Effect/optimization of stabilization gas atmosphere

The stabilization of lignin fibers with and without H3PO4 was studied under air

or N2 atmosphere. It is known that stabilization of pure lignin fibers have to be carried

out at lower heating rate in oxidizing atmosphere to prevent the fusion of lignin fibers (Figure 4.1b) [14,15,18]. In presence of inert atmosphere a complete fibers fusion is produced, even at low heating rates (Figure 4.1c), indicating that the stabilization of lignin fibers necessarily requires oxidation to increase the glass transition temperature and prevent fusion [18]. The use of H3PO4 in the initial solution, as it was observed in

Chapter 3, allows the faster stabilization of lignin fiber under oxidizing or inert or atmosphere (Figure 4.1e and 4.1f) due to the formation of phosphate esters in the lignin structure. This extraordinary effect opens the way for a fast and simplified manufacture of CFs by using only one atmosphere.

Figure 4.1. SEM images of as spun, stabilized lignin fibers under air and inert atmosphere. Pure lignin fibers (a-c), phosphorus-containing lignin fibers (d-f)

a) AF

b) SF-0.08-200-50

c) SF-0.08-200-50-N

Optimization of the thermo-stabilization conditions

The stabilization yields and the analysis of surface chemical composition are shown in Table 4.1. The results reveal that, in the absence of H3PO4, the stabilization

under inert conditions (SF-0.08-200-50(N2) sample) reduces the oxygen content in the

as-spun fibers (AF) to the half (ca. 16 wt%O). On the contrary, the stabilized H3PO4/lignin fibers under inert atmosphere (PSF-1-200-1(N2)) show practically the

same chemical composition to that of as-spun ones (PAF) and fibers stabilized under air atmosphere (PSF-1-200-1).

Table 4.1. Stabilization yields and weight surface composition by XPS of as spun and stabilized fibres.

Sample Stabilization yield (%)

Mass surface composition by XPS (wt%) C O P AF − 73.4 26.6 --- SF-0.08-200-50 76 68.0 32.0 --- SF-0.08-200-50(N2) 74 83.6 16.4 --- PAF -- 64.5 28.3 7.2 PSF-1-200-1 83 64.1 29.5 6.4 PSF-1-200-1(N2) 82 63.4 28.3 8.3

The mechanism of thermo-oxidative thermostabilization of lignin has been studied by different authors [15,18,31-35]. The observed weight-losses in stabilization steps have been attributed to condensation and dehydration reactions during cross- linking reactions, the evaporation of occluded ethanol, and the release of some oxygen groups (as CO and CO2) [15,18,33]. On the other hand, it is generally accepted that at

temperatures below 250 ºC, the oxidation of lignin results in an apparent increase in oxygen content [18,33], although this oxygen gain is low since lignin presents an already oxidized structure. Hence, the larger mass loss observed when lignin is treated in N2 atmosphere may be explained considering that under these conditions only

volatilization occurs [36], whereas under air atmosphere the heat-treatment of fibers produces volatilization together with oxygen uptake (mass gain), therefore, showing a lower net mass loss. Lower stabilization yield are observed for pure lignin fibers, in these cases the higher stabilization times produce an increase in the cross linking reactions and on the surface oxidation.

Figures 4.2a and 4.2b show the C(1s) and O(1s) core-level XPS spectra, respectively, of the as-spun lignin fibers (AF), the fibers subjected to thermostabilization heat-treatment under air (SF-0.08-200-50) or N2 atmosphere

(SF-0.08-200-50-N2). The C(1s) spectrum of as-spun fibers (Figure 4.2a) is composed

of two overlapped main contributions at 284.5 eV and 286.0 eV which correspond to C=C/C–C and C–O (C-OH and C-O-C) groups, respectively [14,37]. The presence of these oxygen functionalities agree with the quite symmetric O(1s) spectrum (Figure 4.2b) centered at ca 532.6 eV, associated to oxygen (*) in C-O*H, RO-C=O* and -R-O*-R- groups [15,37]. These spectra features well fit with the molecular structure of lignin and its main (p-hydrophenyl (H), guaiacyl (G), and syringyl (S)) units, with a varying degree of methoxylation [38,39].

Figure 4.2. XPS spectra of C(1s), O(1s) and P(2p) core-levels of as-spun and stabilized fibers under oxidizing or inert atmosphere.

After stabilization in air, the C(1s) of SF-0.08-200-50 spectrum shows a relative decrease in the intensity of C–O contribution (286.0 eV) and the appearance of a new band centered at ca. 288.4 eV with a small tail near 290.0 eV (Figure 4.2a). This spectral feature can be attributed to the formation of –C=O (287.6 eV) and –O–C=O

Optimization of the thermo-stabilization conditions

(at 289 eV) bonds in carbonyl or anhydride, ester and/or carboxylic groups, respectively, these groups are generated after cross linking reaction [15,18,37]. This change is in agreement with the O(1s) broadening towards lower binding energies (530.9 eV in Figure 4.2b), where the carbonyl groups are increased after stabilization. By contrast, the spectrum of fibers stabilized in N2 (SF-0.08-200-50-N2) involves the

decrease in the intensity of C–O contribution (286.0 eV) in C(1s) spectrum (Figure 4.2a) and that of the whole O(1s) spectrum (Figure 4.2b). These results confirm that the heat treatment of lignin fibers in N2 atmosphere only produces the release

(volatilization) of oxygen functional groups, without oxidation or crosslinking reactions, avoiding any possibility of stabilization, and subsequent production of CFs, under these conditions.

XPS characterization for phosphorus containing fibers (Figures 4.2c - 4.2e) reveals slight but meaningful differences between the surface chemistry of the different as-spun fibers. Compared to the pure fibers (AF), the H3PO4/lignin fibers (PAF) show a

lower relative intensity but better defined contribution at 286.0 eV in C(1s) spectrum, together with an increase at 532.6 eV of the O(1s) spectrum. These changes may involve a relative increase in C-O-P species due to the presence of H3PO4. In this sense,

PAF fibers show a symmetric band in the P(2p) spectrum (Figure 4.2e) centered at 134.2 eV, which exactly corresponds with the binding energy reported for C-O-P species in C-O-PO3,(C-O)2PO2 or (C-O)3PO groups [40-43]. These results suggest that

H3PO4 may react with the oxygen functionalities of lignin (C-OH) to form different

phosphates and/or polyphosphates. Paying attention to the stabilization atmosphere, slight differences have been found, having practically identical composition to those of as-spun phosphorus fibers. A slight decrease at 532.6 eV in O1s spectra is found due to the release of C-O-C and C-OH groups. A higher increase in carbonyls groups is observed when O1s spectra of SF-0.08-200-50 and PSF-1-200-1 are compared. In this case, the long stabilization of pure lignin fibers (more than 80 hours) favors the carbonyl groups generation. These groups have been found the responsible to avoid the fiber fusion in carbonization step [14,15,18], but when H3PO4 is added to the initial

solution, phosphates esters generated are enough to prevent the fibers fusion. Furthermore, the P(2p) bands of both stabilized fibers are centered approximately at the same binding energy to that of PAF (Figure 4.2e), although the spectrum of fibers stabilized in air (PSF) is slightly shifted towards lower binding energies, producing

some phosphorus in a more reduced state as in C-PO bonds, as a consequence of dehydration and cross-linking reactions.

Differential scanning calorimetry (DSC) was used to study the influence of phosphorus functionalities in the thermal behavior of the electrospun lignin fibers. This technique can quantitatively analyze both physical transitions and chemical reactions.

The particularly interesting but complex thermal behavior of lignin, including Alcell® lignin, has been previously studied [44-47]. It is well known that these thermal properties remarkably depend on the history of the sample (extraction/purification treatments, storage temperature and storage time, heating and/or cooling treatments, etc.). In this sense, the thermal behavior of the electrospun lignin fibers is expected to be greatly influenced by the experimental steps followed for their production (the procedure for the preparation of the ethanolic solution and the electrospinning process) and, therefore, somehow different to that of the pristine Alcell® lignin powder.

Figure 4.3 depicts the DSC thermograms of the as-spun pure and H3PO4-containing lignin fibers under different atmospheres. As it can be observed,

independently of the presence of H3PO4 or the used atmosphere, the thermograms show

two-three clear subsequent heat-exchange processes that are attributed to characteristic physical transitions on plastic polymeric materials, namely, glass transition, crystallization and melting [48,49]. The corresponding glass transition temperature (Tg), crystallization temperature (Tc) and melting temperature (Tm) were recorded as the midpoint temperature of the corresponding heat flow transitions and are collected in Table 4.2.

Independently of the used atmosphere, the glass transition of both the as-spun H3PO4–free and H3PO4–containing lignin fibers occur at low temperatures, ranging

between 57-63 ºC (Table 4.2). These Tg values are lower compared to those reported for Organosolv lignins (between 90-130 ºC) in literature [45-47], highlighting the importance of the stabilization treatment to produce lignin-based CFs. Nevertheless, some clear effects of H3PO4 and the atmosphere on this process can be observed: (i) the

Tg values of both type of lignin fibers in air atmosphere are higher than in inert conditions; (ii) differences are higher for the fibers without H3PO4; (iii) in air

Optimization of the thermo-stabilization conditions

H3PO4 the glass transition is accompanied by a considerably higher heat exchange and

enthalpy relaxation [49-50].

After glass transition, further heating induces crystallization of the different lignin fibers (Figure 4.3). This process is assigned to the rearrangement/ordering of polymer chains and, as observed in the Figure 4.3, it is strongly influenced by the introduction of H3PO4 in the fibers. In particular, the presence of H3PO4 hinders this

structural transition in both air and N2 atmospheres, so that the lignin fibers need to

achieve considerably higher temperatures (around extra 60 ºC) to experience it (Table 4.2). Moreover, the heat release associated to this process is, like in the case of glass transition, much higher when H3PO4 is incorporated in the fibers. On the other

hand, the influence of atmosphere in this process seems to be negligible for both lignin and lignin/H3PO4 fibers.

Figure 4.3. DSC of as-spun lignin (AF) and lignin/H3PO4 (PAF) fibers under (a) air or (b) N2 atmosphere

(HR = 2 ºC·min-1).

Table 4.2. Glass transition (Tg), crystallization (Tc) and melting (Tm) temperatures calculated from DSC measurements and heat flows (integral).

Sample Tg (ºC) Integral (J/g) Tc (ºC) Integral (J/g) Tm (ºC) Integral (J/g) AF-air 63.13 -5.39 107.43 -2.64 141.67 4.26 AF-N2 56.63 -5.22 107.27 -1.56 140 -0.49 PAF-air 60.67 -49.86 167 -6.1 -- -- PAF-N2 59.07 -35.30 165.80 -45.16 -- --

Finally, the DSC curves of pure lignin fibers show a third heat exchange process after crystallization, which has been attributed to polymer melting. In the case of fibers

stabilized in air, the process occurs at ca. 142 ºC and is found endothermic; while in inert conditions it is observed as an exothermic process at practically the same temperature (ca. 140 ºC). By contrast, the presence of H3PO4 seems to suppress (at least

below 250 ºC) or to delay (at higher temperatures) the melting transition.