**01**
First, Let's Clarify: What are Aromatic and Aliphatic Compounds?
We all know that the essence of chemical reactions lies in the behavior of electrons. Atoms have electrons in their outer shells, and the formation of covalent bonds essentially involves atoms sharing electron pairs. In molecules we encounter daily, such as aliphatic compounds like cyclohexane, methane, and polyethylene, their electrons are mostly **localized**—confined to specific atoms or specific bonds.
However, the situation is completely different for **aromatic compounds**. Their electrons are not localized but are **delocalized**: they are no longer confined to a single bond but are distributed over a larger area. Let's look at the definition of aromaticity: Aromatic compounds are those that contain at least one cyclic conjugated system composed of delocalized π electrons and that obey Hückel's rule.
To understand this with a simpler analogy, aromatic compounds are like an "electron-sharing paradise." To establish this paradise, several conditions must be met simultaneously:
- Must be a **ring** (closed-loop structure): So electrons can "run in circles."
- The ring must be **planar**: If the ring twists and turns, the electron "racetrack" isn't smooth.
- Every atom in the ring must participate in electron sharing: Holding hands to form a continuous electron cloud.
A typical representative is **benzene**—a six-membered ring where electrons are evenly distributed across the entire ring, forming a conjugated electron cloud.
In contrast, electrons in aliphatic compounds remain localized; it's more like "everyone driving their own car," without forming a shared large racetrack.
**02**
Getting to the Essence: Benzene Ring vs. Ordinary Ring – Why Such a Big Difference?
Isn't a benzene ring just a ring? What's the fundamental difference between it and cyclohexane? The key lies in **electron behavior**.
**Benzene Ring:** 6 π electrons are in a delocalized state, forming a stable "π electron cloud." Let's first explain the electronic state of the benzene ring. In the benzene ring, the 6 π electrons are not confined to a specific C=C double bond but are delocalized across the entire six-membered ring. This means the electron cloud is evenly distributed above and below the aromatic ring, forming a ring-shaped "π electron cloud." In other words, the electrons expand from the local bond range to a larger system (the entire ring). They don't "leave the domain" but rather "enlarge the domain."
Some one might ask: So what if electrons are delocalized and shared? What does that have to do with performance? In our impression, the benzene ring is almost synonymous with rigidity. In fact, this "rigidity" is precisely bestowed by electron delocalization. This mainly involves two aspects: **energy distribution + structural constraints.**
**(1) Energy Averaging**
If electrons were localized on double bonds, it would create differences in single and double bond lengths. In a delocalized state, electrons are evenly distributed → all six C–C bonds have identical lengths, the system energy is lower, and the entire ring naturally tends toward "symmetry + flatness."
**(2) Constraining Effect of the π Electron Cloud**
In the benzene ring, the 6 π electrons are shared collectively, forming a highly symmetric, planar ring-shaped electron cloud. It acts like a "ring-shaped shield" covering the entire ring. Attempting to this delocalization (e.g., forcing it into 3 isolated C=C double bonds) would significantly raise the system energy. Therefore, the benzene ring is "locked by the electron cloud" and cannot freely rotate like alkanes.
**(3) Resulting Manifestation**
The benzene ring structure is fixed as planar with equal bond lengths; it cannot easily be stretched or compressed. Introducing benzene rings into polymers "locks" the chain segment mobility, manifesting as increased material rigidity and a corresponding rise in the glass transition temperature (Tg).
**03**
Aromatic vs. Aliphatic in Polyurethanes
The backbone of polyurethanes is formed by the polycondensation of **disocyanates** (such as HDI, MDI, IPDI, TDI, etc.) with polyols. The type of disocyanate determines whether the polyurethane skeleton is primarily **aromatic** or **aliphatic**, significantly influencing the material's properties and application scenarios.
**Aromatic Polyurethanes (Typical: MDI, TDI)**
- **Strong mechanical properties:** Typically exhibit high modulus and high tensile strength, suitable for load-bearing or structural applications.
- **Low cost:** High degree of industrialization, with relatively low raw material and processing costs, leading to widespread application.
- **Main drawback – Prone to yellowing:** Aromatic rings can act as chromophores. Under UV irradiation, photo-oxidation occurs, forming larger conjugated systems (chromophores) that absorb the short-wavelength end of visible light (blue-violet light), visually manifesting as yellowing.
- **Typical applications:** Shoe soles, automotive interiors, structural components, etc., where strength is required and exposure to strong UV is infrequent.
**Aliphatic Polyurethanes (Typical: HDI, IPDI)**
- **Good yellowing resistance:** Strong weatherability; transparent products resist yellowing even with long-term outdoor use.
- **Excellent flexibility and weatherability:** Perform better in applications requiring long-term exposure and high transparency, such as coatings, optical films, outdoor seals.
- **Drawbacks:** Higher raw material costs, stricter processing requirements, and generally slightly lower mechanical strength compared to aromatic systems.
- **Typical applications:** Optical films, outdoor coatings, transparent TPU, etc., where colorfastness, weatherability, and appearance are critical.
**Material Selection and Design Considerations**
**Select Materials Based on the Use Environment**
- **Indoors, structural parts:** Aromatic polyurethanes offer high cost-effectiveness and can be prioritized.
- **Outdoors, transparent, and optical applications:** Prioritize aliphatic polyurethanes to reduce later maintenance and replacement costs.
**Modification and Anti-Aging Strategies**
- **For aromatic polyurethanes:** Add UV absorbers, hindered amine light stabilizers (HALS), etc., to inhibit photo-oxidation and delay yellowing.
- **For aliphatic polyurethanes:** If further hydrolysis resistance or improved durability is needed, strategies like fluorination, addition of hydrolysis-resistant agents, or increasing crystallinity to reduce moisture penetration can be used.
**Molecular Structure Optimization**
- A common strategy is **copolymerization/blending**, combining aromatic and aliphatic monomers in specific ratios to balance strength, weatherability, and cost. For example, a combination of MDI and HDI can achieve both mechanical performance and improved weatherability/appearance.
- Additionally, fine-tuning final properties is possible through **segment design** (soft/hard segment ratio, molecular weight, crosslinking degree) and adding **fillers/plasticizers**.
**Summary in One Sentence**
- **Aromatic** = Strong, rigid, inexpensive, but **fears sunlight** and prone to yellowing.
- **Aliphatic** = Stable, weather-resistant, transparent, but **more expensive** and slightly weaker in strength.
**Final Words**
Have you noticed a pattern? Many material properties can actually be traced back to the most fundamental molecular structure—the π electrons of the benzene ring, the flexibility of chain segments, the number of hydrogen bonds...
So, the next time you see a material "yellowing," "becoming brittle," or "suddenly failing after working fine," don't just focus on the phenomenon. Think one level deeper: Is there a "cause" buried within its structure?