Coarsening in Multiphase PP Copolymer Systems

It was argued above that phase separation from a one-phase melt was not typically relevant to systems based on impact polypropylenes. Further, it was shown how the initial particle size distribution is created in an extruder, due to efficient dispersive mixing. Also, it was shown that this initial particle size distribution did not tend to be greatly modified in subsequent processing steps, namely part fabrication. 

However, it was mentioned that coarsening of the dispersed phase particles is a potentially important effect in systems in which the two-phase melt experiences, at least partly, a quiescent state. 

The demixing of the immiscible components in an initially one-phase melt proceeds rapidly due to a thermodynamic instability, this is spinodal decomposition, or a thermodynamic metastability (i.e., nucleation and growth of a new phase). However, this distinction of the character of the liquid-liquid phase transition is largely academic for the purposes of this work, because in either case the system rapidly produces droplets of a new phase (i.e., for off-critical mixtures). The demixing process itself was not studied in detail in this work. However, further details of this process may be found elsewhere (21). The production of droplets of a new phase in these immiscible systems is extremely rapid compared with the subsequent coarsening of the system to the final morphology. 

The coarsening of the phase-separated system can occur by two mechanisms. Particle diffusion, collision, and coalescence is one mechanism. Particle diffusion occurs in the quiescent melt by Brownian motion of the particles. Another mechanism is evaporation and condensation, called Ostwald ripening. Ostwald ripening occurs by molecular diffusion of the minor component, which primarily makes up the minor phase particles, through the matrix phase. This results in evaporation of particles smaller than a critical radius by diffusion of the minor component out of these and growth of particles larger than the critical radius by condensation of the diffusing molecules into these. 

The slower coarsening process is of interest because it occurs on a timescale comparable to melt processing procedures, such as pelletization, injection molding, film blowing, and the Uke. Coarsening can occur by coalescence or Ostwald ripening, or by both simultaneously. 

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