Molecular dynamics simulation of organic–inorganic copolymers based on methacryl-POSS and methyl methacrylate
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 (SiO1.5)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, Tg, 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.
Section snippets
Material synthesis
(Isobutyl)7Si8O12(propyl methacrylate) and (cyclohexyl)7Si8O12(propyl methacrylate), denoted iBuPOSS and CyPOSS, respectively, were purchased from Hybrid Plastics (Hattiesburg, MS). iBuPOSS and CyPOSS are crystalline compounds. The melting temperature for iBuPOSS is 109 °C, but no fusion was observed before degradation for CyPOSS. iBuPOSS was used as received, but CyPOSS was purified by recrystallization from methanol in order to eliminate residual trisilanol POSS that was evidenced by MALDI-TOF
Systems studied
Two series of random copolymers have been studied:
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The first series is based on MMA and isobutyl-POSS with molar fraction of POSS of 2.5, 5 and 10 mol%.
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The second series is based on MMA and cyclohexyl POSS with molar fraction of POSS of 2.5, 5 and 10 mol%.
The hybrid copolymers were built as a single chain with degree of polymerization equal to 80; they are atactic with 50% of meso dyads, a value close to the experimental ones determined using 13C NMR. Depending on the copolymer considered, there
Copolymer characterization
The composition, molecular weight, density and glass transition temperature of the copolymers produced are summarized in Table 1. We noted that the mol% of POSS in the final copolymer was systematically less than the mol% in the feed, even if the difference is weak. Such a trend has already been observed [5], [20]; this effect is directly related to the reactivity ratio difference: from the data provided by Hybrid Plastics [21], the reactivity ratio for POSS and MMA is 0.584 and 1.607 when R is
Conclusions
The objectives of this molecular modelling study was to investigate the influence of POSS introduced as pendant groups onto PMMA chains, on the intermolecular interactions and on the mobility at a molecular level. Despite some unavoidable differences between experimental and simulated systems, in particular concerning the morphologies generated, the following conclusions can be drawn:
- (i)
A very good agreement between simulated predicted X-ray scattering intensities and experimental ones proved the
Acknowledgments
This work was supported by the French Ministry of Education and by the European Community through Research Training Network funded under the 5th framework programme, under contract HPRN-CT-2002-00306 “NBB-HYBRIDS”.
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