Preparation And Properties Of Polyelectrolyte Microcapsules Fabricated By In-situ Coacervation

Microcapsules have been widely used in many fields such as pharmaceutical, cosmetic,food as well as agricultural industries.Nowadays,more intense requirements have been proposed upon microcapsules.There is a boost in the fundamental investigation and real application of hollow polyelectrolyte microcapsules fabricated by Layer-by-Layer(LbL)technique these days due to its advantages such as well-controlled size and shape,finely tuned wall thickness,etc. Functional multilayer microcapsules can also be fabricated by introducing functional "building blocks" into the microcapsules.But multilayer microcapsules also have the disadvantages of low efficiency and time consuming.In this work we focus on finding a simple and efficient rout to fabricate polyelectrolyte microcapsules while maintaining the advantages of LbL type multiplayer capsules,and subcequently imposing this kind of microcapsule with new functions by either introducing novel functionalized polyelectrolytes or revising the fabrication route.Firstly,in order to overcome the tedious and time-consuming fabrication process of LbL assembly,a facile pathway to fabricate microcapsules by in situ polyelectrolyte coacervation on poly(styrene sulfonate)(PSS)doped CaCO₃ particles are proposed.After only one layer of poly(allylamine hydrochloride)(PAH)was adsorbed on the PSS-doped CaCO₃ templates,intact complex microcapsules are successfully obtained after core-removal by disodium ethylene diamine tetraacetate dihydrate(EDTA).UV-vis,Raman,scan force microscope andζ-potential analysis confirme the fabrication mechanism of this kind of microcapsules.There is no PSS left in the capsule interior and the capsule wall is composed of random polyelectrolyte complex.The complex microcapsules have good thermal stability,but the elasticity modulus detected by osmotic induced invagination method is 140 Mpa,only half of the value(290 MPa)of the orderly assembled PAH/PSS microcapsules.After assembling one layer of PAH-g-PNIPAAm on the template,microcapsules containing PNIPAAm were obtained.Compared with ordinary microcapsules,this kind of microcapsules shows better base stability at high temperatures than PSS/PAH complex microcapsules,and the release rate decreases when the temperature reach above the lower critical solution temperature(LCST)of PNIPAAm.Since the capsules fabricated by polyelectrolyte coeacervation has a relatively low mechanical strength,we crosslink the microcapsules by glutaraldehyde(GA). UV-vis spectrum confirms that the crosslinking reaction proceeds fast at the beginning and then slows down.The crosslinking degree can be tuned by the crosslinking time. Tthe elasticity modulus of the microcapsule walls is found to 200MPa after 2h crosslinking,which are 1.4 times larger than that of the original one(140MPa).The crosslinked microcapsules also show good stability in 0.01M NaOH solution,and poorer permeability for large fluorescent probe.Finally,microcapsules with charge-controlled permeation for electrolytes are fabricated by an in situ coacervation method in the existence of polyelectrolyte during core removal.A layer of PAH was adsorbed onto PSS-doped CaCO₃ microparticles, followed by crosslinking the PAH layer with GA.After treated with PAH,the crosslinked particles were subjected to core removal by EDTA in the presence of PAH. This kind of microcapsules show a unique feature of charge-controlled permeation for electrolytes:negatively charged probes such as fluorescein sodium and even fluorescein isothiocyanate labeled dextran were completely rejected,but positively charged ones such as rhodamine 6G(Rd6G)and tetramethylrhodamine isothiocyanate labeled dextran(TRITC-dextran)were attracted.The same phenomenon was observed for microcapsules obtained by using PAH-g-PNIPAAm as the polycation,and the release rate of this kind of microcapsules decreases when the temperature reach above the LCST of PNIPAAm.