Developing a qualitative chain-level description of polymer mechanical properties

In many polymer products, better mechanical properties are achieved through nonequilibrium structures acquired from specific processing procedures. Today sustainable (biodegradable, recycled) polymers cannot replace PE, PP and PET because of their inadequate mechanical characteristics. Sustainability of polymers remains an empty promise unless we develop a basic chain-level understanding mechanical behavior of polymeric materials in terms of what structures are required for desirable mechanical properties. By reviewing the available phenomenology concerning processing-structure-property relationship in the literature we emphasize that (a) preservation and enhancement of chain networking are essential, (b) non-equilibrium structures such as geometric condensation (GC) of the chain network can be generated during processing to promote toughness and ductility. We will describe the concept of crystalline chain network (CCN), suggest that predrawing of semicrystalline polymers can result in GC envisioned previously for glassy polymers, and show why molecular strategies such as strain-induced nano-confined crystallization preserves CCN to render certain sustainable polymers such as poly(lactic acid) superbly ductile and strong.