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Organomodification of montmorillonite and its effects on the properties of poly(butylene succinate) nanocomposites.

INTRODUCTION

Polymer/organoclay nanocomposites are emerged as a promising area for technological development in materials research, which broaden the usage of various polymers in a wide range of applications. As early as in year 1950, Carter et al. (1) demonstrated the use of organomodifiedlayered silicates in the reinforcement of elastomer. Then, polymer/organoclay nanocomposites based on Nylon 6 were firstly commercialized by Toyota Group (2). To date, nanocomposites with various polymer matrices have been reported, such as polypropylene (3), polyethylene (4), poly(ethylene terephthalate) (5), (6), epoxy (7), polyurethane (8), polylactic acid (9), (10), etc. In recent years, much attention has been devoted to the nanocomposites based on biodegradable polymers and organoclay, due to their potential to minimize the environmental pollution associated with the disposal of synthetic, nonbiodegradable polymers.

Poly(butylene succinate) (PBS) is a biodegradable aliphatic thermoplastic polyester, which was invented in the early 1990s by Showa Highpolymer in Japan. It is a competitive material against other biodegradable plastics due to its superior mechanical properties, high chemical resistance, availability, and relatively lower cost. However, certain properties of PBS, such as softness, poor gas barrier properties, and low melt strength, have restricted its potential in various applications. In this research, an organoclay namely organomontmorillonite (OMMT) was incorporated into the PBS matrix to produce a "green" nanocomposite, with the intention of obtaining materials with improved properties suitable for a wide range of applications, such as packaging applications.

Basically, pristine montmorillonite (MMT) is hydrophilic, whereas most polymers are hydrophobic and thus, except a few hydrophilic polymers (11-13), most polymers are not compatible with pristine MMT. To render MMT miscible with other polymers, it is required to modify the clay surface by exchanging the cations initially present in the interlayer with organic cations surfactants, such as alkyl-ammoniums (14-16). Their long aliphatic tails, attached with their cationic head to the surface of the negatively charged clay, result in a larger interlayer spacing (2), (15), (17), (18). To date, numerous researchers have been reported on the PBS/OMMT nanocomposites filled by various types of OMMT (19-21). Someya et al. (22) discovered that properties of PBS nanocomposites might vary by the utilization of different organic cations surfactants during clay modification. Cationic surfactants that are widely used in the preparation of organoclay are octadecylamine (ODA), 12-aminolauric acid, and stearylamine salts, etc. Study on the MMT modified with hexadecyltrimethylammonium bromide (HTAB) is common (23), (24) but the study on the incorporation of this OMMT into PBS is rather limited. In this work, pristine sodium-MMT was modified with different HTAB content, that is, 1.0 cation exchange capacity (CEC) and 1.5 CEC/g of clay. Characterizations of the OMMT were performed through X-ray diffraction (XRD), Fourier transform infrared (FUR) spectroscopy, scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) techniques. In our previous work, the optimum OMMT loading was determined at 2 wt%, where the enhanced mechanical and rheological properties of PBS nanocomposites were observed (25). Hence, 2 wt% of HTAB-modified OMMT was incorporated into PBS matrix and the properties of PBS nanocomposites were studied. In addition, maleic anhydride-grafted PBS (PBS-g-MA) was used as compatibilizer in the nanocomposites, by reason of their polar functional group that will improve the filler--matrix interfacial interactions (26). Mechanical, morphological and thermal properties of the PBS nanocomposites were presented. Besides that, the properties of the nanocomposites prepared by using laboratory synthesized HTAB-modified OMMT and commercial ODA-modified OMMT were compared.

EXPERIMENTAL

Materials

PBS (Bionolle #1020) was obtained from Showa High-polymer Co. (Japan) with the melt flow index value of 25 g/10 min (190[degrees]C, 2.16 kg) and the melting temperature of 115[degrees]C. The pristine sodium MMT (Cloisite[R] [Na.sup.+]) with CEC of 92.6 mequiv./100 g was provided by Southern Clay Products, Texas. The commercial OMMT (Nanomer[R] 1.30TC, Nanocor) with CEC of 110 mequiv./100 g, which containing MMT (70 wt%) intercalated by ODA (30 wt%) was used in this study. HTAB was obtained from Fluka Analytical (Denmark). Dicumyl peroxide (DCP; Aldrich Luperox[R]) from Sigma--Aldrich was used as initiator in the grafting process of PBS-g-MA. Maleic anhydride (MA) was supplied by R&M II Chemicals.

Preparation of OMMT

The OMMT were prepared according to a standard ion exchange procedure by using HTAB as cationic surfactant. First, the pristine sodium MMT was dried at 80[degrees]C for 24 h. Hexadecyltrimethylammonium chloride solution is prepared by adding the desired amount of 0.04 N hydrochloric acid (HCI) and 0.04 N HTAB in 1000 ml distilled water. Two different contents of HTAB were incorporated, which are 6.75 g and 10.125 g, corresponding to 1 CEC/g and 1.5 CEC/g of the clay (labeled as HM10 and HM15, respectively). The mixture was stirred at 80[degrees]C for 1 h using a magnetic stirrer. At the same time, a suspension of 20 g of MMT in 1000 ml distilled water was prepared using magnetic stirrer. The suspension of clay was then added into the hexadecyltrimethylammonium chloride solution and stirred at 80[degrees]C for 1 h using a magnetic stirrer. The obtained precipitate was collected by vacuum filtration. Next, hot water (60[degrees]C) was added and followed by stirring for 1 h to remove salt. This process was repeated several times until no chloride and bromide traces were detected by addition of 0.1 mol silver nitrate (AgN[O.sub.3]). The resultant precipitate was subsequently dried in an oven at 80[degrees]C for 24 h. After that, it was ground in a mortar, sieved to obtain the powders, and dried in an oven at 80[degrees]C until a constant weight is obtained.

Synthesis of PBS-g-MA

PBS-g-MA was produced via one-step reactive grafting process. First, PBS, MA, and DCP were physically premixed in the composition of 100, 10, and 1.5 phr, respectively. The reactive grafting process was performed in an internal mixer (Haake Polydrive R600, Germany) at 135[degrees]C for 7 min, at a rotation speed of 50 rpm. The obtained PBS-g-MA was then purified by refluxing in chloroform for 4 h, and the hot solution was filtered into cold methanol. Next, the precipitated polymer was then washed with methanol for several times to remove any unreacted reagents, followed by drying in an oven at 60[degrees]C for 24 h. The degree of grafting in PBS-g-MA is 4.84% as determined through the titration process.

Preparation of Nanocomposites

The PBS/OMMT nanocomposites were prepared via melt-mixing process by using an internal mixer (Haake PolyDrive R600, Germany) at 135[degrees]C for 5 min at a rotation speed of 50 rpm. Various compounds were prepared as shown in Table 1. The nanocomposites were filled by laboratory synthesized HTAB-modified OMMT (HM10 and HM15) and commercial ODA-modified OMMT (DM). Dumbbell specimens were then prepared using compression molding machine (GT 7014-A30C, Taiwan) at 135[degrees]C and 3 min.

TABLE 1. Sample designations of PBS and the nanocomposites.

Component  PBS  PBS/  PBS/  PBS/  PBS/  PBS/   PBS/
(wt%)            MMT   2DM  HM10  HM15    MA  HM10/
                                                 MA

PBS        100    98    98    98    98    95     93

MMT         --     2    --    --    --    --     --

HM10        --    --    --     2    --    --      2

HM15        --    --    --    --    --    --     --

DM          --    --     2    --    --    --     --

PBS-g-MA    --    --    --    --    --     5      5


FTIR Spectroscopy. FTIR analysis was carried out at ambient temperature by using Perkin-Elmer Spectrum One FTIR Spectrometer. It was performed through the scanning wave number from 4000 [cm.sup.-1] to 550 [cm.sup.-1] with 32 scanning. times.

EDX Analysis. EDX (EDAX Falcon System, EDAX) was performed to analyze the elements presented in the OMMT. It is an attachment of SEM that allows quantitative compositional analysis.

X-Ray Diffraction. XRD analysis was carried out with PANalytical X'pert Pro Mrd PW2040 XRD diffractometer (Netherlands) in a scan range from 2[degrees] to 10[degrees] and 2.0[degrees]/min scanning rate. The X-ray source was Cu K[alpha] radiation with a wavelength ([lambda]) of 0.154 nm.

Thermogravimetric Analysis. Thermogravimetric Analysis (TGA) was conducted in the Perkin-Elmer Pyris 6 TGA Analyzer from room temperature to 700[degrees]C at the heating rate of 10[degrees]C/min under nitrogen atmosphere.

Characterizations of Nanocomposites

Mechanical Properties. Tensile test was conducted using universal testing machine (Instron 3366, Instron Co.) at 23 [+ or -] 2[degrees]C and 50 [+ or -] 5% relative humidity, according to ASTM D638 (Type IV) with a gauge length of 50 mm and a cross-head speed of 5 mm/min. Flexural test (three-point bending) was performed on a universal testing machine (Instron 3366, Instron Co.) at 23 [+ or -] 2[degrees]C and 50 [+ or -] 5% relative humidity, according to ASTM D790 with a support span length of 50 mm and a cross-head speed of 5 mm/min. Charpy impact test was carried out for both notched and unnotched specimens using Pendulum Impact Machine (Zwick Roe11 Group, Germany) according to ASTM D6110 with pendulum energy of 7.5 J. Five measurements were carried out on each sample.

Scanning Electron Microscopy. The tensile fractured surface was observed under a field-emission scanning electron microscope (Zeiss LEO Supra 35VP, Germany). Before observations, the samples were sputter coated with a thin layer of gold to avoid electrical charging during examination.

Transmission Electron Microscopy. The morphologies of the nanocomposite samples were observed using an Energy Filter Transmission Electron Microscope (TEM: Zeiss Libra 120, Netherlands) operated at 200 kV. The ultrathin sections of sample with the thickness 70-80 um were prepared via ultramicrotomy technique using Reichert Ultramicrotome Supernova.

Differential Scanning Calorimetry. The differential scanning calorimetry (DSC) analysis was carried out using Perkin-Elmer DSC-6 machine in a nitrogen atmosphere. Sample was heated from 30[degrees]C to 150[degrees]C at a heating rate of 10[degrees]C/min. The sample was then cooled from 150[degrees]C to 30[degrees]C at a same heating rate. Next, second heating was performed from 30[degrees]C to 150[degrees]C. Finally, it is cooled to 30[degrees]C. Degree of crystallinity ([X.sub.e]) was calculated using the following equations.

For pure PBS,

[X.sub.e] = [[DELTA][H.sub.c]]/[[DELTA][H.sub.m.sup.0]] x 100% (1)

where [DELTA][H.sub.c] = crystallization enthalpy of sample; [DELTA][H.sub.m.sup.0] = melting enthalpy of 100% crystalline PBS (110.3 J/g).

For polymer nanocomposites,

[X.sub.c] = [[DELTA][H.sub.c]]/[[DELTA][H.sub.m.sup.0] (1 - [W.sub.f])] x 100% (2)

where [W.sub.f] = weight fraction of OMMT in the nanocomposite.

RESULTS AND DISCUSSION

Characterizations of OMMT

FTIR Spectroscopy. FTIR spectra of pristine sodium MMT and OMMT are demonstrated in Fig. 1. The peak at 3621 [cm.sup.-1] was corresponding to the 0--H stretching band of hydrogen-bonded water. A sharp peak that related to the interlayer water deformation vibrations was observed at 1638 [cm.sup.-1] (27). The band at 1035-1051 [cm.sup.-1] was attributed to the in-plane Si-O stretching of layered silicates. The peaks at 916 [cm.sup.-1] and 798 [cm.sup.-1] were due to the Al--Al--OH and Al--Mg--OH bending vibrations, respectively (24), (28).

Introduction of HTAB into the MMT caused the occurrence of an additional peak at 3018-3032 [cm.sup.-1], which was assigned to the typical N--H stretching band. Two additional peaks that corresponded to the asymmetric and symmetric stretching of C[H.sub.2] were found at 2921-2928 [cm.sup.-1] and 2851 [cm.sup.-1] after organomodification of sodium MMT. A band at 1489 [cm.sup.-1] is related to the bending vibration of N--H groups in HTAB (29). Besides that. the FTIR spectra in the region of 721-722 [cm.sup.-1] reflected the disordered hexagonal packing of layered silicates, where the alkyl chains rotate freely along the longitudinal axis (24). Hence, the FTIR spectra reveal the success in the preparation of HTAB-modified MMT at 1.0 CEC and 1.5 CEC of HTAB content.

EDX Analysis. The SEM micrographs of pristine MMT and OMMT are presented in Fig. 2. To identify the elements present in the samples, EDX technique was employed on specific point regions as marked in Fig. 2. The chemical composition MMT and OMMT by EDX is shown in Fig. 3. The principal components of MMT such as Si, Al, Mg, and 0 were detected in all the clay samples. Furthermore, the presence of Na is observed in the pristine sodium MMT. After organomodification of MMT, the C peak appeared, accompanied with the vanishing of Na peak. This confirmed that most of the [Na.sup.+] cations on the clay surface have been exchanged by the alkylammonium cations. In addition, the higher C content in HM15 when compared with HM10 reveals a higher amount of HTAB component in the organoclay, as shown in the embedded table in Fig. 3. Although nitrogen (N) is present in HTAB, the quantity is relatively low to be detectable through EDX.

Element    Wt%    At%

OK       44.66  57.89
NaK      03.57  03.22
MgK      02.37  02.02
ALK      13.20  10.15
SiK      36.20  26.73

Element    Wt%    At%

CK       32.51  44.96
OK       33.03  34.30
MgK      01.58  01.08
ALK      08.97  05.52
SiK      23.91  14.14

Element    Wt%    At%

CK       41.47  54.05
OK       31.03  30.36
MgK      00.75  00.48
ALK      08.72  05.06
SiK      18.03  10.05


X-Ray Diffraction. The XRD diffractograms of the pristine sodium MMT and OMMT (Fig. 4) were analyzed to determine the corresponding dm spacing of the clay. The XRD spectrum of pristine MMT exhibits a broad peak at 2[theta] = 6.08[degrees], corresponding to a [d.sub.001] spacing of 1.45 nm. The [d.sub.001] peaks tend to shift toward a lower 2[theta] values, associated with an increment in the peak intensity after organomodification of the pristine MMT. The HM10 and HM15 organoclay show the 2[theta] values at 4.76[degrees] and 4.57[degrees], in line with [d.sub.001] spacing of 1.86nm and 1.93nm, respectively. The enhancement of the [d.sub.001] spacing indicates the effective intercalation of HTAB cations into the MMT layers (30).

Thermogravimetric Analysis. Organomodification of MMT was also confirmed by TGA. Figure 5 expresses the thermal decomposition of pristine sodium MMT and OMMT in terms of weight as a function of temperature, while the derivative weight loss curves are shown in Fig. 6. Decomposition of pristine sodium MMT occurs in two steps: (a) desorption of water from the clay interlayer spaces at 40-90[degrees]C and (b) dehydroxylation of structural OH units from the MMT in the temperature range of 530-690[degrees]C. A similar observation was reported by Magaraphan and Lilayuthalert (31). For the OMMT, four weight loss steps were observed. The first step occurs around 35-100[degrees]C due to the loss of residual water trapped in the organoclay. The second step occurs at approximate 170-340[degrees]C is assigned to the decomposition of the cationic surfactant. The third step is found from the small peak appeared at the temperature about 300-420[degrees]C as seen from Fig. 6. This is attributed to the surfactant molecules that adhere on the surface of MMT. Generally, the cationic surfactant undergoes ion exchange with the [Na.sup.+] ions and attach to the MMT surface via electrostatic interactions. In this work, 1.0 CEC and 1.5 CEC of cationic surfactant were incorporated. This exceeds the CEC of clay, which causes the excessive surfactant molecules to adsorb on the MMT surface by van der Waals force (32). The forth step that occurred at 510-640[degrees]C shows the dehydroxylation of structural OH units from the MMT.

Besides that, TGA result provides the information of water content and exchange efficiency of cationic surfactant on the clay surface (33). In comparison to the OMMT, pristine MMT demonstrates a higher desorption of water, with approximate 4.2% of weight loss, while HM10 and HM15 exhibits 2.1% and 2.0% of water weight loss, respectively. This reveals that the HTAB-modified organoclay is less hydrophilic than the pristine sodium MMT. The exchange efficiency of cationic surfactant on clay was determined from the second weight loss step in the TGA curves. It is found that HM1O and HM15 contain 11.8% and 20.9% of HTAB, respectively. The incorporation of higher amount of HTAB during the modification process improved the exchange efficiency. This result also indicates a successful modification of MMT using HTAB as cationic surfactant.

Characterizations of Nanocomposites

Mechanical Properties. The mechanical properties of PBS nanocomposites are shown in Table 2. It is worth noting that the mechanical properties of the nanocomposites filled with HM10 and HM15 is apparently higher than that of the nanocomposite filled with pristine sodium MMT. This is attributed to the improved filler--matrix interactions through the interactions between the amine groups in HTAB and the carbonyl groups in PBS by forming hydrogen bonding. Furthermore, the organomodification process on the MMT facilitates the penetration of the polymer into the clay galleries, leading to better filler dispersion and yields higher mechanical properties (22), (34), (35). This is verifiable through the XRD and TEM analysis.

TABLE 2. Mechanical properties of PBS and the nanocomposites.

                         Compound

Properties         PBS    PBS/2DM    PBS/MMT    PBS/HM10    PBS/HM15

Tensile

Strength (MPa)    32.6    33.6 [+    25.4 [+  34.0 [+ or  32.1 [+ or
                 [+ or      or -]      or -]     -] 0.38      -] 2.0
                    -]        1.2        2.0
                   2.7

Modulus (MPa)   589 [+  631 [+ or  616 [+ or   661 [+ or   655 [+ or
                 or -]     -] 9.1      -] 25      -] 8.4       -] 21
                   6.8

Elongation at     10.9    12.9 [+    5.38 [+  16.7 [+ or  13.3 [+ or
break (%)        [+ or      or -]      or -]      -] 1.2     -] 0.75
                    -]        2.4       0.39
                   2.6

Flexural

Strength (MPa)    33.3    36.1 [+    28.0 [+  37.5 [+ or  36.0 [+ or
                 [+ or      or -]      or -]      -] 1.2     -] 0.56
                    -]       0.91       0.91
                   1.9

Modulus (MPa)   570 [+  619 [+ or  659 [+ or   683 [+ or   671 [+ or
                 or -]      -] 11      -] 16       -] 23       -] 18
                    14

Imparl

Notched           8.78    9.35 [+  8.3 [+ or  13.8 [+ or  12.9 [+ or
(kJ/[m.sub.2])   [+ or      or -]     -] 2.0      -] 1,4      -] 1.1
                    -]        1.8
                   1.4

Unnotched       104 [+  108 [+ or    99.1 [+   116 [+ or    112.0 [+
(kJ/[m.sub.2])   or -]     -] 8.1      or -]       -] 11   or -] 3.5
                   2.3                   4.3

Properties       PBS/MA    PBS/HM10/MA

Tensile

Strength (MPa)  29.0 [+  39.4 [+ or -]
                  or -]           0.45
                    2.2

Modulus (MPa)    581 [+   684 [+ or -]
                  or -]             23
                     14

Elongation at   6.67 [+    14.1) [+ or
break (%)         or -]        -] 1.32
                    1.1

Flexural

Strength (MPa)  32.1 [+  40.3 [+ or -]
                  or -]           0.56
                    1.3

Modulus (MPa)    598 [+   694 [+ or -]
                  or -]            8.1
                     21

Imparl

Notched         6.21 [+  15.7 [+ or -]
(kJ/[m.sub.2])    or -]            1.3
                   0.45

Unnotched       84.9 [+   121 [+ or -]
(kJ/[m.sub.2])    or -]            2.5
                    1.8


In our previous study, commercial ODA-modified OMMT (DM) was incorporated into PBS (25) and the optimum properties were obtained at 2 wt%. It can be Observed that the tensile strength and flexural strength of this nanocomposite is comparable with those filled by HM, while the impact strength was well improved in the nanocomposites filled by HM. From the impact properties, it is noted that the unnotched impact strength far higher than the notched impact strength. This means that PBS nanocomposites are sensitive to notches.

Moreover, PBS/HM 10 and PBS/HM15 nanocomposites demonstrate higher modulus and elongation at break, when compared with PBS/2DM nanocomposites. The better reinforcement effect of HM could be associated with the better filler dispersion, as confirmed in the XRD results that will be discussed later. The presence of trimethyl groups in HTAB caused a steric hindrance effect on the adjacent clay layers, and facilitated the filler dispersion when an external shearing force was applied during mixing process (22), (35). The nanocomposite with higher degree of exfoliation possesses high aspect ratio of organoclay, which offers higher surface area to interact with the polymer matrix. This subsequently improves the mechanical properties. Besides that, PBS/HM1O nanocomposite shows better mechanical properties when compared with PBS/HM15 nanocomposite. It has been discussed earlier that when high amount of cationic surfactant being incorporated, the excessive surfactant molecules tend to adsorb on the MMT surface by van der Waals force (32). According to Chavarria et al. (36), these excessive surfactant molecules lead to a higher hydrophilicity of organoclay, which may reduce the affinity between the organoclay and the low polarity polymer such as PBS.

On the other hand, the effects of compatibilizer on the PBS/HM nanocomposites were studied by incorporating PBS-g-MA as compatibilizer into the PBS/HM1O nanocomposites. It can be seen that the addition of PBS-g-MA into PBS decreased the tensile, flexural and impact strength, as well as the elongation at break of PBS. The low molecular weight of PBS-g-MA is believed to responsible for this reduction. In a contrary, the modulus of PBS was enhanced by adding PBS-g-MA, as a consequence of the higher modulus of PBS-g-MA due to its high stiffness. However, a remarkable improvement was observed in the mechanical properties of PBS nanocomposites after addition of PBS-g-MA. This is due to the combination effects of improved organoclay dispersion and interfacial interactions between polymer and organoclay (37-39). This shows that the improvement in mechanical properties after addition of PBS-g-MA is due to the compatibilization effects, but not the PBS-g-MA itself. In the PBS/MA blend, a significant reduction in the elongation at break was observed after the addition of PBS-g-MA into PBS, but this effect was rather limited for the compatibilized nanocomposites. The incorporation of PBS-g-MA into the nanocomposites did not cause a significant reduction in the elongation at break. It is believed that the better interfacial interaction between polymer and organoclay has compensated the negative effect of PBS-g-MA on the elongation at break. From the results of mechanical properties, it can be concluded that the HTAB-modified OMMT filled nanocomposites gives overall better mechanical properties when compared with the ODA-modified OMMT-filled nanocomposites.

X-Ray Diffraction. The structure of PBS nanocomposites were investigated by XRD analysis and displayed in Fig. 7. The DM, MMT, HM10, and HM 15 show the [d.sub.001] spacing of 2.79 nm, 1.45 nm, 1.86 nm, and 1.93 nm, respectively. From Fig. 7, it is noticeable that the peaks tend to broaden and shift toward a lower value of 2[theta] after the formation of nanocomposites. A broad shoulder is observed in the XRD patterns of PBS/MMT and PBS/ HM15 nanocomposites at 2[theta] = 5.03[degrees] and 4.19[degrees], in proportion to a [d.sub.001] spacing of 1.76 nm and 2.11 nm. This indicates the formation of partially exfoliated and partially intercalated structures in the nanocomposites (35), (40), (41). The disappearance of characteristic diffraction peak in the XRD patterns of PBS/HM10 and PBS/HMIO/MA nano-composites also suggests the delamination and dispersion of the clay nanolayers within the polymer matrix via the formation of an exfoliated structure (35), (40).

The commercial organoclay, DM exhibits higher initial [d.sub.001] spacing when compared with the modified organoclay in our work, HM. This is mainly due to the larger amount of the surfactant in DM (~30 wt%). However, the XRD results show that the PBS/HM1O nanocomposite demonstrates a better exfoliation of organoclay compare to the PBS/2DM nanocomposite. Although the initial [d.sub.001] spacing of HM is lower, the bulky methyl side chains of HTAB facilitate the diffusion of polymer chains inside clay galleries when shearing force was applied during melt-mixing process, further spreading the clay layers apart (22). Therefore, the characteristic of cationic surfactant in organoclay is an important factor to determine the structure of PBS nanocomposites.

Scanning Electron Microscopy. Tensile fractured surface of PBS and its nanocomposites are viewed under SEM and presented in Fig. 8. On the fractured surface of PBS/2DM nanocomposites (Fig. 8a), the clay dispersion is generally good but clay aggregates and cavities are noticeable at certain area. Large agglomerates of MMT and clear separation between the MMT and PBS matrix are detectable in PBS/MMT (Fig. 8b), as a consequence of poor compatibility and inhomogeneous filler dispersion.

Conversely, Fig. 8c--e shows an improved compatibility between the OMMT and the PBS matrix, in which a better filler--matrix adhesion was observed. The fibrillate morphology is still presented in the PBS/HMIO/MA nanocomposites. This means that the addition of PBS-g-MA does not greatly affect on the fracture mode of the compatibilized nanocomposite. Hence, the changes in elongation at break were rather limited after compatibilization, as discussed earlier. Comparing to PBS/2DM nanocomposite, PBS/HM nanocomposites show a better filler--matrix adhesion with lower number of cavities on the fractured surface.

Transmission Electron Microscopy. TEM images PBS nanocomposites are shown in Fig. 9. From Fig. 9a, it shows a mixed region of intercalated clay platelet stacks, and individual exfoliated platelets in the PBS/2DM nano-composites. In Fig. 9b, it can be seen clearly that PBS/MMT exhibits poor clay dispersion, where intercalated clay layers stacks can be observed in addition to large clay agglomerates. TEM images of the nanocomposites prepared from HM (Fig. 9c--e) demonstrated an improved OMMT dispersion. In PBS/HM10 and PBS/HM15 nanocomposites, the organoclay is highly exfoliated into single platelets in the PBS matrix, where few intercalated clay layers stacks are still detectable. This suggests that these nanocomposites composed of a mixture of intercalated and exfoliated structures. This also confirmed the improvement in OMMT dispersion after organomodification of clay.

From Fig. 9e, it shows that the compatibilization has contributed to a more homogenous OMMT dispersion, in which higher degree of exfoliation was achieved. PBS-g-MA is able to intercalate between the silicate galleries, further separate the clay platelets apart. Meanwhile, it is also noted that PBS/HM10 nanocomposites exhibit better clay dispersion when compared with the PBS/2DM nanocomposites. This is in line with the observations in XRD analysis and mechanical properties.

Differential Scanning Calorimetry. Figure 10 shows the DSC thermograms of PBS and its nanocomposites with different organoclays, and the results are summarized in Table 3. Pure PBS and PBS/2DM nanocomposites show two distinct peaks (marked as "x" and "y") in the heating scans as observed in Fig. 10a, which is assigned to the melting of two populations of crystal lamella in PBS. The "x" melting endotherm is corresponds to the melting of the original crystallites formed at the isothermal crystallization temperature; whereas the "y" melting endotherm reveals the melting of the recrystallized crystals (42), (43). In PBS/MMT and PBS/HM nanocomposites, the "x" peak is found to slightly shill toward a higher temperature (~106-108[degrees]C), and appear as a superimposed feature with a shoulder at the left side of the "y" peak. The higher [T.sub.m, x] of PBS/MMT nanocomposite is resulted from its higher [x.sub.c], as shown in Table 3. In the case of PBS/HM nanocomposites, the improved interactions between PBS matrix and the organoclay may also yields a higher [T.sub.m, x] (44), (45). This is also observable in the compatibilized nanocomposites.

TABLE 3. DSC results of PBS and the nanocomposites with
different organoclays.

Compound     [T.sub.m, x]  [T.sub.m, y]     [T.sub.c]  [T.sub.c]
             ([degrees]C)  ([degrees]C)  ([degrees]C)        (%)

PBS                 102.2         1 2.4          85.9       57.6

PBS/2 DM            102.1         113.4          85.6       55.7

PBS/MMT             106.1         113.3          69.1       59.7

PBS/HM10            108.4         113.9          71.9       56.0

PBS/HM15            108.5         114.1          73.6       56.1

PBS/HM10/MA         108.7         113.5          75.2       57.3


The crystallization behavior of PBS was investigated through the DSC cooling scans, as presented in Fig. 10b. The incorporation of DM into PBS does not affect to the [T.sub.c] and [T.sub.m], while the crystallization peaks is significantly shifted toward a lower [T.sub.c], by the incorporation of MMT and HM into PBS. This shows two different crystallization behaviors between DM and HM. From the results, it shows that MMT could act as a nucleating agent to enhance the [X.sub.c] of PBS/MMT composite. Hence, this may alter the kinetics of crystallization of PBS, where a lower [T.sub.c] was observed. This is in line with the studies reported by Borsig et al. (46) on the PP/organoclay nanocomposites. They claimed that the MMT layers could act as obstacles to lower the rate of radial growth of lamellas, but promote the crystallization through "bridges" among the nuclei on the silicate particles. Hence, the presence of MMT slows down the crystallization rate, but provides more nucleating sites for the crystallization to occur. After organomodification of MMT, the improved filler-- matrix interactions and filler dispersion slightly decreased the [x.sub.c]. Nevertheless, due to the characteristics of PBS-g-MA as a heterogeneous nucleating agent, the [X.sub.c] of the compatibilized PBS/HM nanocomposite is higher than that of the uncompatibilized one.

CONCLUSIONS

The organoclay modified by HTAB was found to provide better reinforcement effect to PBS nanocomposites when compared with the pristine sodium MMT and commercial ODA-modified organoclay. This was mainly due to the trimethyl groups in HTAB, which could facilitate the filler dispersion. The organoclay modified at 1.0 CEC HTAB content was showing a better reinforcement effect in PBS nanocomposites than the one modified at 1.5 CEC HTAB content, due to the higher hydrophilicity of OMMT at excessive surfactant content. It was also observed that improvements in the mechanical properties were obtained in the compatibilized PBS/HM nanocomposite, indicating the functionality of PBS-g-MA as compa-tibilizer in this nanocomposite system. In addition, the filler dispersion and filler--matrix interactions were improved after modifying the MMT and incorporation of PBS-g-MA, as observed under TEM and SEM. The improved filler--matrix interactions in PBS/HM nanocomposites led to an increased in [T.sub.m] and a decreased [x.sub.c]. Hence, by concluding all the observations, the optimum performance of PBS nanocomposites was obtained in compatibilized HM lilled nanocomposites. This shows the potential of HTAB-modified organoclay to be applied in the production of polymer nanocomposites, as an alternative to replace the conventional ODA-modified organo-clay.

Correspondence to: Dr. Z.A. Mohd Ishak; e-mail: zarifin@eng.usm.my or zarifin.ishak@gmail.com

Contract grant sponsor: USM Research University Cluster; contract grant numbers: 100I/PKT/8640012; contract grant sponsor: USM Incentive; contract grant number: 1,001/PBAHAN/8021011; contract grant sponsor: USM Postgraduate Research; contract grant number: 1001/PBAHAN/8044004; contract grant sponsor: USM Fellowship.

DOI 10.1002/pen.23460

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

[C] 2013 Society of Plastics Engineers

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Y.J. Phua, (1), (2) W.S. Chow, (1), (2) Z.A. Mohd Ishak (1), (2)

(1) Cluster for Polymer Composites, Engineering and Technology Research Platform, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

(2) School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia
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Author:Phua, Y.J.; Chow. W.S.; Ishak, Z.A. Mohd
Publication:Polymer Engineering and Science
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Geographic Code:9MALA
Date:Sep 1, 2013
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