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Development and properties evaluation of bio-based PLA/PLGA blend films reinforced with microcrystalline cellulose and organophilic silica.

INTRODUCTION

Biodegradable polymers from renewable sources have been extensively investigated for replacing petroleum-derived polymers commonly used in various applications [1-3].

The advantages of these materials over conventional polymers obtained from nonrenewable sources include biodegradability. Biocompatibility, low toxicity, and low cost [4-7].

The most important and popular class of biodegradable polymers are aliphatic polyesters, with poly(lactic acid) (PLA) and its copolymers being important examples of this category.

PLA, in addition to its biodegradability and biocompatibility, exhibits excellent thermal and mechanical properties, and superior transparency of the processed materials [8-15].

Products based on PLA can be recycled after their use either by re-melting and reprocessing or by hydrolyzing to lactic acid, which is its basic building unit [16].

Owing to these properties. PLA has been used in various medical applications such as surgical implants, tissue culture, restorable surgical sutures, wound closure, controlled release systems, and has, in the last few years, gained attention as a food packaging material [8-17].

Another reason for using PLA in a large number of applications is the possibility of modifying its properties and improving its performance.

The mechanical and degradation properties of PLA can be modified by controlling parameters such as its chain size and degree of crystallinity, by the addition of comonomers, copolymers, and fillers as well as by controlling the manufacturing process [17], One of the promising materials that can be used to improve the properties of the PLA is cellulose, which is the most abundant renewable and biodegradable natural polymer. Cellulose can be produced from commercial sources such as wood pulp and cotton linters [18]. The isolated cellulose should be submitted to a partial acid hydrolysis process in order to produce microcrystalline cellulose. During the process of acid hydrolysis, the noncrystalline region is preferentially hydrolyzed to produce a cellulosic material with high crystallinity.

Microcrystalline cellulose can be applied for modifying the mechanical and thermal stability properties of PLA without affecting the transparency of the matrix [19, 20].

However, for obtaining optimal properties of the polymer/ cellulose materials, a good dispersion of the filler in the polymeric matrix is required.

The chemical compatibility between cellulose and the polymer plays a major role in its dispersion in the polymer matrix and the adhesion between these phases.

The aim of this work was to obtain materials based on PLA/ poly(lactic acid-co-glycolic acid)(PLGA) blends with microcrystalline cellulose and/or organophilic silica and characterize them using conventional techniques such as X-ray diffraction (XRD). differential scanning calorimetry (DSC) and tensile testing, and an unconventional technique, relaxometry for determining proton spin-lattice relaxation time ([T.sub.1] H).

For this purpose, hybrid materials were prepared by solution casting in the form of a film. Organophilic silica was added to improve the compatibility among other components of the system. since the use of cellulose materials with hydrophobic polymers such as PLA induces a weak interaction and results in poor filler-matrix adhesion. Similarly, PLGA was used to improve the filler dispersion in polymeric systems. Films with an improved flexibility can be fabricated using PLGA without causing significant loss of the desired material properties.

EXPERIMENTAL

Materials

Nature Works[TM] 2002D PLA in pellet form was supplied by Nature works, PLGA 430471 85:15 (laclide:glycolide) pellets were purchased from Sigma-Aldrich, MCC ph102 powder was obtained from Viafarma, and R972, an organophilic silica treated with dimethyldichlorosilane based hydrophilic silica, with a specific surface area of 130 [m.sup.2]/g, was supplied by Evonik Industries.

The molecular structures of these materials arc represented in Table 1.

Production of hybrid materials

In order to fabricate the films, separate dispersions of PLA/ PLGA and the filler in their respective concentrations were prepared. After mechanically stirring each solution for 24 h, the dispersions of polymers and fillers were mixed together for 24 h, and were cast into plates and kept in an oven in order to eliminate the solvent.

Eight films with different formulations comprising PLA/ PLGA. MCC, and/or R972 were prepared (see Table 2). Besides the films based on PLA/PLGA blends, two more films were prepared, each containing only one of these polymers to understand the behavior of the polymeric mixtures.

Characterization

X-ray Diffraction. XRD was performed using a Rigaku diffractometer with CuK[alpha] radiation ([lambda] = 0.154 nm, 40 kV, 120 mA) at room temperature, scanning over a 2[theta] range of 2-40[degrees] with a 0.05 step, at a rate of 1[degrees]/min. XRD provides information on the effect of MCC and/or R972 on the crystallinity of the polymeric matrix. Crystallinity measurements were obtained by XRD deconvolution. All calculations were performed using Fityk software and the peaks were deconvoluted using Gaussian peak functions.

Differential Scanning Calorimetry. DSC measurements were performed with a TA instrument (Q1000) in nitrogen atmosphere. A first heating scan at a rate of 20[degrees]C/min from room temperature to 150[degrees]C was employed to eliminate the residual water and solvent. The second scan was carried out at heating rate of 10[degrees]C/min from 0 to 200[degrees]C. DSC analysis was employed to evaluate the miscibility of PLA/PLGA blend.

Nuclear Magnetic Resonance. The relaxation time was analyzed in a Maran Ultra low-field nuclear magnetic resonance (NMR) spectrometer (Oxford Instruments), using an 18 mm NMR tube, operating at 23 MHz for hydrogen nucleus. The pulse sequence used to obtain data on spin lattice relaxation time was inversion-recovery (recycle delay--180[degrees]--[tau]--90[degrees]--acquisition data), and a 90[degrees] pulse of 4.7 [micro]s was calibrated automatically using the software package of the instrument. The amplitude of free induction decay (FID) was sampled for 40 t data points, ranging from 0.01 to 5,000 ms, using four scans for each point. The samples were analyzed at 27[degrees]C. The relaxation values and relative intensities were obtained by fitting the experimental data using a WINFIT software. Distributed exponential tits on plots of relaxation amplitude versus relaxation time were performed using a WINDXP software. Both the software packages were provided with the NMR spectrometer.

The relaxation time allows the evaluation of molecular dynamics of the samples according to changes in its values and domain curves. We evaluated this parameter from the data obtained from a single exponential decay of the magnetization.

Mechanical Properties. Tensile testing was carried out with an Instron universal testing machine (model 4204). Modulus and tensile strength of the films were determined to evaluate the effect of MCC and/or R972 on the mechanical behavior of hybrid materials. The tests were performed according to guidelines outlined in ASTM D882, which is the standard test method for tensile properties of thin sheeting and film less than 1 mm (0.04") thick. Five specimens of each film type were tested.

RESULTS AND DISCUSSION

X-ray diffraction

XRD patterns obtained for PLA and PLGA samples in a pellet and film form are shown in Fig. 1 and those for the PLA/ PLGA blends are given in Fig. 2.

XRD pattern of PLA pellet shows that the polymer is semicrystalline in nature. Mainly, two diffraction peaks were observed at 2[theta] values of 16.9 and 19.4[degrees], and a third weak peak was observed around 22.6[degrees]. These diffraction peaks are characteristics of PLA and are in consistent with XRD patterns recorded in other studies [21-23]. On the other hand, no peaks were observed in the XRD patterns of PLA film that was molded by solution casting, indicating an amorphous nature.

These results show that the morphology of these materials depends on the type of polymer molding used. The films were produced by solution casting, which does not favor the formation of ordered polymer chains due to the presence of the solvent molecules, thereby inhibiting the development of erystallinity.

Samples of PLGA in a film and pellet form were also analyzed by XRD (Figu. 1) and were shown to be amorphous in nature, as no diffraction peaks were observed. Although PLA and PGA homopolymers are crystalline in nature, the PLGA copolymer is found to be amorphous in different lactic acid/glycolic acid ratios. According to Kumar and Banker [24], PLGA copolymers containing less than 85% of glycolide in their compositions are amorphous in nature.

XRD analysis of the films of PLA/PLGA and PLA/PLGA/ R972 did not show the presence of any diffraction peaks. On the other hand, all the mixtures containing MCC are found to exhibit diffraction peaks. XRD patterns of the films containing MCC exhibit peaks corresponding to cellulose and PLA. From the literature, diffraction peaks of microcrystalline cellulose are observed at approximately 14.7, 16.6, and 22.6[degrees] [25, 26]. The peak at 22.6[degrees], which can be attributed to both PLA and cellulose, becomes more pronounced with the addition of MCC in comparison with the XRD curve obtained from the PLA pellet analysis. Therefore, according to XRD patterns of the films, MCC might be acting as a nucleating agent for crystallization.

Similar observations have been reported previously for different kinds of polymers such as PLA [27, 28], polycaprolactone (PCL) [29, 30], Polyvinyl alcohol) (PVA) [31, 32], poly(3-hydroxibutyrate) (PHB) [33], and poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) [34, 35].

The degree of erystallinity in our samples as obtained from the XRD deconvolution method is presented in Table 3.

The addition of cellulose not only promoted the erystallinity in the films, but also caused a gradual increase in it with the increasing of MCC content.

With the same MCC content, the erystallinity values of the PLA/PLGA films with silica (R972) were found to be higher than those of the films without silica, except for the systems with 7 wt% of MCC, for which results were quite similar with or without silica.

These results indicate that the addition of silica improved the compatibility between the polymer matrix and cellulose in the PLA/PLGA films and R972 can aid in reducing cellulose aggregations, as the hydroxyl groups in silica can interact with the hydroxyl groups present at the surface of the cellulose particles.

Therefore, an improved dispersion of these particles occurs as the number of spherulite nuclei increases, resulting in an enhancement of the erystallinity of the film. Additionally, R972 is an organically modified silica, on which functional groups derived from the insertion of dimethyl dichloro silane are present that can interact with the organic chains of PLA, promoting an improvement in the adhesion between the phases.

Pei et al. [36] reported that the nucleating effect of the cellulose is enhanced if a homogenous cellulose dispersion in the poly(lactic acid) matrix can be achieved.

The degree of erystallinity of the semicrystalline polymeric matrix determines the stiffness, mechanical and fracture behavior of these materials.

Differential scanning calorimetry

PLA and PLGA polymers as well PLA/PLGA blends were examined using DSC. The results of the analysis (the values of glass transition temperature ([T.sub.g]) and melting temperature ([T.sub.m])) are summarized in Table 4.

The DSC results for PLA indicated the occurrence of two dominant transitions located approximately at 58 and 155[degrees]C, corresponding to the glass transition temperature and the melting temperature, respectively. This shows that the polymer PLA is semicrystalline in nature, in consistent with the XRD analysis.

In the thermal analysis of PLGA, only one dominant transition at 46[degrees]C was observed, corresponding to the [T.sub.g] of the copolymer. Tm could not be determined for this material due to its amorphous nature.

Similarly, the DSC curves of the PLA/PLGA blend exhibit only one dominant transition (51[degrees]C) corresponding to [T.sub.g]. The fact that the blend exhibits a single [T.sub.g] at a value that is in intermediate to the [T.sub.g]s of the two polymeric constituents indicates that PLA and PLGA are miscible at a ratio of 4:1. Similar behavior was observed for all the PLA/PLGA blends with fillers. Although XRD analysis indicated an amorphous nature of the PLA/PLGA and PLGA/PLGA/R972 films, DSC analysis of these samples showed the presence of a [T.sub.m]. This could be due to the presence of semicrystalline PLA component in the polymeric blend.

Even though a slight variation was observed in the [T.sub.g] values of the different films, a trend in the changes in [T.sub.g] could not be obtained, as the standard deviation of the equipment itself is 2[degrees]C.

It was observed that the [T.sub.m] values for films with 5 and 7 wt% of MCC are higher than that of the PLA/PLGA blend without MCC. Tm is directly related to the size of the crystalline domains, which can be influenced by the presence of cellulose as well as the solvent and the processing method employed. These results indicate that the size of the crystalline domains was modified in the matrices.

The term miscibility has been largely used to describe polymer blends whose behavior is similar to that of a single-phase system [37, 38].

The definition given by Olabisi et al. [38] indicates that the term miscibility does not relate to ideal molecular mixing; however, the level of molecular mixing is adequate to yield macroscopic properties expected for a material with only one phase.

According to Kalogeras and Brostow [39], glass transition temperature as a function of composition can provide information on the miscibility of polymeric blends, which virtually decides all properties of these materials.

Immiscibility between two polymeric components in the blends could lead to phase segregation, which frequently occurs in immiscible blends, and this could affect the material performance by affecting its properties such as the tensile strength.

Although the results reveal the miscibility of the PLA/PLGA systems, NMR analysis was carried out to confirm this observation, and found that the NMR results are in good agreement with the DSC, as explained in the "Nuclear magnetic resonance" section.

Nuclear magnetic resonance

NMR studies for measuring a proton spin-lattice relaxation time (T|H) have been recently employed to observe changes in the molecular mobility of different polymer systems. T,H can provide important and detailed information about the compatibility between the matrix and the dispersed phase, the homogeneity of the materials as well as the dispersion and distribution of the fillers [23].

In this work. [T.sub.1]H measurement was employed to evaluate the influence of the addition of microcrystalline cellulose and/or organophilic silica on the molecular mobility of the PLA/PLGA blends and the domain distribution curves were obtained to examine the homogeneity of the blends.

[T.sub.1]H determined from monoexponential fit of all the materials are displayed in Table 5. The distribution domains curves obtained for the polymer and the fillers are given in Fig. 3. and those for the PLA/PLGA blends are displayed in Fig. 4.

A [T.sub.1]H value of 700 ms was obtained for the PLA pellet, whereas the PLA film gave a value of 646 ms. The low [T.sub.1]H value for the PLA film can be attributed to a higher molecular mobility in the PLA film as compared to the PLA pellet. This loss of crystallinity during film formation was also observed in the XRD analysis ("X-ray diffraction" section).

High molecular mobility means that there are larger spaces between the chains, facilitating the movement characteristic of flexible materials. Conversely, low molecular mobility means that the chains are constrained and. consequently, the space between the molecular chains is smaller, resulting in the creation of a more rigid material [40, 41].

PLGA pellet exhibits a lower [T.sub.1]H value (625 ms) than the PLA pellet (700 ms). This variation can be attributed to the amorphous nature of PLGA, resulting in a higher molecular mobility compared to PLA, which has a semicrystalline morphology as discussed in the XRD section. Moreover, the presence of glycolic acid mers within the copolymer increases the space between the PLGA chains since glycolic acid has a low miscibility in lactic acid and causes irregularities in the PLGA chains. This increases the molecular mobility, thereby reducing T|H. PLA and PLGA are miscible because the quantity of lactic acid mers is much higher than the glycolic acid mers in the PLGA. Thus, PLA chains can interact with the PLGA chains with their lactic acid segments.

Even though the [T.sub.1]H values of both PLA/PLGA/R972 and PLA/PLGA films are similar due to the amorphous structure, the samples containing silica exhibited a [T.sub.1]H value slightly higher than the one without silica. This result can be attributed to the rigid nature of R972 that do not change the crystallinity profile of the matrix, but reduces the molecular mobility of the films, which increases the relaxation time of hydrogen nuclei. resulting in a higher [T.sub.1]H value.

Similar behavior was observed in the case of films with equal amounts of MCC and varying amounts of R972. All the films containing both MCC and R972 exhibited a higher [T.sub.1]H than those found for the corresponding films with only MCC. In this case, the increase in the [T.sub.1]H values for systems containing both MCC and R972 can be attributed to the rigid nature of silica and the increase in crystallinity of the films due to the presence silica, as seen by XRD (Table 2).

It was seen that all the proportions of MCC used in this study were enough to cause changes in the molecular dynamic behavior of the films, as noted by the progressive reduction in the [T.sub.1]H values with gradual increase of MCC ratio.

Although a high [T.sub.1]H value is expected for cellulose, as the material has a high stiffness, elevated tensile modulus, degree of crystallinity and low molecular mobility, a low [T.sub.1]H value was observed for MCC (Table 5). This effect can be attributed to water absorption by cellulose, due to the presence of hydroxyl groups in its structure. The absorption of water drastically influences the physical properties of cellulose [42, 43].

Taylor et al. [44] have investigated the interaction of water with cellulose and its influence on the nuclear spin dynamics in cotton using a 1H and 13C solid-state NMR techniques. According to these authors. The 1H spin-lattice relaxation time varies with water content.

The sorption of moisture allows some of the secondary bonds between cellulosic OH groups to be replaced by hydrogen bonds of these hydroxyl groups with water molecules. When sufficient moisture is sorbed, promoting these interactions between celulose and water, some of the solid matrix becomes plasticized. and occurs the mobilization of some segments of the cellulose matrix, giving rise to a high molecular mobility and low relaxation times [45]. The effect seen in the MCC was also observed in the blends in which MCC was added (Table 5).

To complement the molecular dynamic study by NMR anc to get a better idea about the homogeneity of the prepared blends, [T.sub.1]H domains curves were obtained (Figs. 3 and 4).

Figure 3 shows that only one domain is observed for isolated polymers (PLA and PLGA) and MCC, indicating a homogeneous structure. The displacement of the domain to lower time; observed for MCC confirms a higher molecular mobility as compared to PLA and PLGA. The enlargement on base line observed for PLA represents its lower molecular mobility, as evident from its higher [T.sub.1]H value compared to PLGA (Table 5).

As seen in Fig. 4A, only one domain was observed in the case of PLA/PLGA blends. This indicates the homogeneity of the system and confirms the data obtained from DSC experiment wherein only one [T.sub.g] value was found, indicating its miscibility ("Differential scanning calorimetry" section).

The addition of isolated R972 in the PLA/PLGA caused a small enlargement in the base line of the domain compared to the film without R972. This behavior can be attributed to the changes in the molecular mobility, which is observed by an increase in the [T.sub.1]H value (Table 5).

Two domains were observed for all the blends containing MCC, as seen in Fig. 4C and D. The domain with highei intensity corresponds to the PLA/PLGA matrix. This domair is responsible for controlling the hydrogen nucleus relaxation process. The lower intensity domain is attributed to MCC This domain at lower times, which can be attributed to the presence of water molecules as explained, presented lower intensity corresponding to the smaller proportion of MCC in the blend.

According to the [T.sub.1]H domain curves, an increase in the MCC content is found to promote the homogeneity in the blends. This can be attributed to the fact that the domains becomes closer with the progressive addition of MCC (Fig. 4C and D).

The data obtained by domain curves show that a better filler distribution/dispersion was observed in the PLA/PLGA blends containing 7 wt% of MCC regardless of the presence of R972.

Mechanical properties

The tensile strength and the tensile modulus of each film were determined to evaluate the improvements in the mechanical behavior due to MCC and/or R972 incorporation.

Figure 5 represents the results for the tensile strength of the films based on PLA/PLGA blends.

An increase in the tensile strength values were observed for the PLA/PLGA/MCC system in comparison to that observed for the PLA/PLGA film. The tensile strength values for the films with 3 and 5 wt% of MCC are similar and the increase in their tensile strength was very small. The film with 7 wt% of MCC exhibited a higher tensile strength value.

PLA/PLGA/MCC systems exhibited tensile strength values of 26.5, 27, and 29.3 MPa for films with 3, 5, and 7 wt% of MCC, indicating an ~3, 5, and 14% increase, respectively, in comparison with the values obtained for PLA/PLGA without fillers.

PLA/PLGA/MCC/R972 films with 3, 5, and 7 wt% of MCC showed an increase in the tensile strength values of about 16, 25. and 27%, respectively, in comparison with the values for PLA/PLGA/R972. These values are larger than those found for similar systems without R972.

The tensile strength values of the PLA/PLGA/5MCC/R972 and PLA/PLGA/7MCC/R972 films are similar. Hence, 5 wt% of MCC content was found to be optimal in the presence of R972.

Frone et al. [46] reported that for a series of composites based on PVA and nanofibers of cellulose isolated from MCC by ultrasonication, the highest improvement in the tensile strength was achieved for samples with 5 wt% of cellulose added to the PVA matrix.

The optimal cellulose content required for improving the mechanical properties of nanocomposites based on polyurethane was also found to be 5 wt% by Wu and colleagues in 2007 [47]. Nanocomposites containing 5 wt% of cellulose exhibited a tensile strength that is approximately seven times larger than that of the pure polymer matrix.

Oka [48] has reported the preparation of materials based on PLA reinforced with lactic acid-g-hydrolyzed microcrystalline cellulose particles. In this study, the cellulosic filler loadings were varied from 5 to 15 wt%. A high value for tensile strength was achieved with 5 wt% of lactic acid-g-hydrolyzed MCC. The tensile strength value remained similar for the material with 10% of the filler, and then decreased with 15%. The author attributed this increase on tensile strength of composites of the PLA and cellulose microcrystalline to the improvement of filler dispersion and filler-matrix interactions, which was induced by surface polymerization of PLA on the cellulose particles. Thus, this method to produce the composites led to an improvement of filler dispersion and increase of its effective volume fraction in the composite and resulted in an increased reinforcement with the same filler.

Alternatively, some studies have shown that the ideal content of cellulosic fillers within a material may vary depending on the polymer-cellulose system under study.

Zimmermann et al. |49] in their work on nanocomposites of poly(vinyl alcohol) and cellulose microfibrils isolated from sulphite pulp, observed that the addition of 5 wt% of cellulose did not increase the tensile strength or stiffness of the PVA composites. Therefore, a minimum filler content is required to induce strong interactions between the polymer and fibrils.

Rahman et al. [50] have produced biocomposite films made from PVA reinforced with crystalline cellulose. The study showed that tensile strength increased with an increase in cellulose content up to 9 wt%. Above this percentage, the tensile strength started to decrease. The authors associated this behavior to the aggregation of cellulose in the composite.

Zulkifli et al. [51] have found that in the case of polypropylene/MCC composites incorporated with maleic anhydride-grafted polypropylene (MAPP) as coupling agent, an increment in tensile strength was observed when the MCC content was varied from 2 to 40%. The addition of MAPP induced an improvement in the tensile strength via an enhancement of the interfacial adhesion between the MCC fiber and the polypropylene matrix.

The tensile strength showed notably higher values for films containing both MCC and silica. However, the tensile strength values for PLA/PLGA and PLA/PLGA/R972 films are found to be similar. This result indicates that the addition of silica alone was not enough for promoting an increase on the tensile strength.

On the other hand, the highest value for tensile strength was obtained for PLA/PLGA-based films when both types of fillers were added concomitantly.

The observed increase in tensile strength indicates a better stress transfer from the matrix to the reinforcement at the filler-polymer interface.

The addition of cellulose does not improve the tensile strength significantly, which can be attributed to an insufficient cellulose-matrix interaction. Although PLA contains polar groups that can promote the polymer-cellulose interactions by hydrogen bond formation between hydroxyl groups of cellulose and carbonyl groups of PLA, it seems to be insufficient to guarantee a strong interaction of cellulosic filler-matrix to promote a significant increase in the mechanical properties. This might probably be attributed to the difficulty in forming a uniform dispersion of the cellulose with a polar surface in nonpolar plastics.

Thus, a method to improve this interaction and to obtain a better dispersion of cellulose particles is essential.

Interfacial interactions can be improved by several physical and/or chemical treatment methods, such as the use of ultrasound, surfactants, coupling agents, and chemical modification of the surface of cellulose (using grafting polymerization or by admicellar polymerization). In this study, we used modified silica to improve the tensile strength.

Figure 6 shows the results for the tensile modulus of PLA/ PLGA films.

An increase in the tensile modulus of the films was observed with an increase in the MCC content. For PLA/PLGA/MCC systems, the tensile modulus achieved an increase of 14, 30, and 42% for films with 3, 5, and 7 wt% of MCC, respectively, compared with the PLA/PLGA without fillers.

Similarly, PLA/PLGA based films with R972 also exhibited an increase in the tensile modulus with the addition of MCC. An increment of 27, 34, and 31% was obtained for films with 3, 5, and 7 wt% of MCC, respectively, compared to the values found for PLA/PLGA/R972.

In the case of films with the same cellulose content and different silica content, films containing R972 exhibited higher values of tensile modulus for films with 3% and 5% of MCC. However, films with 7% of MCC exhibited similar values for this property.

These results on the tensile modulus are in accordance with the results on the degree of erystallinity measurements, since the degree of erystallinity has a significant influence on the mechanical properties. Crystallinity affects secondary intcrmolecular bonding within a material, thereby increasing the tensile modulus with an increase in erystallinity. This secondary bonding is much less prevalent in amorphous regions because of the inherent chain misalignment. However, for crystalline regions that are characterized by an ordered and parallel arrangement of molecular chains, extensive secondary bonds typically exist between the adjacent chain segments.

Mathew et al. [52] reported the reinforcement of PLA with three different cellulose-based materials, namely microcrystalline cellulose, wood flour and wood pulp. Addition of microcrystalline cellulose resulted in samples with an improved tensile modulus and dynamic modulus, but a low tensile strength. They attribute the improvement in the mechanical properties to the nucleating ability of the microcrystalline cellulose.

Wu et al. [47] have prepared polyurethane-based elastomeric composites with microcrystalline cellulose and obtained high values for modulus, strength, and strain to failure due to a strong interfacial interaction between the matrix and the microcrystalline cellulose.

Thus, in order to improve the tensile modulus, an effective compatibility between phases in the filler-matrix system is essential.

Abdelmouleh et al. [53] have produced materials based on unsaturated polyester and phenolic resin reinforced with cellulose fibers using organoalkoxisilanes as coupling agents and observed an increase in their tensile modulus.

Thus, the use of R972 alone did not produce a notable increase in the tensile modulus. However, when both silica and cellulose were used concomitantly, the tensile modulus exhibited a higher value compared to other films. This phenomenon can be attributed to the synergic effect of the fillers MCC and R972 and a good interaction between the components of the films. Silica (R972) reduced the interfacial tension between PLA and MCC, which probably occurred because the hydroxyl groups on the silica surface interacted with cellulose through hydrogen bonds, thereby increasing the interaction between silica and PLA, due to the organophilic nature of the silica used.

CONCLUSIONS

Biodegradable materials reinforced with microcrystalline cellulose were successfully prepared by solvent casting, and the effect of cellulose and organophilic silica on their properties was investigated.

XRD studies of the materials demonstrated that cellulose acts as a nucleating agent for the crystallization of PLA/PLGA blends. Although PLGA is an amorphous material, it could not prevent the nucleation in the semicrystalline PLA.

Use of silica as a filler did not affect the crystallinity of the polymeric matrix. However, when silica and cellulose were added together, a pronounced increase in the crystallinity was observed because of the synergic effects of the fillers. This effect can be attributed to a strong interaction between silica and cellulose.

NMR analysis indicated that cellulose and silica have opposite effects on [T.sub.1]H parameter. While cellulose decreases [T.sub.1]H. silica is found to increase it.

The domain distribution curves revealed that PLA/PLGA with a ratio of 4:1 arc homogeneous, which was confirmed by DSC analysis that exhibited a single [T.sub.g].

The organophilic silica used in this study did not act as a reinforcement filler in the films. However, it enhanced the reinforcement effect of microcrystalline cellulose. Hence, when both the filers were used together, improved mechanical performance was obtained.

Thus, our study indicates that a better dispersion of cellulose in the polymer matrix in the presence of silica increases the particle-matrix interaction.

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Fernanda Abbate dos Santos, (1,2) Gisele Cristina Valle Iulianelli, (1,2) Maria Ines Bruno Tavares (1,2)

(1) Instituto de Macromoleculas Professora Eloisa Mano--Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil

(2) Centro de Tecnologia Bloco J--Cidade Universitaria llha do Fundao, Rio de Janeiro, RJ, CEP 21945-970, CP 68525, Brazil

Correspondence to: F.A. dos Santos; e-mail: labbate@ima.ufrj.br

Contract grant sponsor: CAPES; contract grant sponsor: FAPERJ.

DOI 10.1002/pen.24447

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. XRD patterns of PLA and PLGA. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. XRD patterns of PLA/PLGA-based films. [Color ligure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. [T.sub.1]H Domain distribution curves for MCC and polymers in pellet form. [Color figure can be viewed at wileyonlinelibrary.com]

FIG. 4. [T.sub.1]H domain distribution curves for PLA/PLGA-based films. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Tensile strength for the films of PLA/PLGA/MCC and PLA/ PLGA/MCC/R972. (Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Tensile modulus for the films PLA/PLGA/MCC and PLA/PLGA/ MCC/R972. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: TABLE 1. Molecular structures of the materials.
TABLE 2. Formulations of PLA/PLGA films.

Polymer matrix (wt)   Fillers (%)      Film

2g PLA/PLGA,            --             PLA/PLGA
4:1 ratio             3% MCC           PLA/PLGA/3MCC
                      5% MCC           PLA/PLGA/5MCC
                      7% MCC           PLA/PLGA/7MCC
                      3% R972          PLA/PLGA/R972
                      3% MCC+3% R972   PLA/PLGA/3MCC/R972
                      5% MCC+3% R972   PLA/PLGA/5MCC/R972
                      7% MCC+3% R972   PLA/PLGA/7MCC/R972

TABLE 3. Degree of erystallinity (%) obtained lor the materials.

Material                Crystallinity (%)

PLA pellet              30 [+ or -] 0.4
PLGA pellet             Amorphous
PLA film                Amorphous
PLGA film               Amorphous
PLA/PLGA                Amorphous
PL A/PLG A/3 MCC        18 [+ or -] 1.7
PLA/PLGA/5MCC           30 [+ or -] 0.9
PLA/PLGA/7MCC           40 [+ or -] 1.7
PLA/PLGA/R972           Amorphous
PLA/PLG A/3 MCC/R972    25 [+ or -] 0.8
PLA/PLGA/5 MCC/R972     37 [+ or -] 0.9
PLA/PLGA/7MCC/R972      41 [+ or -] 0.5

TABLE 4. Measurements of [T.sub.g] and [T.sub.m] of the polymers
and blends.

Sample                 [T.sub.g]       [T.sub.m]
                      ([degrees]C)   ([degrees]C)

PLA pellet                 58             155
PLGA pellet                46             --
PLA/PLGA                   51             153
PLA/PLGA/3MCC              49             154
PLA/PLGA/5MCC              50             153
PLA/PLGA/7MCC              49             157
PLA/PLGA/R972              50             157
PLA/PLGA/3MCC/R972         47             153
PLA/PLGA/5 MCC/R972        49             158
PLA/PLG A/7MCC/R972        49             158

TABLE 5. [T.sub.1]H values for the materials.

Stab                 [T.sub.1]H (ms) [+ or -] 2%

MCC                              186
PLA pellet                       700
PLGA pellet                      625
PLA film                         646
PLA/PLGA                         630
PLA/PLGA/3MCC                    589
PLA/PLGA/5MCC                    567
PLA/PLGA/7MCC                    537
PLA/PLGA/R972                    639
PLA/PLGA/3MCC/R972               593
PLA/PLGA/5MCC/R972               578
PLA/PLGA/7MCC/R972               547
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Author:Santos, Fernanda Abbate dos; Iulianelli, Gisele Cristina Valle; Tavares, Maria Ines Bruno
Publication:Polymer Engineering and Science
Article Type:Report
Date:Apr 1, 2017
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