13 Polymer Physicals Properties Flashcards
Structure: Polymer architecture
Properties: Crystallinity, viscosity,
mechanical properties
Structure: Molecular weight and
distributions
Properties: Melting point, viscosity,
solubility
Structure: Additives (Plasticizers)
Properties: Mechanical properties
Structure: Stereoregularity
properties: Crystallinity, solubility
Polymer architecture
- linear
- Comp
- Branched
- Star
- Dendritic
- Crosslinked
High vs low density
Linear=high
branched = low
Molecular weight affects
affect solubility, strength, viscosity, among other
properties.
Number average molecular weight (𝑴𝒏)
sum( number of polymer chains of a given mass (Ni) *molecular weigh (Mi)/sum
use colligative properties (i.e. osmotic
pressure) to determine MW we obtain a
number-average molar mass.
Weight average molecular weight (𝑴𝒘)
=sum(NiMi^2)/sum(NiMi)
If light scattering measurements are used to
determine MW, a weight average (Mw
) is
obtained. Mw
is biased towards higher
molecular weight polymers.
Mv
viscosity average molecular weight. Useful
to compare viscosity of polymer chains.
Mz
centrifugation average molecular weight.
Useful for describing mechanical properties.
Rank all the M
Mz > Mw > Mv > Mn.
number of molecules is opposite
What is PDI
Polydispersity = Mw/Mn
Provides a measure of the heterogeneity of
polymer lengths.
Degree of poymerization
DPn = Xn = Mn/M0
Number average number of repeat units in chain
what is M0
MW monomer
MW and mechanical strength
MW below certain point poly no mechanical strength
MW increases
beyond that point, mechanical strength
increases rapidly
At a given chain length,
the increase in MW does not significantly
change the mechanical strength of the material
Mw and processing
- Large MW increases mechanical
strength/viscosity. (Increased viscosity undesirable for melt extrusion/injection molding
.
– Broad MW favor low
softening point.
– Narrow MW distribution favors crystallization.
Crystalline vs. amorphous domains polymers
- Polymers have
crystalline and amorphous
domains. - # domains impactsmechanical strength
Glass transition temperature (Tg)
- Temperature below which a material has
properties similar to that of a glass (rigid,
brittle). - Above Tg
the material is elastic.
Tuning Tg
Plasticizers interfere with intermolecular
interactions between the chains,
- plasticizers reduce theTg
Tg vs Tm
Tm involves change in latent
heat where Tg does not.
Thermoplastics
amorphous polymer with a
Tg
above room temperature. Polymer can be
reversibly shaped by heating while applying a
stress before cooling.
Elastomer
An amorphous polymer with a Tg
below room temperature
Thermosets
Soft solid or liquid that crosslinks
upon heating (a process known as curing). After
curing the shape of the polymer is irreversibly
se
machine that measures stress-strain
Instron Machine
Stress-strain plots
Stress (N cm-2, MPa) vs. elongation or strain (ΔL/L)
Modulus:
Ultimate strength or Tensile strength:
Ultimate elongation:
Elastic elongation:
- The initial slope of the stress-strain curve.
- The stress
required to rupture a sample. - The maximum elongation
before the sample ruptures. - The maximum elongation in the
elastic region
Stress-Strain Curves and Deformation
When a stress is applied,
the material will
experience a reversible
change shape (strain)
until the elastic limit.
If the stress increases,
the change of shape is
irreversible.
Tacticity
Defines stereoregularity of neighboring C centers
isotactic, syndiotactic, and atactic.
Other structure – property relationship backbone
- C-C: High torsional flexibility (low Tg)
- C=C: Restricted rotation; chemical sensibility; electrical conductivity.
Ring: Restricted rotation (high Tg); chemical resistance. - ring w/ alkene: Restricted rotation; colored; electrical conductivity.
-C-O- High chain flexibility; stable. - S: Chain flexibility; sensitive to oxidation.
-Si-O: Very high chain flexibility (very low Tg); stable to heat and oxidation
Other structure – property relationship side groups
Side groups affect solubility, flammability,
hydrophobicity or reactivity.
– Bulky side groups (i.e. biphenyl) raise Tg
and
make the material stiff.
Crosslinking:
– Decreases solubility.
– Enhances thermal stability.
Strength and extensibility
High strength and low extensibility is obtained
by combinations of high crystallinity,
crosslinking, or polymer structure (rigid chains)
Low strength and high extensibility is obtained
from polymers with low crystallinity, limited or
no crosslinking, and low Tg
Strength decreases and is eventually lost near Tg
