The mechanooptical behavior and structural organization processes in melt miscible PEN-PEI blends have been investigated in the rubbery state as influenced by blend composition, stretching temperature, and deformation rate. This was accomplished using a spectral birefringence system integrated in a specially built uniaxial stretching machine 1,2 where real-time birefringence, true stress, and true strain are monitored during the course of the deformation. Three distinct stress-optical regimes have been observed with an additional glassy component. The final structure and deformation behavior of the blends have been mapped out in a dynamic phase diagram showing that the material undergoes three critical structural transitions. At low temperatures near Tg the polymer remains in a nematic-like state, and orientation-induced crystallization occurs only above a certain stretching temperature. At intermediate temperatures the liquid-liquid (Tll) transition occurs wherein the material transforms from a "structured liquid" to a "true liquid" state at (1.08Tg (K)) exhibited by the disappearance of the initial glassy component as the material becomes devoid of the segmental correlations. At higher temperatures, where the relaxation process dominates and where the thermal induced crystallization is still suppressed, the material was found to remain in the amorphous state even after being stretched to large deformation levels.
The effects of nanoparticle concentration
and processing conditions on the relaxation behavior of PEN nanocomposites
are investigated using mechano optical techniques where birefringence,
true stress and true strain are measured while subjecting the polymer
films to uniaxial deformation followed by relaxation. For this purpose,
two different stretching temperatures were employed to stretch the
films containing 0.5 and 2 wt % nanoparticles: one above and the other
below the T
ll (liquid–liquid transition).
Increasing the temperature as well as the addition of nanoparticles
suppresses the spontaneous deformation processes leading to sharp
increase in true strain. An instantaneous stress drop is observed
during relaxation below the T
ll corresponding
to the initial glassy component observed in the stress-optical behavior
during stretching. This stress is attributed to the presence of segmental
rigid correlations that were not broken during the stretching stage.
This behavior was found to be absent above the liquid–liquid
transition as they are already melted. Nanoparticles were found to
act as suppressors of crystallinity during stretching. Their presence
reduces likelihood of strain crystallization as they reduce the relaxation
of oriented chains into favorable registry with each other to crystallize.
During relaxation, the presence of nanoparticles was found to increase
the crystallinity. Their presence increases the population of oriented
amorphous chains during deformation that relax into favorable registry
with each other leading to increase in crystallinity.
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