High-performance polylactide biocomposites reinforced with cellulose nanofibers
Polylactide (PLA) is a semi-crystalline, thermoplastic, bio-based polymer that exhibits attractive properties (e.g., good processability, UV stability, and gloss). This material, however, also has some shortcomings (such as low heat resistance) that limit the applicability of PLA, and the properties of PLA therefore need to be improved. There are a number of promising methods that have previously been considered as ways to solve the problems associated with PLA, including copolymerization, blending, and the use of fillers.1, 2 Indeed, with the rising importance of ‘green’ materials for protecting the environment, the possibility of using natural fibers, e.g., cellulose nanofibers (CNFs), as reinforcements in PLA composites has attracted a large amount of interest. It can be difficult, however, to achieve good dispersion/distribution of hydrophilic CNFs within the hydrophobic PLA matrix.
To date, several efforts have been made to prepare PLA/CNF composites (with low CNF loadings) that have enhanced properties, but these were not highly successful because of the presence of CNF agglomerates within the matrices.3–9 Attempts to combat this issue include the use of a solvent in the preparation of the composites to prevent the formation of the CNF agglomerates/entanglements.3–6, 10 Alternatively, chemical modifications can be used to lower the hydrophilicity of the CNFs.7–9
In our work,10 we have used a solvent-cast method to prepare PLA biocomposites that are reinforced with CNFs, and have considered the effects of the CNFs on various properties of the PLA. In the first part of our study, we used scanning electron microscopy to investigate the state of nanofiber dispersion/distribution within the composites. A micrograph of the freeze-dried CNFs—see Figure 1(a)—illustrates the web-like structure of the nanofibers. We also observe fair dispersion/distribution of the nanofibers within the PLA matrix, as shown for the PLA/2CNF sample (i.e., PLA containing 2wt% CNFs) in Figure 1(b). As can be seen in Figure 1(a), fiber bundles are present when the CNFs are initially dispersed in their most appropriate medium (water). We therefore expected—since PLA has a lower polarity than water—the presence of some fiber bundles in our composite samples. In our proposed process for the fabrication of the PLA/CNF biocomposites, we use an efficient solution method to ensure a uniform dispersion/distribution of the fibers within the PLA matrix. In this method, we prepare suspensions of the CNFs in a solution of PLA in dimethylformamide, and we thus have no need for a compatibilizer.
We have also studied the rheological properties of our biocomposites. For example, we plot the complex viscosity (η*) versus the complex modulus (G*) of our samples in Figure 2 so that we can determine the apparent yield stress, which is indicative of the quality of the CNF dispersion. For the PLA/0.25CNF and PLA/0.5CNF samples (i.e., PLA containing 0.25 and 0.5wt% CNFs, respectively), we do not observe any significant increase in η* as G* decreases, whereas the other composites (with higher CNF content) show an obvious apparent yield stress. This results from the reduction of the PLA chain mobility, which is caused by the interactions between the nanofibers (and possibly between the polymer chains and the nanofibers). Our results therefore indicate that rheological percolation in this system occurs at a CNF concentration of 0.5–1wt%. We have also previously reported a similar percolation threshold range for a PLA/cellulose nanocrystal system.11 To quantify the apparent yield stress we observe in the samples, we have used the modified Carreau-Yasuda model.10 We find that this model fits the data for the PLA and the PLA/CNF composites very well (see Figure 2).
As it is possible for fillers to affect the transparency of the polymer,10 we have qualitatively and quantitatively compared (see Figure 3) the transmittance of the neat PLA film with that of the PLA/5CNF film (PLA containing 5wt% CNFs). The transparency of both films is quite similar and we do not observe any visual agglomerates (even for the sample containing the highest CNF content, i.e., PLA/5CNF), which confirms the good dispersion of the nanofibers within the biocomposites. We also find that the values of transmittance for the PLA and PLA/5CNF films differ by a maximum of only 5% in the visible light range. Moreover, the PLA/5CNF film shows good transparency since the transmittance is greater than 75%.10, 12 With these excellent transparency characteristics, we demonstrate that our PLA/CNF composites are suitable for many applications (e.g., in optics and optoelectronics) where polymers filled with other nanoreinforcements are not appropriate (because of their lack of transparency).10, 13
The mechanical properties, in terms of the normalized Young modulus and tensile strength, of our biocomposites are represented in Figure 4. We find that both parameters increase significantly with increased CNF content, i.e., the Young modulus and tensile strength of the sample containing 5wt% CNFs are 50 and 31% higher, respectively, than the neat PLA sample. These improvements may be caused by the good interactions between the matrix and the nanofibers, and by the presence of a strong network of nanofibers. This is because such interactions are advantageous for transferring the load from the matrix to the fibers,10, 14 and between the fibers.10
The results of our dynamic mechanical thermal analysis (i.e., the normalized storage modulus values at 25 and 70°C) are presented in Figure 5. We observe a large enhancement upon addition of the nanofibers into the PLA, particularly at high temperatures. This enhancement arises because of the good CNF dispersion/distribution, as well as the entangled network of nanofibers within the matrix.6, 10,15 We achieve an increase of 51 and 264% in the storage modulus of PLA by incorporating 5wt% CNFs at 25 and 70°C, respectively. Indeed, as the storage modulus of the matrix decreases significantly at high temperatures, the reinforcement effect of the nanofibers becomes even more obvious.3, 10 With our methodology, the use of PLA can thus be extended to applications where its low modulus at high temperatures has previously raised challenges.
In summary, we have used a solution method to prepare PLA/CNF composites with good dispersion/distribution of the hydrophilic CNFs within the hydrophobic matrix. The results of our investigation demonstrate that this approach can be used to enhance the properties of PLA and make it more suitable for a variety of applications and processes where high heat resistance and transparency are of great importance. Examples include optical, optoelectronics, automotive, and packaging industries (as water/milk bottles and biodegradable plastic bags), as well as pipes and products that are exposed to high temperatures, and in film-blowing and blow-molding processes.10 In the next stage of our research we will use this preparation technique to produce a concentrated masterbatch. We will then use melt-mixing to prepare PLA/CNF composites on an industrial scale.
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