Influence on hydrogen bonding efficiency of structural modification

Secondary interactions have a strong influence in crystallization or self assembling process of materials in general. Very often in macromolecular structures, having amide motifs, the presence of hydrogen bonding governs physical and mechanical behavior. In hydrogen bonded polymers, structural organization is mainly prevailed by density of hydrogen bonding and length of aliphatic or aromatic units between the hydrogen bonding motifs. In nature, hydrogen bonded polymers such as proteins, use water with ions as a solvent, and depending upon nature of ions its higher order organization is directed. For an example, spider spins the silk-protein web using ionic water, where the desired tensile strength depends on the amount of amorphous component in the semi-crystalline silk-protein. During spinning process of the protein, the presence of water and ions facilitate processing by shielding and de-shielding of the hydrogen bonding. In this thesis, we attempt to unravel the process of shielding and de-shielding of the hydrogen bonded motifs in synthetic polymers and model compounds. It is also known that hydrogen bonding in water at elevated temperatures, i.e. above its boiling point, is strongly suppressed thus drastically improving the mobility of the water molecules. The presence of ions, either kosmotropic or chaotropic, also influences the hydrogen bonding leading to the increase or decrease in the boiling point. With the help of solid state NMR and molecular dynamics simulations influence of hydrogen bonding in water, in the presence of monovalent and divalent ions, has been investigated. It is demonstrated that in the presence of smaller ions, irrespective of charge, water can form hydration shell due to higher interaction potentials and thus display hydrophilicity. Whereas, the larger ions cause perturbation in the hydrogen bonding efficiency between the water molecules and exhibit hydrophobic nature. The molecular insight on the structural variation is summarized in Chapter 2 of the thesis. Understandings of the water-ion interaction (chapter 2) were then applied to study the dissolution process of synthetic aliphatic polyamides. The time resolved NMR studies were performed to follow the molecular origin of the dissolution process of polyamide (PA46) in water with or without ions. During dissolution process of PA46 in water, close to the dissolution temperatures of PA46, two distinct 1H resonances from water were observed. One of the two resonances was associated with water in the vicinity of PA46 and the other to the bulk state of water. On further heating, the signal from water associated with PA46 dominated. This sudden change in water environments suggested that water molecules, which have escaped the dense hydrogen-bonded network of bulk water, can diffuse into the structure of PA46, and thus trigger the dissolution of PA46. The observed dissolution temperature was more than 100 °C below the melting temperature of the polymer, without any chemical degradation. On cooling, recrystallization of PA46 from aqueous solution showed incorporation of water molecules into the polymer structure. Introduction of chaotropic salts based on Hofmeister ions (LiI, CaI2) in different concentrations resulted in overall weakening of the hydrogen-bonded network of the bulk water. On heating, depopulation of the hydrogen bonding between the water molecules took place and resulted in decreased chemical shift. This reduced hydrogen-bond efficiency between the water molecules facilitated dissolution of PA46 at much lower temperatures compared to pure water and consequently resulted in complete suppression of crystallization even at room temperature. These influences of ion size in the structural organization of water molecules and on the dissolution process are described in Chapter 3 of this thesis. Further, in general, interaction of water with amide motifs was investigated. For the purpose couple of oxalamides based model compounds were synthesized (a) Diethyl 4,5,14,15-tetraoxo-3,6,13,16-tetraazaoctadecane-1,18-dioate (ßala oxa (CH2)6 oxa ßala ) and (b) N1,N2-bis (3-methoxypropyl) 2,11-dioxo-3,10-diaza-1,12-dodecanamide(Meo oxa (CH2)6 oxa Meo. These compounds are used as model systems to overcome the ambiguity of chain folding in synthetic polyamides. The structural and conformational changes have been followed with the help of X-ray diffraction and NMR studies. The study revealed that, prior to melting two phase transitions are present in the melt as well as in the water crystallized model compounds. These low and high temperature phase transitions were associated with crystal-to crystal modification resulting from the conformational changes at low temperature, and crystallographic changes at high temperature. The two transitions are associated with the crystal to crystal transformations. Changes in the molecular conformational of the model compounds, during the low temperature phase transition, were due to the temperature induced release in the constrained environment of the end groups. The high temperature phase transition was associated with the introduction of gauche conformers in the aliphatic units between the oxalamide motifs. These results are summarized in Chapter 4 of this thesis. Finally to investigate the influence of rigid and flexible segments on the hydrogen bonding efficiency, poly (amide-aramide)s were synthesized. Chapter 5 of this thesis describes the influence of varying methylene unit length on thermal properties. Melting in the poly (amide-aramid)s was feasible in the poly (amide-aramid) having at least eight methylene units between the rigid aramid motifsPolymers having methylene segments below eight degraded prior to melting. A detailed structural investigation using solid-state NMR, FTIR spectroscopy, WAXD, in combination with crystallographic modelling (Cerius software) revealed the cause of lower melting temperature in poly(amide-aramide)s having eight methylene units. These investigations additionally helped in understanding the influence of methylene segment incorporation on the crystal packing and ring dynamics of the aromatic component. Specifically, the presence of longer methylene segments provided an additional degree of freedom, originating from increasing chain flexibility at elevated temperatures which eventually resulted in melting of the polymer.