Introduction
Differential scanning calorimetry is a thermal analysis technique used to study the phase transition behavior of materials by measuring the heat difference between the sample and the reference object under the temperature control of the program. As a kind of material with liquid crystal orderliness and polymer processing characteristics, the phase transition temperature of liquid crystal polymer is a key parameter that determines the application performance of the material. The purpose of this paper is to elaborate the principles, methods and data analysis points of determining the phase transition temperature of liquid crystal polymers by differential scanning calorimetry.
Rationale
When the differential scanning calorimeter is working, the sample and the inert reference are placed in an independent crucible and heated or cooled under the same temperature program. When a sample undergoes a phase change, such as a liquid crystal polymer changing from a crystalline state to a liquid crystal state, or a liquid crystal state to an isotropic liquid state, it will be accompanied by heat flow changes. The relationship between heat flow difference and temperature is recorded by the instrument, which is the DSC curve. Phase transition temperatures are usually determined by curve feature points, such as the start point, peak point, or extrapolation point.
The heat flow difference ΔQ is related to the change in heat capacity and can be expressed as:
ΔQ = Cp × (dT/dt)
where Cpis the heat capacity, and dT/dt is the heating rate. The multiple phase changes of liquid crystal polymers often manifest as multiple endothermic or exothermic peaks.
Test methodology
Instrument calibration is required before testing, and reference materials such as indium and tin are commonly used to calibrate the temperature and thermal enthalpy. Sample preparation should be representative, usually a few milligrams of powder or sections, evenly placed in a crucible. The selection of test conditions should consider the material characteristics: the heating rate is often 5-20°C/min, and the nitrogen atmosphere is protected to prevent oxidation. For a clear phase change signal, multiple cycles of heating-cooling can be performed to eliminate thermal history.
Data analysis
Typical liquid crystal polymer DSC curves may show glass transitions, crystal-liquid crystal state transitions, and liquid crystal-isotropic transitions. The determination method of each transition temperature is as follows: the vitrification transition takes the midpoint of the curve step; The first-order phase transition is like melting to take the peak temperature, which can also be combined with the start point analysis. For overlapping peaks, they can be processed with peak splitting software. The transition enthalpy is calculated by integrating the peak area using the formula:
ΔH = K × A / m
where K is the calibration factor, A is the peak area, and m is the sample mass.
| Change type | D-curve features |
| Vitrification transformation | Baseline steps, heat capacity mutations |
| Crystalline - liquid crystal state | endothermic peaks, which may be accompanied by enthalpy changes |
| Liquid crystal state - isotropic state | endothermic peaks, usually with a small enthalpy value |
Influencing factors
The rate of temperature rise affects the peak shape and temperature: an increase in the rate may cause the peak temperature to shift towards high temperature and the peak shape to widen. Large sample volumes are prone to temperature gradients, so it is recommended to use small mass samples. Sample morphology such as crystallinity and orientation will affect peak position and intensity. Under atmospheric conditions, oxidizing atmospheres may trigger decomposition reactions and interfere with phase transition signals.
Application examples
Taking an aromatic polyester liquid crystal polymer as an example, DSC tests show that there is a glass transition step around 120°C, an endothermic peak that changes from crystalline to nematic liquid crystal at 280°C, and an endothermic peak that changes from liquid crystal state to isotropic state at 350°C. Through multiple cycle tests, the reversibility of each transition can be confirmed and the material processing temperature window can be informed.
Summary
Differential scanning calorimetry is an effective tool for characterizing the phase transition temperature of liquid crystal polymers. Accurate transition temperature and thermodynamic parameters can be obtained through optimized test conditions and rigorous data analysis. These data are of reference significance for understanding the structure-performance relationship of liquid crystal polymers and guiding the synthesis and processing of materials.
References
1. General principles of thermal analysis technology, national standardization guidance documents.
2. Thermal analysis and measurement methods of polymer materials, industry technical reports.
3. Research on the thermal behavior of liquid crystal polymers, International Journal of Materials Science paper.
4. Differential scanning calorimeter operation manual, instrument technical documentation.