To understand the properties of materials, their phase structure must be established. It has been difficult in previous work to visualize the heterogeneous cross-linked structure of methacrylate-based networks. In this work, nano-sized phases with worm-like features were detected in the surfaces of model crosslinked methacrylate copolymer containing hydrophobic/hydrophilic co-mono-mers using tapping mode atomic force microscopy/phase imaging technique. The effects of different surface-contact covers on the height and phase-contrast images of model resin surfaces were also studied. Based on the experimental data, the identification of phase domains was proposed.
β-Nitroalcohols (βNAs) are promising corneoscleral crosslinking agents for the treatment of diseases such as keratoconus and myopia. Although it is believed that formaldehyde is released from the crosslinking reactions of βNAs, the mechanism by which βNAs react with amine-functionalized polymers has yet to be known. In this study, we present the reaction mechanism of the βNA crosslinking. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) data provide strong evidence that formaldehyde is released during the reaction. Catalytic studies show that sodium bicarbonate (NaHCO3) and salmon testes DNA accelerate the reaction while hydroxynitrile lyase from Arabidopsis thaliana decelerates the crosslinking reaction. These results suggest that βNAs are potential self-administered crosslinking agents for future clinical use.
Novel urethane shape-memory polymers (SMPs) of significant industrial relevance have been synthesized and characterized. Chemically crosslinked SMPs have traditionally been made in a one-step polymerization of monomers and crosslinking agents. However, these new post-polymerization crosslinked SMPs can be processed into complex shapes by thermoplastic manufacturing methods and later crosslinked by heat exposure or by electron beam irradiation. Several series of linear, olefinic urethane polymers were made from 2-butene-1,4-diol, other saturated diols, and various aliphatic diisocyanates. These thermoplastics were melt-processed into desired geometries and thermally crosslinked at 200°C or radiation crosslinked at 50 kGy. The SMPs were characterized by solvent swelling and extraction, differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), tensile testing, and qualitative shape-recovery analysis. Swelling and DMA results provided concrete evidence of chemical crosslinking, and further characterization revealed that the urethanes had outstanding mechanical properties. Key properties include tailorable transitions between 25 and 80°C, tailorable rubbery moduli between 0.2 and 4.2 MPa, recoverable strains approaching 100%, failure strains of over 500% at T(g), and qualitative shape-recovery times of less than 12 seconds at body temperature (37°C). Because of its outstanding thermo-mechanical properties, one polyurethane was selected for implementation in the design of a complex medical device. These post-polymerization crosslinked urethane SMPs are an industrially relevant class of highly processable shape-memory materials.
A new flame retardant polycarbonate/magnesium oxide (PC/MgO) nanocomposite, with high flame retardancy was developed by melt compounding. The effect of MgO to the flame retardancy, thermal property, and thermal degradation kinetics were investigated. Limited oxygen index (LOI) test revealed that a little amount of MgO (2 wt %) led to significant enhancement (LOI = 36.8) in flame retardancy. Thermogravimetric analysis results demonstrated that the onset temperature of degradation and temperature of maximum degradation rate decreased in both air and N2 atmosphere. Apparent activation energy was estimated via Flynn-Wall-Ozawa method. Three steps in the thermal degradation kinetics were observed after incorporation of MgO into the matrix and the additive raised activation energies of the composite in the full range except the initial stage. It was interpreted that the flame retardancy of PC was influenced by MgO through the following two aspects: on the one hand, MgO catalyzed the thermal-oxidative degradation and accelerated a thermal protection/mass loss barrier at burning surface; on the other hand, the filler decreased activation energies in the initial step and improved thermal stability in the final period.
Electrically conductive adhesives (ECAs) have been explored as a
SnPb solder alternative for attaching encapsulated surface mount
components on rigid and flexible printed circuits. However, limited
practical use of conductive adhesives in surface mount applications is
found because of the limitations and concerns over current commercial
ECAs. One critical limitation is the significant increase of joint
resistance with SnPb finished components under 85°C/85% relative
humidity (RH) aging. Conductive adhesives with stable joint resistance
are especially desirable. In this study, a novel conductive adhesive
system based on epoxy resins has been developed. Conductive adhesives
from this system show very stable joint resistance with SnPb finished
components during 85°C/85% RH aging. One ECA selected from this
system has been tested here and compared with two popular commercial
surface mount conductive adhesives. ECA properties studied included:
cure profile, glass transition temperature (T<sub>g</sub>), bulk
resistivity, moisture absorption, die shear adhesion strength, and shift
of joint resistance with SnPb metallization under 85°C/85%RH aging.
It was found that, compared to the commercial conductive adhesives, our
in-house conductive adhesive had higher T<sub>g</sub>, comparable bulk
resistivity, lower moisture absorption, comparable adhesion strength
and, most importantly, much more stable joint resistance. Therefore,
this conductive adhesive system should have better performance for
surface mount applications than current commercial surface mount
Recently there has been considerable interest in hydrophilic/hydrophobic patterned surfaces because they serve as important templates for selective deposition of various materials. We report a novel and simple method for the creation of hydrophilic/hydrophobic patterned surfaces using soft UV irradiation (365 nm wavelength). The method employed a photoinitiated hydrosilylation reaction of vinyl terminated polydimethylsiloxane with H-Si groups catalyzed by platinum(II) acetylacetonate. In UV-irradiated regions, the photo hydrosilylation reaction occurred to form hydrophobic regions. In unirradiated regions, the remaining H-Si groups were converted to HO-Si groups in the presence of aqueous sodium hydroxide to form hydrophilic regions. The photoinitiated hydrosilylation reaction completed within a little over 1 min, which was confirmed by water contact angle measurements and reflection-absorption spectroscopy. The value of the water contact angle for the hydrophilic regions was about 10deg and that for the hydrophobic regions was about 103deg. The success of pattern formation at the micron scale was confirmed by scanning electron microscope.
Phosphorus containing polyamides and copolyamides were prepared from 1-(dialkoxyphosphinyl)methyl-2,4- and -2,6-diaminobenzenes. The polymers produced were characterized by infrared and proton nuclear magnetic resonance spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. Their thermal properties were compared with those of the corresponding common polyamides. In addition, by determining the limiting oxygen index value of some of these polyamides, their fire resistance was evaluated.
Fire resistant compositions were prepared using 1-di(2-chloroethoxy-phosphinyl)methyl-2,4- and -2,6-diaminobenzene (DCEPD) as a curing agent for typical epoxy resins such as EPON 828 (Shell), XD 7342 (Dow), and My 720 (Ciba Geigy). In addition, compositions of these three epoxy resins with common curing agents such as m-phenylenediamine (MPD) or 4,4'-diaminodiphenylsulphone (DDS) were studied to compare their reactions with those of DCEPD. The reactivity of the three curing agents toward the epoxy resins, measured by differential calorimetry (DSC), was of the order MPD DCEPD DDS. The relatively lower reactivity of DCEPD toward epoxy resins was attributed to electronic effects.
A study was carried out of the surface recession (etching) of thin films of plasma-polymerized tetrafluoroethylene (PPTFE), polytetrafluoroethylene (PTFE), and ion-beam sputter-deposited polytetrafluoroethylene (SPTFE), exposed to atomic oxygen [O(3P)] downstream from a nonequilibrium radio-frequency O2 plasma. At 22°C, the etch rates for PTFE, SPTFE, and PPTFE were in the ratio of 8.7 : 1.0. A thin, conformal coating of PPTFE (etch rate of 0.3 nm/h at 22°C) was found to protect an underlying, cast film of a reactive polymer, cis-1,4-polybutadiene (etch rate of 0.13 mg/cm2 h ≡ 1300 nm/h at 22°C), against O(3P) attack for the time required to fully etch away the PPTFE coating. From ESCA analysis, PTFE exhibited only minor surface oxidation (uptake of 0.5 atom % O) upon etching, its F/C ratio decreasing slightly from 2.00 to 1.97; PPTFE exhibited considerable surface oxidation (uptake of 5.9 atom % O) and a decrease in F/C ratio from 1.30 to 1.23; and SPTFE exhibited a surface oxidation (uptake of 2.2 atom % O) intermediate between those of PTFE and PPTFE, with a decrease in F/C ratio from 1.73 to 1.67. The O(3P)-induced etching of PTFE had an activation energy of 3.8 Kcal/mol, but no activation energy value was obtained for PPTFE which gave a nonlinear Arrhenius plot apparently because of thermally induced thinning above 50°C.
The sorption of argon, carbon dioxide, and nitrogen on two heat shield composites (SLA-561 and SLA-561V) and on the SLA components was measured over the pressure range of 0.001 to 760 torr and in the temperature range of 30 to 50 C. The sorption of the gases by both the composites and the components varied directly with pressure. The sorption of CO2 by the phenolic spheres and the silicone elastomer and of Ar by the silicone elastomer varied inversely with temperature. The mechanism involved in the gas sorption was primarily absorption.
Cellulose acetate butyrate (CAB) membranes gave high salt and urea rejection with a water flux of about 3 gfd (gallons/ft2 · day) during hyperfiltration at 600 psig. Evidence was obtained which indicated that the CAB membranes used in this work were asymmetric. Membrane heat treatment increased urea rejection significantly while salt rejection was invariant, and water flux decreased. An increase in feed solution temperature caused a significant increase in water flux and a small decrease in urea and salt rejection. Increasing the pressure increased water flux and urea and salt rejection. During a 400-hr life test, the water flux decreased by about 25% while urea rejection increased and salt rejection was invariant. The influence of pressure, membrane heat treatment, and compaction during CAB membranes life testing on urea and salt rejection provided evidence that these two solutes were rejected by somewhat different mechanisms. Salt rejection was consistent with a solution–diffusion mechanism for membrane transport and uncoupled flow while changes in urea rejection with pressure, membrane heat treatment, and compaction during life testing suggested that urea was at least partially rejected by membrane exclusion resulting from geometric factors.
Complementing an earlier paper which utilized an energy balance criterion for a continuum mechanics analysis of adhesive failure in a pressurized blister at the interface of an elastic material and a rigid substrate, the analysis is extended to include an additional elastic interlayer between them. An infinite lateral-length elastic plate strip bonded through a Winkler elastic foundation to a rigid substrate is assumed, in which the plate is separated from the adhesive layer by internal pressure. It is found that the important design parameters are the tensile modulus-to-thickness ratio of the adhesive layer and the adhesive fracture energy of separation of the respective materials. The results provide a basis for investigating changes in the chemical microstructure of the adhesive.
Reverse osmosis (RO) composite polymer membranes were prepared from the organic monomers 3-butenenitrile, propyleneimine, 4-vinylpyridine, and allylamine by plasma polymerization in a radio-frequency discharge. RO performance data (water flux and salt rejection) were obtained as a function of discharge power and deposition time. Membranes were subject to RO testing at 45°C for up to 150 hr to investigate membrane deterioration. No appreciable degradation was observed. Water flux increased significantly with time at 45°C, and improved salt rejection at 45°C was observed in most cases.
This paper reports the variation of the etch rate of polymers in the afterglow of a radio frequency discharge in oxygen as a function of total flow rate in the range 2-10 cu cm (STP)/min. The measurements were made at ambient temperature with the O(P-3) concentration held essentially constant. Results are reported on three polymers: cis-polybutadiene, a polybutadiene with 33 percent 1,2 double bonds, and a polybutadiene with 40 percent 1,2 double bonds. It has been observed that the etch rate of these polymers decreases significantly with increasing flow rate, strongly suggesting that the vapor-phase products of polymer degradation contribute to the degradation process.
Recent developments in research on polyimides for high temperature applications have led to the synthesis of many new polymers. Among the criteria that determines their thermal oxidative stability, isothermal aging is one of the most important. Isothermal aging studies require that many experimental factors are controlled to provide accurate results. In this article we describe a statistical plan that compares the isothermal stability of several polyimide resins, while minimizing the variations inherent in high-temperature aging studies.
A study has been carried out of the performance and chemical characteristics of composite reverse osmosis membranes prepared by plasma polymerization of allylamine in a radio-frequency electric discharge. It has been shown that membranes can be prepared which simultaneously exhibit a high rejection for sodium chloride and a high water flux. The primary factors influencing the quality of the membranes are the choice of substrate material, the deposition time, and the power supplied to the discharge. Variations in rejection and flux as a function of applied pressure indicate that water flows through the membrane by both diffusive and bulk flow. A reduction in rejection and an increase in flux are observed when membranes are operated for prolonged periods or at higher temperatures (up to 60°C). Elemental analysis of plasma-polymerized allylamine shows that it can be represented by the stoichiometry C3H3.8N0.9O0.1. Infrared spectra show evidence for NH, CN, CN, and CH bond vibrations. ESCA spectra of the polymer surface show that the surface contains substantial amounts of both nitrogen and oxygen and that the nitrogen is present as either a nitrile or an imine group but not as an amine group. ESCA spectra of membranes used for reverse osmosis show that the surface loses nitrogen and gains oxygen with time and that this phenomenon is accelerated at higher operating temperatures. A decrease in rejection and an increase in flux accompanies these changes. It is postulated that most of the nitrogen in the polymer is present in the form of RR′CNH or RR′CNR″ type structures. The loss of nitrogen and gain in oxygen observed in the ESCA spectra of membranes run at elevated temperatures is explained by the hydrolysis of the proposed structures.
Samples of poly(etheretherketone) (PEEK) neat resin and APC-2 carbon fiber composite were subjected to various heat treatments, and the effect of quenching and annealing treatments was studied by wide-angle X-ray scattering. It is found that high-temperature treatments may introduce disorder into neat resin and composite PEEK when followed by rapid cooling. The disorder is metastable and can revert to ordered state when the material is heated above its glass transition temperature and then cooled slowly. The disorder may result from residual thermal stresses.
A simple analytical form of induced anisotropy of heat conductivity of initially isotropic polymer solids results from employing the simplified theory of the three-chain model of the non-Gaussian network. The analytical form appears to be valid up to a stretch ratio of 2.65, which is the limit of existing experimental data. The effect of induced anisotropy on the temperature distribution, due to the large deformations, is illustrated for a highly expanded spherical shell and a cylindrical tube under a steady-state heat flow using the derived analytical form of the strain-dependent heat conductivity.
Two similar polyimide systems were synthesized and characterized. The only structural difference was a sulfide linkage in the anhydride-derived portion of the first system vs. a sulfone linkage in the second. Their physical, mechanical, melt-flow and thermal properties, and their resistance to some of the more common solvents were determined. The flow properties of these polyimides indicate a potential for melt processability.
Various polymers were exposed to atomic oxygen O(3P) and comparisons made of their stability. The rates of reaction and mass removal for polyphosphazenes exposed to atomic oxygen were found to be significantly lower than for other polymers evaluated in this study. Surface analysis of polyphosphazene films indicated that O(3P) induces rearrangements in the phosphorus-nitrogen backbone of the polymer, resulting in cross-linking and cleavage of some pendant groups that are then replaced by oxygen.
The effect of thermal history on the tensile properties of polyetheretherketone neat resin films was investigated at different test temperatures (125, 25, and -100) using four samples: fast-quenched amorphous (Q); quenched, then crystallized at 180 C (C180); slowly cooled (for about 16 h) from the melt (SC); and air-cooled (2-3 h) from the melt (AC). It was found that thermal history significantly affects the tensile properties of the material below the glass transition. Fast quenched amorphous films were most tough, could be drawn to greatest strain before rupture, and undergo densification during necking; at the test temperature of -100 C, these films had the best ultimate mechanical properties. At higher temperatures, the semicrystalline films AC and C180 had properties that compared favorably with the Q films. The SC films exhibited poor mechanical properties at all test temperatures.
The preparation and magnetism of three supramolecular complexes, DABT-Ni2+/PAA, DABT-Cu2+/PAA and DABT-Fe2+/PAA, which were obtained via electrostatic interactions of the metal complexes of 2,2′-diaminos-4,4′-bithiazole (DABT-Ni2+, DABT-Cu2+and DABT-Fe2+) and poly(acrylic acid) (PAA) were described for the first time. FT-IR and elemental analysis were used to characterize the structures of the obtained materials. The magnetic behavior was examined as a function of magnetic field strength at 4 K and as a function of temperature (4–300 K). It was found that DABT-Ni2+/PAA and DABT-Cu2+/PAA exhibit ferromagnetic properties, while DABT-Fe2+/PAA exhibits an antiferromagnetic property. The corresponding multilayer films were also constructed on the quartz substrate. UV–vis spectra and AFM images were applied to characterize these thin films. The result indicates a process of uniform assembling.
High-molecular weight, high-temperature, highly optically transparent films suitable for electronic and aerospace applications were prepared based on a novel methylene-bridged dianhydride 3,3-prime bis(3,4-dicarboxyphenoxy) diphenylmethane dianhydride (PDMDA). All films had lower dielectric constants and significantly higher optical transparency than the commercial Kapton H film, with air-cured films having slightly higher dielectric constants and lower optical transparency than vacuum films, due to oxidative cross-linking.
The desorption of mixtures of ethane and butane at atmospheric pressure from low-density polyethylene was investigated over the temperature range from 20 to 60°C. Desorbed penetrants were continuously trapped in glass tubes immersed in liquid nitrogen, and composition was determined as a function of time by means of gas chromatography. The ratio of the quantity of desorbed gas at any time t, qt, to the quantity at complete desorption, q∞, was used to determine diffusion coefficients and solubility constants. The diffusion coefficients for both ethane and butane increase with increasing butane concentration in the temperature interval investigated. The solubility of both penetrants can be correlated by Henry's law at 40, 50, and 60°C. However, at 20 and 30°C. the solubility constant for both penetrants increases with increasing butane concentration. This trend is consistent with experimental observations for single-component diffusion and solubility of several hydrocarbons in polyethylene, where increasing concentration of penetrant plasticizes the polymer, resulting in increasing diffusion coefficients and solubility constants.
Carborane substituted polyphosphazenes were prepared by the thermal polymerization of phenyl-carboranyl penta chlorocyclotriphosphazene. Successive isothermal vacuum pyrolyses were conducted on the polymer and examined for structural changes by infrared spectroscopy. The degradation products were ascertained by gas chromatography-mass spectrometric analysis. It was found that the presence of the carborane group improves the thermal stability of the polymer by retarding the ring chain equilibrium processes of decomposition.
The crosslinking or curing reaction of polystyrylpyridine (PSP) has been studied by means of thermal reactions of its model compounds. Compounds 2,6-distyrylpyridine, 4-stilbazole, and deuterated 4-stilbazole were pyrolyzed at 200-325 C both in air and under vacuum in a sealed tube. The major pyrolysis products were diarylethane and stilbene, and were characterized by gas chromatography-mass spectrometry. Major dimeric products were naphthalene or quinoline derivatives. Mechanisms for the pyrolysis are suggested, and a crosslinked structure for cured PSP is proposed based on the thermal reaction products of model compounds.
The chemorheological properties of a commercial Hercules 3501-6 resin system were studied under isothermal curing conditions between 375 and 435 K. For cure temperatures ≥ 385 K, the storage modulus curing curves, G′(t), exhibited abrupt changes in slope which occurred at various times depending on the curing temperatures. These slope changes were attributed to the onset of gelation. Crossover points between G′(t) and G″(t) curves were observed for curing temperatures ≥ 400 K. The gelation and the crossover points obtained from the chemorheological measurements, therefore, defined two characteristic resin states during cure. Although gelation theory did not predict them correctly, the degree of cure in each of these states was approximately the same for every cure temperature. The temperature dependency of the viscosities in the characteristic resin states, and the rate constants of increase in moduli at different stages of curing were analyzed. Various G′(t) and G″(t) isothermal curing curves were also shown to be capable of being superimposed to one another by the principle of time-temperature superposition. The resultant shift factors at(T) and aη(T) were shown to follow Arrhenius type relationships. Values of the activation energy suggested that the reaction kinetics, instead of the diffusion mechanism, was the limiting step in the overall resin advancement for cure at temperatures ≥ 385 K.
The stress-relaxation modulus of an unfilled ethylene vinyl acetate polymer at three different degrees of crosslinking was measured at 15 temperatures over a temperature span of about 160 C. At each temperature, the time response was measured for at least three decades of time. From these data it was possible to construct a master curve for each degree of crosslinking. The time-temperature shift factors, alpha(T), were found to be related to temperature by the relation log alpha(T) = -A(T - 273), where the parameter A has an average value of 0.234 for the three materials.
To aid the development of a characterization methodology for future polymers as tough composite matrix materials with at least known and hopefully controlled crystallinity, a model composite system based upon poly(ethylene terephthalate) (PET) has been studied. Since PET, a semicrystalline polyester, is one of the best characterized polymers, use can be made of the many crystallinity studies which have been made in the past as an aid in the subsequent analysis. The present report is concerned with a determination of whether or not an estimation of crystallinity may be obtained by study of the wide angle x-ray scattering (WAXS) of PET in the presence of the carbonaceous fibrous reinforcement, with the simulation made reasonable by keeping the resin content and the crystallinity low.
The glass transition temperatures of polystyrene, poly(methyl methacrylate), and copolymers prepared from their respective monomers were determined by using a volume dilatometer. The glass transition temperatures of polystyrene and poly(methyl methacrylate) were found to be 82 and 104°C., respectively. The glass transition temperatures obtained for the copolymers were between the values determined for each of the homopolymers and were a monotonic function of the polymer composition. The experimental values agree with predictions in the literature based on interpolation formulae. The effects of changes in composition and radiation-induced crosslinking on the glass transition of the copolymer system are described.
The effect of 60Co γ-radiation on the thermal conductivity of polypropylene (PP) has been studied over the temperature range 0–160°C. for radiation doses of 600 and 1800 Mrad. The conductivity of unirradiated specimens rises from 4.5 × 10−4 cgs units (cal./cm.-sec.-°C.) at 0°C. to 4.8 × 10−4cgs units at 80°C. and subsequently decreases with temperature to a value of about 3.1 × 10−4cgs units at 160°C. Upon irradiation to 600 Mrad the thermal conductivity is lowered over the 0–150°C. temperature range. Above 90°C. the conductivity decreases with temperature and becomes relatively constant at 3.4 × 10−4 cgs units from 120 to 160°C. Differential scanning calorimeter (DCS) measurements from 30 to 200°C. show that irradiation to 600 Mrad lowers the energy associated with crystalline melting and shifts the endotherm melting peak from about 160 to 105°C. Irradiation to 1800 Mrad results in additional lowering of the thermal conductivity over the 50–160°C. range, a further decrease in area of the endothermic peak and a shift of its maximum peak position to about 75°C. The effects of radiation on the thermal conductivity of polypropylene are compared and correlated with the observed effects of radiation on the dynamic mechanical behavior.
Due to the strict regulations on the usage of heavy metals as the additives in the coating industries, the search for effective organic corrosion inhibitors in replace of those metal additives has become essential. Electrically conducting polymers have been shown to be effective for corrosion prevention but the poor solubility of these intractable polymers has been a problem. We have explored a polyaniline/4-dodecylphenol complex (PANi/DDPh) to improve the dissolution and it has been shown to be an effective organic corrosion inhibitor. With the surfactant, DDPh, PANi could be diluted into the coatings and the properties of the coatings were affected. Emeraldine base (EB) form of PANi was also found to be oxidized by the hardener. The oxidized form of polyaniline provides improved corrosion protection of metals than that of emeraldine base since the value of the standard electrode potential for the oxidized form of PANi is higher than that of EB. Additionally, the surfactant improves the wet adhesion property between the coating and the metal surface.
A study was made of the behavior of radicals generated by Co-60 gamma radiation in the epoxy system tetraglycidyl-4,4'-diaminodiphenyl methane (TGDDM) cured with 4,4'-diaminodiphenyl sulfone (DDS). The molar ratio of TGDDM to DDS was varied in the epoxy samples, and they were prepared under the same curing conditions to obtain various extents of crosslinking. ESR spectrometry data suggest that the rate of decay of radicals is related to inhomogeneities in the resin, with radicals in the highly crosslinked regions having long decay times. The inhomogeneities are thought to be due to statistical variation associated with the complex crosslinking reactions or to difficulties in mixing the reactants.
The water and salt transport properties of ionizing radiation crosslinked poly(vinyl alcohol) (PVA) membranes were investigated. The studied membranes showed high permeabilities and low selectivities for both water and salt. The results were found to be in accord with a modified solution-diffusion model for transport across the membranes, in which pressure-dependent permeability coefficients are employed.
A spontaneous luminescence is reported when epoxy resin samples are heated in air. This phenomenon is very sensitive to the nature of the atmosphere. The same treatment in nitrogen leads to an extinction of the luminescence. The emission process is restored when samples are kept for a sufficient time in air. In order to better understand this phenomenon, we have investigated the luminescence of the elementary constituents of the epoxy (resin and hardener) when heated in air and nitrogen, as well as during resin curing in the same atmospheres. It appears that the emission process is linked with the presence of oxygen. Although the kinetics of the luminescence can differ depending on the nature of the sample (cured resin, resin during curing, liquid components), the emission spectra are the same during resin curing and upon heating of the cured resin and hardener. The emission spectrum of the base resin is different. It is concluded that the light results from a chemiluminescence process during oxidation. Comment: p. 18
The thermal degradation of several polyimidazopyrrolone (pyrrone) films was studied in air and in vacuum over the range of 100–1000°C. by thermogravimetric analysis (TGA), with the use of both isothermal heating and programmed heating rates of 2, 3, 5, and 7.5°C./min. At pressures of 10−6 torr or less, maximum weight losses average 30% at 800°C. Rates of volatilization and activation energies were derived to provide comparison between these ladder-type polymers. Mass spectrometric analysis of the pyrolysis gases evolved under vacuum conditions showed that CO, CO2, and H2O were the primary volatile products and that they were formed throughout the period of weight loss. Approximate correlation between changes in weight and changes in the total pressure for the vacuum tests indicates that mass spectrometric results could provide quantitative as well as qualitative data. The importance of sample history prior to heating and of sample geometry in developing meaningful and reproducible TGA results is aptly demonstrated. The ability of these materials to absorb readily 5–7 wt.-% of water under ambient conditions and the effect of this property upon weight loss measurements are shown.
The measurement of the ultimate properties of elastomers is characterized by variability in the data. For example, when a sufficient number of specimens is tested, distributions in the values of the stress-at-break, strain-at-break, and time-to-break are commonly obtained. It is pointed out that such variability can be rationalized on the basis of variations in both the degree of crosslinking and in the size of naturally occurring flows present in the elastomer.
A number of studies have been reported in the literature on the polymerization and thermal decomposition of epoxide resins. Lee1 and Anderson2 have both studied the thermal decomposition of epoxy resins, and they concluded that the characteristic exothermic peak (which can occur anywhere between 300° and 400°C) is caused at least partially by some reaction of the epoxide group. We have been investigating the thermal decomposition of an aromatic polyether resin which is produced by curing the diglycidyl ether of bisphenol A (Epon 825) with the catalytic agent trimethoxyboroxine (Fig. 1). DTA studies of the polyether in an inert atmosphere of N2 showed exothermic peaks at approximately 390°, 430°, and 470°C, with the major exotherm being the one at 430°C. Our investigation has shown the important role played by low molecular weight epoxides in these exothermic reactions.
SBR, unfilled and filled with glass beads, MT, and HAF carbon blacks, was tested for tearing energy, rupture values in simple tension, tear diameter, and strain distribution at four rates and seven temperatures. The energy density to failure at the tear zone Wt was obtained from the tearing energy τ and the natural tear diameter d using a modified Rivlin and Thomas relationship Wt ≈ τ/d. This was then compared with the nominal energy density at rupture in simple tension W. It is shown that always Wt > W (sometimes Wt/W = 10), that Wt is subject to a smaller statistical scatter than W, and that Wt is more amenable to the WLF type of superposition than W. It is concluded that Wt and not W is the strength-determining property. Where W data permitted superposition, it is followed the WLF equation. It is presumed that so would Wt. τ, although more superposable than W, showed a bigger shift factor than that dictated by WLF, the difference being the result of the temperature dependence of the diameter. The reinforcing effects of the various fillers are also discussed. It is shown that the carbon black fillers increase both Wt and d. Glass beads have only a small effect on d and none on Wt.
The kinetics of the bulk thermal polymerization of phenyl glycidyl ether induced by trimethoxyboroxine were investigated. Infrared absorption spectroscopy and gel permeation chromatography were used to follow the course of polymerization, while proton and boron-2 NMR spectroscopy were used to support the kinetic model developed. The postulated mechanism involves a fast-initiated, non-stationary cationic polymerization with five elementary steps, including spontaneous and monomer transfer as well as a termination reaction. The trimethoxyboroxine was found to be incorporated into the structure of the cured polymer. Tile polymerization was followed at several temperatures and with several ratios of initial concentrations of trimethoxyboroxine to phenyl glycidyl ether.
A strain energy function of the Valanis-Landel type, W equals w( lambda //1) plus w( lambda //2) plus w( lambda //3), is shown to be applicable to styrene-butadiene rubber (SBR) materials having varying crosslink densities v//e. A previously obtained functional form of the strain energy derivative w prime ( lambda ), normalized by dividing by v//e, is confirmed by one of the validity check plots in which a single curve represents the whole body of large-deformation test results for all degrees of biaxiality and crosslink density.
Rate of molecular bond rupture is successfully correlated by a Griffith-type energy balance to the strain energy release rate during ozone cracking of rubber. Rate of bond rupture is determined from electron paramagnetic resonance (EPR) measurements. The rate of strain energy release is determined from stress–elongation measurements during stress relaxation, creep, and cyclic loading tests. To compare with macroscopic crack studies, it was assumed that each ruptured bond created a given amount of fracture surface. Numerical agreement could be obtained by assuming each broken bond results in the production of an area of approximately 10−13 cm2. Using the surface energy density determined from stress relaxation tests in an energy balance gives surprisingly accurate predictions of expected behavior in creep and cyclic loading tests. There is a one-to-one correspondence between the rate of crack growth (bond rupture) and rate of energy release from strain and external work in all cases. It is proposed that such correlations give credence to a Griffith-type approach to environmental cracking which it did not have previously.
Discussion of a problem encountered in the processing of viscoelastic materials that is caused by the presence of a hydrodynamic instability in the extrusion of polymer melts. The importance of the so-called Weissenberg number in determining the onset of the melt fracture is examined using classical linearized hydrodynamic stability analysis. It is shown that the simple shearing flow of a viscoelastic fluid becomes unstable at a critical value of the Weissenberg number. Implications for the extrusion processing of polymers are reviewed.
BDSDA/APB, a novel linear polyethersulfideimide, was synthesized using siloxane units as flexible linkages in the backbone in an attempt to improve use properties and processability. The effect of these flexible linkages on molecular weight buildup, flexural strength and modulus, glass transition temperature, and melt-flow properties was determined.
Our previously reported one-step method of simultaneously polymerizing and molding pyrrone monomers to produce high-strength, high-modulus, dense, thermally stable moldings has been studied in more detail and improved by using crystalline 1 : 1 salts of the monomers. Interrelationships between molding powder characteristics and molding conditions required to achieve optimum polymer properties were investigated. Typical properties of the moldings obtained from the salt of pyromellitic anhydride and 3,3′-diaminobenzidine were: density, 1.29 g/cc; diametral tensile strength, 10,700 psi; ultrasonic modulus, 1.3 × 106 psi; hardness, 80 Rockwell B; and thermal stability in air (at 0.5°C/min), 400°C. Quick TGA methods, applied to the salts for estimating the thermal stability of molded polymers, before actual polymerization and molding, have been developed. No problems associated with gas release are observed when molding conditions have been optimized.
Twelve polyimides which differ systematically in chemical structure were investigated in nitrogen through the temperature range −190° rlhar2; 500°C by torsional braid analysis. The degradative regions were also examined in nitrogen by thermogravimetric analysis. Relationships between chemical structure/thermal history, processibility, thermomechanical behavior, and weight loss are discussed. A logical thermal program for converting the precursor polyamic acid solutions to solid polyimides was developed. High-temperature, thermally induced chemical reactions could be regulated so as to preferentially freeze out longer-range relaxations and extend the glass state behavior to well above its original load-limiting Tg. Materials made from more flexible molecules had lower glass transitions, softened more through the Tg, and had simpler damping spectra and lower thermal stability than materials made from more rigid molecules. A commercially available polyimide film and polyimide-forming varnish of undisclosed structures were examined by torsional pendulum and torsional braid analyses, respectively. The thermomechanical spectra of the film and cured varnish were similar to the spectra of one structural type of polyimide.