Temperature and Polymers

Now that we have a better understanding of the material structure we will dive into its thermal properties to understand its behavior as a function of the temperature. In order to do that, we first need to define the test which will reveal the thermal properties of a polymer: DSC.

DSC definition

Differential scanning calorimetry (DSC) is a type of thermal analysis in which a specimen is placed within a chamber and the amount of heat required to continually increase the internal temperature of the chamber is measured. This form of analysis is designed to pinpoint the temperatures at which the specimen undergoes certain state transitions e.g. Glass transition, crystallization, and melting, by documenting how a polymer reacts to the gradual heat increase via its level of energy absorption and release.

DSC

Glass transition temperature (Tg)

The glass transition temperature can be found in all polymers, it refers to the temperature at which a polymers physical state transitions from glass (hard & brittle) to rubbery (soft & flexible). The Tg is usually used to highlight the highest working temperature of an amorphous polymer.

Crystallization temperature (Tc)

Crystallization happens between Tg and Tm (melting temperature). It is the process of polymer molecules aligning to form crystals. The crystallization temperature is the point at which the polymers crystalize at the highest speed.

Melting temperature (Tm)

The melting temperature is the point at which the crystalline domains of a semi- crystalline polymer starts to melt/deform. Amorphous polymers do not have a defined melting temperature.

Decomposition temperature (Td)

The decomposition temperature is the temperature at which a material begins to deteriorate, meaning that the backbone of the polymer begins to break down.

Notes about the above graph and definitions

A simple way to understand it is that the heat(energy) injected in the chamber will be used to increase the internal temperature, however if the sample (polymer) inside the chamber absorbs some thermal energy for structural realignment, more heat will be needed to be injected to continuously increase the temperature at a constant rate.

Referring to the graph below, at the beginning a constant amount of heat is applied to the system to increase the temperature at a certain rate. At Tg (glass transition temperature), we can notice that more heat is required to increase the temperature at this same rate, this is because the sample will absorb some thermal energy to break its non-covalent bonds and make the polymers move more freely (resulting in the material becoming soft).

After this phase transition, the sample will have a higher heat capacity, so the system will still require a constant amount of heat to be injected to increase the system temperature at the same rate, but this amount will be higher than before Tg. The energy continuously absorbed by the sample will make the polymer microstructure move more and more freely (excite them). At Tc (crystallization temperature), the polymer chain of the sample will have enough free movement to form crystals. The sample will then release energy (heat) which means that we need to inject less heat to the system to increase its temperature.

The reason is that the crystals structure (a more ordered structure) is coming from a more disordered structure, which will require less energy, thus the release of the extra energy. Once the crystals are formed, no more energy will be released from the sample to the system. However, soon after creating the crystals, at Tm (melting temperature), the polymer chains will continue gaining energy(movement) which will excite them too much and make them break the crystal structure, thus absorbing energy from the system, thus needing to inject more energy in the system to continue increasing the temperature at a constant rate. After breaking all the crystals, the sample will not require any additional energy from the system. This explains the two opposite spikes at Tc and Tm. At Td (decomposition temperature), the sample will start to decompose, meaning that covalent bonds will start to be broken, the sample will lose its heat capacity and thus less heat will be needed to increase the system temperature.

Now that the thermal transitions and behavior of polymers in function of the temperature are better understood, we can use this knowledge to explain some of the 3D printing phenomena: