Comparison of polymeric particles synthesised using scCO2 as the reaction medium on the millilitre and litre scale
Graphical abstract
Introduction
Polymeric particles have many applications, for example as additives in paint or to help influence properties of materials such as impact modifiers, [[1], [2], [3], [4]] which require a range of particle sizes and compositions. The most common methods of synthesising such particles are through emulsion, mini-emulsion, dispersion and suspension polymerizations [5]. These polymerizations are typically carried out using volatile organic compounds (VOCs) and water as solvents [6]. However, as we become more concerned with the environmental impact associated with chemical processes, there has been an increased interest in alternative, greener solvents. Attention has turned to using supercritical fluids (SCFs) as the reaction medium [[7], [8], [9]]. SCFs have unique physical properties, exhibiting diffusion coefficients similar to a gas whilst having liquid-like densities. Supercritical carbon dioxide (scCO2) is a particularly promising alternative for polymerizations; CO2 is inexpensive, non-toxic, non-flammable and is readily available in high purity, with an easily accessible critical point (Tc =31.1 °C, pc =73.4 bar) [10]. The tuneable density and thus solvation power is a key property that can be exploited for polymer synthesis and processing. Many non-polar and some polar molecules with low molecular weights are readily soluble in scCO2, including several monomers. However, it is a very poor solvent for most high molecular weight polymers under mild conditions (<100 °C, <1000 bar) [9]. Exceptions include various fluorinated polymers and siloxanes as well as a few conventional polymers such as poly(vinyl acetate) [11,12] and some poly(vinyl alkanoates) [13,14]. As a result of this, there are very few examples of homogeneous polymerizations in scCO2 [9,[15], [16], [17]].
However, for heterogeneous polymerization processes this insolubility is a prerequisite for the process. The difference in solubility also aids purification, for example, the extraction of low molecular weight contaminants or fractionation of polymers or copolymers by molecular weight and/or composition is possible [18]. These properties, coupled with the fact that scCO2 reverts to a gaseous state upon depressurisation, thereby eliminating energy-intensive drying steps, make it a desirable solvent for the synthesis of polymeric particles. There are many examples of precipitation [19,20] and emulsion [21] polymerizations in scCO2, but it is dispersion polymerization which has proven to be the most successful [22]. Several different polymerizations including free radical [22], cationic [23,24], oxidative coupling [9], ring-opening [[25], [26], [27], [28]], and polycondensation [[29], [30], [31], [32]] have been carried out using scCO2 as the solvent, with vinyl monomers being the most heavily investigated.
In the last 25 years research on scCO2 as the reaction medium for dispersion polymerization has expanded significantly. In 1994 DeSimone reported the first example of free radical dispersion polymerization in scCO2 on the 10 mL scale [33]. Uniform spherical particles with average diameters in the range of 1.2–2.5 μm were confirmed via scanning electron microscopy (SEM) analysis. [34] These results encouraged DeSimone and other groups to investigate the free radical dispersion of not only MMA but also styrene using a variety of stabilisers [9,33,35,36]. A particularly important component of the reaction is the stabiliser as this dictates the size of the particles that are produced [6,34]. Particle size is a significant parameter for the end application of the particles [37].
Research into polymerizations using siloxane based surfactants such as poly(dimethyl siloxane) (PDMS) has increased. Silicone based stabilisers are generally cheaper than fluorinated based stabilisers. In 1996 Shaffer et al. reported the synthesis of high molecular weight PMMA particles on the 10 mL scale via dispersion polymerization in scCO2 using methacrylate terminated PDMS and AIBN (65 °C, 340 bar, 4 h) [38]. Others have reported the use of the same type of PDMS macromolecule stabilisers for dispersion polymerization in scCO2, finding that both polymer yield and molecular weight were strongly influenced by mixing phenomena, particularly when higher concentrations of the radical initiator were used [8,[39], [40], [41], [42]]. However, all of these polymerization were carried out at relatively small scale (< 100 mL).
In recent years, increasingly more sophisticated chemistries have been reported using scCO2 as a reaction medium. Three of the most widely used controlled/living radical polymerization techniques: nitroxide-mediated radical polymerization (NMP), atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer polymerization (RAFT), have been used in the synthesis of many novel particle-based polymer materials. All three mechanisms have been successful in synthesising well-defined particles with narrow size distributions [[43], [44], [45], [46], [47]].
These techniques give access to more complex structures including cross-linking, [48] inorganic nanoparticles [49], and more recently block copolymer particles with internal phase separation [50,51]. Through varying the chemical nature of the second block and degree of polymerization a range of nanostructures from lamella, cylindrical, and spherical have been observed. Interestingly, the relative CO2-philicity of the two blocks can influence the observed volume fraction and the phase separation, leading to non-equilibrium morphologies [52]. These structured particles have been used to produce controlled porosity [53] and structured hierarchical inorganic materials [54] all of which show potential for applications including drug delivery [55], optical electronics [56,57], and thermoplastic elastomers [50].
Despite these numerous advances, scale remains a fundamental hurdle, which has thus far limited the applicability of dispersion polymerization in scCO2 for the production of polymer particles commercially. Specifically, the typical vessels used for their synthesis are 150 mL or below, producing approximately 10 g per batch [31,58,59]. Although extraction vessels designed for high pressure can have very large volumes and are capable of reaching pressures of 300 bar at 40 °C, those developed to date are unsuitable for batch reactions as their tubular dimensions are better designed for flow systems [60]. The focus of this work was to reproduce a successful 60 mL reaction on the 1 L scale. Herein we describe the design of a pilot-scale 1 L autoclave and subsequent use for the free radical dispersion polymerization of MMA in scCO2 on a >150 g scale. The synthesis of PMMA particles was chosen as a proof of concept reaction because it has been extensively reported in the literature [40,41,56,58,59,61].
Section snippets
Materials
Methyl methacrylate (MMA, ProSciTech, 99 %), 2,2′-Azobis(isobutryronitrile) (AIBN, Sigma Aldrich, 98 %), and methacrylate terminated polydimethylsiloxane (PDMS-MA, 150−200 cP, ABCR GmbH & Co.) were all used as received. All reactions were carried out in SCF grade 4.0 CO2 (≥99.99 %, BOC special gases).
60 mL autoclave
High pressure reactions were performed in a 60 mL high pressure autoclave built in-house, previously used for dispersion polymerizations [8,39,50,51,56]. All pipework used was Swagelok SS316, HiP
Batch 60 mL reactions
In a typical reaction, MMA (10 mL, 93.5 mmol) was deoxygenated by bubbling with argon for 30 min. AIBN (1 wt% wrt MMA, 0.094 g, 0.57 mmol) and PDMS-MA (5 wt% wrt MMA, 0.468 g, 0.05 mmol) were separately flushed with argon for 30 min. The autoclave was deoxygenated by purging with CO2 for 30 min at 1−2 bar. The MMA was combined with the AIBN/PDMS-MA and injected into the autoclave via a syringe under a positive pressure of CO2. The autoclave was sealed, pressurised to 48 bar, and heated to 65 °C
Scanning electron microscopy
Scanning Electron Microscopy (SEM) was performed on a Phillips XL30 microscope and used to analyse the particle morphology. Particles were washed by centrifuging in dodecane three times (10 min, 4000 rpm) to remove residual stabiliser, before being dispersed in dodecane onto a glass slide and dried prior to coating in platinum. Particle size was calculated from SEM images as the average diameter of 100 particles.
Size exclusion chromatography
Size exclusion chromatography (SEC) was performed in THF (HPLC grade, Fisher
Batch reactions
Initial studies used a simple one-pot batch synthesis method. Both the 60 mL and 1 L scale afforded a free-flowing powder upon depressurisation. The reaction conditions were performed as a duplicate at both scales and comparable conversions, molecular weights and particle sizes were obtained for repeat reactions (Table 1). These parameters were also similar when compared between the 60 mL and 1 L reactions. The difference between the yield (%) and conversion (%) is attributed to the fact that
Conclusions
Design and implementation of a pilot-scale (1 L) autoclave was successful. The set-up was used to synthesise PMMA via free radical dispersion polymerization. The size of the particles produced was varied using different loadings of stabiliser. Batch conditions produced discrete, well-defined particles that were comparable on both scales. All conditions using the two-stage reaction method on the 1 L scale also produced comparable particles to the 60 mL scale. The slight difference in particle
Declaration of Competing Interest
There are no conflicts of interest with this manuscript
Acknowledgements
The authors thank M. C. Dellar, P. Fields, R. Wilson, D. Litchfield, K. Hind and J. Warren for their technical and engineering input. Thanks also to Dr Thomas Bennett and Dr Vincenzo Taresco for scientific discussions. The authors also gratefully acknowledge the University of Nottingham Nanoscale and Microscale ResearchCentre (NMRC) for access to their instrumentation.
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