Chemorheological analysis and model-free kinetics of acid catalysed furfuryl alcohol polymerization Nathanael Guigo, Alice Mija,* Luc Vincent and Nicolas Sbirrazzuoli Received 25th May 2007, Accepted 24th July 2007 First published as an Advance Article on the web 8th August 2007 DOI: 10.1039/b707950h The complete curing of furfuryl alcohol (FA), was studied by chemorheological analysis and model-free kinetics under isothermal and non-isothermal modes. Polymerization of FA under acidic catalysis involves complex reactions, with several steps (such as condensations and Diels–Alder cycloadditions). To account for the polymerization complexity, kinetic analysis of DSC data was performed with a model-free isoconversional method. The obtained E a -dependencies were closely-correlated with the variation of complex viscosity during curing. Linear condensations are predominant during the early curing stage and are followed by two distinct stages of branching cycloadditions. Gelation and vitrification, identified by rheometric measurements, were associated with a decrease of the overall reaction rate that becomes controlled by diffusion of small oligomers. Before vitrification, the rate of crosslinking is limited by the mobility of longer polymer chains and diffusion encounters a large energy barrier due to the cooperative nature of the motions, leading to higher E a values. Introduction Vegetable biomass represents a sustainable solution to replace petroleum-based chemicals. In particular, hemicellulose con- stitutes an important source of monomers such as furfuryl alcohol (FA). This latter is obtained by conversion of furfural and can react with Brønsted or Lewis acidic catalysts to form a black, cross-linked macromolecule. 1 At an industrial scale, FA is usually converted into prepolymers and these furanic resins represent excellent eco-friendly precursors for wood impreg- nation 2 and for elaborating composite materials 3,4 or wood adhesives. 5 Currently, for this type of resin, the control of prepolymerization is an important issue and requires careful application of processing conditions. A perfect knowledge of the chemorheological behaviour of the reactive polymer sys- tem during processing is very important in order to determine optimum process parameters, adapted to the aimed applica- tion. As it is the case for many thermosets, viscosity of FA prepolymers is strongly dependent on the evolution of tem- perature and the extent of conversion during curing. The variation of the viscosity is also a key parameter which may govern the chemical reactions at the microscopic scale. The polymerization reaction of furfuryl alcohol has been previously studied under different experimental conditions. 6–13 The reported step-growth curing mechanisms can be separated into two stages. In the first stage, under acid catalysis, the methylol group of one furan ring condenses with the C5 position of another furan ring with dehydration. 1 Furan rings connected by methylene linkages create linear oligomers, 14 as shown in Scheme 1. In the second stage, these linear oligomers are cross-linked into black materials. It is postulated 3,6,10,15,16 that these oligomers are branched together mostly due to Diels–Alder cycloadditions between the furan rings (diene) and the dihydrofuranic cycles (dienophile). Despite these previous investigations, the overall kinetics of this complex curing process (i.e., linear growth of chains and crosslinking) still remain imperfectly understood. According to Milkovic, 17 the overall mechanism involves many steps that are likely to have different activation energies. The contribution of these steps into the overall cure rate should generally vary with both temperature and extent of curing. This means that the effective activation energy determined from the overall rate measure- ments is likely to be a function of these two variables. A kinetic study based on empirical models could be ineffective if the reaction models are unknown. For this reason, an alternative solution is to use model-free isoconversional methods. These methods require no hypotheses on the reaction mechanism and allow for evaluation of the apparent activation energy as a function of the extent of conversion. Consequently, changes in curing mechanisms are associated with the variation of appar- ent activation energy. The objective of the present study is to get a better under- standing of the curing behaviour of FA and to highlight changes in mechanisms during both resinification and cross- linking. For this purpose, the curing of FA catalyzed with maleic anhydride (MA) was investigated by infrared spectro- scopy (IR), rheometry and by differential scanning calorimetry (DSC) under both isothermal and non-isothermal conditions. DSC data were treated with an advanced isoconversional method in order to yield the dependence of activation energy on conversion. To our knowledge, this work reports the first chemorheological study combining model-free kinetics and rheological data for the study of FA polymerization. In particular, we demonstrate that this type of analysis allows us to obtain consistent results from isothermal and non- Thermokinetic Group, Laboratory of Chemistry of Organic and Metallic Materials C.M.O.M., Institute of Chemistry of Nice, University of Nice—Sophia Antipolis, 06108 Nice Cedex 2, France. E-mail: mija@unice.fr This journal is  c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 5359–5366 | 5359 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics