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Polymer beads. Image Credit: Shutterstock.com/Meaw_stocker

Crystalline Polymers: Definition, Properties, Uses, and Advantages

Xomety X
By Team Xometry
October 18, 2023
 14 min read
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Crystallization of polymeric materials occurs when its polymer chains have become locally aligned into an ordered pattern. These regions are referred to as lamellae. The crystallization has various drivers. These triggers may generate irregular and localized crystal regions that are not aligned with each other or they may align them into a nearly uniform unidirectional structure. These regions can be extensive and interconnected or they may be localized and small.

When lamellae constitute at least 10% of the volume of the polymer, the material can be described as semi-crystalline. When they represent 80% or more of the mass, the material is considered to be crystalline. Molecules within crystalline polymers are more closely packed than in disordered materials, so crystalline materials are denser.

This article will discuss crystalline polymers along with their definition, properties, uses, and advantages.

What Is a Crystalline Polymer?

A crystalline polymer is a type of long-chain organic material characterized by the presence of lamellae, which are ordered zones of aligned molecular chains. Crystalline polymers have highly structured regions and can in some cases be entirely composed of a single crystal aligned on one axis. The degree of crystallinity and mutual alignment is highly affected by the crystallization process, which can be triggered and driven by a variety of mechanisms.

You can, for example, heat-treat a polymer component by holding it at a temperature between the glass transition temperature (Tg) and the melting point to let its molecules shift and crystalize. This can be a very slow process. It is difficult to maintain dimensional stability this way for extended periods, so this method typically only gives you a limited degree of molecular alignment. 

Some polymers naturally “drift” towards crystallization over time. High molecular motility and elevated temperatures can induce this, resulting in either progressive strengthening or embrittlement.

By controlling the flow of liquid polymer in the manufacturing process, you can influence local and bulk crystallization. A restricted outlet for the molten polymer can generate highly crystalline material. This can create a uniaxial high-strength region that can, for example, be used as a living hinge. Extrusion of polymer can induce molecular alignment, driving a generalized change that makes the material tougher on the primary axis. Extension can similarly align molecule chains. Such extension can be uniaxial (simple stretching), biaxial (X and Y axis combined stretching), or essentially annular (such as when vacuum-forming a dome). These result in differing forms of molecular alignment.

How Are Crystalline Polymers Made?

Some polymers can be manufactured to be highly crystalline while others can’t. In either case, though, the manufacturing and handling methods largely determine the final percentage of crystalline lamellae. 

1. Intrinsic Chemistry and Polymerization Process: Polymers with highly linear polymeric chains are the easiest to crystalize. High-density polyethylene (HDPE) polymer chains, for example, have very few side branches, so the chains can move and align with relative ease. Low-density polyethylene (LDPE), despite being so chemically similar to HDPE, can only achieve limited crystallization because its chains have so many side branches. 

However, the degree of crystallization is still highly variable. Isolated HDPE lamellae within a more amorphous structure can be induced to grow, or they can be prevented from doing so. The “native” crystallization of cast HDPE is around 60%, but it can be raised to 80% by heat-treating the material. Many polymers display some degree of crystalline behavior in their natural state.

2. Manufacturing and Handling Variations: Manufacturing methods also impact crystallinity. Some can even induce crystalline structure in non-crystalline polymers.  

Extrusion generates crystallinity in many polymers. By pushing the material through a narrow die, you force many of its molecular chains into alignment. Extruded polymers thus take on a fibrous appearance and work well as living hinges. An everyday example of flow-induced crystallization is the lid hinge on a toothpaste tube. After being extruded, the hinge also receives a mechanical pinch to keep it compact and minimize internal shear. 

Drawing a polymer will also induce some crystallization in a similar way to extrusion. The fibers get extended — often to over double their initial length — so the internal polymer chains slide into alignment with one another. This crystallization is why drawn polymers are stronger than unprocessed materials. Polymer stock can also be drawn down along multiple axes to create orientated or biaxially oriented films.

Vacuum forming can induce crystallinity. Stretched portions of the item become more crystalline while domed areas take on biaxial orientations. 

Some polymers can be heat treated during their raw-material production phase to give them crystalline properties from the start. Temperature gradients and localized cooling will help control the crystals’ orientation. 

How Do Crystalline Polymers Differ From Regular Polymers?

Many polymer chains can be crystallized. Materials like: nylon, syndiotactic polypropylene, isotactic polystyrene, and acetal (POM) are nearly always crystalline. 

Other thermoplastic polymers are intrinsically amorphous and cannot form crystals. Examples include: polymethyl methacrylate (PMMA/Acrylic), polystyrene (PS), polycarbonate (PC), polysulfone (PSU), polyvinyl chloride (PVC), and acrylonitrile butadiene styrene (ABS). Their molecular structures remain randomized and nonlinear enough that they cannot be aligned. Thermoset plastics are all considered low- or non-crystalline materials, though researchers are working to create semi-crystalline versions. 

To learn more, see our guide on Polymer Material.

Is There a Difference Between Crystalline Polymers and Semi-Crystalline Polymers?

Yes, there is a difference between crystalline and semi-crystalline polymers. A polymer is called crystalline if more than 80% of its polymer chains are aligned. Semi-crystalline polymers exhibit anywhere from 10% to 80% crystalline alignment. Materials that fall below the 10% mark are known as amorphous. 

The spectrum of crystallinity is caused by side branching and randomness within the polymers’ molecular structure. Some polymer chains inherently tend to align with one another while others come in nonlinear shapes and can’t align. Material processing can also make a polymer more crystalline. For example: molding, extruding, drawing, or heat treatment all help polymers crystallize. 

What Are the Characteristics of Crystalline Polymers?

Crystalline thermoplastic polymers differ significantly from amorphous ones. 

  1. Crystalline thermoplastics are typically denser. It’s especially evident when comparing chemically similar materials with different polymer chain structures. LDPE and HDPE have respective densities of 0.92 g/cm3 and 0.96 g/cm3, for example. 
  2. A crystalline structure makes for better tensile strength. For example, LDPE has a tensile strength of 10-15 MPa, whereas HDPE reaches over 21 MPa despite being a nearly identical chemical. 
  3. Hardness is generally higher for this class of polymers.
  4. Crystalline and semi-crystalline polymers tend to be opaque or slightly translucent.
  5. Melting points tend to be more sharply defined as the degree of crystallization increases.
  6. These polymers tend to shrink more as they solidify because they change from a disordered liquid state to a highly ordered solid state. 
  7. They are less chemically accessible and therefore more resistant to organic solvents and acids.
  8. Crystalline polymers aren’t as permeable because of their closer-packed atomic structure.

Is Crystalline Polymer Weldable?

Yes, crystalline polymers are weldable. The most common welding technique used on plastics is ultrasonic welding or the application of focused high-frequency vibration. This technique works well with semi-crystalline and even highly crystalline polymers. However, heat dissipates into the parts more easily than for amorphous polymers. Sound waves travel further in crystals before being transformed into heat. This makes the welding more challenging and requires higher contact pressure and more vibration energy.

Laser transmission welding (LTW) is an emerging technology that loses less energy to the polymer’s broader structure. 

Thermal welding, meanwhile, is well-studied and practical for most amorphous and semi-crystalline polymers. There has been less study of the thermal welding of highly crystalline polymers. However, anecdotal evidence reports moderate success with thermal welding, even in advanced hot-melt glue applicators.

An important thing to remember is that crystallinity is often extrinsic — in other words, it is imposed on the material through post-production processes. The weld will likely alter crystallinity in its heat-affected zone so that the zone’s properties will shift. 

Do Crystalline Polymers Rust?

No, crystalline polymers do not oxidize. They can, however, degrade under the influence of UV radiation, organic solvents, and some organic acids. Crystalline polymers resist these forces better than non-crystalline ones, though. 

What Are the Physical Properties of Crystalline Polymers?

The typical physical properties of crystalline polymers are:

  1. Crystalline polymers tend to melt more suddenly than amorphous ones. 
  2. The close molecular packing in the crystalline lattice results in higher density in crystalline regions than in the amorphous volume.
  3. They are generally stiffer, stronger, and tougher than other polymers. However, they become more brittle, which can offset some of the advantages. 
  4. Highly crystalline polymers are opaque, although distributed lamellae can still allow good transparency.
  5. Crystalline polymers generally exhibit good thermal stability and resistance to high temperatures. The onset of weakening in the intramolecular Van der Waals forces scales with the molecules’ proximity to one another. 

What Are the Chemical Properties of Crystalline Polymers?

The typical chemical properties of such polymers are:

  1. The polymer chains are arranged in an orderly, repeating fashion, generally straight rather than tangled. Crystalline regions are defined by this regularity. 
  2. Polymer crystals are robust in terms of intermolecular bonds. This makes them stronger and more resilient than the equivalent semi-crystalline or amorphous forms of the polymer.
  3. The bond integrity makes these polymers less susceptible to solvents, organic acids, alkalis, and radiation attacks than their less crystalline equivalents.

What Are Crystalline Polymers Composed Of?

All thermoplastics consist of long-chain molecules or repeating units of simple chemicals called monomers. These monomers are strung together (polymerized) to form chains. The nature of the polymerization process and the intrinsic chemical properties of the monomers define the regularity or complexity of the resulting molecule. Crystalline polymers are composed of unbranched chains or chains with only minor branching. 

What Are the Uses of Crystalline Polymers?

Crystalline polymers have many uses. Most engineering plastics such as POM (polyoxymethylene), HDPE, and many nylons are tough and resilient polymers. They offer great dimensional stability, good machining properties, respectable temperature tolerance, and durability against wear. This makes them ideal for engineering components and high-stress parts in machine construction, vehicle components, and street furniture applications. They are ideal for sliding and rotating bearing surfaces, particularly when paired with hard materials such as hardened and precision-ground stainless steel.

The chemical resilience of many crystalline thermoplastics lets them handle aggressive chemical environments. This allows fuel line components to be made using liquid crystal polymer, acetal (POM) copolymer, and fluoropolymers, all of which are essentially crystalline. 

What Are Some Examples of Crystalline Polymers?

Most engineering polymers are at least semi-crystalline. They often have similar properties. 

1. Polypropylene (PP)

Polypropylene (PP) is not generally thought of as an engineering plastic, being quite soft and ductile, but it is very versatile. It belongs to the polyolefin group and is highly crystalline. This material consists of repeated propylene monomer units. Its chemical formula is (C3H6)n where “n” is the repeat step that forms it into a chain molecule. It is formed from a chemically simple monomer and possesses no side branching. 

Polypropylene is widely exploited for its chemical resistance to solvents and aggressive/reactive chemicals. It is formed into containers and pipes in chemical processing/storage, food/medical storage, and packaging.

To learn more, see our guide on Polypropylene Material.

2. Polybutylene Terephthalate (PBT)

Polybutylene terephthalate (PBT) is a thermoplastic engineering polymer from the polyester family. It has beneficial mechanical, thermal, and electrical properties. PBT is a linear polyester consisting of repeating butylene terephthalate units. Its chemical formula is (C10H10O4)n, where "n" represents the number of repeated units in the polymer chain. The melting temperature ranges between 225 and 260 °C. Since it doesn’t absorb water, PBT is  ideal for piping and use in humid and wet conditions. PBT is electrically insulative and does not react with most solvents or other chemicals. 

To learn more, see our guide on Polybutylene Terephthalate.

3. Polyethylene (PE)

Low-density polyethylene is highly amorphous and is generally not viewed as an engineering material. However, high-density polyethylene (HDPE) is very crystalline and sees significant use in engineering settings. HDPE is composed of minimally side-branched chains of ethylene monomers. Its chemical formula is (C2H4)n, where "n" represents the number of repeated units in the polymer chain. It is much stronger and more rigid than its low-density form. 

HDPE resists acids, alkalis, and organic solvents far better than most polymers. There’s little wonder why it gets used alongside corrosive substances. HDPE can withstand temperatures ranging from -50 to 120 °C without significant deformation, making it a useful material in power generators, engine bays, and aggressive chemical processing environments. And the material barely absorbs any moisture throughout that whole temperature range. The polymer is highly resistant to ultraviolet (UV) radiation, making it suitable for outdoor applications. It is also an excellent electrical insulator that’s ideal for high-voltage applications.

To learn more, see our guide on Polyethylene.

Is Polyoxymethylene (POM) a Crystalline Polymer?

Yes, Polyoxymethylene (POM) is a chemically simple macromolecule that does not form side branches. This makes the resulting material highly crystalline.

What Are the Advantages of Crystalline Polymers?

Compared to other polymers, crystalline ones are:

  1. Generally stronger, tougher, and more dimensionally stable.
  2. More heat tolerant.
  3. Generally easy to machine.
  4. More rigid.

What Are the Disadvantages of Crystalline Polymers?

Some disadvantages of crystalline polymers include:

  1. Less ductile and can display brittle behavior, particularly under high strain rates. Intramolecular movement within a crystalline structure is much more restricted than in amorphous material. Strain can generate microfractures that amass and compound over time. 
  2. Are generally opaque. They won’t work if you need a translucent material. 
  3. Are generally more expensive than their amorphous equivalents.
  4. Their chemical stability makes them hard to solvent-weld or paint. And though ultrasonic welding works, it’s more difficult than for amorphous polymers.

Are Crystalline Polymers Expensive?

Yes, crystalline polymers typically cost more than amorphous ones — largely because less of the raw material gets produced worldwide. For example, the price of LDPE is around $1,750 per tonne, whereas HDPE sells for around $2,100 per tonne. 

An exception to this higher-cost principle is polypropylene (PP) which is among the cheapest polymers out there and gets used in huge quantities — as much as 80 million tonnes p.a. The simplicity of the monomer, the relative ease of polymerization, and the huge production volumes make this material around$1,200 per tonne.

Are Crystalline Polymers Hard To Find?

No, crystalline polymers are not hard to find. They are among the most common polymer materials and are widely manufactured and supplied across the planet. Sourcing is simple, even in remote locales, because they’re so fundamental to virtually all technologies and products.

What Is the Difference Between Crystalline Polymers and Amorphous Polymers?

Crystalline polymers are those in which the proportion of lamellae (crystal regions) make up 80% or more of the structure. Amorphous polymers, on the other hand, are those whose structure is below 10% crystalline lamellae.

What Is the Difference Between Semi-Crystalline Polymers and Amorphous?

Semi-crystalline polymers are those whose structure consists of between 10% and 80% crystalline regions. Amorphous structure contains less than 10% crystalline regions, and their polymers are highly disordered. 

Summary

This article presented crystalline polymers, explained them, and discussed their properties and uses. To learn more about crystalline polymers, contact a Xometry representative.

Xometry provides a wide range of manufacturing capabilities and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.

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Xomety X
Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.