Intercalation-induced changes in DNA supercoiling observed in real-time by atomic force microscopy

Atomic force microscopy (AFM) has been employed to observe in real-time and in an aqueous environment the process of ethidium bromide induced supercoiling in individual DNA plasmid molecules. Image data reveal both the onset and the progressive presence of plectonemic DNA supercoiling. In addition, significant molecular motion of the surface adsorbed DNA is observed. These data illustrate the potential of AFM in the time-resolved study of biomolecular processes, and hence, provide new insights into biomolecular structure and function.     

Effects of evaporation velocity and film thickness on poly(3‐hexylthiophene) thin films processed from aggregate dispersions in binary solvent mixtures

Aggregate dispersions of P3HT in two series of solvent mixtures, chloroform:dichloromethane and toluene:dichloromethane, are used to study the impact of the evaporation velocity and film thickness on the P3HT films processed using two spin‐coating speeds (1000 rpm and 2000 rpm). The structural order and surface morphology were investigated with UV/Vis absorption spectroscopy and atomic force microscopy techniques. There is no evidence that the characteristics of the liquid phase P3HT dispersions impact the structures of the films, which is in agreement with a previous study of drop cast P3HT films that were dried over much longer time periods. An association is observed between the extent of aggregation in the liquid phase and the thickness and surface roughness parameters of the films. However, the structural order does not correlate with the thickness of the films, which was previously reported for polymer films processed from amorphous polymer solutions in pure organic solvents. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 330–343     

Concurrent formation of two different type precipitation-free zones during the initial stage of homogenization

Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy were used to study the microstructure evolution of Al–Cu–Mg alloy during the initial stage of homogenization. It was found that two types of precipitation-free zones (PFZs) can form concurrently: one near grain boundaries and the other at the grain centres. Depth profile analyses of solute concentrations and dislocation-loop distributions strongly suggested that the formations of the two type of PFZs are different, due solely and exclusively to solute and vacancy depletion, respectively. A mechanism model was proposed to explain the concurrent formation of the two different type of PFZs during the initial stage of homogenization.     

Thermosensitive graphene nanocomposites formed using pyrene‐terminal polymers made by RAFT polymerization

Thermosensitive graphene‐polymer composites have been prepared by attaching poly(N‐isopropylacrylamide) (PNIPAAm) onto the basal plane of graphene sheets via π‐π stacking. Pyrene‐terminated PNIPAAm was synthesized using reversible addition fragmentation chain transfer (RAFT) polymerization via a pyrene‐functional RAFT agent. Aqueous solutions of the graphene‐polymer composites were stable and thermosensitive. The lower critical solution temperature (LCST) of pyrene‐terminated PNIPAAm was measured to be 33 °C. When the pyrene‐functional polymer was attached to graphene the resultant composites were also thermosensitive in aqueous solutions exhibiting a reversible suspension behavior at 24 °C. Atomic force microscopy (AFM) analysis revealed that the thickness of a graphene‐PNIPAAm (M_n: 10,000 and PDI: 1.1) sheet was ～5.0 nm. The surface coverage of polymer chains on the graphene basal plane was calculated to be 7.2 × 10^?11 mol cm^?2. The graphene‐PNIPAAm composite material was successfully characterized using X‐ray photoelectron spectroscopy (XPS), attenuated total reflection infrared (ATR‐IR) spectroscopy, and thermogravimetric analysis (TGA). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 425–433, 2010     

High‐molecular‐weight symmetrical multiblock copolymers: Synthesis by RAFT polymerization and characterization

High‐molecular‐weight (MW) symmetrical multiblock copolymers, based on the hydrophobic monomers styrene (Sty) and methyl methacrylate (MMA), and the more polar monomer, 2‐vinyl pyridine (2VPy), were prepared using stepwise reversible addition‐fragmentation chain transfer polymerization. All copolymers shared a common poly(ethylene glycol) (PEG) midblock, introduced as a bifunctional macromolecular chain transfer agent. In total, five ABA triblock copolymers, five ABCBA pentablock terpolymers, and two ABCDCBA heptablock quaterpolymers (comprising four different monomer types) were synthesized. The MWs of the multiblock polymers were determined using gel permeation chromatography (GPC) and proton nuclear magnetic resonance (¹H NMR) spectroscopy, with the latter values being closer to the theoretically expected, whereas GPC MW distributions were relatively narrow, broadening with the number of blocks. The compositions of the synthesized polymers, as determined by ¹H NMR spectroscopy, were also close to the expected values. Finally, films cast from chloroform solutions of the pentablock terpolymers P2VPy‐b‐PSty‐b‐PEG‐b‐PSty‐b‐P2VPy, PSty‐b‐PMMA‐b‐PEG‐b‐PMMA‐b‐PSty, and P2VPy‐b‐PMMA‐b‐PEG‐b‐PMMA‐b‐P2VPy examined using transmission electron microscopy exhibited PSty and PMMA cylinders (first two) and lamellae (third terpolymer). © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4957–4965     

Multiarm star triblock terpolymers via sequential double click reactions

Multiarm star triblock terpolymers were obtained by using two different click reactions sequentially: Cu(I) catalyzed azide–alkyne and Diels–Alder. The synthetic strategy is described as follows: (poly(methyl methacrylate))_n‐(polystyrene)_m‐poly(divinyl benzene)) ((PMMA)_n‐(PS)_m‐polyDVB) multiarm star diblock copolymer was first obtained from an azide–alkyne click reaction of (alkyne‐PS)_m‐polyDVB multiarm star polymer with α‐anthracene‐ω‐azide PMMA (anth‐PMMA‐N₃), followed by a Diels–Alder click reaction of the anthracene groups at the star periphery with α‐maleimide poly (tert‐butyl acrylate) (PtBA‐MI) or α‐maleimide poly(ethylene glycol) (PEG‐MI) leading to target (PtBA)_k‐(PMMA)_n‐(PS)_m‐polyDVB and (PEG)_p‐(PMMA)_n‐(PS)_m‐polyDVB multiarm star triblock terpolymers. The hydrodynamic diameter of individual multiarm star triblock terpolymers were measured by dynamic light scattering (DLS) to be ～24–27 nm in consistent with the atomic force microscopy (AFM) images on silicon substrates. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1557–1564, 2010