Forces in Polymers

These forces result from slight displacement of electrons and nuclei in covalent molecules. They take place in the proximity of electrostatic fields associated with the dipoles from other molecules. Such displacement causes interaction between the induced dipoles and the permanent dipoles creating induction forces. The energy of the induction forces is small and not temperature-dependent. 

The third force is a result of different instantaneous configurations of the electrons and nuclei about the bonds of the polymeric chains. These are time-varying dipoles that average out to zero. They are polarizations arising from molecular motions. 

Polymer Cohesion/5 A chain length (kcal/mole) Repeat unit 

The temperatures and the flexibility of polymeric molecules govern both the sizes of molecular segments that can be in motion and the frequencies at which that can occur. This in turn determines the rate at which polymer molecules respond to molecular stresses. In flexible polymers, if the thermal energy is sufficiently high, large segments can disengage and slip past each other quite rapidly in response to applied stress. This is a property of all elastomers. 

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When describing the effect of an external force, we must first define the force itself. A lay person s definition of a force is the amount of effort to get the desired effect. As scientists, we need a more precise definition of force. With a precise definition we can understand and quantify the effect of an applied force on a polymeric material. The mathematical definition of force is the work (which is a form of energy) required to move an object over some distance. Another way to define a force is in terms of the acceleration it creates when applied to some object of a mass m. In our everyday experiences, the first explanation is a simple idea to relate to. When we push a stalled car we exert a force on it. We could easily quantify the force from the weight of the car, the slope of the hill it is sitting on, and how far we must push it. Once we begin to talk about forces in polymer systems, the ideas become a bit more complicated. For example, the force required to open a bag of candy is defined by the work required to deform the bag until it ruptures by overcoming the intermolecular forces which hold the plastic together. 

The driving force in polymer synthesis is the search for new polymers with improved properties to replace other materials of construction. Polymers are lightweight and can be processed easily and economically into a wide range of shapes and forms. The major synthetic efforts at present are aimed at polymers with high temperature, liquid crystal, conducting, and nonlinear optical properties [Maier et al., 2001 Sillion, 1999]. There is an interrelationship between these efforts as will become apparent. 

The primary valence bonds and intermolecular forces in polymer samples are exactly the same as those in any other chemical species, but polymers form strong... 

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