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The renewed interest in the flow induced crystallization is due to the availability of
new experimental tools that can yield insight into molecular characteristics that
control the crystallization pathways adopted by a stressed polymer melt. Most of the
studies have concentrated on shear flow, which is also the kind of flow used in this
work. The ultimate goal is to develop a tool that is able to simulate, improve and even
optimize polymer synthesis at the industrial conditions, because optimization of
properties of polymers in industry is still done by expensive and time consuming trial
and error methods, which are based on experience.
1.1. Work done
In this work, the effects of a shear flow applied during crystallization on the
morphology evolution and on the kinetics of isothermal crystallization of two iPP have
been studied experimentally.
The group of Material Technology of the Department of Mechanical Engineering of the
University of Eindhoven - TU/e, The Netherlands (Prof. G.W.M. Peters) developed a
Multi Pass Rheometer (MPR) equipped with a in house designed slit flow cell that can
be used to perform flow experiments at processing conditions, i.e. high pressures, high
shear rates, high cooling rates (but also isothermally) and (multiple times) reversed
flow. This device is, as a result, an experimental setup to study in-situ and ex-situ
structure and morphology development of polymers with a control over the processing
conditions and shear history.
The aim of this thesis is to contribute to the understanding of the relation between
molecular properties, processing conditions and final morphology of polymers using
this powerful tool and investigating the flow-induced crystallization in melts.
The MPR has been used to perform isothermal shear flow experiments on two
different iPP that are well characterized both rheologically and thermally. The resulting
micro-structure of the samples has been analyzed by in situ measurements like
turbidity and birefringence measurements and by optical microscopy (OM) and FT-IR
measurements (ex-situ measurements) and the results are summarized in this work.
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To describe the evolution of molecular orientation present over the thickness of the
samples at the end of shear flow a Maxwell model has been used.
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2. The isotactic polypropylene
The isotactic polypropylene (iPP) is one of polyolefinic polymers more widely used. It
has an excellent combination of low cost and great versatility about properties,
applications and recyclability.
The polypropylene is obtained by polymerization of propylene, made by cracking of
hydrocarbons of high molecular weight.
Polypropylene can be made with different tacticities. Most polypropylene we use is
isotactic. This means that all the methyl groups are on the same side of the chain.
Figure 2.1 Schematic drawing of isotactic polypropylene
But sometimes we use atactic polypropylene or syndiotactic polypropylene. Atactic
means that the methyl groups are placed randomly on both sides of the chain;
syndiotactic means that the substituents have alternate positions along the chain.
The isotactic polypropylene is highly crystalline and its melting point is about 165°C at
atmospheric pressure and its glass transition temperature is about -15°C. It is a
thermoplastic, colorless and translucent, rigid polymer and it has good dielectric
characteristics and high resistance to chemical agents.
The types of crystal phases found in isotactic polypropylene are α, β and γ.
In all crystalline phases in isotactic polypropylene, the chain adopts a 3i-helix, a three-
fold helix which indicates that it takes three monomer units to make one helical turn.
The helix can be either right (R)- or left (L)- handed, with a period of 6.5 .
Furthermore, the orientation of the C-CH3 bond with respect to the chain axis can be
either up or down.
It has been demonstrated that the crystalline structures can coexist in a sample, but
every spherulite is made of one single phase, so we can talk about α-spherulite, β-
spherulite and γ-spherulite.
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Figure 2.2 Chain conformations of isotactic polypropylene. Right(R) - and left (L)-handed 31-helices in their up (up)
and down (dw) configuration.
2.1. The α phase
The most common crystal form in iPP is the α phase having a monoclinic crystal
structure. The α phase was the first to be discovered and characterized (Natta and
Corradini, 1960). The density of this phase is about 0.936 g/cm3 .
2.2. The β phase
The β-phase is normally observed in the presence of nucleating agents or under
specific conditions like a strong imposed orientation. The β phase was first noticed by
Keith et al. (1959). Only recently this phase has been recognized as a crystal phase with
a “frustrated” chain packing within a hexagonal unit-cell (Dorset et al., 1998) and
Meille et al., 1994)). The density of this phase is 0.9105 g/cm3.
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2.3. The γ phase
Crystallization of iPP in the γ phase strongly depends on specific aspects of the
molecular structure. For example, the γ phase is observed in less stereo-regular
isotactic materials, in very low molecular weight samples (3000 g/mol) and in certain
types of random copolymers. The γ phase is also observed on crystallization at
elevated pressures, in that situation even in highly isotactic samples, independent of
molar mass. Morrow and Newman (1968) proposed the crystal structure of the γ
phase to be triclinic. Bruckner and Meille (1989) showed that the γ phase has an
unusual crystal structure with a non-parallel chain conformation in an orthorhombic
unit-cell. The density of this phase is 0.933 g/cm3.
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3. Birefringence and turbidity
Birefringence, or double refraction, is the decomposition of a ray of light into two rays
(the ordinary ray and the extraordinary ray) when it passes through a material. This
effect can occur only if the structure of the material is anisotropic (directionally
dependent). If the material has a single axis of anisotropy, it is uniaxial. When light
propagates through a birefringent medium, the state of polarization will be changed;
the phase difference of the two orthogonal field components of the light beam will be
changed; the refraction index in one principle direction will be different than the other
two directions. The optical axis in a birefringent material is defined in this direction.
Suppose that an incident - field propagates perpendicular through a birefringent
medium. Three different situations can be defined:
The optical axis is parallel to the propagation direction of the incident -field.
The two orthogonal field components will experience the same retardance.
The optical axis is perpendicular to the propagation direction of the incident
-field. The two orthogonal field components will experience a different
retardance and will coincide in space.
All the other case. The two orthogonal field components will experience a
different retardance and will not coincide in space.
In the last two cases, an ordinary and an extraordinary wave can be defined. The
ordinary wave is perpendicular to the plane described by the propagation direction of
the incident -field and the optical axis. The extraordinary wave is perpendicular to
the ordinary wave and parallel with the optical axis.
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Figure 3.1 Definition of the ordinary and extraordinary wave in a birefringent material, oa: optical axis, e:
extraordinary wave, o: ordinary wave
Birefringence can be formalized by assigning two different refractive indices to the
material for different polarizations. The birefringence magnitude is then defined by:
where no and ne are the refractive indices for polarizations perpendicular (ordinary)
and parallel (extraordinary) to the optical axis respectively.
Due to the different refraction indices, the ordinary and extraordinary wave will travel
with different velocity through the medium. On exit of the medium, the two waves will
have different phases. The phase difference or phase retardance δ is related to Δn
according to:
where d is the sample thickness and λ is the laser wavelength.
δ is calculated from
where and are the intensity of light through crossed and parallel polarizers
respectively.
So Δn is a measure of the anisotropy in the sample:
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Since the function y = arcsin x has a codomain of −π/2 ≤ y ≤ π/2, known the laser
wavelength and the thickness of the sample, it is possible to evaluate the maximum
theoretical value for .
Polymer chains subjected to flow will orient in flow direction and this orientation
causes birefringence.
Therefore, the degree of turbidity can be used to qualitatively monitor the progress of
crystallization. It is calculated as the ratio of the total intensity at a given time and the
initial total intensity.
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4. FT-IR spectroscopy
A dichroic material is a material in which light rays in different polarization states
travelling through it are absorbed by different amounts so experience a varying
absorption.
Infrared dichroism involves the application of polarized infrared beam to the analysis
of the material. This is a method for determining the orientation of the polymer and
for estimating the crystallinity index in a non destructive measurement.
An infrared beam is transmitted through a sample to obtain transmission spectra. A
transmission spectrum shows the absorbance, A, over a range of wave number, ν.
Some peaks are assigned to the amorphous phase, other to the crystalline phase and
some are insensitive to structure. This is because different frequencies will absorb in
different phases of the polymer structure.
Absorption in the infrared region of the spectrum is a function of the internal energy of
the molecule. The molecules absorb energy according to quantum rules and a
continuous spectrum of energy absorption is not observed, but instead energy is
absorbed only at discrete frequencies.
When absorption occurs in this energy region, it changes the rotational and vibrating
energy levels of the molecules producing rotational and vibrating motion of the atoms
constituting the molecules. Since the rotational energy changes are smaller than the
vibrating energy changes, the observed spectra can be considered as a measure of the
vibrating modes of energy absorption of the molecules, i.e. a vibrating spectrum.
To understand how to correlate this energy change with the structure of the molecule,
see (Samuels, 1981).
The crystallinity index can be estimated by analysis of FT-IR absorbance spectra
applying Lambert and Beer’s law to selected peaks.
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