Struik model

The fractional free volume,/, calculated as a function of aging time using the Struik model can then be used in the following correlation to give gas permeability ... 

Figure 3.10 Influence of film thickness on physical aging and relaxation rates, (a) Effect of aging time on oxygen permeability coefficients in PSF films ranging from 465 nm to 20 nm in thickness. Lines were generated from the modified Struik model, (b) Dependence of and Tg, from Kim et al., on PSF film thickness. Lines drawn to guide the eye in Figure 3.8(b). Reproduced with permission of Elsevier. ...

The modified Struik model, as applied to these ultra-thin PSF films, used the same initial conditions for each film, i.e. at a short, finite time following the quench from above Tg, permeability was independent of film thickness. Figure 3.11 presents the model predicted O2 permeability coefficients for the PSF films, including aging times 1 h. 

Influence of aging time and film thickness on the predicted oxygen permeability behavior of the PSF films studied based on the modified Struik model. Reproduced with permission of Elsevier. ... 

In the 1970s a model for semi-crystalline polymers was presented by Struik (1978) it is reproduced here as Fig. 2.13. The main feature of this model is that the crystalline regions disturb the amorphous phase and reduce its segmental mobility. This reduction is at its maximum in the immediate vicinity of the crystallites at large distances from the crystallites will the properties of the amorphous phase become equal to those of the bulk amorphous material. This model is similar to that of filled rubbers in which the carbon black particles restrict the mobility of parts of the rubbery phase (Smith, 1966). 

The main consequence of this reduced mobility is an extension of the glass transition region towards the high temperature side it will show a lower and an upper value, viz. Tg(L) and Tg(U), the values of the undisturbed amorphous region and that of regions with reduced mobility. By means of this model, Struik could interpret his measurements on volume relaxation (physical ageing) and creep in semicrystalline materials. 

In the previous section we found H 3Y,. Moreover, Struik (1991) developed a model that takes account of the intermolecular forces between adjacent molecules and relates the yield stress to the elastic modulus E which predicts ... 

Struik [3] originally proposed a method to model physical aging through the use of a momentary creep master curve obtained from a series of short term creep tests performed at various aging times. The momentary creep master curve was then used in conjunction with the effective time theory to predict long term creep in a polymer in the presence of physical aging. 

The viscous-elastic model, also called the thermo-elastic model, was proposed by Struik (1990). The model assumes that the thermal elastic parameters are constant and independent of temperature below as well as above the soUdihcation temperature T, and that the only discontinuous changes occur at Tj. Usually, the material is assumed to be hquid and to sustain no stresses above T, while below Ts the material is assumed elastic and able to sustain stresses. Under this assumption we have... 

The KAHR model can be extended to predict mechanical response shift factors ate if relationship between ate (or v) could be ascertained. Based on the findings of Struik [111] and McKenna [89], the logarithmic ate data appear to be well represented by a straight line versus 5 (or v) for materials that do not reach... 

Due to the universality in all glasses, physical aging can be theoretically explained in a straightforward way based on the free-volume concept. As proposed by Struik, This is the basic and rather obvious idea that the transport mobility of particles in a closely packed system is primarily determined by the degree of packing of the system or by its inverse measure, viz. the free volume [2]. The idea could date back to 1943 when Alfrey et al. proposed that the isothermal aging below Tg can be attributed to the diffusion of free volume holes from the interior of polymers into the surface [34]. This free volume diffusion model (FVDM) was developed by Curro et al. [35] to quantitatively analyze the volume relaxation experiments of poly(vinyl acetate) [36, 37]. The motion of free volume holes can be described by a diffusion equation ... 

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