Polymer Materials - Chemistry/Physics

Polymer

a substance composed of molecules which have long sequences of 1 or more species of atoms or groups of atoms linked to each other by primary (usually covalent) bonds; formed through polymerization

Polymerization

the process of polymer molecules (monomers) linking together

3 Types of Polymer Skeletal Structure

linear, branches, or network/cross-linked

Linear - Polymer Skeletal Structure

a chain with 2 ends; most simple form

Branched - Polymer Skeletal Structure

side chains/branches of significant length are bonded to the main chain at branch points; essentially linear but w/ side chains/branches

Network/cross-linked - Polymer Skeletal Structure

3d structure with each chain connected to all the others

2 Types of Thermo-polymers

thermoplastics and thermosets

Thermoplastics

linear and branched

can be melted upon heat

semicrystalline or amorphous

think plastic - melts

Thermosets

cross-linked

rigid or rubbery

intractable → can’t be melted

think set = does not move so does not melt/reshape (will just decompose or carbonate)

Homopolymers

a polymer that results from the polymerization of a SINGLE monomer; consists of mostly a single type of repeating unit

ex. -A-A-A-A-A-A-A-, -[A]-_n (A → poly-A) with n being the degree of polymerization (Polyethylene or PE)

n

the degree of polymerization

Copolymer

polymers whose molecules contain MORE than 1 type of repeat unit

multiple types: statistical (subtype random), alternating, graft, and block

Statistical Copolymers

the sequential distribution of the repeat units obeys known statistical laws

Random Copolymer

a special tyoe of statistical copolymer; means true random

Alternating Copolymer

repeat units have a pattern

What do statistical/alternating copolymers generally have?

properties intermediate to those of the corresponding homopolyers

Graft Polymer

branched polymers with then branch(es) having different chemical structure from the main chain; repeat unit as the main chain and so are the branches just a different kind

Block Polymer

repeat exists in long sequences or blocks of the same type; sections of each repeat

What may graft and block copolymers have?

unique properties that are not intermediate to those of the corressponding homopolymers unlike statistical/alternating ones

Block Polymer Picture

can be used to make specifc designs

Types of Synthesis

step-growth polymerization and chain-growth polymerization

Step-growth Polymerization

polymer chain grows step-wise by reactions that can occure between any 2 molecular species; very small to large; will consume most of the monomers bc all react w/ each other; aka condensation

Chain-growth Polymerization

polymer chain grows only by reaction of monomers with a reactive end-group on the growing chain; think of it growing like adding individual beads to a string; initiator starts reaction; very fast

Condensation - Step-growth Polymerization

forms polyesters and polyamides/nylons using OH (alc) or NH₂ (amine) groups

Polyester Formation

condensation (step-growth)

ester: alc + carboxylic acid

polyester: multiple esters

used for grafts and sutures

Polyamides/Nyon Formation

condesnation (step-growth)

amide: amine + carboxylic acid

polyamide: multiple amides

primary type of suture material

Branching and Crosslinking

monomers have functionality > 2; essentially multiple reaction sites like diacid R(COOH)₂ and triol R’(OH)₃

Types of Chain Growth Polymerization

free radical polymerization and ionic polymerization

Free Radical Polymerization

a type of chain growth; polymer molecule grows by addition of monomer to a terminal free-radical reactive site (active center); steps: initiation → propagation → termination (combination + disproportionation) → chain transfer

Free Radical

independently-existing species with unpaired e- and are normally highly reactive

Active Center

the free-radical reactive site

Ionic Polymerization

anionic and cationic; ion in center

Free Radical Polymerization Steps

1. initiation

2. propagation

3. termination

4. chain transfer to solvent, initiators, monomers, polymers etc.

Initiation - Free Radical Polymerization Step 1

use either thermolysis or photolysis to start reaction to form initiator which then reacts w/ the monomer

Thermolysis

delta; ex. Benxoyl peroxide - great one bc becomes a free radical

Photolysis

hv; ex. Azobisisobutyronitrile (AIBN) → absorbes photo energy and easy to contol bc can just cover the parts you don’t want polymerized like resin

Propagation - Free Radical Polymerization Step 2

Termination - Free Radical Polymerization Step 3

2 types: combination (2 free radicals bond) and disproportionation (radicals make new bonds in molecules they are already in)

Chain Transfer - Free Radical Polymerization Step 4

this can be to a solvent, initiator, monomer, polymer etc.; one ends but kickstarts another usually

Anionic/living Polymerization

active center has an ionic charge; no inherent termination process bc no chain transfer → progating polymer chain retains active carbonionic end-groups so when more monomers are added, the chain continues to grow; can be used to make block copolymers

Anionic/living Polymerization Picture

Nomenclature

can be source based like poly (monomer name) or structure based like poly (chemical structure of the repeat unit)

Molar Mass

M = n*M₀

n - degree of polymerization (# of repeat units)

M₀ - molar mass of repeat unit

Molar Mass Distribution

usually polymers consist of macromolecules with a range of molar mass; wide distribution

M_n (avg.) - # avg. Molar Mass

sum of all molecular weight divided by their total number of molcules

Ni - # of molecules with Mi

Mi - molecular weight

wi - weight fraction of all molecules with Mi

M_w (avg.) - weight avg. Molar Mass

sum of the products of the molar mass of each fraction multiplied by its weight fraction

Polydispersity Index (PDI)

= M_w (avg,) / M₀ (avg.)

higher this = more widely distributed it is

= 1 → monodisperse

> 1 = for most cases

X_n (avg.)

#-avg. degree of polyermization

= M_n (avg.) / M₀

X_w (avg.)

weight-avg. degree of polymerization

= M_w (avg.) / M_o

Molecular Weight Detemination

end group anaylsis (M_n (avg.)), gel permeation chromatography (GPC), or mass spectroscopy

End Group Analysis (M_n (avg.))

weigh sample, divide by the weight of the end group to get the # of strands

Gel Permeation Chromatography (GPC)

size exclusion; dilute polymer soln. passes a column of porous beads, high MW molecules are excluded from the beads and elute first, low MW molecules pass through through the pores of the beads, elute later; essentially large can’t get into the pores so come out first/faster; most commonly used in polymers??

Mass Spectroscopy

bombard w/ some kind of energy (ionize it/put a charge to it) and then based on the deflection of charge, it hits different parts of the target; not commonly used in polymers

Uniqueness of Polymers

chain entanglement, summation of intermolecular forces, and time scale of motion

Summation of Intermolecular Forces

this is huge even with the weaker van der waals; will affect properties

Time Scale of Motion

would think have fast movement but bc of chain entanglement and summation of intermolecular forces ends up they move slowly

Shape of Polymer Chain and Amorphous State

rotation of the bond (diff. bonds have diff. rotational property) → chain bending and twisting → random coil → end to end distance is much smaller than the total chain length; can get entangled with the neighboring chain;

What is amorphous polymer like?

a pot of spaghetti, but with a much higher ratio of length/diameter

Bond Rotational Properties

C-C → most rotation

C=C → resisting rotation due to pi bond

C=-C → further restriction of rotation bc of 2 pi bonds

benzene ring → very stiff

Polyethylene Crystal Unit Cell

extended planar zigzag conformation; c axis always along the chain direction; orthohombic unit cell’ 2 chains per unit cell

Crystallinity

long-chain molecules can not be crystallized completely since most polymers are semi-crystalline or amorphous

Crystallinity (C%)

anything that prevents chain alignment or discourages interchain bonding will decrease this; molecular structure, intermolecular force, and processing conditions affect this; increasing this = higher mechanical strength

Molecular Structure Requirements for Crystallization

should be no defects (bracnhes, cross-links, or excesssive end groups) → lead to disturbances within the crystal or rejection from the crystal

size/disposition of a side group is important

Side Group Disposition - Irregular

then must be small so that the polymer can crystallize; ex. poly(vinyl alcohol) and poly(vinyl fluoride)

Side Group Size - Large/Significant SIze

can still crystallize as long as it is disposed regulary and symmetrically; ex. isotactive and syndiotactic vinyl polymers

Isotatic

side group is every other C and all on the same side = highly crystalline

Syndiotactis

side group is every other C but orientation alternates every one = highly crystalline

Atatctic

group is every other C but random distributed so hard to pack into a crystal = amorphous
+

Intermolecular Force + Crystallinity

ny;on 6,6 forms H-bonds between the carbonyl O’s and the amide hydrogens → allows chains to line up in an ordely fashion to form fibers; higher intermolecular force = high liklihood of crystallizing

Processing Conditions

quenching, annealing, and drawing; even w/ amporhous these can increase/decrease the chance of crystallization

Processing Conditions - Quenching

fast cooling process that causes crystallinity to fall bc the polymer has very little time to move around; type of thermo-processing

Processing Conditions - Annealing

lower temp. but still high enough for the chains to move so higher crystallinity; type of thermo-processing

Processing Conditions - X-ray Diffraction PIc of unannealed vs annealed

Processing Conditions - Drawing

moce amorphous structure through a tight space so mpre oriented = increases crystallinity

Melting Transition

occurs when enough energy for the overall translational chain motion to overcome secondary bonds, disrupting long range orders; inject heat into system but ata certain point, the temp. of the polymer stays the same (crystalline to this state)

Glass Transition

occurs when there is enough energy to cause molecular motion around the polymer backbone

Glass Transition Temperature (Tg)

the temp. at which a glassy polymer softens into a viscous liquid or rubbery phase; temp. where chains in amorphous (disordered) regions of the polymer gain enough thermal energy to cause molecular motion around the backbone

Tg > room temp.

this makes glassy materials

Tg < room temp.

this makes rubbery materials

Factors that Influence Tg

backbone flexibility, pendant group, intermolecular forces (strength), crosslinking, and plasticizer

Tg Influencer - Backbone Flexibility

flexible = low Tg (-C-C- or -C-O-) and rigid = high Tg (C=C or C=-C)

Tg Influencer - Pendant Group

steric hindrance → less flexible → high Tg

ex. Polyethylene (PE) = -125 C, Polypropylene (PP) = -20 C (has an extra methyl group compared to PE), and PS = 100 C (has a benzene ring)

Tg Influencer - Intermolecular Forces

higher forces = higher Tg bc stronger bonds

Tg Influencer - Crosslinking

causes higher TG bc locks chains together

Tg Influencer - Plasticizer

this lowers Tg

The same molecular characteritics can do what?

rise and lower Tg and Tm: Tg = (0.5~0.8) Tm

Differential Scanning Calorimetry (DSC)

use to find the thermal transition probed; find Tg by measuring the heat that goes in the systme and then measures the change

Differential Scanning Calorimetry (DSC) - Amorphous

if amorphous → will only show Tg

Differential Scanning Calorimetry (DSC) - Semicrystalline Polymer

has 3 T’s: Tg first then Tc (cooling?) and then Tm (melting)

Temperatire Dependence of Mechanical Properties Picture

Segmented Polyurethane

essentially different parts contain different physical/mechanical properties due to the makeup

Polyurethane

used for prosthetic spinal implants, speciality balloon.probe catheters, artifical hybrid pancreas, intra-aortic balloons, cardiac-assist devices, vascular grafts and stents, pacemaker/neurostimulator leads, and biodegradable polymer implants

Different Types of Polymers

fibers, glassy, semi-crystalline, and estamomer

Stress-Strain Behavior - Ductile Polymer

Ductile-Brittle Transition

as increase strain rate or decrease temp. = more brittle; ex. silly putty

Viscoelastic Behavior

a combo of both creep and stress relaxation; ex. polymers and biological tissue; strain rate must be reported when reporting measured mechanical moduli

Creep - Viscoelastic Behavior

a time dependent extension under a load (not an immediate stretch)

Stress Relaxation - Viscoelastic Behavior

a time dependent decrease in stree at a fixed strain (stretch polymer, keep it there, and then measure the load it is experiencing)

Creep and stress relaxation are results of what?

viscous flow