Ultrathin polymer film formation by collision-induced cross-linking of adsorbed organic molecules with hyperthermal protons

A new synthetic approach for the formation of ultrathin polymer films with customizable properties was developed. In this approach, the kinematic nature of proton collisions with simple organic molecules condensed on a substrate is exploited to break C-H bonds preferentially. The subsequent recombination of carbon radicals gives a cross-linked polymer thin film, and the selectivity of C-H cleavage preserves the chemical functionalities of the precursor molecules. The nature and validity of the method are exemplified with theoretical results from ab initio molecular dynamics calculations and experimental evidence from a variety of characterization techniques. Its applicability is demonstrated by the synthesis of ultrathin polymer films with precursor molecules such as dotriacontane, docosanoic acid, poly(acrylic acid) oligomer, and polyisoprene. The approach is fundamentally different from conventional chemical synthesis as it involves an unusual mix of physical and chemical processes including charge exchange, projectile penetration, kinematics, collision-induced dissociation, inelastic energy transfer, chain transfer, and chain cross-linking.     

Interpretation of contact angle measurements on two different fluoropolymers for the determination of solid surface tension

Contact angle measurements with a large number of liquids on the semi-fluorinated acryl polymer EGC-1700 films are reported. The surface tension was determined to be gammasv=13.84 mJ/m2 from contact angles of octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS). Inertness of these two liquids makes them ideal for determination of surface tension of low-energy fluoropolymers. On the other hand, contact angles of many other liquids deviated somewhat from a smooth contact angle pattern that represents the EGC-1700 surface tension. It is argued that noninertness of the molecules of these liquids gives rise to specific interactions with the polymer film, causing the deviations. Furthermore, contact angles of a series of n-alkanes (n-hexane to n-hexadecane) showed systematic deviations from this curve, similar to the trend observed for n-alkanes/Teflon AF 1600 systems studied earlier. Adsorption of vapor of short-chain liquids onto the polymer film caused their contact angles to fall above the gammasv=13.84 mJ/m2 curve, and a parallel alignment of molecules of the long-chain n-alkanes in the vicinity of the solid was the explanation for the deviation of their contact angles below it. It is found that vapor adsorption effect is more significant in the case of Teflon AF 1600, while the alignment of liquid molecules close to the surface is more pronounced for EGC-1700.     

Measuring inter-DNA potentials in solution

Interactions between short strands of DNA can be tuned from repulsive to attractive by varying solution conditions and have been quantified using small angle x-ray scattering techniques. The effective DNA interaction charge was extracted by fitting the scattering profiles with the generalized one-component method and inter-DNA Yukawa pair potentials. A significant charge is measured at low to moderate monovalent counterion concentrations, resulting in strong inter-DNA repulsion. The charge and repulsion diminish rapidly upon the addition of divalent counterions. An intriguing short range attraction is observed at surprisingly low divalent cation concentrations, approximately 16 mM Mg2+. Quantitative measurements of inter-DNA potentials are essential for improving models of fundamental interactions in biological systems.     

AC Poling study of lead zirconate titanate/vinylidene fluoride-trifluoroethylene composites

It is experimentally known that a polymer matrix phase in a composite of ferroelectric particles dispersed in a ferroelectric polymer can be polarized by using a few cycles of an ac field, without causing much disturbance to the state of polarization of the inclusion particles. This paper attempts to investigate this special poling process for a typical ferroelectric composite system of lead zirconate titanate (PZT) ceramic particles in a vinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) copolymer matrix, upon the application of 10 Hz ac fields of various amplitudes. Compared to a copolymer sample, the composite samples can be polarized at a lower field, and hence show a larger remanent polarization at the same poling field. Among the composites, the observed remanent polarization increases with increasing ceramic volume fraction. These experimental observations can be understood by a simple model, in which space charges are allowed to accumulate at the particle-matrix interfaces because of the electrical conductivity of the constituents. At a fast switching poling field of 10 Hz, the calculation shows that conductivity and charge accumulation effects in the composite are only minimal. Accordingly, although the PZT phase as well as the copolymer phase are both polarized under the ac field, the ceramic phase is only polarized to about 10% when the copolymer phase is almost fully polarized. Thus, one can still use an ac field to polarize only the copolymer phase of the composite without altering the polarization state of the ceramic phase significantly. PACS 77.22.Ej; 77.84.Lf     

Inhibitory activity and conformation changes of soybean trypsin inhibitors induced by ultrasound

The inhibitory activities, sulfhydryl bonds and far-UV circular dichroism (CD) spectra of Kunitz and Bowman–Birk soybean trypsin inhibitors (KTI and BBTI) were measured before and after ultrasound treatments. The differences between KTI and BBTI in conformation changes and resistance to ultrasound were observed. The secondary structures of KTI were found to be composed of β-sheet (22.5%), β-turn (16.2%) and random coils (61.4%) while that of BBTI composed of only β-sheet (52.6%) and random coils (47.4%). KTI lost its inhibitory activity more quickly than BBTI, proportionally to the ultrasound amplitudes and sonication durations used. Relevant synchronous phenomena observed included the inactivation of KTI, significant rise in sulfhydryl content and corresponding conformation changes, in which there were dramatic decreases in both β-turn and random coil contents and increase in the β-sheet structure over the entire sonication duration and ultrasonic amplitude scale used by the study. Ultrasound affected the activities and conformations of KTI and BBTI by exerting an influence on their disulfide bonds. For KTI, up to 55% of inhibitory activity could be inactivated, at which about 71.5% of disulfide bonds were altered and the [θ]_200nm value was changed from native −2545 deg cm² dmol⁻¹ to −1827 deg cm² dmol⁻¹. Whereas for BBTI, far-UV CD spectra, β-sheet and random structures were barely affected, only about 5.29% of disulfide bonds were found altered and the [θ]_200nm value was changed only from native −797 deg cm² dmol⁻¹ to −700 deg cm² dmol⁻¹. It is concluded that ultrasound inactivates KTI by inducing a reduction in the disulfide bonds and then changes the conformations.