Chain relaxation capability

Brostow W. The Chain Relaxation Capability. Ch. 5. In performance of Plastics. Ed. W. Brostow, Hanser, Munich-Cincinnati, 2000. 

At the reduced stress equal to 0.30 the conversion is complete, while the three LC regions in the material (in the middle and on both sides) are easily visible. We have thus also seen the LC reinforcement in action until the reduced stress of 0.20 was exceeded, it was the LC region in the middle which protected the cis conformations on the left side from extension to the trans form. The same results confirm also the basic rule of the mechanical behavior of polymeric materials related to the chain relaxation capability (CRC) [19—22] a polymeric material will relax if it only can. None of the mechanical energy coming from outside has been spent on destructive processes. 

We shall briefly summarize MD simulations of systems of PLC chains. First, the chains systems have to be constructed. There are various ways of generating them, and the mechanical behavior is little if at all influenced by the generation procedure. It is influenced, however, by the vacancies, or the amount of free volume as predicted by the chain relaxation capability (CRC) theory [125]. The problem is to create a realistic bulk polymer system. A procedure originally developed by Mom [126] has been modified to achieve a more realistic representation of polymeric chains [127-129]. 

Relaxational and Destructive Processes Chain Relaxation Capability... 

Chain Relaxation Capability. What is the key factor of deciding whether a material will serve—rather than deform and fracture into pieces To answer this, we need to remember that polymer-based materials are viscoelastic. The face each polymer shows to the observer—elastic, viscous flow, or a combination of both—depends on the rate and duration of force application as well as on the nature of the material and external conditions inclnding the temperature. Later there will be a more detailed discussion of the natnre of viscoelasticity. At this point let us stress that properties of viscoelastic materials vary with time—while for elastic materials time plays no role at all. 

The question whether the component will survive in fact hinges on Ur- The energy Ur is related to the chain relaxation capability (CRC) which has been defined (2-4) as the amount of external energy dissipated by relaxation in a unit of time per unit weight of pol5nner. 

Chain relaxation capability depends strongly on free volume, that is the amoimt of space in which polymer chain segments can move aroimd and relax. This connection will be explored and used to advantage later on in this article. 

Sufficient free volume is essential for these processes. An increase in Vf results in an increase in the number of conformational changes in the chains enhancing the chain relaxation capability (CRC) (Brostow and Macip 1989 Doolittle 1951 Akinay et al. 2001). Brostow and Macip (1989) defined CRC as the amount of external energy dissipated by a relaxation process in a unit of time and unit mass of the polymer. The excess energy that cannot be dissipated by relaxational processes can go into destructive processes (irreversible changes in response to an applied force and/or temperature such as bond fracture and plastic deformation). The theory of the chain relaxation capability is discussed in detail in several references (Akinay et al. 2002 Brostow and Kubat 1996 Brostow 2000 Brostow et al. 2000 Brostow et al. 1999a, b). 

Brostow, W. (2000), Chain Relaxation Capability in Performance of Plastics, Hanser/ Gardner, Munich/Cincinnati, Chapter 5. 

The quantity RllM Y is Rg in A, a measure of chain stiffness. For example, polycarbonate, with (RpM y = 0.457, is stiffer than polystyrene, which has a value of 0.275. The importance of these quantities lies in their relation to physical and mechanical behavior. Both melt and solution viscosities depend directly on the radius of gyration of the polymer and on the chain s capability of being deformed. The theory of the random coU (Section 5.3), strongly supported by these measurements, is used in rubber elasticity theory (Chapter 9) and many mechanical and relaxation calculations. 

Polycarbonate s high toughness is based on its relaxation potential at low temperatures. Any accumulation of foreign molecules in individual chain segments influences relaxation capability and thus viscoelastic behavior. For example, methylene chloride or chloroform accumulates on the carbonate group, which is the reason for polycarbonate s good solubility in these media. 

Above the -relaxation process, the 2,4-TDI/PTMO polymer displayed a short rubbery plateau at a storage modulus of about 5 MPa while 2,6-TDI/PTMO was capable of crystallization, as evidenced by the ac-loss process. This difference in dynamic mechanical properties demonstrates the effect of a symmetric diisocyanate structure upon soft-segment properties. As previously discussed, single urethane links can sometimes be incorporated into the soft-segment phase. The introduction of only one of these diisocyanate molecules between two long PTMO chains inhibits crystallization if the diisocyanate is asymmetric. In the case of a symmetric diisocyanate, soft-segment crystallization above Tg can readily occur. The crystals formed were found to melt about 30°C below the reported melting point for PTMO homopolymer, 37°-43°C (19), possibly because of disruption of the crystal structure by the bulky diisocyanate units. 

The Fickian diffusion described above is relatively easy to analyze, and demonstrates the capabilities of AW devices for monitoring transient uptakes. However, Fickian diffusion in polymers is the exception rather than the rule. A wide variety of transient responses have been observed, often due to the long time constants tequiied for relaxation of the polymer chains upon absorption of species into the film [93,95]. A detailed discussion of these trends is beyond the scope of this book, and the reader is referred to the polymer literature for these details... 

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