Stress-induced crystallization rate

In order to demonstrate the predictability of the above model, the macroscopic behavior of the SMPF is estimated and the prediction is compared with the test result. In the following, three such predictions and/or comparisons are made. The first one investigates the effect of the heating rate on fully constrained stress recovery. The second one evaluates the effect of the amount of the amorphous phase and the crystalline phase on the stress-strain behavior under cyclic tension. The last one examines the growth of the crystalline phase due to stress induced crystallization. In all cases, the SMPF has a diameter of 0.04 mm. The material parameters for the stress recovery and strain hardening modeling as well as for the amorphous and crystalline subphase modules and for crystalline phase shp systems are summarized in Tables 5.2 to 5.4, respectively. 

In the present paper a kinetic theory of crystal nucleation is considered for the polymers subjected to time-dependent deformation rates, with transient effects of the chain relaxation. The considerations provide a theory useful in modelling fast polymer processing with stress-induced crystallization, like high-speed melt spinning, melt blowing, electro-spinning, etc. 

The same test conditions were used to demonstrate the effect of stressing on the amorphous formulation (Fig. 15.9), which showed lower rate and extent of dissolution of the amorphous formulation when stressed by heat and humidity. The diminished dissolution observed in the stressed sample was determined to be due to stress-induced crystallization of the amorphous product (Shah et al. 2013). 

Crystallization in step-growth polymers such as polyesters and nylons is known to assist their subsequent solid-state polymerization because exclusion of reactive end-groups from crystalline domains enhances their effective concentration in the amorphous domains [14,15]. However, the condensation reaction between the last fraction of end-groups may be hindered by crystallization [16, 17]. The possibility and rate of crystallization can also be enhanced by processes that enhance orientation, such as shearing and fiber drawing [18]. For example, partial replacement of terephthalic units with isophthalic units in PET reduces crystallinity, so that no crystallization in seen in 70 30 random poly(ethylene terephthalate-co-ethylene iso-phthalate) under quiescent conditions. However, heating its amorphous fiber above its Tg under a moderate tensile force results in rapid stress-induced crystallization [19]. The reduction in crystallization by copolymerization has been employed to enhance drawability of melt-spun polyester and polyamide fibers [20]. 

Equation 9.28 can be used to describe the kinetics of non-isothermal crystallization process imder quiescent conditiom However, the crystallization process in the spinning filament is non-quiescent, and the molecular orientation developed imder the tensile stress affects the crystallization rate. Therefore, the traditional non-isothermal crystallization rate, K T), must be replaced with the non-isothermal, stress-induced crystalhzation rate, K T,J), where / is the orientation factor. K T,J) also is called the total crystallization rate. With the total crystallization rate, Equation 9.28 can be rewritten to give ... 

The original Doufas-McHugh (1,2) two-phase microstmctural/constitutive model for stress-induced crystallization (SIC) is validated for its predictive capability using on-line Raman crystallinity and spinline tension data of two Dow homopolymer polypropylene resins. The material parameters -inputs to the model - are shown to be obtained from lab scale material characterization data oscillatory shear (DMS), rheotens and DSC. The same set of two SIC material parameters are shown to be able to predict the crystallinity profiles along the spinline and tension very well overall. The model captures quantitatively the effect of take-up speed, throughput and MFR on crystallization rate due to SIC. 

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