Elasticity molecular nature

These deductions from basic facts of observation interpreted according to the rigorous laws of thermodynamics do not alone offer an insight into the structural mechanism of rubber elasticity. Supplemented by cautious exercise of intuition in regard to the molecular nature of rubberlike materials, however, they provide a sound basis from which to proceed toward the elucidation of the elasticity mechanism. The gap between the cold logic of thermodynamics applied to the thermoelastic behavior of rubber and the implications of its... 

Portions of polymer molecules which are in crystalline regions have overall dimensions and space-filling characteristics that arc determined by the particular crystal habit which the macromoleculc adopts. Here, however, we are concerned with the sizes and shapes of flexible polymers in the amorphous (uncrystallized) condition. It will be seen that the computation of such quantities provides valuable insights into the molecular nature of rubber elasticity. 

As alluded to in the introduction to this entry, the LE theory was conceived for small molecule LCs while molecular theories are intended for LCPs. LC molecules retain their equilibrium orientation distribution. LCPs are susceptible to disturbances to their distribution function T (m) its temporal relaxation gives rise to molecular viscoelasticity, while its spatial gradient produces distortional elasticity. A natural question is whether the molecular theories reduce properly to the continuum LE theory in the limiting case of an undisturbed orientation distribution. This situation arises in the weak flow limit where the flow is weak De < . 1) and spatial distortions are small ([V l [Pg.2962]

In the previous chapter the molecular nature of high elasticity was considered. It was shown that the force-deformation characteristics were almost, but not entirely, independent of the polymer type, so long as the material was rubbery at the particular temperature or over the range of temperatures considered. 

In earlier chapters the chemical structure of natural rubber, the molecular nature of high elasticity and, in outline, the general dependence of properties on structure were considered. 

Quite obviously such an assumption is at odds with our knowledge of the atomic and molecular nature of materials but is an acceptable approximation for most engineering applications. The principles of linear elasticity, though based upon the premise of a continuum, have been shown to be useful in estimating the stress and strain fields associated with dislocations and other non-continuum microstructural details. 

The semi-resilient tapes are more elastic in nature and are usually based on regular butyl rubber. Both reinforcing and extender pigments are used to maintain a balance between compressibility and elasticity. The higher molecular weight polybutenes are used as plasticizers and tackifiers because of their better aging properties. A typical formulation for semi-re-... 

To illustrate this linear combination, various analogical models with no relationship with the molecular nature of the phenomena were proposed, identifying a polymer with a combination of springs and dashpots. The spring can be stfained without inertia and thus reflects a purely elastic mechanical behavior whereas dash-pots, which are pistons moving in cylinders filled with a viscous liquid, cannot respond instantaneously to a stress. These two elements were thus associated under various combinations to simulate the response of a viscoelastic body to a mechanical stress. 

Radiation probes such as neutrons, x-rays and visible light are used to see the structure of physical systems tlirough elastic scattering experunents. Inelastic scattering experiments measure both the structural and dynamical correlations that exist in a physical system. For a system which is in thennodynamic equilibrium, the molecular dynamics create spatio-temporal correlations which are the manifestation of themial fluctuations around the equilibrium state. For a condensed phase system, dynamical correlations are intimately linked to its structure. For systems in equilibrium, linear response tiieory is an appropriate framework to use to inquire on the spatio-temporal correlations resulting from thennodynamic fluctuations. Appropriate response and correlation functions emerge naturally in this framework, and the role of theory is to understand these correlation fiinctions from first principles. This is the subject of section A3.3.2. 

Oberhauser et al., 1998] Oberhauser, A. F., Marszalek, P. E., Erickson, H., and Fernandez, J. The molecular elasticity of tenascin, an extracellular matrix protein. Nature. In Press. 

A number of high molecular weight polyisoprenes occur in nature which differ from natural rubber in that they are essentially non-elastic. As with natural rubber they are obtained from the latex of certain plants but they differ in that they are either frani-l,4-polyisoprenes and/or are associated with large quantities of resinous matter. 

Tackifying resins enhance the adhesion of non-polar elastomers by improving wettability, increasing polarity and altering the viscoelastic properties. Dahlquist [31 ] established the first evidence of the modification of the viscoelastic properties of an elastomer by adding resins, and demonstrated that the performance of pressure-sensitive adhesives was related to the creep compliance. Later, Aubrey and Sherriff [32] demonstrated that a relationship between peel strength and viscoelasticity in natural rubber-low molecular resins blends existed. Class and Chu [33] used the dynamic mechanical measurements to demonstrate that compatible resins with an elastomer produced a decrease in the elastic modulus at room temperature and an increase in the tan <5 peak (which indicated the glass transition temperature of the resin-elastomer blend). Resins which are incompatible with an elastomer caused an increase in the elastic modulus at room temperature and showed two distinct maxima in the tan <5 curve. 

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