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1. Calculate the average width and thickness of the rubber band and its crosssectional area. Assuming that no systematic change in the L0 values was observed, calculate the average unstressed length, and the volume of the sample.
2. For each isothermal data set (manual and instrumental), plot the force
versus relative elongation and construct a smooth curve through the points. On
each plot locate the 
value used in the isostrain experiment
(henceforth
symbolized as 0). Evaluate
as the slope of the tangent line at
that point. Use
equation 25 to calculate the cross-sectional area at this elongation.
Calculate Young's modulus at each temperature. If the manual apparatus and TMA
were used at a common temperature, compare the results obtained by the two
methods, and explain any significant difference.
3. For the 25oC TMA data,
determine the force corresponding to 0 Determine
the elongation produced by this force on each of the other isothermal plots.
Prepare a plot of versus T;
use its slope and
at 25oC to calculate 
at this temperature.
(If the manual method is used at more than one temperature,
a similar calculation can also be performed using that data.)
4. For the isostress TMA experiment plot versus temperature,
and evaluate the
slope at 25oC. Use this result to calculate
in the same manner as in
step 3.
5. For the isostrain experiment plot force versus temperature and evaluate
as the slope at 25oC.
Calculate from this result and the
length of the unstressed band.
6. Calculate the average value of ,
obtained in steps 3-5, and try
to
identify the cause(s) of significant variations in the results. Discuss the
value
of (especially the sign) in terms of the
molecular model for the elongation of an elastomer.
7. Using the data for each of the isothermal data sets(manual and instrumental)
and the cross-sectional area of the unstretched band, prepare plots of 
versus
.
Calculate Ne at each temperature and the average Ne.
Use equation 31 and the corrected cross-sectional area to calculate Young's
modulus at each temperature for equal to
0
Compare the Y values to those calculated in
step 2.
8. Assume that the sample is composed of poly(cis-isoprene) molecules, each containing 5000 monomer units, with disulfide crosslinks at 100 bond intervals (25 isoprene units between cross-links). If the density of the polymer is 0.91 g/cm , calculate a value for the number of chain segments per unit volume, and compare it to the Ne obtained experimentally. Discuss the validity of this comparison [3,4].
The author first learned of the "weights and ruler" stress/strain experiment during a workshop on polymer chemistry conducted by Dr. John P. Droske of the University of Wisconsin at Stevens Point. Dr. Droske continues to be active in promoting the incorporation of polymer concepts in the chemistry curriculum. Development of the present instrumental version of the elastomer experiment was made possible by a grant from the NSF-ILI program for the thermal analysis system. The author also acknowledges the assistance of Ensign Stephen E. Musson and Mr. Jeff Groh, who both helped with the initial experimental design, and Dr. Julianne Harmon for her helpful comments on the manuscript.
1) Nash, L. K. J. Chem. Educ., 1979, 56 , 363.
2) Flory, PaulJ. "Principles of Polymer Chemistry", (Cornell University Press, Ithaca,NY, 1953) Chapters X and XI.
3) Mark, J. E. J. Chem. Educ., 1981, 58 ,898.
4) Aklonis, J. J.; MacKnight, W. J. "Introduction to Polymer Viscoelasticity", Second Edition, (Wiley-interscience, New York, 1983) Chapters 5 and 6.
5) Bader, M. J. Chem. Educ., 1981, 58 ,285.
The instrumental version of this experiment requires a thermal analysis system containing a TMA module. See Addendum
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