Polyimides have been extensively used in the electronics industry [1] as coatings, films, or adhesives and also in composite applications [2] as a primary or secondary structural material. When incorporated with organic or inorganic fibers, including carbon fibers and glass fibers, they can potentially be applied as a sizing material for high-temperature uses. In advanced applications, one of the most desirable advantages of polyimides, especially the aromatic polyimides, is their excellent high-temperature performance. Their thermal characteristics, including high temperature stability and cure behavior, are critical to successful processing and to the final properties of the resulting polyimide. For aerospace and aircraft applications, advanced polymer materials must successfully maintain their properties at high service temperatures over a long exposure time [3-5].

The important parameters influencing the heat resistance of a polymeric material are its glass transition temperature (T_g), melting point (T_m), thermal stability, and etc., which may depend on its thermal history. Accordingly, polymers that are used to fulfill such high temperature performance should exhibit a high T_g or T_m and have excellent thermal stability. Even though some polyimides meet the high temperature requirement, their uses have been limited due to processing difficulties, such as poor solubility in common solvents, the release of volatiles, brittleness, and high processing temperature [6,7].

Aromatic polyimides, especially acetylene or ethynyl-terminated polyimides, have been extensively studied over the past two decades as candidate resins for advanced applications due to their unique combination of properties [8-12]. In the 1990s a variety of phenylethynyl-terminated imide oligomers for high-temperature aerospace and aircraft applications, known as phenylethynyl-terminated imide (LaRC PETI), were developed at the National Aeronautics and Space Administration (NASA) Langley Research Center, USA [13-19]. These materials have broader processing windows and thermal oxidative stability than simple ethynyl-terminated materials, and consequently have many practical advantages as a potential material for coatings, adhesives, films, and composite matrix resins [20,21].

Among the LaRC PETI series, it was found that a PETI-5 version with a molecular weight of 2500 g/mol exhibits an excellent combination of processing, toughness, thermal, mechanical, physical, and chemical performances at elevated temperature [22].

Phenylethynyl-terminated polyimides require high temperature treatment to completely remove solvents, and also to convert the oligomeric precursor into cured polyimide. The LaRC PETI-5 amide acid oligomer is imidized via cyclodehydration between the amide and carboxylic acid groups to form the corresponding LaRC PETI-5 polyimide on heating, as indicated in Fig. 1 (hereinafter, LaRC PETI-5 will simply be referred to as PETI-5). During thermal cure this material undergoes a complex reaction involving crosslinking and chain extension of the phenylethynyl groups located on the imide polymer chain ends, without releasing volatiles [21-23].

The neat PETI-5 resin is very viscous and may be applied to the surface of various substrates as a sizing, coating, matrix prepreg, or adhesive. Because processing generally requires the resin to be exposed to a prescheduled temperature and time cycle, to optimize resin processing and properties it is necessary to understand the chemical changes that occur in the PETI-5 during imidization, curing, and consolidation.

Fourier transform infrared (FTIR) spectroscopy is a powerful tool used to monitor chemical changes that occur during thermal treatment. The FTIR method has also been successfully used in many research areas to study the curing [25], thermal aging [26], crosslinking reaction [5], semi-interpenetrating networks [27,28], as well as structure identification [29,30] of polyimides.

The imidization of PETI-5 has been monitored by examining the disappearance and appearance of the most characteristic infrared absorption bands, and by monitoring the variation of representative amide acid and imide peaks observed via imidization, during cure of PETI-5 at different temperatures. In Fig. 2, the ΔT (%) values determined from amide, amine, and carboxylic acid groups are shown to decrease with increasing temperature, and are constant above 250℃. The ΔT (%) values were calculated from the difference in transmittance values between the absorption maximum and the minimum in close proximity to the characteristic absorption peak. All the values are close to zero at 350℃. This indicates that the conversion of the phenylethynyl-terminated amide acid oligomer into the corresponding imide polymer proceeds rapidly at around 200℃, becomes less reactive with temperature, and is complete around 250℃. The reduction of the ΔT value for the amide group is remarkable and the value approaches close to zero around 220℃. The amide groups in the oligomer are substantially consumed first during the imidization reaction.

FTIR analysis also provides useful information on the extent of conversion of poly(amide acid) into polyimide through imidization [31,32]. The extent of imidization can be determined from the heights of the absorption bands for imides I and III, normalized to the height of the absorption band for ‘as-received’ PETI-5, at 1374 and 1777 cm^–1, respectively. In Fig. 3, the imidization reaction initiates around 100℃, increases linearly up to about 200℃, and then proceeds slowly to 250℃. This can be explained by the fact that below 200℃ the solvent in the neat resin does not play an important role in the reaction but still contributes to stabilizing the conformation of amide acid.

The values of the extent of imidization determined for PETI-5 at various temperatures are similar to a polyimide system with different monomers, studied by Ishida et al. [33]. A small decrease in the extent of reaction after 250℃ is probably due to a lack of sensitivity in detecting the vibrational segmental motion in the fully imidized molecular chains using FTIR spectroscopy, resulting from an increase in the local viscosity in the sample.

In summary, the phenylethynyl end-group reaction of PETI-5 can be well monitored by inspecting the disappearance and appearance of FTIR absorption bands of characteristic chemical groups, and it was found that the phenylethynyl-terminated amide acid was completely converted to polyimide at about 250℃.

The extent of cure of thermosetting resins can often be determined from differential scanning calorimetry (DSC) data, in terms of the heat of reaction. Fig. 4 exhibits the variation in the extent of cure of PETI-5 exposed to two different thermal histories, individual and cumulative cure conditions, and as a function of temperature, respectively. The extent of cure can be calculated from the changes in the exothermic peak areas. The cumulatively cured material exhibits a higher extent of cure than the one cured individually over the entire temperature range up to complete cure, with the exception of those cured at 300℃. It should also be noted that the material starts to cure slowly and partially at a temperature lower than the imidization temperature. The cure accelerates around the imidization temperature. The extent of cure of the cumulatively cured sample in the imidization temperature region (200℃–250℃) is about twice that of the individually cured sample. The thermal history of the imidization reaction may significantly influence the extent of cure of the partially cured resin. The extent of cure increases most in the range of 300℃–350℃, where the reaction of the phenylethynyl end-group predominates.

The T_g increases very slowly with increasing cure time below 300℃. This indicates that such temperatures do not significantly influence the chain stiffness of the polymer even after an extended period of time. However, the T_g increases rapidly with time above 330℃, showing that the T_g already has reached a higher temperature (about 280℃) than the temperature required for full imidization. Similar to the tendency of the T_g change, the extent of cure increases gradually with temperature up to 300℃ after 60 min isothermal cure, as seen in Fig. 5. Such a large increase between 300℃ and 350℃ in the isothermally cured sample is consistent with the result obtained from the cumulatively cured sample. Upon thermal cure, temperatures greater than the imidization temperature lead to a higher T_g and extent of cure. Therefore, a cure temperature of around 350℃ in air produces a fully cured PETI-5 within an hour.

In summary, the curing behavior of PETI can be monitored either by a dynamic heating method (individual or cumulative cure) or by an isothermal method. It is concluded that the T_g of PETI-5 increases with increasing cure time as well as cure temperature, and heat treatment of PETI-5 at 350℃ for 1 h in air is sufficient for full curing.

Many investigators have utilized a variety of analytical methods to examine the imidization and/or cure of ethynylor phenylethynyl-terminated imide polymers, including, for example, FTIR [19,24,35], DSC [19,23,36], thermogravimetric analysis (TGA) [23], high performance liquid chromatography (HPLC) [4], and solid-state ¹³C nuclear magnetic resonance (NMR) [37] methods. Nonetheless, each of the approaches has limitations. FTIR measurement may restrict further monitoring of the cure behavior of PETI-5 occurring above the full imidization temperature, even though the presence and absence of the FTIR absorption band of the phenylethynyl end group can be successfully detected, depending on the thermal cure history. This is because the glass transition temperature and chain stiffness of the imide polymer increase with increasing degree of cure and the local mobility of the chain segments is largely restricted while being consolidated [24]. DSC may also limit further understanding of the imidization behavior occurring in the early stage, during consolidation, even though it provides useful information on cure behavior [23]. Solid-state ¹³C NMR can provide chemical identification of the molecular structure of cured polyimides, but it cannot give detailed information for monitoring imidization and cure [37].

Dynamic mechanical analysis (DMA) is a very powerful technique for obtaining information on the thermo-mechanical behavior of polymers, and is based on the temperature dependence of storage modulus, loss modulus and tan δ. The glass transition temperature and other physical transition temperatures can be determined from the peak of the tan δ or loss modulus, which are sensitive to molecular mobility, stiffness behavior, phase transformation and morphological change [38]. Owing to its analytical advantage in providing molecular information and ease of use, the DMA technique has been widely used to explore the thermal mechanical behavior of not only viscoelastic polymer materials [5], blends [39] and polymeric composites [40,41] but also at the interface or sizing interphase [42-44] between fiber and matrix in a composite system. Also, the reaction behavior can be observed using this method, because changing thermal conditions can change molecular states during the transformation from amide acid to fully cured polyimide.

Imidization and cure reaction behaviors have been simultaneously monitored during cumulative and isothermal cure processes, based on the dynamic mechanical behavior of a phenylethynyl imide oligomer material impregnated in a glass fabric composite system, using DMA. The chain stiffness, and the corresponding storage modulus of PETI-5, increases with increasing cure temperature as the amide acid transforms into imide oligomer and then polyimide. The dynamic mechanical property of PETI-5 strongly depends on thermal history, especially above 300℃.

The three distinguishable peaks in the tan δ curves, as shown in Fig. 6, indicate the glass transition temperature, the imidization temperature, and the cure reaction temperature from each peak, respectively. The completion and incompletion of the imidization or cure reaction occurring during cumulative and isothermal cure processes is demonstrated by the absence and presence of an imidization peak, or a cure reaction peak. The degree of cure increases with a higher rate, above 250℃, and then greatly increases near full cure temperature due to the rapid development of a three dimensionally crosslinked network structure. With the cumulative or isothermal curing process, the PETI-5 imide oligomer can be converted to a fully cured polyimide at 350℃ for 1 h, as indicated in Fig. 7.

In summary, it was found that impregnating PETI-5 in a glass fabric, and its thermal cure history, significantly influenced the dynamic mechanical properties of the fiber sizing material and accordingly, the fiber-reinforced composite material.

Carbon fiber/bismaleimide (BMI) composite materials are currently utilized for commercial and military aircraft and aerospace applications [46]. However, these materials have some critical issues, such as brittleness and loss of properties due to microcracking and thermal degradation. The level of fibermatrix adhesion for carbon/BMI composites has been measured using a variety of adhesion tests, and it has been found that in general the adhesion is very low [47]. Therefore, achieving better adhesion in carbon/BMI composites has become an important research objective [48].

Adhesion, or the interfacial strength between fibers and matrix, is critically important in a carbon/polymer composite material because the ultimate mechanical properties and performance of the material strongly depend on it [49]. There have been considerable efforts to improve the interfacial properties of carbon fiber-reinforced polymer matrix composites. They include changes in fiber surface energy [50], surface morphology [51], surface chemistry [52] and resin chemistry [53], and fiber sizing [54]. Sizing is normally used to provide better adhesion between fibers and matrix in a composite system [55] as well as to improve the handleability and reduce fiber damage during fabrication of the prepreg and composite [56]. The proper sizing material has high thermal stability and durability, and these properties are desirable because a beneficial interphase may be created if there is appropriate miscibility between the sized material and matrix resin. Optimizing the sizing interphase and sizing process is, of course, necessary to obtain the desired ultimate properties of a composite.

In general, the experimental methods used to characterize the fiber-matrix interphase have been based on single fiber-matrix interfacial measurements, such as the single fiber fragmentation test, single fiber pull-out test and microbonding test; or they have been based on direct interfacial measurements of the actual composite laminate, such as the transverse tensile test, short-beam shear test, and micro-indentation test. Excellent review articles describing these experimental methods and the measurement of fiber-matrix adhesion in composites have been published elsewhere [49,57-59].

PETI-5 oligomer has been applied as a sizing material onto the surfaces of polyacrylonitrile (PAN)-based carbon fiber tows and woven carbon fabrics, to introduce an interphase between the fibers and matrix in carbon/BMI composites. The adhesion between the fibers and matrix was largely enhanced by the PETI-5 interphase. For evaluating the fiber-matrix adhesion of fiberreinforced plastics, interfacial shear strength and/or interlaminar shear strength (ILSS) tests can often provide very useful information [60-62]. Short-beam shear test results have indicated that when PETI-5 was sized at 150℃, the ILSS of a 2-directional carbon/BMI composite was markedly improved, by about 35% and 66%, respectively, in comparison to an unsized counterpart, as seen in Fig. 8. The adhesion enhancement strongly depends not only on the presence and absence of the PETI-5 sizing interphase, but also on sizing temperature, because both factors critically influence the physical and chemical state of the sizing material.

In summary, sizing the PET-5 at 150℃ prior to composite processing is appropriate to increase the interfacial and ILSSs of carbon fiber reinforced BMI composites.

Like layered silicates, graphite is a layered mineral having a large aspect ratio. It consists of a huge number of graphene layers held together by weak van der Waals forces. Theoretically, single crystal graphite has an elastic modulus of over 1 TPa, which is many times greater than nanoclay. Unlike clay, graphite is thermally and electrically conductive. Expandable graphite, which is composed of natural graphite flake intercalated with acid, can be expanded up to hundreds of times its initial volume at high temperature, resulting in separation of the graphene sheets at the nanoscopic level along the c axis of the graphene layers [64-67]. The extent of expansion may depend on the heat-treatment temperature, processing method, the amount of intercalant, and the stage index of intercalant [67,68]. The intercalated graphite flake material is also referred to as a graphite intercalation compound [68,69].

Fig. 9 shows the thermal expansion behavior of a single intercalated graphite flake as a function of temperature, as studied by thermomechanical analysis (TMA) [63]. The result reveals that the onset temperature of expansion is about 160℃–170℃. The expansion takes place most rapidly in the range of 220℃–240℃, and the expansion ratio is approximately 20–35, which is much less than expected. The following three reasons explain why. First, during the measurement the flake is substantially loaded by the TMA probe itself, by a minimum normal force of 0.005 N in the direction opposite to the expansion. Therefore, the expansion is significantly suppressed under the probe during the measurement. Second, the expansion proceeds in a twisting and bending configuration, not in a single straightforward direction. Third, the TMA instrument normally cannot successfully monitor extremely rapid expansion, meaning it is measuring just a partial range of the expansion, not the full range.

Much attention has been paid to carbon nanoparticle-based polymer materials with high performance and/or functionality, for the purpose of improving the polymer properties and extending their applications [70-72]. By utilizing layered reinforcements with a large aspect ratio, such as graphite flake, which can be exfoliated and dispersed in a polymer matrix as nanoplatelets, polymer nanocomposites have exhibited improved mechanical, thermal and barrier properties, depending upon the type of nanoplatelets [73-75].

One approach to further improving polymer properties involves preparing polymer composites that are composed of a thermally stable polymer matrix reinforced with exfoliated graphite nanoplatelets (xGnP) [77-79]. Nano-sized graphite flakes can be expanded and exfoliated and then pulverized to produce reinforcement with pristine characteristics.

The dynamic mechanical and thermal properties of PETI-5 composites reinforced with three types of xGnP having an average particle size of 1 µm, 7 µm, and 19 µm have been studied. The xGnP samples were prepared by ultrasonication and vibratory ball-milling techniques, respectively. As listed in Table 1, the DMA results demonstrate that reinforcing the PETI-5 resin with the xGnP particles increased its storage modulus, both through a composite reinforcing mechanism, and by decreasing the flexibility of the PETI-5 molecular chain in the composite. The storage modulus of cured PETI-5 polymer was significantly enhanced by the incorporation of xGnP in the matrix. The storage modulus of PETI-5/xGnP composites increases with increasing xGnP content, and also by decreasing the average particle size from 19 µm to 7 µm and to 1 µm. The improvement in storage modulus is also greater in the temperature range above the T_g than below the T_g.

The coefficient of thermal expansion (CTE) of PETI-5/xGnP composites is slightly lower than that of cured PETI-5 in the temperature region below the glass transition, as indicated in Fig. 10. In the glass transition region where enhanced thermal expansion takes place, the thermal expansion of the composites significantly decreases with increasing xGnP content. A lower CTE value was obtained with smaller average particle size xGnP, because it allowed better dispersion of the xGnP in the composite matrix.

In summary, the dynamic mechanical properties, the T_g and the CTE of xGnP-incorporated polymer composites all depend on the particle size, pulverization method, and concentration of PETI-5.

In recent years, many research efforts have been devoted to studying the effect of carbon nanoparticles, such as xGNP and carbon nanotubes, on the mechanical, thermal, and/or electrical properties of various polymers [80-88].

Fig. 11 displays the variations in the electrical resistivity of PETI-5 and PETI-5/xGnP composites as a function of xGnP content. Resistivity is gradually decreased by increasing xGnP. It seems that the percolation threshold occurs at 5 wt%. This result is consistent with an earlier study on xGnP-polypropylene composites processed by injection molding and compression molding, as reported by Kalaitzidou et al. [82]. Also, the incorporation of small sized (1 µm) xGnP into PETI-5 helps to reduce the amount of xGnP needed to reach the percolation threshold, because the presence of small xGnP particles increases the number of xGnP-PETI-5 interfaces, increasing the number of contact points between the xGnP and PETI-5. At 20 wt%, the electrical resistivity was decreased to about 3 × 10³ ohm·cm.

In summary, the incorporation of xGnP into PETI-5 has a significant effect on decreasing electrical resistivity, suggesting that an optimal concentration of xGnP should be determined.

Recently, a large number of papers have reported studies to modify carbon nanoparticles in efforts to improve their dispersibilty in polymer, as well as in solutions, and ultimately to increase targeted properties of polymers [91,92]. The modification of carbon nanoparticles has often focused on chemical modification or functionalization, to covalently attach specific functional groups to their surfaces and edges [93-96].

Amine-terminated poly(butadiene-co-acrylonitrile) (ATBN) is a liquid rubber with amine groups at the ends, and butadiene and acrylonitrile groups in the repeating unit. Due to their many advantages, liquid rubbers, including ATBN or carboxyl-terminated poly(butadiene-co-acrylonitrile) (CTBN), have been utilized to make brittle polymers more ductile, and in particular to toughen thermosetting polymers such as epoxy, unsaturated polyester, vinyl ester, and phenolic resins [97-99].

Epoxy resins have versatile applications due to their excellent mechanical, thermal, and electrical, and adhesive properties. However, their brittleness restricts other potential uses. ATBN has been utilized as an impact modifier of epoxy for this reason [97,100,101]. In many cases, a relatively large amount of ATBN, higher than 10 wt%, has been used to toughen epoxy, resulting in a considerable decrease in mechanical and thermal properties [101,102].

Other work has been motivated by the hypothesis that incorporating a small amount of xGnP particles coated with ATBN into a thermosetting polymer would contribute to toughening the thermosetting polymer, if the elastomeric coating performed well. Fig. 12 shows the effect of ATBN coating concentration on the impact strength of uncoated and ATBN-coated xGnP/epoxy composites. The xGnP (here referred to as EGN) content was fixed at 1 wt% of the epoxy content. The impact strength of the cured epoxy composites was gradually increased by increasing the ATBN coating concentration, indicating a toughening effect. The maximum impact strength was obtained when the xGnP was coated with 10 wt% ATBN, which resulted in an improvement of 18% compared to epoxy. 10 wt% ATBN coated onto 1 wt% xGnP corresponds to only 0.1 wt% ATBN against the epoxy amount. The result indicates that a very small amount of rubber component used in the xGnP/epoxy system can play a considerable role in increasing the toughness of the epoxy resin.

In summary, xGnP nanoparticles can be physically modified by coating with a liquid rubber ATBN, and the effect of the coating is well demonstrated by multiple analytical approaches. In particular, incorporating the ATBN coated nanoparticles into epoxy resin considerably enhances the impact toughness of xGnP/epoxy composites, with only a small ATBN coating concentration.

This paper provides an extensive overview of fundamental studies on PETI-5 and xGnP, and applied studies on relevant composite materials.

The paper described imidization and thermal curing behaviors based on phenylethynyl end-group reactions, using thermal analyses and FTIR spectroscopy, to review the characterization, properties, and processing of PETI-5 as a high-temperature sizing material for carbon fiber-reinforced polymer matrix composites. Simultaneous monitoring of the imidization and cure reactions of PETI-5 impregnated on a glass fabric composite was studied using DMA. The role of PETI-5 in enhancing the interfacial adhesion between carbon fibers and the BMI matrix, as well as the dynamic mechanical properties of carbon/BMI composites, were discussed.

The effect of incorporated xGnP on the properties of PETI-5 matrix composites was also stressed. The thermal expansion behavior of intercalated graphite flake, preliminarily monitored using the TMA technique, was reported. The dynamic mechanical and thermal properties, the fracture surface topography, and the electrical resistivity of PETI-5 composites reinforced with xGnP were described. It was noted that physically modifying xGnP particles by coating the carbon nanoparticles with ATBN liquid rubber, and then incorporating the coated xGnP into epoxy resin, significantly influenced the Izod impact strength of the xGnP/epoxy composites.