A COMPARATIVE STUDY OF CHAIN DYNAMICS OF DI-AND TRI-BLOCK COPOLYMERS IN SEMIDILUTE SOLUTION IN A NON-SELECTIVE SOLVENT

The chain dynamics of a pair of diblock poly(styrene-b-butadiene) (PS210-b-PB960) and triblock poly(styrene-b-butadiene-b-styrene) (PS200-b-PB1815-b-PS200) copolymers in both dilute and semidilute toluene solutions has been comparatively studied by dynamic laser light scattering. As expected, the mutual diffusion of individual chain changes into a fast cooperative diffusion of the chain segments ('blobs') between two neighboring entanglement points for both the copolymers as the solution changes from dilute to semidilute. Further increases of the concentration lead to a second slow relaxation mode. For the triblock chains, there exists an additional middle relaxation between the fast and the slow modes. The concentration (c) dependent study of the average characteristic decay time of the fast mode ((tau(f))) reveals that 1/(tau(f)) - c(-a) with 0.33 < alpha < 0.44, much smaller than 0.75 predicted or 0.72 observed for linear homopolymer chains in good solvent. It implies that the solvent quality of toluene for PB might not be as good as that for PS. Due to such a difference in solubility, it is reasonable to speculate that the PB and PS blocks are transiently segregated in semidilute solution. The relaxation of these transient PB and PS richer domains leads to the observed slow relaxation. Such a speculation is supported by the appearance of an additional slow relaxation mode in the study of polyisoprene-b-polystyrene-b-polyisoprene in semidilute solution in cyclohexane, a non-selective solvent, in which we alternated the solubility difference by a variation of the solution temperature.

THE ADSORPTION OF LINEAR POLY(N-ISOPROPYLACRYLAMIDE)CHAINS ON SURFACTANT-FREE POLYSTYRENE NANOPARTICLES

The adsorption of linear poly(N-isopropylacrylamide) (PNIPAM) chains on surfactant-free polystyrene (PS) nanoparticles was used as a model system to study the hydrophobic adsorption of polymer on the surface, because the hydrophobility of PNIPAM can be continuously varied by a small temperature change. The adsorption was investigated by a combination of static and dynamic laser light scattering (LLS) measurements. In static LLS, the absolute excess scattered light intensity led to the amount of PNIPAM adsorbed on the surface. In dynamic LLS, the hydrodynamic thickness of the adsorbed PNIPAM layer was accurately measured. For a given particle concentration, the adsorption increases as the PNIPAM concentration and the incubation temperature increase. The average density of the adsorbed PNIPAM layer is reciprocally proportional to the number of the PNIPAM chains on the surface, revealing a simple scaling of the chain density distribution. The adsorption follows the Langmuir's isotherm. The enthalpy change estimated from the adsorption at 25 degrees C and 30 degrees C is slightly positive, indicating that the adsorption involves the coil-to-globule transition of the chains on the surface.

Nonlinearity of the Dipole Moment Surface and Intensities Anomaly of CHCl_(3)

Relative absorption intensities of the Fermi resonance polyads of isolated C-H chromophore for the CHC13 molecule are calculated by one-dimensional dipole moment surface which had been obtained by the ab initio density functional method B3PW91 with 6-311++G(3df,2pd)basis set,and agree very well with the experimental results.It is shown that the nonlinearity of the dipole moment surface in the vicinity of the equilibrium configuration is responsihle for the intensities anomaly,i.e.the unusual strong intensities of the second Fermi resonance polyad.

Quantum Beat Spectroscopy of SO_(2)in the 235-224nm Range

Using time-resolved laser-induced fluorescence(LIF)spectroscopy,the fluorescence quantum beat decay was observed in C(1B_(2))←X(1A_(1))transition of SO_(2)molecule.The experimental results show that only the v_(3) mode and its combination band v_(1)+v_(2)+v_(3) excitation exist the beat decay.Analyzing the result,we attribute the quantum beat states to the coupling of the v3 vibration mode of C state with the high vibrational levels of the electronic ground state.

Theoretical Scanning Tunnelling Microscopy Images of Metal (Fe, Co, Ni and Cu) Phthalocyanines

The scanning tunnelling microscopy (STM) images of isolated iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), nickel phthalocyanine (NiPc) and copper phthalocyanine (CuPc) are simulated theoretically. All the simulated STM images show submolecular structures and reproduce well the features of the experimental images. The results show that there is a strong dependence of the STM images on the ion valence configuration of the metal ion. At the small tip bias voltages of less than 0.5 V, the central metal ions in NiPc and CuPc appear as holes in the molecular images, while they are the highlighted bumps in FePc and CoPc. The simulated images are interpreted by the fact that both FePc and CoPc systems have a significant dz2 character near the Fermi level while the NiPc and CuPc systems do not. Moreover, we predict that the central nickel ion for NiPc appears as a highlighted point when the tip bias voltage is larger than 0.7 V.

Theoretical Study of Extraction of a Single Al Atom from Al(111) Surface by Scanning Tunneling Microscopy W Tip

Based on the discrete variational method with the local-density-functional approximation,the cluster models were chosen to simulate the extraction of a single Al atom from Al(111)sample surface by a scanning tunneling microscopy W tip with and without external bias voltages.The calculated results show that the tip-sample effects of W tip on the extraction of an Al atom are very important.The Al atom can be extracted from the Al(111)surface by the pure W tip without external bias,as the tip approaches the sample within 10 a.u.(5.3Å).When the separation between the tip and the sample is larger than 10 a.u.,a positive bias must be applied to the sample.The extracted Al atom is expected to have net positive charge when it escapes from the Al(111)sample surface.The corresponding critical field is about 0.6V/Å.This value is in good agreement with experimental results.

Scanning Tunnelling Microscope Tip-Induced Reconstruction on Si(111)√3×√3 R30°-Ag Surface

The reconstruction process of the Si(111)√3 ×√3 R30°-Ag surface is studied by using a scanning tunnelling microscope at 78K.By applying a strong interaction between the tip and the surface,a tip-induced reconstruction corresponding to the mergence of two Si(111)√3 ×√3 R30°-Ag domains is observed.Based on the inequivalent trimers(IET)model,this reconstruction process is attributed to a transition between the clockwise and counterclockwise IET domains.With this transition,the honeycomb-chained-trimer Si(111)√3 ×√3 R30°-Ag anti-phase boundary disappears and changes to the IET structure.

How Does a Polymer Brush Repel Proteins?

The captioned question has been addressed by the steric effect; namely, the adsorption of proteins on a surface grafted with linear polymer chains decreases monotonically as the grafting density increases. However, there is no quantitative and satisfactory explanation why the adsorption starts to increase when the grafting density is sufficiently high and why polyethylene glycol（PEG） still remains as one of the best polymers to repel proteins. After considering each grafted chain as a molecular spring confined inside a ＂tube＂ made of its surrounding grafted chains, we estimated how its free energy depends on the grafting density and chain length, and calculated its thermal energy-agitated chain conformation fluctuation, enabling us to predict an adsorption minimum at a proper grafting density, which agrees well with previous experimental results. We propose that it is such a chain fluctuation that slows down the adsorption kinetically.