Succinic Acid Based Biodegradable Thermoplastic Poly(ester urethane)

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Succinic acid-based biodegradable thermoplastic poly(ester urethane) elastomers: effects of segment ratios and lengths on the physical properties Shao-Long Li, Jian-Bing Zeng, Fang Wu, Yang Yang, and Yu-Zhong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 03 Jan 2014 Downloaded from http://pubs.acs.org on January 12, 2014

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Industrial & Engineering Chemistry Research

Succinic Acid-Based Biodegradable Thermoplastic Poly(ester urethane) Elastomers: Effects of Segment Ratios and Lengths on the Physical Properties

Shao-Long Li, Jian-Bing Zeng*, Fang Wu, Yang Yang, Yu-Zhong Wang*

Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric

Materials

(Sichuan),

Sichuan

University,

Chengdu

610064,

China.

Email:

[email protected]; [email protected]; Fax: +86-28-85410755; Tel: +86-28-85410755.

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Abstract:

Biodegradable

thermoplastic

poly(ester

urethane)

(PEU)

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elastomers

containing

poly(diethylene glycol succinate) (PDGS) and poly(butylene succinate) (PBS) were synthesized by chain-extension of dihydroxyl terminated PDGS and PBS precursors with 4,4'-methylenediphenyl diisocyanate as a chain extender. The structure, molecular weight, and physical properties of the PEUs were investigated by 1H NMR, GPC, DSC, WAXD, DMA, and tensile tests. The results suggest that the compositions affect the physical properties more significantly than the segment lengths. The PEU containing 28.2 wt% showed the best mechanical properties with ultimate strength and elongation at break of 41 MPa and 1503%, respectively. Both the storage modulus and Young’s modulus increased significantly with increasing PBS segment content, which was reasonably ascribed to the increasing degree of crystallinity. The hysteresis value increased with PBS segment content while decreased slight with lengths of both hard and soft segments, which were also attributed to the different crystallization behaviors of the PEUs.


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1. Introduction Because of the resources and environmental problems, bio-based and biodegradable polymers have drawn considerable interests for use as biomaterials and daily life materials;1-3 however, most of them are hard, brittle, and semi-crystalline plastics.4-6 Elastic biodegradable polymeric materials with well tuned physical properties are usually designed for tissue engineering and drug delivery applications due to their biodegradability and biocompatibility.7-18 In contrast, less attention has been given to the potential of biodegradable polyurethane elastomers (BPUEs) for common elastic materials. For the tissue engineering and drug delivery applications, high strength for BPUEs is not required,19,20 while the high strength is helpful to expand the field of BPUEs conventional applications. The previous investigations usually focused on the biomaterials application,21-27 thus less work has been done to develop high strength BPUEs.28, 29 BPUEs are usually built from the reaction of a macroglycol, a polyisocyanate, and a chain extender.30-32 The structures of BPUEs can be regulated by the choice of starting materials and thus the properties can be possibly tailored. When the final BPUE is a linear polymer, the starting reagents are a difunctional polyol, a diisocyanate, and a difunctional chain extender, and the resulting polymer is a thermoplastic multiblock segmented BPUEs.33-34 The polyol forms the so-called soft segment (SS), whereas the reaction of the diisocyanate and the chain extender constitutes the so-called hard segment (HS). The SSs are usually biodegradable aliphatic polyesters with molecular weight in the range of several hundred to several thousand, and the BPUEs are usually called poly(ester urethane)s (PEUs). The mechanical properties of the BPUEs are dependent on the natures of the both SS and HS. The HSs and SSs are usually immiscible and cause phase separation. The SSs form the continuous matrix phase which determines the elastomeric behavior of BPUEs, whereas HSs form the dispersed phases providing physical crosslinks and mechanical strength for BPUEs. The mechanical properties of BPUEs can be tuned by adjusting the SS/HS ratio, the chemical structures, and the chain sequence.19, 20, 35



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Succinic acid-based aliphatic polyesters have attracted more and more attention due to their excellent biodegradability, thermal stability, and mechanical properties.36 In recent years, succinic acid has been successfully produced from biobased resources, which would further stimulate the development of those polymers. Poly(butylene succinate) (PBS) and poly(ethylene succinate) (PES) are the typical succnic acid-based polyesters since the both showed relative high melting temperature, and they are regarded as good candidates to replace conventional plastics such as polyethylene and polypropylene due to their comparable mechanical properties.36-39 Up to date, the application of succinic acid-based polyesters has been dominantly limited to plastic materials. Less work has been done to develop new application of those polymers. In this study, a series of novel biodegradable segmented thermoplastic PEU elastomers based on succinic acid is designed and synthesized by chain extension reaction of dihydroxyl terminated PBS (HO-PBS-OH) and PDGS (HO-PDGS-OH) precursors with MDI as a chain extender. The amorphous PDGS forms the soft segments of the PEU, whereas the reaction of crystalline PBS with MDI forms hard segments. The effects of segment ratios and lengths on the miscibility, crystallization, mechanical properties, and elastic behaviors of the PEUs are systematically investigated.

2. Experimental section 2.1 Materials. Succinic acid, diethylene glycol and 1,4-butanediol with AR grade were purchased from Kelong Chemical Corporation (Chengdu, China) and were used without further purification. Tetra-isopropyl titanate (AR grade) bought from Sigma-Aldrich Corporation was dissolved in anhydrous toluene to prepare 0.2 g/mL solution. 4,4'-Methylenediphenyl diisocyanate (MDI, AR grade) from Alfa Aesar Corporation was used as received. All other chemicals with AR grades were used without any treatment. 2.2 Synthesis of HO-PDGS-OH and HO-PBS-OH precursors. HO-PDGS-OH with certain molecular weight was prepared by a two-step procedure including esterification and following 4 ACS Paragon Plus Environment

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polycondensation. Typically, 1.2 mol diethylene glycol and 1 mol succinic acid were added into a 500 mL three-necked round-bottom ﬂask equipped with mechanical stirrer, water separator, and nitrogen inlet pipe. The esterrification was implemented at 180 °C for 4 h, then the catalyst tetra-isopropyl titanate with 0.1 wt% of the total amount of reactants was introduced into the ﬂask and the polycondensation was continued at 220 °C with vacuum of 30 Pa for predetermined time. The same procedure was used to prepare HO-PBS-OH precursor. 2.3 Synthesis of the PEU elastomers. The PEU elastomers were synthesized by chain extension reaction of HO-PDGS-OH and HO-PBS-OH using MDI as a chain extender in bulk. Typically, HO-PDGS-OH and HO-PBS-OH with predetermined amounts were put into a two-necked flask which was vacuumed and purged with nitrogen for three times. Then the flask was immersed in a 150 °C silicone oil bath. The reactants were stirred with a mechanical stirrer when they were completely molten, then the predetermined amount of MDI was added to the flask. The chain extension reaction was carried out at 150 °C for 2 hrs. The resulting polymer was cooled down to room temperature and purified by dissolving in chloroform then precipitating in excess methanol. The precipitates were washed with methanol and then vacuum dried at 40 °C to constant weight. 2.4 Nuclear magnetic resonance (NMR) spectroscopy. 1H NMR spectra for all the samples were recorded on a Bruker AC-P 400 MHz spectrometer at ambient temperature in CDCl3 solution with tetramethylsilane as the internal reference. The chemical structures and the number average molecular weights of HO-PDGS-OH and HO-PBS-OH, and the compositions and chemical structures of the PEU elastomers were determined from their 1H NMR spectra. 2.5 Gel permeation chromatography (GPC). GPC was performed on a Waters instrument, which is equipped with a model 1515 pump, a Waters model 717 autosampler, and a model 2414 refractive index detector. CHCl3 and polystyrene were used as the eluent and standard, respectively. The flow rate of eluent and the concentration of samples were 1.0 mL/min and 0.25 mg/mL, respectively. The experiments were carried out at 35 °C. 5 ACS Paragon Plus Environment


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2.6 Differential scanning calorimeter (DSC). A TA DSC-Q200 differential scanning calorimeter was utilized to study the glass transition temperature (Tg), cold crystallization temperatures (Tcc) and enthalpy (∆Hcc), and melting temperature (Tm) and enthalpy (∆Hm) of the PEU elastomers. The samples with around 5 mg were first melted at 140 °C for 3 min to eliminate thermal history, then quenched to -50 °C, and finally reheated to 140 °C at a heating rate of 10 °C/min. All the processes were carried out under a nitrogen atmosphere. The heating scans were recorded for data analysis. 2.7 Wide-angle X-ray diffraction (WAXD). Wide-angle X-ray diffraction patterns of the samples were recorded with an X-ray diffractometer (Philips X’Pert X-ray diffractometer) with Cu Ka radiation. The equipment was operated at room temperature with a scan rate of 2°/min scanning from 10° to 40°. 2.8 Dynamic mechanical analysis (DMA). Thermomechanical properties of the PEU elastomers were tested with a dynamic mechanical analyzer (DMA Q800, TA Instruments, USA) using a tensile resonant mode at a heating rate of 3 °C/min from -50 to 70 °C and at a frequency of 1 Hz. 2.9 Mechanical property measurement. The stress-strain and hysteresis tests of the PEU elastomers were performed on an Instron Universal Testing Machine (Model 4302, Instron Engineering Corporation, Canton, MA) at a crosshead speed of 200 mm/min and room temperature. The specimens were prepared by hot pressing and cutting with a dumbbell-shaped cutter. The thickness and width of the specimens were 0.4 mm and 4 mm, respectively, and the length of the sample between the two pneumatic grips of the testing machine was 24 mm. Five measurements were carried out for each sample, and the average results were reported. For hysteresis tests, the samples were stretched to 300% elongation at a crosshead speed of 200 mm/min and then immediately reversing the crosshead at the same speed. All the tests were carried out at room temperature.

3 Results and discussion 3.1 Synthesis and characterization of the PEU elastomers. The PEU elastomers containing PDGS and PBS segments were synthesized by chain extension reaction of HO-PDGS-OH and HO-PBS-OH 6 ACS Paragon Plus Environment

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with MDI as chain extender, as shown in Scheme 1. It is well-known that hydroxyl is more reactive than carboxyl when reacting with isocyanate group. In order to get high chain extension efficiency and high-molecular-weight PEU elastomers, the precursors should be mostly terminated with hydroxyl group. Therefore, HO-PDGS-OH and HO-PBS-OH should be prepared prior to synthesizing the PEU elastomers. Both HO-PDGS-OH and HO-PBS-OH were synthesized by direct polycondensation of succinic acid with the corresponding diol, as described in Section 2.2. Previous investigations suggest that when the feed mol ratio of diol to succinic acid was 1.2:1 the obtained aliphatic polyester precursors were almost terminated with hydroxyl groups.40, 41 Thus, the same feed ratio was employed to synthesize HO-PDGS-OH and HO-PBS-OH precursors. In order to study the effect of segment length of PDGS and PBS on the properties of the resulting PEUs, three HO-PDGS-OH and three HO-PBS-OH precursors with different molecular weights were prepared by controlling the polycondensation time. The chemical structure and number average molecular weight (Mn) of the precursors were determined by 1H NMR. Figs. 1a and b show the 1H NMR spectrum of HO-PDGS-OH and HO-PBS-OH, respectively. For HO-PDGS-OH, the shifts occurring at 2.67 (δHa), 3.70 (δHb), and 4.25(δHc) ppm with the same integral areas are reasonably assigned to three different methylene protons of the repeating unit. The signal of the methylene connected with the terminal hydroxyl group was observed at 3.60 ppm (δHc’). In the case of HO-PBS-OH, the shifts of the three different methylene protons in the repeating unit were observed at 1.71 (δHe), 2.62 (δHd) and 4.12 (δHf) ppm, respectively, and the methylene protons connected to the terminal hydroxyl group were observed at 3.68 (δHf’) ppm. The Mn of the precursors can be calculated from the integral areas of the terminal protons and those of the protons of the repeating units. The detailed information for calculating the Mn of precursors can be found in our previous study.42,43 The Mn values of the three HO-PDGS-OH precursors were 2420, 3800, and 4840 g/mol, respectively, and those of the three HO-PBS-OH precursors were 3130, 4390, and 5510 g/mol, respectively. The feed mol ratio of precursors to diisocyanate plays a very important role in synthesizing high-molecular-weight PEU elastomers by chain-extension reaction. Our previous study suggests that 7 ACS Paragon Plus Environment


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the Mn of precursors obtained from NMR analysis (Mn,NMR) was effective to determine the amount of diisocyanate, and high-molecular-weight PEU was achieved when the feed mol ratio of precursors (based on Mn,NMR) to diisocyanate was 1:1.42,43 Therefore, the same calculation was used to determine the amount of MDI used in synthesizing high-molecular-weight PEU elastomers containing PDGS and PBS segments in this study.


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A series of PEU elastomers with various PDGS and PBS contents and lengths were synthesized. For property comparison, PEU containing only PDGS or PBS segment were synthesized by chain extension reaction of HO-PDGS-OH or HO-PBS-OH with MDI. They were named as PDGSU and PBSU, respectively. For the PEU elastomers, they are coded as PDxxByyU-z, where D and xx represent PDGS segment and the initial two numbers of its molecular weight, B and yy denote PBS segment and the initial two numbers of its molecular weight, and z represents feed content of PBS. For example, PD38B43U-3 denotes PEU synthesized from HO-PDGS-OH and HO-PBS-OH with Mn of 3800 and 4390 g/mol, and the feed weight ratio of HO-PBS-OH to HO-PDGS-OH was 3:7. The chemical structures and compositions of the synthesized PEU elastomers were characterized by 1H NMR. Fig. 2 shows the 1H-NMR spectrum and the peak assignment of a typical PEU. All the characteristic resonance signals belonging to PDGS and PBS can be observed in the 1H NMR spectrum of the PEU. It is worth noting that the shifts belonging to the terminal methylene protons of both HO-PDGS-OH and HO-PBS-OH precursors disappeared, which suggests that the hydroxyl groups of the both precursors consumed completely during chain extension reaction. In addition, the three different protons incorporated by MDI can be observed at the shifts of 7.30 (δHg), 7.10 (δHh) and 3.88 (δHi) ppm. The results suggest that PEU was successfully synthesized. The compositions of the PEU including the weight fraction of PDGS, PBS, and MDI can be calculated from the peak areas and corresponding proton numbers according to the following equations:

.

(1)

. . .

.

(2)

. . .

(3)

1

where I4.25, I4.12 and I3.88 are the peak areas of methylene protons of diethylene glycol residue linked with carbonyl in the PDGS block, methylene protons of 1,4-butanediol residue linked with carbonyl in the PBS block, and methylene protons of the MDI residue in the copolymers, respectively. The numbers 188, 172 and 252 are the molecular weights of repeating units of PDGS, PBS, and residual MDI units, respectively. The compositions of polymers are listed in Table 1. The weight fraction of each component in the PEU (F) was very close to that in the feed ratio (f), which was a strong evidence for the successful synthesis of the PEU. 9

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The molecular weight plays an important role in the mechanical properties of biodegradable polymers. The molecular weight and polydispersity index (PDI) of the PEUs were determined by GPC in chloroform at 30 °C with polystyrene as a standard. The number average molecular weight (Mn), weight molecular weight (Mw), and PDI are shown in Table 1. Except for PBSU which shows Mw value of 27.8×104 g/mol all the other samples show very high molecular weights with Mw of more than 30.0×104 g/mol, which should be sufficient for the PEUs to provide excellent mechanical properties. All the samples show comparable PDI of around 2.0~2.9, which is very good to make property comparisons of the different samples. 3.2 Thermal transition and crystallization behaviors. The thermal transition and crystallization behaviors of the PEUs were characterized by DSC. The glass transition temperature (Tg), cold crystallization temperature (Tcc) and enthalpy (∆Hcc), and melting temperature (Tm) and fusion enthalpy (∆Hm) were obtained from the heating scans. The results are shown in Figs. 3 and 4 (the values are in Table S1). Fig. 3 shows the DSC heating scans of PEUs with different segment lengths. All the samples showed only a single Tg, which may suggest that PDGS and PBS segments are miscible in the amorphous phase. When the length of one segment is fixed, Tg decreased slightly, Tcc decreased significantly, and Tm increased slightly with increase in length of the other segment. For example, when the length of PDGS was 3800 g/mol, Tg decreased from -19.1 °C to -19.4 °C and -19.9 °C, Tcc decreased from 51.4 °C to 37.6 °C and 24.0 °C, and Tm increased from 97.8 °C to 98.5 °C and 106.9 °C with PBS segment length increased from 3130 g/mol to 4390 and 5510 g/mol; while when the length of PBS segment was fixed at 4390 g/mol, Tg decreased from -19.1°C to -19.4°C and -21.6 °C, Tcc decreased from 41.6 °C to 37.6 °C and 24.3 °C, and Tm increased from 97.1 °C to 98.5 °C and 100.5 °C with PDGS segment length increased from 2420 g/mol to 3800 and 4840 g/mol. The decrease of Tg should be attributed to the decreasing amount of rigid MDI with increase of segment of both segments. It is well known that the lower Tcc indicates higher crystallization capacity. Therefore we can conclude that the crystallization capacity of the PEUs increased with increasing segment length. It is easy to understand that the crystallization capacity of PEUs increased with PBS segment since the crystallization of the PEU arises from PBS segment. The chain regularity of the crystalline PBS segment increased with its length, which then improved the crystallization capacity of the PEUs. 10


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While PDGS segment is amorphous, why does the crystallization capacity of the PEUs also increased with the increasing length of the amorphous PDGS segments? The possible reason is that the rigid MDI segments in PEUs decreased with increasing length of PDGS segments, thus the chain mobility and flexibility of PEUs increased, then showing improved crystallization capacity. The increase of Tm with both segments resulted from the formation of more perfect crystals with improved crystallization capacity. The values of ∆Hcc and ∆Hm also showed similar changing trend to crystallization capacity and melting temperature with increasing length of both segments, i.e., increasing with lengths of both segments. Fig. 4 shows the DSC heating scans of PEUs with different compositions. PDGSU shows no crystallization and melting peaks with a Tg of -14.6 °C, while PBSU shows good crystallization and melting peak with Tg, Tcc and Tm of -38.6, -9.4 and 114.4 °C, respectively. All the PEUs show a single Tg which decreased with increase in content of PBS segment, suggesting that PDGS and PBS segments are miscible in amorphous phase. The Tg value of miscible system could be evaluated by the fox equation: 1/Tg(fox)=F1/Tg1+F2/Tg2 where Tg1 and Tg2 are the Tg of the individual segment, and F1 and F2 are the weight fractions of corresponding components.44,45 In this study, the values of Tg1 and Tg2 were -14.6 and -38.6 °C, respectively. The Tg(fox) and Tg(exp) (obtained by experiment) versus FPDGS are graphically shown in Fig. 5. It can be found that the Tg(exp) is systemically higher than Tg(fox) for all the PEUs, which was also caused by the incorporated MDI which is rigid structure thus to increase Tg of the samples. The cold crystallization and melting peaks can be observed for all the PEUs except PD38B43U-1 due to the limit amount of crystallizable PBS segments. The value of Tcc decreased gradually with increase in content of PBS segments, suggesting that the crystallization capacity of PEUs increased. It is reasonable since the concentration of crystallizable segments increased. The Tm of the PEUs increased slightly with the content of PBS segments. It is worth noting that the Tm of PBSU is much higher than those of PEUs, which was ascribed to the disturbed crystal perfection by incorporation of PDGS segments. The values of both ∆Hcc and ∆Hm increased significantly with the content of PBS segments, suggesting that the degree of crystallinity of the PEUs increased, which would affect the mechanical properties strongly. The crystal structures of PEUs were characterized by WAXD. Fig. 6 shows the WAXD patterns of 11



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PEUs with different compositions. The pattern of PDGSU shows character of amorphous polymer with bread-like peak at 2θ degree of around 21°, while the pattern of PBSU shows character of crystalline polymer with the three main characteristic diffraction peaks at 2θ of 19.7°, 21.9°, and 22.6°, corresponding to (020), (021), and (110) planes, respectively.46 All the other PEUs showed similar diffraction peaks to that of PBSU. The location of the characteristic diffraction peaks almost kept unchanged with compositions, suggesting that the crystal structure is the same as that of PBSU. However, the peak intensities are much weaker than those of PBSU, and increased gradually with the content of PBS segments, as an indication of increasing degree of crysallinity, which can be calculated through deconvolution of crystalline and amorphous peaks in the WAXD pattern using the peak separation software, and the detailed analytical procedure can be found in our previous paper.45 The values of degree of crystallinity for PD38B43U-1, PD38B43U-2, PD38B43U-3, PD38B43U-4, and PD38B43U-5 are 5.8%, 7.9%, 11.5%, 14.2%, and 18.6%, respectively, and that of PBSU was calculated to be 45%. The effects of PBS and PDGS segment lengths on the crystal structure and degree of crystallinity of the PEUs were also investigated, there was almost no difference in their diffraction patterns, thus for brevity the diffraction patterns are not shown. And the degree of crystallintiy of PEUs with weight content of PDGS to PBS of 7:3 was all around 11.5% regardless of the segment lengths. Therefore, we can conclude that the degree of crystallinity of PEUs depends more on segment ratios than on the segment lengths. 3.3 Dynamic mechanical analysis. DMA was used to study the temperature dependence of mechanical properties of the PEUs. Fig. 7 shows the temperature dependence of the storage modulus for PEUs with different compositions from -50 °C to 67 °C. All the samples showed typical elastomer behaviors, with a low glass transition temperature and a wide temperature range rubbery plateau. The storage modulus decreased magnitude of several GPa to less than 150 MPa over the αa-relaxation range of -10-20 °C. The αa-relaxation was associated with the micro-Brownian motion of the PEUs accompanied by the glass transition, where they changed from a glassy state to a rubbery state. It is worth noting that the reinforcement in storage modulus can be found for both glassy state and rubbery state with increase in the content of PBS segments. The modulus of PDGSU in rubbery state was only 4 MPa, while those of PD38B43U-1, PD38B43U-2, PD38B43U-3, PD38B43U-4, and PD38B43U-5 are 9, 21, 56, 85, and 132 MPa, respectively. The improvement in storage modulus in rubbery state makes the 12


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elastomers stronger to resist external damage, which would then widen the application of the elastomers. The reinforcement was attributed to the increased degree of crystallinity with increase in PBS segment content. It is known that the rubbery modulus is attributed to the cohesive energy density of the chains and the stiffness of the elastomers.47, 48 In addition, the crystalline phase that dispersed in the rubbery amorphous matrix can work as physical crosslink which would also reinforce the mechanical properties. The cohesive energy density, stiffness and degree of physical crosslink all increased with increasing PBS segments content. They are responsible for the reinforcement of the storage modulus of the PEUs. The effects of PDGS and PBS segment lengths on the dynamic mechanical properties of the PEUs were also investigated. Fig. 8 shows the temperature dependence of the storage modulus for PEUs with different segment lengths from -50 °C to 67 °C. The storage modulus in both glassy and rubbery states changes slightly with lengths of the both segments. It is reasonable because the ratio of soft segment to hard segment is almost identical for those samples, and they showed almost the same degree of crystallinity, thus possess comparable dynamic mechanical properties. 3.4 Mechanical properties. The mechanical properties of the PEUs were investigated with a tensile test. Fig. 9 shows the stress-stain curves of the samples with different segment ratios, and Table 2 summarizes their mechanical properties. PBSU shows a typical stress-strain curve of semi-crystalline plastic, with an obvious yielding point. The ultimate strength and elongation at break of PBSU were 41.5 MPa and 634%, respectively. While all the other samples showed stress-strain curves of typical elastomer, with high elongation and no obvious yielding point. It is known that the elongation at break of a PEU is usually dependent on the nature of the soft segments while the Young’s modulus and tensile strength are usually determined by the natures of the hard segments. In this study, the soft segments of the PEUs are amorphous PDGS segments and the hard segments are formed by the reaction of crystalline PBS and MDI. As a matter of fact, the content of MDI is limited, thus its contribution to the hard segments might be negligible. Therefore, the mechanical properties of the present PEUs are almost determined by PBS segments. PDGSU is a very soft material with the values of Young’s modulus, ultimate strength, and elongation at break of 3.0 MPa, 5.1 MPa, and 1282%, respectively. The Young’s modulus increased gradually with the content of PBS segments, which should be attributed to the increased stiffness of the samples with increasing 13



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content of crystalline PBS segments. PD38B43U-5 shows the highest Young’s modulus with the value of 101.7 MPa, which is 33 times higher than that of PDGSU. The change of Young’s modulus versus the content of PBS segments is similar to the storage modulus determined by DMA. The ultimate strength of the PEUs increased significantly with increasing PBS segment content. When only 9.5 wt% PBS segments were incorporated, the ultimate strength increased to 31 MPa, 6 times higher than that of PDGSU. The highest ultimate strength with the value of more than 40 MPa was reached when the content of PBS segment was 28.5 wt%. The ultimate strength decreased slightly with further increase in the content of PBS segments. The ultimate strength is very important to evaluate the application of the elastomers, however, the influencing factors for the ultimate strength seems very complicated. Its value is not so efficient to assess the effect of the composition on the strength of the samples. Therefore, the stress at a given strain was analyzed to investigate the effect of PBS segment content on the strength of the PEUs, as listed in Table 2. For the three different strains, the stress increased gradually with PBS segment contents, showing similar trend to the Young’s modulus. The elongation at break increased first and then decreased with the content of PBS segments. The maximum value occurred for PD38B43U-3 with the value of 1516%. The changes of mechanical properties can be ascribed to the change in degree of crystallinity of the PEUs accompanied by PBS content. The crystals of PBS segments provide physical crosslinks for the PEUs. The physical crosslink should play similar role to chemical crosslink in determining mechanical properties of elastomers. The degree of physical crosslink (i.e., the number of physical crosslink points) must increase with the degree of crystallinity of PBS segment. Usually, the mechanical properties of elastomer are dependent on its degree of crosslink. High degree of crosslink provides high strength and modulus but low elongation at break, whereas low degree of crosslink provides high elongation at break but low strength and modulus. Proper degree of crosslink provides a good balance between strength/modulus and elongation at break. Except for the physical crosslink effect, the crystalline PBS segment provides PEUs with stiffness by decreasing the amorphous soft segment contents, which would also affect the mechanical properties of the elastomers, increasing modulus but decreasing flexibility. The effects of PDGS and PBS segment lengths on the mechanical properties of the PEUs were also investigated; and the data are listed in Table 2. We can conclude from the results that the mechanical properties changes slightly with varying lengths of the both segments when the weight ratio of PDGS 14


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to PBS was fixed. This can also be explained by the degree of crystallinity of the samples, since the five samples with different PDGS and PBS segment lengths possess almost the same degree of crystallinity as discussed in Section 3.2. 3.5 Hysteresis behavior of the PEUs. Hysteresis behavior plays an important role in practical applications of elastomers. Tensile hysteresis is caused by energy losses which occur in polymers when they are repeatedly stretched and released. Many factors affect the hysteresis such as stretching rate, temperature, amount and number of extension cycles. During the stretching process the equilibrium structure or morphology of the polymer is often distinctly changed or destroyed thus leading to a major difference in the hysteresis behavior of the first versus the second or any later deformation cycle. After removing the load, which is usually fast after stretching, the polymer chains generally cannot completely regain their original configurations. This results in an energy loss, also termed as tensile hysteresis. The hysteresis value can be calculated by 49 Hysteresis =

AL − AR ×100% AL

(4)

where AL and AR indicate the area under the loading curve and the area under the recovery curve, respectively. In this study, we investigated the tensile hysteresis of the PEUs at a strain of 300% with a constant rate of 200 mm/min. Four cycles have been carried out for all the samples. The composition dependence of hysteresis of PEUs was investigated firstly, and the results are summarized in Fig 10 and Table S2. All samples showed a high hysteresis during the first cycle with the value of more than 35%, and as expected that the hysteresis value decreased gradually with the increasing cycle times. For the PEUs containing both PBS and PDGS segments, the hysteresis value increased with the PBS segment content. The hysteresis values of the PEUs are comparable or even lower than those of PPG-based, PTMO-based polyurethanes and silicon-urea copolymers,49,50 suggesting that the present PEUs showing comparable or even better elastic properties in comparison with those conventional elastomers. It is known that the hysteresis mainly arises from partial destruction in ordered structure of hard segments.49 In this study, the ordered structure of hard segments was the crystals formed by PBS hard segments. The crystals underwent deformation during stretching. PEUs showed more perfect crystals and higher degree of crystallinity with increasing PBS segment content, i.e., more ordered structure of 15



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hard segments. During stretching to the same strain, the PEUs with more ordered structure are disrupted more significantly. It is difficult for the crystalline phase to recovery after disruption. Therefore the PEUs with higher degree of crystallinity showed higher hysteresis. It is worth noting that hysteresis value of PDGSU falls in between those of PB38D43U-2 and PB38D43U-3. The ordered structure of hard segment also provides driving force for the recovery of segmented PEUs. For PDGSU, the content of hard segments is very limited, only 7.4 wt% (seeing FMDI in Table 1), therefore, the interaction among the hard segments namely the driving force for recovery was very weak, thus showing relatively higher hysteresis. In addition, we can find that the stress at a given strain increased with increase in PBS segment content, which is in accordance with the results obtain in Section 3.4 and also ascribed to the increased degree of crystallinity. The effect of segment lengths on hysteresis behaviors of the PEUs was also investigated, and the results are summarized in Fig.11 and Table S2. All the samples with similar compositions but different segment lengths showed analogous hysteresis, which is mainly due to the fact that they have similar degree of crystallinity. However, if we observe carefully we can find that the values of hysteresis decreased slightly with increasing lengths of both segments. This phenomenon could also be reasonably ascribed to the different perfection of the crystals with increased block lengths. It is known that the crystals go to be more perfect with increasing segment lengths for segment copolymers, and the more perfect crystals should provide stronger driving force for recovery after unloading thus to decrease the hysteresis value.

Conclusions Biodegradable thermoplastic PEU elastomers containing PDGS and PBS segments were successfully synthesized by chain extension reaction of HO-PDGS-OH and HO-PBS-OH using MDI as a chain extender. The PEUs showed very high molecular weight with Mw of more than 30×104 g/mol. PDGS and PBS segments are miscible in the amorphous phase of the PEUs. The amorphous PDGS forms the soft segments while the crystalline PBS and MDI constitute the hard segments of the PEU elastomer. The effects of segment ratios and lengths on the physical properties of the PEUs were investigated systematically. The results suggest that the segment ratio affects the physical properties of PEUs more significantly than segment lengths. The crystallization capacity, melting 16


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temperature, and fusion enthalpy increased gradually with PBS segment content while changed slightly with lengths of both segments. The stress at a given strain increased gradually while the elongation at break increased first and then decreased with PBS segment content, and the hysteresis also increased. Both the storage modulus and Young’s modulus were reinforced significant with increasing PBS segment content. The hysteresis increased gradually with PBS segment content. The effect of segment lengths on the physical properties was less prominent than that of segment ratios. The results are mainly attributed to the different crystallization behaviors of PEUs with increasing PBS segment content. The PEUs with excellent comprehensive properties are supposed to find many applications in elastic materials. Acknowledgement This work has been supported by National Natural Science Foundation of China (51373107 and 51121001), the Specialized Research Fund for the Doctoral Program of Higher Education (20110181130008), and the Program for Changjiang Scholars and Innovative Research Teams in University of China (IRT 1026).

Supporting Information The data for thermal properties, crystallization behavior, and Hysteresis of the PEU elastomers. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table 1 Designation, composition and molecular weight of PEUs containing PDGS and PBS segments. fa (wt%)

Sample

fPDGS

PDGSU

92.9

PD38B43U-1

84.5

PD38B43U-2

fPBS

Fb (wt%) fMDI

Molecular weight Mn×10-4

Mw×10-4

(g/mol)

(g/mol)

7.4

12.7

32.3

2.7

FPDGS FPBS FMDI

PDI

7.1

92.6

9.4

6.1

84.0

9.5

6.5

23.7

48.4

2.0

75.2

18.8

6.0

74.8

19.0

6.2

19.0

46.1

2.4

PD38B43U-3

65.9

28.2

5.9

65.3

28.5

6.2

16.4

40.0

2.4

PD38B43U-4

56.5

37.7

5.8

56.0

38.1

5.9

18.4

43.0

2.3

PD38B43U-5

47.1

47.1

5.8

46.5

47.6

5.9

16.2

38.8

2.4

PD24B43U-3

64.3

27.5

8.2

63.9

28.1

8.0

12.8

32.1

2.5

PD48B43U-3

66.4

28.5

5.1

66.1

28.9

5.0

12.8

33.8

2.6

PD38B31U-3

64.9

27.8

7.2

64.7

28.0

7.3

17.7

49.0

2.8

PD38B55U-3

66.2

28.4

5.4

65.8

28.7

5.5

17.0

49.0

2.9

94.6

5.4

94.8

5.2

11.3

27.8

2.5

PBSU a

Weight fraction in the feed ratio.

b

Weight fraction in the resulting PEU determined by NMR calculating.

23



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Table 2 Tensile properties of the PEUs. Sample

Ultimate strength Elongation at break Young’s modulus Stress at a given strain (MPa) (MPa)

(%)

(MPa)

100%

PDGSU

5.1±0.5

1282±12

3.0±0.2

0.9±0.1 1.1±0.1

1.4±0.1

PD38B43U-1

31.0±1.2

1517±35

9.5±0.6

2.1±0.1 2.9±0.1

4.0±0.2

PD38B43U-2

38.4±5.4

1516±168

17.4±1.5

3.3±0.3 5.2±0.3

7.4±0.4

PD38B43U-3

41.0±1.9

1503±69

38.9±2.2

5.5±0.2 7.5±0.2

9.8±0.3

PD38B43U-4

36.4±0.9

1293±15

55.7±2.8

6.8±0.2 8.9±0.3

11.7±0.3

PD38B43U-5

30.4±2.0

971±33

101.7±3.8

9.6±0.3 11.7±0.3 15.5±0.3

PD24B43U-3

39.1±1.0

1491±45

31.3±1.5

4.8±0.7 7.1±0.2

9.0±0.3

PD48B43U-3

40.9±0.9

1288±2

37.0±2.1

5.5±0.2 7.8±0.3

10.6±0.6

PD38B31U-3

36.2±2.5

1533±50

34.2±2.4

4.6±0.2 6.2±0.4

8.6±0.3

PD38B55U-3

39.2±1.1

1501±62

36.7±1.8

5.3±0.4 7.4±0.7

9.6±1.3

24


300%

500%

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Scheme 1 Synthesis of PEU containing PDGS and PBS segments.

25



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Fig. 1 1H NMR spectra of HO-PDGS-OH (A) and HO-PBS-OH (B).

26


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Fig. 2 1H NMR spectrum of PEU containing PDGS and PBS segments.

27



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Fig. 3 DSC heating scans of PEUs with different segment lengths from the amorphous state at a heating rate of 10 °C/min.

28


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Fig. 4 DSC heating scans of PEUs with different compositions from the amorphous state at a heating rate of 10 °C/min.

29



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Fig. 5 Glass transition temperature of the PEUs as a function of FPDGS.

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Fig. 6 WAXD patterns of PEUs with different composition.

31



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Fig. 7 Temperature dependence of storage modulus for PEUs with different compositions.

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Fig. 8 Effect of segment lengths on storage modulus of the PEUs.

33



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Fig. 9 Stress-strain curves of PEUs with different compositions.

34


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Fig. 10 Hysteresis curves of the PEUs with different compositions. The 1st (●), 2nd (○), 3rd (■) and 4th (□) cycles are displayed.

35



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Fig. 11 Hysteresis curves of the PEUs with different segment lengths. The 1st (●), 2nd (○), 3rd (■) and 4th (□) cycles are displayed.

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For TOC only

37


Succinic Acid Based Biodegradable Thermoplastic Poly(ester urethane)

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