Journal of Science: Advanced Materials and Devices (2017) 245e254 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Enhanced mechanical and thermal properties of polystyrene nanocomposites prepared using organo-functionalized NieAl layered double hydroxide via melt intercalation technique Kelothu Suresh, Manish Kumar, G Pugazhenthi*, R Uppaluri Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India a r t i c l e i n f o a b s t r a c t Article history: Received 22 March 2017 Received in revised form May 2017 Accepted 10 May 2017 Available online 17 May 2017 The article reports upon the preparation and characterization of organo-functionalized NieAl layered double hydroxide (LDH)-polystyrene (PS) nanocomposites Initially, pristine NieAl LDH was synthesized via the co-precipitation technique and was subsequently treated using sodium dodecyl sulfate to obtain organo-functionalized NieAl LDH (ONieAl LDH) PS nanocomposites were fabricated by melt intercalation using a twin screw extruder in presence of ONieAl LDH nanofiller (1, 3, 5, and wt.%) The PS nanocomposites were characterized for their structural, thermal and mechanical properties The dispersion and morphology of the obtained PS nanocomposites were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) Mechanical and thermal properties of the PS nanocomposites as a function of LDH content were examined by tensile tests, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) The XRD and TEM results revealed the formation of an exfoliated structure of the PS nanocomposite with wt.% ONieAl LDH loading The maximum improvements of the mechanical and thermal properties of the nanocomposites with ONieAl LDH loading over pristine PS included tensile strength ¼ 34.5% (1 wt.%), thermal decomposition temperatures (T15%) ¼ 27.4 C (7 wt.%), and glass transition temperature (Tg) ¼ 4.3 C (7 wt.%) The PS nanocomposites possessed higher mechanical strength and thermal degradation resistance compared to the pristine PS The activation energy (Ea) and reaction mechanism with respect to thermal degradation of the pristine PS and its nanocomposites were evaluated by the Coats-Redfern and Criado model, respectively © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Ni-Al LDH Polystyrene Nanocomposites Tensile test Thermal degradation kinetics Introduction Layered double hydroxides (LDHs) constitute an important class of hydrotalcite materials Typically, LDH are expressed as [M2ỵ1x M3ỵx(OH)2]xỵ(Am)x/m yH2O, with M3ỵ, M2ỵ and Am designated for trivalent cations (e.g Mn3ỵ, Al3ỵ, Cr3ỵ, Ga3ỵ), divalent cations (e.g Ni2ỵ, Mg2ỵ, Co2ỵ, Cu2ỵ) and interlayer anion (e.g NOÀ , CO3 , ClÀ, OHÀ) Being inexpensive to fabricate and environmentally friendly, such materials are of serious interest Hence, several authors targeted their applications such as catalysts and catalyst precursors [1,2], adsorbents and ion exchangers [3,4], electrode modifiers [5], optical materials [6], precursors for preparing CO2 adsorbents [7], fire retardant additives [8], host materials to * Corresponding author Fax: ỵ91 361 2582291 E-mail address: pugal@iitg.ernet.in (G Pugazhenthi) Peer review under responsibility of Vietnam National University, Hanoi facilitate drug delivery [9], additives in cement [10] and polymer/ LDH nanocomposites [11] Very recently, their potential as nanofillers for preparing polymer nanocomposites has received considerable attention due to their significantly enhanced properties such as thermal stability, mechanical properties, flame retardency and impermeability to gas [8,12] compared with polymer matrix Usually, nanocomposites are fabricated using in-situ polymerization, solution intercalation and melt intercalation methods [13] Compared to all other mentioned methods, melt intercalation is promising due to high productivity, low cost, environmentally benign (elimination of organic solvents) and ease to adapt conventional polymer processing techniques (e.g.: extrusion and injection moulding) A brief outline of fabrication aspects associated to polymer nanocomposites can be presented as follows Paul et al [14] and Yeh et al [15] prepared clay based PS nanocomposites using solution intercalation technique and in-situ thermal polymerization method, respectively The authors inferred that the nanocomposites http://dx.doi.org/10.1016/j.jsamd.2017.05.003 2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 246 K Suresh et al / Journal of Science: Advanced Materials and Devices (2017) 245e254 possessed enhanced thermal stability due to nanofiller incorporation in the polymer matrix Adopting melt intercalation method, Limpanart et al [16] prepared clay based conventional and intercalated PS nanocomposites Solvent blending coupled with sonication was adopted by Morgan et al [17] for similar nanocomposite fabrication The authors inferred that in the solvent blending process, sonication contributed significantly towards exfoliation of the nanocomposite structure Adopting emulsion polymerization method, Noh and Lee [18] prepared the PS/clay nanocomposites (30 wt.% MMT) and observed that glass transition temperatures (Tg) of the nanocomposites is C higher than that of the pristine PS PS/ ZneAl LDH nancomposites were prepared by He et al [19] using solvent blending technique For the nanocomposites with wt% ZneAl LDH loading, crystallization temperature was C higher than that of the polymer For similar nanocomposites prepared by Qiu et al [20] using solution intercalation method, considering 50% weight loss as a reference point, an enhancement of 17 C was reported for thermal stability of 20 wt.% inorganic loaded nanocomposites in comparison with the pristine PS Du et al [21] obtained nylon 6/MgeAl LDH nanocomposites via melt intercalation of nylon into part of organomodified MgeAl LDH interlayers Their XRD results demonstrated that too high LDH loading makes it difficult for LDH layers to undergo exfoliation Chen and Wang [22] prepared polypropylene (PP) composites by melt blending method and studied the thermal decomposition kinetics of the nanocomposites They utilized Coats-Redfern and Criado model to measure the activation energy and determine the reaction mechanism of PP nanocomposites, respectively Given significant fabrication research emphasis in the inorganic-polymer nanocomposites, it can be observed that the synthesis of PS/ONieAl LDH nanocomposites using melt intercalation has not been conducted till date The work primarily focuses upon sodium dodecyl sulfate based modification of pristine NieAl LDH for the enhancement of its compatibility with the polymer matrix The subsequent section addresses the experimental methods adopted towards the synthesis of said PS nanocomposite for targeted enhancement in thermal and mechanical properties the first step involved calcination of 2.5 g of pristine NieAl LDH powder at 500 C (using Box furnace) with 10 C/min heating rate and atmospheric pressure The calcined gray colored powder was dissolved in 120 mL of 2.5 g SDS containing solution and was refluxed for 12 h at 80 C After this step, the residue was separated using a centrifuge and washed thoroughly using Millipore water to remove adhering SDS Finally, after drying at 70 C, the ONieAl LDH sample was grounded as powder to serve as a starting material for PS nanocomposite fabrication 2.3 Preparation of PS/ONieAl LDH nanocomposites A co-rotating twin-screw extruder was used to prepare PS/ ONieAl LDH nanocomposites using melt compounding method Initially, the moisture from pristine PS and ONieAl LDH was removed by drying the grounded samples for 16 h at 60 C and 70 C, respectively Further, the ONieAl LDH (1, 3, and wt.% with respect to pristine PS) was dispersed in 100 mL methanol (see Table 1) The solution was sprayed on a precise quantity of pristine PS pellet for ensuring uniform distribution of ONieAl LDH on the pristine PS matrix Subsequent drying step for 24 h facilitated methanol evaporation from the samples The coated PS pellets were fed to the extruder (Model: ZV-20 HI-Torque; Make: Specifiq Engineering and Automats, India) that was operated at temperature values of 185, 195, 210 and 200 C, for feed, metering, compression and adaptor zones, respectively After operation, extrudate was thoroughly quenched with water at ambient condition and was cut into pellets and dried using hot air oven at 60 C for 24 h Thereafter, dried pellets were fed to injection molding equipment (Model: 180 High Pressure; Make: JSW, Japan) that was operated at 195e210 C This step was followed to achieve samples for mechanical characterization studies As controls and reference set, a clean PS sample was also prepared using similar procedure without ONieAl LDH In summary, the experimental investigations allowed fabrication of samples designated as NieAl LDH, PS, PSNL 1, PSNL 3, PSNL and PSNL to represent nickel-aluminum LDH, polystyrene, PS/ONieAl LDH (wt.%), PS/ONieAl LDH (wt.%), PS/ONieAl LDH (wt.%), and PS/ONieAl LDH (wt.%), respectively Experimental 2.1 Materials PS polymer was procured from National Chemicals Ltd., Gujarat (India) Nickel nitrate (Ni(NO3)2$6H2O), sodium nitrate (NaNO3), aluminum nitrate (Al(NO3)3$9H2O), sodium hydroxide (NaOH), sodium dodecyl sulfate (SDS), methanol (CH3OH) were purchased from Merck India Ltd 2.2 Preparation of ONieAl LDH The pristine NieAl LDH was prepared with co-precipitation technique The solution mixture containing nickel, aluminum and sodium nitrate was synthesized with the molar ratio of 2:1:2, respectively Thereby, M NaOH solution was added to the mixture under constant stirring condition to enhance the solution pH to 10 Eventually, stirring was continued for 16 h at ambient temperature condition After this step, filtration was carried out and the retained precipitate was washed thoroughly using Millipore water (Model: Elix 3, Milli-Q; Make: M/s Millipore, USA) up to achieving neutral pH condition of the washed solution After 16 h of room temperature stabilization, the precipitate was collected in a petridish followed by drying for 16 h at 60 C For further use, dried pristine LDH was thoroughly grinded to result as a powder The prepared pristine NieAl LDH was organo-functionalized through regeneration method to obtain ONieAl LDH To so, 2.4 Characterization X-ray diffractometer (Model no: D8 Advance; Make: Bruker, Germany) facilitated with Cu-Ka radiation (0.15406 nm wavelength) and Ni filter at room temperature was used to obtain XRD profiles of ONieAl LDH, pristine PS and PS nanocomposites Further, the structural morphology of the PS nanocomposites was evaluated with transmission electron microscopy (TEM) analysis, using TEM instrument (Model: JEM 2100; Make: JEOL, Japan) operated at 200 kV accelerating voltage Fourier transform infrared (FTIR) analyser (Model: IRAffinity1; Make: Shimadzu, Japan) was used to record FTIR spectra at room temperature in the range 400e4000 cmÀ1 Mechanical characterization studies involved evaluation of impact strength, tensile, and flexural properties of the nanocomposites Specimens having dimensions of 168 Â 13 Â mm3 of pristine PS and the nanocomposites were evaluated for tensile strength and Table Preparation chart for PS/ONieAl LDH nanocomposites Sample LDH loading (wt.%) LDH (g) PS (g) PS PSNL PSNL PSNL PSNL 10 30 50 70 1000 990 970 950 930 K Suresh et al / Journal of Science: Advanced Materials and Devices (2017) 245e254 modulus using ASTM D 638 method and INSTRON (M 3382, UK) universal testing machine The nanocomposites along with pristine PS specimens of dimension of 126 Â 13 Â mm3 were taken for flexural tests were carried out using universal testing machine INSTRON (M 3382, UK) and ASTM D790 test method Impactometer (M/s Tinius Olsen, USA) was used to conduct impact strength measurements (specimens having dimension of 62 Â 13 Â mm3) For each case, six specimens were tested and average value has been reported Thermal stability of the prepared samples was determined under nitrogen atmosphere with a flow rate of 60 mL/min using high temperature thermo-gravimetric (TG) analyser (Model no: TGA 851e/LF/1100; Make: Mettler Toledo, Switzerland) at 10 C/min heating rate and 30e700 C temperature range DSC instrument (Model no: 1; Make: Mettle Toledo, Switzerland) operated at C/ heating rate under nitrogen environment (and 40 mL/min) was used to determine glass transition temperature of pristine PS and other nanocomposite samples Results and discussion 3.1 XRD analysis XRD analysis facilitates the evaluation of the degree of intercalation and exfoliation in LDHs based nanocomposites In such materials, these features are dependent upon several factors such as LDH composition, organic modifier's chemical nature and method of fabrication For intercalated nanocomposites, d-spacing (defined as d003 or 003 peak is shifted to lower angle) is higher than that of the original LDH The exfoliated nanocomposites exhibit well separated inorganic layer from one another with no peaks corresponding to basal plan (003) and good distribution of nanofillers in the polymer matrix The intercalation extent of LDH in PS is based on the LDH interlayer spacing evaluated using Bragg's law (gallery height or d003 spacing determined using expression nl ¼ 2dsin q, where n ¼ and l correspond to X-ray wave length (1.5406 Å)) [23] Fig depicts XRD diffractograms of ONieAl LDH, pristine PS and PS/ONieAl LDH nanocomposites The XRD patterns convey that for ONieAl LDH, d003 peak exists at 6.54 with a basal spacing of 1.35 nm (see Fig 1(a)) The amorphous nature of pristine PS is characteristically reflected in two small halos that are centered at 2q values of 10 and 20 [24] No peaks corresponding to d003 at lower angles can be observed for all PS nanocomposite samples (Fig 1(cef)), thereby conveying that the LDH layers might be 247 exfoliated or delaminated in the PS matrix However, to further confirm such ambiguity, electron microscopy based analysis is to be considered for the nanocomposite morphology 3.2 TEM analysis For the PS nanocomposites, the extent of dispersion and distribution of ONieAl LDH fillers in the polymer matrix has been investigated using TEM microscopy Fig depicts the LDH layers microstructure in the PS matrix Fig 2(a) depicts TEM image for PSNL sample and the pertinent dark lines and bright region signify LDH galleries and PS matrix, respectively Black arrows have been presented in the figure to indicate LDH layers Based on the image analysis, it can be inferred that LDH layers got totally exfoliated in the matrix and not exhibit ordered stack structure The TEM image of PSNL sample depicted in Fig 2(b) involves arrows and circle mark to signify exfoliated and intercalated structure, respectively Thus, the TEM image of the sample conveys that the sample refers to the partially exfoliated structure Similar morphology with mixed exfoliation characteristics was reported by Alansi et al [25] for PS/LDH nanocomposites with 4% loading of inorganic filler (MgeAl LDH) The TEM image of PSNL and PSNL nanocomposites depicted in Fig 2(c) and (d), respectively constitute circles indicating intercalated regions and stacked LDH layers This conveys that the intercalated structure is attained at higher doping of the nanofiller Similar intercalated structures have been reported by Qiu et al [20] for PS/ZneAl LDH nanocomposites for higher values of inorganic filler loading 3.3 FTIR analysis The FTIR analysis involved comparative assessment of FTIR spectra of PS nanocomposites and pristine PS Fig depicts the spectra obtained from FTIR analysis for ONieAl LDH, pristine PS and PS nanocomposites For ONieAl LDH (Fig 3(a)), lattice vibration bands have been observed at 400-800 cmÀ1 (NieO, AleO, OeAleO modes) [26] The ONieAl LDH has a characteristic peak at 1063 cmÀ1 (asymmetric vibration (vo S]O) of sulfate from dodecyl sulfate anion), 1218 cmÀ1 (symmetric vibration (vS]O) of sulfate from dodecyl sulfate anion) [27], 2850 cmÀ1 (stretching vibration of CH3 found in the modifier SDS), 2920 cmÀ1 (stretching vibration of CH2 found in the modifier SDS), 3528 cmÀ1 (OeH stretching vibration of metal hydroxide layer and interlayer water molecules) [25] Fig 3(b) affirmed upon the existence of several absorption bands in pristine PS These refer to the wavelengths of 698 cmÀ1 (monosubstituted benzene), 1368, 1453 cmÀ1 (vibrational mode of CH2 bending), 1496, 1504 cmÀ1 (C]C bending vibration), 2930 cmÀ1 (aliphatic CeH stretching vibration), 3070 cmÀ1 (aromatic CeH stretching vibration) Compared with ONieAl LDH (Fig 3(a)) and pristine PS (Fig 3(b)), the PS nanocomposite (PSNL 1, PSNL 3, PSNL and PSNL 7) samples (Fig 3(cef)) exhibit fewer new absorption peaks at 1218 cmÀ1 (symmetric vibration of sulfate from dodecyl sulfate anion), 3528 cmÀ1 (OeH stretching vibrations) and 400e800 cmÀ1 (lattice vibration bands (NieO, AleO, OeAleO modes)) The FTIR analysis of various samples and their comparative assessment indicates that LDH layers are well dispersed in the PS polymer matrix Similar trends have been reported by Wang et al [28] for MMT dispersed in PS polymer matrix 3.4 Tensile properties Fig XRD patterns of (a) ONieAl LDH, (b) pristine PS, (c) PSNL 1, (d) PSNL 3, (e) PSNL and (f) PSNL nanocomposites A primary goal of LDH nanofiller reinforcement into the PS matrix is to enhance mechanical properties of the nanocomposites in conjunction with the pristine PS As LDH loading increases, tensile strength and modulus of nanocomposites increases This is 248 K Suresh et al / Journal of Science: Advanced Materials and Devices (2017) 245e254 Fig TEM images of (a) PSNL 1, (b) PSNL 3, (c) PSNL and (d) PSNL nanocomposites Fig FTIR spectra of (a) ONieAl LDH, (b) pristine PS, (c) PSNL 1, (d) PSNL 3, (e) PSNL and (f) PSNL nanocomposites due to the incorporation of LDH into the composite system which enhances stiffness and rigidity of polymer nanocomposites A sharp enhancement in tensile modulus for very small LDH loading (1 wt.%) can be observed in the figure However, the subsequent enhancement in tensile modulus was not significant upto wt.% LDH loading In this regard, it can be observed that Genity et al [29] conveyed that the filler modulus, loading and aspect ratio are three main factors to enhance the composite modulus A composite with high stiffness needs filler particles possessing higher combinations of modulus and aspect ratio at higher filler loading In line with this hypothesis, in the carried out work, with minimal LDH loading (1 wt.%), significant enhancement in tensile modulus of the PS/ ONieAl LDH nanocomposites has been possible This is possibly due to higher combinations of aspect ratio and LDH modulus Fig depicts the tensile strength and modulus profiles of PS and PS/ LDH nanocomposites Compared to pristine PS and all other nanocomposites, PSNL sample exhibited highest tensile strength of 39.01 MPa (36.29 MPa for PSNL 3, 32.30 MPa for PSNL 5, 30.22 MPa for PSNL and 29.01 MPa for PS samples) Stronger interfacial interactions (between ONieAl LDH and PS) facilitated due to maximum exfoliation in the structure are responsible for maximum tensile strength values at lower LDH loading (1 wt.%) Similar trends have been reported by Li et al [30] for ZnO nanoparticles embedded polyurethane polymer composites The marginal reduction in tensile strength for higher loading of inorganic nanofiller (3e7 wt.%) is possibly due to non-uniform distribution of LDH layers in the polymer along with agglomeration and segregation to thereby allow formation of weak spots in the nanocomposites Similar behavior has been reported by Uthirakumar K Suresh et al / Journal of Science: Advanced Materials and Devices (2017) 245e254 249 polymer nanocomposite materials Chow et al [33] also inferred upon the enhancement in flexural strength for nanocomposites compared with pristine polymer Fig depicts trends in flexural modulus of PS and PS nanocomposite Similar trends have been obtained for flexural modulus, given the fact that flexural strength of both pristine PS and PS nanocomposites underwent similar variations with variant concentration of nanofiller loading The PSNL nanocomposite possessed 12% higher flexural modulus value than that of the PS polymer PSNL 3, and samples with higher ONieAl LDH loading did not indicate even significant enhancement Reasons for such trends are similar to those presented for the flexural strength 3.6 Impact strength Fig Tensile properties of pristine PS and PS nanocomposites et al [31] for clay loaded PS polymer nanocomposites Further, Tanniru et al [32] elaborated that clay filler aggregation in polyethylene nanocomposites facilitates concentration of stress field at the aggregates thereby triggering easy crack propagation and rapid premature failure The tensile modulus profiles of pristine PS and its nanocomposites have been demonstrated in Fig In comparison with pristine PS, the tensile modulus for all PS nanocomposites exhibit significant enhancement with the highest tensile enhancement for PSNL sample (23% higher value) PSNL samples with higher inorganic filler loading (3e7 wt.%) exhibit marginally lower tensile strength than PSNL sample Reasons for the same are similar to those presented for the tensile strength profiles 3.5 Flexural properties Fig depicts trends in flexural strength and modulus of pristine PS and PS nanocomposites (1e7 wt.% ONieAl LDH) The trends confirm that PS/ONieAl LDH nanocomposites exhibit improved tensile strength in comparison with pristine PS While pristine PS possessed 54.08 MPa flexural strength, PSNL possessed highest flexural strength of 68.95 MPa followed by PSNL (64.46 MPa), PSNL (59 MPa) and PSNL (55.59 MPa) samples This is due to homogeneous and uniform distribution of LDH in the polymer matrix that resulted in maximum exfoliation in the nanocomposite structure at wt.% loading of the inorganic nanofiller At higher loading, LDH agglomeration might have reduced the flexural strength Also, several factors such as extent of intercalation, orientation and distribution of LDH platelets in stress/load direction have been reasoned to influence the flexural properties of Fig Flexural properties of pristine PS and PS nanocomposites Fig shows impact strength profiles for PS and PS/ONieAl LDH nanocomposites Compared to pristine PS, the impact strength for all PS nanocomposites enhanced significantly The impact strength for PS/ONieAl LDH sample with wt.% LDH loading is 22% higher in comparison with pristine PS Yuan et al [34] reported 50% enhancement in impact strength for 0.32 wt.% nanofiller loaded PSm-MWCNT nanocomposites Similarly, Yilmazer and Ozden [35] opined that 1.6 wt.% organoclay loaded PS nanocomposites possessed 4% higher impact strength than that of pure PS For PSNL sample uniform and homogenous distribution of LDH in the polymer matrix is reasoned to be crucial for measured enhancement in the impact strength For higher loading of ONieAl LDH cases (3, and wt.%), impact strength did not enhance significantly compared to PS samples and was lower than that of the PSNL sample Non-uniform distribution of inorganic filler and agglomeration are possible reasons for the reduction in impact energy absorption for the samples with higher inorganic loading 3.7 DSC analysis The DSC analysis was conducted to evaluate the effect of nanofiller on the molecular mobility of PS chains in the polymer nanocomposite This is typically reflected in the glass transition temperature (Tg) of the sample For various samples including pristine PS and PS composites, results obtained from DSC analysis have been presented in Fig and the Tg has been evaluated as the value corresponding to inflection point of onset and end-set temperature profiles For PS, PSNL 1, PSNL 3, PSNL and PSNL 7, the Tg values are 68.1, 70.5, 71.2, 72.1 and 72.4 C, respectively This indicates that ONieAl LDH addition into the PS matrix enhanced Tg Fig Impact strength of pristine PS and PS nanocomposites 250 K Suresh et al / Journal of Science: Advanced Materials and Devices (2017) 245e254 Fig DSC analysis of pristine PS and PS nanocomposites gradually with increasing inorganic filler content For the PSNL sample, the Tg value is 4.3 C higher that of the pristine PS sample This is probably due to the restriction or hindrance of polymer chain motion in the nanocomposite that has been brought forward due to inclusion of the inorganic nanofiller Similar insights have been presented by Zidelkheir et al [36] who inferred that the 10 wt.% clay-PS nanocomposites possessed C higher Tg values than the pristine PS polymer 3.8 TGA analysis TGA analysis was conducted to evaluate the comparative thermal degradation/stability of PS/ONieAl LDH nanocomposites in comparison with pristine PS Fig (a) depicts TGA profiles of pristine PS, ONieAl LDH and all nanocomposite samples For the ONieAl LDH sample, significant weight loss from room temperature up to 200 C is due to evaporation of physically adsorbed and interlayer water molecules (Fig 8(1)) Subsequent weight loss from 200 to 350 C is due to decomposition of interlayer dodecyl sulfate Above 350 C, weight loss occurred due to LDH degradation to NieAl oxides Considering 15% mass loss as reference point, the thermal decomposition temperature of ONieAl LDH is 639 C, which is in agreement with the value reported in the literature [37,38] For pristine PS sample (Fig 8(2)), thermal degradation occurs at 345e445 C and residue did not exist above 480 C Fig 8(3e6) depicts TGA curves of PSNL 1, PSNL 3, PSNL and PSNL nanocomposite samples The PS nanocomposite TGA profiles involved simpler variations in comparison to the pristine PS TGA profiles This is due to PS nanocomposite degradation in two stages namely first stage decomposition (135e330 C) and second stage decomposition (330e460 C) The first stage decomposition occurred due to evaporation of physically adsorbed and interlayered water molecules and thermal degradation of sodium dodecyl sulfate molecules The second stage involved thermal decomposition of polymer molecules to form charred (black) residues The TGA curves also exhibit thermal degradation rate of PSNL 1, 3, and samples and convey that these are slower than that of the pristine PS sample This is due to significant interactions between pristine PS and LDH layers that contribute towards higher diffusional resistance for oxygen and volatile compounds [8] Beyond 500 C, the TGA curves for all samples are flat thus indicating that only inorganic constituents are left behind in the sample Quantitative information with respect to mass loss at various temperature values has been presented in Table for pristine PS and all polymer nanocomposite samples It could be inferred from Table that the degradation temperature (Td) corresponding to 15% mass loss (T15%) of PS nanocomposites samples depicts significant enhancement with respect to pristine PS, thereby indicating increased thermal stability Moreover, the enhancement can be observed to be proportional to ONieAl LDH content in the PS polymer The T15% for pristine PS is 358 C, which got further enhanced by 11, 20.1, 25.3 and 27.4 C for PSNL 1, 3, and samples respectively The observed enhancement in higher degradation temperature is due to the incorporation of the inorganic nanofiller in PS polymer matrix [39] Among all nanocomposites, PSNL exhibited highest thermal stability (Table 2) Such behavior is reasoned due to the presence of barrier effect of LDH lamellar layers which limit emission of produced degradation gases and heat transmission and eventually result in improving thermal stability of nanocomposite materials In the nanocomposite, volatile products have to take a long and tortous path around impermeable clay platelets that are distributed in PS matrix as compared to pristine PS, where the diffusion of volatile products is much easier Table Thermal degradation temperatures of pristine PS and PS nanocomposites Sample Temperature at 15% weight loss (T15) C DT15% ( C) Tmax ( C) PS PSNL PSNL PSNL PSNL 358.0 369.0 378.1 383.3 385.4 e 11.0 20.1 25.3 27.4 415.7 418.1 420.5 421.7 422.5 Fig (a) TGA curves of (1) ONieAl LDH, (2) pristine PS, (3) PSNL 1, (4) PSNL 3, (5) PSNL and (6) PSNL nanocomposites (Inset shows the TGA profiles between 335 and 435 C) (b) TGA derivative of (1) pristine PS, (2) PSNL 1, (3) PSNL 3, (4) PSNL and (5) PSNL nanocomposites K Suresh et al / Journal of Science: Advanced Materials and Devices (2017) 245e254 Fig 8(b) illustrates DTG curves of PS and PS nanocomposites The peaks in Fig 8(b) convey the maximum degradation temperature (Tmax) of the prepared samples It could be inferred from Fig 8(b) that Tmax peaks of the DTG curves of PS nanocomposites exhibit shift towards higher temperature as compared to pristine PS, thus indicating enhancement in thermal stability The Tmax for pristine PS, PSNL 1, PSNL 3, PSNL and PSNL nanocomposites have been evaluated to be 415.7, 418.1, 420.5, 421.7, and 422.5 C, respectively For the PSNL nanocomposite, Tmax is 6.8 C higher than that of pristine PS (see Table 2) Thus, both TGA and DTG analysis based trends essentially confirm upon enhanced thermal 251 stability of the polymer due to the incorporation of LDH layers in the polymer matrix 3.9 Kinetics analysis 3.9.1 Coats-Redfern method for kinetics analysis The kinetic analysis of thermal degradation process for pristine PS, PSNL 1, PSNL 3, PSNL and PSNL nanocomposites has been conducted to understand degradation behavior of composites in comparison with pristine PS Coats-Redfern method [40] was deployed for the kinetic analysis Compared to other kinetic Fig Determination of kinetic parameters by plots of the left part in Equation (1) against 1/T using Coats-Redfern method 252 K Suresh et al / Journal of Science: Advanced Materials and Devices (2017) 245e254 Table Thermal degradation kinetics of pristine PS and PS nanocomposites obtained from Coats-Redfern method Sample Ea (kJ/mol) A PS PSNL PSNL PSNL PSNL 88.1 108.5 125.4 132.2 134.8 7.95 4.70 5.99 2.01 3.15 Â Â Â Â Â 105 107 108 109 109 n R2 IPDT ( C) 0.2 0.4 0.4 0.4 0.4 0.996 0.997 0.997 0.998 0.998 379.1 387.1 401.1 408.9 413.2 models, the method requires only single but not multiple heating data Relevant model expressions in the method are presented as follows: n¼1 lnð1 À aÞ AR 2RT Ea ¼ ln À À ln À Ea bEa RT T2 (1a) ns1 À ð1 À aÞ1Àn ln ð1 nịT ! ẳ ln AR 2RT Ea 1À À Ea bEa RT (1b) where, Ea, n, A, R and T refer to apparent activation energy, order of reaction, pre-exponential factor, gas constant and absolute temperature, respectively Using the Coats-Redfern method, for each TG curve corresponding to a precise heating rate, kinetic parameters (Ea, n and A) can be obtained To so, the left side expressions in Equations (1a) and (1b) have been plotted with respect to T1 for visualization as a straight line plot with slope and intercept as Activation Energy and pre-exponential factor, respectively In this regard, it is important to note that the intercept is determined by considering the expression À 2RT as To determine the value of n, different values of n Ea Fig 10 Determination of the thermal degradation mechanism by plotting Z(a) versus a using Criado model: (a) pristine PS, (b) PSNL 1, (c) PSNL 3, (d) PSNL and (e) PSNL nanocomposites K Suresh et al / Journal of Science: Advanced Materials and Devices (2017) 245e254 253 have been assumed and regression coefficient of plots corresponding to various n values have been compared and the n value for which best regression coefficient value was obtained has been inferred as the n value for the measured data Fig depicts linear fitness plots of Coats-Redfern model for PS, PSNL 1, PSNL 3, PSNL and PSNL nanocomposites and different n values The obtained kinetic parameter values including A, Ea and n for the prepared samples are enlisted in Table The evaluated Ea values for PS, PSNL 1, PSNL 3, PSNL and PSNL samples are 88.1, 108.5, 125.4, 132.2, and 134.8 kJ/mol Thus, the activation energy for nanocomposite degradation enhanced by 20.4e46.7 kJ/ mol in comparison with the value obtained for pristine PS This is due to the enhanced thermal stability of the nanocomposite samples 3.9.2 Criado method for reaction mechanism determination Criado method [41] involves reaction mechanism determination using Ea, A and n evaluated from Coats-Redfern method Relevant expressions for the solid-phase reactions are as follows: b da Ea Zaị ẳ gaị eRT A dt Zaị ẳ da Ea Ea eRT Pxị dt R where, Pxị ẳ (2) (3) ex x3 ỵ 18x2 ỵ 86x ỵ 96 x x4 ỵ 20x3 ỵ 120x2 ỵ 240x ỵ 120 In the above equations, while Equation (2) is used to obtain master Z(a) versus a curves [42], Equation (3) is used to represent experimental Z(a) versus a curve Thereby, comparative assessment of master Z(a) versus a curve with experimental Z(a) versus a curve to predict pertinent reaction mechanism of thermal degradation process Analysis was conducted for pristine and all nanocomposite samples Figs 10(aee) illustrate master and experimental Z(a) versus a curves for PS, PSNL 1, PSNL 3, PSNL and PSNL samples For pristine PS (Fig 10(a)), the experimental curve is in good agreement with the master curve of Z(F1), thus indicating that the pristine PS thermal degradation process follows F1 reaction mechanism (random nucleation of one nucleus on individual particle) For the nanocomposite samples (Figs 10(bee)), the initial phase involved thermal decomposition as per F1 reaction mechanism (a < 0.4) followed with gradual transition to A4 mechanism (nucleation and growth for a ¼ 0.7e0.9) Thus, it is apparent that the thermal degradation mechanism involved a shift at higher temperature 3.10 Integral procedural decomposition temperature Integral procedure decomposition temperature (IPDT) method [43] was used for thermal stability evaluation of prepared pristine PS and PSNL 1, PSNL 3, PSNL and PSNL nanocomposite samples Relevant expressed as presented as follows: IPDT Cị ẳ B N Tf Ti ỵ Ti where, N ẳ Bẳ (4) R1 þR2 Þ ðR1 þR2 þR3 Þ; ðR1 þ R2 Þ ; ðR1 Þ where N, Tf and Ti refer to area ratio of total experimental curve specified by the total TGA thermogram, the final and initial experimental temperature Fig 11 depicts a graphical Fig 11 Schematic diagram of Doyle's method for determining IPDT representation of typical TGA thermogram in terms of three areas (R1- R3) Using parameters of these regions, the IPDT values for all samples were determined using Equation (4) The IPDT values for PS, PSNL 1, PSNL 3, PSNL and PSNL samples are 379.1, 387.1, 401.1, 408.9 and 413.2 C Thus, PS composites possess higher IPDT values in comparison with pristine PS and hence comparatively improved thermal stability Among all samples, PSNL possessed highest IPDT values (Table 3) Conclusion Melt intercalation facilitated in twin screw extruder equipment was followed in this work to fabrication PS nanocomposites with variant constitution of ONieAl LDH (1, 3, and wt.%) XRD and TEM analyses affirmed good dispersion of ONieAl LDH layers in the PS matrix TGA analysis affirmed enhanced thermal decomposition temperature for samples with enhanced inorganic nanofiller content Compared to pristine PS, enhancement in tensile strength, flexural strength and impact strength is 34.5%, 27.5% and 22%, respectively With 15% mass loss as reference point, thermal decomposition temperature of PS nanocomposites has been determined to be 11.0e27.4 C higher than that of the pristine PS polymer The DSC graphs revealed marginal improvement in the glass transition temperature (2.4e4.3 C) for the nanocomposites The Coats-Redfern method based activation energy values have been estimated to be 20.4e46.7 kJ/mol higher than that of the pristine PS Reaction mechanism of nanocomposites using Criado method indicated F1 reaction mechanism at lower degradation temperature followed with A4 reaction mechanism at higher temperature The IPDT value of PS nanocomposites increases with an increase in the ONieAl LDH concentration confirmed by TGA data In summary, PS/ONieAl LDH nanocomposites possessed superior mechanical strength and thermal stability parameters in comparison with pristine PS References [1] B.M Choudary, M.L Kantam, V Neeraja, K.K Rao, F Figueras, L Delmotte, Layered double hydroxide fluoride: a novel solid base catalyst for CeC bond formation, Green Chem (2001) 257e260 [2] B.M Choudary, S Madhi, N.S Chowdari, M.L Kantam, B Sreedhar, Layered double hydroxide supported nanopalladium catalyst for Heck-, Suzuki-, Sonogashira-, and Stille-type coupling reactions of chloroarenes, J Am Chem Soc 124 (2002) 14127e14136 [3] Y You, H Zhao, G.F Vance, Adsorption of dicamba (3,6-dichloro-2-methoxy benzoic acid) in aqueous solution by calcined-layered double hydroxide, Appl Clay Sci 21 (2002) 217e226 [4] N Kozai, T Ohnuki, S Komarneni, Selenium oxyanions: highly selective uptake by a novel anion exchanger, J Mater Res 17 (2002) 2993e2996 254 K Suresh et al / Journal of Science: Advanced Materials and Devices (2017) 245e254 [5] J Qiu, G Villemure, Anionic clay modified electrodes: electron transfer mediated by electroactive nickel, cobalt or manganese sites in layered double hydroxide films, J Electroanal Chem 428 (1997) 165e172 [6] K Takagi, T Shichi, H Usami, Y Sawaki, Controlled photocycloaddition of unsaturated carboxylates intercalated in hydrotalcite clay interlayers, J Am Chem Soc 115 (1993) 4339e4344 [7] Q Wang, Z Wu, H.H Tay, L Chen, Y Liu, J Chang, Z Zhong, J Luo, A Borgna, High temperature adsorption of CO2 on Mg-Al hydrotalcite: effect of the charge compensating anions and the synthesis pH, Catal Today 164 (2011) 198e203 [8] C Nyambo, P Songtipya, E Manias, M.M.J Gasco, C.A Wilkie, Effect of MgAllayered double hydroxide exchanged with linear alkyl carboxylates on fireretardancy of PMMA and PS, J Mater Chem 18 (2008) 4827e4838 [9] A.C.S Alcantara, P Aranda, M Darder, E.R Hitzky, Bionanocomposites based on alginate-zein/layered double hydroxide materials as drug delivery systems, J Mater Chem 20 (2010) 9495e9504 [10] J Plank, D Zhimin, H Keller, F Hossle, W Seidl, Fundamental mechanisms for polycarboxylate intercalation into C3A hydrate phases and the role of sulfate present in cement, Cem Concr Res 40 (2010) 45e57 [11] F Leroux, J.P Besse, Polymer interleaved layered double hydroxide: a new emerging class of nanocomposites, Chem Mater 13 (2001) 3507e3515 [12] V Realinho, M Antunes, D Arencon, A.I Fernandez, J.I Velasco, Effect of a dodecylsulfate-modified magnesium-aluminum layered double hydroxide on the morphology and fracture of polystyrene and poly(styrene-coacrylonitrile) composites, J Appl Polym Sci 111 (2009) 2574e2583 [13] S.S Ray, M Okamoto, Polymer/layered silicate nanocomposites: a review from preparation to processing, Prog Polym Sci 28 (2003) 1539e1641 [14] P.K Paul, S.A Hussain, D Bhattacharjee, M Pal, Preparation of polystyreneclay nanocomposite by solution intercalation technique, Bull Mater Sci 36 (2013) 361e366 [15] J.M Yeh, S.J Liou, C.G Lin, Y.P Chang, Y.H Yu, C.F Cheng, Effective enhancement of anticorrosive properties of polystyrene by polystyrene-clay nanocomposite materials, J Appl Polym Sci 92 (2004) 1970e1976 [16] S Limpanart, S Khunthon, P Taepaiboon, P Supaphol, T Srikhirin, W Udomkichdecha, Y Boontongkong, Effect of the surfactant coverage on the preparation of polystyrene-clay nanocomposites prepared by melt intercalation, Mater Lett 59 (2005) 2292e2295 [17] A.B Morgan, J.D Harris, Exfoliated polystyrene-clay nanocomposites synthesized by solvent blending with sonication, Polymer 45 (2004) 8695e8703 [18] M.W Noh, D.C Lee, Synthesis and characterization of PS-clay nanocomposite by emulsion polymerization, Polym Bull 42 (1999) 619e626 [19] F.A He, L.M Zhang, F Yang, L.S Chen, Q Wu, New nanocomposites based on syndiotactic polystyrene and organo-modified ZnAl layered double hydroxide, J Polym Res 13 (2006) 483e493 [20] L Qiu, W Chen, B Qu, Structural characterisation and thermal properties of exfoliated polystyrene/ZnAl layered double hydroxide nanocomposites prepared via solution intercalation, Polym Degrad Stab 87 (2005) 433e440 [21] L Du, B Qu, M Zhang, Thermal properties and combustion characterization of nylon 6/MgAl-LDH nanocomposites via organic modification and melt intercalation, Polym Degrad Stab 92 (2007) 497e502 [22] Y Chen, Q Wang, Thermal oxidative degradation kinetics of flameretarded polypropylene with intumescent flame-retardant master batches in situ prepared in twin-screw extruder, Polym Degrad Stab 92 (2007) 280e291 [23] J.M Hwu, T.H Ko, W.T Yang, J.C Lin, G.J Jiang, W Xie, W.P Pan, Synthesis and properties of polystyrene-montmorillonite nanocomposites by suspension polymerization, J Appl Polym Sci 91 (2004) 101e109 [24] R Botan, T.R Nogueira, F Wypych, L.M.F Lona, In situ synthesis, morphology, and thermal properties of polystyrene-MgAl layered double hydroxide nanocomposites, Polym Eng Sci 52 (2012) 1754e1760 [25] A.M Alansi, W.Z Alkayali, M.H Al-qunaibit, T.F Qahtan, T.A Saleh, Synthesis of exfoliated polystyrene/anionic clay MgAl-Layered double hydroxide: structural and thermal properties, RSC Adv (2015) 71441e71448 [26] L Wang, S Su, D Chen, C.A Wilkie, Variation of anions in layered double hydroxides: effects on dispersion and fire properties, Polym Degrad Stabil 94 (2009) 770e781 [27] F.R Costa, A Leuteritz, U Wagenknecht, D Jehnichen, L Haubler, G Heinrich, Intercalation of Mg-Al layered double hydroxide by anionic surfactants: preparation and characterization, Appl Clay Sci 38 (2008) 153e164 [28] H.W Wang, K.C Chang, J.M Yeh, S.J Liou, Synthesis and dielectric properties of polystyrene-clay nanocomposite materials, J Appl Polym Sci 91 (2004) 1368e1373 [29] P.M McGenity, J.J Hooper, C.D Paynter, A.M Riley, C Nutbeem, N.J Elton, J.M Adams, Nucleation and crystallization of polypropylene by mineral fillers: relationship to impact strength, J M Polym 33 (1992) 5215e5224 [30] J.H Li, R.Y Hong, M.Y Li, H.Z Li, Y Zheng, J Ding, Effects of ZnO nanoparticles on the mechanical and antibacterial properties of polyurethane coatings, Prog Org Coat 64 (2009) 504e509 [31] P Uthirakumar, M.K Song, C Nah, Y.S Lee, Preparation and characterization of exfoliated polystyrene/clay nanocomposites using a cationic radical initiator-MMT hybrid, Eur Polym J 41 (2005) 211e217 [32] M Tanniru, Q Yuan, R.D.K Misra, On significant retention of impact strength in clay-reinforced high-density polyethylene (HDPE) nanocomposites, Polymer 47 (2006) 2133e2146 [33] W.S Chow, L.Y Khim, A.T Kang, Flexural properties of polystyrene/organomontmorillonite masterbatch composites, J Reinf Plast Compos 27 (2008) 255e261 [34] J.M Yuan, Z.F Fan, X.H Chen, X.H Chen, Z.J Wu, L.P He, Preparation of polystyreneemultiwalled carbon nanotube composites with individual-dispersed nanotubes and strong interfacial adhesion, Polymer 50 (2009) 3285e3291 [35] U Yilmazer, G Ozden, Polystyrene-organoclay nanocomposites prepared by melt intercalation, in situ, and aasterbatch methods, Polym Compos 27 (2006) 249e255 [36] B Zidelkheir, S Boudjemaa, M.A Goad, B Djellouli, Preparation and characterization of polystyrene/montmorillonite nanocomposite by melt intercalative compounding, Iran Polym J 15 (2006) 645e653 [37] S Lv, W Zhou, H Miao, W Shi, Preparation and properties of polymer/LDH nanocomposite used for UV curing coatings, Prog Org Coat 65 (2009) 450e456 [38] F Malherbe, J Besse, Investigating the effects of guest-host interactions on the properties of anion-exchanged Mg-Al hydrotalcites, J Solid State Chem 155 (2000) 332e341 [39] B.A Bhanvase, D.V Pinjari, P.R Gogate, S.H Sonawane, A.B Pandit, Synthesis of exfoliated poly(styrene-co-methyl methacrylate)/montmorillonite nanocomposite using ultrasound assisted in situ emulsion copolymerization, Chem Eng J 181e182 (2012) 770e778 [40] A.W Coats, J.P Redfern, Kinetic parameters from thermogravimetric data, Nature 201 (1964) 68e69 [41] J.M Criado, J Malek, A Ortega, Applicability of the master plots in kinetic analysis of a non-isothermal rate, Thermochim Acta 147 (1989) 377e385 [42] E Urbanovici, C Popescu, E Segal, Improved iterative version of the CoatsRedfern method to evaluate non-isothermal kinetic parameters, J Therm Anal Calorim 58 (1999) 683e700 [43] C.D Doyle, Estimating thermal stability of experimental polymers by empirical thermogravimetric analysis, Anal Chem 33 (1961) 77e79