Exam 2 Study Guide Flashcards
What are some practical concerns with mechanical testing of polymers compared to metals?
Polymers are temperature sensitive, strain rate dependent and time dependent which all need to be considered when mechanically testing them.
What are some modifications engineers take to overcome mechanical concerns for polymers?
- Reinforcement with fillers/fibers
- Crosslinking
- Drawing
How does a polymer deform when crystalline domains are present?
- undeformed structure
- amorphous regions elongate
- crystalline regions align
- crystalline block segments separate
- fibrillar structure
Give an example of a commercial polymer commonly used in each region of viscoelastic behavior.
Glassy region = PMMA sheets used in windows
Tg = not common for polymers to be used at their Tg because it is an unsteady point
Rubbery plateau = a rubber used in tires
Rubbery flow = silly putty
Liquid flow = 3D printing, gel, paint
What are the factors in aging of a polymer?
- Temperature
- Stress (mechanical stress)
- Environment (UV radiation)
How does time-temperature superposition model aging?
- TTS models aging by treating aging like temperature
- TTS models aging by using short term, high temperature data to predict long-term behavior at lower temperatures
What does DMA measure and how is the data presented/interpreted?
- DMA measures the viscoelastic behavior of polymer (storrage & loss modulus & tan delta)
- Data presented by stress strain curve
What are the possible testing scenarios within the DMA?
- Compressive
- Tension
- Shear
- Flexure
- Cup & plate
What types of samples/sample shapes would work for these different set ups?
- For tension: dog bone shape
- For compression: cylindrical shape
How is Tg measured by DSC compared to DMA?
DSC gives a finite Tg value through heat flow while DMA gives a range for Tg from mechanical data and is more sensitive
What is the difference between creep and stress relaxation?
Creep = change in STRAIN as a function of time usually under constant LOAD and temperature
Stress Relaxation = change in STRESS as a function of time usually under constant DEFORMATION and temperature
When could creep and stress relaxation be a problem for some polymers?
Creep = pipes being under load and permanently deforming
Stress Relaxation = seals and rubber bands with their stress fading overtime and being loose
What does the four element model describe and how could an engineer use this information?
- It is a combination of Maxwell and Kelvin elements and it is used to describe the viscoelastic deformation of polymers in simple terms
- Engineers use it as a way to mathematically predict behavior
