Mechanical Properties of Polymers

Characterizing Polymers
Mini-tutorial

First step in characterizing a polymer:
Use your senses (take pictures to document)What color? Does it fluoresce Transparent or opaque?Homogeneous in appearance?Solid or liquidTacky or sticky or brittle or toughMass compare with theoretical yield

Describe the material below

Describe the material below

Second, try and dissolve the polymer in different solvents
Water, ethanol, benzene, methylene chloride, tetrahydrofuran, acetonitrile, hexane, acetone, diethyl ether, dimethyl sulfoxide, N-methyl pyrrolidone (NMP)Leave it at room temp overnight. Look for swelling if not dissolved.Boil solvent for 4 hours.If it doesnt dissolve its probably cross-linked or really crystalline

Types of Polymers & solubility
Wont dissolve
Will dissolve

Third, Structural Characterization of soluble polymers

1H & 13C & 29Si Nuclear Magnetic Resonance and infrared spectroscopyMolecular weight by gel permeation chromatographyComposition by combustion analysesX-ray diffraction on film or powderViscosity of dilute solutions- shape of polymer

X-ray diffraction
Semi-crystalline polymer shows diffraction rings
amorphous polymer shows diffuse band

Or third, Structural Characterization of insoluble polymers
Harder to characterizeSolid state 1H & 13C & 29Si Nuclear Magnetic Resonance and infrared spectroscopyComposition by combustion analysesX-ray diffraction on film or powder

Morphological Characterization of polymers

If opaque or transluscent, SEM and optical microscopy (bifringence)-crystalline or amorphous & more.Fracture polymer and look at fracture surfacesLook for phase separation (like immiscible block copolymers)Look for long range orderLook for pores

Thermal characterization of polymers
Thermal gravimetric analyses (TGA) determines decomposition temperature

Differential scanning calorimetry (DSC) detects phase changes (melting or glass transition temperatures) or chemical reactions

DSC analysis

Thermal gravimetric analysis

Mechanical characterization of polymers
Stress-strain curves:Youngs modulus (brittleness)Tensile strength-pull sample appart Flexural strength- bend until it breaksCompressive strength-crush sample Dynamic mechanical analyses (same info as above but with cyclic application of stress or strain.Generate modulus temperature curvesFatigue studies to predict failure under cyclic stress

Elongation by extension of neck
Chains in neck align along elongation direction: strengthening


Stress-Strain Analysis
Tensile strength = pull sample appart

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Stress relaxation test:
--strain to eo and hold.--observe decrease in stress with time.
Relaxation modulus:
Data: Large drop in Er for T > Tg.
(amorphouspolystyrene)
Sample Tg(C) values:
PE (low Mw)PE (high Mw)PVCPSPC
-110- 90+ 87+100+150
Adapted from Fig. 15.7, Callister 6e. (Fig. 15.7 is from A.V. Tobolsky, Properties and Structures of Polymers, John Wiley and Sons, Inc., 1960.)
Selected values from Table 15.2, Callister 6e.
TIME DEPENDENT DEFORMATION

Not every polymer needs all of these analyses, but structure is the most basic and important
Known (described in literature) polymers need less structural characterization. Often just IR and Mw from GPC.

New polymers need complete structural characterization: NMR, IR, Combustion analysis, GPC, solubility, glass transition temp and/or melting point.

Morphological and Mechanical studies are dependent on research interests.
If you are interested in strong polymers, then morphological, mechanical & thermal studies are importantOther applications requiring morphological, mechanical & thermal studies would include preparation and testing of 1) membranes, 2) coatings, 3) paint, 4) polymer foams, 5) organic photovoltaics, 6) OLEDs, 7) adhesives and 8) low friction coatings

Other polymer properties that are important for specialty polymers
Coatings, packaging, membranes, Photovoltaics & OLEDs; gas and water permeability should be measured.Battery and fuel cell membranes: gas & water permeability and ion conductivityDielectrics, wiring insulation: dielectric and electrical conductivityFabrics & building materials: fire resistanceAny polymer used in sunlight or radiation or in the presence of chemicals: resistance to oxidative degradation.