che541_basic concepts and definitions
DESCRIPTION
Polymer sciTRANSCRIPT
HISTORICAL OVERVIEW Age of Polymers
1839 Vulcanization of natural rubber 1868 Celluloid 1909 Bakelite
References: 1. Flory, P.J., "Principles of Polymer Chemistry",
Cornell University Press, Ithaca, NY(1953). 2. Brydson, J.A., "Plastics Materials", 3rd edition,
Butterworths, London (1975). 3. Stahl, G.A., "A Short History of Polymer
Science", ACS Symposium Series 175, American Chemical Society, Washington D.C. (1981).
4. Mark, H.F., "Polymer Chemistry: The Past 100
Years", C&EN, p.176 (April 6, 1976) 5. Bolton, E.K., I/EC, 34(1), 53-58 (Jan. 1942)
History and uses of nylon.
BASIC CONCEPTS AND DEFINITIONS
Polymer: Many parts
1920 Staudinger
Large molecule (macromolecule) made up of smaller repeating units
Polymers have molecular weights varying from
5x103 to several millions Examples:
Natural polymers: Cellulose: polysacharide Proteins: polyaminoacids DNA: polynucleotide
Synthetic polymers:
Polyethylene (PE) Polyvinylchloride (PVC)
Cl Cl Cl Cl Cl
Monomer: Only one part
A small molecule which reacts with other molecules of the same or different type to form a polymer
Example:
Styrene Polystyrene (PS)
Oligomer: A few parts
Low molecular weight ( < 5000 )
Repeat unit: Segment of the chain or macromolecule which repeats at regular intervals
Examples: Polymethylmethacrylate Polypropylene Polyvinylchloride
Functionality # of sites on monomer molecule available for bonding to other molecules under polymerization conditions
Degree of polymerization (DP)
Number of repeat units in the polymer molecule
( MW )chain = DP * ( MW )monomer
Homopolymer: Polymer derived from one type of monomer
Copolymer: Contains structural units of two or
more different monomers Random - AABABABBBABABABABB - Alternating- ABABABABABABABABAB - Block - AAABBBBBBAAAABBBBB - Graft - AAAAAAAAAAAAAAAAAA -
B B B B
NOMENCLATURE OF POLYMERS Polymer nomenclature has been largely a matter of custom without any system being universally accepted. It is not unusual for a polymer to have several different commonly used names due to different nomenclature systems.
IUPAC nomenclature systems are currently under consideration
Based on source: applicable to polymers
synthesized from a single monomer
"Poly"monomer Examples: Polyacrylonitrile
Poly(carbonate)
Based on structure: applicable to polymers synthesized from two monomers
"Poly" (name of structural grouping attached to the parent compound)
The parent compound is the particular member of the class of the polymer - the particular ester, amide, urethane and so on.
Examples Poly(ethylene terephthalate) Poly(hexamethylene adipamide)
MOLECULAR ARCHITECTURE
Linear polymers: Each repeating unit is linked only to two polymers
-- CH2 - CH2 - CH2 --
Branched polymers: Same repeating units can be linked to more than two others, and form branches which are not present in the monomers
-- CH2 - CH - CH2 -- CH2 CH2
Network polymers: Inter-connected polymers of
extremely high MW
They can be formed: a) directly from monomers
e.g. polyester b) by cross-linking linear or
branched polymers (e.g. vulcanization of rubber)
CHAIN STRUCTURE Basic chain properties:
Molecular weight and its distribution Conformation Configuration
Examples: Configurations Head-to-tail
Head-to-head
Conformations Random flight or freely jointed chain For a random walk process Then:
∑=r
rPrr )(22
22
222
23
)exp()(
nlb
rbArrP
=
−=
r−
22 nlr =
• Root-mean-square of end-to-end distance
• Real chains
lnr 2/12/12 =
22 Cnlr =
CLASSIFICATION OF POLYMERS 1. Method of formation
Step-growth or condensation
- Monomers with reactive groups (e.g. carboxyls) - Slow reaction
Chain or addition
- Vinyl monomers (e.g. monomers with C=C ) - Fast reaction
2. Morphology
Amorphous: Disordered state; chains in random coil arrangement; no sharp melting point
Examples: PS / PVAc
Crystalline: Ordered state; chains exist in well-defined shapes; sharp melting point; chemical and geometric regularity required
Examples: Linear PE / Nylon
Polymers can be found in two states reflecting the shapes (conformations) of polymeric chains
Amorphous Crystalline
In the amorphous state polymer chains have random conformations associated with the flexibility of chemical bonds along the chains
An important concept associated with the amorphous state is that of physical entanglements When more than one polymer molecules with random conformation coexist, entanglements can result Chain entanglements are largely responsible for the transfer of forces between chains in spite of the absence of bonds
force force
Chain at rest Entanglement
Some polymeric chains are capable of crystallizing Crystallinity implies a regular ordering of the repeat units into unit cells on a very small scale, lamellae on a larger scale and spherulites on an even larger scale
lamellae
spherulites
In the crystallization process, solid material starts forming at discrete points called nuclei The solid material then grows as spherulites until the whole melt has turned into a solid consisting of adjacent fully grown spherulites
Depending on the number of nuclei involved, the crystallized material may feature many small spherulites or few large ones
fine coarse
Polymers are never perfectly crystalline materials
A semi-crystalline polymer can be viewed as a combination of crystals (lamellae) and amorphous material in-between
Degree of crystallinity (DC)
Properties depend on DC and structure Examples: Transparency / Toughness
3. Mechanical behaviour
Why do polymers form useful materials?
i) Large molecules ii) High intermolecular forces iii) Entanglements
Elastomers: Recover instantly from extensive stretching (up to 100 %) Low initial modulus (up to 1000 psi) Modulus increase with decreasing temperature
Fibres: High initial modulus (106 psi)
Low extension at break Deformation only partially recoverable
Plastics: Intermediate modulus (104 - 105 psi)
Breaking strain varies from a few percent (e.g. PS) to 400% (PE) Elastic recovery generally very small
Elastomers Flexible chains with weak intermolecular
forces, do not crystallize. Recoverability due to cross-linking
Fibres Stiff chains with strong
intermolecular forces, highly crystallizable
Plastics Variable chain stiffness, generally weak
intermolecular forces, usually crystallize (due to regular, symmetric chain structures)
4. Processing characteristics
Thermoplastic: Can be heated and softened to a viscous fluid which can be extruded or moulded; hardens on cooling and retains the shape imposed; temperature limit, degradation; can be heated and reformed repeatedly; linear or branched not cross-linked materials; will dissolve.
Examples: PE, PP, PVC, PTFE, PS
Thermosetting: Reactive materials; polymerization and processing usually done in the same step; 2 components, prepolymer & curing agent; once shaped can not be reformed via heating; can not be dissolved.
Examples: Epoxies (adhesives), phenol-
formaldehyde resins
Commodity Resins Olefinics Vinylics Styrenics Acrylonitrilics Acrylics Misc. Copolymers & Polyalloys Cellulosics Engineering Resins Polyamides Polyesters Polycarbonates Sulfone Polymers Imide Polymers Ether-oxide polymers Ketone Polymers Fluoropolymers Misc. Heterochain polymers
THERMAL TRANSITIONS Change of state with temperature
Glass transition temperature (Tg)
Amorphous polymers
Virtual cessation of local molecular motions
Chains become stiff; material becomes glass-like
Macroscopic effects: discontinuity of temperature derivatives of enthalpy, entropy & specific volume
Effect of molecular weight
Effect of chemical structure
Effect of chain topology Copolymerization
Branching & X-linking
POLYMER STRUCTURE AND PHYSICAL PROPERTIES
Properties Small deformations Large deformations e.g. electrical e.g. tensile strength optical melt properties yield stress impact strength
Properties involving large deformations depend primarily on the long-chain nature of polymers and the configuration; MW, MWD, branching
Properties involving small deformations are
influenced by factors determining the manner in which chain atoms interact at small distances; symmetry, steric effects, ability to crystallize, polar groups