When comparing polymers, we often encounter the property of crystallinity. While it is tempting to think that all polymers are alike in structure, the degree of crystallinity in turn has impact on the more visible properties of the plastic, such as its softness, its brittleness, and even its machinability.
Crystalline Polymers
Crystalline polymers have regions where molecular chains are arranged in a highly ordered, repeating pattern. These crystalline zones coexist with amorphous regions, but the extent of crystallinity determines the material's overall behaviour. Examples include PTFE, PEEK, UHMWPE, PVDF, PA66, PA12, PPS, and PCTFE.
Amorphous Polymers
Amorphous polymers lack ordered regions, with their molecular chains arranged randomly. This structure imparts unique mechanical and optical properties, often favouring transparency and flexibility. Examples include PEI, Polyimide, and PVC.
Key Property Differences
1. Mechanical Properties
-
Crystalline Polymers: High crystallinity contributes to superior mechanical strength, stiffness, and dimensional stability. For instance:
-
PTFE: Although it is a soft polymer, PTFE is in fact semi-crystalline. PTFE exhibits exceptional tensile strength and low friction due to its crystalline structure.
-
PEEK: Its high crystallinity ensures toughness, making it suitable for demanding environments.
-
UHMWPE: Noted for its impact resistance, thanks to its long molecular chains and crystalline regions.
-
-
Amorphous Polymers: Tend to be more flexible, less brittle, and more resistant to crazing or stress fractures. For example:
-
PEI (Polyetherimide): Offers excellent toughness while maintaining dimensional stability.
-
Polyimide: Known for its flexibility and ability to withstand thermal cycling.
-
2. Thermal Properties
-
Crystalline Polymers: Sharp melting points due to their ordered structure. For example:
-
PVDF: Crystalline regions allow it to retain strength and stability at high temperatures.
-
PA66: Displays a distinct melting point, enabling predictable thermal behavior.
-
-
Amorphous Polymers: No sharp melting point, transitioning from rigid to rubbery over a temperature range. For example:
-
PVC: Softens gradually, allowing for versatile processing.
-
PEI: Exhibits high glass transition temperatures (Tg), maintaining rigidity under heat.
-
3. Chemical Resistance
-
Crystalline Polymers: Excellent chemical resistance due to their tight molecular packing.
-
PTFE: Among the most chemically inert materials, ideal for harsh chemical environments.
-
PPS: Offers resistance to solvents, acids, and bases.
-
-
Amorphous Polymers: Typically, more prone to chemical attack, though some, like Polyimides, exhibit high chemical resilience.
4. Optical Properties
-
Crystalline Polymers: Usually opaque due to light scattering by crystalline regions.
-
UHMWPE and PTFE: Generally white or translucent.
-
-
Amorphous Polymers: Often transparent.
-
PEI: Commonly used in optical applications due to its clarity.
-
5. Density and Shrinkage
-
Crystalline Polymers: Higher density and greater shrinkage during cooling due to crystallization.
-
Amorphous Polymers: Lower density and minimal shrinkage, providing better dimensional control during molding.
Applications of Crystalline vs. Amorphous Polymers
Crystalline Polymers
-
PTFE: Used in non-stick coatings, gaskets, and seals, thanks to its low friction and chemical resistance.
-
PEEK: Applied in aerospace, medical implants, and high-temperature components.
-
UHMWPE: Favoured for wear-resistant applications like artificial joints and conveyor belts.
-
PVDF: Popular in piping and chemical processing due to its durability.
-
PA66 and PA12: Used in automotive and textile applications for their mechanical strength and heat resistance.
-
PPS: Common in electrical components and automotive parts for its dimensional stability and resistance to high temperatures.
Amorphous Polymers
-
PEI: Frequently used in aerospace and medical applications for its heat resistance and transparency.
-
Polyimide: Ideal for electronics and aerospace, particularly in flexible circuits and insulation.
-
PVC: Utilized in construction (pipes, fittings) and healthcare (tubing) due to its versatility and cost-effectiveness.
Ease of Processing
Crystalline Polymers
Processing crystalline polymers requires precise control over cooling rates to ensure proper crystallization. Their higher melting points often demand advanced equipment and techniques. For example:
-
PTFE: Cannot be melt-processed; requires sintering or compression moulding.
-
PEEK: Demands high processing temperatures (300–400°C).
-
PPS: Crystallizes rapidly, necessitating careful cooling to prevent defects.
Amorphous Polymers
Amorphous polymers are generally easier to process due to their gradual softening and lower shrinkage. For example:
-
PVC: Easily extruded or injection moulded into a wide range of shapes.
-
PEI: Processable with standard injection moulding techniques, despite its high glass transition temperature (Tg).
-
Polyimide: While challenging due to its high thermal stability, amorphous grades simplify processing for thin films.
Comparing Specific Polymers
Property/Polymer |
PTFE |
PEEK |
POM |
PVDF |
PA66 |
PA12 |
PPS |
Polyimide |
UHMWPE |
PEI |
PCTFE |
PVC |
Type |
Crystalline |
Crystalline |
Crystalline |
Crystalline |
Crystalline |
Crystalline |
Crystalline |
Amorphous |
Crystalline |
Amorphous |
Crystalline |
Amorphous |
Tg (°C) |
~127 |
~143 |
~-60 |
~-35 |
~70 |
~45 |
~90 |
~250 |
~-100 |
~215 |
~45 |
~85 |
Melting Point |
~327 |
~343 |
~175 |
~177 |
~255 |
~180 |
~280 |
None |
None |
None |
~210 |
None |
Applications |
Seals, Filters |
Medical, Aerospace |
Gears, Bearings |
Pipes, Films |
Auto Parts |
Hoses, Tubes |
Electronics |
Circuits, Films |
Wear Parts |
Aerospace |
Cryogenics |
Pipes, Sheets |
Processing |
Difficult |
Moderate |
Easy |
Moderate |
Moderate |
Easy |
Moderate |
Difficult |
Moderate |
Moderate |
Difficult |
Easy |
Conclusion
The choice between crystalline and amorphous polymers depends on the specific requirements of an application. Crystalline polymers, with their strength and chemical resistance, excel in demanding environments, while amorphous polymers provide versatility, clarity, and ease of processing.
High-performance polymers such as PTFE, PEEK, and UHMWPE dominate applications requiring durability and chemical inertness. In contrast, amorphous options like PEI and Polyimides are favoured for electronics and optical applications.
Understanding these differences helps manufacturers and designers select the right material for their needs, balancing performance, cost, and processability.
Read More
1. Exploring Polyetherimide (PEI) - The No Fuss Polymer
2. PTFE in Filtration: The Uses and Benefits of Porous and Microporous PTFE Materials
3. Ultra-High-Molecular-Weight Polyethylene (UHMWPE): The Polymer that Just Won't Wear Itself Out