3.1. Composite processing
The mechanism of grafting CNC onto PLA (PLA-g-CNC) relies on reactive extrusion through the chemistry of ring-opening polymerization (ROP). In this process, L-lactide is grafted onto a PLA molecule, initiated by a catalyst system involving Sn(Oct)2. Additionally, the introduction of DCP facilitates decomposition, generating free radicals that readily react with hydroxyl groups during the PLA and CNC grafting mechanism. Furthermore, LA is associated with the dimer ring-opening of Sn(Oct)2, and the resulting free hydroxyl radicals act as protective agents for the polymer matrix, as reported by Kalia et al. (2011).
Catalyst systems and the activation of free radicals act as coupling and dimerization agents. The release of free radicals from DCP, coupled with LA associated with the catalyst system, establishes a balanced equilibrium influencing the propagation rate and polymer degradation of PLA. The grafting process was carried out at a rotational speed of 60 rpm and a working temperature of 180 °C.
The grafting mechanism was meticulously assessed through FTIR analysis, as illustrated in
Figure 1. Dhar et al. established a crucial vibration coupling moment between PLA and CNC polymers, as exemplified by the σ vibration band at 3333 cm⁻¹. This band signifies the stretching of the unsaturated carbon skeleton construction of CNC (-C-C-) and the presence of hydroxyl groups (-OH) [
16]. This correlation is extrapolated into the ion interaction between PLA-g-CNC, indicating a chemistry affinity achieved through reactive extrusion (
Figure 2c and 2d).
The effective grafting process, as discovered by Jacobsen et al. and Dhar et al., involves the ring-opening polymerization (ROP) of L-lactide, initiated from available hydroxyl groups onto the CNC surface. The equimolar composition of the catalyst system and the inclusion of DCP as coupling agents significantly contribute to this reaction [
25,
39]
FTIR results suggest the presence of a grafting polymer complement, indicating effective interactions between chain segments and functional groups of PLA and CNC during the polymer condensation process. This is illustrated in
Figure 1a and 1c, where band minimization is demonstrated as a result of incorporating PCAL in an equimolar composition (
Figure 1b and 1d).
Furthermore,
Figure 1b and 1c display a reduction in the vibrational stretch band corresponding to -OH, suggesting the exclusion of this molecule from the process, confirming and maximizing the hydroxyl molar ratio.
Resonance magnetic hydrogen analysis (1H NMR) of the PLA and PLA-g-CNC samples, as depicted in
Figure 2 (a) and 2 (b), respectively, serves to illustrate the efficiency of the grafting process. It is noteworthy that new peaks emerge at 3.68 ppm and within the range of 4.1 to 4.5 ppm. The newly observed peak at 3.68 ppm corresponds to the protons of the methyl group formed in the PLA-g-CNC structure. The presence of these protons has been previously reported in the work of Dhar et al., where a combination of PLA and CNC was also employed [
15,
16,
40].
The peaks within the range of 4.2 and 4.4 ppm correspond to the methylene protons in the cellulose structure, providing conclusive evidence of the presence of CNC in the grafted structure010; Dufresne 2017). In the 1H NMR spectra of pure PLA (
Figure 2 (a)), the peak at 3.68 ppm was not identified, and the peak at 1.5 ppm, representing the -CH
3 protons, exhibited significant changes compared to the 1H NMR spectra of PLA-g-CNC. The 1H NMR results align with those obtained by FTIR, as the -CH
3 band was observed close to 2900 cm⁻¹.
3.2. Field emission scanning electron microscopy (FESEM)
The morphology of CNC is illustrated in
Figure 3. The CNC suspension underwent dilution and sonication before freeze-drying. The cellulose distribution exhibited excellent dispersion with a Gaussian profile. Nanocrystal cellulose displayed a monomodal distribution, with nanometric dispersion ranging from ≈ 5 to ≈ 400 nm. Notably, the maximum dispersion was observed at ≈ 200 nm, as demonstrated in
Figure 3f and 3g. The needle-like structure of CNC, resulting from acid hydrolysis (H
2SO
4), revealed a whisker-like geometry. The measurement of CNC was conducted through FESEM analysis and sequential image analysis, involving the counting of approximately 400 particles using image analysis software, ImageJ. The results indicated an average diameter of ≈ 11.38 nm and a length of ≈ 237.89 nm, as shown in
Figure 3a.
The cellulose nanocrystals used in this study had approximate dimensions of ≈ 40 nm in diameter and ≈ 200 nm in length, as shown in Figures 3d and 3e. The distribution of CNCs throughout the PLA matrix was mostly random, although some areas of higher concentration agglomeration and a diffuse crystal dispersion were observed in micrographs 3e in make γ and θ.
The effect of matrix concentration has an interaction with Vander Waals force, forcing a zone ramification. To prevent agglomeration of CNCs in the PLA matrix and maintain the transparency of the films obtained, researchers used a dilution in dialysis membrane with a continuous flow of water to obtain the neutralized material, followed by probe sonication before freeze-drying, together with associate coupling agent.
CNCs are commonly hydrophilic and tend to agglomerate by Vander Walls interaction during the drying process, which hinders the thermal and mechanical properties of the nanocomposites. To address this issue, researchers investigated the use of DCP and LA associated with t(Sn(Oct)2)/(P(C6H5)3) to incorporate into PLA/CNC compounding by reactive extrusion, forming a polymer condensation in a PLA-g-CNC interfacial adhesion of components.
Biopolymers, such as semi-crystalline PLA, have shown better physical, chemical, and mechanical properties and are suitable for variable melting processing routes by extrusion methodology, injection molding, and/or torque rheometer. PLA is one of the main candidates to substitute oil-based synthetic polymers due to its low cost of production and biodegradability, low density, low cost of production, and reactive surface.
These nanocrystals originate from renewable and abundant sources, such as vegetables (e.g. wood, cotton, bacterial cellulose) and animals (e.g. tunicates). An important characteristic of CNC is its reinforcement for the PLA matrix. However, the hydrophobic matrix surface interaction of PLA-g-CNC nanocomposites must be improved to enhance these properties.
The addition of natural biodegradable nanofillers, such as CNC, has been shown to improve mechanical properties and confer a biodegradable biopolymer characteristic. Controlling the geometric dimensions, the polymer nano-charger interactions, and incorporating a small number of nanoparticles (usually less than 10% by weight) can change the properties of the material, such as improving mechanical strength and thermal resistance.
Biopolymers, such as PLA, have been used as a suitable and promising alternative to oil-based synthetic polymers due to their low cost of production, biodegradability, low density, and reactive surface. PLA has stood out for its physical, chemical, and mechanical properties, making it one of the main candidates to substitute oil-based synthetic polymers. However, the difficulty of dispersing CNC in the PLA matrix has been reported by other researchers.
CNCs are commonly hydrophilic and tend to agglomerate by Vander Walls interaction during the drying process, which hinders the thermal and mechanical properties of the nanocomposites. To address this issue, researchers have investigated the use of DCP and LA associated with (Sn(Oct)2)/(P(C6H5)3) to incorporate into PLA/CNC compounding by reactive extrusion, forming a polymer condensation in a PLA-g-CNC interfacial adhesion of components.
The improved dispersion and compatibility of the CNC nanocrystals with PLA have resulted in improved mechanical properties, such as Young’s modulus and tensile strength.
3.3. Thermogravimetric analysis (TGA)
Thermogravimetric (TG) curves of neat PLA and biopolymer nanocomposites are shown in
Figure 4. The addition of coupling agents (DCP, LA, and Sn(Oct)
2/P(C
6H
5)
3) modified the arrangement of the curves. The TG curves of samples with CNC also showed a modified profile. The derivative thermogravimetric (DTG) curves revealed a modification in the degradation parameters, indicating a change in the mass composition, possibly due to an increase in the biopolymer mass and a slower mass loss rate at higher heating temperatures. This suggests an increase in the force of interaction between the polymerized molecules.
The polymerization system consists of a chain conjugation that converts a pi (π) bond to a sigma (σ) bond, as shown in
Figure 2. This coupling also increases the molecular mass and hydrogen bonding interactions. The weight loss stages of the composites, shown in
Figure 4, were identified as a loss of macromolecular mass and a slow and steady event, which is attributed to the strong interaction between the formed products.
The thermogravimetry results shown in
Figure 4 suggest a slight improvement in thermal stability in the presence of CNC and DCP (PD and PC). This is due to polymeric propagation, which is confirmed by the possible polymer incorporation shown in
Table 2. Improvements in thermal stability are obtained when C=C and C-C bonds are formed between PLA and CNC. The mechanism of ring-opening polymerization (ROP) is well-documented in the literature.
As shown in
Figure 1, the vibrating moments β and γ have a minimized band frequency around 3010 cm
-1 and 3090 cm
-1, respectively. This suggests that the present process activates the polymer system in two phases: electrophilic substitution and chain propagation, as observed during extrusion.
Activation energy is the minimum energy required to initiate a reaction mechanism. Dicumyl peroxide (DCP) is a crosslinking agent that forms radical molecules and induces an induction period (Salmieri et al., 2014; Dhar et al., 2016b; Kargarzadeh et al., 2017).
A multifunctional radical crosslinked with coupling agent LA (0.5 wt.% and 1 wt.%) and (Sn(Oct)2)/P(C6H5)3 was used to generate a high molecular weight polymer through an extrusion reactive process. This polymer showed an increased degradation temperature and mass incorporation ratio.
3.4. Differential scanning calorimetry (DSC)
Results from the differential scanning calorimetry (DSC) analysis of all samples are presented and shown in
Table 3. The glass transition temperature (Tg) was determined to assess the dimensional stability of the polymer and nanocomposites and is revealed in
Figures 5a and 5b, located in the region denoted by σ, ranging approximately between 40 to 60 °C.
PLA is a semi-crystalline polymer conducted, a study on the structural properties of nanocrystalline cellulose, focusing on the uniform distribution of methyl groups in the alpha carbon (-CH3) through reactive extrusion in their research system. This methyl group is a representative part of the cellulose backbone, and the peptidic bond represents lateral groups in PLA-g-CNC, as observed in a melting peak.
A modest reduction in the melting temperature (Tm) of certain compositions, when compared to neat PLA (as indicated in
Table 3), was observed in the temperature range of approximately 145 to 153 °C. This observation was determined at positions β and γ, revealing two consecutive temperature transitions, as depicted in
Figures 5a and 5b. The decrease in crystalline domains can lead to the appearance of peaks at different temperatures.
The presence of double peaks (denoted as peaks β and γ) suggests the coexistence of two crystalline structures with a minimal difference in enthalpy transition. This phenomenon is observed through a displacement in the transition phase, where the α-crystal occurs at a temperature of approximately 148 °C, and the α'-crystal at a temperature of about 152 °C. The α-crystals exhibit a higher level of organizational crystalline structure, along with a higher fusion temperature compared to the α'-crystals, as illustrated in
Figure 5a. This testing process at elevated temperatures requires an extended period for the atomic organization and rearrangement of crystalline structures from a solid state to a liquid interface.
This aligns with the degradation temperature presented in
Table 2. The most favorable growth patterns of α-crystals were observed in samples where double peaks were absent, and the melting temperature (Tm) ranged from 148 to 152 ºC, excluding PDAL1_0.0045, PDAL0.5_0.0023, and PCDAL0.5_0.0023. by Jonoobi M. et al., the introduction of LA to the system resulted in the emergence of double Tm peaks, indicating that the interaction of these components contributed to a reduction in the content of α-crystals.
Furthermore, both LA and CNC, despite the addition of DCP, elevated the crystallization temperature (Tc) of neat PLA. This enhancement can be attributed to the improved mobility of ungrafted chains, surpassing the activation energy required for crystal formation.
The increase in the degree of crystallinity (XC) for PLA/LA, with or without CNC and/or DCP, when compared to neat PLA, is attributed to the enhanced mobility of both unmodified and grafted chains within the amorphous fraction of PLA. The improved dispersion and interfacial adhesion of CNC throughout the matrix, as observed in
Figure 5a, support the grafting hypothesis. This hypothesis is further substantiated by the presence of LA/(Sn(Oct)
2/P(C
6H
5)
3) catalyst and the polymerization chain formation.
In samples where LA was incorporated with Sn(Oct)
2/P(C
6H
5)
3 along with PLA determination, an increase in the degree of crystallinity was observed, as demonstrated from PCAL0.5_0.0023 to PCAL2_0.0023, indicating a synergistic nucleation effect. This led to a noticeable weakening (fragile characteristic) of PLA, as illustrated in
Figure 5b. A similar behavior was evident in PDAL2_0.009, PCAL2_0.009, and PCDAL2_0.009.
3.5. Mechanical characterization
Mechanical results and characterizations are presented in Figure 8 and Figure 9, with values for Young's modulus and tensile strength shown in Table 4. Neat PLA specimens exhibited Young's modulus (E) ranging from 1.65 GPa and maximum tensile strength (σ_max) of 20.31 MPa. In the standard PC formulation, Young's modulus remained almost unchanged at 1.61 GPa, and the tensile strength was 18.93 MPa.
Additionally, the results for the standard PD formulation showed increases in both E and σ_max, by 46% to 54%, respectively, with average values of 2.54 GPa and 29.57 MPa, as demonstrated in
Figure 6. Goffin A. L. et al. conducted a study on the increment of coercive force, attributing it to crosslink interactions in the reaction mechanism, resulting in macromolecule formation and enhanced nanocellulose crystallinity.
CNC and DCP were simultaneously introduced into the system (PCD) without causing changes in both Young's modulus (E) and maximum tensile strength (σ_max) when compared to neat PLA. In contrast, samples PDAL1_0.0045 and PDAL0.5_0.0023, where coupling agents DCP and LA up to 1.0 wt.% were associated with a catalyst system (Sn(Oct)2/P(C6H5)3), exhibited an approximate 50% increase in both E and σ_max. This supports the justification for the combined use of LA/ DCP catalyst. Additionally, this combination enhanced thermal properties, reducing thermal degradation, as confirmed by TGA. Samples PCAL1_0.0045 and PCAL0.5_0.0023, with an increment of 1 wt.% LA associated with a catalyst system (Sn(Oct)2/P(C6H5)3), CNC, and DCP, displayed an increase in both Young's modulus and tensile strength.
Furthermore, the improvement in mechanical properties can be attributed to the formation of crosslinking bonds, along with the presence of grafted structures such as PLA-g-CNC. This suggestion is supported by the FESEM micrographs shown in Figures 3d and 3e.