Rational Composite Design

A number of papers studied polyethylenes in PLA as deliberately phase-separated areas to provide better toughness [24], But without compatibility with the PLA, these are likely to be sources of weakness rather than strength. Hence, there is a trend to use block copolymer compatibiUzers such as PLA/PE. As long as the PE is at a low level [25], although if the PE is above 1% the polymer cannot claim to be to composting standards, merely that it is bio-based.  

A helpful review of glass transition temperatures in PLA and its blends provides a good summary of many of these issues [26]. 

With a relatively young polymer such as PLA, it is possible to put a new product out into the marketplace with a potentially serious technical liability it might break unexpectedly in use. Over the years, end users for other polymers have learned by trial and (sometimes disastrous) error to avoid the problem, but thermodynamics can help us avoid the error by rational processes. The problem is that a plastic part in contact with a particular liquid might crack for no apparent reason. The crack will appear at a point of stress in the part, even though mechanical tests will have shown that the stresses pose no risk at all. This phenomenon is called environmental stress cracking (ESC). 

To say that a polymer will not dissolve in a solvent is to say that the free energy of the polymer and solvent system is lower than that of polymer plus solvent. But if some extra energy is supplied, then the balance can change. Physical stress on a part is sufficient to add that extra energy and therefore allow the hitherto stable polymer and solvent system to become the weak polymer plus solvent, which then breaks. 

It is hard to quantify that statement, but fortunately we do not have to. If a solvent is inside the polymer HSP sphere, then we do not have an ESC problem because it is already obvious that the solvent and polymer are incompatible. In other words, a good solvent does not give ESC (i.e., long-term damage under stress) because the short-term damage is readily seen in short-term tests. On the other hand, if a solvent is well outside the sphere, then experience shows that extra stress is insufficient to overcome the basic thermodynamics. 

---

An often-quoted limitation of rational drug design is the lack of biodiversity and chemical space (the various possible chemical compositions in nature. 

One-dimensional nanostructured polymer composite materials include nanowires, nanorods, nanotubes, nanobelts, and nanoribbons. Compared to the other three dimensions, the first characteristic of one-dimensional nanostructure is its smaller dimension structure and high aspect ratio, which could efficiently transport electrical carriers along one controllable direction, thus is highly suitable for moving charges in integrated nanoscale systems (Tran et al., 2009). The second characteristic of one-dimensional nanostructure is its device function, which can be exploited as device elements in many kinds of nanodevices. With a rational synthetic design, nanostructures with different diameters/... 

The objective of a rational reactor design scheme is the development of a polymer reactor configuration optimal in some sense. The measures of product polymer properties typically available to the reactor engineer are the molecular weight distribution, expressed in terms of the distribution function itself or in terms of the moments of this distribution, and the composition and sequence distributions in the case of copolymers. 

One of the features of composites is the importance of their internal structure. It is well known that the same volume fractions of the main components, but arranged differently in space, may prodnce completely different materials. That observation leads to material design and optimization in cement-based materials. The rational composition of layers, pores, fibres and particles may respond perfectly and in the most efficient way to particular requirements of exploitation or to prodnction technology. That way of thinking may help to tailor-make concrete and to improve the design and execntion of concrete structures. 

The role of kinetic and reactor modeling is crucial in the continued advancement of these catalysts as they are optimized for specific applications. We have described different mechanisms for SCR for feed compositions spanning the standard to fast to NO2 types. Convergence to the correct mechanisms is essential if predictive mechanistic-based kinetic models are to be developed. To date the kinetic models have been of the global variety. While these are usefiil for reactor optimization, microkinetic models are needed to guide rational catalyst design and the discovery of new catalyst formulations. 

Even in situations where atomic level information is directly available from experiments, [111] the complexity of t) ical experimental conditions may hinder the establishment of knowledge that can lead to rational materials design. In fact, a much simpler approach can be useful and the effects of changing e.g., the atomic composition in a less complex situation may generate the needed insight. This is exactly where simulations hold the key and the inability to fully simulate the real world complexity can be turned to an advantage, since it can provide an opportunity to understand why materials perform differently. 

How many organizations do what we could really call direct rational solution of the composite structure design problem — very, very few. Perhaps only in some very restricted design areas do people feel that they can use a mathematically oriented optimization approach. That situation is unfortunate, but changing. 

The system lithium-zinc-germanium was investigated again to afford additional compositional and structural information useful to the exploitation and rationalization of the electrochemical results and to the design of new syntheses. 

Given that the pharmacological and biophysical properties of recombinant GABAa receptors have been shown to depend critically on their subunit composition, much effort has been directed towards understanding the assembly of native receptors. This could provide a rational basis for the design of compounds able to interact with specific... 

Zeolites are formed by crystallization at temperatures between 80 and 200 °C from aqueous alkaline solutions of silica and alumina gels in a process referred to as hydrothermal synthesis.15,19 A considerable amount is known about the mechanism of the crystallization process, however, no rational procedure, similar to organic synthetic procedures, to make a specifically designed zeolite topology is available. The products obtained are sensitive functions of the reaction conditions (composition of gel, reaction time, order of mixing, gel aging, etc.) and are kinetically controlled. Nevertheless, reproducible procedures have been devised to make bulk quantities of zeolites. Procedures for post-synthetic modifications have also been described.20 22... 

Physical models of fuel cell operation contribute to the development of diagnoshc methods, the rational design of advanced materials, and the systematic ophmization of performance. The grand challenge is to understand relations of primary chemical structure of materials, composition of heterogeneous media, effective material properties, and performance. For polymer electrolyte membranes, the primary chemical structure refers to ionomer molecules, and the composition-dependent phenomena are mainly determined by the uptake and distribuhon of water. 

The composition of a mixture need not be given in terms of the mole fractions of its components. Other scales of concentration are frequently used, in particular, when one of the components, say. A, can be designated as the solvent and the other (or others), B, (C,...) as the solute (or solutes). When the solute is an electrolyte capable of dissociation into ions (but not only for such cases), the molal scale is often employed. Here, the composition is stated in terms of the number of moles of the solute, m, per unit mass (1 kg) of the solvent. The symbol m is used to represent the molal scale (e.g., 5 m = 5 mol solute/1 kg solvent). The conversion between the molal and the rational scale (i.e., the mole fraction scale, which is related to ratios of numbers of moles [see Eq. (2.2)] proceeds according to Eqs. (2.32a) or (2.32b) (cf. Fig. 2.4) ... 

---