Preparation and properties of PVC/PMMA-g-imogolite nanohybrid via surface-initiated radical polymerization
Graphical abstract
Introduction
Since the discovery of carbon nanotubes (CNTs), polymer nanohybrids have attracted considerable attentions due to their unique mechanical, thermal, optical, and electric properties [1], [2]. In the past few decades, a variety of nanofillers have been developed for nanohybrids preparation. Among them nanotubes attract special research interests, as they usually have high mechanical strength, low density, and large aspect ratios [3], [4]. CNT exhibits the highest overall properties in the nanotube family, including the mechanical, thermal, and electric features, whereas, it still has some drawbacks, e.g., nanohybrids containing CNT usually appear non-transparent, because CNT is colored due to its conjugated system. In recent years, an increasing interest has been paid to clay mineral-based nanotubes which possess the advantages of both nanotubes and clay minerals [5], [6]. Nano-sized clay minerals are considered to be the potential candidate nanofillers for next generation because they are nature friendly and abundant [7], [8]. Halloysite is an important commercialized clay nanotube and has been investigated as a new type of additive for various polymers, due to its unique tubular structure and improved properties of the resultant nanohybrids [9], [10].
Imogolite is another clay nanotube with a much smaller size compared with halloysite. It forms a single-walled nanotube by the curling of a gibbsite-like sheet, presenting a SiOH-functionalized interior and an AlOH-functionalized exterior [11]. The external diameter of imogolite nanotube is about 2 nm, and the internal diameter is about 1 nm. Natural imogolite was first discovered in volcanic ash soil in 1962 [12], which consists of tube bundles with lengths varying from hundreds of nanometers to micrometer scale and overall diameters of several tens of nanometer. However, the abundance of natural imogolite is low. As an alternative, synthetic imogolite was first reported by Farmer et al. in 1977 [13], who revealed that the synthetic imogolite exhibited properties similar to those of the natural one. The average length of the synthetic imogolite nanotubes is approximately 100 nm [14], [15]. Due to the unique structure and surface properties of imogolite, numerous applications have been reported in the past few decades, including shape selective catalyst [16], gas storage [17], building blocks for self-assembly [18], [19], [20], filler for organic/inorganic hybrid materials [21], [22], [23], [24], contaminant removal [25], and scaffold for cell culture [26]. Recently, new synthetic protocols toward large quantity production of imogolite were explored and several new synthetic processes have been developed [27], [28]. These efforts could potentially stimulate new interests in imogolite research and open up possibilities for large-scale applications.
The application of imogolite in transparent nanohybrids has been demonstrated in our previous studies, where two preparation methods were used [21], [22]. One is in-situ polymerization of organic monomer in imogolite dispersion and the other is in-situ synthesis of imogolite in polymer solution, while the latter one is only suitable for water soluble polymers. Both of these two methods allow the formation of binary nanohybrids, where the matrix and the grafted polymers (if exist) are the same polymer. However, the preparation of a ternary imogolite nanohybrid has not been reported so far. In this work, the authors prepared a ternary imogolite nanohybrid by grafting the first polymer from imogolite surface, and then blending this polymer grafted imogolite with the second matrix polymer. PMMA was grafted from the external surface of imogolite nanotubes and poly(vinyl chloride) (PVC) was used as a matrix polymer. PVC is one of the most widely used thermoplastic resins due to its numerous advantages, including inherent fire safety. However, PVC is vulnerable to deformation at environmental temperatures above 40 °C. PMMA is generally used to blend with PVC in order to improve the thermomechanical properties of the resultant blend materials. Moreover, PVC and PMMA are miscible polymers through hydrogen bonding [29], thus a strong interface between PVC matrix and imogolite can be expected if imogolite is grafted with PMMA chains. We expected that the inclusion of PMMA-g-imogolite can improve the performance of PVC at elevated temperatures. In addition, the influence of environmental temperature on the interfacial adhesion performance of the prepared nanohybrid is studied. Scheme 1 shows the schematic illustration for the preparation of PVC/PMMA-g-imogolite nanohybrid.
Section snippets
Materials
Imogolite was synthesized according to the method introduced by Farmer et al. [13] Tetraethyl orthosilicate and aluminum chloride (AlCl3·6H2O) were purchased from Tokyo Chemical Industry (TCI) and Sigma–Aldrich, respectively, and used as received. Copper(II) bromide (99.999%, Aldrich), N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, TCI), and ascorbic acid (AA, WAKO) were used as received without purification. Methyl methacrylate (WAKO) was distilled before polymerization. PVC, with a
Results and discussion
Various ATRP initiators that can be fixed on inorganic surfaces have been synthesized by several groups [32], [33], [34], [35], [36], [37]. Among them, surface-attachable groups have almost exclusively been alkoxy- or chlorosilanes. However, organosilanes are not suitable for the modification of imogolite, because surface modification with organosilanes usually needs a dried condition to prevent unfavorable side reactions. Whereas imogolite is a very hydrophilic material with an
Conclusions
In summary, poly(methyl methacrylate) grafted imogolite clay nanotubes were successfully prepared by surface-initiated ARGET ATRP of MMA under a relatively mild condition. ATRP initiator was immobilized onto imogolite surface by developing an amphiphilic surface-attachable ATRP initiator, BMPOPO4(NH4)2, which carries both an ATRP initiator moiety and a surface-attachable phosphate group. BMPOPO4(NH4)2 is soluble in water, which is important for the homogenous modification of imogolite
Acknowledgments
The authors acknowledge the financial support of a Grant-in-Aid for Scientific Research (A) (No. 19205031) from Japan Society for the Promotion of Science. The present work is also supported by a Grant-in-Aid for the Global COE Program, “Science for Future Molecular Systems” from the Ministry of Education, Culture, Science, Sports and Technology of Japan.
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