Y-Transition

Determination of the glass-transition temperature, T, for HDPE is not straightforward due to its high crystallinity (16—18). The glass point is usually associated with one of the relaxation processes in HDPE, the y-relaxation, which occurs at a temperature between —100 and —140° C. The brittle point of HDPE is also close to its y-transition. 

With DMA the effect of temperature on the modulus can be studied. By increasing the temperature from -150 to 300°C, one encounters several transitions in PA (Fig. 3.1). There is a transition at about —120°C, the y-transition, which is due to the mobilization of methylene units. There is also a transition at —30°C, which is present in wetted aliphatic PA this is due to non-H-bonded amide units and is termed the /J-transition. At about 50°C the glass Uansition (Tg) (a-transition) of the aliphatic polyamides PA-6 and PA-6,6 occurs. At this Uansition, the modulus is lowered considerably. For partially aromatic PA, the Tg occurs above 100°C. The last transition is the flow temperature, at which temperature the material melts the flow temperature and the melt temperature, as measured by DSC, correspond well. The modulus is a measure of dimensional stability and increases with crystallinity and filler content (Fig. 3.12). 

The influence of the decay scheme on the retention (through differences in the percent conversion of y-transitions) was demonstrated by comparison of the -decay products of Pb and Pb in Pb(CgH5)3Cl. The retention of Bi in Bi(CgH5)3Cl2 was 17—19% and of Bi about 50%. According to Nefedov, this isotope effect is directly proportional to the conversion coefficients of the two isotopes. Corresponding to the complement of the conversion coefficient, 1—a, the molecular structure should be preserved to the extent of 80% for the two isotopes. The probability of chemical reaction for change or preservation of molecular structure is the same for the two cases. 

Bi decays by emission of n, = 1.16meV with no y-transition. Thus, the only influences are those of mechanical recoil from the fj emission (maximum recoil energy 3.5 eV) and the ionization due to collective electron excitation (shakeoff). 

For X = Br, Cl and F, the observed retentions are 73%, 33% and 5% respectively. Simultaneous with the decrease of retention, the (CgH5)2PoX2 increased. Since no y transition occurs, this series clearly must reflect the influence of the halogen electronegativity on the cumulative electron excitation and the shakeoff. Evidently fluorine is least able to compensate for shakeoff and thus preserve the original structure ... 

A further complicating factor is that many low-energy y-transitions are converted, that is, their energy is released through the expulsion of one or more atomic electrons. This gives rise to the Auger cascade, which will be described below. 

The best-studied system is Pb(CHj)4. This can easily be prepared using 22-yr Pb, which decays to °Bi by branched jS-emission—maximum j3-energies 15 keV (81%) and 61 keV (19%). The crossover y-transition is 14% converted. The results of various studies 22, 21) show retention of bonding in some 70-80% of the °Bi formed in the gas phase, and a similarly high yield in dilute solutions. Curiously, the yield of °Bi(CH3)4 fell to 18% at a mole fraction of 0.05 and rose again on further dilution (21). Adloff (2) has studied the f decay of PbPh4, with comparable results. 

The partial pressures of disilane and trisilane are shown in Figure 32, for the same process conditions as in Figure 31. Both partial pressures increase as a function of pressure. Around the a-y transition at 30 Pa the disilane partial pressure increases faster with increasing pressure, as can be seen from the deviation from the extrapolated dashed line. The disilane partial pressure amounts to about 1 % of the total pressure, and the trisilane partial pressure is more than an order of magnitude lower. Apparently, in the y -regime the production of di- and trisilane is enhanced. 

Andujar et al. [246] have used SE to study the effects of pressure and power on the properties of a-Si H. In their system the a-y transition occurs just above 0.2 mbar at a power of 10 W. They found that the density of the material is decreased on going from the a- to the y -regime, as is deduced from the decrease in the imaginary part of the dielectric function ei-... 

The influence of power variation on the material properties is in agreement with the trends observed by Andujar et al. [246], who studied the a-y transition in pure silane discharges at 13.56 MHz. Further, this has also been observed for pure silane discharges at 50 MHz by Meiling et al. [423],... 

As can most clearly be seen in Figure 46d, the a-y transition occurs at a pressure of about 0.3 mbar for these experimental conditions. The impedance of the plasma, as well as the optical emission from the plasma, changes on going through the transition. The depletion of SiHq during deposition was already shown and... 

In the presented range of pressure variation. Hamers [163] also has studied the influence of the substrate temperature on the plasma and the material. It was found that in the temperature range of 200 to 300°C the trends of the bias voltage, the plasma potential, and the growth rate as functions of pressure all are the same, while the absolute magnitude depends on the temperature. The trends in material properties are similar to the ones reported above at a temperature of 200°C the material quality is worse than at higher temperatures. The a-y transition occurs at a lower pressure than at a temperature of 250°C. This has been observed before [248]. 

The presented material quality results are similar to results reported by others [119, 120, 280, 475], although those frequency series were obtained in the a-regime at constant low pressures. Here, good quality homogeneous layers were deposited at the a-y transition region. Especially the low power results compare well with results by others. 

Table 4.4 Direction-dependent terms 0) for Ml and E2 nuclear y-transitions (see, e.g., [47])...

Fig. 7.43 Graphical representation of isomer shifts of the 6.2 keV y-transition in Ta for tantalum compounds and for dilute impurities of Ta in transition metal hosts (from [186])...

There are two y-transitions in Pt amenable to the Mossbauer effect - the 130 keV transition between the 5/2 excited state and the 1/2 ground state and the 99 keV transition between the first excited 3/2 state and the ground state. Figure 7.70 shows the simplified decay scheme of Pt. The relevant nuclear data may be taken from Table 7.1 (at the end of the book). 

The first Mossbauer measurements involving mercury isotopes were reported by Carlson and Temperley [481], in 1969. They observed the resonance absorption of the 32.2 keV y-transition in (Fig. 7.87). The experiment was performed with zero velocity by comparing the detector counts at 70 K with those registered at 300 K. The short half-life of the excited state (0.2 ns) leads to a natural line width of 43 mm s Furthermore, the internal conversion coefficient is very large (cc = 39) and the oi pj precursor populates the 32 keV Mossbauer level very inefficiently ( 10%). 

Dynamic Mechanical Properties. The dynamic mechanical properties of branched and linear polyethylene have been studied in detail and molecular interpretation for various transitions have already been given, although not necessarily agreed upon in terras of molecular origin.(52-56) Transitions for conventional LDPE (prepared by free radical methods) when measured at low frequencies, are located around +70°C, -20°C and -120°C and are assigned to o, 5, and y transitions respectively. (53) Recently Tanaka et al. have reported the dynamic mechanical properties for a sample of HB which was also prepared by anionic polymerization, but contrary to our system the hydrogenation of the polybutadiene was carried out by a coordinate type catalyst.(12) The transitions reported for such a polymer at 35 Hz are very similar to those of LDPE.(12)... 

When the fragments X and Y approach one another in the X. .. H. .. Y transition state, their outer electron shells begin to repel one another. It is to be expected that the repulsion will be stronger when the radii of the atoms X and Y is larger. The examples presented in Table 6.7 confirm this conclusion. 

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