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Rheoreversible Polymeric Organogels: The Art of Science for Art Conservation Emiliano Carretti,†,‡ Luigi Dei,*,† Azzurra Macherelli,†,§ and Richard G. Weiss| Department of Chemistry & CSGI Consortium, University of Florence, via della Lastruccia, 3 I-50019 Sesto Fiorentino, Florence, Italy, and Department of Chemistry, Georgetown University, 37th and O Streets, NW, Washington, DC 20057-1227 Received February 24, 2004. In Final Form: June 22, 2004 A new category of gels where gelification and breaking of the gels are chemically induced is presented. In particular, the latent gellant polyallylamine produced stable gels with some organic solvents after reaction with CO2 at room temperature, giving the gellant polyallylammonium carbamate. The rheological behavior switches from solution-type to gel-type. After weak acid-catalyzed displacement of CO2, the gel character disappears in a few seconds, making these polymeric organogels rheoreversible by a simple chemical action. This “intelligent” chemical switch between solution-type and gel-type rheological behavior has been exploited to clean pictorial surfaces in art conservation. In fact, during the cleaning procedure, there is a need for the gel supporting the cleaning solvent to have a very high viscosity. After cleaning has been successful, there is a strong necessity to reduce the viscosity, to better eliminate traces of the gellant that must be completely removed from the work of art. In the present study, we show that the art of science, in the sense of designing new physicochemical systems exploiting the “science palette”, can lead to an improvement in the techniques used to protect and conserve the results of the “artists’ palette”.
Introduction Gel technology has advanced rapidly during the past decade and has become an increasingly popular component of many products and processes. One reason for this increased interest is that gels offer rather unique avenues to new materials with controllable physical properties and applicability. Since 1980, gels have been successfully exploited as tools in art conservation as a medium to control the contact of liquid cleaning agents with the surfaces of easel paintings. Today, in this field, they represent one of the most important frontiers to be explored.1,2 The development of new categories of systems useful in conservation has been possible with the progress of gel technology in many other fields. For instance, low-temperature-processable nanoglues based on carbon-silica composite aerogels have been used to prepare opaque or low-reflectivity coatings.3 Gels based primarily on organic molecules have been used to improve the consistency of foods,4 to design new drug delivery systems (such as that based on ascorbic acid derivatives5 or those utilizing artificial proteins that hold the promise of controlling the rate and timing for release of encapsulated molecular and cellular species6), and to improve the recovery of crude oil from wells via “hydraulic fracturing”.7,8 In addition, polymers with variable macro* To whom correspondence should be addressed. E-mail: dei@ csgi.unifi.it. Fax: +39 0554573036. † University of Florence. ‡ E-mail:
[email protected]. § E-mail:
[email protected]. | Georgetown University. E-mail:
[email protected]. (1) Wolbers, R. C. Notes for workshop on new methods in the cleaning of paintings; CCI: Ottawa, Canada, 1999. (2) Wolbers, R. C. Restoration ’92: conservation, training, materials and techniques: latest developments; IIC Publications: London, 1992. (3) Morris, C. A.; Anderson, M. A.; Stroud, R. M.; Merzbacher, C. I.; Rolison, D. R. Science 1999, 284, 622. (4) Sun, Y.; Hayakawa, S. J. Agric. Food Chem. 2002, 50, 1636. (5) Palma, S.; Manzo, R. I.; Allemandi, D.; Fratoni, L.; Lo Nostro, P. J. Pharm. Sci. 2002, 91, 1810. (6) Petka, W. A.; Harden, J. L.; McGrath, K. P.; Wirtz, D.; Tirrell, D. A. Science 1998, 281, 389. (7) Abdelhaye, Y.; Daccord, G.; Duval, F.; Louge, A.; Van Damme, H. C. R. Acad. Sci., Ser. IIb: Mec., Phys., Chim., Astron. 1997, 325, 221.
porosity9 and coatings with controlled wetting properties (alumina based, transparent, superhydrophobic coating films) have been prepared through sol-gel methods that involve a combination of microstructural and chemical approaches.10 Sol-gel technology has also been used to synthesize many materials with well-defined nanoscopic features11 that are important in the ceramics industry,12 that yield magnetic nanoparticles13-15 or siloxane thin films,16 and that are dielectric materials for microelectronic applications or precursors for the preparation of inorganic oxide particles (silica, phosphosilicate, borosilicate, and borophosphosilicate).16 In the conservation of works of art, problems related to the direct application of pure organic solvents to painted surfaces (such as swelling of the binding media and original varnish layers, dispersion over unintended surface areas, and diffusion to internal layers) can be reduced drastically when the liquids are applied in the form of organogels.17,18 However, the current gel technology is not without its own problems: removal of polymer (gellant) residues after the cleaning action often involves application of some of the gelled solvent as a neat liquid;17 even with such treatments, some polymer (gellant) residues usually (8) Collias, D. I.; Prud’homme, R. K. J. Rheol. 1994, 38, 217. (9) Cooper, A. I.; Wood, C. D.; Holmes, A. B. Ind. Eng. Chem. Res. 2000, 39, 4741. (10) Tadanaga, K.; Morinaga, J.; Matsuda, R.; Minami, T. Chem. Mater. 2000, 12, 590. (11) Matsumoto, T.; Murakami, Y.; Takasu, Y. J. Phys. Chem. B 2000, 104, 1916. (12) Shafi, K. V.; Ulman, A.; Lai, J.; Yang, N.-L.; Cui, M.-H. J. Am. Chem. Soc. 2003, 125, 4010. (13) Holmes, S. M.; Girolami, G. S. J. Am. Chem. Soc. 1999, 121, 5593. (14) Liu, C.; Zou, B.; Rondinone, A. J.; Zhang, Z. J. J. Am. Chem. Soc. 2001, 123, 4344. (15) Escribano, P.; Beltran, H.; Cordoncillo, E.; Garcia-Belmonte, G.; Ruiz, L.; Gonzalez-Calbet, J. M.; West, A. R. Chem. Mater. 2001, 13, 415. (16) Bagley, B. G.; Quinn, W. E.; Khan, S. A.; Barboux, P.; Tarascon, J.-M. J. Non-Cryst. Solids 1990, 121, 454. (17) Wolbers, R. C. Cleaning Painted Surfaces, Aqueous Methods; Archetipe: London, 2000. (18) Dorge, V. The Getty Conservation Institute Newsletter 2000, 15, 16.
10.1021/la0495175 CCC: $27.50 © 2004 American Chemical Society Published on Web 08/21/2004
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remain on or beneath the painted surface because the high viscosity of the gels makes them difficult to remove. Recently, we reported the synthesis and characterization of new organogels19 in which the gellant is polyallylamine (PAA) in its ammonium carbamate form (PAA‚ CO2).20 Here, we describe a study of the rheological behavior of such a new class of polymeric organogels with the aim of developing an “intelligent” system that can switch from solution-type to gel-type rheological behavior by simple chemical action. The main target was the successful utilization of these gels for art conservation, a primary goal in developing the PAA‚CO2 systems. Perhaps more importantly, we demonstrate that this gellant is easily and completely removed from the painted surfaces by a chemically benign step that exploits the rheoreversibility of the gels. As indicated in Scheme 1, our approach is to induce gelation by bubbling CO2 through a 4 wt % PAA solution (part A), a chemical reaction that leads to strong electrostatic interactions among chains (via the newly created ammonium and carbamate groups, as shown in part B21). The gel is subsequently returned to its low-viscosity solution state by another benign chemical step, the addition of weak aqueous acid to cause decarboxylation of the PAA‚CO2. The gel (part B) can be applied directly onto a painted surface. After a predetermined contact time, a weak vinegar (aqueous acetic acid) solution is added “in situ” to effect (1) chemical decarboxylation of the PAA‚CO2 and (2) concomitant return of the viscoelastic material to a free-flowing liquid. From a technical and technological perspective, Scheme 1 represents a completely new approach to cleaning the surfaces of paintings; the chemically induced modulation of the viscosity of the system facilitates enormously its complete removal from a cleaned surface.
of sample needed, almost 25 mL). All the measurements were performed at a temperature of 25 °C (Peltier control system). Scanning electron microscopy (SEM) images were obtained by means of a Philips 515 instrument after graphitization of the samples with a JEOL JEE 4B apparatus. The energy-dispersive X-ray (EDX) spectra were recorded using a microprobe coupled with the SEM microscope. Fourier transform infrared (FTIR) spectra were obtained in the transmittance mode using a BioRad FTS-40 spectrometer with a 4 cm-1 resolution and 256 scans by means of KBr pellet. Typically, gels are applied to the surface of the soiled areas of the paint with a small synthetic brush. Then, they are stirred on the surface for 5-10 s and adsorbed onto a dry cotton swab. The area thus treated is finally rinsed with mineral spirits that are allowed to evaporate to dryness. In this case, the gel has been left in direct contact with the paint for 60 s before the PAA is decarboxylated by the addition of a few microliters of a 0.05 M CH3CO2H aqueous solution. In the last step, the liquified mixture is wiped away with a dry cotton swab.
Results and Discussion
Poly(allylamine hydrochloride) (PAA‚HCl; Mw ) 60 000), acetic acid, and 1-pentanol were purchased from Aldrich. Rheological measurements were performed on a Paar Physica UDS 200 rheometer working in controlled shear stress. For the gel samples, the plate-plate geometry was the following: diameter, 2.5 cm; thickness of the gap, 1 mm; with these conditions, the total amount of the gel in the cell was ∼2 mL. The equilibrated sample was introduced into the gap by using a spatula; then, four measurements were collected with high reproducibility (the maximum deviation was ∼1%). For the PAA solution and for the solution obtained by acidification of the gel, due to the drastic decrease of the viscosity, a double gap measuring system has been chosen (diameter, 5 cm; thickness of the gap, 1 mm; amount
The method for the preparation of the PAA is the one based on the neutralization of its ammonium chloride.20 Because the PAA solution before the addition of CO2 (Scheme 1A) shows clearly a Newtonian behavior and the horizontal asymptote viscosity values of the PAA solution (3.38 mPa s) and the pure solvent (3.35 mPa s)22 are virtually the same, the polymer chains cannot be involved extensively in a three-dimensional network. The formation of the gel (Scheme 1B) upon the addition of CO2 is very fast and was indicated visually by two factors: (1) the appearance changed from uncolored and transparent to opaque and white and (2) the free-flowing liquid did not fall to the bottom of the container tube (diameter, 4 mm) when it was inverted for at least 3 min.20 Quantitative evidence for gelation was obtained from rheological measurements. The flow curve of the gel at equilibrium (Figure 1) is typical of a non-Newtonian system. Furthermore, the viscosity of the PAA‚CO2 system is ∼1010 that of the PAA solution (3.5 mPa s).23 The viscoelasticity and the gel character of the system are confirmed by the frequency dependence of the rheological parameters reported in Figure 2. Both elasticity and energy dissipation occur; furthermore, G′ and G′′ are relatively insensitive to the frequency within the range investigated, and the storage modulus (G′) is always much higher than the loss component (G′′) of the complex modulus (G*). The applied strain was adjusted to 0.2%: with this condition, the system was in the linear regime of deformations, allowing the linear viscoelastic theory to apply.
(19) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (20) Carretti, E.; Dei, L.; Baglioni, P.; Weiss, R. G. J. Am. Chem. Soc. 2003, 125, 5121. (21) George, M.; Weiss, R. G. J. Am. Chem. Soc. 2001, 123, 10393.
(22) Dean, J. A. Lange’s Handbook of Chemistry; McGraw-Hill: New York, 1985. (23) Ferry, J. D. Viscoelastic Properties of Polymers, 3rd ed.; Wiley: New York, 1980.
Experimental Section
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Figure 1. Flow curve at room temperature for the PAA‚CO2 based gel after complete CO2 bubbling and before acid-catalyzed decarboxylation.
Figure 3. (A) A XIVth century painting from the National Gallery in Siena, Italy. The black square toward the bottom indicates the area selected for cleaning (removal of an external layer applied long after the original painting was completed). (B) Grazing light image of the area treated with the PAA‚CO2/ 1-pentanol based gel. Figure 2. Rheological properties as a function of the oscillation frequency of the PAA‚CO2 based gel. The displacement was 10%.
The rheoreversibility of the system is clear from the flow curve, taken on the PAA‚CO2/1-pentanol gel sample responsible for Figure 1, after the addition of 200 µL of 0.05 M aqueous acetic acid. PAA is reformed, presumably in its protonated (ammonium) form, PAA‚H+, and a complete loss of the gel character is observed. The Newtonian behavior is almost identical to that of the initial PAA/1-pentanol solution (from the flow curve, the viscosity value is around 3.8 mPa s). 1-Pentanol is commonly employed as a solvent for cleaning varnishes from paintings.24 The efficiency and utility of the PAA‚CO2/1-pentanol gel as a cleaning agent for the removal of the nonoriginal superimposed materials have been tested on the surface of a XIVth century wood painting (Figure 3A). PAA‚CO2 gelates a large number of liquids besides 1-pentanol,20 including some that are useful cleaning agents such as 1-methyl-2-pyrrolidone and others that are short-chained primary alcohols.24 1-Pentanol was the solvent selected here due to the robustness of its PAA‚ CO2 gel and the large body of information available about its cleaning properties. As a calibration for further studies, 1-pentanol alone was used as the cleaning agent; based on the IR spectrum of the material extracted by the
1-pentanol (Figure 4) and the elemental analyses by EDX (vide infra), the outermost layer of the painting is mainly a protective lacquer put onto the original painting by past conservation interventions.25 An additional comparison of the cleaning efficiency was made with a 2 wt % poly(acrylic acid)/1-pentanol gel2 because this polymer is employed most frequently today as the gellant in formulations for cleaning in cultural heritage conservation
(24) Horie, C. V. Materials for Conservation; Butterworth: London, 1987.
(25) Price, B.; Pretzel, B. IRUG Spectral Database; IRUG: Philadelphia, 2000.
Figure 4. FTIR spectrum of (A) the residue from the 1-pentanol applied to the painting surface and (B) a reference spectrum of a natural lacquer.25
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Figure 5. Flow curve (A) and rheological properties as a function of the oscillation frequency (B) for the 2 wt % poly(acrylic acid)/1-pentanol gel at room temperature.
Figure 6. SEM micrographs (A and B) and EDX spectra (C and D) of the painted surface before (A and C) and after (B and D) application of the PAA‚CO2/1-pentanol gel. The space bars in parts A and B are 100 µm.
(despite the complications alluded to previously).17 Figure 5 displays data from a rheological investigation of the poly(acrylic acid)/1-pentanol gel. Both the flow curve and the frequency sweep profiles in Figure 5A are typical of a gel (G′ higher than G′′), but the gel character is less pronounced than that in the corresponding PAA‚CO2 sample. Aliquots of both the acrylic acid based and PAA‚ CO2 based gels were applied with a brush to the surface of the painting, and the total contact time was 60 s.20 Removal of the PAA‚CO2 gel onto adsorbent cotton20 was preceded by the addition of a few drops of 0.05 M aqueous acetic acid (to effect the chemistry described in Scheme 1). The viscosity of the aliquot on the painting decreased very rapidly after the acid was added (visual observation), allowing fast and complete absorption of the liquid by the cotton. By contrast, a mixture of organic solvents was necessary to remove the detectable vestiges of the poly(acrylic acid) based gel, and its viscosity remained high.
Figure 3 shows the painting and the region on it selected for cleaning (black square, Figure 3A) and the grazing light image of the same area after cleaning (Figure 3B). The main target of this project was to test the power of PAA‚CO2 based gels as selective cleaning agents for easel paintings. To determine the utility of these rheoreversible gels as new tools in art conservation and restoration, we have applied them to the surface of a medieval panel with superimposed layers of a lacquer from an a posteriori conservation treatment. Figure 3B indicates that the PAA‚ CO2/1-pentanol based gel was successful in removing the nonoriginal glossy surface layer; furthermore, the “aesthetic issue” of the application is almost the same as the one usually obtained by the application of a poly(acrylic acid) based gel. The grazing light image in Figure 3B clearly shows that the shining effect is completely removed. At the same time, the HgS based underlying paint layer (see Figure 6D) is still opaque due to the presence of other
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layers, presumably casein based, which naturally still have to be investigated. The morphology of the painted surface before cleaning is shown in Figure 6A; a smooth layer with some cracks and defects is evident and is similar to micrographs of other painting surfaces with an old superimposed layer.26 After application and removal of the PAA‚CO2/1-pentanol gel, the surface texture (Figure 6B) is like that of a painting not altered by an organic surface layer.27 These micrographs demonstrate the ability of the rheoreversible PAA‚CO2 gel system to remove the external layers that were superimposed on the original painting. Furthermore, no traces of PAA could be detected on the cleaned surface by FTIR spectroscopy after removal of the gel. EDX spectra were recorded before and after treatment with the PAA‚CO2/1-pentanol gel in order to determine the composition of the inorganic components of the uncolored material removed from the surface of the painting. This analysis is based on the difference between the compositions of the surface before (Figure 6C) and after (Figure 6D) cleaning with the gel. Figure 6C is the EDX spectrum of the very thin external layer (before cleaning). The presence of aluminum is consistent with the FTIR based attribution (Figure 4) of the organic part of the surface layer to a lacquer applied by conservators in the past to protect and refresh the painting.28 Moreover, the presence of lead infers the presence of white lead pigment, added to increase the drying rate of the probable binder, linseed oil. The potassium may be attributed to KAl(SO4)2‚ 12H2O, used as the salt to precipitate Al2O3‚nH2O;27 the expected sulfur peak should be completely masked by the strong peak of lead. The EDX spectrum of the cleaned surface no longer gives evidence of any of these elements (26) Hedley, G.; Odlyha, M.; Burnstock, A.; Tillinghast, J.; Tillinghast, C. Preprints to the ICC Congress, Cleaning, Retouching and Coatings; IIC Publications: London, 1990; pp 98-105. (27) Burnstock, A.; White, R. The effects of selected solvents and soaps on a simulated canvas painting. In Preprints to the ICC Congress, Cleaning, Retouching and Coatings; IIC Publications: London, 1990; pp 111-118. (28) Webb, M. Lacquer, technology and conservation: a comprehensive guide to the technology and conservation of Asian and European lacquer; Butterworth-Heinemann: Oxford, U.K., 2000.
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(Figure 6D). In their place are the expected peaks of Hg (from the red pigment HgS) that is prevalent at the newly exposed paint surface. Conclusions The most important aspect of this work is the development and testing of the new gellant system for art conservation. The rheoreversible PAA‚CO2 gels offer a completely new approach to the cleaning of painted surfaces. The ability to convert the gel into a low-viscosity liquid at will by the addition of a small amount of a benign second agent, dilute vinegar, represents an enormous conceptual advantage of these gels over those used traditionally. The polymeric components of the PAA‚CO2 gels can be removed as easily as the solvent components because their three-dimensional gel network is not present when they are removed. This obviates the frequent necessity with classical polymeric gels of adding additional organic solvent or applying hard mechanical action to ensure removal from a cleaned surface. Finally, we note that development of these rheoreversible gel systems was born from a wedding of material science and art. The art of science can be applied to art. In this case, they meet at a chemistry/materials science interface. There are others. Our application of the modern tools and concepts of materials science to art conservation is ongoing. For instance, poly(N-methyl vinylamine) and its ammonium carbamate gels are being investigated currently as another potentially useful system for art conservation, and others are planned. Acknowledgment. The authors gratefully thank Dr. A. M. Guiducci of the National Gallery in Siena, Italy who authorized the tests on the XIVth century painting and Mr. D. Rossi, conservator, for the assistance during the tests. Thanks are also due to Prof. P. Baglioni and Dr. M. George for helpful discussions. R.G.W. thanks the U.S. National Science Foundation for its support of the Georgetown portion of this research. Financial support from Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase, CSGI, is gratefully acknowledged. LA0495175
