Temperature orientation factor

Figure 13.3 also shows the orientation factors of the crystalline and amorphous regions as a function of take-up speed, which is pronounced in the case of a branched PET polymer. The shift towards increased freezing temperatures in branched polymer samples seems to be an indicator of higher elasticity (Figure 13.4). 

The isothermal pyrolysis in the presence of air proceeds at a much faster rate and higher weight losses are obtained as compared to vacuum pyrolysis at the same temperature. The first order rate constant obtained is linearly related to the expression [%LOR + o-(% crystallinity)]//o with a degree of correlation r = 0.923, where a is the accessible surface fraction of the crystalline regions according to Tyler and Wooding [501], and / is the orientation factor. No correlation could be found with DP due to very rapid depolymerization. The fact that the rate is inversely proportional to the orientation and that it decreases with the increase in the thickness of the fibers indicates that the rate of the diffusion of the oxygen into the fibers controls the kinetics and that oxidation is the predominant process in air pyrolysis. 

The approach taken in this section will be to consider in turn the empirical factors which govern the occurrence of molecular fracture and its consequences. The incidence of main-chain fracture in polymers under tensile stress is dependent on a wide range of variables, including polymer chemical structure, physical structure or morphology, additives, environment, time, temperature, orientation and physical state, as well as the more obvious variables of stress and strain. 

In spite of the above diversity of oriented crystalline morphologies, Samuels has shown that the structural state can sometimes be adequately characterized by the crystalline and amorphous orientation factors (37). For polypropylene samples prepared with different draw ratios, draw temperatures, shrinkage temperatures, etc., simple property correlation with these two orientation factors was observed ".. . these results suggest that different fabrication processes are simply different paths along which the sample is moved to equivalent structural states. Thus, general structure-property correlations are achieved by concentrating on the final structural state of the sample and not on the path by which the state was reached." Where applicable, this Is a most useful approach however, when radically different fabrication processes and radically different morphologies are compared, the definition of "structural state" must include more subtle features than the crystalline and amorphous orientation factors. 

Its lower limit is of the order of E-ll s. for small molecules and, often, of the order of E-2 to E+4 s. for polymers at room temperature. Since this orientation polarisability, aro, involves physical movement of parts of the macromolecules it is not surprising that, apart from the temperature, other factors which determine the molecular mobility affect it. 

FIGURE 24 (a) Evolution of the orientational order parameter, S, of the lamellar microstructure in a PS-PMMA diblock (MW = 31,000) in a 15 kV/cm electric field and at three temperatures. In (b) the time axis is multiplied by the rheological temperature shift factor, a-r- (Adapted from Ref. 64.)... 

In analyzing the effect of the polymer concentration on magnetic field-induced changes in phase transition temperatures, two factors should be taken into account. First, as concentration increases, the number of macromolecules capable of orientation in the magnetic field grows as a consequence, Tph should increase. Second, a rise in the polymer concentration in solution facilitates densification of the fluctuation network of entanglements. This impedes the occurrence of orientation processes and weakens the effect of the magnetic field. On the whole, the concentration dependence of AT is apparently described by a curve with a maximum. 

It is, thus, observed that the initial orientation factor and the temperatures of crystallization or annealing are the two important parameters. It was also observed that the rates of strain-induced crystallization are higher by an order of magnitude than the crystallization rate from isotropic polymer melt. 

In (6), fa is the molecular orientation factor, K, and 0D are the maximum crystallization rate, maximum rate temperature, and crystallization half-width, respectively, and C is the stress-induced crystallinity coefficient (see [7]). 

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