Polyethylene-like glass transition

Polyethylene is a semicrystalline polymer. It means that at ambient temperatures the polymer consists of two rather distinct fractions, or phases—crystalline and amorphous. The amorphous part of polyethylene, which is a sort of rubbery at ambient temperatures, becomes a glass-like at a certain transition temperature, the so-called glass transition point. For polyethylene the glass transition point varies from very low to low (from -130 to 20°C), thus making the plastic ductile at common temperatures. The lower glass transition point (y-transition) is always present in the range of -130 to -100°C, the higher one (P-transition, at —20°C) is manifested not in all PE materials. To complicate the picture even more, we can notice that there is one more transition in polyethylenes, called a-transition, commonly found between 10 and 70°C, and it is associated with crystallinity of PE. For WPC the last two transitions (a- and P-) are of little importance. 

With plastics there is a certain temperature, called the glass transition temperature, Tg, below which the material behaves like glass i.e. it is hard and rigid. As can be seen from Table 1.8 the value for Tg for a particular plastic is not necessarily a low temperature. This immediately helps to explain some of the differences which we observe in plastics. For example, at room temperature polystyrene and acrylic are below their respective Tg values and hence we observe these materials in their glassy state. Note, however, that in contrast, at room temperature, polyethylene is above its glass transition temperature and so we observe a very flexible matoial. When cooled below its Tg it then becomes a hard, brittle solid. Plastics can have several transitions. 

It is well known from diffusion theory that different types of polymers have different diffusion behaviours. For example, the polyester type polymers like poly(ethylene terephthalate) (PET), poly(ethylene naphthalate) (PEN) and polycarbonate (PC) as well as rigid poly(vinyl chloride) (PVC), which have a high glass transition temperature, are low diffusive polymers. The migration of potential contaminants in these polymers will result in low migration values. In contrast, polyolefins like high density polyethylene (HDPE), polypropylene (PP) or low density polyethylene (LDPE), which... 

Similarly, the glass transition of polyethylene is well below ambient temperature, and if the polymer was amorphous, it is likely that it would be a viscous liquid at room temperature. It is in fact, a tough, leathery, or semirigid plastic because it is highly crystalline and the crystallite cross-links impart a high modulus and increased strength to the polymer between 188 and 409 K, a very useful temperature range. 

The rate dependence of fatigue strength demands carefiil consideration of the potential for heat buildup in both the fatigue test and in service. Generally, since the buildup is a function of the viscous component of the material, the materials that tend toward viscous behavior will also display sensitivity to cyclic load frequency. Thus, TPs, particularly the crystalline polymers like polyethylene that are above their glass-transition temperatures, are expected to be more sensitive to the cyclic load rate, and highly crosslinked plastics or glass fiber reinforced TS plastics are much less sensitive to the frequency of load. 

The diffusion theory takes the view that polymers in contact may interdiffuse, so that the initial boundary is eventually removed. Such interdiffusion will occur only if the polymer chains are mobile (i.e. the temperature must be above the glass transition temperatures) and compatible. As most polymers, including those with very similar chemical structures such as polyethylene and polypropylene are incompatible, the theory is generally only applicable in bonding like linear... 

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