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TPU bio-blends
Journal Pre-proofs A Comprehensive Study on Shape Memory Behavior of PEG Plasticized PLA/TPU Bio-Blends S. Boyacıoğlu, M. Kodal, G. Ozkoc PII: DOI: Reference:
S0014-3057(19)31817-8 https://doi.org/10.1016/j.eurpolymj.2019.109372 EPJ 109372
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
9 September 2019 8 November 2019 14 November 2019
Please cite this article as: Boyacıoğlu, S., Kodal, M., Ozkoc, G., A Comprehensive Study on Shape Memory Behavior of PEG Plasticized PLA/TPU Bio-Blends, European Polymer Journal (2019), doi: https://doi.org/ 10.1016/j.eurpolymj.2019.109372
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A Comprehensive Study on Shape Memory Behavior of PEG Plasticized PLA/TPU Bio-Blends
S. Boyacıoğlu, M. Kodal and G. Ozkoc*
Department of Chemical Engineering, Kocaeli University, Kocaeli 41380, Turkey
*Corresponding
Author:
Guralp Ozkoc, Ph.D. Tel: +90 262 303 3540 Fax: +90 262 303 3550 E-mail: [email protected]
Abstract In this study, the shape memory behavior of poly(ethylene glycol) (PEG) plasticized poly(lactic acid)/thermoplastic polyurethane (PLA/TPU) blends was investigated as functions of PLA/TPU ratio, plasticizer molecular weight and programming conditions. The mechanical, thermal and morphological properties of polymer blends were characterized by tensile tests, differential scanning calorimeter (DSC) and scanning electron microscopy (SEM). It was found that the addition of plasticizers decreased the glass transition and cold crystallization temperature of PLA phase. As the molecular weight of PEG increased, the plasticization efficiency decreased. The shape memory tests showed that the higher the TPU content, the higher the recovery ratio between 40°C-55°C temperatures ranges. The maximum total recovery was obtained at 60°C for 20/80 PLA/TPU blends. The decreasing molecular weight of PEG maximized the shape recovery values. It was demonstrated that the samples were able to lift stresses up to 245 kPa during shape recovery depending on the temperature and composition. Moreover, the biocompatibility and cytotoxicity tests revealed that plasticized PLA/TPU blends are non-toxic and safe to use in-vivo conditions.
Keywords: Shape memory polymers; poly (lactic acid); thermoplastic polyurethane; poly(ethylene glycol); plasticizers; biomedical materials
1. Introduction Recently, shape memory materials such as alloys, ceramics and polymers have attracted attention because of their various applications. Lightweight, high performance to price ratio, biocompatibility and processability make shape memory polymers (SMPs) highly useful for many applications such as smart coatings, textiles, packaging, biomedical etc. [1, 2]. Per definition, SMPs change their shape with an external force such as light, chemicals, heat and magnetic field etc, however; one of the commonly studied type is the thermally responsive polymer systems [3]. Thermally induced SMPs show a characteristic transition temperature (Ttrans), which is responsible from shape changing. When SMPs are heated above the Ttrans, they are able to deform to a temporary-shape under an applied force. Then a cooling step below the Ttrans is applied to fix the temporary shape. In order to have a recovery from the final state, the applied stress is needed to be removed and the polymer must be heated above its Ttrans. Tg and/or melting temperature (Tm) of the polymer can be taken as the Ttrans [4-6]. To overcome the solid waste pollution resulted from the petroleum-based plastics, ecofriendly, compostable and biodegradable polymers are under focus. Poly(lactic acid) (PLA) is the most widely studied biodegradable polymer due to its degradability, relatively better physical properties and cost efficiency [7]. As a comparison with other well-known biodegradable polymers such as polycaprolactone (PCL), PLA has some advantageous properties, such as high strength, high modulus, transparency, processability and commercially availability [8, 9]. Moreover, PLA exhibits shape memory behavior. The PLA’s shape memory behavior depends on the presence of crystalline and amorphous phases. The crystalline phase behaves as the rigid phase, which is responsible for the energy storage during the temporary shape. Furthermore, the amorphous phase is acting as switching phase [10]. On the other hand, its brittleness is a limiting disadvantage for PLA [9]. Thus, it needs to be plasticized prior to usage to improve the flexibility and ductility of PLA. The commonly used plasticizers for PLA are citrate esters, oligomeric lactic acid, poly(ethylene glycol) (PEG), and poly(propylene glycol) [11]. Sheth et al. focused on the investigation of PLA/PEG ratios. They showed that when PEG amount was lower than 50%, the blend had higher elongation and lower modulus. However, higher PEG ratio increased the crystallinity and results in decreasing the elongation and increase of the modulus [12]. In another study, Sungsanit et al. found that increasing PEG (1000 g/mole) concentration enhanced the crystallinity and impact resistance of PLA. Moreover, the composites showed lower Tg, viscosity and stiffness than PLA [13]. In a study of Ozkoc et al., PLA/PEG films having 0, 3, and 5% organoclay (Cloisite 30B) were prepared using a lab-compounder connected to a film-
casting device. The glass transition temperature (Tg) of PLA decreased nearly 30°C with the addition of 20 wt% of PEG. Moreover, the plasticization effect of PEG was slightly deteriorated by the addition of clay [7]. In a recent study of our research group, it was aimed to investigate the mechanical, thermal and morphological properties of reactive and nonreactive polyhedral oligomeric silsesquioxanes (POSS) reinforced and PEG (8000 g/mole) plasticized PLA to obtain a balance of toughness to stiffness. We found that Tg of pure PLA decreased when compounded with PEG. Moreover, the POSS addition into PLA/PEG also decreased the Tg of PLA. This indicated that POSS molecules acted as a co-plasticizing agent in the polymer matrix [14]. Thermoplastic polyurethane (TPU) is considerable as one of the well-known polymer due to its good mechanical, biocompatibility and biodegradability properties. TPU consists two different segments, i.e. hard and soft segments. Hard-segments can be considered as the physical crosslinks and soft-segments give elasticity to TPU [15]. The soft segments of TPU, which contains polyester or polyether, are expected to be well compatible with PLA. Thus, TPUs are potent material to toughen the PLA [16]. In a study from the literature, Feng and Ye investigated the properties of PLA/TPU blends with various compositions. Their results showed that PLA exhibited flexible nature by the addition of TPU. They mentioned that the blends were partially miscible because of the hydrogen bonding between the PLA and TPU [17]. In our previous study, 1,4 phenylene diisocyanate (PDI) was used to improve the compatibility of PLA/TPU blends via reactive melt blending. The results indicated that the presence of PDI in the PLA/TPU blends improved the tensile properties of the blends and enhanced the desired compatibilization of PLA and TPU [16]. Zhao et al. reported the enhanced compatibility of TPU toughened PLA blends in the presence of 4,4-methylene diphenyl diisocyanate (MDI) via melt blending [18]. TPUs and TPU based polymer blends exhibit shape memory behavior. Ajili et al. [19] focused on the shape memory behaviors of the polyurethane/polycaprolactone (PU/PCL) to be used as stents. They stated that PU/PCL (70/30 wt%) blend showed excellent biocompatibility and shape recovery near the human body temperature. Song et al. [20] investigated the optimizing the shape recovery ratio of the biocompatible TPU/PLA blends by the changing composition and programming temperature. The programming temperatures were selected as 25°C, 37°C and 70°C. They found that the shape fixity ratio was increased by increasing of PLA content and programming temperature. Lai and Lan [21] investigated the effects of pre-deformation temperature (25°C, 80°C and 120°C) on the shape memory properties of PLA/TPU blends. They found that the increasing in TPU ratio in the blend caused the increasing of shape recovery ratio up to 93.4±0.4% at 160°C. In a recent study, Wei and coworkers stated that TPU phase in PLA/TPU blend exhibited stronger driving force. Moreover,
addition of CB and carbon nanotubes (CNTs) to the PLA/TPU showed better shape recovery properties [22]. In another recent study, Sun et al. pointed out that PLA/TPU SMP system including PEG presented enhanced shape-fixing capability after deformation at room temperature in the presence of single-walled carbon nanotubes [23]. In the current study, we have investigated the shape memory behavior of PLA/TPU blends by plasticizing the PLA with using poly(ethylene glycol) (PEG) to tune the shape recovery near the human body temperatures for the potential applications in biomedical areas. To the best of our knowledge, this study is the first attempt to plasticize the PLA in a PLA/TPU blend to tune the shape memory properties of PLA/TPU blends. In order to plasticize PLA, various molecular weights of PEG such as 1000 g/mol, 8000 g/mol and 35000 g/mol were used as the plasticizers. The PLA/TPU ratio was varied as 80/20, 60/40, 50/50, 40/60 and 20/80% by weight. The various shape memory experiments were conducted to understand the phenomena, such as the effect of deformation rate, programming temperature, counter weight during recovery and the thermocyclic test. In addition, the physical properties of the blends were also examined. In addition, the biocompatibility was evaluated by in-vitro cell culture tests. 2. Experimental 2.1. Materials PLA (trade name: PLI005) was purchased from NaturePlast Company. Its density, melting temperature and MFI are 1.25 g/cm3, 145-150°C and 10-30 g/10 min at 190°C/2.16 kg, respectively. The caprolactone diol based thermoplastic polyurethane was kindly obtained from Lubrizol Company. Its hardness is 77 Shore-A. Poly(ethylene glycol) (PEG, Mn, avg = 1000, 8000 and 35000 g/moles) (Sigma–Aldrich) was used as the plasticizer. 2.2. Processing Before the processing, PLA and TPU were dried in an oven under vacuum at 65°C for 24 h. A laboratory scale 15 mL twin-screw extruder (Xplore Instruments MC 15, The Netherlands) was used for compounding the polymers. Firstly, PLA was plasticized via PEGs. For PLA/PEG blends, the screw speed, barrel temperature and residence time were 100 rpm, 180°C and 1 min, respectively. The PLA/PEG blends were chopped as granules. Secondly, plasticized PLA and TPU blends were prepared via melt mixing. In this case, the screw speed, barrel temperature and residence time were 100 rpm, 190°C and 3 min, respectively. Argon was continuously supplied into the barrel to prevent the thermo-oxidative degradation. After compounding process, the melt
was transferred to the 12 mL laboratory injection-molding device (Xplore Instruments IM 12, The Netherlands). The melt and mold temperatures were 190 °C and 30 °C, respectively. The pressure during injection molding was set to 10 bars. 2.3. Characterization of thermal and mechanical properties and phase morphology Differential scanning calorimeter (DSC) analysis was performed using Mettler Toledo DSC1 Star System in the nitrogen atmosphere. The samples were cooled to -50°C with a cooling rate of 10°C/min and heated to 250°C with a heating rate of 10°C/min. The tensile properties were measured with an Instron (Model 3345) universal testing machine according to ISO 527. The crosshead speed was of 10 mm/min. Five samples were tested and then the average values were reported. Phase morphologies of plasticized PLA/TPU blends were investigated with scanning electron microscopy (SEM, JEOL JSM-6060). The samples were cryogenically fractured in liquid nitrogen and sputter coated with gold prior to analysis. 2.4. Investigation of shape memory properties The programming process of the samples was carried out as follows: I.
Ttrans was determined as Ttrans=45°C from the glass transition temperatures of plasticized PLA types. The oven was heated to 45°C and then the samples were placed into the oven. To provide homogenous heat distribution on the sample, they were kept in the oven for 2 min.
II. III.
The sample was deformed (i.e. elongated) to 100% at Ttrans=45°C using a 5.7 MPa load. The sample was cooled to the room temperature under load for 10 min for homogenous heat distribution.
IV.
Then the load was removed. The temporary shape was obtained after the mentioned steps above.
The testing of shape memory properties carried out as follows: I.
The sample length was measured and the sample was placed into the water tank equipped with a controller. It was heated from 40°C to 60°C with 5°C intervals.
II.
The change in sample lengths were measured at 40°C, 45°C, 50°C, 55°C and 60°C. Below 40°C, the shape recovery properties was not observed.
The shape recovery ratio (Rr) was calculated as follows (Eq.1): Ɛ𝑢(𝑁) ― Ɛ𝑝(𝑁)
(1)
𝑅𝑟 = Ɛ𝑢(𝑁) ― Ɛ𝑝(𝑁 ― 1)
where Ɛ𝑢(𝑁) is strain after deformed and removal load, Ɛ𝑝(𝑁) is recovered strain and Ɛ𝑝(𝑁 ― 1) is permanent strain [3]. Representative pictures related to the shape memory experiments were illustrated in Fig. 1.
(I)
(II)
Fig. 1. Representative pictures illustrating (I) the programming of the samples and (II) testing of the samples 2.5 Evaluation of Biocompatibility and Cytotoxicity In-vitro biocompatibility was performed using mouse fibroblast cells (L929). The number of viable cells present in a cell suspension was determined according to trypan blue exclusion assay. L929 fibroblast cells were routinely cultured in high glucose containing Dulbecco’s Modified Eagle Media (DMEM, Sigma Aldrich) supplemented with 10% fetal bovine serum (FBS, Sigma Aldrich) and 50 mg/ml of streptomycin (Sigma Aldrich) in 50 ml-plastic culture flasks. They were subcultured in every 4 days. The cells at logarithmic growth phase were treated with 0.2% trypsine0.02% EDTA (Sigma Aldrich), washed with DMEM containing 10% FBS, collected by centrifugation at 1000 rpm for 3 min and re-suspended in DMEM containing 10% FBS at 2.5x105 cells/ml. The cells were seeded onto each dish at 5x104cells/dish and cultivated at 37°C in a humidified 5% CO2 incubator. After incubation for a defined period, the cells on the scaffolds were
detached by the 0.2% trypsin treatment and counted by hemocytometer for cell viability measurement by trypan blue exclusion assay. The trypan blue exclusion assay is based on the principle that the live cells possess intact cell membranes that exclude certain dyes, such as trypan blue, whereas dead cells do not. In this test, a cell suspension obtained from the medium that was previously in contact with scaffold was mixed with the dye and then they were visually examined for up-taking or excluding the dye. 3. Results and Discussion 3.1. Thermal and Mechanical Properties Thermal properties of PLA, plasticized PLA and plasticized PLA/TPU blends having 50% TPU were summarized in Table 1. The results showed that neat PLA has glass transition temperature (Tg) and melting temperature (Tm) of 58.1 and 152.4°C, respectively. The Tg of PLA is rather high as compared to the human body temperature; therefore, PEG at 10 wt% was used as a plasticizer to decrease the Tg. It was observed that the addition of PEG to the PLA decreased the Tg of PLA about 15-20C depending on the molecular weight of PEG. The reduction of the Tg of PLA is due to the decreasing the intermolecular interactions of PLA chains in the presence of PEG molecules. The molecular weight of PEG has an influence on the plasticization of PLA. As the molecular weight increased, the reduction in Tg of PLA decreased. This is due to the fact that as the molecular weight of the PEG increased, the miscibility became lower with PLA [24]. As the 50PLA/50TPU blends are considered, the Tg of PLA phase decreased about 4C with the addition of TPU. This is possibly due to the shift in the Tg of the PLA towards the Tg of the polyester soft segment of TPU, which is below room temperature. This shift is also a proof of the compatibility between PLA and TPU [16]. Addition of 10%PEG to the PLA in the PLA/TPU blends reduced the Tg of the PLA phase in the blends depending the molecular weight of the PEG. The reduction in Tg of PLA phase in the blends was lower in comparison to plasticized PLA. This can be attributed to the dilution of PEG during secondary processing with TPU since the soft segment of TPU and PEG can have a possible compatibility, and hence some of the PEG can migrate through TPU phase. The cold crystallization exotherm was not observed for neat PLA, however for all plasticized PLA and plasticized PLA/TPU blends, the cold crystallization temperature (Tcc) was observed. This was due to the fact that the addition of PEG to the PLA decreased the melt viscosity, which promoted PLA chains to crystallize. The Tcc value was similar in the case of PEG1000 and PEG8000, but slightly higher in the case of PEG35000 since its plasticization efficiency, the ability to decrease the viscosity, was lower. Similar to plasticized blends, the PLA/TPU blends also showed the cold
crystallization behavior during heating due to the fact that the TPU phase acted as a nucleating agent for PLA. Nucleating role of TPU for PLA was also reported in the literature [25, 26]. Unplasticized PLA/TPU blend exhibited a cold crystallization temperature of 103.3°C belonging to PLA phase. The incorporation of PEG to the PLA in the PLA/TPU blends reduced the cold crystallization temperature of the PLA due to the decreasing melt viscosity that allows PLA chains crystallize earlier. A gradual increase was observed in the cold crystallization temperature parallel to the increasing molecular weight of the PEG. This is due to the weakening plasticization influence of the PEG on PLA with molecular weight [7]. The melting point of the PLA phase did not affect by either the plasticization or blending with TPU. In addition, the melting point of TPU was not observed under the experimental conditions of DSC analysis. Table 1. Thermal properties of neat PLA, plasticized PLA and plasticized-PLA/TPU blends Materials Neat PLA 10%PEG1000-PLA 10%PEG8000-PLA 10%PEG35000-PLA 50PLA/50TPU 10%PEG1000-50PLA/50TPU 10%PEG8000-50PLA/50TPU 10%PEG35000-50PLA/50TPU
Tg of PLA phase (°C) 58.1 37.4 41.1 42.4 54.9 46.0 48.7 51.7
Tcc of PLA phase (°C) 91.1 91.5 95.1 103.3 95.8 97.8 101.4
Tm of PLA phase (°C) 152.4 152.1 153.6 153.2 152.9 152.6 151.9 152.3
In order to understand the effects of blending with TPU and addition of plasticizers on the mechanical properties of PLA, tensile tests were performed for 50/50 PLA/TPU blends. The elastic modulus, elongation at break and yield strength data were tabulated in Table 2. As can be seen from Table 2 that the addition of TPU to PLA resulted in a drop in elastic modulus and yield strength but increase in elongation at break. This improved ductility is due to the inherent ductility of the TPU with respect to PLA. TPU is a block copolymer consisting of alternating sequences of hard and soft segments. The soft block, consists of a polyol and an isocyanate, gives the flexibility and elastomeric property of a TPU. Therefore, when TPU was added into PLA, the flexibility of the blend increased and the brittle characteristics of PLA decreased. Feng and Ye [17] observed the transition of PLA from brittle fracture to ductile fracture with the addition of TPU into PLA and suggested a partially miscible system due to the hydrogen bonding between the TPU and PLA phases. The addition of plasticizers having different molecular weights to the 50/50 PLA/TPU blend led to further decrement in stiffness and yield strength, but increment in elongation of the blends due to the decreasing intermolecular interaction between two phases. It was obtained that
as the molecular weight of PEG increased the flexibility of the blend decreased. This is due to the decreasing compatibility of PEG with PLA as the molecular weight of PEG increases [24]. Table 2. The mechanical properties of neat PLA, PLA/TPU and plasticized PLA/TPU blends
3713±432
Elongation at break (%) 4.58±0.0
Yield Strength (MPa) 56.8±2.5
(0%PEG) 50PLA/50TPU
1463±775
261.5±0.7
34.3±1.2
(10%PEG1000) 50PLA/50TPU
1048±120
656.7±1.2
22.9±1.2
(10%PEG8000) 50PLA/50TPU
1088±187
362.5±1.6
25.3±1.9
(10%PEG35000) 50PLA/50TPU
1348±12
261.7±0.8
29.0±3.0
Materials
Elastic Modulus (MPa)
Neat PLA
3.2. Investigation of shape memory behavior Fig. 2. shows the shape memory behavior of 80/20, 60/40, 50/50, 40/60 and 20/80 unplasticized PLA/TPU blends. One should note that the blends were programmed at 45°C by deforming 100%. Independently from the composition, all the blends exhibited higher recovery ratios as the test temperature increased. This is due to the fact that the switching phase in the blends is the PLA and the switching occurred due to the relaxation of the PLA chains. Therefore, as the temperature increases, the relaxation of the PLA chains will be more favorable. There are two distinct zones visible in the plots: i.) the TPU dominant zone and ii.) PLA dominant zone. The borderline between two zones is the glass transition temperature of PLA, which is around 55-60C. When the TPU content increased in the PLA/TPU blends, the recovery ratio improved in the first (lower temperature) zone. The improvement in shape recovery ratios of the blends, having higher TPU content (i.e. 80% and 60%) below the glass transition temperature of PLA where the PLA chains are frozen, is due to the presence of the soft segments of TPU. The soft polyester segments are still mobile below the Tg of PLA [7] and acted as the switching phase. At lower contents of TPU below Tg of PLA, the shape recovery was lower due the diluted contribution to the restoring force. In addition, for the blends of 20PLA/80TPU and 40PLA/60TPU, TPU might form the continuous phase and therefore it dominated the behavior of the blends. As the test temperature of shape recovery exceeded the Tg of PLA, i.e. 55C and 60C, the blends having higher PLA contents exhibited higher recovery ratios. The maximum recovery ratio of 97% was obtained for the blend containing the highest content of PLA (80PLA/20TPU) at the highest test temperature. Similar observations was obtained by Song et al. They stated that the higher the content of PLA, the higher the total recovery ratios [20]. The decreasing amount of switching phase (PLA) resulted
in a decrease of recovery ratio. The similar findings were also found by Dhollander et al. and Wang et al. [27, 28].
TPU dominant zone
PLA dominant zone
Fig. 2. Shape memory behavior of 80/20, 60/40, 50/50, 40/60 and 20/80 unplasticized PLA/TPU blends between 40-60°C (Blends were programmed at 45°C and deformed 100%) Shape memory behavior of plasticized and unplasticized PLA/TPU blends were given in Fig. 3 with respect to the blend composition and plasticizer molecular weight. It was clearly observed that for all PLA/TPU ratios, unplasticized blends showed the highest total recovery, but they demonstrated the lowest recovery ratio for lower temperature because of high Tg of PLA. The plasticization improved the low temperature recovery of the blends, especially for the ones having higher PLA content. For the blends of 80/20, 60/40 and 50/50, the recovery ratio below 50C increased with the addition of plasticizer. Moreover, the recovery ratios were improved by the decreasing molecular weight of the plasticizer. This is due to the plasticization ability of the lower molecular weight plasticizers. For the blends of 40/60 and 20/80, the significance of the plasticizer molecular weight was lost probably due to the phase inversion of the blends where the continuous phase became TPU rather than PLA. As the PLA/TPU ratio is taken into consideration, it was observed that the increasing amount of TPU in the blend improved the recovery ratio of the plasticized blends especially at lower temperatures due to the contribution of the hard segments of the TPU that are able to increase stored energy to improve recovery ratios. The recovery range of the blends, based on the temperature, was enhanced by the addition of TPU. Song et al. obtained similar results in their recent work on the PLA/TPU blends. They found that the recovery range
was broadened by the addition of TPU in to the PLA matrix [20]. At higher temperatures, i.e. above 50C, plasticized blends exhibited lower recovery ratios and the significance of PEG type was lost. The difference in recovery ratio for unplasticized and the plasticized blend was found maximum in 80/20 blends. This difference became smaller as the TPU content increased in the blend.
(a)
(b)
(c)
(d)
(e)
Fig. 3. Shape memory behavior of unplasticized and 1000, 8000, 35000 g/mole PEG plasticized PLA/TPU (a) 80/20, (b) 60/40, (c) 50/50, (d) 40/60 and (e) 20/80 (One should note that the blends were programmed at 45°C and deformed 100%) The influence of the deformation rate in the programming step has been investigated on the PEG1000 plasticized 50/50 PLA/TPU blends (Fig. 4). The samples were first programmed at 45C than recoveries were measured at different temperature ranging from 40C to 60C. Five different deformation rates i.e. 30%, 75%, 100%, 150% and 200%, were investigated. As can be seen from the Fig. 4 that the effect of deformation rate did not follow a trend. The glass transition temperature of PLA phase in PEG 1000 plasticized 50/50 bend is 46C, which is higher than the test temperature of 40C; therefore the contribution of PLA chains are very limited at this temperature. The shape recovery ratios increased above 45°C with decreasing deformation rates. The samples that deformed 30% during the programming showed the highest recovery ratios, while the ones deformed 200% showed the lowest. It was observed that the blends were able to recover fully to their original shape when they were programmed at lower deformation rates. It can be explained that the amorphous segments of PLA/TPU blends are able to compensate a certain amount of elastic deformation, but after a threshold, due to the extensive plastic deformation in the amorphous phases, the recovery ratios lowered.
Fig. 4. The influence of the deformation rate in the programming step on the PEG1000 plasticized 50/50 PLA/TPU blends
It is seen from Fig. 5 that the PEG1000 containing blend of 20/80 PLA/TPU exhibited higher shape recovery ratios than PEG1000 containing 50/50 PLA/TPU, regardless of deformation rate i.e. 75% and 150%. As discussed before, the shape recovery ratios increased with increasing TPU content. This is due to the fact that the hard segments of TPU acted as physical crosslinks which were the energy storage points during deformation [21].
(a)
(b)
Fig. 5. Shape recovery of PEG1000 plasticized 50/50 and 20/80 PLA/TPU blends with between 40-60°C with 5°C intervals (The blends programmed at 45°C and deformed (a) 75% and (b) 150%). 3.2.4. The effects of load on recovery ratio In order to judge the shape recovery behavior of the blends under load, the 50/50 and 20/80 PLA/TPU blends containing PEG1000 were tested under loads of 0, 10, 50, 120, 150 and 200 g (corresponding stresses to 0, 12, 60, 150, 185 and 245 kPa, respectively). The test samples were programmed at 45°C and deformed 100%. The weights were attached to the bottom of the samples during testing. As seen from Fig. 6a, as the stress increased, the recovery ratio decreased as expected. The samples without weight showed more than 70% total recovery, while the samples under load showed more than 50% total recovery. This means that the blends are able to carry stress up to 245 kPa with only small amount of recovery loss. Fig. 6b-d shows the recovery ratio of PEG1000 plasticized 50/50 PLA/TPU and 20/80 PLA/TPU blends under 10 g, 120 g and 200 g load with respect to temperature. It was clearly seen that the higher the content of TPU, the higher the stress carrying capacity is, due to the hard segments of TPU. It was reported similar findings in the literature that PEG has a positive effect on a 20% PLA and 80% TPU system: more than 80% Rr can be achieved
under 5% and 10% PEG content [23, 29]. Moreover, Liu et al. stated that lower amount of TPU also caused a decrease in the shape recovery ability, which is consistent with our results [30].
(a)
(c)
(b)
(d)
Fig. 6. (a) PEG1000 plasticized 50/50 PLA/TPU with various weights (0, 10, 50, 120, 150 and 200 g) and, PEG1000 plasticized 50/50 PLA/TPU and 20/80 PLA/TPU blends with (b) 10 g, (c) 120 g and (d) 200 g weights. The blends programmed at 45°C and deformed 100%. 3.2.5. The effects of thermocyclic tests on recovery ratio In order to investigate the cyclic shape memory behavior of plasticized 20/80 PLA/TPU blends, the test was repeated three times. It was possible to observe that the recovery ratio was higher in the second cycle than the first cycle (Fig. 7). D’hollander et al. reported that the recovery ratio improved with increasing repeated cycles [27]. However, when the third cycle was repeated, the recovery ratios were reduced at all temperatures. This may be associate to the change of the microstructural properties of the polymer blends due to the plasticizer migration/diffusion or change in the phase morphology.
Fig. 7. Thermocyclic test results of PEG1000 plasticized 20/80 PLA/TPU blends programmed at 45°C and deformed 25%. 3.2.6. The effects of PEG ratio and programming temperature on recovery ratio In order to investigate the ratio of plasticizer on the shape recovery behavior of the blends, 0, 10 and 20% PEG containing 50/50 PLA/TPU blends were investigated (Fig. 8). The blends were programmed at 45°C and deformed 100%. The high content of PEG resulted in a sharp reduction in the Tg of PLA, the switching polymer. While non-plasticized blends showed nearly 5% recovery at 45°C, the 20% PEG containing blends showed more than 45%. This increment proved the effects of plasticizers on the shape memory behavior of PLA/TPU system in order to be actuated at lower temperatures.
Fig. 8. The recovery ratios of 50/50 PLA/TPU blends containing 0, 10, 20% PEG The recovery ratios of 50/50 PLA/TPU blends containing 20% PEG1000 was represented in Figure 9 in order to compare the effects of programming temperature of 45°C and 60°C. It was found that the higher the programming temperature, the higher the recovery ratio was. For example, the shape recovery ratios improved from 45% to over 65-70% at 45°C.
Fig. 9. The recovery ratios of 20%PEG1000 plasticized 50/50 PLA/TPU blends programmed at 45°C and 60°C
3.3. The phase morphology of PLA/TPU
Fig. 10. SEM pictures of (a1 and a2) 10%PEG1000 plasticized PLA (b1 and b2) unplasticized 50/50 PLA/TPU (c1 and c2) 10%PEG1000 plasticized 50/50 PLA/TPU (d1 and d2) 10%PEG1000 plasticized 80/20 PLA/TPU with different magnifications (5000x and 10000x)
The SEM pictures of PEG1000 plasticized PLA, plasticized and unplasticized 50/50 PLA/TPU blends and plasticized 80/20 PLA/TPU blends are represented in Fig. 10. It was seen for PEG1000 plasticized PLA that there was no phase separation indicating a fully miscible polymer mixture (Fig. 10a). In 50/50 blends, the morphology of the blends was seemed to be a two-phase morphology with a possible continuous phase of TPU and dispersed PLA phase, since the melt viscosity of TPU was expected to be higher than that of plasticized and unplasticized PLA (Fig. 10b and Fig. 10c). Therefore, the small dentations seen on the surface can be associated to the PLA dispersed phase. However, when the PLA content increased to 80%, a phase inversion occurred and resulted in a PLA continuous phase with dispersed TPU domains, as can be seen small droplets in PLA matrix in Fig.10d. 3.4. The cytotoxicity and biocompatibility of plasticized PLA/TPU blends L929 mouse fibroblast cell line was used in in-vitro culture studies to estimate the biocompatibility and cytotoxicity of PLA/TPU blends. The cell growth and proliferation were checked using an inverted light microscope (Olympus CKX 41) while the cultivation was going on cell-culture plate. The cell-line morphologies at 3rd, 5th and 7th days are shown in Fig. 11. It can be seen in the first seeding stage that the cells were globular and round whereas they looked like elongated or spinal during proliferation. In the 3rd day, despite the growth of the cells, there was not enough proliferation. It was observed that the cell proliferation and layout were decided to be enough at the end of 5th day. On the other hand, in the 7th day, the cell growth and the proliferation could be inhibited due to the contact inhibition. Therefore, the comparison was done on the 5th day for the PLA/TPU samples in cell-culture studies.
Fig. 11. The cell line morphology of L929 a) 3rd day (10X), b) 5th day (10X), c) 7th day (10X), d) 3rd day (40X) The cytotoxicity test was employed to judge if there was any toxicity of the components of polymer blends, such as matrices or plasticizer (PEG). A glass-slide as the control and, some selected PLA/TPU blends were investigated. The film samples of blends were located onto the hemocytometer without any leakage. The medium was then dyed with trypan blue. The percentage of dead cells was calculated from trypan blue exclusion assay test and viable cell ratio was calculated accordingly. The percentage of viable cells was shown in Fig. 12. According to ISO 10993-5, percentages of cell viability above 80% are considered as non-cytotoxic; within 80%– 60% weak; 60%–40% moderate and below 40% strong cytotoxicity, respectively [31]. It is seen that the cell viability on lamella and on all the blends were greater than 80%. This means that no evidence of cytotoxicity was observed from the blends of PLA/TPU.
Fig. 12. Cell viability of L293 mouse fibroblast cell line on the selected blend compositions 4. Conclusion In this study, we have focused on the understanding shape memory behaviors of PEG plasticized PLA/TPU blends as a function of PEG molecular weight, PLA/TPU ratio, programming temperature and applied stress. It was found that higher TPU composition caused higher recovery ratios for the non-plasticized PLA/TPU between 40-55°C range. In addition, higher content of PLA increased the shape fixity ratios. All plasticized PLA/TPU blends exhibited better shape recovery at lower temperatures with respect to the unplasticized opponents. The lowest molecular weight PEG provided the highest recovery at lower temperatures. It was seen that the deformation ratio was rather important to gain higher recovery ratios. It was obtained that the lowest deformation rates resulted in higher shape recovery ratios for all recovery temperature. Moreover, it was found that the samples were capable to lift weights up to 200 g by losing only a small amount of recovery. As expected, 20/80 PLA/TPU blends showed higher recovery for the all weights due to the strong elasticity of TPU. The biocompatibility and cytotoxicity experiments revealed that both plasticized PLA and TPU were non-toxic and safe to use in-vivo conditions. As a conclusion, we suggest that the plasticized PLA/TPU blends with lower molecular weight PEG (i.e. 1000 g/mole) with an optimum PLA/TPU composition can be a promising candidate for in-vivo application near human body temperature.
References [1] D. Ratna, J.K. Kocsis, Recent advances in shape memory polymers and composites: a review, J. Mater. Sci. 43 (2008) 254–269. [2] B. Yan, S. Gu, Y. Zhang, Polylactide-based thermoplastic shape memory polymer nanocomposites, Eur. Polym. J. 49 (2013) 366-378. [3] S. Xu, J. Song, Thermal Responsive Shape Memory Polymers for Biomedical Applications, 1st ed., InTech, China, 2011. [4] N. Choi, A. Lendlein, Degradable shape-memory polymer networks from oligo[(L-lactide)-ranglycolide]dimethacrylates, T. Roy. Soc. Chem. 3 (2007) 901-909. [5] E. Zini, M. Scandola, P. Dobrzynski, J. Kasperczyk, M. Bero, Shape memory behavior of novel (L-Lactide-glycolide-trimethylene carbonate) terpolymers, Biomacromolecules 8 (2007) 36613667. [6] K.M. Lee, P.T. Knight, T. Chung, P.T. Mather, Polycaprolactone-POSS chemical/physical double networks, Macromolecules 41 (2008) 4730-4738. [7] G. Ozkoc, S. Kemaloglu, Morphology, biodegradability, mechanical, and thermal properties of nanocomposite films based on PLA and plasticized PLA, J. Appl. Polym. Sci. 114 (2009) 2481– 2487. [8] A. Demirbas, Biodegradable plastics from renewable resources, Energ. Source. Part A 29 (2007) 419-424. [9] I. Pillin, N. Montrelay, Y. Grohens, Thermo-mechanical characterization of plasticized PLA: Is the miscibility the only significant factor?, Polymer 47 (2006) 4676–4682. [10] M. Keramati, I. Ghasemi, M. Karrabi, H. Azizi, M. Sabzi, Incorporation of surface modified graphene nanoplatelets for development of shape memory PLA nanocomposite, Fiber Polym. 17 (2016) 1062-1068. [11] S. Gumus, G. Ozkoc, A. Aytac, Plasticized and unplasticized PLA/organoclay nanocomposites: short- and long-term thermal properties, morphology, and nonisothermal crystallization behavior, J. Appl. Polym. Sci. 123 (2012) 2837–2848. [12] M. Sheth, R.A. Kumar, V. Dave, R.A. Gross, S.P. McCarthy, Biodegradable polymer blends of poly(lactic acid) and poly(ethylene glycol), J. Appl. Polym. Sci. 66 (1997) 1495–1505.
[13] K. Sungsanit, N. Kao, and S.N. Bhattacharya, Properties of linear poly(lactic acid)/polyethylene glycol blends, Polym. Eng. Sci. 52 (2012) 108-116. [14] M. Kodal, H. Sirin, G. Ozkoc, Effects of reactive and nonreactive POSS types on the mechanical, thermal, and morphological properties of plasticized poly(lactic acid), Polym. Eng. Sci. 54 (2014) 264–275. [15] H. Hong, J. Wei, Y. Yuan, F.P. Chen, J. Wang, X. Qu, C.S. Liu, A novel composite coupled hardness with flexibleness—polylactic acid toughen with thermoplastic polyurethane, J. Appl. Polym. Sci. 121 (2011) 855–861. [16] S. K. Dogan, E. A. Reyes, S. Rastogi, G. Ozkoc, Reactive compatibilization of PLA/TPU blends with a diisocyanate, J. Appl. Polym. Sci. 131 (2014) 1–10. [17] F. Feng, L. Ye, Morphologies and mechanical properties of polylactide/thermoplastic polyurethane elastomer blends, J. Appl. Polym. Sci. 119 (2011) 2778–2783 [18] F. Zhao, H.X. Huang, S.D. Zhang, Largely toughening biodegradable poly(lactic acid)/thermoplastic polyurethane blends by adding MDI, J. Appl. Polym. Sci. 132 (2015) 42511. [19] S.H. Ajili, N.G. Ebrahimi, M. Soleimani, Polyurethane/polycaprolactane blend with shape memory effect as a proposed material for cardiovascular implants, Acta Biomaterialia 5 (2009) 1519–1530. [20] J.J. Song, H.H. Chang, H.E. Naguib, Biocompatible shape memory polymers actuators with high force capabilities, Eur. Polym. J. 67 (2015) 186-198. [21] S.M. Lai, Y.C. Lan, Shape memory properties of melt-blended polylactic acid (PLA)/thermoplastic polyurethane (TPU) bio-based blends, J. Polym. Res. 20 (2013) 1-8. [22] Y. Wei, R. Huang, P. Dong, X.D. Qi, Q. Fu, Preparation of polylactide/poly(ether)urethane blends with excellent electro-actuated shape memory via incorporating carbon black and carbon nanotubes hybrids fillers, Chinese J. Polym. Sci. 36 (2018) 1175-1186. [23] Y.C. Sun, M. Chu, M. Huang, O. Hegazi, H.E. Naguib, Hybrid electroactive shape memory polymer composites with room temperature deformability, Macromol. Mater. Eng. 304 (2019) 1900196.
[24] M. Kodal, H. Sirin, G. Ozkoc, Long- and short-term stability of plasticized poly(lactic acid): effects of plasticizers type on thermal, mechanical and morphological properties, Polym. Bull. 76 (2019) 423-445. [25] S.M. Lai, W.L. Wu, Y.J. Wang, Annealing effect on the shape memory properties of polylactic acid (PLA)/thermoplastic polyurethane (TPU) bio-based blends, J. Polym. Res. 23 (2016) 99. [26] H.Y. Mi, M.R. Salick, X. Jing, B.R. Jacques, W.C. Crone, X.F. Peng, L.S. Turng, Characterization of thermoplastic polyurethane/polylactic acid (TPU/PLA) tissue engineering scaffolds fabricated by microcellular injection molding, Mater. Sci. Eng. C Mater. Biol. Appl. 33 (2013) 4767-4776. [27] S. D’Hollander, G.V. Assche, B.V. Mele, F.D. Prez, Novel synthetic strategy toward shape memory polyurethanes with a well-defined switching temperature, Polymer 50 (2009) 4447–4454. [28] W. Wang, P. Ping, X. Chen, X. Jing, Shape memory effect of poly(L-lactide)- based polyurethanes with different hard segments, Polym. Int. 56 (2007) 840–846. [29] S. Cai, Y.C. Sun, J. Ren, H.E. Naguib, Toward the low actuation temperature of flexible shape memory polymer composites with room temperature deformability via induced plasticizing effect, J. Mater. Chem. B 5 (2017) 8845-8853. [30] T. Liu, R. Huang, X. Qi, P. Dong, Q. Fu, Facile preparation of rapidly electro-active shape memory thermoplastic polyurethane/polylactide blends via phase morphology control and incorporation of conductive fillers, Polymer 114 (2017) 28-35. [31] International Organization for Standardization. ISO 10993 5: 2009. Biological evaluation of medical devices, Part 5: Tests for in vitro cytotoxicity. International Organization for Standardization: Geneva, Switzerland, 2009.
LIST OF TABLE CAPTIONS
Table 1. Thermal properties of neat PLA, plasticized PLA and plasticized-PLA/TPU blends Table 2. The mechanical properties of neat PLA, PLA/TPU and plasticized PLA/TPU blends
LIST OF FIGURE CAPTIONS
Fig. 1. Representative pictures illustrating (I) the programming of the samples and (II) testing of the samples Fig. 2. Shape memory behavior of 80/20, 60/40, 50/50, 40/60 and 20/80 unplasticized PLA/TPU blends between 40-60°C (Blends were programmed at 45°C and deformed 100%) Fig. 3. Shape memory behavior of unplasticized and 1000, 8000, 35000 g/mole PEG plasticized PLA/TPU (a) 80/20, (b) 60/40, (c) 50/50, (d) 40/60 and (e) 20/80 (One should note that the blends were programmed at 45°C and deformed 100%) Fig. 4. The influence of the deformation rate in the programming step on the PEG1000 plasticized 50/50 PLA/TPU blends Fig. 5. Shape recovery of PEG1000 plasticized 50/50 and 20/80 PLA/TPU blends with between 40-60°C with 5°C intervals (The blends programmed at 45°C and deformed (a) 75% and (b) 150%) Fig. 6. (a) PEG1000 plasticized 50/50 PLA/TPU with various weights (0, 10, 50, 120, 150 and 200 g) and, PEG1000 plasticized 50/50 PLA/TPU and 20/80 PLA/TPU blends with (b) 10 g, (c) 120 g and (d) 200 g weights. The blends programmed at 45°C and deformed 100% Fig. 7. Thermocyclic test results of PEG1000 plasticized 20/80 PLA/TPU blends programmed at 45°C and deformed 25% Fig. 8. The recovery ratios of 50/50 PLA/TPU blends containing 0, 10, 20% PEG Fig. 9. The recovery ratios of 20%PEG1000 plasticized 50/50 PLA/TPU blends programmed at 45°C and 60°C Fig. 10. SEM pictures of (a1 and a2) 10%PEG1000 plasticized PLA (b1 and b2) unplasticized 50/50 PLA/TPU (c1 and c2) 10%PEG1000 plasticized 50/50 PLA/TPU (d1 and d2) 10%PEG1000 plasticized 80/20 PLA/TPU with different magnifications (5000x and 10000x) Fig. 11. The cell line morphology of L929 a) 3rd day (10X), b) 5th day (10X), c) 7th day (10X), d) 3rd day (40X) Fig. 12. Cell viability of L293 mouse fibroblast cell line on the selected blend compositions
Dear Editor;
We have no conflict of interest with any person or organization.
Best regards; Dr. Guralp Ozkoc
Graphical abstract
Highlights
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The shape memory behavior of PEG-plasticized PLA/TPU blends was investigated as a function of blend ratio, plasticizer molecular weight and programming conditions.
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The shape memory tests showed that the higher the TPU content, the higher the recovery ratio was.
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The decreasing molecular weight of PEG maximized the shape recovery values.
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The samples are able to carry stresses up to 245 kPa during shape recovery depending on the temperature and composition.
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The biocompatibility and cytotoxicity tests revealed that the blends are non-toxic and safe to use in-vivo conditions.
S0014-3057(19)31817-8 https://doi.org/10.1016/j.eurpolymj.2019.109372 EPJ 109372
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
9 September 2019 8 November 2019 14 November 2019
Please cite this article as: Boyacıoğlu, S., Kodal, M., Ozkoc, G., A Comprehensive Study on Shape Memory Behavior of PEG Plasticized PLA/TPU Bio-Blends, European Polymer Journal (2019), doi: https://doi.org/ 10.1016/j.eurpolymj.2019.109372
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A Comprehensive Study on Shape Memory Behavior of PEG Plasticized PLA/TPU Bio-Blends
S. Boyacıoğlu, M. Kodal and G. Ozkoc*
Department of Chemical Engineering, Kocaeli University, Kocaeli 41380, Turkey
*Corresponding
Author:
Guralp Ozkoc, Ph.D. Tel: +90 262 303 3540 Fax: +90 262 303 3550 E-mail: [email protected]
Abstract In this study, the shape memory behavior of poly(ethylene glycol) (PEG) plasticized poly(lactic acid)/thermoplastic polyurethane (PLA/TPU) blends was investigated as functions of PLA/TPU ratio, plasticizer molecular weight and programming conditions. The mechanical, thermal and morphological properties of polymer blends were characterized by tensile tests, differential scanning calorimeter (DSC) and scanning electron microscopy (SEM). It was found that the addition of plasticizers decreased the glass transition and cold crystallization temperature of PLA phase. As the molecular weight of PEG increased, the plasticization efficiency decreased. The shape memory tests showed that the higher the TPU content, the higher the recovery ratio between 40°C-55°C temperatures ranges. The maximum total recovery was obtained at 60°C for 20/80 PLA/TPU blends. The decreasing molecular weight of PEG maximized the shape recovery values. It was demonstrated that the samples were able to lift stresses up to 245 kPa during shape recovery depending on the temperature and composition. Moreover, the biocompatibility and cytotoxicity tests revealed that plasticized PLA/TPU blends are non-toxic and safe to use in-vivo conditions.
Keywords: Shape memory polymers; poly (lactic acid); thermoplastic polyurethane; poly(ethylene glycol); plasticizers; biomedical materials
1. Introduction Recently, shape memory materials such as alloys, ceramics and polymers have attracted attention because of their various applications. Lightweight, high performance to price ratio, biocompatibility and processability make shape memory polymers (SMPs) highly useful for many applications such as smart coatings, textiles, packaging, biomedical etc. [1, 2]. Per definition, SMPs change their shape with an external force such as light, chemicals, heat and magnetic field etc, however; one of the commonly studied type is the thermally responsive polymer systems [3]. Thermally induced SMPs show a characteristic transition temperature (Ttrans), which is responsible from shape changing. When SMPs are heated above the Ttrans, they are able to deform to a temporary-shape under an applied force. Then a cooling step below the Ttrans is applied to fix the temporary shape. In order to have a recovery from the final state, the applied stress is needed to be removed and the polymer must be heated above its Ttrans. Tg and/or melting temperature (Tm) of the polymer can be taken as the Ttrans [4-6]. To overcome the solid waste pollution resulted from the petroleum-based plastics, ecofriendly, compostable and biodegradable polymers are under focus. Poly(lactic acid) (PLA) is the most widely studied biodegradable polymer due to its degradability, relatively better physical properties and cost efficiency [7]. As a comparison with other well-known biodegradable polymers such as polycaprolactone (PCL), PLA has some advantageous properties, such as high strength, high modulus, transparency, processability and commercially availability [8, 9]. Moreover, PLA exhibits shape memory behavior. The PLA’s shape memory behavior depends on the presence of crystalline and amorphous phases. The crystalline phase behaves as the rigid phase, which is responsible for the energy storage during the temporary shape. Furthermore, the amorphous phase is acting as switching phase [10]. On the other hand, its brittleness is a limiting disadvantage for PLA [9]. Thus, it needs to be plasticized prior to usage to improve the flexibility and ductility of PLA. The commonly used plasticizers for PLA are citrate esters, oligomeric lactic acid, poly(ethylene glycol) (PEG), and poly(propylene glycol) [11]. Sheth et al. focused on the investigation of PLA/PEG ratios. They showed that when PEG amount was lower than 50%, the blend had higher elongation and lower modulus. However, higher PEG ratio increased the crystallinity and results in decreasing the elongation and increase of the modulus [12]. In another study, Sungsanit et al. found that increasing PEG (1000 g/mole) concentration enhanced the crystallinity and impact resistance of PLA. Moreover, the composites showed lower Tg, viscosity and stiffness than PLA [13]. In a study of Ozkoc et al., PLA/PEG films having 0, 3, and 5% organoclay (Cloisite 30B) were prepared using a lab-compounder connected to a film-
casting device. The glass transition temperature (Tg) of PLA decreased nearly 30°C with the addition of 20 wt% of PEG. Moreover, the plasticization effect of PEG was slightly deteriorated by the addition of clay [7]. In a recent study of our research group, it was aimed to investigate the mechanical, thermal and morphological properties of reactive and nonreactive polyhedral oligomeric silsesquioxanes (POSS) reinforced and PEG (8000 g/mole) plasticized PLA to obtain a balance of toughness to stiffness. We found that Tg of pure PLA decreased when compounded with PEG. Moreover, the POSS addition into PLA/PEG also decreased the Tg of PLA. This indicated that POSS molecules acted as a co-plasticizing agent in the polymer matrix [14]. Thermoplastic polyurethane (TPU) is considerable as one of the well-known polymer due to its good mechanical, biocompatibility and biodegradability properties. TPU consists two different segments, i.e. hard and soft segments. Hard-segments can be considered as the physical crosslinks and soft-segments give elasticity to TPU [15]. The soft segments of TPU, which contains polyester or polyether, are expected to be well compatible with PLA. Thus, TPUs are potent material to toughen the PLA [16]. In a study from the literature, Feng and Ye investigated the properties of PLA/TPU blends with various compositions. Their results showed that PLA exhibited flexible nature by the addition of TPU. They mentioned that the blends were partially miscible because of the hydrogen bonding between the PLA and TPU [17]. In our previous study, 1,4 phenylene diisocyanate (PDI) was used to improve the compatibility of PLA/TPU blends via reactive melt blending. The results indicated that the presence of PDI in the PLA/TPU blends improved the tensile properties of the blends and enhanced the desired compatibilization of PLA and TPU [16]. Zhao et al. reported the enhanced compatibility of TPU toughened PLA blends in the presence of 4,4-methylene diphenyl diisocyanate (MDI) via melt blending [18]. TPUs and TPU based polymer blends exhibit shape memory behavior. Ajili et al. [19] focused on the shape memory behaviors of the polyurethane/polycaprolactone (PU/PCL) to be used as stents. They stated that PU/PCL (70/30 wt%) blend showed excellent biocompatibility and shape recovery near the human body temperature. Song et al. [20] investigated the optimizing the shape recovery ratio of the biocompatible TPU/PLA blends by the changing composition and programming temperature. The programming temperatures were selected as 25°C, 37°C and 70°C. They found that the shape fixity ratio was increased by increasing of PLA content and programming temperature. Lai and Lan [21] investigated the effects of pre-deformation temperature (25°C, 80°C and 120°C) on the shape memory properties of PLA/TPU blends. They found that the increasing in TPU ratio in the blend caused the increasing of shape recovery ratio up to 93.4±0.4% at 160°C. In a recent study, Wei and coworkers stated that TPU phase in PLA/TPU blend exhibited stronger driving force. Moreover,
addition of CB and carbon nanotubes (CNTs) to the PLA/TPU showed better shape recovery properties [22]. In another recent study, Sun et al. pointed out that PLA/TPU SMP system including PEG presented enhanced shape-fixing capability after deformation at room temperature in the presence of single-walled carbon nanotubes [23]. In the current study, we have investigated the shape memory behavior of PLA/TPU blends by plasticizing the PLA with using poly(ethylene glycol) (PEG) to tune the shape recovery near the human body temperatures for the potential applications in biomedical areas. To the best of our knowledge, this study is the first attempt to plasticize the PLA in a PLA/TPU blend to tune the shape memory properties of PLA/TPU blends. In order to plasticize PLA, various molecular weights of PEG such as 1000 g/mol, 8000 g/mol and 35000 g/mol were used as the plasticizers. The PLA/TPU ratio was varied as 80/20, 60/40, 50/50, 40/60 and 20/80% by weight. The various shape memory experiments were conducted to understand the phenomena, such as the effect of deformation rate, programming temperature, counter weight during recovery and the thermocyclic test. In addition, the physical properties of the blends were also examined. In addition, the biocompatibility was evaluated by in-vitro cell culture tests. 2. Experimental 2.1. Materials PLA (trade name: PLI005) was purchased from NaturePlast Company. Its density, melting temperature and MFI are 1.25 g/cm3, 145-150°C and 10-30 g/10 min at 190°C/2.16 kg, respectively. The caprolactone diol based thermoplastic polyurethane was kindly obtained from Lubrizol Company. Its hardness is 77 Shore-A. Poly(ethylene glycol) (PEG, Mn, avg = 1000, 8000 and 35000 g/moles) (Sigma–Aldrich) was used as the plasticizer. 2.2. Processing Before the processing, PLA and TPU were dried in an oven under vacuum at 65°C for 24 h. A laboratory scale 15 mL twin-screw extruder (Xplore Instruments MC 15, The Netherlands) was used for compounding the polymers. Firstly, PLA was plasticized via PEGs. For PLA/PEG blends, the screw speed, barrel temperature and residence time were 100 rpm, 180°C and 1 min, respectively. The PLA/PEG blends were chopped as granules. Secondly, plasticized PLA and TPU blends were prepared via melt mixing. In this case, the screw speed, barrel temperature and residence time were 100 rpm, 190°C and 3 min, respectively. Argon was continuously supplied into the barrel to prevent the thermo-oxidative degradation. After compounding process, the melt
was transferred to the 12 mL laboratory injection-molding device (Xplore Instruments IM 12, The Netherlands). The melt and mold temperatures were 190 °C and 30 °C, respectively. The pressure during injection molding was set to 10 bars. 2.3. Characterization of thermal and mechanical properties and phase morphology Differential scanning calorimeter (DSC) analysis was performed using Mettler Toledo DSC1 Star System in the nitrogen atmosphere. The samples were cooled to -50°C with a cooling rate of 10°C/min and heated to 250°C with a heating rate of 10°C/min. The tensile properties were measured with an Instron (Model 3345) universal testing machine according to ISO 527. The crosshead speed was of 10 mm/min. Five samples were tested and then the average values were reported. Phase morphologies of plasticized PLA/TPU blends were investigated with scanning electron microscopy (SEM, JEOL JSM-6060). The samples were cryogenically fractured in liquid nitrogen and sputter coated with gold prior to analysis. 2.4. Investigation of shape memory properties The programming process of the samples was carried out as follows: I.
Ttrans was determined as Ttrans=45°C from the glass transition temperatures of plasticized PLA types. The oven was heated to 45°C and then the samples were placed into the oven. To provide homogenous heat distribution on the sample, they were kept in the oven for 2 min.
II. III.
The sample was deformed (i.e. elongated) to 100% at Ttrans=45°C using a 5.7 MPa load. The sample was cooled to the room temperature under load for 10 min for homogenous heat distribution.
IV.
Then the load was removed. The temporary shape was obtained after the mentioned steps above.
The testing of shape memory properties carried out as follows: I.
The sample length was measured and the sample was placed into the water tank equipped with a controller. It was heated from 40°C to 60°C with 5°C intervals.
II.
The change in sample lengths were measured at 40°C, 45°C, 50°C, 55°C and 60°C. Below 40°C, the shape recovery properties was not observed.
The shape recovery ratio (Rr) was calculated as follows (Eq.1): Ɛ𝑢(𝑁) ― Ɛ𝑝(𝑁)
(1)
𝑅𝑟 = Ɛ𝑢(𝑁) ― Ɛ𝑝(𝑁 ― 1)
where Ɛ𝑢(𝑁) is strain after deformed and removal load, Ɛ𝑝(𝑁) is recovered strain and Ɛ𝑝(𝑁 ― 1) is permanent strain [3]. Representative pictures related to the shape memory experiments were illustrated in Fig. 1.
(I)
(II)
Fig. 1. Representative pictures illustrating (I) the programming of the samples and (II) testing of the samples 2.5 Evaluation of Biocompatibility and Cytotoxicity In-vitro biocompatibility was performed using mouse fibroblast cells (L929). The number of viable cells present in a cell suspension was determined according to trypan blue exclusion assay. L929 fibroblast cells were routinely cultured in high glucose containing Dulbecco’s Modified Eagle Media (DMEM, Sigma Aldrich) supplemented with 10% fetal bovine serum (FBS, Sigma Aldrich) and 50 mg/ml of streptomycin (Sigma Aldrich) in 50 ml-plastic culture flasks. They were subcultured in every 4 days. The cells at logarithmic growth phase were treated with 0.2% trypsine0.02% EDTA (Sigma Aldrich), washed with DMEM containing 10% FBS, collected by centrifugation at 1000 rpm for 3 min and re-suspended in DMEM containing 10% FBS at 2.5x105 cells/ml. The cells were seeded onto each dish at 5x104cells/dish and cultivated at 37°C in a humidified 5% CO2 incubator. After incubation for a defined period, the cells on the scaffolds were
detached by the 0.2% trypsin treatment and counted by hemocytometer for cell viability measurement by trypan blue exclusion assay. The trypan blue exclusion assay is based on the principle that the live cells possess intact cell membranes that exclude certain dyes, such as trypan blue, whereas dead cells do not. In this test, a cell suspension obtained from the medium that was previously in contact with scaffold was mixed with the dye and then they were visually examined for up-taking or excluding the dye. 3. Results and Discussion 3.1. Thermal and Mechanical Properties Thermal properties of PLA, plasticized PLA and plasticized PLA/TPU blends having 50% TPU were summarized in Table 1. The results showed that neat PLA has glass transition temperature (Tg) and melting temperature (Tm) of 58.1 and 152.4°C, respectively. The Tg of PLA is rather high as compared to the human body temperature; therefore, PEG at 10 wt% was used as a plasticizer to decrease the Tg. It was observed that the addition of PEG to the PLA decreased the Tg of PLA about 15-20C depending on the molecular weight of PEG. The reduction of the Tg of PLA is due to the decreasing the intermolecular interactions of PLA chains in the presence of PEG molecules. The molecular weight of PEG has an influence on the plasticization of PLA. As the molecular weight increased, the reduction in Tg of PLA decreased. This is due to the fact that as the molecular weight of the PEG increased, the miscibility became lower with PLA [24]. As the 50PLA/50TPU blends are considered, the Tg of PLA phase decreased about 4C with the addition of TPU. This is possibly due to the shift in the Tg of the PLA towards the Tg of the polyester soft segment of TPU, which is below room temperature. This shift is also a proof of the compatibility between PLA and TPU [16]. Addition of 10%PEG to the PLA in the PLA/TPU blends reduced the Tg of the PLA phase in the blends depending the molecular weight of the PEG. The reduction in Tg of PLA phase in the blends was lower in comparison to plasticized PLA. This can be attributed to the dilution of PEG during secondary processing with TPU since the soft segment of TPU and PEG can have a possible compatibility, and hence some of the PEG can migrate through TPU phase. The cold crystallization exotherm was not observed for neat PLA, however for all plasticized PLA and plasticized PLA/TPU blends, the cold crystallization temperature (Tcc) was observed. This was due to the fact that the addition of PEG to the PLA decreased the melt viscosity, which promoted PLA chains to crystallize. The Tcc value was similar in the case of PEG1000 and PEG8000, but slightly higher in the case of PEG35000 since its plasticization efficiency, the ability to decrease the viscosity, was lower. Similar to plasticized blends, the PLA/TPU blends also showed the cold
crystallization behavior during heating due to the fact that the TPU phase acted as a nucleating agent for PLA. Nucleating role of TPU for PLA was also reported in the literature [25, 26]. Unplasticized PLA/TPU blend exhibited a cold crystallization temperature of 103.3°C belonging to PLA phase. The incorporation of PEG to the PLA in the PLA/TPU blends reduced the cold crystallization temperature of the PLA due to the decreasing melt viscosity that allows PLA chains crystallize earlier. A gradual increase was observed in the cold crystallization temperature parallel to the increasing molecular weight of the PEG. This is due to the weakening plasticization influence of the PEG on PLA with molecular weight [7]. The melting point of the PLA phase did not affect by either the plasticization or blending with TPU. In addition, the melting point of TPU was not observed under the experimental conditions of DSC analysis. Table 1. Thermal properties of neat PLA, plasticized PLA and plasticized-PLA/TPU blends Materials Neat PLA 10%PEG1000-PLA 10%PEG8000-PLA 10%PEG35000-PLA 50PLA/50TPU 10%PEG1000-50PLA/50TPU 10%PEG8000-50PLA/50TPU 10%PEG35000-50PLA/50TPU
Tg of PLA phase (°C) 58.1 37.4 41.1 42.4 54.9 46.0 48.7 51.7
Tcc of PLA phase (°C) 91.1 91.5 95.1 103.3 95.8 97.8 101.4
Tm of PLA phase (°C) 152.4 152.1 153.6 153.2 152.9 152.6 151.9 152.3
In order to understand the effects of blending with TPU and addition of plasticizers on the mechanical properties of PLA, tensile tests were performed for 50/50 PLA/TPU blends. The elastic modulus, elongation at break and yield strength data were tabulated in Table 2. As can be seen from Table 2 that the addition of TPU to PLA resulted in a drop in elastic modulus and yield strength but increase in elongation at break. This improved ductility is due to the inherent ductility of the TPU with respect to PLA. TPU is a block copolymer consisting of alternating sequences of hard and soft segments. The soft block, consists of a polyol and an isocyanate, gives the flexibility and elastomeric property of a TPU. Therefore, when TPU was added into PLA, the flexibility of the blend increased and the brittle characteristics of PLA decreased. Feng and Ye [17] observed the transition of PLA from brittle fracture to ductile fracture with the addition of TPU into PLA and suggested a partially miscible system due to the hydrogen bonding between the TPU and PLA phases. The addition of plasticizers having different molecular weights to the 50/50 PLA/TPU blend led to further decrement in stiffness and yield strength, but increment in elongation of the blends due to the decreasing intermolecular interaction between two phases. It was obtained that
as the molecular weight of PEG increased the flexibility of the blend decreased. This is due to the decreasing compatibility of PEG with PLA as the molecular weight of PEG increases [24]. Table 2. The mechanical properties of neat PLA, PLA/TPU and plasticized PLA/TPU blends
3713±432
Elongation at break (%) 4.58±0.0
Yield Strength (MPa) 56.8±2.5
(0%PEG) 50PLA/50TPU
1463±775
261.5±0.7
34.3±1.2
(10%PEG1000) 50PLA/50TPU
1048±120
656.7±1.2
22.9±1.2
(10%PEG8000) 50PLA/50TPU
1088±187
362.5±1.6
25.3±1.9
(10%PEG35000) 50PLA/50TPU
1348±12
261.7±0.8
29.0±3.0
Materials
Elastic Modulus (MPa)
Neat PLA
3.2. Investigation of shape memory behavior Fig. 2. shows the shape memory behavior of 80/20, 60/40, 50/50, 40/60 and 20/80 unplasticized PLA/TPU blends. One should note that the blends were programmed at 45°C by deforming 100%. Independently from the composition, all the blends exhibited higher recovery ratios as the test temperature increased. This is due to the fact that the switching phase in the blends is the PLA and the switching occurred due to the relaxation of the PLA chains. Therefore, as the temperature increases, the relaxation of the PLA chains will be more favorable. There are two distinct zones visible in the plots: i.) the TPU dominant zone and ii.) PLA dominant zone. The borderline between two zones is the glass transition temperature of PLA, which is around 55-60C. When the TPU content increased in the PLA/TPU blends, the recovery ratio improved in the first (lower temperature) zone. The improvement in shape recovery ratios of the blends, having higher TPU content (i.e. 80% and 60%) below the glass transition temperature of PLA where the PLA chains are frozen, is due to the presence of the soft segments of TPU. The soft polyester segments are still mobile below the Tg of PLA [7] and acted as the switching phase. At lower contents of TPU below Tg of PLA, the shape recovery was lower due the diluted contribution to the restoring force. In addition, for the blends of 20PLA/80TPU and 40PLA/60TPU, TPU might form the continuous phase and therefore it dominated the behavior of the blends. As the test temperature of shape recovery exceeded the Tg of PLA, i.e. 55C and 60C, the blends having higher PLA contents exhibited higher recovery ratios. The maximum recovery ratio of 97% was obtained for the blend containing the highest content of PLA (80PLA/20TPU) at the highest test temperature. Similar observations was obtained by Song et al. They stated that the higher the content of PLA, the higher the total recovery ratios [20]. The decreasing amount of switching phase (PLA) resulted
in a decrease of recovery ratio. The similar findings were also found by Dhollander et al. and Wang et al. [27, 28].
TPU dominant zone
PLA dominant zone
Fig. 2. Shape memory behavior of 80/20, 60/40, 50/50, 40/60 and 20/80 unplasticized PLA/TPU blends between 40-60°C (Blends were programmed at 45°C and deformed 100%) Shape memory behavior of plasticized and unplasticized PLA/TPU blends were given in Fig. 3 with respect to the blend composition and plasticizer molecular weight. It was clearly observed that for all PLA/TPU ratios, unplasticized blends showed the highest total recovery, but they demonstrated the lowest recovery ratio for lower temperature because of high Tg of PLA. The plasticization improved the low temperature recovery of the blends, especially for the ones having higher PLA content. For the blends of 80/20, 60/40 and 50/50, the recovery ratio below 50C increased with the addition of plasticizer. Moreover, the recovery ratios were improved by the decreasing molecular weight of the plasticizer. This is due to the plasticization ability of the lower molecular weight plasticizers. For the blends of 40/60 and 20/80, the significance of the plasticizer molecular weight was lost probably due to the phase inversion of the blends where the continuous phase became TPU rather than PLA. As the PLA/TPU ratio is taken into consideration, it was observed that the increasing amount of TPU in the blend improved the recovery ratio of the plasticized blends especially at lower temperatures due to the contribution of the hard segments of the TPU that are able to increase stored energy to improve recovery ratios. The recovery range of the blends, based on the temperature, was enhanced by the addition of TPU. Song et al. obtained similar results in their recent work on the PLA/TPU blends. They found that the recovery range
was broadened by the addition of TPU in to the PLA matrix [20]. At higher temperatures, i.e. above 50C, plasticized blends exhibited lower recovery ratios and the significance of PEG type was lost. The difference in recovery ratio for unplasticized and the plasticized blend was found maximum in 80/20 blends. This difference became smaller as the TPU content increased in the blend.
(a)
(b)
(c)
(d)
(e)
Fig. 3. Shape memory behavior of unplasticized and 1000, 8000, 35000 g/mole PEG plasticized PLA/TPU (a) 80/20, (b) 60/40, (c) 50/50, (d) 40/60 and (e) 20/80 (One should note that the blends were programmed at 45°C and deformed 100%) The influence of the deformation rate in the programming step has been investigated on the PEG1000 plasticized 50/50 PLA/TPU blends (Fig. 4). The samples were first programmed at 45C than recoveries were measured at different temperature ranging from 40C to 60C. Five different deformation rates i.e. 30%, 75%, 100%, 150% and 200%, were investigated. As can be seen from the Fig. 4 that the effect of deformation rate did not follow a trend. The glass transition temperature of PLA phase in PEG 1000 plasticized 50/50 bend is 46C, which is higher than the test temperature of 40C; therefore the contribution of PLA chains are very limited at this temperature. The shape recovery ratios increased above 45°C with decreasing deformation rates. The samples that deformed 30% during the programming showed the highest recovery ratios, while the ones deformed 200% showed the lowest. It was observed that the blends were able to recover fully to their original shape when they were programmed at lower deformation rates. It can be explained that the amorphous segments of PLA/TPU blends are able to compensate a certain amount of elastic deformation, but after a threshold, due to the extensive plastic deformation in the amorphous phases, the recovery ratios lowered.
Fig. 4. The influence of the deformation rate in the programming step on the PEG1000 plasticized 50/50 PLA/TPU blends
It is seen from Fig. 5 that the PEG1000 containing blend of 20/80 PLA/TPU exhibited higher shape recovery ratios than PEG1000 containing 50/50 PLA/TPU, regardless of deformation rate i.e. 75% and 150%. As discussed before, the shape recovery ratios increased with increasing TPU content. This is due to the fact that the hard segments of TPU acted as physical crosslinks which were the energy storage points during deformation [21].
(a)
(b)
Fig. 5. Shape recovery of PEG1000 plasticized 50/50 and 20/80 PLA/TPU blends with between 40-60°C with 5°C intervals (The blends programmed at 45°C and deformed (a) 75% and (b) 150%). 3.2.4. The effects of load on recovery ratio In order to judge the shape recovery behavior of the blends under load, the 50/50 and 20/80 PLA/TPU blends containing PEG1000 were tested under loads of 0, 10, 50, 120, 150 and 200 g (corresponding stresses to 0, 12, 60, 150, 185 and 245 kPa, respectively). The test samples were programmed at 45°C and deformed 100%. The weights were attached to the bottom of the samples during testing. As seen from Fig. 6a, as the stress increased, the recovery ratio decreased as expected. The samples without weight showed more than 70% total recovery, while the samples under load showed more than 50% total recovery. This means that the blends are able to carry stress up to 245 kPa with only small amount of recovery loss. Fig. 6b-d shows the recovery ratio of PEG1000 plasticized 50/50 PLA/TPU and 20/80 PLA/TPU blends under 10 g, 120 g and 200 g load with respect to temperature. It was clearly seen that the higher the content of TPU, the higher the stress carrying capacity is, due to the hard segments of TPU. It was reported similar findings in the literature that PEG has a positive effect on a 20% PLA and 80% TPU system: more than 80% Rr can be achieved
under 5% and 10% PEG content [23, 29]. Moreover, Liu et al. stated that lower amount of TPU also caused a decrease in the shape recovery ability, which is consistent with our results [30].
(a)
(c)
(b)
(d)
Fig. 6. (a) PEG1000 plasticized 50/50 PLA/TPU with various weights (0, 10, 50, 120, 150 and 200 g) and, PEG1000 plasticized 50/50 PLA/TPU and 20/80 PLA/TPU blends with (b) 10 g, (c) 120 g and (d) 200 g weights. The blends programmed at 45°C and deformed 100%. 3.2.5. The effects of thermocyclic tests on recovery ratio In order to investigate the cyclic shape memory behavior of plasticized 20/80 PLA/TPU blends, the test was repeated three times. It was possible to observe that the recovery ratio was higher in the second cycle than the first cycle (Fig. 7). D’hollander et al. reported that the recovery ratio improved with increasing repeated cycles [27]. However, when the third cycle was repeated, the recovery ratios were reduced at all temperatures. This may be associate to the change of the microstructural properties of the polymer blends due to the plasticizer migration/diffusion or change in the phase morphology.
Fig. 7. Thermocyclic test results of PEG1000 plasticized 20/80 PLA/TPU blends programmed at 45°C and deformed 25%. 3.2.6. The effects of PEG ratio and programming temperature on recovery ratio In order to investigate the ratio of plasticizer on the shape recovery behavior of the blends, 0, 10 and 20% PEG containing 50/50 PLA/TPU blends were investigated (Fig. 8). The blends were programmed at 45°C and deformed 100%. The high content of PEG resulted in a sharp reduction in the Tg of PLA, the switching polymer. While non-plasticized blends showed nearly 5% recovery at 45°C, the 20% PEG containing blends showed more than 45%. This increment proved the effects of plasticizers on the shape memory behavior of PLA/TPU system in order to be actuated at lower temperatures.
Fig. 8. The recovery ratios of 50/50 PLA/TPU blends containing 0, 10, 20% PEG The recovery ratios of 50/50 PLA/TPU blends containing 20% PEG1000 was represented in Figure 9 in order to compare the effects of programming temperature of 45°C and 60°C. It was found that the higher the programming temperature, the higher the recovery ratio was. For example, the shape recovery ratios improved from 45% to over 65-70% at 45°C.
Fig. 9. The recovery ratios of 20%PEG1000 plasticized 50/50 PLA/TPU blends programmed at 45°C and 60°C
3.3. The phase morphology of PLA/TPU
Fig. 10. SEM pictures of (a1 and a2) 10%PEG1000 plasticized PLA (b1 and b2) unplasticized 50/50 PLA/TPU (c1 and c2) 10%PEG1000 plasticized 50/50 PLA/TPU (d1 and d2) 10%PEG1000 plasticized 80/20 PLA/TPU with different magnifications (5000x and 10000x)
The SEM pictures of PEG1000 plasticized PLA, plasticized and unplasticized 50/50 PLA/TPU blends and plasticized 80/20 PLA/TPU blends are represented in Fig. 10. It was seen for PEG1000 plasticized PLA that there was no phase separation indicating a fully miscible polymer mixture (Fig. 10a). In 50/50 blends, the morphology of the blends was seemed to be a two-phase morphology with a possible continuous phase of TPU and dispersed PLA phase, since the melt viscosity of TPU was expected to be higher than that of plasticized and unplasticized PLA (Fig. 10b and Fig. 10c). Therefore, the small dentations seen on the surface can be associated to the PLA dispersed phase. However, when the PLA content increased to 80%, a phase inversion occurred and resulted in a PLA continuous phase with dispersed TPU domains, as can be seen small droplets in PLA matrix in Fig.10d. 3.4. The cytotoxicity and biocompatibility of plasticized PLA/TPU blends L929 mouse fibroblast cell line was used in in-vitro culture studies to estimate the biocompatibility and cytotoxicity of PLA/TPU blends. The cell growth and proliferation were checked using an inverted light microscope (Olympus CKX 41) while the cultivation was going on cell-culture plate. The cell-line morphologies at 3rd, 5th and 7th days are shown in Fig. 11. It can be seen in the first seeding stage that the cells were globular and round whereas they looked like elongated or spinal during proliferation. In the 3rd day, despite the growth of the cells, there was not enough proliferation. It was observed that the cell proliferation and layout were decided to be enough at the end of 5th day. On the other hand, in the 7th day, the cell growth and the proliferation could be inhibited due to the contact inhibition. Therefore, the comparison was done on the 5th day for the PLA/TPU samples in cell-culture studies.
Fig. 11. The cell line morphology of L929 a) 3rd day (10X), b) 5th day (10X), c) 7th day (10X), d) 3rd day (40X) The cytotoxicity test was employed to judge if there was any toxicity of the components of polymer blends, such as matrices or plasticizer (PEG). A glass-slide as the control and, some selected PLA/TPU blends were investigated. The film samples of blends were located onto the hemocytometer without any leakage. The medium was then dyed with trypan blue. The percentage of dead cells was calculated from trypan blue exclusion assay test and viable cell ratio was calculated accordingly. The percentage of viable cells was shown in Fig. 12. According to ISO 10993-5, percentages of cell viability above 80% are considered as non-cytotoxic; within 80%– 60% weak; 60%–40% moderate and below 40% strong cytotoxicity, respectively [31]. It is seen that the cell viability on lamella and on all the blends were greater than 80%. This means that no evidence of cytotoxicity was observed from the blends of PLA/TPU.
Fig. 12. Cell viability of L293 mouse fibroblast cell line on the selected blend compositions 4. Conclusion In this study, we have focused on the understanding shape memory behaviors of PEG plasticized PLA/TPU blends as a function of PEG molecular weight, PLA/TPU ratio, programming temperature and applied stress. It was found that higher TPU composition caused higher recovery ratios for the non-plasticized PLA/TPU between 40-55°C range. In addition, higher content of PLA increased the shape fixity ratios. All plasticized PLA/TPU blends exhibited better shape recovery at lower temperatures with respect to the unplasticized opponents. The lowest molecular weight PEG provided the highest recovery at lower temperatures. It was seen that the deformation ratio was rather important to gain higher recovery ratios. It was obtained that the lowest deformation rates resulted in higher shape recovery ratios for all recovery temperature. Moreover, it was found that the samples were capable to lift weights up to 200 g by losing only a small amount of recovery. As expected, 20/80 PLA/TPU blends showed higher recovery for the all weights due to the strong elasticity of TPU. The biocompatibility and cytotoxicity experiments revealed that both plasticized PLA and TPU were non-toxic and safe to use in-vivo conditions. As a conclusion, we suggest that the plasticized PLA/TPU blends with lower molecular weight PEG (i.e. 1000 g/mole) with an optimum PLA/TPU composition can be a promising candidate for in-vivo application near human body temperature.
References [1] D. Ratna, J.K. Kocsis, Recent advances in shape memory polymers and composites: a review, J. Mater. Sci. 43 (2008) 254–269. [2] B. Yan, S. Gu, Y. Zhang, Polylactide-based thermoplastic shape memory polymer nanocomposites, Eur. Polym. J. 49 (2013) 366-378. [3] S. Xu, J. Song, Thermal Responsive Shape Memory Polymers for Biomedical Applications, 1st ed., InTech, China, 2011. [4] N. Choi, A. Lendlein, Degradable shape-memory polymer networks from oligo[(L-lactide)-ranglycolide]dimethacrylates, T. Roy. Soc. Chem. 3 (2007) 901-909. [5] E. Zini, M. Scandola, P. Dobrzynski, J. Kasperczyk, M. Bero, Shape memory behavior of novel (L-Lactide-glycolide-trimethylene carbonate) terpolymers, Biomacromolecules 8 (2007) 36613667. [6] K.M. Lee, P.T. Knight, T. Chung, P.T. Mather, Polycaprolactone-POSS chemical/physical double networks, Macromolecules 41 (2008) 4730-4738. [7] G. Ozkoc, S. Kemaloglu, Morphology, biodegradability, mechanical, and thermal properties of nanocomposite films based on PLA and plasticized PLA, J. Appl. Polym. Sci. 114 (2009) 2481– 2487. [8] A. Demirbas, Biodegradable plastics from renewable resources, Energ. Source. Part A 29 (2007) 419-424. [9] I. Pillin, N. Montrelay, Y. Grohens, Thermo-mechanical characterization of plasticized PLA: Is the miscibility the only significant factor?, Polymer 47 (2006) 4676–4682. [10] M. Keramati, I. Ghasemi, M. Karrabi, H. Azizi, M. Sabzi, Incorporation of surface modified graphene nanoplatelets for development of shape memory PLA nanocomposite, Fiber Polym. 17 (2016) 1062-1068. [11] S. Gumus, G. Ozkoc, A. Aytac, Plasticized and unplasticized PLA/organoclay nanocomposites: short- and long-term thermal properties, morphology, and nonisothermal crystallization behavior, J. Appl. Polym. Sci. 123 (2012) 2837–2848. [12] M. Sheth, R.A. Kumar, V. Dave, R.A. Gross, S.P. McCarthy, Biodegradable polymer blends of poly(lactic acid) and poly(ethylene glycol), J. Appl. Polym. Sci. 66 (1997) 1495–1505.
[13] K. Sungsanit, N. Kao, and S.N. Bhattacharya, Properties of linear poly(lactic acid)/polyethylene glycol blends, Polym. Eng. Sci. 52 (2012) 108-116. [14] M. Kodal, H. Sirin, G. Ozkoc, Effects of reactive and nonreactive POSS types on the mechanical, thermal, and morphological properties of plasticized poly(lactic acid), Polym. Eng. Sci. 54 (2014) 264–275. [15] H. Hong, J. Wei, Y. Yuan, F.P. Chen, J. Wang, X. Qu, C.S. Liu, A novel composite coupled hardness with flexibleness—polylactic acid toughen with thermoplastic polyurethane, J. Appl. Polym. Sci. 121 (2011) 855–861. [16] S. K. Dogan, E. A. Reyes, S. Rastogi, G. Ozkoc, Reactive compatibilization of PLA/TPU blends with a diisocyanate, J. Appl. Polym. Sci. 131 (2014) 1–10. [17] F. Feng, L. Ye, Morphologies and mechanical properties of polylactide/thermoplastic polyurethane elastomer blends, J. Appl. Polym. Sci. 119 (2011) 2778–2783 [18] F. Zhao, H.X. Huang, S.D. Zhang, Largely toughening biodegradable poly(lactic acid)/thermoplastic polyurethane blends by adding MDI, J. Appl. Polym. Sci. 132 (2015) 42511. [19] S.H. Ajili, N.G. Ebrahimi, M. Soleimani, Polyurethane/polycaprolactane blend with shape memory effect as a proposed material for cardiovascular implants, Acta Biomaterialia 5 (2009) 1519–1530. [20] J.J. Song, H.H. Chang, H.E. Naguib, Biocompatible shape memory polymers actuators with high force capabilities, Eur. Polym. J. 67 (2015) 186-198. [21] S.M. Lai, Y.C. Lan, Shape memory properties of melt-blended polylactic acid (PLA)/thermoplastic polyurethane (TPU) bio-based blends, J. Polym. Res. 20 (2013) 1-8. [22] Y. Wei, R. Huang, P. Dong, X.D. Qi, Q. Fu, Preparation of polylactide/poly(ether)urethane blends with excellent electro-actuated shape memory via incorporating carbon black and carbon nanotubes hybrids fillers, Chinese J. Polym. Sci. 36 (2018) 1175-1186. [23] Y.C. Sun, M. Chu, M. Huang, O. Hegazi, H.E. Naguib, Hybrid electroactive shape memory polymer composites with room temperature deformability, Macromol. Mater. Eng. 304 (2019) 1900196.
[24] M. Kodal, H. Sirin, G. Ozkoc, Long- and short-term stability of plasticized poly(lactic acid): effects of plasticizers type on thermal, mechanical and morphological properties, Polym. Bull. 76 (2019) 423-445. [25] S.M. Lai, W.L. Wu, Y.J. Wang, Annealing effect on the shape memory properties of polylactic acid (PLA)/thermoplastic polyurethane (TPU) bio-based blends, J. Polym. Res. 23 (2016) 99. [26] H.Y. Mi, M.R. Salick, X. Jing, B.R. Jacques, W.C. Crone, X.F. Peng, L.S. Turng, Characterization of thermoplastic polyurethane/polylactic acid (TPU/PLA) tissue engineering scaffolds fabricated by microcellular injection molding, Mater. Sci. Eng. C Mater. Biol. Appl. 33 (2013) 4767-4776. [27] S. D’Hollander, G.V. Assche, B.V. Mele, F.D. Prez, Novel synthetic strategy toward shape memory polyurethanes with a well-defined switching temperature, Polymer 50 (2009) 4447–4454. [28] W. Wang, P. Ping, X. Chen, X. Jing, Shape memory effect of poly(L-lactide)- based polyurethanes with different hard segments, Polym. Int. 56 (2007) 840–846. [29] S. Cai, Y.C. Sun, J. Ren, H.E. Naguib, Toward the low actuation temperature of flexible shape memory polymer composites with room temperature deformability via induced plasticizing effect, J. Mater. Chem. B 5 (2017) 8845-8853. [30] T. Liu, R. Huang, X. Qi, P. Dong, Q. Fu, Facile preparation of rapidly electro-active shape memory thermoplastic polyurethane/polylactide blends via phase morphology control and incorporation of conductive fillers, Polymer 114 (2017) 28-35. [31] International Organization for Standardization. ISO 10993 5: 2009. Biological evaluation of medical devices, Part 5: Tests for in vitro cytotoxicity. International Organization for Standardization: Geneva, Switzerland, 2009.
LIST OF TABLE CAPTIONS
Table 1. Thermal properties of neat PLA, plasticized PLA and plasticized-PLA/TPU blends Table 2. The mechanical properties of neat PLA, PLA/TPU and plasticized PLA/TPU blends
LIST OF FIGURE CAPTIONS
Fig. 1. Representative pictures illustrating (I) the programming of the samples and (II) testing of the samples Fig. 2. Shape memory behavior of 80/20, 60/40, 50/50, 40/60 and 20/80 unplasticized PLA/TPU blends between 40-60°C (Blends were programmed at 45°C and deformed 100%) Fig. 3. Shape memory behavior of unplasticized and 1000, 8000, 35000 g/mole PEG plasticized PLA/TPU (a) 80/20, (b) 60/40, (c) 50/50, (d) 40/60 and (e) 20/80 (One should note that the blends were programmed at 45°C and deformed 100%) Fig. 4. The influence of the deformation rate in the programming step on the PEG1000 plasticized 50/50 PLA/TPU blends Fig. 5. Shape recovery of PEG1000 plasticized 50/50 and 20/80 PLA/TPU blends with between 40-60°C with 5°C intervals (The blends programmed at 45°C and deformed (a) 75% and (b) 150%) Fig. 6. (a) PEG1000 plasticized 50/50 PLA/TPU with various weights (0, 10, 50, 120, 150 and 200 g) and, PEG1000 plasticized 50/50 PLA/TPU and 20/80 PLA/TPU blends with (b) 10 g, (c) 120 g and (d) 200 g weights. The blends programmed at 45°C and deformed 100% Fig. 7. Thermocyclic test results of PEG1000 plasticized 20/80 PLA/TPU blends programmed at 45°C and deformed 25% Fig. 8. The recovery ratios of 50/50 PLA/TPU blends containing 0, 10, 20% PEG Fig. 9. The recovery ratios of 20%PEG1000 plasticized 50/50 PLA/TPU blends programmed at 45°C and 60°C Fig. 10. SEM pictures of (a1 and a2) 10%PEG1000 plasticized PLA (b1 and b2) unplasticized 50/50 PLA/TPU (c1 and c2) 10%PEG1000 plasticized 50/50 PLA/TPU (d1 and d2) 10%PEG1000 plasticized 80/20 PLA/TPU with different magnifications (5000x and 10000x) Fig. 11. The cell line morphology of L929 a) 3rd day (10X), b) 5th day (10X), c) 7th day (10X), d) 3rd day (40X) Fig. 12. Cell viability of L293 mouse fibroblast cell line on the selected blend compositions
Dear Editor;
We have no conflict of interest with any person or organization.
Best regards; Dr. Guralp Ozkoc
Graphical abstract
Highlights
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The shape memory behavior of PEG-plasticized PLA/TPU blends was investigated as a function of blend ratio, plasticizer molecular weight and programming conditions.
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The shape memory tests showed that the higher the TPU content, the higher the recovery ratio was.
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The decreasing molecular weight of PEG maximized the shape recovery values.
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The samples are able to carry stresses up to 245 kPa during shape recovery depending on the temperature and composition.
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The biocompatibility and cytotoxicity tests revealed that the blends are non-toxic and safe to use in-vivo conditions.