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

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%

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

• 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.
  
  

a)     10 nm

b)     50 nm

c)    100 nm

d)      1 micron

e)     10 microns

f)    100 microns

g)      1 mm

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.

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 transfer
A 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.

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Last Update- July 8, 1995- wld

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