Raman spectroscopic investigation of tannin-furanic rigid foams☆
Introduction
One of the most innovative polymeric materials proposed in the last years are tannin-based rigid foams (see Fig. 1), [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13] which can be produced from naturally occurring resources through different formulations of compounds. In the case of tannin-furanic rigid foams these are condensed tannin (e.g. Mimosa tannin or Quebracho tannin) [14], [15], furfuryl alcohol [14], [15], [16], [17], [18], and an acid as polymerization catalyst.
The density of these blackish appearing foams generally governs their physical properties [1], [13]. Compression resistance (a tannin-furanic foam of 200 kg/m3 density has a mechanical compression resistance of up to 1 MPa) and water absorption (a lighter foam of 50 kg/m3 can absorb water more than seven times the dry weight of the foam) are properties, among others, which have already been studied in depth [1], [2], [5], [9], [11], [12], [13].
Further basic characterization of tannin-furanic foams was carried out considering more application-oriented properties [2], [8], [11], [13]. These foams have very low thermal conductivity (0.030–0.040 W m−1 K−1). They can easily resist strong acidic and basic conditions, hence showing remarkable chemical resistance. Moreover, tannin-furanic foams show high fire resistance, extreme fireproof behavior being obtainable using phosphoric acid as polymerization catalyst [2].
According to these interesting properties, several applications for these materials have been investigated or proposed [3], [4], [19], e.g. as insulation material for eco-sustainable buildings (green building technology) [5], [11], or as adsorbent for wastewater treatment [12]. Present developments include the use of tannin as precursor for carbon polymerized High Internal Phase Emulsions (carbon polyHIPE or carboHIPEs) [20].
The most important precursor for the formation of the tannin-furanic foam polymer is the tannin itself [13], [14], [15], [21], [22], [23], [24], in the present case Mimosa tannin, which is a representative for condensed tannins. It is a complex oligomer mostly composed of flavan-3-ol repeating units, in this case predominantly prorobinetinidin flavonoids, having a great amount of aromatic carbons (resorcinolic A-ring and pyrogallic B-ring) and of hydroxyl groups [21].
Characterizations using MALDI-ToF [6], [7], [16] and solid state 13C NMR [11], [25], [26] were performed on tannin-furanic foams in order to find information about the copolymerization between flavonoid groups in tannin, furanic groups derived from furfuryl alcohol, and formaldehyde used as hardener, showing that the skeleton of these polymers is constituted by flavonoids and furans randomly arranged during the condensation reaction that produces them [1], [7]. Additional information was deduced regarding the dimensional arrangement with a networked crosslinking and the presence of some methylene bridges.
Since Raman spectroscopy [27], [28] revealed itself also advantageous in order to analyze flavonoids [29], [30], [31], [32], [33], tannins and wood extracts rich in flavonoids [34], [35], and tannin-impregnated wood species [36], or in order to discriminate the content of different flavonoids and carotenoids in petals of pansy cultivars [37], and in order to distinguish between recent and fossil resins [38], or between lignin parts and leather parts in wood-leather panels [39], we decided to apply this spectroscopic technique to receive additional information on the chemical rearrangement of flavonoids and furans after formation of the tannin-furanic rigid foam, and possibly establish a link to results obtained by infrared spectroscopy in similar materials [12], [40], [41], and by Raman spectroscopy in sp2 carbon-based materials [42], [43], [44], [45].
It turned out that the analysis of tannin-furanic foams by Raman spectroscopy is a non-trivial task; on the one hand, due to their strong light absorbing property and their sponge-like structure it is not possible to use high laser intensities (in particular in the near-infrared spectral region), because this would result in thermal degradation of the sample area exposed to laser light; on the other hand, the polymeric structure contains sp2 bonded atoms and a rather high hydrogen content, which cause, depending on the laser excitation wavelengths used (in our case 1064 nm, 532 nm, and 455 nm), the appearance of a variable degree of photoluminescence.
We will demonstrate that the use of multi-wavelength Raman spectroscopy for the tannin-furanic foam investigation can help to somewhat unscramble the structure of tannin-furanic foam.
Section snippets
Polymeric sample preparation
Furfuryl alcohol polymerization was accomplished according to the following procedure [6], [7], [40]: 2 g of furfuryl alcohol and a few drops of 65% solution of p-toluensulphonic acid in water were let react in a test tube. After two minutes of constant stirring the original yellow color (see e.g. Refs. [18], [46]) turned into black and, within a few seconds, a highly exothermic polymerization reaction took place. After reaction completion, the black polymer was scratched out from the test tube
Results and discussion
For translating spectral information into chemical information, we analyzed the Raman bands of the precursor molecules and extrapolated the information obtained in order to interpret the more complex polymeric structures of our tannin-furanic foam. Additionally, simulated Raman spectra calculated on the basis of basic molecular structures for the polyfurfuryl homo-polymer and for the tannin-furanic hetero-polymer have been compared with the experimental results, yielding a reasonable agreement.
Conclusions
Complementary to previous infrared and Raman investigations of the tannin-furanic foam [12], [40], the use of multi-wavelength Raman spectroscopy at 455 nm, 532 nm and 1064 nm has helped to obtain some additional information on this polymer-like sp2 carbon-based material resulting from the hetero-polymerization between Mimosa condensed tannin and furfuryl alcohol. We find reasonable agreement between the experimental Raman spectra obtained at 455 nm and at 532 nm laser excitation and the calculated
Acknowledgements
The authors thank Franz Renz and Nicola Hüsing for advice and fruitful discussions. Prof. Dage Sunholm from the University of Helsinki is acknowledged for the generous provision of computational resources. Additionally, our thanks go to Massimiliano Rocchia from Thermo Fisher Scientific for giving us the opportunity to use the Thermo DXRxi Raman imaging microscope equipped with the 455 nm laser.
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Selected paper from 8th International Conference on Advanced Vibrational Spectroscopy, 12–17 July 2015, Vienna, Austria.