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Direct fluorination of poly(vinyl trimethylsilane) and poly(phenylene oxide)
Journal of Fluorine Chemistry 93 (1999) 129±137
Direct ¯uorination of poly(vinyl trimethylsilane) and poly(phenylene oxide) a
A.P. Kharitonova,*, Yu.L. Moskvina, V.V. Teplyakovb, J.D. Le Rouxc
Institute of Energy Problems of Chemical Physics (Division) of the Russian Academy of Sciences, Chernogolovka, Moscow region 142432, Russia b Topchiev Institute of Petrochemical Synthesis of the Russian Academy of Sciences, Leninskii Prospect, 29 Moscow, Russia c Innovative Membrane Systems Inc., 189 Dean Street, Norwood, MA 02062, USA Received 12 June 1997; accepted 14 September 1998
Abstract Fundamental features of the direct ¯uorination of poly(vinyl trimethylsilane) (PVTMS) and poly(phenylene oxide) (PPO) have been investigated. The in¯uence of the ¯uorination conditions (¯uorinating mixture composition, treatment duration) and polymer molecular weight on the rate of formation of the ¯uorinated layer and on physical±chemical properties (chemical composition, density, refractive index, absorption spectra in the visible and UV, surface energy) was investigated. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Direct ¯uorination; Poly(vinyl trimethylsilane); Poly(phenylene oxide)
1. Introduction Direct ¯uorination is a well-known method for surface modi®cation of polymers [1±3]. This paper continues our investigations on the direct ¯uorination of polymer ®lms. The following polymers have been previously investigated by Kharitonov et al: poly(ethylene terephthalate) (PET) [4], polystyrene (PS) [5±10], poly(4-methyl-pentene-1) (PMP) [11], poly(vinyl trimethylsilane) (PVTMS), poly(vinyl trimethylsilane)±polydimethysiloxane block-copolymer (PVTMS-PDMS) [10,12,13], poly(phenylene oxide) (PPO) [13], polycarbonatesiloxane (PCS) [10,14,15] and blockcopolymer of polysulfone and polybutadiene (``SERAGEL1'') [15,16]. In this paper fundamental features of the direct ¯uorination of PVTMS and PPO are described. 2. Experimental PPO was supplied by Aldrich1 (USA) (MW 244 000 and MW 34 000). PVTMS (MW 1 000 000) was supplied by Kuskovo's Plant (Russia). PPO has only C±C, conjugated and C±H bonds. On the contrary PVTMS has also C±Si bonds and its behavior under the action of ¯uorine differs
*Corresponding author. Fax: +7-095-1378-258; e-mail: [email protected]
very much from PPO:
Two types of samples were used: free ®lms (thickness 5± 50 mm) and ®lms deposited from solution onto zinc selenide supports (thickness 2±10 mm). Zinc selenide is stable to ¯uorine and is transparent over the range 20 000± 500 cmÿ1. Fluorination has been carried out in closed vessels (volume 300±1000 cm3). The ¯uorine used had less than 0.1% impurities (mainly oxygen). A polymer sample was inserted into the closed vessel, evacuated to (2± 3)10ÿ2 Torr during 0.5±1 h to remove oxygen and water vapors. Then the closed vessel was ®lled with ¯uorinating mixture and the sample was treated during the necessary time interval. After treatment the vessel was evacuated. Fourier IR spectrometer model 1720 (Perkin Elmer) was used to measure IR spectra. The special reaction vessel equipped with zinc selenide windows was designed to measure the IR spectra during the ¯uorination or in vacuum. In this case the sample was not subjected to atmospheric moisture action. Refractive indices were measured by refractometer RF-454B (Lomo, Russia). To measure the surface energy the method proposed in [17] was used. The spectra in the visible and UV spectral ranges were measured by spectrometer Specord UV±VIS (Karl Zeiss Jena, Germany).
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S0022-1139(98)00278-4
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3. Results and discussion To study the in¯uence of direct ¯uorination on the chemical composition of polymers we used IR spectroscopy. Identi®cation of the virgin PVTMS bands was made on the basis of [18±21]: C±Si(CH3)3 ± 747 cmÿ1, Si±CH3 ± 831, 1247, 1259 and 1403 cmÿ1, tertiary H ± 1338 cmÿ1, CH3 and CH2 ± 2899 and 2953 cmÿ1, respectively (Fig. 1, curve 1). The main characteristic features of the spectra of PVTMS treated with undiluted ¯uorine (spectrum was measured in vacuum) are as follows. All the bands of CH, CH2 and CH3 are absent from the spectra of ¯uorinated PVTMS monitored in vacuum (Fig. 1, curve 2); hence all the hydrogen atoms are substituted for ¯uorine. A strong system of overlapping diffuse bands between 1000 and 1400 cmÿ1 is due to C±Fx absorption. Band 848 cmÿ1 can be attributed to the Si±F bond [21±24]. 1010 cmÿ1 band is probably due to C±SiF3 [23]. Band 1860 cmÿ1 with shoulder at 1760±1770 cmÿ1 similar to [1,4,5,7] was attribgroup). uted to the presence of C=O bond (>C=O or The origin of this group is due to oxygen (0.1 vol%) in the ¯uorine. The intensity of this band increases with the content of oxygen in the ¯uorinating mixture (see Fig. 3). A weak band between 3600 and 3000 cmÿ1 range is due to associated HF (see below). The spectra of ¯uorinated PVTMS are substantially changed under the action of atmospheric moisture (Figs. 1±3). A very strong diffuse 2500±3600 cmÿ1 band appears. Bands at 1860 and 1760±1770 cmÿ1 (C=O) are shifted to 1740 cmÿ1 and 1630 cmÿ1. Bands 1010 (C±SiF3) and 848 cmÿ1 (Si±F) virtually disappear. A new 840 cmÿ1 band (possibly Si±OH) appears. The intensity of a diffuse 2500±3600 cmÿ1 band ®rst increases with time but then decreases when the reaction vessel is depressurized and a ®lm is exposed to atmospheric moisture. The action of a dried N2:O2 80:20 mixture (air simulant) does not in¯uence and Si±F groups the spectra. It is well known that
Fig. 1. IR spectra of a virgin thin PVTMS film (thickness 2 mm; curve 1) deposited onto a ZnSe support and totally fluorinated film (curve 2). Spectrum 2 was measured in vacuum. Reaction vessel with the film was evacuated for 3 h before the spectrum measurement. Curve 3: the same film treated with moist air for 60 min. Curve 4: fit to the same film after additional evacuation for 100 min. All the spectra have the same scale.
Fig. 2. (log T)/F (T ± transparency of the film, F (mm) ± thickness of the fluorinated layer) vs wavenumber (frequency) (cmÿ1) for PVTMS treated with a fluorine±oxygen mixture. Ratio of partial oxygen pressure pO to fluorine pressure pF is equal to 0.001 (undiluted fluorine), 0.2 and 1 for spectra 1, 2 and 3 respectively. All spectra were measured in vacuum.
show a tendency to hydrolysis: H2 O HF and Si±FH2OSi±OHHF. Hence absorption over the 2500±3600 cmÿ1 range may be attributed both to hydroxyl OH (possibly C±OH and Si±OH groups) and associated hydrogen ¯uoride HF. The 2500±3600 cmÿ1 band should be accompanied with a shift of C=O band to lower wavenumbers as is observed in our experiments. The hydrolysis of a SiF3 group would result in Si(OH)3 formation but this group is unlikely to be stable. One possible transformation of a Si(OH)3 group is as follows: j
j
ÿSi OH3 !ÿ Si ÿO H2 O !ÿ Si O H2 O j j OH OH
Fig. 3. Spectrum of the PVTMS film on ZnSe support treated with F2±O2 mixture ( pO/pF1). Spectra 1 was measured in vacuum, spectra 2 and 3 show the film exposed to atmospheric moisture for 5 min and 3 days, respectively.
A.P. Kharitonov et al. / Journal of Fluorine Chemistry 93 (1999) 129±137
Crosslinking through formation of Si±O±Si and Si±O±C bonds can also take place. However, we have not observed absorption of the above bands (1020±1090 cmÿ1, very strong Si±O stretching [24]) and we were not able to identify a 730 cmÿ1 band. So we believe that Si±O±Si and Si±O±C bonds are hardly formed and that the end product of the hydrolysis of the ±SiF3 group is a ÿ Si O j OH group rather than Si(OH)3. To measure the spectrum of associated hydrogen ¯uoride a direct experiment has been carried out. Flat-parallel PVTMS ®lm deposited onto a ZnSe support was introduced into the reaction vessel and treated with ¯uorine. The treatment duration was more than enough to ¯uorinate the ®lm completely (i.e. through all its thickness). The thickness of the starting ®lm was measured by an interference method (see Eq. (11) below) and the duration of treatment was calculated from Eq. (1) (see below). The degree of the ®lm transformation was checked by IR spectroscopy. Then the reaction vessel was evacuated for 5 h and ®lled with HF at 30 Torr pressure. In 30 min the reaction vessel was opened (HF was not removed from the vessel) and the spectra of the ®lm saturated with HF were monitored at 2, 5, 70 and 95 min (Fig. 4). HF diffuses from the sample to the atmosphere, so the absorption at 3700± 2700 cmÿ1 drops with time. Curve 6 in Fig. 4 represents that the spectrum of hydrogen ¯uoride (HF)n associates inside the ¯uorinated PVTMS calculated as a difference of spectra 5 and 1. bands under the action of moist Transformation of air is shown in Fig. 3. The PVTMS ®lm cast onto the ZnSe support was treated with a F2±O2 mixture (pO/pF1). Spectrum 1 was measured in vacuum. Then the reaction vessel was opened and the ®lm was subjected to atmospheric moisture. Spectra 2 and 3 show the ®lm exposed to atmospheric moisture for 5 min and 3 days, respectively.
Fig. 4. IR spectra of PVTMS film on ZnSe support treated with undiluted fluorine through all its thickness (curve 1). Before testing the film was evacuated for 5 h and subjected to moist air for 1 day. Then the film was treated with 30 Torr HF for 30 min and was removed from the reaction vessel. Spectra 2±5 were measured in air 2, 5, 70 and 95 min after withdrawn of the film from the vessel. Curve 6: difference of spectrum 2 and 1.
131
The 1860 and 1760 cmÿ1 bands are transformed into 1740 and 1630 cmÿ1 bands which can be attributed to ±COOH absorption [25]. After three days exposure to atmospheric moisture, PVTMS ®lm treated with ¯uorine was again inserted into the reaction vessel and treated with undiluted F2 (virgin ®lm-spectra 1 in Fig. 1). The spectrum of this ®lm (measured in vacuum) was practically identical to the spectrum 2 in Fig. 1. This is not surprising because hydroxyl groups are substituted for ¯uorine atoms under ¯uorination [1]. On the basis of IR spectra we have estimated the residual amount of ±Si-containing groups in PVTMS treated with ¯uorine and then exposed to atmospheric moisture. For this reason we have compared the areas under the band 747 cmÿ1 (±Si(CH3)3 vibration) (spectrum 1, Fig. 1)) and under the 840 cmÿ1 band (vibration of ±Si(OH)3 group) (spectra 3, Fig. 1)) and we have estimated that the amount of undisrupted C±Si bonds is close to 7±8% as compared to the Si content in the virgin PVTMS. This value is only a very rough estimate. Another estimation of this value made below gave a similar value 164% for residual Si-containing groups. In the PVTMS ®lm (thickness of the virgin ®lm is more than 1 mm) treated with ¯uorine or ¯uorine±oxygen mixture all the hydrogen atoms are substituted with ¯uorine, most of the C±Si bonds are broken and in vacuum the ®lm consists of the following fragments: ±CF2±CF2± (844 vol%) and ±CF2±CF(SiF3)± (164 vol%). Under the action of atmospheric moisture the latter fragments are mainly transformed into ±CF2±CF Si(O)(OH) groups. The case of PPO is not as complex as compared to PVTMS (Fig. 5). All the hydrogen atoms are substituted with ¯uorine, conjugated bonds in phenyl rings are saturated with ¯uorine and C=O bonds are also formed. The direct ¯uorination of both PVTMS and PPO has the following characteristic features. The visible spectrum of polymer ®lms treated with ¯uorine has remarkable interference features (Fig. 6). Similarly to other polymers studied [4±12] this phenomenon ®ts to the case where the ¯uorine treated polymer consists of layers of mainly ¯uorinated polymer and virgin (untreated) polymer which are separated by a very thin transition layer (Fig. 7). The direct
Fig. 5. IR spectra of a virgin thin PPO film (curve 1) deposited onto ZnSe support and totally fluorinated PPO film (curve 2). Spectrum 2 was measured in vacuum.
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Fig. 6. Transmission T (%) of a polymer film treated with fluorine vs wavenumber (cmÿ1) in the visible region of spectra.
Fig. 8. Fluorinated layer thickness F (mm) against the square root of fluorination duration t (s) for HMW PPO (molecular weight 244 000). Fluorine and helium partial pressures (Torr, in parentheses) are as follows: (14.7; 0) (curve 1), (29.4; 0) (curve 2), (58.8; 0) (curve 3), (147; 0) (curve 4) and (7.35; 728) (curve 5). Treatment temperature 2941 K.
Fig. 7. Cross-section of fluorine treated polymer. F, B and V are fluorinated, boundary and virgin layers respectively.
¯uorination is so-called surface treatment, i.e. the reaction proceeds as a diffusion-limited process. Almost all the chemical reactions occur only inside this transition boundary layer and the physical±chemical properties (density, refraction index, chemical composition, etc.) of the polymer are mainly changed inside this layer: The thickness b of this layer is small as compared to the thickness F of the ¯uorinated layer and must be much less than 0.1 mm. The thickness of the ¯uorinated layer formed on the surface of modi®ed polymer can be controlled over the range 0.1± 10 mm by a combination of the ¯uorine partial pressure and treatment duration. The method of kinetic interference spectroscopy developed by Kharitonov et al. has permitted continuous measuring of the dependence of the ¯uorinated layer thickness F on time without interruption of the ¯uorination [5,7,10,13,15,16,24]. The dependence of the ¯uorinated layer thickness F for PPO and PVTMS are represented in Figs. 8 and 9. The F (mm) value depends on ¯uorination duration t (s) as the square root: F A t0:5 b
(1)
The plot of A value vs pF is shown in Fig. 10. For PVTMS and PPO b A and the following correlations are valid (pF is measured in Torr): for PVTMS;
t0:5 0:55 10ÿ2 pF F 1:44 10ÿ3 p0:61 F
for PPO MW 244000; for PPO MW 34000;
F 0:92 10ÿ3 p0:76 t0:5 F F 2:17 10ÿ3 p0:53 t0:5 F
Fig. 9. Fluorinated layer thickness F (mm) of fluorine treated PVTMS vs square root of fluorination duration t (s). Fluorine pressure is equal to 14,7; 29.4, 44.1, 58.8, 88.2 and 147 Torr for curves 1±6 correspondingly. Treatment temperature 2941 K.
Fig. 10. Dependence of A (mm.sÿ0.5) value (see Eq. (1): FA.t0.5b) on fluorine pressure pF (Torr) for PPO. Treatment conditions: undiluted fluorine, treatment temperature 2941 K. Curves 1 and 2 fit to molecular weights 244 000 and 34 000, respectively.
where MW is the molecular weight of PPO. The kinetics of the formation of ¯uorinated layer for PPO depends on the molecular weight of the polymer. The limiting stage of the direct ¯uorination is ¯uorine penetration through the ¯uorinated layer to the untreated polymer, i.e. the direct ¯uorination process is diffusion-
A.P. Kharitonov et al. / Journal of Fluorine Chemistry 93 (1999) 129±137
133
limited and can be characterized by the ¯uorine permeability P through the ¯uorinated polymer layer. The balance between the amount of ¯uorine consumed in the reaction zone and the amount of ¯uorine passed through the ¯uorinated layer can be expressed by the following relation: B dF =dt D dcF =dx
(2)
where D is the diffusion coef®cient of ¯uorine for the ¯uorinated polymer; cF is the concentration of ¯uorine in the ¯uorinated polymer; axis x is directed into the ®lm perpendicular to its surface (on the ®lm surface x0); and the experimentally determined coef®cient B is the amount of ¯uorine (cm3 under standard conditions) necessary to form 1 cm3 of the ¯uorinated polymer divided by 1 cm3 of the ¯uorinated polymer. Within an in®nite reaction zone (that is b F) cF(xF) cF(x0). When the physicochemical properties of the ¯uorinated layer are constant along the axis x then B dF =dt D cF 0=F DS pF =F P pF =F (3) where S is the solubility coef®cient of ¯uorine in the ¯uorinated polymer and P is the permeability coef®cient for ¯uorine in the ¯uorinated polymer (cF(x0)SpF and PDS). If the partial ¯uorine pressure is constant during ¯uorination and P remains unchanged during ¯uorination, then F 2=B P pF t0:5 A t0:5
(4)
We have shown experimentally that the presence of helium does not affect the rate of the direct ¯uorination of PVTMS and PPO when the partial pressures of these gases in the ¯uorinating mixture are ten times greater than the ¯uorine partial pressure. However, oxygen inhibits the process (Fig. 11). As mentioned above for each ¯uorinating mixture FAt0.5b. For the case of treatment with F2±O2 mixtures the value of A (mm sÿ0.5) depends not only on ¯uorine partial pressure but also on the concentration of oxygen in the ¯uorinating mixture.
Fig. 11. Dependence of A (mm sÿ0.5) value (see Eq. (1): FA.t0.5b) on fluorine partial pressure pF (Torr) for PPO (MW244 000). Curves 1±5 correspond to the following concentration (vol%) of oxygen in fluorinating mixture: <0.1%, 10%, 20%, 33.3% and 50%.
Fig. 12. Visible transmission spectra (T%) of PVTMS film. Treatment duration increases for the curves 2±5, respectively. (cmÿ1) ± wavenumber. 1 ± spectrum of a virgin (untreated) film.
The density of polymers is greatly increased by the action of ¯uorine and usually varies over the range 1.6±2 g cmÿ3 [4,7,10,13]. It is dif®cult to measure the density directly, and hence the following procedure was applied. PVTMS ®lms were cast on a solid support of sapphire and then removed from it. Films were treated with ¯uorine from both sides. Only ¯at-parallel free ®lms were selected. The transmission spectrum of those ®lms exhibited interference in the visible and near UV (Fig. 12). From these data the thicknesses of the virgin and ¯uorinated layers can be measured (see below). Fluorine treated ®lm presents a three-layer structure (Fig. 13) and consists of two layers of ¯uorinated polymer (F1 and F2) and of a layer of unmodi®ed (virgin) polymer (V). Interference phenomenon arises from interference of the following light beams: (1) passed through a ®lm without re¯ections and twice re¯ected from surfaces F1-air and F2air (layer VF1F2), (2a) passed through a ®lm without re¯ections and twice re¯ected from surfaces F1-air and F1V and twice re¯ected from surfaces F2-air and F1-V (layer VF2), (3b) passed through a ®lm without re¯ections and twice re¯ected from surfaces F1-air and F2-V (layer VF1) and (4) passed through a ®lm without re¯ections and twice re¯ected from surfaces F2-V and F1-V (layer V). As was subsequently found, the thickness of the ¯uorinated layers F1 and F2 are the same, therefore we shall designate a ¯uorinated layer by a symbol F and we shall operate only with layers F, VF, V2F and V. In this case the dependence of intensity of light I, passed through ¯uorinated ®lm, on wavenumber (frequency) is described by the following
Fig. 13. Cross-section of a polymer film treated with fluorine. V, F1 and F2 are virgin and fluorinated layers, respectively.
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Fig. 14. The Fourier-image |f| of the direct Fourier transformation of the transmission spectra (see Fig. 12) of polymer film treated with fluorine.
V 4:95 ÿ 0:875 F for the film deposited onto a solid support and treated
expression: I Kf1 a1 sin2 nV V 2nF F a2 sin2 nF F
(5)
a3 sin2 nV :V nF F a4 sin 2 nV V g where K, a1, a2, a3 and a4 are constants, nV and nF- the refractive indices of unmodi®ed and ¯uorinated polymer, V and F are the thicknesses of unmodi®ed and ¯uorinated polymer, respectively. To measure the thickness of layers F1, F2 and V simultaneously we have applied a discrete Fourier transformation to the transmission spectra of the treated ®lms. The kinetic interference method [5,7,10,13,15,16,26] was applied to monitor the thickness of the ¯uorinated layer F. This method allows measurement of the dependence of F on time during the course of the process, without interruption. The Fourier-image |f| of the direct discrete Fourier-transformation is shown in Fig. 14. The maxima 1, 2 and 3 ®t to the equivalent thicknesses eq of layers F, VF and V2F, respectively. The true thickness of the layers can than be calculated as follows: nF F Feq ;
eq nF F nV V VF ;
eq 2nF F nV V V2F
(6)
Filtering allowed removal of ``noise'' and different characteristic frequencies of the transmission spectrum of a ¯uorinated ®lm were measured. The results are shown in Fig. 15. The points indicated in Fig. 14 can be described by the following equation: V 11:54 ÿ 2 F 1:72
Fig. 15. Dependence of the thickness V (mm) of an unmodified part of PVTMS film on a double thickness of a fluorinated layer 2F (mm).
from a single side
(9)
where F (mm) is the thickness of the ¯uorinated layer on one side of the polymer ®lm. A free ®lm expands under ¯uorination more signi®cantly as compared to a ®lm deposited on a solid support. The general distinction between PVTMS and PPO under direct ¯uorination can be seen when comparing Eqs. (7)± (9). Upon ¯uorination PVTMS ®lm is compressed (thickness of the treated ®lm is smaller than the thickness of the virgin (starting) ®lm). On the contrary PPO ®lms increase their volume upon ¯uorination (thickness of the treated ®lm is larger than the thickness of the virgin ®lm). Simultaneously with measurement of thicknesses V and F each sample was weighed after each ¯uorination stage. The total mass of the sample mV2mF depends linearly on the thickness of the ¯uorinated layer F (Fig. 16). Sample areas before and after treatment were the same. The mass of the ¯uorinated part of the sample 2mF (calculated as the difference between the total mass and the mass of the unmodi®ed part of the sample) depends linearly on F. Hence the density F of the ¯uorinated polymer is constant during the ¯uorination when F>0.1 mm and F1.73 g cmÿ3. In these calculations we have used the density V measurements of virgin PVTMS ®lms over the
(7)
The coef®cient 2 on the right-hand side of Eq. (7) ®ts to the treatment of the free ®lm from both sides; hence the 2F value corresponds to the total thickness (2FFF) of the ¯uorinated polymer layers. From Eq. (7) it is evident that the ¯uorination of PVTMS ®lms results in compression of the virgin ®lm by a factor of 1.72. Similar relations to Eq. (7) have been experimentally obtained for PPO (MW244 000): V 5:19 ÿ 0:761 2 F for the free film treated from both sides
(8)
Fig. 16. Dependence of the total mass m (mg) (mmV2mF) (curve 1) and the mass of a fluorinated part mF (curve 2) of the PVTMS sample treated with fluorine on the thickness of a fluorinated layer F (mm).
A.P. Kharitonov et al. / Journal of Fluorine Chemistry 93 (1999) 129±137 Table 1 Density (g cmÿ3) of virgin rV and fluorinated F polymers (PTFE ± polytetrafluoroethylene) Polymer
V
F
PET [4, 10, 13] PS [7, 10, 13] PVTMS PPO (MW244 000) PTFE
1.46 1.05 0.85 1.07 ±
1.75 2.05 1.73 1.72 2.12±2.28
range 2±30 mm (for the virgin PVTMS ®lm with thickness 10 mm rV0.90 g cmÿ3) [27]. Values of density of virgin and ¯uorinated polymers are shown in the Table 1. As was shown above the ¯uorinated polymer subjected to atmospheric moisture action consists mainly of ±CF2±CF2± and
fragments. From the data shown in Fig. 16 it is possible to make a very rough estimate that the amount of residual Sicontaining groups does not exceed 164%. Fluorine treatment in¯uences the surface energy of the polymers. Measurements of the total surface energy s and its polar sp and dispersion sd components have been carried out as described in [17]. The dependence of the s and
sp values on the thickness of the ¯uorinated layer dF for PVTMS are shown in Fig. 17. Both s and sp at ®rst rise with dF but then fall. The s and sp for dF0.1±0.2 mm are close (slightly higher) as compared to their values for dF0.7±2 mm. This means that the majority of chemical transformations took place in the upper surface ¯uorinated layers during the time interval needed for the boundary of the reaction zone (in which the main chemical transformations occur) to move 0.1±0.2 mm inside from the upper surface. For the virgin PVTMS we have measured the following values (mJ mÿ2 or dyne cmÿ1): s23.6,
Fig. 17. Dependence of the total surface energy s (curve 1) and its polar component sP (curve 2) (mJ mÿ2) on the thickness of the fluorinated layer dF(mm) for PVTMS films treated with undiluted F2.
135
sp 1:9 and sd 21:7. Sugden [28] used the parachor H: H s1=4 M=
(10)
where s, and M are correspondingly total energy, density and the mass of the monomeric unit of a polymer. Parachor H is an additive value and is equal to the sum of group or atomic contributions. The atomic contribution HSi of a Si atom to the total energy can be calculated on the basis of [29]: HSi13.9. We have measured the s value for a thick ¯uorinated (treated with undiluted ¯uorine) PVTMS ®lm (F1 mm): s221. We have found that the ¯uorinated (with undiluted ¯uorine) PVTMS ®lm subjected to atmospheric action can be considered to consist of the following fragments: ±CF2±CF2± (844 vol%) and ±CF2± CF(SiOOH)± (164 vol%). The density of ¯uorinated PVTMS ®lm 1.73 g cmÿ3 (see Table 1). The s value of a ¯uorinated PVTMS ®lm (F1 mm) can be calculated from (Eq. (10)): s21.20.3. So there is quite good agreement of experimentally measured ( s221) and calculated s values. Another method can be applied to measure the F value. The transition (or absorption) spectrum of ¯uorinated polymer ®lms in the visible and near UV spectral regions have interference features and consist of a set of minima and maxima when frequency is measured in cmÿ1 (Fig. 6) and the F value can be calculated from the equation F 2 nF ÿ1
(11)
where (cmÿ1) is the distance between two neighboring interference maxima or minima. The main disadvantage of this method is that ¯uorination must be interrupted to carry out each measurement. This method can also be used to measure the dependence of the refractive indices of virgin and ¯uorinated polymers on wavelength or wavenumber in the visible and near IR by the equation: nF 2 F ÿ1
(12)
The quantitative value of the refractive index at a selected wavelength can be measured by an ordinary refractometer. A polymer ®lm ¯uorinated through all its thickness was used for this purpose. As an example the dependence of refractive index on wavenumber for a virgin PVTMS is shown in Fig. 18. Spectra of virgin and ¯uorinated polymers in the visible and near UV are shown in Fig. 19. Refractive indices of ¯uorinated polymers differ from those of virgin polymers and depend on the composition of the ¯uorinating mixture (Fig. 20). As mentioned earlier [1,3,5,7,10,13] C=O groups are introduced into the polymer structure by the treatment with a ¯uorine±oxygen mixture. Oxygen content in the ¯uorinating mixture in¯uences the rate of formation of the ¯uorinated layer on the surface of PPO and also decreases the permeability of ¯uorine through the ¯uorinated layer to the untreated layer (see Eq. (4)). The concentration of oxygen in F2±O2 mixtures was varied from 0.1 vol% (undiluted
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Fig. 21. k value (Eq. (12)) vs concentration V (vol %) of oxygen in F2±O2 mixture for PPO (MW244 000)
Fig. 18. Refractive index n of virgin (1) and treated with undiluted fluorine (2) PVTMS film deposited on a solid support vs wavenumber (cmÿ1).
Fig. 19. Transmittance T(%) of a virgin PVTMS film (1) and treated throughout (i.e. totally fluorinated film) with undiluted fluorine (2).
Fig. 20. Refractive index of PPO treated with F2±O2 mixture (curves 1 and 2 correspond virgin PPO MW 34 000 and 244 000, respectively) vs ratio of fluorine and oxygen partial pressures pF/pO in the fluorinating mixture.
¯uorine) to 50 vol%. For all the mixtures used the correlation FAt0.5d was valid and d At0.5. For each F2±O2 mixture coef®cient A depends on pF as follows (see Fig. 11): A C pF k
(13)
where C is constant for any F2±O2 mixture. It is seen that there are two distinct ranges of the oxygen concentration: below 10 vol% the k value is close to 0.76 but above 20 vol% the k value drops to 0.46±0.53 (Fig. 21).
Similarly to our earlier results [5,7,10,13] we have attributed the inhibiting in¯uence of oxygen to the formation of C=O groups. It is known [30] that insertion of C=O groups decreases the permeability of polymers. To verify the suggestion that C=O group formation results in P decrease we have evaluated C=O concentrations in oxy¯uorinated PPO. To evaluate the concentration of C=O groups we applied IR spectroscopy as described in [5,7,10,13]. Absorption coef®cients (measured in vacuum) of thin ¯at-parallel PMMA (polymethylmethacrylate) ®lms (thickness 1±5 mm) cast on to ZnSe plates and treated with ¯uorine were used as reference data. It should be noted that C=O groups are also present in untreated PMMA (one group per monomer unit). Thicknesses of untreated and ¯uorinated layers were measured by interference spectroscopy. We have made two suppositions: (1) the concentration NC of C=O groups in untreated and ¯uorinated with undiluted ¯uorine PMMA in vacuum coincide with each other and are equalR to one C=O group per monomer unit, and (2) d values ( F ÿ1 log Tbackground =T , where T is the transmission value) ofR ¯uorinated PMMA and PPO coincide with each other ( d value is proportional to the C=O group concentrations). We Rhave shown experimentally that the difference between the d values of untreated and ¯uorinated PMMA in vacuum is less than 20%. Also R we have shown experimentally that the difference of d values for oxy¯uorinated PPO measured in vacuum and in a moist atmosphere does not exceed 20%. We have measured the dependence of the C=O group concentrations in situ by the following method. The IR spectra of the sample were monitored by an FTIR spectrometer (duration of a measurement of a Rsingle spectrum was 10 s) during the ¯uorination and the d value was R calculated for various treatment times. The d value was shown to be proportional to t0.5 (Fig. 22) and hence proportional to the F value. This means that the total of C=O groups in a ¯uorinated polymer is proportional to the F value and the concentration (i.e. amount of the C=O groups divided by the ¯uorinated layer thickness) C=O groups is ®xed in the course of treatment. The concentration of C=O groups depends weakly on pF for treatment with a mixture of constant oxygen concentration. The number NC of C=O groups per monomer unit of ¯uorinated PPO was estimated to depend on the R value (RpF/pO) as follows
A.P. Kharitonov et al. / Journal of Fluorine Chemistry 93 (1999) 129±137
Fig. 22. Total amount NC=O of C=O groups (arbitrary units) vs square root of the treatment duration t (s). Free PPO (MW244 000) was treated with F2±O2 mixture (20 volume % of O2). IR spectra were measured in the course of treatment
Fig. 23. The concentration NC of C=O groups vs ratio R of fluorine pF and oxygen pO partial pressures for PPO (MW244 000)
(Fig. 23): NC 2 1= 1 033 R
(14)
Acknowledgements The results of A.P. Kharitonov and his co-workers described in this publication were made possible in part by the grant no. 96 1277 from the INTAS and by grant no. 94 03 08197a of the Russian Foundation of the Basic Research. References [1] R.J. Lagow, J.L. Margrave, Progr. Inorg. Chem. 26 (1979) 162. [2] J. Jagur-Grodzinski, Progr. Polym. Sci. 17 (1992) 361.
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[3] M. Anand, J.P. Hobbs, I.J. Brass, in: R.E. Banks, B.E. Smart, J.C. Tatlow (Eds.), Organofluorine Chemistry: Principles and Commercial Applications, Plenum Press, 1994, p. 469. [4] A.P. Kharitonov, Yu.L. Moskvin, G.A. Kolpakov, Sov. J. Chem. Phys. 4 (1987) 877. [5] A.P. Kharitonov, Yu.L. Moskvin, Proceedings of the Second International Conference on Fluorine on Coatings, Salford, UK, 1994, Paper 13.. [6] G.A. Kolpakov, S.I. Kuzina, A.P. Kharitonov, Yu.L. Moskvin, A.I. Mikhailov, Sov. J. Chem. Phys. 9 (1992) 2283. [7] A.P. Kharitonov, Yu.L. Moskvin, Chem. Phys. Rep. (The former Sov. J. Chem. Phys.) 13 (1994) 818. [8] A.P. Kharitonov, Yu.L. Moskvin, Chem. Phys. Rep. (The former Sov. J. Chem. Phys.) 13 (1994) 830. [9] S.I. Kuzina, A.P. Kharitonov, Yu.L. Moskvin, A.I. Mikhailov, Russian Chem. Bull. 45 (1996) 1623. [10] A.P. Kharitonov, Popular Plastics and Packaging 42 (1997) 75. [11] A.P. Kharitonov, Yu.L. Moskvin, G.A. Kolpakov, Patent of the Russian Federation No. 1 754 191, filed 26 July 1990. [12] V. Frolov, V.V. Teplyakov, A.P. Kharitonov, Proceedings of the Second International Conference on Fluorine in Coatings, Salford, UK, 28±30 September 1994, Conference Papers, Paper 22. [13] A.P. Kharitonov, Yu.L. Moskvin, V.V. Teplyakov, J.D. Le Roux, Proceedings of the Third International Conference on Fluorine in Coatings ± II, Munich, Germany, 24±26 February 1997, Conference Papers, Paper 35. [14] A.P. Kharitonov, Yu.L. Moskvin, L.N. Kharitonova, M.N. Tulskii, A.A. Kotenko, Mendeleev Commun. No. 3 (1994) 91. [15] A.P. Kharitonov, Yu.L. Moskvin, L.N. Kharitonova, A.A. Kotenko, M.N. Tulskii, Kinetics and Catal. 35 (1994) 792. [16] A.P. Kharitonov, Yu.L. Moskvin, L.N. Kharitonova, A.A. Kotenko, M.N. Tulskii, Polym. Sci., Ser. B 37 (1995) 307. [17] D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1740. [18] Yu.Ya. Mikhailenko, Khimia I prakticheskoe primenenie kremniiorganicheskih soedinenii. Trudy conferencii (in Russian), Leningrad, 1958, p. 59. [19] Ya.I. Ryskin, M.G.Voronkov, Khimia I prakticheskoe primenenie kremniiorganicheskih soedinenii. Trudy conferencii (in Russian), Leningrad, 1958, p. 42. [20] D.S. Kim, Y.H. Lee, J. Electrochem. Soc. 141 (1994) 3562. [21] A. Lee Smith, Applied Infrared Spectroscopy, Wiley, New York, 1979. [22] P. Favia, G. Caporiccio, R. D'Agostino, J. Polym. Sci., Part A 32 (1994) 121. [23] J. Sakata, M. Yamamoto, I. Tajima, J. Polym. Sci., Part A 26 (1988) 1721. [24] Termodinamichiskie Svoistva Individual'nyh veschestv (Thermodynamical Properties of Individual Substances) (in Russian). Moscow, 1979, vol. 2, Book 1, p. 220.. [25] G. Socrates, Infrared Characteristic Group Frequences, 2nd ed., Wiley, New York, 1994. [26] A.P. Kharitonov, Yu.L. Moskvin, G.A. Kolpakov, Polym. Sci. USSR 27 (1985) 739. [27] A.P. Kharitonov, V.V. Teplyakov, Polym. Sci., submitted for publication. [28] S. Sugden, J. Chem. Soc. 125 (1924) 1177. [29] O.R. Quayle, Chem. Revs. 53 (1953) 539. [30] M. Salame, Polym. Preprints 8 (1964) 137.
Direct ¯uorination of poly(vinyl trimethylsilane) and poly(phenylene oxide) a
A.P. Kharitonova,*, Yu.L. Moskvina, V.V. Teplyakovb, J.D. Le Rouxc
Institute of Energy Problems of Chemical Physics (Division) of the Russian Academy of Sciences, Chernogolovka, Moscow region 142432, Russia b Topchiev Institute of Petrochemical Synthesis of the Russian Academy of Sciences, Leninskii Prospect, 29 Moscow, Russia c Innovative Membrane Systems Inc., 189 Dean Street, Norwood, MA 02062, USA Received 12 June 1997; accepted 14 September 1998
Abstract Fundamental features of the direct ¯uorination of poly(vinyl trimethylsilane) (PVTMS) and poly(phenylene oxide) (PPO) have been investigated. The in¯uence of the ¯uorination conditions (¯uorinating mixture composition, treatment duration) and polymer molecular weight on the rate of formation of the ¯uorinated layer and on physical±chemical properties (chemical composition, density, refractive index, absorption spectra in the visible and UV, surface energy) was investigated. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Direct ¯uorination; Poly(vinyl trimethylsilane); Poly(phenylene oxide)
1. Introduction Direct ¯uorination is a well-known method for surface modi®cation of polymers [1±3]. This paper continues our investigations on the direct ¯uorination of polymer ®lms. The following polymers have been previously investigated by Kharitonov et al: poly(ethylene terephthalate) (PET) [4], polystyrene (PS) [5±10], poly(4-methyl-pentene-1) (PMP) [11], poly(vinyl trimethylsilane) (PVTMS), poly(vinyl trimethylsilane)±polydimethysiloxane block-copolymer (PVTMS-PDMS) [10,12,13], poly(phenylene oxide) (PPO) [13], polycarbonatesiloxane (PCS) [10,14,15] and blockcopolymer of polysulfone and polybutadiene (``SERAGEL1'') [15,16]. In this paper fundamental features of the direct ¯uorination of PVTMS and PPO are described. 2. Experimental PPO was supplied by Aldrich1 (USA) (MW 244 000 and MW 34 000). PVTMS (MW 1 000 000) was supplied by Kuskovo's Plant (Russia). PPO has only C±C, conjugated and C±H bonds. On the contrary PVTMS has also C±Si bonds and its behavior under the action of ¯uorine differs
*Corresponding author. Fax: +7-095-1378-258; e-mail: [email protected]
very much from PPO:
Two types of samples were used: free ®lms (thickness 5± 50 mm) and ®lms deposited from solution onto zinc selenide supports (thickness 2±10 mm). Zinc selenide is stable to ¯uorine and is transparent over the range 20 000± 500 cmÿ1. Fluorination has been carried out in closed vessels (volume 300±1000 cm3). The ¯uorine used had less than 0.1% impurities (mainly oxygen). A polymer sample was inserted into the closed vessel, evacuated to (2± 3)10ÿ2 Torr during 0.5±1 h to remove oxygen and water vapors. Then the closed vessel was ®lled with ¯uorinating mixture and the sample was treated during the necessary time interval. After treatment the vessel was evacuated. Fourier IR spectrometer model 1720 (Perkin Elmer) was used to measure IR spectra. The special reaction vessel equipped with zinc selenide windows was designed to measure the IR spectra during the ¯uorination or in vacuum. In this case the sample was not subjected to atmospheric moisture action. Refractive indices were measured by refractometer RF-454B (Lomo, Russia). To measure the surface energy the method proposed in [17] was used. The spectra in the visible and UV spectral ranges were measured by spectrometer Specord UV±VIS (Karl Zeiss Jena, Germany).
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S0022-1139(98)00278-4
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3. Results and discussion To study the in¯uence of direct ¯uorination on the chemical composition of polymers we used IR spectroscopy. Identi®cation of the virgin PVTMS bands was made on the basis of [18±21]: C±Si(CH3)3 ± 747 cmÿ1, Si±CH3 ± 831, 1247, 1259 and 1403 cmÿ1, tertiary H ± 1338 cmÿ1, CH3 and CH2 ± 2899 and 2953 cmÿ1, respectively (Fig. 1, curve 1). The main characteristic features of the spectra of PVTMS treated with undiluted ¯uorine (spectrum was measured in vacuum) are as follows. All the bands of CH, CH2 and CH3 are absent from the spectra of ¯uorinated PVTMS monitored in vacuum (Fig. 1, curve 2); hence all the hydrogen atoms are substituted for ¯uorine. A strong system of overlapping diffuse bands between 1000 and 1400 cmÿ1 is due to C±Fx absorption. Band 848 cmÿ1 can be attributed to the Si±F bond [21±24]. 1010 cmÿ1 band is probably due to C±SiF3 [23]. Band 1860 cmÿ1 with shoulder at 1760±1770 cmÿ1 similar to [1,4,5,7] was attribgroup). uted to the presence of C=O bond (>C=O or The origin of this group is due to oxygen (0.1 vol%) in the ¯uorine. The intensity of this band increases with the content of oxygen in the ¯uorinating mixture (see Fig. 3). A weak band between 3600 and 3000 cmÿ1 range is due to associated HF (see below). The spectra of ¯uorinated PVTMS are substantially changed under the action of atmospheric moisture (Figs. 1±3). A very strong diffuse 2500±3600 cmÿ1 band appears. Bands at 1860 and 1760±1770 cmÿ1 (C=O) are shifted to 1740 cmÿ1 and 1630 cmÿ1. Bands 1010 (C±SiF3) and 848 cmÿ1 (Si±F) virtually disappear. A new 840 cmÿ1 band (possibly Si±OH) appears. The intensity of a diffuse 2500±3600 cmÿ1 band ®rst increases with time but then decreases when the reaction vessel is depressurized and a ®lm is exposed to atmospheric moisture. The action of a dried N2:O2 80:20 mixture (air simulant) does not in¯uence and Si±F groups the spectra. It is well known that
Fig. 1. IR spectra of a virgin thin PVTMS film (thickness 2 mm; curve 1) deposited onto a ZnSe support and totally fluorinated film (curve 2). Spectrum 2 was measured in vacuum. Reaction vessel with the film was evacuated for 3 h before the spectrum measurement. Curve 3: the same film treated with moist air for 60 min. Curve 4: fit to the same film after additional evacuation for 100 min. All the spectra have the same scale.
Fig. 2. (log T)/F (T ± transparency of the film, F (mm) ± thickness of the fluorinated layer) vs wavenumber (frequency) (cmÿ1) for PVTMS treated with a fluorine±oxygen mixture. Ratio of partial oxygen pressure pO to fluorine pressure pF is equal to 0.001 (undiluted fluorine), 0.2 and 1 for spectra 1, 2 and 3 respectively. All spectra were measured in vacuum.
show a tendency to hydrolysis: H2 O HF and Si±FH2OSi±OHHF. Hence absorption over the 2500±3600 cmÿ1 range may be attributed both to hydroxyl OH (possibly C±OH and Si±OH groups) and associated hydrogen ¯uoride HF. The 2500±3600 cmÿ1 band should be accompanied with a shift of C=O band to lower wavenumbers as is observed in our experiments. The hydrolysis of a SiF3 group would result in Si(OH)3 formation but this group is unlikely to be stable. One possible transformation of a Si(OH)3 group is as follows: j
j
ÿSi OH3 !ÿ Si ÿO H2 O !ÿ Si O H2 O j j OH OH
Fig. 3. Spectrum of the PVTMS film on ZnSe support treated with F2±O2 mixture ( pO/pF1). Spectra 1 was measured in vacuum, spectra 2 and 3 show the film exposed to atmospheric moisture for 5 min and 3 days, respectively.
A.P. Kharitonov et al. / Journal of Fluorine Chemistry 93 (1999) 129±137
Crosslinking through formation of Si±O±Si and Si±O±C bonds can also take place. However, we have not observed absorption of the above bands (1020±1090 cmÿ1, very strong Si±O stretching [24]) and we were not able to identify a 730 cmÿ1 band. So we believe that Si±O±Si and Si±O±C bonds are hardly formed and that the end product of the hydrolysis of the ±SiF3 group is a ÿ Si O j OH group rather than Si(OH)3. To measure the spectrum of associated hydrogen ¯uoride a direct experiment has been carried out. Flat-parallel PVTMS ®lm deposited onto a ZnSe support was introduced into the reaction vessel and treated with ¯uorine. The treatment duration was more than enough to ¯uorinate the ®lm completely (i.e. through all its thickness). The thickness of the starting ®lm was measured by an interference method (see Eq. (11) below) and the duration of treatment was calculated from Eq. (1) (see below). The degree of the ®lm transformation was checked by IR spectroscopy. Then the reaction vessel was evacuated for 5 h and ®lled with HF at 30 Torr pressure. In 30 min the reaction vessel was opened (HF was not removed from the vessel) and the spectra of the ®lm saturated with HF were monitored at 2, 5, 70 and 95 min (Fig. 4). HF diffuses from the sample to the atmosphere, so the absorption at 3700± 2700 cmÿ1 drops with time. Curve 6 in Fig. 4 represents that the spectrum of hydrogen ¯uoride (HF)n associates inside the ¯uorinated PVTMS calculated as a difference of spectra 5 and 1. bands under the action of moist Transformation of air is shown in Fig. 3. The PVTMS ®lm cast onto the ZnSe support was treated with a F2±O2 mixture (pO/pF1). Spectrum 1 was measured in vacuum. Then the reaction vessel was opened and the ®lm was subjected to atmospheric moisture. Spectra 2 and 3 show the ®lm exposed to atmospheric moisture for 5 min and 3 days, respectively.
Fig. 4. IR spectra of PVTMS film on ZnSe support treated with undiluted fluorine through all its thickness (curve 1). Before testing the film was evacuated for 5 h and subjected to moist air for 1 day. Then the film was treated with 30 Torr HF for 30 min and was removed from the reaction vessel. Spectra 2±5 were measured in air 2, 5, 70 and 95 min after withdrawn of the film from the vessel. Curve 6: difference of spectrum 2 and 1.
131
The 1860 and 1760 cmÿ1 bands are transformed into 1740 and 1630 cmÿ1 bands which can be attributed to ±COOH absorption [25]. After three days exposure to atmospheric moisture, PVTMS ®lm treated with ¯uorine was again inserted into the reaction vessel and treated with undiluted F2 (virgin ®lm-spectra 1 in Fig. 1). The spectrum of this ®lm (measured in vacuum) was practically identical to the spectrum 2 in Fig. 1. This is not surprising because hydroxyl groups are substituted for ¯uorine atoms under ¯uorination [1]. On the basis of IR spectra we have estimated the residual amount of ±Si-containing groups in PVTMS treated with ¯uorine and then exposed to atmospheric moisture. For this reason we have compared the areas under the band 747 cmÿ1 (±Si(CH3)3 vibration) (spectrum 1, Fig. 1)) and under the 840 cmÿ1 band (vibration of ±Si(OH)3 group) (spectra 3, Fig. 1)) and we have estimated that the amount of undisrupted C±Si bonds is close to 7±8% as compared to the Si content in the virgin PVTMS. This value is only a very rough estimate. Another estimation of this value made below gave a similar value 164% for residual Si-containing groups. In the PVTMS ®lm (thickness of the virgin ®lm is more than 1 mm) treated with ¯uorine or ¯uorine±oxygen mixture all the hydrogen atoms are substituted with ¯uorine, most of the C±Si bonds are broken and in vacuum the ®lm consists of the following fragments: ±CF2±CF2± (844 vol%) and ±CF2±CF(SiF3)± (164 vol%). Under the action of atmospheric moisture the latter fragments are mainly transformed into ±CF2±CF Si(O)(OH) groups. The case of PPO is not as complex as compared to PVTMS (Fig. 5). All the hydrogen atoms are substituted with ¯uorine, conjugated bonds in phenyl rings are saturated with ¯uorine and C=O bonds are also formed. The direct ¯uorination of both PVTMS and PPO has the following characteristic features. The visible spectrum of polymer ®lms treated with ¯uorine has remarkable interference features (Fig. 6). Similarly to other polymers studied [4±12] this phenomenon ®ts to the case where the ¯uorine treated polymer consists of layers of mainly ¯uorinated polymer and virgin (untreated) polymer which are separated by a very thin transition layer (Fig. 7). The direct
Fig. 5. IR spectra of a virgin thin PPO film (curve 1) deposited onto ZnSe support and totally fluorinated PPO film (curve 2). Spectrum 2 was measured in vacuum.
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Fig. 6. Transmission T (%) of a polymer film treated with fluorine vs wavenumber (cmÿ1) in the visible region of spectra.
Fig. 8. Fluorinated layer thickness F (mm) against the square root of fluorination duration t (s) for HMW PPO (molecular weight 244 000). Fluorine and helium partial pressures (Torr, in parentheses) are as follows: (14.7; 0) (curve 1), (29.4; 0) (curve 2), (58.8; 0) (curve 3), (147; 0) (curve 4) and (7.35; 728) (curve 5). Treatment temperature 2941 K.
Fig. 7. Cross-section of fluorine treated polymer. F, B and V are fluorinated, boundary and virgin layers respectively.
¯uorination is so-called surface treatment, i.e. the reaction proceeds as a diffusion-limited process. Almost all the chemical reactions occur only inside this transition boundary layer and the physical±chemical properties (density, refraction index, chemical composition, etc.) of the polymer are mainly changed inside this layer: The thickness b of this layer is small as compared to the thickness F of the ¯uorinated layer and must be much less than 0.1 mm. The thickness of the ¯uorinated layer formed on the surface of modi®ed polymer can be controlled over the range 0.1± 10 mm by a combination of the ¯uorine partial pressure and treatment duration. The method of kinetic interference spectroscopy developed by Kharitonov et al. has permitted continuous measuring of the dependence of the ¯uorinated layer thickness F on time without interruption of the ¯uorination [5,7,10,13,15,16,24]. The dependence of the ¯uorinated layer thickness F for PPO and PVTMS are represented in Figs. 8 and 9. The F (mm) value depends on ¯uorination duration t (s) as the square root: F A t0:5 b
(1)
The plot of A value vs pF is shown in Fig. 10. For PVTMS and PPO b A and the following correlations are valid (pF is measured in Torr): for PVTMS;
t0:5 0:55 10ÿ2 pF F 1:44 10ÿ3 p0:61 F
for PPO MW 244000; for PPO MW 34000;
F 0:92 10ÿ3 p0:76 t0:5 F F 2:17 10ÿ3 p0:53 t0:5 F
Fig. 9. Fluorinated layer thickness F (mm) of fluorine treated PVTMS vs square root of fluorination duration t (s). Fluorine pressure is equal to 14,7; 29.4, 44.1, 58.8, 88.2 and 147 Torr for curves 1±6 correspondingly. Treatment temperature 2941 K.
Fig. 10. Dependence of A (mm.sÿ0.5) value (see Eq. (1): FA.t0.5b) on fluorine pressure pF (Torr) for PPO. Treatment conditions: undiluted fluorine, treatment temperature 2941 K. Curves 1 and 2 fit to molecular weights 244 000 and 34 000, respectively.
where MW is the molecular weight of PPO. The kinetics of the formation of ¯uorinated layer for PPO depends on the molecular weight of the polymer. The limiting stage of the direct ¯uorination is ¯uorine penetration through the ¯uorinated layer to the untreated polymer, i.e. the direct ¯uorination process is diffusion-
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133
limited and can be characterized by the ¯uorine permeability P through the ¯uorinated polymer layer. The balance between the amount of ¯uorine consumed in the reaction zone and the amount of ¯uorine passed through the ¯uorinated layer can be expressed by the following relation: B dF =dt D dcF =dx
(2)
where D is the diffusion coef®cient of ¯uorine for the ¯uorinated polymer; cF is the concentration of ¯uorine in the ¯uorinated polymer; axis x is directed into the ®lm perpendicular to its surface (on the ®lm surface x0); and the experimentally determined coef®cient B is the amount of ¯uorine (cm3 under standard conditions) necessary to form 1 cm3 of the ¯uorinated polymer divided by 1 cm3 of the ¯uorinated polymer. Within an in®nite reaction zone (that is b F) cF(xF) cF(x0). When the physicochemical properties of the ¯uorinated layer are constant along the axis x then B dF =dt D cF 0=F DS pF =F P pF =F (3) where S is the solubility coef®cient of ¯uorine in the ¯uorinated polymer and P is the permeability coef®cient for ¯uorine in the ¯uorinated polymer (cF(x0)SpF and PDS). If the partial ¯uorine pressure is constant during ¯uorination and P remains unchanged during ¯uorination, then F 2=B P pF t0:5 A t0:5
(4)
We have shown experimentally that the presence of helium does not affect the rate of the direct ¯uorination of PVTMS and PPO when the partial pressures of these gases in the ¯uorinating mixture are ten times greater than the ¯uorine partial pressure. However, oxygen inhibits the process (Fig. 11). As mentioned above for each ¯uorinating mixture FAt0.5b. For the case of treatment with F2±O2 mixtures the value of A (mm sÿ0.5) depends not only on ¯uorine partial pressure but also on the concentration of oxygen in the ¯uorinating mixture.
Fig. 11. Dependence of A (mm sÿ0.5) value (see Eq. (1): FA.t0.5b) on fluorine partial pressure pF (Torr) for PPO (MW244 000). Curves 1±5 correspond to the following concentration (vol%) of oxygen in fluorinating mixture: <0.1%, 10%, 20%, 33.3% and 50%.
Fig. 12. Visible transmission spectra (T%) of PVTMS film. Treatment duration increases for the curves 2±5, respectively. (cmÿ1) ± wavenumber. 1 ± spectrum of a virgin (untreated) film.
The density of polymers is greatly increased by the action of ¯uorine and usually varies over the range 1.6±2 g cmÿ3 [4,7,10,13]. It is dif®cult to measure the density directly, and hence the following procedure was applied. PVTMS ®lms were cast on a solid support of sapphire and then removed from it. Films were treated with ¯uorine from both sides. Only ¯at-parallel free ®lms were selected. The transmission spectrum of those ®lms exhibited interference in the visible and near UV (Fig. 12). From these data the thicknesses of the virgin and ¯uorinated layers can be measured (see below). Fluorine treated ®lm presents a three-layer structure (Fig. 13) and consists of two layers of ¯uorinated polymer (F1 and F2) and of a layer of unmodi®ed (virgin) polymer (V). Interference phenomenon arises from interference of the following light beams: (1) passed through a ®lm without re¯ections and twice re¯ected from surfaces F1-air and F2air (layer VF1F2), (2a) passed through a ®lm without re¯ections and twice re¯ected from surfaces F1-air and F1V and twice re¯ected from surfaces F2-air and F1-V (layer VF2), (3b) passed through a ®lm without re¯ections and twice re¯ected from surfaces F1-air and F2-V (layer VF1) and (4) passed through a ®lm without re¯ections and twice re¯ected from surfaces F2-V and F1-V (layer V). As was subsequently found, the thickness of the ¯uorinated layers F1 and F2 are the same, therefore we shall designate a ¯uorinated layer by a symbol F and we shall operate only with layers F, VF, V2F and V. In this case the dependence of intensity of light I, passed through ¯uorinated ®lm, on wavenumber (frequency) is described by the following
Fig. 13. Cross-section of a polymer film treated with fluorine. V, F1 and F2 are virgin and fluorinated layers, respectively.
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Fig. 14. The Fourier-image |f| of the direct Fourier transformation of the transmission spectra (see Fig. 12) of polymer film treated with fluorine.
V 4:95 ÿ 0:875 F for the film deposited onto a solid support and treated
expression: I Kf1 a1 sin2 nV V 2nF F a2 sin2 nF F
(5)
a3 sin2 nV :V nF F a4 sin 2 nV V g where K, a1, a2, a3 and a4 are constants, nV and nF- the refractive indices of unmodi®ed and ¯uorinated polymer, V and F are the thicknesses of unmodi®ed and ¯uorinated polymer, respectively. To measure the thickness of layers F1, F2 and V simultaneously we have applied a discrete Fourier transformation to the transmission spectra of the treated ®lms. The kinetic interference method [5,7,10,13,15,16,26] was applied to monitor the thickness of the ¯uorinated layer F. This method allows measurement of the dependence of F on time during the course of the process, without interruption. The Fourier-image |f| of the direct discrete Fourier-transformation is shown in Fig. 14. The maxima 1, 2 and 3 ®t to the equivalent thicknesses eq of layers F, VF and V2F, respectively. The true thickness of the layers can than be calculated as follows: nF F Feq ;
eq nF F nV V VF ;
eq 2nF F nV V V2F
(6)
Filtering allowed removal of ``noise'' and different characteristic frequencies of the transmission spectrum of a ¯uorinated ®lm were measured. The results are shown in Fig. 15. The points indicated in Fig. 14 can be described by the following equation: V 11:54 ÿ 2 F 1:72
Fig. 15. Dependence of the thickness V (mm) of an unmodified part of PVTMS film on a double thickness of a fluorinated layer 2F (mm).
from a single side
(9)
where F (mm) is the thickness of the ¯uorinated layer on one side of the polymer ®lm. A free ®lm expands under ¯uorination more signi®cantly as compared to a ®lm deposited on a solid support. The general distinction between PVTMS and PPO under direct ¯uorination can be seen when comparing Eqs. (7)± (9). Upon ¯uorination PVTMS ®lm is compressed (thickness of the treated ®lm is smaller than the thickness of the virgin (starting) ®lm). On the contrary PPO ®lms increase their volume upon ¯uorination (thickness of the treated ®lm is larger than the thickness of the virgin ®lm). Simultaneously with measurement of thicknesses V and F each sample was weighed after each ¯uorination stage. The total mass of the sample mV2mF depends linearly on the thickness of the ¯uorinated layer F (Fig. 16). Sample areas before and after treatment were the same. The mass of the ¯uorinated part of the sample 2mF (calculated as the difference between the total mass and the mass of the unmodi®ed part of the sample) depends linearly on F. Hence the density F of the ¯uorinated polymer is constant during the ¯uorination when F>0.1 mm and F1.73 g cmÿ3. In these calculations we have used the density V measurements of virgin PVTMS ®lms over the
(7)
The coef®cient 2 on the right-hand side of Eq. (7) ®ts to the treatment of the free ®lm from both sides; hence the 2F value corresponds to the total thickness (2FFF) of the ¯uorinated polymer layers. From Eq. (7) it is evident that the ¯uorination of PVTMS ®lms results in compression of the virgin ®lm by a factor of 1.72. Similar relations to Eq. (7) have been experimentally obtained for PPO (MW244 000): V 5:19 ÿ 0:761 2 F for the free film treated from both sides
(8)
Fig. 16. Dependence of the total mass m (mg) (mmV2mF) (curve 1) and the mass of a fluorinated part mF (curve 2) of the PVTMS sample treated with fluorine on the thickness of a fluorinated layer F (mm).
A.P. Kharitonov et al. / Journal of Fluorine Chemistry 93 (1999) 129±137 Table 1 Density (g cmÿ3) of virgin rV and fluorinated F polymers (PTFE ± polytetrafluoroethylene) Polymer
V
F
PET [4, 10, 13] PS [7, 10, 13] PVTMS PPO (MW244 000) PTFE
1.46 1.05 0.85 1.07 ±
1.75 2.05 1.73 1.72 2.12±2.28
range 2±30 mm (for the virgin PVTMS ®lm with thickness 10 mm rV0.90 g cmÿ3) [27]. Values of density of virgin and ¯uorinated polymers are shown in the Table 1. As was shown above the ¯uorinated polymer subjected to atmospheric moisture action consists mainly of ±CF2±CF2± and
fragments. From the data shown in Fig. 16 it is possible to make a very rough estimate that the amount of residual Sicontaining groups does not exceed 164%. Fluorine treatment in¯uences the surface energy of the polymers. Measurements of the total surface energy s and its polar sp and dispersion sd components have been carried out as described in [17]. The dependence of the s and
sp values on the thickness of the ¯uorinated layer dF for PVTMS are shown in Fig. 17. Both s and sp at ®rst rise with dF but then fall. The s and sp for dF0.1±0.2 mm are close (slightly higher) as compared to their values for dF0.7±2 mm. This means that the majority of chemical transformations took place in the upper surface ¯uorinated layers during the time interval needed for the boundary of the reaction zone (in which the main chemical transformations occur) to move 0.1±0.2 mm inside from the upper surface. For the virgin PVTMS we have measured the following values (mJ mÿ2 or dyne cmÿ1): s23.6,
Fig. 17. Dependence of the total surface energy s (curve 1) and its polar component sP (curve 2) (mJ mÿ2) on the thickness of the fluorinated layer dF(mm) for PVTMS films treated with undiluted F2.
135
sp 1:9 and sd 21:7. Sugden [28] used the parachor H: H s1=4 M=
(10)
where s, and M are correspondingly total energy, density and the mass of the monomeric unit of a polymer. Parachor H is an additive value and is equal to the sum of group or atomic contributions. The atomic contribution HSi of a Si atom to the total energy can be calculated on the basis of [29]: HSi13.9. We have measured the s value for a thick ¯uorinated (treated with undiluted ¯uorine) PVTMS ®lm (F1 mm): s221. We have found that the ¯uorinated (with undiluted ¯uorine) PVTMS ®lm subjected to atmospheric action can be considered to consist of the following fragments: ±CF2±CF2± (844 vol%) and ±CF2± CF(SiOOH)± (164 vol%). The density of ¯uorinated PVTMS ®lm 1.73 g cmÿ3 (see Table 1). The s value of a ¯uorinated PVTMS ®lm (F1 mm) can be calculated from (Eq. (10)): s21.20.3. So there is quite good agreement of experimentally measured ( s221) and calculated s values. Another method can be applied to measure the F value. The transition (or absorption) spectrum of ¯uorinated polymer ®lms in the visible and near UV spectral regions have interference features and consist of a set of minima and maxima when frequency is measured in cmÿ1 (Fig. 6) and the F value can be calculated from the equation F 2 nF ÿ1
(11)
where (cmÿ1) is the distance between two neighboring interference maxima or minima. The main disadvantage of this method is that ¯uorination must be interrupted to carry out each measurement. This method can also be used to measure the dependence of the refractive indices of virgin and ¯uorinated polymers on wavelength or wavenumber in the visible and near IR by the equation: nF 2 F ÿ1
(12)
The quantitative value of the refractive index at a selected wavelength can be measured by an ordinary refractometer. A polymer ®lm ¯uorinated through all its thickness was used for this purpose. As an example the dependence of refractive index on wavenumber for a virgin PVTMS is shown in Fig. 18. Spectra of virgin and ¯uorinated polymers in the visible and near UV are shown in Fig. 19. Refractive indices of ¯uorinated polymers differ from those of virgin polymers and depend on the composition of the ¯uorinating mixture (Fig. 20). As mentioned earlier [1,3,5,7,10,13] C=O groups are introduced into the polymer structure by the treatment with a ¯uorine±oxygen mixture. Oxygen content in the ¯uorinating mixture in¯uences the rate of formation of the ¯uorinated layer on the surface of PPO and also decreases the permeability of ¯uorine through the ¯uorinated layer to the untreated layer (see Eq. (4)). The concentration of oxygen in F2±O2 mixtures was varied from 0.1 vol% (undiluted
136
A.P. Kharitonov et al. / Journal of Fluorine Chemistry 93 (1999) 129±137
Fig. 21. k value (Eq. (12)) vs concentration V (vol %) of oxygen in F2±O2 mixture for PPO (MW244 000)
Fig. 18. Refractive index n of virgin (1) and treated with undiluted fluorine (2) PVTMS film deposited on a solid support vs wavenumber (cmÿ1).
Fig. 19. Transmittance T(%) of a virgin PVTMS film (1) and treated throughout (i.e. totally fluorinated film) with undiluted fluorine (2).
Fig. 20. Refractive index of PPO treated with F2±O2 mixture (curves 1 and 2 correspond virgin PPO MW 34 000 and 244 000, respectively) vs ratio of fluorine and oxygen partial pressures pF/pO in the fluorinating mixture.
¯uorine) to 50 vol%. For all the mixtures used the correlation FAt0.5d was valid and d At0.5. For each F2±O2 mixture coef®cient A depends on pF as follows (see Fig. 11): A C pF k
(13)
where C is constant for any F2±O2 mixture. It is seen that there are two distinct ranges of the oxygen concentration: below 10 vol% the k value is close to 0.76 but above 20 vol% the k value drops to 0.46±0.53 (Fig. 21).
Similarly to our earlier results [5,7,10,13] we have attributed the inhibiting in¯uence of oxygen to the formation of C=O groups. It is known [30] that insertion of C=O groups decreases the permeability of polymers. To verify the suggestion that C=O group formation results in P decrease we have evaluated C=O concentrations in oxy¯uorinated PPO. To evaluate the concentration of C=O groups we applied IR spectroscopy as described in [5,7,10,13]. Absorption coef®cients (measured in vacuum) of thin ¯at-parallel PMMA (polymethylmethacrylate) ®lms (thickness 1±5 mm) cast on to ZnSe plates and treated with ¯uorine were used as reference data. It should be noted that C=O groups are also present in untreated PMMA (one group per monomer unit). Thicknesses of untreated and ¯uorinated layers were measured by interference spectroscopy. We have made two suppositions: (1) the concentration NC of C=O groups in untreated and ¯uorinated with undiluted ¯uorine PMMA in vacuum coincide with each other and are equalR to one C=O group per monomer unit, and (2) d values ( F ÿ1 log Tbackground =T , where T is the transmission value) ofR ¯uorinated PMMA and PPO coincide with each other ( d value is proportional to the C=O group concentrations). We Rhave shown experimentally that the difference between the d values of untreated and ¯uorinated PMMA in vacuum is less than 20%. Also R we have shown experimentally that the difference of d values for oxy¯uorinated PPO measured in vacuum and in a moist atmosphere does not exceed 20%. We have measured the dependence of the C=O group concentrations in situ by the following method. The IR spectra of the sample were monitored by an FTIR spectrometer (duration of a measurement of a Rsingle spectrum was 10 s) during the ¯uorination and the d value was R calculated for various treatment times. The d value was shown to be proportional to t0.5 (Fig. 22) and hence proportional to the F value. This means that the total of C=O groups in a ¯uorinated polymer is proportional to the F value and the concentration (i.e. amount of the C=O groups divided by the ¯uorinated layer thickness) C=O groups is ®xed in the course of treatment. The concentration of C=O groups depends weakly on pF for treatment with a mixture of constant oxygen concentration. The number NC of C=O groups per monomer unit of ¯uorinated PPO was estimated to depend on the R value (RpF/pO) as follows
A.P. Kharitonov et al. / Journal of Fluorine Chemistry 93 (1999) 129±137
Fig. 22. Total amount NC=O of C=O groups (arbitrary units) vs square root of the treatment duration t (s). Free PPO (MW244 000) was treated with F2±O2 mixture (20 volume % of O2). IR spectra were measured in the course of treatment
Fig. 23. The concentration NC of C=O groups vs ratio R of fluorine pF and oxygen pO partial pressures for PPO (MW244 000)
(Fig. 23): NC 2 1= 1 033 R
(14)
Acknowledgements The results of A.P. Kharitonov and his co-workers described in this publication were made possible in part by the grant no. 96 1277 from the INTAS and by grant no. 94 03 08197a of the Russian Foundation of the Basic Research. References [1] R.J. Lagow, J.L. Margrave, Progr. Inorg. Chem. 26 (1979) 162. [2] J. Jagur-Grodzinski, Progr. Polym. Sci. 17 (1992) 361.
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[3] M. Anand, J.P. Hobbs, I.J. Brass, in: R.E. Banks, B.E. Smart, J.C. Tatlow (Eds.), Organofluorine Chemistry: Principles and Commercial Applications, Plenum Press, 1994, p. 469. [4] A.P. Kharitonov, Yu.L. Moskvin, G.A. Kolpakov, Sov. J. Chem. Phys. 4 (1987) 877. [5] A.P. Kharitonov, Yu.L. Moskvin, Proceedings of the Second International Conference on Fluorine on Coatings, Salford, UK, 1994, Paper 13.. [6] G.A. Kolpakov, S.I. Kuzina, A.P. Kharitonov, Yu.L. Moskvin, A.I. Mikhailov, Sov. J. Chem. Phys. 9 (1992) 2283. [7] A.P. Kharitonov, Yu.L. Moskvin, Chem. Phys. Rep. (The former Sov. J. Chem. Phys.) 13 (1994) 818. [8] A.P. Kharitonov, Yu.L. Moskvin, Chem. Phys. Rep. (The former Sov. J. Chem. Phys.) 13 (1994) 830. [9] S.I. Kuzina, A.P. Kharitonov, Yu.L. Moskvin, A.I. Mikhailov, Russian Chem. Bull. 45 (1996) 1623. [10] A.P. Kharitonov, Popular Plastics and Packaging 42 (1997) 75. [11] A.P. Kharitonov, Yu.L. Moskvin, G.A. Kolpakov, Patent of the Russian Federation No. 1 754 191, filed 26 July 1990. [12] V. Frolov, V.V. Teplyakov, A.P. Kharitonov, Proceedings of the Second International Conference on Fluorine in Coatings, Salford, UK, 28±30 September 1994, Conference Papers, Paper 22. [13] A.P. Kharitonov, Yu.L. Moskvin, V.V. Teplyakov, J.D. Le Roux, Proceedings of the Third International Conference on Fluorine in Coatings ± II, Munich, Germany, 24±26 February 1997, Conference Papers, Paper 35. [14] A.P. Kharitonov, Yu.L. Moskvin, L.N. Kharitonova, M.N. Tulskii, A.A. Kotenko, Mendeleev Commun. No. 3 (1994) 91. [15] A.P. Kharitonov, Yu.L. Moskvin, L.N. Kharitonova, A.A. Kotenko, M.N. Tulskii, Kinetics and Catal. 35 (1994) 792. [16] A.P. Kharitonov, Yu.L. Moskvin, L.N. Kharitonova, A.A. Kotenko, M.N. Tulskii, Polym. Sci., Ser. B 37 (1995) 307. [17] D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1740. [18] Yu.Ya. Mikhailenko, Khimia I prakticheskoe primenenie kremniiorganicheskih soedinenii. Trudy conferencii (in Russian), Leningrad, 1958, p. 59. [19] Ya.I. Ryskin, M.G.Voronkov, Khimia I prakticheskoe primenenie kremniiorganicheskih soedinenii. Trudy conferencii (in Russian), Leningrad, 1958, p. 42. [20] D.S. Kim, Y.H. Lee, J. Electrochem. Soc. 141 (1994) 3562. [21] A. Lee Smith, Applied Infrared Spectroscopy, Wiley, New York, 1979. [22] P. Favia, G. Caporiccio, R. D'Agostino, J. Polym. Sci., Part A 32 (1994) 121. [23] J. Sakata, M. Yamamoto, I. Tajima, J. Polym. Sci., Part A 26 (1988) 1721. [24] Termodinamichiskie Svoistva Individual'nyh veschestv (Thermodynamical Properties of Individual Substances) (in Russian). Moscow, 1979, vol. 2, Book 1, p. 220.. [25] G. Socrates, Infrared Characteristic Group Frequences, 2nd ed., Wiley, New York, 1994. [26] A.P. Kharitonov, Yu.L. Moskvin, G.A. Kolpakov, Polym. Sci. USSR 27 (1985) 739. [27] A.P. Kharitonov, V.V. Teplyakov, Polym. Sci., submitted for publication. [28] S. Sugden, J. Chem. Soc. 125 (1924) 1177. [29] O.R. Quayle, Chem. Revs. 53 (1953) 539. [30] M. Salame, Polym. Preprints 8 (1964) 137.