PEEK Insulation Film Crystallinity Analysis: Testing Methods, Standards, and Applications

PEEK insulation film crystallinity analysis is essential for verifying the thermal, mechanical, and dielectric performance of polyetheretherketone films used in high-temperature electrical and electronic insulation. By quantifying the degree of crystallinity through DSC, XRD, and density testing, manufacturers can control film quality, optimize annealing processes, and guarantee long-term reliability in demanding applications. This article explains the sample types, testing items, analytical methods, and typical applications involved in PEEK film crystallinity characterization.

Technical Overview

PEEK (polyetheretherketone) is a semi-crystalline aromatic thermoplastic widely used as a high-performance insulation film in flexible electronics, aerospace wiring, electric motors, and energy storage systems. The polymer exhibits a glass transition temperature (Tg) of approximately 143 °C and a melting temperature (Tm) of approximately 343 °C, with a continuous-use temperature rating of up to 260 °C. Depending on the melt-processing conditions and thermal history, the degree of crystallinity of PEEK film can range from nearly amorphous (below 10%) to approximately 40%.

For insulation-grade film, crystallinity is one of the most critical quality attributes because it directly governs mechanical strength, dielectric strength, heat-shrinkage behavior, dimensional stability above Tg, and resistance to solvents and chemicals. Rapid quenching during film extrusion or casting produces a largely amorphous film with high flexibility, while controlled cooling or post-annealing produces a semi-crystalline film with improved stiffness, barrier properties, and thermal endurance. Crystallinity analysis therefore serves as the core analytical link between film-processing parameters and end-use electrical performance.

• Melt temperature and residence time during extrusion influence nucleus formation and crystal perfection.
• Cooling rate is the dominant factor: fast quenching suppresses crystallization, while slow cooling promotes spherulitic growth.
• Annealing between Tg and Tm (typically 200–300 °C) increases crystallinity and eliminates latent cold crystallization.
• Uniaxial or biaxial stretching induces strain crystallization and molecular orientation, which alters both XRD and DSC responses.
• Fillers, pigments, and nucleating agents shift crystallization kinetics and must be corrected for in enthalpy calculations.

Testing Samples

Neat Unfilled PEEK Insulation Film

Standard commercial-grade PEEK film in typical insulation thicknesses of 5–250 μm, produced by slit-die extrusion, blown-film, or solvent-free casting processes. These samples represent the as-delivered state and are analyzed first without any thermal preconditioning so that the original crystallinity is preserved.

Quenched Amorphous Film

Film chilled rapidly on a cold roll immediately after extrusion, exhibiting low crystallinity (often below 10%) and a distinct cold-crystallization exotherm in the first DSC heating scan. These samples are used to evaluate crystallization potential and residual process stress.

Annealed Semi-Crystalline Film

Film subjected to a defined annealing profile above Tg, targeting a crystallinity of approximately 30–35%. These samples are used to verify annealing uniformity across web width, roll length, and through the film thickness.

Oriented Stretched Film

Uniaxially or biaxially oriented film in which strain-induced crystallites and chain orientation coexist. Analysis of these samples requires diffraction-based methods to separate crystallinity from orientation effects.

Filled, Pigmented, and Modified Films

PEEK films containing colorants, PTFE lubricants, or mineral fillers. The filler weight fraction must be determined (for example by ash testing or TGA) so that the polymer-fraction enthalpy can be corrected during DSC crystallinity calculation.

Sample preparation requirements: DSC specimens are die-cut discs of 3–10 mg; XRD specimens are flat pieces of at least 10 mm × 10 mm free of wrinkles; density specimens must be void-free. All samples are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours, and no drying above Tg is permitted unless the thermal history study specifically requires it.

Testing Items

• Degree of crystallinity (Xc, %) by DSC, XRD, and density methods.
• Melting temperature (Tm) and melting enthalpy (ΔHm), including detection of double melting peaks.
• Glass transition temperature (Tg) and the width of the transition region.
• Cold-crystallization temperature (Tcc) and cold-crystallization enthalpy (ΔHcc).
• Secondary melting or annealing peaks that reveal low-temperature thermal treatment or physical aging.
• Crystalline diffraction peak positions (2θ), peak widths, and estimated crystallite size by XRD.
• Bulk density and crystallinity derived from the two-phase density model.
• Crystallinity uniformity across the web, along the machine direction, and through the film thickness.
• Filler content correction and polymer mass fraction for filled films.

Testing Methods

Differential Scanning Calorimetry (DSC)

DSC is the primary reference method, performed under nitrogen purge at a heating rate of 10 °C/min over a range from ambient to approximately 400 °C in accordance with ISO 11357 and ASTM D3418. The first heating scan reflects the as-processed crystallinity of the film, while a second heating scan after controlled cooling evaluates the intrinsic crystallization capability of the material. The mass fraction crystallinity is calculated as: Xc (%) = [(ΔHm − ΔHcc) / ΔHm°] × 100 / (1 − w), where ΔHm° is the melting enthalpy of a 100% crystalline PEEK reference taken as 130 J/g, and w is the weight fraction of non-melting filler. Double melting peaks are interpreted with care, as they usually reflect melt-recrystallization during the scan or two crystal populations of different perfection rather than two separate phases.

Wide-Angle X-ray Diffraction (XRD/WAXS)

XRD provides a non-thermal measure of crystallinity that leaves the sample unaltered, making it ideal for oriented and quenched films. Using Cu Kα radiation, semi-crystalline PEEK shows characteristic reflections at approximately 18.7°, 20.6°, 22.7°, and 28.7° (2θ) superimposed on a broad amorphous halo near 20°. After peak deconvolution and baseline subtraction, the crystallinity index is obtained from the ratio of the integrated crystalline peak area to the total scattering area. Peak half-width analysis further yields apparent crystallite size, and pole-figure measurements on oriented film quantify the Herman orientation factor.

Density Gradient Column Method

Bulk density is measured in a calibrated density gradient column following ASTM D1505 or by hydrostatic weighing per ISO 1183. Crystallinity is then derived from the two-phase model: Xc (%) = [ρc × (ρs − ρa)] / [ρs × (ρc − ρa)] × 100, where ρs is the measured sample density, ρa = 1.263 g/cm³ for fully amorphous PEEK, and ρc = 1.400 g/cm³ for the perfect crystal. The method is fast and precise, but results are invalidated by voids, moisture, or filler content.

Supplementary Techniques

FTIR crystallinity indices based on crystallinity-sensitive bands relative to an internal reference band enable fast surface mapping and through-thickness profiling using micro-ATR. Small-angle X-ray scattering (SAXS) resolves lamellar long period and lamellar thickness, which correlate with annealing conditions. Polarized light microscopy on microtomed cross-sections reveals spherulite size and morphology distribution across the film thickness.

Cross-Validation and Reporting

Because DSC, XRD, and density methods probe crystallinity through different physical properties, values may differ by a few percentage points. Professional reports always state the method, scan conditions, enthalpy reference, filler correction, and whether first-heat or second-heat data are quoted. Cross-validation between at least two methods is recommended for lot release of safety-critical insulation film.

Applications

• Flexible printed circuits: crystallinity control balances the flexural endurance of cover films against the heat resistance required during reflow soldering.
• Aerospace and high-temperature wire insulation: a stable, well-developed crystalline phase ensures cut-through resistance and dielectric integrity at continuous operating temperatures up to 260 °C.
• Electric vehicle traction motors: PEEK films with optimized crystallinity serve as slot liners, phase insulation, and hairpin conductor wraps combining thin-gauge toughness with thermal class H+ performance.
• Lithium-ion battery insulation: precisely crystallized films provide barrier properties and dimensional stability in cell-level barrier and encapsulation layers.
• Acoustic diaphragms: controlled crystallinity tuning allows the modulus and damping of PEEK membranes to be engineered for miniature speaker applications.
• Semiconductor carrier and release films: annealed, low-shrinkage films maintain flatness through repeated high-temperature processing.
• Transformer, capacitor, and general electrical insulation where chemical resistance combined with stable dielectric properties is required.

FAQ

What is the typical crystallinity of PEEK insulation film?

Rapidly quenched film is nearly amorphous (usually below 10%), standard extruded film falls between 15% and 30%, and properly annealed film reaches approximately 30–35%. Values above 40% generally indicate very slow cooling or prolonged high-temperature annealing and may reduce film toughness.

Why is 130 J/g used in the DSC calculation?

This is the accepted melting enthalpy of a 100% crystalline PEEK reference crystal, established from studies of polymer of well-defined crystalline content. Dividing the measured net enthalpy by this reference converts the enthalpy directly into a mass-fraction crystallinity.

Which method gives the most accurate crystallinity value?

DSC is the internationally recognized reference method because it follows ISO 11357 and ASTM D3418 and directly measures the crystalline enthalpy. XRD is preferred for oriented or very thin films because it avoids thermal alteration, while density measurement offers excellent precision for routine quality control on unfilled, void-free film.

Why does my DSC curve show two melting peaks?

Double melting behavior in PEEK is usually caused by melt-recrystallization during the heating scan, in which imperfect crystals melt, recrystallize, and remelt at a higher temperature. It can also indicate two crystal populations produced by the processing and annealing history, and a small low-temperature peak can reveal a secondary annealing treatment.

How does crystallinity affect the dielectric properties of the film?

The dielectric constant of PEEK remains close to 3.2–3.3 across the normal crystallinity range, but higher crystallinity improves dielectric strength, reduces high-temperature deformation, and greatly enhances resistance to solvent swelling and chemical attack, all of which are decisive for long-term insulation reliability.

How can film crystallinity be increased after extrusion?

Annealing the film under slight tension at 200–300 °C, that is, above Tg but well below Tm, allows additional crystallization and crystal perfection to develop. The annealing temperature and time are optimized by DSC monitoring until the cold-crystallization exotherm disappears and the target crystallinity plateau is reached.

What sample size is required for crystallinity testing?

For DSC, a die-cut disc or folded section of 3–10 mg is sufficient. XRD requires a flat specimen of at least 10 mm × 10 mm, and density testing requires several void-free pieces of a few tens of milligrams each. Full-thickness and full-width mapping studies require additional specimens sampled systematically across the film web.