Lecture: Microstructures in polymers

Textbook: Plastics: Materials and Processing (Third
Edition), by A. Brent Young (Pearson, NJ, 2006).
Structure and Properties of Engineering Polymers
Lecture: Microstructures in Polymers
Nikolai V. Priezjev

Microstructures in Polymers
• Gas, liquid, and solid phases, crystalline vs. amorphous structure, viscosity
• Thermal expansion and heat distortion temperature
• Glass transition temperature, melting temperature, crystallization
• Polymer degradation, aging phenomena
• Molecular weight distribution, polydispersity index, degree of polymerization
• Effects of molecular weight, dispersity, branching on mechanical properties
• Melt index, shape (steric) effects
Reading: Chapter 3 of Plastics: Materials and Processing by A. Brent Strong
https://www.slideshare.net/NikolaiPriezjev

Gas, Liquid and Solid Phases
Increasingdensity
At room temperature
Solid or liquid?

Pitch Drop Experiment
http://smp.uq.edu.au/content/pitch-drop-experiment
Pitch, before and after being hit with a hammer.
In 1927, Professor Parnell at UQ heated a
sample of pitch and poured it into a glass
funnel with a sealed stem. Three years were
allowed for the pitch to settle, and in 1930 the
sealed stem was cut. From that date on the
pitch has slowly dripped out of the funnel, with
seven drops falling between 1930 and 1988, at
an average of one drop every eight years.
However, the eight drop in 2000 and the ninth
drop in 2014 both took about 13 years to fall.
It turns out to be about 100 billion times more
viscous than water!
Pitch (derivative of tar) at room T feels like solid
and can be shattered by a hammer. But, the
longest experiment shows that it flows!

Liquid phases: polymer melt vs. polymer solution
Semi-dilute solution:Dilute solution: Polymer melt:
Chains in solvent do not
overlap:
-- Can consider that each
polymer chain acts in
isolation.
In semi-dilute solutions
concentration of polymer
chains is sufficient for
chains to just overlap.
Entangled/unentangled
No solvent/only polymer
chains
Reptation—motion of long linear, entangled macromolecules amorphous polymers.
High viscosity Low viscosity
T up, viscosity down

Liquid Crystal Phases
Isotropic phaseNematic phaseSmectic A
Liquid crystal displays:
OFF state ON state
No positional but
orientational order!
director
Stiff backbone

Liquid Crystal Polymers
Characteristics:
• These are a class of aromatic
polymer.
• Extremely unreactive and inert.
• Highly resistant to fire.
Discotic nematic liquid crystals
Kevlar, the most widely used
body armor is made up of
intertwined liquid crystal
polymers.

Copolymers
two or more monomer polymerized
together
• random – A and B randomly vary
in chain
• alternating – A and B alternate in
polymer chain
• block – large blocks of A alternate
with large blocks of B
• graft – chains of B grafted on to A
backbone
A – B –
random
block
graft
alternating

Block Copolymer Microstructures
a Lamella structure. b Double gyroid
(bicontinuous) structure. c Cylindrical dispersion
structure. d Spherical dispersion structure.

(Di) Block Copolymer Microstructures
Block copolymer microstructures and the phase diagram for the polystyrene-
polyisoprene diblock copolymer system. The metastable perforated lamellar
microstructure is illustrated in the upper right corner of the phase diagram.
x, the relative volume
fractions of the dissimilar
segments
N, molecular weight
Structure and order in soft matter: symmetry transcending length scale. Ward and Horner CrystEngComm, 2004, 6, 401-407

Triblock Terpolymers Microstructures
https://wiesner.mse.cornell.edu/res_nanomaterials.htm
Tri-block terpolymers may be
thought of simply as a diblock
copolymer (AB) upon which a
third block (C) is grown.

Polymer Crystallinity
Crystallization in linear polymers: achieving a
very regular arrangement of the mers
Induction of crystallinity
● cooling of molten polymer
● evaporation of polymer solution
● annealing  heating of polymer at a specific temperature
● drawing  stretching at a temperature above Tg
 Increased Density
 Increases Stiffness (modulus)
 Reduces permeability
 Increases chemical resistance
 Reduces toughness
Effects of crystallinity:
Semi-crystalline
Amorphous

Polymer Crystallinity
Crystalline morphologies
• Spherulite  aggregates of small fibrils in a radial pattern (crystallization under
no stress)
• Drawn fibrillar  obtained by drawing the spherulitic fibrils
• Epitaxial  one crystallite grown on another; lamella growth on long fibrils; the
so-called shish-kebab morphology (crystallization under stirring)
Crystalline polymers (vs amorphous polymers)
 tougher, stiffer (due to stronger
interactions)
 higher density, higher solvent
resistance (due to closely packing
morphology)
 more opaque (due to light scattering
by crystallites)

Thermal Expansion and Heat Distortion Temperature
Aromatic content, crosslink density, crystallinity, secondary bonding can raise T at which distortion occurs.
Test apparatus (dilatometer) used for measuring the
coefficient of thermal expansion (CTE=size/C).
Deflection Under Load test to
determine heat distortion temperature
(HDT). Test time: ~10,000 hours!
Changes in length compared to original length
(Δℓ/ℓ0) called linear expansion.

The Glass Transition Temperature, Tg
• The glass transition, Tg, is temp. below
which a polymer OR glass is brittle or
glass-like; above that temperature the
material is more plastic.
• The Tg to a first approximation is a
measure of the strength of the secondary
bonds between chains in a polymer; the
stronger the secondary bonds; the
higher the glass transition temperature.
Polyethylene Tg = 0°C;
Polystyrene = 97 °C
PMMA (plexiglass) = 105 °C.
Since room temp. is < Tg for PMMA, it is
brittle at room temp.
For rubber bands: Tg = - 73°C….

First-order and second-order phase transitions (I)
The classification of phase transitions proposed by Ehrenfest is based on the behavior of G
near the phase transformation.
First-order phase
transition: first
derivatives of G
are discontinuous.
Second-order phase
transition: first
derivatives of G are
continuous, but second
discontinuous.
G = H - TS

First-order and second-order phase transitions (II)
First-order phase
transition: first
derivatives of G
are discontinuous.
Second-order phase
transition: first
continuous, but second
discontinuous.
G = H - TS

The Glass Transition Temperature, Tg
Glass transition temperature of a
polymer is the temperature at which
there is enough thermal energy for
the polymer chains to move freely
(wiggle around).
Differential scanning
calorimetry (DSC)
method
Thermomechanical analysis
(TMA) = volume expansion
Dynamic Mechanical Analysis
(DMA): response to oscillatory shear

Glass Transition and Melting Point for Thermoplastics

Glass Transition and Melting

Thermal Properties of Selected Plastics

Melting temperature, Tm: Influence of Structure

Factors that affect the glass transition temperature, Tg
Molecular Weight
Adapted from A. Abou Elfadl et al. From Rouse to Fully Established Entanglement Dynamics: A Study of Polyisoprene by Dielectric
Spectroscopy 2010 Macromolecules, 43, 3340-3351.
Flory–Fox equation
Polyisoprene

Backbone Flexibility
Pendant Groups
This backbone is so flexible
that polydimethylsiloxane has
Tg way down at -127o C!
The backbone is very stiff. It's so
rigid that it doesn't have a Tg!
You can heat it to over 500o C
and it will still stay in the glassy
state. It will decompose from all
the heat before it lets itself
undergo a glass transition!
Adamantine act like a hook that
catches on nearby molecules and
keeps the polymer from moving.
But big bulky pendant groups can lower the Tg, too.
Pendant groups limit how closely the polymer chains
can pack together. The further they are from each
other, the more easily they can move around. This
lowers the Tg, in the same way a plasticizer does.
C10H16

Phase Transitions and Structure
Tm and Tg increase with
• Increasing intermolecular forces
• Increasing intra- and intermolecular barriers to chain
rotation
• Bulky, stiff backbone and side groups
• Shorter flexible side groups
Adapted from R.F. Boyer, 1963 Rubber Chem. Technol. 36, 1303-1421;
L.H. Sperling, 1992 Introduction to Physical Polymer Science, 2nd Ed, pp 303-381.

Crystallization, Melting, Glass Transition
Crystallization: crystalline nuclei form and grow, chains align and order. Crystallization
rates can be defined from the same type of S-curves we saw for metals - can be described
by the Avrami equation: y = 1 – exp(-k tn)
Polypropylene (PP)
TH
T
r
m
mSL






 12
Tm ~160 to 166 °C

Crystallization, Melting, Glass Transition
Dependence of melting and glass transition temperatures and polymer properties on
molecular weight.

Polymer Degradation
• Polymer degradation is a change in the properties like tensile strength, color,
shape, etc.
• Polymer-based product under the influence of one or more environmental
factors such as heat, light or chemicals such as acids, alkalis and some salts.
Main factors: solar radiation, temperature, moisture, oxygen. These changes are
usually undesirable, such as cracking and chemical disintegration of products.
Solar radiation:
 Physical changes resulting from exposure to the environment are initiated by
chemical bond breaking reactions caused by the absorbed light.
 The ultraviolet portion of solar energy, with the shortest wavelengths often
having the greatest effect.
 Solar absorptivity is closely related to color, thus samples of different colors
will reach different on-exposure temperatures.

UV radiation
 The lower boundary in Earth’s atmosphere solar UV spectrum is caused by ozone
shielding. Ozone O3 layer.
 UV radiation can be classified as near, far or extreme UV but it is also possible to
classify UV radiation in terms of UVA, UVB and UVC

Polymer Degradation (cont.)
Photoinduced degradation:
• Short wavelength UV radiation causes
yellowing; long wavelength UV (penetrate
deeper in the material) is primarily responsible
for degradation of physical properties, such as
tensile strength and impact strength.

Thermal degradation:
• Chain-growth polymers like poly(methyl methacrylate) PMMA can be
degraded by hemolysis at high temperatures to give monomers, oils,
gases and water.
• For example the PVC eliminates HCl, under 100–120 °C.
• CH2(Cl)CHCH2CH(Cl) → CH=CH-CH=CH + 2HCl Hydrochloric acid
Biological degradation:
• Biodegradable plastics can be biologically degraded by microorganisms to
give lower molecular weight molecules. To degrade properly biodegradable
polymers need to be treated like compost and not just left in a landfill site
where degradation is very difficult due to the lack of oxygen and moisture.
• The mechanism of biodegradation is by anaerobic processes, where oxygen is not
present.

Moisture:
 Moisture, in combination with solar radiation, contributes significantly to the
weathering of many polymeric materials
 Mechanical stresses imposed when moisture is absorbed or desorbed and to
the chemical participation of moisture in the chemical evolution cause
weathering
 The span of time over which the precipitation occurs and the frequency of
wetness are important in the weathering of materials
 Water absorption in the surface layers
produces a volume expansion which places
mechanical stress on the dry subsurface layers.
 Drying out of the surface layers would lead to
a volume contraction.
 The hydrated inner layers resist this contraction,
leading to surface stress cracking.

Physical Ageing in Polymers
Polymer ageing may involve physical ageing without chemical reaction occurring; chemical
changes such as crosslinking during curing of a thermoset; thermal conditioning at elevated
temperature; photochemical ageing, as occurs in weathering.
Density versus ageing time at room
temperature of samples extracted,
respectively, from the skin and the core
of an injection moulded polystyrene bar.
Ageing took place in a density column at
23 °C.
Volumetric relaxation after
rapid cooling
11days

• Molecular weight, M: Mass of a mole of chains.
Low M
high M
Not all chains in a polymer are of the same length
— i.e., there is a distribution of molecular weights
polydispersity
Molecular Weight
Department of Mechanical and Materials Engineering Wright State University

Molecular Weight Distribution
Most polymers are polydisperse — they contain more than
one chain length
— i.e., there is a distribution of molecular weights
Arrhenius equation
)/( RTE
Aerate 


Molecular weight of ethylene C2H4 = 2 x 12 + 4 x 1 = 28 g/mole
Molecular weight of polyethylene 1000 x C2H4 =1000 x 28 g/mole = 28,000 g/mole

Mass Distribution in Low-MW Polystyrene
Adapted from “The Characterization of Polystyrene Oligomers by Field-desorption Mass Spectrometry”, K. Rollins
et al., 1990 Rapid Commun. Mass Spectrom., 4, 355-359

The average distribution of chain masses can be described in more than one
way:
– Mn, the number-average molecular weight
– Mw, the weight-average molecular weight
– Mz, the z-average molecular weight
– Mz ≥ Mw > Mn
– PDI, polydispersity index, is the ratio of the weight-average molecular weight to the
number-average molecular weight
Low M
high M

Example: average mass of a class
Student Weight
mass (lb)
1 104
2 116
3 140
4 143
5 180
6 182
7 191
8 220
9 225
10 380
What is the average
weight of the students in
this class:
a) Based on the number
fraction of students in
each mass range?
b) Based on the weight
fraction of students in
each mass range?
Molecular Weight Calculation

Solution: The first step is to sort the students into weight ranges. Using
40 lb ranges gives the following table:
weight number of mean number weight
range students weight fraction fraction
Ni Wi xi wi
mass (lb) mass (lb)
81-120 2 110 0.2 0.117
121-160 2 142 0.2 0.150
161-200 3 184 0.3 0.294
201-240 2 223 0.2 0.237
241-280 0 - 0 0.000
281-320 0 - 0 0.000
321-360 0 - 0 0.000
361-400 1 380 0.1 0.202
Ni NiWi
10 1881
total
number
total
weight
Calculate the number and weight
fraction of students in each weight
range as follows:

xi 
Ni
Ni

wi 
NiWi
NiWi
For example: for the 81-120 lb range

x81120 
2
10
 0.2
117.0
1881
011x2
12081 w
Molecular Weight Calculation (cont.)

Mn  xiMi  (0.2 x 110 0.2 x 142+ 0.3 x 184+ 0.2 x 223+ 0.1 x 380) =188 lb
weight mean number weight
range weight fraction fraction
Wi xi wi
mass (lb) mass (lb)
81-120 110 0.2 0.117
121-160 142 0.2 0.150
161-200 184 0.3 0.294
201-240 223 0.2 0.237
241-280 - 0 0.000
281-320 - 0 0.000
321-360 - 0 0.000
361-400 380 0.1 0.202
Mw  wiMi  (0.117 x 110 0.150 x 142+ 0.294 x 184
+ 0.237 x 223+ 0.202 x 380) = 218 lb
Mw  wiMi  218 lb
Molecular Weight Calculation (cont.)

Mn: Number-Average Mol. Wgt.
The number-average molecular weight (molar mass) of a polymer containing Ni
molecules of mass Mi is the arithmetic mean of the molar mass distribution:
Mw: Weight-Average Mol. Wgt.
The weight-average molecular weight (molar mass) is the sum of the products of
the molar mass of each fraction multiplied by its weight fraction (wi).
In terms of wi or numbers of molecules, Mw is

Mz: Z-Average Mol. Wgt.
The z-average molecular weight (molar mass) is:
Mz is especially sensitive to the presence of high-MW chains.
PDI: Polydispersity index
The molecular weight distribution, or polydispersity index, is the ratio of the weight-
average molecular weight to the number-average molecular weight:
The polydispersity index of a monodisperse polymer is 1.00.
The polydispersity index increases as the polymer distribution broadens.

Example:
You have a polymer sample that contains the following
molecules:
What are Mn, Mw, and the polydispersity index?

Example:

Here are:
10 chains of 100 molecular weight
1347
5402010
)100005()100040()50020()10010(
Mn 



5390
)100005()100040()50020()10010(
)100005()100040()50020()10010(
M
2222
w 



4
M
M
sityPolydisper
n
w

Molecular Weight and Dispersion - an example:
Find Mn, Mw, polydispersity index:

Polydispersity index (PDI) is
a measure of the distribution
of molecular mass in a given
polymer sample. PDI=1,
many chains with the same
length (monodisperse).
Otherwise, polydisperse
PDI>1.
Bimodal MWD
1 Da (Dalton) = 1 g/mol
1 Da = 1.660×10–27 kg
Number average:
break, yield, and
impact strength
increase

Degree of Polymerization, DP
DP = average number of repeat units per chain
ii mfm
m


:followsascalculatedisthiscopolymersfor
unitrepeatofweightmolecularaveragewhere
C C C C C C C CH
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
C C C C
H
H
H
H
H
H
H
H
H( ) DP = 6
mol. wt of repeat unit iChain fraction
m
M
DP
n


Melt Index
Simple melt index test
T
For example: PE 190C/1.0kg
Not intrinsic or fundamental property
of polymer melt, rather convenient
and easy method for expressing flow
properties useful for processing.
High melt index = low molecular
weight; low melt index = high
molecular weight.
Weighted Plunger
Barrel
Molten
Pellets
Extrudate
Orifice
Heater Band
Melt Flow Index
# grams of flow per 10 minutes

Effects of molecular weight, dispersity, branching
• The molecular weight, dispersity and branching has a significant effect on the mechanical
and physical bulk properties of polymers. In general, a higher molecular weight improves
the mechanical properties, that is, break, yield, and impact strength increase. However, a
higher molecular weight also increases the melt and glass transition temperature as well
as the solution and melt viscosity, which makes processing and forming of the polymeric
material more difficult.
• The dispersity has the opposite effect; a wider molecular weight distribution lowers the
tensile and impact strength but increases the yield strength, or in other words, a lower
dispersity (narrower distribution) leads to better mechanical properties. The low-molecular
weight portion of the distribution has a similar effect as a plasticizer, that is, it reduces the
brittleness and lowers the melt viscosity which improves the processability, whereas the
high-molecular weight portion causes processing difficulties because of its huge
contribution to the melt viscosity.
• Branching is another important performance parameter. In general, branching lowers the
mechanical properties. For example, it decreases the break and yield strength. The effect
on toughness is less clear; if the length of the branches exceed the entanglement weight it
improves the toughness, otherwise it lowers the impact strength. Branching also lowers the
brittleness, the melt temperature, the melt and solution viscosity and increases the
solubility. In conclusion, the processability improves with increasing degree of branching.

Shape (Steric) Effects
The effects of the shape or size of the atoms or groups of atoms are called steric effects.
Various methods for representing the 2-
chloropropane (C3H7Cl) molecule:
reduced crystallinity => tensile strength,
Tg lower. But hindered movement.

Main physical properties of polymers
1 - Primary bonds: the covalent bonds that
connect the atoms of the main chain
2 - Secondary bonds: non – covalent bonds
that hold one polymer chain to another
including hydrogen bond and van der Waals
(dipole –dipole) attraction
3 - Crystalline polymer: solid polymers with
high degree of structural order and rigidity
4 - Amorphous polymers: polymers with a low
degree of structural order
5 - Semi – crystalline polymer: most polymers
actually consist of both crystalline domains
and amorphous domains with properties
between that expected for a purely crystalline
or purely amorphous polymer
6 - Glass: the solid form of an amorphous
polymer characterized by rigidity and
brittleness
7 – Crystalline melting temperature (Tm):
temperature at which crystalline polymers melt
8 - Glass transition temperature (Tg ):
temperature at which an amorphous
polymer converts to a liquid or amorphous
domains of a semi crystalline polymer melt
9 – Thermoplastics (plastics): polymers that
undergo thermally reversible conversion
between the solid state and the liquid state
10 - Thermosets: polymers that continue
reacted at elevated temperatures
generating increasing number of crosslinks
such polymers do not exhibit melting or
glass transition
11 - Liquid–crystalline polymers: polymers
with a fluid phase that retains some order
12 - Elastomers: rubbery, stretchy
polymers the effect is caused by light
crosslinking that pulls the chains back to
their original state

Summary
Reading: Chapter 3 of Plastics: Materials and Processing by A. Brent Strong
• Gas, liquid, and solid phases, crystalline vs. amorphous structure, viscosity
• Thermal expansion and heat distortion temperature
• Glass transition temperature, melting temperature, crystallization
• Polymer degradation, aging phenomena
• Molecular weight distribution, polydispersity index, degree of polymerization
• Effects of molecular weight, dispersity, branching on mechanical properties
• Melt index, shape (steric) effects

Lecture: Microstructures in polymers

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