Track-etched membrane: dynamics of pore formation

Nuclear Instruments and Methods in Physics Research B 84 (1994) 331-336 North-Holland

Track-etched

membrane:

LiWNl B

Beam Interactions with Materials 8 Atoms

dynamics of pore formation

E. Ferain * and R. Legras Unite‘ de Physique et de Chimie des Hauts Polym&es, Universite’ Catholique de Louvain, Croix du Sud I, 1348 Louvain-la-Neuue, Belgium

Received 10 September 1993 and in revised form 8 October 1993

The dynamics of pore formation during etching of heavy ion (Ar 9+ - 4.5 MeV/amu) irradiated bisphenol-A polycarbonate (PC) and polyethylene terephthalate (PET) films is determined by a conductivity cell. This work presents the theoretical basis of this method and describes the experimental procedure. The obtained results allow the determination of the track (V,) and bulk (V,) etch rates, and an estimate of the damage zone diameter in PC before etching.

The irradiation of most polymeric materials with energetic heavy ions leads to the creation of linear

narrow paths of intense damage called tracks. Tracks may be revealed by a properly chosen chemical agent that preferentially attacks the latent track [1,2]. Prolonged etching allaws the formation of pores with diameters of several microns leading to thin f < 50 &ml planar m~~o~ltratjon membranes with a very sharp cutoff. There exist many applications for these membranes [3-61. E.g., their very sharp cutoff allows their use in the filtration field, but they are also useful as in vitro substrates in cell biology [7]. Pore formation dynamics during etching can be studied by electrical conductivity measurements [8-f4]. This method allows to relate the pore diameter with the etch time, and allows to determine track (V,) and bulk etch rates. First, this work presents the theoretical basis of this conductivity method and describes the used apparatus. Second, typical curves of the dynamics of the pore formation are shown.

2. I. Materials Two types of polymer film are used: a 25 micron thick bisphenol-A polycarbonate (PC) film from Gen-

* Corresponding author. 0168-583X/94/$07.00

era1 Electric (Lexan-A) and a 23 micron thick polyethylene terephthalate (PET) film from DuPont (Mylar-A); they arc both used as received

2.2. fkauy ion irradiation and W illumination Heavy ions fAr9’) are generated in the ion source (CIctopus) of the accelerator of the Centre de

Recherches du Qclotron at Louvain-la-Neuve and accelerated to 180 MeV. The irradiation is carried out under vacuum (lo-* mbar) at room temperature. The maximum used fluence rate is 4 X 10’ ions cm-2s-1S During irradiation, the polymer film is pulled through the scanned ion beam at constant speed; the irradiation fluence ior desired pore density) is determined from the tluence rate and from the fifm velocity. The film is pulled over a bending roll to increase the angular spread of the tracks, and thus raise the selectivity of the microfiltration membrane by avoiding parallel double tracks. After the irradiation, the film is stored in dark at room temperature. To increase the selectivity of the etching, the heavy ion irradiated film is illumjnated in air with 4 Philips black light 360 nm UV lamps which are 5 cm apart. During this ~l~um~nat~on, the film is situated 10 mm from the lamps and the temperature at the surface of the films reaches 50°C. Etching is performed with an NaOH aqueous solution at a controlled temperature (50.0 or 60.7 + O.OS’C). A surfactant (Dowfax 2A1 - Dow Chemical) is added at 0.1 ~01% to improve the wetting of the film. The resuiting micro~ltration membranes are analysed with scanning electron (Hitaachi S570) and optical (Leitz Wetzlar) microscopes connected to an image

8 1994 - Elsevier Science B.V. All rights reserved

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E. Ferain, R. Legras /Nucl. Ins@. and Meth. in Phys. Res. B 84 (1994) 331-336

332

analyser (Kontron MB): the open area of the membrane and the pore density are determined. 2.3. Electrical conductivity measurement 2.3.1. Principle The electrical membrane resistance CR,) of the etch medium filled pores is measured as a function of etch time. On the basis of Ohm’s law, this resistance is proportional to the electrical resistivity of the solution (p) and to the pore length (0, and inversely proportional to the open area of the membrane (S); this open area depends on the pore density (N), on the analysed film surface area (A) and on tbe pore diameter Cd):

(1) This relation allows the calculation of the pore diameter from the measured value R,. Eq. (1) is based on the following assumptions: i) All the pores are cylindrical; otherwise, the electrical resistivity is given by R,=---

4P

+1/2

ANP

/

-

dz

-L/Z d’(z)

’

where d(z) is the pore diameter at the axial distance z. If the pore shape is a “diabolo”, an effective diameter can be calculated according to Eq. (21. The relation between R, and the effective pore diameter is then given by Eq. (3), where d, is the inner pore diameter and d, is the outer pore diameter: R,=-

4P AN?r d&

with

d,, = a.

If the effective diameter cd,,) is measured by electrical conductivity measurement and if the outer diam-

heavy ion ir&diated Fig. 1. Representation

film

of etch device used for conductivity measurement.

eter (d,) is observed by SEM, the calculation of the inner diameter Cd,) and the evaluation of the shape of the pore is possible. ii) All pores are identical; otherwise, the calculated effective diameter is a mean value. The pore diameter disparity may be due (a) to the heterogeneity of the polymer film, (b) to the variation of the linear energy density deposited by the ions in the film, and (c) to the disparity between the track lengths. iii) The pore diameter must be much smaller than the pore length (I/d > 100); for larger pores, the distortion of the current streamlines at the pore entrance must be taken into account. So it has been shown [12] that the pore length (I) has to be replaced by an effective pore length (I’ = I -I-0.82d) to take this additional resistance into account. iv) Finally, the resistivity of the electrolyte inside the pores is equal to the resistivity of the etch solution; in other words, the electrical conductivity of the pore walls and the variation of the solution electrical conductivity during etching are negligible. 2.3.2. Etch deuice The film to analyse is clamped between two half-cells (Fig. 1) each consisting of two concentric glass cylinders. The inner cylinder contains the etch solution (electrolyte), and the temperature of this solution is controlled by water circulating in the outer cylinder. The area of the analysed film is defined by a diaphragm (diameter = 0.45 cm) made of two Teflon rings. The inner cylinder has three inputs for (a) fitting a thermometer, (b) fitting a square platinum electrode (1 cm2>, and (c) filling the cell with preheated etch solution. Also, an output for draining the etch solution is installed. 2.3.3. Electrical measurements (Fig. 2) A square wave generator (model 182-A Wavetek; output resistance r0 = 50 a> imposes a square wave (STl) to one platinum electrode; the used amplitude and frequency are, respectiveIy, 1 V and 1000 I-Iz to avoid electrode polarisation. The other platinum electrode is grounded by a small resistance (r, = 1 a). The electrical resistance of the conductivity cell is (R, + R,), where R, is the electrical resistance of the electrolyte between the film and the electrodes. The impedance analyser measures the value (I&,) of the terminal voltage (ST21 at the end of each halfperiod (r/2); V,,, is then only a function of the resistive component of the electrical circuit (rO + R, + R, + r,) and is independent of the capacitive component (C) of the cell conductivity if discharging of (C) is completed during (7/2). A “cut signal” (ST3) defined by the user by means of the impedance analyser defines the measurement period of the terminal voltage (ST2).

333

E. Ferain,R Legras/Nucl. Instr. and Meth. in Phys. Res. B 84 (1494) 331-336 OSCILLOSCOPE

illI_.IN

RECORDER

ELECTRICAL CIRCUIT

I_,._.______________~...._.....,..~~”.,.............

Fig. 2. ~~~duct~ty measurement fr, = generator ouput resistance; I?, = electrical resistance of track pores; R, = electrical resistance of etectrolyte between film and electrodes; C = capacity of cell; V,,, = output voltage; ST1 - generator output voltage; ST2 = terminal voltage; ST3 = trigger signal).

An oscilloscope (Tektronix) is used for the visuaIisation of the signals STI, STZ and ST3. Finally, V,, is recorded as a function of etch time. 23.4. C&bra tion For calibration, the cell is replaced by resistances of known value and the relation between the recorded voltage (V,,) and the resistance value is established {Fig. 3).

The knowledge of the voltage (V,,,,) allows the determination of the resistance (rO + R, + R, + r,) by means of the calibration curve. A parallel measurement without membrane (R, = 0) allows the calculation of the resistance R,. The resistance R, can thus be determined and the mean effective pore diameter calculated by means of Eq_ (3).

The smallest observable resistance is defined by (r, -+R, f r,f and is limited by electrode polarisatism. The maximum observable resistance depends on the time constant of the system. Thus maximum and mini-

103

102

lo’ 100

L.....

102

“,..,.I

1

103

“....,I

~’

IO4

“...‘.*

CALIBRATION RESISTANCE &2)

Fig. 3.

~alibrat~~~ curve of conductivity ceil.

105

1

334

E. Ferain, R. Legras /Nucl.

Instr. and Meth. in Phys. Res. B 84 (1994) 331-336

Table 1 Maximum and minimum observable pore diameters with conductivjty cell as a function of pore density; values given for SN NaOH aqueous solution at 60°C and for 25 pm thick film Pore density [cm-*]

Maximum observable pore diameter

Minimum observable pore diameter

h-4

bml

IO3 104

10s 106 10’

0.15 0.50

6’

0.50 0.15 0.05

0

0

mum observable pore diameter pore density (Table 1).

are a function of the

3. Results and discussion A first measurement was performed with a 25 micron thick polycarbonate film. The irradiation fluence (N) was 1.3 X lo5 cm-‘, the UV illumination time was 120 min and the etching was carried out at 60.7”C with SN NaOH aqueous solution containing surfactant (0.1 vol%). The variation of the measured electrical conductivity (l/R,) as a function of etch time is plotted in Fig. 4. The mean effective pore diameter is then calculated with Eq. (3) and is plotted as a function of etch time in Fig. 5. For short etch times, no electrical conductivity is detected; this incubation time corresponds to the first pore breakthrough. Thereafter, the increase of the electrical conductivity corresponds to the enlargement of the first pores and to the breakthrough of additional pores. Finally, all latent tracks are converted to pores.

TIME

bin)

Fig. 4. Measured electrical ~onductivi~ (l/R,) as a fundion of etch time (PC fiim; irradiation fluence = 1.3X lo5 cm-‘; W illumination time = 120 min; etching at 60.7”C with 5N NaOH aqueous solution containing 0.1 ~01% surfactant).

J!

B

(.

5

‘.

1

1“. 10

. 1 15

ETCH

TIME fmin)

”

“I”,

‘I

20

‘I

“1

25

30

Fig. 5. Calculated mean effective pore diameter as a function of etch time (PC film; irradiation fluence= 1.3X10’ cm-*; W illumination time = 120 min; etching at 60.7”C with 5N NaOH aqueous solution containing 0.1~01% surfactant).

The increase of the conductivity is further on only due to the enlargement of the pores. The extrapolation to zero of the conductivi~ in Fig. 4 gives the breakthrough time t, (point A), which is equal to 3.2 min. The calculation of the average track etch rate (V,) can then be defined by K=

film thickness

= 2 ii

Il;mmin = 3.9 [J,m/min].

2G Fig. 5 shows that the enlargement growth rate of the pore diameter becomes constant for etch times above 10 min (point B). The curve slope corresponds then to the bulk etch rate (V,) and is equal to 0.0094 pm/min. This value is comparable to that determined by weight measurements (0.010 pm/min) 1151. Fig. 6 shows the etch rate as a function of mean effective pore diameter determined from Fig. 5. It appears that the etch rate becomes almost constant for diameters above 0.1 p,rn (point C). Assuming that all

0 ETCH

‘.

0.1

0.2

0.3

0.4

MEAN EFFECTIW PORE DIAMETER(pm) Fig. 6. Etch rate as a (PC film; irradiation tion time = 120 min; ous solution

function of mean effective pore diameter fluence = 1.3~ 10’ cm-‘; UV illuminaetching at 60.7”C with 5N NaOH aquecontaining 0.1 ~01% surfactant).

E. Ferain, R. L.egras/Nucl. Instr. and Meth. in P&s. Res. B 84 fI994/ 331-336

pores are perforated at the same time, the damage zone of the track has a diameter of 0.1 Wm. Nevertheless, for diameters above 0.03 pm (point D), the etch rate is no more than 0.03 pm/min, and thus 100 times lower than the etch rate of the core track (V, = 3.9 pm/mini. The major part of the damage is therefore localised in a very narrow region having a diameter of approximately 0.03 km; the damage density quickly decreases outside this region and finally becomes nearly zero at a distance from the core track higher than 0.05 pm. In another type of measurement, we observed the influence of an intermediate UV illumination on the pore formation dynamics. The used film is polyethylene terephthalate; the irradiation fluence is 1.5 x lo5 crns2 and the etching is carried out at 5O.o”C with 5N NaOH aqueous solution containing surfactant (0.1 ~01%). The UV illumination times are 30, 60, 120, 240 and 480 min. Fig. 7 displays the mean effective pore diameter as a function of etch time for the chosen UV treatments. The most important difference between the obtained curves is the breakthrough time: it decreases with increasing UV illumination time (Fig. 8). It seems also that the bulk etch rates are similar in the five curves: +0.0023 ~m/min; this is comparable with the values determined by weight measurements: 0.00230 and 0.00236 km/min, respectively after 30 and 120 min UV irradiation [15]. Similar results are obtained with PC film. Fig. 8 displays the track etch rate (V,) as a function of UV illumination time (irradiation fluence = 3 X lo8 cmW2; etching carried out at 40°C with a 3.75N NaOH solution (water 60%/methanol 40%) containing surfactant (0.1 ~01%)). Similar to PET film, V, increases with increasing UV illumination time. This effect has been

UV

ILL~~AT~O~

335

TIME bin)

Fig. 8. Track etch rate (V,> as a function of UV illumination time (PC film; irradiation fluence = 3 x lo8 cm-‘; etching at 40°C with 3.75N NaOH solution (water 60%/methanol 40%) containing 0.1 vol% surfactant).

explained in ref. [16]. It has been shown that a wellchosen UV source modifies the track compounds without increasing the bulk etch rate of PC. The action of UV treatment is attributed to chain rupture and radical formation reacting with 0, and creating new chain ends; moreover, the formed fragments possess a high polarity improving the diffusion of the etch solution along the ion path, and an acidic character leading to a high sensitivity to the etch process. All these modifications improve the track etch rate.

4. Error estimate 4.1. Etch time error The etch times plotted in Figs. 4 and 7 are averages from 3 or 4 different measurements respectively for PC and PET. The relative error varies from 1 to 10% of the mean value.

calculated a 0.2 E 3 0.21: P w

12

3

4.2. Error of mean effective diameter (Table 1)

5

4

Y5 E 0.10 B 0.05 ki ~~~ ~

"0

10

20

30 40 59 60 70 ETCH TIME hid

80

90 109

Fig. 7. Mean effective pore diameter as a function of etch time for different UV illumination times (1: 480 min; 2: 240 min; 3: 120 min; 4: 60 min; 5: 30 min). Breakthrough points (t,) are respectively 14.9, 18.4, 24.6, 34.9 and 52.2 min (PET film; irradiation fluence = 1.5 x lo5 cm-*; etching at 50°C with 5N NaOH aqueous solution containing 0.1 ~01% surfactant).

Derived from the pore density N: it is measured by SEM on the sample used for the electrical conductivity measurement; it is estimated from the number of pores present in 10 different areas of the sample. The relative error varies from 6 to 10% of the mean value. Derived from the pore length: the mean effective pore diameter is calculated taking the thickness of the film (e) as pore length (il. As during the irradiation the film is pulled over a cylindrical roll, the pore length varies from (e) to a value dependent on the roll curvature and height of the irradiation beam. E.g., if a roll with a diameter of 120 cm is used and for a vertical beam width of 5 cm, the maximum pore length is 27.5 km for a film with a thickness of 25 km. The calcu-

336

lated mean effective derestimated.

E. Ferain, R Legras / Nucl. Instr. and A4eth. in Phys. Res. B 84 (1994) 331-336

diameter

is therefore

weakly un-

5. Conclusions The method described here allows the determination of characteristic etch rates, Vt and VB, of a heavy ion irradiated polymer film. It can be used to establish relations between, on the one hand, V, and VP and, on the other hand, the characteristic parameters of the ionic irradiation (ion mass and energy), the chemical treatment (nature, concentration, temperature, etc.) and possible intermediate treatments (e.g. UV illumination). Recently [17], we have considered to prepare microfiltration membrane from poly(ary1 ether ether ketone) (PEEK) film to increase membrane properties as heat and solvent resistance; moreover, the surface of this polymer film may be easily modified with covalent linked chemical groups which could lead to interesting biological uses. We determined the modifications induced by heavy ion irradiation and UV illumination in PEEK film, and defined etch conditions for the membrane realisation. The conductivity method could be useful for the optimisation of the etch parameters.

References [l] P.B. Price and R.M. Walker, J. Appl. Phys. 33 (1962) 3407. [2] R.L. Fleischer, P.B. Price and R.M. Walker, in: Nuclear Tracks in Solids (Univ. of California Press, 1975).

[3] G.N. Flerov and VS. Barashenkov, Sov. Phys. -Usp. 17 (1975) 783. [4] Commission of the European Communities, in: Future Industrial Prospects of Membrane Processes (Elsevier, London, New York, 1989). [.5] E. Ferain and R. Legras, Nuclear tracks in polymer films and membrane preparation, 23rd Congr. National du GFP, Pau (1993) p. 155. [6] J.C. Bisconte and P. Pouppez, La cytomttrie sur filtre, Biofutur 120 (1993) 1. [7] T. Sergent-Engelen, C. Halleux, E. Ferain, H. Hanot, R. Legras and Y.-J. Schneider, Improved cultivation of polarized animal cells on culture inserts with new transparent polyethylene terephthalate or polycarbonate microporous membranes, Biotechnol. Techn. 4 (1990) 89. [8] C.P. Bean, M.V. Doyle and G. Entine, J. Appl. Phys. 41 (1970) 1454. [9] J.A. Quinn, J.L. Anderson, W.S. Ho and W.J. Petzny, Biophys. J. 12 (1972) 990. [lo] G. Guillot and F. Rondelez, J. Appl. Phys. 52 (1981) 7155. 1111A.P. Yu, Nucl. Tracks 6 (1982) 115. S.K. Mahna and L.V. Sud, Appl. Ra1121 SK. Chakarvarti, diat. Isot. 37 (1986) 1089. 1131W.-D. Mittmann, G. Siegmon, R. Beaujean and W. Enge, Proc. 11th Int. Conf. on Solid State Nuclear Track Detectors, Bristol, UK, 1981 (Pergamon, 1982) p. 727. 1141G. Schnoor, H. Schiitt, R. Beaujean and W. Enge, Proc. 11th Int. Conf. on Solid State Nuclear Track Detectors, Bristol, UK, 1981 (Pergamon, 1982) p. 51. entre les ions lourds [151 E. Ferain, Etude de l’interaction Cnergktiques et les polymtres, Ph.D. Thesis, Universitk Catholique de Louvain (1991). [I61 E. Ferain and R. Legras, Nucl. Instr. and Meth. B 82 (1993) 539. [171 E. Ferain and R. Legras, Nucl. Instr. and Meth. B 83 (1993) 163.