Chain structure entanglements

The chain structure (length, entanglement, branching, tacticity,...) of polymers is discussed further in the next chapter (Chapter 4) by Beaucage and Kulkarni. 

In addition to terminal chains and entanglements, there are other types of network imperfections. Figure 6-2 shows that if a short chain were crosslinked only once, the crosslink is a wasted one because the chain cannot support elastic stress. Also, if a crosslink forms an intrachain loop, it is again an ineffective crosslink. Unfortunately, owing to its very complexity, it is at present impossible to completely characterize the network structure of an elastomer. 

The behavior of NIPAAm around its phase transition temperature has not been fully uncovered. A possibility is, as explained earlier, that as the temperature is increased, the water molecules which formed a elathrate structure around the hydrophobic methyl groups, will unfreeze. This in turn allows the polymer chains to entangle, crosslink, and form a gel, as seen in Fig. 11.2. 

Polymers are light in weight because of their low density and large volume. This is due to their long, coiled and entangled chain structure. 

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