Crystalline polymers

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Crystalline polymers Vibrational
spectroscopy of polymers
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Crystalline Polymer molecules show some degree
of ordering. Depending on molecular symmetry
(tacticity), molecular weight (kinetics) and
branching, etc.
Lamella thickness 100 200 Å
Lamellar growth direction 10 µm
Glassy molecules in a random coil conformation.
For example fully amorphous PMMA and PPO
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10 mm x 10 mm
Microscopy image of a crystal of high density
poly(ethylene) - viewed while looking down at
the lamella.
Lamella grows outwards
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Polymers with some symmetry are usually
polycrystalline. They are usually never
completely crystalline but have some amorphous
regions and packing defects.
Several crystals of isotactic poly propylene G.
Ellis, M. A. Gómez and C. Marco
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Why is the lamellar crystal a basic unit?
competition between polymer chain stretching and
coiling on one hand and on reduction in free
energy for crystal formation on the other hand
determines lamellar thickness.
t-1 is a microscopic frequency, Dg is negative by
definition
Calculating the maximum net rate for a crystal of
thickness l gives an estimate of the optimal
thickness (fastest growing).
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Lamellar thickness of PE grown from a melt
Lamellar Thickness
? melt crystallization ? annealing
Surface energy
Equilibrium melting Temperature
Enthalpy of fusion per unit volume
Crystallization temperature
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Micellar Structures
When diblock copolymes are asymmetric, lamellar
structures are not favoured.
Instead the shorter block segregates into small
spherical phases known as micelles.
Interfacial energy cost g(4pr2)
Reduced stretching energy for shorter block
Density within phases is maintained close to bulk
value.
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Exotic Morphologies
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Molecular vibrations vibrational spectroscopy
Restoring force (the bond) Fk(X2X1)
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Quantum Vibrational Motion
molecular motion is quantized vibrational
quantum levels (quantum number v)
energy absorbed is energy difference between
two levels for SHO, spacing is same between ALL
adjacent levels.
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Anharmonic Oscillator
real molecules, vibrations close to being
harmonic. relaxes the selection rules
(overtones and combination bands) distorts the
intensities of the transitions changes energy
levels so that they come closer together as you
go up the vibrational ladder. bond can break
not so with SHO.
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Types of Vibrations
molecular dipole moment must change during a
vibration to be IR active. this oscillating
dipole interacts with the oscillating E-M field
of the photon, leading to absorption.
Bending Vibrations
Stretching Vibrations

symmetric
anti-symmetric
rocking
scissoring
twisting
wagging
In-Plane
Out-of-Plane
Changes in bond length
Changes in bond angle
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Vibrational spectroscopy
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Infrared is Rovibrational Spectroscopy
Wavelengths between 0.8 µm to 1 mm.
Associated with changes in nuclear motion
(vibrations and rotations). In gas phase,
rotational transitions are resolved in liquid
phase, they are broadened. Usually only focus on
vibrational character. Energy is usually
reported in wavenumbers (cm-1) also proportional
to frequency
Near IR 0.8 - 2.5 µm 12800 - 4000 cm-1
Mid-IR 2.5 - 50 µm 4000 - 200 cm-1
Far IR 50 - 1000 µm 200 - 10 cm-1
most commonly studied
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The Raman Spectrum
A complete Raman spectrum consists of a
Rayleigh scattered peak (high intensity, same
wavelength as excitation) a series of
Stokes-shifted peaks (low intensity, longer
wavelength) a series of anti-Stokes shifted
peaks (still lower intensity, shorter
wavelength) spectrum independent of excitation
wavelength (488, 632.8, or 1064 nm)
Spectrum of CCl4, using an Ar laser at 488 nm.
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Origin of Raman Effect - Classical
The oscillating electric field of the excitation
light.
The induced dipole moment from this oscillating
field.
The molecular polarizability changes with bond
length.
The bond length oscillates at vibrational
frequency.
Hence the polarizability oscillates at same
frequency.
Substitute.
Remember trig identity.
Induced dipole has Rayleigh, Stokes, and
anti-Stokes Components.
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Raman vs. IR
Infrared Spectroscopy
Raman Spectroscopy
Absorption
Scattering
Interaction
Polychromatic
Monochromatic
Excitation
Frequency measurement
Absolute
Relative
Activity
Dipole moment change
Polarizability change
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Band intensity
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Compare IR and Raman
Spectra of PETN explosive. From D.N. Batchelder,
Univ. of Leeds
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Group frequencies

• For large molecules - vibrations of some
  functional groups acts like independent
  oscillators i.e. always found at same
  frequencies

Group cm-1
- C-H 2900-3000
- CO 1700
- C-F 1100
- O-H 3600
- C-C - 900
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Factors affecting group frequencies

• Center of Symmetry (i)determine IR active or
  Raman active.
• For IR active vibration an oscillating electric
  dipole must be generated.
• For Raman active vibration a change in
  polarizability of the molecule is produced, which
  gives rise to induced dipole.

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Symmetry factor

• A molecule having a center of symmetry (i) has no
  permanent dipole moment, so a vibration symmetric
  to i (sym. mode) does not generate oscillating
  dipole and therefore it is an IR inactive
  vibration.
• But vibration anti-symmetric to i (anti-sym.
  mode) generates transient oscillating dipole, so
  it will be IR active vibration and show up in the
  spectrum.

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Mutual exclusion rule

• Symmetric mode vibration usually gives rise to
  Raman scattering which causes changes in
  polarizability of molecule, and it is a Raman
  active vibration, showing up in the Raman
  spectrum.
• Mutual exclusion rule For IR and Raman spectra,
  some lines missing in one would show up in the
  other, due to different symmetry requirement for
  each spectra. Thus, the information from IR data
  is complementary to that obtained from Raman.

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General rule of symmetry

• Symmetry in molecule reduces the normal modes of
  vibrations and simplifies spectrum.
• CO2 sym. stret. 1340 cm-1 , IR
  inactive, but Raman active.
• assym. stret. 2349 cm-1 , IR
  active.
• IR vs. Raman mutual exclusive.
• Symmetry IR shows only assym. stret.

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Symmetry simplify spectra
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Mechanical coupling

• Interaction between two vibrational modes through
  common atom or common bond Such two identical
  groups are linked (or fused) by a common atom or
  a common bond.
• Induce mixing and redistribution of energy
  states, yielding new energy levels, one being
  higher and one lower in frequency.

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Degrees of freedom

• Always 3 N degrees of freedom (N number of
  atoms in molecule) with 3N-6 (-5) vibrational
  degrees of freedom.
• For polymers in practice we have 3n-6 modes
  (nnumber of atoms in repeat unit) instead of 3N-6

?Good news
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Quick IR Analysis Algorithm
Infrared spectra It is important to remember
that the absence of an absorption band can often
provide more information about the structure of a
compound than the presence of a band. Be careful
to avoid focusing on selected absorption bands
and overlooking others. Use the examples linked
to the table to see the profile and intensity of
bands. Remember that the absence of a band may
provide more information than the presence of an
absorption band. Look for absorption bands in
decreasing order of importance 1. the C-H
absorption(s) between 3100 and 2850 cm-1. An
absorption above 3000 cm-1 indicates CC, either
alkene or aromatic. Confirm the aromatic ring by
finding peaks at 1600 and 1500 cm-1 and C-H
out-of-plane bending to give substitution
patterns below 900 cm-1. Confirm alkenes with an
absorption at 1640-1680 cm-1. C-H absorption
between 3000 and 2850 cm-1 is due to aliphatic
hydrogens. 2. the carbonyl (CO) absorption
between 1690-1760cm-1 this strong band indicates
either an aldehyde, ketone, carboxylic acid,
ester, amide, anhydride or acyl halide. The an
aldehyde may be confirmed with C-H absorption
from 2840 to 2720 cm-1. 3. the O-H or N-H
absorption between 3200 and 3600 cm-1. This
indicates either an alcohol, N-H containing amine
or amide, or carboxylic acid. For -NH2 a doublet
will be observed. 4. the C-O absorption between
1080 and 1300 cm-1. These peaks are normally
rounded like the O-H and N-H peak in 3. and are
prominent. Carboxylic acids, esters, ethers,
alcohols and anhydrides all containing this
peak. 5. the CC and CN triple bond absorptions
at 2100-2260 cm-1 are small but exposed. 6. a
methyl group may be identified with C-H
absorption at 1380 cm-1. This band is split into
a doublet for isopropyl(gem-dimethyl) groups. 7.
structure of aromatic compounds may also be
confirmed from the pattern of the weak overtone
and combination tone bands found from 2000 to
1600 cm-1.
This is a little recipe from Stanislau State
University (California).
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Some Raman Advantages
Here are some reasons why someone would prefer to
use Raman Spectroscopy. Non-destructive to
samples (minimal sample prep) Higher
temperature studies possible (dont care about IR
radiation) Easily examine low wavenumber
region 100 cm-1 readily achieved. Better
microscopy using visible light so can focus more
tightly. Easy sample prep water is an
excellent solvent for Raman. Can probe sample
through transparent containers (glass or plastic
bag).
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A Raman disadvantage - Fluorescence
Spectrum of anthracene. A using Ar laser at
514.5 nm. B using NdYAG laser at 1064 nm. Want
to use short wavelength because scattering
depends on 4th power of frequency. BUT Want to
use long wavelength to minimize chance of
inducing fluorescence.
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What is it good for?

• Composition
• Co-ordination
• Conformation

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Endgroups - example
CH2 and CH3 stretching and bending modes in
saturated hydrocarbons. With increasing chain
length the absorption from CH2 groups increases.
Introduction to Mol. Spect. Academic press 1970
. Molecules in motion. A presentation created by
Dr Alexander Brodin