Molecular architecture polymer molecules

Next let us consider the differences in molecular architecture between polymers which exclusively display viscous flow and those which display a purely elastic response. To attribute the entire effect to molecular structure we assume the polymers are compared at the same temperature. Crosslinking between different chains is the structural feature responsible for elastic response in polymer samples. If the crosslinking is totally effective, we can regard the entire sample as one giant molecule, since the entire volume is permeated by a continuous network of chains. This result was anticipated in the discussion of the Bueche theory for chain entanglements in the last chapter, when we observed that viscosity would be infinite with entanglements if there were no slippage between chains. 

How does the molecular architecture of the bisphenol molecule affect the physical properties of the final polycarbonate polymer ... 

Covalent polymer networks or (Class II) crosslinked macromolecular architecture polymers rank among the largest molecules known. Their molecular weight is given by the macroscopic size of the object for instance, a car tire made of vulcanized rubber or a crosslinked layer of protective coating can be considered one crosslinked molecule. Such networks are usually called macronetworks. On the other hand, micronetworks have dimensions of several nanometers to several micrometers (e.g. siloxane cages or microgels). 

Another important concept from organic chemistry that has an impact on polymer structure is that of isomerism. Recall that structural isomers are molecules that have the same chemical formula, but different molecular architectures. For example, there are two different types of propyl alcohols, both with the same formula, depending on where the —OH functional group is placed on the carbon backbone (see Figure 1.53). Another type of isomerism results in stereoisomers, in which the functional groups are... 

Macromolecules are very much like the crystalline powder just described. A few polymers, usually biologically-active natural products like enzymes or proteins, have very specific structure, mass, repeat-unit sequence, and conformational architecture. These biopolymers are the exceptions in polymer chemistry, however. Most synthetic polymers or storage biopolymers are collections of molecules with different numbers of repeat units in the molecule. The individual molecules of a polymer sample thus differ in chain length, mass, and size. The molecular weight of a polymer sample is thus a distributed quantity. This variation in molecular weight amongst molecules in a sample has important implications, since, just as in the crystal dimension example, physical and chemical properties of the polymer sample depend on different measures of the molecular weight distribution. 

The structural complexity of synthetic polymers can be described using the concept of molecular heterogeneity (see Fig. 1) meaning the different aspects of molar mass distribution (MMD), distribution in chemical composition (CCD), functionality type distribution (FTD) and molecular architecture distribution (MAD). They can be superimposed one on another, i.e. bifunctional molecules can be linear or branched, linear molecules can be mono- or bifunctional, copolymers can be block or graft copolymers, etc. In order to characterize complex polymers it is necessary to know the molar mass distribution within each type of heterogeneity. 

Diblock and triblock copolymers are usually synthesized anionically, which inherently limits the possible blocks and homopolymers. However, the anionic route gives very good control over molecular weight, molecular architecture and polydispersity and is ideally suited for a systematic study of the effect of connector molecules. Furthermore, the problems frequently encountered during the anionic synthesis have been ironed out for many common polymers and the synthesis, although tedious, can be considered a routine procedure. 

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