Letter pubs.acs.org/NanoLett
Instantaneous Directional Growth of Block Copolymer Nanowires During Heterogeneous Radical Polymerization (HRP) Chunliang Lu and Marek W. Urban* Department of Materials Science and Engineering, Center for Optical Materials and Engineering Technologies (COMSET), Clemson University, Clemson, South Carolina 29634-0915, United States S Supporting Information *
ABSTRACT: Polymeric nanowires that consist of ultrahigh molecular weight block copolymers were instantaneously prepared via one-step surfactant-free heterogeneous radical polymerization (HRP). Under heterogeneous reaction and initiator-starvation conditions, the sequential copolymerization of hydrophilic and hydrophobic monomers facilitates the formation of amphiphilic ultrahigh molecular weight block copolymers, which instantaneously assemble to polymeric nanowires. As polymerization progresses, initially formed nanoparticles exhibit the directional growth due to localized repulsive forces of hydrophilic blocks and confinement of the hydrophobic blocks that adopt favorable high aspect ratio nanowire morphologies. Using one-step synthetic approach that requires only four ingredients (water as a solvent, two polymerizable monomers (one hydrophilic and one hydrophobic), and water-soluble initiator), block copolymer nanowires ∼70 nm in diameter and hundreds of microns in length are instantaneously grown. For example, when 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) and styrene (St) were copolymerized, high aspect ratio nanowires consist of ultrahigh (>106 g/mol) molecular weight pDMAEMA-b-St block copolymers and the presence of temperature responsive pDMAEMA blocks facilitates nanowire diameter changes as a function of temperature. These morphologies may serve as structural components of the higher order biological constructs at micro and larger length scales, ranging from single strand nanowires to engineered biomolecular networks capable of responding to diverse and transient environmental signals, and capable of dimensional changes triggered by external stimuli. KEYWORDS: Block copolymers, polymer nanowires, heterogeneous radical polymerization (HRP)
M
molecular reagents (macroinitiator or chain transfer agents)14−18 as well as thermal self-assembly (PITSA)19 induced by lower critical solution temperature (LCST) are alternatives for producing fused micelles and wormlike morphologies, but time-consuming synthesis and purification procedures are the limiting factors. In contrast, the synthesis of high molecular copolymer nanoparticles dispersed in an aqueous media have been known for many years,5,20−22 but the main limitations are not only morphology control. Although recent studies resulted in phase-separated core− shell,23,24 hollow,25 Janus,26,27 gibbous,28 and tubular29 shapes, the challenge is to synthesize high aspect nanowires composed of high molecular weight block copolymers using efficient and robust process. In these studies, we describe an instantaneous generation of ultrahigh aspect ratio high molecular weight (>106 g/mol) block amphiphilic copolymer nanowires using heterogeneous radical polymerization (HRP) in aqueous media. The uniqueness of this process is that it requires water (solvent), two polymerizable hydrophilic and hydrophobic monomers, and water-soluble initiator.
orphologies formed by amphiphilic compounds such as surfactants and lipids are fundamental building blocks of soft matter and have been of interest and technological importance for many decades.1 Their unique static and dynamic properties2 as well as diversified morphologies offer numerous applications3 that have been extended to self-assembly of amphiphilic block copolymers.4 Exhibiting enhanced stability, the resulting wormlike polymeric entities offer numerous applications, including nanocarrier drug delivery systems, therapeutics, lithographic and optoelectronic applications, templating materials, or sensing devices, to name just a few.5 Because polymer moieties offer more versatile control of properties, wormlike polymers have been exclusively obtained by self-assembly of block copolymers synthesized by living cationic/anionic/ring-opening or controlled radical polymerizations (CRPs).6−13 Typical self-assembly of these materials is achieved by solubilizing one block, thus facilitating rearrangement of the other block to form desirable morphologies.10 Although these solvent displacement methods facilitate thermodynamic and kinetic control over final morphologies,6 multistep synthesis and purification as well as time-consuming assembly process along with low concentration (usually
