Properties of nanocomposites

1.1.5.1 Thermal stability and flammability

The thermal stability of polymeric materials is usually studied by thermogravimetric analysis (TGA). The weight loss due to the formation of volatileproducts after degradation at high temperature is monitored as a function of temperature. When the heating occurs under an inert gas flow, a non-oxidative degradation occurs, while the use of air or oxygen allows oxidative degradation of the

samples. Generally, the incorporation of clay into the polymer matrix was found to enhance thermal stability by acting as a superior insulator and mass transport barrier to the volatile products generated during decomposition.

Jin and co-worker investigated thermal property of polymer-clay nanocomposites by

TGA and cone calorimetry. [37] The thermal stability of the nanocomposite is enhanced

relative to that of virgin polystyrene and this is shown in Figure 1-12. Typically, the onset

temperature of the degradation is about 50 ^oC higher for the nanocomposites than for

virgin polystyrene.

Recently clay nanocomposites were found to impart a substantial level of flame retardancy. The flame retardancy effect appears to originate from the clay’s ability to contribute to char formation. This char layer forms an insulative layer to slow down heat transfer and retards movement of gases to feed the flame. One invariably finds that nanocomposites have a much lower peak heat release rate (PHRR) than the virgin polymer. The peak heat release rate for polystyrene and the three nanocomposites are also

shown graphically in Figure 1-13. P16-3 means that the nanocompoite was formed using

3 % of P16 clay with polystyrene. The peak heat release rate falls as the amount of clay

was increased. The suggested mechanism by which clay nanocomposites function

involves the formation of a char that serves as a barrier to both mass and energy transport.

[38] It is reasonable that as the fraction of clay increases, the amount of char that can be

formed increases and the rate at which heat is released is decreased. There has been a

suggestion that an intercalated material is more effective than is an exfoliated material in

fire retardancy. [37]

1.1.5.2. Dimensional stability

Dimensional stability is critical in many applications. For example, if the layers of a microelectronic chip have different thermal or environmental dimensional stabilities, then residual stresses can develop and cause premature failure. Poor dimensional stability can also cause warping or other changes in shape that affect the function of a material. Nanocomposites provide methods for improving both thermal and environmental dimensional stability. The possible mechanism by which nanofillers can affect the coefficient of thermal expansion (CTE) of a polymer has also been observed in traditional fillers.

The dimension stability of nanocomposites was studied by Zeng and Lee. [39]

Figure 1-14 shows the shape changes of injection molded PS and PS/clay nanocomposites

under the aforementioned thermal cycle (50 ^oC, 1 h; 75 ^oC, 1 h; 105 ^oC, 1 h; and 135 ^oC,

1h). The original sample shape is shown in the first row. Pure PS and the extruded

PS/20A (dimethyl dehydrogenated tallow ammonium montmorillonite, 20A)

nanocomposite are shown in the second row for comparison. The third row shows the

in-situ polymerized pure PS, PS/20A, and PS/MHABS

(2-methacryloyloxyethylhexadecyldimethylammonium bromide, MHABS)

nanocomposites. All the nanocomposites contain 5 wt % of clay. In the absence of clay,

the sample shrank greatly, and the shape became highly irregular. Dimension stability at

elevated temperature was improved significantly when 5 wt % of clay was present in the

in-situ polymerized nanocomposites, as shown in the third row. The exfoliate

PS/MHABS exhibited the best dimensional stability. After the heating cycle, although the

sample shrank to a certain extent, the original shape and surface smoothness remained. It

is noteworthy that the PS/20A nanocomposite prepared by extrusion compounding did

not show much improvement in dimension stability at elevated temperature, as compared

to the in-situ polymerized PS/20A nanocomposite with the same clay content.

1.1.5.3 Gas barrier properties

In many applications the gas barrier properties of polymers is critical. This is especially true in the food packaging industry. In many food packages the oxygen ingress determines the shelf life of the food in the package. For carbonated drinks the egress of carbon dioxide is the issue. It was recognized early in nanocomposite development that the high aspect ratios of clays could impart barrier to the composite. Clays are believed to increase the barrier properties by creating a maze or

“tortuous path” (Figure 1-15) that retards the progress of the gas molecules through the

matrix resin.

Nielsen [40] proposed a very simple model for the effect of platy materials on relative barrier performance. This model is commonly referred to as the tortuous path model. Figure 1-16 exhibits the effect of aspect ratio and clay loading on

relative gas permeability of a composite utilizing this model. It can be seen that relatively low clay loadings can change the gas permeability greatly. There are a number of nanocomposites that come close to fitting the predictions of this simple model. Lan et al. [41] observed in polyimide nanocomposites that the relative permeability for O2, CO2, and H2O fit the tortuous path model reasonably well but measurements on ethyl acetate exhibited a very large dependence on relative humidity. The relative permeability of ethyl acetate at 0% RH was 0.19 and at 50%

RH it was 0.09. In contrast the pure polyimides more than double its permeability going from 0% to 50% RH. Chaiko and Leyva [42] reported that in a polypropylene wax composite that they observed 62 fold decreases in oxygen permeability at 5%

weight loading of clay.

1.1.5.4 Mechanical properties

1.1.5.4.1 Dynamic mechanical analysis (DMA)

Dynamic mechanical analysis (DMA) measures the response of a given material to an oscillatory deformation (here in tension–torsion mode) as a function of temperature. DMA results are composed of three parameters: (a) the storage modulus (G'); (b) the loss modulus (G''); and (c) tanδ; the ratio (G"/G'); useful for determining the occurrence of molecular mobility transitions, such as the glass

transition temperature (Tg). [43]

The cyclic deformation of PS/MMT nanocomposites as a function of temperature was measured by DMA. The temperature dependence of storage modulus and tanδ were shown in Figure 1-17 and 18, respectively. The storage modulus of PS/MMT nanocomposites were greater than that of pure PS and monotonically increased with the clay content in both the glassy and rubbery regions. However, the improvements in the rubbery region were much greater than those in the glassy region. This behavior indicates that the restricted segmental motions at the organic-inorganic interface are due to large aspect ratios of the clay platelets, and the polymer chains were also well confined inside the clay galleries at the nanoscale level. [44,45] The storage modulus of PS/MMT-3 was 1.2 times higher than that of pure PS, which is comparable to the earlier reported data (1.4 times improvement). [44] The T

gs of the nanocomposites were estimated from the peak values of tanδ, which were shifted towards higher temperature with increasing the clay content. These results indicate that nanoscale clay platelets strongly restrict the polymer segmental motions, resulting in the significant increase in T

g. This improvement in T

g is higher than those of other researchers even though the smaller clay content was used in this experiment. [46,47]

The effects of clay loadings on tensile properties of the PS/MMT nanocomposites are shown in Figure 1-19. The tensile strength and Young’s modulus

were significantly enhanced in the presence of the small contents of clay, while the elongation at break was reduced with increasing the clay content. The increase in tensile strength was attributed to the stronger interfacial adhesion between PS and the clay platelets. However, the enhancement of modulus was reasonably ascribed to the high resistance exerted by the clay platelets against the plastic deformation and the stretching resistance of the oriented polymer backbones in the galleries. The improvement of tensile strength in PS/MMT-3 compared to pure PS was ~47 %, which is greater than the earlier reported value in the literature (~21 %) for PS/MMT nanocomposite with 3 wt% MMT prepared by melt blending. [45] Similarly, the enhancement of Young’s modulus in PS/MMT-3 compared to pure PS was ~25 %, which is much greater than the reported value (7.4 % improvement for PS/MMT nanocomposite with 5 wt% clay prepared by emulsion polymerization). [48] However, the elongations at break were reduced with increasing the clay content. Similar results were earlier reported. For example, the reduction of elongation at break in PS/MMT nanocomposite with 4.4 wt% MMT prepared by melt blending was reported to ~26 %.

[44]

1.1.5.4.2 Tensile properties

The tensile modulus of a polymeric material has been shown to be remarkably

improved when nanocomposites are formed with layered silicates. N6 nanocomposites prepared through the in situ intercalative ring opening polymerization of 1-caprolactam, leading to the formation of exfoliated nanocomposites, exhibit a drastic increase in the tensile properties at rather low filler content. The main reason for the drastic improvement in tensile modulus in N6 nanocomposites is the strong interaction between matrix and silicate layers via formation of hydrogen bonds, as shown in Figure 1-20. In the case of nanocomposites, the extent of the improvement of the modulus depends directly upon the average length of the dispersed clay particles, and hence the aspect ratio. Figure 1-21 represents the dependence of the tensile modulus E measured at 120 °C for exfoliated N6 nanocomposites with various clay content, obtained by the in situ intercalative polymerization of 1-caprolactam in the presence of protonated aminododecanoic acid-modified MMT and saponite.

Moreover, the difference in the extent of exfoliation, as observed for N6-based nanocomposites synthesized by the in situ intercalative polymerization of 1-caprolactam using Nat-MMT and various acids, strongly influenced the final modulus of the nanocomposites.