Phase Behavior of Magnesium Stearate Blended with Polyethylene

Apr 12, 2010 - Heating the blend to below the LAM melting point melts the co-crystals and allows pure, ... A generalized phase diagram for MgSt blende...
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Phase Behavior of Magnesium Stearate Blended with Polyethylene Ionomers Katsuyuki Wakabayashi# and Richard A. Register* Department of Chemical Engineering Princeton UniVersity, Princeton, New Jersey 08544-5263

The phase behavior in blends of magnesium salts of long-chain fatty acids with magnesium-neutralized ethylenebased ionomers is described. Anhydrous magnesium stearate and magnesium stearate-palmitate, collectively termed MgSt(Pm), show a common phase behavior, consisting of a crystalline lamellar mesophase (LAM) bilayer phase at low temperatures, a liquid crystalline hexagonal (HEX) phase at intermediate temperatures, and a molten disordered (DIS) phase at high temperatures. Once heated into the HEX or DIS phase, neat MgSt(Pm) is kinetically prevented from reforming the LAM phase upon cooling. Blending an ionomer into MgSt(Pm) destabilizes the HEX phase in favor of DIS; an intimately mixed DIS phase is formed above the LAM and ionomer melting points, where the ionomer acts as an effective solvent for the MgSt(Pm). Quenching this solution forms co-crystals that incorporate alkyl tails from the fatty acid salt and ethylene sequences from the ionomer. Heating the blend to below the LAM melting point melts the co-crystals and allows pure, well-ordered LAM MgSt(Pm) to crystallize from the solution. A generalized phase diagram for MgSt blended with ionomers is presented to describe the observed behavior. Introduction Fatty acid salts, also known as metal soaps, are salts of saturated or unsaturated long-chain R-carboxylic acids, typically derived from renewable animal and plant sources.1–4 Depending on the carboxylic acid chain length and the neutralizing cation type, these salts are used in a wide range of applications: as binding agents in pharmaceutical tablets, as heat-stabilizing and antistatic agents for plastics such as poly(vinyl chloride), and as the base for cosmetic powders. The amphiphilic nature of fatty acid salts also renders them useful as driers and thickeners in paints, inks, and lubricating greases, waterproofing agents in textiles, and as slip and release agents in rubber and plastics processing, where weak interactions with most polymers allow fatty acid salts to bloom to the surface. If, instead, the polymer is designed to interact strongly with the fatty acid salt, compatible blends can be obtained wherein large amounts of salt can be homogeneously incorporated, strongly modifying the polymer’s melt rheology and solid-state properties. Ionomers,5–7 which are polymers with a low level of covalently bound ionic functional groups, have been blended with fatty acid salts in several studies;8–14 the metal soaps serve as highly effective, nonfugitive plasticizers for the ionomers, often greatly enhancing melt processability.15 When a crystallizable fatty acid salt such as zinc stearate (n-octadecanoate; saturated C18 salt) is blended with rubbery ionomers such as sulfonated ethylene-propylene-diene terpolymer (SEPDM), an additional benefit is the increase in room-temperature tensile strength8–10 and Young’s modulus9 provided by the reinforcing salt crystals. Our previous work12 has shown that magnesium stearate and semicrystalline ethylene-based ionomers form cocrystals that strongly reinforce the material, while sodium stearate crystallizes out from the same ethylene-based ionomer to form a phase-separated composite structure. Such mechanical reinforcement of ionomers by incorporating metal soaps nicely complements reinforcement by inorganic additives such as layered silicates (clays), as described by Paul and co-workers.16–18 * To whom correspondence should be addressed. E-mail: [email protected]. # Present address: Department of Chemical Engineering, Bucknell University, Lewisburg, Pennsylvania 17837.

As much as these metal carboxylates influence the behavior of polymers to which they are added, they themselves also exhibit different properties upon blending. The structure and phase transformations of fatty acid salts have been investigated for over a century;19 yet, their phase behavior is still not fully elucidated, because the behavior varies greatly, depending on the length of the chain and the metal cation type, as well as the hydration state.2,4,20–22 For the widely used magnesium stearate,23–28 contradictions between literature reports often reflect uncontrolled evaporation of water in the temperature range of the polymorphic transitions. In this paper, we investigate the structure and phase behavior of magnesium stearate and a magnesium stearate/palmitate (n-hexadecanoate; saturated C16 salt) mixture, in blends with amorphous and semicrystalline magnesium-neutralized, ethylene/(meth)acrylic acid/n-butyl acrylate (E/(M)AA/nBA) ionomers, where the ionomer acts as a nonvolatile “solvent” for the fatty acid salt. Unlike most polymer-polymer blends,29,30 where weak interpolymer interactions yield phase-separated domains of nearly pure components, the intimate interactions between the ionomer and the fatty acid salt strongly affect the latter’s phase behavior (melting point, polymorph favored kinetically and at equilibrium)sthese considerations are important when these salts are used as modifiers for ionomers. Experimental Section Materials. The polymer, metal carboxylate, and blend samples were provided by DuPont Packaging and Industrial Polymers, except as noted below. The parent terpolymers were synthesized by high-pressure free-radical polymerization of ethylene (E) with methacrylic or acrylic acid ((M)AA) and n-butyl acrylate (nBA),7 yielding a statistical arrangement of the three monomers; the acid and nBA contents were determined by titration and infrared spectroscopy, respectively. The terpolymers are identified by a sample code (e.g., E/9AA/45nBA), indicating the nature (AA or MAA) and weight percentage of the acid monomer and n-butyl acrylate monomer (9 wt % AA and 45 wt % nBA, in this example). Blends were prepared with different grades of magnesium stearate, and following three different processing routes that have

10.1021/ie100109e  2010 American Chemical Society Published on Web 04/12/2010

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been previously described, to demonstrate that these are not critical to the findings herein. The blends with E/9MAA/23nBA were made by first melt-neutralizing 50% of the acid functional groups of the parent terpolymer with magnesium hydroxide in a twin-screw extruder, followed by separate melt-mixing of magnesium stearate (MgSt) from Witco Chemical (Grade D; 94% stearate, 3% palmitate) in a twin-screw extruder at 230-250 °C; different stearate contents (0, 2.5, 5, 10, and 40 wt %) are denoted by x in the sample code E/9MAA/23nBA50Mg:xMgSt. By contrast, a blend based on E/9AA/45nBA was prepared in a Haake mixer at 230 °C, where the terpolymer was blended, in a 59:41 w/w ratio, with a stearic-palmitic acid mixture from J.T. Baker (triple-pressed grade; min. 40% stearic, min. 40% palmitic, min. 90% stearic and palmitic combined) and sufficient magnesium hydroxide to neutralize 100% of the acid functional groups in both the polymer and fatty acid, in situ;12 the resulting blend is labeled E/9AA/45nBA-100Mg: 41MgSt(Pm). Lastly, E/8AA/16nBA was mixed with stearic acid from Witco (Hystrene 9718; 94% stearic acid, 3% palmitic acid), in a 65/35 w/w ratio, and was intentionally “over-neutralized” to 114% of stoichiometric with magnesium hydroxide in a twinscrew extruder at 230-250 °C. The resulting blend is denoted E/8AA/16nBA-114Mg:35MgSt. For comparison, the unblended magnesium salt of the stearic-palmitic acid mixture, denoted MgSt(Pm), was prepared by neutralizing the acid with the stoichiometric amount of dibutyl magnesium (Aldrich Chemical, dissolved in heptane) in tetrahydrofuran solution and drying in a fume hood. MgSt anhydrate powder was prepared by drying the as-received MgSt at 95 °C in a vacuum oven for 48 h. Prior to testing, the blends were compression-molded into sheets 0.2-0.5 mm thick at 150 °C using a PHI hot press, followed by a quench to room temperature. Molded sheets, and meltquenched MgSt anhydrate, were stored at room temperature in a desiccator over CaSO4 at room temperature for set aging times. Characterization. Both small-angle and wide-angle X-ray scattering (SAXS and WAXS, respectively) were employed, utilizing CuKR radiation from a PANalytical PW3830 X-ray generator. SAXS data were acquired with an Anton Paar compact Kratky camera, incorporating a custom hotstage for precise temperature control,31 and an MBraun OED-50M position-sensitive detector. The data were reduced to the desmeared absolute intensity (I/IeV) vs the magnitude of the scattering vector q, as described previously;32 q ) (4π/λ) sin θ, where λ is the X-ray wavelength (0.15418 nm) and θ is half the scattering angle. Room-temperature WAXS data were acquired with a Philips-Norelco wide-range goniometer retrofitted with an Advanced Metals Research graphite focusing monochromator for the diffracted beam, and a Philips Electronic Instruments scintillation detector. Variable-temperature WAXS data were acquired using a W. H. Warhus pinhole camera (Statton type)33 with hotstage; Kodak storage phosphor image plates were used to record the two-dimensional scattering patterns, which were read with a Molecular Dynamics PhosphorImager SI. The isotropic 2D patterns were azimuthally averaged to generate one-dimensional (1D) traces of intensity vs 2θ. Thermal characterization was conducted in a dry N2 environment on a Perkin-Elmer Model DSC7, equipped with an intracooler and Perkin-Elmer Pyris 1 software. Specimens (5-10 mg) were cut from compression-molded sheets or taken directly from the metal soap powders and crimped into standard

Figure 1. DSC thermograms for two MgSt specimens: (s) as-received (dihydrate), (- - -) vacuum-dried at 95 °C for 48 h (anhydrate), (- · · -) immediate rescan of the anhydrate after quenching from the 240 °C melt. The short trace at the top is that for the as-received MgSt, with the heat flow scale expanded by a factor of 10, showing the weak endotherm that corresponds to the HEX f DIS transition.

aluminum pans. All data presented herein were acquired on heating at 10 °C/min. Results and Discussion MgSt Hydration States. The hydration states and phase behavior of neat MgSt warrant some discussion at the outset, given the sometimes conflicting reports in the literature. MgSt exists in three different states, namely, anhydrate, dihydrate, and trihydrate;23–28 the latter two states have traditionally been the structures of principal interest, because of their lubricating properties.34 The hydrated forms of the long-chain alkyl salts are crystalline, although the packing of the alkyl tails is not sufficiently regular to unambiguously determine the crystal structure;27 however, they clearly possess a lamellar mesostructure with a surfactant bilayer motif.27,35 The anhydrate is easily obtained26 by heating to 95 °C, although higher temperatures can also induce transformations of the mesostructure, as discussed below. If the anhydrate is not exposed to higher temperatures, it shows a lamellar mesostructure very similar to that of the hydrated phases.20,27 However, the alkyl tail packing is significantly less regular: a hexatic arrangement with rotational symmetry along the chain axis, known as the rotator structure.12,27,36 As confirmed below by X-ray diffraction, the as-received specimen is the dihydrate,24 and as seen in the solid and dotted curves in Figure 1, the DSC melting thermograms of the as-received and anhydrous MgSt differ significantly. Decomposition of the dihydrate, transitioning to the anhydrous state with rotator structure, occurs at ∼100 °C;26 this transition is absent in the thermogram of the anhydrous sample. The endotherms near 120 °C in both curves correspond to melting of the anhydrate rotator structure. However, recovery of these endotherms, once the material is melted, is a very slow process, as will be discussed further below; the immediate reheat in Figure 1 shows only a broad endotherm peaking at 32 °C. MgSt Polymorphism. When MgSt is used as an additive in melt-processed polymers, the water of hydration would normally be removed (e.g., in a vented extruder); therefore, the remainder of this paper will focus on the anhydrate. In the preceding section, we stated that the endotherm near 120 °C corresponds to destruction of the lamellar (LAM) mesophase, and melting of the ordered (rotator) arrangement of the alkyl tails; this structural assignment is proven by SAXS and WAXS in Figures 2 and 3, respectively. Figure 2a shows variable-temperature SAXS patterns of the anhydrate during both an initial heat and

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Figure 2. (a) Hotstage SAXS patterns of MgSt anhydrate, at the temperatures indicated, collected consecutively from bottom to top; patterns are shifted vertically to avoid overlap. (b) DSC thermograms (same as Figure 1), where the points represent the temperatures of the SAXS patterns: (s) first heating scan, (- · -) immediate rescan after quenching from the 240 °C melt. These thermograms are intended as a general guide; the DSC temperature ramp is much faster than the stepwise heating employed in SAXS.

Figure 3. (a) WAXS patterns of MgSt, acquired with Statton camera. Bottom pattern: as-received dihydrate, at room temperature. Top four patterns: anhydrate at temperatures indicated, collected consecutively from bottom to top; patterns are shifted vertically to avoid overlap. (b) DSC thermograms of the anhydrate (same as in Figure 1), where the points represent the temperatures of the WAXS patterns: (s) first heating scan, (- · -) immediate rescan after quenching from the 240 °C melt. These thermograms are intended as a general guide; the DSC temperature ramp is much faster than the stepwise heating employed in WAXS.

a reheat, and the corresponding DSC thermograms are shown in Figure 2b. At 70 °C, the lamellar structure is clearly evident, with a primary peak at q* ) 1.37 nm-1 and higher-order peaks

at 2q* and 3q*, where the layer spacing is d ) 2π/q* ) 4.59 nm. At 100 °C, near the start of the melting endotherm, a fraction of the LAM mesophase melts, replaced by a disordered (DIS) structure that exhibits peaks near 2 and 4 nm-1. (Note that the 10 °C/min heating rate in DSC is much faster than the stepwise heating in SAXS; each SAXS pattern requires 15 min to collect. Moreover, the low thermal conductivity of powdery MgSt likely leads to some thermal lag in the DSC measurement, so precise correspondence between the two panels should not be expected.) At 110 °C, the SAXS pattern is essentially all DIS. We interpret this DIS pattern to indicate an inverse micellar phase, with salt headgroups aggregating within a hydrocarbon matrix, but without long-range order; the SAXS pattern is similar to that for other dense assemblies of hard spherical particles without long-range order, such as colloids37 and block copolymer micelles.38 As the specimen is further heated to 120 and then to 150 °C, however, a second set of narrow reflections emerges, with a primary peak at q* ) 2.45 nm-1 and secondary peaks at 3q* and 2q*, indicating the formation of the hexagonal (HEX) mesostructure.35 This suggests that the DIS structure present at 110 °C is not the equilibrium phase, but that the formation of highly ordered mesostructures such as HEX is impeded by the strong associations between salt groups. Upon further heating to 200 °C, the HEX structure is destroyed, replaced again by DIS. A very small endotherm (enthalpy 0.3 J/g) is visible in the DSC trace at 173 °C, which we associate with the HEX f DIS transition; this same small endotherm is also visible in DSC scans of the as-received dihydrate (Figure 1, where the endotherm is expanded for clarity), since the dihydrate converts to anhydrate well before 173 °C. The DIS phase appears stable at 200 °C; we did not observe the second HEX structure reported by Spegt and Skoulios35 over a narrow temperature range spanning 200 °C. Cooling the sample to room temperature does not regenerate the LAM structure; instead, a mixture of majority DIS and minority HEX is obtained. Upon reheating, only the DIS + HEX mixture is observed, again converting to all DIS by 200 °C. Corresponding hot-stage WAXS results for the anhydrate are shown as the top four traces in Figure 3a; for comparison, the bottom trace in Figure 3a shows the pattern for the as-received dihydrate.26 At 100 °C, the anhydrate shows a sharp peak at 2θ ) 21° from the rotator structure, superimposed on a broader hump centered at 2θ ) 19° from a disordered packing of alkyl chains. At 130 °C, only the broad hump remains, indicating the loss of rotator order when the LAM phase melts. Quenching to room temperature (RT2) regenerates a relatively narrow peak, but this peak is significantly broader than the peak at 100 °C in the initial heat, indicating a packing of the alkyl tails, which is less ordered than the initial rotator structure. Moreover, reheating to 80 °Cspast the endotherm visible in the DSC reheat tracesyields only the broad WAXS hump characteristic of fully molten alkyl tails. Therefore, the low-temperature endotherm in once-melted MgSt corresponds to the loss of order in the alkyl tails; the DSC reheat data in Figures 1-3 were taken in standard pans (not airtight), immediately after quenching from the 240 °C melt, so the endotherm cannot correspond to the loss of residual water, as may be the case for other preparations.26,27 Taken together, the results in Figures 2 and 3 indicate that the events detected by DSC in the anhydrate principally reflect changes in the state of order of the alkyl tails, with the HEX f DIS transformation near 173 °C yielding a thermal event that is barely detectable. Figure 4 shows that aging the once-melted MgSt anhydrate at room temperature in a sealed desiccator leads to a progressive,

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Figure 4. (a) DSC heating thermograms of MgSt anhydrate, taken at different times after quenching from the 240 °C melt. Peak melting temperature of partially ordered alkyl tails occurs at 32-50 °C, depending on the thermal history. (b) WAXS profiles of MgSt anhydrate, acquired with powder diffractometer: (top) after drying an as-received specimen at 95 °C, showing a sharp peak at 2θ ) 21.7°, which is characteristic of the rotator structure; and (bottom) after 1 h, 4 days, and 7 days of roomtemperature aging, following a quench from the 150 °C melt.

but very slow, development of order in the alkyl tails. This progression is evident by the increase in the endotherm’s peak temperature from 32 °C to 50 °C, and in the growth of a narrow component in the WAXS pattern near 2θ ) 21.5°. However, even after a week of aging, the structure is still considerably less ordered than prior to melting. A highly regular packing of the alkyl tails is undoubtedly frustrated by the mesostructure (mixed DIS + HEX, see top SAXS pattern in Figure 2a); in the LAM structure present before heating, the alkyl chains can attain an all-trans conformation (as needed for the rotator structure), while maintaining a uniform density throughout the hydrocarbon regionsa balance not possible with the HEX or DIS mesostructure. After 3 days of aging at room temperature, following a quench from the melt, a higher-temperature endotherm at 100 °C reappears (Figure 4a), suggesting that a fraction of the alkyl tails has achieved an extent of order approaching that in the original rotator structure. These trends strongly suggest that the initial LAM structure is the equilibrium polymorph at room temperature (and for all temperatures below 110 °C), but they also suggest that its formation from the DIS + HEX mesostructure is exceedingly slow; in this case, rearrangement is hampered not only by ionic associations, but also by the reduction in mobility due to the partial ordering of the alkyl tails. MgStPm vs MgSt. Depending on the natural source of the fatty acid, and any subsequent refining, “stearate” can contain substantial proportions of other fatty acid salts, particularly palmitate (Pm). Previous reports have suggested that inconsistent reports of MgSt polymorphs have resulted from differences in “grade” (especially St:Pm ratio).24 While the results in Figures 1-3 were for an almost-pure MgSt (94% St), here we show that an anhydrous, roughly equimolar mixture of MgStPm behaves quite similarly. The solid curves in Figure 5 correspond to the MgStPm after preparation and air-drying at room

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Figure 5. (a) DSC heating thermograms of MgStPm: (s) as prepared and (- · -) immediate rescan after quenching from the 200 °C melt. (b) WAXS profiles of MgStPm, acquired with powder diffractometer: (s) as prepared and (- · -) after quenching from the 200 °C melt.

temperature. The WAXS pattern shows a relatively narrow peak at 2θ ) 21.5° (although not quite as narrow as that for the MgSt anhydrate; see Figure 4) and a series of peaks which can be indexed to a LAM mesophase with q* ) 1.32 nm-1 (higherorder peaks at 2q*, 3q*, and 5q*). The less-ordered packing of the alkyl tails could reflect the different methods of preparation of the MgStPm and MgSt, but could also be due simply to the additional disorder induced by intimately mixing alkyl chains of different lengths. Heating this material produces a complex endotherm, with principal peaks at 77 and 104 °C. The similarity to the behavior of MgSt (Figure 1) suggests a common explanation: that the lower-temperature endotherm reflects dehydration to the anhydrate, while retaining the LAM mesophase and ordered packing of the alkyl tails, while the highertemperature endotherm reflects the destruction of both. The SAXS data in Figure 6 support this interpretation: upon heating to 90 °C, the LAM structure is largely retained, but with an increase in q* to 1.54 nm-1, reflecting a decrease in the bilayer spacing. These SAXS measurements were conducted under helium flow in a cell that was not leak-tight, so the water of hydration should be removed once evolved. Heating through the second endotherm, to 120 °C, leads to the complete loss of order and the formation of the DIS structure. Subsequent heating to 150 and then to 170 °C leads to the progressive emergence of a HEX structure, which dominates the pattern at 170 °C. The WAXS pattern of the MgStPm quenched from the melt (Figure 5b) indicates a disordered packing of the alkyl tails, and the DSC reheat (Figure 5a) shows a broad endotherm at 25 °C. The results for both MgSt and MgStPm suggest that the equilibrium mesostructure just above the melting point (∼110 °C) is HEX, but this structure is relatively slow to form. Interestingly, HEX formation seems to be significantly faster in MgStPm than MgSt, suggesting that some polydispersity in chain length can facilitate rearrangement into a regular mesostructure. Upon further heating, the HEX structure transforms to DIS (by 200 °C for MgSt). Given the sluggish formation kinetics of the ordered mesostructures, no attempt was made to pinpoint these transition temperatures more precisely. We also

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Figure 6. (a) Hot-stage SAXS patterns of MgStPm, at the temperatures indicated, collected consecutively from bottom to top; patterns are shifted vertically to avoid overlap. (b) DSC thermogram (same as initial heat in Figure 5a), where the points represent the temperatures of several of the SAXS patterns. The thermogram is intended as a general guide; the DSC temperature ramp is much faster than the stepwise heating employed in SAXS.

note that the ordered mesophases (LAM, HEX) in MgStPm (Figure 6a) are all characterized by a single set of reflections, indicating that the stearate and palmitate chains are intimately mixed into a single structure,27 despite their 12% difference in chain length. If phase separation into relatively pure MgSt and MgPm is thermodynamically favoredsas we might expect, at least for the LAM structure, where pure MgSt is able to achieve a high degree of rotator-like order in its alkyl tailssthen it too must be kinetically suppressed. Consequently, for the remainder of the discussion, MgSt and MgStPm will be used and treated interchangeably, denoted collectively as MgSt(Pm). MgSt(Pm) in Solution with Ionomer. The morphology and mechanical properties of selected blends of ethylene-based ionomers with MgSt(Pm) have been reported previously.12 The ionomer:MgSt(Pm) blends exhibit multilevel co-assembly: the ionic groups of the salt co-aggregate with the ionic groups of the ionomers in the melt, while long ethylene sequences in the ionomer slowly co-crystallize with the alkyl tails of the fatty acid into a rotator structure upon cooling to room temperature and aging for a period of days. The solid curve in Figure 7a is a heating thermogram of such a well-aged E/8AA/16nBA114Mg:35MgSt blend; the endotherm at 59 °C was previously found12 to reflect melting of the rotator co-crystals. The SAXS data in Figure 7b show that, at room temperature, this material exhibits the broad peak characteristic of these co-crystals, near q* ) 1.2 nm-1. Upon heating to 65 °C, a peak at q* ) 0.8 nm-1 emerges, which disappears upon further heating to 80 °C and leaves only a single peak at q* ) 2 nm-1, which is characteristic of the DIS structure. This lower-q peak visible between 60 °C and 80 °C suggests that ethylene crystallites form as the ionomer:salt co-crystals melt; when all the crystals, including the ethylene crystallites, melt at ∼90 °C, a homogeneous DIS blend phase is recovered. Figure 8 shows analogous data for a blend with an ionomer of high termonomer content, which by itself is incapable of ethylene sequence crystallization. The solid curve in Figure 8a

Figure 7. (a) DSC thermogram of an E/8AA/16nBA-114Mg:35MgSt blend, aged at room temperature for 10 days prior to heating. (b) Hot-stage SAXS profiles of the same blend, acquired consecutively: (f) 25 °C, ([) 60 °C, (9) 65 °C, (2) 80 °C, and (b) 90 °C. Note the low-q peak from polyethylene crystallites visible at 65 °C.

Figure 8. (a) DSC thermograms of two specimens of an E/9AA/45nBA100Mg:41MgStPm blend, aged at room temperature for 10-18 days prior to examination: (s) heated normally from 0 to 130 at 10 °C/min, (- · -) annealed at 70 °C for 2 h prior to being heated from 0 to 130 at 10 °C/min. (b) Hotstage SAXS profiles of the same well-aged blend, acquired consecutively on heating from room temperature: (f) 60 °C, ([) 70 °C, (2) 80 °C, (9) 100 °C, and (b) 120 °C. Third-order reflection visible at 80 °C is indicated.

is a heating thermogram of a well-aged E/9AA/45nBA-100Mg: 41MgStPm blend; the endotherm at 67 °C corresponds to melting of the rotator co-crystals.12 It is interesting that the peak melting point of the co-crystals, which incorporate sequences from an otherwise-amorphous ionomer, is slightly higher than that for co-crystals incorporating sequences from moderately

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crystalline ionomers (59 °C, Figure 7a). Hotstage SAXS data in Figure 8b reveal the development of an intermediate structure upon heating the well-aged ionomer:MgStPm blend. Between 60 and 70 °C, the initial broad peak begins to narrow, and by 80 °C, the SAXS pattern shows a sharp peak at q* ) 1.38 nm-1, with a clear third-order peak at 3q*, as for the LAM mesophase in pure MgSt(Pm) (see Figures 2 and 6). At 100 °C, most of this LAM structure has melted, and the pattern is dominated by a broad peak at q* ) 2.2 nm-1 from a DIS structure; by 120 °C, the LAM structure has completely converted to DIS, with no structural transitions occurring at higher temperatures. The ionomer may be viewed as a “solvent” for the MgSt(Pm). At high temperatures, where neat MgSt(Pm) shows a HEX phase with molten alkyl tails, the two components are intimately mixed in the blend, with the ionic groups from the two components forming mixed aggregates in an organic matrix.12 Above 120 °C, only the DIS structure is present in the blend, as in the pure ionomer,12 because the random distribution of sequence lengths between the ionomer’s ionic groups disfavors the formation of highly regular mesophases such as HEX. However, at lower temperatures, there is still a driving force for MgSt(Pm) to “crystallize out” from the blend into an ordered LAM structure, thereby achieving an enthalpically favorable packing of the soap’s alkyl tails. To explore this process in more detail, we annealed a specimen in the DSC at 70 °C to promote crystallization of the MgSt(Pm), and to better mimic the slower, stepwise heating used to acquire the SAXS data in Figure 8b. The broken curve in Figure 8a is the heating thermogram of this blend immediately following 2 h of 70 °C annealing, which reveals two melting events: the major peak, at 102 °C, corresponds to melting of the LAM MgStPm crystals which formed during annealing, while the smaller peak at 91 °C corresponds to the melting of residual ionomer:soap co-crystals (presumably, reflecting crystals at the more-stable end of the distribution, which were not melted by the 70 °C annealing). Recall that neat anhydrous MgSt(Pm), once melted, does not re-form the LAM structure on any reasonable time scale. Yet, surprisingly, blending MgSt(Pm) with an ionomersdespite the ionomer’s extremely high melt viscositysprovides enough mobility for the MgSt(Pm) to re-form these LAM crystals. Although Figure 8b clearly shows that MgSt(Pm) crystals are stable in this blend at 80 °C, they do not form by simply quenching the blend from the melt to 80 °C, as gauged by several hours of examination by time-resolved SAXS. For this crystallization process to occur at a measurable rate at 70-80 °C, the alkyl tail:ethylene co-crystals first must form (by quenching and annealing at a lower temperature, e.g., room temperature); a fraction of these co-crystals appear to provide “seeds” for the crystallization of LAM MgSt(Pm) crystals, which form after a subsequent annealing at the higher temperatures. Phase Diagram of Ionomer:MgSt(Pm). These results prompt us to construct a phase diagram for MgSt(Pm) blended with an ionomer, for which we acquired additional data on the E/9MAA/23nBA-50Mg:xMgSt series; Figure 9 shows hot-stage SAXS data for x ) 40 and x ) 10. At 40 wt % MgSt (panel (a)), the sequence of SAXS patterns is very similar to that in Figure 8b, with a narrow peak at q* ) 1.40 nm-1 (and at 3q*) emerging at 80 °C, characteristic of the MgSt LAM structure; these peaks then disappear just above 100 °C as a DIS phase forms. At 10 wt % MgSt (panel (b)), by contrast, the SAXS patterns show no sign of the MgSt LAM structure at any temperature; the patterns resemble those of unblended semicrystalline ethylene-based ionomers, with a peak at q ) 0.5 nm-1 and an ionic aggregate hump at q ) 2 nm-1, both quite similar

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Figure 9. Hot-stage SAXS profiles of two blends, previously aged at room temperature for more than two months, acquired consecutively on heating from room temperature to the temperatures indicated: (a) E/9MAA/23nBA50Mg:40MgSt (at (f) 60 °C, ([) 80 °C, (9) 90 °C, (2) 100 °C, and (b) 150 °C) and (b) E/9MAA/23nBA-50Mg:10MgSt (at (f) RT, ([) 60 °C, (9) 70 °C, and (2) 85 °C).

Figure 10. Phase diagram for the anhydrous MgSt:E/(M)AA/nBA ionomer system. (Abbreviations: LAM, lamellar mesophase; HEX, hexagonal mesophase; and DIS, disordered melt.) Within the LAM + DIS region, mass fractions of LAM and DIS phases (or LAM and PX phases, below the dashed lines) are expected to be dictated by the lever rule in the usual fashion, while the HEX region is intended to represent a uniform phase which crosses to DIS at higher temperatures or ionomer contents. Open circles connected by solid line, dividing the DIS and PX regions, correspond to melting points measured in the E/9MAA/23nBA-50Mg:xMgSt series; however, this boundary depends on the ethylene content for such statistical terpolymers; for comparison, two dashed lines are shown, which represent the hypothetical DIS-PX phase boundaries for blends based on ionomers of higher ethylene content (higher melting point).

to what is observed for unblended E/9MAA/23nBA ionomers. The shift from q* ) 0.6 nm-1 to lower q with increasing temperature is indicative of progressive melting of polyethylene crystals in the ionomer.39 The blend samples of E/9MAA/ 23nBA-50Mg:0MgSt,:2.5MgSt, and:5MgSt (not shown) all behave in the same manner. Therefore, it is apparent that a minimum of 10 wt % MgSt dissolves into the ionomer at these temperatures (ca. 70 °C). A schematic phase diagram is presented in Figure 10. On the left axis (pure MgSt anhydrate), the stable phases are LAM, HEX, and DIS with increasing temperature. Direct measurement of the phase behavior at low ionomer contents is complicated

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by the practical difficulty of homogeneously dispersing a small quantity of ionomer in MgSt. However, we infer that the solubility of ionomer in the LAM MgSt crystals is insignificant, given the structural similarity between the crystals that form in the blends (see Figures 8 and 9) and those in pure MgSt (see Figure 2). The solubility of ionomer in the MgSt HEX phase should be somewhat larger but still small, as the sequence disorder in the ionomer will quickly destabilize the HEX phase vs the DIS phase. Therefore, the HEX region is shown as a narrow region in Figure 10, extending up to ca. 10 wt % ionomer; again, measuring the actual width of this region would be practically challenging. At the right of the diagram, the open points connected by a solid line correspond to the melting points of the primary polyethylene crystals (PX) in the E/9MAA/ 23nBA-50Mg:xMgSt series (x ) 0, 2.5, 5, 10). However, this polymer has a relatively low ethylene content and, therefore, a short average ethylene sequence length and a low melting point; the ethylene content is, of course, adjustable at the time of polymerization, with most ionomers having a higher melting point.40 The two dashed curves lying above the solid curve are intended to represent the DIS-PX phase boundary for blends containing two more conventional ionomers of higher ethylene content and melting point. At some temperature and composition, the dashed curve intersects the solid curve corresponding to the top of the LAM + DIS dome, in a eutectic-like fashion; at this point, polyethylene crystals (PX) and MgSt LAM crystals melt simultaneously to a DIS phase. However, the analogy with well-studied eutectic systems, such as metal alloys, is somewhat misleading, because considerable DIS material exists in the PX region: these are low-crystallinity polymers, and the intercrystalline amorphous regions (with a characteristic dimension of 10 nm) contain ionic aggregates, as in the melt (DIS).41 In other words, the material is not all crystallized below the apparent eutectic temperature; microscopically, one could find regions in the blend consisting of MgSt LAM, crystalline polyethylene, and amorphous organic material containing ionic aggregates (from the amorphous fraction of the ionomer plus some dissolved MgSt). In practice, the structure observed below this intersection point appears dominated by kinetically favored (although likely nonequilibrium) co-crystals between the two components, complicating elucidation of the phase diagram; note that even polymers such as E/9AA/45nBA, which cannot crystallize on their own (i.e., where the intersection point is below room temperature), easily form co-crystals in the composition midrange (see Figure 8). Conclusions Anhydrous MgSt and MgStPm show a common phase behavior: a lamellar mesophase (LAM) bilayer structure, with rotator order of the alkyl tails, up to ∼110 °C; an ordered HEX phase with molten alkyl tails up to ∼190 °C; and a DIS phase at higher temperatures. The reversibility of these transitions is severely impeded by the strong associations between the polar headgroups, and at low temperatures, the order that develops in the alkyl tails, such that these transitions are practically irreversible in neat MgSt(Pm). Mg-neutralized E/(M)AA/nBA ionomers are fully miscible with MgSt(Pm) in the melt, forming co-assembled ionic aggregates constituting the DIS phase; the inherent sequence disorder in the statistical terpolymer ionomer destabilizes formation of the ordered HEX phase. However, LAM MgSt(Pm) can crystallize out of “solution” with the ionomer, although neat MgSt(Pm) is kinetically impeded from recrystallizing into LAM. Formation of LAM MgSt(Pm) crystals in the blend is facilitated by crystal “seeds” which form by co-

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ReceiVed for reView January 17, 2010 ReVised manuscript receiVed March 19, 2010 Accepted March 30, 2010 IE100109E