Article pubs.acs.org/Langmuir
Adsorption of Hydrophobically End-Capped Poly(ethylene glycol) on Cellulose Susanna Holappa,† Katri S. Kontturi,*,† Arto Salminen,‡ Jukka Seppal̈ a,̈ ‡ and Janne Laine† †
Department of Forest Products Technology, Aalto University School of Chemical Technology, P.O. Box 16300, FI-00076 Aalto, Finland ‡ Department of Biotechnology and Chemical Technology, Aalto University School of Chemical Technology, P.O. Box 16100, FI-00076 Aalto, Finland S Supporting Information *
ABSTRACT: Adsorption of poly(ethylene glycol), hydrophobically endcapped with octadecenylsuccinic anhydride (OSA-PEG-OSA), on an ultrathin film of cellulose has been studied by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM). Normally, PEG does not adsorb on cellulosic surfaces, but the use of the telechelic hydrophobic modification was found to promote adsorption. The influence of the conformation of the polymer in solution prior to adsorption and the subsequent properties of the adsorbed layer were investigated. The adsorption experiments were done at concentrations below and above the critical association concentration. The adsorption of OSA-PEG-OSA on cellulose was observed to occur in four distinct stages. Because of the amphiphilic nature of cellulose, further adsorption experiments were performed on hydrophobic (polystyrene) and hydrophilic (silica) model substrates to illuminate the contribution of hydrophobic and hydrophilic factors in the adsorption phenomenon. As expected, the kinetics and the mechanism of adsorption were strongly dependent on the chemical composition of the substrate.
1. INTRODUCTION Cellulose is the most abundant biopolymer on earth, and substantial research efforts are currently invested in its utilization as a nanomaterial1−3 and as a source for transportation fuels.4 Particularly in the nanosized form, the interaction of cellulose and other polymers is relevant for a variety of applications, including renewable composites,5−8 templates for nanomaterials,9,10 and green electronics.11,12 Polyelectrolytes are probably the most investigated species of polymers whose interactions have been monitored and utilized with cellulose.13−17 The interactions are usually long-range electrostatic forces based on the attraction between cationic polyelectrolytes and a cellulosic substrate that bears anionic charges due to, for example, remainders of hemicellulose from its native origin.16 Cellulose is, however, an amphiphilic polymer with a hydrophobic pyranose ring and hydrophilic hydroxyl groups, which has been recently restated in the discussion concerning the difficult solubility of cellulose.18 The amphiphilic nature suggests that probably more elaborate interactions than were previously thought of can be utilized between cellulose and other polymers. Yet there are very few efforts to probe these interactions systematically.19−21 This paper presents a fundamental study on cellulose and a telechelic amphiphilic polymer, namely poly(ethylene glycol) (PEG), that has been modified by end-capping with hydrophobic C18 segments (octadecenylsuccinic anhydride, OSA). Their interactions have been examined by following the adsorption of OSA-PEG-OSA on cellulose with quartz crystal microbalance with dissipation monitoring (QCM-D) which is © XXXX American Chemical Society
capable of in-situ monitoring of adsorption and analyzing the viscoelastic properties of the adsorbed layer. Typically, adsorption of hydrophobically end-capped amphiphilic polymers on different surfaces has been observed to occur on multiple stages, possibly with different mechanisms at different stages, resulting in different kinds of underlayer and overlayer structures.22,23 The adsorption mechanism of OSA-PEG-OSA on cellulose is particularly interesting because it is a wellestablished fact that PEG as such does not adsorb on cellulose at all.24,25 The fundamental approach was further complemented by investigating the adsorption of OSA-PEG-OSA also on hydrophilic silica and hydrophobic polystyrene (PS) surfaces. Because telechelic OSA-PEG-OSA bears resemblance to a triblock copolymer structure, correlations with the properties of triblock copolymers were discussed. The results make a contribution to the current discussion on the amphiphilic nature of cellulose and the peculiar properties of this biopolymer.26,27 The fundamental knowledge is likely to facilitate the further utilization of cellulose in several applications within modern materials science.
2. EXPERIMENTAL SECTION 2.1. Materials. 2.1.1. OSA-PEG-OSA. The amphiphilic telechelic polymer OSA-PEG-OSA, poly(ethylene glycol) end-capped with C18 groups, was synthesized by ring-opening reaction of 2-(1-octadecenyl)Received: July 1, 2013 Revised: September 17, 2013
A
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succinic anhydride with the hydroxyl end groups of PEG as has been described in detail earlier.28 The molecular weight of PEG was 6000 g/ mol. The chemical structure of OSA-PEG-OSA is presented in Figure 1. Dissociation of the carboxylic group of the OSA chain brings slight anionic charge in the polymer in neutral aqueous solutions.
and (iv) viscoelastic properties are independent of frequency. The modeling was conducted with Q-Tools software (Q-Sense Ab, Gothenburg, Sweden), assuming constant values for fluid density (1.05 g cm−3), fluid viscosity (1.3 × 10−3 N s m−2), and density of the adsorbed layer (1.15 g cm−3). 2.2.3. Atomic Force Microscopy (AFM). A Nanoscope IIIa multimode scanning probe microscope (version V6.13 R1, Digital Instruments Inc., Santa Barbara, CA) was used to determine the topography of the dried polymer layers. Drying of the films was accomplished with N2 gas flow instantly after the QCM-D experiments. The AFM images were scanned in tapping mode in air at 25 °C using silicon cantilevers. No image processing except flattening was done. At least three areas on each sample were measured.
Figure 1. Chemical structure of the OSA-PEG-OSA.
3. RESULTS AND DISCUSSION In this study we examined the adsorption of a hydrophobically end-capped poly(ethylene glycol) OSA-PEG-OSA onto cellulose, silica, and PS substrates. First, the concentration dependency of the intermolecular interactions was determined. Second, the adsorption process was monitored online with QCM-D, and the properties of the adsorbed films were looked into, concentrating first on the influence of the substrate and then on the influence of the polymer concentration. 3.1. Solution Properties. The association of hydrophobically end-capped hydrophilic polymers as well as BAB-type block copolymers can be divided in three different kinds of conformation regimes in aqueous solutions: (1) unimers, which may form loops via intramolecular interaction of the hydrophobic end blocks; (2) flower-like micelles; (3) networks, formed via bridging between micelles with dangling associative ends.33−35 In this study the regimes (1) unimers and (2) micelles were of interest. Here, the critical association concentration (cac) was determinedkeeping however in mind that the cac is not a sharp transition for polymeric amphiphiles36−38 and the obtained value is dependent on the analytical method to some extent. The dynamic light scattering intensity and the surface tension values of the OSA-PEG-OSA solutions are presented in Figure 2. Based on the data, the cac values of 0.1 and 0.5 g/L were obtained by light scattering and surface tension, respectively. The relatively small cac value is characteristic for neutral amphiphilic polymers, indicating that
Polymer Solutions. Despite OSA segments being insoluble in water, the solutions could be prepared by directly dissolving the polymers in water because the OSA segments represented only a small fraction in the copolymer composition. Stock solutions of 5 and 0.5 g/L were used for the determination of the critical association concentration (cac) and QCM-D measurements, respectively, and were allowed to dissolve for 24 h. The stock solutions were diluted and allowed to stabilize for 24 h prior use. 0.1 mM NaCl was used as background electrolyte in all experiments. 2.1.2. Other Chemicals. Trimethylsilyl cellulose (TMSC) was synthesized from cellulose powder from spruce (Fluka) as described in ref 29. NaCl (Merck) was used as received. Milli-Q water (resistivity 18.2 MΩ cm) was used throughout the study. 2.1.3. QCM-D Crystals. The QCM-D crystals (purchased from QSense AB) were so-called AT-cut quartz crystals with thickness of 0.3 mm, fundamental frequency of f 0 ≈ 5 MHz, and a sensitivity constant of C = 0.177 mg m−2 Hz−1. The silica crystals prepared via silica vapor deposition and the polystyrene crystals prepared by spin coating on gold were delivered by the manufacturer. The cellulose crystals were prepared by applying 30 layers of TMSC by the Langmuir−Schaefer technique on PS crystal and subsequently hydrolyzing the TMSC to cellulose by HCl vapor.14 Prior to the QCM-D measurement, the cellulose films were allowed to swell in the solvent overnight. 2.2. Methods. 2.2.1. Solution Properties. The aggregation behavior of the amphiphilic polymer was investigated by determining the critical association concentration (cac) with the surface tension of the solutions measured with a tensiometer (Sigma 70, KSV Instrument, Helsinki, Finland) and the scattering intensity by dynamic light scattering at a scattering angle of 173° and a wavelength of 633 nm (N5 submicrometer particle size analyzer, Beckman Coulter Inc., Miami, FL). Dynamic light scattering was also utilized for determination of the particle size. 2.2.2. Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D). Adsorption and the properties of the adsorbed polymer layer were studied with QCM-D (Q-Sense D300, Q-Sense Ab, Gothenburg, Sweden). The principle of the technique has been reported in detail in the literature.30,31 In short, the resonance frequencies and the dissipation factors of a quartz crystal coated with the adsorbent film are monitored as a function of time at the fundamental frequency 5 MHz and its several overtones. From these data, the adsorbed mass and rheological nature of the adsorbed layer can be analyzed. The modeling of viscoelastic properties of the layers in this study is based on the model presented by Voinova et al.32 In this model, the adsorbed layer is represented by a single Voigt element, which can be described using a frequency-dependent complex equation for the rigidity modulus. The real part of the complex modulus represents the elastic component of the modulus and corresponds to the changes in resonance frequency detected with QCM-D. The imaginary part of the modulus, on the other hand, represents the viscous part of the modulus and corresponds to the dissipation of energy also detected by QCM-D.32 Because of the simplicity of the model, we have to expect that (i) quartz crystal is purely elastic, (ii) the surrounding solution is purely viscous and Newtonian, (iii) the adsorbed film is uniform in thickness and density,
Figure 2. Light scattering intensity and surface tension as a function of OSA-PEG-OSA concentration in aqueous solution. The solid lines are added as a guide to the eye, and the arrows point to the turning points of the slopes of the curves, indicating the critical association concentration (cac) of the polymer. B
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the influence of individual anionic charges in each OSA segment on the self-organization of the OSA-PEG-OSA is likely to be insignificant. The hydrodynamic diameter of the micelles, analyzed with DLS, was 20.9 ± 1.3 nm in the concentration range 0.5−5 g/L. This corresponds to the particle size determined earlier for the same polymer, 19.8 nm at 10 g/L.28 3.2. Effect of Substrate. The influence of the chemical composition of the substrate on the adsorption mechanism and the properties of the adsorbed layer were investigated. 3.2.1. Adsorption. As discussed in the previous section, conformation of the polymer in the solution is determined by the interactions between the polymer chains and the solvent.33,34,39 The conformation affects the affinity of the polymer toward the substrate.40 At the same time, the chemical and physical properties of the substrate are critical for the adsorption process.40−44 The substrates used in this study show hydrophilic (silica), hydrophobic (polystyrene), and amphiphilic (cellulose) characters. The hydrophilic component of the amphiphilic cellulose substrate stems from the presence of numerous hydroxyl groups in the structure, whereas the hydrophobic component stems from hydrophobic pyranose rings that form the backbone of the molecule. OSA-PEG-OSA and the corresponding homopolymer PEG (Mw 6000 g/mol) were adsorbed on the three different substrates. The values of changes in frequency (Δf) and dissipation (ΔD) detected with QCM-D after adsorption and rinsing of the adsorbed layers with buffer are summarized in Table 1. The concentration of the polymer solutions was 0.05
Figure 3. Changes in frequency (Δf) and dissipation (ΔD) as a function of time during adsorption of PEG homopolymer (Mw 6000 g/mol) on cellulose, hydrophilic silica, and polystyrene from 0.05 g/L solution. The logarithmic time scale is chosen to emphasize the kinetics of the initial adsorption stage.
QCM-D curves Δf and ΔD for adsorption of OSA-PEG-OSA from 0.05 g/L solution onto cellulose, hydrophilic silica, and PS are presented in Figure 4. The graphs show that the kinetics are less straightforward than those of the adsorption of homopolymer PEG and indicate that the adsorption mechanism depends markedly on the chemical composition of the substrate. Characteristically for the adsorption of amphiphilic substances,23,48−50 adsorption on each substrate can be considered to occur in several regimes (called here stages I− IV). In short, the initial adsorption stage (stage I) is fast, and in this regime adsorption is typically diffusion controlled and independent of distribution of associative groups along the adsorbing molecule. Stage II, on the other hand, is typically slower and can be defined as polymer adsorption with rearrangement; after stage I all the available surface sites are occupied, and rearrangement of the adsorbed molecules must occur before any additional molecules can adsorb. Stages III and IV are generally described as activation barrier controlled adsorption and rearrangement without further adsorption. The specific features of the adsorption process of OSA-PEG-OSA on the three different substrates are discussed more closely in the following. Cellulose. Although neither of the segments in OSA-PEGOSA has a strong affinity for cellulose, a large amount of the OSA-end-capped polymer was observed to adsorb (Table 1 and Figure 4). The adsorption curves of OSA-PEG-OSA on the cellulose surface (Figure 4a) shows that the initial adsorption of OSAPEG-OSA unimers in stage I occurs relatively slowly and induces a Δf of about −10 Hz and ΔD of 1.0 × 10−6, indicating formation of a rather rigid layer. (The adsorbed layer can be considered rigid when ΔD is
