Influence of spatial constraints in non-crystalline regions on the polymer dynamics in semi-crystalline polyethylene: a solid state NMR study

A broad variety of solid state NMR techniques were used to investigate the chain dynamics in several polyethylene (PE) samples, including ultrahigh molecular weight PEs (UHMW-PEs) and low molecular weight PEs (LMW-PEs). Via changing the processing history, i.e. melt/solution crystallization and drawing processes, these samples gain different morphologies, leading to different molecular dynamics. Due to the long chain nature, the molecular dynamics of polyethylene can be distinguished in local fluctuation and long range motion. With the help of NMR these different kinds of molecular dynamics can be monitored separately. In this work the local chain dynamics in non-crystalline regions of polyethylene samples was investigated via measuring 1H-13C heteronuclear dipolar coupling and 13C chemical shift anisotropy (CSA). By analyzing the motionally averaged 1H-13C heteronuclear dipolar coupling and 13C CSA, the information about the local anisotropy and geometry of motion was obtained. Taking advantage of the big difference of the 13C T1 relaxation time in crystalline and non-crystalline regions of PEs, the 1D 13C MAS exchange experiment was used to investigate the cooperative chain motion between these regions. The different chain organizations in non-crystalline regions were used to explain the relationship between the local fluctuation and the long range motion of the samples. In a simple manner the cooperative chain motion between crystalline and non-crystalline regions of PE results in the experimentally observed diffusive behavior of PE chain. The morphological influences on the diffusion motion have been discussed. The morphological factors include lamellar thickness, chain organization in non-crystalline regions and chain entanglements. Thermodynamics of the diffusion motion in melt and solution crystallized UHMW-PEs is discussed, revealing entropy-controlled features of the chain diffusion in PE. This thermodynamic consideration explains the counterintuitive relationship between the local fluctuation and the long range motion of the samples. Using the chain diffusion coefficient, the rates of jump motion in crystals of the melt crystallized PE have been calculated. A concept of "effective" jump motion has been proposed to explain the difference between the values derived from the chain diffusion coefficients and those in literatures. The observations of this thesis give a clear demonstration of the strong relationship between the sample morphology and chain dynamics. The sample morphologies governed by the processing history lead to different spatial constraints for the molecular chains, leading to different features of the local and long range chain dynamics. The knowledge of the morphological influence on the microscopic chain motion has many implications in our understanding of the alpha-relaxation process in PE and the related phenomena such as crystal thickening, drawability of PE, the easy creep of PE fiber, etc.

Tuning of magnetic exchange interactions between organic radicals through bond and space

The dissertation entitled "Tuning of magnetic exchange interactions between organic radicals through bond and space" comprises eight chapters. In the initial part of chapter 1, an overview of organic radicals and their applications were discussed and in the latter part motivation and objective of thesis was described. As the EPR spectroscopy is a necessary tool to study organic radicals, the basic principles of EPR spectroscopy were discussed in chapter 2. rnAntiferromagnetically coupled species can be considered as a source of interacting bosons. Consequently, such biradicals can serve as molecular models of a gas of magnetic excitations which can be used for quantum computing or quantum information processing. Notably, initial small triplet state population in weakly AF coupled biradicals can be switched into larger in the presence of applied magnetic field. Such biradical systems are promising molecular models for studying the phenomena of magnetic field-induced Bose-Einstein condensation in the solid state. To observe such phenomena it is very important to control the intra- as well as inter-molecular magnetic exchange interactions. Chapters 3 to 5 deals with the tuning of intra- and inter-molecular exchange interactions utilizing different approaches. Some of which include changing the length of π-spacer, introduction of functional groups, metal complex formation with diamagnetic metal ion, variation of radical moieties etc. During this study I came across two very interesting molecules 2,7-TMPNO and BPNO, which exist in semi-quinoid form and exhibits characteristic of the biradical and quinoid form simultaneously. The 2,7-TMPNO possesses the singlet-triplet energy gap of ΔEST = –1185 K. So it is nearly unrealistic to observe the magnetic field induced spin switching. So we studied the spin switching of this molecule by photo-excitation which was discussed in chapter 6. The structural similarity of BPNO with Tschitschibabin’s HC allowed us to dig the discrepancies related to ground state of Tschitschibabin’s hydrocarbon(Discussed in chapter 7). Finally, in chapter 8 the synthesis and characterization of a neutral paramagnetic HBC derivative (HBCNO) is discussed. The magneto liquid crystalline properties of HBCNO were studied by DSC and EPR spectroscopy.rn