Four novel high performance soft thermoplastic polyurethane elastomers utilizing methylene bis(4-cyclohexylisocyanate) as a hard segment, 1,4 butanediol as a chain extender and modified low crystallinity carbonate copolymer as a soft segment were synthesized. The samples were characterized by infrared spectroscopy (FTIR), tensile, elongation, hardness, abrasion resistance and atomic force microscopy (AFM). SAXS data shows evidence of an interdomain "center-to-center" distance of 45Å. DSC traces show evidence of one glass transition temperature and a weak melting region. DMA analysis reveals a low temperature secondary relaxation and the glass to rubber transition followed by a rubbery plateau. All samples demonstrated the ability to maintain excellent physical and mechanical properties in hardness below 70 Shore A. Thermoplastic polyurethanes in this study do not possess surface tackiness usually observed in soft polyurethanes. Biocompatability testing showed no toxicity of these samples as indicated by USP Class VI, MEM Elution Cytotoxicity and Hemolysis toxicology reports. This novel type of polyurethane material targets growing markets of biocompatible polymers and can be utilized as peristaltic pump tubing, balloon catheters, enteral feeding tubes and medical equipment gaskets and seals. Polyimides are a family of engineering polymers with temperature stability, high polarity and solvent resistance. These high-performance materials are used in aerospace applications, in the production of semi-dry battery binders, and in a host of other high temperature demanding situations. However, their glass transition and melt temperatures are characteristically very high and close to one another, making them difficult to melt process and limiting them to thin film formulations from their polyamic acid precursors. Here, a new series of thermoplastic polyether-polyimides (PE-PIs) are synthesized by incorporating a polyetherdiamine monomer to reduce rigidity and break up an otherwise fully aromatic backbone as seen with most conventional polyimides. It will be shown that control of the stoichiometric ratio between the aromatic 4,4'-methylenebis(2,6-dimethylaniline) and aliphatic polyetherdiamines relative to PMDA (pyromellitic dianhydride), along with the molecular weight of the polyetheramine, can be used to tune the Tg to best balance between temperature performance and processability.

This research addresses and illuminates a little understood region of miscible polymer mixtures and demonstrates a new means of separating wax from such blends. The method, termed Deformation Induced Phase Segregation potentially eliminates need of toxic processing solvents for wax removal or recovery in these types of blends. Previous theories of polymer combinations address them exclusively as solutions or as blends, two independent classes having very different behaviors. This study provides bridge connecting these two classes by identifying crossover points between them and the behaviors exhibited therein. The blends of this form were found to be semi-miscible, forming a homogenous phase in the melt but a two-phase system in the solid, with the rheological behavior influenced by the polymer's molecular weight and architecture. It also demonstrates practical promise of this regime by introducing a mechanical compression process to separate the wax phase from such a type of blend. This process potentially permits production of ultra-high molecular weight polyethylene (UHMwPE) films and fibers by melt processing, thereby obviating need of otherwise essential but expensive and environmentally unfriendly toxic solvents.

The present thesis is focused on rational design and synthesis of π-conjugated donoracceptor-donor (D-A-D) type oligomers and D-A type copolymers. Thesis is organized in seven chapters, apart from introduction remaining six chapters are grouped into two parts (A and B). Part A deals with Chapters 2, 3, 4 and Part B contains chapters 5, 6 and 7. A brief discussion on the content of individual chapters is provided below. Chapter 1 discusses the introduction to organic solar cell with operating principles and effect of spinodal decomposition on stability of the devices is presented. The status and literature related to the improvement of life time of the organic solar cells by self-assembly approach has been explored. In addition, design and synthesis of the fluorine substituted π-conjugated organic semiconductors for n-type OFETs and OLED has been discussed. Part A This part of the thesis attempt to address some of the challenges listed below (1) Investigation of miscibility of binary components in bulk heterojunction solar cells through H-bonding approach. (2) Synthesis of new low band gap molecular semiconductors having H-bonding sites. (3) Fabrication of bulk heterojunction solar cell devices using these new molecules and exploring the photovoltaics performance. Chapter 2, donor-acceptor-donor (D-A-D) concept has been employed to design low band gap oligomers named as TTB. Barbiturate functional group has been utilized to explore the concepts of supramolecular chemistry. It is shown that, TTB molecule self-organizes via intermolecular H-bonding between barbituric acid units. Interactions between the oligothiophene subunits were also found to be important, affording nanoribbons that were observed by atomic force and transmission electron microscopy. The applicability of TTB for organic electronic applications was investigated by fabricating organic field-effect transistors (OFETs) and organic photovoltaic device. The crystalline nanoribbons were beneficial in understanding the phase morphology of PCBM and TTB blend. Chapter 3, the self-assemble property of TTB was disrupted by the substitution of methyl group on the nitrogen of the barbituric acid moiety. The optical and electrochemical properties of the new derivative have been investigated by UV-Visible spectroscopy, photoluminescence spectroscopy and cyclic voltammetry. Further investigations on the effect of self-assembly on organic solar cells were carried out by fabricating BHJ and OFET. The results proved that the self-assembly within the donor moieties led to complete phase separation between the donor and acceptor which had an adverse effect on the photovoltaic performance. Chapter 4, the conjugation of TTB was extended by the synthesis of two new copolymers by polymerizing with two oliogothiophene (terthiophene and benzobithiophene) derivatives with different donating strength. The investigation of photophysical and electrochemical properties of copolymers were studied by varying the donating strength. As we increase the donating strength of oligothiophenes, the intramolecular charge transfer band of DA copolymers was red shifted. Further, density functional theory (DFT) calculation of these materials was carried out to get insight into their photophysical properties. Part B This part of the thesis attempt to address some of the challenges listed below (1) Investigation of fluorine substituted organic semiconductos like 2,2’ bithiazole and pheanthroimidazole. (2) Synthesis of pentafluoro phenyl appended derivatives of 2,2’ bithiazole and pheanthroimidazole. (3) Fabrication of OFETs and OLEDs using these new molecules and elucidated the device performance with molecular structure. Chapter 5, pentafluorophenyl appended 2,2’-bithiazole derivatives were synthesized. The single crystal x-ray diffraction studies shows the unusual strong type-II F•••F interactions within the distance of 2.668 Å, at an angle of 89.14° and 174.15°. It also shows the usual type-I F•••F interaction within the distance of 2.825Å, at an angle of 137.38° and 135.93°. Upon bromination type-II Br•••Br interaction was observed and the packing was further stabilized by S•••Br interactions. The conjugation was further extended with different aromatic and heteroaromatic substituents and synthesized the star shaped structure. The band gap as well as the electronic energy levels was tuned by substituting various aromatic and heteroaromatic substituents. These star shaped derivatives shows electron mobilities in the order of 10-4 to 10-3cm2/Vs. Chapter 6, Novel D-A copolymers were synthesized by Stille condensation of electron acceptor fluorinated phenanthroimidazole with electron donors like terthiophene and benzobithiophene. Prior to that insoluble pentafluoro phenyl phenanthroimidazole was Nalkylated in presence of DMF which concurrently resulted in C-F activation of the pentafluoro phenyl moiety. As we increase the donor strength from benzobithiophene to terthiophene the absorbance spectra was red shifted from 446 nm to 482 nm in solution and 455 nm to 484 nm in solid state. The band gap of these copolymers was found to be 2.4 eV for PIBDT and 2.2 eV for PIDHTT from the absorbance spectra. The photoluminescence data shows that these materials are promising for the yellow colour as well as orange colour displays, of narrow wavelength range (FWHM 40 nm for PIBDT and 35 nm for PIDHTT), which can be achieved just by the manipulation of donor moieties in the copolymers. The preliminary electroluminiscence data shows high brightness of 888cd/m2 (orange luminescence) for PIDHTT and 410cd/m2 (yellow luminescence) for PIBDT. Chapter 7, Acenaphtho[1,2-b]quinoxaline based donor–acceptor type low band gap conjugated copolymers were synthesized by Stille coupling reaction with the corresponding oligothiophene derivatives. The optical properties of the copolymers were characterized by ultraviolet-visible spectrometry while the electrochemical properties were determined by cyclic voltammetry. The band gap of these polymers was found to be in the range of 1.8-2.0 eV as calculated from the optical absorption band edge. The intense charge transfer band in absorption spectra shows the significant effect of acceptor in the copolymers. X-ray diffraction measurements show weak π–π stacking interactions between the polymer chains. The OFET devices fabricated using these co-polymers showed dominant p-channel transistor behavior with the highest mobility of 1×10-3cm2/Vs.

The main focus of this thesis is in understanding the role of phase separation and microstructure in determining the physical properties of manganites. We also aim to be able to tune certain material properties using appropriate control mechanisms. For this, an understanding of the local electronic properties of manganites is essential. We thus set out to study the local electronic states in manganites using a highly sensitive probe: the scanning tunneling microscope (STM). The chapter 1 of the thesis gives an introduction to manganites, and of how manganites are susceptible to various perturbations due to closely lying ground states and an intricate interplay of their charge, spin and lattice degrees of freedom. Chapter 2 of this thesis gives a detailed account of various experimental methods used in the current investigation. In particular, we describe the design and fabrication of a variable temperature ultra-high vacuum scanning tunneling microscope (UHV-STM) which was used to carry out spatially resolved measurements on various manganite systems. This chapter also describes sample fabrication techniques by which strain and microstructure of thin films can be controlled. Other characterization techiniques, such as tranport and magnetotransport measurements, are also described in detail. Chapter 3 presents our investigation of the role of microstructure and phase separation on the DOS and local electronic properties of manganite thin films. We describe various spatially resolved STM/STS measurements carried out on La0.67Sr0.33MnO3 and La0.67Ca0.33MnO3 films having different micrsotructure and varying degrees of phase separation. We also present a theoretical model used in interpreting STS data to account for finite temperature effects and explain the existing data in this context. We use this model to gain insight into the behaviour of the DOS at EF near the MIT where thermal smearing can often give rise to misleading inferences. Chapter 4 presents our investigation on the density of states in a typical charge ordered manganite system, Pr1-xCaxMnO3. We describe STS measurements carried out on this system to study the occurrence and evolution of the charge ordering (CO) gap as a function if temperature as well as tunneling current. We report the observation of destabilization of the CO gap using tunnel current injection by an STM tip. Chapter 5 presents our investigation into the controlled and localized “nanoscale” phase separation in Pr1-xCaxMnO3 (PCMO) using an STM tip. The investigations were carried out on PCMO single crystal and PCMO epitaxial films. Our results raise the possibility of nano-fabrication of metallic nanoislands in a CO matrix using an STM tip. We demonstrate some examples of this and also raise the relevance of intrinsic phase separation in this context. We show that the “melting” of CO using tunnel current injection by an STM tip is analogous to the magnetic field-induced melting of CO on a microscopic scale. Chapter 6 summarizes the important results of this thesis work and suggests the scope for future experiments.

The thesis entitled “In situ crystallography and charge density analysis of phase transitions in complex inorganic sulfates” consists of six chapters. Structural changes exhibited by ferroic and conducting materials are studied as a function of temperature via in situ crystallography on the same single crystal. These unique experiments bring out the changes in the crystal system resulting in subtle changes in the complex polyhedra, distortions in bond lengths and bond angles, rotation of sulfate tetrahedral around metal atoms, phase separations and charge density features. The results provide new insights into the structural changes during the phase transition in terms of coordination changes, variable bond paths and variability in electrostatic potentials while suggesting possible reaction pathways hitherto unexplored. Chapter 1 gives a brief review of the basic features of structural phase transitions in terms of types of phase transitions, their mechanisms and related properties and outlines some of the key characterization techniques employed in structural phase transition studies like single crystal diffraction, thermal analysis, conductivity, dielectric relaxation, Raman spectroscopy and charge density studies. Chapter 2 deals with the group of compounds A3H(SO4)2, where A= Rb, NH4, K, Na which undergoes ferroelastic to paraelastic phase transitions with increase in temperature. Crystal structures of these compounds have been determined to a high degree of accuracy employing the same single crystal at room temperature at 100K and at higher temperatures. The data collection at 100K allows the examination of the ordered and disordered hydrogen atom positions. Rb3H(SO4)2 show two intermediate phases before reaching the paraelastic phase with increase in temperature. However, in case of (NH4)3H(SO4)2 and K3H(SO4)2, the paraelastic phase transition involves a single step. Chapter 3 deals with variable temperature in situ single crystal X-ray diffraction studies on fast super protonic conductors AHSO4, where A= Rb, NH4, K to characterize the structural phase transitions as well as the dehydration mechanism. The structure of KHSO4 at room temperature belongs to an orthorhombic crystal system with the space group symmetry Pbca and on heating to 463K it transforms to a C centered orthorhombic lattice, space group Cmca. The high temperature structure contain two crystallographically independent units of KHSO4 of which one KHSO4 unit is disordered at oxygen and hydrogen sites an shows a remarkable increase of sulfur oxygen bond distance – 1.753(4)Å. On heating to 475K, two units of disordered KHSO4 combine and loose one molecule of water to result in a structure K2S2O7 along with an ordered KHSO4 in a monoclinic system [space group P21/c]. On further heating to 485K two units of ordered KHSO4 combine, again to lose one water molecule to give K2S2O7 in a monoclinic crystal system [space group C2/c]. In the case of RbHSO4, both the high temperature structural phase transition and a serendipitous polymorph have been characterized by single crystal X-ray diffraction. The room temperature structure is monoclinic, P21/n, and on heating the crystal insitu On the diffractometer to 460K the structure changes to an orthorhombic system [space group Pmmn]. On keeping the crystallization temperature at 80°C polymorph crystals of RbHSO4 were grown. In case of NH4HSO4 both the room temperature and high temperature structures are structurally similar to those in RbHSO4, but the transition temperature is found to be 413K. Chapter 4 deals with the crystal structure, ionic conduction, dielectric relaxation, Raman spectroscopy phase transition pf a fast ion conductor Na2Cd(SO4)2. The structure is monoclinic, space group C2/c, and is built up with inter connecting CdO6 octahedra and SO4 tetrahedra resulting in a framework structure. The mobile Na atoms are present in the framework, resulting in a high ionic conductivity. The conductivity measurement shows two phase transitions one at around 280°C, which was confirmed later from DTA, dielectric relaxation, high temperature powder diffraction and Raman spectroscopy. Chapter 5 describes the structure and in situ phase separation in two different bimetallic sulfates Na2Mn1.167(SO4)2S0.33O1.1672H2O and K4Cd3(SO4)5.3H2O. These compounds were synthesized keeping them as mimics of mineral structures. The structure of Na2Mn1.167(SO4)2S0.33O1.1672H2O is trigonal, space group R . The stiochiometry can be viewed as a combination of Na2Mn(SO4)22H2O resembling the mineral Krohnkite with an additional (Mn0.167S0.333O1.167) motif. On heating the parent compound on the diffractometer to 500K and keeping the capillary at this temperature for one hour, a remarkable structural phase separation occurs with one phase showing a single crystal-single crystal transition and the other generating a polycrystalline phase. The resulting single crystal spots can be indexed in a monoclinic C2/c space group and the structure determination unequivocally suggests the formation of Na2Mn(SO4)2, isostructural to Na2Cd(SO4)z. The mechanism follows the symmetry directed pathway from the rhombohedral → monoclinic symmetry with the removal of symmetry subsequent to the loss of the two coordinated water molecules. In case of K4Cd3(SO4)5.3H2O the structure belongs to the space group P21/n at room temperature and on heating to 500K and holding the capillary at this temperature for 60 minutes as before, the CCD images can be indexed in a cubic P213 space group after the phase separation, generating K2Cd2(SO4)3, belonging to the well known Langbeinite family, while the other phase is expected to be the sought after K2Cd(SO4)2. The possible pathways have been discussed. Chapter 6 reports the charge density studies of phase transitions in a type II langbeinite, Rb2Mn2(SO4)3. The structure displays two different phases, cubic at 200K, orthorhombic at 100K respectively. After multiple refinements it is found that there are significant differences in the actual bond path (Rij) and the conventional bond length. In the cubic phase the distortions in sulfate tetrahedral are more than in the orthorhombic phase which could be the expected driving force for the phase transition to occur. Appendix contains reprints of the work done on the structures of the following: a) Rb2Cd3(SO4)3(OH)2.2H2O: structural stability at 500 K b) Structure of (NH4)2Cd3(SO4)4.5H2O c) Structure of Rb2Cd3(SO4)4.5H2O