Clay platelets alter the thermal properties of Nylon
In recent years, filler-based nanocomposites have attracted great interest in both the academic and industrial arenas because of the remarkable improvements in the physical properties that can be achieved when comparing the materials to conventional composites.1–4 Clay is one of the most affordable fillers that has shown promise in nanocomposites. Indeed, clay can enhance the physical properties—such as stiffness, thermal stability, crystallization rate, and barrier properties—of the polymers. One such natural clay, montmorillonite, has a ‘platelet’ structure with average dimensions of 1nm in thickness and 50–100nm in diameter. These dimensions, coupled with the negative charges on the clay's surface, constrain the polymer chains to the surface of the clay, which greatly affects the polymer properties.
Nanocomposites composed of Nylon 6 (PA) and montmorillonite clay are thought to have enhanced physical properties compared with the virgin polymer because the high exfoliation of the clay provides a large aspect ratio, which results in a greater surface for interaction with the polymer. This interaction is normally mediated through the amide groups of the Nylon. Furthermore, good interfacial compatibility between clay layers and PA matrix considerably alters the crystalline phases, rheologies, and mechanical properties of the PA polymer.5 Furthermore, this interfacial compatibility can be detected by a shift of amide group vibrations, which have been confirmed using Fourier transform IR spectroscopy.6
Generally, PA exhibits polymorphic structures containing two stable crystalline phases: monoclinic α- and γ-phase. These phases arise via the transformation of intermolecular hydrogen bonds, which are sensitive to their thermal histories and are crucially related to its physical properties. Typically, PA comprising the α-crystalline phase has a melting point of 220°C whereas that of γ-crystalline phase melts at 210°C. Indeed, the interactions of the clay with the polymer matrix as well as the thermal annealing process used are important factors in controlling the crystallization and crystalline transformation of the resulting nanocomposites.7 Most experiments focus on the crystallization of PA/clay nanocomposites under various solid-state annealing processes. During these processes, the crystalline phase of PA changes from α- to γ-phase under the influence of the clay. That is, the exfoliated clay influences the crystalline structure because the clay platelets hinder the movement of the polymer chain.
Although much research has been dedicated to examining the thermal properties of nanocomposites under the solid-state annealing process, very few studies investigated the melt-state annealing process, in which annealing occurs above the melting point of the polymer. In the case of PA, melt-state annealing would involve operation above both the melting points of the polymer's crystalline phases, in the range of 220–250°C. Generally, the melting point of virgin polymer does not change with melt-state annealing. However, the presence of nanoparticles of clay can affect the polymer in such a way that the melting point is raised and the crystalline structure is altered.
Without melt-state annealing, the thermal properties of PA/clay exhibit an endothermic peak appearing at 220°C corresponding to the melting point of the α-crystalline phase (see Figure 1). The addition of clay induces the transformation from the α- to γ-crystalline phase because the interfacial interaction between the PA and clay promotes the formation of γ-crystalline phase. This is evidenced by the two melting peaks that simultaneously appear in the PA/clay composites. The peak at 220°C corresponds to the α-crystalline phase whereas the other (at 213°C) is attributed to the γ-crystalline phase. After melt-state annealing at 230°C for 30min, the thermal properties of the PA/clay nanocomposite exhibit two melting peaks: one endothermic peak (Tmγ) at 213°C, and another new endothermic peak (Tmx) at 245°C. The intensity of the new endothermic peak becomes stronger with increased clay loading under the same melt-state annealing conditions.
We also examined the elastic modulus (E′) and loss factor (tan δ) of PA and PA/clay nanocomposites in non-annealing and after melt-state annealing (see Figure 2). After melt-state annealing, PA demonstrated little change in E′, with only a slight shift toward a higher modulus. However, melt-state annealing generates a higher modulus for the PA/clay composites that is 10% by weight clay (PA/clay-10) above the glass transition temperature (Tg) rather than below Tg. Tg shows slight shifts within several degrees to higher temperature for melt-state-annealed samples. The β relaxation is constrained in the case of the melt-state-annealed PA/clay-10 compared with non-annealed samples. This can be attributed to the constraint of the amide groups by the clay. An increase in stiffness is also reflected in the reduction in the peak height of tan δ.
The novelty of this study is that a new endothermic peak is observed in PA/clay nanocomposites under melt-state annealing, which is dependent on the annealing process and the extent of the clay loading. We have shown that melt-state annealing is a new approach to enhance the thermal properties and mechanical properties of Nylon 6 via the improvement of the interaction between clay and the polymer. In the future, we plan on developing this research into the area of fiber-reinforced composites (FRP) containing clay. Clay, as nanoscale reinforcement, has the potential to enhance the performance of FRP composites.
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