Crystalline surface free energy of linear polyethylene as estimated from acombination of crystal thickness distribution and DSC melting curve shapes

The surface free energies of linear polyethylene samples having different m olecular characteristics and thermal histories were determined, based on el ectron microscopy observation and differential scanning calorimetric analys is. On the electron microscopy photograph of a given sample, over one thous and lamellae were randomly selected and the crystalline thickness of each l amella was measured. The lamellar number having the comparable crystalline thickness was summed up for every I nm thickness. The relative probability of lamellae thus obtained was plotted as a function of crystalline thicknes s. Such a crystal thickness distribution curve was converted into a tempera ture scale through the Thomson-Gibbs equation. Comparison between these cal culated profiles and measured DSC melting curves gave the surface free ener gy of crystalline lamellae. For the melt-crystallized sample, it has a cons tant value of 94 +/-2 erg/cm(2), independent of the sample molecular weight , molecular weight distribution and crystallization conditions. In contrast , for the solution-crystallized sample, the surface free energy was 88 +/-2 erg/cm(2) even when the lamellar reorganization effect on DSC melting beha vior had been eliminated.