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Light-Induced Catalyst and Solvent-Free High Pressure Synthesis of High Density Polyethylene at Ambient Temperature Matteo Ceppatelli*, Roberto Bini
The combined effect of high pressure and electronic photo-excitation has been proven to be very efficient in activating extremely selective polymerisations of small unsaturated hydrocarbons in diamond anvil cells (DAC). Here we report an ambient temperature, large volume synthesis of high density polyethylene based only on high pressure (0.4–0.5 GPa) and photoexcitation (∼350 nm), without any solvent, catalyst or radical initiator. The reaction conditions are accessible to the current industrial technology and the laboratory scale pilot reactor can be scaled up to much larger dimensions for practical applications. FTIR and Raman spectroscopy, and X-ray diffraction, indicate that the synthesised material is of comparable quality with respect to the outstanding crystalline material obtained in the DAC. The polydispersity index is comparable to that of IV generation Ziegler-Natta catalysts. Moreover the crystalline quality of the synthesised material can be further enhanced by a thermal annealing at 373 K and ambient pressure.
1. Introduction Polyethylene (PE) is a model system in polymer chemistry as the prototype of an infinite mono-dimensional alkane chain. PE is also one of the most important and largely produced polymers in the world. It is available in a variety
Dr. M. Ceppatelli, Prof. R. Bini ICCOM-CNR, Institute of Chemistry of OrganoMetallic Compounds, National Research Council of Italy, Via Madonna del Piano 10, I-50019, Sesto Fiorentino, Firenze, Italy E-mail: [email protected]fi.it; [email protected] Dr. M. Ceppatelli, Prof. R. Bini LENS, European Laboratory for Non-Linear Spectroscopy, Via N. Carrara 1, I-50019, Sesto Fiorentino, Firenze, Italy Prof. R. Bini Dipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze, Via della Lastruccia 3, I-50019, Sesto Fiorentino, Firenze, Italy Macromol. Rapid Commun. 2014, 35, 787−793 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
of grades with different thermal and mechanical properties and is employed in a wide range of applications, spanning from thermoplastics and packaging to implant medical prostheses. The large scale synthesis of PE is currently based on the catalytic polymerisation of ethylene, the starting monomeric unit, and is typically performed in the gas phase or solution requiring high temperature and/or high pressure.[1] PE, usually obtained as a mixture of amorphous and crystalline domains, is selected and classified on the basis of the length and branching of the polymeric chains, which affect the density and the properties of the material.[2] On a molecular scale, the density is determined by the packing of the polymeric chains. As far as the trans conformation along the –C–C– backbone is preserved, the chains are indeed linear and a compact packing, giving rise to a crystalline high density PE (HDPE), is expected. However, if a gauche conformation along the –C–C– backbone occurs, the alkane chains bend, losing linearity, and compact packing is prevented by the folded conformation. Gauche defects and chain branching represent
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DOI: 10.1002/marc.201300919
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critical issues for compact chain packing, resulting in a low density PE (LDPE) containing large amorphous domains. Due to its mechanical resistance, the thermodynamically stable crystalline orthorhombic HDPE is the preferred form of the polymer for realizing long term duration materials, such as medical prostheses for implantation in the human body. High pressure chemistry of simple molecular systems in the GPa range has recently emerged as a new research area, providing new insight into the behaviour of matter under non-conventional conditions and opening up attractive possibilities for innovative high pressure materials synthesis.[3–17] Highly crystalline orthorhombic HDPE has been synthesised in a diamond anvil cell (DAC) at ambient temperature and a few tenths of GPa, in the total absence of solvents, catalysts and radical initiators.[18,19] In this case, the extreme efficiency and selectivity of the polymerisation results from the combined effect of pressure and photo-excitation. The optical excitation of the ethylene molecules to the first electronic excited state (1B1υ) by two-photon absorption of near-UV wavelengths corresponds to the transition of an electron to the first anti-bonding molecular orbital. The electron density redistribution associated with the reduction of the bond order is responsible for the ethylene molecule assuming a twisted conformation, changing the symmetry from the D2h to the D2d point group. In particular, the elongation of the ethylene molecule along the C–C bond and the lowering of the rotational barrier for the two –CH2 groups, in combination with the increase in the density and the reduction of the intermolecular distances induced by the pressure, favour the activation and linear propagation of the polymerisation. A compact packing of the polymeric chains to give a crystalline high density polyethylene is then ensured by pressure (ref.[18] and references therein). Nevertheless, to date, the importance of this result in terms of technological applications has been limited by the extremely small sample volume allowed by the DAC (a cylinder of approximately 150 μm diameter and 50 μm height), which is able to produce only ∼1 × 10–3 mm3 sized PE samples (∼1 × 10–3 mg). However, the relatively low pressure required for the activation of the reaction and the two-photon absorption of near-UV photons,[18] not requiring drastic vacuum UV conditions as is the case for one-photon absorption processes, represent reaction conditions accessible with current industrial technology, thus opening up the possibility of upscaling this process to large volume apparatuses, according to the principles of Green Chemistry.[20] In this paper, we report a large volume light-assisted high pressure polymerisation of ethylene at ambient temperature in the total absence of solvents, catalysts and radical initiators. Using this method, HDPE samples as large as several mm3 (5–20 mg) could be synthesised. Furthermore, in contrast to the DAC
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method, there are virtually no technical limitations for scaling up the dimensions of the laboratory scale pilot reactor, designed and realised for this synthesis, towards practical applications.
2. Experimental Section Ethylene (2.5 supplied by Rivoira in 50 L bottles at a pressure of 82.4 bar) was first filtered using a high pressure filter from Nova Swiss. This blocked dust and metallic particles and prevented damage to the booster and intensifier employed for the compression (see Results and Discussion). The reaction was photoactivated using the UVML emission of an Ar ion laser (Coherent Sabre Innova) with the highest energy emissions at 351 and 364 nm. The polyethylenes, whose FTIR and Raman spectra were employed as a reference, were purchased from Aldrich, with the exception of those labelled as HDPE and LDPE, which were provided by Stryker Orthopaedics. Stycast 2850 FT epoxy glue (Emerson & Cuming) was prepared according to the manufacturer instructions using Catalyst 24LV and allowed to dry for 24 h before use. Thermal annealing was performed using a resistive heater. The sample was placed on an aluminium support and temperature was measured by means of a K type thermocouple.
2.1. Optical Spectroscopy The samples employed for the IR and Raman analysis originated from both the external and the internal parts of the synthesised polymeric pellets and the results did not show any dependence on the section of the polymer investigated. The FTIR spectra were recorded from approximately 100 μm thick slices, cut from the recovered samples, to avoid the saturation of the absorption signal. The FTIR spectra were acquired using a Bruker IFS 120 HR interferometer, with a resolution of 1 cm–1. The Raman spectra were measured using the 647.1 nm line of a Kr ion laser source (Coherent Innova 300). The scattered radiation was collected in a back scattering geometry with a microscope objective (Mitutoyo), spatially filtered, dispersed by a single stage monochromator equipped with a 900 groove mm–1 grating (Acton Trivista 555), and revealed by a nitrogen cooled CCD detector (Princeton Instruments), with a resulting instrumental resolution of 1.5 cm–1
2.2. ADXRD Angle dispersed X-ray diffraction patterns (ADXRD) of the synthesised PE were measured using synchrotron radiation on beamline ID09A of the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). In the synchrotron experiment, the monochromatic beam (λ = 0.4108 Å) was focussed on the sample with a spot size of about 30 μm. The diffracted light was revealed by an image plate (MAR345) and the diffraction patterns were analysed and integrated by means of FIT2D computer code (A. P. Hammersley, FIT2D) to obtain the 1D intensity distribution as a function of the 2Θ scattering angle.
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Figure 1. Experimental set-up for synthesising large volume high density PE (LVPE) using only pressure and optical excitation, in the total absence of solvents, catalysts and radical initiators. a) Technical drawing showing the high and low pressure sections. b) Image of the experimental set-up shown in a. c) Image of the overall experimental set-up. d) Image of the high pressure cell with the supports for the diamond windows (hollow cylindrical supports, threaded caps and copper o-rings). e) Technical drawing of the high pressure cell.
2.3. HTGPC The weight average molecular weight Mw and the number average molecular weight Mn of the polymers were evaluated by high temperature gel permeation chromatography (HTGPC) with a Waters GPC 2000 system equipped with a set of three columns (Styragel HT6, HT5, and HT3), and a refractive index detector. The analyses were performed at 413 K using 1,2,4-trichlorobenzene as a solvent with an elution time of 1 mL min–1 and standard polystyrene as the reference.
3. Results and Discussion A pilot reactor, able to operate at ambient temperature up to 0.7 GPa, while providing optical access to the sample from the near-UV to the FIR, was designed and realised for this purpose. The experimental set-up, reported in Figure 1, has a modular structure, which allows the separation of a low pressure and a high pressure section. This design ensures the possibility of refilling the cell and allows for portability of the system, while maintaining the operating pressure. The first section is able to pre-compress ethylene up to 0.07 GPa (∼700 bar) and is composed of a single-stage contaminant-free air-driven gas booster compressor with an air-driven-piston/gas-driven-piston area ratio of 75 (Haskel mod. AG-75). A gas purifier and a mechanical filter are mounted before the gas booster to ensure the ethylene purity and prevent damage to the gas booster. The second section, able to operate up to 0.7 GPa (∼7 kbar), is composed of a manual pressure intensifier (Nova Swiss) and is connected to the high pressure cell, where diamond windows
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provide optical access to the sample from the near-UV to the far-IR. The gas booster, valves, tubing, pressure gauge and pressure intensifier were purchased from commercial suppliers, but the cell and all its mechanical components were specifically designed and manufactured. The body and all the other components of the cell were made of marval and gilded to prevent corrosion and chemical interaction with ethylene and with ethanol, which was used to clean the system. The high pressure cell is equipped with IIa type circular diamond windows of 4.00 mm diameter and 1.00 mm thickness, to ensure transparency down to near-UV wavelengths, while preserving the highest mechanical resistance. Each diamond window is mounted on a hollow cylindrical support and then held in place by a threaded hollow cap screwed to the cylindrical support. The effective area of the diamond windows providing optical access to the sample had a diameter of 2.0 mm. The two cylinders with windows are then inserted along the optical axis into seats finely machined on the body of the cell. The sealing of the cylinder with respect to the body of the cell is ensured by copper gaskets. A critical issue for the system concerns the sealing of the two diamond windows. The seats of the diamond windows were specifically polished to be flat and parallel with respect to the diamond, so that sealing should be ensured by the internal pressure of the cell. However, due to the small diameter of the diamond windows and to the non-perfectly normal force exerted by the thread of the cap on the windows, we found that gluing the windows onto the seat was a necessary step for a perfect seal. We tried different kind of glues and found
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Figure 2. Images of the recovered LVPE sample still attached to the support for the diamond window (a, b and c) and free standing (d).
that Stycast 2850 FT + Catalyst 24LV from Emerson & Cuming was the best choice to ensure the sealing of the diamond windows and avoid sample contamination. Ethylene was first compressed at ambient temperature by a gas booster to the supercritical fluid state (∼700 bar, 298 K), where a density comparable to the liquid is achieved.[21,22] It was then further compressed by the manual pressure intensifier to reach several tenths of GPa. The pre-compression stage is mandatory for achieving a sufficiently high density of ethylene for the manual pressure intensifier to operate. Once high pressure conditions (P ≤ 0.5 GPa) were achieved, ethylene was irradiated using the near-UV multi line (UVML) emission of an Ar ion laser centered at ∼350 nm to induce the polymerisation. In the case of the DAC polymerisation, 100 mW of UVML radiation (∼350 nm) were focused on a 150 μm diameter sample, with a resulting power density of 5.7 × 106 W m–2.[18,19] In this case, due to the larger sample area (10–20 times larger), the homogeneous irradiation of the sample with the same power density as in the case of the DAC would have required more than 20 W of near-UV radiation, which is an exceedingly high power for laboratory gas lasers. For this reason, to maximise the power density of our laser source, the 2.0 mm diameter of the output beam from the Ar ion laser, matching the effective diameter of the windows, was used without any focusing optics. We performed several experiments employing laser powers ranging from 0.5 to 1.9 W (corresponding respectively to a power density of 1.6 × 105 W m–2 and 6.0 × 105 W m–2), observing that an
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output laser power of ∼1 W (3.2 × 105 W m–2) was required to trigger the reaction and that irradiation of 20–25 h duration was typically necessary to convert all the ethylene. These power density values are one order of magnitude smaller than those employed with the DAC.[18,19] The reaction was monitored by FTIR spectroscopy, following the decrease in the intensity of the ethylene absorption bands. The pressure drop due to the volume contraction of the sample during the polymerisation was compensated by regulating the pressure in the cell through the manual intensifier in order to maintain constant pressure conditions. After the reaction, the polymer was recovered by first unloading the unreacted fluid monomer from the tubes, and then by removing the window holder using a specially designed extractor device. Images of a recovered sample are shown in Figure 2. The disk-like shaped recovered PE (diameter ∼4 mm, height ∼2 mm, 18 mg) presents a thicker concentric region of smaller diameter corresponding to the optically accessible sample volume directly in contact with the diamond windows. Whereas longer samples could be simply produced using a longer cell, samples of larger diameter would require larger and consequently thicker diamond windows, whose cost would grow exponentially. However, the duration of the windows against the cost of purchase, development, regeneration and disposal of catalyst should be taken into account. The synthesised PEs were characterised by vibrational optical spectroscopy (FTIR and Raman), angle dispersive
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28000
LDPE, indicates the absence of the amorphous fraction (branching and gauche amorphous δ(CH ) 0.8 LVPE 24000 defects) suggesting a high degree of LVPE crystallinity in LVPE. UHMWPE 20000 The crystallinity was confirmed by UHMWPE 0.6 the X-ray diffraction data reported DAC 16000 in Figure 4. The X-ray diffraction pattern also highlights the presence of DAC 0.4 HDPE 12000 a mixture of two crystal phases: the orthorhombic Pnam, stable at ambient HDPE 8000 conditions, and the monoclinic A2/m, 0.2 stable at high pressure above 14 GPa.[23] LDPE 4000 The latter phase is reported to be progauche LDPE duced by battering orthorhombic sam0.0 0 ples, or more generally when the crys1300 1350 1400 1000 1200 1400 1600 talline polymer is subjected to stress.[24] -1 -1 Wavenumber / cm Raman shift / cm Since all the diffracted intensity is Figure 3. Comparison among the FTIR (left panel) and Raman (right panel) spectra of difessentially located on the low angle different polyethylenes in selected frequency regions: LVPE (large volume PE) synthesised fraction peaks (2Θ < 7°), a careful fit of in this study using high pressure and photo-excitation; UHMWPE (ultra high molecular the most significant diffraction lines weight PE) from Aldrich; DAC (diamond anvil cell PE) synthesised in a diamond anvil (the 010, 200, -210 monoclinic and 110, cell;[18] HDPE (high density PE) provided by Stryker Orthopaedics; LDPE (low density PE) provided by Stryker Orthopaedics. Peak assignments of the relevant signals (δ(CH3) 200 orthorhombic) was performed to bending due to chain terminations and branching, gauche defects and the amorphous determine the relative amounts of the contribution) are also indicated. The thickness of the samples is ∼50 μm for the PE syn- orthorhombic, monoclinic and amorthesized in the DAC and ∼100 μm for the other PEs. The spectra were vertically transphous fractions. The deconvolution of lated for representative purposes and the values reported on the ordinate scales are the observed diffraction lines allowed intended only for relative comparison of the intensities of the spectra. the relative amount of the different phases to be determined. The amount of orthorhombic phase in the synthesised samples was X-ray diffraction (ADXRD) and high temperature gel perfound to range between 54% and 68%, and that of the meation chromatography (HTGPC). The FTIR and Raman monoclinic phase between 22% and 37%, whereas the spectra provide information about the crystalline quality amount of amorphous fraction was estimated to be of the material and give direct evidence about chain conalways of the order of 10%. However, this value may be formation, branching and termination. In Figure 3 (left largely overestimated (even by 50%) due to the broadness panel), a comparison of the infrared spectra of different of the amorphous signal, which can be revealed only by polyethylenes in the frequency region 1300–1425 cm–1 is the fitting analysis. shown. Here theabsorption bands relative to the vibrational modes involving gauche conformations and High temperature gel permeation chromatography branching (tertiary carbon atoms) are expected. In par(HTGPC) was employed to determine the molecular ticular, despite a slight intensity difference related to the weight and the weight distribution of the polymer chains. different sample thickness, the infrared absorption proThe ratio of the weight average molecular weight Mw to file of the large volume PE (LVPE) synthesized in this study the number average molecular weight Mn is called the is identical to those of the DAC synthesized PE and of the polydispersity index (PDI). The smaller the PDI value, the HDPE. It is worth to remark that the absorption band at sharper the distribution of the molecular weights and 1378 cm–1, assigned to the antisymmetric bending mode the greater the polymer homogeneity. The polydispersity index of polymers obtained with IV generation Zieglerof the -CH3 groups (umbrella mode) and thus related to Natta homogeneous catalysts is 1–2 in laboratory synthe chain length and branching in terms of number of thesis, whereas it is 16 for industrial heterogeneous cataterminations, is clearly visible in the LDPE, whereas is lysts.[25] In this case, the HTGPC analysis provided a value absent in LVPE. In Figure 3 (right panel) are also reported the Raman spectra of different polyethylenes in the freof Mw = 720 000 g mol–1 and a value of Mn = 446 000 g quency region between 1000–1600 cm–1, where specmol–1, giving PDI = 1.62, which, considering the radical tral information related to the crystalline vs amorphous nature of the electronic excitation and the small cross secfraction of the samples can be gained. In particular, the tion of the two-photon absorption process, could be comabsence of the broad Raman bands in the 1050-1100 cm–1 patible with the occurrence of some “livingness”. These results indicate that the synthesised LVPE, besides being and 1300–1330 cm–1 frequency regions, observable in FTIR
amorphous
Raman
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Counts
A
3
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highly crystalline and therefore of very high density, also has a narrow molecular weight distribution. Finally, further attention was dedicated to increasing the amount of the crystalline fraction with respect the amorphous one and to enhancing the crystalline quality of the synthesised samples. On the basis of high pressure and high temperature experiments performed in our laboratory on PE in the diamond anvil cell,[18,19,23,26] we have found that the amorphous and monoclinic domains can be efficiently converted into orthorhombic ones by thermal annealing cycles at 6–7 kbar and 530–550 K. Since these conditions cannot presently be realised with our large volume cell, we simply heated the LVPE to about 373 K for a few hours at ambient pressure. The effects of this treatment can be readily observed by comparing the FTIR spectra acquired before and after the thermal annealing reported in Figure 4. In particular the infrared spectra clearly indicate a reduction of the amorphous fraction absorptions due to gauche defects in the 1325–1400 cm–1 frequency region, at 1444 cm–1 and at 1469 cm–1,[18,27,28] a narrowing of both the orthorhombic (doublets at 719 and 731 cm–1 and at 1462 and 1472 cm–1) and monoclinic (shoulders at 717 cm–1 and at 1474 cm–1)[26] crystalline components, an intensification of the orthorhombic bands with respect to the monoclinic ones, which remain substantially unaltered by the thermal annealing, and an inversion of the intensity ratio between the components of the orthorhombic doublets assigned to the C–H rocking (719 and 731 cm–1) and scissoring modes (1462 and 1472 cm–1) of polyethylene.[27] This can be better appreciated from the spectral deconvolution of the frequency region corresponding to the C–H scissoring modes reported in the inset of Figure 4. According to the mathematical fit of the absorption profile of the scissoring region, the amount of amorphous fraction after annealing can be estimated from the integrated areas of the two broad absorptions at 1444 and 1469 cm–1 as ∼45% of the value before the annealing, which was already low. All the absorption bands assigned to the crystalline phases sharpen after annealing and the integrated areas of the two orthorhombic components (1462 and 1472 cm–1) increase by a factor of ∼1.3, whereas that of the monoclinic band remains substantially unaltered. Moreover the intensity ratio between the high (1472 cm–1) and low (1462 cm–1) frequency components, belonging to the orthorhombic C–H scissoring modes, increases according to the theoretically predicted value.[27] The absence of a significant conversion of the monoclinic phase into the orthorhombic one is not surprising, considering the mild annealing conditions, and further confirms the efficiency of the annealing procedure through the hexagonal phase.[23,26] As expected, the integrated absorption of the broad and weak band at 1455 cm–1, assigned to the C–H bending mode of –CH3
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Figure 4. Upper panel: X-ray diffraction pattern from a recovered LVPE, indicating the coexistence of the orthorhombic (Pnam) and monoclinic (A2/m) crystal phases. The Miller indices of the orthorhombic (red) and monoclinic (blue) phases and the corresponding cell parameters are also reported. Lower panel: Room temperature FTIR absorption spectra of LVPE synthesised in this study before (bottom trace) and after (top trace) annealing at 373 K for 2 h. The spectra are vertically shifted and the absorbance units in the ordinate axis are intended only for comparing the relative intensity of the bands in the two spectra. The inset shows the fit of the absorption profile ranging between 1400 and 1500 cm–1 before and after annealing, showing the behavior of the different components: 1444 cm–1 amorphous (a);[28] 1455 cm–1 δ (CH3) bending (δ);[28] 1462 cm–1 orthorhombic (o);[28] 1469 cm–1 amorphous (a);[28] 1472 cm–1 orthorhombic (o);[28] 1474 cm–1 monoclinic (m);[26] (only one component is expected for the A2/m monoclinic phase due to the non-centro-symmetric site symmetry).
groups and related to the chain terminations, remains constant after annealing.
4. Conclusion In this paper we reported a large volume high pressure synthesis of high density polyethylene based only on pressure and photo-excitation, occurring at ambient temperature in the total absence of solvents, catalysts and radical initiators. In particular we realised a laboratory scale pilot reactor, able to replicate the reaction conditions and the
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results obtained in the DAC on micrometric samples.[18] This reactor could be further scaled up for applicative purposes because the required pressure is accessible to the current industrial technology and the two-photon absorption mechanism using near-UV exciting wavelengths allows unpractical vacuum UV conditions to be avoided, even if, at the moment, high pressure photo-induced reactions on large volume systems are still very challenging. Furthermore, the absence of other chemicals offers the advantage of eliminating costs of disposal, developing and purchasing of solvents and catalysts. The crystalline quality of the synthesized HDPE, which can be further enhanced by thermal annealing at 373 K and ambient pressure, makes the material particularly suitable for technological applications. Acknowledgements: The authors would like to acknowledge Dr. Marco Frediani, Dr. Marco Candelaresi, Prof. Paolo Foggi and Dr. Luca Fontana for helpful discussions during the realization of the experimental set up. The authors would like to futher thank Dr. Marco Frediani for providing access to the HTGPC equipment. The authors would like to thank the beamline ID09A at the European Synchrotron Radiation Facility (ESRF) in Grenoble for the ADXRD measurements. This work was supported by Stryker Orthopaedics, Laserlab-Europe (EU-FP7 284464) and the Italian Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR). Received: December 20, 2013; Revised: January 30, 2014; Published online: February 18, 2014; DOI: 10.1002/marc.201300919 Keywords: ethylene polymerisation: green chemistry; high pressure chemistry; photochemistry; polyethylene [1] D. B. Malpass, Introduction to Industrial Polyethylene - Properties, Catalysts, Processes, Wiley, New York 2010. [2] A. J. Peacock, Handbook of Polyethylene - Structures, Properties and Applications, Marcel Dekker, Inc., New York 2000. [3] V. Schettino, R. Bini, M. Ceppatelli, L. Ciabini, M. Citroni, Chemical reactions at very high pressure in Advances in Chemical Physics, Vol. 131 (Ed. S. A. Rice), Wiley, New York 2005, pp. 105–242. [4] V. Schettino, R. Bini, Chem. Soc. Rev. 2007, 36, 869.
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[5] L. Ciabini, M. Santoro, F. A. Gorelli, R. Bini, V. Schettino, S. Raugei, Nat. Mater. 2007, 6, 39. [6] W. Grochala, R. Hoffmann, J. Feng, N. W. Ashcroft, Angew. Chem. Int. Ed. 2007, 46, 3620. [7] M. Santoro, F. A. Gorelli, R. Bini, G. Ruocco, S. Scandolo, W. A. Crichton, Nature 2006, 441, 857. [8] M. I. Eremets, A. G. Gavriliuk, I. A. Trojan, D. A. Dzivenko, R. Boehler, Nat. Mater. 2004, 3, 558. [9] M. Citroni, M. Ceppatelli, R. Bini, V. Schettino, Science 2002, 295, 2058. [10] M. Ceppatelli, A. Serdyukov, R. Bini, H. J. Jodl, J. Phys. Chem. B 2009, 113, 6652. [11] R. Bini, Acc. Chem. Res. 2004, 37, 95. [12] M. Ceppatelli, R. Bini, V. Schettino, Proc. Natl. Acad. Sci. USA 2009, 106, 11454. [13] M. Ceppatelli, R. Bini, V. Schettino, J. Phys. Chem. B 2009, 113, 14640. [14] M. Ceppatelli, R. Bini, V. Schettino, Phys. Chem. Chem. Phys. 2011, 13, 1264. [15] M. Ceppatelli, R. Bini, M. Caporali, M. Peruzzini, Angew. Chem. Int. Ed. 2013, 52, 2313. [16] M. Ceppatelli, S. Fanetti, R. Bini, J. Phys. Chem. C 2013, 117, 13129. [17] M. Santoro, F. A. Gorelli, R. Bini, J. Haines, A van der Lee, Nat. Commun. 2013, 4, 1557. [18] D. Chelazzi, M. Ceppatelli, M. Santoro, R. Bini, V. Schettino, Nat. Mater. 2004, 3, 470. [19] D. Chelazzi, M. Ceppatelli, M. Santoro, R. Bini, V. Schettino, J. Phys. Chem. B 2005, 109, 21658. [20] P. Anastas, J. Warner, Green Chemistry: Theory and Practice, Oxford University Press, New York 1998. [21] J. Smukala, R. Span, W. Wagner, J. Phys. Chem. Ref. Data 2000, 29, 1053. [22] K. Leonhard, T. Kraska, J. Supercrit. Fluids 1999, 16, 1. [23] L. Fontana, D. Q. Vinh, M. Santoro, S. Scandolo, F. A. Gorelli, R. Bini, M. Hanfland, Phys. Rev. B 2007, 75, 174112. [24] K. E. Russell, B. K. Hunter, R. D. Heyding, Polymer 1997, 38, 1409. [25] L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 2000, 100, 1253. [26] L. Fontana, M. Santoro, R. Bini, D. Q. Vinh, S. Scandolo, J. Chem. Phys. 2010, 133, 204502. [27] G. Zerbi, G. Gallino, N. Del Fanti, L. Baini, Polymer 1989, 30, 2324. [28] S. Krimm, Fortschr. Hochpolym.-Forsch. 1960, 2, 51.
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Communication
Light-Induced Catalyst and Solvent-Free High Pressure Synthesis of High Density Polyethylene at Ambient Temperature Matteo Ceppatelli*, Roberto Bini
The combined effect of high pressure and electronic photo-excitation has been proven to be very efficient in activating extremely selective polymerisations of small unsaturated hydrocarbons in diamond anvil cells (DAC). Here we report an ambient temperature, large volume synthesis of high density polyethylene based only on high pressure (0.4–0.5 GPa) and photoexcitation (∼350 nm), without any solvent, catalyst or radical initiator. The reaction conditions are accessible to the current industrial technology and the laboratory scale pilot reactor can be scaled up to much larger dimensions for practical applications. FTIR and Raman spectroscopy, and X-ray diffraction, indicate that the synthesised material is of comparable quality with respect to the outstanding crystalline material obtained in the DAC. The polydispersity index is comparable to that of IV generation Ziegler-Natta catalysts. Moreover the crystalline quality of the synthesised material can be further enhanced by a thermal annealing at 373 K and ambient pressure.
1. Introduction Polyethylene (PE) is a model system in polymer chemistry as the prototype of an infinite mono-dimensional alkane chain. PE is also one of the most important and largely produced polymers in the world. It is available in a variety
Dr. M. Ceppatelli, Prof. R. Bini ICCOM-CNR, Institute of Chemistry of OrganoMetallic Compounds, National Research Council of Italy, Via Madonna del Piano 10, I-50019, Sesto Fiorentino, Firenze, Italy E-mail: [email protected]fi.it; [email protected] Dr. M. Ceppatelli, Prof. R. Bini LENS, European Laboratory for Non-Linear Spectroscopy, Via N. Carrara 1, I-50019, Sesto Fiorentino, Firenze, Italy Prof. R. Bini Dipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze, Via della Lastruccia 3, I-50019, Sesto Fiorentino, Firenze, Italy Macromol. Rapid Commun. 2014, 35, 787−793 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
of grades with different thermal and mechanical properties and is employed in a wide range of applications, spanning from thermoplastics and packaging to implant medical prostheses. The large scale synthesis of PE is currently based on the catalytic polymerisation of ethylene, the starting monomeric unit, and is typically performed in the gas phase or solution requiring high temperature and/or high pressure.[1] PE, usually obtained as a mixture of amorphous and crystalline domains, is selected and classified on the basis of the length and branching of the polymeric chains, which affect the density and the properties of the material.[2] On a molecular scale, the density is determined by the packing of the polymeric chains. As far as the trans conformation along the –C–C– backbone is preserved, the chains are indeed linear and a compact packing, giving rise to a crystalline high density PE (HDPE), is expected. However, if a gauche conformation along the –C–C– backbone occurs, the alkane chains bend, losing linearity, and compact packing is prevented by the folded conformation. Gauche defects and chain branching represent
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DOI: 10.1002/marc.201300919
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critical issues for compact chain packing, resulting in a low density PE (LDPE) containing large amorphous domains. Due to its mechanical resistance, the thermodynamically stable crystalline orthorhombic HDPE is the preferred form of the polymer for realizing long term duration materials, such as medical prostheses for implantation in the human body. High pressure chemistry of simple molecular systems in the GPa range has recently emerged as a new research area, providing new insight into the behaviour of matter under non-conventional conditions and opening up attractive possibilities for innovative high pressure materials synthesis.[3–17] Highly crystalline orthorhombic HDPE has been synthesised in a diamond anvil cell (DAC) at ambient temperature and a few tenths of GPa, in the total absence of solvents, catalysts and radical initiators.[18,19] In this case, the extreme efficiency and selectivity of the polymerisation results from the combined effect of pressure and photo-excitation. The optical excitation of the ethylene molecules to the first electronic excited state (1B1υ) by two-photon absorption of near-UV wavelengths corresponds to the transition of an electron to the first anti-bonding molecular orbital. The electron density redistribution associated with the reduction of the bond order is responsible for the ethylene molecule assuming a twisted conformation, changing the symmetry from the D2h to the D2d point group. In particular, the elongation of the ethylene molecule along the C–C bond and the lowering of the rotational barrier for the two –CH2 groups, in combination with the increase in the density and the reduction of the intermolecular distances induced by the pressure, favour the activation and linear propagation of the polymerisation. A compact packing of the polymeric chains to give a crystalline high density polyethylene is then ensured by pressure (ref.[18] and references therein). Nevertheless, to date, the importance of this result in terms of technological applications has been limited by the extremely small sample volume allowed by the DAC (a cylinder of approximately 150 μm diameter and 50 μm height), which is able to produce only ∼1 × 10–3 mm3 sized PE samples (∼1 × 10–3 mg). However, the relatively low pressure required for the activation of the reaction and the two-photon absorption of near-UV photons,[18] not requiring drastic vacuum UV conditions as is the case for one-photon absorption processes, represent reaction conditions accessible with current industrial technology, thus opening up the possibility of upscaling this process to large volume apparatuses, according to the principles of Green Chemistry.[20] In this paper, we report a large volume light-assisted high pressure polymerisation of ethylene at ambient temperature in the total absence of solvents, catalysts and radical initiators. Using this method, HDPE samples as large as several mm3 (5–20 mg) could be synthesised. Furthermore, in contrast to the DAC
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method, there are virtually no technical limitations for scaling up the dimensions of the laboratory scale pilot reactor, designed and realised for this synthesis, towards practical applications.
2. Experimental Section Ethylene (2.5 supplied by Rivoira in 50 L bottles at a pressure of 82.4 bar) was first filtered using a high pressure filter from Nova Swiss. This blocked dust and metallic particles and prevented damage to the booster and intensifier employed for the compression (see Results and Discussion). The reaction was photoactivated using the UVML emission of an Ar ion laser (Coherent Sabre Innova) with the highest energy emissions at 351 and 364 nm. The polyethylenes, whose FTIR and Raman spectra were employed as a reference, were purchased from Aldrich, with the exception of those labelled as HDPE and LDPE, which were provided by Stryker Orthopaedics. Stycast 2850 FT epoxy glue (Emerson & Cuming) was prepared according to the manufacturer instructions using Catalyst 24LV and allowed to dry for 24 h before use. Thermal annealing was performed using a resistive heater. The sample was placed on an aluminium support and temperature was measured by means of a K type thermocouple.
2.1. Optical Spectroscopy The samples employed for the IR and Raman analysis originated from both the external and the internal parts of the synthesised polymeric pellets and the results did not show any dependence on the section of the polymer investigated. The FTIR spectra were recorded from approximately 100 μm thick slices, cut from the recovered samples, to avoid the saturation of the absorption signal. The FTIR spectra were acquired using a Bruker IFS 120 HR interferometer, with a resolution of 1 cm–1. The Raman spectra were measured using the 647.1 nm line of a Kr ion laser source (Coherent Innova 300). The scattered radiation was collected in a back scattering geometry with a microscope objective (Mitutoyo), spatially filtered, dispersed by a single stage monochromator equipped with a 900 groove mm–1 grating (Acton Trivista 555), and revealed by a nitrogen cooled CCD detector (Princeton Instruments), with a resulting instrumental resolution of 1.5 cm–1
2.2. ADXRD Angle dispersed X-ray diffraction patterns (ADXRD) of the synthesised PE were measured using synchrotron radiation on beamline ID09A of the European Synchrotron Radiation Facility (ESRF) in Grenoble (France). In the synchrotron experiment, the monochromatic beam (λ = 0.4108 Å) was focussed on the sample with a spot size of about 30 μm. The diffracted light was revealed by an image plate (MAR345) and the diffraction patterns were analysed and integrated by means of FIT2D computer code (A. P. Hammersley, FIT2D) to obtain the 1D intensity distribution as a function of the 2Θ scattering angle.
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Figure 1. Experimental set-up for synthesising large volume high density PE (LVPE) using only pressure and optical excitation, in the total absence of solvents, catalysts and radical initiators. a) Technical drawing showing the high and low pressure sections. b) Image of the experimental set-up shown in a. c) Image of the overall experimental set-up. d) Image of the high pressure cell with the supports for the diamond windows (hollow cylindrical supports, threaded caps and copper o-rings). e) Technical drawing of the high pressure cell.
2.3. HTGPC The weight average molecular weight Mw and the number average molecular weight Mn of the polymers were evaluated by high temperature gel permeation chromatography (HTGPC) with a Waters GPC 2000 system equipped with a set of three columns (Styragel HT6, HT5, and HT3), and a refractive index detector. The analyses were performed at 413 K using 1,2,4-trichlorobenzene as a solvent with an elution time of 1 mL min–1 and standard polystyrene as the reference.
3. Results and Discussion A pilot reactor, able to operate at ambient temperature up to 0.7 GPa, while providing optical access to the sample from the near-UV to the FIR, was designed and realised for this purpose. The experimental set-up, reported in Figure 1, has a modular structure, which allows the separation of a low pressure and a high pressure section. This design ensures the possibility of refilling the cell and allows for portability of the system, while maintaining the operating pressure. The first section is able to pre-compress ethylene up to 0.07 GPa (∼700 bar) and is composed of a single-stage contaminant-free air-driven gas booster compressor with an air-driven-piston/gas-driven-piston area ratio of 75 (Haskel mod. AG-75). A gas purifier and a mechanical filter are mounted before the gas booster to ensure the ethylene purity and prevent damage to the gas booster. The second section, able to operate up to 0.7 GPa (∼7 kbar), is composed of a manual pressure intensifier (Nova Swiss) and is connected to the high pressure cell, where diamond windows
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provide optical access to the sample from the near-UV to the far-IR. The gas booster, valves, tubing, pressure gauge and pressure intensifier were purchased from commercial suppliers, but the cell and all its mechanical components were specifically designed and manufactured. The body and all the other components of the cell were made of marval and gilded to prevent corrosion and chemical interaction with ethylene and with ethanol, which was used to clean the system. The high pressure cell is equipped with IIa type circular diamond windows of 4.00 mm diameter and 1.00 mm thickness, to ensure transparency down to near-UV wavelengths, while preserving the highest mechanical resistance. Each diamond window is mounted on a hollow cylindrical support and then held in place by a threaded hollow cap screwed to the cylindrical support. The effective area of the diamond windows providing optical access to the sample had a diameter of 2.0 mm. The two cylinders with windows are then inserted along the optical axis into seats finely machined on the body of the cell. The sealing of the cylinder with respect to the body of the cell is ensured by copper gaskets. A critical issue for the system concerns the sealing of the two diamond windows. The seats of the diamond windows were specifically polished to be flat and parallel with respect to the diamond, so that sealing should be ensured by the internal pressure of the cell. However, due to the small diameter of the diamond windows and to the non-perfectly normal force exerted by the thread of the cap on the windows, we found that gluing the windows onto the seat was a necessary step for a perfect seal. We tried different kind of glues and found
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Figure 2. Images of the recovered LVPE sample still attached to the support for the diamond window (a, b and c) and free standing (d).
that Stycast 2850 FT + Catalyst 24LV from Emerson & Cuming was the best choice to ensure the sealing of the diamond windows and avoid sample contamination. Ethylene was first compressed at ambient temperature by a gas booster to the supercritical fluid state (∼700 bar, 298 K), where a density comparable to the liquid is achieved.[21,22] It was then further compressed by the manual pressure intensifier to reach several tenths of GPa. The pre-compression stage is mandatory for achieving a sufficiently high density of ethylene for the manual pressure intensifier to operate. Once high pressure conditions (P ≤ 0.5 GPa) were achieved, ethylene was irradiated using the near-UV multi line (UVML) emission of an Ar ion laser centered at ∼350 nm to induce the polymerisation. In the case of the DAC polymerisation, 100 mW of UVML radiation (∼350 nm) were focused on a 150 μm diameter sample, with a resulting power density of 5.7 × 106 W m–2.[18,19] In this case, due to the larger sample area (10–20 times larger), the homogeneous irradiation of the sample with the same power density as in the case of the DAC would have required more than 20 W of near-UV radiation, which is an exceedingly high power for laboratory gas lasers. For this reason, to maximise the power density of our laser source, the 2.0 mm diameter of the output beam from the Ar ion laser, matching the effective diameter of the windows, was used without any focusing optics. We performed several experiments employing laser powers ranging from 0.5 to 1.9 W (corresponding respectively to a power density of 1.6 × 105 W m–2 and 6.0 × 105 W m–2), observing that an
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output laser power of ∼1 W (3.2 × 105 W m–2) was required to trigger the reaction and that irradiation of 20–25 h duration was typically necessary to convert all the ethylene. These power density values are one order of magnitude smaller than those employed with the DAC.[18,19] The reaction was monitored by FTIR spectroscopy, following the decrease in the intensity of the ethylene absorption bands. The pressure drop due to the volume contraction of the sample during the polymerisation was compensated by regulating the pressure in the cell through the manual intensifier in order to maintain constant pressure conditions. After the reaction, the polymer was recovered by first unloading the unreacted fluid monomer from the tubes, and then by removing the window holder using a specially designed extractor device. Images of a recovered sample are shown in Figure 2. The disk-like shaped recovered PE (diameter ∼4 mm, height ∼2 mm, 18 mg) presents a thicker concentric region of smaller diameter corresponding to the optically accessible sample volume directly in contact with the diamond windows. Whereas longer samples could be simply produced using a longer cell, samples of larger diameter would require larger and consequently thicker diamond windows, whose cost would grow exponentially. However, the duration of the windows against the cost of purchase, development, regeneration and disposal of catalyst should be taken into account. The synthesised PEs were characterised by vibrational optical spectroscopy (FTIR and Raman), angle dispersive
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LDPE, indicates the absence of the amorphous fraction (branching and gauche amorphous δ(CH ) 0.8 LVPE 24000 defects) suggesting a high degree of LVPE crystallinity in LVPE. UHMWPE 20000 The crystallinity was confirmed by UHMWPE 0.6 the X-ray diffraction data reported DAC 16000 in Figure 4. The X-ray diffraction pattern also highlights the presence of DAC 0.4 HDPE 12000 a mixture of two crystal phases: the orthorhombic Pnam, stable at ambient HDPE 8000 conditions, and the monoclinic A2/m, 0.2 stable at high pressure above 14 GPa.[23] LDPE 4000 The latter phase is reported to be progauche LDPE duced by battering orthorhombic sam0.0 0 ples, or more generally when the crys1300 1350 1400 1000 1200 1400 1600 talline polymer is subjected to stress.[24] -1 -1 Wavenumber / cm Raman shift / cm Since all the diffracted intensity is Figure 3. Comparison among the FTIR (left panel) and Raman (right panel) spectra of difessentially located on the low angle different polyethylenes in selected frequency regions: LVPE (large volume PE) synthesised fraction peaks (2Θ < 7°), a careful fit of in this study using high pressure and photo-excitation; UHMWPE (ultra high molecular the most significant diffraction lines weight PE) from Aldrich; DAC (diamond anvil cell PE) synthesised in a diamond anvil (the 010, 200, -210 monoclinic and 110, cell;[18] HDPE (high density PE) provided by Stryker Orthopaedics; LDPE (low density PE) provided by Stryker Orthopaedics. Peak assignments of the relevant signals (δ(CH3) 200 orthorhombic) was performed to bending due to chain terminations and branching, gauche defects and the amorphous determine the relative amounts of the contribution) are also indicated. The thickness of the samples is ∼50 μm for the PE syn- orthorhombic, monoclinic and amorthesized in the DAC and ∼100 μm for the other PEs. The spectra were vertically transphous fractions. The deconvolution of lated for representative purposes and the values reported on the ordinate scales are the observed diffraction lines allowed intended only for relative comparison of the intensities of the spectra. the relative amount of the different phases to be determined. The amount of orthorhombic phase in the synthesised samples was X-ray diffraction (ADXRD) and high temperature gel perfound to range between 54% and 68%, and that of the meation chromatography (HTGPC). The FTIR and Raman monoclinic phase between 22% and 37%, whereas the spectra provide information about the crystalline quality amount of amorphous fraction was estimated to be of the material and give direct evidence about chain conalways of the order of 10%. However, this value may be formation, branching and termination. In Figure 3 (left largely overestimated (even by 50%) due to the broadness panel), a comparison of the infrared spectra of different of the amorphous signal, which can be revealed only by polyethylenes in the frequency region 1300–1425 cm–1 is the fitting analysis. shown. Here theabsorption bands relative to the vibrational modes involving gauche conformations and High temperature gel permeation chromatography branching (tertiary carbon atoms) are expected. In par(HTGPC) was employed to determine the molecular ticular, despite a slight intensity difference related to the weight and the weight distribution of the polymer chains. different sample thickness, the infrared absorption proThe ratio of the weight average molecular weight Mw to file of the large volume PE (LVPE) synthesized in this study the number average molecular weight Mn is called the is identical to those of the DAC synthesized PE and of the polydispersity index (PDI). The smaller the PDI value, the HDPE. It is worth to remark that the absorption band at sharper the distribution of the molecular weights and 1378 cm–1, assigned to the antisymmetric bending mode the greater the polymer homogeneity. The polydispersity index of polymers obtained with IV generation Zieglerof the -CH3 groups (umbrella mode) and thus related to Natta homogeneous catalysts is 1–2 in laboratory synthe chain length and branching in terms of number of thesis, whereas it is 16 for industrial heterogeneous cataterminations, is clearly visible in the LDPE, whereas is lysts.[25] In this case, the HTGPC analysis provided a value absent in LVPE. In Figure 3 (right panel) are also reported the Raman spectra of different polyethylenes in the freof Mw = 720 000 g mol–1 and a value of Mn = 446 000 g quency region between 1000–1600 cm–1, where specmol–1, giving PDI = 1.62, which, considering the radical tral information related to the crystalline vs amorphous nature of the electronic excitation and the small cross secfraction of the samples can be gained. In particular, the tion of the two-photon absorption process, could be comabsence of the broad Raman bands in the 1050-1100 cm–1 patible with the occurrence of some “livingness”. These results indicate that the synthesised LVPE, besides being and 1300–1330 cm–1 frequency regions, observable in FTIR
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A
3
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highly crystalline and therefore of very high density, also has a narrow molecular weight distribution. Finally, further attention was dedicated to increasing the amount of the crystalline fraction with respect the amorphous one and to enhancing the crystalline quality of the synthesised samples. On the basis of high pressure and high temperature experiments performed in our laboratory on PE in the diamond anvil cell,[18,19,23,26] we have found that the amorphous and monoclinic domains can be efficiently converted into orthorhombic ones by thermal annealing cycles at 6–7 kbar and 530–550 K. Since these conditions cannot presently be realised with our large volume cell, we simply heated the LVPE to about 373 K for a few hours at ambient pressure. The effects of this treatment can be readily observed by comparing the FTIR spectra acquired before and after the thermal annealing reported in Figure 4. In particular the infrared spectra clearly indicate a reduction of the amorphous fraction absorptions due to gauche defects in the 1325–1400 cm–1 frequency region, at 1444 cm–1 and at 1469 cm–1,[18,27,28] a narrowing of both the orthorhombic (doublets at 719 and 731 cm–1 and at 1462 and 1472 cm–1) and monoclinic (shoulders at 717 cm–1 and at 1474 cm–1)[26] crystalline components, an intensification of the orthorhombic bands with respect to the monoclinic ones, which remain substantially unaltered by the thermal annealing, and an inversion of the intensity ratio between the components of the orthorhombic doublets assigned to the C–H rocking (719 and 731 cm–1) and scissoring modes (1462 and 1472 cm–1) of polyethylene.[27] This can be better appreciated from the spectral deconvolution of the frequency region corresponding to the C–H scissoring modes reported in the inset of Figure 4. According to the mathematical fit of the absorption profile of the scissoring region, the amount of amorphous fraction after annealing can be estimated from the integrated areas of the two broad absorptions at 1444 and 1469 cm–1 as ∼45% of the value before the annealing, which was already low. All the absorption bands assigned to the crystalline phases sharpen after annealing and the integrated areas of the two orthorhombic components (1462 and 1472 cm–1) increase by a factor of ∼1.3, whereas that of the monoclinic band remains substantially unaltered. Moreover the intensity ratio between the high (1472 cm–1) and low (1462 cm–1) frequency components, belonging to the orthorhombic C–H scissoring modes, increases according to the theoretically predicted value.[27] The absence of a significant conversion of the monoclinic phase into the orthorhombic one is not surprising, considering the mild annealing conditions, and further confirms the efficiency of the annealing procedure through the hexagonal phase.[23,26] As expected, the integrated absorption of the broad and weak band at 1455 cm–1, assigned to the C–H bending mode of –CH3
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Figure 4. Upper panel: X-ray diffraction pattern from a recovered LVPE, indicating the coexistence of the orthorhombic (Pnam) and monoclinic (A2/m) crystal phases. The Miller indices of the orthorhombic (red) and monoclinic (blue) phases and the corresponding cell parameters are also reported. Lower panel: Room temperature FTIR absorption spectra of LVPE synthesised in this study before (bottom trace) and after (top trace) annealing at 373 K for 2 h. The spectra are vertically shifted and the absorbance units in the ordinate axis are intended only for comparing the relative intensity of the bands in the two spectra. The inset shows the fit of the absorption profile ranging between 1400 and 1500 cm–1 before and after annealing, showing the behavior of the different components: 1444 cm–1 amorphous (a);[28] 1455 cm–1 δ (CH3) bending (δ);[28] 1462 cm–1 orthorhombic (o);[28] 1469 cm–1 amorphous (a);[28] 1472 cm–1 orthorhombic (o);[28] 1474 cm–1 monoclinic (m);[26] (only one component is expected for the A2/m monoclinic phase due to the non-centro-symmetric site symmetry).
groups and related to the chain terminations, remains constant after annealing.
4. Conclusion In this paper we reported a large volume high pressure synthesis of high density polyethylene based only on pressure and photo-excitation, occurring at ambient temperature in the total absence of solvents, catalysts and radical initiators. In particular we realised a laboratory scale pilot reactor, able to replicate the reaction conditions and the
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results obtained in the DAC on micrometric samples.[18] This reactor could be further scaled up for applicative purposes because the required pressure is accessible to the current industrial technology and the two-photon absorption mechanism using near-UV exciting wavelengths allows unpractical vacuum UV conditions to be avoided, even if, at the moment, high pressure photo-induced reactions on large volume systems are still very challenging. Furthermore, the absence of other chemicals offers the advantage of eliminating costs of disposal, developing and purchasing of solvents and catalysts. The crystalline quality of the synthesized HDPE, which can be further enhanced by thermal annealing at 373 K and ambient pressure, makes the material particularly suitable for technological applications. Acknowledgements: The authors would like to acknowledge Dr. Marco Frediani, Dr. Marco Candelaresi, Prof. Paolo Foggi and Dr. Luca Fontana for helpful discussions during the realization of the experimental set up. The authors would like to futher thank Dr. Marco Frediani for providing access to the HTGPC equipment. The authors would like to thank the beamline ID09A at the European Synchrotron Radiation Facility (ESRF) in Grenoble for the ADXRD measurements. This work was supported by Stryker Orthopaedics, Laserlab-Europe (EU-FP7 284464) and the Italian Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR). Received: December 20, 2013; Revised: January 30, 2014; Published online: February 18, 2014; DOI: 10.1002/marc.201300919 Keywords: ethylene polymerisation: green chemistry; high pressure chemistry; photochemistry; polyethylene [1] D. B. Malpass, Introduction to Industrial Polyethylene - Properties, Catalysts, Processes, Wiley, New York 2010. [2] A. J. Peacock, Handbook of Polyethylene - Structures, Properties and Applications, Marcel Dekker, Inc., New York 2000. [3] V. Schettino, R. Bini, M. Ceppatelli, L. Ciabini, M. Citroni, Chemical reactions at very high pressure in Advances in Chemical Physics, Vol. 131 (Ed. S. A. Rice), Wiley, New York 2005, pp. 105–242. [4] V. Schettino, R. Bini, Chem. Soc. Rev. 2007, 36, 869.
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[5] L. Ciabini, M. Santoro, F. A. Gorelli, R. Bini, V. Schettino, S. Raugei, Nat. Mater. 2007, 6, 39. [6] W. Grochala, R. Hoffmann, J. Feng, N. W. Ashcroft, Angew. Chem. Int. Ed. 2007, 46, 3620. [7] M. Santoro, F. A. Gorelli, R. Bini, G. Ruocco, S. Scandolo, W. A. Crichton, Nature 2006, 441, 857. [8] M. I. Eremets, A. G. Gavriliuk, I. A. Trojan, D. A. Dzivenko, R. Boehler, Nat. Mater. 2004, 3, 558. [9] M. Citroni, M. Ceppatelli, R. Bini, V. Schettino, Science 2002, 295, 2058. [10] M. Ceppatelli, A. Serdyukov, R. Bini, H. J. Jodl, J. Phys. Chem. B 2009, 113, 6652. [11] R. Bini, Acc. Chem. Res. 2004, 37, 95. [12] M. Ceppatelli, R. Bini, V. Schettino, Proc. Natl. Acad. Sci. USA 2009, 106, 11454. [13] M. Ceppatelli, R. Bini, V. Schettino, J. Phys. Chem. B 2009, 113, 14640. [14] M. Ceppatelli, R. Bini, V. Schettino, Phys. Chem. Chem. Phys. 2011, 13, 1264. [15] M. Ceppatelli, R. Bini, M. Caporali, M. Peruzzini, Angew. Chem. Int. Ed. 2013, 52, 2313. [16] M. Ceppatelli, S. Fanetti, R. Bini, J. Phys. Chem. C 2013, 117, 13129. [17] M. Santoro, F. A. Gorelli, R. Bini, J. Haines, A van der Lee, Nat. Commun. 2013, 4, 1557. [18] D. Chelazzi, M. Ceppatelli, M. Santoro, R. Bini, V. Schettino, Nat. Mater. 2004, 3, 470. [19] D. Chelazzi, M. Ceppatelli, M. Santoro, R. Bini, V. Schettino, J. Phys. Chem. B 2005, 109, 21658. [20] P. Anastas, J. Warner, Green Chemistry: Theory and Practice, Oxford University Press, New York 1998. [21] J. Smukala, R. Span, W. Wagner, J. Phys. Chem. Ref. Data 2000, 29, 1053. [22] K. Leonhard, T. Kraska, J. Supercrit. Fluids 1999, 16, 1. [23] L. Fontana, D. Q. Vinh, M. Santoro, S. Scandolo, F. A. Gorelli, R. Bini, M. Hanfland, Phys. Rev. B 2007, 75, 174112. [24] K. E. Russell, B. K. Hunter, R. D. Heyding, Polymer 1997, 38, 1409. [25] L. Resconi, L. Cavallo, A. Fait, F. Piemontesi, Chem. Rev. 2000, 100, 1253. [26] L. Fontana, M. Santoro, R. Bini, D. Q. Vinh, S. Scandolo, J. Chem. Phys. 2010, 133, 204502. [27] G. Zerbi, G. Gallino, N. Del Fanti, L. Baini, Polymer 1989, 30, 2324. [28] S. Krimm, Fortschr. Hochpolym.-Forsch. 1960, 2, 51.
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