Ananta Kumar Mishra    - research student -

Abstract: Modification of surface of clay platelets by ionic and covalent modification techniques renders it to be easily dispersed in polymers like, Thermoplastic Polyurethane (TPU). Only ionic or covalent modification techniques in isolation are not sufficient to achieve uniform nanoscale dispersion of Laponite (synthetic hectorite nanoclay) in TPU. Hence, the dual modification of Laponite (both ionic and covalent) is performed and the effects of the modification on the morphology, thermal and rheological behaviors of the TPU-modified clay nanocomposites have been studied. The degree of exfoliation of clay platelet in TPU matrix is found to be higher for dual functionalized nanoclays compared to their singly modified counterparts. Interestingly, dual modified Laponite clays prepared by using two different techniques (ionic followed by covalent and covalent followed by ionic) exhibit different morphology and properties. The dual modified clays significantly alter the equilibrium morphology of TPU. The storage modulus of the dual modified Laponite-TPU nanocomposite in the glassy region (at -60oC) and in the rubbery region (+98oC) is improved by 172.8% and 85%, respectively as compared to the neat TPU. Similarly, the onset of degradation is found to be enhanced by 28.7oC as compared to the neat TPU.

Abstract: Novel Thermoplastic Polyurethane (TPU)-dual modified Laponite clay nanocomposites were prepared by ex-situ and in-situ techniques. Two types of modified clays used in this work differ from each other by the number of active functional groups (tethering). Modified nanoclays are characterized by FTIR, Solid State NMR, XRD and TGA. Structural differences in the modified clays lead to novel tubular, elliptical and spherically aggregated morphologies of clays together with the hard segments of TPU. Changes in such morphology result in the difference in segmental relaxation, mechanical and rheological properties of the nanocomposites. In-situ prepared nanocomposites register inferior properties as compared to their ex-situ counterparts. The percent improvement in tensile strength and elongation at break of the ex-situ prepared nanocomposites with the modified clay having lesser tethering are found to be 67% and 208%, respectively. Thermal stability is enhanced by 35 oC as compared to that of the neat TPU.

Abstract: The effect of nanoclays (modified Laponite and Cloisite) on the dynamic modulus, dynamic viscosity, and relaxation time of thermoplastic polyurethane (TPU)-based nanocomposites is studied by using the dynamic mechanical rheometer in strain,t emperature, frequency sweeps, and stress relaxation experimental modes. Cloisite20A preferentially associates to the soft domains and dodecylamine-modified LaponiteRD favors the hard domains of the TPU, where as cetyl trimethyl ammonium bromide-modified LaponiteRD doesnot show any preference and gets distributed both in the hard and soft domains randomly. Cloisite-based nanocomposites, having longer diskette size,possess greater dynamic modulus and viscosity than Laponite-based ones. The change in modulus and viscosity of the nanocomposites over the range of frequencies registers a completely different behavior at different temperature regimes depending on the size of diskettes of the nanoclays and their distribution (before and after the softening of the harddomains). Addition of clay is found to increase the elastic component of stress relaxation of the TPU at 120oC. This behavior is more prominent in the case of Cloisite-based nanocomposites as compared to their Laponite-based counterparts. The morphology correlates well with the dynamic rheological properties of these nanocomposites.

Abstract: Thermoplastic polyurethane (TPU) is a versatile polymer exhibiting many engineering applications. In this article, two varieties of clay (Cloisite and Laponite RD) have been used to prepare TPU-based nanocomposites. They differ in, chemical composition, hydrophobicity, aggregation tendency, and dispersibility in a particular solvent. A detailed investigation of the thermal, morphological, and rheological behavior reflects the affinity of Cloisite towards the soft segment, whereas it is the hard segment for modified Laponite. The maximum improvement in onset degradation temperature has been observed to be 17.5 and 8.3 °C for Cloisite and Laponite, respectively. Five percent Cloisite-filled sample shows optimum storage modulus in the glassy region where as it is the 10% filled sample at the rubbery region. However, the trend remains indifferent both in rubbery and glassy regions for Laponite, and properties have been found optimum for 3% filled sample. To explore the behavior in the terminal and flow regions, dynamic rheological experiments were performed in low shear rate. Variation in dynamic rheological properties can be explained well on the basis of the combination of partly exfoliated, intercalated, and aggregated structures of the nano clay inside the TPU matrix, depending on their nature and preferential association with different segments.

Abstract: The thesis aims to explore the structure-property relationship between the TPU and the TPU-modified Laponite clay nanocomposites prepared following solution mixing, ex-situ and in-situ techniques by using various modified clays. Laponite clay has been modified by ionic, covalent and dual modification techniques. Initially, modified clays were dispersed in the commercial TPU by using simple solution mixing technique. Finally, the most suitable grade of the modified Laponite clay was used for the preparation of TPU-clay nanocomposite by using ex-situ and in-situ techniques using the in-house synthesized TPU as a matrix. Effect of tethering with functional modifiers has also been compared by using another grade of modified Laponite prepared in a similar technique. The modified Laponite clays have been found to act as hard domain markers. Improvements in technical properties including the thermal stability have been observed to be a strong function of the degree of dispersion of the modified nanoclays in the TPU matrix. Novel tubular and elliptical morphologies have been evolved by using dual modified Laponite clays in combination with the hard domains of the TPU. The thermal stability of the nanocomposite has been found to increase by 35 oC, as compared to the neat TPU. The tensile strength has been observed to increase by 67% as compared to the neat TPU with merely 3% clay content. Increased numbers of tethering in the modified clays have been observed to be detrimental to the improvements in technical properties of the TPU-clay nanocomposite systems. In the system studied ex-situ prepared nanocomposites have been found to offer better improvements in technical properties as compared to the in-situ prepared nanocomposites.