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. 2020 Nov 9;5(45):29284–29291. doi: 10.1021/acsomega.0c04070

Toughening of Poly(l-Lactide) with Branched Polycaprolactone: Effect of Chain Length

Xiangming Yang †, Shuaibo Liu †, Erlei Yu †,‡,*, Zhong Wei †,‡,*
PMCID: PMC7675962  PMID: 33225159

Abstract

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In this work, a series of branched polycaprolactone (BPCL) samples with different ε-caprolactone (CL) chain lengths were synthesized and used to toughen poly (lactic acid) (PLA). The spherical structure increased the free volume, facilitating the free movement of the PLA chain segment and increasing the ductility. In addition, the hydrogen bonds between the multi-terminal hydroxyl group of BPCLx and PLA improved the interaction between them. The glass-transition temperatures (Tg) and crystallization temperatures (Tc) of the blends were significantly lower than those of PLA, and these temperatures increased with the chain length of polycaprolactone. BPCLx increased the crystallization rate of PLA through heterogeneous nucleation. A longer chain length of CL increased the mutual entanglement in the blends, reduced the hydrogen bonding between BPCLx and PLA, and increased the entanglement of BPCLx chains. When the chain length of CL was 6, the impact strength and elongation at break of the PLA/BPCL blends exhibited an increase of 151.72 and 465.8%, respectively, as compared with PLA.

1. Introduction

Owing to the limited sources of petroleum-based polymer materials and the significant environmental pollution created by their waste, the search for renewable and biodegradable environmentally friendly polymer materials has become a research focus in the field of materials.1 Poly (lactic acid) (PLA) has good biocompatibility and biodegradability in the human body and the environment.2 At the same time, it has high strength, modulus, transparency, and processability, and it has been widely used in the biomedical and food packaging fields. However, PLA resin is hard and brittle with a poor impact resistance, which limits its application in many fields especially for applications requiring plastic deformation under high stress.3 Therefore, the toughening modification of PLA has become a popular topic in the field of materials.

Considerable efforts have been made to toughen PLA, such as through blending, copolymerization, and plasticization.4 Because flexible polymer has high flexibility, which can dilute the rigid PLA and thereby improve the ductility of PLA chains. Compliant polymers, such as elastomers (e.g., polyurethane elastomers5 and polyamide elastomers6) and polymers with flexible-chain segments (e.g., polypropylene glycol,7,8 polyethylene glycol,9 and polycaprolactone (PCL)),10 are the most commonly used for blending modification. To improve the toughness of PLA without affecting its biodegradability, biodegradable materials are preferred.11

As a fully biodegradable semi-crystalline polyester, PCL is primarily used to toughen PLA due to its low glass-transition temperature (Tg, approximately −60 °C) and excellent molecular chain flexibility.12 However, like other problems faced by PLA with pliable polymer toughening, the interface compatibility between PLA and PCL is poor, which makes the interface adhesion force between the two phases poor. Consequently, macroscopic phase separation occurs easily, resulting in poor performance of the material.13 The toughness of the blended PCL and PLA was not significantly improved, and the strength and modulus decreased, which was mainly attributed to the poor interface compatibility. To improve the compatibility, various methods, such as adding block or random polymer compatibilizers,14−17 adding inorganic compatibilizers,18,19 and interfacial crystallite cross-linking,20 have been studied.

Recently, polymers with different branched structures have also been used to toughen PLA due to their advantages of low melt viscosities and large numbers of end functional groups and long branched chains that are prone to chain entanglement. Doganci et al.21 synthesized a star-shaped PCL (SP) using POSS as the initiator and found that the tensile toughness and impact toughness were improved, which was attributed to the PCL segment and star structure. Long-branched chain structures exhibit high shear thinning and high zero shear viscosities, which can increase the melt strength.22,23 For polymers with the same molecular weight, a higher branching degree of the polymer provides it with a smaller hydrodynamic volume and radius of gyration,24 and a large number of terminal hydroxyl groups allow the polymer to form hydrogen bonds easily, increasing the binding ability with the substrate.25 Lin et al.26 fused hyperbranched polyester with PLA and found that when the content of added hyperbranched polyester was 2%, the tensile strength increased from 59 to 70 MPa, and the elongation at break increased from 2 to 8%. Bhardwaj et al.27 reported that a hydroxyl functional hyperbranched polymer was in situ cross-linked with a polyanhydride in the PLA matrix, which improved the toughness and elongation at break by ∼570 and ∼847%, respectively, compared to those of the unmodified PLA.

To improve the performance of PCL for toughening PLA, the introduction of hyperbranched polyester structures into PCL is a very effective method. However, to the best of our knowledge, this method has not been studied. In our previous work,28 branched polycaprolactone (BPCL) was blended with PVC, and it was found that the elongation at break increased significantly without reducing the tensile strength, which was very beneficial for toughening the PLA. In this work, hyperbranched polyesters were synthesized by glycidol and succinic anhydride, and branched PCLs with different branch chain lengths (BPCLx, x represents the degree of polymerization of a single PCL arm in BPCL) were obtained by using hyperbranched polyesters to initiate the ring-opening polymerization of the caprolactones. The synthesis method is simple, and the raw materials glycidol and succinic anhydride are biodegradable in the environment.29,30 BPCLx was blended with PLA with a 10 wt % content in a HAAKE rheometer at high speeds to explore the improvement of the interface compatibility and mechanical properties of PLA/BPCLx by introducing the same amount of additive and a branched structure. Since the length of the caprolactone chain affects the branched structure and the viscosity, we studied the effects of different chain lengths on PLA.

2. Results and Discussion

2.1. Impact Analysis

Figure 1A shows the variation of the PLA impact toughness after the addition of BPCL with different long-branched chains. The addition of BPCLx prevented the merging of microcracks in the PLA matrix and forming destructive cracks, which resulted in the curved propagation and bifurcation of cracks and increased the impact energy.48 However, if the particles were too large, it promoted the formation of cracks, thereby reducing the impact energy.27 As shown in Figure 1A, when BPCLx was blended with PLA, it showed a significant improvement in the impact strength compared to that of only PLA and increased with the increase in the chain length of CL. However, when the chain length was above 6, the impact strength decreased, but it was still higher than the compact toughness of PLA. This was because when the chain length was short, BPCLx could penetrate the PLA chain and act as a plasticizer in the polymer.32 It can be seen from Figure 1B that the OH peak (3500 cm–1) of PLA is very sharp, but the peak of BPCL6 and PLA/BPCL6 is broadened and the peak of C=O moves toward a low wave number, indicating the H-bonding in BPCLx and PLA/BPCL6.41 The C=O stretching band observed at around 1750 cm–1 in BPCL and PLA/BPCL consists of two peaks: one is free C=O and another is hydrogen-bonded C=O. With the increase in the flexible chain segment, BPCLx became easier to entangle with the PLA chain, and the terminal hydroxyl group formed hydrogen bonds with the PLA chain, which is conducive to the uniform distribution of PCL chains. However, as the molecular weight and chain length continued to increase (x > 6), the hydrogen bond density began to decrease, the segregation power became larger, and the long BPCLx chains entangled with each other, resulting in larger particle sizes which were not conducive to energy transfer33−35 and lead to the decrease of impact strength.

Figure 1.

Figure 1

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(A) Impact strength of PLA and PLA/BPCLx polymers. (B) FT-IR spectra of PLA/BPCL6, PLA, and BPCL6.

2.2. Tensile Analysis

The impact test results showed that the addition of BPCLx could effectively improve the brittleness of the PLA. To further explore the effect of BPCLx on the mechanical properties of PLA, the tensile properties of the blended materials were tested. The spherical structure increased the free volume, which facilitated the free movement of the PLA chain segments and increased the ductility. As shown in Figure 2A and Table 1, when the chain length was 4, the elongation at break increased to 23.2%. This occurred because the addition of BPCL4 lowered the interaction between chains to let the PLA chains slide over each other.21,36 The shorter the CL chain was, the closer to the spherical structure of BPCLx and the greater the effective free volume provided to the PLA segment was, which correspondingly increased the mobility of the PLA segment. However, longer chains increased the entanglement of the chains between the blends but reduced the hydrogen bonding between the PLA and BPCLx, forming an entangled BPCLx chain and leading to the reduction of the elongation at break values of the blends. As shown in Figure 2B, both the tensile strength and the modulus remain unchanged.

Figure 2.

Figure 2

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(A) Stress against the strain of PLA and PLA/BPCLx blends. (B) Tensile strength and elastic modulus of blends of PLA and PLA/BPCLx blends.

Table 1. Detailed Mechanical Data of PLA and PLA/BPCLx Blends Obtained from Both Tensile and Impact Measurements.

  elongation at break (%) tensile strength (MPa) elastic modulus (MPa) impact strength (J/m)
PLA 4.18 59.49 1829.47 9.27
PLA/BPCL4 23.20 43.31 1488.23 17.46
PLA/BPCL6 6.58 45.18 1487.52 23.34
PLA/BPCL7 5.40 48.35 1534.71 12.76
PLA/BPCL8 5.96 46.95 1543.75 14.29
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2.3. Differential Scanning Calorimetry Analysis

Figure 3A shows that there were two exothermic peaks for PLA and three exothermic peaks for PLA/BPCLx blends. The exothermic peaks in the lower temperature of the blends are the Tm of BPCLx as shown in Figure 3B and Table 2. The exothermic peak at about 100 °C was the cold crystallization peak of PLA, and the exothermic peak near the melting peak corresponded to the recrystallization of the low-perfection crystal into a more perfect crystalline form.37,38 Pure PLA had an exothermic peak at 102.0 °C, indicating that pure PLA had a low crystallinity and a low cold recrystallization ability. When the chain length was less than 6, because the branched structure hindered the crystallization of the PCL chain segment, it had a dilution effect on the crystallization of PLA and inhibited the crystallization of the PLA chain segments; the decrease in Tc showed that as a nucleating agent, BPCLx promoted the heterogeneous crystallization rate of PLA, but it did not affect the crystal morphology of the PLA, and the branched structure of BPCLx reduced the crystallinity of PLA and increased its amorphous characteristics.39,40 However, when the chain length was greater than 6, the crystallinity tended to increase as shown in Table 2. This occurred due to the BPCLx chains entangling with each other, which caused the interaction between the PLA chains to be shielded, making the polymer segments easier to move and increasing the crystallinity.41 When the relative molecular mass is small, the entanglement of a molecular chain itself or between the segments is small, and the energy and free volume required for the movement of the molecular segments are small.42 Therefore, Tg increases with the chain length of CL. During the temperature rising process, a segment with a relatively low molecular weight can undergo a “glass transition” through its own adjustment earlier, and therefore will have a lower Tg.43−45 A sample with a larger relative molecular mass requires more free volume and a higher temperature and therefore exhibits a higher Tg.

Figure 3.

Figure 3

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Differential scanning calorimetry (DSC) curves of (A) PLA/BPCLx and (B) BPCLx blends.

Table 2. DSC Data of BPCLx, PLA and PLA/BPCLx Blends.

  Tg Tc Tm ΔHm ΔHC XC
BPCL4     33.2 45.8   33.7
BPCL6     43.9 55.1   40.5
BPCL7     44.6 52.2   38.4
BPCL8     47.4 58.4   42.9
PLA 62.8 102.00 168.30 45.95 10.20 38.44
PLA/BPCL4 61.4 93.00 167.90 36.86 3.65 35.71
PLA/BPCL6 60.2 91.80 168.00 40.55 3.82 39.50
PLA/BPCL7 59.2 95.40 168.10 41.28 3.42 40.90
PLA/BPCL8 57.6 98.50 167.90 40.64 2.46 41.05
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2.4. Thermogravimetric (TG) Analysis

As shown in Table 3 and Figure 4B, the temperature at which BPCLx began to lose weight (Ton) was between 180 and 250 °C. As the chain length of CL increased, Ton also increased. The shorter the chain was, the closer it was to a spherical structure. However, the branched structure reduced the thermal stability of PCL. After blending with PLA, the initial weightlessness temperature of the blend material was between 290 and 300 °C, which was similar to or even higher than that of pure PLA, indicating that hydrogen bonding occurred between BPCLx and PLA. The thermal weight loss curve of the blended material in Figure 4A shows two thermal weightlessness platforms. According to Figure 4B, the thermal decomposition of BPCLx mainly occurred in the range of 290–340 °C,46 and the second thermal weight loss stage (340–400 °C) was related to the thermal weight loss of PLA. Meanwhile, the thermal stability of the blended material increased with the increase in the chain length.

Table 3. TG Data of BPCLx, PLA, and PLA/BPCLx Blends.

  Ton T5% T10% T50% weight %
BPCL4 182.75 300.25 312.75 352.75 3.24
BPCL6 195.91 298.41 310.91 345.91 2.43
BPCL7 205.69 300.69 313.19 345.69 1.51
BPCL8 251.73 301.73 311.73 341.73 3.16
PLA 294.02 331.52 341.52 361.52 5.04
PLA/BPCL4 291.43 301.43 304.93 321.21 2.01
PLA/BPCL6 296.38 300.73 305.85 324.56 3.45
PLA/BPCL7 288.88 305.85 312.64 326.33 4.56
PLA/BPCL8 299.09 309.09 314.42 329.68 2.33
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Figure 4.

Figure 4

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(A) Thermogravimetric (TG) curves of PLA and PLA/BPCLx blends. (B) TG curves of BPCLx.

2.5. Scanning Electron Microscopy Analysis

To further investigate the effect of BPCL with different chain lengths on PLA, scanning electron microscopy (SEM) tests were performed on the impact sections. Figure 5 shows that the impact fracture surface of pure PLA was flat, without any heterogeneity or plastic deformation, and it showed significant brittleness. Compared with pure PLA, the surface of the PLA/BPCLx blends exhibits plenty of cavities resulting from debonding between the PLA matrix and dispersed BPCLx particles. The rough surfaces with rich shear zones and significant matrix deformation are characteristics of large-scale plastic deformation through shear yielding, indicating a gradual transition from brittle impact fracture to ductile impact fracture.47 The formation of a rough surface can absorb a large amount of impact energy and thus result in high impact strength. Multiple silver streaks and shear yield are two main mechanisms for improving the impact toughness with rubber-modified polymers. BPCLx was dispersed as a stress concentration point in the PLA matrix, which triggered the shear yield of the PLA matrix. This process absorbed a large amount of energy, which in turn hindered crack propagation and effectively improved the toughness of PLA. However, when the chain length of CL was over 8, the particles became larger and the toughness decreased, proving that agglomeration occurred.

Figure 5.

Figure 5

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SEM micrographs of the impact fractured surface of PLA and PLA/BPCLx blends.

2.6. Toughening Mechanism

Rubber-toughened modified polymers mainly follow the cavitation mechanism of rubber particles and multiple silver-grain and shear band theory.48 PLA is a semi-crystalline glassy polymer that undergoes brittle deformation due to cracks. The crack volume fraction (Vf) plays a crucial role in determining the tendency of cracks to fail catastrophically.49,50 The real stress σt acting on the cracked fiber is inversely proportional to the volume fraction Vf of the crack, which is given as follows

2.6.

where σ∞ is the applied stress, and λ is the crack propagation ratio. As the volume fraction of the cracks increases, the true stress applied to a single crack will be reduced. As shown in Figure 6, the phenomenon of stress whitening appeared in the tensile spline. BPCLx particles acted as a stress concentration agent, causing multiple cracks in the material, and the crack volume increased greatly. The increase in the crack volume reduced the actual stress on each crack, which helped the steady growth of voids. Generally, blends that broke in a brittle manner only showed stress whitening at the notches, while stress whitening was almost invisible in the crack propagation area. However, the specimens that fractured in a ductile manner often showed a stress whitening zone that was 1 to 2 mm thick and penetrated the entire fracture surface.51 From Figure 5, we can find that there are plenty of cavities on the surface. According to the toughing mechanism, to prevent the localization of strain, cavities formed in the matrix alter the triaxial stress state and favors the formation of shear bands ultimately leading to shear yielding of matrix.52

Figure 6.

Figure 6

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Tensile morphology of PLA and PLA/BPCLx blends.

3. Conclusions

BPCL with different chain lengths of CL was successfully synthesized and melt-blended with PLA at a mass ratio of 10%. When the chain length was short, BPCLx had a high fluidity, and as a plasticizer, the elongation at break was significantly improved. When the chain length of CL increased, because of the increase in the entanglement and weakening of the hydrogen bonding, the toughness increased first and then decreased. When mixed with BPCL6, the impact strength and the elongation at break of the blended material increased by 151.72 and 465.8% compared to pure PLA, respectively, with a slight decrease in the tensile strength. The DSC results showed that the addition of BPCLx promoted the rearrangement of PLA chains, which reduced the glass transition and crystallization temperatures (Tc) and increased the crystallinity. The thermogravimetric results showed that the initial decomposition temperature of the mixture was higher than that of BPCLx, proving that hydrogen bonding occurred between BPCLx and PLA. The effect of branched PCL with different chain lengths on PLA was examined in this work, providing a possible method for designing PLA/BPCLx materials.

4. Materials and Methods

4.1. Materials

PLA was obtained from Nature Works (product 4032D); this polymer is characterized by a 2% d-lactic acid content with an overall density of 1.24 g/cm3; its number average molecular weight and weight average molecular weight are 1.11 × 105 and 1.71 × 105 g/mol, respectively. BPCLx (x = 4, 6, 7, 8) was obtained by the combination method of this project as shown in Scheme 1,28 and the molecular weights are shown in Table 4. The 1H NMR spectra of HE and BPCLx are shown in Figures S1 and S2, respectively. x and the degree of branching was determined according to the method used in our previous research.28

Scheme 1. Synthetic Procedure of BPCLx.

Scheme 1

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Table 4. Molecular Parameters of BPCLx.

  Mn Mw PDI DB
HE 546 583 1.07 0.47
BPCL4 7089 9269 1.31  
BPCL6 7814 17247 2.21  
BPCL7 9062 11243 1.24  
BPCL8 10730 23325 2.17  
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4.2. Sample Preparation

Before blending, PLA and BPCLx (x = 4, 6, 7, 8) were dried for 12 h in a vacuum drying oven at 80 and 35 °C, respectively. Next, 10 wt % BPCLx and PLA were fused and blended in a HAKKE rheometer (HAKKE 90, Germany). The blended materials were obtained by blending at 180 °C and a rotor speed of 60 rpm for 6 min, after which they were cut with scissors and cooled to room temperature. Injection molding is beneficial for improving the ductility of blended material.31 The obtained pellets were injection molded into the required standard splines in a micro-injection molding machine. The injection pressure was 5 MPa, the furnace temperature was 180 °C, and the mold temperature was 30 °C. The blends of PLA and BPCL with different chain lengths are denoted as PLA/BPCLx (x = 4, 6, 7, 8). For the samples to have the same thermal history, PLA must be prepared under the same blending conditions.

4.3. Characterization of BPCLx and PLA/BPCLx Blends

1H NMR spectra (400 MHz) of HE and BPCLx were recorded with a Bruker AVANCE spectrometer in chloroform-d and dimethyl sulfoxide-d. The molecular weight was tested in THF by a Waters 2695 Alliance gel permeation chromatograph equipped with a PL-Gel column (Waters AQ3, Massachusetts, USA) based on polystyrene standards at 40 °C.

The notched Izod impact strength was measured following the ASTM-D256 standard. The spline size was 80 × 10 × 4 mm, the notch depth was 2.54 mm, and the radius was 0.25 mm. The impact strength was tested using an impact meter. The test temperature was 23 °C, and the pendulum power was 1 J. The impact strength of the sample was recorded. Each chain length sample was tested five times, and the final average value was taken.

The mechanical tests of the blends were performed on a universal material testing machine. Based on the ISO527-1 standard, dumbbell-shaped samples were selected, a loading rate of 10 mm/min was employed, and the test was conducted at room temperature. The tensile strength, elongation at break, and elastic modulus of the sample were recorded; samples with each chain length were tested five times, and the final average value was taken.

Under a nitrogen atmosphere, the sample was first heated from room temperature to 190 °C and held at this temperature for 3 min. It was then cooled to 0 °C and held at this temperature for 5 min. Finally, the sample was heated again to 190 °C. The heating and cooling rates were all 10 °C/min. To minimize the thermal history, the Tg, cold Tc, and melting temperature (Tm) were obtained from the secondary heating curve, and the crystallinity XC was calculated by the following formula

4.3.

where ΔHC is the crystallization enthalpy, ΔHm is the melting enthalpy, ΔHm0 is the melting enthalpy of PLA or BPCLx at 100% crystallization, which is generally 93.6 J/g for PLA and 136 J/g for BPCLx, and Φf is the mass fraction of PLA or BPCLx in the blend.

Thermal stability of the polymer blends was determined by using the thermal gravimetric analyzer (STA449F3 MaiaJupiter (NETZCH, Germany), The sample amounts were in the range of 8–10 mg, the heating procedure was: 20 °C/min heating from room temperature to 700 °C in N2 atmosphere, the measurement range of pure BPCL is 25–600 °C, and the measurement temperature range of blend with PLA is 25–800 °C.

The morphology of the impact section of the sample was observed using SEM. The sample adhered to the sample table and was sprayed with gold. The morphology of the impact section was observed to compare the toughening effect of BPCL with different chain lengths..

Acknowledgments

We gratefully acknowledge financial support from Science and Technology Major Project of Xinjiang Bingtuan (2019AA003), Shihezi University “Double First Class” Science and Technology Cooperation Project (SHYL-GH201901), Yangtze River Scholars and Innovation Team Development Plan of Ministry of Education (IRT_15R46), Achievement Transformation and Technology Extension Project of Shihezi University (CGZH201710), Outstanding young scientific and technological personnel training plan of Shihezi University(2013ZRKXYD02).

Glossary

NOMENCLATURE

PLA

poly(lactic acid)

HE

hyperbranched polyester

DB

degree of branching

BPCL

branched polycaprolactone

NMR

nuclear magnetic resonance

DSC

differential scanning calorimetry

TG

thermogravimetric analysis

SEM

field emission scanning electron microscopy test

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04070.

  • 1HNMR spectra of HE and BPCLx, DSC curves of BPCLx, and the impact strength of PLA and PLA/BPCLx blends (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c04070_si_001.pdf (274.6KB, pdf)

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