Emulsion polymerisation 69

3.1.2.1 Surfactant assisted emulsion polymerisation

One of the most common and suitable techniques to synthesise polymer nanospheres is radical initiated emulsion polymerisation as it offers good control over polydispersity and size with the potential to generate sub-100 nm nanospheres.[177-179] Latex synthesis is based on a heterogeneous reaction using a monomer, surfactant, an initiator and a dispersion medium, such as water. A typical surfactant used for a variety of polymer emulsion polymerisations is sodium dodecyl sulphate (SDS).[180, 181]

In the case of PS synthesis, styrene is dispersed in a water phase. In a first step, the surfactant emulsifies parts of the monomer by micelle formation. The micelles are then infiltrated by the monomer. Free radicals are formed in the heating step by initiator decomposition based on single bond homolysis. The radical attacks a dissolved monomer and initiates a chain reaction leading to oligomers in the water phase. In a next step the still reactive oligomers enter the bigger monomer-swollen micelles and continue the polymerisation eventually forming a colloidal sphere. The terminal active centres are fed by dissolved monomer from the aqueous phase until either all the monomer is used or the radical site becomes quenched in one of the possible side reactions.[182, 183] Typical initiators are anionic persulfates such as potassium persulfate (KPS) and cationic 2,2’- azobis(2-methylpropion amidine) dihydrochloride (AMPAD). The surfactant and the charged remains of the initiator on the particle surface lead to colloidal sphere stabilisation in the dispersion. The molecular structure of the different compounds and product can be seen in Figure 3.1.

In the case of nanosphere templating in OPVs, surfactants have the ability to form a residue film on the templated organic interface after organic solvent treatment. A clean D/A interface is vital for efficient OPV device operation, hence a remaining surfactant film can severely disturb device operation. Although dialysis removes residues, surfactant and unreacted monomer in the latex, it may cause sphere instability and particle agglomeration. Also, surfactants and residues can disturb sphere self-assembly leading to blockage of the voids in between the particles. Complete removal of surfactants and reaction residues is very difficult to achieve.[184]

Chapter 3: Nanosphere synthesis

Figure 3.1 Molecular structures of compounds used in emulsion polymerisation: a) styrene, b) PS, c)

NaSS, d) AMPAD, e) KPS, f) SDS.

3.1.2.2 Surfactant-free emulsion polymerisation

To produce ‘cleaner’ polymer nanospheres, surfactant-free radical initiated emulsion polymerisation was developed, using just monomer, a dispersion medium, and the initiator.[185, 186] _{Surfactant-free emulsion polymerisation typically leads to particles} with diameters in the range 200 nm to 1000 nm which are stabilised by their ionic surface charge.[187] Smaller particles are hard to achieve due to the absence of additional particle stabilising surfactant.

The reaction mechanism is similar to a general emulsion polymerisation with the main difference of micelle formation by preformed oligomers. Oligomers are short polymer chains generated in the first polymerisation step. They are similar in structure and function to surfactants consisting of a long hydrophobic tail which assembles inside the micelle, and an ionic head which faces the aqueous phase providing sphere stability. After micelle formation by assembled oligomers, monomer and primary radicals diffuse into the micelles and polymerise. With continuous sphere growth, the particle surface charge decreases leading to instability. To regain stability particles coagulate benefiting from the favourable surface-to-volume growth dependence as observed for regular

SO3Na

n

emulsion polymerisation. By fusing two or more particles the new particle gains in surface charge density. The charged surface groups prevent the nanospheres from aggregation through vdW attraction using electrostatic repulsion to keep the particles dispersed. The charged spheres form an electrical double layer to maintain charge neutrality in close proximity of the particles in solution.

To form colloidal crystals with long range order the polydispersity of nanospheres has to be in the range of 4-8 % in diameter distribution, which correlates with a PDI of <0.05. [188, 189] In surfactant-free emulsion polymerisation nanosphere synthesis there are many parameters which determine particle size and polydispersity. Some of the parameters interfere with each other making precise size control very difficult. Size control of small particles <100 nm is particularly challenging.[190]

Amongst the parameters affecting the nanosphere size in a synthesis are reaction temperature, stirring speed, as well as monomer, initiator and additive concentrations.

With a temperature above 60 ºC the initiator decomposition rate is high enough to be eliminated as the reaction rate limiting factor, which otherwise can lead to uneven particle growth with a large size distribution. With increased initiator decomposition rate more nuclei are formed which therefore reduces the average sphere size. A higher monomer concentration ultimately leads to a higher solid content, but also increases the particle size due to larger nuclei and more monomer per nuclei.[191] The initiator concentration defines the number of seed nuclei provided for particle growth, but also sets the concentration of micelle forming oligomers. Hence, an increase in initiator concentration with all other parameters kept constant causes a decrease in particle size but is also linked to a higher PDI due to secondary nucleation.[186] _{However, with} increasing initiator concentration the ionic strength of the solution increases and thins the vital electrical double-layer which prevents the particles from coagulation during the growth process. This effect leads to an increase in particle size and also polydispersity. In the literature both processes were observed.[186, 191, 192]

To synthesise small nanospheres, styrene-4-sulfonic acid sodium salt (NaSS) can be added in low quantities as a monomer building block.[190] _{NaSS is based on a styrene} monomer which is functionalised with a sulfonic acid substituent for increased surface charge in anionic systems. The molecular structure of NaSS is shown in Figure 3.1.

Chapter 3: Nanosphere synthesis Unlike surfactants, NaSS copolymerises with styrene and adds charged functional groups to the otherwise hydrophobic polymer chain and therefore increases the surface charge. With even small concentrations of NaSS the particle size was significantly reduced due to promoted nucleation but also the additional surface charges.[193] Xue et al. achieved particle sizes of about 40 nm with NaSS and a continuous monomer feed.[194] NaSS helps to form smaller particle sizes but can also lead to high polydispersity. Similar to an increased initiator concentration, the ionic strength of the solution can be raised with added NaSS and can therefore cause particle coagulation and larger particle sizes. Therefore, a balance between low particle diameter, latex stability and acceptable polydispersity has to be found.

The entire synthesis is very oxygen sensitive due to initiator quenching, which explains why the synthesis is performed under N2. However, oxygen can also be used to quench the polymerisation in an early stage, when the particle size is still small. By not allowing full conversion the solid content is assumed to be very low and purification needs much more care due to vast amounts of remaining monomer.