Crystalline PEEK Research Articles

Direct evidence of interfacial crystallization preventing weld formation during fused filament fabrication of poly(ether ether ketone)

Poly(ether ether ketone) (PEEK) is a high-performance, semicrystalline thermoplastic that has attracted significant interest for material extrusion additive manufacturing techniques such as fused filament fabrication (FFF) but remains beset by poor weld strength. Here, it is observed that under typical processing conditions that surface crystallization prevents effective weld formation between printed layers, leading to weak and brittle welds. Utilizing atomic force microscopy, differential scanning calorimetry, bulk tensile fracture testing, and mode III tear testing, appropriate processing conditions and post-print annealing conditions have been developed to improve the weld strength of FFF-PEEK. After printing PEEK in an amorphous state through careful control of the thermal gradients during printing a two-step annealing procedure yields crystalline PEEK welds that are 6–8 times stronger than welds in FFF-PEEK that crystallized during printing. • Fused filament fabrication of PEEK leads to weak and brittle welds with printed fibers. • Atomic force microscopy is used to map the weld microstructure of PEEK and is correlated with measurements of weld strength. • Interfacial crystallization between fibers inhibits weld formation, reducing the strength of fused filament fabricated PEEK. • A printing and annealing procedure reduces interfacial crystallization, improving PEEK weld strength from 7 MPa to 37 MPa.

Strong Interface Construction of Carbon Fiber-reinforced PEEK Composites: An Efficient Method for Modifying Carbon Fiber with Crystalline PEEK.

In order to improve the poor solvent resistance and poor temperature resistance caused by traditional sizing agents, crystalline poly(ether ether ketone) (PEEK) is introduced to the interfacial phases of carbon fiber (CF) reinforced PEEK composites by a soluble precursor named PEEK-1,3-dioxolane. By changing the soluble precursor molecular weight and concentration in the sizing solution, the content of PEEK coated on the CF fiber surface can be controlled and the different interfacial properties of the PEEK composites can be obtained. The results shows that, with this method, crystalline PEEK can be completely coated on the CF surface, and the interfacial shear strength of the PEEK composites increases from 43.42 to 83.13 MPa. Due to none of any soluble compounds in the PEEK composites, the interfacial layer is well preserved under organic solvents and hygrothermal conditions, and the interfacial shear strength (IFSS) of the PEEK composites maintained above 85.4% and 90.44%, respectively. Scanning electron microscope clarifies that the mechanism of interface enhancement comes from a better wetting of crystalline PEEK on the fiber surface. Additionally, the sizing system of this investagation has the potential commercial value because of no toxic reagent (such as 2,4,5-trichloro-1-hydroxy-benzene or concentrated sulfuric acid) is required during sizing.

The chemistry and topography of stabilized and functionalized graphene oxide coatings

Graphene oxide (GO) thin films and coatings are regarded as superior in quality to other materials especially for biomedical applications. However, the lack of stability and understanding of their structure and defects hinder their use in value added applications. Here, we describe our successful attempt at stabilizing, reducing and functionalizing GO through multiple plasma treatments with polymerizing (to deposit a crosslinking and compressing layer of diamond like carbon, DLC) and non‐polymerizing precursors (H2, O2, and N2). The hybrid GO and DLC coatings on semi crystalline PEEK were evaluated using AFM, SEM, and XPS. The GO deposited layer showed roughness around 70 nm and, despite care, resulted in several wrinkles and particle aggregations. The hybrid coatings conformed to the roughness and crystalline features of PEEK. XPS showed that the DLC layer cross‐linked the GO nano‐flakes while not completely masking which enable the partial exposure of GO. The GO‐DLC hybrid interface is higher in thickness than the PEEK‐GO and is dominating the overall thickness of the hybrid structure ≈13 ± 1 μm. XPS measurements showed that the often unstable CO functional groups on the surface of the hybrid coating can be reduced by effective plasma treatment. Plasma treatments also generated CO functional groups that probably originated from the decomposed carboxyl groups. The plasma treatment also contributed to the reduction of GO. Treatment with H2 was more effective in oxygen reduction than with the N2, however, treatment with N2 increased the reactants on GO as N2 is heavier tending to deposit more on a surface. Plasma treatment with O2 increased the surface oxygen content further and hence more defects on the hybrid surface.

Radiation-induced graft polymerization of functional monomer into poly(ether ether ketone) film and structure-property analysis of the grafted membrane

Radiation-induced graft polymerization of sulfo-containing styrene derivatives into crystalline poly(ether ether ketone) (PEEK) substrates was carried out to prepare thermally and mechanically stable polymer electrolyte membranes based on an aromatic hydrocarbon polymer, so-called “super-engineering plastics”. Graft polymerization of the sulfo-containing styrene, ethyl 4-styrenesulfonate (E4S) into a high crystalline PEEK substrate (degree of crystallinity: 32%) hardly progressed, whereas graft polymerization into a low crystalline PEEK substrate (degree of crystallinity: 11%) gradually progressed, achieving a grafting degree of more than 50% after 72 h. Oxygen radicals appeared in the ESR spectra of irradiated PEEK films, indicating that graft polymerization initiates from the phenoxy radicals generated by scission of PEEK main chains and proceeds so as to yield block type grafts. The PEEK-based polymer electrolyte membrane (PEM) converted by aqueous hydrolysis of grafted films exhibited mechanical strength (100 MPa), being 88% of the original PEEK substrates. These mechanical properties of PEEK-based PEM are much higher than those of graft-type fluorinated PEM reported previously and almost three times higher than that of Nafion (35 MPa). Wide- and small-angle X-ray scattering (WAXS and SAXS) indicated that the graft polymerization was accompanied with recrystallization of the amorphous phase of PEEK substrate, the well known solvent-induced recrystallization of amorphous PEEK solids, to form a weak lamellar structure with 8 nm spacing. Complementary SAXS and small-angle neutron scattering (SANS) observations clearly showed that the graft-type PEEK membranes possessed ion channel domains with the average distance of 13 nm, being larger than that of Nafion. Furthermore, there was a micro-structure in the ion channels with the average distance of 1.8 nm.

Dielectric Response of Aramid Fiber‐Reinforced PEEK

AbstractDielectric spectroscopy was applied for the first time to aramid fiber‐reinforced PEEK, wherein the effect of the fiber on the dielectric response was examined for both amorphous and crystalline poly(ether ether ketone) (PEEK) over wide temperature and frequency ranges. Whereas the temperature behavior of the dielectric losses of the materials exhibited the typical α and β processes of PEEK, the specific effect of the fibers in the crystalline PEEK was revealed in shifting the α process to a higher temperature. The unique effect of the fibers was expressed by a significantly higher activation energy and lower dielectric strength for the α relaxation, reflecting a higher constraint level that is imposed by the fiber. It is proposed that this additional constraint is associated with fiber generated transcrystallinity. Scanning electron micrograph of transverse fracture surface of crystallized unidirectional aramid fiber‐reinforced PEEK.magnified imageScanning electron micrograph of transverse fracture surface of crystallized unidirectional aramid fiber‐reinforced PEEK.

Solid state and mechanical properties of injection molded poly(ether ether ketone)/polyarylate blends

AbstractBlends of poly(ether ether ketone) (PEEK) and bisphenol‐A polyarylate (PAr) were directly prepared during the plasticization step of an injection molding machine and their solid‐state and mechanical behaviors were studied. Despite the fact that PEEK‐rich blends were apparently miscible by differential scanning calorimetry (DSC), from the dynamic‐mechanical analysis (DMTA) results all the blends were composed of (a) a crystalline PEEK phase, (b) a practically pure amorphous PEEK phase, and (c) a PAr‐rich phase richer in PEEK as the PEEK content in the blends increased. Annealed blends showed a poor mechanical performance in the PAr‐rich region, but the PEEK‐rich blends showed additive modulus of elasticity and tensile strength, and ductility and impact strength values similar to those of the highest of the two pure components. All the as‐molded low‐crystalline blends presented a synergistic behavior in the modulus of elasticity, as well as, surprisingly, in ductility and impact strength in the intermediate and slightly majority PEEK compositions. The different mechanical response of the components in fine dispersed phases and in macroscopic tensile specimens may account for the observed results.

Disordering of crystalline PEEK upon mechanical treatment

AbstractIt has been established that crystalline PEEK can become completely amorphous upon rolling at room temperature. This effect was verified by data of differential scanning calorimetry. X‐ray diffraction and density measurements. In contrast to quenched amorphous PEEK samples, cold crystallization in rolled amorphous samples is shifted to lower temperatures and starts just with the onset of the glass transition. This shift, along with the decrease in the heat of cold crystallization from 21 J/g (for quenched amorphous sample) to 11 J/g, suggest some retention of paracrystalline structure in rolled amorphous samples.

Crystallization behavior and mechanical properties of poly(aryl ether ether ketone)/poly(ether imide) blends

AbstractCrystallinity and mechanical properties of blends with different amounts of semicrystalline poly(aryl/ ether ether ketone) (PEEK) and amorphous poly(ether imide) (PEI) polymers have been studied. The blends, prepared by melt mixing, have been investigated by differential scanning calorimeter (DSC) to analyze the miscibility between the components and the final crystalline content. Moreover, for the 20/80 PEEK/PEI blend, crystallization in dynamic and isothermal conditions has been carefully investigated in order to find proper conditions for maximum development of crystallinity. Mechanical tests (static and dynamic) have been performed to evaluate the properties of the as‐molded and crystallized blends and to compare them with those of crystalline PEEK and amorphous PEI neat resins. Finally, a few SEM observations have been performed to compare the fractured surface of the blend with those of the pure constituents.

The effect of crystallinity on the scratch hardness of poly(ether ether ketone)

This paper describes a study of the surface properties of the poly(ether ether ketone) (PEEK) using a scratch hardness technique. A comparison of the hardness values determined for amorphous and crystalline PEEK shows that the crystalline polymer is harder. In addition, comparison of the scratch frictional behaviour indicates several friction mechanisms apply for this polymer.

Uniaxial Draw of Poly(ether ether ketone)/Poly(ether imide) Blends by Solid-State Coextrusion

Amorphous and crystalline blends of poly(ether ether ketone) (PEEK) and poly(ether imide) (PEI) have been drawn uniaxially by solid-state coextrusion over the whole composition range. The amorphous blends have been drawn at 125 o C which is below the T g s of these compatible blends, and the crystalline blends have been drawn at 125 o C and also at 225 o C, which is above the single T g . Reported are the effect of draw on T g , PEEK crystallization, and the respective orientations of PEEK and PEI in the drawn blends. The T g s of the miscible amorphous PEEK/PEI blends are depressed up to 6 o C by draw, but their densities are slightly increased, contrary to conventional free volume theory. The density of the crystalline PEEK drawn at 125 o C exhibits a minimum at an extrusion draw ratio (EDR) of about 2.0, whereas that of the crystalline PEEK drawn at 225 o C increases monotonically with EDR. The densities of the crystalline blends drawn at both 125 and 225 o C decrease monotonically with EDR. These observations are attributable to the interplay between the destruction of the lower-melting PEEK crystals and the increasing packing in the amorphous regions on drawing. The effect of draw on the crystallization kinetics of PEEK and in PEEK/EI blends has been evaluated by DSC. PEEK in the drawn blends crystallizes faster on cold crystallization. On the other hand, when the samples were annealed in the melt followed by crystallization (melt crystallization), the drawn PEEK crystallizes slower than the undrawn PEEK. The orientations of PEEK and PEI in the amorphous drawn blends were studied by infrared dichroism. It is found that the orientations of both PEEK and PEI at a given draw ratio increase with increasing PEEK content in the blends

Permeability of PEEK to carbon dioxide

AbstractSorption and diffusion of carbon dioxide were measured in amorphous and crystalline poly(aryl‐ether‐ether‐ketone) or PEEK, at different temperatures. The sorption follows Henry's law at all the investigated temperatures, and the reduction of sorption in the crystalline sample corresponds to the reduction of the amorphous phase as derived from X‐rays. Due to a more tortuous path, the diffusion parameter in the crystalline sample is lower than in the amorphous one, whereas the activation energy of the diffusion process is similar for both the samples. The permeability to carbon dioxide was very low, comparable, or lower than other rigid polymers. © 1994 John Wiley & Sons, Inc.

Effects of methylene chloride sorption on the properties of poly(ether ether ketone)

AbstractSorption of methylene chloride by poly(ether ether ketone) (PEEK) has been studied for both amorphous and highly crystalline polymer. After the determination of sorption and desorption curves, the crystallinity of the two materials after desorption was determined both by density and X‐ray measurements. The experimental results indicate the existence of solvent‐induced crystallization in initially amorphous PEEK and a virtual lack of this process in highly crystalline PEEK. In the latter case, the observed density increase is attributed to solvent compression and a decrease in free volume. The mechanical behavior of both PEEKs is consistent with their crystallinity levels. The mechanical behavior of both PEEKs before and after sorption allows us to discern the separate effects of the two processes to which the presence of methylene chloride in PEEK gives rise, i.e., plasticization and solvent‐induced crystallization. © 1994 John Wiley & Sons, Inc.

The essential work of plane-stress ductile fracture of poly(ether-ether ketone) thermoplastic

The total work of fracture in a ductile material is not a material constant and the linear elastic fracture mechanics is not appropriate. Only the work performed per unit area in the fracture process zone, called the specific essential work of fracture, is a material constant for a given specimen thickness. The results of an experimental investigation on the essential work of fracture of a crystalline and a non-crystalline poly(ether-ether ketone) (PEEK) films are reported. Single-edge notched specimens were used to determine the specific essential work by extrapolating the straight-line relationship between the specific work of fracture and ligament length to zero ligament length. In this way, the specific essential work of fracture for the crystalline PEEK film of thickness 0.1 mm was measured to be 65.02 kJ m−2 and for the non-crystalline film of thickness 0.25 mm was measured as 62.71 kJ m−2. Advancing crack opening displacements (COD) have also been analysed and the specific essential work of fracture calculated from the COD values compared extremely well with those results obtained from the intercept of specific work of fracture versus ligament length.

Irradiation effects on aromatic polymers: 1. Gas evolution by gamma irradiation

The effects of gamma irradiation on aromatic polymers, such as polyimides (Kapton, Upilex-R and Upilex-S), poly(aryl ether ether ketone) (PEEK), poly(aryl ether sulphone) (PES), bisphenol A type Udel poly(aryl sulphone) (U-PS) and poly(aryl ester) (U-Polymer), were investigated based on gas evolution. The radiation resistance in terms of gas evolution was in the following order: Upilex-R ⩾ Kapton > PEEK > PES > Upilex-S ⪢ U-PS > U-Polymer. The G values of total gases from these aromatic polymers were 1 100 th to 1 1000 th of the G values from aliphatic polymers. The major component gases were: H 2 and N 2 for polyimides; CO 2 and CO for PEEK; CO 2, CO and SO 2 for polysulphones; and CO and CO 2 for U-Polymer. The influence of crystallinity on the behaviour and yield of gas evolution was investigated in PEEK. Crystalline PEEK gave lower yield compared to amorphous PEEK. The radiolysis mechanism of gaseous products was discussed based on the structures of the aromatic polymers.

The crystallinity of poly(ether ether ketone)

Differences have been observed between the enthalpies of crystallization and fusion of quenched ‘amorphous’ poly(ether ether ketone) (PEEK) which have been attributed to residual crystallinity produced during quenching. Comparison with samples of PEEK crystallized under well defined conditions reveals that these apparent differences arise from changes in the heat capacity of the crystalline solid and the liquid alone and there is no need to invoke abnormal crystalline structure, or the presence of residual crystallinity produced on quenching to explain the apparent differences. Comparison is made of the degree of crystallinity as measured by d.s.c., WAXS and density from which the enthalpy of fusion of totally crystalline PEEK at 437 K was determined to be 122.5 ± 1.5 J g −1.

The absorption of organic liquids in poly(aryl‐ether‐ether‐ketone) [PEEK

AbstractThe absorption and subsequent desorption of benzene, toluene, carbon disulfide, and chloroform in amorphous and 27% crystalline poly (aryl‐ether‐ether‐ketone) (PEEK) were determined. At 35°C, the equilibrium weight gain (solubility) of benzene, toluene, chloroform, and CS2 are 23.5, 19.8, 51.2, and 21.2 wt%, respectively. The initial weight gain is linear with root‐time and pseudodiffusion constants for absorption into amorphous PEEK ranging from 0.35 to 9.85 x 10‐12m2/s were calculated. The desorption processes are two‐step and are controlled by the Tg of the penetrant‐resin mixture. The rate of diffussion into the crystalline material is extremely slow; crystalline PEEK reaches saturation (12.5 wt%) after immersion in CS2 (35°C) for several hundred hours but, even after 1300 h immersion, the other fluids do not reach saturation.

Crystallization of poly(ether ether ketone) oriented by solid-state extrusion

Semi crystalline and amorphous PEEK were drawn at 154 and 149°C, respectively, using the solid-state extrusion technique. The crystal orientation and total orientation of drawn PEEK were determined. The relationships between deformation history and crystallization have been investigated. Previously drawn PEEK was crystallized nonisothermally and isothermally from the melt. Results show that the crystallization of drawn PEEK is slower than that of the undrawn initial films. An orientation-induced crystallization was found during the solid-state extrusion of amorphous PEEK films at 149°C

The preparation of poly(aryl ether ketones) by the use of trifluoromethanesulfonic acid

Two synthetic routes were used for the preparation of aromatic poly(ether ketones): Friedel-Crafts acylation and aromatic nucleophilic substitution. The first route involved the reaction of diphenyl ether and terephthalic acid dichloride in trifluoromethane-sulfonic acid. The addition of trifluoromethanesulfonic anhydride or phosphorus pentoxide resulted in increased molecular weights. The second synthetic route involved the synthesis of an amorphous tert -butyl-substituted poly(ether ketone) prepolymer. Subsequently, the tert -butyl groups were removed by a reversed Friedel-Crafts alkylation reaction yielding the crystalline polymer PEEK. The dealkylation reaction was performed in trifluoromethanesulfonic acid. The addition of a tert -butyl group acceptor resulted in high dealkylation conversions.

Absorption of low-molecular-weight penetrants by poly(aryl ether ketone): 2. Bromoform

The diffusion of bromoform vapour into amorphous and crystalline poly(aryl ether ketone) (PEEK) has been investigated from the increase in weight with time. The kinetics of diffusion into amorphous material follows case III diffusion, i.e. w = kt n, where w is the fractional weight increase at time t and the n is 0.7. This is interpreted as plasticization by the bromoform followed by crystallization. Evidence for this mechanism can be seen in the changes in the dynamic mechanical thermal analysis of the materials. Desorption of the bromoform from the saturated material appears to follow case I with n = 0.3–0.4 rather than 0.5. Crystalline PEEK absorbs only 1% by weight of bromoform, and unlike the amorphous polymer this has no detectable effect on material properties.

Viscoelastic effects in poly(ether ether ketone) (PEEK) and PEEK-based composites

Viscoelastic effects occuring in amorphous and semicrystalline poly (ether ether ketone) (PEEK) and in a PEEK-based composite (APC2) have been analyzed by means of transient (creep) and dynamic mechanical tests, in the range of temperature covering the main viscoelastic regions. In the sub- T g region the non-linear approach to viscoelastic phenomena, i.e. creep, is related to the volume relaxation, and the effect of thermal treatments (physical aging) on the polymer amorphous phase of the three systems is presented. In the transition region just above T g , the amorphous PEEK undergoes cold crystallization and, as a consequence, passes from a liquid-like to a solid-like state. The isothermal crystallization kinetics and the changes in viscoelastic properties are analyzed by means of dynamic mechanical measurements and compared with calorimetric tests. Above T g the mobility of chains is sufficient to allow the free volume to reach its equilibrium level rapidly at any temperature. The principle of time-temperature superposition is applicable in the conventional fashion using a Williams, Landel and Ferry equation. The shift factors obtained for the crystalline PEEK and for an eight-ply unidirectional laminate are compared.