Units 1 & 2 Flashcards
What gives a polymer its properties?
Chemical structure + Physical structure
Polymers differ considerably in what?
amount of crystallinity
Crystallinity requires the polymer molecules to be able to….
pack into a regular repeating array
Building Block Unit
Mer
_____________ can significantly limit potential crystallinity.
Structural irregularities
The higher the degree of irregularity, the ____ the polymer is able to crystallize.
less
All polymers have a ________ over which they can crystallize.
limited range of temperature
A polymer cannot crystallize ____ its melting temperature, or ____ its glass transition temperature.
above
below
Why can’t a polymer crystallize above its melting point?
the polymer molecules have too much energy to “settle down”
Define melting temperature
the breakup of crystallites
Why can’t a polymer crystallize below its glass transtion temperature?
the polymer molecules do not have enough energy to move sufficiently to pack into a crystallite
For some plastics, the range for crystallization to occur is ______ than Tm to Tg.
considerably narrower
What happens if a plastic is cooled very quickly from the melt to a temperature below its crystallization temperature range? 1
What happens to the same plastic if held within the crystallization temperature range for an extended period of time? 2
- It will develop little or no crystallinity
2. It may become highly crystalline
Chemical Regularity - Branches
at branch points the polymer won’t “fit” into the crystal lattice
Chemical Regularity - Cross-links
not only interfere with the fit, but also restrict the ability of molecular segments to move in order to assume a regular order
What is the linear homopolymer of ethylene?
HDPE
What is the branched homopolymer of ethylene?
LDPE
What is the crystallinity of HDPE?
65-90%
What is the crystallinity of LDPE?
40-60%
HDPE vs LDPE
By decreasing the crystallinity, the presence of the branches results in….
less efficient packing of the molecules into space, decreasing the density of the plastic
In the vast majority of materials, the spacing between adjacent molecules is _____ in crystalline areas than in amorphous areas.
smaller (water is an exception)
The structure affects how much crystallinity ___ develop. Processing affects how much crystallinity ___ develops.
can
actually
For a given polymer, increasing crystallinity increases….
Sealing temperature
Tensile strength
Stiffness
For a given polymer, increasing crystallinity decreases…
Elongation
Permability
Impact strength
Tear strength
In general, significant structural irregularity will prohibit crystallinity. Why is LDPE an exception?
it has very small side groups (hydrogens only)
HDPE, LDPE, and LLDPE are all _____ polymers formed by what?
addition
free radical polymerization reactions from a monomer with a double bond
HDPE - Initiation step
formation of the free radical and the addition of the first monomer
HDPE Polymerazation - How is the initial generation of a free radical usually accomplished?
by the decompostion of an initiator, which is a relatively unstable compound such as peroxide, hydroperoxide or azo compound
Sometimes _____ or _____ are used to generate free radicals.
UV radiation
High temperatures
HDPE Polymerazation - What happens after the initiator free radical is formed?
the addition of the first monomer unit occurs
How does branching occur?
the free radicals participate in chain transfer reactions
Chain transfer reaction are favored by….
high energy states in the free radicals (high temperature)
The rate of the propagation reaction generally _____ as temperature increases.
increases
How is LDPE made?
under conditions of high temperature and high pressure
Why does LDPE have lots of short branches and some long ones?
the free radical is most likely to collide with a nearby part of the same molecule
How is HDPE made?
under milder conditions than LDPE, where chain transfer reactions are much less likely to occur
*requires a catalyst
In most cases, two different monomeric units _____ into the same crystalline arrangment.
will not fit
The presence of a comonomer ______ crystallinity.
disrupts
Isomorphous
when 2 different structures will fit into the same crystalline latice
LLDPE is a _______ copolymer, with comonomers of ______
linear polyethylene
butene, hexene or octene
How is LLDPE made?
essentially the same conditions as HDPE
using a catalyst to provide a lower energy route for polymerization
How are HDPE and LLDPE different?
differences in crystallinity
How are LLDPE and LDPE similar?
equal density
equal percent crystallinity
How are LDPE and LLDPE different?
molecular weight and molecular weight distribution
What is required for molecules to pack into a crystal lattice?
Stereochemical regularity and structural regularity
What does stereochemistry refer to?
the arrangement of molecules in space
Chiral Carbon
a carbon attached to 4 different groups, therefore it can exist in two mirror image forms, which cannot be interconverted without breaking primary bonds
What are the different stereochemical forms?
Isotactic
Syndiotactic
Atactic
What refers to the pattern of bonding within the molecule - what atoms are attached to what, and in what position?
Configuration
What refers to the current arrangement in space?
Conformation
_____ changes every time there is rotation about a bond.
Conformation
______ cannot be changed without disrupting and reforming primary chemical bonds.
Configuration
In ______ PP, when the PP molecule is laid out in its fully-extended-chain conformation, the position of the methyl group alternates from one side of the chain to the other.
Syndiotactic
In ______ PP, when the PP molecule is laid out in its fully-extended-chain conformation, all the methyl groups are on the same side of the chain.
Isotactic
In ______ PP, when the PP molecule is laid out in its fully-extended-chain conformation, the position of the methyl group is random
Atactic
______ polymers are generally amorphous.
Atactic
What is the PP we use in packaging?
linear isotactic
_____ PP is a soft, weak material which is not useful to us as a packaging material.
Atactic
Sources of structural irregularity:
- branching (or, even worse, cross-linking)
- departure from the “normal” head-to-tail pattern of monomer addition
- presence of comonomers (except if we have an alternating copolymer)
_____ may be present if we have chiral carbons in our polymer, even if the polymer is chemically (structurally) regular.
Stereochemical irregularity
A chiral carbon in a polymer molecule can be recognized as one that has _____ side groups attached to it.
2 different
How does crystallinity affect polymer properties?
Increases tensile strength, stiffness, and barrier
The coordinated crankshaft-type motion of 10-50 atoms.
Segmental mobility
What is the basic requirement for segmental mobility?
that there be rotation around single bonds in the polymer backbone
Which conformation has the lowest energy state?
Staggered conformation, where all the side groups are, on average, farthest apart
Why is the eclipsed conformation at a higher energy level than the staggared conformation?
As there is rotation around the single C-C bound causes side groups to move closer to each other
Polymers with double bonds in the backbone -
If we look at rotation about the single bond adjacent to the double bond, there is ________ with rotation
less interference
Polymers with double bonds in the main chain tend to have ____ Tg’s, and are _______.
low
soft and flexible
What group are elastomers usually found?
polymers with double bonds
The presence of a double bond means that the material …
retains some reactivity, permitting subsequent cross-linking and other modification, if desired.
What do ring structures in the backbone of a polymer do?
put rigid links but provide no appreciable effect of easing rotation around adjacent single bonds like double bonds do
Polymers containing _____tend to be quite stiff, and have high Tg and Tm.
backbone rings
What does the presence of O or N in the main chain tend to do?
increase the flexibility of a polymer structure, lowering its Tg (and Tm, if applicable)
What must we consider whenever we have atoms other than C and H in the polymer?
the effects of the magnitude of secondary forces on properties
Below Tg, there is ______ mobility of the polymer molecules.
no significant
What happens to the polymer above Tm?
we have mobility everywhere (liquid flow)
Large side groups ______ the mobility (conformational flexibility) of the molecule.
decrease
What are the 3 main types of primary bonds?
covalent, ionic and metalic
Which type of bonds involve sharing of electrons between the 2 atoms being bonded.
covalent
Which type of bonds involve the transfer of electrons from one atom to another?
ionic (the molecular fragments are then joined together by electrostatic attraction)
_____ bonds occur when electrons are very loosely held, and are therefore relatively free to wander away from their originating atom.
metallic
Covalent bonds can be further subdivided into…
non-polar and polar bonds
Non-polar Bonds
electron sharing is equal
Polar Bonds
electron sharing is unequal
Bonds involving _______ are generally polar.
C and O, N, Cl and F
What is the prime determinant of the chemical reactivity of the polymer?
the strength of the primary bonds
What determines the basic spacing of the atoms in the molecule?
the bond length
If a substance had no secondary bonds….
it would behave like an ideal gas
Types of secondary bonds
dispersion forces
induction forces
dipole forces
hydrogen bonds
Forces due to electostatic attractions between atoms with equal and opposite electric charges.
Dipole
_______ forces occur when a polar molecule induces dipoles in surrounding nonpolar molecules.
Induction
_______ forces are the attraction between neighboring molecules that results from instantaneous configurations of electrons and nuclei.
Dispersion
Intermolecular attractions are proportional to ____
r^-6, where r is the distance between the atoms
One of the ways in which crystallinity influences polymer properties is by achieving a _______ packaging of the atoms, and hence _____.
tighter
increasing intermolecular forces
In order for segmental mobility to occur, not only must we have enough energy for ______, we must also have enough energy to ______.
rotation within the polymer backbone
disrupt the secondary bonds
In general, lower conformational flexibility, for instance, associated with side groups, would tend to ____.
increase Tm
Increasing the spacing (in the crystal framework) acts to _______ the strength of secondary bonds in the cystallite, which acts to _____ Tm.
decrease
lower
_______ is a measure of the strength of secondary forces.
cohesive energy density
Polymers which have intermolecular hydrogen bonds are often ________ by the presence of water.
strongly affected
example: PVOH, which interacts so strongly that it is soluble in water
Polymers with significant hydrogen bonding are usually ________ when dry than in high moisture conditions.
better barriers
ex: nylon and ethylene vinyl alcohol
Primary bonds are inportant in determining properties such as ___________ and _________.
thermal stability
chemical reactivity
Most of the performance properties that we care about are determined predominantly or exclusively by the ______.
secondary bonds
primary bonds can have an important indirect effect
The more polar groups we have (or the more polar the groups are), the _________ the resulting intermolecular forces.
stronger
