Kinetically Controlled Photoinduced Phase Separation for Hybrid Radical/Cationic Systems

Heterogeneous Acrylic Resins with Bicontinuous Nanodomains as Low-Modulus Flexible Adhesives

Adhesives play a critical role in the assembly of electronic devices, particularly as devices become more diverse in form factors. Flexible displays require highly transparent and rapidly recoverable adhesives with a certain stiffness. In this study, novel structured adhesives are developed that incorporate bicontinuous nanodomains to fabricate flexible adhesives with low moduli. This structure is obtained via polymerization-induced microphase separation using a macro chain transfer agent (CTA). Phase separation is characterized using small-angle X-ray scattering, transmission electron microscopy, and dynamic mechanical analysis. By optimizing the length of the macro CTA, an adhesive with both hard and soft nanodomains is produced, resulting in exceptional flexibility (strain recovery = 93%) and minimal modulus (maximum stress/applied strain = 7 kPa), which overperforms traditional adhesives. The optimized adhesive exhibits excellent resilience under extensive strain, as well as strong adhesion and transparency. Furthermore, dynamic folding tests demonstrate the exceptional stability of the adhesive under various temperature and humidity conditions, which is attributed to its unique structure. In summary, the distinct bicontinuous phase structure confers excellent transparency, flexibility, and reduced stiffness to the adhesive, rendering it well-suited for commercial foldable displays and suggesting potential applications in stretchable displays and wearable electronics.

Effect of Block Copolymer Self-Assembly on Phase Separation in Photopolymerizable Epoxy Blends

Directing self-assembly of photopolymerizable systems is advantageous for controlling polymer nanostructure and material properties, but developing techniques for inducing ordered structure remains challenging. In this work, well-defined diblock or random copolymers were incorporated into cationic photopolymerizable epoxy systems to investigate the impact of copolymer architecture on self-assembly and phase separated nanostructures. Copolymers consisting of poly(hydroxyethyl acrylate)-x-(butyl acrylate) were prepared using photoiniferter polymerization to control functional group placement and molecular weight/polydispersity. Prepolymer configuration and concentration induced distinctly different effects on the resin flow and photopolymerization kinetics. The diblock copolymer self-assembled into nanostructured phases within the resin matrix, whereas the random copolymer formed an isotropic mixture. Rapid photopolymerization and ambient temperature conditions during cure facilitated retention of the self-assembled phases, leading to considerably different composite morphology and thermomechanical behavior. Increased loading of the diblock copolymer induced long-range ordered cocontinuous structures. Even with nearly identical prepolymer composition, controlled nanophase separation resulted in significantly enhanced tensile properties relative to those of the isotropic system. This work demonstrates that controlling phase separation with a block copolymer architecture allows access to nanostructured photopolymers with unique and enhanced properties.

Interfacial energy as an approach to designing amphipathic surfaces during photopolymerization curing

Photopolymerization induced phase separation (PIPS) is a platform capable of creating heterogeneous materials from initially miscible resin solutions, where both the reaction's governing thermodynamics and kinetics significantly influence the resulting phase composition and morphology. Here, PIPS is used to develop materials in a single photopolymerization step that are hydrophobic on one face and hydrophilic on the other. These two faces possess a water contact angle difference of 50°, bridged by a bulk-scale chemical gradient. The impact of the PIPS-triggering inert additive is investigated by increasing the loading of poly(methyl methacrylate) (PMMA) in an acrylonitrile/1,6-hexanediol diacrylate comonomer resin. The extent of phase separation in the sample network depends on this loading, with increasing PMMA corresponding to macroscale domains that are more chemically and mechanically distinct. A significant period between the onsets of phase separation and reaction deceleration, determined using in situ FT-IR, facilitates this enhanced phase segregation in PMMA-modified samples. Spatially directed domain formation can be further promoted using multiple interface types in the sample mold, here, glass and stainless steel. With multiple interface types, interfacial rearrangements to minimize surface energy during resin photopolymerization result in a hydrophobic face that is nitrile-rich and a hydrophilic face that is nitrile-poor (e.g., acrylate-rich). Using this strategy, patterned wettability on a single face can also be engineered. This study illustrates the capabilities of PIPS for complex surface design and in applications requiring stark differences in surface character without sharp interfaces.

Compatibility versus reaction diffusion: Factors that determine the heterogeneity of polymerized adhesive networks

OBJECTIVES

Heterogeneity and phase separation during network polymerization is a major issue contributing to the failure of dental adhesives. This study investigates how the ratio of hydrophobic crosslinkers to hydrophilic comonomer (C/H ratio), as well as cosolvent fraction (ethanol/water) influences the degree of heterogeneity and proclivity for phase separation in a series of model adhesive formulations.

METHODS

Twelve formulations were investigated, with 4 different C/H ratios (7:1, 2.2:1, 1:1, 0.5:1) and 3 different overall cosolvent fractions (0, 10 and 20 wt%). The heterogeneity and phase behavior were characterized using Fourier Transform Infrared Spectroscopy (FT-IR), dynamic mechanical analysis (DMA), small-angle x-ray scattering (SAXS) and atomic force microscopy (AFM).

RESULTS

In resins without cosolvent, all characterizations confirm reduced heterogeneity as C/H ratio decreases. However, when 10 or 20 wt% of cosolvent is included in the adhesive formulation, a higher degree of heterogeneity and even distinct phase separation with domains ranging from a few hundreds of nanometers to a few micrometers in size form. This is particularly noticeable at lower C/H ratios, which is surprising as HEMA is commonly considered a compatibilizer between hydrophobic crosslinkers and aqueous (co)solvents.

SIGNIFICANCE

Our experiments demonstrate that formulations with lower C/H ratio and thus a lower viscosity experience later onsets of diffusion limitations during polymerization, which favors thermodynamically driven phase separation. Therefore, to determine or predict the resulting phase structure of adhesive materials, it is necessary to consider the kinetics and diffusion constraints during the formation of the polymer network and not just the compatibility of resin constituents.

Light-Mediated Polymerization Catalyzed by Carbon Nanomaterials

Carbon nanomaterials, specifically carbon dots and carbon nitrides, play a crucial role as heterogeneous photoinitiators in both radical and cationic polymerization processes. These recently introduced materials offer promising solutions to the limitations of current homogeneous systems, presenting a novel approach to photopolymerization. This review highlights the preparation and photocatalytic performance of these nanomaterials, emphasizing their application in various polymerization techniques, including photoinduced i) free radical, ii) RAFT, iii) ATRP, and iv) cationic photopolymerization. Additionally, it discusses their potential in addressing contemporary challenges and explores prospects in this field. Moreover, carbon nitrides, in particular, exhibit exceptional oxygen tolerance, underscoring their significance in radical polymerization processes and allowing their applications such as 3D printing, surface modification of coatings, and hydrogel engineering.

From Unregulated Networks to Designed Microstructures: Introducing Heterogeneity at Different Length Scales in Photopolymers for Additive Manufacturing

Photopolymers have been optimized as protective and decorative coating materials for decades. However, with the rise of additive manufacturing technologies, vat photopolymerization has unlocked the use of photopolymers for three-dimensional objects with new material requirements. Thus, the originally highly cross-linked, amorphous architecture of photopolymers cannot match the expectations for modern materials anymore, revealing the largely unanswered question of how diverse properties can be achieved in photopolymers. Herein, we review how microstructural features in soft matter materials should be designed and implemented to obtain high performance materials. We then translate these findings into chemical design suggestions for enhanced printable photopolymers. Based on this analysis, we have found microstructural heterogenization to be the most powerful tool to tune photopolymer performance. By combining the chemical toolbox for photopolymerization and the analytical toolbox for microstructural characterization, we examine current strategies for physical heterogenization (fillers, inkjet printing) and chemical heterogenization (semicrystalline polymers, block copolymers, interpenetrating networks, photopolymerization induced phase separation) of photopolymers and put them into a material scientific context to develop a roadmap for improving and diversifying photopolymers' performance.

Hierarchically Structured Metal-Organic Framework Polymer Composites for Chemical Warfare Agent Degradation

Metal-organic frameworks (MOFs) have captured the imagination of researchers for their highly tunable properties and many potential applications, including as catalysts for a variety of transformations. Even though MOFs possess significant potential, the challenges associated with processing of these crystalline powders into usable form factors while retaining their functional properties limit their end use applications. Herein, we introduce a new approach to construct MOF-polymer composites via 3D photoprinting to overcome these limitations. We designed photoresin composite formulations that use polymerization-induced phase separation to cause the MOF catalysts to migrate to the surface of the printed material, where they are accessible to substrates such as chemical warfare agents. Using our approach, MOF-polymer composites can be fabricated into nearly any shape or architecture while retaining both the excellent catalytic activity at 10 wt % loading of the MOF components and the flexible, elastomeric mechanical properties of a polymer.

Effect of Macromolecular Structure on Phase Separation Regime in 3D Printed Materials

In this study, the fabrication of 3D-printed polymer materials with controlled phase separation using polymerization induced microphase separation (PIMS) via photoinduced 3D printing is demonstrated. While many parameters affecting the nanostructuration in PIMS processes are extensively investigated, the influence of the chain transfer agent (CTA) end group, i.e., Z-group, of macromolecular chain transfer agent (macroCTA) remains unclear as previous research has exclusively employed trithiocarbonate as the CTA end group. Herein, the effect of macroCTAs containing four different Z-groups on the formation of nanostructure of 3D printed materials is explored. The results show that the different Z-groups lead to distinct network formation and phase separation behaviors between the resins, influencing both the 3D printing process and the resulting material properties. Specifically, less reactive macroCTAs toward acrylic radical addition, such as O-alkyl xanthate and N-alkyl-N-aryl dithiocarbamate, result in translucent and brittle materials with macrophase separation morphology. In contrast, more reactive macroCTAs such as S-alkyl trithiocarbonate and 4-chloro-3,5-dimethylpyrazo dithiocarbamate produce transparent and rigid materials with nano-scale morphology. Findings of this study provide a novel approach to manipulate the nanostructure and properties of 3D printed PIMS materials, which can have important implications for materials science and engineering.

A Practical Primer: Raman Spectroscopy for Monitoring of Photopolymerization Systems

Photopolymerization systems provide compelling advantages for industrial applications due to their fast reaction kinetics, wide selection of monomers for physical property development, and energy-efficient initiation via illumination. These same advantages can present challenges when attempting to monitor these reactions or characterize their resulting polymers; however, Raman spectroscopy can provide the flexibility and resolution needed. In this overview, Raman spectroscopy is compared to common characterization techniques, such as photo-differential scanning calorimetry and infrared spectroscopy, highlighting advantages of Raman spectroscopy. Examples are provided of how Raman spectroscopy has been used to monitor photopolymerizations and to provide insight on the impact of monomer chemistry and processing conditions, as well as paired with other techniques to elucidate physical properties. Finally, practical tips are provided for applying Raman spectroscopy and microscopy in photopolymerization systems.

CuAAC-methacrylate interpenetrating polymer network (IPN) properties modulated by visible-light photoinitiation

Interpenetrating polymer networks (IPNs) are a class of materials with interwoven polymers that exhibit unique blended or enhanced properties useful to a variety of applications, ranging from restorative protective materials to conductive membranes and hydrophobic adhesives. The IPN formation kinetics can play a critical role in the development of the underlying morphology and in turn the properties of the material. Dual photoinitiation of copper-catalyzed azide-alkyne (CuAAC) and radical mediated methacrylate polymerization chemistries enable the manipulation of IPN microstructure and properties by controlling the kinetics of IPN formation via the intensity of the initiating light. Specifically, azide and alkyne-based polyethylene glycol monomers and tetraethylene glycol dimethacrylate (TEGDMA) were polymerized in a single pot to form IPNs and the properties were evaluated as a function of the photoinitiating light intensity. Morphological differences as a function of intensity were observed in the IPNs as determined by thermomechanical properties and phase-contrast imaging in tapping mode atomic force microscopy (AFM). At moderate intensities (20 mW/cm2) of visible light (470 nm), the TEGDMA polymerization gels first and therefore forms the underlying network scaffold. At low intensities (0.2 mW/cm2), the CuAAC polymerization can gel first. The ability to switch sequence of gelation and IPN trajectory (simultaneous vs. sequential), affords control over phase separation behavior. Thus, light not only allows for spatial and temporal control over the IPN formation but also provides control over their thermomechanical properties, representing a route for facile IPNs design, synthesis, and application.

Multimorphic Materials: Spatially Tailoring Mechanical Properties via Selective Initiation of Interpenetrating Polymer Networks

Access to multimaterial polymers with spatially localized properties and robust interfaces is anticipated to enable new capabilities in soft robotics, such as smooth actuation for advanced medical and manufacturing technologies. Here, orthogonal initiation is used to create interpenetrating polymer networks (IPNs) with spatial control over morphology and mechanical properties. Base catalyzes the formation of a stiff and strong polyurethane, while blue LEDs initiate the formation of a soft and elastic polyacrylate. IPN morphology is controlled by when the LED is turned "on", with large phase separation occurring for short time delays (≈1-2 min) and a mixed morphology for longer time delays (>5 min), which is supported by dynamic mechanical analysis, small angle X-ray scattering, and atomic force microscopy. Through tailoring morphology, tensile moduli and fracture toughness can be tuned across ≈1-2 orders of magnitude. Moreover, a simple spring model is used to explain the observed mechanical behavior. Photopatterning produces "multimorphic" materials, where morphology is spatially localized with fine precision (<100 µm), while maintaining a uniform chemical composition throughout to mitigate interfacial failure. As a final demonstration, the fabrication of hinges represents a possible use case for multimorphic materials in soft robotics.

3D Printed Solid Polymer Electrolytes with Bicontinuous Nanoscopic Domains for Ionic Liquid Conduction and Energy Storage

Solid polymer electrolytes (SPEs) offer several advantages compared to their liquid counterparts, and much research has focused on developing SPEs with enhanced mechanical properties while maintaining high ionic conductivities. The recently developed polymerization-induced microphase separation (PIMS) technique offers a straightforward pathway to fabricate bicontinuous nanostructured materials in which the mechanical properties and conductivity can be independently tuned. In this work SPEs with tunable mechanical properties and conductivities are prepared via digital light processing 3D printing, exploiting the PIMS process to achieve nanostructured ion-conducting materials for energy storage applications. A rigid crosslinked poly(isobornyl acrylate-stat-trimethylpropane triacrylate) scaffold provided materials with room temperature shear modulus above 400 MPa, while soft poly(oligoethylene glycol methyl ether acrylate) domains containing the ionic liquid 1-butyl-3-methylimidazolium bis-(trifluoromethyl sulfonyl)imide endowed the material with ionic conductivity up to 1.2 mS cm^-1 at 30 °C. These features make the 3D-printed SPE very competitive for applications in all solid energy storage devices, including supercapacitors.

Development of Proanthocyanidin-Loaded Mesoporous Silica Nanoparticles for Improving Dental Adhesion

Dentin biomodification is a promising approach to enhance dental tissue biomechanics and biostability for restorative and reparative therapies. One of the most active dentin tissue biomodifiers is proanthocyanidin (PAC)-rich natural extracts, which are used in the dental bonding procedure in combination with resin-based adhesives (RBAs). This study aimed to investigate the use of mesoporous silica nanoparticles (MSNs) for the sustained delivery of PACs for dentin biomodification as a novel drug-delivery system for dental applications. The effects of the incorporation of MSN functionalized with 3-aminopropyltriethoxysilane (APTES) and loaded with PAC into an experimental RBA were assessed by characterizing the material mechanical properties. In addition, the immediate and long-term bonding performance of an experimental resin-based primer (RBP) containing MSN-APTES loaded with PAC was also evaluated. For that, different formulations of RBA and RBP were prepared containing 20% w/v MSN-APTES loaded with PAC before or after functionalization (MSN-PAC-APTES and MSN-APTES-PAC, respectively). The incorporation of MSN-APTES-PAC did not negatively impact the degree of conversion or the overall mechanical properties of the RBA. However, adding MSN-PAC-APTES resulted in inferior mechanical properties of the experimental RBA. In the adhesion studies, APTES-functionalized MSN was successfully added to an experimental RBP for drug-delivery purposes without compromising the bond strength to the dentin or the failure mode. Interestingly, the sequence of surface functionalization with APTES resulted in differences in the bonding performance, with better long-term results for RBP containing MSN loaded with PAC after functionalization.

Designing Nanostructured 3D Printed Materials by Controlling Macromolecular Architecture

Nanostructured polymeric materials play important roles in many advanced applications, however, controlling the morphologies of polymeric thermosets remains a challenge. This work uses multi-arm macroCTAs to mediate polymerization-induced microphase separation (PIMS) and prepare nanostructured materials via photoinduced 3D printing. The characteristic length scale of microphase-separated domains is determined by the macroCTA arm length, while nanoscale morphologies are controlled by the macroCTA architecture. Specifically, using 2- and 4- arm macroCTAs provides materials with different morphologies compared to analogous monofunctional linear macroCTAs at similar compositions. The mechanical properties of these nanostructured thermosets can also be tuned while maintaining the desired morphologies. Using multi-arm macroCTAs can thus broaden the scope of accessible nanostructures for extended applications, including the fabrication of actuators and potential drug delivery devices.

Nano- to macro-scale control of 3D printed materials via polymerization induced microphase separation

Although 3D printing allows the macroscopic structure of objects to be easily controlled, controlling the nanostructure of 3D printed materials has rarely been reported. Herein, we report an efficient and versatile process for fabricating 3D printed materials with controlled nanoscale structural features. This approach uses resins containing macromolecular chain transfer agents (macroCTAs) which microphase separate during the photoinduced 3D printing process to form nanostructured materials. By varying the chain length of the macroCTA, we demonstrate a high level of control over the microphase separation behavior, resulting in materials with controllable nanoscale sizes and morphologies. Importantly, the bulk mechanical properties of 3D printed objects are correlated with their morphologies; transitioning from discrete globular to interpenetrating domains results in a marked improvement in mechanical performance, which is ascribed to the increased interfacial interaction between soft and hard domains. Overall, the findings of this work enable the simplified production of materials with tightly controllable nanostructures for broad potential applications.

Hybrid Free-Radical/Cationic Phase-Separated UV-Curable System: Impact of Photoinitiator Content and Monomer Fraction on Surface Morphologies and Gloss Appearance

Simultaneous photopolymerization of radical and cationic systems is one strategy to generate polymer network architectures named interpenetrating polymer networks (IPNs). In these hybrid systems, phase separation and final polymer morphology are ultimately governed by thermodynamic incompatibility and polymerization kinetics. This behavior is quite complex, as numerous factors can affect polymerization kinetics including monomer/oligomer viscosity and structure, light intensity, photoinitiator content and absorbance, cross-linking, vitrification, etc. In this work, the impact of photoinitiator concentration and monomer fraction on surface morphologies in a hybrid radical/cationic phase-separated system was examined. Wrinkles formed on the surface of photopolymerized films depend on the polymerization rate and acrylate/epoxy ratio. This phenomenon is partially explained by the rapid polymerization rate associated with the development of an epoxy matrix and a smaller acrylate domain. The size and shape of the wrinkles can be controlled by varying formulation parameters (mainly, composition) and photoinitiator content. It was possible to create surface roughness and consequently decrease the gloss by controlling the polymerization kinetics and phase-separated morphology. This study demonstrates that the morphology, polymerization kinetics, and film properties (e.g., gloss, transparency) can be manipulated with the ratio of the acrylate/epoxy mixture and the photoinitiator content.

Controlling phase separated domains in UV-curable formulations with OH-functionalized prepolymers

Modification of photocurable radical systems with high molecular weight prepolymers enables access to a wide array of polymer structures and properties.

NIR-sensitized hybrid radical and cationic photopolymerization of several cyanines in combination with diaryliodonium bis(trifluoromethyl)sulfonyl imide

A series of cyanines exhibiting absorption between 750 and 930 nm reacted after NIR excitation with the bis(t-butylphenyl) iodonium cation comprising the [(CF3SO2)2N]− anion (NTf2)−, resulting in the generation of free radicals and conjugate acids.

Controlling network morphology in hybrid radical/cationic photopolymerized systems

Hybrid formulation chemistry was used to internally control the reaction rate differences between radical and cationic photopolymerizations leading to a tailorable array of polymer morphologies and mechanical properties.

Nanostructure Control in 3D Printed Materials

Currently, there are no straightforward methods to 3D print materials with nanoscale control over morphological and functional properties. Here, a novel approach for the fabrication of materials with controlled nanoscale morphologies using a rapid and commercially available Digital Light Processing 3D printing technique is demonstrated. This process exploits reversible deactivation radical polymerization to control the in-situ-polymerization-induced microphase separation of 3D printing resins, which provides materials with complex architectures controllable from the macro- to nanoscale, resulting in the preparation of materials with enhanced mechanical properties. This method does not require specialized equipment or process conditions and thus represents an important development in the production of advanced materials via additive manufacturing.

NIR-Sensitized Cationic and Hybrid Radical/Cationic Polymerization and Crosslinking

NIR‐sensitized cationic polymerization proceeded with good efficiency, as was demonstrated with epoxides, vinyl ether, and oxetane. A heptacyanine functioned as sensitizer while iodonium salt served as coinitiator. The anion adopts a special function in a series selected from fluorinated phosphates (a: [PF₆]⁻, b: [PF₃(C₂F₅)₃]⁻, c: [PF₃(n‐C₄F₉)₃]⁻), aluminates (d: [Al(O‐t‐C₄F₉)₄]⁻, e: [Al(O(C₃F₆)CH₃)₄]⁻), and methide [C(O‐SO₂CF₃)₃]⁻ (f). Vinyl ether showed the best cationic polymerization efficiency followed by oxetanes and oxiranes. DFT calculations provided a rough pattern regarding the electrostatic potential of each anion where d showed a better reactivity than e and b. Formation of interpenetrating polymer networks (IPNs) using trimethylpropane triacrylate and epoxides proceeded in the case of NIR‐sensitized polymerization where anion d served as counter ion in the initiator system. No IPN was formed by UV‐LED initiation using the same monomers but thioxanthone/iodonium salt as photoinitiator. Exposure was carried out with new NIR‐LED devices emitting at either 805 or 870 nm.

Development of High-Resolution Three-Dimensional-Printed Extracellular Matrix Scaffolds and Their Compatibility with Pluripotent Stem Cells and Early Retinal Cells

Purpose: Widely used approaches for retinal disease modeling and in vitro therapeutic testing can be augmented by using tissue-engineered scaffolds with a precise 3-dimensional structure. However, the materials currently used for these scaffolds are poorly matched to the biochemical and mechanical properties of the in vivo retina. Here, we create biopolymer-based scaffolds with a structure that is amenable to retinal tissue engineering and modeling. Methods: Optimal two-photon polymerization (TPP) settings, including laser power and scanning speed, are identified for 4 methacrylated biopolymer formulations: collagen, gelatin, hyaluronic acid (HA), and a 50/50 mixture of gelatin/HA, each with methylene blue as a photoinitiator. For select formulations, fabrication accuracy and swelling are determined and biocompatibility is evaluated by using human induced pluripotent stem cells and rat postnatal retinal cells. Results: TPP is feasible for each biopolymer formulation, but it is the most reliable for mixtures containing gelatin and the least reliable for HA alone. The mean size of microscaffold pores is within several microns of the intended value but the overall structure size is several times greater than the modeled volume. The addition of HA to gelatin scaffolds increases cell viability and promotes neuronal phenotype, including Tuj-1 expression and characteristic morphology. Conclusion: We successfully determined a useful range of TPP settings for 4 methacrylated biopolymer formulations. When crosslinked, these extracellular matrix-derived molecules support the growth and attachment of retinal cells. We anticipate that when combined with existing patient-specific approaches, this technique will enable more efficient and accurate retinal disease modeling and therapeutic testing in vitro than current techniques allow.