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Plastics as Optical Materials.

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Received: October 28, 1974 [A 154 IE]
German version: Ange-. Chem. 91,20 (1979)
The original manuscript was dedicated to Gerold Schwarzunbach on the occasion of his 70th birthday. Owing to various
unforeseen circumstances simultaneous publication of the
German and English versions has been delayed.
[ I ] R. Wiiikler, Doctoral Thesis, Wien-Cattingen
[2] H 4 . Beiiesi. J . H . Hildebraiid, J. Am. Chem. Soc. 71, 2703 (1949).
[3] M. Di.xon, E. C. Webb. Enzymes. 2nd Edit., Longmans, London 1965.
141 J . ‘I‘ Edsall, J . W)muii: Biophysical Chemistry, Vol. I. Academic Press,
New York 1958.
[5] n.ElgPU, L. D e M u e y r , Tech. Org. Chem. M l l b , 895 (1963); see also
G. G . Hummrs in: Techniques of Chemistry. Vol. VI.’2. Wiley, New
York 1974, p. 63.
[6] P ./oh, Ann. Chiin. Paris 9. 113 (1928).
[7] F. ./. C. Rorsotri. H . Rossorri: The Determination of Stability Constants,
McGraw-Hill. New York 1961.
[8] M . Eigeti, G . K i i r t x , K . Tamm, Z . Elektrochem. Ber. Bunsenges. Phys.
Chem. 57. 103 (1953).
[9] h l . Eigen, K . Tumm. 2 . Electrochem. Ber. Bunsenges. Phys. Chem.
66. I07 ( I 962).
[lo] G. Cxv/in.skL, Theor Exp. Biophys. 2. 69 (1969).
[I I ] D. Thusius in C. Sudron. Dynamic Aspect of Conformational Changes
in Biological Macromolecules. Reidel, Dordrecht 1973, p. 271.
[12] T M . J o i i n in R. Chrn, H . Edellioch. Biochemical Fluorescence:
Concepts, Dekker, New York 1975, p. 305.
[ 131 G . Brier. S. Busu. P. Srhusrer, to be published; cf. also G . Beier, Doctoral
Thesis. Vienna 1975.
[I41 H . W Cliatig. E . Neumann, Proc. Nat. Acad. Sci. U S . 73, 3364 (1976).
[15] a ) W Burgermeister. Th. Wirland, R . Winkler. Eur. J. Biochem. 44,
305 (1974); b) P. B. Chock, F . Eggers, M . Eigrii, R . Winkler, Biophys.
Chem. 6,239 ( I 977).
1161 J . Meixiirv, Kollold-Z. 134, 3 (1953).
[17] G . Scliwar:enhach. H . Flaschka: Die komplexometrische Titration. 5th
Edit., Enkc Verlag, Stuttgart (1965).
[18] J . Bjerruin. Chem. Rev. 50, 381 (1950).
[I91 G. Scatchard. Am. Acad. Sci. 51, 660 (1949); cf. also Ref. [4].
[20] H . Sund, K . Markau, R . Kobersteiri. Biol. Macromol. Ser. 7: “Subunits
in Biological Systems”, Vol. C. Marcel Dekker, New York 1975.
[21] J . Muiid, J . Wpnuit, J . P. Changeux, J. Mol. Bioi. 12, 88 (1965).
[22] D. E. Koshlontl, G. Nemerhy, D. Filmer, Biochemistry 5 , 365 (1966).
[23] M . Eigen, G. Ilgeffrirz, to be published.
[24] H . !l Mulmstudr, Anal. Chem. 26, 1348 (1954); 27, 1757 (1955); P.
Deitrhuy, ihid. 20, 1212 ( 1 948).
[25] L. Bruhacher, personal communication.
[26] G Oster, A. Perelson, A . Katchalsky, Q. Rev. Biophys. 6, 1 (1973).
[27] M . Eiyeri in F. 0. Schmirr er a/.: Neurosciences, Third Study Program,
The MIT Press, Cambridge, Mass. 1973. p. XIX.
Plastics as Optical Materials
By Helmut Dislich[’]
The principal material used in optics is glass. Plastics can only gain acceptance in optics
if they have properties not encountered with glass or if a desired article can be produced
more rationally from a plastic. An example of the first case is provided by UV light guides
consisting of a quartz glass core surrounded by a plastic cladding; there is no glass with a
sufficiently low refractive index. Examples of the second case are viewfinder optics for camergs
and lenses for sunglasses and industrial safety spectacles.
1. Introduction
“Plastics in optics”-when considering this topic it is inadvisable to start from the annual production of 1000 tons
because even the potentially interested reader is likely to
feel his interest wane at the thought of such unusually low
production figures. The turnover of high-value products made
possible by new plastics and plastics technologies would
appear to attract much more attention. In many cases the
user is unconscious of the application of plastics, unless he
notices it by way of the price which may be very low (example:
high quality viewfinder optics) or very high (example: spectacles worn after removal of a cataract or UV fiber optics).
In the former case, use of plastics is dictated by the market,
in the second case by the optical and other properties of
the plastics.
The most comprehensive report about the use of plastics
for optical purposes appeared in 1945[’].At that time, however,
many of the plastics of interest nowadays were unknown.
Widely acclaimed surveys were subsequently authored by
[*] Dr. H. Dislich
JENAer Glaswerk Schott & Gen., Mainz
Postfach 2480, D-6500 Mainz (Germany)
Angew. C
/ieii7. I i i t .
Ed. Engl. 18. 49-59(1979)
Schreyer[’ -41. Optical properties are listed in Landolt-BornAn exceptional wealth of detail is given in a review
by Torbin and Darninov[’].
In the present article, the optical properties of plastics will
be correlated with their chemical structure wherever possible.
Rather exotic plastics hardly known outside the realm of
optics sometimes prove particularly interesting. Mass produced plastics-in optics, these include poly(methy1 methacrylate) o r polycarbonate-are
of interest with regard to the
possibilities of working them. Combinations of plastics with
glasses were frequently the logical consequence of studies
on the optical scope of plastics.
Any consideration of optics compels us to look at glass,
because glass was, and still is, the classical material of optics,
and any plastics making an inroad into this field will be
measured against glasses. In view of the well established position ofglass, seriousconsideration must be given to the question
of the motivation for the intense work being done on plastics
for optical purposes, particularly by an author in a glass-house
who, as the saying goes, may not throw stones with impunity.
There are two simple answers:
1) A desired article can be produced more rationally from
plastics ;
2) some plastics have-urgently
shown by glasses.
2. Glasses and Plastics
In this article, glass is taken to mean a fused inorganic
product, and plastic an organic high polymer, produced, for
example, by polymerization, polycondensation, or polyaddition of low-molecular starting materials. We shall restrict
our attention to glass-clear, preferably more recent plastics.
2.1. Comparison of Optical Properties
1 Ipml --+-
Essential properties always employed as initial selection
criteria are the spectral transmittance, the refractive index
nd, and the Abbe number
nF. nc = refractive indices for the F and C lines, respectively.
Fig. 2. Spectral transmittance ol ( 1 ) poly(methy1 methacrylate) (PlexigIass*
201 with U V absorber), 3 mm; (2) poly[p-(p-h~droxy-c*,r-dimethylbenzyl)phenyl hydrogen carbonate] (Makrolon, Lexan ” ), 3 mm; ( 3 ) poly(diethy1ene
glycol diallyl biscarhonate)(CR39),3 m m ;(4)poly(methy1 methacrylate) (pure).
3 mm.
“monomethylated quartz glass”, which is suggestive of its
high transmittance resembling that of quartz glass.
2.1 . I . Transmittance
In the case of glasses the ranges of high transmittance
extend from the ultraviolet region (silica glass) via the visible
(many optical glasses often having very high transmittance
owing to extremely high homogeneity and purity) to the
infrared region (chalcogenide glasses)[’].
The transmittance ranges are somewhat more limited in
the case of plastics. Apart from the good transmittance in
the visible range, the UV transmittance of poly(methy1 methacrylate), methylpolysiloxanes, and perfluorinated polymers
deserves mention; some perfluorinated polymers, e. g. the
copolymer of tetrafluoroethylene and hexafluoropropylene
(Teflon FEP@),are also transparent to IR up to about 6 p m
(Figs. 1 and 2).
As a result of their high IR transmittance, fluorinated
polymers can be used as embedding agents in IR spectroscopy.
FEP@,which is glass-clear as thin films, owes its special position as a largely non-crystalline, and hence non-scattering,
perfluorinated polymer (in contrast to polytetrafluoroethylene)
to the branching arising from copolymerization, which inhibits
The reflection losses of plastics having higher refractive
indices, such as polystyrene or polycarbonate, can be reduced
by coating with a layer of material having a lower refractive
index such as MgFz and their transmittance consequently
enhanced“’], as is common practice with glasses.
2.1.2. The nd-vd Diagram
The “optical position” of transparent plastics and glasses” I ]
is normally represented in a nd - diagram. Figure 3 shows
some examples.
The plastics region overlaps with the glass region but
appears more extensive at the bottom. The extremely low
refractive indices of Teflon FEP@(point 19) and methylpolysiloxane (point 5) permit their use as optical coatings for silica
glass fibers and rods[I” 13]; no alternative glass is foreseen.
Correlation of the “optical position” with the chemical structure of polymers leads to simple relationships with which
new plastics always comply. Directed syntheses of tailormade
products would be readily feasible. In practice, however, one
0.L 0.5
Fig. 1 . Spectral transmittance of ( 1 ) silica glass (Suprasil W), 1.Omm; (2)
methylpolysiloxane (Glass Resin 650). 1.04 m m ; (3) polytetrafluoroethylenehexafluoropropylene copolymer (Teflon FEP? 0.02 mm.
A monomethyl polysiloxane (“Glass Resin 6 5 0 ) produced
mainly from rnethyltrialkoxy~ilane[~~,
can be designated as
Anyew. Chrm. I n t . Ed. E t i y l . 18.49-59 (1979)
Fig. 3. Glass region and plastics region in the i i d - v d diagram. ---:present
present limit of the region of transparent
limit of the glass region;
plastics. A quartz glass, 0 plastics 1-19. 1 = styrene-acrylonitrile copolymer,
2 = poly(o-diallyl phthalate), 3 =poly(methyl cx-chloroacrylate). 4 =methylphenylpolysiloxane,
5 = methylpolysiloxane,
6 = poly[p-(p-hydroxy-a,%dimethylbenry1)phenyI hydrogencarbonate], 7 = poly(methy1 methacrylate),
8 = poly(diethy1ene glycoldiallyl biscarbonate),9 =cellulose acetate. 10= cellulose propionate, 1 1 =polystyrene, 12 = poly-N-vinylcarbazole, 13 = poly(cyc1ohexyl methacrylate), 14 = poly(viny1 chloride), 15 =epoxy resin (Araldite Cy
206), I h = poly(2.2.2-trifluoroisopropyl methacrylate), 17 =methyl methacrylate-2-methylstyrene copolymer, 18 =methyl methacrylate-acrylonitrile copolymer. I9 = tetrafluoroethylene-hexafluoropropylenecopolymer.
searches through known materials, applying possible corrections by copolymerization.
Highly aromatic plastics, such as polystyrene, poly(ary1 carbonates), or even poly-N-vinylcarbazole, have high refractive
indices and low Abbe numbers; fluorinated, and especially
perfluorinated, plastics have low refractive indices and high
Abbe numbers.
By means of the Lorentz-Lorenz equationI2. ‘‘I
t e r n p e r a t ~ r e “ The
~ ~ . drawback of being less hard or less resistant to wear is less serious because it can be at least partly
overcome by mechanical protection or coating.
The greater impact resistance of plastics can be of advantage
since the danger of fracture is reduced. Their low thermal
conductivity can lead to deformation on fast changes in temperature.
The effects of these properties are accurately known[41;
it may therefore be predicted whether the required tolerances
can be met for any given application.
The main plastics used are polymers and copolymers of
methyl methacrylate, styrene, diethylene glycol diallyl biscarbonate, and acrylonitrile, as well as polycarbonate, cellulose
acetate, and cellulose acetobutyrate. Polymethylpentene TPX
should also be mentioned in passing. Let us turn our attention
specifically to poly(methy1 methacrylate) (PMMA) and polystyrene among this relatively sparse selection. Their optical
positions (see Fig. 3) are sufficiently different to lead to
achromatism in an optical system. PMMA then corresponds
to the crown glasses and polystyrene to the flint glasses.
2.2. Comparison of Production Techniques
2.2.1. Glasses
According to the ASTM definition, glass is “an amorphous
inorganic usually transparent or translucent substance formed
by cooling and solidifying a molten mass without crystallization”, and has been prepared in this way for some 4000 years.
The ingredients of glass, frequently oxides, are mixed and
heated until they melt (usually above 1OOO”C), reactions
between the components leading to formation of the glass
melt. O n cooling, the melt solidifies to a glass without crystallizing. This cooling can be carried out in a mold. The optical
component (lens, prism, disk, etc.) is then fashioned from
the casting by grinding and polishing.
2.2.2. Plastics
R M=molar refractivity, A4 = molecular weight, d = density, n = refractive index
it is possible to estimate the limits of plastics, including hypothetical polymers: as far as can be seen nowadays no polymers
will become available with refractive indices much in excess
of rid = 1.73 or below nd= 1.33. Plastics fail to reach the center
of the glass region. Thus the terms of competition between
optical glasses and transparent plastics are defined, as are
the limitations of their combinations.
2.1.3. Effects ofThermal and Mechanical Properties of Plastics
on Optical Properties
The lower specific gravity of plastics ( z1.0 to 1.5) often
gives them an advantage over glasses ( z2.2 to > 3). The coefficient of linear thermal expansion of plastics is roughly ten
times greater than for glasses; this is a disadvantage since
the optical properties become strongly temperature dependent.
The refractive index of a plastic decreases with increasing
Chem. Int Ed. Eiiyl. 18. 49-59 (1979)
The starting materials are monomers which are transformed
into polymers by polymerization, polycondensation, or
polyaddition. These reactions take place at relatively low temperatures. It is of considerable importance that conversion
into the polymeric state can occur in the mold (casting process)
and so immediately lead to the finished component. As an
example we can consider the polymerization of diethylene
glycol diallyl biscarbonate to give spectacle lenses as described
in Section 3.1.2. This “one-step process” is by no means highly
rationalized because polymerization must be performed slowly
and under careful control in order to ensure formation of
a homogeneous optical component of reproducible geometry.
The mold, which is usually rather complicated, is blocked
during this time.
The relatively short treatment given to plastic optical components produced by injection molding or similar processes
should not disguise the fact that this is actually the largest
market. Several reports have already been published on this
topic. Combined Optical Industries Ltd. in England were
pioneers in this field.
Whenever possible the polymer is converted into granules
and shaping is by standard injection molding processes. Thus
lenses and prisms for the viewfinder optics of a camera are
molded from polyfmethyl methacrylate), as are reflectors,
Fresnel lenses, and many other similar objects. Without doubt
this is still the most rational method for the production of
large numbers of objects.
Injection molding of lenses requires a high, but nowadays
attainable, degree of precision; the accuracy obtainable with
ground and polished glass lenses is greater. The material
used for the injection mold and the quality of its surface
play an important role along with other parameters. A recent
development is the use of glass molds[”].
Injection pressing, stamping, and extrusion are included
in this section as broadly similar processes starting from a
polymeric material. For example, pressing and stamping
techniques are used to manufacture thick-walled lenses of
large diameter, reflector optics, traffic signals, mirrors, and
aspherical optics.
Apart from injection molding, machining of plastic blocks
is used for the production of large Fresnel lenses which can
hardly be manufactured from glass.
Adhesive layers of epoxy resins, consisting of resin and
curing agent (amine or anhydride), are often used to join
two halves of a component. Alkylpolysiloxanes are used mainly
as layers on substrates. The starting material is often methyltrialkoxysilane, which is hydrolyzed by atmospheric moisture,
according to
CH3Si(OR)3+ 3 HzO t CH3Si(OH)3+ 3 ROH
With increasing temperature, methylsiloxanes are formed by
The method of choice for manufacturing a given product
depends upon the technical possibilities and requirements,
and also on the manufacturing runs involved[’ ’I.
2.2.3. Synthesis of Glasses by Polycondensation
While the production techniques for glasses and plastics
do show a certain resemblance (thermoplastic processing),
this does not appear to apply to the synthesis of glasses
and of plastics. It therefore appeared most challenging to
synthesize a glass by one of the principles used in the synthesis
of plastics, i. e. by polycondensation.
An analogy was seen in the synthesis of a silica glass[’*]
according to
which proceeds via polyorgano(hydroxy)siloxanes.
In fact, numerous oxide glasses-and also crystals-containing many elements from various positions in the periodic
table can be synthesized without having to go through the
molten phase if the highly reactive and invariably hydrolyzable
alkoxides of several elements are allowed to react with one
another in alcohol. Hydrolysis and polycondensation ultimately lead to the pure multicomponent oxide, i.e. a glass“ ’I.
This reaction is illustrated for the known glass Duran 50
in Figure 4[201.
+ nB(OH), + pAI(OR), + qNaOR + rKOR +
Fig.4. Synthesisof a borosilicate glass Duran 50 by polycondensation (schematic).
This approach can be exploited for the production of thin
oxide layers by a dipping process[21!
3. Plastics for Optical Applications
Glass still reigns supreme in the field of optics, and will
continue to do so. Nevertheless, there are narrow yet interesting areas which plastics have already conquered and will
conquer in the future. Information supplementing that in I’
will be found in[22-241.In the following, examples will be
given in which plastics are chosen because of their optical
properties such as transmittance or refractive index, because
of more rational production methods, or even because of
their non-optical properties such as specific gravity, impact
resistance, ease of coloring.
Once again the reader is reminded that poly(methy1 methacrylate) plays a dominant role among plastics, and has even
been described as “organic glass”. However, all this is so
well known (see, e.9. [’I)
that it will only be mentioned
in passing.
3.1. Spectacle Lenses
3.1 .l. Thermoplastics
Most people have had close contact with lenses made of
plastics such as poly(methy1 methacrylate), cellulose acetate,
or polycarbonate in sunglasses and safety spectacles.
Rational production and impact resistance are of advantage.
A disadvantage of thermoplastics is their sensitivity to scratching and wiping. Attempts have been made in the last five
years to reduce the sensitivity to wiping by aftertreatment.
One approach employed vacuum condensation with inorganic
materials[26]-often glass-and another involved dip coating,
preferentially with methylpolysiloxanes. Many of these developments were apparently promoted by the Glass Resin 650
of Owens Illinois[91,and led to specific formulations by various
companies (Degussa, Toray, Daicel, Resart, Rohm, etc.).
Du Pont’s Abcite@ layers employ a copolymer of partly
hydrolyzed vinyl acetate and tetrafluoroethylene which is
crosslinked with a silicic ester. Curing is performed thermally
or with hard radiation[”]. The search for new markets
for large transparent plastic sheets as a substitute for glass,
e . g . in buses as is promoted by present legislation in the
USA, has had a stimulating effect on this very active field.
The prospects[281of producing integral spectacles-frame
and lenses injection-molded as a single unit-are very interesting, in order to meet the needs of those countries where normal
medical care will remain impossible for years to come owing
to the lack of any infrastructure. A lens which roughly satisfies
requirements is said to be far better than no aid at all.
Angrir. Chem. I t i f . Ed. E i i g l . 18. 49-59 i l Y 7 9 )
3.1.2. Duroplastics
The properties of poly(diethy1ene glycol diallyl biscarbonate)
are ideal for spectacle lenses-whether correcting or non-correcting. Apart from its very good optical quality, it has the
best wiping properties of all the transparent plastics produced
as bulk solids. Its optical data (see 8 in Fig. 3) lie in the
glass region and thus permit true competition between glass
and plastic. The refractive index nd = 1.499 can be modified
as desired by copolymerization. The specific weight of 1.32
is roughly half that of glasses, which is of great advantage
in lenses of high focal power; it shatters far less readily than
the usual glasses.
The material developed by Pittsburgh Plate G l a s ~ [ ’ ~in
the 1940s (CR 39, from “Columbia Resin 3 9 ) has long been
accepted on the market and certainly promoted by legislation
in the USA. On the other hand, the spectacle glass industry
has taken up the challenge and marketed silicate glasses hardened by ion exchange which comply with legal requirements,
as defined in a ball-impact test.
Of greater significance is the negative cosmetic effect in
the case of plastics. Lenses produced from the lower refractive
index material CR39 must have a greater curvature then those
made of more refractive glass. Highly refractive but lighter
glass, which has recently become available by replacement
of the heavy element lead by titanium[301,will probably lower
the market potential of CR39 in some aread3’1.
Moreover, trends in fashion play a role in the spectacle
market. Thus allyl carbonate (there are many trade names
corresponding to the multitude of manufacturers) is coming
to the fore because of numerous possibilities of bulk and
surface coloring by diffusion processes. A diffusion technique
is also used to produce a continuously changing transmission
from the bottom to the top of the lens (graded spectacles).
Glass is an excellent substance for making photochromic lenses
whose transmission adjusts to the brightness of incident light.
Despite all improvements to CR39 (e.9. condensed SiOz
coating), glass remains far more resistant to scratching, and
lower weight becomes significant only with very thick spectacle
lenses. Wearers of plastic spectacles must still forego the comfort of photochromism. Photochromic plastics remain reversible for only very limited periods.
CR39 monomer is synthesized from allyl alcohol, phosgene,
and diethylene glycol and subjected to crosslinking polymeri-
zation at the two allylgroups with the aid of a dialkyl peroxydicarbonate, e. g. diisopropyl- (IPP) or dicyclohexyl peroxydicarbonate.
The monomer is introduced, together with 3-4 %, of peroxide into a mold consisting of two glass half-molds with an
optically perfect internal surface separated by an elastic spacer
ring. On polymerization, induced by heating, the volume contracts by 1 4 % ; the two half-molds to which the polymer
tends to adhere-a problem in its own right-follow the
contracting mass. There results a lens which requires no aftertreatment provided that it is of low focal power. In the range
of medium focal power, it proves possible to impart the final
curvature to only one side. The other receives its ultimate
shape by grinding and polishing. At high focal powers, deviations from the shape of the mold are so large that only
a rough casting can be produced by polymerization. Polyfocal
lenses can also be produced by p~lymerization[~
High-quality sunglass lenses are produced analogously from
CR39. Polymerization may take hours. Since the mold is
tied during this time, considerable costs result. For this reason,
non-correcting lenses are preferably produced from large flat
sheets of CR39 about 2mm thick which are then sawn and
worked similarly to glass. The disks are softened by heating
and pressed or sucked into a curved mold; they retain this
shape on cooling. When in use, their temperature should
not be allowed to attain values leading to restoration of the
original flat shape.
The UV stability of CR39 and the UV opacity of a spectacle
lens can be adjusted in the finished product by several minutes’
immersion in a boiling aqueous suspension of UV absorber
(Fig. 5).
R‘ = R2 = H, R3 = OH
R’ = R2 = R3 OH;
R’ = OH, R2 = R3 = OCH,
Fig. 5. Examples of UV absorbers based o n hydroxyphenylbenzotriazole
and hydroxybenzophenone.
C Hz =CH-C H2-0-C-O-C
H2-C H2,
CH2=C H-C H2-0-C-0-C
C R 39-Monomer
( CH3)2C H-O-C-O+-C-O-CH
Angrn.. Ciirm. Inf Ed. Enyl. i X , 49-59 (19791
(CH, ),
The absorber diffuses a few pm into the surface where
it is retained so strongly that it cannot be removed by
The depth of penetration is controlled by the nature of the
substituent(s) at C-4 (and C-4’), and the position of the UV
absorption edge by the number and position of the O H groups.
This is a very apt example of “doubly tailored molecules’’.
The advantage of this method lies in its versatility, speed,
and effective protective action (UV barrier), and also in the
saving in expensive UV absorber and avoidance of a certain
yellow tinge observed on bulk UV stabilization of optical
material. The process can also be applied to other plastics.
Choice of an inert solvent, which may not lead to swelling
of the plastic, is of critical importance; water is often ideal
for this
CR39 will probably retain its distinctive role mainly in
the case of correcting glasses, variations being possible by
copolymerization. Preliminary studies on epoxy resins have
revealed various advantages with regard to rates of curing
and coloring; a serious disadvantage is the sensitivity of the
surface to wiping which is greater than that of CR39 and
this has to be overcome by coating[35! Epoxy resin lenses
can be cast in plastic molds to which they do not adhere‘36!
3.2. Other Products‘”. 24, 3 7 , 3 8 1
3.2.1. Photographic Optics
The use of plastics [poly(methyl methacrylate), polycarbonate (which is increasingly replacing polystyrene), and polystyrene-acrylonitrile copolymers] in the image-forming optical
system of a camera is limited to simpler cameras. The imageproducing units in high-quality cameras remain a domain
of glass owing to the temperature dependence of optical properties. The viewfinder optics of even expensive equipment
is nowadays made almost entirely of plastic (lenses, Fresnel
lenses, etc.). Practically the only exception is the bulky pentaprism of single lens reflex cameras. The narrow tolerances
of the angles and the planarity of the surfaces are unattainable
with plastics. A possible solution could lie in the use of a
hollow prism.
A balanced mixture of plastic and glass is used in camera
production. However, a trend toward the use of plastics cannot
be denied, especially in “integral plastic optics” in which the
optical components as well as mounting and centering components are produced in one piece. A classical example is
the “Polaroid Land Super Swinger Camera”, in which the
transition to plastics is reported to have incurred cost saving
of up to 95 % for the optical system[161.Viewed realistically
savings of 30 % can be expected. It is of considerable significance that aspherical plastic lenses are very easy to produce;
just one such lens is said to replace several spherical glass
lenses in many applications. (Pro’s and con’s are compiled
3.2.2. Lighting Optics
Lighting optics is a field of application for plastics provided
that high temperatures are not involved, such as occur in
studio lighting[4!
3.2.5. Polarization Optics
Poly(viny1 alcohol) and poly(viny1 acetate) films colored
with iodine and stretched have long been in use[’l. Plane
polarizers made of plastic have also been described for the
infrared and ultraviolet spectral regi0ns~~~1.
3.2.6. Filter Optics
Apart from colored transparent plastic disks, photographic
filters are also made in sandwich form, consisting of a colored
gelatin or cellulose acetate filter between glass layers, e. g.
Kodak Wratten filters[401.The variety of spectral transmission
and the possibility of precise adjustment are considerable
(for recent improvements see Section 4).
Protective welding spectacles having the required high IR
absorption also belong in this Section. Again, plastics are
seeking to penetrate into a domain of glass. Cast PMMA
is used for protective glasses for electric welding, while injection-molded cellulose propionate is employed in those for
autogeneous welding. It would appear that only American
Cyanamid produces organic IR absorbers that can be used-to
a limited extent-in practice. Protective glasses for welding
are often coated with a transparent layer of gold to reflect
strong IR radiation.
Goggles affordingprotection against laser light are produced
from plastics by Glendale Optical Corp., USA. Dyes having
extremely high extinction coefficients are incorporated into
the plastics[41!
Narrow bandpass filters have been developed for the region
between 2000 and 3000A[421. For this purpose, organic
absorbers are dissolved in partly condensed methylpolysiloxane which is then cured. This plastic was selected as “host
material” owing to its high UV transmission.
3.2.7. Grating Optics
Original diffraction gratings are produced with extreme
precision by scratching of glass and metal and may bear
up to several thousand lines per millimeter. Plastic replicas
of the valuable originals are made, e. g. gelatine replicas, which
in turn serve as masters for the production of polyester replicas.
Styrene or methyl methacrylate may also be polymerized in
contact with the original gratings. The fidelity of reproduction
is excellent (see also Section 4).
3.2.8. Fresnel Optics
3.2.3. Strain Optics
Transparent plastics having various strain-optical constants
have proved of value[’’.
Fresnel lenses, prisms, and mirrors are produced both by
injection and by machining; these techniques are suitable
for plastics. The principal material used is poly(methy1 methacrylate).
3.2.9. Optical Cements
3.2.4. Signal Optics
Plastics (frequently PMMA owing to the resistance to weathering often required) have cornered a considerable market
and have replaced glass (e.y. automobile reflectors, warning
signs, traffic signs). The colored disks of traffic lights can
also be produced from polycarbonate by injection molding.
Good flow properties of plastics permit highly rational production methods.
The classical optical cement, Canada balsam, can be replaced by plastics. Presumably the epoxy resins already used
for this purpose will find wider use. Examples are cemented
glass filter packs. Such cements can be formulated as desired
in considerable variety from numerous combinations of resin
and curing agent. Their good adhesion to glass, attributable
to chemical reaction of the Si-OH groups of the glass surface
with the epoxy or OH groups of‘ the resin, can be further
Aiigew. Chem. Inr. Ed. Enyl. 18. 49-59 ( 1 9 7 9 )
enhanced by admixture of specific adhesion promoters (silicon
compounds having “glass-functional” and “resin-functional”
groups). On cementing lenses with non-thermoplastic cements,
the elegant possibility of “fine adjustment” in the warm state
is renounced.
with the core, and the difference between the refractive indices
n1 -n2 should be as large as possible for it determines the
aperture angle 2x0, according to
where no is the refractive index of the surrounding medium.
Although light guide optics, and especially fiber optics,
Apart from hard plastics used as contact lenses, particular
a domain of glass, the contribution of plastics can no
interest attaches to the hydrogels developed by W i ~ h t e r l e [ ~ ~ ’
be overlooked.
which permit the production of soft lenses[441.Crosslinking
- vd diagram (Fig. 3) shows the availability of several
copolymerization of hydroxymethyl methacrylate with, e.g.,
industrially accessible “pairs of plastics”. Thus
diethylene glycol dimethacrylate, reduces the water solubility
fibers (nd = 1.59)have been cladded with polyof the former to water swellability.
(methyl methacrylate) (nd= 1.49).
H3C. Q
Better known are the Crofon” light guide fibers manufacCH~=~-&-O-CH,-CH,
by Du P ~ n t ‘ ~ ’which
are produced from poly(methy1
H3C 0
I 1
extrusion and stretching. The
cladding is applied continuously in a coating bath. Since
the PMMA preferentially used as core material for optical
reasons already has a relatively low refractive index, none
Silicone rubber with a hydrophilized surface has also
of the mass-produced plastics can serve as cladding (with
recently been used as a contact lens
Its high
a necessarily lower refractive index). Use is therefore made
permeability to oxygen represents a considerable advantage.
of fluorinated esters of acrylic and methacrylic acid[491(see
Section 4.1).
3.2.1 1. Laser Optics
Sheath and core should not absorb nor scatter light, i.r.
Even though numerous laser effects of rare earth chelates
should be noncrystalline or only slightly crystalline. According
in solid solution are known, e.g. in PMMA, as are other
to Schreyerr41, scattering losses account for the major part
laser-active organic compounds (metal phthalocyanines and
of losses in plastic light guides. Absorption losses are effective
dyes), we have to assume that the significance of “plastic
mainly in the ultraviolet and the infrared spectral region.
lasers” made in this way will remain limited to special scientific
Optical contact is said to be good in Crofon@.
problems. Some 50 organic dyes provide overlapping coverage
The development of fibers for communications has led to
of the range between 3900 and 10000A, and this range of
further optimization of plastic light guides. Meanwhile, glassvariation is where their strength lies. Plastics have proved
glass communications fiber optics having extremely low losses
of value as “host material” for laser dyes, e . g . rhodamine
of only a few dB/km are available[50!
G in polyurethane f i l m ~ [ ~ ~ * ~ ’ ] .
Plastics cannot meet these extreme requirements, at least
as core material. Use of plastics as sheath material is considered
3.2.12. Aspherical Optics
in Section 4.1. Whereas CrofonO cables exhibit losses of 1100
dB/km, values of 470 dB/km have been reported for the new
While the production of aspherical glass lenses requires
PFX fiber produced by Du P ~ n t [ ’ ~ ] .
time-consuming working of individual components, an aspherPlastic light guides are used mainly for control and illuminaical steel mold will provide large numbers of aspherical plastic
lenses. Theeconomicadvantagesofplasticlenses become particPlanar
waveguides made of plastics have proved of interest
ularly significant with very large lenses (680 mm diafor
optical systems[461.To this end, thin films are
meter)[’‘. 281.
produced from solution or by plasma polymerization.
Although the energies in the plasma are so high that only
3.3. Light Guide Optics
relatively inert structures have a chance of survival, this underinvestigated field would appear to offer numerous possibilities.
In light guides, a rod or fiber of transparent material with
Alkylpolysiloxanes and fluorinated polymers have been exama high refractive index (nl) is surrounded with a transparent
ined in more detail; they afford glass-clear films of low
cladding of lower refractive index (nz). Light falling on the
end surface is conducted through the guide as a result of
3.2.10. Contact Lenses
total reflection (Fig. 6).
4. Glass-Plastics Combinations for Optical Applications
Fig. 6 . Principle of light guide optics
In order to ensure high light transmission and large aperture
angle, the core material should have excellent transmittance,
the likewise transparent coating should be in excellent contact
Aiigc,w. Chem. I n t . Ed. Engl. 18. 49-59 (1979)
4.1. Light Guide Optics
4.1.1. UV Fiber Light Guides
The fundamentals are presented in Section 3.3. Fiber optics
have long been used for conducting visible light; until recently,
however, conduction of UV light was possible only by classical
methods using lenses and prisms made of silica glass. UV
fiber optics cannot be made from combinations of glass with
glass or plastic with plastic; only glass-plastic combinations
offer this possibility.
The only material transparent down to wavelengths of
200 nm that can be drawn out to fibers is silica glass having
refractive indices of n546 = 1.460,11365= 1.475,and n25 = 1.506.
A glance at Figure 3 shows that there is no glass with a
sufficiently low refractive index to serve as a possible cladding
material, while plastics have such refractive indices. Since
this cladding material should also be UV transparent and
nonscattering, two readily accessible materials appear likely
candidates from Figure 1, uiz. methylpolysiloxane and Teflon
FEP: in more general terms, alkylpolysiloxanes and extensively, if possible completely, fluorinated polymers insofar as
they are glass-clear. The wavelength dependence of the refractive indices of Teflon FEP is shown in Figure 7L521.
loss fibers for transmission of information by the same prin~ i p l e [ ’ ~581.
- The light to be conducted is usually 800 to 1060nm radiation from a laser source; the excellent transparency
of certain silica glasses is exploited and a low-refractive-index
material is therefore required as coating; the same problem
has already been solved in the case of UV fiber optics Accordingly, the same plastics (fluoropolymers and alkylpolysiloxanes) are described as cladding for these (step index) communications fibers. Other plastics are unsuitable. Examples are
shown in Figure 9.
C H z=CH F
Vinyl fluoride
Vinylidene fluoride
Perfluorovinyl methyl
rnethacry late
1. LO
Fig. 9. Fluorinated monomers for optical cladding.
133 L
2 lnml
Fig. 7. Refraction by a Teflon film as a function of wavelength.
O n this basis, JENAer Glaswerk Schott & Gen., Mainz,
produced UV fiber optics[”
which provided the first
means of conveniently handling UV light, and represent a
new tool for use in medical technology, medicine, and
numerous technical and scientific applications. The spectral
transmittance is shown as an essential property in Figure
8. Further information will be found in company literature[’ l . 531 which also gives details concerning development.
To our knowledge the resulting fibers having exceptionally
low losses of a few dBjkm are not in technical use. We can
expect considerable investment in scientific and technical development in this “hotly contested field in the near future.
The two classes of compounds are fixed; however, variations
are possible within these classes.
In conclusion, it should be mentioned that glass-plastic
combinations are not essential in this field. The best prospects
exist for glass-glass systems, in the form of gradient fibers.
It is interesting to note that after achieving 470 dB/km with
their PFX fiberL5’], Du Pont has switched to the glass-plastic
area and so far achieved 80dB/km[591. The core of these
fibers consists of undoped silica glass and the cladding of
an unnamed polymer. The price of this PFX-S-108-R02 cable
is presently 5 dollars per meter.
4.2. Grating Optics
Fig. 8. Spectral transmittance of flexible U V light guides made of silica glass
coated with Teflon FEP. Length of light guide: (a) 500mm. (b) 1000mm,
(c) 1800mm.
4.1.2. Communications Fibers
After this, to our knowledge the first, application of coatings
made of transparent plastic on glass core material‘’ *’ 31, developments were reported which were aimed at producing low-
The reader is referred to Section 3.2.7 for a description
of the production of plastic gratings. Owing to the high thermal
expansion of plastics, the line separations are dependent upon
temperature. Some glass ceramics show practically no thermal
expansion over a wide range of temperatures. If a drop of
monomeric diethylene glycol diallyl biscarbonate is polymerized between a block of a glass ceramic and an original glass
grating (Fig. 10)one obtains a plastic grating which is likewise
not subject to expansion. In order to separate the original
grating from the replica, the former is “siliconized”, i. e. treated
with methylchlorosilanes, prior to copying (Fig. 11). This procedure is rendered possible by the Si-OH groups of the
glass. These groups are also responsible for the very firm
adhesion between the plastic and the glass ceramic by copolymerization of “grafted” vinyl groups (Fig. 1 1)[601. There results
a plastic lattice some km thick which is chemically bonded
to the substrate.
Aiiyerr Chem. Iirt.
E d . Enyl. 18. 49-59 ( 1 9 7 9 )
catalysts, and polymerized between two glass sheets separated
by spacer strips. Owing to the ready oxidation of many dyes,
azodiisobutyronitrile is used as initiator in place of the usual
peroxides. These composites produced as large sheets are
so firm, that filters can be produced with conventional glassworking machines[621.Thus maximum planarity and scratchresistance are ensured, as is resistance to climatic, and even
tropical, conditi~ns[~~~-properties
not found with plastic
On the other hand, plastics offer ideal means of introducing
absorbing materials which are impossible with glass. For
Fig. 10. Bonding of a grating of polymeric diethylene glycol diallyl biscarbonate
to a glass support. 1 =glass block, 2=vinylation of the surfaces, 3=drop
of diethylene glycol diallyl biscarbonate, 4 =original glass grating with a
methylated surfwe, 5 =polymerization of the drop after spreading, 6 =plastic
grating bonded to the glass block.
Carrier glass
CH3-C 0-0
fast binding
no adhesion
Fig. 1 I . Polymerization of diethylene glycol diallyl biscdrbonate (“ally1 carbonate”) between the vinylated surface of a glass support and the methylated
surface of a glass grating (schematic).
This exampler6”, which is not employed in practice, illustrates the chemistry of a glass-plastic composite and its potential.
example, a series of extremely sharp-edged UV suppression
filters can be produced with extremely narrow tolerance with
regard to the wavelength of the absorption edges, and having
very low intrinsic fluorescence (Fig. 13)[641.
4.3. Filter Optics
Optical filters for photographic or scientific purposes can
be made from glass or plastic. Glass filters generally meet
all the optical requirements made of them and stability requirements. Certain limitations d o apply to their spectral scope,
particularly if other properties are desired simultaneously,
e. y. lack of fluorescence.
Plastics have almost unlimited spectral possibilities, but
are less robust in use and prone to scratch. Moreover, plastic
filters cannot be produced with the same high degree of planarity as ground and polished glass filters.
Fig. 12. Glass-plastic composite filter. A =cover glass, B = plastic composite
layer, YTY high polish.
2InmlFig. 13. UV suppression filter. K V 370 to KV 418 signify the values of
internal transmission =0.50.
A glass-plastic composite filter as shown in Figure 12 combines the advantages of both materials. In their production,
methyl methacrylate is treated with soluble dyes, silane adhesion promoters of the kind described in Section 4.2, and
A n y m Chem. I n t . E d . Enyl. 18, 49-59 ( 1 9 7 9 )
Such combinations are accordingly suitable as UV suppression filters, as fluorescence suppression filters, as filters for the
study of excitation conditions, and as conversion
While plastic is protected by glass in these filters, we shall
now consider cases in which inorganic material requires protection by plastic. This is the case with IR-transparent materials
such as single crystals or polycrystals of alkali or alkaline
earth halides. These products, which are often very expensive,
are sensitive to moisture. They can be protected by coatings
which, however, must also be IR-transparent.
For example, a polycrystalline lithium fluoride disk, which
is transparent down to about 5-6 pm acquires excellent protection against water from a film of Teflon FEP just a few
pm thick, which is applied as a dispersion, without losing
Films applied in this way enclose
its IR
LiF disks so well that they can be kept for years under
water. The protective layer of Teflon FEP can be also applied
by sputtering (see, e.g. [671). As a rule, however, this leads
to far-reaching structural changes.
While thanking many colleugues f o r their kind support, I
would like to mention two co-workers specifically. Karl-Heinz
Wiesner hus promoted the entire development of plastics tliunks
to his dedicated engagement and understanding, and Paul Hinz
has done the samefor the unorthodox glass synthesis und the
coating processes. The author wishes to express his particulur
thanks to them and their co-workersfor more than ten years
of cooperation.
Received: January 23, 1978 [A 254 [ E l
German version: Angen. Chem. 91, 52 (1979)
4.4. Insulating Windows
It may appear surprising to find mention of windows in
an article about optical materials. And yet, in these days
of rising energy prices, a window no longer merely fulfills
the function of mechanical protection against the whims of
nature through which light falls and people can look out.
Its protective function has meanwhile also become optical
in nature, e. g. sunshielding glasses in high-rise buildings which
reduce the load on air conditioning plant on sunny days
or conversely prevent the escape of longwave IR radiation
in winter. Reflecting coatings best serve both purposes, e. g.
layers o f TiOz at interference layer thicknesses or of metals
such as Au, Ag, C U [ ~ Of
~ ] .course, this is a domain of glass.
Triply-glazed windows, of which one pane may also bear
a reflecting coating, have very low heat transfer coefficients
(k values). The third pane, which leads to considerably greater
weight and greater thickness of the overall system, can be
replaced by a coated plastic film. T h i s a doubly-glazed window
is transformed into a triply-glazed window without any significant increase in weight; coating of the film and carefully
chosen gas filling of the intermediate spaces leads to very
low heat transfer and-where
desired-affords protection
against sunlight[h9! Table 1 shows the astonishing extent to
which the k value of such a “film window” can be reduced.
The k value of 0.65 kcal m- h- K corresponds to a 0.5-m
solid brick wall.
Table 1 . k values or window glazing.
Kind of glazing
k [kcal m
Single glazing
Double glazing ( I 2 mm air)
Triple glazing (2 x 12 mm air)
Double glazing (12mm air) (gold)
Double glazing (12 mm krypton) (gold)
Film window (metal) (2 x 6 mm air)
Film window (metal) (2 x I2mm air)
Film window (metal) (2 x 6 m m krypton)
Film window (metal) (2 x 8 mm krypton)
I .46
h-’ K - ’ 1
Such windows have been made on an experimental basis;
however, many questions still surround their use in practice.
It was the author’s intention to close a rather sober account
of plastics in optics with a somewhat futuristic section, in
order to show that plastics can often provide solutions for
freshly occurring problems in spite of all their limitations.
It is not the material itself which should be placed in the
foreground, but the solution of problems. The question of
the material-plastic or glass-is admittedly important, but
nevertheless secondary.
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Physik, Chemie. Astronomie, Geophysik und Technik. 6th Edit. Springer, Berlin 1957, Vol. 4. Part 3, pp. 925ff.
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Preprints Vol. I
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Int. Ed. Engl 12, 439 (1973).
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& Gen.
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& Gen.
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H. Sckriider, Opt. Acta 9, 249 (1962).
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Glaswerk Schott & Gen.
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[41] A . E . Sherr er a / ,AD-A00951 1 , National Technical Information Service,
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[45] P. Frweberg, Suddtsch. Optiker-Ztg. 2, 1 (1976).
[46] H . P. Webrr, W J . Tomlinsnn, E. A . Chandross, Opt. Quant. Electronics
7, 465 (1975).
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Sci 5, 373 (1975).
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[52] W G[,flckrn, personal communication.
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dienyl)ferriostannanes['] and some cyclopentadienyl compounds of divalent and tetravalent tinr21. We have now been
able to isolate for the first time such an ion as the tetrafluoroborate and to elucidate its structure by X-ray crystallography.
Treatment of bis(pentamethylcyclopentadieny1)tin ( 1 )['I
with tetrafluoroboric acid leads to formation of pentamethylcyclopentadiene and the salt (CH3)SCSSn+BF: (2), whose
cation exists in the form of an axially symmetric pentagonal
bipyramidal nido-cluster :
Synthesis and Structure of the nido-Cluster
By Peter Jutzi, Franz Kohl, and Carl Kriiger[*]
Species of the type R5CsSn+ have previously only been
observed in the mass spectra of dicarbonyl(q 5-cyclopenta[*I Prof. Dr. P. Jutzi. DipLChem. F. Kohl
Institut fur Anorganische Chemie der Universitit
Am Huhland, D-8700 Wiirzburg
Dr. C. Kruger
Max-Planck-lnstitut fur Kohlenforschung
Lembkestrasse 5, D-4330 Miilheim-Ruhr 1 (Germany)
[**I Thls work was supported by the Deutsche Forschungsgemeinschaft
and the bonds der Chemischen Industrie.
Angew. Cheni. Int. Ed. Engl. 18 ( 1 9 7 9 ) No. 1
Compound (2), which is isolable as colorless crystalline
needles, is air- and moisture-sensitive and rapidly turns violet
on exposure to daylight. On the other hand, no decomposition
is observed over several weeks when it is stored at -20°C
and in the dark. (2) is readily soluble in strongly aprotic
solvents (acetonitrile, nitromethane, dichloromethane). 'HNMR (in CH2CI,): 6CH3=2.15 (s); "B-NMR (in CD3CN,
ext. BF3.0Et2): 6BF,.= -0.53 (s); I9F-NMR (in CD3CN,
ext. CFC13): 8BF; = 146.8 (s). Conductivity (in CH3N02,
20°C): A = 127.4 R - ' cm2 mol-' (1.035 x
M) [comparative measurement on C6H5CH2N(C2H5):CI- : A = 130.5 W '
cm'mol-' (1,010x
M)]. Molecular weight determination
(cryoscopic in HMPT): 175 [ M , for (2) = 340.71.
The orbital and electron requirements for clusters are predicted from MO considerations and established rules[31.The
new cation in (2) can be placed quite neatly in the series
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