Do you need Size Exclusion chromatography (SEC) & Gel Permeation Chromatography (GPC) in Additive Manufacturing?

Gel Permeation Chromatography (GPC) is a widely used technique that separates macromolecules such as proteins and polymers based on size. As researchers and additive manufacturers continue to push the boundaries of performance and demand more complex material properties, the evolution of the technique has enabled a better understanding of the key indicators that lead to a high performance printed product.

What is Additive Manufacturing?

Additive manufacturing (AM) is a technology that uses three-dimensional computer models to print parts by building the component layer by layer. For metallic products, the traditional method used for manufacturing metal parts is subtractive, whereby milling machines form the part from a solid block of metal.

In the AM process, high-precision electron beams or lasers move at high speeds to selectively melt layer upon layer of metal, tens of microns thick. Highly complex parts are rapidly formed with novel functionality using less material than other methods. Multiple fields use AM, including construction, prototyping, biomechanical, and others, to produce prostheses individually adapted to humans and animals.

Types of Additive Manufacturing Processes

Powder Bed Fusion (PBF), like Selective Laser Sintering (SLS), uses a laser to selectively fuse thin layers of powder particles (usually metal, polymer, or ceramic). Thermoplastic polymers such as nylon are well suited for use in PBF as they are processed reliably due to their semi-crystalline nature, which provides a distinct melting point. The wide temperature working window between melting (during heating) and subsequent crystallisation (via cooling) makes nylon the choice polymer.

Stereolithography is one of the first additive manufacturing or 3D printing technologies developed. Initially, parts manufacturers used the process to create polymeric prototypes, but now it is also used in final-part production. In stereolithography, a large tank or vat of photopolymer resin (composed of oligomers, monomers, and photoinitiators) undergoes cross-linking upon exposure to Ultraviolet (UV) or Visible (Vis) light. A support platform moves the cured object upward or downward layer by layer to form the final product. The tensile stiffness and elasticity of the solid product are essential for additive manufacturers to analyse and ensure consistent quality. Controlling the oligomers’ molecular weight distribution, structure, and proportion of photoinitiator used achieves optimisation. These properties also affect the photopolymer formulation’s rheology and viscosity.

Fused Deposition Modelling (FDM) uses a material-extrusion technique where a thermoplastic filament is drawn through a nozzle, heated to its melting (or glass transition) point, and then deposited layer by layer to cool and harden, repeating the process until the 3D structure is complete. Assessing the melt characteristics and determining the structure of the polymers (i.e. molecular weight distribution, molecular density, and degree of branching) is critical in developing novel feedstock material with unique mechanical properties that are also printable.

Material/binder jetting uses a liquid binding agent to join the metal, ceramic, or polymer powder particles rather than melting or fusing with a laser or electron beam used in PBF. This process forms a green part removed from the printer with solidification via a secondary de-binding or sintering step. Accurate determination of molecular weight and structure of polymeric powders and binders is required to optimise final component properties.

What are the main challenges of Additive Manufacturing techniques?

The leading challenge additive manufacturers face relates to the quality of the final product made, which is highly dependent on understanding the quality of the feed material. Selecting high-quality metal or polymer powders highly spherical and free from satellites or deformed/ agglomerated particles can reduce variation and prevent cracking, distortion, weakness, and poor surface finishes of final products. However, high-quality materials are relatively expensive, which contribute to high build costs. Although the ability to recycle the unused material can save on costs, reusing the polymer powder can age it and cause unfavourable structural changes. By accurately characterising the molecular properties, such as the molecular weight, molecular size, and size distribution of the bulk polymer and polymeric structure (branching, crystallinity), manufacturers can optimise specific AM processes and prevent processability issues that impact the quality of the final component.

Why is particle size and structure important for 3D printing?

Understanding key properties such as particle shape, structure, particle size, and particle size distribution in the powders is essential. These properties can impact the powder’s packing density, flowability, and compressibility. Each characteristic ensures uniformity and must be optimised to create a product free from defects such as pores, cracks, inclusions, residual stresses, and unwanted surface roughness. Irregularly shaped particles tend to increase interparticle friction and decrease flowability while the preferred smoother, more regular-shaped particles flow more easily. As particle size decreases, the forces of attraction between particles increases. Optimisation of flowability and packing density occurs as finer particles increase density by filling the gaps left by larger ones. Therefore, measuring particle shape and size distribution impacts the powder material properties; it is vital for ensuring the feed material is suitable for an application.

One of the primary characterisation techniques for analysing polymers used in additive manufacturing is size exclusion/gel permeation chromatography (SEC/GPC). This technique enables a better understanding of macromolecular characteristics, such as particle size and structure of the feed material, the effects on powder reusability, and ultimately, the final product’s quality.

What is Size Exclusion Chromatography (SEC) or Gel Permeation Chromatography (GPC)?

SEC or GPC is a liquid chromatography technique that separates polymers according to their size (hydrodynamic volume) to measure molecular size and structure. GPC or SEC involves separating the sample as it passes through a porous chromatography column. Larger molecules unable to penetrate the pores are excluded and thus travel through the column faster than smaller molecules, allowing separation based on size.

GPC or SEC can be used to measure molecular weight (MW), molecular weight distribution, intrinsic viscosity, and the hydrodynamic size of macromolecules. The inherent viscosity measurements combined with the molecular weight identify structural differences between samples.

What is absolute Molecular Weight (MW)?

The MW of a polymer is the sum of the atomic weights of the individual atoms that comprise a molecule. It indicates the average length of the bulk resin’s polymer chains. There are different kinds of molecular weight: Number average molecular weight (Mn), weight average (Mw), and z-average molecular weight (Mz). Various techniques can measure each MW moment (Mn, Mw, Mz). For instance, osmotic pressure calculates the number of molecules present and provides average molecular weight regardless of their shape or polymers’ size. In comparison, SEC or GPC in a single measurement provides complete and accurate MW distribution characterisation while also providing structural information. GPC or SEC determines the polymer or biopolymers’ absolute molecular weight and branching degree by measuring light scattering at various angles as a concentration function.

The molecular weight (MW) and molecular size play a key role in determining the mechanical, bulk, and solution properties, determining how the polymer material will behave during processing as a final product. For AM, selecting the correct polymer MW is a balance between printing ease and final-product performance. Low MW polymers exhibit low viscosity and offer better flow properties with fewer stresses. As MW and cross-links increase, so do polymer strength, brittleness, melt temperature, and viscosity, but solubility decreases.

Why use a multi-detection SEC or GPC system?

A conventional GPC or SEC system setup usually consists of only an isocratic pump and a detector, either Refractive Index (RI) or Ultraviolet (UV). This setup provides only a concentration profile of the size-separated sample and relative MW. The calibration standards contain a polymer mixture of known MW correlated against the RI traces in the calibration process. However, this calibration plot is accurate only if the standards’ intrinsic viscosity is identical to that of the sample. Only polymers of the same MW with equivalent intrinsic viscosity will elute at the same rate, a significant limitation when gathering precise data for the detailed comparison of relatively similar polymers when the calibration standards are sub-optimal for the polymers of interest.

In contrast, the Malvern Omnisec GPC/SEC system employs the universal calibration technique to address this limitation. It uses highly informative multiple detection regimes to directly and accurately measure MW. This process includes a concentration detector (RI or UV-Vis), a multi-angle light-scattering detector (RALS/LALS/MALS), plus a self-balancing viscometer that enables the measurement of structural features such as branching or conformation. Multiple detectors provide additional information about a sample when simultaneously evaluating a single injection. This information includes Absolute MW and MW moments; Intrinsic Viscosity (IV), hydrodynamic radius (Rh), the radius of gyration (Rg), dn/dc calculated value, sample concentration, and recovery, to name a few. The Rh of a sample is the radius of a sphere with the same mass and density of the sample based upon molecular weight and intrinsic viscosity. Rg represents the root mean square distance of the molecule’s components from the molecule’s mass centre. Both provide valuable molecular size information. Plotting the MW measured directly from the light scattering detector against the IV measured from the viscometer detector produces a Mark-Houwink plot to illustrate the relationship between molecular structure and molecular weight.

The pioneering work from Viscotek, a market leader in GPC, led to the Omnisec system from Malvern. For the last two decades, the system has continued to evolve. Today, it is the most advanced GPC system for measuring absolute molecular weight, molecular size, intrinsic viscosity, branching, and other structural parameters.

What is the power of multi-detection GPC/SEC?

In a complete GPC/SEC system, OMNISEC integrates the separations unit and all four detectors with inter-detector tubing temperature control.

RI detector: This is the most common detector for any GPC or SEC system. RI detectors are referred to as concentration detectors because the difference in refractive index between the sample solution and the solvent is proportional to the sample’s concentration. It provides a dn/dc value, the refractive index increment, which is essential because it is the link that translates the raw RI signal to sample concentration. Knowing the concentration allows the calculation of all molecular parameters, including absolute molecular weight and IV.

UV-VIS PDA detector: UV-VIS detectors are also concentration detectors but require the sample to have a chromophore and absorb light at a detectable wavelength between 190 – 900nm.

Capillary differential viscometer: First invented by Max Hanley in 1984, this unique viscometer measures the changing solution viscosity to calculate the sample’s intrinsic viscosity (structure). The viscometer detector uses four capillaries, a delay column, and two transducers (DP and IP). It has a user-replaceable self-balancing bridge that helps to reduce downtime and maintenance requirements. The viscometer works by directly measuring the specific viscosity by subtracting the solvent’s contribution in a balanced capillary bridge. When used with a concentration detector, it will measure the IV distribution of any polymer.

Static Light Scattering (SLS) detectors: Light scattering occurs when a photon from an incident beam is absorbed by a macromolecule and re-emitted in all directions. The intensity of light scattering measures MW and Rg described by the Rayleigh theory. Small molecules less than 10 – 15nm in radius will scatter light evenly in all directions and are known as isotropic scatterers. Large molecules with an Rg of more than 15nm (radius) and high MW are anisotropic scatters. They have multiple scattering points and tend to scatter more light in different directions with different intensities. A Debye plot models this angular dependence of samples scattering and is used to determine the MW andRg at every data slice within the chromatogram using multi-angle light scattering.

There are four types of SLS instruments:

• Low Angle (LALS) measures the intensity of light scattering very close to the Zimm plot’s axis or very close to 0°. The calculated MW will be very close to the actual MW therefore ideal for anisotropic scatterers such as large polymers.
• Right Angle (RALS) measures the intensity of light scattering at 90° and with sample concentration provides the measurement of MW for molecules, <15nm (radius) in size, ideal for proteins. Low molecular weight polymers are isotropic scatterers. The resulting partial Zimm plot is flat with a zero slope; therefore, it is unsuitable for these smaller materials. Isotropic scatterers, smaller than 10 – 15nm in radius, will scatter light evenly in all directions, enabling only the MW measurement.
• For large polymers with an Rg >15nm that exhibit angular dependency in the light they scatter, a Multi-Angle (MALS) detector makes it possible to determine molecular size Rg in addition to MW. A conformation plot (plot of Rg against MW) allows the measurement of any structural differences between the samples.

The LS detectors’ high sensitivity enables molecular weights measurements as low as 200 Da or injection masses as low as 100 ng of material. This sensitivity measures low molecular weight samples, novel polymers with low dn/dc, or tiny amounts of precious samples. An RI detector combined with light scattering and viscometer detectors provides the sample’s exact concentration at each data slice using the sample’s dn/dc value to calculate the absolute molecular weight intrinsic viscosity.

By housing all the detectors together in a single compartment, the distances between them can be kept to an absolute minimum, reducing the level of band-broadening and tailing. Additionally, the use of a single temperature-controlled compartment for detectors and all connecting tubing means the temperature can be elevated for polymer applications to reduce the viscosity of certain mobile phases such as DMSO (dimethyl sulphoxide). Combining all of these factors makes Malvern Omnisec Reveal the most-advanced multi-detection platform for analysing natural and synthetic polymers.

Whether you’re looking for the Malvern Omnisec Reveal or another scientific instrument to assist your addictive manufacturing, our team has the expertise to match your research scope to the right analytical instruments. Contact us for more information.

References

• Without a doubt: the future is additive | Materials Talks (materials-talks.com)
• Polymers in Additive Manufacturing: The Future of Part Production | Malvern Panalytical
• From Polymer to Powder: Investigating the Additive Manufacturing Recycling Process | Malvern Panalytical
• Optimizing the macromolecular properties of natural polymers for 3D-bioprinting applications | Malvern Panalytical
• Understanding the effect of polymer structure on its bulk properties: How to optimize for end-use application | Malvern Panalytical
• https://www.malvernpanalytical.com/en/learn/knowledge-center/technical-notes/TN150116-Introduction-to-OMNISEC-REVEAL.html?