M. VALVERDE DE LA VIRGEN

The temperature effect on the viscoelastic properties was studied at four melt temperatures ranging from 180 to 210 °C. Figures 7.7 and 7.8 shows an example of the storage modulus (G’) of PP 5 and PP 7 at four temperatures. The elastic nature of the PP-PEGM nanocomposite decreases with increasing temperature at all frequencies. Similar behavior was found for PP and PP-PEGM (MB) nanocomposites. Information regarding temperature and frequency dependence of material properties is fundamental for the design of some advanced materials. Knowledge of the material properties over a range of temperatures is essential for the design of products or specimens in commercial applications. Frequency affects the vibration performance of polymers and thus, of polymeric matrix composite parts. Therefore, for optimal design and analysis it becomes indispensable to determine the actual material properties over the range of operational temperatures and frequencies. Figures 7.7 and 7.8 show decrease in storage modulus with increasing temperature. The

small change observed in storage modulus suggests changes not only in matrix properties but also changes in the clay-matrix interface.

100 1000 10000 100000 1000000 0.1 1 10 100 ω(rad/s) G' (Pa) 180 °C 190 °C 200 °C 210 °C

Figure 7.7 Temperature dependence of storage modulus for PP-PEGM(MB) nanocomposites – 5 phr 100 1000 10000 100000 1000000 0.1 1 10 100 ω(rad/s) G' (P a) 180 °C 190 °C 200 °C 210 °C

Figure 7.8 Temperature dependence of storage modulus for PP-PEGM(MB) nanocomposites – 7 phr

7.1.4.1 Calculation of the activation energy for PP-PEGM (MB) nanocomposites Arrhenius relationship was used to analyze the data and obtain the activation energy for nanocomposites at four different temperatures 180, 190, 200 and 210 °C respectively. The results indicated that the activation energy for flow can be used to differentiate filler ratios and rank their temperature susceptibility in a quantitative manner. Viscous flow in any liquid can be regarded as a thermally activated rate process where molecules must overcome an energy barrier to move to an adjacent vacant site. Henry Eyring modeled the concept of an activation energy barrier to flow. When a liquid flows, layers of liquid molecules slide over each other and intermolecular forces cause resistance to flow. The viscosity and temperature relationship can then be modeled using an Arrhenius Equation.

RT E_f

Ae

=

η

It is more useful to rewrite the equation as

lnη

=

E

/RT

+lnA

A plot of ln η versus (1/T) gives a straight line with a slope of Ef/R. The concept of

activation energy was applied to study the properties of nanocomposites, recently. The typical activation energy determined for PP was around 40 kJ/mol [201].

PP PP 1 PP 2 PP 5 PP 7

slope 5110.80 6898.70 7619.60 7916.84 8222.69

Ef(kJ/mol) 42.49 57.36 63.35 65.82 68.37

Table 7.2 Slope and activation energy values for PP and PP-PEGM(MB)nanocomposites Interactions between individual particles within fillers, as well as among fillers, hinder relative motion between material planes, modifying the solid-state and the melt-state behavior of the host polymer. In highly filled polymer based composites, filler interactions are so strong that relevant solid like yield phenomena can be observed even at temperatures above the melting temperature of the polymer. This behavior is often attributed to the existence of a filler network that spans large sections of the polymer matrix [206]. When the particle is at least in one dimension, filler approaches the mean radius of gyration of

host polymer chains, a large fraction of polymer is in contact with the filler, and any chain is able to interact simultaneously with more than one particle. The relative motion among chains may be retarded by the polymer confinement between nanoparticle and aggregate surfaces [202]. Addition of nanoscopic fillers of high anisotropy instead of conventional reinforcing agents allows PP nanocomposites to exhibit interesting structure-property relationships and promising applications. That means, large interface surface are possible for low clay loading, leading to improvement of barrier properties and others (discussed in chapter 9). The dispersion problem due to strong particle interaction of nanofillers has limitations. The driving force of the intercalation originates from the strong hydrogen bonding between the MA groups of the PP and the oxygen groups of the silicates. The interlayer spacing of the clay increases and the interactions of the layers are weakened. If the miscibility of clay and PP is good enough for dispersion at the molecular level, a higher degree of intercalation and/or exfoliation of the clay should take place. It should be noted that PEGM increases the chance of miscibility of clay in PP matrix. Manias et al proposed that an important feature in designing surfactants to promote exfoliation was the choice of a surfactant which has a good compatibility with the PP matrix [6]. Nonionic-surfactants like poly- (ethylene glycol) (PEG) is clayophilic in character and is known to readily intercalate unmodified clays [8, 9].

6 7 8 9 10 0.00206 0.00208 0.0021 0.00212 0.00214 0.00216 0.00218 0.0022 0.00222 1/T (1/K) ln η 0 PP PP 1 PP 2 PP 5 PP 7

Figure 7.9 Arhennius plot of Zero shear viscosity as a function of temperature

It can be inferred from Figure 7.9 that the flow activation energy of nanocomposites was higher than that of pure PP, which is consistent with the results of the other researchers [201]. The higher flow activation energy causes a larger energetic barrier for segmented motions in the confined space and thus causes an increase of storage and loss modulus. The increasing flow activation energy was strengthened because of the hydrogen bonding between the polar functional group of PP-g-MA and oxygen group of MMT. Similar results were reported by researchers elsewhere [201].