Finally, someone has clearly explained the "plasticizer" of polymers!

What is the essence of plasticizers?

Let's start with the name. The so-called "plasticizer" is something that can make materials more "plastic." Essentially, it is a type of...Small molecules or oligomersIt can insert between the polymer chains, reducing the forces between them, allowing the segments to move more easily.

You can imagine it as a group of dancers (polymer chains) who are initially holding hands tightly, making their movements stiff and their dancing not flexible at all. Then, some "little lubricants" (plasticizers) come in quietly and slip between them, loosening their grip, and suddenly their dance movements become flexible.

From a molecular perspective, polymer chains are originally held together by hydrogen bonds, Van der Waals forces, and other interactions, making chain segments almost immobile at insufficient temperatures. The introduction of plasticizers acts like "inserting wedges."The binding energy between chain segments is weakened.The glass transition temperature (Tg) decreases, the segmental motion window opens earlier, and the material softens.

The essence of plasticizers can be summarized in one sentence:

It is a "molecular lubricant" that reduces the interaction force between chain segments and increases the freedom of movement of the chain segments.

What is the mechanism of action of plasticizers?

We often say that plasticizers make materials softer, but this statement is too general. In fact, from the molecular to the electronic level, its mechanism can be understood in at least three dimensions.

1. Thermodynamic Dimension: Free Volume Effect

The polymer chains are originally packed tightly, similar to a crowded subway during rush hour where people can barely move. When plasticizer molecules infiltrate, it's like "shoving in a few balloons," pushing the crowd apart. This creates more "gaps" (free volume) between the chain segments, significantly increasing the freedom of movement for the chains.
The direct result: the glass transition temperature Tg decreases, and the material becomes soft at a lower temperature.

Keywords: increase in free volume → decrease in activation energy barrier → decrease in Tg

2. Dimension of intermolecular forces: weakening the cohesive energy density

The hardness of polymers originates from intermolecular attractions such as hydrogen bonds, dipole-dipole interactions, and π-π stacking. These forces tightly bind the chains together.
When plasticizer molecules are introduced, on one hand, they "dilute" the attraction between polymer chains (similar to adding a bit of oil to glue), and on the other hand, they themselves can form some "weaker but flexible" interactions with the chains.

In PVC, the dipole of the –C=O (ester group) interacts with the –C–Cl segments, breaking the original pattern of "tight traction between Cl–Cl."

In polyamides, plasticizers weaken the hydrogen bond network, making the segments less rigid.

From the perspective of energy,Plasticizers reduce the cohesive energy density of the system.Make the chains "stick less tightly" together.

3. Electronic Behavior Dimension: Local Electron Cloud Perturbation

The attraction between polymer chains essentially comes from the distribution and interaction of electron clouds.

In polar chains (such as PVC, PA), the uneven distribution of electron clouds results in a large dipole moment and strong mutual attraction.

The introduction of polar groups of plasticizers (such as ester groups and hydroxyl groups) can couple with the dipoles of these segments, effectively "reshuffling" the electron distribution and weakening the original strong interactions.

In other words, the plasticizer acts as an "electronic buffer" between molecules: it interacts with the polymer chain segments using its own electron cloud, dispersing the originally tight dipole-dipole and hydrogen bond couplings.

This is also why different plasticizers have vastly different effects on different materials.Only when the electronic behavior matches can the plasticizer truly "insert and stay."

4. Dynamic Dimension: Shortened Segmental Relaxation Time

If you further examine the dynamic mechanical properties (DMA) of the material, you will find that after adding a plasticizer, the α relaxation peak (corresponding to the main chain segment motion) shifts to a lower temperature, and the peak broadens. What does this indicate?
The energy barrier for chain segment motion is reduced, and the material's...Relaxation time distributionBroader. In other words, plasticizers not only make the chain segments mobile but also enable chain segments of different lengths and in different environments to become active.

How to choose plasticizers for different material systems?

This is where many people encounter pitfalls. A plasticizer is not a "universal key"; it must match your material system. Otherwise, the effect may be insignificant, or it could lead to the major issues of "migration or precipitation."

Several Core Principles:

Compatibility First
A plasticizer must be compatible with your polymer chain. For example, PVC and phthalates are a good match because their polarities are compatible. However, if you use a non-polar hydrocarbon plasticizer (such as white oil) with PVC, it may not mix in at all and will eventually precipitate out.

Operating condition matching
Is your product used at room temperature or for long-term use at high temperatures? Is it used in air, or in oil, water, or solvent?

High temperature conditions: require low-volatility plasticizers (such as epoxy macromolecular plasticizers).

Contact with oil media: consider the risk of dissolution.

Medical and food contact: require low migration and low toxicity, such as citrate esters and bio-based plasticizers.

Physical Properties Requirements
Some systems need to be flexible but also resistant to low temperatures, so it is necessary to choose plasticizers with a low glass transition point and good cold resistance.
Some systems require flame retardancy, and functional plasticizers containing phosphorus and chlorine can be selected.

Processing requirements
Plasticizers also affect melt viscosity and processing flow. If your system is too viscous, adding a little low-molecular-weight plasticizer can make processing smoother. However, don't forget that it may also bring the risk of exudation.

Translate the above content into English and directly output the translation without any explanation.When choosing a plasticizer, you should look for one that "can enter, stay, and doesn't run around."

04 How to quickly determine if a plasticizer is suitable? (The most important point)

This is also the issue that everyone is most concerned about. Many people make samples immediately after a formula is released, only to find out from customer feedback that there is a problem with the plasticizer. In fact, it can be quickly verified in advance.

Several practical methods:

Accelerated migration testing

High temperature aging (e.g., 70°C, 1000 hours), check if the surface is sticky or exuding oil.

Soak in solvent, measure weight change, and observe how much migration occurs.

Vacuum oven, simulating low-pressure evaporation.

DSC Test
The more significant the decrease in Tg, the stronger the effect of the plasticizer added; if there is almost no change, it may indicate poor compatibility.

Infrared or NMR characterization
You can observe whether there are significant interaction peaks between the plasticizer and the matrix. If there is little interaction, the probability of future migration is high.

Rule of thumb
The most straightforward principle is "like dissolves like": choose polar plasticizers for polar systems and non-polar plasticizers for non-polar systems. Don't try to "force it," or there will be endless troubles.

In Conclusion

Many people think that plasticizers are minor components that can solve problems by simply adding any one of them. However, the reality is quite the opposite:The plasticizer is one of the key points for the system to remain stable in service.。

I have always told my group members:

Don't wait until the product fails and customers complain to ask "why did the plasticizer precipitate?"

At that time, you will realize that not only have you lost the trust of your clients, but more importantly, the effort you spent months developing has all been in vain.

So,It is essential to conduct accelerated evaluation and validation of plasticizers in the early stages.Don't be afraid of trouble; if you do it one day earlier, you'll have ten times less trouble.