Thermal and Crystallization Behavior of PLA/PLLA-Grafting Cellulose Nanocrystal

PLLA-modified cellulose nanocrystals (CNC) were produced from commercial CNC by tin-catalyzed polymerization of lactide in presence of CNC. FTIR spectroscopy demonstrated that the result of the reaction produced the grafting of PLLA chains onto CNC surface (CNC-g-PLLA). Films of poly(lactic acid) (PLA) and PLA/CNC nanocomposites (with non-modified CNC and CNC-g-PLLA) containing 0.5% and 5% (w/w) of the nanofillers were prepared by casting in chloroform solution and the crystallization behavior and thermal properties investigated. All nanocomposites had similar thermal stability when analyzed by TGA analyses under an inert nitrogen atmosphere. Addition of both types of CNC influenced crystallization, the higher crystallization rate being observed for 5% (w/w) CNC. Nanocomposites with 5% (w/w) CNC-g-PLLA had the strain resistance of PLA improved in the rubbery state. PLLA-modification of CNC surface increased the crystallization of PLA in PLA/CNC nanocomposites and improved the rigidity at temperatures above the glass transition, properties which are desirable for hot drinking application.


Introduction
Accumulation of plastic objects and particles as plastic bottles, bags and microbeads in the Earth's environment has adversely affected wildlife habitat and humans. Thus, in recent years, there has been a growing interest in the development of environmentally friendly materials, as well as a need to replace synthetic polymers by biodegradable polymer materials, such as poly(lactic acid) (PLA), Materials Sciences and Applications especially in packaging [1] [2]. Cellulose-based materials are interesting biodegradable fillers and have shown to be able to improve the properties of PLA by affecting the crystallinity, and mechanical and thermal properties [3] [4]. In this sense, in films for packaging applications, nanosized cellulose like celullose nanocrystals (CNC) can be an interesting option since improvement of PLA mechanical properties can be reached at low filler content [5] [6] [7] [8]. Nevertheless, mixing of CNC and PLA faces many obstacles related to interfacial incompatibility due to hydrophilic nature of CNC surface and hydrophobic nature of PLA chains. A good way to make compatible CNC with PLA is to graft PLA chains on CNC surface, considering that this CNC surface modification can, in principle, increase the interaction between the filler and the matrix. In addition, such a kind of composite can potentially result in 100% compostable materials [9].
In this work, we report the crystallization and thermal behavior of PLLA-grafting CNC synthesized by in-situ polymerization of L-lactide with the presence of CNC as well as PLA/CNC nanocomposites films prepared by Solution Casting Technique. The effect of reaction conditions on these properties of CNC-PLLA nanomaterials is also reported.

Materials
Film grade PLA Ingeo 2003D (M w = 185.000 g•mol −1 ) from NatureWorks was used as received. Commercial cellulose nanocrystals were furnished by Cellu-Force (FribriaCellulose, Brazil). L-lactide (Purasorb L) was supplied by Corbion, Netherland. Tin II octanoate (Aldrich, USA) was used as received and toluene and chloroform were purified by distillation.

Cellulose Nanocrystal Surface Modification with Lactide (CNC-g-PLLA)
Commercial CelluForce CNC was vacuum dried at 80˚C for 12 h. CNC-g-PLLA nanoparticles were prepared by in-situ ring-opening polymerization of L-lactide.
Initially, lactide was introduced into a round bottom glass flask and heated at 100˚C up to the complete melting. CNC and a 15% (v/v) solution of tin II octanoate in chloroform were added to the flask which was magnetically stirred. The temperature was then raised to 110˚C and the reaction system maintained at this temperature for 2 h 30 min under nitrogen atmosphere. At the end of reaction, the material was solubilized in chloroform , and precipitated in ethanol, resulting in a solid containing probably PLLA grafted onto CNC particles (CNC-g-PLLA) and PLLA. The solid product was dissolved in chloroform and centrifugated to separate CNC-g-PLLA from free PLLA. This procedure was repeated 3 times.
CNC-g-PLLA was collected after a final centrifugation. in the step-scan mode with a 2θ angle ranging from 4˚ to 50˚ with a step of 0.04 and scanning time of 5.0 min.

Nanocomposite Films Characterization
Thermogravimetric analyses were performed on a TGA Q500 TA Instruments under inert atmosphere (nitrogen flow, 10 mL/min) from room temperature to 600˚C. Crystallinity and melting behavior of the films were evaluated using a DSC Q-1000 (TA Instruments). Samples were first heated from 20˚C to 200˚C at 20˚C/min and held for 3 min to erase the thermal history. Subsequently, it was cooled at 50˚C/min to the prescribed crystallization temperature (100˚C) and held for enough time to fully crystallize. Samples were then, cooled at 20˚C/min to 20˚C and held for 3 min. Finally, the sample was heated again to 220˚C at 10˚C/min to obtain the melting temperature (T m ). The analyses were carried out under nitrogen atmosphere using a flow rate of 20 mL/min. Figure 1 shows the representative TEM image of commercial CNC. As expected, CNC showed a rod-like shape reported in the literature [10]. CNC showed a mean length of 107 ± 36 nm.

Nanocrystals Morphology, Structure and Thermal Features
In-situ polymerization of L-lactide on hydroxylated CNC surface may produce PLLA chains covalently bonded to CNC (CNC-g-PLLA) ( Figure 2).      200) and (040) planes of cellulose I crystalline structure, respectively [11]. CNC-g-PLLA showed the PLLA crystalline peaks, which confirms the presence of PLLA chains bonded onto the surface of the nanocrystals. Degree of crystallinity of 68% and 70% for CNC and CNC-g-PLLA was found, respectively. These values are in accordance with expected values for CNC reported in the literature [14], which consider small variations in the degree of crystallinity to variation of acid concentration used to produce nanocrystals from cellulose hydrolysis. For CNC-g-PLLA, presence of crystalline peaks of PLLA seems to cause the apparent decrease in the intensity of CNC crystalline peaks due to the decrease of CNC content in the modified filler. According to Cetin et al. [15], during the progress of modification reaction, internal crystalline regions can be modified.
TG curves of PLA, CNC and CNC-g-PLLA are shown in Figure 5. TG curve of CNC presented an initial weight loss of 6% which was attributed to water evaporation, while for CNC-g-PLLA this weight loss was 50% lower. Considering   According to the supplier, the commercial CNC used in this work was obtained by sulfuric acid hydrolysis. This information could be confirmed by the DTG curve of the CNC, which showed three stages of weight loss, the first around 50˚C probably due to evaporation of absorbed water. The others two are related to the cellulose structure, with sulfate moieties accelerating the weight loss of glycoside components [16].
It is important to emphasize that although the weight loss profile of CNC and CNC-g-PLLA are similar, CNC showed higher thermal stability since the onset temperature is up to 100˚C higher.
For CNC-g-PLLA it is possible to observe two well defined stages of weight loss, in addition to the water loss. The first one is related to the glycoside components and the second one manifested as a shoulder seems to be related to sulfate groups still present in the structure, overlapped with weight loss attributed to lactide, since it occurs at the same temperature range for PLLA [16] [17].

Thermal Transitions of PLA/CNC Films
Films of PLA and PLA/CNC nanocomposites (PLA/CNC and PLA/CNC-g-PLLA) were prepared by casting and the thermal behavior investigated by DSC. Figure   6 presents the DSC traces obtained from the first heating of these films, while Figure 7 shows the DSC traces of a second heating run, after a first heating and subsequent quenching of the samples. Table 1     The first DSC heating run of all films ( Figure 1) showed a glass transition and a melting peak, indicating the semi-crystalline nature of the materials. No cold crystallization was observed, suggesting that these materials present the high X c which is commonly found for PLA casting films. In fact, the results of Table 1 show that addition of both unmodified and modified-CNC increased X c , the fact noted using either DSC or XRD. The melting peaks were monomodal (T m , 150˚C -154˚C), excepting for the nanocomposite with the higher content of unmodified CNC (5% w/w), where a shoulder appears at lower temperature.  were performed under an inert nitrogen atmosphere. It must be emphasized the presence of a peak around 300˚C for the samples with 5% (w/w) filler, which seems to be related to the filler degradation present in higher amount.

Thermal Stability of PLA/CNC Films
The crystallinity of the nanocomposites was evaluated by X-ray diffraction.
According to Figure

Crystallization Behavior of PLA/CNC Films
The crystallization kinetics of the films was investigated and results of relative crystallinity versus crystallization time are presented in Figure 10.  showed the higher crystallization rate. The highest crystallization rate was attained for the nanocomposite containing 0.5% (w/w) of unmodified CNC.

Dynamic Mechanical Behavior of PLA/CNC Films
The effect of CNCs on the mechanical behavior of PLA was investigated by DMA. The behavior of the storage modulus (E') as function of temperature is shown in Figure 11 for films of PLA and PLA/CNC nanocomposites containing 0.5% and 5% (w/w) of non-modified and PLLA-modified CNC. All films showed the typical semi-crystalline behavior with high modulus below the glass transition, followed by an abrupt decrease in E' after the glass transition temperature (T g ), i.e., in the rubbery state.    It is also important to mention that after T g , the majority of the samples showed a progressive increase in E' with increasing temperature which indicated the cold crystallization phenomenon (Figure 11). Only the nanocomposite containing 5% (w/w) CNC-g-PLLA did not show this behavior, demonstrating that this nanocomposite film had a high degree of crystallinity.
Spinella et al. [18] used the Tan d peak profile to verify the effect of surface top chemistry on the interaction between CNCs and PLA. According to the authors, since Tan δ is related to polymer chain relaxation, when the interaction between the filler and matrix increases, it is expected that the Tan δ peak is shifted to higher temperatures, the peak intensity decreases, and the transition is broadened. Results of Tan δ versus temperature for PLA and the PLA/CNC nanocomposites investigated in this work are presented in Figure 12. While no decrease in the peak intensity was noted for the composites with 0.5% and 5% (w/w) CNC, an intensity decrease was observed for both composites containing  the PLLA-modified CNC. In addition, it is clearly seen that Tan δ peak is broad for the composite containing 5% (w/w) CNC-g-PLLA, which suggests the better adhesion of the filler with the PLA matrix.

Conclusion
In this work, modification of commercial cellulose nanocrystals was performed