CA1049177A - Amphipathic copolymer dispersions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Concentrated fluid dispersions of amphipathic block or graft copolymer in a hemi-solvent liquid are converted to homogeneous solid masses by heating to a temperature above the glass transition temperature of the components of the copolymer which are insoluble in the hemi-solvent, and then cooling to below that temper-ature. The process is suitable for coating and potting techniques.

Description

10~9~77 This invention relates to concentrated, fluid dis-persions of amphipathic copolymer aggregates in hemi-solvent liquids, more particularly to a process for converting such dispersions into homogeneous solids without the intervention of arly chemical change.
In our Canadian Patent No. 908892, issued on ~ugust 29, 1972 there is described and claimed a process of preparing in a hemi-solvent a dispersion of substantially spherical aggregates . of an amphipathic copolymer which contains at least 5% by weight of each of two types of polymeric component of mole-cular weight at least 500 whieh have different solubility charaeteristies. The process comprises heating the amphipathie copolymer in the presence of the hemi-solvent to a temperature above the environmental glass transition temperature (the ; ~:
"environmental Tg") of the polymeric ccmponent of the copoly-mer which is insoluble in the hemi-solvent, the proportion of copolymer which is heated with the hemi-solvent being such - :
that on cooling to a temperature below the environmental Ty of the insoluble component the final mixture of eopolymer and :.
20 . hemi-solvent is completely miscible with added hemi-solvent ~ :~
liquid which is a good solvent for the polymerie component which ::
; is solu~le ln the hemi-solvent present in the final mixture. :~
The dispersions so obtained are proposed in Patent No. 908892 for use in the formulation of eoating compositions ` because of their generally low viscosity, due, it is believed, to the existence of the amphipathic copolymer in the dis-I persions in the form of stable, approximately spherical -, aggregates, in which the polymeric components which are insoluble in the hemi-solvent medium cluster together to form a core, this .
, ~ '"

i~ .
~ - 2 -, , , . . . :
: . : ,, 31L0~9~7~

core being surrounded by the associated soluble polymeric componen-ts o~ the copolymer. The dispersions are particularly suitable for use in coating compositions when the aggregated insoluble components of the copolymer are at temperatures below their environmental Tg, that is to say, when they are in the solid or glassy state, since under these conditions it is believed that the aggregates are not in equilibrium with individual copolymer molecules in the hemi-solvent, in the manner of micelles, but have stable, permanent identities.
It is an essential condition that, in preparing these dis-persions by heating the two constituents together at a temper-ature above the environmental Tg of the insoluble components, a certain critical concentration of copolymer in hemi-solvent is not exceeded. If this concentration is exceeded above the environmental Tg, the mixture of copolymer and hemi-solvent, on cooling to below that temperature, no longer appears miscible with added hemi-solvent, such addition re-sulting instead in the formation of separate phases so that the product is useless as a constituent of a coating composition.
It is believed that under these conditions the insoluble com-;, ~,.:,:, ' ponents are not aggregated to form the cores of identifiable, stable particles but are grossly entangled. It is, however, disclosed in Patent No. 908892 that dispersions having a :
concentration of copolymer in hemi-solvent which is above the critical concentration just referred to can be produced by preparing a dispersion of concentration below the critical value in the manner described above, and then removing part of the hemi-solvent, for example, by distillation, provided that during the distillation or other treatment the li~uid phase '~.' .',~ , ...
. .
_ 3 -.. ~ ,.
' ' , . ... ..

1~49177 remains a hemi-solvent and the temperature is not allowed to rise above the environmental Tg of the insoluble components.
Such a concentrated dispersion also contains the amphipathic copolymer in spherical aggregate form and, like the relatively more dilute dispersions discussed previously, is comparatively low in viscosity.
We have now found that certain of these concentrated dispersions are very suitable for use in the formulation of casting or potting compositions, by virtue of the fact that they may be converted into solid materials by a simple heat treatment involving no chemical change.
- According to the present invention we provide a pro- -cess for converting into a homogeneous solid mass a concentrated fluid dispersion of spherical aggregates of an amphipathic block or graft copolymer in a hemi-solvent as hereinafter defined, the process comprising the steps of (i) heating the concen~
trated dispersion, without significant change in the concentra-' - tion of the copolymer in the hemi-solvent, to a temperature above the environmental glass transition temperature o the 20 components of the copolymer which are insoluble in the hemi- -solvent and ~ subsequently cooling the resulting fluid composition to a temperature below the said transition temper-ature.
By an amphipathic copolymer we mean a block or grat - .
copolymer as defined in Patent No. 908892, that is to say a block or graft copolymer comprising at least two polymeric ~.
~-. components of molecular weight at least 500, the said compo-nents having such differences in their chemical nature that they differ significantly from each other in their solubility charac-teristics.
. '.

, 4 ' '~ ' ' : ' ~, .': ' , ,:: . . , ~49~77 A hemi-solvent liquid. is defined as a liquid or mixture of liquids which is a good solvent for one polymeric component and a non-sol~ent or precipitant for another polymeric component of an amphipathic copolymer By a good solvent we mean a solvent which is better than a theta solvent, the nature of a theta solvent being discussed in "Polymer Handbook" (Ed. Brandrup and Immergut, Interscience 1966) For the purposes of the present invention, amphiphatic copolymers are further defined as being copolymers in which the weight ratio of the components insoluble in a given hemi-solvent to the components soluble in that hemi-solvent is not less than 50:50. :
By a concentrated, fluid dispersion of an amphipathic block or graft copolymer in a hemi-solvent is meant in the present context a dispersion wherein the concentration of the copolymer in the hemi-solvent is not less than 20% by weight . and is such that when the dispersion is heated.without ~` :
significant loss of hemi-solvent to a temperature above the environmental glass transition temperature of the insoluble ~omponents of the copolymer and is subsequently cooled to a temperature below the said transition temperature, the ~:
dispersion is transformed into a homogeneous, solid mass. .
The term environmental glass transition temperature" .:
used herein (abbreviated to "environmental Tg") means ~e . temperature at which the insol~bLe component of the copolymer passes from the glassy to the non-glassy state, or vice versa, ~ in the environment constituted by the hemi-solvent with which ; it is in contact. In general, the environmental Tg will be substantially below the Tg as normally determined for the material of the insol.uble components on samples of pure high : ~ .
: ........

' ' '.'' ' ' ' ' , ' ' '. ' : ' ' , ' ' ' ' 1049~1L7~
molecular weight bulk polymer. This is because (1) the hemi-solvent liquid may contain minor proportions of liquid, which, alone, would be a good solvent for the insoluble com-ponents; (2) in many cases the molecular weight of ~he insoluble components is relatively :Low; and (3) the soluble components of the copolymer are also present. Normally the environmental Tg of the material of the insoluble components measured in bulk should be at least 20C, and preferably at least 50C, above the maximum ambient temperature to which the dispersion will be subjected during storage. Exact determination of the envi-ronmental Tg may be carried out on polymer of the same molecular weight and composition as the insoluble polymeric components present in the amphipathic copolymer. This polymer must have been equilibrated in the relevant hemi~solvent environment. The determination may be made by differential thermal analysis or similar wellknown techniques.
As is explained in the specification of Patent No.
908,892, relatively low concentration dispersion of amphipathic copolymer in hemi-solvent are fluid at temperatures both above and below the environmental ~g of the insoluble components. In contrast, as will be understood from the foregoing description, analogous compositions of concentration above the critical value are fluid dispersions above the environmental Tg and are homo-geneous solids below that temperature. The exact minimum value of the concentration which is required in order that the present ., : .
; invention may be performed varies from one amphipathic copolymer to another, according to its structure and molecular weight, and ~ -~
also varies with the particular hemi-solvent employed. The higher the molecular weight and the higher the weight fraction ' , 31 (~49~77 of the insoluble component, the lower will be this limiting concentration. When the molecular weight of the amphipathic copolymer is of the order of 10,000 and. the proportion of soluble component approaches the upper limit of 50% o~ the total weight of the molecules, the limiting concentration may be as high as 50%. In the case of copolymers having a molecular weight of several thousands and having a ratio of insoluble to soluble components of 3:1, the limiting concentration may be in the region of 20%. :
In view of the foregoing considerations, it is preferred that the copolymer dispersions used in the process :
of the invention should have a weight ratio of insoluble components to soluble components of the amphipathic copolymer of at least 67:33. It is also preferred that the total molecular weight of the copolymer should.not exceed 200,000.
However,for any particular combination of amphipathic copolymer - and hemi-solvent, the minimum concentration of copolymer required to give the benefits of the present invention is .
readily established by means of a series of simple tests. ::.
It is further preferred that ~he weight ratio of insoluble to soluble components in the amphipathic copolymer should not be greater than 80:20.
The amphipathic copolymer, when prepared, should be ~.
as free as possible from any ungrafted polymer species insoluble in the hemi-solvent en~ironment.
'I In selecting suitable hemi-solvents to be used with any given amphipathic copolymer, it is necessary to appreciate that the difference in solubility of the two components will generally derive from differences in polarity~
Therefore, the principle to be observed is that "like dissolves like"; that is polar polymeric components are dissolved by ~7~

liquids of similar polarity, or a highly polarizable liquid, while non-polar polymeric components are solvated by non-polar liquids. Those liquids which will or will not dissolve polymeric components of given polarity are well known to those skilled in the art and are illustrated for example in British Patent Specificatio~ ~o~ 1,052,241 and in page IV~ -~
185-234 of "Polymer Handbook" (Ed. Brandrup and Immergut, Interscience 1966).
For example, non-polar liquids such as aliphatic or cyclo-aliphatic hydrocarbons or long chain alcohols will dissolve non-polar polymeric components such as:
copolymers of ethylene and copolymers of propylene;
polymers of vinyl stearate;
polymers of an ester of along chain alcohol with acrylic or methacrylic acid;
polyesters derived from a long chain hydroxy carboxylic acid;
but will not dissolve polar polymeric components such as:
polymers of an ester of a short chain alcohol with acrylic or methacrylic acid;
polyesters of a short chain hydroxy carboxylic acid;
polyvinyl chloride;
polyacrylonitrile.
Conversely, polar liquids will not dissolve non-polar polymeric components such as those listad above, but they will dissolve polar polymeric components such as those listed above.
The preparation of graft or block copolymers from such polymeric components is well known to those skilled in the art and reference may be made to British Patent ~: ~
,: :
~
.
, : .. , :

".` ` ~)4~17~ ~
Specification No. 1,122,397 and to "Graft Copolymers" (Battaerd .
and Tregear, Interscience 1967) and "Copolymerisation" (Ham, Interscience 1964).
When the amphipathic copolymer is a graft copolymer, it is preferred that it should be of the type in which a ¦
polymeric backbone has grafted on to it a plurality of polymeric side-chains, and that the backbone should constitute the insoluble component of the copolymer an~ the side chains the soluble components. Where the amp~ipathic copolymer is a 10 block copolymer, it may be advantageous in certain cases if .
it is of the ABA type, where the A blocks constitute the soluble components and the B`block the insoluble component As already indicated, the concentrated dispersions used in the process o~ the invention ma~ be prepared by (i) heating an amphipathic block or gra~t copolymer as herein- .
before defined with a hemi-solvent as hereinbefore defined to a temperature above the environmental Tg of the polymeric components o the copolymer which are insoluble in the hemi-solvent, the proportion of hemi-solvent to copolymer employed 20 being such that the resulting ~luid composition on cooling to - .
a temperature below the said environmental Tg forms a stable, fluid precursor dispersion of substan~ially spherical : ag~regates o~ the copolymer, and (ii) xemoving from the said precursor dispersion at a temperature below the environmental Tg a su~icient proportion o hemi sol.vant to give a dispersion having a copolymer concentration which is not less than 20% by ~.
weight and is such that when th~ said concentrated _ .

. . ' ..
.
'' ' '"

'.'':' ' ~ ' . .
.

..
. .

1~9~77 dispersion is heated without significant loss of hemi-solvent to a temperature above the environmental Tg and is subsequently cooled to a temperature below the environmental Ty, the disper-sion is transformed into a homogeneous, solid mass.
Preferably the removal of part of the hemi-solvent from the precursor dispersion first obtalned, in order to pro-vide the final, concentrated dispersion, is carried out by distillation, if necessary under reduced pressure so that the hemi-solvent can be volatilised without the environmental Tg of the insoluble components being exceeded. It is, however, essential to ensure that, as such volatile constituents of the dispersion are removed, the remaining li~uid phase of the dis-persion must still be a hemi-solvent for the amphipathic copoly-mer.
The temperature to which the amphipathic copolymer is initially heated with the hemi-solvent may be only just above that at which the insoluble components cease to be in the glassy form. However, the rate at which the copolymer becomes dispersed in the hemi-solvent depends upon the extent to which the temper-ature used exceeds the environmental Tg of the insoluble compo-nents and it is, therefore, preferred that the copolymer is heated to a temperature of up to 100C above the environmental Tg.
With reference to the foregoing discussion of the in-, fluence of temperature and concentration upon the nature of compositions comprising an amphipathic copolymer and a hemi-solvent, and also to the discussion in Patent ~o. 908,892, it may be surmised that, in preparing the concentrated dispersions to be used in the process of the invention, the fact that the removal of part of the hemi-solvent from the relatively -~

~ 10 - ,.. .

10~9~77 dilute precursor dispersion is performed at a temperature at which the insoluble components of the copolymer are in the glassy state means that the microparticles o~ copolymer present have no possibility of re-arranging their structure into that which they would otherwise possess under the conditions of relatively high concentration, namely a structure which is not one of essentially spherical particles but is believed, as already stated, to be a disordered arrangement in which the insoluble components may be grossly entangled with one another.
HoweverD as soon as the concentrated dispersion is heated to a temperature above the environmental Tg in step (i) of ~he present process, such a re-arrangement of structure can occur because ofthe mobility of the insoluble components. On cooling below the environmental Tg, in step (ii) of the process, the postulated disordered, entangled structure persists and the composition becomes an apparently homogeneous, solid mass. It retains this form indefinitely, so long as the lower temperature is maintained, even in prolonged contact with added hemi-solvent.
In converting the concentrated, fluid dispersion to a solid product according to the process hereinabove de~ined, it is preferred that the dispersion should be heated to a temperature which is very appreciably above the environmental Tg of the insoluble components of the copolymer, for example at least 100C. above that temperature. In general, this preferred temperature of heating will be higher than the temperature to which the amphipathic copolymer and hemi-solvent ;~
are heated in the initial preparation of the relatively dilute precursor dispersion. This preference is based on the fact that the greater the difference between the environmental Tg , . .
::
:: ,' , , . - . . , :., , , ', ,;
, . , ~ .
.

and the temperature at which conversion to the disordered struc-ture is e~fected, the greater is the rigidity and strength of the solid mass obtained on cooling. This effect is believed to be attributable to the existence of a higher energy barrier towards transition, from the ordered, essentially spherical aggregated structure of the copolymer molecules to the disordered, entangled structure than that which operates in the reverse transition.
~s a matter of convenience, the temperature to which the concentrated, fluid dispersion is heated in order to convert it to a solid product may be chosen to be at least 100C above the Tg for the bulk polymer of which the in-soluble components of the copolymer consist~ rather than as stated above. Since the environmental Tg of the insoluble components will, as explained earlier, always be lower than the bulk Tg of these components, a heating temperature so chosen will be amply adequate to effect the desired transition between the ordered and the disordered structures Provided this preferred form of the process is carried out, therefore, the experimental determination of the environmental Tg of the insoluble components can be replaced by the rather more straightorward determination of the corresponding bulk Tg, for example by differential scanning calorimetry, where that information i9 not already available from published data.
From the foregoing description it will be evident ~
that the present invention provides a means of rapidly trans- -forming low viscosity fluids which are permanently stable when kept at normal temperatures, by a simple heating and cooling process, into homogeneous solid masses without any change whatever in their chemical composition. The process of the invention is, therefore, admirably suitable for .. .

Lg~L77 casting and potting techniques, ~or example t~e encapsulation and protection of electronic equipment and other articles of a delicate nature. The absence of any chemical r~action from the process means that there is no risk of attack upon the articles being so treated by reactive chemical species, nor is there any risk of local overheating due to exothermic effects. In carrying out such casting or encapsulating operations, some form of containing mould will normally be employed and two alternative methods are available. In the first method, the concentrated fluid dispersion is poured into the mould(in which any article to be encapsulated has already been placed) at ambient temperature; the mould and contents are then heated to a temperature above the environmental : Tg and subsequently allowed to cool. In the second method, the ..
concentrated fluid dispersion is separately heated to above the environmental Tg and is then poured into the mould and allowed to cool. Where other considerations permit, the first method is to be preferred since the viscosity of the concentrated dispersion at ambient temperature is mormally lower than that 20 of the same composition at a temperature above the environmental -Tg, due to the disordered structure of the latter. If the .~ second method is to be employed, it is desirable to heat the :
dispersion to a higher temperature than is employed in the ~irst method, in order to offset this viscosity increase The invention is illustrated by the following Examples, in which parts and percentages are by weight unless otherwise stated~ ' . ' '~

-13- :

, 9~7~

In this Example there was employed an amphipathic graft copolymer prepared in known manner having a backbone of poly(methyl methacrylate) and side chains of poly~12-hydroxy-stearic acid) in a weight ratio of 75/25 respectively. The molecular weight of the copolymer was approximately 56,000 and of the side chains approximately 1700. Hence the average number of side chains per molecule was 8. The bulk Tg of the backbone was about 100C.
To a gelatinous solid consisting of 40 parts of the above graft copolymer mixed with 60 paxts of high-boiling aliphatic hydrocarbon, which contained large irregular aggregates, were added 280 parts of medium-boiling aliphatic hydrocarbon(boiling point approximately 160C). Both hydro-carbons were hemi-solvents for the copolymer. The temperature of the mixture was raised to 160C. and maintained at that temperature for one hour with stirring to form a dispersion of aggregates in hydrocarbon in which t~e poly-(hydroxystearic acid) component was soluble. The dispersion was allowed to ~0 cool to 80C. to form micro,-particles. A~ this stage, the polymer concentration was 11%. The dispersion was concentrated by removing medium-boiling aliphatic hydrocarbon by distillation under reduced pressure at temperatures below 80C. (i,eO below the environmental Tg of the poly(methyl methacrylate) backbone~.
The resulting concentrated dispersion had a polymer content of 40%; a viscosity of approximately 2 poises at 25C.
A portion of the concentrated, fluid dispersion was poured into a glass mould, which was then slowly heated until : -,, , . : : . .

-the temperature of the contents reached approximately 140C.
(i e above the environmental Tg of the poly (methyl methacrylate)backbone~; the mould and contents were then allowed to cool to room temperature The mould was found to contain a translucent, slightly opalescent gelatinous solid which was immobile and could not be poured. The combined weight of the mould and its contents was the same before and after the heat treatment.
Another portion of the concentrated fluid dispersion 10 was treated in the manner just described, except that the temperature of the contents of the mould was raised to 170C.
before cooling. In this case the mould was found to contain a clear, wax-like solid. There was again no loss in weight . of material during the heat treatment.
EX~MPLE 2 .
.: ; .:: ., In this Example there was employed an amphipathic graft copolymer, prepared in known manner having a backbone of poly(methyl methacrylate). and side-chains of poly ~ -(12-hydroxystearic acid) in a weight ratio of 70:30. The 20 molecular weight of the copolymer was approximately 47,000 (peak, as determined by gel phase chromatography)and that of the side-chains 2,400. The bulk Tg of the backbone was about 100C. 20 parts of this graft copolymer and 180 parts of aliphatic hydrocarbon (boiling range 205-215C,) were heated at reflux temperature for 30 minutes; the temperature was.
then lowered to 80C and aliphatic hydrocarbon removed by ~ :
distillation under reduced pressure until the xesidue had a pol~mer content of 45%. The concentrated, fluid dispersion . .
so obtained was then heated momentarily to reflux temperature, 30 at which it had the consistency of a mobile syrup; no .

~9~
significant evaporation of the hemi-solvent hydxocarbon occurred during this operation. The hot syrup was then poured into a mould and allowed to cool. The resulting product was a clear, rubbery solid which could not be poured and which was free from voids, bubbles or cracks.

In this Example, the rubbery solid obtained as described in Example 2, consisting of 45% graft copolymer and 55% hemi-solvent, was employed as starting material. This solid was broken up into irregular fragments of about 1 cm.
diameter. .
In a first experiment, a portion of the solid : .
fragments was covered with sufficient hemi-solvent aliphatic hydrocarbon (boiling range 100-120C.) to give a polymer ~-concentration in the total mixture of 8.5%. The mixture was left at room temperature and examined at intervals over a .
period of three years. ~o change in ~he appearance of the ~ :
. ~ . .
material was observed over the whole of this period, the fragments of solid being unaffected and the hemi-solvent remaininglas separate, fluid phase.
In a second experiment, a further portion of the solid fragments was mixed with hemi-solvent as in the first experiment to give a polymer concentration of 8.5%. The - ~ .
mixture was then heated at 90-95C. for 30 minutes with stirring and allowed to cool. The product was an opalescent, fluid and homogeneous dispersion of spherical micro-particles~
of copolymer which were completely stable and showed no tendency to flocculate or aggregate, either when subjeatad to high shear or when stored at room temperature for three years.

' .

. :
: .~ . . .. :,, , :~

~4g~77 In a third experiment, a portion of the stable dispersion obtained in the second experiment was concentrated by removal of the lower boiling hemi-solvent by distillation at 80C. under sub-atomospheric pressure The product obtained had a polymer concentration of 45%, but, although similar to chemical composition to the solid fragments used in the first experiment, it was a fluid dispersiion, having a viscosity of 0.8 po~ise at 25C. when measured at a shear rate of 104 sec. l and showing no tendency to flocculate ~:
l0 or aggregate. When a portion of the dispersion was dilu.ted. : -with the low boiling aliphatic hydrocarbon to a polymer content of 8.5%, it resembled in all respects t~e product from the second experiment described above~
; In a further experiment,a portion of the dispersion of polymer content 8.5% produced in the second experiment . .
was concentrated by removing hemi-solvent by distillation under atmospheric pressure until the polymer content was 45%. The final temperature of the concentrated dispersion was 205C. ~. -On cooling to room temperature, the product obtained was a -' 20 clear, rubbery solid which was identical in appearance to ; the product described in Example 2.

.

~ ' .

' - ,: .. , . :
. . , : ' . ' .:

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A process for converting, into a homogeneous solid mass, a concentrated, fluid dispersion of self-stable, substantially spherical aggregates of an amphipathic block or graft copolymer in a hemi-solvent, the amphipathic copolymer comprising at least two polymeric components of molecular weight at least 500 which differ significantly from each other in their solubility characteristics, the hemi-solvent being a liquid or mixture of liquids which is a good solvent for one of the said polymeric components and a non-solvent for another of the said components and the weight ratio in the amphipathic copolymer of the components insoluble in the hemi-solvent to the components soluble in the hemi-solvent being not less than 50:50, wherein the concentration of the amphipathic copolymer in the hemi-solvent is not less than 20% by weight, said process comprising the steps of (i) heating the concentrated dispersion without significant change in the concentration of the copolymer in the hemi-solvent to a temperature above the environmental glass transition temperature of the insoluble components of the copolymer and (ii) subsequently cooling the resulting fluid composition to a temperature below the said transition temperature.

2. A process as claimed in Claim 1, wherein the weight ratio of the insoluble components to the soluble components of the amphipathic copolymer used is at least 67:33 but not greater than 80:20.

3. A process as claimed in Claim 1, wherein the total molecular weight of the amphipathic copolymer is not greater than 200,000.

4. A process as claimed in Claim 1, 2 or 3, wherein the amphipathic copolymer is a graft copolymer having a polymeric backbone constituting the component insoluble in the hemi-solvent and, grafted on to the backbone, a plurality of polymeric side-chains constituting the components soluble in the hemi-solvent.

5. A process as claimed in Claim 1, 2 or 3, wherein the amphipathic copolymer is a block copolymer of the ABA type, where the A blocks constitute the components soluble in the liquid or mixture of liquids and the B block the insoluble component.

6. A process as claimed in Claim 1, 2 or 3, wherein the concentrated fluid dispersion of amphipathic copolymer is heated in step (i) to a temperature at least 100°C above the environmental glass transition temperature of the insoluble components of the copolymer.

7. A process as claimed in Claim 1, 2 or 3 wherein the concentrated fluid dispersion of amphipathic copolymer is heated in step (i) to a temperature at least 100°C above the bulk glass transition temperature of the insoluble components of the copolymer.