An Environmental Friendly Fluorinated Oligoamide for Producing

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An Environmental Friendly Fluorinated Oligoamide for Producing Non-Wetting Coatings with High Performance on Porous Surfaces Mara Camaiti, Leonardo Brizi, Villiam Bortolotti, Alessandra Papacchini, Antonella Salvini, and Paola Fantazzini ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09440 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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ACS Applied Materials & Interfaces

An Environmental Friendly Fluorinated Oligoamide for Producing Non-Wetting Coatings with High Performance on Porous Surfaces Mara Camaiti^,§, Leonardo Brizi⊥,§, Villiam Bortolotti#, Alessandra Papacchiniǂ, Antonella Salviniǂ and Paola Fantazzini⊥,§,*

^

Institute of Geosciences and Earth Resources, National Research Council

Via G. La Pira 4, 50121 Florence, Italy

§

Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, P.za del

Viminale 1, 00184 Rome, Italy

⊥

Department of Physics and Astronomy, University of Bologna, Viale Berti Pichat 6/2, 40127

Bologna, Italy

#

Department DICAM, University of Bologna, Via Terracini 28, 40131 Bologna, Italy

ǂ

Department of Chemistry “Ugo Schiff”, University of Florence, Via della Lastruccia 3-13,

50019 Sesto Fiorentino (Florence), Italy

ACS Paragon Plus Environment

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KEYWORDS: Hydrophobic effect, wettability, surface chemistry, perfluoropolyethers, oligo(ethylenesuccinamide), Magnetic Resonance Imaging.

ABSTRACT: The change in surface wettability changes of many materials are receiving increased attention in recent years. It is not too hard to fabricate resistant hydrophobic surfaces through products bearing both hydrophobic and reactive hydrophilic end groups. More challenging is to obtain resistant non-wetting surfaces through non-covalent reversible bonds. In this work a fluorinated oligo(ethylenesuccinamide), soluble in benign solvent for operators and environment,

has

been

synthesized.

It

contains

two

opposite

functional

groups

(perfluoropolyether segments and amidic groups) (SC2-PFPE), that provide water repellence while hydrophilicity is retained. Its performance has been tested on a porous calcarenite and investigated by Magnetic Resonance Imaging, water capillary absorption and vapor diffusivity tests. The results demonstrate that SC2-PFPE modifies the wettability of porous substrates in a drastic and durable way, and reduces the vapor condensation inside the pore space, due to the perfluoropolyether segments that act at the air/surface interface.

1. INTRODUCTION The change in surface wettability has received increasing attention because of the relevant role that hydrophobic surfaces play in a wide range of applications, from water absorption, transport and diffusion at macro1 and microscale level,2 to textiles modifications,3 oil-water separation,4-6


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prevention of bacterial and dust adhesion on a surface, 7,8 surface protection9 including stone treatments.10-16 To fabricate hydrophobic surfaces, products with low-energy interaction with water are used to create a “low-energy surface”. Compounds containing alkyl and fluorinated groups are considered the best candidates to produce hydrophobic surfaces because of their non-polar structures that strongly repel water, with a consequent increase in the contact angle. 7 In the case of a surface with high roughness which is also water repellent, super hydrophobic properties can be achieved.17 Surface modifications through multifunctional interfaces have recently received great interest due to the possibility to simultaneously carry out two functions, e.g. water repellency and enzyme catalysis.18 Moreover, molecules bearing two different end groups (hydrophobic and hydrophilic) have been used to create hydrophobic or super hydrophobic surfaces through the reaction of the hydrophilic end group with the material of the surface.19, 20 In this study, we aimed to change the wettability of hydrophilic surfaces of porous media (typically rocks) through a molecule bearing both hydrophobic and hydrophilic groups, in order to protect them against the chemical-physical processes of degradation caused by water. Since the protection of stone surfaces mainly involves historical buildings and other stone artifacts, the molecule must fulfill some requirements, primarily it must be inert toward the substrate. Therefore, our attention was devoted to obtain adhesion to the rock through durable and reversible non-covalent bonds (e.g. hydrogen bonding or dipolar interaction). Acrylic, alkyl alkoxy silanes and perfluoropolyethers are the most common compounds used to change the surfaces wettability in the field of stone conservation, but their performance and


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properties do not completely satisfy some basic requirements. Acrylics show low stability (mainly photo-oxidation reactions) 21 and not durable adhesion to the substrate.22 Alkyl alkoxy silanes, thanks to the presence of alkyl groups (typically –CH3), create highly hydrophobic surfaces with very high contact angles, but the hydrophilic end groups react with the rock making these compounds not more soluble after application and polymerization (not reversible treatments).23 Perfluoropolyethers (PFPE) have many of the properties required to an ideal protective compound, such as high stability, water repellency and low surface tension. However, the basic PFPE, proposed24 and applied on some historical buildings25,26 since the 1980s, showed a weak interaction with the polar medium, shortening the duration of the protective action.27 PFPE containing polar groups (typically amidic groups) showed better hydrophobic properties and improved the durability of the water repellency. This behavior was explained by the interaction with the stone through dipolar interaction between the –C=O-NH– groups and the polar surface.28 Unfortunately, these compounds are soluble only in chlorofluorocarbons (CFC) and in supercritical CO2,29 therefore their use as protective agents for historical stone artifacts has been abandoned since 1995. A partially fluorinated terpolymer based on acrylic-methacrylic chains grafted with monofunctionalized PFPE was also proposed as substitute of PFPE because of its solubility in common organic solvents, its good performance as stone protective agent, and its economic viability. 30,31 The improved photo-oxidative stability and hydrophobic effect of this terpolymer, compared to those of acrylic-methacrylic polymers, were attributed to the pendant perfluoropolyether segments that rearrange themselves at the air-polymer interface through surface segregation during the evaporation of the solvent and the coating formation.31,32


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However, the hydrophobic effect of this terpolymer with pendant perfluoropolyether segments is limited to the external treated surface due to the size of its macromolecules (medium-high average molecular weight) which hinders the distribution and penetration inside the porous structure. In this work, a new low average molecular weight hydrophobic agent containing short pendant perfluoropolyether segments linked to an oligo(ethylenesuccinamide) chain (SC2-PFPE), soluble in environmental friendly solvents, was synthesized in order to achieve high hydrophobic effect, good penetration and uniform distribution into the pore space and durable adhesion to the polar substrate. The hydrophobic property of SC2-PFPE was tested by Magnetic Resonance Imaging (MRI) of 1

H nuclei of water, and compared with a poly(hexafluoropropene-co-vinylidene fluoride)

(N215), a fluorine-containing commercial product. Nuclear Magnetic Resonance, in fact, is widely applied to study the behavior, including interaction, of fluids inside porous media.33, 34 In particular, MRI has been demonstrated to be a valid non-destructive and non-invasive technique for monitoring the porous structure and the water absorption in porous materials, as well as for evaluating the hydrophobic effect and spatial distribution of compounds to be used in protective treatments.35-43 The results found here by MRI are in agreement with those obtained by standard tests, mainly used in the field of stone protection,44, 45 with the further advantage of visualizing the localization and the distribution of water inside the pore space of the samples. To confirm that the water repellency effect and the inhibition of water uptake of the porous material are due to the hydrophobic property of the compound and not to the pores blockage, the


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residual vapor flow through the porous space (vapor diffusivity) has been also evaluated after the treatment following a standard method.44 A highly porous biocalcarenite (Lecce stone), used as construction and decorative material, was the porous substrate to test the performance of SC2PFPE. 2. EXPERIMENTAL SECTION 2.1. Materials. Diethyl succinate (99%), ethylenediamine (99.5%), anhydrous ethyl alcohol (99.5%), anhydrous tetrahydrofuran (99.9%), 2-propanol (99.8%) and anhydrous diethyl ether (99.8%) were purchased from Sigma-Aldrich (Milan, Italy) and used as supplied. Poly(hexafluoropropene-co-vinylidene fluoride) (N215, a fluoroelastomer with average Mw = 125,000 g/mol) was kindly supplied by Solvay-Solexis (Milan, Italy). The monocarboxylic perfluoropolyether acid (CF3-O-(C3F6O)m-(CF2-O)-CF2CH2COOH) (PFPE-acid) (molecular weight from end group titration = 880 g/mol) and trichlorotrifluoethane (CFC 113) were kindly supplied by Ausimont S.p.A. (Milan, Italy). The ethyl ester of PFPE-acid (PFPE-ester) was obtained with a 100% yield, according to a method reported in the literature,28 by heating the acid in the presence of a great excess of ethyl alcohol (molar ratio 1:30) and periodically removing the water produced in the condensation process by distilling the azeotropic mixture. The product was characterized by FT-IR, 1H-NMR, 13C-NMR and 19F-NMR (refer to Supporting information: Materials characterization). Samples (5x5x2 cm3) of Lecce stone, a biocalcarenite with porosity accessible to water PH2O= 39% and grain size distribution between 100 and 200 µm,46 were used to test the performance of the surface wetting modification agents. The porespace of this stone is macroscopically very homogeneous but there is a substantial range of pore and pore-channel sizes: Mercury Injection Porosimetry gives a range from 0.01 µm to a few micrometers, with a sharp peak at just over 2 µm. 37 Lecce stone is a suitable rock model to be


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used in testing the performance of hydrophobic compounds by MRI. Moreover, the behavior of these compounds applied on Lecce stone is similar to that observed on other natural and artificial materials with medium-high porosity (e.g. tuffs, bricks, ceramic, concrete). Hydrophobic treatments on rocks with low or very low porosity, such as marbles, usually show the same efficacy than on other stones. However, the surface wettability change on these rocks is much more influenced by the application method (e.g. use of brush instead of imbibition or spray, kind of solvent, concentration) than on Lecce stone or other highly porous stones. This fact may limit the use of certain hydrophobic products on very low porosity stones. 2.2. Preparation of SC2-PFPE. The synthesis was performed, for the first time, in two subsequent steps (scheme 1). First step - synthesis of SC2 (1). SC2 was synthetized in a 20 ml screw-cap Sovirel® tube. Diethyl succinate (S) dissolved in tetrahydrofuran (THF) in the ratio 1:1 (v/v) was added at room temperature to ethylenediamine (C2), under nitrogen atmosphere and with magnetic stirring. Then the temperature was raised to 90 °C for 7 hours. The molar ratio S:C2 was 1:2. The product precipitated as it was formed because of its insolubility in THF. The white solid was purified under nitrogen atmosphere by washing it with diethyl ether for three times, then it was dried at reduced pressure and stored at room temperature under nitrogen atmosphere (diethyl succinate conversion 98%). SC2 was characterized by FT-IR, 1H-NMR and 13

C-NMR (refer to Supporting information: Materials characterization, Figures S1-S3). Average

molecular weight estimated by 1H-NMR: 330g/mol. Second step - synthesis of SC2-PFPE (2). In a two-necked flask, equipped with a reflux condenser and a dropping funnel, the PFPE-ester, mixed with ethyl alcohol in the ratio 1:4 (w/w), was slowly added to SC2 at room temperature under nitrogen atmosphere and with magnetic stirring. Then the mixture was heated at 80°C for 25 hours. The molar ratio SC2:PFPE-ester was 1:1.9. The white waxy solid obtained, insoluble at


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room temperature in ethyl alcohol, was decanted and purified by washing it with CFC 113 and then with 2-propanol, and finally dried at reduced pressure (yield 67%). SC2-PFPE was characterized by FT-IR, 1H-NMR and 19F-NMR (refer to Supporting information: Materials characterization, Figures S4-S6).

Scheme 1. Synthesis of products 1 (SC2) and 2 (SC2-PFPE) based on the condensation reaction of ester and amine groups.

2.3. Instruments for materials characterization. Transmission infrared spectra were recorded using a Perkin-Elmer spectrometer, mod. System 2000 on neat oligomer films casted on KBr windows. The spectra were collected from 370 to 4000 cm-1 using a DTGS detector, with 4 scans and 2 cm-1 spectral resolution. 1H-NMR and 13C-NMR spectra were recorded with a Varian Mercury Plus 400 spectrometer on D2O or CD3OD/D2O (95/5 w/w) solutions. 19F-NMR spectra were recorded with a Varian VXR 200 spectrometer on CD3OD/D2O (95/5 w/w) solutions. 2.4. Coating procedure. The surface modification agents were applied on only one 5x5 cm2 surface of the prismatic stone samples using SC2-PFPE and a fluoroelastomer (N215), as a reference. SC2-PFPE was applied both as 0.5% (w/w) solution in a mixture of 2-propanol:H2O (70:30, w/w), and as 1% (w/w) suspension in a mixture of 2-propanol:H2O (70:30, w/w). N215


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was applied as 1% (w/w) ethyl acetate solution. For all the products, the amount of solution/suspension to be applied was calculated considering a final theoretical amount of active compound on the stone surface of 10 g/m2. The solution/suspension was deposed on the stone sample by a pipette. The solvent was evaporated at laboratory conditions and then the stone specimens were dried in desiccator to determine the mass of active product actually applied before subjecting them to the tests for performance evaluation. Three samples for each mixture were prepared. All of them were used for the water capillary absorption test and then two were used for the vapor diffusivity test. One of the two specimens used for the evaluation of vapor diffusivity was later used for the MRI analysis. 2.5. Magnetic resonance imaging. MRI images were collected using Artoscan (Esaote S.p.A., Genova, Italia), a tomograph based on a 0.2 T permanent magnet, operating at about 8 MHz for 1

H nuclei, on the 5x5x2 cm3 samples. Multi-slice Spin-echo sequences were used to obtain at the

same time a number of adjacent axial sections (5x2 cm2) on each sample sufficient to cover all the sample (thickness of each slice = 5 mm, gap between slices = 1 mm, pixel size = 0.78 × 0.78 mm2, Repetition Time (TR) = 900 ms, Echo Time (TE) = 10 ms). Each sequence was repeated eight times in order to maximize the signal/noise ratio within a reasonable measurement time. Particular attention was devoted to avoid electronic saturation of the signal and to ensure that the signal intensity was determined on the same dynamic range from sample to sample. Bright regions in MRI images reveals the presence of water (the signal is due to the 1H nuclei magnetization). The signal in each pixel is proportional to the water amount in the corresponding voxel only if the nuclear magnetization can to reach the equilibrium after each acquisition sequence. In porous media, wide and multimodal distributions of longitudinal (T1) and transverse (T2) relaxation times can be observed due to wide distributions of pore sizes, and these


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distributions change under conditions of partial saturation. The choice of TE = 10 ms, due to instrumental limitation, and TR = 900 ms, due to the need to optimize the compromise between measurement duration and reduction of water evaporation, did not allow us to detect the signal at very short T2 and very long T1.37 In order to overcome this unavoidable drawback, all the images of the treated samples were compared with those of untreated samples and quantitative analyses of water content were made only by mass measurements. MRI images were taken at increasing intervals of time during capillary water absorption, from one hour up to a week. Images were analyzed by an in-house software in order to quantify the water signal (in arbitrary units) inside the samples, in a scale that was the same for all the samples. The algorithm also created the signal profile to represent the total signal at each height along the shortest side of the sample. The profile was obtained by the sum of the pixel values, for each row of 6 adjacent internal slices. In order to characterize the spatial distribution of water in the stone, a three-dimensional view of the sample was reconstructed starting from the two-dimensional slices, by implementing an algorithm under the Enthought Canopy environment, a Python academic free distribution for scientific computing. 2.6. Coating performance evaluation. The water absorption inhibition on stone was evaluated following the UNI-EN 15801-2010 method.35, 45 The WIE was calculated from the mass of liquid water absorbed by the 5x5 cm2 surface in 30 minutes and 1 hour, before (A0) and after (A1) coating, according to Eq. (1). A0 and A1 are the average values computed from the data found in three consecutive tests performed after the samples were dried before each measurement. =

∙ 100

(1)


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The procedure used for the evaluation of WIE was followed also during the MRI study of the kinetics of liquid water absorption through the treated and the untreated face of the samples. After a fixed time of water absorption inside the box, and before being introduced in the tomograph, the samples were weighed and immediately wrapped up in a plastic film in order to minimize water evaporation during imaging measurements. The plastic film, as previously checked, did not contribute to the MRI signal. After image acquisitions, the samples were immediately put again in absorption. This procedure was repeated at regular time intervals and stopped when the samples reached constant weight (typically after 7 days). The residual vapor flow through the coated porous medium “vapor diffusivity” has been evaluated following the UNI-EN 15803-2010 method44 measuring the mass mi of vapor flowed every 24 hours up to 7 days. The average value on the 7 days of the vapor mass (g) flowed in 24

hours ( ∑ ∙ ) through the surface area A (cm2) was computed and used to find the vapor diffusivity (D) of each stone sample. The percentage residual diffusivity, RD (%), was calculated from the sample diffusivity after treatment (D1) and the average value of diffusivity of 4 untreated samples (D0), as given in Eq (2):

RD = ∙ 100

(2)

The maximum sample standard deviation achieved for RD was 1% if one considers for D1 and D0 the same sample. The sample standard deviation for D0 values measured on the untreated samples was 2% (0.0005 g/cm2), therefore the maximum standard deviation for RD, calculated considering D0 as the average value of four untreated samples, was about 4%.


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3. RESULTS AND DISCUSSION 3.1. Synthesis and characterization of SC2-PFPE. The synthesis of a new oligosuccinamide containing low molecular pendant perfluoropolyether segments was performed in two steps, according to Scheme 1. Product 1 (SC2) was obtained by condensation of diethyl succinate and ethylenediamine. The molar ratio di-ester:di-amine was 1:2 in order to privilege the formation of oligomers with –NH2 terminal groups, which are the active sites in the grafting reaction with short perfluoropolyether chains. The synthesis of product 2 (SC2-PFPE) was also based on condensation between an ester and an amine group (SC2). The condensation processes in the synthesis of 1 and 2 were monitored by FT-IR spectroscopy. The reaction progress was estimated by the decreasing of the signals ascribed to the stretching vibration of the C=O bond of ester (at 1730 cm-1 for di-ethyl succinate and at 1793 cm-1 for the mono ethyl ester of PFPE). The reactions were considered completed when the band of the ester has disappeared. The structure of the products has been confirmed by the presence of the bands due to Amide I (1635-1638 cm-1 in succinamide chain, and 1705 cm-1 in the PFPE grafted chain) and Amide II (1556-1560 cm-1), as well as the band ascribed to CF stretching in PFPE (1400-900 cm-1). An exhaustive characterization of the products has also been performed by 1H-, 13C- and 19F-NMR. 1H-NMR was also used to determine the average molecular weight of SC2; the signal intensity of the singlets at 3.32 ppm (-CO-NH-(CH2)2-NH-CO-) and at 2.55 ppm (-NH-CO-(CH2)2-CO-NH-), and of the triplet at 2.75 ppm (CO-NH-CH2-CH2-NH2) was used for the calculation. An expected low molecular weight of 330 g/mol was found. The corresponding NMR and FT-IR spectra are reported in Supporting information (Figures S1-S6). As the use of the fluorinated oligoamide is related to its solubility in non-toxic and environmentally benign solvents, the dissolution of SC2PFPE in some solvents was tested. The results are reported in Table 1, along with the solubility


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of SC2 and PFPE ester. SC2-PFPE showed insolubility in water and trichlorotrifluoroethane (CFC 113), while SC2 was soluble in highly polar solvents (i.e. water) and PFPE ester in 2propanol and CFC 113. At the same time, SC2-PFPE showed good solubility in polar solvents in hot conditions (typically around the boiling point of the solvent). However, the fluorinated oligoamide remained partially soluble (about 50%) when the hot solution was cooled down to room temperature. This behavior allowed us to apply the product at room temperature on the stone surface as dilute solution. The best solvent was 2-propanol or mixtures of 2propanol/water. In particular, the mixture 2-propanol:water 70:30 w/w can be considered a good solvent for commercial distribution and application of SC2-PFPE (e.g. as protective agent for stone artifacts). The product used as solution was called SC2-PFPEsol. The mixture containing the soluble and insoluble SC2-PFPE was called SC2-PFPEsusp.

Table 1. Solubility (%) of 10 mg of product in 1 g of solvent at room temperature (typically 25°C, cold) and around the boiling point of the solvent (hot). Solvent

SC2-PFPE

SC2

PFPE-ester

cold

hot

cold

hot

cold

hot

Acetone

0

0

0

0

nd

nd

Tetrahydrofuran

0

10

0

15

nd

nd

Ethyl alcohol

0

60

0

20

0

100

Water

0

0

100

100

0

0

2-propanol

50*

100

0

20

100

100

2-propanol:H2O (70:30)

50*

100

0

70

0

0

2-propanol:H2O (90:10)

50*

100

0

40

nd

nd

CFC 113 0 0 0 0 100 100 *This percentage was found when the hot solution was cooled down to room temperature; nd = not determined.


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3.2. Water repellence and vapor diffusivity by standard methods. Water repellence was firstly determined through the standard method of capillary absorption mainly used in the field of stone protection. This method was applied to verify the water absorption inhibition, as well as the adhesion stability of the fluorinated oligoamide when applied on a polar substrate (i.e. Lecce stone, a carbonate sedimentary rock). The wettability changes of porous surfaces are expressed as the percentage reduction of water capillary uptake at fixed times, as given in Equation (1) where WIE is the water absorption inhibition efficacy of a coated porous substrate calculated on the basis of the mass of water absorbed before (A0) and after (A1) the application of the coating. Moreover, the effect of the coating on the diffusivity of water vapor in treated stone was investigated following the “cup method”,44 by measuring the mass of vapor flowed (“vapor diffusivity”) during a fixed period of time through a section of treated and untreated samples. The results are expressed as percentage residual diffusivity (RD). In Table 2 the WIE values at different water capillary absorption times (30 minutes and 1 hour) on Lecce stone samples are reported. The results show that the water repellence obtained with SC2-PFPE, both as solution and suspension, is much higher (> 90%) than that obtained with N215 (average value ~ 50%). Moreover, N215 shows lower reproducibility of the measurements (not only on the same sample, but also among samples) as compared with SC2-PFPE, and a tendency of WIE to decrease from 30 minutes to 1 hour. The low reproducibility of the WIE measurements for N215, on the same sample, can be due to a non-homogeneous distribution of the fluoroelastomer on the external surface of the samples and/or inside the pores with a rearrangement of the polymeric film, poorly adhered at the surface, during the drying step from measurement to measurement. The difficulty to obtain a homogeneous distribution of the product


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may explain the high WIE variability among samples as well. The high hydrophobic effect achieved on highly porous surfaces treated with SC2-PFPE is also visible in the photographs of Figure 1, where the behavior of water droplets was captured after 5 s and 30 s from the deposition on untreated and treated stone surfaces.

Figure 1. Photographs of water droplets on Lecce stone samples captured after 5 s (a) and 30 s (b) from the water droplets deposition. In the left the uncoated stone surface, in the right the stone surface was coated with 10 g/m2 of SC2-PFPE susp. Sample sizes 5x5x2 cm3.

The water repellency of the samples treated with SC2-PFPEsusp appears slightly higher than that with SC2-PFPEsol, the amount of product applied being about the same. The small number of measurements and the small differences between the average values do not allow to assign a statistical significance to this observation, but this is just what it could be expected considering the presence or the absence of undissolved product in the treatment mixture. Indeed, the not dissolved product causes a preferential precipitation on the external surface, while the dissolved


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one can penetrate inside the porous material. Thus, for SC2-PFPEsusp we expect a higher percentage of the applied product to concentrate on the external surface, while SC2-PFPEsol may easily enter into the porous medium and spread in depth inside the entire pore space of the stone sample. Despite the drastic wettability change of SC2-PFPE, the diffusion of vapor through the porous medium is not greatly influenced by the presence of the hydrophobic compound. Indeed, RD of the treated stones is high (90% and more) not only for N215, but also for SC2-PFPE. In particular, it is worthy to observe that for SC2-PFPEsusp and N215 the values exceed 90%. Although the diffusion of vapor through SC2-PFPE and N215 has not been measured, it is possible to state that it is at least two or three orders smaller than rocks. In fact, perfluoropolyethers (liquid compounds) show a gas diffusion coefficient of about 10-6 cm2/s, 47 while the water vapor effective diffusion coefficient in Lecce stone is 1.8·10-2 cm2/s (refer to Supporting information: Materials characterization). Therefore, the RD values of the treated stone are related to the fraction of the pore space still accessible to the water vapor after the treatment. Using the simple piston-like model for the ingress of the product inside the stone (refer to: Computation of the pore space volume reduction in Supplementary Information), it is possible to argue that the observed RD values are not consistent with a very low penetration depth. However, the RD values for SC2-PFPFsol appear to be slightly lower than for SC2PFPFsusp. This would suggest a concentration of the product on a thinner superficial layer for SC2-PFPFsol. This conclusion is not consistent with the WIE values, which are slightly lower for SC2-PFPEsol than for SC2-PFPEsusp. In order to solve this apparent conflict, we have to conclude that the piston-like model is not enough and other phenomena should be taken into account. Namely, we have to consider that the water vapor diffusion in porous systems is


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assisted and increased by the presence of thin films of liquid water,48,49 which is favored by the presence of hydrophilic pore surfaces. In the case of SC2-PFPEsusp, where a lower penetration depth is expected, the vapor can easily condensate inside the pores and diffuse in a way similar to that of an untreated sample until the protected superficial layer is met (for sake of clarity, it is useful to remember that during the diffusion measurement the vapor enters from the untreated face). If the pores are not blocked, water can quickly evaporate as a consequence of the high vapor gradient concentration between the dry environment and the stone. On the contrary, in the case of SC2-PFPEsol, where a higher penetration depth is expected, a hydrophobic effect may occur along several pore walls inside the stone causing a reduced vapor condensation than in hydrophilic pores. Therefore, in the samples treated with SC2-PFPEsol, where the condensation is less, a reduced vapor diffusivity is expected, although pores are not blocked.


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Table 2. Water absorption inhibition efficacy, WIE, at 30 minutes and 1 hour, and residual vapor diffusivity, RD, of Lecce stone samples coated with SC2-PFPE and N215. Sample

Coating agent

APL_1 APL_15 APL_4

SC2-PFPE sol

Amount applied (g) $

APL_18 APL_8

SC2-PFPE susp

1 hour

0.041 ± 0.001

95.2 ± 0.7

93.3 ± 0.9

86

0.031 ± 0.001

95.5 ± 0.4

94.5 ± 0.6

nd

0.035 ± 0.001

90.9 ± 2.2

88.3 ± 1.9

92

93.9 ± 1.5

92.0 ± 1.9

0.040 ± 0.001

96.9 ± 0.1

96.5 ± 0.2

93

0.032 ± 0.001

94.6 ± 1.0

94.7 ± 0.9

nd

0.040 ± 0.001

97.0 ± 0.2

96.5 ± 0.3

102&

96.2 ± 0.78

95.9 ± 0.6

0.034 ± 0.001

70.7 ± 6.3

64.3 ± 4.8

90

0.028 ± 0.001

35.2 ± 19.5

29.9 ± 17.0

nd

0.026 ± 0.001

53.5 ± 4.6

48.1 ± 4.2

111&

53.1 ± 10.2

47.4 ± 9.9

8.6 ± 3.2

6.5 ± 2.2

0.0 ± 4.7

-0.8 ± 2.7

7.2 ± 3.2

-2.5 ± 1.6

Average value APL_6 APL_21 APL_14

N215

Average value APL_12 APL_16 APL_17

NT

RD (%)

30 minutes

Average value APL_2

WIE (%)

APL_26 13.5 ± 4.5 10.1 ± 4.0 WIE are expressed as average values with sample standard deviations; nd = not determined; NT = not treated sample; $ These amounts of applied products correspond to values of 10 – 15 g/m2; &

As reported in Experimental Section the sample standard deviation for RD is influenced by the

variability of the vapor diffusivity (D0) values in four untreated samples. Therefore, RD values may be found higher than 100%.


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3.3. MRI analysis. MRI analyses were performed with water absorption through both the treated and the untreated stone surface, giving rise to unambiguous results. Indeed, if the hydrophobic product would be concentrated on the treated external surface, the absorption through the treated face would produce dark images (typically black), because the hydrophobic compound would block the ingress of liquid water. On the contrary, the absorption carried out through the untreated face would determine bright images, similar to those of the not treated samples. The hydrophobic treatments performed in this work never gave rise to the extreme condition exposed above, and the different behavior and properties of SC2-PFPE and N215 were clearly demonstrated through the comparison of the MRI images acquired after water absorption through both the treated and the untreated face. Figure 2 reports the images of an internal slice of the specimens during the kinetics of water absorption through the treated surface. The images of the samples coated with SC2-PFPE show a more prolonged resistance to water uptake than that treated with N215. In fact, the images corresponding to the samples treated with SC2-PFPEsol and SC2-PFPEsusp appear dark (almost black) up to 2 hours and 4 hours of water absorption, respectively, while those of the sample treated with N215 show a slow but progressive ingress of water already after 1 hour of absorption. These images are consistent with the WIE values reported in Table 2. After longer absorption times (up to 192 hours) water can penetrate and it is distributed in the entire pore space of the sample for all the treatments, confirming the substantial absence of closed pores, in accordance with the high RD values (Table 2). Moreover, the amount of the absorbed water appears lower in the treated samples than in the untreated one, by comparing the gray levels of the images with the look up table (LUT). Quantitative data on the mass of the absorbed water over time (Figure 3) confirm the information obtained by the images: the untreated sample


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absorbs more water (about 15-20%), and faster, than the treated samples. The higher amount of water absorbed by the untreated sample cannot be due to a different intrinsic water uptake ability of this sample. Indeed, the water capillary test performed on the same samples before the treatment showed that the same untreated specimen (sample APL_16) absorbed less amount of water than the other samples (Table S2 Supporting information). The different kinetics of absorption of the samples treated with SC2-PFPEsol and SC2-PFPEsusp, as compared with N215, shown in the inset of Figure 3 are equally consistent with the WIE data in Table 2. Moreover, the comparison between SC2-PFPEsol and SC2-PFPEsusp agrees with and confirms the previous discussion of the WIE results, concerning the more homogeneous spread of the former compared with the latter. In agreement with the behavior shown in Figures 2 and 3, also the images of water absorption through the untreated face (Figure 4) show slower and less water absorption for the treated samples than the untreated one, both at short (1 hour) and long times (168 hours) of absorption. All the hydrophobic treatments give the same trend in terms of water mass (Figure 5), also for short absorption times (0-5 hours), but the images show some features, that the mass measurements cannot reveal. Indeed, the images of samples treated with N215 and SC2-PFPEsol allow to localize zones with lower water content. In the case of N215 a darker region at about 5 mm from the treated surface, well visible at long absorption times (≥ 24 hours) (Figures 2 and 4), suggests that near the treated face there is a concentration of the polymer, evidencing a poor propensity of the dissolved polymer to diffuse. In the case of SC2-PFPEsol a darker region is visible at about 5 mm from the untreated face. This region is already visible after 1 hour of water absorption if the test is performed through the untreated face (Figure 4), and after longer times (4 hours), when it is carried out through the treated face (Figure 2). This effect is observed in the


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entire sample, not only in the section shown in Figures 2 and 4. Figure 6 is a three-dimensional view of this specimen in false colors, reconstructed from all the adjacent slices, after 4 hours of water absorption through the untreated face. This non-homogeneous distribution of the product is justified by a high penetration depth of SC2-PFPEsol, in agreement with the discussion of the results shown in Table 2. Indeed, the images show an accumulation of the product at the face opposite to the treated one due to the limited thickness of the specimen (2 cm). The accumulated compound slightly reduces the water ingress in this sample from the untreated face, consistently with the lower value of water absorbed during the first hours, shown in the inset of Figure 5. The kinetics of water ingress shown in Figures 2 to 5 can be well interpreted by a different distribution of the hydrophobic product in different classes of pore sizes. As known, the velocity of water ingress is governed by pore sizes determining lowest velocities and highest heights reached for water in the smallest pores. On the basis of the molecular dimensions of the products, it is expected that N215 preferentially enters the largest pores, while SC2-PFPE the medium and large ones. Therefore, the velocities for water ingress into N215 and SC2-PFPEcoated samples should be different, through both the treated and the untreated face. Indeed, when the absorption is performed through the treated face, the velocity of the capillary rise is higher in the case of the N215 treatment than in the SC2-PFPE one (inset of Figure 3). Therefore, the water absorption occurs through the small and medium pores for N215, and only through the smallest pores for SC2PFPE. When the absorption takes place through the untreated face (inset of Figure 5), the velocity of the rising process is similar for all the coated samples and consistent with the untreated sample. However, it is worthy to observe that the SC2-PFPEsol coating gives rise to a slightly lower velocity than N215 and SC2-PFPEsusp, as a result of the ingress of water through


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small-medium sized pores. Therefore, the results shown in Figures 2 - 6 suggest that SC2PFPEsol, located at about 5 mm from the untreated face, is mainly distributed and effective only in medium-large pore sizes. The high hydrophobicity of SC2-PFPE, as suggested by the low wettability of the treated surfaces, is responsible for lowering the concentration of liquid water in the region of the hydrophobic agent accumulation, as well as for reducing the vapor condensation. This last consideration, if applied to the diffusivity test (Table 2), justifies the lower trend of RD observed for SC2-PFPEsol treatment in respect to the other two treatments. The high RD values observed for all the treatments are in agreement with the images of Figure 2, showing that all the treatments determine neither an accumulation of the products at the external treated surface, nor a blockage of the pores.

Figure 2. MRI images of an internal section of the Lecce stone samples, over time (h) of water absorption from the treated face. NT is the untreated sample (APL_16). N215 is the sample treated with the fluoroelastomer (APL_6), SC2-PFPEsol and SC2-PFPEsusp are the samples treated with SC2-PFPE in solution (APL_1) and in suspension (APL_2), respectively. The LUT represents the signal intensity in arbitrary units. Axial sections size (5x2 cm2) on each sample.


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Figure 3. Absorbed water mass relative to the dry mass (%) entered from the treated face of the samples of Figure 2 as a function of time.

Figure 4. MRI images of an internal section of the same Lecce stone samples in Figure 2, over time of water absorption from the untreated face. Axial sections size (5x2 cm2) on each sample.


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Figure 5. Absorbed water mass relative to the dry mass (%) entered from the untreated face of the samples of Figure 2 as a function of time.

Figure 6. Three-dimensional reconstructed MRI image of the Lecce stone sample treated with SC2-PFPEsol after 4 hours of water absorption through the untreated face. Sample size 5x5x2 cm3. (FIGURE 7 MOVED TO SUPPLEMENTARY)

Figure S7 of Supporting Information confirms that the different hydrophobic effect given by the three coatings shown in Figs. 2 and 4 is common to the entire sample, not only to the internal slices represented.


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3.4. Adhesion stability. The effectiveness of the dipolar interaction to give a stable adhesion between the polar groups (-CO-NH-) of the fluorinated oligosuccinamide and the rock was evaluated by subjecting coated and uncoated stone specimens to artificial ageing by repeated wet-dry cycles. During a prolonged contact with water, competitive hydrogen bonds or dipolar interaction water/coating and rock/coating can be formed causing a detachment of the polymeric material from the external and internal surfaces of the stone with a loss of the hydrophobic properties of the coated surfaces. Likewise, the repeated wet-dry cycles, occurred during the capillary water absorption test, can cause processes of dissolution, migration and recrystallization of more or less soluble stone components acting as degradation factors. If the hydrophobic chains have a correct position, the non-covalent bonds between the polymeric material and the rock will be protected, so a good and persistent adhesion is guaranteed, and the natural degradation processes of the stone are expected to be reduced in the coated stone as compared with the uncoated one. Figure 7 shows the WIE values at 30 minutes of water absorption for treated and untreated stone samples subjected to different conditions of wet-dry cycles, evaluated after 3 years from the beginning of the experiment (dark color), compared with the corresponding values before any wet-dry cycle (light color) (the values are reported in Table S3, Supplementary Information). Two different ageing treatments, consisting of repeated wet-dry cycles as detailed in the caption of Figure 7, were used: weak (WA) and strong (SA) ageing. Fig. S8 shows the comparison between 30 minutes and 1 hour of water absorption. For the untreated samples, a drastic increase of water uptake with negative values of WIE, is observed when subjected to protracted contact with water (SA). In the case of WA an increase of water uptake is also observed, but it is less evident than in the previous case and it is mainly limited to short times (Figure S8a).


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Figure 7. Water Inhibition Efficacy (WIE) of SC2-PFPE and N215 after different ageing conditions and at different time of water capillary absorption (wca) at 30 minutes. Dark colors for WIE measured just after treatment and light colors for WIE measured at 3 years after treatment. WA = Weak Ageing corresponding to 3 wet-dry cycles with wca up to 1 hour for each cycle. SA = Strong Ageing corresponding to 1 WA plus 2 wet-dry cycles with wca up to 7 days for MRI analysis followed by 2 wet-dry cycles with water saturation under vacuum and 4 wetdry cycles with wca up to 1 hour. After vacuum saturation the specimens were left wet for 5 days before drying them. After the last wet-dry cycle, all the samples were conditioned in the dark under laboratory conditions for about 2 years.

The treated samples generally show a lower ageing effect than the untreated samples. The ageing effect, visible by the reduction of the WIE values, for the treated samples subjected only to WA is very low, both at 30 minutes and 1 hour. The higher ageing effect for N215, in respect to SC2-PFPE, is mainly justified by its lower hydrophobicity, which causes a higher water uptake and lower stability of the coating. When a SA is carried out, the ageing effect is more evident for all the treatments. However, for SC2-PFPEsol, that was demonstrated to have a better penetration and distribution than both SC2-PFPEsusp and N215, the degradation process, caused


26

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by protracted contact with water (SA), is less effective than in the other two treatments, where the hydrophobic product acts mainly at the external surface reducing the protection of the stone in depth. In any case, the WIE of SC2-PFPE after ageing remained relatively high (>45%), indicating a stable adhesion of the product on the surface. The better performance of SC2PFPEsol in respect to SC2-PFPEsusp is in agreement with the higher availability of the polar amidic groups to give dipolar interactions in dissolved molecules than in molecules in suspension.

4. CONCLUSIONS The new oligo(ethylenesuccinamide) containing low molecular pendant perfluoropolyether segments, easily synthesized by two-step poly-condensation reactions, shows suitable characteristics to be used as non-wetting coating, mainly for highly porous and polar substrates where a drastic wettability change is obtained with very low amount of applied product. Its solubility in hot alcohols or hydro-alcoholic solvents makes possible its application on many surfaces without any environmental negative impact. The permanence of a partial solubility of SC2-PFPE when cooled down to room temperature also allows the application on stone materials for uses in field work, as needed in the case of materials of interest for Cultural Heritage. On the other hand, its insolubility, or very low solubility in many common solvents at room temperature, can be considered a great advantage when the coated material must have solvent resistance. The presence of low molecular pendant perfluoropolyether segments greatly improves the water repellence in respect to other fluorinated compounds (e.g. fluoroelastomers) demonstrating


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that the only presence of fluorine in the molecule structure is not enough to give high hydrophobicity. The high hydrophobic properties are due to the orientation of the fluorinated segments at the air/stone interface, as a consequence of their strong segregation due to low surface free energy.31, 32, 50, 51 On the other hand, the amphipathic character of this molecule makes possible the dipolar interactions of the not fluorinated amidic groups of the oligo(ethylenesuccinamide) with the polar substrate, thus giving a good permanence of the product on the surface also after ageing (wet-dry cycles). At the same time, the presence of dipolar interactions instead of covalent bonds, can provide the reversibility of the coating treatment when suitable solvents are used in hot conditions. This property is of relevant importance mainly in the field of Cultural Heritage to ensure the reversibility of the treatment. Moreover, the low molecular weight helps the diffusion inside the porous substrate, reducing the possibility of the product deposition in a thin thickness with probable pore blockage. As a consequence, the vapor diffusivity is only slightly reduced in respect to the uncoated material due to the lower vapor condensation inside the pore structure. This study demonstrates that the combination of MRI and of standard methods is able to show the distribution of the hydrophobic products in certain classes of pores, which helps to explain the small changes in liquid water and vapor diffusivity due to the coating.


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ASSOCIATED CONTENT Supporting Information. This material is available free of charge on the ACS Publication website at http://pubs.acs.org. Materials characterization Computation of the pore space volume reduction Figure S1. FT-IR spectrum of SC2. Figure S2. 1H-NMR spectrum of SC2 in D2O. Figure S3. 13C-NMR spectrum of SC2 in D2O. Figure S4. FT-IR spectrum of SC2-PFPE. Figure S5. 1H-NMR spectrum of SC2-PFPE in CD3OD/D2O (95/5 w/w). Figure S6. 19F-NMR spectrum of SC2-PFPE in CD3OD/D2O (95/5 w/w). Figure S7. MRI images of the internal slices with corresponding signal profiles (obtained on the entire sample) of the same Lecce stone samples reported in Figures 2 and 4, after 4 hours of water capillary absorption from the untreated (left) and the treated (right) face. Table S1. Calculated reduction of the volume of the pore space for different stone thicknesses due to the presence of the hydrophobic product. Table S2 - Water mass per dry mass (%) absorbed by the samples of Figures. 2 and 4 before any coating. Table S3. – Ageing Effect on Lecce stone samples coated with SC2-PFPE and N215 due to different artificial ageing stresses.

AUTHOR INFORMATION Corresponding Author *Address: Department of Physics and Astronomy, University of Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy. E-mail: [email protected]. Phone +39 (051) 2095119. Fax +39 (051) 2095047.


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Funding by Centro Enrico Fermi – Rome (Italy) for the project “Magnetic Resonance techniques for Cultural Heritage”, and by Regione Toscana within the framework of the POR-FESR 20072013 program (TeCon@BC project, cod. 57476) are acknowledged. REFERENCES (1)

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Table of Contents Graphic

Synopsis A fluorinated oligo(ethylenesuccinamide) has been developed for giving high hydrophobic properties to building stones and other porous surfaces, without changing their water vapor diffusivity. The new product, soluble in benign solvents for operators and environment, provides non-covalent bonds with polar surfaces giving durable and reversible treatments. The low molecular weight allows a good penetration inside the porous substrate reducing the condensation of vapor in the porous structure.


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An Environmental Friendly Fluorinated Oligoamide for Producing

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