Structure Mediation and Properties of Poly(l-lactide)/Poly(d-lactide) Blend Fibers

Poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) blend as-spun fibers (50/50, wt.%) were prepared by melt spinning. Structure mediation under temperature and stress and properties of poly(l-lactic acid)/poly(d-lactic acid)(PLLA/PDLA) as-spun fibers were investigated by wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). The results show that highly oriented stereocomplex (SC) crystals can be formed in PLLA/PDLA blend fibers drawn at 60 °C and annealed at 200 °C. However, at drawn temperature of 80 °C, only lower oriented SC crystals can be formed. For PLLA/PDLA blend fibers drawn twice at 60 °C (PLLA/PDLA-60-2), the crystallinity of SC crystals increases with annealing temperature in the range of 200 to 215 °C, while the degree of orientation decreases slightly. When the annealing temperature is 210 °C, the crystallinity and orientation of SC crystals in PLLA/PDLA-60-2 fibers reach 51% and −0.39, respectively. Moreover, PLLA/PDLA-60-2-210 fibers exhibit excellent heat-resistant property even at 200 °C. The results indicate that the oriented PLLA/PDLA blend fibers with high SC crystals content can be regulated in a short time.


Introduction
Poly(lactic acid) (PLA) has comparable strength with polypropylene, yet it has the advantages of biodegradability [1] and biocompatibility, and so has already been applied as a biomedical material [2,3]. PLA can be made into lightweight fabric which is soft, comfortable, anti-microbial, etc. [4].
However, the poor heat resistance property of PLA materials is detrimental for application in the textile industry [5,6]. Many practical studies have been done by researchers including: Blending with nucleating agents to improve the crystallinity of PLA materials [7,8]; Adding crosslinking agents to form cross-linked structures [9,10]; Adding a certain proportion of PDLA to form SC crystals [11,12]. Among these methods, the SC crystal has attracted great interest in the improvement of PLA heat resistance. The racemic stereocomplex structure was firstly reported by Ikada et al. in the equimolar mixtures of PLLA and PDLA [13]. The melting temperature of SC crystals is about 50 • C higher than that of α crystals in PLLA or PDLA homopolymers (homo-crystals), exhibiting superior heat resistance [14,15]. The enhanced thermal stability of SC crystals provides the possibility to prepare poly(lactic acid) (PLA)-based materials with better dimensional stability at high temperatures [16][17][18]. Therefore, it is industrially promising to prepare PLA fibers with a high content of SC crystals.

Measurements
The melting and crystallization behavior of PLLA/PDLA blends was examined with differential scanning calorimetry(DSC) (TA Instruments, Inc., Newcastle, DE, USA). The instrument was calibrated with indium before measurements. Temperature scans were performed under nitrogen atmosphere. Liquid aluminum crucibles were used. The weight of all samples varied between 5 and 10 mg. The samples were heated to 260 • C and the heating curves were recorded. The heating rate was 10 • C min −1 .
The as-spun fibers were placed on a Linkam LNP95 stretching hot stage (Linkam Scientific Instruments, Ltd., Tadworth, UK), heated to 60 • C or 80 • C at a rate of 30 • C min −1 , annealed for 1 min, and then the fibers were stretched to 2 times at the speed of 150 µm s −1 . After being drawn, the samples were heated to 200 • C annealing for 20 min or 210/215 • C for 1 min, and subsequently cooled to 40 • C. The cooling/heating rate was 5 • C min −1 . The temperature/draw procedure for in situ wide-angle X-ray scattering (WAXS) is shown in Figure 1. The codes of PLLA/PDLA blend fibers under different conditions are shown in Table 1. In situ WAXS measurements were carried out on X-ray diffractometer(XRD) (Bruker, karlsruhe, ND, USA). The wavelength of the radiation source was 0.154 nm. The scattering patterns were collected by a MAR CCD (MAR-USA) detector, which had a resolution of 2048 × 2048 pixels (pixel size = 79 × 79 mm 2 ). The sample-to-detector distance was 85.6 mm and the image acquisition time was 60 s. The samples were tested for every 10 • C during heating and cooling process. All the images were corrected for background scattering, air scattering and sample absorption.
The one-dimensional (1D) diffraction intensity for each 2θ was obtained by integration over the azimuthal range (60 • -120 • ) of the 2D diffraction images. To estimate the fraction of different phases, the WAXS intensity profile was deconvolved into several Gaussian profiles to represent all of the visible scattering peaks and the amorphous halo. The relative fraction of different phases (X) of the samples was calculated by the following equations: with indium before measurements. Temperature scans were performed under nitrogen atmosphere. Liquid aluminum crucibles were used. The weight of all samples varied between 5 and 10 mg. The samples were heated to 260 °C and the heating curves were recorded. The heating rate was 10 °C min −1 .
The as-spun fibers were placed on a Linkam LNP95 stretching hot stage (Linkam Scientific Instruments, Ltd., Tadworth, UK), heated to 60 °C or 80 °C at a rate of 30 °C min −1 , annealed for 1 min, and then the fibers were stretched to 2 times at the speed of 150 μm s −1 . After being drawn, the samples were heated to 200 °C annealing for 20 min or 210/215 °C for 1 min, and subsequently cooled to 40 °C. The cooling/heating rate was 5 °C min −1 . The temperature/draw procedure for in situ wideangle X-ray scattering (WAXS) is shown in Figure 1. The codes of PLLA/PDLA blend fibers under different conditions are shown in Table 1. In situ WAXS measurements were carried out on X-ray diffractometer(XRD) (Bruker, karlsruhe, ND, USA). The wavelength of the radiation source was 0.154 nm. The scattering patterns were collected by a MAR CCD (MAR-USA) detector, which had a resolution of 2048 × 2048 pixels (pixel size = 79 × 79 mm 2 ). The sample-to-detector distance was 85.6 mm and the image acquisition time was 60 s. The samples were tested for every 10 °C during heating and cooling process. All the images were corrected for background scattering, air scattering and sample absorption.
The one-dimensional (1D) diffraction intensity for each 2θ was obtained by integration over the azimuthal range (60°-120°) of the 2D diffraction images. To estimate the fraction of different phases, the WAXS intensity profile was deconvolved into several Gaussian profiles to represent all of the visible scattering peaks and the amorphous halo. The relative fraction of different phases (X) of the samples was calculated by the following equations: where I α , I sc , and I amor stand for the integral intensities of the peaks of α crystals, SC crystals and amorphous phase, respectively [32]. The orientation factor of the (110)/(200) crystal plane is calculated according to the Hermans formula [33], where Φ is the angle between the normal direction of the crystal plane and the fiber axis.
The fibers were placed in an oil bath with different temperatures for 1 min. The original length (L 0 ) and the final length (L 1 ) were measured and the shrinkage S was calculated by the following equation:

The Initial Structure of PLLA/PDLA As-Spun Fibers
WAXS intensity profile of PLLA/PDLA as-spun fibers is shown in Figure 2a. No reflection peak is observed in the WAXS profile, and only an amorphous halo appears, indicating amorphous phase in the PLLA/PDLA as-spun fibers. As shown in Figure 2b, the endothermic peak at 59.92 • C is glass transition and thermal enthalpy relaxation. The sharp exothermal peak at 90.64 • C is assigned to cold crystallization. Zhang et al. [34] reported that α' crystals would generate below 120 • C, which was less stable than α crystals. The small exothermic peak at 157.32 • C corresponds to the transition from α' to α crystals [34,35]. The two endothermic peaks at 173.33 • C and 224.20 • C are assigned to the melting peaks of α crystals and SC crystals, respectively. From these results, it can be concluded that the prepared as-spun fibers are amorphous.

Crystal Formation of Fibers with Different Initial Structures during Heating and Cooling Process
It is known that the initial structures of PLLA/PDLA blend fibers influence the structure evolutions of fibers upon heating and cooling. To acquire the different initial structures, the stretching temperatures of 60 and 80 • C were selected. The crystallization variation of PLLA/PDLA blend fibers with different initial structure during heating and cooling process is shown in Figure 3. The unstretched as-spun fibers, PLLA/PDLA-60 and PLLA/PDLA-80, exhibited similar behavior during the heat treatment. The initial structure was almost amorphous. During heating, α crystals with a low degree of orientation appeared gradually. SC crystals, with a negligible degree of orientation, appeared at 200 • C. Upon cooling, isotropic α crystals formed again. These results show that the orientation of fiber is lost although the amount of SC crystals increases in the fibers.
Polymers 2018, 10, x FOR PEER REVIEW 5 of 11 in the pattern, demonstrating that the high orientation can be maintained at 200 °C in PLLA/PDLA-60-2 fibers. When PLLA/PDLA-60-2 fibers were cooled to 150 °C, the reflections of (110)/(200) assigned to α crystals appeared at 16.1°, and formed α crystals that were also highly oriented.  For PLLA/PDLA-80-2, diffraction of (110)/(200)α is observed at 16.1 °. The PLA segments can move to form oriented α crystals because the drawing temperature (80 °C) is higher than Tg of PLA. When the fibers are heated to 100 and 150 °C, only α crystals are formed in PLLA/PDLA blend fibers. With temperature up to 200 °C for 20 min, diffraction arc of SC crystals is observed at 11.6°. However, SC crystals reflections are obtuse, suggesting that the formed SC crystals are less oriented. The contents of SC crystals, α crystals and orientation factor of the PLLA/PDLA fibers are listed in Tables  2 and 3. The result shows that PLLA/PDLA-60-2 has higher SC crystals content and degree of orientation, which is attributed to the higher orientation of PLA segments in PLLA/PDLA-60-2 at low drawing temperature. When the fibers are annealed at 200 °C, the higher PLA oriented segments improve crystallization rate of SC crystals, thus suppress the relaxation of oriented PLA segments and form highly oriented SC crystals. The content of SC crystals in PLLA/PDLA-60-2 fibers reaches more than 30% (Table 2), and the value of ƒ(110)sc reaches −0.40 (Table 3). These results suggest that oriented SC crystals with higher content can be formed in the PLLA/PDLA-60-2 fibers in this unique treatment process. in the pattern, demonstrating that the high orientation can be maintained at 200 °C in PLLA/PDLA-60-2 fibers. When PLLA/PDLA-60-2 fibers were cooled to 150 °C, the reflections of (110)/(200) assigned to α crystals appeared at 16.1°, and formed α crystals that were also highly oriented.   Tables  2 and 3. The result shows that PLLA/PDLA-60-2 has higher SC crystals content and degree of orientation, which is attributed to the higher orientation of PLA segments in PLLA/PDLA-60-2 at low drawing temperature. When the fibers are annealed at 200 °C, the higher PLA oriented segments improve crystallization rate of SC crystals, thus suppress the relaxation of oriented PLA segments and form highly oriented SC crystals. The content of SC crystals in PLLA/PDLA-60-2 fibers reaches more than 30% (Table 2), and the value of ƒ(110)sc reaches −0.40 (Table 3). These results suggest that oriented SC crystals with higher content can be formed in the PLLA/PDLA-60-2 fibers in this unique treatment process. For PLLA/PDLA-80-2, diffraction of (110)/(200)α is observed at 16.1 • . The PLA segments can move to form oriented α crystals because the drawing temperature (80 • C) is higher than T g of PLA.
When the fibers are heated to 100 and 150 • C, only α crystals are formed in PLLA/PDLA blend fibers. With temperature up to 200 • C for 20 min, diffraction arc of SC crystals is observed at 11.6 • . However, SC crystals reflections are obtuse, suggesting that the formed SC crystals are less oriented. The contents of SC crystals, α crystals and orientation factor of the PLLA/PDLA fibers are listed in Tables 2 and 3. The result shows that PLLA/PDLA-60-2 has higher SC crystals content and degree of orientation, which is attributed to the higher orientation of PLA segments in PLLA/PDLA-60-2 at low drawing temperature. When the fibers are annealed at 200 • C, the higher PLA oriented segments improve crystallization rate of SC crystals, thus suppress the relaxation of oriented PLA segments and form highly oriented SC crystals. The content of SC crystals in PLLA/PDLA-60-2 fibers reaches more than 30% (Table 2), and the value of ƒ(110) sc reaches −0.40 (Table 3). These results suggest that oriented SC crystals with higher content can be formed in the PLLA/PDLA-60-2 fibers in this unique treatment process.

Effect of Annealing Temperature on Crystallization of PLLA/PDLA Blend Fibers
The annealing temperature is a key factor to regulate the formation of SC crystals in the PLLA/PDLA fibers. In our previous study [38], the PLLA/PDLA (50/50) film were annealed at 185-225 • C for 30 min. The results suggest that the fraction of SC crystals increased from 16% to 35% with increasing annealing temperature. It is believed that SC crystals formed at the interface acting as a barrier layer so as to hinder the further diffusion of the chains. When the annealing temperature increases to melt the "SC crystal barrier wall", the enhanced chain diffusion between PLLA and PDLA enrichment region will improve the formation of SC crystals. Therefore, the effect of annealing temperature on the crystallization behavior of PLLA/PDLA-60-2 fibers needs further exploration to obtain a higher content of SC crystals with orientation.
The 2D WAXS patterns of the PLLA/PDLA-60-2 fibers upon heating are shown in Figure 4. When the PLLA/PDLA-60-2 fibers are heated to 220 • C, SC crystals of PLLA/PDLA-60-2 are partially melted and the orientation decreases because the melting temperature of SC crystals is lower than 220 • C. Further increasing temperature to 225 • C, the crystals are completely melted. In order to maintain the crystal orientation of PLLA/PDLA-60-2 fibers during annealing and cooling process, the annealing temperatures of 210 and 215 • C are selected.

Effect of Annealing Temperature on Crystallization of PLLA/PDLA Blend Fibers
The annealing temperature is a key factor to regulate the formation of SC crystals in the PLLA/PDLA fibers. In our previous study [38], the PLLA/PDLA (50/50) film were annealed at 185-225 °C for 30 min. The results suggest that the fraction of SC crystals increased from 16% to 35% with increasing annealing temperature. It is believed that SC crystals formed at the interface acting as a barrier layer so as to hinder the further diffusion of the chains. When the annealing temperature increases to melt the "SC crystal barrier wall", the enhanced chain diffusion between PLLA and PDLA enrichment region will improve the formation of SC crystals. Therefore, the effect of annealing temperature on the crystallization behavior of PLLA/PDLA-60-2 fibers needs further exploration to obtain a higher content of SC crystals with orientation.
The 2D WAXS patterns of the PLLA/PDLA-60-2 fibers upon heating are shown in Figure 4. When the PLLA/PDLA-60-2 fibers are heated to 220 °C, SC crystals of PLLA/PDLA-60-2 are partially melted and the orientation decreases because the melting temperature of SC crystals is lower than 220 °C. Further increasing temperature to 225 °C, the crystals are completely melted. In order to maintain the crystal orientation of PLLA/PDLA-60-2 fibers during annealing and cooling process, the annealing temperatures of 210 and 215 °C are selected. The effect of annealing temperature on the crystallization of PLLA/PDLA-60-2 fibers is shown in Figure 5. When PLLA/PDLA-60-2 fibers are heated to 210 or 215 °C for 1 min, the diffraction of SC crystals becomes sharper and stronger, which indicates the higher crystallinity of SC crystals. When the fibers are cooled down to the room temperature slowly, the diffraction of α crystals is very weak. The oriented SC crystals are the main crystal structure in the fibers. The fractions of α crystals and SC crystals in the PLLA/PDLA-60-2 fibers treated with different processes are estimated by the The effect of annealing temperature on the crystallization of PLLA/PDLA-60-2 fibers is shown in Figure 5. When PLLA/PDLA-60-2 fibers are heated to 210 or 215 • C for 1 min, the diffraction of SC crystals becomes sharper and stronger, which indicates the higher crystallinity of SC crystals. When the fibers are cooled down to the room temperature slowly, the diffraction of α crystals is very weak. The oriented SC crystals are the main crystal structure in the fibers. The fractions of α crystals and SC crystals in the PLLA/PDLA-60-2 fibers treated with different processes are estimated by the deconvolution of the WAXS intensity profiles ( Figure 6). During the cooling process, the content of SC crystals in PLLA/PDLA-60-2 fibers increases when the temperature is higher than 180 • C. The maximum content of SC crystals in both PLLA/PDLA-60-2-210 and PLLA/PDLA-60-2-215 fibers is about 50% ( Table 4). The content of α crystals is very low, indicating that high content of SC crystals in the fibers suppresses the formation of α crystal [39,40]. As shown in Table 4, with the annealing temperature of 200 and 215 • C, the content of SC crystals increases from 32% to 51%, and the value of ƒ(110) sc decreases from −0.40 to −0.36. Furthermore, the melting temperature of SC crystals reaches up to 231.70 • C with annealing temperature up to 215 • C (Figure 7). According to the results of crystallinity and orientation of SC crystals, the annealing temperature of 210 • C is optimal.
Polymers 2018, 10, x FOR PEER REVIEW 7 of 11 deconvolution of the WAXS intensity profiles ( Figure 6). During the cooling process, the content of SC crystals in PLLA/PDLA-60-2 fibers increases when the temperature is higher than 180 °C. The maximum content of SC crystals in both PLLA/PDLA-60-2-210 and PLLA/PDLA-60-2-215 fibers is about 50% ( Table 4). The content of α crystals is very low, indicating that high content of SC crystals in the fibers suppresses the formation of α crystal [39,40]. As shown in Table 4, with the annealing temperature of 200 and 215 °C, the content of SC crystals increases from 32% to 51%, and the value of ƒ(110)sc decreases from −0.40 to −0.36. Furthermore, the melting temperature of SC crystals reaches up to 231.70 °C with annealing temperature up to 215 °C (Figure 7). According to the results of crystallinity and orientation of SC crystals, the annealing temperature of 210 °C is optimal.   Polymers 2018, 10, x FOR PEER REVIEW 7 of 11 deconvolution of the WAXS intensity profiles ( Figure 6). During the cooling process, the content of SC crystals in PLLA/PDLA-60-2 fibers increases when the temperature is higher than 180 °C. The maximum content of SC crystals in both PLLA/PDLA-60-2-210 and PLLA/PDLA-60-2-215 fibers is about 50% ( Table 4). The content of α crystals is very low, indicating that high content of SC crystals in the fibers suppresses the formation of α crystal [39,40]. As shown in Table 4, with the annealing temperature of 200 and 215 °C, the content of SC crystals increases from 32% to 51%, and the value of ƒ(110)sc decreases from −0.40 to −0.36. Furthermore, the melting temperature of SC crystals reaches up to 231.70 °C with annealing temperature up to 215 °C (Figure 7). According to the results of crystallinity and orientation of SC crystals, the annealing temperature of 210 °C is optimal.

Heat-Resistant Properties of PLLA/PDLA Blend Fibers
The heat resistance property of PLA fibers was investigated by placing PLLA-130 (annealing at 130 °C for 30 min) and PLLA/PDLA-60-2-210 (annealing at 210 °C for 1 min) fibers into the oil bath. The shrinking results of PLLA-130 and PLLA/PDLA-60-2-210 fibers at different temperatures are listed in Table 5 and Figure 8. The shrinkage percentage of PLLA-130 fibers reaches 48% at 150 °C. When the temperature is 200 °C, PLLA-130 fibers are molten, while PLLA/PDLA-60-2-210 fibers maintain the original length without significant shrinkage. Thus, high content of SC crystals endows the fibers with better heat resistance. When the temperature reaches 210 °C, the PLLA/PDLA-60-2-210 fibers also begin to shrink because this temperature is already close to the melting temperature of SC crystals. PLLA/PDLA-60-2-210 fibers are completely molten at 220 °C.

Heat-Resistant Properties of PLLA/PDLA Blend Fibers
The heat resistance property of PLA fibers was investigated by placing PLLA-130 (annealing at 130 • C for 30 min) and PLLA/PDLA-60-2-210 (annealing at 210 • C for 1 min) fibers into the oil bath. The shrinking results of PLLA-130 and PLLA/PDLA-60-2-210 fibers at different temperatures are listed in Table 5 and Figure 8. The shrinkage percentage of PLLA-130 fibers reaches 48% at 150 • C. When the temperature is 200 • C, PLLA-130 fibers are molten, while PLLA/PDLA-60-2-210 fibers maintain the original length without significant shrinkage. Thus, high content of SC crystals endows the fibers with better heat resistance. When the temperature reaches 210 • C, the PLLA/PDLA-60-2-210 fibers also begin to shrink because this temperature is already close to the melting temperature of SC crystals. PLLA/PDLA-60-2-210 fibers are completely molten at 220 • C.