Chain slippage

A deformation due to slippage of polymer molecules past one another (viscous deformation Dvisc)- H often assumed that such viscous deformation rates do not change with time if the applied stress is constant. However, in long-term deformations chemical and morphological changes may occur which affect the rate of chain slippage. 

Both tear resistance and hysteresis increase on incorporation of silica, but the effect is less pronounced as compared to the stress-strain properties. Tension set of the ZnO-neutralized m-EPDM system is low (around 20%) and incorporation of filler causes only a marginal increase in set due to chain slippage over the filler surface, as previously discussed. Measurement of physical properties reveal that there occurs an interaction between the filler surface and the polymer. Results of dynamic mechanical studies, subsequently discussed, support the conclusions derived from other physical properties. 

The introduction of cross-links to inhibit chain slippage was discovered by Goodyear in 1839. He accomplished this through the addition of sulfur to NR. Shortly after this, an accelerator, zinc (II) oxide, was used to speed up the process. Other additives were discovered often through observation and trial-and-error so that today s elastomers have a number of important additives that allow them to perform demanding tasks. 

Molecular motion Bond Side groups Main chain Main chain-large Chain slippage stretch and bending... 

Figure 3.2 A mechanical force causes a loss of an entanglement in a covalent polymeric network by one of two major mechanisms process A, chain slippage via reptation process...

Figure 3.3 Entanglement response to a mechanical force in a supramolecular system process A, chain slippage via reptation process B, chain scission via breaking of the supramolecular bond.

At higher mobility of polymer chains within the fibrils, another mechanism for fibril breakdown can happen chain slippage allowing chain disentanglement and fibril creep till rupture. Such a mechanism emphasises the time... 

It is worth noticing that this chain slippage mechanism applies both to CSCs and CDCs. The only difference concerns the MW of the fibril polymer chains. Indeed, in the case of CDCs, this is the bulk polymer MW since only disentanglement has occurred. In contrast, for CSCs, the fibril polymer MW is lower than that of the bulk polymer, nevertheless, the average MW of chain fragments in the fibril is higher when the bulk polymer MW is larger. 

As already mentioned, the introduction of rigid CMI cycles within the PMMA chain hinders the cooperativity of the p transition motions. This effect plays against the chain slippage and, thus, the occurrence of SDZs. 

In the temperature range c, the molecular process is the creep of the fibrils in the only present CDCs, due to the easier chain slippage occurring when approaching Ta. 

The role of fJ> transition motions in the fibril chain slippage is supported by the observation that in the xTyl -y series, the temperature range where toughness is independent of MW extends to - 20 °C/0°C, which also corresponds to the end of the fJ> transition at 1 Hz, of these compounds. 

At higher temperatures, fibrils fail by chain slippage, which is a MW-dependent process. Indeed, even for CSCs, the MW of the chains within the fibrils increases with the initial MW of the sample. Thus, the craze stability is directly related to the polymer MW. When dealing with CDCs, the break of fibril by chain slippage is a direct function of the material MW. 

For the lower MW BPA-PC(18), increasing temperature makes the chain slippage within the fibrils easier and easier, decreasing their failure time and, by the way, decreasing toughness. 

The low temperature range is characterised by a toughness independent of MW. It extends to a temperature where all the /3 transition motions are not still active. Consequently, the lack of chain mobility within the craze fibrils avoids chain slippage and the craze fibrils fail by chain scission. 

Molecular motion Side groups Main chain gradual Main chain (Segmental ) large- scale mobility Chain slippage... 

In this expression, q as been substituted for convenience by q = s0/ (ftyo)m with so and yo being below-Tg parameters in Eq. 3, and ft a nondimensional constant. The deformation in the molten state is generally believed to involve chain slippage and temporary entanglements between the moving chains resulting in the non-Newtonian viscosity [ 14]. The details of the deformation process are lumped into the parameters q (or ft) and m so that no back stress contribution is considered above Tg a = a and r = s/l/la a. 

Molecular mechanisms for stress-softening are also discussed. It is shown that this phenomenon is not related to the chain slippage or to a conversion of a "hard" adsorbed phase to a soft one. The obtained results assume that the stress-softening in silicon rubbers is caused by two possible reasons changes in the positions of filler particles relative to the direction of stretching at the first deformation and by a re-distribution of the topological hindrances. It is shown that the tensile strength at break as a fiinction of temperature is closely related to the chain dynamics at the elastomer-filler interface. 

Xmax, clustering around the line X z = 0.6 Xn,a. In all cases however Xdz is less than X of crazes in the same polymer. This observation indicates that the chain scission/chain slippage necessary for fibrillation in a craze modifies the entanglement network (increases the effective X ,an of the polymer fibrils) and so increases X above... 

Glassy polymer-diluent mixtures deformed in a temperature range close to are susceptible to exhibit a cavitational mode of plasticity at hi stresses and strains. Activation of this mechanism in mixtures of polycarbonate with esters of the phthalic acid results in extensive fibrillation and stress whitening of the material. There is strong evidence that the diluent plays an important role in enhancing chain slippage, which is required for the formation of craze fibrils. One of the most fundamental problems which is still unsolved is the elucidation of the molecular mechanism by which diluents become active. 

The deformation is assumed to result in chain slippage but some chain entanglement will influence the mechanical response. This feature is assumed to be lumped into the parameters q and m so that no back stress contribution appears above Tg. Therefore, the driving stress reduces to a = o and the equivalent shear stress x in (4) is that of the Cauchy stress o. 

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