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Adsorption of Polypropylene and Polyethylene on Liquid Chromatographic Column Packings 2004, 59, 461–467

T. Macko1,&, J. F. Denayer2, H. Pasch1, L. Pan3, J. Li3, A. Raphael1 1 German Institute for Polymers (Deutsches Kunststoff-Institut), Schlossgartenstr. 6, D64289 Darmstadt, Germany; E-Mail: [email protected] 2 Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2,1050 Brussels, Belgium 3 Department of Chemistry and Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854-8087, USA

Received: 1 December 2003 / Revised: 17 December 2003 / Accepted: 6 January 2004 Online publication: 2 April 2004

Abstract A group of zeolites and a 3D nanoporous metal-organic material RPM-1 were tested as column packings for adsorption of isotactic polypropylene and linear polyethylene from dilute solutions. It was found that polyethylene is fully or partially retained from thermodynamically good solvents (1,1,2,2-tetrachloroethylene, 1,4-dimethylbenzene, diphenylether, 1,2-dichlorobenzene and 1,3-dichlorobenzene) at temperatures of 115 C or 140 C, when a specific type of zeolite ˚ has been used as the column packing. Polypropylene was fully retained with pore sizes 5–6 A ˚ , when diphenylether was used as the mobile in another type of zeolite with pores of 7–12 A phase. As far as known, this is the first system sorbent - mobile phase, where adsorption of polypropylene was observed.

Keywords Column liquid chromatography Adsorption chromatography Zeolite adsorbents Polypropylene and polyethylene

Introduction Polyethylene (PE) and polypropylene (PP) are, according to the volume of their industrial production, the most produced synthetic polymers [1]. Copolymers of PE and PP with various comonomers are produced industrially too. Moreover, a large number of new copolymers have been prepared in various laboratories [2–4]. For the analytical characterization of polyethylene, polypropylene and copolymers, several fractionation methods, spectroscopic and calorimetric methods

Original DOI: 10.1365/s10337-004-0228-6 0009-5893/04/04

are used [5]. Fractionation methods used are crystallization fractionation [6] [CRYSTAF], temperature rising elution fractionation [7] [TREF], fractional precipitation [8] and liquid chromatography. Among all existing liquid chromatographic analytical methods, however, only size exclusion chromatography [9,10] (SEC) has been established for polyolefins. SEC of polyolefins is performed exclusively at high temperatures, as high as 135–145 C, because of the low solubility of polyethylene and polypropylene at low temperatures. Adsorption chromatography of polyethylene and

polypropylene, if made available, could create a new way of analytical separation of polyolefin materials. The adsorption of synthetic polymers on chromatographic stationary phases depends very sensitively on their chemical composition and architecture [11], which is utilized in interactive liquid chromatography of such compounds [12–14]. Although many new homo- and copolymers based on polyethylene and polypropylene have been prepared in recent times [2–4], efficient separation methods for such materials do not exist. Adsorption of polyolefins from liquids onto adsorption column packings has never been reported. Inspired by the experimental work of Denayer et al. [15], we have recently found a strong retention of polyethylene with molar masses between 1 kg mol)1– 500 kg mol)1 on a column packed with a specific zeolite [16, 17]. Polyethylene was fully retained in ZSM-5, a zeolite with pore sizes between 5–6 A˚, when decalin [16], 1,1,2,2-tetrachloroethane or 1,3,5-trimethylbenzene [17] was used as the mobile phase at 140 C. Adsorption of polyethylene to the zeolite is supposed to be the primary reason for its strong retention. Zeolites are crystalline materials built up of SiO4 and AlO4 tetrahedra, joint together in regular arrangements to form an open structure with pores of molecular dimensions [18, 19]. Their unique chemical and physical properties have made them highly capable in adsorption of

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gaseous and liquid substances and in catalysis of numerous chemical reactions [19]. Moreover, some zeolites are used for catalytic degradation of polyethylene and polypropylene at temperatures as high as 300–500 C [20–22]. In this connection, interactions between polyolefins and zeolites have not yet been studied using liquid chromatography. On the other hand, sorption of alkanes, i.e. oligomers of polyethylene, in zeolites from liquids has been studied experimentally. A number of authors [23–26] have shown that the amount of adsorbed n-alkanes under adsorption equilibrium (expressed in mol of sorbate per gram of zeolite) from a liquid (i.e., carbon tetrachloride and 1,1,2,2-tetrachloroethane [23]; benzene [24]; isooctane [24]; trimethylbenzene [25, 26]) decreases as the number of carbon atoms increases. The largest nalkane studied was C20 [24]. Denayer et al. [15] have shown that retention time of C7AC16 alkanes increased when n-hexane or n-octane was used as the mobile phase with certain zeolites as column packing. Contrarily, the retention time of C5AC16 n-alkanes decreased, when n-hexane, ndecane, tetradecane or heptadecane were used as mobile phases and silica gel C18, as column packing [27]. The silica gel used [27] had a mean pore size of 150 A˚. Quite differently; the pores of zeolites are uniform and a size of 5–15 A˚, which is comparable to the size of the analyte molecules entering the cavities. In these very small pores, the adsorption potential indeed becomes larger as the pore size decreases. This is because in such small pores, adsorbed molecules feel the force field from the whole pore, so the smaller the pore, the smaller the distance between molecule and pore walls, and the higher the attractive forces. For non-polar molecules, such as n-alkanes, the adsorption can completely be ascribed to dispersion forces where the sorbent–sorbate interactions are additive and proportional to the polarizability of the sorbate [18]. For n-alkanes, the mean polarizability of the molecules increases with the chain length [18]. While adsorption of linear and branched alkanes with small molar masses is a well known phenomenon, adsorption of alkanes with much higher molar masses, i.e. polyethylene or polypropylene, has not yet been extensively studied. In the present paper, new experimental data concerning the adsorption behaviour of polyethylene

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and polypropylene are presented. In addition to the zeolites, a thermally stable, 3D nanoporous metal-organic material RPM-1 was also employed as a sorbent.

Experimental Solvents 1,4-dimethylbenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,2,4-trichlorobenzene, diphenylether, decalin, 2-ethyl-hexyl acetate, cyclohexyl acetate, (2-butoxyethyl)acetate, isoamyl acetate, and ethylene glycol monobutyl ether, all of synthesis quality (Merck, Darmstadt, Germany) and 1,1,2,2-tetrachloroethylene (Sigma-Aldrich, Steinheim, Germany) were used in this study.

Polymers Linear polyethylene standards were purchased from Polymer Standards Service, Mainz, Germany. Their peak molar masses were 1.08; 2.03; 22; 66; 145 kg mol)1, polydispersities were in the range 1.11–1.72. Isotactic polypropylene samples of 32.6 kg mol)1 and 57 kg mol)1 (polydispersity 2.6 and 2.1, respectively) were prepared in the Department of Chemistry, University of Stellenbosch, South Africa [28]. Polypropylene with an average molar mass of 200 kg mol)1, polyethylene 260 kg mol)1 and 500 kg mol)1 were obtained from PSD Polymers, Linz, Austria, with polydispersities around 3. Ultra high molar mass polyethylene ‘‘Chirulen’’ with an average weight molar mass of 3790 kg mol)1 and a polydispersity of 24 was product of Ticona, Oberhausen, Germany. Polymer samples were dissolved at 140 and 160 C, the time of dissolution for the polymer samples varied between 1–3 h. The polymer concentration was around 1 mg mL)1. Dissolution of the samples was performed using a heater and shaker (Model PL-SP 260, Polymer Laboratories, Shropshire, England). An antioxidant was not added to the sample solution because the antioxidant (Irganox 1010) was detected as a significant interfering peak in the chromatogram. It was assumed that the extent of polymer degradation is small and may be neglected. Chromatographia 2004, 59, April (No. 7/8)

Column Packings HPLC columns were prepared by manual dry-packing of the zeolite crystals into stainless steel columns with an internal diameter of 4.6 mm. All zeolites (Silicalite, SH-300, CBV-780, CP814E) were dried before use by heating in air atmosphere from room temperature to 450 C at a rate of 4.5 C min)1, and staying for 4 h at 450 C. The sorption properties of the used column packing materials were described by Denayer et al. [15].

Silicalite A column with a length of 150 mm contained silicalite (Alsi-Penta Zeolithe GmbH, Schwandorf, Germany), with a Si / Al ratio of 400. The pore system of silicalite contains linear channels with a free pore diameter of 5.6 · 5.3 A˚, intersecting with sinusoidal channels, having a free diameter of 5.5 · 5.1 A˚. This material has a particle size larger than 10 lm.

ZSM-5 (SH-300) The second column with a length of 150 mm, contained zeolite SH-300 (AlsiPenta), a ZSM-5 material with a Si/Al ratio of 150. The pore system of SH-300 is identical to that of silicalite. SH-300 has an average particle size of about 10 lm.

USY (CBV-780) A column with a length of 50 mm was filled with an Ultra Stable Y zeolite, USY CBV-780 (Zeolyst Int. Valley Forge, PA, USA), with a Si / Al ratio of 40. This zeolite contains free cavities with a diameter of 12 A˚. These cavities are connected through windows with a size of 7.3 A˚. This zeolite also contains larger mesopores, with a size between 40 A˚ and 400 A˚ [29]. Most of the mesopores are present as cavities rather than cylindrical pores connecting the external surface with the interior of the crystallite [29]. The CBV-780 crystals have an irregular spherical morphology, with an average particle size of 0.5 lm.

Beta (CP814E) Zeolite CP 814E Beta ((Zeolyst Int., Valley Forge, PA, USA), with a Si/Al Original

ratio of 12.5 has a complex structure. The channel system is constituted of three interconnecting pore systems. Two 12-ring linear channels (5.7 · 7.5 A˚) in different crystallographic directions intersect partially. A third, sinusoidal channel (5.6 · 6.5 A˚) is formed by these intersections. The cavities have a diameter of 7.6 A˚. Crystals of this zeolite with a size of about 1 lm were packed in a column with a length of 50 mm and an internal diameter of 4.6 mm I.D.

RPM-1 In addition to the aforementioned inorganic zeolites, a thermally stable, 3D nanoporous metal-organic material RPM1 (Rutgers Recyclable Porous Materials) recently synthesized in Rutgers University [30] was also tested. Chemical composition [Co3 (biphenyldicarboxylate)3 (4,4’-bipyridine)]. 4(dimethylformamide).H2O corresponds to this sorbent [30]. Channels in RPM-1 contain large diameter cages (approximately 11 · 11 · 5 A˚) and smaller windows (triangular, with an effective maximum dimension of about 8 A˚). The surface of the pore structure is formed by aromatic rings in various orientations. This sorbent has demonstrated a strong capability in sorption of some hydrocarbons (n-hexane, cyclohexane) [30]. The RPM-1 has a crystallic structure, however, in difference to zeolites, this material can be dissolved in water. This allow a full recovery of polymer chains synthetised (or inreversibly adsorbed) in the sorbent. After evaporation of water, crystals of the sorbent are again formed, i.e. the material is recyclable [30]. Before carrying out HPLC measurements, the metal-organic material RPM-1 was activated by heating to 180 C for 2 h. As soon mentioned, RPM-1 is highly sensitive to water or water vapour. The framework will collapse if the sample is exposed to moisture for some time. As a result, its colour turns from purple to white. In order to avoid access of water, RPM-1 was mixed with decalin after activation and packed into a column as suspension. The column packing was stabilized by pumping of decalin at a flow-rate 1 mL min)1.

HPLC Assembly A high temperature liquid chromatograph Waters 150 C (Waters, Milford, Original

All experimental results are summarized in Table 1. Depending on the type of adsorbent used and the mobile phase, different behaviour was observed. Polypropylene was not retained in the majority of tested systems. However, when diphenylether was used as the mobile phase, full retention of polypropylene in a column packed with zeolite CBV-780 was found. Therefore, after injection of both polyethylene and polypropylene

standards dissolved in diphenylether into the column packed with zeolite CBV-780, only peaks of unretained polyethylene samples are seen in the chromatogram (Fig. 1a). This indicates that polypropylene is fully retained within this column packing. Similar chromatograms as shown in Fig. 1 were obtained with several other sorbent - mobile phase systems (Table 1). However, polyethylene was retained and polypropylene was eluted in this case. For evaluation of polymer recovery, the same polymer solutions with identical polymer concentrations were injected with and without column. Measurements without column and with diphenylether as the mobile phase have shown that the ELSD response, i.e. the area of the polyethylene peaks decreases with molar mass (Fig. 1b), explaining the decrease of peak heights with increasing molar masses in Fig. 1a. Such a strong dependence of the ELSD response on molar mass was not yet reported in the literature (see Review [32]). However, the responses of polypropylene and polyethylene obtained by use of other mobile phases do not depend on molar mass to a such large extent. When polyethylene was injected into column SH-300 using 1,3-dichlorobenzene as the mobile phase, polyethylene peaks decreased with molar mass of polyethylene (Fig. 2a), similarly as in Fig. 1. Without column, the peaks of both polyethylene and polypropylene have approximately the same height (Fig. 2b), because the concentration of polymer in the injected solution was similar (about 1 mg mL)1). The decreased recovery of polyethylene in this case is evident after comparison of chromatograms in Figs. 2a and 2b. This is due to a partial adsorption of polyethylene within the column packing. Polyethylenes with smaller molar masses are more strongly retained in the column, thus their recovery and peak heights are smaller (Fig. 2a), than polyethylenes with higher molar masses. This phenomenon was also observed for polyethylene in other mobile phase–zeolite systems [16, 17] and can be explained that smaller polyethylene chains be adsorbed to a larger extent. Smaller polyethylene chains may penetrate deeper into the pores as a result of a faster diffusion rate and the fact that short chains may change their conformation easier. Consequently, the smaller poly-

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USA) was connected to an evaporative light scattering detector (ELSD), model PL-ELS 1000 (Polymer Laboratories, Shropshire, England). The nebulizer temperature, the evaporator temperature and gas flow rate varied depending on the solvent used. Parameters recommended for the ELSD by producer were used. The temperature of the autosampler and the column oven was maintained at 115, 135 or 140 C, depending on the boiling point of the solvent used, as the mobile phase. Aliquits of 50 lL or 100 lL of the polymer solution was injected. The flow rate of the mobile phase was 1.0 mL min)1.

Determination of Critical Diameters The critical diameter of a molecule is the diameter of the smallest cylinder that can circumscribe the molecule [31]. If a molecule has smaller critical diameter than the zeolite pore size, it is supposed that the pore volume of the zeolite is sterically accessible to it. Models of molecules and macromolecules were created by computer simulations using the software ‘‘Materials Studio’’ (Accelrys Inc., San Diego, USA). The computer simulations were done with forcefield methods using the COMPASS forcefield. When simulating molecular dimensions, bond angles, bond lengths and van der Waals radii of atoms were appropriatelly considered. Critical diameters were estimated for an equilibrium conformation of a molecule. In the case of a polymer chain, a prolonged conformation of the chain was considered. Influence of both temperature and pressure on bond lengths and radii was neglected. Furthermore the influence of interactions between molecule and pore walls on critical diameter was not considered as well.

Results and Discussion

Table 1. Survey of elution behavior of polypropylene and polyethylene standards

1

Sorbent

Mobile phase

Isotactic Polypropylene

Linear Polyethylene

Zeolite SH-300 Si/ Al = 150 Pores 5-6 A˚

1,1,2,2-tetrachloroethylene1 Diphenylether 1,4-dimethylbenzene2 1,2-dichlorobenzene 1,3-dichlorobenzene

Eluted Eluted Eluted Eluted Eluted

Fully retained Fully retained Fully retained Fully retained Partially retained

Zeolite Silicalite Si/ Al = 400 Pores 5 – 6 A˚

1,1,2,2-tetrachloroethylene1 Diphenylether 1,4-dimethylbenzene2 1,2-dichlorobenzene 1,3-dichlorobenzene



Zeolite CP814E Beta Si/Al = 12.5 Pores 5.6 – 7.5 A˚

1,1,2,2-tetrachloroethylene1 Diphenylether 1,4-dimethylbenzene2 1,2-dichlorobenzene 1,3-dichlorobenzene Dekalin 1,2,4-trichlorobenzene 2-ethyl-hexyl acetate

Eluted Eluted Eluted Eluted Eluted Eluted Eluted Eluted

Fully retained Fully retained Fully retained Fully retained Fully retained Fully retained Fully retained Eluted

Zeolite CBV-780 Si/Al = 40 Pores 7.3 A˚, mesopores

1,1,2,2-tetrachloroethylene1 Diphenylether 1,4-dimethylbenzene2 1,2-dichlorobenzene 1,3-dichlorobenzene Isoamyl acetate2 2-butoxyethyl acetate Ethylene glycol monobutyl ether

Eluted Fully retained Eluted Eluted Eluted Eluted Eluted Eluted

Eluted Eluted Eluted Eluted Eluted Not soluble Not soluble Not soluble

Metal-organic sorbent RPM-1 Channels with cages (approximately 11 · 11 · 5 A˚) and windows (triangular, approximately 8 A˚)

1,1,2,2-tetrachloroethylene1 Diphenylether 1,4-dimethylbenzene2 1,2-dichlorobenzene 1,3-dichlorobenzene Isoamyl acetate2 Decalin 1,2,4-trichlorobenzene Cyclohexyl acetate 2-butoxyethyl acetate

Eluted Eluted Eluted Eluted Eluted Eluted Eluted Eluted Eluted Eluted

Eluted Eluted Eluted Eluted Eluted Not soluble Eluted Eluted Eluted Not soluble

Measured at 110 C; 2Measured at 135 C

ethylene chains may be adsorbed in larger extent in the narrow pores than the larger chains. Adsorption of polyethylene and polypropylene from diphenylether is different and depends on the column packing. Polyethylene is fully retained from diphenylether on both SH-300 and CP814E, while polypropylene is eluted under these conditions (Table 1). Using zeolite CBV-780, polyethylene is eluted in diphenylether, while polypropylene is fully retained (Fig. 1). Thus, a selective removal of either polyethylene or polypropylene from mixtures of both polymers is probably possible. For verification that really only polyethylene or only polypropylene is eluted from the column after injection of polymer blends, additional measurements will be necessary because peaks of both polyethylene and polypropylene samples eluate at the same elution volumes and in addition, the ELSD response is not specific only to one of these polymers. Polyethylene standards up to 145 kg mol)1 or 500 kg mol)1 and polypropylene up to 200 kg mol)1 were fully retained with some of the tested column packings. Additionally, we have injected ultra high molecular weight polyethylene (average weight molar mass 3790 kg mol)1) into the column packed with SH-300, using 1,2-dichlorobenzene and decalin as mobile phases. Even with such a high molecular weight, the sample was fully retained within the column packing. In all cases polyethylene and polypropylene were dissolved at high temperature and hence precipitation of polyethylene and polypropylene on the column packing should be ruled out. Therefore, adsorption on the surface of the crystals or in the pores should be responsible for complete retention of polyethylene and polypropylene. Earlier studies already demonstrated that polymers may be fully adsorbed from thermodynamically good solvents on sorbents [11]. However, such systems are relatively rare. For example, for polystyrene, a well studied polymer, full adsorption was found on porous glass in b Fig. 1. Chromatograms of polyethylene (PE) and polypropylene (PP) samples injected in diphenylether as mobile phase. Temperature: 140 C. Flow rate: 1 mL min)1. a) Column packing: CBV-780, 50 · 4.6 mm. b) No column. Notice: The ELSD response for polyethylene depends pronouncedly on molar mass

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tetrachloromethane [33], but also on the following systems: tetrachloromethane/ silica gel [34], methylcyclohexane/PS/ DVB gel [35], isooctane/silica gel C18 [36], trichloroethylene/nonporous silica [37] and N,N-dimethylformamide/silica gel C18 [38], 1,2,4-trichlorobenzene/ porous glass [39]. As shown here, several sorbent/solvent systems allowing full retention of polyethylene were discovered in a relatively short time using zeolites with microporous structure as adsorbents. Adsorption of polymers might occur on the surface of the zeolite particles, at the pore entrance (pore mouth), or inside the pores. The structure of the pores of zeolite SH-300 and silicalite is schematically shown in Fig. 3. A solute may enter the pores of a zeolite when its critical diameter is smaller than the pore diameter. Critical diameters of used solvents and polymers are summarized in Table 2. A critical diameter of 4.8 A˚ was reported for normal alkanes by Jasra et al. [26] and a value of 4.3 A˚ was claimed by Richards and Rees [40]. A value of 4.9 A˚ for n-hexane was found elsewhere [41]. As seen, there is no consensus about the critical molecular size. The published values are close to the value we calculated for polyethylene monomer (Table 2). A polyethylene chain in equilibrium conformation has a zigzag structure. A larger critical diameter corresponds to such a conformation (Table 2). According to its critical diameter (Table 2), a linear polyethylene chain in its prolonged conformation should be able to penetrate into pores with diameters of 5–6 A˚ and larger, while the polypropylene chain due to its steric requirements fit only in larger pores. This is in agreement with our observations (Table 1), i.e. polyethylene and polypropylene are retained only in zeolites with pores, which are sterically accessible for the chains in a prolonged conformation. According to the critical diameters of the solvents used (Table 2) it seems that several of them can not penetrate inside the pores of ZSM-5 and silicalite. This would mean that these solvents do not compete with the polymer chains in the pores for adsorption sites. Nevertheless, the calculation of the critical diameter does not take into account changes of conformation of the solvent molecules under influence of the pore walls. Thus it cannot be completely ruled out that solvent molecules might, after alteration Original

Fig. 2. Chromatograms of polyethylene and polypropylene samples injected in 1,3-dichlorobenzene as mobile phase. a) Column packing: SH-300, 150 · 4.6 mm. b) Without column. Temperature: 140 C. Flow rate: 1 mL min)1

of their conformation, penetrate the pores. However, adsorbed polymer chains may block pore entrances to such an extent that solvent molecules can not diffuse into pores. As a result, desorption of chains may be very difficult or even impossible. Really, up to now we have not found a possibility for desorption of retained polyethylene or polypropylene from the described zeolites, when only the mobile phase (desorption supporting liquid) was varied. It has been shown that n-alkanes in the range C12-C22 adsorbed in zeolite may be recovered by extraction with n-pentane [42]. Full desorption of n-dodecane has required 10 h extraction. The rate of desorption was slower for longer n-alkanes then for shorter, i.e. n-docosane has required 100 h extraction. Difficulties with desorption of both polyethylene and polypropylene give rise an assumption about penetration of polymer chains into zeolite pores. Polymers being adsorbed on the external surface of sorbent particles should be desorbed more easily. The retention of polypropylene seems to be more specific than the retention of polyethylene (Table 1). Polypropylene is fully retained only from diphenylether on zeolite CBV-780. Polar solvents like

isoamyl acetate, (2-butoxyethyl)-acetate and ethylene glycol monobutyl ether prevent adsorption of polypropylene on this zeolite. Also polypropylene is not retained also from cyclohexanone [16], 2ethyl-1-hexanol [16], cyclohexyl acetate [17] and 2-ethyl-hexylacetate [17] on a CBV-780 column packing. Moreover, diphenylether is a rather poor solvent for polyethylene and polypropylene. A specific poor solvent might, however, promote adsorption of a polymer on a well chosen adsorbent [11].


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Fig. 3. Schematic of the structure of zeolite SH-300 indicating straight and zigzag channels

Table 2. Critical diameters of polymers and solvents Substance

Critical diameter [A˚]

Polyethylene (monomer) Linear polyethylene Polypropylene (monomer) Isotactic polypropylene 1,1,2,2-tetrachloroethylene Diphenylether 1,4-dimethylbenzene 1,2-dichlorobenzene 1,3-dichlorobenzene Decalin 1,2,4-trichlorobenzene Isoamyl acetate 2-ethyl-hexyl acetate Cyclohexyl acetate Ethylene glycol monobutyl ether 2-butoxyethyl acetate 1,3,5-trimethylbenzene Cyclohexanone

4.46 5.28 5.19 7.83 6.59 7.34 6.69 7.34 7.34 7.41 7.32 6.49 8.64 6.80 5.65 5.71 8.43 6.73

The adsorption of polyethylene and polypropylene seems to be very sensitive to specific surface properties of the studied sorbents. For example, silicalite and SH-300 have identical pore diameters, but silicalite does not retain polyethylene from the tested solvents (Table 1), while SH-300 retains polyethylene in several cases. Probably, the different chemical composition of both materials is the cause of the different behaviour in adsorption (silicalite has a lower Al content, and contains a low concentration of Na as compensation cation). CP814E (Beta) has slightly larger pores than SH-300 and retains PE fully from both 1,3-dichlorobenzene and 1,2,4trichlorobenzene, contrary to SH-300. Zeolite CP814E retains polyethylene most strongly from all tested materials (Table 1). Si/Al ratios between the used zeolites are substantially different and it is known that this ratio markedly governs stationary phase hydrophobicity. Both silicalite ( Si/Al ¼ 400) and CBV-780 (Si/Al ¼ 40) do not adsorb polyethylene (Table 1), on the other hand, adsorption of polyethylene increases in order SI-300 (Si/Al ¼ 150), CP814E (Si/Al ¼ 12.5). It means that there is not a clear correlation between the adsorption and Si/Al ratio. At present, we do not have specific explanation for the mentioned differences in adsorption properties of the zeolites. Among the tested sorbents, silicalite and RPM-1 do neither adsorb polyethylene nor polypropylene from the tested liquids. A partial adsorption of polyethylene on silicalite from decalin has been described elsewhere [16]. The pores in RPM-1 have a highly irregular pore

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shape due to the orientation of the aromatic rings that form the surface of the pore structure [30]. Possibly, the solvents are adsorbed to a much larger extent than the non-polar polyethylene and polypropylene chains on RPM-1 as a result of the chemical nature of this material. It could also be suggested that the polyethylene and polypropylene chains do not match the irregular shape of the pores of RPM1. Using silica gel or silica gel with chemically bonded octadecyl groups with an average pore size of 100 A˚, Berek et al. [43, 44] have observed increased retention of macromolecules, which should be, according to their hydrodynamic diameter, sterically fully excluded from the pore volume. The authors [43, 44] suggested that macromolecules change their conformation, de-coil, and ‘‘reptate’’ into the pores and are adsorbed in a ‘‘flower-like conformation’’ [45, 46]. According to this suggestion, the whole chain does not enter into the pore. The de-coiled part of the macromolecule, entering a narrow pore looks like a stem while the rest of the coil, localized around the pore mouth (pore entrance), resembles the crown of a flower. We believe that this hypothesis could also be valid for polymers in zeolites. Polymer chains in their linear conformation could be sucked partially into the narrow zeolite pores, as a result of the favourable attractive interactions between the pore walls and the chain. The used zeolites have about 10 - 660 times smaller pore sizes, then silica gels, which are commercially available with the average pore size from 60 to 4000 A˚. With decreasing pore diameter, the heat Chromatographia 2004, 59, April (No. 7/8)

of adsorption, generated through interaction of the adsorbate with the adsorbent, increases [47]. Thus, as illustrated in this paper, zeolites enable a more pronounced adsorption of polymers than many another sorbents. Such strong retention of polymers in narrow channels in the case of attractive interactions between polymer and pore walls was forecasted by Gorbunov and Skvortsov [48]. They have shown on the basis of theoretical calculations that an increase of molar masses of polymers at constant pore diameter (when pore diameter radius of gyration) leads to an exponential increase of the partition coefficient for polymers. Attractive interactions between polymer chains and the pore surface are prerequisite for such effect.

Conclusion Several new sorbent - mobile phase systems enabling strong retention of polyethylene were experimentally found. Moreover, the first chromatographic system supporting full retention of polypropylene was identified. Several thermodynamically good solvents and one poor solvent, diphenylether, promote adsorption of polyethylene, while adsorption of polypropylene, was found only from diphenylether. Polyethylene and polypropylene were adsorbed only on column packings with pores, sterically accessible for polymer chains in prolonged conformation. It is supposed that this is due to the high adsorption potential in very small pores of the zeolites. Polymer chains are probably partially retained inside of the pores and partially near to pore entrance. Furthermore, in some of the tested chromatographic systems, polyethylene chains are fully retained by the column packing, while polypropylene chains are not retained. In one system, in which polypropylene is retained, polyethylene is eluted. This difference in elution behaviour could enable removal of the first or the second polymer by selective adsorption from mixtures of both polymers. Desorption of retained polymers was, however, not yet reached, when only mobile phases were varied. This may be explained by blocking of pores with adsorbed chains. Clearly, there are many unanswered questions regarding the diffusion and adsorption of the chain molecules in Original

confining media, such as zeolites and nanoporous metal-organic materials. Evaluation of both the extent of penetration of the polymer chains inside of the pores and the possibilities for their desorption, requires additional studies.

Acknowledgement J. F. Denayer is grateful to the F.W.O. Vlaanderen for a fellowship as postdoctoral researcher. T. Macko expresses his appreciation to Mr. Christoph Brinkmann (German Institute for Polymers, Darmstadt, Germany) for his support in handling the chromatograph Waters 150 C. The financial support by BMBF (Germany) and Basell GmbH (Project Code 03C035YA) is gratefully acknowledged.

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