Langmuir 1994,10, 3323-3327
3323
Control of Contact Charge in Polymers with Ionomers D. Fenzel-Alexander, P. Brock, and A. Diaz* IBM Almaden Research Center, K931801, 650 Harry Road, San Jose, California 95120 Received January 20, 1994. I n Final Form: June 14, 1994@ The contact charge developed between powders of a copolymer-ionomer blend and ferrite beads correlates well with the chemical structure ofthe ions. There is a clear correspondencebetween the sign ofthe charge acquired by the blend and the sign ofthe bonded ion in the ionomer. When both ions are bonded, the charge is small. These charging results support the ion transfer mechanism for charging. The measured charge is less than the value calculated from the total amount of surface ions, and there is a small difference in the magnitude of the charge between the various ionomer blends. A higher charging capacity is observed with the ionomers with a mobile H+, OTs-, or I-, while a lower charging capacity is observed with the pyridinium, BUD+,and Na+ ion. This result is attributed to ion pairing which reduces the charging capacity of the ions. It is proposed that only dissociated ions transfer across the contact to produce charge. With this model, it is estimated that only 10%of the ion pairs in the surface water layer are dissociated. This indicates that the ion pair dissociation equilibrium constant in the surface water layer is lower than in bulk water.
Introduction
~cH~-cH~+H~-cH),
~CH~-CH#CH~-CH),
The sign of the contact charge which develops between a polymer containingions and a second dissimilar surface can be reasonably predicted based on the relative mobilities of the cations and the anions.’-14 For example when the ions are provided by an ionomer, the ionomerlpolymer blend consistently acquires the sign of the covalently bonded ion. This result has been obtained with several ionomers with different ion types. Examples of ionomers with bonded cations and mobile anions which induce a positive charge in the resin are ionomers of poly(styreneco-methylvinylpyridinium tol~enesulfonates)~-’([PlPyMe+ OTs-1, ionomers with arylphosphonium arylsulfonate ion^,',^,^ e.g., poly(styrene-co-methyldiphenylstyrylphosphonium OTs-)l ([PI-PhPPhzMe+ OTs-), and a n ionomer from a modified octene-maleic anhydride copolymer with pendent 3-aminopropylene(trimethylammonium) groups, ([P]-(CH2)3NMe3+MeOS03-).15 The latter is one of several materials studied with variations in both the polymer structure and the mobile ion, including MeOSOs-, OTs-, C1-, and Br-. Abstract publishedinAdvance ACSAbstracts, August 15,1994. (1)Bugner,D.E.;Anderson, J. H.ACSNationalMeeting,LosAngeles CA, 1988;Polymer Reprints 29,p 463. (2)Anderson, J. H.; Bugner, D. E. 4th International Congress on Non-Impact Printing Technologies, New Orleans, LA, 1988;p 79. (3)Diaz, A. F.; Fenzel-Alexander, D.; Miller, D. C.; Wollmann, D.; Eisenberg, A. 63rd Colloid and Surface Science Symposium, ACS, Seattle, WA, 1989. (4)Diaz, A. F.; Fenzel-Alexander, D.; Miller, D. C.; Wollmann, D.; Eisenberg, A. J. Polym. Sci., Polym. Lett. Ed. 1990,28, 75. (5)Diaz, A. F.; Fenzel-Alexander, D.; Wollmann, D.; Eisenberg, A. J. Polym. Sei., Part B: Polym. Phys. 1991,29,1559. ( 6 )Diaz, A. F.; Wollmann, D.; Dreblow, D. C h e n . Muter. 1991,3, 997. (7)Wollmann, D.; Dreblow, D.; Eisenberg, A.; Diaz, A. Chem. Mater. 1991,3,1063. (8)Anderson, J.H.; Bugner, D. E. US Patent 4,837,391, June 6,1989. (9)Anderson,J. H.; Bugner, D. E.; DeMejo, L. P.; Sutton, R. C.; Wilson, J. C. US Patent 4,837,392, June 6,1989. (10)Alexandrovich, P. S.; DeMejo, L. P.; Wilson, J. C. US Patent 4,837,393, June 6,1989. (11)Watanabe, M.; Nagase, H. US Patent 4,883,735, Nov. 28,1989. (12)Guay, J.; Ayala, J. E.; Diaz, A. Chem. Mater. 1991,3, 1068. (13)Birkett, K.L.;Gregory, P. Dyes Pigm. 1986,7,341. (14)Mizes. H.A.: Conwell. E. M.: Salamida. D. P. ADD^. Phrs. - Lett. 1990,56,1597. ’ (15)Gruber, R. J.; Bolte, S. B.; Agostine, D. US Patent 4,415,646, November 15,1983. (16) Diaz, A.;Fenzel-Alexander, D.; Wollmann, D.; Barker, A. J . Langmuir 1992,8,2698. @
06
09
CH3 I +OTs-
PhzPCH30Ts-
[PI-PyMe+OTs-
[PI-PhPPh2Me+OTsCH3
-(-W-#j C 1 \\ HN 0
I
CH2
I
CH2
I
CH2
I +
CHs-N-CH3
I
CH3 OTs[P]-(CHp)3NMe3+MeOS03-
Examples of ionomers with bonded anions which induce a negative charge in the resin are partially sulfonated polystyrene and the corresponding sodium salt,6 [PlPhS03- H+ (or Na+), and styrene-2-acrylamido-2-methylpropanesulfonic acid copolymer,ll [PI-CONH-C(CH& CH2S03-Hf. The latter is called a n “inner salt’’because both ions are presumably “anchored”;however, the proton is mobile and can transfer out of the polymer whether it rests initially on the amide nitrogen or on the sulfonate oxygen.
CHp
I
CH2
I+
__
CH3-N-CH3
I
CH3 SO~-H+ [P]-PhS03- H+
CH30SO-3 [P]-CONH-C(CH3)2CH2SO3-H+
0743-7463/94/2410-3323$04.50/00 1994 American Chemical Society
Fenzel-Alexander et al.
3324 Langmuir, Vol. 10, No. 9, 1994
When the ions in the polymer sample are provided by a molecular salt, the sign of the charge acquired by the polymer usually coincides with the sign of the bigger i ~ n . Examples ~ ~ - ~ of~ these are the Ph3PMe+ OTs- and PbP OTs- which induce a positive charge1J6and the metal azobenzene complexes which induce a negative charge.13J7 These observations plus the detection of the transferred ion on the second ~ u r f a c e ~were - ~ explained by a model based on the transfer of ions across the contact interface.l8 In this model it is assumed that only ions in the surface region of the materials are important for transfer and the charge results from the final distribution of ions between the two surfaces after contact and separation. Thus, for ion i with a surface concentration T I , the equilibrium and the equilibrium constant distribution is (1- firt is K,. For the case where the charge develops between two different particles (powders and beads), and one of them contains ions, e.g., a polymer blend containing a salt or a n ionomer, the charge q1produced by ion i is related to rtas in eq 1where n is the number ofparticles per bead, z, is the charge on the ion, F is Faraday's constant, a is the surface area of the particle, and A is the surface area of the bead. That is, the charge is basically related to cf, r. This is the charge for both particle types and not a s shown in eqs 2 and 3 (and correspondingly eqs 7 and 8) in ref 18. Finally, K, is equal to [(f)lA]l[(l - filna] from the distribution of ions and to (qlA)l(nqlna) from charge neutrality.
-ort
distinction between dissociated ions and ion pairs (eq 3) is the observed linear dependence between QIM and [MXI". This relationship is derived by defining the concentration ofthe dissociated ions [M+Iand [x-I in terms of [MXI for the bulk ions as in eq 4. The charging response KD
M+ X-
E-]= [M'I
+ X-
(3)
= (~[MX1)1'2
(4)
M+
to ion concentration is completely different with molecular salts. With molecular salts the charge does not respond monotonically to the ion concentration but instead rises a maximum value then decreases. The decrease is often due to contamination of the second surface by the transfer of paired ions, a situation which does not occur as easily with ionomers. In this report we present the charge response ofpolymer blends containing ionomers either with a bonded cation or anion or with both ions bonded and compare the results with eq 2 from the ion transfer model. A comparison is made between the charging behavior of blends containing ionomers and the charging behavior of blends containing a n ionomer pair (two oppositely charged ionomers), where both ions are bonded and unable to transfer across the interface to generate charge. With this combination of materials, the applicability of this model for predicting the charge acquired by the blends with ions is further demonstrated.
Experimental Section Materials. Powders of the blends of a (65135)styrene butyl methacrylate random copolymer (S-co-BMA)containing 1-5% of the corresponding ionomer were available from a previous study.3-7 The ionomers used were poly(styrene-co-methylvi-
For a blend containing a salt, M+ X-,ZM = -zx, and the charge will depend on the relative values of KM and Kx for the two ions as in eq 2. Inspection of this equation quickly reveals that the highest charge will result when either KMor KXis zero, and the lowest charge will result when they are equal. This outcome was previously demonstrated in the series listed in Table 1 of ref 6 and Table 2 of ref 19. In practice these limits may be compromised by ionic impurities. To a first approximation, the effect of additional ions appears as additional terms in eq 2 . In this discussion, M+ X- can also represent the ionomers [PI-M+ X- or [PI-XM+ where one ion is bonded to the polymer. With ionomers, the sign of the resulting charge in the blend is dictated by the relative mobilities of the two ions in the salt, more than by the ion distribution between the two surfaces, Ki.3-7J6J8 With ionomers the charge (measured as QIM, charge1 mass of particles) increases monotonically with the [ion] but not linearly as in the above equations. The deviation from linearity was attributed ion pairing and a model was presented which related the charge to the concentration of dissociated ions, M+ or X- (eq 3).16 According to the model, only dissociated ions on the surface of the polymer transfer to the second surface to produce charge separation. Ions which are ion paired will not produce charge. They will either not transfer or transfer as a pair, and in either case will not produce a net charge. With ionomers where one of the ions is bonded to the polymer, the ion pairs do not transfer. Consistent with the (17) (a) Macholdt, H.-T.; Sieber, A. Dyes Pigm. 1988,9, 119. (b) Macholdt, H.-T.; Sieber, A. SPIE/SPSE Conference Proceedings, May 1987, 89. (18)Diaz, A. F.; Fenzel-Alexander, D. Langmuir 1993,9, 1009.
nylpyridinium toluenesulfonate), [PI-PyMe OTs (520 pmollg), the partially sulfonatedpolystyrenes,[Pl-PhSO3- H+(440pmoll g), and the corresponding sulfonate salts: tributylammonium (660 pmollg), lutidinium (700 pmollg), 4-ethylpyridinium(420 pmollg), ammonium (440 pmollg), and sodium (440 pmollg) cations. The blends were prepared by melt-mixingeachionomer with the styrene-butyl methacrylatecopolymer and the powders were obtained by milling and particle size classification. The particles are nominally 9-10 pm in diameter, the width of the size distributionat half height is 5-6pm and
