Reaction-induced Microphase Separation Research Articles

Reaction-induced microphase separation in thermosetting blends of epoxy resin with poly(methyl methacrylate)-block-polystyrene block copolymers: Effect of topologies of block copolymers on morphological structures

Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) block copolymers with linear and tetra-armed star-shaped topological structures were synthesized via sequential atomic transfer radical polymerization (ATRP). With pentaerythritol tetrakis(2-bromoisobutyrate) as the initiator, the star-shaped block copolymers with two sequential structures (i.e., s-PMMA-b-PS and s-PS-b-PMMA) were prepared and the arm lengths and composition of the star-shaped block copolymers were controlled to be comparable with those of the linear PS-b-PMMA (denoted as l-PS-b-PMMA). The block copolymers were incorporated into epoxy resin to access the nanostructures in epoxy thermosets, by knowing that PMMA is miscible with epoxy after and before curing reaction whereas the reaction-induced phase separation occurred in the thermosetting blends of epoxy resin with PS. Considering the difference in miscibility of epoxy with PMMA and/or PS, it is judged that the reaction-induced microphase separation occurred in the systems. The design of these block copolymers allows one to investigate the effect of topological structures of block copolymers on the morphological structures of the thermosets. By means of atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS), the morphology of the thermosets was examined. It is found that the nanostructures were formed in the thermosets containing l-PMMA-b-PS and s-PS-b-PMMA block copolymers. It is noted that the long-range order of the nanostructures in the epoxy thermosets containing l-PMMA-b-PS is obviously higher than that in the system containing s-PS-b-PMMA. However, the macroscopic phase separation occurred in the thermosetting blends of epoxy resin with s-PMMA-b-PS block copolymer.

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.

Reaction-Induced Microphase Separation in Epoxy Thermosets Containing Poly(ε-caprolactone)-block-poly(n-butyl acrylate) Diblock Copolymer

Reaction-induced microphase separation in epoxy thermosets containing an amphiphilic diblock copolymer was investigated in this work. To this end, the diblock copolymer poly(ε-caprolactone)-block-poly(n-butyl acrylate) (PCL-b-PBA) was synthesized via the combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). The block copolymer was incorporated into epoxy thermosets. Before the curing reaction all the subchains of the diblock copolymer were miscible with the precursors of epoxy resin. After curing, only the PBA blocks were separated out whereas the PCL blocks remained miscible with epoxy resin. Such a reaction-induced microphase separation results in the formation of ordered nanostructures in the thermosets. The nanostructures in the thermosets were investigated by means of field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), and dynamic mechanical analysis (DMA). It is found that, depending on the concentration of the diblock copolymer, the thermosets can displayed spherical particles or lamellar nanostructures. The SAXS curves with the multiple scattering maxima suggests that the thermosets possess long-range ordered nanostructures.

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 ...

Curing and Morphology Studies of Thermosets from Blends of a Main Chain Liquid Crystalline Polyester and Diepoxide/Diamine

AbstractSummary: Blends of the commercial liquid‐crystalline polyester Rodrun LC‐3000 (60–90 wt.‐%) with a bisphenol A‐diglycidyl ether based diepoxide (DOW D.E.R.330) and an aromatic diamine (MCDEA) prepared in a twin‐screw extruder have been compression‐moulded and cured either isothermally at 260 °C or in a temperature ramp between 160 and 230 °C. The blends were investigated with SEM and thermal analysis (DSC, DMTA). Blends with 80% Rodrun and less cured at 260 °C and the blend containing 60% Rodrun cured in a temperature ramp showed macro‐phase separation followed by reaction‐induced micro‐phase separation (RIPS) both in the Rodrun‐rich and in the epoxy‐rich macro‐separated phases. Blends containing 90, 80 and 70% Rodrun moulded at 160 °C and cured in the temperature ramp showed only RIPS and a morphology rather similar to that of the uncured blends that was most likely co‐continuous; the blend with 90% Rodrun cured at 260 °C showed RIPS and a dispersed epoxy phase in a Rodrun matrix. Phase composition has been determined by extraction of the soluble fraction and chemical analysis.SEM showing the reaction induced micro‐phase separation for sample REA90/10i.magnified imageSEM showing the reaction induced micro‐phase separation for sample REA90/10i.

Kinetics and Mechanism of the Late Stage of Cure Reaction-induced Microphase Separation

Reaction-induced phase separation in oligomer modified epoxy resin cured by tertiary amine as a catalyst was studied. Evolution of the reaction system morphology was observed in situ by optical microscopy technique. Intensity of the coalescence that proceeds at the late stage of phase separation depends on conditions of the cure, i., formulation and temperature. The final morphology of the reaction system was shown to depend on a chemical mechanism of the cure process, first of all, polycondensation or polymerization reaction paths.

Model of Heterophase Polymer Formation from Reacting Blends at Phase Decomposition Induced by Thermoset Cure

The model of cure reaction-induced microphase separation proceeding via nucleation and growth mechanism with moderate reaction rate and far enough from the critical point is proposed. The classical frequency of elementary acts of nucleation was adopted. The model of particle growth with equilibrium boundary conditions and the conventional linear diffusion equation was developed to describe the concentration changes both inside and outside the particle. This provides a new insight on the ways of control of polymer morphology generated.