Amorphous polymers motion

In the glassy amorphous state polymers possess insufficient free volume to permit the cooperative motion of chain segments. Thermal motion is limited to classical modes of vibration involving an atom and its nearest neighbors. In this state, the polymer behaves in a glass-like fashion. When we flex or stretch glassy amorphous polymers beyond a few percent strain they crack or break in a britde fashion. 

The properties of a rubbery amorphous polymer form a continuum ivith those of the polymer in its molten state. Rubbery amorphous polymers exhibit the same range of motions as molten polymers, but they happen much slower, due to reduced thermal motion and the associated decrease in free volume. 

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. 

D. Axelson For the polyethylenes, at least, there is a major effect of morphology on linewidth. This is going to make more difficult a detailed description of the dynamics of the low frequency motion relative to a completely amorphous polymer. 

The flexibility of amorphous polymers above the glassy state, where segmental mobility is possible, is governed by the same forces as melt viscosity and is dependent on a wriggling type of segment motion in the polymer chains. This flexibility is increased when many methylene groups (—CH2—) or oxygen atoms (—O—) are present. Thus, the flexibility of aliphatic polyesters usually increases as m is increased ... 

The flexibility of amorphous polymers is reduced drastically when they are cooled below a characteristic transition temperature called the glass transition temperature (Tg). At temperatures below Tg there is no ready segmental motion and any dimensional changes in the polymer chain are the result of temporary distortions of the primary covalent bonds. Amorphous plastics perform best below Tg but elastomers must be used above the brittle point, or they will act as a glass and be brittle and break when bent. 

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. 

Because of the kinetic energy present in the molecule, amorphous flexible polymer chains are usually in constant motion at ordinary temperatures. The extent of this wiggling-like segmental motion decreases as the temperature is lowered in this reversible process. The temperature at which this segmental or micro-Brownian motion of amorphous polymers becomes significant as the temperature is increased is called the glass transition temperature, Te The term free volume is used to describe the total vplume occupied by the holes. 

A polymer may be amorphous, crystalline, or a combination of both. Many polymers actually have both crystalline and amorphous regions, i.e., a semicrystalline polymer. The Tg is a transition related to the motion in the amorphous regions of the polymer [3,8,9], Below the Tg, an amorphous polymer can be said... 

A general concept for describing all kinds of order in chain molecules, ranging from crystalline order to liquid crystalline order and then to order in oriented and isotropic amorphous polymers, is introduced in the third article written by Pieper and Kilian. After the presentation of the basic concept, experimental results obtained on different polymers including phases with rotatory segmental motion are discussed. 

An amorphous polymer is one with no crystallites. If the attractive forces between the chains are weak and if the motions of the chain are not in... 

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. 

The approach developed in this paper, combining on the one side experimental techniques (dynamic mechanical analysis, dielectric relaxation, solid-state 1H, 2H and 13C NMR on nuclei at natural abundance or through specific labelling), and on the other side atomistic modelling, allows one to reach quite a detailed description of the motions involved in the solid-state transitions of amorphous polymers. Bisphenol A polycarbonate, poly(methyl methacrylate) and its maleimide and glutarimide copolymers give perfect illustrations of the level of detail that can be achieved. 

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