Temperature Terms Links

The first and most important thermodynamic potential we need is entropy. One way to define entropy would be to simply say that the Z-term in Equation (3.14) does indeed exist, where entropy is called S, and Z = —S. This provides a useful analogy between pressure-volume and temperature-entropy, and we will see these terms linked together in many equations. They represent work and heat energy in many processes we will be considering. 

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

Whilst the term thermosetting plastics arose out of the fact that early products of this type were cross-linked by subjecting the intermediate-stage materials to elevated temperature, the term is also widely used where cross-linking takes place at normal ambient temperatures. 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

Temperatures at off-centre locations within the solid body can then be obtained from a further series of charts given by Heisler (Figures 9.17-9.19) which link the desired temperature to the centre-temperature as a function of Biot number, with location within the particle as parameter (that is the distance x from the centre plane in the slab or radius in the cylinder or sphere). Additional charts are given by Heisler for the quantity of heat transferred from the particle in a given time in terms of the initial heat content of the particle. 

In the reality of an agricultural ecosystem, successful plant recovery after stress is linked with several agronomic variables including the length of the effective growing season. If recovery is delayed it may then take place in an unsuitable environment, in terms of photoperiod or temperature. Thus, in... 

The above equations are linked by the reaction rate term rA, which depends on concentration and temperature. 

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