Entropic phase separation in polymer--vitrimer melts
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, with a phase diagram qualitatively similar to that of dissociative systems. We furthermore find that --- for equal-length chains in the limit of highly functionalised vitrimers --- entropic phase separation sets in as soon as two function sites per chain are cross-linked to other chains, such that only loosely cross-linked networks are stable.