J. Phys. Chem. B 2006, 110, 23719-23728
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In Situ and Frontal Polymerization for the Consolidation of Porous Stones: A Unilateral NMR and Magnetic Resonance Imaging Study Noemi Proietti,† Donatella Capitani,*,† Sara Cozzolino,‡ Massimiliano Valentini,§ Enrico Pedemonte,# Elisabetta Princi,# Silvia Vicini,# and Anna Laura Segre† Institute of Chemical Methodologies, CNR, Research Area of Rome, 00016 Monterotondo Stazione, Rome, Italy, SMAArt Center and Department of Chemistry, UniVersity of Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy, Agricultural Research Council, Experimental Institute for Plant Nutrition, Instrumental Center of Tor Mancina, Via della NeVe Km 1, 00016 Monterotondo, Rome, Italy, and Department of Chemistry and Industrial Chemistry, UniVersity of Genoa, Via Dodecaneso 31, 16146, Genoa, Italy ReceiVed: May 25, 2006; In Final Form: September 25, 2006
Consolidation treatment of porous materials was performed by in situ and frontal polymerization of acrylic monomers inside a porous stone. To study the penetration of the polymer inside the stone and its consolidating effects we used water as a contrast agent, detecting its penetration using unilateral NMR and magnetic resonance imaging. All data obtained on differently treated stones were compared with corresponding ones obtained analyzing both untreated stones and stones simply painted with a well-known polymeric protective agent. In situ polymerization of acrylic monomers inside porous stones has been demonstrated to be an extremely powerful consolidating method, whereas thermally initiated frontal polymerization seems less efficient. In both cases the optimal choice of monomers is still open and requires further study. Our data indicate that unilateral NMR represents an inexpensive and simple technique for the non-invasive observation of the water uptake and of the effect of consolidation procedures in porous materials.
Introduction Water infiltration is one of the main causes of damage and degradation of porous materials such as stones and bricks. In a porous structure water often acts as a transporting agent for aggressive pollutants that cause corrosion. In addition, freezing and thawing cycles of water may cause fractures inside the porous structure. These days, the use of consolidating products, either organic or inorganic, is usually accepted to reduce the water infiltration.1-3 Moreover, suitable organic polymers are preferable because of their protective action. Some properties of polymeric materials, such as their stability on aging4,5 and the possible effect of their degradation products on the treated stones, are not yet fully understood. Moreover, some specific properties, such as the adhesion of the polymeric film to the porous material, must be improved: in many cases a slight stress is sufficient enough to cause the detachment of the polymeric film from the porous material. It is also crucial that the polymeric film does not entrap the water inside the porous structure. In fact, in the presence of water, thermal gradients may cause pressure gradients whose cycles can disaggregate the structure of the material. The large size of macromolecules is the main cause of the incompatibility between the polymer and the porous structure of a stone,6,7 since it hinders its penetration into the smallest pores (i.e., micropores, L < 20 Å) and confines the polymer to the stone surface. To achieve a deeper penetration of the polymeric consolidating agents, we have carefully studied two consolidation †
Institute of Chemical Methodologies, CNR. University of Perugia. § Experimental Institute for Plant Nutrition. # University of Genoa. ‡
procedures using polymeric materials introduced into the stones by in situ and frontal polymerization, respectively.7-14 In both methods the monomer is introduced inside the stone by capillarity, and thereafter it is in situ or frontally polymerized. Due to the small size of the monomer molecule, it is possible to foresee that a deeper penetration into the stone will occur. Note that the use of water repellent acrylic monomers also warrants a good protective effect.15 Both frontal and in situ polymerization procedures may affect the structure of any porous material. Therefore, these procedures should be applied only when serious damage is already evident. It should be noted that the possible effect of the polymer on the stability of the porous structure is still not fully understood. This work is aimed at investigating and characterizing the penetration depth of acrylic systems into a porous stone, named “Pale Finale”,16 widely used in churches and buildings in northern Italy. In this Article, the consolidating and protective effects of acrylic polymers introduced using in situ and frontal polymerization are compared to those obtained using a polymeric commercial product, Paraloid B72,5,15 a protective agent widely used for the protection of porous materials. To evaluate the performance of all these methods we used unilateral NMR, an innovative nondestructive NMR technique able to characterize not only the water content in porous materials but also its NMR relaxation values, strictly connected to the pore size distribution.17-21 In this way, the performance of each treatment is indirectly investigated. The main advantage of the unilateral NMR technique is that it is not only portable but can also be performed directly on large objects such as monuments, churches, and in general any building. Many traditional analytical techniques, such as porosimetry or gravimetric methods and also NMR, both imaging and
10.1021/jp063219u CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2006
23720 J. Phys. Chem. B, Vol. 110, No. 47, 2006 relaxometry, allow the amount of water adsorbed in porous materials to be evaluated. All these methods require sampling, whereas with the unilateral NMR instrument sampling is eliminated. Unilateral NMR has been previously used as a diagnostic tool for studying early degradation in cellulose-based materials22,23 and detachment of the pictorial film from the plaster in frescos.24 In this Article the results obtained with unilateral NMR are compared with the corresponding ones obtained by magnetic resonance imaging (MRI), a powerful technique that is an excellent tool for visualizing the presence of mobile species in any material.25 Besides, to have a better insight into the mobility of the water within the pores and to evaluate the effect of diffusion, relaxation times measured by unilateral NMR have been compared to the corresponding ones obtained in a homogeneous magnetic field. Experimental Section Materials. Acrylic monomers (butyl methacrylate, ethyl acrylate, and 1,6-hexanediole diacrylate) were purchased from Aldrich. The polymerization initiator (AIBN, 2,2′-azobisisobutyronitrile) was supplied by Fluka; Paraloid B72 was supplied by Rohm and Haas. Deionized water was used throughout the work. The stone used in this study was chosen according to two basic requirements: heterogeneous porosity, to reproduce a material degraded by physical-chemical agents; easy to find and cut in any shape. A calcareous sedimentary stone of biological origin easily found in Liguria, Italy, called “Pale Finale” stone,16 was chosen because it fulfills the above-mentioned requirements This stone is found in four different formations, differing in physical characteristics, composition, as well as in impurities and fragments content. The variety “Pale” of the Finale stone, with a porosity of about 40%, was chosen; the specimen size was 5 × 5 × 2 cm3. In Situ and Frontal Polymerization. Two different systems were polymerized in situ: a mixture of butyl methacrylate (BMA)/ethyl acrylate (EA) 75/25 wt % and a pure monomer 1,6-hexanediole diacrylate (HDDA). Frontal polymerization was carried out only with HDDA. In situ polymerization was performed in acetone solution (20% v/v). The experimental procedure consists of three steps. (i) Absorption: the polymerizing system (monomer or monomers mixture, initiator, and solvent) is absorbed from the specimen by capillarity. This absorption was achieved by placing the sample on a thick layer of cotton soaked in the reaction solution. The operation was carried out in 4 h at 4 °C, in the absence of light. (ii) Polymerization: performed for 24 h at 50 °C because at this temperature AIBN allows a good conversion suitable for our application to be reached.26 (iii) Purification: traces of solvent and unreacted monomers, still present after the polymerization, were removed over several days by air evaporation. Frontal polymerization consists of two steps.(i) Absorption: the absorption was performed as described above. (ii) Polymerization: this was carried out by heating only one side of the sample, placing a side of the stone on a hot plate (T = 200 °C). To avoid spontaneous polymerization, after the formation of an ascending polymerization front the sample was immediately removed from the hot plate.9 After each consolidating treatment, the amount of polymer inside the stone (∆M %), was calculated according to eq 1, and its value is reported in Table 1:
Proietti et al. TABLE 1: Protective Efficiency (EP) of Consolidating Treatments and Amount of Polymer in the Stone (∆M %) Pale Finale stone
EP (%)
∆M (%)
treated with Paraloid B72 polymerized in situ with BMA/EA polymerized in situ with HDDA frontally polymerized with HDDA
70 99 94 45
0.27 5.5 6.0 8.0
∆M % ) [(Pf - Pi)/Pi] × 100
(1)
where Pi and Pf are the weights of the specimens before and after the treatment, respectively. To compare the performance of in situ and frontal polymerization with that obtainable from the traditional technique of stone restoration, a 3 wt %/vol solution of Paraloid B72 in acetone was painted six times on each side of the stone. Methods. Three different techniques were used to study the effect of the consolidating treatment on the Pale Finale stone, namely, capillary water absorption, unilateral NMR, and MRI. All measurements were carried out on five dry and wet specimens of Pale Finale stone in accordance with the Normal Protocol:27 untreated sample (blank); sample polymerized in situ with (BMA)/(EA); sample polymerized in situ with HDDA; sample frontally polymerized with HDDA; sample treated with Paraloid B72. Capillary Water Absorption. The determination of capillary water absorption was carried out using the gravimetric absorption technique, in accordance with the Normal Protocol.27 The stone specimen was placed on a 1 cm thick filter paper pad partially soaked in deionized water, with the treated surface in contact with the pad. The amount of water absorbed by capillary rise was determined by weighing the specimen after 10, 20, 30, 40, 50 min, 1, 2, 4, 6, 24, 48, 72, and 96 h to obtain the wet specimen mass Mi (∆M ( 0.0001 g). The Normal Protocol is a European Union validated protocol27 used to evaluate the protective and consolidating treatments in stone conservation. The water capillary absorption test was used to study the kinetics of the water absorption. The amount of water absorbed by the stone was calculated using this simple gravimetric test, and consequently, the degree of protection of different procedures was evaluated. The amount of absorbed water Qi, at the time ti per surface unit, is defined as follows:
Qi ) (Mi - M0)/S
(2)
where M0 is the dry specimen mass (g), Mi is the specimen mass (g) at the time ti (s), and S is the contact surface (cm2). The Qi values (g/cm2) are plotted against the square root of the time (s1/2), to give the capillarity absorption curve. The result can also be expressed as the protective efficiency EP%:
EP% ) [(Q0 - Qt)/Q0] × 100
(3)
where Q0 and Qt are the average values of the amount of water absorbed after 1 h by the untreated and the treated stone, respectively.27 Unilateral NMR. All measurements were performed on both untreated and treated, dry, partly wet, and saturated Pale Finale stones, using a commercial unilateral NMR “ProFiler” from Bruker Biospin Italy. Unilateral NMR allows the NMR signal to be detected at different depths within the stone. Three pretuned probeheads were used. The 1 mm probehead, operating at 18.153 MHz, allows the measurement to be performed on a 1 mm thick slice
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Figure 1. Capillary water absorption curves of untreated Pale Finale stone (b), Pale Finale stone treated with Paraloid B72 (0), frontally polymerized with HDDA([), polymerized in situ with HDDA (]), and polymerized in situ with BMA/EA (2).
of sample; hence, the measurement is performed to 1 mm in depth from the surface. The 3 mm probehead, operating at 17.3 MHz, allows the measurement to be performed on a 2.5-3.5 mm slice of sample. The 5 mm probehead, operating at 16.3 MHz, allows the measurement to be performed on a 4.5-5.5 mm slice of sample. The maximum echo signal corresponding to a π/2 pulse was obtained with a pulse width of 3 µs in the case of the 1 mm probehead, 6 µs in the case of the 3 mm probehead, and 12 µs in the case of the 5 mm probehead. The dead time was 15 µs for the 1 and 3 mm probeheads and 17 µs for the 5 mm probehead. Spin-spin relaxation times, T2, were measured using the CPMG sequence28a,b according to a previously published procedure.22,29 The echo time, 2τ, was 100 µs; 2048 echoes were collected. The number of scans used for obtaining the CPMG decays was optimized to obtain a signal/noise higher than 100 in all cases. Single Hahn echo28c measurements were performed with an echo time of 20 µs. Since in an inhomogeneous field T2 values can be heavily affected by diffusion,30,31 to evaluate the effect of the diffusion, CPMG measurements were also performed at different echo times, namely 2τ ) 100, 200, 300, 400, 500, 600, and 800 µs. Spin-lattice relaxation times, T1,28d were measured with the aperiodic saturation recovery sequence according to a previously published procedure.32 Because in all cases T1 values were found to be shorter than 350 ms, a recycle delay of 2 s was always used. To check the value of the relaxation times measured with unilateral NMR, some measurements were also performed with a standard NMR relaxometer. 1H NMR Relaxometry. 1H low-resolution NMR measurements were carried out at 20 MHz on a commercial spectrometer Spinmaster 2000 (SM 2000) Stelar, Mede (Pavia, Italy). Stones were carefully cut to fit into the standard 10 mm NMR tube; the height of samples was kept well within the NMR coil (1 cm). The 90° pulse width was 3.5 µs, and the dead time of the instrument was 8 µs. Spin-lattice relaxation times, T1, were measured with the aperiodic saturation recovery sequence.28d Spin-spin relaxation times, T2, were measured with the CPMG sequence,28a,b The echo time, 2τ, was 100 µs; 8000 echoes were collected. Analysis of NMR Relaxometric Data. Saturation recovery data were calculated as previously described.32 The echo decays obtained applying the CPMG sequence were treated as multi-
Figure 2. (a) Capillary water absorption of a Pale Finale stone treated with Paraloid B72. (b) Hahn echo obtained on a Pale Finale stone treated with Paraloid B72 at the beginning of the water absorption process, t ) 0 (solid line), after 10 min (dash-dot line), after 1 h (dotted line), after 2 h (dash-dot-dot line), and after 4 h (medium dash line). (c) Amount of absorbed water vs the intensity of the 1H NMR signal, in the case of the Pale Finale stone treated with Paraloid B72.
exponential decays:
Y ) C0 +
[] -t
∑i Wi exp T
2i
i ) number of components (4)
where C0 is the offset value, Wi is the spin density of the ith component, and T2i is the spin-spin relaxation time of the ith component. Distribution of Relaxation Times. The distribution of relaxation times was obtained by numerical inversion of the Fredholm integral using a software within the Matlab (The MathWorks) framework.17,33 In the resulting distribution, the abscissa provides the relaxation time value and the integral corresponds to the normalized spin density.
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Figure 3. (a) Hahn echoes obtained with the 1 mm probehead in the case of dry Pale Finale stones; only the signal due to the polymer and to the residual moisture is detected. (b) Hahn echoes obtained with the 1 mm probehead on wet Pale Finale stones; the signal is due both to the polymer and to the absorbed water. (c) Residual Hahn echoes obtained from the difference between Hahn echoes reported in (b) and the corresponding ones reported in (a); the signal is due only to the absorbed water. (d) Residual Hahn echoes obtained from the difference between Hahn echoes performed on wet stones and on dry stones with the 3 mm probehead; the signal is due only to the absorbed water. (e) Residual Hahn echoes obtained from the difference between Hahn echoes performed on wet stones and on dry stones with the 5 mm probehead; the signal is due only to the absorbed water.
The algorithm used to obtain the inversion of the experimental data was previously tested on simulated and experimental data: 17 no artifacts were observed in the obtained distribution for signal/noise higher than 10. MRI. MRI measurements were performed on a Bruker AVANCE 300 MHz spectrometer, equipped with a cylindrical birdcage single-tuned nucleus (1H) coil probehead with an inner diameter of 60.0 mm. Samples were left to equilibrate in the MRI magnet for 30 min before measuring. The image was reconstructed using the water signal. 3D gradient echo fast imaging (3D-GEFI),
multislice multiecho (MSME), 2D single-point imaging (2DSPI) experiments, ge3D, m_msme_ortho and m_spi_2D, respectively (Bruker library), were performed according to standard procedures.34,35 In 3D-GEFI measurements, which generate echoes only by gradient pulses and give as output a 3D reconstruction of the sample, the field of view was 70.0 × 70.0 × 70.0 mm3, the matrix size was 128 × 128 × 128 data points, the spectral width was 100 kHz, the echo and repetition times were equal to 1.472 and 60.0 ms, respectively, the number of scans was four, and the excitation pulse shape was a sinc3. Data were processed to
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TABLE 2: Signal/Noise Values Obtained with Unilateral NMR Using the 1, 3, and 5 mm Probeheads Pale Finale stone
S/N
1 mm probehead untreated treated with Paraloid B72 frontally polymerized with HDDA polymerized in situ with HDDA polymerized in situ with BMA/EA
13 11 2.8 2.0 0.2
3 mm probehead untreated treated with Paraloid B72 frontally polymerized with HDDA polymerized in situ with HDDA polymerized in situ with BMA/EA
12 12 3.7 0.62 0.62
5 mm probehead untreated treated with Paraloid B72® frontally polymerized with HDDA polymerized in situ with HDDA polymerized in situ with BMA/EA
2.5 3.0 0.24 0.21 0.01
obtain images with a size of 128 × 128 × 128 data points and a field of view of 70.0 × 70.0 × 70.0 mm3; the processing mode was FT_MODE, the latter was complex_FFT, and spikes were eliminated. In MSME experiments, which produce echoes via a spinecho-based sequence, the field of view was 70.0 × 70.0 mm2, the matrix size was 128 × 128 data points, the spectral width was 100 kHz, the echo and repetition times were 5.519 and 3000.0 ms, respectively, the number of scans was eight, and the excitation pulse shape was a sinc3. Data were processed to obtain images with a matrix size of 128 × 128 data points and a field of view of 70.0 × 70.0 mm2; the processing mode was FT_MODE, the latter was complex_FFT, and spikes were eliminated. In 2D-SPI measurements, which allow the detection of signals with T2 of the order of microseconds and use phase gradients acquiring only the first point, two different sets of parameters were used, the first one to detect the signal when T2 is faster than 100 µs and the second one when T2 is faster than 500 µs. In the first case the field of view was 70.0 × 70.0 mm2, the matrix size was 64 × 64 data points, the filter width was 1250 kHz, the number of scans was four, the repetition time was 100 ms, and the dephase time, i.e., the delay between the rf pulse and the acquired point, was 92 µs. Data were processed to obtain images with a matrix size of 64 × 64 data points and a field of view of 70.0 × 70.0 mm2; the processing mode was FT_MODE, the latter was complex_FFT, and spikes were eliminated. In the second case of the 2D-SPI the field of view was 70.0 × 70.0 mm2, the matrix size was 128 × 128 data points, the filter width 1250 kHz, the number of scans was four, the repetition time was 100 ms, and the dephase time was 183 µs. Data were processed to obtain images with a size of 128 × 128 data points and a field of view of 70.0 × 70.0 mm2; the processing mode was FT_MODE, the latter was complex_FFT, and spikes were eliminated. Results and Discussion In Figure 1 the water capillary absorption curves obtained regarding untreated and treated Pale Finale stones are shown: the amount of adsorbed water per surface unit is reported as a function of the square root of the hydration time. Note that the untreated Pale Finale stone quickly absorbs a great amount of water, whereas in all treated stones, at short hydration times, absorption is greatly reduced.
Figure 4. (a) T2 decays measured with the 1 mm probehead using the CPMG sequence. (b) T2 decays measured with the 3 mm probehead using the CPMG sequence. Untreated Pale Finale stone (b), stone treated with Paraloid B72 (0), stone frontally polymerized with HDDA ([), stone polymerized in situ with HDDA (]) and stone polymerized in situ with BMA/EA (2).
The protective efficiency values (EP %), calculated according to eq 3, are reported in Table 1. The most efficient protective treatment is the in situ polymerization of BMA/EA (EP ≈ 99%), whereas using the HDDA monomer a slight decrease in the protective effect is observed (EP ≈ 94%). In the case of frontal polymerization the protective effect further decreases, down to EP ≈ 45%. The stone treated with Paraloid B72 shows a peculiar behavior. At short hydration times (1 h or less), the EP value is ≈ 70%, which means that the protective action is rather good (see Figure 1). However, at longer hydration times, the Paraloid B72 treatment no longer shows a protective effect. In fact, after 4 h, the amount of absorbed water is about the same as the amount found in the case of untreated Pale Finale stone. Consequently, the simple treatment with Paraloid B72 is efficient only at short hydration times because it affects the kinetics of the water absorption only in the initial stage. Monitoring the amount of water in heterogeneous porous material is a critical and difficult task. The problem of evaluating the water content is even more critical when measurements must be performed in situ without damaging the object under study. Bearing in mind these facts excludes most analytical techniques; therefore, the best choice is unilateral NMR. In addition, unilateral NMR also gives information both on porosity and on
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TABLE 3: T2 Values, Measured with Unilateral NMR, Obtained Fitting the Experimental Data with a Multiexponential Decay Pale Finale stone
Wa (%)
T2a (ms)
untreated treated with Paraloid B72 polymerized in situ with BMA/EA polymerized in situ with HDDA frontally polymerized with HDDA
10 ( 2 28 ( 2 61 ( 5 51 ( 4 40 ( 4
1 mm probehead 0.440 ( 0.050 0.290 ( 0.050 0.96 ( 0.050 0.255 ( 0.020 0.586 ( 0.030
untreated treated with Paraloid B72 polymerized in situ with BMA/EA polymerized in situ with HDDA frontally polymerized with HDDA
35 ( 3 62 ( 5 83 ( 6 60 ( 5 71 ( 6
3 mm probehead 0.231 ( 0.050 0.127 ( 0.040 0.328 ( 0.050 0.305 ( 0.020 0.328 ( 0.020
the water distribution within the solid matrix, which is not the case when a gravimetric method or electric measurements are used. The kinetic of the water uptake by capillary absorption was monitored using unilateral NMR performing Hahn echo measurements. In Figure 2a the capillary water absorption curve of the Pale Finale stone treated with Paraloid B72 is shown. We chose to perform Hahn echo measurements at the beginning of the water absorption, t ) 0, after 10 min, t ) 10′, after 1 h, t ) 1 h, after 2 h, t ) 2 h, and after 4 h, t ) 4 h. These points have been evidenced with circles (see Figure 2a). The corresponding Hahn echoes are shown in Figure 2b. It is worth noting that the amount of the absorbed water matches well the intensity of the Hahn echo. A linear relationship exists between the amount of absorbed water and the intensity of the corresponding Hahn echo which is directly proportional to the number of water molecules within the sensible volume of the probehead (see Figure 2c). It is therefore obvious that the unilateral NMR method is highly suitable for monitoring the amount of absorbed water in a porous material. Single Hahn echo measurements performed with the 1 mm probehead on treated and untreated Pale Finale stones before and after the water uptake are shown in Figure 3, parts a and b, respectively. The same number of scans and the same experimental conditions were always used. The untreated dry Pale Finale stone shows a very weak signal, i.e., an intensity of about 13%; the stone treated with Paraloid B72 also shows a rather weak signal, about 20%. The intensity of the echo definitely increases in all dry stones frontally or in situ polymerized. Since the stones are dry, the observed signal is due to the polymer used in the treatment. In the stone frontally polymerized with HDDA the intensity of the signal is 66%, whereas in the stones polymerized in situ with HDDA and with BMA/EA the intensities are 67% and 44%, respectively. After the water uptake (see Figure 3b) both the untreated stone and the stone treated with Paraloid B72 show a very intense signal, mostly due to the absorbed water, with an intensity of 89% and 88%, respectively. In the case of the stone frontally polymerized with HDDA the intensity of the signal is 76%, while in the stones polymerized in situ with HDDA and with BMA/EA the intensities are 80% and 47%, respectively. In both cases, the proton signal is due to the sum of the water signal plus the signal of the polymer. As a consequence, to evaluate the efficiency of the hydrophobic treatment, the intensity of the echo measured in the dry stones was subtracted from the intensity of the echo measured in the corresponding wet stones (data shown in Figure 3c). After subtraction, the intensity of the Hahn echoes reported in Figure 3c is only due to the water signal. Both the untreated stone and the stone treated with Paraloid B72 show a very high signal; hence, in this case, the treatment with Paraloid B72 does not impair the water uptake.
Wb (%)
T2b (ms)
Wc (%)
T2c (ms)
58 ( 5 38 ( 3 39 ( 4 22 ( 2 22 ( 2
11.4 ( 0.5 15.0 ( 0.5 15.0 ( 0.5 4.0 ( 0.1 7.9 ( 0.2
32 ( 3 34 ( 3
54 ( 4 60 ( 4
27 ( 3 38 ( 4
26 ( 2 33 ( 2
14 ( 2 13 ( 2
4.7 ( 0.5 6.9 ( 0.4
20 ( 2 13 ( 2
4.06 ( 0.2 2.5 ( 0.2
51 ( 5 25 ( 2 17 ( 2 20 ( 2 16 ( 2
16 ( 1 21 ( 1 14 ( 1 21 ( 1 29 ( 2
All these results are in accordance with the water capillary absorption data previously reported. The intensity of the echo is strongly reduced in stones treated with frontal and in situ polymerization. Among these stones, the weakest signal is found after in situ polymerization with BMA/EA. Again, these results are in accordance with the water capillary absorption curves. As a consequence, the most efficient hydrophobic treatment is the in situ polymerization of the BMA/ EA system. The Hahn echo measurements were also performed using 3 mm and 5 mm probeheads (see Figure 3, parts d and e, respectively). In these figures we report the difference between the intensity of the Hahn echo measured on wet stones and the intensity measured in the corresponding dry stones. The intensity of the Hahn echo shown in Figure 3, parts d and e, is only due to the absorbed water. The obtained results are in accordance with the corresponding ones obtained with the 1 mm probehead. Again the treatment with Paraloid B72 does not impair the water uptake, the intensity of the signal being the same as the one measured in the untreated stone. Again, the in situ polymerization is found to be the most efficient treatment. In Table 2 the signal/noise obtained with 1, 3, and 5 mm probeheads are reported. It is worth noting that the highest signal/noise is obtained in both the cases of the untreated Pale Finale stone and the stone treated with Paraloid B72, indicating a rather larger amount of confined water. The signal/noise is remarkably reduced in the case of stones treated with frontal and in situ polymerization. Moreover, the lowest signal/noise is obtained after treating the stone with in situ polymerization of BMA/EA (see Table 2). This means that the copolymer fills up the porous structure impairing any water absorption. It is well-known that relaxation times, in particular the transverse spin-spin relaxation time T2, depend on the degree of confinement of a fluid (water) within a porous structure.21,36,37 Because the T2 decay rate depends on the surface-to-volume ratio, water in small pores relaxes rapidly, whereas water in large pores relaxes more slowly.36 The measurement of T2 of a fluid confined in a porous matrix allows the pore size distribution and the effect of the hydrophobic treatment on the capillary water absorption properties to be studied.36,38-40 Further on, the relaxation decays measured on wet untreated Pale Finale stone and wet Pale Finale stone submitted to different treatments will be compared. The treatment being the only difference among the samples, any change in the T2 decays is due to variations in water absorption properties. Since water present in large pores exhibits longer T2 relaxation times, a shorter T2 in a treated sample simply means that large pores are completely filled with the polymer. It should be noted that, using the CPMG sequence for measuring T2, only the water component is observed, whereas the rigid polymeric matrix is disregarded. To summarize, the faster the T2 decay, the more
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Figure 6. T2 distributions of a wet untreated Pale Finale stone. The distributions were obtained inverting the CPMG decays measured in a homogeneous field (dotted line) and measured with the unilateral NMR instrument (solid line), respectively.
Figure 5. T2 decays measured with the 1 mm probehead using the CPMG sequence with different echo times 2τ [2τ ) 100 (O), 200 (b), 300 (9), 400 (2), 500 ([), 600 (]), and 800 µs (\)]. (a) Untreated Pale Finale stone; (b) stone treated with Paraloid B72; (c) stone polymerized in situ with BMA/EA.
efficient is the treatment. In Figure 4a the T2 experimental decays measured with the 1 mm probehead on all treated and untreated wet stones are compared. The T2 values were obtained by fitting the experimental data to eq 4, see also Table 3. T2 values are the time constants of the decay itself. The slowest decay is observed in the case of the untreated stone and in the case of the stone treated with Paraloid B72, whereas the fastest decay is observed in the case of stones after the polymerization in situ. This observation is in accordance with the intensity of Hahn echo which corresponds to the amount of water. A slow decay
corresponds to water confined in large pores. On the contrary, a fast T2 decay corresponds to water confined in small pores, such as that found in stones enforced with in situ polymerization. The stone treated by frontal polymerization shows an intermediate behavior. All NMR relaxometric data are in accordance with the capillary absorption curves, confirming the poor efficiency of the hydrophobic treatment with Paraloid B72, the excellent efficiency of the in situ polymerization, and the good efficiency of the frontal polymerization. A similar T2 trend was found when the same measurements were performed using the 3 mm probehead, see Figure 4b. When parts a and b of Figure 4 are compared, a net shortening of the decay is observed in the case of the stones treated with frontal and in situ polymerization, whereas in the untreated stone and in the stone treated with Paraloid B72 the decay shortening is not so evident. It is evident from this observation that, after frontal and in situ polymerization, large pores are filled with the polymer and the amount of adsorbed water in the deeper layers, 2.5 ÷ 3.5 mm, is much less than that absorbed in the first few layers of the stone. Therefore, the use of unilateral NMR, coupled with 1, 3, and 5 mm probeheads, allows the extent of the penetration of the water inside the material and its distribution to be evaluated. The magnetic field produced with the unilateral NMR devices is rather inhomogeneous. Therefore, the T2 spin-spin relaxation time is influenced by molecular self-diffusion.41 As a result the measured T2 value strongly depends on the interpulse duration used in the CPMG pulse sequence.42 This effect is evident in Figure 5a where the T2 decay measured as a function of the interpulse duration τ in the untreated and wet stone is shown: the shorter the τ the slower the decay. With the use of a long τ value, the echo decay and its time constant T2 are drastically shortened due to diffusion effects. To minimize this effect we performed all T2 measurements using the shortest interpulse duration possible,42 which is 50 µs. However, it must be taken into account that even using τ ) 50 µs the measured T2 value might be shorter than the “real” T2 value. Note that a net T2 shortening as a function of the interpulse duration is also observed in the wet stone treated with Paraloid B72 (see Figure 5b); this effect is greatly reduced in the wet stone treated with a frontal polymerization. The diffusion effect is completely absent in the stone treated with in situ polymerization; in this case, due to the treatment, the amount of absorbed
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Figure 8. Signal/noise (S/N) obtained with the unilateral NMR using the 1 mm (solid line), 3 mm (dotted line), and 5 mm probehead (dashed line), vs the S/N obtained performing a 2D-SPI experiment with a dephasing time of 183 µs. In all cases the S/N has been normalized with respect to its highest value. Untreated Pale Finale stone (O), stone treated with Paraloid B72 (0), frontally polymerized with HDDA (]), polymerized in situ with HDDA (\) and polymerized in situ with BMA/EA (4).
Figure 7. Column a: MRI images obtained by performing a 2D-SPI experiment with a dephasing time of 92 µs. Column b: MRI images obtained by performing a 2D-SPI experiment with a dephasing time of 183 µs. Column c: MRI images obtained by performing an MSME experiment with an echo time of 5 ms. Row 1: untreated Pale Finale stone. Rows 2, 3, 4, and 5: Pale Finale stones treated with Paraloid B72, frontally polymerized with HDDA, polymerized in situ with HDDA, and polymerized in situ with BMA/EA.
water is highly reduced by the presence of the polymer within the pores (see Figure 5c). The degree of T2 shortening due to diffusion effects may be evaluated by performing T2 measurements in a homogeneous field, see Table 4. As expected, the T2 values of wet untreated stone measured in a homogeneous field are definitely longer than those obtained using the unilateral NMR instrument.43 Due to the high inhomogeneity of the Pale Finale stone, and to ensure a better comparison with relaxometric bulk methods,29 T2 measurements were performed on all six sides of the specimen that had been previously wetted according to the Normal Protocol, using the unilateral NMR relaxometer. The CPMG decay, obtained on each side, was inverted to obtain the corresponding T2 distributions.17 When the arithmetic average of these distributions is calculated, an average T2 distribution of all six sides was obtained. This procedure allows the data obtained by unilateral NMR and those obtained in a homogeneous field to be compared. In fact, in a homogeneous field the recorded proton signal is obtained from the whole
specimen, whereas with the unilateral NMR relaxometer the recorded proton signal is obtained from just one side of the specimen. In Figure 6 the average distribution obtained in both cases are compared. As expected, due to the diffusion effect, the distribution obtained with the unilateral NMR is shifted toward shorter T2 values as compared to that obtained in a homogeneous field. However, both distributions show a similar trend, see Figure 6. The measurement of the spin density maps of mobile molecules in a rigid matrix is one of the most straightforward applications of MRI.34,35 In principle, the amplitude of the NMR signal should give a quantitative measurement of the spin density and, therefore, of the content of mobile molecules confined in a porous structure.44 However, fluids in porous media exhibit a range of relaxation times depending on the pore size;36 moreover, magnetic susceptibility effects may affect the relaxation time values. Therefore, in obtaining spin density images one must be aware of the spin relaxation effects on the signal amplitude. Indeed, the possibility of using various contrast schemes enables a variety of methods to be used to study fluids in porous media. Hence, the experimental parameters for obtaining an image of a wet stone must be properly chosen in order to detect water molecules having different transverse relaxation times, T2. Figure 7 shows the MRI images of all treated and untreated Pale Finale stones. The images reported in column a were obtained by performing SPI experiments with a very short dephasing time, 92 µs. Therefore, these images were reconstructed using the signal of water molecules characterized by very short T2 values (
