Research ArticleSurface Modification of Sodium MontmorilloniteNanoclay by Plasma Polymerization and Its Effect onthe Properties of Polystyrene Nanocomposites

Rosa Idalia Narro-Céspedes ,1 María Guadalupe Neira-Velázquez ,2

Luis FernandoMora-Cortes,1 Ernesto Hernández-Hernández,2

Adali Oliva Castañeda-Facio,1 María Cristina Ibarra-Alonso,1

Yadira Karina Reyes-Acosta,1 Gustavo Soria-Arguello,2 and José Javier Borjas-Ramos2

1Departamento de Polımeros, Facultad de Ciencias Quımicas, Universidad Autonoma de Coahuila,Blvd. Venustiano Carranza y Jose Cardenas Valdez, 25280 Saltillo, COAH, Mexico2Departamento Sıntesis de Polımeros, Centro de Investigacion en Quımica Aplicada (CIQA), Blvd. Enrique Reyna Hermosillo,Colonia San Jose de los Cerritos, 25294 Saltillo, COAH, Mexico

Correspondence should be addressed to Rosa Idalia Narro-Cespedes; rinarro@uadec.edu.mxand Marıa Guadalupe Neira-Velazquez; guadalupe.neira@ciqa.edu.mx

Received 24 January 2018; Accepted 19 March 2018; Published 23 April 2018

Academic Editor: Alessandro Pegoretti

Copyright © 2018 Rosa Idalia Narro-Cespedes et al. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Sodium montmorillonite nanoclay (Na+-MMT) was modified by plasma polymerization with methyl methacrylate (MMA) andstyrene (St) as monomers and was denominated as Na+-MMT/MMA and Na+-MMT/St, respectively. This plasma modifiednanoclay was used as reinforcement for polystyrene (PS) nanocomposites that were prepared by melt mixing. Pristine andmodifiedNa+-MMTnanoclay were analyzed by the dispersion in various solvents, Fourier transform infrared spectroscopy (FTIR),thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results confirmed a change in hydrophilicity ofthe modified Na+-MMT, as well as the presence of a polymeric material over its surface. The pristine PS/Na+-MMT and modifiedPS/Na+-MMT/MMA and PS/Na+-MMT/St nanocomposites were studied with X-ray diffraction (XRD), differential scanningcalorimetry (DSC), and TGA, as well as mechanical properties. It was found that the PS/Na+-MMT/St nanocomposites presentedbetter thermal properties and an improvement in Young’s modulus (YM) in compared to PS/Na+-MMT/MMA nanocomposites.

1. Introduction

Research and development of polymeric materials with bet-ter properties have led to the reinforcement of polymericmatrices with particles at nanometer scale, which facilitatesthe exchange and transference of properties between thephases [1, 2]. The success of nanostructures as reinforcementagents is because of the intrinsic properties of each typeof nanoparticle and low levels of loadings compared toconventional reinforced composites and, above all, becauseof their high surface area-volume ratio.

Na+-MMT has been one of the reinforcement agentsmost used in polymeric and other matrices to obtain

nanocomposites with unique properties such as a gas andliquid barrier, flame retardant, and mechanical improve-ment [2–9]. Na+-MMT is a swelling clay with three layers,which has a great surface and an elevated capacity forcation exchange; with Na+ and Ca2+ as typical exchangeablecations, for this, it is also used to remove componentsin contaminated waters [6–10]. Other different nanostruc-tures than clays like carbon nanotubes (CNTs), noble metalnanoparticles, nanofibers, and so forth are equally used toobtain polymeric nanocomposites [11–17]. However, one ofthe best advantages of Na+-MMT nanoclay is that it is amaterial that is found in great abundance in the five conti-nents, for which its cost-performance ratio is considerably

HindawiJournal of NanomaterialsVolume 2018, Article ID 2480798, 13 pageshttps://doi.org/10.1155/2018/2480798

2 Journal of Nanomaterials

low with respect to many other nanometer-scale structures[10].

On the other hand, even if the physical and performanceproperties of nanocomposites can be improved upon addi-tion of nanostructure reinforcement agents, the experimentalvalues of force and efficiency are still lower than theoret-ical predictions because of aggregation and compatibilityproblems between the interfacial union of nanoparticles andpolymeric matrices. Within the most used techniques topromote the interaction between phases is the ion exchangeof the dispersive phase with organophilic cations [18–20].Another line of investigation is the use of a third substancethat serves as a phase compatibilist [21, 22]. Afterwards is themixture process. In this stage, there is in situ polymerization,solution mixing, and melt blending as the main techniquesof nanocomposite preparation which have revealed favor-able levels of dispersion [23, 24]. Particularly polymer/claycomposites are generally divided in three types: conventionalcomposite materials, where the clay acts as a normal rein-forcement material [23, 25], intercalated nanocomposites,where a small quantity of polymer moves into the galleryspaces of the clay [26, 27], and exfoliated nanocomposites,where the sheets of clay are completely dispersed in a polymermatrix [27, 28].

In the improvement of the dispersive phase, a recentalternative is polymerization on the nanoparticle by surfacemodification with plasma [5, 16, 17, 29, 30]. Plasma presentscertain advantages in this area compared to conventionalsurface modification methods. It is a relatively simple, rapid,dry, and environment-friendly technique that is used tomodify the surface of a great variety of materials. Even ifthis technique is normally used to modify the surface ofdifferent polymeric substrates, it has been used with successto modify different nanoparticles and other surfaces likezinc oxide, iron oxide, aluminum oxide, titanium oxide,carbon nanofibers (CNFs), CNTs, nanoclays [5, 7, 15, 16, 31–36], and quartz [37, 38]. Treatment with plasma can add ananometric film that covers nanoparticles, modifying theirsurface properties. If the plasma modification is carriedout by using a monomer (plasma polymerization), this cangenerate a polymeric coating like the specific polymericmatrix over the nanoparticle with a desired increase in sur-face interaction, promoting an improvement in the adhesionbetween phases and the transference of properties betweenthem [15, 16]. For example, Jarvis and Majewski [37, 38]have recently investigated changing the surface of quartzparticles from hydrophobic to a hydrophilic character byusing plasma treatment of allylamine. Longer polymerizationtimes increased the number of positively charged groups andtheir hydrophilic behavior.

Reported works using plasma modification of nanoclaysand other nanoparticles to prepare polymer composites havepresented favorable improvements in particle dispersion aswell as thermal, mechanical, and gas barrier properties [5,15, 16, 39–41]. According to Ramos-de Valle et al., thesedispersion improvements have allowed the investigation ofthe effects of styrene plasma modified CNFs to investigatethe effects on the tensile properties of polystyrene/CNFcomposites [5]. In this report, the plasma treatment greatly

enhances the nanofiber dispersion within the PS matrix byhelping the nanofiber compatibilization, which in turn highlyincreased the tensile modulus.

However, in the reported investigations, there have beenno references that include the comparative study of modifiedNa+-MMT with St (Na+-MMT/St) and with MMA (Na+-MMT/MMA) added to PS for the purpose of evaluating thedifferences between the modifications on the two modifiednanoclays and their interaction in the PS matrix.

2. Experimental

2.1. Materials. The used polymer was general purpose PS,with a molecular weight, 𝑀𝑤 = 168.000 g/mol, and a glasstransition temperature (𝑇𝑔) of 98

∘C.TheNa+-MMTnanoclaywas acquired from Southern Nanoclay Products, Inc., andpresented the following characteristics: purity = 99.5%, tac-toid size = 2–13 𝜇m, sheet width = 1 nm, and sheet longitude= 150–600 nm. The monomers that were used for plasmapolymerization were MMA (CH2=C(CH3)COOCH3) andSt (C6H5-CH=CH2). Said monomers were acquired fromSigma-Aldrich with a purity of 99.9%.

2.2. Plasma Polymerization. Na+-MMT was surface-modi-fied by plasma polymerization using MMA monomer andSt monomer. To achieve this, the MMA and St monomerswere polymerized in a plasma reactor at low pressure. Themodification of Na+-MMT with these monomers was donein order to change the hydrophilic nature of the nanoclay tohydrophobic and thus promote greater compatibility with thePS matrix which is hydrophobic to obtain thus nanocompos-ites with better mechanical properties.

The plasma reactor used for modification of Na+-MMTwas composed mainly of a power controller coupled to aradiofrequency generator of 13.56MHz, a vacuum pump, acontrolled gas flow valve, and a 500mL glass flask, whichwas used as a reactor. A copper wire that acts as aninductive electrode was placed around the glass flask andone end connected to the radiofrequency generator (AdvanceEnergy RFX600). Figure 1 illustrates the plasma reactor.The process of plasma polymerization of MMA and St tomodify Na+-MMT was done as follows: 1.5 g of Na+-MMTwas introduced in the glass flask and the vacuum pump wasturned on. The initial internal base pressure of the systemwas 10 Pa. Afterwards, the MMA or St monomer flow washeld until the pressure of the system was kept constant at17 Pa. This pressure leads to a constant flow of MMA or Stat 1.25 cm3/min. The Na+-MMT nanoclay was modified atdifferent plasma power intensities (50, 100, and 150W) andeach one applied the following times: 1, 1.5, and 2 h. Thisreaction was done with constant agitation bymagnetic stirrerto obtain a surface modification of Na+-MMT with plasmapolymers of polymethyl methacrylate (PMMA) or PS.

2.3. Preparation of Nanocomposites. The nanocompositeswere prepared in a Brabender mixture chamber using CAM-type rotators at 190∘C with 60 rpm during 10min. In allcases, 75mL chamber was filled to 93% of its capacity,(which is 70mL). The PS/Na+-MMT nanocomposites were

Journal of Nanomaterials 3

Cold trap

Vacuum pump

Controllingvalve

Pressure sensor

Monomer

Radio frequencygenerator

Glass reactor

Figure 1: Illustration of the plasma reactor used for the surface modification of Na+-MMT.

prepared with a 3%wt. concentration of Na+-MMT and 97%of PS. The mixing procedure was the following: the PS wasintroduced to the mixing chamber. When the PS was melted,the pristine Na+-MMT and modified Na+-MMT (Na+-MMT/MMA and Na+-MMT/St) were introduced during aperiod of 2min; after this, the mixing was continued during8 more min to complete the mixing time of 10min. Thenthe nanocomposites were extracted, ground, and moldedby compression molding to obtain plaques of 150 × 150 ×3mm3. Finally, using the ASTM-D638 norm, the sampleswere cut for mechanical testing. The rest of the materialwas used for thermal characterization of the PS/Na+-MMTnanocomposites.

2.4. Solvent Dispersion. The pristine Na+-MMT and mod-ified Na+-MMT nanoclays were examined by dispersionin various solvents with different solubility parameters (𝛿):water (H2O), chloroform (CHCl3), and tetrahydrofuran(THF), with 𝛿 of 23.5, 9.52, and 9.21, respectively. In thisprocedure, 3mg of the Na+-MMT was placed in 10mL ofeach solvent and stirred. After 5min, pictures were taken toobserve the dispersion grade.

2.5. FTIR Spectroscopy. A Nicolet Magna 550 FT-IR spec-trophotometer was used to obtain infrared spectra andobserve the functional groups present in the pristine Na+-MMT and modified Na+-MMT nanoclays. The sampleswere previously dried in a vacuum oven at 100∘C during12 h. Each sample was supported with potassium bromide(KBr).

2.6. Scanning Electron Microscopy (SEM). The micrographicandmorphological analysis of the nanoclay samples (pristineand plasma treated) was carried out by SEM in a fieldemission scanning electron microscope (FESEM, HitachiSU8010) at a voltage of 3 kV for the pristine, while for theplasma modified nanoclay it was of 10 kV.

2.7. Thermogravimetric Analysis. Thermal stability of Na+-MMT and the nanocomposites was obtained with aQ500 thermogravimetric instrument at a heating velocity

of 10∘C/min within the range of 25–600∘C in nitrogenatmosphere at 50mL/min andwithin the range of 600–800∘Cin oxygen atmosphere to favor the oxidation process.

2.8. X-Ray Diffraction. The effect of pristine and plasmamodified Na-MMT on the structural properties of PS wasdetermined using Wide Angle X-ray Diffraction (WAXD)in a Siemens D500. Pristine and plasma treated Na-MMTwere also analyzed by WAXD. The equipment consists ofa Cu anode with a wavelength of 1.5406 A. The sampleswere analyzed in a range of 2𝜃 from 2 to 35∘. The runswere performed with a pitch size of 0.5 and time of5.0 sec.

2.9. Differential Scanning Calorimetry. Thermal analysis ofthe nanocomposites was also carried out by DSC analysis ofTA Instruments model MDCS-2920. A small amount of thenanocomposite of about 10mg was used for each experiment.The samples were heated twice; the first heating was usedto erase the thermal history of the sample. The heating ratewas 10∘C/min. During the second heating, the samples wereheated to a temperature of 180∘C at 10∘C/min. The glasstransition (𝑇𝑔) temperature of the sample was obtained fromthe thermograms.

2.10. Mechanical Properties. The tensile-strain propertieswere studied with a Universal machine model 3M-10,equipped with a charging cell of 450 kg at an extensionvelocity of 5mm/min. All of the tests were done in accor-dance with the ASTM-D638 norm. All of the samples weremodified for 40 h in a 50% relative humidity environment. Sixsamples were tested for each set of data and the average wasreported.

2.11. UV Spectroscopy. The UV analysis of the pristine PSand the nanocomposites weremeasured (300–800 nm)with aUV-visible Spectrophotometer (UV-Vis-NIR Cary 5000); thebackground was run in air. Films of neat PS and PS/MMTnanocomposites of ∼300 𝜇m thickness were used for opticalclarity measurements.

4 Journal of Nanomaterials

Pristine 1 h 1.5 h 2 h

T = 0 mim ．；+-MMT/MMA

(a)

Pristine 1 h 1.5 h 2 h

．；+-MMT/MMAT = 5 mim

(b)

Pristine

．；+-MMT/MMA

T = 0 mim

Modified

50W 100W 150W

(c)

Pristine

．；+-MMT/MMA

T = 5 mim

Modified

50W 100W 150W

(d)

Figure 2: Water dispersions of pristine Na+-MMT and Na+-MMT/MMAmodified by plasma at 50W for 1, 1.5, and 2 h: (a) before agitationand (b) after 5min of agitation. Water dispersions of pristine Na+-MMT and Na+-MMT/MMA modified by plasma for 1.5 h at differentplasma powers: (c) before agitation and (d) after 5min of agitation.

3. Results and Discussion

3.1. Characterization of Nanoclay

3.1.1. Dispersion Tests. For the dispersion tests, 𝛿 of thepolymers and solvents were used in this work. 𝛿 is a measureof the attractive strength between molecules of the material;it allows the prediction ofmiscibility. It is also called cohesionparameter, a term which is often preferred to solubilityparameter when referring to nonliquid materials such aspolymers [42–44]. This is a qualitative estimation to theinteraction degree between materials, since the organic filmsdeposited on the Na+-MMT using MMA and St monomersthat crosslink due to plasma polymerization cause the modi-fied Na+-MMT to interact in solvents with similar solubilityparameter. This study is expected to see an increase ininteraction between the nanoclay modified with plasma andthe matrix, when 𝛿 of both phases are the same. In previousstudies, this polymeric coating has already been confirmed onother different nanomaterials using plasma polymerization[5, 15, 39].

In Figure 2, the dispersions in water at room temperatureof pristine Na+-MMT and of modified Na+-MMT/MMAbefore (Figure 2(a)) and after agitation (Figure 2(b)) bothwith different times of plasma polymerization are presented.In these images, we can observe that the pristineNa+-MMT ishighly hydrophilic and disperses well in water after agitation.

This is due to the high content of Na+ cations present inthe clay which have been hydrated (solvated with water,Figure 2(b)) [27, 28, 45]. ForNa+-MMT/MMA,with differenttime of plasma modification, it is observed that they stayon the surface of the water before and after agitation, whichidentifies their hydrophobic property; the presence of thepolymer on the surface of Na+-MMT indicates a differencein 𝛿 between the coating of PMMA (𝛿 = 9.25) and H2O (𝛿 =23.5), which avoids the diffusion of water between the sheetsand consequently their interaction with the sodium ions.In this figure, a part of Na+-MMT/MMA were sedimented,which indicated that a fraction of them agglomerated duringthe modification and are denser than water.

According to the results of Figures 2(a) and 2(b), there isno significant difference due to the time treatments betweenthe behaviors of the modified Na+-MMT nanoclay. Thissuggests that the treatment time only will increase thethickness of polymer films (PMMA or PS) deposited on thesurface of Na+-MMT. Figures 2(c) and 2(d) show that Na+-MMT/MMA treated at 50, 100, and 150W remains on thewater surface before and after agitation. It is noted that allthemodifiedNa+-MMTnanoclays treated at different plasmapowers have the same hydrophobicity; this is due to thefact that the plasma coatings probably present very similarchemical structures at any power used in the reaction.

Figure 3 shows the dispersion in water at room tempera-ture of pristine Na+-MMT and Na+-MMT modified with St

Journal of Nanomaterials 5

Pristine

．；+-MMT/St

Modified

50W 100WT = 0 min

(a)

T = 5 min

Pristine

．；+-MMT/St

Modified

50W 100W

(b)

Figure 3: Water dispersions of pristine Na+-MMT and Na+-MMT/St modified by plasma for 1.5 h: (a) before agitation and (b) after 5min ofagitation.

Pristine

．；+-MMT/MMA

T = 5 mim

chloroform

50W 100W 150W

(a)

Pristine

T = 5 min chloroform

50W 100W

．；+-MMT/St

(b)

．；+-MMT/MMA

T = 5 minTHF

Pristine 50W 100W 150W

(c)

T = 5 mim THF

Pristine 50W 100W

．；+-MMT/St

(d)

Figure 4: Chloroform dispersions after 5min of agitation of (a) pristine Na+-MMT and Na+-MMT/MMA and (b) pristine Na+-MMT andNa+-MMT/St modified by plasma for 1.5 h. THF dispersions after 5min of agitation of (c) pristine Na+-MMT and Na+-MMT/MMA and (d)Na+-MMT and Na+-MMT/St modified by plasma for 1.5 h.

plasma (Na+-MMT/St) in water before agitation (Figure 3(a))and after 5min of agitation (Figure 3(b)) both at 50 and 100W.According to the images, Na+-MMT/St is more hydrophobicthan that with the coating of PMMA; this can be due toPS presenting a solubility parameter (𝛿 = 9.12) lower thanPMMA (𝛿 = 9.25). Also, unlike PMMA, the coating of PSdoes not present electron fluctuations (instantaneous dipoles)because of the absence of molecules with free electronpairs that can present interaction with the hydrogen ofwater.

Figure 4 shows the vials that contain the pristine Na+-MMT, Na+-MMT/MMA, and Na+-MMT/St dispersed inCHCl3 (Figures 4(a) and 4(b)) and THF (Figures 4(c) and4(d)). These figures show that the pristine Na+-MMT nan-oclay dispersed in CHCl3 is deposited on the bottom almostimmediately, while those modified with MMA plasma or Stplasma stay dispersed after 5min of resting; meanwhile thepristine and modified Na+-MMT dispersed in THF show agood interactionwith the solvent (Figures 4(c) and 4(d)).Thisis because the THF is a dipolar aprotic protophilic solvent,

6 Journal of Nanomaterials

1.0

0.8

0.6

0.4

0.2

0.0

Tran

smitt

ance

4000 3500 3000 2500 2000 1500 1000

Wavenumber (＝Ｇ−1)

(b)

(a)

Al-OH

O-H

O-H

C-H C=O

Si-OH

．；+-MMT

．；+-MMT/MMA 100 W

Figure 5: FTIR spectra of (a) pristine Na+-MMT and (b) Na+-MMT/MMAmodified by plasma for 1.5 h.

which means it is capable of accepting protons, given theunshared electron pairs of the oxygen atomwhich give Lewis-base features and enable the THF interaction with a widerange of compounds.

In general, it was observed that the dispersion of Na+-MMT/MMA or Na+-MMT/St was poor with solvents of highpolarity like water 𝛿 = 23.5, while their dispersion was betterin a medium of low polarity such as THF (𝛿 = 9.52) orchloroform (𝛿 = 9.21), as 𝛿 of PMMA and PS are 9.25 and9.12, respectively. 𝛿 are very similar to the last two solventscausing a good dispersion between them [5, 15, 39].

The dispersion tests do not give a quantity of Na+-MMT modification degree; however, they give a clear ideaon whether the modification of Na+-MMT nanoclay wasaccomplished or not.

3.1.2. FTIR Spectroscopy. In Figure 5, the FTIR spectra ofpristine Na+-MMT and Na+-MMT/MMA are shown. Inthese spectra, the previously reported characteristic signals ofnanoclay can be observed [36, 46].The signal at 3640 cm−1 isattributed to the Al-OH bond stretching, while the presenceof absorbed humidity is observed at 3440 cm−1. The band at1640 cm−1 corresponds to the bending of the OH proceedingfrom the water situated on the surface layers of the nanoclay.The bands that are found in 1117 cm−1 and 1039 cm−1 aregenerated by the stretching of the Si-O bond out of andwithin the plane, respectively. Comparing spectra “a” and “b”from Figure 5, it can be observed that there is a decrease inintensity of the signals of Al-OH and OH, indicating a slightdehydration anddehydroxylation of the nanoclay;meanwhilethe band located at 2947 cm−1 is due to the stretching of theC-H bond. Finally, the presence of PMMA on the surfaceof Na+-MMT/MMA nanoclay is defined by the carbonyl(C=O) group signal of the PMMA ester at 1745 cm−1 and a

1.0

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0.6

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4000 3500 3000 2500 2000 1500 5001000

Wavenumber (＝Ｇ−1)

(b)

(a)

Al-OH

O-H

O-H

C-H

C-H

C-H

=C-H aromatic

C=C

Si-OH

．；+-MMT/St

．；+-MMT

100 W

Figure 6: FTIR spectra of (a) pristine Na+-MMT and (b) Na+-MMT/St modified by plasma for 1.5 h.

band corresponding to the C-O stretching between 1300 and1050 cm−1.

In Figure 6, the spectra of pristine Na+-MMT (describedpreviously) and Na+-MMT/St are shown. Comparing thesespectra, it can be observed that there is an increase in the Al-OH/OH signal ratio in Na+-MMT/St (Figure 6(b)) regardingNa+-MMT/MMA in Figure 5, which is caused by the changein chemical environment of the Na+-MMT nanoclay by thepresence of PS. In relation to the deposit of the plasmapolymer, a low intensity band can be seen at 3025 cm−1corresponding to the aromatic C-H bond and a band at1600 cm−1 corresponding to aromatic C=C bond. The bandssituated at 2919 cm−1 and 2849 cm−1 refer to the C-H andCH2 bonds, with which the presence of these characteristicgroups of PS on the surface of Na+-MMT/St nanoclay can beconfirmed.

Even if the presence of characteristic bands of PMMAandPS can be appreciated by infrared, it is important to point outthat plasma polymerization occurs only on the surface of thenanoparticle. Figures 5 and 6 confirmed the presence of thecoatings of plasma polymers of PMMA and PS on the surfaceof the nanoclay.

3.1.3. Thermogravimetric Analysis. In Figure 7, the thermo-grams of pristine Na+-MMT, Na+-MMT/MMA, and Na+-MMT/St nanoclays are shown. According to Ros et al. (2002)[47] and Toebes et al. (2004) [48], weight loss between50 and 160∘C is mainly attributed to humidity loss. Thedifference in humidity loss between the pristine Na+-MMTand modified Na+-MMT nanoclay can be attributed to thedecrease in water absorption of the modified nanoclay dueto the presence of the plasma coating at the surface of Na+-MMT/St or Na+-MMT/MMA; the plasma coating acts asa water barrier, and coated nanoclay absorbs less amountof water. Na+-MMT/MMA exhibit an average (𝑋) weightloss of 8.25% between 200 and 400∘C; this is related to the

Journal of Nanomaterials 7

X200−400∘C = 8.25%

．；+-MMT

100

98

96

94

92

90

88

86

84

82

80

78

Wei

ght l

oss (

%)

Temperature (∘C)

5.8%200

∘C

200∘C

6.5%10.7%

13.3%

400∘C

400∘C

0 100 200 300 400 500 600 700 800

X200−400∘C = 9.99%

．；+-MMT/MMA 100 W

．；+-MMT/St 100 W

Figure 7: TGA curves of pristine Na+-MMT and Na+-MMT/MMAand Na+-MMT/St modified for 1.5 h.

thermal degradation of the polymeric coating deposited onthe Na+-MMT/MMA nanoclay. For the sample modifiedwith St, the average weight loss of Na+-MMT/St is slightlylarger (9.99%) than the one observed in Na+-MMT/MMA(8.25%) at the same temperature interval; this result indicatesthat the thickness of the plasma coating deposited on Na+-MMT/St was slightly higher than the coating deposited onNa+-MMT/MMA.

3.1.4. Morphological Analysis by SEM. SEM micrographs ofpristine and treated nanoclays are presented in Figure 8.Figure 8(a) shows an image of Na+-MMT without treatment,where it is possible to appreciate the leaf-shaped morphologyof the nanoclay. The coating with plasma polymer covalentlybound to Na+-MMT and this is what is observed in Figures8(b) and 8(c) for both Na+-MMT/MMA and Na+-MMT/St,respectively. The observed roughness at the surface of thenanoclay, clearly confirms that plasma coatings of PMMAand PS have been deposited on the surface of Na+-MMT.Said images indicate that the thickness of the coating isnanometric; by means of a standard measurement it couldbe calculated that the thickness for the Na+-MMT/MMA is∼10 nm,while for theNa+-MMT/St it is∼15 nm in someparts,while in others it is ∼20 nm, exhibiting that the deposit ofthe coating is not completely homogenous on the surface.Guadalupe et al. (2008) [15] have already shown in theirwork that the 100W treatment for 30 minutes resulted in thecrosslinking of PMMA by plasma but on the surface of theCNF and that it was also confirmed by SEM.

3.2. Characterization of Polymer Nanocomposites

3.2.1. X-Ray Diffraction Analysis. Figure 9 shows the X-ray diffractograms of PS/Na+-MMT, PS/Na+-MMT/MMA,and PS/Na+-MMT/St nanocomposites. In the X-ray diffrac-tograms, all PS nanocomposites present only two defined

peaks, which are shown at 10 and 20∘ of 2𝜃which correspondto the pristine Na+-MMT according to the JCPDS cards (29-1498); the polymer, in this case PS, is amorphous and does notpresent any signal.

In samples of PS/Na+-MMT/MMAand PS/Na+-MMT/Stnanocomposites, the results indicate that there is no evi-dence of a significant level of exfoliation or intercalationof Na+-MMT modified by plasma in the PS/Na+-MMTnanocomposites, possibly because the coating formed byplasma on the nanoclay does not allow the introduction ofthe St or MMA polymer in-between the nanoclay galleries.This, however, does not contradict the surface interactionresults demonstrated by the article which are attributed tothe compatibility of the coating of the nanoclayNa+-MMT/Stwith the PS matrix, as well as the shear stress during meltmixing [39].

3.2.2. Thermal Stability. For the thermogravimetric analy-sis, the temperatures of degradation for all polymers wereobtained at 10% loss in weight as can be seen in Figure 10,where for the PS/Na+-MMT/St nanocomposite it is 402.9∘C,for PS/Na+-MMT/MMA nanocomposite it is 387.5∘C, andfor the PS it is 377.5∘C. These results are in accordance withprevious reports, where nanoparticles have been mixed withpolymers in order to improve their thermal properties suchas CNFs modified with plasma methyl methacrylate in aPMMAmatrix [15] or nanoclays dispersed in a polypropylenematrix [23]; however, in none of them, there is mention ofnanocomposites formulated by the materials combinationand methodologies of this investigation, giving a new sci-entific contribution about polymeric nanocomposites, espe-cially those formed by plasma modification of the dispersedphase.

The results obtained by TGA show an improvement inthermal stability of PS in presence of modified or unmodifiedNa+-MMT. It is clear that the incorporation of the nanoclayand its dispersion by mixing give the thermal stability to thePS, but the polymerization with St plasma on the surfaceof the Na+-MMT presents a better interaction with the PSmatrix, since both present the same functional group (St) thatinteract within themselves, provoking a better compatibilitybetween both phases, promoting an improvement in thedispersion of the analyzed sample. Consequently, it causessignificant improvement in the transfer of thermal propertiesfrom the Na+-MMT/St to the PS polymer, as shown inFigure 10.

Referring to pristine PS/Na+-MMT, an improvement inthermal stability is observed when compared to that ofPS and PS/Na+-MMT/MMA; this suggests that the pris-tine Na+-MMT nanoclay has a better dispersion in thePS matrix than the PS nanocomposite prepared with Na+-MMT/MMA nanoclay (PS/Na+-MMT/MMA). This can beexplained by the increase in repulsion energy of the PS andNa+-MMT/MMA nanoclay compared to the pristine Na+-MMT nanoclay, provoking more agglomeration of the Na+-MMT/MMA, lowering the transfer of thermal properties.The increase in repulsion energy is due to the difference inthe solubility parameter, as mentioned in Dispersion Tests(Section 3.1.1).

8 Journal of Nanomaterials

(a)

10

nm

Coatin

g

(b)

CoatingCoating

20nm

15 nm

(c)

Figure 8: SEM micrographs of (a) pristine Na+-MMT, (b) Na+-MMT/MMA plasma coated at 150W for 1.5 h, and (c) Na+-MMT/St coatedat 150W for 1.5 h.

Nor

mal

ized

inte

nsity

(au)

5 10 15 20 25 30

2()

PS/．；+-MMT

PS/．；+-MMT/MMA W 1.5 h100

PS/．；+-MMT/St W 1.5 h100

Figure 9: X-ray diffraction patterns of pristine PS/Na+-MMT,PS/Na+-MMT/MMA, and PS/Na+-MMT/St nanocomposites.

The appreciable difference of weight loss between 450and 500∘C for the PS and the PS/Na+-MMT nanocompositesconfirms the presence of the Na+-MMT nanoclay. In thenanocomposites, it is observed that around 465∘C the weight

100

90

80

70

60

50

40

30

20

10

0

300 325 350 375 400 425 450 475 500

382.5∘C

402.9∘C

377.5∘C

3% Weight

PS

Wei

ght l

oss (

%)

PS/．；+-MMT

PS/．；+-MMT/MMA W 1.5 h100

PS/．；+-MMT/St W 1.5 h100

Temperature (∘C)

Figure 10: TGA curves of PS and pristine PS/Na+-MMT, PS/Na+-MMT/MMA, and PS/Na+-MMT/St nanocomposites.

loss is completely ceased, due to the thermal resistance of thenanoclay, leaving around 3% of Na+-MMT weight.

3.2.3. Glass Transition Temperature. In Figure 11, the DSCthermograms for pristine PS and PS/Na+-MMT nanocom-posites are shown. Since PS is amorphous, it does not present

Journal of Nanomaterials 9

Hea

t flow

(a.u

.)

60 80 100 120 140

100∘C

103.9∘C

PSPS/．；

+-MMTPS/．；

+-MMT/MMA W 1.5 h100

PS/．；+-MMT/St W 1.5 h100

Temperature (∘C)

Figure 11: Curves obtained by DSC of PS and pristine PS/Na+-MMT, PS/Na+-MMT/MMA, and PS/Na+-MMT/St nanocompos-ites.

a melting point, so upon analysis we can only observe itsglass transition temperature (𝑇𝑔), which is around 100∘C[49]. The most marked shift of 𝑇𝑔 is shown by the PS/Na+-MMT/St composite, where 𝑇𝑔 increases approximately from100.1∘C to 103.9∘C with respect to the pristine PS. Theendothermic slope shift is due to the interaction of the PSmolecules with the nanoclay that has beenmodifiedwith a PScoating thanks to St plasmamodification, which provokes thepolymerization upon Na+-MMT. 𝑇𝑔 shift observed confirmsthe interfacial adhesion between the PS polymericmatrix andthe modified Na+-MMT nanoclay. This phenomenon is notobserved with the PS/Na+-MMT and PS/Na+-MMT/MMAnanocomposites, which demonstrates the lack of interfacialinteraction, as explained in Figure 14. The results aboutthe increase of 𝑇𝑔 are similar to previous investigations ofnanocomposites with clay/polymer formulations [49].

3.2.4. Mechanical Properties. Figure 12 shows the tensile-strain diagrams of PS/Na+-MMT, PS/Na+-MMT/MMA, andPS/Na+-MMT/St nanocomposites. PS is a polymer that doesnot suffer large deformations [5]; since it is a rigid polymer,it only suffers elastic deformation, so Figure 12 presents thechanges that occur in the slope in deformation percentages,and the values of deformation were lower than 2%. In thisresearch, increment values of 0.2% deformation were inten-tionally chosen to make evident the instantaneous changes inthe tensile stress, as can be seen in Figure 12. This indicatesthat the presence of Na+-MMT in the PS matrix modifies theYM and the elastic deformation percentage.

The nanocomposites presented a gradual increase in theYM and a decrease in the deformation compared to PS. Thiseffect is most notable in the PS/Na+-MMT/St nanocompositethat is shown in Figures 12 and 13. This is due to thesurface modification of the Na+-MMT nanoclay with Stplasma which provoked a better dispersion and a greater

PSPS/．；

+-MMTPS/．；

+-MMT/MMA W 1.5 h100

PS/．；+-MMT/St W 1.5 h100

40

35

30

25

20

15

10

5

0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Tens

ile S

tress

(MPa

)

Strain (%)

Figure 12: Tensile-strain diagram of PS and pristine PS/Na+-MMT,PS/Na+-MMT/MMA, and PS/Na+-MMT/St nanocomposites.

4500

4000

3500

3000

2500

2000

1500

1000

500

0

Youn

g’s M

odul

us (M

Pa)

PSPS/MMT

PS/MMT/MMAPS/MMT/St

50W 100W

2852.1

3621.1

27

%

30.5

%

3729.7

3783

34

%

23

%

32

%

3494.2

3766.4

Figure 13: Young’s modulus for PS and pristine PS/Na+-MMT andPS/Na+-MMT/MMA PS/Na+-MMT/St nanocomposites at a timetreatment of 1.5 h.

interfacial interaction between this modified nanoclay andthe PS polymeric matrix owing to the similarity and com-patibility of the St monomer to PS. Meanwhile the MMA-modifiedNa+-MMTnanoclay was not as compatible with thepolymeric matrix. These results are similar to those obtainedfrom the TGA, where the PS/Na+-MMT/St nanocompositepresents the highest thermal stability, and, in this analysis,PS/Na+-MMT/St nanocomposite presents higher YM too.This increase in the slope upon the addition of the nanoclayis in accordance with previous reports [5, 15, 16].

In Figure 13, the calculations of YM obtained for thePS/Na+-MMT/MMA and PS/Na+-MMT/St nanocompositesare reported. The PS/Na+-MMT/St nanocomposite modifiedat 100W for 1.5 h has an increase of 32% in its YM com-pared to PS; meanwhile PS/Na+-MMT/MMA and pristine

10 Journal of Nanomaterials

O O O OO O

．；+-MMT

．；+-MMT

．；+-MMT

．；+-MMT/MMA

．；+-MMT/St

MMA

Plasma

Plasma

St

Melting

Melting

Mix

mix with PS

Melting

mix with PS

PS/．；+-MMT

PristinePristine

PS/．；+-MMT/MMA

MMA coatingMMA coating

PS/．；+-MMT/St

St coating St coating

+

PS matrix

+

PS matrix

+

PS matrix

= Incompatible

= Alike

= Completelycompatible

PS

PS

PS

Hydrophilic

Hydrophobic

Hydrophobic

Hydrophobic

Less hydrophobic

More hydrophobic

(a)

PS-MMT/S interfacial interaction scheme Specimen

PS Plasmapolymerization

Clay

Polystyrene

Intercalation(b)

Figure 14: (a) Mechanism of interfacial interaction of PS/Na+-MMT, PS/Na+-MMT/MMA, and PS/Na+-MMT/St nanocomposites and (b)representation of interfacial adhesion between Na+-MMT/St and PS matrix.

PS/Na+-MMT have an increase of 23 and 27%, respectively.In Figure 13, it is notable that the best results are presentedfor the Na+-MMT nanocomposites modified at 50W, wherethe YM has an increase of 34 and 30.5% for PS/Na+-MMT/Stand PS/Na+-MMT/MMA, respectively, at a time treatmentof 1.5 h. However, the data suggests after examining resultsthat the modifications with plasma at 100 or 150W do notaffect noticeably the YM and thermal stability of the studiednanocomposites; however, it is recommended to use thelower parameter of plasma power (50W), which is optimalin terms of energy savings.

A proposed mechanism of the modification and inter-action of pristine Na+-MMT and plasma modified nanoclay(Na+-MMT/MMA, Na+-MMT/St) with the matrix of PSis represented by a schematic in Figure 14, in which the

compatibility between the different composites is brieflyexplained.

3.2.5. Optical Properties. Theoptical properties of pristine PSand PS/Na+-MMT nanocomposites were measured with aUV-visible Spectrophotometer. The results are presented inFigure 15; it is appreciated that pristine PS has the higheroptical clarity, as expected; however, the presence of untreatedand plasma treated MMT affected the optical clarity ofthe nanocomposites as it is shown in Figure 15. But it isimportant to point out that PS nanocomposites containingthe plasma treated MMT (PS/Na+-MMT/MMA 100W andPS/Na+-MMT/St 100W)presented higher optical clarity thanPS nanocomposites containing untreated MMT (PS/Na+-MMT); this somehow indicates that treated MMT particles

Journal of Nanomaterials 11

3000

10

20

30

40

50

60

70

80

90

100

% T

rans

mitt

ance

400 500 600 700 800Wavelength (nm)

PSPS/．；

+-MMTPS/．；

+-MMT/MMA W100

PS/．；+-MMT/St W100

Figure 15: UV-visible transmission spectra of pristine PS andPS/Na+-MMT, PS/Na+-MMT/MMA 100W, and PS/Na+-MMT/St100W nanocomposites.

presented higher dispersion in the PSmatrix, and as a result ofthis dispersion these nanocomposites presented better opticalclarity. In conclusion, the plasma treatment of MMT with Stand MMA increased the compatibility towards PS, and as aresult these nanocomposites presented better dispersion andoptical properties.

4. Conclusions

The Na+-MMT nanoclay was modified by plasma polymer-ization with MMA and St as monomers. According to thestudies of dispersion and thermal properties as well as theFTIR carried out, the presence of organic material (plasmapolymer) over modified Na+-MMT was found. It was clearthat the PS or PMMA plasma coating deposited on the Na+-MMT via plasma treatment changes the hydrophilic charac-ter of the nanoclay into hydrophobic character. The modifiedNa+-MMT showed poor dispersion in high polarity solvents,whereas it showed a better dispersion in low polarity solvents.On the other hand, when comparing the effects of the typeof monomer used for the modification of the nanoclay, it wasfound that the coating formed by each type ofmonomer playsa fundamental role; the St monomer polymerizes to PS overNa+-MMT and promotes a greater affinity between the PSmatrix and the nanoclay. This greater interfacial interactionand dispersion of the Na+-MMT nanoclay modified by Stplasma in the PS matrix gave way to an improvement inthermal stability as well as an increase in 34% of YM for thePS/Na+-MMT/St. In the PS/Na+-MMT/St nanocomposites,the plasma modified Na+-MMT nanoclay with St is scatteredin the PS polymeric matrix due to the intercalation thatwas done. A greater interaction between the phases andbetter scattering are two of the most important factorsfor the improvement of nanocomposite properties. It wasconcluded that the properties of the modified PS/Na+-MMT

nanocomposites were significantly affected by the type ofmonomers used.

Data Availability

Data of this paper are available on request; please contactRosa Idalia Narro-Cespedes (email: rinarro@uadec.edu.mx)for any more information.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The funding for the publication of themanuscript is providedby the Faculty of Chemistry of Autonomous University ofCoahuila (UAdeC) (Grant no. 287245) (biomaterials basedon ceramics) and by the Mexican Council of Science andTechnology (CONACYT) (Grant no. 280425) (LANI-Auto).The authors wish to thank B. Huerta, G. Mendez-Padilla,S. Torres-Rincon, J. Felix, E. Saucedo-Salazar, M. Lozano-Estrada, M. C. Gonzalez-Cantu, J. Rodrıguez-Velazquez, J.F. Zendejo-Rodrıguez, A. Cardenas-Quinones, H. Prado-Lopez, and E. Hurtado-Suarez for their valuable techni-cal support in the preparation and characterization of thenanocomposites. The authors thank Luis Felipe MedinaVallejo for helping with the grammar of this article.

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