Monday, 31 December 2018

Dynamic Mechanical Analysis (DMA) as a Polymer Material Selection Tool



Dynamic mechanical analysis (DMA) provides insights into the mechanical behavior of a material under different conditions. Therefore, it is a powerful tool for polymeric materials selection. In this blog post, I show you why and how you can benefit of it. Polymeric materials are based on long chains of repeating units with inherent high molecular weight. In thermoplastics, the polymer chains are to certain extent entangled. However, they are not chemically bonded to one other, which means they have certain degree of mobility. For instances, disentanglement of the polymer chains can lead to failure of a polymer-based part.

What is DMA?

At its core, DMA is a thermomechanical analytical method that estimates the viscoelastic properties of a given material. It allows you to gain insights into the temperature and time dependency of the tested material. This is achieved by measuring the modulus as a function of temperature, time, or frequency. As a result you will obtain the storage or elastic modulus (E’), the loss or viscous modulus (E’’), and the tangent of the phase angle delta (E’/E’’). In this blog post, I will focus especially on temperature-dependency. During testing some sort of deformation i.e. tension, shear, compression, torsion, or flexure, is applied on the sample material and the modulus is measured. As a result, the material’s elastic modulus (E’) can be plotted against different temperatures. This can be helpful to evaluate the mechanical properties of an existing polymeric part over its service temperature range as well as to decide upon most suitable material to choose according to the application service temperature demands.

What happens on a molecular level?

Most molecular transitions can be made transparent by DMA. For understanding this better, we can have a look at the so-called crankshaft model which explains the molecular chains as a series of joints with emphasis on the change of free volume [2]. There are also more detailed models such as the Doi-Edwards model.


In Figure 1, I sketched the different types of molecular motions and how they are linked to the different temperature transitions. Main movements are slipping of chains, i.e. rotation, bending, and stretching. The gamma transition (Tγ) is the first transition away of the solid state where the first movements of atoms are noticed. With increasing temperature, a secondary transition, also called beta transition (Tβ) happens. Here, 4 - 8 backbone atoms are able to move. Polymers can develop toughness due to increased movement of side chains. With further increase of the temperature the alpha transition (Tα), also called glass transition (Tg). In this case, more than 40 backbone atoms are able to move and mechanical properties drop by one to two powers of ten.

Figure 1: Crankshaft model and type of molecular motions [2].


DMA in action: a useful tool for better decision making in polymer material selection


Let’s have a look at some results of DMA. Figure 2 shows the elastic modulus as a function of temperature for an amorphous polymer such as polycarbonate (PC) and a semi-crystalline polymer such as Nylon (e.g. PA 6). Amorphous polymers show a steep drop in modulus in the region of the glass transition temperature. This is for PC 147°C. For semi-crystalline polymers, such as PA 6 there is also a decrease in modulus at the glass transition temperature, in this case 47 °C. However, in contrast to PC some degree of mechanical strength can be retained from 80 to 180 °C until the crystalline melting temperature of 220 °C is eventually reached.

Figure 2: DMA of an amorphous and a semi-crystalline polymer [1].


Figure 3 shows the elastic modulus as a function of temperature of highly glass fiber filled polyamidimide (PAI), polyphenylene sulfide (PPS) and a thermoset (phenolic). For a room temperature application, PPS and phenolic seem to be the best choice judging from their higher elastic modulus and thus mechanical strength. However, when the service temperature of your application is, for example 130°C a different choice may be more suitable. In this temperature range, PAI is in the lead keeping its high modulus up to 260°C.


Figure 3: DMA of highly filled PAI, PPS and phenolic [1].

Overarching, the most suitable polymer for a certain application obviously depends on the specific application requirements, respectively needed modulus at a certain service temperature or range thereof. As such, the whole DMA curve of a material should be considered during the selection process. In summary, DMA allows you assessing materials’ properties under different circumstances. Material selection has a lot to do with thinking in relationships of time-dependency and temperature-dependency behaviors which can be shown better graphically. Single point data on materials’ technical data sheets (e.g. modulus value at 23 ºC) can lead to misjudgment and negatively impact the material selection process.


Thus, whenever in a discussion with a material supplier or design engineer, discuss how selected materials behave over a wide range of temperatures for instances.


I hope this method is useful and helps you in your material selection.


Thanks for reading & till next time!
Greetings,
Herwig Juster


New to my Find Out About Plastics Blog – check out the start here section


Literature:
[1] M. Sepe - Thermal Analysis of Polymers, Rapra Technology, Shawbury, U.K., 1999
[2] PerkinElmer - Dynamic Mechanical Analysis Basics: Part 2 Thermoplastic Transitions and Properties

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