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Fully bio-based polymer blend of polyamide 11 and Poly(vinylcatechol) showing thermodynamic miscibility and excellent engineering properties
Polymer 181 (2019) 121667
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Fully bio-based polymer blend of polyamide 11 and Poly(vinylcatechol) showing thermodynamic miscibility and excellent engineering properties
T
Takayuki Hiraia,∗, Jumpei Kawadaa, Mamiko Naritaa, Taiji Ikawaa, Hisaaki Takeshimab, Kotaro Satohb, Masami Kamigaitob a
Material and Processing Department, Polymer Processing and Mechanics Laboratories, Toyota Central R&D Laboratories, Inc., 41-1 Yokomichi, Nagakute, 480-1192, Japan b Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan
HIGHLIGHTS
polymer blends of polyamide 11 and poly(vinyl catechol) are prepared. • Bio-based miscibility is observed, with T shifting and a smooth surface. • Thermodynamic polyamide 11 with 15 wt% poly(vinyl catechol) increases the T by 26 °C. • Blending • The tensile properties and flowability are also improved by blending. g
g
ARTICLE INFO
ABSTRACT
Keywords: Polyamide 11 poly(vinylcatechol) Bio-based polymer Polymer blend Miscibility Glass transition temperature
Fully bio-based polyamide 11 (PA11) and poly(vinylcatechol) (PVCa) blends prepared by melt mixing demonstrate thermodynamic miscibility and excellent engineering properties. The glass transition temperature (Tg) of PA11 increases upon blending with PVCa; an 85/15 wt% PA11/PVCa blend exhibits a Tg 23–26 °C higher than that of PA11 devoid of additives. Morphological observations revealed that the PA11/PVCa blends do not phaseseparate, confirming the homogeneity of PA11 and PVCa. Good chemical resistance of the PA11/PVCa blends was confirmed, with the blends resisting morphological changes even after immersion in methanol, which is a good solvent for PVCa. Tensile testing revealed that the PA11/PVCa blends have higher moduli and strengths than PA11. A PA11/nonpolar polystyrene blend was also examined by the same experimental procedure, which revealed that strong hydrogen bonding between PA11 and PVCa is the primary reason for the miscibility and excellent performance of PA11/PVCa blends.
1. Introduction The use of renewable resources offers a solution to the problem of fossil fuel depletion [1–7]. Bio-based polymers produced from natural resources have received much attention for realizing a sustainable material supply [1–7]. Poly(lactic acid) (PLA), polyamide 11 (PA11), poly(hydroxybutyrate) (PHB), and poly(butylenesuccinate) (PBS) are typical examples of bio-based polymers, which have already been applied to products such as packaging, electronics, and automobile parts [2]. However, the properties of bio-based polymers are typically inferior to those of petroleum-based polymers, which restricts their applications [6]. PA11 synthesized from castor bean plants is an industrial bio-based polymer; however, its glass transition temperature (Tg) is lower than ∗
that of conventional polyamide 6 (PA6). The Tg of a polymer determines its acceptable operating temperature, and increasing the Tg of PA11 is necessary to widen its applicability and replace petrochemicalbased PA6. Polymer blending is an attractive method for producing new polymer materials without the need for new synthetic plants; moreover, the properties of polymer blends can be controlled by adjusting their composition ratios [8]. Numerous attempts have been made to enhance the properties of bio-based polymers by polymer blending [8–12]. To improve the Tg by polymer blending, the constituent polymers need to be thermodynamically miscible, because the Tg of a miscible system shifts from that of the constituent polymers. However, most polymer combinations are immiscible because of their small entropic benefits [13]. Miscible polymer blends can be obtained with sufficient
Corresponding author. E-mail address: [email protected] (T. Hirai).
https://doi.org/10.1016/j.polymer.2019.121667 Received 23 May 2019; Received in revised form 12 July 2019; Accepted 18 July 2019 Available online 05 September 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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exothermic interactions between polymer chains, such as hydrogen bonding [14–17]. There are some studies on miscible polymer blends including biobased polymers [18–20]. Zhang et al. reported that PLA/poly(methylmethacrylate) (PMMA) blends showed thermodynamic miscibility and a single Tg between 56 °C for PLA and 104 °C for PMMA [18]. This indicates that the Tg of PLA can be increased by blending with miscible PMMA. However, incorporating petrochemical-based PMMA decreases the amount of bio-based polymer in the blend. Fully bio-based polymer blends such as PLA/PA11 [21,22] and PLA/PBS [23,24] have been reported; however, these blends are intrinsically immiscible. To our knowledge, a PLA/low-molecular weight PHB blend is the only example of a fully bio-based miscible polymer blend; unfortunately, PLA/PHB blends have a low Tg due to the low Tg of PHB [8,25,26]. In previous studies on petrochemical-based polyamide blends, it has been reported that polyamides and phenol-containing polymers such as phenol novolac or poly(vinylphenol) are miscible [27–32]. These studies suggested that the driving force for such miscibility is hydrogen bonding between the carbonyl groups of polyamide and the phenolic hydroxyl groups [27–32]. Miscible polymer blends of PA11 and other phenol-containing polymers are also expected to be fabricated. One of the most promising candidates for the formation of strong hydrogen bonds with PA11 is poly(vinylcatechol) (PVCa). PVCa has two phenolic hydroxyl groups per monomer unit in a poly(styrene)-like (PSt-like) structure, as shown in Fig. 1. One of our research groups recently reported the synthesis of PVCa from naturally occurring compounds such as ferulic and caffeic acids [33,34], which are abundant in rice bran and coffee beans, respectively. These acids were easily decarboxylated in the presence of triethylamine into styrene monomers (i.e., 4-vinylguaiacol (4VG) or vinylcatechol), which were radically polymerized after protection of the phenolic groups to yield PVCa after deprotection. In this study, fully bio-based PA11/PVCa blends were prepared by melt mixing. The miscibility of the blends was studied by Tg measurements and morphological observations. Furthermore, to evaluate the industrial applicability of the blends, their rheological behavior, chemical resistance, and mechanical properties were investigated. A PA11/PSt blend was also examined by the same experimental procedure to assess the influence of the polar functional groups of PVCa on the miscibility and other properties of the PA11/PVCa blends.
Table 1 Sample designations and constituent weight fractions. Sample designation
PA11 (wt%)
PVCa (wt%)
PSt (wt%)
PA11 PA11/PVCa5 PA11/PVCa15 PA11/PSt15
100 95 85 85
0 5 15 0
0 0 0 15
Fig. 2. (a) Time evolution of viscosities during melt mixing. The encircled numbers represent the GPC measurement points. (b) GPC curves of pristine PA11, melt-mixed PA11, and the polymer blends.
Fig. 3. DSC curves during the first cooling scan of melt-mixed PA11 and polymer blends. The dotted line shows the peak temperature of PA11 crystallization (based on the exothermic peak for PA11).
2. Experimental section 2.1. Preparation of Poly(vinylcatechol) Protected 4VG was synthesized by referring to a reported procedure [33]. Triethylsilane (94 mL, 0.590 mol) and a toluene solution of tris (pentafluorophenyl)boron (7.5 mL, 40 mM) were stirred in toluene (114 mL). Then, commercially available 4VG (synthesized via the decarboxylation of ferulic acid (40 mL, 0.295 mol)) was diluted in toluene (40 mL) and slowly added to the prepared triethylsilane solution. The silylation reaction proceeded at 0 °C for 12 h, and bis(triethylsilyl)-
Fig. 1. (a) Molecular structures of PVCa and PSt. (b) Schematic diagram of hydrogen bonding between PVCa and PA11.
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Fig. 4. DSC curves during the second heating scan of (a) PVCa and PSt before melt mixing, and (b) melt-mixed PA11 and polymer blends.
Fig. 5. Viscoelastic behavior of melt-mixed PA11 and polymer blends. Temperature dependencies of (a) E′ and (b) tan δ.
protected vinylcathechol (TES2VC) was obtained after washing with water three times and drying. Nitroxide-mediated polymerization of TES2VC was conducted by using N-tert-butyl-N-(2-methyl-1-phenylpropyl)-O-(1-phenylethyl)hydroxylamine (PhEt-TIPNO) as the initiator. TES2VC (22.1 ml, 56.4 mmol) was added to PhEt-TIPNO (0.2038 g, 0.626 mmol) under an argon atmosphere, and polymerization was performed at 110 °C for 66 h. The number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of poly(TES2VC) were experimentally determined as 17,200 and 1.13, respectively (Fig. S1). Finally, PVCa was obtained after deprotection of the silyl groups of poly(TES2VC) with concentrated HCl in tetrahydrofuran. Successful deprotection was confirmed by nuclear magnetic resonance spectroscopy (Fig. S2).
the same conditions. PA11 devoid of additives was also melt-mixed to obtain homo-PA11 with the same process history. While mixing, the pressures of the melted polymers were measured at two points in the extruder, and time revolutions of viscosities were calculated based on the measured pressure losses (Fig. S3). 2.3. Gel permeation chromatography (GPC) The molecular weight changes upon melt mixing were measured using gel permeation chromatography (GPC; HLC-8220, TOSOH, Japan) with a PMMA standard. Hexafluoroisopropanol (HFIP) solutions of pristine PA11, melt-mixed PA11, and three polymer blends (PA11/ PVCa5, PA11/PVCa/15, and PA11/PSt15), each at 0.1% w/v, were subjected to gel permeation chromatography at a flow rate of 0.8 mL/ min for 100 mL at 40 °C.
2.2. Blend preparation The PA11 used in this study was a natural-grade PA11 (Rilsan BMNO, ARKEMA, France). PA11 and PVCa were vacuum-dried at 80 °C for 12 h and melt-mixed using a micro compounder (HAAKE Minilab, Thermo Fisher Scientific, Germany). The sample designations and weight fractions of the fabricated polymer blends are listed in Table 1. Melt-mixing under nitrogen flow was conducted at a cylinder temperature of 230 °C and a screw speed of 200 rpm for a mixing time of 5 min. PA11 and PSt (Aldrich, Mw = 35,000) were melt-mixed under
2.4. Differential scanning calorimetry (DSC) PA11 and the polymer blends were evaluated using differential scanning calorimetry (DSC; Q1000, TA Instrument, USA). Samples of approximately 10 mg were placed in aluminum pans and sealed, and measurements were carried out between −50 and 230 °C at a heating rate of 10 °C/min and cooling rate of 20 °C/min under nitrogen flow. The crystallization behavior was evaluated during the first cooling run,
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Fig. 6. SEM images of fracture surfaces of untreated blends and blends immersed in methanol or toluene for 15 min. Scale bar, 10 μm.
rectangular specimens (35 × 5 × 0.5 mm) were obtained after cold pressing. The specimens were vacuum-dried at 80 °C for 8 h prior to any measurement. DMA measurements were carried out between −100 and 200 °C at a heating rate of 5 °C/min and a frequency of 10 Hz in a dynamic mechanical analyzer (DVA-225, ITK, Japan). The temperature dependencies of storage modulus (E′) and loss modulus (E″) were obtained from these measurements. Here, Tg was considered the peak temperature of tan δ (= E"/E′). 2.6. Scanning electron microscopy (SEM) The cryo-fractured surfaces of the polymer blends were fabricated by fracturing rectangular specimens in liquid nitrogen. Then, the specimens were immersed in methanol (good solvent for PVCa) [33] or toluene (good solvent for PSt), and ultrasonicated for 15 min. The fracture surfaces devoid of treatment or after immersion in an organic solvent were sputter-coated with platinum and observed by scanning electron microscopy (SEM; S–3600N, Hitachi, Japan) at an accelerating voltage of 15 kV. 2.7. Transmission electron microscopy (TEM) Rectangular specimens of PA11/PVCa15 and PA11/PSt15 were cryo-microtomed into thin films with a thickness of 100 nm. The PA11 in the samples was stained by ruthenium tetroxide, and transmission electron microscopy (TEM; JEM-2100F, JEOL, Japan) observations were carried out at an accelerating voltage of 120 kV.
Fig. 7. TEM images of stained thin films of (a) PA11/PVCa15 and (b) PA11/ PSt15. The areas indicated by black boxes are shown at higher magnification in the right-hand-side images.
and the crystallization temperature (Tc) was determined as the peak temperature of the exothermic peak. The Tg was taken as the mid-point of the change in heat capacity during the second heating run to ensure the samples had consistent thermal histories.
2.8. Tensile testing The melt-mixed PA11 and polymer blends were molded into rectangular specimens (50 × 10 × 0.5 mm) by the same procedure as that used to prepare the DMA sample. Tensile testing was conducted using a universal testing machine (model 5566, Instron, USA) after vacuum drying the specimens at 80 °C for 8 h. The grip distance was 20 mm, and the deformation speed was set to 2 mm/min. The tensile modulus was defined as the slope of the obtained stress–strain curves between 0.5%
2.5. Dynamic mechanical analysis (DMA) The extruded samples were hot-pressed at 230 °C for 1 min in a laboratory compression machine. Then, to increase the degree of crystallinity, the pressed samples were annealed at 150 °C for 1 min. Finally,
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Fig. 8. Mechanical properties and deformation behavior of PA11 and polymer blends. (a) Stress–strain curves of specimens. (b) Tensile moduli of specimens calculated from stress–strain curves. Error bars represent standard deviations (n = 3). (c) Appearances of specimens after tensile testing, which reveal the deformation history of each specimen.
and 2.5% strain.
PA11/PVCa blends slightly decreased during melt-mixing, and a loading of only 5 wt% PVCa maintained the low viscosity of PA11. From an industrial viewpoint, these results imply that the PA11/PVCa blends have superior flowabilities to PA11. The molecular weights of PA11 in PA11/PVCa5 (encircled 3) and PA11/PVCa15 (encircled 4) were almost the same as that of pristine PA11 or decreased during melt mixing. The results suggest that PVCa inhibits the post-condensation reaction. The viscosity of PA11/PSt15 was low in the early stage of mixing due to the low viscosity of PSt at 230 °C. In the latter half of the melt-mixing process, the viscosity of PA11/PSt15 increased. This is considered to be caused by the post-condensation of PA11, because the molecular weight of PA11 in the PA11/PSt15 blend (encircled 5) increased after 5 min of mixing, which is similar to that observed for homo-PA11.
2.9. Fourier transform infrared spectroscopy (FT-IR) Infrared (IR) spectra of homo polymers (PA11, PVCa, and PSt) and polymer blends (PA11/PVCa15 and PA11/PSt15) were recorded with a Fourier transform infrared spectrometer (FT-IR AVATAR360, Nicolet, USA). The measurements were conducted under attenuated total reflectance (ATR) mode for 32 scans at a resolution of 2 cm−1. 3. Results and discussion 3.1. Rheological behavior and molecular weight change The viscosities of PA11 and the polymer blends during the meltblending process are shown in Fig. 2(a). PA11 devoid of additive became viscous; its viscosity doubled after 5 min of mixing. The molecular weight distributions are shown in Fig. 2(b), and the results for pristine PA11 (encircled 1) and melt-mixed PA11 (encircled 2) reveal that the molecular weight of PA11 increased after melt mixing. The reason for the increase in viscosity during melt mixing was considered to be a post-condensation reaction, which has been observed during the meltmixing of polyamides [35,36]. On the other hand, the viscosities of the
3.2. DSC measurements Fig. 3 shows the DSC curves during the first cooling scan. Exothermic peaks of PA11 crystallization were observed for all samples, and the PA11/PVCa blends showed lower Tc than PA11. In a miscible system, the crystalline component pushes out the non-crystalline component during crystallization [37,38], so a lowered Tc is a typical characteristic of such blends [32,39]. On the other hand, PA11/PSt15
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higher than those of PA11. The Tg of PA11/PVCa15 is ca. 26 °C higher than that of PA11, as well as higher than that of conventional PA6 (Fig. S5). PA11/PSt15 shows a bi-modal behavior, with its peak appearing at a lower temperature that corresponds to the Tg of PA11. The softening point and Tg of PA11/PSt15 observed at the higher temperature were confirmed to be very close to those of PSt (Fig. S6); that is, PA11/PSt15 shows the typical behavior of an immiscible polymer blend. 3.4. SEM observations SEM images of untreated specimens and specimens after immersion in an organic solvent are shown in Fig. 6. The insets show the appearance of PVCa or PSt solely immersed in each solvent. Untreated PA11/PVCa15 had a smooth surface that remained unchanged even after immersion in methanol, which is a good solvent for PVCa, as shown in the inset image. Toluene caused no change in PA11/PVCa15 because PVCa is insoluble in toluene. On the other hand, untreated PA11/PSt15 shows a sea–island structure; the island phase was determined to be PSt from the composition ratio. After immersion in methanol, which is a poor solvent for PSt, some PSt particles precipitated during ultrasonication due to few interactions between PA11 and PSt. After immersion in toluene, the PSt phases were extracted from the surface, and many holes with sizes of 1–20 μm are observed. Incorporating an amorphous polymer into a crystalline polymer often causes a loss of chemical resistance [13], as shown in the PA11/PSt15 blend. However, the PA11/PVCa blends maintained the good chemical resistance of PA11, which is considered to be due to strong interactions between PA11 and PVCa. 3.5. TEM observations The morphologies of PA11/PVCa15 and PA11/PSt15 were observed by TEM, as shown in Fig. 7. PA11/PVCa15 (Fig. 7(a)) exhibits a homogeneous morphology without clear interfaces. In contrast, PA11/ PSt15 (Fig. 7(b)) exhibits a phase-separated morphology, and the PSt phase is larger than the observation field. At high magnification, the lamellar structure of the PA11 crystals can be observed in PA11/ PVCa15, and the PA11 phase in PA11/PSt15. In PA11/PVCa15, dark regions are present between bundles of lamellae, which are considered to be condensed PVCa that was rejected during PA11 crystallization [37,38].
Fig. 9. ATR-IR spectra of different polymers and blends between wavenumbers of 1680 and 1580 cm−1 (amide I band region). (a) PA11, PA11/PVCa15, and PVCa, and (b) PA11, PA11/PSt15, and PSt.
shows almost the same Tc as PA11 due to its immiscibility. PA11 retains its crystalline structure even in blends, and constant values of scattering peaks were observed in wide angle X-ray scattering (Fig. S4). The DSC curves acquired during the second heating scans are shown in Fig. 4. The Tg of PVCa and PSt are 181.6 and 61.0 °C, respectively (Fig. 4(a)). The extremely high Tg of PVCa was considered to be caused by inter- and intra-molecular hydrogen bonding. Fig. 4(b) shows that the Tg shifted in the PA11/PVCa blends. The Tg of PA11/PVCa15 is ca. 23 °C higher than that of PA11. In comparison, the PA11/PSt15 blends show a broad transition region, which is caused by immiscibility and the similar Tg of PA11 and PSt. The broadness of the DSC curve made the Tg analysis difficult; however, PA11 did not exhibit Tg shifting due to blending with PSt, as shown in Fig. 4(b).
3.6. Tensile testing The stress–strain curves, tensile moduli, and appearances of the specimens after tensile testing are shown in Fig. 8. The PA11/PVCa blends exhibited a higher yield strength than PA11. In particular, PA11/PVCa15 showed a ca. 20% higher yield strength than PA11 (Fig. 8(a)). Fig. 8(b) revealed that the tensile modulus of PA11 improved upon blending with PVCa; PA11/PVCa15 shows a ca. 24% higher tensile modulus (1.04 GPa) than PA11 (0.84 GPa). On the other hand, PA11/PSt15 shows almost the same modulus (0.86 GPa) as PA11. Thus, specific inter-molecular hydrogen bonding between PA11 and PVCa was considered to be the reason for the good mechanical properties of the PA11/PVCa blends. However, the strains at breakage of the PA11/PVCa blends were lower than that in PA11, and a widespread oriented region was generated only in PA11 (Fig. 8(c)). Strain hardening induced by oriented crystallization is the primary reason for the good elongation of PA11. As shown in the DSC curves of the first cooling scan (Fig. 3), the addition of the second polymer disrupts the crystallization of the crystalline polymer; PVCa prevents PA11 from crystallizing and elongating.
3.3. DMA measurements The temperature dependencies of E′ are shown in Fig. 5(a). The softening point of PA11 increased upon blending with PVCa; that is, the PA11/PVCa blends retained the high E′ of the glassy state at a higher temperature than PA11 did. PA11/PSt15 showed two softening points (ca. 50 and 75 °C); the softening point at the lower temperature overlapped with that of PA11. The temperature dependencies of tan δ presented in Fig. 5(b) show that the Tg of the PA11/PVCa blends were
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In contrast, PA11/PSt15 showed an extremely low yield strength and strain at breakage due to blending with PSt. The PA11/PSt15 specimens had large whitened regions after the tensile test, which indicate gap generation at the PA11 and PSt interface [40]. These phenomena are caused by a lack of interaction between the immiscible PA11 and PSt.
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3.7. FT-IR measurements Fig. 9 shows IR spectra between wavenumbers of 1680 and 1580 cm−1. The amide I band of PA11, which originates from C]O stretching, can be observed in the region. This band has been widely used to estimate the extent of hydrogen bonding, and a shift of the amide I band toward lower frequencies is reported to be a direct observation of hydrogen bonding [29,41,42]. The spectrum of PA11/ PVCa15 in Fig. 9(a) shows a slightly shifted amide I band toward low frequencies compared with that of PA11. The increased absorption around 1600 cm−1 is considered to be caused by superposition of the spectrum from PA11 and PVCa, or a contribution from the signal of hydrogen-bonded C]O. These results supply direct observations of hydrogen bonding between PA11 and PVCa. On the other hand, the amide I band of PA11/PSt15, shown in Fig. 9(b), reveals almost the same spectrum as that of PA11 due to the absence of hydrogen bonding. 4. Conclusions Bio-based polymer blends of PA11 and PVCa are thermodynamically miscible and show higher Tg values than pristine PA11. The Tg of PA11 increased upon blending with PVCa, and a loading of 15 wt% PVCa led to an increase of over 20 °C in the Tg compared to that of PA11. The driving force for the miscibility of PA11 and PVCa is considered to be hydrogen bonding between the polar functional groups of PA11 and PVCa, which was directly observed by the FT-IR measurement. The strong hydrogen bonding also contributes to the good performance of the PA11/PVCa blends. PVCa prevented the postcondensation of PA11 and lowered its viscosity. The PA11/PVCa blends exhibited smooth surfaces under SEM, with little extraction even after immersion in a good solvent for PVCa. Furthermore, PA11/PVCa (85/ 15 wt%) showed over 20% higher yield strength and tensile modulus than PA11. The PA11/PVCa blend is a rare example of a miscible blend entirely synthesized from renewable resources, and its attractive properties are promising for broadening the applicability of PA11. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declarations of interest None. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.121667. References [1] F.A. Kucherov, L.V. Romashov, K.I. Galkin, V.P. Ananikov, Chemical transformations of biomass-derived C6-furanic platform chemicals for sustainable energy research, materials science, and synthetic building blocks, ACS Sustain. Chem. Eng. 6
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T. Hirai, et al. ma020940z. [26] Y. Kikkawa, T. Suzuki, M. Kanesato, Y. Doi, H. Abe, Effect of phase structure on enzymatic degradation in poly(L-lactide)/atactic poly(3-hydroxybutyrate) blends with different miscibility, Biomacromolecules 10 (2009) 1013–1018 https://doi. org/10.1021/bm900117j. [27] M.W. Huang, K.J. Zhu, E.M. Pearce, T.K. Kwei, The modification of nylon-6 by a phenol-formaldehyde resin, J. Appl. Polym. Sci. 48 (1993) 563–573 https://doi. org/10.1002/app.1993.070480401. [28] F.Y. Wang, C.C.M. Ma, H.D. Wu, Hydrogen bonding in polyamide toughened novolac type phenolic resin, J. Appl. Polym. Sci. 74 (1999) 2283–2289 https://doi. org/10.1002/(SICI)1097-4628(19991128)74:9%3c2283::AID-APP18%3e3.0. CO;2-X. [29] J. Hartikainen, O. Lehtonen, T. Harmia, M. Lindner, S. Valkama, J. Ruokolainen, K. Friedrich, Structure and morphology of polyamide 66 and oligomeric phenolic resin blends: molecular modeling and experimental investigations, Chem. Mater. 16 (2004) 3032–3039 https://doi.org/10.1021/cm0401591. [30] P.T. Huang, T.K. Kwei, E.M. Pearce, S.V. Levchik, Blends of nylon-6 with phenolcontaining polymers, J. Polym. Sci. A 39 (2001) 841–850 https://doi.org/10.1002/ 1099-0518(20010315)39:6%3c841::AID-POLA1057%3e3.0.CO;2-7. [31] P.T. Huang, J.L. Lee, S.C. Chiu, T.K. Kwei, E.M. Pearce, Modification of nylon 6 by phenol-containing polymers, J. Appl. Polym. Sci. 73 (1999) 295–300 https://doi. org/10.1002/(SICI)1097-4628(19990711)73:2%3c295::AID-APP16%3e3.0. CO;2-O. [32] T. Hirai, K. Fukumori, T. Ikawa, T. Oda, Polyamide 6 and high molecular weight phenol novolac blend having excellent mechanical properties in humid conditions, J. Thermoplast. Compos. Mater. 32 (2019) 778–794 https://doi.org/10.1177/ 0892705718780195. [33] H. Takeshima, K. Satoh, M. Kamigaito, Bio-based functional styrene monomers derived from naturally occurring ferulic acid for poly(vinylcatechol) and poly(vinylguaiacol) via controlled radical polymerization, Macromolecules 50 (2017)
4206–4216 https://doi.org/10.1021/acs.macromol.7b00970. [34] H. Takeshima, K. Satoh, M. Kamigaito, Scalable synthesis of bio-based functional styrene: protected vinyl catechol from caffeic acid and controlled radical and anionic polymerizations thereof, ACS Sustain. Chem. Eng. 6 (2018) 13681–13686 https://doi.org/10.1021/acssuschemeng.8b04400. [35] S. Acierno, P. Van Puyvelde, Rheological behavior of polyamide 11 with varying initial moisture content, J. Appl. Polym. Sci. 97 (2005) 666–670 https://doi.org/10. 1002/app.21810. [36] P. Marechal, R. Legras, J.M. Dekoninck, Postcondensation and oxidation processes in molten polyamide 6, J. Polym. Sci. A 31 (1993) 2057–2067 https://doi.org/10. 1002/pola.1993.080310812. [37] Y. Okabe, H. Murakami, N. Osaka, H. Saito, T. Inoue, Morphology development and exclusion of noncrystalline polymer during crystallization in PVDF/PMMA blends, Polymer 51 (2010) 1494–1500 https://doi.org/10.1016/j.polymer.2010.01.055. [38] H. Saito, B. Stuhn, Exclusion of monocrystalline polymer from the interlamella region in poly(vinylidene fluoride)/poly(methyl methacrylate) blends, Macromolecules 27 (1994) 216–218 https://doi.org/10.1021/ma00079a032. [39] Z.B. Qiu, T. Ikehara, T. Nishi, Miscibility and crystallization in crystalline/crystalline blends of poly(butylene succinate)/poly(ethylene oxide), Polymer 44 (2003) 2799–2806 https://doi.org/10.1016/S0032-3861(03)00149-6. [40] O.G. Ersoy, N. Nugay, Effect of inorganic filler phase on mechanical and morphological properties of binary immiscible polymer blends, Polym. Bull. 49 (2003) 465–472 https://doi.org/10.1007/s00289-003-0127-2. [41] M.M. Coleman, D.J. Skrovanek, J.B. Hu, P.C. Painter, Hydrogen-bonding in polymer blends. 1. FTIR studies of urethane-ether blends, Macromolecules 21 (1988) 59–65 https://doi.org/10.1021/ma00179a014. [42] E. Yilgör, I. Yilgör, E. Yurtsever, Hydrogen bonding and polyurethane morphology. I. Quantum mechanical calculations of hydrogen bond energies and vibrational spectroscopy of model compounds, Polymer 43 (2002) 6551–6559 https://doi.org/ 10.1016/S0032-3861(02)00567-0.
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Fully bio-based polymer blend of polyamide 11 and Poly(vinylcatechol) showing thermodynamic miscibility and excellent engineering properties
T
Takayuki Hiraia,∗, Jumpei Kawadaa, Mamiko Naritaa, Taiji Ikawaa, Hisaaki Takeshimab, Kotaro Satohb, Masami Kamigaitob a
Material and Processing Department, Polymer Processing and Mechanics Laboratories, Toyota Central R&D Laboratories, Inc., 41-1 Yokomichi, Nagakute, 480-1192, Japan b Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan
HIGHLIGHTS
polymer blends of polyamide 11 and poly(vinyl catechol) are prepared. • Bio-based miscibility is observed, with T shifting and a smooth surface. • Thermodynamic polyamide 11 with 15 wt% poly(vinyl catechol) increases the T by 26 °C. • Blending • The tensile properties and flowability are also improved by blending. g
g
ARTICLE INFO
ABSTRACT
Keywords: Polyamide 11 poly(vinylcatechol) Bio-based polymer Polymer blend Miscibility Glass transition temperature
Fully bio-based polyamide 11 (PA11) and poly(vinylcatechol) (PVCa) blends prepared by melt mixing demonstrate thermodynamic miscibility and excellent engineering properties. The glass transition temperature (Tg) of PA11 increases upon blending with PVCa; an 85/15 wt% PA11/PVCa blend exhibits a Tg 23–26 °C higher than that of PA11 devoid of additives. Morphological observations revealed that the PA11/PVCa blends do not phaseseparate, confirming the homogeneity of PA11 and PVCa. Good chemical resistance of the PA11/PVCa blends was confirmed, with the blends resisting morphological changes even after immersion in methanol, which is a good solvent for PVCa. Tensile testing revealed that the PA11/PVCa blends have higher moduli and strengths than PA11. A PA11/nonpolar polystyrene blend was also examined by the same experimental procedure, which revealed that strong hydrogen bonding between PA11 and PVCa is the primary reason for the miscibility and excellent performance of PA11/PVCa blends.
1. Introduction The use of renewable resources offers a solution to the problem of fossil fuel depletion [1–7]. Bio-based polymers produced from natural resources have received much attention for realizing a sustainable material supply [1–7]. Poly(lactic acid) (PLA), polyamide 11 (PA11), poly(hydroxybutyrate) (PHB), and poly(butylenesuccinate) (PBS) are typical examples of bio-based polymers, which have already been applied to products such as packaging, electronics, and automobile parts [2]. However, the properties of bio-based polymers are typically inferior to those of petroleum-based polymers, which restricts their applications [6]. PA11 synthesized from castor bean plants is an industrial bio-based polymer; however, its glass transition temperature (Tg) is lower than ∗
that of conventional polyamide 6 (PA6). The Tg of a polymer determines its acceptable operating temperature, and increasing the Tg of PA11 is necessary to widen its applicability and replace petrochemicalbased PA6. Polymer blending is an attractive method for producing new polymer materials without the need for new synthetic plants; moreover, the properties of polymer blends can be controlled by adjusting their composition ratios [8]. Numerous attempts have been made to enhance the properties of bio-based polymers by polymer blending [8–12]. To improve the Tg by polymer blending, the constituent polymers need to be thermodynamically miscible, because the Tg of a miscible system shifts from that of the constituent polymers. However, most polymer combinations are immiscible because of their small entropic benefits [13]. Miscible polymer blends can be obtained with sufficient
Corresponding author. E-mail address: [email protected] (T. Hirai).
https://doi.org/10.1016/j.polymer.2019.121667 Received 23 May 2019; Received in revised form 12 July 2019; Accepted 18 July 2019 Available online 05 September 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.
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exothermic interactions between polymer chains, such as hydrogen bonding [14–17]. There are some studies on miscible polymer blends including biobased polymers [18–20]. Zhang et al. reported that PLA/poly(methylmethacrylate) (PMMA) blends showed thermodynamic miscibility and a single Tg between 56 °C for PLA and 104 °C for PMMA [18]. This indicates that the Tg of PLA can be increased by blending with miscible PMMA. However, incorporating petrochemical-based PMMA decreases the amount of bio-based polymer in the blend. Fully bio-based polymer blends such as PLA/PA11 [21,22] and PLA/PBS [23,24] have been reported; however, these blends are intrinsically immiscible. To our knowledge, a PLA/low-molecular weight PHB blend is the only example of a fully bio-based miscible polymer blend; unfortunately, PLA/PHB blends have a low Tg due to the low Tg of PHB [8,25,26]. In previous studies on petrochemical-based polyamide blends, it has been reported that polyamides and phenol-containing polymers such as phenol novolac or poly(vinylphenol) are miscible [27–32]. These studies suggested that the driving force for such miscibility is hydrogen bonding between the carbonyl groups of polyamide and the phenolic hydroxyl groups [27–32]. Miscible polymer blends of PA11 and other phenol-containing polymers are also expected to be fabricated. One of the most promising candidates for the formation of strong hydrogen bonds with PA11 is poly(vinylcatechol) (PVCa). PVCa has two phenolic hydroxyl groups per monomer unit in a poly(styrene)-like (PSt-like) structure, as shown in Fig. 1. One of our research groups recently reported the synthesis of PVCa from naturally occurring compounds such as ferulic and caffeic acids [33,34], which are abundant in rice bran and coffee beans, respectively. These acids were easily decarboxylated in the presence of triethylamine into styrene monomers (i.e., 4-vinylguaiacol (4VG) or vinylcatechol), which were radically polymerized after protection of the phenolic groups to yield PVCa after deprotection. In this study, fully bio-based PA11/PVCa blends were prepared by melt mixing. The miscibility of the blends was studied by Tg measurements and morphological observations. Furthermore, to evaluate the industrial applicability of the blends, their rheological behavior, chemical resistance, and mechanical properties were investigated. A PA11/PSt blend was also examined by the same experimental procedure to assess the influence of the polar functional groups of PVCa on the miscibility and other properties of the PA11/PVCa blends.
Table 1 Sample designations and constituent weight fractions. Sample designation
PA11 (wt%)
PVCa (wt%)
PSt (wt%)
PA11 PA11/PVCa5 PA11/PVCa15 PA11/PSt15
100 95 85 85
0 5 15 0
0 0 0 15
Fig. 2. (a) Time evolution of viscosities during melt mixing. The encircled numbers represent the GPC measurement points. (b) GPC curves of pristine PA11, melt-mixed PA11, and the polymer blends.
Fig. 3. DSC curves during the first cooling scan of melt-mixed PA11 and polymer blends. The dotted line shows the peak temperature of PA11 crystallization (based on the exothermic peak for PA11).
2. Experimental section 2.1. Preparation of Poly(vinylcatechol) Protected 4VG was synthesized by referring to a reported procedure [33]. Triethylsilane (94 mL, 0.590 mol) and a toluene solution of tris (pentafluorophenyl)boron (7.5 mL, 40 mM) were stirred in toluene (114 mL). Then, commercially available 4VG (synthesized via the decarboxylation of ferulic acid (40 mL, 0.295 mol)) was diluted in toluene (40 mL) and slowly added to the prepared triethylsilane solution. The silylation reaction proceeded at 0 °C for 12 h, and bis(triethylsilyl)-
Fig. 1. (a) Molecular structures of PVCa and PSt. (b) Schematic diagram of hydrogen bonding between PVCa and PA11.
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Fig. 4. DSC curves during the second heating scan of (a) PVCa and PSt before melt mixing, and (b) melt-mixed PA11 and polymer blends.
Fig. 5. Viscoelastic behavior of melt-mixed PA11 and polymer blends. Temperature dependencies of (a) E′ and (b) tan δ.
protected vinylcathechol (TES2VC) was obtained after washing with water three times and drying. Nitroxide-mediated polymerization of TES2VC was conducted by using N-tert-butyl-N-(2-methyl-1-phenylpropyl)-O-(1-phenylethyl)hydroxylamine (PhEt-TIPNO) as the initiator. TES2VC (22.1 ml, 56.4 mmol) was added to PhEt-TIPNO (0.2038 g, 0.626 mmol) under an argon atmosphere, and polymerization was performed at 110 °C for 66 h. The number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of poly(TES2VC) were experimentally determined as 17,200 and 1.13, respectively (Fig. S1). Finally, PVCa was obtained after deprotection of the silyl groups of poly(TES2VC) with concentrated HCl in tetrahydrofuran. Successful deprotection was confirmed by nuclear magnetic resonance spectroscopy (Fig. S2).
the same conditions. PA11 devoid of additives was also melt-mixed to obtain homo-PA11 with the same process history. While mixing, the pressures of the melted polymers were measured at two points in the extruder, and time revolutions of viscosities were calculated based on the measured pressure losses (Fig. S3). 2.3. Gel permeation chromatography (GPC) The molecular weight changes upon melt mixing were measured using gel permeation chromatography (GPC; HLC-8220, TOSOH, Japan) with a PMMA standard. Hexafluoroisopropanol (HFIP) solutions of pristine PA11, melt-mixed PA11, and three polymer blends (PA11/ PVCa5, PA11/PVCa/15, and PA11/PSt15), each at 0.1% w/v, were subjected to gel permeation chromatography at a flow rate of 0.8 mL/ min for 100 mL at 40 °C.
2.2. Blend preparation The PA11 used in this study was a natural-grade PA11 (Rilsan BMNO, ARKEMA, France). PA11 and PVCa were vacuum-dried at 80 °C for 12 h and melt-mixed using a micro compounder (HAAKE Minilab, Thermo Fisher Scientific, Germany). The sample designations and weight fractions of the fabricated polymer blends are listed in Table 1. Melt-mixing under nitrogen flow was conducted at a cylinder temperature of 230 °C and a screw speed of 200 rpm for a mixing time of 5 min. PA11 and PSt (Aldrich, Mw = 35,000) were melt-mixed under
2.4. Differential scanning calorimetry (DSC) PA11 and the polymer blends were evaluated using differential scanning calorimetry (DSC; Q1000, TA Instrument, USA). Samples of approximately 10 mg were placed in aluminum pans and sealed, and measurements were carried out between −50 and 230 °C at a heating rate of 10 °C/min and cooling rate of 20 °C/min under nitrogen flow. The crystallization behavior was evaluated during the first cooling run,
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Fig. 6. SEM images of fracture surfaces of untreated blends and blends immersed in methanol or toluene for 15 min. Scale bar, 10 μm.
rectangular specimens (35 × 5 × 0.5 mm) were obtained after cold pressing. The specimens were vacuum-dried at 80 °C for 8 h prior to any measurement. DMA measurements were carried out between −100 and 200 °C at a heating rate of 5 °C/min and a frequency of 10 Hz in a dynamic mechanical analyzer (DVA-225, ITK, Japan). The temperature dependencies of storage modulus (E′) and loss modulus (E″) were obtained from these measurements. Here, Tg was considered the peak temperature of tan δ (= E"/E′). 2.6. Scanning electron microscopy (SEM) The cryo-fractured surfaces of the polymer blends were fabricated by fracturing rectangular specimens in liquid nitrogen. Then, the specimens were immersed in methanol (good solvent for PVCa) [33] or toluene (good solvent for PSt), and ultrasonicated for 15 min. The fracture surfaces devoid of treatment or after immersion in an organic solvent were sputter-coated with platinum and observed by scanning electron microscopy (SEM; S–3600N, Hitachi, Japan) at an accelerating voltage of 15 kV. 2.7. Transmission electron microscopy (TEM) Rectangular specimens of PA11/PVCa15 and PA11/PSt15 were cryo-microtomed into thin films with a thickness of 100 nm. The PA11 in the samples was stained by ruthenium tetroxide, and transmission electron microscopy (TEM; JEM-2100F, JEOL, Japan) observations were carried out at an accelerating voltage of 120 kV.
Fig. 7. TEM images of stained thin films of (a) PA11/PVCa15 and (b) PA11/ PSt15. The areas indicated by black boxes are shown at higher magnification in the right-hand-side images.
and the crystallization temperature (Tc) was determined as the peak temperature of the exothermic peak. The Tg was taken as the mid-point of the change in heat capacity during the second heating run to ensure the samples had consistent thermal histories.
2.8. Tensile testing The melt-mixed PA11 and polymer blends were molded into rectangular specimens (50 × 10 × 0.5 mm) by the same procedure as that used to prepare the DMA sample. Tensile testing was conducted using a universal testing machine (model 5566, Instron, USA) after vacuum drying the specimens at 80 °C for 8 h. The grip distance was 20 mm, and the deformation speed was set to 2 mm/min. The tensile modulus was defined as the slope of the obtained stress–strain curves between 0.5%
2.5. Dynamic mechanical analysis (DMA) The extruded samples were hot-pressed at 230 °C for 1 min in a laboratory compression machine. Then, to increase the degree of crystallinity, the pressed samples were annealed at 150 °C for 1 min. Finally,
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Fig. 8. Mechanical properties and deformation behavior of PA11 and polymer blends. (a) Stress–strain curves of specimens. (b) Tensile moduli of specimens calculated from stress–strain curves. Error bars represent standard deviations (n = 3). (c) Appearances of specimens after tensile testing, which reveal the deformation history of each specimen.
and 2.5% strain.
PA11/PVCa blends slightly decreased during melt-mixing, and a loading of only 5 wt% PVCa maintained the low viscosity of PA11. From an industrial viewpoint, these results imply that the PA11/PVCa blends have superior flowabilities to PA11. The molecular weights of PA11 in PA11/PVCa5 (encircled 3) and PA11/PVCa15 (encircled 4) were almost the same as that of pristine PA11 or decreased during melt mixing. The results suggest that PVCa inhibits the post-condensation reaction. The viscosity of PA11/PSt15 was low in the early stage of mixing due to the low viscosity of PSt at 230 °C. In the latter half of the melt-mixing process, the viscosity of PA11/PSt15 increased. This is considered to be caused by the post-condensation of PA11, because the molecular weight of PA11 in the PA11/PSt15 blend (encircled 5) increased after 5 min of mixing, which is similar to that observed for homo-PA11.
2.9. Fourier transform infrared spectroscopy (FT-IR) Infrared (IR) spectra of homo polymers (PA11, PVCa, and PSt) and polymer blends (PA11/PVCa15 and PA11/PSt15) were recorded with a Fourier transform infrared spectrometer (FT-IR AVATAR360, Nicolet, USA). The measurements were conducted under attenuated total reflectance (ATR) mode for 32 scans at a resolution of 2 cm−1. 3. Results and discussion 3.1. Rheological behavior and molecular weight change The viscosities of PA11 and the polymer blends during the meltblending process are shown in Fig. 2(a). PA11 devoid of additive became viscous; its viscosity doubled after 5 min of mixing. The molecular weight distributions are shown in Fig. 2(b), and the results for pristine PA11 (encircled 1) and melt-mixed PA11 (encircled 2) reveal that the molecular weight of PA11 increased after melt mixing. The reason for the increase in viscosity during melt mixing was considered to be a post-condensation reaction, which has been observed during the meltmixing of polyamides [35,36]. On the other hand, the viscosities of the
3.2. DSC measurements Fig. 3 shows the DSC curves during the first cooling scan. Exothermic peaks of PA11 crystallization were observed for all samples, and the PA11/PVCa blends showed lower Tc than PA11. In a miscible system, the crystalline component pushes out the non-crystalline component during crystallization [37,38], so a lowered Tc is a typical characteristic of such blends [32,39]. On the other hand, PA11/PSt15
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higher than those of PA11. The Tg of PA11/PVCa15 is ca. 26 °C higher than that of PA11, as well as higher than that of conventional PA6 (Fig. S5). PA11/PSt15 shows a bi-modal behavior, with its peak appearing at a lower temperature that corresponds to the Tg of PA11. The softening point and Tg of PA11/PSt15 observed at the higher temperature were confirmed to be very close to those of PSt (Fig. S6); that is, PA11/PSt15 shows the typical behavior of an immiscible polymer blend. 3.4. SEM observations SEM images of untreated specimens and specimens after immersion in an organic solvent are shown in Fig. 6. The insets show the appearance of PVCa or PSt solely immersed in each solvent. Untreated PA11/PVCa15 had a smooth surface that remained unchanged even after immersion in methanol, which is a good solvent for PVCa, as shown in the inset image. Toluene caused no change in PA11/PVCa15 because PVCa is insoluble in toluene. On the other hand, untreated PA11/PSt15 shows a sea–island structure; the island phase was determined to be PSt from the composition ratio. After immersion in methanol, which is a poor solvent for PSt, some PSt particles precipitated during ultrasonication due to few interactions between PA11 and PSt. After immersion in toluene, the PSt phases were extracted from the surface, and many holes with sizes of 1–20 μm are observed. Incorporating an amorphous polymer into a crystalline polymer often causes a loss of chemical resistance [13], as shown in the PA11/PSt15 blend. However, the PA11/PVCa blends maintained the good chemical resistance of PA11, which is considered to be due to strong interactions between PA11 and PVCa. 3.5. TEM observations The morphologies of PA11/PVCa15 and PA11/PSt15 were observed by TEM, as shown in Fig. 7. PA11/PVCa15 (Fig. 7(a)) exhibits a homogeneous morphology without clear interfaces. In contrast, PA11/ PSt15 (Fig. 7(b)) exhibits a phase-separated morphology, and the PSt phase is larger than the observation field. At high magnification, the lamellar structure of the PA11 crystals can be observed in PA11/ PVCa15, and the PA11 phase in PA11/PSt15. In PA11/PVCa15, dark regions are present between bundles of lamellae, which are considered to be condensed PVCa that was rejected during PA11 crystallization [37,38].
Fig. 9. ATR-IR spectra of different polymers and blends between wavenumbers of 1680 and 1580 cm−1 (amide I band region). (a) PA11, PA11/PVCa15, and PVCa, and (b) PA11, PA11/PSt15, and PSt.
shows almost the same Tc as PA11 due to its immiscibility. PA11 retains its crystalline structure even in blends, and constant values of scattering peaks were observed in wide angle X-ray scattering (Fig. S4). The DSC curves acquired during the second heating scans are shown in Fig. 4. The Tg of PVCa and PSt are 181.6 and 61.0 °C, respectively (Fig. 4(a)). The extremely high Tg of PVCa was considered to be caused by inter- and intra-molecular hydrogen bonding. Fig. 4(b) shows that the Tg shifted in the PA11/PVCa blends. The Tg of PA11/PVCa15 is ca. 23 °C higher than that of PA11. In comparison, the PA11/PSt15 blends show a broad transition region, which is caused by immiscibility and the similar Tg of PA11 and PSt. The broadness of the DSC curve made the Tg analysis difficult; however, PA11 did not exhibit Tg shifting due to blending with PSt, as shown in Fig. 4(b).
3.6. Tensile testing The stress–strain curves, tensile moduli, and appearances of the specimens after tensile testing are shown in Fig. 8. The PA11/PVCa blends exhibited a higher yield strength than PA11. In particular, PA11/PVCa15 showed a ca. 20% higher yield strength than PA11 (Fig. 8(a)). Fig. 8(b) revealed that the tensile modulus of PA11 improved upon blending with PVCa; PA11/PVCa15 shows a ca. 24% higher tensile modulus (1.04 GPa) than PA11 (0.84 GPa). On the other hand, PA11/PSt15 shows almost the same modulus (0.86 GPa) as PA11. Thus, specific inter-molecular hydrogen bonding between PA11 and PVCa was considered to be the reason for the good mechanical properties of the PA11/PVCa blends. However, the strains at breakage of the PA11/PVCa blends were lower than that in PA11, and a widespread oriented region was generated only in PA11 (Fig. 8(c)). Strain hardening induced by oriented crystallization is the primary reason for the good elongation of PA11. As shown in the DSC curves of the first cooling scan (Fig. 3), the addition of the second polymer disrupts the crystallization of the crystalline polymer; PVCa prevents PA11 from crystallizing and elongating.
3.3. DMA measurements The temperature dependencies of E′ are shown in Fig. 5(a). The softening point of PA11 increased upon blending with PVCa; that is, the PA11/PVCa blends retained the high E′ of the glassy state at a higher temperature than PA11 did. PA11/PSt15 showed two softening points (ca. 50 and 75 °C); the softening point at the lower temperature overlapped with that of PA11. The temperature dependencies of tan δ presented in Fig. 5(b) show that the Tg of the PA11/PVCa blends were
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In contrast, PA11/PSt15 showed an extremely low yield strength and strain at breakage due to blending with PSt. The PA11/PSt15 specimens had large whitened regions after the tensile test, which indicate gap generation at the PA11 and PSt interface [40]. These phenomena are caused by a lack of interaction between the immiscible PA11 and PSt.
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3.7. FT-IR measurements Fig. 9 shows IR spectra between wavenumbers of 1680 and 1580 cm−1. The amide I band of PA11, which originates from C]O stretching, can be observed in the region. This band has been widely used to estimate the extent of hydrogen bonding, and a shift of the amide I band toward lower frequencies is reported to be a direct observation of hydrogen bonding [29,41,42]. The spectrum of PA11/ PVCa15 in Fig. 9(a) shows a slightly shifted amide I band toward low frequencies compared with that of PA11. The increased absorption around 1600 cm−1 is considered to be caused by superposition of the spectrum from PA11 and PVCa, or a contribution from the signal of hydrogen-bonded C]O. These results supply direct observations of hydrogen bonding between PA11 and PVCa. On the other hand, the amide I band of PA11/PSt15, shown in Fig. 9(b), reveals almost the same spectrum as that of PA11 due to the absence of hydrogen bonding. 4. Conclusions Bio-based polymer blends of PA11 and PVCa are thermodynamically miscible and show higher Tg values than pristine PA11. The Tg of PA11 increased upon blending with PVCa, and a loading of 15 wt% PVCa led to an increase of over 20 °C in the Tg compared to that of PA11. The driving force for the miscibility of PA11 and PVCa is considered to be hydrogen bonding between the polar functional groups of PA11 and PVCa, which was directly observed by the FT-IR measurement. The strong hydrogen bonding also contributes to the good performance of the PA11/PVCa blends. PVCa prevented the postcondensation of PA11 and lowered its viscosity. The PA11/PVCa blends exhibited smooth surfaces under SEM, with little extraction even after immersion in a good solvent for PVCa. Furthermore, PA11/PVCa (85/ 15 wt%) showed over 20% higher yield strength and tensile modulus than PA11. The PA11/PVCa blend is a rare example of a miscible blend entirely synthesized from renewable resources, and its attractive properties are promising for broadening the applicability of PA11. Funding This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declarations of interest None. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.121667. References [1] F.A. Kucherov, L.V. Romashov, K.I. Galkin, V.P. Ananikov, Chemical transformations of biomass-derived C6-furanic platform chemicals for sustainable energy research, materials science, and synthetic building blocks, ACS Sustain. Chem. Eng. 6
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