Plasticization of poly(styrene-b-isobutylene-b-styrene) ionomers by acid salts of 2-ethylhexyl-p-dimethylaminobenzoate.
Ionomers are polymers containing up to about 15 mol% of ionic comonomers. Several morphological models for ionomers have been presented that explain various aspects of ionomer behavior such as their reduced melt flow, increased solvent resistance and enhanced toughness, especially at higher temperatures. One of the most recent, presented by Eisenberg et al. (1), describes the morphology of random ionomers as follows: Ionic groups phase separate from the nonpolar polymer to form aggregates of 2-8 ion pairs, termed multiplets. These multiplets act as ionic crosslinks and are surrounded by matrix polymer chains that experience a reduced mobility because of their firm attachment at the ionic multiplet interface. The region surrounding the multiplets is termed the region of restricted mobility and extends a distance on the order of the persistence length of the polymer from the multiplet surface. As the ion content increases, more multiplets are formed and the regions of restricted mobility begin to overlap and eventually constitute a separate phase large enough to exhibit its own glass transition temperature.
Because of the multiphase nature of ionomers, consisting of ionic domains dispersed within a non-polar matrix, plasticization of ionomers has been investigated using both non-polar and polar compounds. Several studies (2-7) have shown that nonpolar plasticizers can plasticize both the ionic phase and the nonionic matrix polymer, while some polar plasticizers are able to selectively plasticize the ionic regions as a result of their preferential segregation into the polar ionic multiplets.
In this study, salts of 2-ethylhexyl-p-dimethylaminobenzoate (ODAB) were examined as ionic plasticizers in poly(styrene-b-isobutylene-b-styrene) (PS-PIB-PS) block copolymer ionomers. PS-PIB-PS block copolymers are thermoplastic elastomers, i.e., they behave as crosslinked rubbers at low temperatures but can be processed as thermoplastics at higher temperatures. This behavior is the result of a microphase separated morphology consisting of glassy polystyrene domains dispersed in a rubbery polyisobutylene matrix. These domains act as physical crosslinks, which can be weakened to facilitate processing by heating to temperatures above the [T.sub.g] of PS. By lightly sulfonating the PS blocks, another type of thermally reversible crosslink, ionic aggregation, is introduced within the PS domains. The ionic multiplets act as crosslinks within the PS domains, even at temperatures well above the [T.sub.g] of PS, resulting in improved high temperature performance but decreased processability. It was expected that the ODAB salts, owing to their polar nature, would be preferentially incorporated into the ionic aggregates resulting in their preferential plasticization without plasticization of either the PIB phase or the PS matrix. The efficacy of these compounds as plasticizers was determined using dynamic mechanical analysis. Depression of the flow temperature was accepted as an indication of preferential plasticization of the ionic aggregates.
Synthesis of the parent PS-PIB-PS block copolymer, by living carbocationic polymerization using the sequential monomer addition method, has been described (8). The PIB mid-block peak molecular weight ([M.sub.P]) was 53,000 g/mol as determined by size exclusion chromatography. The PS blocks were 9100 g/mol, as calculated from the PIB molecular weight and the wt% PS (25.5) determined using proton NMR.
The chemicals 2-ethylhexyl-p-dimethylaminobenzoate (ODAB) and ionic ODAB derivatives were used as received from First Chemical Corporation. The derivatives used were the hydrochloric acid, sulfuric acid, stearic acid, p-toluenesulfonic acid, and methane sulfonic acid salts and the methyl iodide quaternary ammonium salt.
Zinc stearate (tech. grade), acetic anhydride, concentrated sulfuric acid and tetrachloroethylene were used as received from Aldrich Chemical Co.
The polystyrene blocks of the triblock copolymer were lightly sulfonated with acetyl sulfate in refluxing methylene chloride using a modification of the method described by Thaler (9). A small portion of a solution of the sulfonated polymer (5 wt % in xylenes with 1-2% (v/v) n-hexanol as polar cosolvent) was then titrated to a thymol blue endpoint using 0.05N methanolic KOH. The result indicated that 4.7% of the PS repeat units had been sulfonated. The remainder of the sample solution was then fully neutralized by adding the appropriate amount of methanolic KOH solution.
Table 1. TGA Results Showing Temperature st 50% Weight Loss for Each Plasticizer Studied. Plasticizer Temp. at 50% Wt. Loss ODAB 248 ODAB HC1 242 ODAB PTSA 271 ODAB stearate 272 ODAB [H.sub.2]S[O.sub.4] 202 ODAB Mel 235 ODAB MSA 256 Zinc stearate 405
[TABULAR DATA FOR TABLE 2 OMITTED]
The PS-PIB-PS ionomer-plasticizer blends were prepared by dissolving 2.0 g of the ionomer in 30 ml of tetrachloroethylene with a small amount (1-3 mL) of n-hexanol as a polar cosolvent. Plasticizers were then added to the ionomer solution either directly or as a measured volume of a 10 wt% solution of the plasticizer in tetrachloroethylene. The ionomer solutions were then poured into DuPont Teflon-lined pans, tightly covered with aluminum foil, and placed into a 50 [degrees] C oven to allow slow evaporation of the solvent over a five to seven day period. The films were then further dried in a 80-90 [degrees] C vacuum oven for three days. Film thickness ranged from 0.8 to 1.4 mm.
Dynamic Mechanical Analysis
Dynamic mechanical analysis (DMA) was performed using a Seiko Instruments SDM5600 Viscoelasticity Analysis System with the DMS 210 Tension Module. Rectangular samples with gauge lengths of 20 mm and having cross-sectional areas of 8-13 [mm.sup.2] were utilized. Samples were subjected to a 1 Hz cyclic tensile deformation as temperature was ramped from -120 to 350 [degrees] C at a rate of 5 [degrees] C/min.
Thermogravimetric Analysis (TGA) was performed on each of the plasticizers using a DuPont Instruments 951 Thermogravimetric Analyzer. Samples were heated from 25 to 500 [degrees] C at 10 [degrees] C/min in a nitrogen atmosphere.
RESULTS AND DISCUSSION
Thermal Stability of ODAB Salts
Figure 1 shows typical TGA results and Tab/e 1 lists the temperature at 50% weight loss for the ODAB derivatives and for zinc stearate. All ODAB derivatives exhibited poor thermal stability compared with zinc stearate. However, several of them are stable within the range of anticipated processing temperatures, and it is probable that their stability will be greater when blended with the ionomer.
Dynamic Mechanical Properties of Plasticized Ionomer Films
Dynamic mechanical analysis was performed on films to determine the effect of plasticizer addition on the glass transition temperatures ([T.sub.g]) of the PIB and PS phases and to compare the effectiveness of each plasticizer in weakening the ionic crosslinks. A depression in [T.sub.g], as indicated by tan [Delta] peak position, indicated the presence of plasticizer in the corresponding phase. A drop in the storage modulus below approximately [10.sup.5] Pa and a sharp rise in tan [Delta] indicated the onset of flow and provided a measure of the degree of ionic plasticization. The temperature at which tan [Delta] = 3 was observed to correspond well with the drop in modulus and was therefore used to rate the effectiveness of each plasticizer. This temperature was also expected to parallel practical processing temperatures for the given ionomer/plasticizer compositions.
Figures 2 and 3 show representative E[prime] vs. temperature and tan [Delta] vs. temperature curves, respectively, for films containing 5.0 [+ or -] 0.2 wt% plasticizer. Table 2 lists [T.sub.g]s of the PIB and PS phases (peak position, tan [Delta]) and the temperature at which tan [Delta] = 3 (T.sub.tan] [Delta] = 3). The degree to which the latter was depressed by a given plasticizer ([Delta][T.sub.tan] [Delta] = 3) was used as a measure of the degree of ionic plasticization. Thus, also listed in Table 2 for each plasticizer are their equivalent weights, the actual concentration (wt%) at which each plasticizer was used, and the ratio, ODAB (stearate) moieties/ionic group. From these quantities, an efficiency rating, also listed in Table 2, was calculated as [Delta][T.sub.tan] [Delta] = 3/mol ODAB moiety/mol ionic group. Zinc stearate and the sulfuric acid salt of ODAB were both added neat to the ionomer solutions. This resulted in larger deviations from 5.0 wt% than were seen with the plasticizers that were added as 10% solutions. Results for the ODAB methyl iodide quaternary ammonium salt are not shown because this compound was insoluble in the ionomer solutions and films.
All films, including the block copolymer precursor, showed a similar drop in storage modulus at approximately -55 [degrees] C which corresponds to the [T.sub.g] of the PIB phase. Table 2 shows that the [T.sub.g] of the PIB phase in each sample was unaffected by the ionic plasticizer, indicating that the plasticizers are not present in the PIB phase and are therefore confined to the PS ionomer phase. The E[prime] curve obtained for the ionomer control was typical of a block copolymer ionomer. The rubbery plateau modulus was very similar to the nonionic precursor until the glass transition temperature of the PS phase was approached near 100 [degrees] C. As the temperature was raised further, the non-ionic precursor showed a precipitous decline in modulus, whereas the ionomer control displayed a very gradual decrease in modulus over the range 120-330 [degrees] C, with softening finally occurring at about 330 [degrees] C. In contrast to this behavior, the modulus dropped rather sharply at lower temperatures, i.e., 100-250 [degrees] C, for the films containing ODAB derivatives.
The higher glassy and rubbery plateau moduli seen in samples containing the HCl and MSA salts of ODAB appear to be typical of block copolymer ionomer samples that possess near-equilibrium morphologies as a result of ionic plasticization and/or solution casting under optimized conditions (10). This effect provides further evidence of selective plasticization of the ionic domains. The nonplasticized sample has apparently formed a non-equilibrium morphology as a result of the mobility restriction imposed by the ionic crosslinks. This results in fewer ionic clusters and more isolated ion pairs within the matrix which do not contribute significantly to modulus enhancement.
The ionic plasticization effect can be seen more readily in Fig. 3 as a sharp upturn in the tan [Delta] curves. In the ionomers, this upturn is believed to coincide with the onset of the ionic cluster phase glass transition, which is followed immediately by flow. The effectiveness of a given plasticizer was judged by the extent to which the temperature of this transition, taken to be the temperature at which tan [Delta] = 3, was lowered relative to the unplasticized ionic precursor ([Delta][T.sub.tan [Delta] = 3]). As shown in Table 2, the most effective compounds tended to be the smaller organic acid salts of ODAB such as methane sulfonic or p-toluenesulfonic. When considering only [Delta][T.sub.tan [Delta] = 3], the HCl and [H.sub.2]S[O.sub.4] salts appeared to be as effective as the organic acid salts. However, the efficiency factors shown in Table 2 clearly show that on a molar basis, the methane sulfonic and p-toluenesulfonic acid salts were the most effective plasticizers. It should also be noted that the PS [T.sub.g] was lowered by about 15 [degrees] C for all ODAB derivatives but not by zinc stearate. This reflects the well-known behavior of zinc stearate, as described by Duvdevani et al. (4), to phase separate as small crystallites that can act as reinforcing filler below their melting point of [approximately equal to] 117 [degrees] C. Therefore, within the temperature regime of the PS [T.sub.g], it is unavailable for plasticization; at higher temperatures it becomes available as an ionic plasticizer, albeit a rather inefficient one. ln contrast, the ODAB derivatives were liquids at room temperature and therefore could not act as reinforcing filler. The octyl benzoate tails of ODAB are apparently well extended into the PS phase and cause depression of the [T.sub.g]. The stearic acid salt of ODAB was ineffective as an ionic plasticizer, suggesting that it is less ionic than the salts of the stronger acids; however, it did plasticize the PS phase, lowering its [T.sub.g] by 14 [degrees] C.
Figures 4 and 5 show E[prime] vs. temperature and tan [Delta] vs. temperature, respectively, for films in which varying ratios of potassium hydroxide and ODAB were used to neutralize the ionomer. This method represents an in situ plasticization of the ionomer since it essentially forces protonated ODAB into the ionic domains as a counterion for the bound sulfonate groups of the ionomer. By substituting H-ODAB for varying amounts of the potassium counterions, a range of softening temperatures similar to those of the various ODAB salts was seen. Table 3 summarizes the DMA results for ionomers neutralized with potassium and H-ODAB. In all cases, except for the block copolymer and the non-neutralized, sulfonated precursor, samples that exhibited [TABULAR DATA FOR TABLE 3 OMITTED] larger shifts in the PS [T.sub.g] also exhibited larger shifts in the tan [Delta] = 3 temperature. The lowering of the PS [T.sub.g] seen in some samples indicates slight plasticization of the non-ionic PS phase, while the much larger shift in the temperature at which tan [Delta] = 3 indicates preferential plasticization of the ionic clusters.
Several organic and inorganic acid salts of ODAB were shown to be effective ionic plasticizers within PS-PIB-PS block copolymer ionomers; most of these compounds were more effective than zinc stearate at the level studied (5 wt%). Using ODAB to partially neutralize the ionomers was also an effective means of providing ionic plasticization. These results suggest that the modulus-temperature behavior or the processing temperature of these ionomers may be tailored to suit specific applications by choosing the correct plasticizer and level of incorporation.
The research upon which this material is based was supported by First Chemical Corporation of Pascagoula, Mississippi, the National Science Foundation through Grant No. EPS-9452857, and The University of Southern Mississippi.
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|Author:||Storey, Robson F.; Baugh, Daniel W.|
|Publication:||Polymer Engineering and Science|
|Date:||Jul 1, 1999|
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