Polymer analysis Flashcards
Molecular weight vs molar mass
Molecular weight:
mass of molecule
Molar mass:
mass of a mole of substance
Number average (Mn)
Average molecular weight based on the total number of polymer chains in a sample. (Not very accurate representation)
Mn = sum(Ni*Mi) / sum(Ni)
Mi = molecular weight of each chain
Ni = # polymer molecules with molecular weight Mi
Weight average (Mw)
average molecular weight based on the contribution of each chain to the total mass of the sample.
Mw = sum(NiMi^2) / sum(NiMi)
Mi = molecular weight of each chain
Ni = # polymer molecules with molecular weight Mi
Size average (Mz)
reflects the contribution of very large chains to properties like tensile strength and viscosity.
Mz = sum(NiMi^3) / sum(NiMi^2)
Mi = molecular weight of each chain
Ni = # polymer molecules with molecular weight Mi
Polydispersity index (PDI)
measures the distribution of molecular weights in polymer sample (how uniform chain lengths)
PDI = Mw / Mn
PDI = 1:
MONODISPERSE, all chains same lenght (in practice chains are POLYDISPERSED, Mw > Mv > Mn)
PDI > 1:
chain length becomes increasingly varied
Molecular Weight Distribution (MWD)
Discribes the full range of molecular weights in polymer sample.
Narrow MWD:
better consistancy in processing and properties
Wide MWD:
More complex behaviours but greater toughness
Size Exclusion Chromography (SEC)
Measuring technique for all weight classifications.
First disolve polymer then filter in through crosslinked beads with small pores. This seperates molecules by size, highest molecular weight come through first. Light scattering detector at the end counts # of molecules passing.
Degree of Polymerization (DP)
number of monomeric units in molecule of polymer
DP = Mw,polymer / Mw,monomer
Higher DP = longer chains
Differential Thermal Analysis (DTA)
Measures the difference in temp between test and reference sample. Deviation from sample temp indicates phase change (energy absorbsion/rejection)
Note: Not accurate for measuring enthalpy of transition
Differential Scanning Calorimetry (DSC)
Measures the energy required to maintain zero temp change between test and reference samples.
Process is good for measuring enthalpy change.
Thermo-mechanical Analysis (TMA)
Measures deformation of substance as a function of temperature or time.
Thermo-gravimetric Analysis (TGA)
Identifies different components of polymetric material by analyising thermal degredation of sample. Works by sensitive measurment of weight changes.
Can observe polymer phases, fillers, reinforcement, moisture content and chemical reactions as different elements will burn off at different temps.
Dynamic Mechanical Analysis (DMA)
Uses oscillating force on material and measures response. In polymers there is phase lag between stress and strain.
Elastic response is IN-PHASE
Viscous response is OUT-OF-PHASE
Note: Can use different testing geometrys depending on material application e.g. bending, tension, shear.
Tg of Miscible Blends
Can use the fox equation to predict Tg of miscible blends.
1/Tg = m1/Tg1 = m2/Tg2
Material that deviates from the fox equation indicates poor miscibility
Response changes with frequency
Low Frequency:
Material has time to relax and respond. Flow dominates and is more liquid like - Damping behaviour.
High Frequency:
limits material to elastic behaviour. Chains lock up and cant strech out. Elastic dominates and more solid like - spring behaviour.
