Stress-induced melting

First of all the term stress-induced crystallization includes crystallization occuring at any extensions or deformations both large and small (in the latter case, ECC are not formed and an ordinary oriented sample is obtained). In contrast, orientational crystallization is a crystallization that occurs at melt extensions corresponding to fi > when chains are considerably extended prior to crystallization and the formation of an intermediate oriented phase is followed by crystallization from the preoriented state. Hence, orientational crystallization proceeds in two steps the first step is the transition of the isotropic melt into the nematic phase (first-order transition of the order-disorder type) and the second involves crystallization with the formation of ECC from the nematic phase (second- or higher-order transition not related to the change in the symmetry elements of the system). 

It was shown that the stress-induced orientational order is larger in a filled network than in an unfilled one [78]. Two effects explain this observation first, adsorption of network chains on filler particles leads to an increase of the effective crosslink density, and secondly, the microscopic deformation ratio differs from the macroscopic one, since part of the volume is occupied by solid filler particles. An important question for understanding the elastic properties of filled elastomeric systems, is to know to what extent the adsorption layer is affected by an external stress. Tong-time elastic relaxation and/or non-linearity in the elastic behaviour (Mullins effect, Payne effect) may be related to this question [79]. Just above the melting temperature Tm, it has been shown that local chain mobility in the adsorption layer decreases under stress, which may allow some elastic energy to be dissipated, (i.e., to relax). This may provide a mechanism for the reinforcement of filled PDMS networks [78]. 

In practice, many fabrication processes take place under non-isothermal, non-quiescent and high-pressure conditions. Mechanical deformation and pressure can enhance the crystallisation as well as the crystal morphology, by aligning the polymer chains. This leads to pressure-induced crystallisation and to flow-induced or stress-induced crystallisation, which in fact is the basis for fibre melt-spinning (see Sect. 19.4.1)... 

It is important to point out that this coil-stretch transition mechanism for interfacial slip does not require the adsorbed chains to stay permanently anchored to the wall. The polymer melt adsorption only needs to be strong enough to keep adsorbed chains tethered onto the surface for as long as it takes for them to undergo the stress-induced coil-stretch transition. Approximately, this residence... 

However, for polymers with low degree of crystallization it was assessed that the melting point was depressed at high shear stresses (4). Shear Induced melting has been theoretically predicted from non-equilibrium molecular dynamic simulations (5,... 

SIMS has also been used to determine the As concentration profile after liquid state diffusion from laser melted spin-on arsenic glass. In this case laser diffusion was desireable because It avoided the thermal stress-induced damage to large thinned wafers that could occur during heating In a furnace. (41). 

For this comparison, a melt-spinning process was chosen. Each special thermoplastic process influences the structure and thus the properties of the obtained polymer samples differently. This is particularly pronounced for fibers, since especially melt spinning is a process which makes extremely high demands on the deformation ability of the polymer melts at high deformation speeds. Particularly the tensile stress within the fiber formation zone is a very important factor to reach a high orientation of the macromolecules along the fiber axis and a stress-induced crystallization. This crystallization should be discussed in relation to PLA and PHB multifilaments, and at the same time the general property spectrum of these polymers should be represented. 

The annealing procedure for PTFE entails heating the stock shape 10°C above its service temperature (always below 327°C, melting point of PTFE). It should be held at temperature at the rate of 25 min/cm of thickness. Stresses are relieved during this operation. At the end of the hold time, the part should be cooled slowly to room temperature. After rough cutting the part to about 300-500 pm, re-annealing will help remove stresses induced by the tool, before the final cut is made. 

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