clay nanocomposites

Review Journal of Reinforced Plastics and Composites 30(5) 446–459 ! The Author(s) 2011 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684411399132 jrp.sagepub.com

A review: polystyrene/clay nanocomposites Artee Panwar1, Veena Choudhary2 and D.K. Sharma1

Abstract This article reviews the literature reports based on polystyrene nanocomposites using nanoclay as filler. The use of various clay surfactants and different processing conditions, i.e., in situ polymerization, melt intercalation and solution casting used for the preparation of nanocomposites and its effect on the properties and morphology is also reviewed.

Keywords polystyrene, nanocomposites, organoclay, melt blending, solution casting, intercalation

Introduction Polymer nanocomposites are defined as the combination of polymer matrix and the additives having nanometer dimensions. Polymer nanocomposites have attracted great interest from academicians and industrialists due to their outstanding properties like improved mechanical strength, water and oxygen barrier, dimensional stability, thermal stability, flame retardancy, scratch and wear resistance, chemical resistance, optical, magnetic, and electrical properties. The improved characteristics of the polymer nanocomposites as compared to macro- and microcomposites are due to high aspect ratio and large surface area of nanofillers. With the nanodispersion of fillers, a remarkable improvement in the properties of polymers can be achieved even with a considerably small loading. In these days, various nanoadditives have been used for the development of polymer nanocomposites. These may be one-dimensional including carbon nanotubes, fibers, and cellulose whiskers; two-dimensional including layered silicates; and three-dimensional including spherical particles like silica, latex, metallic particles, etc. For the synthesis of these nanofillers, specialized methods are required which involve mixing of reactants at the atomic level. These methods are:1

2. Sol–gel synthesis: This method involves the production of quite high ultrapure materials at atomic scale and offers the advantage of tailoring the composition. Sol–gel is the most viable method for the production of homogenous alloys and composites in an efficient and cost-effective manner. 3. Polymerized complex method: In this technique, metal ions are first chelated to form complexes and are then polymerized to form a gel. Among other chemical processes, this method is the most suitable due to homogenous dispersion of cations in the polymer network. 4. Chemical vapor deposition: In chemical vapor deposition, the substrate is exposed to a volatile precursor which then reacts or decomposes on the surface of substrate and is then obtained as the desired deposit. Materials are collected in various forms such as monocrystalline, polycrystalline, amorphous, and epitaxial. The nanoparticles synthesized by this method are silicon, silicon carbide, carbon nanotubes, carbon nanofibers, etc. 5. Microwave synthesis: Microwave-assisted non-aqueous sol–gel technique is widely used for the synthesis of metallic nanoparticles. The size of the

1

Centre for Energy Studies, Indian Institute of Technology, India. Centre for Polymer Science and Engineering, Indian Institute of Technology, India. 2

1. Supercritical hydrothermal synthesis: This process is used for the synthesis of metal oxide nanoparticles. During supercritical hydrothermal synthesis, water is used as a solvent due to its high dielectric constant.

Corresponding author: Veena Choudhary, Centre for Polymer Science and Engineering, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Email: [email protected]

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nanoparticles can be controlled by choosing the appropriate reaction conditions including temperature, reactant amount, and the reaction time. This type of synthesis is also used for the fabrication of carbon nanotubes and fibers. 6. High-energy ball milling process: High-energy ball milling used for the synthesis of nanoparticles is of three types: (a) Mechanical alloying – during mechanical alloying, mixtures of powders are milled together and then material transfer takes place to obtain homogenous alloy. (b) Mechanical milling – in this method, powders with uniform composition are milled together as no material transfer occurs. (c) Mechano-chemical synthesis – this process is similar to mechanical alloying, but its specialty lies in the fact that here chemical reaction between the powders takes place during the milling process at low temperature which is quite much far from equilibrium conditions. High-energy ball milling does not surely produce particles with homogenous nanosize. There may be some contamination as well due to wear and tear of milling media and container which depends upon various factors including milling time, intensity, and atmosphere. However, this method is highly advantageous due to large-scale production of nanoparticles with cost effectiveness. Various polymer nanocomposites have been developed using different types of nanofillers to achieve different type of properties, e.g., nanoclays are used for the enhancement of mechanical, barrier, and fire properties; carbon nanotubes and fibers are used to have improved tensile strength and conductivity; metal nanoparticles are used to prepare antimicrobial polymers, etc. There has been a great interest to study the polymer-layered silicates after the successful preparation of Nylon 6/clay hybrid by Toyota group.2 Researchers have developed many methods to prepare polymer clay nanocomposites in which incorporation of the layered silicates at molecular level into the polymer matrix has been achieved. The modified silicate is added to the polymer either by in situ method,3,4 solution blending,5 or melt blending.6 Friedlander and Grink7 were the first to report intercalation of polystyrene (PS) inside the clay galleries. Since then many researchers have been working on the preparation of PS clay nanocomposites and attempts are also being made to prepare exfoliated nanocomposites. In exfoliated nanocomposites, there is separation of individual layers of clay, whereas in intercalated, there is insertion of polymer chains into the layered structures with a few nanometers of repeat distance. There is general agreement in the literature that exfoliated

systems show better properties than the intercalated systems. However, it is quite difficult to achieve required improvement in the properties of polymer nanocomposites due to poor dispersion of nanofillers inside the polymer matrix. For this reason, various methods for the development of polymer nanocomposites are being tried which include various processing techniques and modification of polymer and nanomaterials using different surfactants. This article aims at elaborating various methods used to prepare PS clay nanocomposites using various surfactants to modify the clay minerals along with polymer.

General concepts Clays and their modification Clay is the most widely investigated material for the production of polymer nanocomposites. It is quite much preferred for the synthesis of polymer nanocomposites as it is cheap, easily available, and most importantly, environment-friendly material. There are two types of clays found in nature which are expanding and non-expanding clays. The expanding clays are phyllosillicates, smectite, and montmorillonite (MMT), and the non-expanding clays are talc, mica, and kaolin. To prepare nanocomposites, clay selection is made on the basis of its moderate surface charge, i.e., cation exchange capacity and layer morphology. The most commonly used clays for the formation of nanocomposites are montmorillonite, hectorite, and saponite. Montmorillonite has cation exchange capacity 110 meqiv/100 g, hectorite has 120 meqiv/100 g, and saponite has 86.6 meqiv/100 g. These clays are miscible with hydrophilic polymers only such as poly(ethylene oxide) and poly(vinyl alcohol). To make them compatible with hydrophobic polymers, it is necessary to convert these clays into organophillic. This can be achieved by exchanging the cation present in the clay layers with the cationic surfactants such as quaternary alkylammonium and alkylphosphonium ions. The alkylammonium and alkylphosphonium ions lower the surface energy of inorganic silicates and thus improve its wetting characteristics with the polymer. The cations present inside the clay galleries can be exchanged by two methods. These are: 1. Cation exchange reaction: In cation exchange reaction, the interlayer cation of the clay mineral is exchanged with cationic surfactants in aqueous solution. 2. Solid-state reaction: In solid-state reactions, the organic molecule is intercalated in the clay layers without the use of solvent. The advantage of this


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method over the previous method is that due to absence of the solvent, it is environmentally benign and therefore more suitable for industrialization. Many cationic modified montmorillonites have been commercially available as well.8,9 Ogawa et al.10 carried out first cation exchange reaction of clay minerals with ammonium ions.

Polymer nanocomposite formation Currently, polymer nanocomposites are prepared by three main techniques – in situ polymerization, melt intercalation, and solution casting. These three processes may be used individually or in combination with each other so as to achieve desired structure of nanocomposites.

In situ polymerization In situ polymerization is a widely used method for the preparation of polymer nanocomposites. In this method, clay is dispersed in the monomer which enables the entry of monomer inside the clay galleries. During the formation of nanocomposites, monomer first enters inside the clay galleries and then polymerization reaction occurs between the clay layers. This method is thought to be the most promising one to obtain exfoliated structure as it provides liberty to choose an appropriate surfactant and polymerization technique so as to get better dispersion of clay inside the polymer matrix. PS is generally polymerized by radical, cationic, or anionic polymerization among which radical polymerization is the most commonly used method. In radical polymerization, bulk, solution or suspension polymerization may be followed. Further, while choosing the appropriate surfactant, many factors should be considered. The surfactant used to modify the clay should be reactive so that it can react with the monomer and thus gets properly attached to the polymer. And second, surfactant should also contain some bulky groups like long alkyl chains or tetrahedral structures which will increase the interlayer spacing to a larger extent. Various researchers have worked upon many types of surfactants to prepare PS/clay nanocomposites. Table 1 lists different methods which have been used to prepare PS/clay nanocomposites by in situ polymerization method along with surfactants to modify the clay.

mixed with the polymer in molten state using different processing techniques including: single- or doublescrew extrusion, internal mixers, and manual mixing. The shear and extrusion applied by the processing instruments help to provide better dispersion of the clay into the polymer matrix. The high processing temperature is another factor which is considered to play a role in exfoliation of clay. It is thought that the high temperature allows proper mixing of clay and polymer. However, there is a limitation of clay being unstable as the organic ion used to modify the clay may decompose at higher temperature. This would decrease the interlayer space and therefore reduces the affinity of clay toward the polymer. Hence, it is essential to study the thermal stability of the surfactants first and then use them in the formation of polymer nanocomposites. In melt intercalation, different commercial polymers can be used which may not be suitable for in situ or solution polymerization. Further, this method is environmentally benign because it does not involve any solvent use. It is also one of the best compatible methods with current industrial processing equipments like extruder and injection molding. Table 2 lists various surfactants used to prepare PS clay nanocomposites by melt intercalation and their morphological data. The processing techniques along with temperature specifications are also mentioned.

Solution casting In solution casting, clay is immersed into the suitable solvent which tends to penetrate into the clay galleries resulting in their expansion. The important benefit of solution casting over melt intercalation is the presence of less viscosity which allows the polymer molecules to reach the surface of platelets quite easily. However, in solution casting, the solvent gets adsorbed on the clay surface and thus it is quite necessary for the polymer molecule to get adsorbed on the clay surface in order to replace the solvent. PS is a non-polar polymer and this method is considerably much suitable for the preparation of nanocomposites from weak polar polymers. Table 3 lists various solvents which have been used for the preparation of PS nanocomposites along with dispersion techniques. The morphology of the nanocomposites is also mentioned.

Melt intercalation

Characterization techniques and results Mechanical properties

It is one of the most widely used methods for the preparation of polymer nanocomposites and is also commercially acceptable. In melt intercalation, clay is

The increase in mechanical properties of nanocomposites has attracted researchers from all across the world toward this new class of materials.


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Table 1. List of PS/clay nanocomposites prepared by in situ method Clay surfactant

Clay type and content

Polymerization technique

Sodium dodecyl sulfate

Lap

Emulsion

Benzyldimethyltetradecyl ammonium chloride Octadecyltrimethyl ammonium bromide Dodecyl imidazolium salt Hexadecyl imidazolium salt Octadecyl imidazolium salt 0 2,2 -Azo bis(2-(1-2-hydroxyethyl)-2-imid azolin-2-yl)propane)dihydrochloride monohydrate g-Methacryloxypropyltrimethoxysilane –

Bentonite MMT MMT MMT MMT MMT

Emulsion Bulk Bulk Bulk Bulk Bulk

VMT MMT

Bulk

Vinyl functionalization Oligomeric polyoxypropylene derivative

MCM-48 MMT

–

Claytone APA MMT

Bulk Solution Bulk Solution

(11-Acryloyloxyundecyl)dimethyl(2-hydroxyethyl) ammoniumbromide (hydroxyethyl surfmer)

Bulk

Phenylacetaphenone dimethylhexadecyl Ammonium salt Pyridine hexadecyl bromide

MMT

Bulk

MMT

Bulk

Quinoline hexadecyl bromide

MMT

Bulk

Octadecyltrimethylammonium bromide Poly(dimethylsiloxane)

Sap Ca MMT MMT MMT MMT MMT MMT MMT MMT MMT MMT

Miniemulsion Bulk Emulsion Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk

MMT MMT MMT MMT MMT MMT MMT

Bulk Bulk Bulk In situ Bulk Bulk Bulk

Oligomeric trimethyl ammonium salt Oligomeric triethyl ammonium salt Oligomeric dimethyl hexadecyl ammonium salt Octadecyl amine Hexadecyltrimethylammonium bromide Benzalkonium chloride Vinylbenzylalkyldimethylammonium chloride 2,2-Azobis (2-methyl-N-[2-N,N,Ntributylammonium bromide)-ethyl propionamide (ABTBA) 3-Sulfopropyl methacrylate CTAB CPC ABTBA N,N-Dimethyl-n-hexadecyl-(4-vinylbenzyl) ammonium chloride Triphenylhexadecylstibonium trifluoromethylsulfonate N,N-dimethyl-n-hexadecyl-(4-hydroxymethylbenzyl) ammonium chloride

Morphology

Ref.

Partially exfoliated

11

12

Intercalated Intercalated Intercalated Intercalated Intercalated

13

Exfoliated Partially exfoliated Intercalated Exfoliated Exfoliated Intercalated

15

Mixed (intercalated + exfoliated) Intercalated

20

Immiscible/ intercalated Immiscible/ Intercalated Intercalated Exfoliation Intercalated Intercalated Intercalated Intercalated Intercalated Intercalated Intercalated Exfoliated Intercalated + Exfoliated

21

Intercalated Exfoliated Intercalation Intercalated – Intercalated Intercalated + exfoliated

28

13 13 13 14

16

17 18

19

20

22 23 24 25

26

27

29

30 31 32 33

(continued)


450


Table 1. Continued Clay surfactant N,N-dimethyl-n-hexadecyl-(4-vinylbenzy;) ammonium chloride N-Hexadecy; trophenylphosphonium chloride Tris-[2-(dimethyloctadecylammonium chloride) iso-propyl] phosphate CTAB Magnesium sulfate Aluminum sulfate Vinylbenzyldimethyldodecylammonium chloride CTAB Toluene-2,4-di-isocyanate Vinylbenzyltrimethylammonium bromide – Benzyldimethyl tetradecylammonium chloride N,N-dimethyloctadecylamine



Morphology

MMT

Bulk

Exfoliated

MMT

Bulk

MMT

Bulk

MMT MMT

In situ Emulsion

Intercalated + exfoliated Intercalated + exfoliated Exfoliation –

MMT MMT MMT MMT MMT MMT

Bulk Emulsion Emulsion Solution Emulsion Emulsion

Exfoliation Intercalation Intercalated Intercalated Intercalated –

36

MMT

Emulsion

Intercalated + exfoliated Exfoliated Exfoliated

42

Exfoliated Intercalated Intercalated Intercalated Intercalated Intercalated Partially intercalated Partially intercalated Exfoliated Intercalated Intercalated Exfoliated Intercalated Intercalated Intercalated Exfoliated Intercalated Intercalated Immiscible Intercalated Exfoliated Intercalated Exfoliated Exfoliated

43

N,N-dimethyloctadecylamine + 4-vinylbenzyl chloride N,N-dimethyloctadecylamine + polyhedral oligomeric silsesquioxane Cl compound Ar-vinylbenzyltrimethylammonium chloride Allyl-triphenyl-phosphonium chloride Tetradecylammonium flourohectorite Octadecylammonium montmorillonite Dioctadecyldimethylammonium montmorillonite Vinylbenzyltrimethyl ammonium Hexadecyltrimethylammonium bromide

MMT MMT

Emulsion Emulsion

MMT MMT Hectorite MMT MMT MMT MMT

Mini-emulsion Emulsion Solution Solution Solution Solution Emulsion

Hexadecyltrimethylammonium bromide

MMT

Emulsion

Vinylbenzyldimethylhexadecyl ammonium Dimethylbenzyl-hydrogenated tallow ammonium Octadecyltributyl phosphonium Vinylbenzyldimethylhexadecyl ammonium Dimethylbenzyl-hydrogenated tallow ammonium Octadecyltributyl phosphonium Dimethylbenzyl-dihydrogenated tallow ammonium Methacryloyloxyethylhexadecyl dimethylammonium Pentylcarbazole – dimethylhexadecylammonium Decarbazole – dimethylhexadecylammonium Decylcarbazole methyldidecylammonium Hexadecyltriphenyl phosphonium Vinylbenzyldimethylhexadecyl ammonium Vinylbenzyldimethylhidroxyethyl ammonium Hexadecyl pyridinium Vinylbenzyldimethylhidroxyethyl ammonium

MMT MMT MMT Synthetic clay Synthetic clay Synthetic clay MMT MMT MMT MMT MMT MMT MMT MMT MMT MMT

Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk

Ref.

34 35

37 38 39 40 41

44 45

46 47

48

49

50

51

52

53

54

(continued)


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Table 1. Continued Clay surfactant



Vinylbenzyl ammonium Vinylbenzyldimethyldodecylammonium chloride Alkoxyamine derivative Diphenylethylene derivative

MMT MMT MMT MMT

Emulsion Bulk Bulk Solution

Diphenylethylene derivative

MMT

Solution

Monocationic free radical initiator

MMT

Solution

Bicationic free radical initiator

MMT

Solution

Morphology

Ref.

Immiscible Exfoliated Exfoliated Partially exfoliated Partially exfoliated Partially exfoliated Exfoliated

55 56 57 58

59

60

MMT, montmorillonite; FH, flourohectorite; LAP, laponite; VMT, vermiculite; SAP, saponite; and Ref. reference.

Conventional composites also show improvement in mechanical properties as compared to neat polymers, however, the amount of filler is relatively considerably large which is 40–50%. To achieve better and desired improvement in mechanical properties, it is required to prepare exfoliated structures rather than intercalated structures. Various groups have shown that exfoliated structures give better improvement in mechanical properties. He et al.17 showed that PS/silylated MCM-48 nanocomposite particles when incorporated into the PS matrix improved the tensile strength by 70–560% and young modulus by 7–10 times. However, they observed a decrease in elongation at break. Uthirakumar et al.27 observed 50% improvement in Young’s modulus with 5 wt.% of clay into PS added by solution blending. Zhu et al.53 reported that with 3% addition of VB-16-, OH-16-, and P-16-modified clays, there was 300%, 120%, and 90% improvements in tensile strength at break, respectively. However, they did not find any change in elongation at break by the addition of OH-16- and P-16-modified clays. With VB16 clay, elongation at break was increased by 45%. Tseng et al.54 studied the effect of organically modified clay on the flexural and impact strengths of PS. They showed that VBDEAC modified clay showed better improvement in flexural modulus and strength than unmodified clay. Some researchers have also claimed a decline in mechanical properties by the addition of layered silicates into the PS matrix. Burmistr et al.68 prepared PS nanocomposites with pure bentonite and modified bentonite and studied their tensile strength, sharpy impact, and elongation at break. PS/clay nanocomposites when prepared with pure bentonite showed decrease in tensile strength and sharpy impact, whereas increasing trend of up to 2% was observed when the nanocomposites were prepared with modified bentonite. Similar results were observed for elongation at

break with purified as well as modified clay. Su et al.71 reported decrease in % elongation for nanocomposites prepared with PBD-modified clay using PS and high-impact polystyrene (HIPS). It has been observed that mechanical properties largely depend upon the morphology of the nanocomposite prepared. For immiscible or intercalated structures prepared, a decrease in the tensile strength was noticed which could be due to the formation of voids in the structure. Due to the formation of voids, the interaction between the polymer and clay gets weakened and thus the cohesive force between the matrix particles is reduced which causes a drop in the tensile strength and % elongation. In case of PS clay nanocomposites, most of the structures formed are of intercalated type and thus significant improvement in mechanical properties has not been observed.

Thermal properties Inorganic clays are quite stable at higher temperature. The weight loss observed in most of the clays is only 5–7% at 800 C, which is due to the presence of various forms of water molecules inside the clay galleries. It is known that the organic treatment reduces the thermal stability of clay; however, enhanced thermal stability of polymers has been observed when using organically treated clays in small amounts. Samakande et al.14 showed that T10 was increased from 362 C for neat PS to 390 C (4.50% experimental loading of VA060-modified clay), 364 C (14.20% experimental loading of VA060-D3-modified clay), and 409 C (8.80% experimental loading of VA060B3-modified clay) for various PS nanocomposites prepared. Tang et al.,15 however, studied the thermal stabilities of vermiculite/PS nanocomposites with different loadings of modified clay prepared by bulk co-polymerization. They concluded that the thermal


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Table 2. List of PS/clay nanocomposites prepared by melt blending Clay type

Polymer form

Processing technique

Processing temperature ( C)

Morphology

Ref

MMT

PS

Twin-roll mill

170

Intercalated

61

MMT

ABS

Twin-roll mill

190

Intercalated

MMT MMT MMT MMT

ABS ABS HIPS PS

Mixer Mixer Mixer Mixer

190 190 190 190

Intercalated Intercalated Intercalated Intercalated

MMT

ABS

Mixer

190

Intercalated

MMT

HIPS

Mixer

190

Intercalated

MMT

PS

Single-screw extruder

140/175/ 180/185

Dimethyl benzyl-hydrogenated tallow Dimethyl-dehydrogenated tallow Ferrocene ion Ferrocenium ion Octadecylamine

MMT MMT MMT MMT MMT

PS PS PS PS PS

Plasticorder

190

Immiscible

64

200–220

–

65

Octadecylamine

MMT

MA-grafted PS

200–220

–

MMT

PS

Single-screw extruder Single-screw extruder Twin-screw extruder

Tg + 50

Immiscible

MMT

PS + tetra-octyl ammonium SPS PS + tetra-decyl ammonium SPS PS + tetra-butyl ammonium SPS PS

Clay surfactant Benzyldimethyl-hydrogenated tallow ammonium halide Benzyldimethyl-hydrogenated tallow ammonium halide 1,3-Dihexadecyl-3H-benzimidazole-1-ium 1,3-Dihexadecyl-3H-benzimidazole-1-ium 1,3-Dihexadecyl-3H-benzimidazole-1-ium 2-Methyl-1,3-dihexadecyl-3Hbenzimidazol-1-ium 2-Methyl-1,3-dihexadecyl-3Hbenzimidazol-1-ium 2-Methyl-1,3-dihexadecyl-3Hbenzimidazol-1-ium –

MMT MMT Trimethyloctadecyl ammonium

MMT

62

63

66

Partially Exfoliated Partially Exfoliated ntercalated Twin-screw extruder

SMA-2 SMA-6 SMA-8 SMA-14 SMA-25 SAN-25-MA SAN-31-MA SAN-25-MA SAN-2

220

Immiscible

67

Immiscible Immiscible Immiscible Slightly intercalated Slightly intercalated Slightly intercalated Slightly intercalated Slightly intercalated Immiscible (continued)


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Table 2. Continued Clay type

Clay surfactant

Polymer form



Morphology

200

Slightly intercalated Slightly intercalated Slightly intercalated Intercalated

SAN-13.5 SAN-25 SAN-31 Polymeric quaternary ammonium salts

MMT

PS

Octadecyl ammonium

MMT

HIPS

Alkyl carbazole salt Di-alkyl carbazole salt Tallow alkyl ammonium salt Vinylbenzyl-grafted polybutadiene (PBD) ammonium salt

MMT MMT MMT MMT

PS PS PS PS

MMT

HIPS ABS PS

MMT

PS

Exfoliated

MMT

PS

Exfoliated

MMT

PS

Intercalated

MMT

PS

Exfoliated

MMT

s-PS

Manual mixing

290

FM

PS

MicroCompounder

FM MMT

PS PS

MMT

N,N,N-trimethylpolystyryl ammonium chloride N,N-dimethyl-N-benzylpolystyryl ammonium chloride N,N-dimethyl-N-hexadecylpolystyryl ammonium chloride 1,2-Dimethyl-3polystyrylimidazolium chloride Triphenylpolystyryl phosphonium chloride Hexadecyl-imidazolium Dihexadecyl-imidazolium 2-Phenyl amine Amine-terminated-PS Di(hydrogenated tallowalkyl)dimethyl ammonium chloride Dimethylbenzyl-hydrogenated tallow ammonium Dimethylbenzyl-hydrogenated tallow ammonium Octadecylammonium Dioctadecyldimethyl ammonium bromide Dodecylammonium Octadecylammonium Dodecylammonium Tetradecylammonium

Ref

68

Disk-screw extruder Twin-screw extruder Internal mixer

190–210

Intercalated

69

180

Intercalated

52

Internal mixer Internal mixer

180 200

Intercalated Immiscible

70

190

Immiscible Immiscible Exfoliated

Internal Mixer

71

72

73

200

Exfoliated Exfoliated Intercalated

Internal mixer

175

Intercalated

75

PS

Annealing

210

Intercalated

76

MMT

PS

Internal mixer

210

Intercalated

FH MMT

PS PS

Annealing Annealing

160 165

Intercalated Intercalated

77

FH FH FH FH

PS PS PS + PS3Br PS

Annealing

155

79

Annealing + double-screw extruder

150–170

Intercalated Intercalated Intercalated Intercalated

74

78

45

(continued)


454


Table 2. Continued

Clay surfactant

Clay type

Polymer form



Dioctadecyldimethylammonium Octadecylammonium Octadecylammonium Octadecylammonium Octadecyltrimethylammonium Dioctadecyldimethylammonium Dioctadecyldimethylammonium Dioctadecyldimethylammonium Dioctadecyldimethylammonium Dioctadecyldimethylammonium Dioctadecyldimethylammonium Dioctadecyldimethylammonium Dioctadecyldimethylammonium Dioctadecyldimethylammonium Octadecyltrimethylammonium

MMT SAP FH FH MMT SAP FH MMT MMT MMT FH FH FH MMT

PS PS PS PS PS PS PS PS + PVCH PS + PS3Br PS + PVP PS + PVCH PS + PS3Br PS + PVP PS

Annealing

170

Twin-screw extruder

180

Exfoliated Immiscible Immiscible Intercalated Intercalated Intercalated Intercalated Immiscible Immiscible Intercalated Intercalated Immiscible Immiscible Immiscible Intercalated

MMT

PS + SOZ Star-shaped PS

Annealing

220

Exfoliated

Octadecyltrimethylammonium Dimethylbenzyl-hydrogenated tallow ammonium Dimethyl-dihydrogenated tallow ammonium Dimethyl-dihydrogenated tallow ammonium Dimethyl-dihydrogenated tallow ammonium Dimethyl-dihydrogenated tallow ammonium Decyldimethylimidazolium Hexadecyldimethylimidazolium Protonated aminododecanoic acid Ammonium-terminated PS Tetraethylammonium bromide Tetrabutylammonium bromide CTAB

MMT

Morphology

Ref 80

81

82

Exfoliated 200

Intercalated

51

PS

Twin-screw extruder Annealing

165

Immiscible

83

MMT

PS

Internal mixing

200

Intercalated

84

MMT MMT MMT

PS PS PS+Epoxy Resin PS

Twin-screw Extruder Manual mixing

180

85

NO

Immiscible Intercalated Exfoliated

Internal mixer

200

Exfoliated

87

PS

Twin-screw extruder

160–210

Intercalated

88

MMT

PS

MMT

Synthetic clay MMT MMT

86

Intercalated + exfoliated Intercalated

MMT

MMT, montmorillonite; FH, flourohectorite; LAP, laponite; VMT, vermiculite; and SAP, saponite.

stability of the nanocomposites prepared with coupling agent between the nanofillers and the polymeric matrices were better than the pure polymeric materials and those without coupling agents. Zang et al.16 synthesized three polystyryl quaternary ammonium surfactants and showed that PS nanocomposites prepared with modified montmorillonite obtained from reaction of chloromethyl PS were the most stable.

Chigwada et al.25 reported that T10 was increased from 361–397 C by the addition of 10% ter-clay. Essawy et al.29 observed that PS clay nanocomposites with CPC-modified clay were more stable above 400 C temperature than neat PS and PS/CTAB-modified clay nanocomposites. Uthirakumar et al.30 showed 35 C improvement in Tonset by the addition of just 1 wt.% clay. Chen et al.34 also used CTAB-modified clay and


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Table 3. List of PS/clay nanocomposites prepared by solution casting Clay surfactant

Clay type

Polymer modification

Solvent

1-2-Dimethyl-3-nhexadecylimidazolium cation

MMT

PS

Chlorobenzene Sonication

Dimethyl, hydrogenated tallow, 2-ethylhexyl, quaternary ammonium methylsulfate Dioctadecyl dimethylammonium Octadecyl trimethylammonium Dioctadecyl dimethylammonium Dimethylbenzyloctadecil ammonium

Fluorinated synthetic mica MMT PS MMT PS MMT PS FH Lap Florinated PS synthetic mica

Dimethyl-dehydrogenated tallow ammonium MMT Methyl tallow bis-2-hydroxyethylammonium Dimethyl-dihydrogenated tallow ammonium Methyl tallow bis-2-hydroxyethylammonium Dimethyl-dihydrogenated tallow ammonium Methyl tallow bis-2-hydroxyethylammonium Pentylcarbazole dimethylhexadecylammonium MMT

observed 11 C increase in Tonset by the addition of 5 wt.% clay. Manzi-Nshuti and Wilkie,64 in their studies on ferric- and ferrocenium-modified clay nanocomposites, observed that addition of 3 wt.% of 1,10 -bis (nhexadecyl) ferrocene-modified clay improved T10 from 346 C to 418 C. Su et al. prepared nanocomposites with PS, HIPS, and acrylonitrile butadiene styrene (ABS) and showed that improvement in thermal stability was comparatively better in PS matrix followed by ABS and HIPS. Some authors have also studied the effect of different preparation methods on the thermal properties of PS nanocomposites. Chigwada et al.20 observed that thermal stability of PS was better improved when modified clay was added during bulk polymerization compared to melt blending method. During their studies on nanocomposites prepared using carbazole salts,52 they noticed that T10 was more in case of nanocomposites prepared using bulk method rather than melt blending. Some authors have shown that the addition of clay has increased Tonset temperature, but no further effect on thermal degradation process was caused.68 Giannakas et al.,90 however, studied the effect of different solvents on the thermal stability of nanocomposites. They showed that T10 and T50 was better improved in nanocomposites prepared using CCl4 solvent rather than CHCl3.

Dispersion technique Morphology Exfoliation

Ref. 89

Intercalation 90

Intercalated Chloroform

Stirring

Toluene

Stirring

Toluene

Stirring

PS THF + water PS PS-t-COOH PS-t-COOH PS-t-COONa PS PS Toluene

Sonication Stirring

Sonication

Intercalated

91

92

Intercalated

93

Immiscible Immiscible Immiscible Immiscible Exfoliated Exfoliated Exfoliation

94

95

Rheological properties It is considerably important to understand the rheology of the polymers after the addition of nanofiller as it governs the processability of the nanocomposite. Flow behavior of the nanocomposite also relates to its morphology. Considerably few studies have been carried out about the rheology of PS clay nanocomposites. Dazhu et al.69 studied in detail the rheological studies of PS/clay nanocomposites prepared with organically modified clay. They studied the activation energy of PS at various crosshead speeds (0.06–20 cm/ min) with varying amounts of clay. There was no definite trend observed for activation energy. They also studied the flow behavior index at different temperatures for various nanocomposites and observed a decrease in flow behavior index along with increasing amount of organoclay. This could be due to the stress caused in the matrix caused by the addition of nanofiller. Therefore, it would be significant to carry out rheological studies in order to improve the processing conditions of nanocomposites.

Oxygen and water permeabilities The permeability of nanocomposite membrane depends upon many factors such as nanofiller content,


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dimensions of the filler, and orientation and dispersion states of the nanofiller. The decrease in permeability is caused by the tortuous path toward the diffusing gas molecules. Various researchers have studied the role of different types of nanofillers on the barrier properties of PS. Giannakas et al.90 prepared nanocomposites using solvent blending and concluded that nanocomposites showed 23–54% and 15–44% reductions in water permeability when prepared with CCl4 and CHCl3, respectively. They have also concluded that the water permeability decreases with increasing clay content. They observed that the best barrier property was observed in PS/clay nanocomposite prepared with 10% of CTAB-exchanged clay with 3CEC.

Conclusion PS is a commercial and widely used polymer, thus, it is quite important to consider many parameters when it is used to prepare nanocomposites. PS/clay nanocomposites have been prepared by many techniques including in situ method, melt blending, and solution casting. Among these, most of the nanocomposites having exfoliated structures have been prepared by in situ method. In situ method is an advantageous method as it provides intercalation of monomer inside the clay galleries which leads to polymerization inside the galleries itself. Successful exfoliated structures have been obtained with this method by the appropriate choice of cationic initiators or other reactive cations. Melt blending, on the other hand, seems to be quite promising method due to its commercial acceptance. Further, new procedures like modification of polymers and addition of compatibilizers have created a platform for the successful formation of exfoliated structures by this method. Solution casting technique has also been used for the preparation of PS nanocomposites. However, use of large amount of solvent reduces its industrial viability. PS nanocomposites prepared by various methods have shown improved mechanical and thermal properties in addition to enhanced properties like flame retardancy and oxygen permeability. Also, there is a need to address issues like lower thermal stability of modified clays and industrial viability of the process used to prepare nanocomposite. In order to commercialize the production of nanocomposites, a great deal of novel and relevant methods are required to achieve full exfoliation and thus desired improvement in properties of PS.

Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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