Chiang Mai J. Sci. 2021; 48(2) : 323-331http://epg.science.cmu.ac.th/ejournal/Contributed Paper

Property Enhancement of Polylactide Monofilaments by Cold-Drawing Process and Solvent Infusion Jirawan Jindakaew [a], Chalita Ratanatawanate [b], Takeshi Kikutani [c], and Pakorn Opaprakasit*[a] [a] School of Bio-Chemical Engineering and Technology, Sirindhorn International Institute of Technology (SIIT),

Thammasat University, Pathum Thani 12121, Thailand. [b] Environmental Nanotechnology Research Team, Nanohybrids and Coating Research Group, National

Nanotechnology Center, National Science and Technology Development Agency, Pathum Thani 12120, Thailand.[c] School of Materials and Chemical Technology, Department of Materials Science and Engineering, Tokyo Institute

of Technology, Tokyo, 152-8550, Japan.

*Author for correspondence; e-mail: pakorn@siit.tu.ac.thReceived: 18 September 2020

Revised: 23 January 2021Accepted: 27 January 2021

ABSTRACT Post treatments of polymeric products after fabrication, especially improvements of their

crystallinity, are commonly applied to further enhance properties of the materials. In this study, a cold-drawing process in various environments is developed for poly(L-lactide) (PLLA) monofilaments, whose solvent-infusion mechanism is examined. Multiple necking behavior was observed when the filaments were drawn in water and ethanol, compared to a single-neck deformation when the experiment was conducted in air. The number of necks was also dependent on the drawing media conditions, in which the number of necks decreased with an increase in the draw ratio (DR). The number of necks of the filaments drawn in water and ethanol increased with a further increase in the drawing speed. The drawing treatments in ethanol lead to a decrease in the yield and drawing stresses. and an increase in the natural draw ratio (NDR) of the monofilaments. The results also indicate that ethanol is effectively infused into the PLLA filaments, leading to further chain arrangements. The process has high potential for promoting the crystallization of filaments in cold drawing conditions, leading to enhancement in the mechanical properties.

Keywords: polylactide, solvent infusion, cold drawing, post treatment, induced crystallization

1. INTRODUCTION Biopolymers have increasingly attracted vast

attentions in both research and commercial usages, as these are produced from renewable resources and can be decomposed after use, without imposing of negative impacts to the environment [1-4]. The environmental and sustainability issues surrounding petroleum-based polymeric materials have become increasingly serious, due to their shortage availability

and unsustainable waste disposal processes. Alternatively, bio-based polymers have played a vital role to replace these materials in various applications [5-7]. In this regard, polylactide (PLA) is of great interest, due mainly to its outstanding performances, biodegradability, and renewability [8-10]. Recently, PLA is widely fabricated in a form of filaments by melt spinning for use in

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textile industry [11]. However, some properties of the polymer need to be optimized to improve its mechanical properties and durability, and also provide specific functions. The material’s low degree of crystallization and slow crystallization rates are among major concerns [12-14].

Post treatments of polymer products after fabrication, e.g., drawing, lead to the material’s structural rearrangements and improvements of their mechanical properties. In addition, solvent-drawing process further provides unique features, especially a multiple neck phenomenon on the fiber’s surfaces [15]. It was reported that certain solvents can induce crystallization of polymeric filaments. For example, the immersion of nylon filaments in benzene or acetone solvents led to the formation of multiple necks upon drawing [16]. A process for functionalization of poly(ethylene terephthalate) (PET) filaments during cold drawing of the fibers in organic solvents was developed [17]. In this process, additional chemicals, e.g., dye were infused into the PET filaments together with ethanol solvent, while drawing the filaments. The infusion of the organic solvent caused structural rearrangement, leading to the formation of multiple necks [18-19]. Furthermore, structural changes of PET fibers were examined by drawing the fibers in n-propanol. As a result, porous structures with well-developed interfacial surface (craze structure) were observed [20].

In the drawn part of filaments, solvent crazing of polymer deformation proceeded up to relatively high strains (with an applied tensile stress). The development of free surface on crazing involves breakdown of certain fractions of the polymer chains. This effect is most pronounced at low temperatures, where molecular mobility is inhibited [21-22]. Solvent crazing provides a forced delivery of the modifying solvents or additives to a continuous developing interfacial surface area as nano-porous network structure. Then, the infusion phenomenon occurred via a fast mode of transfer through a viscous flow. To immobilize the additive in structure of a

polymer, the polymer and the low molecular mass component do not necessarily have to contain active functional groups which are capable of any mutual interaction [23-25].

In this work, a process for post treatments of poly(L-lactide) (PLLA) monofilaments is developed by a solvent-infusion cold-drawing process, employing water and ethanol media. Effects of types of media, drawing speeds, and drawing ratios on properties of the resulting filaments are investigated. Mechanisms of the solvent infusion into the fibers are examined. The process has high potential for applying in textile industry, as this can provides significant cost reduction by decreasing its processing steps and energy consumption, compared to other heat-treating processes.

2. MATERIALS AND METHODS 2.1 Materials

Poly(L-lactide), PLLA pellets (ρ = 1.24 g/cm3), with the optical purity 96% L-lactide and melt flow rate: 6 g/10 min (PLA grade LX175), were obtained from Corbion, Netherlands. PLLA filaments were prepared by drying the polymer pellets under vacuum condition, at 60 oC for 8 h. The material was then melted and spun at 230 oC, using a single-hole spinneret with a 0.5 mm diameter. The throughput rate was kept at 5 g/min. The resulting filaments were taken-up on a bobbin at a velocity of 1 km/min by using a winder, which was located at 3.3 m down from the exit of the spinneret.

2.2 Filament Drawing MethodThe cold drawing of as-spun PLLA filaments

was performed at different drawing speeds (3-200 mm/min), and various draw ratios (DR1.2 - DR6) at room temperature. The initial sample length of the monofilament was 3 mm. The drawing treatments were conducted in different environmental conditions, i.e., air, water, and ethanol by using a horizontal mini tensile tester, as illustrated in Figure 1. After the treatments,

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the filaments were dried at room temperature for 1 day to completely remove ethanol or water residues from the filaments surface.

2.3 Characterization of As-spun and Drawn Filaments

Structures and surface morphology of the filaments before and after the drawing treatments were examined by an optical microscope (Olympus

BH-3) using an immersion liquid, with a refractive index of 1.58. The yield point and drawing stress on the drawn filaments obtained from different conditions were examined with a mini tensile tester. Crystalline structures of the filaments treated at various drawing conditions were examined by a wide-angle X-ray diffractometer (WAXD), equipped with a CCD detector (Rigaku). Thermal properties of the original and treated filaments

(a) Top view

(b) Side view

Fig 1 Schematic diagram of the filament-drawing process, employing a horizontal mini tensile tester:

(a) top view and (b) side view.

Figure 1. Schematic diagram of the filament-drawing process, employing a horizontal mini tensile tester: top view (a) and side view (b).

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were characterized by differential scanning calorimetry (DSC, Q100, TA Instruments) at a constant heating and cooling rate of 10 oC/min, under nitrogen atmosphere.

3. RESULTS AND DISCUSSION3.1 Drawing Behavior

The as-spun PLLA monofilaments were cold drawn at room temperature, with a constant drawing speed of 60 mm/min in different environments. The optical micrographs showing drawn and undrawn parts of the filaments, as a function of the drawing ratios, i.e., DR1.2, DR1.4, DR1.6 and DR1.8, are compared in Figure 2. A single-neck deformation was generated and propagated for the drawing process in air, while the unique multiple-neck features were observed when the filaments were drawn in water and ethanol. In a previous study on PET fibers, the formation of multiple necks was observed only in the drawing process using organic solvents, because of the polymer’s high energy barrier [17]. Although the drawn filaments in water exhibit the multiple neck features, the neck patterns were not as well-defined, compared to those observed from the drawn filaments in ethanol. The number of necks decreased with an increase in the draw ratio (DR), in which the neck patterns started to disappear at a draw ratio of DR2 or higher. An important parameter of drawn filaments, a natural draw ratio (NDR), is examined. This is defined as a draw ratio of filaments after necking at the onset of the strain hardening. When the drawing speed was increased, the number of necks increased until reaching the NDR, which is reflected by a disappearance of the necks. After this point, the subsequent continuously-drawing forces are applied to the drawn parts. The NDR values of the filaments show an increasing trend from the drawing process in air to water and ethanol, respectively. It was also speculated that the increase in the number of necks leads to a decrease in the speed of the individual neck propagation, which is an important parameter in the solvent-infusion

phenomenon. This change stops at the filament’s NDR, due to the disappearance of the neck features. Therefore, NDR is a key factor in this study which determine the degree of solvent-infusion into the filaments.

Tensile’ s stress-strain curves of the filaments were recorded during the drawing process in different environments, as shown in Figure 3. The curves of all monofilaments being stretched in air or solvents (water and ethanol) exhibit a unique yield point and NDR, reflecting their necking behavior. After that, further elongation proceeded with lower forces, indicating plastic deformation on the PLLA structure. The filaments drawn in air showed the highest yield stress and drawing stress, but the lowest NDR. A reduction in the yield stress and the drawing stress was a significant consequence from the drawing process in water and ethanol. The PLLA filaments drawn in ethanol showed the lowest yield stress and drawing stress, compared to those in air and water. The filaments drawn in ethanol exhibited the highest value of NDR at a length of 10 mm, compared to those drawn in air and water (around 6 mm). This evidence confirms the infusion of ethanol into the filaments, which cannot be observed in water, even though multiple necks are also generated. This is likely because of the lower barrier energy of the filaments, similar to those observed in PET fibers [17-19]. The interfacial free energy between the fibers and the medium surrounding them plays a key role in the neck formation during the drawing process. As the interfacial free energy of ethanol and PLLA is lower than those of air/PLLA and water/PLLA, this makes it easier to increase the surface area of the fibers, leading to a formation of multiple necks.

3.2 Structural Changes in FilamentsThe infused organic solvents also affect the

crystallization behaviors of polymeric filaments, as a result from induced chain orientation during the cold drawing process. To obtain insights into the mechanisms of this behavior, wide angle

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Filament drawing in air

Filament drawing in water

Filament drawing in ethanol

Fig 2 Optical micrographs of PLLA monofilaments, illustrating the drawn and undrawn parts after

being cold drawn at various conditions (DR1.2-DR1.8)

Magnification

4x

10x

Magnification

4x

10x

Magnification

4x

10x

Figure 2. Optical micrographs of PLLA monofilaments, illustrating the drawn and undrawn parts after being cold drawn at various conditions (DR1.2-DR1.8).

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X-ray dffraction (WAXD) patterns of the original as-spun and drawn filaments in different media were examined, as shown in Figure 4. The results indicate that the crystalline structures of the undrawn filament and the profile for filaments drawn in air and water did not significantly change. Only the amorphous halo pattern was observed, similar to that of the as-spun fibers. In

contrast, rearrangements of amorphous PLLA chains occur when the filaments were drawn in ethanol, leading to an induced crystallization. The evidence of crystalline reflections proceeds only when the filament is stretched in the organic solvent, whereas the intensity of the amorphous halo decreased.

Fig 3 Tensile behaviors of PLLA monofilaments upon cold drawing in various media at different drawing

ratios (DR): (a) DR1.5 and (b) DR4

Fig 4 WAXD patterns of undrawn filaments and those after drawing treatments in different media.

(a) (b)

Figure 3. Tensile behaviors of PLLA monofilaments upon cold drawing in various media at different drawing ratios (DR): DR1.5 (a) and DR4 (b).

Figure 4. WAXD patterns of undrawn filaments and those after drawing treatments in different media.

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Thermal properties of the corresponding samples were examined by DSC, whose thermograms are compared in Figure 5. The results on thermal properties of the materials are summarized in Table 1. The fiber samples were drawn in various media with a constant drawing speed of 60 mm/min and DR4, compared with the untreated monofilament. Only the fibers drawn in ethanol shows an endothermic peak at around 80 °C, due to an evaporation of ethanol molecules remaining in the filaments. This indicates the infusion of ethanol molecules into the filaments during the cold-drawing process. The content of ethanol in the filaments was estimated at around 4% by considering its heat of vaporization (393 kJ/kg). It

is noted, however, that ethanol can be completely removed from the filament at room temperature for a certain period or only slightly increasing the temperature, the evaporation can be accelerated. During the application of stress in organic media, the solvent molecules surrounded the filaments and partly penetrated into the polymer filament, leading to an increase in the volume, i.e, swelling. This, in turns, causes the fibers to create the craze structure for negatively pressed and further absorb the surrounding media to the polymer matrix. The material at craze tip is considered to be plastically deformed. Since the extent of the instability is related to the surface energy of the interfaces, when an environmental liquid is allowed to flow into

Fig 5 DSC thermograms of: (a) undrawn filament, and the filaments drawn in

(b) air, (c) water, and (d) ethanol

(a) (b)

(c) (d)

Figure 5. DSC thermograms of: undrawn filament (a), and the filaments drawn in air (b), water (c), and ethanol (d).

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the craze in place of air, the surface energy of the interface is lowered and this effectively accelerates the meniscus instability. In addition, the DSC thermogram of the fibers drawn in ethanol did not show any cold crystallization peak, compared to the other 3 samples. The undrawn sample and those drawn in air and water exhibited ΔHc values of 21.2, 5.7 and 10.3 J/g, respectively (Table 1). This clearly indicates that ethanol molecules are infused into the filaments. As ethanol possesses hydroxyl groups that can form hydrogen bonding with the carbonyl groups of PLLA, this leads to induced crystallization of the polymer chains during the cold drawing process, as reported earlier [26]. An enhancement in crystallinity (Xc) of the filaments drawn in ethanol was observed at 43.2 J/g, compared to 28.3 J/g of the undrawn counterpart. Lower degree of the crystallinity improvement was observed in the filaments drawn in air and water (34.0 and 36.0 J/g). The results are also in good agreement with those observed from WAXD results.

4. CONCLUSIONS Effect of solvent infusion during the cold-drawing process of PLLA monofilaments was investigated. The drawing process in water and ethanol leads to the formation of multiple necks on the fibers, whereas a single neck deformation was observed when the filaments were drawn in air. The increase in the drawing speed leaded to an increase in the number of necks on the as-spun fibers. A reduction in the yield stresses

was observed when the filaments were drawn in ethanol, while the natural draw ratio is increasing. The number of necks were decreased by increasing the draw ratio, which disappeared at a draw ratio of at least DR2. A diffusion of organic solvent into the filaments accompanied by a propagating necking, enabling the solvent molecules to penetrate into the polymer matrix during the drawing process, as called an infusion phenomenon. The cold drawing of PLLA filaments in ethanol leads to an induced crystallization of the fibers at room temperature. This process has high potential for applying in textile industry. This induced crystallization process provides significant cost reduction by decreasing its processing steps and energy consumption, compared to other heat-treating processes. Furthermore, the process can be applied in functionalization of polymeric fibers using modifying agents or drugs as well as organic solvents.

ACKNOWLEDGEMENTS The authors are grateful for supports from the Thailand Advanced Institute of Science and Technology (TAIST) 2018 - Tokyo Tech Student Exchange Program in Japan 2019, Thailand Graduate Institute of Science and Technology (TGIST) 2019 scholarship, and the Center of Excellence in Materials and Plasma Technology (CoE M@P Tech), Sirindhorn International Institute of Technology (SIIT), Thammasat University.

Table 1. Thermal properties of undrawn PLLA filament, and the filaments drawn in different media, derived from DSC thermograms.

Filament samples Tm (oC) ΔHm (J/g) ΔHc (J/g) Xc = ΔHm- ΔHc

(a) Undrawn 174.5 49.6 21.2 28.3(b) Drawn in air 174.7 39.7 5.7 34.0(c) Drawn in water 173.8 46.3 10.3 36.0(d) Drawn in ethanol 175.0 43.2 0 43.2

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