Largely improved toughness of poly(lactic acid) by unique electrospun fiber network structure of thermoplastic polyurethane

Accepted Manuscript Largely improved toughness of poly(lactic acid) by unique electrospun fiber network structure of thermoplastic polyurethane Haibin Sun, Jing Hu, Xue Bai, Zhipeng Zheng, Zhanbin Feng, Nanying Ning, Liqun Zhang, Ming Tian PII:

S0142-9418(17)31265-5

DOI:

10.1016/j.polymertesting.2017.10.012

Reference:

POTE 5210

To appear in:

Polymer Testing

Received Date: 1 September 2017 Accepted Date: 9 October 2017

Please cite this article as: H. Sun, J. Hu, X. Bai, Z. Zheng, Z. Feng, N. Ning, L. Zhang, M. Tian, Largely improved toughness of poly(lactic acid) by unique electrospun fiber network structure of thermoplastic polyurethane, Polymer Testing (2017), doi: 10.1016/j.polymertesting.2017.10.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Material Properties

Largely Improved Toughness of Poly(lactic acid) by Unique Electrospun Fiber Network Structure of Thermoplastic Polyurethane Haibin Sun1, Jing Hu1, Xue Bai1, Zhipeng Zheng1, Zhanbin Feng1,

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Nanying Ning1,2,3,*, Liqun Zhang1,2,3, Ming Tian1,2,3,*

1. State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

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2. Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China

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3. Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 10029, China. Abstract Oriented thermoplastic polyurethane (TPU) fiber and fiber network were first prepared by electrospinning. The as-prepared TPU fiber or fiber network was then pre-fixed in poly(lactic acid) (PLA)/TPU composite to improve the toughness of PLA. For comparison purpose, TPU/PLA

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composites with sea-island morphology were also prepared by traditional solution blending and mechanical blending. The results show that the toughness of PLA is greatly increased by the special pre-fixed oriented TPU fibers even at a low content, and the toughness is further increased by the TPU

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fiber network. Our results indicate for the first time that the toughening effect of special TPU fibers or fiber network is much better than that of traditional TPU with sea-island morphology. This study

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provides guidance to largely improve the toughness of PLA by designing the special phase morphology of TPU.

Key words: Mechanical properties; Poly(lactic acid); Phase morphology; Toughening; Electrospinning

ACCEPTED MANUSCRIPT 1 INTRODUCTION The increasing concern about plastic pollution and the dwindling oil resources have greatly expedited the promotion of biodegradable materials [1, 2]. Because of its good biocompatibility, biodegradability and high mechanical strength, as well as excellent processability [3, 4], PLA exhibits

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great potential for application in many fields such as biomedical devices, packaging and the automotive industry [4-6], and thus has attracted much attention in recent years. However, the inherent brittleness of PLA significantly restricts its wider application [7, 8].

Over the past decade, extensive efforts have been made aiming at improving the toughness of PLA,

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such as copolymerization with other monomers, addition of plasticizers, blending with flexible polymers or rubbers [9-11]. Among these strategies, blending PLA with rubber is often recognized as

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the most common and effective strategy [10, 11]. Typically, three toughening mechanisms have been proposed for rubber toughened PLA blends, shear yielding, multiple crazing and their combination [12-14]. The general steps for toughening starts with stress concentration, which usually leads to cavitation of the blends, cavitation then initiates shear yielding or crazing, which leads to large plastic deformation of matrix to dissipate a large amount of fracture energy. Shear yielding is the main source

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for energy dissipation, and is regarded as the key to toughen polymer blends. Both shear yielding and crazing can occur during deformation, and which is dominant is determined by the intrinsic properties of the brittle polymer [15, 16].

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The toughening effect of rubber on PLA can be influenced by interfacial adhesion, particle size, ligament thickness, volume fraction, physical and chemical characteristics of the rubber, which have

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been widely studied [17, 18]. However, the influence of the phase morphology of the rubber on the toughness of PLA still lacks in-depth understanding. TPU is a good candidate for the toughening of PLA [19]. Many studies regarding TPU toughening PLA are focused on the miscibility or interfacial interaction [19-21]. Usually, high content of TPU (higher than 30 wt%) is needed to effectively improve the toughness of PLA [22, 23], which is against energy saving and cost-reduction. Electrospinning is a simple and effective method for fabricating nano-fibers [24] [25]. In this study, oriented TPU fiber and TPU fiber network were first prepared by electrospinning. TPU/PLA composite with oriented fiber and fiber network were then prepared by solution casting. For comparison purpose, TPU/PLA composite with sea-island structure was also prepared by traditional solution blending and mechanical blending. The influence of different phase morphology of TPU on the toughness of PLA

ACCEPTED MANUSCRIPT was studied. The aim is to greatly improve the toughness of PLA by using for the first time a special TPU microstructure. An additional aim is to get in-depth understanding of the effect of phase morphology of TPU on the toughness of PLA. 2 EXPERIMENTAL

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2.1 Materials PLA (PLA-4032D) was purchased from Nature Works LLC., USA. TPU (TPU-1185A) was supplied by BASF Co. Ltd., Beijing, China. Solvents are all analytical grade. Tetrahydrofuran (THF), N,N-Dimethylformamide (DMF), and trichloromethane (CHCl3) were provided by BUCT Chemical Co.

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Ltd., Beijing, China. 2.2 Preparation and characterization

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Oriented TPU fibers and fiber network

TPU was first dissolved in a mixed solvent (THF: DMF=7:3 by volume ratio). The solution was then fed into a syringe. A rotating cylinder with a linear rate of 700 rpm was used for collecting fibers. After collecting the samples for 24 hours, an oriented TPU membrane with the thickness of 0.1mm was achieved. Network structure was achieved by changing the direction of the aluminum surface vertically

PLA/TPU composites

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every 20mins during electrospinning.

PLA/TPU composites with oriented TPU fiber or networks were prepared as follows. PLA was

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dissolved in CHCl3 with different concentration. Then, the electrospun TPU membrane was fixed in a PTFE box and 5 ml of PLA/CHCl3 solution was added. After evaporation of CHCl3, the membrane was

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turned over and another 5 ml of the PLA/CHCl3 solution was poured. Finally, the composite membrane was dried under vacuum at 40

until reaching constant mass. The as-prepared composites were

designated as electrospun oriented fiber composite (EOF) and electrospun fiber network composite (EFN).

In addition, PLA/TPU composites with common spherical morphology were prepared by mechanical blending and solution blending according to previous study [22], and the as-prepared composites were designated as MB and SB, respectively. The morphology of composites was observed by using a S-4800 scanning electron microscopy (SEM) at 5 kV. The samples were first coated with platinum before SEM observations. Tensile tests were carried out using a tensile-testing machine (Instron1185, Instron Corporation,

ACCEPTED MANUSCRIPT American), at a crosshead speed of 10 mm/min, at room temperature according to the JIS K7113 test method. At least five specimens were tested for each sample. Tensile toughness, the area below the strain-stress curves, was calculated to better represent material toughness. 3 RESULTS AND DISSCUSSION

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3.1 Phase morphology of TPU in PLA/TPU composites The phase morphologies of PLA/TPU composites are shown in Fig. 1. The SEM images were taken after PLA in the composites was etched by CHCl3. As expected, a sea-island structure is formed in both MB and SB composites, where the TPU spheres with a diameter of about 3 µm are dispersed in PLA

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matrix (see Fig. 1(a) and (b)). TPU fibers with a diameter of 800 nm were prepared by electrospinning (see Fig. 1(c)). These fibers exhibit smooth surface and are highly orientated. Oriented fiber structure

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and fiber network structure are observed in EOF composites and EFN composites, respectively (see Fig. 1(d) and (e)). Obviously, these TPU fibers and fiber network were not damaged during preparation. 3.2 Mechanical properties

The stress-strain curves of PLA/TPU composites are shown in Fig. 2 and the corresponding mechanical properties are shown in Table 1. Pure PLA shows quite low elongation at break (ε) and

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tensile toughness, indicating a typical brittle fracture. All the PLA/TPU composites show increased ε and tensile toughness, indicating the improvement of toughness of PLA by adding TPU. Both ε and tensile toughness are slightly improved by adding 20 wt% or 40 wt% of TPU in MB composite,

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indicating limited toughening effect. The toughening effect of TPU on PLA in SB is obviously better than that in MB. However, the toughening effect of TPU with sea-island morphology on PLA is still

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limited in SB, especially at low TPU content. Interestingly, both ε and tensile toughness are largely increased by adding TPU fibers in EOF composite, and the ε and tensile toughness are further increased by adding TPU fiber network in EFN, indicating greatly improved toughening effect of TPU fibers or fiber network on PLA. For example, the ε and tensile toughness of EFN composites with 20% TPU increases to 230% and 34.9 MPa, which are 250-fold and 125-fold increase over that of pure PLA, individually. Obviously, the toughening effect of TPU fiber network on PLA is quite good even at low TPU content (20 wt%). In addition, all the PLA/TPU composites show decreased tensile strength and modulus. 3.3 Toughening mechanism Fig. 3 shows the SEM images of tensile fractured surfaces of pure PLA and TPU/PLA composites.

ACCEPTED MANUSCRIPT The tensile fracture surface of pure PLA is very smooth (see Fig. 3(a)), indicating a typical brittle fracture behavior. Significant deformation of PLA matrix is observed in all the TPU/PLA composites. Some TPU spheres are deformed in MB composite (see Fig. 3(b)), which can only adsorb some energy during deformation, leading to the limited improvement of toughness. Many TPU spheres are detached

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from the PLA matrix in SB composites (see Fig. 3(c)), indicating a cavitation phenomenon, which is the typical trigger of shear yielding or crazing for rubber toughed materials. The cavitation and plastic deformation induce energy dissipation and, therefore, leads to the improvement in tensile toughness [26, 27]. Thus, the toughening effect of TPU on PLA in SB is better than that in MB. In addition, ligament

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thickness also plays an important role in the final toughness for the composites with sea-island morphology. At low TPU content, the toughness of both MB and SB composites are poor when the

effectively improve the toughness of PLA.

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average ligament thickness is above the critical value. Thus, a high content of TPU is required to

Unlike SB or MB composites, EOF and EFN composites were designed and prepared with a pre-fixed oriented long fiber or fiber network structure. Broken fibers remain oriented perpendicular to fracture surface in EOF composites. The interface between TPU fibers and PLA matrix is blurred,

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indicating good adhesion. This is ascribed to the large aspect ratio of TPU fiber which increases the contact area between PLA and TPU. Thus, stress can be easily transferred from PLA to TPU. More importantly, the pre-fixed TPU long fiber structure is beneficial for the load transfer and thus inducing

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debonding cavitation at the PLA/TPU interfaces during stretching, leading to extensive energy dissipation, and thus largely improved toughness, even at low TPU content. For EFN composites, the

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network can work as a whole when stress is applied, because fibers perpendicular to the fracture surface are connected by fibers parallel to fracture surface. Thus, the stress can be distributed among fiber network, leading to the better toughening effect.

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0%

Preparation methods

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MB

SB

EOF

EFN

MB

SB

EOF

EFN

Tensile strength (MPa)

46.0 ± 0.7

26.0 ±0.2

26.5 ± 0.5

17.7 ± 0.4

18.3 ± 0.3

17.7 ± 0.2

14.3 ± 0.1

16.3 ± 0.2

16.4 ± 0.2

Modulus (MPa)

21800 ± 1120

905 ± 63

752 ± 85

221 ± 27

Elongation at break (%)

0.8 ± 0.6

6.3 ± 1.6

20 ± 1.7

200 ± 8.5

Tensile toughness (Jm-3)

0.25 ± 0.09

1.1 ± 0.3

5.3 ± 0.8

31.3 ± 4.5

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TPU content

40 wt%

226 ± 14

324 ± 42

311 ± 43

260 ± 18

124 ± 9

230 ± 7.1

10.7 ± 3.2

120 ± 4.1

230 ± 8.6

270 ± 9.3

1.3 ± 0.2

16.2 ± 2.5

32.6 ± 4.4

33.6 ± 2.1

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20 wt%

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Table 1 Mechanical properties of TPU/PLA composites prepared by different methods

34.9 ± 4.3

ACCEPTED MANUSCRIPT 4 CONCLUSIONS PLA/TPU composites with different phase morphology were prepared to improve the toughness of PLA. The results show that TPU/PLA composites prepared by electrospinning with either oriented fiber or fiber network structure exhibit significantly improved tensile toughness even at low TPU content (20

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wt%). Our results indicate for the first time that the toughening effect of special TPU fibers or fiber network is much better than that of traditional TPU with sea-island morphology at the same TPU content. This study provides guidance to largely improve the toughness of PLA by designing the special phase morphology of TPU.

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Acknowledgement:

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We gratefully acknowledge the National Natural Science Foundation of China (Grant No. 51525301, 51521062) for financial supports. Reference

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Fig. 1 SEM images of PLA/TPU composites with 20 wt% TPU prepared by different methods after etching for 10min. (a) mechanical blending (MB); (b) solution blending(SB); (c) electrospun TPU

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fibers; (d) electrospun oriented fibers (EOF); (e) electrospun fiber network(EFN)

Fig. 2 Stress-Strain curves of various TPU/PLA composites

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Fig. 3 SEM images of tensile fractured surfaces of pure PLA and TPU/PLA composites with 20 wt%

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TPU prepared by different methods (a) pure PLA; (b) mechanical blending; (c) solution blending; (d)

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electrospun oriented fiber; (e) electrospun fiber network

ACCEPTED MANUSCRIPT 1 TPU/PLA composites with different phase morphology were prepared. 2. The toughness of Poly (lactic acid) was significantly improved by controlling the morphology of TPU. 3 The toughening mechanism of TPU/PLA composites with different phase morphology of TPU was studied. 4 TPU/PLA composites exhibit significantly improved tensile toughness at low TPU content (20 wt%). 5 Our results indicate for the first time that the toughening effect of special TPU fibers or fiber network

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is much better than that of traditional TPU with sea-island morphology at the same TPU content.