The Decomposition Process of Melamine Formaldehyde Cores: The Key Step in the Fabrication of Ultrathin Polyelectrolyte Multilayer Capsules

Melamine Formaldehyde Core Decomposition as the Key Step Controlling Capsule Integrity: Optimizing the Polyelectrolyte Capsule Fabrication

“…[8,20] During the core-decomposition process, the concentration of the decomposition products inside the capsules becomes almost instantaneously quite large, leading to an osmotic pressure difference between the bulk and the capsule interior that is the driving force causing capsule swelling. The counterion pairs between the oppositely charged polyelectrolyte molecular chains provide a hyper-crosslinked multilayer structure.…”

“…A larger permeability would be favorable for releasing the tension. As indicated before, [20] FCS yielded similar diffusion coefficients, D, when MF cores reacted with HCl at pH values ranging from 0.7 to 1.5.According to P = D/d, where P is the permeability coefficient and d is the capsule wall thickness, MF decomposition products produced at pH 0.7-1.5 should have similar permeation through the capsule wall. However, at lower pH value, e. g., pH 0.7, a very large core-decomposition rate is expected.…”

“…Instead of decomposition, MF resin undergoes a further polymerization on exposing to acid solution with a pH value higher than 1.7. [20] It is clear that acid can catalyze both the decomposition of lNCH 2 OCH 2 Nl linkages to form soluble oligomeric products, lNCH 2 OH, and, further, the polymerization of either of the oligomeric products or lNCH 2 OCH 2 Nl to form lNCH 2 Nl linkages which cannot be degraded by acid anymore. The rates of these competing reactions depend on the pH value, i.e., decomposition will surpass polymerization at lower pH value resulting in total core decomposition.…”

“…By using Route 1, i. e., when aliquots of the same amount of the coated core suspension is injected into a large amount of HCl solution, the MF cores are exposed to the optimal pH value and are decomposed immediately. Due to the large ratio of acidic groups to cores, the pH value remains almost constant during the entire core-removal process, which lasts generally around 20 s. [20] Therefore, this sequence can produce intact capsules with good quality. However, when Route 2 is employed, i.e., adding a large amount of acid solution to the same amount of coated Scheme 2.…”

“…If these entities do not permeate through the wall sufficiently quickly, the osmotic pressure may increase above rupture limit and the capsules may break. [20] Even if they do not break, the deposition of core material at the wall has to be controlled, [6,20] because this may affect its physicochemical properties. This problem is especially serious since typically 100 times more core than wall material is initially present.…”