Curing behavior and toughening performance of epoxy resins containing hyperbranched polyester

The effects of the hyperbranched polyester with hydroxyl end groups (HBPE‐OH) on the curing behavior and toughening performance of a commercial epoxy resin (diglycidyl ether of bisphenol A, DGEBA) were presented. The addition of HBPE‐OH into DGEBA strongly increased its curing rate and conversion of epoxide group due to the catalytic effect of hydroxyl groups in HBPE‐OH and the low viscosity of the blend at curing temperature. The improvements on impact strength and critical stress intensity factor (or fracture toughness, K_1c) were observed with adding HBPE‐OH. The impact strength was 8.04 kJ m^?1 when HBPE‐OH reached 15 wt% and the K_1c value was approximately two times the value of pure epoxy resin when HBPE‐OH content was 20 wt%. The morphology of the blends was also investigated, which indicated that HBPE‐OH particles, as a second phase in the epoxy matrix, combined with each other as the concentration of HBPE‐OH increased. Copyright © 2004 John Wiley & Sons, Ltd.     

Investigation of the Curing Mechanism in the GPTS/APTS Binder System

The acid catalyzed sol-gel reaction in the mixed binder system, 3-glycidoxypropyltrimethoxysilane (GPTS)/3-aminopropyltriethoxysilane (APTS) was investigated and one step and two step synthesis process were compared. Hydrolysis product was observed using the ¹H, ¹³C NMR and Raman spectra. Especially, based on the Raman spectra, epoxy ring opening was observed, varying the ratio of GPTS to APTS. The two step process made clear sol, while the one step process resulted in a milky suspension. According to the Raman spectra, the epoxy ring opening reaction kinetics proceeded slower in the two step process than one step process. Throughout the two step process, it was possible to apply the binder for the coating of substrate.     

Curing of urea-formaldehyde resins on awood substrate

TG-DTA analysis method was used to study the curing behaviour of urea-formaldehyde (UF) adhesive resins in the presence of a wood substrate. The cure process was followed using a Setaram labsys^TM instrument in flowing nitrogen atmosphere by varying the ratio of resin and wood. Resin cure was catalysed with 2% of NH₄Cl. Curing tests were performed in the open standard platinum crucibles and in the sealed glass capsules. To characterise the reactivity of curing system, the peak temperatures in DTA curve and the mass loss values in TG curve were taken as the apparent indices. The main attention was paid to phenomena which actually take place in curing of UF resins during manufacturing of particleboards. Reactivity of the curing system depends mostly on methylol content of resin and can be adequetly evaluated by the maximum temperature of exothermic peak. The wood substrate has a substantial influence on the resin and water diffusion in system causing the changes in water/resin separation and water evaporation conditions. The water movement in curing adhesive joint was a confusing parameter in determining the peak positions. The rate of mass loss on a wood substrate is higher as compared to curing UF resin alone.     

Novel sulphur-containing banana-shaped liquid crystal molecules

In this preliminary report we present the mesomorphic properties and electro-optic investigations of three new 'banana-shaped' mesogens. The materials are structurally similar to Niori's the original bent-core materials but possess alkylthio substituents in place of alkoxy substituents. Microscopic investigations revealed an unusual 'spiral-domain texture', similar to that observed in the 'B₇' phase of related materials. Further investigations support our earlier observations that the materials display antiferroelectric switching in this phase, in two of the new materials studied. The nature of this phase, in comparison with the 'switchable' (B₂) and 'chiral-domain' (B₄) phases of the original materials, is discussed.     

Studies on the electro-optical and the light-scattering properties of PDLC films with the size gradient of the LC droplets

Polymer dispersed liquid crystal (PDLC) films with the size gradient of the LC droplets were prepared based on the epoxy/acrylate hybrid polymer matrix. The ultraviolet (UV) intensity gradient was induced by the UV-absorbing dye over the thickness of the samples. Taking advantage of the difference between the epoxy monomers and acrylate monomers in polymerisation rates and the UV intensity gradient, the gradient distribution of the LC droplet size was formed in PDLC films. The effect of the size gradient of the LC droplets on the electro-optical and the light-scattering properties of PDLC films was investigated. The results showed that due to the size gradient distribution of the LC droplets, PDLC films could exhibit the strong light scattering in the UV-visible-near infrared (VIS-NIR) region. Consequently, it provides a potential approach for modulating NIR light transmittance.     

Silicon‐containing trifunctional and tetrafunctional cyanate esters: Synthesis,cure kinetics,and network properties

The synthesis and physical properties of new silicon‐containing polyfunctional cyanate ester monomers methyl[tris(4‐cyanatophenyl)]silane and tetrakis(4‐cyanatophenyl)silane, as well as polycyanurate networks formed from these monomers are reported. The higher crosslinking functionality compared to di(cyanate ester) monomers enables much higher ultimate glass transition temperatures to be obtained as a result of thermal cyclotrimerization. The ability to reach complete conversion is greatly enhanced by cocure of the new monomers with di(cyanate ester) monomers such as 1,1‐bis(4‐cyanatophenyl)ethane. The presence of silicon in these polycyanurate networks imparts improved resistance to rapid oxidation at elevated temperatures, resulting in char yields as high as 70% under nitrogen and 56% in air in the best‐performing networks. The water uptake in the silicon‐containing networks examined is 4–6 wt % after 96 h of immersion at 85 °C, considerably higher than both carbon‐containing and/or di(cyanate ester) analogs. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 767–779     

Synthesis and application of cinnamate-functionalized rubber for the preparation of UV-curable films

A butyl rubber derivative that can be cured upon exposure to UV light in the absence of additional chemical additives was developed. This polymer was prepared by the reaction of hydroxyl-functionalized butyl rubber with cinnamoyl chloride to provide a cinnamate functionalized rubber. The cinnamate content was varied by starting with derivatives prepared from butyl rubber containing either 2 or 7 mol% isoprene. The kinetics of the cross-linking was studied by UV–visible spectroscopy and it was found to vary according to the film thickness. The changes in gel content and volume swelling ratio with irradiation time were dependent on the cinnamate content. Toxicity studies suggested that the cross-linked materials do not leach toxic molecules. The approach was also applied to obtain cross-linked films of butyl rubber-poly(ethylene oxide) graft copolymers, leading to surfaces that resisted the adhesion and growth of cells. Thus the approach is versatile and is of particular interest when non-leaching coatings of cross-linked butyl rubber are desired for biomedical or other applications.     

A New Approach for the Synthesis of Miktoarm Star Polymers Through a Combination of Thiol–Epoxy “Click” Chemistry and ATRP/Ring‐Opening Polymerization Techniques

A new approach was developed for synthesis of certain A₃B₃‐type of double hydrophilic or amphiphilic miktoarm star polymers using a combination of “grafting onto” and “grafting from” methods. To achieve the synthesis of desired miktoarm star polymers, acetyl protected poly(ethylene glycol) (PEG) thiols (M_n = 550 and 2000 g mol^?1) were utilized to generate A₃‐type of homoarm star polymers through an in situ protective group removal and a subsequent thiol–epoxy “click” reaction with a tris‐epoxide core viz. 1,1,1‐tris(4‐hydroxyphenyl)ethane triglycidyl ether. The secondary hydroxyl groups generated adjacent to the core upon the thiol–epoxy reaction were esterified with α‐bromoisobutyryl bromide to install atom transfer radical polymerization (ATRP) initiating sites. ATRP of N‐isopropylacrylamide (NIPAM) using the three‐arm star PEG polymer fitted with ATRP initiating sites adjacent to the core afforded A₃B₃‐type of double hydrophilic (PEG)₃[poly(N‐isopropylacrylamide)] (PNIPAM)₃ miktoarm star polymers. Furthermore, the generated hydroxyl groups were directly used as initiator for ring‐opening polymerization of ε‐caprolactone to prepare A₃B₃‐type of amphiphilic (PEG)₃[poly(ε‐caprolactone)]₃ miktoarm star polymers. The double hydrophilic (PEG)₃(PNIPAM)₃ miktoarm star polymers showed lower critical solution temperature around 34 °C. The preliminary transmission electron microscopy analysis indicated formation of self‐assembly of (PEG)₃(PNIPAM)₃ miktoarm star polymer in aqueous solution. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 146–156     

Liquid crystalline epoxy resin with improved thermal conductivity by intermolecular dipole–dipole interactions

To address tremendous needs for developing efficiently heat dissipating materials with lightweights, a series of liquid crystalline epoxy resins (LCEs) are designed and synthesized as thermally conductive matrix. All prepared LCEs possess epoxies at the molecular side positions and cyanobiphenyl mesogenic end groups. Based on several experimental results such as differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction, it is found that the LCEs exhibited liquid crystalline mesophases. When LCE is cured with a diamine crosslinker, the cured LCE maintains the oriented LC domain formed in the uncured state, ascribing to a presence of dipole–diploe and π–π interactions between cyanobiphenyl mesogenic end groups. Due to the anisotropic molecular orientation, the cured LCE exhibits a high thermal conductivity of 0.46 W m^?1 K^?1, which is higher than those of commercially available crystalline or amorphous epoxy resins. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 708–715