Thermodynamics of flow‐induced phase separation in polymers. II. The effect of molecular weight distribution

Abstract

SynopsisA thermodynamic analysis of flow-induced phase transformations in polymers is presented. Calculations are based on the assumption that stresses on the polymer coils lower their effective flexibility. This leads to the possibility of a spontaneous liquid separation in which the more concentrated phase contains chains having a lower flexibility relative to those in the more dilute phase. The effects of molecular weight distribution are included, and calculations demonstrate that reducing the flexibility also enhances fractionation in that the more concentrated phase is preferentially enriched with the higher molecular weight fraction of the original polydisperse system. These results offer a quantitative basis for understanding flow-induced structure formation and phase transformation phenomena in polymers in general and the phenomena of precursor formation and polymer fractionation in flow-induced crystallization in particular. Application to the latter phenomenon is also discussed.

The kinetics of flow‐induced crystallization from solution

“…( 4 ) takes the following form -In( 1 -4 ) = k't' ( 5 ) in which where N is the number concentration of heterogeneous nuclei, u is their activation rate, and k is the Avrami coefficient in the high-flow rate region. Equation ( 6 ) shows that under these conditions the overall Avrami coefficient, k', takes a form which can be expressed as a product of the principal temperature-dependent functions, G and u, with the flow rate function B which should be much less temperature dependent. Such separability greatly simplifies the analysis.…”

“…and al are 1.35 and 3.76 X The purely temperature-dependent Avrami constant, k , can be determined from k' at any flow rate using eqs. ( 6 ) and ( 8 ) . Figure 9 presents results for both polyethylenes and the polypropylene on a min/cm, respectively.…”