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

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

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

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

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

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

Matrix: Polymers Classification

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

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

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

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

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)

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

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

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 1.1-1.2 0.9 Young’s modulus (GPa) 3.0 1.4-2.8 2.2-2.4 1.4-1.9 Tensile strength (MPa) 60-70 45-70 25-38 Ductility (%) 30-100 90-110 100-600 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

Matrix: Polymers Classification- Thermosets Thermosetting polymers (thermosets): Over ¾ of all PMCc are thermosetting polymers Covalent crosslinks (~ 10 - 50% 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.

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

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

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

Matrix: Polymers Classification- Thermosets

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.

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

Matrix: Polymers- Properties Stress Strain Behavior Moduli of elasticity for polymers are ~ 10 MPa - 4 Gpa (compare to metals ~ 50 - 400 GPa) Tensile strengths are ~ 10 - 100 MPa (compare to metals, hundreds of MPa to several GPa) Ductility for polymer can be up to 1000 % in some cases (< 100% for metals)

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

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

polymers- Mechanism of deformation of semicrystalline polymers

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

Further Reading: Text Book: Reference Book: Composite Materials: Engineering and Science (pages 168-179). Reference Book: Introduction to Materials (材料概论) pages 211-239 Other reference: Lecture note 3