Ind. Eng. Chem. Res. 2000, 39, 65-71
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MATERIALS AND INTERFACES Synthesis, Characterization, and Interfacial Behaviors of Poly(oxyethylene)-Grafted SEBS Copolymers Jiang-Jen Lin,*,† I-Jein Cheng,† Chou-Nan Chen,† and Chang-Chin Kwan‡ Department of Chemical Engineering, National Chung-Hsing University, Taichung 402, Taiwan, and Department of Applied Chemistry, Providence University, Taichung County, Taiwan
Poly(oxyalkylene) blocks (POA) were synthetically grafted onto polystyrene-b-poly(ethylene/ butylene)-b-polystyrene (SEBS) by the reaction of the maleated SEBS and various poly(oxyalkylene)amines. Structurally, the prepared comb-branch copolymers consist of a hydrophobic SEBS triblock backbone (approximate 45 000 MW with ∼29 wt % of styrene content) and multiple poly(oxyethylene) (POE) or poly(oxypropylene) (POP) pendants (in the range of 2000-6000 MW for each pendant). The hydrophilicity was probed by measuring the polymer surface resistivity from 1011.4 to 108.1 Ω/sq, depending on the pendant structures. In toluene or hexane, these copolymers formed very stable multiple emulsions with water as observed by an optical microscope. The copolymers were capable of reducing interfacial tension of toluene/water, in one example, from 31 to 7.4 dyn/cm by adding the copolymer at 3 × 10-2 g/100 mL concentration. At very low concentration (3 × 10-4 g/100 mL), these copolymers exhibited high efficiency in reducing interfacial tensions up to 17.5 dyn/cm, but slowly reaching the equilibrium over a 10-h period. In comparison, the commercially available Span 60 surfactant (HLB ) 4.7) at the same concentration can only reduce to 28.5 dyn/cm. These phenomena are rationalized by the collective noncovalent bond interactions, predominantly the π-π interaction of the polystyrene block with toluene and the hydrogen bonding of the POE segments with water molecules. Introduction Copolymers consisting of chemically different blocks1-5 may exhibit properties such as self-assembly, micellization, metal complexation, absorption, molecular association, and so on. The ability of forming microstructures could render the polymers a wide spectrum of industrial applications5-7 including polymer blend compatibilizers, polymeric surfactants, pigment and inorganic solid dispersants, associative thickeners, drug delivery systems, and so on. One of the most intriguing properties is that such copolymers with specific blocks of repeating units may self-assemble into supramolecular materials.2 For example,8 the diblock copolymer of poly(phenylquinoline)-b-polystyrene could automatically arrange into high regularity of hollow sphere having an aggregation number of over 108. In another example,9,10 triblocks of oligo(styrene-b-isoprene-b-biphenyl) was found to have the ability of self-organizing into a highly oriented thin film. In nanostructural scale, they were observed to be mushroom-shaped supramolecules. Fundamentally, the self-assembling supramolecules are formed through the collective behaviors of noncovalent bonds among molecules, mainly including hydrogen * To whom correspondence should be addressed. Telephone: +886-4-285-2591. Fax: +886-4-285-4734. E-mail: jjlin@ dragon.nchu.edu.tw. † National Chung-Hsing University. ‡ Providence University.
bonds, electrostatic interactions (such as ionic and dipole-dipole), aromatic π-stacking, and van der Waals forces.11 Particularly, the chemical properties derived from the hydrogen bond association are ubiquitous in polymeric materials and in biological systems. Copolymers containing poly(ethylene glycol) (PEG) segments were found to have many applications because of their versatile forms of secondary structures involving hydrogen bonding and dipole-dipole electrostatic interaction.7 The materials can therefore exhibit the natures of hygroscopicity, semicrystalline, metal ion complexation, and so on. For example, PEG-incorporated poly(ethylene terephthalate) gave rise to hydrophilic and watersoluble copolymers.12 When adsorbed onto polyester fibers, the amphiphilic copolymers could enhance the surfactancy of nonionic or anionic surfactants. In relating to the poly(oxyethylene) hydrogen bonding, we previously prepared13,14 a family of poly(oxyethylene)amine segmented copolyamides which exhibited the property of electrostatic dissipating. The property can be tailored by controlling the weight amount of the incorporated poly(oxyethylene) (POE) segments which could facilitate the charge transferring in the solid state through the absorption of water molecules and segmental mobility. In this paper, we describe the preparation of a series of POE-grafted poly(styrene-b-ethylene/ butylene-b-styrene) (SEBS) and their properties in a solid and in solution. In this system, the SEBS backbone is considered as the hydrophobe capable of delivering inter- and intramolecular π-π interaction as well as
10.1021/ie9905456 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/30/1999
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Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000
hydrophobic effect. By grafting of the various lengths of POE pendants onto the SEBS backbone, the hydrophilic nature was introduced in different degrees through the hydrogen bond association. In the solid phase, the hydrophilicity mediated by hydrogen bonding with water was probed by measuring the surface resistivity. The different structures of the comb-branch amphiphilic copolymers were further correlated to their surfactancy in solutions. Their surfactancy, investigated in toluene/ water and in hexane/water, allows us to probe the relative importance of the hydrogen bonding and the π-π stacking in the system.
Table 1. Solubility of SEBS Derivativesa modified SEBS
toluene
cyclohexane
chloroform
DMF
H2O
SEBS-g-MA SEBS-g-MA/M2070 SEBS-g-MA/ED2001 SEBS-g-MA/D2000 SEBS-g-MA/M3000 SEBS-g-MA/ED6000
++ ++ ++ ++ ++ +
++ + ++ ++ + -
++ ++ ++ ++ ++ -
-
-
a + +, completely soluble; +, partially soluble; -, insoluble (10mg sample in 1 mL of solvent).
Table 2. Surface Resistivity of Poly(oxyalkyllene)amine-Modified SEBS
Experimental Section Preparation of the Poly(oxyalkylene)-Grafted Polystyrene-b-poly(ethylene/butylene)-b-polystyrene Copolymers. (1) Preparation of the SEBS-gMA/M2070 Adduct (1:1 Molar Ratio). To a 250-mL three-necked round-bottomed flask, equipped with a mechanical stirrer, nitrogen inlet-outlet lines, a thermometer, and a Dean-Stark trap, the maleated polystyreneb-poly(ethylene/butylene)-b-polystyrene copolymer (i.e., Shell Kraton 1901X, 2 wt % maleic anhydride, 30.0 g, 6.0 mmol of MA) and toluene/2-propanol (126 mL/14 mL) were placed with SEBS-g-MA. While the mixture was heated, stirred, and dissolved at 80-90 °C, methoxy-poly(oxyethylene-oxypropylene)-2-propylamine (i.e., Jeffamine M2070, ca. 2000 MW, 12.6 g, 6.0 mmol) was added in one portion. The reactants were further maintained at 120 °C for 3-4 h. During the process, most of the toluene/2-propanol was removed through a Dean-Stark trap. The crude product was then poured and dispersed in a large quantity of water with agitation. The product in toluene was further extracted by deionized water several times, rotary-evaporated, and dried to give a light-colored solid. The FT-IR (in KBr) showed the characteristic absorption at 1109 cm-1 (VS, C-O-C of POA), 1644 cm-1, and 1560 cm-1 (W, amide). A portion of products was dissolved in toluene and spincoated onto a glass plate (2.6 × 7.6 cm2) which was then dried in an oven at 80 °C over 12 h. The coated glass was conditioned in an atmosphere of 50% relative humidity before measuring the surface resistivity. The reading was 109.1 Ω/sq. (2) Preparation of the SEBS-g-MA/ED2001 Adduct (1:1 Molar Ratio). By use of similar procedures and equipment to the above example, the mixtures of SEBS-g-MA (30.0 g, 6.0 mmol of MA) in toluene/2propanol (126 mL/14 mL) and poly(oxypropylene)-bpoly(oxyethylene)-b-poly(oxypropylene) bis(2-aminopropyl ether) of 2000 MW (i.e., Jeffamine ED2001, 12 g, 6.0 mmol) were converted into the SEBS-g-MA/ED2001 graft copolymer. The product was extracted with water, dried, and identified by FT-IR (in KBr). The FT-IR (in KBr) showed the characteristic absorption at 1106 cm-1 (VS, C-O-C of POA), 1644 cm-1, and 1577 cm-1 (W, amide). A portion of products in toluene was coated on a glass plate. The surface resistivity was measured to be 108.1 Ω/sq. (3) Preparation of SEBS-g-MA/M1000 (1:1 Molar Ratio) in a Sodium Complex. With the same procedures, the SEBS-g-MA/M1000 copolymer was prepared from SEBS-g-MA (20.0 g, 4.0 mmol of MA) in toluene/ IPA (82.6 mL/10.4 mL) and methoxy-poly(oxyethyleneoxypropylene)-2-propylamine (i.e., Jeffamine M1000, 20 g, 4.0 mmol). During the workup procedure, the crude product was extracted with water and 8 wt % aqueous
modified SEBSa SEBS SEBS-g-MA ED2001 ED6000 SEBS-g-MA/M3000 SEBS-g-MA/M3000 SEBS-g-MA/M2070 SEBS-g-MA/M1000 SEBS-g-MA/M1000/Na+ SEBS-g-MA/ED2001 SEBS-g-MA/ED2001 SEBS-g-MA/D2000 SEBS-g-MA/ED6000 SEBS-g-MA/DAP SEBS-g-MA/M2070(180 °C)c
molar ratio
1:1 2:1 1:1 1:1 1:1 1:1 2:1 1:1 1:1 1:1 1:1
weight fractionb (%)
surface resistivity (10x Ω/sq)
100 100 37.5 23.1 28.6 16.7 16.7 28.6 14.3 28.6 54.5 2.7 28.6
11.4 10.6 10.0 9.8 8.1 8.6 9.1 9.1 7.3 8.3 8.6 10.0 9.0 10.6 9.0
a SEBS-g-MA: polystyrene-b-poly(ethylene/butylene)-b-polystyrene copolymer. M-1000: methoxy-poly(oxyethylene-oxypropylene)-2-propylamine at ∼1000 MW. M-2070: methoxy-poly(oxyethylene-oxypropylene)-2-propylamine at ∼2000 MW. M-3000: methoxy-poly(oxyethylene-oxypropylene)-2-propylamine at ∼3000 MW. DAP: N,N-dimethylaminopropylamine H2NCH2CH2CH2N(CH3)2. D-2000: poly(oxypropylene)-bis(2-aminopropyl ether) at ∼2000 MW. ED-2001: poly(oxypropylene)-b-poly(oxyethylene)-bpoly(oxypropylene)-bis(2-aminopropyl ether) ∼2000 MW. ED6000: poly(oxypropylene)-b-poly(oxyethylene)-b-poly(oxypropylene)bis(2-aminopropyl ether) ∼6000 MW. b Weight fraction (%): amine/ (SEBS + amine). c Reaction temperature at 120 °C, except noted.
sodium hydroxide. The resultant copolymer, dissolved in toluene, was spin-coated onto a glass plate which was then dried in an oven at 80 °C over 12 h. The coated glass was conditioned and the surface resistivity was measured to be 109.1 Ω/sq. Similarly, other POA-amine-grafted SEBS copolymers were prepared, as summarized in Table 2. In one case, the SEBS-g-MA/M2070 adduct was made under a higher temperature of 180 °C. The IR analysis indicated the formation of cyclic imide at the characteristic absorption of 1703 and 1800 cm-1. (4) Analyses. FT-IR was recorded by a Perkin-Elmer Paragon 500 FT-IR spectrometer. The thermal analysis was carried out by a Seiko SII model SSC/5200 differential scanning calorimeter at a heating rate of 10 °C/min. Surface resistivity was measured by a ST-3 model (Simco Co.) tester according to the ASTM method D257-93. Interfacial tensions were examined at 28 °C by a Kruss K10 tensiometer equipped with a LAB Thermo Cool LTC7D. The emulsion solution was examined by a 500X magnification of optical microscopy (Olympus, model BHT). Results and Discussion Syntheses and Structures of SEBS-g-MA/POA Copolymers. The poly(oxyalkylene)amines were grafted
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 67 Scheme 1. Representative Reaction and Product Structure of Maleated SEBS and Amine
Figure 1. Representative structures of SEBS-g-MA and POAamine adducts.
onto the SEBS triblock backbone according to the reaction described in Scheme 1. The reaction involved the reactive intermediate, SEBS-g-MA, and a family of poly(oxyalkylene)amines. In this study, the poly(oxyalkylene) (POA) groups can be divided in two classes, the hydrophobic poly(oxypropylene) (POP) and highly hydrophilic poly(oxyethylene) (POE). The poly(propylene glycol)-bis(2-aminopropyl ether) at ca. 2000 MW (i.e., Jeffamine D2000) is water-insoluble. The POE-rich amines include water-soluble, difunctional poly(oxypropylene)-b-poly(oxyethylene)-b-poly(oxypropylene)-bis(2aminopropyl ether) of 2000 and 6000 MW as well as a series of monofunctional methoxy-poly(oxyethylene)-bpoly(oxypropylene)-2-propylamine of 1000, 2000, and 3000 MW (i.e., Jeffamine M1000, M2070, and M3000, accordingly). These POA-amines are commercially available products, presumably prepared from a series of block ethoxylation/propoxylation and followed by the catalyzed amination.15,16 Each of these POA-backboned primary amines is readily grafted onto the reactive moiety of succinic anhydride in SEBS-g-MA. The SEBSg-MA has a well-defined structure consisting of an average 9 succinic anhydride moieties on each SEBS backbone of approximately 8000 MW for PS endblocks and 29 000 MW for an EB midblock. The amidation of SEBS-g-MA with POA-amines afforded the amido acids initially and subsequently the corresponding cyclic imides. During the reaction, the conversion can be easily monitored by the characteristic absorption in the carbonyl region of FT-IR for the formation of the amido acid intermediates and thermally more stable cyclic imides.17 These products can be isolated in powder form by precipitating out from toluene/water phases, except the SEBS-g-MA/ED6000 which formed an unusually stable emulsion in toluene/water. The structures of the SEBS-g-MA/POA copolymers are conceptually presented
in Figure 1. In the use of monoamines, the structure is presumably a comb-branch shape containing a SEBS backbone and multiple POA pendants. When diamines were involved in the synthesis, the products could be deviated from a comblike shape to a mixture of combbranch and network copolymers. Some of their solubility tests are summarized in Table 1. It appears these copolymers are soluble in toluene, cyclohexane, and chloroform, but insoluble in DMF or water. In the case of high MW of POE-amine (ED6000), the copolymer is sluggishly soluble in toluene. Hydrophilicity and Conductivity of the Copolymers. The amphiphilic graft copolymers consisting of polypropylene backbones and POE pendants were previously reported.18 Their surface resistivities were measured and correlated with the hydrophilicity. It was found that the presence of POE pendants facilitated the moisture absorption from the atmosphere and consequently enhanced the ability of electrostatic dissipation. The structural variations in the composition of POE segments and the degree of crystallinity are important factors in controlling the hydrophilic properties. For practical applications19,20 for antistatic agents, the hydrophobic backbone can further provide the compatibility for the host materials such as polypropylene. In this study, the newly prepared copolymers are consisted of the polystyrene-b-poly(ethylene/butylene)-b-polystyrene backbone which is targeted at materials such as polystyrene, acrylonitrile-butadiene-styrene, and other structurally analogous polymers. The surface resistivities of these prepared SEBS-g-MA/POA adducts are summarized in Table 2. In controlled experiments, it was shown that the SEBS-g-MA starting material had an observed 1010.6 Ω/sq, slightly lower than the reading of 1011.4 Ω/sq for SEBS. The incorporation of the POE pendants rendered the copolymers with even lower surface resistivity. As low as 108.1 Ω/sq was observed
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Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 Table 3. Surface Tensions (dyn/cm) of SEBS-Derived Amphiphilic Copolymers at Different Concentrationsa in toluene (g/100 mL)
in hexane (g/100 mL)
surfactants (concentration)
0.03
0.24
0.03
0.24
SEBS-g-MA/M2070 SEBS-g-MA/ED2001 SEBS-g-MA Span 60b none (blank)
31.0 30.8 31.0 29.9 (31.0)
30.8 30.0 30.5 30.3
20.2
20.3
19.9 (20.3)
19.9
a Measured by tensiometer in dyn/cm at 28 °C. b Span 60: Sorbitan monostearate (HLB ) 4.7).
Figure 2. Noncovalent bonding interaction in the toluene/water phase.
in using M3000 at a 1:1 molar ratio of MA to amine. Comparisons among the analogous amines of 1000, 2000, and 3000 MW reveal that the change in resistivity (109.1-108.1 Ω/sq) seems to be correlated to the weight fraction of the introduced POE groups. Surface resistivity of 108.6 versus 108.3 Ω/sq was measured when using the POE diamine (ED2001) at 1:1 and 2:1 molar ratios or the calculated POE weight fractions of 28.6 and 14.3 wt %, respectively. In comparison with SEBS-g-MA/ M2070 (monoamine), the SEBS-g-MA/ED2001 (diamine) appeared to have lower resistivity, 109.1 versus 108.3 Ω/sq. This may reflect the difference in the terminal functionalities by using ED2001 versus M2070. Thepresence of the -NH2 terminus instead of the -OCH3 contributed to lowering resistivity. The structural difference between the POE and the POP was noticed. The SEBS-g-MA/ED2001 at 108.3 Ω/sq was compared to the analogous SEBS-g-MA/D2000 having a reading of 1010.0 Ω/sq. The difference in hydrophilicity between oxyethylene and oxypropylene is therefore evidenced. Furthermore, the presence of POE pendants is essential because the N,N-dimethylaminopropylamine- (DAP-) modified SEBS copolymers had a relatively high surface resistivity (1010.6 Ω/sq). The hydrogen bond interaction with moisture is rationalized to be the main reason for enhancing conductivity, as depicted in Figure 2. The partial ionization to possess the charged protons may be the medium for electron transferring. This is indirectly evidenced that the addition of sodium ions through the complexation21-23 with POE or amido acids actually increased conductivity up to 107.3 Ω/sq. The factor of ionic conduction is involved in the water molecule association with POE segments. The effectiveness of increasing the hydrophilicity and the consequent surface conductivity appeared to be proportionate to the weight fraction of POA pendants, more accurately, the POE contents. It is noticed that the block composition of average EO/PO unit ratios are 19/3 for Jeffamine M1000, 32/10 for M2070, and 49/8 for M3000. Therefore, with the same weight fraction of POA, M3000 contributes more to the polymer hydrophilicity than M2070, because of its higher POE weight fraction in the molecule. This trend was shown by the results of SEBS-g-MA/M2070 (1:1 molar ratio or 28.6 wt %) and SEBS-g-MA/M3000 (2:1 molar ratio or 23.1 wt %), as 109.1 versus 108.1 Ω/sq. However, the ED6000derived SEBS-g-MA/ED6000 (1:1 molar ratio or 54.5 wt %) afforded a relatively low conductivity (109.0 Ω/sq). The difunctional hydrophilic amines contain 39/5 and 138/4 unit ratios of oxyethylene/oxypropylene per mole for ED2001 and ED6000, respectively. Particularly, in the case of ED6000, the high molecular weight of POE
segments gave rise to the relatively high degree of crystallinity, which reduced the polymer segmental mobility and hence the electrostatic charge transferring. This can be explained by their thermal analyses. In a differential scanning calorimetry analysis, SEBS-g-MA/ ED6000 exhibited a segmental melting temperature apparently higher than other analogues. On heating, SEBS-g-MA/ED6000 showed a Tm at 51.1 °C as a result of the segmental POE melting, compared to 33.9 °C for SEBS-g-MA/ED2001. The low segmental mobility23 of the matrix polymer may increase the barrier for electrostatic transferring. For reference, the starting POE amines (ED2001 and ED6000) in pure form have a relatively high surface resistivity (109.8-10.0 Ω/sq), indicating the low mobility in crystalline form. Surface Tension and Interfacial Tension. The SEBS-g-MA/POA in hydrocarbon solvents exhibited the unique surfactancy due to the distinct hydrophilic and hydrophobic functionalities in the same polymer strain. Three types of different functional blocksspoly(styrene), poly(ethylene/butylene), and poly(oxyalkylene)sconstitute the amphiphilic SEBS-g-MA/POA copolymers. These graft copolymers are hydrophobic in nature and soluble in organic solvents such as toluene and chloroform, but they are insoluble in water (Table 1). The high degree of hydrophobicity was evidenced by the measurement of their surface tensions in toluene and hexane. As shown in Table 3, when dissolved in toluene or hexane, the copolymers consistently showed no effect on the surface tension in the concentration ranging from 0.03 to 0.24 g/100 mL. Similar to the commercial Span 60 surfactant (HLB ) 4.7), the copolymers appeared to be too hydrophobic to interact with air. However, the hydrophilicity was demonstrated in hydrocarbon/water by measuring their interfacial tensions. The POA pendants in the SEBS copolymers demonstrated a strong affinity with water as measured in toluene/water, showing very low interfacial tensions. Summarized in Table 4, the interfacial tension of the blank toluene/ water was measured to be 31.0 dyn/cm and the unmodified SEBS in the concentration of 0.24 g/100 mL was 29.8 dyn/cm. The SEBS-g-MA was capable of lowering the tension slightly to 19.5 dyn/cm because of the presence of succinic anhydride or acid moieties. The association of succinic acids and water through hydrogen bonding could be the driving force for lowering the surface energy. This is in contrast with the measurement for the unmodified SEBS copolymer, which contains no polar functionality and therefore shows no effect on the toluene/water interfacial energy throughout the concentration examined. The driving force through the noncovalent bonding was tremendously enhanced by the presence of the POA pendants in the modified SEBS. In Table 4, a series of POA-amines-grafted SEBS-g-MA copolymers were measured to be around
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 69 Table 4. Interfacial Tension (dyn/cm) of SEBS-Derived Copolymers in Toluene/Watera concentration (g/100 mL) 0.24 0.12 0.06 0.03 0.003 0.0003 0.00003
SEBSb
SEBS-g-MAb
29.8 30.9 30.7 30.3
19.5 20.5 22.2 23.3
SEBS-g-MA/ M2070b
SEBS-g-MA/ D2000b
SEBS-g-MA/ ED2001b
5.0 6.4 7.0 7.4 13.8 16.3 23.0
7.5 7.6 8.0 7.9 11.2 17.5 24.6
9.5 11.4 10.8 12.1 15.6 19.0
a Measured by tensiometer in dyn/cm at 28 °C; blank concentration: 31.0 dyn/cm. b SEBS, unmodified poly(styrene)-b-poly(ethylene/ butylene)-b-polystyrene; SEBS-g-MA, maleated SEBS; SEBS-g-MA/M2070 and others, adducts at 1:1 molar ratio.
Figure 3. Interfacial tension of SEBS copolymers in toluene/water.
5-9.5 dyn/cm at 0.24 g/100 mL in toluene. Even in the very low concentration of approximately 3 ppm, both SEBS-g-MA/M2070 and SEBS-g-MA/D2000 still showed a low reading of 16-17 dyn/cm. To clearly demonstrate the efficacy, the interfacial tensions are plotted against their corresponding concentration in logarithm in Figure 3. All of these data were recorded at the end of 5 h after water was introduced as the second phase. The high efficiency of these copolymers for lowering interfacial tension in toluene/water was observed. It was also noted that the SEBS-g-MA/ED6000 grafted copolymer was less effective than the ED2001-derived analogue, perhaps because of the unbalance between the MW of the pendant with the SEBS hydrophobe. The 6000 MW for each pendant (or totally 54.5 wt %) is less suitable than the 2000 MW analogues. Furthermore, the M2070derived SEBS (SEBS-g-MA/M2070) at 120 °C enabled the lowering of the interfacial tension more than the analogue prepared at 180 °C. This is attributable to the difference in the polarity between amido acid and cyclic imide linkages in the structures. Noncovalent Bonding. The lowering of the interfacial energy can be explained by the relative noncovalent bonding between the copolymers and toluene/water environment. At least two kinds of distinct noncovalent bonds were interacting in the amphiphilic copolymerss the water/POA hydrogen bond associations and the toluene/SEBS π-π interactions. Both forces imposing on the SEBS-g-MA/POA copolymers rendered the amphiphilic property in toluene/water. The interactions are conceptually represented by Figure 2. The π-π aromatic interaction between polystyrene units and toluene as well as the hydrogen bonding between POA and water
Table 5. Comparison of Interfacial Tension (dyn/cm) in Toluene/Water and in Hexane/Water at Different Surfactant Concentrationsa surfactants (concentration) SEBS-g-MA/M2070 Span 60b none (blank)
in toluene/water (g/100 mL)
in hexane/water (g/100 mL)
0.03
0.24
0.03
0.24
7.4 9.4 (31.0)
5.0 3.8
20.2 7.3 (44.8)
21.2 3.9
a Measured by tensiometer in dyn/cm at 28 °C. b Span 60: Sorbitan monostearate (HLB ) 4.7).
are depicted. Interestingly, the importance of π-π interaction can be indirectly evidenced by comparing the interfacial tension in toluene/water and in hexane/ water, as shown in Table 5. The SEBS-g-MA/M2070 exhibited a very low interfacial tension in toluene/water, but only slight reduction in hexane/water. In hexane, the amphiphilic copolymer is mainly associated by the hydrophobic effect rather than by the π-π interaction in toluene. The importance of the π-π interaction was observed. However, for the hydrogen bonding in water, the difference between D2000- and ED2001-derived copolymers was insignificant according to the results in Figure 3. Self-assembling Film. It is also interesting to note the observation of a polymer-aggregating film during the measurement of interfacial tension in toluene/water. In the cases of SEBS-g-MA/M2070 and /ED2001 at high concentrations (0.12-0.24 g/100 mL), a thin film formation in the phase boundary was observed. The nature of this self-assembled film will be further characterized and reported separately.
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Figure 4. Decrease of interfacial tension in toluene/water over time.
Figure 5. Emulsion of toluene/water by SEBS-g-MA/ED6000 at 3.75 g/100 mL (magnification, 500×).
Kinetics of Forming Interphase Film. In a low concentration (3 ppm), the SEBS-g-MA/M2070 had an initial interfacial tension of 28.9 dyn/cm in toluene/ water within 30 min. However, the tension continuously dropped to reach the equilibrium of 17 dyn/cm after 39 h. For comparison, the commercially available Span 60 surfactant of HLB ) 4.7 (sorbitan monostearate) reached equilibrium at 28.8 dyn/cm after 2 h (Figure 4). The amphiphilic nature of such a high MW copolymer allowed slow diffusion from toluene to the water phase by the integrated forces between the π-π interaction and the hydrogen bonding. Emulsion. Besides the property of lowering interfacial tension in toluene/water and in hexane/water, the copolymers were capable of forming an emulsion mixture. Among these SEBS-derived copolymers, the POE of 6000 MW grafted SEBS (i.e., SEBS-g-MA/ED6000) formed a very stable emulsion in toluene/water by a simple mixing process. This type of emulsion was formed at a wide range of toluene/water/copolymer ratios and stable for months. One of these emulsions was observed and recorded by using a optical microscope (Figure 5). The average size of the emulsion particles
was estimated as 20 µM with some aggregations of 200300 µM in diameters. Conclusion The comb-branch copolymers consisting of a hydrophobic SEBS triblock and multiple hydrophilic POA pendants can be prepared. The multiple POE or POP pendants in the range of MW 1000-6000 rendered these amphiphilic copolymers the nature of hydrophilicity. In the solid state, the surface resistivity of these materials reached 1010-108 Ω/sq. The decrease of resistivity is proportional to the MW, the weight content, the termini of the hydrophilic pendants and their segmental mobility in the polymer matrix. The low MW pendant was less effective. In solutions, the POA-grafted copolymers were capable of emulsifying toluene/water. The interfacial tensions in toluene/water were measured to be as low as 5.0-9.5 dyn/cm at 0.24 wt % concentration. In a very low concentration such as 3 ppm, the interfacial tension slowly reached equilibrium. The comparison between their surfactancy in toluene and in hexane demonstrated the importance of the π-π interaction of
Ind. Eng. Chem. Res., Vol. 39, No. 1, 2000 71
polystyrene units with toluene and the hydrogen bonding of POE segments with water.
(12) Yang, C.; Rathman, J. F. Adsorption-Solution Structure Relationships of PET/POET Polymeric Surfactants in Aqueous Solutions. Polymer 1996, 37, 4621.
Acknowledgment
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Received for review July 22, 1999 Revised manuscript received September 17, 1999 Accepted September 21, 1999 IE9905456
