Oriented Self-assemble Pattern Construction And Crystallize/Melting Behavior Of Block Copolymer In Ultrathin Films

Block copolymer films have potential applications in microelectronic devices,sensors,and biomedical materials because of their self-assemble ability to fabricate micro/nano structures.The morphology of block copolymer films can be controlled by the additional flow field and its own crystallization behavior.In this study,the self-assemble structures of the amphiphilic block copolymer poly?styrene?-b-poly?2-vinyl pyridine??PS-b-P2VP?,crystalline block copolymers poly??-caprolactone?-b-poly????-lactide??PCL-b-PLLA?and poly?ethylene oxide?-b-poly??-caprolactone?-b-poly????-lactide??PEO-b-PCL-b-PLLA?have been studied.An ordered alignment induced by dewetting was fabricated in PS-b-P2VP films.By analyzing the effect of the annealing time,the width of contact line and the growth behavior of the dewetting holes on the microdomain orientation of PS-b-P2VP film,the formation mechanism of the oriented nanostructures induced by the dewetting flow field was investigated and verified.Furthermore,the melting behavior of self-assembled nanostructures in PCL-b-PLLA and PEO-b-PCL-b-PLLA was investigated by using in-situ hot-stage atomic force microscope and wide-angle X-ray scattering analysis technology.By analyzing the sequential melting process of three block crystals during heating,the formation of self-assembled nanostructures were discussed.The main results are as follows:1.The dewetting behavior of the film can be used to control the orientation of the cylindrical mesophases in PS-b-P2VP films.In the course of dewetting enabled by solvent vapor annealing,cylinders aligned parallel to the dewetting contact line with the size of the ordered domain increasing in the direction normal to the dewetting front.We observed a power law increase of the orientation parameter?f_o? for the P2VP cylinders with increasing time(t_grow)of hole growth:f_o=t_grow^1/3.Arrays of precisely aligned cylinders were achieved in long filaments generated by the coalescence of adjacent dewetting holes.The degree of order of the aligned P2VP cylinders at the contact line was enhanced by increasing the annealing time as well as by decreasing the distance to a neighboring contact line,i.e.,in narrow filaments generated by dewetting.Alignment of cylindrical mesophases was considered to be driven by the dewetting flow field and by minimization of the elastic energy of the P2VP cylinders.2.The melting behavior of self-assembled structure in PCL-b-PLLA and PEO-b-PCL-b-PLLA thin films has been investigated by using in-situ hot stage AFM.PCL-b-PLLA and PEO-b-PCL-b-PLLA thin films have obvious layered nanostructures after crystallization.For the difference of melting temperature for each crystal layer,during the heating process,PEO crystals first melted;the PCL lamellae disappeared at 60°C when PCL crystals melt.At that temperature,the only remaining crystals are formed by the PLLA block.According to the change of the lamellar structure and the adhesion,we found that in the PCL-b-PLLA,the PLLA lamellar layer and the PCL lamellar layer are alternately arranged,and only one PCL lamellar layer exists between two PLLA lamellae.In PEO-b-PCL-b-PLLA,one or more PEO lamellae and PCL lamellae are inserted between PLLA lamellae.It is considered that the isothermal crystallization of the PLLA block is induced first when cooling from the melt.Then,PLLA lamellae form a template,whereupon cooling the other two blocks can crystallize within the PLLA interlamellar spaces.