Experimental 74

The general procedure of emulsion polymerisation in either a round-bottle flask or a reactor involved a degassing phase, polymerisation initiation and conversion.[196, 197]

First the monomer styrene (Fluka, purum, monomer, ≥ 99 % (GC), 0.005 % 4- tert-butylcatechol) and the required volume of deionised, prior degassed water were poured together in the reaction vessel. Depending on the type of synthesis other additives including surfactant SDS (BDH Laboratory supplies), co-monomer NaSS (Aldrich, technical, ≥ 90 %) and pH buffer sodium hydrogen carbonate (NaHCO3) (Aldrich, > 99.5 %) were added. The vessel was then sealed with septums to provide a closed system and the solution was degassed by excess N2 injection below the solution surface through an inserted needle for about 30 minutes under constant stirring. After completion of the degassing stage the reaction solution was heated up to 70 ºC followed by initiator injection from a syringe. The initiator was pre-dissolved in a few millilitres of distilled water. The nitrogen syringe was then withdrawn above solution level. In these experiments two radical generating initiators of opposite charge were used, anionic KPS (K2S2O8, Aldrich, 99 %) and cationic AMPAD (Aldrich, 97%). A first indicator of a working initiator is the change in appearance of the dispersion from transparent to opaque white. This change is due to particle formation of <50 nm in diameter which happens within the first 20 minutes to one hour. The reaction was stirred for at least 4 hours to reach full conversion. After cooling to room temperature the nanosphere dispersion was filled into dialysis tube membranes (Sigma-Aldrich, Dialysis Tubing, Cellulose membrane, 33 mm diameter). The latex was dialysed against distilled water for up to two weeks. For efficient monomer and additive removal the water was changed daily. Upon completion the product was stored in air-tight bottles in the dark.

3.2.1.1 Round-bottom flask

In the first experiments a 250 mL round-bottom flask with a magnetic stirrer bar and an oil bath heater was employed for PS nanospheres synthesis. In these experiments

different compounds and parameters for radical initiated emulsion polymerisation of monodisperse nanospheres were screened. The temperature was monitored by a temperature probe immersed in the oil bath and controlled by a temperature controller linked to the hot plate. The stirring speed during the reaction was held at ca. 200-300 rpm. A total of five reaction recipes were screened in this way.

In reaction 1 (R1) to 3 (R3), deionised water (90 mL), styrene (10.0 g, 0.10 mol) and KPS initiator (0.05 g - 0.10 g, 0.2 x 10-3 mol - 0.4 x 10-3 mol) were used (see Table 3.1). To reduce the particle size and to probe the potential of a surfactant, SDS (0.50 g, 1.7 x 10-3 mol) was added in (R3).

Table 3.1 Summary of the experimental parameters of synthesis (R1) to (R3) using KPS with different

synthesis conditions. The synthesis was carried out in a round-bottom flask.

React. H2O [mL] Styrene [mol L-1_] KPS [10-3 _{mol L}-1_] NaSS [10-3 _{mol L}-1_] SDS [10-3 _{mol L}-1_] (R1) 90 1.07 (10.0 g) 2.1 (0.05 g) 5.4 (0.10 g) Surfactant-free (R2) 90 1.07 (10.0 g) 4.1 (0.10 g) 5.4 (0.10 g) Surfactant-free (R3) 90 1.07 (10.0 g) 2.1 (0.05 g) 5.4 (0.10 g) 19.3 (0.50 g)

In reaction 4 (R4) and 5 (R5), deionised water (100 mL), styrene (1.0 g, 0.01 mol) and AMPAD (0.16 g, 0.6 x 10-3 mol) were used. A technique to reduce the particle size involves reaction quenching by sudden O2 injection, and is used in (R5). The exact details for each reaction are summarised in Table 3.2.

Chapter 3: Nanosphere synthesis

Table 3.2 Summary of the experimental parameters of synthesis (R4) and (R5) using AMPAD with

different synthesis conditions. The experiments were carried out in a round-bottom flask.

React. H2O [mL] Styrene [mol L-1] AMPAD [10-3 mol L-1] Quenching

(R4) 100 0.10 (1.0 g) 5.9 (0.16 g) -

(R5) 100 0.10 (1.0 g) 5.9 (0.16 g) O2

3.2.1.2 Reactor

In later experiments a 250 mL reactor with an overhead four-blade stirrer and a water heater aggregate (HAAKE K41) has been used to allow better control over parameters such as temperature and stirring compared to synthesis in the round-bottom flask. The reactor setup is shown in Figure 3.2. Typical stirring speeds were between 250 and 350 rpm.

Figure 3.2 Photograph of the reactor setup for emulsion polymerisation of PS latex. Overhead stirrer

N₂supply

Hot water inlet Water outlet

Drain

In reaction 6 (R6) to 8 (R8) deionised water (180 mL), styrene (2.0 g – 10.0 g, 0.02 mol - 0.10 mol), AMPAD (0.10 g - 0.30 g, 0.4 x 10-3 mol - 1.1 x 10-3 mol) or KPS (0.05 g, 0.2 x 10-3 _{mol) with NaSS (0.10 g, 0.5 x 10}-3 _{mol) were used to study full} conversion emulsion polymerisation with an anionic and a cationic initiator in a reactor. In experiments 9 (R9) to 19 (R19) deionised water (180 mL), styrene (20.0 g, 0.19 mol), KPS (0.10 g, 0.4 x 10-3 mol), NaHCO3 (0.10 g, 1.2 x 10-3 mol) and NaSS (≤0.40 g, ≤1.9 x 10-3 mol) were used to study full conversion emulsion polymerisation. In this set of experiments all parameters were kept constant only varying the concentration of NaSS to regulate particle size and PDI. These experiments were all based on the anionic initiator KPS. The details for each reaction (R6) to (R19) are listed in Table 3.3.

Table 3.3 Summary of the experimental parameters of synthesis (R6) to (R8) using AMPAD and KPS as

well as (R9) to (R19) using KPS with varying NaSS concentration. All reactions were carried out in a reactor. React. H2O [mL] Styrene [mol L-1_] AMPAD [10-3 _{mol L}-1_] KPS [10-3 _{mol L}-1_] NaSS [10-3 _{mol L}-1_] (R6) 180 0.53 (10.0 g) 2.0 (0.10 g) - - (R7) 180 0.11 (2.0 g) 6.1 (0.30 g) - - (R8) 180 0.53 (10.0 g) - 1.0 (0.05 g) 2.7 (0.10 g) (R9) to (R19) 180 1.07 (20.0 g) - 2.1 (0.10 g) 0.0 (0.00 g) to 10.8 (0.40 g)

3.2.2

Sphere size characterisation and solid content measurement

Dynamic light scattering (DLS)

The nanosphere dispersion was analysed after dialysis with DLS to determine the Z-average diameter and PDI. The dispersion was either used as synthesised or slightly diluted to satisfy the scattering conditions. Sample dispersions were analysed in disposable plastic cuvettes at 25 ºC after at least 10 minutes of temperature equilibration.

Chapter 3: Nanosphere synthesis Typically, data acquisition times of 20 seconds per measurement with 10 measurements per run, averaged over 10 runs, were used to determine particle diameter and PDI.

Electron microscopy (EM)

For selected latexes additional size analysis by TEM or SEM was carried out to determine the sphere diameter of the dry particles, which is slightly lower than the average diameter determined by DLS. An average number of at least 30 particles were measured for each reaction using the software Image Tool 3.00.

Solid content measurement

The determination of solid content mass fraction of a latex was performed gravimetrically. A measured amount of dispersion was weighed on a metal tray with a defined mass and put in an incubator over night to evaporate the solvent. By weighing the tray again with just the remaining dry solid content, the solid mass fraction can be calculated.