Preparation of bio-based nanocomposite emulsions: Effect of clay type

Progress in Organic Coatings 74 (2012) 660–666

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Preparation of bio-based nanocomposite emulsions: Effect of clay type Karuntarut Sermsantiwanit a , Siriwan Phattanarudee a,b,∗ a b

Program of Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Patumwan, Bangkok 10330, Thailand Department of Imaging and Printing Technology, Faculty of Science, Chulalongkorn University, Patumwan, Bangkok 10330, Thailand

a r t i c l e

i n f o

Article history: Received 6 June 2011 Received in revised form 1 September 2011 Accepted 29 September 2011 Available online 14 December 2011 Keywords: Nanocomposite emulsion Biopolymer Montmorillonite Organoclay Emulsification–diffusion method Microsphere

a b s t r a c t Bio-based nanocomposite latexes were prepared by emulsification–diffusion method. Two kinds of silicate, sodium and organo-modified montmorillonites, were compared and clay concentrations were varied at 1–6 wt%. The resultant particles were characterized in terms of particle size, particle morphology, and zeta potential. Silicate dispersion in the polymer matrix was evaluated by X-ray diffractometry and transmission electron microscopy, while thermal properties of the resultant nanocomposite films were also investigated by differential scanning calorimetry and thermogravimetry. The average particle sizes of the emulsions were found in the range of 20–30 ␮m, whereas zeta potentials of the particles were obtained in the range from −37 to −60 mV, depending on the silicate type and content. X-ray diffraction patterns combined with micrographs indicated that the silicates were effectively distributed in the polymer matrix where extensive exfoliation and partial intercalation were present. Thermal properties of the nanocomposite films were significantly improved compared to those of the unfilled film. Thermal stability tended to increase in accordance with clay content for both types of silicate, in which the highest shift of degradation temperature was obtained with the sodium montmorillonite filled nanocomposite film at the maximum loading content (6 wt%). © 2011 Elsevier B.V. All rights reserved.

1. Introduction Biopolymers are of growing research interest. One of the most is polylactic acid (PLA), produced from renewable resources, and its properties are benign to the environment [1]. Polylactic acid is a linear aliphatic thermoplastic polyester produced by the ringopening polymerization of lactic acid monomer, which is obtained from the fermentation of corn, potato, sugar cane, and sugar beet [2]. Owing to its biodegradable property, it is often employed in preparation of bio-plastic for producing compostable bags, food packaging, and disposable tableware. However, its strength and thermal stability are often insufficient for wider industrial applications. Addition of a small amount (1–10 wt%) of nanofillers to polymers forming polymer layered silicate nanocomposites has gained increasing attention in the last decades because of their remarkably improved mechanical, thermal, barrier, and flame retardance properties compared to the unfilled matrix [3,4]. Among the commonly used layered silicates, montmorillonite (MMT) is the most familiar species of smectide group, which consists of two fused silica tetrahedral sheets sandwiching an edge-shared

∗ Corresponding author at: Department of Imaging and Printing Technology, Faculty of Science, Chulalongkorn University, Patumwan, Bangkok 10330, Thailand. Tel.: +66 2 2185568; fax: +66 2 2553021. E-mail address: [email protected] (S. Phattanarudee). 0300-9440/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2011.09.033

octahedral sheet of either magnesium or aluminum hydroxide [5]. MMT has been frequently employed in nanocomposite systems because of its high aspect ratio and high surface area [3]. In general, unmodified layered silicates in their pristine state are hydrophilic, creating intrinsic incompatibility with hydrophobic engineering polymers. Modification of the natural clay is often done through cationic-exchange processes between the interlayer cations (Na+ or Ca2+ ) of clay minerals and quaternary alkylammonium or alkylphosphonium cations to render compatibility and improve interfacial interaction between phases [6,7]. After being dispersed in the polymeric matrix, two idealized polymer/silicate structures are expected: intercalated and exfoliated. The highest mechanical property increments are favorably obtained with the exfoliated nanocomposites in which the silicate layers are fully delaminated and individually dispersed through out the continuous phase, while the intercalated structure is obtained through moderate polymer penetration, resulting in interlayer spacing enlargement and generating a modest improvement. In the field of nanocomposite emulsions, earlier findings showed that the nanocomposite latexes provided better mechanical, thermal, and anticorrosive properties than the neat polymer [8–12]. Diaconu et al. synthesized high solid content of poly(methyl methacrylate-co-butyl acrylate)/montmorillonite nanocomposite latexes by in situ emulsion polymerization, in which the materials exhibited improved mechanical properties (tensile strength and modulus) and reduced water vapor permeability [8]. Lai et al.

K. Sermsantiwanit, S. Phattanarudee / Progress in Organic Coatings 74 (2012) 660–666

prepared a series of advanced environmentally friendly anticorrosive latexes by effectively dispersing Na+ -montmorillonite into water-based polyarylate (i.e. vinyl acrylic terpolymers) [9]. It was found that the resultant latexes at low clay loading coated on the cold rolled steel coupons were remarkably superior in anticorrosion efficiency to those of the neat polyacrylate based on electrochemical measurements of corrosion potential and corrosion current. The nanocomposite films (at 1 wt% of clay) showed about ∼50% and ∼12% reduction in H2 O and O2 permeability, respectively. Moreover, the onset of thermal decomposition of the nanocomposites shifted slightly toward the higher temperature range than that of the neat material, which confirmed the enhancement of thermal stability of exfoliated polymer. Recently, Yilmaz et al. investigated the effect of clay types on emulsion polymerization of acrylate/montmorillonite nanocomposites [10]. The latexes obtained had fine particle sizes varying from 94 to 174 nm with a very narrow distribution. Mechanical and thermal analyses showed that the elastic moduli and thermal stabilities of the materials were enhanced. The clay type used during the polymerization influenced both the minimum emulsifier content to obtain stable latexes and silicate dispersion stability: intercalation/exfoliation. It was concluded that the clay having moderate hydrophobicity required lower amount of emulsifier and showed better compatibility with the polymer, resulting in good mechanical and thermal properties. Papageorgiou et al. studied the effect of filler type on nonisothermal crystallization of PLA nanocomposites prepared with fumed silica, montmorillonite, and oxidized multi-walled carbon nanotubes by solvent evaporation method [13]. The nanofillers were found to be effective nucleating and reinforcing agents. Wu and Yang also focused on preparation method of nanocomposite material with a biodegradable polyester and montmorillonite [14]. The clay dispersion duration was varied from 2 to 6 h at a temperature between 50 and 90 ◦ C. The contents of clay were added in a range of 0.01–10 wt%. It was shown that a direct dispersion of clay in an organic solvent yielded the nanocomposite material with greatly improved performance, such as heat resistance and operating temperature. Hence, the current research interest is to prepare novel polylactic acid/montmorillonite nanocomposite latexes using an emulsification–diffusion method. The effects of clay types, sodium and organo-modified montmorillonites, at various concentrations (1–6 wt%) were investigated and compared. The resultant nanocomposite latexes and films were evaluated in terms of colloidal and thermal properties. The prepared products have a potential to be used as novel bio-based nanocomposite latexes for environmentally friendly coating or binder applications.

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lauryl sulfate (SLS) from Fluka, lauryl alcohol from Fluka, and ethyl acetate from Merck served as surfactants, co-surfactant, and solvent, respectively, during the emulsion preparation. All chemicals were used as received. 2.2. Preparation of nanocomposite emulsion containing organo-modified montmorillonite Polylactic acid solution was initially prepared by dissolving polylactic acid pellets (at 6 wt%) in ethyl acetate, a solvent. To achieve a good dispersion of clay in the solution, a certain amount (1–6 wt% based on PLA weight) of organoclay (Cloisite 30B) was separately dispersed in ethyl acetate using a high speed homogenizer (T10B, IKA Labotechnik) for 5 min, and magnetic stirred for 6 h. The polylactic acid solution, clay dispersion, and lauryl alcohol at 1.17 mL/g (based on polymer weight) were then mixed and used as an oil phase. Then, the oil phase (50 mL) was gently poured into an aqueous phase (100 mL) containing a mixture of surfactants, PVA at 1.08 × 10−4 mM, and anionic surfactant (SLS) at 6.94 mM. The mixture of oil in water (1:2, v/v) was homogenized for 3 min, and then subjected to a magnetic stirring for 4 h. The resultant emulsion was continuously stirred at 76 ◦ C for 15 min to evaporate the remaining solvent. 2.3. Preparation of nanocomposite emulsion containing sodium montmorillonite A similar preparation method as mentioned previously was followed except that the sodium montmorillonite (Na+ -MMT) was initially dispersed in a small volume of deionized water through 5 min of homogenization, and the dispersion was then mixed with the surfactant solution and continuously stirred for 2 h. The emulsion particles were formed by dispersing the oil phase (polylactic acid solution and lauryl alcohol) into the clay-surfactant phase using the same mixing process as described in the previous section. 2.4. Preparation of film Dry powder of the obtainable latexes (approximately 15 g) was preheated at 185 ◦ C for 2 min, then pressed under a load of 5 kg for 1 min at 185 ◦ C. The thickness of final film was in the range of 0.2–0.5 mm.

2. Experimental

2.5. Polymer characterization

2.1. Materials

The resultant emulsion particles were characterized in terms of particle size, morphology, and zeta potential. The average size of the prepared particles was measured by a mastersizer (MAF 5000, Malvern Instruments). The latex was diluted with deionized water before the analysis. The reported particle size was an average of three measurements. Particle morphology was viewed by using a scanning electron microscope (SEM) (JSM-5410LV, JEOL). Before the measurement, the microparticles were freeze-dried and finely spread over a stab, and later coated with gold. Zeta potential of the particles was analyzed by using a Zetasizer (Nanosizer, Malvern Instruments). A sample was diluted with deionized water and sonicated for 3 min prior to the measurement. The reported data were averaged from three repeated measurements. The silicate dispersion in polymer matrix was investigated by X-ray diffraction (XRD) corporating with TEM micrographs of the nanocomposite films. The XRD pattern of the nanocomposites was obtained by a Bruker AXS, ˚ model D8 Discover, equipped with Cu K␣ radiation ( = 1.5418 A). The angle was varied in a range of 1–40◦ with a scanning rate of

Polylactic acid (PLA 4042D, MW 74,000 g/mol) was supplied by Natureworks. Sodium montmorillonite (Na+ -MMT or Cloisite Na+ ) and organo-modified montmorillonite with bis(2-hydroxyethyl)methyl (hydrogenated tallow alkyl) ammonium cations (Cloisite 30B) were obtained from Southern Clay Products. The details related to these types of clay are listed in Table 1 according to the manufacturer. Poly(vinyl alcohol) (PVA, MW 93,000 g/mol, 98–99% hydrolysis) from Ajax Finechem, sodium Table 1 Characteristics of montmorillonites. Filler

Ammonium cation

Modifier concentration (meq/100 g clay)

Cloisite Na+ Cloisite 30B

Na+ (C18 H37 )–N+ (C2 H4 OH)2 CH3

– 90

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0.02◦ /step. The (d0 0 1 ) basal spacing of the silicate layer was determined by using the Bragg’s equation as shown below [15]: 0 1 sin 

(1)

where d0 0 1 is the inter-planer distance of (0 0 1) diffraction face ˚  is Bragg angle of the reflection (◦ ), and n is a whole num(A), ber, representing the order of diffraction, taken as one in the calculation. For TEM experiment, ultrathin samples were sectioned by an ultra-microtome (model MTX 75500, RMC) equipped with a diamond knife. Then, the ultrathin sections were observed in a high-resolution transmission electron microscope (TEM, model JEM-2100, JEOL) to analyze clay dispersion characteristic in the PLA matrix. The thermal properties of nanocomposite films were characterized by a differential scanning calorimeter (DSC, Mettler Toledo DSC822) and a thermogravimetric analyzer (TGA, Mettler Toledo TGA/SDTA 851). For DSC measurement, the prepared film was heated from 25 ◦ C to 200 ◦ C at a heating rate of 10 ◦ C/min. The measurement was performed under a nitrogen atmosphere. The %crystallinity can be calculated using Eq. (2) [16]. c =

Hm × 100 Hm ◦ × [1 − (wt%MMT /100)]

(2)

where Hm is the specific melting enthalpy of the sample, Hm ◦ is the melting enthalpy of the 100% crystalline polymeric matrix (146.0 J/g for polylactic acid) [17], and wt % MMT is the weight percentage of the montmorillonite. For TGA experiment, the sample was heated from 30 ◦ C to 800 ◦ C at a heating rate of 20 ◦ C/min under a nitrogen atmosphere. Weight loss of the sample was measured as a function of temperature. 3. Results and discussion 3.1. Particle size The resultant latex particles were prepared by the solvent evaporation process, in which the polymer is dissolved in a suitable water immiscible solvent, and the resultant solution or dispersion is subsequently emulsified in an aqueous continuous phase to form discrete droplets [18]. To enable the formation of microspheres, the organic solvent must first diffuse into the aqueous phase and then evaporate at the water/air interface. During the solvent evaporation process, the microspheres harden. This method has been used extensively to prepare PLA and PLGA microspheres containing many different drugs [18–21]. Several parameters have been investigated, including drug solubility and loading, solvent type, polymer concentration, emulsifier content, ratio of organic/aqueous phase volumes, and rate and duration of agitation during emulsification [22–24]. These variables can influence the microsphere formation and properties. In the current system, a small amount of lauryl alcohol was also mixed in the polymer solution to prevent the oil phase from diffusing into the aqueous phase so that the stability of the droplets is maintained [25]. A previous study has shown the effect of long-chain alcohols

0

1

2

3

4

5

6

7

-10

Zeta potential (mV)

n = 2d0

Montmorillonite (%wt) 0

-20 -30 -40 -50 PLA

-60

Cloisite Na+ Cloisite 30B

-70

Fig. 1. Zeta potentials of the unfilled PLA particles and the nanocomposite latexes prepared with Cloisite 30B and Cloisite Na+ at various contents.

(Cn OH for n = 8–18) on the partitioning of sodium dodecyl sulfate (SDS) to the oil/water interface in oil-in-water macroemulsions [26]. The results suggested a chain-length compatibility effect, in which the maximum amount of SDS partitioned to the interface and the minimum in droplet size for emulsions were reached when lauryl alcohol was added to the oil. The results of particle size are included in Table 2, where an average particle size of approximately 28 ␮m for the unfilled polylactic acid emulsion was found, and the sizes obtained with the nanocomposite latexes are in a slightly smaller range of 23–27 ␮m. Interestingly, with 1 wt% of silicate, the smallest average sizes are produced from both types of montmorillonite. The results correlate well with the highest zeta potentials (absolute values) obtained at the same concentration (Fig. 1). The average size tends to increase slightly according to the clay loading. The reason might be the partitioning of the surfactants between the silicate and the droplet surface. It is observed that the zeta potential increases (smaller negative numbers) with increasing silicate content, suggesting lesser amount of surfactants adsorbed on the droplet surface; hence, particles of larger sizes are formed. 3.2. Zeta potential The zeta potential of unfilled particles was found to be −47 mV, resulting mainly from the adsorption of the surfactants (Fig. 1). The negative value indicates electrostatic repulsive force between the particles. Previous findings reported that the surface charge of Na+ -MMT in aqueous solution is always negative and pH independent [27], and the zeta potential of the Na+ -MMT dispersion in the absence of surfactant was found to be −41.8 mV [28]. A comparison between two types of MMT, at low silicate concentrations (1–3 wt%), shows that the emulsions containing Cloisite 30B have a higher zeta potential (absolute value) on the particles than the ones containing Cloisite Na+ . A similar result was observed with the acrylate/montmorillonite nanocomposite latexes (3 wt% of clay), in which the emulsion prepared with Cloisite

Table 2 Particle size of the unfilled and nanocomposite particles and thermal properties of the resultant films. Substances

Particle size (␮m)

Unfilled PLA 27.9 PLA + Cloisite Na+ (wt%) 23.4 1 26.1 3 27 6 PLA + Cloisite 30B (wt%) 23.6 1 3 24.5 26.5 6

Tg (◦ C)

Tc (◦ C)

Tm1 (◦ C)

Tm2 (◦ C)

c (%)

T10 (◦ C)

T50 (◦ C)

45.8

81.7

135

149.3

13

191

362

53.7 46.1 54.6

101.5 80.6 100.5

143 130 143

150.5 145.6 152.5

11.6 12 13.9

220 224 248

375 373 381

47.5 47.2 48.5

85.4 84.1 95.1

134 135 139

151 149 149.5

12.2 11.5 11.7

225 229 240

375 375 377

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Fig. 2. Particle morphology of the latex particles (A) unfilled PLA, (B) PLA with Cloisite 30B (at 1 wt%) and (C) PLA with Cloisite Na+ (at 1 wt%).

30B also exhibited a higher zeta potential compared to the latex prepared with Cloisite Na+ [10]. Especially, at 1 wt% of clay, the highest values are noted at −60.5 mV with Cloisite 30B and −51.8 mV with Cloisite Na+ , which is considerably higher with respect to the zeta potential of the unfilled particles. This might be attributed to the self-arrangement of the silicate layers within the polymer particles, affecting the surface potential. This behavior is identical to Pickering phenomenon where the silicate platelets are capable of stabilizing emulsions by adsorption onto the droplets [10,29,30]. However, the potential tends to increase with increasing clay concentration, which is possibly due to lesser amount of surfactants present on the particle surface as previously described.

electron microscopy (TEM) to evaluate the state of the silicate dispersion in the polymer matrix. When the interlayer spacing of clay is extended relative to its original spacing, it is called intercalation. In case the interlayer of the silicate is completely disrupted, it is called exfoliation. The XRD patterns of the unfilled polylactic acid and nanocomposites analyzed from the dried powder samples are shown in Figs. 3 and 4. It is seen that the diffraction patterns of Cloisite 30B and Cloisite Na+ have their characteristic ˚ and 7.24◦ (d0 0 1 = 12.19 A), ˚ peaks at 2 = 4.83◦ (d0 0 1 = 18.27 A) respectively. For the latex without clay addition, the prominent

3.3. Particle morphology E

3.4. Morphology of the nanocomposites The morphology of the nanocomposites prepared from the latexes was analyzed by X-ray diffraction (XRD) and transmission

D

Intensity (a.u.)

The SEM micrographs of the resultant particles are displayed in Fig. 2. The morphology of the unfilled and nanocomposite microspheres appears to be spherical with different sizes. It is also noticed that smaller sizes and fewer particles remained in the samples prepared with Na+ -MMT and organoclay, in which some particles have already coalesced, forming partial smooth film in some areas. This phenomenon suggests that incorporating layered silicate in the emulsion might help facilitating film formation of the particles.

C

B

A

1

2

3

4

5

6

7

8

9

10

2 Theta (degree) Fig. 3. XRD patterns of (A) Cloisite Na+ , (B) unfilled PLA and PLA nanocomposites containing Cloisite Na+ at (C) 1 wt%, (D) 3 wt% and (E) 6 wt%.

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E D

Intensity (a.u.)

C B

A 1

2

3

4

5

6

7

8

9

10

2 Theta (degree) Fig. 4. XRD patterns of (A) Cloisite 30B, (B) unfilled PLA and PLA nanocomposites containing Cloisite 30B at (C) 1 wt%, (D) 3 wt% and (E) 6 wt%.

peaks positioning at close proximity to 2.5◦ , 4.9◦ , and 7.4◦ are noticeable, which suggested the peak pattern of surfactant (SLS) [8]. The first highly intensive peak at the lowest angle is estimated to be owing to the reflection from (1, 0, 0) plane and the other peaks represent the higher order reflection of the same plane [31]. Similar surfactant diffraction patterns are also present in both nanocomposites prepared with Cloisite Na+ and Cloisite 30B. Moreover, additional peak at a slightly lower region to 2.5◦ appears, suggesting surfactant intercalation in the silicate layers. Besides the peaks of surfactant, there are no XRD peaks, representing both silicates within 2 range of 1–10◦ . Such results potentially indicate dominance of exfoliated silicate layers achieved in the PLA matrix. Other reports related to the PLA/MMT nanocomposites with Cloisite 30B prepared by solution casting and melt compounding also suggested effective exfoliation pattern in the nanocomposites [7]. At higher 2 range (10–30◦ ), the diffraction peaks of the unfilled PLA and the nanocomposites (the results were selected at 6 wt% from both types of silicate to be representatives) exhibit a similar narrow strong crystalline peak centered at 2 = 16.6◦ that is associated with ␣-crystalline structure as illustrated in Fig. 5. In addition to the XRD analysis, the high resolution TEM images of the nanocomposites were also taken to correlate with the XRD results. The images shown in Fig. 6 contain dark lines, indicating the cross section of clay sheet ca. 1 nm thickness, and gray background, representing the polymer matrix. The gap between two adjacent lines is known as the interlayer spacing or gallery. The images of the nanocomposites (at 6 wt% of silicate) reveal a coexistence of

partially disordered, exfoliated platelets of clay layers and partially intercalated tactoids. The degree of nanoclay dispersion in the continuous phase plays a crucial role in the degree of enhancement of the polymer properties, in which a higher amount of exfoliation achieved gives rise to higher mechanical properties because of higher surface interaction present in exfoliated nanocomposites with respect to intercalated materials.

C

Intensity (a.u.)

Fig. 6. TEM micrographs of the polylactic acid/montmorillonite nanocomposite films prepared at 6 wt% of (A) Cloisite Na+ and (B) Cloisite 30B.

3.5. Thermal properties B

A

0

5

10

15

20

25

30

35

40

2 Theta (degree) Fig. 5. XRD patterns of (A) unfilled PLA and PLA nanocomposites containing 6 wt% of (B) Cloisite Na+ and (C) Cloisite 30B.

DSC and TGA experiments were employed to investigate the thermal property and stability of the resultant nanocomposites. All thermal characteristic values are presented in Table 2. Typically, the glass transition temperature (Tg ) is a complex phenomenon depending on a number of factors such as chain flexibility, molecular weight, branching, cross-linking, intermolecular interactions, and steric effects [32]. It was found that Tg tends to increase according to the silicate concentration, and the nanocomposites containing Cloisite Na+ exhibit a higher Tg than those prepared

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C

Endo

B

A

0

20

40

60

80

100

120

140

160

180

200

Temperature (oC) Fig. 7. DSC thermograms of (A) unfilled PLA and PLA nanocomposites with (B) 6 wt% Cloisite 30B and (C) 6 wt% Cloisite Na+ .

with Cloisite 30B (except at 3 wt%, which might be caused by a random silicate distribution in the sample). By adding the silicate, Tg is elevated from 45.8 ◦ C (in case of the unfilled PLA) to 54.6 ◦ C, by 8.8 ◦ C (at 6 wt% of Na+ -MMT). A rise in Tg can be attributed to the restricted segmental motions of the PLA chains at polymer–clay interface. It indicates a good interfacial adhesion between polylactic acid and silicate particles [33]. A higher degree of increment in Tg is observed with the sodium type, suggesting that the dispersion of silicate in the polymer matrix might possibly achieve a higher degree of exfoliation, generating more polymer–clay interfacial interaction compared with the organoclay. Others also reported the increase in Tg with silicate content in the PLA/clay nanocomposite systems [34]. Melting points (Tm s) of the samples were evaluated from the position of the maximum in the endothermic peaks of DSC thermograms (as shown in Fig. 7). The bimodal melting temperatures are noticeable. It was reported that the double melting behavior in polylactic acid can be linked to the formation of different crystal structures [35–37]: the ␣-form (pseudo-orthorhombic, pseudo-hexagonal or orthorhombic [37]), melting at a higher temperature, and the ␤-form (orthorhombic or trigonal) that melts in correspondence of the endotherm at a lower temperature. The melting temperatures of the nanocomposites are comparable to that of the unfilled PLA, suggesting that incorporation of nanoclay did not much affect the melting point. Di Lorenzo reported that poly(l-lactic acid) is a polymorph material that presents several crystal modifications, in which its crystals can grow in several structural modifications, characterized by different helix conformations and cell symmetries that are arranged upon different thermal and/or mechanical treatments [38]. The multiple melting peaks occur when imperfect crystals that have melted at a lower temperature recrystallize to give crystals at a higher temperature [39]. Increasing the clay content, regardless of its organic modification type, tends to reduce the heat of fusion; hence, the percent of crystallinity is decreased. This can be explained by the patterns of silicate dispersion in the PLA matrix, i.e. partial intercalation/or exfoliation, which consequently hinders the mobility and flexibility of the polymer chain to fold and arrange crystallization [7,40]; thus, there is an increase in crystallization temperature (Tc ) of the nanocomposites relative to that of the unfilled PLA. Interestingly, the highest thermal enhancement of the nanocomposite is attained with Cloisite Na+ at 6 wt%, showing the highest Tg, Tm , and %crystallinity. In general, the incorporation of clay into the polymer matrix is found to effectively enhance thermal stability by acting as a superior insulator and mass transport barrier to the volatile products generated during decomposition [41]. The thermal degradation temperatures at 10% and 50 wt% loss (T10 and T50 ) of the nanocomposites at various filler concentrations are summarized in Table 2. A drastic increase in thermal stability is obviously achieved with the filled polymers, especially at the onset temperature (T10 ), in

Fig. 8. TGA thermograms of (A) unfilled PLA and PLA nanocomposites with (B) 6 wt% Cloisite 30B and (C) 6 wt% Cloisite Na+ .

which the temperatures are considerably shifted by 30–60 ◦ C. The highest temperature is 248 ◦ C with 6 wt% Cloisite Na+ compared to that of the unfilled PLA at 191 ◦ C. Correspondingly, the thermal stability of the silicated materials at T50 is improved as well. The temperature is increased from 362 ◦ C for the unfilled PLA to 381 ◦ C for the nanocomposite with 6 wt% Na+ -MMT (temperature shifted by 19 ◦ C), which is the highest increment. A similar thermal improvement is gained with the organoclay nanocomposites where T50 is shifted by 15 ◦ C at the highest loading. The increase in thermal stability could be caused by an ablative reassembling of the silicate layers, which may occur on the surface of the nanocomposites, creating a protective barrier. In addition, the decomposition might also be delayed as a result of exfoliated silicate layer in the polymer matrix [42]. The TGA results of the PLA nanocomposites obtained with the resultant emulsions are comparable to other reports where the PLA/clay nanocomposites were prepared by melt compounding [43,44]. Krishnamachari et al. found that the PLA nanocomposites containing Cloisite 30B (1–3 wt%) exhibited temperatures at maximum weight loss in a range of 376–394 ◦ C with respect to the neat PLA at 349.3 ◦ C [44]. The TGA thermograms of the nanocomposites (at 6 wt%) compared with the unfilled PLA are illustrated in Fig. 8. The two distinct regions between 200–300 ◦ C and 400–500 ◦ C are caused by the degradation temperatures of PVA. Upon heating, the polymer begins a rapid chain-stripping elimination of H2 O [45–47]. From the current study, it is seen that the composite material containing sodium MMT tends to give better thermal stability with higher degradation temperature shift than the organo-modified type when the clay loading is increased. 4. Conclusions The polylactic acid/montmorillonite nanocomposite emulsions were successfully prepared by using two types of silicate, sodium and organo-modified, via emulsification–diffusion method. The average particle sizes of the resultant latexes were found within 20–30 ␮m, and the zeta potentials were obtained in a range of −37 to −60 mV, depending on the silicate type and concentration. SEM micrographs revealed that the microspheres attained a spherical shape with different sizes. XRD and TEM experiments indicated that the silicate layers were extensively exfoliated and partially intercalated in the PLA matrix. As a result, thermal properties of the silicate filled polymers were significantly enhanced compared to the unfilled PLA. The highest thermal stability was achieved with the nanocomposite containing Na+ -MMT at the highest concentration (6 wt%).

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Acknowledgements The authors would like to thank the National Research Council of Thailand for granting this project, and Unilever Thai Trading Company for particle size measurement.

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