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)
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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
Clay type and content
Polymerization technique
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
Clay type and content
Polymerization technique
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
Processing technique
Processing temperature ( C)
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)
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Table 2. Continued
Clay surfactant
Clay type
Polymer form
Processing technique
Processing temperature ( C)
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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