Strong and thermal-resistance glass fiber-reinforced polylactic acid (PLA) composites enabled by heat treatment

Accepted Manuscript Strong and thermal-resistance glass fiber-reinforced polylactic acid (PLA) composites enabled by heat treatment

Guilong Wang, Dongmei Zhang, Bo Li, Gengping Wan, Guoqun Zhao, Aimin Zhang PII: DOI: Reference:

S0141-8130(19)30443-X https://doi.org/10.1016/j.ijbiomac.2019.02.020 BIOMAC 11672

To appear in:

International Journal of Biological Macromolecules

Received date: Revised date: Accepted date:

18 January 2019 3 February 2019 3 February 2019

Please cite this article as: G. Wang, D. Zhang, B. Li, et al., Strong and thermal-resistance glass fiber-reinforced polylactic acid (PLA) composites enabled by heat treatment, International Journal of Biological Macromolecules, https://doi.org/10.1016/ j.ijbiomac.2019.02.020

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ACCEPTED MANUSCRIPT Strong and thermal-resistance glass fiber-reinforced polylactic acid (PLA) composites enabled by heat treatment

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Guilong Wanga,*, Dongmei Zhanga, Bo Lia, Gengping Wanb, Guoqun Zhaoa,**, Aimin

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials

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a

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Zhanga,***

(Ministry of Education), Shandong University, Jinan, Shandong 250061, China Key Laboratory of Chinese Education Ministry for Tropical Biological Resources, Hainan

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b

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University, Haikou, Hainan 570228, China

* Corresponding authors. E-mail addresses: [email protected] (G. Wang), [email protected] (G. Zhao), and [email protected] (A. Zhang).

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ACCEPTED MANUSCRIPT 1. Introduction Polylactic acid (PLA) is a biopolymer derived from renewable resources such as starch and sugar [1,2]. Due to its excellent performances in biocompatibility and

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biodegradability, PLA has been already widely used in biomedical fields such as implant

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devices, tissue scaffolds, and internal sutures [3,4]. Moreover, being an aliphatic

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thermoplastic polyester, PLA exhibits a high modulus, high strength, and good clarity, as

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well as outstanding processing ability. In the past decades, with increasing in concerns about the pollution problems resulted from petroleum-based polymers [5–7] and also the

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continuously decreasing in production cost, PLA has gradually shown broad prospects in

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replacing the traditional polymers such as polypropylene (PP), polyethylene, and

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polystyrene (PS), due to its excellent performances in renewability and ideal carbon cycle excluding petroleum resources [9–10]. However, PLA shows some obvious drawbacks

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including its brittleness and poor thermal-resistance, which significantly limits its further

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wide usage in many fields. Thus, to enhance PLA’s mechanical performance and thermal

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resistance have always been a hot topic in both industrial and academic areas, and many efforts have been devoted in the past decades [11]. The poor crystallization ability of PLA is the root cause of its poor mechanical performance in toughness and thermal resistance. Thus, most of the efforts on enhancing PLA are focused on enhancing the crystallization PLA. Introducing nucleating agents has been demonstrated to be an effective method for promoting crystallization of semi-crystalline polymers. Among available nucleating agents, fibers shows some obvious 2

ACCEPTED MANUSCRIPT superiorities because it can not only effectively promote crystallization but also significantly enhance polymer matrix. The fibers used for reinforcing PLA can be divided into two major categories: nature fibers and synthetic fibers. In recent years, the PLA

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composite reinforced with various nature fibers has been studied a lot because of its

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obvious advantages in biodegradability and sustainability [12–14]. The commonly used

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natural fibers include ramie, flax, hemp, jute, kenaf, and abaca. A major drawback of

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nature fiber is its relatively poor compatibility performance with PLA matrix, which leads to the unsatisfactory mechanical performance of composite. As a result, surface modification

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is usually required to improve fiber dispersion and interfacial bonding strength between

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fiber and PLA matrix [15–17]. Moreover, nature fiber generally shows strong hydrophilicity,

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which results in the relatively poor durability of composite [18,19]. In addition, the reinforcing effect of natural fibers is usually not very satisfactory, and the natural fiber

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reinforced composite even shows poorer mechanical performance than neat PLA,

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particularly in terms of toughness and ductility [20–22].

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Compared with natural fiber, synthetic fiber shows much stronger mechanical properties. As a result, the synthetic fiber reinforced PLA composite typically shows superior mechanical properties than the natural fiber reinforced one [23–25]. Among synthetic fibers, glass fiber is the most popular one used for reinforcing polymers due to its outstanding mechanical properties, good heat resistance, and low cost [26–30]. Moreover, the glass fibers made from silicate glass, phosphate-based glass or borate-based glass have good biological activity, which can be used for produce composites that are fully 3

ACCEPTED MANUSCRIPT degradable [31]. By now, there have been a large number of studies on using glass fibers to reinforce polymers. It has been reported that the major factors affecting the mechanical properties of the glass fiber reinforced composite include the content of fiber, the aspect

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ratio of fiber, and the distribution of fiber, as well as the interaction between fiber and

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matrix [24,32–34]. Due to the poor wettability of glass fiber, the interfacial bonding

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strength between fiber and matrix is usually rather unsatisfied. Thus, surface modification

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of glass fiber is required to achieve good bonding between fiber and polymer matrix. It is worth noting that most of the past research on glass fiber reinforced composite are

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focused on the traditional petroleum-based polymers, such as PP, polyamide (PA) and

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polyethylene terephthalate (PET), however very few attentions have been paid on glass

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fiber reinforced PLA composite [24,32]. In a small amount of research with PLA as matrix, the reinforcement effects of glass fiber and natural fiber were compared, and it was found

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that glass fiber reinforced PLA composite exhibited much better mechanical properties in

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strength, rigidity and toughness than cellulose fiber reinforced PLA composite [35,36]. In

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another study, the surface glass fiber was modified with a layer of graphene to improve reinforcing effect, and it was found that the tensile strength can be significantly improved not only due to the improved interfacial bonding strength but also due to the improved interfacial crystallization [37]. Recently, Varsavas and Kaynak reported that glass fiber reinforcement led to significant increases in strength and elastic modulus values of PLA, and the optimum glass fiber content was 15 wt% [38]. So far, the effect of GF on the crystallization behavior and mechanical properties of PLA is still unclear, and hence a 4

ACCEPTED MANUSCRIPT systematic study is required to clarify the mechanism. Heat treatment is an effective method for improving the mechanical properties of semi-crystalline polymer, particularly for those with poor crystallization ability, like PLA.

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Srithep et al. investigated the effects of heat treatment on the mechanical properties of

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neat PLA [39]. It was reported that an increase of over 17% and 26% in tensile strength

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was achieved at an annealing temperature of 80 C for 30 min and 65 C for 31 h,

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respectively. Chen et al. studied the effects of heat treatment on the thermal and mechanical properties of ramie fabric-reinforced PLA composite [40]. It was found that

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heat treatment at 115 C for 1 h led to significant increase in both strength and heat

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resistance of the neat PLA and its composites. Zhou et al. investigated the temperature

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dependence of PLA mechanical properties, and the results demonstrated that a higher annealing temperature led to increased tensile strength and modulus [41]. Lv et al.

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investigated the effect of annealing on the thermal properties of PLA/starch blends. It was

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demonstrated that annealing was beneficial to increase crystallinity, however it shows little

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effect on thermal stability [42]. Recently, Deng et al. reported that thermal annealing above the glass transition temperature led to super-toughened and high-thermal resistance PLA binary blend, and it was argued that the toughening mechanism was the formation of negative pressure on the elastomeric particles dispersed within the PLA matrix during the quench process and thermal annealing thereafter [43]. To the best of our knowledge, there is rarely any public reports about the heat treatment on the crystallization and mechanical properties of the glass fiber reinforced PLA composite. 5

ACCEPTED MANUSCRIPT Toward developing high-strength and temperature-resistant biological materials, we took an experimental study to investigate the effect of heat treatment on the mechanical properties of PLA and PLA/GF composites in this study. Firstly, PLA/GF composites with

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different GF contents were prepared using a twin-screw compounding machine.

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Afterwards, injection molding experiments were conducted to produce the standard

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samples for testing. Thereafter, the standard PLA and PLA/GF samples were isothermally

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treated under different conditions. Finally, tensile, flexural, and impact tests were employed to evaluated the mechanical properties of the treated and untreated samples.

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To clarify the reinforcing mechanism, scanning electronic microscope (SEM) was used to

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observe the micromorphology, and both differential scanning calorimetry (DSC) and X-ray

composite.

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2. Experimental section

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diffraction (XRD) were conducted to analyze the crystallization of PLA and PLA/GF

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2.1. Materials and sample preparation

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An injection molding grade PLA, Ingeo™ biopolymer 3052D, was supplied by NatureWorks LLC, Minnetonka, MN, USA. The PLA has a density of 1.24 g/cm 3, a D-lactide content of 4%, and a melt flow index of 14 g/10 min (@210 C/2.16 kg). It has an average molecular weight Mw of 128.6 kg/mol and Mn of 84.3 kg/mol. Commercial glass fiber (GF), ECS303A, was purchased from Chongqing Polycomp International Corporation, Chongqing, China. The GF has an average diameter of about 13 μm and a pre-compounded length of about 4 mm. In order to improve the interaction between GF 6

ACCEPTED MANUSCRIPT and PLA, the surface of GF was modified with 3-aminopropyl methyl dimethoxy silane. The PLA in pellet form and the short chopped glass fibers with different doses (0 wt%, 5 wt%, 10 wt%, 15 wt%, and 20 wt%) were compounded using a twin-screw extruder with

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a screw diameter of 21 mm and length to diameter ratio of 36. In order to avoid the effect

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of any moisture, both PLA and glass fiber were dried at 80 C for 8 h by using a vacuum

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oven. In order to reduce the breakage of glass fiber resulted from the strong shear at the

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first several zones of barrel during compounding, glass fibers were fed into barrel through a side feeder at the zone 4 of barrel after the PLA was completely melted. The profile

from

the

hopper

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temperature

to

die

was

set

to

be

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160°C–185°C–190°C–190°C–190°C–185°C–165°C. The rotation speed of screw was set

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at 100 rpm, and the feed rate was maintained at 3.5 kg/h. Extrudate in strand shape was first cooled in a water-cooling bath and then chopped using a pelletizer that has a

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diameter of about 2.5 mm and a length of 4–5 mm.

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After compounding, the extruded materials were dried once again at 80 C for 8 h to

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remove any moisture. Thereafter, a 68 tons injection molding machine, XL-650, was employed to mold the dried materials into the standard specimens for multipurpose testing. The specimen for tensile test has dimensions of 150 mm × 12.7 mm × 3.2 mm. The specimen for flexural test possesses dimensions of 130 mm × 12.7 mm × 3.2 mm. The dimensions of the notched impact test specimen are 63.5 mm × 12.7 mm × 3.2 mm. In injection molding, the major processing parameters including melt temperature, mold temperature, injection speed, packing pressure and packing time were set to be 190 C, 7

ACCEPTED MANUSCRIPT 45 C, 20 cm3/s, 5.0 MPa, and 3.0 s, respectively. The injection molded specimens without any obvious defects were used for further characterization and analysis. Some of the injection molded specimens underwent additional isothermal heat

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treatment at 80 C and 100 C, respectively, for varying lengths of time. Prior to isothermal

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heat treatment, the specimens were placed at room temperature (23 ± 2 C) for more than

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two weeks to stabilize their structure and property. Thereafter, the specimens were

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transferred to a hot oven under the protection of nitrogen environment at a set temperature for a period of time. To evaluate the temperature uniformity within the

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specimen during heat treatment, the Biot number (Bi) was calculated with the following

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equation:

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(1)

where h is the heat convection coefficient at the surface of the specimen, L is the

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thickness of the specimen, and k the thermal conductivity of the specimen. During heat

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treatment in the oven, the heat transfer at the surface of specimen can be taken as air free

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convection, and hence h has a typical value of about 15 W/(m2·K) [44]. In addition, the PLA specimens have a thickness of 3.2 mm and a typical thermal conductivity of about 200 mW/(m·K) [45]. Thus, Bi has a value of 0.24, which is much less than 1, and it indicates that the temperature gradient within specimen during heat treatment is relatively small. Consequently, the crystallization within the specimen should be uniform during heat treatment.

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ACCEPTED MANUSCRIPT 2.2. SEM analysis A scanning electronic microscope (SEM), JSM-7800F, JEOL, Japan, was employed to investigate the morphology of the PLA/GF composite and the impact fracture surface of

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the injection molded sample. To prepare the specimen for observing the morphology of the

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PLA/GF composite, the prepared PLA/GF composite pellets were firstly molded into a

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rectangular plate by hot compression and then cryofractured in liquid nitrogen. Prior to

automatic sputter coater.

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2.3. Differential scanning calorimetric analysis

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SEM observation, the fracture surfaces were coated with a thin layer of gold using an

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A Q2000 differential scanning calorimeter (DSC), TA instruments, USA, was used to

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analyze the crystallization of the heat-treated and untreated injection-molded PLA specimens. For a typical DSC test, the sample with a weight of about 10 mg was cut from

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PLA specimen, which was then sealed in aluminum pans. Afterwards, DSC scan was

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performed at a temperature range from 25 C to 200 C, and run at a heating rate of 10 C/min under the protection of nitrogen environment. For some samples, a

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heating–cooling–heating cycle scan in a temperature range of 25–200 C was conducted in order to investigate the effect of glass fiber on the crystallization of PLA. Both the heating and cooling rates were set to be 10 C/min, and an isothermal treatment at 200 C for 10 min was applied right after the first heating to erase the thermal history of sample. The DSC data was analyzed using TA Instruments’ Universal Analysis program. The glass transition temperature (Tg) was taken as the midpoint of the glass transition temperature 9

ACCEPTED MANUSCRIPT range. The cold crystallization temperature (Tc) and melting temperature (Tm) were taken as the peak temperatures of cold crystallization and melting, respectively. Based on the thermograms obtained from the first heating scan, the sample’s degree of crystallinity (Xc)

(2)

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was calculated as follows:

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where Hm is the melting enthalpy, Hc is the cold crystallization enthalpy, Hm0 is the

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melting enthalpy of the fully crystalline PLA, taken as 93J/g [46,47], and w is the weight fraction of PLA in the composite.

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To evaluate the effect of glass fiber on the crystallization behavior of PLA, the

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crystallization kinetics of the neat PLA and PLA/GF composite in heating process were

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analyzed by using the Jeziorny-modified Avrami model [48]. According to the model, the relative degree of crystallinity as a function of a definite period of time is described as:

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(3)

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where Xt is the relative degree of crystallinity at the period of time t in the crystallization

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process, k is the crystallization kinetic constant for crystal nucleation and growth, and n is the Avrami exponent which is diagnostic of the crystallization mechanism. Taking the log of both sides of Eq. (3), the modelling equation can be rewritten as: (4) Based on Eq. (4), the Avrami exponent (n) and the crystallization kinetic constant (k) can be determined by plotting ln[-ln(1-Xt)] versus lnt. To quantitatively analyze the effect of glass fiber on crystallization, the crystallization half-time (t1/2), which is defined as the time 10

ACCEPTED MANUSCRIPT required to develop 50% of the final crystallinity (Xt = 1/2), was calculated as: (5) According to the Jeziorny-extended Avrami model, the crystallization kinetic constant

(6)

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(k) should be corrected using the heating rate as follows:

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where k is the corrected crystallization kinetic constant, and Φ is the heating rate.

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2.4. X-ray diffraction characterization

To further analyze the crystalline structure of sample, XRD was performed using a

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Bruker D8 Advance X-Ray Powder Diffraction system. In measurement, the generator was

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set up at 45 kV and 35 mA, using Cu-Kα radiation (λ = 1.542 Å) as the X-ray source,

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together with a Ni-filter to extract the Kα radiation. The scan was performed at room temperature (23 ± 2 C) over a range of scattering angles (2) of 5–30 with a scanning

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speed of 2.5/min. The data was collected with a 2 step of 0.02. Upon completion of the

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scan, data was analyzed using the Bruker Diffrac Suite Eva software package (Bruker,

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Billerica, Massachusetts). Based on the XRD profiles, the samples’ degree of crystallinity (Xc) can also be evaluated using the following equation [49]: (7)

where Ic and Ia are the integrated intensities scattered over a suitable angular interval by the crystalline and the amorphous phases, respectively, and K is a correction factor for the disorder in the crystalline phase.

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ACCEPTED MANUSCRIPT 2.5. Mechanical performance test According to ASTM D638, a computer controlled Instrons 3366 universal testing machine, equipped with a load cell of 10 KN, was used to evaluate the tensile properties of

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the heat-treated and untreated composites. The measurement was conducted at room

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temperature (23 ± 2 C) with a crosshead speed of 10 mm/min. According to the ASTM

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D256, notched Izod impact tests were further conducted to evaluate the samples’

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mechanical properties at a much higher deformation rate. The impact energy and impact speed used in measurement were 2.75 J and 3.50 m/s, respectively.

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Prior to any testing, all of the samples were placed in standard testing conditions to

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avoid the effects of environmental fluctuations on the testing results. For each case, at

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least five samples were evaluated, and the average would be used for analysis. 3. Results and discussions

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3.1. Composite morphology

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Fig. 1 shows the micro morphology of the PLA//GF composites without any heat

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treatment. It can be seen that there is not any obvious agglomeration of glass fibers on the fracture surface, which indicates a uniform distribution of glass fibers within the PLA matrix. The uniform distribution of GF is essential to ensure a good mechanical properties of the PLA/GF composite. In addition to the GF distribution, the interfacial bonding between GF and PLA matrix also plays a significant role in determining the composite mechanical properties. Thus, the organic silane coupling agent was applied improve the interaction between GF and PLA. Fig. 2 shows the comparison of the micromorphology of the 12

ACCEPTED MANUSCRIPT PLA/GF composites with and without coupling agents. For the composite without any coupling agent (Fig. 2a), the glass fiber shows a very smooth surface, which indicates a weak interaction between GF and PLA matrix. In the presence of coupling agent, it can be

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observed in Fig. 2b that the glass fiber is covered with a layer of material, which indicates

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a good interfacial bonding between GF and PLA matrix.

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Figure 1. SEM images of the micro morphology of PLA/GF composites without any heat

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treatment: (a) 5 wt%, (b) 10 wt%, (c) 15 wt%, and (d) 20 wt%.

Figure 2. The micro morphology of the PLA/GF composites (a) with and (b) without coupling agent.

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ACCEPTED MANUSCRIPT 3.2. Effect of glass fiber on the crystallization of PLA DSC analysis was performed to investigate the thermal behavior of PLA and PLA/GF composites. Fig. 3 shows the DSC thermograms of PLA and PLA/GF composites during

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the second heating stage. With increasing temperature from 50 C to 170 C, each DSC

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curve possesses three thermal characteristics: (a) a glass transition temperature (Tg) near

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60 C, (b) a cold crystallization peak (Tc), and (c) an endothermic melting peak (Tm). It is

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worth noting that the neat PLA has a single melting peak while the PLA/GF composites show two distinct melting peaks (Tm1 and Tm2). For the neat PLA, it only has one melting

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peak, and hence its Tm1 equals to Tm2. The values of the glass transition temperature, cold

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crystallization temperature, melting temperature, specific melting enthalpy, and the

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calculated degree of crystallinity are listed in Table 1. From Fig. 3 and Table 1, it can be found that all the samples have almost the same glass transition temperature of about 60

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C, indicating the presence of glass fiber has little effect on the chain mobility of PLA. This

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chain segment.

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is as expected because the size of glass fiber is too macro to affect the mobility of polymer

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Figure 3. DSC thermograms of PLA and PLA/GF composites in the second heating stage

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Regarding cold crystallization, it can be seen from Fig. 1 that neat PLA shows a wide cold crystallization peak, indicating its slow crystallization behavior. Obviously, the

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presence of glass fiber significantly narrows the cold crystallization peak. The cold

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crystallization peak becomes narrower with increasing in the fiber content. This clearly

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demonstrates the positive effect of glass fiber in promoting the crystallization of PLA.

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Moreover, the cold crystallization peak obviously shifts to low temperatures in presence of glass fiber, and this is another indication that glass fiber can greatly promote the

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crystallization of PLA due to its outstanding nucleating effect. From Table 1, it can be seen that Tc is dramatically decreased from 127.0 C of neat PLA to 114.0 C by introducing 5 wt% of GF. However, with further increasing in GF content, Tc just shows a slight drop. Table 1. DSC data of PLA and PLA/GF composites in the second heating stage Sample

Tg (C)

Tc (C)

Tm1 (C)

Tm2 (C)

Hm (J/g)

Xc (%)

PLA

61.15

126.99

154.48

154.48

19.88

21.24

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114.04

150.70

156.45

28.10

31.60

PLA/GF-10

60.64

111.96

149.30

156.04

27.07

32.13

PLA/GF-15

61.45

113.00

150.06

156.71

25.82

32.45

PLA/GF-20

60.29

110.70

149.85

156.89

25.07

33.48

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PLA/GF-5

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With further increasing in temperature, the crystals generated during cold crystallization will finally be melted. As a result, a melting peak appears in the DSC

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thermograms, as Fig. 1 shows. It can be seen that the neat PLA shows a single melting

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peak, and its melting temperature (Tm) is about 154.5 C. In contrast, PLA/GF composites show a double melting peak, and the higher the GF content is, the more distinct the double

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melting peak is. All PLA/GF composites have similar low and high melting peaks which are

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around 150.0 C and 156.5 C, respectively. Compared with the neat PLA, PLA/GF

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composites can crystallize at much lower temperatures, however, these crystals

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commonly show less perfect structure and are much thinner than the crystals generated at higher temperatures during cold crystallization. As a result, PLA/GF composites shows a

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melting peak at relatively low temperatures. However, these thinner crystals can melt and recrystallize to generate more perfect and thicker structure before the second melting peak [50]. With increasing in GF content, more thin crystals transfer into thick crystals during the heating process, which results in a more obvious high-temperature melting peak. Based on the melting peak, the specific melting enthalpy can be determined, which can be further used for calculating the degree of crystallinity using Eq. (2), and the results

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ACCEPTED MANUSCRIPT are listed in Table 1. It can be seen that the degree of crystallinity is significantly increased from 21.2% to 31.6% by blending 5 wt% glass fiber. However, with further increasing in GF content, the degree of crystallinity of PLA only shows a very gentle increasing trend.

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To further analyze the effect mechanism of GF on the crystallization behavior of PLA,

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the crystallization kinetics of PLA and PLA/GF composite during heating were analyzed.

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Fig. 4a shows the crystallized fraction percentage trends of PLA and PLA/GF composites

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over time. It can be seen that each curve shows a typical ‘S’ style, which can be divided into three sections. The first stage represents the crystallization induction period during

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which crystallization is very slow. The second stage is the rapid growing period of crystal,

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during which crystals grow up quickly. The last stage is the end of crystallization and

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hence it shows a slow crystallization increase. Clearly, the inclusion of GF accelerates crystallization and hence shortens the time period required to finish crystallization.

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Increasing glass fiber content leads to faster crystallization due to the outstanding effect of

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GF on promoting crystallization.

Figure 4. Crystallization kinetics analysis of PLA and PLA/GF composite: (a) relative

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ACCEPTED MANUSCRIPT crystallinity as a function of time, and (b) ln[-ln(1-Xt)] versus lnt.

Fig. 4b plots the Avrami double-log plots for the neat PLA and PLA/GF composites. Overall, all the materials exhibit a linear relation between lnt and ln[-ln(1-Xt)]. By

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regressively fitting these linear relationships using Jeziorny-modified Avrami method, the

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values of the Avrami parameters (n, k, and k ) and t1/2 derived from the Avrami plots for all the materials were evaluated, and the results were summarized in Table 2. Clearly, the

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presence of GF increases the value of the crystallization kinetic constant (k and k ). For

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instance, k increases from 0.540 to 0.725 and 0.925 after the addition of 5 and 20 wt% glass fibers, respectively. This is attributed to the strong heterogeneous nucleation effect

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of glass fibers [51,52]. Meanwhile, t1/2 decreased with an increasing GF content due to an

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increased crystal nucleating effect [48]. In addition, it is found in Table 2 that the values of

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n reduces gradually with increasing GF content. It indicates that the presence of GF

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should affect the dimensional growth of the crystal under non-isothermal conditions. Glass fibers might to certain extent limit the free growth of crystals [37].

Sample

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Table 2. DSC data of PLA and PLA/GF composites in the second heating stage k

k

t1/2 (s)

n

PLA

0.069

0.540

134.81

2.851

PLA/GF-5

0.247

0.725

87.08

2.772

PLA/GF-10

0.272

0.741

84.42

2.741

PLA/GF-15

0.361

0.791

77.57

2.540

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ACCEPTED MANUSCRIPT PLA/GF-20

0.711

0.925

59.33

2.271

3.3. Effect of heat treatment on the crystallization of PLA and PLA/GF composites PLA and PLA/GF composites were thermally treated at 80 C and 100 C,

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respectively, for different time periods. After heat treatment, all the materials were

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analyzed using DSC. Fig. 5 shows the DSC thermograms of the neat PLA and PLA/GF composites after thermally treated at 80 C. Table S1 in the Supporting Information gives

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the major thermal characteristics of the materials. In general, when the heat treatment

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period is relatively short, all the materials exhibit a cold crystallization peak which is

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followed by a melting peak.

Figure 5. DSC thermograms of (a) neat PLA, (b) PLA/GF-5, (c) PLA/GF-10, (d) PLA/GF-15, and (e) PLA/GF-20 after thermally treating at 80 C for different time periods.

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ACCEPTED MANUSCRIPT With increasing the time period of heat treatment, it is observed from Fig. 5 and Table S1 that the cold crystallization peak (Tc) tends to shift down to lower temperatures, and becomes narrower and weaker. When the heat treat time is long enough, the cold

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crystallization peak finally disappears. It illustrates that heat treatment can promote cold

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crystallization but reduce the amount of crystallization during heating in DSC test. The

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reason is that heat treatment can induce crystallization, and in turn the induced crystals

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can accelerate the cold crystallization in DSC test. Since crystallization has taken place during heat treatment, the amount of cold crystallization in the subsequent DSC test is

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reduced. When the heat treatment period is long enough to achieve sufficient

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crystallization, the cold crystallization in the DSC test will disappear. By comparing Fig.

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5a–e, it can be found that increasing GF content can shorten the time period required to achieve sufficient crystallization state during heat treatment due to the enhanced

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heterogeneous nucleating ability. For instance, the required heat treatment period for neat

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PLA to achieve sufficient crystallization or to make cold crystallization disappear is 90 min,

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however, it is shortened to 60 min and 30 min after the addition of 10 and 20 wt% glass fibers, respectively. Additionally, it can be observed in Fig. 6e that the DSC thermograms of PLA/GF-20 after a long heat treatment show a small crystallization peak prior to the major melting peak, as marked with the red arrow in Fig. 6f. It is attributed to the transformation of some unperfected crystals like α

crystal generated during heat

treatment into more perfect crystals like α crystal [53]. Based on the cold crystallization enthalpy and the melting enthalpy, the degree of 20

ACCEPTED MANUSCRIPT crystallinity of the neat PLA and PLA/GF composites after heat treatment can be calculated using Eq. 2, and the calculation results are given in both Fig. 6a and Table S1. Without heat treatment, the neat PLA and PLA/GF composites all have similar but very low

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degree of crystallinity, indicating the extremely poor crystallization ability of PLA matrix.

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With increasing in the heat treatment period, the degree of crystallinity increases gradually

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for all the materials, which indicates the positive effect of heat treatment on promoting

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crystallization. It is worth noting that the growth of the degree of crystallinity is much more pronounced with increasing in the GF content. For instance, the degree of crystallinity of

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the neat PLA only shows a very gentle increase after 10 min heat treatment, while the

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degree of crystallinity of the PLA/GF composite with 15 wt% GF dramatically increases to

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25% after the same heat treatment. It clearly demonstrate the outstanding ability of GF in accelerating crystallization during heat treatment. After a long heat treatment period, the

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difference in the degree of crystallinity of the neat PLA and PLA/GF composites shows a

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decreasing trend. However, the composite with a higher GF content still has a higher degree of crystallinity.

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In addition to the heat treatment at 80 C, the injection molded neat PLA and PLA/GF composite samples were also isothermally treated at 100 C for various time periods in order to investigate the effect of heat treatment temperature. After that, the thermal behavior of the treated samples were also analyzed using DSC. Fig. S1 shows the DSC thermograms, and Table S2 lists the major thermal characteristics and the calculated degree of crystallinity. Notably, after heat treatment at 100 C for merely 10 min, the cold 21

ACCEPTED MANUSCRIPT crystallization has gone from the DSC thermograms for all the materials. It indicates all the samples have already crystallized sufficiently after 10 min annealing. In contrast, the annealing time required to achieve relatively sufficient crystallization is 90 min for the neat

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PLA annealed at 80 C. Thus, it is concluded that increasing heat treatment temperature

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promotes crystallization significantly. Based on Eq. (2), the degree of crystallinity of the

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samples annealed at 100 C was calculated, and the data is given in Table S2 and also

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plotted in Figure 6b. It can be seen that the degree of crystallinity of all the materials increases dramatically after 10 min heat treatment, and then almost maintains constant

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with further increasing the heat treatment time. Moreover, increasing GF content leads to

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increased degree of crystallinity. In addition, it is worth noting that, for each material, the

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maximum degree of crystallinity that can be achieved under annealing at 100 C is similar with that can be achieved under annealing at 80 C. However, a longer annealing period is

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needed at lower temperatures due to the poorer mobility of polymer chains [54,55].

Figure 6. Degree of crystallinity of the neat PLA and PLA/GF composites after heat treatment at (a) 80 C and (b) 100 C, acquired based on DSC data.

22

ACCEPTED MANUSCRIPT To acquire further information on the crystallization behavior of the neat PLA and PLA/GF composites, the crystalline structure of all the materials after heat treatment was characterized using XRD. Fig. 7 shows the XRD patterns of all the prepared neat PLA and

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PLA/GF composite samples annealed at 80 C for various time periods. In general, all the

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original samples without heat treatment do not exhibit any visible diffraction peak, which

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indicates an amorphous phase. This is not very consistent with the DSC analysis data,

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based on which the degrees of crystallinity of all the samples before heat treatment are not zero although their values are very small. This is because the material undergoes

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relatively complex structural evolution during the heating of the DSC test, such as cold

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crystallization, melt recrystallization, etc., which can cause uncertainties in the final

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analysis results evaluated based on the DSC data. After heat treatment, all the samples exhibit diffraction peaks at around 2 = 14.8, 16.8, 19.0, and 23, which are in

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agreement with the α crystal form of PLA. With increasing in the heat treatment time, the

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diffractions peaks becomes stronger. This reveals that the crystallinity of all the materials

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increases gradually by prolonging the heat treatment at 80 C. This variation trend of crystallinity is consistent with the results obtained by DSC, as Fig. 6a shows. Based on the XRD patterns, the degree of crystallinity can also be evaluated using Eq. (7), and the calculated data is plotted in Fig. 8a. The overall trend of crystallinity of the reaction in Figure 8 is consistent with the results in Figure 7, although there are some differences in specific values. From Fig. 8a, it is clear that a higher GF addition leads to enhanced crystallization after heat treatment. Moreover, all the materials annealed at 100 23

ACCEPTED MANUSCRIPT C were also characterized using XRD, and the acquired XRD patterns are plotted in Fig. S2. It is observed that the samples treated under 100 C under exhibit much stronger diffraction peaks than those samples treated under 80 C. This confirms that a higher

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annealing temperature leads to enhanced crystallization of all the materials. Fig. 8b shows

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the degree of crystallinity of the samples treated at 100 C, evaluated based on the XRD

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data. For all the materials, a 10 min heat treatment at 100 C leads to a sharp increase of

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the degree of crystallinity. However, further increasing the heat treatment time shows little effect on the degree of crystallinity. This phenomenon is also in agreement with the DSC

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analysis results, as Fig. 6b shows. However, the XRD data in Fig. 8b shows all the

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materials have a similar degree of crystallinity after annealing at 100 C, while the DSC

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data in Fig. 6b shows that increasing GF content leads to increased degree of crystallinity. Moreover, by comparing Fig. 8b with Fig. 8a, it is observed that the samples treated at a

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high temperature of 100 C show a higher degree of crystallinity than the samples treated

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at a low temperature of 80 C, even though the latter has a longer heat treatment time. It

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can be ascribed to the fact that molecular chains have much stronger mobility at higher annealing temperature, which results in faster and more crystallization [39,56]. It is expected that the enhanced crystallinity contributes to improving the mechanical performance of the neat PLA and PLA/GF composites, which will be verified in the following section by testing the materials’ mechanical properties.

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Figure 7. XRD profiles of (a) neat PLA, (b) PLA/GF-5, (c) PLA/GF-10, (d) PLA/GF-15, and

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(e) PLA/GF-20 after thermally treating at 80 C for different time periods.

Figure 8. Degree of crystallinity of the neat PLA and PLA/GF composites after heat treatment at (a) 80 C and (b) 100 C, acquired based on XRD data.

3.4. Mechanical properties In order to assess the mechanical behavior of the neat PLA and PLA/GF composites, 25

ACCEPTED MANUSCRIPT tensile, flexural, and notched Izod impact tests were conducted. Fig. 9 shows the tensile properties of the neat PLA and PLA/GF composite samples annealed at 80 C for various time periods. Overall, when the weight percentage of GF increases, tensile strength and

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tensile modulus increase gradually, while elongation at break reduces gradually, under the

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same heat treatment condition. It indicates the inclusion of GF contributes to both strength

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and rigidity, but deteriorates ductility. For instance, with the addition of 20 wt% GF, the

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tensile strength is increased from 43.81 MPa of the neat PLA to 81.67 MPa of the PLA/GF composite, with an increasing of 86.4%. Meanwhile, the tensile modulus is increased by

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60.8%, while the elongation at break is merely sacrificed by 21.1%. It is inferred that the

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high strength of GF combined with the good interfacial bonding between GF and PLA

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matrix leads to the improved strength and modulus. Moreover, it can also be seen from Fig. 9 that heat treatment improves tensile

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strength, tensile modulus and elongation at break, for both the neat PLA and the PLA/GF

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composites. A longer heat treatment leads to more pronounced improvement in the tensile

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mechanical properties. For instance, the tensile strength, tensile modulus, and elongation at break of the neat PLA are improved by 60.4%, 34.3% and 18.4%, respectively, after annealing at 80 C for 90 min. After the same conditions of heat treatment, the tensile strength, tensile modulus, and elongation at break of the PLA/GF composite with 20 wt% GF are increased by 20.6%, 35.8% and 13.9 %, respectively. As demonstrated in Section 3.3, heat treatment leads to significantly increased crystallization of the PLA matrix, which in turn contributes to the improved mechanical properties, because the crystalline PLA 26

ACCEPTED MANUSCRIPT phase is stronger and stiffer than the amorphous PLA phase. Moreover, it was reported

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that the fine crystals induced by annealing leads to improved ductility of PLA [57,58].

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Figure 9. Tensile properties of the neat PLA and PLA/GF composites annealed at 80 C for

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various periods: (a) tensile strength, (b) tensile modulus, and (c) elongation at break.

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The tensile properties of the neat PLA and PLA/GF composite samples annealed at 100 C for various time periods were also measured, and the results are plotted in Fig. 10.

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In general, the effects of GF and heat treatment on tensile strength and tensile modulus,

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obtained from Fig. 10, are very similar with those obtained from Fig. 9. However, the samples annealed at 100 C exhibit higher tensile strength and modulus than those

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annealed at 80 C. This is ascribed to the enhanced crystallization at a higher annealing temperature. It is worth noting that heat treatment at 100 C leads to reduced elongation at break for all the materials, and the phenomenon is quite different from that in the case with the annealing temperature of 80 C. Thus, the samples annealed at 100 C exhibit lower elongation at break than those annealed at 80 C. It is induced that the enhanced crystallization at higher annealing temperature leads to the decreased elongation at break 27

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because large crystals generated at high annealing temperatures show brittle nature [59].

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Figure 10. Tensile properties of neat PLA and PLA/GF composites annealed at 100 C for

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various periods: (a) tensile strength, (b) tensile modulus, and (c) elongation at break.

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Fig. 11 shows the measured flexural properties of the neat PLA and PLA/GF

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composite samples annealed under various conditions. It is observed that the inclusion of GF leads to enhanced flexural strength and modulus. The higher the GF content is, the

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more pronounced the improvement is. Moreover, heat treatment also contribute to flexural

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strength and modulus. A longer heat treatment period results in enhanced flexural strength

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and modulus. As discussed for the tensile properties, the improved flexural performance can also be attributed to the enhanced crystallization by heat treatment. By increasing annealing temperature, crystallization is accelerated and enhanced, which in turn leads to more pronounced improvement in flexural strength and modulus.

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Figure 11. Flexural properties of the neat PLA and PLA/GF composite samples annealed

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at (a) 80 C and (b) 100 C for various time periods.

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Further, notched Izod impact tests were performed to evaluate the mechanical

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properties of the neat PLA and PLA/composite samples under a high deformation rate. The presence of GF leads to significant increasing in impact strength. For instance, the

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impact strength of the sample without heat treatment can be increased from 30.9 J/m for neat PLA to 33.4 J/m, 39.7 J/m, 44.7 J/m, and 51.9 J/m by adding 5 wt%, 10 wt%, 15 wt%, and 20 wt% GF, respectively. For all the materials, heat treatment at 80 C or 100 C benefits impact strength. The improved impact toughness by heat treatment could be attributed to the following reasons. First, heat treatment increased crystallization of PLA matrix which contributes to the impact toughness [2,60]. As shown in Fig. S3, the fracture

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ACCEPTED MANUSCRIPT surface of the annealed sample becomes rougher compared with that of the pristine sample. Second, heat treatment and its induced crystallization are beneficial for improving the interfacial bonding strength between GF and PLA matrix. Since pulling out is the major

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failure mechanism of GF during impact fracture, as demonstrated by Fig. S3, the improved

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interfacial bonding will definitely increase the energy consumption during pulling out glass

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fibers, and hence the impact strength is increased.

Figure 12. Impact strength of the neat PLA and PLA/GF composite samples annealed at

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(a) 80 C and (b) 100C for various time periods.

Moreover, in order to evaluate the effects of GF and heat treatment on the thermal

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resistance of the neat PLA and PLA/GF composites, the heat deflection temperatures (HDT) of the pristine injection molded samples and the injection molded samples annealed at 80 C and 100 C for 30 min were measured. As shown in Fig. 13, the samples without heat treatment show a similar HDT of about 50 C, indicating little effect of the inclusion of GF on the thermal resistance of PLA. In contrast, after heat treatment either at 80 C or 100 C, all the samples show dramatically increased HDT, in particular

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ACCEPTED MANUSCRIPT for PLA/GF composite. For instance, the HDT of neat PLA is increased to about 100 C, while the HDT of PLA/GF composite is increased to about 150 C. Clearly, the greatly improved thermal resistance is owing to the significantly enhanced crystallization after

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than the neat PLA, the former exhibits higher thermal resistance.

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heat treatment [37,60]. Because the PLA/GF composite high higher degree of crystallinity

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Figure 13. HDT of the injection molded neat PLA and PLA/GF composite samples before

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4. Conclusions

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and after heat treatment.

In summary, we took an experimental study to reinforce PLA by blending GF and applying isothermal heat treatment. The pristine injection molded samples are amorphous regardless of GF content due to the poor crystallization ability of PLA matrix. GF shows little effect on the glass transition temperature of PLA matrix because it is too macro to affect the mobility of molecular segment. However, GF can indeed significantly accelerate

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ACCEPTED MANUSCRIPT and increase the crystallization of PLA during heat treatment. Increasing temperature can effectively shorten the annealing time required to finish crystallization. Enhanced crystallization after heat treatment leads to significant improvement of mechanical

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properties of the neat PLA and PLA/GF composites. Moreover, GF exhibits little effect of

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thermal resistance, and all materials regardless of GF content show a similar HDT of 50

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C. However, after heat treatment at 100 C for 30 min, the HDT of neat PLA is increased

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to about 100 C, while the HDT of PLA/GF composite is increased to about 150 C. Heat treatment-induced crystallization is responsible for the significantly enhanced thermal

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resistant performance. This study is of great significance for the development of

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high-performance PLA products, which show outstanding mechanical and thermal

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resistance properties. Acknowledgements

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The authors are grateful to the National Natural Science Foundation of China (NSFC,

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Grant No. 51875318, 21706046), the Young Scholars Program of Shandong University

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(Grant No. 2017WLJH23), and the Fundamental Research Funds of Shandong University for the funding support. References

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