1. Subject: Composite Materials Science and Engineering Subject code: 0210080060
Prof. C. H. XU
School of Materials Science and Engineering
Henan University of Science and Technology
Chapter 3: Matrix-Polymers

2. Matrix: polymers Contents Molecules Classification of polymers
Thermoplastics
Thermosets
Elastomers
Mechanical Properties

3. Matrix: Polymers Molecules
Polymer means ‘many parts’
Organic compounds, containing C, H, O, N, Si
Intramolecular bonds (along the chain): covalent bonds
Intermolecular bonds (between chains): van der Waals bonds or hydrogen bonds

4. Matrix: Polymers molecular weight
Distribution of molecular weights for a typical polymer

5. Matrix: Polymers molecular structures
Linear polymer
Long chains are flexible and may be thought of as a mass of noodles.
Branched polymer
Side-branch chains are connected to main chain.
Crosslinked polymer
Adjacent linear chains are jointed one to another at various positions by covalent bonds. Many of the rubber, elastic materials are crosslinked.
Network polymers
Trifunctional mer units form three-dimensional networks. A highly crosslinked polymer is classed as network polymer.
Linear
Branched
Crosslinked
Network
cross-linked

6. Matrix: polymers copolymer
Copolymers
Composed of two mer units
Rubber: often copolymer structure

7. Matrix: Polymers Classification

8. Matrix: Polymers Classification- thermoplastics
Thermoplastic polymers (thermoplastics):
soften reversibly when heated (harden when cooled)
At elevated temperatures inter-chain bonding is weakened allowing deformation at low stresses.
Most thermoplastics are linear polymers and some branched structures

9. Matrix: Polymers Classification- thermoplastics
Polymer crystallinity:
Packing of molecular chains to produce an ordered atomic array
Often partially crystalline
Where s is the density of a specimen; a is the density of amorphous polymer and c is the density of the perfectly crystalline polymer.
Arrangement of molecular chains in a unit cell for polyethylene

10. Matrix: Polymers Classification- thermoplastics
Crystallization process
Form small plate-like lamellae of the folded crystalline to spherulite
A transmission photomicrograph showing the spherulite structure of polyethylene

11. Matrix: Polymers Classification- thermoplastics
Polymerization: synthesis of the large molecular weight polymer.
Raw materials for synthetic polymers are coal, petroleum products.
Methods
Addition
Condensation

12. Matrix: Polymers Classification- thermoplastics
Addition polymerization:
Three stages: initiation, propagation and termination. e.g. Hydrocarbon ethylene (C2H4) gas
(a) Initiation: Catalysts (R.) with a unpaired electron are added to ethylene gas under suitable temperature and pressure.
(b) Propagation: monomer units become attached to one another in succession to produce the chain molecule
(c) Termination:
(a)
(b)
(c)

13. Matrix: Polymers Classification- thermoplastics
Condensation polymerization
Usually there is small by-product that is then eliminated
Significantly slower than addition polymerization
Often form trifunctional molecules that can form cross-linked and network polymers

14. Matrix: Polymers Classification- thermoplastics
H C
C C
H CH3
Polymethyl methacrylate (PMMA) O
OCH3 n
H H
C C
H CH3
Polypropylene n
聚丙烯
Polycarbonate 聚碳酸酯

15. Matrix: Polymers Classification- thermoplastics
Properties of thermoplastics
Properties
Acrylic
(PMMA)
Nylon
(6,6)
Polycarbonate 聚碳酸酯
Polypropylene 聚丙烯
Density (Mg/m3)
1.2
1.1
0.9
Young’s modulus (GPa)
3.0
Tensile strength (MPa)
60-70
45-70
25-38
Ductility (%)
30-100
90-110
Fracture toughness K1c (MPa m1/2)
1.5
Thermal Expansion (10-6 K-1)
50-90 90
Glass trans. Temp (℃)
90-105
150
Melting point (℃)
261
175

16. Matrix: Polymers Classification- Thermosets
Thermosetting polymers (thermosets):
Over ¾ of all PMCc are thermosetting polymers
Covalent crosslinks (~ % of mers) formed during curing (固化). Curing involves application of heat, pressure or addition of a catalyst (a curing agent).
Cross-linking hinder bending and rotations. Thermosets are harder, more dimensionally stable, and more brittle.
harden permanently when heated.

17. Matrix: Polymers Classification- Thermosets
Polyester Resins (聚酯树脂)
Unsaturated linear polyester
Initiator (R): Organic peroxide initiator
Curing agent: styrene (crosslinking monomer)
Cheap,
low viscosities → easy fabrication (room temperature)
Shrinkage (4-8%) during curing
Dominate the market

18. Matrix: Polymers Classification- Thermosets
Epoxy Resins (环氧树脂)
Linear polymer
Curing agent (hardener): polyamides, polyamines, diamine
More expensive and more viscous than Polyester Resins
A major advantage: curing in two or more stages (pre-preg in manufacture composite)
Shrinkage 1-5% during curing

19. Matrix: Polymers Classification- Thermosets
Phenolic resins (酚醛树脂)
The oldest of the thermosets
Low cost and good balance of properties
Good fire resistance
Stringent fire
Polyimide (聚酰亚胺)
More expensive and less widely used
Withstand relatively service temperature

20. Matrix: Polymers Classification- Thermosets

21. Matrix: Polymers Classification- Elastomers
Light crosslink is required for elastomers.
It can be achieved by vulcanization - an irreversible chemical reaction usually at high temperatures, and usually involving the addition of sulfur compounds.
Sulfur atoms bond with double-bonded C in chain backbones and form the bridge cross-links.

22. Matrix: Polymers – Properties Stress Strain Behavior
The description of stress-strain behavior is similar to that of metals
The stress-strain behavior can be brittle, glass-like (A), plastic (B), and highly elastic (C)
(A) network polymer, amorphous
(B) semi-crystal polymer
(C) totally elastic (rubber-like elasticity). Crosslinked polymer. This class of polymers - elastomers

23. Matrix: Polymers- Properties Stress Strain Behavior
Moduli of elasticity for
polymers are ~ 10 MPa - 4 Gpa
(compare to metals ~ GPa)
Tensile strengths are
~ MPa
(compare to metals, hundreds of MPa to several GPa)
Ductility for polymer can be up to 1000 % in some cases (< 100% for metals)

24. polymers- Mechanism of deformation of semicrystalline polymers
The macroscopic deformation involves necking. Neck gets stronger since the deformation aligns the chains and increases local strength in the neck region (up to 2-5 times) Þ neck is expanding along the specimen.
Different from ductile metals where the deformation is confined in the initial neck region.
Tensile stress-strain curve for a semi-crystalline polymer. Specimen contours at several stages of deformation are included

25. polymers- Mechanism of deformation of semicrystalline polymers
Elastic deformation:
Basic mechanism of elastic deformation is elongation (straightening) of chain molecules in the direction of the applied stress. Elastic modulus is defined by elastic properties of amorphous and crystalline regions.
Plastic deformation
the interaction between crystalline and amorphous regions. Stages of plastic deformation:
elongation of amorphous tie chains
tilting of lamellar crystallites towards the tensile axis
separation of crystalline block segments
stretching of crystallites and amorphous regions along tensile axis

26. polymers- Mechanism of deformation of semicrystalline polymers

27. Polymers- Properties Elastomers
Elastomers can be deformed to very large strains and then spring back elastically to the original length, a behavior first observed in natural rubber. To be elastomeric, the polymer needs to meet several criteria:
Resistance to crystallization (elastomers are amorphous)
Relatively free chain rotations (unstressed elastomers have coiled/twisted structure – uncoil during deformation)
Certain degree of cross-linking (achieved by vulcanization) that increases resistance to plastic deformation

28. Further Reading: Text Book: Reference Book:
Composite Materials: Engineering and Science (pages ).
Reference Book:
Introduction to Materials (材料概论) pages
Other reference:
Lecture note 3