Dynamic Mechanical Analysis (dma)

Dynamic mechanical analysis (DMA) Dynamic mechanical analysis (abbreviated DMA, also known as dynamic mechanical spectroscopy) is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers. The samples may be presented in a variety of forms including bars, strips, discs, fibers and films. Even powders can be tested when suitable containment is arranged.

Principle of DMA A sinusoidal stress is applied and the strain in the material is measured, allowing one to determine the complex modulus. The temperature of the sample or the frequency of the stress are often varied, leading to variations in the complex modulus; this approach can be used to locate the glass transition temperature of the material, as well as to identify transitions corresponding to other molecular motions.

Material Selection Materials having long molecules, such as synthetic and natural polymers, are immediate candidates, but are by no means the only ones. Any material that forms a glass will have viscoelastic behavior. Typically these are referred to as ‘amorphous’, in that they have no regular crystalline structure. There are also large numbers of such materials that are classified as semi-crystalline (and by implication, semi-amorphous).

Viscoelastic behavior The opposite end of the spectrum to elastic behavior is viscous behavior. This is readily evident in liquids where they flow in response to an applied force. Some materials behave between the elastic and viscous regime. Such materials are described as viscoelastic and many glass forming or amorphous materials fall into this category.

Viscoelastic properties of materials Polymers composed of long molecular chains have unique viscoelastic properties, which combine the characteristics of elastic solids and Newtonian fluids. The solidlike and liquidlike behavior of polymer can be modeled mechanically with combinations of springs and dashpots.

Cont….. Some materials behave between the elastic and viscous regime. Such materials are described as viscoelastic and many glass forming or amorphous materials fall into this category. For example, the continuous increase in length with time under the influence of a constant applied force defines a phenomenon called ‘creep’.

Usage of DMA instruments There are four main areas of DMA use: • Molecular structure characterization • General material analysis • Food and biomedical testing • Derivation of engineering data

Molecular structure characterization • Molecular structure characterisation is the main reason that DMA was developed. The ability to explore the molecular structure via a simple thermal scanning test, requiring a relatively small amount of sample (0.5–2 g), gave the polymer chemists a powerful characterisation tool. generally the apparatus is more expensive, harder to use, experiments take longer and specimens frequently require special preparation. This is why DMA has emerged as the dominant tool for the structural evaluation of polymers.

General material analysis The vast expansion of the use of DMA has occurred in the field of polymeric material analysis. First, DMA is one of the best techniques for assessing the amorphous content of a material. It is important to know how much amorphous material is present in a number of situations. Since DMA is sensitive to molecular structure it is frequently used to check one sample against another that is meant to be the same.

Food and biomedical testing An important and growing sector of DMA application is within the food and bioscience sector. Many samples are measured for the reasons given above, namely the determination of Tg, checking similarity of samples. Generally, the techniques are similar to those required for the analysis of polymers. However, the temperature range is usually less (−50–200◦C) and the water content frequently plays a pivotal part in the sample’s properties.

Derivation of engineering data A more specialized area of use of DMA is the derivation of engineering data. DMAs are capable of generation of modulus and damping factor (tan δ) data over a wide range of frequency and temperature. In such applications great care has to be taken that meaningful data are generated. Other mechanical testing equipment is normally used to source static modulus or viscosity data, but DMA is usually the only source when high frequency or damping factor data are required.

Dynamic moduli of polymers The viscoelastic property of a polymer is studied by dynamic mechanical analysis where a sinusoidal force (stress σ) is applied to a material and the resulting displacement (strain) is measured. Viscoelastic polymers have the characteristics in between where some phase lag will occur during DMA tests.

Applications Measuring glass transition temperature Amorphous polymers have different glass transition temperatures, above which the material will have rubbery properties instead of glassy behavior and the stiffness of the material will drop dramatically with an increase in viscosity. At the glass transition, the storage modulus decreases dramatically and the loss modulus reaches a maximum.

Polymer composition Varying the composition of monomers and crosslinking can add or change the functionality of a polymer that can alter the results obtained from DMA. DMA can also be used to effectively evaluate the miscibility of polymers.

Types of analyzers • forced resonance analyzers force the sample to oscillate at a certain frequency and are reliable for performing a temperature sweep.

• free resonance analyzers measure the free oscillations of damping of the sample being tested by suspending and swinging the sample.

Test modes • Temperature sweep A common test method involves measuring the complex modulus at low constant frequency while varying the sample temperature. A prominent peak in Tan(δ) appears at the glass transition temperature of the polymer.

• Frequency sweep A sample can be held to a fixed temperature and can be tested at varying frequency. Peaks in Tan(δ) and in E’’ with respect to frequency can be associated with the glass transition.