Temperature dependence hole size

With the Monte Carlo method, the sample is taken to be a cubic lattice consisting of 70 x 70 x 70 sites with intersite distance of 0.6 nm. By applying a periodic boundary condition, an effective sample size up to 8000 sites (equivalent to 4.8-p.m long) can be generated in the field direction (37,39). Carrier transport is simulated by a random walk in the test system under the action of a bias field. The simulation results successfully explain many of the experimental findings, notably the field and temperature dependence of hole mobilities (37,39). 

Meltzer et al. (2001) showed direct evidence for long-range interactions between lanthanide impurity ions in embedded nanocrystals with the TLS s of the matrix based on hole burning data. The samples were prepared as follows monoclinic 23 run Y2C>3 0.1% Eu3+ nanocrystals, produced by condensation after laser evaporation, were dispersed in a siloxane polymer. Fig. 13 shows the temperature dependence of the width of spectral holes (khb), which were burned in the 5Do <- 7Fo transition of Eu3+ ions located on the c sites (582.8 nm)innanociys-tals embedded in polymer, in comparison with the case of the free, as-prepared nanocrystals having the same size. The embedded nanocrystals exhibit a drastic increase in hole width and a very different power-law behavior for the temperature dependence of the hole widths relative to similar isolated nanoparticles. The 7 3 temperature dependence of the hole width... 

The reaction of an amorphous polymer with a solvent is dependent upon diffusion of the attacking reactant. If the polymer is at a temperature above its Tg there is a constant cooperative segmental motion that favors diffusion from hole to hole in the polymer mass wherein hole formation in a polymer above its Tg is related to the relative mobility of the diffusing molecules. The rate of diffusion is dependent on the temperature, the molecular size, and the extent of the compatibility of the corrosive molecules with those of the polymer (36). 

Therefore, the engineer needs to determine the maximum nipple height, the temperature-dependent modulus, and the molding vacuum to solve for the permissible diameter of the vent holes. After determining the correct vent hole size, the value is rounded down to the next smallest drill size. 

The glass transition can then be visualized as the onset of coordinated segmental motion made possible by an increase of the holes in the polymer matrix to a size sufficient to allow this type of motion to occur. This is manifest as a change in the specific volume due solely to an increase in the free volume and is shown schematically as the crosshatch area in Figure 12.5, where the broken line indicates the temperature dependence of Vg-... 

Muramatsu, M., Okura, M., Kuboyama, K., Ougizawa, T., Yamamoto, T., Nishihara, Y., Saito, Y., Ito, K., Hirata, K., and Kobayashi, Y, Oxygen permeability and free volume hole size in ethylene-vinyl alcohol copolymer film temperature and humidity dependence, Radiat. Phys. Chem., 68, 561-564 (2003). 

Temperature dependence of the free-volume hole size... 

Experimental data from our laboratories will be shown for an extensive series of amorphous polymers with glass transitions between Tg = 200 and 500 K. We discuss the temperature dependence of the hole-size distribution characterized by its mean and width and compare these dependencies with the hole fraction calculated from the equation of state of the Simha-Somcynsky lattice-hole theory from pressure-volume-temperature PVT) experiments [Simha and Somcynsky, 1969 Simha and Wilson, 1973 Robertson, 1992 Utracki and Simha, 2001]. The same is done for the pressure dependence of the hole free-volume. The free-volume recovery in densified, and gas-exposed polymers are discussed briefly. It is shown that the holes detected by the o-Ps probe can be considered as multivacancies of the S-S lattice. This gives us a chance to estimate reasonable values for the o-Ps hole density. Reasons for its... 

We assume that the o-Ps hole-size distribution above Tg directly mirrors the thermal density fiuctuation. This allows us to extract information on the length scale of the dynamic heterogeneity in polymers. Using a fluctuation approach, the temperature dependency of the volume of the smallest representative freely fluctuating subsystem can be estimated. Limits of this interpretation for polymers with a high structural disorder, which already appears in the glass, are discussed. 

TEMPERATURE DEPENDENCE OF THE FREE-VOLUME HOLE SIZE 431... 

It is interesting to compare the hole sizes for polymers of different Tg values and different chemical structure. Such a comparison is made in Figure 11.5, showing plots of the temperature-dependent mean hole volume, ( ua ), and the standard deviation, <7ft, of the hole volume distribution for a large collection of polymers with Tg values between 200 and 500 K. We have grouped the polymers under discussion into... 

In this part we present an example for the pressure dependence of hole-size distribution. In our experiments we could not vary the temperature but keep it constant at room temperature. Details of experiments are described by Goworek [2007]. Figure 11.7a displays the mean, T3 (= (T3)), and standard deviation, <73, of o-Ps lifetime distribution and the o-Ps intensity h of PIB at 296 K as a function of pressure, P [Kilburn et al., 2006]. All of these parameters exhibit an exponential-like decrease with increasing pressure, h shows a small hysteresis which can be attributed to positron irradiation effects. T3 decreases from 2.0 ns at P = 0.1 MPa to 0.83 ns at 1.3 GPa and from 0.45 ns to about 0.05 ns. The low value of T3 =0.83 ns is possibly the lowest o-Ps lifetime observed until now for polymers. A lifetime of 0.5 ns is the theoretical limit of the pickoff annihilation for disappearing hole sizes [see Eq. (11.3)]. In polytetratluoroethylene (PTFE) a second, medium o-Ps lifetime of about 1 ns has been resolved and attributed to o-Ps annihilation in the densely packed polymer crystals [Dlubek et al., 2005d]. 

Figure 11.10b shows plots of the temperature and pressure dependence of the volumes in PFE rubber [Dlubek et al., 2004e, 2005a]. The slope of the Vf versus hole size curve is constant and the same in isothermal compression and isobaric thermal expansion experiments. The same behavior was observed for PD3 [Kilburn et al., 2006b]. The temperature-dependent macroscopic volume V shows a behavior that is parallel to Vf. 

Region II occurs between Tg and the knee in the temperature dependency of <7/ ,themeanhole-sizedispersion,at7]tCT.Both (u/,) anda increaselinearlywith the temperature, (u varies parallel to the specific free volume Vf, so that the specific number of o-Ps holes, N h = Vf vh), is constant. From (Vsv> oc hh it follows that the mean size of the fluctuation subsystem decreases almost quadrat-ically with T. At a certain temperature, here 1.3 Tg, the subvolume (Vsv) becomes small and constant, resulting in the knee in a. At Tg the subvolume is occupied by several small holes which are needed for the cooperative rearrangement of mers (the a-process). 

Figure 2. Temperature dependence of the size of o-Ps holes obtained from the...

Creep testing is a test to determine the strain-time dependence of a material under a constant load. This test is also used to determine the long-term behavior of the material luider combination of load and temperature. For ceramic matrix composites, when this testing is done in air, there is an added complexity of environmental exposure. A total of 12 creep tests were done on straight-sided tensile bars with two different central hole sizes (2.286 and 4.572 mm) at 1204 C. 

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