Structure and relaxation in amorphous materials studied by solid state NMR and computer simulations

Date of Completion

January 2005

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

This thesis is aimed at investigating the structure and relaxation in amorphous materials using solid state NMR and computer simulations. Amorphous solids, unlike crystalline materials, lack long range order. Also, structure in amorphous materials has to be expressed through distribution functions instead of concise structural parameters. This leads to complications in characterizing the structure and relaxation in these materials. Solid state NMR spectroscopy proves very useful in this regard, as it directly probes the molecular environment and the nearest neighbor influences. With the growing computing capabilities and hardware improvements, computer simulations provide an alternative route to probe the amorphous nature of materials in an effective manner. ^ Unlike crystalline materials, where it is well known that plastic deformation is a result of formation and propagation of dislocations, the molecular mechanism of plastic deformation in amorphous solids has not been completely understood. On probing this problem further, it is evident from the literature that plastic deformation can be interpreted as a strain-induced glass transition. Several similarities arise between plastic deformation, which is a mechanical process, and glass transition which is a thermal process. Chapter 3 probes this concept, by comparing thermal relaxation of a supercooled liquid and deformation induced relaxation in a glass by using molecular dynamics simulations. The decay induced in the self part of the intermediate structure factor by these relaxation processes is compared. ^ Alq3 is widely used as an electron transport layer in organic light-emitting diodes (OLEDs). The structure and morphology of the Alq 3 layer directly influences the device properties. Characterizing the morphology of this layer has been a problem owing to the amorphous nature of the Alq3 deposits. We have used, for the first time, 27Al solid state NMR spectroscopy to characterize the structure and morphology of these Alq3 layers. ^ Alq3 exists as two different isomers, facial and meridianal, both varying by the symmetry of the arrangement of the ligands around the Al atom. Identifying these isomers will go a long way in characterizing the morphology of Alq3 films. In chapter 4, 27Al solid state NMR is used to clearly identify the isomers of Alq3. It has been found that the rate at which the Alq3 layers are deposited affects the performance of the resulting device. The structural disorder in these films are characterized by 27Al NMR in chapter 5. ^ Many organic solids are globally anisotropic, and their physical properties depend on the non-uniform distribution of molecular orientation. Several macroscopic probes are sensitive to the molecular order, such as optical birefringence or mechanical anisotropy. But solid state NMR spectroscopy provides molecularly resolved picture of structural order. Finally, in chapter 6, a new methodology for characterizing orientation behavior in organic solids, based on MAS deuterium rotor-synchronized NMR is introduced. This improves on a previously existing MAS rotor-synchronized NMR technique by using deuterium instead of 13C. This deuterium technique improves the sensitivity of the spectrum several folds. Combined with the conjugate orthogonal function (COF) approach which is used to extract the order parameters, this methodology can be used to extract orientation information from wide ranging organic solids. ^

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