Glass transition temperature repeating unit structure

Cao C, Lin Y (2003) Correlation between the glass transition temperatures and repeating unit structure for high molecular weight polymers. J Chem Inf Comput Sci 43 643-650... 

Mattioni BE, Jurs PC (2002) Prediction of glass transition temperatures from monomer and repeat unit structure using computational neural networks. J Chem Inf Comput Sci 42 232-240... 

Linear polyethylene (hdpe), which has a repeating unit of 4-CHr CH24 , is a typical thermoplastic with a glass transition temperature Tt below room temperature. Its mechanical properties permit its use as both a plastic and a fiber. From a structural viewpoint, this polymeric hydrocarbon is the least complex of all polymers. 

The Backbone. The linear inorganic backbone imparts an unusual combination of properties. First, perhaps unexpectedly in view of the unsaturated structure, the skeletal bonds have a low barrier to torsion (perhaps as low as 0.1-0.5KcaF repeating unit), which becomes translated into one of the most flexible backbones known throughout polymer chemistry. This means that some polyphosphazenes have glass-transition temperatures (Tg) as low as -100 °C. It also means that, in the absence of microcrystallinity, numerous polymers of this type are rubbery elastomers. This is a key property for... 

Figure 1.1. Calculation of glass transition temperature Tg and density p at room temperature of polystyrene as an example of the application of group contribution techniques, (a) The structure of a polystyrene repeat unit, (b) Fonnal breakdown of this structure into two "groups", (c) Each group makes a contribution to the molecular weight M of the repeat unit, to the "molar glass transition function" Yg, and to the amorphous molar volume V [3a], M, Yg and V arc sums of these contributions ...

Polyethylene is an inexpensive and versatile polymer with numerous applications. Control of the molecular structure leads to low density (LDPE), linear low density (LLDPE) and high density (HDPE) products with corresponding differences in the balance of properties. Some typical properties are Glass Transition Temperature, Tg = -78° C Melting Temperature, Tm = 100° C Amorphous density at 25° C = 0.855 g/cc Crystallinity at 25° C = 1.00 g/cc Molecular weight of repeat unit = 28.05 g/mole. The basic structure of PE is as follows ... 

The Tg of polymers is very much a function of chain structure. Repeat units which introduce stiffness into the backbone chain or bulky side groups pendant from the backbone tend to increase the observed Tg. Strong interchain forces, such as hydrogen bonding, also reduce the crankshaft-like motion of the polymer backbone at any given temperature and tend to increase the Tg. The glass transition temperature is often termed the a-transition. If the temperature is lowered below that of the Tg, temperatures are reached where the rotation of side groups - and eventually, at even lower temperatures, the vibrational behaviour of bonds - are frozen . These lower transition temperatures are known as j - and y-transitions respectively. 

Sketch of the PDMS chain, showing some structural information relevant to its high flexibility. Because of the difference in bond angles d = 37°, 0" = 70°). The all-trans configuration closes to Figure 1.1 after 360°/(70°-37°) = 11 repeat units. The torsional barrier for rotations about the skeletal bonds is very low, which accounts for the high dynamic flexibility and low glass transition temperature. Reproduced by permission of John Wiley and Sons. 

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