A study on the degradation of polylactic acid in the presence of phosphonium ionic liquids

A study on the degradation of polylactic acid in the presence of phosphonium ionic liquids

Polymer Degradation and Stability 94 (2009) 834–844 Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: ww...

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Polymer Degradation and Stability 94 (2009) 834–844

Contents lists available at ScienceDirect

Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab

A study on the degradation of polylactic acid in the presence of phosphonium ionic liquids K.I. Park a, M. Xanthos b, * a b

Material Science and Engineering Program, New Jersey Institute of Technology, Newark, NJ 07102, USA Otto H. York Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of Technology, Newark, NJ 07102, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 October 2008 Received in revised form 22 January 2009 Accepted 27 January 2009 Available online 13 February 2009

In this overview study, two ionic liquids (IL) with different anions (decanoate, tetrafluoroborate) but with the same phosphonium-based cation that showed promising plasticizing/lubricating behavior in polylactic acid (PLA) were screened for their effects on the polymer degradation under thermomechanical, thermo-oxidative (at 160  C), hydrolytic (100% humidity, 60  C), conditions, and during soil immersion. Depending on the particular medium and conditions used, degradation was followed by changes in molecular weight, melt viscosity, sample weight and appearance, morphology, crystallinity, acid number, and pH. The effects of the IL containing a decanoate anion were more pronounced on lubrication and also on degradation as evidenced by reduced melt viscosities and accelerated thermomechanical, isothermal, hydrolytic, and soil degradation. The IL containing the tetrafluoroborate anion showed higher thermal stability compared with the IL containing decanoate anion as also confirmed from thermal degradation rate constants which were calculated from random chain scission statistics. Accelerated hydrolytic degradation was observed in PLA containing the tetrafluoroborate based IL but to a lesser extent than the decanoate based IL. The catalytic role of the decanoate anion in hydrolytic degradation was confirmed through experiments with model compounds. X-ray diffraction (XRD) data on the materials exposed to soil degradation provided evidence that the initially amorphous polymer attained a certain degree of crystallinity as a result of the significant MW reduction. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Ionic liquids Polylactic acid Thermal/hydrolytic degradation Additives

1. Introduction Ionic liquids (ILs) are organic salts with low melting temperatures, typically below 100  C. They usually consist of nitrogen or phosphorous containing organic cations; corresponding anions are halides, tetrafluoroborate, hexafluorophosphates, etc. Recently, ILs have been shown to be potentially environmentally-benign solvents due to their low volatility, low melting points, a broad temperature liquid range, high-temperature stability, low flammability, and compatibility with a variety of organic and inorganic materials [1]. So far, ILs have been mostly used as ‘‘greener’’ alternatives to conventional organic solvents for chemical synthesis [2] but also evaluated in bio-processing operations, catalysis, gas separation or as electrolytes in electrochemistry; however, academic and industrial research efforts have paid little attention to the use of ILs as performance additives in polymer formulations. ILs can be used advantageously to overcome the lower thermal stability of conventional organic modifiers that are used to enhance

* Corresponding author. E-mail address: [email protected] (M. Xanthos). 0141-3910/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2009.01.030

dispersion, wetting, and compatibility of inorganic particles, including layered silicates such as montmorillonite (MMT), carbon nanotubes and other nanoparticles, in polymers and other organic media; the high thermal stability of ILs as measured by TGA in different atmospheres by various investigators and the ability to control their hydrophilic/lipophilic balance through proper selection of the constituent ions are some of their major attributes. ILs containing long-chain cations (mostly imidazolium or phosphonium-based) have recently been evaluated [3] as alternatives to conventional alkylammonium-base modifiers, the latter characterized by relatively low thermal stability at melt processing temperatures. 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 [4]. Thus, some phosphonium, imidazolium and ammonium based ILs have been evaluated as non-volatile

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plasticizers and as external or internal lubricants in several polymers including PVC [5], PMMA [6], and polyamides [2]. Biodegradable polymers and, in particular aliphatic polyesters of the poly(hydroxyacid)-type such as polylactic acid (PLA), have been investigated for both biomedical and consumer applications [7]. For such applications, it is of major importance to understand the degradation characteristics of these polymers, particularly their hydrolytic degradation, which may occur in the human body, or accompany soil or composting biodegradation. PLA degradation has been found to be dependent on a range of factors, such as molecular weight, crystallinity [7,8], purity, temperature, pH [9], presence of terminal carboxyl or hydroxyl groups, water permeability, and additives acting catalytically that may include enzymes, bacteria or inorganic fillers [10]. In most cases, however, degradation is relatively slow; loss of strength is usually measured in weeks, with complete degradation often taking years [8]. Acceleration of hydrolytic degradation, important for PLA products used in biomedical applications or to be composted or landfilled, has so far been achieved in several ways including high degradation temperatures and high acidity degradation media [11]. Toxicity research studies including work on aquatic systems, microorganism toxicology, cytotoxicity, and tests on animals have recently received broad attention and the commonly accepted notion that ionic liquids have low toxicity has been shown to be incorrect [12]. The complexities involved in the toxicological causes of ILs suggest that more work is needed to achieve a convincing data pool for the environmental and human risk assessment of ILs. This, in addition to their lack of biodegradability, would restrict the use of most ILs as plasticizers in biodegradable polymers; on the other hand, understanding of the effects of adding ILs having different chemical structures on the properties and degradation characteristics of aliphatic polyesters would encourage further research efforts to design benign, degradable and bio-renewable ILs [12]. The present 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 [13], the present article focuses on their effects on the hydrolytic and thermal degradation rates of PLA and attempts to separate the individual effects of the cations and anions by using model compounds. Blends to be tested after exposure to thermomechanical, isothermal oxidative, hydrolytic or soil degradative conditions are based on an amorphous PLA polymer that is mixed with two phosphonium ILs having the same long-chain hydrophobic cations but different anions. 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 [14]. The modified PLA samples are prepared at various ratios by melt-blending and solvent casting, and characterized for uniformity in the distribution of the IL in the PLA matrix and changes in its glass transition temperature (Tg). Depending on the particular medium and conditions used, degradation is followed by changes in molecular weight, melt viscosity, sample weight and appearance, morphology, crystallinity, acid number and pH. It should be emphasized that environmental toxicity issues were not considered in the selection of the ILs used in the present work.

Trihexyl tetradecyl phosphonium decanoate, [THTDP][DE], MW 655.13 was purchased from Sigma–Aldrich and is designated as (IL1); it is a yellowish gel, immiscible with water and has a density of about 0.885 g/cc as per information for the equivalent CYPHOS IL 103 product (Cytec Industries). Trihexyl tetradecyl phosphonium tetrafluoroborate, [THTDP][BF4], MW 570.68, was purchased from Sigma–Aldrich and is designated as (IL-2); it has a melting point of 37  C, appears as white flakes, is immiscible with water and has a density of about 0.93 g/cc as per information for the equivalent CYPHOS IL 111 product (Cytec Industries). Structures of IL-1 and IL2 are shown in Fig. 1. Sodium montmorillonite (MMT–Naþ), used in this study was obtained from Southern Clay Products Inc. (CAS# 1318-93-0, trade name: CloisiteÒ–Naþ). MMT–Naþ contains 4–9% moisture and sodium ions in the basal spacing without added organic modifiers. Its cationic exchange capacity (CEC) has been reported as 92.6 meq/100 g clay. 2.2. Sample preparation 2.2.1. Melt-blending PLA pellets were first processed until steady torque was achieved. IL-1 or IL-2 were then added at three different concentrations up to 5 wt% and mixed for 10 min. This direct addition method resulted in a decrease of the torque to zero when the ILs were added, regardless of the concentration used, 5 or 2 wt%. In order to achieve high-quality mixing, IL-1 or IL-2 was dissolved in EtOH/H2O (4:1) and PLA pellets were added in this solution until thoroughly immersed for 24 h. The solvent was removed by heating at 70  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 pre-mix contained 10 wt% IL. Different weight percentages of ILs in PLA were used. To produce 1 wt% and 5 wt% of IL-1 or IL-2 in PLA, proper amounts of PLA pellets were added along with the pre-coated pellets in the mixer bowl and melt blended at 50 rpm and 160  C for 10 min under nitrogen until the torque stabilized. After melt processing, PLA and PLA/5 wt% IL-2 samples were transparent and had brown color; PLA/5 wt% IL-1 was dark brown probably as the result of excessive degradation. The mixtures produced were then compression molded in a hydraulic compression press for 2 min at 180  C into 10 mm thick films. 2.2.2. Preparation of nanoclay modified with IL-1 IL-1 was added at 2 the stoichiometric amount of the cationic exchange capacity (CEC) (3.65 g respectively in 250 ml solution) of MMT–Naþ. IL-1 was dissolved in ethanol (200 ml) and deionized water (50 ml) solution by vigorous stirring for 10 min at room

2. Experimental 2.1. Materials Polylactide polymer pellets 4060D purchased from NatureworksÒ had a reported glass transition temperature of 58  C.

835

Fig. 1. Structures of ionic liquids, IL-1 and IL-2.

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temperature. MMT–Naþ (3 g) was then added to the solution. The cationic exchange reaction was performed at 60  C under reflux. The modified MMT was collected by vacuum filtration (Genuine Whatman filter paper, 7 cm diameter, 1.6 mm pore size). The modified MMT was washed many times in the funnel with EtOH/ H2O solution and then three more times with vigorous stirring in a 500 ml (EtOH 80%, H2O 20%) beaker in order to remove residual anions. The finally filtered MMT (designated as MMT-1) was dried at room temperature for 24 h and then at 90  C for 24 h under vacuum. The final products were ground with a mortar and pestle and dried at 90  C for 24 h under vacuum once again. 2.2.3. Solvent casting 5 wt% (based on PLA) of IL-1 or IL-2 was dissolved in chloroform and PLA pellets were added in each solution. Model compounds containing only decanoic acid (DECA) and MMT/phosphonium cation (MMT-1) were also prepared in order to separate the effect of the anion from that of the cation in IL-1. To keep the same concentration of decanoate anions and phosphonium cations as in IL-1, 1.6 wt% of DECA and 50 wt% of MMT-1 were dissolved/ dispersed, respectively in the PLA solution. As a control, an MMT slurry containing 50 wt% clay based on PLA was also produced. After homogenization, the polymer/additive solution was poured into glass petri dishes and kept for 48 h under the hood. After most of the chloroform had evaporated, the remaining solids were dried under vacuum. The thicknesses of the dried films were 1 mm with a diameter of 7.5 cm.

Table 1 EDX elemental analysis of PLA/5 wt% IL-1 surface. Element

Weight %

C O P Cl Total

56.66 41.59 0.20 0.32 100.00

for predetermined periods of time at which time the pH of the medium was measured. Samples were withdrawn from the PBS after 144 h and washed with distilled water followed by drying in the vacuum oven for a week at room temperature. 2.5. Soil degradation

Isothermal degradation experiments were carried out at 160  C for 7 days in a ventilated oven in air. An initial amount of 2 g of each sample (PLA, PLA/5 wt% IL-1, PLA/5 wt% IL-2, IL-1, and IL-2) from the melt mixing process was ground to a powder and placed in uncovered glass jars. All samples were weighed every 2 h during the first day and then weighed at intervals of 24 h for seven days.

Biodegradability in soil was determined by visual observation of the pre-cut 1 cm  1 cm specimens with 1 mm thickness buried in the soil. The soil compost contains 0.21% nitrogen (0.12% ammonia– nitrogen, 0.09% nitrate nitrogen), 0.07% of P2O5, and 0.14% of K2O. Measured pH of the soil was 8.1 at 25  C. Sterilized soil (pH 6.8) was prepared by heating up the above soil at 200  C for 24 h in the oven followed by storing in a vacuum desiccator. Plans to measure % weight changes were not materialized since the recovery of the degraded fragments was difficult after the degradation. The buried specimens were incubated at 60  C. Relative humidity was kept constant at 30–35% and measured by a moisture meter (HH2, Delta T, Cambridge, England). Each specimen was removed from the soil after burial for 1, 2, 3, 4 weeks, respectively. The specimens were swiped to remove soil on the surface and dried at 40  C in a vacuum oven overnight. Another set of samples were placed in the sterilized soil in the same manner to compare their visual appearance with those of samples degraded in the unsterilized soil. X-ray diffraction (XRD) measurements were conducted to compare the crystallinity of the PLA control to that of a sample after 4 weeks of burial in soil.

2.4. Hydrolytic degradation

2.6. Characterization

PLA, PLA/5 wt% IL-1, PLA/5 wt% IL-2, PLA/1.6 wt% DECA, PLA/ 50 wt% MMT, and PLA/50 wt% MMT-1 processed by solvent casting were compression molded using a circular plate at 160  C. Specimens with 30 mm diameter and 1.5 mm thickness were then cut into four equal pieces. Specimens were immersed into small flasks filled with 60 ml of 1 M phosphate buffer solution (PBS) at pH 7.4. The flasks were allowed to stand in a thermostatted oven at 60  C

2.6.1. Energy dispersive X-ray scattering (EDX) Elemental analysis was performed by EDX on the surfaces and cross-sections of samples examined by SEM. The distribution of IL1 and IL-2 in the PLA matrix was investigated by analyzing for elements such as P and F present in the cations of IL-1 and IL-2 and in the anion of IL-2, respectively. The P/F ratio was calculated and compared with the theoretical ratio of P/F in the case of IL-2.

2.3. Thermal degradation

Fig. 2. (a) SEM micrograph of the surface of PLA/5 wt% IL-1 and (b) EDX elemental mapping of phosphorous element for the same area.

K.I. Park, M. Xanthos / Polymer Degradation and Stability 94 (2009) 834–844

837

Fig. 3. (a) SEM micrograph of the surface of PLA/5 wt% IL-2, (b) EDX elemental mapping of phosphorous for the same area, (c) EDX elemental mapping of fluorine for the same area.

2.6.3. Helical barrel rheometer The Helical Barrel Rheometer (HBRÔ) is a portable online viscosity measuring instrument with a single screw that measures pressure drop across a flight at a given shaft speed [15]. The data set can be converted to determine viscosity as a function of shear rate. Viscometric behavior of PLA, PLA/5 wt% IL-1, and PLA/5 wt% IL-2 was determined by the HBRÔ over a period of 70 min at constant shaft speed. To eliminate air in the feed compound and stabilize it before measuring viscosity, 20 g of PLA and PLA pellets pre-coated with 5 wt% IL-1 and 5 wt% IL-2 were fed and compressed for a period of 1 min. Measurements were taken at 160  C and low speed (5 RPM) in order to minimize excessive thermomechanical degradation.

2.6.4. Differential scanning calorimetry (DSC) PLA, ILs and their compounds were characterized by DSC scans (TA Instruments, QA 100 analyzer) from 0  C to 150  C at a heating rate of 10  C/min to determine glass transition temperatures.

10000 PLA PLA/1 wt IL-2 PLA/10 wt IL-2

Apparent viscosity (Pa-s)

2.6.2. Capillary rheometry Apparent viscosities of PLA and its IL compounds were determined using a capillary rheometer (Instron, Model 4204) in the shear rate range of 15 s1 to 38 s1 at 190  C.

1000

100

Table 2 EDX elemental analysis of PLA/5 wt% IL-2 surface. Element

Weight%

C O F P Total

55.29 43.74 0.96 0.28 100.00

10

10

100

1000

Apparent shear rate (1/s) Fig. 4. Apparent viscosity vs. apparent shear rate curves of PLA and PLA/IL-2 blends by capillary viscometry at 190  C.

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1.0

Heat Flow (W/g)

0.5

0.0 PLA/5 wt

IL-1

PLA/5 wt PLA/10 wt

IL-2 IL-2

-0.5

-1.0 PLA/10 wt

IL-1

PLA

-1.5 0 Exo Up

20

40

60

Temperature (C)

80

100 Universal V3.9A TA Instrum

Fig. 5. DSC heating curves, comparison of PLA vs. PLA containing different wt% of IL-1 and IL-2.

2.6.5. Gel permeation chromatography (GPC) GPC [Waters Co., RI detector, PLgel 5 mm MIXED-C, 300  7.5 mm (PL1110-6500) column with THF as eluent at 1.0 ml/min, and 35  C] was used to determine the changes of the PLA molecular weight after hydrolytic or thermal degradation from THF solutions of samples prepared by melt mixing or solvent casting. Calibration of the GPC was based on polystyrene standards. 2.6.6. Scanning electron microscopy (SEM) The morphologies of PLA and its IL compounds were investigated by SEM (JSM-5410LV; JEOL Ltd., Tokyo, Japan). Cross-sections of the specimens were examined after fracturing in liquid nitrogen. 2.6.7. Acid number–pH Acid numbers (mg of KOH/g of sample) of PLA, PLA/5 wt% IL-1, PLA/5 wt% IL-2 (prepared by solvent casting) before and after six days of hydrolytic degradation were determined by titration using 0.05 N KOH solution with phenolphthalein as an indicator in chloroform. Acid numbers of 0.016 wt% IL-1, and IL-2 in chloroform were also measured. pH changes in the PBS medium as a function of time were also recorded. 2.6.8. X-ray diffraction (XRD) Philips PW3040 diffractometer (Cu Ka radiation l ¼ 1.5406 Å, generator voltage ¼ 45 kV, current ¼ 40 mA) was used to evaluate the crystallinity of the PLA before and after soil degradation. The scanned 2q range was from 5 to 45 . 3. Results and discussion 3.1. Characterization of blends 3.1.1. IL Miscibility-dispersion SEM micrographs and corresponding EDX mapping analysis (Fig. 2) for PLA/5 wt% IL-1 film surfaces showed no agglomerates and uniform distribution of the phosphorus element present in the IL cations (Table 1). More accurate uniformity in the distribution of the IL-4 anion could not be ascertained by EDX due to the absence of a characteristic element other than C, H, O. However, from calculations for both cations and anions based on the wt% concentration of the P element, the total amount of IL-1 in PLA was

approximately 4.36 wt% which is close to the initial concentration of IL-1 (5 wt%) in the PLA. Considering that EDX results were obtained in various regions of the sample and then averaged, IL-1 appears to be well dispersed and also partly dissolved in the PLA matrix. It should be noted that XRD could be applied to detect the miscibility state of the ILs (crystalline or amorphous) in the PLA matrix; however, based on our prior experiments, XRD spectra of ILs could not provide much detailed information. SEM micrograph of a PLA/IL-2 surface and a corresponding mapping for phosphorous (cation) and fluorine (anion) elements are shown in Fig. 3. Note that the light boron element cannot be detected by EDX. The results suggest uniform distribution of IL-2 on a small scale (and partial dissolution) although the presence of coarser agglomerates is still a possibility. As shown in Table 2 the P:F ratio is 0.29, which is close to the theoretical value of 0.36 calculated from the structural formula of IL-2. This suggests that there is no segregation of cations or anions on the surface of PLA/ 5 wt% IL-2. Elemental EDX mapping of ILs has been used by other authors to determine uniformity of their distribution in a polymer matrix. Sanes et al. [16] prepared polymer/10 wt% IL blends using polystyrene, PS, and 1-ethyl-3-methyl imidazolium tetrafluoroborate, [EMIM]þ[BF4]. EDX results of the mixture on fractured surfaces showed a heterogeneous phase distribution where in dark grey regions attributed to the IL, elements such as fluorine, nitrogen, etc. were detected. This non-uniform IL distribution may be due to either the high concentration of IL in the PS or the poor miscibility of PS and [EMIM]þ[BF4]. 3.1.2. Rheological characterization Fig. 4 contains viscosity/shear rate curves for PLA and PLA/IL-2 samples prepared by melt mixing. It is evident that IL-2 produces significant reductions in the PLA apparent viscosity, proportional to its increasing concentration. The apparent viscosity of PLA/IL-2 (10 wt%) is 57.3 Pa s while that of PLA is 531.6 Pa s at 384 s1 and 190  C. In addition, while PLA/IL-2 (10 wt%) exhibits a marked negative dependence of apparent viscosity vs. apparent shear rate in the range relevant for processing, the viscosity of PLA decreases only slightly when the shear rate increases. Viscosity reduction was also observed for the PLA/IL-1 system at 1 wt% IL-1. However, viscosity measurements at any of the other IL1 concentrations used (5 wt% and 10 wt%) were not possible, even

K.I. Park, M. Xanthos / Polymer Degradation and Stability 94 (2009) 834–844

PLA PLA/5 wt PLA/5 wt

1600

Shear viscosity (Pa.s)

1400

IL-2 IL-1

839

Table 3 Number average MW, Mn, changes of melt processed PLA, PLA/5 wt% IL-1 and PLA/ 5 wt% IL-2 during isothermal degradation up to 50 h at 160  C. Thermal treatment

Sample Mn

1200 Before melt processing/thermal degradation After melt processing After melt processing and 4 h heating After melt processing and 6 h heating After melt processing and 8 h heating After melt processing and 19 h heating After melt processing and 50 h heating

1000 800 600

PLA

PLA/IL-1

PLA/IL-2

133,000 (pellets) 113,000 104,000 96,400 92,000 88,600 85,800





39,000 – 21,000 17,000 19,000 –

127,000 97,000 78,000 90,000 57,000 52,000

400 200 0

3.2. Thermomechanical degradation 0

10

20

30

40

50

60

70

Time (min) Fig. 6. Comparison of shear viscosity changes of PLA, PLA/5 wt% IL-1 and PLA/5 wt% IL2 over a period of 70 min as measured by the Helical Barrel Rheometer at 160  C and 5 rpm.

with a smaller diameter die due to very low values as a result of lubrication and possibly thermal degradation effects. The occurrence of lubrication is consistent with reports that carboxylic acid (including decanoic acid) additives in thermoplastics migrate to the steel walls of the capillary instrument and induce slippage by the formation of a lubricating layer; also, with our batch mixing results where torque could not be measured when ILs were added directly in the molten PLA. 3.1.3. Glass transition temperature Tg determination experiments for the PLA/IL blends confirm that both ILs are miscible or partly miscible with PLA. This is related to the decrease of the PLA Tg from 60  C to lower values depending on the type and concentration of ILs as shown in Fig. 5. 10 wt% IL-1 lowers Tg to about 45  C, which is at the same level as that attained with PEG conventional plasticizers [17]. The effect of IL-2 on the Tg of PLA is less pronounced. Note that no transition temperature was detectable by DSC for either IL, even at temperatures as low as 30  C. Miscibility and plasticization could also be confirmed from the transparency of the flexible plasticized films prepared by meltblending and in the case of PLA/IL-2 by the solvent casting method.

Fig. 6 shows HBR shear viscosity data at 160  C for PLA, PLA/ 5 wt% IL-1, and PLA/5 wt% IL-2 obtained over a time period of 70 min. The results clearly indicate that PLA/5 wt% IL-1 has the lowest viscosity, which decreases continuously during the entire experimental period. After the first 20 min at 160  C, no shear viscosity data could be collected, as was also the case for the earlier batch mixer experiments at any IL-1 concentration higher than 1 wt%. This behavior can be attributed to the presence of decanoate anions that enhance lubrication and also catalyze excessive thermal degradation (in spite of the low RPM) within the first 20 min at 160  C. In the case of PLA and PLA/5 wt% IL-2, in agreement with the results of the capillary viscometry, PLA/IL-2 still shows a lower viscosity than PLA but also a reduced tendency towards degradation. Thus, both IL-1 and IL-2 reduce viscosities of PLA/ILs during melt processing, acting as internal lubricants and improving flow charateristics; however, this behavior is accompanied by thermomechanical degradation to different degrees. 3.3. Isothermal degradation 3.3.1. Weight loss In isothermal heating at 160  C in air, with melt mixed samples, a significant observation was that up to 45 h PLA/IL-2 was the most stable sample while PLA/IL-1 had lost up to 38% of its weight (Fig. 7). This could be due to the inherent lower thermal stability of IL-4 or a very pronounced catalytic degradative effect. By contrast,

PLA/IL-1 0.004 100 90

)

0.003

1/DP

Weight percent (

80 70

0.002 60 PLA PLA/5 wt IL-2 IL-1 PLA/5 wt

50 40 30

0

20

40

PLA/IL-2 IL-1 0.001

PLA

IL-2 60

80

100

120

140

Time (hr) Fig. 7. Weight loss of PLA, IL-1, IL-2, PLA/5 wt% IL-1, and PLA/5 wt% IL-2 as a function of thermal degradation time at 160  C in air.

0

2x104

4x104

6x104

Time (sec) Fig. 8. Thermal degradation data up to 19 h, 68400 s for PLA, PLA/IL-1, and PLA/IL-2 based on 1/DP and Eq. (3.1).

840

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Fig. 9. (a) PLA, PLA/5 wt% IL-2 and PLA/5 wt% IL-1 before hydrolytic degradation. (b) PLA, PLA/5 wt% IL-2 and PLA/5 wt% IL-1 after 144 h of hydrolytic degradation.

PLA and IL-2 were quite stable throughout the entire experiment of 140 h. IL-1 showed a similar trend to PLA/IL-2 with the exception of a slightly steeper slope (higher weight loss). 3.3.2. Molecular weight changes Table 3 contains GPC molecular weight data, for unprocessed PLA pellets, processed PLA, and its blends with IL-1 and IL-2, before and after isothermal degradation at 160  C. Molecular weights in the table are expressed as Mn (number average MW). The data were most often based on single point measurements on degraded, small samples, having complex and possibly non-uniform composition; as a result, some observed discrepancies are as expected. Overall, however, reasonable trends can be observed. As expected, the MW of PLA decreases somewhat after melt processing in the batch mixer and by about 27% after 50 h of additional isothermal heating at 160  C. The PLA/5 wt% IL-1 undergoes the most rapid degradation after melt processing and the polymer MW decreases further to very low values after 19 h isothermal heating. MW distributions as defined by Mw/Mn ratios for melt processed PLA and PLA/IL-1 samples are 1.71, 1.58 respectively and become narrower after isothermal degradation (for example, 1.42 and 1.12 respectively, after 8 h). Although the PLA/5 wt% IL-2 has an abnormally high MW value after melt processing, the polymer MW began decreasing upon isothermal heating more rapidly than that of the unmodified PLA. The data are in agreement with the differences in thermal stability of the three materials discussed in Sections 3.1.2 and 3.2. The data of Table 3 show that the Mn of as received PLA pellets was 133,000, which is only slightly higher than the Mn of the PLA in its mixture with IL-2 after melt processing (127,000). This suggests that PLA degradation in the presence of ILs took place largely during melt mixing, rather than in the pre-coating step from H2O/EtOH solution. 3.3.3. Degradation rate constant Emsley et al. [18] used the following equation that accounts for the relationship between degree of polymerization and time, based on random chain scission model:

1=DPt  1=DP0 ¼ kt

(3.1)

where DPt is degree of polymerization at time t, DP0 is initial degree of polymerization, and k is degradation rate constant. Calculated 1/DP values in this work based on Mn, are plotted vs. degradation time according to Eq. (3.1) in Fig. 8. All systems obey a reasonable linearity up to 19 h which allows the calculation and comparison of the initial rate constants. The derived k values were 8.5  106 s1, 3.05  104 s1, and 3.42  105 s1, for PLA, PLA/IL-1, and PLA/IL-2, respectively. The corresponding R2 values were 0.88, 0.99, and 0.96. Thus, the calculated rate constant values were higher for both IL modified PLA than the unmodified polymer. The high R2 values also confirm that the thermal degradation of PLA in the presence of the ILs occurs by the same chain scission mechanism that was assumed in the derivation of Eq. (3.1). 3.4. Hydrolytic degradation 3.4.1. Visual observations PLA, PLA/5 wt% IL-1 and PLA/5 wt% IL-2 sheets used for hydrolytic degradation were initially transparent in agreement with the amorphous structure of the PLA and the assumed PLA/IL miscibility as suggested by our Tg data (Fig. 5). In the PBS degradation medium, at 60  C, the surface of all samples became rapidly white in the order: PLA/5 wt% IL-1 > PLA/5 wt% IL-2 > PLA; this is presumably due to the crystallization of the amorphous PLA as its MW decreases as will be shown below. For all samples, differentiation between surface and interior was clearly detectable, surface being white while interior being transparent. This may be due to the neutralization of the carboxylic end groups located at the surface by the external buffer solution and the diffusion of the soluble oligomers from the surface [19]. In the case of PLA/5 wt% IL-1, after 144 h, full breakdown took place and residues remained in the form of a white powder. Both PLA and PLA/5 wt% IL-2 samples after 144 h, as shown in Fig. 9, became rough on their surface and very fragile when bent.

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841

Fig. 10. (a) SEM micrographs of samples before immersion in PBS [(a), (c), (e)] and after 144 h of immersion in PBS [(b), (d), (f)]. PLA: (a) and (b); PLA/IL-1: (c) and (d); PLA/IL-2: (e) and (f).

3.4.2. Morphology The morphological changes due to the incubation in PBS were visualized by SEM micrographs of specimens collected at certain time periods. Fig. 10 shows SEM micrographs of PLA, PLA/IL-1, and PLA/IL-2 before and after 144 h of hydrolytic degradation. Both PLA/ IL-1 and PLA/IL-2 samples display rough surfaces and porosity compared to PLA. The pictures, clearly, reveal that all surfaces of the PLA/IL blends were degraded. The effects are much more pronounced for the PLA/IL-1 sample that is characterized by accelerated degradation. Average pore sizes increased in the order of PLA, PLA/IL-2, and PLA/IL-1 (0.3 mm, 1 mm, 5 mm), confirming the rapid acceleration of degradation of PLA by both IL-1 and IL-2, but in particular IL-1. It is well known that hydrolytic degradation causes an increase in the number of carboxylic acid chain ends, which are known to autocatalyze ester hydrolysis [20]. In the case of IL-1, the higher degree of degradation appears to be due to the presence of decanoate anions/decanoic acid in the hydrolysis medium.

3.4.3. Molecular weight MW changes of melt mixed samples after 144 h of hydrolytic degradation confirm that both IL-1 and IL-2 accelerate the degradation of PLA (Table 4); however, as in the case of thermal degradation, the effects of IL-1 are much more pronounced. Peak molecular weights, Mp, obtained by GPC after 144 h were 65,000 (PLA) vs. 3500 (PLA/IL-1), and 37,000 (PLA/IL-2). As also mentioned above, it is known that PLA degradation is acid catalyzed, particularly in the presence of carboxylic groups [2]; this could explain the significant effect of the decanoate anion. On the other hand, the BF 4 anion of IL-2 could also accelerate the degradation by forming HF in water [21]. 3.4.4. Model compounds To investigate the relative contributions of the cation and the anion to the degradation of PLA, model compounds (PLA/DECA, PLA/MMT, and PLA/MMT-1) were prepared. The effects of the cations and anions on the polymer peak MW after 144 h hydrolytic

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Table 4 Peak MW, Mp, changes of PLA, PLA/5 wt% IL-1 and PLA/5 wt% IL-2 before and after hydrolytic degradation of 100% humidity at 60  C (144 h).

8

Treatment

7

Sample Mp PLA/IL-1

PLA/IL-2

169,000 155,000 65,000

N/A 63,000 3,500

N/A 186,000 37,000

degradation at 60  C are shown in Table 5. Unmodified MMT appears to have little, if any, catalytic effect on the polymer degradation. The presence, however, of the phosphonium cation on the MMT (MMT-1) although having little effect on the polymer MW of the ‘‘as prepared’’ samples does have a significant effect on the samples subjected to hydrolytic degradation. Decanoic acid and its anion contribute somewhat to degradation of both the ‘‘as prepared’’ samples and much more to the ones subjected to hydrolytic degradation. IL-1 containing a combination of phosphonium and decanoate ions appears to catalyze degradation of both the ‘‘as prepared’’ samples and also those subjected to hydrolytic degradation. In summary, both phosphonium cations and decanoate anions play a significant role in the PLA degradation with the catalytic effect more pronounced for the decanoate anion. 3.4.5. pH and acid number Fig. 11 shows the pH of the degradation medium containing PLA and its blends at different incubation times. In general, the pH decreases due to polymer degradation involving autocatalysis, resulting in increase of –COOH concentration. In the case of PLA/ 5 wt% IL-1, as mentioned before, the presence of decanoic acid in the degradation medium could further catalyze and accelerate the hydrolysis of ester bonds. Similarly, for PLA/5 wt% IL-2 the byproduct (HF) could also decrease the pH value of the degradation medium [21] although, in this case, it is understood that HF could react with the silicate glass of the container, thus affecting the pH reading. The pH of the PBS medium containing immersed PLA and PLA/ 5 wt% IL-2 remained stable for the first 3 days whereas for the PLA/ 5 wt% IL-4 system it dropped down to pH 3.5 in 5 days. In the case of PLA, pH decreased slightly between 10 and 12 days followed by rapid decrease to 4 between 13 and 16 days, due to formation of acidic oligomers/monomer. The pH change of PLA/5 wt% IL-1 was the most rapid among the samples, in agreement with the MW changes of PLA/5 wt% IL-1 after 6 days of hydrolytic degradation. Table 6 contains data on as received PLA pellets and melt processed PLA before and after hydrolytic degradation, and similar data for melt mixed PLA/IL-1 and PLA/IL-2 compounds. Acid numbers of as received ILs are also shown for comparison. Acid numbers for PLA/ILs combinations do not obey law of mixtures equations since the data for PLA before hydrolytic degradation refer to unprocessed pellets rather than melt mixed controls. The acid numbers of PLA, PLA/5 wt% IL-1, and PLA/5 wt% IL-2 increased as a result of the formation of acidic oligomers, lactic acid monomer, Table 5 Peak MW, Mp, changes of PLA, PLA/MMT, PLA/MMT-1, PLA/IL-1, and PLA/DECA prepared by solvent casting method after 144 h of hydrolytic degradation of 100% humidity at 60  C. Sample

Mp control

Mp after 144 h of hydrolytic degradation

PLA PLA/MMT PLA/MMT-1 PLA/IL-1 PLA/DECA

162,000 168,000 157,000 29,000 111,000

89,000 80,000 35,000 750 20,000

PLA PLA/ IL-1 PLA/ IL-2

6

pH

None After melt processing After melt processing followed by 144 h hydrolytic degradation

PLA

5

4

3

2 0

5

10

15

20

25

30

Days Fig. 11. pH changes of PBS medium containing different immersed PLA samples during hydrolytic degradation.

as well as the contributions of the individual ILs. The initial value of PLA/5 wt% IL-1 is relatively higher than that of other samples due to the contribution of the more acidic IL-1. The system PLA/5 wt% IL-1, contributing to the most rapid degradation, showed again the largest increase of acid number after immersion.

3.5. Soil degradation 3.5.1. Visual observations Soil degradation in a non-sterilized soil is a complex process that predominantly involves hydrolytic and bacterial degradation. PLA/ILs blends were already cracked and crumbled after 14 days, and recovery of the degraded fragments was difficult after that time period. For all samples, color change to opaque white started on the 7th day as in the case of hydrolytic degradation. This is due to crystallization promoted by the formation of low MW polymer during degradation. Fig. 12 shows pictures of PLA and PLA/ILs blends degraded in soil and sterilized soil. The buried specimens of PLA/ILs blends were very fragile and some parts were broken in the washing process that prevented the measurement of weight loss. In the case of specimens in soil, the PLA/ILs blends were expected to be degraded by both the action of water and enzymatic interactions while the degradation process in the sterilized soil system was expected to be controlled by moisture only. The surface of the matrix PLA appears unchanged except for the color change and shrinkage after the burial-test (Fig. 12b,c). In the case of PLA/ILs blends, many cracks were observed especially for the PLA IL-1 in soil. The cracks were thought to be formed by the excessive degradation of the polymer and shrinkage of the sample due to PLA

Table 6 Acid numbers before and after hydrolytic degradation of 100% humidity at 60  C. Sample

Acid number (control)

Acid number (after 144 h of hydrolytic degradation)

PLA PLA/5 wt% IL-1 PLA/5 wt% IL-2 IL-1 IL-2

2.21 (pellets) 7.57

9.2 35.6

5.02

8.4

7.15 2.52

N/A N/A

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843

Fig. 12. Photographs of neat PLA and PLA/ILs blends before and after burial in soil or sterilized soil (s.soil) for 28 days: (a) PLA before burial, (b) PLA after burial in soil, (c) PLA after burial in s.soil, (d) PLA/IL-1 before burial, (e) PLA/IL-1 after burial in soil, (f) PLA/IL-1 after burial in s.soil, (g) PLA/IL-2 before burial, (h) PLA/IL-2 after burial in soil, (i) PLA/IL-2 after burial in s.soil.

crystallization as a result of its MW reduction. Such cracks were not observed in the case of PLA and PLA/IL-2 blend in both soil and sterilized soil. In general, samples degraded in sterilized soil were less opaque than samples degraded in the ‘‘as received’’ soil.

200 PLA control PLA 4 weeks in soil

Intensity

150

100

3.5.2. Crystallinity Fig.13 shows the X-ray spectrum of PLA before and after 4 weeks of degradation in non-sterilized soil. The spectrum of the initial polymer is typical of an amorphous polymeric material. After 4 weeks of degradation, a peak at 17 and a second narrow peak at 18.5 appear, indicating morphological changes. The peaks indicate that amorphous PLA crystallized during/after soil degradation due to its lower molecular weight since degradation gives shorter chains that are more mobile than longer ones, and more susceptible to crystallize. This is supported by the results of Miyajima et al. [22] who studied the hydrolytic degradation of amorphous PLA prepared by quenching. They obtained similar peaks after 16 h immersion in pH 7.4 PBS at 37  C. Since PLA/IL-1 and PLA/IL-2 also became whitish after 4 weeks, it appears that crystallization of the amorphous polymer in these blends also takes place. The results of this section are directly applicable to the case of hydrolytic degradation in Section 3.4.1 where initially transparent samples became opaque after immersion.

4. Conclusions

50

0

0

5

10

15

20

25

30

35

40

Two theta Degrees Fig. 13. XRD spectra of PLA before and after 4 weeks of soil degradation.

45

In this study, two ILs with different anions (decanoate, tetrafluoroborate) but with the same phosphonium-based cation were evaluated as thermal/hydrolytic and soil degradants for PLA. Both ILs were well dispersible and partly miscible with PLA at 5 wt% content as evidenced by SEM and Tg characterization, as well as from EDX. The effects of the IL containing the decanoate anion (IL-1) were more pronounced on lubrication and degradation as evidenced by reduced melt viscosities and accelerated thermomechanical,

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isothermal, hydrolytic and soil degradation. The IL containing the tetrafluoroborate anion increased the thermal stability of PLA, at least up to 40 h at 160  C in air, and accelerated its hydrolytic degradation, but to a lesser extent than the decanoate based IL. The catalytic role of the decanoate anion in hydrolytic degradation was confirmed through experiments with model compound. Hydrolytic chain scission of ester bonds catalyzed by carboxylic groups of PLA and IL-1, and HF formed from the IL-2 are the suggested mechanisms for the hydrolytic degradation based also on results of pH and acid number changes; it appears, however, that the presence of the phosphonium cations contributes also to accelerated degradation. SEM micrographs confirm that both ILs accelerate hydrolytic degradation by increasing pore size. In the ‘‘as received’’ soil, PLA and their IL blends (particularly the PLA/IL-1 blend) appears to degrade faster than in a sterilized soil, presumably as a result of additional enzymatic degradation. XRD data on the material exposed to soil degradation provides evidence that the initially amorphous polymer attains a certain degree of crystallinity as a result of the significant MW reduction. Data on the toxicity of ILs and their environmental impact is an absolute necessity to broaden their range of applications. Given the potential toxicity of most ILs towards living organisms, possible applications for the studied PLA/ILs systems could be as additives in antibacterial coatings or in insecticide/pesticide containing devices. It follows that further experiments are recommended in order to optimize the rates of hydrolytic/biodegradation of the PLA and the rates of the controlled release of the ILs from the PLA matrix.

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