Polymer Testing17 (1998) 395–401 1998 Elsevier Science Ltd. All rights reserved

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MATERIAL CHARACTERIZATION

Structural Characterization of the Polymeric Lattice ofFurfuraldehyde Resins

R. Sanchez,a* J. Rieumonta & M. I. Bruno Tavaresb

aAdvanced Material Laboratory, Polymer Section, North Fluminense State University, Campos,Rio de Janeiro, Brazil

bMacromolecular Institute, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil

(Received 3 September 1997; accepted 24 November 1997)

ABSTRACT

Furfuraldehyde resins were synthetized in bulk and in nitromethane solution withsulphuric acid as a catalyst. A comparison between both resins obtained was perfor-med, using solid state nuclear magnetic resonance and thermal analysis. Cross polar-ization and magical angle spinning spectra and variable contact-time experimentsallowed observation of the difference in their polymer lattice domains associatedwith different molecular mobilities. Crosslinking could be ascertained through spinrelaxation time indicating a more stiff and regular structure for the resin obtainedin solution. 1998 Elsevier Science Ltd. All rights reserved

1 INTRODUCTION

The integral utilization of sugar cane process wastes as raw materials is aprocess considered as a self sustained non polluting industry. The furfural-dehyde is a by-product from the sugar cane bagasse which produces resinswith an excellent chemical stability, low swelling, insolubility and possesinga heterogeneous chemical structure.

Recently, furfuraldehyde resins have been tested as adsorbents, looking formaterials to be used with silica for reverse phase in liquid chromatography.1–3

*To whom correspondence should be addressed.

395

396 R. Sanchez et al.

With this purpose in mind, a furfuraldehyde resin was obtained in bulk (FHB)that presented a suitable chemical structure,4 but its irregular morphology didnot help to improve its adsorption perfomance. FHB was characterized by anindirect method such as the thermodegradative technique. It was based onpyrolysis coupled to a gas chromatograph-mass spectrometer (Py/GC/MS) andto Fourier transform infrared equipment (Py/GC/MS) as detectors.5 Thus, theidentification of the pyrolysis products suggested the main polymericsequences of the FHB.4

The reticulated lattice of the FHB stems from trifurylic sequences becausethe furfuraldehyde molecule behaves as a tri-functional monomer and othersthat come from the ring opening. A “pure” trifurylic polymer web wouldresemble a dense grid. Some ring openings have been detected as a result ofthe uncontrolled exothermicity of the resinification under bulk experimentalconditions. This fact contributes to decrease the stiffness of the polymericlattice because aliphatic sequences are introduced. On the other hand thechemical composition is heterogenous due to the presence of several types ofcarbonyl groups and hydroxyl-methynic as a consequence of the occurrenceof those side reactions.

In this context, a new furfuraldehyde resin was obtained in solution (FHS)with the aim of improving its adsorption perfomance. The solvent used poss-eses a dielectric constant similar to furfuraldehyde with the purpose of con-ducting the reaction by the same chemical scheme but eliminating undesirableheating in order to reach a regular morphology with a suitable adsorption per-fomance.

We report here the structural comparison of the FHB and FHS resins usingnuclear magnetic resonance spectroscopy and thermal analysis. The results ofthese investigations will be referred and compared to our previous thermodeg-radative results.

2 EXPERIMENTAL

2.1 Sample preparation

The resins FHB in bulk and FHS in solution were obtained from fresh vacuumdistilled furfuraldehyde (from Merck synthesis reagent) under stirring withsulphuric acid (5%) at 60°C.1,6 Nitromethane was used as solvent.7 Resinswere purified in a soxhlet apparatus with dichloromethane and ethanol con-secutively for 20 hours and dried under vacuum (10−4 Pa) for 4 hours.

397Structural characterization of the polymeric lattice of furfuraldehyde resins

2.2 Thermal analysis

A Rigaku Thermoflex TG 8110 DSC instrument was used. Samples weighing8 mg were used at a heating rate of 10°C/min.

2.3 NMR measurements

Solid state NMR spectra were obtained on a VARIAN VXR 300 spectrometeroperating at 299·9 MHz and 75·4 MHz for1H and13C respectively. The Hart-man-Hahn CP matching and dipolar decoupling field was| 40kHz, whilst a5 msec. 90° pulse for the carbon-13 nucleus was used. All experiments weredone at ambient probe temperature and performed using gated high powerdecoupling. The13C was carried out in the cross-polarization mode with magicangle spinning (CP/MAS) and variation contact time (VCT). A range of con-tact times was established as 50–8000msec. Proton T1r was determined fromthe intensity attenuation of13C peaks with increasing contact time. Carbon-13 spectra are referred to the chemical shift of the methyl group carbons ofhexamethyl benzene (17·3 ppm).

3 RESULTS AND DISCUSSION

Previously, the identification of precursors of the polymeric sequences presentin the FHB resin and their relative abundance was established by thermodeg-radative combined techniques as discussed above. Its structure is characterizedby the presence of trifurylic and ring opened sequences in a crosslinked web(Scheme 1)5 responsible for non-solubility and chemical stability.

There is no reason to expect quite a different chemical structure for theFHS resin. Thus, the reason for turning to a spectroscopic approach is toprovide a more detailed analysis at the molecular level. Selective NMR experi-

Scheme 1. Representative structure of furfuraldehyde resins proposed by NMR and thermode-gradative techniques.

398 R. Sanchez et al.

220 200 180 160 140 120 100 80 60 40 20 0 ppm

Fig. 1. Solid state carbon-13 NMR spectra of FHB resin.

ments8–10 based either on differences in chemical shifts or relaxation times,offer the possibility of measuring some structural features related to crystallineor amorphous phases, components of a blend or copolymer, different layersin a laminated film,11 branching or crosslinking.12 In this context the CP/MASspectra for both resins were recorded (Figs 1 and 2), supporting a similarstructure for both resins except for the lesser intensities of the C-2, C-5 carbonatoms of the furan ring and the appearance of a signal at 80 ppm, probablydue to a hydroxyl-methynic signal in the FHB spectrum that is missing forthe FHS resin.

All other common features of the spectra are: the methylenic carbon atomsCH2 appear at 43·2 ppm, the methynic CH at 58·1 ppm and signals from C-2 and C-5 at 158·8 ppm and C-3 and C-4 at 116·2 ppm carbon atoms of thefuran ring are present. Signals from the carbonylic carbons (C= O) with adifferent environment appear around 170–220 ppm. The difference givenabove indicates that ring opening plays a more important role in the FHB resin.

From the variable contact-time experiment (VCT) the proton spin-latticerelaxation times in the rotating frame was obtained for both resins. Thesetimes depend on the segmental molecular mobility of the system and were

200 150 100 50 0 ppm

Fig. 2. Solid state carbon-13 NMR spectra of FHS resin.

399Structural characterization of the polymeric lattice of furfuraldehyde resins

TABLE 1Proton spin-lattice relaxation time in the rotating frame of furfuraldehyde resins

T1r (msec)Furfuraldehyde resins CH2 C3C4 C2C5

41·2 (ppm) 116·2 151·5

FH (bulk) 3·2 11·5 5·7FH (solution) 3·0 9·6 4·6

able to detect homogenity relative to domains of small chain segments up to2 nm. Table 1 shows the T1r values for some proton hydrogens linked todifferent resolved carbon atoms as assigned in the structure of the furan resins.As a rule, slower relaxation takes place on the lattice of the FHS resin. Thus,the synthetic procedure succeded in producing a more regular lattice for thisresin. Thus, this resin resembles an intricated trifuryl web in comparison tothe FHB that possesses more ring opened sequences between the trifurylicones, giving a more flexible waving lattice.

Figure 3 shows the distribution form of the carbon-13 decays during thecontact-time experiment. From the figure it is clear that more than onemicrodomain is present and also a high concentration of signals at short con-tact-times (rigid domains). The signals of the FHS are located at shorter timesthan for FHB.

Other significative differences in the spin-lattice relaxation times is relatedto the furan rings whose mobility is more affected because they are associatedto the domain of the tridimensional trifurylic structure. However, the methyl-ene groups are mainly associated with the ring opened structures and itschange in mobility is less dramatic.

The thermogravimetric curve (Fig. 4) for FHS around 100°C showed asimilar behaviour to FHB.13 It presents a defined endothermic transitionaccompanied with mass loss of 6% of the total weight sample in comparisonwith FHB loss already reported (5·6%). It was identified as corresponding to

Fig. 3. The FHS carbon-13 decay signal during the contact time experiment.

400 R. Sanchez et al.

Fig. 4. DSC-TG thermogram of FHS resin.

water adsorption.7 However, some differences could be observed on the onsetof the mass loss (around 180°C) for the FHS consisting of a smoother andmore uniform trend than FHB for which the weight loss was around 160°C.These facts are in agreement with a more regular structure for FHS resin.

4 CONCLUSIONS

The synthesis of furfuraldehyde resins could be conducted in order to obtaina more regular chemical structure by avoiding or decreasing side reactionssuch as ring opening. Those colateral reactions were shown by solid statecarbon-13 magnetic resonance to be related to more flexible microdomains inthe polymeric lattice. Relaxation times clearly indicate a more intricated poly-meric web for the furfuraldehyde resin obtained in solution.

401Structural characterization of the polymeric lattice of furfuraldehyde resins

ACKNOWLEDGEMENTS

The authors are grateful to PETROBRAS/CENPES/DIQUIM, for the NMRfacilities and UENF/FENORTE for the financial support of this work.

REFERENCES

1. Sanchez, R., Ph.D. thesis, National Center for Scientific Research (CENIC), Hav-ana, Cuba, 1988.

2. Sanchez, R. and Herna´ndez, C., 2nd Brazilian Polymer Congress, 1, 1993, 70–72, Sao Paulo.

3. Sanchez, R. and Herna´ndez, C.,Eur. Polym. J., 1994,30, 51–54.4. Sanchez, R., Herna´ndez, C. and Kerestury, G.,Eur. Polym. J., 1994,30, 43–50.5. Sanchez, R., Herna´ndez, C., Jalsovszky, G. and Czira, G.,Eur. Polym. J., 1994,

30, 37–42.6. Sanchez, R., Cuban Patent No. 21644, 1986.7. Sanchez, R., Neves, S. and Almeida, J. R.,MRS Symposium Proceedings, 1996,

431, 533–537.8. Tavares, M. I. B., Castro, W. P. and Costa, D. A.,J. Appl. Polym. Sci., 1995,

55, 1165–1171.9. Amorin, M. C. V., Oliveira, C. M. F. and Tavares, M. I. B.,J. Appl. Polym. Sci.,

1996,61, 2245.10. Komoroski, R. A.,High Resolution NMR Spectroscopy of Synthetic Polymers in

Bulk. VCH Publishers, Inc., Deerfield Beach, Florida, 1986.11. Clayden, N. J., Eaves, J. G. and Croot, L.,Polymer, 1997,38(1), 159–162.12. Stejskal, E. O. and Memory, J. D.,High Resolution NMR in the Solid State.

Oxford University Press, New York, 1994.13. Sanchez, R., Herna´ndez, C., Rosado, A. and Fajardo, D.,Ciencias Quı´micas,

Review CENIC, 1988,19, (1–2–3).