Supercritical drying has distinct advantages in generating microcellular materials. The dimensional stability of the polymer is not affected on drying because the supercritical process does not go through the two phase path and therefore the effect of capillary forces is absent. This helps in maintaining the morphology of the final polymer structure and better control over cell size. Organic microcellular foams were prepared by polymerizing directly in a near-critical fluid and pursuing the supercritical drying in the same reactor. The critical variables are the choice of a diluent with a strong enough solvent power to stabilize the polymer matrix, but with a low enough critical temperature to permit critical point drying without damage to the polymer matrix.

TECHNICAL FIELD 
This invention relates to the preparation of microcellular copolymer foams 
by supercritical fluid processing or near-critical diluent processing. 
BACKGROUND OF THE INVENTION 
Polymer foams are materials made by generating void spaces inside a bulk 
polymer, resulting in substantially reduced density. When these void 
spaces are interconnected, the material is characterized as open-celled. 
If these voids are discrete and not connected, the material is 
characterized as closed-celled. The nature of these cells and the cell 
size determine many properties of the polymer. For example, light weight 
and low thermal conductivity are the typical properties of a conventional 
foam. The density of conventional styrofoam is in the range of 0.02-0.2 
g/cm.sup.3 with closed cells between 50-100 .mu.m in diameter. 
When the cell size is less than 10 .mu.m, the foams are referred to as 
microcellular foams. Several remarkable properties have been noted for 
microcellular foams. First the strength/weight ratio for a closed-cell 
microcellular foam has been shown to be 5-6 times higher than for a 
macrocellular foam. Second, the high pore volume and high surface area of 
open-celled aerogels suggest applications as catalyst supports. The 
variability of the cell size and surface chemistry suggest novel 
applications as membranes or controlled release supports. Finally, when 
cell sizes are smaller than 0.040 .mu.m, the materials become transparent 
while retaining their low densities (0.05-0.10 g/cm.sup.3 ) and relatively 
low thermal conductivities. 
The key to making a microcellular foam is to focus on the mode of phase 
separation. Phase separation in conventional foaming occurs when the 
bubble forms and inflates in a manner that is difficult to control 
resulting in non-uniform foams. Synthesizing a microcellular foam requires 
gaining significantly greater control over the phase separation process. 
The method by which the lowest densities and smallest pore sizes have been 
obtained is reaction induced phase separation with critical point drying. 
Critical point drying was first applied to foam drying of aerogels, and 
subsequently applied to an organic resorcinol-formaldehyde foam. 
Probably the most versatile preparation technique involves thermally 
induced phase separation (TIPS) of polymer solutions. In this technique, a 
polymer solution is quenched in order to induce phase separation, either 
through liquid-liquid phase separation or polymer crystallization. 
When the TIPS process results in the formation of a continuous polymer-rich 
phase, two additional processing steps can lead to a microcellular foam. 
First, the morphology of the phase-separated solution is preserved either 
through vitrification or crystallization of the polymer. This step 
preserves the small-scale morphology of the demixed solution. Next, the 
solvent is removed through freeze-drying or supercritical extraction. 
The TIPS process is a general method whose primary requirement is polymer 
solubility. Low-density microcellular foams have been prepared with TIPS 
using many different polymers, including atactic polystyrene, isotactic 
polystyrene, poly(4-methyl-1-pentene), polyacrylonitrile, and 
water-soluble polymers such as (carboxymethyl)cellulose, poly(acrylic 
acid) and dextran. 
One significant limitation of the commercially viable processes is that 
pores are produced that are generally closed-celled and poorly controlled 
in pore size and morphology. 
SUMMARY OF THE INVENTION 
This invention relates to the preparation of microcellular copolymer foams 
by supercritical fluid processing or near-critical diluent processing. 
It is a first object of this invention to synthesize a microcellular 
epoxy-based copolymer foam of bisphenol-A/tetraethylenepentamine using a 
solvent/non-solvent solution followed by supercritical drying. 
It is a second object of this invention to synthesize a microcellular 
methacrylate-based copolymer foam of methylmethacrylate and ethyleneglycol 
dimethacrylate using a solvent/non-solvent solution followed by 
supercritical dryxg. 
It is a third object of this invention to polymerize and dry a 
microcellular foam in a single reactor by utilizing a near supercritical 
fluid diluent.

DETAILED DESCRIPTION OF THE INVENTION 
Reactants 
The monomers methylmethacrylate (MMA) (98%) and ethylene glycol 
dimethacrylate (EGDMA) (99%) were purified by successive washing with 10% 
w/w sodium hydroxide (NaOH) solution to remove the free radical inhibitor. 
The inhibitor-free monomers were washed in distilled and deionized water 
to remove the NaOH. The residual water in the monomers were removed by 
adding a small amount of magnesium sulfate (MgSO.sub.4). After 12-15 
hours, the monomers were filtered out. 
The monomers Bisphenol-A, (Epon.TM.-829), commercially available from Shell 
Chemical Co., and tetraethylene pentamine were used as is. 
Solvents/Non-solvents Diluents 
The solvents, methylisobutylketone (MIBK) and toluene were reagent grade. 
The non-solvents, butanol, methanol and heptane were also reagent grade. 
Freon-22, carbon dioxide (CO.sub.2) and propane were used as received 
without further purification. 
Initiators 
Benzoylperoxide (BPO), reagent grade, and tertbutylperoxy pivalate (TBPP) 
(75%) in mineral spirits were used as initiators for the methacrylates. 
Polymerizations 
The methacrylate polymerizations were free-radical polymerizations. These 
reactions involve three steps, initiation, propagation and termination. 
The initiation step yields a reactive species which is a free radical. 
This free radical then propagates by reacting with the monomers and later 
terminates to produce the polymer. In free radical polymerizations, high 
molecular weight polymer is formed immediately and the molecular weight of 
the polymer is relatively unchanged during the polymerization although the 
overall percent conversion of monomer to polymer increases with reaction 
time. Therefore, the early stages of the polymerization consist of dilute 
solutions of very long chains dissolved in a monomer and diluent mixture. 
This stage of polymerization is distinctly different from a condensation 
polymerization where the entire solution polymerizes at the same rate and 
the solution consists of a large number of short oligomeric chains. 
In this application, the following set of abbreviations will be employed: 
______________________________________ 
MMA methyl methacrylate 
PMMA polymethyl methacrylate 
EGDMA ethyleneglycol dimethacrylate 
MIBK methylisobutyl ketone 
TBPP tert-butylperoxy pivalate 
BPO benzoyl peroxide 
TP tetraethylpentamine 
______________________________________ 
A schematic representation of the copolymerization of MMA with difunctional 
EGDMA is provided below. An additional complexity of the copolymer 
methacrylate system is the substantial crosslinking via the addition of 
ethylene glycoldimethacrylate (EGDMA). It has been shown that the density 
of the final product decreases monotonically as the percentage EGDMA 
increases to 40 wt %. 
##STR1## 
The epoxy polymerizations, by contrast, are condensation polymerizations. 
In the instant case, this indicates that the polymerizations occur by 
reaction of the epoxide with the amine, with sufficient activation energy 
(i.e. temperature) to subsequently react. This type of polymerization, 
tends to be slower and more uniform in conversion than free radical 
mechanisms. In rudimentary schematic form, one example of this type of 
condensation polymerization may be characterized as: 
##STR2## 
Since the polymerizations are carried out in dilute solutions, the time for 
complete polymerization and complete crosslinking is longer than for 
free-radical polymerization, and the solution chemistry, (i.e. phase 
behavior), is significantly different. The long chains from the free 
radical polymerization tend to precipitate out of solution before 
extensive intermolecular crosslinking, simply due to size. The formation 
of polymeric sols in the epoxy systems, on the other hand, is closely 
related to the degree of crosslinking. 
Methacrylate Microcellular Foam Polymerization with Supercritical Drying 
MMA and EGDMA are copolymerized in a diluent and cured at about 
50.degree.-75.degree. C. As used in this application, diluent is intended 
to mean a solvent which has a relatively strong interaction with the 
polymer and non-solvent means a diluent which has a weak interaction. 
Strong solvents are capable of completely dissolving the monomer and 
non-crosslinked polymer. They are also capable of swelling the crosslinked 
polymer, often to remarkably large proportions depending on the strength 
of the interaction and degree of crosslinking. Strong non-solvents cannot 
dissolve or swell the polymer and function to promote phase separation of 
the polymer from solution. 
Toluene is an example of an acceptable diluent. The ratio of MMA/EGDMA 
should be between 20:1 to 1:1 with 3:2 being near optimal. The ratio of 
diluent/monomer should be between 10:1 to 1:3, where monomer refers here 
to both MMA and EGDMA. After maintaining the solution at constant 
temperature between 50.degree.-75.degree. C. for 24-48 hours, gelation 
takes place. The sample is removed from the oven and allowed to cool to 
room temperature. After cooling, the sample is placed in a high-pressure 
reactor which is cooled to 10.degree.-15.degree. C. and filled with liquid 
carbon dioxide or other near-critical liquid. 
After 4-8 hours, the high-pressure reactor is vented enough to remove some 
of the liquid contents, but not so much as to expose the polymeric 
material to vapor. The reactor is then refilled with near-critical liquid. 
This process is repeated several times until the diluent in the polymer 
has been totally removed and replaced by near-critical liquid. The 
high-pressure reactor and its contents are then raised to a supercritical 
temperature (45.degree. C. for carbon dioxide is sufficient) while 
maintaining the pressure well above the critical pressure. After holding 
the reactor and its contents at those conditions for 30-60 minutes, the 
vapors are vented until the pressure drops to ambient pressure and the 
reactor is opened and polymeric product collected. 
EXAMPLE 1 
Specifically, 0.005 g of BPO or 0.005 ml TBPP were used. The polymerization 
was performed at 60.degree. C. in an 11 ml glass ampule provided with a 
screw cap. The volume of MMA+EGDMA was 5 ml. The ratio of monomer to 
solvents was 1:1. The polymerization was allowed to proceed to about five 
times the half-life period of the initiators. The half-life of BPO is 10 h 
at 73.degree. C. and for TBPP is 10 h at 55.degree. C. After 
polymerization, the samples were cooled back to room temperature. The 
glass ampule was then carefully broken and the samples removed. The 
samples were swollen in toluene for 24 h. The degree of swelling gave a 
measure of crosslinking in the polymer. The polymers were then ready for 
washing and supercritical drying. 
For washing, the gels were placed in a high pressure reactor. The air 
trapped in the reactor was slowly removed by opening the exit valve and 
simultaneously filling the reactor with liquefied carbon dioxide at 900 
psi and 8.degree. C. The temperature of the reactor was controlled to 
within .+-.4.degree. C. 
The gels were kept immersed in liquefied carbon dioxide by opening the exit 
valve. In this way, the sample was always immersed in liquid CO.sub.2. 
Thus, the diluents in the pores of the gel were replaced by CO.sub.2. Five 
flushes were conducted in 24 h to complete the washing phase. 
At the end of this phase, the inlet and outlet valves of the pressure 
reactor were closed and the temperature of the water bath raised to 
45.degree. C. The critical point of CO.sub.2 is 1,100 psi at 31.degree. C. 
During heating, care was taken to insure that the pressure did not go 
beyond 1,500 psi. The CO.sub.2 was released at pressures greater than 
1,200 psi. After 6 h, the reactor was brought to atmospheric pressure 
isothermally at 45.degree. C. the apparatus was then cooled and the 
samples removed. 
The results for various copolymer microcellular materials are found in 
Tables I-IV. The ratios of solvent/non-solvent as well as the choice of 
solvents and non-solvents used are summarized in columns 2-3. The density 
of the copolymer microcellular foams prepared by subsequent air drying of 
the product are tabulated in column 4. The corresponding densities of the 
same material, which has been dried under supercritical conditions is 
listed in column 5 for comparative purposes. 
TABLE I 
______________________________________ 
Drying 
Super- 
EGDMA Toluene Butanol Air critical 
(ml) (ml) (ml) (g cm.sup.-3) 
(g cm.sup.-3) 
______________________________________ 
1 5 0 1.15 0.96 
1.5 5 0 0.91 0.75 
2 5 0 0.80 0.64 
1.5 4 1 0.87 0.79 
1.5 2 3 0.98 0.80 
1.5 1 4 0.78 0.69 
______________________________________ 
TABLE II 
______________________________________ 
Drying 
Super- 
EGDMA Toluene Methanol Air critical 
(ml) (ml) (ml) (g cm.sup.-3) 
(g cm.sup.-3) 
______________________________________ 
1 1 4 1.11 0.83 
1.5 2 3 0.93 0.79 
______________________________________ 
TABLE III 
______________________________________ 
Drying 
Super- 
EGDMA Toluene Heptane Air critical 
(ml) (ml) (ml) (g cm.sup.-3) 
(g cm.sup.-3) 
______________________________________ 
1 0 5 0.58 0.55 
1.5 4 1 0.68 0.65 
1.5 3 2 0.57 0.57 
2 4 1 0.64 0.61 
2 2 3 0.57 0.56 
______________________________________ 
TABLE IV 
______________________________________ 
Drying 
Super- 
EGDMA MIBK Butanol Air critical 
(ml) (ml) (ml) (g cm.sup.-3) 
(g cm.sup.-3) 
______________________________________ 
1 5 0 0.86 0.80 
1 4 1 1.12 0.84 
1 3 2 1.11 0.83 
1 2 3 1.05 0.83 
1.5 4 1 0.83 0.70 
1.5 3 2 0.81 0.72 
1.5 2 3 0.77 0.69 
2 4 1 0.75 0.67 
2 3 2 0.74 0.68 
2 2 3 0.76 0.66 
______________________________________ 
The data obtained from the experiments indicates that supercritical drying 
results in about a 15% reduction in density relative to air drying. Methyl 
isobutyl ketone (MIBK) is not as effective as toluene in decreasing the 
density and pore size. Increasing the amount of EGDMA generally decreases 
the density of the final material. In cases where EGDMA was not included, 
the materials almost always collapsed. Neither butanol nor methanol had a 
significant advantage in decreasing the density. Among the non-solvents, 
heptane gave the lowest density. 
As seen in FIGS. 1 and 2, scanning electron micrographs show a bead-like 
structure. The magnitude of the void space between beads depends on the 
type of solvent/non-solvent combination. Additionally, SEM photographs 
reveal that the pores are interconnected rather than closed-celled and 
that the pores are small and relatively uniform. 
When heptane was used as the non-solvent, the samples did not show any 
improvement on supercritical drying (i.e., the densities of the samples 
were nearly the same irrespective of the method of drying employed). The 
SEM micrographs of the samples that used heptane showed macrocellular 
structure and the average cell size was an order of magnitude higher than 
that obtained when the other non-solvents were used. 
This is clearly seen in FIG. 1, which is a typical SEM micrograph for the 
heptane runs. The cell size is greater than 10 .mu.m. FIG. 2 shows another 
typical SEM micrograph for the case where only toluene was used. The 
smallest cells obtained for the MMA+EGDMA system were 1 .mu.m. 
There is a distinct difference in the cell structure for the two cases. It 
appears to seem that supercritical fluid (SCF) percolates through the 
interconnected pores and the lack of a discrete phase change helps reduce 
capillary forces in the pores, thus preserving the polymer network. The 
high diffusivity of the SCF solvent facilitates solvent removal. 
Pore size is dependent on the affinity between the polymer and solvent 
used. Larger pores are formed when the affinity goes down. While not 
wishing to be bound by theory, it is believed that this is due to easier 
phase separation. The affinity between polymer and solvent can be 
estimated using the Flory interaction parameter and Hildebrand solubility 
parameter. As the solubility parameter goes down, the pores become larger. 
This is probably the reason for the macrocellular bead-like structure 
obtained in the case of heptane. It should be noted that the pore size did 
not increase when the solvent's solubility parameter was significantly 
larger than that of the polymer. Naturally, this discussion assumes that 
the final structure is primarily controlled by the nature of the 
equilibrium phase diagram. Because of the long times involved in the 
solvent replacement and subsequent drying, it is believed that the 
kinetics do not determine the structure of the microcellular product. 
For the toluene runs, the density decreased with an increase in amount of 
crosslinker used. It was observed that the best results were obtained when 
EGDMA represented 40% of the monomeric liquid. When a higher percentage of 
EGDMA was used, the material cracked extensively during polymerization. 
Roughly a 30% reduction was possible at 40% EGDMA. The benefit of 
supercritical drying was another 15% reduction in density. The use of 
non-solvents in the diluent did not lead to any significant advantage. 
Methacrylate Microcellular Foam Polymerization with Near Supercritical 
Process Conditions 
Direct polymerizations in near-critical solvents to synthesize 
microcellular foams were also performed. As used in this application, a 
near-critical solvent is one that has a critical temperature low enough 
that it can be exceeded in the supercritical fluid drying process without 
damaging the substrate foam. In this way, the polymerization, washing, and 
drying was converted into a polymerization and drying process in a single 
reactor. Previous attempts to apply a similar process to 
resorcinol-formaldehyde aerogels resulted in substantial changes to the 
polymer product. 
Polymerization in supercritical fluids is a relatively recent field. The 
polymers which have been obtained have generally been variants on the 
original high pressure polyethylene process or the polymers have been of 
relatively low molecular weight (4,000 or less). As for crosslinking 
during polymerization, this has apparently led to precipitation of the 
polymer from the supercritical fluid phase. 
The experiments were performed with MMA as the monomer and EGDMA as the 
crosslinker and either Freon-22 or propane as the supercritical solvent. 
The monomer/solvent ratio was fixed at 1:1, but the ratio of 
monomer/crosslinker was varied. TBPP was used as the initiator. 
The initiator concentration was 0.1% of the monomer weight in all 
experiments. Therefore, 0.005 ml of TBPP was used in all experiments. The 
reaction temperature was 70.degree. C. and 1,000 psig. This is below the 
critical temperature and above the critical pressure of either solvent and 
provides an optimum level of free radicals to the system. The critical 
temperatures and pressures of the solvents are given in Table V. 
Examples of other supercritical solvents which could be used are diethyl 
ether, methyl chloride, trimethylamine, chloropentafloroacetone, perfluoro 
acetone, ethyl chloride, ethyl fluoride, methyl formate, and acetaldehyde. 
In fact, almost any solvent with a relatively low critical temperature 
(&lt;200.degree. C.) is a candidate. The pertinent restriction which is 
applied to the solvent is that it be within its supercritical range in the 
phase diagram during the drying stage. It is not essential that the 
solvent be in this region of the phase diagram during the polymerization. 
TABLE V 
______________________________________ 
Solvent T.sub.c (K) P.sub.c (MPa) 
T.sub.b (K) 
______________________________________ 
Propane 369.8 4.25 231.1 
Freon-22 369.8 4.97 232.4 
______________________________________ 
The polymerization time was set at five times the half-life period of the 
initiator. The half-life of TBPP at 70.degree. C. is 100 minutes. The time 
was set to ensure that the reaction went to high conversions. 
Consequently, the polymerization was allowed to proceed for about 8 hours 
before the conditions were changed for the drying step. 
EXAMPLE 2 
Each experiment could be divided into two stages; (a) polymerization stage, 
and (b) drying stage. The polymerization was performed at 70.degree. C. 
and 1,000 psig. At these conditions, the diluents could be classified as 
near-critical liquids. The pure and dry monomers and initiator were loaded 
into the pressure reactor and the diluent added later. 
After loading the monomers, the high pressure reactor was cooled to a 
temperature below the boiling point of the solvent at atmospheric pressure 
using dry 
The boiling points of Freon-22 and propane are-40.8.degree. C. and 
-42.1.degree. respectively. A measured amount of the solvent which was 
previously collected as a liquid in a beaker was then added to the monomer 
mixture. A plug was installed into the end cap of the pressure reactor and 
immediately tightened to seal that end of the apparatus. The reactor was 
then placed in an oven and connected to the high pressure generator 
through a quick connect. The reactants were then pressurized to 1,000 psig 
quickly and held constant as the temperature increased to 70.degree. C. 
The polymerization was allowed to proceed for about 8 hours at about 
70.degree. C. before heating it up for the drying step. 
The temperature of the system was then raised to 100.degree. C. at the 
constant pressure of 1,000 psig. This temperature is above the critical 
temperature of either solvent and below the glass transition temperature 
of polymethyl methacrylate which is 105.degree. C. Although the 
crosslinked material had different thermal properties from PMMA, the glass 
transition temperature of PMMA was regarded as safe with respect to 
undesirable side effects. PMMA is known to depolymerize at high 
temperatures. The condition were maintained at 100.degree. C. and 1,000 
psig for about 6 hours. The pressure was then gradually reduced to 
atmospheric pressure at a temperature of 100.degree. C., by backing out 
the piston of the high pressure generator. The polymer was then removed 
after cooling the apparatus to room temperature. The results of the 
experiments are summarized in Table VI. 
TABLE VI 
______________________________________ 
Sample % EGDMA Diluent Density 
______________________________________ 
1 10 freon 0.910 
2 20 freon 0.646 
3 30 freon 0.508 
4 40 freon 0.408 
5 60 freon 0.470 
6 80 freon 0.596 
7 90 freon 1 
8 10 propane 0.378 
9 20 propane 0.700 
10 30 propane 0.753 
11 40 propane 1.017 
12 60 propane 1.200 
______________________________________ 
As seen from Table VI and FIGS. 3-4, the morphology of polymers prepared in 
propane (FIG. 3) is different from the morphology of equivalent polymers 
prepared in freon-22 (FIG. 4). In each case, the structure of the polymer 
prepared in the more polar freon is smaller than the structure of the 
polymer prepared in non-polar propane. 
Unexpectedly, increasing crosslinking does not always lead to a lower 
density. As shown in Table VI, increased crosslinking leads to lower 
density in the freon systems up to 40 wt % EGDMA, but higher densities 
result at 60 wt % and 80 wt % EGDMA. A similar trend is evident in the 
propane systems, but the minimum density appears at only 10 wt % EGDMA. 
While not wishing to be bound by theory, it is speculated that higher 
crosslinking leads to earlier phase separation and "squeezing" diluent out 
of the polymer phase. The resulting polymer would then be relatively dense 
because it never solubilizes enough diluent to permit a density reduction 
when drying. 
An advantage of the supercritical fluid process is that conditions and 
concentrations can be adjusted in order to tailor the morphology to a 
specific application. The major benefit of supercritical fluid processing 
is that the entire reaction can occur in one reactor. 
While the above discussion has focused on the methacrylate system, the 
procedure is general, and with minor variations, can be applied to other 
systems. In extending this procedure to other co-polymerization systems, 
it is envisioned that other supercritical fluids may be needed to be 
employed as solvents, and the need may arise, where it is critical to 
incorporate non-solvents into the polymerization, such as was described 
previously. 
Epoxy Microcellular Foam Polymerization with Supercritical Drying 
The epoxy system is especially complex because the epoxy monomer, 
bisphenol-A (Epon-829 resin) and tetraethylene pentamine curing agent are 
chemically very different and their solubilities are different in 
different solvents. Therefore, the choice of an optimum solvent and 
non-solvent is very important. An important factor considered for the 
selection of diluent mixture was the solubility parameter of the epoxy and 
that of the diluent mixture. The solubility parameters of the epoxy and 
diluents are given in Table VII. 
TABLE VII 
______________________________________ 
Solubility Solubility 
Compound 
Parameter Compound Parameter 
______________________________________ 
Epoxy 9.17 Butanol 11.40 
Toluene 8.90 Toluene/Butanol 
10.15 
(50/50) 
MIBK 8.58 MIBK/Butanol 9.99 
(50/50) 
______________________________________ 
As shown in the table, the solubility parameters of the diluent mixtures 
that resulted in the lowest density foam were close to that of the epoxy. 
A number of other solvent/non-solvent mixtures with similar solubility 
parameters were tried, but in all cases, either a precipitate or a dense, 
hard gel was formed. Butanol was found to be the most important 
non-solvent, not just because of its role in phase separation, but because 
of its role as a catalyst to the crosslinking reaction of the epoxy. Since 
the crosslinking reaction was carried out in very dilute solution, the 
rate of crosslinking was very slow. The catalytic effect of butanol 
increased the rate of crosslinking such that a network of high molecular 
weight was formed before the polymer could phase separate out, and a foam 
was obtained. Ethanol and propanol, which can also act as catalysts, were 
tried as non-solvents, but were too strong. The rate of phase separation 
was faster than the rate of crosslinking and a precipitate formed. These 
observations suggest that the porous structure is formed only if the rate 
of crosslinking and the rate of phase separation are balanced. 
The most important factor in determining the morphology of the product is 
the time of phase separation, which depends on the degree of crosslinking. 
The phase separation of the crosslinked polymer occurs either by 
macrosyneresis (deswelling of the gel) or by microsyneresis (formation of 
a dispersion of the separated diluent and the gel phases). 
Microsyneresis prevails in lightly crosslinked gels, with slow relaxation 
times, while deswelling is dominant in highly crosslinked gels. Although 
the dispersed phase is unstable initially, it gradually becomes fixed 
through subsequent gel crosslinking. In several experiments, a combination 
of macro- and micro- syneresis occurs, and depending on the prevailing 
method of phase separation, two different kinds of morphologies were 
obtained. 
These morphologies are shown in FIG. 5. The beaded morphology was obtained, 
when microsyneresis took place, whereas the bigger cellular morphology was 
obtained, when deswelling took place. 
The diluent must be a relatively strong solvent for the polymer and soluble 
in the comonomer solution. One key to the process is adjusting the ratio 
of hydrogen bonding in the diluent to obtain the highest possible dilution 
ratio for which viable products can be synthesized. The ratio of 
bisphenol-A/TP should be between 10:1 to 1:1, with 7:1 being near optimal. 
The ratio of diluent/monomer should be between 9:1 and 3:1, where monomer 
refers to both bisphenol-A and TP. No initiator is added because the 
polymerization begins immediately upon combination of the epoxy solution 
with the TP solution. 
After maintaining the solution at constant temperature between 
40.degree.-50.degree. C. for 5-7 days, gelation takes place, and 
crosslinking becomes extensive. Depending on the choice of diluent, the 
removal of diluent is near critical, or by exchanging the diluent for a 
suitable near critical solvent with subsequent supercritical drying. 
Supercritical drying is effected by holding the reactor containing the 
polymer samples and near-critical diluent at a pressure of roughly two 
times the critical while raising the temperature from subcritical to about 
5-10% above the critical temperature. After holding the reactor and its 
contents at those conditions for some time, 30-60 minutes, the vapors are 
vented until the pressure drops to ambient pressure and the reactor is 
opened and the polymeric product is collected. The product is a 
low-density polymer which appears to be smooth and uniform, even it is 
comprised of cross-linked polymer with void spaces of about 0.1 .mu.m in 
diameter. 
Scanning electron micrographs reveal that the pores are interconnected 
instead of being closed-celled and that the pores are small and relatively 
uniform. 
The results of a series of experimental runs are summarized in Table VIII. 
The densities, and porosities when available, are given for a number of 
solvent/non-solvent diluent systems. 
TABLE VIII 
______________________________________ 
Monomer 
Sam- initial Poro- 
ple conc. Density sity Toluene 
BuOH MIBK 
______________________________________ 
101 10.0 0.20 0.83 40% 60% 
102 12.5 0.27 0.78 40% 60% 
103 15.0 0.32 0.73 40% 60% 
104 17.5 0.39 0.68 40% 60% 
105 20.0 0.44 0.63 40% 60% 
106 22.5 0.50 0.58 40% 60% 
107 25.0 0.55 0.54 40% 60% 
108 10.0 0.19 0.83 50% 50% 
109 12.5 0.29 0.75 50% 50% 
110 15.0 0.39 0.66 50% 50% 
111 17.5 0.48 0.58 50% 50% 
112 20.0 0.54 0.53 50% 50% 
51 10.0 0.16 0.86 50% 50% 
52 15.0 0.23 0.80 50% 50% 
53 20.0 0.33 0.71 50% 50% 
54 25.0 0.43 0.63 50% 50% 
55 30.0 0.54 0.53 50% 50% 
56 40.0 0.72 0.37 50% 50% 
______________________________________ 
The variation of the density of the foams with different variables is shown 
in Table VIII. The density of the foam increases with an increase in the 
polymer concentration, since the solids content increases. The density 
shows a linear variation with the initial concentration for foams with all 
different solvents. Comparison of the densities with the same initial 
concentration, but different composition of solvent/non-solvents is shown 
in FIG. 6. As shown in the figure, the density is higher with a lower 
non-solvent content. The time of phase separation is delayed with a lower 
non-solvent content, the phase separation takes place at a higher degree 
of crosslinking, and hence, for the same initial concentration, the 
density is higher. 
The morphology of these foams was determined using scanning electron 
microscope. The SEM's of the foams are shown in FIG. 7 (a-c). The 
microcellular foams show the beaded morphology, with a bead size less than 
1 .mu.m. The foam has an interconnected structure. The holes in FIG. 7b 
are the solvent droplets that phase separated, but could not diffuse out 
of the gel. This morphology was obtained at high epoxy concentrations. 
In general, it is observed that the structure is more beaded with higher 
non-solvent diluent, which also explains lower density. A striking feature 
about all these morphologies is that, although the porosity and the 
density of the foams vary with dilution ratio and the diluent mixture 
composition, the size of the beads or the pores is almost in the same 
region of 0.1 .mu.m. It is believed that changes in the morphology can 
only be developed by making changes in the chemistry of the system. 
DISCUSSION 
While the foregoing discussion has been limited to two component (i.e. 
copolymer) systems, it is envisioned that the process and synthetic steps 
described would be applicable to homopolymers with at least two reactive 
sites. It is essential that one of the reactive sites effect the 
polymerization reaction and that a second reactive site be capable of 
effecting the crosslinking reaction. In this way, it is possible to 
synthesize a rigid microcellular foam from a homopolymer solution. 
In order to maximize the synthetic potential of the present technique, it 
is critical to elucidate the mechanism of the formation of the microporous 
structure, as it is formed in-situ. Previous scanning electron micrographs 
clearly indicate that the microcellular foams are comprised of tiny beads 
of polymer from 0.01-1.0 .mu.m in diameter. To control the morphology, it 
is important to know whether the beads are formed during the 
polymerization or during the drying stage. Dynamic light scattering is 
ideal for studying this phenomenon. 
Dynamic Light Scattering 
MMA and EGDMA were used as the comonomers in this study. Freon-22 was used 
as the diluent. TBPP was used as the free radical initiator. To prepare a 
typical reaction mixture, MMA and EGDMA were added to a 40 ml high 
pressure reactor in the proportions of 12 ml of MMA and 8 ml of EGDMA. 20 
.mu.l of TBPP were added and the high-pressure reactor attached to tubing 
with an open-shut valve. The reactor and contents were cooled in ice and 
connected to a supply of Freon-22 kept at room temperature. The freon 
valves were opened and Freon allowed to condense into the bomb for 10-20 
minutes. The valves were closed, the freon tank disconnected and the 
high-pressure reactor and its contents refrigerated until use. To charge 
the high pressure light scattering cell, the high-pressure reactor was 
raised to room temperature and the scattering cell was cooled to 5.degree. 
C. 
The high pressure light scattering cell was similar to conventional light 
scattering cells except that it was made of 3/4" pyrex and the bottom was 
open to a mercury resevoir that permitted regulation of the pressure. The 
height of mercury in the cell was adjusted to keep reactive solution out 
of the cell's mercury reservoir while keeping the mercury well below the 
light path. The high-pressure reactor was inverted and connected to the 
scattering cell and the monomer plus diluent solution was allowed to 
condense into the cell for 10-20 minutes. Valves were sealed and capped 
and the pressure was adjusted to about 400 psig at room temperature. The 
temperature in the cell was raised to the designated value by heating 
tape. 
Time-resolved scattering intensities were measured at 90.degree. angle 
using a Thorn-EMI photomultiplier tube and a Brookhaven Instruments 
amplifier/discriminator integral with the phototube housing. The data was 
analyzed using a Brookhaven Instruments Corporation BI-2030AT correlator 
using Brookhaven Instruments NNLS fitting software to estimate the 
particle size distribution as a function of reaction time. 
Dynamic light scattering detects the presence and diffusion coefficients of 
disperse inhomogeneities in a bulk fluid. Typically, diffusion 
coefficients can be measured for dispersions ranging in size from 1 nm-10 
.mu.m. 
In general, without being constrained to any particular theory, it is 
believed that there are at least three possible mechanisms by which 
monomer solution may evolve into a macroscopic bulk microporous material. 
First, the system may consist of steadily growing primary particles which 
grow until they fill the entire solution. The particle size histograms of 
such a system would show a broad polydisperse population of particles with 
the peak slowly moving to higher sizes. 
Second, the system could grow in stages whereby small particles are 
generated, then flocculated into large particles that are eventually too 
big to grow, then a new population of small particles evolves and begins 
to flocculate. The particle size distributions in this case would appear 
as waves of particles size peaks when considered as a function of time. 
A third possibility would be that the primary particles grow to a certain 
size then stop growing until the concentration of particles becomes so 
great that the particles percolate at a gelation point and convert from 
disperse sols to the macroscopic material in a very short time. In this 
case, very monodisperse populations of the largest, but still small, 
particles, would be expected. However, the scattering count would change 
as more scatterers evolve until the solution gels. 
Table IX summarizes the light scattering data for the methacrylate free 
radical polymerization. The reactions were terminated at the times 
indicated above columns 2-4. Table X summarizes the light scattering data 
for the condensation polymerization. As with the previous table, the 
reactions were terminated at the times indicated above columns 2-5. 
TABLE IX 
______________________________________ 
MMA + EGDMA Free-Radical Polymerization 
particle population density % relative to 
diameter most populous particle size 
(nm) 30 min. 42 min. 43 min. 
______________________________________ 
1.0 60 0 0 
1.5 100 0 0 
2.0 60 0 0 
2.5 0 0 0 
3.0 0 66 0 
3.5 0 100 0 
4.0 0 66 0 
6.0 0 0 0 
7.0 0 0 0 
10.0 0 17 0 
13-14.0 0 34 39 
16-18.0 0 43 79 
21-22.0 0 34 100 
24-27.0 0 17 79 
32-34.0 0 0 39 
42.0 0 0 0 
56.0 0 0 0 
______________________________________ 
TABLE X 
______________________________________ 
Bisphenol-A + TP Condensation Polymerization 
particle population density % relative to 
diameter most populous particle size 
(nm) 1.0 hr. 1.5 hr. 3.5 hr. 
5.0 hr. 
______________________________________ 
1.0 70 71 74 0 
1.3 100 100 100 0 
1.7 70 71 74 0 
2.0 0 0 29 0 
2.5 0 0 0 0 
3.0 0 0 0 0 
3.5 0 0 0 0 
4.0 0 16 0 0 
4.5 0 38 0 0 
5.0 0 50 0 0 
5.5 0 38 0 0 
6.0 0 0 0 0 
7.0 0 0 0 0 
9.0 0 0 0 67 
10.0 0 0 16 100 
11.0 0 0 21 67 
12.0 0 0 16 0 
13.0 0 0 6 0 
15.0 0 0 0 0 
______________________________________ 
As is evident from Table IX, the most plausible scenario for free radical 
polymerization is the third scenario described above. The initial 
appearance of the particles at .about.1 nm probably corresponds to the 
polymer backbone. The peak at 3 nm probably corresponds to assemblies 
which have been minimally crosslinked. The peak at .about.22 nm 
corresponds to the final beads of polymer which make up the polymer matrix 
of the polymer foam. The significance of this data lies in the observation 
that the primary particles are very small, and that they do not 
flocculate, but rather percolate at gelation. There probably is some 
transition in particle size, but there is not a large enough population of 
these intermediate size particles to show up in the scattering 
measurement. 
Table X indicates a similar mechanism for the condensation polymerization, 
but the primary particles are much smaller. 
This all appears to indicate that certain particle sizes are more favored 
than others, and that the polymerization occurs by rapidly populating 
these favored sizes until three-dimensional connectivity occurs 
(percolation). This indicates that these same particles are preserved in 
the macroscopic material even through the supercritical drying process. 
Thus, the drying process would appear to have little adverse impact on the 
morphology of the macroscopic material. 
This light scattering data can be cross-referenced with data on the total 
intensity and gelation times for the epoxy polymerizations. FIG. 8(a) 
shows a polymerization where gelation occurs before a large number of 
primary particles have been generated. The intensity tracks the 
concentration of primary particles whereas viscosity tracks gelation. FIG. 
8(b) shows a case where gelation occurs shortly after the creation of a 
large number of primary particles. The polymer in FIG. 8(a) is a hard 
dense gel, whereas the polymer in FIG. 8(b) is a microcellular foam. 
As shown in the phase diagram given in FIG. 9, when the concentration of 
solvent or monomer is too high, a hard dense gel is formed. Additionally, 
when the concentration of non-solvent is too high, a precipitate is 
formed. When the combination is falls within the appropriate region, a 
microcellular foam is formed. 
With this chemistry, it is now possible to describe a methodology for 
producing a microcellular foam, and conditions which will optimize the 
chemistry and resultant foam morphology. The first step is to devise a 
chemistry which generates primary particles which are small, because the 
size of these primary particles controls the limit of the ultimate pore 
size. Secondly, a solvent environment must be generated which causes these 
primary particles to separate just before gelation initiates. If the 
separation occurs too soon, a precipitate will form, and if separation 
occurs too late, a hard, dense gel will form. It is critical that the 
solvent environment be strong enough to swell the primary particles to as 
great an extent as possible after they have separated into their 
inhomogeneous regions in solution. 
While in accordance with the patent statutes, a best mode and preferred 
embodiment have been described in detail, the invention is not limited 
thereto, rather the invention is measured by the scope of the attached 
claims.