Self-assembled films of cellulose nanofibrils and POAE

Self-assembled films of cellulose nanofibrils and POAE

(Parte 1 de 2)

Self-assembled films of cellulose nanofibrils and poly(o-ethoxyaniline)

Eliton S. Medeiros & Luiz H. C. Mattoso & Rubens Bernardes-Filho & Delilah F. Wood & William J. Orts

Abstract Nanostructured films of poly(o-ethoxyaniline) (POEA) alternated with cellulose nanofibrils (CnF) were successfully produced by self assembly (SA) at different pH values and investigated by atomic force microscopy and ultraviolet-visible spectroscopy. Results show that it was possible to build up films by alternating POEA and CnF layers with relatively precise architectural control by controlling the number of layers and pH. Film thickness had a dependence on pH which is a combination of the effects of the deposited amount for each POEA layer and the pH at which the absorption of the cellulose nanofibrils was carried out. Comparison of alternated layers of POEA and CnF with multi-immersions of POEA at different pH values, as measured by the ratio between slopes of the straight lines of deposited amount of polymer versus the number of self-assembled layers, shows that alternate deposition at pH 2 has a fourfold increase in the slope. Alternatively, at pH 5, there is no significant difference whether the deposition is alternated (POEA–CnF) or not (POEA).

Keywords Cellulose nanofibrils . Poly(o-ethoxyaniline) . Conducting polymers . Self-assembly

Introduction

Although cellulose nanofibrils (also known as microcrystalline cellulose, cellulose whiskers, nanofibrils, and nanocrystals) have been studied and industrially used since early 1960s [1–3]; these nanostructures have gained more scientific attention only recently due to the search for nanomaterials from renewable resources. Moreover, cellulose is the most abundant form of biomass with an estimated annual production of 1.5×1012 ton [4–7], and its nanofibrils exhibit remarkable physical and rheological properties [6, 8, 9].

Similarly, conducting polymers have gained more scientific attention over the last two decades because of their unique combination of properties of both metals and plastics, which has enabled applications in devices such as sensors and biosensors [10], batteries [1], light emitting diodes and electroluminescent displays [12, 13], and coatings [6, 14, 15]. In most of these applications, the polymer is used in the form of thin films which are usually produced by self assembly, spin coating, Langmuir-Blodgett, or casting [16–18]. Therefore, control of film architecture is of paramount importance to attaining film reproducibility, durability, and performance.

Self-assembly has been used to build up multilayered films by means of electrostatic attraction forces, stereocomplex formation, host–guest interactions, and hydrogen and covalent bonds [16]. This has allowed different types of materials to be assembled into thin films with a relatively precise control of architecture by adjusting process conditions such as nature and concentration of materials, pH, ionic strength, immersion time, dopant counter-ion, and number of layers. Furthermore, self-assembly offers several advantages over other techniques used for thin film formation since the substrate can take any form and size,

Colloid Polym Sci (2008) 286:1265–1272 DOI 10.1007/s00396-008-1887-x

E. S. Medeiros: L. H. C. Mattoso : D. F. Wood: W. J. Orts (*) United States Department of Agriculture, Bioproduct Chemistry and Engineering Unit, Western Regional Research Center, 800 Buchanan St., Albany, CA 94710, USA e-mail: bill.orts@ars.usda.gov

E. S. Medeiros : L. H. C. Mattoso : R. Bernardes-Filho Laboratório Nacional de Nanotecnologia para o Agronegócio, Embrapa Instrumentação Agropecuária, Rua XV de Novembro, 1452, São Carlos 13560-970, Brazil the deposition time is independent of the substrate area, and there are no requirements for additional equipments and/or clean rooms [17–21].

In this work, nanostructured multilayer thin films of poly (o-ethoxyaniline) and cellulose nanofibrils were successfully produced by self-assembly at different pH values, and their structure was investigated by atomic force microscopy (AFM) and ultraviolet-visible (UV-Vis) spectroscopy.

Experimental Polymer synthesis and characterization

Poly(o-ethoxyaniline) was chemically synthesized using freshly distilled o-phenetidine (1.0 M, Sigma Aldrich). Ammonium persulfate (APS; 0.25 M, Mallinckrodt) and hydrochloric acid (HCl; 1.0 M, Fischer Chemicals) were used as oxidant and dopant, respectively. The monomer to oxidant ratio was kept at 4:1 with polymerization carried out at room temperature for about 3 h. The polymerization was monitored by measuring the open-circuit potential (Voc) of the reaction using a platinum (working) electrode versus a saturated calomel (reference) electrode (SCE). Subsequently, the poly(o-ethoxyaniline) (POEA) powder obtained was removed from reaction medium, filtered under vacuum, exhaustively rinsed with acetone to remove unreacted monomers and oligomers, and dried in vacuum for 24 h.

Fourier transform infrared spectroscopy (FTIR) analysis of the polymer was recorded on a Perkin-Elmer 1000 IR spectrometer, spectral window ranging from 400 to 4,0 cm-1 and resolution of 4 cm-1. Samples were prepared using KBr pellets containing about 2 wt.% of the polymer powder. UV-Vis spectra were recorded on a Shimadzu UV-Visible spectrophotometer model UV 1700 PharmaSpec from 200 to 1,100 nm.

Extraction of cellulose nanofibrils

Extraction of cellulose nanofibrils was carried out according to the methodology described by Favier et al. [2]. Briefly, 10% w/v short cellulose fibers (Fibrous cellulose powder CF11, Whatman Int. Ltd., England) was stirred vigorously with 60 wt.% sulfuric acid preheated and maintained at 60 °C for 45 min. The mixture was, then, quenched in cold water and stirred. The resulting dispersion was centrifuged, decanted, and washed with water continuously until the pH was above 1.5 and a stable dispersion formed. The resulting solution was water dialyzed in dialysis tubing (Spectrapor 2; MWCO 12,0–14,0; 45 m flat width) for 4–5 days until the pH of the solution was between 5.5 and 6.5. Cellulose nanofibrils were recovered as a dispersion of approximately 1.16 g/100 mL water from dialysis.

Preparation of solutions of POEA and nanofibrils

POEA solutions were prepared by dissolution of POEA in deionized water under stirring for overnight. The final concentration of polymer was c.a. 1.0×10−3 mol L−1 (0.611 g L-1), based on its doped tetramer (see Scheme 1) and the pH of each solution was adjusted to 2, 3, 4, and 5 using 1.0 M HCl and/or 0.1 M NH4OH solutions. The dispersion of cellulose nanofibrils was diluted to make 0.611 g L-1 solutions whose pH values were also adjusted to 2, 3, 4, and 5 using 1.0 M HCl and/or 0.1 M

NH4OH solutions.

Film growth and characterization

Glass slides (10×10×1 m) used for film deposition were previously cleaned with hot piranha solution, H2SO4/H2O2 these slides were extensively rinsed with deionized water.

Films were deposited by self-assembly onto glass slides using the polymer solutions with pH adjusted as previously described. The isosbestic points of POEA (HCl) in water were determined by UV-Vis spectrophotometry from 10–3 M POEA solutions at different pH values and the kinetics of adsorption was carried out by measuring the absorbance at 455 nm for fixed periods of time, ranging from 0 up to 30 min, until absorbance reached a constant value.

The amount of material adsorbed, Γ (μg/cm2), was calculated according to the Beer-Lambert law, which

Cl Cl

Scheme 1 Chemical structure of a repeating HCl-doped tetramer that constitutes poly (o-ethoxyaniline)

provides a relationship between absorbance, A (a.u.), molar absorptivity ε (m2/g), path length b (m), and molar concentration c (g/L) by

A ¼ b"ici where the subscript i refers to poly(o-ethoxyaniline) or cellulose nanofibrils (CnF).

Based on the kinetic studies, both POEA and POEA alternated by CnF (POEA–CnF) films were built up according to the following procedure: (1) deposition of a POEA layer for 10 min; (2) rinsing for 15 s in deionized water at the same pH of the deposition polymer solution to remove loosely adsorbed polymer chains; (3) drying gently with a nitrogen flow; (4) measurement of film absorbance at 455 nm by UV-Vis spectrophotometry; and (5) deposition of a CnF layer followed by the rinsing (step 2) and drying (3) steps. For multi-immersion films of POEA only steps 1 through 4 were repeated, whereas POEA–CnF multi-layered films were assembled by repeating steps 1 through 5. In each case, films containing ten layers (monolayers for POEA and bilayers for POEA–CnF films) were produced.

AFM Characterization

AFM images of the top layer of self-assembled films with ten immersions of POEA and ten bilayers of POEA–CnF were obtained with a Topometrix TMX 2010 Discoverer Atomic Force Microscope in the noncontact mode, using probes with a nominal tip radius of 20 nm and a spring constant of 5 N/m. The obtained images were processed using Gwydion© 2.1 data analysis software. Line profile analyses of AFM images also enabled determination of nanofibril dimensions.

Results and discussion

Figure 1 shows Voc as a function of the polymerization time of POEA using a two-electrode configuration. It can be observed that there is a steep increase in Voc immediately after addition of APS to monomer in acidic medium, followed by a maximum (0.59 V, SCE) between 15 and 35 min, and then a decrease to around 0.32 V due to the completion of the polymerization reaction after approximately 70 min. The peak, with maximum of about 0.59 V, is associated with pernigraniline oxidation state, which is the fully oxidized state of polyaniline and derivatives. According to the literature, pernigraniline acts as an oxidizing agent to remaining monomers and oligomers, while being itself reduced to the half-oxidized emeraldine oxidation state [23–25]. In this stage, Voc decays and the polymer formed precipitates as a dark green-colored powder, characteristic of the doped emeraldine salt state.

The FTIR spectrum of as-synthesized POEA/HCl powder in Fig. 2 displays absorption peaks characteristic of polyaniline in the emeraldine HCl-doped state. This result is in accordance with those reported in the literature [26–29]a s described below. The absorption peaks between 3,450 and 2,950 cm-1 are assigned to symmetric H stretching of the N– Ha nd C–H bondings; the peaks at 1,490 and 1,560 cm-1 are assigned, respectively, to symmetric C=C stretching of the quinoid and benzenoid aromatic rings; the peaks at 1,260 and 1,135 cm-1 are assigned to (C–N)+ symmetric stretching of secondary amines bonded to aromatic groups and –NH+=, therefore, indicating that the polymer is in its doped form; and absorption peaks at 1,400 and 1,300 cm-1 are, respectively, attributed to in-plane angular stretching of C– Ha nd C–N of secondary amines bonded to aromatic rings. The region between 1,245 and 1,030 cm-1,r espectively assigned to symmetric and asymmetric stretching of the Car–

V oc (V)

Polymerization time (min) Addition of APS

Fig. 1 Open circuit potential of (Voc in Volts versus SCE) as a function of polymerization time for POEA, indicating the addition of APS to the aniline solution, both in aqueous 1 M HCl

Transmittance (%)

Wavenumber (cm )

Fig. 2 FTIR spectrum of the as-synthesized POEA–HCl powder

O–C (ar = aromatic), is characteristic of o-ethoxy groups of POEA; the region between 800 and 450 cm-1 is characteristic of 1,4-disubstituted benzene rings, indicating that growth of polymer chains is predominantly linear.

The UV-Vis spectra of POEA in water at various pH values are shown in Fig. 3. These spectra show three absorption bands whose wavelength of the peak maxima depend on pH, e.g., for pH 3, these maxima are located at approximately 350, 430, and 740 nm. The absorption at 350 nm is characteristic of π→π* electronic transitions of the benzenoid rings of POEA; the absorptions bands at 430 and 740 nm are associated with excitonic (n→π*) transitions (electronic transitions from the highest occupied molecular orbitals (HOMO) of the benzenoid rings to lowest unoccupied molecular orbitals (LUMO) of the localized quinoid rings) of the polarons [30]. The peak at 740 nm shifts toward lower wave numbers with increasing pH due to the decrease in doping level. This reduces both the intensity of the polaronic band and the number of polarons. These spectra also show two isosbestic points at 374 and 455 nm. The isosbestic point at 455 nm was used to monitor the adsorption of POEA and POEA alternated with CnF.

Figure 4 shows the graph of the amount of adsorbed material (Γ) versus deposition time (t) for the first layer of POEA at pH 2, 3, 4, and 5. These results show that both the stabilization time for the deposition of the first layer and the amount of polymer deposited depend on solution pH. Since more than 90% of the adsorbed amount is deposited in the first 10 min of immersion, as observed by the decrease in slope of the Γ vs. t curve, this value was chosen as the deposition time to be used for building multilayered films. The longer stabilization time required for the highest and lowest pH investigated (pH 5 and 2) is, respectively, due to the lower attraction forces between weakly charged molecules and the substrate and electrostatic repulsion forces between highly charged POEA molecules, which prevent a faster diffusion of the polymer chains from the bulk solution to the substrate. Nevertheless, by keeping the immersion time fixed for each layer, one may assure that each immersion contributes with approximately a same amount of polymer, as it will be shown later.

Polymer molecules can assume different conformations depending on factors such as pH, backbone rigidity, concentration, and polymer–solvent interactions [31–35]. At lower pH values, polyelectrolyte molecules assume an extended or rod-like conformation as a consequence of the electrostatic repulsion forces between doped sites within the same polymer backbone, whereas at higher pH, they can assume a coiled conformation as a result of the lower electrostatic repulsion forces [36]. When a polymer is adsorbed on a surface charged with oppositely charged sites, it may neutralize the charges on the surface or even overcompensate for them [37, 38].

The number of charged sites of a molecule which can interact with an oppositely charged layer is dependent on pH and chain flexibility. Thus, the driving forces for deposition are a balance of (1) attraction forces, specifically between charged molecules and the oppositely charged substrate, (2) repulsion forces between polymer chains with the same type of charge, and (3) chain conformation and flexibility that depends on pH, as previously mentioned. Therefore, rather than a linear trend of increasing thickness with pH, there is a small range of pH in which the thickness reaches a maximum value. This occurs when the surface charge density experienced by an adsorbing, fully charged polyelectrolyte drops below the critical value needed to overcome chain coiling entropy effects [16, 2].

(Parte 1 de 2)

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