Description:TITLE OF THE INVENTION

“A PROCESS FOR SYNTHESIZING ALUMINIUM ENCAPSULATED FERROCENYL HOLLOW POLYMER NANOCAPSULES”

FIELD OF INVENTION
[0001] The present invention relates to the field of advanced materials and nanotechnology. More specifically, the invention relates to a process for the synthesis of aluminium encapsulated ferrocenyl hollow polymer nanocapsules.
BACKGROUND OF THE INVENTION
[0002] In the realm of materials science and nanotechnology, there is a persistent demand for innovative approaches to synthesize functional nanomaterials with enhanced properties and applications. Among these materials, nano-catalysts, particularly metal nanoparticles, are at the forefront of advancements in heterogeneous catalysis due to their highly active surface area and unique electronic properties. These catalysts play a crucial role in facilitating various chemical reactions, offering enhanced reaction rates, selectivity and efficiency compared to their bulk counterparts. The high surface area-to-volume ratio of metal nanoparticles ensures that a larger fraction of atoms are available for catalysis, significantly boosting their activity.

[0003] Moreover, an appealing and efficient approach has been followed significantly to enhance the efficacy of a metal nanoparticle which is to encapsulate them and that becomes highly advantageous amidst harsh conditions by diminishing the possible aggregation and sintering of the particle. Several reports are there presenting encapsulated metal-based nano-catalysts preparation and among themselves accomplishing an encapsulated structure of nano-aluminium catalyst is highly desirable to prevent them from the reduction of their catalytic activity in real-world applications.

[0004] In addition, Aluminium nanoparticles (Al-NP) always remain a subject of interest in a variety of fields like pyrotechnics, propellants, and explosive industries (Prog. Energy Combust. Sci. 2016, 57, 75–136) because of their high combustion enthalpy, easy availability, low toxicity, and good stability. Al-NPs in the form of powder or flakes can be used to increase the energy and flame temperature in rocket propellants and enhance air blasts when added to the propellant formulation (Defence Technology 2018, 14, 5, 257-265). However, the small sizes of aluminium nanoparticles make them susceptible to excessive oxidation due to the deposition of a thick layer of oxide coating of Al2O3 ranging from 1.7 nm to 6 nm in size (Orient. J. Chem. 2014, 30, 4, 1941-1949). Thus, an effective passivation of Al-NP is needed nowadays but the success is quite limited.

[0005] Nyapete et al. disclosed an in-situ synthesis of aluminium nanoparticles inside the cavity of hollow polymer nanocapsules with the help of a novel titanium (IV) benzyloxide initiator that was synthesized and trapped inside the nanocapsule.

[0006] Stratakis et al. disclosed the generation of aluminium nanoparticles via the ablation of bulk Al in liquids with short laser pulses. The synthesized aluminium nanoparticles had an average size ranging from 10 nm to 60 nm depending on the experimental conditions.

[0007] Murlidharan et al. disclosed the isolation and stabilization of aluminium nanoparticles by the room temperature reaction between SiCl4 and LiAlH4 in the presence of poly(vinylpyrrolidone) (PVP) under sonication.
[0008] Numerous studies have explored different methods for the synthesis and stabilization of aluminium nanoparticles, which are particularly attractive due to their abundance, low cost, and excellent catalytic properties. Key references and patents in this area include:

[0009] US9011572B1 discloses the stabilization of aluminium nanoparticles after the decomposition of an alane precursor in the presence of a catalyst and an organic passivation agent and show great stability of the particles in the air but they are highly reactive with water; later they produce hydrogen by reaction of aluminium particles with water.

[0010] WO2012047345A2 discloses a method for preparing a core comprising Al-NP with the following passivation of an epoxide-based oligomer coating.

[0011] Propellants are the fundamental source of supplying chemical energy in rockets and burn rate catalysts (BRCs) are the most important components of propellants where ferrocene-based materials act as an active and best choice as burn rate catalysts because of their good compatibility with an organic binder, better ignitability with the propellant and good homogeneities in distribution (Journal of Organometallic Chemistry 2018, 872, 40-53). Generally, ferrocene-based polymers enhance the burn rate of propellants and perform excellent thermite reactions with aluminium powder (J Appl Polym Sci. 2011, 119, 5, 2517-2524). As compared to raw nanoaluminium (nAl) powders, ferrocene-based nAl composites help in catalyzing ammonium perchlorate (AP) decomposition, improving the performance of thermal and combustion reactions as well. Therefore, a polymer composite material containing ferrocene as a burn rate catalyst and aluminium as a metallic fuel is the need of the hour for enhancing the efficiency of rocket composite solid propellants. Hu et al. (Chem. Eng. J. 2020, 394, 124884) synthesized fluoro-containing ferrocene compounds and coated them on the surface of nanoaluminium to enhance performances in fields like anti-immigration and better combustion.

[0012] Hollow polymer nanocapsules (HPNs) have been already widely subjected to numerous applications in a multitude of arenas like the biomedical field, nanocatalysis, controlled-drug delivery models, nanoreactors, etc reason being their excellent physical properties; herein, we aim to explore them to encapsulate a metallic fuel within the cavity of a burn rate catalyst and use them as an additive in CSPs. Synthesis of a hollow capsule at the nano/micro dimension involves sacrificing a template coated with heavily crosslinked stable polymer (Chem. Mater. 2007, 20, 848–858; Macromolecules 2010, 43, 1792–1799) that depends on the choice of the monomer and the crosslinking agent used. One of these syntheses by the colloidal templating method goes back to Caruso’s seminal paper in 1988 (Science, 1998, 282, 1111–1114) where hollow silica and silica polymer spheres of size around 1000 nm were synthesized by assembling silica nanoparticles and polymers on the surface of colloids and then removing the colloidal template by either calcination or exposure to solvents. Besides several reports are there which involve grafting of silica nanoparticle surfaces by surface-initiated LRP techniques to synthesize hollow polymer nanocapsules (HPNs) (Macromolecules 2005, 38, 18, 7867–7871; Acc. Chem. Res. 2014, 47, 1, 125–137).

[0013] Among several references, Voit et al. synthesized hollow polymeric nanocapsules with the size of 450 nm or 900 nm (Macromolecules 2011, 44, 8351–8360) via reversible addition fragmentation chain transfer (RAFT) polymerization technique taking [poly (tert-Butyl methacrylate)–co-poly(2,3-dimethylmaleic imidopropyl methacrylate)-b-poly(2-hydroxypropypl methacrylamide)] (PtBMA-co-PDMIPM-b-PHPMA) as the polymeric shell where DMIPM is the crosslinker which crosslinked upon exposure to UV light making the nanocapsules stable. On subsequent treatment with HF, the silica core got etched out forming stable hollow nanocapsules.

TECHNICAL PROBLEM WITH KNOWN PRIOR ART

[0014] Traditionally, various ferrocene derivatives have been employed as burn rate catalysts (BRCs) in solid propellants. However, these derivatives often result in unstable combustion due to their tendency to migrate and volatilize, leading to storage stability issues such as reduced shelf life, limited storage capacity, and poor aging behavior. Additionally, these BRCs can cause electrostatic discharge during propellant fabrication, posing safety risks.

[0015] Further, Aluminium nanoparticles (Al-NP) are used as metallic fuels to enhance energy and flame temperature in rocket propellants. However, Al-NP tends to form a layer of Al2O3 upon exposure to air and water vapor. Over time, this oxide layer can thicken, reducing the active aluminium content and thereby diminishing the effectiveness of the propellant's steady burning rate.

[0016] In conventional Al/Fc thermites, short-chain ferrocene-based derivatives migrate easily to the propellant surface during storage. Long-chain ferrocene-based derivatives, such as ethylene diamine-based Fc or amine ester dendrimers functionalized Fc, resolve migration issues but often result in non-homogeneous physical mixing with nano aluminium, leading to suboptimal reaction efficiency and energy output.

[0017] Previous inventions have proposed dual-functional additives, such as fluoro-containing long-chain ferrocene-coated nano aluminium (nAl), to address these concerns. However, there remains significant potential to enhance the efficiency of these systems by increasing the ferrocene content and improving the in-situ encapsulation of nAl to achieve higher active aluminium content compared to existing solutions.

[0018] For instance, nanorattles have been developed where a titanium (IV) benzyloxide initiator is synthesized within hollow polymer nanocapsules, and Al-NPs are prepared in-situ within the capsules. However, these nanocapsules, which include hydrophobic acrylate monomers and self-assembled surfactant vesicles, do not contribute to burn rate properties in propellant formulations. Consequently, separate burn rate catalysts are still required.

[0019] Therefore, there is a need for a ferrocene-based burn rate catalyst with enhanced shelf life. The development of a simple, cost-effective, and scalable synthetic method for creating a dual-functional framework—combining both fuel and burn rate catalyst capabilities—with precise control over structural and compositional properties is essential. This would improve efficiency and expand applications, particularly for use in solid composite propellants (CSPs), addressing the issues identified above.

[0020] Additionally, there is a continued need to improve the efficiency of such dual-functional systems to achieve the highest active aluminium content compared to prior art.
OBJECTIVES OF THE INVENTION
[0021] The principal objective of the present invention is to solve the problems of the prior arts.

[0022] An objective of the present invention is to provide a process to create aluminium encapsulated hollow polymer nanocapsules (Al-HPNs) with a ferrocenyl shell and provides variable aluminium (Al) and iron (Fe) content in it.

[0023] Another objective of the present invention is to provide a process to create aluminium encapsulated hollow polymer nanocapsules (Al-HPNs) in a less-toxic, less corrosive and reduced chemical reliance method and with good yield (95%).

[0024] Yet another objective of the present invention is to provide a process to create aluminium encapsulated hollow polymer nanocapsules (Al-HPNs) which can act as burn rate modifier cum fuel with ferrocene acting as burn rate catalyst and aluminium nanoparticle acting as a metallic fuel in composite solid propellants (CSPs) to improve their burn rate.

[0025] Yet another object of the present invention is to provide a process that is simple and scalable, has economic viability, and which can be produced in an industry.
SUMMARY OF THE INVENTION
[0026] This section provides a general summary of the disclosure and is not a comprehensive disclosure of the full scope of all its features.

[0027] One aspect of the present invention is a process for the synthesis of aluminium encapsulated hollow polymer nanocapsules of Formula IV. Said method comprises: synthesizing a precursor substrate material by reacting silica nanoparticles core with an activated RAFT agent, a coupling agent and an organic solvent at a temperature range of 0°C to 2°C to obtain RAFT-modified silica nanoparticles (SiNP-RAFT) as a substrate for initiating polymerization reaction followed by polymerizing two ferrocene monomers with a crosslinker, an initiator, an organic solvent in presence of N2 atmosphere at 60°C to 65°C to obtain a polymer, rp(FpDqVr)-g-SiNP of Formula I having repeat units of F, D and V on the SiNP-RAFT surface. The obtained polymer of Formula I was dissolved in a polar protic organic solvent and sonicated for a certain time, exposed to UV light for 24 to 48 hours under stirring conditions to obtain a crosslinked polymer, CL-rp(FpDqVr)-g-SiNP of Formula II. The crosslinked polymer of Formula II is treated with a weak acid, an organic solvent, and a phase transfer catalyst at room temperature to etch out the silica core to form hollow polymer nanocapsules, HL-rp(FpDqVr) of Formula III, followed by dispersing the hollow polymeric nanocapsules of Formula III in a degassed non-polar solvent and reacting with a reducing agent such as lithium aluminium hydride to reduce either SiCl4 or AlCl3 under nitrogen environment at 55°C to 60°C for a certain time to obtain aluminium encapsulated hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr) of Formula IV with a variable Al and Fe content.

[0028] In another aspect of the present invention, the Fe content (wt %) in aluminium encapsulated hollow polymer nanocapsules (Al-HPNs) (Formula IV) is varied by simply altering the chain length (mole ratio) of the two ferrocene monomers while polymerizing them on the SiNP-RAFT surface while synthesizing Formula I.

[0029] In yet another aspect of the present invention, the coupling agent is selected from the group consisting of N,N'-dicyclohexylcarbodiimide (DCC), N,N'-diisopropylcarbodiimide (DIC), or any other suitable coupling agents.

[0030] In further aspects of the present invention, the organic solvent is selected from the group consisting of dichloromethane, tetrahydrofuran, dimethylformamide, or any other suitable organic solvents.

[0031] In yet another aspect of the present invention, the weak acid used for etching the silica core is selected from the group consisting of hydrofluoric acid, ammonium fluoride, or any other suitable weak acids.

[0032] In yet another aspect of the present invention, the phase transfer catalyst is selected from the group consisting of tetrabutylammonium bromide, cetyltrimethylammonium bromide, or any other suitable phase transfer catalysts.

[0033] In yet another aspect of the present invention, the initiator used for polymerization is selected from the group consisting of azobisisobutyronitrile (AIBN), benzoyl peroxide (BPO), or any other suitable initiators.

[0034] In yet another aspect of the present invention, the sonication time for the dissolution of the polymer structure of Formula I ranges from 10 minutes to 2 hours.

[0035] In yet another aspect of the present invention, the UV light exposure time ranges from 24 to 48 hours.

[0036] In yet another aspect of the present invention, the reducing agent used for the aluminium encapsulation is selected from the group consisting of lithium aluminium hydride, sodium borohydride, or any other suitable reducing agent.

[0037] In yet another aspect of the present invention, the Al content is 13.20% Al, and the Fe content is 3.15%.

[0038] In yet another aspect of the present invention, the hollow polymer nanocapsules with ferrocenyl shells are used as nano-catalysts in various industrial applications.

[0039] In yet another aspect of the present invention, the ferrocene monomers are ferrocene carboxylic acid hydroxyethyl methacrylate (FCA-HEMA) and vinyl ferrocene (VFc).

[0040] In yet another aspect of the present invention, the crosslinker is 2,3-dimethylmaleic imidopropyl methacrylate (DMIPM).

[0041] In yet another aspect of the present invention, a free RAFT agent is used to get better control over the polymerization process.

[0042] In yet another aspect of the present invention, the polar protic organic solvent is selected from the group consisting of methanol, ethanol, water, or a combination of two or more of these solvents.

[0043] In another aspect of the present invention, the non-polar solvent is selected from the group consisting of toluene, tetrahydrofuran (THF), or 1,4-dioxane.

[0044] In yet another aspect of the present invention, an aluminium encapsulated hollow polymer nanocapsules for use in solid composite propellant comprising Formula IV with a ferrocenyl shell and an aluminium nanoparticle as a metallic fuel with a variable Al and Fe content to form a burn rate catalyst.

[0045] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which numerals represent like components.

BRIEF DESCRIPTION OF DRAWINGS
[0046] In the figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label with a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description applies to any one of the similar components having the same first reference label irrespective of the second reference label.

[0047] Figure 1 illustrates the GPC traces of the polymer, rp(FpDqVr)-g-SiNP (Formula I).

[0048] Figure 2 illustrates the comparison of the TGA analysis plot of the precursor SiNP-RAFT, the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II) and the hollow polymer nanocapsules, HL-rp(FpDqVr) (Formula III).

[0049] Figure 3 illustrates DLS plots of the precursor SiNP-RAFT, the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL- rp(FpDqVr) (Formula III).

[0050] Figure 4 illustrates the FE-SEM images of the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL- rp(FpDqVr) (Formula III).

[0051] Figure 5 illustrates the TEM image of the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL- rp(FpDqVr) (Formula III).

[0052] Figure 6 illustrates the IR spectrum of the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL- rp(FpDqVr) (Formula III).

[0053] Figure 7 illustrates the BET analysis plot of the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL- rp(FpDqVr) (Formula III).

[0054] Figure 8 illustrates the CLSM images of the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL- rp(FpDqVr) (Formula III).

[0055] Figure 9 illustrates the FE-SEM and TEM images of the aluminium-encapsulated hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr) (Formula IV).

[0056] Figure 10 illustrates the IR plot comparing the spectrum of the aluminium encapsulated hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr) (Formula IV) with the hollow polymer nanocapsules, HL-rp(FpDqVr) (Formula III).

[0057] Figure 11 illustrates the TGA analysis plot comparing the degradation of the aluminium-encapsulated hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr) (Formula IV) with the controlled aluminium sample.

[0058] Figure 12 illustrates the PXRD data of the aluminium encapsulated hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr) (Formula IV) compared with the controlled aluminium sample.

DETAILED DESCRIPTION OF THE INVENTION
[0059] The following description discloses the preferred embodiments of the present invention. However, it will be clear from this description of the invention that the invention is not limited to these illustrated embodiments but may include a variety of modifications and embodiments thereto. Therefore, the present description should be seen as illustrative and not limiting.

[0060] While the invention is susceptible to various modifications and alternative constructions, it should be understood, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims.

[0061] At the outset, for ease of reference, certain terms used in this application and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent.

[0062] Further, the present techniques are not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present claims.

[0063] The articles “a” and “an” as used herein mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of “a” and “an” does not limit the meaning to a single feature unless such a limit is specifically stated. The article “the” preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used. The adjective “any” means one, some, or all indiscriminately of whatever quantity.

[0064] The present invention relates to a novel and efficient method for synthesizing aluminium-encapsulated hollow polymeric nanocapsules. The process is designed to be simple and effective, allowing for precise control over the synthesis of these advanced nanomaterials. Moreover, the steps of synthesis are subjected to variations than those described in the present disclosure specifically.

[0065] In an embodiment of the present invention, a process for the synthesis of aluminium encapsulated hollow polymer nanocapsules of Formula IV comprises of the following steps: a.) synthesis of RAFT-modified silica nanoparticles as a substrate for initiating polymerization reaction; b.)polymerization of ferrocene monomers with repeating units on the SiNP-RAFT surface of Formula I, i.e., rp(FpDqVr)-g-SiNP; c.) crosslinking of polymer of Formula II, i.e., CL-rp(FpDqVr)-g-SiNP; d.) formation of Hollow Polymer Nanocapsules (HPNs) of Formula III, i.e., HL-rp(FpDqVr) and d.) encapsulation of aluminium into the HPNs to obtain an aluminium encapsulated HPNs, Al-NP/HL-rp(FpDqVr) of Formula IV with a variable Al and Fe content.

[0066] The process begins with the synthesis of a precursor substrate, where silica nanoparticles (SiNP) are functionalized with a Reversible Addition-Fragmentation Chain Transfer (RAFT) agent. The RAFT agent is activated and reacts with the silica nanoparticles in the presence of a coupling agent and an organic solvent. This reaction occurs at a low temperature (0°C to 2°C) to yield RAFT-attached silica nanoparticles referred to as SiNP-RAFT to form a precursor substrate for the polymerization reaction.

[0067] In another embodiment, the RAFT agent used has an activity control group (Z) selected from a range of functional groups and a free radical leaving group (R). The specific functional groups can be selected from various chemical moieties as needed for the process.

[0068] In one embodiment, the preferable RAFT (reversible addition-fragmentation chain transfer) agent has the structure:

wherein the Z is the activity control group selected from the following functional groups
or and;
R is the free radical leaving group selected from a group comprising the following functional groups
, , , or .

[0069] In another embodiment, the coupling agents include examples but are not limited to carbodiimides such as dicyclohexylcarbodiimide (DCC) or diisopropylcarbodiimide (DIC), or 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC, EDAC or EDCI).

[0070] In one embodiment, the organic solvents include examples but are not limited to 1,4-dioxane, hexanes, dichloromethane, tetrahydrofuran (THF), toluene, or a mixture of two or more solvents thereof.

[0071] In one embodiment, the Ferrocene monomers comprise carboxylic acid hydroxyethyl methacrylate (FCA-HEMA) and vinyl ferrocene (VFc). Said monomers are polymerized on the SiNP-RAFT surface along with a crosslinker 2,3-dimethylmaleic imidopropyl methacrylate (DMIPM). The polymerization is carried out in the presence of an initiator selected from a group of azobisisobutyronitrile (AIBN), tert-butyl methacrylate (TBMA), or 4,4'-Azobis(4-cyanovaleric acid) (ACV), some free RAFT agent, an organic solvent, and under a nitrogen atmosphere at temperatures between 60°C and 65°C. This process yields the polymer, rp(FpDqVr)-g-SiNP of Formula I having repeating units of F, D, and V on the precursor substrate SiNP-RAFT surface.

[0072] In another embodiment, the free RAFT agent added to the polymerization mixture has the following structure:

where the Z is the activity control group selected from the following functional groups.
or and;

R is the free radical leaving group selected from the following functional groups
, , , or ;

wherein the organic solvents include examples but are not limited to 1,4-dioxane, N,N-dimethylformamide (DMF), or dimethyl sulfoxide (DMSO).

[0073] The polymer, rp(FpDqVr)-g-SiNP of Formula I is dispersed in a polar protic solvent and subjected to sonication for a certain time. The dispersion is then irradiated with UV light for 24 to 48 hours while stirring, leading to the formation of a crosslinked polymer, CL-rp(FpDqVr)-g-SiNP of Formula II.

[0074] In another embodiment, polar protic solvents include examples but are not limited to methanol, ethanol, water, or a combination of two or more of these solvents.

[0075] The cross-linked polymer, CL-rp(FpDqVr)-g-SiNP of Formula II is dispersed in an organic solvent along with a phase transfer catalyst. A weak acid is added to remove the silica core from the crosslinked polymer, resulting in hollow polymer nanocapsules, HL-rp(FpDqVr) of Formula III;

[0076] In an embodiment, the organic solvents include examples but are not limited to Dichloromethane, DMF, DMSO, chloroform, 1,4-dioxane, or a combination thereof.

[0077] In another embodiment, phase transfer catalysts are selected from tetrabutyl ammonium bromide (C16H36BrN), benzyltriethylammonium chloride (C13H22ClN), methyltricaprylammonium chloride (C25H54ClN) or methyltributylammonium chloride (C13H30ClN).

[0078] In another embodiment, weak acids include examples but are not limited to Oxalic acid [(CO2H)2], chloroacetic acid (ClCH2CO2H), or benzoic acid (C6H5COOH).

[0079] The hollow polymer nanocapsules, HL-rp(FpDqVr) obtained after treatment with phase transfer catalyst are dispersed in a non-polar solvent and degassed with nitrogen. A reducing agent is added to reduce either SiCl4 or AlCl3, and the mixture is treated in an ultrasonic water bath at 55°C to 60°C to obtain aluminium-encapsulated hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr) of Formula IV with a variable Al and Fe content.

[0080] In a preferred embodiment, aluminium encapsulated hollow polymer nanocapsules (Al-HPNs) of Formula IV comprise 13.20% of Al and 3.15% of Fe content.

[0081] In one embodiment, non-polar solvents include examples but are not limited to toluene, tetrahydrofuran (THF), or 1,4-dioxane.

[0082] In yet another embodiment, the reducing agent includes metal hydride but is not limited to lithium aluminium hydride (LiAlH4) or sodium borohydride (NaBH4).

[0083] In one embodiment, the present invention provides aluminium encapsulated hollow polymer nanocapsules comprising Formula IV with a ferrocenyl shell and an aluminium nanoparticle as a metallic fuel with a variable Al and Fe content to form a burn rate catalyst.

[0084] In another embodiment, the present invention provides aluminium encapsulated hollow polymer nanocapsules for use in a solid composite propellant.

EXAMPLES
[0085] The following examples have been given to illustrate and give a more detailed explanation of the claimed present invention. The examples are given to exemplify the present invention and they are not to be considered to limit the scope for the present invention.
EXAMPLE 1: Synthesis of the polymer, rp(FpDqVr)-g-SiNP (Formula I):
General procedure for the synthesis of the polymer, rp(FpDqVr)-g-SiNP:
[0086] In a two-neck round-bottom flask, 50 mL of dry tetrahydrofuran (THF) was introduced, followed by nitrogen bubbling to create an inert atmosphere. To the flask, 3 grams of amine-modified silica nanoparticles (SiNP-NH2) were added and dispersed for several minutes. Subsequently, 0.375 grams (1.375 mmol) of an activated RAFT agent, CPDB-NHS, was added. The CPDB-NHS was prepared by activating 4-cyanopentanoic acid dithiobenzoate (CPDB) with N-hydroxysuccinimide (NHS). The reaction mixture was maintained at 0°C under a nitrogen atmosphere and in the dark for 16 hours.

[0087] Following the reaction, the product was precipitated by adding a mixture of cyclohexane and diethyl ether (4:1 v/v). The solid product was recovered by centrifugation at 3000 rpm for 10 minutes. To remove unreacted RAFT agents, the precipitate was redispersed in THF, followed by precipitation from a cyclohexane and diethyl ether mixture (4:1 v/v), and recovered by centrifugation at 3000 rpm for 10 minutes. This purification process was repeated three times. The final product, CTA (RAFT) functionalized silica nanoparticles, designated as SiNP-CPDB, was dried under vacuum at room temperature for 24 hours.

[0088] For the polymerization step, a Schlenk tube was charged with 600 mg (23.47 µmol) of SiNP-CPDB, along with calculated amounts of FCA-HEMA (1173.7 mg for a 60K chain length target), VFc (1173.7 mg for a 60K chain length target), DMIPM (469.46 mg for a 60K chain length target), free CPDB (6.558 mg, 23.47 µmol), AIBN (1.92 mg, 11.735 µmol), and 6 mL of dry 1,4-dioxane. The molar ratio of the reactants was maintained as [SiNP-CPDB]:[free CPDB]:[AIBN] = 1:1:0.25. The Schlenk tube was sealed and subjected to three freeze-pump-thaw cycles. After returning to room temperature, the tube was placed in a preheated oil bath set to 60°C, and polymerization was allowed to proceed for 13 hours. After the reaction, it was quenched with liquid nitrogen.

[0089] The reaction mixture was then precipitated using a mixture of dichloromethane and 1,4-dioxane and the solid product was recovered by centrifugation at 3000 rpm for 20 minutes. This precipitation step was repeated three times. The obtained polymer was dried under vacuum for 24 hours, resulting in the polymer, rp(FpDqVr)-g-SiNP of Formula I.

Polymer – rp(FpDqVr)-g-SiNP (Formula I)

Fig. 1 depicts GPC traces of the polymer, rp(FpDqVr)-g-SiNP (Formula I). The trace displays a peak or peaks corresponding to the polymer's molecular weight distribution and its effectiveness. For rp(FpDqVr)-g-SiNP, the trace may show a single peak or multiple peaks depending on the polymer's polydispersity. As shown in Fig. 1, a single peak in the GPC trace indicates a polymer with narrow molecular weight distribution and low polydispersity, confirming successful synthesis and uniform product characteristics.

The polymer produced exhibited a yield of 85%. The number average molecular weight (Mn) of the polymer was 54,662 g/mol, with a polydispersity index (PDI) of 1.19. The loading percentage on the SiNP-CPDB was determined to be 10.2%. Characterization using transmission electron microscopy (TEM) indicated a particle size of 932 nm, while dynamic light scattering (DLS) measurements revealed a size of 1236 nm. Fourier-transform infrared spectroscopy (FTIR) analysis showed absorption peaks at 1068 cm?¹ and 1642 cm?¹. The material had a surface area of 3.929 m²/g, a total pore volume of 0.059 cm³/g, and an average pore size of 0.18 nm.
EXAMPLE 2: Synthesis of the crosslinked polymer CL-rp(FpDqVr)-g-SiNP (Formula II)
General procedure for the synthesis of the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP:
[0090] 40 mg of the synthesized polymer was dissolved in 4 mL of ethanol within a 10 mL flask. The solution was sonicated for 30 minutes. Following sonication, the flask was placed in a UV chamber, stirred, and exposed to UV light for 24 hours. The resulting particles were then collected by centrifugation at 3000 rpm for 10 minutes and subsequently dried under vacuum for 24 hours.

Crosslinked polymer – CL-rp(FpDqVr)-g-SiNP (Formula II)

[0091] The crosslinked polymer, identified as CL-rp(FpDqVr)-g-SiNP, demonstrated a yield of 90% and a loading percentage on SiNP-CPDB of 8.6%. Characterization results revealed that the polymer particles had a size of 989 nm as determined by transmission electron microscopy (TEM) and 1554 nm as measured by dynamic light scattering (DLS). Fourier-transform infrared spectroscopy (FTIR) identified absorption peaks at 1068 cm?¹ and 1650 cm?¹. The polymer exhibited a surface area of 7.120 m²/g, a total pore volume of 0.064 cm³/g, and an average pore size of 0.32 nm.
EXAMPLE 3: Synthesis of the hollow polymer nanocapsules (HPN), HL-rp(FpDqVr) (Formula III)
General procedure for the synthesis of the hollow polymer nanocapsules (HPN), HL-rp(FpDqVr):
[0092] A total of 35 mg of the crosslinked polymer was dissolved in a mixture of 3 mL of dichloromethane (DCM) and N,N-dimethylformamide (DMF) in a 3:1 volume ratio within a Teflon vial. To this solution, a catalytic amount of tetrabutylammonium bromide, a phase transfer catalyst, was added and stirred for 5 minutes. Subsequently, 1 mL of 48% hydrofluoric acid (HF) solution was introduced, and the reaction was allowed to proceed for 10 hours at room temperature. Following the reaction, the product was neutralized using a saturated sodium bicarbonate solution and then precipitated in diethyl ether. The resulting particles were collected and dried under a vacuum for 24 hours.

Hollow polymer nanocapsules – HL-rp(FpDqVr) (Formula III)

[0093] Fig. 2 depicts a comparative Thermogravimetric Analysis (TGA) plot for different materials involved in the synthesis process: the precursor SiNP-RAFT, the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL-rp(FpDqVr) (Formula III). It analyzes the thermal properties, stability, and composition of these materials by measuring weight changes as a function of temperature to ensure their suitability to work as an effective metallic fuel and burn rate catalyst.

[0094] Figure 3 summarizes the Dynamic Light Scattering (DLS) plots for different samples: the precursor SiNP-RAFT, the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL-rp(FpDqVr) (Formula III) to characterize the size, distribution, and stability of the above-mentioned components based on the light scattering caused by their Brownian motion.

[0095] Figure 4 showcases the Field Emission Scanning Electron Microscopy (FE-SEM) images of the following materials: the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL-rp(FpDqVr) (Formula III). It shows the high-resolution images of the sample surfaces, providing detailed morphological information.

[0096] Figure 5 shows the TEM image of the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL- rp(FpDqVr) (Formula III).

[0097] Figure 6 depicts the Infrared (IR) spectra for three different samples: the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL-rp(FpDqVr) (Formula III). The IR spectra provide information about the functional groups and chemical bonds present in the samples.

[0098] Figure 7 depicts the BET analysis plot of the polymer, rp(FpDqVr)-g-SiNP (Formula I), the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL- rp(FpDqVr) (Formula III).

[0099] Figure 8 depicts the CLSM images of the crosslinked polymer, CL-rp(FpDqVr)-g-SiNP (Formula II), and the hollow polymer nanocapsules, HL- rp(FpDqVr) (Formula III).

[0100] The processed hollow polymer nanocapsules exhibited a yield of 70%. Characterization results showed that the particles had a size of 500 nm as determined by transmission electron microscopy (TEM) and 909 nm as measured by dynamic light scattering (DLS). Fourier-transform infrared spectroscopy (FTIR) analysis revealed an absorption peak at 1654 cm?¹. The material displayed a surface area of 59.842 m²/g, a total pore volume of 0.12 cm³/g, and an average pore size of 3.96 nm.
EXAMPLE 4: Synthesis of the aluminium encapsulated hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr) (Formula IV)
General procedure for the synthesis of the hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr):
[0101] A 50 mL three-neck round-bottom flask was equipped with a nitrogen gas inlet and outlet. Into this flask, 20 mL of degassed toluene was added, followed by the introduction of 20 mg of synthesized hollow polymer nanocapsules. Next, 75 mg of lithium aluminium hydride (LiAlH4), equivalent to 1.976 mmol, was suspended in the solution, and 0.4 mL of silicon tetrachloride (SiCl4), or 4.355 mmol, was added. The reaction was conducted at 60°C for 3 hours to ensure the removal of low-boiling silanes and other gaseous by-products. The weight ratio of LiAlH4 to hollow polymer nanocapsules (HPN) was maintained at 3:1 to achieve a complete yield of the metal-polymer nanocomposite. After 3 hours, the reaction was quenched with excess methanol. The resulting solution was filtered to obtain aluminium-encapsulated hollow polymer nanocapsules (Al-NP encapsulated HPN). The product was then washed with methanol to remove any residual impurities and dried under vacuum for 24 hours. A similar procedure was performed without the addition of HPN to produce aluminium nanoparticles, which were used as a blank for comparative analysis.

Aluminium encapsulated hollow polymer nanocapsules – Al-NP/HL-rp(FpDqVr) (Formula IV)

[0102] Figure 9 depicts the FE-SEM and TEM images of the aluminium-encapsulated hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr) (Formula IV).

[0103] Figure 10 depicts the IR plot comparing the spectrum of the aluminium encapsulated hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr) (Formula IV) with the hollow polymer nanocapsule, HL-rp(FpDqVr) (Formula III).

[0104] The resulting product exhibited a yield of 95%. Fourier-transform infrared spectroscopy (FTIR) analysis indicated an absorption peak at 1742 cm?¹. The weight gain of the material was measured at 8.02%.

[0105] Fig. 11 depicts TGA analysis for a detailed comparison of the thermal degradation characteristics of aluminium-encapsulated hollow polymer nanocapsules versus control aluminium samples, providing insights into how metal encapsulation influences thermal stability and decomposition behavior.

[0106] Fig. 12 depicts PXRD analysis for comparing the structural characteristics of aluminium encapsulated hollow polymer nanocapsules with control aluminium samples. It provides valuable information on crystallinity, phase composition, and structural changes induced by aluminium encapsulation, which can be critical for understanding the efficiency of the encapsulated aluminium properties and performance.

[0107] Various embodiments of the present invention provide potential advantages that are realizable by the invention including the following.

[0108] Unlike other processes in the art, the present method provides a novel approach to create aluminium-encapsulated hollow polymer nanocapsules with controlled properties. This method is advantageous due to its simplicity, efficiency, and the precise control it offers over the synthesis process, leading to high-quality nanocapsules suitable for various applications.

[0109] Another competitive advantage of the present invention is to provide a simple and cost-effective way to prepare reactive metal-polymer composite i.e., Al-NP encapsulated with hollow polymer nanocapsules with (Al-HPNs) ferrocenyl shell is easily scalable up to industrial scale, which is targeted to employ as a burn rate catalyst cum fuel in composite solid propellants (CSPs).

[0110] An advantage of the present invention is the ease and affordability of accessing the starting materials, including metal halides, acids, catalysts, reducing agents, and organic solvents. Additionally, the process described is straightforward and facilitates the efficient purification of products after each synthesis step.

[0111] The methodology of the present invention allows for modification of the ferrocene-based monomer structure to address specific needs, such as mitigating the passivation effects of Al2O3. For example, incorporating fluorine into the ferrocene-based polymer system can significantly enhance the passivation of in-situ synthesized aluminium nanoparticles (Al-NP).

[0112] The invention offers a solution to migration-related issues by utilizing a ferrocene-based polymer and its resultant meta-polymer nanocomposite. Unlike prior methods that use Al2O3-coated nanoscale aluminium (nAl) as a starting material, this invention enables the in-situ synthesis of Al-NP within the hollow spaces of ferrocene polymer nanocapsules, providing a more effective approach to prevent the aerial oxidation of Al-NP.

[0113] The metal-encapsulated polymer structure of the invention prevents the coalescence of metal nanoparticles, thus protecting the metallic nanoparticles from deactivation. This stabilization of the active metal species against sintering enhances the catalytic activity.

[0114] The present invention also achieves a high yield of the metal-polymer composite, i.e., aluminium encapsulated hollow polymer nanocapsule (Al-HPNs).

[0115] In contrast to other synthesis methods, embodiments of the present invention prevent the coalescence of metal nanoparticles in the encapsulated polymer structure. This approach protects the metallic nanoparticles from deactivation by stabilizing the active metal species against sintering, thereby improving catalytic performance.

[0116] An additional benefit of the invention is its ability to modify the ferrocene-based monomer structure to meet various requirements, such as alleviating the passivation effects of Al2O3. For instance, incorporating fluorine into the ferrocene-based polymer system can significantly impact the passivation of in-situ synthesized aluminium nanoparticles (Al-NP).

[0117] Although the present invention has been illustrated and described herein with reference to preferred embodiments, it will be readily apparent to those of ordinary skill in the art that other embodiments may perform similar functions and/or achieve similar results. All such equivalent embodiments are within the spirit and scope of the present invention, are contemplated thereby, and are intended to be covered by the following claims.

[0118] The advantages set forth above, and those made apparent from the foregoing description, are efficiently attained. Since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description as illustrative and not in a limiting sense.

[0119] It is also to be understood that the following claims are intended to cover all the generic and specific features of the invention herein described, and all statements of the scope of the invention that, as a matter of language, might fall there within.

[0120] The advantages set forth above, and those made apparent from the foregoing description, are efficiently attained. Since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
, Claims:We Claim:
1. A process for synthesis of aluminium-encapsulated hollow polymer nanocapsules of Formula IV, comprising:
(a) synthesizing a precursor substrate material by reacting silica nanoparticles core with an activated RAFT agent, a coupling agent, and an organic solvent at a temperature range of 0°C to 2°C to obtain a RAFT modified silica nanoparticles (SiNP-RAFT) as a substrate for initiating polymerization reaction ;
(b) polymerizing two ferrocene monomers with a crosslinker, an initiator, an organic solvent, and an N2 atmosphere at 60°C to 65°C to obtain a polymer, rp(FpDqVr)-g-SiNP of Formula I having repeating units F, D and V on the SiNP-RAFT surface;
(c) dissolving the polymer of Formula I in a polar protic organic solvent and sonicate for a certain time, exposing to UV light for 24 to 48 hours under stirring conditions to obtain a crosslinked polymer, CL-rp(FpDqVr)-g-SiNP of Formula II;
(d) treating the crosslinked polymer of Formula II with a weak acid, an organic solvent, and a phase transfer catalyst at room temperature to etch out the silica core to form hollow polymer nanocapsules, HL-rp(FpDqVr) of Formula III;
(e) dispersing the hollow polymeric nanocapsules of Formula III in a degassed non-polar solvent and reacting with a reducing agent such as lithium aluminium hydride to reduce either SiCl4 or AlCl3 under a nitrogen environment at 55°C to 60°C for a certain time to obtain aluminium encapsulated hollow polymer nanocapsules, Al-NP/HL-rp(FpDqVr) of Formula IV with a variable Al and Fe content.
2. The process as claimed in claim 1, wherein the Fe content (wt %) in aluminium encapsulated hollow polymer nanocapsules (Al-HPNs) (Formula IV) is varied by simply altering the chain length of the two ferrocene monomers while polymerizing them on the SiNP-RAFT surface while synthesizing Formula I.
3. The process of claim 1, wherein the coupling agent is selected from the group consisting of N,N'-dicyclohexylcarbodiimide (DCC), N,N'-diisopropylcarbodiimide (DIC), or any other suitable coupling agents.
4. The process of claim 1, wherein the organic solvent is selected from the group consisting of dichloromethane, tetrahydrofuran, dimethylformamide, or any other suitable organic solvents.
5. The process of claim 1, wherein the weak acid used for etching the silica core is selected from the group consisting of hydrofluoric acid, ammonium fluoride, or any other suitable weak acids.
6. The process of claim 1, wherein the phase transfer catalyst is selected from the group consisting of tetrabutylammonium bromide, cetyltrimethylammonium bromide, or any other suitable phase transfer catalysts.
7. The process of claim 1, wherein the initiator used for polymerization is selected from the group consisting of azobisisobutyronitrile (AIBN), benzoyl peroxide (BPO), or any other suitable initiators.
8. The process of claim 1, wherein the sonication time for the dissolution of the polymer structure of Formula I ranges from 10 minutes to 2 hours.
9. The process of claim 1, wherein the UV light exposure time ranges from 24 to 48 hours.
10. The process of claim 1, wherein the reducing agent used for the aluminium encapsulation is selected from the group consisting of lithium aluminium hydride, sodium borohydride, or any other suitable reducing agents.
11. The process of claim 1, wherein the Al content is 13.20% Al and Fe content is 3.15%.
12. The process of claim 1, wherein the hollow polymer nanocapsules with ferrocenyl shells are used as nano-catalysts in various industrial applications.
13. The process of claim 1, wherein the ferrocene monomers are ferrocene carboxylic acid hydroxyethyl methacrylate (FCA-HEMA) and vinyl ferrocene (VFc).
14. The process of claim 1, wherein the crosslinker is 2,3-dimethylmaleic imidopropyl methacrylate (DMIPM).
15. The process of claim 1, wherein the polymerization process is controlled by a free RAFT agent.
16. The process of claim 1, wherein the polar protic organic solvent is selected from the group consisting of methanol, ethanol, water, or a combination of two or more of these solvents.
17. The process of claim 1, wherein the non-polar solvent is selected from the group consisting of toluene, tetrahydrofuran (THF), or 1,4-dioxane.
18. An aluminium-encapsulated hollow polymer nanocapsules for use in solid composite propellant comprising Formula IV with a ferrocenyl shell and an aluminium nanoparticle as a metallic fuel with a variable Al and Fe content to form a burn rate catalyst.