Polymer Chemistry Exam 4 Flashcards
At VERY short range, what is the strongest intermolecular force?
Repulsive forces due to Pauli Exclusion
What intermolecular force do ALL long polymer chains have a lot of?
London Dispersion
For a single interaction, rank the following in terms of strength:
Hydrogen bonding
Coulombic interaction
Dipole-Dipole interactions
Strongest of the three: Coulombic
Medium: Hydrogen bonding
Weakest of the three:
Dipole-Dipole
Why don’t long polymers have a boiling point like other smaller molecules?
In order to boil a chemical, you must overcome its intramolecular interactions. The intramolecular interactions are additive, so they scale by the number of atoms present in a molecule. A polymer has so many atoms that it requires less energy to break covalent bonds than it does to overcome the intramolecular interactions. Polymers decompose before they can enter the gas phase.
What sort of temperatures is annealing done at?
Below the melting point, but above the glass transition temperature.
Irregularities in an otherwise crystalline material are called
Crystal defects
The process of removing crystal defects with thermal energy is called what?
Annealing
What does it mean to be amorphous?
Without a regular predictably repeating structure (e.g. a glass)
Why do polymers have a tough time annealing?
A single batch might have chains of many different lengths, branching, and compositions. The can never pack well if they don’t have a regular structure. Even if a polymer does have a very regular structure, to anneal it you need to give it enough energy to overcome the intramolecular forces around the defects, but not destroy existing crystals. Since polymers have very strong intramolecular forces, that is a tough ask.
In terms of chemical structure, what makes something an elastomer like rubber?
- Crosslinked.
- Above Tg but below Tm
- Stress is directly proportional to strain (via Young’s modulus) for large amounts of strain
Why might a single polymer melt at a variety of different temperatures?
A polymer crystal will melt at the temperature it was formed at. A single polymer which has the ability to crystallize can do so at any temperature between the reference Tg and Tm values. It just takes longer time to crystallize at hotter temperatures
For the formation of crystals, what is the relationship between temperature, degree of super-cooling, crystal size, and quantity of crystals, and time it takes to crystallize?
The temperature in question must be between the looked up Tm and Tg values. That said…
Hotter temperatures =
Lower degree of super cooling =
Takes longer to form crystals =
Fewer crystals form =
Crystals end up bigger
When a sample interacts with light differently depending on the orientation of the sample or light, what is that called?
Birefringence
What is it called when the molecules in the sample cannot rotate freely, but they can slide past each other?
A liquid crystal
What are thin sheets of crystals called?
Lamellae
When several thin sheets of crystals arrange themselves into round structures, what are those structures called?
Sphereulites
What equation relates the thickness of a crystal to the temperature which it melts?
The Thompson-Gibbs Equation (See end of lecture 21)
Which is more difficult for polymers: Nucleation, or crystal growth
Nucleation
A crystal defect that is an empty site in the crystal structure is called what?
A vacancy defect
A crystal defect where one atom replaces the usual atom in the crystal structure is called what?
A substitutional defect
A crystal defect where an atom is found between the usual lattice sites is called what?
An interstitial defect
When two or more atoms of opposite charges are missing from a crystal structure nearby each other, what is that called?
This pair of vacancy defects is called a Schottky Defect
When an atom moves from its usual location in a crystal lattice and ends up nearby between lattice sites, what sort of defect is this?
This pair of vacancy and interstitial defects is known as a Frenkel defect.
When half a plane of atoms is wedged in between two planes of a crystal structure, what is this sort of defect called?
An edge defect
When one plane of atoms is partially twisted relative to the adjacent plane, what is this sort of crystal defect called?
A screw dislocation
What is the term for where two crystalline regions meet?
Grain boundary
What is the difference between homogeneous and heterogeneous nucleation?
Homogeneous occurs directly from solution or neat.
Heterogeneous is when the nucleation site forms on a
pre-existing surface.
Heterogeneous nucleation occurs slightly faster than homogeneous at elevated temperatures. At colder temperatures, homogenous slightly dominates.
How do you get a liquid into the glass phase
Cool it below Tg, and do it so quick that it doesn’t have time to nucleate any crystals
What is the most important thing when comparing melting points of different polymers:
Enthalpy or Entropy?
Entropy.
A polymer with a more rigid backbone will have what sort of melting point?
It will want to stay as a solid. It will be hard to melt. It will have a HIGHER melting point.
Single-bonded non-carbon atoms in the backbone of a polymer chain tend to do what to the polymer’s melting point
Those different bond angles give the backbone added flexibility. The chain wants to be in the liquid phase more. It melts easier. The melting point is LOWER
Bulky side chain groups near the chain tend to do what to the melting point of the polymer
Their steric hinderance prevent the chain from bending as much. The chain is more rigid and wants to stay in the solid state. It is harder to melt. It will have a HIGHER melting point.
Very long flexible side chains on the main backbone tend to do what to the polymer’s melting point
This added flexibility tends to make the polymer want to become a liquid easier. It melts at a LOWER temperature.
What is a plasticizer?
An additive, usually a small molecule like a solvent, used to DECREASE the melting point of a polymer, and make it a little more pliable when it is in the solid state
If two polymers have about the same amount of flexibility, and one has more intramolecular forces than the other, which one would have the higher melting temperature?
The one with stronger intramolecular forces would melt at a hotter temperature.
For a given polymer made by addition polymerization, why do short chains melt at much lower temperatures than longer chains?
The endgroups disrupt the crystal packing, making it easier to melt the crystal.
The shorter chains having fewer intramolecular than longer chains is also a reason, but its not as important.
When will a copolymer have a higher glass transition temperature than homopolymers of its constituents?
If the two monomers have extraordinarily strong intramolecular forces, it stabilizes the glass phase more than either homopolymer’s glass phase.
When will a copolymer have a lower glass transition temperature than homopolymers of its constituents?
Trick question. It never would.
Ain’t I a stinker!
:-P
When the two monomers have vastly different chemistries and strong repulsive interactions (e.g. one is very polar and the other is nonpolar), but they are assembled into a copolymer, how does the glass transition temperature of that copolymer compare to homopolymers of the individual monomers?
THE glass transition temperature of the copolymer??? THE??? Nossir! It won’t just have ONE glass transition temperature. The two monomers will phase segregate and each will form a glass phase at a different temperature (roughly equivalent to the temperatures their homopolymers become glass at)
Crystallization is what, energetically speaking? Endothermic or Exothermic?
It takes energy put in to melt a crystal. So when a crystal FORMS, it gives off energy. Exothermic.
Vitrification (forming a glass) is what energetically speaking? Endothermic or Exothermic?
It takes energy put in to turn a glass into a liquid. So forming a glass gives off energy. Exothermic.
If you were to measure the number average molecular weight of a polymer via a colligative property, which property is best for that?
Osmotic pressure.
Name two good ways to measure the number average molecular weight of a polymer.
End group analysis
Osmotic pressure
Refractive index
Size Exclusion Chromatography
What is another name for size exclusion chromatography? (SEC)
Gel permeation chromatography (GPC)
Name two good ways to measure the weight average molecular weight of a polymer
Ultracentrifugation
Light Scattering
Neutron Scattering
Size Exclusion Chromatography
In size exclusion chromatography, which polymer chains elute from the column first?
The ones with larger hydrodynamic radii
(roughly speaking, the longer chains)
When forces are elongations are perpendicular to the applied area, what is that called?
Tensile
What is Young’s modulus?
The ratio of tensile stress over tensile strain
What is Poison’s ratio
The ratio of relative tensile elongation to relative width contraction
What is the bulk modulus
The ratio between applied pressure and relative volume change.
When a force causes polymer chains to align and makes it more easier for the sample to elongate, what is that called?
Necking
Forces and deformation parallel to the applied plane are called what?
Shear
Tensile stress causes maximum shear stress in which planes?
Planes that are 45 degrees from the plane the tensile stress was applied to.
What is the difference between elastic and viscous behavior?
Elastic behavior is quick and any strain is reversed once the stress is removed.
Viscous behavior is time dependent and is not reversed once the stress is removed.
Are most polymers viscous or are most polymers elastic?
Uh….. Yes?
Trick question! Most polymers exhibit both those characteristics. Polymers are viscoelastic.
What is viscosity?
The ratio of the shear stress to the RATE of shear strain
What is the shear modulus?
The ratio of shear stress to the maximum shear strain
Give an example of a shear thinning non-Newtonian fluid
Ketchup. It takes a large amount of shear stress to get it to move slowly, but a much tinier amount to get it to flow much quicker. The faster the ketchup moves, the lower its viscosity.
