Effect of Clay Modification on Photo-oxidation of Polyethylene/Clay Nanocomposites

Transcription

1 Effect of Clay Modifi cation on Photo-oxidation of Polyethylene/Clay Nanocomposites Effect of Clay Modification on Photo-oxidation of Polyethylene/Clay Nanocomposites Zhongzhong Qian, Shimin Zhang and Mingshu Yang* CAS Key Laboratory of Engineering Plastics, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing , China Received: 9 January 2008 Accepted: 12 April 2008 SUMMARY In this paper, effect of clay modification and the resultant clay dispersion and orientation structure on the photo-oxidation of polyethylene has been investigated upon ultraviolet exposure using the technique of infrared spectroscopy. Polyethylene/clay nanocomposites have been prepared using different clay modification. Both the pristine MMT clay and CTAB ammonium intercalated clay were modified via grafting reaction of silane with hydroxyl groups on the clay edge. The grafting modification leads to decreased photo-oxidation rate compared with that of neat PE, while the pristine MMT and ammoniums accelerate the photo-oxidation rate of the corresponding composites. Effect of the dispersion and orientation structure of clay in polymer matrix has also been discussed. In addition, the influence of photo-oxidation behavior on crystallization and morphology of the composites was studied by means of DSC and SEM. 1. INTRODUCTION In the past decade, polymer/clay nanocomposites have attracted significant attention both in scientific research and industrial exploitation 1-3. Compared with the conventional filled polymers, this new-generation of composite presents considerable improved properties at a rather low clay loading (usually less then 5 wt.%), such as enhanced mechanical properties, increased heat distortion temperature, improved thermal stability and barrier properties, and also good flame retardancy 2-5. Environmental stability is critical to polymer/clay nanocomposites in their processing and application. There are many factors influencing the degradation of polymeric materials, such as photo-irradiation, thermodegradation, oxidation, hydrolization etc., and the combinations. Within the * Corresponding author. Tel: ; Fax: ; yms@iccas.ac.cn. Smithers Rapra Technology, 2008 various factors, the photo-oxidation plays an important role. There are some works in the literatures concerning the photo-oxidative degradation of polymer/clay nanocomposites In the previous work of our group, the photo-oxidation behaviors of polyamide 6 8, polyethylene 9,10 and polypropylene 11 nanocomposites have been investigated. The rate of photo-oxidative degradation of these nanocomposites is much faster than that of neat polymers, which is ascribed to several factors including nature of clay (interlayer cations, acidic sites on surface) and interlayer ammonium ions. The structures of composites, the compatibilizers and antioxidants used in processing also have influence on the photo-oxidation behaviors 6,7,11,12. Polyethylene is one of the most widely used polyolefin polymers. Due to their nonpolar backbone, the aim to obtain PE nanocomposites by melt compounding process remains a challenge 13,14. Even though the organoclay is hydrophobic after the ion exchange modification, the ammonium is thermodynamically immiscible with polyethylene matrix 15,16. Another modification approach involving direct grafting reaction to form covalent bonds with clay has recently attracted much attention There are some silanol groups (Si-OH) located on the edge of clay due to the broken edge and crystalline defects This route uses chloro or alkoxyl silanes as modifiers, which were grafted to the clay edge via condensation with the silanol groups. The first attempt to use the edge grafting modification to improve the miscibility between organoclay and polyethylene has been reported by Zhao et al. 18,26. The clay was grafted by chlorosilane and then ion-exchanged with ammonium, showing improved dispersion state in polyethylene nanocomposites. Zhang et al. 27 have also reported a grafting process of chlorosilane with the ammonium intercalated clay controlled by the presence of base, and found better dispersion than the composite of PE and ion-exchanged clay. Recently, we have investigated Polymers & Polymer Composites, Vol. 16, No. 8,

2 Zhongzhong Qian, Shimin Zhang and Mingshu Yang the grafting reaction on the clay edge and found the increased dispersion of organoclay with PE matrix. An alkylsilane, Dodecyltrimethoxylsilane, was used to graft on the edge of the ammonium-intercalated clay. Effect of grafting modification on the dispersion state and orientation structure, and mechanical and thermal properties will be described elsewhere. In this paper, we describe the effect of clay modification on photo-oxidation of PE/clay nanocomposites. In order to evaluate the influence of nature of clay surface, ammoniums and grafting modification, several types of clay, pristine MMT, silane grafted MMT (DS-MMT), ammonium intercalated MMT (CMT) and silane grafted CMT clays (DMTs), were used to melt compound with PE. The photooxidation behavior was investigated by means of infrared spectroscopy. It is shown that the grafting modification of clay leads to decreased photooxidation rate compared with that of neat PE, while the pristine MMT and ammoniums accelerate the photooxidation rate of the corresponding composites. Effect of the dispersion and orientation structure of clay in polymer matrix has also been discussed. In addition, the influence of photo-oxidation behavior on crystallization and morphology of the composites was studied by means of DSC and SEM. 2. EXPERIMENTAL 2.1 Materials Sodium montmorillonite (Na-MMT) used in experiments, with cation exchange capacity (CEC) of 90 meq/100 g, was obtained from Zhangjiakou Qinghe Chemical Factory. Cetyltrimethyl ammonium bromide (C 16 H 33 N + (CH 3 ) 3 Br, denoted as CTAB) was purchased from Beijing Chemical Reagent Co., China. Dodecyltrimethoxylsilane (C 12 H 25 Si(OCH 3 ) 3, denoted as DS) was obtained from Hubei Wuhan University Silicone New Materials Co., Ltd. All reagents were used as received. Lowdensity polyethylene, LDPE 1I2A, was supplied by Yanshan Petrochemical Co. Ltd., Beijing, China. It has a density of g/cm 3 (23 C) and a melt flow index of 2.0 g per 10 min (2.16 kg at 190 C). 2.2 Surface Modification of Clay The organophilic montmorillonite (organoclay) was prepared via ionexchange reaction using CTAB alkylammonium. For this purpose, sodium montmorillonite (10 g) was dispersed in an ethanol/water mixture (100/100 vol/vol, 200 ml) by stirring at room temperature. Then CTAB (14.9 g, 120% of the CEC) was added to the clay suspension under stirring and the reaction mixture was stirred at 80 C for 8 h. At the end of the reaction time, the suspension was filtered and the product was washed repeatedly with the mixture of ethanol/water. The product was denoted as CMT. Prior to the grafting reactions with clay, required amount of DS silane was firstly added into a 100 ml mixture of ethanol/water (90/10, v/v) to pre-hydrolyze for 1 h. The ph of the mixture solution was adjusted to 4 by addition of acetic acid. Then 10 g of the organoclay, CMT, was added into the mixture and the suspension was stirred vigorously at 80 C for 24 h. The products were filtered and washed by mixture of ethanol/water. The silane modified clay products (DMT) were denoted as LDMT, MDMT, HDMT, corresponding to low (0.61 mmol/g), moderate (1.53 mmol/g) and high (3.06 mmol/g) silane concentration used in the grafting reaction, respectively. All the clay samples were dried at 80 C for 48 h in a vacuum, ground into powder, and sifted through a 200-mesh sieve (maximum diameter of 75 μm). 2.3 Preparation of Nanocomposites The nanocomposites were prepared by premixing PE and clay samples and then melt blending in a HAAKE Rheomix 600 internal mixer (Mess- Technic GmbH, Germany) equipped with a chamber of 60 ml and a couple of Sigma-shape counter-rotating blades at 190 C for 5 min at a rotation speed of 70 rpm. In order to control the morphology of nanocomposites, the melt blending condition was changed to facilitate the orientation and/or the random uniform dispersion of clay. The PE/MDMT-HO nanocomposites sample with high orientation structure was prepared by melt blending at a higher rotation speed of 90 rpm. The PE/MDMT-E sample with intercalation/exfoliation structure was prepared according to literature 28.. The MDMT clay was firstly melt blended with PE at a rotation speed of 10 rpm for 2 min to facilitate the diffusion of PE chains, and then at 70 rpm for 3 min to promote the dispersion. Several compounds were prepared and their compositions are listed in Table 1. The resultant compounds were compression molded to 1 mm thickness plates, which were used for measurements of XRD, TEM and SEM. In order to minimize the effect of the pre-oriented shear field, the small chip samples were mixed and melted at 190 C for 5 min, and then heat compressed in a steel frame of dimensions mm between two brass plates at 190 C and 10 MPa for 5 min. The sample was cooled to room temperature. The thin films of ca. 40 μm in thickness used for UV exposure tests were prepared by compression-molding the pellets of the compounds at 190 C with a pressure of 10 MPa. 2.4 Ultraviolet (UV) Exposure Tests Ultraviolet (UV) exposure tests were carried out in a WFH-201-B canned UV exposure instrument (Wenzhou Analytic Apparatus Co., China). The working temperature was controlled at 35±2 C and the wavelength of radiation was 300 nm. The photooxidation rate was followed by 536 Polymers & Polymer Composites, Vol. 16, No. 8, 2008

3 Effect of Clay Modifi cation on Photo-oxidation of Polyethylene/Clay Nanocomposites Table 1. The PE compounds, their composition and melt blending condition Sample code Clay CTAB Melt condition Type Silane amount (mmol/g) ammonium (wt %) PE 190 C, 5 min, 70 rpm PE/CTAB 0.75 PE/Na-MMT Na-MMT PE/DS-MMT DS-MMT 1.53 PE/CMT CMT PE/LDMT LDMT 0.61 PE/MDMT MDMT 1.53 PE/HDMT HDMT 3.06 PE/MDMT-HO MDMT C, 5 min, 90 rpm PE/MDMT-E MDMT C, 2 min, 10 rpm and then 3 min, 70 rpm measuring carbonyl absorbance through FT-IR per 48 h upon UV exposure, for a period h. The FT- IR measurements were performed on a Perkin-Elmer System 2000 infrared spectrum analyzer in the wave-number range of cm Characterization of Modified Clays and Nanocomposites The grafting reaction of silane and clay was detected by Fourier-transform Infrared (FT-IR) measurements, which were performed on a Perkin- Elmer system 2000 infrared spectrum analyzer using KBr pellets. The thermogravimetric analysis (TGA) was performed on a Perkin-Elmer 7 series analyzer with a heating ramp of 20 C/min in nitrogen atmospheres in the temperature range of C. Wide-angle X-ray diffraction (XRD) patterns were obtained from a Rigaku D/max 2400 diffractometer with Cu Kα radiation (λ = nm, 40 KV, 120 ma) at room temperature. The diffractogrames were scanned in the 2θ range of Transmission electron microscopy (TEM) was carried out on Hitachi (Japan) H-800 microscope at an acceleration voltage of 100 kv. Scanning electron microscopy (SEM) was carried out on a JEOL S-4300F field emission scanning electron microscope at 5 KV. DSC experiments were carried out on a Perkine-Elmer DSC-7 thermal analysis system in nitrogen atmosphere at a heating rate of 10 C/min. The percentage crystallinity was calculated from DSC results using the following relation: %Crystallinity = H f(observed) ( 1+ w) 100 H f(100%crystalline) where w is the weight percent of organoclays in composites, H f is the enthalpy of the material obtained from DSC curves and H f(100%crystalline) is the enthalpy of 100% crystalline LDPE taken from the literature as 285 J/g RESULTS AND DISCUSSION 3.1 Clay Modification The various modified clays studied in this work were characterized using FTIR, TGA and XRD. The chemical reaction between hydrolyzed DS and the silanol groups on clay edge can be confirmed by FT-IR spectrum. Figure 1a shows the FT-IR spectra of CMT and DMT clays with different amount of grafted silane. The band around 3627 cm -1 corresponds to the stretching vibration of surface hydroxyl groups (AlAlOH, AlMgOH and isolated Si-OH). Another broad band at about 3450 cm -1 is ascribed to the -OH stretching vibration of the adsorbed water and the hydrogenbonded surface Si-OH 18,20,23,26.. After the grafting reaction of silanes, it can be observed that the intensity of both -OH stretching vibration bands at about 3627 cm -1 and about 3450 cm -1 decreased obviously with the increasing silane concentration. Thermogravimetric analysis (TGA) provides another approach to investigate the grafting reaction. Figure 1b shows the TGA and DTG curves of CMT and DMT clays. The mass loss in the region of C is due to the decomposition of organic portion attached on clay surface. The DTG curve of CMT shows two major peaks at 270 C and 335 C with a shoulder around 400 C, corresponding to the decomposition of physically Figure 1. (a) FT-IR spectra and (b) TGA & DTG curves of CMT and DMT clays Polymers & Polymer Composites, Vol. 16, No. 8,

4 Zhongzhong Qian, Shimin Zhang and Mingshu Yang adsorbed and intercalated ammonium, respectively 22,30. For the DMT clays, a new peak at around C is attributed to the decomposition of the chemically grafted silane 17,22, and shifted to higher temperature as the silane concentration increased. In addition, the decomposition peak of physically adsorbed ammonium at 270 C become weakened for LDMT, and finally disappeared for MDMT and HDMT. This is due to the grafting reaction occurred on the edge of clay surface, replacing the physically adsorbed ammonium 27. Thus, as the silane concentration increased, the onset temperature of degradation and the organic content increased as follows: 228 C and 17.8% for CMT, 260 C and 18.3% for LDMT, 300 C and 19.5% for MDMT and 305 C and 22.2% for HDMT, respectively. Unfortunately, the grafted silane contents can not be obtained from TGA because of the overlapping of the degradation steps for ammonium and silane. The XRD patterns for CMT and DMT are shown in Figure 2a. It is clear that the d-spacing of DMT clays almost have no difference with CMT. This is in agreement with TGA results, indicating the grafting reaction was mainly occurred on the edge of clay platelets. 3.2 Structure of the Composites Figure 2 shows the X-ray diffraction (XRD) patterns for the organoclays (CMT and DMT clays) and their corresponding PE nanocomposites (PE/CMT and PE/DMTs nanocomposites). The DMT clays show quite similar diffraction patterns with CMT. In the case of PE/CMT, the d-spacing decreased from 1.83 nm of the CMT powder to 1.76 nm of the composite, and the peak profile have no change after melt blending, indicating that the ammonium intercalated clay is immiscible with PE and only microcomposite was formed. The decrease of the d-spacing is due to the surfactant degradation during melt processing 31. The XRD patterns for the polyethylene/dmt clays nanocomposites, however, present a strong diffraction peak with high intensity at about 4.8 (~1.8 nm) and broad peaks at lower diffraction angle. The emergence of new peaks at 3.48 (2.54 nm) for PE/LDMT, 3.2 (2.76 nm) for PE/MDMT and 2.94 (3.00 nm) and 1.48 (5.96 nm) for PE/HDMT indicate the formation of intercalation structure and suggest that the extent of dispersion and exfoliation is increased with increasing grafted silane. It should be noted that the peaks at 4.8 exhibit a rather distinct high intensity profile comparing with that of the corresponding clay and the PE/ CMT composite. The peaks at 9.7 (0.91 nm) and 14.7 (0.61 nm) correspond to the second order (002) and third order (003) of the first reflection, as shown in the inset of Figure 2b. It has been reported that the orientation of the clay particles is expected to result in the high intensity of the (001) reflection 32-34, and the presence of higher rational orders indicate the ordered structure of clay platelets, which have also been reported in some other clay nanocomposites 35, 36. Furthermore, the intensity of the (00l) reflection peaks increases steadily as the amount of silane used for modification increases, indicating the preferred orientation derives from the surface grafting of silane. Among these PE nanocomposites, PE/ MDMT-HO exhibits highest 00l reflection peaks, indicating the highest extent of the orientation. In contrast, the diffraction peak of clay in PE/MDMT-E almost disappears, as shown in Figure 2, in which only very broad peaks can be observed. This indicates the formation of partially intercalated and partially exfoliated structure, instead of orientated structure. Since XRD alone is not enough to determine the structure of the nanocomposites, TEM micrograph is used to directly observe the dispersion state of clay layers, as shown in Figure 3 and Figure 4. It is observed that large aggregation of clay dispersed in micron-scale in Figure 3a for the PE/ CMT composite, without intercalation and orientation of the clay particles. After surface grafting, the DMT clays exhibit quite more uniform dispersion than CMT in polyethylene and obvious orientation morphology, as shown in low magnification TEM images (Figure 3b-d). High magnification TEM images (Figure 3e-h) give more information about the local dispersion state of clay layers. The Figure 2. XRD patterns for (a) the clay powder samples, (b) PE/clay nanocomposites between the diffraction range of 1-25 and (c) PE/clay nanocomposites between the diffraction range of 1-8. The inset in (b) indicates the 10 times magnified patterns between diffraction range of Polymers & Polymer Composites, Vol. 16, No. 8, 2008

5 Effect of Clay Modifi cation on Photo-oxidation of Polyethylene/Clay Nanocomposites Figure 3. Low magnification TEM images of (a) PE/CMT, (b) PE/LDMT, (c) PE/MDMT and (d) PE/HDMT nanocomposites, and high magnification TEM images (e-h) of the above nanocomposites, respectively (a) (b) (c) (d) (e) (f) (g) (h) PE/CMT composite shows large randomly distributed aggregates of the clay tactoids, while the PE/ DMT nanocomposites show highly orientation of the stacked clay layers and some intercalated/exfoliated clay sheets. From both the low and high magnification TEM images, it can be clearly observed that the degree of the orientation, the aspect ratio and the extent of exfoliation/intercalation of clay particles increase steadily as the amount of silane increases, which is in agreement with the result of XRD. Figure 4a and b present the TEM images of PE/MDMT-HO, in which high degree of orientation clay layers can be clearly observed. While the TEM images of PE/MDMT-E, as shown in Figure 4c and d, suggest the uniform clay dispersion throughout PE matrix. Especially in the high magnification image (Figure 4d), the intercalated and exfoliated clay layers are present, which is also consistent with XRD result. The orientation of clay in PE/DMT Figure 4. TEM images of (a) and (b) PE/MDMT-HO, (c) and (d) PE/MDMT-E at low and high magnification, respectively (a) (c) (b) (d) Polymers & Polymer Composites, Vol. 16, No. 8,

6 Zhongzhong Qian, Shimin Zhang and Mingshu Yang samples is attributed to the grafted alkylsilane, which decreases surface energy of the organoclay and increases the miscibility with the non-polar polyethylene matrix. The increased interaction between clay and polymers leads to slippage of the clay particles and then the peeling of clay layers during melt processing 37,38, which corresponds to the preferred orientation and intercalation/exfoliation of clay layers. In Figure 3f-h, it can be clearly observed that some intercalated or exfoliated clay layers were peeled away from the oriented clay tactoids. The intercalation/exfoliation structure of PE/MDMT-E is formed due to diffusion of PE chains into clay galleries at static or at least under mild shearing conditions. Thereafter, when shear intensity is increased, a higher degree of delamination of the various overlapped clay layers was obtained 28. Figure 5. FTIR spectra of (a) neat PE and (b) PE/MDMT-E nanocomposite before and after 336 h UV irradiation 3.3 Infrared Analysis The photo-oxidation of the films upon UV exposure was characterized by measuring carbonyl absorbance through FT-IR spectra. When exposed under UV irradiation, polyethylene suffers degradation and reveals a new broad absorption band at around 1714 cm -1, which belongs to a mixture of several carbonyl species, such as lactone (near 1780 cm -1 ), ester (around 1735 cm -1 ), ketone (at 1715 cm -1 ), and carboxylic acid group (about 1700 cm -1 ) 39. The area of carbonyl band of FT-IR spectrum of each specimen can be obtained separately and considered as the quantity of the carbonyl. Figure 5a shows the FTIR spectra of neat PE before and after 336 h UV irradiation. As shown in the figure a new broad band appears at about 1714 cm -1 after 336 h UV irradiation, which belongs to a mixture of different carbonyl species. This indicates the photo-oxidation of PE molecules under UV exposure. Figure 5a shows the FTIR spectra of PE/MDMT-E nanocomposite before and after 336 h UV irradiation. The absorption bands at 1036 and 1094 cm -1 are Si-O stretching of MMT. The bands at 516 and 463 cm -1 are assigned to Al-O stretching and Si-O bending of MMT. Similar to the case of neat PE, a new carbonyl band at about 1714 cm -1 appears after 336 h UV irradiation for the PE nanocomposite. But the intensity of the carbonyl band is quite higher than that of neat PE, indicating the different degree of photooxidation before and after addition of organoclay. For a qualitative description of the extent of photo-oxidation in this research, the evolutions of the FTIR spectrum in the wavenumber range of cm -1 were examined. The film samples were measured by FT-IR per 48 h upon UV exposure for a total irradiation time up to 336 h. Figure 6 show the typical evolution of the infrared spectra of neat PE and PE clay composites in carbonyl region with different UV exposure time. It can be observed that the intensity of carbonyl band increases steadily with the increasing exposure time. The other samples show similar FT-IR spectra under UV irradiation. The intensity of the carbonyl band of all compound samples grows faster than that of neat PE, except for the PE/DS-MMT composite. However, the shape of the oxidation bands is similar to that observed for neat PE. It is suggested that the addition of clays and corresponding modification method do not change the mechanism of photo-oxidation of the PE matrix. By calculating the area of carbonyl band in function of the exposure time, we can determine the photo-oxidation rate of different PE composites. 540 Polymers & Polymer Composites, Vol. 16, No. 8, 2008

7 Effect of Clay Modifi cation on Photo-oxidation of Polyethylene/Clay Nanocomposites Figure 6. Evolution of the infrared spectra with different UV exposure time in carbonyl region ( cm -1 ) of film samples: (a) neat PE, (b) PE/Na-MMT, (c) PE/DS-MMT, (d) PE/CMT, (e) PE/MDMT, (f) PE/MDMT-E 3.4 Effect of the Surface Grafting Modification on Photo-oxidation Figure 7 shows the area of carbonyl band in function of the exposure time for the film samples of neat PE, PE/ Na-MMT and PE/DS-MMT. These samples have similar dispersion state, as shown in Figure 7. Comparing with neat PE, the PE/Na-MMT composite exhibits increased photo-oxidation rate. This result is consistent with previous reports. It has been reported 30 that the complex crystallographic structure and habit of clay minerals result in some catalytic active sites, such as Bronsted acidic sites like the weakly acidic SiOH and strongly acidic bridging hydroxyl groups at the layer edge of the silicate, unexchangeable transition metal ions in the galleries, and crystallographic defect sites within the layers. All these active sites can accept single electrons from donor molecules and form free radical, leading to the oxidation of the polymer matrix. In the case of PE/DS- MMT composite, the photo-oxidation rate is obviously slower than that of neat PE and PE/Na-MMT composite. This probably due to that the grafting reaction of silane leads to decreased Figure 7. Variations in the area of carbonyl band during the photo-oxidation of films of PE, PE/Na-MMT and PE/DS-MMT composites amount of active sites on clay surface and the alkyl chain of silane covered on clay surface blocks the interaction between active sites and PE matrix. Meanwhile, barrier property of the clay particles against the UV light during exposure can also slow down the photo-oxidation rate. 3.5 Effect of the Ammonium on Photo-oxidation In order to understand the effect of ammoniums, we compared the photo-oxidation rate of neat PE, PE composites containing clays without ammoniums, such as Na-MMT, DS-MMT, with PE nanocomposites containing clays intercalated modified with CTAB, such as CMT and DMTs. To evaluate the effect of ammoniums separately, PE/CTAB compound was prepared by direct melt blending PE with CTAB ammonium. Figure 8 shows the area of carbonyl band in function of the exposure time for the film samples of neat PE, PE/Na- MMT, PE/DS-MMT, PE/CMT, PE/ LDMT, PE/MDMT composites and PE/CTAB compound. It is obvious that Polymers & Polymer Composites, Vol. 16, No. 8,

8 Zhongzhong Qian, Shimin Zhang and Mingshu Yang Figure 8. Variations in the area of carbonyl band during the photo-oxidation of films of PE and composites including clays modified with different methods Figure 9. Variations in the area of carbonyl band during the photo-oxidation of films of PE composites with different dispersion state the photo-oxidation rate of composites have significant increase when the clays were intercalated modified with ammonium, regardless of the dispersion state and modification method. As a control sample, the photo-oxidation rate of PE/CTAB compound has very similar behavior with the PE/OMMT composites. This indicates the influence of ammonium plays a major role during photo-oxidation compared with clay activate surface, which only has slight influence. The decomposition of alkyl ammonium salts in the clay interlayer is known to take place following the Hoffman elimination mechanism. The resultant substances are ammonia, corresponding olefin and an acidic site on layered silicate. During the melt processing and UV exposure period, the decomposition of the ammonium ions in the galleries happened partially. The decay products, such as the acidic sites and corresponding olefin, can both lead to the formation of free radical and accelerate the photooxidation of polymer matrix under UV irradiation. It should be noted that about 20% of the cation exchange capacity of clay resides on the edge of the platelet. It has been reported that the ammonium adsorbed on the clay edge exhibits poor stability 27, which is also shown in TGA date of modified clays (Figure 1). The PE/ LDMT composite has almost similar dispersion state with the PE/CMT composite, while the former presents decreased photo-oxidation rate. This probably due to the grafted silane replaces the adsorbed ammoniums on the edge of clay platelet, reducing the catalysis influence of the ammonium and slowing down the photo-oxidation rate. In general, the catalysis effect of ammonium dominates the photooxidation process, even though the clay surface was grafted with silane. The extent of the influence depends on the nanocomposites structure, as will be discussed as follow. 3.5 Effect of the Nanocomposites Structure on Photo-oxidation Figure 9 presents the area of carbonyl band in function of the exposure time for the film samples with different dispersion state and structure. The PE/ CMT conventional composite shows very fast photo-oxidation rate, which is due to the catalysis of ammoniums, as has been discussed above. After the clay edge was grafted with silane, we found the orientation structure has obvious influence on the enhancement of photo-oxidation stability, which is clearly exhibited in the case of PE/ DMTs nanocomposites. In particular, PE/MDMT-HO nanocomposite, which has highest orientation degree among the PE nanocomposites, presents slowest photo-oxidation rate. This is probably due to the barrier properties of clay layers, especially in the case of orientation structure, can lead to hindering of UV light during exposure. On the other hand, it can be observed that the PE/MDMT-E nanocomposite, which has best dispersion state (intercalation and exfoliation) among all composites, exhibits the highest photo-oxidation rate. This is firstly due to that the ammoniums in the clay interlayer are exposed to PE matrix and can directly catalyze the photo-oxidation process when the nanocomposite has a uniformly dispersed structure. Secondly, in the case of PE/MDMT-E, the clay layers were randomly dispersed and there is no obvious barrier effect on photooxidation. 542 Polymers & Polymer Composites, Vol. 16, No. 8, 2008

9 Effect of Clay Modifi cation on Photo-oxidation of Polyethylene/Clay Nanocomposites However, one can observe from Figure 9 that with increasing of both the orientation and dispersion degree, PE/LDMT, PE/MDMT and PE/HDMT nanocomposites present very similar photo-oxidation behavior. As has been discussed, the increased silane amount leads to reduced ammoniums adsorbed on clay edge and increased orientation degree. Both factors can slow down the photo-oxidation rate. Meanwhile, the increased silane amount also leads to more intercalated and exfoliated clay layers, resulting in increased catalysis effect of the interlayer ammoniums. The contrary effects lead to similar photo-oxidation behavior. This indicates that the barrier effect of clay orientation structure and catalysis of ammonium play competitive roles during photo-oxidation process. 3.6 DSC Results DSC measurements were used to investigate the effect of photooxidation on the crystallinity of PE and nanocomposites. Figure 10 shows the second round heating scan curves of the samples at a heating rate of 10 C/ min. It is apparent that there is no obvious change in melting temperature after 336 h UV exposure, but the area under the melting endotherm changes after UV exposure. This is due to that the photo-oxidation occurs mainly in the amorphous regions, while the crystalline regions remain unaffected. The characterization data obtained from DSC is listed in Table 2. The neat PE shows obvious decrease of crystallinity, which is due to the cross-linking of molecules upon UV exposure. The PE/DS-MMT composite, which has slow photo-oxidation rate, also shows decreased crystallinity. For the PE composites containing organoclays, the crystallinity has obvious increase with different extent, depending on the clay modification methods and composites structure. It has been reported that the increase in crystallinity of polyethylene upon UV irradiation is due to the chain scission in the amorphous region 29. The chain scission leads to low molecular weight segment, which can increase the crystallization rate or act as nucleating agent. It is clear that the change of crystallinity after UV exposure is consistent with the photo-oxidation rate of the samples. For instance, PE/CMT and PE/MDMT-E, which have very fast photo-oxidation rate, exhibit obvious increase of crystallinity. PE/MDMT- HO, which has slowed-down photooxidation rate due to the orientation structure, exhibit only slight increase of crystallinity. 3.7 Morphological Characterization The effect of the UV exposure on morphology of the film samples was investigated using scanning electron microscopy. Figure 11 shows the SEM images of neat PE and PE composites before and after 336 h UV exposure. The film samples have a smooth surface before UV irradiation. However, the roughness of films surface has an obvious increase after 336 h UV irradiation. The extent of the damage was dependent on the photo-oxidation rate. In the case of high photo-oxidation rate, such as the films of PE/CMT and PE/MDMT-E, there are some cracks formed after UV irradiation. 4. CONCLUSION In the present work, polyethylene/clay nanocomposites have been prepared using different clay modification methods. Both the pristine MMT clay and CTAB ammonium modified clay were successfully modified via grafting reaction of alkylsilane with hydroxyl groups on the clay edge. The improved miscibility between organoclays and PE matrix leads to improved dispersion state and high orientation structure. The photo-oxidative behavior of the PE clay nanocomposite has been investigated upon ultraviolet exposure. The photo-oxidation rate of PE/clay Figure 10. DSC heating scans of Neat PE and PE composites before and after 336 h UV exposure Table 2. DSC data and crystallinity of neat PE and PE composites before and after 336 h UV exposure Sample H f (J/g) T m ( C) Crystallinity (%) 0 h 336 h 0 h 336 h 0 h 336 h PE PE/Na-MMT PE/DS-MMT PE/CMT PE/MDMT PE/MDMT-HO PE/MDMT-E Polymers & Polymer Composites, Vol. 16, No. 8,

10 Zhongzhong Qian, Shimin Zhang and Mingshu Yang Figure 11. SEM images of film samples before (left column) and after (right column) 336 h UV exposure. a, a ) neat PE, b, b ) PE/Na-MMT, c, c ) PE/DS- MMT, d, d ) PE/CMT, e, e ) PE/MDMT-HO and f, f ) PE/MDMT-E nanocomposite is much faster than that of neat PE, which is due to the catalysis of surface properties of clay and ammonium during photo-oxidation process. The effect of clay modification and the resultant clay dispersion and orientation structure on the photooxidation has been investigated: 1. Effect of the grafting modification. The catalytic active sites including the complex crystallographic structure and habit of clay minerals can accelerate the photo-oxidation rate. After surface grafting modification with silane, the amount of these active sites decreased and the alkyl chain of silane covered on clay surface blocks the interaction between active sites and PE matrix. 2. Effect of the intercalated ammonium. Compared with the clay surface active sites, ammonium plays a major role in catalyzing the photo-oxidation process. The decomposition of ammonium can lead to catalytic acidic sites created on the layers and correspondingly olefin generated. All these photoresponsive groups and catalytic active sites can accelerate the photo-oxidation rate. 3. Effect of nanocomposites structure. The clay orientation structure has obvious influence on the enhancement of photooxidation stability, which is due to the barrier effect of the oriented clay layers. While the uniform dispersion structure leads to obvious acceleration of photo-oxidation, due to both the catalysis effect of intercalated ammonium and no obvious barrier effect of clay. The barrier effect of clay orientation structure and catalysis of ammonium have contrary influence and play competitive roles during photooxidation process. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No and ). 544 Polymers & Polymer Composites, Vol. 16, No. 8, 2008

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