Reversed trends for the isotropic-nematic transition density in confined semiflexible polymer solutions with repulsive or ideal nanoparticle additives*

We investigated how nanoparticle additives with extremely different inter-particle interactions affected the phase transition of strongly confined semiflexible polymer (SFC) solutions. We used GPU-accelerated Langevin dynamics simulation to explore how adding purely repulsive hard nanoparticles (HNP) and non-interacting ideal nanoparticles (INP) affected inter-polymer alignment and the isotropic-nematic transition. Polymers are modeled as semi-flexible chains with beads of diameter s_m=1, persistence length P=20 and contour length L=25. The NPs are spheres with s_p =2.5. The mixture is confined in a thin slit of height H=4.
We observed that adding low volume fraction $\phi_p$ of HNP dispersed the polymer matrix. disrupted inter-polymer alignement, and upshifted the critical volume fraction $\phi_cr$. In contrast, INP induced depletion attraction between polymers, enhanced polymer alignment, and always facilitated the I-N transition. For moderate or high $\phi_p$, hard NPs would crystalize and phase separate from the nematic polymer micro-domain. On the other hand, ideal NPs formed a liquid micro-domain that decreased in size as the monomer volume fraction $\phi_m$ increased.