On the Effect of Particle Size, Shape, Concentration, and Aggregation

Apr 25, 2013 - R&I Centre Brussels, Solvay Central Laboratories, Neder Over .... Daniel J. Beltran-Villegas , Ronald G. Larson , Ubaldo M. Córdova-Fi...
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On the Effect of Particle Size, Shape, Concentration, and Aggregation on the Flow-Induced Crystallization of Polymers Michelle D’Haese,† Florentin Langouche,‡ and Peter Van Puyvelde†,* †

Laboratory of Applied Rheology and Polymer Processing, Department of Chemical Engineering, Leuven Material Research Centre, Katholieke Universiteit Leuven, W. de Croylaan 46 B-3001 Leuven, Belgium ‡ R&I Centre Brussels, Solvay Central Laboratories, Neder Over Heembeek, Rue de Ransbeek 310 B-1120 Brussels, Belgium S Supporting Information *

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.

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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 co-workers21−25 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

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 crystallization. Both the addition of particles and the application of flow can influence the crystallization process in semicrystalline 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 the seminal work of Binsbergen.4 Another important step in explaining the nucleating ability of particles toward polymers, was taken by Wittmann and Lotz5−8 with the introduction of the observation of epitaxial nucleation. According to this approach, 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 Kumuraswamy for an overview9). Nowadays, it is widely accept © 2013 American Chemical Society

Received: February 14, 2013 Revised: March 24, 2013 Published: April 25, 2013 3425

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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 synthesized by forced hydrolysis of iron(III) perchlorate in the presence of urea.46−48 The dimensions of the obtained hematites are given in Table 1. The aspect ratio of the hematite particles is relatively limited

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, e.g., refs 30−33. 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.34,35 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.36−38 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 particles28,33,39 or anchoring of polymer chains on the particle surface.29,32,40−45 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. On the basis of 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.

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Table 1. Dimensions (with Absolute and Relative (%) Variation) of the Different Hematites av. aspect ratio (c/a) particle length (c) [nm] particle width (a = b) [nm]

1 286 ± 77 (27%) 286 ± 77 (27%)

1.8 148 ± 13 (9%) 81 ± 4 (5%)

3.6 268 ± 27 (10%) 74 ± 5 (6%)

due to the synthesis procedure. Here, iron(III) perchlorate is mixed with urea, sodium dihydrogen phosphate and deionized water. This mixture is kept in a convection oven during 24 h in order to age. Subsequently, the mixture is allowed to cool to room temperature after which it is centrifuged for 30 min at a certain speed. The speed is higher as the aspect ratio is larger, since the larger the aspect ratio, the more difficult it becomes to sediment a sufficient amount of particles. However, even at speeds of 10000 rpm it proved impossible to collect a workable amount of 6.2 aspect ratio particles and hence, we will limit the discussion of the results to particles with an aspect ratio up to 3.6. Finally, a commercially available 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 1 μm. 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). No extra stabilizers were added during mixing. 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 by solution blending. The hematite particles were dried after synthesis and then ultrasonically dispersed in xylene. Subsequently, the iPP was solvent-mixed with the hematite-containing xylene at 135 °C, using magnetic stirring and ultrasonification. Finally, the solvent is removed by evaporation. Since the final amount of hematite is dried before solution mixing, its exact weigth can be ascertained after drying, and the necessary amount of iPP to be added can later be determined accurately. 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. 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:

MATERIALS AND METHODS

An isotactic polypropylene (iPP) (grade 76207, Mw = 517 kg/mol, polydispersity index PDI = 8, Tm = 165 °C) 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, Mw = 671 kg/mol, PDI = 9.32, Tm = 164 °C), 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 and 500 nm particles are relatively spherical and the 1 μm particles

I = IDC + Isin sin(4ωt ) + Icos cos(4ωt ) 3426

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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 Δn′ Δn′ =

λδ′ 2πd

tration 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

(3.5)

with λ the laser light wavelength (670 nm), d the sample thickness, and δ′ the optical retardation calculated as

|sin δ′| =

Isin 2 + Icos 2 IDC

(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).49−53 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 (102−104 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.49

Figure 1. Example of the effect of the partial melting temperature on the crystallization peak temperature as prescribed by the NE-protocol.

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. 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. On the basis of these experiments, the nucleating efficiency is defined as:55

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RESULTS AND DISCUSSION 1. Effect of Particles on Crystallization: The 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 selfnucleation of the polymer. This is the essential idea behind the nucleation efficiency (NE) scale for nucleating agents,54,55 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 nonisothermal 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/min 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 nonisothermally cooled, these fragments act as a (perfect and perfectly dispersed) nucleating agent, increasing the peak temperature Tc,nuc in exothermal crystallization. This is called self-nucleation of the polymer. As the temperature for partial melting is lowered, the concen-

NE = 100

Tc,NA − Tc,pure ΔTNA = 100 Tc,nuc MAX − Tc,pure ΔT

(1)

in which ΔT is the difference between the nonisothermal crystallization peak temperatures of the pure (minimally nucleated) and the maximally (self-) nucleated polymer. ΔTNA is the difference between the nonisothermal 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.55 In previous work,30 this methodology has been used to characterize the nucleating efficiency of mixtures of iPP and 1 wt % 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 NE-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 3427

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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 might 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. However, this reasoning is rather hypothetical since no direct proof of preferential crystallization on the lateral or end parts of the particles is available. Nevertheless, the results in Figure 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 parameter. Hence, using the NE-methodology a clear and consistent relationship can be established regarding nucleation efficiency. In addition, this relativelysimple methodology provides a richness of important information since it clearly delineates 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 extensively, both with the sandwich cell used in this work30,49−53 and with other shear geometries.59−64 Birefringence experiments with the sandwich cell can be used to obtain both information about the kinetics and morphology (amount of crystalline order as reflected in the level of birefringence) (see also previous studies30,49,50). 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 or 0.4 s 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 before, e.g. ref 50, 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. The variation of the birefringence is due the existence of fewer primary nuclei at low shear rate that are oriented with their chain axis in the shear direction. However, as was stated

Figure 2. Dependence of the nucleating efficiency on particle concentration for mixtures of iPP with the 500 nm zinc oxide (with trend line to guide the eye) (top) and on particle aspect ratio for mixtures of iPP with 2 wt % of hematite (bottom).

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 %),20,56,57 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.56 and in the work on particle aggregation of Pukansky et al.,57,58 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. 3428

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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.

Figure 3. Birefringence Δn′ 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).

is enhanced. At intermediate shear rates (200−350 s−1), a plateau seems to be present after which a final acceleration of the crystallization process by shear occurs. It was shown by Baert et al.50 and by D’Haese et al.30 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.53 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. 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 and 1 wt % of zinc oxide particles. The latter is consistent with the nucleating efficiencies determined using the NE scale (see Figure 2) that also pointed toward 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 NE 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,

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by for instance Bassett, also growth phenomena might interfere. Indeed, when a low density of primary lamellae is present, a higher probability of lamellar branching is present, which will reduce the overall birefringence. On the contrary, a high density of nuclei on any shish formed, or a high concentration of shishes will impose parallel growth of lamellae that cannot branch and hence a highly birefringent respons is expected. The latter observations are well in line with longstanding observation by for instance Keller and co-workers.66,67 Note 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,68 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. In addition, the possible presence of βspherulites would also contribute to a large extent to the total birefringence and possibly interfere with the observations. Wide angle X-ray diffraction (WAXD) on samples recuperated from the sandwich cell, as a function of shear rate, has been performed in order to observe the possible development of βphase (see Supporting Information). From these experiments, no characteristic β-phase peak could be discerned for our experimental window. In addition, polarized optical microscopy (POM) images were taken. If β-spherulites would be present, highly birefringent spots should be visible. However, no such spots were observed pointing toward the absence of a significant amount of β-phase. 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 particlecontaining 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 (0−200 s−1), the kinetics 3429

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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,

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.

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 (nonelongated) 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). 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 flow-induced 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 degree of orientation 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

Figure 5. Dependence of the characteristic time of crystallization (top) and value of degree of orientation (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 1 wt % of zinc oxide with diameter 35 nm (dots), 200 nm (gray squares), and 1 μm (downward triangles).

the dependence of the degree of orientation (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. 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). Thus, a correlation between the crystallization time and birefringence in flow-induced crystallization of iPP and its blends are expected. To investigate this, the characteristic value of the degree of orientation 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 3430

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Figure 7. Dependence of characteristic time of crystallization (time to minimum of intensity, left) and characteristic value of degree of orientation (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).

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. Second, it was observed above that when samples of the same (matrix) polymer display similar crystallization kinetics, a similar degree of orientation will be found in them. In other words, a similar nucleation density will also lead to a similar orientation. As the crystalline degree of orientation 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 is 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 cooperated53 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 degree of orientation 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 wellcontrolled flow conditions. The particles are no ideal model systems, but they are still very simple: random (not elongated), crystalline, nonporous 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. This observation is not necessarily general since also a third contribution to flow-induced crystallization of particle-containing polymers can come into play, resulting from an interplay between the flow and the particles. This is termed a “synergetic effect”. We will briefly demonstrate two examples of possible synergetic interferences. When such synergetic effects are mentioned in this context in literature, they mostly concern stress amplification of the flow by the particles28,33,39 or anchoring of polymer chains on the particle surface.29,42,40−45 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 3431

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denoting a possible 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), which is insufficient to fully orient the particles in the flow field. However, even a short shear pulse leads already to a small but significant deviation from a mere additivity rule and hence can be seen as a synergetic effect. The precise mechanism responsible for this deviation can however not be inferred from these experiments. Another example of synergetic effects of the presence of particles and flow on crystallization is the flow-induced breakup of particle aggregates, which will 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.57 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−carbonate-containing 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 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 nonisothermal 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 flow-induced 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.

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 degree of orientation 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 7), 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. The latter observation is identical to the observations made in the previous section. These data can now be used to construct a kineticsmorphology master curve as before, which is shown in Figure 8.

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 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 birefringence end of the range, where all mixtures display the same crystallization behavior, as is visible from Figure 8. 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 degree of orientation are slightly higher for the mixtures with elongated hematites than for the pure iPP and the iPP containing spherical hematite, 3432

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anisotropy and the state of aggregation of the particles may cause synergetic effects between particles and flow.

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ASSOCIATED CONTENT

S Supporting Information *

Characteristic values of kinetics as a function of shear rate and X-ray diffraction intensity as a function of scattering angle. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(P.V.P) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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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REFERENCES

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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).

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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. The crystallization rate dependences on particle size, shape, and particle concentration have been established. For these cases, the nucleation efficiencies have been determined using the NE 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 was found to be 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. On the basis of this, as the degree of orientation 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. However, the additivity rule of flow and the presence of particles is not a general feature. Possible deviations of this additivity rule and by extension, the master curve, have been put forward: both the 3433

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