8.3 Morphology of semicrystalline polymers

Semicrystalline polymers have a unique two-phase structure with crystalline and amorphous regions. This blend gives them a special mix of properties like strength from the ordered parts and flexibility from the disordered areas.

Understanding the morphology of these polymers is key. We'll look at how factors like chain folding, tie molecules, and degree of crystallinity affect their behavior. We'll also explore techniques used to study their structure.

Morphology of Semicrystalline Polymers

Morphology of semicrystalline polymers

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• Semicrystalline polymers have a two-phase structure consisting of crystalline regions interspersed with amorphous regions
  • Crystalline regions have highly ordered, densely packed polymer chains arranged in a regular, repeating pattern (polyethylene)
    • Impart strength, rigidity, and chemical resistance to the polymer
  • Amorphous regions have disordered, randomly arranged, and entangled polymer chains with no specific orientation (rubber)
    • Provide flexibility, ductility, and impact resistance to the polymer
• The ratio of crystalline to amorphous regions varies depending on factors such as polymer type, molecular structure, and processing conditions (cooling rate)

Degree of crystallinity

• Degree of crystallinity represents the fraction of the polymer that exists in the crystalline state
  • Quantified as a percentage of the total polymer volume or mass
• Increasing the degree of crystallinity enhances strength, stiffness, density, chemical resistance, and melting temperature (nylon)
• Decreasing the degree of crystallinity improves flexibility, elasticity, impact resistance, and optical clarity (low-density polyethylene)
• Degree of crystallinity is influenced by polymer structure (tacticity, copolymerization) and processing conditions (cooling rate, annealing)

Chain folding and tie molecules

• Chain folding occurs in the crystalline regions, where polymer chains fold back and forth upon themselves to form ordered, lamellae-like structures
  • Lamellae have a typical thickness of 10-20 nm
  • Chain folding enables efficient packing of chains in the crystalline regions (polyethylene terephthalate)
• Tie molecules are polymer chains that connect different crystalline regions by extending through the amorphous region
  • Provide mechanical strength, integrity, and stress distribution to the polymer
  • Prevent crack propagation by bridging crystalline domains (polypropylene)
• The presence of chain folding and tie molecules contributes to the unique balance of strength (crystalline regions) and flexibility (amorphous regions) in semicrystalline polymers

Characterization of polymer morphology

• X-ray diffraction (XRD) provides information about crystalline structure and degree of crystallinity
  • Sharp, intense diffraction peaks indicate crystalline regions, while broad, diffuse peaks represent amorphous regions
  • Degree of crystallinity is estimated from the relative intensities of crystalline and amorphous peaks
• Microscopy techniques offer visual insights into polymer morphology at different scales
  1. Optical microscopy visualizes larger-scale features like spherulites but has limited resolution
  2. Scanning electron microscopy (SEM) reveals high-resolution surface morphology, including spherulites, lamellae, and phase separation
  3. Transmission electron microscopy (TEM) provides the highest resolution for imaging internal structure, directly visualizing lamellae, chain folding, and tie molecules (requires thin samples < 100 nm)
• Differential scanning calorimetry (DSC) measures thermal transitions (melting, glass transition) and estimates degree of crystallinity based on melting enthalpy