Random thermal degradation can usually be described as a first-order reaction (loss of weight as a parameter) if the decomposition products are volatile. Chain depolymerization has been extensively studied by Simha and Wall [19]. The two factors that are important for the course of the depolymerization are:
(1) the reactivity of depropagating radical and
(2) the availability of a reactive hydrogen atom for chain transfer.
All polymers containing α-hydrogens (such as polyacrylates, polyolefins, etc.) give poor yields of monomer; conversely, polymethacrylates and p-α-methylstyrenes five high yields of monomer, due to the blocking of chain transfer by the α-methyl group. Poly(tetrafluoroethylene) gives high yields of monomer because the strong C-F bonds are resistant to transfer reactions.
Also this type of degradation can be described by an overall quasi-first-order reaction, but the kinetic scheme may be complicated. Besides the rate constant two other parameters can be obtained by kinetic analysis:
initiation of
y probabilit
transfer of
y probabilit constant
transfer
the = k
tr=
(2-55)transfer) on
(terminati of
y probabilit
n propagatio of
y probabilit length
chain kinetic
the
kin= +
= Λ
(2-56)For polyethylene
Λ
kin≈ 0
(no monomer produced); for poly(methyl methacrylate)Λ
kin≈ 200
(nearly 100 % monomer produced).
General Reference
D. W. Van Krevelen, Properties of Polymers, 3rd ed., chapter 21, Amsterdam: Elsevier (1990).
F. A. Bovey, and F. H. Winslow, Macromolecules An Introduction to Polymer Science, chapter 7, New Jersey: Bell Laboratories (1979).
2.5 Conclusion
This chapter provided the background knowledge which is necessary for designing base materials of POF. Firstly, we reviewed the importance of radical polymerization in bulk. In this method, we can obtain highly pure polymers without any impurities and contaminants. From the overview of intrinsic three factors affecting attenuation of POF, especially from the Morse potential theory for absorption and the fluctuation theory for light scattering, we can say one of the effective ways to obtain polymers with high transparency is fluorination. Substituent with fluorine reduces the refractive index, thus leads to less scattering, and also reduces the vibrational absorption. To obtain a high Tg, cohesive properties of polymer molecular structure should be considered.
Molecular interactions decrease the mobility of chains, according to the kinetic theory, which must contribute to an increase of Tg. The size of certain atoms or the geometry of some substituents may contribute to stiffen the chain backbone and cause an increase of Tg. However, it is still hard to theoretically predict Tg of polymers from chemical structures and the clear guideline for calculating or expecting Tg has not been established. Heat decomposition of polymers is divided into two types: depolymerization and random decomposition. Both degradations can be characterized by the breaking of the weakest bond and is consequently determined by a bond dissociation energy. While the mechanism of thermal degradation is too complicated to estimate accurately, the important thing we have to note is the bond dissociation energy.
References
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3. W. Groh, and A. Zimmerman, “What is the lowest refractive index of an organic polymer?,”
Macromol., 24, 6660 (1991).
4. T. Kaino, M. Fujiki, and K. Jinguji, “Preparation of plastic optical fibers,” Rev. Electrical Comm.
Lab., 32, 478 (1984).
5. F. Urbach, “The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids,” Phys. Rev., 92, 1324 (1953).
6. A. Einstein, “Theorie der Opaleszenz von homogenen Flüssigkeiten und Flüssigkeitsgemischen in der Nähe des kritischen Zustandes,” Ann. Phys., 33, 1275 (1910).
7. Y. Koike, N. Tanio, and Y. Ohtsuka, “Light scattering and heterogeneities in low-loss poly(methyl methacrylate) glasses,” Macromol., 22, 1367 (1989).
8. N. Tanio, Y. Koike, and Y. Ohtsuka, “Inherent light scattering losses by amorphous optical polymer glasses,” Polym. J., 21, 259 (1989).
9. Y. Koike, S. Matsuoka, and H. E. Bair, “Origin of excess light scattering in poly(methyl methacrylate) glasses,” Macromol., 25, 4807 (1992).
10. P. Debye, “A photoelectric instrument for light scattering measurements and a differential refractometer,” J. Appl. Phys., 17, 392 (1946).
11. P. Debye, and A. M. Bueche, “Scattering by an inhomogeneous solid,” J. Appl. Phys., 20, 518 (1949).
12. P. Debye, H. R. Anderson, and H. Brumberger, “Scattering by an inhomogeneous solid. 2. The correlation function and its application,” J. Appl. Phys., 28, 679 (1957).
13. T. G. Fox, and P. J. Flory, “Second-order transition temperatures and related properties of polystyrene. Influence of molecular weight,” J. App. Phys., 21, 581 (1950).
14. Y. Gnanou, and M. Fontanille, Organic and physical chemistry of polymers, chapter 11, New Jersey: Wiley-Interscience (2008).
15. M. Gordon, and J. S. Taylor, “Ideal copolymerization and the second–order transitions of synthetic rubbers,” J. Appl. Chem. USSR, 2, 493 (1952).
16. S. L. Madorsky, and S. Straus, “High temperature resistance and thermal degradation of polymers,” S. C. I. Monograph, 13, 60 (1961).
17. V. V. Korshak, “The chemical structure and thermal characteristics of polymers,” Israel Program
for Scientific Translations, Jerusalem (1971).
18. C. Arnold, “Stability of high-temperature polymers,” J. Polym. Sci., Macromol. Rev., 14, 265 (1979).
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Chapter 3
-Poly(2,2,2-Trifluoro(chloro)ethyl Methacrylate)-
3.1 Preface
In chapter 2, basic knowledge for designing POF base materials were summarized and influential factors were described in detail. Previous studies have provided a clear guideline to decrease the attenuation of POF; substituent with fluorine reduces the refractive index, thus leads to less scattering, and also reduces the vibrational absorption, whereas the way of increasing thermal properties such as Tg and Td has not become clear enough. Therefore, as the first approach to prepare a low-loss and thermally stable GI POF for home network, we started this work from the investigation of an effective method for obtaining polymers with low attenuation.
As mentioned in section 1.4, there have been previous reports on low-loss POFs consisting of perdeuterated PMMA [1, 2] and perfluorinated polymer (CYTOP®) [3-7]. They have no C-H bonds in its structures, which is the major factor of attenuation at the emission wavelength of the light source
[8, 9]
, thus exhibit excellent low-loss characteristics. However, the synthesis procedures of monomers are complicated and hence the fibers are overpriced for general consumers. That was the biggest problem. On the other hand, surprisingly few studies have so far been made at partial substitution. Actually such a low attenuation property is not necessary for the very short reach networks in houses, and the attenuation of up to 200 dB/km is acceptable.
Compared to all hydrogen substituted polymers, a partial substitution can be obtained by simple procedures and is more cost effective. We believed that the critical factor for reducing attenuation should be not the per-substitution but the amount of C-H bonds per unit volume of polymers, that is, we expected that enough low-loss characteristics would be obtained by partial substitutions. To clarify the validity of this hypothesis, we investigated two kinds of methacrylates:
poly(2,2,2-trifluoroethyl methacrylate) and poly(2,2,2-trichloroethyl methacrylate). Both