Polymeric materials, molecular engineering

As one tries to write down an analysis of the developments in the block polymers area, one realizes very soon that it is going to consist of a series of variations on a theme a theme which is the increasing ly stronger reality, in our everyday scientific life, of what can be now really called "the molecular engineering of polymers properties", i.e. the possibility to control, through precise (although sometimes small) modifications of molecular structures, the final bulk properties and macroscopic behaviour of polymeric materials. In other words, one deals there with a very acti ve version of the studies on structure-properties inter relationships, a question which by the way has always been a focal point among the many diversified interests of Professor Mark. 

The methods utilized to measure the viscoelastic functions are often close to the stress patterns occurring in certain conditions of use of polymeric materials. Consequently, information of technological importance can be obtained from knowledge of these functions. Even the so-called ultimate properties imply molecular mechanisms that are closely related to those involved in viscoelastic behavior. Chapters 16 and 17 deal with the stress-strain multiaxial problems in viscoelasticity. Application of the boundary problems for engineering apphcations is made on the basis of the integral and differential constitutive stress-strain relationships. Several problems of the classical theory of elasticity are revisited as viscoelastic problems. Two special cases that are of special interest from the experimental point of view are studied viscoelastic beams in flexion and viscoelastic rods in torsion. 

In this chapter an explanation is presented of certain engineering aspects that are important in understanding the mechanical properties of wood. Individual factors such as growth, environment, chemicals, and use can greatly affect the physical and mechanical properties of the wood material. A theoretical model is presented to explain the relationship between physical properties and chemistry of wood at three distinct levels macroscopic or cellular, microscopic or cell wall, and molecular or polymeric. These three levels and their implications on material properties must be understood to relate both wood chemistry and wood engineering from a materials science standpoint. When this is accomplished, the treatment and processing of wood and wood products can be controlled to yield more desirable and uniform properties. 

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