3.1. Properties of Azidated Glycerol
To produce AG, a chemical reaction of two steps was involved; namely, tosylation and azidation reactions. Glycerol is a polyhydric alcohol, and by having three free hydroxyls (OH) groups, it can undergo many chemical reactions, and some compounds can be used to produce other derivatives. It has been found that the alcohol hydroxyl group (poor leaving group) can be converted into a good leaving group by replacing the hydroxyl group with a tosyl group [
13,
14,
15]. Para-toluenesulphonyl chloride (p-TsCl) has been widely used as a tosylating agent over tosyl anhydride and p-tolunenesulphonic acid [
16]. p-TsCl contains sulphur which is bound to two oxygens and a chlorine. Oxygen and chlorine are more electronegative than sulphur, so sulphur has a partial positive charge. Meanwhile, the oxygen in the hydroxyl group of glycerol has a partial negative charge, so it is attracted to the sulphur atom. Therefore, the oxygen atom will attack the sulphur atom, and the chlorine atoms are removed from the p-TsCl. However, hydrogen atoms are still attached to oxygen and usually, triethylamine (TEA) is used as a strong nucleophilic base. TEA will accept hydrogen atoms and form an amine salt (by-product). Additionally, at the end of the reaction, the final product produced is called tosylated glycerol. The mechanism of the tosylation reaction is illustrated in
Figure 2.
After then, the tosylated glycerol was reacted with sodium azide (NaN
3); this chemical reaction is called an azidation process. The azidation mechanism to produce AG from tosylated glycerol is shown in
Figure 3. This reaction involved the replacement of the tosylate leaving group with the azide ion (N
3−) in sodium azide to produce an azido compound. The final product, which is AG, is the main material that will be used as a stabilizer in the NRL colloid. Some of the chemical properties were characterized to observe their properties and how they affect the colloid stability of NRL.
The azide group induces a dipole moment by partial displacement of the nitrogen (N) atom, which is the most electronegative in the AG molecule. The AG produced from the azidation process is neutral in pH due to the balance of nitrogen atoms charges (+ve and –ve charges), but it is a polar solvent.
The presence of a negative (−ve) charge on one of the N atoms in the AG is highly attractive to the positive (+ve) charge in the stern layer of a latex particle, thus forming a dipole moment interaction. This dipole moment interaction is crucial because it ensures that the AG molecules form a layer around the latex particles. This layer increases the colloidal stability of the latex. During freezing, the whole latex will be frozen because the water in the latex serum is frozen.
On the other hand, the water entrapped by the AG around the latex particles will not be frozen due to the anti-freezing properties of glycerol. However, during thawing processes, the frozen latex serum will thaw and the latex particles will be released into the latex without undergoing coalescence. Thus, the frozen latex is said to be regenerated again on thawing. The AG functions not only as a stabilizer but also as an anti-freeze agent. For another explanation, the mechanism of the stabilization of latex particles by an ionic stabilizer during freezing has been explained by Cockbain and co-workers, 1969 [
7].
3.1.1. FTIR
The presence of different functional groups of glycerol, tosylated glycerol, and azidated glycerol were analysed by FTIR and their transmittances were compared in
Figure 4. In general, a broad peak at the wavenumber range 3000–3700 cm
−1 detected the presence of the hydroxyl group for all materials. The TSC of AG produced in this study was 80%, so the transmission intensity of the AG spectrum around 3500 cm
−1 was higher than glycerol due to the other 20% of the aqueous medium containing an OH group. After modification with tosyl chloride, the absorbance peak in the range of 1150–1085 cm
−1 is related to C–O, as the C–OH of hydroxyl stretching in glycerol has been replaced with C–O stretching of the aliphatic ether in tosylated glycerol. After the modification of tosylated glycerol with sodium azide, the appearance of a peak in the range of 2160–2120 cm
−1 corresponded to the N=N=N stretching, thus confirming the introduction of an azide group into the AG molecule.
3.1.2. Morphology of AG
The morphological behaviour of semi-crystalline structures of AG is shown in
Figure 5. AG is a highly water-soluble suspension when mixed with water. However, at 80% (highly concentration) TSC, it will form aggregations that result from the high density of OH groups in AG molecule; therefore, it has a high tendency to form strong associations with itself by hydrogen bonding forces, as shown in the optical microscope image in
Figure 5b.
3.2. Effect of Variable Loading of AG on the Colloidal Properties f NRL before and after the Freeze–Thaw Processes
Figure 6 shows a TEM image of the NRL particles at 0 phr of AG (NRL without stabilizer). It is observed that grey rings, which are the membrane layers derived from protein lipid, surround the particles. The rubber particles are believed to be covered by some proteins and phospholipids, concerning the colloidal stability of natural rubber latex [
17]. Phospholipids are strongly adsorbed to the surfaces of the rubber particles and are believed to be the intermediate by which the proteins are anchored to the rubber particles [
18].
The TEM images of AG-stabilized NRL particles before freezing with variable loadings of 0.1 phr, 0.2 phr, 0.3 phr, 0.4 phr, and 0.5 phr are shown in
Figure 7. The particles can be differentiated with two different colour tones which are darker grey and light grey. The darker particles are the latex rubber particles due to the staining process before testing. Osmium tetroxide stains the double bonds of the rubber molecules; hence, all of the rubber particles appear black/darker. Since the latex particles were all stained under the same conditions, the difference in morphology as revealed by micrographs can be attributed to the presence of latex rubber particles with a small percentage of AG at lower loading. Meanwhile, the presence of AG molecules (light grey region) was observed at higher AG loadings. This finding provides fresh evidence of the presence of adsorption of AG molecules around the latex particles. The obvious presence of AG molecules was observed starting at 0.2 phr of AG loading. It showed that the AG molecules started to encapsulate the latex particles even at low stabilizer loading. On the other hand, we also observed the formation of interparticle bridging of AG molecules in between the rubber latex particles at 0.4 phr and 0.5 phr of AG loading, as shown in
Figure 7d,e.
A clear interparticle bridging of AG molecules can be seen at higher AG loadings (0.5 phr), as shown in
Figure 7e. The latex rubber particles are obstructed from being close to each other by the interparticle bridging of AG molecules. Interparticle bridging has also been observed in colloid systems with high stabilizer concentrations [
19]. It is noted that the AG structures were aggregated together to form interparticle bridging, thus acting similar to a barrier obstructing the close approach of latex rubber particles.
Interparticle bridging by adsorbed molecules gives rise to a strong association between particles. For interparticle bridging to occur, a single molecule must become simultaneously adsorbed on the surfaces of two neighbouring particles. This may occur if the molecule is of sufficiently high molecular mass to bridge the gap between particles, as well as to become firmly adsorbed at the particle surfaces. However, it is also possible that interparticle bridging will occur by association in the interparticle region between separate molecules, some of which are adsorbed on one particle and some on another. In any event, molecules must become adsorbed on the particle surfaces. This implies that the particle surfaces have vacant adsorption sites, or that the adsorption tendency of the molecule is sufficient to displace some of the molecules already adsorbed at the surface. The immediate consequence of interparticle bridging by an adsorbed molecule is effectively that each particle is immobilized relative to its neighbour, and thus it imparts a structure to the latex [
3].
Figure 8 shows the TEM images of latex particles containing variable loadings of AG after the freeze–thaw processes. From the observations, the morphological behaviour of the regenerated latex containing AG molecules was not affected much by freezing (compared with TEM images of NRL before freezing in
Figure 7). This observation suggests that the freezing condition does not much affect the morphological behaviour of NRL containing AG as a stabilizer system. AG molecules are able to protect the latex colloid during freezing and after the freeze–thaw processes.
Figure 9 shows the mean diameters of the stabilized latex particles before freezing and after the freeze–thaw processes with variable loadings of AG. In general, the mean diameters of stabilized latex particles after freeze–thaw processes were higher compared to the stabilized latex particles before freezing. Under controlled conditions, some of the latex particles coalesced during freezing, and after the thawing process, these coalesced particles did not separate, thus increasing the mean diameter of the latex particles.
Table 2 shows the zeta potential values of NRL containing AG as a stabilizer system before freezing and after the freeze–thaw processes (regenerated NRL). In general, the zeta potential value is used to predict the stability of colloids such as NRL when stabilizers are added. The zeta potential of NRL is always negative due to the negative charge of proteins and carboxylic groups surrounding the rubber particles. NRL without stabilizers (0 phr) has a zeta potential value of −1.88 mv; an increase or decrease in this value indicates coalescence or repulsion, respectively, among the rubber particles. Nanoparticles with a zeta potential between −10 and +10 mV are considered approximately neutral. If there was no stabilizer present in the NRL (0 phr), this indicates that NRL with zeta potential of −1.88 mV was not yet deprotonated [
20].
After freeze–thawing, the neutral charge of NRL without a stabilizer (0 phr) (zeta potential value between −10 mV and +10 mV, in this case the zeta potential is −1.88 mV) may indicate the inability to maintain latex stabilization after the freeze–thaw cycle, as shown by the presence of a big lump in
Figure 10. At 0.1 phr of AG, the latex that was anionically stabilized before freezing has become neutral after the freeze–thaw cycle (zeta potential value between −10 mV and +10 mV). As the AG was increased to 0.2 phr, more AG can stabilize the particles anionically, albeit there is still a reduction in the zeta potential after the freeze–thaw cycle (from −59.11 mV to −56.27 mV). Ammonia presence however was not enough to deprotonate the AG at high phrs of AG (0.3 and above). For AG at 0.4 and 0.5 phr, stabilization, however, was provided by the possible bridges formed between particles by aggregating AG, as shown by the TEM images in
Figure 7 and
Figure 8.
A loading of 0.2 phr AG gave the best results before freezing and after the freeze–thaw processes, as it had zeta potentials of −59.63 mV and −56.27 mV before freezing and after freeze–thawing, respectively (nanoparticles with zeta potentials of greater than +30 mV or less than −30 mV are considered strongly cationic or strongly anionic, respectively).
As a control, the NRL without any stabilizer was frozen and thawed. After thawing, it was observed that a big lump of rubber was formed, as shown in
Figure 10. The formation of the rubber lump is due to the coalescence of NRL particles after the freeze–thaw process. During freezing, the aqueous medium of NRL is frozen and ice crystals form. NRL particles are trapped in these crystals and are forced to come closer together, thus disrupting the protein cloud surrounding the NRL particles. This broken protein cloud makes the NRL particles more prone to coalesce and form a big lump of rubber after thawing.
Figure 11 shows the colloidal properties of NRL containing variable loadings of AG before freezing and after the freeze–thaw processes. In general, addition of AG into NRL was not significant in influencing the TSC of the regenerated latex (freeze–thaw latex). The same trends also are present in the TSC after the freeze–thaw processes.
Figure 12 shows the effect of variable loadings of AG on the pH of NRL before freezing and after the freeze–thaw processes. The pH value of pure glycerol is 5.08 (acidic), meanwhile for AG (80% TSC AG), it is 7.15. On the other hand, the pH value of the 50% diluted NRL before adding any stabilizer was 9.85. The results shown in
Figure 12 indicate that the addition of AG did not affect the pH values of the colloid, even at higher loadings. The pH values were slightly reduced after the freeze–thaw processes, which may due to disruption of the protein cloud that influences the negative charge of latex particle.
Figure 13 shows the mechanical stability time (MST) of NRL containing variable loadings of AG before freezing and after the freeze–thaw processes. In general, increasing AG loading up to 0.5 phr slightly increased the MST from 699 s to 828 s for 0.1 phr to 0.4 phr, respectively. This may be due to the interaction of a negative charge of the N
3 of the azide group in the AG molecule being more attracted to the positive charge of the stern layer and not disrupting the proteins around the latex particles, thus protecting the rubber latex from faster flocculation. The rates of change in the MST of the regenerated NRL containing AG for each loading was low, especially from 0.2 phr to 0.5 phr of AG. These results show that AG was able to maintain colloid stability after the freeze–thaw process by a low rate of changes in the MST. The formation of a thicker water-bound layer surrounding the latex particle after the addition of AG was able to maintain the colloid stability. This result is explained by the interaction of AG with the nitrogen atoms of proteins surrounding the latex particles. The presence of a thicker/higher water-bound layer able to protect the latex particles at lower temperatures thus maintains the stability of the regenerated NRL colloid at the higher speed of shear.
The viscosity of NRL containing variable loadings of AG as a stabilizer is lower compared to NRL without AG (0 phr). According to Blackley (1997) [
3], the addition of a stabilizer was to slow down the increases in the viscosity of the NRL while mechanical agitation continued. Additionally, the slight increase in latex viscosity while increasing AG loading provides evidence of the effects in the interfacial region between the particle and the aqueous phase. The interparticle bridging by AG molecules at higher loadings can increase the viscosity of the latex due to the dispersion medium being entrapped in the particle agglomerates, thereby increasing the volume fraction of the dispersed phase in the latex [
3].
The viscosity of NRL colloids containing 0.4 phr and 0.5 phr of AG after freeze–thawing reduced compared to the NRL before freezing, as shown in
Figure 14. This behaviour shows that at higher AG loadings, the particles of regenerated NRL are free to move in the colloid without any restrictions.