Glass rubber molecular motion

At normal temperatures, the rubber molecular chains are in a constant state of thermal motion, they are constantly changing their configuration, and their flexibility makes them reasonably easy to stretch. It is to be noted that as the temperature is lowered the chains become less flexible and the amount of thermal motion decreases. Eventually, a low temperature, the glass transition temperature, is reached, where all major motion of the chains ceases. The material no longer has the properties which make it a rubber, and it behaves as a glass. 

The CDCs occurring at high temperatures involve molecular motions associated with the glass-rubber transition, in such a way that the temperature interval (T - Ta) plays an important role. Given the differences in Ta values in the xTy ]-, series (Table 8), the calculations for the various copolyamides are performed by considering the intercept of cry(T - Ta) and <7q c(T - Ta), as shown in Fig. 97. 

Due to the high polarity of these polymers the location of the fluorine atoms in the aromatic ring play an important role on the molecular motions below glass-rubber transition. For this reason the knowledge of the mean square dipole moment per repeating unit, (/u2)/x, which is calculated by means of the Guggenheim- Smith equation [173-175] ... 

Before looking closer at the glass rubber transition, we shall try to visualize what happens on a molecular scale when a polymer is heated, starting from zero Kelvin. At T = 0 K the chains are at absolute rest. No thermal motions occur everything is... 

As already mentioned earher (see Section 7.1.1), because of its high glass-transition temperature, that is, T = 208°C, PPO has to be melt-processed at elevated temperatures. As a result, degradation of the polymer may occur at such temperatures (particularly through oxidation reactions at the methyl substituents), furthermore, upon cooling from the liquid to the rubber state, two unwanted events can take place (i) the polymer crystallizes and (ii) the molecular motions are frozen and the rubbery polymer turns to a glass. As a consequence, the material becomes brittle and cannot be used for practical applications. Fortunately, PPO exhibits unusual and remarkable blending properties [36]. 

The transition from the glass to the rubberlike state is an important feature of polymer behavior, marking as it does a region where dramatic changes in the physical properties, such as hardness and elasticity, are observed. The changes are completely reversible, however, and the transition from a glass to a rubber is a function of molecular motion, not polyma- structure. In the rubberhke state or in the melt, the chains are in relatively rapid motion, but as the temperature is lowered, the movement... 

The extreme separation width shown with arrows in Fig. 8.2 gradually narrows and steeply drops with an increase in temperature. The temperature dependence of the extreme separation width for each specimen is shown in Fig. 8.3. The steep drop is caused by a micro-Brownian type molecular motion reflecting the glass-rubber transition. We can estimate a transition temperature of the molecular motion, Ts.omT where the extreme separation width is equal to 5.0 mT. The Ts.omT values of the spin-labeled PS at the chain end (Ts.omi.e) and the inside sites (Ts.omxi) are 423 and 435 K, respectively. The Ts.omx of the spin-labeled PS at the chain end is thus lower... 

The transition temperature, Ts.omT was observed to be higher than the glass-rubber transition temperature (Tg) for the PS sample. This can occur if the rate of motion at Tg is too slow for the averaging of the anisotropy of the hyperfine splitting. In other words the spin labeling method (ESR) detects the molecular motion at a higher frequency than the thermal analysis. 

The molecular motion of a segment at a chain end is more rapid than that in an inner chain site. The fact is clarified by the selective spin labeling in an amorphous polymer. The molecular weight dependencies of glass-rubber transitions at two kinds of sites, the chain end and inner chain sites give a free volume size of micro Brownian motion. 

As a consequence it is obvious that polymer dispersity will have an influence on surface segregation. Smaller chains in the samples will migrate at the interfaces [62]. Tanaka et al. used scanning force microscopy in order to investigate the surface molecular motion of PS films. It was revealed that the surface was in a glass/rubber transition state at 293 K due to the surface segregation of the lower molecular weight chains of a polydisperse blend (compared with 373 K in the bulk) [63]. 

Polymeric solids exhibit a wide spectrum of local molecular motion. In the classical subdivision into thermosets, rubbers and thermoplastics, both the thermosets and the thermoplastics below the glass transition temperature would exhibit transverse relaxation time on the sub-millisecond timescale, the so-called solid regime of NMR. It is these materials which require specialised solid state imaging methods rather than the elastomeric materials where... 

Before reviewing in detail the fundamental aspects of elastomer blends, it would be appropriate to first review the basic principles of polymer science. Polymers fall into three basic classes plastics, fibers, and elastomers. Elastomers are generally unsaturated (though can be saturated as in the case of ethylene-propylene copolymers or polyisobutylene) and operate above their glass transition temperature (Tg). The International Institute of Synthetic Rubber Producers has prepared a list of abbreviations for all elastomers [3], For example, BR denotes polybutadiene, IRis synthetic polyisoprene, and NBR is acrylonitrile-butadiene rubber (Table 4.1). There are also several definitions that merit discussion. The glass transition temperature (Tg) defines the temperature at which an elastomer undergoes a transition from a rubbery to a glassy state at the molecular level. This transition is due to a cessation of molecular motion as temperature drops. An increase in the Tg, also known as the second-order transition temperature, leads to an increase in compound hysteretic properties, and in tires to an improvement in tire traction... 

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