Crystals from Spaghetti
Polymer crystallization is the remarkable process by which long, tangled chain molecules organize into ordered lamellar structures. Unlike small-molecule crystals, polymer crystals are always imperfect — chains fold back and forth, creating thin lamellae (~10 nm thick) separated by amorphous regions of chain loops and tie molecules. The resulting semicrystalline structure gives polymers their unique combination of stiffness (from crystals) and toughness (from amorphous tie molecules).
Spherulite Growth
Crystallization begins at nucleation sites — either heterogeneous (on dust, pigment particles, or nucleating agents) or homogeneous (from random chain fluctuations). From each nucleus, lamellar crystals grow radially outward, splaying and branching to form spherical superstructures called spherulites. The simulation visualizes these growing spherulites in real time, showing how they nucleate, expand, and eventually impinge on their neighbors to fill the entire volume.
Avrami Kinetics
The overall crystallization kinetics follow the Avrami equation: Xc(t) = 1 - exp(-K·t^n). The sigmoidal shape arises from the interplay between nucleation (which provides new growth fronts) and impingement (which consumes available amorphous material). The Avrami exponent n encodes the dimensionality of growth and nucleation mechanism — for 3D spherulites with pre-existing nuclei, n ≈ 3. The simulation plots the Avrami curve alongside the growing microstructure.
Processing and Properties
Injection molders and film producers control crystallinity through cooling rate, nucleating agents, and annealing conditions. Fast quenching freezes chains before they can crystallize, producing transparent amorphous material (like PET bottles). Slow cooling allows extensive crystallization, producing opaque, rigid parts (like HDPE containers). Nucleating agents create many small spherulites, improving both clarity and mechanical properties — a trick used in clarified polypropylene for food packaging.