Challenges for industrialization of miniemulsion polymerization
Introduction
Miniemulsion polymerization [1], [2], [3], [4], [5], [6], [7], [8], [9] seems to be the perfect technique to synthesize complex materials that cannot be produced otherwise. Claimed applications of materials synthesized by means of miniemulsion polymerization include adhesives [10], [11], [12], [13]; anti-reflection [14], anticorrosive [15], [16] and UV resistant [17] coatings; anticounterfeiting [18]; textile pigments [19]; bio-based polymer dispersions [20]; gene and drug delivery [22], [23], [24], [25], [26], [27]; anti-viral therapy [28]; low viscosity high solids dispersions [29], [30], [31]; chemosensors [32], [33], [34], [35]; polyolefin waterborne dispersions [36], [37], [38], [39]; catalyst supports [40]; enzymatic polymerization [41]; controlled free radical polymerization [42], [43], [44], [45], [46], [47], [48], [49], [50]; responsive materials [51], [52]; photoswitchable fluorescent particles [53]; encapsulation [54], [55], [56], [57], [58]; polymer fillers [59]; polymeric photoresists [60]; light emitting diodes [61]; night-vision displays [62]; multicolor optical coding [63]; ultrabright fluorescent polymer nanoparticles [64]; single photon emission quantum dots [65]; tissue engineering [56], [66], [67], [68]; energy storage [69], [70], [71], [72], [73]; glass and ceramics coatings [57]; DNA separation [26], [74]; surface-enhanced Raman scattering substrates [75]; self-healing agents [58] and dielectric elastomer actuators [76]. However, 40 years after the pioneering work of Ugelstad et al. [1] the presence of miniemulsion polymerization in commercial products is still scarce.
This article reviews advances in the field, discusses the reasons for this delay and analyzes the challenges that have to be overcome in order to fully use this process in commercial practice.
Section snippets
Industrial constraints for miniemulsion polymerization
The performance of the materials mentioned above is determined by the characteristics of the particles: particle size and size distribution; polymer functionality and architecture; molecular weight distribution (MWD); number, type and relative amount of the phases; particle composition distribution; and particle morphology (including the characteristics of the surface of the particles). In addition for biomedical applications, biocompatibility is a must. These materials are
Miniemulsification
Miniemulsification adds complexity and cost (investment, energy consumption) to the process. Therefore, at first sight, phase inversion emulsification is an attractive way to produce miniemulsions as it does not involve the use any special emulsification device and the energy consumption is modest. Transitional phase inversion involves an induced change of the surfactant affinity [83], [84] that, among other ways, can be achieved by changing the temperature. However, in order to obtain small
Droplet nucleation
For most formulations, the miniemulsion consists of a dispersion of composite droplets colloidally stabilized by surfactants. Because of cost and performance reasons, the amount of surfactant is limited, so that the droplets are not completely covered by surfactant, and there are no micelles in the system. The objective in miniemulsion polymerization is to transform the composite droplets into composite polymer particles minimizing the heterogeneity in particle composition. The challenges
High monomer conversion/minimizing the residual monomer
Polymerizations rarely proceed until completion and therefore a certain amount of monomer remains in the product after polymerization (typically in the range of thousands parts per million range in conventional emulsion polymerization [237]). The problem is more acute when polymer–polymer hybrids are prepared by polymerization of miniemulsion droplets containing preformed resins as in this case unacceptable high concentrations of residual monomer due to a limiting monomer conversion have been
Controlling polymer functionality and architecture
Miniemulsion polymerization has been used to synthesize polymers and polymer hybrids by both chain growth polymerization (free radical, controlled free radical, anionic, cationic and coordination) and step growth polymerization (polyaddition and polycondensation). However, not all of them are equally promising for industrial implementation.
Particle morphology
The most attractive feature of miniemulsion polymerization is that it opens the possibility of synthesizing complex colloidal materials. Particle morphology is a key characteristic of these materials as it determines performance. Experimental evidence reported in literature shows that widely different particle morphologies are attainable by this technique [10], [17], [18], [24], [54], [55], [65], [68], [151], [165], [181], [245], [301], [302], [303], [304], [305]. Industrialization of
Inverse miniemulsion polymerization
In inverse miniemulsions, submicron droplets of solutions of highly hydrophilic monomers are dispersed in a continuous hydrophobic medium. Although solvents as formamide, dimethyl sulfoxide and dimethyl formamide can in principle be used [360] to prepare the monomer solutions, water is the most commonly used solvent [361], [362], [363], [364]. Similar to the inverse emulsion polymerization [365], [366], [367], low HLB surfactants are used to colloidally stabilize the system [361], [362], [368]
Summary
The main achievements and challenges associated to the steps involved in the synthesis of materials by miniemulsion polymerization are summarized in the following.
Acknowledgments
Diputación Foral de Gipuzkoa, University of Basque Country (UFI 11/56), Basque Government (GVIT373-10 and Etortek Nanoiker IE11-304) and Ministerio de Economía y Competitividad (CTQ2011-25572) are grateful acknowledged for their financial support.
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