On the effect of particle size, shape, concentration and aggregation on the
flow-induced crystallization of polymers
Michelle D’Haese°, Florentin Langouche*, Peter Van Puyvelde°#
°Katholieke Universiteit Leuven
Leuven Material Research Centre
Department of Chemical Engineering
Laboratory of Applied Rheology and Polymer Processing
W. de Croylaan 46
B-3001 Leuven (Belgium)
*Solvay Central laboratoriesR&I Centre Brussels
Neder Over Heembeek
Rue de Ransbeek 310
B-1120 Brussels (Belgium)
#corresponding author: peter.vanpuyvelde@cit.kuleuven.be
abstract
In this work, the combined effect of particles and flow on the crystallization of polymers has been
investigated, using a rheo-optical approach.
The effect of particle size, concentration, shape and
aggregation on the kinetics of flow-induced crystallization is investigated. It is shown that at high enough
flow rates, flow will dominate the nucleation process, independent of the presence of particles, whatever
their size, shape and concentration. Moreover, a framework will be presented to explain the additive
effect of the presence of particles and flow on polymer crystallization. Finally, possible deviations from
this additive rule (“synergetic effects”) will be discussed.
Introduction
In industrial applications, polymers typically contain an additive package, such as for instance particulate
fillers, to selectively change their properties. In addition, during processing, they are subjected to complex
and intense thermo-mechanical treatments after which the material
solidifies through for instance
crystallization. Both the addition of particles or the application of flow can influence the crystallization
process in semi-crystalline polymers.
Both may enhance the nucleation process, which results in an acceleration of the crystallization kinetics
and a finer final morphology. When taking a closer look at the effect of particles, their surfaces may
provide a number of sites for heterogeneous nucleation1 which is dependent on many factors: the particle
size, concentration, shape, the dispersion quality, its nature (chemical nature, crystal lattice, surface
characteristics and modification) and above all on the specific interaction with the polymer.2,3
Heterogeneous nucleation at an edge (also called tertiary nucleation) has a lower free energy barrier than
nucleation on a flat surface (secondary nucleation) as less melt / crystal specific surface needs to be
created.1 This effect of surface topography on nucleation was further elaborated in for instance the
seminal work of Binsbergen.4 Another important step in explaining the nucleating ability of particles
towards polymers, was taken by Wittmann and Lotz5-8 with the introduction of the theory of epitaxial
nucleation. According to this theory, a certain degree of dimensional and structural lattice matching
between the crystal lattices of the nucleating agent and polymer is necessary for nucleation.
As to the effect of flow on nucleation phenomena, this has been the subject of many studies in literature
(see for instance Kumuraswamy for an overview11). Nowadays, it is widely accept that flow will (locally)
enhance polymer chain orientation, which is conducive to the formation of crystalline nuclei.9-11
Moreover, sufficiently intense flow may cause a structural transition from an isotropic spherulitic
morphology to highly oriented shish-kebab structures.11-13 One of the mechanisms proposed for the
nucleation of these structures involves the aggregation of shear-induced point-like nuclei, leading to a row
of these nuclei which then form a shish, on which kebab lamellae can subsequently nucleate.10,14,15 This
idea was recently further elaborated by Mykhaylyk et al.16,17
Next to nucleation, crystalline growth is an important step in polymer crystallization. The influence of
particles or flow on the spherulitic growth rate however, is generally less important or even negligible.
During crystallization, dispersed particles must be rejected and/or occluded by the growing spherulites,
which may disturb spherulite growth.18 However, at low particle concentrations and undercooling, the
slowing down of the spherulitic growth rate is often negligible.19 As to flow, according to some reports it
is not significantly influenced by flow,9,20 whereas observations during fiber-pull experiments by Haudin
and Monasse and coworkers21-25 showed that the spherulite growth rate may in some cases, depend on the
shear conditions and the molecular weight of the polymer.
Most of the studies on flow-induced crystallization have been carried out on relatively pure polymers,
without the deliberate addition of particles. Although the combined effect of shear flow and particles on
the crystallization of polymers has already received some attention in literature, not many systematic
studies have been performed so far. However, some general features could be concluded from these
reports. An often-made observation in studies concerning low- to moderate-shear conditions20,26-28 and in
some studies involving more intense shear29,30 is that the influence of flow on the crystallization kinetics is
less significant for polymers containing a nucleating agent. Byelov et al.31 generalized this observation
and concluded that the effect of flow will only be important when its contribution to the nucleation density
is of the same order of magnitude as the total nucleation density under quiescent conditions. Recently,
D’Haese et al.30 conducted an experimental study over a large shear range and directly evidenced a shift in
dominance from the influence of particle nucleation to the influence of flow on polymer crystallization
behavior with increasing shear intensity. Another omnipresent observation is that the presence of particles
enhances oriented crystallization.30,32-42 This enhancement can be accomplished in several ways. In the
case of strongly nucleating fibers or platelets, nuclei formed on the surface are less restricted in the
direction perpendicular to it, and will hence form a typical columnar layer which is known as
transcrystallinity.43,44 When flow is applied onto a polymer containing such fillers, these become oriented
in the flow direction, and hence template oriented crystallization through lamellar growth perpendicularly
onto the fibril surface, even if flow-induced orientation or stretch of the matrix is absent or weak.45,46
47
The enhancement of orientated crystallization in particle-containing polymers has also often been ascribed
to a reinforcing of the flow effect by the particles, resulting from a specific interaction or synergy between
the particles and the surrounding polymer matrix. The suggested mechanisms for this synergy include
stress amplification of the flow by the particles28,48,49 or anchoring of polymer chains on the particle
surface.29,50-56 The former term indicates the reinforcing of the flow effect on the polymer kinetics and
morphology by local stresses generated around the particles during the application of flow. The second
hypothesis explains this reinforcing effect as the result of decreased chain mobility due to the anchoring of
the chains onto the particles. However, the difficulty in identifying synergetic effects is to accurately take
into account the simultaneous presence of the nucleation effect of the particles.
Although some important aspects of flow-induced crystallization of particle-containing polymers have
already been discerned, a comprehensive framework for its interpretation is still missing. Based on a
systematic and relatively broad set of experiments concerning a wide shear range and variation in particle
characteristics, such a framework will be proposed in this paper.
Materials and Methods
An isotactic polypropylene (iPP) (grade 76207) obtained from Borealis GMBH was used for most of the
experiments presented in this paper. For the mixtures with calcium carbonate, the polymer used was a
commercial iPP (grade 34225-1), also supplied by Borealis GmbH. Commercially available zinc oxide
particles with average diameters of 35 nm, 200 nm, 500 nm and 1 µm, obtained from Umicore Group Zinc
Chemicals, were selected to investigate the effect of particle diameter and concentration on the polymer
crystallization. These zinc oxides all possess the same surface crystal lattice and have a high purity.
However, they differ in shape and surface roughness. Whereas the 35 nm particles are rather oblong, the
200 nm and 500 nm particles are relatively spherical and the 1 µm particles are irregular in shape,
consisting of aggregated rod- and spike-like particles, which leads to a very rough surface.
To investigate the influence of particle shape on polymer crystallization, particles with different aspect
ratios were self-synthesized by forced hydrolysis of iron (III) perchlorate in the presence of urea. 57-59 The
dimensions of the obtained hematites are given in Table 1.
av. aspect ratio (c/a) [ ]
1
1.8
3.6
particle length (c) [nm]
286 ± 77 (27%)
148 ± 13 (9%)
268 ± 27 (10%)
particle width (a=b) [nm]
286 ± 77 (27%)
81 ± 4 (5%)
74 ± 5 (6%)
Table 1: Dimensions (with absolute and relative (%) variation) of the different hematites
Finally, a commercial aggregated calcium carbonate filler was supplied by Borealis GmbH (Austria). The
calcium carbonate is precipitated calcium carbonate (PCC). These particles are coated with calcium
stearate and have a particle size of about 80 nm, but form aggregates of about 1µm. It has been shown in a
previous study20 that the calcium stearate coating is subject to significant degradation at temperatures
around 220°C. To avoid this effect, the material was not heated above 200°C in any of the reported
experiments.
Mixtures of iPP grade 76207 with the different zinc oxides and mixtures of iPP grade 34225-1 with 3 wt%
of calcium carbonate were made by means of a twin-screw mini-extruder (DSM-Research, The
Netherlands). The mini-extruder consists of a mixing chamber with a capacity of 15 ml and two corotating conical screws. A recirculation channel within the mixing chamber allows for a variation of the
residence time. The screw speed was set at 100 rpm and the material was allowed to recirculate for 5 min
at 220°C. To avoid degradation, N2 was circulated through the extruder during mixing. It was verified by
means of scanning electron microscopy (SEM) that a good dispersion of the particles was obtained in all
cases.
In the case of the hematite particles, mixtures were prepared along a solution route. The hematite particles
were dried after synthesis and then ultrasonically dispersed in xylene. Subsequently, the iPP was solventmixed with the hematite-containing xylene at 135°C, using magnetic stirring and ultrasonification. Again,
SEM was used to ensure a proper dispersion of the hematite in the iPP matrix.
Differential Scanning Calorimetry experiments have been performed with a DSC Q2000 (Universal
V4.3A TA Instruments) to determine the nucleating efficiency of the particles (explanation nucleating
efficiency, see further).
The flow-induced crystallization has been studied by means of birefringence measurements. The
experiments were performed using an optical train consisting of a laser modulated by means of a rotating
half wave plate (OAM module, TA Instruments) and at the other side of the shear cell, a circular analyzer
and a photo detector. For the analysis of the signal measured by the detector, a decomposition of the
intensity signal of the following form is used:
I  I DC  I sin sin(4t )  I cos cos(4t )
with ω the angular speed of the rotating half wave plate and Isin and Icos the amplitudes of the sine and
cosine component in the signal.The information contained in this signal is expressed in terms of the
transmitted intensity IDC, which is normalized with respect to the initial value of the intensity I0 or
IDC(t=0), and of the birefringence ∆n’:
n ' 
 '
2 d
(3.5)
with λ the laser light wavelength (670 nm), d the sample thickness and δ’ the optical retardation calculated
as:
sin  ' 
2
2
I sin
 I cos
I DC
(3.6)
The optical train was combined with a sandwich type sliding-plates shear cell (abbreviated as “sandwich
cell” in this text), developed at Solvay’s Central Laboratory in Brussels (Belgium). 32,33 In this setup, the
sample is sheared between two parallel glass plates, ensuring a constant shear rate throughout the sample.
The glass plates are incorporated in sample holders, which are placed in oppositely moving conditioning
blocks. The conditioning blocks are independently heated using circulating oil baths and PT100
controllers, resulting in a temperature precision of the order of 0.1°C. Apertures are provided in the
equipment for laser light transition, allowing a view of the velocity - vorticity plane of the sheared sample.
The setup allows achieving shear rates up to 1500 s-1 and deformations up to a few hundred shear units,
which is in the order of the shear rates realized in the outer layers of injection molded products (10 2 – 104
s-1). The unit is sufficiently rigid to withstand the normal forces generated during shear, even in case of
highly viscous polymers. More detailed information about the signal processing and the setup can be
found in the dedicated paper by Langouche.32
Results and discussion
1. Effect of particles on crystallization: the Lotz nucleation efficiency scale
Prior to considering the influence of particles on flow-induced polymer crystallization, it is important to
characterize their influence on the quiescent crystallization process, or in other words their nucleating
ability. From a quantitative point of view it is interesting to situate this ability with respect to two
references: a lower limit (i.e. the pure polymer), but also an upper limit which is the maximally nucleated
polymer. To attain this maximum nucleation density of the polymer, a nucleating agent ideal in terms of
dispersion, chemical and crystallographic interactions should be used. This can be achieved in an
unexpectedly simple way: by self-nucleation of the polymer. This is the essential idea behind the Lotz
efficiency scale for nucleating agents,62,63 an elegant, but often forgotten protocol to determine the
nucleating ability of particles in a polymer/particle mixture.
This method realizes self-nucleation of the polymer by using a non-isothermal DSC protocol. After a high
temperature annealing step (in this study: at 210°C), a standard (semi)crystalline state is created by
cooling the polymer at a fixed rate of 10°C/min63 to a temperature low enough to ensure that the
crystallization process of the sample is entirely finished (for iPP, 50°C is adequate). This crystallization
experiment is characterized by the DSC peak temperature Tc,pure and represents the crystallization of the
pure polymer. Then the fully crystallized polymer is partially melted in order to create stable crystal
fragments. When non-isothermally cooled, these fragments act as a (perfect and perfectly dispersed)
nucleating agent, increasing the peak temperature Tc,nuc in exothermal crystallization. This is called selfnucleation of the polymer. As the temperature for partial melting is lowered, the concentration of residual
crystalline fragments increases and finally saturates, in this example at a partial melting temperature of
about 166°C (see Figure 1). When cooled from the saturation temperature, the corresponding
crystallization peak occurs at the highest temperature attainable by this procedure, Tc,nuc MAX (in this study:
at about 144°C) (remark that annealing at 210°C precedes each partial melting experiment). This state
represents the maximally (self-)nucleated polymer.
Figure 1: example of the effect of the partial melting temperature on the crystallization peak temperature as prescribed by the
Lotz protocol.
Annealing and cooling a particle-containing polymer will result in a DSC peak temperature Tc,NA which
allows situating the nucleation by the particles with respect to the lower and upper nucleation limit. Based
on these experiments, the Lotz nucleating efficiency is defined as:63
NE  100.
Tc, NA  Tc, pure
TNA
 100.
T
Tc, nuc MAX  Tc, pure
(1)
in which T is the difference between the non-isothermal crystallization peak temperatures of the pure
(minimally nucleated) and the maximally (self-) nucleated polymer. TNA is the difference between the
non-isothermal crystallization peak temperatures of the pure polymer and the polymer with a certain
percentage of particles. For nucleating agents, values of NE between 5 and 70% are typically found.63
In previous work,30 this methodology has been used to characterize the nucleating efficiency of mixtures
of iPP and 1wt% of zinc oxides (same particles as in the current study) with different diameters (35 nm,
200 nm, 500 nm and 1 µm). The puzzling problem in that study was that no link could be found between
the nucleating efficiency and the specific surface of the particle diameter. This was attributed to
differences in the shape and surface characteristics (irregularities, roughness) of the different zinc oxides.
This observation corroborates earlier evidence of the importance of particle surface topography on
nucleation. Steps, ridges, cracks or holes in the surface form thermodynamically advantageous sites for
nucleation because they provide a forming nucleus with a larger part of its surface than a flat particle
surface (so-called tertiary nucleation).1,4
Here, the Lotz-scale is further used for investigating the effect of particle concentration and aspect ratio
on the nucleation efficiency.
In Figure 2 (top), the dependence of nucleating efficiency on the
concentration of the 500 nm zinc oxide is shown. From this figure, it can be seen that the effect of
concentration on the nucleation efficiency already saturates at quite low concentrations of about 1 wt%
(only 0.16 vol%). It has been reported previously that the effect of particles on nucleation saturates at
(much) lower concentrations than those commonly used in commercial products (20-30 wt%),20,64,65
however, the precise mechanism underlying this saturation with particle concentration is still unclear. This
mechanism, though, is clearly not related to the ability of a polymer to nucleate onto a foreign surface: for
the 500 nm zinc oxide saturation occurs at a nucleating efficiency of 6%, but this is not the maximum
attainable nucleating efficiency for this polymer. Hence, it must be caused by the particles and/or their
organization in the polymer matrix. It has been suggested by Supaphol et al.64 and in the work on particle
aggregation of Pukansky et al.,65,66 that the saturation is due to aggregation of the particles at higher
particle contents. SEM images of the iPP containing 2 wt% of 500 nm zinc oxide (not shown here) indeed
seem to indicate an increased tendency to aggregation compared to, for instance, the mixture containing
0.5 wt% of the zinc oxide. However, it has to be remarked that the local SEM visualization does not allow
any precise estimate of the extent of the aggregation so that no final conclusion can be drawn from these
experiments.
Figure 2: Dependence of the nucleating efficiency on particle concentration for mixtures of iPP with the 500 nm zinc oxide (with
trend line) (top) and on particle aspect ratio for mixtures of iPP with 2 wt% of hematite (bottom)
The correlation between particle shape and nucleating efficiency was also investigated, using mixtures of
iPP and 2 wt% of hematite particles with different aspect ratios (1, 1.8 and 3.6). As can be seen from
Figure 2 (bottom), a clear link exists between these parameters. This correlation cannot simply be
attributed to an increased contact surface area between the polymer and particles with increasing aspect
ratio since, as can be deduced from Table 1, the particles with aspect ratio 1.8 and 3.6 have similar
estimated specific surface areas. In contrast, their nucleating efficiencies are significantly different.
According to the same reasoning already mentioned when discussing the role of surface irregularities in
nucleation, the relatively flat flanks of the particles may be thermodynamically the most favorable location
for nucleation on the elongated particles, as these provide a forming nucleus with a larger part of its
surface than the tip locations. Hence, the more elongated a particle is, and consequently the more
relatively flat surface it possesses compared to strongly curved surface (close to the tips), the more
possible nucleation sites may be present. Although this reasoning is rather hypothetical, the results in 2
(bottom) again seem to point to an important role of particle topography in determining nucleating
efficiency.
As a general conclusion, it can be stated that rather than characterizing the influence of particles on
polymer crystallization using a particle property, such as diameter, concentration or shape, the nucleating
efficiency itself is the most meaningful and universally valid characteristic. Hence, using the Lotzmethodology a clear and consistent relationship can be established between the measured nucleation
efficiency and other experiments, related to the determination of crystallization kinetics. In addition, this
relativelysimple methodology provides a richness of important information since it clearly demonstrates
the boundaries of optimal nucleation conditions.
2. Effect of flow, reference case: the unfilled polymer
The influence of flow on the crystallization of pure polymers has been documented quite well, both with
the sandwich cell used in this work30,32-36 and with other shear geometries.37-42 Birefringence experiments
with the sandwich cell can be used to deduce both information about the kinetics and morphology (amount
of crystalline order as reflected in the level of birefringence) (see also previous studies 30,32,33). Here we
will mainly focus on the kinetics of the crystallization process. A typical evolution of the birefringence is
given in Figure 3. Initially, during the application of flow, the birefringence increases, due to an
increased molecular orientation. At some point (after reaching 100 strain units in the experiment shown in
Figure 3), the flow is stopped and birefringence eventually relaxes. At longer times, due to crystallization,
the optical anisotropy increases again and finally a certain degree of birefringence is obtained. From the
figure, a characteristic crystallization time can be determined as the inflection point of the birefringence
curve. It was shown beforee.g.33 that this inflection point corresponds to the minimum in transmitted
intensity, and hence this time-scale can easily be identified.
A second characteristic that can be
determined from this figure is the degree of crystalline order in the sample, which is defined as the
birefringence level reached at the intersection point of the two tangent curves shown in Figure 3.
Figure 3: Birefringence ∆n’ as a function of time after a shear step at a shear rate 217 s-1 and strain of 100 units for
crystallization at 145°C of pure iPP with indication of the characteristic crystallization time (dashed arrow) and level of
orientation (full arrow).
Remark that care has to be taken in interpreting the birefringence data in terms of crystalline orientation.
To ensure that the birefringence is really caused by oriented polymer crystals, and not random
depolarization,77 it is best to take into account only experiments at shear rates which result in sufficiently
strongly oriented crystal structures. Hence, in this study only the birefringence characteristic values of
experiments at shear rates above 200 s-1 are considered, when discussing crystalline order.
3. Combined effects of particles and flow: additivity effects
Using the same methodology as for the pure polymer, the dependence of the kinetics and morphology of a
particle-containing polymer on shear rate (at a fixed strain) can be determined. Through the definition of
the characteristic values, this dependence can be visualized in a single graph, an example of which is
shown in Figure 4 for the kinetics of iPP containing different concentrations of zinc oxide. The evolution
of the characteristic crystallization time as a function of shear rate follows a similar route as reported by
many authors. As can be seen, at moderate levels of shear rate, the kinetics is enhanced. At intermediate
shear rates, a plateau seems to be present after which a final acceleration of the crystallization kinetics
occurs. It was shown by Baert et al.8 and by D’Haese et al.9 that the transition between the two
accelerations was associated with an upturn of the birefringence curve during flow which was attributed to
the dominant presence of highly oriented structures.36 Finally, after the plateau, a further acceleration of
the crystallization process occurs. Both the pure iPP and the mixtures of iPP with different concentrations
of zinc oxide display these regimes , as can be observed from Figure 4
Figure 4: Dependence of the characteristic time of crystallization on shear rate at a strain of 100 units for pure iPP (circles) and
for crystallization at 145°C of iPP with 0.1 wt% (gray upward triangles), 0.5 wt% (dots) and 1 wt% (squares) zinc oxide
The effect of adding particles can clearly be observed at the lowest shear rates. For instance, when 0.1
wt% is added the characteristic time scale for crystallization is significantly lowered. However, at these
low shear rates, the effect of the presence of particles saturates since almost no difference is observed
between samples containing 0.5 wt% and 1 wt% of zinc oxide particles. The latter is consistent with the
nucleating efficiencies determined using the Lotz efficiency scale (see Figure 2) that also pointed towards
a saturation of the nucleation effect .
In any case, irrespective of the particle concentration, the difference between the characteristic values for
the pure polymer and the different mixtures diminishes with increasing shear rate, denoting a competition
between the particle and flow contributions to the nucleation density. From the graph, it can be seen that
at sufficiently high shear rates, flow will dominate the crystallization kinetics entirely.
The results reported in Figure 4 are in line with the results reported by D’Haese et al.30 on the effect of
particle size on the flow-induced crystallization. In that study, zinc oxide particles of various sizes were
used. In line with the Lotz-analysis, a clear difference in nucleating ability of the different particles was
found (ranging from 4% up to 22% for the 1 µm particles). This corresponded to a difference in
characteristic crystallization times at low shear rates. However, at large enough shear rates, no difference
in crystallization times was found anymore. The present study leads to the same conclusion: a low shear
rates, the nucleating ability of the particles dominates whereas at high shear rates, the nucleation
phenomenon is dominated by flow.
Next to the characteristic crystallization times, also the level of crystalline order can be compared. As was
discussed above, only experiments above 200 s-1 are considered here to exclude possible depolarization
effects. An example of the evolution of the crystalline order with shear rate is illustrated here for zinc
oxide particles with different sizes. As can be seen in Figure 5, the dependence of the level of order
(derived from the birefringence measurements) on shear rate (at the same fixed strain) mirrors the
dependence of the kinetics (also shown in this figure for comparison). At moderate levels of shear rate, a
clear difference in crystalline order can be discerned whereas at higher shear rates, the crystalline order is
solely determined by flow.
Figure 5 Dependence of the characteristic time of crystallization (top) and value of level of order (bottom) on shear rate at a
strain of 100 units for crystallization at 145°C of pure iPP (circles) and of mixtures of iPP and 1wt% of zinc oxide with diameter
35 nm (dots), 200 nm (gray squares) and 1 µm (downward triangles)
In the previous sections, the kinetics and morphology resulting from shear-induced crystallization of a
pure polymer and its mixtures have been considered separately. It was already remarked that their
dependence on shear rate (at a fixed strain) is mirrored (see Figure 5). However, one might wonder
whether stronger links exist between the kinetics and morphology of the pure iPP and its mixtures. To
investigate this, the characteristic value of the level of order can be plotted as a function of the
characteristic time of crystallization, effectively eliminating the shear rate (see Figure 6). From this plot, it
can be seen that there is a remarkable correspondence between the characteristic values for all materials.
This is not only the case at high shear rates (where the results for all mixtures are similar anyway), but
also at the level of the plateau and before it, down to the limit for the birefringence results (at a shear rate
of about 200 s-1), where, as can be seen in Figure 5, significant differences definitely do exist. Figure 6
suggests the existence of a kinetics-morphology master curve for a polymer and its mixtures with random
(non-elongated) particles like the zinc oxides, and this within a nucleating efficiency range up to at least
20% (the maximum NE in this study, for the 1 wt% 1 µm zinc oxide).
Figure 6 Dependence of the characteristic value of birefringence (maximum of the birefringence curve) on characteristic time of
crystallization (time to minimum of intensity) for crystallization at 145°C of pure iPP and all its mixtures with zinc oxide from
experiments at different shear rates and at a strain of 100 units
In the previous paragraphs, the appearance of the upturn in flow birefringence was described. In the
master curve shown in Figure 6, the range of variation of the characteristic time and birefringence value
associated with this upturn is given by the two dash-dot lines. As can be seen, this upturn shows up at a
quite narrow band of time and birefringence level, compared to the full range of variation shown in Figure
6.
The observations and analyses made above combine into a comprehensive framework for the flowinduced crystallization of particle-containing polymers.
This framework is based on the additive
competition between the influences of particle nucleation and flow on polymer crystallization, resulting in
an absolute dominance of flow at high shear rates irrespective of particle presence, which was already
clear from Figure 4 and 5. ‘Additive’ means that nucleation by particles and flow-induced contribute
independently to the total nucleation density. Indeed, particles contribute
a fixed amount of
(heterogeneous) nuclei whereas flow contributes a variable amount of (homogeneous) nuclei. As the
flow intensity increases, so does the amount of flow-induced nuclei. Addition also implies that the level of
orientation is solely determined by the flow conditions, and not influenced in any way by the presence of
the particles (e.g. no synergetic reinforcement of the orientation by the particles). The shear rate / strain
combination determines whether and to what extent polymer chains can be stretched and hence whether
and to what extent oriented nuclei will be formed. The nuclei then template the subsequent crystalline
growth and hence the final level of order built up in flow-induced crystallization.
Returning to the master curve, first, it has been checked that after the shear pulse, the spherulite growth
rate is independent of both the presence of particles and the preceding flow, and hence solely determined
by the polymer characteristics. Thus it follows that when samples of the same (matrix) polymer have the
same nucleation density after cessation of shear flow, they will display the same crystallization kinetics.
Secondly, it was observed above that when samples of the same (matrix) polymer display similar
crystallization kinetics, a similar level of order will be found in them. In other words, a similar nucleation
density will also lead to a similar morphological level of order. As the crystalline level of order is
determined by both the amount of nuclei and the extent of their orientation, this suggests that the
concentration of oriented nuclei plays a much more important role than the exact extent of their
orientation as determined by shear rate and strain. A slightly smaller number of nuclei that are slightly
more oriented will hence lead to a similar level of orientation as a larger amount that are slightly less
oriented. It should be remarked that the nucleation density in iPP at the shear conditions investigated in
this work can be assumed to be very high indeed.
Regarding the appearance of an upturn in the flow birefringence, it was shown in Figure 6 that it occurs at
a similar combination of crystallization kinetics and morphology for the iPP and all its mixtures used in
this work. According to the reasoning above, this indicates a similar nucleation density on the occurrence
of the upturn. This is in agreement with the conclusions from a previous article to which the authors
cooperated36 concerning the onset of oriented crystallization, where it was shown that the birefringence
technique will only detect oriented precursor formation when the concentration of these nuclei reaches a
certain concentration. As this threshold concentration for detection is a constant, the appearance of an
upturn in the flow birefringence will indeed correspond to a similar characteristic crystallization time and
level of order for all mixtures of particles with the same matrix polymer. The shear rate at which the
upturn is observed in the experiments at constant strain is linked to the nucleating efficiency of the
particles: the nucleation efficiency characterizes the contribution of the particles to the total nucleation
density, and hence also how many additional shear-induced nuclei are necessary to reach the detection
concentration. Remark that this implies the assumption that the heterogeneous nuclei can form starting
points for oriented nuclei.
The experiments on which this framework is based are certainly limited. Only one matrix polymer was
used. The maximum possible shear rate range for this polymer was investigated, but only at one strain and
under very well-controlled flow conditions. The particles are no ideal model systems, but they are still
very simple: random (not elongated), crystalline, non-porous and not coated, with rather low nucleating
efficiencies. However, in view of the deliberately generalizing nature of the trends identified in the
behavior for these materials, which form the basis for this framework, it seems likely that they are also, to
a certain extent at least, valid and useful in a larger context- in less well-defined flow conditions and for
more complicated composites.
The framework can also be used as an instrument to detect (real) synergetic effects, as will be pointed out
in the next section. It is difficult to unambiguously establish synergetic effects in particle-containing
polymers due to the simultaneous presence of the nucleation effect of the particles. However, applying the
framework sketched above allows filtering out the nucleation effect of the particles: the presence of a
significant synergetic effect would cause deviations from the additive master curve and, in the case of
birefringence experiments, from the fixed combination of kinetics and morphology at the appearance of an
upturn in the flow birefringence.
4. Breaking the additivity rule: possibilities for ‘synergetic effects’
The focus of the previous chapter was on the additive competition between the influences of particle
nucleation (at low shear) and flow (at high shear) on polymer crystallization. However, a third
contribution to flow-induced crystallization of particle-containing polymers is possible, resulting from an
interplay between the flow and the particles. This is termed a “synergetic effect”.
When such synergetic effects are mentioned in this context in literature, they mostly concern stress
amplification of the flow by the particles28,48,49 or anchoring of polymer chains on the particle surface.29,5056
As mentioned earlier, the former term indicates the reinforcing of the flow effect on the polymer
kinetics and morphology by local stresses generated around the particles during the application of flow.
The second hypothesis explains this reinforcing effect as the result of decreased chain mobility due to the
anchoring of the chains onto the particles. However, an interplay between flow and particles with an effect
on polymer crystallization can go further than the two effects mentioned above. In the case of elongated
particles, flow can for instance cause orientation of the particles, influencing polymer crystallization
through a more powerful variant of stress amplification produced by the movement of the particles. To
investigate this, mixtures of iPP and hematite with different aspect ratios are used. As can be seen in
Figure 7, the dependence of the characteristic values of crystallization kinetics and level of order on shear
rate for these materials are qualitatively similar to those of the iPP / zinc oxide mixtures. As the hematites
have rather low nucleating efficiencies concentrated in a narrow range (see Figure ) the differences in
crystallization kinetics are not very large, even during quiescent crystallization and at low shear rates.
Nevertheless, at these low shear rate values, a significant decrease of the characteristic crystallization time
with increasing aspect ratio can be observed. At high shear rates, flow again dominates the crystallization
kinetics and aspect ratio will not play a role anymore.
These data can now be used to construct a kinetics-morphology master curve, which is shown in Figure 8.
The black circles and dots represent the pure iPP and the iPP containing the spherical hematite. They can
be seen to collapse nicely onto one curve. However, the gray and white triangles representing the mixtures
with elongated hematites seem to deviate somewhat from the black master curve: they are systematically
above it except at the highest end of the range, where all mixtures display the same crystallization
behavior, as is visible from Figure 8.
Figure 7: Dependence of characteristic time of crystallization (time to minimum of intensity, left) and characteristic value of level
of order (maximum of the birefringence curve, right) on shear rate at a strain of 100 units for crystallization at 143°C of pure iPP
(black circles) and mixtures of iPP and 2 wt% of hematite with aspect ratio 1 (black dots), 1.8 (gray upward triangles) and 3.6
(open downward triangles)
Figure 8: Dependence of the characteristic value of birefringence (maximum of the birefringence curve) on characteristic time of
crystallization (time to minimum of intensity) for crystallization at 143°C of pure iPP and all its mixtures with hematite from
experiments at different shear rates and at a strain of 100 units
The characteristic time at which the upturn is observed is still the same for all mixtures, which is logical in
view of the underlying mechanism: the upturn is detected at a fixed concentration, and particles nor flow
influence crystalline growth rate. However, the values of the characteristic level of order are slightly
higher for the mixtures with elongated hematites than for the pure iPP and the iPP containing spherical
hematite, denoting a synergetic reinforcing of the enhancement of orientation.
The observed differences, though systematic, are very small. This is quite logical, because of the shortness
of the shear pulses applied and the high viscosity of the polymer at the crystallization temperature (about
11 000 Pa.s). Though the time needed to obtain significant orientation of the particles in the flow direction
is larger than the shearing times used in the sandwich cell experiments, during the latter the particles
experience a short push. This slight movement of the elongated particles may be enough to cause the
observed, equally slight synergetic effect.
Another example of synergetic effects of the presence of particles and flow on crystallization is the flowinduced break-up of particle aggregates, which may influence the nucleation behavior of the particles. A
calcium-carbonate-containing iPP was used to investigate this. Some particles are very prone to
aggregation, which is the reason why they are often coated. In the case of calcium carbonate and iPP,
which cannot chemically interact on their own, a logical choice of surface treatment is using an
amphiphilic coupling agent such as calcium stearate, which reduces the particle-particle interaction and
the surface tension with respect to iPP.65 Nevertheless, the calcium carbonate particles used in the
experiments presented below, which have an average diameter of about 80 nm, form aggregates of about 1
µm. Indications that shear may be able to break up these aggregates were already reported in a previous
paper:20 microscopy experiments indicated an increased nucleation density in the calcium-carbonatecontaining iPP when an experiment is preceded by (other experiments involving) shear flow.
To distinguish the extra heterogeneous nucleation from aggregate breakup from flow-induced
homogeneous nucleation, a kinetics-morphology master curve can be constructed. However, as the change
in particle organization is permanent, a faster and more flexible way to fathom it can be employed. For
this purpose, sandwich experiments were performed consisting of a series of experiments at a constant
shear rate and varying strain and a series at varying shear rate and constant strain. The values of shear rate
and strain were chosen low enough to avoid stretching of the polymer molecules, which might lead to
annealing-resistant shish nuclei. The samples recuperated after these experiments were used in DSC
experiments: the samples were melted, annealed and crystallized again. Both non-isothermal and
isothermal crystallization experiments were performed and all samples were subjected to identical
experimental procedures. For the pure polymer, no significant differences were found when comparing the
crystallization behavior of samples that had originally (in the sandwich cell) undergone quiescent or flowinduced crystallization. However, for the calcium carbonate samples that were subjected to shear in the
sandwich cell, the crystallization kinetics after annealing are accelerated with respect to the crystallization
kinetics of the quiescently crystallized sample. This effect, as a function of shear rate at constant strain and
as a function of strain at constant shear rate, is illustrated in Figure 9. Expressed in terms of nucleating
efficiency: the sample that has undergone no shear has an NE of 9.8%, while the NE for the sheared
samples is 17.2% for (which corresponds to a difference of somewhat more than 2°C in the crystallization
peak temperatures). Hence, a shear-induced increase in heterogeneous nucleation can cause a significant
synergetic effect in the flow-induced crystallization of polymers containing particle aggregates.
Figure 9: DSC measurements of isothermal crystallization at 145°C of samples of iPP containing 3 wt% of calcium carbonate
recuperated from the sandwich cell after quiescent crystallization or crystallization at constant strain (left) or constant shear rate
(right) (shear conditions indicated in the graph)
Conclusions
In this paper, a systematical investigation of the combined effect of the presence of particles and flow on
the crystallization of polymers has been conducted. Hereto, the dependence of size, shape and particle
concentration has been examined. For these cases, the nucleation efficiencies have been determined using
the Lotz protocol. It has been shown that at low values of the shear rate, heterogeneous nucleation by the
particles accelerates the crystallization kinetics. However, at sufficiently large values of the shear rate,
flow will dominate the nucleation and hence the kinetics, independent of particle size, shape and
concentration. A generic framework, based on the additivity of the presence of particles and flow, is
presented to explain how both effects contribute to the crystallization process. Hereto, a kinetics -
morphology- master curve is proposed and validated. Based on this, as the crystalline level of order is
determined by both the amount of nuclei and the extent of their orientation, it is suggested that the
concentration of oriented nuclei plays a much more important role in the amount of crystalline order than
the exact extent of their orientation as determined by shear rate and strain.
Possible deviations of this
additivity rule and by extension, the master curve, have been put forward: both the shape and the state of
aggregation of the particles may cause synergetic effects between particles and flow.
Acknowledgements
The authors thank Borealis GmbH and the Umicore Group for providing the materials for this study and
Solvay for allowing the use of their facilities at Solvay R&I Centre Brussels.
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