Physical aging of an amorphous polyimide: Enthalpy relaxation and mechanical property changes

The physical aging behavior of an isotropic amorphous polyimide possessing a glass transition temperature of approximately 239°C was investigated for aging temperatures ranging from 174 to 224°C. Enthalpy recovery was evaluated as a function of aging time following sub‐T_g annealing in order to assess enthalpy relaxation rates, and time‐aging time superposition was employed in order to quantify mechanical aging rates from creep compliance measurements. With the exception of aging rates obtained for aging temperatures close to T_g, the enthalpy relaxation rates exhibited a significant decline with decreasing aging temperature while the creep compliance aging rates remained relatively unchanged with respect to aging temperature. Evidence suggests distinctly different relaxation time responses for enthalpy relaxation and mechanical creep changes during aging. The frequency dependence of dynamic mechanical response was probed as a function of time during isothermal aging, and failure of time‐aging time superposition was evident from the resulting data. Compared to the creep compliance testing, the dynamic mechanical analysis probed the shorter time portion of the relaxation response which involved the additional contribution of a secondary relaxation, thus leading to failure of superposition. Room temperature stress‐strain behavior was also monitored after aging at 204°C, with the result that no discernible embrittlement due to physical aging was detected despite aging‐induced increases in yield stress and modulus. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1931–1946, 1999     

Effect of the network structure on thermal and mechanical properties of mesogenic epoxy resin cured with aromatic amine

The epoxy resin containing a typical mesogenic group such as biphenol was cured with catechol novolak and aromatic diamines which have neighboring active hydrogens. In the biphenol-type epoxy resin cured with catechol novolak, 4,4′ diaminodiphenylmethane, and p-phenylenediamine (PPD), the glass-rubber transition almost disappeared, and thus a very high elastic modulus was obtained in the high temperature region. It is clear that the thermal motion of the network chains is significantly suppressed in these cured systems. In addition, in the PPD-cured system, a characteristic pattern like a schlieren texture was clearly observed under the crossed polarized optical microscope. Thus we conclude that the mesogenic group contained in the epoxy molecule is oriented in the networks when the mesogenic epoxy resin is cured with phenols and diamines which have neighboring active hydrogens. On the other hand, the biphenol-type resin cured with 3,3′,5,5′-tetraethyl-4,4′-diamino diphenylmethane (TEDDM) showed a well-defined glass-rubber transition and, thus, a low rubbery modulus. In this cured system, no characteristic pattern was observed under the crossed polarized light. These results show that the large branches, such as ethyl groups on the network chains, prevent the orientation of network chains which contain the mesogenic group. © 1997 John Wiley & Sons, Inc.     

Effect of new melamine-terephthaldehyde resin modified graphene oxide on thermal and mechanical properties of PVC

In this study a new melamine-terephthaldehyde resin modified graphene oxide was synthesized and used as a reinforcement of poly(vinyl chloride) (PVC). Characterization, morphology, thermal and mechanical properties of the nanocomposites were examined by means of attenuated total reflectance-Fourier transform infrared spectroscopy, X-ray diffraction, field emission-scanning electron microscopy, thermogravimetric analysis, differential scanning calorimeter and tensile properties. The first hydrochloric acid releasing data of poly(vinyl chloride) was removed by incorporation of the modified graphene oxide as compare to the neat polymer. The temperatures at 2 wt% losses, main decomposition temperatures, maximum decomposition temperatures, also shift to higher temperature in the corresponding nanocomposites as compared to the neat PVC. The tensile strength and elongation at break of the nanocomposite films was increased as compared to the neat PVC. The interesting results in crystallinity of PVC were observed with adding 5 wt% of the modified graphene oxide.     

Effect of a hyperbranched polymer over the thermal curing and the photocuring of an epoxy resin

In this study, the authors study by calorimetry the influence of hyperbranched polyester Boltron^®H40 on the thermal curing and the photocuring of a diglycidyl ether of bisphenol epoxy resin (DGEBA) using ytterbium (III) trifluoromethanesulfonate and triarylsulfonium hexafluorantimonate as thermal and photo cationic initiators, respectively. In the dynamic thermal curing at different heating rates, the authors have seen a decelerating effect when H40 is added to DGEBA, the system with 10% of H40 being the slowest. An isoconversional method has been used to determine the apparent activation energy of the thermal curing. In the isothermal photocuring at low temperatures, the authors have also appreciated a decelerating effect on adding H40, obtaining a minimum conversion when the H40 proportion is 15%. However, at high temperatures, the photocuring process can be accelerated at the first part of this process. This behavior is a consequence of the temperature dependence of H40 solubility in DGEBA, the viscosity of the system, and the hydroxyl-induced chain-transfer reaction. The values found of the maximum glass transition temperature in the thermal curing and in the photocuring, show that H40 is not completely solubilized in the reacted system.     

Effect of thermal treatment on the characteristics of electrospun PVDF−silica composite nanofibrous membrane

Composite nanofibrous membranes were prepared by the electrospinning and the thermal treatment from poly(vinylidene fluoride) (PVDF)-tetramethyl orthosilicate (TMOS) blend solutions. The average diameter of nanofibers was reduced with increasing the concentration of TMOS in the solution due to the decrease of the solution viscosity. The EDX spectra confirmed the presence of TMOS on the external surface of the composite nanofibrous membrane. The porosity of membranes was effectively enhanced by the introduction of electrospinning technique. However, the mechanical properties, thermal stability and hydrophobicity were not markedly amplified. Thus the thermal treatment of the composite membranes was carried out, leading to the enormous enhancement of the mechanical properties and hydrophobicity. In addition, XRD results revealed that the crystal structure of PVDF in the composite membranes transformed from α-phase to β-phase due to the formation of silica particles by the thermal treatment.     

Effect of alkali treatment on mechanical and thermal properties of Kenaf fiber-reinforced thermoplastic polyurethane composite

In this study, a composite of thermoplastic polyurethane reinforced with short Kenaf fiber (Hibiscus cannabinus) was prepared via melt-blending method using Haake Polydrive R600 internal mixer. Effect of various sodium hydroxide NaOH concentrations, namely 2, 4 and 6% on tensile, flexural and impact strength was studied. Mean values were determined for each composite according to ASTM standards. Tensile, flexural and impact strength negatively correlates with higher concentrations of NaOH. Scanning electron microscope (SEM) was used to examine the surface of both treated and untreated fibers as well as fracture surface of tensile specimens. Morphology of treated and untreated fibers showed a rougher surface of treated fibers. It also showed that some of high concentrations of NaOH treated fibers have NaOH residues on their surface. This was confirmed by energy dispersive X-ray point shooting performed on the same SEM machine. Morphology of surface of fracture indicated that untreated composite had a better adhesion. Treated and untreated fibers as well as composites were characterized using Fourier transform infrared spectroscopy (FTIR). FTIR of treated fibers showed that NaOH treatment resulted in removal of hemicelluloses and lignin. FTIR also showed that untreated composite has more H-bonding than all treated composites. Thermal characteristic studies using thermogravimetry analysis and differential scanning calorimetry showed that untreated composite was more thermally stable than treated composites.     

Effect of atmospheric-pressure plasma on adhesion characteristics of polyimide film

In this work, the effect of atmospheric-pressure plasma treatments on surface properties of polyimide film are investigated in terms of X-ray photoelectron spectroscopy (XPS), contact angles, and atomic force microscopy (AFM). The adhesion characteristics of the film are also studied in the peel strengths of polyimide/copper film. As experimental results, the polyimide surfaces treated by plasma lead to an increase of oxygen-containing functional groups or the polar component of the surface free energy, resulting in improving the adhesion characteristics of the polyimide/copper foil. Also, the roughness of the film surfaces, confirmed by AFM observation, is largely increased. These results can be explained by the fact that the atmospheric-pressure plasma treatment of polyimide surface yields several oxygen complexes in hydrophobic surfaces, which can play an important role in increasing the surface polarity, wettability, and the adhesion characteristics of the polyimide/copper system.     

Formation of separating layers under conditions of the thermal aging of sorbents modified by fluorinated polyimide

Thermogravimetry, elemental analysis, low-temperature nitrogen adsorption, high-resolution electron microscopy, and gas chromatography are used to study the effect of the content of perfluorinated polyimide when used as a stationary phase for modifying Chromosorb P NAW diatomite supports and aluminum oxide, and the effect of thermal aging conditions on changes in their texture and chromatographic characteristics. It is shown that Chromosorb P NAW + 5 wt % of polyimide (PI) adsorbent thermally aged at 700°C in a flow of inert gas exhibits properties of carbon molecular sieves, while aluminum oxide impregnated with 10 wt % of PI and thermally aged at 250°C allows us to selectively separate permanent and organic gases, as well separate saturated and unsaturated hydrocarbons.     

Effect of resin infiltration on fumed silica-based thermal insulations

The thermal conductivities and the strengths of fumed silica-based thermal insulations prepared by the dry process and the resin infiltration method have been studied. Infiltration of the insulation samples prepared by dry processing of mixtures, consisting of fumed silica, ceramic fiber and a SiC opacifier, was performed to strengthen their binding structure. The high temperature thermal conductivities for the insulation samples obtained by the resin infiltration method were similar to those for the samples prepared by the dry process. On the other hand, the bending strengths for the former were superior to those for the latter. However, the resin-infiltrated samples were more brittle than the samples obtained by the dry process due to the occurrence of rigid bonding between the fibers and the infiltrated resin in the porous insulations. The thermosetting resin solution infiltration method appeared to be applicable to producing a dust-free fumed silica-based thermal insulation.     

Effect of homologous nano-composites on the thermal degradation of the silicone resin

Effect of homologous of nano-composites on the thermal degradation of the silicone resin was researched based on graphene oxide (GO)/polyhedral oligomeric silsesquioxane (POSS). First, the amino-POSS was grafted onto the GO surface (GO/POSS) via the amide bond. Second, GO/POSS was incorporated into the silicone with active epoxy group via chemistry grafting. The reaction kinetics of the thermal decomposition of the epoxy–silicone resin based on nano-composite homologous effect is developed. The initial decomposition temperature of the modified silicone resin is improved by 77.2°C. At high temperatures, GO/POSS-modified silicone molecular end forms homologous nano-structures, which can restrain silicone future degradation. The developed strategy has potential to restrain the degradation of the polymer molecular chain.     

Low-temperature thermal decomposition of an epoxy resin

The thermal decomposition of the epoxy resin system, MY720 (70 wt % tetraglycidyl diaminodiphenyl methane), cured with Eporal (diamino diphenyl sulfone), was investigated at 125–215°C. The primary decomposition product was propenal. Several other aldehydes were observed; however, they are formed during cure and are not thermal decomposition products. The apparent activation energy for the production of propenal was 67 kJ/mol (16 kcal/mol). Possible mechanisms for the source of this compound are discussed.     

Effect of ionizing radiation on the dynamic mechanical properties of an epoxy and a graphite fiber/epoxy composite

TGDDM/DDS epoxy and T300/NARMCO 5208 composite specimens were exposed to 0.5 MeV electrons to dose levels up to 10,000 Mrad, and the effects of radiation on the mechanical properties were characterized using dynamic mechanical analysis (DMA). In nominally cured specimens there remain unreacted epoxide groups because the epoxy system vitrifies during the cure, preventing additional reaction. DMA shows that ionizing radiation continues the reactions of epoxide groups. Also, the ultimate glass transition is shown to decrease monotonically with radiation dose. The room temperature elastic modulus of the epoxy increases slightly with radiation, but at temperatures exceeding 100°C there is a slight decrease with radiation. There is a dynamic loss phenomena associated with the composites, not seen in the epoxy, that is thought to be due to the interphase region between the fiber and the matrix.     

Study of the curing of an acetylene-terminated polyphenylene resin with thermal expansion measurements

The curing of an acetylene-terminated polyphenylene resin (Hercules H-Resin) was followed by thermal expansion measurements. This approach proved useful in optimizing the curing conditions of the resin. Curing the polymer in air led to the formation of carboxyl groups, whereas curing under nitrogen did not. The thermal expansion coefficient is a minimum (32 ppm deg^?1) for a cure cycle of 250°C for 30 min, followed by 350°C for 30 min. Heating at temperatures above 350°C led to degradation of the crosslinked polymer and an increase in the thermal expansion coefficient.     

Effect of carbon nanotubes and their dispersion on thermal curing of polyimide precursors

The process of thermal imidization reaction is significant for temperature and time control in the polyimide industry. Here, we report the effect of carbon nanotubes and their states of dispersion on the thermal imidization of the precursor films of polyimide (poly(amic acid)) for the first time. The curing process was followed by measuring Fourier transform-infrared (FT-IR) spectra, fluorescence spectra, thermogravimetric-differential scanning calorimeter (TG-DSC) properties and the refractive indices of films. It was found that by evenly dispersing 1 wt% of carbon nanotubes assisted by a dispersant in the poly(amic acid),the full imidization temperature of the polyimide can be reduced from 300 °C to 250 °C. Different states of distribution of CNTs were observed by light microscopy and scanning electron microscopy, and proved that a better dispersion of carbon nanotubes dramatically enhanced the speed of imidization. Moreover, the DSC results showed that lower decomposition temperature of poly(amic acid) could be obtained with more uniform distribution of carbon nanotubes, which means the process of cyclodehydration of the poly(amic acid) was accelerated.     

Comparative mechanical property characterization of three indirect composite resin materials compared with two direct composites

Various new indirect composite materials have been developed with required advantages. In this study three indirect composite material (Artglass, Belleglass HP, Targis) were tested for flexural strength, fracture toughness, wear resistance and hardness against Filtek P60 and Z‐100. Five specimens of each material were fabricated according to the manufacturer's directions. The flexural strength and fracture toughness was measured using the bending test. The wear test was performed to accelerated wear in a toothbrushing apparatus. Vickers hardness was measured for each of the tested materials. The statistical tests used for flexural strength, fracture toughness, wear and hardness were One‐way ANOVA and Kruskal–Wallis test. The level of statistical significance chosen was p = 0.05. Results of the study showed that Filtek P60 was superior to the other composites in all tests. Significant differences were found among the materials. The differences in flexural strength, fracture toughnes, wear and hardness may have been due to differences in chemistry or method of polymerization of the composites. Copyright © 2003 John Wiley & Sons, Ltd.     

Study of the physical aging of an epoxy/cycloaliphatic amine resin modified with abs

Using differential scanning calorimetry (DSC) we have studied the physical aging of an epoxy resin based on the diglycidyl ether of bisphenol A (DGEBA) modified by two different contents of an acrylonitrile-butadiene-styrene (ABS) and cured with 1,3-bisaminomethylcyclohexane (1,3-BAC). Samples fully cured were annealed at temperature of 125°C for periods of time of 72 and 120 h, to determine the process of physical aging. The apparent activation energy for the enthalpy relaxation, Dh*, is determined as the sample is heated at 10°C min^-1 following cooling at various rates through the glass transition region. DSC studies suggested that the presence of thermoplastic inhibits the process of relaxation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of gamma-ray irradiation on the mechanical strength of polystyrene-ferrocene composite

Thin films of polystyrene-ferrocene (PS---Fe) composite were irradiated with various doses of gamma irradiation ranging from 4·1 to 25·3 Mrad. The Vickers hardness number (H_v) was calculated for pure and irradiated specimens. It was used to analyse mechanical strength at a saturation load of 100 g. The mechanical strength was found to increase with irradiation dose up to 12 Mrad. Brittleness of the composite PS-Fe was found to occur at a higher dose of 25 Mrad as compared to 15 Mrad for polystyrene. Further, fracture toughness, brittleness index and yield strength were calculated for various irradiated specimens consequent to crack propagation in the specimens.