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High Internal Phase Emulsion Foams: Copolymers and Interpenetrating Polymer Networks.

MICHAEL S. SILVERSTEIN [*]

High internal phase emulsion (HIPE) copolymer and interpenetrating network foams were prepared from 2-ethylhexyl acrylate (EHA), styrene (S) and divinylbenzene (DVB) using a unique process. The morphologies, thermal properties and dynamic and static mechanical properties of these foams were investigated. The glass transition temperatures and damping properties of the EHA/S copolymer foams vary with its composition. IPN foams with very broad tan [delta] peaks were obtained. The damping properties of IPN foams were tailored through changing copolymer composition and monomer composition. The IPN foams based on a copolyrner foam and styrene had a broader tan [delta] peak, a higher glass transition temperature and a higher modulus than the copolymer foams of similar overall styrene contents. It is therefore possible to prepare novel damping foams based on polyHIPE foams through the synthesis of interpenetrating polymer networks.

INTRODUCTION

A high internal phase emulsion (HIPE) is defined as an emulsion in which the dispersed phase occupies more than 74% of the volume, the maximum packing fraction of monodispersed spheres. Dispersed phase volume fractions above 0.74 can involve polydispersed spheres or the deformation of monodispersed spheres into polyhedra separated by a thin continuous film [1]. A continuous monomer phase can be polymerized to yield a low density foam, a polyHIPE, following the removal of the dispersed phase [2]. PolyHIPEs have an open-cell morphology and have demonstrated advantageous structural properties and liquid absorbency, prompting their use as absorbents [3-5], in ion exchange systems [6-8], and as heat resistant structural foams [9-13]. The application of polyHIPE as damping materials has not been explored in depth.

Individual homopolymers are effective damping materials over a limited temperature range. A wider temperature range for damping materials can be obtained using polymer blends [14]. Incompatible polymer blends exhibit separate glass transitions that are characteristic of the individual polymers. As the degree of molecular mixing is increased through increasing polymer compatibility, the individual transitions broaden and merge into a single transition. The compatibility of two polymers can be augmented by creating interpenetrating polymer networks (IPN) [15-19]. Sequential IPNs are synthesized by swelling a crosslinked polymer I with monomer II, containing a crosslinking comonomer and an initiator, and polymerizing and crosslinking monomer II.

This paper focuses on the production of polyHIPE with broad damping peaks through the synthesis of IPN. The unique technique used to prepare the IPN foams involves synthesizing a polyHIPE foam (polymer I), swelling it with monomer II, crosslinking comonomer and monomer soluble initiator, enhancing diffusion and swelling by imbibing water at elevated temperatures and then polymerizing and crosslinking monomer II.

EXPERIMENTAL

Materials

The monomers used were styrene (S, purity [greater than] 99%, Fluka Chemie) and 2-ethylhexyl acrylate (EHA, purity [greater than] 98%, Aldrich Chemical Company, Inc.). The crosslinking comonomer was divinylbenzene (DVB, which includes 40% ethylstyrene isomers, Riedel-de Haen). All monomers were used without removing their inhibitors. The initiators were potassium peroxodisulfate ([K.sub.2][S.sub.2][O.sub.8], purity [greater than] 99%, Riedel-de Haen) for HIPE polymerizations (water soluble) and benzoyl peroxide (BPO, purity [greater than] 97%, Fluka) for IPN polymerization (monomer soluble). The emulsifiers had different hydrophilic lipophilic balances (HLB): sorbitan monooleate (SMO, Span 80, HLB = 4.3, less viscous) and sorbitan monolaurate (SML, Span 20, HLB = 8.6, more viscous), Sigma-Aldrich Chemie Company. Calcium chloride hydrate ([CaCl.sub.2]*2[H.sub.2]O, chemical pure, A. C. S. company) was the electrolyte used to stabilize the HIPE.

Preparation of PolyHIPE Foams

The copolymer compositions and synthesis recipes are listed In Table 1. The oil phase (11.5 g) consisted of the comonomers (styrene, DVB and EHA) and emulsifiers (Span 80 or Span 20). The aqueous phase (100 g) consisted of deionized water, initator and electrolyte. A stable PS polyHIPE was produced using 10% DVB and Span 80. The EHA-based polyHIPE with 10% DVB shrank on drying; 20% DVB was needed to synthesized a stable HIPE. Throughout this work 1 g DVB was used for each gram S and 2 g DVB was used for each gram EHA, both for the polyHIPE and the IPN. For simplicity's sake, hereafter, the use of S indicates S-co-DVB and use of EHA indicates EHA-co-DVB in the ratios described above. The DVB in the EHA-co-S copolymers thus varied from 20% to 10%, based on their ERA/S. Span 20, with its higher HLB (8.6) was used for the less hydrophobic monomers (EHA/S [greater than or equal to] 40/60). Span 80, with its lower HLB (4.3) was used for the more hydrophobic monomers (EHA/S [less than or equal to] 20/80).

The comonomers and emulsifier were stirred in a beaker for 5 minutes with a magnetic stirrer. Then, while stirring continued, 100 g of the aqueous solution (0.2% [K.sub.2][S.sub.2][O.sub.8] and 0.5% [CaCl.sub.2]*[H.sub.2]O) was added slowly, over a period of 30 min. This yielded a HIPE with a water-to-oil ratio of about 9 to 1. The HIPE was then stirred for 10 min using a mechanical stirrer. 20 ml of the aqueous phase was then was added on the surface of the HIPE to prevent the surface from becoming dry, and the beaker was covered with a plastic film. The HIPE was placed in a convection oven at 60[degrees]C overnight for polymerization. The polymerized HIPE was a firm, porous polymer filled with water. The water-filled polymer was removed from the beaker and dried in a convection oven at 80[degrees]C until a constant weight was achieved (about 24 hr). The weight of the dried polymer corresponded to the total weights of the comonomers and emulsifier.

Preparation of IPN Foams

The IPN foams were prepared by taking a polyHIPE foam, swelling it with a monomer solution (monomer, crosslinking comonomer and initiator) and, then, polymerizing and crosslinking. The method used to swell the polyHIPE foam with the monomer solution is described below. The compositions and processing conditions for the IPN foams are listed in Table 2.

The monomer solution was added to a sample of dried foam and was absorbed immediately. The foam was then immersed in a beaker of deionized water and a weight was used to keep it beneath the water surface. The beaker was placed in a vacuum oven at room temperature to remove the air entrapped in the foam and to allow the water to penetrate. A vacuum of 5 x [10.sup.3] Pa was applied for 15 min and then released. This vacuum/release cycle was applied three times. The beaker was then kept at room temperature for 24 hr and, then, in a convection oven at 40[degrees]C for 24 hr. Monomer II was then polymerized and crosslinked in the connection oven (70[degrees]C, 24 hr). The foam was then removed from the beaker and dried in the manner described previously. Copolymer foams B, C, D and E could be swollen uniformly in this manner. The increase in foam mass was approximately equal to the mass of the monomers added.

Characterization

The densities of the foams were determined from weight and volume measurements. Dynamic mechanical thermal analysis (DMTA) temperature sweeps were performed in compression using a Rheometrics MK III DMTA at a frequency of 1 Hz at 3[degrees]C/min on 4 mm x 4 mm x 4 mm cubes. The full width at half maximum (FWHM) of the tan [delta] peak was calculated using a program written by one of the authors.

The compressive stress-strain measurement curves were performed at 25[degrees]C using the Rheometrics MK III DMTA on 4 mm x 4 mm x 4 mm cubes. The measurements were carried out until an equipment-related force limitation was reached. The modulus of the specimen ([E.sub.f]) was calculated using the highest slope at low strains. At least two specimens were used for each foam and the variation In modulus was approximately 1%. The modulus for the bulk polymer ([E.sub.p]) is related to [E.sub.f] using Eq 1 [20].

[E.sub.p] = [([d.sub.p]/[d.sub.f]).sup.2] [E.sub.f] (1)

where [d.sub.f] and [d.sub.p] are the foam and polymer densities, respectively. The exact densities of the copolymers are not known, but are expected to be quite similar. [E.sub.p] was calculated by assuming a constant density for all the polymers, [d.sub.p] = 1 g/[cm.sup.3].

The foam structure was characterized from cryogenic fracture surfaces using high resolution scanning electron microscopy (HRSEM, LEO 982, Zeiss) with uncoated specimens and an accelerating voltage of 2 kV.

RESULTS AND DISCUSSION

Dimensional Stability

The PEHA, copolymer A and copolymer B foams shrunk initially during the drying process and then re-expanded when the drying was almost complete. This shrinkage is related to the capillary forces exerted by the water on the foam during the drying process. The PS, copolymer D and copolymer E foams did not shrink markedly. The copolymer C foam shrunk and did not recover its original size. It was then annealed at 110[degrees]C, whereupon it re-expanded. The shrinking and expansion occur in foams whose [T.sub.g] was expected to be low, below the drying temperature, but not in foams whose [T.sub.g] was expected to be high, above the drying temperature. The capillary force driving shrinkage is stronger than the force exerted by the crosslinked network trying to maintain its original shape. At lower [T.sub.g]s the capillary forces predominate until drying is complete and then the recovery forces dominate. At high [T.sub.g]s the capillary force is not strong enough to deform the foam. At an intermediate [T.sub.g] (foam C) the capillary force is strong enough to deform the foam. The recovery force, however, is not strong enough to re-expand the foam. Molecular motion is easier at higher temperatures and then the recovery forces re-expand the foam.

Copolymer Foams

Morphology

The morphologies of the PS, PEHA, and copolymer A, B, C and E foams are seen in Fig. 1. The foams have an open-cell structure with spherical cells whose diameters vary from 1 to 10 [micro]m, depending on the composition. The cell walls are perforated with circular pores whose diameters range from 0.3 to 3 [micro]m, depending on the composition. The foams containing Span 20 as an emulsifier (i.e. PEHA, A, B and C) have a bimodal distribution of cell diameters, as has been observed in other systems [21]. For PEHA (Fig. 1a), the average diameter of the large cells is about 10 [micro]m; the small cells have an average diameter of about 2 [micro]m. The cell diameters become more uniform with increasing monomer hydrophobicity (increasing styrene content). The foams with Span 80 have a monodisperse cell size distribution. The PS foam (Fig. 1f) has cells 4 [micro]m in diameter perforated with pores from 1 to 3 [micro]m in diameter. Foam E, EHA/S = 20/80, has cells 9 [micro]m in diameter that are more spherical than t he PS cells and pores, 1.5 to 3 [micro]m in diameter, which are more circular than the PS pores. This change in cell size and shape may reflect the effect of EHA on the interfacial tension.

The PS and PEHA foam densities were both 0.10, as expected from the HIPE composition, in spite of the differences in structure (Fig. 1). The densities of the copolymer foams (Table 1) lie between 0.10 and 0.11, also reflecting the 90% aqueous phase in the HIPE.

The low viscosities at low water-to-oil ratios indicate that a stable water-in-oil emulsion is formed. For HIPE with Span 20, the emulsion becomes highly viscous at water-to-oil ratios of about 5 to 1. This high viscosity is related to the higher HLB and higher viscosity of Span 20. The more hydrophilic emulsifier reduces the driving force that tends to minimize the interfacial area of the dispersed aqueous phase and this can yield a significant increase in interfacial area, and high viscosities, at high water-to-oil ratios. The smaller cells in the bimodal cell size distribution are retained from the small water droplets formed in the early stages of HIPE synthesis, when the high shear stress of mixing the low viscosity HIPE breaks up larger droplets. The larger cells are formed in the later stages of HIPE synthesis when mixing the more viscous HIPE does not generate sufficient shear stress to break up larger droplets. The foam structure thus reflects the interfacial tension and viscosity of the HIPE. The vi scosity remains low following addition of the aqueous phase for the HIPE with the more hydrophobic Span 80. The more hydrophobic emulsifier enhances the driving force that tends to minimize the interfacial area, to maintain the discrete nature of the aqueous phase within the continuous organic phase and to yield the low viscosity. Since the shear stress of mixing remains high throughout, the water droplet diameter, and, therefore, the cell size, remains monodispersed.

Glass Trans Won Temperatures

The variation of copolymer [T.sub.g] with composition is seen in Fig. 2 (the [T.sub.g] was determined from the compressive E" maximum). The [T.sub.g]s for the crosslinked PEHA and PS, -36[degrees]C and 123[degrees]C, respectively, are close to the values expected from the literature [22]. The Fox equation (Eq 2) is commonly used to describe the relationship between copolymer composition and glass transition temperature [23]. The experimental data fall between the curve generated using the Fox equation (Eq 2) and the straight line describing the arithmetic average, as expected for copolymers.

1/[T.sub.g] = [w.sub.1]/[T.sub.g1] + [w.sub.2]/[T.sub.g2] (2)

where [w.sub.i] is the weight fraction of the component whose [T.sub.g] is [T.sub.gi] (i = 1, 2).

The variation of tan [delta] with temperature for the homopolymer and copolymer foams is seen in Fig. 3. The PS tan [delta] peak is high and narrow while the PEHA peak is low and broad. The smaller, broader peak for the PEHA foam can be related to the molecular structure of PEHA and the higher DVB content in the PEHA foam. PEHA has long side groups and therefore has a broader distribution of segmental motions in its glass transition temperature. Copolymerization with 20% DVB can yield an inhomogeneous distribution of segmental compositions owing to the expected large reactivity differences between DVB and EHA [the reactivity ratios for styrene and octyl acrylate are [r.sub.1] = 0.39 and [r.sub.2] = 0.01 (24)]. The variations of tan [delta] peak height and FWHM with composition are seen in Fig. 4. The height of the tan [delta] peak increases with increasing styrene content, while the FWHM decreases with increasing styrene content. This demonstrates that it is possible to vary the position and range of the poly-HIPE glass transition temperature by varying the comonomer composition.

Mechanical Properties

The compressive stress-strain curves of the copolymer foams are seen in Fig. 5. The maximum force measurable in compression limited these experiments; the samples did not fail on reaching this force. Composition has a marked influence on the mechanical properties of the foams. The stress-strain curves for the less rigid foams (PEHA, A, B and C) can be divided into 3 regions: linear elastic, plateau, and bulk compression. The modulus increases and the plateau length decreases with increasing styrene content. Only a linear elastic region is observed for the more rigid PS and E (not shown) foams.

[E.sub.p] was calculated by substituting the experimental [E.sub.f] and [d.sub.f] into Eq 1. The variation of [E.sub.p] with composition is seen in Fig. 6, along with an arithmetic average. [E.sub.p] falls far below the arithmetic average, in spite of the fact that the foam densities are almost identical. This is also the case for the copolymer E foam, whose structure Is very similar to that of the PS foam. The variation of modulus for the copolymer is thus more complex than that for bulk copolymers.

IPN Foams Based on Copolymer B

Foam Morphology

The densities of the IPN foams are listed in Table 2. The foam's volume increased significantly on swelling in styrene and this swollen volume was maintained during polymerization. The IPN foams 3B/1S, 2B/1S and 1B/1S had densities of around 0.10 g/[cm.sup.3], somewhat lower than the density of foam B, 0.11 g/[cm.sup.3]. These foams did not shrink noticeably during drying. Foam 1B/2S shrank somewhat during drying and did not recover following drying. The density of IPN foam 1B/2S, 0.20 g/[cm.sup.3], was about twice that of the other IPN foams. Polymerization of styrene within the cells or the reduction in relative cell volume would yield such an increase in density.

The cryogenic fracture surfaces of IPN foams based on foam B and styrene are seen in Fig. 7. The structures of the larger cell walls for the 1B/1S and 1B/2S foams are seen in Fig. 7a and 7b, respectively. The structure of the larger cell wall for the 1B/1S foam is quite similar to that seen for foam B. The cells and pores are slightly larger for the IPN foam, reflecting a monomer-swollen foam whose structure was fixed by polymerization and crosslinking. The density is slightly less than that of foam B for a 100% increase in mass, indicating that the foam did, indeed, swell to twice its volume (as observed visually). The larger pore and cell sizes reflect this increase in volume. The polymerized and crosslinked styrene seems to have been incorporated within copolymer B and not within the water-filled cells. These results are consistent with the formation of an IPN.

The cells of IPN foam 1B/2S were partially filled by fibrils of a second polymer phase (Fig. 7b). The polymer fibrils were especially profuse in the medium size cells (ca. 20 [micro]m). The formation of polymer fibrils within the empty cells and the contraction of the foam during drying contributed to the significant increase in density. The emulsifier that remained in foam B plays an important role in the formation of this fibrillar structure. Following the saturation of the swollen foam B with styrene, the excess monomer and residual emulsifier interacted with the aqueous phase. A water-in-oil emulsion, with a HIPE-like structure, was formed within the water-filled cells. The fibrils (Fig. 7b) are actually the cell walls of a HIPE structure developing within the cell. The medium-sized cells seem to have had an optimal cell volume for the development of this HIPE structure.

Glass Transition Temperatures

The variation of the [T.sub.g]s derived from the E" peaks with the amount of styrene used in swelling is seen in Fig. 8. The [T.sub.g] of 17[degrees]C for 3B/1S is similar to the 16[degrees]C [T.sub.g] of foam B; no significant influence of styrene incorporation is observed. The IPN foams 2B/1S, 1B/1S and 1B/2S (first peak of a curve with 2 E" peaks) have similar [T.sub.g]s, increasing from 65[degrees]C to 75[degrees]C with increasing styrene content. The second E" peak of foam 1B/2S is at 128[degrees]C, slightly higher than the [T.sub.g] of the PS foam (123[degrees]C). The two [T.sub.g]s for 1B/2S reflect a two-phase morphology: one phase is an IPN similar to 1B/1S; the other phase is crosslinked polystyrene fibrils within the cells of the foam.

The variation of tan [delta] with temperature for various IPN foams based on copolymer foam B and styrene is seen in Fig. 9. 1B/2S seems to exhibit two tan [delta] peaks. These two peaks are more pronounced in the variation of E" with temperature (not shown) for lB/2S. The tan [delta] peak height and FWHM were taken from the second peak. There is only one pronounced peak for the other IPN foams. The variation of the tan [delta] peak height and FWHM with the amount of styrene used for swelling is seen in Fig. 10. The peak height decreased somewhat and then increased markedly when taken from the second, predominant, lB/2S peak. The IPN foams exhibit a lower FWHM than foam B except for lB/1S, whose FWHM of 84[degrees]C is larger than that of PEHA (74[degrees]C). The extraordinarily wide FWHM reflects the continuous distribution of compositions typically found in IPN of incompatible polymers. The two lB/2S peaks taken together also span a considerable temperature range.

The properties of the IPN foams based on copolymer foam B swollen with styrene can be compared with copolymer foams of similar overall compositions (i.e., EHA/S ratios) using DMTA. IPN and copolymer foams with overall EHA/S of 30/70 and 20/80 are compared in Fig. 11a and 11b, respectively. The behavior of IPN and copolymer foams with similar compositions is quite different. The copolymers exhibit sharp tan [delta] peaks at a temperature that reflects their composition, with E' decreasing rapidly at this temperature. The IPN tan [delta] peaks are broader, smaller and shifted to higher temperatures. IPN 1B/1S at exhibits a very small tan [delta] peak that spans a very wide temperature range and the decrease in E' at high temperatures is quite gradual. E' reaches 1 MPa at 97[degrees]C for copolymer D and at 130[degrees]C for IPN 1B/1S (70% styrene). The formation of an IPN thus extends the high temperature range of the foam by about 60[degrees]C. An exten ded high temperature range of about 50[degrees]C Is seen for the IPN 1B/2S (80% styrene) in Fig. lib. The IPN structure is reflected in the relatively low peak height and high FWHM.

Mechanical Properties

The compressive stress-strain curves of the IPN foams are seen in Fig. 12. The addition of styrene increases the modulus and the height of the plateau, while decreasing the length of the plateau. No plateau was observed for the 1B/2S IPN. Thus, while foam B reaches compressive strains greater than 60% at -1.2 MPa, 1B/2S reaches strains of only 1%. [E.sub.p] for both the IPN foams and the copolymer foams, as a function of the overall styrene content, is seen in Fig. 6. In each case, the modulus of the IPN foam is greater than that of the copolymer foam with a similar overall styrene content. This higher modulus reflects the presence of the stiffer crosslinked PS network entangled within the copolymer B network in the IPN. The calculation of [E.sub.p] for 1B/2S is for comparison only, since it ignores the two-phase nature and the distinctly different cellular structure of the foam.

IPN Foam Based on Copolymer E

MorpholDgy

The morphology of 1E/1EHA lERA is quite similar to that of foam E (Fig. 13). The density of 1E/1EHA, 0.13 g/[cm.sup.3], is slightly higher than that of foam E (0.11 g/[cm.sup.3]) (Table 3). This increase in density, as opposed to the decrease in density for IPN based on foam B and styrene, reflects the differences between foams E and B. The volumetric swelling per mass monomer is, therefore, significantly higher for the lower [T.sub.g] foam B in styrene than for the higher [T.sub.g] foam E in EHA. Whereas foam B expands immediately on the addition of styrene, there is no observable change in foam E on the addition of ERA. During polymerization at 70[degrees]C, however, foam E does expand somewhat. Polymer fibrils, reflecting the formation of a HIPE within the water-filled cells, are also seen for 1E/2EHA (Fig. 13). The formation of EHA fibrils within the cells yields the increase in density to 0.16 g/[cm.sup.3]. 1E/2EHA shrank less than 1B/2S during drying and, therefore, is less dense than 1B/25.

Glass Transition Temperatures

The [T.sub.g]s of these IPN foams, listed in Table 3, were derived from the E" peaks. IPN 1E/ lEHA exhibits both a high [T.sub.g] (93[degrees]C) similar to that of foam E (96[degrees]C), and a low [T.sub.g] (-l7[degreees]C). The [T.sub.g] of -l7[degrees]C is significantly higher than that of PEHA (-38[degrees]C) and thus represents IPN domains in which the PEHA and copolymer E networks are Intimately mixed. IPN 1E/2EHA exhibits a [T.sub.g] typical of PEHA (-47[degrees]C) as well as [T.sub.g]s typical of a PEHA-rich IPN (-17[degrees]C) and a PS-rich IPN (80[degrees]C). The significant overlapping of these peaks reflects a continuous variation in IPN composition.

The variation of tan [delta] with temperature for IPN foams lE/lEHA and 1E/2EHA is seen In Fig. 14. Some characteristics of the tan [delta] peaks are listed in Table 3. Tan [delta] curves for the IPN foams based on foam E have two peaks, unlike the single peak seen for most of the IPN foams based on foam B (Fig. 9). The lower temperature peak corresponds to a rubbery phase and the higher temperature peak corresponds to a glassy phase. For IPN 1E/lEHA, the first peak is barely discernable and the second peak is quite similar to that of foam E. The first peak for IPN 1E/2EHA is at a lower temperature and is more pronounced, reflecting presence of PEHA. It is impossible to discern a distinct shoulder in the tan [delta] curve that corresponds to the shoulder seen in the E" curve. The second tan [delta] peak for IPN 1E/2EHA is smaller, broader and at a lower temperature than that for copolymer E, as expected for an IPN phase. In fact, for IPN 1E/2EHA, there is only a slight decrease in the value of tan [delta] with temperature following the lower temperature peak, yielding a broad tan [delta] range of about 160[degrees]C. The FWHM of the second peak for the IPN foams based on foam E are listed in Table 3. The FWHM for 1E/2EHA is more than twice that of foams E and 1E/lEHA.

The more limited swelling of the glassy polymer explains some of the differences between IPN based on foam E and a relatively inferior solvent (EHA) and IPN based on the more rubbery foam B and a relatively superior solvent (styrene). Specifically, while 1B/1S exhibits a single, very broad tan [delta] peak, lE/1EHA exhibits two less broad tan 8 peaks.

Mechanical Properties

The stress-strain curves for the IPN foams based on copolymer E are seen in Fig. 15. The compressive moduli of the foams and the polymer moduli are listed in Table 3. [E.sub.p] decreases with the amount of EHA added, as expected. The higher foam modulus for 1E/1EHA seen in Fig. 15 reflects its higher density and does not reflect its [E.sub.p]. IPN 1E/1EHA also exhibits a distinct plateau, reflecting its higher EHA content. The absence of a distinct plateau for 1E/2EHA may reflect its higher density, which would limit the collapsibility of the foam structure and limit the length of the plateau region.

Overall, for foam/monomer ratios below 1/1, IPN foams were successfully prepared from polyHIPE foams without the formation of a second phase. The damping and mechanical properties of the foams were thus modified without a significant change in density.

CONCLUSIONS

A series of copolymer foams was prepared from high internal phase emulsions. Interpenetrating polymer network foams were prepared by adding monomer and crosslinker and then imbibing water. The morphologies and thermal and mechanical properties of the foams were then investigated.

The foam structure consisted of cells, 2 to 10 [micro]m in diameter, whose walls were perforated with pores 0.5 to 2 [micro]m in diameter. The foam density was approximately 0.1 g/[cm.sup.3].

The foam morphology was mainly determined by the HLB and viscosity of the emulsifier used. Span 20 yields a bimodal cell size distribution while Span 80 yields a monodispersed cell size distribution.

IPN with a density of about 0.1 g/[cm.sup.3] result for foam/monomer ratios below 1/1, when a rubbery foam is swollen in styrene. IPN based on a less swellable glassy polymer had a somewhat higher density. IPN with polymer fibrils within the cells and with relatively high densities resulted for the foam/monomer ratio of 1/2, as the excess monomer formed a HIPE within the water-filled cells.

The rubbery PEHA with 20% DVB had a low, broad tan [delta] peak while the glassy PS with 10% DVB had a high narrow peak. The shape and position of the copolymer tan [delta] peaks fell between those of the homopolymers. The IPN had relatively broad and low damping peaks compared to the copolymers. The IPN from high monomer/foam ratios exhibited a second [T.sub.g]. The formation of a S/B IPN extends the high modulus temperature range by 60[degrees]C as compared to a copolymer foam with a similar overall styrene content. The compressive moduli of the IPN are higher than those of copolymer foams with similar overall styrene contents, reflecting the rigidity of the IPN's PS network.

The mechanical and damping properties of these foams can thus be tailored through comonomer composition and through IPN synthesis.

ACKNOWLEDGMENTS

The partial support of the Technion VPR Fund and the Goldsmith Fund for Visiting Scientists is gratefully acknowledged.

(*.) Corresponding author.

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[Graph omitted]

[Graph omitted]

[Graph omitted]

[Graph omitted]

[Graph omitted]

[Graph omitted]

[Graph omitted]
Table 1

Recipes and Densities of the Copolymer Foams.


Foam PEHA

EHA/S 100/0
Styrene, g 0
EHA, g 8.0
DVB, g 2.0
Span 20, g 1.5
Span 80, g 0
Aqueous phase 100g (0.2 g [K.sub.2][S.sub.2][O.sub.8],
 0.5 g [CaCl.sub.2], and 99.3 g deionized
 water)
Foam density, g/[cm.sup.3] 0.10



Foam A B C D E PS

EHA/S 80/20 60/40 40/60 30/70 20/80 0/100
Styrene, g 1.8 3.6 5.4 6.3 7.2 9.0
EHA, g 6.4 4.8 3.2 2.4 1.6 0
DVB, g 1.8 1.6 1.4 1.3 1.2 1.0
Span 20, g 1.5 1.5 1.5 0 0 0
Span 80, g 0 0 0 1.5 1.5 1.5
Aqueous phase


Foam density, g/[cm.sup.3] 0.10 0.11 0.11 0.11 0.11 0.10
Table 2

Recipe and Densities of the IPN Foams.


 Foam 3B/1S 2B/1S 1B/1S 1B/2S 1E/1EHA

Original foam B B B B E
Monomer S S S S EHA
Foam/monomer, wt/wt 3/1 2/1 1/1 1/2 1/1
Overall S content, % 55 60 70 80 40
IPN foam density, g/[cm.sup.3] 0.10 0.10 0.11 0.20 0.13




 Foam 1E/2EHA

Original foam E
Monomer EHA
Foam/monomer, wt/wt 1/2
Overall S content, % 27
IPN foam density, g/[cm.sup.3] 0.16
Table 3

Properties of IPN Foams Based on Foam E and EHA.


Foam E 1E/1EHA 1E/2EHA

[T.sub.g], [degrees]C 96 -17, 93 -47 (-17, 80) [*]
Tan [delta] peak height 0.85 0.84 0.32
Tan [delta] peak FWHM, [degrees]C 20.1 20.0 45.2
[E.sub.f], MPa 10.0 11.0 8.0
[E.sub.p], MPa 769 512 409 [**]



(*)From shoulders of the [E.sup.*] curve.

(**)This calculation is for comparison only, since it ignores the two-
phase nature and the distinctly different cellular structure of the
foam.
COPYRIGHT 2001 Society of Plastics Engineers, Inc.
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Author:TAI, HUIWEN; SERGIENKO, ANATOLY; SILVERSTEIN, MICHAEL S.
Publication:Polymer Engineering and Science
Geographic Code:1USA
Date:Sep 1, 2001
Words:5763
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