Mechanical Properties And Structure Of Polymer/Montmorillonite Nanocomposites With Nacre-Like Layered Microstructures

For the past decade, some synthetic polymer materials with highly organized stratified morphology, which entitles the materials with much improved toughness and strength, have attracted a lot of academic interest. However, generating a microscopically stratified morphology, which at the same time is aligned on a macroscopic length scale, has turned out to be difficult, and still represents a great challenge in engineering commercially attractive materials in large quantities. Our researches showed that pressure-induced-flow processing (PIF-processing) in the solid state could result in an ordered arrangement of alternating layers constructed by hard and soft segments in some polymeric materials, and lead to markedly enhanced mechanical properties. In this article, we obtained polyamide 6/montmorillonite (PA6/ MMT) and polypropene (PP)/MMT hybrids with characteristic nacre-like layered microstructures by applying PIF-processing on the injection molding samples in the solid state. The mechanical performances of the samples including the impact strength, the tensile strength and the elongation at break were simultaneously much improved after PIF-processing. Various methods of characterization such as transmission electron microscope (TEM), scanning electron microscope (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and dynamic mechanical analyzer (DMA) were used to study the microscopic morphologies and the mechanisms of molecular relaxation. A series of consequence is as follows:1. Experimental results of the mechanical properties showed that the impact and tensile strengths of both PA6/MMT and PP/MMT nanocomposites, which differs much from each other in the polarity of the polymer matrix, could be improved by PIF-processing. The two yielding points as shown in the tensile stress-strain curves implies the formation of new microstructures or morphologies, which strongly depend on the conditions of PIF-processing. The addition of MMT weakens the toughing effect of PIF-processing, but facilitates the improvement of the elongation at break.2. The enhancement of the mechanical properties could be directly related to the alteration in microscopic morphologies. The nacre-like highly stratified morphologies characteristic of "brick and mortar" structures which arises from the deformation and orientation of spherulites during PIF-processing were believed to be responsible for the excellent mechanical performances. Generally speaking, the higher the processing temperature and pressure, the smaller the MMT concentration, the more obvious is the improvement of mechanical properties. The regular and parallel orderly alignment of MMT in the matrix could well indicate the changes of microscopic morphologies.3. XRD measurements along different direction further confirm the contribution of rotation and orientation of crystalline structures to the formation of layered structures. For PA6/MMT composites with low MMT concentration, the slippage or rotation of the broken lamellae leads to obvious orientation of lamellae along the PIF loading direction, resulting in the formation of layered structures. For PA6/MMT composites with high MMT concentration, a crystal transformation fromγcrystal toαcrystal happens during PIF-processing, leading to markedly different mechanical performances from those of low MMT concentration systems. A similar behavior of crystal crushing and orientation could be observed in PP/MMT composites, without any crystal transformation.4. The different effects of PIF-processing on the melting behaviors of PP/MMT and PA6/MMT systems stem from the crystal transformation and the different interactions between the matrix molecules. The changes of melting behavior could also be adjusted by the processing temperature. However, the improvement of mechanical performances due to enlarged crystallinity is rather limited in comparison with the contribution of layered structures.5. DMA measurements provide abundant information on the inner structures and morphologies. Taking the PA6/MMT system as an example: a new relaxation peak between T_g and T_m occurs after PIF-processing, which results from the much hindered movement of some molecules in the amorphous region. As to the influence of PIF-processing on the final morphologies, it appears that higher pressures facilitate the deformation and orientation of spherulites, while a higher processing temperature favors the relaxation of matrix molecules, which helps to simultaneously improve the impact and tensile performances. 6. The "brick and mortar" model and the "energy-dissipating paths" as the key strengthening and toughening mechanisms in nacre can also well explain the reasons of the ultra-high toughness and simultaneously improved tensile properties in polymer matrix-MMT nanocomposites with PIF-processing.