Effect of preparation method on the properties of poly(methyl methacrylate)/mesoporous silica composites

Emergent Materials (2019) 2:363–370 https://doi.org/10.1007/s42247-019-00057-1 ORIGINAL ARTICLE Effect of preparation method on the properties of poly(methyl methacrylate)/mesoporous silica composites M. A. Sibeko 1 & M. L. Saladino 2 & F. Armetta 2 & A. Spinella 3 & A. S. Luyt 4 Received: 6 March 2019 / Accepted: 13 September 2019 / Published online: 6 November 2019 # The Author(s) 2019 Abstract The preparation method of a polymer composite and the filler loading are amongst the factors that influence the properties of the final composites. This article studies the effect of these factors on the thermal stability and thermal degradation kinetics of poly(methyl methacrylate) (PMMA)/mesoporous silica (MCM-41) composites filled with small amounts of MCM-41. The PMMA/MCM-41 composites were prepared through in situ polymerisation and melt mixing methods, with MCM-41 loadings of 0.1, 0.3, and 0.5 wt.%. The presence of MCM-41 increased the thermal stability of PMMA/MCM-41 composites prepared by melt mixing, but in the case of the in situ polymerised samples, the MCM-41 accelerated the degradation of the polymer. As a result, the activation energy was low and less energy was required to initiate and propagate the degradation process of these composites. The small-angle X-ray scattering (SAXS) measurements showed that the preparation method of the composites had no influence on the pore size of MCM-41, but the PMMAs used in the two methods both had shorter chains than the MCM-41 pore size. This allowed the polymer chains to be trapped inside the pores of the filler and be immobilised, as was observed from nuclear magnetic resonance (NMR) spectroscopy. The immobilisation of the polymer chains was more significant in the in situ polymerised samples. Keywords Poly(methyl methacrylate) (PMMA) . Mesoporous silica (MCM-41) . In situ polymerisation . Melt mixing . Thermal degradation kinetics . 13C {1H} CP-MAS-NMR 1 Introduction Organic–inorganic hybrid materials, especially polymer matrices with inorganic nanoscale building blocks, have drawn extensive attention of researchers, mostly because they combine superior mechanical and thermal properties of inorganic phases with the flexibility and processability of organic polymers [1, 2]. The comprehensive performance of these * A. S. Luyt 1 Department of Chemistry, University of the Free State (Qwaqwa Campus), Private Bag X13, Phuthaditjhaba 9866, South Africa 2 Dipartimento Scienze e Tecnologie Biologiche, Chimiche e Farmaceutiche – STEBICEF and INSTM UdR – Palermo, Università di Palermo, Viale delle Scienze pad.17, I-90128 Palermo, Italy 3 ATeN Center, Università di Palermo, Via F. Marini 14, I-90128 Palermo, Italy 4 Center for Advanced Materials, Qatar University, PO Box 2713, Doha, Qatar composites depends on many factors, such as the intrinsic properties of the polymers, the preparation method of the composites, the dispersion of the nanoparticles in the polymer matrix, and the interfacial compatibility between the nanoparticles and the polymer matrix. Amongst the well-studied nanoscale materials such as nanoclays, nanofibers, and carbon nanotubes, mesoporous molecular sieves are a new class of nanoscale materials which possess large surface areas and tuneable pore sizes between 2 and 50 nm [3–6]. Mesoporous silicas have different pore sizes and structures which can be differentiated into hexagonal mesoporous silica (MCM-41), cubic MCM-48, hexagonal SBA-15, wormhole framework MSU-J, hexagonally ordered MSU-H, and mesocellular silica foam MSU-H [7–9]. Although mesoporous molecular sieves have been widely used in other applications, their use as polymer additives has attracted less attention. Only recently, nano-mesoporous silica has been used as an additive with the goal of enhancing the mechanical and thermal properties of polymers. The pores on the surface of the silica provide the possibility of incorporating diverse organic guest species, including polymers, into their 364 ordered mesoporous structures [4–6]. A polymer can be introduced inside the mesopores by melt compounding or through in situ polymerisation of organic monomers, depending on the mesoporous size, the molecular weight, the structure of the polymer, and the physical or chemical interactions. The microstructure of the interface between the matrix and pore openings of the fillers can be easily tailored. It has been reported that the polymer in the nano-sized pores, extending along the channels to the openings, can not only enhance the miscibility through entanglement and inter-diffusion between the matrix and the particulate, but can also strongly supress the aggregation of the fillers [9–12]. Only a few studies reported on the improvement of poly(methyl methacrylate) (PMMA) properties with the addition of mesoporous silica, prepared through different polymerisation methods [6, 8, 13]. Run et al. [14] prepared PMMA/mesoporous molecular sieve (MMS) composites by an in situ polymerisation method with filler contents of 2 and 10 wt.%. An increase in thermal stability and glass transition temperature of PMMA was observed with an increase in MMS loading. The elevated decomposition temperature of PMMA was attributed to the strong interaction between the MMS particles and the matrix. Zhang et al. [15] evaluated the effect of MSU-F silica at 5 and 10 wt.% loading on the thermal and mechanical properties of PMMA. The composites were prepared through batch emulsion polymerisation and compression moulding. The degradation of PMMA occurred in two degradation steps: the first step was attributed to the unzipping of chains starting at both the vinylidene end groups and the weak head-to-head linkages, while the second step was associated with chains undergoing degradation through random chain scission. The PMMA/MSU-F composites displayed mainly the second decomposition stage, as the presence of silica reduced the vinylidene end group and head-tohead linkages. However, the thermal stability of PMMA increased with increasing MSU-F content, which was attributed to the radical scavenging role of the silica and the intrinsic stiffening of the polymer chains. Mohammadnezhad et al. [16] incorporated PMMA into amine-functionalised MCM41 by ultrasonic irradiation. The thermal stability of PMMA increased by 50 °C with the addition of 2 wt.% MCM-41, which was associated with the improvement in the interfacial interaction between the two components. So far, all the papers in the literature relating to PMMA/MCM-41 composites were focused on the effect of MCM-41 at high contents, ranging from 1 to 10 wt.% [6, 8, 14, 15], on the thermal properties of PMMA. This motivated this study where we evaluated the influence of lower MCM41 contents on the properties of PMMA. To the best of our knowledge, there is no study that compared the effect of MCM-41 at low content (0.1–0.5 wt.%) and prepared through different methods, on the structure, thermal stability, and degradation kinetics o (...truncated)