Rationale
Differential scanning calorimetry is a thermal analysis technique that measures the heat difference between a sample and a reference object with temperature under program temperature control. When a sample undergoes a physical or chemical transition, such as a melting, crystallization, or glass transition, heat is absorbed or released, resulting in a temperature difference between it and the inert reference. The instrument compensates for the energy to keep the temperature of the two the same, and the compensated energy is the heat flux rate (dQ/dt), which directly reflects the thermal effect of the sample. This technique provides a quantitative data basis for studying the thermal properties of materials.
Resin blend compatibility evaluation
The compatibility of resin blends refers to the ability of each component to achieve molecular or chain level dispersion in the blending system to form a homogeneous phase structure. Thermodynamically, compatible systems usually represent a single glass transition temperature (Tg）。 For partially compatible or incompatible systems, two or more T's corresponding to each pure component may be observedg, or its TgA certain degree of closeness occurred. Therefore, the accurate determination of the glass transition temperature and its changes of the blend by differential scanning calorimetry is the key basis for judging its compatibility. The transition temperature can be extrapolated by extrapolating the starting temperature (Te) or midpoint temperature (T.)mg) to characterize.
Experimental methods
Experiments are usually conducted in sealed crucibles under the protection of an inert atmosphere. Samples should be thoroughly dried and accurately weighed before testing. Typical temperature procedures include: first heating at a constant rate above the melting point of each component to eliminate thermal history, constant temperature followed by rapid quenching, and then a second temperature scan to record the glass transition. The glass transition is manifested as a heat capacity step, and its width and shape also contain compatibility information.
For binary blending systems, the relationship between glass transition temperature and component content can often be fitted and analyzed by the Gordon-Taylor equation:
Tg = (w1Tg1 + k w2Tg2) / (w1 + k w2)
Among them, TgThe glass transition temperature of the blend, w1、w2is the mass fraction of the component, Tg1、Tg2It is the glass transition temperature of pure components, and k is the fitting parameter, which is related to the interaction strength between components. The closer the k value is to 1, the better the compatibility.
Thermal behavior characteristics of blending system
The following table lists the characteristics of differential scanning calorimeters for resin blends under different compatibility states:
| Compatibility status | Thermal behavior characteristics |
| Fully compatible | Presents a single and sharp vitrification transition step, TgIt is between pure components and is in line with the prediction of the Gordon-Taylor equation. |
| Partially compatible | A widened vitrified transition region appears, or two T's close to each otherg, indicating that there is phase separation but the phase zone size is small. |
| Incompatible | Two independent glass transitions were clearly observed, with temperatures that were basically the same as those of the pure components, respectively, indicating a clear phase separation. |
Influencing factors
The preparation process of the blend (e.g., solution blend, melt blend), molecular weight of the components, temperature rise rate, and thermal history of the sample can all affect the test results. For example, a faster quenching rate may inhibit the phase separation process, resulting in the observation of apparent compatibility. Therefore, it is necessary to strictly control the conditions in the experimental design, and combine multiple temperature rise and fall cycles and annealing treatment to study the stability of the phase behavior. When analyzing the data, the step height, width, and temperature of the glass transition should be comprehensively examined to obtain a comprehensive conclusion about compatibility.
Conclusion
Differential scanning calorimetry is an effective and commonly used tool for studying the compatibility of resin blends. By accurately measuring the glass transition temperature and its changes with composition, the interaction and phase structure between blending components can be qualitatively and semi-quantitatively evaluated. The method is relatively simple to operate, requires a small amount of sample, and has good reproducibility, which provides an important theoretical basis and data support for the design and performance optimization of polymer blended materials.
References
1. Ulrik. Thermal Analysis Application of Polymer Materials. Chemical Industry Press.
2. Standard test method: Plastic differential scanning calorimetry to determine the glass transition temperature.
3. Gordon M, Taylor J S. Ideal Copolymers and the Second-Order Transitions of Synthetic Rubbers. Journal of Applied Chemistry.