Impact of PVDF Copolymerization and Homopolymerization on Properties

The copolymerization and homopolymerization structures of polyvinylidene fluoride (PVDF) significantly influence its properties, including crystallinity, mechanical performance, thermal stability, chemical resistance, and electrical characteristics.

Below is a detailed analysis:

1. Crystallinity

Homopolymer PVDF:

Composed solely of vinylidene fluoride (VDF) monomers, its highly regular molecular chains facilitate high crystallinity (typically 50%–70%). High crystallinity enhances mechanical strength and solvent resistance but may reduce flexibility.

Major crystal phases: β-phase (strongest piezoelectricity), α-phase (most common), γ-phase (thermally stable).

Copolymer PVDF:

The introduction of comonomers (e.g., hexafluoropropylene HFP, trifluoroethylene TrFE) disrupts chain regularity, reducing crystallinity (can drop to 20%–40%).

Effects:

- Lowers melting point (e.g., VDF-HFP copolymer melts 20–30°C lower than homopolymer).

- Increases amorphous regions, improving flexibility and transparency.

- Certain comonomers (e.g., TrFE) promote β-phase formation, enhancing piezoelectric/ferroelectric properties.

2. Mechanical Properties

- Homopolymer PVDF:

High modulus and tensile strength but greater brittleness (especially at low temperatures).

- Copolymer PVDF:

Improved flexibility and elongation at break, making it suitable for impact-resistant or bendable applications (e.g., flexible films, cable sheaths).

3. Thermal Properties

- Homopolymer PVDF:

Higher melting point (~170–175°C) and heat deflection temperature, suitable for high-temperature environments.

- Copolymer PVDF:

Lower melting point (e.g., VDF-HFP copolymer: ~140–160°C), easing processing but reducing thermal stability.

4. Chemical Resistance

- Homopolymer PVDF:

Dense crystalline regions provide superior chemical resistance (e.g., against acids, alkalis, and organic solvents).

- Copolymer PVDF:

Increased amorphous content may reduce resistance to certain solvents (e.g., ketones, esters), though fluorinated comonomers can help retain chemical stability.

5. Electrical Properties

- Homopolymer PVDF:

High β-phase content enhances piezoelectric/ferroelectric performance, ideal for sensors and energy storage devices.

- Copolymer PVDF:

- VDF-TrFE copolymers: Promote β-phase formation, significantly increasing piezoelectric coefficients (d₃₃ up to ~30 pC/N).

- VDF-HFP copolymers: May exhibit higher dielectric constants, suitable for capacitor dielectrics.

6. Processability

- Homopolymer PVDF:

Higher melting point and melt viscosity require more demanding processing conditions.

- Copolymer PVDF:

Improved flow properties, facilitating easier injection molding, extrusion, or film formation.

Application Scenarios

- Homopolymer PVDF:

Applications requiring high strength, thermal resistance, or chemical stability (e.g., chemical piping, battery separators, high-purity containers).

- Copolymer PVDF:

Applications needing flexibility, piezoelectricity, or low-temperature processing (e.g., flexible sensors, medical films, wire coatings).

By adjusting copolymerization, PVDF properties can be finely tuned for diverse applications. The choice depends on balancing crystallinity, flexibility, thermal stability, and functional properties (e.g., electroactivity).