Macromolecular behavior

Phase behavior and morphology in conventional polymers are heavily dependent on the thermal history of the sample, as is obvious to anyone even remotely familiar with macromolecules. Polymer liquid crystals (PLCs) are clearly subject to similar constraints by virtue of their macromolecular identity. In addition, a number of thermal properties are specific to PLCs as a result of the interaction between macromolecular behavior and the molecular ordering characteristic of LC mesophases. This chapter focuses on just such features of thermal history, as revealed by the interplay of kinetic and thermodynamic factors observed in thermotropic polymers. 

By explaining the experimental and mathematical basis for the theories and models used to define macromolecular behavior, Physical Chemistry ol Macromolecules demonstrates how these techniques and models can be applied to analyze and predict the properties of new polymeric materials. 

The value for the most isotactic fraction is -1-160 against the value -h240 calculated for a left handed helical chain of poly(4-methyl-l-pentene).2i Cooperativity is a critical feature of macromolecular behavior. Studies of the dependence of chirooptical properties on the degree of tacticity in homopolymers and the mole fraction of chiral monomers in copolymers has shed considerable light on the nature of cooperativity in linear macromolecules. 

In the past [4-6] it was common to characterize amphiphiles according to their major performance in food systems (1) emulsification and stabilization, (2) protein interactions, (3) polysaccharide complexation, (4) aeration, and (5) crystal structure modification of fats. Such classifications correlate the surfactant chemical structure to its interaction (chemical or physical) with substrates such as fats, polysaccharides, and proteins. It was confirmed fhat certain surfactants interact molecularly with macromolecules, forming complexes and/or hybrids, and alter the macromolecular behavior at the interface. Such activity is an important new contribution of cosurfactants to the surface performance of other surfactants [7]. Such interactions are sometimes a very important contribution of amphiphiles to food systems. 

Historically, theoretical treatments have resorted to simple phenomenological models of polymeric materials. In the framework of statistical mechanics, polymeric chains are at a first stage considered to consist of independent elements or segments. The principal property of macromolecular behavior taken into account with this representation is the flexibility of the chains. With non-interacting monomeric units having uncorrelated directions, it is straightforward to show that the chains acquire random-walk behavior. 

There are several reasons that Newton-Raphson minimization is rarely used in mac-romolecular studies. First, the highly nonquadratic macromolecular energy surface, which is characterized by a multitude of local minima, is unsuitable for the Newton-Raphson method. In such cases it is inefficient, at times even pathological, in behavior. It is, however, sometimes used to complete the minimization of a structure that was already minimized by another method. In such cases it is assumed that the starting point is close enough to the real minimum to justify the quadratic approximation. Second, the need to recalculate the Hessian matrix at every iteration makes this algorithm computationally expensive. Third, it is necessary to invert the second derivative matrix at every step, a difficult task for large systems. 

It is the interplay of universal and material-specific properties which causes the interesting macroscopic behavior of macromolecular materials. This introduction will not consider scales beyond the universal or scaling regime, such as finite element methods. First we will give a short discussion on which method can be used under which circumstances. Then a short account on microscopic methods will follow. The fourth section will contain some typical coarse-grained or mesoscopic simulations, followed by some short general conclusions. 

Successful systems design and fabrication depend on understanding the connections between microscale phenomena and macroscale behavior of materials. For example, with sufficient insight into intermolecular interactions, appropriate models, and the computational power of supercomputers, it may be possible to predict changes in macromolecular configurations when loads are imposed on polymers or changes in the properties of a material as a result of... 

This chapter draws a comprehensive picture of what has been done in the field of dendrimers with polymeric cores putting emphasis first on synthetic issues and then on experiments investigating the aggregation behavior of these intruiging macromolecules both in the solid state and on surfaces. Additionally, experiments will be described which show that some of these dendrimers can be considered cylindrical molecular objects. The macromolecules treated in this chapter may be considered as either dendrimers with polymeric core or alternatively dendronized polymers (or polymers with appendent dendrons) depending on whether one sees them from the vantage point of an organic or macromolecular chemist. 

R.G. Alamo and C. Chi, Crystallization behavior and properties of polyolefins. In Y. Morishima, T. Norisuye and K. Tashiro (Eds.), Molecular Interactions and Time-Space Organization in Macromolecular Systems, Springer, New York, 1999, p. 29. 

These principles of macromolecular organization replaced the naive vision of some of the early molecular biologists, who tried to reduce each complex structure and function of organisms directly to one (or a limited number of) proteins or other macromolecules. The best example of this simplistic reductionist approach was the efforts aiming in the sixties at discovering the molecules of memory - molecules that allegedly encoded the memories or behaviors. Many articles and books1 were devoted to this search, without any success. 

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