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Polymers

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Crazing - Orientation of polymer chains across the opening of a 'crack-like' feature. The work of crazing is 1000 times larger than the surface work to create a ... – PowerPoint PPT presentation

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Title: Polymers


1
Polymers
  • Macromolecule that is formed by linking of
    repeating units through covalent bonds in the
    main backbone
  • Properties are determined by molecular weight,
    length, backbone structure, side chains,
    crystallinity
  • Resulting macromolecules have huge molecular
    weights

2
Polymers
  • Terminology
  • mer a unit
  • monomer one unit
  • dimer two units
  • trimer three units
  • tetramer four units
  • polymer many units
  • pre-polymer growing towards being a polymer
  • oligomer few units fixed in size
  • homopolymer polymer of fixed mer type

HOMOPOLYMER
3
Polymers
  • Terminology (contn)
  • copolymer polymers of two mer types
  • random -B-A-B-A-B-B-A-
  • alternating -A-B-A-B-A-B-A-
  • block -A-A-A-A-B-B-B-
  • heteropolymer polymers of many mer types

COPOLYMER
4
Polymers Molecular Weight
  • i degree of polymerization ( of monomer units)
  • Mi i x Mm
  • Mi molar mass of polymer molecule i
  • Mm molecular weight of monomer
  • Typically all chains are not equally long but
    display a variation
  • monodisperse equal chain lengths is specific to
    proteins
  • polydisperse unequal length specific to most
    synthetic molecules
  • Therefore we need to define an average
    molecular weight
  • number average, Mn
  • weight average, Mw

5
Polymers Molecular Weight
  • number average, Mn
  • weight average, Mw

Ni of molecules with degree of polymerization
of i Mi molecular weight of i
6
Polymers Molecular Weight
  • Ratio of Mw to Mn is known as the polydispersity
    index (PI)
  • PI is a measure of the breadth of the molecular
    weight
  • PI 1 indicates Mw Mn, i.e. all molecules have
    equal length (monodisperse)
  • PI 1 is possible for natural proteins whereas
    synthetic polymers have 1.5 lt PI lt 5
  • At best PI 1.1 can be attained with special
    techniques
  • EXERCISE Draw the molecular weight distribution
    for PI 1, PI 2, and PI 4

7
Polymers Molecular Weight
  • Biomedical applications 25,000ltMnlt100,000 and
    50,000ltMwlt300,000
  • Increasing molecular weight increases physical
    properties however, decreases processibility

8
Polymers
  • Types of polymers
  • Thermoplastic Polymers that flow when heated
    thus, easily reshaped and recycled. This property
    is due to presence of long chains with limited or
    no crosslinks. (polyethylene, polyvinylchloride)
  • Thermosetting Decomposed when heated thus, can
    not be reformed or recycled. Presence of
    extensive crosslinks between long chains induce
    decomposition upon heating and renders
    thermosetting polymers brittle. (epoxy and
    polyesters)
  • Elastomers Intermediate between thermoplastic
    and thermosetting polymers due to presence of
    some crosslinking. Can undergo extensive elastic
    deformation (natural rubber, silicone)

9
Polymers
10
Polymers
11
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12
Polymers
  • Polymers can be either amorphous or
    semi-crystalline
  • Tacticity, i.e. arrangements of substituents
    around the backbone, determines the degree of
    crystallinity
  • Atactic polymers are amorphous
  • Isotactic and syndiotactic may crystallize
  • Crytallinity depends on
  • size of side groups (smaller, ?crystallinity)
  • regularity of chain
  • Increased crystallinity enhances mechanical
    properties

13
Polymer Synthesis
  • Two common methods of polymerization
  • Condensation polymerization (or stepwise
    addition)
  • Addition reaction (or chain polymerization)
  • Condensation Two monomers react to form a
    covalent bond usually with elimination of a small
    molecule such as water, HCl, methanol, or CO2.
    Reaction continues until one type of reactant is
    used up.
  • Addition Monomers react through stages of
    initiation, propagation, and termination.
  • initiators such as free radicals, cations, anions
    opens the double bond of the monomer which
    becomes active and bonds with other such monomers
  • rapid chain reaction propagates in this fashion
  • reaction is terminated by another free radical or
    another polymer

14
Polymer Synthesis Condensation
  • phenol-formaldehyde results in condensation of a
    water molecule
  • nylon (polyamide) an organic acid reacts with an
    amine to form an amide. HCl condenses

15
Polymers Synthesis Addition
  • Termination may occur by
  • two radicalized polymers reacting
  • another radicalized monomer
  • one initiator (alkoxy radical, OR, in this case)

16
Condensation vs. Addition
  • Addition
  • Difficult to control molecular weight
  • Undesirable branching products
  • Condensation
  • Molecular weight closely controlled
  • Polydispersity ratios close to unity can be
    obtained

17
Polymers Deformation
Stress
I
II
III
IV
Strain
I. Chain unfolding, unwinding, unwrapping or
uncoupling (low energy) II. Chain sliding (low
energy) III. Bond stretching, side group ordering
(high energy) IV. Bond breaking (high energy)
18
Polymers Deformation
Ceramics
Metals
Stress
Polymers
Strain
  • Lower elastic modulus, yield and ultimate
    properties
  • Greater post-yield deformability
  • Greater failure strain

19
Polymers Viscoelasticity
  • Dependency of stress-strain behavior on time and
    loading rate
  • Due to mobility of chains with each other
  • Crosslinking may affect viscoelastic response

20
Polymers Thermal Properties
  • In the liquid/melt state enough thermal energy
    for random motion (Brownian motion) of chains
  • Motions decrease as the melt is cooled
  • Motion ceases at glass transition temperature
  • Polymer hard and glassy below Tg, rubbery above Tg

linear amorphous
21
Viscoelastic Deformation Glassy Materials and
Tg Rigid Brittle Solid below Tg Viscous
Deformable liquid above Tg
? is the viscosity of the fluid
22
Viscoelastic Modulus versus Temperature
23
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26
  • Fracture of Polymers
  • Thermoset and Thermoplastic materials below Tg
    behave as brittle solids and fail by cracking.
    The cracks are sharp.
  • Crystalline Thermoplastic Resins above Tg yield
    and undergo ductile failure.
  • Noncrystalline Thermoplastic resins above Tg
    undergo Crazing and Cracking.
  • Crazing - Orientation of polymer chains across
    the opening of a crack-like feature. The work
    of crazing is 1000 times larger than the surface
    work to create a crack.
  • Cracking then occurs down the middle of the
    craze.

27
Craze Formation
28
Crack Propagation Through Crazed Area
29
Cracking Example in Polymers
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