Introduction
PEEK is an engineering plastics material characterized as an aromatic thermoplastic; its main chain contains a repeating unit consisting of a ketone bond and two ether bonds. It has high mechanical strength, is flame retardant, and has good electrical properties as well as good resistance to heat, impact, acid and alkali, hydrolysis, abrasion, fatigue, irradiation, etc. It can be used as a high-temperature-resistant structural material and electrical insulating material, but also as a compositereinforcing material when combined with glass fiber or carbon fiber, offering wide applications in the aerospace, medical device (as artificial bone to repair bone defects) and other industrial fields.
PEEK shows the typical behavior of semi-crystalline polymer materials; its Crystallinity / Degree of CrystallinityCrystallinity refers to the degree of structural order of a solid. In a crystal, the arrangement of atoms or molecules is consistent and repetitive. Many materials such as glass ceramics and some polymers can be prepared in such a way as to produce a mixture of crystalline and amorphous regions.crystallinity and crystalline morphology are greatly influenced by the thermal history during processing, which then affects its properties, such as mechanical or optical properties. Therefore, studying the CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization and melting process of PEEK is of great practical significance.
Temperature-Modulated DSC (TM-DSC)
TM-DSC is an expansion of the traditional differential scanning calorimetry (DSC) technique. This technique superimposes a sinusoidal temperature wave on the linear temperature ramp, which yields a corresponding oscillating heat-flow curve of the sample. This oscillating heat-flow curve is then separated into two extra curves: the reversing and non-reversing heat-flow curves. Thermal effects related to the change in a material’s heat capacity are on the reversing curve; these typically include the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition, the Curie transition, secondorder Phase TransitionsThe term phase transition (or phase change) is most commonly used to describe transitions between the solid, liquid and gaseous states.phase transitions, and the change in heat capacity before and after reactions. Kinetic effects will be on the non-reversing curve, the reaction rate of which depends on the temperature and conversion rate, but not on the heating rate; e.g., cold CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization, cross-CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization, Curing (Crosslinking Reactions)Literally translated, the term “crosslinking“ means “cross networking”. In the chemical context, it is used for reactions in which molecules are linked together by introducing covalent bonds and forming three-dimensional networks.curing effects, etc. For polymers, TM-DSC is usually used to separate the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition with superimposed thermal effects such as enthalpy RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation, cross-linking Curing (Crosslinking Reactions)Literally translated, the term “crosslinking“ means “cross networking”. In the chemical context, it is used for reactions in which molecules are linked together by introducing covalent bonds and forming three-dimensional networks.curing, and solvent volatilization; a more accurate Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition temperature can then be obtained.
Applying TM-DSC in melt and CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization is complex and controversial. It’s proven that the melting effect cannot be separated as either reversible or nonreversible effects alone, and the separation result varies with test parameters; this is because melting is not a pure heat capacity effect or kinetic effect. However, some related publications have proven that TM-DSC is still useful in this research field; e.g., on the non-reversing curve, one can often observe an extra ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermal peak, which is frequently attributed to the recrystallization of a secondary crystalline phase. Those secondary crystals melt at lower temperatures; then, the free polymer chains attach to the surface of primary crystal grains where they recrystallize and release heat.
Note
Secondary crystal: usually with small grains, relatively imperfect lattice structure, somewhat disorderly molecular chain arrangement, and relatively lower Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting temperature
Primary crystal: usually with thicker plates, more complete crystal structure, well-arranged molecular chains, and higher Melting Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting temperature
In this Application Note, TM-DSC was used to study the Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition, cold CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.crystallization and melting, recrystallization and remelting processes of PEEK film samples.
Measurement Conditions
The sample was a PEEK film. Sample preparation (figure 1) consisted of punching out a series of small discs of film (approx. 5 mg) using a punching device, inserting them into an aluminum Concavus® crucible, and covering the crucible with a slide-in lid (the slide-in lid is an embedded crucible lid which can press onto the loose film firmly in order to improve the thermal contact).
The test atmosphere wasN2 (50 ml/min), and TM-DSC was chosen as the test mode.
Measurement Results
The thermal effects of the sample included two stages:
1st stage: below 210°C; Glass Transition TemperatureThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or rubbery.glass transition and cold CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released. crystallization
2nd stage: above 210°C; melting, recrystallization and remelting
Different modulation parameters were used for the two stages to obtain better results:
Parameters in the1st stage: heating from 100°C to 210°C at 2 K/min, period 30 s, amplitude 0.5 K.
Parameters in the2nd stage: heating from 210°C to 400 °C at 2 K/min, period 60 s, amplitude 0.32 K.
The raw TM-DSC signals are shown in figure 2.
The results of the glass transition and cold CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released. crystallization are presented in figure 3. The RelaxationWhen a constant strain is applied to a rubber compound, the force necessary to maintain that strain is not constant but decreases with time; this behavior is known as stress relaxation. The process responsible for stress relaxation can be physical or chemical, and under normal conditions, both will occur at the same time. relaxation peak (peak 143.4°C) and the cold CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released. crystallization peak (peak 161.5°C) are shown on the non-reversing DSC curve (red curve). The glass transition (Tg 143.8°C (midpoint)) can be seen on the reversing DSC curve (green curve). Besides that, the reversing curve also shows a slight drop (0.043 J/g*K) in the Specific Heat Capacity (cp)Heat capacity is a material-specific physical quantity, determined by the amount of heat supplied to specimen, divided by the resulting temperature increase. The specific heat capacity is related to a unit mass of the specimen.specific heat capacity during cold CrystallizationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released. crystallization.
This is due to more molecular chains being bounded to the crystalline region after cold crystallization, so the vibrational freedom of the chains decreases, and then the Specific Heat Capacity (cp)Heat capacity is a material-specific physical quantity, determined by the amount of heat supplied to specimen, divided by the resulting temperature increase. The specific heat capacity is related to a unit mass of the specimen.specific heat capacity decreases.
The results of melting, recrystallization and re-melting are presented in figure 4. The total DSC curve (blue curve) just shows a huge EndothermicA sample transition or a reaction is endothermic if heat is needed for the conversion.endothermic peak (peak 344.9°C), as well as a minor ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic peak (270°C (peak temperature)). More information can be found after separation of the total DSC curve into the reversing DSC curve (green curve) and non-reversing DSC curve (red curve). There is a broad EndothermicA sample transition or a reaction is endothermic if heat is needed for the conversion.endothermic peak (342.7°C (peak temperature)) on the reversing DSC curve, which contains melting of the secondary crystals, remelting after recrystallization of the secondary crystals, and melting of the primary crystals [1]. The EndothermicA sample transition or a reaction is endothermic if heat is needed for the conversion.endothermic peak (346.6°C) on the non-reversing DSC curve represents the melting of a portion of the primary crystals [1]. In addition, the ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic peak (peak 329.2°C) on the non-reversing DSC curve corresponds to recrystallization after melting of the imperfect secondary crystals [1]. The heat-flow signals of the EndothermicA sample transition or a reaction is endothermic if heat is needed for the conversion.endothermic effect of melting and the ExothermicA sample transition or a reaction is exothermic if heat is generated.exothermic effect of recrystallization partially overlapped, so it is possible that the area of each peak is smaller than the actual value.
Conclusion
Using the TM-DSC method, it was possible to separate the reversing and non-reversing thermal effects. For the PEEK sample, more information about melting, crystallization and remelting was gleaned.