The glass transition temperature (Tg) is one of the most critical thermal parameters for polyimide (PI) insulation films, defining the temperature region where the material transitions from a rigid, glassy state to a softened, rubbery state. Accurate Tg determination by DSC, DMA, or TMA is essential for qualifying PI films used in flexible printed circuits, aerospace wiring, motor insulation, and high-temperature electronics. This technical guide explains the measurement principles, applicable standards, sample requirements, testing procedures, and typical results for PI film glass transition temperature determination.
Technical Overview
Polyimide (PI) insulation films are high-performance aromatic polymers synthesized from tetracarboxylic dianhydrides (such as PMDA or BPDA) and aromatic diamines (such as ODA or PPD), typically produced via a polyamic acid precursor followed by thermal imidization. These films combine exceptional thermal stability, outstanding dielectric strength, chemical resistance, radiation resistance, and mechanical toughness over a wide temperature range, making them the material of choice for demanding electrical insulation applications.
The glass transition temperature (Tg) is the temperature region in which the amorphous polymer changes from a hard, glassy state to a more compliant, rubbery state as chain segments gain molecular mobility. Below Tg, PI film exhibits high modulus, low coefficient of thermal expansion (CTE), and stable dielectric behavior. Above Tg, the storage modulus may drop by one to three orders of magnitude, the CTE increases significantly, and dielectric and barrier properties shift. Consequently, Tg represents a fundamental design boundary for structural and mechanical performance, even though PI films can continue to function as electrical insulation at temperatures above Tg due to their high thermal stability.
For common commercial PI films, Tg values typically range from approximately 300°C to 420°C depending on polymer chemistry and the measurement technique used. PMDA-ODA based films (Kapton HN type) generally show a DSC Tg near 360–390°C, while highly rigid BPDA-PPD based films (Upilex-S type) often exhibit a very weak or undetectable DSC step, requiring DMA measurement instead. Filled, modified, or copolymer films may show Tg anywhere between 250°C and 400°C, which is why accurate and method-specific determination is essential.
Testing Samples
Standard PMDA-ODA Polyimide Films
Conventional amber-colored PI films with thicknesses from 12.5 μm to 125 μm, including 25 μm and 50 μm grades widely used in flexible printed circuits and cable insulation. These films normally display a measurable but relatively small heat capacity step at Tg in DSC and a clear tan delta peak in DMA.
High-Modulus BPDA-Based Films
Low-CTE films designed for flexible copper-clad laminates and chip packaging applications. Their rigid, highly ordered chain structure yields an extremely small specific heat change at the glass transition, so DMA (mechanical) methods are strongly recommended for reliable Tg determination.
Specialty and Modified Films
Corona-resistant PI films containing inorganic nano-fillers for inverter-duty motor insulation, black or pigmented films, thermally conductive filled films, fluoropolymer-coated composite films, adhesive-coated coverlay films, and unknown or recycled lots requiring material identification and lot-to-lot quality verification.
Specimen Forms
Acceptable sample forms include free-standing film sheets, slit tape, strips cut from laminates, and specimens die-punched from supplied rolls. Specimens must be clean, flat, free of creases, and representative of the lot being evaluated.
Testing Items
The following measurement items are typically included in a PI film Tg characterization program:
- Glass transition temperature by DSC (onset, midpoint, and endset temperatures of the heat capacity step)
- Glass transition temperature by DMA (tan delta peak temperature and onset of storage modulus decline at a specified frequency)
- Glass transition temperature by TMA (inflection point of the dimensional expansion curve)
- Complete storage modulus (E') and tan delta spectra as a function of temperature
- CTE values below and above Tg
- Detection of enthalpic relaxation peaks and residual moisture or solvent effects
- Comparison of first-heating and second-heating scans to evaluate thermal history
Testing Methods
Differential Scanning Calorimetry (DSC)
DSC detects Tg as a reversible step in the heat flow baseline caused by the change in specific heat capacity. Testing follows ASTM E1356 or ISO 11357-2. A typical procedure uses 5–10 mg of film punched into discs, a nitrogen purge of 20–50 mL/min, and a heat–cool–reheat cycle at 10–20°C/min from room temperature up to 420–450°C. The second heating scan is normally reported to erase prior thermal history. Because the ΔCp step of fully aromatic PI is small, slower heating rates, optimized baseline subtraction, and experienced interpretation are required to distinguish the true transition from baseline drift or incipient degradation.
Dynamic Mechanical Analysis (DMA)
DMA is the most sensitive method for PI films and follows ASTM E1640 or ISO 6721-11. Film strips (typically 5–6 mm wide and 30–40 mm long) are tested in tension mode with a gauge length of 10–15 mm, a frequency of 1 Hz, a strain amplitude of 0.05–0.1%, and a heating rate of 3–5°C/min. Tg is reported as the tan delta peak temperature, along with the onset temperature of the storage modulus drop. When results at multiple frequencies are required, activation energy of the relaxation can also be derived.
Thermomechanical Analysis (TMA)
TMA determines Tg from the inflection point in the dimensional change curve as the material expands at a higher rate above Tg, following ASTM E1545 (expansion method) or ASTM E1824 (penetration method). A small static force and a 5–10°C/min heating rate under nitrogen are typical. TMA simultaneously provides CTE values on both sides of the transition, which is valuable for packaging and laminate design.
Sample Conditioning and Reporting
PI films absorb 1–3% atmospheric moisture, which plasticizes the polymer and can shift the apparent Tg. Specimens should therefore be conditioned consistently, for example by drying at 120–150°C for 1–3 hours or conditioning at 23°C/50% RH, and the conditioning state must be documented. Final reports must state the test method, heating rate, frequency (for DMA), specimen mass or dimensions, conditioning history, and the specific Tg criterion used (onset, midpoint, or tan delta peak), because Tg values from different methods are not interchangeable.
Applications
Tg determination of PI insulation films supports a wide range of engineering and quality functions, including:
- Qualification and incoming inspection of PI film for flexible printed circuits, flexible copper-clad laminates, and coverlay materials
- Verification of corona-resistant PI films used in inverter-fed traction motors, generators, and form-wound coil insulation
- Material qualification for aerospace wire and cable insulation subjected to high ambient temperatures
- Transformer interlayer insulation, bushings, and high-temperature barrier films
- EV battery busbar and cell insulation, where thermal margins are safety-critical
- Research and development of new polyimide chemistries, copolymers, and nano-composite films
- Failure analysis and assessment of thermal aging, physical aging, or moisture exposure in service-aged films
- Lot-to-lot consistency control and supplier comparison in semiconductor and electronics manufacturing
FAQ
Why do DSC and DMA give different Tg values for the same PI film?
The two techniques measure different physical responses. DSC detects the thermodynamic heat capacity step, while DMA measures the mechanical relaxation associated with the onset of segmental mobility, which is frequency-dependent and typically occurs at a higher temperature. For PI films, DMA tan delta peak temperatures commonly appear 20–50°C higher than DSC midpoint values. The method and criterion must always accompany the reported number.
Can every PI film be measured by DSC alone?
No. Highly rigid, fully aromatic structures such as BPDA-PPD films show an extremely small heat capacity change at Tg, often within the noise of the DSC baseline. In these cases, or when the transition overlaps with degradation exotherms, DMA is the recommended reference method because the modulus drop is large and unambiguous.
What are typical Tg values for common PI films?
PMDA-ODA films generally show DSC midpoints near 360–390°C and DMA tan delta peaks near 380–420°C. BPDA-based low-CTE films are typically reported above 350°C by DMA. Filled corona-resistant films and modified copolymers may range from about 250°C to 400°C depending on formulation.
Does absorbed moisture affect the measured Tg?
Yes. Water acts as a plasticizer for polyimide and can depress the apparent Tg by several degrees to tens of degrees and broaden the transition. Standardized drying or conditioning before testing, combined with reporting the conditioning state, ensures comparable results between laboratories.
How is Tg related to the continuous operating temperature of PI film?
Tg is not the same as the maximum continuous use temperature or the thermal class (temperature index) defined by long-term thermal endurance testing. PI films retain excellent dielectric performance above Tg, but mechanical modulus, dimensional stability, and creep resistance degrade. Designers should use Tg as the limit for load-bearing and dimensional stability requirements and rely on thermal endurance data for insulation life assessment.
How much sample is required for testing?
DSC typically requires 5–10 mg of film, equivalent to a few square centimeters of 25 μm material, while DMA requires strips approximately 30–40 mm long and 5–6 mm wide. TMA specimens of a few millimeters square are sufficient. All specimens should be clean, flat, and free of surface contamination.
Why is the second heating scan used for DSC reporting?
The first heating scan reflects the thermal and mechanical history of the film, including residual stress, moisture, and enthalpic relaxation peaks that can distort or shift the apparent transition. Cooling in a controlled manner and recording the second heating scan yields an intrinsic, reproducible Tg characteristic of the material itself.