Polyethylene glass transition

Carbon Cha.in Backbone Polymers. These polymers may be represented by (4) and considered derivatives of polyethylene, where n is the degree of polymeriza tion and R is (an alkyl group or) a functional group hydrogen (polyethylene), methyl (polypropylene), carboxyl (poly(acryhc acid)), chlorine (poly(vinyl chloride)), phenyl (polystyrene) hydroxyl (poly(vinyl alcohol)), ester (poly(vinyl acetate)), nitrile (polyacrylonitrile), vinyl (polybutadiene), etc. The functional groups and the molecular weight of the polymers, control thek properties which vary in hydrophobicity, solubiUty characteristics, glass-transition temperature, and crystallinity. 

Plasticizers and Processing Aids. Petroleum-based oils are commonly used as plasticizers. Compound viscosity is reduced, and mixing, processing, and low temperature properties are improved. Air permeabihty is increased by adding extender oils. Plasticizers are selected for their compatibihty and low temperature properties. Butyl mbber has a solubihty parameter of ca 15.3 (f /cm ) [7.5 (cal/cm ) ], similar to paraffinic and naphthenic oils. Polybutenes, paraffin waxes, and low mol wt polyethylene can also be used as plasticizers (qv). Alkyl adipates and sebacates reduce the glass-transition temperature and improve low temperature properties. Process aids, eg, mineral mbber and Stmktol 40 ms, improve filler dispersion and cured adhesion to high unsaturated mbber substrates. 

The size of the group attached to the main chain carbon atom can influence the glass transition point. For example, in polytetrafluoroethylene, which differs from polyethylene in having fluorine instead of hydrogen atoms attached to the backbone, the size of the fluorine atoms requires the molecule to take up a twisted zigzag configuration with the fluorine atoms packed tightly around the chain. In this case steric factors affect the inherent flexibility of the chain. 

Inclusion of double bonds will stiffen the chain at the point of inclusion but at the same time may increase the flexibility of adjacent bonds. The net effect may therefore be to reduce the glass transition temperature and this appears to occur in 1,4-polybutadiene when compared with polyethylene. 

The acetal polymer moleeules have a shorter backbone (—C—O)—bond and they pack more closely together than those of polyethylene. The resultant polymer is thus harder and has a higher melting point (175°C for the homopolymer). The position of the glass transition is a subjeet of debate since at least two transitions in addition to the melting point are discernible. The true glass transition is usually associated with the temperature at which movement of segments of about 50-150 baekbone atoms becomes relatively easy, in the... 

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. 

The most common backbone structure found in commercial polymers is the saturated carbon-carbon structure. Polymers with saturated carbon-carbon backbones, such as polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polyacrylates, are produced using chain-growth polymerizations. The saturated carbon-carbon backbone of polyethylene with no side groups is a relatively flexible polymer chain. The glass transition temperature is low at -20°C for high-density polyethylene. Side groups on the carbon-carbon backbone influence thermal transitions, solubility, and other polymer properties. 

In contrast to the applications previously described in which alkanesulfonates are used in polymers with a high glass transition temperature (PVC, polystyrene, and ABS), in antistatic-modified polyethylene articles the antistatic agent is able to continue migrating to the surface over a long period of time. Thus, a more permanent antistatic effect is achieved. 

The Tg of P-plastomers changes as a function of ethylene content. The Tg decreases with increasing ethylene content, primarily due to an increase in chain flexibility and loss of pendant methyl residues due to incorporation of ethylene units in the backbone. It is well known that PP has a Tg of 0°C, and polyethylene a Tg< —65°C. The addition of ethylene to a propylene polymer would therefore be expected to decrease the Tg, as is observed here. A secondary effect would be the reduction in the level of crystallinity associated with increasing ethylene content, which is expected to reduce the constraints placed upon the amorphous regions in proximity to the crystallites. Thus, an increase in ethylene content will result in a lower T as well as an increase in magnitude and a decrease in breadth of the glass transition. 

The flow of liquid hydrocarbons can be enhanced by introducing into the stream a nonagglomerating suspension of ultra-high molecular weight polyethylene [490,1647] in water with small amounts of surfactant. The finely divided polyethylene is prepared by polymerization and then cryogrinded below the glass-transition temperature. 

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