Single crystal morphology of star-branched polyesters with crystallisable poly(ε-caprolactone) arms

Star-branched polymers consisting of poly(ε-caprolactone) (PCL) attached to third generation dendrimer, hyperbranched and dendron cores have been studied together with linear PCL analogues. The degree of polymerisation of the PCL arms of the star-branched polymers ranged from 14 to 81. Single crystals were grown from dilute solution and studied by transmission electron microscopy. Single crystals of linear PCL were multilayer hexagons with flat or slightly curved {110} and {100} faces. These single crystals were larger along 010] than along 100]. Single crystals of star-branched PCL showed the same basic shape, but with many crystallographic and irregular steps on the lateral crystal faces. The width of the micro-faces was typically 100-300 nm. These single crystals were more extended along 100] than along 010]. It is proposed that the high fold surface free energy and the constrained character of the star-branched polymers favour the formation of steps on the growth faces. Globular polycrystalline aggregates were also observed. They originated from a more concentrated polymer phase following phase separation of the solution. In the case of the star-branched polymers, lamellar branching was observed with a 30° angle between the crystals arms.