Diblock copolymer brushes

Surface Rearrangement of Diblock Copolymer Brushes—Stimuli Responsive Films... 

Abstract This article reviews results from our group of the synthesis and characterization of diblock copolymer brushes. Results from the literature are also covered. We report a wide variety of diblock compositions and compare the miscibility of the two blocks with the tendency to rearrange in response to block-selective solvents. Also, we describe the types of polymerization methods that can be utilized to prepare diblock copolymer brushes. We have compared the molecular weight of free polymer and the polymer brush based on results from our laboratory and other research groups we have concluded that the molecular weight of the free polymer and that of degrafted polymer brushes is similar. 

In this review, synthesis of block copolymer brushes will be Hmited to the grafting-from method. Hussemann and coworkers [35] were one of the first groups to report copolymer brushes. They prepared the brushes on siUcate substrates using surface-initiated TEMPO-mediated radical polymerization. However, the copolymer brushes were not diblock copolymer brushes in a strict definition. The first block was PS, while the second block was a 1 1 random copolymer of styrene/MMA. Another early report was that of Maty-jaszewski and coworkers [36] who reported the synthesis of poly(styrene-h-ferf-butyl acrylate) brushes by atom transfer radical polymerization (ATRP). 

This was the first report using ATRP and sequential monomer addition. Hydrolysis of these diblock copolymer brushes yielded poly(styrene-fc-acrylic acid) brushes. 

The first diblock copolymer brushes synthesized in our group were made by a combination of carbocationic polymerization and ATRP (Scheme 1) [46]. Zhao and co-workers [47] synthesized diblock copolymer brushes consisting of a tethered chlorine-terminated PS block, produced using carbocationic polymerization, on top of which was added a block of either PMMA, poly(methyl acrylate) (PMA) or poly((Ar,M -dimethylamino)ethyl methacrylate) (PDMAEMA), synthesized using ATRP. The thickness of the outer poly(meth)acrylate block was controlled by adding varying amounts of free initiator to the ATRP media. It has been reported that the addition of free initiator is required to provide a sufficiently high concentration of deactivator, which is necessary for controlled polymerizations from the sur-... 

Table 1 Summary of the properties of diblock copolymer brushes...

Diblock copolymer brush structure Thickness Thickness of tethered of outer block (nm) block (nm) Polymerization technique Refs. 

Scheme 2 Synthesis of surface-immobilized diblock copolymer brush (Si/Si02//PS-fc-PMMA) using reverse atom transfer radical polymerization and ATRP...

To make further use of the azo-initiator, tethered diblock copolymers were prepared using reversible addition fragmentation transfer (RAFT) polymerization. Baum and co-workers [51] were able to make PS diblock copolymer brushes with either PMMA or poly(dimethylacrylamide) (PDMA) from a surface immobihzed azo-initiator in the presence of 2-phenylprop-2-yl dithiobenzoate as a chain transfer agent (Scheme 3). The properties of the diblock copolymer brushes produced can be seen in Table 1. The addition of a free initiator, 2,2 -azobisisobutyronitrile (AIBN), was required in order to obtain a controlled polymerization and resulted in the formation of free polymer chains in solution. 

We reported the synthesis of Si/Si02//PS-h-poly(acrylate) tethered diblock copolymer brushes [31,32,46,47]. The properties of these diblock brushes were studied using water contact angles, ellipsometry. X-ray photoelectron spectroscopy (XPS), FTIR spectroscopy and atomic force microscopy (AFM). For a sample with a 26 nm PS layer and a 9 nm PMMA layer, the advanc-... 

To better understand the time-scale of diblock brush reorganization, we prepared semifluorinated diblock copolymer brushes where the outer block (at the air interface) was a semifluorinated block composed of poly(pentafluoro-... 

Fig. 4 Dependence of the advancing water contact angle on annealing temperature for PS-based diblock copolymer brush layers (filled squares) Si/Si02//PS- -PHFA, (filled triangles) Si/Si02//PS- -PPFA, (filled diamonds) Si/Si02//PS- -PPFS. Lines added as guide for the eye...

Besides ATRP, RAFT polymerization can also be employed to graft hydrophilic polymer brushes from the PVDF membrane surface. The PVDF membrane was firstly functionalized with hydroxyl groups, which can react with 4,4 -azobis(4-cyanopentanoic acid) (ACPA) to produce the activated PVDF membrane to allow the subsequent graft copolymerization of MMA or PEGMA in the presence of a CTA via RAFT polymerization [143]. The modified membrane can be further graft copolymerized with DMAEMA to produce the diblock copolymer brushes on the PVDF membrane surface. 

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