Reaction-induced Microphase Separation Research Articles

Controlling block copolymer one-dimensional self-assembly in polymeric matrices.

The synthesis of one-dimensional (1D) nanostructures in polymeric matrices has become the focus of much research, as the presence of these highly anisotropic domains determines the transport behaviour and mechanical properties of the resulting nanostructured polymers. In this work, 1D PEO nanocrystals were synthesized in situ from polystyrene-b-polyethylene oxide (PS-b-PEO) self-assembly in a polystyrene matrix. For this, three different block copolymers (BCP) were employed: L-BCP (PS = 32 000 Da and PEO = 11 000 Da), M-BCP, (PS = 59 000 Da and PEO = 31 000 Da), and H-BCP, (PS = 102 000 Da and PEO = 34 000 Da). The formation of 1D nanocrystals starts with the reaction-induced microphase separation of BCP during styrene photopolymerization at room temperature. Then, as matrix polymerizes, the primary crystalline micelles aggregate via epitaxial crystallization by end-to-end coupling. The morphology of the resulting nanocrystals was highly dependent on the BCP employed. While L-BCP self-assembles into 1D ribbon-like nanocrystals, M-BCP macro-phase separates and, H-BCP self-assembles into short disk-like nanocrystals. This dissimilar behavior was mainly associated to the length of the stabilizing corona block. In the case of H-BCP, it was found that 1D self-assembly occurred when the conditions for core thickening were given, that is, when a non-reactive period was introduced in the cure cycle. During such a period, core thickening clears the lateral surface of the nanocrystals, allowing end-to-end coupling. The driving force for crystallization was also modified. An increase in undercooling resulted in an elevated nucleation rate and accelerated crystal growth. This led to a narrower size distribution of shorter 1D nanocrystals. This knowledge will enable the synthesis of customized 1D nanocrystals in a thermoplastic matrix, through the precise selection of the BCP formulation and curing conditions.

Nanostructure transformation in epoxy/block copolymer composites with good mechanical properties

Fabricating high performance thermosets through the formation of specific nanostructures via suitable design of block copolymer has become a new trend in recent years. While the correspondence between nanostructures and block copolymer was rarely reported, especially in the field of reaction-induced microphase separation. In our work, three linear diblock copolymers poly(ε-caprolactone)-block-polystyrene (PCL-PS) with similar molecular weights but different compositions were successfully synthesized. Then, they were implanted into the epoxy matrix to prepare nanostructured composites. The effects of PCL-PS concentration and composition on the organization and transformation of nanostructures in resin matrix were systemically investigated. Meanwhile, the relationship between macro properties of epoxy composites and the morphology of nanostructures was also researched. We hope readers can get some theoretical advice from our work for constructing specific nanostructures in epoxy composites depending on the self-assembly method of block copolymer.

Synthesis of well-defined PMMA-b-PDMS-b-PMMA triblock copolymer and study of its self-assembly behaviors in epoxy resin

PMMA- b -PDMS- b -PMMA block copolymers can mediate different reaction-induced microphase separation mechanism with different curing agents and formed ordered nanospheres. • A mild condition to conduct chain extension of Br-PDMS-Br with MMA via effective SARA ATRP was demonstrated and obtained a well-defined PMMA- b -PDMS- b -PMMA tBCP. • Then the tBCP shows its good compatibility with respect to epoxy resin, owing to the presence of compatible PMMA segments. • Through in-situ SAXS measurements, we found the tBCP underwent reaction-induced microphase separation (RIMPS) mechanism cured by 4,4′-methylenedianiline (MD) but self-assembly (SA) mechanism cured by phenol novolacs (PN). • We inspected the interesting mechanism variations, compatibility, and microphase morphology by using FT-IR, DSC, TGA, TEM, and SAXS of the obtaining MD- and PN-cured composites, mainly including nanospheres morphology in approximately 30 nm. With the evolutions of reversible deactivation radical polymerizations (RDRPs), including atom transfer radical polymerization (ATRP), reversible addition-fragmentation chain transfer (RAFT) polymerization, and nitroxide-mediated polymerization (NMP), one technique can be noted that supplemental activator and reducing agent (SARA) ATRP can conduct controlled/living radical polymerization fashion with a ppm level of catalyst. In addition, polydimethylsiloxane (PDMS) has wide ranges of applications in surfactants, sealants, and impact-relief materials. In this study, we first prepared a well-defined PDMS ended with α,ω-isobutyryl bromide (i.e., Br-PDMS-Br) macroinitiator (MI). Then chain extension of Br-PDMS-Br with methyl methacrylate (MMA) via SARA ATRP was conducted. Mild chain extension conditions were demonstrated and we successfully achieved controlled/living radical polymerization. A well-defined PMMA- b -PDMS- b -PMMA tBCP (named as ABA: M n, GPC = 49770; PDI = 1.44) comprising two external epoxy-philic blocks and a middle soft block was thus acquired. The ABA was further blended with diglycidyl ether of bisphenol-A (DGEBA) epoxy monomer and two crosslinkers of 4,4′-methylenedianiline (MD)/phenol novolacs (PN) to construct nanostructures driven by curing reactions. The cured epoxy thermoset (ET)/ABA composites showed good thermal properties and well-compatible behaviors (i.e, T g = ca. 100 °C and T (5 wt% loss) = ca. 270–335 °C) characterized by the measurements of differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). We further analyzed the two composite systems by utilizing Fourier transform infrared spectroscopy (FT-IR), (in-situ) small-angle X-ray scattering (SAXS), and transmission electron microscope (TEM). With different curing agent, interestingly, we observed a reaction-induced microphase separation (RIMPS) mechanism in MD-cured system but a self-assembly (SA) mechanism in PN-cured system. Furthermore, the PN-cured composites can create ordered nanospheres in a range of 27–41 nm.

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.

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.

The effect of reaction-induced micro-phase separation of block copolymer on curing kinetics of epoxy thermosets

The curing kinetics of epoxy system modified with Poly(e-caprolactone)-block-Polystyrene (PCL-b-PS) diblock copolymer was investigated by differential scanning calorimetry(DSC). PCL-b-PS was synthesized Via the combination of ROP and ATRP, then incorporated into epoxy to access the nanostructured thermosets. The results of TEM, SAXS and DSC demonstrated the occurrence of Reaction-induced Micro-phase Separation. Kinetic studies showed that the PCL-b-PS block copolymer delayed the curing reactions of epoxy system. The occurrence of Reaction-induced Micro-phase Separation had no significant effect on the total heat of reaction ∆H and the total activation energy, but resulted in a higher activation energy at the beginning of curing. The increase in activation energy at the initial stage of curing was related to the size and distribution of the dispersed phase. It is expected that the investigation of cure kinetics of EP/PCL-b-PS could provide more theoretical basis for the preparation of block copolymer modified epoxy resin.

Synthesis and microphase separation behavior of random, mixed cylindrical brush copolymers bearing polystyrene and poly(ε-caprolactone) side chains

A series of mixed, random cylindrical brush copolymers bearing polystyrene (PS) and poly(ε-caprolactone) (PCL) side chains were synthesized via the combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). These novel cylindrical brush copolymers have been characterized by means of nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC) and differential scanning calorimetry (DSC). It was found that the mikto-armed cylindrical brush copolymers were microphase-separated in bulks and that the morphologies were dependent on the mass ratios of PS to PCL side chains. One of the cylindrical brush copolymers was employed to incorporate into epoxy thermoset to investigate effect of the mikto-armed cylindrical brush architecture on the reaction-induced microphase separation behavior. Depending on the concentration of the cylindrical brush in epoxy, the thermosets can display the morphologies with the spherical, worm-like and lamellar PS microdomains dispersing in continuous thermosetting matrices.

Photophysical and dielectric properties of nanostructured epoxy thermosets containing poly(N-vinylcarbazole) nanophases

In this contribution, we reported the preparation of the nanostructured epoxy thermosets containing poly(N-vinylcarbazole) (PVK) microdomains. Toward this end, we first synthesized poly(N-vinylpyrrolidone)-block-poly(N-vinylcarbazole)-block-poly(N-vinyl pyrrolidone) (PVPy-b-PVK-b-PVPy) triblock copolymer via sequential reversible addition-fragmentation chain transfer/macromolecular design via the interchange of xanthate (RAFT/MADIX) process; then this triblock copolymer was incorporated into epoxy to obtain the nanostructured thermosets. It was found that reaction-induced microphase separation occurred in the mixtures of this triblock copolymer with epoxy. The formation of the PVK microdomains in epoxy thermosets were investigated by means of transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). Owing to the creation of PVK nanophases, the nanostructured epoxy thermosets became light-emissive under ultraviolet irradiation. The photoluminescent behavior is attributable to the formation of the excimers within the PVK microdomains. Compared to control epoxy, the nanostructured thermosets significantly displayed the enhanced dielectric constants due to the formation of PVK nanophases. The enhanced dielectric constants resulted from the dominant contribution of electron polarization and interfacial polarization with the formation of PVK nanophases.

Flexible Epoxy Resin Formed Upon Blending with a Triblock Copolymer through Reaction-Induced Microphase Separation.

In this study, we used diglycidyl ether bisphenol A (DGEBA) as a matrix, the ABA block copolymer poly(ethylene oxide–b–propylene oxide–b–ethylene oxide) (Pluronic F127) as an additive, and diphenyl diaminosulfone (DDS) as a curing agent to prepare flexible epoxy resins through reaction-induced microphase separation (RIMPS). Fourier transform infrared spectroscopy confirmed the existence of hydrogen bonding between the poly(ethylene oxide) segment of F127 and the OH groups of the DGEBA resin. Small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy all revealed evidence for the microphase separation of F127 within the epoxy resin. Glass transition temperature (Tg) phenomena and mechanical properties (modulus) were determined through differential scanning calorimetry and dynamic mechanical analysis, respectively, of samples at various blend compositions. The modulus data provided evidence for the formation of wormlike micelle structures, through a RIMPS mechanism, in the flexible epoxy resin upon blending with the F127 triblock copolymer.

Hyperbranched polyurethane as a highly efficient toughener in epoxy thermosets with reaction-induced microphase separation

Improvements in the toughness and thermal properties without affecting other mechanical properties in the hyperbranched polyurethane modified epoxy were demonstrated.

A reactive polystyrene-block-polyisoprene star copolymer as a toughening agent in an epoxy thermoset

A polystyrene-block-polyisoprene ((PS-b-PI)3) star polymer was synthesized by photochemical reversible addition fragmentation chain transfer (RAFT) polymerization. The obtained star polymer was epoxidized and used as a toughening agent in an epoxy thermoset. The incorporation of the epoxidized star polymer resulted in the formation of nanostructures and it was fixed by a crosslinking reaction. The formation of nanostructures in the thermosets follows the mechanism of reaction-induced microphase separation. The mechanical properties such as toughness and tensile strength were considerably increased due to the nanostructures formed by reactive blending.

Cylindrical brush copolymer bearing polystyrene-block-poly(ε-caprolactone) diblock side chains: Synthesis via a sequential grafting-from polymerization approach and its formation of fibrillar nanophases in epoxy thermosets

In this contribution, we reported the synthesis of a core-shell cylindrical brush copolymer with poly(2-hydroxyethyl methacrylate) (PHEMA) backbone and polystyrene-block-poly(ε-caprolactone) (PS-b-PCL) diblock side chains [denoted PHEMA-g-(PS-b-PCL)n] with the combination of atom transfer radical polymerization, the Huisgen 1,3-dipolar cycloaddition between azido and alkynyl groups and the ring-opening polymerization via a sequential “grafting from” polymerization approach. The cylindrical brush block copolymer was characterized by means of 1H nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC), atomic force microscopy (AFM) and differential scanning calorimetry (DSC). It is found that the fibrillar nanophases were formed with the length of about 400 nm and the diameter of about 15 nm via reaction-induced microphase separation mechanism while this cylindrical block copolymer brush was incorporated into epoxy thermosets. The fibrillar nanophases were characterized by means of transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS) and dynamic mechanical thermal analysis (DMTA). The formation of the fibrillar nanophases was interpreted on the basis of the constraint of cylindrical brush architecture of copolymer on reaction-induced microphase separation behavior.

Reaction-induced microphase separation in DDS-cured TGDDM thermosets containing PCL-b-PES-b-PCL triblock copolymer

Schematic diagram of micro-phase separation during the curing reaction.

Structure and properties of urea-formaldehyde resin/polyurethane blend prepared via in-situ polymerization

UF/PU blends with improved toughness were preparedvia in situpolymerization. PU participated in the polymerization reaction of UF. The reaction-induced microphase-separation indicated the energy-dissipation toughening mechanism.

Toughening of Epoxy Thermoset with Polystyrene-block-polyglycolic Acid Star Copolymer: Nanostructure–Mechanical Property Correlation

A novel amphiphilic polystyrene-block-polyglycolic acid three-arm star copolymer (PS-b-PGA)3 was incorporated in epoxy thermoset to show reaction-induced microphase separation. The diblock copolymer was prepared by atom-transfer radical polymerization followed by chain end modification, and toward the end ring-opening polymerization was utilized to attach the polyglycolic acid moiety. The formation of polystyrene-block-polyglycolic acid was confirmed by using Fourier transform infrared analysis, gel permeation chromatography, and 1H NMR. The microphase separation of block copolymer resulted in the formation of ordered nanodomains in the epoxy thermoset. It was found that the block copolymer in epoxy thermoset displayed reaction-induced microphase separation behavior as evidenced by transmission electron microscopy, scanning electron microscopy, and atomic force microscopy. The tensile strength and the toughness of the thermoset were increased by the incorporation of PGA, which has high glass transition and melting temperatures.

Controlling Nanodomain Morphology of Epoxy Thermosets Modified with Reactive Amine-Containing Epoxidized Poly(styrene-b-isoprene-b-styrene) Block Copolymer

Controlling nanodomain morphology of nanostructured epoxy thermosets is critical to modulate the mechanical properties of the cross-linked matrix. In this contribution, we demonstrate that this can be achieved by using a suitable block copolymer containing an epoxy soluble block with the ability to react toward the epoxy system during curing. For this purpose we designed an epoxidized poly(styrene-b-isoprene-b-styrene) block copolymer incorporating amine-reactive functionalities (eSIS-AEP) in the epoxidized block as modifier for an epoxy system, which allowed the formation of nanostructured thermosets with controlled spherelike nanodomain morphology. The eSIS-AEP was obtained in two steps from poly(styrene-b-isoprene-b-styrene) (SIS) block copolymer by controlled epoxidation of the olefinic block followed by partial oxirane ring-opening reaction using 1-(2-aminoethyl)piperazine as nucleophile. Before the curing reaction it was observed that poly(styrene) blocks self-assembled to form ordered spherelike nanostructures in blends of eSIS-AEP with epoxy precursors. Since the amine-reactive moiety was incorporated to the block copolymer so that it could react toward diglicidyl ether of bisphenol A (DGEBA) at a similar temperature than the DGEBA/hardener reaction, the epoxy miscible block of eSIS-AEP (ePI-AEP) was able to react with DGEBA during curing. Once the cross-linked network was formed, the initially obtained spherelike nanodomains were preserved, indicating that no reaction-induced microphase separation of ePI-AEP subchains occurred. A completely different scenario was ascertained for epoxidized SIS block copolymer, which conducted to nonspherical nanodomains due to the uncontrolled epoxidized poly(isoprene) demixing process during the curing reaction. These results demonstrate the importance of the epoxy soluble block being reactive toward the epoxy precursors to control the morphology of the obtained nanostructure.

Formation of nanostructures in thermosets containing block copolymers: From self-assembly to reaction-induced microphase separation mechanism

In this work, we investigated the effect of formation mechanisms of nanophases on the morphologies and thermomechanical properties of the nanostructured thermosets containing block copolymers. Toward this end, the nanostructured thermosets involving epoxy and block copolymers were prepared via self-assembly and reaction-induced microphase separation approaches, respectively. Two structurally similar triblock copolymers, poly(ε-caprolactone)-block-poly(butadiene-co-styrene)-block-poly(ε-caprolactone) (PCL-b-PBS-b-PCL) and poly(ε-caprolactone)-block-poly(ethylene-co-ethylethylene-co-styrene)-block-poly(ε-caprolactone) (PCL-b-PEEES-b-PCL) were synthesized via the ring-opening polymerization of ε-caprolactone (CL) with α,ω-dihydroxyl-terminated poly(butadiene-co-styrene) (HO-PBS-OH) and α,ω-dihydroxyl-terminated poly(ethylene-co-ethylethylene-co-styrene) (i.e., HO-PEEES-OH) as the macromolecular initiators, respectively; the latter was obtained via the hydrogenation reduction of the former. Both the triblock copolymers had the same architecture, the identical composition and close molecular weights. In spite of the structural resemblance of both the triblock copolymers, the formation mechanisms of the nanophases in the thermosets were quite different. It was found that the formation of nanophases in the thermosets containing PCL-b-PBS-b-PCL followed a reaction-induced microphase separation mechanism whereas that in the thermosets containing PCL-b-PEEES-b-PCL was in a self-assembly manner. The different formation mechanisms of nanophases resulted in the quite different morphologies, glass transition temperatures (Tg's) and fracture toughness of the nanostructured thermosets.

Poly(methyl methacrylate)-block-poly(N-vinyl pyrrolidone) diblock copolymer: A facile synthesis via sequential radical polymerization mediated by isopropylxanthic disulfide and its nanostructuring polybenzoxazine thermosets

In this contribution, we reported a facile synthesis of poly(methyl methacrylate)-block-poly(N-vinyl pyrrolidone) (PMMA-b-PVPy) diblock copolymers via sequential radical polymerizations mediated by isopropylxanthic disulfide (DIP). It was found that the radical polymerization of N-vinyl pyrrolidone (NVP) mediated by DIP was in a controlled and living manner. In contrast, the polymerization of methyl methacrylate mediated by DIP displayed the behavior of telomerization, affording xanthate-terminated PMMA with a good control of molecular weights while the conversion of monomer was not very high. The xanthate-terminated PMMA can be successfully used as the macromolecular chain transfer agent for the polymerization of NVP via RAFT/MADIX process and thus PMMA-b-PVPy diblock copolymers can be successfully synthesized via sequential radical polymerization mediated by isopropylxanthic disulfide. One of these diblock copolymers was incorporated into polybenzoxazine and the nanostructured thermosets were obtained as evidenced by transmission electron microscopy, small angle X-ray scattering, and dynamic mechanical thermal analysis. The formation of nanostructures in polybenzoxazine thermosets was ascribed to a reaction-induced microphase separation mechanism.

Hydrogen Bonding-Mediated Microphase Separation during the Formation of Mesoporous Novolac-Type Phenolic Resin Templated by the Triblock Copolymer, PEO-b-PPO-b-PEO

After blending the triblock copolymer, poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (PEO-b-PPO-b-PEO) with novolac-type phenolic resin, Fourier transform infrared spectroscopy revealed that the ether groups of the PEO block were stronger hydrogen bond acceptors for the OH groups of phenolic resin than were the ether groups of the PPO block. Thermal curing with hexamethylenetetramine as the curing agent resulted in the triblock copolymer being incorporated into the phenolic resin, forming a nanostructure through a mechanism involving reaction-induced microphase separation. Mild pyrolysis conditions led to the removal of the PEO-b-PPO-b-PEO triblock copolymer and formation of mesoporous phenolic resin. This approach provided a variety of composition-dependent nanostructures, including disordered wormlike, body-centered-cubic spherical and disorder micelles. The regular mesoporous novolac-type phenolic resin was formed only at a phenolic content of 40–60 wt %, the result of an intriguing balance of hydrogen bonding interactions among the phenolic resin and the PEO and PPO segments of the triblock copolymer.