Applied Clay Science 67–68 (2012) 83–90

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Applied Clay Science

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A novel LDH nanofiller intercalated by silsesquioxane for preparing organic/inorganichybrid composites

Yan Yuan, Wenfang Shi ⁎CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Jinzhai Road 96, Hefei,Anhui 230026, PR China

⁎ Corresponding author. Tel.: +86 551 3606084; fax:E-mail address: wfshi@ustc.edu (W. Shi).

0169-1317/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.clay.2012.08.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 February 2012Received in revised form 24 July 2012Accepted 4 August 2012Available online 5 September 2012

Keywords:Layered double hydroxide (LDH)SilsesquioxaneUV curingNanocomposite

A novel layered double hydroxide nanofiller (LDH-SA) was synthesized through intercalating thesilsesquioxane-based aminoundecanoic acid (SAD) into MgAl-based LDH, following by grafted with thesemiadduct of IPDI and 2-hydroxyethyl acrylate (IPDI-HEA). SAD was obtained by the reaction of semiadductisophorone diisocyanate-aminoundecanoic acid (IPDI-AD) with the pre-synthesized silsesquioxane hybrid polyol(SOH). Themolecular structureswere characterized by FT-IR and 1HNMR spectroscopy. LDH-SAwas blended intoan acrylic resin and UV irradiated, obtaining an exfoliated polymer/LDH nanocomposite which possesses the com-bined enhancement in thermal stability brought by both LDH and silsesquioxane. The formed UV curednanocomposite possessed the exfoliated microstructure from the transmission electron microscope (TEM) andHR-TEM observations. The thermal and mechanical properties were greatly enhanced. The onset decompositiontemperature of nanocomposite with 5% LDH-SA loading increased by 45 °C from TGA analysis, while the tensilestrength and pencil hardness were enhanced to 9.4 MPa and 2 H from 7.2 MPa and 3B, respectively, comparedwith the pure polymer.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Polymer/layered crystal inorganic nanocomposites have drawnmajor attention among organic/inorganic nanocomposites for theiradvantaged mechanical and thermal properties, wonderful chemicalstability, reduced flammability and so on (Becker et al., 2011;Costache et al., 2006; Maag et al., 2000; Pojanavaraphan andMagaraphan, 2010; Qiu et al., 2006). Therefore, the organic-intercalated layered double hydroxide (LDH) nanomaterials havebecome a hot topic and have been widely studied in scientific and in-dustrial fields in recent years, for example, used as photoinitiatorcomplexes in UV curing systems, flame retardant additives in coat-ings, as well as drug containers and catalyst carriers (Chakraborti etal., 2011; Evans and Duan, 2006; Frache et al., 2011; Hu et al., 2011;Tong et al., 2010; Yuan and Shi, 2010). The formed polymer/LDHnanocomposites possess the combined advantages of LDH and poly-mers. The LDHs are a class of anion clay materials containing hydrox-ides of different kinds of metal cations and anions intercalated intohydrated interlayer for supplying the balanced negative charge(Khan and O'Hare, 2002). They can be represented by the ideal for-mula [M2+

1−xM3+x (OH)2 ]x+An−

x/n·mH2O, where x means the

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content of M3+ in the total element weight of the formula dividedby the molar mass of M3+ when OH−'s value set as 2, M2+ andM3+ are divalent and trivalent metal cations, such as Mg2+, Al3+, re-spectively, and An− is an exchangeable anion, such as CO3

2−, SO42−,

and NO3− (Zhao et al., 2011).

Recent years, a new hybrid reagent, silsesquioxane complex, wasintroduced to prepare organic/inorganic nanocomposites for its en-hancement in physical and chemical properties, and especially inthermal properties (Cheng et al., 2009; Choi et al., 2003; Li et al.,2001; Pyun and Matyjaszewski, 2001; Yoshimoto and Takahiro,2004; Zhao and Schiraldi, 2005). Silsesquioxane refers a term of mat-ters with the empirical formula of RnSinO1.5n−x(OH)2x, and can formladder and cage-like or polymeric structure (Cheng et al., 2009).There are some studies combining LDH with silsesquioxane as adual-nanofiller in nanocomposites. Yei and coworkers used ami-nopropylisobutyl polyhedral oligomeric silsesquioxane (POSS) as asurfactant to prepare polystyrene/montmorillonite (MMT) nanocom-posites (Yei et al., 2004). However, the limited distance of 0.5 nm be-tween Si-Si in POSS resulted in a small increase of 0.4 nm in d-spacingbetween layers after intercalated by POSS, almost acting as the traditionallong chain alkyl ammoniumsalt surfactant. Furthermore, the complicatedand time-consuming procedure in the synthesis of POSS makes its appli-cation limited in laboratory research.

Therefore, several large-scale silsesquioxane-based hybrid re-agents have been produced in a brief process with lower cost(dell'Erba and Williams, 2007; dell'Erba et al., 2003, 2004). Mori andcoworkers described the synthetic route for a kind of cage-type

84 Y. Yuan, W. Shi / Applied Clay Science 67–68 (2012) 83–90

silsesquioxane with hydroxyl groups on the outermost surface,possessing 12–18 Si atoms and an average particle size of 2.7 nm(Mori et al., 2003, 2004, 2007). In this regard, the larger scalesilsesquioxane is priority to be chosen as a surfactant for preparingpolymer/LDH nanocomposites with unique properties. Firstly, a com-mon surfactant, aminoundecanoic acid (AD), was reacted withisophorone diisocyanate (IPDI) to obtain the semiadduct IPDI-AD,followed by reacted with the pre-synthesized silsesquioxane hybridpolyol (SOH), obtaining a new anion surfactant silsesquioxane-based aminoundecanoic acid (SAD). Then SAD was intercalated intoMgAl-LDH through the coprecipitation method, preparing LDH-SADwith expending layer spacing.

Moreover, the UV irradiation technology was introduced to preparepolymer/LDH nanocomposites for its “clean, green and efficient” advan-tages (Zahouily et al., 2001, 2002; Zammarano et al., 2005). In our group,a novel UV cured acrylated silane-intercalated polymer/LDH nanocom-positewas prepared bymodifying LDHwith both sodiumdodecyl sulfate(SDS) and [3-(methyl-acroloxy)propyl]trimethoxysilane (KH570), fol-lowed by UV irradiation after blended into an acrylate system (Yuanand Shi, 2011). Compared with the pure polymer, the mechanical andthermal properties of UV cured nanocomposites showed enhancementsin different extents, as well as hardness. Herein, LDH-SAD was furtherreacted with the semiadduct of IPDI and 2-hydroxyethyl acrylate(IPDI-HEA), obtaining UV curable acrylate-modified LDH and named asLDH-SA. The LDH-SA was blended into an acrylic resin in the presenceof 1-hydroxycyclohexyl-phenyl ketone as a photoinitiator, and then ex-posed to amedium pressuremercury lamp, forming a completely exfoli-ated polymer/LDH nanocomposite. The photopolymerization kinetics,mechanical and thermal properties, and microstructures of nanocom-psite were all examined in details.

2. Experimental

2.1. Materials

Glycidol and aminoundecanoic acid (AD) were purchased fromSigma-Aldrich Co. (USA) and used as received.3-Aminopropyltriethoxysilane (KH550) supplied by NanjingYudeheng Fine Chemical Co. (Nanjing, China), isophoronediisocyanate (IPDI) supplied by Shanghai First Reagent Co. (Shang-hai, China), and 2-hydroxyethylacrylate (HEA) supplied by EternalChemical Co. (Taiwan) were distilled under reduced pressure beforeuse. Hydrofluoric acid (46–48% HF aqueous solution), di-n-butyltindilaurate (DBTDL), p-hydroxyanisole, N,N-dimethylacetamide(DMAC), Mg(NO3)2·6H2O, Al(NO3)3·9H2O, sodium hydroxide (NaOH),toluene were all purchased from Sinopharm Chemical Reagent Co.(Shanghai, China) and used as received except for DMAC, which wasused after drying over 4-Å molecular sieves. The detailed synthesis andcharacterization of semiadduct (IPDI-HEA) of IPDI and HEA were de-scribed in our laboratory (Asif and Shi, 2003). EB270, an aliphatic ure-thane diacrylate with an unsaturation concentration of 1.33 mmol g−1

and a molar mass of 1500 g mol−1, tripropylene glycol diacrylate(TPGDA) and 1,6-hexamethyldiol diacrylate (HDDA) were supplied byCytec Industries Inc. (USA). 1-Hydroxycyclohexyl-phenyl ketone(Runtecure 1104) used as a photoinitiator was provided by RuntecChemicals Co. (Changzhou, China).

2.2. Measurements

The Fourier transfer infrared (FT-IR) spectra were recorded usinga Nicolet MAGNA-IR 750 spectrometer with a KBr disk. The ProtonNuclear Magnetic Resonance (1H NMR) spectra were recorded withan AVANCE 300 Bruker spectrometer using tetramethylsilane as aninternal reference and D2O as a solvent. The X-ray diffraction (XRD)analysis was performed using a Rigaku D/Max-rA rotating anodeX-ray diffractometer equipped with a Cu Kα tube and Ni filter (γ=

0.1542 nm). The transmission electron microscope (TEM) and highresolution TEM (HR-TEM) micrographs were obtained using Hitachi(Tokyo, Japan) H-800 and JEOL-2011 instrument, respectively, oper-ated both at an acceleration voltage of 200 kV. The samples wereultramicrotomed with a diamond knife on a LKB Pyramitome togive 60-nm thick slices.

The photopolymerization rate was monitored in air by a CDR-1differential scanning calorimeter (DSC) (Shanghai Balance Instru-ment Co., Shanghai, China) equipped with a UV spot cure systemBHG-250 (Mejiro Precision Co., Japan). The incident light intensityat the sample pan was measured to be 2.4 mW cm−2 with a UVpower meter. The unsaturation conversion (Pt) was calculated bythe formula, Pt=Ht/H∞, where Ht is the heat effect within t (s),the H∞ is the heat effect of 100% unsaturation conversion. For calcu-lating the H∞, the value, H0=86 kJ mol−1, for the heat of polymer-ization per acrylate unsaturation was taken. The DSC curve wasnormalized by the weight (g) of sample. The polymerization rateis defined by J g−1 s−1, named the variation of enthalpy (J g−1)per second.

The thermogravimetric analysis (TGA) was performed on aShimadzu TGA-50 H thermoanalyzer. In each case a 10-mg samplewas examined under a N2 flow rate of 6×10−5 m3 min−1 at aheating rate of 5 °C min−1 from room temperature to 600 °C. Thetensile storage modulus (E′) and tensile loss factors (tanδ) were mea-sured by a dynamic mechanical thermal analyzer (Diamond DMA, PECo., USA) at a frequency of 10 Hz and a heating rate of 5 °C min−1 inthe range of −100 to 200 °C with the sheets of 25×5×1 mm3. Thecrosslink density (νe) as themolar number of elastically effective networkchain per cube centimeter of the film, was calculated from the storagemodulus in the rubbery plateau region according to: νe=(E′/3RT),where E’ is the elastic storage modulus, R is the ideal gas constant, andT is the temperature in K.

The mechanical properties were measured with an Instron Uni-versal tester (model 1185, Japan) at 25 °C with a crosshead speed of25 mm min−1. The dumb-bell shaped specimens were preparedaccording to ASTM D412-87. Five samples were analyzed to deter-mine an average value in order to obtain the reproducible result.The pendulum hardness was determined in Persoz mode in secondsby using a QBY pendulum apparatus (Tianjin Instrument Co., China).The pencil hardness was determined using a QHQ-A pencil hardnessapparatus (Tianjin Instrument Co., China).

2.3. Preparation of acrylated silsesquioxane-intercalated LDH (LDH-SA)

2.3.1. Preparation of silsesquioxane hybrid polyol (SOH)The SOH was prepared according to the procedure developed by

Mori et al. (2003). 110.5 g (0.5 mol) KH550 was dropped slowly into74.08 g (1.0 mol) glycidol at ice cooling, and reacted for 1 h at 25 °Cunder stirring. The formed adduct N,N-di(2,3-dihydroxypropyl)-(aminopropyl)triethoxysilane was then dissolved in 700 mL of meth-anol, following by addedwith 21.03 g HF aqueous solution (3.225 wt.%) ,and then stirred for 4 h at 25 °C. After removed methanol, ethanol andwater, and dried at 80 °C in vacuum for 72 h, the silsesquioxane hybridpolyol (SOH) was obtained in a glassy solid at room temperature (yield97%).

2.3.2. Preparation of silsesquioxane-based aminoundecanoic acid (SAD)A mixture of 8.04 g (0.04 mol) AD, 8.88 g (0.04 mol) IPDI and a

catalyst amount of DBTDL (0.1 wt%) were dispersed in 40 mL ofdried DMAC with vigorously stirring for 5 h at 40 °C, forming the in-termediate semiadduct IPDI-AD. Then the IPDI-AD was droppedslowly in 73.4 g (0.02 mol) SOH solution of DMAC (1.5 wt%) atroom temperature for 1 h, and reacted at 60 °C for 5 h. After driedat 80 °C in vacuum for 72 h to remove DMAC, a new anionsurfactant silsesquioxane-based aminoundecanoic acid (SAD) wasobtained.

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2.3.3. Preparation of SAD-intercalated LDH (LDH-SAD)The coprecipitation method was used to intercalate SAD into LDH

according to the literature (Yuan and Shi, 2010). 45.16 g (0.01 mol) SADwas dissolved in 200 mL of NaOH aqueous solution (0.4 wt.%). 5.12 g(0.02 mol) Mg(NO3)2·6H2O and 3.75 g (0.01 mol) Al(NO3)3·9H2Owere first dissolved in 50 mL of deionized water, then dropped into theabove SAD solution with vigorously stirring at room temperature, andaged at 80 °C for 24 h. The pH value of the reactant was maintained at10 by adding 1 M NaOH solution. All the operations were done underN2 atmosphere to avoid carbonate anions. The formed slurrywas filtered,

Scheme 1. Preparation route and structural illustration of

and washed by deionized water repeatedly to eliminate unreacted SAD,obtaining the SAD-modified LDH aqueous suspension, named LDH-SAD.

2.3.4. Preparation of acrylate-modified LDH (LDH-SA)A proper amount of the above aqueous suspension containing

3.0 g LDH-SAD was dispersed in 200 mL of toluene, and then theazeotropic distillation was performed to remove water from the sus-pension, obtaining a toluene suspension of LDH-SAD. An excessamount of previously synthesized IPDI-HEA (0.1 mol) was dropped

LDH-SA and UV cured polymer/LDH nanocomposite.

Fig. 3. FT-IR spectra of LDH-SAD and LDH-SA-48.Fig. 1. FT-IR spectra of SOH and SAD.

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slowly into the above toluene suspension with 0.1 wt% DBTDL and1000 ppm p-hydroxyanisole at room temperature, and vigorouslystirred at 80 °C for different times (24 h and 48 h) under N2 atmo-sphere. After filtrated and repeatedly washed with toluene to removeunreacted IPDI-HEA, the acrylate-modified LDH toluene suspensionsnamed as LDH-SA-24 and LDH-SA-48 for the reacting time of 24 hand 48 h, respectively, were obtained.

2.4. Preparation of UV cured polymer/LDH nanocomposite

2.4.1. Preparation of UV curable urethane diacrylate/LDH-SA blendThe primary UV curable resin formulation utilized in this study

was a 7:2:1 mixture of EB270:HDDA:TPGDA. The above synthesizedLDH-SA was added into the resin with the respective concentrationof 1, 3 and 5 wt.%, and stirred for 48 h to achieve the complete disper-sion. After toluene was removed under vacuum, a radical fragmentalphotoinitiator, Runtecure 1104, (3 wt.%) was added and stirred for 2 h.All the operations were performed in dark to prevent any unexpected

Fig. 2. 1H NMR spectra of SOH and SAD.

polymerization. The pure organic formulation as reference was obtainedby EB270:HDDA:TPGDA (7:2:1) with 3 wt.% Runtecure 1104.

2.4.2. UV curingThe above obtained UV curable blends were exposed to a medium

pressure mercury lamp (1 kW, FusionUV systems, USA)with the bandconveyer speed of 2.2 m min−1 at the incident light intensity of30 mW cm−2 on the samples. Moreover, the UV cured polymer/LDHnanocomposites containing the LDH-SA with the respective concentra-tion of 1, 3 and 5 wt.% were named as LS1, LS3 and LS5, respectively.

3. Results and discussion

3.1. Preparation and characterization of SOH and SAD

The SOH was synthesized according to the literature (Mori et al.,2003) possessing a diameter of about 2.7 nm and 58 terminal hydrox-yl groups. Theoretically, two terminal hydroxyl groups of SOH werereacted with the semiadduct IPDI-AD, obtaining an aminoundecanoicacid based on silsesquioxane (SAD) as a new anion surfactant. Thesynthetic route of SAD is shown in Scheme 1.

FT-IR and 1H NMR measurements were performed to determinethe molecular structures of SOH and SAD. As shown in Fig. 1, thebroad absorption peak from 3000 to 3800 cm−1 in the FT-IR spec-trum of SOH is assigned to the O\H stretching vibration for the ter-minal hydroxyl group. In addition, a sharp peak between 2840 and2940 cm−1 due to the C\H stretching vibration in the alkyl chain

Fig. 4. XRD patterns of LDH-SAD, LDH-SA-24 and LDH-SA-48.

Fig. 6. Photopolymerization rates and unsaturation conversion in UV cured pure poly-mer and polymer/LDH nanocomposites.

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and a strong absorption band around 1030–1150 cm−1 correspond-ing to Si\O\Si stretching are present. Moreover, the same character-istic peaks occur in the spectrum of SAD except for the new peaksobserved at 1739 cm−1 for C_O and 1567 cm−1 for N\H, indicatingthe appearance of urethane structure from IPDI-AD and ester struc-ture from AD in SAD. This reveals the successful adduction of IPDI-AD to SOH.

The 1H NMR spectra of SOH and SAD are shown in Fig. 2. The char-acteristic peaks for SOH are clearly seen at 0.4–0.9 (−SiCH2–), 1.4–1.9(−SiCH2CH2–), 2.4–3.2 (−NCH2–), 3.4-4.1 (−OCH2–, –OCH–) ppm(Mori et al., 2003). In the spectrum of SAD, new peaks are representedat 0.3–0.6 (−C(CH3)CH2C(CH3)2–), 1.2–1.4 (−CH2(CH2)nCH2–), 1.5–1.8(−CH2CH(NHCOO)CH2–), 2.4–3.2 (−CCH2NHCOO–, –CH2CH2NHCOO–,–CH2CH2COOH), and 3.5–4.2 (−CH2CHNHCOO–) ppm, indicating thesuccessful synthesis of SAD.

3.2. Preparation and characterization of LDH-SAD and LDH-SA

The LDH was intercalated by SAD by the coprecipitation methodand then modified by IPDI-HEA, obtaining the acrylated LDH-SA.

The FT-IR spectra of LDH-SAD and LDH-SA are represented inFig. 3. In both spectra, the broad absorption peak between 3600 and3200 cm−1 attributed to the O-H stretching vibration, characteristicpeaks at 2840 and 2940 cm−1 assigned to –CH2 and –CH3 groupsand a strong absorption peak around 1030–1150 cm−1 correspond-ing to Si\O\Si stretching in SAD chain are observed. Besides, twonew peaks at 1719 and 810 cm−1 occur in the LDH-SA spectrumassigned to the acrylate group from IPDI-HEA, indicating the efficientreaction between hydroxyl and isocyanate group.

Fig. 4 shows the XRD patterns for LDH-SAD, LDH-SA-24 andLDH-SA-48. The LDH-SAD presents a basal spacing of 3.48 nm (2θ=2.5°), which is greatly larger than the spacing of 0.78 nm for pristineLDH-MgAl (Yuan and Shi, 2010). Moreover, the spacing of 3.48 nm isconsistent with the addition of the nanoparticle size of 2.7 nm forSOH and the original spacing of LDH-MgAl. This proves SAD was suc-cessfully intercalated into LDH. The LDH-SA-24 possesses the similarbut weaker XRD diffraction compared to LDH-SAD. Moreover, the dif-fraction peak shows much weaker in the pattern for LDH-SA-48. Thiscan be explained that as IPDI-HEA grafted into the interlayer ofLDH-SAD, the spacing of layers enlarged until the layers lost their or-dered structure.

The successful synthesis of modified LDH was further confirmedby TGA in N2 atmosphere, as shown in Fig. 5. In the TGA curves, theweight loss below 200 °C is related to the loss of physically adsorbedand interlayer water. At above 200 °C, the weight loss of 32.4% forLDH-MgAl is ascribed to the dehydroxylation of LDH layer and theelimination of NO3− and CO3

2−. While the weight loss of 67.2% and

Fig. 5. TGA curves of LDH-MgAl, LDH-SAD and LDH-SA-48 in N2.

76.3% for LDH-SAD and LDH-SA-48, respectively, is attributed to theintercalated SAD and IPDI-HEA. After heating to 600 °C, the residualchars for LDH-MgAl, LDH-SAD and LDH-SA-48 were 53.7%, 27.9%and 21.5%, respectively. The contents of intercalated SAD andIPDI-HEA into LDH interlayer can be estimated as 34.8% and 9.1%, re-spectively, by calculating from the weight loss based on three curvescarefully.

3.3. Photopolymerization behavior of urethane acrylate/LDH-SA-48blend

Fig. 6 shows the photopolymerization rates (Rpmaxs) at the peakmaximums and the final unsaturation conversion (Pf) in the curedpolymer/LDH films compared with acrylate resin without LDH-SA-48 addition. The Rp

maxs of pure polymer, LS1, LS2 and LS3 are 4.2,3.9, 4.2 and 4.5 s, respectively, which is different from the normal sit-uation. Usually, the Rp

max decreases slightly with increasing LDH load-ing in the blend, owing to the increased viscosity and diluted doublebond concentration compared with the pure acrylate resin. However,in this study, the acrylate groups which were intercalated intoLDH-SA-48 have partly counteracted the effects from the diluted dou-ble bond concentration and the increased viscosity, as a small amountof LDH-SA-48 (1 wt.%) was blended, resulting in the highest Rpmax forLS1 sample among the nanocomposites. Even more,the Rp

max of LS1 ishigher than that of the pure polymer. This might be explained thatthe lower double bond concentration in the organic formulationconsisting of EB270 with a molar mass of 1500 g mol−1 was com-pared with the LDH-SA nanofiller modified by IPDI-HEA. The same

Fig. 7. XRD patterns of UV cured polymer/LDH nanocomposites.

Fig. 8. TEM (a) and HR-TEM (b) micrographs of UVcured polymer/LDH nanocomposites at 5 wt.% LDH loading.

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situation happens in the curves of Pf in the UV cured pure polymerand LS1 film. However, as the LDH-SA-48 loading increased further,the Rp

max values for LS3 and LS5 decrease, and that of LS5 is the lowestamong the nanocomposites. This trend is corresponds to the usualcomposite systems. On the contrary, the Pf of LS5 is slightly higherthan that of the pure polymer and LS3. This usually happens to theself-acceleration reaction systems of acrylates.

3.4. Morphology of the UV cured nanocomposite

The structural illustration of the UV cured polymer/LDH nano-composite is shown in Scheme 1. The exfoliated structure formation ofnanocomposite during theUV irradiation can be confirmed by XRDmea-surement, as shown in Fig. 7. The absence of XRD diffraction is observedfor all the nanocomposites at 1, 3 and 5 wt.% LDH-SA-48 loadings.

Fig. 8 shows the TEM and HR-TEM micrographs of LS5 samplewith 5% LDH-SA-48 loading. The dark line represents the LDH platelet.The fine dispersion of LDH in the polymer matrix was observed in theTEM photograph (Fig. 8a) under the lower magnification. While theHR-TEM observation (Fig. 8b) displays the random orientation andlost of order structure of LDH layers, indicating the formation of total-ly exfoliated microstructure of LDH in the UV cured nanocomposite. Itcan be explained that the acrylate groups intercalated in LDHinterlayer take part in the UV-introduced photopolymerization inhigher conversion, resulting in the generation of complete exfoliationhappened to LDH-SA-48 easily.

Fig. 9. TGA curves of UV cured polymer and polymer/LDH nanocomposites in N2.

3.5. Thermal properties of the UV cured nanocomposite

TGA is one of the most important techniques to evaluate the ther-mal decomposition behavior of nanocomposite. The TGA curves of UVcured polymer/LDH nanocomposites in comparison with that of purepolymer without LDH addition are shown in Fig. 9. It can be seen thatthe thermal decomposition performance of all nanocomposites issimilar with the pure polymer except for the raised onset decomposi-tion temperature as the LDH loading increased. The temperature of 5%mass loss is defined as the onset decomposition temperature, and228, 244, 252 and 272 °C, therefore, for the pure polymer, LS1, LS3and LS5, were determined, respectively. Besides, the char residuesdisplay the same trend with 2.2%, 4.5%, 8.7% and 12.4% for the purepolymer, LS1, LS3 and LS5, respectively. Both demonstrate the en-hancement of the thermal stability with the addition of LDH-SA-48,exhibiting even a higher enhancement than normal LDH addition.This can be explained by the thermostable char brought by siliconin SOH and the formed exfoliated nanostructure of LDH composite(Cheng et al., 2009).

The dynamic mechanical thermal analysis (DMTA) was utilized tofurther investigate the thermal property of nanocomposite. The stor-age modulus (E’) and tanδ curves of pure polymer and nanocom-posites are shown in Fig. 10. The detail data are listed in Table 1.The E′ value and the crosslinking density (XDL) increase withLDH-SA loading increasing, achieving the highest value of 44.0 MPaand 4.51×10−3 mol/cm−3 for the film at 5% LDH-SA loading com-pared with 33.4 MPa and 3.40×10−3 mol/cm−3 of pure polymer,

Fig. 10. DMTA curves of UV cured polymer and polymer/LDH nanocomposites.

Table 1DMTA results of UV cured polymer and polymer/LDH nanocomposites.

Sample E′ (MPa) Tg (°C)

Pure polymer 33.4 63.0LS1 35.7 63.5LS3 43.7 64.1LS5 44.0 64.9

Table 2Mechanical properties and hardness of UV cured polymer and polymer/LDHnanocomposites.

Sample Tensile strength(MPa)

Elongation atbreak (%)

Persozhardness (s)

Pencilhardness

Pure polymer 7.23±0.08 16.2±0.2 78 3BLS1 7.72±0.13 16.5±0.3 82 2BLS3 8.15±0.05 16.0±0.2 95 HBLS5 9.41±0.07 17.1±0.4 105 2H

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respectively. This can be explained by the uniform dispersion of LDHin the polymer matrix, resulting in the increase of effective crosslinkingin the nanocomposite. The glass transition temperature (Tg) of acrosslinked material can be determined as the peak temperature oftanδ curve. As listed in Table 1, the Tg values follow the same trendwith E’ and XDL, showing a slight increase compared with the purepolymer with LDH-SA-48 loading increasing. As well known, the inor-ganic addition in an organic material would restrict the motion of mac-romolecular chain, which would make the requisite thermal energy forthe occurrence of glass transition in the hybrid material higher, leadinghigher temperature to achieve the transition. In this case, the exfoliationmorphology of LDH in the polymer matrix and the higher XDL ofnanocomposites both restrict the segmental motion near the organic–inorganic interface, resulting in a tiny rise in Tg from 63.0 °C of thepure polymer to 64.9 °C of LS5.

3.6. Mechanical properties and hardness of the UV cured nanocomposite

The mechanical properties of UV cured nanocomposites are listedin Table 2. The tensile strength increases along with the increase ofLDH loading, which is in agreement with the results obtained fromthe DMTA measurements. Moreover, the percent elongation atbreak almost remains unchanged for all the nanocomposites in com-parison with the pure polymer, owing to the toughening effect intro-duced by silicon of SOH in the LDH interlayer counteracts therestriction from the exfoliated LDH to the polymer chains. Moreover,the pendulum and pencil hardness show great enhancement from78 s and 3B for the pure polymer to 105 s and 2 H for LS5,respectively.

4. Conclusion

A novel LDH nanofiller LDH-SA was synthesized through interca-lated MgAl-based LDH with the pre-synthesized anion surfactantSAD and grafted with the semiadduct IPDI-HEA. The polymer/LDHnanocomposite was obtained by blended LDH-SA into an acrylateresin, following by irradiated using a UV lamp. The XRDmeasurementand TEM observation were performed to confirm the exfoliated micro-structure of the nanocomposite. The mechanical and thermal properties,as well hardness were enhanced in different extents compared with thepure polymer, indicating that the prepared polymer/LDH nanocompositewith LDH-SA addition possesses the combined advantages of both LDHand silsesquioxane.

Acknowledgements

This work was supported by the National Natural Science Foundationof China (grant no. 50973100).

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