Preparation and characterization of CNC-CA
The preparation route and reaction mechanism of CA modified CNC-OSO3H are presented in Scheme 1. Large amount of hydroxyl group on the surface of CNC-OSO3H reacted with carboxyl group of CA and sulfonate of CNC-OSO3H was replaced by carboxyl group during the reaction. The FTIR spectra of CNC-CA and CNC-OSO3H were provided in Fig. 1a. It is worth noting that all CNC-CA were washed thoroughly to eliminate all residual reactants before analysis. As shown in Fig. 1a, in the FTIR spectra of CNC-CA, there is a new band appeared at 1725 cm-1 which corresponds to the C = O vibration from the ester groups of the carboxylic acids (Yu et al. 2014). Compared with the spectrum of CNC-OSO3H, the band at 812 cm-1 which corresponds to C-O-S vibration disappeared in CNC-CA (Wang et al. 2020). Those evidences suggest that the CA have been grafted successfully on CNC-OSO3H and most of the sulfonate on the surface of CNC-OSO3H was removed during the modification process. The removal of sulfonate was also verified by EDS elemental analysis, as shown in table S1, after the surface modification, no sulfur element can be detected.
The crystalline structure of CNC-CA and CNC-OSO3H were analyzed by X-ray diffractions. Both CNC-CA and CNC-OSO3H show diffraction peaks at 2θ of 14.8°, 16.4° and 22.7°, corresponding to the typical X-ray diffraction pattern of form-I crystal of cellulose. Based on the above investigation, it is confirmed that CNC-CA has same crystalline structure as CNC-OSO3H. The application of CNC-OSO3H in polymer compounds is limited due to the poor heat resistance. Here the thermal degradation of CNC-CA and CNC-OSO3H was compared by thermogravimetric analysis (TGA). As shown in Fig. 1c. and d, CNC-CA displays a markedly enhanced thermal stability compared with CNC-OSO3H. The maximum weight-loss rate temperature of CNC-CA is 354°C, which is 143°C higher than that of CNC-OSO3H. According to the FTIR results of the CNC-CA and CNC-OSO3H, it is deduced that removal of the sulfate groups is a crucial factor in improving the thermal stability of CNCs.
The morphology of CNC-OSO3H and CNC-CA were observed by using TEM (shown in Fig. 2a and b). The individual cellulose nanocrystals of both CNC-OSO3H and CNC-CA can be clearly distinguished. Compared with CNC-OSO3H, the surface of CNC-CA is much more blurry because the surface of CNC-CA is covered with small molecules of citric acid. Because of the strong hydrogen bond interaction between the carboxyl groups of citric acid, CNC-CA is more likely to agglomerate after drying. The average particle sizes of CNC-OSO3H and CNC-CA were estimated by dynamic mechanical analyses (DLS), the values are around 187 nm and 161 nm, respectively. The decrease of the size of cellulose nanocrystals after modification may be due to the further etching of the amorphous region on the surface of CNC-OSO3H by CA.
As mentioned in the experimental section, after surface modification of CNC-OSO3H, un-grafted CA can be separated and reused. To examine the effectiveness of recycled CA, CNC-CA are prepared by using pristine CA, first and second recycled CA as the modifiers. The results are shown in Fig. S1a and b. It can be observed that all these three samples can be stably dispersed in deionized water. In addition, the similar FTIR spectra of these three kinds of CNC-CA indicates that recycled CA can also modify CNC-OSO3H effectively.
Mechanical Properties of XSBR/CNC-CA Nanocomposites
In order to study the effect of dual crosslinking network on the mechanical properties of XSBR nanocomposites, the tensile tests were performed. The tensile curves and mechanical property parameters of neat XSBR, X/mC-H and X/mC-D nanocomposites containing 1, 3, 5, 7 and 9 wt% CNC-CA are shown in Fig. 3. For neat XSBR, the stress increased slowly until break, and achieved tensile strength and strain at failure of 2.56 MPa and 389%, respectively. For the X/mC-H nanocomposites, even though there are some improvement by adding CNC-CA into the XSBR matrix, the improvement is quite limited (shown in Fig. 3a). XSBR is not crosslinked by covalent bonds in the X/mC-H nanocomposites, and the role of hydrogen bond is very limited. The tensile strength of X/mC-H nanocomposites is far lower than the requirement of industrial application. Therefore, covalent crosslinking of the nanocomposites is an essential step to further improve its properties. Here, end-epoxy group PEGDE was used as crosslinking agent to realize chemical crosslinking of nanocomposites through the reaction between epoxy group of PEGDE and carboxyl group of XSBR and CNC-CA. As expected, the mechanical properties of X/mC-D nanocomposites show a significant improvement compared to the X/mC-H ones (as shown in Fig. 3b). For example, with only 5 wt% CNC-CA addition, tensile strength and strain at failure of X/5C-D nanocomposites increase to 8.59 MPa and 456%, respectively, which are even superior to that of X/9C-H nanocomposites. When the content of CNC-CA increases to 9 wt%, the X/9C-D nanocomposites show tensile strength of 12.65 MPa, which are nearly five times that of the neat XSBR, and it’s valuable that X/9C-D keeps same elongation at break with pure XSBR. Moreover, from the charts of Fig. 3c. and d., it can be seen that the tensile stresses of the nanocomposites with dual crosslinking networks are nearly double of that with only hydrogen bond, moreover, the fracture energy of the samples with dual crosslinking networks is also significantly improved.
Dynamic mechanical performance of XSBR/CNC-CA Nanocomposites
The temperature dependence of the storage modulus (E’) and the loss tangent (tan δ) for neat XSBR and X/mC-D nanocomposites with different CNC-CA contents are shown in Fig. 4a and Fig. 4b. A successive increase of the E’ values can be observed by increasing the amount of CNC-CA (as shown in Fig. 4a.). For example, the storage modulus at -70°C increases from ∼1049 to 3083 MPa with the addition of CNC-CA from 0 to 9 wt% in the XSBR matrix. The enhancement in modulus below glass transition temperature is a good evidence for the strong reinforced tendency of CNC-CA in the XSBR matrix. Meanwhile, the improvement of crosslinking degree brought by the dual crosslinking network (the covalent bond networks and the H-bond networks) also contributes to the improvement of the modulus.
Figure 4b. shows the variation of tanδ as a function of temperature for the neat XSBR and X/mC-D nanocomposites. XSBR exhibits a peak tanδ value at a temperature around − 3°C, which corresponds to the glass transition temperature (Tg) of XSBR. While the Tg of X/mC-D nanocomposites gradually decreases to lower temperature (to -6°C) with the increase of CNC-CA content. Two effects might decrease the Tg value: one is, as reported by Cao et. al, the hydrogen bond formed between filler particles and matrix may act as an internal plasticization resulting in the shifting of Tg to a lower temperature with increasing filler content (Cao et al. 2013); another is, excessive 2-methylimidazole might act as plasticizer in the X/ mC-D nanocomposites because the amount of 2-methylimidazole used is proportional to the CNC-CA content in the nanocomposite. However, further research is needed to clarify this question. Besides, the intensity of tan δ, which indicates the damping properties, decreases with the increase of CNC-CA content, which is mainly due to the decrease of matrix content and the enhancement of filler’s restriction effect on the chain mobility of rubber matrix.
The dispersion of CNC-CA in XSBR/CNC-CA Nanocomposites
The uniform dispersion of fillers in the matrix plays a key role on improving the properties of nanocomposites. The aggregation of fillers in the matrix forms stress concentration points and has adverse effects on the properties of the nanocomposites.
Figure 5a-f. illustrate the SEM images of cryogenically fractured surfaces of the neat XSBR and X/mC-D nanocomposites containing 1, 3, 5, 7 and 9 wt% CNC-CA, respectively. Compared to the micrograph of neat XSBR, the morphology of CNC-CA in the XSBR matrix can be easily identified. The CNC-CA appears as white dots and their concentration on the fracture surface of the nanocomposites increases directly with the increase of the CNC-CA loading. A uniform distribution of CNC-CA in the XSBR matrix can be observed in all X/mC-D nanocomposites (as shown in Fig. 5b-f). It is obvious that those CNC-CA dots are partial embedded in XSBR matrix, demonstrating good compatibility between the fillers and matrix. Furthermore, no gaps, voids or pull-out cracks are observed on the surface, indicating excellent interfacial adhesion between the CNC-CA and XSBR matrix. This excellent compatibility and interface adhesion can be ascribed to the hydrogen bond between XSBR and CNC-CA and the formation of ester group among PEGDE, XSBR and CNC-CA. This uniform distribution and excellent interfacial adhesion are anticipated to play an important role on the improvement of the mechanical performance of the resulting X/mC-D nanocomposites.
Swelling properties and solubility of XSBR/CNC-CA Nanocomposites
The water and toluene uptake ratios of X/mC-D nanocomposites containing 1, 3, 5, 7 and 9 wt% CNC-CA were conducted to investigate the hydrophilicity and crosslinking degree. The X/mC-D nanocomposites were immersed in deionized water or toluene for 72 h at 60°C and weighed. Figure 6. shows the water uptake ratios of neat XSBR and X/mC-D nanocomposites. The water uptake ratios of X/mC-D nanocomposites with CNC-CA content ≤ 7 wt% was much lower than that of neat XSBR. This may be attributed to the dual crosslinking network which makes the components of nanocomposites compact more densely and restricts the chain mobility of the rubber matrix, subsequently inhibits the diffusion of water in the nanocomposites effectively. But, the hydrophilicity of X/mC-D nanocomposites increases with the increase of hydrophilic CNC-CA content, especially when the CNC-CA content > 7 wt%, there is a dramatic increase of water uptake ratios with increase of CNC-CA content. As can be seen from Fig. 6, the water uptake ratios of X/9C-D is almost twice that of neat XSBR. This phenomenon is due to the dual role of CNC-CA in X/mC-D nanocomposites. On one hand, the formation of hydrogen bond between CNC-CA and XSBR molecular chains inhibits the movement of XSBR molecular chains and reduces the possibility of hydrogen bond formation between XSBR molecular chains and water, resulting in the decrease of hydrophilicity of X/mC-D nanocomposites. But, due to hydrogen bonds have saturability, when CNC-CA content reaches a certain level, the number of hydrogen bond between CNC-CA and XSBR molecular chains remains unchanged with increase of CNC-CA. So, the effect of hydrogen bond on the hydrophilicity of X/mC-D nanocomposites doesn’t increase when CNC-CA content reaches a certain level. On the other hand, CNC-CA has the inherent hydrophilicity, more CNC-CA, more hydrophilicity is introduced in the X/mC-D nanocomposites. As a result of the above two aspects, water uptake ratios of X/mC-D nanocomposites increase with increase of CNC-CA content and has an interrupt increase when the content of CNC-CA > 7%. On the contrary, the toluene absorption ratios decrease with the increasement of CNC-CA content, which should be attributed to the increased hydrophilicity caused by the introduction of CNC-CA and the dual crosslinking network restricts the chain mobility of XSBR.
In order to confirm the formation of dual crosslinking network, the X/mC-D and X/mC-H nanocomposites were immersed in DMF/toluene mixed solution for 72 h at 60°C and ultrasound treating for 1h to check the swelling and dissolution behavior. As shown in Fig. S2, all the X/mC-H nanocomposites sheets (upper) and neat XSBR disintegrated and dissolved in the DMF/toluene mixed solvent. In sharp contrast, the shape of all the X/mC-D nanocomposites sheets (below) keep well. The above experimental phenomena further prove that PEGDE has an effective crosslinking effect on the XSBR/CNC-CA nanocomposite.
Structural Characterization and proposed dual crosslinking networks of XSBR/CNC-CA nanocomposites
Structural Characterization of XSBR/CNC-CA nanocomposites
FTIR is a powerful tool for studying the intermolecular interactions, hydrogen bonds and chemical reactions. The FTIR spectra of neat XSBR, PEGDE, XSBR/PEGDE, CNC-CA, and CNC-CA/PEGDE are shown in Fig. 7. In order to make the results clear, here, the mass ratios of XSBR to PEGDE and CNC-CA to PEGDE were set to be 1:1, and the samples were treated at 110°C for six hours. As shown in Fig. 7, the three peaks of 3055, 1252, 854cm− 1 in the FTIR spectrum of PEGDE are the characteristic peaks of epoxy group. Compared to the FTIR spectra of single XSBR and PEGDE, a new band at 1725 cm− 1, which corresponds to the C = O vibrations of ester group, appeared in the XSBR/PEGDE blends as shown in Fig. 7a. This reveals that the carboxyl groups of XSBR reacted with the epoxy groups of PEGDE and formed β-hydroxyl esters. The spectra of CNC-CA/PEGDE blends, is shown in Fig. 7b. The three characteristic peaks of PEGDE disappeared in the FTIR spectrum of CNC-CA/PEGDE blends, which means that the carboxyl group of CNC-CA and the epoxy group of PEGDE are esterified. From the above analysis results, we confirmed that the epoxy group on PEGDE can react with carboxyl group of both XSBR and CNC-CA, therefore, PEGDE can effectively crosslink XSBR/CNC-CA nanocomposites.
The existence of hydrogen bonds in X/mC-D nanocomposites was also confirmed by FTIR. As shown in Fig. S3, the intensity of the band between 3500 − 3250 cm− 1 corresponding to the hydrogen bond became stronger after compounding the CNC-CA with XSBR, indicating there is a strong hydrogen bond interaction between XSBR and CNC-CA. According to the above results, it can be concluded that dual crosslinking networks of the covalent bond networks and the hydrogen bond networks have been formed in XSBR/CNC-CA nanocomposites, both contribute to improve the mechanical properties of XSBR/CNC-CA nanocomposites.
The proposed model of dual crosslinking networks for crosslinked XSBR/CNC-CA nanocomposites
Based on the above discussion, it can be concluded that dual crosslinking networks featured of hydrogen bonds network and covalent bonds network have been formed in X/mC-D nanocomposite. The esterification of epoxy group of PEGDE with carboxyl group of XSBR and CNC-CA is shown in Scheme S1, the illustration of hydrogen bonds network and dual crosslinking network is shown in Scheme 2. Compared with the ionic bond and covalent bond, hydrogen bond is much weaker so that its ability on the improvement of the mechanical properties of XSBR/mC-H is very limited. By further constructing covalent bonds in the X/mC-D nanocomposites using PEGDE as crosslinking agent, the rubber chains and CNC-CA are held together tightly. The combination of the two networks can effectively improve the compatibility and interface adhesion between CNC-CA and XSBR, as well as the interactions between XSBR-XSBR, which is beneficial to the improvement of the physical and mechanical properties of XSBR/CNC-CA nanocomposites.