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Langmuir 1994,10, 592-596
Effect of Surface Moisture on Contact Charge of Polymers Containing Ions S. Pence, V. J. Novotny, and A. F. Diu' IBM Almaden Research Center, K93l801, 650 Harry Road, Sun Jose, California 95120 Received May 28, 1993. In Final Form: November 3 , 1 9 9 P
The effect of humidity on the contact charge which develops between the ion-containing polymeric powders and steel beads was examined. The powders are a blend of a styrene-butyl methacrylatecopolymer and minor amounts of the acid form of a partially sulfonated Polystyrene ionomer or the corresponding the contact charge on the blends is effectively that of the sodium salt. At 0% relative humidity (RH), host copolymer. It rises as the humidity increases from 0 to 20-40% RH and then decreases at the higher humidities. A higher contact charge is attainedwith the acid form of the ionomer than with the corresponding sodium salt which in turn charges higher than the copolymer. The surface water layer thickness of the blends was measured by ellipsometry and also found to increase with humidity. The optimal charge is observed in the humidity range where about one monolayer of water adsorbs on the polymer and oxide surfaces. The decrease in charge at the higher humidities is attributed to the ineffective mixing between the powder and the beads during rolling, plus charge "run-off" due to reduction of the surface electrical resistance as detected by ac dissipation. The contact charge is attributed to the mobilization of dissociated ions (from the ionomer) in the surface water layer followed by ion transfer to the second surface during the contact. Introduction In our previous studies on contact charging with polymer blends containing ionomers, it was shown that the sign of the charge which develops on the blend can be anticipated by considering which ion of the ion pair is bonded and immobilized.'" These results lend support to the model that materials containing ions charge largely by ion transfer between the contacted surfaces. For example, poly(styrene-co-methylvinylpyridiniumtoluenesulfonate), [Pl-PyMe+OTs-, which has bonded cations and mobile anions, induces a positive charge to the polymer blend. On the other hand, the partially sulfonated polystyrene, [P]-PhSOs-H+, and the corresponding sodium salt, [P]-PhSOs-Na+,4 which have bonded anions and mobile cations, induce a negative charge to the blend. Similar results have been reported in previous patents with ionomers containing other ion types such as protons or arylphosphonium or alkylammonium cations and MeOS03-, OTs-, C1-, or B r a n i ~ n s . ~With - l ~ the polymer blends containing ionomers, the charge increases monotonically with the ion content. However the response is not linear and the deviation from linearity has been attributed to the importance of ion pairing feq 1).12 A model for the observed dependence of the charge on ion concentration was developed on the basis that only Abstract published in Advance ACS Abstracts, January 15,1994. (1)Diaz, A. F.;Fenzel-Alexander, D.; Miller, D. C.; Wollmann, D.; Eisenberg, A. 63rd Colloidand Surface ScienceSymposium,ACS, Seattle, WA, 1989. (2)Dim, A.; Fenzel-Alexander, D.; Miller, D. C.; Wollmann, D.; Eisenberg, A. J. Polym. Sci., Polym. Lett. Ed. 1990,28,75. (3)Diaz, A.; Fenzel-Alexander, D.; Wollmann, D.; Eisenberg, A. J. Polym. Sci., Part B Polym. Phys. 1991,29,1559. (4)Diaz, A. F.;Wollmann;D., Dreblow, D. Chem. Mater. 1991,3,997. (5)Wollmann, D.; Dreblow, D.; Eisenberg, A,; Diaz, A. Chem. Mater. 1991, 3, 1063. (6)Guay, J.; Ayala, J. E.; Diaz, A. Chem. Mater. 1991,3,1068. (7)Anderson, J. H.;Bugner, D. E. US Patent 4,837,391,June 6,1989. Wilson, (8)Anderson, J.H.;Bugner,D.E.;DeMejo,L.P.;Sutton,R.C.; J. C. US Patent 4,837,392,June 6,1989. (9)Watanabe, M.; Nagase, H. US Patent 4,883,735,Nov 28, 1989. (10)Mizes, H.A.; Conwell, E. M.; Salamida, D. P. Appl. Phys. Lett. 1990,56,1597. (11)Gruber, R. J.; Bolte, S. B.; Agostine, D.; US Patent 4,415,646, November 15,1983. (12)Diaz, A.; Fenzel-Alexander, D.; Wollmann, D.; Barker, A. J. Langmuir 1992,8,2698.
[P]-PyMe+OTs-
[P]-PhSOs-H+
dissociated ions, M+ or X- (eq 11, on the surface of the polymer transfer to produce charge separation between the two surfaces. In eq 1,M+ X- can represent [PI-M+Xor [Pl-X-M+ depending on which ion is "anchored" to the polymer. M+X- M+ + X(1) Differences are observed in the magnitude of the charge which have not been adequately explained. The magnitude of the charge produced by [PI-PyMe+ OTs- and [Pl-PhS03-H+ is nearly equal but greater than the charge produced by [Pl-PhS03-Na+. This variation has been attributed tothe difference in hydration of the Na+cation, since humidity is known to affect the charge.l"l6 In particular, surface moisture will have an affecton the ionic equilibria associated with the charge transfer process, such as, ion pair dissociation, ion pair aggregation and ion distribution between the surfaces. Although surface moisture has been implicated in the charging process with materials containing i0ns,~3-'~ and without ions,16understanding of the role of surface water in the charging process requires a quantitative study of the water content and its influence on the surface properties. Apart from the water layer thickness, ion concentrations and mobilities alsoneed to be determined to better describe the charge transfer (13)Mataui, N.; Oka,K.; Inaba, Y.The Sixth International Congress on Advances in Non-Impact Printing Technologies Black & White and Color, 1990;p. 45. (14)Mateui, N.;Oka,K.; Inaba, Y.The Sixth International Congress on Advances in Non-Impact Printing Technologies Black & White and Color, 1990; p 123. (15)Wollmann, D.; Diaz, A., SPIE/SPSE Conference on Electronic Imaging Science and Technology, 1991, San Jose, p 111. (16)Folan, L.M.; Arnold, S.; O'Keeffee, T. R.; Spock, D. E.; Schein, L. B.; Diaz, A. F. J. Electrost. 1990,25,155.
0743-7463/94/2410-0592$04.50/0 0 1994 American Chemical Society
Effect of Humidity on Contact Charge
Langmuir, Vol. 10, No. 2, 1994 593
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Figure 2. Plot of Q/Mvs relative humidity for the powders of the ionomer/polymerblends containing: (A) [PI-PhSOs-Na+with (a) 4.4,(b) 13.1,and (c) 22 pmol/g ions and (d) the polymer only; (B) [P]-PhSOs-H+with (a) 4.4,(b) 13.1,and (c) 22 pmollg ions. The charge was activated with steel beads in every case. W
Figure 1. Diagram of the Q/Mmeasurement setup and the Faradaycage used for totalblow-off: (a)BNC electricalf e e d t h , (b) stainless steel ground shield; (c) stainless steel sample chamber; (d) Teflon washer; (e) stainless steel screen cap; (f) Teflon centering cap.
mechanism. The surface moisture on inorganic oxides has been measured quantitatively at different humidities, and the parallel quantitative measurements are needed with the materials used in contact charging studies.
Experimental Section Materials. Powders of the blends of a (65/35)styrene-butyl methacrylaterandom copolymer (S-co-BMA) containing 1-5 wt % of an ionomer were available from previous studies.'" The ion concentrationsin the blends were 4.4,13.3,and 22.1 pmol/g with the partially sulfonated polystyrene, [PI-PhSOs-H+ and 4.4,13.1,and 21.9pmovgwiththe partially sulfonatedpolystyrene sodium salt, [P]-PhSOs-Na+. These blends were prepared by melt-mixingthe variouspolymers and the correspondingpowders were generated by milling and size classification. The powders had a mean particle diameter of 10 pm. Charging. Mixturesof the ionomer/polymerpowder (30mg) and irregularly shaped steel beads with 140 pm diameter (1 g) were rolled to induce the charge using a Faraday cage of the type shown in Figure 1. These cages were designed to allow easy disassembling and cleaning of the complete unit. The sample chamber (1.6cm i.d. by 4cm length), end-caps, and ground shield (4.5cm i.d.) were fabricated from stainlesssteel. Teflon washers were used to hold the 200mesh (44pm diagonalopening)stainless steel screen in place in each end cap. The Teflon centering caps provided a 1.3 cm separation between the sample cage and the ground shield. The mixtures were rolled for 30 min at 276 rpm with the cage positioned in a humidity controlled environment. This rotation rate was greater than the 60 rpm rate used in our previous studies with rolling cans (7.5 cm i.d.);l" however the linear velocity at the rolling surface was the same for both the cage and the cans. With these rotation rates the charge data with this Faraday cage reproducedthe data with the rolling cans. The powder was then separated from the beads and blown out of the cage using a dry air stream. The charge, Q,of the beads remainingin the cage was measured using a Keithly electrometer Model 616,and the mass was measured gravimetricallyto 0.1 mg resolution. With a carefully conditioned sample chamber a reproducibility of 1 pC/g in Q/M can be achieved. For the measurements madebetween 20and 60% relative humidity (RH),
the reproducibility was within 0.8% for our samples. For the measurements made at 0 and 90% RH, the reproducibilitywas within 3% Surface Moisture. Thin f i i of the ionomer/polymer blends were prepared by spin coating a 3 wt % solution of the powder in chloroform/methanolonto a silicon wafer at 8000 rpm. The film thicknesses obtained were ca. 100 nm. The film samples were placed in a humidity chamber constructed of stainless steel and Teflon, and the humiditywas controlledby bubblingnitrogen through deionized water. The humidity was varied in 10% RH increments and the chamber was allowed to stabilize for 30 min before each measurement. The amount of surface moisture adsorbed onto the surface of the ionomer/polymer film was determined by using a Rudolph Research i-1000 ellipsometer equipped with a rotating analyzer and fixed polarizer (RAFP) and capable of 0.1 8, resolution. The adsorption layer thickness was calculated using a two-layer structure of water and polymer films on silicon. This simplified analysis includes native silicon dioxide with the polymer layer which is an adequate approximation because the refractive indices of silicon dioxide and polymer are similar, 1.46 and 1.52,respectively. Surface Resistivity. The same thin films of the ionomer/ polymer blends used for ellipsometry were employed for surface resistivity measurements. A gold contact pattern with a 1 cm width was vapor-deposited on the sample surface using a 60pm diameter wire as a mask to create a gap between the gold contacts. The blends with the higher ion concentrationswere particularly susceptibleto electricalshorts even though inspection of the gap under a microscope at 500X magnification did not reveal any defects. The samples were placed in a controlled humidity chamberand the surfaceresistancewas obtainedby ac dissipation using an EG&G PAR potentiostat Model 273 and an EG&G LIA Model 5210. The impedance was measured between 5 and l@ Hz and using a 500-mV amplitude.
.
Results The Q/Mvaluesfor all the powder samples show a strong dependence on RH as seen in Figure 2. The samples containing [PI-PhS03-Na+ or [PI-PhSOS-H+ charge negatively as previously reported4and the acid form produces a higher charge. The charge increases between 0 and 2040% RH, then decreases at the higher humidities. The charge measurements made at 0% RH have the poorest reproducibility because of the difficulty in removing the last traces of water from the samples. Nevertheless, the charges at 0 % RH are effectively those of the S-co-BMA,
Pence et al.
594 Langmuir, Vol. 10, No. 2, 1994
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Figure 4. Schematic model of water bridge between polymer particles and metal beads. In this schematic, M+is the mobile ion in the ionomer, e.g., Na+ or H+, and X- is the attached ion.
V U
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Figure 3. Plot of water layer thickness vs relative humidity for the thin films of the ionomer/polymer blends containing: (A) [P]-PhSOs-Na+with (a) 4.4, (b) 13.1,and (c) 22 pmol/g ions and (d) the polymer only; (B) [P]-PhS03-H+ with (a) 4.4, (b) 13.1, and (c) 2.2 pmol/g ions and (d) the polymer only.
i.e., with no significant change due to the ions. Above 60% RH the powder/bead mixtures did not flow freely and tended to clump. Under these conditions, a film of “caked” powder forms on the walls of the can during the rolling period. This effect is stronger at the higher humidities. This clumping clearly reduces the powder flow and the number of contacts and must be partly responsible for the lower charge levels. The unblended S-co-BMApowder behaves similarly. On the other hand, unblended polystyrene behaves quite differently where the charge increases monotonically with RH from -1 pC/g at 0-20 % RH to -10 pC/g at 60 % RH to -30 pC/g at 90 % RH. Thus, the charge-humidity behavior of S-co-BMA is present in the response of the blends. The average surface water layer thickness was measured with ellipsometry. As seen in Figure 3, there is always water on the surface of the polymers except, possibly, at 0% RH. However, some water must remain on the surface of the polymers even at 0 % RH because the samples were not exposed to the extensive bake-out procedures often required to remove the last traces of moisture. The water layer thickness increases almost linearly with RH with all the blends. With the [PI-PhS03-Na+, the thickness increases slightly with the ion content in the blend. With the [PI -PhS03-H+, no differences are observed between the blends and only a small difference is seen between the blends and the copolymer. Since a monolayer of water is reported to be 2.5-3.5 A thick, and the optically measured monolayer thickness is estimated to be 2.7 A, then the thicknesses measured here correspond to
