Langmuir 1992,8, 2269-2273
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Analysis of the Microporosity of Nascent Polyethylene: 129XeNMR and High-Resolution Adsorption M. A. Ferrero,? S. W. Webb,$ Wm. C. Conner, Jr.,*’t J. L. Bonardet,! and J. Fraissards Chemical Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003, General Electric Corporate Research, P.O. Box 8, Schenectady, New York 12345, and Laboratoire de Chimie des Surfaces, URA CNRS 1428, Universite P. et M . Curie, Place Jussieu, Paris 75252, France Received October 15, 1991. In Final Form: June 18, 1992
During the initial stages of ethylene polymerization, the product polymer forms on the surface and eventually fills the void space of the catalyst. In later stages of polymerization, the catalyst fragments and the surface accessible to the gas phase decreases precipitously. These studies focus on the initial polymer formed on the surface to determine if ita morphology is different than the polymer formed at higher yield. Several earlier studies had suggested that the structure of the polymer formed on the catalyst during the initial stages of polymerization is different from the rest of the bulk polymer structure. High resolution adsorption (HRADS) and lzsXeNMR experiments were used to confirm the existence of a organic microporousstructure formed by the polymer and sorbed on the catalyst surface. Both independent techniques, HRADS and lz9XeNMR, performed on low yield catalyst/polymer samples, detected some microporosity that disappeared at higher polymer yield or higher polymer loading of the catalyst.
Introduction Nascent polyethylene structures have been studied by several workers.’+ Most of the studies were focused on Ziegler-Natta supported catalysts, although some studies have also been reported over silica supported catalysts. Several workers have demonstrated the existence of a fibril structure in recently synthesized polyethylene. Direct electron microscopy observations by Chanzy and Marchessaultl on thin films of nascent polyethylene from Ziegler-Natta catalyst crystallites showed that polymer chains form “ribbons” which connect small “polyp” structures of size 100nm containing fragments of the original catalyst crystallites. They claimed that the fracture and breakage of the catalyst crystallites were caused by the movement of the growing polymer. The fragments are pushed away and this movement generates the ribbon structure. The chain morphology is similar to that observed in stress-induced crystallization. When there is extensive fracture of the catalyst, a polyp-and-ribbon-like structure is obtained. Blaisand Manley? using optical and electron microscopy and X-ray and electron diffraction, observed a fibrillar structure that according to Chanzy and Marchessaultl could probably be associated with the ribbons they had observed. The fibrils are of -20-100 nm in diameter. They claimed the molecules are folded within lamellae analogous to polymer single crystals. They also observed that the fibrils form a coiling helix macrostructure. They proposed that the function of the active site was to coordinate the insertion of monomer to produce stereoregularity and to facilitate conformational orientation of the chains to enable rapid and complete crystallization in
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t
University of Massachusetts.
t General Electric Corporate Research.
Universite P. et M. Curie. (1)Chanzy,H. D.;Marchessault, R. H. Macromolecules 1969,2,108. (2)Blais, P.;Manley, R. S. J. J. Polym. Sci. Part A-1 1968,6,291. (3)Ingram,M.; Schindler, P. Makromol. Chem. 1967,11,267. (4) Lippman, R.D. A.; Norrirrh, R. G. V. Proc. R. SOC.1963,A275,310. (5)Mihailov, M.; Minkova, L.; Nedkov, E. Makromol. Chem. 1979, 180, 2351. (6)Minkova, L.;Velikova, M.; Damyanov, D. Eur. Polym. J. 1988,24, 661.
a microporous matrix adjacent to the polymerization site. They conclude that crystallization takes place simultaneously with synthesis and the active surface exerts a strong influence on both the polymer stereoregularity and the chain orientation. Electron microscopy and small-angle X-ray scattering studies by Ingram and Schindler3 also detected a fibrillike chain morphology which they found to be dependent on the medium of polymerization. In homogeneous systems (the catalyst and polymer are both soluble), fibrillar macrostructures were not observed and the spherulite crystal sizes were much smaller, 6-8 nm. They claimed the large-scale fibrillar structures are caused by the incompatibility of the polymer with the bulk phase. The small-dimension crystalline structures were detected for polymer grown in each medium. Lippman and Norrish4 proposed that the polymer coverage of the catalyst takes place in the form of “towers” based on their kinetics studies. They suggested that a fibrous growth of polyethylene with the polymer chains extending upward from the catalyst surface leaves enough space between the fibrous structure for the monomer to reach the active site. Therefore there would be no diffusion control of the reaction. Mihailov et al.s studied the structure of high molecular weight nascent polyethylene. They found experimental evidence that the structure “consists of crystalline chain lamellaewith an increased amount of tie molecules between them as well as extended chain crystals”. They found that is reasonable to compare the nascent morphology fibrous structure, which does not have a high degree of crystallinity, “with the less-perfect crystalline material obtained in solutions by stirring or by crystallization from the melt and extending”. Minkova et studied the structure of nascent polyethylene over silica supported titanium and vanadium catalysts. They found that the catalyst type substantially affects the morphology of the polymer. They also observed that the structure obtained on a supported titanium consists of rounded 3-pm particles connected by fibrils. For the vanadium catalyst the aggregates are -50-200
0743-7463/92/2408-2269$03.00/00 1992 American Chemical Society
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2270 Langmuir, Vol. 8, No. 9,1992 pm, but higher magnifications showed small particles of about 2 pm tightly connected by fibrils. In our prior studies7 of the initial stages of polymerization, we had found that the surface area of the catalyst measured by nitrogen sorption at 77 K (and employing BET analysis) decreased from >lo0 m2/g for the initial catalyst to less than 20 m2/gas 1.7 g of polymer/g of catalyst are formed. Further, the pore volume decreases from >1.7 scm3/gto less than 0.2 scm3/gat the same yield, as found from mercury intrusion/retraction porosimetry. Neither conventional sorption analyses nor mercury porosimetry are capable of detecting micropores less than 2 nm in radius; other approaches are needed to determine if microporosity is present. Recently two techniques have been developed that are sensitive to pore dimensions between 0.4 and 2 nm and have been applied for the analysis of the pores found in crystalline zeolites. In each case a gas is sorbed or allowed to diffuse within the pores. In high-resolution adsorption (HRADS), the sorption of a gas is analyzed at pressures below 10% of the saturation pressure, and in 129XeNMR, Xe is used to diffuse through the pores and thus to probe the dimensions and electrical environment as reflected in its nuclear spectra. These two techniques were employed in this study of very low yield polymer particles to detect the presence of a porous polymeric (possibly crystalline) phase. The microporositywas postulated by these earlier studies and suggested by the work of Webb8on mesopore and surface area characterization of the polymer. Results from both different techniques seem to confirm the presence of a microporous structure, formed during the initial stages of the polymerization and present until polymer yields slightly above 150 mg/g. At higher yields this microstructure is occluded by the subsequent polymer formed.
Experimental Section High-Resolution Adsorption (HRADS). The catalyst/ polyethylene samples were evacuated until a residual pressure of -loa Pa, at a temperature of 80 "C before the HRADS experiments. Higher temperatures are often employed for the analyses of inorganic solids; however, the melting point of the polyethylene is 120 "C and the samples should not approach this temperature without the possibilityof changing the polymer morphology. The nitrogen adsorption isotherms at 77 K were obtained in an Omicron Omnisorp 360 as currently manufactured by Coulter Instruments. The HRADS experiment involves the addition of a certain amount of the sorption gas (0.3-1.5 scm3) and waiting for the equilibrium (about 8-15 min.) before the next dose is admitted to the sample. The analysisof the micropore size distribution from the sorption data was done using the H~rvath-Kawazoe~ model,developed for the analysisof the pores in active carbonsand used also for zeolites. Note that the absolute value of the pore dimensions employing this model may not be precise when applied to pores of varying shape; however, the relative dimensions do follow the general relationship reflected in this model.1° Xenon Adsorption Isotherms. Before any adsorption of xenon, the samples were pretreated under vacuum (1W2 Pa) for 12 h at 373 K. Adsorption isotherms were obtained by a volumetricmethod at 300 K; adsorption equilibrium was reached very rapidly (t < 5 min).
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(7) Weist, E. L.; Ali, A. H.; Naik, B.; Conner, W. Macromolecules 1989,
22, 3244. Conner, W.; Webb, S. W.; Weist, E. L.; Chiovetta, M. G.;
Laurence, R. L. Can. J. Chem. Eng. Silver Anniversary Issue 1991,69 665. (8) Webb, S. W. Ph.D. Thesis, Chemical Engineering Department,
University of Massachusetts a t Amherst, Sep. 1990. (9) Horvath, G.;Kawazoe, K. 1. Chem. Eng. Jpn. 1983,16,470. (10) Conner, W.;Ferrero, M.;Webb, S. W.; Springuel-Huet, M. A. and Fraissard,J.,"TheUseof IBXe NMRand HRADS for the Characterization of Zeolites" submitted to Langmuir 1992.
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"Xe NMR of Adsorbed Xenon. lBXe NMR spectra were obtained with a Brucker CXP 100 spectrometer. The resonance frequency of xenon is 24.90 MHz. With the amount of adsorbed xenon being low, the acquisition time for a spectrum is long a t ambient temperature. The number of scans necessary to obtain agood spectrum can be greater than 105. However, the relaxation time of adsorbed xenon is short and the pulse delay between two scans is then 0.5 s.
Results High-ResolutionAdsorption (HRADS). The results from the HRADS experiment on low yield polyethylene particles are shown in Figure 1. The micropore size distributions of the nitrogen-accessible surfaces of the polymer particles is represented. Volume sorbed was significant ( l e 3 0 scm3/g). A broad peak a t 1-1.2 nm is observed and is attributable to the interparticle surfaces of the silica solid. However, a peak that develops at 0.60.7 nm for yields of 100-150 mg/g, and disappears at higher yields, may be due to the formation of a porous polymer on the catalyst surface. This is the first experimental evidence for the presence of an adsorbed layer of porous microcrystalline polymer created during nascent polymerization. The persistence of the 1.0-1.2-nm peak to high polymer yields demonstrates that polymer accumulation does not occur on the entire surface area of the catalyst. It probably occurs adjacent to the active chromium sites actually making polymer. It should be noted that the pore distributions shown in Figure 1 are based on the fraction of pores found and, indeed, the totalmicroporosity is decreasing as polymer is formed. As an example, the initial porosity for pores less than 20 nm is 0.0858 cm3/g. This decreases to 0.0702 cm3/g at a yield of 0.1 and to 0.0312 cm3/g at a yield of 0.5 cm3/g. Xenon Adsorption Isotherms. The amount of xenon adsorbed at equilibrium decreases when the loading of polyethylene on the catalyst increases (asshown in Figure 2), whatever the equilibrium pressure (0 < P < 170 P a ) . The formation and the growth of polymer cause a progressive blockage of the pores of the support. 129XeN M R of Adsorbed Xenon. Figure 3 shows spectra at 300 K of xenon sorbed at the same temperature on six catalyst/polyethylene samples, the polyethylene loading ranging from 0 to 11 g/g. In the case of fresh medium porous volume (MPV) catalyst, the resonance line is broad, symmetrical, and shifted 50 ppm downfield of xenon gas. The chemical shift is independent of the equilibrium pressure (Figure 4). When the yield of polyethylene is very low (YO.l), at 300 K we observe only one broad and asymmetric line at
Langmuir, Vol. 8, No. 9,1992 2271
Microporosity of Nascent Polyethylene
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200 ppm but the broad intense line at 50 ppm completsly disappears. At last, for the highly loaded samples (Y1.5 and Y 11)NMR spectra show two sharp lines: one near 0 ppm, and the other, very downfield shifted at 205 ppm.
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Figure 3. Spectra of adsorbed xenon at 300 K on different catalysta/polyethylenesamples of yield ranging from 0 to 11 g/g.
about 50 ppm. At lower temperature, the line is highly shifted and narrower and becomes symmetrical (Figure 5). For a loading of 0.15 g/g (Y0.15) a second line, sharp and almost not shifted (0 < 6 < 2 ppm), appears. At the same time, the intensity of the 50 ppm line decreases and it becomes again symmetrical. Finally if we increase the detection sensitivity, a very weak signal begins to appear in the 200 ppm region. In the case of the Y 0.33 sample, the spectrum consists essentially of a sharp signal near 0 ppm, the intensity of which increases. As previously, a small signal appears at
Discussion In the case of the Y 0 sample, extrapolation of the 6 = f(nxe)curve to zero pressure gives a value of 52 f 3 ppm. If this value has the same meaning as in microporous systems like zeolites,ll it leads to an average pore diameter of about 1.4 nm for cylindric pores. Nitrogen sorption experiments detected an average mesopore size of about 20 nm diameter and a small quantity of micropores of 1 nm diameter for the silica catalyst/support. The narrowing and the increasing shift with decreasing temperature and the pressure insensitivity of the shift for the Y 0 sample (Figure 4) may be interpreted as the result of a fast exchange between adsorbed xenon atoms and free inter(11)Springuel-Huet,M. A.; Demarquay, J.; Ito,T.;Fraissard,J. Stud. Surf. Sci. Catal. 1987, 37,183.
Ferrero et al.
2272 Langmuir, Vol. 8, No. 9,1992 141 ppm
6 (ppm)
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Figure 5. Xenon-129 NMR spectra at differenttemperatures (K) for the Y 0.1 polyethylene sample with 1020xenon atoms/g sorbed at 300 K. porous xenon atoms as has been shown by Conner et al.12 for xenon interaction with several silica samples. The observed linear variation of 6 withtemperature is the result of the change in the relative concentrations of these two xenon species. Consistent with this interpretation, the line at 50 ppm downfield for the Y 0 sample corresponds to Xe interacting with the silica surface. The characteristics of the shift are similar those found for xenon interactingwith a rough silica surface;however,the relation between 6 and pore size for the silica support is different from the relation for zeolites. For samples highly loaded with polymer (Y1.5 and Y 111,the shape of the spectra shows that Xe is no longer able to interact with the surface of the MPV catalyst because the surface is now blocked by the polyethylene. The sharp signal shifted far downfield can be attributed to xenon atoms “absorbed” (dissolved) in the amorphous region of the polymer. Indeed, it has been observed by Stengle and Williamson13that the resonance line of xenon dissolved in a liquid alkane is shifted 150-180 ppm downfield from the free gas and that a linear relationship exists between chemical shift and solvent density. This shift is considered to be the result of instantaneous electric moments arising in the dilute xenon which induce a reaction field in the solvent (polyethylene) affecting the magnetic shielding of the solute (xen0n1.l~If we plot the 205 ppm value in the previous relationship, we find a polyethylene density of about 0.87-0.88 g/scm3, which is the usual value for the amorphous region of a low-density polyethylene. This value is the same as that found by Stengle and Williamson15in their study of xenon absorbed (dissolved) in solid polymers. On the other hand, interactions of xenon gas with the walls of macrocavitiesformed by polyethylene particles or with the carbon chain in motion lead to a marked decrease of the relaxation time of xenon that explains the appearance of the 0 ppm line, the shift, and the line width which is insensitive to the temperature. This line is not observed in the fresh MPV (12) Conner, W. C.;Weist, E. L.; Ito, T.;Fraissard, J. J.Phys. Chem. 1989,93,4138. (13) Stengle, T.R.; Reo, N. V.; Williamson, K. L. J.Phys. Chem. 1981, 8.5. - - , 3772. - . .-. (14) Rummens, F. H. A. Chem. Phys. Lett. 1978,31,596. (15) Stengle, T.R.;Williamson, K. L. Macromolecules 1987,20,1428.
catalyst and in samples at low yields of polymer. The same phenomenon has been observed in highly coked Y zeolites.16 For the Y 0.33 sample, the sharp line near 0 ppm is due to the interactions of xenon gas with the carbon chain but the amount of polyethylene formed on the catalyst is yet too low to observe easily the 205 ppm line of “absorbed” xenon; although, if the detection sensitivity is increased, it is possible to see a very weak signal at 200 ppm as is shown in Figure 3. As in the other highly loaded samples (Y1.5 and Y 111, none of the pores of the support are accessible to xenon and the broad line at 50 ppm, characteristic of adsorbed xenon in the pores, does not appear. Sample Y 0.15 spectrum presents two lines: (a)the broad line at about 50 ppm and (b) the sharp line near 0 ppm showing that there are areas where xenon interacts with polyethylene particles. There are two possible interpretations for the broad line at about 50ppm. It may represent xenon sorbed on the support which has not yet been covered by polyethylene or it may represent sorption in the polymer micropores. The answer is related to the rough silica surface which is being covered with a polymer layer that blocks further interaction of xenon with the silica. It is highly possible that at this yield, sorption on the silica support itself is completelyblocked due to polymer coating of the surface. For a yield of 0.15 g/g and for a uniform covering of the available surface area, a polymer thickness of about 0.6-0.75 nm will be present. If this is the case, there will be no NMR line characteristic of interaction with the rough silica surface. The effect will be similar to that observed by Conner et al.I7 when a silica surface was coatedwith water: No xenon NMR limes were observed when water was preadsorbed on the surface. If this phenomenon is occurring for Y 0.15 sample, only the line corresponding to sorption in the polymer micropores (detected by HRADS,see Figure 1)will be present in its NMR spectrum at about 50 ppm. That is exactly what (16) Barrage, M. C.;Bonardet, J. L.; Fraiseard, J. Catal. Lett. 1990, 5, 143. (17) Conner, W. C.; Weist, E. L.; Ito, T.;Chen, Q.; Springuel-Huet, M. A.; Fraiaeard, J. In Fundamentals ofddsorption; Mersmann and Scholl, E&.; Engineering Foundation: New York, 1991; 977. (18) Springuel-Huet, M. A,; Fraiasard, J. Chem. Phys.Lett. IWS, 154, 299.
Microporosity of Nascent Polyethylene
is observed; only one line shifted a little downfield at 50 ppm appears. The fact that the chemical shifts are quite the same for Y 0 and Y 0.15 samples is due to the multiple exchanges which are existing between adsorption sites and the gas phase. The existence of a broad and asymmetrical line for the Y 0.1 sample suggests several possible explanations: (a) There is a chemical shift anisotropy due to a factor of form as has been shown in AIP04-11 zeolites;18 (b) there is a heterogeneous distribution of pore sizes when polyethylene begins to fill or to block the pores; (c) there are two overlapping lines corresponding to chemical exchange between xenon sorbed on the remaining porosity of the support, xenon within the polymer micropores, and the gas phase. This hypothesis agrees with the micropore size distribution (Figure l),which shows the appearance of a microporosity a t about 0.6-0.7 nm, while the total porosity dramatically decreases. When the temperature decreases (Figure 5), the resonance line becomes at first more symmetric, i.e., a t 250 K, we only observe a shoulder at 55 ppm with a narrower symmetric line at 81 ppm. At lower temperatures, this shoulder disappears and the signals are shifted strongly downfield and are symmetric. Thus we find a 6 = f(T, variation analogous to that observed with the Y 0 sample. We conclude that at low temperature the xenon is preferentially sorbed in the narrowest pores, i.e. within the micropores of the nascent polyethylene. We suggest hypothesis c is the most probable as discussed below. This also seems to agree with the results from HRADS. For the Y 0.1 sample some microporosity was evident at about 0.7 nm (Figure 1). We suggest that this gives rise to the line corresponding to sorption in the polymer pores in the NMR spectrum of this sample. This microporosity is not present in the HRADS of the initial catalyst support, appears at low yield, and disappears at Y 0.33. The NMR line corresponding to sorption into polymer micropores is not present in the initial silica/ support, appears in the Y 0.1 sample (see Figure 3), overlapped with the line for sorption on the silica, and disappears for Y 0.33. The Xe NMR results for the sample without polymer (Y 0) and for the sample with yield 0.15 are most surprising. The NMR signals seem quite similar and yet the amount of polymer filling the pores is dramatically different. It may only be coincidence that the chemical shifts are so similar under these measurement conditions. Our inter-
Langmuir, Vol. 8, No. 9,1992 2273
pretation is thus quite speculative but is supported by the sorption measurements. Further studies will be necessary to test this interpretation. The existence of microporosity formed during the initial stages of polymerization on the surface of the catalyst poses several questions. Does the microporous polymer remain as a coating to the surface and subsequent polymer grows to coat this underlayer with an impervious polymer phase@)? Does the microporous polymer overlayer convert to an less-permeable phase as it is pushed from the surface by continued polymer growth? Is the microporous overlayer a transient-metastable phase that becomes impervious to nitrogen at 77 K, and probably to other sorbents, as the yield increases? The basic question as it relates to the transport of monomer to the active catalytic sites is whether a microporous surface layer is present near the surface as more polymer is formed. If a microporous layer is maintained adjacent to the catalyst surface, then transport is controlled by inhibited diffusion through the pores and polymer overlayer to a boundary layer adjacent to the surface where more rapid transport in two dimensions is afforded. Otherwise, if a microporous layer is not maintained, transport is restricted to solubility/diffusion through the isomorphic polymer. We hypothesize that a microporous overlayer is maintained above the catalyst surface and in this manner diffusion of monomer in two dimensions above the surface is facilitated. Thus, transport of monomer to the active sites does not become a rate controlling process in the polymerization. This conclusion is based on the fact that the rate of polymerization remains constant from modest to high polymer yields. The monomer insertion rate continues at a turnover frequency above l/site/atm*s (O.lOl/site/MPa*s)throughout the polymerization to over lo4 gpolymer/gcatalyst yield. Monomer transport is not evidently controlling for the more active, state-of-the-art, commercial catalysts.
Acknowledgment. This research was supported by the National Science Foundation under Grant CTS-8921381 and the CNRS through their support of the Laboratoire de Chimie des Surface 6 Universite P. et M. Curie. This research was initiated during a NATO grant for cooperative research. Registry No. Polyethylene, 9002-88-4.
