Ultrasonic Treatment Enhanced Surface Modification of Titanium Oxide by Tailor-Made Surface-Active Polymers

The surface modification of titanium dioxide in aqueous dispersions of specially tailor-made periodic acrylic acid/isobutylene copolymers, poly(acrylic acid)/polystyrene graft copolymers, and hydrophobically modified polyethyleneoxide urethane (HEUR) by ultrasonic treatment was studied. The pigment surface modification by the above copolymers was comparatively investigated regarding conventional adsorption as contrasted to an ultrasonic treatment assisted procedure. The course and efficiency of the polymer adsorption onto the pigment surface was quantified by electrokinetic sonic amplitude measurements. The higher efficiency of the pigment surface coating by the copolymers as achieved by ultrasonic treatment varies with the copolymer architecture and is a consequence of ultrasonically induced pigment surface activation.


Introduction
The homogeneous dispersing of pigments is im-portant in coatings formulations. For producing aq-ueous pigment dispersions, many approaches have been described in literature [1 -3]. It was shown [4 -6] that highly stable dispersions are obtained by ul-trasonic treatment of pigment slurries in aqueous so-lutions of polymer surfactants. Recent studies [7,8] by employing electrokinetic sonic amplitude (ESA) measurements have revealed that the origin of the improved stabilization of the pigment dispersion is due to first the generation of a finer dispersion of the pigment by the mechanical, i.e., ultrasonic treatme-nt, and second the formation of a thicker layer of the polymeric additive around the pigment; the better encapsulation of the pigment by the stabilizer pol-ymer is a consequence of the ultrasonically induced activation of the pigment surface.
In this work, we report on the dispersing of TiO 2 in aqueous solutions of amphipolar copolymers of well-defined structures by ultrasonic treatment. Th--ese copolymers are characterized by special sequ-ences of hydrophilic and hydrophobic units which result in specific pigment-polymer interactions and stabilizing properties [9 -11]. The effect of the ult-rasonic treatment on the deposition of the stabilizer copolymer is discussed in the context of the various copolymer architectures.

Techniques
For the preparation and investigation of the TiO 2 pigment dispersions, amphipolar copolymers were predissolved in water, and then the powdery pigment was added. After premixing with a laboratory stirrer (700 rpm for 10 min), the system was treated with ultrasound for 2 min with an ultrasonic generator Br-anson Sonifier B-12 with actual power of 1,5 W/cm 2 .
The pigment-copolymer interaction and the for-mation of the polymeric adsorption layer were inv-estigated by ESA measurements as described earlier [7,11]. The particle size was measured by both ESA and by using a Malvern Instruments Nanosizer [7]. Instrumental uncertainties have not exceeded 1%. For sedimentation measurements, the obtained di-spersions were poured into test-tubes and the sedi-mentation kinetics were monitored by following the border line between the turbid and the transparent zones; the refraction index of the supernatant liquid was measured by standard optical devices.

Results and discussion
Acrylic acid (AA) based periodic copolymers P([IB-AA]-r-AA) consisting of alternating isobu-tylene (IB) -AA sequences [IB-AA]m interrupted by (AA)n sequences, and graft copolymers PMAAg-PS consisting of poly(methacrylic acid) (PMAA) backbone and polystyrene (PS) grafts of various st-ructure were already shown to act as stabilizers of TiO 2 aqueous dispersions [9,10]. In these studies the dispersions had been prepared by conventional stirring, and then the pigment-polymer interaction and their stability were measured.

TiO 2 -(IB-AA-r-AA) System
As already mentioned above, the acrylic acid (AA)/isobutylene (IB) copolymer consists of seque-nces of alternating AA-IB units and homosequences of AA as illustrated in the chemical formula 1: The alternating AA-IB sequence is relatively hy-drophobic as compared to the strongly hydrophilic (AA)n homosequence. Earlier studies of the copoly-mer-pigment interaction by means of the ESA meth-od had revealed that the saturation concentration SC increases with decreasing AA content and that the (AA)n homosequences act as anchor to the pigment, and electrosteric stabilization is predominantly ach-ieved through loops of alternating AA-IB sequences [9,10].
The effect of ultrasonic treatment on the copo-lymer-pigment interactions has been investigated exemplarily for the water soluble copolymer (P(IB-AA-r-AA)-1) containing 82 mole-% AA comonom-er which in earlier work [9] was shown to be one of the most effective dispersion stabilizing copolymer of this series. The comparative ESA measurements of the aqueous dispersion of TiO 2 (without as well as with different amounts of added copolymer as st-abilizer) for systems without and with ultrasonic tr-eatment can give quantitative information about the process of polymer adsorption. This is reflected first from the dependence of the dynamic mobility μ on the relative polymer concentration [11] as shown in Fig. 1, and second from the comparison of this dep-endency for the system without and with ultrasonic treatment (Fig. 1, curves 1 and 2).
The increase in the dynamic mobility with inc-reasing polymer concentration (relative to the TiO 2 concentration) reflects the increase of the zeta pot-ential, and the reaching of a plateau means that an adsorption equilibrium has been reached (see Fig. 1, curve 1). This is primarily a consequence of the co-mplete occupation of positively charged sites on the TiO 2 surface by negatively charged carboxylate gro-ups of the (AA)n homosequences; the high number of ionizable carboxylic groups in the alternating IB-AA sequence loops of the adsorbed copolymer are responsible for an increased charge density of the copolymer coated pigment as compared to the pure TiO 2 pigment. The same observation holds true for the ultrasonically treated system (Fig. 1, curve 2).
The comparison of the dynamic mobility vs. co-polymer concentration curves of the ultrasonically treated system with the untreated system ( Fig. 1) reveals the distinct effect of ultrasonic treatment si-milarly as already observed for other systems [7,8]. when ultrasonification was applied. The polymer concentration (relative to TiO 2 mass) are also refe-rred to as the so-called saturation concentration SC which is defined as the polymer concentration above which the dynamic mobility does only little increase [11]; SC is approximately 0,3 wt.-% [9,10] and is given by the intercept of the tangents to the initial sharp increase and to the approaching of the plateau (see Fig. 1). In this context it must be emphasized that the in-itial dynamic mobility of the pristine TiO 2 is alrea-dy much higher in the presence of ultrasonic action (Fig. 1, curve 2) which illustrates the already initial-ly higher zeta potential as compared to the untreated system. A common feature of both the ultrasonically treated and untreated system is that further addition of polymer beyond the SC does not appreciably ch-ange the dynamic mobility meaning that polymolec-ular adsorption doesn't appear to occur.
The effect of ultrasonic treatment on the colloid-al stability of the aqueous TiO 2 dispersions, stabili-zed by the IB/AA copolymers of different monomer sequence periodicity, is given in Tab. 1.  First it is evident that, irrespective of the constit-ution of the copolymer, the ultrasonification substa-ntially improves the dispersion stability as reflected from the comparison of the sedimentation half times of the non-treated and treated systems. The data also infer that there is an optimal copolymer structure with regard to the mole fraction of alternating IB-AA sequences and AA homosequences (P(IB-AA-r-AA)-4)); this is in accordance with previously disc-ussed constitutional effects [9].

TiO 2 -PMAA-g-PS System
The effect of the ultrasonification on the adsor-ption of the amphipolar graft copolymers PMAAg-PS consisting of poly(methacrylic acid) (PMAA) backbone and polystyrene (PS) grafts onto TiO 2 is phenomenologically similar as obse-rved for the IB/AA copolymers: the change in the dynamic mobility is much more pronounced for the ultrasonically treated dispersion than for the non-tr-eated system (see Fig. 2). The mechanism of the copolymer adsorption via a sequential mono-and bilayer formation followed by the deposition of solloids has already been disc-ussed elsewhere [10]. Again, the adsorption of the graft copolymer ultimately means an increase of the charge density through the carboxylic groups of the PMAA backbone; this is reflected by the increased dynamic mobility, and thus a ξ-potential upon addi-tion of the graft copolymer. If one were to attribute a higher statistical weight to the data point at 0,5 wt.-% in Fig. 2, curve 1, and at 1 wt.-% in Fig. 2, curve 2, respectively, a curve with a shoulder in this concentration range would result; such a curve cou-ld be tentatively explained by the transition from the bilayer covered to the solloid covered pigment.

TiO 2 -HEUR System
The addition of the nonionic HEUR consisting of hydrophobic alkyl end blocks and a hydrophilic poly(ethylene oxide) center block, which is usually employed as an associative thick-ener for aqueous latex dispersion coatings formul-ations, leads to a decrease of the dynamic mobility  of the system when adsorbing on the TiO 2 surface [14]. This is due to the shielding of the charges pr-esent on the TiO 2 pigment surface by the hydroph-obic hydrocarbon end blocks of the HEUR triblock copolymer.
In comparison to the ionic (M)AA based copoly--mers discussed above, it is obvious that the adsorp-tion of the non-ionic amphipolar HEUR must result in a decrease of the ξ-potential since no ionic groups are introduced which, as a net result, can overcomp-ensate the shielding of the charged TiO 2 by polymer adsorption (Fig. 3). The saturation concentration for the non-treated sample (SC about 0,15 %) is relatively small and in accordance with earlier results [13]. In contrast to this, the saturation concentration is much higher for the ultrasonically treated sample (SC about 12 %). This clearly shows the dual effect of the ultrasonific-ation, i.e., the improvement of the particulate pristine pigment dispersion (which also means an increase of the total pigment surface area) and the simultaneous creation of a highly activated pigment surface.
Correlation of ESA data with the thickness of the coating on the pigment particle Comparing the values of saturation concentrati-on SC without and after ultrasonic treatment, shows that the amount of polymer adsorbed on the particle surface significantly increases upon ultrasonificati-on. However, as already mentioned above, the dec-rease of the pristine particle size by the application of the ultrasonic power, and consequently the inc-reased total surface area, automatically leads to an increasing of the amount of polymer deposition.
In order to get some quantitative information ab-out the polymer adsorption as revealed from the ESA data, and to correlate the amount of polymer adsorb-ed with the dispersing conditions, calculations of the surface area of the pristine pigment particle and of the polymer coated particle were done for both the systems without and with ultrasonic treatment and related to the amount of adsorbed polymer.
In these calculations a spherical shape of the pi-gment particle was assumed, and that the maximum polymer adsorption is indicated by the saturation concentration SC; this saturation concentration is reached when the dynamic mobility vs. relative pol-ymer concentration curve ( Fig. 1 -3) becomes more or less parallel to the abscissa and doesn't change much for increasing polymer concentration [13].
First, the total surface area Stot of the particles with surface area Spart is obtained from the measu-red average particle diameter dpart and the particle number density in the dispersion. Since the saturat-ion concentration SC as obtained by ESA measure-ments corresponds to the total mass of amphiphilic polymer that is adsorbed on the dispersed particles, the increased particle diameter (due to the adsorb-ed polymer) and thus the thickness of the polymer adsorption layer can be calculated as well. For the background and the equations on which the calcul-ations of the data compiled in Table 2 are based it is referred to the literature [7].
The comparison of the experimentally measured diameter dpart of the polymer coated particles whi-ch were obtained by application of ultrasonification (column 3 in Tab. 2) and without ultrasonification (column 2 in Tab. 2) shows that smaller particles resulted from the ultrasonification; this was to be expected since ultrasonic treatment is known to cau-se a breaking of agglomerates/aggregates present in pigment slurries.
Whereas the particle surface area Stot increases upon ultrasonic treatment by about a factor of 1,2 (minimum) and up to a factor of about 12 (see col-umn 4 of Tab. 2), the saturation concentrations SC as derived from the experimental curves Fig. 1 -3 is 4 to 80 times higher for the ultrasonically treated systems as compared to the non-treated systems (see column 6 of Tab. 2); in other words, the SC ratio is about three to seven times larger than the Stot ratio (compare columns 4 and 6 in Tab. 2). This indicates that the amount of polymer adsorbed per unit of the particle surface after ultrasonic treatment is higher as compared to non-treated samples.
The thicknesses of the adsorbed polymer layer as obtained for the different amphipolar copolymers under conditions of ultrasonic treatment in compar-ison to the systems without ultrasonic treatment are compiled in Tab. 3. The thicker adsorption layers observed for the ultrasonically treated systems conf-irm that an activation of the pigment surface occurs by the action of ultrasonic power. Table 2 Effect of ultrasonic treatment on the particle diameter dpart, and effect of ultrasonic treatment on the increase of the particle surface area (Stot) as well as on the saturation concentration (SC) of added amphipolar copolymer as expressed by the corresponding Stot and SC ratio; the indexes 1 and 2 refer to the non-treated (1) and ultrasonically treated (2) Table 3 Thicknesses d of the adsorption layer of the amphipolar copolymer on TiO 2 for ultrasonically treated and non-treated dispersions as calculated on the basis of the saturation concentration SC obtained from the ESA measurements (see Table 2) Thickness d of the adsorption layer without ultrasonic treatment, nm.

Amphipolar Copolymer
Thickness d of the adsorption layer after ultrasonic treatment, nm.
Ratio between the thicknesses of trea-ted and non-treated HEUR C16-PEO14000-C16 3 11 3,7 A general conclusion that can be drawn from the data compiled in Tab. 3 is that the increasing of the thickness of the polymer adsorption layers upon ul-trasonic treatment is confirmed from both the info-rmation obtained from the ESA measurements and from the calculations of the particle surface.
The almost tenfold increase of the particle diam-eter dpart as compared to the diameter of the start-ing material in case of the HEUR additive without ultrasonification (see Tab. 2) can only be explained by agglomerate formation. The fact that the meas-ured particle diameter of the HEUR coated TiO 2 as obtained with ultrasonic treatment is even smaller than the specified diameter of 0,3 μm of the starti-ng material (see Exp. Part) drastically illustrates the effect of the ultrasonic power. As a consequence of this pronounced effect of ultrasonification, the larg-est increase in the particle surface area is obtained (ratio S tot2 /S tot1 = 11,9).
The particle diameter and surface area of the TiO2 dispersions stabilized by PMAA-g-PS do not show such a pronounced difference between the ul-trasonically treated and non-treated system and is also smaller than for the P(IB-AA-r-AA) polymer (see Tab. 2). This is also reflected from Fig. 2 whi-ch shows that the saturation concentration is already reached for comparatively low polymer concentra-tions which are much lower than measured for the other polymers (see Fig. 1 and 3).
This behaviour of the TiO 2 /PMAA-g-PS system can be explained by the micelle adsorption model, i.e., the deposition of polymer micelles (core of PS grafts and shell of PMMA backbone) onto the pol-ymer double layer initially formed around the pig-ment particle [10,11]: First, the hydrophilic, poly-electrolyte polyacrylic acid chains anchor through the carboxylic groups on positive sites of the TiO 2 surface resulting in the formation of a polymer mo-nolayer with a relatively apolar surface, followed by deposition of further graft copolymer through interaction of the PS grafts with the firstly formed monolayer. Subsequently, micelle adsorption and la-yer thickening takes place, finally resulting in larger particles than observed with the other amphipolar copolymers.

Conclusions
ESA measurements have shown that ultrasonic treatment of TiO 2 pigment dispersions in aqueous solutions of amphipolar copolymers significantly ef-fects the pigment surface modification and enhances the dispersion stability. The effect of the ultrasonic treatment is dual in that not only a finer particle dis-persion but also an activation of the particle surface is achieved in the first place. The activated surface allows for a better copolymer-pigment interaction leading to firmly bonded and thicker polymer ads-orption layers. Thus the ultrasonic treatment in co-mbination with the employing of specially designed amphipolar copolymers opens new prospectives to modify the pigment surface more efficiently.