Chapter 1
Overview of Structure of
Polymers
Ming Wei, Carol Barry, Joey Mead
Department of Plastics Engineering
University of Massachusetts Lowell

Introduction
• The mechanical properties of polymers depend on the chemical compositions and structures of the polymer at the molecular and supermolecular levels
•
•
•
•

Molecular weight (how long is the chain)
Chain flexibility (Tg)
Chain regularity (can it crystallize?)
Strength of intermolecular forces

• Before start, let’s see some elementary ideas concerning the properties of polymers. Outline
1. What are polymers? Is the polymer a thermoplastic or thermoset?
2. What is the polymer’s molecular weight and molecular weight distribution?
3. What is the polymer’s morphology?
4. What are the polymer’s characteristics?
5. What additives have been incorporated into the resin?

1. Polymers
Polymers - long chain molecules

Features of polymeric materials
• Chain entanglement
• Time scale of motion
• Summation of intermolecular forces Thermoplastics
Thermoplastics
• Polymers with linear or branched chains
• May be dissolved or remelted

LDPE

HDPE
LLDPE

thermoplastic particles heat

plastic melt grind

cool

plastic product Thermosets
12-15% of plastics resins1

Thermosets
• During processing covalent bonds (cross-links) are formed between polymer chains
• Cross-links prevents the polymer from dissolving or melting

heat

thermoset resin plastic
“melt”

cure

plastic product grind

1

M. Sepe, Injection Molding Magazine, September (1999).

Types of Thermoset Polymers
Thermoset resins
• e.g., epoxy, polyester, urethane, phenolics, urea formaldehyde
• Relatively short polymer chains
• Develop high levels of cross-linking

1

cross-linked phenolic1

http://www.iupac.org/didac/Didac%20Eng/Didac04/Content/POL20.htm

Types of Thermoset Polymers
Thermoset rubber
• e.g., nitrile rubber, butyl rubber
• Very long polymer chains
• Develop moderate levels of cross-linking

1

http://www.matter.org.uk/glossary/detail.asp?dbid=81

Types of Thermoset Polymers
Lightly cross-linked polymers
• e.g., cross-linked PE (XLPE)
• Modified thermoplastic resins
• Contain < 20% cross-links

http://www.specialchem4polymers.com/documents/indexa bles/contents/37/images/peroxrubberp2_3image4.gif 2. Molecular Weight
• Length of polymer chains can be varied
• Not all chains have exactly the same length

Molecular weight
• Average chain length
Molecular weight distribution
• Range of chain lengths

http://wwwcp.tphys.uni-heidelberg.de/Polymer/day1/p5.htm

Molecular Weight (MW)
Number-average molecular weight
• Indicates mean chain length
• Correlates with modulus, Tg, etc.

Number of chains

Mn
Mw
Mz

Length of polymer chain

Weight-average molecular weight
• Counts longer chains
• Correlates with viscosity and impact strength z-average molecular weight
• Counts longest chains
• Related to melt strength (ability to stretch the molten polymer)
Measured with gel permeation chromatography (GPC) or dilute solution viscosity

Molecular Weight, continued
Molecular weight of thermoplastics inferred from melt flow
• i.e., melt index (MI or MFI)
– Mass of material that flows through a barrel and die
(capillary) using a specified weight and time
– Units: g/10 min. or dg/min.

http://www.mgaresearch.com/Temp_Materials_Site/Temp_Materials_Site/extrusion.htm http://cappa-d.mcmaster.ca/facilities/images/melt-index.GIF Molecular entanglement
 Increases viscosity
 e.g., PMMA sheet
 Increases melt strength
 e.g., pipe and blown film grade PE’s
 Increases impact strength

Intermolecular attraction and end groups
• Affects tensile and flexural properties • Influence transition temperatures Properties which depend on:

Effects of Molecular Weight
Molecular
entanglement

End groups or intermolecular attraction

Molecular Weight

Effect of MW on Properties of PC
Property
Melt index

Units

High MW Low MW

g/10 min.

4

15

Flow length1

mm

495

668

Yield strength

MPa

61.4

60.0

%

6.7

6.5

Tensile modulus