Elsevier

Chemical Engineering Science

Volume 195, 23 February 2019, Pages 1021-1027
Chemical Engineering Science

A molecular dynamics approach to nanostructuring of particles produced via aerosol cationic photopolymerization

https://doi.org/10.1016/j.ces.2018.10.047Get rights and content

Highlights

  • Description of transport phenomena in aerosol droplets via molecular simulations.

  • Mechanistic understanding of the link between formulation and particle structuring.

  • Design of formulations which produce particles with different structural properties.

  • Control of nano-structuring of micro-particles via molecular simulations.

Abstract

Structuring of particles obtained by aerosol photopolymerization is here described from both the experimental and the simulation point of view. Transport phenomena occurring within aerosol droplets are studied in order to identify the key issues that must be carefully designed when considering particles structuring. In particular, phase separation and diffusion of the reacting species were evaluated using molecular dynamics simulations, allowing the identification of a series of formulation parameters such as the composition of a mixed solvent (hexadecane and 2-octanone) or the addition of a chain transfer reagent, which are crucial for the control of phase separation and, thus, of the final morphology of the microparticles. The results were compared with morphology analyses obtained from the experimental work. We also found that molecular simulations were useful for the decoupling of the effects of different solvents which were thus treated separately. The interplay between phase separation and gelation rates was also found to be crucial in the structuring process. For example, the ratio between hexadecane and a chain transfer reagent seemed to strongly affect the final morphology of micro-particles.

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.

References (22)

  • P.J. Flory

    Principles of polymer chemistry

    (1953)
  • Cited by (5)

    • A kinetic model of gas-particle mass transfer in aerosol: Application to phase state in aerosol

      2020, Powder Technology
      Citation Excerpt :

      Thus, in order to get the dynamic evolution of aerosol, it is necessary to couple more equations into these models. Different from the thermodynamic equilibrium models, the molecular dynamic simulation (MD) could be used to illustrate the transient change of a finite number of molecules [37–42]. The MD method emphatically accounts for the inter- and intra-molecular forces, and it could extract the information of energy change in nucleation, growth and aggregation of molecules.

    • Continuous aerosol photopolymerization to coat de-agglomerated nanoparticles

      2020, Chemical Engineering Journal
      Citation Excerpt :

      To address these limitations, we built upon the principals of aerosol photopolymerization. Several studies have shown that irradiating sprayed monomer droplets with UV light leads to formation of micro- and nano-sized polymeric particles [28–30]. This technique was subsequently refined to produce micro-scale spherical nanoparticles loaded with inorganic nanoparticles such as zinc oxide [31,32].

    View full text