Polymers Final

polymer solution

polymer is solute (substance being dissolved), some low-molecular weight species are the solvent (doing dissolving)

good solvent

-X < 1/2, B > 0

-solvent molecules separate

B > 0

-good solvent
-monomers on different chains are happy enough to be surrounded by solvent
-when two coils approach to each other, there is steric repulsion (excluded volume) and the chains separate

poor solvent

-Χ > 1/2, B < 0

-polymer chains prefer self interaction

B < 0

-poor solvent

-polymer is barely able to stay dissolve in solution

-monomers find it energetically favorable to be close to other monomers

-when two coils approach each other, there is a tendency to cluster

-all solvent molecules excluded from polymer chain that is coiled, can’t penetrate polymer

theta solvent

-Χ = 1/2, B = 0

-"not-very-good" solvent

-excluded volume and relatively unfavorable solvent-solute interactions cancel one another in the net

-Following Flory-Huggins, it's a θ solvent

interaction parameter (chi - Χ)

-represent exchange energy per molecule normalized by thermal energy kT

-used in enthalpy (H) of mixing for RST

-small Χ or negative allows chains to mix

-lower X → smaller H → more likely to dissolve

solubility parameter (delta - δ)

materials with similar energy of vaporization would have similar intermolecular force, therefore might intend to be soluble with each other

N

-contributes to entropy (S)

-if small, limited contribution

chi N > 10.5

-for block polymer

-indicates polymers will be phase separated

-chi from H (enthalpy) contribution, N from S (entropy) contribution

chi N > 2

-for polymer blends

-indicates polymers will be phase separated

gauche state

preferred configuration for more flexibility because ready to change conformation

molecular weight

-_ must be similar to favor mixing and ensure compatibility in solution

-high _ makes it dissolving of polymer harder

Regular Solution Theory

-description of mixtures of non-polar small molecule liquids with positive or zero mixing enthalpy
-relatively simple statistical model to express free energy of mixing for a binary solution of two
components

regular solution theory where one of the components is a polymer

Flory Huggins Theory

ENTROPY (S) of mixing, S > 0 favors mixing

adopt same lattice model as RST but:
-each lattice site has one solvent molecule
-each polymer occupies N lattice sites
-N is proportional to degree of polymerization
-monomer unit was same volume as solvent

uses volume fractions instead of molar fractions (phi, Φ)

polymer-polymer mixtures

-enthalpy contribution does NOT allow for polymer mixing in this case due to N → infinity

-when N → infinity, prevents mixing because S → 0

entropy (S)

-a measure of the disorder of a system
-higher _ favors mixing

enthalpy (H)

-total energy/heat of a system

-considers internal change

-high _ opposes spontaneous mixing

-opposes mixing when Χ > 0

Tg

-temperature at which an amorphous polymer transitions from a brittle state to a ductile state

-at this temperature, enough energy for polymer chains to slide past each other

-not a thermodynamic property of polymer, can adopt a range of values

factors that affect Tg

-flexibility of polymer backbone (more flexible = lower), means can accommodate to additional change like temperature

-steric effects of pendant groups, more mobility = lower

-configurational isomerism, inefficient packing causes drop

increase free volume

-poly dispersity

-low molecular weight

-plasticizers

-branches

-residual solvent

-inefficient packing

stress vs strain for a semicrystalline polymer

-shows both elastic and plastic deformation regions

-Initially, the material exhibits linear elastic behavior, followed by yielding and eventual necking before fracture.

glassy state of polymers

-amorphous solid→ no long-range order or symmetry in the packing of molecules

-kinetically trapped

-over some range of temperature, molecular motion will become so slow, that an equilibrium packing of molecules can’t be attained during experiment → sample has undergone _

-each polymer has this

Strain and Tg

-polymer chains locked up in place under Tg, not enough thermal energy to allow chains to slide past, strain prior to fracture is small

-large stresses are dissipated by fracture of covalent bonds, only small strain can be recovered

-smaller stress can be dissipated by local rotation of bonds, sub-Tg relaxation mechanism that contributes to toughness of polymer sample

-glassy polymer still tougher, absorbs more energy prior to fracture than ceramic in general

glass transition of semi-crystalline

-changes at Tg less drastic since crystalline regions unaffected

-between Tg and Tm, this polymer is composed of rigid crystallites dispersed in a rubbery amorphous matrix

-at Tm, crystallites melt leading to a viscous state

glass transition of amorphous

-heated from low T region, volume expands at constant rate

-at Tg, rate of expansion increases abruptly; change in physical behavior from hard brittle, glassy solid below to a soft rubbery region above

-further heating changes from rubbery to viscous liquid

mechanical properties of glassy polymers (1)

-becomes stretchy before starts to deform permanently

-entropic elasticity is the dominant restorage force

-~0-10% strain

mechanical properties of glassy polymers (2)

-stretching a little bit more but slowly, it yields

-breaking the crystalline domain apart and realign them along the strain axis

mechanical properties of glassy polymers (3)

stretching really fast, it will break and won’t extend very much

mechanical properties of glassy polymers (4)

-suspend it and wait for a day, it will get to the point of ultimate strain

-creep, which is associated with viscoelasticity of polymer samples

-it doesn’t store any mechanical energy, they just dissipate all

creep

time-dependent deformation at elevated temperature and constant stress

viscosity

-measure of a fluids resistance to flow

-high = thicker fluid, resists motion

viscoelasticity

-property of materials that exhibit both viscous and elastic characteristics

-when deformed have time-dependent strain behavior that combines elastic recovery with energy dissipation

time-temperature superposition

-a measurement at a certain temperature and time (or frequency) is equivalent to a measurement at a lower temperature and longer time

-experimental time too short → lower temperature to make response long

glassy state

-below Tg

-locked polymer chains

rubber state

-occurs from Tg to Tm

-chain conformations are available

-directly observed shape deformation

sticky fluid

-occurs after Tm

-all polymer chains can slide past each other

-deformation cannot recover

T and S

always positive for dissolving process, system becomes more chaotic after mixing (favors spontaneous mixing)

metal reaction to heat

-heat stretches the metal lattice, causing expansion and changes in properties

-heat enables vibration of molecules → expands

rubber reaction to heat

-rubber is entropic controlled

-stretch out rubber band → heats up

-contract → cools down

unique properties of rubber materials

-entropy (S) driven; in a stretched state, entropic degree of freedom has been reduced

-large deformation

-totally recover after removing stress

creep (for viscoelasticity)

-stress is suddenly applied to a polymer

-resulting strain is a function of time

stress relaxation

-stress is suddenly applied to a polymer

-resulting stress is monitored as a function of time showing dissipating energy

glassy polymer viscoelasticity

-behaves like a real solid

-E > 1 GPa

leathery polymer viscoelasticity

-requires time to return to original shape

-time dependence to its elasticity

rubber polymer viscoelasticity

-time dependence partially goes away

viscous polymer viscoelasticity

polymer molecular mechanism when stretched

-involves chain slippage and realignment under stress, allowing for energy dissipation and recovery

-chain realignment

metal molecular mechanism when stretched

-involves dislocation movement and slip systems, leading to deformation and strain hardening -accumulation of dislocations

phase diagram (chi vs deltaG/RT)

-chi small → H ignore, G depends on S>0

-chi large → H term produces a local maximum in G which can lead to phase separation or instability in the system

phase diagram (chi vs T)

tensile response: elastomer

-no yielding, deformation largely recoverable

-stress increases monotonically up to point of failure

-non-crystalline polymer above Tg has rubber elasticity

-polymer network, gels have rubber elasticity

-rubber elasticity unique to polymeric materials

viscoelasticity of polymers