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Macromolecules with Non-uniform Solubilities Ц Heterogels and Heteropolymers.

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Macromolecules with Non-uniform Solubilities - Heterogels
and Heteropolymers [*]
BY PROF. DR. CH. SADRON
CENTRE DE RECHERCHES SUR LES MACROMOLfiCULES (CNRS), STRASBOURG (FRANCE)
It is possible to produce chain-like macromolecules A-B, with A and B parts which have
different solubilities in a given solvent. In solvents, depending on the concentration, the
insoluble parts of these non-uniformly soluble macromolecules form micelles, aggregates,
cylinders, or layers, while the soluble parts and the solvent fill up the gaps. The structure
of these heterogels is destroyed by stirring or heating, but it can be fixed by polymerizing
the solvent.
1. Introduction
If a chain-like copolymer derived from monomers a
and b contains only short stretches in which several
units of either a or b are linked together in sequence,
then the molecule behaves practically like a homogeneous chain. If, however, the copolymer contains long
parts that are built up alternately. from only a or b,
its properties can be described, more or less accurately,
by a combination of the properties of the homopolymers
a and b.
It is interesting to see how the properties of such a copolymer vary as a function of the length and statistical
distribution of the homogeneous sequences (a)i and (b)j.
It can be assumed that a molecule with two long sequences a-a-a ....a-b....b-b-b, - in brief, A-B, - possesses
solubility characteristics in each particular range which
are similar to those of a chain made up from pure a or b,
respectively. Part A of the copolymer will therefore behave towards a solvent S just like a homopolymer made
up of a, and part B just like a homopolymer made up of
b. The molecule A-8 has therefore heterogeneous solubility properties and is described here as being “nonuniformly soluble”.
between the molecules of the solute and of the solvent as well
as by temperature. Generally speaking, for very good solvents, i.e. for loosely tangled macromolecules, E has values
of the order of J/3. There is a temperature for each solvent a t
which E equals zero.
2. Micellar Solutions a n d Heterogels
Classical soaps are examples of molecules with nonuniform solubility. The polar end-group is readily soluble in water, for instance, while the hydrocarbon
residue is not.
If part A of a copolymer A-B is soluble in a solvent S,
and part B is insoluble, and if A forms only a small proportion of the macromolecule, the copolymer A-B will
be insoluble in S. Solubility can only be attained by increasing the ratio mA/mB. In other words, solubility
depends on the relationship between the total mass
MA-B of the molecule A-B and the masses of its parts
mA and mB. If A-B is completely soluble in S and if the
two parts of the molecule behave effectively as if they
existed independently, the soluble part A should form
a loosely tangled ball and B a more or less compact
nucleus (Fig. 1).
In the following, only molecules with two sequences
(A-B) will be dealt with. Binary copolymers with more
complex structures, e.g. with three or four sequences
(A-B-A or A-B-A-B), have also been investigated, but
they are not essentially different from copolymers of
the type A-B.
The most likely shape for an open-chain molecule in solution
is that of a tangled string that is looser in a good solvent and
tauter in a bad one. Theory and the interpretation of experimental results give the following relationship for the
mean square of the distance between the ends of a long
flexible chain-like molecule of mdSS M :
M
1 + C
?f = b z (m)
Fig. 1. Monomolecular micelle of a chain molecule with non-uniform
solubility. The less soluble part of the molecule (thick line) forms the
conipact nucleus of a loose tangle of the more soluble part of the
molecule (thin line).
M
The value of b depends on the length of the units of the
macromolecule, m is the mass of these units, E is determined
by the second virial coefficient of the solution. The value of E
is therefore influenced by the thermodynamic interaction
[*] Lecture given at the XIVth International Plastics Congress,
which took place together with the 12th International Engineering
Exhibition, from Sept. 27th to 29th, 1962 in Turin (Italy).
248
Monomolecular micelles of this kind can only be observed at high dilutions, as the insoluble parts of several
molecules tend to associate with increasing concentration. Above a certain concentration which depends on
the chemical nature of A, B, and S and on the ratio by
weight MA-B :(mA/m,,), aggregates are formed in
Angew. Chem. internat. Edit.
Vol. 2 (1963) / No. 5
solution. These consist of nuclei which are formed from
the B parts of several molecules and which are surrounded by loose tails of A parts. The suspension thus
consists of multimolecular micelles.
If the concentration is still higher, the aggregates organize
themselves in a regular structure. The insoluble parts B
form parallel layers or parallel cylinders of homogeneous
material, dispersed in an interstitial medium lormed by
the solution of A in S (Fig. 2). In general one observes
cylinders (Fig. 2a) at lower and layers (Fig. 2b) at higher
concentrations. Both structures are actually mesomorphous gels.
The solubilities of the polyoxyethylene and the polystyrene chains differ in most solvents. If, for instance,
80 parts of a copolymer consisting of 41 % polystyrene
(A) and 59 % polyoxyethylene (B) (aggregate molecular
weight: 13600) are dissolved in I00 parts of butyl phthalate, which is a good solvent for polystyrene, a gel with a
layer structure is formed at room temperature. The polyoxyethylene parts, which are only slightly soluble in
butyl phthalate, form crystalline layers, 105 A thick.
The polystyrene parts and the solvent are interspersed
in layers 110 A thick.
If 60 parts of this copolymer are treated with 100 parts
of nitromethane as solvent at room temperature, a mesomorphous gel of parallel cylinders is formed. These
cylinders consist of the polystyrene parts of the copolymer, which are insoluble in nitromethane, while the
space between them is occupied by the solution of the
polyoxyethylene parts in nitromethane (Fig. 2a). The
distance between the axes of any two cylinders is 163 A,
the diameter of the cylinders is 90 A.
In both cases, therefore, heterogels are obtained in the
form of highly viscous liquids. As can be seen in the
polarization microscope (Fig. 3), they are made up of
Ibl
Fig. 2. Structure of a copolymer of type A-B made from polystyrene
and polyoxyethylene (a) in nitromethane (cylindrical structure) and (b)
in butyl phthalate (layer structure). Nitromethane dissolves preferentially the polyoxyethylene part, butyl phthalate the polystyrene part.
polystyrene part
_ _ _ _ -polyoxyethylene
_
part
solvent.
o
Thus, with increasing concentration, the following
shapes arise in succession : monomolecular micelles,
multimolecular micelles or aggregates, organized cylindrical structures, and finally organized layer structures.
At some concentrations, several shapes can exist side
by side. Moreover, their succession is influenced by
temperature.
An example of a copolymer A-B with non-uniform solubility is the linear macromolecule consisting of a polystyrene (A) and a polyoxyethylene chain (B). The degree
of polymerization of A and B can vary between some
tens and several hundreds. The reaction takes place according to a process described by Richards and Szwurc
HI.
Polystyrene is prepared by anionic polymerization at -80 "C
in homogeneous phase, [solvent: tetrahydrofuran; initiator:
phenylisopropylpotassium ( r ) ] . A chain terminating in a
carbanion is obtained (2). On addition of ethylene oxide, (3)
is formed first. The solution is then heated in small portions
under pressure to 80 "C for 48 hours.
Fig. 3. Anisotropic micro-ranges of a liquid heterogel under the
polarization microscope.
small, markedly anisotropic ranges. The X-ray diagram
of this type of heterogel shows a black coloration at the
centre, which can be ascribed to the presence of small
ranges each with a uniform layer or cylindrical structure
[3-51.
3. Heteropolymers
a) Non-ionic Heteropolymers
HsC6-C(CH1)2-K
- CHz--CH(CsHs)-K
-
,*
8
-CHZ-CH(GHS)
+ KO
CH~-CH(C~H~)-CHZ-CHZ-OK
11)
(2)
13)
The copolymer obtained is washed in order to remove any
homopolymer formed by a side reaction.
This process can also be applied to the preparation of
copolymers of the type B-A-B. An indirect method [2] has to
be used for the preparation of A-B-A and (A-B), types.
[l] D . H. Richards and M . Szwarc, Trans. Faraday Soc. 55, 1644
(1959).
[2] G. Finaz, P . Rempp, and J. Parrod, Bull. SOC. Chim. France
1962,262.
Angew. Chem. internat. Edit. VoI. 2 (1963)
/ Nu.5
Mesomorphic heterogels areunstable. Agitation or heating destroys their structure. If the solvent is replaced by
a polymerizable monomer and the latter polymerized,
the gel's strucLure is fixed. A solid polymer of high
stability is formed, with the heterogeneous character of
[3] A. Skoulios, G. Finaz, and J. Parrod, C. K. hebd. Seances
Acad. Sci. 251, 739 (1960).
[4] A. Skoulios and G . Finaz, C. R. hebd. Stances Acad. Sci. 252,
3467 (1961).
[5] A. Skuulios and G. Finaz, J. Chlrn. physique 1962, 473.
249
a system A-B/S. After polymerization of S, the polymer
contains as inclusions the micelles, aggregates, cylinders,
or layers that had existed in the gel before polymerization. A heteropolymer with organized structure [6] is
thus obtained.
Some examples are shown in Table 1. Experience shows
[7] that polymerization does not cause disintegration of
the gel structure, but only a minor alteration of the
structural parameters.
Interesting results were obtained by combination of the
potassium salt of 10-(p-ethy1phenyl)undecanoic acid
with styrene [8,9]. The styrene was added to the soap at
15 “C until saturation was reached. A band structure,
made up of the carboxyl groups, was obtained (Fig. 4).
Table 1. Heteropolymers formed from a copolymer of 41 % polystyrene
and 59 % polyoxyethylene. With the exception of acrylic acid, the
monomeric solvent dissolves preferentially the polystyrene part.
Monomeric solvent
Gel structure
Polymerization
initiated by
Methyl methacrylate
Isoprene
Vinyl acetate
Acrylonitrile
Propylene oxide
layer
layer
layer
layer
layer
Acrylic acid
cylindrical
ultraviolet light
BF3
ultraviolet light
ultraviolet light
ultraviolet light plus
azo-bis-isohutyronitrile
ultraviolet light
If, for example, 32 parts of the polystyrene~polyoxyethylene
copolymer are mixed with 100 parts of acrylic acid, which
preferentially dissolves the polyoxyethylene part, a gel with
cylindrical structures is formed. The cylinders consist of the
polystyrene parts, the intervening space is filled by the
polyoxyethylene/acry1ic acid mixture. If a thin layer of this
gel is irradiated between two mica plates with ultraviolet
light, after addition of some azo-bis-isobutyronitrile as
photosensitizer, solidification of the layer takes place within
about 48 hours, without any change in its mesomorphous
structure (diameter of cylinders 105 A, inter-axial distance
between neighboring cylinders 152 A).
The cylindrical structure has thus been fixed. It is now
stable to mechanical wear and also resists temperature
changes. If the sample is heated, the structure is retained
up to about 250 OC,where it begins to melt and decompose. I t appears that the structure is reformed on cooling, in part at least. Generally speaking, large structures
fixed by the polymerization are less stable than fixed
cylindrical structures.
b) Ionized Heteropolymers
A molecule A-B with non-uniform solubility can also
consist, for example, of a non-ionic part A, a hydrocarbon chain, and an ionizable part B which may be, for
instance, polyacrylic acid, polyphosphoric acid, or sulfonated polystyrene. If a monomer is chosen as solvent
S in which A is soluble and B is insoluble, the relationships will resemble those in non-ionic heteropolymers.
An example is a soap molecule, consisting of a long fatsoluble chain - even if this is relatively short, compared
with the parts of non-ionic copolymers - in which the
ionized part is confined to a single group, i. e. the carboxyl group.
[6] Ch. Sadron, Chimie Pure et Appliquee 4, 347 (1962).
[7] G. Finaz, A . Skoulios, and Ch. Sadron, C. R. hebd. Seances
Acad. Sci. 253,265 (1961).
250
Fig. 4. Band structure of a soap.
o
-___
carboxyl group
hydrocarbon residue.
The intervening space was taken up by the hydrocarbon
chains and the styrene. This structure persists after polymerization of the styrene, as is shown by its X-ray diagram (Fig. 4). The product is white and intensely hygroscopic.
4. Conclusions
Two types of heteropolymers can be distinguished, viz.
isotropic heteropolymers, i.e. homopolymers in which
a multiplicity of insoluble small nuclei are included, and
anisotropic or organized heteropolymers with visible
micro-ranges which possess sub-microscopic layer or
cylindrical structures. The structures of the compounds
investigated were completely stable.
The method discussed above makes it possible to prepare
solid high molecular-weight polymers in which particles
are dispersed, the material, shape, size, and concentration of the dispersate being variable within wide limits.
Heteropolymers with anisotropic lamellar or cylindrical
structures are distinguishable by their dielectric and
optical properties. It would be a great advance if it became possible to enlarge the zones of uniform structure,
by analogy to seeding of single crystals. Furthermore,
the construction of organized polymers with an “insoluble” ionized phase might result in electrolytic condensers or i n amplifiers with special characteristics.
Beyond that, the construction of very thin threads or
films or of porous substances might be conceivable.
Heteropolymers might perhaps also be used as the basis
for explosives or solid propellants.
Received, November 22nd, 1962
[A 275/78IEI
[8] F. Husson, J . Herz, and V. Luzzati, C. R. hebd. Seances Acad.
Sci. 252, 3290 (1961).
[9] J. Herz, F. Husson, and V . Luzzati, C. R. hebd. Seances Acad.
Sci. 252, 3462 (1961).
Arigew.
Claem. interticit. Edit. I Vol. 2 (1963) / No. 5
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