Self-Assembly Characteristics of a Crystalline-Amorphous Diblock Copolymer in Nanoscale Thin Films

Abstract

A diblock copolymer of crystalline polyethylene (PE) and amorphous poly(methyl methacrylate) (PMMA), PE69-b-PMMA(92), was synthesized; this polymer is thermally stable up to 270 degrees C. The morphological structures of thermally annealed nanoscale thin films of the copolymer were investigated in detail at various temperatures by using in-situ grazing incidence X-ray scattering (GIXS) with a synchrotron radiation source. Quantitative GIXS analysis found that the PE and PMMA blocks undergo phase separation to produce a vertically oriented hexagonal PE cylinder structure in the PMMA matrix that is very stable up to around 100 degrees C (which is the onset temperature of PE crystal melting and PMMA glass transition); over the range 100-200 degrees C, slight variations with temperature in the cylinders' dimensions and orientation were observed. Furthermore, the PE block chains of the cylinder phase crystallize and undergo crystal growth along the cylinders' long axes; however, these lamellar crystals do not stack properly because of the limited space along the cylinders' short axes. As a result, the overall crystallinity is very low. The crystallization of the PE block chains in the diblock copolymer thin film is severely restricted in the diblock architecture by the confinement effects of the limited cylinder space and the anchoring of one end of the PE chain to the cylindrical wall interface. Surprisingly, however, in a nanoscale thin film the PE homopolymer forms a highly ordered lamellar structure; the lamellae are well stacked along the out-of-plane of the film, even though the crystallization is confined by the air and substrate interfaces. This well-ordered and oriented lamellar structural morphology does not arise in melt-crystallized PE bulk specimens.