Contributions of the nanovoid structure to the kinetics of moisture transport in epoxy resins

Absorbed moisture can degrade the physical properties of an epoxy resin, jeopardizing the performance of an epoxy‐based component. Although specific water–epoxy interactions are known to be very important in determining transport behavior, the role of network topology is not clear. In this article, a series of epoxies in which the topology is systematically varied (and the polarity held constant) is used to explore how topology influences the kinetics of moisture transport. The topology is quantified via the positron annihilation lifetime spectroscopy technique in terms of the size and volume fraction of electron density heterogeneities 5–6 Å in diameter, a dimension comparable to the 3‐Å kinetic diameter of a water molecule. Surprisingly, the volume fraction of such nanopores does not affect the diffusion coefficient (D) of water in any of the resins studied. For temperatures at and below 35 °C, there is a mild exponential dependence of D on the average nanopore size observed. Otherwise, the kinetics of moisture transport do not appear to depend on the nanopores. However, the initial flux of moisture into the epoxy does appear to correlate with the intrinsic hole volume fraction. That this correlation persists only in the initial stages of absorption is partially understood in terms of the ability of the water to alter the nanopore structure; only in the initial stages of uptake are the nanopores, as quantified in the dry state, relevant to transport. The role of specific epoxy–water interactions are also discussed in terms of transport kinetics. The lack of a correlation between the topology and transport suggests that polar interactions, and not topology, provide the rate‐limiting step of transport. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 776–791, 2000     

Free volume and water uptake in a copolymer hydrogel series

A copolymer of 2-hydroxyethyl methacrylate (HEMA) with 2-ethoxy ethyl methacrylate (EEMA) was synthesized and the molecular mobility, free volume, and density properties examined as a function of composition. These properties were correlated with the equilibrium water uptake in order to determine which of the properties were most influential in causing high water sorption, as these materials are suitable candidates for hydrogel systems. It was found that the polar HEMA repeat unit results in a rigid, glassy sample at room temperature due to the high degree of hydrogen bonding between chains whereas high EEMA content leads to rubbery samples with subambient glass transition temperatures. The free volume properties on the molecular scale measured by positron annihilation lifetime spectroscopy (PALS) showed that higher HEMA content led to smaller, fewer holes and a lower free volume fraction than EEMA. Therefore the high water uptake of HEEMA-containing copolymers is largely related to the high polarity of the HEMA unit compared to EEMA, despite the low content of free volume into which the water can initially diffuse. Trends in density with copolymer composition, as measured on a macroscopic level, differs to that seen by PALS and indicates that the two techniques are measuring different scales of packing. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 463–471, 1998     

Nanovoid relaxation in a series of copolyester glasses under cyclic loading using synchronous PALS

This study examines if correlation between a specific molecular motion and the growth and relaxation of nanovoids, which are precursors to crazes, can be established. A novel technique, positronium annihilation lifetime spectroscopy (PALS) synchronized with cyclic stress, is reported. In this technique the positronium annihilation signal was accumulated in sixteen channels corresponding to sixteen phases of the sinusoidal load, which allowed statistically significant data for nanovoid generation and relaxation due to the fluctuating stress to be accumulated. This technique was applied to a series of copolymers of poly(ethylene terephthalate) (PET) and poly(1,4-cyclohexylenedimethylene terephthalate) (PCT). Previous studies have shown that the cyclohexylene rings in the main chain of PCT actively undergo chair-boat-chair conformational transitions in the glassy PCT at around room temperature. The PET-co-PCT series was also chosen for this study because the crazing stress increases systematically with the cyclohexylene content. The synchronized PALS data are consistent with the hypothesis that transient nanovoids generated by the cyclic stress in polymers containing more cyclohexylene rings relax more readily than those with fewer rings. The results also correlate well with the crazing stress in this series of copolymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1410–1417, 2007     

The moisture absorption characteristics of crosslinked vinyl terminated polyethers compared with epoxies

Processable and practically useful thermosetting resins are almost invariably highly polar. For this and other reasons they can absorb considerable moisture, which affects their physical, electrical and mechanical properties. Crosslinked vinyl terminated oligomeric polyethers (VTPs) of very low polarity are described and their moisture absorption characteristics are compared with those of a standard amine cured Epikote 828® epoxy resin cured with metaphenylene diamine (MPD). The equilibrium moisture absorption of the vinyl terminated polymers is about one-tenth of that of the epoxy system, but their diffusivity is higher. The dependence of the equilibrium uptake on temperature, relative humidity, specimen thickness and oligomer proportions is discussed. The effect of moisture saturation on the d.c. volume resistivity is considered. Moisture prevents the resin from obeying Ohm's law.     

Moisture transport studies on newly developed aromatic and aromatic/aliphatic copolyester thin films

In polymers for microelectronics applications, moisture is known to have a deleterious effect upon device reliability. In this paper, the moisture transport behaviors of a newly developed family of all-aromatic and aromatic/aliphatic copolyester thermosetting films were described. The moisture uptake as a function of temperature, relative humidity, sample thickness, and processing conditions were presented via conjugate moisture sorption tests.¹ It was found that the post curing near but below T_g resulted in an increase in both total moisture uptake and diffusion coefficient due to the effect of physical aging and the generation of sample defect volume. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1025–1035, 1998     

Physical aging of epoxy polymers and their composites

Exposure to extended periods of sub‐T_g temperatures causes physical changes in the molecular structure of epoxy resins and epoxy‐based materials to occur. These physical aging mechanisms include the reduction in free volume and changes to the molecular configuration. As a result, mechanical, thermodynamical, and physical properties are affected in ways that can compromise the reliability of epoxy‐based engineering components and structures. In this review, the physical changes in the molecular structure of epoxies are described, and the influence of these changes on the bulk‐level response is detailed. Specifically, the influence of physical aging on the quasistatic mechanical properties, viscoelasticity, fracture toughness, thermal expansion coefficient, volume relaxation, enthalpy relaxation, endothermic peak temperature, fictive temperature, and moisture/solvent absorption capability is reviewed. Also discussed are relationships between relaxation functions, crosslink density, composite reinforcement, and epoxy/copolymer blending and the physical aging response of epoxies. Finally, the concepts of thermal and mechanical rejuvenation are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Measurement of free-volume hole size distribution in Nafion-117 using positron annihilation spectroscopy

We report a new result of free-volume hole size distribution in water and ethanol-swollen Nafion-117 polymer. With the increase in water content, free-volume hole size decreases, but overall the volume fraction increases. The hole size distribution in dry polymer is seen to be distinctly different from hydrated membranes. The narrow and symmetric distribution in hydrated membrane as compared to dry membrane is believed to be a consequence of crosslinking due to cluster formation. In alcohol-swollen membranes, on the other hand, not only are the free-volume size and fraction seen to be higher, the hole size distribution is seen to be broader compared to dry or hydrated membranes, indicating the effect of penetration of alcohol into the hydrophobic backbone region. We have also examined our results vis-a-vis reported gas diffusion studies in Nafion in the framework of existing free-volume model. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 983–989, 1998     

Connecting free volume with shape memory properties in noncytotoxic gamma‐irradiated polycyclooctene

The free volume holes of a shape memory polymer have been analyzed considering that the empty space between molecules is necessary for the molecular motion, and the shape memory response is based on polymer segments acting as molecular switches through variable flexibility with temperature or other stimuli. Therefore, thermomechanical analysis (TMA) and positron annihilation lifetime spectroscopy (PALS) have been applied to analyze shape recovery and free volume hole sizes in gamma‐irradiated polycyclooctene (PCO) samples, as a noncytotoxic alternative to more conventional PCO crosslinked via peroxide for future applications in medicine. Thus, a first approach relating structure, free volume holes and shape memory properties in gamma‐irradiated PCO is presented. The results suggest that free volume holes caused by gamma irradiation in PCO samples facilitate the recovery process by improving movement of polymer chains and open possibilities for the design and control of the macroscopic response. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1080–1088     

Positron annihilation lifetime measurements of mechanically fatigued polystyrene samples

Early stages of cyclic fatigue‐loaded polystyrene (PS) specimens were investigated by positron annihilation lifetime spectroscopy (PALS) at a maximum stress amplitude of 15 MPa. PALS yields information about the average unoccupied hole volume. A linear increase in the ortho‐positronium (o‐Ps) lifetime was observed in a range from 0 to 50,000 cycles. This increase occurs homogeneously distributed at different positions along a sample of 170 mm. The average unoccupied void volume increases by 1.2%. On the other hand, the o‐Ps intensity shows no systematic change upon cycling. The results suggest a homogeneous and linear increase in free volume prior to craze formation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1991–1995, 2008     

Synthesis,physical characterization,and acetone sorption kinetics in random copolymers of poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalate)

Random copolymers of poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) were synthesized by melt condensation. In a series of thin, solvent cast films of varying PEN content, acetone diffusivity and solubility were determined at 35°C and an acetone pressure of 5.4 cm Hg. The kinetics of acetone sorption in the copolymer films are well described by a Fickian model. Both solubility and diffusivity decrease with increasing PEN content. The acetone diffusion coefficient decreases 93% from PET to PET/85PEN, a copolymer in which 85 weight percent of the dimethyl terephthalate in PET has been replace by dimethyl naphthalate 2,6-dicarboxylate. The acetone solubility coefficient in the amorphous regions of the polymer decreases by approximately a factor of two over the same composition range. The glass/rubber transition temperatures of these materials rise monotonically with increasing PEN content. Copolymers containing 20 to 80 wt % PEN are amorphous. Samples with <20% or >80% PEN contain measurable levels of crystallinity. Estimated fractional free volume in the amorphous regions of these samples is lower in the copolymers than in either of the homopolymers. Relative free volume as probed by positron annihilation lifetime spectroscopy (PALS) decreases systematically with increasing PEN content. Acetone diffusion coefficients correlate well with PALS results. Infrared spectroscopy suggests an increase in the fraction of ethylene glycol units in the trans conformation in the amorphous phase as the concentration of PEN in the copolymer increases. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2981–3000, 1998     

The hydrogen bond and free volume property of poly(ether‐urethane) irradiated by neutron

Poly(ether‐urethane) (PEU) was irradiated by neutron in different atmospheres. The hydrogen bonding interaction was analyzed by Fourier transform infrared (FTIR), and the microstructure of PEU had been investigated by positron annihilation lifetime spectroscopy (PALS). The gas products were detected by gas chromatography after irradiation. The results demonstrated that the irradiation led to more hydrogen bonded carbonyl in PEU, smaller relative free volume fraction, and narrower free volume distribution. It suggested that increasing hydrogen bonds would result in the collapse of free volume. The irradiation induced micro‐phase merging together and the presence of oxygen would accelerate this tendency, which was revealed by PALS. All the results indicated that the chain relaxation led to more hydrogen bonds, and the hydrogen bonding interaction suppressed the free volume. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 381–388, 2010     

尼龙6/碳纳米管纳米复合材料自由体积特性的正电子湮没研究

采用正电子湮没寿命谱技术研究了尼龙6/碳纳米管纳米复合材料的自由体积特性。实验结果发现碳纳米管对纳米复合材料的自由体积孔洞尺寸影响甚微,而自由体积孔洞数目和相对自由体积分数均随碳纳米管含量的增加而明显减小。导致这种减小的原因可能来自两方面,其一是由于碳纳米管和基质聚合物间的相互作用限制了高分子链段运动;其二是碳纳米管填充增强了尼龙6基体结晶性能。此外,力学性能研究表明,碳纳米管在复合材料中较均匀的分散和较好的界面接触可以提高材料的力学强度,而自由体积分数的减小则使材料的韧性变差。     

Intermolecular interactions on amine‐cured epoxy matrices with different crosslink densities. Influence on the hole and specific volumes and the mechanical behavior

The architecture of an epoxy matrix was modified by curing the resin with mono‐/diamine mixtures having identical chemical structures. Both hole volume and specific volume variations were studied by positron annihilation lifetime spectroscopy and pressure‐volume‐temperature/density measurements, respectively. The average hole volume of the networks at room temperature slightly increased when the monoaminic chain extender content increased. The increment in the intermolecular interactions between functional groups of the networks chains, due to the less hindered nitrogen introduced by the monoamine, appears to be the responsible for the observed behavior. Besides, only small variations on the specific volume were observed on increasing the monoamine content, which points out that for a cured epoxy system, the chemical structure of the curing agent is mainly responsible for chain packing in the networks. On the other hand, intermolecular interactions between chains were considered as the key factor for fixing stiffness and strength. Thus, it was observed that the increase of the intermolecular interactions with the monoamine content produced a decrease in the sub‐T_g small‐range cooperative motions, which increased the low‐deformation mechanical properties at temperatures between β and α relaxations. This conclusion could be applied to previous investigations with epoxy matrices not fully crosslinked (nonstoichiometric or noncompletely cured formulations). Finally, it was found that fracture properties do not significantly depend either on the hole volume or on the intermolecular interactions. Fracture properties are more dependent on the crosslink density and the glass transition temperature. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1240–1252, 2009     

A study on free volume of acrylonitrile‐styrene copolymer and methyl methacrylate‐styrene copolymer by using positron annihilation

The o‐Ps lifetime τ₃ and the intensity I₃ of ST‐AN copolymers and ST‐MMA copolymers have been determined by using the positron annihilation technique. The average free volume hole radius R is estimated according to Tao's and Eldrup's model. The result shows that the average free volume hole size mainly attributes to lateral group volume and polarity of macromolecular chain as well as polymerizing temperature, and the o‐Ps intensity I₃ to the effect of the lateral group volume and the polarity. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 465–472, 1999     

Evolution of nanometer voids in polycarbonate under mechanical stress and thermal expansion using positron spectroscopy

Positron Annihilation Lifetime Spectroscopy (PALS) measurements were conducted on polycarbonate subjected to either thermal expansion or to tensile and compressive strains. It was found that thermal expansion affected both the nanometer hole size and the hole number density, whereas mechanical stress affected mainly the size of existing holes, and did not generate or eliminate holes in the quasielastic deformation region. The effect of stress on yield and postyield behavior of this glassy material was also investigated. The deduced hole volume fraction of this polymer at 25°C was 6.8 ± 0.5% from the thermal expansion experiment and 7.2 ± 1.2% from the mechanical loading experiment. When the specimen was under compression, the hole volume fraction was found to continuously decrease. This can be considered as evidence of the inability of the free volume concept in explaining the yield behavior of glassy polymers. ©1995 John Wiley & Sons, Inc.     

Positron annihilation spectroscopy study of moisture absorption in protective epoxy coatings

Position annihilation spectroscopy (PAS) was used to measure the relative free-volume fraction of protective epoxy coatings before and after exposure to liquid water at room temperature. The relative free volume fraction determined before water exposure was compared to the equilibrium water uptake of each coating and a correlation was found. The relative free-volume fraction of the epoxy coatings decreased slightly after water exposure. This decrease is contrary to the free volume theory of plasticization, but is consistent with the antiplasticization process. Larger decreases in the relative free volume fraction were sought by repeating the water uptake experiments with nitrobenzene which in the bulk, liquid form quenches ortho-positronium (o-Ps). Since the o-Ps lifetime remained approximately constant and the o-Ps intensity decreased after nitrobenzene absorption, nitrcbenzene is believed to be inhibiting the formation of o-Ps in the epoxy free volume cavities. Larger decreases in the relative free volume fractions were found after nitrobenzene exposure than after water exposure. These larger decreases are due to the fact that nitrobenzene is a better inhibitor of o-Ps formation than water in the epoxy free volume cavities. Larger volume fractions of nitrobenzene were absorbed by the coatings than water and were interpreted to be due to interactions between nitrobenzene and the epoxies. © 1993 John Wiley & Sons, Inc.     

Monte Carlo simulation of diepoxides and monoepoxides cured with amines

The diepoxide–monoepoxide–diamine curing systems are investigated with a Monte Carlo simulation. The dependence of the molecular weight distribution (MWD), gel fraction, and cycle rank of the polymers on the differences in the epoxy reactivities and the contents of the monoepoxide as a reactive diluent are discussed. Before gelation, the MWD of the curing systems with a lower content of the monoepoxide is broader than the MWD of the curing systems with a higher content, and it leads to a lower critical conversion. The gel fraction and cycle rank of the polymers decrease with an increasing amount of the diluent. Even fully cured, the system with a 0.6 epoxy molar fraction of the monoepoxide still has a large fraction of sol, about 49%. Although the various reactivities of the monoepoxide result in different ways of forming gels during curing, the final gel fractions are always near 100% as long as the epoxy molar fraction of the diluent is no more than 0.2. The profiles of the molecular weights of the polymers calculated by the model are in agreement with the experimental data. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1857–1868, 2002     

Free volume from molecular dynamics simulations and its relationships to the positron annihilation lifetime spectroscopy

The free volume micro-structural properties of propylene glycol obtained by means of molecular dynamics simulations have been investigated and compared with the experimental data from positron annihilation lifetime spectroscopy (PALS). The results are also compared to those recently obtained on glycerol. The bulk microstructures of the samples have been analyzed in the temperature range 100–350 K with a probe-based procedure for exploring the free volume cavities of the microstructures. The basic free volume property, i.e., mean cavity volume, is compared with the hole volume data from PALS. A comparison between calculated and experimental data suggests the existence of a threshold volume for the smallest cavity detectable by PALS, which may be ascribed to fast local motions of the matrix constituents. At high temperatures the cavity analysis reveals the formation of an infinite cavity, i.e., percolation phenomenon. The onset temperatures of the percolation process in propylene glycol and glycerol are found to be close to the characteristic PALS temperature \(T^{\rm L}_{\rm b2}\) , where a pronounced change in the PALS response occurs, as well as to the characteristic dynamic Schönhals temperature \(T^{\rm SCH}_{\rm B}\) , and Stickel’s temperature \(T^{\rm ST}_{\rm B}\) , marking a dramatic change in the primary α properties.     

Molecular packing in amorphous carbohydrate matrixes

The molecular packing of bidisperse matrixes of amorphous carbohydrates consisting of a fractionated maltopolymer supplemented with various amounts of the disaccharide maltose is investigated by combining Positron Annihilation Lifetime Spectroscopy (PALS) with specific volume measurements. The maltopolymer-maltose blends are equilibrated at a range of water activities between 0 and 0.75 at 25 degrees C in order to investigate the effect of water content and carbohydrate molecular weight distribution on the size of the molecular free volume holes in both the glassy and rubbery states. In the rubbery state, the size of the intermolecular holes is only very weakly dependent on the carbohydrate molecular weight, provided that the carbohydrate blends are analyzed at the same water content. In contrast, in the glassy state, significant differences in the size of the free volume holes are observed between the various blends at constant water content. Both the specific volume and the hole volume decrease with increasing maltose content, initially rapidly up to a maltose content of about 40 wt % on total carbohydrate. In addition, we find that the role of water as a plasticizer and matrix constituent is a complex one. At very low water contents, water acts by filling the free volume holes between the carbohydrate molecules. This hole-filling mechanism could well be related to the phenomenon of anti-plasticization observed before. At higher water contents, corresponding generally to water activities above 0.11 at 25 degrees C, water conversely increases the average hole volume in the carbohydrate matrixes, most likely caused by water interfering with the hydrogen bonding between the carbohydrate molecules, leading to a local expansion of the molecular packing.