The behavior of polypropylene nanocomposite fibers and films under uniaxial deformation in a partial and fully molten state was investigated. A fiber that exited the die was found to contain orientation gradient in the radial direction that was preserved even after solidification without application of a take up. The shearing effect in the die resulted in a band of oriented outer layers in which broad surfaces of the clay particles became parallel to the surface of the fibers. The polymer phase trapped between these particles exhibited moderate to high preferential orientation levels. Conversely, in the core low levels of preferred orientation were found in both the clay and the polymer phases. Upon application of take up, the presence of clay particles substantially enhanced the orientation of amorphous and crystalline phases in PP/PPgMA (maleic anhydride modified polypropylene) fibers. This was due to the substantial decrease in chain relaxation in the proximity of the clay platelets and enhancement of orientation in the polymer phase in the vicinity of particles that created amplified deformation field by their relative motions. Measurements of the clay orientation in the melt-spun fibers as they undergo “confined melting” in constrained state revealed that these naturally anisotropic nanoplatelets contributed positively to the birefringence of the fibers.
A hybrid real-time spectral birefringence technique depolarized light intensity method was used to capture the mechanistic changes that take place during heating, stretching, holding and cooling cycles of PP/PPGMA and nanocomposite films in a partially molten state. During the heating stage, the birefringence and the degree of melting of the as-cast films were determined by real-time depolarizing light intensity technique. The results indicated the initial fraction of crystallites, which govern the deformation behavior of the PP films, remained in dynamic equilibrium with the molten phase prior to the deformation after the initial transient stage. Throughout the uniaxial deformation of PP/PPgMA at this “mushy” (partially molten) state, the true stress-birefringence results showed three-regime stress optical behavior. Regime I was characterized by the slight decrease of birefringence with stress. At this small strain level, the original spherulites deformed and some of these superstructures were destroyed, resulting in broken and deformed lamellar remnants. Regime II showed a rapid rise of birefringence accompanied by orientation and stress induced crystallization. During this stretching microfibrillar morphology developed. Polymer chains approached their limit of extensibility in Regime III, where birefringence approached a plateau. After the deformation, the film was held in a stretched state at the processing temperature to simulate the holding stage. During this period, birefringence and strain increased, while stress decreased. This stage was found to involve the growth of lamellae, primarily in the transverse direction on the previously developed fibrillar morphology. During cooling, the stress initially decreased, and birefringence and strain showed significant development. Following this stage, the thermal and stress-induced crystallization under the influence of substantial contraction forces developed.
The results of the experimental investigation on as-cast nanocomposite films indicated the nanoplatelet poles are highly oriented perpendicular to the plane of the film. This leaded to a uniplanar texture in PP crystalline as well as clay phase. During the heating cycle, real-time depolarized light intensity and independent IR spectroscopy measurements revealed the presence of clay particles reduced significantly the rate of PP/PPgMA melting. The uniaxial stretching of nanocomposite films at partially molten states (T<165°C) resulted on the typical three-regime behavior of PP/PPgMA blend. Throughout the deformation, the nanoclay had two effects: (1) Reduced the decrease of birefringence in the earlier stages of the deformation (Regime I) and (2) the molecular orientation/strain induced crystallization was considerably enhanced as compared to the PP/PPgMA blend. These two effects were attributed to the suppression of molecular relaxation in the presence of clay particles. The nanocomposite films were found to exhibit a more affine deformation.