Response of adsorbed polystyrene layers to desorption conditions

Adsorption of macromolecules onto solids plays an important role in many technologies, especially those relying on suspensions, since adsorbed polymer alters the interparticle forces, which largely govern the suspension’s properties.1,2 The technological importance of the adsorption/desorption process has generated interest in understanding the details in model systems. A number of reports have been published on the adsorption of polystyrene onto model surfaces with weak or moderate segmental sticking energies such as metals, silica, glass, and mica.3-7 For these systems, a phenomenological understanding of how coverage and thickness develop on a clean solid surface has resulted. These studies indicate that unusually long time scales (days) are needed for adsorbed layers to fully equilibrate. Further, adsorption appears to be nearly irreversible insofar as desorption kinetics into pure solvent are imperceptibly slow. At present, there is no clear molecular-level understanding of these long time scale dynamic features in adsorbed layers. In this communication, the response of a partially formed adsorbed layer to desorption conditions is followed by a fluorescence label technique. All experiments described herein were performed on a silicapolystyrene system using a hydrated porous silica. Polystyrene labeled with fluorescent active groups is first adsorbed from a good solvent, and we follow the changes in spectral features of the fluorophore as a function of time after exchanging the bulk phase with a pure solvent. Our results show that the adsorbed layer responds dramatically to replacement of bulk polymer solution with pure solvent, though there is no perceptible desorption. Systematic trends in the response are found with changes in the pure solvent and with the layer’s preparation history. A complete, systematic study of these effects should lead to a clearer molecular-level view of adsorbed layer dynamics, especially the very long time scale features. Experimental Procedure. Fluorescence labeling of polystyrene (PS) (molecular weight 45 730, PD ) 1.05) was achieved by the aminomethylation method.8 The functionalization of PS throughout the course of reactions was characterized and confirmed by both IR and NMR. The total conversion to the amine derivative was confirmed by the absence of phthalimide peaks in the aromatic region of the NMR spectra. Pyrenebutyric acid was converted to its acid chloride with thionyl chloride, and the acid chloride was reacted with the amine derivative in the presence of triethylamine to attach the fluorophore. NMR and UV were used to determine the average number of pyrene groups in a chain. The labeling density of PS was 1 pyrene/300 monomer units. (∼1.6 labels per chain). In a typical experiment, silica (nominal surface area ) 272 m2/g, pore size ) 57 A, obtained as a gift from Dr. Walter Mahler of Dupont) was first dried at room temperature in a vacuum oven overnight (1 Torr) and then stored in a desiccator until use. No special precautions were taken to exclude moisture from the substrate during subsequent procedures. Consequently, the surface has a layer of physisorbed water. A known amount of silica (∼300 mg) was transferred to a flask containing 2 mL of pure chloroform (CHCl3). The solvent was allowed to fully wet the adsorbate by agitating it in a mechanical agitator overnight. A volume of polymer solution at a known concentration was then added, and the adsorption was allowed to proceed on a mechanical agitator for a fixed time at ambient temperature. The solution was then decanted, and the silica washed 3 times with pure CHCl3 on the agitator. “Desorption” of the adsorbed layer was then carried out by transferring the washed solid to a cuvette and adding pure solvent. The steady-state emission spectra of the labeled polymer on the quiescent silica suspension were then recorded as a function of desorption time using a SPEX Fluorolog (DM1680B) in frontface mode. The spectra were obtained by exciting at 332 nm and collecting between 360 and 600 nm. The excimer to monomer intensity ratios are reported by taking the ratio of the peak heights at 480 and 380 nm, respectively. In every case, there was no detectable fluorescence signal in the supernatant solvent, indicating no measurable desorption into the bulk after exchange. Results and Discussion. An adsorbed layer is first formed by contacting the clean solid with PS in CHCl3 solution. Subsequently, the bulk is exchanged with pure solvent; we refer to this step as desorption hereafter, although no measurable desorption occurs. The initial structure of the adsorbed layer was varied by varying the adsorbing conditions. Figure 1 shows the kinetics of adsorption of the labeled PS onto the substrate from a 1 g/L solution in CHCl3. Clearly, a layer obtained by adsorbing in a 1 g/L PS/CHCl3 solution for 15 min differs from one obtained by adsorbing for 1 or 20 h. Figure 2 shows the emission spectra obtained at different time intervals during “desorption” in CHCl3 of an adsorbed layer formed from a 1 g/L solution for 0.5 h. An obvious feature of these spectra is the relative increase in excimer emission intensity with time. It has to be noted here that a 1 g/L solution of this polymer in CHCl3 shows very little excimer. Clearly, the concentration of the fluorophore is higher in the adsorbed layer than in the bulk polymer solution, favoring excimer † Department of Chemical Engineering, Material Science and Mining Engineering. ‡ Department of Chemistry. Figure 1. Adsorption kinetics for PS(45.7K) from a 1g/L solution in chloroform. Coverage was determined by depletion of the polymer from the bulk using UV spectroscopy. 5505 Macromolecules 1996, 29, 5505-5507