Research paperSynthetic talc as a new platform for producing fluorescent clay polyurethane nanocomposites
Graphical abstract
Introduction
Over the past fifteen years, the development of inorganic-filler polymer nanocomposites has stimulated extensive research in both industry and academia (Hussain et al., 2006; Paul and Robeson, 2008). The nature of nanofillers, their immobilization and the structuration of the polymer matrix around them lead to strong changes in the physical and chemical properties, and govern all possible applications. In this context, clay polymer nanocomposites (CPN) are of special practical and commercial significance, since their strength, fire retardation and chemical stability are markedly enhanced with respect to conventional polymers and polymer composites (Da Silva et al., 2013; Galimberti et al., 2013; Gürses, 2015; Shunmugasamy et al., 2015; Taheri and Sadeghi, 2015). They may also display additional specific properties, in particular fluorescence, after introduction of appropriate organic compounds (Aloisi et al., 2010; Esposito et al., 2010; Diaz et al., 2013; Hao et al., 2014; Zhong et al., 2017). A smart way to ensure good dispersion of the photoactive agent into the CPN is to modify the clay filler by organic molecules before introduction into the polymer. To do so, two strategies can be considered. The most popular one requires synthesis efforts. It consists of covalently labeling the nanoclay with various dye molecules, as reported for montmorillonite linked to fluorescein, rhodamine and anthracene derivatives (Aloisi et al., 2010; Esposito et al., 2010; Diaz et al., 2013). Although it would be easier to implement, the non-covalent approach has seldom been used. For instance, the characteristic property of swelling clays, i.e. the replacement of interlayer inorganic cations by cationic organic molecules (Suzuki et al., 2011; Bujdák et al., 2011; Felbeck et al., 2013; Ley et al., 2015), has only recently been exploited to obtain a fluorescent CPN (Zhong et al., 2017). Besides, the non-covalent approach has not been extended to non-swelling clays, which do not contain inorganic charge-exchange cations, probably because they are known to interact very weakly with organic compounds.
The aim of the present work was to introduce fluorescence into CPN by using a non-swelling clay modified by a very simple and effective process. The clay is expected to confer both mechanical and spectroscopic properties to the polymer. Talc polyurethane nanocomposites were considered as the continuation of other works performed on this type of materials (Dias et al., 2015; Dos Santos et al., 2015; Prado et al., 2015). Polyurethanes (PU) are formed by hard and soft segment blocks that can be tailored by varying their chemical composition to enable a wide variety of applications such as coatings, foams, elastomers and biomaterials (Li et al., 2012; Amela-Cortes et al., 2015). The addition of talc particles decreases gas permeability, and improves corrosion resistance (Kantheti et al., 2015). However, in this system, the use of natural talc (NTlc) presents some disadvantages because talc particles cannot be ground homogenously below one micron without losing their structure and becoming amorphous (Dumas et al., 2013, Dumas et al., 2015; Yousfi et al., 2013; Dos Santos et al., 2015). In contrast, synthetic talc (STlc), like the one developed in the team, has well-defined chemical composition, high purity, crystallinity, particle size and layer thickness control. Among numerous applications that have recently been reviewed (Claverie et al., 2018), STlc has been used successfully as nanofiller in CPN, in particular for the reinforcement of PU. The good compatibility of organic and inorganic phases has been attributed to hydrogen bonding interactions between the OH groups of the filler and the PU chain (Dumas et al., 2013, Dumas et al., 2015; Dias et al., 2015; Dos Santos et al., 2015; Prado et al., 2015). Moreover, studies have shown that STlc has a remarkable capacity to adsorb colored and fluorescent organic molecules (Aymonier et al., 2017a, Aymonier et al., 2017b). Berberine chloride (Fig. 1) was chosen as the fluorescent dye to be incorporated into the talc particles. This compound is emissive in organic phases (Díaz et al., 2009; Zhang et al., 2014), constrained media (Megyesi and Biczók, 2006; Gade and Sharma, 2014) and in the solid state (Soulié et al., 2016). Its fluorescence properties are closely dependent on the micro-environment and provide useful information in this regard. The incorporation into polymers has scarcely been investigated (Gade and Sharma, 2014). Besides, berberine is a natural alkaloid of wide therapeutic interest, and it can be used to mimic the behavior of drugs in systems designed for biomedical applications (Soulié et al., 2015; Duval and Duplais, 2017). Synthetic talc fillers were therefore synthesized, loaded with berberine, and introduced into PU matrices. A comparison was made with NTlc. Here, it is of significance to note that, for the sake of simplicity, all clay polymer composites have been called “nanocomposites” and abbreviated CPN, even if the inorganic filler is of micrometric size. At each stage, the compounds were characterized by various analytical methods. Special attention was brought to homogeneity because the simple dispersion of inorganic particles in a polymer matrix often leads to phase segregation, which is an obstacle in the preparation of CPN (Amela-Cortes et al., 2015). Present studies showed that modified STlc fillers can be used as original platforms to produce fluorescent CPN. This simple concept allows potential applications to be envisioned in various fields: materials for optics, light-emitting devices, and controlled released of drug molecules.
Section snippets
Preparation
The PU synthesis was performed by reacting poly(caprolactone) diol (PCL, MM = 2000 g/mol, Sigma–Aldrich) and 1,6-hexamethylene diisocyanate (HDI, Merck) in a molar ratio of NCO/OH of 1:1. Dibutyl tin dilaurate (DBTDL, Miracema-Nuodex Ind.) was used as catalyst (0.1% w/w) and methyl ethyl ketone (MEK, P.A., Merck) as solvent (about 50 mL). Commercial natural talc was obtained from IMERYS (Talc de Luzenac). STlc was prepared as described elsewhere (Le Roux et al., 2013). Fluorescent talcs were
Preparation of talc-berberine hybrids and CPN
Fluorescent STlc fillers were prepared via hydrothermal synthesis according to a procedure patented by Le Roux et al. (2013). In a first step, the talc precursor was formed by reacting sodium metasilicate pentahydrate with magnesium acetate tetrahydrate in proportion that fits the ratio Si/Mg = 4/3, in the presence of a catalyst. The second step turned the amorphous prototalc into well-crystallized nano-sized talc gel by subjecting it to thermal treatment under high pressure for a specified
Conclusions
New fluorescent CPN, which incorporate natural and synthetic talcs as fillers, have been obtained and characterized by various methods. It was proposed that the Mg-OH groups situated on the edges of the talc layers interact with the PU chains. Thanks to these interactions, the talc particles were well dispersed into the PU matrix even at high filler content, without addition of coupling agents or compatibilizers. The onset temperatures were increased in the presence of STlc1 and NTlc fillers
Acknowledgements
GD and MP thanks CAPES for their PhD scholarship. SE acknowledges CNPq for DT grant (number 303467/2015-0).
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