Investigation On Molecular Mobility Of Block Copolymer Brush Containing Crystallizable Side Chain In Nanoconfinement

With the rapid development of nanoscience and nanotechnology,the size of polymer materials is getting smaller and smaller.When the size of polymer material was reduced to the dimension of polymer chain,the molecular dynamics of polymer chains will deviate from the bulk,but the reason remains unclear so far.Polymer brushes are ultrathin polymer films consisting of polymer chains that are attached at one end to an interface.It was well known that the conformation of polymer chains was determined by grafting density,and will be changed from‘‘mushroom or pancake''conformation at low grafting densities to the‘‘brush''conformation at high grafting densities.Moreover,it is reported that the conformation of polymer chains is closely related to the relaxation dynamics.Therefore,polymer brushes are an ideal model to investigate polymer chain dynamics under nanoconfinement.In this study,we have systematically investigated the melting,crystallization and glass transition dynamics of block copolymer brushes.The study can be devided into two parts.One is concerning on the melting behavior of poly(octadecyl methacrylate)(PODMA)brush which is chemically tethered onto an underlying amorphous brush.Another aspect is about the glass transition temperature(T_g)of poly(methyl methacrylate)(PMMA)covered by an upper layer of polymer chain with higher mobility.The conclusions are summarized as follows:(1)Block copolymer brushes,such as poly(methyl methacrylate)-block-poly(octadecyl methacrylate)(PMMA-b-PODMA),poly(methyl acrylate)-block-poly(n-butyl methacrylate)(PMMA-b-PBMA)and poly(methyl acrylate)-block-poly(octadecyl methacrylate)(PMA-b-PODMA),were synthetized by surface initiated atom transfer radical polymerization(SI-ATRP).The grafting density was controlled by the amout of initiator tethered on the substrate.It is found that the thickness of block copolymer brushes is linear with the polymerization time,indicating that the polymerization process is controllable.The PMMA-b-PODMA,(PMMA-b-PBMA)and PMA-b-PODMA brushes with different grafting densities and thicknesses are obtained successfully.(2)The melting behavior of PODMA brushes which were chemically tethered on the underlying PMMA or PMA brush was investigated by ellipsometer.It was shown that the melting temperature(T_m)of PODMA brush increases with increasing grafting density and thickness of the brushes.More importantly,the T_m of PODMA brush tethered on PMA brush with T_g about 3?is higher than that tethered on PMMA brush with T_g about 110?.The crystalline morphology of the block copolymer brushes was furthermore investigated by atomic force microscopy.Results show that crystalline of upperlayer PODMA brush becomes more perfect with increasing the grafting density.It is also interesting that the crystalline of upperlayer PODMA brush tethered on PMA brush is more perfect than that tethered on PMMA brush.Those results indicate that the melting behavior of upper layer of PODMA brush can be affected by the grafting density and the molecular mobility of the underlying amorphous brushes.The PODMA brushes with the higher grafting density and tethered onto the highly mobile underlying amorphous brush(e.g.PMA)were more easily crystallized and form perfect spherulites,which enhance the melting temperature of PODMA brush finally.(3)The glass transition temperature(T_g)of PMMA brush covered with a chemical linked upper overlayer is studied by ellipsometer.It was found that the T_g of bare PMMA brush without the upper overlayer increases significantly with decreasing of thickness.Surprisingly,it is shown that T_g value of PMMA brush with an upper chemical linked overlayer of PODMA or PBMA is much smaller than that of bare PMMA brush.In a control experiment,we investigated the T_g of PMMA brush covered by upper overlayer(e.g.PBMA)without chemical linking to further clarify the role of chemical linking point.It was found that the T_g of PMMA brush covered by a PBMA film is higher than that chemical linked by PBMA brush.Obviouely,the results show that the chemical linking point plays a role of transferring mobility along with the macromolecule chain.The enhanced mobility of the upper PODMA or PBMA layer with low T_g was effectively transfer to the underlying PMMA chains,resulting in the large T_g reduction of PMMA brush.Our study provides a new method to modulate the molecule dynamics of polymer brush by chemical linking a second chain with high mobility.