Polymer Structures

What is a polymer?

Compound made of many repeating small units

Mer / repeat unit

the small unit that repeats along the chain

monomer

reactive small molecule before polymerization

degree of polymerization, DP

how many mers are in one chain

marcomolecule

the whole long chain molecule made up of polymerized monomers

what kind of bonds are along the polymer backbone?

covalent C-C bonds

what kind of bonds are between polymer chains?

secondary bonds (van der waals forces), weak bonds hold the chains together

linear chain architecture

polymer chains held together by VDW forces

long separate chains packed together

branched chain architecture

side branches prevent polymer chains from packing together closely, leads to lower density

crosslinked chain architecture

chains are joined together with some covalent bonds, leads to a rubbery texture

network chain structure

heavily crosslinked polymer chains

molecule is very rigid

how does chain length impact your material?

longer chains, more tangling, more flexible

eg. candle wax has smaller chains than plastic bags, bags are more flexible

how does DP impact melting point?

for solids: longer chains means more tangling, stronger IMFs, higher melting point

DP formula

DP = Mn / m

Mn = number average molecular weight

m = molar mass of one repeat unit

Number average weight, Mn

average weighting each chain equally (by number of molecules)

Weight average, Mw

average weight by the mass of each chain (bigger chains count more)

Degree of polymerization, DP

DP = Mn / mass of each mer

how many repeat units per chain

Polydispersity Index, PDI

PDI = Mw / Mn

spread of distribution

is only equal to 1 if all chains are identical

When are Mn and Mw used?

Mn: strength, chain end chemistry, DP

Mw: melt viscosity, melt strength

Thermoplastic

softens and flows when heating, can be remelted and reshaped, recyclable by melting

separate chains held by secondary bonds

Thermoset

sets permanently when cured, cannot be remelted, not recyclable by melting

estimate % crystallinity from density

% crystallinity = p_c(p_s - p_a) / [p_s(p_c - p_a)] x 100

glass transition Tg

below tg amorphous regions are rigid and glassy, above it they become soft and rubbery

melting Tm

the temperature at which ordered crystalline regions lose their order and flow

what transitions does each polymer have?

amorphous - tg only

semicrystalline - tg and tm

network - tg only (never melts)