<p>When starch granules are gelatinized, many complex structural changes occur as a result of large quantity of water being absorbed. The enlargement of granule sizes and the leaching out water-soluble macromolecules contribute to the viscoelasticity. Starch pasting behavior greatly influences the texture of a variety of food products such as canned soup, sauces, baby foods, batter mixes etc. It is important to characterize the relationship between the structure, composition and architecture of the starch granules with its pasting behavior in order to arrive at a rational methodology to design modified starch of desirable digestion rate and texture. Five types of starch used in this study were waxy maize starch (WMS), normal maize starch (NMS), waxy rice starch (WRS), normal rice starch (NRS) and STMP cross linked normal maize starch. Evolution of volume fraction φ and pasting of 8% w/w starch suspension when heated at 60, 65, 70, 75, 80, 85 and 90 °C were characterized by particle size distribution and G’, G” in the frequency range of 0.01 to 10 Hz respectively. As expected, granule swelling was more pronounced at higher temperatures. At a fixed temperature, most of the swelling occurred within the first 5 min of heating. The pastes exhibited elastic behavior with G’ being much greater than G”. G’ increased with time for waxy maize and rice starch at all times. G’ and G’’ were found to correlated only to the temperature of pasting and not change much with the rate of heating. For WMS, WRS and STMP crosslinked NMS, G’ approached a limiting value for long heating times (30 min and above) especially at heating temperatures of 85°C and above. This behavior is believed to be due to the predominant effect of swelling at small times. For normal maize and rice starch, however, G’ reached a maximum and decreased at longer times for temperatures above 80 °C due to softening of granules as evidenced by peak force measurements. For each starch sample, the experimental data of G’ at different heating temperatures and times could be collapsed into a single curve. The limiting value of G’ at high volume fraction was related to granule size and granule interfacial energy using a foam rheology model. The interfacial free energy of granules were obtained from contact angle measurements and was employed to evaluate the limiting G’. The experimental data of G’ for all starches when subjected to different heating temperatures and times were normalized with respect to the limiting value at high volume fractions. The master curve for normalized G’ was employed to predict the evolution of G’ with time for different starches which was found to agree well with experimental data of storage modulus. A mechanistic model for starch swelling that is based on Flory Huggins polymer swelling theory was employed to predict the evolution of volume fraction of swollen granules. The model accounts for the structure and composition of different types of starches through starch-solvent interaction as quantified by static light scattering, gelatinization temperature and enthalpy of gelatinization, porosity and its variation with swelling and crosslinking of starch molecules within the granule from equilibrium swelling. Consequently, one could predict the evolution of texture of these starch suspension from the knowledge of their swelling behavior. Expressing the limiting storage modulus of complete swelling (volume fraction approaching unity) of starch suspension in terms of foam rheology, we were able to normalize the storage modulus of different types of starches with respect to its limiting value which is found to fall into a master curve. This master curve when employed along with the swelling model resulted in the successful prediction of development of texture for different types of starches. The above methodology can quantify the effects of structure and composition of starch on its pasting behavior and would therefore provide a rational guideline for modification and processing of starch-based material to obtain desirable texture and rheological properties.</p> <p> </p>

Resilin is a disordered elastomeric protein and can be found in specialized regions of insect cuticles. Its protein sequence, functions and dynamic mechanical properties vary substantially across the species. Resilin can operate across the frequency range from 5 Hz for locomotion to 13 kHz for sound production. To understand the functions of different exons of resilin, we synthesize recombinant resilin-like hydrogels from different exons, and investigate the water content and dynamic mechanical properties, along with estimating surface energies relevant for adhesion. The recombinant resilin-like hydrogel has 80wt% water and does not show any sign of tack even though it satisfies the Dahlquist criterion. Finally, doubly shifted dynamic moduli master curves are developed by applying the time-temperature concentration superposition principle (TTCSP), and compared to results obtained with natural resilin from locusts, dragonflies and cockroaches. The resulting master curves show that the synthetic resilin undergoes a prominent transition, though the responsible mechanism is unclear. Possible explanations for the significant increase in modulus include the formation of intramolecular hydrogen bonds, altered structural organization, or passing through a glass transition, all of which have been reported in the literature for polymeric materials. Results show that in nature, resilin operates at a much lower frequency than this glass transition frequency at room temperature. Moreover, recombinant resilins from different clones have comparable resilience with natural resilin, though the modulus is around 1.5 decades lower. Results from the clones with and without chitin binding domains (ChBD) indicate that the transition for the clone without ChBD occurs at lower frequencies than for those with the ChBD, perhaps due to the disordered nature of the clone without ChBD. Atomistic molecular modeling is applied on the repetitive motifs of resilin and different elastomeric proteins to better understand the relationship between elastomeric behavior and amino acid sequences. Results show that the motifs form a favorable bent conformation, likely enabled by glycine's lack of steric hindrance and held in place through intramolecular hydrogen bonds. During Steered Molecular Dynamic (SMD) pulling of these motifs, the hydrogen bonds break and they reform again when the peptides are released to move freely, returning to similar bent conformations. The transition seen in the master curves of recombinant resilins might be due to either these intramolecular hydrogen bonds or to glass transition behavior, though evidence indicates that the transition probably due to the glass transition. What we learned from the synthesized recombinant resilin and simulating the repetitive motifs of resilin may be applicable to the biology and mechanics of other elastomeric biomaterials, and may provide deeper understanding of their unique properties. / Ph. D.

Les propriétés des polymères sont intéressantes pour des applications pneumatiques, de verres organiques, de joints, … Leurs propriétés mécaniques sont néanmoins mal comprises. Dans ce manuscrit, le comportement mécanique du PMMA et du CR39 est étudié en fonction du temps.Il en résulte des courbes maîtresses à partir d’expérience de relaxation de contrainte et de fluage d’indentation. D'autre part, le comportement mécanique au contact est analysé sur des expériences de fluage et recouvrance d’indentation et de rayures analysées pour la première fois dans cette thèse. Finalement, des lois de comportements sont construites, leurs précisions sont comparées grâce à des calculs par éléments finis aux expériences en contact. / Polymer's low weight, deformability and easy manufacturing make them attractive materials for tire, organic glasses, sealing applications … Their mechanical properties are nonetheless poorly understood. In particular, two fields are searched over this thesis: time dependency and contact behavior for two transparent polymer: PMMA and CR39. The mechanical behavior time dependency is observed by the construction of stress relaxation and contact master curves. The mechanical contact behavior is analyzed by indentation creep and recovery experiments. Moreover the immediate scratch recovery is measured in the thesis. Finally, the uniaxial data is used to build constitutive laws, which accuracy is compared by Finite Element Modeling to contact tests.