From cellulosic based liquid crystalline sheared solutions to 1D and 2D soft materials

Liquid crystalline cellulosic-based solutions described by distinctive properties are at the origin of different kinds of multifunctional materials with unique characteristics. These solutions can form chiral nematic phases at rest, with tuneable photonic behavior, and exhibit a complex behavior associated with the onset of a network of director field defects under shear. Techniques, such as Nuclear Magnetic Resonance (NMR), Rheology coupled with NMR (Rheo-NMR), rheology, optical methods, Magnetic Resonance Imaging (MRI), Wide Angle X-rays Scattering (WAXS), were extensively used to enlighten the liquid crystalline characteristics of these cellulosic solutions. Cellulosic films produced by shear casting and fibers by electrospinning, from these liquid crystalline solutions, have regained wider attention due to recognition of their innovative properties associated to their biocompatibility. Electrospun membranes composed by helical and spiral shape fibers allow the achievement of large surface areas, leading to the improvement of the performance of this kind of systems. The moisture response, light modulated, wettability and the capability of orienting protein and cellulose crystals, opened a wide range of new applications to the shear casted films. Characterization by NMR, X-rays, tensile tests, AFM, and optical methods allowed detailed characterization of those soft cellulosic materials. In this work, special attention will be given to recent developments, including, among others, a moisture driven cellulosic motor and electro-optical devices.

Biological soft materials

The soft side of life: Recent insights into the self-assembly of biological materials, including proteins, DNA, lipids, and blood cells, are reviewed. The particular focus is on applying concepts from soft-matter physics and chemistry to understand structural self-organization.

Healable supramolecular polymeric materials

This chapter details the design, synthesis and evaluation techniques required to produce healable supramolecular materials. Key developments in supramolecular polymer chemistry that laid down the design concepts necessary to produce responsive materials are summarized. Subsequently, select examples from the literature concerning the synthesis and analysis of healable materials containing hydrogen bonding, π−π stacking and metal–ligand interactions are evaluated. The last section describes the most recent efforts to produce healable gels for niche applications, including electrolytes and tissue engineering scaffolds. The chapter also describes the design criteria and production of nano-composite materials that exhibit dramatically increased strength compared to previous generations of supramolecular materials, whilst still retaining the key healing characteristics.

Nanomechanical and phononic properties of structured soft materials

Significant interest in nanotechnology, is stimulated by the fact that materials exhibit qualitative changes of properties when their dimensions approach ”finite-sizes”. Quantization of electronic, optical and acoustic energies at the nanoscale provides novel functions, with interests spanning from electronics and photonics to biology. The present dissertation involves the application of Brillouin light scattering (BLS) to quantify and utilize material displacementsrnfor probing phononics and elastic properties of structured systems with dimensions comparable to the wavelength of visible light. The interplay of wave propagation with materials exhibiting spatial inhomogeneities at sub-micron length scales provides information not only about elastic properties but also about structural organization at those length scales. In addition the vector nature of q allows, for addressing the directional dependence of thermomechanical properties. To meet this goal, one-dimensional confined nanostructures and a biological system possessing high hierarchical organization were investigated. These applications extend the capabilities of BLS from a characterization tool for thin films to a method for unravelingrnintriguing phononic properties in more complex systems.

Low-molecular-weight gels: from the rational design to the application of versatile soft materials

Low-molecular-weight (LMW) gels are a versatile class of soft materials that gained increasing interest over the last few decades. They are made of a small percentage, often lower than 1.0 %, of organic molecules called gelators, dispersed in a liquid medium. Such molecules have a molecular weight usually lower than 1 kDa. The gelator molecules start to interact after the addition of a trigger, and form fibres, whose entanglement traps the solvent through capillary forces. A plethora of LMW gelators have been designed, including short peptides. Such gelators present several advantages: the synthesis is easy and can be easily scaled up; they are usually biocompatible and biodegradable; the gelation phenomenon can be rationalised by making small variation on the peptide scaffold; they find application in several fields. In this thesis, an overview of several peptide based LMW gels is presented. In each study, the gelation conditions were carefully studied, and the final materials were thoroughly investigated. First, the gelation ability of a fluorinated phenylalanine was assessed, to understand how the presence of a rigid moiety and the presence of fluorine may influence the gelation. In this context, a method for the dissolution of sensitive gelators was studied. Then, the control over the gel formation was studied both over time and space, taking advantage of either the pH-annealing of the gel or the reaction-diffusion of a hydrolysing reagent. Some gels were probed for various applications. Due to their ability of trapping water and organic solvents, we used gels for trapping pollutants dissolved in water, as well as a medium for the controlled release of either fragrances or bioactive compounds. Finally, the interaction of the gel matrix with a light-responsive molecule was assessed to understand wether the gel properties or the interaction of the additive with light were affected.

Exploring oligo(p-phenylenevinylene) gelators as sensors and stimuli responsive materials

Gelation provides a unique medium, which often induces organization of molecules resulting in the modulation of their optical, morphological and electronic properties thereby opening a new world of fascinating materials with interesting physical properties at nano- meso- and macroscopic levels. Supramolecular gels based on linear π-systems have attracted much attention due to their inherent optical and electronic properties which find application in organic electronics, light harvesting and sensing. They exhibit reversible properties due to the dynamic nature of noncovalent forces. As a result, studies on such soft materials are currently a topic of great interest. Recently, researchers are actively involved in the development of sensors and stimuli-responsive materials based on self-assembled π-systems, which are also called smart materials. The present thesis is divided into four chapters

Effect of the addition of antimicrobial agents on shore a hardness and roughness of soft lining materials

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Biomimetic soft matter

Biomaterials are often soft materials. There is now growing interest in designing, synthesizing and characterising soft materials that mimic the properties of biological materials such as tissue, proteins, DNA or cells. Research on biomimetic soft matter is therefore a developing theme with important emerging applications in biomedicine including tissue engineering, diagnostics, gene therapy, drug delivery and many others. There are also important basic science questions concerning the use of concepts from colloid and polymer science to understand the self-assembly of biomimetic soft materials. This issue of Soft Matter presents a selection of extremely topical articles on a diversity of biomimetic soft matter systems. I thank the contributors for this quite remarkable collection of papers, which report many fascinating discoveries and insights.

Tetrafenilciclopentadienon-derivati ed analoghi: sintesi, "self-assembly" e caratterizzazione reologica

Gels are materials that are easier to recognize than to define. For all practical purpose, a material is termed a gel if the whole volume of liquid is completely immobilized as usually tested by the ‘tube inversion’ method. Recently, supramolecular gels obtained from low molecular weight gelators (LMWGs) have attracted considerable attention in materials science since they represent a new class of smart materials sensitive to external stimuli, such as temperature, ultrasounds, light, chemical species and so on. Accordingly, during the past years a large variety of potentialities and applications of these soft materials in optoelectronics, as electronic devices, light harvesting systems and sensors, in bio-materials and in drug delivery have been reported. Spontaneous self-assembly of low molecular weight molecules is a powerful tool that allows complex supramolecular nanoscale structures to be built. The weak and non-covalent interactions such as hydrogen bonding, π–π stacking, coordination, electrostatic and van der Waals interactions are usually considered as the most important features for promoting sol-gel equilibria. However, the occurrence of gelation processes is ruled by further “external” factors, among which the temperature and the nature of the solvents that are employed are of crucial importance. For example, some gelators prefer aromatic or halogenated solvents and in some cases both the gelation temperature and the type of the solvent affect the morphologies of the final aggregation. Functionalized cyclopentadienones are fascinating systems largely employed as building blocks for the synthesis of polyphenylene derivatives. In addition, it is worth noting that structures containing π-extended conjugated chromophores with enhanced absorption properties are of current interest in the field of materials science since they can be used as “organic metals”, as semiconductors, and as emissive or absorbing layers for OLEDs or photovoltaics. The possibility to decorate the framework of such structures prompted us to study the synthesis of new hydroxy propargyl arylcyclopentadienone derivatives. Considering the ability of such systems to give π–π stacking interactions, the introduction on a polyaromatic structure of polar substituents able to generate hydrogen bonding could open the possibility to form gels, although any gelation properties has been never observed for these extensively studied systems. we have synthesized a new class of 3,4-bis (4-(3-hydroxy- propynyl) phenyl) -2, 5-diphenylcyclopentadienone derivatives, one of which (1a) proved to be, for the first time, a powerful organogelator. The experimental results indicated that the hydroxydimethylalkynyl substituents are fundamental to guarantee the gelation properties of the tetraarylcyclopentadienone unit. Combining the results of FT-IR, 1H NMR, UV-vis and fluorescence emission spectra, we believe that H-bonding and π–π interactions are the driving forces played for the gel formation. The importance of soft materials lies on their ability to respond to external stimuli, that can be also of chemical nature. In particular, high attention has been recently devoted to anion responsive properties of gels. Therefore the behaviour of organogels of 1a in toluene, ACN and MeNO2 towards the addition of 1 equivalent of various tetrabutylammonium salts were investigated. The rheological properties of gels in toluene, ACN and MeNO2 with and without the addition of Bu4N+X- salts were measured. In addition a qualitative analysis on cation recognition was performed. Finally the nature of the cyclic core of the gelator was changed in order to verify how the carbonyl group was essential to gel solvents. Until now, 4,5-diarylimidazoles have been synthesized.

Radiometric Method for Emissivity Retrieval in High Reflective Materials

High reflective materials in the microwave region play a very important role in the realization of antenna reflectors for a broad range of applications, including radiometry. These reflectors have a characteristic emissivity which needs to be characterized accurately in order to perform a correct radiometric calibration of the instrument. Such a characterization can be performed by using open resonators, waveguide cavities or by radiometric measurements. The latter consists of comparative radiometric observations of absorbers, reference mirrors and the sample under test, or using the cold sky radiation as a direct reference source. While the first two mentioned techniques are suitable for the characterization of metal plates and mirrors, the latter has the advantages to be also applicable to soft materials. This paper describes how, through this radiometric techniques, it is possible to characterize the emissivity of the sample relative to a reference mirror and how to characterize the absolute emissivity of the latter by performing measurements at different incident angles. The results presented in this paper are based on our investigations on emissivity of a multilayer insulation material (MLI) for space mission, at the frequencies of 22 and 90 GHz.

Three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films

A process for preparing three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films is introduced. Samples are made from commercial FEP films by means of laser cutting, laser bonding, electrode evaporation, and high-field poling. The observed dielectric-resonance spectra demonstrate the piezoelectricity of the FEP sandwiches. Piezoelectric d (33) coefficients up to a few hundred pC/N are achieved. Charging at elevated temperatures can increase the thermal stability of the piezoelectrets. Isothermal experiments for approximately 15 min demonstrate that samples charged at 140A degrees C keep their piezoelectric activity up to at least 120A degrees C and retain 70% of their initial d (33) even at 130A degrees C. Acoustical measurements show a relatively flat frequency response in the range between 300 Hz and 20 kHz.

pH Influence on the stability of foams with protein-polysaccharide complexes at their interfaces

Food foams such as marshmallow, Chantilly and mousses have behavior and stability directly connected with their microstructure, bubble size distribution and interfacial properties. A high interfacial tension inherent to air/liquid foams interfaces affects its stability, and thus it has a direct impact on processing, storage and product handling. In this work, the interactions of egg albumin with various types of polysaccharides were investigated by drop tensiometry, interfacial rheology and foam stability. The progressive addition of egg albumin and polysaccharide in water induced a drop of the air-water surface tension which was dependent on the pH and polysaccharide type. At pH 4, that is below the isoeletric point of egg albumen (pI = 4.5) the surface tension was decreased from 70 mN/m to 42 mN/m by the presence of the protein, and from 70 mN/m to 43 mN/m, 40 mN/m and 38 mN/m by subsequent addition of xanthan, guar gum and kappa-carrageenan, respectively. At pH 7.5 the surface tension was decreased from 70 mN/m to 43 mN/m by the simultaneous presence of the protein and kappa-carrageenan. However, a higher surface tension of 48 and 50 mN/m was found when xanthan and guar gum were added, respectively, when compared with carrageenan addition. The main role on the stabilization of protein-polysaccharide stabilized interfaces was identified on the elasticity of the interface. Foam stability experiments confirmed that egg-albumin/kappa-carrageenan at pH below the protein isoeletric point are the most efficient systems to stabilize air/water interfaces. These results clearly indicate that protein-polysaccharide coacervation at the air/water interface is an efficient process to increase foam stability. (C) 2009 Elsevier Ltd. All rights reserved.

Characterization of optically driven fluid stress fields with optical tweezers

We present a controlled stress microviscometer with applications to complex fluids. It generates and measures microscopic fluid velocity fields, based on dual beam optical tweezers. This allows an investigation of bulk viscous properties and local inhomogeneities at the probe particle surface. The accuracy of the method is demonstrated in water. In a complex fluid model (hyaluronic acid), we observe a strong deviation of the flow field from classical behavior. Knowledge of the deviation together with an optical torque measurement is used to determine the bulk viscosity. Furthermore, we model the observed deviation and derive microscopic parameters.

Modifying fish gelatin electrospun membranes for biomedical applications: cross- linking and swelling behavior

Development of suitable membranes is a fundamental requisite for tissue and biomedical engineering applications. This work presents fish gelatin random and aligned electrospun membranes cross-linked with glutaraldehyde (GA). It was observed that the fiber average diameter and the morphology is not influenced by the GA exposure time and presents fibers with an average diameter around 250 nm. Moreover, when the gelatin mats are immersed in a phosphate buffered saline solution (PBS), they can retain as much as 12 times its initial weight of solution almost instantaneously, but the material microstructure of the fiber mats changes from the characteristic fibrous to an almost spherical porous structure. Cross-linked gelatin electrospun fiber mats and films showed a water vapor permeability of 1.37 ± 0.02 and 0.13 ± 0.10 (g.mm)/(m2.h.kPa), respectively. Finally, the processing technique and cross-linking process does not inhibit MC-3T3-E1 cell adhesion. Preliminary cell culture results showed good cell adhesion and proliferation in the cross-linked random and aligned gelatin fiber mats.

Rheology of the cytoskeleton as a fractal network

We model the cytoskeleton as a fractal network by identifying each segment with a simple Kelvin-Voigt element with a well defined equilibrium length. The final structure retains the elastic characteristics of a solid or a gel, which may support stress, without relaxing. By considering a very simple regular self-similar structure of segments in series and in parallel, in one, two, or three dimensions, we are able to express the viscoelasticity of the network as an effective generalized Kelvin-Voigt model with a power law spectrum of retardation times L similar to tau(alpha). We relate the parameter alpha with the fractal dimension of the gel. In some regimes ( 0 < alpha < 1), we recover the weak power law behaviors of the elastic and viscous moduli with the angular frequencies G' similar to G" similar to w(alpha) that occur in a variety of soft materials, including living cells. In other regimes, we find different power laws for G' and G".