A fundamental study on the non-linear mechano-optical behavior of polyethylene naphthalate, its blends with polyetherimide and its nanocomposites

by Kanuga, Karnav.

Abstract (Summary)
Non-linear mechano-optical behavior and structural organizational process of polyethylene naphthalate (PEN)/polyetherimide (PEI) blends as well as polyethylene naphthalate nanocomposites are investigated under uniaxial deformation above their respective glass transition temperatures. A uniaxial stretching machine fitted with a spectral birefringence setup and a laser micrometer was used to monitor real-time the birefringence, true stress as well as true strains developed in the material during deformation. Offline characterization techniques such as WAXD, DSC and FT-IR were used to unravel and augment the understanding of the structural development mechanisms occurring in the material. The stress-optical behavior of PEN/PEI blends as well as PEN nanocomposites under large deformation revealed that there are three distinct stressoptical regimes with an additional glassy component appearing at low temperatures. 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 Liquid-Liquid (Tll) transition occurs wherein the material transforms from a ‘structured liquid’ to a ‘true liquid’ state at (1.08 Tg(°K)) exhibited by the disappearance of the initial glassy component as the material becomes devoid of the iii segmental correlations. At higher temperatures, where the relaxation process dominates and where the thermally induced crystallization is still suppressed, the material was found to remain in amorphous state even after being stretched to large deformation levels. The nanocomposites were found to undergo two critical structural transitions: i) Nematic-crystalline transition wherein the material stretched below a certain temperature does not undergo orientation-induced crystallization but develops a highly ordered nematic state. ii) Liquid-Liquid (Tll) transition wherein the material transforms from a ‘fixed liquid’ to a ‘true liquid’ state at 1.25 times Tg(°C) or 1.07 times Tg(°K), exhibited by the disappearance of the initial glassy component as the material becomes devoid of the inherent structure due to segmental correlations. While unfilled PEN and PEN/PEI blends remains amorphous at high temperatures due to high relaxation rates combined with suppressed thermal crystallizability, the presence of nanoplatelets was found to facilitate strain induced crystallization even at such high temperatures primarily as a result of its suppression of relaxation mechanism in its sphere of influence. The addition of nanoparticles in PEN did not change the stress-optical constant of PEN but with further deformation the birefringence developed decreased with the addition of nanoparticles due to lower crystallinity and amorphous orientation in the material. Subjecting these nanocomposites to further relaxation showed that distinct differences were observed above and below the liquid-liquid transition temperature (Tll). Relaxation below Tll exhibits an instant stress recovery as the segmental correlations remaining unbroken during stretching, elastically relax during stretching which is absent above Tll. Addition of nanoparticles suppresses the relaxation rate in PEN and even though addition iv of nanoparticles suppresses the development of crystallinity during stretching they no longer play that role during relaxation. v
Bibliographical Information:


School:The University of Akron

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:polyethylene nanostructured materials thermoplastics


Date of Publication:

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