In Situ Homologous Polymerization of l-Lactide Having a

Aug 10, 2018 - Seung Hyuk Im†‡ , Youngmee Jung‡§ , and Soo Hyun Kim*†‡§. † KU-KIST Graduate School of Converging Science and Technology,...
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In Situ Homologous Polymerization of L‑Lactide Having a Stereocomplex Crystal Seung Hyuk Im,†,‡ Youngmee Jung,‡,§ and Soo Hyun Kim*,†,‡,§ †

KU-KIST Graduate School of Converging Science and Technology, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea ‡ Center for Biomaterials, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea § Korea University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea

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ABSTRACT: Polylactide (PLA) is one of the most commonly used biodegradable polymers for various fields such as biomedical and renewable industries. This material must undergo molding processes to achieve the desired shape in the final manufactured product, which are typically thermal- or solvent-based. Unfortunately, the mechanical and thermal properties of PLA, such as the strength and crystallinity, are inevitably damaged during the molding process due to the slow crystallization rate. To overcome this limitation, a simple, novel, one-pot, self-accelerating method for the in situ self-nucleating (ISN) polymerization of L-lactide is developed. This strategy results in the simultaneous polymerization and self-nucleation of L-lactide crystallites upon addition of a self-nucleating agent. The results show that ISN-PLA experiences an acceleration effect on the crystallization with stereocomplex polylactide (sc-PLA) as the nucleating agent and has enhanced mechanical and thermal properties, despite repetitive thermal processing. The ISN polymerization of L-lactide can save considerable time and cost by using the one-pot process composed of both polymerization and self-nucleation. This study suggests a novel strategy for manufacturing industrial products with high strength and crystallinity based on PLA as a promising biopolymer.

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INTRODUCTION Biodegradable polymers have become widely utilized in various fields such as biomedical and renewable industries.1−3 One of the most promising and commonly used biodegradable polymers is polylactide (PLA), which has highly desirable properties, including biocompatibility, biodegradability, mechanical strength, and renewability. During manufacturing, PLA must undergo a molding process, which can be thermalor solvent-based, depending on the specific shape required.4,5 However, shape molding of PLA has been shown to degrade its mechanical and thermal properties, such as its strength and crystallinity, due to the slow crystallization rate.6−12 To overcome the current limitations, numerous secondary methods have been developed. One of the most common methods is to make composite materials with improved properties, including the crystallization rate, heat resistance, and mechanical strength, through addition of a nucleating agent (NA) to the polymer matrix.13,14 To maximize its © XXXX American Chemical Society

efficacy, the NA must have (1) a higher melting point than the polymer matrix, (2) a high thermal and chemical stability, (3) good miscibility with the matrix, (4) good biocompatibility and nontoxicity with regards to the human body, (5) a simple and economic method of processing, (6) high efficacy in only a small quantity, and (7) a favorable interfacial interaction and lattice match with the matrix. There are numerous methods for modifying the surface charge of NA to improve its interaction with the matrix, for example, by the incorporation of positively charged molecules15,16 or surface functionalization by the addition of a covalent group (i.e., −OH, −COOH, and −F) or an amino group.17,18 The resulting decrease in the free energy barrier for nucleation is very important to increase the nucleating effect of NA and promote crystallization kinetics. Received: July 10, 2018 Revised: July 28, 2018

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fore, our current study suggests a novel approach for the industrial production of a high-strength PLA biopolymer suitable for broad applications by overcoming the current limitation of the slow crystallization rate of PLLA

However, although the methods described above for NA modification can improve the nucleation effect, the lattice matching between the NA and the matrix polymer cannot be artificially modified because the crystal structure is an inherent property of the material. Therefore, most NA-modifying processes cannot improve the lattice matching for improvement of the nucleating effect. Hence, self-nucleation, which occurs by a homologous substance, is an ideal method. Because the NA and matrix polymer have identical chemical composition and crystal lattices, optimal lattice matching and interfacial interaction can be achieved.19−22 Ultimately, selfnucleation is the best strategy to obtain the greatest nucleation effect. However, current self-nucleation methods have not been shown to have any significant effect on crystallization in practical applications and are very difficult and uneconomical. This is because self-nucleation ideally requires that the crystallites or nuclei of the NA not be melted during melting of the matrix polymer during thermal processing. However, the processing temperature window that meets this requirement is very narrow due to the similar thermal properties of the NA and matrix polymer. Thus, self-nucleation methods have been difficult to apply to industry as they are easily affected by the external environment. Ming-Bo Yang et al. reported that when PLLA crystallites (hPLLA) with a relatively high melting point (187 °C) were added as a NA to a PLLA matrix (lPLLA) with a low melting point (169 °C), the crystallization rate of the PLLA material was accelerated by the difference in the melting temperature (Tm) between the PLLA resins.23,24 Despite the novelty of this approach, it is not easily applicable in industrial processing, as the small difference in the Tm between the matrix and the NA (18 °C) can result in damage of the properties of the NA during processing and handling. A conventional strategy for NA addition, including that for selfnucleation, should be composed of a two-step process of matrix polymerization and physical blending with the NA. This can be complex, costly, and time-consuming, requiring a suitable thermal or solvent process for blending of the two different materials. Therefore, this study focused on the development of the novel in ISN polymerization of L-lactide to maximize the potential of self-nucleation as an ideal strategy. This novel strategy resulted in simultaneous polymerization of L-lactide and self-nucleation of crystallites by the addition of self-NA. In this reaction, stereocomplex PLA (sc-PLA) nanoparticles, which were composed of L (levorotatory) and D (dextrorotatory) enantiomers, were able to induce selfnucleation as a homologous NA of PLLA. In a previous study, our team developed a novel strategy utilizing supercritical fluid technology for the production of sc-PLA nanoparticles.25,26 This strategy is an economic, efficient, and feasible method for polymerization and stereocomplexation of polylactides derived from monomers and produced a sc-PLA product with high mechanical and thermal tolerance. sc-PLA particles facilitate thermal processing due to their melting point (>230 °C) being higher than that of the homopolymer (PLLA and PDLA), preventing damage to the physical properties of the particle during the repeated heat process. Therefore, in situ selfnucleated (ISN-)PLA, which is produced by ISN polymerization, could have an accelerating effect on the crystallization induced by sc-PLA acting as the NA and can result in a product with excellent mechanical properties despite the harsh thermal process used. Thus, ISN polymerization is a novel method that could save significant time and cost due to the one-step process of polymerization and self-nucleation. There-

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EXPERIMENTAL SECTION

Materials. ISN-PLA was synthesized by ring-opening polymerization (ROP) of L-lactide monomers (Medichem Co., Korea) and scPLA particles in a one-pot process, termed ISN polymerization. PLLA was also synthesized by normal ROP polymerization using the same protocol without the addition of sc-PLA particles. Stannous octoate [Sn(Oct)2] and 1-dodecanol were utilized as a catalyst and an initiator with concentrations of 2 × 10−4 and 2 × 10−4 mol per mol of Llactide, respectively. The ratio of the monomer, initiator, and catalyst (M:I:C) was 1250:1:1. ISN-PLA was polymerized at 140 °C for 30 h in a glass ampule under vacuum. The product was purified by precipitation in methanol and vacuum-dried for 24 h. The molecular weights (M̅ n and M̅ w) of the polymerized PLLA and ISN-PLA were measured by gel permeation chromatography (GPC). Stereocomplex Formation. sc-PLA particles were fabricated by the continuous supercritical fluid (SCF) process.25 The reactor for stereocomplexation was cooled to below room temperature (RT) in a vacuum to provoke vigorous transfer of the PLLA and PDLA from the polymerization units. The unit for stereocomplexation was then filled with additional dimethyl ether (DME) to reach a pressure of 5 MPa at RT and allow for formation of sc-PLA particles. After the reaction, the unit was chilled to evaporate the DME by opening the reactor. Scanning Electron Microscopy (SEM) Analysis. The surface morphology of ISN-PLA and PLLA films was observed by SEM (HITACHI S-4200, Japan) with an acceleration voltage of 15 kV. The samples were sputter-coated with Pt with the current set at 15 mA for 60 s. Fourier Transform Infrared (FTIR). FTIR spectra were recorded with a NicoletTM iS10 (Thermo Fisher Scientific Inc., USA) with a smart iTR accessory. The samples were measured in the 600−4000 cm−1 range with a 4 cm−1 resolution. Dynamic Light Scattering (DLS). sc-PLA particles were suspended in isopropanol (0.3 mg/mL), and the size and polydispersity index (PDI) of the specimens were measured by DLS (ELS-Z, Otsuka Electronics, Japan) and defined as (width/ mean)2. Nonisothermal and Isothermal Crystallization. Specimens of around 5 mg were first heated to 200 °C at a rate of 100 °C/min and held at 200 °C for 3 min to reach a stable temperature and then cooled to 30 °C at a rate of 2.5 °C/min. The samples were then reheated to 200 °C at a rate of 10 °C/min to analyze the nonisothermal crystallization behavior and melting behavior. For isothermal crystallization, specimens of around 5 mg were first heated at 200 °C for 3 min and subsequently cooled to the corresponding isothermal crystallization temperatures of 100, 110, 120, 130, and 140 °C at a rate of 40 °C/min and held until completion of the crystallization. In addition, the half-crystallization time (t0.5) was calculated to determine the overall crystallization kinetics. This value, which is a key parameter for characterizing the accelerating effect for crystallization, was gained from a graph of the relative degree of crystallinity [Xc(t)] at Xc(t) = 50%. For this experiment, Xc was defined by the following equation:27 X(t ) =

∫0

t

ij dH yz jj zz dt / k dt {

∫0

∞ i dH y

jj zz dt = ΔH /ΔH j z ∞ t k dt {

(1)

Nucleation efficiency (NE): To evaluate the capacity of sc-PLA particles as self-nucleating agents, the NE was estimated by cooling a specimen from the melt. The NE was calculated using the following equation, originally designed by Wittmann et al.:19,20

NE (%) = Tc − Tcmin /Tcmax − Tcmin × 100 B

(2)

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Figure 1. (a) Schematic of the process of in situ self-nucleating (ISN) polymerization with L-lactide and sc-PLA (top) and the distribution of scPLA particles in the PLLA matrix (bottom). (b) Schematic illustration of continuous SCF technology. V1, V2, V3, and V4 represent pressure valves 1, 2, 3, and 4. T1, T2, and T3 represent temperature controllers 1, 2, and 3. P and S mean pressure gauge and magnetic stirrer, respectively. (c) Image of sc-PLA powder produced by the SCF process (left) and comparison of melting peak between PLLA and sc-PLA (right). where Tmin is the crystallization temperature of the neat polymer and c is the maximum crystallization temperature of the self-nucleated Tmax c polymer. Wide-Angle X-ray Diffraction Analysis (WAXD). WAXD analysis was conducted with a Rigaku D/MAX-2500/PC X-ray diffractometer (Rigaku Co., Japan) at room temperature. The WAXD data were collected from 5° to 35°. Differential Scanning Calorimetry (DSC). DSC analysis was performed on a Q10 DSC (TA Instruments, USA) using nitrogen as a purge gas. The weight of the specimens was around 10 mg. The software provided by the manufacturer was used for analyzing the resulting thermograms. The equipment was periodically calibrated with indium standards. The Xc after the DSC measurements was calculated according to the equation

Xc (%) = ΔHm/ΔH ° × 100%

image of the sc-PLA powder produced by the SCF process and DSC thermograms of PLLA and sc-PLA. The graph indicates that the sc-PLA particles have a significantly higher melting point than PLLA. We confirmed that ISN-PLA was successfully polymerized by the ISN method and had the same morphology as the normal PLLA product (Figure 2a, left). To compare the difference between ISN-PLA and PLLA in detail, the surfaces of two products prepared as a solvent-cast film were characterized by SEM (Figure 2a, right). As shown in the SEM image, the surface of the ISN-PLA film was bumpy while the PLLA film had a smooth surface. It was assumed that the sc-PLA affected the crystal arrangement in ISN-PLA during the solvent-casting procedure of the polymer-dissolved solution for making the film. The molecular weight and PDI of ISN-PLA and PLLA were also measured, as shown in Table 1. ISN-PLA had slightly higher molecular weight (M̅ n and M̅ w) values than PLLA. The weight percent (wt %) of the added NA did not appear to affect the molecular weight of the specimens, as ISNPLA with a high wt % (10 wt %) of sc-PLA was also polymerized. Figure 2b shows FTIR spectra of ISN-PLA, scPLA, PDLA, and PLLA. The peaks at 2890, 2946, and 2997 cm −1 were assigned to Cα-H stretching modes, CH 3 symmetric stretching, and CH3 asymmetric stretching, respectively. The peaks between 1000 and 1300 cm−1 represent absorption bands of the ester bond, and the peak at 1040 cm−1 represents the −C−O− stretching vibration. The intensity of the peak corresponding to the −C−O−C− stretch of sc-PLA was significantly stronger than that for ISN-PLA and about twice as strong as that for the homopolymers due to the L and D forms. The FTIR spectra did not differ between the homopolymers and ISN-PLA, indicating that they had the same molecular structure and composition. 1H NMR spectroscopy showed that ISN-PLA had the same peaks as PLLA, confirming that they had the same number and type of functional groups. Thus, the product of ISN-PLA ISN

(3)

where ΔHm is the enthalpy of melting and ΔH° (the theoretical melting enthalpy of 100% crystalline PLA) = 93.1 J/g. Measurement of the Mechanical Properties. ISN-PLA and PLLA films were prepared using a solvent-casting method. ISN-PLA and PLLA were dissolved with a polymer concentration per CHCl3 of 60 mg/mL, and the solution was uniformly poured onto a glass plate. It was dried for about 120 h under vacuum at RT. The mechanical properties of the films were measured with a 5966 universal testing machine (Instron Co., USA) using a tensile extension speed of 10 mm/min and a length of 30 mm between the benchmarks of the specimens.

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RESULTS AND DISCUSSION Novel Method for ISN Polymerization. Figure 1a shows a schematic of the ISN polymerization of L-lactide. As shown in the figure, PLA was polymerized by ring-opening upon the addition of the L-lactide monomer and sc-PLA nanoparticles as a self-nucleating agent to the reactor. Figure 1b shows a schematic of the SCF process, which produced sc-PLA particles as the self-nucleating agent. The self-nucleating scPLA was composed of L and D enantiomers to induce rapid nucleation and regular arrangement of the crystal for ISN polymerization of the L-lactide monomer. Figure 1c shows the C

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Figure 2. (a) Images of polymerized products for ISN-PLA and PLLA (left) and SEM images of ISN-PLA and PLLA surfaces (right). (b) Graphical analysis of ISN-PLA, sc-PLA, PDLA, and PLLA by FTIR. (c) XRD spectra of PLLA, ISN-PLA 0.1 wt %, and 4 wt % (left) and 1H NMR spectra of ISN-PLA and PLLA (right).

Table 1. Molecular Weights of the Polymerized Products for PLLA and ISN-PLA at Various Conditions specimen

sc-PLA (size/PDI)

M:I:C

sc-PLA (wt %)

Mn (g mol−1)

Mw (g mol−1)

PDI

PLLA ISN-PLA

• 700 nm/0.5

1250:1:1 1250:1:1

0 0.001 0.1 0.5 2 4 10

84 000 99 000 96 000 117 000 99 000 96 000 118 000

145 000 156 000 164 000 179 000 174 000 184 000 167 000

1.737 1.577 1.724 1.525 1.760 1.728 1.414

D

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PLA on ISN-PLA was found to increase with decreasing NA particle size. Following polymerization of a biodegradable polymer, thermal processing is required for manufacturing the final product for industrial applications. This could result in damage to the molecular structure, such as the crystallinity, and subsequently affect the physical properties of the material. This could restrict application of the biodegradable polymer due to loss of its desirable mechanical and thermal properties. To determine the industrial applicability of ISN-PLA as a novel material, we measured DSC thermograms of ISN-PLA for repeated thermal and cooling cycles, with heating up to 200, 220, 230, and 250 °C and cooling down. As shown in Figure 4a, for four cycles at 200 °C, the melting peak attributable to

polymerized with sc-PLA nanoparticles had the same composition as the homopolymers. The crystal growth for PLLA and the ISN-PLA product with 0.1 and 4 wt % sc-PLA was measured by XRD (Figure 2c). ISN-PLA with 0.1 wt % had larger diffraction reflections at 2θ = 16.7° and 19.08°, indicating growth of the α-crystal rather than the PLLA group. Upon increasing the sc-PLA content to 4 wt % to induce growth of the α-crystal, the intensity of the scattering reflections at 2θ = 12.4° and 20.6° corresponding to sccrystallites continuously increased. Effect of sc-PLA Nanoparticles on the Crystallization of ISN-PLA as a Self-Nucleating Agent. The size of the scPLA particles added for ISN polymerization as a self-NA could affect the distribution and surface interaction between the PLLA matrix and the NA. Thus, we analyzed whether the size of the sc-PLA particles affected the crystallization of ISN-PLA. sc-PLA particles with various size were produced by continuous SCF processing. As shown in Figure 3a, sc-PLA

Figure 4. (a) Comparison of thermograms for PLLA and ISN-PLA at 4 wt % for the fourth cycle of repetitive thermal processing. (b) Identification of sc-PLA used as a nucleating agent in ISN-PLA at 0.5, 1, 2, 4, and 10 wt %.

Figure 3. Effect of the size of sc-PLA particles on crystallization of ISN-PLA. (a) DLS histograms of the sc-PLA particles made by the continuous SCF technique with various sizes. (b) Enthalpy of Tcc and Tc-H2 of sc-PLA particles with sizes of 700, 1400, 2300, and 3900 nm.

the PLLA group was divided into two peaks, indicating breakage of the crystal in the polymer. In contrast, no splitting of the melting peak of ISN-PLA with 4 wt % sc-PLA was observed, despite also being heated at 200 °C for four cycles. This indicated that cold crystallization was induced by the selfNA upon cooling. To illustrate the difference between the ISNPLA and PLLA groups, the magnified figure in Figure 4a shows that the melting peak of PLLA was divided into two peaks due to breaking of the well-ordered crystal and a decrease in the crystallinity that occurred during the third heating cycle at 230 °C. The crystallinity of PLLA was completely destroyed during the fourth heating cycle at 250 °C due to thermal decomposition of PLLA. In contrast, ISN-PLA did not show any significant reduction of the melting peak for the fourth cycle at 230 °C, indicating retention of the well-ordered crystal. In fact, the melting peak of ISN-PLA for the fourth cycle showed less damage than did that of PLLA for the third

particles with sizes of 700, 1400, 2300, and 3900 nm were produced. We also investigated the dependence of the enthalpy of crystallization temperature on cooling (Tcc) and crystallization temperature on reheating (Tc-H2) on the size of the particles added to ISN-PLA. Figure 3b shows that crystallization upon cooling was observed for PLLA, whereas ISNPLA showed an increase in the enthalpy of Tcc as the particle size was reduced. In contrast, the enthalpy of Tc-H2 showed incomplete crystallization on reheating for the second heating process as the particle size was reduced, and the ISN-PLA group with the smallest particle size of 700 nm did not show any Tc-H2 peak due to complete crystallization induced by the self-NA upon cooling. As a result, the nucleating effect of scE

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Macromolecules cycle at 250 °C. Thus, ISN-PLA was able to retain its original properties, including the amount and type of polymer crystal, for heating temperatures of 200, 220, 230, and 250 °C over four heating cycles. This indicated that the final product composed of ISN-PLA can retain its thermal properties during the harsh heating process; thus, it has great potential as a material for industrial polymers. In addition, comparison of the thermal behavior for two heating cycles of the ISN-PLA and PLA composites prepared by the normal solvent-blending method, which involved physically blending PLLA with sc-PLA particles, revealed significant differences in the Tcc, Tc-H2, and Tm peaks (Figure 5).

Tg point of all groups decreased slightly after the second cycle for heating and cooling. All ISN-PLA groups had a higher Tm than PLLA before and after processing, and no differences in the cooling rate were observed (Figure S1b). Furthermore, the Tm point of PLLA was slightly decreased upon loss of the crystal structure, whereas the Tm of ISN-PLA was not damaged after the second cycle for heating and cooling. On the first cycle for heating and cooling, the enthalpy of Tm for ISN-PLA at all sc-PLA wt % was slightly higher than for PLLA, although the difference was not significant (Figure S1c). PLLA also exhibited a significant decrease in the enthalpy of Tm as well as a loss of the molecular chain and crystal structure induced by the thermal processing following melt processing. This was due to incomplete crystallization of the PLLA during the fast cooling process, due to its slow crystallization rate. In contrast, ISN-PLA at all wt % underwent extensive crystallization induced by the nucleating effect of sc-PLA, despite rapid cooling. The high crystallinity of ISN-PLA was also retained even for cooling rates of 15, 20, and 40 °C/min. These results showed that ISN-PLA has the potential to overcome the current limitation of the slow crystallization of PLLA that causes a loss of the crystallinity after thermal processing and thus could be a promising solution to prevent the loss of the thermal properties of PLA. The secondary crystallization on reheating (Tc-H2) of the product was also analyzed to investigate how the high degree of crystallinity in ISN-PLA was maintained during the rapid cooling after melt processing. As shown in Figure S1d, ISN-PLA exhibited crystallization on cooling for all cooling rate conditions, whereas PLLA did not exhibit any Tcc. The Tcc point of all ISN-PLA samples increased as the cooling rate decreased from 40 to 5 °C/min and increased as the wt % of sc-PLA was increased, with ISN-PLA with a wt % of 4 giving the highest Tcc point. A higher Tcc point indicates more rapid crystallization during the cooling process. Even ISN-PLA with an extremely small sc-PLA wt % of 0.001 exhibited cold crystallization with a rapid cooling rate, induced by the nucleating effect of sc-PLA, while PLLA did not shown any cold crystallization for all conditions and all cooling rates, due to its slow crystallization rate. The enthalpy of Tcc increased as the wt % of sc-PLA increased from 0.001 to 4 but decreased for a wt % of 10. Thus, the nucleating effect of sc-PLA in ISN-PLA was optimized at 4 wt %. Furthermore, with a slower cooling rate, ISN-PLA showed increased cold crystallization due to the additional time available for crystallization. Interestingly, ISNPLA with a sc-PLA wt % of 1 and 4 had a Tcc sufficient for crystallization, despite the fast cooling rate. This showed that the novel ISN-PLA could effectively promote slow and extensive crystallization even during rapid cooling, which is a current limitation for PLLA. The Tc-H2, which refers to growth of the α′-crystal, was also investigated to determine how ISNPLA could retain its high crystallinity after melt-processing and exhibit good crystallization during rapid cooling, unlike PLLA. Figure S1e shows that all ISN-PLA samples had a Tc-H2 peak around 160 °C, which is near the Tm, in contrast to PLLA. The enthalpy of the Tc-H2 peak increased as the sc-PLA wt % and cooling rate increased, indicating that the α′-crystal, which is a disordered type of α-crystal, was present in ISN-PLA for fast cooling, in comparison with PLLA, for which growth of the α′-crystal did not occur.28−30 This result shows that ISN-PLA could maintain its high crystallinity even after the continuous melting and cooling processes, unlike PLLA. For ISN-PLAs with a sc-PLA wt % above 0.1, growth of the α′-crystal did not

Figure 5. DSC thermograms of PLA composite and ISN-PLA with scPLA of 2 wt %.

Although both the ISN-PLA and PLA composites were composed of a PLLA matrix mixed with sc-PLA particles, ISNPLA exhibited a larger Tcc peak than did the PLA composite during the cooling process. Additionally, ISN-PLA did not show crystallization on reheating (Tc-H2) for the second heating cycle, unlike the PLA composite, and ISN-PLA had a higher enthalpy of Tm than the PLA composite for all cycles. As shown in Figure 4b, we increased the sc-PLA content in the PLLA matrix to 0.5, 1, 2, 4, and 10 wt % to determine the amount at which the self-NA was detectable in the PLLA matrix. Consequently, the DSC thermogram showed an increasingly larger sc-PLA melting peak as the wt % was increased from 2 to 10. As the wt % of sc-PLA was increased from 2 to 10, the melting peak became more prominent. No melting peak of sc-PLA was observed with the wt % of the selfNA below 1, and it could only be identified in the thermograms at a wt % higher than 2. Thermal processing, composed of consecutive heating and cooling cycles, is essential for manufacturing the industrial product. Thus, a rapid cooling process to reduce the fabrication time is essential for achieving high productivity. However, the biodegradable PLA commonly used in various fields is limited by low productivity, as the slow rate of crystallization precludes rapid cooling. Therefore, enhancing the rate of crystallization to allow for fast cooling is imperative to improve the productivity and applicability of the PLA material. To achieve this goal, the thermal properties of ISN-PLAs processed with various cooling rates of 5, 10, 15, 20, and 40 °C/min were analyzed before and after melt processing. In the first cycle for heating and cooling, the glass transition temperature (Tg) did not differ between all specimen groups with various cooling rates (Figure S1a). The Tg point of ISN-PLA with a cooling rate of 5 °C/min decreased by a lesser extent than it did for PLLA, though the F

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Figure 6. (a) Relative crystallinity from the DSC test as a function of time for samples isothermally crystallized at 100, 110, 120, 130, and 140 °C. (b) Half-crystallization time (t0.5) obtained from Xc(t) = 50% against the content of sc-PLA. (c) Nucleation efficiencies of sc-PLA as a self-NA on ISN-PLA.

occur at a relatively slow cooling rate of 5 °C/min. Additionally, ISN-PLAs with a sc-PLA wt % of 1 and 4 retained their high crystallinity without growth of the α′-crystal for a cooling rate below 10 °C/min. This is due to sufficient time for the induction of ordered crystallization at these conditions. In contrast, a fast cooling rate was able to induce growth of the α′-crystal over the ordered α-crystal in ISN-PLA because it did not allow for sufficient time for ordered crystallization to occur during cooling. Because of its slow crystallization, PLLA was not even able to undergo disordered crystallization of the α-crystal during cooling. Thus, sc-PLA particles in ISN-PLA acted as a nucleating agent regardless of the cooling rate and were able to promote well-ordered αcrystal growth during slow cooling and disordered α-crystal growth during fast cooling. We measured both the relative crystallinity [X(t)], which is the degree of crystallinity relative to the maximum achievable crystallinity, and the t0.5, which is the time to reach half the maximum crystallinity, to analyze the crystallization behavior with various wt % of sc-PLA and the change of the crystallization rate of ISN-PLA for isothermal crystallization at temperatures of 100, 110, 120, 130, and 140 °C. Figure 6a shows a plot of the crystallization kinetics of ISN-PLA with various wt % of sc-PLA heated at fixed temperatures of 100, 110, 120, 130, and 140 °C. The ISN-PLAs with 1 and 4 wt % sc-PLA showed the fastest crystallization rate for all temperature conditions, and ISN-PLA with 4 wt % sc-PLA exhibited the fastest crystallization for high temperatures of 130 and 140 °C. At temperatures of 100 and 110 °C, at which crystallization

of PLA generally occurs, PLLA did not undergo any crystallization until 10 min, while ISN-PLA with 4 wt % scPLA took just 1.5 min to reach maximum crystallinity. All ISNPLAs with various wt % of sc-PLA achieved a X(t) of 100% within 30 min of heating at 140 °C, even though the temperature can disrupt crystallization as it is close to the Tm of PLA. Only the ISN-PLA sample with 4 wt % sc-PLA exhibited a constant slope of the crystallization rate until 100%, whereas the other groups showed a change in the slope of the rate, induced by crystal damage. As shown in Figure 6b, the t0.5 value for ISN-PLA gradually decreased with increasing wt % of sc-PLA for all temperatures (100, 110, 120, 130, and 140 °C). Moreover, ISN-PLA with 4 wt % sc-PLA had the shortest t0.5 time for all heating temperatures. The time required for Xc(t) to reach 50% increased as the heating temperature was increased. For example, ISN-PLA with 0.01 wt % sc-PLA had a t0.5 value of 1.95 min at 100 °C, but this value steadily increased to 1.95, 4.43, 6.81, and 16.49 min for heating temperatures of 110, 120, 130, and 140 °C, respectively. ISNPLA with 4 wt % sc-PLA had the shortest t0.5 value at all heating temperatures. ISN-PLA with 4 wt % sc-PLA took only 0.5 min to reach half-crystallinity at 100 °C, and ISN-PLA with an extremely small sc-PLA wt % of 0.001 took only 2.34 min to reach half-crystallinity. This result showed that ISN-PLA with 4 wt % sc-PLA experienced the highest accelerating effect of crystallization among all ISN-PLA samples with various wt % of sc-PLA, which agrees with the above results. To quantify the nucleating effect, the NE parameter was calculated as shown in Figure 6c. Consequently, the NE value was almost proporG

DOI: 10.1021/acs.macromol.8b01471 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules tional to the sc-PLA content for a wt % between 0 and 4, but above 10 wt %, the NE decreased slightly. ISN-PLA with 4 wt % sc-PLA had the highest NE value (100%), and ISN-PLA with only 0.001 wt % sc-PLA had a value of 41%. However, the NE for ISN-PLA with 10 wt % decreased because the high scPLA content did not have an accelerating effect but actually interrupted crystallization in ISN-PLA. As shown in the Supporting Information, the time to reach the crystallization point of ISN-PLA decreased with increasing wt % of sc-PLA for all temperatures, while PLLA did not show a crystallization peak until 30 min (Figure S2a). All ISN-PLA groups exhibited a crystallization peak within 35 min at temperatures of 130 and 140 °C, which are considerably higher than the normal crystallization temperature of PLA, and ISN-PLA with 4 wt % sc-PLA exhibited the fastest crystallization and sharpest crystallization peak at the same conditions. As shown in Figure S2b, the melting peak of PLLA product was significantly diminished due to insufficient crystallization during the melting and cooling process. In contrast, ISN-PLAs for all sc-PLA wt % did not show any reduction in the Tm, which was over 10 °C higher than that for PLLA after melt processing. The melting point of ISN-PLA gradually increased, and the area of the Tm of ISN-PLA with 1 and 4 wt % sc-PLA was the largest. In addition, all ISN-PLA samples with various wt % of sc-PLA exhibited a Tcc peak between 115 and 130 °C, while PLLA did not shown any Tcc peak for cooling (Figure S2c). The enthalpy of Tcc for ISN-PLA increased with increasing scPLA wt %, and ISN-PLA with 4 wt % sc-PLA had the highest enthalpy and Tcc point, in accordance with the preceding figures. As a result, sc-PLA particles added to ISN-PLA at 4 wt % were proficient in encouraging crystallization as the best selfNA. To investigate the mechanical properties of ISN-PLA with a self-NA, films of ISN-PLA with various wt % of sc-PLA were measured by a UTM before and after thermal annealing. As shown in Figure 7a, the value of the tensile strength did not

show any significant difference between PLLA and ISN-PLAs with 0.001, 0.1, 0.5, 1, and 4 wt % sc-PLA for nonthermal annealing, with ISN-PLAs having the smaller value. However, the tensile strength of all ISN-PLAs improved considerably after thermal annealing, unlike that for thermally annealed PLLA. The value of the Young’s modulus also did not show any considerable difference between PLLA and all ISN-PLAs before thermal annealing and greatly increased for ISN-PLAs after thermal annealing (Figure 7b). In contrast, the elongation of all specimens markedly decreased upon thermal annealing (Figure 7c). The S−S curve shows that annealed ISN-PLA had significantly increased tensile strength due to the nucleating effect of the self-NA in ISN-PLA after thermal annealing, in contrast with annealed PLLA (Figure 7d). That is, nonannealed ISN-PLA films made by the solvent-casting method did not have a higher tensile strength than PLLA due to the absence of stimulation by heating. In contrast, annealed ISNPLA had a dramatically increased tensile strength due to acceleration of crystallization by the nucleating effect of sc-PLA after thermal processing. These results showed the potential for overcoming the current limitation for many industrial products based on biodegradable polymers, namely, the significant loss of mechanical strength after thermal processing.

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CONCLUSIONS

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ASSOCIATED CONTENT

In conclusion, this study developed a novel method for the in situ simultaneous one-pot polymerization and self-nucleation of L-lactide. The novel ISN-PLA material had a wide temperature window for thermal processing and a stable nucleating effect even at high temperature (250 °C) and during repetitive heat cycling compared with other NAs because it utilized sc-PLA, which has a high thermal stability, as a self-NA for the PLLA matrix. This advantage allowed for the original crystallinity and mechanical strength of the material to be retained even after thermal processing at high temperatures, which is essential in manufacturing the final product for various industries. This allows for the application of polylactide, which is normally limited by its inferior mechanical properties despite being highly biocompatible and biodegradable, to various fields based on biodegradable polymers, such as implantable devices, drug delivery, and ecofriendly products. Additionally, this study has shown that the novel ISN polymerization method is simple to perform, economical, and effective for self-nucleation when compared to normal PLA composites blended with sc-PLA and other current methods. Our further study will focus on developing novel functional materials that can be loaded with substances such as cancer drugs and markers in the gap between enantiomeric L- and D-forms of polylactide chains in sc-PLA.

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01471. Various thermal properties for PLLA and ISN-PLA (Figure S1); DSC heat flow, melting behaviors, and crystallization on cooling of PLLA and ISN-PLA (Figure S2); list of thermal properties for PLLA and ISN-PLA (Table S1) (PDF)

Figure 7. Comparison of the mechanical properties between nonannealed and thermally annealed specimens on condition of 120 °C for 2 h. (a) Tensile strength, (b) Young’s modulus, (c) elongation, and (d) strain−stress curves of PLLA and ISN-PLA films with sc-PLA at 0.001, 0.1, 0.5, 1, and 4 wt %. H

DOI: 10.1021/acs.macromol.8b01471 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

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AUTHOR INFORMATION

Corresponding Author

*(S.H.K.) E-mail [email protected], Tel +82-2-958-5343. ORCID

Soo Hyun Kim: 0000-0002-8391-3686 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Korea Institute of Science & Technology (KIST) project (2E27930) and the KU-KIST Graduate School of Converging Science and Technology Program.

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DOI: 10.1021/acs.macromol.8b01471 Macromolecules XXXX, XXX, XXX−XXX