Phase changes of Poly(alkoxyphosphazenes), and their behavior in the presence of oligoisobutylene.
Polyphosphazenes are hybrid inorganic-organic macromolecules in which the properties are determined by both the inorganic phosphorus-nitrogen backbone and the organic side groups. The backbone imparts properties such as fire resistance, thermo-oxidative stability, macromolecular flexibility, and low glass transition temperatures, while the side groups control solubility and many other properties including an influence on overall molecular flexibility. Several hundred different polyphosphazenes are now known, each with a combination of different properties and solubilities. The blend-type interactions of polyphosphazenes with classical polymers that provide opportunities for polymer-polymer coordination such as polyesters [1, 2] and polyelhers have been studied as part of efforts to integrate the sometimes unique properties of polyphosphazenes with those of classical organic polymers. However, relatively little has been reported about the interactions of polyphosphazenes with low or medium molecular weight hydrocarbon polymers or oligomers that are widely used in the field of lubricants and sealants.
Oligo - and poly-isobutylenes (1) are important liquids, gums, and elastomers in technology. They also have a number of structural and physical properties that are similar to several alkoxy-substituted linear polyphosphazenes (2) (Fig. 1). For example, they are all amorphous macro-molecules with liquid-, gum-, or elastomerlike morphology, and with low glass transition temperatures. They are characterized by unusual high chain flexibility due to the low barrier to torsion of the backbone bonds. In the case of alkoxy - and alkyl ether-substituted polyphosphazenes, this chain flexibility is accentuated by the high torsional mobility of short chain alkoxy or alkyl ether side chains. The solubility of one polymer in another or of a polymer in an oligomeric liquid is favored by enthalpy-lowering interactions between the structural components of the two macromolecules. However, given the well-known entropy-driven disincentives for mixing that exist when two high polymers are brought together, the overall properties of multicomponent systems depend on the ability of polymers with similar structures and similar characteristics to form homogeneous mixtures.
Polyoleflns such as polyisobutylene [3-5], polypropylene [6, 7], polybutylene , polyethylene , and polybutadiene  form blends with related classical organic polymers. This blending effectiveness has been studied using techniques such as NMR [3, 5, 10], differential scanning calorimetry (DSC) , small angle neutron scattering (SANS) , wide angle x-ray scattering (WAXS) , melt Theological measurements , and dynamic mechanical thermal analysis . These techniques are used to determine the extent to which polymer mixtures form miscible, compatible, or immiscible polymer systems. In the case of DSC experiments, the following criteria are employed to determine the character of a oolvmer mixture: (1) Miscible blends are characterized by a single glass transition (Tg), and are the result of the two polymers interacting at the microscale level; (2) compatible blends cause shifts of both parent Tg's as a result of weak polymer-polymer interactions; and (3) immiscible blends are identified by no shift in the Tg's of the parent polymers. Most polymer blends are favored by polymer chain entanglements and low molecular weights, both of which reduce the entropy required to mix two macromolecules . Low molecular weight polymers or oligomers can act as a solvent for another polymer to form miscible blends and may also be characterized using the above criteria. For example, low molecular weight polyisobutylene is a good solvent for polyethylene .
[FIGURE 1 OMITTED]
With these facts in mind, it was of interest to examine the ability of low or medium molecular weight oligoisobutylene to serve as a solvent for and a blend component with polyphosphazenes. Interest in such systems arises from the fire-resistance of some polyphosphazenes, and the low Tg and cation-coordination ability of others, quite apart from the fundamental scientific issues involved.
In this work, polyphosphazenes were synthesized with side groups such as propoxy, pentoxy, hexoxy, octoxy, isostearyloxy, and 2-(2-methoxyethoxy)ethoxy (MEE). The lengths of the alkyl side chains were varied to determine the influence of this feature on the solubility and miscibility of the polyphosphazenes in low molecular weight polyisobutylene. Concurrently, these same alkoxy side groups were combined with cosubstituent MEE side groups to form mixed-substituent polyphosphazenes. The solubilities of the polyphosphazenes in oligoisobutylene were determined at room temperature (25 [degrees] C) and at elevated temperature (80 [degrees] C). After cooling, these solutions/ blends were then examined by differential scanning calorimetry (DSC) to monitor any changes in Tg. The polyphosphazenes were also studied by means of thermogravi-metric analysis (TGA) to estimate their potential thermal stability in mixed polymer/oligomer systems
Reagents and Equipment
Polymer synthesis reactions were carried out under a dry argon atmosphere using standard Schlenk line techniques. Tetrahydrofuran (EMD) was dried using solvent purification columns . Propanol, pentanol, hexanol, octanol (Sigma-Aldrich), and 2-(2-methoxyethoxy)ethanol (Acros) were dried over calcium hydride, distilled, and stored over molecular sieves. Isostearyl alcohol, oligoiso-butylene (Mw = 880 g/mol, PDJ = 1.02) (Penreco), and a 60% sodium hydride dispersion in mineral oil (Aldrich) were used as received. Poly(dichlorophosphazene) was prepared via the thermal ring-opening polymerization of recrystallized and sublimed hexachlorocyclotriphosphazene (Fushimi Chemical Co., Japan) in evacuated Pyrex tubes at 250 [degrees] C. This was then used as a reactive macro-molecular intermediate for reactions with sodium alkox-ides. 3IP and (H NMR spectra were obtained with a Bruker 360 WM instrument operated at 145 and 360 MHz, respectively. Glass transition temperatures were measured with a TA Instruments Q10 differential scanning calorimetry apparatus with a heating rate of 10 [degrees] C/ min and a sample size of ca. 10 mg. Gel permeation chro-matograms (GPC) were obtained using a Hewlett-Packard HP 1100 gel permeation chromatograph equipped with two Phenomenex Phenogel linear 10 columns and a Hewlett-Packard 1047A refractive index detector. The samples were eluted at 1.0 mL/min with a 10 mM.solution of tetra-n-butylammonium nitrate in THF. The elution times were calibrated with polystyrene standards. Thermal degradation estimates were obtained using a Perkin Elmer TGA7 Thermogravametric Analyzer with Perkin Elmer TAC 7/DX Thermalanalysis Controller with a heating rate of 10 [degrees] C/min.
Synthesis of Poly(alkoxyphosphazenes) (Polymers 3-6)
Polymers 3-6 were synthesized in a similar manner following previous synthetic procedures [13, 14]. The resulting polymers were characterized by [P.sup.31] P and [H.sup.11] H NMR spectroscopy to confirm full replacement of the chlorine atoms. The final yields were 80-90% based on the reactive intermediate poly(dichlorophosphazene).
Synthesis of Poly(diisostearyloxyphosphazene) (Polymer 7)
Polymer 7 was synthesized using the following procedure. Poly(dichlorophosphazene) (2.00 g, 17.3 mmol) was dissolved in dry THF (200 mL). Isostearyl alcohol (18.67 g, 69.0 mmol) was treated with 60% NaH (2.08 g, 51.9 mmol) suspended in THF (50 mL). The alkoxide solution was then added to the poly(dichlorophosphazene) solution, and the mixture was stirred under reflux for 48 h. It was then concentrated via rotary evaporation and precipitated into methanol. The product was further purified by precipitation twice from THF into ethanol and water. Final structural characterization was by [P.sup.31] P and [H.sup.1] H NMR spectroscopy. The overall yield was 85%.
Synthesis of Poly[bis(2 -(2 -methoxyethoxy)ethoxy)phosphazene] (Polymer 8) (MEEP)
Polymer 8 was prepared by a method described previously [14, 15, 16].
Synthesis of Poly((alkoxy)[alkoxy.sub.1](2-(2-methoxyethoxy)ethoxy) [alkoxy.sub.1] phosphazenes) (Polymers 9-13)
Polymers 9-13 were synthesized by similar procedures. The synthesis of polymer 9 is described as an example. Poly(dichlorophosphazene) (5.00 g, 43.1 mmol) was dissolved in dry THF (500 mL). 2-(2-Methoxyethoxy)ethanol (6.21 g, 51.7 mmol) was treated with 60% NaH (1.72 g, 43.1 mmol) suspended in THF (150 mL). This solution was added dropwise to a dilute solution of poly(dichloro-phosphazene) over 1 h. Propanol (3.62 g, 60.3 mmol) was added to 60% NaH (2.07 g, 51.7 mmol) suspended in THF (150 mL). The sodium propoxide solution was added to the polymer mixture, which was stirred for 24 h under reflux. The final polymer solution was concentrated via rotary evaporation and was precipitated into methanol. The product was further purified by precipitation twice from THF into ethanol and then from THF into water. The final product purity was confirmed by 31P and [H.sup.1] H NMR spectroscopy. The overall yields were 75-80%.
The solubility of the polyphosphazenes in oligoisobuty-lene was determined using the following method. Oligoi-sobutylene (25 g) was placed in a mechanical mixer at 25 [degrees] C. The polyphosphazene (1 wt%) was added, and the mixture was stirred. Following complete dissolution another 1 wt% polyphosphazene was added until the solubility limit was reached at room temperature. After the limit of room temperature solubility was reached, the polymer solution was then heated to 80 [degrees] C, and solubility testing resumed until the solubility limit was reached at 80 [degrees] C
RESULTS AND DISCUSSION
Synthesis of Polyphosphazenes
Two variants of the polyphosphazene side group structural arrangement were examined. First, polymers 3-7 are single-substituent species with increasing lengths of the hydrocarbon component in the alkoxy side chains, culminating with the isostearyloxy derivative 7. Polymers 3-6 have been described in previous reports . Polymer 7 is new. Table 1 shows the 31P and 1H NMR shifts and thermal transitions of these polymers. Polymer 7 is a pale yellow elastomer (Tg - 73 [degrees] C) that is soluble in THF.
TABLE 1. Characterization data for polymers 3-13, OIB and PIB. Polymer [P.sup.31]P [H.sup.1]H T[T.sub.g] Molecular NMR (ppm) NMR (ppm) ([degrees] Weight (a) C) (g/mol) 1 (OIB) - 0.9(6H), -87 880 1.1 (2H) 2 (PIB) - 0.9(6H), -65 890,000 1.1 (2H) 3 -7.3 0.9(6H), -99 710,000 1.6(4H), 3.9(4H) 4 -5.4 0.9(10H), -102 530,000 1.2(4H), 1.6(4H), 3.9(4H) 5 -5.3 0.9(6H), -104 1,347,000 1.3(12H), 1.6(4H), 3.9(4H) 6 -7.0 1.1 (6H), -104 2,030,000 1.5(20H), 1.9(4H), 4.1(4H) 7 -7.8 3.3(3H), -80 1,132,000 (MEEP) 3.5(2H), 3.62(2H), 3.66(2H), 4.0(2H) 8 -11 3.2(5H), -67 2,970,000 3.4(3H), 3.5(4H), 3.6(2H), 4.0(2H), 4.2(2H) 9 -5.7 0.8(3H), -74 131,000 1.2(4H), 1.6(2H), 3.3(3H), 3.5(4H), 3.6(2H), 4.0(4H) 10 -5.3 1.2(9H), -87 296,000 1.6(2H), 3.5(7H), 3.6(2H), 3.8(4H) 11 -5.2 1.2(3H), -86 1,009,000 1.6(10H), 1.9(2H), 3.5(2H), 3.6(3H), 3.8(4H), 3.9(2H), 4.1(2H) 12 -5.2 0.9(12H), -65 217,000 1.3(44H), 1.6(4H), 1.7(4H), 3.9(2H) 13 -5.3 1.1(9H), -66 / -80 1,779,000 1.6(24H), 1.9(3H), 3.5(4H), 3.8(4H), 4.1(2H) Polymer PDI (b) Solubility in OIB at 80[degrees] C (wt/wt%) 1 (OIB) 1.02 - 2 (PIB) 1.33 - 3 2.40 1 4 1.75 5 5 1.83 9 6 1.95 11 7 1.98 - (MEEP) 8 1.86 3 9 1.83 0 10 3.22 0 11 1.79 0 12 1.82 5 13 1.93 21 (16 @ RT) (a)The molecular weights approximate to M[M.sub.w] (b) PDI was determined by M[M.sub.w]/M[M.sub.n]
Second, a series of mixed-substituent polyphosphazenes, 9-13, were prepared, all of which contained the methoxyethoxyethoxy (MEE) side group together with the same alkoxy groups utilized in the single-substituent mac-romolecules. The syntheses of 9-13 are illustrated in Scheme 1. All five mixed-substituent polymers were obtained by the reactions of poly(dichlorophosphazene) with the appropriate alkoxide and etheric nucleophiles. Random substituent distributions were favored by the initial addition of 2-(2-methoxyethoxy)ethoxide dropwise to a dilute, stirred solution of poly(dichlorophosphazene) over a 1 h period, followed by the addition of the second alkoxide to the polymer solution. The molecular structures were confirmed by [P.sup.31] P and [H.sup.1] H NMR spectroscopy. All these mixed-substituent polymers are pale yellow gums that are soluble in THF.
Glass Transition Temperatures
DSC analysis was used to identify second-order transitions of all 10 polyphosphazenes, oligoisobutylene, and polyisobutylene. The values are shown in Table 1. All the polymers except 13 had a single glass transition temperature (T [T.sub.g]) in the range of -65 [degrees] C to - 104 [degrees] C, and only one 31P NMR signal that indicated random distribution of the different side groups in the mixed-substituent polymers. By any standard these indicate a high order of molecular flexibility, similar in many ways to poly(dimethylsiloxane) or polyisobutylene. However, polymer 13 had two T [T.sub.g] 's, at - 66 [degrees] C and -80 [drgees] C, even though it gave only a single 31P NMR signal. Possible reasons for this dual glass transition temperature are discussed below.
The thermal transition behavior of polymer 13 is unusual for polyphosphazenes. Polymer 13 in a solution of deuterated tetrahydrofuran gives a single 3lP NMR signal at -5.3 ppm, which suggests that nearly every phosphorus atom bears one isostearyloxy and one 2-(2-methoxye-thoxy)ethoxy side group. In other words, there were no [P.sup.31] P signals detected that would be consistent with the presence of two side groups of the same type on a given phosphorus atom or of a blocky structure. Because [P.sup.31] P NMR spectroscopy is sensitive only down to 5% concentrations of a given species, this suggests that at least 95% of the repeating units have a randomized side group disposition. This structure should generate a polymer that has a single glass transition (T [T.sub.g]) in a DSC experiment. However, this is not the case as seen in Fig. 2. Polymer 13 has two second-order transition temperatures, at - 66 [degrees] C and - 80 [degrees] C. Two separate transitions suggest the existence of a blocky structure with a different T [T.sub.g] for each block, the existence of a two-domain morphology, or the onset of two different types of coordinated side group motions. Polymer 13 is not a block copolymer as indicated by the single [P.sup.31] P NMR signal. The possibility of micro- or nano-scale domain segregation cannot be discounted. However, the marked differences in hydrophobicity of the two different types of side groups suggest that the side groups, though randomized, are organized in an unusual manner. Thus, isostearyloxy groups are associated with their counterparts, as are the alkyl ether side groups, and the cross combination interactions are minimized. A possible orientation is illustrated in Fig. 3. Each individual domain would affect the backbone mobility in a different way, thus causing two second order transitions to appear in the DSC trace (see Fig. 2).
[FIGURE 2 OMITTED]
Solubility and Miscibility of Polymers 3-13 in Oligoisobutylene (OIB)
The solubilities of polymers 3-13 in oligoisobutylene depend on the side groups linked to the polyphosphazene chains. The presence of alkoxy groups with the longer hydrocarbon chains allows solvation type interactions with oligoisobutylene due to the similar polarities of the different constituents. It also leads to chain entanglements between OIB and the polyphosphazene alkoxy side groups. Conversely, MEEP (8) is insoluble in OIB probably due to the dissimilar polarities. However, when both long carbon chain alkoxy and 2-(2-methoxyethoxy)ethoxy side groups are present (9-13) the solubility increases.
The solubilities of the single-substituent poly(dialkoxy-phosphazenes) in OIB are summarized in Table 1. The solubility is low, but it increases to a small extent as the lenglh of the hydrocarbon side chain is increased. For example, the solubility of poly(dipropoxyphosphazene) (3) is only 1 wt/wt% in OIB at 80 [degrees] C, but poly(dipentoxy-phosphazene) (4) has a solubility of 5 wt/wt%, and so on. However, at some point a solubility threshold is reached and further increases in alkoxy chain length cause a plateau or a decrease in solubility. This is demonstrated by the behavior of poly[bis(isostearyloxy)phosphazene] (7), which has a solubility of only 3 wt/wt% in OIB. It is likely that, beyond a certain side group length, the side groups on the polyphosphazene have a greater affinity for their counterparts along the same chain or on adjacent phosphazene chains than with the OIB molecules. Moreover, increases in side group length in the polyphosphazene cause increased intramolecular side group entanglements, thus making it sterically harder for solvation by oliaoisobutvlene to occur.
[FIGURE 3 OMITTED]
The solubility of the mixed-substituent poly[(alkoxy)[ poly[(alkoxy).sub.1] (2-(2-methoxyethoxy)ethoxy)[(2-(2-methoxyethoxy)ethoxy).sub.1] phosphazenes] (9-13) in OIB is also dependent on the length of the alkoxy cosubstituent group. Thus, cosubstituent polyphosphazenes with short alkoxy cosubstituents such as propoxy, pen-toxy, or hexoxy, together with the methoxyethoxyethoxy component, are completely insoluble in OIB, as shown in Table 1. Moreover, poly[bis (2-(2-methoxyethoxy)ethoxy)-phosphazene] (MEEP) (8) is also completely insoluble in OIB.
[FIGURE 4 OMITTED]
However, the influence of the 2-(2-methoxyethoxy)ethoxy side groups on the insolubility of the cosubstituted polymers in OIB is counteracted as the length of the alkoxy cosubstituent side group is increased. Thus, a slightly enhanced solubility in OIB (5 wt/wt% solubility at 80 [degrees] C) is achieved for the mixed-substituent polymers once the alkoxy cosubstituent side chain reaches the length of the octoxy group (polymer 6).
By contrast, the solubility of the cosubstituted polymer with both isostearyloxy and 2-(2-methoxyethoxy)ethoxy side units (polymer 13) is much higher than that of any of the other macromolecules. Polymer 13 has a solubility in OIB of 16 wt/wt % at room temperature and 21 wt/wt% at 80[degrees] C. The 2-(2-methoxyethoxy)ethoxy side groups may affect solubility in two ways: first, by reducing the intramolecular interactions between the isostearyloxy side chains, which are probably responsible for the insolubility of 7 in OIB; and second, by increasing the free volume in 9, which enhances the opportunities for intermolecular interactions between the isostearyl units and OIB.
[FIGURE 5 OMITTED]
The solubility studies were accompanied by differential scanning calorimetry experiments of the mixtures to examine the interactions between the different polyphos-phaGenes and OIB. Polyphosphazenes mixed with OIB rely on London dispersion forces and chain entanglements to form miscibie systems. DSC studies were carried out with solutions of polymer 6 in OIB as a representative of the poly(dialkoxyphosphazene) series and of polymer 13 in OIB representing the poly[(alkoxy)j(2-(2-methoxye-thoxy)ethoxy)lphosphazene] group. The DSC characterization of polymer 6 initially in 5 wt/wt% OIB is shown in Fig. 4. The separate glass transition temperatures (7g) of both 6 and OIB were detected at -104 [degrees] C and -86 [degrees] C, respectively. This indicates that, in spite of the solubility at room temperature or at 80 [degrees] C, the system underwent phase segregation by the time the temperature had been reduced to -80 [degrees] C or -100 [degrees] C. This is consistent with the trend of the solubility being reduced as the temperature is lowered. It also reflects the low molecular weight of the oligoisobutylene since a high polymeric isobutylene would presumably have a lower tendency to phase separate, at least over a short time scale.
[FIGURE 6 OMITTED]
The presence of 2-(2-(methoxyethoxy)ethoxy) side groups might be expected to yield immiscible blends with OIB due to the hydrophilic nature of the etheric side group. However, the DSC characterization of mixed-sub-stituent polymer 13 dissolved in OIB (15 wt/wt%) suggests that the system does not completely phase-segregate at low temperatures. Figure 5 shows a shift of the two parent peaks of polymer 13 from - 66 [degrees] C and - 80 [degrees] C to one peak at -86 [degrees] C in the mixture. This is accompanied by a negligible shift in the peak of pure OIB from -87 [degrees] C to - 86 [degrees] C (see Fig. 5). These results suggest that, although the overall solubility of this polymer in OIB is reduced at the lowest temperatures, some interactions between the two components remain even at - 90 [degrees] C. If this interpretation is correct, the 2-(2-(methoxyethoxy) ethoxy side group is probably responsible for the remaining interactions between the two components.
[FIGURE 7 OMITTED]
Repulsion between the two types of side groups in 13 is believed to be the phenomenon that affects the solubility of 13 in OIB. As shown in Fig. 6, polymer 13 may be enthalpy-driven to form segregated oriented domains. Thus the isostearyloxy groups prefer to interact with other hydrophobic units rather than with 2-(2-methoxyethoxy)ethoxy) units. The introduction of OIB may induce an entropy-driven rearrangement in molecular conformation. This might favor the partial dissolution of 13 in OIB due to the increased interactions between the isostearyl units and OIB and, at the same time, cause the change in the second-order transition temperatures detected experimentally. For comparison, a blend of 15 wt% mixed-substituent polymer 13 with high molecular weight polyisobutylene (PIB) 2 cast from THF resulted in an immiscible blend. Therefore, the interaction and dissolution phenomenon only occur when polymer 13 is dissolved by oiigoi-sobutylene.
Thermal Decomposition of Polymers 3-13 and OIB
Thermo-gravimetric analysis (TGA) was used to monitor the thermal behavior of polymers 3-13, OIB, and BIB. As shown in Fig. 7a and b, the onset of 10 wt% weight loss of poly(dialkoxyphosphazenes) and poly((alkoxy)i(2-(2-methoxyethoxy)ethoxy)]phosphazenes) occurs at least 100[degrees]C higher than that for OIB and at the same temperature as PIB. Polymer 13 has the highest 10 wt% loss (248 [degrees] C), compared to the 10 wt% loss of OIB at 123 [degrees] C and 10 wt% loss of PIB at 269 [degrees] C. Mass spectrometric analysis of the products liberated from OIB at 123[degrees]C or higher indicates that the principal volatile products are butylene (major) and propylene (minor). This suggests that OIB and PIB undergoes depolymerization, unlike the polyphosphazenes studied here which decompose semirandomly at high temperatures. If these weight loss data truly reflect relative thermal stabilities, the results suggest that polymers 3-13 may be useful as additives to OIB for a number of applications such as lubricants or cooling oils for low to medium temperature applications.
One of the unusual characteristics of most of the polyphosphazenes described here is their low solubility in oli-goisobutylene. Even the single-substituent polymer with octoxy side groups, a species with a highly aliphatic hydrocarbon-like side chain structure, has a solubility of only 11% at 80 [degrees] C. Moreover, an extension of the side chains to isostearyloxy actually reduces the solubility to less than 2%. The solubility of polymers with both hydrophilic methoxyethoxyethoxy and linear alkoxy side groups, as in polymer 10, increases with the length of the (hydrophobic) alkoxy side chain to give a maximum solubility of 21% in OIB, even though the presence of alkyl ether groups alone results in total insolubility. These results suggest that a great deal of the behavior of these polymers may be explained by the interactions within the side group system on each phosphazene chain rather than on their interactions with a second component such as oli-goisobutylene. This possibility is reinforced by the appearance of two separate second-order low temperature transitions for the polyphosphazene that bears randomly substituted methoxyethoxyethoxy and isostearyloxy side groups (13), a phenomenon that is ascribed to phase segregation brought about by the two different types of side groups undergoing preferential association with their counterparts rather than with the cosubstituents that have a different character.
This work was supported by Penreco Inc.
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Arlin L. Weikel, David K. Lee, Nicholas R. Krogman, Harry R. Allcock
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802
Correspondence to: H.R. Allcock; e-mail: firstname.lastname@example.org
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|Author:||Weikel, Arlin L.; Lee, David K.; Krogman, Nicholas R.; Allcock, Harry R.|
|Publication:||Polymer Engineering and Science|
|Date:||Sep 1, 2011|
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