Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to porous membranes made from
aliphatic thermoplastic polyamide materials.
Synthetic polymeric membranes are used for separation
of species by dialysis, electrodialysis, ultrafiltration,
cross flow filtration, reverse osmosis and other similar
techniques. One such synthetic polymeric membrane is
disclosed in Australian Patent Specification No. 505,494 of
Unisearch Limited.
The membrane forming technique disclosed in the
~nisearch Patent is broadly described as being the
controlled uni-directional coagulation of the polymeric
material from a solution which is coated onto a suitable
inert surface. The first step in the process is the
prieparation of a "dope" by dissolution of a polymer. This
is said to be achieved by cutting the hydrogen bonds (which
link the molecular chains of the polymer together) with a
solvent. After a period of maturation, the dope is then
cast onto a glass plate and coagulated by immersion in a
coagulation bath which is capable of diluting the solvent
and annealing the depolymerised polymer which has been
used. According to the one example given in this
specification, the Rdop~ consisted of a polyamide dissolved
in a solvent which comprised hydrochloric acid and ethanol.
In another membrane forming technique, the liquid
material out of which the membrane is cast is a colloidal
suspension which gives a surface pore density that is
significantly increased over the surface pore density of
prior membranes.
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According to that technique, a thermoplastic material
having both relatively non-crystalline and relatively
crystalline portions i5 dissolved in a suitable solvent
under conditions of temperature and time which cause the
relatively non-crystalline portions of the thermoplastic
material to dissolve whilst at least a portion of the
relatively crystalline portion does not dissolve but forms a
colloidal dispersion in the solvent. The colloidal
dispersion and solvent (i.e. the "dope") is then coated onto
a surface as a film and thereafter precipitation of the
dissolved thermoplastic portion is effected to form a porous
membrane.
Such aliphatic polyamide membranes suffer from
disadvantages which limit their commercial usefulness and
applicability. For example, they exhibit dimensional
instability when drying and may shrink by up to 7%0 Thus,
it is essential that they be kept moist prior to and after
use. Furthermore, it has not been possible to generate
chemical derivatives of the membrane matrix which restricts
the situations to which the membrane may be applied.
Another disadvantage is that such polyamide membranes
are fundamentally unstable and eventually become brittle on
storage. The instability has been carefully investigated by
I.R. Susantor of the Faculty of Science, Universitas
Andala~, Padang, Indonesia with his colleague Bjulia~ Their
investigations were reported at tha ~Second A.S.E.A.N. Food
Wàste Project Conference", Bangkok, Thailand 11982) and
included the following comments regarding brittleness:
"To anneal a membrane, the thus prepared membrane
(according to ~ustralian Patent ~o. 505,494 using Nylon 6
yarn) is immersed in water at a given temperature, known as
the annealing temperature, T in degrees Kelvin. It is
allowed to stay in the water a certain length of time,
called the annealing time. For a given annealing
temperature, there is a m~ximum annealing time, t~b) in
minutes, beyond which further ann~aling makes the membrane
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brittle. Plotting lN l/t~b) versus l/T gives a straight
line. From the slope of this line it can be concluded that
becoming brittle on prolonged annealing is a pro~ess
requiring an activation ener~y of approximately 10.4
kilocalories/mole. From the ma~nitude of this ~ctivation
energy, which is of the order of van der Waals forces, the
various polymer fragments are probably held together by
rather strong van der Waals forces or hydrogen bond(s)."
We have confirmed that the brittleness is due to a
recrystallization of water-solvated amorphous polyamide. In
some cases (such as polyamide 6) brittleness occurs within
48 hours of immersion in distilled water (pH7) at 80C.
Colorimetric -NH2 end group analysis has shown that there is
no significant hydrolysis of the amide groups during this
time. As would be expected, the rate of embrittlement is
catalysed by dilute acids teg: pH of 1.0) due to nitrogen
protonation and subsequent solvation. This effect explains
the apparently low a~id resistance of the polyamide
membranes. However colorimetric determination of both -NH2
end groups and -COOH end groups has shown that the effect is
due to crystallization rather than acid catalysed
hydrolysis.
That most of the brittleness is due to physical effects
rather than chemical decomposition or chemical solvation (at
least for dilute acids) is shown by the extreme
embrittlement caused on standing 5 minutes in absolute
ethanol.
The problem of crystallization of the aliphatic
polyamide material can be overcome by cro~s-linking portions
of he polyamide through the rea~tion of a bis-aldehyde with
the membrane matrix as is described in our Canadian Patent
Application 507,896, filed April 29, 1986 "Cross Linked Porous Me~branes .
~ owever, the chemical derivatives of such
cross-linked polyamide membranes are limited to those which
can be prepared in water and thus those membranes can not be
used to provilde derivatives which do not lend themselve~ to
agueous syn~hesis su~h as the ester of ~-hydroxybenzaldehyde.
Furthermore, the density of derivatives prepared in water
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may not be as large as desired. For example, the up-take of
resorcinol in the glutaraldehyde ~ro~s-l inked membrane of
example 2 of our above m~rltioned Cana~.an Patent
Appli~:ztion was only 0.2~ ~f the dry weight of the membrane.
S It i~ an ob~ec~ of this invention 'co provide aliphatic
polyamide porou~ membranes whi~h lend them~elve to the
preparation of chemical d0rivatives which ~re not readily
~v~ilable by agyeou~ ~ynthe~i~ and to increa~ea density of
derivatives which o~herwi~e may be prepare~ ln water.
According to the invention there i~ provided a
polymeric porous membrane compri~inq a ~e~brane matrix made
from an ~liph~ti~ thermoplastic polyamide ~aterial which has
both relatively non-~ry~talline and relatively ~ry~talline
portion~ ~oined together by relatively non-erystalline
portions charac~eri3ed ln that at least some of the
relatively non-erys~alline portions of ghe masnbrane are
reacted with an a~id halide of ba~i~ity above one to provide
within the membrane the following type of ~tructures:
X
--C~2 ~ CH2 --
o
where X i3 the a~id radical of the Acid halide of ba~icity
abo~e one.
Preferably, th~ acid halide i~ deriYed from an ~romatic
carboxylic a~id or an aromatic derivative of a
chloro~ ne. ~ar~eu~rly preerred acid h~lld~ are
terephth~loylchlori~e, isophthaloyl~hloride, ~nd the
reaction product of an exc~s~ o a dichloro~ilane with a
diph~nol.
The utll:ity of the acid halide tre~t~d polyamide
membrane~ of l:he invention may be further improved by
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reacting the free end of the terminal acid halide chain with
a phenol of basicity above one (such as resorcinol) so that
the free end of the acid halide becomes a phenol ester as
follows:
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where R is a substituent consisting of or containing at
least one phenolic group.
Apart from resorcinol, the phenolic component may be a
phenol derivative such as 2,2-bis(4-hydroxyphenyl)propane.
The resultant phenolic ester may be cross-linked by
reaction with an aldehyde such as with glutaraldehyde. A
small amount of formaldehyde may be additionally used as the
final aldehyde link particularly if free ends are further
reacted with resorcinol. The original polyamide membrane
thus becomes a block co-polymer of polyamide/aromatic
polyester/phenol-aldehyde, all with little effect on the
ori~inal porosity3
The terminal aromatic acid chloride intermediate
membranes are particularly useful to foxm derivatives so
that the membranes can react with bioloyical products such
as -NH2 or -COOH terminated proteins. The resulting
products may be used to isolate pure products by affinity
c~romatography.
The invention also provides a method of preparing a
porous membrane which comprises the steps of:-
(i~ dissolving an aliphatic thermoplastic polyamide
which has both relatively non-crystalline and
relatively crystalline portions into an acidic,
hydrolytic solvent under conditions of
temperature and time which cause the relatively
non~crystalline portions of the polyamide to
dissolve while at least a part of the relatively
crystalline portions of the
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polyamide do not dissolve, but, Eorm a colloidal
dispersion in said solvent,
(ii) forming said colloidal dispersion and an acidic
hydrolytic solvent into a film and thereafter
causing precipitation of at least part of the
dissolved non-crystalline portions in the film to
form a porous membrane matrix, and,
(iii) reacting the membranes matrix with an acid halide
of basicity above one to provide within the
membrane the following types of structures:-
CH2 N _ Cl CH2 -
O
where X is the acid radical of the acid halide of
basicity above one.
EXAMPLE 1
An acidic~ hydrolytic solvent (A) was prepared by
mixing 225 ml of 6.67N hydrochloric acid with 15 ml of
anhydrous ethanol. 90 grams of 50 dtex 17 filament
polyamide 6 with lO9S twists per metre (which constitutes
the thermoplastic starting material) was added to solvent
(A) held at a temperature of 22C over a period of less than
15 minutes~
The dope of the poiyamide 6 and solvant (~) was then
left to mature for 24 hours at a temperature of 22C during
which the relatiYely non crystalline portions of the
polyamide 6 dissolved as did no more than ~0~ oE the
relatively crystalline portions of the polyamide 6 with the
remaining relatively crystalline portion dispersing in the
solvent.
After mat:uration, the dope was then spread as a film of
about 120 micron thick on a clean glass plate. The coated
plate was placed in a water bath where precipitation
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of the dissolved portions of the polyamide was effected
within 3 minutes. The membrane had a water permeation rate
of 330L/M2.h at 100 kPa pressure. It shrank 7~ on drying
and crystallized to a brittle sheet on heating at pE17 for 48
h~urs at 80C. It dissolved readily and completely in 7N
hydrochloric acid.
A sheet ~B) of the above membrane was dried at 60C to
constant weight. A portion of the sheet tB) weighing 144 g
was soaked in 1 L of petroleum spirit containing 20 g of
isophthaloylchloride as the acid halide and S0 g of powdered
potassium carbonate for 20 hours at 22C. Three independent
analyses (chloride ion formed, weight increase of the sheet
and loss from the petroleum spirit) showed that 18 g to 19 g
of isophthaloylchloride had reacted. This ~heet (C) was
washed with petroleum spirit and dried at 60C in dry aix.
A 35 g portion of membrane (C) was reacted with 0.lM
sodium carbonate for 15 minutes and 60C giving copious
carbon dioxide and chloride ion. The membrane was then
washed with water and gave a water permeation rate at 100
kPa pressure of 313L/M~h. After soaking in 2N hydrochloric
acid and washing the rate was 303 L/M2h and the membrane
- stained blue with methylene blue showing abundant free
carboxylic acid groups.
The rest of the sheet - membrane (C) - was reacted with
2L of aqueous solution containing 20 g resorcinol at p~ 9.0
with sodium carbonate to become membrane (D). Analysis
showed the up-take of 5.6 g of resorcinol. The membrane (D)
was washed and showed a water permeation rate of 150L/M2h at
100 kPa pressure. Membrane (D) was much more resistant than
the original membrane to 5N hydrochloric acid. A portion of
membrane (D) was stained deep orange by a solution of p-
nitro-benzenediazonium tetrafluoroborate showing the
presence of large amounts of resorcinol derivative.
An 88 g portion of membrane ~D) was heated for 4 days
at 60C with a 2.2~ w/v solution of glutaraldehyde at pH 4.0
and then washed to give a membrane (E). Mem~rane (E) showed
a 2.5% change in length when dried, then wetted. The
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permeation rate was now 272L/M2h. The ra~istance to acid
was improved. The presence of free -C~0 group~ ~a~ proven
by t~e inten~e violet formed with fuch~in - NaHS03
reagent. The latter te~t i8 due to the combination of the
-CH0 group~ in mrmbrane (E) with Na~S03 to form a
hydroxysulphonic acid derivative. A further indication of
copious -CH0 group~ wa~ the up-take of 2,4 -
dinitrophenylhydrazine from 3N hydrochlorie acid to form the
deep yellow 2,4 - dinitrophenyl-hydrazone.
1~ Each 100 g of dry original membran~ had ~equentially
ta~cen up 12.6 g isophthaloylchloride, ~4 g r~sorcinol aDd
4 . 0 g glutaraldehyde.
13XA~PLE 2
The procedures of EXAMPL~ 1 were ~epe~ted using
terephthaloylchloride ~8 the acid halide instead of
i~ophthaloylchloride and gav~ very ~imllar result3 e~cept
that at that ~tage (C) in ~XAMPI.l~ 1 th~ mambrane was
relatively ~tif f .
3 3
A solution of 2.97 g of 2,2 - b~s(4-hydro~cyphenyl)
propane in 10 ml of d~ pyridine was poured onto 3 . 22 g of
stirred dimethyldichlorosilane thereffl precipitating
pyrid1ne hydrochloride as wa~te.
Th~ acia chlorid~ ~o ~orm~d wa~, in effec~, a ~ilicon
analogua of an aromatic bis(acid chloride) a~ chemical
con~iderations ~ugge~t that it had the following ~tructure:
~l~Ma2)~i-o-c6 ~4~ctMe)2 C6~4 ~ 2
A port~on o~ the dry starting membrane (8) as in
EX~MPLE 1 was added to the resultant solution and heated 1
hour at 60C. The resultant sheet was washad ~n pyridine,
then ethanol, then methylenechlorlde and dried. The
permaation rate of N/10 caustic soda at 100 kPa prsssure
both before and after treatment wa 169L/MZh. One
unexpected difference was that
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the treatment doubled tannic acid ad~orption. The
explanation is that the treatment conferred silicic acid
derivative end-group8 on the membrane.
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