Kinked molecular chain

Considering a fiber or thread of nylon-66, which is an unoriented glassy polymer, its modulus of elasticity is about 2,000 MPa (300,000 psi). Above the Tg its elastic modulus drops even lower, because small stresses will readily straighten the kinked molecular chains. However, once it is extended and has its molecules oriented in the direction of the stress, larger stresses are required to produce added strain. The elastic modulus increases. 

Elastomers Rubbery materials. The restoring force comes from uncoiling or unkinking of coiled or kinked molecular chains. They can be highly stretched. 

Quite large elastic strains are possible with minimal stress in TPEs these are the synthetic rubbers. TPEs have two specific characteristics their glass transition temperature (7 ) is below that at which they are commonly used, and their molecules are highly kinked as in natural TS rubber (isoprene). When a stress is applied, the molecular chain uncoils and the end-to-end length can be extended several hundred percent, with minimum stresses. Some TPEs have an initial modulus of elasticity of less than 10 MPa (1,500 psi) once the molecules are extended, the modulus increases. 

Figure 2. The physical model of local strain produced by nucleation of a pair of molecular kinks along the molecular chain (a) before and (b) after yielding.

Actually, crosslinks control the molecular packing and indeed significantly affect the elastic modulus of the material. As the intermolecular energy of kink formation is also determined by elastic modulus, the yield stress will definitely vary with modulus and thus the cross -linking density. In other words, crosslinks may not seriously affect the activation segment configuration in the molecular chain but will indirectly control the yield stress. 

Figure 5.5 shows a few saturated, trans, and cis fatty acid using molecular models. Note the extended saturated versus kinked cis chains. This is the major factor in the solidihcation of the former acids and the resistance of the latter to solidify. And this is the major difference in their impact on our health. 

The introduction of kinked linkages into the polymer backbone effectively reduces the regularity of the molecule and lowers the melting temperature. However, the incorporation of kinked units has an unfavorable influence on the liquid crystallinity because the kinks disrupt the molecular linearity. Frequently used kinked monomers include isophthalic acid (with the meta linked core angle of 120°), 2,5 substituted thiophene (with a core angle of 148°), and so on. The induction of kinks into the molecular chain tends to lower the thermal stability of theLCPs [4,19]. 

As shown in Sect. 3, kinks exist in the predicted concentrations (Figs. 12 and 13), but are relatively immobile in the simulations, and thus relaxation and annealing are more heavily linked to the skeletal vibrations of the molecular chains than the motion of conformational defects, although the presence of the kink defects seems to play an important role in the activation of chain diffusion (see Figs. 16-18). 

Isophthalic acid (lA) is a monomer extensively employed to modify LCPs because its cost is low and the meta linkage can induce a kink into the molecular chain. The resultant pol5nner has a lower Tm. However, the meta linkage also has a detrimental effect on the stability of the LC phase because it will disturb the LC character if its percentage is too high. In the thin-film polymerization of ABA/acetoxy acetanilide (AAA)/IA system, the critical meta-linked lA content is 26 mol% at 280°C, which means the liquid crystal phase can only be formed when lA content is lower than 26% and crystallization occurs once the lA content is higher than this critical point (45). 

Phthalic acid (PA) has an ortho linkage which can also introduce a kink into the molecular chain. However, it is seldom utilized to modify the LCPs because the hquid crystal phases are not stable for the systems containing PA units (46). Recently, it was found that, in the early stage of thin-film polymerization, both ANA/AAA/PA and ANA/AAA/LA systems form liquid crystal phase when the PA or lA content is 20%. However, with the further reaction going on, the ANA/AAA/PA system crystallizes, whereas the ANA/AAA/IA system remains in the LC state (46). 

An average way of taking into account those constraints is to assume that the chain is trapped into a virtual tube, envelope of all the obstacles which directly surround it (dotted line of (Fig. 2c)), a picture first proposed by Edwards. The chain can move freely along the curvilinear axis of the tube, but it cannot escape laterally. At any time, by fluctuations of its local kinks, the chain leaves some parts of the initial tube, and the chain extremities define new portions of the tube. The detailed statistical description of de Gennes leads to definite predictions for the molecular weight dependences of both the relaxation times and the diffusion coefficient of the chain, which we shall present in a simplified version. 

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