Roles of Proton in the Formation of Particles: Soap‐free Emulsion Polymerization of Styrene using AIBN and Potassium Persulfate as Initiators

Abstract

In this paper, the hydrophobic effect or hydration of hydrophobe was firstly introduced into the mechanism of polystyrene particles formation. Negatively charged surface of hydration shell was proposed due to the ordered structure of water molecules. Consequentially the concentration of hydrophobic chains depended on pH because protons destroyed the hydration shell, thereby extruded hydrophobic chains from the H-bonds networks. The soap-free emulsion polymerization of styrene by using AIBN (azodiisobutyronitrile) and KPS (Potassium Persulfate), respectively was performed at various pHs. It was proved that protons played a crucial role on the formation of PSt (polystyrene) particles. By using AIBN, particles were not prepared at pH 2.0 (HAc), pH 4.0 (HCl) and pH 2.0 (HCl), whereas by using KPS, particles were obtained regardless to pH. It indicated that anions of sulfate greatly enhanced the tolerance of hydration shell to the suffering from protons. Meanwhile, the roles of polymerization in the monomer phase, aqueous phase and growing particles were elucidated. By using AIBN, ca. 30 % PSt with M n (Numeral average molecular weight) = 300-1500 formed in the monomer phase at pH 6.8, which dissolved in water at pH 8.0 and pH 10.0. By using KPS, ca. 20 % and 30 % conversion of St were found in the aqueous phase at pH 2.0 (HCl) and pH 10.0 (NaOH), respectively. At pH 2.0 (HCl), oligomers generated in the aqueous phase were protonated, whereas at pH 10.0 (NaOH), they were soluble due to the seriously suppressed dissociation of water. At other pHs, oligomers captured protons, thereby relieved particles and interface from the suffering of protons. As the concentration of oligomers increased, oligomers precipitated to form clusters due to the increase of average number of protons on the surface of oligomers hydration shell, which successively incorporated into the growing particles, thus enlarged PDI (Polydispersity index) of MW (Weight-average molecular weight) distribution in particles. By using AIBN, owing to the limited partition of AIBN in the interfacial layer of St phase, the flat-roof peaks of GPC curves were observed, namely that the weight fraction of M i was constant. It implies that the freeradicals are living in the growing particles.

Roles of Proton in the Formation of Microspheres: Precipitation Polymerization of Styrene in Alcohols by Using AIBN as Initiator

“…Here, it should be remarked that the deposited PSt quickly re-dissolved soon after DC was deposed. This result is the basis that we postulated the theory of hydration cage incurring entropic charge in the previous paper [35] and alcoholization cage incurring entropic charge that will be discussed in this paper.…”

“…However, the percentage of low MW portion was about 8.7 %, much smaller than ∼ 30 % in water (Table 2), and also the number-averaged molecular weight was 32,750, larger than 24,100 in water. As we discussed in the previous paper, [35] the smaller MW (1,500-300) was ascribed to the high concentration of AIBN in the monomer phase. Hence, these results indicated that the concentration of AIBN in the monomer phase was 2, aside the shape of GPC diagrams and the smaller MW in MeOH, EtOH and IPA, the main MW was ∼ 4,400, ∼ 5,000 and ∼ 6,600, which were much smaller than MWs (∼ 24,000 to 33,000) in water and EG, respectively.…”

“…An concept of surplus negative charges (entropic charges) is suggested incurred by the cage structure of hydration shell, thus the hydration shell of hydrophobe possesses an affinity for protons that are the destroyers of cage constituted by hydrogen bonds (HD). [35] It is supported by the slow dynamics of HD in the hydration cage. However, because the structure of cage is dynamic, the affinity does not mean that the proton will be fixed on the cage by adsorption, but the probability that protons appear on the cage is higher than that anywhere of bulk water.…”

“…The reason of flat-roof like peak has be interpreted in the previous paper. [35] It indicated that EG is similar to water due to the low pK a . However, the percentage of low MW portion was about 8.7 %, much smaller than ∼ 30 % in water (Table 2), and also the number-averaged molecular weight was 32,750, larger than 24,100 in water.…”

“…Inspired by the positive charges of the amphiphobic segments in the diblock copolymers, [33,34] in the previous paper, [35] a new principle for the stabilization of polymeric particles/microspheres was proposed on the basis of achievements in the water dynamics and hydration of hydrophobic molecules. An concept of surplus negative charges (entropic charges) is suggested incurred by the cage structure of hydration shell, thus the hydration shell of hydrophobe possesses an affinity for protons that are the destroyers of cage constituted by hydrogen bonds (HD) [35] . It is supported by the slow dynamics of HD in the hydration cage.…”