Mutual Alignment of Block Copolymer−Magnetic Nanoparticle Composites in a Magnetic Field

Abstract

For the past ∼30 years, polymer nanocomposites (PNCs) 1 and block copolymers (BCPs) 2 have represented two extremely active areas of research within the polymer science community. It was inevitable, then, that the two topics should overlap; block copolymer/nanoparticle composites represent a relatively new and exciting field in materials research. A large portion of such work has, to date, sought to employ the well-ordered microphaseseparated structures available in block copolymers as scaffolds to direct the spatial arrangement of the nanofiller. 3,4 Indeed, effective control over the spatial location and/or orientation of the nanofiller particles in PNCs offers great possibilities for dramatically improved composite properties. 1 In the block copolymer field, great strides have been made in developing techniques to globally orient the BCP microdomains using external fields 5 (e.g., shear 6 or electric fields 7 ) or surfaces, [8][9][10][11] even further enhancing the possibility for the creation of nanocomposites with precisely defined morphology.In this Communication, we present a general method for creation of aligned block copolymer/nanoparticle (BCP/NP) composites, although via something of an inverse approach: an external (magnetic) field is used to define the spatial orientation of rodshaped magnetic NPs, which then serve as structure-directing agents for neighboring BCP domains. Specifically, the oriented NPs, which are incorporated at a concentration of only a few percent, present templating surfaces for the alignment of cylindrical block copolymer nanodomains. In a sense, our approach is an extrapolation of the graphoepitaxy technique, which can be used to template BCP cylinder orientation in thin films, 10,11 to thicker films, and eventually into the bulk. The ability of rod-shaped NPs to nucleate coaxially oriented BCP cylinders has recently been established. 12,13 There has also been a fair amount of work using magnetic fields to orient block copolymers, although it is necessary to use copolymers which contain moieties or semicrystalline domains with appreciable magnetic susceptibility. 5,[14][15][16][17] In the absence of such a condition, we introduced the rod-shaped nanoparticles, which are capable of being magnetically aligned. Using this kind of scheme to induce reorientation in lyotropic liquid crystal/nanomagnet composites has also been demonstrated. 18,19 The composite under study consisted of a polystyrene-blockpoly(2-vinylpyridine) (PS-P2VP) diblock copolymer, which forms cylindrical microdomains of P2VP in a PS matrix, 20 and spindletype hematite (R-Fe 2 O 3 ) particles, which were synthesized by the forced hydrolysis of Fe(ClO 4 ) 3 , according to procedures previously described by Ocaña et al. 21 The particles, an example of which is shown in the transmission electron microscopy (TEM) image in Figure 1a, had an average diameter of 55 ( 6 nm and an average long axis length of 330 ( 38 nm (see the Supporting Information, Figure S1). Such hematite particles are weakly ferromagnetic and hav...

Macroscopic Alignment of Lyotropic Liquid Crystals Using Magnetic Nanoparticles

“…The lower order parameter of the mesophase orientation compared to the order parameter of the magnetic nanoparticles obtained from a 2D order parameter on the microscopy images is understood to arise from the decreasing orientational correlations between the grain boundaries and mesophase domains. Nonetheless, the amount of orientational order obtained by the present procedure is by far superior to the magnetic‐induced order reported for other examples of microphase separated complex fluids such as block copolymer–nanoparticle hybrids 25…”

Block copolymer–nanoparticle composites: Structure, functional properties, and processing

“…For example, the application of magnetic field was found successful in improving the alignment of needle-like hematite (α-Fe 2 O 3 ) NPs in cylindrical structure of PS-P 2 VP BCPs (see Fig. 18) [335].…”

Orientational Behavior of Ellipsoidal Silica‐Coated Hematite Nanoparticles Integrated within an Elastomeric Matrix and its Mechanical Reinforcement

“…Few examples can be found in the literature of integrating anisometric nanoparticles within a polymer matrix, and most of those are polymer/clay nanocomposites [15][16][17] or block copolymer/carbon nanotube nanocomposites, [18,19] with improvement of mechanical properties, but still with aggregation of the nanoparticles in clusters. Two recent successful works on the integration of wires into a liquid-crystalline polymer network, [20] and ellipsoidal nanoparticles into block copolymers [21] were also reported.…”