Cross-Reference To Related Application [0001] This application claims the benefit of priority of United States Patent Application No. 62/270,218, filed 21 December 2015, the content of it being hereby incorporated by reference in its entirety for all purposes.

Technical Field

[0002] The present disclosure relates to a method for synthesizing hollow poly(vinylidene difluoride) microspheres. The present disclosure also relates to hollow poly(vinylidene difluoride) microspheres and a film comprising a plurality of such hollow poly(vinylidene difluoride) microspheres.

Background

[0003] Within the last decade, research into microsphere technologies has progressed to encompass a range of applications in fields such as pharmaceuticals, biological sensors, photonics and more recently, superhydrophobic materials. Further, microspheres with central voids may have desirable compressive, acoustic, and tensile properties when organized into syntactic foams. In view of these traits, research endeavours have focused on enhancing control of microspheres with the aim of efficient, reliable fabrication of micro objects possessing desirable bulk or unit performance. In this regard, microspheres of several polymers have been realized. For example, fluorinated polystyrene (PS) microspheres have been prepared through distillation precipitation polymerization; poly-epsilon-caprol acetone (PCL) microspheres have been studied as a degradative delivery vehicle with production methods including emulsion solvent extraction, hot melt, solution-enhanced dispersion and spray drying.

[0004] One particular intriguing methodology for microsphere growth may be non- solvent induced phase separation (NIPS). NIPS is typically a process in which a ternary solution of solvent, non-solvent and polymer components separate into a bi-phase system of polymer rich and polymer poor regions. The introduction of a non-solvent, miscible with existing solvent, serves as the driving force behind the separation, creating a solvent/non-solvent phase which may be allowed to evaporate from the system. By controlling various external parameters, percolation activity of the polymer poor phase may be altered to introduce unique surface morphologies. Recently, NIPS has been realized as a growth procedure for polymer microspheres. In a particular study, combined membrane emulsification with NIPS were utilized to produce polyethersulfone (PES) microspheres. In another study, a coupled method of NIPS and electrospray technology were leveraged for fabricating hierarchically porous polymethyl methacrylate (PMMA) microspheres capable of superhydrophobic performance.

[0005] One useful application of microspheres may be the production of syntactic foams. Syntactic foams may be regarded as composite materials consisting of a continuous metal, polymer and/or ceramic phase with intermixed hollow particles. There are typically two methodologies for syntactic foam fabrication. These comprise (1) hollow spheres for embedment and (2) inorganic nanoparticles as bubble nucleation sites. One example of (2) may be the use of low density polyethylene (LDPE) with hectorite nanoparticles to improve bulk mechanical and thermal properties of the polymer. In another approach, embedded hollow spheres showed several energy absorptive properties with voids in the micro/nano regime; the hollow particles seem to influence stiffness, fracture toughness, impact and vibrational damping. Additionally, density and coefficient of thermal expansion may be lowered due to reliefs in the material. These materials may be involved at macro-scale applications, such as servicing aircraft and improving constructive materials, to micro-scale ones e.g. used for piezoelectric transducers, microfluidic filters and acoustic insulation.

[0006] Typically, the foam properties may be manipulated via tuning the relative size, distribution, strength and volume fraction of the microsphere in the surrounding matrix. An example of such a microsphere used in syntactic foams may be an air vesicle composed of a glass microsphere due to the ease of synthesis with polymers, ceramics or metals. Other works have also attempted to leverage on glass microspheres to improve dielectric constant and thermal conductivity of traditional epoxy. However, these glass microspheres tend to have poor shear properties. As a result, polymer microspheres have been proposed as suitable replacements. Studies involving polymer microspheres have utilized hollow polymeric (amino resin) microspheres to increase compression, tensile and shear strength of a phenolic resin by 2, 7, and 3.3 times, respectively. Unfortunately, the limits of these materials rely heavily on the properties of the microspheres. Thus, it may be highly desirable to widen the range of available substituents in the design of syntactic foams and other vesicle based materials. Many microsphere formations, whether polymer based or otherwise, have been synthesized but these conventional methods commonly require the need for a precursory template. These methods tend to have a general format of template, adsorption, shell, and degrade in its production process sequence, which tend not to be highly scalable. Alternate fabrication methods of polymeric microspheres are thus sought for procedural scalability.

[0007] Thus, there is a need to provide for a method to make microspheres, wherein the method and microspheres are capable of ameliorating one or more of the above limitations. There is also a need to provide such microspheres in syntactic foams/films or usable as superhydrophobic materials which can ameliorate one or more of the above limitations and possess desirable traits as mentioned above.

[0008] There is a further need to provide for a scalable method having an ease of manufacturing tunable polymeric microspheres that are chemically resistant, thermally stable and have low surface energy.

Summary [0009] In one aspect, there is a method for synthesizing hollow poly(vinylidene difluoride) microspheres comprising the steps of: (i) adding a first non-solvent to a polymer solution comprising poly(vinylidene difluoride) dissolved in an organic solvent to form a ternary mixture without inducing phase separation of the poly(vinylidene difluoride); and (ii) contacting the ternary mixture with a second non- solvent to induce phase separation to thereby form the hollow poly(vinylidene difluoride) microspheres. [0010] Advantageously, the method is capable of ameliorating one or more of the limitations as described above. The method is also scalable for convenient manufacturing of tunable polymeric microspheres that are chemically resistant, thermally stable with low surface energy. The method serves as a technique to create rough and hollow polymeric microspheres without requiring any post-synthesis processing such as mixing two chemicals prior to application (epoxy-like) or vaporizing low boiling point solvents to expand the spheres. This technique removes such steps from the manufacturing process in addition to allowing for the use of PVDF as a material.

[0011] In another aspect, there is a hollow poly(vinylidene difluoride) microsphere comprising a hollow core and a shell layer surrounding the hollow core, wherein the shell layer consists of poly(vinylidene difluoride) and comprises an external surface having no apertures that extend from the external surface towards the hollow core. Advantageously, the hollow poly(vinylidene difluoride) microsphere as disclosed herein imparts chemical resistance, thermal stability, low surface energy, acoustic insulation and superhydrophobicity.

[0012] In another aspect, there is a film comprising a plurality of hollow poly(vinylidene difluoride) microspheres obtained by the method as defined above dispersed on at least one surface of a substrate. The beneficial properties possessed by the film, such as the advantageous effects as mentioned above, arise from the method or technique as disclosed herein.

Brief Description of the Drawings [0013] In the drawings, like reference characters generally refer to like parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present disclosure are described with reference to the following drawings, in which:

[0014] Figure la is a scanning electron microscopy (SEM) image showing poly(vinylidene difluoride) microspheres of 1.3 μπι diameter, formed with 0.29 wt% of water loaded in the polymer solution, on an aluminum surface. Inset image shows a 5 μΐ sessile droplet on a surface having the microspheres with a water contact angle of 163°. The scale bar represents 5 μιη.

[0015] Figure lb is a SEM image showing poly(vinylidene difluoride) microspheres of 820 nm diameter, formed with 0.58 wt% of water loaded in the polymer solution, on an aluminum surface. Inset image shows a 5 μΐ sessile droplet on a surface having the microspheres with a water contact angle of 167°. The scale bar represents 5 μιη.

[0016] Figure lc is a SEM image showing poly(vinylidene difluoride) microspheres of 650 nm diameter, formed with 1.72 wt% of water loaded in the polymer solution, on an aluminum surface. Inset image shows a 5 μΐ sessile droplet on a surface having the microspheres with a water contact angle of 170.5°. The scale bar represents 5 μιη.

[0017] Figure Id is a SEM image showing poly(vinylidene difluoride) microspheres of 330 nm diameter, formed with 2.83 wt% of water loaded in the polymer solution, on an aluminum surface. Inset image shows a 5 μΐ sessile droplet on a surface having the microspheres with a water contact angle of 161°. The scale bar represents 5 μιη.

[0018] Figure 2 shows a plot depicting the relationship between water contact angle (WCA) and sphere diameter versus loaded water as a percentage of the total mass of the ternary mixture of water/dimethylformamide (DMF)/poly(vinylidene difluoride). There is an inverse relationship between sphere size and WCA for a given spin angular velocity and submersion bath temperature. The peak WCA measured was 170.5 ± 0.9° for a sphere diameter of 0.652 ± 0.065 μιη.

[0019] Figure 3a is a SEM image of hollow poly(vinylidene difluoride) microsphere with a large hollow center and other small voids present across the cross-section of the sphere. The scale bar represents 500 nm.

[0020] Figure 3b is a SEM image of a porous, membrane-like film shown relative to the hollow poly(vinylidene difluoride) microspheres. The scale bar represents 3 μπι. Figure 3b demonstrates water not added in a low humidity environment (e.g. 30% relative humidity (RH) in the atmosphere) leads to no formation or poor formation of microspheres. Instead, a porous membrane-like film tends to form. This shows the necessity of providing moisture to create the spherical morphology. The inset shows a 5 μΐ sessile droplet on the porous membrane-like film.

[0021] Figure 3c is a SEM image of smooth film casted on glass shown relative to a hollow poly(vinylidene difluoride) microsphere. [0022] Figure 3d is a SEM image of hollow poly(vinylidene difluoride) microspheres having wrinkled and rough surface. The inset shows a 5 μΐ sessile droplet having a WCA of 170°. The scale bar represents 1 μιη.

[0023] Figure 3e is a SEM image of hollow poly(vinylidene difluoride) microspheres smaller in size and having a smoother surface compared to microspheres of figure 3d. The inset shows a 5 μΐ sessile droplet having a lower WCA compared to the inset of figure 3d. The scale bar represents 500 nm.

[0024] Figure 3f is a SEM image of large agglomerated hollow poly(vinylidene difluoride) microspheres having a smoother surface compared to microspheres of figure 3e. The inset shows a 5 μΐ sessile droplet with lower WCA compared to the inset of figure 3e. The scale bar represents 3 μπι.

[0025] Figure 4a shows a plot of coating angular velocity (in rotation per minute (RPM)) against thickness, indicating a very tight control over film thickness ranging from 6 μπι to 16 μπι, independent of other variables. The blue series represented by the circular markers correspond to 1.72 wt% water loading while the red series represented by the square markers correspond to 0.58 wt% water loading.

[0026] Figure 4b shows a plot of film thickness against WCA showing an increase in WCA with increasing film thickness. The increase in film thickness results in a higher surface area due to the deeper pores formed, thereby resulting in an increase of WCA. The blue series represented by the circular markers correspond to 1.72 wt% water loading while the red series represented by the square markers correspond to 0.58 wt% water loading.

[0027] Figure 5 shows a WCA against hysteresis and slide angle plot indicating a strong linear decrease in adhesion force with increasing contact angle. The large negative slope for the hysteresis correlation signifies the droplets are in a wetting or Wenzel state through all contact angles.

[0028] Figure 6a shows a photograph of a dry polymer film floating in water in a 100 ml beaker indicating the bulk density is less than 1.

[0029] Figure 6b shows a photograph of a wet polymer film floating in water in a 100 ml beaker indicating the bulk density is less than 1.

[0030] Figure 7 shows a depth profile focusing from the bottom (leftmost image) to the top (rightmost image) of a microflow channel filled with water (specific gravity (SG) is 1), polystyrene spheres (SG is about 1.05) and hollow poly(vinylidene difluoride) microspheres made according to the present method as disclosed herein. The polystyrene spheres are in focus at the bottom of the channel observable in the first image. As the focal plane is moved up vertically, the polystyrene spheres go out of focus and the agglomerations of poly(vinylidene difluoride) hollow microspheres come into focus. This implies that the polystyrene spheres are denser than water while the hollow poly(vinylidene difluoride) microspheres are less dense than water. The scale bar represents 75 μπι.

[0031] Figure 8a shows a SEM image indicating large spheres (2.2 μπι). The scale bar represents 2.5 μπι as created by using ethanol as the non-solvent.

[0032] Figure 8b shows a SEM image indicating smaller spheres (600 nm). The scale bar represents 1.5 μπι as created by using ethanol as the non-solvent.

[0033] Figure 9 shows a plot of the sphere diameter as a result of the ethanol loading quantity.

Detailed Description

[0034] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0035] Embodiments described in the context of the present method are analogously valid for the hollow poly(vinylidene difluoride) microspheres and a film/foam comprising a plurality of such hollow poly(vinylidene difluoride) microspheres, and vice versa.

[0036] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0037] The expression "microsphere" or its grammatical variants thereof, such as "microspheres" or "microspherical particles" etc., in the context of the present disclosure, refers to at least substantially spherical particles having a mean diameter in the range of 50 nm to 15 μπι, or preferably a mean diameter in the range of 100 nm to 10 μπι. The term "diameter" may be taken as an average or mean diameter.

[0038] In the context of the present application, the phrase "organic solvent" refers to a liquid that is carbon based and is capable of dissolving a polymer. The organic solvent may be polar or non-polar. The organic solvent may be miscible with water.

[0039] The word "substantially" does not exclude "completely" e.g. a composition which is "substantially free" from Y may be completely free from Y. Where necessary, the word "substantially" may be omitted from the definition of the invention.

[0040] In the context of various embodiments, the articles "a", "an" and "the" as used with regard to a feature or element include a reference to one or more of the features or elements.

[0041] In the context of various embodiments, the term "about" or "approximately" as applied to a numeric value encompasses the exact value and a reasonable variance.

[0042] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

[0043] As used herein, the phrase of the form of "at least one of A or B" may include A or B or both A and B. Correspondingly, the phrase of the form of "at least one of A or B or C", or including further listed items, may include any and all combinations of one or more of the associated listed items.

[0044] Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

[0045] The present disclosure relates to a method for synthesizing hollow poly(vinylidene difluoride) microspheres. The method may comprise the steps of (i) adding a first non-solvent to a polymer solution comprising or consisting of poly(vinylidene difluoride) dissolved in an organic solvent to form a ternary mixture without inducing phase separation and/or decomposition of the poly(vinylidene difluoride), and (ii) contacting the ternary mixture with a second non-solvent to induce phase separation and/or decomposition to thereby form the hollow poly(vinylidene difluoride) microspheres.

[0046] Poly(vinylidene difluoride), known interchangably as polyvinylidene difluoride, polyvinylidene fluoride or abbreviated as PVDF, is a fluoropolymer which tends to be chemically resistant and hence compatible for use with several commerical solvents, acids and bases. PVDF is thermally stable compared to other polymers as it tends not to thermally decompose or melt until temperature reaches about 250°C or 180°C, respectively. PVDF tends to be hydrophobic in nature and may be used in applications that require such a property to repel or remain immiscible with water.

[0047] Typically, syntactic fluoropolymer based foams may contain hollow glass microspheres encapsulated in a PVDF resin. However, by using pure PVDF hollow microspheres made according to the present disclosure, the glass microspheres may be removed or replaced. In another instance, the advantages of PVDF over polystyrene microspheres may include better chemical resistance, thermal stability and low surface energy. Because the method or technique described in the present disclosure is scalable, syntactic foams incorporating the hollow PVDF microspheres obtained by the present method may be created for naturally mold resistant thermal and sound insulation, shock dispersing military armor, or even used in applications such as spray on superhydrophobic coatings.

[0048] To synthesize the hollow PVDF microspheres, the present method adds an initial amount of the first non-solvent to a polymer solution formed from a polymer dissolved in a solvent. The polymer may comprise or consist of PVDF. The solvent may be an organic solvent. The initial addition of the first non-solvent to a solution of the PVDF polymer and its solvent in the present method does not cause phase separation and/or decomposition of the dissolved PVDF polymer. This means the dissolved polymer does not precipitate from its solvent in step (i). Particularly, decomposition in the context of step (i), or even step (ii), refers to spinodal decomposition. Spinodal decomposition may comprise the rapid unmixing of two components to form two coexisting phases e.g. a polymer rich phase and the solvent rich phase. Spinodal decomposition may also be taken as a form of phase separation in which there may be no barrier to nucleation and there may be spontaneous demixing into two phases e.g. the polymer rich phase and the solvent rich phase. Where a mixture comprises two or more components, when spinodal decomposition occurs, the mixture of materials or components may separate in a thermodynamically favourable way without changing the chemical structure of molecules of each of the individual components or materials. Nucleation in growth may also be avoided when the first non-solvent is added in step (i) of the present method. Nucleation in growth may be taken as a type of phase separation where there is an energy barrier to a phase change.

[0049] Where the term "decomposition" is used with reference to non-solvents, or more particularly in the context of introducing the first and second non-solvents to the polymer solution or ternary mixture, respectively, this term would refer to spinodal decomposition and not other forms of decomposition e.g. thermal decomposition, unless specified otherwise.

[0050] Regardless of the phase separation mechanism, the addition of the first non- solvent in step (i) may be critical as this step operates near the phase transition boundary in order to create microspheres in the range of 50 nm to 15 μπι, or preferably 100 nm to 10 μπι, or more preferably 200 nm to 6 μπι. If the first non-solvent is added to the solvent before PVDF, the subsequently added PVDF may not be able to dissolve adequately or completely in a non-solvent/solvent binary mixture. Consequently, PVDF microspheres may not form. Further, there may be a risk where the polymer precipitates if the first non-solvent is added before the polymer is added to or dissolved in the solvent. Another risk where the rate of polymer solvation becomes compromised or decreased may also be present if the first non-solvent is added to the solvent before the PVDF polymer, even though thermodynamically, the polymer may still dissolve into solution but the rate may be detrimentally affected.

[0051] Accordingly, the present method of initially adding the first non-solvent to the polymer solution helps to form smaller microspheres as a result of operating near the phase transition boundary when adding a non-solvent to a polymer solution.

[0052] In step (i) of the present method, the first non-solvent may comprise water or alcohol. The first non-solvent in step (i) may be the same or different from the second non-solvent used in step (ii) of the present method. The water used as the first non- solvent may be deionized water, reverse osmosis water or even tap water. The alcohol used as the first non-solvent for step (i) may be any alcohol even if it may be used to induce phase separation and/or decomposition of the polymer from the ternary mixture in step (ii). The alcohol may comprise ethanol. Any of the first non-solvents, particularly water or ethanol, may be added to the polymer solution.

[0053] According to the present method, the polymer solution comprising PVDF may be formed by dissolving PVDF in an organic solvent at a temperature of up to 200°C, 150°C, 100°C or 50°C etc. As a non-limiting example, the polymer solution comprising PVDF may be formed by dissolving PVDF at 80°C for 3 hours. When DMF is used as the solvent, the polymer solution may be formed by dissolving PVDF in DMF at a temperature of up to 150°C. Increasing dissolution temperature or the higher dissolution temperatures may be used to accelerate dissolution of the polymer or PVDF into the solvent. Hence, the dissolution time may decrease by increasing temperature. The dissolution time and temperature may also depend on the solvent used.

[0054] The polymer solution may be a binary solution because it comprises two components, namely PVDF and the organic solvent for PVDF. The PVDF may be in the form of powder, pellets or other forms suitable for dissolution in an organic solvent. In some instances, the PVDF dissolved in the organic solvent may form 20 weight percent (wt%) of the polymer solution, wherein the wt% is based on total weight of the polymer solution. This implies the remaining composition of the binary solution may consist of the organic solvent. In some instances, the dissolved PVDF may be present in the polymer solution in the range of 0 wt% to saturation (about less than 30 wt%), or up to 25 wt%, 20 wt%, 15 wt%, 10 wt% or 5 wt%, wherein the wt% may be based on total weight of the polymer solution). The range of PVDF dissolved in the polymer solution may also form up to 30 wt% of the polymer solution, wherein the wt% is based on total weight of the polymer solution. The wt% of PVDF dissolved in the polymer solution may depend on temperature of the solvent. The amount of PVDF present in the polymer solution may affect the quantity of first non-solvent added in step (i) of the present method.

[0055] The organic solvent may comprise a polar aprotic solvent with a normal boiling point higher than 100°C. The normal boiling point refers to the temperature at which a liquid boils under atmospheric pressure. In some embodiments, the organic solvent may be selected from the group consisting of dimethylformamide (DMF), dimethyl sulfoxide (DMSO), nitromethane and propylene carbonate. In some embodiments, the organic solvent used to dissolve PVDF may comprise DMF.

[0056] In the present method, the first non-solvent may be added to the binary solution (i.e. the polymer solution comprising PVDF dissolved in an organic solvent) without causing phase separation and/or decomposition (i.e. spinodal decomposition) of the dissolved PVDF. Water or alcohol (e.g. ethanol) may act as a first non-solvent for PVDF and may cause PVDF to precipitate. However, in the method as disclosed herein, when water is added in step (i) of the present method, a ternary mixture is formed and the PVDF does not solidify from the organic solvent. A ternary mixture may be a mixture formed from only three components. The three components may form a single phase. The three components may also be in their liquid state. As a non-limiting example, a ternary mixture may be formed by adding a first non-solvent to a polymer dissolved in an organic solvent. Such a ternary mixture may be known as a polymer/solvent/non-solvent mixture or system. The first non-solvent may be miscible with the organic solvent for dissolving PVDF or to drive the reaction. The first non- solvent and the organic solvent may be taken as miscible if they mix in a binary solution and form a single phase. In the context of the present application, the ternary mixture may refer to a mixture comprising PVDF as the polymer, DMF as the solvent and water or ethanol as the non-solvent i.e. PVDF/DMF/water or PVDF/DMF/ethanol ternary mixture or system.

[0057] In embodiments where the first non-solvent is water, the water added in step (i) may be up to or not more than 5 wt%, 4 wt%, 3 wt%, 2 wt%, 1 wt% or 0.5 wt% of the ternary mixture, wherein the wt% of the water added may be based on total weight of the ternary mixture. Hence, for instance, the amount of water added as the first non- solvent in step (i) may be up to or not more than 5 wt% or 3 wt%.

[0058] In embodiments where the first non-solvent is ethanol, the ethanol added in step (i) may be up to or not more than 25 wt%, 20 wt%, 15 wt%, 10 wt%, 5 wt% or 1 wt% of the ternary mixture, wherein the wt% of ethanol added may be based on total weight of the ternary mixture. Hence, for instance, the amount of ethanol added as the first non-solvent in step (i) may be up to or not more than 25 wt%. [0059] The amount of first non-solvent added may depend on the polymer concentration in the binary polymer solution and if too much of the first non-solvent is added in step (i), the solution may gel and form a solid due to the solvent being removed from the solvation of the polymer. The solvent may preferentially mix with the first non-solvent to minimize the free energy of the solution. As mentioned above, the amount of first non-solvent added in step (i) to eventually form the microspheres in step (ii) of the present method may be performed near the phase transition boundary of the ternary mixture such that the amount of first non-solvent added in step (i) of the present method leads to a stable ternary mixture.

[0060] In some embodiments, the first non-solvent added may comprise between 0.1 wt% and 25 wt% (inclusive) of the ternary mixture, and wherein the wt% of the first non-solvent may be based on total weight of the ternary mixture. As mentioned above, the capacity of the first non-solvent in the ternary mixture may be relative to the polymer concentration. Advantageously, by operating along the transition phase boundary with high polymer concentration in the range as disclosed above, it may be easier to control the phase separation and/or decomposition kinetics.

[0061] After forming the ternary mixture, step (ii) of the present method may be carried out to contact the ternary mixture with the second non-solvent to complete the formation of hollow PVDF microspheres.

[0062] In some instances, the contacting of step (ii) may be carried out by spraying the ternary mixture, perhaps directly after formation of ternary mixture, into the second non-solvent to induce phase separation and/or decomposition (i.e. spinodal decomposition) to obtain the hollow PVDF microspheres. The ternary mixture may be sprayed into water, steam or onto a surface and then contacted with steam.

[0063] As mentioned above, the second non-solvent may be the same or different from the first non-solvent. For example, the first non-solvent used in step (i) may comprise an alcohol but the second non-solvent used in step (ii) may be water, or vice versa.

[0064] The sprayed ternary mixture may remain in the second non-solvent for 15 seconds to 12 hours, 30 seconds to 12 hours, an hour or any other duration within the specified range. Doing so may cause PVDF to instantaneously precipitate into hollow microspheres. The spheres may then float to the top and be separated using known froth flotation techniques. The loose microspheres may be post-processed using known polymer molding techniques such as injection molding or casting to form a foam or film incorporating the microspheres. This method as disclosed herein advantageously eliminates use of resins and blowing agents, many of which may be CFCs and are currently banned in several countries due to global warming concerns. For superhydrophobic applications, the ternary mixture or solution may simply be sprayed onto a desired surface and allowed to dry. The advantageous properties may be due to pure PVDF and surface texture.

[0065] In other instances, the method may further comprise a step of dropcasting or atomizing the ternary mixture onto a substrate before the contacting of step (ii). Advantageously, atomization may be used to cover large areas of spraying or surface areas of existing infrastructures. Where the ternary mixture is dropcasted onto a substrate, the substrate comprising the dropcasted ternary mixture may be spun at 500 to 3000 rotation per minute (rpm) for 5 seconds to 5 minutes. For instance, the substrate may be spun for 30 seconds. The duration of spinning may depend on the humidity.

[0066] In some instances, the substrate comprising the dropcasted ternary mixture may be submerged in the second non-solvent for 15 seconds to 12 hours to carry out the contacting of step (ii). In some instances, the submersion in the second non-solvent may occur for 30 seconds or even 1 hour to contact the ternary mixture with the second non- solvent e.g. water. The duration of submersion may depend on the temperature of the second non-solvent.

[0067] In some instances, the substrate comprising the atomized ternary mixture may be submerged into the second non-solvent for 30 seconds to 12 hours, or even 1 hour, to carry out the contacting of step (ii). In other instances, the submersion or spray duration may depend on the temperature of the second non-solvent and vice versa. Atomization may be carried out by spraying the ternary mixture through any nozzle capable of generating droplets. The droplets may comprise the ternary mixture which may form the hollow PVDF microspheres when contacted with the second non-solvent.

[0068] In the present method, the first and second non-solvent may independently comprise water or alcohol. In other words, the second non-solvent used to induce phase separation and/or decomposition (i.e. spinodal decomposition) in step (ii) of the present method may comprise water or alcohol independent of the first non-solvent used in step (i). The alcohol may comprise or consist of ethanol. The water non-solvent used in step (i) and/or (ii) may comprise or consist of reverse osmosis water, deionized water, tap water or may be in the form of steam. The water may be in the form of a water bath. The water non-solvent used in step (ii) may have a temperature between 0 and 100°C, 20 to 90°C, 25 to 85°C, 35 to 85°C, 45 to 85°C, 55 to 85°C, 65 to 85°C, 75 to 85°C, 80 to 85°C, 90 to 95°C or any other temperature value or range falling within any of these specified ranges. The water non-solvent used in step (ii) may also be at 100°C or 95°C. The temperatures as specified may be under atmospheric pressure. In some instances, the water to induce phase separation and/or spinodal decomposition in step (ii) may be present as vapour at atmospheric pressure and 100°C.

[0069] In some instances, the method as disclosed herein may further comprise the step of mixing the ternary mixture for 10 seconds to 12 hours or even at least 5 minutes before dropcasting the ternary mixture onto the substrate. Any suitable duration may be applied as long as it helps to ensure a uniform ternary mixture is formed. The mixing may be carried out by vortex mixing or any other suitable mixing means.

[0070] In the present disclosure, there is also a hollow poly(vinylidene difluoride) microsphere comprising a hollow core and a shell layer surrounding the hollow core, wherein the shell layer comprises or consists of poly(vinylidene difluoride) and the shell layer also comprises or consists of an external surface having no apertures that extend from the external surface towards the hollow core. Where the shell layer consists of PVDF, it means no other polymers may be used in the present method and the microsphere may be a pure PVDF microsphere because the shell only consists of PVDF. The shell layer comprising or having no apertures may be taken as the shell layer may not have any holes on its surface and/or may not be porous.

[0071] The size or diameter and roughness may be an important parameter of the PVDF hollow microspheres. Differences in diameter and roughness may have an effect on hydrophobicity of the microspheres. The roughness may also have effects on the packing fraction of microspheres after sintering. This may result in a lower density after the sintering of the microspheres into a bulk object and smaller particles may give higher structural integrity of the sintered object.

[0072] The present method enables the formation of microspheres with uniform diameter distribution of less than 10 μπι, depending on the amount of the first non- solvent added. The hollow PVDF microsphere may comprise a diameter of 0.05 μπι to 15 μηι, 0.1 μηι to 10 μηι, 0.2 μηι to 6 μηι, 0.25 μηι to 5.5 μηι, 0.3 μηι to 5 μηι, 0.5 μηι to 5 μηι, 1 μηι to 5 μηι, 2 μηι to 5 μηι, 3 μηι to 5 μηι, 4 μηι to 5 μηι or any other diameter value or range falling within any of these specified ranges. In the context of present application, diameter refers to the longest distance taken between two points on the external surface of the shell layer measured through the center of the microsphere. The microsphere may be completely or at least substantially spherical.

[0073] In some instances, the shell layer may comprise an external surface which may be uneven, flat or a combination of both. In some instances, the shell layer may comprise an external surface having a surface roughness of 10 or up to 10. Accordingly, a film or foam consisting of the microspheres as described herein may have a surface roughness of 10 or up to 10.

[0074] The use of PVDF to construct the hollow PVDF microspheres as disclosed herein adds to the hydrophobicity of PVDF itself and hence the hollow PVDF microsphere becomes superhydrophobic. As mentioned above, the diameter and surface roughness may be two variables controlled in the present disclosure. They may be both correlated to structure and total surface area, which may be further correlated to the contact angle and thus the degree of superhydrophobicity. The hollowness of the spheres may be responsible for the lower specific mass. Accordingly, the hollow PVDF sphere may be superhydrophobic. Superhydrophobicity may be present when the water contact angle comprises a range between 90° and 180°. Particularly, superhydrophobicity may be indicated by a water contact angle between 150° and 180°.

[0075] In the present disclosure, there is also a film or foam composed of or comprising a plurality of hollow PVDF microspheres obtained by the method as disclosed above. The hollow PVDF microspheres may be dispersed on the film or foam surface or within the film or foam. The film or foam may be made of PVDF or solely PVDF. The hollow PVDF microspheres may also be dispersed on at least one surface of a substrate. The substrate may comprise or consists of PVDF.

[0076] The film or foam may comprise a water contact angle of 90° to 180°, 160 to 171° or any other water contact angle value or range falling within the specified ranges. The film or foam may comprise a water contact angle of 110°, between 90° to 180°, between 110° and 180°, between 90° and 150°, between 110° and 150°, or between 150° and 180°. [0077] In some instances, the plurality of hollow PVDF microspheres may be arranged to form structures that extend away from the poly(vinylidene difluoride) surface or the at least one surface of the substrate. The structures may be finger-like. The height, which may also be termed as the depth, of these structures may be the same or less than the thickness of the film or foam, or the structures that extend away from the poly(vinylidene difluoride) surface or the at least one surface of the substrate, may comprise a depth or height which may be the same as the thickness of the film or foam. The thickness may be 0.1 to 100 μιη, 4 to 20 μιη, 4 to 20 μιη, 5 to 20 μιη, 10 to 20 μιη, 15 to 20 μπι, 5 to 15 μπι, 5 to 10 μπι, 6 to 16 μπι or any other thickness value or range falling within these specified ranges. In some instances, the film or foam may comprise a thickness of up to 100 μπι or a range of 0.1 to 100 μπι.

[0078] The film or foam may be superhydrophobic.

[0079] According to the present disclosure, the method provides tunability of the sphere diameter. The microsphere's diameter may be tuned from any values in the range of 0.2 to 6 um by varying the first non-solvent concentration in the binary solution. The surface roughness may be tuned by varying the temperature of the second non-solvent used to contact the ternary mixture so as to obtain the hollow PVDF microspheres. The density may also be varied by tuning the reaction parameters such as the second non- solvent concentration in the ternary mixture and/or the second non-solvent (i.e. water) temperature of step (ii).

[0080] Advantageously, the present method, the PVDF hollow microspheres and the film or foam enjoy ease of manufacturing. Unlike conventional microsphere synthesis techniques, the present method does not require the expansion of encapsulated low boiling point solvents to induce the hollow structure. The roughened surface texture may be induced during the present method itself.

[0081] The present method to create the spheres may be scaled indefinitely by spraying the polymer solution into a non-solvent bath. The hollow microspheres may then be collected from the surface of the non-solvent for processing.

[0082] The hydrophobic nature of PVDF combined with the high specific surface area of the microspheres or the structures formed from such microspheres on a film or foam provides low surface energy, thereby allowing for a high degree of water repellency which imparts additional benefits such as microbial resistance and self-cleaning properties.

[0083] While the methods described above are illustrated and described as a series of steps or events, it will be appreciated that any ordering of such steps or events are not to be interpreted in a limiting sense. For example, some steps may occur in different orders and/or concurrently with other steps or events apart from those illustrated and/or described herein. In addition, not all illustrated steps may be required to implement one or more aspects or embodiments described herein. Also, one or more of the steps depicted herein may be carried out in one or more separate acts and/or phases.

Examples

[0084] The present disclosure relates to a method for synthesizing polyvinylidene difluoride (PVDF) microspheres. The present disclosure also relates to PVDF microspheres. The PVDF microspheres as disclosed herein were particularly synthesized using a modified non-solvent induced phase separation (NIPS) method. The diameter of the microspheres was controlled from 0.25 to 4 μπι by varying the amount of water added to the PVDF ternary solution. The microsphere roughness was modulated by changing the phase separation temperature and the film thickness was augmented by varying the spin cast rotation per minute (RPM or rpm). The microspheres were shown to be hollow upon solidification and their synthesis are discussed in terms of relative diffusion rates during instantaneous demixing. The microspheres as disclosed herein are capable of applications as chemically resistant superhydrophobic coatings as well as for syntactic foams usable to disperse shock in battle armor, act as a sound-proof barrier for residential and commercial insulation and/or decrease the weight of molded polymeric materials.

[0085] While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced. [0086] Microsphere Synthesis - Example 1

[0087] Example 1 A: Microsphere Synthesis - Spin Cast Synthesis

[0088] A 20 wt% polymer solution was created by dissolving PVDF pellets (Sigma- Aldrich) into Dimethylformamide (DMF) (Sigma-Aldrich) in a 80°C water bath for 3 hours. Quantities of water ranging from 0 to 3 wt% of total mass were loaded into the polymer solution. The polymer solution was then vortex mixed for 5 minutes to ensure uniformity. 50.0 μΙ_, of the polymer solution was then dropcasted onto a substrate and spun at variable RPM (500 to 3000 RPM for 30 seconds). Directly after spinning, the substrate was submerged in deionized water at temperatures ranging from 25°C to 85°C, albeit any other suitable water temperature range as disclosed above may be used. The substrate was left submerged for 30 seconds. After removing the samples from water, they were air dried.

[0089] Example IB: Microsphere Synthesis - Bulk Spray Synthesis

[0090] The dissolved PVDF/DMF/water solution can also be sprayed through an atomizing nozzle onto substrates that can immediately be submerged into or sprayed with warm water for 30 seconds to form superhydrophobic coatings. Additionally, in the bulk synthesis of hollow microspheres for applications in syntactic foams, the solution can be sprayed into a water bath where hollow microspheres instantaneously precipitate. The spheres then float to the top and may be separated (similar to froth flotation techniques). The loose microspheres can then be post-processed using polymer molding techniques such as injection molding or casting etc. This technique eliminates the use of resins and blowing agents, many of which are CFCs and are currently banned in several countries, one of which is United States of America, due to global warming concerns.

[0091] Example 1C: Microsphere Synthesis - Ethanol as Non-Solvent

[0092] Experiments were performed with ethanol as the non-solvent under the same application conditions as water with the same resulting spheres as when water is used as the non-solvent. Ethanol allows for further reducing the viscosity of the solution due to its higher solubility in the mixture with the same resultant sphere properties before hitting the gelation point. This viscosity reduction will make spray application more feasible than the addition of water. [0093] Scanning electron micrographs indicating (figure 8a) large spheres (2.2 μπι) and (figure 8b) small spheres (600 nm) were created by loading differing quantities of ethanol. These experiments have been confirmed over various trials with both ethanol and water as a non-solvent. Ethanol loading requirements to generate these sphere sizes via spinodal decomposition can be seen in figure 9. Trial 2 is repeated using the conditions of trial 1 except for different ethanol loadings. Figure 9 shows the sphere diameter as a result of the ethanol loading quantity. As the ethanol loading increases, the sphere size decreases with a trend similar to that of water. The spheres can be reduced down to 600 nm with this method. More ethanol is required to achieve similar sphere diameters using this method when compared to water (about 50 times more). There is an added benefit of decreasing the ternary solution viscosity by adding ethanol. It allows for easier manufacturing, particularly in spray applications.

[0094] Microsphere Properties - Examples 2 to 7

[0095] Example 2: Microsphere Properties - Effect of Water Loading on Microsphere Size

[0096] Figures la to Id contain scanning electron micrographs (SEM) of PVDF microspheres that were casted onto aluminum substrates and the SEM images show PVDF microspheres formation at increasing loaded water amounts. Sphere sizes range from about 1 μπι in figure la to about 0.3 μπι in figure Id. Inset images are 5 μΐ sessile droplets on each respective surface. Contact angles range from 160° to 171 °. The amount of water loaded into each polymer solution was 0.29 wt%, 0.58 wt%, 1.72 wt%, and 2.83 wt% for figures la to Id, respectively. The wt% of water loaded is based on the ternary mixture. As the water content added to the PVDF solution increases, sphere size decreases. Microspheres with diameters ranging from 3 μπι down to 0.25 μπι were fabricated using modified NIPS technique and a 20 wt% PVDF solution. The behaviour observed with regard to size dependency on water added is consistent with reported results. By adding more water, the polymer degree of saturation increases and a larger number of smaller particles participate upon submersion in the water bath.

[0097] Example 3 : Microsphere Properties - Relationship of Sphere Size, Water Contact Angle and Water Loading

[0098] Figure 2 shows a plot of water contact angle and sphere diameter versus loaded water as a percentage of total mass. Figure 2 also demonstrates the quantitative relationship between sphere size and loaded water fraction, where the diameter decreases with higher water mass loading. The sessile drops' water contact angles (WCAs) are also shown for each water loading and sphere size in the insets of Figure 1, where the sessile drop contact angle initially increases with water mass loading.

[0099] Based on figure 2, an inverse relationship between sphere size and WCA for a given spin angular velocity and submersion bath temperature is observed. The peak WCA measured was 170.5 ± 0.9° for a sphere diameter of 0.652 ± 0.065 μιη. The WCA and sphere size are inversely proportional to each other until the sphere size reaches about 0.5 μπι, when the contact angle begins to decrease with decreasing sphere size. This trend can be explained by an increase in film specific surface area and surface roughness (r) given by the following equation: r = ^AsL

^A cs

[00100] where ASL is the surface area that contacts a liquid wetting the surface and Acs is the cross-sectional area of the substrate. Under wetting conditions, the contact angle for a rough hydrophobic surface will increase according to the equation shown below:

cos θ _α = r cos Θ

[00101] where θ _α is the apparent contact angle on a rough surface and Θ is the contact angle on a smooth flat surface.

[00102] Example 4: Microsphere Properties - Factors Influencing Surface Roughness and Microsphere Arrangement

[00103] The measured decrease in contact angle at small sphere sizes indicates that the water mass loading fraction also control other properties, such as sphere surface roughness and microsphere arrangement, which has a greater effect on the overall surface roughness than the sphere size. This may be observed from figures 1, 2 or 3. Given a monolayer sphere array, for example, the surface roughness is π/2 regardless of the sphere diameter. This is based on the surface area of a surface that consists of hemispheres and is not associated with the water loading. This is used to show that the roughness is primarily given by the roughness of each sphere along with how the spheres are arranged on a surface. Consequently, sphere size has a relatively small direct contribution to surface roughness. [00104] PVDF dissolved in DMF has a large swelling coefficient of 1.43. This indicates that the spheres are significantly larger in the swollen state than after the DMF diffuses out. After the films of various water fractions are casted onto the substrate and submerged into heated water, the spheres precipitate out of the solution and the outside surface of the microspheres immediately solidifies as DMF preferentially diffuses outward into the water. As the DMF continues to diffuse out of the sphere, the polymer solidifies onto the interior walls from the outer radius to the center, resulting in a hollow sphere. Simultaneously, the DMF diffuses outward faster than the polymer can diffuse inward, thereby resulting in voids, similar to Kirkendall voids in semiconductors. These combined effects in varying degrees result in a vacuum inside the sphere and a compressive pressure acting on the outside of the sphere, causing it to collapse while still in a partially swollen state, wrinkling the already solidified exterior surface. The voids form as a function of the controlled input parameters i.e. the amount of polymer, non-solvent in step (i), and the temperature of the non-solvent used to induce phase separation or decomposition of PVDF in step (ii). The range of these parameters are as described above. Poorly formed or collapsed spheres tend to result from lower PVDF weight percentage e.g. 1 wt%. At 20 wt% loading of PVDF, the spheres did not give away to the pressure. The wt% referred to in these instances are based on the total weight of the binary solution.

[00105] Figure 3a shows a scanning electron micrograph of spheres that solidified asymmetrically with a large hollow center and other small voids across the cross- section. Because this sphere lost its pressure difference due to the cavity collapsing, the sphere, particularly its surface, remained smooth and did not wrinkle.

[00106] As larger spheres tend to radially collapse to a greater extent compared to smaller spheres, the outside surface will become more wrinkled and rough (figure 3d) than spheres of smaller size (figure 3e). In other words, the larger spheres experience a higher change in diameter when going from a swollen state to solid polymer which may cause the appearance effect.

[00107] Both of the films created with different water loading fractions in figures 3d and 3e were submerged in 80°C water immediately after spin casting. The larger amount of water added to film (see figure 3 e) increased the polymer degree of saturation and formed a larger quantity of smaller nuclei while smaller quantity of water added to the film for figure 3d formed a smaller quantity of larger nuclei. Upon submersion into hot water, DMF rapidly diffuses out, forcing a transition from a swollen gel state to a solid, hollow, shrinking sphere. The key parameter for synthesis of superhydrophobic PVDF microspheres is control over the counter diffusion rate of water/DMF in and out of the polymer, respectively. This rate increases with temperature and water concentration resulting in instantaneous demixing and rough textured spheres by using high temperature water baths. In contrast, lower temperature water baths result in less textured spheres and lower contact angles.

[00108] Referring to figure 2, it can be observed there is at least one factor for maximizing contact angle, one of which is water loading. The other being the temperature of the non-solvent. For the latter factor, the temperature should be maximized without boiling. The maximum contact angle according to figure 2 demonstrates that water loading not need be maximized.

[00109] If a film is allowed to dry slowly in 50% humidity and at room temperature air, large 5 μπι smooth spheres are formed (figure 3f) with even lower contact angles. Further reducing the humidity below 30% results in a porous membrane-like film (figure 3b) instead of microspheres because the binodal line of the PVDF/DMF/water ternary phase diagram is never crossed. If both polymer solution and the atmosphere are anhydrous, the resulting smooth film will have the bulk contact angle value of PVDF at 86°. A smooth film casts onto float glass from the melt is shown in figure 3c for comparison.

[00110] In summary, each of figure 3 shows a SEM image of (a) hollow PVDF microspheres; (b) a porous film and (c) a smooth film shown relative to the microspheres; surface area decreasing from (d) rough microspheres to (e) small spheres to (f) large agglomerated spheres, corresponding to a decrease in contact angle. Observably, surface area increases with an increased solvent/non-solvent diffusion rate resulting in high WCAs of 170° for the rough sphere films.

[00111] Example 5: Microsphere Properties - Effect of Film Thickness on Surface Roughness

[00112] Another method to increase the surface roughness of a film comprised of the microspheres as disclosed herein is to increase the film thickness by reducing the spin cast angular velocity. Figure 4a shows a plot of film thickness versus angular velocity with a linear decrease in thickness as RPM ascends independently of the water loading fraction prior to spin casting. Upon film submersion and instantaneous demixing, finger-like macro arrangements of the spheres are formed due to the high miscibility of DMF and water. The depths of these finger-like structures are proportional to the film thickness, which result in a higher specific surface area for thicker films spun at a lower angular velocity. The trend of increasing WCA with film thickness is plotted in figure 4b for films with different water loading fractions submerged in an 80°C water bath. The trend is the same regardless of the water loading fraction, which indicates a stronger dependence on macro finger dimensions rather than sphere size when evaluating surface roughness due to differing water loading fractions.

[00113] Example 6: Microsphere Properties - WCA Hysteresis and Slide Angle Measurements

[00114] WCA hysteresis and slide angle measurements indicate a strong correlation with the WCA (figure 5a). There is a linear decrease in both slide angles and hysteresis with an increase in WCA. The large constant negative slope indicates the droplet wets the surface through the entire range of contact angles. This also means the droplets are in Wenzel state through all contact angles. The Wenzel state is a fully wetted state of a roughened surface. Alternatively, the droplets may or may not demonstrate Cassie- Baxter state i.e. which is based on a composite interface, typically with air as the composite. This is expected due to the low hydrophobic contact angle of 86° for smooth PVDF. The wetting-nonwetting transition point can be predicted according to the following equation:

-1

E* = = 1

r cos Θ

[00115] where E ^* is the relative energy between the nonwetting and wetting droplet state. If E ^* is greater than 1, the droplet will be in the wetting state, and E ^* less than 1 predicts that the droplet will be nonwetting. The theoretical transition point to a nonwetting state occurs once the surface roughness is larger than 11.47, much larger than that of the microspheres. Combining this with the correlated hysteresis data and WCA angles that drastically increase with pore depth suggests the film remains in a wetted state across all parameters in this study.

[00116] The synthesis technique as disclosed herein allows for a large range of control. Surface roughness and sphere size can be independently controlled, and consequently, various properties can be optimized, including density. The tenability of the method as disclosed herein opens the door for a plethora of applications that require chemically resistant light-weight polymeric materials.

[00117] Example 7: Microsphere Properties - Sphere Density and Hollowness

[00118] Due to the superhydrophobic nature of films comprising the PVDF microspheres as disclosed herein, accurately determining the density of the microspheres through submersion methods is difficult. The films keep a thin layer of air trap at all surfaces and appear much more buoyant than their density suggests. Figure 6a is a photograph of a dry film floating in water. The film was submerged multiple times, ultrasonicated, and vortex mixed, yet it still does not wet. Figure 6b is a photograph of a film that was wet with a small quantity of acetone and then immediately submerged in water. In this case, although the film has been wet, it still floats. This indicates the spheres are hollow with a bulk density less than 1.

[00119] The film also floats in hexane which has a specific gravity of 0.6. This indicates the spheres have a bulk density less than 0.6. After 2 days of floating in hexane, the film sank to the bottom of the jar due to hexane displacing the air by diffusing into the sphere cavity.

[00120] MicroChannel experiments were performed to isolate the spheres in a channel without air to remove the surface tension effects on the apparent density of the hollow microspheres. The depth profile in figure 7 shows the microspheres float in water while polystyrene spheres do not. This further indicates the microspheres are hollow.

[00121] The depth profile in figure 7 is shown from the bottom (leftmost image) to the top (rightmost image) of a microflow channel filled with water (i.e. specific gravity (SG) is 1), polystyrene spheres with an SG of about 1.05 and hollow PVDF microspheres made according to the embodiments as disclosed herein. The polystyrene spheres are in focus at the bottom of the channel as observed in the first image. As the focal plane is moved up vertically, the polystyrene spheres go out of focus and the agglomerations of PVDF hollow microspheres come into focus. This implies the polystyrene spheres are denser than water while the PVDF microspheres are less dense than water.

[00122] Commerical Applications - Example 8

[00123] Example 8: Commerical Applications [00124] The microspheres obtained by the method as disclosed herein are usable for the manufacture of syntactic foams that can be used as mold resistant heat and acoustic insulation, anti-shock military battle armor and/or to reduce the weight of bulk polymeric materials. Additionally, the microspheres as disclosed herein can be used as a spray on superhydrophobic coating due to their low surface energy.

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