Heterogeneous Radical Synthesis

One-Step Synthesis of Amphiphilic Ultrahigh Molecular Weight Block Copolymers by Surfactant-Free Heterogeneous Radical Polymerization

Ultrahigh molecular weight (>106 g/mol) amphiphilic block copolymers were synthesized using one-step surfactant-free heterogeneous radical polymerization (SFHRP). The polymerization initially involves formation of water-soluble homopolymer blocks, followed by copolymerization of a hydrophobic monomer, resulting in ultrahigh molecular weight block copolymers. Facilitating heterogeneous reaction conditions and continuous supply of an initiator controls the process. Using this synthetic approach, we synthesized amphiphilic block copolymers of poly(2-(N,N-dimethylamino)ethyl methacrylate)-block-poly(n-butyl acrylate) (pDMAEMA-b-pnBA), pDMAEMA-block-p(tert-butyl acrylate) (pDMAEMA-b-tBA), and pDMAEMA-block-polystyrene (pDMAEMA-b-pSt) with molecular weights of 1.98E6, 1.18E6, and 0.91E6 g/mol, respectively. These ultrahigh molecular weight block copolymers can self-assemble in nonpolar solvents to form thermochromic inverse polymeric micelles as well as other shapes and exhibit many potential applications.

Tri-Phasic Size- and Janus Balance-Tunable Colloidal Nanoparticles

These studies show synthesis of triphasic size- and Janus balance (JB)-tunable nanoparticles (JNPs) utilizing a two-step emulsion polymerization of pentafluorostyrene (PFS) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) and n-butyl acrylate (nBA) in the presence of poly(methyl methacrylate (MMA)/nBA) nanoparticle seeds. Due to built-in second-order lower critical solution temperature (II-LCST) transition of p(DMAEMA/nBA) copolymer, macromolecular segments collapse when temperature increases from 30 to 45 °C, resulting in size and shape changes. The p(DMAEMA/nBA) and p(MMA/nBA) phases within each JNP assume concave, flat, or convex shapes, forcing p(PFS/nBA) phase to adopt convex, planar, or concave interfacial curvatures, respectively. As a result, the JB can be tuned from 3.78 to 0.72. The presence of pH-responsive DMAEMA component also facilitates the size and JB changes due to protonation of the tertiary amine groups of p(DMAEMA/nBA) backbone. Synthesized in this manner, JNPs are capable of stabilizing oil droplets in water at high pH to form Pickering emulsions, which at lower pH values release oil phase. This process is reversible and can be repeated many times.

Nano-Structured Colloidal Particles

Utilization of bioactive molecules to prepare unique colloidal morphologies ranging from hollow or non-spherical particles to nanotubes and coalesced films is of significant importance. The use of colloidal synthesis facilitates various morphologies that often mimic Mother Nature resulting in unique film properties.

Shape evolution control of phase-separated colloidal nanoparticles

These studies illustrate controlled shape synthesis of two distinct phase-separated copolymers within one colloidal nanoparticle which consists of poly(methylmethacrylate) (p-MMA)/n-butylacrylate (nBA) and poly(nBA)/pentafluorostyrene (p-PFS) phases. Using sequential free radical polymerization, particle morphologies ranging from acorn to ellipsoidal, core–shell, and spherical were produced by adjusting the glass transition temperature (Tg) via compositional gradients during copolymerization. These studies show for the first time that in order to achieve desirable asymmetric particle shapes, the Tg as well as interfacial surface tension between two copolymers within one nanoparticle should be maintained during and after polymerization.

Color- and shape-tunable colloidal nanoparticles

Color- and shape-tunable nanoparticles consisting of phase-separated copolymers within each particle were synthesized using emulsion copolymerization synthesis. Each particle consists of two phases: (1) poly(methyl methacrylate/n-butylacrylate) (p(MMA/nBA)) phase, which is responsible for a particle shape control, and (2) poly(n-butylacrylate/pentafluorostyrene/N,N-dimethyl aminoazoaniline methacrylate) (p(nBA/PFS/DMAAZOAMm)) phase, facilitating tunable color changes and fluorescence emission induced by acidic/basic environments. Since these particles are capable of coalescence, these studies also show simple nano-deposition process will result in [similar]200–300 nm width nanowires with “polka-dot” morphologies, offering many diversified applications ranging from high-resolution inexpensive display technologies to nano-sensing devices, or multi-drug and multi-transient delivery systems.

Expandable Temperature-Responsive Polymeric Nanotubes

These studies show the synthesis of temperature-induced reversibly expandable nanotubes that were prepared by polymerization of N-isopropylacrylamide (NIPAAM) in the presence of biologically active 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (DC8,9PC) diacetylenic phospholipids (PL). As a result, thermally responsive poly-NIPAM-phospholipid nanotubes (PNNTs) were prepared. Polymerization reactions occur within hydrophilic regions of PL bilayers, whereas PL hydrophobic zones facilitate transport and supply of the monomer for polymerization. The unique feature of PNNTs is that, above 37 °C, the outer diameter (OD) as well as the wall thickness (WT) shrink by 20 and 55%, respectively, whereas the inner diameter (ID) increases by 16%. This behavior is attributed to the PNIPAM backbone buckling induced by local rearrangements within PL bilayered morphologies. The presence of acetylenic moieties along the PL bilayers in PNNTs provides an opportunity for irreversible “locking” of designable dimensions, which is facilitated by the formation of cross-linked PNNTs (CL-PNNTs).

Magnetic Nanotubes from Phospholipids

Due to many applications ranging from biomedical devices to metamaterials and others, tailored magnetic nanomaterials continue to be of technological importance. We developed a prototype of ferromagnetic nanotubes (FMNTs) from biologically active nanotube-forming phospholipid templates. Using simple redox reactions and thermal treatments we can produce FMNTs that consist of magnetite/carbon/magnetite concentric nanotubes with the amorphous carbon phase sandwiched between the two magnetite layers. Their magneto-electric properties can be tailored, depending upon desired applications and needs.

Morphological control of ferromagnetic nanotubes

Although magnetic nanotubes exhibit many potential applications ranging from drug or gene delivery to bioseparation, catalysis, electromagnetic or magneto-optic devices, and many others, their developments are at relatively infant stages. Recently, we utilized biologically active phospholipids (PLs) as templates to produce ferromagnetic magnetite/carbon/magnetite concentric nanotubes (FMNTs) in which simple synthetic efforts resulted in the formation of an electrically conductive carbon layer sandwiched between ferromagnetic magnetite phases, thus offering magnetic and electric attributes combined in one nanotube. Since the geometry and size of each of the FMNT components are critical factors in controlling desirable magnetic and electrical properties, control of the wall thickness as well as the diameter and length is of particular interest. This study reports the development of a simple synthetic approach to control the geometry of FMNTs by changing the concentration levels of the reactants and solvent conditions. Using this approach we synthesized concentric carbon-magnetite nanotubes with a variable thickness of 6, 10, and 60 nm conductive carbon layers and 12 to 45 nm magnetic magnetite layers. As a result, saturation magnetization values could be incrementally tuned from 40 to 79 emu g−1.