Architecture of polymeric superstructures: Locking of cubic lattices by graft polymerization from photofunctional core-shell polymer microspheres

Abstract

It is well known that submicron-size colloidal spheres form a body-centered cubic (BCC) or face-centered cubic (FCC) lattice in an aqueous solution [1–5]. More recently, Asher et al. [6] have developed an approach to permanently lock in the crystalline colloidal array ordering in a hydrogel matrix. We have also constructed polymeric superstructures by free radical polymerization of N-vinylpyrrolidone (VP) monomer as a matrix, after polypyrrole microspheres were arranged on a BCC lattice in VP monomer solution [7]. More recently, we demonstrated that the core-shell polymer microspheres [8] and highly armed star polymers [9] led to hierarchical structure transformation of the cubic lattices. That is to say, these radial branched polymers formed a lattice with a BCC structure near the overlap threshold (C∗). In the bulk of the film, this structure changed to a FCC lattice. After such branched polymers formed a cubic lattice in polymerizable vinyl monomer, the polymeric superstructures were locked permanently into an ordered lattice in a solid matrix by means of free-radical polymerization [10, 11]. This technique is one of the best methods for creating nanoscopic polymeric superstructures composed of three or two phase-separated microdomains. However, the microspheres exhibiting relatively large particle diameter were packed with irregular arrangement in places due to polymerization-induced phase separation. In this communication, we report the architecture of polymeric superstructure films formed by locking of polymer matrix by means of graft polymerization from photofunctional core-shell polymer microspheres. The well-defined polystyrene-block-poly(4-vinylpyridine) (PS-block-P4VP) diblock copolymer (SV: Mn = 7.27× 104, P4VP block 26.7 mol%) was prepared by the sequential anionic polymerization technique. The details concerning the synthesis, purification and characterization of diblock copolymers have been given elsewhere [12]. Fig. 1a shows the transmission electron microscopy (TEM) photograph of SV specimen cast from toluene. The dark portions are the P4VP phases selectively stained with OsO4. The morphological structure showed the dispersed P4VP spheres in a PS matrix. Core-shell polymer microspheres (SV-M) were synthesized by crosslinking the segregated chains in P4VP spherical microdomains with 1,4-dibromobutane (DBB) vapor at room temperature. The details concerning the synthesis and the characterization of such microspheres have been given previously [13]. Characteristics of the SV-M microsphere are listed in Table I. The crosslink density of P4VP cores was 30.3 mol%. Fig. 1b shows a TEM photograph of SV-M cast from 1.0 wt% toluene solution. This specimen corresponds to the restructural film from the core-shell microspheres. It is found from this photograph that dark crosslinked P4VP cores (average diameter Dc = 30.2 nm) with a narrow size distribution are dispersed in a PS matrix. It was also mentioned previously from small-angle Xray scattering (SAXS) measurements that P4VP cores were packed with a FCC arrangement in a bulk film [8]. The hydrodynamic diameter (DH = 96.9 nm) of microspheres was determined by means of dynamic light scattering (DLS). The C∗ [14] of the SV-M in benzene was calculated to be 11.9 wt% from the DH and total molecular weight of the microsphere. Photofunctional core-shell microspheres (SV-MP) were prepared as follows. The PS shell of parent