Effect of biodegradable poly(ethylene adipate) with low molecular weight as an efficient plasticizer on the significantly enhanced crystallization rate and mechanical properties of poly(L-lactide)

Abstract

The slow crystallization rate and low elongation at break are the main drawbacks of biodegradable poly(L-lactide) (PLLA) from a viewpoint of practical application. Such disadvantages of PLLA have been successfully resolved by blending with low molecular weight biodegradable poly(ethylene adipate) (PEA) in the present work. As a biodegradable plasticizer, PEA not only formed fully miscible and biodegradable polymer blends with PLLA but also apparently reduced the glass transition temperature of the blends. The increased chain mobility favored the nonisothermal cold and melt crystallization behaviors, increased the spherulitic growth rate, and accelerated the overall isothermal melt crystallization process of the blends; however, the blends had the same crystallization mechanism and crystal structures as neat PLLA. One of the exciting results was that the elongation at break value of an 80/20 PLLA/PEA blend was dramatically increased; therefore, for a wider practical application, the preparation of PLLA-based materials with fast crystallization rate, good mechanical properties, and complete biodegradability could be accomplished by the blending with a small amount of PEA with low molecular weight as an efficient plasticizer.

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Introduction

Relative to other biodegradable polyesters, poly(L-lactide) (PLLA) has received more research interest, as the monomer for the preparation of PLLA can be obtained from renewable resources, thereby reducing the dependence on chemical resources.^1–11 Because it is biodegradable and biocompatible, PLLA has found many end uses as both environmentally friendly materials and biomedical materials; however, the brittleness and slow crystallization rate are the main drawbacks of influencing its wider practical applications.^9–14 In order to improve the ductility, the blending with ductile polymers is one of the most efficient and convenient ways of regulating the mechanical properties of PLLA.^12–14 In previous work, we found that poly(ethylene succinate) (PES) may obviously improve the elongation at break of the PLLA/PES blends without losing the biodegradability.¹³

Depending on the D-lactic acid unit, molecular weight, and crystallization conditions, PLLA does not crystallize or may only crystallize very slowly. Polymer crystallization consists of nucleation and crystal growth processes. To achieve a faster crystallization rate of PLLA, the addition of a nucleating agent is one of the most convenient choices. Till now, a number of nucleating agents have been found to be effective in accelerating the crystallization processes of PLLA-based materials under both nonisothermal and isothermal crystallization conditions. Some typical nucleating agents include polyglycolide, orotic acid, uracil, cyanuric acid, etc.^15–19 In addition, some nanofillers are efficient nucleating agents for the PLLA-based nanocomposites, such as clay, polyhedral oligomeric silsesquioxanes, graphene, graphene oxide, multi-walled carbon nanotubes, and layered metal phosphonate.^20–32 Many more nucleating sites may usually be provided in the presence of nucleating agent, thereby increasing the nucleation density of PLLA spherulites and accelerating the crystallization processes.^15–32

In addition, the blending with a plasticizer is another effective way to promote the crystallization behavior of PLLA. Till now, some compounds or polymers may act as typical plasticizers for the plasticization of PLLA, such as triacetin, tributyl citrate, acetyl tri-n-butyl citrate, oligomeric lactic acid, triphenyl phosphate, poly(ethylene glycol), poly(propylene glycol), and poly(3-methyl-1,4-dioxan-2-one).^33–39 In most cases, the presence of plasticizers can lower the glass transition temperature of PLLA-based materials; therefore, the increased chain mobility of the blends may enhance the crystallization behaviors of PLLA by mainly increasing the spherulitic growth rate and overall crystallization rate.^33–39 It is interesting to note that the plasticizers can not only increase the ability of PLLA to crystallize under different crystallization conditions but also favor the plastic deformation of PLLA by significantly increasing the elongation at break and decreasing the elastic modulus as well as the yield strength.

The aim of this work is to study whether poly(ethylene adipate) (PEA) with low molecular weight may act as an efficient plasticizer of PLLA. In the present work, we prepared the PLLA/PEA blends with 10 and 20 wt% PEA to overcome the brittleness and slow crystallization rate of PLLA. The reasons why we chose low molecular weight PEA as a plasticizer are as follows. First, PEA has a low glass transition temperature value, which is comparable to that of poly(ethylene glycol), thereby making it a suitable candidate as a plasticizer. Second, PEA is miscible with PLLA, and no obvious phase separation occurs in the PLLA/PEA blends.⁴⁰ Third, PEA is also biodegradable; therefore, the blending with PEA must still keep the biodegradability of PLLA. The research results are expected to be interesting and important from both polymer crystallization and practical application of PLLA-based materials viewpoints.

Experimental

PLLA (M_n = 9.56 × 10⁴ g mol⁻¹, M_w = 1.42 × 10⁵ g mol⁻¹) was kindly provided by Professor Yongjin Li of Hangzhou Normal University, China. PEA (M_n = 1.86 × 10³ g mol⁻¹, M_w = 2.69 × 10³ g mol⁻¹) was synthesized in our laboratory.⁴¹ The PEA sample used in this study had a glass transition temperature (T_g) of around −50 °C.

Through a solution and casting process, the following three samples, including neat PLLA (100/0), 90/10, and 80/20 (weight ratio of PLLA to PEA) were prepared using chloroform as a solvent.

A TA Instruments differential scanning calorimeter (DSC) Q100 with a Thermal Analyst 2000 was used for the thermal analysis of neat PLLA and the PLLA/PEA blends under different conditions. For each sample, any previous thermal history must first be erased by annealing at 190 °C for 3 min. A heating rate of 20 °C min⁻¹ was used to study the nonisothermal cold crystallization behaviors of the melt-quenched samples from the amorphous state, which had been reached by quenching to −80 °C at 60 °C min⁻¹ after erasing any previous thermal history. A cooling rate of 5 °C min⁻¹ was used to investigate the nonisothermal melt crystallization behavior, while a range of 125 to 132.5 °C was used to study the isothermal melt crystallization kinetics. The subsequent melting behaviors were studied at 20 °C min⁻¹ after the isothermal melt crystallization processes at indicated crystallization temperature values for the determination of equilibrium melting point.

The spherulitic morphology and growth rates of neat and blended PLLA were studied with a polarized optical microscope (POM) (Olympus BX51) equipped with a temperature controller (Linkam THMS 600).

A Rigaku D/Max 2500 VB2+/PC X-ray diffractometer was used to study the crystal structures of neat and blended PLLA, which was operated 40 kV and 200 mA.

An Instron 1185 tensile testing machine was used to measure the mechanical properties of neat and blended PLLA at 20 mm min⁻¹.

Results and discussion

Miscibility of PLLA/PEA blends

As shown in Fig. 1, the melt-quenched samples of neat PLLA and the PLLA/PEA blends were heated from the amorphous state at 20 °C min⁻¹. Neat PLLA has a T_g of 62.5 °C. The PLLA/PEA blends only exhibit a single T_g below that of neat PLLA. The T_g values are significantly decreased to be 46.8 and 30.8 °C for the 90/10 and 80/20 samples, respectively. In brief, the blending with a small amount of PEA has obviously reduced the T_g values of the blends, indicating PLLA and PEA are miscible.

Fig. 1 The melt-quenched samples of neat and blended PLLA were heated at 20 °C min⁻¹ from the amorphous state.

The depression of equilibrium melting point of PLLA/PEA blends can also confirm whether the two components are miscible or not.⁴² The equilibrium melting point (T^o_m) value is usually estimated by the Hoffmann–Weeks equation:

T_m = ηT_c + (1 − η)T^o_m (1)

where η represents a measure of the stability and T_m corresponds to the apparent melting point at crystallization temperature (T_c).⁴³ Fig. 2 displays the Hoffman–Weeks plots for all the three samples. Neat PLLA has a T^o_m of 186.4 °C, while the 90/10 and 80/20 samples have the T^o_m values of 182.2 and 173.9 °C, respectively. Relative to neat PLLA, the T^o_m values of the blends were reduced, confirming again that PLLA is thermodynamically miscible with PEA.⁴² In summary, PLLA is miscible with PEA, because the blends exhibit not only the single composition dependent T_g but also a depression of T^o_m.

Fig. 2 Hoffman–Weeks plots of neat and blended PLLA.

Nonisothermal crystallization behavior studies of PLLA/PEA blends

The effects of the blending with PEA on the nonisothermal crystallization behaviors of the blends were investigated with DSC. As shown in Fig. 1, the nonisothermal cold crystallization behaviors were investigated for the melt-quenched samples of neat and blended PLLA at 20 °C min⁻¹. Neat PLLA has a cold crystallization peak temperature (T_ch) of 136.1 °C with a cold crystallization enthalpy (ΔH_ch) of 12.4 J g⁻¹. The 90/10 blend has a T_ch of 114.8 °C with a ΔH_ch of 39.5 J g⁻¹, while the 80/20 sample has a T_ch of 86.9 °C with a ΔH_ch of 29.8 J g⁻¹. From the aforementioned results, the blends show smaller T_ch values than neat PLLA; moreover, the blends exhibit greater ΔH_ch values than neat PLLA. By using the heat of fusion of 93 J g⁻¹ (100% crystalline PLLA),⁴⁴ neat PLLA shows a crystallinity value of 13.3%. Considering the PLLA composition, the 90/10 and 80/20 samples show the normalized crystallinity values of 47.2% and 40.1%, respectively. In brief, the nonisothermal cold crystallization behaviors of PLLA have been apparently enhanced in the blends, which should arise from the obviously increased mobility of PLLA chains because of the smaller T_g values of the blends.

Fig. 3 illustrates the DSC cooling traces of neat PLLA and its blends at 5 °C min⁻¹. Both neat and blended PLLA crystallized, showing the broad crystallization exotherms. All the three samples show the nonisothermal melt crystallization peak temperature (T_p) values of around 98–99 °C, regardless of the PEA composition. However, the blending with PEA has significantly influenced the crystallization enthalpy (ΔH_c) and crystallinity values. Neat PLLA shows a ΔH_c value of only 6.0 J g⁻¹; however, the 90/10 and 80/20 samples exhibit the ΔH_c values of 32.7 and 33.7 J g⁻¹, respectively. Considering the PLLA composition, the normalized crystallinity values are increased from 6.5% for neat PLLA to 39% and 45% for the 90/10 and 80/20 samples, respectively. The blending with PEA slightly affects the T_p values but significantly increases the ΔH_c and crystallinity values of the blends; therefore, a small amount of PEA may obviously enhance the nonisothermal melt crystallization behavior of the blends. In brief, both the nonisothermal cold and melt crystallization behaviors of PLLA have been significantly enhanced after the blending with a small amount of PEA in their blends.

Fig. 3 DSC cooling traces of neat and blended PLLA at 5 °C min⁻¹.

Overall isothermal melt crystallization kinetics of neat and blended PLLA

The isothermal melt crystallization kinetics of neat and blended PLLA was also studied at different T_c values. For all the three samples, the crystallization time became longer with increasing T_c. At the same T_c, the blends finished crystallization within shorter time than neat PLLA; moreover, the crystallization time of the 80/20 blend was shorter than that of the 90/10 sample, indicating that the former crystallized faster than the latter.

The Avrami equation was applied to analyze the crystallization processes of neat and blended PLLA. The relative crystallinity (X_t) develops with crystallization time (t) as follows:

1 − X_t = exp(−ktⁿ) (2)

where n is the Avrami exponent and k is the crystallization rate constant.^45,46 Fig. 4 displays the Avrami plots of neat and blended PLLA, which were crystallized at different T_c values ranging from 125 to 132.5 °C. Regardless of T_c, all the three samples show the almost parallel straight lines; therefore, the Avrami method is suitable to analyze the isothermal melt crystallization processes in this work.

Fig. 4 The Avrami plots of (a) 100/0, (b) 90/10, and (c) 80/20.

The n and k values were calculated from Fig. 4. For comparison, Table 1 lists all the related parameters for all the three samples crystallized at different T_c values. Regardless of T_c and the PEA composition, the n values vary slightly between 2.3 and 2.6, suggesting the samples may all crystallize according to a three-dimensional truncated sphere growth with athermal nucleation mechanism.⁴⁷ For each of the three samples, the k values increase with decreasing T_c; therefore, an acceleration of crystallization rate may occur at lower T_c because of larger supercooling when they have the same n values. To make a comparative study of the crystallization rates of neat and blended PLLA, the crystallization half-life time (t_0.5) values were determined as follows:

Table 1 Summary of crystallization kinetics parameters of neat PLLA and the PLLA/PEA blends

Tc (°C)	100/0			90/10			80/20
	n	K (min^−n)	t0.5 (min)	n	K (min^−n)	t0.5 (min)	n	K (min^−n)	t0.5 (min)
125	2.4	7.74 × 10^−4	16.88	2.5	2.66 × 10^−3	9.42	2.6	9.52 × 10^−3	5.09
127.5	2.3	7.05 × 10^−4	19.52	2.4	1.55 × 10^−3	12.97	2.6	5.96 × 10^−3	6.08
130	2.3	4.98 × 10^−4	23.65	2.4	1.44 × 10^−3	15.10	2.6	5.08 × 10^−3	6.53
132.5	2.3	4.50 × 10^−5	25.22	2.5	5.96 × 10^−4	16.90	2.6	2.49 × 10^−3	8.65

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Table 1 also includes all the t_0.5 values. The t_0.5 values increase with an increase in T_c for each of the three samples, indicative of a slower crystallization rate at higher T_c. At the same T_c, the blends show smaller t_0.5 values than neat PLLA; moreover, the 80/20 blend shows a smaller t_0.5 value than the 90/10 sample. Accordingly, the reciprocal of t_0.5 (1/t_0.5) values were used to represent the isothermal melt crystallization rate.

Fig. 5 shows the T_c dependence of 1/t_0.5 for neat and blended PLLA. Relative to neat PLLA, the blending with PEA has accelerated the isothermal melt crystallization process of PLLA in their blends at all T_c values. In conclusion, the blends have the same crystallization mechanism as neat PLLA; moreover, they crystallize faster than neat PLLA, after the blending with a small amount of PEA.

Fig. 5 T_c dependence of 1/t_0.5 for neat and blended PLLA.

Spherulitic morphology and growth rates of neat and blended PLLA

The spherulitic morphology and growth rates should also be studied for neat PLLA and the PLLA/PEA blends, because they may affect their final physical properties as well as biodegradation behaviors. Fig. 6 displays the POM images of neat and blended PLLA after crystallizing at 125 °C. Fig. 6a illustrates neat PLLA crystallized through a typical spherulitic growth with clear boundaries. The 90/10 and 80/20 samples present similar spherulitic morphologies as neat PLLA. Fig. 6b and c display that the whole spaces are filled with PLLA spherulites, suggesting that PEA must reside in the interlamellar and interfibrillar regions of PLLA spherulites.^48,49

Fig. 6 Spherulitic morphology images of (a) 100/0, (b) 90/10, and (c) 80/20 crystallized at 125 °C.

Fig. 7 summarizes the measured spherulitic growth rates for all the three samples in a wide T_c range. Fig. 7 clearly shows that both neat and blended PLLA exhibit a bell-shape spherulitic growth rates within the investigated T_c range; moreover, with increasing the PEA composition the temperature values corresponding to maximum growth rates shift slightly downward to low temperature range. At the same T_c, neat PLLA shows slower spherulitic growth rate than blended PLLA; moreover, the spherulitic growth rate of an 80/20 sample is faster than that of a 90/10 sample. It is obvious that the blending with a small amount of PEA with low molecular weight favors faster spherulitic growth rates of the blends. The blending with PEA with low molecular weight significantly reduces the T_g values of the blends, especially at higher PEA content, and consequently increases the mobility of polymer chains of the blends; therefore, the ability to crystallize has been apparently enhanced for the blends, compared to neat PLLA.

Fig. 7 Variation of spherulitic growth rates with T_c for neat PLLA and the PLLA/PEA blends.

Crystal structure study of neat and blended PLLA

Fig. 8 illustrates the WAXD patterns of neat and blended PLLA, which were crystallized at 125 °C. When PLLA is crystallized from the melt, it may crystallize in order α modification at T_c > 120 °C or α′ modification at T_c < 100 °C, respectively; moreover, the formation of the former is thermodynamically preferential, while that of the latter is kinetically favored.^50,51 In this research, neat and blended PLLA were crystallized at 125 °C; consequently, α form crystals must be developed for all the samples. Neat PLLA exhibits two main diffraction peaks at 16.51° and 18.87°, corresponding to (200)/(110) and (203) planes, respectively.^50,51 The PLLA/PEA blends also present the similar WAXD patterns as that of neat PLLA, regardless of the PEA composition; therefore, the crystal structures of PLLA remain unmodified in the blends. In addition, neat PLLA shows a crystallinity value of around 50%, while the 90/10 and 80/20 samples show the crystallinity values of around 44% and 40%, respectively. In brief, the blends have the same crystal structures as neat PLLA; however, the blending with PEA slightly decreases the crystallinity values of the blends.

Fig. 8 WAXD patterns of neat and blended PLLA.

Enhanced mechanical properties of PLLA/PEA blends

The influence of the blending with PEA on the mechanical properties of the PLLA/PEA blends was further studied. Fig. 9a displays the typical stress–strain curves of neat and blended PLLA, and Fig. 9b illustrates the enlarged stress–strain curves of all of the samples within the strain range of 0 and 15%. From Fig. 9, the tensile mechanical properties were determined.

Fig. 9 (a) Stress–strain curves of entire strain range and (b) magnification of 0–15% strain of neat and blended PLLA.

Table 2 lists the tensile mechanical properties of the three samples. From Table 2, the blends show smaller Young's modulus and tensile strength values than neat PLLA; however, they exhibit greater elongation at break values than neat PLLA. The details are as follows.

Table 2 Summary of the tensile mechanical properties of neat and blended PLLA

Samples	Young's modulus (MPa)	Tensile strength (MPa)	Elongation at break (%)
100/0	2311.6 ± 129.0	52.9 ± 0.2	4.8 ± 0.1
90/10	1252.3 ± 32.6	42.4 ± 3.7	8.2 ± 1.2
80/20	474.3 ± 14.1	13.5 ± 0.9	284.4 ± 11.1

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Neat PLLA has a Young's modulus of 2311.6 ± 129.0 MPa, a tensile strength of 52.9 ± 0.2 MPa, and an elongation at break of 4.8 ± 0.1%. For the PLLA/PEA blends, the Young's modulus values are reduced to be around 1252.3 ± 32.6 and 474.3 ± 14.1 MPa for the 90/10 and 80/20 samples, respectively; moreover, the tensile strength values are also decreased to be 52.9 ± 0.2 and 42.4 ± 3.7 MPa for the two samples with low and high PEA contents. One of the exciting results is that the elongation at break values have been significantly improved in the blends, relative to neat PLLA. For example, the elongation at break value is obviously increased to be around 8.2 ± 1.2% for a 90/10 blend from 4.8 ± 0.1% for neat PLLA; moreover, it is further tremendously improved to be around 284.4 ± 11.1% for an 80/20 sample. In other words, relative to that of neat PLLA, the elongation at break values have been increased by around 70% and 5800% for the 90/10 and 80/20 samples. Neat PLLA is brittle while the PLLA/PEA blends are very ductile, especially for the 80/20 blend; therefore, novel fully biodegradable PLLA-based materials with excellent ductile mechanical properties may be achieved through a simple polymer blending. On the basis of the chemical structures of PLLA and PEA, it is unlikely to show strong or weak chemical interactions between both of the components in their blended state. Therefore, the enhanced elongation at break should be attributed to the efficient plasticizer effect of PEA, which has actually lowered the glass transition temperature and improved the chain mobility of the blends.

Conclusions

In this work, the blending with a small amount of low molecular weight biodegradable PEA was used to overcome the disadvantages of slow crystallization rate and low elongation at break of PLLA, without losing the biodegradability. PEA was found to be miscible with PLLA, because the blends exhibited not only the single composition dependent T_g but also a depression of T^o_m. The chain mobility of the blends was increased by the obvious reduction of glass transition temperature values, thereby enhancing the crystallization behaviors of the blends under different crystallization conditions. First, the nonisothermal cold crystallization behaviors were obviously enhanced in the blends, as the cold crystallization peak temperature was decreased and the crystallinity values were increased. Second, the blending with PEA also favored the nonisothermal melt crystallization behaviors, as evidenced by the apparent increased crystallinity values, although the variation of nonisothermal melt crystallization peak temperatures was slight between neat PLLA and the PLLA/PEA blends. Third, the blends crystallized faster than neat PLLA during the overall isothermal melt crystallization process, as evidenced by the shorter crystallization half-time, indicating the blending with PEA also accelerated the overall isothermal melt crystallization processes of the blends. Fourth, the blending with PEA was also found to obviously enhance the growth rates of PLLA spherulites in the blends. The enhanced crystallization behaviors of the blends must be related to the increased chain mobility of PLLA, which was induced by the blending with PEA as an efficient plasticizer. After the blending with PEA, the blends show the same crystallization mechanism and crystal structures as neat PLLA. Moreover, the mechanical properties of the blends were also found to be improved, after the blending with a small amount of PEA. The elongation at break value of the 80/20 blend was dramatically increased to be around 284%, which was significantly than that of neat PLLA. Through a blending with a small amount of low molecular weight PEA, the wider practical application of PLLA-based materials would be achieved after overcoming their disadvantages of slow crystallization rates and unsatisfied mechanical properties.

Acknowledgements

This research was supported by the National Natural Science Foundation, China (51221002 and 51373020).

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