DYNAMIC MECHANICAL ANALYZER (DMA)

BYDAVID SEHGAL

What is DMA?

DMA is a measuring instrument which is used to determine the dynamic characteristics of materials.

It applies a dynamic oscillating force to a sample and analyze the material’s response to that cyclic force.

Basically, DMA determines changes in sample properties resulting from changes in five experimental variables: temperature, time, frequency, force, and strain.

Movable frame (main unit)

Fixed frame or base

Temperature control

DMA+450 MODEL

Construction of DMA

Upper part of DMADMA+450 MODEL

How does DMA work?

The basic principle of this instrument is to exert a dynamic excitation of known amplitude and frequency to a specimen of known dimensions.

The measurement of strains and dynamic forces yields the specimen's stiffness.

From the known geometry, one can derive mechanical properties of the material, such as modulus and loss factor or damping.

The presence of the thermal chamber allows us to perform test at different temperatures and thus determining materials' glass transition temperature.

Which materials can be analyzed with DMA?DMA instrument can be used to characterize mechanical and/or thermal

properties of a great numbers of materials: Polymers Elastomers Composites Metals and metallic alloys Ceramics, glass Adhesives Bitumen (solid and pasty) Paint and varnish (gels or films) Cosmetics (gels, spray….) Oils Biomaterials Leather, skin hair….

Mechanical testing

Configuration of specimen and specimen holder for different tests in DMA

Contd……

Compression platesTension jaws for film

Tension jaws for bars

Tension jaws for bars

Plane shear for films

Plane shear Shear for liquid Shear for pasty material

Dual cantilever Three point bendingDMA+450 MODEL

Viscoelasticity :- Viscoelastic materials exhibit characteristics of both viscous and elastic materialsEx.- Elastomers, polymers etc. Viscosity resistance to flow (damping)Elasticity ability to revert back to original shape

Elastic vs viscoelastic response

Glass Transition Temperature (Tg)Definition: Transition from bond stretching to long range molecular motionFlow TemperatureDefinition: point at which heat vibration is enough to break bonds in crystal lattice

Theoretical basis for DMA

Continued….

sinusoidally applied stress

measured strain

phase lag between applied stress and measured strain

Complex dynamic modulus (E*)• Ratio of applied stress to measured strain E* = E’ + iE” = SQRT(E’2+ E”2) Storage modulus (E’)• Energy stored elastically during deformation• “Elastic” of “viscoelastic”• E’= E* cos

Loss modulus (E’’)• Energy loss during deformation• “Visco” of “viscoelastic”• E” = E* sin

Loss tangent (tan ) or damping or loss factor• shows the ability of material to dissipate the energy• Tan = E’’/E’

Continued….

If phase lag is zero then E*= E’ material is purely elastic If phase lag is 90 degree then E* = E” material is purely viscous If phase lag is between 0 90 degree then E* = E’ + iE” material is viscoelastic

Working of DMA

Contd……

1. Preparation of specimen Depending on the material to analyze, the specimen can be prepared in different ways: Molding, Cutting

As a general rule, common specimen dimensions range from a few millimeters to a few centimeters. The use of a caliper is then advised. The use of a micrometer is preferred to measure film thickness.

Cutting

Venire caliper micrometer

2. Selection of specimen holder On the basis of - The nature of the material - specimen shape

Correspondence between specimen holder and material of specimen

DMA+450 MODEL

Materials specimen holder

Elastromer (cylinder or bar) compression plates, plane shear

Elastomer (band) tension jaws for bars, tension jaws for films, shear jaws for films

Polymer compression plates, tension jaws for bars, three point bending, dual cantilever bending, plane shear, shear jaws for films

Polymers (films) tension jaws for films, shear jaws for films Polymers (fibers) tension jaws for fibers

Pasty bitumen shear for pasty material, shear for liquid materials

Metals, metallic alloys, ceramics three point bending, dual cantilever bending

Continued…..

Correspondence between specimen holder and specimen shape

DMA+450 MODEL

3. Installation of the selected specimen holder

4. Installation of the prepared specimen into the specimen holder inside thermal chamber

5. Start temperature, finish temperature, and step

6. Application of dynamic excitation (stress or strain) on the specimen by dynamic shaker through entire temperature range

7. Then DMA records the response of specimen and determines: E’, E”, Tan

8. Identify transition temperatures based on noticeable changes in curves

Result

50 100 150 200 250 30010

100

1000

Temperature/oC

Sto

rage

Mod

ulus

/MPa

Loss

Mod

ulus

/MPa

Tan

Del

ta

10

100

Tg=213oC

0.0

0.1

0.2

0.3

0.4

0.5

DMA Graph

Storage modulus (E’):elastic property

Loss modulus (E”) :viscous property

Loss tangent (tan )

A typical response from a DMA shows both modulus and Tanδ. As the material goes through its glass transition, the modulus reduces and the Tanδ goes through a peak.

Tg indicated by major change in curves: Large drop in log E’ curve and Peak in Tanδ curve

Testing of elastomer by DMA Specimen material Elastomer Specimen shape BarTest Compression testSpecimen holder Compression plates

Glass transition temperature

Thank you