantages not capable with single phase blends [29]. Me-
chanical compatibility refers to an average property of
the blend constituents in the final blend mechanical
properties. Impact modification is a well known virtue of
many phase-separated blends but other properties; in-
cluding rheology, environmental stress, rupture resis-
tance, opacity, flammability, cost reduction, specific film
properties, adhesion and performance of coating can be
achieved via specific combination of immiscible blends.
In order to achieve mechanical compatibility in phase-
separated polymer blends, different approaches have
been proposed, demonstrated and in many cases com-
mercialized. These methods include: addition of graft or
block copolymer, interpenetrating polymer network, re-
active extrusion, addition of polymeric interfacial agents
and minor addition of acid and base compatibilized units.
Reactive extrusion is an area that has attracted consider-
able interest as a facial means of compatilizing polymer
blends, which offers limited performance as simple
blends. This technology initially, emerged with “super
tough” polyamides, where maleic anhydride grafted onto
an ethylene propylene rubber allowed for graft formation
with terminal –NH2 groups of polyamide. The resultant
graft structure stabilized the interface between the un-
grafted constituents and allowed for the desire impact
Copyright © 2011 SciRes. MSA
Rheological Properties of Polymers: Structure and Morphology of Molten Polymer Blends
In engineering polymer blends, a number of advances
in the technology and the commercial areas have been
realized in the past decades. New commercial polymers
have been manufactured by various combination of
pre-existing polymers. One of the major areas has been
the polyester and polycarbonate (polybutyleneterephtha-
late/polycarbonate; polyarylate/polycarbonate; cyclo-
hexane dimenthanol-based polyesters/polycarbonate and
polyethyleneterephthalate/polycarbonate). The emergence
of polymeric material for the future auto panels resulted
in a large number of potential candidates, based almost
exclusively on engineering polymer blends [30]. The
most recent addition to the engineering polymer field is
the ethylene carbon monoxide alternating copolymers,
initially introduced by Shell. The commercial polymer is
highly crystalline and believed to contain small amount
of propylene to reduce the crystalline melting point in
order to allow a broad window of process ability. There
are several areas involving extensive research endeav-
ours, presently been investigated that are relevant to
polymer blends of the future: These areas include, but
not limited to: molecular composites, liquid crystalline
polymer blends, blends that involve electrically conduc-
tive polymers, blends containing biodegradable polymers
and theoretical studies that involve the prediction of
polymer phase behaviour. Another interesting area in-
volves the computation model predictions of polymer
Due to the two-phase or multiphase nature and from
the macroscopic point of view, the rheological behaviour
of incompatible (or heterogeneous in general) polymer
blends must be dealt with from phenomenological (i.e.
fluid mechanics) consideration. Conversely, the rheo-
logical behaviour of miscible polymer blends can be
dealt with (or interpreted) using the molecular approach.
While there are few miscible polymer blends that have
met with commercial success, numerous investigators
have reported on the rheological behaviour of miscible
polymer blends [31]. Multi-phase polymer blend systems
examine the current state-of-the-art, challenges and fu-
ture prospects in the field of polymer blends. The
hand-picked selection of topics and expert contributors,
make this survey of phase morphology in polymer blends
an outstanding resource for anyone involved in the field
of polymer materials design. It is now well known that
morphology is a major parameter to control final proper-
ties of immiscible polymer blends. Predicting this mor-
phology from the mixing conditions is the next step to
develop and produce new performing materials. A uni-
versal acceptable theory for describing the evolution of
morphology of multi-phase immiscible polymer systems
may still be far off, thus polymer blends are becoming
more exciting interesting research discipline, in both in-
dustrial and academic laboratories around the world. It is
obvious that research and development on polymer
rheology is purely fundamental and serves long-term
7.2. Current Research Status
The developments of melt rheology of polymer blends
depend largely on the understanding of the struc-
ture-properties relationship of the materials which re-
quires multiscale model to predict the material properties.
The current research in the modeling and simulation of
polymer blends are largely limited to length and time
scale. However it should be noted that some efforts have
been made to develop multiscale strategies for the pre-
diction of multiscale level structure, properties and proc-
essing performance of polymer blends. Some of these
works will be mentioned. Bouhlel et al. [32] recently
used Biot’s model to describe the quasi-static deforma-
tion of a continuous porous medium, saturated by in-
compressible Newtonian fluid at a microscopic scale.
The microscopic description involves four effective
properties, i.e. the effective elasticity tensor, Biot’s ef-
fective tensor, the coefficient β and the permeability ten-
sor. The numerical results obtained from their work were
compared with bounds, self-consistent estimation, exact
expansion and experimental result available in the litera-
ture on ceramic and metals. Balazs and co-workers [33]
recently combined DFT with SCF to calculate phase be-
haviour of clay-based polymer nanocomposites and other
polymer nanoparticles mixtures. In this model, the ther-
modynamic behaviour and interaction information
among various components are obtained from the SCF
model which served as input to a DFT to calculate the
phase behaviour. Chu et al. [34] proposed a modified
model of Chen’s for the calculation of the relative vis-
cosity of BR-SBS blends of different compositions. The
agreement between the theoretical prediction and ex-
perimental results was satisfactory. Edwards et al. [35]
isothermally studied the dynamic and creep behaviour of
80ln15Pb5Ag and 50Sn50Pb over nine decades of time
and frequency. 80ln15Pb5Ag was examined at –6, 21,
and 50˚C, while 50Sn50Pb was examined at 21˚C. Vis-
coelastic behaviour as a function of temperature and
time/frequency was observed at strain less than 10-5. At
the small strain level employed in the study, the alloys
exhibit linear viscoelasticity rather than the viscoplastic-
ity observed at larger strain. Viscoelasticity was subject
to constitutive modeling. The creep followed a superpo-
sition of a stretched exponential and power law, in time
and an Arrhenius form, in temperature. Behaviour was
thermorheologically simple over at least nine decades of
true time and frequency.
Copyright © 2011 SciRes. MSA
Rheological Properties of Polymers: Structure and Morphology of Molten Polymer Blends39
7.3. Opportunities and Challenges
An overview of technological developments involving
rheological properties of polymer, structure and mor-
phology of molten polymer blends clearly indicates that
the field expands at unprecedented rates. The guiding
principles behind this progress are efficiency, functional-
ity and precision of these materials. However, effective
implementation of morphology of molten polymer blend,
structure and its effects on rheology directly depends on
the degree of understanding of their behaviour and prop-
erties. In this regard, challenges arise due to the sensitiv-
ity of the polymers to variations in their fabrication,
time- and temperature-dependent effects and the
non-linearity of their coupled mechanical, physical and
chemical properties. A problem of immediate signifi-
cance stems from the fact that polymer blend materials
are often dependent on the morphology and control of the
materials. In fact, these materials are considered as pri-
mary candidates for distributed control of compliant
structures. The inherent properties of polymers combined
with their control functions tend to produce new and un-
explored effects that should be fully understood. It is also
of importance that polymers perform their functions in
dynamic environments over a wide range of temperatures
and frequencies. Such conditions tend to accelerate their
degradation and, consequently, alter their functional re-
sponse through electromechanical couplings. Besides the
experimental investigations required in addressing the
outlined problem areas, there is a need to advance the
computational rheology of polymers. In this regard, it is
important that modeling and simulation are treated as an
integral part of design and manufacturing processes.
The prediction of the end-use properties of polymeric
products is faced with some daunting challenges. The
current process simulation approach, which is based on
the continuum mechanics of non-Newtonian fluids, must
be combined with models describing macromolecular
conformations, relaxation and polycrystalline morpholo-
gies. The various types of constitutive models, whether
continuum [36] or reptation [37], have had very limited
successes in predicting the unusual rheological phenom-
ena exhibited by polymeric liquids, even under isother-
mal conditions. Determination of heat transfer coeffi-
cients and modeling of flow-induced crystallization [38],
are necessary for the eventual prediction of properties of
films and other extruded products. Numerous other
problems remain unresolved in other polymer processes,
such as the prediction of shrinkage, warpage and stress
cracking in injection molding. The goal of precise prop-
erty prediction is likely to remain a challenge for a con-
siderable length of time. However, new technologies,
even without detailed scientific understanding, are likely
to play a significant role in the field of polymers. These
include: nanocomposites with exceptional properties,
conductive plastics for electronics, self-assembly proc-
esses for the creation of special polymeric structures and
fabrication of biomaterials and polymer-based tissue en-
gineering. In the immediate future, continuing progress
in the field of smart polymers will depend on the inten-
sity of research efforts directed towards the development
of polymer rheology systems with enhanced adaptive
capabilities. Concurrently, it is critical to develop formu-
lation of advanced theoretical models for the simulations
of the rheological behaviour of polymer. On this basis,
the unprecedented opportunities offered by generating
mathematical models for simulating rheological charac-
teristics of polymers, will continue to stimulate further
technological inquisition.
8. Conclusion Remarks
This review provides the collection of knowledge based
on what is published in the scientific literature. Devel-
opments involving rheological properties of polymer,
structure and morphology of molten polymer blends viv-
idly show that the field of polymer technology rapidly
expands at higher rates. Different polymer blends may
represent a feasible method for exploiting some of these
developments. Some modeling techniques used for the
prediction of mechanical properties of polymer blends
have been reviewed in this paper. Future research is ex-
pected to be focused on the generation of advanced
mathematical models for simulating rheological proper-
ties of polymers, in order to stimulate further techno-
logical progress
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Glossary of Some Rheological Terms [39]
Rheology—The science of deformation and flow of
Rheometer—An instrument for measuring rheologi-
cal properties.
Flow—A deformation of which at least part is
Elasticity—Reversible stress/strain behaviour.
Modulus—In rheology, this is the ratio of a compo-
nent of stress to the component of strain. Pa.
Model—An idealized relationship of rheological
behaviour in mathematical, electrical or mechanical
Relaxation time—The time taken for the strain in a
material that obeys the Kelvin model to reduce to 1/e
(0.368) of its original equilibrium value after the
removal of stress. Generally, it is the time required
for an exponential variable to decrease to 1/e of its
original value.
Yield stress—The stress corresponding to the transi-
tion from elastic plastic deformation. σy, Pa.
Stress—A force per unit area, Pa
PPE Polyphenylene Ether
PS Polystyrene
PE Polyethylene
PP Polypropylene
PA6 Polyamide 6
SMA2 Styrene-maleic anhydride
PVPh Poly (vinylphenol)
PVP Poly (vinylpyrrolidone)
DSC Differential scanning calorimeter
FTIR Fourier transforms infrared spectroscopy
NMR Nuclear magnetic resonance
PS GMA-poly (styrene-ran-glycidy
MB Melt blending
SOM Solution blending followed by an
oscillatory shearing at a molten state
GPC Gel permeation chromatography
SEM Scanning electron microscopy
TEM Transmission electron microscopy
PIB Polyisobutylene
SBS Styrene-butadiene-styrene
PDMS Polydimethylsiloxane
DMF Dimethylformamide
PCAM Painter-coleman association model
PMMA Poly methyl methacrylate
SAN24 Styrene-co-acrylonitrile
EVA Ethylene-vinyl acetate
FENE Finitely extensible nonlinear elastic
LCP Liquid crystalline polymer
LCST Lower critical solution temperature
IGC Inverse gas chromatography
PB Polybutadiene
SC Self consistence function
DFT Density function theory
MC Monte Carlo
MD Molecular Dynamic
G Storage modulus
G Loss modulus