Obtaining and Characterization of Biodegradable Composites Reinforced with Microcrystalline Cellulose Fillers

In recent years, there has been a growing discussion about the problems related to the massive use of many synthetic plastic materials, which inevitably leads to an increase in environmental pollution caused by the inappropriate disposal of these materials. In this sense, biodegradable materials have been a subject of great interest, as they are a real alternative to replace these materials and tackle this issue. In this work, fully biodegradable composites were prepared by solution casting method. Microcrystalline cellulose (MCC) and treated microcrystalline cellulose (TMCC) were separately incorporated into biodegradable PLA and PHB matrices at ratios of 3, 5 and 7 wt% and the properties of the obtained biocomposites were evaluated by TGA, DSC, XRD and TD-NMR. From thermal analyses, it was seen that TMCC resulted in better thermal stability and 3 wt% of filler, in general, promoted a more pronounced thermal improvement. Furthermore Tg, Tc and Tm remained practically unchanged after MCC and TMCC addition. From XRD it was seen that the cellulose fillers influence in different ways the matrices, promoting increase or decrease in the degree of crystallinity. Finally, the results obtained by TD-NMR showed a decrease in the T 1 H values for all prepared biocomposites, indicating a good dispersion of the cellulose fillers in the matrices and pointed that the systems containing 3 wt% of cellulose fillers were the most homogeneous formulations.


Introduction
In the last few decades, there has been a progressive increase in the production and use of petroleum-based polymers [1] [2]. The systematic growth of these synthetic materials production and their post-use disposal have resulted in a massive polymeric waste generation. This panorama can both contribute to fossil fuel depletion and environmental pollution. Consequently, the management of polymeric waste accumulation is becoming a major concern in terms of preserving the environment and sustainable development [3] [4].
In this regard, materials made of polymers from natural renewable sources or bio-based polymers, have been developed to replace traditional petroleum-based materials in many applications and, consequently, reduce the polymer waste problem and the global dependence on fossil sources [5]. Biodegradable polymers have important advantages over those obtained from petroleum such as biodegradability, biocompatibility, low toxicity, sustainability, among others [6] [7] [8] [9]. However, the wide application of these polymers is still a challenge due to inherent limitations on their performance in comparison to conventional polymers, which in general exhibit better mechanical properties and thermal stability [10].
Many studies have shown that the development of bio-based materials with natural reinforcement fillers, such as cellulose, starch, and chitin, appears as a promising strategy to provide the improvement of their properties [11]. Cellulose particularly has been largely employed to produce these fillers, granted being the most abundant biopolymer in nature and available from a wide variety of sources, such as plants and microorganisms [12] [13] [14] [15] [16]. In addition, the application of cellulose as the dispersed phase in biodegradable polymer matrices promotes in many cases enhanced performance concerning several factors, including barrier, thermal, and mechanical properties.
There are many reports about the reinforcing effects of cellulose-based filler or nanofiller incorporated into biodegradable polymer matrices [17]- [33], which demonstrate that the addition of low percentages of this type of fillers has positive effects on the structural properties of these polymers. However, obtaining optimal properties requires appropriate dispersion of cellulose particles within the polymer matrix. Ultrasonication treatment has been commonly used as a mechanical method to improve cellulose particles and nanoparticles dispersion in different solvents and polymer matrices [34]- [40].
In the present work, fourteen fully biodegradable films were developed by the solution casting method, in which seven were made with PLA, and the other seven with PHB. Microcrystalline cellulose was incorporated in these matrices at the ratios of 3, 5 and 7 wt%. Besides comparing the influence of cellulose proportion, the present study also aimed at comparing the effect of the incorporation of microcrystalline cellulose without any treatment (MCC) and after being treated by ultrasonication (TMCC), since there are very few studies showing this comparison. The biocomposites obtained were characterized by conventional techniques, namely thermogravimetric analysis (TGA), differential scanning ca-lorimetry (DSC), X-ray diffraction (XRD) and by an unconventional technique, Time Domain-Nuclear Magnetic Resonance (TD-NMR). This technique, despite providing relevant information on the molecular dynamic behavior and homogeneity at the molecular level of hybrid materials such as composites and nanocomposites [41] [42] [43] [44], which contribute to the understanding of many properties of materials, has been little explored for the study of composite material. Moreover, scanning electron microscopy (SEM) was carried out to evaluate the effect of ultrasonication treatment on the morphology of microcrystalline cellulose filler.

Materials
The poly(3-hydroxybutyrate) (PHB) was supplied by PHB Indústria do Brasil, polylactic acid (PLA) was supplied by Nature works. Microcrystalline cellulose (MCC) ph102 powder was supplied by Viafarma. Treated microcrystalline cellulose (TMCC) was obtained by ultrasonication treatment of the MCC. Chloroform (CHCl 3 ) used as solvent was purchased from Merck.

Preparation of the Microcrystalline Cellulose
MCC was used as received, while TMCC underwent an ultrasonication treatment to verify a possible size reduction and shape modification and to investigate the influence of these parameters on the properties and homogeneity of the prepared systems. Ultrasonication treatment was carried out in an Eco-sonics equipment from Ultronique Company, Desruptor model, at a frequency of 20 kHz and potency of 500 W. Aqueous suspensions containing MCC at 1 wt% were subjected to sonication at 25˚C for 60 min.

Preparation of the Biocomposites Systems
Biodegradable composites films were obtained by solution casting method, utilizing PHB and PLA as matrices and two types of cellulosic filler: MCC and treated MCC (TMCC). To obtain the cellulosic biocomposites systems, solutions of PHB/CHCl 3 and PLA/CHCl 3 at 10% w/v were separately prepared under magnetic stirring for 24 hours at room temperature. The PHB solution was subjected to an additional hour under magnetic stirring at 50˚C to complete polymer solubilization. After the polymer solutions preparation step, MCC/CHCl 3 and TMCC/CHCl 3 dispersions at concentrations of 3, 5 and 7 wt% (filler/polymer) were prepared by magnetic stirring for 30 min at room temperature followed by sonication bath for 30 min. Immediately after, the polymer solutions were added to the cellulosic fillers dispersions and the resulting mixtures were kept under constant magnetic stirring for 1 hour at room temperature. The materials obtained were poured into glass Petri dishes and placed in the exhaust hood at room temperature for at least seven days to eliminate all residual solvent. The materials obtained were coded as described in Table 1

Characterization of the Microcrystalline Cellulose Fillers
Scanning Electron Microscopy (SEM) technique was employed for direct investigation of the morphology of microcrystalline cellulose before and after ultrasonication treatment. The MCC and TMCC were analyzed in powder form, as samples were placed on a double-sided adhesive carbon tape after being sputter-coated with gold for 4 min. Images were taken with Hitachi TM3030 Plus scanning electron microscope (SEM) applying an accelerating voltage of 1 -15 kV.

Characterization of the Biocomposites
Thermal stability of the materials was evaluated by thermogravimetric analysis

Thermogravimetric Analysis (TGA)
Two parameters were measured by thermogravimetric analysis to study the thermal stability of the prepared materials: the initial degradation temperature (Tonset) and temperature of maximum degradation rate (Tmax). The Tonset values were obtained from TG curves and denote the lowest temperature at which mass variation of the material occurs. In turn, the Tmax values were obtained from the peak of each DTG curve, which refers to the temperature where the degradation speed occurs more sharply. The results obtained from this analysis are denoted in Table 2 and Figure 2.
According to the results displayed in Table 2 and Figure 2 it was seen that for PHB biocomposites systems, the addition of MCC at 7 wt% and TMCC at 5 and 7 wt% did not interfere significantly on the thermal stability of the PHB matrix, while  Tmax, respectively. This result indicates that for PHB composites the best proportion of cellulose filler was 3 wt% and that the ultrasonication treatment of this filler was important, making the improvement of this property more effective.
For PLA biocomposites, the results (Table 2 and Figure 2) showed that the addition of 5 and 7 wt% of MMC did not cause significantly change on the thermal stability of the PLA matrix, while the addition of 3 wt% promoted an improvement on the thermal performance of the PLA matrix, with an increase of 12˚C on Tonset and 7˚C on Tmax. For the samples prepared with TMCC, the best system composition was with TMCC 5 wt%, exhibiting an increase of 18˚C on Tonset and 12˚C on Tmax. Comparing the results obtained for the prepared systems with MCC and TMCC, it was clear that the treated microcrystalline cellulose (TMCC) promotes a better thermal performance for both biodegradable matrices studied in this work.

Differential Scanning Calorimetry (DSC)
The glass transition temperature (Tg), the melting temperature (Tm) and the crystallization temperature (Tc) obtained from DSC are denoted in Table 3. According to results, the glass transition temperature (Tg), the melting temperature (Tm) and the crystallization temperature (Tc) found for PHB were 3.6˚C, 174˚C and 48˚C, respectively and for PLA were 62˚C, 174˚C and 99˚C, respectively. Analyzing the results obtained for biocomposites prepared in this study with both biodegradable matrices and both cellulose filler types, it was possible to notice that the addition of MCC or TMCC did not affect these thermal properties in any of the proportions studied, maintaining practically unchanged these thermal characteristics Tg, Tm and Tc of the PHB and PLA matrices.   Figure 3 and Figure 4 [46]. PHB, PHB/MCC and PHB/TMCC systems revealed a similar crystalline profile corresponding to orthorhombic crystal planes (Figure 3). It was observed two strong scattering intensity peaks detected around 2θ 13˚ and 17˚, which is assigned to the (020) and (110) planes of the orthorhombic unit cell, respectively. Other weaker reflections located at around 22.5˚, 26˚, 27˚ and 31˚ correspond to (111), (121), (040) and (002), respectively. Furthermore, all the samples presented a small amount of orthorhombic β-form crystal with zig-zag conformation, as revealed by the reflection of the (021) plane located at around 2θ = 20˚ [47] [48] [49]. Compared to pure PHB, some PHB/MMC and PHB/TMCC systems presented slight changes in peak intensities, which can be attributed to changes in the crystalline profile. The calculated degree of crystallinity for studied materials is denoted in Table 4. It was found that for PHB/MCC systems, the formulation containing 5 and 7 wt% of MCC presented an increase of 5.8% and 6.6% on degree of crystallinity, respectively, while PHB/MCC3 did not present significant changes. On the other hand, PHB/TMCC formulations showed opposite behavior, such that the formulations containing 5 and 7 wt% of TMCC maintained the degree of crystallinity practically unchanged and the PHB/TMCC3 formulation presented an increase of 7.3% on this parameter. This result suggests that MCC and TMCC influence differently on the PHB matrix. The increase in the peak intensity located at 2θ = 22.5˚ observed for systems containing 5 and 7 wt% of MCC and TMCC is attributed to (002) plane of the cellulose fillers, which are present in higher proportions for these compositions.

Time Domain-Nuclear Magnetic Ressonance (TD-NMR)
TD-NMR is an unconventional technique that provides information about molecular dynamic behavior and homogeneity of materials at the molecular level.
The molecular dynamic of the materials was evaluated through hydrogen spin-lattice relaxation time (T1H) and their homogeneity were investigated by fitting domain distribution curves. These plots provide information on homogeneity, according to the number of curves present in the plot and these curves' base width. Regarding relaxation measurements, it was observed, in general, a tendency of progressive decrease in the T 1 H values with the increase in microcrystalline cellulose proportion, regardless of the type (Table 5). This behavior was observed for both PHB and PLA matrices, which indicates that both MCC and TMCC promoted an increase on the molecular freedom of polymer chains. This points to a good dispersion of these fillers, which presumably led to a reduction of the inter-and intramolecular interaction within the polymer chains. Comparing the systems prepared with MCC and TMCC at the same filler proportions, for PHB biocomposites it was observed that all systems prepared with TMCC had lower T 1 H values, indicating a more important change on the PHB matrix. This result was clearly observed for PHB systems containing 5 wt% of cellulose filler, where PHB/TMCC5 presented a more pronounced decrease of T 1 H value (565 ms) compared to PHB/MCC5 (582 ms), which suggests a better dispersion when TMCC is used in this proportion.
PLA biocomposites presented a different behavior, showing higher T 1 H values for the biocomposites prepared with TMCC at 3 and 7 wt% and lower T 1 H value for PLA/TMCC5. This result indicated that for systems containing 3 and 7 wt% of cellulose filler, the MCC was more effective on the dispersion and for system containing 5 wt% of filler, the TMCC presented greater influence, as seen by the lowest T 1 H values found, when compared to systems containing the same proportion of cellulose, which indicates a better cellulose filler dispersion. The results obtained for T1H values indicate that MCC and TMCC act differently on the filler dispersion when added in the PHB or PLA matrices. Regarding to homogeneity at

Conclusion
Treated and untreated microcrystalline cellulose were separately incorporated into the PHB and PLA matrices in order to develop fully biodegradable composites. Important properties of the prepared biocomposites were evaluated, considering the effect of type and proportion of microcrystalline cellulose incorporated. The main results showed that MCC and TMCC influenced differently in PHB and PLA matrices. It was found that TMCC filler promoted better thermal stability for both studied matrices and that MCC and TMCC caused slight but discontinuous changes in the degree of crystallinity of the studied systems. TD-NMR technique provided consistent information about the homogeneity achieved in the prepared systems, indicating that lower proportions of cellulose fillers result in more homogeneous systems, which justifies the better thermal stability for both 3 wt% compositions. The results found in this work suggest that PLA and PHB biocomposites containing microcrystalline cellulose as dispersed phase are promising alternatives to replace non-biodegradable synthetic polymers. Further analyses to evaluate mechanical and barrier properties, as well as swelling tests are some prospects to point out the potential applications of these materials.