Plasticizing Effects of Imidazolium Salts in PMMA - American Chemical

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Chapter 37

Plasticizing Effects of Imidazolium Salts in PMMA: High-Temperature Stable Flexible Engineering Materials Mark P. Scott, Michael G. Benton, Mustafizur Rahman, and Christopher S. Brazel* Department of Chemical Engineering, University of Alabama, Tuscaloosa, AL 35487

The unique, environmentally-sound applications of ionic liquids (ILs) are well-known, and include use as green solvents for a variety of chemical processes. Because ILs are stable liquids over a wide temperature range, they offer technological advantages over some chemicals used in their liquid phase, such as plasticizers, where polymer flexibility can be enhanced. Common problems with plasticizers include evaporation and leakage from the surface, instability at high temperatures, lack of lubrication at low temperatures, migration within the polymer, and toxicity. We have addressed couple of these issues using poly(methyl methacrylate), PMMA. Systems studied include bulk PMMA, and PMMA plasticized with butyl methylimidazolium hexafluorophosphate [bmim ][PF -], hexyl methyl imidazolium hexafluorophosphate [hmim ][PF -], and a traditional plasticizer, dioctyl phthalate (DOP). Experiments indicate that high temperature stability is improved significantly by replacing DOP with an IL. The effect of IL on glass transition temperature and elastic modulus were also determined. Ionic liquids as plasticizers may revolutionize the usage of flexible polymers at high temperatures, without brittleness or loss of mechanical strength. +

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© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Background Plasticizers are polymer additives used to improve the flexibility, processability and workability of plastics. Four million tons of plasticizers are produced annually worldwide . Most of these plasticizers are based on three classes of compounds: phthalates, adipates and trimellitates, with dioctyl phthalate (DOP) accounting for more than 50% of all plasticizers used. Ideally plasticizers should exhibit most of the following characteristics: low volatility, low leachability, high and low temperature stability, thermodynamic compatibility with polymers, low cost and minimal health and safety concerns . However, current plasticizers have relatively small thermal working ranges (due to evaporation or solidification) and have limited lifetimes in products (due to volatility and leaching).


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Room temperature ionic liquids (ILs) meet many of the same requirements of plasticizers and offer the potential for improved thermal and mechanical properties. ILs, being salts with melting points of below ca. 100 °C (and reportedly as low as -96 °C), can be used as solvents under reaction conditions similar to conventional organic liquids. ILs possess a wide liquid temperature range, in some cases in excess of 400 °C . They have unique properties that have led to their investigation in a wide range of applications, including electrochemical and separation processes " , and as solvents for chemical and biochemical synthesis " . The principal interests in using ILs is either (1) to utilize their non-volatile behavior in developing environmentally-responsible systems (as replacement solvents for volatile organic compounds, for example), or (2) to give a technological advantage for the creation of novel materials, unique reactions, or more economical processes. Because of their non-volatile nature and the wide temperature range where ILs can be used as liquids, they provide unique opportunities when used in polymer systems. As plasticizers, ILs have several advantageous properties, as they are nonflammable, have high thermal stability, exhibit no measurable vapor pressure, and can be designed to be highly solvating for specific organic and inorganic compounds. In addition, the DLs may provide environmentally responsible alternatives to traditional plasticizers since the lifetimes of the flexible plastics are increased (reducing landfill waste) and less plasticizer will be released into the environment due to leaching or volatility during a material's use. ILs have been shown to be compatible with certain polymers and free radical polymerizations have been successfully carried out in fbmim ][PF '], although isolation and separation of polymer and IL have proven difficult " . As plasticizers, ILs offer better mechanical and thermal properties, and may expand the temperature range whereflexiblepolymers can be used. 3

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In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Objective The objectives of this study were to determine the effects of ILs as plasticizers on material properties of poly(methyl methacrylate), PMMA. The moduli of elasticity were determined as a function of plasticizer type and concentration, and the effect of plasticizer content on glass transition temperatures and thermal stability of PMMA samples at elevated temperatures was measured.

Experimental PMMA was synthesized using specific volume ratios of methyl methacrylate (Acros, Fairlawn, NJ) with DOP (Sigma, St. Louis, MO), butyl methylimidazolium hexafluorophosphate [bmim^fPFo] or hexyl methylimidazolium hexafluorophosphate [hmim ][PF "] (both ILs were prepared on campus as described elsewhere ). Inhibitor was removed from M M A prior to the reactions by passing the monomer through a dehibiting ion exchange column (Aldrich, Milwaukee, WI). Samples were prepared either with 1 mol % ethylene glycol dimethacrylate, EGDMA (Aldrich), or without crosslinking agent. Nitrogen was passed through the monomer solution to remove dissolved oxygen prior to addition of 1 wt % azobisisobutyronitrile, AIBN (Aldrich). The polymers were formed between two siliconized glass sheets separated by Teflon® spacers to provide samples of uniform thickness for mechanical testing. The reactions were carried out at 55 °C for 24 hours. Bulk polymers and samples containing up to 50 vol % of plasticizer were synthesized. Dogbone and rectangular-shaped samples were cut for mechanical analysis and long term thermal exposure at 170 °C. Small pieces (approximately 10 mg) of polymers were collected for differential scanning calorimetry, DSC, and thermogravimetric analysis, TGA (Models 2920 MDSC and 2950 TGA, both TA instruments, Newcastle, DE). Due to the plasticizer incorporation method, all plasticizer contents are volume percentages based on the feed ratios to the reaction. +

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Initial mechanical tests were conducted on dogbone-shaped samples at room temperature using an Instron Automated Materials Testing System (Model 4465, Canton, MA). A strain rate of 5 mm/min was applied while stress was measured. Mechanical tests were conducted as a function of temperature using a Rheometrics Solids Analyzer (RSA II, Rheometrics Inc., Piscataway, NJ). Here, elastic moduli were measured using an incremental strain rate of 0.06%/min at temperatures from 25 to 200 °C. Mass loss was monitored in three ways: (1) by


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TGA during a temperature ramp of 10 °C/min to 350 °C, (2) by TGA when held isothermally at 250 °C for 25 minutes, and (3) over an extended period of time held isothermally at 170 °C in an oven. TGA experiments were conducted using approximately 10 mg polymer samples, while the third method utilized rectangular-shaped samples with approximate dimensions of 40 χ 15 χ 1.4 mm and 1 g weight. DSC experiments were conducted to determine glass transition temperatures, using a heating/cooling cycle to erase sample thermal history before heating at 10 °C/min to 150 °C.

Results & Discussions Uniform plasticized PMMA samples were successfully polymerized with [bmim ][PF "j and [hmim ][PF ] containing up to 50 vol % plasticizer. Syntheses of PMMA samples containing up to 50 vol % DOP were also attempted, but samples containing 40 vol % DOP or greater phase separated during polymerization. Therefore, DOP-containing samples had from 10 to 30 vol % of the plasticizer only. +

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Elastic Moduli The elastic modulus is the prime consideration in determining the general utility of a polymer. The elastic moduli of PMMA samples (with crosslinking agent) decreased with increasing amounts of plasticizer added, regardless of whether the plasticizer was DOP or an IL (Table 1). This was not unexpected, but the similarity of the curve for the ILs compared to DOP indicates that these ILs are equally good at plasticizing PMMA, with the added benefit that the ILs have lower vapor pressure. Additionally, the ability to incorporate greater amounts of IL into the PMMA samples allows the creation of plastics that have glass transition temperatures at or below room temperature. The elastic behavior of plasticized PMMA was also determined as a function of temperature, by generating the master curves for each polymer. Figure 1 shows the effect of adding [bmim ][PF ] to PMMA formulations in successively increasing quantities. The slope of the leathery region (between the glassy region at low temperatures and the rubbery plateau above the glass transition, T , temperatures) is nearly constant, but as the IL content is increased, the T drops. The behavior of PMMA plasticized with 30 vol % [hmim^lWV] nearly matched the moduli of DOP-plasticized PMMA (Figure 2). Because BLs were successfully incorporated into PMMA at a higher concentrations than DOP (and 50 vol % is only the highest tested, not necessarily a maximum IL content), ILs allow better control of polymer mechanical properties. +

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472 Table 1. Effect of Plasticizer and Plasticizer Content on Elastic Moduli of PMMA Samples as Determined by Instron at Room Temperature


Plasticizer Content (vol %) 0 10 20 30 40 50

Elastic Modulus for PMMA Plasticized with [bmim ][PF ](MPa) 535 515 314 152 13.8 * 0.386 DOP concentration was above solubility limit to form uniform samples Elastic Modulus for PMMA Plasticized with DOP (MPa) 535 521 340 207

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Dynamic mechanical tests were conducted on samples containing each of the three plasticizing agents to compare the universal curves of the ILplasticized PMMA with that of the samples containing DOP (Figure 2). P M M A with 30 vol % [hmim^tPFô] followed the behavior of PMMA with 30 vol % DOP both in magnitude of elastic modulus and the temperature dependence. This indicates that the ILs can do as good a job at plasticizing PMMA as the standard plasticizing agent, while the ILs offer improved high temperature stability.

Thermal Stability Low volatility is one of the most important characteristics of plasticizers. It must remain in the polymer over for the lifetime of the product, and may be subjected to a range of temperatures. ILs are particularly intriguing in this aspect as they typically have no detectable vapor pressure which enables the formation of flexible materials with significantly extended lifetimes. TGA experiments were used to elucidate the mass loss of bulk and plasticized PMMA. [bmim ][PF ] and DOP were also tested in bulk. DOP has a boiling point of 380 °C and is highly volatile at elevated temperatures. In experiments where approximately 10 mg samples of DOP and [bmim ][PF ] were subjected to a 10 °C/min ramp, 95 wt% of the DOP evaporated by the time the temperature reached 300 °C, while only 1.15 wt % of the EL was lost in the same temperature ramp (see data points at 100 % plasticizer content in Figure 3). Plasticized PMMA with 10 to 50 vol % of each plasticizer (DOP and [bmim ][PF ]) were also tested for thermal stability (Figure 3). With increasing concentrations of [bmim ][PF ], the PMMA samples actually became more thermally stable than PMMA by itself. This is in stark contrast to the loss in mass of the DOPplasticized PMMA. 17

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Figure L Effect of[bmim ][PF ] Content in Plasticized PMMA on the Master Curve Representing the Mechanical Behavior Over a Range of Temperatures 6

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Figure 2. Effect of 30 vol % [hmim ][PF ], DOP and [bmim+][PF ] as Plasticizers for PMMA on the Master Curve 6


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Figure 3. Thermal stability of[bmim*][PF ] and DOP plasticized PMMA samples in TGA experiments 6

Because the high temperature stabilities tested above are not realistic for the long-term use of flexible plastic, a similar comparison between [bmim ][PF ] and DOP as plasticizers for PMMA was carried out at 170 °C over a period of more than a month (Figure 4). Here, unplasticized PMMA was shown to be the most thermally stable over 42 days, while there was nearly 5 wt % mass loss in the same time period for PMMA plasticized with either 20 or 30 vol % [bmim ][PF *]. This accounts for a significant portion of the IL that was incorporated, as 20 to 25 wt % of all of the IL incorporated had evaporated in this time period. Still, the DLs are much more promising as plasticizers at elevated temperatures than DOP. DOP-plasticized PMMA lost 75 wt % of the plasticizing agent in only 22 days, dropping 15 wt % of total mass for a sample plasticized with 20 vol % [bmim ][PF ], and 24 wt % for PMMA plasticized with 30 vol % [bmim ][PF *]. Further work may focus on other ILs, since the hexafluorophosphate anion in known to decompose to products including HF at high temperatures through hydrolysis. +

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Glass Transition Temperature The glass transition temperature (T ) is important in designing polymeric materials because it draws the distinction between the hard, glassy region and the rubbery plateau in the master curve, thereby determining the temperature range where a polymer will be commercially useful. Plasticizers are often used to lower the glass transition temperature of polymers to make polymers workable g



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Figure 4. Comparison of Long Term Thermal Stability of DOP- and [bmim* ][PF ]-Plasticized PMMA 6

and flexible at low temperatures. The T of PMMA dropped with the addition of DOP, but only up to 52.44 °C, since DOP was incompatible with formulations containing 40 vol % DOP or greater (Table 2). The two ILs tested, [bmim ][PF " ] and [hmim ][PF ], on the other hand, lowered the T to as low as 10 °C using up to 50 vol% IL in PMMA. Thus, ILs can provide a wider range of material properties, and may be able to drop the glass transition temperature of PMMA significantly below 0 °C with increased concentrations. g

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Table 2. Effect of Plasticizer Type and Concentration on the Glass Transition Temperature of Bulk and Plasticized PMMA. Errors shown represent the standard deviation for three experiments. Plasticizer content (%) 0 10 20 30 40 50

DOP 116.7 (+/-0.26) 85.09 66.68 (+/- 1.88) 52.44 (+/- 3.37) Not compatible Not compatible

T.CQ [bmim ][PF ] +

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116.7 (+/-0.26) 93.87 71.99 (+/- 1.36) 48.25 (+/- 2.24) 28.71 21.64 (+/- 6.31)

[hmimltPFV] 116.7 (+/-0.26) —

68.28 (+/- .43) 40.98 (+/-1.42) —

9.49 (+/- 0.48)


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Conclusions +

From this on-going work, it has been found that [bmim ][PF "] and [hmim ][PF ] are able to provide better thermal and mechanical properties to PMMA than does the traditional plasticizer, DOP. Yet PF * anions have their drawback of decomposing at high temperatures to yield HF. Low and moderately high temperature applications are being investigated for them. For additional low temperature applications, ionic liquids such as hexyl and octyl derivatized salts would be advantageous as these ILs have melting points below -70 °C. Better compatibility of ILs with PMMA than DOP allowed incorporation of over 50 vol % [bmim ][PF ], which in turn led to a wider range of glass transition temperatures, and may allow PMMA to be used as a flexible material in subzero (Celsius) conditions. This indicates a promising development in the field of flexible polymeric materials that withstand a large range of working temperatures, with minimal evaporation or leaching. 6

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Acknowledgments This research was supported by National Science Foundation grants NSFCTS0086874 and a University of Alabama SOMED research grant. The authors also acknowledge Dr. David Nikles and Dr. Mark Weaver for the use of testing equipment and Matthew Reichert for preparation of ILs.

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Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Rogers, R. D.; Mayton, R.; Sheff, S.; Wierzbicki, Α.; Davis Jr., J. H., Chem. Commun., 2001; 135. Welton, T., Chem. Rev., 1999; 99, p 2071. Holbrey, J. D.; Seddon, K. R., Clean Prod. Proc., 1999; 1, p 223. Wasserscheid, P.; Keim, W., Angew. Chem., Int. Ed., 2000; 39,p 3772. Sheldon, R., Chem. Comm., 2001, p 2399. Gordon, C. M., Appl. Catal. A, 2002; 222, p 101. Harrisson, S.; Mackenzie, S.; Haddleton, D. M . , Polym. Prepr., 2002; 43(2), p 883. Benton, M . G.; Brazel, C. S., Ind. Eng. Chem.; American Chemical Society: Washington, DC, 2001; 221, p 165. Hong, K.; Zhang, H.; Mays, J. W.; Visser, A. E.; Brazel, C. S.; Holbrey, J. D.; Reichert, W. M . ; Rogers, R. D., Chem. Commun., 2002; p 1368 - 1369. Huddleston, J.G.; Visser, A.E.; Reichert, W.M.; Willhauer, H.D.; Broker, G.A.; Rogers, R.D. Green Chem. 2001, 3, 156. Pernak, J.; Czepukowicz, Α.; Pozniak, R., Ind. Eng. Chem. Res., 2001; 40, 2379.


Plasticizing Effects of Imidazolium Salts in PMMA - American Chemical

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