People study the changes of molecular structure, aggregation structure and molecular motion of substances by detecting the changes of thermophysical properties of samples with temperature or time. ?
The most widely used thermal analysis instruments are power compensation DSC, heat flow DSC, differential thermal DTA, thermogravimetric TG and so on. Under the control of temperature program, DSC studies the changes of physical quantities (Δ q and Δ h) of a substance with temperature, that is, it controls the changes of temperature through the program, and measures the relationship between the power difference (heat flow) between the sample and the reference and the temperature when the temperature changes.
Under the same conditions, the sample with phase change and the reference sample without phase change and any thermal effect are heated or cooled isothermally. When the sample changes phase, there will be a temperature difference between the sample and the reference sample.
A group of differential thermocouples placed under it generate a temperature difference potential u δ t, which is amplified by a differential thermal amplifier and sent to a power compensation amplifier. The power compensation amplifier automatically adjusts the current of the compensation heating wire, so that the temperature difference between the sample and the reference tends to zero, and the temperatures of the two are always consistent. This compensation heat is the thermal effect of the sample and is displayed on the recorder in the form of electric power.
Extended data:
DH/dt varies discontinuously, so there is a baseline shift in the thermal spectrum. From the point of view of molecular motion, the glass transition is related to the microscopic Brownian motion of molecular segments in amorphous polymers or amorphous parts of crystalline polymers. Below the glass transition temperature, the motion is basically frozen. After reaching Tg, the moving live wave heat capacity increases and the baseline moves to the heat absorption side.
The determination of glass transition temperature is based on the deviation of baseline on DSC curve, and a step appears. Usually, the intersection of the tangent of the curve leading edge and the baseline is used to determine Tg.
The factors affecting Tg include chemical structure, relative molecular weight, crystallinity, cross-linking and curing, sample historical effects (thermal history, stress history, annealing history, morphological history) and so on.
Polymers with rigid backbone or large side groups will have higher TG; Polymers with strong inter-chain attraction are not easy to swell and have high TG; Loose side groups hang on the molecular chain, which makes the molecular structure loose, that is, it increases the free volume and reduces the Tg.
References:
Baidu encyclopedia -DSC principle