clay nanocomposites

Polymer Degradation and Stability 94 (2009) 1571–1588

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Effect of extrusion and photo-oxidation on polyethylene/clay nanocomposites N.Tz. Dintcheva a, S. Al-Malaika b, *, F.P. La Mantia a a b

` di Palermo, Viale delle Scienze, ed. 6, 90128 Palermo, Italy Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita Polymer Processing and Performance Research Unit, School of Engineering and Applied Science, Aston University, Aston Triangle, Birmingham, B4 7ET, UK

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 March 2009 Received in revised form 28 March 2009 Accepted 6 April 2009 Available online 23 April 2009

Polyethylene (a 1:1 blend of m-LLDPE and z-LLDPE) double layer silicate clay nanocomposites were prepared by melt extrusion using a twin screw extruder. Maleic anhydride grafted polyethylene (PEgMA) was used as a compatibiliser to enhance the dispersion of two organically modified monmorilonite clays (OMMT): Closite 15A (CL15) and nanofill SE 3000 (NF), and natural montmorillonite (NaMMT). The clay dispersion and morphology obtained in the extruded nanocomposite samples were fully characterised both after processing and during photo-oxidation by a number of complementary analytical techniques. The effects of the compatibiliser, the organoclay modifier (quartenary alkyl ammonium surfactant) and the clays on the behaviour of the nanocomposites during processing and under accelerated weathering conditions were investigated. X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), rheometry and attenuated reflectance spectroscopy (ATR-FTIR) showed that the nanocomposite structure obtained is dependent on the type of clay used, the presence or absence of a compatibiliser and the environment the samples are exposed to. The results revealed that during processing PE/clay nanocomposites are formed in the presence of the compatibiliser PEgMA giving a hybrid exfoliated and intercalated structures, while microcomposites were obtained in the absence of PEgMA; the unmodified NaMMT-containing samples showed encapsulated clay structures with limited extent of dispersion in the polymer matrix. The effect of processing on the thermal stability of the OMMT-containing polymer samples was determined by measuring the additional amount of vinyl-type unsaturation formed due to a Hoffman elimination reaction that takes place in the alkyl ammonium surfactant of the modified clay at elevated temperatures. The results indicate that OMMT is responsible for the higher levels of unsaturation found in OMMT-PE samples when compared to both the polymer control and the NaMMT-PE samples and confirms the instability of the alkyl ammonium surfactant during melt processing and its deleterious effects on the durability aspects of nanocomposite products. The photostability of the PE/clay nanocomposites under accelerated weathering conditions was monitored by following changes in their infrared signatures and mechanical properties. The rate of photooxidation of the compatibilised PE/PEgMA/OMMT nanocomposites was much higher than that of the PE/ OMMT (in absence of PEgMA) counterparts, the polymer controls and the PE–NaMMT sample. Several factors have been observed that can explain the difference in the photo-oxidative stability of the PE/clay nanocomposites including the adverse role played by the thermal decomposition products of the alkyl ammonium surfactant, the photo-instability of PEgMA, unfavourable interactions between PEgMA and products formed in the polymer as a consequence of the degradation of the surfactant on the clay, as well as a contribution from a much higher extent of exfoliated structures, determined by TEM, formed with increasing UV-exposure times. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Clay–PE nanocomposites Processing Photo-oxidation Polyethylene-graft-maleic anhydride

1. Introduction Clay-reinforced polymer nanocomposites (PNCs) have attracted great interest in the recent past due to the low nanofiller * Corresponding author. E-mail addresses: [email protected] (N.Tz. Dintcheva), s.al-malaika@ aston.ac.uk (S. Al-Malaika), [email protected] (F.P. La Mantia). 0141-3910/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2009.04.012

concentration typically required (1–5% compared to 30–40% for conventional fillers and fibres) and the abundance and low cost of the nanoclay fillers combined with significant benefits in terms of the performance properties of the resultant nanocomposites, and especially fire retardancy, thermal and barrier properties [1–5]. The main type of nanoclay that has attracted most attention in polymer nanocomposites is based on the natural mineral 2:1 double layered silicate clays, particularly the plate-like montmorillonite (MMT)

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having both octahedrally and tetrahedrally substituted layered silicates with a negative surface charge that is balanced by interlayer alkali or alkali earth cations which are exchangeable by other cations; the layer dimensions are w1 nm thick with lateral dimensions of up to few microns, hence the very high aspect ratio of nanoclays, Scheme 1 [4]. It is generally accepted that to achieve PNCs with good performance properties, the clay platelets must be sufficiently dispersed with exfoliated (fully delaminated to individual clay platelets and uniformly dispersed in the polymer matrix), intercalated (expanded clay layers with increased interlayer spacing but non-uniformly dispersed), or a mixture of both exfoliated and intercalated (hybrid) morphologies [6]. However, the nanoscale dimension, the high aspect ratio and the hydrophilic character of the MMT clays make their dispersion during processing in hydrophobic non-polar polymers such as polyolefins quite difficult. To address this issue, the surface chemistry of MMT is normally modified through ion exchange reactions with organic cation surfactants, e.g. quaternary ammonium salts having typically long organic chains (referred to as quats) in order to obtain a more organophilic MMT (referred to as OMMT) clays. The organically modified clays experience reduced inter-platelet interactions as manifested by the expanded galleries which enable the diffusion of polymer resins between the layers giving rise to increased separation and, ultimately, to delamination of the clay platelets [6–9]. The quat surfactants (intercalants) are expected to promote nanoclay–polymer interactions because of increased compatibility and their tendency to bring about intercalation of the clay particles. The process of dispersion of OMMT in polyolefins can be further improved by the addition of a compatibilising agent, for example a polyolefin graft copolymer containing maleic anhydride or ethylene-(meth)acrylic acid comonomers (high or low molar mass depending on the level of the anhydride or acid functions) [10–12]. The dual purpose of these agents is to provide both compatibility with the polymer matrix and enhanced interaction with the organically-modified clays. Although both the quats and the compatibilisers are known to enhance the dispersion of the clays in polyolefin resins, they are expected also to affect both processing and performance of polymer nanocomposites. For example, the overall thermal stability of alkyl ammonium ions is low under typical polymer melt processing conditions of high temperature and shear and, consequently, this would affect both the extent of exfoliation (which influences

mechanical properties) and the durability of the nanocomposite product [13,14]. It has been suggested, based on thermogravimetric (TG) studies, that the thermal decomposition of quaternary alkyl ammonium ions, which is known to occur through a Hoffman elimination mechanism, starts at about 180  C and that the main decomposition occurs between 200 and 500  C [13,15–17]. This issue is further complicated by the additional use of compatibilisers in PNC which introduce a further problem with respect to the stability of the nanocomposite system in spite of their apparent beneficial effects in improving nanoclay dispersion and distribution. Overall, the extent of thermal degradation of the organoclay, and the effect of maleic anhydride-based compatibilisers, can be expected to be influenced by a combination of factors including the structure of the quat surfactant, the level of MA grafting in the compatibiliser, the type of the polymer matrix, the processing temperature and mechanical (shear) stress experienced by the nanocomposite during melt processing [18–20]. A large volume of literature is available on the thermal stability of quaternary ammonium ions in OMMT-nanocomposites at high temperatures >300  C from TG studies. However, limited data are available on their effect, and that of MA-based compatibilisers, at the lower melt processing temperatures typically used for producing polyolefin nanocomposites (e.g. 200  C for PE), and much of the literature on processing of polyolefin nanocomposites tends to focus mainly on the clay morphology and final properties of the nanocomposites but not on their long term thermal stability [21–24]. For outdoor applications, e.g. interior and exterior automotive parts, it is very important that clay nanocomposite materials possess both thermal and photo-oxidative stability. Clay–polymer nanocomposites based on polyolefins, however, have been shown not to have sufficient photo-oxidative stability to fulfil this role [25–27]. To address this, issues relating to compatibilisers, organic modifiers (surfactants such as quats), clay reactivity and clay– polymer interfacial chemistry, and their roles on the photostability of polyolefins need to be better understood and controlled. In this work, the effect of melt processing (mainly by extrusion) and that of maleic anhydride-containing-polyethylene (PE–g–MA) compatibiliser on the extent of dispersion of different clays in PE-based nanocomposites was examined using a number of characterisation methods including spectrometric (FTIR–ATR), diffractometric (WAXD), microscpic (TEM, SEM) as well as rheological (rotational rheometry) and mechanical tests. The influence of the organic modifiers in OMMT clays during melt processing, including the extent of their Hoffman elimination reaction, was also investigated and compared to that of the unmodified natural clay, i.e. the pristine NaMMT. The effect of accelerated weathering on the photooxidative stability and morphology of the nanocomposites in the presence and absence of the compatibiliser PEgMA was also studied. 2. Experimental 2.1. Materials

Scheme 1. Structure of 2:1 layered silicates [4].

The polyethylene blend used in this work was based on a 50:50 w/w blend of a Ziegler–Natta linear low density polyethylene, zPE, (LLDPE, FG106, ex. Polimeri Europa) and a metallocene LLDPE polyethylene, mPE (Exceed 1010B, ex. Exxon); the blend is referred to here as PE. Table 1 gives data on the density, molecular weight information, melt flow (MFI) and viscosity for the two polymers as provided by the manufacturers. The nanofillers used are all Southern Clay products and their characteristics are given in Table 2. The maleic anhydride–graft–polyethylene (code: PE–g–MA) compatibiliser used is a Crompton product (Polybond 3009: 1 wt% MA, MFI190  C/2.16 kg ¼ 5 g/10 min). Three commercial stabilisers were used Irganox 1010 and Irganox MD-1024

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Table 1 Characteristics of the polyethylene polymers used (obtained from manufacturers). Polymer code

MFI (190  C/2.16 g) g/10 min

Mw g/mol

Mw/Mn

Branching index

Newtonian viscosity Pa*sec

Comments

zPE mPE PE (code for the blend)

0.98 1.01 –

122,000 104,000 –

3.96 2.8 –

3 2.8 –

1.7  104 1.18  104 –

– – 50:50 w/w zPE : mPE blend

(Ciba Specialty Chemicals) and THT 6460 (Cytec) and were added at concentrations of 0.2:0.2:0.3% (wt/wt), respectively. 2.2. Polymer processing and sample preparation The PE blend was melt compounded with each of the nanofillers, in the presence or absence of the compatibiliser PEgMA, in a co-rotating intermeshing twin-screw extruder, L/D ¼ 35, D ¼ 19 mm (ex. OMC, Italy), using a shear stress screw profile, see Fig. 1. The temperature profile used was 120 C–140 C–160 C– 170 C–180  C–180  C (die), with residence time of 100 s at 250 rpm. The extrudates were water cooled and granulated first before processing again in a single screw extruder (temp profile: 120–140–160–190  C; screw speed 70 rpm) equipped with a Brabender film-blowing unit. The blown film thickness was about 70 microns. Table 3 gives the composition of the extruded samples used. For selected samples (where mentioned), an internal mixer (Brabender torque rheometer) was also used to examine the effect of processing temperature using rotor speed of 60 rpm. Films were also prepared by compression moulding at 180  C for rheometric tests. 2.3. Accelerated weathering exposures The nanocomposite films and their unfilled counterparts were exposed to accelerated weathering conditions in a QU-V chamber containing eight UV-B lamps. The exposure cycle conditions was 8 h of light at T ¼ 55  C followed by 4 h condensation at T ¼ 35  C. 2.4. Polymer characterizations 2.4.1. FT-IR analysis A Fourier Transform Infrared Spectrometer (Spectrum One, Perkin Elmer) was used to record IR spectra on single screw extruded blown films using 8 scans at a resolution of 1 cm1. The relative concentration of functional groups was determined from their peaks absorption area index (absorbance peak area of the group ratioed to that of a reference absorption peak area at 2019 cm1). The actual concentration of the vinyl group was quantified based on its IR absorption, determined from deconvoluted peak at 912 cm1

(see Fig. 10b later) using an extinction coefficient obtained from model compounds (absorption at 908 cm1, extinction coefficient value of 3908 ¼ 122  7 l/mol cm) as described previously [28]. Measurements were obtained from the average of triplicate samples with a calculated maximum experimental error (relative standard deviation) of around 5%. In order to deconvolute the composite band profile that appears in the region 860–960 cm1 of the clay-containing samples, curve-fitting analyses of the vinyl region in the IR was performed using OriginLab Pro 7.5 software with at least three parameters considered; (a) number of peaks, (b) their position, (c) the form of the baseline. The detection and location of the different individual components of the vinyl peak and that of the clay contribution were initially obtained by visual inspection of the IR bands on the basis of literature studies and also based on IR spectra made on the pure polymers and the pristine clays (OMMT and NaMMT). Subsequently, detailed peak finding, assignment and confirmation were ascertained following a derivative analysis of the experimental spectral curves. Inspection of the plots confirmed the presence of the peaks as suggested by the presence of the negative peaks in the second derivative. After 200 ‘fit-interaction’ an overlapping experimental IR curves with the ‘interacted’ mathematical curves was verified; the absorption of the deconvoluted peak at 912 cm1 was used, subsequently, to calculate the vinyl concentration as described above. 2.4.2. X-ray analyses Wide Angle X-ray Diffraction (WAXD) was performed on compression moulded (at 180  C) 2 mm plaques from twin screw extruded granules to examine the nanocomposites structure (intercalated or exfoliated), the position, shape and intensity of the basal reflections of the silicate layers. Diffraction spectra were obtained over a 2q range of 2–10 using a Siemens D were monitored using 500 Krystalloflex 810 diffractometer used in the reflection mode, with Cu–Ka radiation at l ¼ 0.1542 nm and a scan rate of 1.0 deg/min. Quantification of the changes in the interlayer distances on going from the pristine clays to non-compatibilised and compatibilised clay-containing PE nanocomposites was carried out according to Bragg’s formula, see equation (1) where, n is an integer, q is the angle in incidence of x-ray beam.

Table 2 Nanofillers used and their characteristics obtained from manufacturers. Code

layered silicate nanofiller

Type

Quat surfactant (organic modifier)

Quat conc. meq/100 g clay

d001 nm

Density g/cm3

NF

Nanofill SE 3000

Bentonite

–

3.82

–

CL15

CloisiteÒ 15A

Montmorillonite

Not disclosed by the manufacturer but our analysis suggests it to be dimethyl-distearyl-ammonium cation Dimethyl-dihydrogenated tallow-quaternary ammonium chloride; 2M2HT

125

3.15

1.66

92.6

1.17

–

CH3 TH

N

+

HT Cl⎯ ⎯

CH3 NaMMT

CloisiteÒ Naþ

Natural montmorillonite

None

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Fig. 1. Screw profile of O.M.C. co-rotating intermeshing twin-screw extruder (L/D ¼ 35, D ¼ 19 mm).

d001 ¼ nl=ð2sin qÞ

(1)

From WAXD measurements, the number of clay platelets per average stack with the interlayer distance d001 was also calculated for these samples using the formula given in equation (2) [29], where t is given by the:

N ¼ 1 þ t=d001

(2)

Scherrer’s formula, t ¼ 0.9 l/(B1/2 cos qb), where l is the wavelength, B1/2 ¼ q1–q2 (in radians) at half peak height (lmax/2), qb ¼ (q1þq2)/2. The chemical composition of the clays was determined by energy dispersive X-ray (EDX) attached to a scanning electron microscope and measurements were done on cryogenically fractured thin films. 2.4.3. Morphological characterisation by transmission (TEM) and scanning (SEM) electron microscopy The nanocomposite structures were further evaluated by qualitatively characterising the degree of dispersion of the clays in selected nanocomposite samples using TEM. TEM measurements were recorded on a ZEISS EM 900 transmission electron microscope which was used for the extruded (twin screw extruded and compression moulded) samples (with acceleration voltages of 50 and 80 KeV), whereas a Philips CM120 900 transmission electron microscope was used for QUV-exposed blown films (from single screw extrusion). The measurements were done on ultrathin (70 nm) sections which were obtained at low temperature (around 110 to 120  C) using a LEICA ultra microtome equipped with a diamond knife. The SEM analysis was performed on cryogenically fractured and gold sputtered surfaces of thin blown films (from single screw extrusion) using a Philips (Netherlands) ESEM XL30 scanning electron microscope.

Table 3 Composition of all investigated nanocomposite films. No

Samples

Composition % wt/wt

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

zPE–mPE (PE) PE/NF PE/CL15A PE/NaMMT PE/NaMMT PE/PEgMA PE/PEgMA PE/PEgMA PE/PEgMA/NF PE/PEgMA/CL15A PE/PEgMA/CL15A PE/PEgMA/CL15A PE/PEgMA/NaMMT PE/PEgMA/NaMMT PE/PEgMA/NaMMT PE/PEgMA/NaMMT

(50/50) (50/50)/5 (50/50)/5 (50/50)/5 (50/50)/2.5 (50/50)/2 (50/50)/5 (50/50)/10 (50/50)/5/5 (50/50)/2/5 (50/50)/5/5 (50/50)/10/5 (50/50)/2/5 (50/50)/5/5 (50/50)/10/5 (50/50)/5/2.5

2.4.4. Rheological characterisation Rheological tests were conducted on compression moulded (at 180  C) 2 mm samples (prepared from twin screw extruded granules) in shear flow using a Rheometrics RDA II in a plate-plate mode (plates diameter ¼ 25 mm) at a test temperature of 180  C, in a frequency range of 0.1–500 rad/s with 5% strain. 2.4.5. Mechanical characterisation Tensile properties (Young’s modulus, tensile strength and elongation at break) were measured under ambient conditions according to ASTM test method D882 using an Instron tensometer model 3365. The specimens were cut from 70 mm blown films in the machine direction. For modulus measurements, the speed used was 1 mm/min until a deformation of 10%, which was increased thereafter to 500 mm/min until break. The data reported are the average values obtained by analyzing the results of ten tests per sample with a reproducibility of 5%. 3. Results and discussion 3.1. Characterisation of the clays and the effect of processing on nanocomposites characteristics Fig. 2 shows the atomic percentages (and their concentrations in part per billion, ppb; P/B) of the various elements in both the NF and CL15 clays. It is clear that both OMMT clays contain a small amount of about 2.2–2.3 ppb iron. The catalytic activity iron is detrimental to the photo-oxidative stability of polymers, see reaction Scheme 1 , and nanocomposites, see later. Melt processing of nanocomposites based on hydrophobic polymers such as polyolefins and hydrophilic nanoclays presents a major challenge. In addition to compatibility issues between the polymers and clays, both the processing parameters such as temperature, screw configuration, shear rate, residence time, and the nature of the clay organic modifiers (e.g. type of quat) have marked effects on the thermal stability of nanocomposites and the degree of dispersion and exfoliation of the clays in the polymer matrix; good dispersion is vital to achieve good performance properties and to secure a place for nanocomposite products in the market nanocomposites [14,19,22]. In polyolefins-clay nanocomposites, the presence of a compatibiliser, particularly those based on maleic anhydride–graft–polyolefins, has been shown

.

ROOH + M+n

RO + −OH + Mn+1

ROOH + Mn+1

ROO + +H + Mn+n

2 ROOH

.

M+n / Mn+1

.

.

ROO + RO + H2O

Reaction Scheme 1. Catalytic activity of Feþ2/Feþ3 in hydrocarbon polymers.

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Fig. 2. EDX of (a) OMMT clay CL15, and (b) of the OMMT clay NF.

[18,19] to give better dispersion and adhesion between the silicate layers and the polymer matrix than in their absence. However, such maleated polymers would further complicate the behaviour of these nanocomposites during processing, and the structure of the maleated polymer, e.g. its molar mass and the maleic anhydride content, would also have a major effect on the characteristics of the nanocomposites [18]. We have, therefore, evaluated the effects of the organic modifier and the compatibiliser PE–g–MA (containing 1% MA) on PE during melt processing in a twin screw extruder on the extent of dispersion of three different clays; two organically modified (OMMT: CL15 and NF) and a non-modified pristine clay (NaMMT). Fig. 3 shows the x-ray diffraction patterns and Table 4 gives the calculated data from WAXD measurements obtained for the neat non-modified NaMMT and the modified OMMT (NF and CL15) clays, as well as in polymer nanocomposites (PNC) containing these clays in the presence and absence of the compatibilising agent PEgMA. The data shows that the basal spaces (gallery distances) in both sets of nanocomposite samples (with OMMT and NaMMT; compatibilised and non-compatibilised PNCs) have increased compared to that in the uncompounded OMMT or the pristine NaMMT clays indicating that the polymer does intercalate into the clay interlayers, though to different extent in the presence and absence of the organic modifier. It can also be seen from Fig. 3 that the OMMT-filled samples show several well defined clay diffraction peaks, whereas the NaMMT-filled samples show much wider and less distinctive clay diffraction peaks. These results show also the effect of the compatibiliser (PE–g– MA) on the extent of clay dispersion. The OMMT-filled

compatibilised nanocomposite samples (e.g. see the CL15-containing nanocomposites), when compared to the non-compatibilised analogues, show a shift of the main x-ray peaks to lower 2q values and a lower intensity of their diffraction pattern suggesting increased interlayer spacing, see Fig. 3 and Table 4, with the clay stacks becoming more disordered and resulting in a more intercalated and partially exfoliated clay morphology; full exfoliation of clay layers is normally assumed when the diffraction pattern of OMMT nanocomposites do not reveal the diffraction peaks of the clay [30,31]. Calculation of the average number of platelets per stack shows a drop from 2.61 to 2.48 in the CL 15-based compatibilised PNC and confirms the morphology deduced from the XRD results above. It is known that XRD results may not detect agglomerated clay particles thus giving a false impression of delamination. TEM analysis, which gives visual images, is often used in conjunction with XRD to give a more accurate, albeit qualitative, picture of the ‘real’ microstructure of the nanocomposites [32,33]. Fig. 4 shows the TEM micrographs of melt extruded non-compatibilised and compatibilised PE–CL15-containing nanocomposites. It is clear from Fig. 4 that the presence of PEgMA (in PE–PEgMA– CL15) gives rise to a structure having clay particles that are well dispersed in the matrix and is characteristic of a hybrid morphology containing both exfoliated (some single clay platelets are observed) and intercalated clay structures with increased contact surfaces between the delaminated clay platelets and the matrix. On the other hand the non-compatibilised composite PE/ CL15 shows a lower degree of dispersion whereby the OMMT

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b

a

PE/5%CL15

Intensity, a.u

Intensity, a.u

PE/5%NF

PE/PEgMA/5%NF

0

2

4

6

8

PE//PEgMA/5%CL15

0

10

2

4

2θ

6

8

10

2θ

d

c PE//PEgMA/ 5%NaMMT

Intensity, a.u

Intensity, a.u

PE//PEgMA/ 2.5%NaMMT

PE/ 5%NaMMT PE/ 2.5%NaMMT

0

2

4

6

8

10

0

2

4

2θ

6

8

10

2θ

Fig. 3. X-ray diffraction of compatibilised and non-compatibilised nanocomposite films.

Table 4 Main peaks, interlayer distance and number of stacks of the pristine silicates and the nanocomposite films calculated from WAXD analyses. Samples NF PE/5%NF (non-compatibilised) PE/PEgMA/5%NF(Compatibilised) CL15A PE/5%CL15A (non-compatibilised) PE/PEgMA/5%CL15A (Compatibilised) NaMMT PE/5%NaMMT(non-compatibilised) PE/PEgMA/5%NaMMT(compatibilised) PE/2.5%NaMMT(non-compatibilised) PE/PEgMA/2.5%NaMMT(compatibilised)

Pristine filler OMMT-filled Pristine filler OMMT-filled Pristine filler NaMMT-filled

Main peak 2q, deg

Interlayer distance d001, nm

Average no. of platelets/stack N

2.30 2.26 2.28 2.80 2.68 2.60 7.50 5.23 5.13 5.23 5.07

3.84 3.91 3.87 3.15 3.30 3.40 1.18 1.69 1.72 1.69 1.74

3.49 3.09 2.94 3.11 2.61 2.48 3.87 2.95 3.08 3.15 3.06

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Fig. 4. TEM micrographs of (a) PE–CL15 and (b) PE–PEgMA–CL15 melt extruded nanocomposites.

alone cannot be intercalated further by the polymer chains thus forming a microcomposite. These results support the conclusion that PEgMA is more compatible with the organically modified OMMT clay than is the PE matrix alone, thus it plays an important role in affecting the extent of exfoliation in the PE-nanocomposites studied here. The absence of an organic modifier affects adversely the clay dispersion in the polymer matrix as can be observed from SEM analysis. Figs. 5 and 6 show the effect of the pristine (unmodified) NaMMT and the organically modified (OMMT) clays, in the presence and absence of the compatibiliser, on the morphology of the PNC samples. It is clear from these figures that the sample containing the pristine NaMMT, both in presence and absence of the compatibiliser, reveal the presence of intercalated tactoids with dimension in the micron range and are generally larger than those observed with the OMMT-filled samples. Furthermore, in the presence of the compatibiliser, the NaMMT-containing samples show the presence of ‘encapsulated’ NaMMT clay structures, thereby limiting the extent of dispersion and clay– polymer interactions. On the other hand, the organo-modified clays appear to have partially dispersed structures with some aggregates of micrometer sizes especially in the case of NF clays, see Fig. 5. It is also clear from these micrographs that the presence of the compatibiliser (PEgMA) changes the morphology with the number of large particles decreasing and the extent of dispersion increasing in both CL15 and NF. The best dispersion is observed in CL15-containing nanocomposite sample, i.e. PE– PEgMA–CL15, although some re-aggregation of the clay particles is evident. However, the dimensions of these particles, even in the aggregates, remain to be well below a micron and that the silicate platelets are clearly dispersed at the nanometric level, see Fig. 6. These results support the hybrid (intercalated and exfoliated) structure observed from TEM measurements for this nanocomposite, see Fig. 4. SEM result for the compatibilised NF-based nanocomposite sample, i.e. PE–PEgMA–NF, shows an overall clay particle sizes that appear to be larger than those observed with the analogous sample containing CL15 concomitant with a lower degree of dispersion. These results suggest that, in the presence of NF clays, less dispersion and exfoliation is expected; similar results were obtained for the non-compatibilised samples, see Fig. 5. Differences in the degree of dispersion and exfoliation observed in these samples would affect markedly the performance

properties and behaviour of these nanocomposites during inservice conditions. ATR–FTIR has also been shown to be capable of determining the extent of organoclay delamination in polymer–organoclay nanocomposites [34,35]. This is based on measurements in reflectance mode of the clay silicon–oxygen (Si–O) infrared absorption. ATR of pristine montmorillonite powder shows only one broad band in the region 950–1150 cm1, whereas when the clay layers are in a delaminated state, e.g. in aqueous clay dispersions) several distinct absorption bands with somewhat narrow bandwidth appear. The resolved Si–O absorption bands at around 1042 and 1080 cm1 can then be used to monitor the state of dispersion of the layered silicate clays [34,36]. Fig. 7a shows the ATR spectral region of the Si–O bond absorption for the neat OMMT (CL15 and NF) clays; NaMMT show only one broad band as mentioned above. The ATR spectrum of a thin PNC film of OMMT (CL15) compounded in PE extruded in the absence, i.e. sample PE–CL15, or in the presence, i.e. sample PE–PEgMA–CL15, of the compatibiliser show striking differences, see Fig. 7b: in-plane and out-of-plane absorptions of Si–O bond around 1075 and 1045 cm1, respectively, are resolved and separated; better separation was achieved in the presence of the compatibiliser PEgMA (see also the deconvoluted peaks of the compatibilised nanocomposite sample, Fig. 7c). The fact that the Si–O absorption bands which were overlapped in the neat clays have become much more resolved in the compatibilised analogue sample confirms that the presence of PEgMA causes the clay platelets to become more delaminated and better dispersed in the nanocomposite sample. These results are consistent with XRD spectra which showed a decrease in the intensity of the XRD patterns (see Fig. 3) and TEM micrographs which revealed higher degrees of intercalation and exfoliation when PEgMA is present (Fig. 4). The importance of the ATR–FTIR technique is that, in addition to being relatively simple complementary off-line analytical method for assessing the state of dispersion of clays in nanocomposites, it can also be adapted for in-line monitoring of the extrusion process of clay-polymer nanocomposites thus offering the capability of fine-tuning the processing parameters to obtain the optimum dispersion, quality and consistency of clay nanocomposites during production [37]. Rheological properties of polymers provide a fundamental understanding of their melt processing behaviour. The rheological characteristics of nanocomposites are known [18,38,39] to be

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Fig. 5. SEM micrographs of zPE–mPE/Clay samples filled with organophilic and pristine layered silicates at different magnifications.

sensitive to their state of dispersion and the surface characteristics of the dispersed phase. Melt rheology should therefore provide a further methodology to assessing the state of dispersion of the nanofillers during processing of polymer melts. Figs. 8 and 9 show storage modulus, G0 , and complex viscosity, h*, plots as a function of frequency for compatibilised and non-compatibilised samples containing the different clays. At high frequencies the differences between the rheological properties (G0 , h*, and loss modulus G00 , the latter is not shown here) for all the composites having different compositions is shown to be minimal suggesting that their

response is dominated by the polymer matrix only, whereas at low frequencies (lower than 1 rad/s) the differences are more significant indicating a solid-like rheological behaviour. At low frequency, and in the absence of PEgMA (see Figs. 7b and 8b), the different clays show little effect with very small differences in the values of G0 and h* between composites containing organo-modified (OMMT) and unmodified (NaMMT) fillers. However, in the presence of the compatibiliser PEgMA, significant differences are observed between OMMT and NaMMT fillers (Figs. 8a and 9a) which point to the more important role played by the

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1579

Fig. 6. SEM micrographs of zPE–mPE/PEgMA/Clay samples filled with organophilic and pristine layered silicates at different magnifications.

compatibiliser, compared to the organic modifier (quat), in affecting clay dispersion. It is also important to note that the CL15containing nanocomposite sample shows the largest increase in the G0 and h* values at low frequencies compared to the other NFnanocomposite analogue; the unmodified NaMMT-containing sample show the least change in comparison to the unfilled polymer–compatibiliser control sample. Similar results have been observed in other clay–polymer nanocomposite systems with the increase in storage modulus at low frequencies being attributed to percolation of the randomly distributed silicate layers due to their anisotropy [6]. The rheological measurements, therefore, confirm and support the WAXD, TEM and SEM results which show that the state of exfoliation is higher in the compatibilised samples, and that it is highest in the CL15-containing PNC samples. The very high aspect ratio of layered silicates and the extremely high specific surface areas of the clays that are exposed to polymer matrices give rise to a significant improvement in mechanical properties, and in particular the Young’s modulus which relates to the stiffness of the nanocomposites [8,40]. Furthermore, factors that influence the degree of exfoliation such as the clay content, the organic modification of the clay, the addition of a compatibiliser, have also been shown to have a major impact on the modulus and tensile strength of

clay–nanocomposites [41]. To investigate the effects of the organic modification of the clays and the compatibiliser on the mechanical properties of the PE–clay nanocomposites, the tensile properties of composites containing OMMT and NaMMT prepared in the presence and absence of the PEgMA compatibiliser have been compared with those of the neat PE and the PE–PEgMA polymer blend. Examination of the results given in Table 5 shows that in the absence of the compatibiliser, the organoclaycontaining composites PE/NF and PE/CL15, and even that containing the NaMMT, show some improvements in both their Young’s modulus and tensile strength when compared to the polymer in spite of the poor compatibility and clay dispersion in these samples (see for e.g. TEM of PE–CL15 in Fig. 4). It is reasonable to suggest that the enhanced mechanical properties in this case are entirely due to the high aspect ratio of the nanoclay stacks. The improvement in the Young’s modulus and elongation at break of the nanocomposites, however, is more obvious when the compatibiliser PEgMA is added to the polymer matrix, see Table 5, and this is attributed to the better state of dispersion of the organoclays in these samples; this reasoning is supported by the observation that the more exfoliated PE–PEgMA–CL15 nanocomposite gives higher modulus and elongation at break than that of the PE–PEgMA–NF which show a lower extent of dispersion

1580

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A0,085

a

Si-O in-plane

b

Si-O out-of-plane

0,080 0,075

1075

1045

0,070 0,065

CL15 NF NaMMT

0,060 0,055

1020 PE/PEgMA/CL15 PE/CL15

0,050

A

0,045 0,040 0,035

1105

0,030

A

0,025 0,020 0,015 0,010 40 11 20 11 00 10 80 10 60 10 40 10 20 10 00

60

cm-1 960950,7

980

11

80

11

11

11 99

,3

0,0047

cm-1

PE/PEgMA/CL15

c

cm-1

Fig. 7. ATR-FTIR showing the Si–O absorption region of (a) the neat clays, both organically modified and the NaMMT; (b) the surface of thin films of the nanocomposites PE–CL15 and PE–PEgMA–CL15, and (c) the corresponding deconvoluted peaks for the PE–PEgMA–CL15 nanocomposite sample.

(see Fig. 5). Overall, however, although the organoclays give rise to improved stiffness to PE–clay nanocomposites, the significance of the nanoclay reinforcement in polyolefins (PE, PP) is not as great as that generally observed when Nylon 6 is used as the polymer matrix and this is probably due to the lower degree of exfoliation in polyolefins which has been well documented [30,42]. 3.2. Effect of processing on the thermal stability of the nanocomposites Interactions between polymer matrices and the surface of clay layers have been shown in many studies with different polymers to be one of the most important parameters that influences the thermal stability of clay-polymer nanocomposites [3,14,43].

Organoclays modified with quaternary alkyl ammonium (e.g. alkyl ammonium hydroxide) are known to decompose at high temperatures e.g. 200  C, giving rise to a-olefins and amines [44] and the reaction has been shown to occur by the Hoffmann Elimination mechanism [14], reaction 1. It has also been suggested [3,14] that elimination from quat cations that are bonded to layered silicates would result in a substitution of the ammonium linkage on the clay with a hydrogen proton on the bcarbon giving rise to an acidic site in addition to the a-olefin, reaction 2. Once the a-olefin (a vinyl moiety) is formed through thermal degradation of the surfactant on the organoclay, it can contribute to polymer degradation by a number of known reactions [28]. We have therefore examined the effect of processing on the extent of

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1581

Cl H R-CH

CH3 + CH2 N CH2R

R-CH=CH2 + N(CH3)2(CH2R) + HCl -olefin amine

CH3

CH 3 + N CH 2

MMT

CH-(CH 2 )n -CH 3

+ H + N(CH 3)2 (CH 2 R) + CH 2 =CH-(CH 2 )n -CH 3 -olefi n acidic site

-carbon

CH 2 R

(2)

1.E+06

a 1.E+05

1.E+04

G', MPa

H 3C

H

1.E+03

PE-PEgMA PE-PEgMA-5%NF PE-PEgMA-5%CL15 PE-PEgMA-5%NaMMT PE-PEgMA-2.5%NaMMT

1.E+02

1.E+01 1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

Freq, rad/s 1.E+06

b 1.E+05

1.E+04

G', MPa

MMT

(1)

1.E+03

1.E+02

1.E+01 1.E-01

PE PE-5%NF PE-5%CL15 PE-5%NaMMT PE-2.5%NaMMT 1.E+00

1.E+01

1.E+02

1.E+03

Freq, rad/s Fig. 8. Storage Modulus, G0 , of compatibilised (with PEgMA) (a) and uncompatibilised (b) nanocomposite systems.

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N.Tz. Dintcheva et al. / Polymer Degradation and Stability 94 (2009) 1571–1588

1.E+05

a

PE-PEgMA PE-PEgMA-5%NF PE-PEgMA-5%CL15 PE-PEgMA-5%NaMMT

Viscosity, Pa*s

PE-PEgMA-2.5%NaMMT

1.E+04

1.E+03 1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

Freq, rad/s 1.E+05

b

zPE-mPE PE-5%NF PE-5%CL15 PE-5%NaMMT

Viscosity, Pa*s

PE-2.5%NaMMT

1.E+04

1.E+03 1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

Freq, rad/s Fig. 9. Complex viscosity, of compatibilised (with PEgMA) (a) and uncompatibilised (b) nanocomposite systems.

the Hoffman elimination reaction from the organoclay surfactant by measuring the concentration of the vinyl group formed from the OMMT (CL15) in PE nanocomposites produced under the set extrusion conditions (T ¼ 180/190  C) used here, and also after

processing in an internal mixer at different processing temperatures (T ¼ 180–240  C). Fig. 10 shows the FTIR-absorption region 980–840 cm1 for PE–clay samples (a) and the deconvoluted peaks for the PE–CL15 film (b). The deconvoluted spectrum shows clearly

Fig. 10. FTIR spectra in the 960–840 cm1 region of extruded PE-containing different clays (a) and the corresponding deconvoluted peaks for PE–CL15 extruded sample (b).

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1583

Table 5 Main mechanical properties: Young’s modulus, E, tensile strength, TS, and elongational at break, EB of some investigated films in the machine direction. Samples

Young’s modulus E, MPa

TS, MPa

EB, %

Without a Compatibiliser

PE PEþ5%NF PEþ5%CL15A PEþ5%NaMMT PEþ2.5%NaMMT

235 265 270 258 245

23.5 24.5 26.5 24.9 23.7

820 820 825 775 785

Compatibilised with PEgMA

PE–PEgMA blend PE–PEgMAþ5%NF PE–PEgMAþ5%CL15A PE–PEgMAþ5%NaMMT PE–PEgMAþ2.5%NaMMT

242 271 285 273 253

23.7 25.5 26.8 25.1 24.4

840 845 850 810 815

the vinyl group absorption at 912 cm1. The peak at 922 cm1 corresponds to absorption of the clay which has been confirmed from the ATR-FTIR of the NaMMT. Fig. 11 shows the concentration of the vinyl group in the organoclay-modified PE extruded samples (calculated from the absorption of the deconvoluted vinyl peak and quantified based on Lambert–Beer law using IR extinction coefficient for vinyl group obtained from literature model compound studies as described previously [28]). It is clear that the vinyl concentrations in the OMMT samples (PE–NF and PE–CL15) are much higher than that developed in PE extruded under the same conditions (similar trend was observed, not shown here, from vinyl index calculated from the absorption peak at 917 cm1 and corrected for the contribution of the clay by subtracting the vinyl index values of the PE–OMMT samples from that of the non-modified PE–NaMMT). In view of the thermal instability of the alkyl ammonium surfactants, the effect of processing temperature (in an internal mixer) on the extent of vinyl formation from the organoclay was further examined. Fig. 12 show plots of the corrected vinyl index of PE–CL15 (corrected for the clay contribution as noted above) which indicate clearly that the amount of vinyl formed in this OMMT–PE sample at all processing temperatures examined is much higher than that of the control PE matrix and that of the PE–NaMMT. Thus, it is reasonable to assume that the difference is entirely due to the formation of vinyl unsaturation from the organic surfactant on the clay through the Hoffman elimination reaction. However, Fig. 5 also shows that for the PE–CL15 sample, with increasing processing temperature there is a slight decrease in the vinyl concentration. It is known that the melt degradation mechanism of polyethylene involves both crosslinking and chain scission reactions that occur simultaneously in the polymer melt with the relative contribution of each depending on the processing conditions, particularly the melt temperature and the polymer microstructure [28,45,46]. We have shown in different studies [28,47] that in processed (in both internal mixer and in extruder) metallocene and Zeigler LLDPE polymers crosslinking reactions predominate and their extent increase with

C6H13 CH2=CH-CH2-

+

CH2-C-CH . 2CH2

Fig. 11. Vinyl concentration for extruded PE and PE–clay samples (in the absence of compatibiliser).

increasing the processing temperature up to about a temperature of 260  C where chain scission starts to become the more dominant reaction. The slight decrease observed in the vinyl concentration with increasing processing temperature from 180 to 240  C of the OMMT–PE samples must therefore be due to the predominance of crosslinking reactions in the polymer matrix under the increasing processing temperatures. The higher concentration of the terminal double bonds (vinyl unsaturation) formed from the clay-surfactant decomposition would most likely add to the PE macro-alkyl (or alkoxyl or alkylperoxyl) radicals resulting in crosslinking of the polymer via reactions 3 and 4 as was shown before [28,47]. It is also interesting to point out that the level of vinyl unsaturation formed in the extruded PE–NF sample as shown in Fig. 11 is lower than that obtained from the PE–CL15 sample. It has been shown [14] that for nylon 6-organoclay nanocomposites, increasing

C6H13 C.L

CH2-C-CH2CH2

(3)

CH2-CH-CH2

.

CH2=CH-CH 2-

. +

CH2-CH2-CH2

C.L

CH2-CH-CH2 CH2-CH-CH 2 .

(4)

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that the lower extent of vinyl formation observed here in the NF-containing PE is associated with the lower extent of dispersion and exfoliation observed in this sample compared to that in PE–CL15, see Fig. 5. 3.3. Effect of accelerated weathering on the photo-oxidative stability of PE–Clay nanocomposites

Fig. 12. Vinyl index (from FTIR) in samples processed in internal mixer at different temperatures. The concentration in the organoclay (CL15)-containing sample has been corrected for the clay contribution by subtracting from the value of PE–NaMMT.

levels of clay exfoliation enhances the chance of chemical reactions to take place between the organoclay surfaces and the polymer matrix due to the larger amount of organoclay surfaces which are exposed to the polymer and this was suggested to be responsible for thermal degradation of the polymer matrix in melt processed clay-polymer nanocomposites. It is reasonable to assume, therefore,

A

0.778 0.75 0.70 0.65

432-672 384

0.55

336

0.50

288

0.45

240

0.35

192

0.30

144

0.25

96

0.20

48

0.15

A

Time, h

0.60

0.40

The improvement in properties, such as barrier properties and fire retardancy, that can be achieved from polyolefin-clay nanocomposites has been highlighted earlier. However, one drawback of these materials has been shown to be their poor long-term stability in outdoor applications, and yet no enough attention has been given to studies on the durability of polyolefin nanocomposites under outdoor in-service conditions, with particularly limited published work on the long term thermal and photo-oxidative stability of PE-based clay nanocomposites [48–51]. The influence of the nanofillers and the compatibiliser PEgMA on the photo-oxidative stability (under accelerated weathering conditions) of PE–clay nanocomposites is examined here and compared with that of the pristine polymer and the polymer– compatibiliser blend (in the absence of the nanofillers). Fig. 13 shows the effect of PEgMA on the evolution of the carbonyl oxidation products during UV-exposure (in QUV) of CL15-containing sample. It is clear that typical photo-oxidation products of polyethylene, namely carboxylic acids and ketones at 1713 cm1, esters at 1730 cm1, g-lactones at 1780 cm1 also appear during the photo-oxidation of the clay-containing samples nanocomposite

1730

1713

0.40 0.38 0.36 0.34 0.32 0.30 0.28 0.26 0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.01

PE

1781

1641

0.10 0.05 0.00 -0.026 1878.7 1860

1840

1820

1800

1780

1760

1740

1720

1700

1680

1660

1640

0.42

1620

1600 1581.0

Time, h 384

1714

336 288 240 192 144 96

1781

48

1641

0 1740.84

1880.6 1860 1840 1820 1800 1780 1760 1740 1720 1700 1680 1660 1640 1620 1600 1582.2

cm-1

cm-1

A

A

PE-PEgMA-CL15

Time, h 432-672

PE-PEgMA

384

Time, h 336

336

288

288

240

240

192

192

144

144

96

96

48

48

0

cm-1

PE-CL15

1730

cm-1

Fig. 13. FTIR spectra in the carbonyl region as function of irradiation time of film samples photo-oxidised in QUV.

N.Tz. Dintcheva et al. / Polymer Degradation and Stability 94 (2009) 1571–1588

80

50 PE-PEgMA-CL15

PE-PEgMA-CL15

40

PE-PEgMA-NF

60 50

PE

40 PE–CL15 PE-NF

30

PE-PEgMA

20 PE-NaMMT

10 100

200

300

400

500

PE-PEgMA-NF

30

PE

20 PE-PEgMA

PE–CL15 PE-NF

10

PE-PEgMA-NaMMT

PE-PEgMA-NaMMT

0

Hydroxyl Index

Carbonyl Index

70

0

1585

0 600

700

0

100

Photo-oxidation time, h

200

300

400

500

600

700

Photo-oxidation time, h

Fig. 14. Carbonyl and hydroxyl indices of film samples as function of accelerated weathering exposures (QUV).

samples regardless of the presence or absence of the PEgMA. The similarity in the oxidation products of the CL15-containing nanocomposites to that of the pristine PE is in agreement with literature findings that layered silicate clays do not affect the photo-oxidation mechanism of other polymers e.g. PP, EPDM [27,52,53]. While the compatibiliser does not play a role in the overall photo-oxidation mechanism, it does have a dramatic and detrimental effect on the rate of oxidation of polymers containing OMMT. Fig. 14 shows that the rate of photo-oxidation (using carbonyl and hydroxyl indices obtained from IR spectra) in the compatibilised nanocomposite samples (PE–PEgMA–CL15 and PE–PEgMA–NF) is much faster than

A

that of the corresponding samples but prepared in the absence of PEgMA, i.e. samples PE–CL15 and PE–NF. It is also clear from this figure that even in the absence of PEgMA, the OMMT-containing polymers show a higher rate of photo-oxidation than both the pristine polymer, the PE–PEgMA blend, and the non-modified NaMMT-containing polymers. The negative effect of PEgMA on the rate of photo-oxidation of nanocomposites must be due to a combination of factors including its effect on the extent of exfoliation, the photo-oxidative instability of maleic anhydride and the possible interactions between the anhydride group and the clay where both ionic impurities in the

PE-PEgMA

B PE-PEgMA

9

Carbonyl Index

PE 7

5

3

1 Photo-oxidation time, h 0

50

100

150

200

250

300

350

-1

cm-1

A 0,087 0,08

Photo-oxidation time, h

QUV 0h

0,08

40

PE-PEgMA PE

0,08

0,07

PE-PEgMA

0,07 0,07

1741

0,07

1713

0,07 0,06

1794

0,06 0,06

PE

Carbonyl Index

0,08

30

20

10

0,06

Photo-oxidation time, h

0,060 1959, 194 192 190 188 186 184 182 180 178 176 174 172 170 168 166 164 162 160 1587,

cm-1

0

0

100

200

300

400

500

Photo-oxidation time, h

Fig. 15. FTIR spectra in the carbonyl region and rate of photo-oxidation of PE and PEgMA films at different exposure times in QUV.

600

700

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1.20 PE PE-NF

1.00

PE-CL15 PE-NaMMT

0.80

PE-PEgMA PE-PEgMA-NF PE-PEgMA-CL15

0.60 EB (t=t)/ EB (t=0)

PE-PEgMA-NaMMT

0.40 0.20 0.00

0

100

200

300

400

500

600

Photo-oxidation time, h 2.20

PE PE-NF PE-CL15

2.00

PE-NaMMT PE-PEgMA

1.80

PE-PEgMA-NF PE-PEgMA-CL15 PE-PEgMA-NaMMT

E (t=t)/ E (t=0)

1.60 1.40 1.20 1.00

0

100

200

300

400

500

600

Photo-oxidation time, h Fig. 16. Elongation at break (EB) and Young’s Modulus (E) of film samples as function of accelerated weathering exposures (QUV).

clay and the alkyl ammonium surfactant and its decomposition products would play an important role. PEgMA itself, in the absence of the organoclays, seem to give slightly higher rate of photooxidation compared to PE at the early stages of UV-exposure under the QUV conditions used here, see Fig. 15a and b. It is also clear from this figure that after short irradiation times (e.g. 96 h) the carbonyl absorption region of PEgMA containing film shows that the 1714 cm1 peak (due to carboxylic acids) has higher intensity compared to the 1771 cm1 (cyclic anhydride) and the ester peak at 1731 cm1; at longer irradiation times, see Fig. 13, the 1714 and 1735 cm1 peaks become of comparable intensities and are similar to those evolved during the photo-oxidation of the pristine PE sample. The higher intensity of the 1714 peak in the PEgMA after 96 h UV-exposure must be due, at least partly, to the known hydrolysis reaction of the anhydride group (typically absorbing at 1770 cm1) to carboxylic acid (absorption at 1714 cm1) [52]. It may also be due to the photolytic instability of maleic anhydride or, in this case, grafted succinic anhydride, under the UV-exposure conditions, see reaction 5; this instability would contribute to the slightly higher extent of photo-oxidation rate observed in PEgMA compared to PE at the early stages of exposure and may be responsible, at least in part, to the observed higher rate of photo-oxidation of OMMTnanocomposites containing PEgMA.

. O

O

O

h

O O

Photo-induced oxidation

h

. C

O

(5)

. .

H / OH

O

OH OH C O

It is important to point out, however, as is clearly illustrated in Fig. 14, that while the PEgMA-containing-OMMT nanocomposites show a much higher rate of photo-oxidation than when PEgMA is absent, the PEgMA–pristine clay sample (PE–PEgMA–NaMMT) shows a very similar rate of photo-oxidation to the corresponding sample without PEgMA (PE–NaMMT). This suggests, therefore, that the adverse effect of PEgMA on OMMT-nanocomposites which gives rise to the observed faster rate of photo-oxidation must be due, to a large extent, to unfavourable interactions between the anhydride group and the clay under the UV-exposure conditions used. Further complication may arise from reactions with both the alkyl ammonium surfactant (quat) and its thermal degradation products, see reactions (1) and 2, and possibly with the ionic impurities in the clay. The acidic environment and the vinylic unsaturation shown to be formed from the quat through the Hoffman elimination reaction during processing of the nanocomposites (see Figs. 11 and 12 and reactions 1 and 2) would cause further oxidative degradation of PE involving the formation of polymer peroxides. It has been shown that reaction of hydrogen peroxide with maleic anhydride and with cyclic anhydrides (e.g. succinic anhydride) produce peracids [54], see for e.g. reaction 6, which are known to be pro-oxidants. It would be reasonable to invoke similar reactions involving the higher amounts of polyethylene hydroperoxides that would form in the presence of the thermal degradation products of the quat in the OMMT, thus contributing, at least in part, to the much higher levels of photooxidation rate observed in PE–PEgMA–OMMT nanocomposites compared to the case where PEgMA its absent.

O O O

+ HOOH

O CH-C-O-O-H

(6)

CH-C-O-H O succinic peracid

The much higher rate of oxidation of the PE–PEgMA–OMMT nanocomposites is further supported by a similar trend observed in the changes in mechanical properties of these samples. Fig. 16shows a plot for the changes in elongation to break (Eb) and Young’s modulus for PE nanocomposites containing the three different clays, i.e. CL15, NF (OMMT), and NaMMT, prepared in the presence and absence of PEgMA with those of the pristine PE and the PE–PEgMA blend treated under the same conditions. The changes in both Eb and Young’s modulus parallel the rate of photooxidation of the samples with the highest rate of decrease in Eb is seen in the PE–PEgMA–OMMT samples. The effect of the extent of exfoliation of CL15 in PE in the presence of PEgMA, i.e. sample PE–PEgMA–CL15, on its photo-oxidation was also investigated by examining the morphology of the nanocomposite sample using TEM after different periods of accelerated weathering exposures. Fig. 17 shows TEM micrographs for this sample after 48 and 196 h of exposure: at 48 h, when the sample was still within its short photo-oxidative induction period, both exfoliated and intercalated clay structures can be seen clearly. However, upon further UV-irradiation, e.g. after 196 h, where the polymer has clearly become photo-oxidised as reflected by the significant increase in the carbonyl index, the morphology becomes much more exfoliated (with increased contribution from the small hydrophilic products formed during photo-oxidation which would favour increased interaction with clay platelets thereby contributing to their enhanced dispersion and exfoliation) with single platelets having diameter size in the range of 50–120 nm that are seen to be homogenously dispersed. The much higher extent of exfoliation observed at higher irradiation times gives rise to exposure of larger areas of interfacial regions to UV light: this could additionally

N.Tz. Dintcheva et al. / Polymer Degradation and Stability 94 (2009) 1571–1588

1587

Fig. 17. TEM of PE–PEgMA–CL15 after 48 and 96 h of QUV exposures. The corresponding photo-oxidation curve measured from FTIR carbonyl index of the same sample is also shown.

contribute to greater extent of release of photo-oxidation products from the quat into the matrix and resulting in a higher overall level of photo-oxidation as was observed for this sample, see Fig. 14. The contribution of better exfoliation in nanocomposites to a higher extent of their photo-oxidation has also been reported [55]; for example, injection moulded PP–Clay nanocomposite sample showed higher extent of photo-oxidation than its extruded analogue and this was attributed to better exfoliation in the injection moulded

samples. It is important to note, however, that the photostability of clay–PE nanocomposites can be increased significantly in the presence of stabilisers. Fig. 18 shows the stabilising effect of a processing antioxidant used in combination with a UV absorber and metal deactivator (Irganox 1010:THT6460:MD1024) and indicates that the stabilised PE–PEgMA–CL15 nanocomposite gives a much improved stability compared to the unstabilised counterpart which is higher than that of the unstabilised PE.

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References

Fig. 18. Photo-oxidative stability of PE–PegMA–CL15 nanocomposite in the absence and presence of stabilisers.

4. Conclusions Polyethylene nanocomposites were prepared in the presence of organically modified (OMMT:CL15 and NF) and unmodified (NaMMT) layered silicates in the presence or absence of the compatibiliser PEgMA (maleic anhydride-graft-polyethylene) using a co-rotating twin screw extruder (and for selected samples internal mixer was used). The degree of dispersion and morphology of the OMMT clays in the polymer matrix was evaluated by four complementary techniques (XRD, TEM, SEM and ATR–FTIR). The results obtained show that significant improvements in nanocomposite morphology in terms of intercalated and exfoliated nanostuctured silicate platelets when the compatibiliser PEgMA is used. Nanocomposite samples containing CL15 showed hybrid structures with higher levels of exfoliation, smaller clay aggregation and more finely dispersed silicate layers than those obtained in NF-containing analogues. The thermal degradation of the alkyl ammonium surfactant in melt processed nanocomposites was examined by determining the concentration of the vinyl unsaturation formed from the organomodified clay-containing samples compared to the NaMMTcontaining polymer, and its contribution towards the subsequent photo-oxidation rate of compatibilised-OMMT nanocomposites was evaluated. The level of vinyl unsaturation was found to be highest when CL15 clay was used compared to NF, whereas the NaMMT gave much lower level of unsaturation which was similar to that formed in PE processed under the same conditions. The photo-oxidative stability of OMMT–PEgMA containing nanocomposites was lower than the uncompatibilised analogues and these in turn exhibited less stability than the neat polymer or the PE–PEgMA polymer blend. The major cause for the higher level of photo-oxidation of the PEgMA-containing OMMT samples is attributed to unfavourable interactions between the anhydride moiety and the clay, and, in particular, to possible reactions with the thermal degradation products of the alkyl ammonium surfactant that would ultimately result in more polymer peroxidation giving rise to pro-oxidant effects. Other factors that may also contribute to the lower stability of these compatibilised nanocomposites include the photolytic instability of the anhydride ring and the catalytic effect of the metal ion impurities, especially iron, which are present in naturally occurring clays.

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