A molecular dynamics approach to nanostructuring of particles produced via aerosol cationic photopolymerization
Graphical abstract
Introduction
Aerosol photopolymerization is a rather new and interesting technique to produce polymeric particles in the micron and submicron scale (Esen and Schweiger, 1996, Vorderbruggen et al., 1996, Gao et al., 2007). It is a continuous process, which involves the atomization of a monomer solution followed by a droplet-to-particle conversion within a photoreactor (Akgün et al., 2013). Compared to other polymer particles production techniques, aerosol polymerization displays some advantages such as high purity of the product at the reactor outlet. In fact, it does not involve the use of a liquid medium nor the use of surfactants to stabilize the dispersion (Vorderbruggen et al., 1996, Akgün et al., 2013). Moreover, it is possible to produce nanocomposites by dispersing nanosized inorganic material within monomer formulation prior to atomization (Akgün et al., 2014).
As a drawback of this technique, particles structuring can be challenging. This is due to the extremely low amount of time between the liquid aerosol production and its full conversion into solid particles (Akgün et al., 2014). In such conditions the initial confinement of reacting species, in well-defined volumes within droplets, to produce pores or polymeric shells, can be difficult. Various attempts were made to modify the standard method in order to obtain a structured product (Akgün et al., 2014, Esen et al., 1997). In particular, the exploitation of phase separation during radical photopolymerization (Akgün et al., 2014) resulted in the successful production of polymeric capsules or particles characterized by porous structures.
In a previous work, we were able to exploit a similar method also in cationic photopolymerization (Bazzano et al., 2017). Akgün et al. (2015) proposed the cationic mechanism for aerosol polymerization, thus producing full spherical particles. Inducing phase separation, we were able to provide a structure within particles obtained with this reaction pathway. Phase separation was designed by varying the composition of a two-component solution, whereas, in the case of capsules structuring, gelation delay was induced by adding a chain transfer reagent (CTR). Experimental results highlighted the presence of competing mechanisms during particles production, such as diffusion of the growing molecules, polymerization reaction and gelation of the structure. A correct design of these mechanisms is crucial for a successful structuring process (Bazzano et al., 2017).
In order, to gain insight into the mechanisms that control the outcome of the production, molecular dynamics (MD) can be a useful tool. This technique has been successfully applied to the study of polymeric systems in the presence of solvents (Ahlrichs and Dünweg, 1999, Paradossi et al., 2011, Yamamoto, 2004, Di Pasquale et al., 2014) and for the simulation of solvents mixtures Lavino et al. (2018). In our case study, polymer molecules tendency to be solvated in different environments can be calculated by MD simulations, thus providing information on phase separation and diffusion of the reacting species.
In this work, MD simulations at the equilibrium were carried out in order to study the impact of solvent composition on polymer transport properties. Results were then correlated with the morphology of particles produced by using different solvent mixtures. The goal was to gain a better understanding of the controlling mechanisms, which define the final structure in phase-separation-driven particle structuring and, therefore, to help us in the development of better formulations.
Section snippets
Particles synthesis
Particles synthesis was carried out using an experimental setup consisting of a pneumatic atomizer followed by a reactor equipped with six UV lamps (Bazzano et al., 2017). In all runs the light intensity was 5 mW cm−2. A formulation containing monomer, photo-initiator and solvents was previously prepared and poured into the atomizer reservoir. Monomer was tri(ethylene glycol) divinyl ether (DVE3), a bifunctional vinyl resin with a poly(ethylenglycol)-like backbone. Photo-initiator (PI) was a
Results
As previously stated, in aerosol photopolymerization it is difficult to control the product morphology. We tried to achieve this goal by controlling phase separation during the polymerization step. In order to do so, it is crucial to understand how the presence of different solvents can affect the molecule conformation and, thus, both phase separation tendency and mass transport phenomena. Two parameters were taken into account: the stretching/folding tendency of the macromolecule, defined via
Conclusions
In this work, an extensive study of the transport phenomena occurring during polymerization in an aerosol photoreactor was reported. The effect of solvents composition on the behavior of the growing polymer in solution has been studied. The results of simulations, coupled with experimental evidence, clearly stated the importance of phase separation and diffusion mechanisms in particle structuring. With respect to the production of porous particles, two parameters were studied and one of them,
Acknowledgment
The authors also gratefully acknowledge Donato Latorre and Martina Zappitelli for their valuable support in the experimental investigation.
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