The block length effect of polymethyl methacrylate on surface morphology of polystyrene block polymethyl methacrylate copolymers
Abstract
The effect of polymethyl methacrylate (PMMA) block length on the surface morphology of the two block copolymers (BCPs) was investigated by AFM using tapping mode. Two asymmetric polystyrene block polymethyl methacrylate (PS-b-PMMA) copolymers with different PMMA block lengths were synthesized by a novel combination technique; living anionic polymerization and hydroboration autoxidtion reactions. The molar mass of PS block was (MW PS = 90000 g/mol) and the molar mass of PMMA blocks were (MW PMMA =130000 and MW PMMA =180000 g/mol). Thin films of two asymmetric copolymers were prepared by solvent casting (10 mg/5 ml THF) on freshly cleaved mica. The results showed that phase separation do occur and equilibrium morphologies were observed. It was found that BCP with PMMA molar mass (MW PMMA =130000 g/mol) form parallel cylinders (stripes) oriented horizontally to the substrate. While BCP with PMMA molar mass (MW PMMA =180000 g/mol) form hexagonal ordered cylindrical oriented perpendicular and parallel to the substrate.
Keywords
Download Options
Introduction
Multiphase polymer system include a wide range of materials such as composites, block copolymers (BCPs), blends, alloys, and gels, have gained much interest due to their unique properties. Accordingly, characterization of the inter-phase and other properties are crucial for understanding the behavior of these materials.1 BCPs are material in which dissimilar blocks are covalently bonded. Because of this dissimilarity between the copolymer blocks, phase separation and segregation would occur at different molecular aspects. Only BCPs can be defined as multiphase system having stable microstructures, even after many processing cycles.2
An important property of BCPs is their ability to self-assemble into periodic nanostructure.3 Self-assembly of BCPs domains have several technological applications and is of great scientific interest. It is a phenomenon where the components of a system assemble itself spontaneously via an interaction to form a larger functional unit. This spontaneous organization can be due to direct specific interaction and/or indirectly through their environment. BCPs self-assembly have attracted considerable attention for many decades, because it can yield ordered structures in wide range of morphologies, including spheres, cylinders, lamellae and many other complex structures,4 as shown in Scheme 1.
Such structures are promising for applications in many fields of nanotechnology, such as surface pattering, lithography, templating for the fabrication of quantum dots, nanowires, magnetic storage media, nanopores, and silicon capacitors.6 The self-assembly of BCPs is driven by a competition between the positive enthalpy of mixing of the respective block and the tendency of polymers to desire a random coil configuration. Under certain condition of the BCPs microphase separation into well defined domain structures occurs on the length scale of the respective blocks.7
Conclusion
In summary, we have investigated the block length (molar mass) effect of PMMA on surface morphologies of PS-b-PMMA copolymer thin films. BCPs were synthesized by a novel combination techniques; living anionic polymerization and hydroboration autoxidation reactions. Our results showed that microphase separation do occur and equilibrium morphologies were observed. Moreover, it could be concluded that the BCP composition, in particular, affect the microphase morphology. It was found that BCP with PMMA MW PMMA =130000 g/mol form parallel cylinders (stripes) oriented horizontally to the substrate. On the other hand, BCP with PMMA MW PMMA =180000 g/mol form mixed morphology of cylindrical PMMA domains oriented perpendicular and parallel to the substrate.
Considering that some aspects of these thin film systems, in particular understanding their microstructure remains a worthwhile pursuit. Indeed, it is useful to understand all these details of morphology changes in block copolymers under different conditions. In spite of many studies on the phase behavior of block copolymers, the attention paid to their structure property correlation seems to be insufficient, moreover, theoretical descriptions of forming these morphologies is still a significant research focus. Consequently, future work continue to research in this field to focus on these issues.