Polymer Characterization: Microscopic techniques

Question 1

What is microscopy?

Answer 1

A powerful tool for studying the structure and morphology of polymers and composite materials

Question 2

What can be seen with microscopy?

Answer 2

1. failure mode analysis
2. crystal growth and structure
3. measurement of characteristic dimensions

Question 3

What is polarized optical microscopy?

Answer 3

crystalline and amorphous structures interact differently with polarized light. The contrast allows us to distinguish crystalline regions

Works by shining a light through a series of filters (typically 2) one before and 1 after the sample. The first filter ensure only 1 orientation of wavelengths hits and sample and the second is orientated 90 degrees to the first so that only reflected light can reach analyzer

Question 4

What is polarised light?

Answer 4

Light waves oscillate in directions perpendicular to their direction of travel. Polarized light, we filter light so that only light oscillating along 1 axis passes through

Polarized light interacts strongly with birefrigent materials such as crystals - refractive index varies depending on polarization state

Question 5

What can polarized optical microscopy see?

Answer 5

• relative degree of crystallization
• nuclei density
• crystal size
• location of crystals
• crystal orientation and structure

*if nuclei density is very high, can see a textured pattern of many tiny crystals bunched together

Question 6

What are dynamic crystallization experiments?

Answer 6

Used to observe the crystallization process in real time to help determine kinetic parameters of crystal nucleation and growth

*is just polarized optical microscopy done while crystal growth is occuring

Question 7

What is scanning electron microscopy?

Answer 7

bombard sample with electrons instead of light (protons)

• can get micro and nanostructures, but need to coat the sample to protect it from the bombardment (otherwise charge will build up on surface and cause heating/degradation
• samples must be imaged under vacuum (no moisture, degassing, dust, or loose particles)

Question 8

What can we see with SEM?

Answer 8

• observe structure and interfaces in multi-phase systems (can see how well they mix)
• use for failure mode analysis - can see increasing toughness in images of the fractured surfaces (change from brittle failure to ductile failure)
• can also be combined with imaging software to measure size distributions of crystals and count number of nuclei

Question 9

How does SEM work?

Answer 9

electron source fires electron through anode (to repel electron downward) and then goes through lenses until it hits the sample, the secondary electrons bounced back are read by an SE detector)

*TEM works the same only the detector occurs under the sample

Question 10

What is TEM and what is it used for?

Answer 10

Similar to SEM but looks at electrons that pass through

is able to image internal structures (able to see core-shell particles, solution-grown crystals and micelle structures, and epitaxial crystal growth)

Question 11

What is atomic force microscopy?

Answer 11

Uses a scanning probe to tell us about the topography

• highly sensitive and versatile method that can reach sub-nanometer resolution and does not require a vacuum to operate
• measures forces as well as topography, allowing local measurements of deformation and elasticity, and generation of 3D surface maps

Question 12

How does AFM work?

Answer 12

a microscopic actuated cantilever probe with an extremely sharp tip is moved along the sample surface. A laser bounces off the reflective surface of the cantilever and strikes a sensitive photodiode - any deflection of the probe is recorded by the movement of the laser.

Sample is prepped with a microtoming to produce a smooth, flat surface for imagine (may be performed at cryogenic temp to preserve internal structures) (uses a sharp diamond knife)

Question 13

Contact modes of AFM?

Answer 13

Operation depends on the nature of the surface and info trying to get

1. contact mode: direct contact
2. non-contact mode - tip hovers just above the sample (5-10 nm) while oscillation at a set frequency - attractive van der Waals forces deflect the probe
3. tapping mode - oscillates the tip into periodic contact with the surface to minimize sample adhesion and probe damage

Question 14

What can AFM find?

Answer 14

1. topography mapping (in blends or otherwise)
2. phase imagine