Influence of compatibilizer precursor structure on the phase distribution of low density poly(ethylene) in a poly(ethylene terephthalate) matrix.
The increased interest of many research groups in the properties of PET/polyolefins blends is due to the necessity of preparing toughened materials from virgin PET [1-6] or developing new opportunities for postconsumer PET recycling [7-10]. In any case, the blending of these polymeric materials gives rise to multiphase systems, characterized by poor properties due to the high incompability. To compatibilize the system, usually a reactive blending approach, consisting in the use during the processing of immiscible PET/polyolefins blends of properly functionalized polymers that is compatible with the poly-olefin phase and reactive towards the PET terminal groups, is followed. The properly functionalized polymer acts as a compatibilizer precursor, since it gives rise to the formation of a few percent by weight of a polyolefin-PET copolymer in the interphase region of the blend, where the high interfacial repulsion between the two immissible polymers needs to be reduced to allow a finer dispersion of one phase into the other, which is necessary for achieving good mechanical performances (11). The compatibilizer precursors are copolymers such as ethyl-cne-glycidyl methacrylate copolymer (4), (12-14) or polymer modified by radicial functionalization in the melt, such as maleic anhydride (8), (15-20) or glycidyl methac-rylate-functionalized (21-23) polyolefins. In the latter case, both functionalized polyolefins (15), (16), (20) or block copolymers containing polystyreme blocks (23) were reported to efficiently improve the compatibility of PET/ polyolefins blends.
A cheap and sustainable reactive blending method for the compatibilization of PET/polyethlene blends consists of the use of diethyl maleate (DEM)-functionalized polyethlene in the presence of Zn[(OOCC[H.sub.3]).sub.2] (24) and Ti[(OBu).sub.4] trnasesterification catalysts. In fact, in both cases, in spite of the occurrence of side reactions, such as degradation of PET in the Zn-catalyzed system (26) and crossling of the compatibilizer precursor in the Ti-cata-lyzed one (25), the formation of grafted copolymer was clearly evidenced. The effect of the transesterification reaction depends on the structure of the metal derivatives and compatibilizer precursor. In particular, both DEM-functionalized ultralow density polyethylene (ULDPE-g-DEM) (24) and styrene-co-ethylene-butylene-co-styrene block copolymer (SEBS-g-DEM) act as compatibilizer precursors (27).
Final properties of blends are due not only to the reactivity of the compatibilizer precursors, but also to the kind of phase morphology developed during processing. The composition, structure, and rheological properties of immiscible polymer pairs influence much the achieved phase morphology in terms of phase distribution, such as dispersed or cocontinuous, and its size (11), (28)], Moreover, the processing parameters, such as time (29) and temperature (30), affect much the concentration range of dual-phase continuity.
The present work reports about the study of the morphological features of virgin and postconsumer PET/ LDPE blends having a PET matrix through an innovative and sustainable method of reactive extrusion. In fact, the blends were extruded in the presence of different amounts of ULDPE-g-DEM or SEBS-g-DEM as precursors and ZnO as transesterification catalyst.
PET Lighter C88 ([[eta]] = 0.730 d1/g) was purchased from Dow. LDPE (MFR = 0.25 g/10 min; density = 0.922 g/c[m.sup.3)) was kindly supplied by Polimeri Europa. ULDPE (Engage 8150) was purchased from DuPont-Dow. SEBS (Europrene Sol TH 212) with 30% by weight of styrene was kindly supplied by Enichem Elastomers. Post-consumer PET (PETR) colored flakes ([[eta]] = 0.758 d1/g,m [M.sub.g] = 22,300 [+.or. -] 400 g/mol) were purchased from Se. Ri. Plase. Postconsumer LDPE (LDPER) pellets (FTO 0.4, MFR = 0.4 g/10 min) were purchased from Aliplast.
ZnO was a 99.5% purity "Carlo Erba" commercial product. Diethyl maleate was a 97% purity "Aldrich" product, purified by distillation in vacuum. 1,1,1,3,3,3-Hexafluoro-2-propanol was a 99% purity Aldrich commercial product.
A Water GPC-V 200 instrument equipped with both refractive index and viscosimetric detectors was used for the analysis of molecular weight [M.sub.n] and [M.sub.w] dispersity index ([I.sub.d] = [M.sub.w]/[M.sub.n]) and the branching ratio g' defined as the ratio between the intrinsic viscosity of the sample and the calculated intrinsic viscosity of the linear polymer. The analysis was carried out in trichlorobenzene at 145[degrees] C.
Rheological measurements were carried out by using a DSR dynamic rheometer of Rheometrics. Molded discs of polymeric material having a 25 mm diameter and 1 m thickness were used. The viscosity was measured at a temperature or 250[degrees]C and in the frequency 1-100 rad/sec range.
Preparation of Compatibilizer Precursors
The ULDPE (Engage 8150) was functionalized in the 50 ml Brabender mixer with a blade rate of 50 rpm at 200[degrees]C by adding the 10% by weight of DEM and the 1% by weight of DCP. The material was purified by extraction with boiling acetone for 8 h and a 1.3% by weight of extracted fraction was obtained. Then the residual fraction was characterized by IR spectroscopy (31).
The SEBS (27), (32) was functionalized in a 50 ml Bra-bender mixer at 220[degrees]C setting a blade rate of 40 rpm and a blending time of 20 min by adding the 16% by weight of DEM and the 2.5% by weight of DCP. It was purified by extraction with boiling acetone and characterized by IR and NMR spectroscopy.
Preparation of Blends
The blends were prepared by self-extrusion using a DSM 15 [cm.sup.3] laboratory micro-extruder. The mixing temperature was set at 250 [degrees] C, the screws rate at 80 rpm, and nitrogen was flowing during the blending. The time of mixing was 20 or 5 min. The ZnO (0.3% by weight) was added as master batch in LDPE. The master batch was prepared at 270[degrees] C, 50 rpm and 5 min as extrusion time.
Characterization of Blends
Dissolution tests were carried out by using 1,1,1,3,3,3,-hexafluoro-2-propanol (HFIP) to extract the soluble PET and recover the dispersed insoluble LDPE particles. The undispersed LDPE was weighted to evaluate the degree of continuity of the polyolefin phase. As the percentage of added compatibilizer precursor was kept in the range 1-10%, the weight of copolymer formed during the extrusion can be neglected (25-27).
The extruded strands were cryofractured in liquid nitrogen in the parallel and perpendicular direction with respect to the extrusion one. The surfaces were coated with gold and analyzed with a Philips XL20 scanning electron microscope.
The cryosmoothing was performed at a temperature of 100 K using a ultramicrotome (Leica Ultracut UCT) equipped with a glass knife. After the etching in toluene for 5 days at 60[degrees] C, the smoothed surface was analyzed by SEM. For each blend, three images were analyzed to consider at least 200 particles. A quantitative analysis of the size of the dispersed particles was carried out using Leica QWin image analysis software.
The TEM analyses were carried out by using a Zeiss Leo EM 900 microscope. The slice of blends of 70 nm thickness obtained by using an Ultramicrotome Leica Ultracut UCT with a diamond knife equipped with Leica EM-FCS Cryosystem were previously treated with vapors of [OsO.sub.4] or [RuO.sub.4] to evidence, respectively, the double bonds or aromatic groups.
The Characterization of the functionalized polymers allowed observing that both ULDPE and SEBS were slightly degraded during the functionalization and that the functionalization degree of ULDPE-g-DEM, expressed as percentage by weight of grafted diethyl maleate, was higher than the SEBS-g-DEM on (Table 1). The decrease of g' for ULDPE-g-DEM compared to ULDPE was in agreement with a certain extent of branching.
TABLE 1. Results of GPC, rheological, and IR characterization of starting materials. Polymers M(n) (g/mol) [I.sub.d] g' ULDPE 97.800 2.07 0.83 ULDPE-g-DEM 71,500 (a) 4.8 0.52 SEBS 71,700 1.24 0.55 SEBS-g-DEM 55,100 (a) 3.6 0.69 PET 21,200 (b) -- -- LDPE 30,300 (a) 21 0.34 Polymers [eta] * (c) at 100 rad/s G' (c) FD (d) (% by weight) (Pa s) (Pa) ULDPE nd nd ULDPE- g-DEM 711 53.137 6.1 SEBS nd nd -- SEBS- g-DEM 364 28,551 4.5 PET 108 733 -- LDPE 476 38,210 -- (a) By GPC measurements. (b) By capillary viscosimetry. (c) By oscillatory rheological measurements. (d) By FTIR or NMR spectroscopy.
In Fig. 1, the flow curve for low shear rate was reported for the compatibilizer precursors and LDPE. The three polymer showed a non-Newtonian behavior also at the lowest shear flow values. The ULDPE-g-DEM was more viscous than LDPE, while SEBS-g-DEM was the less viscous. LDPE showed a shear-sensitive behavior. This result is in agreement with g' values obtained by the GPC analysis. In fact, the LDPE presents long chain branching that allows a more efficient loss of entanglements at high shear flow. The storage modulus G' allowed evaluating the elasticity of the polymer melt. The most elastic polymer was the ULDPE-g-DEM, as its storage modulus was the highest in the range 2-100 rad/sec as shear rate. The SEBS-g-DEM showed a gel-like behavior (more elastic than the two other polymers) at low frequencies where the contribution of the ethylene-butylene rubber block can affect more the rheological behavior. The polystyrene block affects the high frequency behavior determining the decrease of elasticity.
[FIGURE 1 OMITTED]
Phase Morphology Analysis
PET/LDPE blends 70/30 and 80/20 were prepared by using different amounts ranging from 1 to 10% by weight of compatibilizer precursor Table 2. The time of blending 5 and 20 min and the use of virgin or postconsumer polymers were also varied in the different runs.
The level of adhesion between the phases was evaluated by transmission electron microscopy TEM see Fig. 2. The images obtained for the noncompatibilized blend showed the presence of voids between the LDPE and PET phases, which agree with the lack of adhesion between the phases, resulting in the breakage of the ultra thin slice during the preparation of the sample. On the other hand, the compatibilized blends with either ULDPE-g-DEM or SEBS-g-DEM showed the absence of such voids, suggesting an improvement of the interface adhesion, due to the formation of compatibilizer in the interphase region. Hence, both the reactive extrusion methods resulted effective for the improvement of compatibility.
[FIGURE 2 OMITTED]
The phase distribution in the blends-extruded strand was studied by SEM analysis of the cryogenically broken surfaces both perpendicular and parallel to the flow plane.
By following the trend obtained in the perpendicular to the flow images, the same kind of compatibilizing effect for ULDPE-g-DEM and SEBS-g-DEM can be hypothesized Figs. 3 and 4. On the other hand, in the parallel to the flow view, a different trend can be observed: the addition of ULDPE-g-DEM tends to destroy elongated and interconnected structures, whereas the addition of SEBS-g-DEM tends to stabilize them.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
With the aim of performing a quantitative evaluation of the extent of continuity in the polyolefin phase, dissolution tests were performed. This test is essential to evaluate the shape of the polyolefin phase, as in a dispersion, polyolefin forms a dispersed-like morphology in the PET matrix, while the original shape is preserved in a cocontinuous phase morphology.
The results Table 2 revealed the presence of partial cocontinuity in all the 70/30 PET/LDPE blends see Fig. 5. The percentage of continuity was more than 60% for the reference blends and was higher for the SEBS-g-DEM-compatibilized blends. In 80/20 PET/LDPE reference blend see Fig. 6, the LDPE phase is completely dispersed, but the addition of only 1% 0.05 precursor to LDPE weight ratio of SEBS-g-DEM determined the formation of a polyolefin continuous fraction, whereas a dispersed-like phase morphology was obtained in the ULDPE-g-DEM-compatibilized blends independently on the precursor concentration see Fig. 7.
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
TABLE 2. Composition of blends, extrusion time, continuity of the polyolefin phase, and apparent dispersed phase diameter. Blends PET LDPE Compalibilizer precursor (a) Bl 70 30 B2 70 29 l(ULDPE-g-DEM)) B3 70 25 5(ULDPE-g-DEM) B4 70 20 lO(ULDPE-g-DEM) B5 70 30 -- B6 70 29 l(ULDPE-g-DEM) B7 70 25 5(ULDPE-g-DEM) B8 70 20 lO(ULDPE-g-DEM) B9 70 29 l(SEBS-g-DEM) BIO 70 25 5(SEBS-g-DEM) Bll 70 20 10(SEBS-g-DEM) B12 70 29 l(SEBS-g-DEM) BI3 70 25 5(SEBS-g-DEM) BI4 70 20 lO(SEBS-g-DEM) B15 85 15 -- B16 80 20 -- BI7 80 19 l(ULDPE-g-DEM) B18 80 16.7 3.3(ULDPE-g-DEM) B19 80 13,4 6.6(ULDPE-g-DEM) B20 80 19 l(SEBS-g-DEM) B21 80 16.7 3.3(SEBS-g-DEM) B22 80 13.4 6.6(SEBS-g-DEM) B23 80 (b) 20 (b) -- B24 80 (b) 13.4 (b) 6.6(ULDPE-g-DEM) B25 80 (b) 13.4 (b) 6.6(SEBS-g-DEM) Time Residual Continuity % of Diameter [mu]m (c) Blends min traction % the polyolefin phase Bl 5 18.74 62 2.8 [+ or -] 0.1 B2 5 14.8 49 2.6 [+ or -] 0.1 B3 5 3.89 13 2.17 [+ or -] 0.08 B4 5 14.45 48 1.93 [+ or -] 0.05 B5 20 27.16 90 3.7 [+ or -] 0.2 B6 20 20.96 70 3.7 [+ or -] 0.2 B7 20 4.41 15 1.99 [+ or -] 0.05 B8 20 25.78 86 2.09 [+ or -] 0.06 B9 5 29.97 100 6.0 [+ or -] 0.3 BIO 5 30.28 101 5.4 [+ or -] 0.3 Bll 5 34.78 116 6.6 [+ or -] 0.3 B12 20 29.50 98 2.7 [+ or -] 0.1 BI3 20 30.82 103 4.3 [+ or -] 0.3 BI4 20 31.16 104 3.3 [+ or -] 0.2 B15 20 0 0 1.74 [+ or -] 0.03 B16 20 0 0 1.90 [+ or -] 0.03 BI7 20 0 0 1.60 [+ or -] 0.04 B18 20 0 0 1.32 [+ or -] 0.02 B19 20 0 0 1.20 [+ or -] 0.01 B20 20 4.5 22 2.12 [+ or -] 0.1 B21 20 6.5 32 2.54 [+ or -]0.16 B22 20 7.7 38 2.18 [+ or -] 0.11 B23 20 0 0 2.35 [+ or -] 0.04 B24 20 0 0 1.46 [+ or -] 0.01 B25 20 19.9 66 2.43 [+ or -] 0.07 (a) 0.3% by weight of ZnO was added to all blends prepared with a compatibilizer precursor. (b) Blends prepared with postconsumer PET and LDPE. (c) The values correspond to [D.sub.n] = ([summation] [D.sub.i] >/[summation] [n.sub.i] and the reported error was calculated as the standard deviation. PET, poly ethylene terephthalate; LDPE, low density poly ethylene; SEBS-g-DEM, styrene-b-ethylene-co-1-butene-b-styrene copolymer bearing grafted diethyl succinate groups; ULDPE-g-DEM, ultralow density poly ethylene bearing grafted diethyl succinate groups.
The effect of the time of blending was negligible in the SEBS-g-DEM-compatibilized blends. In fact, the curves obtained for different times of blending tend to overlay especially in the range 1-6% by weight of copolymer precursor. Moreover, a higher mixing time favors the formation of a cocontinuous phase morphology in the reference blends. This cannot be attributed to the degradation of the PET during blending. In fact, the molecular weight [M.sub.n] of PET extracted from the uncompatibilized blends prepared by setting 5 or 20 min as time of mixing, determined by viscosimetry in phenol/tetrachloroethane solution (26), resulted to be very similar 18,800 and 18,600, respectively. The trend of continuity of LDPE/PET 30/70 blends see Fig. 5 compatibilized with ULDPE-g-DEM showed an evident decrease till a minimum point for 5% by weight of compatibilizer precursor and the successive increase of continuity in the 10% by weight compatibilized blends. Moreover, in this latter case, the effect of time of extrusion was not negligible, suggesting the possible influence of the kinetic of formation of ULDPE-g-PET graft copolymer onto the percentage of continuity.
The smoothed and toluene-etched .surfaces of blends were analyzed by SEM, and in the ULDPE-g-DEM-com- patibilized system, a decrease of the diameter see Fig. 8 with the increase of precursor concentration was observed till a limit precursor to LDPE weight ratio value of 0.17. The further addition of ULDPE-g-DEM did not result in a decrease of the diameter. This effect is known to be caused by formation of micelles, as the copolymer chains form micelles in the bulk instead of positioning in the interphase region. In the SEBS-g-DEM-compatibilized system, an increase of the apparent diameter till a maximum value for precursor to LDPE weight radio of 0.17 was observed.
[FIGURE 8 OMITTED]
The distribution curve of ULDPE-g-DEM-compatibilized 80/20 PET/LDPE blends see Fig. 9 was narrower than the uncompatibilized one. A different trend was observed in the presence of SEBS-g-DEM see Fig. 10, which tends to give a polymodal distribution curve, with also modes above 5 [micro]m, in agreement with a more complex morphological structure.
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
Some PET/LDPE/SEBS-g-DEM-g-DEM blends gave a ringlike structure after the dissolution test Fig. 11b. To explain this result, the cryogenically broken surface was analyzed in the center and near to the edge of the extruded strand. The images obtained in this way Fig. 11a showed that the morphology was almost dispersed-like in the center of the extruded strand, while a cocontinuous interconnected structure could be easily noticed close to the edge. Hence, the cocontinuous morphology was preferentially generated in the region near the wall of the die, where the shear flow was lower, and elongated structures (33) were thus more stable.
[FIGURE 11 OMITTED]
Similar results, in terms of SEM characterization, dissolution tests and morphometric analysis, were observed for both virgin and postconsumer PET/LDPE blends.
The results of the diameter analysis agree with the dissolution tests results and with the SEM analysis of the fractured surfaces. In fact, the dispersed phase size decreased for the ULDPE-g-DEM-compatibilized system, whereas an increase of the dispersed phase diameter and a higher level of complexity of its distribution curve were observed in SEBS-g-DEM-compatibilized blends. Hence, the SEBS-g-DEM precursor stabilized a cocontinuous phase distribution, whereas the ULDPE-g-DEM favored a dispersed-like phase morphology. This difference can be explained on the basis of different considerations.
The viscosity [eta] of the SEBS-g-DEM is lower than the ULDPE-g-DEM one. Hence, by assuming the LDPE/compatibilizer precursor as the polyolefin phase, its phase inversion point [empty set] .sub.PO] is described by the Eq. 1.
The lower viscosity of the SEBS-g-DEM can determine the slight shifting of the inversion point towards lower [empty set] .[sub.PO], allowing the extension of cocontinuity. On the other hand, the use of ULDPE-g-DEM determines an increase of the viscosity of the polyolefin phase, with the shifting of [empty set] .[sub.PO] to higher values. This reduces the cocontinuity extent, leading to a higher fraction of dispersed LDPE.
The effect of the use of ULDPE-g-DEM is in good agreement also with the general tendency of a compatibilizer to give a narrower cocontinuity range (28). This effect can contribute to the observed partial collapse of the cocontinuous structure.
On the contrary, in the SEBS-g-DEM-compatibilized blends, the tendencies of the system to selforganize in a cocontinuous structure were observed also in the blends containing only the 1% of compatibilizer precursor. In this case, the viscosity of the polyolefin phase is practically not affected by the presence of the compatibilizer precursor. Hence, the decrease of the viscosity of the LDPE/SEBS-g-DEM is probably not sufficient to explain the results, and the elasticity of the molten phases must also be considered. From this point of view, the SEBS-g-DEM is the less elastic component (11) and it can be deformed more easily to yield elongated structures, the formation of which can allow an easier development of a cocontinuous phase distribution.
Furthermore, the effect of compatibilization and its kinetic should be considered. From this point of view, the SEBS-g-DEM probably stabilizes the metastable elongated structure formed at the beginning of extrusion because of the faster kinetic of copolymer formation in the interfacial region. In fact, differences obtained in different blending times are in agreement with a faster assessment of the phase morphology in the SEBS-g-DEM-compatibilized system, thanks to its capability to migrate in the PET/LDPE interphase region during the blending. This SEBS-g-DEM capability can derive from its lower viscosity with respect to ULDPE-g-DEM allowing an easier polyethylene phase covering. In fact, in the ternary LDPE/P/PET system the viscosity ratio [eta].sub.P]/[eta]ldpe should be less than 1 to favor the encapsulation of LDPE with P (34). The SEBS and functionalized SEBS (35) show these two characteristics in PET matrix blends with a polyethylene dispersed phase.
The level of miscibility of LDPE/SEBS-g-DEM and LDPE/ULDPE-g-DEM can also affect the copolymer formation kinetics. In fact, the lower miscibility of P in the LDPE phase with the simultaneous attaining of a suitable phase distribution can allow a higher concentration of the ester groups grafted on the precursor in the interfacial region. Anyway, the functionalization with DEM of ULDPE modified more the interfacial tension values between it and LDPE than the modification of SEBS. As on the basis of interfacial tension data reported in literature (35), (36), 9[sigma].sub.LDPE]/[[sigma].sub.SEBS-g-DEM] is lower than [sigma].sub.LDPE/ULDPE-g-DEM], and SEM and TEM analysis showed clearly see Fig. 12 that the LDPE/SEBS-g-DEM system consisted of two separated phases, a similar phase distribution can be also hypothesized for the LDPE/ULDPE-g-DEM system.
[FIGURE 12 OMITTED]
The study of phase morphology of extruded virgin or postconsumer PET matrix blends containing LDPE ranging from 20 to 30% by weight in the presence of ZnO and different amount of SEBS-g-DEM or ULDPE-g-DEM as compatibilizer precursor allowed evidencing that the adopted methods of reactive compatibilization were effective for the improvement of phase adhesion. Moreover, in the case of the use of SEBS-g-DEM, cocontinuous phase distributions were preferentially obtained, whereas the use of ULDPE-g-DEM resulted in dispersed ones. The differences were tentatively explained on the basis of the different rheological properties of the precursors, as the SEBS-#-DEM has a lower viscosity and elasticity in the melt than the ULDPE-g-DEM. The former feature influences the phase inversion point, while the latter characteristic makes the SEBS-g-DEM more capable of developing elongated structure, whose presence is necessary for the development of a continuous phase. Moreover, SEBS-g-DEM probably develops a lower interfacial tension in the PET/LDPE/compatibilizer precursor ternary system.
In the SEBS-gDEM-compatibilized blends, the cocontinuous morphology is preferentially generated in the region near the wall of the die, where the shear flow is lower, and elongated structures are thus more stable.
The results obtained in the present study evidence the possibility of modulating the phase morphology of virgin or postconsumer PET/LDPE polymer blends by properly selecting the reactive compatibilization method by keeping into consideration the structure, properties, and reactivity of the compatibilizer precursor.
M.B.C. thanks the Katholieke Universiteit of Leuven for supporting her work. Dr. Giovanna Costa and Dr. Lucia Conzatti, of the CNR-ISMAC of Genova, are thanked for their help in TEM characterization. Prof. Francesco Ciardelli is also thanked for helpful discussion.
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Correspondence to: M.B. Coltelli; e-mail: firstname.lastname@example.org DOI 10.1002/pen.21118
Published online in Wiley InterScience www.interscience. .interscience.wiley.com. c2008 Society of Plastics Engineers
Maria-Beatrice Coltelli, 1 Charef Harrats, 2 Mauro Aglietto, 3 Gabriel Groeninckx 4
1 Centro Italiano Packaging and Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, 56126 Pisa, Italy
2 Eindhoven University of Technology, Chemical Engineering and Chemistry, 5600 MB Eindhoven, The Netherlands.
3 Dipartimento di Chimica e Chimica Industries, Universita di Pisa, 56126 Pisa, Italy
4 Laboratory of Macromolecular Structural Chemistry, Department of Chemistry, Katholieke Universiteit Leuven, B-3001 Heverlee, Belgium
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|Author:||Coltelli, Maria-Beatrice; Harrats, Charef; Aglietto, Mauro; Groeninckx, Gabriel|
|Publication:||Polymer Engineering and Science|
|Article Type:||Technical report|
|Date:||Jul 1, 2008|
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