We use photon correlation imaging, a recently introduced space-resolved dynamic light scattering method, to investigate the spatial correlation of the dynamics of a variety of jammed and glassy soft materials. Strikingly, we find that in deeply jammed soft materials spatial correlations of the dynamics are quite generally ultra-long ranged, extending up to the system size, orders of magnitude larger than any relevant structural length scale, such as the particle size, or the mesh size for colloidal gel systems. This has to be contrasted with the case of molecular, colloidal and granular ''supercooled'' fluids, where spatial correlations of the dynamics extend over a few particles at most. Our findings suggest that ultra long range spatial correlations in the dynamics of a system are directly related to the origin of elasticity. While solid-like systems with entropic elasticity exhibit very moderate correlations, systems with enthalpic elasticity exhibit ultra-long range correlations due to the effective transmission of strains throughout the contact network.
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...
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