How does the chemical structure of a polymer affect its physical properties?”

It is well understood that the chemical structure of molecules directly affects their physical properties. This is particularly true for polymers, where for instance the molecular weight can make a significant difference even though the monomer repeats doesn’t change. Despite these general rules governing understanding of polymer behaviour the reality of synthesizing a given structure to derive particular property is not at all straightforward. Consider for instance polyethylene (PE), perhaps the simplest polymer there is, which consists as the name implies of repeating ethylene links. However, as many people realize, PE is not simply PE, it comes in many ‘flavors’, with names that have been used historically such as high-density polyethylene (HDPE) and so on, which relate to their ability to crystalize. Whilst prediction of polymer behaviour has made significant advancement over the last several decades, much of this work is done at the continuum level and therefore a disconnect still exists between the macroscopic properties and the molecular structure. Consequently, the question that it has still not been possible to answer is, what chemical structure is needed to achieve a particular property. It seems a reasonable question to ask, but is for most cases not possible to answer and industry spend huge amounts of resources both in terms of materials as well as manpower, synthesizing large numbers of polymers trying to find the polymer that matches the desired properties.
In our research we are trying to address this disconnect between the chemical structure and physical properties and in my talk I will give two examples where we are trying to make those connections. The first example is in the area of organic electronics and in particular polymer organic photovoltaic (P-OPVs), where control of phase behaviour through chemical modification of the components of the active layer clearly shows pathways towards achieving high-efficiency devices. The second example will discuss the mechanical properties of PE, and will show how the branch content is critical in determining the factors affecting failure of the PE.