Advanced polymer nanocapsules with enhanced capabilities for controlled-release of payloads
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Abstract
The main objective of this thesis was to prepare polymer nanocapsules with improved capability for controlled-release of payloads. To fulfill this objective, new strategies were developed to (i) hinder non-controlled leakage and (ii) enhance controlled-release of payloads from these nanocapsules. Nanocapsules with either oily or aqueous core containing hydrophilic or hydrophobic payloads were prepared by miniemulsion technique.
In the first part (section 4.1), the release of hydrophilic payloads from polyurea nanocapsules was programmed by varying the osmotic pressure in the core of these nanocapsules. The results exhibited that the initial release of the payloads from the aqueous core was largely dependent on the concentration of osmotic pressure agents. The analysis showed that the correlation between the release of the dye and the concentration of the co-encapsulated salts (i.e. osmotic pressure agents) is non-linear. Further measurements displayed that the swelling of nanocapsules is the responsible mechanism for the observed differences between the release profiles of the dye from the nanocapsules loaded with osmotic pressure generating species and non-loaded ones. To diminish the burst release of the payload, nanocapsules with crosslinked shell were synthesized using crosslinker. Crosslinking had an influence on shell rigidity and, thus, decreased release kinetics by obstructing the swelling of nanocapsules despite of an osmotic pressure. Finally, we illustrated how this strategy can be utilized to induce triggered release or sustained release profiles through two different applications.
In the next set of experiments (section 4.2), the non-controlled leakage of hydrophilic payloads from the core due to osmotic pressure difference could be suppressed by varying the physicochemical properties of nanocapsule membranes thereby decreasing release kinetics. The shell thickness of polyurea nanocapsules was tuned by changing the monomer concentration used for the preparation of these nanocapsules. A significant increase of shell thickness could largely decrease the non-controlled leakage of the hydrophilic payloads. In addition, this significant increment of shell thickness slowed down the release kinetics and thereby suppressed a precisely targeted delivery of the payload in desired time. Therefore, reduction-responsive unit were introduced into the thick shell to enable triggered release of the payload. Triggered-release of the payload was successfully upon application of reducing agent. In this work, the concept of stimulus-induced release of hydrophilic payloads from nanocapsules was demonstrated for redox-responsive nanocapsules.
The concept of using block copolymers responsive to one stimulus for the preparation of stimulus responsive capsules was extended to triblock terpolymers responsive to three independent stimuli (section 4.3). Novel triple stimuli-responsive nanocapsules consisting of PVFc-b-PDMAEMA-b-PMMA triblock terpolymer were prepared by the solvent evaporation process from miniemulsion droplets. The nanocapsules could encapsulate the dye in the core and perfectly protect it from non-controlled leakage. The triggered release of the payload was achieved because the nanocapsules shell was addressable by three different stimuli: pH change, oxidizing agent, and temperature. Alternatively, nanocapsules were synthesized with a mixture of responsive diblock copolymers, where their shell had the same chemical composition in terms of molar amount of blocks with nanocapsules synthesized from triblock terpolymer. However, the microphase separation between the responsive diblock copolymer across the shell hampers the release performance of the payloads from the nanocapsules. This configuration of diblock copolymers across the shell, thus, made these nanocapsules stimulus-responsive instead of triple stimuli-responsive.
Finally, the concept of utilizing pro-active payloads in order to significantly increase the selective release of small size payloads from mesoporous nanocapsules was demonstrated (section 4.4). Silica nanocapsules were synthesized by the miniemulsion technique. The non-controlled release of payload largely decreased the efficiency of these silica nanocapsules for the precise delivery of the payload. To overcome the problem of non-controlled release, a pro-active payload was synthesized. A pro-active payload is defined as a compound that is converted to an active functional molecule in the environment where it is needed. The pro-active payload could not diffuse through the silica shell due to its larger molecular size compared to the payload. The combination of the encapsulation of the pro-active payload and the responsive behavior of the silica nanocapsules led to significant increase in the final released amount of payload while almost the entire payload in the release medium could be selectivity delivered from the silica nanocapsules.