Reaching the unreachable!
Confocal Raman spectroscopy is a highly suitable technique for this, as the laser beam can be focused on a particular spot beneath a material surface, shown in the schematic. We discuss the capabilities of this technique here in 2 demonstration examples.

1. Polyethylene bag
As a first example we have placed a closed polyethylene (LDPE) bag containing poly(ethylene terephthalate) (PET) granules directly under the confocal Raman apparatus. We performed a depth analysis through the bag into the granules, while continuously analyzing the material composition. A clear transition can be seen going from LDPE towards PET in the spectra below. This example also demonstrates another use case for this technique, in which a bag of unknown material could be analyzed without needing to risk opening the bag, as confocal Raman spectroscopy can be used to analyze the material right through the packaging.

2. Glass vial with acetone
The same principle applies to unknown liquids inside a glass vial. In this example we placed a glass vial with acetone directly underneath the Raman microscope. The measurements can be performed through the glass barrier, while continuously analyzing the chemical composition of both the glass vial and the acetone. The spectra below clearly show the transition between the different materials

Confocal Raman spectroscopy can be used to identify unknown substances, particularly when very detailed and local sample analysis is required. With this technique the chemical composition of particles with a particle size down to 1 μm can be analyzed. Moreover, these particles can be analyzed even if they are fully enclosed inside a matrix, as we have demonstrated in the examples.

Have you ever encountered small particulate matter trapped in your product without knowing its origin? Confocal Raman spectroscopy may be the answer. Please feel free to contact us to discuss the possibilities!

We highlight this importance using two common examples, shown in Figure 1, and briefly discuss how we can measure as well as influence this property

Figure 1. Examples of applications where control over thermal expansion is critical. Left: Antenna tower; right: reflector of a car headlight assembly, visible behind the transparent cover.

The first example comprises 5G antennae, which are increasingly important in our daily lives as they transmit data from our phones, cars, and many other devices. In any antenna, data transfer is most efficient when transmitter and receiver wavelengths are matched. Since 5G operates at high frequency (up to 54 GHz), these antennae can be quite small, as the length of an antenna is inversely proportional to the frequency. Such antenna modules are often densely constructed, in which heat from the integrated circuits can build up. Moreover, many of these antennae are exposed to the everyday weather and fluctuating temperatures. As such, the antenna can undergo thermal expansion, which can lead to reduced efficiency or even damage to the antenna due to material warpage. Therefore, the thermal expansion coefficient is a crucial parameter in material selection for this application.

The other example is a car headlight assembly, particularly focusing on the reflector part (behind the transparent cover), which has a main function to direct the light towards the road. These reflectors are molded plastic parts, coated with an aluminum reflective layer. In such an assembly the temperature can vary greatly, not just from ambient conditions but also from heat generated by the lightbulb itself (although with modern LEDs this is less of an issue). However, the plastic, often polycarbonate (PC), expands much more under increasing temperature than the metal coating. It is not hard to imagine how this can result in delamination of the coating from the plastic, causing the reflector to malfunction. This example also demonstrates the importance of considering the thermal expansion behavior of materials, especially when combining them for any application.

A material’s expansion behavior is captured in the coefficient of thermal expansion (CTE), which can be measured using thermomechanical analysis (TMA). TMA is a technique that accurately measures dimensional changes in a sample, as a function of temperature (or time). Expansion can be measured using quartz compression or tensile probes, but other quartz probes (3-point bending, penetration) can also be used to measure heat deflection temperatures or softening points. Our TMA sample holder and quartz compression probe can be seen in Figure 2 below.

Figure 2. Left: Aluminum sample in the TMA holder. The quartz compression probe accurately measures any length changes. The thermocouple on the right side monitors the temperature. Right: TMA measurement data, with CTE values for each material.

To demonstrate differences in thermal expansion for the materials used in a car headlight assembly, we have measured several materials using our PerkinElmer Diamond TMA: aluminum, PC, and glass. The measurement data is also shown in Figure 2.

We can calculate the CTE of the materials from the slope of the curves in Figure 2, as shown, which are in close agreement with values from literature. Additionally, TMA data can also be used to measure the glass transition temperature (Tg) of a polymer sample; for this PC sample Tg was found to be 148 °C.

We clearly see a large difference in CTE for PC and aluminum, demonstrating the issue for the reflector part of our car headlight assembly, and allowing us to think of a solution. A common method to decrease the CTE of polymers is to add fillers with a low CTE, like glass. Therefore, one possible solution would be to make a PC composite with glass fibers, which is cheap and additionally strengthens the polymer. For example, the CTE of a 30 % glass-filled PC was determined as 22 · 10-6 K-1, which is a close match to the CTE of aluminum and would therefore be a suitable composite material for the headlight reflector application.

However, when compounding such composite materials more aspects need to be considered, such as fiber dispersion during compounding or fiber orientation during processing of the parts. This can result in anisotropy in the material properties, which can be expressed as greatly differing CTEs in the machine or transverse direction. As such, careful analysis of CTE is crucial in many aspects of material and product development.

We are happy to support you with TMA measurements for any of your thermal expansion challenges, so please feel free to contact us for further information.