Molecular dynamics simulation of organic–inorganic copolymers based on methacryl-POSS and methyl methacrylate

Abstract

Atomistic molecular dynamics simulations have been performed to investigate the effects of introducing monofunctional polyhedral oligomeric silsesquioxanes (POSSs) as pendant groups on the polymethyl methacrylate backbone. The effect of POSS loading and of the structure of the organic substituents on the seven silicon atoms, isobutyl groups or cyclohexyl rings, was studied. Calculated volume–temperature behaviour and X-ray scattering profiles were compared with experimental results obtained on copolymers synthesized from POSS and MMA. In the limited time scale used for the simulation, no aggregation can be observed. Cohesive energy density was calculated and found to decrease as POSS was copolymerized with MMA, but in the same order of magnitude whatever the organic ligand nature is. Chain packing around the POSS cluster was evaluated through radial distribution functions. The mobility of the POSS clusters was determined via the mean square displacement. The results tend to confirm the idea that POSS incorporated as dangling groups along the macromolecular backbone behave as anchoring groups.

Introduction

For the last decade, hybrid organic–inorganic nanocomposites based on the incorporation of polyhedral oligomeric silsesquioxanes or POSSs™ into polymeric matrices have received a considerable amount of attention. POSS has a compact hybrid structure with an inorganic core made up of silicon and oxygen atoms (SiO_1.5)_n, with n = 8, 10, 12, externally surrounded by non-reactive or reactive polymerizable organic ligands [1], [2]. The non-reactive ligands, R, are generally isobutyl groups, cyclohexyl or cyclopentyl rings and sometimes phenyl rings. The interactions between the organic ligands and the matrix control the initial solubility of the POSS in the medium, and thus the degree of dispersion of POSS and of property modification. POSS can be dispersed on a molecular level (1–3 nm), or as crystalline or amorphous aggregates, which can be in the order of microns in size. Monofunctional POSS has been incorporated into a variety of conventional polymers, by a variety of polymerization techniques such as radical, condensation, ring-opening polymerization…amongst these polymers polymethyl methacrylate (PMMA) [3], [4], [5], [6]. PMMA is a widely used polymer principally due to its high transparency property in visible light. Moreover PMMA is now a matrix of importance in the field of nanocomposite materials [7]. It has been shown in the literature that the chemical nature of the inert organic ligand R plays a major role in the control of the morphologies generated in the hybrid materials, including the possibility to have amorphous or crystalline POSS aggregates; as a consequence the value of the glass transition temperature, T_g, as well as mechanical properties of the copolymers synthesized from POSS and organic monomer depend strongly on the type of POSS considered.

The goal of the present study is to achieve an understanding of the structure–property relationships in atactic poly(MMA-co-POSS) using atomistic molecular modelling, especially molecular mechanics (MM) and molecular dynamics (MD) simulations, in comparison with the experimental data. Molecular dynamics has been shown to be a powerful method of investigation of molecular motion, intra- and intermolecular interactions… This method has been applied successfully to a large range of amorphous thermoplastic polymers, amongst them PMMA [8], [9], and clay-based nanocomposites [10], [11], [12] but to our knowledge only a few studies deal with POSS-based polymer [13], [14], [15]. The strategy chosen falls into two steps: first, to confront experimental and simulated data in terms of X-ray scattering, glass temperature transition…of atactic poly(MMA-co-POSS) and secondly to focus on the influence of the structure of POSS on intermolecular interactions and molecular mobility.