POLYMERS Flashcards
What is a polymer?
A polymer is a compound comprised of long-chain molecules that consist of repeating units, called mers, connected end to end.
What are the three basic categories of polymers?
The categories are (1) thermoplastics, (2) thermosetting polymers, and (3) elastomers.
How do the properties of polymers compare with those of metals?
In general, polymers have lower strength, hardness, stiffness, density, and temperature resistance compared to metals. In addition, polymers have low electrical and thermal conductivity.
What does the degree of polymerization indicate?
The degree of polymerization indicates the average number of mers or repeating units in the polymer molecule.
What is cross-linking in a polymer, and what is its significance?
Cross-linking is the formation of connections between the long-chain molecules in a polymer. It causes the polymer structure to be permanently altered. If the amount of cross-linking is low, the polymer is transformed into an elastomer; if cross-linking is significant, the polymer is transformed into a thermoset.
What is a copolymer?
A copolymer is a polymer made up of two different types of mers, such as ethylene and propylene.
What is a terpolymer?
A terpolymer is a polymer with three different mer types. An example is ABS (acrylonitrile-butadiene-styrene) plastic.
How are a polymer’s properties affected when it takes on a crystalline structure?
Density, stiffness, and melting temperature increase.
Does any polymer ever become 100% crystalline?
No.
What are some of the factors that influence a polymer’s tendency to crystallize?
The factors that influence a polymer’s tendency to crystallize are the following: (1) only linear polymers can form crystals; (2) copolymers do not form crystals; (3) stereoregularity - isotactic polymers always form crystals, atactic polymers never form crystals, and syndiotactic polymers sometimes form crystals; (4) slow cooling from the molten states promotes crystal formation; (5) plasticizers inhibit crystal formation; and (6) stretching the polymer tends to promote crystallization.
Why are fillers added to a polymer?
Fillers are added to increase strength or simply to reduce the cost of the polymer.
What is a plasticizer?
A plasticizer is a chemical added to the polymer to make it softer and more flexible. It is often added to improve the polymer’s flow characteristics for shaping.
In addition to fillers and plasticizers, what are some other additives used with polymers?
Other additives include (1) lubricants - to reduce friction and improve flow; (2) flame retardents; (3) colorants; (4) cross-linking agents, (5) antioxidants, and (6) ultraviolet light absorbers.
Describe the difference in mechanical properties as a function of temperature between a highly crystalline thermoplastic and an amorphous thermoplastic.
A highly crystalline TP retains rigidity during heating until just before its Tm is reached. An amorphous TP shows a significant drop in deformation resistance as its Tg as temperature is reached; it becomes increasingly like a liquid as temperature continues to increase.The nylons are members of which polymer group?
The nylons are members of which polymer group?
Polyamides.
