Bulk Dynamic Measurements

A number of issues relative to the prediction of solids conveying in smooth bore single-screw extruders are exposed from the theoretical fits to the data in Fig. 5.32. First, the data needed to carry out an effective simulation is difficult to take and is very time consuming, and only a few labs have the proper equipment that is, bulk density measurement, dynamic friction data, lateral stress, and solids conveying data. Moreover, care must be taken to develop an accurate representation of the surface temperature for the barrel and screw as a function of the axial position. This would be quite difficult in a traditional extruder with only a control thermocouple to measure the temperature at the midpoint of the barrel thickness. Second... 

In situ dynamic measurements and DDFT simulations to non-bulk PL and lamella morphologies (Figs. 25 and 26) demonstrate that annihilation of topological... 

Threads, clusters, or networks of water molecules have been detected by crystallographic, thermodynamic, and dynamic measurements. They appear to be a common feature of the protein—water interface, which should not be surprising, in view of the extensive hydrogen bonding of the bulk water. Moreover, the water of the interface differs from... 

As already discussed, the size, the shape and the dispersion are characteristic parameters of metal particles and will determine their properties. It is possible to characterize them as follows (i) The size of the metal particles (transmission electron microscopy, (TEM) [79-81], extended X-ray absorption fine structure (EXAFS) [82, 83]), (ii) their structures (X-ray diffraction (XRD), TEM), (iii) their chemical composition (TEM-EDX, elemental analysis), (iv) the chemical state of the surface and bulk metal atoms (X-ray photoelectron spectroscopy (XPS)), Mossbauer [84], thermo-programmed-reduction (TPR)), (v) chemisorption capacity toward probe molecules such as H2, O2, NO and CO (volumetric or dynamic measurements) [85-90]. 

Thus, one has to expect one dielectric relaxation process, assigned to a local motion of the Si-O bond. It corresponds to the dynamic glass transition (a-relaxation) of the bulk polymer. Measured from temperatures between —25 °C to 33 °C this relaxation process shifts from about 1 Hz to about 10 Hz (Fig. 21.4). 

Apparently the differences between Tg determined from our data and from bulk dynamic mechanical measurements are real. This result may reflect confinement, i.e. the presence of substrate and surface separated by a distance ( 20 nm) on the order of the radius of gyration of the polymer random coils. In an attempt to identify the relative importance of substrate and surface, we have preliminarily measured the temperature dependence of fiiction on a PMMA film 250 nm thick. Figure 7. The shape and position of both the secondary and glass transition peak are approximately the same as on the 20-nm film. Figure 6. This supports the concept that excess free... 

In analogy with bulk dynamical mechanical analysis (10Jl a small mechanical modulation can be applied to the contact and the phase and amplitude of the response measured. In SFM this modulation is either a displacement of the sample or of the base of the cantilever. In either case, the amplitude and phase of the response of the probe end of the cantilever beam are measured. The modulation can be applied either normal(/2-i- ) to the surface or parallel to it (9,75-77). The latter is a shear modulation, shown schematically in Figure 2. 

Velocity / Frequency Relationship. The characteristic temperature-dependent line shapes for each polymer, as determined by bulk dynamic mechanical or dielectric experiments from literature (4), were used as a comparison to our fnctional measurements. For the comparison to be valid the literature data must be adjusted to match the same frequency (time-scale) of the friction experiment. The conversion procedure of scan velocity to frequency has been described in previously (7), whereby a contact diameter was calculated to convert the scan velocity to a frequency by simple division. The contact diameter thus allows a gauge for the time the probe tip affects a point on the polymer surface. For the given radius of curvature of 20 nm, applied load=10 nN, adhesive load=15 nN, and assuming a bulk storage modulus, the contact diameter can be estimated by JKR theory to be 18.7 nm. Thus for a scan velocity of 40 pm/sec the equiv ent frequency of measurement is 2000 Hz and all tabulated tanS data used are scaled accordingly to this frequency for comparison. 

Thin Film / Bulk Comparison. In all surface fnction studies we observe the position of the glass-transition temperature lowered somewhat compared to bulk dynamic mechanical measurements. This type of thin polymer film behavior is not surprising since thin films are known to vary considerably from bulk polymer (17). This is a result of chain confirmation differences (8), confinement effects (18), as well... 

The inclusion of values in Table 1 l-III derived from dynamic bulk viscoelastic measurements implies the concept that the relaxation times describing time-de-pendent volume changes also depend on the fractional free volume—consistent with the picture of the glass transition outlined in Section C. In fact, the measurements of dynamic storage and loss bulk compliance of poly(vinyl acetate) shown in Fig. 2-9 are reduced from data at different temperatures and pressures using shift factors calculated from free volume parameters obtained from shear measurements, so it may be concluded that the local molecular motions needed to accomplish volume collapse depend on the magnitude of the free volume in the same manner as the motions which accomplish shear displacements. Moreover, it was pointed out in connection with Fig. 11 -7 that the isothermal contraction following a quench to a temperature near or below Tg has a temperature dependence which can be described by reduced variables with shift factors ay identical with those for shear viscoelastic behavior. These features will be discussed more fully in Chapter 18. 

In dynamic bulk viscoelastic measurements, the deformations are ordinarily exceedingly small and therefore in the linear range of behavior the change in free volume during a cycle of deformation is a very small proportion of the total free volume. However, a large constant hydrostatic pressure can be imposed if desired on the small periodic pressure changes, thereby altering the free volume and hence the relaxation times and the frequency scale of the viscoelastic dispersion. 

Here, we discuss any discernible influence of the adsorbed layer formation process on our dynamic measurement. As noted above, the viscosity of thick PS-SiOx with ho > 4Rg is the same as bulk viscosity [2]. This shows that the polymer chains constituting the adsorbed layer should make insignificant contribution to the total mobility of the films. In that experiment [2], the steady-state effective viscosity qeff.o was established within 1 h at 150 °C for M = 44 to 393 kg/mol. We notice from Fig. 2.7 that this duration is of the same order as the duration of the initial rapid development of the residue film noted above for the same temperature. The fact that the steady-state effective viscosity is established... 

In Fig. III-7 we show a molecular dynamics computation for the density profile and pressure difference P - p across the interface of an argonlike system [66] (see also Refs. 67, 68 and citations therein). Similar calculations have been made of 5 in Eq. III-20 [69, 70]. Monte Carlo calculations of the density profile of the vapor-liquid interface of magnesium how stratification penetrating about three atomic diameters into the liquid [71]. Experimental measurement of the transverse structure of the vapor-liquid interface of mercury and gallium showed structures that were indistinguishable from that of the bulk fluids [72, 73]. 

Small metal clusters are also of interest because of their importance in catalysis. Despite the fact that small clusters should consist of mostly surface atoms, measurement of the photon ionization threshold for Hg clusters suggest that a transition from van der Waals to metallic properties occurs in the range of 20-70 atoms per cluster [88] and near-bulk magnetic properties are expected for Ni, Pd, and Pt clusters of only 13 atoms [89] Theoretical calculations on Sin and other semiconductors predict that the stmcture reflects the bulk lattice for 1000 atoms but the bulk electronic wave functions are not obtained [90]. Bartell and co-workers [91] study beams of molecular clusters with electron dirfraction and molecular dynamics simulations and find new phases not observed in the bulk. Bulk models appear to be valid for their clusters of several thousand atoms (see Section IX-3). 

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