Polyvinyl alcohol based hydrogels as new tunable materials for application in the cultural heritage field

Colloids and Surfaces B: Biointerfaces 188 (2020) 110777

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Polyvinyl alcohol based hydrogels as new tunable materials for application in the cultural heritage field

T

Claudia Mazzuca1, Leonardo Severini1, Fabio Domenici1, Yosra Toumia1, Francesca Mazzotta1, Laura Micheli1, Mattia Titubante1, Benedetta Di Napoli1, Gaio Paradossi1, Antonio Palleschi*,1 Department of Chemical Science and Technologies, University of Rome “Tor Vergata”, Via Della Ricerca Scientifica 1, 00133 Rome, Italy

A R T I C LE I N FO

A B S T R A C T

Keywords: Polyvinyl alcohol Cellulose degradation Cleaning Cultural heritage Hydrogel Paper artwoks Spectroscopy

Hydrogel-based cleaning of paper artworks is an increasingly widespread process in the cultural heritage field. However, the search for tuned (compatible, highly retentive and not perishable) hydrogels is a challenging open question. In this paper, a complete characterization of chemical hydrogels based on polyvinyl alcohol (PVA) crosslinked with telechelic PVA and their remarkable performances as gels for cleaning paper artworks are reported. The rheological properties, porosity, water content of these gels were determined and analyzed as a function of the components concentration during synthesis. Due mechanical and retentive properties, the reported gels are optimum candidates for paper cleaning applications. The efficacy of these PVA-based gels has been demonstrated applying them on the surface of the sheets of several paper artworks, and characterizing the samples before and after the cleaning process by means of a multidisciplinary approach involving spectroscopic and chromatographic tests.

1. Introduction Restoration of paper artworks involves the wet removal of surface dust, cellulosic degradation byproducts and aged glues that are responsible for their browning, smell and aging acceleration. The most common wet cleaning procedure is water immersion, but it may not be the optimal one as it could induce swelling of paper fibers and dissolution of components [1–3]. To overcome these drawbacks, the use of tuned and ad hoc designed hydrogels as paper cleaning agents has been proposed. The design of gels as paper cleaning agents is not simple. To be efficient, the cleaning gels must be compatible with paper, that is, they do not cause swelling, changes in crystallinity, or hydrolysis and oxidation. Their pores must be large enough to absorb, by capillarity, dust and degradation products from paper surface, even if they must induce a much reduced water uptake by paper, thanks to their retention capability. Moreover, their rheological properties must ensure that they remain intact during handling and cleaning processes. Recently, gels based on Gellan Gum or other polysaccharides has been widely characterized and their paper-cleaning efficacy assessed [4–6]. However, hydrogels from natural sources have the disadvantage that they must be freshly prepared just before use and their water release is too high for a wet cleaning of fragile paper artworks. The focus of this study is to solve

a current task, that is, the development of not perishable gels, with suitable mechanical and retentive properties. In this work, a new class of PVA-based gels as candidate for cleaning of paper artworks is presented. The PVA is an interesting polymer in view of its potential application in the cultural heritage field; it is indeed biocompatible, easily modifiable, has good viscoelasticity properties (resistant to mechanical stress) and can form blend hydrogels when combined with both natural and synthetic polymers. Due to its properties, PVA is widely used yet in the biomedical and pharmaceutical field [7–10] Notwithstanding these properties, in the past, only two PVA-based gels for cleaning in the cultural heritage field have been proposed, and none of them for the cleaning of paper artworks. Indeed, they present some drawbacks in their potential application on paper artworks. PVAborax gels, for example, have a high pH (around 9–10) [11,12] and therefore can cause on application on paper samples, cellulose alkaline hydrolysis (the pH of paper is around 7–8) [13,14], while the freezedried or cast-dried PVA are physical gels whose properties are strongly dependent on concentration, freeze-dried cycles and the careful addition of a porogen like polyvinilpirrolidone [15]. Moreover, at the best of our knowledge, the compatibility of these gels on paper materials has not been tested yet. In this work, we present, as cleaning paper agents, chemical gels based on PVA and tel-PVA (where the acronym tel means

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Corresponding author. E-mail address: [email protected] (A. Palleschi). 1 They have contributed equally https://doi.org/10.1016/j.colsurfb.2020.110777 Received 12 August 2019; Received in revised form 12 December 2019; Accepted 4 January 2020 Available online 24 January 2020 0927-7765/ © 2020 Elsevier B.V. All rights reserved.

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modulus (G”) was monitored at RT by performing frequency sweep measurements in the range of 0.1−10 rad/s within a linear regime under a constant strain of 0.5 %. The storage modulus G’ is directly correlated to the crosslinks density, νe/V0, a structural feature of the network [19]:

telechelic, that is, PVA bearing an aldehyde at each chain end). In these systems, the cross-linking agent of PVA is the tel-PVA. The cross-linking reaction thus allows obtaining a chemical PVA network with a homogeneous structure without introducing any molecule different from PVA. These gels are optimum candidates as paper cleaning gels since they have several advantages: these gels are indeed, biocompatible, as they are used for medical purposes [16–18]; they are transparent, stable and not perishable. Importantly, they can be stored in water for months. Moreover, they can be tunable, that is, the final properties of the hydrogel mainly depend on the PVA/tel-PVA ratio. In fact, changes in the compositional ratios permit to tune hydrogels properties (i.e. pore size distribution and mechanical properties) for specific application needs. The characterization of these gels have been performed by means of rheological and fluorescence recovery after photobleaching, (FRAP), measurements, as well as estimation of water content and of retentive properties. Furthermore, their cleaning capability has been assessed by FTIR, SEM and HPLC measurements, as well as pH and colorimetric analysis.

1/3

νe G′ ⎡ ϕ0 ⎤ = ⎥ V0 RT ⎢ ⎣ ϕ2 ⎦

(1)

where ϕ0 and ϕ2 are the polymer volume fractions in the relaxed state, i.e. after cross-linking and before the swelling, and after the swelling, respectively. 2.3.2. FRAP measurements FRAP measurements and fluorescence images were performed on PVA_A and PVA_B gels embedded with different FITC labeled dextrans of molecular weight of 40 KDa, 70 KDa and 150KDa. Experiments were performed in triplicate, at temperature of 25 °C, using confocal laser scanning microscope Nikon Eclipse Ti-E with 20×objective Nikon plan fluor N.A.0.5 equipped with micromotorized stage and 488 nm laser line (26 mW intensity), filtered down to 2 % of maximum intensity for images, before and after fluorescence bleaching to avoid artifact of bleaching during recovery step images [20–22]. Further details are in A1, section 1.2.

2. Experimental 2.1. Materials PVA (fully hydrolyzed), NaIO4, Fluorescein isothiocyanate dextrans (FITC-dextrans), Methylene Blue (MB) and HCl were from SigmaAldrich (Sigma-Aldrich, St. Louis, MO, USA). All reagents were of analytical grade and used without further purification. In all cases, in the preparation of solutions bidistilled water (Millipore, Billerica, MA, USA) was used. New paper samples were from ALBET (ALBET LabScience, Hahnemühle FineArt, Germany) and they are considered as “reference” paper samples. Their characteristics are density, 80 g/m2 and rate of filtration 1196s/100 mL. Real aged paper samples were from “Breviarium Romanum ad usum fratum” belonging to XVIII century, called in the following Breviarium.

2.3.3. Water content in the hydrogels Total water content (TWC) of gels [4] were carried out weighing a gel portion after several days of equilibration in water and then as lyophilized material. TWC is defined as:

TWC =

(wgel − wlyo ) wgel

*100

(2)

where wgel and wlyo are the weights of the wet and lyophilized material respectively. These data were compared with the equilibrium water content (EWC), defined as:

2.2. Hydrogel synthesis

EWC= Hydrogels differing in their PVA and tel-PVA concentrations were prepared and tested. To prepare the gels, protocol reported in literature is followed [16]. Initially, an opportune amount of PVA powder was dissolved in double-distilled water at 70–80 °C, to obtain a 10 %, 20 % or 50 % (w/v) aqueous PVA solution. After complete dissolution, a 2 % (mol/mol) of solid NaIO4, was added at about 60 °C. After 20 min, the solution was cooled at room temperature. To prepare the hydrogels, equal volumes of a 50 %, 20 % or 10 % (w/v) aqueous PVA solution were mixed with 50 %, 20 % or 10 % (w/v) of tel-PVA at 80 °C, to obtain final molar ratios of PVA/tel-PVA equal to 10/5, 5/10, 25/25, or 10/10 hereafter named PVA_A, PVA_B, PVA_C and PVA_D, respectively. PVA_A and PVA_B were prepared maintaining a constant total polymer concentration.. Aqueous HC1 was then added to a final concentration of 0.2 M. The mixture was left for 24 h at room temperature in a PET reaction vessel of desired dimensions for complete gel formation. The gels was then exhaustively washed against double distilled water for several days until the conductivity of water was about 1μS and no PVA residues were present in the washing water (Fig.s A1 and A2). The gels are stable in water for months.

(wgel − wdry ) wgel

* 100

(3)

where wdry is the weight of the dried gel. 2.4. Hydrogel application on paper samples 2.4.1. Hydrogel application and removal To perform the cleaning process, a piece of gel was taken from the stock and, if necessary, cut to obtain the desired size. During the cleaning process paper sample was fully covered with the gel of about 1 cm high. over them was applied a PET film pressed (usually, on a gel of 6 × 6 cm2 and 1 cm height a weight of about 150 g has been applied) to ensure a close contact between the gel and the sample (Fig.s A4-A5). After cleaning, the gel was manually removed in only one step.” 2.4.2. FTIR and pH experiments Spectra of paper samples were acquired on a ThermoScientific (mod. Is50) instrument (Thermo Scientific Inc., Madison WI, USA), equipped with an attenuated total reflectance (ATR) diamond cell for measurement in the 4000–600 cm−1 region, at a resolution of 4 cm−1. For each sample 64 scans were collected. To determine the presence of degradation products in the paper samples and estimate the Oxidation Index (OI), a deconvolution of FTIR band region was performed following the algorithm described by Lojewska and coauthors [23,24]. Further details concerning data processing and analysis are reported in A1, section 1.3. Measurements of pH were carried out on the paper surface [25] by using an Amel Instrument 334-B pHmeter with a combined glass electrode Ag/AgCl and a porous PTFE diaphragm (Crison Instruments, Spain); RSD was 1 %, calculated on three measurements of the same

2.3. Hydrogels characterization 2.3.1. Rheological measurements The mechanical properties of PVA hydrogels were studied by using an AR2000 rheometer (Waters-TA Instruments, Milan, Italy) equipped with a parallel acrylic plate geometry (diameter 20 mm, gap 1 mm) as well as a Peltier unit which allows temperature control. The hydrogel, with a thickness of about 1.0 mm, was laid with care on the lower plate of the rheometer. The variation of the storage modulus (G’) and the loss 2

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paper materials did not give satisfactory results (data not shown), in terms of gel homogeneity, pH compatibility as well as cleaning capability and presence of gels residues on paper after treatment. To this end, further characterization of the last two gels was not performed. The PVA_A and PVA_B samples were completely hydrated up to their highest degree of swelling. In such condition, considering the hydrophilic character of PVA, both types of polymer chains have reached an almost complete stretched state. This allows some average considerations about the meshes size of PVA_A and PVA_B. Under our oxidation conditions, each tel-PVA chain has a number average degree of polymerization, DP, of 47 [18], corresponding to a number average molecular weight of 2070 Da. The intact PVA chain has a number average molecular weight of 38,000 Da. In the case of PVA_A the network is made of 1 PVA intact chain for 10 tel-PVA chains, whereas in PVA_B 1 PVA chain is accompanied by 37 tel-PVA chains. These features are reflected in the average mesh size of the two hydrogels. Assuming the length of 1.3 Å as typical of a vinyl polymer repeating unit, the mesh size is 13 nm and 7 nm for PVA_A and PVA_B, respectively. Rheology provides information about the viscoelastic behavior of a material. Hydrogels that are candidates as paper cleaning materials must be rigid and behave like a gel, because, during the cleaning process, the gel must be handled, applied and removed without leaving residues and/or losing pieces. In addition, the gel must bear a slight pressure [4] for about an hour to ensure the close contact between the gel and the paper to be cleaned without inducing gel brittleness, strong deformation and fragmentation. Therefore, the rheological behavior of the selected systems was studied, and to this end, we performed amplitude sweep tests. As can be seen in Fig. 1, both the storage and the loss moduli, G' and G” respectively, are constant in the systems in a quite large range of applied deformations. This region is called linear viscoelastic region. Moreover, the storage modulus G’ is several times higher than the loss modulus G” in all the investigated systems, indicating that the gels have a solid-like behavior. The crosslinks density, obtainable from the G’ value (eq. 1) provides an indication of the network structure [18,19]. In our case G’ of PVA_B is 4 times higher than the PVA_A one, see Table1 and 2, indicating a corresponding 4-fold higher crosslinks density or a density of chains between two crosslinks in PVA_B. This result corroborates the previous consideration on the number of tel-PVA chains per intact PVA chain, where the mole chain ratio is 1:10 and 1:36 for PVA_ A and PVA_B, respectively. As for the EWC and TWC parameters (eq. 2 and 3,), the slightly decrease in water contents of PVA_B is consistent (Table 1) with the higher chain densities of this network. A further evaluation of the entanglement features of the gels, related

sample. 2.4.3. Colorimetric measurements Colorimetric measurements were performed by using a Konica Minolta CR-200. Colorimetric coordinates in the CIELab color space (L, a*, b*), using a D65 illuminant and a 10° observer, were obtained. Color differences before and after cleaning tests are reported in terms of ΔE, which is the distance between two points in the color space. Results were obtained from three measurements on the same spot [26]. Details about the diffusion of material from the paper to gel were obtained by performing microscope experiments (Celestron, Microcapture Pro, Celestron, Torrance, USA) on paper loaded with Methylene Blue (MB) [4] during cleaning with gel PVA_A or PVA_B. 2.4.4. Scanning electron microscopy (SEM) Scanning Electron Microscopy (SEM) was used to investigate the surface morphology of paper samples, before and after cleaning process. The experiment was performed by using a field emission scanning electron microscope (FE-SEM) (SUPRATM 35, Carl Zeiss SMT, Oberkochen, Germany), using as operating parameters of the instrument 10 keV as gun voltage and a working distance of about 8 mm, while the used detector was the second electron one. Samples were previously metalized to allow electronic conduction on the sample surface. The metallization, (1 min at 25 mA), was performed using a sputter coater (EMITECH K550X, Quorum Technologies Ltd., West Sussex, UK) with a gold target [6]. 2.4.5. HPLC analyses HPLC analyses were performed with a THERMOQUEST instrument (Shimadzu, Kyoto, Japan), equipped with two pumps and an UV/Vis detector LCGA SPD-10A (Shimadzu, Kyoto, Japan). A chromatographic column HPLC Pinnacle II C18, 5 μm; 250 and 4, 6 mm (RESTEK, U.S.A.) was used. The chromatographic analysis was performed on extracts obtained by treating 1 cm2 of every sample with 1 mL of distilled water, stirring on the rotating wheel (Dynal AS, Sweden) overnight at room temperature. The hydrophilic analysis was carried out in isocratic condition using 25 mM phosphate buffer at pH 2.4 and 1 % (v/v) methanol as a mobile phase. The flow rate was 0.7 mL/min, with a loop of 20 μL and a detection wavelength equal to λ =230 nm. The analysis on paper samples was performed before and after cleaning with hydrogel [6]. 2.4.6. Rust residues removal Preliminary experiments concerning the ability of these gels to remove iron foxes from paper are reported. To this end, reference paper (1 × 1 cm) was soaked with a solution containing rust, and dried. Some of them were cleaned with gels. Successively, each uncleaned paper samples and used gels (1 × 1 cm) were incubated in 10 mL of NaSCN 0.1 M at pH = 3 for 24 h [27]. The formation of the reddish complex between Fe3+ and thiocyanate ion in the incubation solution was monitored by following its absorbance at 476 nm. During the incubation period, indeed, the Fe3+ ions uptake by the gel during rusted paper cleaning were released and formed the complex. Data were compared with the absorbance of the solution in which an uncleaned rusted paper sample was incubated. 3. Results and discussion 3.1. Hydrogel characterization Hydrogel characterization concerns only PVA_A (PVA:tel-PVA = 10:5) and PVA_B (PVA:tel-PVA = 5:10) gels, as their formulation was carried out keeping the total polymer concentration constant. In this way, the rheological and transport properties of the hydrogels under study are depended on the average mesh geometry only. Preliminary experiments concerning the compatibility of PVA_C and PVA_D, with

Fig. 1. Frequency sweep for gel PVA_A (red circles) and gel PVA_B (green squares). T = 25 °C (γ = 0.01),. G’= full symbols, G’’ = empty symbols. (For interpretation of the references to colour in the Figure, the reader is referred to the web version of this article). 3

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time is reported. Fittings of these data by a square-root power law equation (F = kt1/2) demonstrate that MB diffusion from paper to gels follows Fick’s law [4,35,36]. This means that during cleaning process, the variations of dimension and chemico-physical properties of gel (but also of paper) were negligible and no anomalous behavior occurred. The evidence that the treatment with the gels does not change paper features was obtained performing preliminary experiments reported in Appendix A, section 2.1, and Fig.s A6-A7 on reference paper samples that is new cellulose paper in a good conservation state. After cleaning, indeed, FTIR spectra, weight and pH values before and after cleaning are comparable, indicating that paper characteristics are not altered by gels treatment. The cleaning efficiency of the gel was assessed using real paper samples, from Breviarium, a paper artwork well characterized in our laboratories. In Fig. 4A, FTIR spectra of paper samples before and after cleaning are reported. They show the typical pattern of cellulose characterized by absorption bands in the region 1500–950 cm–1 due, for instance, to CO and CC stretching, CCH and OCH deformation stretching, COH and HCH bending [4–6,37]. Moreover, bands localized in the 1800–1500 cm−1 region, due to CO] stretching, and attributable to carbonyl and carboxyl groups formed due to cellulose degradation are present [23,24,38]. It should be noted that the intensities of the last bands are higher in the spectrum of uncleaned paper with respect to the PVA_A and PVA_B cleaned ones. This means that the amount of cellulose degradation products on the paper decreases due to cleaning. A deep analysis of this region of the spectra allows to obtain the Oxidation Index (OI), furnishing indication on the oxidation state of cellulose [4,6,23], as reported in 2.4.2 and A1 section 1.4. The OI obtained are higher in uncleaned paper (0.56 ± 0.04) than cleaned ones (0.41 ± 0.04 and 0.48 ± 0.04 for PVA_A and PVA_B), thus confirming the capability of the gels to remove oxided cellulose byproducts. To confirm that the gels are able to absorb the degradation products from paper we performed also the spectra of the gels before and after cleaning of Breviarium samples. Data are reported in Fig.4B,C. As shown, in addition to the characteristic bands of the gels at 1141 and 1090 cm−1 [39], peaks in the 1500–1800 cm−1 region, revealing the presence into the gel of cellulose byproducts, increase even if slightly, after gels application on Breviarium papers. Finally, the pH values of paper samples increased from 6.7 ± 0.1–7.4 ± 0.1 and 7.0 ± 0.1 after cleaning with PVA_A and PVA_B respectively. At the same time, a strong variation on colorimetric value (ΔE) of 3.1 ± 0.3 and 5.4 ± 0.4 of Breviarium samples, due to PVA_A and PVA_B cleaning process occurred. These results demonstrate the ability of these gels to remove patinas and oxidation products that cause browning and acidity of paper [4,26,40]. It should be noted that these data are comparable with that obtained using Gellan gel, (∼5), thus outlining the goodness of the proposed gel [26]. The chromatographic analyses confirmed results reported earlier. Indeed, HPLC measurements on the water extract of untreated paper samples (Fig.4D, black line) the presence of organic salts due to paper degradation. In particular the peaks at 3, 3.5, 5 and 8 min, due to oxalic, lactic, malic and citric salts, respectively, were recognized by comparison with the retention times obtained for their standards [4,6,26]. The same analyses performed on fragment of the pages cleaned with the gels under examination, showed a strong decrease of

Table 1 Values of EWC and TWC, G’ and G” (at 1 rad/s), δ and cross-links densities (νE/ V0) for gels PVA_A and PVA_ B. gel

EWC

TWC

G’(Pa)

G” (Pa)

G’/G”

νE/V0 (mol/m3)

PVA_A PVA_B

95 ± 5 89 ± 5

97 ± 5 91 ± 5

6849.0 28180

37.840 483.00

181 58

279.9 1151.8

to their pores size, was obtained by performing FRAP experiments on gels loaded with FITC-labeled dextrans with different molecular weight (and dimensions) [4]. From these data, it is possible to evaluate the molecular diffusion into the hydrogels and then, the capability of the gels to adsorb and entrap dust and paper degradation products during the paper cleaning process. The representative recovery profiles are shown in Fig. 2 for PVA_A and PVA_B, respectively, and the results of FRAP analysis are summarized in Tables 2. Probing four different gel spots, the increase of the FRAP for all molecular weights of FITC-dextran fractions is monoexponential (Fig. 2, Table 2) indicating an essentially homogeneous network structure. a) ref. [[28]]; b) ref [29]; c) ref. [30]; d) ref [31]. In PVA_A and PVA_B, the fluorescence recovery is monoexponential, indicating a structural homogeneity of the gel at mesoscale (see Fig. 2). In PVA_A, dextran fractions are able to move without sieving effects of the hydrogel meshes and the diffusion behavior is governed by the size of the polymer coil. On the contrary, the mesh size of PVA_B, i.e. 7 nm, influences the diffusion features of the FITC-Dextran fractions as the FRAP measurements show the same recovery times regardless of the intrinsic hydrodynamic size of the dextrans fractions. In PVA_B the sieving effect of the network operates a dramatic change of the diffusion behavior of FITC-dextran fractions. In summary, PVA_B hydrogel acts on the diffusion of the FITC-dextran fractions as a medium with an apparent viscosity higher then water [32–34]. 3.2. Paper cleaning ability The obtained TWC and EWC values (see Table 2) indicate that these gels retain a great water content to be released for cleaning purpose. The retentive properties of these gels were obtained by determining the amount of water released on a porous hydrophilic surface i.e. a paper sample. The releasing properties of PVA_A and PVA_B were evaluated by weighting paper samples before and during gel application. As reported in Fig.s 3A and A6, the weight increase of paper samples (Breviarium and reference ones), due to water uptaken from every gel during 1 h of cleaning process, is about 70 %, significantly lower with respect to the amount of water absorbed by paper during a standard cleaning method, that is immersion in water (more than 300 %) and, at the same time, comparable to that obtained with recently presented chemical gels for paper cleaning [4]. At the same time, during the cleaning process the weight decrease of the gel is less than 10 %, while the dimensions did not vary. This indicates that gel characteristics substantially remain the same during application. This result was confirmed also by optical microscope experiments performed following the diffusion of MB (colored compound representing dust and/or cellulosic by-products) from reference paper sample fully colored with this dye, into PVA_A or PVA_B. In Fig. 3B, the colored height, due to MB, with respect to the gel total height, during

Table 2 Relevant diffusion parameters of FITC-Dextran fractions in PVA_A and PVA_B hydrogels. Estimated average network pore size are 13 and 7 nm respectively. Dw and Dg are the diffusion values in water and in gel respectively. FITC-Dextran (kDa)

Stoke's Radius (nm)

τ (s) PVA_A PVA_B

Immobile fraction PVA_A PVA_B

Dg (μ m2/s) PVA_A PVA_B

Dw (μ m2/s)

40 70 150

∼4.5a ∼6.0b ∼8.0d

– 4.0 ± 0.2 9.0 ± 0.7

0 % 20 % 1 % 30 % 5 % 35 %

- 6.1 ± 0.5 20.2 ± 0.2 5.9 ± 0.4 9.0 ± 0.4 5.9 ± 0.2

∼60a ∼40c ∼20d

13.4 ± 0.8 13.7 ± 0.7 13.8 ± 0.4

4

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Fig. 2. Representative intensity gaussian profile in the bleached region of interest (ROI) calibrated for FRAP analysis (panel A, left); and FRAP profiles together with first order exponential decay fit from PVA_A (A) and PVA_B; (B) samples embedded (from left to right) with 70 kDa, 150 kDa and 40 kDa, 70 kDa and 150 kDa dextrans, respectively.

PVA_B) clean the cover without causing a spreading of inks. Furthermore, a strong pH increase, passing from 5.1 ± 0.2 (before cleaning) to 7.0 ± 0.3 and 7.7 ± 0.3 after PVA_A and PVA_B cleaning procedure occurs. Similarly also a variation of colorimetric values of 4.7 ± 0.9 and 2.7 ± 0.5 (due to PVA_A and PVA_B application, respectively) was obtained. It is interesting to note that the pH and colorimetric variation of the woodblock cover cleaned with Gellan gel or another commercial gel [4,6], are 7.3 ± 0.2 and 5.4 ± 0.2. These results indicate that, even if the water release by PVA_A and PVA_B is much less than the other two gels, cleaning processes performed with these gels are as efficacy as Gellan gel. Finally, preliminary results concerning the ability of these gels to remove iron residues (i. e. rust from old clips, pins or staples) from paper were reported. These experiments were performed on ad hoc prepared reference paper specimens (see section 2.4.6). The removal ability of the gels was tested by measuring the absorbance (at λ=476 nm due to the iron-thiocyanate complex) of the incubation solution. From these data, it is possible to estimate that the PVA_A and PVA_B gels are able to remove from dirty paper, the 84 % and 45 % of the iron content, respectively.

the peaks due to acids, thus confirming that both gels are able to remove various hydrophilic components from the paper surface. The efficacy of these gels as paper cleaning materials was confirmed also by SEM images obtained on Breviarium paper samples before and after cleaning with gels. As shown in Fig. 5, the cellulosic fibers before cleaning appear dirt, full of dust, while after cleaning, the patina are efficiently removed. Moreover, no changes on fibers morphology occurred due to cleaning. These data are confirmed also by optical microscope images indicating that cleaning does not perturb the readability and features of paper samples (Fig.s A8, A9). To assess the cleaning efficacy of our proposed gels, one other question must be taken into account, that is, the presence of gel residues adsorbed on paper. As concern this investigation, first, in FTIR spectra of cleaned paper sample (Fig.4A, A7), the absorption peaks (localized at 1141 and 1090 cm−1) due to PVA_A and PVA_B are not present, indicating that no polymer residues (above the instrumental detection limit) are present. This result is confirmed by HPLC a more sensitive technique. The water extract of PVA_A gels (Fig. A10) shows a chromatographic profile different to those of paper samples, as it is characterized by a main peak (retention times: 4 min), totally absent in the chromatograms obtained from water extracts of paper samples cleaned with the gels. Furthermore, in SEM images (Fig. 5) no contaminants like gel residues are evident in the cleaned sample. The cleaning efficacy of these two gels with ancient and fragile paper artworks was tested also on red and white bicolor geometric woodblock paper cover from XVII century. As reported in Fig. A11, both gels (even if PVA_A is more able than

4. Conclusion In this work, we have characterized not perishable PVA-based hydrogels, testing their use as smart tools for the cleaning of paper artworks. Physico-chemical characterization, indeed, indicates the suitability of the proposed gels for this purpose. They have a solid-like Fig. 3. A) Water uptake of Breviarium paper samples during treatment with PVA_A gel, (green), PVA_B gel (red) and water bath (blue). B) Diffusion data during time of MB from paper to PVA_A (green circles) and PVA_B (red squares) gels. Height represents the length (in percentage) of the colored zone with respect to the total height of the gel. (For interpretation of the references to colour in the Figure, the reader is referred to the web version of this article).

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Fig. 4. A): FTIR spectra of a sample from Breviarium before (black continuous line) and after cleaning process with gel PVA_A (red dashed line) and with gel PVA_B (green dotted line); B) and C): FTIR spectra of dried PVA_A and PVA_B gels before (red dashed line) and after (black continuos line) cleaning process on Breviarium samples. Spectra are normalized to the maximum for clarity. D: Comparison between the chromatograms obtained for water extracts of Breviarium fragments: before (black line) and after PVA_A (red line) and PVA_B (green line) cleaning process. (For interpretation of the references to colour in the Figure, the reader is referred to the web version of this article).

behavior, thus ensuring a uniform contact between the gel and the artwork, and, at the same time, they are not breakable during handling. The entanglements of the chains, although they are different on changing the PVA/tel-PVA ratio, allow a high mobility of very big molecules like dextrans of 8.3 nm of Stokes radius, thus indicating that these gels can absorb easily bulky dust and degradation molecules. Importantly, the highly retentive properties of these gels, avoid fibers swelling or spreading of inks during the wet cleaning.. Their cleaning efficacy has been assessed by using a wide range of techniques investigating properties of references and ancient paper samples before and after cleaning. Finally, preliminary experiments suggest that these gels are able to remove also rust residues.

FM has performed preliminary experiments concerning gels-paper compatibility. MT has performed chromatographic analysis. LM has performed pH measurements and supervised the chromatographic experiments. GP has analyzed all the data concerning the physico-chemical characterization of the gels, and helped FM to choice the PVA/tel-PVA ratios of the gels and in preparing such gels. AP has coordinated all the experiments, analyzed the data, and helped CM to write the article. All the authors discussed the results and commented on the manuscript.

Funding

Declaration of Competing Interest

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

All authors declare no conflicts of interest and no competing financial interest

Contribution of each author

Acknowledgements

CM has written the article and has analyzed experimental results concerning the capability of the gels to clean paper artworks. LS has performed spectroscopic and colorimetric measurements concerning gels-paper compatibility and gels cleaning capability. FD has performed the fluorescence recovery after photobleaching experiments and analyzed the data. YT has performed rheological measurements. BDN has performed scanning electron microscopy experiments.

Authors thank Dr Martina Marconi, Dr. Roberto Ciccoritti and Dr Katya Carbone for technical support.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.colsurfb.2020.110777. Fig. 5. SEM images of Breviarium paper samples before (A) and after cleaning with PVA_A (B) and PVA_B (C) hydrogels.

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