Análisis y discusión de los resultados o de los instrumentos utilizados

CAP III RESULTADOS DE LA INVESTIGACIÓN

3.1. Análisis y discusión de los resultados o de los instrumentos utilizados

4.2.1 Synthesis

Linear particles

Three different syntheses were performed re- sulting in three different batches (PS1–PS3). The PS1 colloids were washed with methanol after the synthesis was complete. This caused the particles to become unstable and a fleece of styrene was formed on top of the dispersion. The PS2 colloids were washed with water and remained stable in dispersion. The colloids had a low polydispersity and their size was determined to be 579±6 nm using TEM

images. However, after a few days, the dispersion had become unstable.

Therefore, a new synthesis PS3 was performed. An optical microscopy image of the linear PS3 particles can be seen in Fig- ure 4.2.2a. The particles are stable in dispersion, only a few clusters were observed. The Zeta potential of PS2 and PS3 are equal, so there has to be another mechanism that has caused the PS2 colloids to become unstable in water. It could be that the difference in the amount of styrene that had not polymerised changed the properties of the dispersion medium and therefore the stability of the colloids. Another explanation would be that the surface properties of the colloids change over time when they are dispersed in water. As can be seen in the TEM image in Fig- ure 4.2.2b, the particles are spherical and have a low polydispersity. There were also some smaller particles, they were removed by washing with water. Using the TEM images, the diameter of the colloids was determined to be 735±8 nm. Because these particles are stable in water and have a low polydispersity, they can be used to create cross-linked PS particles.

Cross-linked PS particles

Different attempts were made to cross-link the linear particles with styrene and DVB, the results are shown in Table 4.1 and in Fig- ure 4.2.3, microscope images of the cross- linked particles are shown. In cross-link ex- periment PS2C1, secondary particles and protrusions were observed. This can be seen in the SEM image shown in Figure 4.2.3a. For the subsequent cross-link experiments, hydroquinone was added in the water phase to prevent the nucleation of these secondary particles. This was successful since the secondary

15 µm

(a)Light microscope image

0.5 µm (b)TEM image

Figure 4.2.2:Optical and electron microscope images showing the linear PS particles.

(a)It can be seen on this light microscope image that most particles are single particles, therefore they are stable in water.

(b)TEM image showing the linear particles. They are spherical and have a low polydispersity. The diameter of the colloids was determined to be735_±8 nm.

particles were no longer observed in the subsequent cross-linking experiments.

Cross-link experiments PS2C1–PS2C3 used the linear PS2 particles as seed particles. These PS2 particles turned out to become unstable in water over time since clusters were formed. This led to a high polydispersity for particles of batch PS2C1–PS2C3. In PS2C1, three different sizes were observed. The sizes were compared to the size of the PS2 seed particles and to each other. It can be seen in Table 4.1 that the three different sizes are due to the seed particles being single colloids, dumbbells or trimers, since the volume of the PS2C1 cross-linked particles is approximately one time, two times and three times as large as the volume of the smallest cross-linked particles found in PS2C1.

For PS2C3, the volume of the particles are approximately one time, two times and four

times as large as the volume of the smallest cross-linked particles found in PS2C3. This might be caused by the higher volume fraction of seed particles used compared to PS2C1. The probability for seed colloids to cluster increases when the volume fraction of the particles increases (see Equation 2.2.6). The cross-linking was successful when stable PS3 seed particles were used. This supports the hypothesis that the polydispersity of the cross-linked particles is caused by the formation of clusters of seed particles. This mechanism could be used to make patchy particles, by mixing different seed particles, aggregating them in a controlled fashion and poly- merising them by the emulsion polymerisa- tion that is used here.

In Figure 4.2.3d a SEM image of the successful cross-linking attempt PS3C1 can be seen. The particles are stable in water and only a small amount of clustering was observed. It 31

1 µm (a)PS2C1 10 µm (b)PS2C2 1 µm (c)PS2C3 1 µm (d)PS3C1

Figure 4.2.3:Microscope images of the cross-linked PS particles.

(a)SEM image showing the cross-linked PS2C1 particles. The particles are polydisperse and secondary particles were formed.

(b)Optical microscope image of the PS2C2 cross-linked particles. They are polydisperse and highly unstable in water.

(c)SEM image showing the cross-linked PS2C3 particles. They are polydisperse, but are spherical and have a smooth surface.

(d)SEM image showing the cross-linked PS3C1 particles. They have arranged in a hexagonal crystal lattice which confirms that the particles have a very low polydispersity and a spherical shape. Their diameter was determined to be1.38_±0.03 µm.

Table 4.1: Summary of the results of the four different cross-link experiments that were performed with linear PS particles.

PS2C1(Figure 4.2.3a)

Particles were very polydisperse and unstable in dispersion. There were smaller particles or protrusions (100 to 200 nm) on these bigger particles.

Particle size(nm) Volume/seed volume Volume/reference volume

0.90 ±0.05 4.0 ±0.6 1

1.10 ±0.05 7.0 ±1.0 1.8 ±0.4

1.30 ±0.05 11.0 ±1.7 3.0 ±0.6

PS2C2(Figure 4.2.3b)

The particles were still very polydisperse and unstable in dispersion. The particles were only observed with an optical microscope. Large clusters had formed, with a diameter of ∼5 µm. The particles were∼1 µm in diameter. Smaller clusters were also observed. PS2C3(Figure 4.2.3c)

The particles were still very polydisperse and unstable in dispersion. There were no smaller particles or protrusions on the particles, the particles were spherical and smooth. Particle size(nm) Volume/seed volume Volume/reference volume

0.70 ±0.05 1.8 ±0.3 1

0.90 ±0.05 3.8 ±0.6 2.1 ±0.5

1.10 _±0.05 6.9 _±1.0 3.9 _±0.8

PS3C1(Figure 4.2.3d)

For crosslink experiments 1-3, the linear PS particles that were used (PS2) had become unstable and had formed small aggregates. A new synthesis was performed and these stable particles (PS3) were used for the final attempt. The crosslinked particles were stable in water and had a low polydispersity.

Particle size(nm) Volume/seed volume

1.38 _±0.03 6.6 _±0.6

can be seen that the particles are spherical and have a smooth surface. Using the SEM images, their diameter was determined to be 1.38±0.03 µm, therefore they can be easily observed using an optical microscope. They have a low polydispersity which is confirmed by the hexagonal crystal lattice that is formed when the dispersion is dried for analysis using the SEM.

Additionally, the Zeta potential of the PS3C1 cross-linked particles was determined to be −32±2 mV. The cross-linked particles are significantly less charged than the linear PS3 particles. This was expected, because in the linear synthesis, KPS is used as initiator, which induces charges at the surface. In the cross-linking step, AIBN was used as initiator. AIBN does not induce charges at the particle surface, so the overall surface charge is low- ered.

Cross-linked PS particles with added TPM Different attempts were made to cross-link the linear particles with styrene, DVB and TPM, the results are shown in Table 4.2 and in Fig- ure 4.2.4 electron microscopy images of the particles are shown. It was observed that the particles were not stable in water since dumbbells and large clusters were formed. Par- ticles of batch PS2CTPM1–PS2CTPM3 were very polydisperse, caused by clustering of the PS2 seed particles. In the SEM image of PS3CTPM1 in Figure 4.2.4d, it can be seen that the particles are spherical but smaller nu- cleated particles were formed on their surface, similar to PS2CTPM1 in Figure 4.2.4a, even though hydroquinone was added to PS3CTPM1 to prevent this from happening. This indicates that the protrusions on the surface are probably droplets of TPM that did not get absorbed by the linear PS particles and was polymerised. Furthermore, the size of the

cross-linked particles with added TPM is ex- actly the same as the size of the cross-linked particles without added TPM. The diameter of the particles was determined using the SEM images to be 1.38±0.02 µm.

Additionally, the Zeta potential of the colloids was measured. It was expected that the surface charge of these cross-linked PS colloids with added TPM would be larger than the surface charge of the cross-linked PS colloids without added TPM. This was expected because of the formation of TPM−_{, as explained}

in subsection 2.1.2. However, the Zeta potential was measured to be−35±2 mV, which is equal to the surface charge of the cross-linked PS colloids without TPM. This further confirms the hypothesis that the TPM is not in- corporated into the particle.

Because the particles had formed aggregates in dispersion, no hexagonal packing was observed in the SEM images. Because these particles did not have a smooth surface and were unstable in water, these particles were not suited for further experiments.

4.2.2 Salting out

Salting out experiments were performed with the PS3C1 particles. In Figure 4.2.5, optical microscope images of the salted out particles for four different ionic strengths are shown. At all four ionic strengths, clusters were formed. For ionic strengths of 0.4 to 1.2M, the amount of clusters and the sizes of the clusters are comparable. However, at an ionic strength of 2.9M the formation of larger clusters was observed.

The number of clusters and the cluster sizes were analysed using the optical microscopy images, the results are shown in Figure 4.2.6. At I = 0.4M, mostly single colloids are ob-

2 µm (a)PS2CTPM1 2 µm (b)PS2CTPM2 2 µm (c)PS2CTPM3 1 µm (d)PS3CTPM1

Figure 4.2.4:Microscope images of the cross-linked PS particles with added TPM.

(a) SEM image showing the PS2CTPM1 particles. The particles were polydisperse and secondary particles were formed.

(b) TEM image showing the PS2CTPM2 particles. They were spherical, there were no secondary particles but they were polydisperse.

(c)SEM image of the PS2CTPM3 particles. They were spherical and had a smooth surface, but were very polydisperse.

(d)SEM image of the cross-linked PS3CTPM1 particles. It can be seen that the particles are spherical and have a low polydispersity. However, their surface is not smooth, secondary particles have formed on the surface, this was also observed using optical microscopy. The particles do not form a hexagonal crystal lattice when dried, because they had already clus- tered into random aggregates in dispersion.

Table 4.2:Summary of the results of the four different cross-link with added TPM experiments that were performed with linear PS particles.

PS2CTPM1(Figure 4.2.4a)

Particles were very polydisperse and unstable in water. Smaller particles or protrusions (100 to 200 nm) had formed on the surface of these bigger particles.

Particle size(nm) Volume/seed volume Volume/reference volume

0.80 ±0.05 6.9 ±1.0 1

0.90 _±0.05 7.0 _±1.0 1.4 _±0.3

1.30 ±0.05 11.0 ±1.7 4.3 ±0.9

PS2CTPM2(Figure 4.2.4b)

The particles were still very polydisperse and unstable in water. There were no smaller particles or protrusions on the surface of the particles and the particles had a spherical shape and a smooth surface.

Particle size(nm) Volume/seed volume Volume/reference volume

1.10 _±0.05 7.0 _±1.0 1

1.30 ±0.05 11.0 ±1.7 1.7 ±0.4

1.50 ±0.05 17.4 ±2.6 2.5 ±0.5

PS2CTPM3(Figure 4.2.4c)

Large flocks of particles had formed in the dispersion. The particles were very polydisperse. The sizes of the particles ranged from 0.7 to 2 µm. The particles were spherical and had a smooth surface.

PS3CTPM1(Figure 4.2.4d)

The particles have a low polydispersity, but are unstable in water. The diameter of the particles was determined to be 1.38±0.02 µm using the SEM images. There were also some smaller particles formed on the particle surface.

Particle size(nm) Volume/seed volume

25 µm (a)I=0.4M 25 µm (b)I=0.9M 25 µm (c)I=1.2M 15 µm (d)I=2.9M

Figure 4.2.5: Light microscope images of the salted out cross-linked PS particles for three different ionic strengths and a waiting time before quenching of 15 min. For ionic strengths of 0.4to1.2M, the amount of clusters and the cluster sizes are comparable. However, at an ionic strength of2.9Mthe formation of larger clusters was observed.

served, with some dumbbells and very few larger aggregates as well. Increasing the waiting timeTw before quenching did not lead to

the formation of larger clusters. At I =0.9M,

it can be seen that increasing the waiting time Tw seems to lead to a slight increase in the

cluster size, but this is statistically irrelevant because the error is too high to confirm this effect. At I = 1.2M, it can be seen that increasing the waiting timeTwdid not increase

the amount or size of the clusters, although at Tw =30 min, clusters of 6 and 7 colloids were

observed, which was not observed at a shorter waiting times.

The reason that the average cluster size does not increase with longer waiting timesTwbe-

fore quenching may be that the particles are at least partially sterically stabilised. During the cross-linking step, PVA was added to sta- bilise the emulsion droplets. Some of this PVA may have been co-polymerised into the polystyrene shell of the colloid. This may cause the colloids to be sterically stabilised as well as charge stabilised.

Using the same data, the different ionic strengths were also compared at the same waiting time and the results can be seen in Figure 4.2.7. For a waiting timeTw = 5 min,

it can be seen that at higher ionic strengths of 0.9M and 1.2M, less single colloids were observed than at an ionic strength of 0.4M. At these ionic strengths, there were also more clusters of two and three colloids. Larger clusters than three were very rare. For a waiting timeTw=10 min, it can be seen that the distri-

bution of the cluster sizes is largely the same as for Tw = 5 min, but at an ionic strength of

1.2M, there is an increase in the number of clusters of four colloids. For a waiting time Tw =15 min, at ionic strength of 0.9 and 1.2M

there are less single colloids and more larger clusters compared to the shorter waiting times 5 and 10 min.

The reason that increasing the ionic strength led to the formation of larger clusters may be that the geometric configuration of the (suspected) copolymerised PVA strands may change under influence of the ionic strength. This might happen when the strands become charged in water because of the hydroxyl group. Which changes the inter-particle dis- tance at which the PVA stops the colloids from aggregating. This could be confirmed by increasing the ionic strength even more, to see whether there is an ionic strength at which the dispersion becomes completely unstable. Also, a different stabiliser for the swelling emulsion could be used to prevent any steric stabilisation. A reasonable first candidate for this would be SDS, because it can washed off with water more easily that PVA, because the PVA is partially embedded in the polymer surface.

Salting out of DPPS particles

The DPPS particles were also salted out. Here, primarily single colloids and a very low number of dumbbells were formed. No larger clusters than dumbbells were observed. The particles are therefore very stable in water, even at an ionic strength of 1M. This was expected, because the DPPS particles are sterically stabilised by PVP. This prevents the particles from clustering, similar to the cross-linked PS particles.

4.2.3 Swelling with toluene

When the mixture of PS clusters and toluene was heated for approximately 15 min, large flocks were observed by eye in the toluene phase floating above the water phase. This indicates that the particles are completely unstable in toluene. The Magsphere PS colloids

1 2 3 4 5 6 7 8 colloids/cluster 0 10 20 30 40 50 60 70 80 90 % of total # colloids

I

=

0.4

M 5 min 10 min 15 min 1 2 3 4 5 6 7 8 colloids/cluster

I

=

0.9

M 5 min 10 min 15 min 1 2 3 4 5 6 7 8 colloids/cluster 0 10 20 30 40 50 60 70 80 90 % of total # colloids

I

=

1.2

M 5 min 10 min 15 min

Figure 4.2.6:Statistical analyses of the cluster sizes after salting out and quenching of the cross- linked PS colloids. Three different ionic strengths and waiting times were used. Increasing the waiting timeTwhas no effect on the size distribution.

I =0.4M: Mostly single colloids and some dumbbells were observed. I =0.9M: Single colloids, dumbbells and trimers were observed.

I =1.2M: Single colloids, dumbbells, trimers and some clusters of four colloids were observed.

colloids/cluster 0 10 20 30 40 50 60 70 80 90 % of total # colloids

T

=

5 min

0.4M 0.9M 1.2M colloids/cluster

T

=

10 min

0.4M 0.9M 1.2M 1 2 3 4 5 6 7 8 colloids/cluster 0 10 20 30 40 50 60 70 80 90 % of total # colloids

T

=

15 min

0.4M 0.9M 1.2M

Figure 4.2.7:Statistical analyses of the cluster sizes after salting out and quenching of the cross- linked PS colloids. Three different ionic strengths and waiting times were used.

T = 5 min: At ionic strength of 0.9 and1.2M, less single colloids and more dumbbells and trimers were observed than at an ionic strength of0.4M.

T = 10 min: The cluster size distribution is largely the same as for 5 min, but for an ionic strength of1.2M, there is an increase in the number of clusters of four colloids.

T =15 min: Compared to the shorter waiting times, at ionic strength of0.9and1.2Mthere are less single colloids and more larger clusters.

15 µm

(a)Light microscope image

10 µm (b)SEM image

Figure 4.2.8:Microscope images of the toluene-swollen clusters.

(a)Optical microscope image of the clusters. Single colloids and clusters of two, three and at least six colloids can be seen.

(b)SEM image of the clusters. No swollen or fused clusters were observed. did not form these large aggregates during

the swelling with toluene. This might be because the Zeta potential of the Magsphere PS colloids is twice as high as the Zeta potential of the PS3C1 particles. Also, toluene is not a good solvent for PVA. Therefore, the steric sta-

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