|
The Notes for
Polymer and Coatings Science-
Chapter One- part two Crystallinity
The degree of crystallinity is measured by X-ray diffraction.
An x-ray diffraction spectrograph consists of a plot of x-ray counts received by a detector vs. the scattering
angle of the detector. The x-ray unit bombards the sample with x-rays, and the detector rotates around the the sample.
To
generalize, a spectrum will have a broad amorphous peak, and if the polymer has crystallinity, this will show up as sharp
peaks superimposed on the big amorphous peak. The spectrum is the sum of crystalline peaks and an amorphous peak.
The
computer performs a mathematical deconvolution from which the true area of the crystalline peaks, and the amorphous peak can
be determined. Notice the read and orange crystalline peaks.
If it is determined that the area of one crystalline peak is 5.0, the area under the other is 10, and the
area under the amorphous peak is 50, then the percent crystallinity is: 5 + 10 15
----------- = ---- = 0.23 23% relative crystallinity
5 + 10 + 50 65
The amount of crystallinity in a polymer depends on the following:
- the secondary valence bonds which can be formed
- the structure of the polymer chain (degree of order)
- the physical treatment of the polymer
If you "tensile pull" a polymer, the chains may straighten out and "orient" and the end result is more crystallinity.
I don't know what polymers this works for. I think polycarbonate is one of them because Lycette reported surprise that on
heating the polycarbonate, in her words, "it exploded." This was attributed to the "deorientation" released the work stress
built into the polymer by the orienting process. I didn't see the "explosion" so I don't know if it was a frightening bang
or just a dull thump.
- the thermal history of the polymer
If you heat a polymer above Tm, it becomes amorphous. If you then cool it slowly, it will crystallize. If you "cold quench"
it, by throwing it into room temperature water, ice water, or some other cold fluid, it may not be as crystalline; there probably
will still be some crystallinity, though.
- the molecular weight of the polymer
 shows an illustration of semicrystalline polymer (type 'crystalline-amorphous' into the find function when you arrive at
the site) Examples of crystallinity values: polyethylene, high density (HDPE) 50- 90%
teflon 95%
poly(vinyl chloride) (PVC) 5%
trans-poly(1,4-butadiene) 80%
cis-poly(1,4-butadiene) 0%
OTHER CRYSTALLINITY INFORMATION:
Linear polyethylene is 90% crystalline
Isotactic polypropylene is 90% crystalline
(Editor
comment: I think the above should read "can be as high as" because we tested an isotactic polypropylene and calculated 71%.
Linear
random poly(ethylene-co-propylene) has 0% relative crystallinity. Thus, the random copolymerization of two monomers which
produce highly crystalline homopolymers produces an amorphous copolymer.
Tacticity (see isotactic, syndiotactic, and
atactic)  is important: Isotactic polypropylene is 90% crystalline, but atactic polypropylene is 0% crystalline. Examples of polymer melt temperatures: poly(ethylene glycol adipate) 45 deg C
poly(ethylene oxide) 66 deg C
poly(propylene oxide) 77 deg C
linear polyethylene 130 deg C
polypropylene 160 deg C
poly(vinylidine chloride (PVC) 210 deg C
poly(chlorotrifluoroethylene 210 deg C
polystyrene 230- 240 deg C
nylon 66 235 deg C
teflon 327 deg C
Molecular weight determination:
- Physical Methods- These methods depend on physical properties.
- freezing point depression- gives M(n), the number average molecular weight.
- boiling point elevation- gives M(n), the number average molecular weight
- osmotic pressure- gives M(n), the number average molecular weight
For freezing point depression, boiling
point elevation, and osmotic pressure methods to work, the polymer must be soluble, and the test samples must be made from
dilute solutions.
- Light Scattering- Light scattering gives M(w), the weight average molecular weight. Light scattering occurs because the dimensions of the polymer molecules are on a similar size scale as the wavelength of
the light. For a field trip, go to an underground nightclub, and watch what happens when they turn on the strobe lights and
then turn on the dry ice "smoke."
![]() ![]()
Full picture available-
University of Southern Mississippi Polymer Science Department photo
 shows a distribution of polymer molecular weights which illustrates that the weight average molecular weight is higher than
the number average molecular weight. (type "following graph" into the find function)
- Sedimentation gives the M(z), z average molecular weight, and is used primarily in biochemistry where polydispersity values are ~1. Sedimentation isn't an option if you're working
with polydisperse polymer samples.
- Viscosity gives the M(v), the viscosity average molecular weight. M(v) is a quick method, but there are two constants in the calculation, K and a, which must be known. To find out what these
constants are, it is necessary to take known standards of the polymer you wish to test, and run viscometry experiments.
So what is the use of viscometry if, by itself, it cannot determine molecular weight? If you are manufacturing a new polymer,
you may spend a lot of money contracting someone to spend a day doing light scattering experiments on your standards. After
that, you can do viscometric determinations of sample lots for quality control.
We now
give you a review of gas chromatography and liquid chromatograph because this will provide a frame of reference for Gel
permeation chromatography, yet another method for determining molecular weight.
GAS CHROMATOGRAPHY-
(GC) is useful for materials that can be vaporized. The three main parts of the GC are in the
injector, column and detector. The sample is injected by a syringe into the injector, and it immediately vaporizes. For capillary
GC systems a splitter permits only a small portion of the sample to enter the column. The carrier gas, usually helium, pushes
the sample onto the column. The columns are packed with a liquid coating over an inert support such as silicone oil or firebrick.
The liquid packing must be high boiling and have a small vapor pressure of less than 1 mm at the maximum temperature used
so that it is not removed from the column.
For a capillary column system the coating materials are coated directly
on the walls of the column. A capillary column is a more efficient system because a longer column will fit into a given volume
of GC interior, owing to the smaller diameter. Capillary column lengths of 100 to 150 feet are common.
The materials
are separated on the column by selective interaction of one material more than another. The material that interacts the most
moves the slowest. The interactions can be absorption or any type of chemical interaction. Even if two substances have the
same boiling point, they can usually be separated, due to the principle that deals with how well the column absorption is.
The
two types of detectors for the GC are thermoconductivity (TC) and flame ionization (FID.) The T.C. detector measures the difference
in the thermal conductivity of the sample verses a reference sample using a Wheatstone bridge to measure the imbalance. The
FID system uses a hydrogen flame to burn the sample and it measures the amount of materials by this method. FID systems are
usually very much more sensitive than TC by a factor of 1000 or more.
Other detectors include electron capture, Halls,
or other specific detectors. The ability to vary the temperature of the column permits a greater range of separation.
LIQUID CHROMATOGRAPHY-
LC or High Performance Liquid Chromatography (HPLC) is useful for materials
that dissolve in solvents. Just as with GC, there is the injector, the column, and the detector. Rather than using a gas,
one uses a solvent as the carrier media. A liquid pump is needed. The pump should be capable of providing uniform, pulse free,
flow of 0.1 to 10 ml/min at pressures up to 6,000 psi. A loop injector is used to introduce the sample into the system.
Reverse
phase columns are prepared by chemically bonding the organic phase to the inert support. This prevents the solvent from
washing the organic phase form the packing. Typical organic phases can be prepared from C_8_SiMe_2_Cl or C_18_SiMe_2_Cl reacting
with SiOH of the silica type substrate. The covalent bond prevents the solvent from removing the organic packing from the
columns.
The three main types of detectors include refractive index (RI), ultraviolet (UV), and electrochemical. The
RI detector measures the difference between a reference solvent and a sample peak as it elutes from the column. Differences
in the 7th decimal place are important, and because of this, temperature control and solvent uniformality are important. The
UV detector covers the range of 200 to 700 nm. It is more sensitive than RI has two other advantages: 1)it can be tuned
to the exact frequency of the material and 2) it is insensitive to changes in solvent composition. The electrochemical
detector works for charges species or ions.
Typical solvent pairs for liquid chromatography include:
- water/methanol
- water/acetonitrile
- chloroform/hexane
and others may be used.
Gel Permeation Chromatography (Size Exclusion
Chromatography) GPC is used to determine the molecular weight of polymers. It can use the same system as an LC. The pump is the same.
Either UV or RI detectors can be used, but most often an RI detector is used. The solvent of choice is THF, but toluene or
m-cresol are alternatives. Almost any solvent can be used that will dissolve polymers. The RI of the solvent must be different
from the RI of the polymer to be analyzed. Solutions with 0.25 to 0.5% (w/v?) are used for the analysis. Pore sizes
of column materials can be (in angstroms) a) 100, b) 500, c) 1000, d) 10,000, e) 100,000, f) 1,000,000, or g) 10,000,000: a)
10 nm
b) 50 nm
c) 100 nm
d) 1 micron
e) 10 microns
f) 100 microns
g) 1 mm
Crosslinked polystyrene can be used for the packing material. Packing materials must be insoluble in the solvent used,
so a solvent for the experiment must be chosen which doesn't dissolve the packing material. This can be a tiresome issue,
because generally, polymers are insoluble in most solvents. The two main type of detectors for GPC include thermoconductivity(TC)
and flame ionization detector (FID). The thermoconductivity detector measures the difference in thermoconductivity of the
sample verses a reference sample using a Wheatstone bridge to determine the imbalance (the difference, I believe.) The flame
ionization detector uses a hydrogen flame to burn the sample and it measures the amount (mass) of materials by this method.
FID systems are usually much more sensitive than TC systems by a factor of 1000 or more.
Note that detectors measure mass, so 5 chains of degree of polymerization equal to 100 give the same signal as 1 chain of degree of polymerization equal to 500. (01 July 95- wld)  After running a GPC experiment you have a graph of detector response as a function of time.  You can run standards of known molecular weight and determine where they appear on a graph of signal vs. time. From
these you can determine a function of molecular weight vs time, and from this you can convert your signal vs. time graph to
a graph of signal vs. molecular weight. From the signal vs. molecular weight graph, you will can calculate molecular
weight (number average, weight average, etc.), taking note to think carefully about what the signal of your detector is measuring.
Addition Polymerization and Addition Polymerization Issues
CHEMISTRY OF VINYL
POLYMERIZATION: There are four intermediates for polymer chemistry:
- C. free radical
- C+ carbocation (used to be called carbonium)
- C- carbanion
- C: carbene
FREE RADICAL PROCESS-
A free radical reaction is a very fast process that takes place in a fraction of a second.
In a free radical polymerization process, the formation of a polymer molecule requires initiation to occur once, and then
propagation to occur thousands of times (technically speaking, initiation could start two polymer molecules.)
Initiation:
ultraviolet radiation
Cl --------------------> 2 Cl.
2
's discussion of free radicals. Propagation: Cl. + CH ---> HCl + CH .
4 3
CH . + Cl ---> CH Cl + Cl.
3 2 3
(type "propagation" into the find function) Termination (3 types):
Termination by dimerization (or combination) CH . + CH . ---> CH -CH (ethane)
3 3 3 3
CH . + Cl. ---> CH Cl (chloromethane)
3 3
(type "combination" into the find function) Termination by disproportionation H H H H H H H
H H H H H H H
| | | | | | | | | | | | | |
H-C-C-C. + .C-C-C-C-H ---> H-C-C=C-H + H-C-C-C-C-H
| | | | | | | | | | | |
H H H H H H H H H H H H
two radicals collide, and one radical gives a proton to the other, but keeps the electron and uses it along with its
free radical to form a pi bond. the radical that receives the proton places it on the radical site. (type "disproportionation" into the find function) Termination by chain transferA chain transfer reagent comes
in and deposites a hydrogen with one electron (H.) on the radical of the growing polymer. The chain transfer reagent will
then start another polymerization. A chain transfer reagent should not be confused with an inhibitor. P. + C-T-H --> P-H + C-T.
Mercaptans, which contain sulfur (skunk spray is a mercaptan), make excellent chain transfer agents. Chain Transfer Reagent Constants- the numbers vary from monomer to monomer, and there is a temperature dependence.
The values below are for methyl methacrylate at 60 C.
water O
carbon tetrabromide 2,700
benzene? (notes are not clear) 0.036
n-butanol 0.25
Head-to-tail; Head-to-head; Tail-to-tail
Hiementz (Hiementz-23) uses the term 'orienticity' to cover this head/tail nomenclature. This topic relates
to addition polymers with one function group per repeat unit. The example is poly(vinyl alcohol):
 A 100% head-to-tail polymer - C - C - C - C - C - C - C - C - C - C - C - C - C - C - C - C
-
| | | | | | | |
OH OH OH OH OH OH OH OH
If the carbon with the hydroxyl group is considered to be "head", then how often does head-to-head polymerization occur?
- C - C - C - C - C - C - C - C - C - C - C - C - C - C - C - C -
| | | | | | | | |
OH OH OH OH OH OH OH OH OH
In the above polymer head-to-head polymerization occured twice, but no tail-to-tail polymerization occured. Head-to-head
orientation can be tested for because there is a selective oxidation reagent, HIO . the reaction is - C - C
- --> - C - OH HO - C -
4 | | || ||
OH OH O O
How to detect head-to-head contact polymerization
HIO4 will cleave poly(vinyl alcohol only between two head-to-head
linkages, and for a given mass of polymer, a titration can be performed to determine how many carboxyl groups there are. From
these calculations there was less than 1% head-to-head polymerization.
Last Update- July 8, 1995- wld
|
