Addition and Condensation Polymerization Processes

37 Preparation of Permselective Membranes by Radiation Grafting of Hydrophilic Monomers into Polytetrafluoroethylene

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.ch037

Films A. CHAPIRO, G. BEX, A. M. JENDRYCHOWSKA-BONAMOUR, and T. O'NEILL Laboratoire de Chimie des Radiations, C.N.R.S., 92-Bellevue, France Acrylic acid (AA) and vinyl pyridine (VP) were grafted into 0.05 and 0.1-mm. thick PTFE films by irradiating the latter immersed in aqueous solutions of the monomer. Selective inhibitors were used to minimize homopolymerization. The kinetics were investigated at different temperatures and radiation dose rates. AA was grafted in 50% aqueous solutions containing ferrous sulfate. VP was grafted in 80% aqueous solutions. The various kinetic results are discussed and compared with earlier data obtained when grafting styrene into PTFE. Polyfunctional membranes containing both AA and VP groups were prepared by grafting the two monomers in two successive steps. The resulting membranes were found to swell much more in water than membranes containing either AA or VP groups alone. A fairly abundant literature covers the graft copolymerization of polytetrafluoroethylene ( P T F E ) . One of the main features which con trols the kinetics of this reaction is the fact that P T F E does not dissolve or swell in any known reagent. Hence, grafting should be limited in principle to the surface of films, fibers, or molded objects made of P T F E (3,6,15). About 10 years ago it was found, however, that under suitable conditions styrene and methyl methacrylate can be grafted in the bulk of P T F E films by irradiating the latter immersed in a monomer or in a monomer solution (2, 4). This reaction was possible by a gradual pene tration of the monomer through the successive grafted layers which swell in the reaction medium; the grafted front slowly moves inward into the 560 Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


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film. The process involves competition between the diffusion of the monomer and its conversion to polymer. Several studies were devoted to such systems using P T F E films and fibers. Grafting was achieved either by the "direct radiation method" (9,14) or after preirradiation (7, 8, 12, 13, 17). Grafting was also found to occur "spontaneously" when P T F E films are immersed in a polymerizing monomer, the reaction being initiated either thermally or by peroxides (JO). Most likely in such an event grafting occurs by chain transfer to the polymer. More recently, we became interested in grafting hydrophilic mono mers into P T F E films, thereby generating permselective membranes ( 5 ). A preliminary report on the kinetics of the grafting of acrylic acid and 4-vinylpyridine was presented a year ago ( 1 ). This chapter is devoted to a detailed kinetic analysis of the reaction. Experimental P T F E films were irradiated with cobalt-60 γ-rays in vacuum-sealed ampoules containing aqueous monomer solutions. The following reaction parameters were investigated: radiation dose and dose rate, reaction temperature, and film thickness. Selective inhibitors were used to mini mize the homopolymerization of the monomer in the solution around the film. Acrylic acid (of Compagnie Nobel-Bozel or Uglior) was used with out further purification. 4-Vinylpyridine (Fluka) was distilled twice under vacuum (b.p., = 58°C. at 12 mm. Hg) and stored in a refrigerator under nitrogen. Small pieces of P T F E , 10 X 20 to 10 X 40 mm. were cut from commercial films 0.1- and 0.05-mm. thick. They were immersed for 24 hours in either chromic acid or methanol at room temperature, washed, and dried under vacuum. Irradiations were performed with cobalt-60 γ-rays from a 1000-curie source. The sealed ampoules were either irradiated in air at 20 °C. or in a water thermostat at different temperatures. Dosimetry was based on the oxidation of ferrous sulfate in 0.8N sulfuric acid, using G(Fe ) = 15.5. After irradiation the grafted films were extracted in methanol for 8 to 16 hours and dried in vacuo. The grafting ratio is expressed by the weight ratio W/W of the grafted film to the original P T F E film. The ratio (W — W )/W is the weight increase in grams per gram of original material. After extensive drying, the films were again swollen in water and thereafter treated with 0.1N HC1 or K O H . Their swelling ratio W /W was determined under these conditions (W is the weight of the swollen film). After drying, the films usually exhibited a weight increase corresponding to the theoretical addition of Κ or HC1 to the active groups. 2

3+

0

0

8

0

8

Results Carboxylic Membranes. P T F E films were irradiated in 50% aqueous solutions of acrylic acid. To prevent homopolymerization, 0.25% ferrous

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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sulfate (Mohr salt) was added to the reaction mixture. These reaction conditions were found to correspond to an optimum in our earlier work (5). I N F L U E N C E O F D O S E A N D D O S E - R A T E . Figure 1 is a plot of the conversion curves obtained with 0.1-mm. thick films at 20°C. with differ ent dose rates. The curves are composed of two portions: an initial rapid process with a short "induction period/' followed by a slower process which sets in fairly suddenly. The curves exhibit a break at a critical degree of grafting which gradually increases with dose rate. No break is observed at the highest dose rate used. This effect has been described (1,5). It seems to result from the following sequence of events. As the grafting proceeds, the P T F E chains uncoil gradually, and the polymer swells gradually in the reaction mixture. A limiting configuration is reached soon, however, which presumably corresponds to an elastic limit of the chains. At this point the grafted films swell only slightly in water, as was shown by direct measurements (I). If the grafting proceeds further, some of the polymeric chains may break under the mechanical forces developed by the swelling. This leads to more scattered results and to a greater swelling of the grafted films.

W

Hours Figure 1. Weight increase of 0.1-mm. thick PTFEfilmsirradiated in 50% aqueous solutions of acrylic acid at 20° C. and different dose rates (shown on curves)


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At higher dose rates, a given grafting ratio is attained by a higher total dose since the rate follows approximately the square root relation ship (see below). As a result, the P T F E chains then carry a larger number of shorter grafted branches, and this presumably helps their uncoiling. Experimental evidence shows that the above described effect vanishes at ca. 1000 rads per minute (see Figures 1, 2, and 4).

1

10

100

Hours

Figure 2. Log-log plot of weight increase of 0.1-mm. thick PTFE films irradiated in 50% aqueous solutions of A A at 20° C. and different dose rates (shown on curves) To determine grafting rates, the conversion curves were plotted on a log-log diagram (see also Refs. 2 and 4). This plot is shown in Figure 2. The initial portions of the curves are straight lines with a constant slope β = 1.25. The weight increase of thefilmsfollows the relationship: (W -W )/W 0

0

= Kt^

which corresponds to a fairly small "autoacceleration index" (4). The instantaneous grafting rate at 10% weight increase derived from the above results is plotted as a function of dose rate in Figure 3 on a log-log diagram. The plot obeys the following relationship: R

01

=

K10A»

At 0 ° , 40°, and 60°C. the dose-rate exponents are approximately the same.



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Figure 3. Influence of dose rate of y-rays on the rate of grafting AA on PTFEfilmsat different temperatures (0.1-mm. thickfilmsexcept when indicated)

Figure 4. Log-log plot of weight increase of 0.05-mm. thick PTFE films irradiated in 50% aqueous solutions of AA at 20°C. and different dose rates (shown on curves)


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Similar experiments were performed with 0.05-mm. thick films. The log-log plot of the conversion curves is shown in Figure 4. Here the autoacceleration index is β = 1.15. The break on the curves occurs at lower grafting ratios than for the 0.1-mm. films. The dose-rate exponent for the instantaneous rate at 10% weight increase is a = 0.45 as shown in Figure 3. I N F L U E N C E O F IRRADIATION T E M P E R A T U R E . Experiments were carried out at temperatures ranging from 0° to 95 °C. and at various dose rates. The following points were clarified in this work: ( 1 ) For a given dose rate the break in the conversion curves occurs at approximately the same conversion regardless of reaction temperature. (2) For 0.1-mm. thick P T F E films, the autoacceleration index—i.e., the slope of the initial portions of the conversion curves in log-log coordi nates—remains approximately the same at all temperatures and dose rates. Figure 5 shows the data obtained at 350 rads/min. The auto acceleration index is 1.25-1.29. The Arrhenius plot of the instantaneous rates at 10% conversion is shown in Figure 6. This plot is based on the ratios Rt/R2o°c. of the rates measured at different temperatures and dose rates with respect to the corresponding rates at 20 °C. It exhibits a break at a temperature of ca. 33 °C. A transition point of P T F E has been re ported at about the same temperature (11, 16). The activation energies are respectively 15.0 and 5.1 kcal./mole below and above 33°C. (3) For 0.05-mm. thick P T F E films the results are more complex. Between 20° and 40°C. the rate increase is approximately the same as for the thicker films, and the conversion curves are parallel (Figure 7).

Figure 5.

Log-log plot of weight increase of 0.1-mm. thick PTFE films irradiated at 350 rads/min. at different temperatures



ADDITION

Figure 6.

\ure 7.

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Arrhenius plot of the rate of grafting AA onto 0.1-mm. thick PTFE films

Log-log plot of weight increase of 0.05-mm. thick PTFE films irradiated at 45 rads/min. at different temperatures


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Above 40 °C. the autoacceleration index increases. The conversion curves obtained at 40°, 5 8 ° , and 73°C. cross, and the yields for a given dose are comparable. At 73° and 95°C. the autoacceleration index is 3.2 and the over-all activation energy determined on the basis of these two curves is 5.5 kcal./mole. The break in the conversion curves occurs at 8-10% grafting. Above this critical value all experimental points obtained between 40° and 95 °C. fall on the same line.


I N F L U E N C E O F " S E L E C T I V E " INHIBITORS. The use of a selective inhibitor

is essential for grafting acrylic acid deeply into P T F E films because this monomer polymerizes with an unusually high rate ( 5 ), and if no inhibitor is present, the monomer is converted under irradiation into a hard block of polymer before grafting has even started on the surface of the film. In our standard experiments described above we used ferrous sulfate which inhibits homopolymerization efficiently. However, its solubility in the reaction mixture is low, 0.25% being close to the saturation point at 20 °C. To explore further the influence of the inhibitor on the kinetics of grafting, experiments were carried out with cupric chloride at con centrations of 0.5, 2,5, and 50 X 10~ M. The results obtained with 0.1-mm. thick P T F E films at 20°C. and a dose rate of 200 rads/min. are shown in Figure 8. In contrast to the data shown in Figure 1, the initial portion of the conversion curves is autoretarded. A plot of these data on a log-log diagram leads to straight lines with slopes less than 1. 2

The grafting rates decrease as the concentration of C u C l rises. This effect demonstrates that the inhibitor not only interferes with homopolymerization but also affects the grafting process except perhaps in its initial stages (see Figure 8). Grafting occurs faster in the presence of FeS0 than at the lowest concentration of CuCL used, but the latter compound inhibits homopoly merization much more efficiently. At 20°C. and 2000 rads/min. the homopolymer formed in 50% aqueous solutions of acrylic acid amounts to 5% after 23 hours, in the presence of 0.5 X 10" M CuCl , whereas more than 20% of homopolymer accumulate after 7.5 hours irradiation in the presence of 0.25% (or 1 X 10" M) FeS0 . Anionic Membranes. Solutions of 4-vinylpyridine (80%) in water were used in this work. Preliminary experiments had shown that most inhibitors affect the grafting process strongly. The kinetics were there fore investigated without any additive. Most experiments were carried out at a single dose rate: 28 rads/min.; a few irradiations were conducted at 157 rads/min. 2

4

2

2

INFLUENCE OF DOSE AND DOSE RATE.

2

4

With 0.1-mm. thick P T F E

films the grafting did not proceed homogeneously, despite the low dose rate used. All grafted films exhibited crumpled edges, as was observed earlier when P T F E films were grafted near the critical conditions, where



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Figure 8. Weight increase of 0.1-mm. thick PTFEfilmsirradi ated at 20°C. and 2000 rads/min. in 50% aqueous solutions of AA containing various amounts of CuCl (concentrations expressed in moles/liter Χ 10 ) 2

2

the reaction becomes diffusion controlled (2,4). Systematic studies were therefore limited to 0.05-mm. thick films. The conversion curves obtained at 20 °C. exhibit marked autoacceleration. Straight lines are again obtained on a log-log diagram, but the slopes are as high as 4.8 (Figure 9). From experiments conducted at the two dose rates investigated the dose-rate exponent is estimated to be a = 0.47. At the higher dose rate the reaction was diffusion controlled, heterogeneous films being obtained at elevated grafting ratios. I N F L U E N C E O F IRRADIATION T E M P E R A T U R E . Experiments were carried

out at 2 0 ° , 3 0 ° , 3 5 ° , 4 0 ° , and 60°C. at 28 rads/min. The conversion curves are plotted in Figure 9 on a log-log diagram. The kinetics change suddenly between 30° and 35°C. Below 30°C. the acceleration index is 4.8; above 35°C. it drops to 1.9. The activation energies, calculated on the basis of the instantaneous rates measured at 10% grafting, were found to be 6.6 kcal./mole in both ranges of temperatures where they could be determined: 2 0 ° - 3 0 ° and 35°-60°C. I N F L U E N C E O F INHIBITORS. Benzoquinone and ferrous sulfate were tried as selective inhibitors. In both cases the rate of grafting was


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reduced strongly. Moreover, in the presence of an inhibitor the conver sion curves of the grafting process at 20 °C. are linear with dose up to high grafting ratios.


Discussion The various data obtained for the kinetics of graft copolymerization onto P T F E films demonstrate that this reaction is complicated by the fact that the rate of diffusion of the monomer may become the controlling factor. It seems interesting at this point to compare and discuss together the results obtained with the different monomers. Table I summarizes the data obtained for autoacceleration indexes (β), dose-rate exponents ( a ) , and over-all activation energies E, with styrene, acrylic acid, and vinylpyridine. Several conclusions can be derived from an examination of these data. Autoacceleration Indexes. The values of β differ widely for the different systems, and moreover, their variation with temperature has a different character depending on the monomer under consideration. " W-W •

w

0

0

20°

1.0

/ 0.1 5

10

50

100

Figure 9. Log-log plot of weight increase of 0.05-mm. thick PTFE films irradiated at 28 rads/ min. in 80% aqueous solutions of 4-VP at different temperatures (shown on curves)


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Table I. Autoacceleration Indexes, β, Dose-Rate Exponents, a, and Over-all Activation Energies, £ , for the Direct Radiation Grafting of Various Monomers into PTFE Films


Monomer

Irradiation PTFE Films, Temperature, °C. mm.

β

20 40 60

2.5-3.2 1.5-1.6 1.0-1.2

Styrene

0.1 0.1 0.1

Acrylic acid (+50% water + 0.25% Mohr salt)

0.1 0.1 0.05 0.05

0-33 33-95 20-40 73-95

1.25-1.29 1.25-1.29 1.15 3.2

4-Vinylpyridine (+ 20% water)

0.05 0.05

20-30 35-60

4.8 1.9

a

E, kcal./mole

Depends on 0.5 0.68 conversion at 0.72 which rates are measured 15 0.45 0.45 5.1 0.45 —15 0.45 —5.5 —0.47

—

6.6 6.6

With both styrene and vinylpyridine, the autoacceleration index decreases as the reaction temperature rises. This effect can be considered normal behavior of polymerizing systems in which the "gel effect" is operative. As the temperature rises, the termination step, which involves the interaction of two polymeric chains in a highly viscous medium, increases in rate, and the over-all reaction tends to become "normal." Ultimately, the stationary-state conditions may eventually apply. This seems to be the case with styrene at 60 °C. where β is almost unity—i.e., where the reaction proceeds with a constant rate. The doserate exponent of this system indicates, however, more complicated kinetics. A difference between these two systems is that in the case of styrene, β decreases gradually as the temperature rises, whereas with vinylpyri dine a sudden change occurs between 30° and 35°C., which seems to be related to the transition point of P T F E . With acrylic acid the situation is very different. For 0.1-mm. thick films β is independent of temperature, whereas for 0.05-mm. thick films β rises above 40°C. At low temperatures β is unusually small. This effect may be ascribed to the use of a selective inhibitor to prevent homopolymerization. It was shown above that in grafting vinylpyridine the autoaccelerated character of the reaction at 20 °C. vanishes in the presence of inhibitors. Moreover, with acrylic acid the autoacceleration index is lower if a more efficient inhibitor (CuCl ) is used, and β decreases as the amount of inhibitor increases (Figure 8). 2

It is likely, therefore, that the autoacceleration would have been more pronounced if acrylic acid could be grafted on P T F E films without


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using any inhibitor. The results obtained at 73° and 95 °C. with the 0.05-mm. thick P T F E films are of interest in this respect. Here β rises to 3.2. This suggests that at elevated temperatures the inhibitor interferes less with the grafting process, and this in turn may be a consequence of the very rapid diffusion of the monomer. Another peculiar feature of the grafting of acrylic acid is the break observed in the conversion curves particularly at low dose rates. The interpretation of this effect proposed above does not account for the fact that no break is observed with other monomers except at much higher grafting ratios (2, 4). These striking differences in kinetics for systems which in principle should exhibit comparable behavior are presumably related to differences in diffusion rates and polymer-polymer and polymer—monomer compatibilities. Little is known at present on the factors which govern these effects and on their influence on the kinetics. Influence of Temperature. The reaction temperature necessarily plays a major role in grafting, where at least two reaction steps (chain propagation and termination) may become diffusion controlled. Indeed, grafting occurs faster at elevated temperatures, but here again each system exhibits a specific behavior. With styrene, the autoacceleration index gradually changes with temperature which makes it impossible to define an activation energy since its apparent value depends strongly on the grafting ratio at which the rates are measured. With both acrylic acid and vinylpyridine the kinetics change sud denly between 30° and 35°C., but the nature of this change is different for the two systems. With acrylic acid and 0.1-mm. thick P T F E films the autoacceleration index remains constant over the entire temperature range investigated ( 0 ° - 9 5 ° C ) , and only the Arrhenius plot exhibits a break at 33°C. This effect can be understood by assuming that the diffusion of the monomer into the partially grafted P T F E films requires a lower temperature coefficient in the P T F E structure prevailing above 33 °C. than in the low temperature phase. For 0.05-mm. thick films a more profound change occurs at about the same temperature, resulting in a change of the autoacceleration index. From the discussion above one may accept that this effect involves a competition between the dif fusion of monomer and inhibitor ( ferrous sulfate ). For vinylpyridine the sudden decrease of β above 30 °C. conforms with a faster termination step resulting from an increased mobility of the growing chains owing partly to a higher swelling of the films and partly to increased thermal motion. Here again, one has to assume easier diffusion of the monomer into the high temperature structure of P T F E .


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J*olyfunctional Membranes


To explore further the characteristic properties of membranes de rived from P T F E films, several attempts were made to graft mixtures of AA and VP or to graft the two monomers in two successive steps. Both techniques failed because a spontaneous reaction occurs when 4-VP is brought in contact with AA (or its polymer), leading to the formation of a reddish-brown polymer which is extracted readily by acidic or alkaline solutions. Membranes containing both types of active groups were prepared successfully, however, by grafting the two monomers, one after the other, provided the intermediate membrane obtained in the first grafting step was neutralized before starting the second grafting operation.

Figure 10.

Percent swelling in water as a function of percent grafting for different membranes based on PTFE 1: 2: 3: 4:

4-VP grafts (free pyridine) AA grafts (free acid) 4-VP grafts (hydrochloride) AA grafts (potassium salt)

Closed circles pertain to difunctional membranes with various compositions

A number of difunctional membranes were thus obtained by either of the two following reaction sequences: (a) Grafting AA, neutralization by KOH, grafting 4-VP. (b) Grafting 4-VP, neutralization by HC1, grafting AA.


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573

The kinetics of these reactions will be described elsewhere. We would like to present here some unusual properties of such difunctional membranes. The grafted films were treated with a solution of 0 . 1 N K O H to convert the carboxylic groups into potassium salt. The resulting membranes swelled to a much larger extent in water than any of the other membranes studied. The swelling data are summarized in Figure 10. The percent swelling (W — W/W X 100) is plotted as a function of percent grafting (W — W /W X 100) for the different types of membranes. The neutralized membranes always exhibit larger swelling ratios than either the acid or the alkaline forms (see also Refs. 1 and 5). For a given grafting ratio the carboxylic membranes swell more than the vinylpyridine grafts. A l l difunctional membranes examined exhibit larger swelling ratios than the neutralized carboxylic membranes. This unusual behavior of difunctional membranes is being investigated further. s


0

0

Literature Cited (1) Bex, G . , Chapiro, Α., Huglin, M . , Jendrychowska-Bonamour, A . M . , O'Neill, T . , J. Polymer Sci., Pt. C 22, 493 (1968). (2) Chapiro, Α., J. Polymer Sci. 34, 481 (1959). (3) Chapiro, Α., Magat, M . , Sebban, J., French Patent 1,130,099 (1956). (4) Chapiro, Α., Matsumoto, Α., J. Polymer Sci. 57, 743 (1962). (5) Chapiro, Α., Seidler, P., European Polymer J. 1, 189 (1965). (6) Chen, W . K. W . , Mesrobian, R. B., Ballantine, D . S., Metz, D . J., Glines, Α., J. Polymer Sci. 23, 903 (1957). (7) Dobo, J., Somogyi, Α., Proc. Tihany Symp. Radiation Chem., Budapest, 1964, p. 195. (8) Dobo, J., Somogyi, Α., Czvikovsky, T . , J. Polymer Sci., Pt. C 4, 1173 (1964). (9) Dobo, J., Somogyi, Α., Lakner, E . , Plaste Kautschuk 7, 393 (1960). (10) Lebel, P., Chen, W . K. W . , Chapiro, Α., J. Polymer Sci., Pt. C 4, 1193 (1964). (11) McCrum, N . G . , J. Polymer Sci. 34, 355 (1959). (12) Munari, S., Vigo, F., Tealdo, G . , Rossi, L . , J. Polymer Sci., Pt. Β 4, 547 (1966). (13) Munari, S., Vigo, F., Tealdo, G . , Rossi, L . , J. Appl. Polymer Sci. 11, 1563 (1967). (14) Odian, G . , Acker, T . , Rossi, Α., Ratchik, E . , J. Polymer Sci. 57, 661 (1962). (15) Restaino, A. J., Reed, W . N . , J. Polymer Sci. 36, 499 (1959). (16) Schultz, A. K., J. Chim. Phys. 53, 933 (1956). (17) Sinitsyna, Ζ. Α., Tsvetkov, Yu. D . , Bagdasaryan, Kh. S., Voevodskii, V . V . , Dokl. Akad. Nauk. SSSR 129, 631 (1959). RECEIVED

March 22,

1968.


Addition and Condensation Polymerization Processes

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