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NMR study of the carbonization process of furfuryl alcohol resin
CRM84223/82/010013$03.iM/0 0 1982 Per@mon Press Ltd.
Carbon Vol. 20. No. I, pp. 13-16. 1982 Printed m Great Rritain.
NMR STUDY OF THE CARBONIZATION PROCESS OF FURFURYL ALCOHOL RESIN S.G~OWINKOWSKI and Z. PAJ~K Institute of Physics, A. Mickiewicz University, 60-780Poznad, Poland (Received 31 March 1981) Abstract-Proton CW and pulse NMR studies for furfuryl alcohol resin carbonized over the temperature range 60-86O”Care performed. The results are interpretedin terms of structural and chemical changes taking place during heat treatment. Heating up to: (i) 240°C results in an increase in molecular weight and crosslinking; (ii) 320°C produces crosslinking sufficient to form a rigid structure; (iii) 380°C and higher leads to hydrogen release and
appearanceof paramagneticcenters.
preparation have been described elsewhere[9].The carprocess was carried out in a quartz tube attached to a vacuum system with pressure lower than perties of molecular sieves as well as some interesting lOa mmHg. After each successive heating the tube was phenomena similar to those observed in superconducting closed to avoid contact with the atmosphere and used as sample for NMR measurements.The temperatures and materials[I-S]. Though the exact mechanism of the carbonization duration times of the heat treatment process are given in process as well as the structural changes involved are as Table 1. yet not known,a scheme of pyrolysis chemistry has been proposed[2].The main changes in structure occur in the 2.2 NMR measurements Proton magnetic resonance spectra were recorded by temperature region 130-45O”C,where a rupture of methylene bridges, opening of furan rings and formation meansof a Bloch type spectrometerRYa 2301(USSR)at of aromatic systems take place. Above 450°C the a Larmor frequency of 40MHz. The experimental remaining methylene group detaches and a highly un- second moments of the absorption lines were calculated saturated aromatic C-H residue capable of forming a as mean values from at least four spectra, at each temperature. crosslinkedaromatic system arises. Valuable information on the molecular dynamics and The spin-lattice relaxation times of protons were structure of solids can be obtained from NMR studies. measured using a pulse spectrometer constructed in this Measurementsof the second moment of the absorption laboratory operating at a frequency of 25MHz[lO].The line[6] and spin-latticerelaxation times in the laboratory relaxation time T, was measured by applying 180°+900 and rotating frames[7,8] enable us to find the micro- pulse sequences (null method). The parameter T,, was scopic parameters characterizing molecular motion and measured by spin-lockingthe nuclear magnetization,i.e. to describe the changes takingplace in the real structure. by applying a 90” pulse followed by a long pulse with Our previous study[9], carried out on a number of phase shifted by 90”from the former usingan r.f. field of furfuryl alcohol resins differentiated by their viscosity H,= 8.OG. values led us to suggest that the marked decrease in The temperature of the sample during NMR second moment of the NMR line and the high tem- measurementswas controlled by a gas-flowcryostat and perature spin-latticerelaxation minima T, and T,, may monitored with a platinum thermometer to an accuracy be assigned to rotational motion of polymer chains. It of +I”. The second moment and spin lattice relaxation was also found that the mobility of the polymer chains times were measuredwith an accuracy of about 10%. slows down when the viscosity of the resin increases. 3. RESULTS This paper reports the results of CW and pulse NMR measurementsof furfuryl alcohol resin heat treated up to Second moment measurementsfor the initialresin and 860°C.The results obtained are interpreted in terms of after successive heating at 120, 180 and 240°C were the expected influence of structural and chemical carried out over a wide temperature range. For resin changes taking place during this process on molecular heated at higher temperatures the measurements were motion. performed at liquidnitrogen and room temperatures,and only in some cases at higher temperatures. 2. EXPERIMENTAL The shapes of the NMR lines recorded depend both on 2.1 Materials the heat treatment and measuring temperatures. For The initial resin used throughout this work charac- resin treated up to 240°Cthe spectra recorded at low terized by the viscosity 7 = 1.4. 106cP at 20°C was temperaturesconsist of one broad component.However, obtained by polycondensation of furfuryl alcohol in the above the temperature where the marked decrease in presence of hydrochloric acid as catalyst. The details of second moment appears (Figs. 1 and 2), they consist of 1. INTRODUCTION
Heat treatment of furfuryl alcohol resin leads to the formation of glasslikeporous carbon, revealing the pro-
bonization
13
S. GLO~INKOWSKI and Z. PAJ~K Table 1. Conditionsof carbonizationprocess Resin
Temperature
Time
PO1
b3
PA
120
120
2
FA
180
180
1
%'A 240
240
1
FA
320
1
320
PA
380
380
1
FA
460
460
1
PA
560
560
1
FA
660
660
1
FA
760
760
1
FA
860
860
1
two components: a broad and a n~ow one. For resin heated at 320°Cand higher all recorded spectra consist only of one, broad component. The temperature dependences of the second moment for the broad component, after its separation from the experimental spectrum[ 1I], for initial resin as well as for resin heated at 120 and 18(r%(Fig. 1) and 240°C(Fig. 2), exhibit two distinct regions: a low temperature region, where a smallgradual decrease in the second moment is observed, and a high temperature region where the decrease in second moment is larger and faster. For resin heated at higher temperatures, up to 560°Cthe second moment is almost independentof the tem~ra~re of meas~ement (Fig. 2). The changesof the second moment as a function of heat treatment temperature for spectra recorded at liquid nitrogen and . -. _temperature _ . __.at room . . . temperature are shown in Fig. 3. In the case of hquid nitrogen measurements,a two step decrease in second moment takes place: small and gradual up to 46O”C,and greater and faster above this temperature. For room temperature, the situation is
77 z -
&I-
+ PFA 2f,G ---r)---PFA L6O _-cm PFA 360 -.-w.- PFA 560 PFA 380
I-
J TEMPERATURE[“C] Fig. 2. Secondmomentof NMRline vs tempera~re for fu~ury~ alcohol resin after heat treatment at: 240, 320, 380, 460 and 56O@c..
somewhatdifferent: at hrst the second moment increases and above 320°Cit decreases. The temperat~e dependences of the spin-lattice relaxation times T1 and Tip measured close to the high temperature minima for initial resin and after its successive heat treatment up to 320°Care shownin Fig. 4. A pronounced feature of the experimental curves is a min~um, which shifts to higher temperatures when the heat treatment temperature increases. A distinct change, namely an absence of I’, and T,, minima, is observed for the resin heated at 320°C.For the resin heated above this temperature only room temperature T, measureI ments were performed. They are shown in Fig. 5 100 0 together with the room temperature values of T, for TEMP~~ATU~ E”C-j resin heated at lower temperat~es, It is worthwh~e to Fig. 1. Second momentof NMRline vs temperaturefor furfuryl mention that each successive heat treatment of the resin alcohol resin PFA and after its heat treatment at 120and 180°C. especially above 3WC, leads to a decrease of the NMR
NMR study of the carbonizationprocessof furfuryl alcoholresin I 0 l
-196°C
2oT
L--I 100 200 300 400 500 600 700 HEAT TREATMENTTEMPERATUREPC]
Fig. 3. Second moment of NMRline recordedat -1% and 20°C for furfuryl alcoholresin vs heat treatment temperature. 100 200 300 Kx) 500 600 700 800
Z” I-+-
I
PFA + PFA 120 l
HEAT TREATMENT TEMPERATURE
v PFA 2LO A PFA 320
[“C]
Fig. 5. Spin-latticerelaxation time T, for furfuryl alcohol resin vs heat treatment temperature.
0 PFA180
pounds having more freedom of motion than higher polymers which are rather rigid and give the broad component. For resin heated up to 240°C the occurrence of a narrow component suggeststhat the low molecularfraction of the polymer still exists. However, no detailed study of this component has been performed. It is necessary to mention that its intensity decreases after each successive heat treatment of the resin. For the broad componentthe effect of the heat treatment process is more pronounced. The temperature dependences of the second moment are similar to those observed for the initial material. The marked decrease of second moment shifts to higher temperatures, when the heat treatment temperature increases. This suggests that rotational motion of the polymeric chains undergoes constraints after each successive heat treatment. These constraints 3 2 4 1OyT [ f’] are confirmed by measurements of the spin-lattice Fig. 4. Spin-latticerelaxationtimes vs reciprocaltemperaturefor relaxation times revealingthe shift of T1and T,,, minima furfuryl alcoholresin and after its heat treatmentat: 120,180,240 to higher temperature with increasing heat treatment and 320°C. temperature. During the first stage of heat treatment, at 12O”C,the decrease in mobility of the polymeric chains during the signal intensity, rendering impossible the measurement heat treatment process results from the increase in moleof the second momentand spin-latticerelaxationtime for cular weightinduced by thermal polycondensationof the resin heat treated above 760°C. resin. This is supported by the fact that the activation energy, calculated according to the Arrhenius relation 4.DISCIJS!SION from known values of the correlation times at the Ti and Broad and narrow components of the NMR absorption r,, minima,rises from 21.6kcal/molefor initialresin, up line for crystalline polymers are attributed to a rigid, to 24.4kcal/mole for resin heated at 120°C.A similar crystalline phase and to a mobile amorphous one, res- change in activation energy for rotational motion of the pectively. The similarsituation often observed in typical order of 2 kcal/molewas observed when the viscosity of amorphous resin can be explained by a distribution of the resin rises from 1.3+103to 1.4- lo6 cP[9]. Successive heat treatment at 180°Cleads to further the molecular weight. It has been assumed[9] that the narrow component of the absorption line of furfuryl stiffeningof the resin. The greater increase in activation alcohol resin originates in low molecular weight com- energy up to 29.7kcallmole and the shift of the minima
1
16
S. GUNINKOWSIU andZ. PAIAK
of relaxation times T1 and T,, as well as the displacement of the marked decrease in second moment to much higher temperatures indicate that an additional mechanismprobably crosslinkinghindering the molecular motion appears. For resin treated at 320°Cthe disappearance of the narrow line and the insignificanttemperature dependence of the second moment and spin-lattice relaxation times suggest that the rotational motion of polymer chains vanishes due to a large number of crosslinks probably related with a chemical process leadingto the release of water and carbon dioxide observed around 340°C[l]. The differences between the temperature at which structural changes are observed in NMR experiment and the temperature of pyrolytic gas release may arise from different heat treatment conditions which are stationary in the former case and dynamicalin the latter case. Heat treatment of resin at higher temperatures which does not influence the temperature dependence of the second moment indicates that the rigidity of the resin remains unchanged. However, each subsequent heat treatment at higher temperature leads to a reduction of the second moment interpreted in terms of interproton distance enlargement due to reduction of the hydrogen content connected with the release of water, methaneand hydrogen[l, 31. Interesting information on the structural changes occurring duringheat treatment can be derived from Fig. 3. At liquid nitrogen temperature the decrease in second moment reflects a reduction of the amount of hydrogen in the sample. The much faster decrease in second moment above 460°Cis probably related to the disappearance of methylene groups[2, 31givingthe main contribution to the second moment. For spectra recorded at room temperature the increase in the second moment can be explained by a stiffening of resin due to molecular weight increase and crosslinking.These effects mask the small decrease of the second moment which should appear due to the reduction of the hydrogen content. The structure of the resin heated above 320°Cis rigid and the decrease of the second moment originates from hydrogen content reduction. Small difference in the values of the second moment observed at liquid nitrogen and room temperatures for resin heated above 320°C (Fig. 3) as well as an insigniticanttemperature dependence of the second moment for resin heated at 320,380, 460 and 560°C(Fig. 2) may arise from oscillationof small parts of crosslinkedresin. The nuclear magnetic relaxation process for resin heated at 320°C and higher is believed to be due to paramagneticcenters. The existence of such centers in resin heated around 300°Chas been reported in EPR studies[1416]. It seems likely that during the relaxation process the spin diffusion mechanismis operative and magnetization is transferred alongthe crosslinkedresin to paramagnetic centres which have good thermal contact with the lattice. In this case the rate of relaxation depends on the con-
centrationof the centres[17].Ifthis mechanismworks for resin heated at higher temperatures, then the change of the relaxation time above 320°C (Fig. 5) reflects the change in paramagnetic centres concentration. The greatest number of paramagneticcentres is observed for resin heated at 560°C.Similarheat treatment temperature dependencesof paramagneticcentres concentrationhave been found in EPR studies[W161. For resin heated in the region 60-320°C(Fig. 5) the relaxation process is controlled by rotational motion of polymer chains and the observed increase in T, values results from a stiffeningof the resin due to crosslinkingand molecular weight increase. 5. CONCLUSION The observed decrease of the second moment as well as evidenced minimaof the proton spin-latticerelaxation times reflect the structural and chemical changes taking place during carbonization process of furfuryl alcohol resin. Heating up to 240°Ccauses an increase in molecular weight and the appearance of a number of crosslinks. This effect which is more pronounced after heating at 32o”C,makes the chains unable to perform any rotational motion and manifests itself by insignificanttemperature dependences of the second moment and spin-lattice relaxation times. Heat treatment at higher temperatures causes releasing of hydrogen which is reflected by a further decrease of second moment as well as the appearance of paramagneticcenters evidenced by changes observed in nuclear magneticrelaxation. Acknowledgements-The authorswishto thankMr. Z. Szubafor carrying out the carbonization process, and Dr. K. Jurga for assistance with relaxation times measurements.The work was supported by Polish Academy of Sciences under Project MR.l-4. IWERENCES
E. Fitzer, W. Schaferand S. Yamada,Curbon 7,643 (1%9). : E. Fitzer and W. Schafer,Carbon 8,353 (1970). 3: N. Ozanne, J. Amielland L. Bonnetain, Bull. Sot. Chim. Fr. 1971(1971). 4. D. L. Trimmand B. J. Cooper,J. Chem.Sot. D, 477(1970). K. Antonowicz,Nolure (London), 247,358(1974). :. J. H. Van Vleck,Phys. Rev. 74, 1168(1948). 1: N. Bloembergen,E. M. Purcell and R. W. Pound, Phys. Reo. 73,639 (1948). G. P. Jones, Phys. Rev. 148,332 (1966). :: S. Gfowinkowskiand Z. Paiak. -* Acta Phvs. I Pal.. A54.’ 411 (1978). 10. Z. Pajak, K. Jurga and J. Jurga, Acta Phys. Pal. A45,837 (1974). 11. N. N. Tikhomirovaand V. V. Voevodskii,Opt.Spectroscopy 7,486(1959). 12. E. V. Orobchenkoand N. Yu. Priyanishnikova,Furanouyje S&y, pp. 110-111.Gosiztekh,Kiev (1963). 13. S. Domanusowaand B. Kalinowski,Koks, Smda, Car, 11,91 (1966). 14. C. Jackson and W. F. Wynne-Jones,Carbon 2,227 (1964). 15. D. Campbell,C. Jackson, H. Marsh and W. F. K. WynneJones, Carbon 4,159 (1966). 16. N. Ozanne and L. Bonnetain, Bull. Sot. Chim. Fr. 1976 (1971). 17. P. G. DeGennes,J. Phys. Chem. Solids 3, 345(1958).
Carbon Vol. 20. No. I, pp. 13-16. 1982 Printed m Great Rritain.
NMR STUDY OF THE CARBONIZATION PROCESS OF FURFURYL ALCOHOL RESIN S.G~OWINKOWSKI and Z. PAJ~K Institute of Physics, A. Mickiewicz University, 60-780Poznad, Poland (Received 31 March 1981) Abstract-Proton CW and pulse NMR studies for furfuryl alcohol resin carbonized over the temperature range 60-86O”Care performed. The results are interpretedin terms of structural and chemical changes taking place during heat treatment. Heating up to: (i) 240°C results in an increase in molecular weight and crosslinking; (ii) 320°C produces crosslinking sufficient to form a rigid structure; (iii) 380°C and higher leads to hydrogen release and
appearanceof paramagneticcenters.
preparation have been described elsewhere[9].The carprocess was carried out in a quartz tube attached to a vacuum system with pressure lower than perties of molecular sieves as well as some interesting lOa mmHg. After each successive heating the tube was phenomena similar to those observed in superconducting closed to avoid contact with the atmosphere and used as sample for NMR measurements.The temperatures and materials[I-S]. Though the exact mechanism of the carbonization duration times of the heat treatment process are given in process as well as the structural changes involved are as Table 1. yet not known,a scheme of pyrolysis chemistry has been proposed[2].The main changes in structure occur in the 2.2 NMR measurements Proton magnetic resonance spectra were recorded by temperature region 130-45O”C,where a rupture of methylene bridges, opening of furan rings and formation meansof a Bloch type spectrometerRYa 2301(USSR)at of aromatic systems take place. Above 450°C the a Larmor frequency of 40MHz. The experimental remaining methylene group detaches and a highly un- second moments of the absorption lines were calculated saturated aromatic C-H residue capable of forming a as mean values from at least four spectra, at each temperature. crosslinkedaromatic system arises. Valuable information on the molecular dynamics and The spin-lattice relaxation times of protons were structure of solids can be obtained from NMR studies. measured using a pulse spectrometer constructed in this Measurementsof the second moment of the absorption laboratory operating at a frequency of 25MHz[lO].The line[6] and spin-latticerelaxation times in the laboratory relaxation time T, was measured by applying 180°+900 and rotating frames[7,8] enable us to find the micro- pulse sequences (null method). The parameter T,, was scopic parameters characterizing molecular motion and measured by spin-lockingthe nuclear magnetization,i.e. to describe the changes takingplace in the real structure. by applying a 90” pulse followed by a long pulse with Our previous study[9], carried out on a number of phase shifted by 90”from the former usingan r.f. field of furfuryl alcohol resins differentiated by their viscosity H,= 8.OG. values led us to suggest that the marked decrease in The temperature of the sample during NMR second moment of the NMR line and the high tem- measurementswas controlled by a gas-flowcryostat and perature spin-latticerelaxation minima T, and T,, may monitored with a platinum thermometer to an accuracy be assigned to rotational motion of polymer chains. It of +I”. The second moment and spin lattice relaxation was also found that the mobility of the polymer chains times were measuredwith an accuracy of about 10%. slows down when the viscosity of the resin increases. 3. RESULTS This paper reports the results of CW and pulse NMR measurementsof furfuryl alcohol resin heat treated up to Second moment measurementsfor the initialresin and 860°C.The results obtained are interpreted in terms of after successive heating at 120, 180 and 240°C were the expected influence of structural and chemical carried out over a wide temperature range. For resin changes taking place during this process on molecular heated at higher temperatures the measurements were motion. performed at liquidnitrogen and room temperatures,and only in some cases at higher temperatures. 2. EXPERIMENTAL The shapes of the NMR lines recorded depend both on 2.1 Materials the heat treatment and measuring temperatures. For The initial resin used throughout this work charac- resin treated up to 240°Cthe spectra recorded at low terized by the viscosity 7 = 1.4. 106cP at 20°C was temperaturesconsist of one broad component.However, obtained by polycondensation of furfuryl alcohol in the above the temperature where the marked decrease in presence of hydrochloric acid as catalyst. The details of second moment appears (Figs. 1 and 2), they consist of 1. INTRODUCTION
Heat treatment of furfuryl alcohol resin leads to the formation of glasslikeporous carbon, revealing the pro-
bonization
13
S. GLO~INKOWSKI and Z. PAJ~K Table 1. Conditionsof carbonizationprocess Resin
Temperature
Time
PO1
b3
PA
120
120
2
FA
180
180
1
%'A 240
240
1
FA
320
1
320
PA
380
380
1
FA
460
460
1
PA
560
560
1
FA
660
660
1
FA
760
760
1
FA
860
860
1
two components: a broad and a n~ow one. For resin heated at 320°Cand higher all recorded spectra consist only of one, broad component. The temperature dependences of the second moment for the broad component, after its separation from the experimental spectrum[ 1I], for initial resin as well as for resin heated at 120 and 18(r%(Fig. 1) and 240°C(Fig. 2), exhibit two distinct regions: a low temperature region, where a smallgradual decrease in the second moment is observed, and a high temperature region where the decrease in second moment is larger and faster. For resin heated at higher temperatures, up to 560°Cthe second moment is almost independentof the tem~ra~re of meas~ement (Fig. 2). The changesof the second moment as a function of heat treatment temperature for spectra recorded at liquid nitrogen and . -. _temperature _ . __.at room . . . temperature are shown in Fig. 3. In the case of hquid nitrogen measurements,a two step decrease in second moment takes place: small and gradual up to 46O”C,and greater and faster above this temperature. For room temperature, the situation is
77 z -
&I-
+ PFA 2f,G ---r)---PFA L6O _-cm PFA 360 -.-w.- PFA 560 PFA 380
I-
J TEMPERATURE[“C] Fig. 2. Secondmomentof NMRline vs tempera~re for fu~ury~ alcohol resin after heat treatment at: 240, 320, 380, 460 and 56O@c..
somewhatdifferent: at hrst the second moment increases and above 320°Cit decreases. The temperat~e dependences of the spin-lattice relaxation times T1 and Tip measured close to the high temperature minima for initial resin and after its successive heat treatment up to 320°Care shownin Fig. 4. A pronounced feature of the experimental curves is a min~um, which shifts to higher temperatures when the heat treatment temperature increases. A distinct change, namely an absence of I’, and T,, minima, is observed for the resin heated at 320°C.For the resin heated above this temperature only room temperature T, measureI ments were performed. They are shown in Fig. 5 100 0 together with the room temperature values of T, for TEMP~~ATU~ E”C-j resin heated at lower temperat~es, It is worthwh~e to Fig. 1. Second momentof NMRline vs temperaturefor furfuryl mention that each successive heat treatment of the resin alcohol resin PFA and after its heat treatment at 120and 180°C. especially above 3WC, leads to a decrease of the NMR
NMR study of the carbonizationprocessof furfuryl alcoholresin I 0 l
-196°C
2oT
L--I 100 200 300 400 500 600 700 HEAT TREATMENTTEMPERATUREPC]
Fig. 3. Second moment of NMRline recordedat -1% and 20°C for furfuryl alcoholresin vs heat treatment temperature. 100 200 300 Kx) 500 600 700 800
Z” I-+-
I
PFA + PFA 120 l
HEAT TREATMENT TEMPERATURE
v PFA 2LO A PFA 320
[“C]
Fig. 5. Spin-latticerelaxation time T, for furfuryl alcohol resin vs heat treatment temperature.
0 PFA180
pounds having more freedom of motion than higher polymers which are rather rigid and give the broad component. For resin heated up to 240°C the occurrence of a narrow component suggeststhat the low molecularfraction of the polymer still exists. However, no detailed study of this component has been performed. It is necessary to mention that its intensity decreases after each successive heat treatment of the resin. For the broad componentthe effect of the heat treatment process is more pronounced. The temperature dependences of the second moment are similar to those observed for the initial material. The marked decrease of second moment shifts to higher temperatures, when the heat treatment temperature increases. This suggests that rotational motion of the polymeric chains undergoes constraints after each successive heat treatment. These constraints 3 2 4 1OyT [ f’] are confirmed by measurements of the spin-lattice Fig. 4. Spin-latticerelaxationtimes vs reciprocaltemperaturefor relaxation times revealingthe shift of T1and T,,, minima furfuryl alcoholresin and after its heat treatmentat: 120,180,240 to higher temperature with increasing heat treatment and 320°C. temperature. During the first stage of heat treatment, at 12O”C,the decrease in mobility of the polymeric chains during the signal intensity, rendering impossible the measurement heat treatment process results from the increase in moleof the second momentand spin-latticerelaxationtime for cular weightinduced by thermal polycondensationof the resin heat treated above 760°C. resin. This is supported by the fact that the activation energy, calculated according to the Arrhenius relation 4.DISCIJS!SION from known values of the correlation times at the Ti and Broad and narrow components of the NMR absorption r,, minima,rises from 21.6kcal/molefor initialresin, up line for crystalline polymers are attributed to a rigid, to 24.4kcal/mole for resin heated at 120°C.A similar crystalline phase and to a mobile amorphous one, res- change in activation energy for rotational motion of the pectively. The similarsituation often observed in typical order of 2 kcal/molewas observed when the viscosity of amorphous resin can be explained by a distribution of the resin rises from 1.3+103to 1.4- lo6 cP[9]. Successive heat treatment at 180°Cleads to further the molecular weight. It has been assumed[9] that the narrow component of the absorption line of furfuryl stiffeningof the resin. The greater increase in activation alcohol resin originates in low molecular weight com- energy up to 29.7kcallmole and the shift of the minima
1
16
S. GUNINKOWSIU andZ. PAIAK
of relaxation times T1 and T,, as well as the displacement of the marked decrease in second moment to much higher temperatures indicate that an additional mechanismprobably crosslinkinghindering the molecular motion appears. For resin treated at 320°Cthe disappearance of the narrow line and the insignificanttemperature dependence of the second moment and spin-lattice relaxation times suggest that the rotational motion of polymer chains vanishes due to a large number of crosslinks probably related with a chemical process leadingto the release of water and carbon dioxide observed around 340°C[l]. The differences between the temperature at which structural changes are observed in NMR experiment and the temperature of pyrolytic gas release may arise from different heat treatment conditions which are stationary in the former case and dynamicalin the latter case. Heat treatment of resin at higher temperatures which does not influence the temperature dependence of the second moment indicates that the rigidity of the resin remains unchanged. However, each subsequent heat treatment at higher temperature leads to a reduction of the second moment interpreted in terms of interproton distance enlargement due to reduction of the hydrogen content connected with the release of water, methaneand hydrogen[l, 31. Interesting information on the structural changes occurring duringheat treatment can be derived from Fig. 3. At liquid nitrogen temperature the decrease in second moment reflects a reduction of the amount of hydrogen in the sample. The much faster decrease in second moment above 460°Cis probably related to the disappearance of methylene groups[2, 31givingthe main contribution to the second moment. For spectra recorded at room temperature the increase in the second moment can be explained by a stiffening of resin due to molecular weight increase and crosslinking.These effects mask the small decrease of the second moment which should appear due to the reduction of the hydrogen content. The structure of the resin heated above 320°Cis rigid and the decrease of the second moment originates from hydrogen content reduction. Small difference in the values of the second moment observed at liquid nitrogen and room temperatures for resin heated above 320°C (Fig. 3) as well as an insigniticanttemperature dependence of the second moment for resin heated at 320,380, 460 and 560°C(Fig. 2) may arise from oscillationof small parts of crosslinkedresin. The nuclear magnetic relaxation process for resin heated at 320°C and higher is believed to be due to paramagneticcenters. The existence of such centers in resin heated around 300°Chas been reported in EPR studies[1416]. It seems likely that during the relaxation process the spin diffusion mechanismis operative and magnetization is transferred alongthe crosslinkedresin to paramagnetic centres which have good thermal contact with the lattice. In this case the rate of relaxation depends on the con-
centrationof the centres[17].Ifthis mechanismworks for resin heated at higher temperatures, then the change of the relaxation time above 320°C (Fig. 5) reflects the change in paramagnetic centres concentration. The greatest number of paramagneticcentres is observed for resin heated at 560°C.Similarheat treatment temperature dependencesof paramagneticcentres concentrationhave been found in EPR studies[W161. For resin heated in the region 60-320°C(Fig. 5) the relaxation process is controlled by rotational motion of polymer chains and the observed increase in T, values results from a stiffeningof the resin due to crosslinkingand molecular weight increase. 5. CONCLUSION The observed decrease of the second moment as well as evidenced minimaof the proton spin-latticerelaxation times reflect the structural and chemical changes taking place during carbonization process of furfuryl alcohol resin. Heating up to 240°Ccauses an increase in molecular weight and the appearance of a number of crosslinks. This effect which is more pronounced after heating at 32o”C,makes the chains unable to perform any rotational motion and manifests itself by insignificanttemperature dependences of the second moment and spin-lattice relaxation times. Heat treatment at higher temperatures causes releasing of hydrogen which is reflected by a further decrease of second moment as well as the appearance of paramagneticcenters evidenced by changes observed in nuclear magneticrelaxation. Acknowledgements-The authorswishto thankMr. Z. Szubafor carrying out the carbonization process, and Dr. K. Jurga for assistance with relaxation times measurements.The work was supported by Polish Academy of Sciences under Project MR.l-4. IWERENCES
E. Fitzer, W. Schaferand S. Yamada,Curbon 7,643 (1%9). : E. Fitzer and W. Schafer,Carbon 8,353 (1970). 3: N. Ozanne, J. Amielland L. Bonnetain, Bull. Sot. Chim. Fr. 1971(1971). 4. D. L. Trimmand B. J. Cooper,J. Chem.Sot. D, 477(1970). K. Antonowicz,Nolure (London), 247,358(1974). :. J. H. Van Vleck,Phys. Rev. 74, 1168(1948). 1: N. Bloembergen,E. M. Purcell and R. W. Pound, Phys. Reo. 73,639 (1948). G. P. Jones, Phys. Rev. 148,332 (1966). :: S. Gfowinkowskiand Z. Paiak. -* Acta Phvs. I Pal.. A54.’ 411 (1978). 10. Z. Pajak, K. Jurga and J. Jurga, Acta Phys. Pal. A45,837 (1974). 11. N. N. Tikhomirovaand V. V. Voevodskii,Opt.Spectroscopy 7,486(1959). 12. E. V. Orobchenkoand N. Yu. Priyanishnikova,Furanouyje S&y, pp. 110-111.Gosiztekh,Kiev (1963). 13. S. Domanusowaand B. Kalinowski,Koks, Smda, Car, 11,91 (1966). 14. C. Jackson and W. F. Wynne-Jones,Carbon 2,227 (1964). 15. D. Campbell,C. Jackson, H. Marsh and W. F. K. WynneJones, Carbon 4,159 (1966). 16. N. Ozanne and L. Bonnetain, Bull. Sot. Chim. Fr. 1976 (1971). 17. P. G. DeGennes,J. Phys. Chem. Solids 3, 345(1958).