Synthesis of Two-phase Polymer Particles in Supercritical Carbon Dioxide

The synthesis of particles with discrete phases using traditional emulsion polymerisation is a well-established process. Phase-separated particles have a wide range of applications, such as in coatings, drug delivery, impact modification and as supports in catalysis. However, as a dry powder is often desired for the end application, post-polymerisation, energy intensive drying steps are usually required for the removal of water. Alternatively, dispersion polymerisation utilising supercritical carbon dioxide (scCO 2 ) as a reaction medium allows for the production of dry, free-flowing powders upon release of the CO 2 . Here, we present the innovative use of scCO 2 to provide a novel and environmentally acceptable route for creating phase-separated particles. Particles containing a high T g poly(methyl methacrylate) (PMMA) phase, combined with a low T g polymer phase of either poly(benzyl acrylate) (PBzA) or poly(butyl acrylate) (PBA), were investigated. Both monomers were added to the reaction after the formation of PMMA seed particles. Benzyl acrylate (BzA) was chosen as a model low T g monomer, with well-defined and detectable functionality when mixed with PMMA. Butyl acrylate (BA) was also used as an alternative, more industrially relevant monomer. The loading of the low T g monomer was varied and full characterisation of the particles produced was performed to elucidate their internal morphologies and compositions.


Introduction
Supercritical carbon dioxide (scCO 2 ) is a sustainable reaction medium for polymerisations and is a promising alternative to conventional solvents. This is because of its tuneable properties, high natural abundance and easily accessible critical point; T c = 31.1 °C and p c = 73.8 bar. 1,2 CO 2 is also inexpensive, non-toxic, non-flammable and is readily available in high purity. These properties, coupled with the fact that scCO 2 reverts to its gaseous state upon depressurisation, eliminating energy intensive drying steps, make it a desirable solvent for polymerisations. 3 In 1992, DeSimone et al. published the first polymerisation utilising scCO 2 as a reaction medium, reporting the synthesis of fluoropolymers. 4 Since then, many different polymerisations including free radical chain growth, cationic chain growth, oxidative coupling, transition metal catalysis and melt phase condensation have been carried out employing scCO 2 as the solvent, with vinyl monomers predominantly being used. 3,[5][6][7][8][9][10] More specifically, scCO 2 has been shown to be a versatile medium for dispersion polymerisation, facilitated by the use of amphiphilic surfactants including fluoro-polymers and polysiloxanes, which are soluble in scCO 2 . 11 Recently, increasingly more sophisticated chemistries have been reported using scCO 2 as a reaction medium. The three most widely used reversible-deactivation radical polymerisation (RDRP) techniques; nitroxide-mediated radical polymerisation (NMP), atom transfer radical polymerisation (ATRP) and reversible addition-fragmentation chain transfer polymerisation (RAFT), have all been used to synthesise well-defined particles with narrow size distributions. [12][13][14][15][16] These techniques can give access to more complex structures including cross-linking, 17 metal nanoparticles, 18 and more recently block copolymer particles with internal phase separation. [19][20][21] Particles with phase-separated internal morphologies are usually synthesised via emulsionbased techniques, typically with size between 60 -700 nm. 22 ' 23 Many different internal morphologies have been achieved using emulsion polymerisation, including core-shell. The changes in internal morphology observed occur by variation of several different parameters, such as manipulation of particle size, monomer ratio and tuning of the reaction medium. 24,25 However, the products are obtained in the form of a latex and energy intensive steps (e.g. spray drying) are needed for the removal of water, to afford a dry powder. 26 Dispersion polymerisation in scCO 2 produces dry, free flowing powders upon release of the CO 2 post polymerisation. Employing dispersion polymerisation also allows for synthesis of particles between 0.5 -5 µm, relatively large in comparison to those produced by emulsion polymerisation. 27 Although there is an energy cost associated with the compression of the CO 2 used in the supercritical reactions, this is an order of magnitude lower that the cost associated with the removal of water. 28 There are several examples of copolymer particles, synthesised in scCO 2 , which exhibit microphase separation to give various internal particle morphologies. 19-21, 29 Cao et al. described the preparation of graft copolymer nanoparticles in scCO 2 from a one-step polymerisation.
Particles of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) were synthesised with the assistance of a synthetic, graft copolymer surfactant consisting of pH-sensitive, poly(dimethylsiloxane)-graft-polyacrylates (PDMS-g-PAA). The polymers obtained were fine, free-flowing powders with monodispersed nano-sized particles being formed. The structure was confirmed as a PNIPAM core coated in a PDMS-g-PAA shell by TEM. 30 When using homopolymers rather than copolymers, a core-shell structure is typically achieved, in which one polymer phase is encased in another. Core-shell polymeric particles are desirable for a wide range of applications such as drug delivery, 31 electrophoretic displays, 32 and as impact modifiers. 33 McAllister et al. reported the synthesis of core-shell particles consisting of poly(2-(dimethyl amino) ethyl methacrylate) (PDMAEMA) and PMMA via a multi-stage dispersion polymerisation. 32 PMMA particles were modified in scCO 2 to give a core-shell morphology with domains of PDMAEMA within PMMA. By contrast, particles synthesised in traditional solvents produced the inverse of this, with a core of PMMA surrounded by a shell of PDMAEMA. The observed difference in structure was attributed to the ability of scCO 2 to plasticise the PMMA, allowing the DMAEMA monomer, and therefore the growing polymer, to penetrate the particles. This plasticisation does not readily occur in traditional solvents and hence, the PDMAEMA remains at the surface of the particles.
Here, we report the synthesis of particles containing both a polymer phase with a low glass transition temperature (T g ); either poly(butyl acrylate) (PBA) or poly(benzyl acrylate) (PBzA), combined with a high T g poly(methyl methacrylate) (PMMA) phase. Both PBzA and PBA exhibit relatively low T g s of 2 °C and -45 °C respectively, as measured by DMA, in comparison to PMMA (144 °C). This value is higher than the T g of PMMA measured by DSC, often reported in literature as 105 °C. 34,35 However, differences between the values obtained from the two analytical techniques do occur because of the intrinsic difference between the static DSC and the dynamic DMA measurement, resulting in the DSC observed T g s being lower in absolute values. 36 The particles were formed by incorporation of the low T g monomers to preformed PMMA particles via a simple two-step free radical polymerisation. BzA was chosen as a model low T g monomer, with well-defined and detectable functionality when mixed with PMMA.
Notably, the PBzA phase can be selectively stained prior to TEM analysis due to the presence of the aromatic group. BA as an alternative low T g monomer was also investigated, as it is more traditionally used in commercial polymers. In both systems, the low T g monomer loading was varied and a series of analytical techniques were used to probe and confirm the morphologies and compositions of the particles produced. Particles that combine both low and high T g polymer phases can be used as impact modifiers. 37

Polymer Synthesis in 60 mL Autoclave
All reactions were performed in a 60 mL high-pressure autoclave built in-house, previously used for dispersion polymerisations. 12,19,39 These experiments can also be performed at a larger scale (1 L). 40

PMMA/PBzA Particles Synthesis
MMA (Table 1) was deoxygenated by purging with argon for 30 minutes. A mixture of AIBN (1 wt% with respect to (wrt) total monomer, 0.0705 g, 0.43 mmol) and PDMS-MA (5 wt% wrt total monomer, 0.3525 g, 0.04 mmol) was separately flushed with argon for 30 minutes. removed, and the autoclave was allowed to naturally cool to room temperature before being depressurised. The resulting products were typically collected from the base of the autoclave as free-flowing white powders ( Figure 1, stages 1, 2 & vent).
As the BzA feed was increased to deliver a loading of 50 wt%, the quality of the particles produced using a one-stage addition of BzA was reduced, with SEM analysis showing high levels of aggregation ( Figure SI-1). In an attempt to improve this, the BzA was added over twostages, as this had previously been reported to reduce agglomeration. 27 The initial stage of the reaction remained the same as described above. The total amount of monomer was increased to 12 mL to allow for sufficient amounts of MMA needed for nucleation in the primary loading ( is 24 hours. 41 As the reaction had been carried out for approximately 24 hours at this point, additional AIBN (0.059 g, 0.33 mmol) was included in the second charge of BzA. As before, the injection induced a small pressure increase. The reaction was left overnight (18 hours), before being cooled to room temperature and depressurised. The resulting products were typically collected from the base of the autoclave as free-flowing white powders ( Figure 1).

Polymer Characterisation
Details of the homopolymers synthesised for analytical comparison are given in the supporting information (SI).

Scanning Electron Microscopy
Scanning Electron Microscopy (SEM) was performed on a Phillips XL30 microscope. Particles were washed by centrifuging in dodecane three times (10 minutes, 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 Scientific) as the eluent at room temperature, using two Agilent PL-gel mixed-D columns in series with a flow rate of 1 mL min -1 . A multi-angle light scattering (MALS, Wyatt Optilab Dawn 8+) detector, along with a differential refractometer (DRI, Agilent 1260), were used for sample detection. The system was calibrated using PMMA standards (molecular weight range: 1,000 -400,000 g mol -1 ).

Dynamic Mechanical Analysis
Measurements were performed on a Triton Technologies (now Mettler Toledo DMA1) dynamic mechanical analyser (DMA) using the powder pocket accessory. The use of this attachment allowed for direct measurement of the synthesised powder with no further sample preparation required. The sample (40 mg ± 5 mg) was weighed into a powder pocket.
Samples were measured at 1 and 10 Hz in single cantilever bending geometry between 25 to 250 °C or -100 to 250 °C depending on the region of interest. The T g was recorded as the peak temperature of the tan δ trace obtained at 1 Hz.

Transmission Electron Microscopy
Transmission electron microscopy (TEM) was used to analyse the internal morphology of the

Nuclear Magnetic Resonance
The conversion and polymer content of each reaction was determined using 1 H nuclear magnetic resonance ( 1 H NMR) spectroscopy. Samples were dissolved in CDCl 3 and analysed using a Bruker DPX 400 MHz spectrometer. For reactions containing PBzA, analysis was performed in acetone-d 6 and tetramethylsilane (TMS) was used as a reference.

Results and Discussion
In order to synthesise phase-separated particles containing both hard and soft domains in scCO 2 , the hard polymer must be synthesised first. It is known that the dispersion polymerisation of low T g monomers in scCO 2 does not readily produce particles. The main reason for this is the fact that the CO 2 readily plasticises the polymer particles, lowering their T g and causing agglomeration. 42,43 For this reason, dispersion polymerisation of the higher T g MMA was performed first, to create a stable seed particle. The second low T g monomer was subsequently added using methodologies that have previously been reported in the literature. 27,32,42,44,45 The initial focus of this research was the synthesis of particles containing a PBzA phase (Table 3), as a model system.   Surprisingly the NMR spectrum also contained trace levels of unreacted MMA, even after the long reaction time (> 48 hours). A possible explanation for this could be that the remaining It should be noted that the observed decrease in particle size, as the soft component increased, is a result of variations in the amount of PDMS-MA stabiliser used (Table 3). This is because the total amount of monomer in the reaction is constant, therefore as the loading of Dynamic mechanical analysis (DMA) was used to establish whether phase separation has occurred within the particles. Measurements were performed using the powder pocket accessory. The use of this attachment allowed for direct measurement of the synthesised powder with no further sample preparation required. It is well known that hard-soft, coreshell particles show two tan δ peaks, with the lower temperature peak corresponding to the soft-rubbery phase and the higher temperature peak corresponding to the hard-glassy phase. 48  For the particles produced using a 9 wt% loading of BzA, one transition was observed ( Figure 4, red trace, 133 °C), similar to the peak observed for pure PMMA (144 °C), suggesting phase separation had not occurred. The small reduction in T g in comparison to PMMA suggests that some blending of the two polymers has occurred. By contrast, as the loading of BzA was increased, a second lower T g peak becomes visible and is attributed to a PBzA rich phase, whereas the high T g peak is attributed to a PMMA rich phase.
A shift in T g from the pure polymers was observed, suggesting that the phases present are not 100% separated but partially blended. However, as the loading of the BzA increases, the low T g peak moves closer to the pure PBzA transition (2 °C), indicating that the soft phase has observed similar trends in DMA data for the alternative two-monomer system (PMMA/PDMAEMA) that produced an internal core-shell like morphology.
To probe the structure and internal morphology of the PMMA/PBzA particles further, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) coupled with preferential staining were used. Preferential staining is a common technique used to distinguish the internal morphology of two component particles. 49 SEM analysis showed that the particle structure was well maintained as the BzA loading increased, with uniform, monodisperse particles being formed ( Figure 5) In TEM analysis, the presence of the phenyl ring in the PBzA allows for preferential staining of this phase with RuO 4 . 50 Pure PMMA particles appeared homogeneous and no visible internal morphology was observed. A dark ring was observed on the surface of the particle, which is attributed to the PDMS-MA stabiliser ( Figure 5). 32 DMA analysis also implies that only one of the phases is present. The particles obtained using a feed of 9 wt% BzA also appear homogeneous with no visible morphology observed in the TEM image ( Figure 5). This again agrees with GPC and DMA analysis. The presence of two distinct, separate phases was faintly visible in particles synthesised with a BzA loading of 27 wt% showing a PMMA core encased in a PBzA shell ( Figure 5).
As the BzA loading was increased further to 36 wt%, the internal morphology begins to change, showing smaller internal domains of PBzA surrounded by a continuous PMMA phase ( Figure 5). A possible explanation is that under the conditions used (207 bar and 65 °C), the PMMA seed particles will be plasticised, 11 allowing for facile penetration of a second monomer/polymer. 32 For the particles synthesised with a loading of 50 wt%, it also appears that the PBzA is apparently has migrated into the PMMA particles. Phase separation in particles synthesised using emulsion polymerisation is a well-studied area and the internal morphology formed is influenced by a combination of thermodynamic and kinetic factors. 24,25 Particles produced under thermodynamic control will lead to a morphology that is at equilibrium and is driven by a minimisation of the interfacial free energy. However, in most cases the internal morphology formed is controlled by kinetics. Three kinetic factors control the morphology; (1) radical penetration into the seed particles during the second stage of the polymerisation, (2) polymer phase-separation and (3) consolidation of the phase domains after phase separation. 24,25,51 In our system, increasing the PBzA concentration certainly has an effect on the morphology and it could be that the higher concentration is enhancing kinetic factors and aiding phase separation. Another factor that could be influencing the observed morphologies is molecular weight. As the loading of BzA is increased, the molecular weight of the PBzA phase formed also increased (indicated by a shift to lower retention time in the GPC trace (Figure 3)). This change in molecular weight could induce the formation of a different morphology, as has previously been reported for block copolymers. 20,24,52 CO 2 -philicity of the monomer and its corresponding polymer could also be an influencing factor on the morphology produce. 20,29,32 For example, a more CO 2 -phobic monomer/polymer would prefer to migrate inside the PMMA particle and thus limit its interaction with CO 2 . In addition, the variation in particle size of the seed PMMA could also be altering the morphology produced. 24,53 Further work is ongoing to understand the driving force for the morphology formed. However, this may be a promising new method to synthesise particles with complex morphologies, which does not require the use of controlled polymerisation techniques to produce block copolymers.
Particles containing phase-separated hard and soft domains have been successfully synthesised, to produce a core-shell structure as well as a micro-phase separated structure consisting of soft spheres in a hard matrix. Partial blending of the PMMA with the PBzA phase increased the T g of the soft domains to above room temperature. Thus, a lower T g soft block was also investigated to ensure the presence of a soft phase at room temperature.
Poly(butyl acrylate) (PBA) was chosen as an alternative, more industrially relevant soft polymer (T g : -45 °C). PBA is traditionally used for a wide range of applications such as impact modification. [54][55][56][57][58] The same two-stage method was used, in which PMMA seed particles were synthesised before the addition of BA. Various loadings of BA were tested (Table 4).    The PBA content of the particles was slightly lower than the feed for both loadings, which reflects the BA conversion of <75%. However, as the feed loading is increased, an increase in PBA content was observed. The loading of BA measured by 1 H NMR post reaction (unreacted monomer + polymer) was similar to the target loading, indicating that the desired loading of BA was achieved (Table 4).
In GPC analysis, the DRI trace for a BA loading of 9 wt% showed a unimodal peak. By contrast, multimodal peaks were observed for the higher loading of 27 wt% (Figure 7). to be incorporated into the particles via the stabilisation mechanism of the dispersion polymerisation. 44 Concentrating on the "PMMA type" peak, the addition of 9 wt% of BA caused a shift to a lower temperature suggesting that the phases present are not completely separated but partially blended (Figure 7). 59 The peak is also broad in comparison to the PMMA particles, with what could be considered as a small lower temperature shoulder peak.
This suggests that there is a large variation in composition, with lower T g s being measured for the material in the sample containing a higher amount of PBA. As the BA loading was increased to 27 wt%, the "PMMA type" peak becomes less well-defined. The peak also shifts to a lower temperature and broadens further, with the shoulder peak becoming slightly more defined.
Focusing on the low temperature region, three peaks are visible. As previously discussed, if a core-shell morphology was present, then a T g peak for both the "soft" PBA phase and the "hard" PMMA phase of the particles would be observed. Although one of these peaks occurs in a similar position to PBA homopolymer, in this system, the crystal melt of the PDMS-MA stabiliser, occurs at -50 °C, which is very close to the expected T g of PBA at -45 °C ( Figure SI-4 in the SI).  It is therefore very difficult to determine if a homogeneous PBA peak was present using thermal analysis. For the 27 wt% loading, a peak at -8 °C was observed suggesting the presence of a PBA rich phase (Figure 7). This peak is at a higher temperature than the PBA homopolymer (-45 °C), which indicates that blending of PMMA with this phase may be occurring. SEM analysis showed that particle structure was maintained as the BA loading was increased  phase were likely to be present and it was possible that the system had formed a particle with a low T g shell and high T g core.

Conclusions
Successful addition of either BA or BzA (low T g ), at various loadings, to preformed PMMA particles was demonstrated. The two-stage reaction technique utilising scCO 2 as the reaction medium was used to synthesise PMMA particles containing a feed of up to 36 wt% BzA. For a loading of 50 wt%, the addition of BzA was split over two charges in order to maintain particle quality. SEM analysis showed that particle structure did not deteriorate significantly as the loading of BzA was increased up to 50 wt%. DMA analysis indicated two separate Tgs, signifying the presence of two phases. This was complemented by GPC analysis, which also suggested the presence of two different species: PMMA and PBzA chains of different PMMA phase was observed. This suggests that the morphology formed is dependent on the concentration of PBzA but may be related to other factors such as particle size and molecular weight.
Moving to a more industrially relevant system, an alternative soft polymer component of PBA was tested. Once again, the SEM analysis indicated that the structure of the particles did not deteriorate as the PBA content increased. At low loadings of BA, DMA suggested that the particles produced were homogeneous, containing one phase as opposed to the desired coreshell structure, containing two phases. Nevertheless, as the BA loading was increased, the DMA analysis suggested the presence of a PBA rich phase, but it is difficult to say for certain because the Tg of the PBA phase and the PDMS-MA occur at very similar temperatures. AFM analysis showed the formation of a network-like structure of on the surface of the particles.
The work presented is a simple and novel method to synthesis phase-separated particles in scCO 2 that does not require any chemical control agents or post-polymerisation drying steps.
Further work and adjustments are needed to enable full control and understanding of the internal morphology produced for each monomer at the various loadings.
instrumentation, in particular Denise Mclean and Nicola Weston for help with the TEM analysis.

Polymer Chemistry Accepted Manuscript
Open Access Article. Published on 08 July 2020. Downloaded on 7/9/2020 9:56:12 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.