Effect of thermal history on the deformation and failure of polyimides (original) (raw)
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Effects of thermal cycling on phenylethynyl-terminated PMDA-type asymmetric polyimide composites
High Performance Polymers, 2018
The effects of thermal cycling on a polymerized monomeric reactant (PMR) type polyimide (TriA X) reinforced with carbon fibers were investigated. Composite specimens were subjected to 2000 thermal cycles between −54°C and 232°C. At 400-cycle intervals, laminates were inspected for microcracks, and glass transition temperature ( T g) and short-beam shear (SBS) strength were measured. The composites did not exhibit microcracks after thermal cycling, although after 2000 thermal cycles, mechanical properties of the matrix declined slightly. The matrix degradation decreased the resistance to microcracking upon further loading. No effects of thermal oxidative aging were observed from thermal cycling, and thermally driven fatigue and creep were identified as the primary and secondary factors inducing mechanical degradation of the matrix. T g of the composites exhibited no change after 2000 cycles, while the SBS strength decreased slightly (3–9%). The results highlight the potential for use...
Effect of Poly(amic diethyl ester) Precursor on Residual Stress Behavior of Aromatic Polyimides
Polymer Journal, 1999
The influence ofpoly(amic diethyl ester) (PAE) and poly(amic acid) (PAA) precursors on the residual stress behavior of polyimide thin films was investigated. Rodlike poly(p-phenylcne pymcllitimide) (PMDA-PDA). pseudo-rodlike poly(p-phcnylene biphenyltetracarboximide) (BPDA-PDA). semiflexible poly(4,4-oxydiphenylene pyromellitimide). and poly(4,4-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA) were used. For polyimides prepared from PAE precursor, residual stress of fully cured films at 400' C were 13.4 MPa for the PMDA-PDA. 19.5 MPa for BPDA-PDA, 30.3 MPa for PMDA-ODA. and 39.5 MPa for BPDA-ODA. For polyimidcs prepared from PAA precursor. residual stress after fully cured at 400 C was-5.1 MPa for the PMDA-PDA, 7.0 MPa for BPDA-PDA, 29.4 MPa for PMDA-ODA, and 40.1 MPa for BPDA-ODA. The effect of different size of precursor on the residual stress behavior was significantly high for rodlike polyimidcs having high chain rigidity and a high intermolecular packing order, but relatively low for the semi-flexible polyimidcs with relatively low chain rigidity. The degree of molecular order for the rigid structure polyimides. PMDA-PDA and BPDA-PDA. was hindered by the bulky ethyl ester group evaporated during thermal imidization. However. the effect of the bulky ethyl ester group was not significant for the flexible structure polyimides, PMDA-ODA and BPDA-ODA. Residual stress was in decreasing order BPDA-ODA < PMDA-ODA < BPDA-PDA < PMDA-PDA polyimidc regardless of the precursor origins. These results indicate that the residual stress behavior in the polyimides depending on precursor origin is significantly related to the morphological structure as well as the chain mobility of the polyimide during thermal imidization.
High Performance Polymers, 2018
The effects of moisture on a polymerized monomeric reactant (PMR)-type polyimide (TriA X) and associated composites were investigated. Water uptake tests were performed on the polyimide at various temperatures and relative humidity levels to investigate moisture absorption behavior. Two-stage moisture absorption was observed, in which the first stage was diffusion controlled, whereas the second stage was moisture plasticization controlled. As exposure temperature increased, the equilibrium moisture content of the polyimide decreased, indicating an exothermic absorption process. The Arrhenius temperature dependence and moisture saturation as functions of temperature and humidity in the neat polymer were determined using curve fitting based on the published mathematical models. Long-term hydrothermal aging at 95 C was conducted on the neat polyimide and associated carbon fiber composites. Reversible hydrolytic reactions and a trace of irreversible hydrolysis were observed in the long-term exposure. The tensile ductility of the neat polyimide and the short-beam shear strength of the composites decreased with increasing aging time, while the tensile strength and modulus and thermal properties of the polyimide exhibited little change after 2000-h aging, demonstrating hydrothermal stability. The decrease in the ductility of the neat polymer after long-term moisture exposure was attributed to the network structure change, driven by hydrolysis and moisture plasticization.
Effects of Thickness on the Residual Stress Behavior of High Temperature Polyimide Films
Polymer Journal, 2000
The effects of thickness on the residual stress behavior of the hinged structure poly(4,4oxydiphenylene pyromellitimide) (PMDA-ODA) and rigid planar structure poly (p-phenylene biphenyltetracarboximide) (BPDA-PDA) polyimide were studied. Depending on the polyimide chemistry, residual stress behavior of polyimide film with thickness variation was significantly different. For fully cured BPDA-PDA polyimide at 400°C, residual stress of polyimide film increased from 5.4 MPa to 45 MPa. For fully cured PMDA-ODA polyimide at 400°C, residual stress of polyimide film with thickness variation changed from 29.1 MPa to 40.2 MPa. The effect of thickness on the residual stress for the rigid structure BPDA-PDA was significant, but relatively not for the semiflexible structure PMDA-ODA. Residual stress behavior was quite related to the morphology of fully cured polyimide as shown W AXD analysis. Higher molecular anisotropy leads to lower residual stress of polyimide film. Residual stress behavior of polyimide films with thickness variation is thus closely related to chain rigidity and chain anisotropy associated with polyimide chemistry and morphological structure.
Improvement of Polyimide Electrical Properties During Short-Term of Thermal Aging
2008 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2008
An atypical enhancement of both the dielectric properties and the static (dc) conductivity in polyimide (PI) films has been observed using isothermal dielectric relaxation spectroscopy at 300°C under air atmosphere during short-term thermal aging (up to 200h). Simultaneously, an increase of the dielectric strength of PI is observed. Despite a close similarity with the typical electrical signature of the crystallization phenomenon occurring in semi-crystalline polymers, DSC measurements have not revealed the presence of an exothermic crystallization peak in PI up to 500°C thus denying this assumption. In this paper, a cross-linking reaction under oxidant atmosphere is therefore proposed for explaining such electrical improvements. Indeed, oxygen diffusion in the polymers' bulk is generally followed by the formation of links between macromolecular chains leading the molecular and charge motions more difficult. This phenomenon, inducing usually an enhancement of the electrical and mechanical properties, is also supported by FTIR chemical changes of PI during aging and theoretical cross-linking mechanisms in presence of oxygen.
Temperature dependence of the properties of vapor-deposited polyimide
Journal of Applied Physics, 2003
The Young's modulus and helium gas permeability of vapor-deposited poly͑4,4Ј-oxy-diphenylenepyromellitimide͒ were measured at cryogenic and elevated temperatures ͑10-573 K͒. The Young's modulus decreased with increasing temperature from 5.5 GPa at 10 K to 1.8 GPa at 573 K. The temperature dependency of the permeability followed the Arrhenius' relationship, with different activation energy for permeation for samples imidized under different conditions. The effect of the imidization conditions on the permeation properties could be explained in terms of morphology/crystallinity as determined by x-ray diffraction techniques. Imidizing in air instead of nitrogen increased the permeability while lowering the activation energy for permeation and crystallinity. Imidizing at higher heating rates ͑in nitrogen͒ resulted in higher permeability, lower activation energy for permeation, and larger and fewer crystallites with better-aligned lattice planes.
Effect of zone annealing on LARC-CPI thermoplastic polyimide
Journal of Applied Polymer Science, 1994
Mechanical properties and structure were studied for undrawn and zone-drawn films of LARC-CPI thermoplastic polyimide. The dynamic modulus of undrawn glassy material ranged from about 1.8 to 4.3 GPa, depending upon the degree of crystallinity. After zone drawing to draw ratio of 3.6–4.0, the dynamic glassy modulus was increased to a maximum of 9.5 GPa. The highest moduli were attained in samples that were multiply zone-drawn. The maximum-achievable draw ratio increased with the maximum drawing temperature, but the semicrystalline nature of the starting material limited the ultimate drawability. For the first time, highly oriented crystalline films were obtained for X-ray diffraction and preliminary crystal structure analysis. The crystal lattice was fit to the orthorhombic crystal system, and the results indicate that the lattice parameters are a = 8.0 ± 0.2 Å, b = 5.9 ± 0.2 Å, and c = 36.5 ± 0.3 Å. The value of the c-axis lattice parameter is very close to the fully extended chain length of the monomer repeat unit. © 1994 John Wiley & Sons, Inc.
2000
The effect of molecular weight on the viscoelastic performance of an advanced polymer (LaRC TM-SI) was investigated through the use of creep compliance tests. Testing consisted of shortterm isothermal creep and recovery with the creep segments performed under constant load. The tests were conducted at three temperatures below the glass transition temperature of five materials of different molecular weight. Through the use of time-aging-time superposition procedures, the material constants, material master curves and aging-related parameters were evaluated at each temperature for a given molecular weight. The time-temperature superposition technique helped to describe the effect of temperature on the timescale of the viscoelastic response of each molecular weight. It was shown that the low molecular weight materials have higher creep compliance and creep rate, and are more sensitive to temperature than the high molecular weight materials. Furthermore, a critical molecular weight transition was observed to occur at a weight-average molecular weight of M w ∼ 25,000 g/mol below which, the temperature sensitivity of the time-temperature superposition shift factor increases significantly. The short-term creep compliance data were used in association with Struik's effective time theory to predict the long-term creep compliance behavior for the different molecular weights. At long timescales, physical aging serves to significantly decrease the creep compliance and creep rate of all the materials tested. Long-term test data verified the predictive creep behavior. Materials with higher temperature and lower molecular weights had greater creep compliance and higher creep rates.
2015
In this research we describe the preparation of polyimide with pyromellitic dianhydride (PMDA) and p-phenylene diamine (PDA) thin films by physical vapor deposition. For this study, FTIR Spectrometer has been used to measure the effect of imidization temperature on the chemical structure of vapor-deposited thin films of the aromatic PI. When temperature increases, a general increase in all the absorption peaks is observed. This suggests that residual PAA monomers continue to be converted into PI. The surface topology of the PI films was further examined by using AFM atomic force microscopy as a function of the imidization temperature at 150,200,250oC for 1 hour each, it can be clearly seen that the surface became rougher with increasing imidization temperature. The thermal stability of polyimide was also improved by using Thermo gravimetric analysis (TGA).
Thermal properties of bulk polyimides: insights from computer modeling versus experiment
Soft Matter, 2014
Due to the great importance for many industrial applications it is crucial from the point of view of theoretical description to reproduce thermal properties of thermoplastic polyimides as accurate as possible in order to establish "chemical structure-physical properties" relationships of new materials. In this paper we employ differential scanning calorimetry, dilatometry, and atomistic molecular dynamics (MD) simulations to explore whether the state-of-the-art computer modeling can serve as a precise tool for probing thermal properties of polyimides with highly polar groups. For this purpose the polyimide R-BAPS based on dianhydride 1,3-bis(3 0 ,4-dicarboxyphenoxy)benzene (dianhydride R) and diamine 4,4 0 -bis(4 00aminophenoxy)biphenyl sulphone) (diamine BAPS) was synthesized and extensively studied. Overall, our findings show that the widely used glass-transition temperature T g evaluated from MD simulations should be employed with great caution for verification of the polyimide computational models against experimental data: in addition to the well-known impact of the cooling rate on the glass-transition temperature, correct definition of T g requires cooling that starts from very high temperatures (no less than 800 K for considered polyimides) and accurate evaluation of the appropriate cooling rate, otherwise the errors in the measured values of T g become undefined. In contrast to the glass-transition temperature, the volumetric coefficient of thermal expansion (CTE) does not depend on the cooling rate in the low-temperature domain (T < T g ) so that comparison of computational and experimental values of CTE provides a much safer way for proper validation of the theoretical model when electrostatic interactions are taken into account explicitly. Remarkably, this conclusion is most likely of generic nature: we show that it also holds for the commercial polyimide EXTEM™, another polyimide with a similar chemical structure.