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Synthesis and characterization of castor oil-segmented thermoplastic polyurethane with controlled mechanical properties
Accepted Manuscript Synthesis and characterization of castor oil-segmented thermoplastic polyurethane with controlled mechanical properties Nguyen Dang Luong, Le Hoang Sinh, Malin Minna, Weisser Jürgen, Walter Torsten, Schnabelrauch Matthias, Seppälä Jukka PII: DOI: Reference:
S0014-3057(16)30491-8 http://dx.doi.org/10.1016/j.eurpolymj.2016.05.024 EPJ 7371
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
17 March 2016 23 May 2016 27 May 2016
Please cite this article as: Luong, N.D., Sinh, L.H., Minna, M., Jürgen, W., Torsten, W., Matthias, S., Jukka, S., Synthesis and characterization of castor oil-segmented thermoplastic polyurethane with controlled mechanical properties, European Polymer Journal (2016), doi: http://dx.doi.org/10.1016/j.eurpolymj.2016.05.024
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Synthesis and characterization of castor oil-segmented thermoplastic polyurethane with controlled mechanical properties Nguyen Dang Luong1,+, Le Hoang Sinh1,2+, Malin Minna1, Weisser Jürgen3, Walter Torsten3, Schnabelrauch Matthias3, Seppälä Jukka 1,* 1
Polymer Technology Research Group, Department of Biotechnology and Chemical Technology,
Aalto University, School of Chemical Technology, P.O. Box 16100, FI-00076 Aalto, Finland 2
Center for Advanced Chemistry, Institute of Research and Development, Duy Tan University,
Da Nang, Vietnam 3
INNOVENT e.V., Biomaterials Department, Pruessingstrasse 27B, D-07745 Jena, Germany
+
The authors are equally distributed
*Corresponding author: Tel: + 358 400 701 142 Fax: +358 9 451 2622. Email: [email protected]
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Abstract Novel castor oil-segmented thermoplastic polyurethane series (PUs) has been prepared and extensively characterized in this study. The castor oil, poly(tetrahydrofuran) polyol, and poly(dimethylsiloxane) have been employed as soft segments in the polymer synthesis with a maximum bio-based content up to 14.1 wt%. We demonstrate here that it is possible to prepare thermoplastic polyurethane containing castor oil via polyaddition approach in which the formation of prepolymer needs to be carried out with the addition of reasonably small amount of castor oil together with excess amount of the diisocyanate in the very first step. This is followed by the addition of the other polyols and finally a low molecular weight chain extender. The developed synthesis resulted in the formation of branched thermoplastic polyurethane with an elongation at break of about 1200%, which is considerably high compared to reported values for polyurethanes containing castor oil as soft segments. We have demonstrated that mechanical properties and flexibility of polyurethane can be significantly altered by both incorporated castor oil and controllable poly(dimethylsiloxane)/poly(tetrahydrofuran) polyol ratio during the synthesis. Especially the synthesized polyurethanes exhibited good biocompatibility and high transparency, which are useful properties for polymers as potential biomaterials. In addition, thermal stability and thermal transition of polyurethanes have been investigated.
Keywords: castor oil; poly(dimethylsiloxane); poly(tetrahydrofuran); thermoplastic polyurethane; mechanical properties; cytocompatibility
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1. Introduction Polyurethane elastomers are classified as one of the most useful subclasses of thermoplastic elastomers, which have been widely used as biomaterials in biomedical applications thanks to their superior mechanical properties and good biocompatibility. For example, PUs have been used as biomaterials in manufacture of cardiac-assist pumps, blood bags, and chronic implants such as heart valves and vascular grafts [1]. In addition to biomaterial applications, PU polymers in general have been used commonly in manufacture of automotive parts, in building and construction, electronics, paints, etc. Wide applicability of PUs is due to their unique properties such as high hardness, high strength and high elongation at break, which can be modified by variety of choices of starting monomers, catalysts, and other reaction conditions. It should be addressed that polyurethane elastomers have mainly been synthesized from fossil fuel based polyols and isocyanates. In general, the polymeric chains are composed of alternating hard isocyanate segments and soft polyol segments, and thus the mechanical, thermal, and adhesive properties of PUs are dependent on the composition and chemical structure of the hard and soft segments. In recent years, option to use of renewable resources for chemical synthesis has become extremely urgent, because exhaustion of petroleum is predictable in near future [2]. Moreover, the use of renewable resources results in reduction of environmental impact with greenhouse gas emission. In this regard, vegetable oils are potential replacement for petroleum as raw materials in polymeric synthesis because they are abundant in various plants and can be extracted by simple/facile methods [3]. In particular, vegetable oils have been becoming increasingly important as renewable resources for the preparation of polyols used in polyurethane industry.
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However, it is noted that most of vegetable oils must be chemically treated to introduce hydroxyl groups that are required in polyurethane synthesis [4]. Fortunately, castor oil naturally contains hydroxyl in its structure and thus it can be directly utilized as polyol in polyurethane synthesis. Furthermore, since castor oil is not edible it could be substituted in the areas where edible oils can be used. These are huge advantages of castor oil compared to other vegetable oils, such as olive oil, soybean oil, and sunflower oil [5-6]. About 90% composition of castor oil is ricinoleic acid with one hydroxyl group on the every 12th carbon and a double bond between the 9th and 10th carbon. This well-defined chemical structure containing available hydroxyl groups enable castor oil to be used as polyols in polyurethane synthesis. The obtained PUs have been used in many applications including coatings, elastomers, thermoplastics, rigid foams, adhesives, etc. [7]. The castor oil-based polyurethane elastomers prepared by one-spot approach in bulk polymerization process have been reported in several studies [5, 8-9]. Unfortunately, the resulted elastomers were insoluble in common organic solvents leading to the fact that toxic catalyst and un-reacted starting materials cannot be removed easily from the crosslinked elastomers [2]. This is due to fact that the hydroxyl functionality of castor oil is 2.7 on average, which could lead to the formation of threedimensional network of polymer structure during polymerization. Several attempts have been emphasized on synthesis of soluble castor-oil based polyurethanes by solution polymerization methods. However, these polyurethanes had quite low mechanical characteristics, especially in elongation at break, in comparison with the commercial polyurethanes utilizing petroleum-based polyols [10-11]. It is worth mentioning that mechanical properties of many thermoplastic PUs need to be improved to fulfil the needs in engineering applications. With this regard, significant advances in 4
polymer nanocomposites have shown that incorporation of nanofillers such as clay, carbon nanotube, and graphene can improve mechanical properties of PUs, especially their Young’s modulus [12]. However, the presence of these fillers in PUs may impair the biocompatibility and pose a risk of releasing the harmful particulate fillers during their uses in biomedical areas. With this regard, many other bio-based materials such as cellulose nanocrystals (CNCs) or chitin nanocrystals have been used as reinforcements in thermoplastic PU. For examples, thermoplastic castor oil-based segmented PU polymer reinforced with these nanocrystals have been synthesized and demonstrated good shape memory properties [13-14]. The improvement in shape memory performance is due to increased crystallinity of the hard segment in the PU. However, it is noted in these studies that a difunctional polyester derived from castor oil was used instead of original castor oil as we used. In another study, it is reported that the addition of CNCs with less than 0.25 wt% increase strength and ductility of the PU composites [15]. In addition, the PU/CNC composite displayed no toxicity via short-term cytotoxicity test. The reason for the improvement in mechanical properties is also originated from the crystallization of both soft and hard segment. Differently, in this study we showed that incorporation of polydimethylsiloxane polyol (PDMS) in a controlled manner as one of the segments in PU macromolecule could significantly alter the mechanical characteristics of the PU. In this report, we use castor oil, PDMS, and poly(tetrahydrofuran) (PTH) as polyols and 4,4′-methylenebis(phenyl isocyanate) (MDI) as diisocyanate and finally 1,4-benzene dimethanol (BDO) as chain extender. It is worthy noticing that PTH can be also bio-based material [16-17]. Biobased poly(tetrahydrofuran) is produced by polymerization of tetrahydrofuran, which is product from fermentation of biorenewable succinate resources [18]. As a demonstration, the synthesized PUs with good biocompatibility, 5
excellent mechanical strength and high transparency have been achieved by the developed approach. It should also be noticed that the PUs contain bio-based materials in this study ranging from 12.2 wt% to 14.1 wt% in which only castor oil is considered as the bio-based material. The promising results obtained by cytocompatibility suggest that the PU synthesized in this study could be considered for biomedical applications. The key to obtain thermoplastic polyurethane in this study was to grow PU macromolecules in a well-controlled manner and to prevent the formation of three-dimensional network structure, which is usually obtained when castor is used in PU synthesis. Additionally, we have demonstrated here that varying the PDMS/PTH polyol ratio can change the mechanical behaviour of the resulted PU significantly. The developed PU and the synthesis approach could be useful in the development of thermoplastic polyurethanes and their utilizations as polymeric biomaterials.
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2. Experimental 2.1. Materials Refined castor oil (CO, hydroxyl number of 2.7), poly(tetrahydrofuran) (PTH, Mn 2000 g/mol), hydroxyl terminated poly(dimethylsiloxane) (PDMS, Mn 550 g/mol), 4,4′-Methylenebis(phenyl isocyanate)
(MDI,
98%),
dibutyltin
dilaurate
(DBTDL,
95%),
anhydrous
N,
N-
dimethylacetamide (DMAc, 99.8%), N,N-Dimethylformamide (DMF, for HPLC, ≥ 99.9%), and methanol were purchased from Sigma-Aldrich. 1,4-benzene dimethanol (BDM, 99%) was obtained from Acros Organics. All chemicals were used as received. 2.2. Synthesis of polyurethane series The recipes for polyurethane series are presented in Table 1. Firstly, castor oil (0.4 mol of OH) and DBTDL (catalyst) was weighted and dissolved in DMAc solvent, which was then fed in to three-neck round bottom flask reactor equipped with a magnetic stirrer and a condenser. An amount of MDI (2.0 mol of NCO) was dissolved in DMAc and injected via syringe into the reactor. Nitrogen bubbling was carried for 30 min to introduce inert environment. The reaction mixture was then heated to 60 oC and kept at this condition for 2 h. A mixture of two polyols (1.0 mol of OH), PTH and PDMS chemicals, was dissolved in DMAc and injected slowly into the reactor. The molar ratios of PDMS/PTH were 45/55, 50/50, 55/45, and 65/35 for each sample. The reaction was then conducted for 2 h in this step. Subsequently, a solution of BDM as chain extender (0.6 mol of OH) in DMAc was injected to the reaction and kept for 15 h. The reaction mixture was cooled down to room temperature, and the polymer was recovered from the reaction mixture by coagulating in methanol with vigorous mixing. The precipitate was collected by filtering and washed several times with methanol to remove impurities. Finally, the product is dried thoroughly in vacuum oven at 60 oC for 24 h.
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For comparison purpose, a PU containing no CO was synthesized, which is denoted as PU control. In the synthesis of the PU control, MDI solution was added in the first step, which was then followed by the addition of PDMS/PTH mixture, and finally the BDM as chain extender. The PDMS/PTH was fixed at 50/50 by weight percent. 2.3. Preparation of polyurethane film After being dried, polyurethane product was dissolved in DMAc at 60 oC to obtain a 10 wt% polymer solution. The resulting polymer solution was poured into glass Petri dish and solvent removed in a vacuum oven at 60 oC. Finally, the transparent polyurethane film was peeled off from glass surface and used for polymer characterizations. The amount of PU solution was calculated to obtain polymer films with thicknesses of about 0.2 mm. 2.4. Characterizations Infrared (IR) spectroscopy was used to study the chemical structure of polyurethanes using a FTIR-ATR-MATTSON 3000 spectrometer. Molecular weight and polydispersity of the polymers were determined by Gel permeation chromatography (GPC) using a Waters 717 plus Autosampler. N,N-dimethylformamide (DMF) was used as the solvent. The calibration curve was constructed using ten polystyrene standards, which run under the same condition. Mechanical properties of casted PU films were investigated on Instron 4204 universal testing equipment at 25 oC and relative humidity of 50%, with speed of 10 mm/min. Five specimens were tested for each sample in the tensile test. Dynamic mechanical analyses (DMA) were carried out by using the TA instrument (Q800) from -80 oC to 100 oC at a heating rate of 5 o
C/min and frequency of 1 Hz. The static elastic recovery properties were evaluated by using
creep mode on the DMA system (Q800). In particular, the samples were applied with a constant stress of 0.25 MPa for 30 min at 30 oC. Then, the force was released to let the samples recover 8
back to original position. The strain recovery (%) was recorded along with recovery time up to 120 min. The weight loss behaviors of synthesized polyurethanes were monitored by thermogravimetry analysis (TGA) using a TA instrument (Q500) with temperature range from 35 o
C to 650 oC at heating rate of 10 oC/min under nitrogen flow. Differential scanning calorimetry
(DSC) tests were performed on a TA instrument (Q2000) with temperature range from -80 oC to 250 oC at a heating rate of 30 oC/min. Optical transparency of the PU films was measured by a UV-vis spectrometer (Unicam UV 8-700), which was compared at 550 nm as an example. The cytotoxicity of PU elastomer was studied by viability staining of fibroblasts growing directly at the materials. The polymer specimens were prepared by cutting the PU film into circular specimens with a diameter of 1 cm and subsequently sterilized in 70 vol.% ethanol, which was then washed with PBS buffer twice for 30 min each time. The PU elastomer specimens and negative control substrates were cultured with the 3T3 mouse fibroblast cells seeded at an initial density of 25,000 cells/cm2. Cell viability was then estimated from fluorescence micrographs of the cells stained after 4 days of culture with 15 µg/ml fluorescein diacetate (FDA, Sigma-Aldrich, Taufkirchen, Germany), staining viable cells green fluorescent after intracellular esterase catalysed cleavage to fluorescein and acetate, and 1:10,000 diluted GelRedTM stock solution (VWR International, Darmstadt, Germany), staining the nuclei of dead cells orange, Fluorescence micrographs were recorded using a fluorescence microscope Observer.Z1m with camera AxioCam HRc and filter sets 10 and 14 (Carl Zeiss Microscopy, Jena, Germany). Images of 3T3 cells growing on polymer specimen and negative control substrates stained after 4 days of testing with fluorescein diacetate (FDA, staining viable cells green
9
fluorescent after intracellular esterase catalysed cleavage to fluorescein and acetate) and GelRedTM (staining the nuclei of dead cells orange).
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3. Results and discussion The synthesis of PUs is illustrated in Scheme 1 showing the formation of thermoplastic polyurethane which we demonstrate as branched polymer network due to branched structure of castor oil molecules. The addition of castor oil in the very first step together with an excess amount of MDI would result in the formation of branched oligomers with free isocyanate groups. This structure subsequently leads to the thermoplastic polyurethane macromolecules upon the addition of PDMS/PTH polyol mixture and BDM extender in the last step. Camera pictures in Figure S1 show the PU products obtained from precipitating the reaction mixtures into methanol solvent. As seen, the PU control sample show fine polymer particles settling down in the methanol beaker. Interestingly, the castor oil-segmented PU 50/50 sample forms polymer fibers immediately when reaction mixture immersing in the methanol, indicating high viscous solution of the high molecular weight polymer solution. The obtained PUs were soluble in DMAc solvent and formed highly viscous and transparent solutions. The polymer solution was used to prepare PU film with high transparency as seen in Fig. 1b. The formation of polyurethane was proved by IR study and the spectra of synthesized PUs show characteristic peaks of the polyurethane (Fig. 1). It should be noted that the carbonyl peak in the castor oil appears at 1740 cm-1, while this peak of urethane in PU samples is shown at lower value of around 1730 cm-1. The N-H bond in urethane is seen in the 3310 cm-1 region, which is not the case for castor sample. In this region of spectrum, castor show a broader peak and at a higher wavenumber range. The bands at 2854 and 2932 cm-1 are assigned for the C-H asymmetric and symmetric stretching bands in CH2 groups [19-20]. Importantly, the absence of the band in the range of 2260 to 2310 cm-1 indicates the completion of urethane formation. The
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peak at 1645 cm-1 can be assigned to the C=C double bond in the castor oil moieties. In addition, benzene ring in MDI segment can be seen clearly by the peak at 1600 cm-1. The mechanical properties of polyurethanes were characterized using tensile test with pulling rate of 10 mm/min. Young’s modulus, tensile and elongation at break of polyurethanes are shown in Fig. 2a-c, respectively. The low molecular weight of PDMS (Mn 550 g/mol), with high backbone stiffness and hindrance, was used to give stiffness of synthesized polymers; whereas high molecular weight of PTH (Mn 2000 g/mol) was employed to provide ductile of polymers. It can be seen in Table 1, the hard segment concentrations increased with increasing PDMS/PTH ratios. Thus, as seen in Fig. 2a, the Young’s moduli increase with higher PDMS/PTH ratios, which can be explained by higher stiffness and hindrance of PDMS chain backbones compare with PTH as well as higher hard segment concentration. This tendency is also applied for tensile strengths with exception for PU 55/45. The elongation at break of PU 50/50 is highest in the series with a value of around 1200 %. Meanwhile, the PU65/35 shows a lowest strain value of ca. 650% but the highest values of tensile strength and Young’s modulus of 24.4 and 23.7 MPa, respectively. The tensile strength and Young’s modulus of PU65/35 are 1.4 and 4.2 times higher compared with that of the PU50/50. More flexible and ductile elastomers are more suitable for biomedical applications. With regard to these aspects, we consider the PU50/50 as the best recipe in investigated series; however, the decision of choosing ingredient for polyurethane synthesis is based on the target application. PU control sample was also tested for comparison. As can be seen in Fig. 2, Young’s modulus of PU control was higher than any castor oil-segmented PU samples. However, tensile strength and elongation at break of this PU film were much lower. The average value of elongation at break of the PU control was only 445%, which is much lower than the average 12
value of 1230% of PU50/50. Typical stress-strain curves for all PU samples presented in Fig 2d highlights the differences among the PU samples in which PU samples containing CO in the macromolecule structure clearly show much higher tensile strength and elongation at break. Segmented PUs in this study are consisting of two-phase segments, which are soft and hard segments and the properties of PUs are strongly dependent on the chemical structure and the amount of these phases as well as the phase separation in the polymer. The soft segments are defined as the polyols while hard segment is the nonpolyol components such as MDI in this study. Tensile test shows that increasing the amount of PDMS could significantly change the mechanical properties of the obtained PU, e.g., Young’s modulus of PU65/35 is ca. five times higher than that of PU45/55. This could provide a way of controlling the mechanical properties of PU by varying the amount of PDMS polyol in the PU synthesis. In this sense, PDMS with different molecular weights could also affect to mechanical performance of PU containing PDMS, which may be useful to be investigated in the future. The elastic recovery behaviors of the synthesized polyurethanes were investigated by using static creep tests. The polyurethane samples were pulled with a constant stress of 0.25 MPa for 30 min at 30 oC. Then, the stress was released. The strain recoveries of samples were recorded as function of recovery time. The static elastic recovery results of polyurethanes were presented in Fig. 3. All polymers exhibited very fast recovery with about 80% recovered strain within 2 min, while the PU control only showed around 60% of strain. Moreover, maximum recoveries of all polymers were higher than 90% recovered strain, which is much higher than the value demonstrated by the PU control having the value of only 70%. On the molecular level of polyurethane structure, the macromolecules are composed of rigid regions that are joined together by soft, flexible segments. The flexible regions allow the polymer to expand and then 13
recover its original shape when the stress is released and at the same time the rigid regions strengthen the polymeric structure. The excellent recovery of the PUs in most cases is due to the chemical and physical cross-linking; however, in this case, it is clear that recovery by chemical crosslinking is not possible because the PUs are well soluble in the organic solvent and thereby being thermoplastic PUs. Therefore, the high recovery of the PUs can be explained by physical crosslinking including strong hydrogen bonding between macromolecules and hard domains that acts as physical crosslinks. The DMA method is considered as the most effective method for measuring the glass transition temperature (Tg) as well as microphase morphology of synthesized polyurethanes. DMA experiments measure the ability of a viscoelastic material to store and dissipate mechanical energy. In principle, the complex Young modulus was measured as the sum of storage modulus (E’) and loss modulus (E’’). The storage modulus E’ quantifies the energy stored elastically by the materials upon deformation, which can provide information regarding the stiffness of the material; whereas the loss modulus E’’ is a measure of the energy which is dissipated as heat during deformation. The loss factor (tan δ) is calculated from the ratio between the loss and storage moduli, illustrating the degree of molecular motion. The DMA analysis results of castor oil-based segmented thermoplastic PUs are presented in Fig. 4, and compared with that of PU control containing no castor oil segment. In Fig. 4a, the storage modulus (E´) of PU increases with increasing of PDMS/PTH molar ratio, especially in temperature range lower than Tg. At low temperature of -75 oC the PUs are in glassy state and thus the moduli are significantly high. At this condition, E’ value of PU65/35 was measured to be 2476 MPa, meanwhile the values for PU45/55, PU50/50, PU55/45 were 1900, 1936, and 2190 MPa, respectively. When temperature was increased to 25 oC, which is well above the Tg, the 14
PUs chains became flexible and thus demonstrating much lower storage modulus as in the case of PU45/55 wherein the E´ is only 6 MPa. The E’ of PU65/35 at this condition was 50 MPa, which is much higher than the E´ of PU45/55, demonstrating higher rigidity of PU65/35 chains. This can be explained by the higher rigidity of PDMS chains in comparison with PTH chains. As a result, the PU with more PDMS in the chain would show higher storage modulus, which is also seen in tensile Young’s modulus. As seen, PU control sample show different behavior compared to the castor oilsegmented PUs in the E’ curve, which is due to the linear structure of PU control in comparison with the branched network of PU series. It shows higher E’ than that of PU65/35 in a temperature ranging between -24 oC and 11 oC, and lower E’ in the rest of the curve. At -75 oC and 25 oC E’ values of PU control was 2356 MPa and 35.13 MPa, respectively, which are lower than that of PU65/35, but higher than any other PU samples. It shows a sharp drop in E’ at temperature of 73 oC, which is not the case for PU series. In DMA method, glass transition temperature (Tg) of the soft phase, is defined as the peak temperature point in the tanδ versus temperature plot. Moreover, the sharpness of this peak was used to evaluate the microphase morphology of polymers. All polymers showed a single tanδ peaks with similar sharpness imply that they have no significant difference in phase separation. Thus, the effects of PDMS/PTH molar ratios on properties of polymers could be attributed to the difference in rigidity and hindrance of PDMS and PTH backbones. The glass transition temperatures were not significant different for the first three polymers in series, which are -37.8 o
C, -37.7 oC, and -38.5 oC for PU 45/55, PU 50/50, and PU 55/45, respectively. With highest
PDMS/PTH molar ratio in PU 65/35, however, the glass transition temperature was around -24.5 o
C, indicating about 14 oC increase in Tg in comparison with other polymers in the series. As
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seen in PU control sample, the peak in tanδ shows at -54 oC, which is much smaller than castor oil-segmented PU series. The higher in Tg of the PU series compared to that of PU control may be explained that the branched network of castor oil-based segmented thermoplastic PUs that restricts the mobility of the soft phase but gives higher recovery properties. The rigidity difference and building order of blocks in polymer chains respond for broad tanδ peaks of all polymers, which agree well with results from DSC result (see Fig. S2, Supporting Information). In Fig. 4c, the shift of Tg of PUs to higher temperature implies that the segmental mobility decreases with the increasing PDMS/PTH ratio, which is in good agreement with the evidence in E’ (Fig. 4a). The thermal stability of polyurethanes was studied by TGA under a nitrogen atmosphere at a heating rate of 10 oC/min. Fig. 5 shows weight loss and derivation of weight loss of polymers as function of temperature. The multiple degradation steps were observed for all polymers, which can be explained by the different thermal stabilities of hard and soft segments in polymer macromolecules. The thermal stabilities of polymers were selected at temperature of 5% weight loss (T5%) and maximum rate decomposition temperatures, corresponding to degradations of hard and soft segments (Tmax1 and Tmax2). Moreover, peaks of maximum rate decomposition temperatures, Tmax1 and Tmax2, were in range of 330 - 333 oC and 399 - 402 oC, respectively. In temperature ranging between around 300 oC and 400 oC, PU 65/35 showed the lowest thermal stability and then PU 55/45. However, initial weight loss, which is T5%, was similar for all PUs, suggesting that thermal degradation starts at the urethane bond which is known to be relatively unstable at temperature starting from 150 oC [21]. In the spectra, the T5% values of polymers were determined to be in a narrow range of 294-299 oC, and this is due to the similarity in the
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chemical structure of macromolecules. Generally speaking, thermal stability of polymers was not significantly affected by changing relative molar ratios of PDMS and PTH in investigated range. The PU control was also tested and demonstrated the T5% at 314 oC, exhibiting a clear different in thermal stability, which again confirms that the polymer structure affect the stability of the urethane linkage to some extent. In addition the Tmax1 and Tmax2 are determined to be 343 o
C and 414 oC, respectively, which are also higher than that of PU series. In this regard, the
lower stability of PU series can be due to the castor oil segment. Nevertheless, the difference in thermal stabilities as we discussed herein among the PU samples is not significant in many applications. We used GPC to measure molecular weight of the PUs, in which DMF was solvent and 10 polystyrene standards were used as reference. We used Gel permeation chromatography (GPC) to measure molecular weight and polydispersity of the castor oil-segmented PUs and PU control. In this analysis, we used DMF as solvent and polystyrene standards for calibration. The result is summarized in Table S2 (Supplementary Material). As can be seen, all castor oilsegmented PU samples demonstrate much higher molecular weight compared with that of the PU control, and at the same time, they show much broader polydispersity. The PU 65/35 shows the highest value of molecular weight and the smallest polydispersity of 4.84 among the castor oilsegmentd PU series. These values explain why it show the highest tensile strength together with highest Young’s modulus, highest storage modulus in the DMA test, and the best recovery performance. In addition, the high molecular weight of the PU series can also be used to explain the exceptionally high elongation at break values. Nevertheless, the broad polydispersity of PU series is understandable, which could be explained by highly complexity in structure of PU series.
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The in vitro cytocompatibility of developed elastomer was evaluated by cytotoxicity testing. The polyurethane elastomer substrates were cultured with the 3T3 mouse fibroblast cell line, which is commonly used to assess cytotoxicity of potential substrates for cell growth. Figure 6 shows fluorescence microscopy images of fluorescein diacetate/ GelRed TM-stained 3T3 cells growing on PU 50/50 elastomer and glass control substrates after 4 days of cell culture. The elastomer substrate showed excellent cell adhesion and proliferation of 3T3 cells, suggesting that the elastomer is a biocompatible material and could be applied in biomedical applications. 4. Conclusion A series of castor oil-segmented thermoplastic polyurethanes were synthesized in this study. The developed PUs have demonstrated high elongation at break, excellent elastic recovery, and good mechanical/thermal characteristics. The mechanical properties of the PUs can be adjusted by varying the PDMS/PTH ratio to obtain PU with better mechanical strength/Young’s modulus or higher elongation at break/flexibility. The cytocompatibility test shows that the PU series could be potentially used in biomaterial applications. Acknowledgements We gratefully acknowledge the financial support from Academy of Finland (grants no. 12137759 and 13272725) and the European Union 7th Framework Programme (ArtiVasc 3D project, GA no 263416).
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References [1] Michael Szycher, Szycher’s Handbook of Polyurethanes, Second Edition 2013, CRC Press. [2] Pfister DP, Xia Y, Larock RC. Recent Advances in Vegetable Oil-Based Polyurethanes. ChemSusChem 2011;4(6):703-17. [3] Desroches M, Escouvois M, Auvergne R, Caillol S, Boutevin B. From Vegetable Oils to Polyurethanes: Synthetic Routes to Polyols and Main Industrial Products. Polym Rev 2012;52:38-79. [4] Petrović ZS. Polyurethanes from Vegetable Oils. Polym Rev 2008;48:109-55. [5] Hablot E, Zheng D, Bouquey M, Avérous L. Polyurethanes Based on Castor Oil: Kinetics, Chemical, Mechanical and Thermal Properties. Macromol Mater Eng 2008;293(11):922-9. [6] Ogunniyi DS. Castor Oil: A Vital Industrial Raw Material. Bioresour Technol 2006;97:108691. [7] Ionescu M. Chemistry and Technology of Polyols for Polyurethanes, 2005, Rapra Technology Limited. [8] Cayli G, Kusefoglu S. A Simple One-Step Synthesis and Polymerization of Plant Oil Triglyceride Iodo Isocyanates. J Appl Polym Sci 2010;116(4):2433–40. [9] Stirna U, Lazdina B, Vilsone D, Lopez MJ, del Vargas-Garcia Carmen M, Suárez-Estrella F, Moreno J. Structure and Properties of the Polyurethane and Polyurethane Foam Synthesized from Castor Oil Polyols. J Cell Plast 2012;48:476-88. [10] Yeganeh H, Mehdizadeh MR. Synthesis and properties of isocyanate curable millable polyurethane elastomers based on castor oil as a renewable resource polyol. Eur Polym J 2004;40(6); 1233-8. [11] Corcuera MA, Rueda L, Fernandez d’Arlas B, Arbelaiz A, Marieta C, Mondragon I, Eceiza A. Microstructure and properties of polyurethanes derived from castor oil. Polym Degrad Stabil 2010;95(11):2175-84. [12] Kim H, Abdala AA, Macosko CW. Graphene/polymer nanocomposites. Macromolecules, 2010;43(16):6515-30. [13] Saralegi A, Gonzalez ML, Valea A, Eceiza A, Corcuera MA. Compos Sci Technol 2014;92:27–33. [14] Saralegi A, Fernandes SCM, Alonso-Varona A, Palomares T, Foster EJ, Weder C, Eceiza A, Corcuera MA. Biomacromolecules 2013;14:4475−82. 19
[15] Rueda L, Saralegi A, Fernandez-d Arlas B, Zhou Q, Alonso-Varona A, Berglund LA, Mondragon I, Corcuera, Eceiza A. Cellulose 2013;20:1819–28. [16] Stanford JL, Still RH, Cawse JL, Donnelly MJ. Polyurethanes from Renewable Resources. In ACS Symposium Series; chapter 30, pp. 424-42. [17] Ma R. Study of the main influence factors on compression set of PTMG based polyurethane elastomer. Adv Mat Res 2013;630:67-70. [18] de Jong E, Higson A, Walsh P, Wellisch M. Product developments in the bio-based chemicals arena. Biofuels, Bioprod Biorefin 2012;6(6):606-24. [19] Ibrahim S, Ahmad A, Mohamed NS. Characterization of novel castor oil-based polyurethane polymer electrolytes. Polymers 2015;7(4):747-59. [20] Rana S, Cho JW, Tan LP. Graphene-crosslinked polyurethane block copolymer nanocomposites with enhanced mechanical, electrical, and shape memory properties. RSC Adv 2013;3:13796-803. [21] Javni I, Petrovic ZS, Guo A, Fuller R. Thermal stability of polyurethanes based on vegetable oils. J Appl Polym Sci 2000;77(28):1723-34.
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Table 1. Summary of recipe for polyurethane syntheses. Diisocyanate Samples
Polyols
(NCO mol)
Diols
(OH mol)
(OH mol)
Bio-based
Hard segment
(a)
concentration
contents (wt.%)
(b)
(%)
MDI
CA
PDMS
PTH
BDM
PU control
2.0
0
0.7
0.7
0.6
0
21
PU45/55
2.0
0.4
0.45
0.55
0.6
12.2
27
PU50/50
2.0
0.4
0.50
0.50
0.6
12.6
28
PU55/45
2.0
0.4
0.55
0.45
0.6
13.1
29
PU65/35
2.0
0.4
0.65
0.35
0.6
14.1
31
(a)
Bio-based contents (%) = (mass of castor oil x 100)/ total mass of monomers.
(b)
Hard segment concentration is defined as the ratio of the mass of the non-polyol components to the
total mass of the PU polymer.
21
Scheme 1. Synthesis route of castor oil-based elastomer (a) and a PU 50/50 film with diameter of 12 cm and a thickness of around 0.2 mm prepared by solvent casting (b).
22
Fig. 1. IR spectra of castor oil, PU control, and the synthesized castor oil-segmented PU series showing their characteristic bands.
23
Fig. 2. Mechanical properties of polyurethanes: tensile stress (a), Young’s modulus (b), and elongation at break (c), and typical stress-strain curves of the PU samples (d).
24
Fig. 3. Elastic recovery properties of polyurethanes.
25
Fig. 4. Overlay plots of the storage modulus (E’) and the loss factor (tanδ) of polyurethanes.
26
Fig. 5. TGA thermograms of polyurethanes (a) and their differential curves (DTG) (b).
27
Fig. 6. Fluorescence micrographs of fluorescein diacetate/GelRedTM -stained 3T3 fibroblast cells growing on polyurethane elastomer and control substrates after 4 days of cell culture. PU50/50 was used as an example in this test.
28
Highlights
A series of castor oil-segmented thermoplastic polyurethanes is developed.
The polyurethanes show good mechanical properties and high optical transparency.
Good cytocompatibility of the polyurethane is evaluated.
29
Table of Content (TOC)
30
S0014-3057(16)30491-8 http://dx.doi.org/10.1016/j.eurpolymj.2016.05.024 EPJ 7371
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
17 March 2016 23 May 2016 27 May 2016
Please cite this article as: Luong, N.D., Sinh, L.H., Minna, M., Jürgen, W., Torsten, W., Matthias, S., Jukka, S., Synthesis and characterization of castor oil-segmented thermoplastic polyurethane with controlled mechanical properties, European Polymer Journal (2016), doi: http://dx.doi.org/10.1016/j.eurpolymj.2016.05.024
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and characterization of castor oil-segmented thermoplastic polyurethane with controlled mechanical properties Nguyen Dang Luong1,+, Le Hoang Sinh1,2+, Malin Minna1, Weisser Jürgen3, Walter Torsten3, Schnabelrauch Matthias3, Seppälä Jukka 1,* 1
Polymer Technology Research Group, Department of Biotechnology and Chemical Technology,
Aalto University, School of Chemical Technology, P.O. Box 16100, FI-00076 Aalto, Finland 2
Center for Advanced Chemistry, Institute of Research and Development, Duy Tan University,
Da Nang, Vietnam 3
INNOVENT e.V., Biomaterials Department, Pruessingstrasse 27B, D-07745 Jena, Germany
+
The authors are equally distributed
*Corresponding author: Tel: + 358 400 701 142 Fax: +358 9 451 2622. Email: [email protected]
1
Abstract Novel castor oil-segmented thermoplastic polyurethane series (PUs) has been prepared and extensively characterized in this study. The castor oil, poly(tetrahydrofuran) polyol, and poly(dimethylsiloxane) have been employed as soft segments in the polymer synthesis with a maximum bio-based content up to 14.1 wt%. We demonstrate here that it is possible to prepare thermoplastic polyurethane containing castor oil via polyaddition approach in which the formation of prepolymer needs to be carried out with the addition of reasonably small amount of castor oil together with excess amount of the diisocyanate in the very first step. This is followed by the addition of the other polyols and finally a low molecular weight chain extender. The developed synthesis resulted in the formation of branched thermoplastic polyurethane with an elongation at break of about 1200%, which is considerably high compared to reported values for polyurethanes containing castor oil as soft segments. We have demonstrated that mechanical properties and flexibility of polyurethane can be significantly altered by both incorporated castor oil and controllable poly(dimethylsiloxane)/poly(tetrahydrofuran) polyol ratio during the synthesis. Especially the synthesized polyurethanes exhibited good biocompatibility and high transparency, which are useful properties for polymers as potential biomaterials. In addition, thermal stability and thermal transition of polyurethanes have been investigated.
Keywords: castor oil; poly(dimethylsiloxane); poly(tetrahydrofuran); thermoplastic polyurethane; mechanical properties; cytocompatibility
2
1. Introduction Polyurethane elastomers are classified as one of the most useful subclasses of thermoplastic elastomers, which have been widely used as biomaterials in biomedical applications thanks to their superior mechanical properties and good biocompatibility. For example, PUs have been used as biomaterials in manufacture of cardiac-assist pumps, blood bags, and chronic implants such as heart valves and vascular grafts [1]. In addition to biomaterial applications, PU polymers in general have been used commonly in manufacture of automotive parts, in building and construction, electronics, paints, etc. Wide applicability of PUs is due to their unique properties such as high hardness, high strength and high elongation at break, which can be modified by variety of choices of starting monomers, catalysts, and other reaction conditions. It should be addressed that polyurethane elastomers have mainly been synthesized from fossil fuel based polyols and isocyanates. In general, the polymeric chains are composed of alternating hard isocyanate segments and soft polyol segments, and thus the mechanical, thermal, and adhesive properties of PUs are dependent on the composition and chemical structure of the hard and soft segments. In recent years, option to use of renewable resources for chemical synthesis has become extremely urgent, because exhaustion of petroleum is predictable in near future [2]. Moreover, the use of renewable resources results in reduction of environmental impact with greenhouse gas emission. In this regard, vegetable oils are potential replacement for petroleum as raw materials in polymeric synthesis because they are abundant in various plants and can be extracted by simple/facile methods [3]. In particular, vegetable oils have been becoming increasingly important as renewable resources for the preparation of polyols used in polyurethane industry.
3
However, it is noted that most of vegetable oils must be chemically treated to introduce hydroxyl groups that are required in polyurethane synthesis [4]. Fortunately, castor oil naturally contains hydroxyl in its structure and thus it can be directly utilized as polyol in polyurethane synthesis. Furthermore, since castor oil is not edible it could be substituted in the areas where edible oils can be used. These are huge advantages of castor oil compared to other vegetable oils, such as olive oil, soybean oil, and sunflower oil [5-6]. About 90% composition of castor oil is ricinoleic acid with one hydroxyl group on the every 12th carbon and a double bond between the 9th and 10th carbon. This well-defined chemical structure containing available hydroxyl groups enable castor oil to be used as polyols in polyurethane synthesis. The obtained PUs have been used in many applications including coatings, elastomers, thermoplastics, rigid foams, adhesives, etc. [7]. The castor oil-based polyurethane elastomers prepared by one-spot approach in bulk polymerization process have been reported in several studies [5, 8-9]. Unfortunately, the resulted elastomers were insoluble in common organic solvents leading to the fact that toxic catalyst and un-reacted starting materials cannot be removed easily from the crosslinked elastomers [2]. This is due to fact that the hydroxyl functionality of castor oil is 2.7 on average, which could lead to the formation of threedimensional network of polymer structure during polymerization. Several attempts have been emphasized on synthesis of soluble castor-oil based polyurethanes by solution polymerization methods. However, these polyurethanes had quite low mechanical characteristics, especially in elongation at break, in comparison with the commercial polyurethanes utilizing petroleum-based polyols [10-11]. It is worth mentioning that mechanical properties of many thermoplastic PUs need to be improved to fulfil the needs in engineering applications. With this regard, significant advances in 4
polymer nanocomposites have shown that incorporation of nanofillers such as clay, carbon nanotube, and graphene can improve mechanical properties of PUs, especially their Young’s modulus [12]. However, the presence of these fillers in PUs may impair the biocompatibility and pose a risk of releasing the harmful particulate fillers during their uses in biomedical areas. With this regard, many other bio-based materials such as cellulose nanocrystals (CNCs) or chitin nanocrystals have been used as reinforcements in thermoplastic PU. For examples, thermoplastic castor oil-based segmented PU polymer reinforced with these nanocrystals have been synthesized and demonstrated good shape memory properties [13-14]. The improvement in shape memory performance is due to increased crystallinity of the hard segment in the PU. However, it is noted in these studies that a difunctional polyester derived from castor oil was used instead of original castor oil as we used. In another study, it is reported that the addition of CNCs with less than 0.25 wt% increase strength and ductility of the PU composites [15]. In addition, the PU/CNC composite displayed no toxicity via short-term cytotoxicity test. The reason for the improvement in mechanical properties is also originated from the crystallization of both soft and hard segment. Differently, in this study we showed that incorporation of polydimethylsiloxane polyol (PDMS) in a controlled manner as one of the segments in PU macromolecule could significantly alter the mechanical characteristics of the PU. In this report, we use castor oil, PDMS, and poly(tetrahydrofuran) (PTH) as polyols and 4,4′-methylenebis(phenyl isocyanate) (MDI) as diisocyanate and finally 1,4-benzene dimethanol (BDO) as chain extender. It is worthy noticing that PTH can be also bio-based material [16-17]. Biobased poly(tetrahydrofuran) is produced by polymerization of tetrahydrofuran, which is product from fermentation of biorenewable succinate resources [18]. As a demonstration, the synthesized PUs with good biocompatibility, 5
excellent mechanical strength and high transparency have been achieved by the developed approach. It should also be noticed that the PUs contain bio-based materials in this study ranging from 12.2 wt% to 14.1 wt% in which only castor oil is considered as the bio-based material. The promising results obtained by cytocompatibility suggest that the PU synthesized in this study could be considered for biomedical applications. The key to obtain thermoplastic polyurethane in this study was to grow PU macromolecules in a well-controlled manner and to prevent the formation of three-dimensional network structure, which is usually obtained when castor is used in PU synthesis. Additionally, we have demonstrated here that varying the PDMS/PTH polyol ratio can change the mechanical behaviour of the resulted PU significantly. The developed PU and the synthesis approach could be useful in the development of thermoplastic polyurethanes and their utilizations as polymeric biomaterials.
6
2. Experimental 2.1. Materials Refined castor oil (CO, hydroxyl number of 2.7), poly(tetrahydrofuran) (PTH, Mn 2000 g/mol), hydroxyl terminated poly(dimethylsiloxane) (PDMS, Mn 550 g/mol), 4,4′-Methylenebis(phenyl isocyanate)
(MDI,
98%),
dibutyltin
dilaurate
(DBTDL,
95%),
anhydrous
N,
N-
dimethylacetamide (DMAc, 99.8%), N,N-Dimethylformamide (DMF, for HPLC, ≥ 99.9%), and methanol were purchased from Sigma-Aldrich. 1,4-benzene dimethanol (BDM, 99%) was obtained from Acros Organics. All chemicals were used as received. 2.2. Synthesis of polyurethane series The recipes for polyurethane series are presented in Table 1. Firstly, castor oil (0.4 mol of OH) and DBTDL (catalyst) was weighted and dissolved in DMAc solvent, which was then fed in to three-neck round bottom flask reactor equipped with a magnetic stirrer and a condenser. An amount of MDI (2.0 mol of NCO) was dissolved in DMAc and injected via syringe into the reactor. Nitrogen bubbling was carried for 30 min to introduce inert environment. The reaction mixture was then heated to 60 oC and kept at this condition for 2 h. A mixture of two polyols (1.0 mol of OH), PTH and PDMS chemicals, was dissolved in DMAc and injected slowly into the reactor. The molar ratios of PDMS/PTH were 45/55, 50/50, 55/45, and 65/35 for each sample. The reaction was then conducted for 2 h in this step. Subsequently, a solution of BDM as chain extender (0.6 mol of OH) in DMAc was injected to the reaction and kept for 15 h. The reaction mixture was cooled down to room temperature, and the polymer was recovered from the reaction mixture by coagulating in methanol with vigorous mixing. The precipitate was collected by filtering and washed several times with methanol to remove impurities. Finally, the product is dried thoroughly in vacuum oven at 60 oC for 24 h.
7
For comparison purpose, a PU containing no CO was synthesized, which is denoted as PU control. In the synthesis of the PU control, MDI solution was added in the first step, which was then followed by the addition of PDMS/PTH mixture, and finally the BDM as chain extender. The PDMS/PTH was fixed at 50/50 by weight percent. 2.3. Preparation of polyurethane film After being dried, polyurethane product was dissolved in DMAc at 60 oC to obtain a 10 wt% polymer solution. The resulting polymer solution was poured into glass Petri dish and solvent removed in a vacuum oven at 60 oC. Finally, the transparent polyurethane film was peeled off from glass surface and used for polymer characterizations. The amount of PU solution was calculated to obtain polymer films with thicknesses of about 0.2 mm. 2.4. Characterizations Infrared (IR) spectroscopy was used to study the chemical structure of polyurethanes using a FTIR-ATR-MATTSON 3000 spectrometer. Molecular weight and polydispersity of the polymers were determined by Gel permeation chromatography (GPC) using a Waters 717 plus Autosampler. N,N-dimethylformamide (DMF) was used as the solvent. The calibration curve was constructed using ten polystyrene standards, which run under the same condition. Mechanical properties of casted PU films were investigated on Instron 4204 universal testing equipment at 25 oC and relative humidity of 50%, with speed of 10 mm/min. Five specimens were tested for each sample in the tensile test. Dynamic mechanical analyses (DMA) were carried out by using the TA instrument (Q800) from -80 oC to 100 oC at a heating rate of 5 o
C/min and frequency of 1 Hz. The static elastic recovery properties were evaluated by using
creep mode on the DMA system (Q800). In particular, the samples were applied with a constant stress of 0.25 MPa for 30 min at 30 oC. Then, the force was released to let the samples recover 8
back to original position. The strain recovery (%) was recorded along with recovery time up to 120 min. The weight loss behaviors of synthesized polyurethanes were monitored by thermogravimetry analysis (TGA) using a TA instrument (Q500) with temperature range from 35 o
C to 650 oC at heating rate of 10 oC/min under nitrogen flow. Differential scanning calorimetry
(DSC) tests were performed on a TA instrument (Q2000) with temperature range from -80 oC to 250 oC at a heating rate of 30 oC/min. Optical transparency of the PU films was measured by a UV-vis spectrometer (Unicam UV 8-700), which was compared at 550 nm as an example. The cytotoxicity of PU elastomer was studied by viability staining of fibroblasts growing directly at the materials. The polymer specimens were prepared by cutting the PU film into circular specimens with a diameter of 1 cm and subsequently sterilized in 70 vol.% ethanol, which was then washed with PBS buffer twice for 30 min each time. The PU elastomer specimens and negative control substrates were cultured with the 3T3 mouse fibroblast cells seeded at an initial density of 25,000 cells/cm2. Cell viability was then estimated from fluorescence micrographs of the cells stained after 4 days of culture with 15 µg/ml fluorescein diacetate (FDA, Sigma-Aldrich, Taufkirchen, Germany), staining viable cells green fluorescent after intracellular esterase catalysed cleavage to fluorescein and acetate, and 1:10,000 diluted GelRedTM stock solution (VWR International, Darmstadt, Germany), staining the nuclei of dead cells orange, Fluorescence micrographs were recorded using a fluorescence microscope Observer.Z1m with camera AxioCam HRc and filter sets 10 and 14 (Carl Zeiss Microscopy, Jena, Germany). Images of 3T3 cells growing on polymer specimen and negative control substrates stained after 4 days of testing with fluorescein diacetate (FDA, staining viable cells green
9
fluorescent after intracellular esterase catalysed cleavage to fluorescein and acetate) and GelRedTM (staining the nuclei of dead cells orange).
10
3. Results and discussion The synthesis of PUs is illustrated in Scheme 1 showing the formation of thermoplastic polyurethane which we demonstrate as branched polymer network due to branched structure of castor oil molecules. The addition of castor oil in the very first step together with an excess amount of MDI would result in the formation of branched oligomers with free isocyanate groups. This structure subsequently leads to the thermoplastic polyurethane macromolecules upon the addition of PDMS/PTH polyol mixture and BDM extender in the last step. Camera pictures in Figure S1 show the PU products obtained from precipitating the reaction mixtures into methanol solvent. As seen, the PU control sample show fine polymer particles settling down in the methanol beaker. Interestingly, the castor oil-segmented PU 50/50 sample forms polymer fibers immediately when reaction mixture immersing in the methanol, indicating high viscous solution of the high molecular weight polymer solution. The obtained PUs were soluble in DMAc solvent and formed highly viscous and transparent solutions. The polymer solution was used to prepare PU film with high transparency as seen in Fig. 1b. The formation of polyurethane was proved by IR study and the spectra of synthesized PUs show characteristic peaks of the polyurethane (Fig. 1). It should be noted that the carbonyl peak in the castor oil appears at 1740 cm-1, while this peak of urethane in PU samples is shown at lower value of around 1730 cm-1. The N-H bond in urethane is seen in the 3310 cm-1 region, which is not the case for castor sample. In this region of spectrum, castor show a broader peak and at a higher wavenumber range. The bands at 2854 and 2932 cm-1 are assigned for the C-H asymmetric and symmetric stretching bands in CH2 groups [19-20]. Importantly, the absence of the band in the range of 2260 to 2310 cm-1 indicates the completion of urethane formation. The
11
peak at 1645 cm-1 can be assigned to the C=C double bond in the castor oil moieties. In addition, benzene ring in MDI segment can be seen clearly by the peak at 1600 cm-1. The mechanical properties of polyurethanes were characterized using tensile test with pulling rate of 10 mm/min. Young’s modulus, tensile and elongation at break of polyurethanes are shown in Fig. 2a-c, respectively. The low molecular weight of PDMS (Mn 550 g/mol), with high backbone stiffness and hindrance, was used to give stiffness of synthesized polymers; whereas high molecular weight of PTH (Mn 2000 g/mol) was employed to provide ductile of polymers. It can be seen in Table 1, the hard segment concentrations increased with increasing PDMS/PTH ratios. Thus, as seen in Fig. 2a, the Young’s moduli increase with higher PDMS/PTH ratios, which can be explained by higher stiffness and hindrance of PDMS chain backbones compare with PTH as well as higher hard segment concentration. This tendency is also applied for tensile strengths with exception for PU 55/45. The elongation at break of PU 50/50 is highest in the series with a value of around 1200 %. Meanwhile, the PU65/35 shows a lowest strain value of ca. 650% but the highest values of tensile strength and Young’s modulus of 24.4 and 23.7 MPa, respectively. The tensile strength and Young’s modulus of PU65/35 are 1.4 and 4.2 times higher compared with that of the PU50/50. More flexible and ductile elastomers are more suitable for biomedical applications. With regard to these aspects, we consider the PU50/50 as the best recipe in investigated series; however, the decision of choosing ingredient for polyurethane synthesis is based on the target application. PU control sample was also tested for comparison. As can be seen in Fig. 2, Young’s modulus of PU control was higher than any castor oil-segmented PU samples. However, tensile strength and elongation at break of this PU film were much lower. The average value of elongation at break of the PU control was only 445%, which is much lower than the average 12
value of 1230% of PU50/50. Typical stress-strain curves for all PU samples presented in Fig 2d highlights the differences among the PU samples in which PU samples containing CO in the macromolecule structure clearly show much higher tensile strength and elongation at break. Segmented PUs in this study are consisting of two-phase segments, which are soft and hard segments and the properties of PUs are strongly dependent on the chemical structure and the amount of these phases as well as the phase separation in the polymer. The soft segments are defined as the polyols while hard segment is the nonpolyol components such as MDI in this study. Tensile test shows that increasing the amount of PDMS could significantly change the mechanical properties of the obtained PU, e.g., Young’s modulus of PU65/35 is ca. five times higher than that of PU45/55. This could provide a way of controlling the mechanical properties of PU by varying the amount of PDMS polyol in the PU synthesis. In this sense, PDMS with different molecular weights could also affect to mechanical performance of PU containing PDMS, which may be useful to be investigated in the future. The elastic recovery behaviors of the synthesized polyurethanes were investigated by using static creep tests. The polyurethane samples were pulled with a constant stress of 0.25 MPa for 30 min at 30 oC. Then, the stress was released. The strain recoveries of samples were recorded as function of recovery time. The static elastic recovery results of polyurethanes were presented in Fig. 3. All polymers exhibited very fast recovery with about 80% recovered strain within 2 min, while the PU control only showed around 60% of strain. Moreover, maximum recoveries of all polymers were higher than 90% recovered strain, which is much higher than the value demonstrated by the PU control having the value of only 70%. On the molecular level of polyurethane structure, the macromolecules are composed of rigid regions that are joined together by soft, flexible segments. The flexible regions allow the polymer to expand and then 13
recover its original shape when the stress is released and at the same time the rigid regions strengthen the polymeric structure. The excellent recovery of the PUs in most cases is due to the chemical and physical cross-linking; however, in this case, it is clear that recovery by chemical crosslinking is not possible because the PUs are well soluble in the organic solvent and thereby being thermoplastic PUs. Therefore, the high recovery of the PUs can be explained by physical crosslinking including strong hydrogen bonding between macromolecules and hard domains that acts as physical crosslinks. The DMA method is considered as the most effective method for measuring the glass transition temperature (Tg) as well as microphase morphology of synthesized polyurethanes. DMA experiments measure the ability of a viscoelastic material to store and dissipate mechanical energy. In principle, the complex Young modulus was measured as the sum of storage modulus (E’) and loss modulus (E’’). The storage modulus E’ quantifies the energy stored elastically by the materials upon deformation, which can provide information regarding the stiffness of the material; whereas the loss modulus E’’ is a measure of the energy which is dissipated as heat during deformation. The loss factor (tan δ) is calculated from the ratio between the loss and storage moduli, illustrating the degree of molecular motion. The DMA analysis results of castor oil-based segmented thermoplastic PUs are presented in Fig. 4, and compared with that of PU control containing no castor oil segment. In Fig. 4a, the storage modulus (E´) of PU increases with increasing of PDMS/PTH molar ratio, especially in temperature range lower than Tg. At low temperature of -75 oC the PUs are in glassy state and thus the moduli are significantly high. At this condition, E’ value of PU65/35 was measured to be 2476 MPa, meanwhile the values for PU45/55, PU50/50, PU55/45 were 1900, 1936, and 2190 MPa, respectively. When temperature was increased to 25 oC, which is well above the Tg, the 14
PUs chains became flexible and thus demonstrating much lower storage modulus as in the case of PU45/55 wherein the E´ is only 6 MPa. The E’ of PU65/35 at this condition was 50 MPa, which is much higher than the E´ of PU45/55, demonstrating higher rigidity of PU65/35 chains. This can be explained by the higher rigidity of PDMS chains in comparison with PTH chains. As a result, the PU with more PDMS in the chain would show higher storage modulus, which is also seen in tensile Young’s modulus. As seen, PU control sample show different behavior compared to the castor oilsegmented PUs in the E’ curve, which is due to the linear structure of PU control in comparison with the branched network of PU series. It shows higher E’ than that of PU65/35 in a temperature ranging between -24 oC and 11 oC, and lower E’ in the rest of the curve. At -75 oC and 25 oC E’ values of PU control was 2356 MPa and 35.13 MPa, respectively, which are lower than that of PU65/35, but higher than any other PU samples. It shows a sharp drop in E’ at temperature of 73 oC, which is not the case for PU series. In DMA method, glass transition temperature (Tg) of the soft phase, is defined as the peak temperature point in the tanδ versus temperature plot. Moreover, the sharpness of this peak was used to evaluate the microphase morphology of polymers. All polymers showed a single tanδ peaks with similar sharpness imply that they have no significant difference in phase separation. Thus, the effects of PDMS/PTH molar ratios on properties of polymers could be attributed to the difference in rigidity and hindrance of PDMS and PTH backbones. The glass transition temperatures were not significant different for the first three polymers in series, which are -37.8 o
C, -37.7 oC, and -38.5 oC for PU 45/55, PU 50/50, and PU 55/45, respectively. With highest
PDMS/PTH molar ratio in PU 65/35, however, the glass transition temperature was around -24.5 o
C, indicating about 14 oC increase in Tg in comparison with other polymers in the series. As
15
seen in PU control sample, the peak in tanδ shows at -54 oC, which is much smaller than castor oil-segmented PU series. The higher in Tg of the PU series compared to that of PU control may be explained that the branched network of castor oil-based segmented thermoplastic PUs that restricts the mobility of the soft phase but gives higher recovery properties. The rigidity difference and building order of blocks in polymer chains respond for broad tanδ peaks of all polymers, which agree well with results from DSC result (see Fig. S2, Supporting Information). In Fig. 4c, the shift of Tg of PUs to higher temperature implies that the segmental mobility decreases with the increasing PDMS/PTH ratio, which is in good agreement with the evidence in E’ (Fig. 4a). The thermal stability of polyurethanes was studied by TGA under a nitrogen atmosphere at a heating rate of 10 oC/min. Fig. 5 shows weight loss and derivation of weight loss of polymers as function of temperature. The multiple degradation steps were observed for all polymers, which can be explained by the different thermal stabilities of hard and soft segments in polymer macromolecules. The thermal stabilities of polymers were selected at temperature of 5% weight loss (T5%) and maximum rate decomposition temperatures, corresponding to degradations of hard and soft segments (Tmax1 and Tmax2). Moreover, peaks of maximum rate decomposition temperatures, Tmax1 and Tmax2, were in range of 330 - 333 oC and 399 - 402 oC, respectively. In temperature ranging between around 300 oC and 400 oC, PU 65/35 showed the lowest thermal stability and then PU 55/45. However, initial weight loss, which is T5%, was similar for all PUs, suggesting that thermal degradation starts at the urethane bond which is known to be relatively unstable at temperature starting from 150 oC [21]. In the spectra, the T5% values of polymers were determined to be in a narrow range of 294-299 oC, and this is due to the similarity in the
16
chemical structure of macromolecules. Generally speaking, thermal stability of polymers was not significantly affected by changing relative molar ratios of PDMS and PTH in investigated range. The PU control was also tested and demonstrated the T5% at 314 oC, exhibiting a clear different in thermal stability, which again confirms that the polymer structure affect the stability of the urethane linkage to some extent. In addition the Tmax1 and Tmax2 are determined to be 343 o
C and 414 oC, respectively, which are also higher than that of PU series. In this regard, the
lower stability of PU series can be due to the castor oil segment. Nevertheless, the difference in thermal stabilities as we discussed herein among the PU samples is not significant in many applications. We used GPC to measure molecular weight of the PUs, in which DMF was solvent and 10 polystyrene standards were used as reference. We used Gel permeation chromatography (GPC) to measure molecular weight and polydispersity of the castor oil-segmented PUs and PU control. In this analysis, we used DMF as solvent and polystyrene standards for calibration. The result is summarized in Table S2 (Supplementary Material). As can be seen, all castor oilsegmented PU samples demonstrate much higher molecular weight compared with that of the PU control, and at the same time, they show much broader polydispersity. The PU 65/35 shows the highest value of molecular weight and the smallest polydispersity of 4.84 among the castor oilsegmentd PU series. These values explain why it show the highest tensile strength together with highest Young’s modulus, highest storage modulus in the DMA test, and the best recovery performance. In addition, the high molecular weight of the PU series can also be used to explain the exceptionally high elongation at break values. Nevertheless, the broad polydispersity of PU series is understandable, which could be explained by highly complexity in structure of PU series.
17
The in vitro cytocompatibility of developed elastomer was evaluated by cytotoxicity testing. The polyurethane elastomer substrates were cultured with the 3T3 mouse fibroblast cell line, which is commonly used to assess cytotoxicity of potential substrates for cell growth. Figure 6 shows fluorescence microscopy images of fluorescein diacetate/ GelRed TM-stained 3T3 cells growing on PU 50/50 elastomer and glass control substrates after 4 days of cell culture. The elastomer substrate showed excellent cell adhesion and proliferation of 3T3 cells, suggesting that the elastomer is a biocompatible material and could be applied in biomedical applications. 4. Conclusion A series of castor oil-segmented thermoplastic polyurethanes were synthesized in this study. The developed PUs have demonstrated high elongation at break, excellent elastic recovery, and good mechanical/thermal characteristics. The mechanical properties of the PUs can be adjusted by varying the PDMS/PTH ratio to obtain PU with better mechanical strength/Young’s modulus or higher elongation at break/flexibility. The cytocompatibility test shows that the PU series could be potentially used in biomaterial applications. Acknowledgements We gratefully acknowledge the financial support from Academy of Finland (grants no. 12137759 and 13272725) and the European Union 7th Framework Programme (ArtiVasc 3D project, GA no 263416).
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References [1] Michael Szycher, Szycher’s Handbook of Polyurethanes, Second Edition 2013, CRC Press. [2] Pfister DP, Xia Y, Larock RC. Recent Advances in Vegetable Oil-Based Polyurethanes. ChemSusChem 2011;4(6):703-17. [3] Desroches M, Escouvois M, Auvergne R, Caillol S, Boutevin B. From Vegetable Oils to Polyurethanes: Synthetic Routes to Polyols and Main Industrial Products. Polym Rev 2012;52:38-79. [4] Petrović ZS. Polyurethanes from Vegetable Oils. Polym Rev 2008;48:109-55. [5] Hablot E, Zheng D, Bouquey M, Avérous L. Polyurethanes Based on Castor Oil: Kinetics, Chemical, Mechanical and Thermal Properties. Macromol Mater Eng 2008;293(11):922-9. [6] Ogunniyi DS. Castor Oil: A Vital Industrial Raw Material. Bioresour Technol 2006;97:108691. [7] Ionescu M. Chemistry and Technology of Polyols for Polyurethanes, 2005, Rapra Technology Limited. [8] Cayli G, Kusefoglu S. A Simple One-Step Synthesis and Polymerization of Plant Oil Triglyceride Iodo Isocyanates. J Appl Polym Sci 2010;116(4):2433–40. [9] Stirna U, Lazdina B, Vilsone D, Lopez MJ, del Vargas-Garcia Carmen M, Suárez-Estrella F, Moreno J. Structure and Properties of the Polyurethane and Polyurethane Foam Synthesized from Castor Oil Polyols. J Cell Plast 2012;48:476-88. [10] Yeganeh H, Mehdizadeh MR. Synthesis and properties of isocyanate curable millable polyurethane elastomers based on castor oil as a renewable resource polyol. Eur Polym J 2004;40(6); 1233-8. [11] Corcuera MA, Rueda L, Fernandez d’Arlas B, Arbelaiz A, Marieta C, Mondragon I, Eceiza A. Microstructure and properties of polyurethanes derived from castor oil. Polym Degrad Stabil 2010;95(11):2175-84. [12] Kim H, Abdala AA, Macosko CW. Graphene/polymer nanocomposites. Macromolecules, 2010;43(16):6515-30. [13] Saralegi A, Gonzalez ML, Valea A, Eceiza A, Corcuera MA. Compos Sci Technol 2014;92:27–33. [14] Saralegi A, Fernandes SCM, Alonso-Varona A, Palomares T, Foster EJ, Weder C, Eceiza A, Corcuera MA. Biomacromolecules 2013;14:4475−82. 19
[15] Rueda L, Saralegi A, Fernandez-d Arlas B, Zhou Q, Alonso-Varona A, Berglund LA, Mondragon I, Corcuera, Eceiza A. Cellulose 2013;20:1819–28. [16] Stanford JL, Still RH, Cawse JL, Donnelly MJ. Polyurethanes from Renewable Resources. In ACS Symposium Series; chapter 30, pp. 424-42. [17] Ma R. Study of the main influence factors on compression set of PTMG based polyurethane elastomer. Adv Mat Res 2013;630:67-70. [18] de Jong E, Higson A, Walsh P, Wellisch M. Product developments in the bio-based chemicals arena. Biofuels, Bioprod Biorefin 2012;6(6):606-24. [19] Ibrahim S, Ahmad A, Mohamed NS. Characterization of novel castor oil-based polyurethane polymer electrolytes. Polymers 2015;7(4):747-59. [20] Rana S, Cho JW, Tan LP. Graphene-crosslinked polyurethane block copolymer nanocomposites with enhanced mechanical, electrical, and shape memory properties. RSC Adv 2013;3:13796-803. [21] Javni I, Petrovic ZS, Guo A, Fuller R. Thermal stability of polyurethanes based on vegetable oils. J Appl Polym Sci 2000;77(28):1723-34.
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Table 1. Summary of recipe for polyurethane syntheses. Diisocyanate Samples
Polyols
(NCO mol)
Diols
(OH mol)
(OH mol)
Bio-based
Hard segment
(a)
concentration
contents (wt.%)
(b)
(%)
MDI
CA
PDMS
PTH
BDM
PU control
2.0
0
0.7
0.7
0.6
0
21
PU45/55
2.0
0.4
0.45
0.55
0.6
12.2
27
PU50/50
2.0
0.4
0.50
0.50
0.6
12.6
28
PU55/45
2.0
0.4
0.55
0.45
0.6
13.1
29
PU65/35
2.0
0.4
0.65
0.35
0.6
14.1
31
(a)
Bio-based contents (%) = (mass of castor oil x 100)/ total mass of monomers.
(b)
Hard segment concentration is defined as the ratio of the mass of the non-polyol components to the
total mass of the PU polymer.
21
Scheme 1. Synthesis route of castor oil-based elastomer (a) and a PU 50/50 film with diameter of 12 cm and a thickness of around 0.2 mm prepared by solvent casting (b).
22
Fig. 1. IR spectra of castor oil, PU control, and the synthesized castor oil-segmented PU series showing their characteristic bands.
23
Fig. 2. Mechanical properties of polyurethanes: tensile stress (a), Young’s modulus (b), and elongation at break (c), and typical stress-strain curves of the PU samples (d).
24
Fig. 3. Elastic recovery properties of polyurethanes.
25
Fig. 4. Overlay plots of the storage modulus (E’) and the loss factor (tanδ) of polyurethanes.
26
Fig. 5. TGA thermograms of polyurethanes (a) and their differential curves (DTG) (b).
27
Fig. 6. Fluorescence micrographs of fluorescein diacetate/GelRedTM -stained 3T3 fibroblast cells growing on polyurethane elastomer and control substrates after 4 days of cell culture. PU50/50 was used as an example in this test.
28
Highlights
A series of castor oil-segmented thermoplastic polyurethanes is developed.
The polyurethanes show good mechanical properties and high optical transparency.
Good cytocompatibility of the polyurethane is evaluated.
29
Table of Content (TOC)
30