Dynamic Mechanical Analysis (DMA) Flashcards
What are the basic principles of DMA?
- Measures mechanical or viscoelastic properties.
- Sinusoidal force applied to sample.
- Resulting displacement measured.
What material is said to be viscoelastic?
Material with both elastic and viscous (time dependent) behavior.
What are the parameters measured in DMA?
- Stiffness (modulus values).
- Tg.
What are the two modulus values and what behaviors do they represent?
- Storage modulus - elastic behavior
- Loss modulus - inelastic behavior
What is the glass transition temperature Tg?
Where crystalline domain loses its structure, or melts.
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Temperature which amorphous domains lose structure mobility of polymer chains - become rigid glasses.
What is the Storage Modulus? give the equation.
- The ability of material to store energy
- elastic properties
- E’(MPa) = stress / strain x cosδ
- Similar to stiffness
What is the loss modulus,what properties does it show, state the equation?
- Heat dissipated from material due to molecular motions
- damping & viscous properties
- E’‘(MPa) = stress / strain x sinδ
- similar to viscosity
What is the complex modulus?
- Material’s resistance to deformation.
- E* = E’ + iE’’
What is the tan δ?
- Ratio of the dissipated energy to energy stored.
- Tan δ = E’’ / E’
- damping properties.
What are the two significant tan δ values?
Tan δ = 0, pure liquid (purely elastic materials)
Tan δ = ∞, δ = 90° (visco-elastic materials)
What is measured by DMA, express units?
S = stiffness (N/m)
k = geometry constant (m-1)
What is calculated by DMA, express units?
Modulus (Pa): E = S/k
How is the geometry constant (k) calculated?
Tension& Compression:
k = A/l
A: cross-sectional area
I: free length
What are the different geometries?
- single cantilever
- Dual cantilever
- 3 point bending
- simple shear
What is the equation for single cantilever geometry?
k = w(t/l)3
What is the equation for dual cantilever geometry?
k = 2w (t/l)3
What is the equation for 3 point bending geometry?
k = w (t/l)3 / 2
What is the equation for simple shear geometry?
k = 2A / t
What is the modulus for rubbery state?
10^6 Pa
What is the modulus for glassy state?
10^9.5 Pa
What is the optimum geometry constant?
10^(-3.5) to 10^(-2) Pa
When do you choose the simple shear geometry and why?
- For rubbers and gels, because of stiff geometry.
- used for samples with modulus 10^7 or less.
When do you choose the tension geometry and why?
Superimpose static force (larger), because sample remains under net tensile force (prevents buckling of sample)
When do you choose the single and dual cantilever, and bending geometry and why?
For samples of thickness at least 2mm.
- Single cantilever: relax thermal stresses
- 3-point bending: high modulus materials - accurate modulus determination.
What are the measurement modes?
- Bending - single cantilever clamp
- Tension - tensile clamp
- 3-point bending
- shear
What is considered when applying strain,what occurs in a linear and non-linear region?
- Small loads and strains (0.1 - 1%)
- Linear viscoelastic region - constant modulus value independent of strain
- Non-linear behavior region - strain increased, modulus drops
How do you check if the material is within its linear viscoelastic region? (for calibration)
Perform strain scan (0.1 - 5%) on glassy material.
If modulus remains constant - measurement is in linear range.
What happens with the levels of vinyl acetate?
- More Vinyl Acetate
- More viscoelastic copolymer
What are applications of DMA-RH?
Measures changes on mechanical properties (modulus, extension) in controlled RH, of:
1. canvas (cellulose)
2. Parchment (collagen)
3. textiles (wool)
4. wood
