Polymers I Flashcards
What is a polymer?
long chain of molecules made up of a series of repeating units
What is mer
repeating unit
What is the backbone of the polymer chain?
A series of atoms joined by
strong covalent bonds, most commonly carbon-carbon
What does the random rotation of covalent bonds in the backbone of a polymer chain lead to?
Bends,
kinks and coils in the polymer chain
What role do the backbone and addition of side groups play in differentiating polymers?
The backbone and addition of side groups distinguish the one polymer
from another
What are plastics?
Synthetic or semi-synthetic
polymers
What have plastics been historically generated from?
Oil or natural gas
What are common sources that plastics are now generated from?
Oil or natural gas
wood fibers
corn
organic materials (banana peels!)
What are natural polymers?
Polymers
derived from existing biological
processes
What are examples of natural polymers?
-Cellulose
-Sugars and starches (polysaccharides)
-DNA
-Rubber
What is cis-polyisoprene derived from?
Sap of some plants and trees
What is unique about cellulose
It’s an organic material used for cell walls of plants and trees.
It’s the most abundant organic material on earth
Which chemical is natural rubber?
cis-polyisoprene
How are polymers differentiated?
By their side groups and/or backbone
chain
What type of impact can small differences in polymers have?
Small differences can greatly
impact properties (melting points,
reactivity, brittleness, stiffness)
What are many properties of polymers linked to?
the polymer chain length
and molecular weight
What happens to the length of polymer chains during polymerization?
Polymer chains grow to different lengths –there is a distribution of chain lengths
What are the 2 ways of defining the molecular weight?
Number-average molecular weight
Weight-average molecular weight
How is the number-average molecular weight determined?
Determined from a histogram of
the number of chains within a given size interval
How is the weight-average molecular weight determined?
Determined from a histogram of
the total mass of the chains within the given size interval
What is true in all cases about the number- vs. weight-average molecular weight?
In all cases, the weight-average molecular
weight will be higher than the number-average.
What is the histogram for the weight fraction more heavily weighted towards?
The histogram for the weight fraction is more heavily
weighted to the longer, heavier chains.
What is another way of expressing average chain size of a polymer?
degree of polymerisation(DP)
What does the degree of polymerization represent?
The average # of mer units in a chain
How is the degree of polymerization calculated?
DP is typically calculated from the number-average molecular weight.
How does the degree of polymerization impact melting points, stiffness, and strength?
Higher DP means:
* Higher melting points
* Higher stiffness
* Higher strength
What are linear polymers?
Polymers with mer units joined end to end in single chains
What are branched polymers?
Polymers with side-branched chains connected to the main ones
How do branched polymers result?
From side reactions that occur during
synthesis of polymer
How do branched polymers affect the chain packing efficiency and polymer density?
Chain packing efficiency is reduced and lowers polymer density
Can polymers with linear structures be branched?
Yes, polymers that form linear structures may also
be branched.
Describe the branching of high-density polyethylene (HDPE)
Low degree of short-chain branching; primarily a linear polymer.
What is the density of HDPE?
The density of greater or
equal to 0.941 g/cm^3
Describe the properties of HDPE
-Low degree of short-chain branching
The mostly linear molecules pack together
well, so intermolecular forces are stronger
than in highly branched polymers.
Describe the commercial applications of HDPE.
Used in products and packaging such as
milk jugs, detergent bottles, butter tubs,
garbage containers, and water pipes.
What proportion of toys are manufactured from HDPE?
One-third of all toys are manufactured
from HDPE
Describe the branching of low-density polyethylene (LDPE)
High degree of short-chain branching + long-chain branching
What is the density range of LDPE?
0.910–0.940 g/cm3
Describe the main properties of LDPE?
LDPE has a high degree of short- and long-chain branching, which means that chains
pack poorly.
It has, therefore, less strong
intermolecular forces, resulting in a lower
tensile strength and increased ductility.
What are the commercial applications of LDPE?
Used for both rigid containers and plastic
film applications such as plastic bags and
film wrap
What happens during crosslinking?
adjacent linear chains strongly
joined to one another
When is crosslinking achieved?
During synthesis of a polymer or by a nonreversible chemical reaction
How is crosslinking often accomplished?
By additive atoms or
molecules that are covalently bonded to
the chains
What is a common property among rubber elastic materials?
Many rubber elastic materials are
irreversibly crosslinked (called
vulcanization)
What is alginate?
A naturally occurring
polymer, typically derived from brown
algae
What are the advantages of alginate?
Low cost, biocompatibility
and mild gelation conditions
How is hydrogel formed upon cross-linking?
The block copolymer of guluronate and
mannuronate residues to create hydrogel. The guluronate cross links with Ca2+ to form an egg box structure.
What is the result of guluronate cross-linking with divalent cations (Ex. Ca2+)
An egg-box structure
Why does molecular weight or degree of polymerization affect the mechanical properties of polymers? (Written Response)
*Greater van der Waals interactions between polymer chains
*Greater thermal energy and stress required to disrupt these bonds
*Giving rise to higher melting point and higher strength (lower ductility)
What are some defining characteristics of polymers?
*Lightweight (density = 0.9 - 3 g/cm^3)
*Low elastic modulus (in bulk form)
*Low fracture toughness (in bulk form)
*Good ductility and specific toughness (in bulk form)
*Highly formable (in bulk form)
*Chemistry is well understood and manipulatable
Which of these substances are polymers?: Hemaglobin, polylactic acid (PLA), DNA, collagen, and silicone.
All of them are polymers.
What is the relationship between the size of a side chain group, bond rotation, and tensile strength?
Larger side groups limit ease of bond rotation (i.e. make it harder), thereby increasing strength
What properties of polymers can decrease ductility and increase strength?
-Degree of crosslinking
-Number-average molecular weight
-Degree of polymerization
Why does the polymer strength decrease due to the degree of polymerization decreasing?
Due to reduced secondary bonding between polymer chains.
What is the relationship between the average number of monomer units in a chain and the melting point of the chain?
As the average number of monomer units in a chain increases, the melting point of the chain increases.
What happens to the ductility when you increase the degree of crosslinking in rubber elastic materials?
The ductility decreases.
What is the relationship between temperature, young’s modulus, ultimate tensile strength, and ductility?
As temperature increases, the young’s modulus and ultimate tensile strength decreases and ductility increases.
How many times can polymers stretch beyond the original length before breaking?
They stretch over ten times the original length before breaking.
Is it possible to grind up and reuse phenol formaldehyde and polyethylene polymers?
It’s impossible to grind up and reuse phenol formaldehyde since it is a thermoset and not amenable to remolding.
It’s possible to grind up and reuse polyethylene since it is a thermoplastic and can be remolded
