Sixun Zheng

Miscibility and mechanical properties of epoxy resin/polysulfone blends

Abstract Bispehnol A-based polysulfone (PSF) was found to be miscible with uncured bisphenol A-type epoxy resin, i.e. diglycidyl ether of bisphenol A (DGEBA), as shown by the existence of a single glass transition temperature ( T g ) within the whole composition range. Miscibility between PSF and DGEBA is considered to be due mainly to entropy contribution. Furthermore, PSF was judged to be miscible with the 4,4′-diaminodiphenylmethane (DDM)-cured epoxy resin (ER) as revealed by the means of differential scanning calorimetry (d.s.c.), dynamic mechanical analysis (d.m.a.) and scanning electron microscopy (SEM). D.s.c. and d.m.a. studies showed that the DDM-cured ER/PSF blends had only one T g . SEM observation revealed that the DDM-cured ER/PSF blends was homogeneous. Both tensile and flexural properties of the DDM-cured ER/PSF blends slightly improved compared to those of the pure DDM-cured ER. Both fracture toughness ( K IC ) and fracture energy ( G IC ) increased by ca . 20% with the addition of PSF to the system. Morphological investigation of the K IC fracture surface suggests typical characteristics of brittle fracture.

Miscibility, morphology and fracture toughness of epoxy resin/poly(styrene-co-acrylonitrile) blends

Abstract Poly(styrene-co-acrylonitrile) (SAN) with 25 wt% acrylonitrile (AN) content was found to be miscible with uncured bisphenol-A-type resin, i.e. diglycidylether of bisphenol A (DGEBA), as shown by the existence of a single glass transition temperature within the whole composition range. Miscibility between SAN and DGEBA is considered to be due mainly to entropy contribution. However, SAN was judged to be immiscible with the 4,4′-diaminodiphenylmethane-cured epoxy resin (DDM-cured ER) as revealed by the means of differential scanning calorimetry (d.s.c.), dynamic mechanical analysis (d.m.a.) and scanning electron microscopy (SEM). It was observed that the DDM-cured ER/SAN blends have two Tgs, which remain almost invariant with composition and are close to those of the pure components, respectively. SEM study revealed that all the DDM-cured ER/SAN blends have a two-phase structure. The fracture mechanics studies indicate that the DDM-cured ER/SAN blends containing 10 wt% give a substantial improvement of fracture toughness KIC. The fracture toughness KIC increases with SAN content and shows a maximum at 10 wt% SAN content, followed by a dramatic decrease in KIC for the cured blends containing 15 wt% SAN or more. SEM investigation of the KIC fracture surfaces indicates that the toughening effect of the SAN-modified epoxy resin is greatly dependent on the morphological structures.

Rapid deswelling and reswelling response of poly(N-isopropylacrylamide) hydrogels via formation of interpenetrating polymer networks with polyhedral oligomeric silsesquioxane-capped poly(ethylene oxide) amphiphilic telechelics.

Hepta(3,3,3-trifluoropropyl) polyhedral oligomeric silsesquioxane-capped poly(ethylene oxide) telechelics (POSS-capped PEO) were synthesized via click chemistry. The POSS-capped PEO amphiphilic telechelics were incorporated into cross-linked poly(N-isopropylacrylamide) (PNIPAAm) to form the structure of physical interpenetrating polymer networks (IPNs). In the organic-inorganic networks, the POSS terminals of POSS-capped PEO telechelics were self-organized into microdomains to act as the physical cross-linking sites of PEO and the physically cross-linked PEO network was interlocked with the PNIPAAm network. It is identified that the organic-inorganic hydrogels resulting from the PNIPAAm network and the POSS-capped PEO telechelics displayed much faster response rates than the plain PNIPAAm hydrogels in terms of swelling, deswelling, and reswelling tests. The improved thermoresponse of hydrogels has been interpreted on the basis of the formation of the specific supramolecular structures in the hydrogels. The synergism from hydrophobic and hydrophilic components (i.e., POSS domains and PEO chains) is responsible for the improvement of hydrogel properties.

Miscibility of epoxy resins/poly(ethylene oxide) blends cured with phthalic anhydride

Abstract Epoxy resins (EP)/poly(ethylene oxide) (PEO) blends cured with phthalic anhydride were studied by differential scanning calorimetry and dynamic mechanical analysis. Single glass transition temperatures were observed for all blends before and after curing, indicating a high degree of miscibility. At the same time, a marked deviation from the empirical equation (e.g. the Fox equation) was noticed after curing. This is ascribed to the dilution effect of the PEO component and participation of PEO in the cure reaction resulting in incomplete crosslinking, i.e. the formation of imperfect crosslinking network structures. Fourier transform infra-red spectroscopy provided strong evidence that there is a specific interaction between the EP and PEO molecules.

Miscibility and phase behavior in thermosetting blends of polybenzoxazine and poly(ethylene oxide)

Abstract Thermosetting polymer blends composed of polybenzoxazine (PBA-a) and poly(ethylene oxide) (PEO) were prepared via in situ curing reaction of benzoxazine (BA-a) in the presence of PEO, which started from the initially homogeneous mixtures of BA-a and PEO. Before curing, the BA-a/PEO blends displayed the single and composition-dependant glass transition temperatures ( T g s) in the entire blend composition, and the equilibrium melting point depression was also observed in the blends. It is judged that the BA-a/PEO blends are completely miscible. The miscibility was mainly ascribed to the contribution of entropy to mixing free energy since the molecular weight of BA-a is rather low. However, phase separation occurred after curing reaction at the elevated temperature, which was confirmed by differential scanning calorimetry (DSC) and scanning electronic microscopy (SEM). It was expected that the PBA-a/PEO blends would be miscible since PBA-a possesses a great number of phenolic hydroxyls in the molecular backbone, which are potential to form the intermolecular hydrogen bonding interactions with oxygen atoms of PEO and thus would fulfill the miscibility of the blends. To interpret the experimental results, we investigated the variable temperature Fourier transform infrared spectroscopy (FTIR) of the blends via model compound. The FTIR results indicate that the phenolic hydroxyl groups could not form the efficient intermolecular hydrogen bonding interactions at the elevated temperatures (e.g. the curing temperatures), i.e. the phenolic hydroxyl groups existed mainly in the non-associated form in the system. Therefore, the decrease of the mixing entropy still dominates the phase behavior of thermosetting blends at the elevated temperature.

Miscibility and mechanical properties of tetrafunctional epoxy resin/phenolphthalein poly(ether ether ketone) blends

Phenolphthalein poly(ether ether ketone) (PEK-C) was found to be miscible with uncured tetraglycidyl 4,4′-diaminodiphenylmethane (TGDDM), which is a type of tetrafunctional epoxy resin (ER), as shown by the existence of a single glass transition temperature (Tg) within the whole composition range. The miscibility between PEK-C and TGDDM is considered to be due mainly to entropy contribution. Furthermore, blends of PEK-C and TGDDM cured with 4,4′-diaminodiphenylmethane (DDM) were studied using dynamic mechanical analysis (DMA), Fourier-transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). DMA studies show that the DDM-cured TGDDM/PEK-C blends have only one Tg. SEM observation also confirmed that the blends were homogeneous. FTIR studies showed that the curing reaction is incomplete due to the high viscosity of PEK-C. As the PEK-C content increased, the tensile properties of the blends decreased slightly and the fracture toughness factor also showed a slight decreasing tendency, presumably due to the reduced crosslink density of the epoxy network. SEM observation of the fracture surfaces of fracture toughness test specimens showed the brittle nature of the fracture for the pure ER and its blends with PEK-C.

Phase behaviour and mechanical properties of epoxy resin containing phenolphthalein poly(ether ether ketone)

Abstract Blends of bisphenol-A-type epoxy resin(ER) and phenolphthalein poly(ether ether ketone) (PEK-C) cured with 4,4′-diaminodiphenylmethane (DDM) were studied using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM). The phase behaviour of the DDM-cured ER/PEK-C blends was greatly dependent on the curing condition and was affected by both the thermodynamic and kinetic factors. The homogeneous DDM-cured ER/PEK-C blends were obtained. The studies of DSC and Fourier-transform infrared (FTi.r.) spectroscopy indicate that there existed some unreacted oxirane rings of ER in the blends, and the curing reaction was incomplete even though the samples of the blends were further post-cured at 250°C. Mechanical measurements show that incorporation of PEK-C slightly decreased both the fracture toughness (KIC and GIC) and the flexural properties, presumably due to the reduced cross-link density of the epoxy network. SEM observation of the surfaces of fracture mechanical measurement specimens indicates the nature of brittle fracture for the plain ER and the blends.

Hepta(3,3,3-trifluoropropyl) polyhedral oligomeric silsesquioxane-capped poly(N-isopropylacrylamide) telechelics: synthesis and behavior of physical hydrogels.

Hepta(3,3,3-trifluoropropyl) polyhedral oligomeric silsesquioxane (POSS)-capped poly(N-isopropylacrylamide) (PNIPAAm) telechelics with variable lengths of PNIPAAm midblocks were synthesized by the combination of reversible addition-fragmentation chain transfer polymerization (RAFT) and the copper-catalyzed Huisgen 1,3-cycloaddition (i.e., click chemistry). The POSS-capped trithiocarbonate was synthesized and used as the chain transfer agent for the RAFT polymerization of N-isopropylacrylamide. The organic-inorganic amphiphilic telechelics were characterized by means of nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). Atomic force microscopy (AFM) shows that all the POSS-capped PNIPAAm telechelics exhibited microphase-separated morphologies, in which the POSS terminal groups were self-assembled into the microdomains and dispersed into the continuous PNIPAAm matrices. The POSS nanodomains could behave as the physical cross-linking sites and as a result the physical hydrogels were formed while these POSS-capped PNIPAAm telechelics were subjected to the solubility tests with water. These physical hydrogels possessed well-defined volume phase transition phenomena and displayed rapid reswelling and deswelling thermoresponsive behavior compared to control PNIPAAm hydrogel.

Nanostructures and Surface Hydrophobicity of Self-Assembled Thermosets Involving Epoxy Resin and Poly(2,2,2-trifluoroethyl acrylate)-block-Poly(ethylene oxide) Amphiphilic Diblock Copolymer

Poly(2,2,2-trifluoroethyl acrylate)-block-poly(ethylene oxide) (PTFEA-b-PEO) amphiphilic diblock copolymer was synthesized via the reversible addition-fragmentation transfer polymerization of 2,2,2-triffluroethyl acrylate with dithiobenzoyl-terminated poly(ethylene oxide) as a chain-transfer agent. The amphiphilic diblock copolymer was incorporated into epoxy resin to prepare the nanostructured epoxy thermosets. The nanostructures were investigated by means of atomic force microscopy, small-angle X-ray scattering, and dynamic mechanical analysis. In terms of the miscibility of the subchains of the block copolymer with epoxy after and before curing reaction, it is judged that the formation of the nanostructures follows the mechanism of self-assembly. The static contact angle measurements indicate that the nanostructured thermosets containing PTFEA-b-PEO diblock copolymer displayed a significant enhancement in surface hydrophobicity as well as a reduction in surface free energy. The improvement in surface properties was ascribed to the enrichment of the fluorine-containing subchain (i.e., PTFEA block) of the amphiphilic diblock copolymer on the surface of the nanostructured thermosets, which was evidenced by surface atomic force microscopy and energy-dispersive X-ray spectroscopy.

Thermosetting polymer blends of unsaturated polyester resin and poly(ethylene oxide). II. Hydrogen-bonding interaction, crystallization kinetics, and morphology

Hydrogen-bonding interaction between the two components of the poly(ethylene oxide) (PEO)/oligoester (OER) blends and the PEO/crosslinked unsaturated polyester resin (PER) blends was found to be an important driving force to the miscibility of these polymer blends. Its strength is approximately as strong as the self-association of hydroxyl groups in either the pure OER or the pure PER. The crystallization kinetics and morphology of PEO in PEO/PER blends was remarkably affected by crosslinking. It was found that the overall crystallization rate of PEO in PEO/PER blends is larger than that in PEO/OER blends at the crystallization temperature investigated, which was considered to be the result of nucleation controlling mechanism. With decreasing PEO content, the regular shape of PEO spherulites turns irregular in PEO/ OER blends, whereas in PEO/PER blends, the birefrigent spherulites turns into dendritic structures. Raising the crystallization temperature favors the formation of dendritic textures in PEO/PER blends.