Entropic phase separation in polymer--vitrimer melts

Abstract

Traditional plastics demand a choice between durability (thermosets) and reprocessability (thermoplastics). Vitrimers are a recent class of polymer network combining both these qualities. Their increased cost of production can be offset by mixing them with a traditional thermoplastic; however, phase separation in such blends can lead to inhomogenous materials. In this paper, we adapt an existing model for the free energy of dissociative polymer networks to their associative, vitrimeric counterpart. We test the accuracy of the model's predictions by comparing them with the results of novel molecular-dynamics simulations. We demonstrate that such melts can undergo phase separation even in the absence of energetic interactions between the components. We find furthermore that the phase diagram of the melts is qualitatively similar to that of dissociative systems, and that the critical degree of conversion for the onset of phase separation depends reciprocally on the number of function sites per vitrimer chain.

Figures (17)

• Figure 1: Anatomy of a vitrimer--homopolymer melt.
• Figure 2: Generic same-chain linking (a) vs nearest-neighbour only (b).
• Figure 3: Phase diagram obtained from \ref{['eq:nnf']} with $M=N=20$, $s=5$ and $k=0.48$, showing the gelation transition, binodal, spinodal, tie-lines and plait point. Data are plotted on "pinched axes" as the phase space is naturally triangular. On these axes, lines of constant $p$ are rays emanating from the left-hand corner, whereas lines of constant $\phi$ connect a value on the vitrimer axis with its complement on the homopolymer axis. Note that we show only a subspace $p\geq0.4$.
• Figure 4: Bond exchange reactions (BER).
• Figure 5: Atom count distribution of vitrimeric beads at four different times during simulation ($\phi=0.5$, $p=0.95$). The entire simulation lasted a total of $t=10^6 \, \tau$, or $10^8$ time steps.