12.3 Orientation and structure development during processing

Polymer processing techniques like extrusion and stretching can align polymer chains and create ordered regions within the material. This molecular orientation and crystallization greatly impact the final properties of the polymer, including strength, transparency, and heat resistance.

Understanding how processing conditions affect polymer structure is crucial for tailoring material properties. Factors like cooling rate, draw ratio, and additives influence the degree of orientation and crystallinity, which in turn determine mechanical, optical, and barrier properties of the final product.

Molecular Orientation and Crystallization in Polymer Processing

Molecular orientation and crystallization concepts

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Top images from around the web for Molecular orientation and crystallization concepts

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  A study on the crystallization behavior and mechanical properties of poly(ethylene terephthalate ... View original

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  Structural, Thermal and Optical properties of PMMA, PEO and PMMA/PEO/LiClO4 Polymer Electrolyte ... View original

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  A study on the crystallization behavior and mechanical properties of poly(ethylene terephthalate ... View original

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• Molecular orientation involves the alignment of polymer chains along a specific direction induced by applied stress or deformation during processing (extrusion, stretching) which affects mechanical (strength, modulus), optical (birefringence, transparency), and thermal properties (melting point, heat resistance)
• Crystallization is the formation of ordered regions (crystallites) within the polymer structure that occurs when polymer chains pack together in a regular, repeating pattern influenced by factors such as cooling rate (slower promotes crystallinity), molecular structure (regular chains crystallize more easily), and nucleating agents (promote crystallization) which affects mechanical (stiffness, dimensional stability), thermal (melting point, heat resistance), and barrier properties (reduced permeability)

Mechanisms of orientation development

• Fiber spinning involves extruding a polymer melt or solution through a spinneret where elongational flow aligns polymer chains along the fiber axis and rapid cooling or solvent evaporation solidifies the oriented structure with higher draw ratios leading to increased orientation and improved mechanical properties (tensile strength, modulus)
• Film formation involves extruding a polymer melt through a flat die or slit followed by stretching the film in the machine direction (MD) and transverse direction (TD) to induce biaxial orientation which improves mechanical properties (strength, modulus) and dimensional stability with common examples including blow film extrusion (trash bags, packaging) and biaxially oriented films (BOPET, BOPP)

Processing-Structure-Property Relationships

Processing conditions vs polymer structure

• Cooling rate affects crystallization with slower cooling promoting higher crystallinity (more ordered structure) while rapid cooling results in a more amorphous structure (less ordered)
• Draw ratio impacts molecular orientation with higher draw ratios increasing alignment of polymer chains and improving tensile strength and modulus along the drawing direction
• Molecular weight and distribution influence entanglement and crystallization with higher molecular weight enhancing orientation and narrower distribution leading to more uniform crystallization and properties
• Additives and fillers modify crystallization behavior with nucleating agents (talc, calcium carbonate) promoting crystallization and reducing spherulite size while plasticizers (phthalates, oils) increase chain mobility and reduce crystallinity

Impact of orientation on properties

• Mechanical properties are enhanced by increased orientation (higher tensile strength and modulus) and crystallinity (greater stiffness and dimensional stability)
• Optical properties can be modified with orientation inducing birefringence (double refraction) and transparency while crystallinity affects light scattering and opacity
• Thermal properties are improved with higher orientation and crystallinity increasing melting temperature and heat resistance while amorphous regions contribute to lower glass transition temperature ($T_g$)
• Barrier properties are enhanced by increased crystallinity reducing permeability to gases (oxygen, water vapor) and liquids while orientation can improve barrier properties in the drawing direction