Photo-induced thiol-ene crosslinked polymethacrylate networks reinforced with Al2O3 nanoparticles
Introduction
Design and synthesis of crosslinked polymeric networks exhibiting tunable material behaviors and properties for desired purposes have attracted continuous efforts in nanoscience and industrial fields. They have been utilized as effective building blocks in the development of a variety of organic materials with multifunctionalities, typically including microfluidic materials [1], [2], self-healable materials [3], [4], [5], surface coatings [6], and many others. These networks crosslinked with covalent crosslinkages are made of organic hydrophobic polymers in the various forms of gels, films, and others. They are synthesized by several methods utilizing a chain-growth polymerization of vinyl monomers in the presence of difunctional monomers as crosslinkers and a step-growth polymerization through polycondensation or polyaddition with polyfunctional monomers. The latter method recently has been considered as an effective means, with the versatility of “click-type” reactions such as Cu(I)-catalyzed azide-alkyne cycloaddition, Diels-Alder cycloaddition, amine-epoxy reaction, and thiol-ene addition [7], [8], [9].
Of these reactions, photo-induced thiol-ene radical addition involves the reaction between a thiol (SH) group and a vinyl group to form a new sulfide linkage upon UV radiation. [10], [11], [12], [13], [14] Due to being simple and orthogonal under mild conditions as well as facile photo-polymerization and no oxygen inhibition, this unique chemistry has been employed to synthesize various crosslinked nanomaterials as hydrogels [15], [16], [17], surface coating materials [18], [19], [20], soft lithographic templates [21], [22], [23], and high-performance nanomaterials [24], [25], [26] for optical, biomedical, and sensing fields. Recently, our research group has reported a new approach utilizing photo-induced thiol-ene addition of high molecular weight methacrylate copolymers (MCPs) having pendant SH and vinyl groups [27], [28]. This approach allowed for the synthesized crosslinked materials providing enhanced and tunable mechanical properties as well as exhibiting balance between rigidity, determined by the carbon-carbon (C
C) single bonds in the polymethacrylate main chains, and flexibility, influenced by the new sulfide (-S-) bonds as crosslinks [29].
Inorganic nanoparticles have been extensively explored as promising building blocks in constructing novel nanomaterials in materials science, nanoscience, and nanotechnology. [30], [31], [32], [33]. A variety of oxide-based nanoparticles (NPs) such as Fe3O4, Fe2O3, ZnO, and MnO have been synthesized due to their specific properties for desired purposes [34], [35], [36]. Of particular interest is aluminum oxide NPs (ALNPs) that are versatile and have wide range applications in industrial and personal care products. They have been formulated to improve physical, optical, and other properties of various polymer matrices such as thermal conductivity [37], [38], anti-bacterial [39], viscosity [40], and mechanical properties [41], [42].
In this work, we have further investigated photo-induced thiol-ene radical addition for reactive blends consisting of functional MCPs of different molecular weights having pendant vinyl (Bene) and SH (Bsh) groups in the presence of varying amounts of ALNPs (see the structures of Bene and Bsh in Fig. 1). Real-time viscoelastic measurements were used to follow thiol-ene crosslinking upon UV irradiation. The formed ALNP-reinforced crosslinked networks through the formation of new sulfide linkages were characterized for nano-scratch resistance as well as thermal stability.
Section snippets
Experimental
Materials. 4-Pentenoyl chloride (PA-Cl, 98%), triethylamine (Et3N, >99.9%), 2,2-dimethoxy-2-phenylacetophenone (DMPAc, 99%), 1-butanethiol (Bu-SH, 99%) and 2,2′-azobis(2-methybutyronitrile) (AMBN, >98%), and a dispersion of aluminum oxide nanoparticles (ALNPs) (20 wt% in isopropanol) from Aldrich, as well as DL-dithiothreitol (DTT, 99%) from Acros Organics were purchased and used as received. Butyl methacrylate (BMA) and 2-hydroxyethyl methacrylate (HEMA) from Aldrich were purified by passing
Results and discussion
Synthesis of Bene and Bsh copolymers with different molecular weights. Fig. 1 illustrates the synthetic scheme. Bene copolymers having 30 mol% pendant vinyl groups with different molecular weights were synthesized by a carbodiimide coupling reaction of a carboxylic acid group of PA-Cl with pendant hydroxyl groups of BOHs. The precursors BOHs were synthesized by solution polymerization of BMA and 30 mol% HEMA through a free radical polymerization mechanism initiated with AMBN in anisole at
Conclusion
Photo-induced thiol-ene radical addition was investigated for reactive blends consisting of functional polymethacrylates having pendant vinyl (Bene) and SH (Bsh) groups with different molecular weights in the presence of ALNPs. UV irradiation through photolysis yielded thiol-ene crosslinked networks through the formation of new sulfide linkage, evidenced by an increase in viscoelastic properties. Scratch resistance and thermal properties were desirably enhanced with an increasing amount of
Acknowledgements
Financial supports from Canada Research Chair (CRC) Award, Korea Research Institute of Chemical Technology (KRICT) and Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program. No. 10067082, ‘Development of scratch self-healable coatings and related process for automotive’ are gratefully acknowledged. JKO is a Tier II CRC in Nanobioscience.
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