Nanocellulose was synthesized according to the method of Candan et al. (2016). The nanocellulose used in this study is bleached softwood kraft pulp. Nanocellulose was string-like particles with a diameter of approximately 20 nm and a length of 1 µm. The nanocellulose used in this study was obtained from bleached wood pulp using the oxidation mediator 2,2,6,6-tetramethyl-1-piperidin-1-oxyl (TEMPO method), which oxidizes some alcohol groups in the cellulose chain to carboxylic acids, and uses ionic repulsion to help separate the fibrils used.

Nanocellulose, pyrolysis oil, which is 5 %, 10 % and 20 % of the mass of nanocellulose, and distilled water were mixed. The resulting mixture was kept in an ultrasonic bath for 30 min. The obtained homogeneous mixture was grown by pouring into petri dishes and removing the water at 40 °C. It also prepared a control sample consisting of only nanocellulose. The contents of the nanocomposites are shown in Table 1 and the visual representing their preparation is shown in Figure 1.

Figure 1:
Design and manufacture of NCPO nanocomposite films.

Table 1:
Formulation of NCPO nanocomposites.

TGA, DSC and DMTA were used to determine the thermal properties of the nanocomposite films. TGA analysis, Şen et al. (2022) was performed under the conditions previously stated in the literature. DSC analysis was performed between 30 °C-350 °C and additionally in nitrogen atmosphere and with a heating rate of 10 °C/min. Determination of viscoelastic properties was carried out in the HITACHI device between 30-250 °C temperatures, 1 Hz frequency and 5 °C/min heating rate. The morphological structure of NCPO nanocomposite films was determined using SEM (TESCAN brand). Before analysis, the surface of the samples was coated with platinum.

The thermal stability of NCPO nanocomposite films was investigated using TGA. The thermal degradation curve obtained because of the characterization is similar to the curve also called the cellulose pyrolysis curve in the literature. It is known that the cellulose pyrolysis curve is affected by chemical and physical effects, impurities, cellulose crystallinity, type of cellulose, the atmosphere measured, temperature and heating time (Radakisnin et al. 2020). When the thermogram for Figure 2 is examined, it is seen that all nanocomposite samples start to lose weight between temperatures between about 49 °C and 118 °C. In addition, the thermal degradation of the nanocellulose started at lower temperatures than obtained the nanocomposites. From this, it appears that pyrolysis oil needs higher temperatures for its thermal cracking. The losses at these temperatures are related to the moisture content of the nanocellulose (Priyadharshini et al. 2019). In the next region, while the volatile substances in the nanocellulose structure were removed at the weight loss, the pyrolysis oil began to decompose. When the literature is examined, the main degradation temperature of nanocellulose is around 200-300 °C, which shows that the results of our study are consistent (Gan et al. 2020). Table 2 shows the summary of data from TGA analyzes of nanocellulose-based nanocomposites. The main decomposition temperature of all samples is generally observed between 319 °C and 350 °C. The degradation at these temperatures is attributed to the thermal degradation of the long chains in the cellulose structure. The amount of ash remaining as a result of nanocellulose degradation is less than the amount of ash left in the nanocomposite samples. The reason for this situation is that nanocellulose decomposes more than pyrolysis oil and turns into a gaseous product. The addition of pyrolysis oil to the nanocellulose caused a partial increase in the degradation temperature. When all the results were examined, the highest thermal stability was observed in the NCPO-20 sample with 20 % pyrolysis oil.

Figure 2:
TGA curves (a) and DTG curves (b) of NCPO

The DSC curve of nanocomposite films reinforced with pyrolysis oil is shown in Figure 3. Two important endothermic peaks are observed around 80 °C and 350 °C. For the peaks observed around 80 °C, the increase in the amount of pyrolysis oil doped in the nanocellulose-based nanocomposite caused a partial shift in the endothermic peaks and an increase in temperature. Since pyrolysis oil is obtained using waste products, it is difficult to detect thermal transitions in produced PO-doped nanocomposites compared to other materials (Rajasekaran et al. 2021). Subsequent endothermic peaks observed around 350 °C indicate the degree of cellulose decomposition. Increasing the decomposition temperature has been associated with higher thermal stability, and this behavior can be attributed to a high degree of material crystallinity (Draman et al. 2014).

Figure 3:
DSC chart of NCPOs.

Table 2:
Thermal properties of NCPO nanocomposite films.

DMTA is frequently used to determine the damping properties of polymer nanocomposites depending on temperature (Kumar et al. 2019, Soudmand et al. 2020). In this study, DMTA analysis was used to determine the rheological properties of NCPO nanocomposites. The variation of the storage module E' as a function of temperature is shown in Figure 4. For pure NC, the initial storage modulus E' was determined to be 0,05 GPa. The effect of pyrolysis oil added to the NC matrix on the storage modulus of nanocomposites is clearly seen in Figure 4. Increasing the amount of pyrolysis oil doped into the nanocomposite resulted in extraordinary increases in the storage modulus values. According to the initial storage modulus values, the highest storage modulus value was observed in the NCPO-20 sample with 0,79 GPa. Moreover, the initial storage modulus of the fabricated nanocomposites was determined as 2,8-fold for NCPO-5, 8,4-fold for NCPO-10, and approximately 20-fold for NCPO-20, in parallel with the increasing amount of pyrolysis oil compared to pure NC. Increasing the temperature from the initial temperature up to 100 °C did not decrease the storage modulus values of the nanocomposite samples. On the contrary, increasing PO amount and temperature resulted in excellent increases in storage modulus values. When all nanocomposite samples were examined, the highest storage modulus was in the NCPO-20 sample at approximately 100 °C 1,48 GPa. Additionally, at around 100 °C, the storage modulus of NCPO-20 was exactly 18 times that of pure NC. A similar situation was observed in the loss modulus (E'') values of nanocomposites. As shown in Figure 5, as with the Storage modulus, temperature and increasing PO amount caused an increase in the loss modulus values. Karabork and Tipirdamaz (2016) investigated its properties by adding pyrolysis oil obtained from waste tires to the mixture of natural and synthetic rubber. DMTA analysis results showed an increase in elastic behavior with increasing PO content. They attributed this increase to increased crosslinking density. Tan δ the loss coefficient is known as a measure of the damping of the material (Cho and Park 2011). Figure 6 shows the tan δ of NCPO nanocomposites as a function of temperature. The tan δ values of all NCPO varied between approximately 0,04 and 0,12. When the loss coefficient in the prepared nanocomposites is compared with pure NC, a general decrease in Tan δ peaks is observed. The tan δ peak height value for NCPO-10 nanocomposite has a minimum value compared to other prepared nanocomposites. The decrease in tan δ peak height indicates that the molecular chain structure of the reinforced PO is hindered by the additives (Thomas et al. 2021, Boonbumrung et al. 2016). The storage modulus and loss modulus values of nanocellulose-based nanocomposites are summarized in Table 3 and Table 4.

Figure 4:
Storage modulus (E') plot of nanocellulose-based nanocomposites.

Figure 5:
Loss modulus (E'') plot of NCPO nanocomposites.

Figure 6:
Tan δ of NCPO nanocomposites.

Table 3:
Storage modulus and viscous-rheological properties of NCPO nanocomposite films

Table 4:
Loss modulus and viscous-rheological properties of NCPO nanocomposite films.

The results acquired from the DMTA analysis revealed that the storage modulus values of the reinforced nanocomposites were higher than that of the control group. All the storage modulus values of the nanocomposite films or control group increases while the temperature increases from 30 °C to around 125 °C. It might be attributed to water removal from the nanocellulose-based materials (Candan et al. 2016, Poyraz et al. 2023). Previous studies done by Candan, Poyraz indicate that lignocellulosic materials have both cell wall water (bound water) and capillary water (free water) (Thybring et al. 2022). When it moves from the structure, the materials are going to be mor stiff. When the temperature increased from around 100 °C to 250 °C, the storage modulus values of all the composites decreased. The reduction rate increases while the PO loading ratio increases.

This analysis was performed to determine the effect of nanocellulose on the morphological surface of the nanocomposite films, the presence of PO and its appearance in the structure. SEM analysis is important in determining the morphological properties of nanocomposite films (Syafiq et al. 2020). Figure 7, different magnifications for (a) NCPO-5 with 5 % PO, (b) NCPO-10 with 10 % PO, and (c) NCPO-20 with 20 % PO at different magnifications (x 5000, x 10000, x 20000, and x 50000) shows SEM micrographs of NCPO nanocomposite films. All SEM images clearly show the effect of TEMPO-oxidized cellulose on NCPO nanocomposite films. Strong repulsive forces affect the nanofibrils within the cellulose cell wall. Increasing the ionic strength in its environment weakens the effects of the repulsive forces. This results in a reduction in fiber width. On the contrary, the repulsive forces become stronger as the ionic strength decreases. The strengthening of the repulsive forces causes the cell walls to separate from each other. This separation results in an increase in lateral dimension (Levanic et al. 2020, Isogai et al. 2011, Fall et al. 2011). The driving force effect mentioned here may explain the images observed in the morphological structure of the NCPO films produced. SEM photographs consist of dense pyrolysis oil dispersed in a nanocellulose matrix. The PO is mixed homogeneously throughout the matrix. The homogeneous mixture is the reason for the strong interfacial bond between the PO and the nanocellulose matrix. Increasing the percentage of pyrolysis oil resulted in the bleached area in the images. In their study, Syafiq et al. (2020) showed the scattering of light by the effect of cavities as the cause of this whitening. Moreover, they reported that the mechanism of the formation of cavities may be due to the mixing method by cavitation of oil. In general, it can be said that increasing the amount of pyrolysis oil reinforced into the nanocomposite leads to a decrease in the pore concentration and size.

Figure 7:
Morphological structure of NCPO nanocomposites.

^♠Corresponding author: mustafa.zor@beun.edu.tr