Patent Publication Number: US-11020723-B2

Title: Degradable microcapsules for porosity reduction

Description:
BACKGROUND 
     The present disclosure relates to microencapsulated polymerizing agents and, more specifically, to microcapsules with degradable shells. 
     Self-healing materials are materials that automatically repair themselves by closing or filling in open spaces caused by damage such as cracks, tears, and punctures. For example, some self-healing materials are polymers that are able to re-form bonds and/or form new crosslinks after damage has occurred. Other self-healing materials use polymerizing agents that are released (e.g., from mechanically ruptured microcapsules) when the material is damaged. The released polymerizing agents then polymerize and/or interact with the material (e.g., by crosslinking or partially dissolving) to repair the damaged area. 
     SUMMARY 
     Various embodiments are directed to a microcapsule that includes a degradable shell and a polymerizing agent encapsulated by the degradable shell. The degradable shell can be made of a photo-degradable or heat-degradable polymer. In some embodiments, the degradable shell polymer is a polyamic acid. Examples of polymerizing agents can include diamines and diacid chlorides. 
     Additional embodiments are directed to a method, which includes selecting a polymerizing agent, encapsulating the polymerizing agent in a degradable shell, and rupturing the degradable shell by exposure to an energy source, such as a laser, a UV radiation source, and/or a heat source. Examples of polymerizing agents can include diamines and diacid chlorides. The encapsulating can include extruding the polymerizing agent through an inner nozzle of a coaxial nozzle of a high-power electrostatic spinning machine, and extruding a degradable polymer through an outer nozzle of the coaxial nozzle. The degradable polymer can be a polyamic acid. In some embodiments, the encapsulating can include preparing an aqueous solution that includes a degradable compound, and dispersing the polymerizing agent in the solution to form an emulsion. The degradable compound can be a resveratrol dimer, a coumarin-derived dimer, or a Diels-Alder adduct. 
     Further embodiments are directed to an article of manufacture, which includes a microcapsule having a degradable shell and a polymerizing agent encapsulated by the degradable shell. Examples of polymerizing agents can include diamines and diacid chlorides. The degradable shell can be made of a photodegradable polymer or a heat-degradable polymer. In some embodiments, the degradable shell is made of a polyamic acid. The microcapsule can have a diameter between approximately 20 μm and 200 μm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a flow diagram illustrating a process of forming rupturable microcapsules that encapsulate polymerizing agents, according to some embodiments of the present disclosure. 
         FIG. 1B  is a flow diagram illustrating a process of reducing a material&#39;s porosity using polymerizing agents encapsulated by rupturable microcapsules, according to some embodiments of the present disclosure. 
         FIG. 2  illustrates schematic diagrams showing a generic example of a microcapsule and a generic example of the microcapsule&#39;s shell polymer, according to some embodiments of the present disclosure. 
         FIGS. 3A-3C  are schematic diagrams illustrating a selective laser sintering (SLS) environment, according to some embodiments of the present disclosure. 
         FIG. 4A  is a chemical reaction diagram illustrating a process of forming a degradable polyamic acid (PAA), according to some embodiments of the present disclosure. 
         FIG. 4B  is a chemical structure diagram illustrating a degradable PAA, according to some embodiments of the present disclosure. 
         FIG. 5  is a schematic diagram illustrating an example of a rupturable PAA microcapsule formation environment, according to some embodiments of the present disclosure. 
         FIG. 6  is a chemical reaction diagram illustrating reactions of degradable molecules that can be used to form degradable shells, according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Self-healing materials are materials that automatically repair themselves by sealing open spaces caused by damage such as cracks, tears, and punctures. Self-healing materials can be repaired by intrinsic or extrinsic polymerizing agents. Intrinsic self-healing is carried out by the material itself. That is, self-healing materials can be made of polymers that are able to re-form bonds and/or form new crosslinks after the material has been damaged. Some self-healing polymers require a stimulus such as heat in order to repair damaged areas (e.g., furan-maleimide based polymers), though others can repair damaged areas without an external energy source (e.g., poly(urea-urethane) and various thiol-based polymers). 
     Extrinsic self-healing is carried out by polymerizing agents (e.g., solvents, polymerizable compounds, catalysts, etc.) that are separated from the bulk material until the material is damaged. For example, polymerizing agents can be encapsulated by microcapsules, which are embedded in the bulk material. When the material is damaged, at least a portion of the microcapsule shells are mechanically ruptured. This releases the polymerizing agents into cracks or other openings caused by the damage. The polymerizing agents then polymerize and/or interact with the polymeric bulk material to repair the damaged area. 
     However, because these microcapsule shells must be mechanically ruptured, the polymerizing agents can only be released when sufficient force is applied. Further, the thickness of the microcapsule shells must be carefully controlled in order to prevent the capsules from breaking during normal handling of the material. These requirements limit the types of applications that can use encapsulated polymerizing agents. For example, mechanically rupturable microcapsules cannot be used to fill in pores or other open spaces in materials that have not been damaged by application of sufficient force. 
     Disclosed herein are polymerizing agents encapsulated by microcapsules having degradable shells, which can be ruptured without application of mechanical force. The shells are made of polymers that degrade when exposed to laser energy, heat/infrared radiation, and/or UV light. Therefore, exposing the microcapsules to energy sources such as these causes the shells to degrade, rupturing the microcapsules. In some embodiments, other types of energy can be used (e.g., microwaves, visible light, X-rays, gamma rays, etc.). The encapsulated polymerizing agents are released when the microcapsules are ruptured. The microcapsules can be used to reduce porosity in various materials, such as plastics, rubbers, and other polymeric materials. When the microcapsules are embedded in a porous material, degrading the microcapsule shells causes the encapsulated polymerizing agents to be released into the material&#39;s pores. Reactions of the polymerizing agents and/or polymers in the porous material result in the formation of a solid polymeric material that fills and seals the pores. 
     In some embodiments, the rupturable microcapsules are used to reduce porosity in materials prepared by selective laser sintering (SLS). SLS is a technique for additive manufacturing wherein layers of a powder (e.g., polyamide particles) are sintered by a laser based on a computer-designed three dimensional (3D) model. The sintering fuses the particles together to form a solid. The laser sinters a layer of particles deposited on the SLS machine platform according to the 3D model pattern. The platform is then lowered, and a next layer of particles is deposited onto the sintered layer and sintered according to the same pattern. These steps are repeated until the printed object is complete. 
     However, SLS produces pores in the fused material, thereby mechanically weakening objects manufactured using this method. For example, laser-sintered polyamides can have about 12-16% porosity. Porosity of a completed SLS-printed object can be reduced by infiltration methods, wherein the object is submerged in a liquid metal or polymer that can be solidified in the object&#39;s exterior pores (e.g., pores near the surface of the object). However, this technique adds additional steps, time, and cost to the production of SLS-printed objects. Further, the infiltration method does not fill pores in the interior of the object. Therefore, the material is still weakened significantly by the interior porosity. Another strategy for reducing porosity involves using slower scan rates and higher laser power, but this results in increased cost and production time as well. 
     However, the rupturable microcapsules disclosed herein can reduce porosity in both the interior and exterior of an SLS-printed object without requiring long scan rates or higher laser powers. The rupturable microcapsules can be combined with SLS powder that is deposited on an SLS machine platform. The deposited layer is sintered, and the microcapsules are ruptured by an applied energy source. This releases the encapsulated polymerizing agents into pores in the fused material. The resulting pore-filling polymer is solidified prior to depositing the next layer of SLS powder and microcapsules. These steps can be repeated until the 3D object is complete. 
       FIG. 1A  is a flow diagram illustrating a process  100  of forming rupturable microcapsules that encapsulate polymerizing agents, according to some embodiments of the present disclosure. Rupturable microcapsules are also referred to as “microcapsules” herein. Process  100  includes the selection of a polymerizing agent. This is illustrated at step  110 . Herein, “polymerizing agent” refers to any encapsulated reagent involved in reactions to seal (e.g., fill in or otherwise repair) open spaces (e.g., pores, cracks, etc.) in polymeric materials. 
     Examples of polymerizing agents can include polymerizable compounds such as dicyclopentadiene (DCPD), norbornene and/or its substituted analogues, cyclooctadiene and/or its substituted analogues, lactones, acrylates, acrylic acids, styrenes, isoprene, butadiene, hydroxyl-functionalized isocyanates, epoxies, polymethylmethacrylate (PMMA) chains, thiols, polydimethylsiloxane, etc. In some embodiments, combinations of microcapsules containing different reagents are used. For example, combinations of separately encapsulated diamines and diacid chlorides can react to form polyamides upon release from their respective microcapsules. Similarly, separately encapsulated diols/diisocyanates or diamines/bischloroformates can form polyurethanes, diols/diacid chlorides can form polyesters, and diamines/diisocyanates can form polyureas. 
     Additional examples of polymerizing agents that can be encapsulated by degradable polymer shells include protic solvents, aprotic solvents, or mixtures thereof. These solvents can include anisole, dimethylsulfoxide (DMSO), nitrobenzene, etc. Solvent-based polymerizing agents work by partially dissolving the polymeric material surrounding a pore. The dissolved area is then repolymerized to fill in the pore. In some embodiments, a polymerizable compound can be combined with a solvent such as a healing solvent or a carrier solvent. This can facilitate reactions between polymerizable compounds and the surrounding polymeric material. 
     The polymerizing agent polymerization reactions can also require reagents such as catalysts or initiators. If so, rupturable microcapsules containing the additional reagents can be used in combination with the microcapsules containing the polymerizable compounds and/or solvents. Some examples of additional reagents can include DCPD/tungsten(VI) chloride or Grubbs&#39; catalyst, polydimethylsiloxane/tin catalyst, epoxy (e.g., epoxy vinyl ester)/peroxide initiators (e.g., benzol peroxide, lauroyl peroxide, methyl ethyl ketone peroxide, tert-butyl peroxide, tert-butyl peroxybenzoate, etc.), aniline-based activators (e.g., 4,4′-methylene bis(N,N-dimethyl aniline), 4,N,N-trimethylaniline, N,N-dimethylaniline, etc.), radical initiators (e.g., benzoyl peroxide or 2,2′-azo-bis-isobutyrylnitrile (AIBN)), ionic initiators (e.g., butyl lithium or aluminum trichloride), or Zeigler-Natta catalysts. Additional examples can include epoxy polymerization catalysts and/or accelerators (e.g., tertiary amines, Lewis acids, boron trifluoride- or boron trichloride-amine complexes, benzyl dimethyl amine (BDMA), imidazoles, boric acid esters, etc.). Combinations of appropriate polymerizing agents are known to persons of ordinary skill in the art. 
     A degradable polymer capable of forming the rupturable microcapsule shell is selected. This is illustrated at step  120 . However, it should be noted that step  120  can be carried out before or at the same time as step  110  in other embodiments. Degradable polymers can include light- and or heat-sensitive polymers. Additionally, various polymers can form microcapsule shells that can be degraded by laser energy. 
     For example, polymers that can form laser-degradable shells can include polyamides, polyurethanes, polyureas, polyesters, urea-formaldehydes, melamine-formaldehyde/melamine-urea-formaldehydes (MF/MUF), etc. The degradable shell can also be made of a polymer that degrades upon exposure to short-wave ultraviolet (UV or UVC) radiation and/or heat. These polymers can be degraded by a laser as well in some embodiments. Polymers such as this include at least one type of repeat unit having light- or heat-sensitive bonds. Examples of light-sensitive monomers that can be used to form these repeat units can include dimers of resveratrol or coumarin, cinnamic esters, and substituted derivatives thereof. An example of a heat-sensitive monomer is a Diels-Alder adduct of a maleimide and a furan. Light-sensitive and heat-sensitive degradable shells are discussed in greater detail with respect to  FIG. 6 . Further, in some embodiments the degradable shell can be made of a polyamic acid (PAA). Examples of degradable PAA shells are discussed in greater detail with respect to  FIGS. 4A-5 . 
     Degradable microcapsules are then prepared from the selected polymerizing agent and degradable polymer. This is illustrated at step  130 . Techniques for microcapsule formation can include emulsion and electrospray techniques. These techniques are discussed in greater detail with respect to  FIGS. 5 and 6 , respectively. In some embodiments, the rupturable microcapsules have diameters of about 20 μm-200 μm. However, the microcapsules can be any appropriate size (e.g., approximately 10 μm-1000 μm in diameter). In other embodiments, the microcapsules can be nanoscale capsules having diameters between about 10 nm and 1000 nm. 
     The size of the microcapsules can be varied by adjusting factors such as stir speed, temperature, and/or concentration during their formation. The microcapsules can be spherical or approximately spherical, depending upon the materials and reaction conditions used in their formation. Various techniques known to persons of ordinary skill in the art can be used to adjust shell thickness, size distribution, and other properties of the microcapsules. An example of a rupturable microcapsule is illustrated in  FIG. 2 . 
     The microcapsules formed in process  100  can be combined with various polymeric materials in order to form self-healing materials or to reduce porosity. For example, the microcapsules can be used to reduce porosity in materials formed via selective laser sintering (SLS). This is discussed in greater detail with respect to  FIG. 1B . Additionally, heat- and/or light-sensitive microcapsules can be used to release polymerizing agents in applications other than laser sintering (e.g., to fill in other porous materials, to supplement polymerizing agents encapsulated by mechanically rupturable microcapsules, etc.). 
       FIG. 1B  is a flow diagram illustrating a process  135  of reducing a material&#39;s porosity using polymerizing agents encapsulated by rupturable microcapsules, according to some embodiments of the present disclosure. A single layer of rupturable microcapsules combined with a powder for SLS is prepared. This is illustrated at step  140 . The layer is prepared by depositing a mixture of rupturable microcapsule and SLS powder onto the bed of an SLS machine. The rupturable microcapsules can be microcapsules prepared in process  100  ( FIG. 1A ). The powder can be any particles appropriate for use in SLS. These SLS powder particles can be made of materials such as polyamides (PA) (e.g., PA-6, PA-11, PA-12, etc.), polyether ether ketones (PEEK), thermoplastic elastomers, polyurethanes, polyether block amides (PEBA), polycarbonates, polystyrenes, etc. However, other materials can be used (e.g., polypropylene, polyethylene, acetonitrile butadiene styrene (ABS), metals, ceramics, etc.). 
     In some embodiments, the layer has a thickness of approximately 0.20 mm. However, the layer can have any appropriate thickness (e.g., about 0.10 mm-0.30 mm). In some embodiments, the weight percent (wt %) of the microcapsules in the layer is about 2.5 wt % or about 5 wt %. However, other microcapsule concentrations can be used (e.g., about 1-2.5 wt %, about 2.5-5 wt %, or about 5-10 wt %). Microcapsules having approximately the same diameters (e.g., about 20 μm-200 μm) as the particles of SLS powder can be used. Microcapsules of different sizes can be used in other embodiments. An example of a microcapsule/SLS powder layer is illustrated in  FIG. 3A . 
     A laser is rastered across areas of the deposited layer, and the microcapsules in the deposited layer are ruptured. This is illustrated at step  150 . Before the laser is applied, the temperature of the deposited layer of microcapsules and SLS powder is raised to just below the SLS powder particles&#39; melting point. Degradable microcapsules having decomposition onset temperatures above the melting point of the SLS powder particles can be selected at step  140  in order to prevent the shells from rupturing before the laser sintering has begun. The laser fuses the SLS particles according to a pattern from a computer-aided design (CAD) file. In some embodiments, the laser is a carbon dioxide (CO 2 ) laser, though other lasers can be used as well (e.g., Nd:YAG lasers, fiber lasers, etc.). Laser power and other parameters can vary, and are selected according to conventional SLS parameters. For example, a laser power between about 9 W and 15 W can be used to sinter PA-12 particles in some embodiments. 
     The laser can also cause the degradable microcapsules in the sintered layer to rupture. When the degradable shells of the microcapsules include light- or heat-sensitive polymers, further microcapsule shell degradation can be accomplished by application of UV light at appropriate wavelengths (e.g., below about 280 nm) and/or heat (e.g., approximately 140-180° C.). In some embodiments, the light- and/or heat-degradable shells are not ruptured by the laser. The microcapsules can instead be ruptured by UV light or heat, respectively, after the laser sintering has occurred. The ruptured microcapsules release their encapsulated polymerizing agents into pores in the surrounding sintered material. 
     The released polymerizing agents form polymers in the pores, which are then cured. This is illustrated at step  160 . Curing solidifies, hardens, and/or toughens the polymers formed within the pores. For example, curing can facilitate formation of crosslinking bonds in a polymer. The curing can be accomplished by application of UV radiation, heat, and/or other curing agents, depending upon the type of polymerizing agent and/or sintered material. In some embodiments, the curing can occur without application of heat and/or UV radiation. For example, polymerizing agents that form suitable polymers without an additional curing agent can be used. In these instances, the curing step can be waiting a suitable amount of time to allow the polymer to solidify within the pores of the sintered layer. Additionally, the curing agent can be a chemical compound (e.g., vinyl-functionalized silicon dioxide (SiO 2 )) released from different ruptured microcapsules in the layer. However curing agents can be added in other ways. For example, microcapsules can be double-walled, wherein each capsule contains a polymerizing agent and another healing or curing agent separated by a rupturable inner wall. These and other techniques for polymerization and curing are known to persons of ordinary skill in the art. 
     When the pore-filling polymer has cured, the bed of the SLS machine is lowered by the thickness of one layer. Process  135  then returns to step  140 , and a next layer of the SLS powder and rupturable microcapsule mixture is deposited onto the sintered layer. Process  135  repeats steps  140 - 160  until the SLS machine has completed printing the design provided by the CAD file or received instructions to stop. After a cooling period (e.g., to room temperature), the printed object is removed from the SLS machine platform. 
       FIG. 2  illustrates schematic diagrams showing a generic example of a microcapsule  200  and a generic example of the microcapsule&#39;s shell polymer  205 , according to some embodiments of the present disclosure. The microcapsule  200  is an example of a rupturable microcapsule prepared in process  100  (illustrated in  FIG. 1A ). The rupturable microcapsule  200  includes a degradable shell  210  encapsulating a polymerizing agent  220  (e.g., a monomer, an initiator or catalyst, and/or a solvent). The shell polymer  205  is a generic example of a degradable copolymer from which the shell  210  can be made. The shell polymer  205  is a copolymer with two different types of monomer repeat units, which are represented by a white box (x block)  240  and a gray box (y block)  250 , where x and y are each integers greater than 1. 
     In some embodiments, the x block  240  and/or y block  250  monomers are photo- or heat degradable. However, the shell  210  can be made of any polymer that can be degraded by a laser, such as a laser from an SLS machine. Examples of these polymers are discussed in greater detail with respect to  FIG. 1A . The polymer  205  can be a block copolymer or another type of copolymer (e.g., an alternating or random copolymer). In some embodiments, the polymer  205  can include more than two types of monomer (see, e.g.,  FIG. 4B ). In other embodiments, the polymer  205  is a homopolymer (where x or y=0). Further, the polymer  205  can be a crosslinked network of monomers and, optionally, additional crosslinking molecules. 
       FIGS. 3A-3C , are schematic diagrams illustrating a selective laser sintering (SLS) environment, according to some embodiments of the present disclosure.  FIG. 3A  illustrates a prepared layer  305  of particles  310  on an SLS machine bed  320 . This layer can be prepared according to the processes of step  140  of process  135  ( FIG. 1B ). The particles  310  include SLS powder particles (gray spheres) and rupturable microcapsules (white spheres). In some embodiments, the layer  305  is approximately 0.2 mm thick, and the particles  310  include PA-12 SLS particles combined with rupturable microcapsules (e.g., ˜2.5-5 wt %). Each microcapsule contains a polymerizing agent. For example, each microcapsule can contain a solvent-based polymerizing agent. In another example, the layer  305  can have a mixture of two sets of microcapsules, one set encapsulating DCPD and the other set encapsulating a catalyst such as Grubbs&#39; catalyst, Shrock&#39;s catalyst, or tungsten(VI) hexachloride with diethylaluminum chloride (WCl 6 /Et 2 AlCl). 
       FIG. 3B  illustrates a layer  325  of sintered material  330  on the SLS machine bed  320 . The sintered material  330  is prepared by rastering a laser source  335  over regions of the prepared layer  305  ( FIG. 3A ) according to a pattern from an input CAD file. This results in fusion of the SLS powder (gray) and rupture of the microcapsules (white) exposed to laser energy from the laser source  335 . Rupture of the microcapsules releases the encapsulated materials, thereby filling accessible pores in the sintered material  330  with polymerizing agents  340 . For example, the released polymerizing agents  340  can include DCPD and Grubbs&#39; catalyst, which can react to form poly-DCPD in the pores of the sintered material  330 . 
       FIG. 3C  illustrates a layer  345  of the sintered material  330  filled in with a polymer  360  formed by the polymerizing agents  340  ( FIG. 3B ) on the SLS machine bed  320 . The polymerizing agents  340  are cured by an energy source  350 , forming a solid polymer  360  that seals the pores in the sintered material  330 . The energy source  350  is moved or reflected across the sintered layer  345 . The energy source  350  can be attached to the same scanning system as the laser source  335  ( FIG. 3B ), or it can be independent of the laser source  330 . The energy source  350  is selected based on the types of polymer  360  and sintered material  330  in the layer  345 . For example, the energy source  350  can provide short-wave (e.g., ˜250 nm) UV radiation, heated air, infrared radiation, etc. Additional techniques for curing the polymer  360  are discussed in greater detail with respect to  FIG. 1B . 
       FIG. 4A  is a chemical reaction diagram illustrating a process  400  of forming a degradable polyamic acid (PAA), according to some embodiments of the present disclosure. The PAA can be used to form a degradable microcapsule shell. A solution of a diamino coumarin dimer  420 , 4,4′-oxydianiline (4,4′-ODA)  430 , and 2,6-diaminoanthraquinone (2,6-DAAQ)  440  in N,N′-dimethylacetamide (DMAc) is prepared, and then cooled to about 0° C. Pyromellitic dianhydride (PMDA)  450  is added to the solution, which is then stirred until a viscous solution forms (e.g., for about 6 hours). The product of this reaction is illustrated in  FIG. 4B . 
       FIG. 4B  is a chemical structure diagram illustrating a degradable PAA  460 , according to some embodiments of the present disclosure. The degradable PAA  460  is formed in process  400  ( FIG. 4A ), and can be stored as a viscous solution at about −20° C. The degradable PAA  460  includes three repeat units, where x, y, and z are integers greater than 1. For example, each of x, y, and z can independently be an integer between 1 and 1,000,000 (e.g., about 1-25,000, about 500-25,000, about 1,000-25,000, about 2,000-25,000, about 5,000-25,000, about 25,000-50,000, about 50,000-500,000, etc.). The starred bond in the y block represents a bond to the starred nitrogen atom in the z block. 
     The coumarin dimer repeat unit (x) is light-sensitive, and allows the polymer to degrade when exposed to UV light at or below approximately 255 nm. In some embodiments the degradable PAA  460  is a low-molecular weight PAA. For example, the number average molecular weight (M N ) of the degradable PAA  460  can be between about 4,000 g/mol and 6,000 g/mol. However, in some embodiments the degradable PAA  460  has another value of M N  (e.g., ˜250 g/mol-450 g/mol, ˜450 g/mol-700 g/mol, ˜700 g/mol-2,000 g/mol, ˜2,000 g/mol-15,000 g/mol, etc.). Higher molecular weights can also be obtained in other embodiments, such as above approximately 25,000 g/mol (e.g., ˜25,000-100,000 g/mol). 
     By adjusting the ratios of diamine monomers (4,4′-ODA, 2,6-DAAQ, and diamino coumarin dimer) in process  400 , the lengths of the x, y, and z blocks can be varied relative to one another. Additionally, if 4,4′-ODA and/or 2,6-DAAQ are left out of process  400 , an analogous photodegradable PAA, where y and/or z=0, can be formed. Further, a non-photodegradable PAA, where x=0, can be formed if the light-sensitive diamino coumarin dimer is omitted from process  400 . In these instances, microcapsule shells made from the non-photodegradable PAA can be ruptured by laser energy. Further, alternative diamine monomers (not shown) can be used in order to form PAA microcapsules having different mechanical and/or chemical properties. 
       FIG. 5  is a schematic diagram illustrating an example of a rupturable PAA microcapsule formation environment  500 , according to some embodiments of the present disclosure. A high-voltage (e.g., ˜5 kV-50 kV) electrostatic spinning machine  505  is used to prepare PAA microcapsules  510  via electrospray techniques. The high-voltage electrostatic spinning machine  505  includes a first syringe  515  and a second syringe  520 , both of which are connected to a coaxial nozzle  530  having inner and outer nozzle diameters of about 0.8 mm and 1.6 mm, respectively. A solution of a degradable PAA (˜18 wt % in DMAc)  540 , such as the degradable PAA  460  illustrated in  FIG. 4B , is placed in the first syringe  515 , and a polymerizing agent  550  is placed in the second syringe  520 . The polymerizing agent  550  can be a liquid polymerizing agent or a solution containing a polymerizing agent. Examples of polymerizing agents that can be used are discussed in greater detail with respect to  FIG. 1A . 
     The degradable PAA solution  540  is extruded through the outer nozzle of the coaxial nozzle  530  at a rate of about 1.250 mL/h while the polymerizing agent  550  is extruded through the inner nozzle of the coaxial nozzle  530  at a rate of about 0.375 mL/h. A voltage of about 20 kV is applied to the extruded materials  540  and  550 , resulting in the formation of microcapsules  510  having degradable PAA  540  shells and polymerizing agent  550  cores. The microcapsules  510  are removed from the electrostatic spinning machine  505 , and heated in an oven (not shown) at about 170° C. The heating causes the PAA shells of the microcapsules  510  to imidize, thereby forming thermally stable microcapsules having polyimide shells (not shown). Herein, “thermally stable” indicates that the microcapsules have decomposition onset temperatures of about 350° C.-400° C. or higher. The thermally stable microcapsules can be combined with SLS powder to form a particle layer (see, e.g.,  FIG. 3A ) at step  140  of process  135  ( FIG. 1A ). 
       FIG. 6  is a chemical reaction diagram illustrating reactions  600 ,  605 , and  610  of degradable molecules that can be used to form degradable shells, according to some embodiments of the present disclosure. Processes  600 ,  605 , and  610  involve the formation of compounds that can be degraded in light- or heat-activated reactions. Degradable polymers for microcapsule shells such as the shell  210  illustrated in  FIG. 2  can be formed by polymerizing these light- or heat-sensitive compounds, optionally with additional monomers. The microcapsule shells can then be produced via emulsion processes. Emulsion is discussed in greater detail below. 
     The compounds illustrated in  FIG. 6  each have one or more functional groups represented by “M”. M can be a functional group bound directly to an illustrated compound. However, M can also include a hydrocarbon bridging group (R) linking the M functional group to the illustrated compound. Examples of M groups can include hydroxyls (—OH or —ROH), amines (—NH 2  or —RNH 2 ), carboxylic acids (—COOH or —RCOOH), isocyanates (—NCO or —RNCO), acid chlorides (—COCl or —RCOCl), chloroformates (—OC(O)Cl or —ROC(O)Cl), vinyls (—CH═CH 2  or —RCH═CH 2 ), and epoxides (—C 2 H 3 O or —RC 2 H 3 O). However, M can represent any appropriate functional group (e.g., propylene carbonate, alkyl halides, esters, alkynes, etc.). In some embodiments, functional groups on one or more M group can result in shells having orthogonal functionality provided by these groups. 
     Examples of R bridging groups can include alkyl groups. Herein, “alkyl” refers to C 1 -C 100  radicals, which can be linear, branched, or cyclic. In these instances, M can be a substituted analogue of an alkyl such as methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, pentyl, hexyl, octyl, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclooctyl. In some embodiments, the alkyls are unsaturated (e.g., alkenes and alkynes). As used herein, the term “substituted” indicates that a hydrogen atom has been replaced with a carbon atom, a heteroatom, or a heteroatom-containing group. For example, a “substituted alkyl” is a radical made of carbon and hydrogen where at least one hydrogen is replaced by a carbon atom or a heteroatom (e.g., an oxygen or nitrogen atom). In the preceding M group examples, the functional groups are substituents that replace hydrogen atoms on the R bridging groups. 
     M can also be a substituted analogue of a cyclic alkyl group. When cyclic, the alkyl groups can be aromatic or non-aromatic. Herein, the term “aromatic” also refers to pseudoaromatic heterocycles, which are heterocyclic substituents that have similar properties and structures (nearly planar) to aromatic heterocyclic ligands, but are not by definition aromatic. Examples of cyclic aromatic alkyls (referred to herein as “aryl groups”) that can be used can include six-carbon aromatic rings (phenyl) and substituted variants thereof (e.g. 2-methyl-phenyl, xylyl, tolyl, etc.), C 4 -C 60  aromatic rings, C 4 -C 20  aromatic rings, etc. The cyclic groups can optionally include heteroatoms (e.g., nitrogen, oxygen, or sulfur) replacing at least one carbon atom in the cyclic structure. 
     In process  600 , resveratrol  615  is dimerized in the absence of electromagnetic radiation having wavelengths shorter than about 300 nm. That is, dimerization is carried out under light having wavelengths longer than approximately 300 nm (e.g., visible and/or long-wave UV light). Herein, “resveratrol” refers to both unsubstituted resveratrol (3,5,4′-trihydroxy-trans-stilbene) and its functionalized derivatives. The dimerization produces a resveratrol dimer  620  that can be retro-dimerized by exposure to electromagnetic radiation at wavelengths below about 260 nm. In process  605 , a coumarin derivative  630  (e.g., 7-amino-4-methylcoumarin) is dimerized in the absence of electromagnetic radiation having wavelengths shorter than about 300 nm. The resulting compound, referred to herein as a “coumarin dimer  640 ”, can be retro-dimerized by exposure to electromagnetic radiation at wavelengths below about 255 nm. In some embodiments, similar reversable dimers (not shown) can be formed from cinnamic esters or various stilbenoids. The photosensitive dimers can be used to form photodegradable microcapsule shells (see below). 
     In process  610 , a maleimide  650  and a furan  660  are reacted in a Diels-Alder reaction to form an adduct  670 . The maleimide  650  and the furan  660  each have M groups, which are independently selected from the aforementioned M groups. M groups without R bridging groups are not selected for the maleimide  650 . An R bridging group on the maleimide  650  is illustrated separately from the M group in  FIG. 6  in order to indicate that M groups on the maleimide  650  each include a hydrocarbon bridging group. It should be noted however that the M group on the furan  660  can optionally include an R bridging group as well. The maleimide  650  also includes a heteroatom represented by “Y”. Y is an oxygen atom or a nitrogen atom. The reaction to form the adduct  670  is carried out under appropriate Diels-Alder reaction conditions, which are known to persons of ordinary skill in the art. The reaction can be reversed by heating the adduct  670  to about 140-180° C. or greater. Therefore, the heat-sensitive adduct  670  can be used to form heat-degradable microcapsule shells. 
     The degradable compounds  620 ,  640 , and  670  can be used to form rupturable microcapsule shells using emulsion techniques. The emulsion techniques are not illustrated in  FIG. 6 . However, the following is an example emulsion procedure: An aqueous solution containing about 2.5 wt % ethylene maleic anhydride (EMA) is prepared, and stirred at room temperature. Urea (e.g., to a final concentration of about 20 g/L), ammonium chloride (e.g., to about 2 g/L), and a degradable compound  620 ,  640 , or  670  (e.g., to about 2 g/L) are then dissolved in the EMA solution at room temperature. A solution of sodium hydroxide (e.g., ˜10 wt %) is then added until the solution reaches approximately pH 3.5. A small amount of 1-octanol (e.g., a few drops per liter) can be added to the solution in order to eliminate surface bubbles. A selected polymerizing agent (e.g., DCPD) is then dispersed in the aqueous phase to form an emulsion. Examples of polymerizing agents that can be used are discussed in greater detail with respect to  FIG. 1A . 
     The emulsion is allowed to stabilize for about 10 min. Then, a formaldehyde solution (˜37 wt %) is added to the emulsion to obtain an approximately 1:2 ratio of formaldehyde to urea. The reaction vessel is then covered, and the emulsion is heated to about 55° C. at a rate of about 1° C./min. The emulsion is agitated in a mixer for about 4 hours at this temperature, and then cooled to ambient temperature when the agitation has ended. This results in a suspension of rupturable microcapsules, which encapsulate droplets of the polymerizing agent in shells formed by crosslinking the degradable compounds  620 ,  640 , or  670 . The microcapsule suspension is then filtered (e.g., with a coarse-fritted filter). The filtered microcapsules can be rinsed with deionized water, and air dried (e.g., for about 24 h-48 h). 
     The compounds described herein can contain one or more chiral centers. Unless otherwise noted, the disclosed structures cover all stereoisomers, conformers, rotamers, isomers, and enantiomers of the represented compounds. Further, polymers or other materials containing the disclosed compounds can include racemic forms of the compounds in addition to individual stereoisomers, as well as mixtures containing any of these. Substituents on the compounds described herein may participate in additional chemical reactions, transformations, or interactions, which can include synthesis, decomposition, single and/or double replacement, oxidation/reduction, acid/base, nucleophilic, electrophilic and radical substitutions, addition/elimination reactions, and polymerization reactions. 
     Where isomers of a named alkyl, alkenyl, alkoxy, aryl, or other functional group exist (e.g., n-butyl, iso-butyl, sec-butyl, and tert-butyl), reference to a member of the group without specifying a particular isomer (e.g., butyl) is intended to include all isomers in the family (e.g., n-butyl, iso-butyl, sec-butyl, and tert-butyl). Further, unless specified otherwise, reference to one member of the group (e.g., n-butyl) includes the remaining isomers in the family (e.g., iso-butyl, sec-butyl, and tert-butyl). 
     Unless otherwise noted, chemical reactions are performed at ambient conditions or under slight heating with no special atmosphere or head space, and may be performed using standard organic solvents to manage mix properties such as viscosity and flow index. Standard procedures for quenching reactions, solvent removal, and purification are performed. Room temperature is between about 15° C. and 25° C. unless otherwise indicated. Ranges (e.g., time, concentration, temperature, etc.) indicated herein include both endpoints and all numbers between the endpoints. Unless specified otherwise, the use of “about,” “approximately,” or a tilde (˜) in connection with a range applies to both ends of the range (e.g., “approximately 1 g-5 g” should be interpreted as “approximately 1 g-approximately 5 g”). Unless otherwise indicated, modifying terms such as “about,” “approximately,” and “˜” indicate +/−10% of a recited value, range of values, or endpoints of one or more ranges of values. 
     The processes discussed herein and their accompanying drawings are not to be construed as limiting. One skilled in the art would recognize that a variety of techniques may be used that vary in conditions, components, methods, etc., which ultimately generate rupturable microcapsules in porous materials. In addition, the conditions can optionally be changed over the course of a process. Further, in some embodiments processes can be added, omitted, or carried out in alternate orders, while still remaining within the scope of the disclosure, as will be understood by a person of ordinary skill in the art. It should also be noted that processes can be carried out by a single entity, or by multiple entities. For example, a first entity may encapsulate polymerizing agents in rupturable microcapsules, and a second entity may fill in pores in laser-sintered materials using these microcapsules.