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Ionic liquids as plasticizers/lubricants for polylactic acid.

INTRODUCTION

Poly(lactic acid), PLA, is a linear aliphatic thermoplastic polyester, produced from renewable resources. PLA has attractive properties such as high strength, thermo-plasticity, and fabricability (l) and in a suitable disposal site will degrade to natural, harmless products, rendering it biodegradable and bioenvironmentally compatible. Biodegradable polymers and, in particular, aliphatic polyesters of the poly(hydroxyacid)-type, have been investigated for both biomedical and consumer applications (2), (3). In both such applications, it is of major importance to understand and control the degradation characteristics of the polymers.

A great variety of techniques and additives have been evaluated to accelerate the hydrolytic and thermal degradation of aliphatic polyesters (4). Ionic liquids, ILs, with suitable anion/cation pairs could be certainly added to the list of potential degradants. ILs are salts composed wholly of ions and are liquid at low temperatures--many at room temperature--and have extremely low volatility. ILs are considered as "Green" alternatives to volatile organic compounds (VOC) in organic synthesis, catalysis, polymerization, bioprocessing operations, liquid-liquid extraction, and gas separation. Research on ILs as polymer additives has so far focused on their potential as nanoclay modifiers (5), (6), their evaluation as nonvolatile plasticizers and external or internal lubricants in several polymers including PVC (7), PMMA (8), and polyamides or epoxy (9). In addition to other approaches to develop novel flexible polymeric materials, ILs may also offer several advantages for use as plasticizers. The low volatility and high temperature stability of many ILs make them useful for applications at elevated temperatures with minimal loss in flexibility and extended material lifetime. The cations and anions in ILs also have a strong affinity for each other, making plasticizer loss by liquid leaching, solid-solid migration, or evaporation much less likely compared to molecular plasticizers (10).

In our earlier preliminary work (11), two phosphonium cation-based ILs with different anions appeared to be potential plasticizers and/or lubricants for PLA. Both ILs lowered the glass transition temperature of PLA and modified its rheological characteristics as evidenced from reduced viscosities and apparent phase separation and lubrication. This work is part of a broader research program whose objectives are to investigate the role of novel additives, including ILs, as process and property modifiers of PLA. In addition to the ILs effects on processability, which were partly addressed by our earlier preliminary data (11) indicating a reduction of shear viscosity, this article focuses on their short and long-term miscibility with PLA, the morphological characteristics of IL/PLA blends, the effects of ILs on the polymer thermal degradation, as well as on properties related to their potential uses as plasticizers-lubricants; these include contact angle, coefficient of friction, and mechanical properties. The choice of the particular IL structures was based on earlier evidence that alkylphosphonium surfactant cations exchanged in the structure of saponite clay tend to accelerate the soil degradation of aliphatic polyesters including PLA (12). It should be emphasized that environmental toxicity issues were not considered in the selection of the ILs used in this work.

EXPERIMENTAL

Materials

Polylactide polymer pellets 4060D purchased from Natureworks[R] had a reported glass transition temperature of 58[degrees]C. Trihexyl tetradecyl phosphonium decanoate, [THTDP][DE], (designated as IL-1), MW 655.13, trihexyl tetradecyl phosphonium tetrafluoroborate, [THTDP][[BF].sub.4], (designated as IL-2), MW 570.68, chloroform (CAS No 67-66-3) were obtained from Sigma-Aldrich. Structures of IL-1 and IL-2 are shown in Fig. 1.

[FIGURE 1 OMITTED]

Sample Preparation

Direct addition of IL-1, IL-2 into the molten PLA in the counter rotating Brabender batch mixer (PL2000, C.W. Brabender) created segregated samples since the ILs tended to concentrate on the surface of the mixer bowl. To address this issue, IL-1 or IL-2 was dissolved in EtOH/[H.sub.2]O (4:1) and PLA pellets were added in this solution until thoroughly immersed for 24 h. The solvent was removed by heating at 70[degrees]C for 24 h in a ventilated oven. The concentrations of IL-1 and IL-2 in the solution were selected so that the dried premix contained 10 wt% IL. Different weight percentages of ILs in PLA were used. To produce samples with 1 wt% and 5 wt% of IL-1 or IL-2, proper amounts of PLA pellets were added along with the precoated pellets in the mixer bowl and melt blended at 50 rpm and 160[degrees]C for 10 min under nitrogen until the torque stabilized. After melt processing, PLA and PLA/5 wt% IL-2 samples were transparent with brown color; PLA/5 wt% IL-1 was dark brown probably as a result of excessive degradation. The produced mixtures were then pressed into films ~10 mm thick for further testing.

Characterization

Differential Scanning Calorimetry and Thermogravimetric Analysis. PLA, ILs, and their compounds were characterized by Differential scanning calorimetry (DSC) scans (TA Instruments, QA 100 analyzer) from 0 to 150[degrees]C at a heating rate of 10[degrees]C/min to determine glass transition temperatures. PLA, ILs, and their compounds were also characterized by dynamic Thermogravimetric analysis (TGA) (TA Instruments, QA 50 analyzer) by heating from room temperature to 500[degrees]C, at a heating rate of 10[degrees]C/min in a nitrogen atmosphere to determine thermal stability.

Scanning Electron Microscopy. The morphologies of PLA and its IL compounds were investigated by Scanning electron microscopy (SEM) (JSM-5410LV; JEOL, Tokyo, Japan). Cross sections of the specimens were examined after fracturing in liquid nitrogen.

Flexural Testing. Test specimens (100 mm length, 13 mm width, and 1 mm thickness) were prepared by pressing melt mixed samples for 2 min at 160[degrees]C. The flexural strength and flexural modulus by three-point bending were measured by a Tinius-Olsen (Lo Cap Universal) testing machine using a span to depth ratio of 16:1, up to 5% deformation/strain (ASTM D-790-07el) at a crosshead speed of 0.5 cm/min. Flexural testing was selected versus tensile testing since it was thought that would give more reproducible results given the brittle nature of the samples, particularly those based on IL-1.

Coefficient of Friction. A dynamic friction coefficient was measured using a mobility/lubricity tester (9505A, Altek Co., Torrington, CT) composed of 1 kg sled (9793A, Altek, Co.) as shown in Fig. 2. A steel sled with 1 kg weight contacts three steel balls on the surface of the sample and measures the friction force resisting the movement of the load cell. Three 1 -mm thick films (PLA, PLA/5 wt% IL-1, and PLA/5 wt% IL-2) were prepared by the melt mixing method followed by pressing for 2 min at 160[degrees]C. The string connected to the load cell and sled was pulled by a motor at the selected speed and the measured force after converting to coefficient of friction (COF) was recorded by a gauge. COF was the average of three measurements. All friction tests were conducted at room temperature at a pulling speed of 20.0 mm/s over a sliding distance of 22.5 cm.

[FIGURE 2 OMITTED]

Contact Angle. One droplet of distilled water was placed on the surface of PLA, PLA/IL-1, and PLA/IL-2 1-mm thick films using a drop shape analysis system (DSA 10MK2, KRUSS, Germany). Contact angle was determined by measuring the angle between the tangent of the drop surface at the contact line and the surface.

RESULTS AND DISCUSSION

Thermal Stability and Activation Energy

Figures 3 and 4 compare the dynamic TGA thermal stabilities under nitrogen of single components and blends produced by melt mixing. In terms of increasing thermal stability, the blend components are classified as IL-2 > PLA pellets > IL-1 (Fig. 3). IL-1 blends have lower thermal stability than PLA/IL-2 blends (Fig. 4) due to the inherent lower thermal stability of IL-1 and both blends have lower thermal stability than the nonprocessed PLA pellets. This suggests that both ILs display catalytic degradative effects to different degrees.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The thermal decomposition activation energy, [E.sub.a], of PLA, PLA/IL-1, and PLA/IL-2 was calculated based on the TGA results. In general, [E.sub.a] is associated with the lowest energy needed for degradation and, thus, high [E.sub.a] indicates high thermal stability. The kinetic parameters given by the Friedman technique are somewhat lower than those derived from the Freeman-Carrol and Chang methods (13). The reason that the Friedman technique was used in this study is that the [E.sub.a] values represent the thermal decomposition behavior in the temperature range from (onset degradation temperature minus (20-40) K) to the onset degradation temperature, in which the linear relation between ln(d[alpha]/dt) and 1/T (Eq. 1) is available (14).

1n (d[alpha]/dt) = 1n(Z) + n 1n(1 - [alpha]) - [[E.sub.a]/RT] (1)

where [alpha] is the weight loss of the polymer undergoing degradation at time t, Z is a frequency factor, and T is temperature in K.

In plotting the TGA data, the selected temperature range was from about 200[degrees]C to a temperature corresponding to 10% weight remaining for each sample. The calculated [E.sub.a] values from Fig. 5 are 176.3 kJ/mol, 84.8 kJ/mol, and 130.3 kJ/mol for PLA, PLA/IL-1, and PLA/IL-2, respectively, suggesting that PLA and PLA/IL-2 have higher thermal stability than PLA/IL-1. High correlation coefficients [R.sup.2] are calculated for each case.

[FIGURE 5 OMITTED]

Solubility Parameters and Miscibility

The group contributions to cohesive energy, [E.sub.coh], and molar volume by the Fedor's method (15), although apparently less accurate than those by Small, Van Krevelen, and Hoy's methods, contained a larger number of structural groups and could, as a result, be used for calculating the solubility parameter, [delta], of IL-1 that contains elements such as phosphorus. The molar volume, V, of IL-1 is calculated from a density of 0.88 g/[cm.sup.3] provided from the MSDS (Materials Safety Data Sheet) by Merck and Co.

[delta] = [square root of ([[E.sub.coh]/V])] (2)

Fedor's method using Eq. 2, gave a solubility parameter value for PLA of 17.9 [MPa.sup.[1/2]] and a very similar value for IL-1 (~17.5 [MPa.sup.[1/2]]). The cohesive energy of groups in IL-1 provided by Fedor and used for the calculation are -[CH.sub.2] (4940 J/mol), -[CH.sub.3] (4710 J/mol), -P (9420 J/mol), and -COO (18,000 J/mol). Molar volumes of groups used for these calculations were obtained from Table 1. The similarity in the solubility parameters suggests that the IL-1 should be miscible with PLA, as also shown in the literature for other potential PLA plasticizers such as triacetin and citrate oligomers having [delta] values approaching that of PLA. It is recognized, however, that the use of the Hansen solubility parameters based on partial parameters corresponding to dispersion forces, dipole-dipole forces and H-bonding would have been more appropriate considering the polar natures of the PLA and the ILs. However, Hansen's data on the phosphorous group necessary for these calculations are not available.
TABLE 1. Group contributions to group molar attraction constant, F
(15).

Values                               Small  Van Krevelen  Hoy

-[CH.sub.3]                           438       420       303.4
-COO-                                 634       512       668
[FORMULA NOT REPRODUCIBLE IN ASCII]    57       140       176


The solubility parameter of IL-2 was calculated using a molar volume of 651 [cm.sup.3]/mol by the same group contribution technique. The cohesive energy of groups provided by Fedors for IL-2 and used for the calculation are -[CH.sub.2] (4940 J/mol), -[CH.sub.3] (4710 J/mol), P (9420 J/mol), B (13810 J/mol), and F (4190 J/mol). The calculated solubility parameter was 17.4 [MPa.sup.[1/2]] which suggests that IL-2 should also be miscible with PLA. It is understood that predictions of miscibility based on solubility parameters are independent of the IL concentration and therefore can only used as approximate predictors of solubility.

The clarity of PLA/5 wt% IL-1, and PLA/5 wt% IL-2 films prepared by melt processing remained unchanged, with only a slight yellowish tint versus the processed neat PLA; this suggests either thermodynamic miscibility of PLA with the ILs or an extremely fine dispersion at this particular IL concentration. Incompatibility of the ILs with the amorphous transparent PLA would be expected to result in opacity due to phase separation.

Scanning Electron Microscopy

The extent of miscibility or dispersion of the ILs in cross-sectional areas of PLA/IL-1 and PLA/IL-2 films (thickness of 0.1 mm) prepared by melt processing was examined by SEM (Fig. 6). Some white aggregates of IL-1 particles are observed in Fig. 6b and fewer aggregates of IL-2 are shown in Fig. 6c. This suggests incomplete dispersion/mixing of the additives at the 5 wt% level in the bulk of the samples.

[FIGURE 6 OMITTED]

The cross section of PLA/5 wt% IL-1 after 1 year showed uniformly dispersed phases of PLA and IL-1 (Fig. 7a and b). By contrast, a distinct sign of phase separation (Fig. 7c and d) of PLA/5 wt% IL-2 after 1 year was detected by SEM resulting in agglomerated white phases on the cross section of the fractured sample. These observations suggest that PLA/5 wt% IL-2 tends to have relatively poor long-term miscibility and compatibility of the IL-2 with the PLA. This may be the result of the presence of the bulky hydrophobic decanoate cation that would restrain the movement and diffusion of IL-1 along the polymer network. Rahman and Brazel (16) have also observed a similar case of phase separation as the one observed in this study. After UV exposure, PVC with 20 wt% [[thtdPh.sup.+]][(Tf)[2N.sup.-]] {Trihexyl (tetradecyl) phosphonium bis(trifluoromethane) sulfonylimide} exhibited phase separation that was visually observable. SEM images of cross sections of the UV exposed sample showed distinct phase separation and layering of the [[thtdPh.sup.+]][(Tf)[[2N.sup.-]] molecules along the sides of the sample.

[FIGURE 7 OMITTED]

Glass Transition Temperature

Glass transition temperature, [T.sub.g], determination experiments by DSC for the PLA/IL blends suggest that both ILs can be considered as plasticizers. This is related to the decrease of the PLA [T.sub.g] from 60[degrees]C to lower values depending on the type and concentration of ILs as shown in Table 2. 10 wt% IL-1 lowers [T.sub.g] to about 45[degrees]C, which is at the same level as that attained with PEG conventional plasticizers (17). The effect of IL-2 on the [T.sub.g] of PLA is less pronounced.

Coefficient of Friction and Contact Angle
TABLE 2. [T.sub.g] comparison of PLA versus PLA containing different
wt% of IL-1 and IL-2.

Sample           [T.sub.g] ([degrees]C]

PLA                      60.7

PLA/5 wt% IL-1           49.5
PLA/10 wt% IL-1          45.1
PLA/5 wt% IL-2           55.9
PLA/10 wt% IL-2          54.5


For all samples, during measurement of COF, the friction force showed a sudden increase to a maximal value (static friction) and then decreased to a steady state value indicating the kinetic friction force. After each measurement, values were averaged to calculate the average kinetic friction force for the run. The resulting COF values of ILs in PLA are compared with that of PLA in Table 3. In the absence of ILs, friction is between the PLA surface and the steel balls and, as a result, the COF is very high (~0.45). This is supported by data of Sanes et al (18). who studied the lubrication of epoxy resins using a phosphonium-based IL as an additive. Two important observations are made: (i) for both PLA/IL-1 and PLA/IL-2 compositions, COF values were significantly lower than that of PLA and (ii) PLA/IL-2 showed a lower COF than PLA-IL-1. Thus, both IL-1 and IL-2 appear to be good internal lubricants, which would eventually tend to migrate to the surface of the PLA matrix.
TABLE 3. Contact angle and coefficient of friction.

Sample          Coefficient of friction (ALTEK)  Contact angle
                                                  ([degrees])

PLA             0.45 [+ or -] 0.07                 71.10
PLA/5 wt% IL-1  0.17 [+ or -] 0.05                 66
PLA/5 wt% IL-2  0.08 [+ or -] 0.02                 58.9


The data shown in Table 3 suggest that the presence of IL-1 and IL-2 decreases the contact angle of water on PLA resulting in increasing hydrophilicity that depends on the type of the anion. The more hydrophilic tetrafluoroborate anions (19) appear to be present on the surface of the PLA film reducing, thus, the contact angle to about 59[degrees]. These experiments suggest that the lower coefficient of friction corresponds to a lower contact angle, in agreement with the results of Hiratsuka et al. (20).

Flexural Properties

Flexural testing provided quantitative information on modulus and strength and qualitative information on ductility-strain at break of the samples. The effects of ILs on the flexural properties of PLA are summarized in Fig. 8. Please note that in the case of PLA/IL-1, the brittle samples broke at about 7.5 N load and the calculated strength is indeed the maximum stress at break that caused brittle fracture. PLA and PLA/IL-2 samples were more ductile and did not break but rather bent excessively at the selected span to depth ratio; thus, the reported strength is calculated from the maximum recorded force.

[FIGURE 8 OMITTED]

As discussed previously, the low [T.sub.g] of PLA/ILs samples could not only be due to plasticization but also to thermo-mechanical degradation. Molecular weight changes, measured by gel permeation chromatography and reported in our earlier publication (21), indicated a significant reduction of the PLA MW during melt processing in the presence of IL-1. Thus, PLA/IL-1 was the most highly degraded sample with dark color, having the lowest [T.sub.g], presumably as a result of the very low polymer MW; it exhibited relatively higher modulus and strength than PLA/IL-2 but failed catastrophically in a brittle manner. PLA/IL-2 underwent less thermo-mechanical degradation, had a higher [T.sub.g], had lower modulus, and was more ductile. By contrast, processed PLA underwent relatively little thermo-mechanical degradation and retained its high modulus and ductility.

CONCLUSIONS

In this study, two ILs with decanoate or tetrafluoroborate anions containing the same phosphonium-based cation were evaluated as potential plasticizers and lubricants for PLA. Both IL-1 and IL-2 were well dispersible and partly miscible with PLA at 5 wt% content as evidenced by SEM and [T.sub.g] data, as well as from solubility parameter calculations. However, in the case of PLA/5 wt% IL-2, phase separation was observed 1 year after melt processing. Different surface characteristics of the PLA/ILs were manifested in differences in COF and contact angle measurements; PLA/IL-2 showed a lower COF toward steel and more hydrophilic characteristic than PLA and PLA/ IL-1. The effects of IL-2 on the mechanical properties of the PLA blend were lower flexural strength and modulus than of IL-1, but higher ductility. Based on the results of this study, IL-2 could be considered as a more desirable multifunctional additive for PLA than IL-1, offering a better balance of adverse (e.g., polymer degradation) versus beneficial (e.g., lubricity) effects.

Further understanding of the biodegradation of PLA/ILs blends will be the topic of future studies. Given the potential toxicity of ILs toward living organisms, further experiments are recommended to separate the biodegradation of PLA from the effects of ILs that could be released from the matrix in a controlled manner. Potential applications for the studied PLA/ILs systems could be additives in antibacterial coatings or insecticide/pesticide containing devices.

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Kuil Park, (1) Jin Uk Ha, (2) Marino Xanthos (2)

(1) Materials Science and Engineering Program, New Jersey Institute of Technology, Newark, New Jersey

(2) Otto H. York Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, New Jersey

Correspondence to: M. Xanthos; e-mail: xanthos@njit.edu

Published online in Wiley InterScience (www.interscience.wiley.com).

[C] 2009 Society of Plastics Engineers

DOI 10.1002/pen.21629
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Author:Park, Kuil; Ha, Jin Uk; Xanthos, Marino
Publication:Polymer Engineering and Science
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Date:Jun 1, 2010
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