Materials Science midterm 2

Poly

Many

meros

Part

oligo

Few

Mono

One

Polymer

A long chain molecule that consists of a large number of repeating units of identical structure.

Single bonds? Bond strength?

Stable

Double bonds? Bond strength?

Metastable

Triple bonds? Bond strength?

Barely stable

Moiety

An interesting subgroup or part of a molecule. Examples include: A generic amino acid, amine, & r-group variety.

Polyethylene

Polypropylene

Polyvinyl chloride (PVC)

Polystyrene

Linear polymer system

Branched polymer system

Crosslinked polymer system

Synthetic polymer examples

Useful rubbers, plastics, & fiber materials

VDW in polymers?

They act as friction, interact move per chain, & make polymers stronger, but harder to process.

Polymer chain rotation

Primary mechanism, giving polymers flexibility, elasticity & ability to conform under thermal energy or mechanical loads.

Polymer chain reptation

The snakelike thermal motion of long, linear polymer chains as they wiggle through a dense network of surrounding chains in a melt or concentrated solution.

Viscosity

A measure of a noncrystalline material's resistance to permanent deformation.

How can viscosity be changed?

↑ molecular weight,↑ concentrations, & stronger intermolecular forces increase it.

Deformation

Dictates how polymers respond permanently orelastically under stress.

Melting

Above Tm, the thermal energy increases chain movement, significantly lowering viscosity.

Molecular weight formula

MolWt=DP*Mo

DP= Degree of polymerization = # of repeat units

Mo= molecular weight of repeat unit

Why does MW matter?

Longer chains → ↑MW

Longer chains mean more covalent bonds per molecule, fewer chain ends.

Longer chains also mean higher viscosity, higher strength, ductility, & toughness.

Number-average molecular weight (Mn)

The average, based on the number of chains in a sample.

Mn=sum(n*M) / sum(n)

Weight-average molecular weight(Mw)

The average, based on the total weight contribution of each chain size.

Mw=sum(W*M) / sum(W)

What does this distribution tell us about A & B?

“A”has less polydispersity than “B”, & “A” would behave differently than “B”.

How can gel permeation chromatography be helpful?

This is the best way to determine Mn and Mw, provides an entire distribution curve, provides n-value & M-value.

Melt flow index

A standard measure of how easily a thermoplastic polymer melts & flows.

Effect of higher MFI on properties

Improves:

Moldability

Shrinkage anisotropy

Swell

Declines:

Tensile strength

Elongation

Impact strength

Creep resistance

Matting properties

Stress cracking resistance

Polymerization

The process by which monomers are linked together to generate long chains.

Addition polymerization

Monomer units attached one at a time to form linear macromolecule. The composition of the resultant is identical to the original reactant monomer.

Three stages of addition polymerization?

Initiation: formed by reaction between initiator® & the monomer unit.

Propagation: the linear growth of the polymer chain by addition of monomer units to actively growing chain molecule.

Termination: there are 2 ways that this could happen. Two growing molecules could link at their active ends or they could react to form two dead chains.

Condensation polymerization

The formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer species.

Most important addition polymerizations are for ______.

Vinyls

Vinyl examples?

Polyethylene, polystyrene,polypropylene, polyvinyl chloride, & poly (methyl methacrylate).

Chirality

Lacking symmetry like a left & right hand.

Some strengths of condensation polymerization?

Offers great range of chemistry possibilities, stronger backbones & hydrogen bonding.

Why do addition polymers cost less?

There isn't byproducts & waste, the building blocks for addition are easier to get, the process is easier, & is much faster.

Why do condensation polymers have more specialized properties?

Contain functional groups, not just carbon-carbon bonds, stronger forces that change properties, & can be more tailored.

Types of copolymers?

Random copolymers

The growing chain shows no preference for either monomer resulting in a completely random sequence with properties Between 2 homopolymers. An example is ethylene -propylene rubber.

Block copolymer.

The propagation rate of like monomers is favored over cross polymerization. Some examples are synthetic rubber or acrylonitrile butadiene styrene (abs).

Thermosets

3D networked polymers with a high degree of chemical crosslinks with severe restriction of chain mobility.

Elastomers

Lightly crosslinked system with flexible chains. Rubbers that can be easily stretched 5x & rapidly return to original shape.

Amorphous

Having a noncrystalline structure

Semi-crystalline

Has crystalline regions dispersed within remaining amorphous material due to the size & complexity of polymers.

Glass transition temperature (Tg)

Kinetic Transition between rubber & glass over a temperature range.

Tg depends on

Molecular weight, thermal history, & chain configurations.

% Crystallinity

[Pc ( Ps - Pa) ]/[ Ps(Pc-Pa) ] *100

Crystals properties of crystallinity

Some higher order crystallinity & melting is a thermodynamic event.

Glass(amorphous) properties of crystallinity

No higher order structure & melting is a kinetic event,

Modulus/temperature curve example

Stearic hindrance affect Tg?

Big moieties that prevent chain rotation.(greater mobility = lower Tg)

Higher Tg

Polarity affect Tg?

Electron cloud preferentially circle some regions of moiety, generate attraction.(greater mobility = lower Tg)

Greater Tg

Chain stiffness affect Tg?

Rigid chain/double bonds prevent rotation(greater mobility = lower Tg)

Greater Tg

DSC curve

Polymer Crystal structure makeup

Fringed micella, folded chains, discontinuous folding

Crystalline region of polymers properties

More dense, stiffer, stronger, less tough, more opaque, more anisotropic, more environmentally resistant & harder to process.

spherulite

An aggregate of ribbon-like polymer crystallizes radiating from a common central nucleation site, the crystallizes are separated by amorphous regions.

Processed xtal morphology