Introduction
In the field of materials science, understanding the glass transition behavior of polymer materials is of general significance for evaluating their thermal properties and stability. As a widely used thermal analysis technique, differential scanning calorimetry can effectively determine the glass transition temperature of materials. The purpose of this paper is to explore how to use differential scanning calorimetry to analyze the vitrification transition of degraded films, and to explain the relevant principles, methods and precautions.
Rationale
The basic principle of differential scanning calorimetry is to measure the change of energy difference between the sample and the reference object with temperature or time under program temperature control. When polymer materials undergo a glass transition, their heat capacity changes stepwise, usually showing a deviation from the baseline on the differential scanning calorimetry curve. This transition temperature is defined as the glass transition temperature, which is an important parameter for characterizing the transition of a material from a glass state to a highly elastic state. For degradable membranes, their molecular chain structure may be altered by environmental factors, which can affect the vitrification transition behavior.
Experimental methods
When performing a glass transition analysis of degradable films, it is necessary to follow a standardized operating procedure. First, a representative sample should be prepared, usually cutting the membrane material into small pieces or powders to ensure good contact with the sample crucible. Sample quality is recommended between 5 and 10 mg to avoid uneven heat conduction. Before the experiment, the instrument needs to be calibrated for temperature and heat, and commonly used calibration materials include indium and zinc. During the test, nitrogen is usually used as a protective atmosphere, the heating rate is mostly set to 10°C/min, and the scanning temperature range should cover the expected glass transition area. It is recommended to perform at least two replicate tests for each sample to confirm the reproducibility of the results.
Data analysis
After obtaining the differential scanning calorimetry curve, the tangent method is usually used to determine the glass transition temperature. The specific steps are as follows: make an extension line at the baseline before and after the transition area, and then make a tangent at the inflection point of the curve, and the temperature corresponding to the midpoint of the intersection of the two extension lines and the tangent line is often reported as the glass transition temperature. For partially degraded membrane materials, the curve may have a widening transition region or multiple transitions, which may indicate phase separation or varying degrees of degradation of the material. Data analysis should be based on the known history of the material for comprehensive judgment.
Influencing factors
Several factors can affect the measurement of the glass transition temperature of the degraded film. Sample preparation methods, such as flatness and particle size, can affect thermal contact. Changes in the rate of rise change the apparent position of the transition temperature, and generally higher rates of rise cause the measured glass transition temperature to move in the direction of high temperature. The thermal history of a material, including annealing or processing conditions, can also have an impact on molecular segment movement. In addition, changes in molecular weight, plasticizer loss, or cross-linked structure formation introduced during the degradation process can significantly alter the glass transition behavior.
Application examples
In the study of packaging materials, differential scanning calorimetry analysis was performed on bio-based polyester films that underwent different UV aging times. The data show that with the prolongation of aging time, the glass transition temperature shows regular changes, which helps to understand the impact of environmental aging on the flexibility of materials and the service temperature window. In the evaluation of agricultural mulching film, the difference in glass transition temperature between the film before and after use can help judge the aging degree and remaining service life of the material.
Notes:
When testing, ensure that the sample does not experience other strong thermal events, such as melting or decomposition, within the test temperature range to avoid interfering with baseline judgment. For severely degraded samples, the baseline of the differential scanning calorimetry curve may drift significantly, and the position of the front and rear baselines needs to be carefully determined. It is recommended to indicate the specific glass transition temperature method, heating rate and atmospheric conditions in the test report to ensure the comparability of the data. Regular maintenance and performance verification of instruments are the basis for ensuring data reliability.
Summary
Differential scanning calorimetry provides an effective means to study the vitrification transformation of degraded films. Through standardized experimental operation and rigorous data analysis, the glass transition temperature reflecting the change of thermal properties of the material can be obtained, which has reference value for evaluating the durability, stability and applicability of the material. In practical applications, material history, test conditions and data interpretation methods should be comprehensively considered to draw reasonable conclusions.
References
1. Overview of the Application of Thermal Analysis in the Characterization of Polymer Materials, Journal of Materials Science and Engineering.
2. Standard test method for polymer vitrification transition, ASTM international standard.
3. Research on the effect of environmental degradation on the thermal properties of polymer films, polymer materials science and engineering.
4. Principles and experimental guidelines of differential scanning calorimetry technology, instrument analysis and application.