Microscopic flow and failure processes in polymer glasses

The microscopic flow and failure processes, the structural parameters controlling these processes, and how such processes are modified by the service environment are presented for three classes of polymer glasses: polycarbonate, polyimides, and epoxies. The microscopic flow characteristics of polycarbonate are controlled by (a) the ease of shear band deformation which depends on free-volume and previous thermal history and (b) surface crazes whose characteristics depend on exposure to organic compounds, thermal history, and to surface crystallization and fabrication stresses. Polyimides deform and fail in the bulk by crazing with extensive fibrillation. The growth and development of shear bands from microvoids in polyimides are discussed. Epoxies predominantly deform and fail by crazing with regions of high crosslink density remaining intact during the flow processes. Craze cavitation and growth are enhanced in epoxies by the presence of absorbed moisture.     

The short and long term performance of polymers in different environments

The behaviour of polymeric solids in contact with different environments is considered. A thermodynamic viewpoint is adopted and worked out in terms of chemical reactions, physical processes and the interaction between the two. It is shown that chemical attack alone does not normally cause serious loss of physical properties, but that a combination of chemical and physical (or physico-chemical) interactions between polymer and environment can lead to rapid deterioration. An understanding of these processes, and their synergistic effects, is essential in designing against environmental failure.     

Corrosion of Phosphorus Oxynitride Glasses in Water and Humid Air

Phosphorus oxynitride (PON) glasses, prepared by remelting phosphate glasses in anhydrous ammonia vapor, are more resistant to dissolution in water and corrosion in humid environments than the parent phosphate glasses. The chemical durability of PON glasses is directly related to their nitrogen content. Solution analyses, pH stat titrations, and hydrogen depth profiles suggest that nitrogen improves the durability of phosphate glasses by cross-linking the polymeric phosphate chains in the network structure.     

Estimating service lifetimes in weathering: an optimistic view

Directly correlating lifetime to coating composition by using standardized, artificial exposures, or even natural exposure, is often very difficult. However, significant progress can be made by breaking down the problem into smaller questions, which can be separately addressed. If one understands the physical parameters that affect end-use properties, then one can also group, and thus correlate, properties according to whether they depend on processes at the surface or in the bulk of a coating, or whether they depend on defects. A scheme is presented that shows how one can use knowledge from analytical physical or chemical materials science in a statistical model related to the “chemical paradigm.” Simple physical models that use this information, about the initial state of the coating and its rate of degradation, can be used to compare the performance of coatings and estimate, simply, service lifetime depending on the property of interest, in the environment of interest. One can see that different properties are sensitive in different ways to the degradation process and decay with a different rate. Thus, although properties may be determined by the same degradation process, and location within a coating, they do not correlate directly. These approaches show how to organize our knowledge of degradation processes, and environments, and be able to make some testable predictions on how coating properties deteriorate. Presented at the 2006 FutureCoat! conference, sponsored by the Federation of Societies for Coatings Technology, in New Orleans, LA, on November 1–3, 2006     

Structural transformations in thermally modified porous glasses

The changes in the structural parameters of porous glasses (pore radius, pore volume, specific surface of pores, structural resistance coefficients) upon heating are investigated as a function of the composition of initial two-phase alkali borosilicate glasses with the use of a number of independent methods, such as the adsorption techniques (water vapor adsorption, mercury porosimetry, thermal desorption of nitrogen), transmission electron microscopy, small-angle X-ray scattering, and membrane conductivity measurements. It is demonstrated that the structural transformations in thermally modified porous glasses are associated with the processes of overcondensation of pores and viscous flow in the silica network.     

Texture and micro-nanostructure of porous silicon oxycarbide glasses prepared from hybrid materials aged in different solvents

The influence of the aging conditions of the preceramic hybrid material on the microstructure of silicon oxycarbide (SiOC) glasses derived therefrom has been highlighted. The textural and structural properties of the glasses are modified by aging the hybrid precursor in different environments. Three solvents have been employed as aging media to produce macroporous SiOC ceramics with porosities in the range between 30 and 70 vol.%.It has been concluded that the polarity and chemical characteristics of the solvent plays an important role on the surface characteristics and structure of the obtained SiOC glass. Raman spectroscopy and Small Angle X-ray scattering reveal the presence of different nanodomain sizes depending on the polymeric fraction in the preceramic network. The free carbon phase developed during the hybrid-to-ceramic conversion turn out to have a high influence on the growth of the silica nanodomains and thus on the nanostructure of the obtained ceramic.     

Degradation kinetics applied to lifetime predictions of polymers

The prediction of the lifetime of a polymer by extrapolation from weight-loss data requires a thorough analysis of degradation kinetics over a wide temperature range. Techniques are described in which entire kinetic spectra are compared among experiments performed at heating rates from 6 deg/min to 9 deg/day. These comparisons permit diagnosing shifts in reaction mechanism, uncoupling of competing processes and more reliably predicting the rate-limiting process at service conditions. These techniques and a method for obtaining initial X kinetic parameters are illustrated for several polymers.     

An Overview of the Structure and Properties of Silicon-Based Oxynitride Glasses

The silicon oxynitride glasses take advantage of nitrogen bonding to attain high elastic modulus, increased softening temperatures and viscosities, greater slow crack growth resistance, and modest gains in fracture resistance. Of the oxynitride glasses, the Si–Y–Al-based oxynitride glasses have been most extensively studied and a degree of success has been achieved in understanding how changes in glass composition affect structural parameters and their relationship with properties. More recent studies have focused on the Si–RE–Me oxynitride glasses, where Me is primarily Al or Mg and rare earth (RE) includes most of the lanthanide series elements. These glasses possess a range of elastic, thermal, mechanical, and optical properties, which can be correlated with the strength of the RE bond in terms of the cationic field strength. However, such correlations require knowledge of not only the RE valence state but also its coordination with the anions. Herein, the current state-of-the-art understanding of the properties and structural parameters of oxynitride glasses and their interrelationships are reviewed.     

Mode I fracture toughness of adhesively bonded joints as a function of temperature: Experimental and numerical study

Adhesives used in structural high temperature aerospace applications must operate in extreme environments. They need to exhibit high-temperature capabilities in order to maintain their mechanical properties and their structural integrity at the intended service temperature. One of the main problems caused by high temperature conditions is the fact that the adhesives have different mechanical properties with temperature. As is known, adhesive strength generally shows temperature dependence. Similarly, the fracture toughness is expected to show temperature dependence.In this work, the Double Cantilever Beam (DCB) test is analysed in order to evaluate the effect of the temperature on the adhesive mode I fracture toughness of a high temperature epoxy adhesive. Cohesive zone models, in which the failure behaviour is expressed by a bilinear traction–separation law, have been used to define the adhesive behaviour and to predict the adhesive P–δ curves as a function of temperature. The simulation response for various temperatures matched the experimental results very well. The sensitivity of the various cohesive zone parameters in predicting the overall mechanical response as a function of temperature was examined as well for a deeper understanding of this predictive method. Also, issues of mesh sensitivity were explored to ensure that the results obtained were mesh independent.     

γ‐Irradiation effects on the thermal and structural characteristics of modified,grafted polypropylene

The effects of both the degree of grafting and γ irradiation on the thermal stability and structural characteristic of polypropylene‐graft‐polyvinylpyrrolidone and polypropylene‐graft‐polyvinylpyrrolidone modified with α‐cyano‐δ‐(2‐thienyl) crotononitrile were investigated. The employed techniques were thermogravimetric analysis, differential thermogravimetry, and X‐ray diffraction. The thermal stability of various polymeric substrates was investigated through the determination of the degradation temperature and activation energy of degradation. The effects of different parameters on the structural characteristics of different films were investigated through the determination of possible changes in the degree of ordering of the polymeric substrates. The results revealed that the thermal stability of the trunk polymer, grafted polymer, and polymer modified by α‐cyano‐δ‐(2‐thienyl) crotononitrile increased progressively with an increasing degree of grafting. The increase was, however, more pronounced for the sample undergoing the lowest degree of grafting. The activation energy of the thermal degradation process remained almost unchanged, and this indicated that the degradation processes of the different films followed almost the same mechanism. The γ irradiation at a dose of 60 kGy of the sulfur‐treated polymeric films [i.e., the polymeric films treated with α‐cyano‐δ‐(2‐thienyl) crotononitrile] reduced their thermal stability. This conclusion was reached by the consideration of the changes observed in the pre‐exponential factor of the Arrhenius equation due to different chemical and γ‐irradiation treatments. The degree of ordering, evidenced by X‐ray diffraction measurements of the trunk polymer, grafted polymer, and modified polymer, suffered a significant drop. This drop was much more pronounced for the sulfur‐containing polymeric materials. The observed drop in the degree of ordering of the polymeric substrates was taken as a measure of the structure collapse due to a certain treatment (degree of grafting and sulfur inclusion). The γ irradiation of the sulfur‐containing polymeric materials greatly increased their degree of ordering, which reached a value greater than that measured for the trunk polymer. Therefore, it was concluded that the thermal stability increased as the degree of ordering decreased. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 506–515, 2006     

Properties of Phosphorus‐Doped Silicon‐Rich Amorphous Silicon Carbide Film Prepared by a Solution Process

Using a polymeric precursor synthesized from a mixture of cyclopentasilane, white phosphorus, and 1‐hexyne, we deposited phosphorus‐doped silicon‐rich amorphous silicon carbide (a‐SiC) films via a solution‐based process. Unlike conventional polymeric precursors, this polymer requires neither catalysts nor oxidation for its synthesis and cross‐linkage. Therefore, the polymeric precursor is sufficiently pure for effective doping and fabricating semiconducting a‐SiC. This study presents the results of a detailed study of the effect of carbon and phosphorus concentrations on the structural, optical, and electrical properties of a‐SiC films. The lowest activation energy for these films is 0.39 eV, which leads to an optical gap and a dark conductivity of 2.1 eV and 10⁹ Ω cm, respectively. Moreover, these films satisfy the Meyer–Neldel rule for thermally activated conductivity, which indicates that white‐phosphorus doping of solution‐processed a‐SiC produces films with the same characteristics as phosphine‐doped vacuum‐processed a‐SiC.     

Effect of Test Environment on Stress-Corrosion Susceptibility of Glass

The stress-corrosion susceptibility of abraded and acid-polished soda-lime and borosilicate glasses in test environments of 6N NaOH, distilled water, and 6N HCI was measured by dynamic fatigue techniques. Dynamic-fatigue data for these glasses agree well with crack-velocity data for the water and 6N NaOH environments. The lack of agreement in the 6N HCI environment suggests that the failure mechanism for glass in HCI is not simply crack propagation by stress corrosion. The agreement in failure predictions based on strength and crack-velocity data in 6N NaOH and water suggests that either set of data may be used for effective design calculations; however, caution should be used when basing strength calculations on crack velocity data in 6N HCI.     

Failure Probability at the Predicted Minimum Lifetime After Proof Testing

Experimental variation in crack growth parameters introduces an uncertainty in the predicted minimum lifetime after proof testing that results in a possibility of failure at times less than the "minimum" lifetime. This failure probability is estimated with a Monte carlo simulation technique. A A example is given of how this failure probability can be made vanishingly small by increasing the proof stress or by decreasing the allowable stress or permissible lifetime in service.     

Determination of the fracture energy in polymeric films by in situ photoelasticimetry on double edge notch specimen

The fracture toughness is key parameters to select polymeric films. The essential work of fracture (EWF) is a phenomenological but efficient way to characterize this resistance to fracture. One can gain valuable information on the resistance to perforation and propagation of flaws. A new technique was developed to better understanding the EWF experiments. A tensile test combined to photoelasticimetry allows following in situ the geometry and amount of plastic deformation on double edge notched specimen. The EWF parameters are determined when the plastic deformation appears constant, so when the fracture energy W_f only contributes to rupture filament. This new methodology requires just a single sample, whereas at least five specimens are required for general method. It will help characterize expensive polymeric films or reveal the heterogeneous behavior, for instance after polymer ageing. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 132, 42854.     

Surface structures of sodium borosilicate glasses from molecular dynamics simulations

Surface plays an important role in the physical and chemical properties of oxide glasses and controls the interactions of these glasses with the environment, thus dominating properties such as the chemical durability and bioactivity. The surface atomic structures of a series of sodium borosilicate glasses were studied using classical molecular dynamics simulations with recently developed compositional dependent partial charge potentials. The surface structural features and defect speciation were characterized and compared with the bulk glasses with the same composition. Our simulation results show that the borosilicate glass surfaces have significantly different chemical compositions and structures as compared to the bulk. The glass surfaces are found to be sodium enriched and behave like borosilicate glasses with higher R (Na₂O/B₂O₃) values. As a result of this composition and associated structure changes, the amount of fourfold boron decreases at the surface and the network connectivity on the surface decreases. In addition to composition variation and local structure environment change, defects such as two‐membered rings and three‐coordinated silicon were also observed on the surface. These unusual surface composition and structure features are expected to significantly impact the chemical and physical properties and the interactions with the environments of sodium borosilicate glasses.     

Ultraviolet‐induced aging of flexographic printing plates studied by thermal and structural analysis methods

The properties of photopolymer printing plates subjected to artificial aging processes were investigated with differential scanning calorimetry and microscopic analysis. The results showed that exposure to extended ultraviolet radiation type C in the prepress process caused changes both in the thermal properties and in the structure of the outermost surface layer. Long exposure to ultraviolet radiation type A in a weathering tester led to structural changes at a deeper level and a simultaneous increase in the glass‐transition temperatures of the polymeric material. Spectroscopic analysis showed that extensive oxidation occurred in the outermost surface layer. This study provides insight into important aging processes of the photopolymer printing plates. The knowledge can be used to predict the lifetime of printing plates and to understand the effects of their aging on print quality. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Specific features of the relaxation processes in As2Se3 films prepared by different methods

The specific features of the processes of charge transfer and accumulation in thin films of arsenic triselenide As₂Se₃ are investigated by measuring the isothermal relaxation of dark electric currents. It is established that the relaxation properties of the films substantially depend on the method used for their preparation, which, apparently, can be associated with the structural features of this class of materials. Moreover, it is confirmed that the relaxation of the electric current in the films of chalcogenide glasses under investigation occurs through a non-Debye mechanism, irrespective of the preparation technique.     

Mechanical strength and wear resistance of protective coatings applied by fluidized bed (FB)

This study deals with the interrelation between the thermo-rheological behaviour of an epoxy-based powder coating system and its mechanical strength and wear endurance.

Matte-finish protective polymeric films deposited by electrostatic fluidized bed (EFB) and conventional hot dipping fluidized bed (CHDFB) on metal substrates were examined. First, the analysis of thermo-rheological behaviour of the epoxy-based powder coating system was detailed. Secondly, the adhesion strength and wear endurance of polymeric films was related to the thermo-rheological behaviour of the starting material formulation. Finally, based on the experimental data, generalized scratch and wear map, in which the overall mechanical performance of the matte-finish polymeric films at different curing levels is reported, was usefully provided.

The experimental findings lead to further advances in the understanding of the mechanisms involved in the establishment of the overall mechanical performances of fluidized bed (FB) deposited polymeric films. They also provide important indications for the settings of curing parameters or preheating temperatures in FB coating processes as well as for the development of new powder coating formulations.     

Degradation of Stressed Optical Fibers in Water: New Worst-case Lifetime Estimation Model

Simultaneously determined results from zero-stress aging and static fatigue measurements in water at 65°, 80°, and 95°C have been combined into a worst-case fiber lifetime model which shows good agreement between predicted and observed failure times. The model shows that the lifetime in a wet environment of the largest crack at normal service strain (0.1%) is nearly identical to the lifetime of the pristine fiber and that the initial strength and the stress corrosion exponent are of minor importance for the lifetime. In a separate experiment, it is shown that degradation in a water-saturated, jelly-filled cable is significantly slower than when fibers are directly immersed in water. For a fast-degrading fiber, evidence of a strength-increasing mechanism is presented.     

Optimization of fused filament fabrication process parameters for mechanical responses of weather-resistant polymer (acrylonitrile styrene acrylate)

Additively manufactured polymeric products for automotive, aerospace, and biomedical applications are usually intended for service in an outdoor environment with high mechanical loading conditions. The strength and sustainability of the products can be significantly degraded due to the outdoor environmental conditions such as UV light, moisture, heat, and so forth. In this research work, a novel weather-resistant polymer (WRP) material, that is, acrylonitrile styrene acrylate (ASA), has been studied. Furthermore, this work aims to study the effect of process parameters and enhance the strength of WRP (ASA) specimens using the FFF process. The optimized process parameters, that is, build orientation (BO), extrusion temperature (ET), layer thickness (LT), and printing speed (PS), were identified based on the tensile and flexural strength using the Taguchi technique and statistical analysis. The best tensile and flexural strengths for the specimen were achieved at both orientations (XYZ and ZXY) TS: 255°C ET, 0.14 mm LT, 50 mm/s PS; and FS: 245°C ET, 0.28 mm LT, 50 mm/s PS, respectively. Regression model was developed to investigate the correlation between the process parameters with tensile and flexural strength. A validation test confirmed the findings, and the error between the actual and predicted values is less than ±10%.