Using transmission XRD in Aeris to measure the crystalline to amorphous ratio in polymers

Introduction

Polymer solids can have a range of mechanical properties, at one extreme they can be very soft and elastic and at the other, they can be immensely strong and rigid. Their mechanical properties can change with external conditions such as temperature and pressure. They can be used for a large range of applications from protective coatings to containers and building materials. 

The science of polymer chemistry and formulation is now sophisticated and complex. In simple terms, polymer manufacture often starts with polymer pellets known as a ‘masterbatch’. These pellets of plastic resin are polymers blended with specific additives. The mechanical properties of the blend during handling and curing, must also conform to the manufacturing method used, e.g., 3D printing, spraying, extruding or pressing. To make the polymer suitable for its required application, the additives will enhance the product’s qualities (for example strength, elasticity, heat resistance, conductivity, UV protection, color, transparency, etc).

Plastic bag manufacture

At the microstructural level, the components of a polymer blend need to be characterized and controlled for optimum performance at various stages in product manufacture. Malvern Panalytical provides several analytical methods used at various stages in polymer research and manufacture including laser diffraction (LD), dynamic light scattering (DLS) and imaging for the size and shape of fillers and additives such as carbon fibers, pigments, and extenders; X-ray fluorescence (XRF) for elemental analysis (batch quality control and impurity detection); size exclusion chromatography (GPC/SEC) for polymer characterization (molecular weight dispersion, branching) and X-ray diffraction (XRD) for fiber texture, crystalline phase analysis and crystalline to amorphous ratio.

In this application note, we focus on how XRD with the Aeris compact diffractometer, can bring real benefits with quick and high-quality measurement of the phase and crystalline-to-amorphous ratio of polymers.

The crystalline to amorphous ratio in polymers

Polymers are materials where the component molecules have joined together (in solution) to form chains and branched structures. The chain size and branching characteristics can be very uniform (showing a narrow size and structure distribution) or they can exhibit a broad range of sizes, measured by GPC as a wide distribution of molecular weight and structure types. In polymer chemistry, additives are used in polymer production and the polymer blend to create the required molecular size distribution. During processing and curing of polymers into their solid form, some polymers remain amorphous whereas others partly crystallize. A typical solid polymer can be a composite of microscopic crystalline and amorphous phases, and it is this combination that imparts particular mechanical properties of the polymer. For example, a higher proportion of crystallinity in the solid produces a harder and more thermally stable material, but it is also more brittle, whereas the amorphous regions provide elasticity and impact resistance.

Schematic illustration of polymer microstructures

An example polymer analysis: plastic bag

Plastic bags typically are made from one of three basic polymer types: high-density polyethylene (HDPE), low-density polyethylene (LDPE), or linear low-density polyethylene (LLDPE). For example, heavy weight glossy shopping bags are LLDPE, while grocery bags are HDPE, and fine garment covers are LDPE.

For the manufacture of plastic bags, small resin pellets are fed into extruders and melted down until they are molten and pliable. They are subsequently extruded (forced/pressed) through a circular die gap by screw drives to form a continuous tube of plastic. Air is introduced to form a ‘bubble’ which inflates and stretches the plastic to the required size and wall thickness. This continuous air-filled tube cools, after which it will be flattened out and wound onto a roll.

It is possible to produce polyethylene products that are either completely recyclable or biodegradable, so they produce the least possible waste, and all the scrap produced during manufacture is recycled into polyethylene granules to be blown into usable film again. [1]

Sample preparation

The Aeris diffractometer supports a wide range of standard full-sized sample holders to hold liquids, powders, and solids. So, polymers can be analyzed at any stage of a production process, from solution to final product. The Aeris measurement can be fully automated and used in a variety of settings from research through to fast quality control.

Aeris supports a wide range of sample holders. Sample loading and batch measurement and analysis can be fully automated.

The best XRD configuration for measuring polymer crystallinity in a plastic bag is in transmission mode and to facilitate this, a plastic bag was folded several times and mounted in a double framed transmission holder. This kept the film truly perpendicular to the beam and the holder is precision made, to always keep the sample at the center of the diffractometer, resulting in the highest data accuracy and the most reproducible results.

Plastic bag mounted in double-framed precision transmission holder

Data collection, results and analysis

Transmission XRD data were collected within 15 minutes per measurement. Two scans were compared. One scan is obtained without the sample and is used in the analysis to account for background effects such as air scattering. The second scan is obtained with the sample in place. 

Using HighScore for data analysis, the background air scatter (red) data were used to model the background of the 15-minute transmission scan so that the remaining intensity, comprising Bragg peaks and broad peaks, could be attributed respectively to the crystalline and amorphous content of the polyethylene.

The scan below clearly shows both a crystalline component, giving rise to sharp Bragg peaks, and an amorphous component, giving rise to broad humps. 

Phase identification of the crystalline part of the polyethylene was performed with Malvern Panalytical’s HighScore Suite using the PDF-4 Organics database. The database pattern 00-060-0986 was the best match, confirming that the sample was polyethylene HD (HDPE).

Using HighScore for data analysis, the background scatter was subtracted from the raw data for a 15-minute transmission scan and the Bragg peaks (in orange) were pattern matched to pattern 00-060-0986 in the PDF4 Organics database (blue), confirming the crystalline phase of HDPE. The remaining intensity (green) can be attributed to scattering from the amorphous phase. 

Crystalline to amorphous ratio

The Plus option of HighScore was used to employ a Rietveld-like refinement method to refine both the crystalline Bragg diffraction and the amorphous scatter. The crystalline phase is modeled as a Rietveld phase, while the amorphous is modeled as a so-called hkl-phase as it is used in a PONKCS approach. The crystalline to amorphous intensities are adjusted in the model via automated pattern fitting until the best fit is obtained. Quantification was achieved using the recently developed Direct Derivation Method (DDM), which allows the standardless quantification of crystalline and amorphous substances alike. The result was that the polyethylene was 33.5% crystalline and 66.5% amorphous.

Conclusions

The Aeris compact XRD, in combination with Malvern Panalytical’s HighScore Plus Suite enables the rapid analysis and measurement of polymer films. This application note covers the identification of the polymer as HDPE and the crystalline-to-amorphous ratio as approximately 1:2.

This serves as an example of the system's capabilities in one aspect of polymer chemistry. Measurements are also possible for liquid phase (using capillaries) or solid phase in transmission, reflection or grazing incidence geometries (GIXRD), 2D measurement of polymer fibers and films.

Furthermore, this example highlights the potential of the standardless Direct Derivation Method, implemented in HighScore, for quantifying the crystalline and the amorphous content in a polymer without prior knowledge.

Further reading

DDM Method:

• H. Toraya, A new method for quantitative phase analysis using X-ray powder diffraction data: direct derivation of weight fractions from observed integrated intensities and chemical compositions of the individual phases, J. Appl. Cryst. (2016), 49, 1508 - 1516
  
• H. Toraya, A new method for quantitative phase analysis using X-ray powder diffraction data: direct derivation of weight fractions from observed integrated intensities and chemical compositions of the individual phases. Corrigendum, J. Appl. Cryst. (2017), 50, 1 of 1
  

PONKCS  

• N.V.Y. Scarlett and I.C. Madsen Quantification of phases with partial or not known crystal structure, Powder Diffraction (2006), 21(4), 278-284.