Solid-state structure and properties of hyperbranched polyols

Abstract

Structure-property relationships were studied in a series of hyperbranched polyesters based on dimethoxypropionic acid with ethoxylated pentaerythritol as the core. In DSC thermograms, all the polyols exhibit a prominent glass transition and a small melting endotherm. It is possible to model the glass transition temperature of hyperbranched polymers by adapting a method used to calculate the glass transition temperature of dendritic polymers. Because the glass transition occurs near ambient temperature, small changes in the glass transition temperature with generation number have a large effect on the mechanical properties. Polyols that are above the glass transition temperature are ductile. Polyols that are below the glass transition temperature are brittle. When deposited from a dilute solution, the polyols form monolayer aggregates of spherical molecules. The aggregates are stabilized by hydrogen bonding of terminal repeat units. The observation of a yield stress indicates that the intermolecular associations provide a level of resistance to deformation. However, because the globular structure does not permit the usual processes of orientation and strain hardening, the neck gradually thins until it fractures at an engineering strain above 100%.