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Inorganic Chemistry

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Organic Chemistry
Inorganic Chemistry

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:
  1. water/methanol
  2. water/acetonitrile
  3. 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.
The above statement has been questioned, and is now under investigation. (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:
    1. C. free radical
    2. C+ carbocation (used to be called carbonium)
    3. C- carbanion
    4. 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 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.







    Last Update- July 8, 1995- wld