Glassy polymers Density

The properties of glassy polymers such as density, thermal expansion, and small-strain deformation are mainly determined by the van der Waals interaction of adjacent molecular segments. On the other hand, crack growth depends on the length of the molecular strands in the network as is deduced from the fracture experiments. 

Annealing can reduce the creep of crystalline polymers in the same manner as for glassy polymers (89,94,102). For example, the properties of a quenched specimen of low-density polyethylene will still be changing a month after it is made. The creep decreases with time, while the density and modulus increase with time of aging at room temperature. However, for crystalline polymers such as polyethylene and polypropylene, both the annealing temperature and the test temperatures are generally between... 

The model system is a cube of glassy polymer with 3D periodic boundaries, filled with chain segments at a density corresponding to the experimental value for the considered polymer. The entire contents of the cube are formed from a single parent chain with the chemical structure of the polymer. The cube can thus be considered as part of an infinite medium, consisting of displaced images of the same chain, as shown on Fig. 58. 

Local density fluctuations occur in penetrant polymer systems both above and below Tg. It is then reasonable to expect that a free-volume diffusion model should also provide an adequate description of the diffusion of small penetrants in glassy polymers. To reach this goal the free-volume model for diffusion of small penetrants in rubbery polymers, second part of Section 5.1.1, was modified to include transport below Tg (64,65,72,91-93). 

In accordance with the equation for the activation energy of diffusion proposed by Meares [31], cohesion energy density (CED) of the polymer has a significant effect on diffusion coefficients of lower hydrocarbons. This is especially typical of mbbery polymers an increase in CED results in reduction of diffusion coefficients. Similar dependencies also apply to glassy polymers [9,32]. For example, Figure 9.8a shows the dependence of diffusion coefficient of propylene on CED of both glassy and mbbery polymers. 

Fig. 53. Fracture mechanisms map for interfaces between glassy polymers reinforced with connecting chains. Failure mechanisms are represented as a function of normalized degree of polymerization N/Ne and normalized areal density of connectors 1/1 ...

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