12.3 Orientation and structure development during processing

Molecular Orientation and Crystallization in Polymer Processing

When polymers are extruded, stretched, or drawn during processing, their chains can align in specific directions and pack into ordered regions. These two structural changes, molecular orientation and crystallization, are the main levers you have for controlling the final properties of fibers and films. Strength, transparency, heat resistance, and barrier performance all trace back to how well-oriented and how crystalline the material becomes during processing.

Molecular Orientation and Crystallization Concepts

Molecular orientation is the alignment of polymer chains along a preferred direction, driven by stress or deformation during processing (extrusion, stretching, drawing). Oriented chains give the material anisotropic properties, meaning it behaves differently depending on direction. Orientation directly affects:

• Mechanical properties: higher tensile strength and modulus along the orientation direction
• Optical properties: birefringence (double refraction) and changes in transparency
• Thermal properties: shifts in effective melting point and heat resistance

Crystallization is the formation of ordered regions called crystallites, where polymer chains pack into regular, repeating arrangements. Not all polymers crystallize equally. Several factors control how much crystallinity develops:

• Cooling rate: Slower cooling gives chains more time to organize, promoting higher crystallinity. Rapid cooling (quenching) traps chains in a disordered, amorphous state.
• Molecular structure: Polymers with regular, symmetric repeat units (like polyethylene or PET) crystallize more readily than irregular or highly branched chains.
• Nucleating agents: Additives like talc or calcium carbonate provide sites where crystallization can begin, speeding up the process and producing smaller, more uniform crystallites (spherulites).

Higher crystallinity increases stiffness, dimensional stability, melting point, and barrier performance (reduced permeability to gases and liquids).

Mechanisms of Orientation Development

Fiber spinning and film formation are the two primary routes for building orientation into a polymer product. Each uses flow and deformation differently.

Fiber Spinning:

1. A polymer melt or solution is forced through a spinneret (a plate with many small holes).
2. The elongational flow through and beyond the spinneret stretches the chains, aligning them along the fiber axis.
3. The fiber is then drawn (pulled) further, increasing the draw ratio, which is the ratio of final length to initial length.
4. Rapid cooling (melt spinning) or solvent evaporation (dry/wet spinning) locks in the oriented structure before chains can relax.

Higher draw ratios produce greater chain alignment, which translates directly to higher tensile strength and modulus along the fiber axis.

Film Formation:

1. A polymer melt is extruded through a flat die (cast film) or an annular die (blown film).
2. The film is stretched in the machine direction (MD) as it's pulled away from the die.
3. For biaxially oriented films, the film is also stretched in the transverse direction (TD), often using a tenter frame.
4. Biaxial stretching produces more balanced properties in both directions, improving strength, modulus, and dimensional stability.

Common examples include blown film extrusion (used for trash bags and packaging) and biaxially oriented films like BOPET (Mylar) and BOPP, which have excellent clarity, strength, and barrier properties.

Processing-Structure-Property Relationships

Processing Conditions and Polymer Structure

The structure you end up with depends heavily on how you process the material. Here are the key variables:

• Cooling rate: Slow cooling favors crystallinity; fast cooling favors an amorphous structure. This is why annealing (holding at elevated temperature) can increase crystallinity after initial forming.
• Draw ratio: Higher draw ratios increase chain alignment. The improvement in tensile strength and modulus is most pronounced along the drawing direction, but properties perpendicular to drawing may decrease (anisotropy).
• Molecular weight and distribution: Higher molecular weight polymers have more chain entanglements, which helps sustain orientation during drawing. A narrower molecular weight distribution leads to more uniform crystallization and more consistent final properties.
• Additives and fillers: Nucleating agents (talc, calcium carbonate) promote crystallization and reduce spherulite size, which can improve both clarity and toughness. Plasticizers (phthalates, oils) increase chain mobility, making processing easier but reducing crystallinity.

Impact of Orientation on Properties

Mechanical: Orientation increases tensile strength and modulus along the draw direction. Crystallinity adds stiffness and dimensional stability. Together, they determine whether a film or fiber can handle load-bearing applications.

Optical: Orientation introduces birefringence, where the refractive index differs depending on the polarization direction of light. Crystallinity tends to increase light scattering and opacity because crystallites and amorphous regions have different refractive indices. Biaxially oriented films can maintain high transparency if spherulites are kept small (via nucleating agents or rapid stretching).

Thermal: Both higher orientation and higher crystallinity raise the effective melting temperature and improve heat resistance. The amorphous regions, by contrast, govern the glass transition temperature ($T_g$), below which the material becomes brittle.

Barrier: Crystalline regions are essentially impermeable to small molecules, so increasing crystallinity reduces permeability to gases (oxygen, water vapor) and liquids. Orientation can further improve barrier properties by creating a more tortuous path for diffusing molecules, particularly in the draw direction.