Creation of Coatings by Temperature-Controlled Polymer Deposition

In this paper, the results of the recent studies on application of lower critical solution temperature (LCST) phenomena of polymer solutions to the surface modification of flat and spherical substrates are reported. It was found that a controlled polymer deposition can be achieved upon prevailing of the LCST, a peculiar surface topology of polymer coating and particles those remarkably effective in stabilizing pigment dispersions. Second, the finding is that the temperature induces polymer deposition as the coprecipitation of a finely dispersed component on the substrate surface. That goes along with the polymer deposition after exceeding of the LCST which in turn allows for the tailored surface modification of pigments in that particulate additi ives can be incorporated into the surface modifying coating.


Introduction
The coating of the substrate with an organic pol-ymer film is often required for application technoloigy reasons. In the light of nanotechnology, coatings exhibiting features on the nanoscale are of increas-ing interest: this includes both coatings on more or less extended flat surfaces as well as the surface moidification of particles.
As to the technologies of film formation, besides the conventional casting from solution (or dispersi-on) by solvent removal, a novel variant is to take ad-vantage of temperature induced critical phenomena of polymer solutions. Especially the polymer depo-sition on a substrate upon exceeding the lower criti-cal solution temperature (LCST) or by cooling bey-ond the upper critical solution temperature (UCST) opens interesting perspectives for the fabrication of coatings with novel surface properties [1][2][3][4].

Experimental
Materials employed for coating experiments were in most cases commercially available poly(o--xyethylene-boxypropylene-b-oxyethylene) 3-block copolymers (P(EO-b-PO-b-EO)) without and with polymerizable end-functionalities (cf. [2]), and for selected comparative studies polystyrene and hyd-rophobically modified poly(oxyethylene) urethane model systems (cf. [3]). Si-wafers were used as sub-strates for film deposition and surface topology studiies by atomic force microscopy (AFM), and particle surface modification was investigated with commeircially available pigments. Turbidity measurements were carried out with a photometer TP 1 from Tep-per Instruments. Instrumental uncertainties have not exceeded 1%. For information regarding the metho-dology of polymer deposition and coatings analysis it is referred to previously published data [2].

Results and Discussion
In order to further explore the effect of the po-lymer concentration on the change in the surface topology of LCST deposited P(EOibiPOibiEO), fiirst the phase diagram was established. Surprisingly, the turbidity measurements revealed two LCSTs in a semi-diluted concentration regime in a polymer weight fraction w 2 between about 0,05 ≤ w 2 ≤ 0,2, whereas only one LCST was observed for lower as well as higher concentrations (Fig. 1). The complete phase diagram is depicted in Fig. 2.
*corresponding author. E-mail: nbulychev@mail.ru It is evident that in an intermediate concentration range, upon temperature increase, a 2-phase regime is passed followed by a single phase window until finally a second LCST is surpassed. The effect of the constitution, e.g., the block length ratio on the phase behavior is also evident from the data depicted in Fig. 2. The separation of the two component wat-er/copolymer system was also followed by a light microscopy. All the systems studied so far showed the features characteristic for nucleation and growth mechanism (Fig. 3), and no indications for spinodal decomposition were obtained. However, differences in the onset and course of separation as well as in the finally found phase piciture was observed depending on the chosen temper-ature in the two-phase regimes; this was particulary distinct in the intermediate concentration regime p o lym e r we ig ht frac tio n w general, the surface topology exhibited similar isla-nd-like relief structures which could be permanently fixed by subsequent crosslinking via the end group functionalities as already reported [1,2,[4][5][6][7][8]. The comparison of the AFM pictures in Fig. 4 further reveals that the surface topology also depends on which point in the phase diagram the coating exper-iment could be started. Different results are obtained for the same polymer concentration in the two sepa-rated 2-phase regimes. The LCST induced polymer deposition can also be used for the surface modification of particles: a coating layer around the particle is formed when the dispersion of the particles in a polymer solution is heated above the LCST of the respective polymer/ solvent system (cf. [2]). Dispersion of such modified particles are extremely stable as demonstrated, e.g., with carbon black.
Another aspect of the temperature induced polym--er deposition is the coprecipitation of a finely disperised component on the substrate surface that goes alo-ng with the polymer deposition after exceeding of the LCST. This opens new perspectives for the tailored surface modification of pigments. Those particulate additives can be incorporated into the surface modif-ying coating: as illustrated in Fig. 5 for a platelet-like pigment, an encapsulation with a nanoscopic polymer layer containing nanoparticles is also possible.

Conclusions
The above studies have shown that the applic-ation of the critical phenomena of polymer/solvent systems to the modification of the surface of extenided planar substrates as well as of particles leads to novel surface characteristics. Ongoing work further explores aspects of the LCST technique for applicaition in coatings technology.