Chapter 8. Analysis and Testing of Polymers

A. Chemical Analysis of Polymers

Mass Spectrometry.  (MALDI, for example)

Gas Chromatography

B. Spectroscopic Methods

I. the Electromagnetic Spectrum
The energy order of different kind of electromagnetic radiation from high to low is
g rays > X rays > Ultraviolet > visible > infrared > microwaves > radio waves.

For any electromagnetic radiation,   e = hn = hc/l ( n = c/l )
where,  e = Energy of a photon
           h = Planck's constant (6.62x10^-34 J·s)
           n = Frequency (Hz or s^-1)
           l = Wavelength (m)
            c =  Speed of light (3.00x10⁸ m/s)

Problem 12.7 Calculate and compare the energy of the following radiation.
    a) A g ray with l = 5.0x10^-11 m
    b) An X ray with l = 3.0x10^-9 m
    c) Ultraviolet light with n = 6.0 x 10¹⁵ Hz
    d) Visible light with n = 7.0 x 10¹⁴ Hz
    e) Infrared radiation with l = 2.0x10^-5 m
    f) Microwave radiation with n = 1.0 x 10¹¹ Hz
 

Method	Reasons of the absorption
IR	Bond stretch and contract, swing back and forth, etc.
UV	Electrons jumping from HOMO to LUMO
X-ray	electron density
NMR	Nuclear spins orientation

Energy (radiation) is absorbed when the frequency of the radiation matches or resonance
with the frequency of the vibration motion (or electron jumping, or nuclear spins, etc.).

II. Interpreting Infrared Spectra

Infrared spectra -- Caused from bond stretching vibration and bending vibration.

Due to a large number of bond stretching and bending motions in one molecule, IR
method is not used to identify a compound, but to identify functional groups in (or
not in) the molecule. Most functional groups have characteristic IR absorption bands.

Table 1. Four general IR regions and their corresponding bond motions of functional
groups..

4000-2500 cm-1	C-H, N-H, O-H bond stretching motions.
2500-2000 cm-1	triple bond stretching motions.
2000-1500 cm-1	double bonds stretching motions.
1500-  400 cm-1	other single bond stretching vibration and bending motions.  Fingerprint region.

Note: The stretching vibration absorbs at higher energy than the C-H bending vibration.

Some characteristic absorption are:

Functional groups	Bond  type	Characteristic bands	Band features
Alkanes	C-H	2850-2960 cm-1	multi peaks, medium to intense depend on amount
Alkenes	=C-H C=C	3020-3100 cm-1,  1640-1680 cm-1	sharp, medium, sharp, medium.
Alkynes	=C-H C=C	3300 cm-1, 2100-2260 cm-1	sharp, intense sharp, medium
Alcohols	O-H	3400-3650 cm-1	broad and intense
Amines	N-H	3300-3500 cm-1	sharp, medium intensity
Aromatic compounds	C-H ring	3030 cm-1,  1450-1600 cm-1,	multi peaks, weak, multi peaks, medium
carbonyl compounds	C=O	1670-1780 cm-1	a sharp peak, intense

Note:
1. The frequency of C-H bond follows: -C-H < =C-H < =C-H
2. The reason for broad intense peak at 3300-3700 is hydrogen bonding, all of the
    following compounds have peaks at that region. H₂O, alcohol (ROH), carboxylic
    acid (RCOOH), phenol (phOH), primary and secondary amines.
 

Dichroism. For any molecular vibration leading to infrared absorption, there is a periodic change in eletric dipole moment. If the direction of this change is parallel to a component of the electric vector of the infrared radiation, absorption occurs; otherwise it does not. In oriented bulk polymers, the dipole-moment change can be confined to specified directions. The use of polarized infrared radiation then leads to absorption that is a function of the orientation of the plane of polarization. This phenomenon is called dichroism and is usually measured as the dichroic ration. the ratio of the optical densities of an absorption band measured with radiation polarized parallel and perpendicular, respectively, to a specified direction in the sample. dichroic ratios depend upon both the degree of orientation and the angle between the direction of the transition moment and the selected direction in the sample (for example, the axial direction in a fiber). They usually range between 0.1 and 1.

Crystallinity. The infrared spectra of the same polymer in the crystalline and amorphous state are different. A) Sharp and splitting peak in crystalline polymer because of intermolecular interactions  B) Specific conformation exist in one but only the other phase (mainly in crystalline state). Example: -OCH2CH2O- is restricted to the all trans conformation in the crystal, but in part others in the amorphous.

III.  Nuclear Magnetic Resonance (NMR) Spectroscopy

Certain nuclei (either atomic number or neutral number are odd) behave like spinning charges particles -- generate a magnetic moment
eg. ¹H, ¹³C, (²H, ¹⁴N, ¹⁹F, ³¹P),  but not ¹²C, ¹⁶O
If we put proton magnet in a very strong, external magnetic field, the protons orient
itself either with (denoted at +1/2) or against (denoted as -1/2) the field.

The stronger magnetic field, the greater the energy separation between the two spins

Molecules of a sample are situated in a magnetic field (gauss); each nucleus is in one
of two spin states that differ in energy by an amount of DE and a few more nuclei have
spin +1/2; If the sample is subjected to electromagnetic radiation E (radio waves about
10-1000 MHz) exactly equal to E, this energy is absorbed by some of nuclei in the +1/2
states. The absorbed energy causes these nuclei to invert or flip their spins to -1/2 states.

The energy absorption by nuclei in a magnetic field is termed nuclear magnetic resonance,
and can be detected by which is so called NMR spectrometer. The study of this absorption
is called NMR spectroscopy.

 A general NMR chart is: The downfield, deshielded side is on the left, and the upfield,
shielded side is on the right. d 10ppm to d 0ppm from the left to the right.
(CH₃)₄Si, TMS, absorbs at higher magnetic field than almost any other H is used as
reference point (d 0ppm).
Information from an ¹H NMR
    1) The number of signals tell how many different kinds of protons are in the molecule.
    2) The relative areas (integration of the peak) tell how many protons of each type there are.
    3) The position, now called chemical shift, tells us the environment -- how shielded the
        proton is.
    4) The split of peaks tell us how many hydrogen are on the adjacent carbon.

The chemical shift of protons.

The position of the peak relative to a standard is called its chemical shift (d).
The position of a standard, (CH₃)₄Si, tetramethylsilane (TMS), is defined as d = 0.000 ppm

The following is a shortened list.

Saturated Hydrocarbon	0.5-2.5
Cl-CHn- or -O-CHn- or -N-CHn, such as ether	2.5-4.5
Aromatic Ar-H	6.5-8.0
Aldehyde -(C=O)-H	10

Note: a) the chemical shift of -OH varies from 0.5 to 6.0, a singlet.
         b) In general, d 1.0 for RCH₃, d 1.3 for R₂CH₂, d 1.7 for R₃CH.

Spin-Spin Splitting of signals.
Signal Splitting of ¹H (the signal are slitting into multiple peaks) widely exist in organic
compounds. For example: propanol has four signals, one triplet, one singlet, one septet,
one triplet peaks.

Two simple cases:

    Let's focus the attention on absorption of energy by H_a
    The magnetic moment of H_b could affect the magnetic field of neighboring nuclei H_a.
    If the spin of H_b is with the external field, the applied field necessary to cause resonance
    is slightly reduced, vise versa. Since 50:50 of H_b spin with and against the external field
    respectively, a 1:1 ratio doublet peak of H_a appears in the NMR spectra. So is H_b.

n+1 rule: protons that have n equivalent neighboring protons show n+1 peaks in NMR.
The distance between peaks in a multiplet is called coupling constant, denoted J (Hz).
In general, J = 0-18 Hz, a typical value for an alkane is J = 6-8 Hz.

 
¹³C NMR spectroscopy

Since ¹²C has no nuclear spin and the abundance of ¹³C is only 1.1%, the NMR spectrum
is very noisy. Average hundreds of individual runs would cut down the noise, but it would
take a extremely long time (every individual spectrum take more than 3 min). The
difficult of detecting ¹³C NMR has been eliminated by a technique called Fourier-transform
NMR (FT-NMR).
In FT-NMR, the sample is irradiated with a radio wave pulse, but not a radio wave scanning,
then the complex signal is mathematically manipulated to NMR absorption peaks that we can
read. Every individual spectrum can be taken in less than a second, and a large number of
spectra can be recorded and stored in a computer, finally the computer adds all the spectra
together. Since noise is random, it sums to zero, while the sample signals add together to
give a stronger signal than a single experiment.

The FT-NMR are also necessary in the detecting ¹H of very dilute samples.

Information from ¹³C NMR.
    1) The number of signals tell how many different kinds of carbons are in molecule.

    2) The chemical shift tell us the environment of the ¹³C.
        Equivalent ¹³C have the same chemical shift. Most 13C chemical shift are between
        0 and 220 ppm downfield from the TMS reference (0 ppm). Some important chemical
        shift of carbons are: CH₃ (d 10-30ppm), CH₂ (d 30-50 ppm), C-O (d 50-90 ppm),
        C=C and aromatic (d 110-140ppm), C=O (d 160-220ppm).

DEPT ¹³C NMR, a new tech to tell the specific carbons.

Electron Paramagnetic Resonance Spectroscopy (EPR):

The principle is similar to NMR, except it is for the detection of free radicals. These species are uniquely characterized by their magnetic moment, arising from the presence of an unpaired electron.

The first new energy levels is  E = hν₀ = gβμ₀H₀

C. X-Ray Diffraction Analysis

The x-ray diffraction method is a powerful tool for investigation orderly arrangements of atoms or molecules (crystals). If the structure are arranged in an orderly array or lattice, the radiation is scattered or diffracted only under specific experimental conditions. Knowledge of these conditions gives information regarding the geometry of the scattering structures. The information obtained described the spatial arrangements of atoms. Two example are as the following:

           polythioetherketone                             poly(1,3-phenylene)

Application to Polymers

The crystal structure of a polymer is usually determined from x-ray patterns of a fiber drawn from the polymer, this pattern is essentially identical to a rotation pattern from a single crystal.

Chain Packing. Tell the degree of crystallinity.

Disorder in the crystal structure

D. Microscopy

Light Microscopy: for a) the texture of solid opaque polymers  b) crystalline materials (using polarized light microscope); c) the difference in refractive index using a phase-contrast microscope. d) thickness of polymer films by using a interference microscopy.

Electron Microscopy and Electron Diffraction:   to observe better resolution of smaller objects. The limitation is that the sample is damaged.

Scanning Electron Microscopy (SEM): A fine beam of electrons is scanned across the surface of an opaque specimen to which a light conducting film has been applied by evaporation. Secondary electrons, backscattered electrons, or x-ray photons emitted when the beam hits the specimen are collected to provide a signal used to modulate the intensity of the electron beam in a television tube, scanning in synchronism with the microscope beam. Because the latter maintains its small size over large distances relative to the specimen, the resulting images have great depth of field and a remarkable three-dimensional appearance. Resolution is currently limited to the order of 100 Å.

Homework:

1. Which has the higher energy, infrared radiation with λ = 1.0x10^-6 m or an X ray with λ = 1.0x10^-9 m?

2. Which has the higher energy, radiation with n=4.0 x10⁹ Hz or λ = 1.0x10^-9 m?

3. How many kinds of nonequivalent protons are present in each of the following compounds?

    A) CH3CH2Br    B) CH3OCH2CH(CH3)2   C) CH3CH2CH2NO2    d) Methylbenzene

4. How many peaks would you expect in the 1H NMR spectrum of 1,4-dimethylbenzene (p-xylene)?

5. Explain how infrared spectroscopy can be used to determine copolymer reactivity ratios. Given an illustrative example and discuss limitations to the method.

6. Show how NMR can be used to a) distinguish between head-to-head and head-to-tail polymerization in polymers (for example, PVC) and b) distinguish between a random copolymer and a mixture of homopolymers.

7. What microscopic methods would be suitable for studing a) the morphology of crystalline polymers    b) surface oxidation in a hydrocarbon polymer    c) the structure of the beads used in a GPC column?