Chain segment mobility

Highly flexible polymers with high chain segment mobility to facilitate polymer chain interpenetration and interdiffusion... 

The changes of the activation energy and activation entropy around the glass transition temperature listed in Tables 5 and 6 are of such importance that they can only be interpreted on the basis of physical changes of the polymeric environment, i.e. an important increase of the chain segment mobility and consequently of the free volume above Tg. The additional increase of the activation energy has to be related with the Ea of viscous flow of the polymer. 

Relationship Between Dipole and Chain Segment Mobility. 177... 

Instead of following the WLF equation, values of the shift factor follow an Arrhenius-type relationship with temperature, indicating-that the chain segment mobility is restricted even in the rubbery state. 

The effects of orientation via mechanical deformation on Tg have been reviewed [65]. Tg increases in those amorphous regions of a semicrystalline polymer that are either attached to crystallites or so close to them that their chain segment mobilities are hindered because of the interference of the crystallites. On the other hand, orientation has little effect on Tg in amorphous regions far away from crystallites as well as in completely amorphous polymers. 

Photochromic processes in a solid matrix such as polymers generally proceed with considerable deviations from solution. This phenomenon is ascribed to particular interactions between the pho-toreactive molecule and the surrounding matrix (J, 2) on a molecular scale 3) as will be shortly illustrated below. It can be infered from these findings that photochromic molecules, on the other hand, should be suitable probes to detect, e. g., particular motions in solid polymers and changes of the overall chain segmental mobility. This is a challenging technique to be applied in the field of polyurethane elastomers, since quite a few questions are still unsolved in this area. 

The fact that in these bulk polymer samples no photoisomerization could be detected has to be attributed only to the severe restrictions of the local chain segmental mobility around the chromophore, i. e., it is due to the predominant incorporation of the azochromophoric units in the hard phase the mobility of chain segments within the hard domain is widely suppressed in this system, cousing a nearly complete immobilization of the chromophore in the hard phase. This behaviour is comparable to systems where the photochrome was incorporated in highly crystalline polyamide or polyimide (2,20,21). 

It should be expected that the influence of the chain segment mobility is most pronounced when the photochrome groups are inserted into a semirigid polymer backbone instead of being attached as mobile side groups in copolymers. Therefore, polyesters have been prepared by condensation of bis-hydroxymethyl-spirobenzopyrans with bisacid dichlorides.followed by polyesterification with bisphenol-A43,46). Thus, a photo-chromic poly(bisphenol-A-pimelate) (polyester XI) of the following formula was obtained. 

As already mentioned in Section 5.5.1, chain-segment mobility becomes frozen in at and below the glass transition temperature. Perfect crystals do not... 

The external plasticizing method is applied most frequently to polyvinyl chloride (PVC). The distance between the macromolecules of the plastics is increased by incorporation of plasticizer molecules [aromatic plasticizers, e.g., tricresyl phosphate (TCP) and aliphatic plasticizers, e.g., dioctyl phthalate (DOP) or dioctyl sebacate (DOS)]. This reduces the secondary bonding forces and increases chain segment mobility. Solvents (including water in polyamides) may also act by inward diffusion in a manner similar to plasticizers [11]. 

Chlorinated polyvinyl chloride has the higher glass transition temperature because the added chlorines restrict chain segment mobility when subjected to thermal energy. 

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