Date of Award

Spring 5-1-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

Committee Chair

Sergei Nazarenko

Committee Chair Department

Polymers and High Performance Materials

Committee Member 2

Jeffrey Wiggins

Committee Member 2 Department

Polymers and High Performance Materials

Committee Member 3

James Rawlins

Committee Member 3 Department

Polymers and High Performance Materials

Committee Member 4

Sarah Morgan

Committee Member 4 Department

Polymers and High Performance Materials

Committee Member 5

Robert Lochhead

Committee Member 5 Department

Polymers and High Performance Materials

Abstract

Delaminated montmorillonite (MMT) clay/ maleic anhydride grafted LLDPE nanocomposite multilayer films with alternating layers of LDPE were produced through multilayer co-extrusion. The MMT concentration within the nanocomposite layers was increased through annealing the films in the melt due to a mismatch in interdiffusion rates of the polymer layers. Analysis of the nanocomposite layers upon annealing revealed that the platelets impinged upon one another resulting in significant improvement in oxygen barrier in the multilayer system, exceeding the results of bulk nanocomposites.

Model analysis demonstrated that increasing the nanoplatelet aspect ratio or initial concentration in the filled layers would lead to even higher barrier. This lead to an investigation into high aspect ratio graphene nanoplatelete composites. Gas and fire barrier, mechanical, and thermal property enhancement of polyethylene/ short stack graphene nanoplatelet (xGnP) composite systems were thoroughly analyzed in bulk composites and multilayers. Through incorporation of a small amount of xGnP in LLDPE, a substantial increase in barrier and mechanical strength was observed, even without polymer/platelet modification. Composite layer shrinkage from controlled interdiffusion in the melt was confirmed and resulted in a decrease in permeability for the coextruded films that reflected barrier properties of composite multilayers with initially more GnPs.

Finally, to better understand clay-polymer interactions, amorphous hyperbranched polyester (HBP)/MMT nanocomposites were studied. With increasing clay content, the composites’ glass transition (Tg) and heat capacity (∆Cp) at Tg showed behavior indicative of the suppression of the HBP’s segmental mobility. This behavior correlated to changes in intercalation interlayer spacing. The fraction of the mobile amorphous fraction (MAF), which contributed to the change in heat capacity at Tg,and corresponding rigid amorphous fraction (RAF) were calculated based on the corresponding ∆Cp. Calculation of interlayer spacings from the ∆Cp data were in agreement with those determined by x-ray diffraction. The results demonstrated that changes in the RAF and MAF content depend on both the clay volume fraction and the clay morphology. Furthermore, a novel approach to estimate MAF and RAF via positron annihilation lifetime spectroscopy (PALS) from the thermal expansivities of free volume hole sizes in the liquid and glassy states was proposed.

ORCID ID

0000-0002-9746-6696

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