The increase in peak intensity(014) is correlated with the degree of crystallinity. This shows an increase in the crystalline region in the polymer composite which can be examined in mechanical properties. From the graph, there might be a chance that (014) and (002) are the characteristic peaks of filler considering PPT 1 is a pure polymer.

Thank you for your response Akash Ghosh

Effect of size, concentration, and nature of fillers on

crystallinity, thermal, and mechanical properties of

polyetheretherketone composites

Marie Doumeng1,2 | Florentin Berthet2 | Karl Delbé1 | Olivier Marsan3 |

Jean Denape1 | France Chabert1

1Laboratoire Génie de Production (LGP),

INP-ENIT, University of Toulouse,

Tarbes, France

2Institut Clément Ader (ICA), CNRS, IMT

Mines Albi, INSA, ISAE-SUPAERO, UPS,

University of Toulouse, Albi, France

3CIRIMAT, INP-ENSIACET, University of

Toulouse, Toulouse, France

Correspondence

France Chabert, Laboratoire Génie de

Production (LGP), INP-ENIT, University

of Toulouse, 47 Avenue d'Azereix, 65016

Tarbes, France.

Email: [email protected]

Funding information

Occitanie region; Université de Toulouse

ABSTRACT

Polyetheretherketone (PEEK) composites exhibit high stiffness, chemical stability,

and heat resistance and they are therefore employed in applications under

severe operating environments. This work aims to provide insight into the effect

of the size, concentration, and type of fillers on the thermal and mechanical

properties of PEEK. A total of 32 composites are used to highlight the influence

of nature (lamellae, such as boron nitride and graphite and silicon carbide and

alumina), size (nano and micrometric), and content (2.5, 5, 7.5, and 10 vol%) of

fillers. The melting temperature and lamellar thickness did not change regardless

of the nature of the filler. The thermomechanical analysis demonstrates that

lamellar fillers form a percolating network and contribute significantly to the

enhancement of the storage modulus. The increase in the storage modulus is proportional

to the filler content, and it is more pronounced for micro composites.

As expected, the percolating network is formed at lower concentrations for lamellar

fillers than for spherical ones. The highest conductivity is achieved with

graphite at 0.823 W m
1 K
1, which is twice that of PEEK for 10 vol%. Moreover,

the use of micrometric fillers results in thermal conductivity enhancement attributed

to the higher amount of efficient hot zones for heat transfer.

KEYWORDS

composites, crystallinity, rheology, thermal conductivity, thermal transition

1 | INTRODUCTION

Polymer composites are manufactured by incorporating

fillers into a polymer matrix. Composites target many

desirable properties including wear resistance, thermal

properties, mechanical strength, and formability, which

cannot be achieved by the pure matrix alone. The properties

of composites are governed by the properties of

each constituent and their interfacial interactions. In

nanocomposites, the contact surface is higher between

the matrix and the filler at a fixed filler loading, which

makes the role of such an interface predominant. This

role is even more dominant as the particle size decreases.

When switching from a micro filler to a nanometric one,

the specific surface increases by a factor of 1000 for

spherical shapes, and many studies have focused on

predicting how the chemistry and morphology of the

matrix synergize with the surface chemistry, size, and

shape of nanoscale fillers to define the properties of the

resulting material.

The properties of polymer composites depend on the

chemical nature, dimensions, and shapes of the fillers.

The other parameters are the volume fractions and distribution

of the fillers in the compound. The size and shape

of the particles and their surface chemistry influence the

interactions at the polymer/filler interface. The volume

of the material affected by these interactions constitutes

the interphase, that is, a volume where macromolecules are

disturbed by the presence of fillers. In this zone, the mobility

of the macromolecules, degree of crystallinity, entanglement

rate, and degree of crosslinking for thermosetting

polymers are different from those of the bulk, which lead

to the gradation of properties between the two areas. It is

commonly assumed that this phenomenon results from the

creation of weak bonds, such as van der Waals and donor–

acceptor interactions, which is the case with hydrogen

bonds1 Numerous studies on polymer fibers presented in

the review by Liu and Kumar2 suggest that the incorporation

of carbon nanotubes (CNTs) enhances mechanical

properties by the formation of a more ordered, more crystalline,

and better-oriented interphase than the rest of the

matrix. This interphase is crucial for the performance of

the material, as it improves the load transfer from the polymer

matrix to the fillers to fully utilize the higher rigidity of

the particles. Further, interfacial interactions improve the

properties of the polymer because the ordered interphase

layer itself will have much better mechanical properties

than the rest of the matrix. Polyaryletherketones (PAEK)

offer a unique combination of properties and ease of

processing, and they are used in highly demanding applications,

such as medical, automotive, and aerospace industries.

Indeed, PAEK are semi-crystalline thermoplastics that

have been launched in the market since the 1980s. Its polymerization

occurs via nucleophilic substitution obtained by

polycondensation between 200C and 400 C. Monomers

were biphenyl(hydroquinone) and a fluorinated aromatic

compound in a polar aprotic solvent (diphenylsulfone).

Fluorinated derivatives are preferred for this synthesis

because of their better reactivity and higher electronegativity

than chlorinated derivatives.3 The resulting polymer

was a copolymer with ether and ketone groups separated

by aromatic rings. PEEK, with an ether/ketone ratio of 2, is

a widely used high-performance thermoplastic. Not only

their heat resistance, chemical resistance, and stiffness are

the highest of the thermoplastics range, but they also have

demonstrated the best tribological properties, which makes

PEEK suitable for moving parts with high friction rates.4

Owing to these convincing properties, PEEK is used as a

matrix in high-performance composites. Despite its strong

performance, other properties can be achieved by adding

fillers to the PEEK matrix.

In previous studies, long fibers—most often carbon

fibers5 or glass fibers4—are associated with the PEEK

matrix. In addition, lamellar or particulate reinforcements

such as graphite,6,7 (Sukumaran, Schlarb, and

Baets), molybdenum disulfide,8 silicon carbide,9 silicon

oxide,10 and titanium silicocarbide11,12 have been incorporated.

In other cases, it has been proved that polymer

blends, through the addition of polyethersulfone13 and

polyetherimide14 fillers, enhance the thermomechanical

properties of PEEK. These results indicate that the

thermomechanical properties of PEEK vary depending

on the filler size,15–19 filler rates,20–24 and filler types.8,25

In terms of the thermal properties of any polymer,

PEEK exhibits a low thermal conductivity λ. In some cases,

such as in milling or drilling, a better heat transfer is suitable

for releasing the heat regenerated by wear. The λ for

PEEK is 0.4 Wm
1K
1. Adding fillers with a higher λ

than the matrix increases the λ of the composites. For

instance, λ reaches 1.2 Wm
1K
1 with 30 vol% of boron

nitride in polyethylene, which is another semi-crystalline

polymer.26 Zhou27 highlighted a higher λ with boron

nitride particles of 0.5 μm compared to 15 μm at the same

volume rate. As the surface energy of the smaller particles

is higher, the absorption energy between the fillers

and matrix increases. Moreover, the thermal conductivities

of the amorphous and crystalline phases of a polymer

are different. Puertolas28 established that the evolution of

λ follows the evolution of the degree of crystallinity. This

stems from the ordering of the polymeric chains; the latter

are well ordered and more compact in the crystalline

phase, which facilitates heat transfer.

Finally, as expected, the mechanical properties of

fillers with a high modulus act as reinforcements, thereby

increasing the storage modulus. For instance, at 50C,

Kadiyala29 noticed an increase of the storage modulus

from 3.01 GPa for the pure PEEK to 4.22 GPa with the

addition of 10 wt% of silicon carbide. When adding a

higher rate, the storage modulus decreases because the

fillers are more agglomerated with an insufficient polymer

to bind them. Its elongation and fracture resistance

decrease with an increase in the filler loading. The presence

of a stronger interface formed between the matrix

and fillers increases the storage modulus.30

Despite these attempts, general conclusions based on

these works are unreliable for two reasons: each work

focuses on one type of filler, implying that it is difficult to

compare the results with each other. The method of elaboration

of composites is different in each study; again, this

can explain the difference in properties observed when comparing

the results.25,31 Even if undeniable knowledge has

been reached through prolific literature on nanocomposites,

these results cannot be transposed to PEEK. Indeed, the

behavior of such high-performance polymers is specific

because of the high rigidity of their macromolecular chains.

In this study, we intend to provide more insight into the

effect of fillers on the properties of PEEK composites by

comparing four fillers and two sizes for each type. We aim

to link the nature, size, and content of the fillers to the crystallinity,

thermal conductivity, and thermomechanical properties

of the PEEK composites. Therefore, PEEK composites

with micrometric and nanometric fillers are prepared by

melt mixing. The fillers are lamellar compounds (boron

nitride and graphite) and ceramic particles (alumina and silicon

carbide). We chose the concentration of fillers between

2.5 and 10 vol%. The lowest concentration at 2.5 vol% is

defined by the aim of reaching the percolation threshold to

enhance the λ of the composites. The highest concentration

at 10 vol% was limited by the increase in viscosity.

Indeed, at a high filler loading, the viscosity of the composite

increased so much that the dispersion of the particles

was not homogeneous. Such blends were excluded

from the samples. This percolation question is crucial for

controlling the properties of the composites. Even if some

models predict the concentration of the percolation

threshold, these models assume the same size and shape

for all particles. The difficulty is that particle sizes are

affected by the preparation process. We measured the initial

size distribution using laser granulometry for the

eight types of fillers. Then, the filler size distribution after

the blending step was calculated by image analysis from

the scanning electron microscopy (SEM) images. The

results indicated a large size distribution. In addition,

the percolation threshold depends on the temperature, as

suggested by Abbasi et al..32 The percolation threshold is

different whether we are interested in mechanical, electrical,

or thermal percolation. The composites were characterized

by diffrential scanning calorimetry (DSC), X-ray

diffractometry (XRD), and density measurements to

assess the degree of crystallinity and morphology. The

viscoelastic properties of the composites were determined

by rheometry, and the thermal properties were measured

using the hot disk method.

2 | EXPERIMENTAL

2.1 | Materials

PEEK granulates grade 450G (Victrex) were used as the

matrix. Some common properties are provided in

the datasheet.33 The fillers used have either particulate

(silicon carbide grade β, alumina grade α) or lamellar

shapes (hexagonal boron nitride and graphite). The references

and sizes of the fillers are listed in Table 1. Two

sizes were selected for each type of filler: microfillers and

nanofillers. The composite samples were labeled as follows:

PEEK–type–size–rate. For size, N indicates

nanofillers, and M indicates microfillers. As an example,

PEEK–BN–N–7.5% refers to a PEEK filled with 7.5 vol%

of nanosized boron nitride.

2.2 | Preparation of polymer composites

Samples were prepared by blending the matrix in the molten

state and the fillers in an internal mixer (Rheomix

600, ThermoFischer) equipped with roller rotors. The

mixing was performed at 360C for 10 min at a rotating

speed of 40 rev min
1, which corresponds to a mean shearing

rate of 40 s
1. Based on relevant information from the

literature,34–36 we assume that inorganic fillers are stable

under the processing conditions (360C for 10min), which

means their chemical structures are not changed.

According to a previous study,,37 there is no change on the

surface of graphite under oxidation at 500C. The composite

material was then removed from the mixer and ground

to reach pellets with a maximum dimension of 2 mm. The

pellets were then molded into 2-mm-thick plates with a

hot press (LAB 800P, Pinette Emidecau Industries). The

processing cycle follows three steps: (1) heating until 360C

at 4 K min
1, (2) temperature stabilization for 5 min, and

(3) cooling to 100C at 4 Kmin
1. A cooling ramp was

selected to achieve the maximum degree of crystallinity of

the polymer. The concentrations of fillers xr were 2.5,

5, 7.5, and 10 vol%, two sizes, and four types of fillers

were studied. A total of 32 composites were prepared to

assess the type, size, and rate of the fillers.

2.3 | Characterization

SEM observations were performed (EVO HD 15 LS, ZEISS)

to measure the filler distribution on the surface of the

matrix. The samples were cut into parts

of 40 mm  30 mm  2mm. The degree of crystallinity

and morphology were determined using three different

techniques: density measurements, DSC, and XRD. The

density ρ of the specimens was measured by hydrostatic

weight or immersion according to ISO 1183-1:2019. Each

sample was weighed at least thrice. The DSC instrument

used was Q200 (TAInstruments). Samples were cut to

obtain approximately 10 mg of material, placed in sealed

aluminum pans, and heated from 80C to 380 C at

10 Kmin
1. A nitrogen flowat 50Kmin
1 was used to prevent

degradation. The temperature was maintained for

1 min at 380C, and then, the sample was cooled from

380C to 80 C at 10 Kmin
1, and a second heating ramp

was applied. For each sample, at least two analyses were

performed to determine the reproducibility of the measurements.

The XRD instrument was X'Pert Panalytical

(Philips). The measurements were made for 2θ between 5

and 40 with a step of 0.017. The diffractometer was

equipped with a copper tube (λ = 0.154 nm) with an intensity

of 40mA and a voltage of 45 kV. The Hot Disk TPS

2500S analyzer is a device for measuring the thermal

conductivity, thermal diffusivity, and thermal capacity of

homogeneous and heterogeneous materials. The probe

with a diameter of 2mm comprises a double nickel spiral

electrically insulated by an insulating Kapton coating.

The heating power was fixed at 20mW for 5 s to raise

the temperature of the studied material by a few

degrees. Each sample, a 3.5-mm-thick plate, is analyzed

five times for reproducibility. Dynamic mechanical

analysis measures the viscoelastic properties and provides

information on the molecular mobility of the

polymer chains. A rheometer (ARES LN2, Rheometrics)

was used in a rectangular torsion configuration. A fixed

deformation of 0.5% at a frequency of 1 Hz was

imposed by a motor at the lower part. The sensor connected

to the upper part measured the resulting stress

in the sample. The sample was heated in a furnace at a

temperature ramp of 3 Kmin
1. Each material was

tested once to determine the linear viscoelastic domain,

that is, in the range where the stress is proportional to

the strain applied by strain sweep at room temperature.

Then, a heating ramp was applied within the linear viscoelastic

domain from room temperature up to 300C.

3 | RESULTS AND DISCUSSION

3.1 | Characterization of fillers

The fillers formed aggregates before the mixing step

(Figure 1). The high shear viscosity of PEEK in the

melted state—approximately 5000 Pa s at the Newtonian

plateau—generates high stresses on the aggregates,

which leads to filler failure and erosion.38–40 It is known

that the dispersion increases with the duration of shear

until the creation of a percolating network, which results

in a drastic increase in the viscosity at low shear.41 The

TABLE 1 References of fillers used in the study. The letter N indicates