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Langmuir 1999, 15, 7080-7083
Notes Adsorption of Polysiloxanes Bearing Anchoring Phenol Groups onto Silica Particles
Scheme 1
Fre´de´ric Tronc, Laurence Lestel, Catherine Amiel, and Sylvie Boileau* Laboratoire de Recherche sur les Polyme` res, CNRS, UMR C7581, 2-8 rue H. Dunant, 94320 Thiais, France Received September 17, 1998. In Final Form: June 11, 1999
Introduction It is often the case that polymers which adsorb very efficiently from solution onto interfaces are excellent surface modifiers. A large number of experimental and theoretical studies on the configuration of polymers at solid/liquid interfaces have been carried out and extensively reviewed.1 The interactions between poly(dimethylsiloxane) (PDMS) and silica have been also extensively studied because fumed silica is widely used as a reinforcing material for PDMS. Apart from adsorption from the melt,2-4 numerous studies on the configuration of PDMS at solid/liquid interfaces have been carried out.5,6 Copolymers made up of dimethylsiloxane segments (DMS) of different lengths, regularly separated by one bisphenol A unit (BPA) (see Scheme 1) have been synthesized successfully in our laboratory.7 The aim of this work is to tailor the interactions of poly(dimethylsiloxane)-based polymers with silica by using this family of copolymers. Adsorption of dilute solutions of these polymers onto nonporous silica particles has been studied. Two types of hydrogen bonding interactions can occur: weak interactions between DMS segments and silanol groups, and strong interactions between the phenol functions of the copolymers and silanol groups of the silica surface. Moreover, adsorption of copolymers having protected BPA functions has been studied in order to give an insight into the role of the relative strengths of the different interactions. Experimental Section Materials. Poly(DMS-alt-BPA)s were obtained by polycondensation using hydrosilylation (Scheme 1), starting from hexamethyltrisiloxane (I, n ) 2, Hu¨ls America Inc.) or hydride-terminated poly(dimethylsiloxane)s (I, n ) 8, 17, Hu¨ls America Inc.) as the Si-H containing monomers, and from diallyl derivatives such as 2,2′-diallyl BPA (II, R d H, Aldrich) and III (R d Si-Me3) in the presence of platinum(0) catalyst (Karstedt’s catalyst, PCO72, Hu¨ls America Inc.) as described previously.7,14 Typical polycondensation reactions (Scheme 1) were performed in CH2Cl2 under nitrogen, with a 1:1 molar (1) Kawaguchi, M. Adv. Colloid Interface Sci. 1990, 32, 1. (2) Cohen-Addad, J. P.; Roby, C.; Sauviat, M. Polymer 1985, 26, 1231. (3) Cohen-Addad, J. P. In Siloxane Polymers; Clarson, S. J., Semlyen, J. A., Eds.; PTR Prentice Hall: Englewood Cliffs, NJ, 1993; Chapter 11. (4) Cosgrove, T.; Turner, M. J.; Thomas, D. R. Polymer 1997, 38, 3885. (5) Auvray, L.; Cotton, J. P. Macromolecules 1987, 20, 202. (6) Auvray, L.; Auroy, P.; Cruz, M. J. Phys. I 1992, 2, 943. (7) Tronc, F.; Lestel, L.; Boileau, S. Polym. Prepr. 1998, 39 (1), 583.
ratio of Si-H to allyl groups. The platinum catalyst was added at 0 °C, and the mixture was then heated at 40 °C. Reactions were followed by monitoring the decrease of the Si-H IR band at 2125 cm-1. Polymers were usually recovered by evaporation of CH2Cl2 and dried under vacuum. Pure poly(dimethylsiloxane) (PDMS) was prepared by cationic ring-opening polymerization of hexamethylcyclotrisiloxane.8 Average molecular weights were determined by size exclusion chromatography (SEC) in THF for the copolymers having free OH groups (PL-gel 5µ mixed C columns) and in toluene for the copolymers having protected OH groups (PL-gel 5µ mixed D columns) on a Waters apparatus, equipped for refractive index detection using a polystyrene calibration curve, and data are reported in Table 1. Colloidal pyrogenic nonporous Aerosil silica 200 (Degussa) was used as the adsorbent (average diameter of the particles, ca. 12 nm; specific surface area, 200 ( 25 m2/g). To remove the physisorbed water, and to obtain a reproducible chemical state of the surface, the silica was heated at 100 °C for 48 h before use. The adsorption solvents were CCl4 (SDS, HPLC grade) and CH2Cl2 (Prolabo, stored over 3 Å molecular sieves). Adsorption. A homogeneous suspension of silica particles was first prepared by ultrasonication of a flask containing 0.2 g of silica and 5 mL of solvent for 30 min. Adsorption experiments were carried out by adding 5 mL of the copolymer solution. The total copolymer concentration was Ct (expressed in weight/volume). The samples were gently mixed by backward and forward motion for 24 h. At this point, equilibrium was assumed to have been reached among the three components of the system. The suspensions were then centrifuged for 20 min at 1000 rpm, and the polymer concentration of the supernatant, Cs, was measured by UV spectroscopy, except for pure PDMS. PDMS concentration in solution was determined by IR using the band at 2850-3020 cm-1 corresponding (8) Jordan, E.; Lestel, L.; Boileau, S.; Cheradame, H.; Gandini, A. Makromol. Chem. 1989, 190, 267.
10.1021/la981274r CCC: $15.00 © 1999 American Chemical Society Published on Web 09/08/1999
Notes
Langmuir, Vol. 15, No. 20, 1999 7081 Table 1. Polymer Characteristics
ref
n
-Ra
Mnb
Ip
p
1 2 3 4 5 6 7 PDMS
2 8 8 8 17 17 17
H H H SiMe3 H H SiMe3
8 500 8 500 9 300 27 300 10 000 10 000 46 200 47 500
3.0 2.5 3.4 4.0 3.1 3.1 2.7 3.0
16 9 10 23 6 6 25
solvent (adsorption and UV)
λ1 (nm)
1 (L‚mol-1‚cm-1
λ2 (nm)
2 (L‚mol-1‚cm-1)
CH2Cl2 CH2Cl2 CCl4 CCl4 CH2Cl2 CCl4 CCl4 CCl4
276 279 279 274 279 279 276
5750 5900 6150 4900 6200 6900 5800
282 284 284 280 284 284 282
5600 5800 6000 4800 6100 6900 5900
a R ) H, poly(DMS-alt-BPA)s with -OH groups; R ) SiMe , protected poly(DMS-alt-BPA)s. b SEC in THF for runs 1-3 and 5 and 6; 3 in toluene for runs 4 and 7 and PDMS.
Figure 1. Adsorption isotherms of poly(DMS-alt-BPA)s onto silica from CCl4: n ) 8 (triangles); n ) 17 (circles); PDMS (diamonds).
Figure 2. Adsorption isotherms of poly(DMS-alt-BPA)s onto silica from CH2Cl2: n ) 2 (crude polymer, squares; purified polymer, stars); n ) 8 (triangles); n ) 17 (circles).
to the stretching vibrations of the CH of Si-CH3. The adsorbed quantity A (mg/m2) of polymer was derived from:
to the saturation plateau.12 Moreover, those polymer samples contain a small amount (
