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

An amphiphilic diblock copolymer, poly(ethylene oxide)-block-poly(?-caprolactone) (PEO-b-PCL) was synthesized via the ring-opening polymerization of ?-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.     

Shape memory properties and unusual optical behaviour of an interpenetrating network of poly(ethylene oxide) and poly(2‐hydroxyethyl methacrylate)

An interpenetrating polymer network (IPN) with shape memory properties was prepared by using poly(2‐hydroxyethyl methacrylate) (PHEMA) and poly(ethylene oxide) (PEO). PHEMA acts as a fixed phase and PEO as a switching phase. The switching action of PEO is due to the reversible process of melting and crystallization. It was observed that the shape recovery of the IPN increases with increasing crosslinker concentration up to an optimum value and decreases thereafter. In addition to the shape memory property, the IPNs show a reversible change in optical properties from translucent to opaque. The change in optical properties is quite different from that observed in a semicrystalline polymer system where the transparency increases as a result of the melting of crystals. This behaviour of the IPN is explained in terms of H‐bonding of PEO with PHEMA. Fourier transform infrared spectroscopy was used to study the H‐bonding between PEO and PHEMA. © 2019 Society of Chemical Industry     

Nanostructured thermosets from epoxy and poly(2,2,2-trifluoroethyl acrylate)-block-poly(glycidyl methacrylate) diblock copolymer: Demixing of reactive blocks and thermomechanical properties

Poly(2,2,2-trifluoroethyl acrylate)-block-poly(glycidyl methacrylate) (PTFEA-b-PGMA) diblock copolymer was synthesized via sequential reversible addition-fragmentation chain transfer (RAFT) polymerization. The reactive diblock copolymer was incorporated into epoxy to obtain the nanostructured thermosets. The morphology of the thermosets was investigated by means of atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS). It is identified that the demixing of the reactive subchain (viz. PGMA) out of epoxy matrix occurred in the process of curing reaction, which exerted a profound impact on the glass transition temperatures of the nanostructured thermosets. The static contact angle measurements showed that the nanostructured thermosets displayed a significant enhancement in surface hydrophobicity as well as a reduction in surface free energy. The improvement in surface properties was attributed to the enrichment of the fluorine-containing block (i.e., PTFEA) of amphiphilic diblock copolymer on the surface of the thermosets, which was further evidenced by surface atomic force microscopy (AFM). The measurement of critical stress intensity factor (K_1C) showed that the fracture toughness of the materials was significantly enhanced by the inclusion of a small amount of PTFEA-b-PGMA diblock copolymer.     

Crystallization and vitrification effect in a poly(styrene)-g-poly(ethylene oxide) graft copolymer

Crystallization and solid state structure of a poly(styrene)-graft-poly(ethylene oxide) (PS-g-PEO) graft copolymer with crystallizable side chains were studied using simultaneous small angle X-ray scattering/wide angle X-ray scattering/differential scanning calorimetry (SAXS/WAXS/DSC). It is found that the glass transition temperature (T_g) of PS main chain is remarkably higher than that of PS homopolymer. The start cooling temperature (T_o) has a great influence on crystallization of the PEO side-chain. When the graft copolymer is cooled from the temperature above T_g, phase separation is suppressed due to the low mobility of the PS main chain and the homogeneous melt is vitrified. The unfavorable conformation of the rigid main chain results in a single crystallization peak and lower crystallinity. When PS-g-PEO is only heated to a temperature lower than the T_g and then cooled, phase separation is retained. Both the PEO side chains with high and low crystallizability can crystallize in the phase-separated state, leading to double crystallization peaks and higher crystallinity. The effect of solvent on crystallization of the graft copolymer was also examined. It is observed that addition of toluene reduces the T_g of the PS main chain and leads to the disappearance of the vitrification effect.     

Crystalline structures in ultrathin poly(ethylene oxide)/poly(methyl methacrylate) blend films

Amorphous poly(ethylene oxide)/poly(methyl methacrylate) (PEO/PMMA) blend films in extremely constrained states are meta-stable and phase separation of fractal-like branched patterns happens in them due to heterogeneously nucleated PEO crystallization by diffusion-limited aggregation. The crystalline branches are viewed flat-on with PEO chains oriented normal to the substrate surface, upon increasing PMMA content the branch width remains invariant but thickness increases. It is revealed that PMMA imposes different effects on PEO crystallization, i.e. the length and thickness of branches, depending on the film composition.     

Morphology and spherulitic growth kinetics in poly(ethylene oxide)/poly(n‐butyl methacrylate) blends

Results of a study concerning the morphology and the spherulite growth rates of poly(ethylene oxide) (PEO) in binary blends with poly(n‐butyl methacrylate) (PnBMA) are reported. Microscopic observations show that blending causes the spherulite structure to become coarser and less birefringent and confirms that the spherulitic growth rates of PEO were reduced by the addition of PnBMA. X‐ray diffraction studies show no change in the unit cell dimensions and a decrease in the degree of crystallinity upon blending. Analysis of the spherulite radial growth rate data by using Lauritzen–Hoffman theory indicates that crystallization in the range of 300 to 330 K occurs solely within regime III. The calculated surface free energy of folding, σ_e, for pure PEO is 57 erg cm^?2 and decreases with increasing the content of PnBMA in the blend. Copyright © 2004 Society of Chemical Industry     

Synthesis and rheological properties of soluble poly(hydroxyethyl methacrylate) and some copolymers

Methods to synthesise soluble poly(2-hydroxyethyl methacrylate) (PHEMA) with varying molar mass were developed. Conversions over 90% were achieved without losing solubility. Steady state tests (range 0.01–590 Pa) and oscillatory tests (range 0.001–40 Hz) were carried out on 40% solutions in dimethyl formamide using a Carri-Med rheometer. Only PHEMA having molar mass over 500000 showed significant shear thinning and viscoelastic properties. The copolymers of 2-hydroxyethyl methacrylate (HEMA) with n-butyl acrylate (BA) and 2-hydroxypropyl acrylate (HPA) showed higher viscosities and viscoelastic properties compared with PHEMA homopolymer prepared under identical conditions. Also, viscosity and viscoelasticity increased with increase in acrylate content in the initial mixture. This was attributed to the higher molar mass of copolymers, which resulted from faster polymerisation rates owing to the inclusion of relatively highly reactive acrylates. However, in the case of HEMA : BA copolymers, first, the viscosity and viscoelasticity increased with increasing BA content and then dropped again, giving a maximum around HEMA : BA 75 : 25. This anomaly was explained by taking the effects of changes in inter-and intra-molecular hydrogen bonding as well as conformational differences caused by inclusion of BA in the PHEMA chain into account.     

Crystal melting and its kinetics on poly(ethylene oxide) by in situ atomic force microscopy

The process of melting in poly(ethylene oxide) (PEO) is followed in real-time at elevated temperatures by atomic force microscopy (AFM) using a simple hot stage apparatus. AFM imaging of the morphology above the onset of melting revealed the dynamics of a complex melting process. The observed melting behavior of PEO is associated with the existence of separate dominant and subsidiary morphological entities. The morphological observations revealed that the melting process is not explained by a mechanism of crystal reorganization (melting-recrystallization-remelting or crystal thickening. The kinetic data shows that the crystal dimensions decrease proportional to time indicating a nucleation controlled melting process. The crystals melt instantaneously on heating and reveal a spread in the rates of melting of the radial {120} faces. This variation in rate of retrogression of the crystals is assumed to be related to a lamellar thickness distribution of the melt grown crystals.     

Spherulitic crystallization in blends of poly(ethylene oxide) and poly(methyl methacrylate)

The crystallization kinetics of binary blends of poly(ethylene oxide) and poly(methyl methacrylate) were investigated. The isothermal spherulitic growth rates were measured by means of a polarized light microscope. The temperature and composition dependence on the growth rates have been analysed. The temperature range studied was from 44° to 58°C. The introduction of poly(methyl methacrylate) into poly(ethylene oxide) resulted in a reduction of the spherulitic growth rate as the proportion of poly(methyl methacrylate) was increased from zero to 40% by weight. Results have been analysed using the theoretical equations of Boon and Azcue for the growth rate of polymer-diluent mixtures. The experimental results are in good agreement with this equation. The temperature coefficient is negative as is the case in the crystallization of bulk homopolymers.     

Phase behavior of aqueous systems containing block copolymers of poly(ethylene oxide) and poly(propylene oxide)

Phase behavior of aqueous systems containing block copolymers of poly(ethylene oxide (PEO) and poly(propylene oxide) (PPO) was evaluated by building up temperature-concentration phase diagrams. We have studied bifunctional triblock copolymers (HO-PEO-PPO-PEO-OH) and monofunctional diblock copolymers (R-PEO-PPO-OH and R-PPO-PEO-OH, where R length is linear C₄ and C_12–14). The cloud points of the polymer solutions depended on EO/PO ratio, polarity, R length and position of the hydrophilic and hydrophobic segments along the molecule. Such factors influence on the solutions behavior was also analyzed in terms of critical micelle concentration (CMC), which was obtained from surface tension vs. concentration plots. Salts (NaCl and KCl) added into the polymer solutions change the solvent polarity decreasing the cloud points. On the other hand, the cloud points of the polymer solutions increased as a hydrotrope (sodium p-toluenesulfonate) was added. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1767–1772, 1997     

Influence of composition and molecular mass on the morphology,crystallization and melting behaviour of poly(ethylene oxide)/poly(methyl methacrylate) blends

Results are reported on the influence of composition and molecular mass of components on the isothermal growth rate of spherulites, on the overall kinetic rate constant, on the primary nucleation and on the thermal behaviour of poly(ethylene oxide)/poly(methyl methacrylate) blends. The growth rate of PEO spherulites as well as the observed equilibrium melting temperatures decrease, for a given T_c or ΔT, with the increase of PMMA content.Such observations are interpreted by assuming that the polymers are compatible in the undercooled melt, at least in the range of crystallization temperatures investigated. Thermodynamic quantities such as the surface free energy of folding σ_e and the Flory-Huggins parameter χ₁₂ have been obtained by studying the dependence of the radial growth rate G and of the overall kinetic rate constant K from temperature and composition and the dependence of the equilibrium melting temperature depression ΔT_m upon composition, respectively.     

Preparation and characterizations of interpenetrating polymer network hydrogels of poly(ethylene oxide) and poly(methyl methacrylate)

Interpenetrating polymer network (IPN) hydrogels based on poly(ethylene oxide) and poly(methyl methacrylate) were prepared by radical polymerization using 2,2‐dimethyl‐2‐phenylacetophenone and ethylene glycol dimethacrylate as initiators and crosslinkers, respectively. The IPN hydrogels were analyzed for sorption behavior at 25°C and at a relative humidity of 95% using dynamic vapor sorption. The IPN hydrogels exhibited a relatively high equilibrium water content in the range of 13–68%. The state of water in the swollen IPN hydrogels was investigated using differential scanning calorimetry. The free water in the hydrogels increased as the hydrophilic content increased. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 258–262, 2003     

Preparation and characterization of graft copolymers of methyl methacrylate and poly(n-hexyl isocyanate) macromonomers

Living poly(n-hexyl isocyanate) (PHIC) was deactivated with methacryloyl chloride to produce methacryl-terminated poly(n-hexyl isocyanate) (PHIC-MA) rodlike macromonomers. Radical copolymerization of methyl methacrylate (MMA) with PHIC-MA was performed using 2,2′-azobis(isobutyronitrile) as an initiator in benzene at 60 °C to prepare poly(methyl methacrylate)-graft-poly(n-hexyl isocyanate) (PMMA-graft-PHIC) graft copolymers. The monomer reactivity ratios of MMA (M₁) and PHIC-MA (M₂) were evaluated as r₁=11.5 and r₂=∼0, exhibiting remarkably lower reactivity of PHIC-MA macromonomer than that of common macromonomers. The resultant graft copolymers were characterized using gel permeation chromatography equipped with low-angle laser light-scattering to determine the molecular weights, and equipped with a refractive index detector and an ultraviolet light detector to estimate a PHIC weight fraction of PMMA-graft-PHIC at the ith elution volume of the GPC chromatogram. There are 2-3 PHIC grafts per PMMA molecule, and the PHIC rodlike chains might be difficult to introduce into the PMMA main chains having higher molecular weights. A specific dimension of PMMA-graft-PHIC in solution was discussed in detail.     

Nonaqueous emulsion copolymerization of ethyl methacrylate/lauryl methacrylate in propylene glycol. II. Adsorption of poly(ethylene oxide)–polystyrene–poly(ethylene oxide) triblock copolymer on latex particles

The adsorption behavior of various poly(ethylene oxide)–polystyrene–poly(ethylene oxide) (PEO‐PS‐PEO) triblock copolymer (TBC) s on acrylic latex particles in propylene glycol was studied. The composition of the PEO‐PS‐PEO triblock polymers varied from 41 to 106 in each PEO block length and from 18 to 41 in the PS block length. The location of the PEO‐PS‐PEO TBC was determined by analyzing for the physically adsorbed amount on the latex surface, the anchored mount on the surface, the entrapped amount inside the particle, and the “free” PEO‐PS‐PEO TBCs in the propylene glycol. A contour graph technique was applied to analyze the experimental data, which showed that a minimum existed for the physically adsorbed portion at a PS block length of about 30 units. When the PS block length was less than 30 units, the adsorption decreased with increasing PS block length, indicating rearrangement of mixed PEO brush and adsorbed PS block. When the PS block was greater than 30 units, the adsorption increased with increasing block length because of the poor solvency of the PS block in the propylene glycol medium, resulting in a collapse of the PS chain. Considering the binding energy between the PS block and the latex particle surface, which governs adsorption, it was hypothesized that a lower block length limit exists, below which no adsorption takes place. The solubility of the PS block in propylene glycol guides the important upper block length limit. The anchored fraction of the block copolymer increased continuously with increasing PS block length in the entire region investigated. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1963–1975, 2001     

Phase behaviour in poly(ethylene oxide-b-t-butyl methacrylate) block copolymers

The lamellar thickness of the poly(ethylene oxide)-poly(t-butyl methacrylate) (PEO-PTBMA) diblock copolymer system, obtained by differential scanning calorimetry and small angle X-ray scattering investigations, is correlated with the degree of polymerization of the amorphous (PTBMA) and crystallizable (PEO) sequences. The non-equilibrium exponents obtained immediately after bulk crystallization are different to those from extrapolated equilibrium results. Within the experimental standard deviations, the theoretical predictions of DiMarzio et al. and of Whitmore and Noolandi could be confirmed. The molecular weights of PEO and PTBMA ranged from 250 to 21000 g mol⁻¹ and from 1500 to 17000 g mol⁻¹, respectively. Both the equilibrium lamellar thickness l and the PEO domain size d_PEO increase with increasing PEO and decreasing PTBMA degrees of polymerization Z according to d_PEO l Z^0.97±0.08_EOZ^{−(0.53±0.19)}_TBMA.     

Preparation of collagen‐immobilized poly(ethylene glycol)/poly(2‐hydroxyethyl methacrylate) interpenetrating network hydrogels for potential application of artificial cornea

To enhance the mechanical strength of poly(ethylene glycol)(PEG) gels and to provide functional groups for surface modification, we prepared interpenetrating (IPN) hydrogels by incorporating poly(2‐hydroxyethyl methacrylate)(PHEMA) inside PEG hydrogels. Formation of IPN hydrogels was confirmed by measuring the weight percent gain of the hydrogels after incorporation of PHEMA, as well as by ATR/FTIR analysis. Synthesis of IPN hydrogels with a high PHEMA content resulted in optically transparent and extensively crosslinked hydrogels with a lower water content and a 6 ～ 8‐fold improvement in mechanical properties than PEG hydrogels. Incorporation of less than 90 wt % PHEMA resulted in opaque hydrogels due to phase separation between water and PHEMA. To overcome the poor cell adhesion properties of the IPN hydrogels, collagen was covalently grafted to the surface of IPN hydrogels via carbamate linkages to hydroxyl groups in PHEMA. Resultant IPN hydrogels were proven to be noncytotoxic and cell adhesion study revealed that collagen immobilization resulted in a significant improvement of cell adhesion and spreading on the IPN hydrogel surfaces. The resultant IPN hydrogels were noncytotoxic, and a cell adhesion study revealed that collagen immobilization improved cell adhesion and spreading on the IPN hydrogel surfaces significantly. These results indicate that PEG/PHEMA IPN hydrogels are highly promising biomaterials that can be used in artificial corneas and a variety of other load‐bearing tissue engineering applications. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Improving the adhesion of poly(ethylene terephthalate) fibers to poly(hydroxyethyl methacrylate) hydrogels by ozone treatment: Surface characterization and pull-out tests

This work reports a methodology to improve the adhesion between poly(ethylene terephthalate) (PET) fibers and poly(hydroxyethyl methacrylate) (pHEMA) hydrogels by treating PET with ozone. The surface chemistry of PET was examined by water contact angle measurements, X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS) and attenuated total reflectance infrared spectroscopy (ATR-IR) yielding information about the chemical functionalities at depths upon 0.6 μm. Ozone treatment introduces several polar groups in the surface of PET through oxidation and chain scission resulting in increased wettability. These groups include mostly carboxylic and anhydride groups and in small extent hydroxyl groups. Atomic force microscopy (AFM) analysis shows that the surface of ozone-treated PET films is fully covered with spherical particles that are removed after washing the film with water. During the washing step carboxylic functionalities were removed preferentially, as demonstrated by XPS and IR analysis. According to pull-out tests, PET monofilaments and bundles treated by ozone had a higher adhesion to pHEMA hydrogels than untreated ones. The apparent interfacial shear strength is 65% higher on pHEMA hydrogel containing an ozonated than an untreated PET monofilament. In addition, the force to pull-out an ozone-treated PET bundle from pHEMA hydrogel is ca. 81% higher than the one observed for the untreated bundle.     

Effects of micelle structures formed in selective solvents on crystallization behaviors of poly(ethylene glycol)-b-poly(styrene) copolymers

Well-defined amphiphilic block copolymers, poly(ethylene glycol) methyl ether-b-poly(styrene) (mPEG-b-PS), in which the PS blocks had different molecular weights, were synthesized by atom transfer radical polymerization (ATRP). Through introduction of selective solvents for the blocks, crystalline and amorphous blocks were self-assembled into different micelle structures in solutions. Atomic force microscopy (AFM) was used to characterize the micelle structures. It was observed that spherical micelles were always formed, whereas lamellar aggregates appeared only in the PS-selective solvent when the molecular weight of the PS block in mPEG-b-PS was low. The crystallizable mPEG blocks were self-assembled into either the core or corona of the micelles formed. The effects of the self-assembled structures on the crystallization behavior of mPEG blocks were then investigated with differential scanning calorimeter (DSC). When the PS molecular weight was much larger than that of mPEG, the result showed that the crystallinity of the mPEG block was lower when mPEG blocks crystallized in the corona than that in the core of the micelles. In this case, when mPEG blocks crystallized in micelle coronae, the micelle core formed by insoluble PS blocks was very big, so mPEG chains had to distribute sparsely in the micelle coronae. It was hard for mPEG chains in one micelle or among different micelles to gather together to crystallize. However, when the PS molecular weight was lower than that of mPEG, the crystallinity of mPEG was higher when the mPEG chains crystallized in the micelle corona, as the core formed by insoluble PS was small and the mPEG chains in the corona were easy to aggregate and crystallize.     

Miscibility and crystallization of poly(ethylene oxide) and poly(ε-caprolactone) blends

Blends of poly(ethylene oxide) (PEO) with poly(ε-caprolactone) (PCL), both semicrystalline polymers, were prepared by co-dissolving the two polyesters in chloroform and casting the mixture. Phase contrast microscopy was used to probe the miscibility of PEOB/PCL blends. Experimental results indicated that PEO was immiscible with PCL because the melt was biphasic. Crystallization of PEO/PCL blends was studied by differential scanning calorimetry and analyzed by the Avrami equation. The crystallization rate of PEO decreased with the increase of PCL in the blends while the crystallization mechanism did not change. In the case of the isothermal crystallization of PCL, the crystallization mechanism did not change, and the change in the crystallization rate was not very big, or almost constant with the addition of PEO, compared with the change of the crystallization rate of PEO.     

Tacticity effects on the miscibility behavior of poly(methyl methacrylate)/poly(ethylene oxide-b-dimethylsiloxane-b-ethylene oxide) blends

The miscibility of a triblock copolymer poly(ethylene oxide)-poly(dimethylsiloxane)-poly(ethylene oxide) with syndiotactic and isotactic poly(methylmethacrylate) wasstudied. Although isotactic poly(methyl methacrylate) (PMMA) was miscible with poly(ethylene oxide) (PEO) in the pure state, it was immiscible with the PEO end blocks in the copolymer. In comparison, the syndiotactic poly(methyl methacrylate) (sPMMA) was miscible with the PEO blocks as indicated by melting point depression, decrease in crystallinity, and slower rate of spherulite growth of PEO. When blends of the triblock copolymer were cooled to low temperatures, the poly(dimethylsiloxane) (PDMS) middle block which resided in the interlamellar region of PEO spherulites also crystallized; the development of PDMS crystals was clearly suppressed at high sPMMA contents.On leave from Union Chemical Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan