European Polymer Journal 46 (2010) 1456–1464

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Contents lists available at ScienceDirect

European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Macromolecular Nanotechnology

Photooxidation of polypropylene/layered double hydroxidenanocomposites: Influence of intralamellar cations

Sunil P. Lonkar a,b,1, Sandrine Therias a,b,*, Nathalie Caperaa c,d, Fabrice Leroux c,d,Jean-Luc Gardette a,b

a Clermont Université, Université Blaise Pascal, Laboratoire de Photochimie Moléculaire et Macromoléculaire, BP 10448, F-63000 Clermont-Ferrand, Franceb CNRS, UMR 6505, LPMM, F-63173 Aubière, Francec Clermont Université, Université Blaise Pascal, Laboratoire des Matériaux Inorganiques, BP 10448, F-63000 Clermont-Ferrand, Franced CNRS, UMR 6002, LMI, F-63173 Aubière, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 December 2009Received in revised form 6 April 2010Accepted 11 April 2010Available online 24 April 2010

Keywords:PolypropyleneLDHNanocompositePhotooxidationDurability

0014-3057/$ - see front matter � 2010 Elsevier Ltddoi:10.1016/j.eurpolymj.2010.04.009

* Corresponding author at: Clermont UniversitPascal, Laboratoire de Photochimie Moléculaire et M10448, F-63000 Clermont-Ferrand, France. Tel.: +33+33 (4) 73 40 77 00.

E-mail address: sandrine.therias@univ-bpclermo1 Permanent address: Division of Polymer Scien

National Chemical Laboratory, Pune 411 008, India.

The influence of layered double hydroxides (LDHs) on the photooxidation of polypropylene(PP)/LDH nanocomposites was studied under irradiation at long wavelengths (k > 300 nm,60 �C and in the presence of oxygen). The influence of hybrid LDHs containing differentdivalent cations (Mg, Zn or both Mg and Zn) on the photooxidation mechanism of PPand on the rates of oxidation of the matrix was characterised based on infrared analysis.The presence of LDHs modifies the photoproducts accumulating in the PP and the ratesof oxidation of PP were changed depending on the divalent cations in the LDH layers.Whereas natural clays, such as montmorillonite (MMt), can lead to a faster degradationof the materials, LDHs (Zn2–Al–DS, for example) appear to have no inductive effect on poly-mer oxidation.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The growing interest for applications of nanocompositematerials in many industrial fields is the driving force forthe development of new polymer matrix-nanofiller formula-tions that offer enhanced properties compared to compositesmade with traditional fibres. Because of the nanometre-sizedispersed particles, the simple addition of a small amount(less than 5%) of organo-clays gives improved mechanical,thermal and physicochemical properties in comparison withpristine polymers or conventional microcomposites. Re-search has been conducted on a wide variety of polymersand on different nanoclays. Mostly focused on cationic clays,

. All rights reserved.

é, Université Blaiseacromoléculaire, BP

(4) 73 40 71 43; fax:

nt.fr (S. Therias).ce and Engineering,

and particularly on montmorillonite and hectorite, clay-based nanofillers have recently been extended to layereddouble hydroxides (LDHs).

Different authors have pointed out the versatility of LDHmaterials for fabricating nanocomposites, [1–3] and differ-ent aspects make LDH materials intriguing as fillers in poly-mers. The lamellar nature of LDHs permits host–guestchemistry and intercalation reactions, which draw consid-erable attention from material designers. The surface cov-ered by hydroxyl groups and the great versatility in thecation and anion exchange with suitable organic moleculesare advantageous for flame retardant applications [4]. Fur-thermore, the presence of hydroxyl groups renders the par-ticles hydrophilic and even allows for hydrogen bonding andthe formation of more elaborate nanostructures [5].

The construction and characterisation of various poly-mer LDH nanocomposites based on LDH hybrid phases withdodecyl sulphate anions (DS) are reported in the literature.Nanocomposites with LDHs based on [Zn–Al–DS] or [Mg–Al–DS] were prepared either via in situ bulk polymerisation

S.P. Lonkar et al. / European Polymer Journal 46 (2010) 1456–1464 1457

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or by the solution intercalation method with polymers suchas polystyrene (PS) [6–9] or PMMA [10–12]. Some papershave reported on polyolefins/LDH nanocomposites, mainlywith polyethylene [13,14], and one study dealt with poly-propylene (PP)/LDH nanocomposites prepared by a meltintercalation process with LDH phases [Zn3Al–DS] [15].Some polymer LDH nanocomposites were obtained withLDHs exchanged with stearate [16,17], and LDHs, such as[Mg3Al–DS], were also used in nanocomposites with EPDM[18] or PET matrices [19].

Polymer matrices based on nanocomposites have gener-ated a significant amount of attention in the recent litera-ture for nanotechnology. Most of the research has focusedon the synthesis, processing and characterisation of nano-composites and on the evaluation of their properties. Manydiverse topics, including composite reinforcement, barrierproperties, flame resistance, electro-optical properties, cos-metic applications and bactericidal properties, have beendiscussed. Surprisingly, only a few papers deal with thelong-term behaviour of these materials, such as the dura-bility of the nanocomposite properties in conditions ofenvironmental ageing. In many cases, clays can acceleratethe degradation processes of polymers under conditionsof UV light exposure. The improved properties obtainedby nanocomposites could be compromised by their poorlong-term stability and durability. Published reportsinclude studies on various polymers (PP, epoxy, EPDM,PS, etc.) [20–27] with mainly organo-montmorillonites(MMt) as the nanofillers. Only four papers have reportedon the photooxidative behaviour of nanocomposites withLDHs [28–31]. The first published [28] article was on thephotooxidative behaviour of PS/LDH nanocomposites withhybrid LDHs, such as Zn2Al SPMA, with a monomeric sur-factant (3-sulfopropylmethacrylate) as the organic anion.The hybrid nanofiller does not modify the photooxidationmechanism of the polymer matrix PS nanocomposites withclays, such as MMt [20,22–25]. Only a slight influence of theLDH on the oxidation rate was observed. The influence ofnanodispersed hydrotalcite on PP photooxidation [29]was studied with [Mg2AlCO3] as the nanofiller, and a reduc-tion of the induction time of oxidation was observed. Thephotostability of PE nanocomposites with LDHs containingintercalated stearate and Mn or Co as divalent cations wasrecently reported. These LDHs act as photodegradants atvery low amount (0.1%).

The present article aims to provide a better under-standing of the photochemical behaviour of clay nano-composites. This work investigated the photooxidation ofnanocomposites based on PP as the polymer matrix (withPP-g-MA as compatibiliser) and on hybrid LDHs with dode-cyl sulphate anions as the nanofiller. The photochemicalbehaviour of these nanocomposites is of prime importanceas durability is the key factor for outdoor applications. Thechemical modifications resulting from photooxidationwere investigated by infrared spectroscopy. The influenceof hybrid LDHs on the mechanism of oxidation of the PPmatrix was characterised. The rates of degradation of thePP/LDH nanocomposites were compared with those ofthe pristine matrix (PP/PP-g-MA), and the influences ofboth the filler content (5% or 10%) and the divalent cations(Zn or Mg or both) of the LDHs are discussed.

2. Experimental

2.1. Materials

The polymer matrix was an isotactic polypropylene (Ex-xon Mobile PP with 2.5–3.5 MFI). The maleic anhydride–grafted PP polymer (PP-g-MA) used as compatibiliser wasa low molecular weight Polybond 3200 (1% MA content,0.91 g/cc density, and Mw of 90,000 g mol�1) obtained fromChemtura Corporation.

Sodium dodecyl sulphate (SDS, CH3–(CH2)11–SO4Na))(Aldrich), ZnCl2, AlCl3, 6H2O and MgCl2, 6H2O (Acros,99%), NaOH (Acros, 97%) were used as received.

Organo-modified clay (OMMt) was supplied by South-ern Clay Products (C20A).

2.2. Preparation of LDH hybrid materials

The LDH hybrid phases were prepared by an exchangereaction of chlorine LDH precursors.

2.2.1. Synthesis of chlorine LDH by coprecipitationThe chlorine layered double hydroxide phases, Mg2–Al–

Cl, MgZn–Al–Cl and Zn2–Al–Cl, were prepared by thecoprecipitation method [32]. The pH was maintained at 9by adding 1 M NaOH. The conditions for the coprecipita-tion performed under nitrogen were identical (T, pH andageing time) for the preparation of the three chlorine pris-tine samples. The resulting powders were washed severaltimes with decarbonated water, and then dried undervacuum.

Abbreviations of LDH:A, Mg2–Al–DS; B, MgZn–Al–DS; C, Zn2–Al–DS.

2.2.2. Exchange reactionA solution containing four equivalents of dodecyl sul-

phate anions (in comparison to the anion exchange capac-ity of the chlorine pristine LDH) was prepared with adecarbonated aqueous solution at 40 �C under nitrogenatmosphere and allowed to stir for 1 h. The desired amountof chlorine LDH was then added and the exchange reactionwas left for 48 h. The resulting powders were washed sev-eral times with a 50:50 v/v mixture of EtOH/H2O and thendried under vacuum.

2.3. PP/LDH nanocomposite preparation

The PP/LDH composites were prepared in two stepsusing a co-rotating tightly intermeshed twin-screw extru-der (Haake Minilab). All material was dried at 80 �C undervacuum prior to mixing.

Step 1. Preparation of master batch

The DS-intercalated LDH was mixed with PP-g-MA in a1:2 weight proportion. The operation temperature wasmaintained at 160 �C for 5 min and at a 200-rpm rotor speedto prepare a master batch of filler in compatibiliser. Allexperiments were performed under an inert atmosphere.

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Step 2. Preparation of nanocomposites

The master batch of PP-g-MA and LDH was added tomolten PP and mixed at 180 � for 5 min, with the otherabove-described parameters kept the same.

The concentration of nanoclays was either 5% or 10%wt./wt (Table 1). The samples were compression-mouldedin the form of films (thickness between 80 and 100 lm,depending on the samples) between PTFE films at200 bar for 2 min at 180 �C.

2.4. Characterisation

The interlayer distance in the layer structure of the LDHphases was determined by X-ray diffraction. Nanocompos-ite films were characterised by X-ray diffraction,transmission electron microscopy (TEM) and Fourier trans-form-infrared spectroscopy (FT-IR).

2.4.1. X-ray diffractionX-ray diffraction experiments were performed with a

Rigaku (Japan) D/max-RB wide angle X-ray diffractometer(WAXD). The operation parameters were Cu Ka radiationat a rotating anode generator operated at a voltage of40 kV and a current of 100 mA. The scanning rate was2#/min at an interval of 0.02�.

2.4.2. Transmission electron microscopy (TEM)Samples for TEM imaging were prepared using a Leica

Ultracut UCT microtome at a thickness of 80–100 nm witha diamond knife at �100 �C. The sections were collectedfrom water on copper 300 mesh carbon-coated grids. TheTEM imaging was done using a JEOL 1200EX electron micro-scope operating at an accelerating voltage of 100 kV. Imageswere captured using a charged couple detector camera andviewed using Gatan Digital Micrograph software.

2.4.3. Fourier transform-infrared spectroscopyInfrared spectra were recorded with a Nicolet 5SX-FTIR

spectrometer using OMNIC software. Spectra were ob-tained using 32 scans and a resolution of 4 cm�1.

2.5. UV irradiation

The samples were irradiated in the form of the films de-scribed above. The UV-light irradiation was carried out un-der polychromatic light with wavelengths higher than

Table 1Composition of nanocomposite samples.

Sample % PP % PP-g-MA % LDH % oMMt

PP 100 0 0 0fPP 89.5 10.5 0 0PPM5 85 10 0 5PPLA5 85 10 5 0PPLB5 85 10 5 0PPLC5 85 10 5 0PPM10 70 20 0 10PPLA10 70 20 10 0PPLB10 70 20 10 0PPLC10 70 20 10 0

300 nm in a SEPAP 12.24 U [33] in the presence of oxygen.This accelerated weathering device was equipped withfour medium-pressure mercury lamps (400 W), and thesamples were placed on a rotating carousel positioned inthe centre. The temperature was regulated at 60 �C andcontrolled by a Pt thermocouple.

3. Results and discussion

3.1. Characterisation of LDH hybrid phases

3.1.1. X-ray diffractionThe crystallinity of the LDH hybrid phases, which were

obtained after an exchange reaction of LDH chlorine phaseswith DS, was characterised by X-ray diffraction. The XRDdiagrams are displayed in Fig. 1. After the exchange reac-tion, the three LDH phases still showed diffraction peakscharacteristic of a layered geometry. The position of thebasal peak (0 0 3) indicated the interlayer spacing betweenthe two metal hydroxide sheets (d003), and was calculatedto be 2.57 nm from the (0 0 3) reflection at 2# = 3.04,which is consistent with the value reported elsewhere[34]. These results indicate that a swollen and intercalatingstructure is formed by inserting the surfactant (DS) intothe host gallery of the LDH platelets. The interlamellar dis-tance of the hybrid LDH phases corresponds to an enlargedinterlayer distance of 0.76 nm for the LDH precursor phasewith chlorine anions [34].

Fig. 1 clearly shows that the interlamellar distance isthe same in the three LDH phases. However, the best crys-tallised phase was with Zn2–Al–DS (C), and the diffractionlines (0 0 l) were enlarged in the phases incorporatingmagnesium (MgZnAl and Mg2Al) with (A) being less crys-tallised. This tendency, which was observed in the stackingdirection, was also observed for the in-plane coherencelength. The diffraction peak of concern located at2# � 61� is characteristic of an intra-layer crystallised do-main and is associated with the cell parameter a. The slightshift to greater 2# values when Zn is substituted for Mg isin agreement with Vegard’s law considering divalentcation radii of Mg2+ < Zn2+. Even if the (1 1 0) and (1 1 3)

10 20 30 40 50 60 70

IInte

nsity

(a.u

.)

2 Theta (2ϑ°)

Zn2-Al-DS (C)

MgZn-Al-DS (B)

Mg2-Al-DS (A)

2 Theta (2ϑ°)

Zn2-Al-DS (C)

MgZn-Al-DS (B)

Mg2-Al-DS (A)

Fig. 1. X-ray diffractograms of LDH hybrid phases. (A) Mg2–Al–DS, (B)MgZn–Al–DS, (C) Zn2–Al–DS.

(a)

(b)

10 20 30 40 50 60 70

PPLA 5

PPLB5

PPLC5

fPP

IInte

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.u.)

2 Theta (2ϑ°)

PPLA 5

PPLB5

PPLC5

fPP

2 Theta (2ϑ°)

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PPLC5

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PPLC5

2 Theta (2ϑ°)

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diffraction lines strongly overlap in cases of large basalspacing, such as for the DS-intercalated phases due to thel Miller index-dependent contribution of (1 1 3), the com-parison of both contributions may be carried out. Indeed,the diffraction linewidth is also slightly broader when Znis substituted by Mg, indicating a larger in-plane coherencedomain for Zn than that for Mg. Consequently, there ispotentially a greater barrier effect in association with thegreater number of platelets. Notably, a less crystallisedstructure is usually considered to be more suitable for sub-sequent delamination by blending with a polymer.

3.1.2. Infrared characterisationThe LDH hybrid phases were also characterised by

infrared analysis. The IR spectra (Fig. 2a) show characteris-tic vibration bands of LDH–DS, m(OH), d(OH) at 3490 cm�1

and 1625 cm�1, respectively. Depending on the divalentcations of each LDH, the vibration band d(O–M–O) appearsat 428 or 448 cm�1 for the LDHs with Zn or Mg, respec-tively. In the LDH phase with both cations, the maximumof the vibration band is at 438 cm�1 (Fig. 2b).

The infrared spectra also allow for observation of thesurfactant functional groups. The symmetric and asym-metric stretching vibrations characteristic of S@O are at1065 and 1215 cm�1.

3.2. Characterisation of PP/LDH nanocomposites

3.2.1. X-ray diffractionThe XRD diagrams of the PP/LDH nanocomposites are

displayed in Fig. 3.

(a)

(b)

4000 3500 3000 2500 2000 1500 1000 500

Zn2-Al-DS (C)

MgZn-Al-DS (B)

Mg2-Al-DS (A)

Abs

orba

nce

Wavenumbers (cm-1)

Zn2-Al-DS (C)

MgZn-Al-DS (B)

Mg2-Al-DS (A)

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0.2

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Wavenumbers (cm-1)

Mg

Zn

ZnMg

Mg2Al-DSZnMgAl-DSZn2Al-DS

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0.2

0.4

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0.8

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Zn

ZnMg

Mg2Al-DSZnMgAl-DSZn2Al-DS

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0.2

0.4

0.6

0.8

Mg

Zn

ZnMg

Mg2Al-DSZnMgAl-DSZn2Al-DS

Fig. 2. IR spectra of LDH hybrid phases. Mg2–Al–DS. (A) MgZn–Al–DS, (B)Zn2–Al–DS (C). (a) In the domain 4000–400 cm�1, (b) in the domain1100–400 cm�1.

2 Theta (2ϑ°)2 Theta (2ϑ°)

(c)

10 20 30 40 50 60 70

PPM 10

PPM 5

oMMt

Inte

nsi

ty (

a.u

.)

PPM 10

PPM 5

oMMt

Fig. 3. X-ray diffractograms of LDH and PP/LDH nanocomposites. (a) fPPand nanocomposites PPLA5, PPLB5 and PPLC5, (b) Zn2–Al–DS and PPLC5.(c) oMMt, PPM5 and PPM10 nanocomposites.

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In the case of PPLB and PPLC5 (Fig. 3a), no noticeablereflections related to the LDH were observed, indicativeof a collapse of the lamellar structure of the stacked LDHand thus the formation of disordered or exfoliated nano-structure within the matrix. The basal spacing of the hy-brid LDH phase Mg2–Al–DS was measured to be 2.57 nm;after dispersion in the PP polymer, the (0 0 3) reflectionof the PPLA5 film is shifted towards lower scattering angles(Fig. 3b). The complete disappearance of the XRD peaksmay reveal either an exfoliation state or the presence of asmall diffracting volume, as observed in the cases of lowfiller loading. Characterisation by TEM is required to con-firm the exfoliation of the LDH platelets.

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Fig. 3c displays the XRD diagrams of both oMMt and PP/oMMT nanocomposites with 5% and 10% loading. In thetwo PP/oMMt nanocomposites, the presence of an XRDpeak at a lower 2h value is indicative of an intercalatedstructure and the basal spacing for oMMt increased withthe intercalation of the polymer into the interlayer regionof MMt.

3.2.2. TEM analysisThe TEM data was required to complement the informa-

tion from XRD to characterise the dispersion of LDH in thepolymer. Fig. 4 shows the morphology of the PP nanocom-posite loaded with 5% LDH Zn2–Al–DS (PPLC5).

Fig. 4a displays the lower magnification image, whichallows for the observation of the global dispersion of theLDH in the polymer, with the dark lines in the micrograph

Fig. 4. TEM images of PPLC5. (a) Low-magnifica

representing the LDH layers. The TEM image at high mag-nification (Fig. 4b) reveals that delamination occurred,resulting in the presence of both single double hydroxidelayers and tactoids with reduced thickness. These materi-als can be described as exfoliated.

3.2.3. Infrared analysisThe IR spectrum of the maleic anhydride–grafted PP

polymer (fPP) shows absorption bands in the carbonyl do-main (Fig. 5a) with maxima at 1714, 1780 and 1850 cm�1.The intense absorption band at 1714 cm�1 arises from theformation of carboxylic acid resulting from the hydrolysisof the anhydride groups [25]. The absorption bands at1780 and 1850 cm�1 are characteristic of cyclic anhydrideand are due to the symmetric and asymmetric C@O anhy-dride stretching, respectively [35]. The presence of both

tion image, (b) high-magnification image.

S.P. Lonkar et al. / European Polymer Journal 46 (2010) 1456–1464 1461

acid and anhydride groups indicates a chemical equilib-rium resulting from the hydrolysis of the anhydride[23,25]. The IR spectrum of the PPM5 nanocomposite(Fig. 5a) is similar to that of fPP, but with a lower intensityof the absorption band at 1780 cm�1. Conversely, the IRspectrum of the PP/LDH nanocomposite (PPLC5) (alsoshown on Fig. 5a) is quite different in the carbonyl domain.A broad absorption band at 1630 cm�1 is observed,whereas the band at 1780 cm�1 attributed to the anhy-dride is completely absent. These differences in the IRspectra strongly suggest that chemical interactions occurbetween the anhydride and the LDH nanofillers.

In the IR spectra of the nanocomposite PP/LDH films(Fig. 5b), the vibration bands d(O–M–O) characteristic ofeach LDH phase were shifted by ±10 cm�1, depending onthe divalent cations, confirming that LDH platelets arepresent in the polymer. The IR spectrum of the PPLC5 sam-ple is different than those with PPLA5 or PPLB5, as thecharacteristic bands of DS are missing (at 1200 and1060 cm-1) and the broad band around 670 cm�1 is shiftedto 600 cm�1. These results suggest a different environment

(a)

(b)

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

Abso

rban

ce

900 1000 1100 1200 1300

Wavenumbe

Abs

orba

nce

0.22

0.26

0.30

1700 1800 1900

Wavenumbers (cm-1

1780 cm-1 1714 cm

0.18

0.14

0.34PPLC5PPM5fPP

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1700 1800 1900

1780 cm-1 1714 cm

0.18

0.14

0.34PPLC5PPM5fPP

Fig. 5. IR spectra of PP/LDH and PP/oMMt nanocomposites before irradi

of the Zn2Al–DS platelets in PP compared to the nanocom-posites with the two other LDH phases.

3.3. Photooxidation of PP/LDH nanocomposites

3.3.1. Infrared analysisThe shape of the IR spectra recorded after exposure of

the various samples to UV light in the presence of atmo-spheric oxygen reveals notable changes of the chemicalstructure of the materials. As shown in Fig. 6a, an increaseof absorbance was observed in two spectral domains in theIR spectra of the PP/LDH nanocomposites, corresponding tothe hydroxyl and carbonyl stretching vibrations domains.Moreover, in the PP samples containing LDH, the shapeof the oxidation bands in the carbonyl domain (Fig. 6a) isdifferent than those with just PP or PP/oMMt (Fig. 6b).Upon irradiation, the IR spectra of PP/LDH develop twobroad absorption bands in the carbonyl region. One bandhas a maximum at 1715 cm�1 and two shoulders at 1735and 1770 cm�1, which are also be observed with pristinePP or the PP/oMMt nanocomposite. In the case of LDH, an-

500 600 700 800

rs (cm-1)

PPLA5PPLB5PPLC5PP

Mg

Zn

ZnMg

1500 1600

)

-1

1630 cm-1

1500 1600

-1

1630 cm-1

ation. (a) fPP, PPM5 and PPLC5, (b) fPP, PPLA5, PPLB5 and PPLC5.

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PPLC5

0

0.1

0.2

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Abso

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1550 1600 1650 1700 1750 1800 1850 1900

Wavenumbers (cm-1)

(b)

PPM 5

0.2

0.4

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0.8

1.0

1.2

Abso

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ce

1550 1600 1650 1700 1750 1800 1850 1900

Wavenumbers (cm-1)

Fig. 6. IR spectra of nanocomposite films during photooxidation at k > 300 nm, 60 �C. (a) PPLC5, (b) PPM5.

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other IR band appears at 1595 cm�1, and is shiftedto1610 cm�1 for longer irradiation times, indicating thatthe PP/LDH nanocomposites develop different photoprod-ucts compared to pristine PP. This peculiar behaviourwas not observed in the case of PP/MMt nanocomposites[23,24]. The presence of LDH can modify the mechanismof polymer oxidation or the final oxidation products inthe material.

The mechanism of PP photooxidation has been exten-sively studied and is considered to be well established[24]. The broad absorption bands with maxima at 1715,1735 and 1770 cm�1 correspond to photoproducts identi-fied as a-methylated carboxylic acids, esters and c-lac-tones, respectively. The additional broad band observed1595 cm�1 in the PP/LDH samples and then shifted at1610 cm�1 was also observed in photooxidation of PP/hydrotalcite nanocomposites [24]. This band was attrib-uted to a carboxylic acid salt produced from the reaction

of the carboxylic acid with hydrotalcite, which is made ofbasic hydroxides. The presence of carboxylates amongthe oxidation photoproducts of the PP/HDL nanocompos-ites indicates that the a-methylated carboxylic acidsformed by oxidation of PP are neutralised in the presenceof HDL. Therefore, the environment is more basic in LDHnanocomposites than in PP or PP/oMMt nanocomposites.The LDH platelets can be characterised by a ‘‘local pH”,which depends on the LDH cations [36].

3.3.2. Rate of degradationThe rates of photooxidation of the various nanocompos-

ites (PP/LDH and PP/MMt) were compared by measuringthe increase of absorbance at 1713 cm�1 with irradiationtime. Because all of the formulations contained maleicanhydride (even fPP), an absorption band was initiallypresent at the same frequency in the spectrum of the sam-ples before irradiation. The increase in absorbance at

(a)

0 10 20 30 40 500

0.2

0.4

0.6

0.8

1.0

Irradiation time (h)

fPPPPM5PPLA5PPLB5PPLC5

ΔΔ(O

D 17

13 c

m-1)

Δ(O

D 17

13 c

m-1)

(b)

0 10 20 30 40 500.0

0.2

0.4

0.6

0.8

1.0

Irradiation Time (h)

fPPPPLA10PPLB10PPLC10

Fig. 7. Rates of photooxidation of PP/LDH nanocomposites. (a) at 5%loading, (b) at 10% loading.

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1713 cm�1 was plotted as the difference from the absor-bance before irradiation as a function of irradiation time.However, because the samples had slightly different thick-nesses, the absorbance values had to be normalised. A cor-rection factor was applied to adjust the characteristicabsorption band at 2722 cm�1 of PP to the same value.Fig. 7a and b shows the variation of absorbance at1713 cm�1 as a function of irradiation time for fPP, PP/MMt and PP/LDH, respectively, loaded at 5% and 10% (w/w) in the nanofiller.

In the case of the fPP film (Fig. 7a), a short inductionperiod of 5 h was observed before the oxidation of PP. Thisdelay is always observed in PP photooxidation, and reflectsthe activity of the remaining antioxidant, which is con-sumed as a sacrificial additive. Once the active form ofthe antioxidant is consumed, oxidation starts and thephotoproducts accumulate in the polymeric matrix.Fig. 7a compares the nanocomposites with 5% clay. ThePP/MMt film degrades much more rapidly than fPP; after20 h of exposure to UV light, the normalised absorbanceis 0.37, but is only 0.22 with fPP. The three LDH nanocom-posites are less degraded than the nanocomposites withMMt. The oxidation rate of the polymeric matrix in PP/LDH depends on the divalent cations present in the LDH.The LDH phases containing Mg degrade faster than thosewith Zn as the divalent cation. However, the Zn-containingLDH phases have a rate of photooxidation almost similar to

that of fPP. The influence of LDH on polypropylene oxida-tion appears to depend on the divalent cations; the pres-ence of the LDH has no influence on the durability of thenanocomposite material, such as with the Zn2–Al–DSLDH. Increasing the loading of the LDH to 10% gives similareffects as the 5% loading, and the kinetic curves shown inFig. 7b reveal the same behaviour as observed in Fig. 7a.The nanocomposites with LDH phases containing Mg2+

(Mg2–Al–DS and MgZn–Al–DS) degrade faster than thosewith Zn2–Al–DS. Nanocomposites with 10% or 5% of Zn2–Al–DS display the same kinetic curve as fPP. This confirmsthat LDH phase containing only Zn2+ has no influence onthe polymer oxidation.

4. Conclusion

Different PP/LDH nanocomposites were successfullyprepared by melt intercalation with hybrid LDH phasesbased on dodecyl sulphate as the organic anion and eitherMg or Zn or both as divalent cations. As evidenced by XRDand TEM analyses, there were some inorganic layer stac-kings, but mainly exfoliated systems were achieved. Thephotooxidative behaviour of the different samples revealedthat the presence of LDH can modify the chemical struc-ture of the photoproducts accumulating in the matrix.The rates of oxidation are influenced by the nanofiller,depending on the divalent cations of the LDH layers. TheLDH phases containing Mg2+ have a prodegradant effecton the nanocomposite material, whereas LDH with onlyZn2+ has no influence on the rate of oxidation of the poly-mer matrix.

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