- Email: [email protected]
Fully biodegradable blends of poly(butylene succinate) and poly(butylene carbonate): Miscibility, thermal properties, crystallization behavior and mechanical properties
Polymer Testing 31 (2012) 39–45
Contents lists available at SciVerse ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material properties
Fully biodegradable blends of poly(butylene succinate) and poly(butylene carbonate): Miscibility, thermal properties, crystallization behavior and mechanical properties Jin Wang a, b, Liuchun Zheng a, Chuncheng Li a, *, Wenxiang Zhu a, Dong Zhang a, Yaonan Xiao a, Guohu Guan a a
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China Graduate University of the Chinese Academy of Academy of Sciences, Beijing 100049, People’s Republic of China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 July 2011 Accepted 8 September 2011
Fully biodegradable poly(butylene succinate) (PBS) and poly(butylene carbonate) (PBC) blends were prepared by melt blending. Miscibility, thermal properties, crystallization behavior and mechanical properties of PBS/PBC blends were investigated by scanning electron microscopy (SEM), phase contrast optical microscopy (PCOM), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and mechanical properties tests. The SEM and PCOM results indicated that PBS was immiscible with PBC. The WAXD results showed that the crystal structures of both PBS and PBC were not changed by blending and the two components crystallized separately in the blends. The isothermal crystallization data showed that the crystallization rate of PBS increased with the increase of PBC content in the blends. The impact strength of PBS was improved significantly by blending with PBC. When the PBC content was 40%, the impact strength of PBS was increased by nearly 9 times. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Biodegradable poly(butylene carbonate) Poly(butylene succinate) Miscibility Crystallization Mechanical properties
1. Introduction It is well known that a primary factor hindering the development of plastics is the pollution caused by petroleum-based plastic waste, especially polyolefins [1]. Aliphatic polyesters, such as poly(lactic acid), poly(butylene succinate), poly(hydroxybutyrate-co-valerate), and polycaprolactone [2–5], exhibit superior biodegradability and good comprehensive properties. Among these biodegradable aliphatic polyesters, PBS, a semicrystalline polymer, has a relative high melting temperature, excellent processing properties and thermal stability [6–8]. However, its insufficient mechanical properties of PBS, especially its poor impact strength, have prevented it from being used in many applications [9]. Therefore, various polymer blends have been * Corresponding author. Tel.: þ86 10 6256 0029; fax: þ86 10 6255 9373. E-mail address: [email protected] (C. Li). 0142-9418/$ – see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2011.09.005
produced in order to improve the physical properties and extend the field of application PBS, such as blending with poly(hydroxybutyrate) [10], poly(vinylidene fluoride) [11], poly(ethylene oxide) [12], poly(butylene terephthalate) [13], etc. Blending PBS with these biodegradable aliphatic polyester or polyethers could enhance the elongation and impact properties of PBS, but simultaneously lower tensile strength. Blending PBS with non-biodegradable polymers might improve the mechanical properties effectively but the degradable properties of the material would be undermined. Aliphatic polycarbonates (APCs), as another important class of biodegradable polymers, are widely used in the areas of packaging materials, drug carriers and tissue engineering for their favorable biodegradability, biocompatibility and non-toxicity [14,15]. Poly(butylene carbonate), which is a very important member of the APC family, is also a semicrystalline polymer with a clear melting point at around 60 C, as well as a glass transition
40
J. Wang et al. / Polymer Testing 31 (2012) 39–45
temperature at 38 C. During previous work, our team successfully synthesized high-molecular-weight PBC for the first time via a successive two-step polycondensation [14]. Due to the flexibility of the polymer chains and its crystallizability, PBC possesses excellent impact resistance and satisfactory tensile strength. Therefore, blending PBS with PBC is expected to improve the impact strength of PBS while hardly sacrificing other properties, and at the same time a fully biodegradable material can be prepared. To the best of our knowledge, no attention has been paid to the blending of PBS and PBC, which may be due to the unavailability of high-molecular-weight PBC. In the present paper, blends of PBS and PBC were prepared and the miscibility, thermal properties, crystallization behavior and mechanical properties of PBS/PBC blends were investigated. 2. Experimental 2.1. Materials and blends preparation The starting polymers used in this study were PBS and PBC, which were synthesized according to the procedures reported previously, respectively [9,14]. The molecular weigh and molecular weight distribution, which were determined on a Waters 2695 GPC gel permeation chromatography using polystyrene as standard and CHCl3 as solution of such materials, are reported in Table 1. All the investigated samples were obtained by means of melt blending. PBC and PBS were dried in vacuo at 40 C for 12 h before mixing. The blending was carried out at 150 C and 50 rpm in a Haake mixer with a blending time of 6 min. After blending, all the samples were cooled to room temperature in air.
and an Olympus (C-5050ZOOM) camera. A Linkam-THMS 600 hot stage was used to control the experimental temperature. The ultrathin film was prepared by hotpressing at 150 C under a pressure of 2.5 MPa for 30 s. For the PCOM observation, the film was heated to 150 C, held there for 5 min to eliminate any thermal history and then cooled to 130 C at a rate of 100 C/min for phase separation. For the POM characterization, the film was heated to 150 C, held there for 5 min to eliminate any thermal history and then cooled to 86 C as quickly as possible for isothermal crystallization. 2.4. Differential scanning calorimetry (DSC) The thermal properties and isothermal crystallization behavior were measured using a Pyris Diamond DSC instrument (Perkin Elmer). The calibration of the temperature was performed using indium as the standard before the measurements. All the measurements were conducted under a high-purity nitrogen atmosphere. The weight of sample was 5 0.2 mg. For the basal thermal parameters, the samples were heated to 150 C at 20 C/min and held there for 5 min before cooling to 100 C at the same rate. After being held at 100 C for 5 min, the samples were heated again to 150 C at 20 C/min and equilibrated there for 5 min. The isothermal crystallization was performed as follows. The samples sealed in aluminum pans were heated to 150 C, held there for 5 min to erase any thermal history and then cooled to the designated crystallization temperature in the range of 80–94 C for isothermal crystallization at a rate of 200 C/min. The curves of heat flow as a function of time were recorded. 2.5. Wide-angle X-ray diffraction (WAXD)
2.2. Scanning electron microscopy (SEM) The phase morphologies of blends were examined by scanning electron microscopy (SEM; JSM-5900LV, JEOL, Japan) at an accelerating voltage of 20 kV. The injectionmolded tensile specimens were cryo-fractured in liquid nitrogen. Since tetrahydrofuran is a good solvent for PBC but a nonsolvent for PBS, it was chosen as the solvent to extract the PBC on the fractured surfaces. All the surfaces were dried and sputter-coated with platinum for 60 s prior to examination. 2.3. Phase contrast optical microscopy (PCOM) and polarized optical microscopy (POM) The phase contrast optical microscopy (PCOM) was carried out using an Olympus (BX51) optical microscope Table 1 Molecular weight and molecular weight distribution of PBS and PBC. Sample
Mn (104)
Mwa (104)
PDIa
PBC PBS
3.5 3.5
8.0 8.1
2.29 2.31
a Abbreviation of weight-average molecular weight and molecular weight distribution, respectively.
WAXD was performed at room temperature with a Ragaku Model D/max-2B diffractometer using Cu Ka radiation (40 kV, 200 mA), and test data were collected from 2q ¼ 10 to 40 at a scanning rate of 8 /min. The samples used for WAXD characterizations were prepared in the following ways. PBS, PBC and PBS/PBC small particles were first placed on a mold with a thickness of 1 mm between two thick teflon sheets to mold at 150 C under a pressure of 1.5 MPa for 30 s. After that, the polymer models were transferred rapidly into an oven at room temperature for isothermal crystallization. 2.6. Mechanical properties testing Tensile and flexural testing was determined with a universal tester (Instron 1122, UK). The tensile properties of the specimens were measured in general accordance with ISO 527 at a crosshead speed of 50 mm/min; the flexural properties were determined in general accordance with ISO 178 at a crosshead speed of 2 mm/min. The impact strength was measured with an impact testing machine (CSI-137C, USA) in general accordance with ISO 180. The results were taken as an average from measurements of at least five specimens.
J. Wang et al. / Polymer Testing 31 (2012) 39–45
3. Results and discussion 3.1. Miscibility of PBS/PBC blends It’s known that many properties of blends are affected by their miscibility. As the Tg of PBS (43 C) is very close to that of PBC (38 C), the miscibility of PBS and PBC was studied by phase morphology in this work. The morphology of the PBS/PBC blends were investigated by means of SEM. Fig. 1 presents the SEM micrographs of tetrahydrofuran-etched fracture of the 90/10, 70/30, 60/40, 50/50, 40/60 and 30/70 PBS/PBC blends specimens. For the 90/10 PBS/PBC blends, the discrete PBC particles with
41
average diameter of about 1 mm were uniformly dispersed in the continuous PBS matrix (Fig. 1a). With increasing PBC content to 30%, the size of the PBC particles increased to 2– 3 mm and the shape became irregular (Fig. 1b). As the PBC content increased to 40–60%, the PBS and PBC gradually formed a bicontinuous structure (Fig. 1c–e). When the PBC increased to 70%, the PBC became the matrix and the PBS became the dispersed phase (Fig. 1f). These features are typical of incompatible polymer blends. Fig. 2 shows phase contrast optical micrographs of 70/ 30, 50/50 and 30/70 PBS/PBC blends melted at 150 C for 5 min and then isothermally annealed at 130 C for 30 min. Based on the information provided in Fig. 2, it is obvious
Fig. 1. SEM images tetrahydrofuran-etched fracture ends of the PBS/PBC blends (a) 90/10 PBS/PBC (b) 70/30 PBS/PBC (c) 60/40 PBS/PBC (d) 50/50 PBS/PBC (e) 40/60 PBS/PBC and (f) 30/70 PBS/PBC.
42
J. Wang et al. / Polymer Testing 31 (2012) 39–45
Fig. 2. Phase contrast optical micrographs of PBS/PBC blends annealed at 130 for 30 min (a) 70/30 (b) 50/50 and (c) 30/70. The scale bar in (a) is 10 mm and is the same for all pictures.
that phase separation occurred after annealing for 30 min at 130 C. We can conclude that the dark part is the PBS-rich domain and the light part is the PBC-rich domain with the PBC phase being 2–3 mm in diameter, as shown in Fig. 2a. The PBC domain phase dimensions increase with increasing PBC content. When the PBC content increased to 70%, phase inversion occurred as shown in Fig. 2c. The distinct phase-separated morphology indicates that the compatibility between PBS and PBC was not good and is consistent with the SEM results.
3.2. Thermal properties of PBS/PBC blends DSC was used to evaluate the thermal properties of PBS/ PBC blends. The heating and cooling results are summarized in Table 2. It can be found from Table 2 that neat PBS has a Tm of 115.5 C, while neat PBC has a Tm of 55.9 C. In the blends, the Tm of PBC and PBS is at around 55.9 C and 115.5 C, respectively, and they are almost unchanged with variation of the composition. This may be a result of the immiscibility
Table 2 Thermal properties of PBS and PBS/PBC blends. PBS/PBC
PBC
PBS
DHma (J/g)
Tma ( C)
100/0 90/10 70/30 50/50 30/70 0/100
Tma ( C)
1st
2nd
1st
2nd
1st
2nd
– – 54.1 55.3 54.1 55.9
– – – – – –
– – 13.1 20.0 28.1 42.1
– – – – – –
115.6 116.0 115.0 116.8 114.3 –
115.5 115.9 115.3 116.5 114.4 –
Tca ( C)
DHma (J/g) 1st
2nd
1st
2nd
1st
2nd
61.5 73.7 74.4 74.4 – –
63.9 69.6 58.5 34.8 23.7 –
73.3 74.6 60.2 37.0 23.9 –
57.9 63.0 53.0 31.5 21.5 –
66.4 67.6 54.5 33.5 21.6 –
57.9 70.0 75.7 63.0 71.6 –
66.4 75.1 77.9 67.0 72.0 –
Xcb (%)
Xc-PBSc (%)
1st Measured from the first DSC heating scan. 2nd Measured from the second DSC heating scan. a The crystallization temperatures (Tc) taken from cooling scan, the melting temperatures (Tm), melting enthalpies (DHm) were registered by DSC. b The crystallinity degree (Xc) was calculated by dividing the observedDHm from the first/second heating trace by the theoretical value (110.4 J/g) for a 100% crystalline PBS. Xc ¼ DHm/DHm*,DHm* ¼ 110.4 J/g. c The degree of crystallinity (Xc) the crystallinity degree relative to PBS (Xc-PBS) was calculated by Xc-PBS ¼ Xc/PBS wt%.
J. Wang et al. / Polymer Testing 31 (2012) 39–45
43
and 22.5 corresponding to (020), (021) and (110) planes [13], respectively. On the other hand, neat PBC showed sharp diffraction peaks at 2q ¼ 20.9 , 21.3 , 24.1 [18]. The WAXD patterns of the blends showed all the diffraction peaks of the neat components and no shifts were observed. The intensity of the diffraction peaks of one component decreases with decrease of its content. The WAXD results indicate that PBS and PBC crystallize separately in the blends; PBS blending with another semicrystalline polymer PBC does not modify the crystal structure in the blends but only reduces the intensity of the diffraction peaks.
3.4. Isothermal crystallization of PBS/PBC blends
Fig. 3. X-ray Diffraction diagram of PBS/PBC blends.
of the components [17]. However, the Tm of PBC is undetectable during the second heating scan, due to the fact the crystallization rate of PBC is very slow and it cannot crystallize during the cooling scan at the scanning rate. For the same reason, the Tc of PBC is also undetectable. Interestingly, the degree of crystallinity relative to PBS (Xc-PBS) increased with the addition of PBC, indicating that the crystallizability of PBS is enhanced by blending with PBC. Tc of PBS also increased by approximately 10 C after adding 10% PBC, but was almost unchanged with further increase of PBC content. This indicates that the crystallization rate of PBS is enhanced to some extent by PBC. Both the enhanced Xc-PBS and crystallization rate will be confirmed by the isothermal crystallization data discussed below. 3.3. Wide-angle X-ray diffraction analysis of PBS/PBC blends WAXD was employed to investigate the crystal structures of the blend samples. Fig. 3 shows the WAXD patterns of PBS/ PBC blends with various blend compositions. Neat PBS showed three strong diffraction peaks at around 19.5 , 21.5
The aforementioned results showed that both the degree of crystallinity relative to PBS and the crystallization rate of PBS are enhanced to some extent by PBC. The isothermal crystallization of PBS/PBC blends was conducted by DSC to confirm the results. The isothermal crystallization of PBS/PBC blends was investigated at 86 C for PBS. The exothermic crystallization curves of neat PBS and PBS/PBC blends at the crystallization temperatures are shown in Fig. 4a. It was found that the curves became sharper and the crystallization time become shorter with increasing PBC content. This indicates that the isothermal crystallization time for PBS at 86 C was shortened by the addition of PBC. The crystallization half-time (t1/2), i.e., the time at which the relative degree of crystallinity achieves 50%, can be determined from Fig. 4. t1/2 is a significant parameter for the discussion of crystallization kinetics as the crystallization rate is proportional to the reciprocal of t1/2. As displayed in Table 3, value of 1/t1/2 of PBS in the blends decreased with increasing PBC content at the same crystallization temperature, suggesting that the PBC evidently enhances the crystallization rate. It agrees well with the results mentioned before. At the same time, the degree of crystallinity relative to PBS, calculated by the melting enthalpy after isothermal crystallization, also increased with addition of PBC, which is consistent with the aforementioned results. As discussed before, PBS and PBC are immiscible, and the interface resulted from phase separated domains may
Fig. 4. (a) Plots of relative crystallinity of PBS crystallized at 86 C versus crystallization time for PBS/PBC blends and (b) the Avrami plots of PBS/PBC blends crystallized at 86 C.
44
J. Wang et al. / Polymer Testing 31 (2012) 39–45
also indicated that the crystallization rate of PBS was obviously increased by blending with PBC.
Table 3 Crystallization kinetic parameters of PBS at 86 C in PBS/PBC blends. PBS/PBC
DHm (PBS) (J/g) Xc-PBS (%) n
K (minn)
t1/2 (min)a
100/0 90/10 70/30 50/50
48.4 45.2 35.7 26.0
0.096 0.46 1.30 1.86
2.57 1.21 0.77 0.67
a
43.8 45.5 46.2 47.1
2.10 2.46 2.29 2.46
3.5. Mechanical properties of PBS/PBC blends
Obtained from Fig. 4a.
provide favorable nucleation sites for the crystallization of PBS. Therefore, the crystallization rate of PBS in the blends was enhanced by lowering the nucleation barrier. Similar results were also reported in PLLA/PCL blends [2,16]. In order to confirm this inference, the spherulitic morphologies of PBS/PBC blends were studied by polarized optical microscopy (POM). The spherulitic morphologies of neat PBS and 70/30 PBS/PBC crystallized for 30 min at 86 C are shown in Fig. 5. Clearly, the number of PBS nuclei significantly increased by adding 30% PBC. To investigate the isothermal crystallization kinetics in detail, the well-known Avrami equation was used. The expression for the equation is as follow:
Xt ¼ 1 expðKt n Þ
(1)
or
lgf ln½1 XðtÞg ¼ nlgt þ lgK
(2)
where X(t) is the relative degree of crystallinity, t is the crystallization time, n is the Avrami exponent, which depends on the nucleation mechanism and growth geometry of the crystals, and K is the crystallization rate constant. The plots of lg{ln[1 X(t)]} versus lg t for the neat PBS and PBS/PBC blends are shown in Fig. 4b. The Avrami exponents n and crystallization rate constants K are listed in Table 3. The Avrami exponent n for PBS changes little with the addition of PBC in the blends at the same isothermal crystallization temperature. This indicates that the crystal growth mechanism of PBS was not affected much after the PBC was added. From Table 3, it could also be seen that K of the PBS blends increases with increasing PBC content. This
As our working concept is to enhance the impact resistance of PBS by blending with PBC, the mechanical properties are of prime interest. Notched Izod impact strength, tensile properties and flexural properties of the PBS/PBC blends are summarized in Fig. 6. The tensile properties and flexural properties (Fig. 6a) of PBS/PBC blends follow a similar trend over the whole composition range. The tensile strength, flexural strength and flexural modulus decrease gradually with increasing PBC content, and have a plateau level when PBC content is between 20% and 70%. The elongation at break (Fig. 6b) increases with increasing PBC content and achieves a maximum value (>400%) when PBC content is 50%, and then decreases. The phenomenon that elongation at break has a maximum value when the PBC content is 50% can be attributed to the formation of the bicontinous structure at this composition, as confirmed by SEM and PCOM. In the crystalline PBS/crystalline PBC blends, PBC has good ductility while PBS has greater rigidity. When the PBC content is less than 30%, the impact strength increases gradually with increasing PBC content, while when the PBC content is more than 30% the impact strength increases sharply with increase of the PBC content. At PBC content higher than 60%, the impact strength tends to be constant (Fig. 6c). It can be observed that the PBS/PBC blends with 40–60% PBC possess comparatively good tensile, flexural and impact properties. This can be attributed to the fact that bicontinous phase morphology has been formed with the addition of 40–60% PBC in the blend. On the other hand, the semicrystalline polymer PBC, which has excellent mechanical properties, can also enhance the PBS crystallizability. This also contributes to the improvement of the properties of the PBS/PBC blends. The blend with 40% PBC possesses superior mechanical properties with impact strength achieving nearly 1000% of that of neat PBS, while the tensile strength is slightly decreased (only 15%).
Fig. 5. POM micrographs of (a) pure PBS and (b) 70/30 PBS/PBC crystallized at 86 C for 30 min from the melt. The scale bar in (a) is 10 mm and is also applied to (b).
J. Wang et al. / Polymer Testing 31 (2012) 39–45
45
Fig. 6. Mechanical properties of PBS/PBC blends (a) tensile strength (b) the elongation at break (c) Notched Izod impact strength.
4. Conclusions A series of PBS/PBC blends were prepared by melt blending. The miscibility, thermal properties, crystallization behavior and mechanical properties of the blends were investigated by SEM, PCOM, WAXD, DSC and tensile, flexural and impact tests. From the experiment results, the following conclusions were reached:
1) SEM and PCOM confirmed that the PBS/PBC blends showed two-phase structure and PBS and PBC were immiscible. 2) The WAXD results showed that the crystal structure of both PBS and PBC was not changed by blending, and PBS and PBC crystallize separately in the blends. 3) The interface of the immiscible blends provided favorable nucleation sites for the crystallization of PBS and thus increased the crystallization rate and degree of crystallinity of PBS in the blends. 4) The impact strength of PBS was improved substantially by blending with PBC. The impact strength of PBS can be improved by 9 times while its tensile strength was slightly decreased.
The PBS/PBC blends with excellent mechanical properties are promising to extend the application of PBS in fields such as biomedicine and pharmaceutics.
References [1] Y. Tokiwa, B.P. Calabia, J. Polym. Environ. 15 (2007) 259–267. [2] J. Lu, Z. Qiu, W. Yang, Polymer 48 (2007) 4196–4204. [3] X. Liu, C. Li, D. Zhang, Y. Xiao, G. Guan, Polym. Int. 55 (2006) 545–551. [4] D.J. Darensbourg, M.W. Holtcamp, Macromolecules 28 (1995) 7577–7579. [5] J. Matsuo, F. Sanda, T. Endo, Macromol. Chem. Phys. 199 (1998) 97–102. [6] B.D. Aln, S.H. Kim, Y.H. Kim, J.S. Yang, J. Appl. Polym. Sci. 82 (2001) 2808–2826. [7] T. Miyata, T. Masuko, Polymer 39 (1998) 1399–1404. [8] E.S. Yoo, S.S. Im, J. Polym. Sci. B. Polym. Phys. 37 (1999) 1357–1366. [9] X. Huang, C. Li, L. Zheng, D. Zhang, G. Guan, Y. Xiao, Polym. Int. 58 (2009) 893–899. [10] Z. Qiu, T. Ikehara, T. Nishi, Polymer 44 (2003) 2503–2508. [11] J.C. Lee, H. Tazawa, T. Ikehara, T. Nishi, Polym. J. 30 (1998) 327–339. [12] Z.B. Qiu, T. Ikehara, T. Nishi, Polymer 44 (2003) 3095–3099. [13] Y.J. Kim, O.O. Park, J. Appl. Polym. Sci. 72 (1999) 945–951. [14] W. Zhu, X. Huang, C. Li, Y. Xiao, D. Zhang, G. Guan, Polym. Int. 60 (2011) 1060–1067. [15] Y. Yamamoto, S. Kaihara, K. Toshima, S. Matsumura, Macromol. Biosci. 9 (2009) 968–978. [16] R. Dell’Erba, G. Groeninckx, G. Maglio, M. Malinconico, A. Migliozzi, Polymer 42 (2001) 7831–7840. [17] L. Jiang, B. Liu, J. Zhang, Eng. Chem. Res. 48 (2009) 7594–7602. [18] V. Pokharkar, S. Sivaram, Polymer 36 (1995) 4851–4854.
Contents lists available at SciVerse ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material properties
Fully biodegradable blends of poly(butylene succinate) and poly(butylene carbonate): Miscibility, thermal properties, crystallization behavior and mechanical properties Jin Wang a, b, Liuchun Zheng a, Chuncheng Li a, *, Wenxiang Zhu a, Dong Zhang a, Yaonan Xiao a, Guohu Guan a a
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China Graduate University of the Chinese Academy of Academy of Sciences, Beijing 100049, People’s Republic of China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 July 2011 Accepted 8 September 2011
Fully biodegradable poly(butylene succinate) (PBS) and poly(butylene carbonate) (PBC) blends were prepared by melt blending. Miscibility, thermal properties, crystallization behavior and mechanical properties of PBS/PBC blends were investigated by scanning electron microscopy (SEM), phase contrast optical microscopy (PCOM), differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD) and mechanical properties tests. The SEM and PCOM results indicated that PBS was immiscible with PBC. The WAXD results showed that the crystal structures of both PBS and PBC were not changed by blending and the two components crystallized separately in the blends. The isothermal crystallization data showed that the crystallization rate of PBS increased with the increase of PBC content in the blends. The impact strength of PBS was improved significantly by blending with PBC. When the PBC content was 40%, the impact strength of PBS was increased by nearly 9 times. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Biodegradable poly(butylene carbonate) Poly(butylene succinate) Miscibility Crystallization Mechanical properties
1. Introduction It is well known that a primary factor hindering the development of plastics is the pollution caused by petroleum-based plastic waste, especially polyolefins [1]. Aliphatic polyesters, such as poly(lactic acid), poly(butylene succinate), poly(hydroxybutyrate-co-valerate), and polycaprolactone [2–5], exhibit superior biodegradability and good comprehensive properties. Among these biodegradable aliphatic polyesters, PBS, a semicrystalline polymer, has a relative high melting temperature, excellent processing properties and thermal stability [6–8]. However, its insufficient mechanical properties of PBS, especially its poor impact strength, have prevented it from being used in many applications [9]. Therefore, various polymer blends have been * Corresponding author. Tel.: þ86 10 6256 0029; fax: þ86 10 6255 9373. E-mail address: [email protected] (C. Li). 0142-9418/$ – see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2011.09.005
produced in order to improve the physical properties and extend the field of application PBS, such as blending with poly(hydroxybutyrate) [10], poly(vinylidene fluoride) [11], poly(ethylene oxide) [12], poly(butylene terephthalate) [13], etc. Blending PBS with these biodegradable aliphatic polyester or polyethers could enhance the elongation and impact properties of PBS, but simultaneously lower tensile strength. Blending PBS with non-biodegradable polymers might improve the mechanical properties effectively but the degradable properties of the material would be undermined. Aliphatic polycarbonates (APCs), as another important class of biodegradable polymers, are widely used in the areas of packaging materials, drug carriers and tissue engineering for their favorable biodegradability, biocompatibility and non-toxicity [14,15]. Poly(butylene carbonate), which is a very important member of the APC family, is also a semicrystalline polymer with a clear melting point at around 60 C, as well as a glass transition
40
J. Wang et al. / Polymer Testing 31 (2012) 39–45
temperature at 38 C. During previous work, our team successfully synthesized high-molecular-weight PBC for the first time via a successive two-step polycondensation [14]. Due to the flexibility of the polymer chains and its crystallizability, PBC possesses excellent impact resistance and satisfactory tensile strength. Therefore, blending PBS with PBC is expected to improve the impact strength of PBS while hardly sacrificing other properties, and at the same time a fully biodegradable material can be prepared. To the best of our knowledge, no attention has been paid to the blending of PBS and PBC, which may be due to the unavailability of high-molecular-weight PBC. In the present paper, blends of PBS and PBC were prepared and the miscibility, thermal properties, crystallization behavior and mechanical properties of PBS/PBC blends were investigated. 2. Experimental 2.1. Materials and blends preparation The starting polymers used in this study were PBS and PBC, which were synthesized according to the procedures reported previously, respectively [9,14]. The molecular weigh and molecular weight distribution, which were determined on a Waters 2695 GPC gel permeation chromatography using polystyrene as standard and CHCl3 as solution of such materials, are reported in Table 1. All the investigated samples were obtained by means of melt blending. PBC and PBS were dried in vacuo at 40 C for 12 h before mixing. The blending was carried out at 150 C and 50 rpm in a Haake mixer with a blending time of 6 min. After blending, all the samples were cooled to room temperature in air.
and an Olympus (C-5050ZOOM) camera. A Linkam-THMS 600 hot stage was used to control the experimental temperature. The ultrathin film was prepared by hotpressing at 150 C under a pressure of 2.5 MPa for 30 s. For the PCOM observation, the film was heated to 150 C, held there for 5 min to eliminate any thermal history and then cooled to 130 C at a rate of 100 C/min for phase separation. For the POM characterization, the film was heated to 150 C, held there for 5 min to eliminate any thermal history and then cooled to 86 C as quickly as possible for isothermal crystallization. 2.4. Differential scanning calorimetry (DSC) The thermal properties and isothermal crystallization behavior were measured using a Pyris Diamond DSC instrument (Perkin Elmer). The calibration of the temperature was performed using indium as the standard before the measurements. All the measurements were conducted under a high-purity nitrogen atmosphere. The weight of sample was 5 0.2 mg. For the basal thermal parameters, the samples were heated to 150 C at 20 C/min and held there for 5 min before cooling to 100 C at the same rate. After being held at 100 C for 5 min, the samples were heated again to 150 C at 20 C/min and equilibrated there for 5 min. The isothermal crystallization was performed as follows. The samples sealed in aluminum pans were heated to 150 C, held there for 5 min to erase any thermal history and then cooled to the designated crystallization temperature in the range of 80–94 C for isothermal crystallization at a rate of 200 C/min. The curves of heat flow as a function of time were recorded. 2.5. Wide-angle X-ray diffraction (WAXD)
2.2. Scanning electron microscopy (SEM) The phase morphologies of blends were examined by scanning electron microscopy (SEM; JSM-5900LV, JEOL, Japan) at an accelerating voltage of 20 kV. The injectionmolded tensile specimens were cryo-fractured in liquid nitrogen. Since tetrahydrofuran is a good solvent for PBC but a nonsolvent for PBS, it was chosen as the solvent to extract the PBC on the fractured surfaces. All the surfaces were dried and sputter-coated with platinum for 60 s prior to examination. 2.3. Phase contrast optical microscopy (PCOM) and polarized optical microscopy (POM) The phase contrast optical microscopy (PCOM) was carried out using an Olympus (BX51) optical microscope Table 1 Molecular weight and molecular weight distribution of PBS and PBC. Sample
Mn (104)
Mwa (104)
PDIa
PBC PBS
3.5 3.5
8.0 8.1
2.29 2.31
a Abbreviation of weight-average molecular weight and molecular weight distribution, respectively.
WAXD was performed at room temperature with a Ragaku Model D/max-2B diffractometer using Cu Ka radiation (40 kV, 200 mA), and test data were collected from 2q ¼ 10 to 40 at a scanning rate of 8 /min. The samples used for WAXD characterizations were prepared in the following ways. PBS, PBC and PBS/PBC small particles were first placed on a mold with a thickness of 1 mm between two thick teflon sheets to mold at 150 C under a pressure of 1.5 MPa for 30 s. After that, the polymer models were transferred rapidly into an oven at room temperature for isothermal crystallization. 2.6. Mechanical properties testing Tensile and flexural testing was determined with a universal tester (Instron 1122, UK). The tensile properties of the specimens were measured in general accordance with ISO 527 at a crosshead speed of 50 mm/min; the flexural properties were determined in general accordance with ISO 178 at a crosshead speed of 2 mm/min. The impact strength was measured with an impact testing machine (CSI-137C, USA) in general accordance with ISO 180. The results were taken as an average from measurements of at least five specimens.
J. Wang et al. / Polymer Testing 31 (2012) 39–45
3. Results and discussion 3.1. Miscibility of PBS/PBC blends It’s known that many properties of blends are affected by their miscibility. As the Tg of PBS (43 C) is very close to that of PBC (38 C), the miscibility of PBS and PBC was studied by phase morphology in this work. The morphology of the PBS/PBC blends were investigated by means of SEM. Fig. 1 presents the SEM micrographs of tetrahydrofuran-etched fracture of the 90/10, 70/30, 60/40, 50/50, 40/60 and 30/70 PBS/PBC blends specimens. For the 90/10 PBS/PBC blends, the discrete PBC particles with
41
average diameter of about 1 mm were uniformly dispersed in the continuous PBS matrix (Fig. 1a). With increasing PBC content to 30%, the size of the PBC particles increased to 2– 3 mm and the shape became irregular (Fig. 1b). As the PBC content increased to 40–60%, the PBS and PBC gradually formed a bicontinuous structure (Fig. 1c–e). When the PBC increased to 70%, the PBC became the matrix and the PBS became the dispersed phase (Fig. 1f). These features are typical of incompatible polymer blends. Fig. 2 shows phase contrast optical micrographs of 70/ 30, 50/50 and 30/70 PBS/PBC blends melted at 150 C for 5 min and then isothermally annealed at 130 C for 30 min. Based on the information provided in Fig. 2, it is obvious
Fig. 1. SEM images tetrahydrofuran-etched fracture ends of the PBS/PBC blends (a) 90/10 PBS/PBC (b) 70/30 PBS/PBC (c) 60/40 PBS/PBC (d) 50/50 PBS/PBC (e) 40/60 PBS/PBC and (f) 30/70 PBS/PBC.
42
J. Wang et al. / Polymer Testing 31 (2012) 39–45
Fig. 2. Phase contrast optical micrographs of PBS/PBC blends annealed at 130 for 30 min (a) 70/30 (b) 50/50 and (c) 30/70. The scale bar in (a) is 10 mm and is the same for all pictures.
that phase separation occurred after annealing for 30 min at 130 C. We can conclude that the dark part is the PBS-rich domain and the light part is the PBC-rich domain with the PBC phase being 2–3 mm in diameter, as shown in Fig. 2a. The PBC domain phase dimensions increase with increasing PBC content. When the PBC content increased to 70%, phase inversion occurred as shown in Fig. 2c. The distinct phase-separated morphology indicates that the compatibility between PBS and PBC was not good and is consistent with the SEM results.
3.2. Thermal properties of PBS/PBC blends DSC was used to evaluate the thermal properties of PBS/ PBC blends. The heating and cooling results are summarized in Table 2. It can be found from Table 2 that neat PBS has a Tm of 115.5 C, while neat PBC has a Tm of 55.9 C. In the blends, the Tm of PBC and PBS is at around 55.9 C and 115.5 C, respectively, and they are almost unchanged with variation of the composition. This may be a result of the immiscibility
Table 2 Thermal properties of PBS and PBS/PBC blends. PBS/PBC
PBC
PBS
DHma (J/g)
Tma ( C)
100/0 90/10 70/30 50/50 30/70 0/100
Tma ( C)
1st
2nd
1st
2nd
1st
2nd
– – 54.1 55.3 54.1 55.9
– – – – – –
– – 13.1 20.0 28.1 42.1
– – – – – –
115.6 116.0 115.0 116.8 114.3 –
115.5 115.9 115.3 116.5 114.4 –
Tca ( C)
DHma (J/g) 1st
2nd
1st
2nd
1st
2nd
61.5 73.7 74.4 74.4 – –
63.9 69.6 58.5 34.8 23.7 –
73.3 74.6 60.2 37.0 23.9 –
57.9 63.0 53.0 31.5 21.5 –
66.4 67.6 54.5 33.5 21.6 –
57.9 70.0 75.7 63.0 71.6 –
66.4 75.1 77.9 67.0 72.0 –
Xcb (%)
Xc-PBSc (%)
1st Measured from the first DSC heating scan. 2nd Measured from the second DSC heating scan. a The crystallization temperatures (Tc) taken from cooling scan, the melting temperatures (Tm), melting enthalpies (DHm) were registered by DSC. b The crystallinity degree (Xc) was calculated by dividing the observedDHm from the first/second heating trace by the theoretical value (110.4 J/g) for a 100% crystalline PBS. Xc ¼ DHm/DHm*,DHm* ¼ 110.4 J/g. c The degree of crystallinity (Xc) the crystallinity degree relative to PBS (Xc-PBS) was calculated by Xc-PBS ¼ Xc/PBS wt%.
J. Wang et al. / Polymer Testing 31 (2012) 39–45
43
and 22.5 corresponding to (020), (021) and (110) planes [13], respectively. On the other hand, neat PBC showed sharp diffraction peaks at 2q ¼ 20.9 , 21.3 , 24.1 [18]. The WAXD patterns of the blends showed all the diffraction peaks of the neat components and no shifts were observed. The intensity of the diffraction peaks of one component decreases with decrease of its content. The WAXD results indicate that PBS and PBC crystallize separately in the blends; PBS blending with another semicrystalline polymer PBC does not modify the crystal structure in the blends but only reduces the intensity of the diffraction peaks.
3.4. Isothermal crystallization of PBS/PBC blends
Fig. 3. X-ray Diffraction diagram of PBS/PBC blends.
of the components [17]. However, the Tm of PBC is undetectable during the second heating scan, due to the fact the crystallization rate of PBC is very slow and it cannot crystallize during the cooling scan at the scanning rate. For the same reason, the Tc of PBC is also undetectable. Interestingly, the degree of crystallinity relative to PBS (Xc-PBS) increased with the addition of PBC, indicating that the crystallizability of PBS is enhanced by blending with PBC. Tc of PBS also increased by approximately 10 C after adding 10% PBC, but was almost unchanged with further increase of PBC content. This indicates that the crystallization rate of PBS is enhanced to some extent by PBC. Both the enhanced Xc-PBS and crystallization rate will be confirmed by the isothermal crystallization data discussed below. 3.3. Wide-angle X-ray diffraction analysis of PBS/PBC blends WAXD was employed to investigate the crystal structures of the blend samples. Fig. 3 shows the WAXD patterns of PBS/ PBC blends with various blend compositions. Neat PBS showed three strong diffraction peaks at around 19.5 , 21.5
The aforementioned results showed that both the degree of crystallinity relative to PBS and the crystallization rate of PBS are enhanced to some extent by PBC. The isothermal crystallization of PBS/PBC blends was conducted by DSC to confirm the results. The isothermal crystallization of PBS/PBC blends was investigated at 86 C for PBS. The exothermic crystallization curves of neat PBS and PBS/PBC blends at the crystallization temperatures are shown in Fig. 4a. It was found that the curves became sharper and the crystallization time become shorter with increasing PBC content. This indicates that the isothermal crystallization time for PBS at 86 C was shortened by the addition of PBC. The crystallization half-time (t1/2), i.e., the time at which the relative degree of crystallinity achieves 50%, can be determined from Fig. 4. t1/2 is a significant parameter for the discussion of crystallization kinetics as the crystallization rate is proportional to the reciprocal of t1/2. As displayed in Table 3, value of 1/t1/2 of PBS in the blends decreased with increasing PBC content at the same crystallization temperature, suggesting that the PBC evidently enhances the crystallization rate. It agrees well with the results mentioned before. At the same time, the degree of crystallinity relative to PBS, calculated by the melting enthalpy after isothermal crystallization, also increased with addition of PBC, which is consistent with the aforementioned results. As discussed before, PBS and PBC are immiscible, and the interface resulted from phase separated domains may
Fig. 4. (a) Plots of relative crystallinity of PBS crystallized at 86 C versus crystallization time for PBS/PBC blends and (b) the Avrami plots of PBS/PBC blends crystallized at 86 C.
44
J. Wang et al. / Polymer Testing 31 (2012) 39–45
also indicated that the crystallization rate of PBS was obviously increased by blending with PBC.
Table 3 Crystallization kinetic parameters of PBS at 86 C in PBS/PBC blends. PBS/PBC
DHm (PBS) (J/g) Xc-PBS (%) n
K (minn)
t1/2 (min)a
100/0 90/10 70/30 50/50
48.4 45.2 35.7 26.0
0.096 0.46 1.30 1.86
2.57 1.21 0.77 0.67
a
43.8 45.5 46.2 47.1
2.10 2.46 2.29 2.46
3.5. Mechanical properties of PBS/PBC blends
Obtained from Fig. 4a.
provide favorable nucleation sites for the crystallization of PBS. Therefore, the crystallization rate of PBS in the blends was enhanced by lowering the nucleation barrier. Similar results were also reported in PLLA/PCL blends [2,16]. In order to confirm this inference, the spherulitic morphologies of PBS/PBC blends were studied by polarized optical microscopy (POM). The spherulitic morphologies of neat PBS and 70/30 PBS/PBC crystallized for 30 min at 86 C are shown in Fig. 5. Clearly, the number of PBS nuclei significantly increased by adding 30% PBC. To investigate the isothermal crystallization kinetics in detail, the well-known Avrami equation was used. The expression for the equation is as follow:
Xt ¼ 1 expðKt n Þ
(1)
or
lgf ln½1 XðtÞg ¼ nlgt þ lgK
(2)
where X(t) is the relative degree of crystallinity, t is the crystallization time, n is the Avrami exponent, which depends on the nucleation mechanism and growth geometry of the crystals, and K is the crystallization rate constant. The plots of lg{ln[1 X(t)]} versus lg t for the neat PBS and PBS/PBC blends are shown in Fig. 4b. The Avrami exponents n and crystallization rate constants K are listed in Table 3. The Avrami exponent n for PBS changes little with the addition of PBC in the blends at the same isothermal crystallization temperature. This indicates that the crystal growth mechanism of PBS was not affected much after the PBC was added. From Table 3, it could also be seen that K of the PBS blends increases with increasing PBC content. This
As our working concept is to enhance the impact resistance of PBS by blending with PBC, the mechanical properties are of prime interest. Notched Izod impact strength, tensile properties and flexural properties of the PBS/PBC blends are summarized in Fig. 6. The tensile properties and flexural properties (Fig. 6a) of PBS/PBC blends follow a similar trend over the whole composition range. The tensile strength, flexural strength and flexural modulus decrease gradually with increasing PBC content, and have a plateau level when PBC content is between 20% and 70%. The elongation at break (Fig. 6b) increases with increasing PBC content and achieves a maximum value (>400%) when PBC content is 50%, and then decreases. The phenomenon that elongation at break has a maximum value when the PBC content is 50% can be attributed to the formation of the bicontinous structure at this composition, as confirmed by SEM and PCOM. In the crystalline PBS/crystalline PBC blends, PBC has good ductility while PBS has greater rigidity. When the PBC content is less than 30%, the impact strength increases gradually with increasing PBC content, while when the PBC content is more than 30% the impact strength increases sharply with increase of the PBC content. At PBC content higher than 60%, the impact strength tends to be constant (Fig. 6c). It can be observed that the PBS/PBC blends with 40–60% PBC possess comparatively good tensile, flexural and impact properties. This can be attributed to the fact that bicontinous phase morphology has been formed with the addition of 40–60% PBC in the blend. On the other hand, the semicrystalline polymer PBC, which has excellent mechanical properties, can also enhance the PBS crystallizability. This also contributes to the improvement of the properties of the PBS/PBC blends. The blend with 40% PBC possesses superior mechanical properties with impact strength achieving nearly 1000% of that of neat PBS, while the tensile strength is slightly decreased (only 15%).
Fig. 5. POM micrographs of (a) pure PBS and (b) 70/30 PBS/PBC crystallized at 86 C for 30 min from the melt. The scale bar in (a) is 10 mm and is also applied to (b).
J. Wang et al. / Polymer Testing 31 (2012) 39–45
45
Fig. 6. Mechanical properties of PBS/PBC blends (a) tensile strength (b) the elongation at break (c) Notched Izod impact strength.
4. Conclusions A series of PBS/PBC blends were prepared by melt blending. The miscibility, thermal properties, crystallization behavior and mechanical properties of the blends were investigated by SEM, PCOM, WAXD, DSC and tensile, flexural and impact tests. From the experiment results, the following conclusions were reached:
1) SEM and PCOM confirmed that the PBS/PBC blends showed two-phase structure and PBS and PBC were immiscible. 2) The WAXD results showed that the crystal structure of both PBS and PBC was not changed by blending, and PBS and PBC crystallize separately in the blends. 3) The interface of the immiscible blends provided favorable nucleation sites for the crystallization of PBS and thus increased the crystallization rate and degree of crystallinity of PBS in the blends. 4) The impact strength of PBS was improved substantially by blending with PBC. The impact strength of PBS can be improved by 9 times while its tensile strength was slightly decreased.
The PBS/PBC blends with excellent mechanical properties are promising to extend the application of PBS in fields such as biomedicine and pharmaceutics.
References [1] Y. Tokiwa, B.P. Calabia, J. Polym. Environ. 15 (2007) 259–267. [2] J. Lu, Z. Qiu, W. Yang, Polymer 48 (2007) 4196–4204. [3] X. Liu, C. Li, D. Zhang, Y. Xiao, G. Guan, Polym. Int. 55 (2006) 545–551. [4] D.J. Darensbourg, M.W. Holtcamp, Macromolecules 28 (1995) 7577–7579. [5] J. Matsuo, F. Sanda, T. Endo, Macromol. Chem. Phys. 199 (1998) 97–102. [6] B.D. Aln, S.H. Kim, Y.H. Kim, J.S. Yang, J. Appl. Polym. Sci. 82 (2001) 2808–2826. [7] T. Miyata, T. Masuko, Polymer 39 (1998) 1399–1404. [8] E.S. Yoo, S.S. Im, J. Polym. Sci. B. Polym. Phys. 37 (1999) 1357–1366. [9] X. Huang, C. Li, L. Zheng, D. Zhang, G. Guan, Y. Xiao, Polym. Int. 58 (2009) 893–899. [10] Z. Qiu, T. Ikehara, T. Nishi, Polymer 44 (2003) 2503–2508. [11] J.C. Lee, H. Tazawa, T. Ikehara, T. Nishi, Polym. J. 30 (1998) 327–339. [12] Z.B. Qiu, T. Ikehara, T. Nishi, Polymer 44 (2003) 3095–3099. [13] Y.J. Kim, O.O. Park, J. Appl. Polym. Sci. 72 (1999) 945–951. [14] W. Zhu, X. Huang, C. Li, Y. Xiao, D. Zhang, G. Guan, Polym. Int. 60 (2011) 1060–1067. [15] Y. Yamamoto, S. Kaihara, K. Toshima, S. Matsumura, Macromol. Biosci. 9 (2009) 968–978. [16] R. Dell’Erba, G. Groeninckx, G. Maglio, M. Malinconico, A. Migliozzi, Polymer 42 (2001) 7831–7840. [17] L. Jiang, B. Liu, J. Zhang, Eng. Chem. Res. 48 (2009) 7594–7602. [18] V. Pokharkar, S. Sivaram, Polymer 36 (1995) 4851–4854.