(148g) Calculation of the Vapor-Liquid Phase Coexistence of Polymer-Grafted Nanoparticles

Authors: 
Mukherjee, S., Vanderbilt University
Haley, J. D., Vanderbilt University
Iacovella, C. R., Vanderbilt University
McCabe, C., Vanderbilt University
Cummings, P. T., Vanderbilt University



It is well known that the aggregation/dispersion of nanoparticles can have a significant impact on the properties of the system and precisely controlling this behavior is crucial for the rational design of novel materials. One approach for controlling the aggregation is to graft polymers to the surface of the nanoparticles. The resulting behavior is therefore dictated by contributions from both the nanoparticles and polymers interactions and significant changes to the behaviorcan be achieved by tuning the relative strength of these interactions. The large number of interconnected parameters of grafted nanoparticles systems (e.g., nanoparticle size and interaction strength, polymer length and grafting density) makes it challenging to predict the behavior of a system a priori. Towards the goal of predictive assembly, we examine the vapor-liquid equilibrium (VLE) of polymer-grafted nanoparticles [1]. The main goal of this work is to explore the balance between nanoparticle-polymer interactions and provide general trends that can be applied to a wide range of systems.  In this work, the VLE is calculated using both molecular simulation via quench dynamics [1,2] and the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR) [3]. A coarse-grained model is used, generic enough to be applicable to a wide range of systems but with parameters informed by recent studies of the interactions between silica nanoparticles [1,4] and the interactions between alkane chains [5]. Quench dynamics simulations and SAFT-VR calculations exhibit similar trends demonstrating a strong connection between the VLE and the grafting architecture. It is observed from both the simulations and theoretical predictions that the critical temperature is reduced as the number of grafts and length of the grafts is increased. The VLE behavior is also investigated by increasing the nanoparticle-nanoparticle strength for both the low and high grafting density systems. We find that the critical temperature increases with the increase in nanoparticle-nanoparticle interaction strength for both these systems, but the effect of increasing interaction is seen to be much more prominent in the low grafting density system. Also the study of VLE of higher molecular weight chains via SAFT-VR reveals that for these systems the polymer grafts shields almost all the accessible surface area of the nanoparticle, thus shielding the strong nanoparticle interactions. As a result, the VLE behavior is dominated by the polymer-polymer interactions. The role of graft topology for systems with low surface coverage is also presented in this work.

References:

1. Iacovella CR, Varga G, Sallai J, Mukherjee S, Ledeczi A, Cummings PT, Theoretical

 Chemistry Accounts, (2013), 132:1315

2. P.J. in' t Veld, M.A. Horsch, J.B. Lechman, G.S. Grest, J Chem Phys 129 (2008).

3. Y. Peng, C. McCabe, Mol Phys 105 (2007) 261-272.

4. C.K. Lee and C.C. Hua, J. Chem. Phys. 132, 224904 (2010).

5. M.G. Martin and J.I. Siepmann, J. Phys. Chem. B 102 (1998).