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Crystallization behavior of polylactide nucleated by octamethylenedicarboxylic di(2-hydroxybenzohydrazide): Solubility influence
Thermochimica Acta 683 (2020) 178447
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Crystallization behavior of polylactide nucleated by octamethylenedicarboxylic di(2-hydroxybenzohydrazide): Solubility influence
T
Zhuyun Zhena, Qian Xinga,*, Rongbo Lib, Xia Dongc a
School of Materials Science and Mechanical Engineering, Beijing Technology and Business University, Beijing 100048, PR China Petrochina Petrochemical Research Institute, Beijing 102206, PR China c Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Polylactide OMBH Crystallization Nucleation effect Solubility
The crystallization ability of polylactide (PLA) was improved by the addition of a hydrazide compound, octamethylenedicarboxylic di(2-hydroxybenzohydrazide) (OMBH). The nucleation efficiency of OMBH significantly depended on its solubility during annealing treatment. The solubility of OMBH increased gradually with increasing the final annealing temperature (Tf) and/or decreasing the addition concentration of OMBH (COMBH). At a certain COMBH (e.g. 0.5 wt%), the crystallization of OMBH shifted to a lower temperature with increasing Tf due to the increased OMBH solubility. This decreased the crystallization temperature, crystallization enthalpy, and isothermal crystallization rate of PLA. Additionally, at a certain Tf (e.g. 200 °C), the crystallization temperature and nucleation efficiency of OMBH increased with its concentration, resulting in the increase of crystallization temperature, crystallization enthalpy, and isothermal crystallization rate of PLA.
1. Introduction Polylactide (PLA) has great potential application in the medical and health fields as surgical suture and tissue engineering scaffolds [1,2], because it possesses excellent biodegradability, biocompatibility, and stiffness [3,4]. However, its crystallization ability including crystallization rate and crystallinity is poor, resulting in a long processing cycle and low thermal dimensional stability, limiting its extensive applications. Typical approaches to regulate the crystallization behavior of PLA include melt blending and copolymerization among others [5–10]. It is very common and facile to tailor the crystallization ability of PLA via melt blending with inorganic fillers including organically modified montmorillonite, carbon nanotubes [11–16], or a second polymer [17–23]. Recently, it has been demonstrated that some low molecular weight organic compounds, such as orotic acid, aliphatic amides, and hydrazide compounds, can also effectively nucleate PLA [24–26]. Generally, these organic nucleating agents contain functional groups such as eOH or eNH2, showing improved intermolecular interaction with PLA compared to their inorganic counterparts. The solubility of organic nucleating agents in PLA matrix depends on the
⁎
addition concentration, chemical structure, and final melting temperature, which greatly influences the nucleation efficiency [27–31]. Fu et al. investigated the formation of PLA crystal superstructure induced by an organic nucleating agent TMC-328 [30]. TMC-328 can completely dissolve into the PLA melt during annealing and subsequently self-organized into fine fibrils during cooling, which served as shish inducing the epitaxial growth of kebab-like structure of PLA. Ma et al. found that oxalamide derivatives self-organized into fibrils with higher nucleation efficiency for PLA [31]. In a previous study, the crystallization rate of PLA was successfully enhanced by adopting a low molecular weight organic compound tetramethylenedicarboxylic di(2-hydroxybenzohydrazide) (TMBH) [26,28]. The effect of TMBH solubility on the crystallization of PLA was also investigated. The nucleation efficiency of TMBH and crystallization ability of PLA decreased gradually with increasing TMBH solubility. Polar eOH and eNH groups within TMBH molecules promoted interactions with PLA, making the family of hydrazide compounds promising nucleating agents for PLA. In this study, another hydrazide compound, octamethylenedicarboxylic di(2-hydroxybenzohydrazide) (OMBH), was selected to regulate the crystallization behavior of PLA. The molecular skeleton of OMBH contains two more methylene groups than TMBH, which may
Corresponding author. E-mail address: [email protected] (Q. Xing).
https://doi.org/10.1016/j.tca.2019.178447 Received 19 June 2019; Received in revised form 4 November 2019; Accepted 5 November 2019 Available online 06 November 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
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(Mettler-Toledo Ltd). The temperature and heat flow were calibrated using indium and zinc as standards. All differential scanning calorimetry (DSC) measurements were performed under a nitrogen atmosphere to prevent potential thermal degradation. The annealing treatment was performed at the final temperature of the melt (Tf) to erase the thermal history of PLA and offer OMBH different dissolution states at 170, 180 and 200 °C. Samples of about 5 mg were weighed and sealed in an aluminium pan, and subsequently heated to Tfs at 20 K/min and held at this temperature for 5 min. For non-isothermal crystallization, samples were first cooled from Tfs to 20 °C and subsequently heated to 200 °C at a scanning rate of 10 K/min. The peak temperature in DSC curves was selected to define the crystallization and melting temperature. During isothermal crystallization, samples were quenched from Tfs to 125 °C at 30 K/min and held at 125 °C until isothermal crystallisation was completed.
Scheme 1. Chemical structure of OMBH (n = 8).
result in different nucleation effects. A series of PLA/OMBH mixtures with different OMBH concentration was prepared via melt blending, and temperature/composition diagrams of the binary system were generated to better understand the dissolution state of OMBH. The solubility or dissolution state of OMBH was regulated by varying its concentration and/or final annealing temperature of the melt. The study was performed to illustrate the effect of OMBH solubility on its nucleation efficiency, especially the crystallization kinetics and crystal structure of PLA. The results provide useful information regarding the optimization of OMBH concentration and the melting temperature of PLA/OMBH mixtures during practical processing.
2.3.2. Polarized optical microscopy The dissolution state of OMBH at different Tfs and its effect on the subsequent nucleation and growth of OMBH and PLA were in situ observed using an Olympus BX51 polarized optical microscope (POM) equipped with a Canon 40D camera system. The temperature was controlled using a Linkam LTS 350 hot stage with liquid nitrogen as a cooling medium. Hot-pressed films with thickness of approximately 30 μm were sandwiched by two glass slides and placed into the hot stage. The temperature program was designed completely consistent with that of DSC measurements. The sample chamber of the hot stage was purged using nitrogen during the experiments to prevent thermal degradation.
2. Experimental 2.1. Materials The PLA used herein was commercial grade 2002D obtained from NatureWorks with 4.6 wt% D-isomer units. The number-average and weight-average molecular weights were 1.5 and 2.0 × 105 g/mol, respectively. The hydrazide compound OMBH was kindly supplied by Shanxi Chemical Research Institute, PR China, as shown in Scheme 1. DSC measurement showed that OMBH is a highly crystallized compound that melts at 209.7 °C and crystallizes at 177.0 °C (10 K/min). The received materials were dried in a vacuum oven for 24 h at 70 °C and stored in a desiccator before use.
3. Results and discussion 3.1. Nucleation effect of OMBH on crystallization DSC curves of pure PLA and its OMBH mixtures during non-isothermal crystallization after annealing at 200 °C are shown in Fig. 1. The crystallization and melting data are summarized in Table 1. Because of their poor crystallization ability, no exothermic signal can be detected during cooling for pure PLA and OMBH-0.1. During subsequent heating, a minor cold crystallization peak was observed at about 130 °C due to the enhanced chain movement. When the OMBH content reached 0.3 wt%, PLA was effectively nucleated and its hot crystallization was induced. Both the crystallization temperature (Tc) and crystallization enthalpy (ΔHc) of PLA increased gradually with increasing OMBH content. However, the crystallinity of PLA still remained very low, leaving many amorphous PLA molecules. In the heating process, OMBH (> 0.1 wt%) promoted the mobility of PLA, and the cold crystallization of PLA shifted to much lower temperatures than in pure PLA and OMBH-0.1, with the enthalpy of crystallization then also increased. The following melting curves of PLA crystals showed
2.2. Sample preparation A series of PLA/OMBH mixtures with different OMBH concentrations (COMBH) was prepared by melt blending through HAAKE PolyLab OS. The mixing temperature, rotor rotating speed, and residue time were set to 200 °C, 50 rpm, and 5 min, respectively. The concentration of OMBH ranged from 0.1 to 1 wt% and the prepared PLA/OMBH mixtures were denoted as OMBH-x, where x is the percentage of OMBH. Pure PLA was also subjected to the melt extrusion process for comparison. 2.3. Characterization 2.3.1. Differential scanning calorimetry The crystallization behaviours of pure PLA and its OMBH mixtures were measured using a DSC 3 differential scanning calorimeter
Fig. 1. (a) DSC cooling and (b) subsequent heating curves of pure PLA and PLA/OMBH mixtures at a scanning rate of 10 K/min after annealing at 200 °C for 5 min. Arrows indicate (a) the crystallization of OMBH and (b) the cold crystallization of PLA. 2
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PLA/OMBH mixtures during non-isothermal crystallization are shown in Figs. 2 and 3. It is evident that PLA and OMBH crystals completely disappeared at 200 °C after annealing for 5 min. For pure PLA and OMBH-0.1, no birefringence can be detected during the entire cooling process due to their poor crystallization ability, while tiny crystallites were observed at approximately 140 °C during heating due to cold crystallization (Fig. 3). When the OMBH content reached 0.3 wt%, the nucleation and crystal growth of PLA occurred during cooling and the birefringence became increasingly prominent during heating. The addition of OMBH accelerated the crystallization of PLA and affected its crystalline structure. For OMBH-0.3 and -0.5, the visual field remained dark upon cooling to 120 °C from the annealing temperature, since OMBH and PLA didn’t crystallize at this temperature. In contrast, soluble OMBH molecules self-assembled to form sheaf-like crystallites for OMBH-1. With further decreasing temperature, PLA crystallized and replicated the sheaf-like structure of OMBH crystals, while a large number of granular crystallites appeared for OMBH-0.3 and -0.5.
Table 1 Crystallization and melting data of PLA during non-isothermal crystallization after annealing at 200 °C. Samples
Crystallization
Cold crystallization
Melting
Tc (°C)
ΔHc (J/g)
Tcc (°C)
ΔHcc (J/g)
Tm (°C)
ΔHm (J/ g)
PLA OMBH-0.1 OMBH-0.3
– – 96.0
– – 1.2
130.6 130.2 113.0
0.8 0.9 24.1
1.3 1.2 25.7
OMBH-0.5
102.2
10.2
108.2
14.8
OMBH-1
102.7
12.6
104.2
12.5
151.6 152.1 148.2/ 152.2 147.7/ 152.2 147.8/ 152.3
25.8 26.0
apparent double melting peaks at about 148.0 and 152.2 °C. The low temperature peak was attributed to the melting of unstable crystals as they were formed at much lower crystallization temperatures. Subsequently, the recrystallization of molten PLA molecules occurred and remelted at the high temperature endothermic peak. This type of meltrecrystallization-remelt behaviour is thermodynamically favored and kinetically possible at low heating rates (e.g. 10 K/min) [32–34]. POM images obtained during evolution of the structure of PLA and
3.2. Temperature/Composition diagrams of PLA/OMBH mixtures Temperature/composition diagrams of PLA/OMBH mixtures are shown in Fig. 4 [35]. For PLA, both the crystallization and melting temperatures were obtained from DSC measurements. For OMBH, the crystallization temperature was obtained from the associated DSC
Fig. 2. POM images obtained during evolution of the structure of PLA and PLA/OMBH mixtures during cooling at 10 K/min after annealing at 200 °C. 3
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Fig. 3. POM images obtained during evolution of the structure of PLA and PLA/OMBH mixtures during heating at 10 K/min after cooling to 20 °C.
Fig. 4. (a) Crystallisation and (b) melting temperature/composition diagrams of the PLA/OMBH binary system. The denotation L refers to liquid, S to solid, P to PLA, and O to OMBH.
cooling scans, while the melting temperature was observed from POM. Both DSC measurements and POM observations were performed from 20 to 200 °C at a scanning rate of 10 K/min. Three regions along the temperature axis can be observed, i.e. liquid, liquid/solid, and solid regions. Both the crystallization and dissolution temperatures of OMBH were higher than that of PLA for each COMBH. It was evident that the solubility and dissolution state of OMBH were mainly determined by
the Tf and COMBH. Complete dissolution of OMBH occurred at approximately 156, 173 and 190 °C for OMBH-0.3, -0.5, and -1, respectively. Therefore, OMBH dissolved completely into PLA at all Tfs for OMBH-0.3, dissolved partially at 170 °C and completely at 180 and 200 °C for OMBH-0.5, and dissolved partially at 170 and 180 °C and completely at 200 °C for OMBH-1.
4
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Fig. 5. DSC cooling (a1, b1, c1) and subsequent heating (a2, b2, c2) curves of PLA/OMBH mixtures at a scanning rate of 10 K/min after annealing at different Tfs for 5 min. (a1, a2) OMBH-0.3, (b1, b2) OMBH-0.5, and (c1, c2) OMBH-1. Arrows within cooling curves indicate the crystallization of OMBH. The inset in (a) magnifies the crystallization region of PLA and OMBH.
Tf cannot change the crystallization ability or crystalline structure of pure PLA. Therefore, DSC curves and POM images after annealing at 200 °C were selected to represent the non-isothermal crystallization behaviour of pure PLA, as shown in Figs. 1–3 and Table 1. DSC cooling and heating curves of nucleated PLA after annealing at different Tfs are shown in Fig. 5. The crystallization temperature and enthalpy of PLA and OMBH are listed in Table 2. Soluble OMBH after annealing treatment precipitated and crystallized prior to PLA during cooling, and subsequently acted as heterogeneous nucleating sites accelerating the crystallization of PLA. It should be noted that the crystallization temperature of OMBH at a certain concentration decreased with increasing Tf because the increased OMBH solubility hindered its self-assembly. This decreased the nucleation efficiency of OMBH with the decrease of crystallization temperature and enthalpy of PLA. It can be seen from Fig. 5 and Table 2 that in OMBH-0.3, OMBH showed the best nucleation efficiency after annealing at 170 °C. Both the crystallization temperature and enthalpy were higher than for samples annealed at 180 and 200 °C, due to the difference in OMBH crystallization temperature. Because the intensity of concentration fluctuation increased with increasing Tf, it became more difficult for
Table 2 Crystallization parameters of OMBH and PLA during non-isothermal crystallization after annealing at different Tfs. Samples
OMBH-0.3 OMBH-0.5 OMBH-1
Tf (°C)
170 180 200 170 180 200 170 180 200
OMBH
PLA
Cold Crystallation
Tc (°C)
Tc (°C)
ΔHc (J/g)
Tcc (°C)
ΔHcc (J/g)
117.5 108.3 107.8 138.0 119.8 119.2 155.6 148.3 138.5
100.8 96.2 96.0 104.7 102.8 102.2 105.1 99.8 102.7
4.7 1.2 1.2 10.7 8.7 10.2 17.1 6.1 12.6
109.2 112.5 113.0 103.8 108.7 108.2 100.0 107.0 104.2
17.4 23.0 24.1 14.4 17.8 14.8 9.3 19.1 12.5
3.3. Effect of OMBH solubility on PLA crystallization 3.3.1. Non-isothermal crystallization No obvious difference in non-isothermal crystallization behaviour of pure PLA was observed after annealing at different Tfs. The variation of 5
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Fig. 6. POM images obtained during evolution of the structure of OMBH-0.3 during cooling at 10 K/min after annealing at different Tfs. (a) Tf = 170 °C, (b) Tf = 180 °C, and (c) Tf = 200 °C.
Fig. 7. POM images obtained during evolution of the structure of OMBH-0.5 during cooling at 10 K/min after annealing at different Tfs. (a) Tf = 170 °C, (b) Tf = 180 °C, and (c) Tf = 200 °C.
OMBH to arrange in an orderly manner after annealing at higher Tfs. Therefore, the crystallization of OMBH shifted to a lower temperature after annealing at 180 and 200 °C compared to that annealed at 170 °C, which decreased the nucleation efficiency of OMBH. POM images obtained during evolution of the structure of OMBH-0.3 during cooling are shown in Fig. 6. After annealing at 170 °C, many granular OMBH crystallites had formed when cooling to 110 °C, while only some smaller crystallites appeared in samples annealed at 180 and 200 °C. With further decreasing temperature, PLA crystallized on the surface of OMBH crystallites, replicating their granular structure and filling the entire visual field at 80 °C. As shown in Fig. 5a2, the cold crystallization of PLA occurred during the subsequent heating process, and the birefringence phenomenon of PLA crystals became increasingly
prominent. The influence of Tf on the structure evolution of OMBH-0.5 during cooling is shown in Fig. 7. Some OMBH crystallites still remained after annealing at 170 °C due to their partial dissolution. These residual OMBH crystallites induced the crystallization of OMBH at a relatively higher temperature around 138 °C. Therefore, many rod-like OMBH crystallites had formed upon cooling to 120 °C. Otherwise, the birefringence phenomenon of OMBH crystals disappeared in POM images after annealing at 180 and 200 °C due to their complete dissolution. During cooling, these soluble OMBH molecules could not crystallize and only a very small amount of OMBH crystallites could be observed at 120 °C. It’s known that only nucleation agents in the crystalline state can exert good nucleation effects, while those in soluble state cannot 6
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Fig. 8. POM images obtained during evolution of the structure of OMBH-1 during cooling at 10 K/min after annealing at different Tfs. (a) Tf = 170 °C, (b)Tf = 180 °C, and (c) Tf = 200 °C.
[36]. Hence, OMBH crystallites formed after annealing at 170 °C exhibited better nucleating effect compared to those of samples annealed at 180 and 200 °C. The following crystallization of PLA replicated the crystalline structure of OMBH, forming quasi rod-like crystals for samples annealed at 170 and 180 °C as well as granular crystals for that annealed at 200 °C. The influence of Tf on the structure evolution of OMBH-1 during cooling is shown in Fig. 8. A large number of rod-like OMBH crystallites remained after annealing at 170 and 180 °C, since the solubility of OMBH was very small in this case. During cooling, soluble OMBH molecules self-assembled to form rod-like crystallites at 155.6 and 148.3 °C for samples annealed at 170 and 180 °C, respectively. In contrast, the birefringence phenomenon of OMBH crystals disappeared after annealing at 200 °C, because they completely dissolved into the PLA melt. Subsequently, the self-assembly of soluble OMBH molecules occurred at 138.5 °C, forming sheaf-like crystallites. Because of its high self-assembly temperature and nucleation density, OMBH within sample annealed at 170 °C exhibited the best nucleation efficiency. The subsequent epitaxial crystallization of PLA replicated the rod-like (Tf = 170 and 180 °C) and sheaf-like (Tf = 200 °C) structure of OMBH crystals. It should be noted that for OMBH-0.3 and -0.5, the nucleation efficiency of OMBH was inversely proportional to Tf. However, for OMBH-1, OMBH after annealing at 200 °C showed improved nucleation efficiency than that annealed at 180 °C, indicated by the increase of crystallization temperature and enthalpy of PLA. This may be attributed to the difference in self-assembly structure of OMBH, since the rod- and sheaf-like crystallites may possess different specific surface areas and active nucleating sites.
crystallization kinetics (Eqs. 1 and 2) [37,38]: 1−Xt = exp(−Ktn)
(1)
1n[−1n(1−Xt)] = 1nK + n1nt
(2)
where Xt is the relative crystallinity at time t, n is the Avrami exponent which depends on the nature of nucleation and growth geometry of the crystals, and K is the overall crystallization rate constant involving both the nucleation and growth rate. The Avrami plots in Fig. 9 show a good linear fitting for all PLA/OMBH mixtures and the values of n and K can be obtained from the slopes and intercepts, respectively. The crystallization half-time (t1/2), defined as the time at which Xt reaches 50 %, was introduced for the analysis of crystallization kinetics. The crystallization kinetics parameters n, K, and t1/2 are listed in Table 3. The value of n lies between 3 and 4, indicating the heterogeneous nucleation and three-dimensional truncated spherulitic growth of PLA. For OMBH-0.3, the value of t1/2 increased linearly with increasing Tf, indicating the decrease of overall crystallization rate of PLA and nucleating efficiency of OMBH. This may be attributed to the decrease of crystallization/self-assemble ability of OMBH caused by the stronger concentration fluctuation at higher Tf. It was determined that OMBH had not crystallized/self-assembled during the quenching stage from Tfs to 125 °C, since it always occurred below 120 °C, as shown in Fig. 5. During the isothermal stage, the crystallization/self-assembly of OMBH required more time with increasing Tf due to the stronger concentration fluctuation, which decreased its nucleation efficiency and significantly influenced its crystalline structure. As shown in Fig. 10a, PLA within the sample annealed at 170 °C had crystallized after isothermal crystallization for 5 min, generating large numbers of granular crystallites. As Tf increased to 180 and 200 °C, the crystals density of PLA decreased obviously and only a few crystals can be observed. For OMBH-0.5, the value of t1/2 and nucleation efficiency of OMBH showed the same variation with Tf as OMBH-0.3. Insoluble OMBH crystals in PLA melt after annealing at 170 °C promoted the crystallization of soluble OMBH molecules. In this case, the crystallization of OMBH had finished during the quenching stage, and then they induced the crystallization of PLA immediately upon cooling to the isothermal temperature. However, the crystallization/self-assembly of soluble OMBH molecules after annealing at 180 and 200 °C had not occurred until cooling to 125 °C. They would spend some time to precipitate from
3.3.2. Isothermal crystallization The isothermal crystallization behaviour of pure PLA and PLA/ OMBH mixtures at 125 °C was investigated to evaluate the influence of OMBH solubility on crystallization rate of PLA. For pure PLA, DSC heat flow remained nearly invariant within the observation time of 90 min and no distinct crystallization exothermic peak was detected. Therefore, the crystallization kinetics could not be calculated. In contrast, the isothermal crystallization rate of nucleated PLA was effectively improved with the appearance of a significant exothermic peak, as shown in Fig. 9. The Avrami equation was employed to analyse the isothermal 7
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Fig. 9. DSC thermograms (a1, b1, c1) and Avrami plots (a2, b2, c2) of the PLA/OMBH mixtures isothermally crystallized at 125 °C after annealing at different Tfs. (a1, a2) OMBH-0.3, (b1, b2) OMBH-0.5, and (c1, c2) OMBH-1.
annealing temperature. They accelerated the crystallization of PLA more effectively compared with OMBH-0.3 and -0.5. After annealing at 200 °C, OMBH completely dissolved into the PLA melt and then selfassembled to form sheaf-like crystals. These sheaf-like OMBH crystals exhibited improved nucleation efficiency compared to those formed after annealing at 180 °C. Therefore, the overall crystallization rate of PLA after annealing at 180 °C was the slowest among all Tfs. Fig. 10 clearly shows that the nucleation efficiency of OMBH at a certain Tf increased with increasing COMBH, whereas it decreased with increasing Tf at a certain COMBH.
Table 3 Isothermal crystallisation parameters of the PLA/OMBH mixtures at 125 °C. Samples
Tf (°C)
n
lnK
t1/2 (min)
OMBH-0.3
170 180 200 170 180 200 170 180 200
3.7 3.8 4.0 3.7 3.0 3.0 3.1 3.8 2.9
−8.3 −9.6 −10.6 −6.6 −5.9 −6.1 −4.7 −7.0 −4.9
7.8 10.6 11.0 5.2 6.1 6.4 3.8 5.8 4.5
OMBH-0.5 OMBH-1
4. Conclusions
PLA melt first and thereafter self-assemble, which decreased the nucleation efficiency and nucleation density of OMBH. Fig. 10b indicates that the overall crystallization rate of PLA decreased and the crystalline structure varied with the increase of Tf. For OMBH-1, the nucleation efficiency of OMBH and the overall crystallization rate of PLA initially decreased as Tf increased from 170 to 180 °C, and subsequently increased slightly as Tf further increased to 200 °C. According to Fig. 5 and Table 2, the crystallization of soluble OMBH already finished during the quenching stage regardless of
In this study, the crystallization ability of PLA was effectively improved by the addition of OMBH. The effect of OMBH solubility on its nucleation efficiency was investigated systematically. The solubility of OMBH increased gradually with increasing the annealing temperature (Tf) and/or decreasing OMBH concentration (COMBH). The crystallization/self-assembly of OMBH shifted to lower temperatures with the increase of its solubility, which decreased its nucleation efficiency. At a certain COMBH (e.g. 0.5 wt%), with the increase of Tf, the crystallization temperature and crystallization enthalpy of PLA decreased in the non8
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Fig. 10. POM images of the crystalline structure of PLA/OMBH mixtures isothermally crystallized at 125 °C for 5 min quenched from different Tfs at 30 K/min. (a) OMBH-0.3, (b) OMBH-0.5, and (c) OMBH-1.
isothermal process, while the overall crystallization time was prolonged during the isothermal crystallization. Additionally, at a certain Tf (e.g. 200 °C), the crystallization temperature and nucleation efficiency of OMBH increased with increasing its concentration, resulting in the increase of crystallization temperature, crystallization enthalpy, and isothermal crystallization rate of PLA. Therefore, it can be concluded that the influence of OMBH solubility on its nucleation effect was rather complicated, showing great dependence on its crystallization temperature and crystalline structure.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We gratefully acknowledge financial supports from the National Natural Science Foundation of China (Grant No. 51403210) and the Beijing Municipal Natural Science Foundation (Grant No. 2192015). References [1] M. Okamoto, B. John, Synthetic biopolymer nanocomposites for tissue engineering scaffolds, Prog. Polym. Sci. 38 (2013) 1487–1503. [2] B. Laycock, M. Nikolic, J.M. Colwell, E. Gauthier, P. Halley, S. Bottle, G. George, Lifetime prediction of biodegradable polymers, Prog. Polym. Sci. 71 (2017) 144–189. [3] G.E. Luckachan, C.K.S. Pillai, Biodegradable polymers- A review on recent trends and emerging perspectives, J. Polym. Environ. 19 (2011) 637–676. [4] J.-M. Raquez, Y. Habibi, M. Murariu, P. Dubois, Polylactide (PLA)-based nanocomposites, Prog. Polym. Sci. 38 (2013) 1504–1542. [5] M.P. Arrieta, E. Fortunati, F. Dominici, J. Lopez, J.M. Kenny, Bionanocomposite films based on plasticized PLA-PHB/cellulose nanocrystal blends, Carbohydr. Polym. 121 (2015) 265–275. [6] R. Veronica Castillo, A.J. Muller, J.-M. Raquez, P. Dubois, Crystallization kinetics and morphology of biodegradable double crystalline PLLA-b-PCL diblock copolymers, Macromolecules 43 (2010) 4149–4160. [7] Y. Ding, B. Lu, P. Wang, G. Wang, J. Ji, PLA-PBAT-PLA tri-block copolymers: effective compatibilizers for promotion of the mechanical and rheological properties of PLA/PBAT blends, Polym. Degrad. Stab. 147 (2018) 41–48. [8] J. Urquijo, G. Guerrica-Echevarria, J. Ignacio Eguiazabal, Melt processed PLA/PCL blends: effect of processing method on phase structure, morphology, and
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Crystallization behavior of polylactide nucleated by octamethylenedicarboxylic di(2-hydroxybenzohydrazide): Solubility influence
T
Zhuyun Zhena, Qian Xinga,*, Rongbo Lib, Xia Dongc a
School of Materials Science and Mechanical Engineering, Beijing Technology and Business University, Beijing 100048, PR China Petrochina Petrochemical Research Institute, Beijing 102206, PR China c Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Polylactide OMBH Crystallization Nucleation effect Solubility
The crystallization ability of polylactide (PLA) was improved by the addition of a hydrazide compound, octamethylenedicarboxylic di(2-hydroxybenzohydrazide) (OMBH). The nucleation efficiency of OMBH significantly depended on its solubility during annealing treatment. The solubility of OMBH increased gradually with increasing the final annealing temperature (Tf) and/or decreasing the addition concentration of OMBH (COMBH). At a certain COMBH (e.g. 0.5 wt%), the crystallization of OMBH shifted to a lower temperature with increasing Tf due to the increased OMBH solubility. This decreased the crystallization temperature, crystallization enthalpy, and isothermal crystallization rate of PLA. Additionally, at a certain Tf (e.g. 200 °C), the crystallization temperature and nucleation efficiency of OMBH increased with its concentration, resulting in the increase of crystallization temperature, crystallization enthalpy, and isothermal crystallization rate of PLA.
1. Introduction Polylactide (PLA) has great potential application in the medical and health fields as surgical suture and tissue engineering scaffolds [1,2], because it possesses excellent biodegradability, biocompatibility, and stiffness [3,4]. However, its crystallization ability including crystallization rate and crystallinity is poor, resulting in a long processing cycle and low thermal dimensional stability, limiting its extensive applications. Typical approaches to regulate the crystallization behavior of PLA include melt blending and copolymerization among others [5–10]. It is very common and facile to tailor the crystallization ability of PLA via melt blending with inorganic fillers including organically modified montmorillonite, carbon nanotubes [11–16], or a second polymer [17–23]. Recently, it has been demonstrated that some low molecular weight organic compounds, such as orotic acid, aliphatic amides, and hydrazide compounds, can also effectively nucleate PLA [24–26]. Generally, these organic nucleating agents contain functional groups such as eOH or eNH2, showing improved intermolecular interaction with PLA compared to their inorganic counterparts. The solubility of organic nucleating agents in PLA matrix depends on the
⁎
addition concentration, chemical structure, and final melting temperature, which greatly influences the nucleation efficiency [27–31]. Fu et al. investigated the formation of PLA crystal superstructure induced by an organic nucleating agent TMC-328 [30]. TMC-328 can completely dissolve into the PLA melt during annealing and subsequently self-organized into fine fibrils during cooling, which served as shish inducing the epitaxial growth of kebab-like structure of PLA. Ma et al. found that oxalamide derivatives self-organized into fibrils with higher nucleation efficiency for PLA [31]. In a previous study, the crystallization rate of PLA was successfully enhanced by adopting a low molecular weight organic compound tetramethylenedicarboxylic di(2-hydroxybenzohydrazide) (TMBH) [26,28]. The effect of TMBH solubility on the crystallization of PLA was also investigated. The nucleation efficiency of TMBH and crystallization ability of PLA decreased gradually with increasing TMBH solubility. Polar eOH and eNH groups within TMBH molecules promoted interactions with PLA, making the family of hydrazide compounds promising nucleating agents for PLA. In this study, another hydrazide compound, octamethylenedicarboxylic di(2-hydroxybenzohydrazide) (OMBH), was selected to regulate the crystallization behavior of PLA. The molecular skeleton of OMBH contains two more methylene groups than TMBH, which may
Corresponding author. E-mail address: [email protected] (Q. Xing).
https://doi.org/10.1016/j.tca.2019.178447 Received 19 June 2019; Received in revised form 4 November 2019; Accepted 5 November 2019 Available online 06 November 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.
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(Mettler-Toledo Ltd). The temperature and heat flow were calibrated using indium and zinc as standards. All differential scanning calorimetry (DSC) measurements were performed under a nitrogen atmosphere to prevent potential thermal degradation. The annealing treatment was performed at the final temperature of the melt (Tf) to erase the thermal history of PLA and offer OMBH different dissolution states at 170, 180 and 200 °C. Samples of about 5 mg were weighed and sealed in an aluminium pan, and subsequently heated to Tfs at 20 K/min and held at this temperature for 5 min. For non-isothermal crystallization, samples were first cooled from Tfs to 20 °C and subsequently heated to 200 °C at a scanning rate of 10 K/min. The peak temperature in DSC curves was selected to define the crystallization and melting temperature. During isothermal crystallization, samples were quenched from Tfs to 125 °C at 30 K/min and held at 125 °C until isothermal crystallisation was completed.
Scheme 1. Chemical structure of OMBH (n = 8).
result in different nucleation effects. A series of PLA/OMBH mixtures with different OMBH concentration was prepared via melt blending, and temperature/composition diagrams of the binary system were generated to better understand the dissolution state of OMBH. The solubility or dissolution state of OMBH was regulated by varying its concentration and/or final annealing temperature of the melt. The study was performed to illustrate the effect of OMBH solubility on its nucleation efficiency, especially the crystallization kinetics and crystal structure of PLA. The results provide useful information regarding the optimization of OMBH concentration and the melting temperature of PLA/OMBH mixtures during practical processing.
2.3.2. Polarized optical microscopy The dissolution state of OMBH at different Tfs and its effect on the subsequent nucleation and growth of OMBH and PLA were in situ observed using an Olympus BX51 polarized optical microscope (POM) equipped with a Canon 40D camera system. The temperature was controlled using a Linkam LTS 350 hot stage with liquid nitrogen as a cooling medium. Hot-pressed films with thickness of approximately 30 μm were sandwiched by two glass slides and placed into the hot stage. The temperature program was designed completely consistent with that of DSC measurements. The sample chamber of the hot stage was purged using nitrogen during the experiments to prevent thermal degradation.
2. Experimental 2.1. Materials The PLA used herein was commercial grade 2002D obtained from NatureWorks with 4.6 wt% D-isomer units. The number-average and weight-average molecular weights were 1.5 and 2.0 × 105 g/mol, respectively. The hydrazide compound OMBH was kindly supplied by Shanxi Chemical Research Institute, PR China, as shown in Scheme 1. DSC measurement showed that OMBH is a highly crystallized compound that melts at 209.7 °C and crystallizes at 177.0 °C (10 K/min). The received materials were dried in a vacuum oven for 24 h at 70 °C and stored in a desiccator before use.
3. Results and discussion 3.1. Nucleation effect of OMBH on crystallization DSC curves of pure PLA and its OMBH mixtures during non-isothermal crystallization after annealing at 200 °C are shown in Fig. 1. The crystallization and melting data are summarized in Table 1. Because of their poor crystallization ability, no exothermic signal can be detected during cooling for pure PLA and OMBH-0.1. During subsequent heating, a minor cold crystallization peak was observed at about 130 °C due to the enhanced chain movement. When the OMBH content reached 0.3 wt%, PLA was effectively nucleated and its hot crystallization was induced. Both the crystallization temperature (Tc) and crystallization enthalpy (ΔHc) of PLA increased gradually with increasing OMBH content. However, the crystallinity of PLA still remained very low, leaving many amorphous PLA molecules. In the heating process, OMBH (> 0.1 wt%) promoted the mobility of PLA, and the cold crystallization of PLA shifted to much lower temperatures than in pure PLA and OMBH-0.1, with the enthalpy of crystallization then also increased. The following melting curves of PLA crystals showed
2.2. Sample preparation A series of PLA/OMBH mixtures with different OMBH concentrations (COMBH) was prepared by melt blending through HAAKE PolyLab OS. The mixing temperature, rotor rotating speed, and residue time were set to 200 °C, 50 rpm, and 5 min, respectively. The concentration of OMBH ranged from 0.1 to 1 wt% and the prepared PLA/OMBH mixtures were denoted as OMBH-x, where x is the percentage of OMBH. Pure PLA was also subjected to the melt extrusion process for comparison. 2.3. Characterization 2.3.1. Differential scanning calorimetry The crystallization behaviours of pure PLA and its OMBH mixtures were measured using a DSC 3 differential scanning calorimeter
Fig. 1. (a) DSC cooling and (b) subsequent heating curves of pure PLA and PLA/OMBH mixtures at a scanning rate of 10 K/min after annealing at 200 °C for 5 min. Arrows indicate (a) the crystallization of OMBH and (b) the cold crystallization of PLA. 2
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PLA/OMBH mixtures during non-isothermal crystallization are shown in Figs. 2 and 3. It is evident that PLA and OMBH crystals completely disappeared at 200 °C after annealing for 5 min. For pure PLA and OMBH-0.1, no birefringence can be detected during the entire cooling process due to their poor crystallization ability, while tiny crystallites were observed at approximately 140 °C during heating due to cold crystallization (Fig. 3). When the OMBH content reached 0.3 wt%, the nucleation and crystal growth of PLA occurred during cooling and the birefringence became increasingly prominent during heating. The addition of OMBH accelerated the crystallization of PLA and affected its crystalline structure. For OMBH-0.3 and -0.5, the visual field remained dark upon cooling to 120 °C from the annealing temperature, since OMBH and PLA didn’t crystallize at this temperature. In contrast, soluble OMBH molecules self-assembled to form sheaf-like crystallites for OMBH-1. With further decreasing temperature, PLA crystallized and replicated the sheaf-like structure of OMBH crystals, while a large number of granular crystallites appeared for OMBH-0.3 and -0.5.
Table 1 Crystallization and melting data of PLA during non-isothermal crystallization after annealing at 200 °C. Samples
Crystallization
Cold crystallization
Melting
Tc (°C)
ΔHc (J/g)
Tcc (°C)
ΔHcc (J/g)
Tm (°C)
ΔHm (J/ g)
PLA OMBH-0.1 OMBH-0.3
– – 96.0
– – 1.2
130.6 130.2 113.0
0.8 0.9 24.1
1.3 1.2 25.7
OMBH-0.5
102.2
10.2
108.2
14.8
OMBH-1
102.7
12.6
104.2
12.5
151.6 152.1 148.2/ 152.2 147.7/ 152.2 147.8/ 152.3
25.8 26.0
apparent double melting peaks at about 148.0 and 152.2 °C. The low temperature peak was attributed to the melting of unstable crystals as they were formed at much lower crystallization temperatures. Subsequently, the recrystallization of molten PLA molecules occurred and remelted at the high temperature endothermic peak. This type of meltrecrystallization-remelt behaviour is thermodynamically favored and kinetically possible at low heating rates (e.g. 10 K/min) [32–34]. POM images obtained during evolution of the structure of PLA and
3.2. Temperature/Composition diagrams of PLA/OMBH mixtures Temperature/composition diagrams of PLA/OMBH mixtures are shown in Fig. 4 [35]. For PLA, both the crystallization and melting temperatures were obtained from DSC measurements. For OMBH, the crystallization temperature was obtained from the associated DSC
Fig. 2. POM images obtained during evolution of the structure of PLA and PLA/OMBH mixtures during cooling at 10 K/min after annealing at 200 °C. 3
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Fig. 3. POM images obtained during evolution of the structure of PLA and PLA/OMBH mixtures during heating at 10 K/min after cooling to 20 °C.
Fig. 4. (a) Crystallisation and (b) melting temperature/composition diagrams of the PLA/OMBH binary system. The denotation L refers to liquid, S to solid, P to PLA, and O to OMBH.
cooling scans, while the melting temperature was observed from POM. Both DSC measurements and POM observations were performed from 20 to 200 °C at a scanning rate of 10 K/min. Three regions along the temperature axis can be observed, i.e. liquid, liquid/solid, and solid regions. Both the crystallization and dissolution temperatures of OMBH were higher than that of PLA for each COMBH. It was evident that the solubility and dissolution state of OMBH were mainly determined by
the Tf and COMBH. Complete dissolution of OMBH occurred at approximately 156, 173 and 190 °C for OMBH-0.3, -0.5, and -1, respectively. Therefore, OMBH dissolved completely into PLA at all Tfs for OMBH-0.3, dissolved partially at 170 °C and completely at 180 and 200 °C for OMBH-0.5, and dissolved partially at 170 and 180 °C and completely at 200 °C for OMBH-1.
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Fig. 5. DSC cooling (a1, b1, c1) and subsequent heating (a2, b2, c2) curves of PLA/OMBH mixtures at a scanning rate of 10 K/min after annealing at different Tfs for 5 min. (a1, a2) OMBH-0.3, (b1, b2) OMBH-0.5, and (c1, c2) OMBH-1. Arrows within cooling curves indicate the crystallization of OMBH. The inset in (a) magnifies the crystallization region of PLA and OMBH.
Tf cannot change the crystallization ability or crystalline structure of pure PLA. Therefore, DSC curves and POM images after annealing at 200 °C were selected to represent the non-isothermal crystallization behaviour of pure PLA, as shown in Figs. 1–3 and Table 1. DSC cooling and heating curves of nucleated PLA after annealing at different Tfs are shown in Fig. 5. The crystallization temperature and enthalpy of PLA and OMBH are listed in Table 2. Soluble OMBH after annealing treatment precipitated and crystallized prior to PLA during cooling, and subsequently acted as heterogeneous nucleating sites accelerating the crystallization of PLA. It should be noted that the crystallization temperature of OMBH at a certain concentration decreased with increasing Tf because the increased OMBH solubility hindered its self-assembly. This decreased the nucleation efficiency of OMBH with the decrease of crystallization temperature and enthalpy of PLA. It can be seen from Fig. 5 and Table 2 that in OMBH-0.3, OMBH showed the best nucleation efficiency after annealing at 170 °C. Both the crystallization temperature and enthalpy were higher than for samples annealed at 180 and 200 °C, due to the difference in OMBH crystallization temperature. Because the intensity of concentration fluctuation increased with increasing Tf, it became more difficult for
Table 2 Crystallization parameters of OMBH and PLA during non-isothermal crystallization after annealing at different Tfs. Samples
OMBH-0.3 OMBH-0.5 OMBH-1
Tf (°C)
170 180 200 170 180 200 170 180 200
OMBH
PLA
Cold Crystallation
Tc (°C)
Tc (°C)
ΔHc (J/g)
Tcc (°C)
ΔHcc (J/g)
117.5 108.3 107.8 138.0 119.8 119.2 155.6 148.3 138.5
100.8 96.2 96.0 104.7 102.8 102.2 105.1 99.8 102.7
4.7 1.2 1.2 10.7 8.7 10.2 17.1 6.1 12.6
109.2 112.5 113.0 103.8 108.7 108.2 100.0 107.0 104.2
17.4 23.0 24.1 14.4 17.8 14.8 9.3 19.1 12.5
3.3. Effect of OMBH solubility on PLA crystallization 3.3.1. Non-isothermal crystallization No obvious difference in non-isothermal crystallization behaviour of pure PLA was observed after annealing at different Tfs. The variation of 5
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Fig. 6. POM images obtained during evolution of the structure of OMBH-0.3 during cooling at 10 K/min after annealing at different Tfs. (a) Tf = 170 °C, (b) Tf = 180 °C, and (c) Tf = 200 °C.
Fig. 7. POM images obtained during evolution of the structure of OMBH-0.5 during cooling at 10 K/min after annealing at different Tfs. (a) Tf = 170 °C, (b) Tf = 180 °C, and (c) Tf = 200 °C.
OMBH to arrange in an orderly manner after annealing at higher Tfs. Therefore, the crystallization of OMBH shifted to a lower temperature after annealing at 180 and 200 °C compared to that annealed at 170 °C, which decreased the nucleation efficiency of OMBH. POM images obtained during evolution of the structure of OMBH-0.3 during cooling are shown in Fig. 6. After annealing at 170 °C, many granular OMBH crystallites had formed when cooling to 110 °C, while only some smaller crystallites appeared in samples annealed at 180 and 200 °C. With further decreasing temperature, PLA crystallized on the surface of OMBH crystallites, replicating their granular structure and filling the entire visual field at 80 °C. As shown in Fig. 5a2, the cold crystallization of PLA occurred during the subsequent heating process, and the birefringence phenomenon of PLA crystals became increasingly
prominent. The influence of Tf on the structure evolution of OMBH-0.5 during cooling is shown in Fig. 7. Some OMBH crystallites still remained after annealing at 170 °C due to their partial dissolution. These residual OMBH crystallites induced the crystallization of OMBH at a relatively higher temperature around 138 °C. Therefore, many rod-like OMBH crystallites had formed upon cooling to 120 °C. Otherwise, the birefringence phenomenon of OMBH crystals disappeared in POM images after annealing at 180 and 200 °C due to their complete dissolution. During cooling, these soluble OMBH molecules could not crystallize and only a very small amount of OMBH crystallites could be observed at 120 °C. It’s known that only nucleation agents in the crystalline state can exert good nucleation effects, while those in soluble state cannot 6
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Fig. 8. POM images obtained during evolution of the structure of OMBH-1 during cooling at 10 K/min after annealing at different Tfs. (a) Tf = 170 °C, (b)Tf = 180 °C, and (c) Tf = 200 °C.
[36]. Hence, OMBH crystallites formed after annealing at 170 °C exhibited better nucleating effect compared to those of samples annealed at 180 and 200 °C. The following crystallization of PLA replicated the crystalline structure of OMBH, forming quasi rod-like crystals for samples annealed at 170 and 180 °C as well as granular crystals for that annealed at 200 °C. The influence of Tf on the structure evolution of OMBH-1 during cooling is shown in Fig. 8. A large number of rod-like OMBH crystallites remained after annealing at 170 and 180 °C, since the solubility of OMBH was very small in this case. During cooling, soluble OMBH molecules self-assembled to form rod-like crystallites at 155.6 and 148.3 °C for samples annealed at 170 and 180 °C, respectively. In contrast, the birefringence phenomenon of OMBH crystals disappeared after annealing at 200 °C, because they completely dissolved into the PLA melt. Subsequently, the self-assembly of soluble OMBH molecules occurred at 138.5 °C, forming sheaf-like crystallites. Because of its high self-assembly temperature and nucleation density, OMBH within sample annealed at 170 °C exhibited the best nucleation efficiency. The subsequent epitaxial crystallization of PLA replicated the rod-like (Tf = 170 and 180 °C) and sheaf-like (Tf = 200 °C) structure of OMBH crystals. It should be noted that for OMBH-0.3 and -0.5, the nucleation efficiency of OMBH was inversely proportional to Tf. However, for OMBH-1, OMBH after annealing at 200 °C showed improved nucleation efficiency than that annealed at 180 °C, indicated by the increase of crystallization temperature and enthalpy of PLA. This may be attributed to the difference in self-assembly structure of OMBH, since the rod- and sheaf-like crystallites may possess different specific surface areas and active nucleating sites.
crystallization kinetics (Eqs. 1 and 2) [37,38]: 1−Xt = exp(−Ktn)
(1)
1n[−1n(1−Xt)] = 1nK + n1nt
(2)
where Xt is the relative crystallinity at time t, n is the Avrami exponent which depends on the nature of nucleation and growth geometry of the crystals, and K is the overall crystallization rate constant involving both the nucleation and growth rate. The Avrami plots in Fig. 9 show a good linear fitting for all PLA/OMBH mixtures and the values of n and K can be obtained from the slopes and intercepts, respectively. The crystallization half-time (t1/2), defined as the time at which Xt reaches 50 %, was introduced for the analysis of crystallization kinetics. The crystallization kinetics parameters n, K, and t1/2 are listed in Table 3. The value of n lies between 3 and 4, indicating the heterogeneous nucleation and three-dimensional truncated spherulitic growth of PLA. For OMBH-0.3, the value of t1/2 increased linearly with increasing Tf, indicating the decrease of overall crystallization rate of PLA and nucleating efficiency of OMBH. This may be attributed to the decrease of crystallization/self-assemble ability of OMBH caused by the stronger concentration fluctuation at higher Tf. It was determined that OMBH had not crystallized/self-assembled during the quenching stage from Tfs to 125 °C, since it always occurred below 120 °C, as shown in Fig. 5. During the isothermal stage, the crystallization/self-assembly of OMBH required more time with increasing Tf due to the stronger concentration fluctuation, which decreased its nucleation efficiency and significantly influenced its crystalline structure. As shown in Fig. 10a, PLA within the sample annealed at 170 °C had crystallized after isothermal crystallization for 5 min, generating large numbers of granular crystallites. As Tf increased to 180 and 200 °C, the crystals density of PLA decreased obviously and only a few crystals can be observed. For OMBH-0.5, the value of t1/2 and nucleation efficiency of OMBH showed the same variation with Tf as OMBH-0.3. Insoluble OMBH crystals in PLA melt after annealing at 170 °C promoted the crystallization of soluble OMBH molecules. In this case, the crystallization of OMBH had finished during the quenching stage, and then they induced the crystallization of PLA immediately upon cooling to the isothermal temperature. However, the crystallization/self-assembly of soluble OMBH molecules after annealing at 180 and 200 °C had not occurred until cooling to 125 °C. They would spend some time to precipitate from
3.3.2. Isothermal crystallization The isothermal crystallization behaviour of pure PLA and PLA/ OMBH mixtures at 125 °C was investigated to evaluate the influence of OMBH solubility on crystallization rate of PLA. For pure PLA, DSC heat flow remained nearly invariant within the observation time of 90 min and no distinct crystallization exothermic peak was detected. Therefore, the crystallization kinetics could not be calculated. In contrast, the isothermal crystallization rate of nucleated PLA was effectively improved with the appearance of a significant exothermic peak, as shown in Fig. 9. The Avrami equation was employed to analyse the isothermal 7
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Fig. 9. DSC thermograms (a1, b1, c1) and Avrami plots (a2, b2, c2) of the PLA/OMBH mixtures isothermally crystallized at 125 °C after annealing at different Tfs. (a1, a2) OMBH-0.3, (b1, b2) OMBH-0.5, and (c1, c2) OMBH-1.
annealing temperature. They accelerated the crystallization of PLA more effectively compared with OMBH-0.3 and -0.5. After annealing at 200 °C, OMBH completely dissolved into the PLA melt and then selfassembled to form sheaf-like crystals. These sheaf-like OMBH crystals exhibited improved nucleation efficiency compared to those formed after annealing at 180 °C. Therefore, the overall crystallization rate of PLA after annealing at 180 °C was the slowest among all Tfs. Fig. 10 clearly shows that the nucleation efficiency of OMBH at a certain Tf increased with increasing COMBH, whereas it decreased with increasing Tf at a certain COMBH.
Table 3 Isothermal crystallisation parameters of the PLA/OMBH mixtures at 125 °C. Samples
Tf (°C)
n
lnK
t1/2 (min)
OMBH-0.3
170 180 200 170 180 200 170 180 200
3.7 3.8 4.0 3.7 3.0 3.0 3.1 3.8 2.9
−8.3 −9.6 −10.6 −6.6 −5.9 −6.1 −4.7 −7.0 −4.9
7.8 10.6 11.0 5.2 6.1 6.4 3.8 5.8 4.5
OMBH-0.5 OMBH-1
4. Conclusions
PLA melt first and thereafter self-assemble, which decreased the nucleation efficiency and nucleation density of OMBH. Fig. 10b indicates that the overall crystallization rate of PLA decreased and the crystalline structure varied with the increase of Tf. For OMBH-1, the nucleation efficiency of OMBH and the overall crystallization rate of PLA initially decreased as Tf increased from 170 to 180 °C, and subsequently increased slightly as Tf further increased to 200 °C. According to Fig. 5 and Table 2, the crystallization of soluble OMBH already finished during the quenching stage regardless of
In this study, the crystallization ability of PLA was effectively improved by the addition of OMBH. The effect of OMBH solubility on its nucleation efficiency was investigated systematically. The solubility of OMBH increased gradually with increasing the annealing temperature (Tf) and/or decreasing OMBH concentration (COMBH). The crystallization/self-assembly of OMBH shifted to lower temperatures with the increase of its solubility, which decreased its nucleation efficiency. At a certain COMBH (e.g. 0.5 wt%), with the increase of Tf, the crystallization temperature and crystallization enthalpy of PLA decreased in the non8
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Fig. 10. POM images of the crystalline structure of PLA/OMBH mixtures isothermally crystallized at 125 °C for 5 min quenched from different Tfs at 30 K/min. (a) OMBH-0.3, (b) OMBH-0.5, and (c) OMBH-1.
isothermal process, while the overall crystallization time was prolonged during the isothermal crystallization. Additionally, at a certain Tf (e.g. 200 °C), the crystallization temperature and nucleation efficiency of OMBH increased with increasing its concentration, resulting in the increase of crystallization temperature, crystallization enthalpy, and isothermal crystallization rate of PLA. Therefore, it can be concluded that the influence of OMBH solubility on its nucleation effect was rather complicated, showing great dependence on its crystallization temperature and crystalline structure.
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