Nanocomposites Derived from Polymers and Inorganic Nanoparticles

Polymers are considered to be good hosting matrices for composite materials because they can easily be tailored to yield a variety of bulk physical properties. Moreover, organic polymers generally have long-term stability and good processability. Inorganic nanoparticles possess outstanding optical, catalytic, electronic and magnetic properties, which are significantly different their bulk states. By combining the attractive functionalities of both components, nanocomposites derived from organic polymers and inorganic nanoparticles are expected to display synergistically improved properties. The potential applications of the resultant nanocomposites are various, e.g. automotive, aerospace, opto-electronics, etc. Here, we review recent progress in polymer-based inorganic nanoparticle composites.

The properties of the nanocomposites are contributed to the properties of the components, shape and volume fraction of the filler, the morphology of the system and the nature of the interphase that sometimes develop at the interface of the two components [8]. The extent of property enhancement depends on many factors including the aspect ratio (length/diameter) of the filler, its degree of dispersion and orientation in the matrix, and the adhesion at the filler-matrix interface [9]. For example, clays have sandwich types of structures with one octahedral Al sheet and two tetrahedral Si sheets, socalled philo-silicate. There are many types of philo-silicates: kaolinite, montmorillonite (MMT), hectrite, saponite, synthetic mica, etc. The MMT consists of stacked silicate sheets with lengths of about 218 nm and 1,230 nm for synthetic mica. It has the same sheet thickness of 1 nm (Figure 1) [10,11].  [11]).
Since the clay particles have extremely high specific surface area (750 m 2 /g), good dispersion even at low clay contents provides an enormous amount of interfacial area [12].The unique nanometer-size of the layered silicates has characteristics such as high aspect ratios, high surface areas, and high strengths [13]. Zinc sulfide (ZnS) has excellent physical properties such as wide bandgap energy of 37 eV at 300 K [14], high refractive index (n = 2.36 at 620 nm) and low absorption coefficient over a broad wavelength range [15,16].
The idea is to incorporate chemically treated spherical nanoparticles with proper size, to attribute into the polymer matrix and disperse them at the nanoscale to minimize light scattering and thus, to attain high transmittance while realizing retention or improvements in some of the material and optical properties [7]. One method to facilitate dispersion of nanoparticles is to coat the nanoparticle with a thin layer of polymer to introduce steric stabilization [17]. By coating the nanoparticle with a thin layer of polymer [7,18] and surface functionalization of the nanoparticles [10,[19][20][21], the van der Waals influence from the nanoparticles can be masked and the compatibility between the hosting organic polymer and inorganic nanoparticle can be improved, thereby facilitating better nanoparticle dispersion and increased loading amount. Table 1 shows synthesis of inorganic nanoparticles from aqueous solutions and Table 2 from nonaqueous solutions [22].
Polymer based nanocomposite synthesis was effected by molecular weight, inorganic particles size and content, properties of inorganic particles.
Maleic anhydride (MA) grafted polyethylene/clay nanocomposites were prepared by simple melt compounding [42]. The exfoliation and intercalation behaviors depended on the hydrophilicity of polyethylene grafted with maleic anhydride and the chain length of organic modifier in the clay. When polyethylene has a higher grafting level of MA than the critical grafting level of MA (0.1 wt %) and the number of methylene groups in alkylamine chain has more than 16, polyethylene/clay nanocomposites are completely exfoliated. The nano-SiO 2 particle/linear low-density polyethylene (LLDPE) was prepared via in situ polymerization [43]. Only slight increase in MW (molecular weight) with the larger particles was evident. It can be observed that increased amounts of SiO 2 resulted in decreased MW. In addition, a slightly broad MWD (molecular weight distribution) was seen in all samples. It was found that the larger particles exhibited higher activity due to fewer interactions between SiO 2 and MAO (methylaluminoxane).
The low molecular weight nylon 6/MMT nanocomposites had regions of intercalated and exfoliated clay platelets, while the medium and high molecular weight nylon 6/MMT nanocomposites revealed well exfoliated structures. As the molecular weight increased, so did the extent of clay platelet exfoliation for the nanocomposites [44,45].
PMMA (poly(methyl methacrylate))/clay nanocomposites were synthesized via a novel pseudodispersion polymerization in scCO 2 (supercritical carbon dioxide) [46]. The fluorinated surfactantmodified clay (10F-clay) can indeed serve as an effective stabilizer for PMMA polymerization in CO 2 and help improve polymer yields compared with conventional hydrocarbon surfactant-modified clay. PMMA/layered silicate intercalated nanocomposites were synthesized using scCO 2 [47]. At clay concentrations approaching 40 wt%, the silicate morphology is homogeneous, because the intercalated silicate structure is thermodynamically limited with regard to clay separation.

Properties of Polymer-based Nanocomposites
Polymer-based nanoparticle nanocomposites were prepared via various processes and showed improved mechanical, thermal and electrical properties. However, the aforementioned properties of resultant nanocomposites were not always improved. For example, if one property changed for the better, another property changed for the worse. When nanocomposites are designed, one needs to take this tendency into account and find the optimum properties for specific applications.

Mechanical properties
Generally, the reason for adding inorganic particles into polymers is to improve its mechanical properties such as the tensile strength, modulus or stiffness via reinforcement mechanisms described by theories for nanocomposites [13,[48][49][50][51]. However, poor compatibility between the polymer matrices and the inorganic particles in nanocomposites prepared by simple physical mixing will create inherent defects which, consequently, result in a deleterious effect on the mechanical properties of the nanocomposites [20,52,53].
Pattanayak et al. prepared polyurethane (PU)-based clay nanocomposites [48]. When clay particles were fully exfoliated, modulus, tensile strength, tear strength and fracture toughness of PU/clay nanocomposites were increased by 110%, 170%, 110%, 120%, 40%, respectively, as compared with pristine PU. Note that such improvements can be attributed to clay-polymer tethering as well as hydrogen bonding between clay particles and the polymer. Also, Lee et al. reported that tensile strength and elongation at break of PU/clay nanocomposites increased with increasing clay content in the range of 1-3 wt %, but when the clay content is higher than 3 wt %, the tensile properties of the nanocomposites decreased slightly (Figure 4a) [50]. A transmission electron micrograph (TEM) of the cross section of a WPU/clay nanocomposite with 5 wt % clay is shown in Figure 4b. This is probably due to some degree of aggregation of the exfoliated clay platelets above the critical content.

(c) (b) (a)
Wang et al. developed a "slurry-compounding" process for the preparation of epoxy/clay nanocomposites, showing that clay uniformly dispersed and oriented randomly in the epoxy matrix ( Figure 5) [13]. Both Young's modulus and the fracture toughness are improved with the incorporation of clay and the sample containing 2.5 wt % of clay shows the highest fracture toughness.
Sarwar et al. reported poly(trimethylhexamethylene terephthalamide)/SiO 2 nanocomposties prepared by a sol-gel process [54]. The values of yield stress and modulus for the composites increased with an increase in the silica content up to 10 wt % and then decreased with higher concentrations of inorganic phase.
Lu et al. made polyimide (PI)/ZnS nanocomposites, which showed that both the modulus and the tensile strength increased but the elongation at break decreased as ZnS load increased [55]. This can be explained by the better interfacial interaction between polymer and inorganic nanoparticles with higher surface area.
Reddy et al. manufactured polypropylene (PP)/nano-silica (NS) nanocomposites [56]. The modulus of PP-NS was much higher than that of pure PP, and that of PP-ENS (epoxy-resin-grafted nano-silica) was higher than that of pure PP, but lower than PP-NS. The tensile strength and elongation at break decreased for PP-NS because of NS aggregation and a lack of interfacial adhesion between the polymer and the filler. However, those of PP-ENS were increased in comparison with PP and PP-NS because of obstruction of the formation of agglomerates by epoxy-resin grafting.
The mechanical properties of nanocomposites, prepared from various polymers and inorganic particles, did not always increase. In some cases, the properties of nanocomposites were decreased by the addition of inorganic particles because of aggregation in polymer matrices. To solve this problem, the load amounts of inorganic particles were optimized or were functionalized with organic material. For example, the tensile strength and elongation at break of poly(ethylene phthalate) (PET)/nano-TiO 2 fiber was slightly decreased as compared to that of controlled PET fiber. It was assumed that the addition of nano-TiO 2 resulted in decreased interaction between the PET macromolecules [20]. Also, the introduction of ZnO nanoparticles into polystyrene (PS) decreased both the tensile strength and elongation to break. This implies that the interfacial adhesion is not strong enough to stand up to large mechanical forces [53], likely because the homogeneous dispersion of nanoparticles was difficult. Nano-sized particles have high surface energy and thus, they are easy to aggregate.

Thermal properties
For structural applications at elevated temperatures, the dimensional stability of low thermal expansion coefficient of these nanocomposites is also very important. The high thermal expansion coefficient of neat polymers causes dimensional changes during the molding process. The changes are either undesirable or, in some cases, unacceptable for certain applications.
Yu et al. showed that PS and aluminum nitride nanocomposite (AlN) were mixed at room temperature and then hot pressed [57]. The thermal conductivity of nanocomposites increased with increasing filler contents. The thermal diffusivity of composites decreases slightly with increasing temperature in the testing range.
Lee et al. reported that HDPE/filler nanocomposites were prepared using a mixer and fillers were used such as Wollastonite, SiC, and BN [58]. The thermal conductivity of HDPE nanocomposites was increased with increasing filler content ( Figure 6). Wang et al. showed that PMMA/SiO 2 and PMMA/ZrO 2 nanocomposites were prepared using a novel nonhydrolytic sol-gel process [59]. The temperatures, where the maximum weight loss was observed by thermal decomposition of PMMA segments, all increased dramatically. The thermal decomposition temperatures of PMMA/SiO 2 and PMMA/ZrO 2 were profoundly improved, because network structure between inorganic and organic components reduces the movement of polymer chains, and inorganic components may retard the attack of the free radicals.
Du et al. prepared nylon 6/MgAl-layered double hydroxide (LDH) nanocomposites via organic modification and melt intercalation [60]. The exothermic peak temperature for pure nylon 6 is increased up to about 14 °C. Also, they prepared LLDPE/MgAl-LDH nanocomposites via melt mixing [61]. When 5 wt % of MgAl(H-DS) (organic modified MgAl-LDH) is added, exfoliated LDH layers increased endothermic peak temperature by about 5 °C. Inorganic particle layers act as nucleating agents, which have a heterogeneous nucleation effect on the crystallization temperature of polymer.
Lu et al. reported that silane-grafted-polyethylene/OMT nanocomposites (VTMS-g-PE/OMT) were prepared by reactive extrusion from linear low density polyethylene (LLDPE), vinyltrimethoxysilane (VTMS), organically modified montmorillonite (OMT) and dicumyl peroxide (DCP) [62]. VTMS-g-PE/OMT showed higher thermooxidative stability than that of pure LLDPE and VTMS-g-PE. The thermooxidative degradation temperature of VTMS-g-PE/OMT for a mass loss of 10% and 50% is about 23 and 16 °C, respectively, higher than that of pure LLDPE. This is because the chemically bonded silicate layers with high aspect ratios can sustain high temperatures and efficiently hinder the heat and mass transfer. [63]. When 50% weight loss was selected as a point of comparison, the thermal decomposition temperature of exfoliated PS/ZnAl-LDH nanocomposites with 10 wt % ZnAl-LDH is 39 °C higher than that of pure PS. The thermal stability of the exfoliated nanocomposites is generally better than that of intercalated composites. TEM images with the different contents of ZnAl (DS) in the PS/LDH nanocomposites sample are shown in Figure 7. The inset in Figure 7c shows an enlarged image of the part marked by a rectangle.

Qiu et al. reported that exfoliated PS/ZnAl layered double hydroxide (PS/ZnAl-LDH) nanocomposites were synthesized by a solution intercalation method
Gyoo et al. prepared nylon 66/clay nanocomposites by melt compounding [31]. T g and T m of nanocomposites showed little change with increased clay content. So, crystallization temperature of nanocomposites was generally increased by about 15 °C relative to that of neat nylon 66. The crystallinity of the nanocomposites was also increased. However, at high concentrations of clay, the rate of crystallization was retarded and reduced the crystallinity.   [64]. Thermal degradation of the aliphatic chains in the PP-EP/EVA matrix can be retarded by an improvement in the dispersion and exfoliation of the silicate layers, which work as a barrier for heat diffusion. Organoclay concentration up to 6 wt % proportional to the retardation of thermal decomposition of nanocomposites (Figure 8).

Electrical properties
Nanocomposites are closely related to the design of advanced devices for electronic and optoelectronic applications. The dimensional scale for electronic devices has entered the nano-range. The utility of polymer/inorganic particle nanocomposites in these areas is quite varied involving many potential applications as well as types of nanocomposites.
Su et al. carried out in situ polymerization of polyaniline (PANi) in the presence of TiO 2 to synthesize PANi/TiO 2 nanocomposites [65]. The resultant nanocomposite films showed appreciable conductivity (1-10 S/cm), which was further increased after thermal treatment 80 °C for 1 h. Mo et al. also prepared PANi/TiO 2 nanocomposites with TiO 2 nanoparticles and colloids, respectively [66]. As the content of TiO 2 increased, the dielectric constant and loss were also increased. The conductivity of nanocomposites was gradually increased as the amount of TiO 2 increased from 1 to 5 wt %.
Olad  [69]. When the mass ratio of SrFe 12 O 19 to pyrrole was less than 1:15, the conductivity (5.65 S/cm) of the PPy/SrFe 12 O 19 nanocomposite was found to be higher than that of the pure PPy (3.29 S/cm) despite the insertion of the insulating SrFe 12 O 19 particles (Figure 9). Yu et al. manufactured hetrocyclically conjugated poly(3-hexylthiophene) (P3HT)/clay nanocomposites via in situ oxidative polymerization [70]. The polarization resistance of nanocomposites was increased from 10 to 1340 KΩ·cm 2 with increasing content of clay nanoparticles to 10 wt %.
Ma et al. reported that polystyrene resin/ZnO nanocomposites were prepared by melt-blending [71]. The surface resistivity falls as the amount of ZnO increases. Adding 30 wt % of ZnO spherical particle and ZnO whisker reduced the surface resistivities of materials from 1.0 × 10 16 to 8.98 × 10 12 Ω/cm 2 and to 9.57 × 10 10 Ω/cm 2 , respectively. The amount of ZnO in polystyrene resin can be gradually increased to form a conductive network.
Xu et al. reported poly(ethylene glycol) (PEG) and poly(ethylene oxide) (PEO)/lithium intercalated tungsten disulfide (Li x WS 2 ) nanocomposites [72]. They were prepared by exfoliation-adsorption technique. The resultant nanocomposites displayed pretty good conductivity in the range of 1 × 10 -2 to 10 -3 S/cm at ambient temperature. The conductivity likely originated from charge transfers.

Optical properties
The optical properties of discontinuous metallic or granular composite films, consisting of metal nanoparticles embedded in a dielectric, have long been of interest [75]. Moreover, as the market of materials for optical applications expands, the need for novel materials with functionality and transparency increases. Polymer-based inorganic nanoparticle nanocomposites show great promise as they can provide the necessary stability and easy processability with interesting optical properties. As described earlier, metal nanoparticles show characteristic plasmon resonance modes during interaction with electromagnetic waves as a result of collective oscillations of free electrons and local enhancement of the electromagnetic field. This phenomenon largely depends on the particle size, shape, and the surrounding dielectric matrix. Particle plasmon resonances occur through absorption energies in the intra-band transitions and can be either dipolar excitation (one surface plasmon), in the case of spherical particles, or multipolar excitation of particles nonspherical in geometry [76].
The transparency of these composites depends upon the size and spatial distribution of inorganic particles in the polymer matrix. Pure polyamide and hybrid films containing various amounts of silica are transparent, the maximum transmittance was found in the hybrid film containing 5 wt % silica contents in the matrix, beyond which the transmittance was gradually decreased [54].
Chai et al. reported that PMMA/La 0.45 Ce 0.45 Tb 0.1 PO 4 nanocomposite exhibited high transparency for visible light and showed strong green emission of Tb 3+ (dominate at 543 nm) upon UV excitation [77]. Also, Wang et al. prepared nanocomposites from PMMA/SiO 2 and PMMA/ZrO 2 using a novel nonhydrolytic sol-gel process [59]. The transmittance of the nanocomposite films in the visible region remained above 95% even at 20 wt % inorganic content and was increased proportionally with decreasing inorganic content.
Avasthi et al. prepared PET/Ag nanocomposite that showed transmission in a narrow band around 320 nm [78]. With the increase in metal volume fraction, the transmission band becomes narrower and the attenuation of transmission intensity at 320 nm is increased because of superimposion of the absorption spectrum of the sputtered PET polymer and Ag nanostructure. The second feature in the UV-visible absorption spectra showed the broad band absorption. The extension from the visible region to IR region was attributed to the three-dimensional network of metal nanostructures.
Wu et al. reported that TiO 2 nanoparticles prepared using sol-gel process were incorporated into the epoxy matrix by mechanical stirring [79]. Epoxy/TiO 2 nanocomposite coating with refractive index of 1.668 can be obtained by adding 30 wt % the TiO 2 nanoparticles into the epoxy resin. Also, all coating with different amounts of TiO 2 exhibit excellent optical transparency of higher than 90%.
Polycarbonate (PC)/alumina nanocomposites showed that the presence of the nanoparticles reduced the overall light transmittance of the nanocomposites [7]. The light transmittance was decreased by increasing the load of the nanoparticles. Furthermore, the poly(styrene-maleic anhydride copolymer (SMA)-coated alumina/PC nanocomposite had higher light transmittance than that of untreated alumina/PC nanocomposite ( Figure 10).

Magnetic properties
Alam et al. prepared PANi/Fe 3 O 4 nanocomposites that showed higher saturation magnetization of 3.2 emu/g at 300 K, revealing a super paramagnetic behavior [80]. Also, Qiaozhen et al. reported that PANi/Fe 3 O 4 @Au nanocomposites were fabricated by in situ polymerization and they showed higher saturation magnetization (M S ) than the undoped PANi (27.35 × 10 -3 emu/g) [81]. Upon decreasing the molar ratio of Au, the M S of nanocomposites increased from 0.17 to 0.88 emu/g. The magnetic property of the nanocomposites is very close to surpramagnetic behavior.  [69]. PPy/nano-SrFe 12 O 19 nancomposites were found with M S and remnant magnetization (M R ). It was found that the morphology of composites changed from sphere-like, conglobulation-like, and arborization-like structures with the increase of the pyrrole/SrFe 12 O 19 mass ratio ( Figure 11).  [84]. The M S was as high as 6.47 emu/g (20 wt % Fe 3 O 4 ) at 300 K ( Figure 12).  Table 3 presents potential applications of polymer-based inorganic particle nanocomposites [4,85]. The advantages of nanoscale inorganic particle incorporation into polymer matrices can lead to a number of applications that the incorporation of the analogous larger scale particles do not allow due to an insufficient property profile for utilization. These areas include barrier properties, membrane separation, UV screening, flame retardation, polymer blend compatibilization, electrical conductivity, impact modification, biomedical applications, etc. Hence, polymer-based inorganic nanoparticle nanocomposites emerging as new materials provide opportunities and rewards creating a new world of interest. Table 3. Potential applications of polymer-based inorganic nanoparticle nancomposites (adapted from Refs. [4] and [85]).

Conclusions
The hybridization of nanoparticles and polymers could improve various properties of resultant nanocomposites. The synergetic enhancements should originate from the specific attribute of each component. Toward an important objective for the development of multi-functional nanocomposites, is that the bulk physical properties should be easy to tailor for different purposes. Specifically, organic polymer-based nanocomposites generally have many advantages such as long-term stability and good processability, and inorganic nanoparticles possess outstanding optical, catalytic, electronic and magnetic properties. By combining the attractive functionalities of both components, resultant nanocomposites could potentially provide many applications in various areas such as automotive, aerospace, opto-electronics, etc.