Study of soft materials

Since the last few years, the study of soft materials with an atomic force microscope (AFM) has shown that it gives much more than just topography. The idea is either to put the tip in contact with the sample and measure the cantilever deflection variations or to vibrate the tip in the vicinity of the sample and to measure changes of the oscillating behavior. 

In this paper we will compare different experiments in contact and with an oscillating tip to show their contribution for the study of soft material. In static contact mode, force curves and friction loops are recorded while in tapping a systematic investigation of approach-retract curves is presented. A model sample is used monodisperse polystyrene films of different molecular weights (MJ bulk mechanical properties and molecular weight dependence of the glass transition temperature. In order to emphasize the inherent difficulties encountered with an AFM, we begin with a detailed discussion of the technical conditions. 

In this section, we will briefly discuss aspherical colloidal particles. Experimental and computational studies of soft materials find significant effects of the particle geometry on the properties of amorphous structures. These include the fractal dimension of colloidal aggregates in diffusion-limited cluster aggregation (Mohraz et al. 2004), percolation threshold of anisometric particles (Garboczi et al. 1995, Yi and Sastry 2004), and maximum packing density of particulate materials (Donev et al. 2004, Mohraz et al. 2004). 

Modification of an AFM to operate in a dynamic mode aids the study of soft biological materials [58]. Here a stiff cantilever is oscillated near its resonant frequency with an amplitude of about 0.5 nm forces are detected as a shift to a new frequency... 

Rheology concerns the study of the deformation and flow of soft materials when they respond to external stress or strain. If the ratio of its shear stress and shear rate is a straight line, the material is termed Newtonian otherwise, it is termed non-Newtonian (Figure 4.3.2(a)). As the slope of the curve is the viscosity rj, a shear-thinning fluid exhibits a reduced viscosity as the shear stress increases, whereas a shear-... 

The application of X-ray scattering for the study of soft matter has a long tradition. By shining X-rays on a piece of material, representative structure information is collected in a scattering pattern. Moreover, during the last three decades X-ray scattering has gained new attractivity, for it developed from a static to a dynamic method. 

X-rays are electromagnetic radiation with short wavelengths of about 0.01 to 10 nm. X 0.15 nm is the typical wavelength for the study of soft condensed matter. Whenever X-rays are interacting with matter, their main partners are the electrons in the studied sample. Thus X-ray scattering is probing the distribution of electron density, p (r), inside the material. 

The current trend towards miniaturization of functional systems and devices has driven the study of confinement effects (finite film thickness and the nature of the binding interfaces) on the fundamental physical properties of soft materials. Rapid developments of novel sensor and lab-on-chip technologies, and of polymer-based stimuli-responsive materials, raise the question of changes in solvent-polymer interactions under confinement. 

The book collects contributions in the field of characterization of materials also, and these are reported in various chapters. In addition to this, a particular chapter is dedicated to interesting new calorimetric approaches to the study of soft-matter three-dimensional organization intended to demonstrate methods able to make a contribution to our understanding of hierarchical porous structures in which matter and void are organized in regular and controlled patterns. 

From a theoretical standpoint, block copolymers are ideal systems for studying many fundamental issues in the thermodynamics and dynamics of self-assembly of soft materials. This is so because of the relatively large length scale of macromolecules, the slow dynamics associated with the relevant structural relaxations, and the ease of control of the molecular characteristics, such as the molecular weight, compositions, and architecture. 

The study of soft condensed matter (polymers, biopolymers and biological materials) is likely to be the area of INS spectroscopy that sees the largest growth in the next decade. The study of model compounds is a thriving area, where the analysis follows the routes described in this book the use of the isolated molecule approximation where intermolecular interactions are weak and ab initio codes for periodic systems where the interactions are significant. Re-analysis of older data is an activity that can yield further insights into the systems. 

The relatively few QM studies of these materials reflect both the structural complexity (including low symmetry) and also the strong scattering associated with the oxygen potential, which makes the generation of reliable and efficient pseudopotentials for PW-LDA calculations difficult. There has been a concerted effort to solve this problem over the last few years which has yielded a new family of ultra-soft pseudopotentials with which oxygen has been described with plane wave cut-offs as low as 500 eV (Vanderbilt, 1990). In the all electron LCAO-HF method the major approximation is associated with the choice of basis set. This has been studied in some detail and reliable basis sets developed (Nada et al., 1990). In the absence of analytic forces, the use of this method to optimize fully the geometry of such complex structures is rare. 

Linear polymers and colloids represent the most well-studied classes of soft materials. These systems have considerable differences in the dynamics and rheological behavior. On one hand, linear polymers develop mainly short-range interactions, meaning that very high concentrations are needed to achieve... 

Computational studies of equilibrium and transport properties of simple models of molecular systems have become an important part of statistical-mechanical research. Such studies include Monte Carlo calculations, which are described by Valleau in Volume 5 and the molecular dynamics calculations described in this chapter and in Chapter 2 by Kushick and Berne. Here we consider the molecular dynamics (MD) method for systems of hard-core particles. Because of the simplicity of the intermolecular interaction, the integration of the classical equations of motion is trivial and the methods used for the study of various material properties are frequently different from those for soft potentials. 

In this section, the basic principles of high-resolution calorimetry and its modes of operation are presented. Moreover, the use of the method in order to study phase diagrams is explained. The theoretical background of various high-resolution calorimetric techniques was developed in the 1960s by independent groups [26-28]. In the years that followed these techniques were used in numerous studies of soft and solid materials, revealing many subtle features of phase transitions and critical phenomena [29, 30]. 

Polymers generally exhibit complex tribological behaviors due to different energy dissipation mechanisms, notably those induced by internal friction (chain movement), which is dependent on both time and temperature. Polymer friction is then governed by interfacial interactions and viscoelastic dissipation mechanisms that are operative in the interfacial region and also in the bulk, especially in the case of soft materials. Friction of a polymer can be closely linked to its molecular structure. The role of chain mobility has been studied in the case of elastomers, based on dissipation phenomena during adhesion and friction processes of the elastomer in contact with a silicon wafer covered by a grafted layer [1-5]. 

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