Material researchers, design engineers, and process engineers face critical decisions about PEEK crystallinity. This guide explains how crystallinity impacts material performance, processing parameters, and application suitability—enabling informed material selection and optimisation decisions.
- For researchers: Comparative frameworks for semi-crystalline vs amorphous polymers with quantitative property correlations.
- For engineers: Material selection criteria with numerical performance thresholds for mechanical, thermal, and chemical properties.
- For processors: Troubleshooting guidance for injection moulding temperature control and crystallinity optimisation.
**Download our PEEK crystallinity comparison chart** - Visual reference guide comparing semi-crystalline and amorphous properties.
The degree of crystallinity has a significant effect on the properties of a polymer. Semi-crystalline polymers like PEEK show an improvement in mechanical properties above the glass transition temperature, Tg, for example, with increased strength, stiffness and toughness compared to amorphous polymers. The crystalline domains essentially act as a kind of reinforcement of the rubbery amorphous phase above Tg, allowing semi-crystalline polymers to be used at higher temperatures.
Additionally, the crystalline regions scatter light to a greater degree, reducing optical transparency. Semi-crystalline polymers are therefore generally opaque, unless the size of the crystallites therein is smaller than the wavelength of visible light, as can happen for example in highly nucleated polypropylenes. Understanding and controlling crystallinity is therefore critical for tailoring the behaviour of polymers for different applications.
Crystals have an ordered and regular structure and have a well-defined melting point—think about ice, salt, sugar, amethyst. Amorphous (from those Greeks again, meaning "without form") materials, like glass, have disordered structures which when heated up gradually soften before becoming viscous liquids.
Are polymers ever fully crystalline? Understanding Semi-Crystalline Polymer Structure.
Polymers can be amorphous or crystalline, depending upon a number of factors including the shape of their repeat unit, their molecular weight and their thermal history.
Crystallinity in polymers is more complex than in other materials because of the long polymer chains. In some areas, the polymer chains align and pack together to form crystals; in other areas, the chains are disordered and cannot pack together, so semi-crystalline polymers tend to have crystalline regions dispersed within amorphous material.
Strictly speaking, crystalline polymers are rarely, if ever, fully crystalline; they are semi-crystalline because there are always some amorphous regions caused by those long chains never fitting together perfectly.
Denser crystalline versus amorphous regions in polymers
The crystalline regions, being more closely packed, are denser than the amorphous regions, so if you know the density of the polymer crystal, you can calculate the degree of crystallinity of a polymer from its overall density. Typically, high-performance polymers of over about 25% crystallinity display the characteristics of a semi-crystalline polymer.
Semi-Crystalline versus Amorphous Polymers: Property Comparison
Generally, amorphous and semi-crystalline polymers have distinct characteristics:
| Amorphous Polymers | Semi-crystalline Polymers |
| Softens with increasing temperature | Well-defined melting point |
| Good formability | Poor formability |
| High transparency | Opaque |
| Poor chemical resistance | Good chemical resistance |
| Bonds well using adhesives | Difficult to bond with adhesives |
| Poor fatigue resistance | Good fatigue resistance |
| Poor wear resistance | Good wear resistance |
| Used mainly in structural applications | Used in bearing, wear & structural applications |
PEEK Carbon-fibre Composite Trauma Plate
In medical device applications for example, PEEK-OPTIMA™ Ultra-Reinforced, a carbon fibre-reinforced PEEK (CFR-PEEK) has attracted interest. The material can offer the strength of metal in a composite polymer, suitable for trauma plates and nails, while remaining lightweight. With its favourable modulus of elasticity, it seems to provide the potential for enhanced stress absorption and load-bearing by allowing the design of a less stiff construct, which may support the healing process. It has also been shown to have a 50 times greater fatigue life than equivalent metal plates.*
Metal-free distal femur fracture fixation plate made with PEEK-OPTIMA Ultra-Reinforced polymer, a carbon fibre composite from Invibio, Victrex medical company. The trauma plate shown is not available for distribution or implantation - © Invibio
Semi-crystalline PEEK Applications Across Industries
Let's take a look at some of the opportunities that semi-crystalline PEEK and PAEK polymers and compounds can open up for different industries:
- In automotive, excellent strength and high heat resistance allow semi-crystalline PAEK materials to be used for structural components and parts under the hood. Due to their stiffness and wear resistance, they are also a great solution for gears, thrust washers and seal rings in transmission and drivetrain applications.
- In the oil and gas industry, PEEK and PAEK-based materials are used effectively in applications such as in sealing systems, electrical connectors, pump components, and other parts that must withstand extreme temperatures, high pressures, and chemically aggressive environments.
- The food industry is also one of the sectors that benefits from using PEEK and PEEK compounds. It can be used to replace metal parts or PFTE-polymers within food processing equipment, as it can improve hygiene and corrosion resistance.
- PAEK polymers and compounds in aerospace have been shown to be an effective solution for structural brackets, interior components, and other applications where weight savings and resistance to burning are critical.
As demonstrated, the unique properties of semi-crystalline PEEK offer a range of benefits in these, and many other challenging applications.
Injection Moulding Temperature Control: Parameters for PEEK Crystallinity
Injection moulded PEEK is typically about 35% crystalline. Proper attention must be paid to the temperature of the injection mould tool to ensure it is hot enough to produce parts of consistent crystallinity (temperatures in the range 170 - 200°C are typically used in our laboratories): a common mistake is to use mould temperatures which are too low and this can result in parts with darker, amorphous skins.
Here's the difference in extremis on some test specimens:
Impact of mould temperature on PEEK processing
On real components which are more complex shapes than test specimens, this sort of appearance is typical if the mould temperature is too low:
PEEK component injection moulded at too low temperature.
So you can see PEEK's crystallinity is influenced by how it is processed and especially by how it is cooled from the melt.
If you cool it really quickly—at more than 700°C/minute—the polymer chains do not get time to order themselves into crystalline domains, so you will make an amorphous PEEK. Victrex does this deliberately for some of its APTIV™ film products used in thermoforming applications. Consistent with the table above, the transparent, amorphous APTIV film softens and is easily formable at temperatures around 180°C, at which point it spontaneously re-crystallises (as the polymer chains are sufficiently free to move and re-organise themselves into crystalline domains at this temperature) and it becomes more opaque. Compare that to crystalline APTIV film, which can only be formed at temperatures above about 330°C.
APTIV PEEK Polymer film. © Victrex
**Need PEEK processing guidance?** Our comprehensive Injection Moulding Guide covers advanced troubleshooting scenarios, grade-specific processing windows, and design-for-manufacturing considerations.
Post-Processing: Secondary Crystallisation of PEEK
Secondary crystallisation of PEEK during annealing—scientific debate ongoing. It is also quite common in some applications—like extruded rods, shapes and plates—to post-anneal the parts, a process which can result in increased crystallinity. Again, annealing takes place at higher temperatures, typically around 230°C, and some re-crystallisation occurs, leading to products with up to 40% crystallinity.
Additive manufactured PEEK parts, made by Selective Laser Sintering, can have around 45% crystallinity or more.
Secondary crystallisation of PEEK during annealing has been debated for many years and even today the detailed mechanism is still the subject of scientific debate!
Frequently asked questions about PEEK crystallinity
What is polymer crystallinity?
Polymer crystallinity is the degree to which polymer chains align into ordered, repeating structures called crystallites. Semi-crystalline polymers contain both crystalline regions (ordered) and amorphous regions (disordered). The crystalline fraction typically ranges from 25-45% in PEEK, significantly affecting mechanical properties, chemical resistance, and optical transparency.
Crystalline regions form when polymer chains pack together in regular arrangements during cooling from the melt. The crystallisation rate depends on cooling speed, molecular structure, and processing parameters like mould temperature.
How do semi-crystalline polymers differ from amorphous polymers?
The comparison table above shows key distinctions. Semi-crystalline polymers like PEEK have well-defined melting points (343°C for PEEK) and maintain strength above their glass transition temperature through crystalline reinforcement. Amorphous polymers gradually soften with increasing temperature and offer better formability but poorer chemical resistance.
For applications requiring wear resistance, chemical resistance, or fatigue performance, semi-crystalline structures provide significant advantages through their ordered molecular domains.
What processing parameters affect PEEK crystallinity?
Three primary parameters control PEEK crystallinity:
- Mould temperature: 170-200°C range produces consistent 35% crystallinity in injection moulding. Temperatures below 150°C create darker, amorphous surface layers.
- Cooling rate: Rates above about 700°C/minute can prevent crystallisation, producing amorphous PEEK but there are limitations with thicker parts where the heat cannot be removed from the centre of the part quickly enough to prevent some crystallisation. Slower cooling allows chain ordering into crystalline domains.
- Annealing conditions: Post-processing, for example, annealing in an oven at 230°C can increase crystallinity up to ~40% through secondary crystallisation, though detailed mechanisms remain scientifically debated. For thick parts, this process can also relieve frozen-in residual stresses.
Additive manufactured PEEK parts made by Selective Laser Sintering can achieve 45%+ crystallinity depending on thermal cycles.
When should I choose semi-crystalline PEEK over amorphous polymers?
Choose semi-crystalline PEEK when applications require:
- Chemical resistance: Crystalline domains provide reduced permeability to aggressive chemicals.
- Wear and fatigue resistance: Ordered structures are harder and can resist mechanical deformation.
- High-temperature performance: Crystalline reinforcement maintains properties above Tg (143°C).
- Bearing applications: Low friction coefficient with excellent wear characteristics.
Choose amorphous polymers when:
- Transparency required: Optical clarity essential (though not achievable with PEEK unless in a thin film format such as in some APTIVTM products).
- Complex forming: Easier thermoforming at lower temperatures (typically above Tg but well below Tm, which induces the parts to crystallise.
- Adhesive bonding: Better surface wetting for bonding applications.
For medical implants, aerospace brackets, oil and gas seals, and automotive transmission components, semi-crystalline PEEK's combination of properties offers optimal performance.
- Data on file at Invibio. Mechanical Benchmark of Carbon Fibre PEEK-OPTIMA™ Ultra-Reinforced vs Ti 6AI-4V Plates undergoing Static Testing per ASTM F382-99 (2008)
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