Electron Paramagnetic Resonance and 1H and 13

Chapter 19 1

Electron Paramagnetic Resonance and H and C NMR Study of Paper 13

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D. Attanasio , D. Capitani , C. Federici , M. Paci , and A. L. Segre 1

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Istituto Chimica dei Materiali and Istituto Strutturistica Chimica, Consiglio Nazionale delle Ricerche, C.P. 10, 00016 Monterotondo Stazione, Roma, Italy Istituto Centrale per la Patologia del Libro, Via Milano 76, 00184 Roma, Italy Dipartamento di Chimica, II U n i v e r s i t àdi Roma, Tor Vergata, Via Ricerca Scientifica, 00136 Roma, Italy 4

High quality antique sheets of paper have been characterized by H NMR relaxation, C CP MAS spectra, and Electron Paramagnetic Resonance spectroscopy. Paper can be regarded as a bi-component material made by cellulose and water plus a small amount of organic and inorganic additives and impurities. Semi-crystalline fibrous cellulose, rich in water, is present as polymorphs I and Ι Amorphous cellulose, with a lower water content, presents a higher amount of paramagnetic impurities and is characterized by quite short H spin-lattice relaxation times and by C resonances with noticeable chemical shifts. "Ad hoc" tailored sequences are able to produce C CP MAS spectra in which the amorphous content of cellulose in paper is quite well observable. The nature of water as fully bound to the cellulose lattice has also been proved. Low-temperature EPR spectra have shown the presence of measurable amounts of different inorganic paramagnetic impurities, such as Fe , Mn , C u often found in different stereochemical environments. The spectra are all, qualitatively, closely similar. However, quantitative data have shown that in paper the state of conservation does not depend on the amount of pseudo-octahedral iron, but is strongly correlated to the concentration of this metal ion in a rhombic stereochemistry and to the presence of even very small amounts of copper. 1

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I n t r o d u c t i o n . It is widely known that paper is the most commonly used writing material. While books in their current form have been produced 0097-6156/95/0598-0333$12.25/0 © 1995 American Chemical Society Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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MULTIDIMENSIONAL SPECTROSCOPY OF POLYMERS

for almost 18 centuries, paper has been used systematically for this purpose only during the last 6 centuries. In Europe , the first kind of paper was made of rags. This was the rule until the first half of 19th century when wood pulp began to be commonly used as a raw material, causing the rapid degradation of paper which has occurred in the last 150 years. Ancient paper, despite being always made with high quality material, can be very damaged and even fully destroyed. With the aim of characterizing paper of all possible types and its degradation, a spectroscopic characterization of paper was attempted, starting from high quality ancient paper. Paper can be considered as a bi-component material made by cellulose and water containing small amounts of organic or inorganic additives or impurities (Paci M . ; Federici C ; Capitani D . ; Perenze N . ; Segre A . L . Carbohydrate Polymers in press). In the present work all the studied samples are unprinted sheets of welldocumented historical origin, spanning between the 15th and 18th century. This choice has many advantages, the main one being the high quality of paper made o f rags, compared to modern paper made of wood- pulp. Another advantage for the study of paper degradation is the possibility of obtaining paper in a good state of preservation, or in poor condition, or even completely destroyed, taken from a homogeneous source, i.e. an old book. As far as this article is concerned, we will have to refer to paper conservation conditions as "destroyed" , "ruined" and "good". We are aware that these adjective do not have scientific meaning, but they will be useful to classify the conservation state as it looks to an observer. Therefore a very brittle paper , breaking as soon as it is touched, usually brown-coloured will be labeled as "destroyed" ; a weakened paper, usually spotted and spoiled , but which keeps its own cohesive features will be labeled " ruined"; while " good" is the paper which shows no alteration marks. Characterization of solid paper was performed as follows: pulsed low resolution N M R relaxation measurements at 57 M H z , high resolution N M R line width and T i measurements at 200 M H z , C NMR spectroscopy with C P - M A S at 100 M H z also with the use of "ad hoc" tailored sequences, and E P R measurements at room and liquid nitrogen temperatures. 1 3

Results and Discussion. Cellulose, the major component of a sheet of paper, is always present as a semicrystalline polymer. The crystalline fraction is present as a mixture of different polymorphous forms always accompanied by a variable amount of amorphous material. While the complex polymorphism existing in cellulose has been demonstrated by l ^ C C P - M A S techniques (/), the structure of two polymorphous forms of

Urban and Provder; Multidimensional Spectroscopy of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

19. ATTANASIO ET AL.

EPR and NMR Study of Paper

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cellulose has been recently solved by electron diffraction (2). The existence o f amorphous cellulose has been recently demonstrated by C P - M A S N M R on materials originating from algae (3). With the purpose of characterizing the material "paper" a full N M R study was performed using both and ^ C spectra. Finally, following the indications of N M R relaxometry, the presence o f paramagnetic centers was studied by E P R spectroscopy. The information obtained by all these techniques is complementary and for the sake of clarity will be discussed separately. Low Resolution Pulsed N M R . A proton free induction decay (FID) of the nuclear magnetization is shown in Figure l a at 5 7 M H z ; in the time domain the fast decaying component is due to cellulose, while the barely observable slow decaying component is due to water; by Fourier transformation o f the F I D into the frequency domain, Figure l b , the cellulose component appears as a broad and noisy hump, while the water component appears as a sharp peak at the top of the broad one. A n appropriate choice of points within the time domain allows the measure of T\ for both components (4). A n aperiodic sequence of saturationrecovery was used with at least 128 different delays (5). In all examined samples, both the water and the cellulose signals exhibit multiple Ί\ relaxations corresponding to at least three different Ί\ components, see Figure 2a and 2b. Thus for both signals a fast , an intermediate and a slow relaxing T\ components can be observed. For all studied samples , the Ί\ values of the long relaxing component of cellulose are shown in Figure 3a, plotted against the corresponding component o f water. A l l these values , without any correction, are well correlated around the y=x line (correlation coefficient larger than 98% ,while the angular coefficient is 1.09 ± 0.06). This finding shows that a very efficient spin diffusion mechanism exists, able to transfer the magnetization from the water pool to the cellulose pool and vice versa. Thus, water is strongly bound to the cellulose and the amount of free water, i f any, is indeed negligible. B y plotting the intermediate T\ value of the cellulose vs. the corresponding value of water, see Figure 3b, it can be seen that all points lie in the lower part of the y=x line; the correlation coefficient is only 68% , and the slope of the best fit straight line is only 0.4±0.1. Thus the spin diffusion process is rather poor, possibly due to the presence of other relaxation mechanisms such as those due to interactions with paramagnetic impurities. Another possible explanation is that the total amount o f water is not enough to be fully efficient in the spin diffusion process. The fastest T i component, both for cellulose and water is of the order o f few milliseconds with a rather high error; thus, the operating relaxation mechanism should be dominated by paramagnetic impurities such as C u and Fe . Indeed, as shown below, an E P R study reveals the presence o f these and other paramagnetic impurities.


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Figure 1. * H N M R at 57 M H z . a); Free Induction Decay in the time domain of a piece of antique paper. The fast decaying component is due to cellulose, while the slow decaying one is due to water. b); Same experiment; the solid line through the experimental points is the best fit curve.


ATTANASIO ET AL.


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t (sec) Figure 2. N M R at 57 M H z . Aperiodic Saturation Recovery experiment on the cellulose component. a)For the sake o f clarity a multi-exponential decay is shown in a semilogarithmic representation. b)Same experiment in a linear scale; the full line trough experimental points results from a best fit procedure.


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T1(ms) water 150

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T1 (ms) cellulose Figure 3 . a) T\ values due to the long relaxing component o f water are reported vs. the corresponding Ύ\ values o f cellulose, b) T\ values due to the intermediate relaxing component o f water are reported vs. the corresponding T j values of cellulose. Solid lines through the experimental points result from a linear regression analysis. A l l T j values were measured at 57 M H z .




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High Resolution Proton N M R Spectra The sharpness o f the water resonance points to the possibility of performing (4) ^ H N M R measurements on a conventional high resolution N M R spectrometer, see Figure 4. Inversion-recovery experiments performed on this peak show again the presence of three spin-lattice values, whose values agree with the corresponding data at 57MHz, see Table I. In this way, due to the efficiency of the spin diffusion process previously observed, simply by measuring the spin-lattice relaxation time o f water on a conventional high resolution spectrometer, it is possible to attain the corresponding value for the slow relaxing component of cellulose. The relative amount of water was measured on sheets of paper showing different degradation. In all sheets of paper which appear in a good state of conservation or only slightly deteriorated the amount of water is the same within 10% , while in all samples which appear in a very bad state of conservation ( strong degradation) a net loss of water can be measured, see Figure 5. B y comparing the proton line width of paper belonging to the same book, but showing a different state of degradation, a definite enlargement can be measured, as reported in Table I. This enlargement may be due to a higher (or different) paramagnetic contribution, as will be discussed in the E P R section. Moreover the net loss of water seems associated with a net degrade of the material; this in turn may be associated with an increase in the amorphous fraction. In all semicrystalline polymers, the higher content of impurities can be found in the amorphous fraction, hence our attention was focused on two different samples, one as a source of cellulose rich in long fibers and highly crystalline (sample 1), the other one as a source of amorphous cellulose (sample 4), this one also with a very low water content. On these two samples * C C P - M A S N M R spectra were performed with the aim of observing spectroscopic differences between sheets having different degradation. 3

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C C P - M A S N M R (100MHz). The C C P - M A S spectra o f two sheets of ancient paper either well preserved or almost completely destroyed do not show major differences. This is due to the fact that long fibers, well packed and corresponding to the crystalline fraction o f the material, cross polarize much better than the amorphous material (6,7); this is well known and it is due to differences in relaxation times between the crystalline fraction and its amorphous counterpart. As a consequence to evidence the " amorphous" component of paper in presence o f a "long fiber" component it is necessary either to enhance the amorphous component or to lower the crystalline one; all this can be actually done with " ad hoc" tailored sequences. The pulse sequence proposed by Torchia (8) and taking advantage of differences in T j relaxation does not seem to give any significant result; likewise a preparatory sequence usually called D E F T (9) which realigns selectively the magnetization of the slow relaxing spins gives only partial


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Figure 4. The H N M R spectrum at 200.13 M H z is reported for a sample o f ancient paper; only the resonance due to the water component can be observed.




Table I. Line width, Area and Τ χ spin-lattice relaxation times of ancient paper samples Sample Line width Area Tl Tl Tl (Hz) water cellulose water 200 MHz 57MHz 15th century 130 149 143 26 40 41 1(north Italy) 2014 96 2.0 good 3.48 3.5 80 15th century 124 73 12 28 18 2 (north Italy) 2014 100 2.0 1.9 deteriorated 3.00 120 114 15th century 220 30 11 53 3 (north Italy) 1307 101 2.0 2.2 3.7 good 69 15th century 70 108 ο 14 4 (Italy 1816 80 23 40 Germany) 1.7 1.4 2,1 destroyed 82 15th century 138 90 37 56 5.8 5 (north Italy) 1683 79 2.7 destroyed 4.1 1.7 21 18th century 30 24 2.3 6.7 9.8 6 (Sardegna) 3759 80 0.8 0.5 0.9 destroyed 18th century 26 17.2 34 4.7 7 (Sardegna) 8.9 3682 72 1.4 0.4 destroyed 1.0 0.6 62 127 71 15th century 3.2 34 18 8 (Italy) 1414 104 2.0 3.4 1.8 very ruined 81 71 98 15th century 4.7 9.3 18 9 (Italy) 1403 101 1.1 2.4 1.4 ruined 89 90 167 15th century 11.0 21 56 10 (Italy) 1120 101 2.1 2.4 4.7 good 72 73 146 15th century 7.55 38.3 22 11 (France) 1704 87 3.09 2.86 very ruined 1.6 101 107 280 15th century 30 27 88 12 (France) 1557 93 2.89 2.9 3.9 very good 81 86 177 15th century 19 25 56 13 (France) 1857 83 2.5 2.0 2.9 destroyed 15th century 227 105 111 49 31 11.0 14 (France) 1683 95 2.1 3.7 2.9 very good 108 99 16th century 169 26 10 33 15 (Germany) 1215 108 3.4 3.2 very good 6.1 108 102 15th century 106 36 8 13 16 (Germany) 1195 no 2.6 1.7 little ruined 4.3 15th century 209 136 129 42 38 20 17 (Germany) 1180 102 very good 6.1 3.9 3.4 Errors on Tj components are:

Electron Paramagnetic Resonance and 1H and 13

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