Amorphous polymers molecular motions

In Sect. 7.2, we described how the morphology of semi-crystalline polymers was made up of coexisting crystalline domains and amorphous regions. Molecular motions of about ten bonds involved in the glass transition can happen in the amorphous phase. They will concern only those parts of the chains lying in this phase. Consequently, semi-crystalline polymers have a glass transition temperature. It has the same dependence on time (frequency) and chemical structure as in totally amorphous compounds. 

In the study of polymers and polymer-based systems such as blends, the probe of choice should be sensitive to segmental motions, on a length scale shorter than those responsible for glass transition. At low temperatures, a small size probe will be sensitive to changes in molecular motion. In amorphous polymers, molecular dynamics is influenced not only by the chemical microstructure but also by other processes such as molecular packing, physical aging and crosslinldng, all of which... 

Some of the first questions that arise when looking at a new group of polymers such as hyperbranched polymers concern the glass transition temperature -what determines it, what molecular motions determine it, is there a difference in Tg for different parts of the molecule Since hyperbranched polymers are almost exclusively amorphous materials, the glass transition temperature will be one of the most important features. 

Based on the results obtained to date, which have been summarized above for several different semicrystalline polymers— linear and low density (branched) polyethylene, polytrimethylene oxide, polyethylene oxide and cis polyisoprene—it is concluded that the relatively fast segmental motions, as manifested in Tq, are independent of all aspects of the crystallinity and are the same as the completely amorphous polymer at the same temperature. Furthermore, it has previously been shown that for polyethylene, the motions in the non-crystalline regions are essentially the same as those in the melts of low molecular weight ii-alkanes. (17)... 

For all the cases cited above, which represent those data for which a comparison can be presently made, there is a direct connection between the critical molecular weight representing the influence of entanglements on the bulk viscosity and other properties, and the NMR linewidths, or spin-spin relaxation parameters of the amorphous polymers. Thus the entanglements must modulate the segmental motions so that even in the amorphous state they are a major reason for the incomplete motional narrowing, as has been postulated by Schaefer. ( ) This effect would then be further accentuated with crystallization. 

On a molecular level, partially crystalline to amorphous polymers are normally used. As the material is heated, Brownian motion occurs resulting in a more random chain arrangement. When a unidirectional force is applied to a resting polymer melt, the chains tend to move away from the applied force. If the applied force is slow enough to allow the Brownian movement to continue to keep the polymers in a somewhat random conformation, the movement of the polymer melt is proportional to the applied stress, i.e., the flow is Newtonian. 

Polymers, Photochemistry and Molecular Motion in Solid Amorphous... 

The Amorphous Phase and Ts. Not all polymers crystallize, and even those that do are not completely crystalline. Noncrystalline polymer is termed amorphous. Four types of molecular motion have been identified in amoiphous polymers. Listed in order of decreasing activation energy-, they... 

Polymethacrylates and polyacrylates have extensively been studied from the viewpoint of relaxations occurring in the glassy state. Though a vast amount of information has been collected to date, even a qualitative interpretation of the relaxation phenomena on a molecular level often remains questionable. This situation exists despite some favorable circumstances, i.e. polymethacrylates are amorphous polymers with comparatively simple molecular motions and it is possible to alter systematically their constitution and prepare various model polymers. 

In another paper in this issue [1], the molecular motions involved in secondary transitions of many amorphous polymers of quite different chemical structures have been analysed in detail by using a large set of experimental techniques (dynamic mechanical measurements, dielectric relaxation, H, 2H and 13C solid state NMR), as well as atomistic modelling. 

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