Semicrystalline

Semicrystalline describes materials that contain both crystalline and amorphous regions. In polymers, this means portions of the chains are arranged into ordered, tightly packed lamellae embedded in an amorphous, disordered matrix. The crystalline regions can organize into spherulites as the material cools, creating a heterogeneous microstructure that influences properties.

The degree of crystallinity is the fraction of material that exists in the crystalline phase, typically expressed as a percentage from 0 to 100. Semicrystalline polymers often fall in a broad range, commonly around 20 to 60 percent, though higher crystallinity is achievable with suitable chemistry and processing. Crystalline regions provide order and density, while amorphous regions offer mobility, impact resistance, and optical properties. The presence and size of crystalline lamellae affect melting behavior and thermal stability.

Crystallinity is governed by molecular structure and processing conditions. Factors include chain regularity and tacticity, molecular weight, cooling rate, annealing, and the use of nucleating agents. Faster cooling usually yields lower crystallinity, while slow cooling or post-processing annealing can enhance it. Characterization methods include differential scanning calorimetry (DSC), which measures heat of fusion to estimate crystallinity; X-ray diffraction (XRD), which reveals crystalline peaks; and density or spectroscopic techniques.

Industrial implications are significant: higher crystallinity generally increases stiffness, heat resistance, and barrier properties, but can reduce toughness and optical clarity. Melting temperature reflects the crystalline fraction, while the glass transition pertains to the amorphous regions. Common semicrystalline polymers include polyethylene, polypropylene, PET, and nylons, though the concept also applies to blends and composites.