Supercritical Carbon Dioxide Assisted Solid-State Grafting Process of

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Ind. Eng. Chem. Res. 2005, 44, 4292-4299

Supercritical Carbon Dioxide Assisted Solid-State Grafting Process of Maleic Anhydride onto Polypropylene Tao Liu,† Guo-Hua Hu,*,‡,§ Gang-sheng Tong,† Ling Zhao,† Gui-ping Cao,† and Wei-kang Yuan*,† UNILAB Research Center of Chemical Reaction Engineering, State Key Laboratory of Chemical Reaction Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China, Laboratory of Chemical Engineering Sciences, CNRS-ENSIC-INPL, 1 rue Grandville, B.P. 451, 54001 Nancy, France, and Institut Universitaire de France, Maison des Universite´ s, 103 Boulevard Saint-Michel, 75005 Paris, France

The work reported in this paper aimed at exploring the advantages of using supercritical carbon dioxide (scCO2) as an environmentally benign solvent and swelling agent for carrying out the grafting process of maleic anhydride onto polypropylene in the solid state. The effects of scCO2 on the melting temperature and melting enthalpy of an isotactic polypropylene (iPP) were investigated first in order to define the upper reaction temperature and CO2 pressure limits. The effects of various factors on the grafted anhydride content and on changes in the molecularscale and microscale structures of the resulting iPP were then investigated. Those factors included the reaction time, monomer and initiator concentrations, reaction temperature, CO2 pressure, and size of the iPP particles. Results showed that the scCO2-assisted solid-state grafting process of maleic anhydride onto iPP did have some scientifically interesting and industrially relevant advantages over the classical solid-state or melt process. Among them, it is worth pointing out that the CO2 pressure itself constituted an additional and sensitive process parameter capable of significantly modifying the overall reaction pathway and the product quality. For example, without CO2, the solid-state grafting process was diffusion-controlled. Under scCO2, it became reaction-controlled. The CO2 pressure could also regulate the anhydride content with ease. On the other hand, the degree of iPP chain scission was not reduced under scCO2 compared to that of the classical melt process. 1. Introduction Free-radical grafting of maleic anhydride (MAH) onto polypropylene (PP) has been an important industrial practice and has been studied by several conventional processes such as the solution process,1 melt process,2,3 and solid-state process.4 Grafting of polar monomers such as MAH onto PP1-5 is a useful postpolymerization method for functionalizing PP without significantly changing the molecular architecture of the polymer backbone. Therefore, the intrinsic properties of PP could be maintained, and its applications could thus be expanded. However, each of these conventional processes has its own drawbacks. For example, a solution process suffers from the problem of using and subsequently removing organic solvents from the final product. A melt process encounters severe PP chain scission. Furthermore, because MAH is slightly soluble in molten PP, efficient mixing is required and the degree of maleation is often limited and difficult to control. The main problem with a solid-state process is that the grafting reaction is controlled by the diffusion of the monomer into the bulk of the polymer and that the final product is often heterogeneous. * To whom correspondence should be addressed. Fax: 33383-32-29-75 (G.-H.H.) or 86-21-6425-3528 (W.-k.Y.). E-mail: [email protected] (G.-H.H.) or [email protected] (W.-k.Y.). † East China University of Science and Technology. ‡ LSGC-CNRS-ENSIC-INPL. § Institut Universitaire de France.

These drawbacks may be overcome by the use of scCO2. The latter has many unique properties such as nonflammable, nontoxic, and relatively inexpensive. An innovative use of scCO2 may open up a bright future for physical and chemical processing of materials. The use of scCO2 as a carrier for delivering small molecules into or extracting small molecules from solid matrixes is a well-established technique.6-11 At the properly chosen densities, scCO2 dissolves nonpolar or not exceedingly polar compounds and plasticizes and/or swells polymer matrixes. In this way, the diffusion resistance is reduced and the solute may be dissolved in the amorphous domains of the bulk polymer at the molecular level. The subsequent release of pressure may lead to the entrapment of the finely dispersed solute in the polymer structure. The density of scCO2, and thus its solvent strength, is continuously tunable from gaslike to liquidlike by changing the temperature or pressure. This provides the ability to control the degree of swelling of a polymer as well as the partitioning of smallmolecule penetrants between the swollen polymer phase and the fluid phase. The low viscosity of a supercritical fluid allows for rapid mass transfer of penetrants into a swollen polymer. Moreover, because CO2 is a gas at ambient conditions, removal of the solvent from the final product is facilitated. This may be used to one’s advantage to extract undesired molecules from the final product. All of these are favorable to modification of polymers by using scCO2 as the swelling agent. The work reported in this paper aimed at exploring the advantages of scCO2 as an environmentally benign

10.1021/ie0501428 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/05/2005

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solvent and swelling agent for carrying out the grafting process of MAH onto PP in the solid state. The effects of scCO2 on the melting temperature and melting enthalpy of an isotactic PP (iPP) were investigated first in order to define the upper reaction temperature and CO2 pressure limits. The effects of various factors on the grafted anhydride content and on changes in the molecular-scale and microscale structures of the resulting iPP were then investigated. Those factors included the reaction time, monomer and initiator concentrations, reaction temperature, CO2 pressure, and size of the iPP particles. 2. Experimental Section Materials. The iPP pellets with an average diameter of 3-4 mm were supplied by Shanghai Petrochemical Co. The mass-average molar mass (Mm) of iPP was 188 700 g/mol. Its polydispersity index was 5.1. The iPP powder (average diameter of smaller than 0.15 mm) used in this study was prepared from the above pellets by grinding with liquid nitrogen as the coolant. MAH (purity: 99.5%) and a dicumyl peroxide (DCP) initiator (chemical grade) were commercially available from Shanghai Chemical Co. CO2 (purity: 99.9%) was obtained from Air Product Co. MAH and CO2 were used as received. DCP was purified from chloroform before use. Assessment of the Upper Temperature and CO2 Pressure Limits. In order for iPP to be in the solid state under the grafting reaction conditions, the reaction temperature should not exceed its melting temperature, Tm. The evolution of Tm as a function of the CO2 pressure was measured with a high-pressure differential scanning calorimeter (DSC) of type NETZSCH DSC 204 HP, Germany. The calorimeter was calibrated by carrying out the measurement of the melting points and the heat of fusion of In, Bi, Sn, Pb, and Zn under ambient and high CO2 pressures, respectively. Before the DSC measurement, the iPP powder was purified by Soxhlet extraction in acetone for 24 h and then dried in a vacuum oven at 90 °C for 10 h. For each DSC measurement, about 10 mg of the iPP powder was heated to 200 °C at a rate of 10 °C/min. The phase behaviors of the CO2 + MAH and CO2 + DCP binary systems were studied by Paulaitis and Alexander,12 Hayes and McCarthy,7 and Galia et al.9 According to their results, under the grafting conditions adopted in this work, the CO2 + MAH + DCP system was expected to be homogeneous. Grafting Reactions. A high-pressure vessel made from stainless steel was used (Figure 1). A magnetic stirring bar was mounted in the bottom of the vessel to ensure mixing. The internal volume of the vessel was 20.39 cm3, calibrated with distilled water by a syringe pump with the magnetic stirring bar in place. The vessel was placed in a homemade electronically controlled temperature oil bath. The temperature of the latter was measured with a calibrated mercury thermometer at an accuracy of (0.02 °C and was controlled at an accuracy of (0.5 °C. The vessel pressure was measured at an accuracy of (0.05 MPa by a pressure transducer of Shenzhen MSI/JL Electronics Co., China. To graft MAH onto iPP, the iPP powder (2.0 g), MAH, and DCP were placed in the vessel. The latter was then sealed and carefully washed with low-pressure CO2. Thereafter, the right amount of CO2 was charged. The CO2 loading was achieved by a DZB-1A syringe pump

Figure 1. Schematic of the experimental setup for the supercritical CO2-assisted solid-state grafting of MAH onto PP.

of Beijing Satellite Instrument Co., China, at an accuracy of 0.01 cm3. The reaction vessel was immersed in the oil bath and heated to the desired reaction temperature. To sample the reaction system, the vessel was quenched with cold and running water and samples were taken out for characterization. Characterization. Before characterization, both virgin and MAH-modified PPs (PP-g-MAH) were all purified by Soxhlet extraction in acetone for 24 h and then dried in a vacuum oven at 90 °C for 10 h in order to eliminate the unreacted MAH and other volatile molecules. Extraction in refluxing acetone for 72 h and then drying in a vacuum oven at 90 °C for 20 h yielded the same result. During the above treatment, the hydrolyzed anhydride group to its corresponding open carboxylic acid group was also completely cyclized. The anhydride content of PP-g-MAH was determined by the titration of the acid groups after complete hydrolysis of the anhydride groups.13,14 In a typical procedure, 0.5 g of a purified PP-g-MAH sample was dissolved in 80 mL of xylene at boiling temperature, with 50 µL of water and the same volume of pyridine added to cause full hydrolysis of the anhydride functions.13 The system was kept refluxing for 3 h before it was titrated at a temperature above 80 °C with 0.01 N ethanolic KOH and phenolphthalein as an the indicator. The PP-g-MAH samples were always fully soluble and did not precipitate during the titration. After the titration, the anhydride content was calculated by the following equation:

GMAH % )

98.06(VPP-g-MAH - Vvirgin)CKOH × 100% 2 × 1000 × m (1)

where CKOH (mol/L) is the molar concentration of the KOH-ethanol standard solution, VPP-g-MAH (mL) is the volume of the KOH-ethanol standard solution needed to titrate the PP-g-MAH sample, Vvirgin (mL) is the volume of the KOH-ethanol standard solution needed to titrate the virgin iPP, m (g) is the mass of the PP-gMAH sample for titration, and 98.06 is the molar mass of MAH. Fourier transform infrared (FTIR) spectroscopy was also used to estimate the anhydride content in PP-gMAH by using a calibration curve. The measurements were made on an EQUINOX 55 FTIR spectrometer system (Bruker Co., Germany). Polymer films used for the FTIR measurements were prepared by compression molding in a laboratory hot press. The molar masses of the virgin PP and PP-g-MAH were measured by gel

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Figure 2. High-pressure DSC diagrams of iPP used in this work at different CO2 pressures during the first melting round.

Figure 3. Dependence of the melting temperature of iPP on the CO2 pressure.

Table 1. Effect of the CO2 Pressure on the Melting Behavior of the Virgin PP Measured by High-Pressure DSCa

of CO2, the molar volume of the repeating unit of the polymer, the interaction parameter, the equilibrium heat of fusion, and the gas constant, respectively. From eq 2, Tm can be written as follows:16

PCO2, MPa

Tm, °C

∆H, J/g

PCO2, MPa

Tm, °C

∆H, J/g

0 1.0 5.0 20.0 40.0

168.4 168.1 167.5 165.3 163.3

98.6 98.1 99.3 92.2 92.7

60.0 80.0 100.0 120.0 140.0

160.8 159.0 156.6 153.9 150.6

88.2 91.9 79.9 97.3 94.2

a

Tm (°C) ) 168.2-0.12P (bar).

permeation chromatography (PL-GPC220, Polymer Lab, U.S.A.) at 150 °C with 1,2,4-trichlorobenzene as a solvent. The thermophysical properties of the virgin PP and PP-g-MAH were measured using DSC of type NETZSCH DSC200 PC under an ambient nitrogen atmosphere. On the first heating round, the sample was heated to 200 °C at a rate of 10 °C/min and then annealed at that temperature for 5 min. After annealing, the system was cooled at a rate of 10 °C/min to 80 °C. It was then heated again to 200 °C at 10 °C/min. 3. Results and Discussion Upper Reaction Temperature and CO2 Pressure Limits. Figure 2 shows the high-pressure DSC curves of iPP under different CO2 pressures. As expected, the DSC curve shifted to the low-temperature side with increasing CO2 pressure. Table 1 shows the melting temperatures and fusion enthalpies of iPP under different CO2 pressures corresponding to the peaks and areas of the DSC curves in Figure 1, respectively. As expected, the melting temperature of iPP decreased with increasing CO2 pressure. However, the fusion enthalpy was little affected by the CO2 pressure. This tends to suggest that CO2 did not modify or dissolve the crystalline regions of iPP. Figure 3 shows that Tm of iPP decreased linearly with increasing CO2 pressure. This is expected from the Flory-Huggins theory. According to this theory, Tm of plasticized semicrystalline polymers with sufficiently high molar massess can be correlated to the volume fraction of the plasticizer (υ1)15 by

(

)( )

( )

1 R V2u 1 1 ) (1 - χν1) Tm Tm° ν1 ∆Hm° V1

(2)

where Tm°, V1, V2u, χ, ∆Hm°, and R are the equilibrium melting temperature of the polymer, the molar volume

Tm )

Tm° 1 + K(ν1 - χν12)

(3)

with K ) (RTm°/∆Hm°)(V2u/V1). Under our conditions, ν1 values are on the order of a few percent at most. Thus, ν1 . χν12 and the above equation could be reduced to

Tm ≈

Tm° ≈ Tm°(1 - Kν1) 1 + Kν1

(4)

According to Fardi et al.,17 the solubility of CO2 in iPP obeys Henry’s law over a CO2 pressure range of 5-20 MPa and the Henry’s constant (H) of the iPP/CO2 system is independent of temperature. Thus, ν1 at P (bar) is

Vr Vr ωHP ωHP Vig Vig Vr ≈ FHP ν1 ) ) Vr ω ω Vig ωHP + F Vig F

(5)

where ω, F, Vig, and Vr are the weight, the specific gravity of the polymer sample employed, the molar volume of the ideal gas ()22 400 cm-3), and the molar volume of the CO2 dissolved in the polymer, respectively. One finally arrives at the following equation:

( )( )

Vr RTm° V2u Tm ) Tm° - Tm° FHP Vig ∆Hm° V1

(6)

The above equation shows that Tm of iPP is expected to decrease linearly with increasing CO2 pressure. The straight line in Figure 3 indicates the upper reaction temperature and CO2 pressure limits. For example, according to this straight line, if the reaction temperature is to be fixed at 150 °C, the CO2 pressure should not exceed 150 bar in order for iPP to be in the solid state. Chemical Characterization of PP-g-MAH. FTIR spectroscopy is frequently adopted to characterize MAH-

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Figure 4. FTIR spectra of the virgin iPP (curve 1), unpurified PP-g-MAH (curve 2), and purified PP-g-MAH (curve 3).

Figure 5. Calibration curve for the anhydride content in PP-gMAH as a function of the ratio between the FTIR absorbance of the peak at 1790 cm-1 and that at 808 cm-1. Relevant coefficient ) 0.9780, and anhydride content % ) 0.5165(A1790/A808).

modified polymers.13 Figure 4 compares the spectra of the virgin iPP (spectrum 1), a PP-g-MAH taken from the reaction vessel after a certain period of time without purification (spectrum 2), and PP-g-MAH subjected to purification (spectrum 3). In spectra 2 and 3, additional peaks appeared compared to spectrum 1. The ones at 1860 and 1790 cm-1 corresponded to the asymmetric and symmetric stretching of the carbonyl bond of the grafted anhydride. The one at 704 cm-1 was due to the out-of-plane bending vibration of the C-H bonds of the anhydride ring. A peak at 758 cm-1 appeared only in spectrum 2 and corresponded to the DCP absorbance. After purification, the unreacted MAH and residual DCP were eliminated from the polymer matrix. As a result, that peak was not found in spectrum 3. The carbonyl bands determined in this study were in good agreement with those of Galia et al.9 To determine the grafted anhydride content in PP-g-MAH by FTIR, a calibration curve was constructed that related the titration results to those of FTIR. It was based on the anhydride content obtained by titration and the ratio between the absorbance of the peak at 1790 cm-1 (A1790) and that of the methyl group at 808 cm-1,18 as shown in Figure 5. Effect of the Size of the iPP Particles on the Grafting Reaction Kinetics. Figure 6 shows the grafted anhydride content as a function of the reaction time for iPP of two different sizes at two different

Figure 6. Effect of the size of the iPP particles on the grafting kinetics. PP/MAH/DCP ) 100/2.5/1.25.

temperatures. The first iPP was in the form of pellets of 3-4 mm in diameter and the second one in the form of powder obtained from the above pellets by grinding. Its diameter was smaller than 0.15 mm. The mass ratio of iPP/MAH/DCP was 100/2.5/1.25. Whatever the size of the iPP particles, the anhydride content first increased with increasing reaction time and then reached an equilibrium in about 5 h at 130 °C and in about 3 h at 150 °C. The fact that the overall grafting kinetics was independent of the size of the iPP particles implies that, under scCO2, MAH and DCP could diffuse into the PP particles so rapidly that the overall reaction was not diffusion-controlled but reaction-controlled. This is in contrast to the classical solid-state process in which the overall reaction kinetics is diffusion-controlled. Thus, the scCO2-assisted solid-state grafting process has an obvious advantage over the classical solid-state process in that the final product obtained by the former process is more uniform in the grafted anhydride content than that obtained by the latter because in the latter the overall reaction kinetics is diffusion-controlled, giving rise to heterogeneity in the grafting reaction. This will be further discussed later. In Figure 6 is also shown the amount of the primary radicals formed from the decomposition kinetics of DCP as a function of time at 130 and 150 °C, respectively. It can be expressed by

I0 - I ) 1 - exp(-kdt) I0

(7)

where I0 is the initial concentration of DCP, I that at time t, and kd the decomposition rate constant. Short of data in scCO2, the decomposition rate constant in benzene was taken as a reference. It was 1.015 × 10-4 s-1 at 130 °C and 9.55 × 10-3 s-1 at 150 °C.19 It is seen that the overall grafting kinetics in terms of the increase in the anhydride content followed more or less the decomposition kinetics of DCP. This supports the above conclusion that the overall grafting kinetics was not diffusion-controlled and implies that it was controlled primarily by the decomposition kinetics of DCP or the primary free-radical formation kinetics. From Figure 6, it is also noted that the final anhydride content at 150 °C was higher than that at 130 °C. The reasons for that difference will be addressed later when the effects of the temperature and CO2 pressure on the final anhydride content are examined.

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Figure 7. Effect of the initial MAH concentration on the anhydride content at 130 °C and 17.5 MPa of CO2 pressure. DCP/ iPP ) 1.25 g/100 g.

Figure 8. Effect of the initial DCP concentration on the anhydride content at 130 °C and 17.5 MPa of CO2 pressure.

Effects of the Monomer and Initiator Concentrations on the Anhydride Content and Molar Masses. Figure 7 shows the effects of the initial MAH concentration on the grafted anhydride content and molar masses of PP-g-MAH after 5 h of reaction at 130 °C and 17.5 MPa. The initial DCP concentration was 1.25 g/100 g of iPP, and the initial MAH concentration was varied from 0 to 6.25 g/100 g of iPP. Initially, an increase in the initial MAH concentration led to a significant increase in the anhydride content. When the initial MAH was somewhere between 2 and 4, the anhydride content reached a maximum. A further increase in the initial MAH concentration brought about a decrease in the anhydride content. A similar phenomenon was observed in the maleation of polyolefins in scCO2.9 As for the effect of MAH on the molar masses of PP-g-MAH, the basic trend is that an increase in MAH led to a slight decrease in Mm and Mn and a slight increase in the polydispersity index, Mm/Mn. Similar trends were reported in the literature for the same system but without scCO2.3 The explanation was that at low MAH concentrations the grafting took place mainly in the radical chain ends arising from the β scission of PP, whereas at high MAH concentration, it occurred mainly at the tertiary carbons of PP.20 Figure 8 shows the effect of the initial DCP concentration on the grafted anhydride content after 5 h of

Figure 9. Effect of the CO2 pressure on the anhydride content in PP-g-MAH at three different reaction temperatures: (9) 130 °C; (O) 140 °C; (2) 150 °C. iPP/MAH/DCP mass ratio ) 100/2.5/ 1.25.

reaction at 130 °C and 17.5 MPa. The initial MAH concentration was 2.5 or 3.75 g/100 g of iPP, and the initial DCP concentration was varied from 0 to 5 g/100 g of iPP. For both initial MAH concentrations, the evolution of the grafted anhydride content followed the same trend. It first increased rapidly with increasing initial DCP concentration up to 1-1.5 g/100 g of iPP and then leveled off. The existence of a plateau for the anhydride content would be related to the fact that the CO2-swollen iPP became saturated with the initiator at some critical concentration. A further increase in the initiator concentration would then have no effect on its concentration in the CO2-swollen iPP. Effects of the Reaction Temperature and Pressure on the Anhydride Content. Figure 9 shows the anhydride content in PP-g-MAH as a function of the CO2 pressure at 130, 140, and 150 °C. The mass ratio of iPP/ MAH/DCP was 100/2.5/1.25, and the CO2 pressure was varied between 10 and 30 MPa. From this figure, one can make two very interesting remarks. First, the three anhydride content versus CO2 pressure curves obtained at the above three temperatures all followed the same trend. Second, at each temperature, with increasing CO2 pressure, the anhydride content first increased, reached a maximum at a certain CO2 pressure, and then decreased. The value of the CO2 pressure corresponding to the maximum in CO2 pressure depended on the reaction temperature. All of these results indicate that besides the process parameters already discussed above, the CO2 pressure constituted an additional and important process parameter that could control the anhydride content in PP-g-MAH to a significant extent and with ease. This is also an important advantage of scCO2assisted solid-state grafting of MAH onto PP over the classical solid-state or melt process. How does one explain the existence of the maxima in the anhydride content as a function of the CO2 pressure? Under our experimental conditions, iPP was in the solid phase (the crystalline domains remained unchanged) but was swollen to a significant extent by scCO2. An increase in the CO2 pressure led to an increase in the specific density of scCO2, which resulted in an increase in its solvation power and in the degree of swelling of the iPP particles. The former was favorable to the partitioning of MAH and DCP in CO2, whereas the latter was favorable to the partitioning of MAH and

Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4297 Table 2. Molar Masses of the Virgin PP and PP-g-MAH Measured by High-Temperature GPCa sample designation 0 (virgin iPP) 1 (T ) 130 °C; PCO2 ) 19.65 MPa) 2 (T ) 130 °C; PCO2 ) 15.95 MPa) 3 (T ) 150 °C; PCO2 ) 13.30 MPa) 4 (T ) 150 °C; PCO2 ) 19.20 MPa) 5 (T ) 190 °C; classical melt grafting) 6 (T ) 210 °C; classical melt grafting) 7 (T ) 130 °C; classical solid-state grafting)

GMAH %

Mm

Mn

0.28 0.42 0.21 0.45 0.37

188 700 108 900 103 100 163 900 104 300 135 500

36 700 22 100 19 300 22 200 21 500 25 700

0.32

142 100

29 000

0.14

120 400

31 400

a The composition to obtain the PP-g-MAH samples was the same: iPP/MAH/DCP ) 100/2.50/1.25 by mass.

DCP in the iPP particles. At a given temperature, those two opposing effects would lead to a maximum in the anhydride content corresponding to a given CO2 pressure. In practice, it is important to determine this “optimum pressure” because there is no need to apply uselessly a CO2 pressure higher than it. Molar Masses of PP-g-MAH. Table 2 shows the mass-average and number-average molar masses, Mm and Mn, of the virgin iPP and PP-g-MAH produced at different grafting conditions. Samples 5 and 6 were produced by the classic melt process in a Haake mixer at 190 °C for 15 min and at 210 °C for 10 min, respectively. Sample 7 was obtained by the classical solid-state grafting process at 130 °C for 8 h using a small amount of xylene as a solvent to help the diffusion of MAH and DCP into the iPP particles. The initial compositions were the same, i.e., iPP/MAH/DCP ) 100/ 2.5/1.25 by mass. Compared to the virgin iPP, the molar masses of all of the PP-g-MAH samples were smaller. Moreover, the scCO2-assisted solid-state grafting process did not show any advantage over the classical solidstate or melt process in reducing the degree of PP chain scission. In fact, the degree of PP chain scission as measured by the molar masses depended mainly on the grafted anhydride content in PP-g-MAH. The higher the grafted anhydride content, the higher the degree of chain scission. Recall that Figure 7 shows that an increase in the initial MAH concentration from 0.5 to 6.25 g with respect to 100 g of PP also led to a slight increase in the degree of chain scission. The absence of the beneficial effect of the scCO2-assisted solid-state free-radical grafting of MAH onto PP on the reduction of the degree of PP chain scission was not expected. It was confirmed by the molar mass distribution curves of three PP-g-MAH samples obtained by three different processes: scCO2-assisted solid-state grafting (sample 4), classical melt grafting (sample 5), and classical solidstate grafting (sample 7) (see Figure 10). All three PPg-MAH samples showed very similar molar mass distributions. Thermophysical Properties of PP-g-MAH. Table 3 shows selected thermophysical properties of the virgin PP and some PP-g-MAH obtained by the scCO2-assisted solid-state grafting process. Figure 11 compares the melting behavior of the virgin iPP with that of PP-gMAH having similar anhydride contents but produced at different reaction temperatures and at a given scCO2 pressure. On the first melting round (Figure 11a), the values of the melting enthalpy, and thus those of the crystallinity of PP-g-MAH, were all higher than those of the virgin iPP. Moreover, the melting temperatures of PP-g-MAH produced at 130 and 140 °C were slightly

Figure 10. Molar masses of the virgin iPP and selected PP-gMAH. Table 3. Temperature and Enthalpy of Fusion Melting and Recrystallization of the Virgin PP and PP-g-MAH Measured by DSC melting round sample designation

GMAH %

Tm, °C

∆H, J/g

0 (virgin iPP) 8 (T ) 130 °C; PCO2 ) 17.05 MPa) 9 (T ) 130 °C; PCO2 ) 23.05 MPa) 10 (T ) 130 °C; PCO2 ) 17.10 MPa) 11 (T ) 140 °C; PCO2 ) 21.70 MPa) 12 (T ) 150 °C; PCO2 ) 24.40 MPa)

0.11 0.32 0.53 0.51 0.54

168.4 167.8 167.3 166.5 168.0 174.8

-98.6 -108.4 -113.0 -116.1 -112.0 -111.2

cooling round T c, °C

∆H, J/g

118.8 101.8 114.9 95.1 115.8 99.3 113.8 97.1 113.5 95.1 113.7 99.7

lower than that of the virgin iPP. The endothermal peak became narrower than that of the virgin PP as the reaction temperature increased. That of PP-g-MAH produced at 150 °C was very different from those of the two others. There were two distinct endothermal peaks instead of one: a broad peak at a lower melting temperature and a sharp one corresponding to a melt temperature of 174.8 °C, with the latter being much higher than that of the virgin PP. A similar phenomenon was reported in the literature concerning the maleation of poly(4-methyl-1-pentene).7 Wide-angle X-ray diffraction analyses (Rigaku D/maX-RB, Japan) were conducted to ascertain whether a new crystalline phase was formed or not. Figure 12 compares the X-ray results between the virgin PP and the above PP-g-MAH produced at 150 °C. Apparently, no new crystalline phase was formed except that some crystals became more regular, as shown by more distinct peaks 4 and 5. Thus, the two endothermal peaks on the DSC diagram for PPg-MAH obtained at 150 °C could be the result of the creation of two distinct populations of more and less regular crystals of the same overall unit cell. Separate experiments showed that under scCO2, at 140-160 °C, PP could undergo recrystallization, leading to an increase in crystallinity, more regular crystals, and higher melting temperature. This is particularly so for the PPg-MAH produced at 150 °C, on which the effect of scCO2induced PP recrystallization was very strong. Its melting temperature reached as high as 174.8 °C. On the cooling round (Figure 11b), the crystallization temperatures of PP-g-MAH containing similar anhydride contents were similar to each other and were much lower than that of the virgin PP. Their enthalpies of crystallization were slightly lower than that of the virgin PP. The decrease in the crystallization temper-

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assisted solid-state grafting process has two main advantages over the classical solid-state process and the classical melt process. The first one is that the CO2 pressure is an extra process parameter capable of easily regulating and controlling the grafted anhydride content of PP-g-MAH. The second one is that it produces PPg-MAH much more uniform in anhydride content than the classical solid-state process does. We have also shown that the scCO2-assisted solid-state grafting process is not capable of reducing the degree of PP chain scission, a major problem facing the classical melt grafting process. This was not expected. Acknowledgment The authors are grateful to the Ministry of Science and Technology of China for the support of a major project for international cooperation (Grant 2001CB711203), to the National Science Foundation of China and PetroChina for the support of a joint project on multiscale methodologies, and to the Association FrancoChinoise pour la Recherche Scientifique et Technique (AFCRST) for the support of PRA Mx04-05. Literature Cited

Figure 11. DSC diagrams of the virgin iPP (curve 0), sample 10 (curve 10), sample 11 (curve 11), and sample 12 (curve 12).

Figure 12. XRD diagrams of the virgin PP and a PP-g-MAH obtained at 150 °C.

ature and in the crystallization enthalpy was likely due to the fact that the grafting reaction disrupted more or less the regularity of the iPP chains. 4. Conclusion We have explored the advantages of using scCO2 as an environmentally benign solvent and swelling agent for carrying out the grafting process of MAH onto PP in the solid state. We have shown that the scCO2-

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Received for review February 4, 2005 Revised manuscript received April 7, 2005 Accepted April 11, 2005 IE0501428