Non-isothermal crystallization kinetics, thermal degradation behavior and mechanical properties of poly(lactic acid)/MOF composites prepared by melt-blending methods

Abstract

In this article, poly(lactic acid)/metal–organic framework composites were prepared by melt-blending methods and the effects of MOFs on the non-isothermal crystallization, thermal degradation and mechanical property of poly(lactic acid) (PLA) were studied by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA) and tensile testing, respectively. The Jeziorny and Mo methods were applied to describe the kinetics of the non-isothermal crystallization process. The results demonstrated that the presence of MOFs accelerated the crystallization rate and increased the crystallinity (X_c) of the PLA/MOF composites. The XRD results confirmed that the addition of MOFs decreased the crystallite size of PLA. The TGA results indicated that MOFs increased the initial thermal degradation temperature of PLA and decreased the thermal stability at high temperature regions. The tensile strength of PLA was increased from 66.23 MPa to 92.44 MPa, 96.03 MPa and 95.25 MPa with MOF loadings of 0.5 wt%, 1 wt% and 1.5 wt%, respectively, but the strain at break was decreased.

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Introduction

In the past decades, environmentally friendly polymers have attracted increasing interest.^1,2 Poly(lactic acid) (PLA) is one of the most promising candidates because it is completely biodegradable and produced from renewable resources.^3–8 The key bottlenecks in extending the use of PLA are the control of its crystallinity and the thermal stability. Understanding the crystallization behavior and thermal degradation of PLA is particularly crucial to its mechanical, barrier, thermal resistance and thermal properties.⁹

Micro- or nano fillers such as talc,¹⁰ halloysite nanotubes,¹¹ cellulose,¹² carbon nanotubes and graphene¹³ are known to act as nucleating agents for the crystallization of PLA. Papageorgiou et al.¹⁴ found that silica, montmorillonite and oxidized multi-walled carbon nanotubes showed some nucleation activity referring to non-isothermal crystallization from the melt or from the glassy state. Picard et al.¹⁵ studied the influence of organo-modified montmorillonite (O-MMT) on PLA non-isothermal crystallization. They found that the incorporation of small amounts of O-MMT within PLA decreased the cold crystallization temperature and increased the crystallinity of PLA, leading to the formation of less perfect and lower-melting crystals. Xiao et al.¹⁶ analyzed crystallization progress of neat PLA and its composites with triphenyl phosphate (TPP) and/or talc crystallized non-isothermally at different cooling rates by differential scanning calorimetry (DSC). Avrami equation and combined Avrami–Ozawa equation were used to describe the crystallization kinetics of PLA and its composites. The results showed that talc acted as nucleating agent accelerating crystallization rate by decreasing the crystallization half-time t_1/2 or rate parameter F(T).

The thermal degradation of PLA is complex. Therefore, it becomes primarily important to investigate the thermal degradation behavior of PLA and its composites. Kim et al.¹⁷ reported the effects of calcium carbonate on the thermal degradation kinetics of PLA/calcium carbonate nanocomposites by different models. They revealed that the activation energy of nanocomposite samples was lower than that of neat PLA due to the basic nature of calcium carbonate and its catalytic effect on depolymerization of the PLA ester bonds. Abe et al.¹⁸ determined that the residual Zn compounds affected the thermal degradation of PLA depending on the Zn atom content. They concluded that the Zn compounds catalyze the thermal degradation of PLA.

Metal–organic frameworks (MOFs) are made by linking inorganic and organic unit. They are porous and the specific surface areas of many kinds of MOFs exceed those of traditional porous materials such as zeolites and carbons.¹⁹ Incorporation of MOFs into a polymer matrix to fabricate MOFs based composites has been intensively studied in recent years.^20,21 Zornoza et al.²² reported that the MOFs particles showed an excellent adhesion with the polysulfone polymer in mixed matrix membranes without any additional compatibilizer: high filler loadings are possible thanks to hydrogen bonding interactions between the MOFs and the polymer. Elangovan et al.^23,24 combined one kind of MOFs which is well-known as H-KUST-1 into the PLA matrix and investigated the mass transport, optical, thermal and mechanical properties of the PLA/H-KUST-1 composites. The results demonstrated that brittleness of the PLLA–MOF composite sample decreased as the amount of MOF in the PLLA increased. Quirós et al.²⁵ successfully included a cobalt-based metal–organic framework, Co-SIM-1, in a PLA electrospun mat. They found that for increasing concentrations of Co-SIM-1, the fibers became less susceptible to bacterial colonization and biofilm formation.

The PLA/MOF composites are potentially applied in catalysis, gas storage and separation, biomedical field etc.^26–29 The final properties such as mechanical and physical properties of PLA/MOF composites are controlled by the crystallization process. However, the effects of MOFs on the non-isothermal crystallization and thermal degradation kinetics of PLA have not yet been evaluated systematically. From a fundamental point of view and obvious market development considerations, it is necessary to understand the crystallization progress of PLA/MOF composites. In this article, MOFs with Zn–O–Zn dinuclear units and multidentate pyridine-2,5-dicarboxylate linkers was synthesized by simple process.²⁹ The non-isothermal crystallization of the PLA/MOF composites was investigated by DSC with Jeziorny model and Mo method. The thermal degradation of the PLA/MOF composites was investigated with TGA and the mechanism of MOFs influence on the thermal degradation of the PLA/MOF composites was proposed. The mechanical property of PLA/MOF composites was studied by tensile testing.

Experimental

Materials

Poly(lactic acid) (PLA 290, injection molding grade) was supplied by Zhejiang Hai Zheng Biological Materials Co. Ltd, China. Pyridine-2,5-dicarboxylic acid (H2PDC) was purchased from Shanghai J&K Technology Co. Ltd, China. Zn(NO₃)₂·6H₂O, Chloroform (99%) and N,N′-dimethylformamide (DMF, 99.5%) were obtained from Sinopharm Chemical Reagent Co. Ltd, China.

Synthesis of MOFs

The MOFs was synthesized as reported.²⁹ Pyridine-2,5-dicarboxylic acid (1.018 g, 6.10 mmol) was dissolved in DMF (100 mL) under ambient conditions. Then, Zn(NO₃)₂·6H₂O (4.718 g, 15.48 mmol) was added into the above solution at room temperature, followed by the addition of a DMF portion (20 mL). The reaction mixture was stirred at 80 °C for 24 h. The resulting white solid was obtained by centrifugation and washed with DMF (100 mL). The precipitate was immersed in DMF (50 mL) for 24 h and then the solvent was replaced, and the product was immersed in DMF (50 mL) over 96 h. After the solvent was removed, the white powdery product was obtained and dried under a vacuum (1 × 10⁻³ Torr) at 20 °C for 8 h, 90 °C for 1 h, and 150 °C for 1 h.

Preparation of PLA/MOF composites

PLA was dried in an oven at 60 °C for 24 h. Then the oven-dried MOFs powder was compounded with PLA in a RM-200A torque rheometer (Hapro Electric Technology Co. Ltd. Harbin, China). The temperature of three heating zones were 177 °C, 177 °C and 177 °C, respectively. In this way, PLA was mixed with the addition of 0, 0.5, 1 and 1.5 wt% MOFs, respectively. For brevity, the samples containing 0, 0.5, 1 and 1.5 wt% MOFs are abbreviated as PLA, PM0.5, PM1 and PM1.5 from now on. SZ-15 micro injection molding machine (Ruiming, China) was used to manufacture suitable shapes of samples for tests at 180 °C.

Measurement and characterization

C, N and H analyses were performed using a Vario EL III analyzer (Elementar, Germany). The Fourier infrared spectra (FT-IR) from 400 to 4000 cm⁻¹ were obtained by FT-IR-8400S spectrometer (Shimadzu, Japan) using KBr pellets with the resolution of 2 cm⁻¹. The morphology of MOFs powder was investigated by a JEM-2100 transmission electron microscope (TEM) (JEOL, Japan). X-ray diffraction (XRD) measurements were investigated by a D8 Advance diffractometer (Bruker, Germany) with Cu Kα radiation (λ = 0.15418 nm), operating at 40 kV and 40 mA. A XP-213 polarizing microscope (Jiangnan, China) was used to observe the morphology of spherulites. The samples were heated from room temperature to 190 °C and held 3 min for melting. Then, the samples were rapidly cooled to 120 °C, and then maintained at 120 °C for 30 min.

DSC measurements were examined in a nitrogen atmosphere by a Q80 DSC Apparatus (TA Corporation, America). The crystallization thermal history of all samples was removed by heating the samples at 190 °C for 5 min. The samples were cooled from 190 °C down to 30 °C at cooling rates of 2, 5 and 10 °C min⁻¹, followed by a re-heating scan up to 190 °C at a heating rate of 10 °C min⁻¹. The sample's mass was approximately 5 mg. Thermogravimetric analyses (TGA) were performed using a DGT-60 thermo-analyzer instrument (Shimadzu, Japan). The samples were heated from 30 °C to 500 °C with heating rates of 10 °C min⁻¹ under nitrogen. The sample's mass was approximately 5 mg. Tensile testing was performed according to ASTMD638 for tensile properties of plastic on a CMT tensile tester (Sans, China). The measurements were carried out with a rate of 10 mm min⁻¹ at room temperature. The values were averaged over five measurements.

Results and discussion

Synthesis and characterization of MOFs

The results of the element analysis show that the element contents of as-synthesized MOFs are 38.24 wt% C, 8.12 wt% N and 3.82 wt% H, respectively, which are in line with the structural formula of MOFs in Scheme 1.²⁹ As shown in the TEM micrograph in Fig. 1, the particle size of MOFs is around 200 nm without the specific shape. The specific surface area and porous structure of the MOFs were determined from the N₂ isotherm adsorption recorded on a Merck ASAP2020 BET surface analyzer. The specific surface area of MOFs is determined as 296.1 m² g⁻¹ by BET method or 331.2 m² g⁻¹ by Langmuir method, and the medium pore width and the total pore volume are 2.17 nm and 0.095 cm³ g⁻¹, respectively. Fig. 2 shows the FT-IR spectra of MOFs. The peak at 3448 cm⁻¹ for MOFs is corresponded to the stretching vibration of –OH in residual water. While the peak at 3140 cm⁻¹ is attributed to the stretching vibration of –CH in pyridine cycle. The peaks at 1614 cm⁻¹ and 1392 cm⁻¹ are attribute to the –C=O stretching vibration and C–O/C–H stretching vibration, respectively.^30,31 The peaks appearing at 468 cm⁻¹ and 418 cm⁻¹ are attributed to the Zn–O and Zn–N stretching vibration, respectively.^32,33

Scheme 1 The synthesis reaction of MOFs.

Fig. 1 The TEM image of MOFs.

Fig. 2 The FT-IR of MOFs.

The morphology of the PLA/MOF composites

Typical brittle fracture of PLA/MOF composites are exhibited in SEM images (Fig. 3). When mixing with low concentration of MOFs (0.5 wt% and 1.0 wt%), the particles are well embedded in the PLA matrix (see the black arrows). No aggregates and void can be seen in the SEM images for PM0.5 and PM1, which suggests favorable dispersibility and compatibility of MOFs. The good dispersion of MOFs particles in PLA matrix may be due to the existing organic frame, the porous structure and high specific surface area of MOFs. On the one hand, there are van der Waals forces between the organic frames and PLA molecules, and on the other hand the porous structure and high specific surface area of MOFs could lead to the adsorption and diffusion of PLA molecules to MOFs.^34,35 However, when the MOFs content rises to 1.5 wt%, agglomeration appears (see the white arrows).

Fig. 3 SEM images of the fracture surface of PLA/MOF composites ((a) PLA, (b) PM0.5, (c) PM1, (d) PM1.5).

The FT-IR of the PLA/MOF composite

FT-IR spectra (Fig. 4) of PLA composites with different content of MOFs are similar to that of neat PLA. The absence of new peaks suggests that the low content of filler used to produce the composite only physically interaction between PLA and MOFs rather than chemical interaction.³⁶ The peak at 1755 cm⁻¹ corresponding to C=O stretching for neat PLA is shift to 1748 cm⁻¹, 1747 cm⁻¹ and 1747 cm⁻¹ for PM0.5, PM1 and PM1.5, respectively. Besides, the peak around 1080 cm⁻¹ corresponding to C–O–C stretching also shifts slightly to lower wavenumber. These shifts of stretching absorption to lower wavenumber were ascribed to the strong interaction between PLA and MOFs.³⁷ In addition, the intensity of C–O–C stretching and C–H bending vibration (1265 cm⁻¹) in the PLA/MOF composites is significantly higher than that of neat PLA, which also indicates the interaction between PLA and MOFs.³⁸

Fig. 4 The FT-IR spectra of PLA/MOF composites.

The crystal of the PLA/MOF composites

Fig. 5 shows XRD patterns of PLA and its composites. The neat PLA exhibits a very strong reflection at 2θ = 16.8° due to diffraction from (110) and/or (200) planes and the less intense peaks at 19.2°, 22.4° and 27.5° attributing to reflections of the (203), (105) and (207) planes, respectively.³⁹ The similar diffraction patterns are observed in PLA/MOF composites, indicating that the incorporation of MOFs does not affect the crystal forms of PLA. However, the bread part of the curve attributed to amorphous phase in the pattern of PLA disappears in the XRD of PM0.5, PM1 and PM1.5, indicating the formation of more crystalline regions. The Table 1 shows the grain sizes of (110) and/or (200) planes for PLA and its composites, which are calculated by Scherrer formula as shown below:

D = 0.89λ/β cos θ

where D is the grain size, λ is the X-ray wavelength (λ = 0.15406 nm), β is the full width at half maximum (FWHM) in radiance and θ is the Bragg angle of diffraction line. The grain size is 34.53 nm for neat PLA, while it is deceased to 22.81 nm, 19.95 nm and 16.64 nm for PM0.5, PM1 and PM1.5, respectively. This indicated that the addition of MOFs particles significantly decreases the crystallite size of PLA. During the crystallization process of PLA/MOF composites from melt, the dispersed MOFs particles act as a nucleating agent and are inclined to absorb PLA chains. Thus, the movability of PLA chains is reduced and the chain packing is increased through the crystallization. Due to the large amount of nuclei and the fast nucleation rate, the crystallite cannot grow large enough to overlap. Accordingly, the crystallite size of PLA/MOF composites would be smaller than that of pure PLA.

Fig. 5 XRD patterns of the PLA/MOF composites.

Table 1 The crystallite size of PLA/MOF composites

Samples	2θ (°)	β (°)	D(110/200) (nm)
PLA	16.85	0.230	34.53
PM0.5	16.89	0.348	22.81
PM1	16.83	0.398	19.95
PM1.5	16.83	0.483	16.64

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The POM images are presented in Fig. 6. It can be seen in Fig. 6 that the spherulites are formed for all four samples, which suggests that the three-dimensional growth of PLA crystals is unchanged. In case of neat PLA, spherulites exhibited high ordered spherulitic texture with size of 30–50 μm. The size of spherulites is significantly decreased with increasing content of MOFs and out of range for POM. In addition, the number of spherulites is increased in presence of MOFs, which indicates a nucleating effect.

Fig. 6 Optical micrographs of neat PLA and PLA/MOF composites ((a) PLA, (b) PM0.5, (c) PM1, (d) PM1.5).

The non-isothermal crystallization of the PLA/MOF composites

Fig. 7 presents the DSC non-isothermal thermograms obtained for PLA and PLA/MOF composites cooled at rates of 2, 5 and 10 °C min⁻¹. Several typical thermal events, including crystallization, glass transition and melting are observed from the cooling and subsequent heating progress. The crystallization peak temperature (T_c), the glass transition temperature (T_g) and the melting temperature (T_m) are listed in Table 2. According to the Table 2, for a given cooling rate, PLA/MOF composites show a higher T_c (∼0.4–1.2 °C) than that of neat PLA. It shows that PLA/MOF composites started to crystallize sooner than neat PLA, which is an indication that the presence of MOFs can act as nucleating agent on the crystallization of PLA. In addition, the T_g and the T_m of PLA/MOF composites show similar increases in composition with that of neat PLA. The increase of T_g may mainly due to the facts that there is no restriction to the chain motion in the neat PLA, while the incorporation of MOFs crystals hinders the chain mobility and the participation of the PLA chains in this transition in the PLA/MOF composites.⁴⁰ In the researches of Nam⁴¹ and Kathuria,⁴² the T_m of the PLA composites were also increased by adding layered silicate and Cu₃BTC₂ metal organic framework, respectively. Gui⁴³ and Zhang⁴⁴ pointed out that the increase of T_m is attributed to the facts that the crystals of PLA/MOF composites is more perfect compared with neat PLA. At cooling rates of 5 and 10 °C min⁻¹, all samples show two melting peaks. The melting peak at lower temperature is due to the melting of crystals with less perfection in the boundary regions, which subsequently recrystallized and remelted at a higher temperature.⁴⁴

Fig. 7 DSC curves of PLA/MOF composites at different cooling rate ((a) 2 °C min⁻¹, (b) 5 °C min⁻¹, (c) 10 °C min⁻¹).

Table 2 The T_c, T_g and T_m of PLA and PLA/MOF composites

Samples	Φ (°C min^−1)	Tc (°C)	Tg (°C)	Tm (°C)
				1	2
PLA	2	121.1	61.8	—	170.9
	5	116.5	62.5	169.6	174.9
	10	111.1	63.5	168.2	175.1
PM0.5	2	122.2	66.8	—	172.4
	5	117.3	65.6	171.1	176.2
	10	112.0	66.3	169.8	171.7
PM1	2	121.6	66.4	—	176.4
	5	116.9	65.1	170.4	175.6
	10	111.7	66.6	169.1	175.8
PM1.5	2	122.1	68.2	—	172.3
	5	117.4	68.2	171.1	176.1
	10	112.3	67.2	169.9	176.6

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The crystallinity (X_c) was determined from the relation as follows:

X_c = ΔH_f/ΔH⁰_f(1 − φ) (1)

where ΔH_f is the melting enthalpy (J g⁻¹), the ΔH⁰_f is the melting enthalpy of 100% crystallized PLA (93.6 J g⁻¹) and φ is the mass fraction for the filler in composites. The calculated X_cs are summarized in Fig. 8. It can be seen that the crystallinity of PLA/MOF composites are higher than that of neat PLA at all cooling rates. By promoting heterogeneous nucleation, the crystallizing ability of PLA was markedly enhanced, leading to a great increase in the degree of crystallinity. However, when the content of MOFs rise to 1.5 wt%, the crystallinity is lower than that of PM1. It may attribute to the aggregation of MOFs particles, which may weaken the effect on crystal growth.⁴⁵

Fig. 8 Crystallinity of the PLA/MOF composites.

The relative degree of crystallinity (X_t) during the non-isothermal crystallization is defined as follows:

where T₀ and T_∞ are the onset and end temperature of the crystallization process, respectively. The crystallization time can be calculated as a function of temperature by the relationship:

t = (T₀ − T)/Φ (3)

where t is crystallization time, T₀ is the onset crystallization temperature (t = 0), T is the temperature at crystallization time at t, and Φ is the value of the cooling rate. The X_t as a function of time for neat PLA and its composites at different cooling rates is illustrated in Fig. 9. It can be seen that all these curves have the same sigmoidal shape, which means that only the lag effect of cooling rate on crystallization is observed. It is evident that the higher the MOFs content, the shorter the time for completing crystallization. The crystallization half-time, t_1/2, defined as the time required to reach 50% of the final crystallinity, is observed from Fig. 9 and listed in Table 3 for each sample. Table 3 shows that at a given cooling rate, the values of t_1/2 for the PLA/MOF composites are lower than that of neat PLA. This suggests that the presence of MOFs accelerates the overall crystallization process.

Fig. 9 Plots of X_t versus t at different cooling rates for the PLA/MOF composites ((a) 2 °C min⁻¹, (b) 5 °C min⁻¹, (c) 10 °C min⁻¹).

Table 3 The parameters of non-isothermal crystallization for the PLA/MOF composites

Samples	Φ (°C min^−1)	t1/2 (min)	Zc	n	Correlation coefficient (γ)
PLA	2	4.47	0.089	3.1	0.96
	5	1.30	0.800	3.2	0.99
	10	1.01	0.959	3.5	0.99
PM0.5	2	3.71	0.112	3.1	0.98
	5	1.18	0.848	3.1	0.99
	10	0.80	1.037	3.1	0.99
PM1	2	3.40	0.132	3.1	0.98
	5	1.12	0.885	3.1	0.99
	10	0.82	1.030	3.3	0.99
PM1.5	2	2.98	0.178	3.0	0.98
	5	1.02	0.934	2.8	0.98
	10	0.78	1.045	3.2	0.99

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Jeziorny method

One of the common approaches to describe the overall non-isothermal crystallization kinetics is the Avrami model,⁴⁶ in which X_t can be expressed in the following form:

1 − X_t = exp(−Z_ttⁿ) (4)

where n is the Avrami crystallization exponent, which is dependent on the nucleation mechanism and growth dimensions; t is the crystallization time, Z_t is the crystallization rate constant, which depends on nucleation and crystal growth. The double-logarithmic form of eqn (4) yields:

ln[−ln(1 − X_t)] = ln Z_t + n ln t (5)

Considering the temperature-dependent character for the nonisothermal crystallization process, the rate constant Z_t was modified by Jeziorny.⁴⁷

ln Z_c = ln Z_t/Φ (6)

where Φ is the cooling rate, Z_c is the corrected Jeziorny crystallization constant and correlated with the crystallization rate. According to eqn (5), n and Z_t can be obtained from the slope and intercept of plot of ln[−ln(1 − X_t)] versus ln t as shown in Fig. 10. And the values of Z_c can be calculated by the eqn (6).

Fig. 10 Avrami plots for the for the PLA/MOF composites ((a) 2 °C min⁻¹, (b) 5 °C min⁻¹, (c) 10 °C min⁻¹).

The values of n and Z_c are listed in the Table 3. It can be seen in the Table 3 that the values of n for neat PLA and PLA/MOF composites range from 2.8 to 3.5, which indicates an athermal and sporadic nucleation process followed by three-dimensional crystal growth. The crystallization rate constant Z_c which describes the crystallization rate of polymer chains is found to increase with increasing cooling rate, indicating an improved crystallization rate. Furthermore, for a given cooling rate, the values of Z_c for PLA/MOF composites are increased as the increasing of MOFs content, suggesting that the crystallization rate of PLA/MOF composites is increased. However, the increase is less pronounced in higher cooling rates. At 2 °C min⁻¹ cooling, the value of Z_c is increased to 0.178 for PM1.5, which is the twice of that for neat PLA (0.089). In contradiction to the X_t, the crystallization rate of PM1.5 is higher than that of PM1. The similar phenomenon were observed in the researches of Barrau et al.⁴⁵ and Wu et al.⁴⁸ Barrau suggested that the filler have little effect on crystal growth and the nucleation effect should be more sensitive. Thus, we can draw a conclusion that the addition of MOFs does not change the crystal forms of PLA, but accelerates the process of crystallization. For further analysis for the crystallization kinetics of PLA and its composites, we use the Mo method.

Mo method

The Mo method,⁴⁹ which combined the Avrami equation with the Ozawa equation, is used to describe the non-isothermal crystallization. Its final form is given as follows:

ln Φ = ln F(T) − α ln t (7)

where F(T) is the Mo modified crystallization rate parameter, α is the ratio of n in the Avrami equation and m in the Ozawa equation exponents related to the crystallization dimension. Obviously, at a given value of relative degree of crystallinity, plotting ln Φ versus ln t should yield a series of straight lines as shown in Fig. 11. Then, the F(T) and α can be obtained from the intercept and slope of the straight lines, respectively. The parameters obtained from the plots of ln Φ versus ln t for all samples are listed in Table 4. As shown in Table 4, for each sample, the value of F(T) is increased with the increase in percentage crystallinity since the motion of molecular chains was slower as the material crystallized and the formation of new crystals became hindered. At a fixed X_t, the F(T) values are decreased with the increasing content of MOFs particles, suggesting that the rates of crystallization are promoted with the containing of MOFs. For all samples, the values of α relating to the crystallization dimension range between 0.92 and 1.15 and slightly increase with the relative degree of crystallinity. These results are in agreement with the results of Jeziorny method.

Fig. 11 Plots of ln Φ versus ln t for different degrees of crystallinity ((a) PLA, (b) PM0.5, (c) PM1, (d) PM1.5).

Table 4 Values of Mo parameters at a fixed value of X_t for the PLA/MOF composites

Samples	Xt (%)	α	F(T)	Correlation coefficient (γ)
PLA	20	0.92	7.23	0.99
	40	0.96	9.26	0.99
	60	0.99	10.84	0.99
	80	1.02	12.56	0.99
PM0.5	20	0.96	6.60	1.00
	40	0.98	8.44	1.00
	60	1.01	9.49	1.00
	80	1.02	11.75	1.00
PM1	20	1.02	6.05	0.99
	40	1.05	7.78	0.99
	60	1.06	9.26	1.00
	80	1.10	11.09	1.00
PM1.5	20	1.08	5.28	0.99
	40	1.10	6.81	0.99
	60	1.12	8.14	0.99
	80	1.15	9.78	1.00

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Influence of MOFs on the thermal stability of PLA

The thermal stability of neat PLA, MOFs and PLA/MOF composites was measured by TGA. The TGA curves of neat PLA, MOFs and PLA/MOF composites at 10 °C min under N₂ are shown in Fig. 12. As shown in Fig. 12, there are three thermal degradation stages for MOFs. The first mass loss occurring at 50 °C could be assigned to the absorbed water and the second mass loss which ranges from 235 °C to 325 °C corresponds to the removal of the coordinated DMF molecule. Such a high temperature of the coordinated solvent molecule removal indicates that the DMF ligand bonds strongly with the metal centre. The third mass loss stage (325–500 °C) could be attributed to the breakdown of pyridine-2,5-dicarboxylate species followed by their pyrolysis. In this stage, MOFs could be destructed and other Zn compounds such as ZnO could be produced.²⁹ The curves of PLA and its composites show one step of mass loss. The temperature for 10% mass loss (T₁₀), the temperature for 50% mass loss (T₅₀) and the temperature at the maximum mass loss rate (T_max) are listed in Table 5. It can be observed in Table 5 that the T₁₀s are increased with addition of MOFs particles into PLA. For PM1, T₁₀ is increased by 4.7 °C. In contrast, T₅₀ and T_max are decreased with the increase of MOFs content. These results demonstrate that MOFs increases the initial thermal degradation temperature of PLA and decreases the thermal stability at high temperature region (above 370 °C). The influence of MOFs on the thermal degradation of PLA is complex. The probable mechanism of MOFs particles' influence on the thermal degradation of the PLA/MOF composite is shown in Fig. 13. According to the results of FT-IR and DSC, from 50 °C to 160 °C, the interface interaction between PLA and MOFs particles as well as the increased crystalline regions hinder the movement of PLA chains, which impedes the thermal degradation of the PLA composites.^50,51 When the temperature is higher than the melt temperature of PLA (160–370 °C), as the second stage, most polymer chains are free to move and the interactions between PLA chains and MOFs are gradually decreased into a non-interacting state. The thermal degradation of PLA is mainly caused by random scissions to generate linear and cyclic oligomers, and the recombination reactions of the cyclic oligomers and linear oligomers also occur, especially at temperatures lower than the boiling point of lactides.⁵² At this temperature region, the reason that MOFs improves the thermal stability of PLA is probably due to hindered out-diffusion of the volatile decomposition products caused by MOF particles as shown in Fig. 13. From 370 °C to 500 °C, the MOFs is largely degraded and other Zn compounds such as ZnO may be produced.²⁹ The thermal degradation of PLA/MOF composites is accelerated since the Zn compounds can be catalysts for the intermolecular transesterification generating linear PLA oligomers and the unzipping depolymerization of cyclic PLA oligomers generating lactides.^18,53

Fig. 12 TGA curves of neat PLA, MOFs and PLA/MOF composites in N₂.

Table 5 TGA data on PLA and PLA/MOF composites

Samples	T10 (°C)	T50 (°C)	Tmax (°C)
PLA	342.1	384.6	388.3
PM0.5	343.5	383.7	386.6
PM1	346.8	381.5	384.1
PM1.5	344.6	376.1	378.5

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Fig. 13 The mechanism of MOFs particles' influence on the thermal degradation of the PLA/MOF composites.

The mechanical properties of the PLA/MOF composites

The results of tensile testing are shown in Fig. 14. It can be observed in Fig. 14 that tensile strength of PM0.5, PM1 and PM1.5 are increased to 92.44 MPa, 96.03 MPa and 95.25 MPa from 66.23 MPa for neat PLA, respectively. In the case of PM1, the tensile strength is increased by 45.0% compared with neat PLA. It is obvious that the addition of MOFs has a significant effect on the tensile strength of composites. The increase in tensile strength could be attributed to the good dispersion of MOFs in matrix, the interfacial interactions between PLA matrix and MOFs as well as the reduction of the segmental motion by the MOFs crystals.⁵⁴ However, the value of elongation at break is decreased with the increase of MOFs content. This can be attributed to the improvement of the crystallization of PLA/MOF composite. In general, crystallization of semi-crystalline polymer results in embrittlement of the polymer and hence decreases the strain at break.⁵⁵

Fig. 14 Tensile strength and elongation at break of PLA and PLA/MOF composites.

Conclusion

Porous and crystalline MOFs was synthesized and the PLA/MOF composites with different contents of MOFs were prepared by melting blending. The SEM images suggest good dispersion of MOFs particles in PLA matrix. The non-isothermal kinetics crystallization of PLA/MOF composites were studied by DSC at different cooling rates using the Jeziorny and Mo methods. The results demonstrated that MOFs acts as nucleation agent accelerating the non-isothermal crystallization process and significantly increasing the crystallinity (X_c) of PLA. The XRD result demonstrated that the presence of MOFs decrease the crystallite size. The TGA results indicate that the presence of MOFs improves the initial thermal degradation temperature of PLA and decreases the thermal stability at high temperature region. The addition of MOFs increased the tensile strength of PLA from 66.23 MPa to 92.44 MPa, 96.03 MPa and 95.25 MPa with content of 0.5 wt%, 1 wt% and 1.5 wt%, respectively.

Acknowledgements

This work was supported by Science and Technology Support Program (Social Development) of Jiangsu Province of China (BE 2013714) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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