Unveiling the Molecular Architecture of Rubber: Structure-Property Relationships

The remarkable properties of rubber, including its high elasticity and resilience, are fundamentally governed by its intricate molecular structure. Understanding these structural characteristics is crucial for tailoring rubber formulations to meet specific performance requirements. The primary structural motifs found in rubber materials include linear, branched, and crosslinked configurations.

I. What are the different Structural Forms of Rubber?

A. Linear Structure

Linear structures represent the dominant form in unvulcanized rubber. Due to the exceptionally high molecular weight of the polymer chains, they exist as randomly coiled and entangled conformations in the absence of external forces. Upon application of an external force, the degree of entanglement is altered, leading to chain extension and alignment. Upon removal of the applied force, the chains readily return to their original coiled state. This phenomenon underlies the characteristic high elasticity observed in rubber materials.

B. Branched Network

Branched structures are formed due to the aggregation of side chains along the main polymer backbone, leading to the formation of gel-like domains known as "gels." The presence of these gels is generally detrimental to both the processing and ultimate performance of rubber compounds. During mixing (mastication), compounding ingredients may have difficulty penetrating the gel regions, resulting in localized areas deficient in reinforcement and crosslinking. These inhomogeneities can act as weak points in the final product.

C. Crosslinked Networks

Crosslinked structures emerge when linear polymer chains are interconnected via covalent or ionic bonds formed between atoms or atomic groups, creating a three-dimensional network architecture. The extent of crosslinking increases as the vulcanization process progresses. As crosslink density builds, the free mobility of chain segments decreases, leading to reductions in plasticity and elongation. Concurrently, improvements are observed in tensile strength, elasticity, and hardness. Furthermore, compression set and solvent swelling are reduced.

II. How Does Rubber Structure affect performance?

The molecular structure of rubber has profound implications for its material properties:

A. Reinforcement Effects

The reinforcing effects of fillers, such as carbon black, are strongly influenced by the rubber's structure, particularly regarding tensile and tear strength. A general trend is that high-structure carbon blacks exhibit greater reinforcement in non-crystalline rubbers at similar particle sizes, leading to higher tensile and tear strength values.

B. Impacts on Electrical Conductivity

The structure of the rubber matrix plays a vital role in determining its electrical conductivity. Branched structures more readily form interconnected conductive pathways within the rubber, enhancing its electrical conductivity.

C. Crosslinking

Crosslinking of polymer chains contributes to the elastic recovery and mechanical stability of the overall rubber network. When crosslinked rubber undergoes deformation under external forces, it exhibits rapid recovery to its original shape, along with improvements in physical-mechanical properties and chemical resistance.