PEI-modified PEEK blends the chemical resistance, toughness, and wear performance of polyetheretherketone with the elevated glass transition temperature and dimensional stability imparted by polyetherimide, making accurate coefficient of linear thermal expansion (CLTE) measurement critical for material qualification. This article provides a comprehensive overview of PEI/PEEK blend CLTE testing, covering sample preparation, test items, standards-compliant methods such as ASTM E831, ISO 11359-2, and ASTM D696, typical test data, and key application areas in semiconductor, aerospace, and precision engineering industries.
Technical Overview
PEI-modified PEEK is a miscible polymer blend composed of polyetheretherketone (PEEK), a semi-crystalline high-performance thermoplastic with a glass transition temperature (Tg) of approximately 143°C and a melting point of approximately 343°C, and polyetherimide (PEI), an amorphous high-performance thermoplastic with a Tg of approximately 217°C. Because PEEK and PEI are thermodynamically miscible over a wide composition range, the blend exhibits a single, composition-dependent glass transition temperature between that of the two neat polymers, along with a modified crystallization behavior, improved stiffness retention at elevated temperature, and tunable dimensional stability.
The coefficient of linear thermal expansion (CLTE, also written as CTE or α) quantifies the fractional change in length of a material per unit temperature change, expressed in ppm/K (10⁻⁶/K) or 1/°C. For unfilled PEEK, the CLTE below Tg typically ranges from 45 to 55 ppm/K, rising sharply to 110–140 ppm/K above Tg as the amorphous phase enters the rubbery state. Introducing PEI raises the effective Tg of the blend and modifies both the low-temperature and high-temperature expansion segments, which is why CLTE testing is a fundamental quality-control and design-input requirement for PEI/PEEK grades used in precision components.
Accurate CLTE data for PEI-modified PEEK is essential for several engineering purposes:
- Predicting dimensional change of components across service temperature excursions and thermal cycling profiles.
- Designing press-fit and clearance dimensions in assemblies with metals, ceramics, or silicon (approximately 2.6 ppm/K).
- Assessing thermal stress at material interfaces in overmolded, bonded, or composite structures.
- Comparing blend ratios, filler systems, and processing conditions during material development and supplier qualification.
- Supporting failure analysis when warpage, loosening, or fatigue cracking is linked to thermal expansion mismatch.
Testing Samples
Sample geometry, orientation, and preparation history strongly influence CLTE results for PEI/PEEK because injection molding induces molecular orientation and through-thickness crystallinity gradients. Specimens should be documented with respect to grade, blend ratio, processing route, and direction of measurement (flow direction, transverse direction, or thickness direction).
Injection-Molded Rectangular Strips
Flat strips of approximately 6–10 mm in length, 3–5 mm in width, and 2–4 mm in thickness are the preferred geometry for thermomechanical analysis (TMA). The two end faces must be parallel and perpendicular to the measurement axis, with surfaces free of sink marks, gates, and flash. Specimens are typically cut from the narrow parallel sections of ISO 527 tensile bars or from dedicated edge-gated plaques.
Machined Coupons from Plates and Rods
For finished-goods verification, coupons can be machined from compression-molded plates, extruded rods, or actual components. Machining must avoid introducing residual stress or heating that could alter crystallinity; low-feed finishing cuts and sharp tooling are recommended. The measurement direction relative to the original molding flow direction must be recorded.
Films and Thin Sheets
PEI/PEEK films with thicknesses from 50 μm to 500 μm can be tested by TMA in extension mode or by stacking plies to achieve adequate specimen height. Film CLTE is highly sensitive to biaxial orientation introduced during extrusion or calendaring, so machine-direction and transverse-direction specimens should be tested separately.
Additively Manufactured Specimens
Fused filament fabrication (FFF) and laser powder bed fusion parts exhibit significant anisotropy. Specimens oriented along the print direction and perpendicular to it should be evaluated, and porosity should be characterized separately because voids depress the apparent expansion response.
Conditioning Requirements
PEI is moderately hygroscopic, and absorbed moisture plasticizes the blend, lowering the effective Tg and distorting the expansion curve near the transition. Unless otherwise specified, specimens should be dried (for example, 3 hours at 150°C or to constant mass in a desiccator) and then conditioned at 23°C and 50% relative humidity for a defined period before testing, with the conditioning state reported alongside the data.
Testing Items
A complete CLTE characterization program for PEI-modified PEEK typically includes the following test items:
- Mean CLTE below Tg, for example over 23–100°C, 23–150°C, or 0–50°C intervals.
- Mean CLTE above Tg, for example over 160–250°C, capturing the rubbery-state expansion.
- Mean CLTE over the full operating range, such as −40°C to 200°C, for thermal-cycling design.
- Glass transition temperature determined from the inflection point of the expansion curve (TMA) for cross-validation against DSC results.
- In-plane versus through-thickness CLTE to quantify anisotropy caused by flow-induced orientation and crystallinity gradients.
- Tangent (instantaneous) expansion coefficient as a function of temperature for finite-element thermal-stress modeling.
- Comparative CLTE across blend ratios, filler loadings (glass fiber, carbon fiber, mineral), or competitor grades.
- CLTE after thermal aging, annealing, or sterilization cycles to verify long-term dimensional stability.
Testing Methods
Two principal techniques are used for CLTE determination of PEI-modified PEEK: thermomechanical analysis and contact dilatometry. Both rely on precise measurement of dimensional change under a controlled temperature program.
Thermomechanical Analysis (ASTM E831 / ISO 11359-2)
TMA is the standard method for plastic CLTE measurement. A specimen with a height between 2 mm and 10 mm is placed on a flat quartz stage, and a low-mass expansion probe applies a small static force, typically 0.02–0.05 N, to maintain contact without deforming the polymer. The furnace is heated at 5°C/min under nitrogen purge (20–50 mL/min), and probe displacement is recorded against temperature. Common test ranges include −50°C to 300°C for full characterization, or focused windows such as 23–150°C and 150–260°C for segment-specific values.
The mean CLTE is calculated as:
α = (L₂ − L₁) / [L₀ × (T₂ − T₁)]
where L₀ is the initial specimen length at the reference temperature, and L₁ and L₂ are the lengths at temperatures T₁ and T₂. Tangent CTE at a specific temperature is obtained from the first derivative of the displacement curve. The instrument is calibrated for temperature using indium and zinc melting-point standards and for expansion using a reference material such as aluminum or sapphire. Typical measurement reproducibility for a well-prepared PEI/PEEK specimen is within ±5% for the sub-Tg segment and ±10% near the glass transition, where the expansion curve is inherently nonlinear.
Contact Dilatometry (ASTM D696)
For applications requiring a classic two-temperature mean CTE, ASTM D696 uses a vitreous silica dilatometer in which the specimen (50–125 mm in length) is held at 23°C and at −30°C using a constant-temperature bath, and the length change is measured with a dial gauge or LVDT. This method offers excellent precision for narrow-interval CTE but is unsuitable for capturing the transition region and is less commonly applied to high-temperature service evaluation of PEI/PEEK.
Recommended Test Sequence
- Verify specimen geometry, parallelism, and drying/conditioning history.
- Measure initial dimensions at 23°C with a calibrated micrometer to 1 μm resolution.
- Perform a conditioning or erasing ramp (for example, heating to just above the maximum test temperature at 5°C/min) when elimination of molded-in residual stress is required, then re-measure the reference length.
- Run the measurement ramp under nitrogen at 5°C/min, recording displacement continuously.
- Evaluate mean CLTE over specified intervals and the Tg inflection point using standardized software routines (tangent-intersection or inflection methods).
- Report at least three replicate specimens per direction, and state the standard, temperature interval, heating rate, probe force, and conditioning state with the results.
Typical Reference Data
Representative mean CLTE values for unfilled PEI/PEEK blends fall between those of the neat polymers: approximately 45–55 ppm/K below Tg and 110–160 ppm/K above Tg, with the step in expansion shifting upward as PEI content increases and the blend Tg moves toward 217°C. Glass-fiber-reinforced grades typically show in-plane CLTE of 20–30 ppm/K, while carbon-fiber-reinforced grades can reach 5–15 ppm/K in the flow direction with substantially higher through-thickness values. Actual results depend on composition, crystallinity, and orientation, so grade-specific measurement is strongly recommended.
Applications
PEI-modified PEEK materials with verified CLTE data are widely deployed where dimensional precision must be maintained under thermal load:
- Semiconductor test sockets, burn-in carriers, and wafer-handling trays, where the polymer must remain stable across repeated excursions above 150°C while minimizing mismatch with silicon and ceramic packages.
- Aerospace brackets, standoffs, insulators, and interior components requiring predictable fit and clearance over −55°C to 160°C service envelopes.
- Precision pump, valve, and bearing components in chemical processing, where expansion data feeds directly into running-clearance design.
- Medical device housings and sterilizable instrument components subjected to autoclave thermal cycling at 121–134°C.
- Electrical connectors, coil bobbins, and high-temperature insulating parts in automotive electrification platforms.
- Films and laminates used as high-temperature dielectric layers and flexible circuit substrates.
FAQ
What CLTE value should I expect for a typical 50/50 PEI/PEEK blend?
For an unfilled miscible blend, expect a mean CLTE of roughly 47–55 ppm/K below the blend Tg, with the transition occurring between 170°C and 195°C depending on composition, and a rubbery-state CLTE of approximately 120–160 ppm/K. Grade-specific testing is essential because crystallinity and thermal history shift these values.
Which standard should be specified on a drawing or purchase specification?
ASTM E831 or ISO 11359-2 (TMA method) is the standard choice for plastics, and the exact temperature interval must always accompany the standard, for example “CLTE, ASTM E831, 23–150°C.” ASTM D696 is acceptable when a classic two-point room-temperature-to-low-temperature value is required.
Why did my specimen show a different CLTE in the flow direction versus the transverse direction?
Molecular and filler orientation during injection molding creates anisotropic expansion. Flow-direction CLTE is usually lower than transverse or through-thickness CLTE, sometimes by 30% or more in reinforced grades. Both directions should be measured when the part has tight tolerance requirements.
Does moisture affect the CLTE result?
Yes. Water absorbed by the PEI phase plasticizes the blend, which can suppress the apparent Tg and alter the expansion curve near the transition. Drying specimens before testing and reporting the conditioning state ensures comparable, reproducible data.
How many specimens are needed?
A minimum of three replicate specimens per measurement direction is standard practice. For anisotropic or reinforced grades, at least two directions (flow and transverse) should be characterized.
Can CLTE testing also determine the glass transition temperature?
Yes. The TMA expansion curve exhibits a change in slope at Tg, and the inflection point of the tangent CTE curve provides a TMA-based Tg that is frequently cross-checked against DSC results for PEI/PEEK blends.
What sample sizes can the laboratory accept?
TMA specimens between 2 mm and 10 mm in height with parallel faces are ideal; dilatometry per ASTM D696 requires longer bars of 50–125 mm. Smaller coupons from finished parts can usually be accommodated by TMA with adapted fixtures, provided the faces are flat and parallel.