Morphological Interpretation of the Evolution of the Thermal Properties

Article pubs.acs.org/Macromolecules

Morphological Interpretation of the Evolution of the Thermal Properties of Polyethylene during the Fragmentation of Silica Supported Metallocene Catalysts Estevan Tioni,†,‡ Vincent Monteil,*,† and Timothy McKenna*,† †

UMR 5265 Laboratoire de Chimie Catalyse Polymères et Procédés (C2P2), LCPP team, Université de Lyon, Univ. Lyon 1, CPE Lyon, CNRS, Bat 308F, 43 Bd du 11 novembre 1918, F-69616 Villeurbanne, France ‡ Dutch Polymer Institute (DPI), PO Box 902, 5600 AX Eindhoven, The Netherlands ABSTRACT: Gas phase ethylene polymerizations on supported metallocene catalysts have been carried out in a stopped flow minireactor for times as short as 0.3 s. By recovering the polymer particles without altering their morphology it was possible to follow the evolution of the thermal properties of the polymer during the fragmentation of the particles. The melting temperature increased with reaction time (from 118 °C at 0.3 s to 131 °C at 180 s), and three distinct crystallization peaks can be observed during the studied time range, centered on 75 °C for very short reactions, 105 °C for longer reactions, and 115 °C (the usual crystallization temperature of HDPE) which becomes predominant only after 30 s of reaction. It is believed that the reason for this behavior is related to the confinement of the nascent polymer in the nanopores of the catalyst support. This confinement perturbs the chain crystallization by limiting the crystal growth. It is shown that measuring the thermal properties of the reactor powder without removing the support fragments allows one to identify in which pores the nascent polymer forms and to follow the progress of particle fragmentation.

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INTRODUCTION The polymerization of ethylene and of propylene on supported catalysts will account for over 70 million tons of polymer in 2011. Given the economic importance of these products, it is essential to have a description of what occurs during the process that is as precise as possible and that links the reaction conditions to the final polymer properties. As is the case in all heterogeneously catalyzed reactions, the act of supporting the active sites of the catalyst on a solid structure will create heat and mass transfer resistances, the importance of which will be linked in many ways to the shape or morphology of the particles.1 For instance, in terms of mass transfer, the more monomer diffusion needs to occur in the polymer layers surrounding the active sites, the greater the possibility is that the reaction will be mass transfer limited.2 It would therefore be of interest to better understand and eventually to be able to model the evolution of particle morphology during the reaction.1 Let us consider what happens to a catalyst particle during polymerization: Any molecules that eventually reach the active sites must be transported into and through the physical structure (pore space/polymer phase) of the particle. For a very short period of time (on the order of 10−1−102 s, depending on the nature of the support), the particle is a continuous inorganic phase with polymer chains growing on active sites located on the surface of its pores. As polymer accumulates at the active sites, the inorganic phase suffers a buildup of stress at different points and very quickly fragments into a series of © XXXX American Chemical Society

unconnected mineral substructures held together by a polymer phase. Throughout the course of this fragmentation step, the particle is transformed into a continuous organic polymer phase, throughout which the active catalytic sites are dispersed on fragments of the original support. Thus, the original twophase structure (mineral and pore space) is converted into a three-phase structure (a mineral phase, usually embedded in a polymer phase, interspersed by the pore space). Ideally, one supported catalyst particle will yield one polymer particle. Fragmentation of the support is necessary in order to provide enough porosity that monomer can continue to diffuse to the active sites inside the particle and to allow the particle to expand. (Under ideal reaction conditions, one catalyst particle of 10−80 μm will grow into a polymer particle as large as 1 or 2 mm in diameter.) Logically, the mechanical properties of the support material will influence the fragmentation step. Studies on the modeling of fragmentation and the evolution of particle morphology3−9 clearly show that the mechanical properties of the polymer play a determining role in how the original particle breaks up and on how the growing particle expands. This is because the mechanical properties influence the way the tensions produced by the polymer accumulation into the pores are stored and released.10 The rate of reaction will also have an impact on the Received: October 14, 2012 Revised: December 14, 2012

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crystal growth. This means that when polymer chains crystallize in a space with characteristic dimensions close to that of the crystallites, crystallinity and crystallization temperature will drop with respect to the unconfined case. For instance, Woo et al.20 measured a crystallinity of 70% for the PE they used in bulk samples and of 30% for the same polymer in 15 nm pores. By performing nonisothermal crystallization studies, the same authors were also able to see different nucleation mechanisms for different pore sizes.21 It is known that PE in bulk crystallizes around 115 °C with a heterogeneous mechanism. Crystallization peaks of polymers confined in pores having diameters from 62 to 110 nm appeared around 80 °C and can be assigned to homogeneous crystallization mechanism which requires high supercooling because of the large critical nucleus size needed for crystal stability. For smaller pores (15−48 nm) a broad crystallization peak spanning from 80 to 110 °C was measured together with a small dependence of the crystallization rate on the temperature. This is a clear sign that heterogeneous nucleation is the dominant mechanism in very small pores. This is evident if we think about the high surface-to-volume ratio of small pores, which increases the chances for the polymer chain to form a nucleus on the pore wall. When studying the early stages of olefin polymerization, it has to be kept in mind that crystallization and generation of polymer chains are simultaneous events. Properties like local temperature and active site concentration can influence the relative ratio between crystallization and polymerization. For instance, Loos, Thüne, and co-workers22−27 studied nascent polymerization on model catalysts and came to the conclusion that the organization of the chains in a specific crystalline morphology (disordered folded chain crystals with entanglements, untangled folded chain lamellae, extended chain crystals) during polymerization is dependent on local kinetic parameters like temperature, active site concentration, or polymerization rate. It is known that during the reaction start-up both temperature and polymerization rate rapidly change.28,29 It is therefore clear that the relationship between the catalyst/polymer structure, process conditions, and final product properties needs to be addressed. From an industrial point of view, the interest starts from the need to control the growth of the catalyst/polymer particle during the polymerization process. So while model catalyst systems such as those cited above provide valuable insight into the processes involved in the evolution of particle morphology, they are less satisfying in terms on being able to understand how the morphology of the polymer particles evolve in more realistic situations. For this, special experimental equipment is required. Previous studies from our group suggest that stopped flow reactors are suitable for this type of study and can provide information on kinetics, morphology, and polymer properties at very short times.28−35 For instance, at reaction times of less than 1 s, Di Martino et al.28,29 found that high-density polyethylene had lower melting temperatures and crystallinity than expected (Tm around 118 °C, crystallinity of 20%). In addition, they found bimodal peaks in the melting curves of DSC thermograms for the same samples. However, after 1 s the values of the melting points and crystalline content of the polymer approached those one expects for HDPE powders. No satisfactory explanation for these results was offered. A detailed study of the evolution of the thermal properties (melting and crystallization temperature and crystallinity) of

fragmentation stage and the resulting particle morphology for similar reasons.5−7 The crystallinity of the polymer will influence the rate indirectly since the diffusion coefficient of the monomer through the polymer layer covering the active sites is strongly dependent on this property.11 In general, until the fragmentation can be considered completed, the growing particle is an assembly of empty pores, partially or completely filled pores, and polymer domains (corresponding to destroyed pores) coexisting with pure support fragments. The environment in which the polymer chains crystallize during the early reaction stages can be very heterogeneous, and as a consequence the measurable crystallization behavior of the polyethylene chains can be highly perturbed. In particular, polymer chains located in pores that have not yet fragmented can experience a space constriction that perturbs their organization into crystalline structures with respect to chains that are free to move without constraints. This peculiar crystallization behavior is known in the literature, and is not only typical of polymer in pores, but can be generalized to most types of macromolecules that evolve in any type of confined space. For example, melting temperature depression is known for thin films on substrates12,13 while homogeneous nucleation and decrease of the crystallization temperature of tens of degrees is known for crystalline mesophases in block copolymers14 and for polymer in droplets.15−18 It is possible to encounter this situation during the early stages of olefin polymerization as the pore diameter in commercial silica supports is typically distributed over an interval which spans from few nanometers to few tens of nanometers, and the thickness of HDPE crystallites is typically around 10−20 nm.19 Useful information for the study of this problem comes from the works of Woo et al.20,21 Rather than producing the polymer in situ, they deposited linear polyethylene with narrow MWD into the pores of different alumina materials with well-defined diameters. First of all, these authors found that the melting temperature of the polymers measured during the second DSC heating step decreases with decreasing pore size. Values of 122 °C were found for pores of 15 nm while the same polymer in bulk melted at 133 °C. The reason for this depression is the confinement created by the pore which has the consequence of reducing the crystal thickness of the polymer once cooled. The crystallite thickness can be calculated from the measured melting temperature using the Gibbs−Thompson equation: ⎡ ⎛σ σ ⎞⎤ 2 σ Tm = Tm0⎢1 − ⎜ 1 + 2 + 3 ⎟⎥ ⎢⎣ L2 L3 ⎠⎥⎦ ρc ΔHm0 ⎝ L1

(1)

where σ1, σ2, and σ3 denote the specific surface free energy of a crystallite and L1, L2, and L3 are the dimensions of the crystallite. The subscripts represent the three orthogonal directions in a chain-folded lamella. T0m is the equilibrium melting temperature of the crystal with infinite thickness (146 °C), ρc is the crystal density (1 g/cm3), and ΔH0m is the heat of fusion per unit mass (288 J/g). In general, the lateral dimensions of a crystal in bulk polymer are much higher than the thickness, so that only the terms with the subscript 1 remain in eq 1. σ1 has the value of 94 mJ/m2. This equation basically predicts that smaller crystals melt at lower temperature. The same authors performed isothermal crystallization analyses on their polymers and showed that when the polymer is confined in pores, crystal nucleation is favored with respect to B

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by a calibration based on polyethylenes of different weight-average molecular weights, and only the RI signal was used for calculations in order to erase any possible artifacts due to experimental errors. The original samples were in fact support/polymer particles that had been separated from the NaCl seedbed by washing at ambient temperature with demineralized water. The exact polymer quantity (varying between 0.5 and 10 mg depending on yield) was calculated using the total sample weight and reaction yield value. The particles were then dissolved in trichlorobenzene at 150 °C for 3 h and filtered before injection into the chromatography columns in order to remove the inorganic support particles. Heterogeneities in the original samples or incomplete removal of the inorganic support could lead to errors in the molecular weight calculations if the viscometer signal was taken as reference. The crystallinity and melting temperatures of the polymer samples were measured by DSC (Mettler DSC 1). Unless otherwise noted, samples were placed in the holder without separation of the polymer phase from the inorganic support. Our intention is in fact to measure the properties of the polymers as they are during the early reaction stages. The inert seedbed was removed beforehand by washing with demineralized water at ambient temperature (this step does not alter the morphology of the particles). The exact polymer quantity was calculated using the total sample weight and yield value. 40 μL holders were used except for reactions with yields lower than 0.2 g/g when 100 μL holders were preferred. Two heating steps were performed from 50 to 150 °C at heating rate of 5 K/min separated by a cooling from 150 to 50 °C at a rate of 20 K/min. Crystallinity of the samples was calculated using a value of 288 J/g for a full crystalline polyethylene. The data reported below are those obtained during the second heating step for the melting and the first cooling step for the crystallization. This will not pose a problem for the interpretation of the results as this paper focuses on the use of the evolution of the crystallization temperature(s) to interpret the progression of the fragmentation of the particle.

ethylene homopolymers and ethylene−butene copolymers produced during the early stages (less than 30 s) of gas phase polymerizations will be presented here. In particular, the effect of the confinement of the growing polymer chains in the fragmenting pores will be demonstrated by following the evolution of the melting and crystallization peaks and of the particle morphology.

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EXPERIMENTAL SECTION

Materials. Ethylene (purity 99.95%), butene (purity >99%), helium (purity >99.999%), and carbon dioxide (purity >99.995%) were purchased from Air Liquide (Lyon, France). Ethylene was passed through three different purification columns before use: a first one filled with reduced BASF R3-16 catalyst (CuO on alumina), a second one filled with molecular sieves (13X, 3A, Sigma-Aldrich), and a last one filled with Selexsorb COS (Alcoa). The other products were used without further purification. Two types of metallocene catalysts supported on silica (Grace 948) were used for these experiments. The zirconocene complexes are EtInd2ZrCl2 and (nBuCp)2ZrCl2. They were used as received (from Sigma-Aldrich) and supported over silica treated with methylaluminoxane (MAO). A 10 wt % MAO solution in toluene purchased from Sigma-Aldrich was used for this study. The impregnation method has been reported earlier.32 The support was Grace 948 silica (from Grace Davidson). It was treated at 200 °C for 4 h under vacuum (10−5 mbar) before use. Before reaching the 200 °C plateau the solid was heated under vacuum at 130 °C for 30 min to remove the water adsorbed on the surface. Nitrogen porosimetry studies were performed to determine the pore volume and pore diameter as well as the specific surface area of the support used. The physical properties of the Grace 948 support are as follows: average particle diameter is 58 μm, the pore volume is 1.6 mL/g, the mean pore diameter is 24 nm, and the specific surface is 270 m2/g. Reactor Technology and Polymerization Procedure. The reactor consists of a packed bed with a diameter of 20 mm and a depth of 10 mm. The chamber is filled with a mixture of seedbed and catalyst in a glovebox to avoid contamination of the contents. The catalyst particles must be highly diluted with inert solids in this type of reactor to ensure good control of the reaction temperature by reducing the quantity of heat produced per unit volume of bed. The inert seedbed used in this work is fine NaCl with an average particle size from 5 to 10 μm. Since the particles are slightly agglomerated, the size of a single object is around 30 μm.33 Thermocouples at the inlet and outlet of the reactor allow us to record the temperature rise of the gas phase as it flows through the bed. Pressure, temperature, and flow rate of the feed are controlled and measured. The duration of the reactions (minimum 0.1 s) is controlled by the opening of solenoid valves, programmed with a logical controller. The minimum time between subsequent actions of the solenoid is 0.1 s. The setup and the polymerization procedure were described in detail in a previous publication, and the interested reader is referred to ref 33 for more information. All reactions were carried out at conditions offering the best possible heat transfer characteristics defined in a previous paper:33 (i) gas velocity of 15−20 cm/s, (ii) 9 bar of total pressure with 33 mol % of helium in the feed, (iii) use of fine NaCl (10−30 μm of single object size) as inert catalyst diluent, and (iv) use of catalyst mass between 30 and 80 mg according to reaction time and activity to optimize the ratio between low heat and enough polymer production. The evolution of activity, temperature, and molecular weights of the polymers discussed in this paper were presented in a companion paper and so will not be presented here.35 Polymer Characterization. The molecular weight distributions were characterized by SEC (Waters, Alliance GPCV 2000). The system was equipped with two detectors (a refractometer and a viscometer) and with three columns (PL gel Olexis 7*300 mm from Varian). Analyses were performed in trichlorobenzene (TCB) at a flow rate of 1 mL/min. The molecular weight distributions were calculated

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RESULTS AND DISCUSSION Several series of experiments were run using both catalyst precursors (EtInd2ZrCl2 and (nBuCp)2ZrCl2), and an analysis of the kinetic, molecular weight distribution, and copolymerization behavior was presented in a companion paper.33,35 These results will not be shown here for the sake of brevity; our focus will be on the thermal properties of the polymer powders during the very early stages of polymerization. As we will see below, the evolution of the thermal properties of all of these powders is similar and is essentially only a function of the reaction time (for the reaction conditions explored here). Representative activity profiles and outlet gas temperatures observed for the polymerization of ethylene (6 bar) in the presence of 3 bar of helium at 80 °C are shown in Figure 1, and the results of the thermal analyses of the resulting reactor powders are shown in Figure 2. It can be seen from Figure 1 that the activities are very high during the initial phase of the reaction and decline relatively rapidly after 10−15 s. Details of this reaction have been discussed in another paper,35 where it has been shown that the rate of reaction observed in the stopped flow reactor after about 15 s is very close to that observed in a semibatch reactor for much longer times. We can also see the outlet gas temperature increases by ∼8 K and then drops as the activity drops to reach values close to the inlet gas temperature. It can be seen from Figure 2 that melting temperature at times below 30 s is much lower than one would expect for HDPE for both catalyst systems and that the evolution of both sets of data is also very similar. When the same catalysts are used at the same temperature (80 °C) and ethylene pressure (6 bar) for longer reaction times in a 2 L gas phase stirred bed C

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ethylene made with one catalyst will likely be valid for the other. These low values of crystallinity and melting temperatures cannot be attributed to unexpected branching of the polymer chains since NMR spectra of three polymers produced with a supported Et(Ind)2ZrCl2 precursor in reactions lasting 0.5, 30, and 3600 s all show ∼2 branches per 1000 carbons for all three samples. We can also exclude the possibility the low melting temperature is due to the molecular weight of the polymer since the number-average molecular weight is constant at approximately 30000 (±8000) right from the beginning of the reaction and does not change even after several tens of minutes of reaction.35 The explanation for the trends in Figures 1 and 2 must therefore lie elsewhere. Figure 3 shows that there is a

Figure 1. Activity profiles (a) and outlet gas temperature (b) for the polymerization of ethylene at 6 bar of ethylene, 3 bar of helium, and an inlet gas temperature of 80 °C. Figure 3. Evolution of the melting temperature as a function of yield for HDPE samples made using a Et(Ind)2ZrCl2 catalyst supported on silica. Maximum reaction time is 75 s.

discernible trend in terms of the evolution of the thermal properties of the polymer samples as a function of yield and that as one accumulates more and more polymer, the melting temperature (shown here) tends to the value that one expects from a bulk powder. This in turn suggests that the evolution of the thermal properties might somehow be linked with changes in the particle structure. The thermal behavior measured in our experiments might possibly be due to the presence of a high quantity of support into the polymer particle. The maximum yield of the experiments presented in the previous paragraph is approximately 1.5 g PE/g catalyst. This means that in the samples analyzed by DSC there is ∼40 wt % of silica. A confirmation of the effect of the presence of the support on the polymer crystallization comes from the spectra shown in Figure 4, where the DSC spectrum of an analysis of the crystallization and melting temperatures of a “reactor powder” (polymer on silica support, taken directly from the reactor with no treatment) is compared to measurements made on two samples from the same original batch but that consist of polymer that has been extracted and separated from the support before the analysis. All three samples come from the same reaction performed using (nBuCp)2ZrCl2 supported on MAO/silica stopped after 5 s at a yield of 0.5 g/g. The extractions were done with boiling xylene (140 °C) for 4 h in a Soxhlet. The polymer in the extraction 1 was recovered after xylene evaporation, while in the case of extraction 2 it was precipitated by methanol addition and filtered. The three upper curves are the crystallization step, while the other three curves represent the second melting of the polymer. The “reactor powder” sample shows a melting temperature of 121 °C which is coherent with what showed previously. The two other samples (without support) show a

Figure 2. Evolution of melting point (a) and crystallinity (b) of polymers produced in experiments shown in Figure 1.

reactor, the resulting HDPE powder melting temperature is ∼131 °C. Crystallinity varies from 30 to 50% at the beginning of the reaction and increases to ∼60% after 30 s. This last value is very close to what is measured on a polymer produced after long reactions. The results of these two figures suggest that both catalyst systems behave in a similar manner and that the polymers made on both catalysts are similar, and it is therefore reasonable that conclusions drawn using analyses on polyD

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Figure 4. DSC thermograms of the cooling and the second heating of a sample of untreated reactor powder and two samples where the support has been separated from the polymer.

Figure 5. DSC thermograms of polymers produced at different reaction times using supported (nBuCp)2ZrCl2 under reference conditions.

melting temperature of 126−127 °C. While these latter results are still somewhat lower than the value found for bulk PE powder, they clearly show that unusual melting temperatures and crystallization values are (at least partially) due to the presence of the support. The crystallization temperature peaks show a similar tendency, with the difference between the extracted and nonextracted values being even more pronounced for this property. In light of this observation, as well as the results of Woo et al.20,21 reported above, it might be more convenient to look at the evolution of the crystallization temperatures in order to determine the cause of the melting point and crystallization point depressions.

In the case of a bulka homopolymer that melts at 131 °C (i.e., a polymer made in a standard laboratory experiment that lasts for several minutes), the Gibbs−Thompson equation (eq 1) predicts a crystal thickness of 18 nm. This value is very close to the average pore size of our support, which is 24 nm. In addition, one of the two lateral dimensions will be greatly reduced if the polymer crystal is formed into a pore. It is quite possible that the polymer chains produced inside the supported particle are highly perturbed by space confinement and that melting temperature depression at low reaction time (low yield) can be attributed to this. This interpretation is also coherent with the data of Woo et al.20,21 E

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During this time range (that is when the polymer is confined in pores), the crystal growth is limited and the melting temperature is depressed. The depression is however less and less important because of the gradual pore fragmentation that decreases the average space confinement of the chains present in our support. The presence of chains that are confined in different environments together with chains that are not confined is visible in the quite broad shape of the melting temperature peaks. As the reaction continues the first pores are gradually fragmented (5 s). There is then a certain amount of polymer chains that goes from a confined environment to a bulk environment. In this case the crystals can nucleate and grow freely to minimize the surface energy, giving a classical crystallization peak at 115 °C. This peak starts to be visible at yield around 0.5 g/g and becomes more and more predominant as the reaction continues because more and more pores are fragmenting. However, due to the decrease in the reaction rate for this time range (>5 s), the increase of the third peak is slow, and at 30 s we can still observe a coexistence of the second and the third crystallization peaks. It is only from yields of 1 g/g that this peak becomes bigger than the others. For the same reasons the melting temperature grows to values closer to the classical values. The reactions lasting 180 s show melting temperatures very close to the bulk values (Figure 5), and only the third crystallization peak is visible. The amount of chains in bulk environment is now predominant (a big part of the pores is already broken), and the situation is closer and closer to what is found after longer reactions. Thermograms showing the evolution of the crystallization and melting curves (second pass) for polymerizations carried out using an (nBuCp)2ZrCl2 catalyst supported on silica particles of different sizes are given in Figure 7. In order to explore the impact of particle size on the evolution of the thermal properties, catalysts were prepared by sieving the original batch of silica and using the same supporting technique on each cut. The sizes of the cuts considered are given in Table 1 along with the specific surface area, pore size, and pore

If this is correct, then the fact that the polymer initially forms inside the pores of unfragmented particle could explain the unexpected thermal properties found at short time. In this case, the DSC thermograms can be used to interpret changes in particle morphology at low yields. DSC thermograms corresponding to homopolymers produced at different reaction times using (nBuCp)2ZrCl2 at reference conditions are shown in Figure 5. Ethylene homopolymers produced using the other metallocene catalysts show very similar results. In the lower part of the figure, the second heating steps are represented. It can clearly be seen that the melting temperature increases with reaction time. The curves corresponding to the cooling step are shown in the upper part of the figure. In general, three crystallization peaks are visible: one appearing for very low reaction times at 75 °C, one appearing for reaction times higher than 2 s spanning the range 90−110 °C, and one appearing for reactions longer than 5 s at 115 °C. The first crystallization peak disappears quickly and is no longer visible for reactions longer than 2 s. The second one is predominant in the time range between 2 and 15 s and becomes less and less visible as the reaction time increases. The third peak is visible from 5 s and becomes more and more important (polyethylene formed after 180 s exhibits only one crystallization peak at 115−120 °C). This peak is the one (and the only one) that can be seen for bulk polymers produced (cf. Figure 4). The temperatures at which the three peaks appear are very close to the values reported by Woo et al.21 in their nonisothermal crystallization studies. If the different peaks represent the crystallization of PE chains in different space confinements, we can then explain this evolution by recalling the process of fragmentation. In the very first instants of the reaction the pores that are more accessible are gradually filled with polymer. These kinds of pores should be on the particle external volume or can be big pores located anywhere in the particle. The polymer confined in such big pores, as shown in the previous paragraph, crystallizes according to a homogeneous nucleation mechanism that requires high supercooling. It is also possible that the tendency for the chains to crystallize following a homogeneous mechanism in this time range is due to the absence of cracks, support fragments, or, more in general, nucleating agents that favor heterogeneous crystallization. Homogeneous crystallization would certainly explain why, at very short reaction times, we measure a crystallization peak at 75 °C. This is visible for yields up to 0.2 g/g. As the reaction goes on also the smaller pores start to be filled. In these pores heterogeneous nucleation is dominant (or in this yield range nucleating agents become more and more numerous due to the beginning of the fragmentation process). As the chains are confined in a small space, the crystal cannot grow freely, and this reduces the crystallization temperature. This could explain the presence of the crystallization peak at 105 °C. Since these pores (smaller than 40 nm) contribute to the majority of the pore volume of our supports, the second crystallization peak becomes rapidly dominant and the first one becomes rapidly negligible. This peak becomes visible at yields between 0.1 and 0.2 g/g and is predominant up to yield values of 1 g/g. The quantity of polymer that can be accommodated in this type of pore is in fact greater than the one that can be present in the larger pores. In addition, it is possible that the disappearance of the first crystallization peak is also partly due to the fragmentation of the pores first filled.

Table 1. Structural Characteristics of the Different Silica Fractions35 particle size (μm)

specific area (m2/g)

pore size (nm)

pore vol (mL/g)

36−45 45−63 >80

267 270 269

24.4 24.6 24.6

1.63 1.66 1.65

volume. It can be seen that the physical characteristics of the silica cuts are all very similar. These catalysts were then used in the specially designed stopped flow reactor as described in Tioni et al.35 and the yields obtained from each of the cuts are shown in Figure 6. This last figures shows that all of the catalysts have very similar yields as a function of time, and therefore it is reasonable to say that they are chemically similar. Selected thermograms obtained for the polymer samples shown in Figure 6 are shown in Figure 7. As mentioned above, it appears that more can be learned from the crystallization curves, so we will focus on these curves first. It can be seen that at very short times (0.7 s) the curves for the three cuts are similar, with a small peak corresponding to the second crystallization temperature can be seen for the largest particle cut. After 2 s, this peak is much larger on the smallest cut, with the low-temperature peak being less evident. In addition, one F

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pronounced than in the other two samples. Comparing the curves at 15 and 30 s, it can be seen that more and more bulk polymer is being formed, but that this is happening more slowly in the largest cut. If our interpretation presented above is correct, the explanation for this is that the smaller particles fragment first. At very short times, the polymer that forms will do so in the outer layers of the supported catalyst particles. As time progresses, most accessible layers of the support have fragmented, and polymer begins to form closer and closer to the center. If we compare a particle of 40 μm to one of 80 μm, the latter will have a volume 8 times greater than the former. This means that much more polymer would need to be produced in the 80 μm particle than in the 40 μm one. At the time scale of these experiments, it appears that polymer is formed in the pores of the larger particles for a longer period of time. (Relatively speaking, there appears to be less bulk polymer in the large particles than in the small ones.) We also studied the evolution of the thermal properties of two copolymers made with each of the catalyst precursors mentioned above.35 The evolution of the melting temperatures as a function of time for copolymers made in the fixed bed reactor at short times is shown in Figure 8. At longer times, the melting point of the copolymer made on the Et(Ind)2ZrCl2 precursor is lower than for the other catalyst (114 °C vs 121

Figure 6. Yield obtained from catalysts with the same composition and different particle size cuts.

can begin to see the initial formation of bulk polymer in the 36−45 μm sample more clearly than with the other samples. After 15 s, all of the crystallization curves have either two modes or one mode with a well-defined shoulder. In the case of the small support size, the third peak continues to be more

Figure 7. DSC thermograms (crystallization and melting curves) for polymers made using (nBuCp)2ZrCl2 catalyst supported on silica particles of different sizes (36−45, 45−63, and >80 μm) at different times. G

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Figure 9. DSC thermograms of homo- and copolymers produced at different reaction times using supported Et(Ind)2ZrCl2 under reference conditions.

crystallinity of the reactor powder are much lower than found in bulk polymers in the case of polymerizations that lasted up to 30 s (the longest time systematically studied in the current work). Crystallization temperatures as low as 70 °C, melting temperatures of 116 °C (vs expected values of over 130 °C for HDPE), and crystalline volume fractions of under 30% were observed for different samples. In addition, it was observed that the crystallization curves measured using DSC showed multiple peaks. All of these unexpected observations were no longer visible after ∼3 min of polymerization under the conditions used to make the polymers analyzed here. We have shown that this behavior is linked to the presence of the support material in the particles since removal of the polymer from the support leads to thermal properties much closer to the expected values. It was also found that an analysis of the crystallization temperature (Tc) provided much more information on the state of the particles than did the melting point data since Tc is more closely linked to the size of the crystals of polymer than is the melting point. In fact, we were able to correlate the evolution of the crystallization temperature(s) with an evolution of the yield. The reason for this behavior can be explained by the presence of a high amount of support in the final particle due to the low yields reached in our stopped flow reactions (maximum 2 g/g). The support/ polymer particle is then just at the beginning of the fragmentation process, and a relevant quantity of polymer is still located into unfragmented pores. This space confinement has the effect to perturb the chain crystallization by limiting the crystal growth. The thermal properties of the polymers produced at very short reaction times can then be used as a kind of sensor to measure the particle morphology evolution. Their implementation in simulations of the fragmentation process might help in lowering the number of assumption of the model and in obtaining results closer to the experimental ones.

Figure 8. Evolution of melting T at reaction start-up in homo- and copolymerization with (nBuCp)2ZrCl2 (a) and Et(Ind)2ZrCl2 (b).

°C) and corresponds to an incorporation of ∼3.2 mol % of butene into the polymer chains. On the other hand, the melting temperature of the copolymer produced using the (nBuCp)2ZrCl2 precursor shows that the polymer chains contain ∼2 mol % of butene. It is interesting to note that the copolymer made with the (nBuCp)2ZrCl2 catalyst with the lower incorporation of butene exhibits a thermal behavior similar to that of the homopolymer. On the other hand, higher comonomer incorporation clearly influences the thermal properties, even at very short times. A more interesting result comes from the DSC analysis of copolymers made with supported Et(Ind)2ZrCl2 which has an increased capability to incorporate the comonomer into the backbone. Using the Gibbs−Thompson equation for this copolymer with a melting point of 113 °C gives a final crystal thickness of about 8 nm. The crystallites in this case should then be less perturbed by the space confinement. Figure 9 shows a comparison between the thermograms of the copolymer and the respective homopolymer for three relevant reaction times. It is clear that the same trend for the crystallization peaks is seen no matter if the comonomer is present or not. This means that also for the copolymers, despite what can be thought by calculating the copolymer crystal thickness, the space confinement due to the pores in the first reaction seconds is still present. However, due to the fact that the crystallites are smaller for the copolymer, the space confinement does not provoke a melting temperature depression. In addition, one should note that, for the same reason, crystallization peaks of the copolymers are shifted of 10 °C toward lower temperatures.

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CONCLUSIONS In this first systematic study of the thermal properties of nascent HDPE powders, it has been shown that both the melting and crystallization temperatures of polyethylene produced on supported catalysts as well as the degree of

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (V.M.); timothy.mckenna@lcpp. cpe.fr (T.M.). H

dx.doi.org/10.1021/ma302150v | Macromolecules XXXX, XXX, XXX−XXX

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Notes

(23) Loos, J.; Lemstra, P. J.; Kimmenade, E. M. E.; van, Höhne, G. W. H.; Thüne, P. C. Polym. Int. 2004, 53, 824−827. (24) Loos, J.; Thüne, P. C.; Niemantsverdriet, J. W.; Lemstra, P. J. Macromolecules 1999, 32, 8910−8913. (25) Thüne, P. C.; Loos, J.; Jong, A. M. de; Lemstra, P. J.; Niemantsverdriet, J. W. Top. Catal. 2000, 13, 67−74. (26) Thüne, P. C.; Loos, J.; Jong, A. M. de; Lemstra, P. J.; Niemantsverdriet, J. W. J. Catal. 1999, 183, 1−5. (27) Thüne, P. C.; Loos, J.; Weingarten, U.; Müller, F.; Kretschmer, W.; Kaminsky, W.; Lemstra, P. J.; Niemantsverdriet, J. W. Macromolecules 2003, 36, 1440−1445. (28) Di Martino, A.; Weickert, G.; McKenna, T. F. L. Macromol. React. Eng. 2007, 1, 229−242. (29) Di Martino, A.; Weickert, G.; McKenna, T. F. L. Macromol. React. Eng. 2007, 1, 165−184. (30) Machado, F.; Lima, E.; Pinto, J. C.; McKenna, T. F. L Polym. Eng. Sci. 2010, 51, 302−310. (31) Olalla, B.; Broyer, J. P.; McKenna, T. F. L. Macromol. Symp. 2008, 271, 1−7. (32) Tioni, E.; Broyer, J. P.; Spitz, R.; Monteil, V.; McKenna, T. F. L. Macromol. Symp. 2009, 285, 58−63. (33) Tioni, E.; Spitz, R.; Broyer, J. P.; Monteil, V.; McKenna, T. F. L. AIChE J. 2012, 58, 256−267. (34) Silva, F. M.; Broyer, J. P.; Novat, C.; Lima, E.; Pinto, J. C.; McKenna, T. F. L. Macromol. Rapid Commun. 2005, 26, 1846−1853. (35) Tioni, E.; Broyer, J. P.; Monteil, V.; McKenna, T. F. L. Ind. Eng. Chem. Res. 2012, 51, 14673−14684.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by Dutch Polymer Institute is gratefully acknowledged. This work is part of the Research Programme of the Dutch Polymer Institute (DPI, Eindhoven,The Netherlands), project no. #636. Special acknowledgments to Mettler Toledo for providing the DSC. We also thank Dr. René Fulchiron of the laboratory IMP (Villeurbanne, France) for useful discussions.

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ADDITIONAL NOTE Note that in this paper any situation for which the chains are free to crystallize independently on what is surrounding them will be called bulk. This can be a much smaller volume than what is normally thought (i.e., a pore big enough to not perturb the crystallization can be considered as “bulk” in this context). a

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dx.doi.org/10.1021/ma302150v | Macromolecules XXXX, XXX, XXX−XXX