Polymers motion produced

Fig. 17 Illustration of the three levels of polymer motion produced with light. At the molecular level light trans-cis-trans isomerization excludes the chromophores from the direction of polarization, at the domain level the polar chromophore movement reorients polar domains and at the mass level macroscopic movement of the polymer can be induced by light illumination [33]...

The attenuation and velocity of acoustic energy in polymers are very different from those in other materials due to their unique viscoelastic properties. The use of ultrasonic techniques, such as acoustic spectroscopy, for the characterization of polymers has been demonstrated [47,48]. For AW devices, the propagation of an acoustic wave in a substrate causes an oscillating displacement of particles on the substrate surface. For a medium in intimate contact with the substrate, the horizontal component of this motion produces a shearing force. In such cases, there can be sufficient interaction between the acoustic wave and the adjacent medium to perturb the properties of the wave. For polymeric materials, attenuation and velocity of the acoustic wave will be affected by changes in the viscoelastic behavior of the polymer. 

Beyond the traditional biocompatibility concerns, which include the effects of leaching and absorption, the greatest obstacle to the use of polymers in the role of articulating surfaces has been wear. The cyclic motion of an opposing implant component or bone against the polymer may produce substantial amounts of wear debris that can then precipitate bpne loss and implant failure. 

The motion producing the y transition involves short chain segments. In many polymeric systems, the y transition is caused by the crankshaft rotation of the methylene (-CH2-) groups on a long polymer chain. Since the y transition involves those molecular segments, it occurs helow the a and P transitions. 

The use of stodiastic tediniques in the study of local polymer motions entails certain approximations in the treatn nt of polymer/solvent interactions. The presence of a spatial delta function in Eq. (3) effectively impli that the solvent is being approximated as a cxHitinuum Thus the solvent strudure per se cannot have an effed on the simulated dynamics. Solvent propoties are usually induded in stochastic simulations only through the friction terms. In principle, the intramolecular potential should be modified by the solvent to produce a potential of mean force [48, 49]. In practice, this is usually not done. As discussed below, recent work has [ffoduced several examples which justify this practice in the absence of specific interadions between the polymer and solvent... 

The contact-induced charge separation does not directly involve relative motion of the two contact materials. However, motion certainly can enhance the formation of static charges. In many cases, robbing two polymer fibers produce temperature gradients. Hot spots often develop due to the friction effect, and charges can move from hot spot to cold surrounding area. 

The mobility of ions in polymeric systems, particularly those that contain multivalent cations, is mainly based on breaking and re-forming ion-chain bonds. For example, in PEO, as in other aprotic solvents, cations are more strongly solvated than anions. In the transitions between different conformational states coupled with chain mobility, ether oxygen-cation bonds will be broken and restored again to produce a new chain conformation. Cation motion and segmental polymer motion are closely coupled in this manner. Cation-chain interaction can involve single polymer chains and also create pseudo-crosslinks between chains. 

SPACEEIL has been used to study polymer dynamics caused by Brownian motion (60). In another computer animation study, a modified ORTREPII program was used to model normal molecular vibrations (70). An energy optimization technique was coupled with graphic molecular representations to produce animations demonstrating the behavior of a system as it approaches configurational equiHbrium (71). In a similar animation study, the dynamic behavior of nonadiabatic transitions in the lithium—hydrogen system was modeled (72). 

Again because of the crosslinks, such brittle behaviour occurs whatever the temperature unlike brittle materials based on linear polymers, there is no temperature at which molecular motion is suddenly freed. In other words, the Tg, if there is one, does not produce dramahc changes in mechanical properties so that the material is changed from one that undergoes brittle behaviour to one that exhibits so-called tough behaviour. 

Aggregation of particles may occur, in general, due to Brownian motion, buoyancy-induced motion (creaming), and relative motion between particles due to an applied flow. Flow-induced aggregation dominates in polymer processing applications because of the high viscosities of polymer melts. Controlled studies—the conterpart of the fragmentation studies described in the previous section—may be carried out in simple flows, such as in the shear field produced in a cone and plate device (Chimmili, 1996). The number of such studies appears to be small. 

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