Reaction-induced Microphase Separation Mechanism Research Articles

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.

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.

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.

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.

Curing Behavior of Epoxy Resin Mixed with Epoxypropoxypropyl Terminated Polydimethylsiloxane

Diglycidyl ether of bisphenol A (DGEBA) and epoxypropoxypropyl terminated polydimethylsiloxane (ETDMS) were mixed in different proportion. The morphology of ETDMS modified epoxy systems was observed by scanning electronic microscope (SEM). Curing kinetics was also studied by differential scanning calorimetry (DSC). It was suggested that the formation of the microstructures followed reaction-induced microphase separation mechanism.

From flexible to mesoporous polybenzoxazine resins templated by poly(ethylene oxide-b-ε-caprolactone) copolymer through reaction induced microphase separation mechanism

When we blended the diblock copolymer poly(ethylene oxide-b-ε-caprolactone) (PEO-b-PCL) with the monomer (3-phenyl-3,4-dihydro-2H-1,3-benzoxazin-6-yl)methanol (PA-OH), Fourier transform infrared (FTIR) spectroscopy revealed that the ether groups of the PEO block were stronger hydrogen-bond acceptors for the OH group of PA-OH than were the CO groups of the PCL block. Thermal curing resulted in the block copolymer being incorporated into the polybenzoxazine resin, forming cylindrical, wormlike, and disordered spherical nanostructures through a mechanism involving reaction-induced microphase separation, as evidenced using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Wormlike micelles of PCL, formed as the second phase in polybenzoxazine at a high aspect ratio, appearing to possess the optimal length scale to result in greater toughness. Mild pyrolysis conditions led to removal of the PEO-b-PCL diblock copolymer and formation of mesoporous polybenzoxazines. This approach also provided composition-dependent nanostructures that were similar to the structures of the non-pyrolyzed samples. Regular mesoporous polybenzoxazine resin was formed only when the polybenzoxazine content was 40–70 wt%. Intriguingly, the structures were affected not only by the balance between the contents of the polybenzoxazine and the diblock copolymer but also by the curing temperature and process.

In-situ Cure and Cure Kinetic Analysis of a Liquid Rubber Modified Epoxy Resin

The in-situ cure and cure kinetics of an epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) polymerized with an anhydride hardener and its mixtures with a liquid polybutadiene rubber having hydroxyl functionality (HTPB) were studied using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) in an isothermal mode. The cure reaction was monitored in-situ by FTIR spectroscopy by observing variation in intensity of epoxy, anhydride, and ester bands. The cure reaction mechanisms by which the network structure of epoxy was developed were discussed. Isothermal mode DSC measurements were performed at selected temperatures. The reaction followed an autocatalytic mechanism, and kinetic analysis was done by a phenomenological model developed by Kamal. Good fits were obtained between the autocatalytic model and the experimental data up to the vitrification state. Afterward, the reaction became diffusion controlled. The reaction during the later stages of cure was explained by introducing a diffusion factor, which agreed well with the kinetic data. The nature of the developing morphology of modified epoxies was analyzed by optical microscopy (OM) and small angle laser light scattering (SALLS) technique. The ultimate morphology of the cured blends was analyzed using scanning electron microscopy (SEM). The cure kinetics has been correlated with the developed morphology to get insight into the mechanism of reaction-induced microphase separation.

Formation of nanophases in epoxy thermosets containing an organic–inorganic macrocyclic molecular brush with poly(ε-caprolactone)-block-polystyrene side chains

In this contribution, we report the synthesis of organic–inorganic macrocyclic molecular brushes with a 24-membered macrocyclic oligomeric silsesquioxane (MOSS) backbone and with poly(e-caprolactone) (PCL) (or poly(e-caprolactone)-block-polystyrene (PCL-b-PS) diblock copolymer) side chains. The macrocyclic molecular brushes were incorporated into epoxy to access the nanostructured thermosets. The nanostructures in the thermosets were investigated by means of atomic force microscopy (AFM), small angle X-ray scattering (SAXS) and dynamic mechanical thermal analysis (DMTA). It is judged that the formation of the nanophases in the thermosets containing the macrocyclic molecular brush with PCL side chains (MOSS[PCL]12) follows the self-assembly mechanism in terms of the immiscibility of the MOSS backbone with epoxy after and before the curing reaction. It was found that the spherical, vesicular and lamellar nanophases were formed in the epoxy thermosets containing the macrocyclic molecular brush with PCL-b-PS side chains (MOSS[PCL-b-PS]12). In contrast to the thermosets containing MOSS[PCL]12 the formation of the nanophases in the thermosets containing MOSS[PCL-b-PS]12 was ascribed to the self-assembly-directed reaction-induced microphase separation mechanism.

Microphase separation in polybenzoxazine thermosets containing benzoxazine-terminated poly(ethylene oxide) telechelics

Benzoxazine-terminated poly(ethylene oxide) telechelics (Ba-terminated PEO) was synthesized and incorporated into polybenzoxazine to obtain the nanostructured thermosets. The morphology of the thermosets was investigated by means of atomic force microscopy (AFM), small angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA). The formation of the nanophase structures in the thermosetting composites was addressed on the basis of the mechanism of reaction-induced microphase separation (RIMPS), which was in marked contrast to the case of the binary thermosetting blends of polybenzoxazine with hydroxyl-terminated poly(ethylene oxide). The occurrence of RIMPS resulted from the copolymerization reaction of the end groups of Ba-terminated PEO telechelics with the precursor of thermosetting matrix (i.e., benzoxazine), which suppressed the occurrence of the macroscopic phase separation. It was found that the formation of the nanostructures has a significant effect on the melting behavior of PEO in the thermosets, thermal transition properties of the PBZ thermosets.

Reaction-induced microphase separation in polybenzoxazine thermosets containing poly(N-vinyl pyrrolidone)- block-polystyrene diblock copolymer

Poly(N-vinyl pyrrolidone)- block-polystyrene diblock copolymer (PVPy- b-PS) was synthesized via sequential reversible radical-fragmentation transfer polymerization with S-1-phenylethyl O-ethylxanthate as a chain transfer agent. The block copolymer was incorporated into polybenzoxazine to access the nanostructures in the thermosets. The nanostructures in the thermosets were investigated by means of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). It was found that disordered and/or ordered PS nanophases were formed in the PBa thermosets. It is judged that the formation of nanophases followed the mechanism of reaction-induced microphase separation in terms of the miscibility of the subchains of the diblock copolymer ( viz. PVPy and PS) with polybenzoxazine after and before curing reaction.

Nanostructures and thermomechanical properties of epoxy thermosets containing reactive diblock copolymer

Polystyrene-block-poly(glycidyl methacrylate) reactive diblock copolymer (PS-b-PGMA) was synthesized via atom transfer radical polymerization (ATRP). The diblock copolymer was characterized using nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). The cured epoxy thermosets with 10–20 nm PS particles were prepared by blending the diblock copolymer with epoxy resin. The nanostructures were examined by means of transmission electronic microscopy (TEM) and small angle X-ray scattering (SAXS). The formation of the nanostructures was caused by the reaction-induced microphase separation mechanism. It is significant that the glass transition temperatures (Tgs) of these epoxy thermosets were increased by the addition of PS-b-PGMA reactive block copolymer as revealed by both differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Morphology and mechanical properties of nanostructured blends of epoxy resin with poly(ɛ-caprolactone)-block-poly(butadiene-co-acrylonitrile)-block-poly(ɛ-caprolactone) triblock copolymer

Abstract Poly(ɛ-caprolactone)- block -poly(butadiene- co -acrylonitrile)- block -poly(ɛ-caprolactone) triblock copolymer was synthesized via the ring-opening polymerization of ɛ-caprolactone with dihydroxyl-terminated butadiene- co -acrylonitrile random copolymer. The amphiphilic block copolymer was used to toughen epoxy thermosets via the formation of nanostructures. The morphology of the thermosets was investigated by means of atomic force microscopy, transmission electronic microscopy and small-angle X-ray scattering. It was judged that the formation of the nanostructures in the thermosets follows the mechanism of reaction-induced microphase separation. The thermal and mechanical properties of the nanostructured thermosets were compared to those of the ternary blends composed of epoxy, poly(butadiene- co -acrylonitrile) and poly(ɛ-caprolactone) with the identical content of the modifiers. It is noted that at the same composition the nanostructured thermosets displayed higher glass transition temperatures ( T g s) than the ternary blends, which was evidenced by dynamic mechanical analysis. The fracture toughness of the thermosets was evaluated in terms of the measurement of critical stress field intensity factor ( K 1C ). It is noted that at the identical composition the nanostructured blends significantly displayed higher fracture toughness than the ternary blends. In addition, the K 1C of the nanostructured thermosets attained the maximum with the content of the modifier less than their counterpart of ternary blending.

Microphase Separation in Thermosetting Blends of Epoxy Resin and Poly(ε-caprolactone)-block-Polystyrene Block Copolymers

The block copolymers, poly(ε-caprolactone)-block-polystyrene (PCL-b-PS) with the linear and star-shaped topological structures were synthesized via the combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). The two block copolymers with the identical block compositions were incorporated into epoxy to access the nanostructured thermosets. The microphase-separated morphologies in the systems were investigated by means of atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS). It is identified that the epoxy thermosets can display the long-ranged ordered nanostructures depending on the concentration of the diblock copolymers. By considering the miscibility of the subchains of the block copolymers with epoxy after and before the curing reaction, it is judged that the formation of the nanostructures followed the mechanism of reaction-induced microphase separation. It is noted that the nanoscaled morphologies of the thermosets are quite dependent on the macromolecular topologies of the block copolymers. For the epoxy thermosets containing the linear PCL-b-PS diblock copolymer, the spherical PS nanophases were arranged into cubic (e.g., bcc, fcc, or simple cubic) lattice whereas the PS nanophases into lamellar lattice while the thermosets contain the tetra-armed PCL-b-PS block copolymer. The difference in nanostructures for the epoxy thermosets has been interpreted on the basis of the restriction of topological structures of the block copolymers on the formation of nanophases.

Epoxy resin containing poly(ethylene oxide)- block-poly(ɛ-caprolactone) diblock copolymer: Effect of curing agents on nanostructures

An amphiphilic diblock copolymer, poly(ethylene oxide)- block-poly(ɛ-caprolactone) (PEO- b-PCL) was synthesized via the ring-opening polymerization of ɛ-caprolactone in the presence of a hydroxyl-terminated poly(ethylene oxide) monomethyl ether. The diblock copolymer was incorporated into epoxy thermosets. It is found that the formation of nanostructures of thermosetting blends is quite dependent on the uses of aromatic amine hardeners. For 4,4′-methylenebis(2-chloroaniline) (MOCA)-cured thermosetting system, the homogeneous morphology was obtained at the compositions investigated. Nonetheless, the nanostructured thermosets were obtained when the blends were cured with 4,4′-diaminodiphenylsulfone (DDS). The differential scanning calorimetry (DSC) showed that the nanostructured thermosets did not displayed any crystallinity although the subchains of the diblock copolymer are crystalline. The nanostructures were evidenced by means of atomic force microscopy (AFM), small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The dependence of morphological structures on the types of aromatic amines for epoxy and PEO- b-PCL thermosetting blends were interpreted on the basis of the difference in hydrogen bonding interactions resulting from the structure of curing agents. Considering the complete miscibility of the subchains ( viz. PEO and PCL) with the precursors of epoxy resin before curing, it is judged that the formation of the nanostructures in the thermosets follows the mechanism of reaction-induced microphase separation, which is in marked contrast to the mechanism of self-assembly, i.e., micelle structures of block copolymers are formed prior to curing, followed by fixing these nanostructures via curing.

Formation of Ordered Nanostructures in Epoxy Thermosets: A Mechanism of Reaction-Induced Microphase Separation

A polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymer was synthesized via the atom transfer radical polymerization (ATRP) of styrene with mono-2-bromoisobutyryl-terminated PEO [PEO−OOCCBr(CH3)2] as a macroinitiator, and the polymerization was mediated by copper(I) bromide (CuBr) and 2,2‘-bipyridine (BPY). The PS-b-PEO diblock copolymer was used to incorporate into epoxy resin to afford the nanostructured epoxy thermosets. Both atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) showed that the long-range ordered nanostructures were obtained via the approach of in situ polymerization in the presence of the diblock copolymer. It was found that the nanophases of PS were arranged into a simple cubic symmetry lattice while the content of diblock copolymer was 40 wt %. In view of the difference in miscibility and phase behavior for the blends of the subchains (i.e., PS and PEO) of the diblock copolymer with epoxy after and before curing, the formation of the ordered nanostructures ...