Polymorphism-controlled Young’s modulus of biobased polymers: Time-dependence due to physical aging

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Young’s modulus of semicrystalline polymers depends on the structure and morphology of both crystals and amorphous structure. While for selected biopolymers in the first part of the project the effect of crystal polymorphism on Young’s modulus has been quantified, suggesting a lower modulus for less ordered and less dense crystal phases, focus of the second project part is the time dependence of the mechanical behavior due to aging. Main objective is the analysis of the time dependence of selected mechanical properties of semicrystalline biopolymers caused by physical aging related to glass relaxation and post-crystallization. It is the goal to identify (primary)-crystallization-related structural parameters which minimize or enhance long-term change of properties. Among these structural parameters, the crystallinity, the pre-dominance of specific crystal polymorphs, or the crystal morphology including the interface-structure between crystals and amorphous surrounding are considered. These parameters impose different constraints to the amorphous phase, changing the glass-relaxation behavior (effective in the glassy state) and post-crystallization characteristics in amorphous regions between crystals grown during primary crystallization (mainly effective at temperatures above the glass transition temperature). A possible influence of chemical aging, which is superimposed on physical aging, is to be evaluated on selected polymers. The research will lead to an improved understanding of physical aging of semicrystalline polymers, in particular by identification of polymorphism- and crystal-morphology-related parameters of the structure. With the emphasis placed on environment-friendly bio-sourced and/or biodegradable polymers, the important demand for further establishment of ecologically neutral materials with a long lifetime is strongly considered.