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Ind. Eng. Chem. Res. 2002, 41, 5934-5940
γ-Radiation-Initiated Polymerization of Vinylidene Fluoride in Dense Carbon Dioxide Alessandro Galia, Giuseppe Caputo,† Giuseppe Spadaro, and Giuseppe Filardo* Dipartimento Ingegneria Chimica Processi e Materiali, Viale delle Scienze, 90128 Palermo, Italy
The γ-ray-initiated batch polymerization of vinylidene fluoride (VF2) has been investigated in dense carbon dioxide under relatively mild operative conditions (T e 40 °C and P < 25 MPa). When the initial VF2 molar concentration was increased from 3.4 to 6.4 mol/L, monomer conversion increased from 20 to 73%; a similar trend was observed for the number-average molecular weight and the molecular complexity of synthesized poly(vinylidene fluoride) (PVDF) as determined from rheological measurements. Under all adopted experimental conditions, a synthesized PVDF polymer was collected in the form of a white powder. Despite the inherent heterogeneous character of the polymerization process, a homogeneous free-radical chain kinetics model can be used to fit kinetic data collected at 30 °C and 3.4 mol/L VF2 loading, with evidence of heterogeneous-phase polymerization being constituted by the onset of an acceleration period. Introduction Chain-radical polymerization of fluorinated vinyl monomers is a key synthetic route for the preparation of high-performance specialty polymers exhibiting the unique combination of excellent chemical resistance, high thermal stability, low dielectric constant and dissipation factor, unusual surface properties, low water absorptivity, excellent weatherability, and low flammability. All of this bonanza arises from the effect of fluorine substituents in the chain, which, on the other hand, leads to a more difficult control of the polymerization process with respect to that observed in the case of hydrocarbon homologues. Owing to the high electrophilicity of fluorinated macroradical species propagating the chain, a strong kinetic competition exists between the addition of fluorinated free radicals to π bonds of unreacted monomer and the chain-transfer reactions involving H-atom donor species.1 Fluoropolymers are usually obtained through heterogeneous (emulsion or suspension) polymerization techniques in aqueous systems. As a consequence of the adopted aqueous initiators, unstable carboxylic acid and acid fluoride end groups are generated which owing to their thermal instability must be removed before processing the polymer melt. A possible way to reduce the level of thermally unstable end groups is the use of aprotic solvent such as chlorofluorocarbons (CFCs) or perfluorocarbons, perfluoroalkyl sulfide, and perfluorinated cyclic amines, which, however, are too expensive and/or not environmentally friendly.2 In this context, a sustainable alternative to such volatile organic compounds (VOCs) is constituted by dense CO2, a low cost nontoxic solvent, inert to chaintransfer reaction in free-radical polymerization processes, which can be easily used both in the liquid state * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: 0039-0916567257. Fax: 00390916567280. † Current address: Dipartimento Ingegneria Chimica e Alimentare, University of Salerno, Via Ponte Don Melillo, 84084 Fisciano, Salerno, Italy.
and in the supercritical region where medium density can be continuously changed from vaporlike to liquidlike values by tuning the intensive parameters T and P without the appearance of an interface. Other thermodynamic and transport properties such as enthalpy, entropy, dielectric constant, viscosity, and self-diffusion coefficient exhibit a similar behavior and can be continuously modulated as a function of density assuming values between those of liquids and gases.3 As a consequence of the nonideal behavior, a fluid component in the supercritical region (SCF) is able to dissolve a solute in larger amount with respect to what could be computed on the base of its vapor pressure and the solubility is strongly dependent on the value of the density, which, in turn, determines the solvating properties of the medium. It must be pointed out that solubilities are generally much lower with respect to those of conventional liquid solvents, but this parameter can become of minor relevance if the performances of the process can be improved thanks to the tunability of solvent properties and the enhanced mass-transfer kinetics due to the lower viscosities and higher diffusion coefficients of SCFs with respect to liquid condensed phases. When CO2 is considered as a polymerization medium, its solvent strength toward the reagents and the products is of major concern. Carbon dioxide is a low dielectric solvent which behaves with good approximation like a hydrocarbon solvent for what concerns the solubility of volatile nonpolar molecules of low molar mass. On the other hand, it is a weak Lewis acid, and it has a significant quadrupole moment which gives a significant contribution to its solubility parameter, thus allowing it to dissolve some polar molecules such as methanol. As a result, CO2 is a good solvent for most vinyl monomers but is an exceedingly poor solvent for most high molar mass polymers. The only classes of polymers which have exhibited good solubility in supercritical CO2 under relatively mild conditions (T < 100 °C and P < 35 MPa) are amorphous fluoropolymers, silicones, and, more recently investigated, polycarbonate-polyether copolymers.4,5 Owing to this solubility consideration, carbon dioxide has been used as a freeradical homogeneous polymerization medium only for
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highly fluorinated amorphous polymers,6 while heterogeneous techniques have been adopted in the case of other vinyl monomers as reported in recent exhaustive reviews.7,8 In addition to solubility consideration, a further relevant feature of dense CO2 to be taken into account is its capability of highly plasticizing polymers, thus lowering their glass transition temperature (Tg). The plasticization effect is particularly important in heterogeneous processes where polymer coagulum, with relatively high local viscosity, are formed during the process. The plasticization effect leads to an increase in the free volume of the polymer chains, which, in turn, increases the value of the diffusion coefficient of the species inside the coagulated particle, thus allowing the progression of the polymerization process up to a high value of monomer conversion. Also, translational and segmental diffusivities of the growing polymer chains are enhanced by this effect, and this leads to a shift in the onset of the gel effect at a higher value of the conversion or to a practical suppression of it with a consequent advantage for the thermal control of the process. Poly(vinylidene fluoride) (PVDF) is the second most important thermoplastic within the fluoropolymer family after poly(tetrafluoroethylene) (PTFE). Even if thermal and chemical stabilities of PVDF are somewhat lower with respect to PTFE, the hydrogenated polymer can be easily processed on conventional equipment, thus leading to an advantageous compromise between quality and price. PVDF homopolymer is largely used in pipes and various equipment for the chemical industry to handle aggressive chemicals. Distribution facilities of ultrapure water and chemicals used in the semiconductor industry are another important market with increasingly severe requirements concerning the level of contamination and surface smoothness in order to avoid microorganism buildup. The synthesis of PVDF in a continuous stirred tank reactor (CSTR) has been recently investigated in supercritical CO2 in order to model the rate of polymerization in the case of a thermally initiated free-radical chain process.9 In the literature are reported numerous examples of free-radical polymerization of vinyl monomers initiated by γ photons10 which allows one to carry out the process in the absence of a chemical initiator whose fragment should be incorporated in polymer chains and under very mild conditions because no thermal activation of the initiation step is required. The γ-initiated heterogeneous polymerization of bulk liquid VF2 has been investigated by Roberts et al.11 Moreover, carbon dioxide has been proved substantially stable in the presence of ionizing radiations,12,13 thus possessing the additional advantage of being chemically inert under radiative conditions up to a high value of absorbed dose. Prompted by these considerations, we have investigated the γ-radiation-initiated polymerization of VF2 in dense CO2 in the attempt of synthesizing high-purity high molecular weight PVDF. Experiments have been carried out under batch conditions with monomer molar concentration ranging from 3.4 to 6.4 mol/L, temperature from 20 to 40 °C, and average density of the reaction system from 0.66 to 0.90 g/mL. A set of experiments has been aimed at investigating the polymerization kinetics at 30 °C, and a model has been developed to fit conversion and molecular weight data.
Experimental Section Materials. Vinylidene fluoride monomer was kindly donated by Solvay Research, Brussels, Belgium; CO2 was Sol 99.998% pure. Both chemicals have been used without further purification. Phase Behavior Investigation Apparatus. To study the phase behavior of the CO2/VF2 mixture in the adopted reaction conditions, a constant-volume view cell was assembled. A stainless steel high-pressure view cell having a volume of 14.5 mL, equipped with two 1 cm thick sapphire windows with a 180° orientation, and 5 cm optical length was used. The temperature control was ensured by inserting the cell in an electronically controlled water bath, while the pressure was recorded by means of a pressure transducer. The cell, equipped with a magnetic stirrer, was pressurized, at room temperature, by an ISCO syringe pump with weighted amounts of VF2 and CO2. The cell was subjected to a thermal cycle consisting of a gradual heating over the transition temperature and a slow gradual cooling to room temperature. Blank experiments with pure CO2 demonstrated that the equilibrium temperature is reached within 3 min. Polymerization Apparatus. The experiments have been carried out in a 43 cm3 AISI 316 jacketed reactor. Prior to charging inside the reactor the desired weighed amount of VF2, the reaction chamber was purged by gaseous CO2. The selected final density of the reaction system, evaluated as (mass of CO2 + mass of VF2)/ reactor volume, was achieved by adding the proper mass of carbon dioxide. Irradiation was performed in a 3000 Ci 60Co irradiation facility which has been described elsewhere14 having an activity of 1000 Ci at the time of the experiments. The temperature and pressure control during irradiation was ensured by a safety and control system based on a PID controller inserted in a cascade control loop. The polymerization was allowed to proceed for a reaction time varying from 2 to 8 h. After the irradiation, the reactor was quenched in an ice/water bath and the gas was bubbled in water in order to trap solid PVDF entrained by the fluid. A white polymer powder was obtained under all adopted experimental conditions. The yield was determined gravimetrically. No purification was necessary before storage of the polymer product. Polymer Characterization. Dynamic mechanical tests were carried out in the melt and in the solid state by a RDA2 Rheometrics analyzer used in the dynamic mode. Tests in the melt state (frequency sweep tests) were carried out with the plate and plate geometry (R ) 12.5 mm) at 200 °C and 5% strain in the angular frequency range 10-1-5 × 102 rad/s. Tests in the solid state were carried out in the torsion mode, with an angular frequency of 31.4 rad/s, in the temperature range of -70 to -10 °C and with a temperature step of 1.5 °C/min. Solubility tests were done in a Soxhlet extractor using N-methylpyrrolidone (NMP), close to its boiling point, as a solvent. The extraction time was 48 h. Molecular weights of polymers were determined through GPC measurements by Solvay Research, Belgium, by using as an eluent solution of N,N-dimethylformamide and 0.1 M LiBr at 40 °C on a WatersAlliance HPLC system with two HR5E and one HR2E columns.
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Table 1. Effect of the Monomer Concentration on the γ-Ray-Initiated VF2 Polymerization in Dense CO2a [VF2], mol/L
initial pressure, MPa
3.4 4.7 6.2
24 19 18
X, %
Mn, kg/mol (MWD)
gel fraction
20.0 42.5 73.0
140 (2.5) 600 (8.7) unsoluble
0% partially soluble 100%
a T ) 40 °C; system density ) 0.9 g/mL; dose rate ) 7 kGy/ min; irradiation time ) 8 h. X: conversion of monomer. Mn: number-average molecular weight. MWD: polydispersity index of molecular weight distribution.
Experimental Results Phase Behavior of the Mixture CO2/VF2. Vinylidene fluoride and carbon dioxide have critical parameters respectively of 30.1 °C, 4.43 MPa and 31.1 °C, 7.38 MPa. Obviously, any polymerization experiments at temperature higher than the CO2 critical temperature are carried out in an initially homogeneous single phase. The initial phase configuration of the reaction system at 20 and 30 °C under adopted composition conditions has been investigated in a fixed-volume view cell. Only at 20 °C and 0.66 g/mL density does the CO2/VF2 mixture exist as a liquid-vapor system. Visual observation shows that the vapor phase is present in the dead volume of the valve and the pressure transducer connected to the cell constituting about 4% of the total volume of the cell. On this basis, it seems reasonable to assume that the liquid phase is the initial main locus of polymerization so that the polymer powder is substantially nucleated in a liquidlike dispersing medium. Under all other experimental conditions, the reaction system is initially constituted by a single phase even if vapor-phase nucleation cannot be excluded as a consequence of composition modification induced by the polymerization process. Precipitation Polymerization. Effect of the Monomer Concentration. Free-radical polymerization of VF2 in the presence of CO2 leads to the formation of polymer insoluble in the reaction system under experimental conditions adopted in this study9,15 so that PVDF synthesis should be attained in a precipitation polymerization. A first set of experiments were carried out at three different values of the monomer molar concentration, taking constant the system density. Table 1 provides the effect of the monomer concentration on its conversion (X) and number-average molecular weight (Mn). Polymerization were carried out with a reaction system density value of 0.9 g/mL and initial VF2 concentrations of 3.4, 4.7, and 6.2 mol/L. Both the conversion of the monomer and the molecular weight of the synthesized polymer increase with the initial monomer molar concentration at a fixed reaction time; moreover, gel extraction measurements on collected polymer samples suggest that the marked increase of the molecular weight of the sol fraction is accompanied by an increased density of cross-links, resulting in an insoluble polymer at the highest VF2 initial concentration investigated. Dynamic-mechanical tests in the melt state have been performed on the synthesized polymer and on linear high-crystallinity commercial PVDF samples having Mn ) 80 kg/mol, Mw ) 226 kg/mol, and Mw/Mn ) 2.83. Only samples obtained at 3.4 mol/L exhibit a Newtonian behavior at low frequencies and a slope of the non-Newtonian portion which, compared with those of
Figure 1. Flow curves of PVDF samples obtained at different initial monomer concentration. Experimental conditions are as in Table 1. The flow curve of a linear commercial PVDF sample (Mn ) 80 kg/mol, Mw ) 226 kg/mol, Mw/Mn ) 2.83) is reported for comparison.
Figure 2. Loss moduli of PVDF samples obtained at different initial monomer concentrations. Experimental conditions are as in Table 1.
reference samples, suggests that a polymer with rather low long-chain branching frequency is obtained. On the other hand, PVDF synthesized in experiments at higher monomer loading exhibits the typical viscosity behavior of branched and cross-linked polymers, characterized by a steep linear decrease of the melt viscosity as the frequency increases in a logarithmic scale (Figure 1). The marked difference in the polymer architecture observed at increased monomer loading and conversion in reaction systems of fixed density could be attributed to the effect of the ionizing radiation which is able to generate free radicals from the polymer itself. In bulk polymerization this reaction becomes relevant at high values of conversion when the polymer concentration in the reaction system is high and can lead to the formation of a cross-linked chain if the macroradical is formed from the scission of a side group of the polymer molecule.11 To collect more information on the effect of the monomer concentration on the properties of the fluorinated polymer, we have analyzed the PVDF samples through dynamic-mechanical tests in the solid state to determine the effect of polymerization conditions on the glass transition temperature. In Figure 2 the loss factor curves obtained in dynamic torsion mode experiments are reported as a function of temperature for samples obtained at monomer concentrations of 4.7 and 6.2 mol/ L. When the temperature is increased, the loss factor curve crosses a maximum which is correlated to the glass transition of the polymer. A lower value of the loss modulus peak can be observed for the sample obtained at 6.2 mol/L, suggesting that the unsoluble PVFD is less
Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002 5937
Figure 3. Conversion and number-average molecular weight (Mn) of PVDF samples obtained at different irradiation times in γ-rayinitiated batch polymerization in dense CO2. Experimental conditions: T ) 30 °C, system density ) 0.8 g/mL; dose rate ) 7 kGy/ min; VF2 initial concentration ) 3.4 mol/L.
able to dissipate energy by viscous damping as a consequence of an increased density of cross-linking, leading to a more rigid structure with less degree of freedom for molecular motion of the chain segment. On the other hand, this difference in the structure seems to have no relevance on the glass transition temperature of synthesized polymers because the loss moduli G′′ peaks occur almost at the same value of temperature for both of the samples. Kinetics of γ-Ray-Initiated VF2 Polymerization. To investigate the kinetics of VF2 polymerization in dense CO2, the reaction progress was monitored by analyzing the conversion and the molecular weight of polymer samples obtained from stopping different reactions at irradiation times from 2 to 8 h. Experiments have been performed with an initial monomer molar concentration of 3.4 mol/L (22% w/w with the adopted density of the reaction medium) so that a completely soluble polymer was synthesized in all polymerization tests. The values of conversion and number-average molecular weight as a function of the irradiation time are plotted in Figure 3. Recently, Charpentier et al.9 have investigated the thermally initiated continuous polymerization of VF2 in supercritical CO2, and they have found that homogeneous free-radical kinetics is able to fit the experimentally determined rate of polymerization despite the heterogeneous nature of the reaction system. On the basis of this observation, we have tried to model our experimental data by adapting a similar approach to a batch polymerization process initiated by γ radiations. In homogeneous radiation chain free-radical polymerization, the initiation step is brought about by the absorption of the radiation energy which triggers several secondary processes, leading to the formation of free radicals. Under the adopted experimental conditions, considering the inertness of CO2 to γ radiations, the hypothesis that free radicals are generated only in the radiolysis of the monomer has been done. This assumption could be questionable in the case of polymerization processes performed at higher monomer loading where bulklike conditions tend to be approximated and, as suggested by the higher complexity of the polymer architecture, the rate of free-radical generation from polymer radiolysis may become significant also considering the higher values of conversion attained at the same dose value (Table 1).
For what concerns chain-transfer reactions, only intramolecular back-biting reactions leading to shortchain branching could be significant because their occurrence cannot be excluded on the basis of the dynamic-mechanical measurements in the melt state. It has been observed that chain-transfer reactions to polymer can lead to a decrease in the value of the effective propagation rate constant as a consequence of the lower reactivity of nonterminal carbon-centered radical species.16 In the case of VF2 polymerization, the difference in reactivity of R1CF2-CH2-CF2• and R1CF2CH•-CF2R2 should be smaller with respect to that of the corresponding hydrocarbon homologues as a consequence of the substituent effect of fluorine. This consideration leads us to neglect as a first approximation the occurrence of any sort of chain-transfer reaction to the polymer. Under the aforementioned hyphotheses, the γ-rayinitiated polymerization processes can be described by the following kinetic scheme in
Initiation step M f 2R• ki
R• + M 98 P•
Rd )
d[R•] ) φI[M] dt
(1)
•
• ] Ri ) d[P dt ) ki[R ][M] ) φI[M] (2)
where in eq 2 is implicit the assumption that the decomposition step 1 is the rate-determining step in the sequence.
Propagation step kp
• Rp = - d[M] dt = kp[P ][M]
P• + M 98 Pn•
(3)
by assuming that radical reactivity is independent of the chain length also in the presence of a γ radiative field and that the monomer consumed in eqs 1 and 2 can be neglected with respect to that used in eq 3
Termination step ktc
Pn• + Pm• 98 Pn+m
combination Rtc ) -
ktd
Pn• + Pm• 98 Pn + Pm
d[Pn•] • • dt ) 2ktc[Pn ][Pm ] (4)
disproportionation
Rtd ) -
d[Pn•] • • dt ) 2ktd[Pn ][Pm ] (5)
In the previous scheme, R• and M represent the initiator radical and monomer, respectively, while P•, Pn•, and Pm• are growing radical chains and Pn+m, Pn, and Pm are dead polymer chains. The factor φ represents the molar rate of production of radicals that are successful in initiating chains by eq 2, per unit monomer molar concentration and exposure dose rate I. The two termination reactions (combination and disproportionation) can be described by a unique kinetic reaction
Rt ) 2kt[Pn•][Pm•] ) 2kt[P•]2 where kt ) ktd + ktc
(6)
It is generally accepted that the concentration of free-
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heterogeneous character of the process. This acceleration effect is often observed in free-radical polymerization of vinyl monomers in a medium where the resulting polymer is not soluble17,18 and is interpreted on the basis of an impeded termination step owing to the coiled state assumed by precipitated polymer chains, leading to an efficient shielding of the active chain end. To further test the homogeneous modeling of VF2 polymerization, we have extended our approach to the fitting of number-average molecular weight data. In this case too the previously described kinetic scheme has been adopted in order to formulate a suitable equation describing the experimental data because again chain-transfer reactions to polymer can be neglected because they do not alter the values of Mn compared to linear chains. The instantaneous kinetic chain length ν can be expressed as the ratio between the rate of polymerization and the rate of initiation that under steady-state conditions for free-radical concentration must equate the termination rate:
ν)
Figure 4. Plot of 1/(1 - X)1/2 versus time. Test of eq 10. The points are experimental data; the line is a linear least-squares fit of the points. (a) Experiments with X < 0.05. (b) Experiments with X > 0.13. The experimental conditions are the same as those shown in Figure 3.
radical species becomes essentially constant very early in the reaction as radicals are produced and consumed at equal rates. Under this steady-state condition, ri ) rt and eqs 2 and 6 may be equated to solve for [P•]:
[P•] )
(
φI [M] 2kt
)
1/2
(7)
When eq 3 is substituted into, the expression of the rate of polymerization is obtained:
Rp ) -
( )
1/2
φI[M] d[M] ) kp dt 2kt
[M] ) kpol[M]3/2
(8)
( )
Rp 1 ) kp Ri 2ktφI
1/2
[M 0(1 - X)]1/2
(11)
ν can be related with the instantaneous degree of polymerization Xn by a linear relationship
Xn ) aν + (1 - a)2ν ) Kν
(12)
where a is a coefficient which takes into account the kinetic competition between combination and disproportionation termination reactions, leading respectively to polymer chains of length equal to 2ν and ν. On the other hand, we can remember that the polymerization rate can be expressed in terms of conversion as in eq 9 that can be rearranged under the following form:
(
)
φI 0 dX ) kp M dt 2kt
1/2
(1 - X)3/2
(13)
By dividing eq 13 by eq 12, we can express the instantaneous degree of polymerization as a function of conversion:
By introducing the conversion, we can write the following mass balance equations which can be integrated over time:
1 dX φI ) (1 - X) Xn dt K
( )
When Mu is denoted as the repeat unit molecular weight, this equation can be integrated under the form
Rp ) M 0
dX φI ) kp dt 2kt
1/2
[M 0(1 - X)]3/2
( )
kp φIM 0 )1+ 2 2kt x1 - X 1
(9)
1/2
t
(10)
According to the model, we have plotted the value of the function 1/(1 - X)1/2 computed from experimental conversion data versus the irradiation time, and we have obtained two different linear portions respectively at low (Figure 4a; X < 0.05) and higher (Figure 4b; X > 0.13) conversion values. Also in our case the polymerization kinetics can be described by a model based on homogeneous chain growth even if the acceleration effect corresponding to the marked variation of the slope of the kinetic expression can be considered to be indirect evidence of the
∫0tMuXn dt ) ∫0X
KMu dX φI 1 - X
(14)
(15)
Considering that by definition the number-average molecular weight is
∫0tMuXn dt ) Mnt
(16)
we can arrange the equation in the form
M nt ) -
KMu ln(1 - X) φI
(17)
We have then plotted our experimental data in the form
Ind. Eng. Chem. Res., Vol. 41, No. 24, 2002 5939 Table 2. Effect of the Temperature on the γ-Ray-Initiated VF2 Polymerization in Dense CO2a temp, °C
initial pressure, MPa
X, %
Mn, kg/mol (MWD)
20 30 40
15 17 19
41 39 42
344 (5.0) 607 (8.8)
a Initial VF concentration ) 4.7 mol/L; system density ) 0.8 2 g/mL; dose rate ) 7 kGy/min; irradition time ) 8 h. X: conversion of monomer. Mn: number-average molecular weight. MWD: polydispersity index of molecular weight distribution.
Figure 5. Plot of Mnt versus -ln(1 - X). Test of eq 17. The points are experimental data; the line is a linear least-squares fit of the points. The experimental conditions are the same as those shown in Figure 3.
of the product Mnt as a function of -ln(1 - X) as shown in Figure 5. Data fit quite well with the linear trend. Experimental kinetic data collected by other researchers9 in the investigation of continuous precipitation polymerization of VF2 in supercritical CO2 suggest the presence of a contaminant acting as an inhibitor in the monomer supplied by Solvay or that the monomer itself can behave as an inhibitor because a highly electrophilic growing macroradical can abstract a hydrogen atom from the vinyl monomer to generate a CF2CH• radical which is claimed to be inactive for practical purposes. Modeling of our experimental results has been performed without considering such transfer reactions, and a quite good fitting has been obtained. The different experimental behavior observed could be dependent on the different adopted initiation method: in our operative conditions, the process is virtually initiated by γ photons which are continuously supplied to the reaction system during the whole time of the synthesis so that any adventitious chain-transfer contaminant could be consumed during the early stage of the polymerization process, leading to an induction period in the conversion versus time curve. On the other hand, if the chain-transfer agent is the monomer itself, a lower value of the steady-state concentration of active radicals should be present during the whole process as a result of the reaction ktr
Pn• + CF2CH2 98 Pn + CF2CH• so that the radical balance becomes
( ) {x 1/2
φI [P ] ) [M] 2kt •
1+
ktr2M 8ktφI
x } ktr2M 8ktφI
(18)
(19)
Even if a too limited number of experimental points have been collected, the good fitting obtained both for conversion and molecular weight dependent data on the basis of a simple homogeneous kinetic model suggests that, under our operative conditions, the contaminant, if present at all, is consumed in a small time scale compared to the whole reaction time or the rate of the chain-transfer reaction to the monomer is so slow that ktr , ktφI and the growing macroradical concentration is substantially unaffected by the process. Effect of Temperature and Medium Density. The effect of the polymerization temperature has been analyzed by carrying out three runs in the temperature range 20-40 °C (Table 2).
Figure 6. Effect of the reaction temperature on the dynamic viscosity of PVDF samples. Experimental conditions are the same as those shown in Table 2. Table 3. Effect of the Reaction System Density on the γ-Ray-Initiated VF2 Polymerization in Dense CO2a density, g/mL
X, %
Mn, kg/mol (MWD)
initial pressure, MPa
0.66 0.80
24.0 29.5
200 (5.7) 219 (4.0)
7 18
a Initial VF concentration ) 3.4 mol/L; T ) 30 °C; dose rate ) 2 7 kGy/min; irradiation time ) 8 h. X: conversion of monomer. Mn: number-average molecular weight. MWD: polydispersity index of molecular weight distribution.
Because an initial [VF2] of 4.7 mol/L was adopted, PVDF samples are partially insoluble and molecular weights quoted in the table refer to the soluble fraction of the polymer. Monomer conversion results were unaffected by the increase of the polymerization temperature, while a marked increment in the number-average molecular weight and polydispersity index of the soluble fraction of polymer has been detected. These polymers too have been characterized through dynamic-mechanical tests in the melt state (Figure 6). All PVDF samples do not exhibit Newtonian behavior at low frequencies, and the strain frequency slopes of the curves increase as the temperature increases, thus suggesting that all of the samples have a complex molecular structure with a pronounced branching and cross-linking degree increasing with the temperature. The precipitation polymerization of VF2 initiated by γ rays was conducted at two different densities, as shown in Table 3. The polymerization was carried out at 30 °C by changing the CO2 amount and taking constant the molar concentration of VF2 at a value of 3.4 mol/L. As the density changes from 0.66 to 0.8 g/mL, the pressure increases from 7 to 18 MPa. Variation of the conversion and molecular weight distribution is so limited that it is questionable if an enhancement of polymerization kinetics as a consequence of pressure increase has been achieved.19
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The yield values suggest that, under investigated fluid mixture compositions, the system density has only a slight effect on the polymerization kinetics. A significant variation can be observed on the molecular weight distribution that decreases with an increase of the density. Conclusion Using γ radiation as the initiating agent, it was possible to perform the batch polymerization of VF2 in dense carbon dioxide under relatively mild conditions (T e 40 °C and P < 25 MPa) in an extremely pure reaction system. Monomer conversions ranging between 17 and 70% were obtained as a function of the initial VF2 molar concentration which significantly affected also the structure of the synthesized polymer chains (molecular weight, molecular weight distribution, and branching). In the domain of investigated operative conditions, the reaction temperature seems to affect the architecture of the PVDF polymer without substantially changing the kinetics of monomer consumption and a slight effect of the reaction system density on the performance of the polymerization was observed. Even if the polymerization process is inherently heterogeneous, under adopted experimental conditions, kinetic data are well described by a homogeneous free chain model as previously reported by other researchers for thermally initiated VF2 polymerization in CSTRbased continuous reaction systems. In our case, however, the heterogeneous phase character of the process is pointed out by the acceleration effect, which is observed at a high enough value of monomer conversion. Acknowledgment Financial support from Brite/Euram Project BE974520 BRPR-CT97-0503 is gratefully acknowledged. Literature Cited (1) Dolbier, W. R. Structure, Reactivity and Chemistry of Fluoroalkyl Radicals. Chem. Rev. 1996, 96, 1557. (2) Romack, T. J.; DeSimone, J. M.; Treat, T. A. Synthesis of Tetrafluoroethylene-Based, Nonaqueous Fluoropolymers in Supercritical Carbon Dioxide. Macromolecules 1995, 28, 8429. (3) Levelt Sengers, J. M. H. Supercritical Fluids: Their Properties and Applications. In Supercritical Fluids Fundamentals and
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Received for review February 12, 2002 Revised manuscript received May 8, 2002 Accepted May 8, 2002 IE020138L
