We report on the correlation of polyelectrolyte chain
dynamics
in polyelectrolyte complexes (PECs) with the deposition mode and chain
mobility of polyelectrolytes (PEs) within layer-by-layer-assembled
(LbL) films. The study was performed using two polyelectrolyte systems:
poly(2-(dimethylamino)ethyl methacrylate)/poly(methacrylic acid) (PDMA/PMAA)
and completely quaternized PDMA (Q100M)/PMAA. Hydrodynamic sizes of
PDMA/PMAA and Q100M/PMAA complexes in solution were followed by fluorescence
correlation spectroscopy (FCS), while three different techniques were
applied to probe the structure and dynamics of the same PE pairs within
LbL films. Specifically, deposition of PEs at surfaces was monitored
by phase-modulated ellipsometry, film internal structureby
neutron reflectometry (NR), and diffusion of assembled chains in the
direction parallel to the substrateby fluorescence recovery
after photobleaching (FRAP). By applying these complementary techniques
to PDMA/PMAA and Q100M/PMAA systems in solution and at surfaces at
various pH values, we found that the dynamics of polyelectrolyte chains
within PECs underwent a prominent pH-dependent transition, and that
this transition in chain dynamics was closely correlated with the
transition between linear and exponential film growth modes. Neutron
reflectometry results confirm that, at the transition point, film
structure changed from layered for linearly depositing films to highly
intermixed for exponentially depositing LbLs. Moreover, FRAP indicated
a several-fold difference in PE lateral diffusion coefficient for
the two different film growth modes. In addition, the pH transition
point was affected by steric restrictions to ionic pairing, and the
pH range of exponential growth and higher chain mobility was wider
for Q100M/PMAA as compared with the PDMA/PMAA system, due to the presence
of a methyl spacer at the amino group, resulting in weaker ionic pairing.
The activity coefficients of eight hydrocarbons in infinitely dilute solution in dinonyl phthalate at 30°C have been evaluated by gas-liquid chromatography (G.L.C.) and compared with values extrapolated from earlier static measurements. When the G.L.C. data are extrapolated to zero flow rate and zero sample size and corrected for gas imperfection, excellent agreer,ent with data obtained by the static technique is observed. The importance of using the correct equation for the gas imperfection correction is stressed, and an approximate method is suggested for deriving the virial coefficients of a mixed vapour from G.L.C. measurements.
The effect of applying a rounded stylus to thin metallic films on glass substrates has been investigated using diamond and steel styli with tip radii of approximately 25 μm and loadings of up to 230 g. The films were vacuum-deposited indium, tin, lead, gold, copper, aluminium, nickel, chromium and molybdenum of various thicknesses up to 3·2 μm. Scanning electron microscope and optical interference microscope observations showed that the process of scratch formation was generally very complex and varied with the film material, indicating that it is not possible to deduce absolute values of adhesion forces using a simple general theoretical model. The method can, however, be used with caution for qualitative comparisons under certain restricted conditions.
As part of an ongoing programme of reference work for surface analysis at the NPL, the problem of surface charging when analysing insulators by static SIMS is discussed. Meaningful analysis requires stabilization of the surface potential to a defined reference point, here taken as the potential that would exist if the insulator was a conductor. In SIMS spectra it is shown that both the absolute and relative intensities of peaks depend on the surface potential. It is also shown that, only by the correct design of an electron flood source, can the surface be stabilized to a given constant reference potential. The relevant design principles are illustrated by measurements on isolated targets which define the characteristics of the flood source. Stabilization is effected to within the f0.5 eV required for static SIMS.
The effect of oxygen, present during film formation, on the structure and superconducting properties of tin films vacuum deposited on glass has been studied for substrate temperatures during deposition in the ranges of 23-30°C and 63-91°C.In both temperature ranges, in general, an increase in oxygen pressure decreases crystallite size and electron mean free path; it increases superconducting critical magnetic field value and width of transition but decreases the superconducting temperature transition width.The higher substrate temperatures produce films with larger crystallites and thin intercrystalline regions; these show hysteresis in the superconducting magnetic field transition and have higher values of critical field and critical current, in excess of those theoretically predicted.
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