Nanostructured Thermosetting Blends of Epoxy Resin and Amphiphilic Poly(ε-caprolactone)-block-polybutadiene-block-poly(ε-caprolactone) Triblock Copolymer

Synthesis and evaluation of novel urethane macromonomers for the formulation of fracture tough 3D printable dental materials

3D printing of materials which combine fracture toughness, high modulus and high strength is quite challenging. Most commercially available 3D printing resins contain a mixture of multifunctional (meth)acrylates. The resulting 3D printed materials are therefore brittle and not adapted for the preparation of denture bases. For this reason, this article focuses on toughening by incorporation of triblock copolymers in methacrylate-based materials. In a first step, three urethane dimethacrylates with various alkyl spacer length were synthesized in a one-pot two-step synthesis. Each monomer was combined with 2-phenoxyethyl methacrylate as a monofunctional monomer and a polycaprolactone-polydimethylsiloxane-polycaprolactone triblock copolymer was added as toughener. The formation of nanostructures via self-assembly was proven by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The addition of the triblock copolymer resulted in a strong increase in fracture toughness for all mixtures. The nature of the urethane dimethacrylate had a significant impact on fracture toughness and flexural strength and modulus of the cured materials. Most promising systems were also investigated via dynamic fatigue propagation da/dN measurements, confirming that the toughening also works under dynamic load. By carefully selecting the length of the urethane dimethacrylate spacer and the amount of block copolymer, materials with the desired physical properties could be efficiently formulated. Especially the formulation containing the medium alkyl spacer length (DMA2/PEMA) and 5 wt% BCP1 (block copolymer), exhibits excellent mechanical properties and high fracture toughness.

Modeling the competition between phase separation and polymerization under explicit polydispersity

The dynamics of phase separation for polymer blends is important in determining the final morphology and properties of polymer materials; in practical applications, this phase separation can be controlled by coupling to polymerization reaction kinetics via a process called 'polymerization-induced phase separation'. We develop a phase-field model for a polymer melt blend using a polymerizing Cahn-Hilliard (pCH) formalism to understand the fundamental processes underlying phase separation behavior of a mixture of two species independently undergoing linear step-growth polymerization. In our method, we explicitly model polydispersity in these systems to consider different molecular-weight components that will diffuse at different rates. We first show that this pCH model predicts results consistent with the Carothers predictions for step-growth polymerization kinetics, the Flory-Huggins theory of polymer mixing, and the classical predictions of spinodal decomposition in symmetric polymer blends. The model is then used to characterize (i) the competition between phase separation dynamics and polymerization kinetics, and (ii) the effect of unequal reaction rates between species. For large incompatibility between the species (i.e. high χ), our pCH model demonstrates that the strength for phase separation directly corresponds to the kinetics of phase separation. We find that increasing the reaction rate k̃, first induces faster phase separation but this trend reverses as we further increase k̃ due to the competition between molecular diffusion and polymerization. In this case, phase separation is delayed for faster polymerization rates due to the rapid accumulation of slow-moving, high molecular weight components.

Toughening of epoxy thermosets by self-assembled nanostructures of amphiphilic comb-like random copolymers

Amphiphilic comb-like random copolymers synthesized from poly(ethylene glycol) methyl ether methacrylate (PEGMMA) and stearyl methacrylate (SMA) with PEGMMA contents ranging between 30 wt% and 25 wt% were demonstrated to self-assemble into various well-defined nanostructures, including spherical micelles, wormlike micelles, and vesicle-like nanodomains, in anhydride-cured epoxy thermosets. In addition, the polymer blends of the comb-like random copolymer and poly(stearyl methacrylate) were prepared and incorporated into epoxy thermosets to form irregularly shaped nanodomains. Our research findings indicate that both the comb-like random copolymers and polymer blends are suitable as toughening modifiers for epoxy. When added at a concentration of 5 wt%, both types of modifiers lead to substantial improvements in the tensile toughness (>289%) and fracture toughness of epoxy thermosets, with minor reductions in their elastic modulus (<16%) and glass transition temperature (<6.1 °C). The fracture toughness evaluated in terms of the critical stress intensity factor (KIC) and the strain energy release rate (GIC) increased by more than 67% and 131% for the modified epoxy thermosets containing comb-like random copolymers.

Nanostructuring Biobased Epoxy Resin with PEO-PPO-PEO Block Copolymer

A biobased diglycidyl ether of vanillin (DGEVA) epoxy resin was nanostructured by poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-PPO-PEO) triblock copolymer. Due to the miscibility/immiscibility properties of the triblock copolymer in DGEVA resin, different morphologies were obtained depending on the triblock copolymer amount. A hexagonally packed cylinder morphology was kept until reaching 30 wt% of PEO-PPO-PEO content, while a more complex three-phase morphology was obtained for 50 wt%, in which large worm-like PPO domains appear surrounded by two different phases, one of them rich in PEO and another phase rich in cured DGEVA. UV-vis measurements show that the transmittance is reduced with the increase in triblock copolymer content, especially at 50 wt%, probably due to the presence of PEO crystals detected by calorimetry.

Generation of One-Dimensional Fibrous Polyethylene Nanocrystals in Epoxy Thermosets

The one-dimensional (1D) polyethylene (PE) nanocrystals were generated in epoxy thermosets via crystallization-driven self-assembly. Toward this end, an ABA triblock copolymer composed of PE midblock and poly(ε-caprolactone) (PCL) endblocks was synthesized via the ring opening metathesis polymerization followed by hydrogenation approach. The nanostructured thermosets were obtained via a two-step curing approach, i.e., the samples were cured first at 80 °C and then at 150 °C. Under this condition, the one-dimensional (1D) fibrous PE microdomains with the lengths up to a couple of micrometers were created in epoxy thermosets. In contrast, only the spherical PE microdomains were generated while the thermosets were cured via a one-step curing at 150 °C. By the use of the triblock copolymer, the generation of 1D fibrous PE nanocrystals is attributable to crystallization-driven self-assembly mechanism whereas that of the spherical PE microdomains follows traditional self-assembly mechanism. Compared to the thermosets containing the spherical PE microdomains, the thermosets containing the 1D fibrous PE nanocrystals displayed quite different thermal and mechanical properties. More importantly, the nanostructured thermosets containing the 1D fibrous PE nanocrystals displayed the fracture toughness much higher than those only containing the spherical PE nanocrystals; the K_IC value was even three times as that of control epoxy.

Molecular insights into the ‘defects’ network in the thermosets and the influence on the mechanical performance

The introduction of ‘defects’ to the thermoset crosslinking network is one of the most applicable strategies for improving the modulus and toughness simultaneously. However, the reinforcement effect disappears when the ‘defects’ proportion exceeds the threshold. The speculated mechanism was that the aggregation and entanglement of the ‘defects’ chains changed the matrix topology, making the stacking structure more compact. However, the ‘defects’ are hardly directly observed in the experiment. As the result, the relationship between the ‘defects’ proportion and the package state of the matrix, and the effect on the material's mechanical performance was not explored. Herein, the network of bisphenol-A diglycidyl (DGEBA) with diethyltoluenediamine (DETDA) as the hardener was constructed using MD simulation, and n-butylamine was decorated on the matrix by replacing a proportion of DETDA acting as the ‘defects’. The results indicated that the aliphatic chains aggregated and entangled at a low concentration, occupying the voids in the rigid aromatic crosslinking structure, thus lowering the free volume. The strong non-bonding interactions drew the matrix segments close together, thus reinforcing the resin. However, the microphases formed by the aliphatic chains no longer filled the voids but created a new free volume and loosened the network when the content increased, which reduced the mechanical performance of the material. The experimental results were consistent with the findings in the simulations. The moduli of the resin increased with the increase in the n-butylamine content first and then declined. The maximum moduli of the thermosets was 3.4 GPa in S₃₀, which was about 25% higher compared with the control; the corresponding elongation at break was 8.9%, which was about 46% improved compared with the control.

The introduction of ‘defects’ to the thermoset crosslinking network is one of the most applicable strategies for improving the modulus and toughness simultaneously.

Long PEO-based nanoribbons generated in a polystyrene matrix through reaction-induced microphase separation followed by a fast crystallization process

A dispersion of elongated nanostructures with a high aspect ratio in polymer matrices has been reported to provide a material with valuable properties such as mechanical strength, barrier effect and shape memory, among others. In this study, we show the procedure to achieve a distribution of elongated crystalline nanodomains in a PS matrix employing the self-assembly of amphiphilic block copolymers (BCP). The selected BCP was polystyrene-block-polyethylene oxide (PS-b-PEO). It was dissolved at 10 wt% in a styrene (St) monomer and the blend was slowly photopolymerized over four days at room temperature, until the reaction was arrested by vitrification. This blend was initially homogeneous and nanostructuration took place in an early stage of the polymerization as a result of the microphase separation (MS) of PEO blocks. Due to its high tendency to crystallize, demixed PEO blocks crystallized almost concomitantly with MS triggering the growing of the nanostructures. Thus, the time window between the onset of crystallization and the vitrification of the matrix was almost four days, allowing all micelles to have the opportunity to couple to a growing nanostructure. As a result, a population of nanoribbons with average lengths surpassing 10 μm dispersed in a PS matrix was obtained. It was demonstrated that these ribbon-like nanostructures are preserved as long as the heating temperature is located below the Tg of the matrix. If the material is heated above this temperature, softening of the matrix allows the breakup of the molten PEO nanoribbons due to Plateau-Rayleigh instability.

Reaction-induced phase transitions with block copolymers in solution and bulk

Reaction-induced phase transitions use chemical reactions to drive macromolecular organisation and self-assembly. This review highlights significant and recent advancements in this burgeoning field.

Morphological evolution and mechanical properties of an “anchor chain” nanodomain structure of a reactive amphiphilic triblock copolymer in epoxy resin

A epoxy-reactive poly(3,4-epoxycyclohexylmethyl methacrylate)-block-poly(dimethylsiloxane)-block-poly(3,4-epoxycyclohexylmethyl methacrylate) (PMETHB-b-PDMS-b-PMETHB) triblock can self-assemble in epoxy resin to form “anchor-chain” nanodomains.

Investigation of Azobenzene Photoisomerization Effect on Morphologies and Properties of Nanostructured Thermosets Involving Epoxy and a Diblock Copolymer

This work highlights the effect of azobenzene photoisomerization on the morphologies and properties of the nanostructured thermosets involving epoxy and a diblock copolymer. First, a diblock copolymer composed of poly(ethylene oxide) (PEO) and poly(6-(4-(4-cyanophenylazo)phenoxy)hexyl methacrylate) (PCPHM) was synthesized, and this diblock copolymer was composed of an epoxy-philic block (i.e., PEO) and an azobenzene moiety-beating block (viz., PCPHM). This diblock copolymer was introduced into epoxy to obtain the nanostructured thermosets via reaction-induced microphase separation approach. To control the configuration of azobenzene moieties of the PCPHM block, the curing reactions were performed in the absence and/or presence of ultraviolet (UV) irradiation, respectively. It was found that, without UV irradiation, the PCPHM microdomains were generated with the trans isomers of azobenzene. Under UV irradiation, however, the PCPHM microdomains were formed with the cis configuration of azobenzene moieties. The ultraviolet-visible light (UV-vis) spectroscopy showed that the trans and cis configurations of azobenzene moieties were significantly fixed with the occurrence of curing reactions. The photoluminescent measurements showed that the nanostructured thermosets with trans-azobenzene moieties can emit fluorescence, which was in sharp contrast to those with cis-azobenzene moieties. The results of small-angle X-ray and atomic force microscopy showed that the nanostructured thermosets with trans and cis isomers of azobenzene moieties had quite different morphologies. It was found that the sizes of the PCPHM microdomains with cis configuration of azobenzene moieties were significantly larger than those with trans configuration. The difference in configuration of azobenzene moieties also resulted in the difference in glass-transition temperatures and dielectric properties of the materials. The results suggest a new approach to modulate the morphologies and physical properties of the nanostructured thermosets by means of photoisomerization of azobenzene moieties.

Fluorescence Enhancement Induced by Curing Reaction in Nanostructured Epoxy Thermosets Containing a Diblock Copolymer

In this work, a novel curing-induced fluorescence (FL) enhancement phenomenon in the nanostructuring process of epoxy thermosets was investigated. Toward this end, a diblock copolymer composed of poly(ethylene oxide) and poly(((4-vinylphenyl)ethene-1,1,2-triyl)tribenzene) (PTPEE) blocks was introduced into epoxy thermosets. Before curing reaction, the mixtures of epoxy precursors with the diblock copolymer only emitted feeble FL under ultra-visible (UV) irradiation. However, photoluminescence was significantly enhanced after the curing reaction was carried out. It was found that the novel FL enhancement phenomenon resulted from the aggregation-induced emission behavior of PTPEE blocks, which was triggered by curing reaction. In the nanostructured thermosets, the fluorophore blocks (viz. PTPEE) of this diblock copolymer were segregated into aggregates, that is, a reaction-induced microphase separation occurred. Owing to the generation of PTPEE microdomains, the epoxy nanocomposites significantly displayed the enhanced dielectric constants due to the promoted contribution from electron polarizations via π-π conjugation in the materials.

Synthesis of well-defined PCL-b-PnBA-b-PMMA ABC-type triblock copolymers: toward the construction of nanostructures in epoxy thermosets

A novel PCL-b-PnBA-b-PMMA was designed and applied to construct ordered nanostructures within epoxy thermosets.

Studying a novel AIE coating and its handling process via fluorescence spectrum

Study the existing state of TPE–EPOXY-1 polymer chains in good solution varied with its concentration.

Organic-Inorganic Nanocomposites via Self-Assembly of an Amphiphilic Triblock Copolymer Bearing a Poly(butadiene-g-POSS) Subchain in Epoxy Thermosets: Morphologies, Surface Hydrophobicity, and Dielectric Properties

Organic-inorganic nanocomposites composed of polyhedral oligomeric silsesquioxane (POSS) and epoxy resin were prepared via self-assembly of an amphiphilic triblock copolymer bearing a poly(POSS) midblock in epoxy thermosets. First, this organic-inorganic amphiphilic triblock copolymer was synthesized via hydrosilylation of heptaphenylhydro POSS with an existing triblock copolymer containing a short polybutadiene midblock. It was found that this novel amphiphilic block copolymer can self-assemble into nanophases in epoxy thermosets. In the presence of preformed nanophases, the curing reaction was performed, and the organic-inorganic nanocomposites containing poly(POSS) microdomains were thus obtained. Compared with plain epoxy, the as-obtained thermosets exhibited enhanced surface hydrophobicity; the enhanced surface hydrophobicity is attributed to enrichment of the POSS component at the surface of the materials. Owing to the formation of poly(POSS) microdomains, the dielectric constants of the materials significantly reduced, whereas the dielectric loss remained almost unchanged.

Nanoporous Gyroid-Structured Epoxy from Block Copolymer Templates for High Protein Adsorbability

Nanoporous epoxy with gyroid texture is fabricated by using a nanoporous polymer with gyroid-forming nanochannels as a template for polymerization of epoxy. The nanoporous polymer template is obtained from the self-assembly of degradable block copolymer, polystyrene-b-poly(l-lactide) (PS-PLLA), followed by hydrolysis of PLLA blocks. Templated polymerization can be conducted under ambient conditions to create well-defined, bicontinuous epoxy networks in a PS matrix. By taking advantage of multistep curing of epoxy, well-ordered robust nanoporous epoxy can be obtained after removal of PS template, giving robust porous materials. The through-hole nanoporous epoxy in the film state can be used as a coated layer to enhance the adsorbability for both lysozyme and bovine serum albumin.