Effects of Polyoxymethylene as a Polymeric Nucleating Agent on the

Article pubs.acs.org/IECR

Effects of Polyoxymethylene as a Polymeric Nucleating Agent on the Isothermal Crystallization and Visible Transmittance of Poly(lactic acid) Xiaojie Guo,† Hongzhi Liu,† Jinwen Zhang,*,† and Jijun Huang*,‡ †

School of Mechanical and Materials Engineering, Materials Science and Engineering Program, Composite Materials and Engineering Center, Washington State University, P.O. BOX 642920, Pullman, Washington 99164, United States ‡ College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing, 100049, People’s Republic of China S Supporting Information *

ABSTRACT: In this work, use of polyoxymethylene (POM) as a polymeric nucleating agent for poly(lactic acid) (PLA) was studied. The compounding was performed using a twin-screw extruder. Effects of POM on isothermal crystallization of PLA at temperatures ranging from 106 °C to 111 °C and visible transmittance of PLA were examined in detail with various techniques, including polarized optical microscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), and ultraviolet−visible (UV-vis) spectroscopy. Banded spherulites were noted when the POM content exceeded 5 wt %. The presence of POM generally resulted in effective reductions in the half time of crystallization and the spherulite size of PLA. However, 1% POM was a threshold for the nucleating effect: at ≤1% POM, no nucleating effect was observed. Fourier transform infrared (FTIR) spectroscopy revealed the strong interaction between the two polymers. The Avrami modeling suggested a three-dimensional crystal growth at all temperatures and loading levels of POM. Although POM does not accelerate the crystallization of PLA as fast as some of the inorganic nucleating agents such as talc, it imparts PLA the nucleating effect without sacrificing transparency.

1. INTRODUCTION Poly(lactic acid) (PLA) is a promising renewable alternative to traditional petroleum-based polymers such as polyethylene, polypropylene, and polystyrene,1 and it is finding applications in biomedical devices and packaging.2 However, two major drawbacks of PLA have severely limited its broad applications: embrittlement and slow crystallization. Over the years, PLA toughening has received intensive investigation and significant progress has been achieved, which was mainly focused on blending of PLA with flexible polymer or elastomer.3−11 The slow crystallization of PLA also limits its applications, because the mechanical and thermal properties of PLA are largely determined by its crystallization behavior. Enhanced crystallinity of polymer usually lead to greater heat resistance, as well as higher modulus and strength. Many substances, such as talc, sodium stearate, calcium lactate,12 zinc citrate complex nanoparticles,13 and montmorillonite clay,14 etc., have been attempted as nucleating agents for PLA. Generally, these nucleating agents exhibit very limited accelerating effects on the crystallization of PLA but bring the issues of opacity and incompatibility. To avoid these problems, introduction of a compatible polymeric nucleating agent is more attractive, compared with the use of inorganic and organic nucleating agents. PLA is an attractive plastic material for food packaging. High transparency is required for some of these packaging applications. Transparency requires minimization of the scattering of light. In principle, any factor causing the scattering of light influences the transparency of materials to some extent. For polymer blends, numerous factors, including roughness, the © 2014 American Chemical Society

volume fraction of the dispersed phase, an optical path length (which is related to the shape of the domains) and the wavelength of light, the size of the dispersed phase, and the refractive indexes of the dispersed phase and matrix, affect light scattering and, subsequently, transparency. Among these factors, the differences in refractive index between the dispersed phase and the matrix, the size of the dispersed phase, and the shapes of the domains are more influential.15,16 Transparency is maintained or improved through matching the refractive index between the components, minimizing the crystal size and uniformly dispersion of nucleating agents (NAs).17,18 Poly(butylene adipate-co-terephthalate) and poly(butylene succinate) were reported to improve the crystallization of PLA, but the improvement was minor compared with the results from using the inorganic/organic counterparts.19,20 We hypothesize that incorporation of a small amount of a polymer with a similar refractive index, as a nucleating agent, is likely to result in a transparent PLA with enhanced crystallization. With this in mind, we chose polyoxymethylene (POM) as a polymeric nucleating agent for PLA, because it has a similar refractive index to that of PLA. In addition, its melting point of 175 °C is close to the typical processing temperature of PLA. For comparison, talc was also employed to nucleate PLA in several cases. The objective of this work is to gain insights into the effects of POM as a polymeric nucleating agent on the Received: Revised: Accepted: Published: 16754

May 22, October October October

2014 1, 2014 2, 2014 2, 2014

dx.doi.org/10.1021/ie502104y | Ind. Eng. Chem. Res. 2014, 53, 16754−16762

Industrial & Engineering Chemistry Research

Article

Model TMS 94). The sample centered between two coverslips was first heated to 200 °C at 50 °C/min and held at 200 °C for 3 min to obtain a thin melt film. Subsequently, the sample was quenched to the predetermined crystallization temperature (130 °C) and then held at 130 °C for observation of the crystal growth and morphology. The crystal growth rate, G (μm/min), is calculated using the following equation:

crystallization and transparency of PLA. Here, the detailed spherulitic development of PLA as a function of time and the concentration of POM and talc is revealed by polarized optical microscopy. In addition, we describe the isothermal crystallization behavior of PLA at a temperature range of 106−111 °C using differential scanning calorimetry (DSC), and we analyze the kinetics of crystallization with the Avrami model. Furthermore, the influences of POM and talc on the transparency of the nucleated PLA are compared through both optical imaging and UV-vis transmittance spectra. Finally, the miscibility of PLA with POM is briefly discussed on the basis of the results of Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). To the best of our knowledge, use of POM as a polymeric nucleating agent for PLA is novel and creative; and this work is based on the original research initiated in Xiaojie Guo’s 2012 Master’s Thesis at Washington State University.21 Coincidently, we noted that another group recently also reported a similar nucleation study of POM on PLA.22

G=

ΔR Δt

(1)

where ΔR is the increase of spherulitic radius and Δt is the time interval. Differential Scanning Calorimetry (DSC). The DSC measurement was performed with a differential scanning calorimeter (Mettler−Toledo, Model DSC 822e). Approximately 5 mg of the sample was crimp-sealed in a 40-μL aluminum pan, and each sample was tested in duplicate. To determine the thermal properties, the sample was first heated to 200 °C at 10 °C/min and then held isothermally for 5 min to erase the thermal history. Next, the sample was cooled to 0 °C at 10 °C/min, and was finally heated to 200 °C at the same rate. To investigate the isothermal crystallization, the sample was first heated to 200 °C at 10 °C/min and held isothermally for 5 min to erase the thermal history. The samples were then cooled from melt (200 °C) to the predetermined crystallization temperatures, i.e., 106, 107, 108, 109, and 111 °C, at a cooling rate of 80 °C/min and held at the crystallization temperatures (106, 107, 108, 109, and 111 °C) isothermally for either 15 or 30 min. Dynamic Mechanical Analysis (DMA). Dynamic mechanical properties of the materials were examined with a DMA Q800 machine (TA Instruments) under a single-cantilever mode at a frequency of 1 Hz. DMA specimens with a size of ∼13 mm × 3 mm × 35 mm were machined from injection-molded bars. A temperature sweep was applied from −100 °C to 150 °C at a rate of 3 °C/min. Ultraviolet−Visible (UV-vis) Spectrometry. Optical transmittance of the sample was measured using a UV-vis spectrophotometer (Perkin−Elmer, Model Lambda 25) with the wavelength ranging from 400 nm to 700 nm. The transmittance spectrum was collected using air as a reference. The thickness of all samples was controlled to be ∼100 μm. Wide-Angle X-ray Diffraction (WAXD). XRD spectra were obtained using an X-ray diffractometer (PANalytical, Model Empyrean). The Cu Kα radiation (λ = 0.15418 nm) source was operated at 40 kV and 40 mA. XRD patterns were recorded from 5° to 90° at a step of 8°/min. The sheets used for the XRD test were prepared from injection-molded bars by melting and then quickly quenching into a thickness of ∼100 μm.

2. EXPERIMENTAL SECTION 2.1. Materials. The PLA used in this study was NatureWorks PLA 2002D, and it has a specific gravity of 1.24 and a melt flow index of 5−7 g/10 min (201 °C, 2.16 kg). The POM was RTP 800 obtained from RTP Company, Winona, MN, USA. The specific gravity of the POM is 1.41. The talc (Magsil 3183A) was supplied by Richard Baker Harrison, Ltd., Trent, U.K., with a specific gravity of 2.75 and an average particle size of 7.6 μm. 2.2. Sample Preparation. Prior to extrusion, PLA, POM, and talc were dried in a convection oven at 85 °C for 24 h. A co-rotating twin-screw extruder (Leistritz ZSE-18), with a screw diameter of 17.8 mm and a screw L/D ratio of 40, was used to disperse POM in the PLA. The screw speed was set at 50 rpm for all runs, and the temperature profile of the barrel was set at 160, 160, 200, 200, 200, 170, 150, and 150 °C from the first heating zone to the die. The extrudate was cooled under water and subsequently pelletized. The samples were formulated with the weight ratios of PLA/POM at 99/1, 98/2, and 95/5. The compositions with 1, 2, and 5 wt % of POM are denoted hereafter as POM1, POM2, and POM5, respectively. Neat PLA, the PLA/talc composites (PLA/talc ratio = 99/1, 98/2, and 95/5, by weight) and PLA/POM blends (PLA/POM ratio = 90/10, 75/25, and 50/50, by weight) were prepared under the same conditions. Before injection molding, the compositions were dried at 85 °C for 1 day in a convection oven. Standard tensile (ASTM D638, Type I) specimens were prepared with an injection molding (Sumitomo SE 50D) machine at a melt temperature of 190 °C and a mold temperature of 40 °C. Prior to characterization, all molded specimens were conditioned at 23 °C and 50% humidity for 7 days. 2.3. Characterization. Fourier Transform Infrared Spectroscopy (FTIR). FTIR absorption spectra were collected using a Thermo Nicolet Nexus 670 spectrometer (Nicolet, Thermo Scientific), equipped with a Smart iTR device (Thermo Scientific) with a pointed pressure tip and a Ge crystal plate in a resolution of 4 cm−1, 32 scans. The specimens used were prepared by melting and then quickly quenching to room temperature. Polarized Optical Microscopy. Crystal growth and morphology were examined using a polarized optical microscope (Olympus, Model BX51) equipped with a hot stage (Linklam,

3. RESULTS AND DISCUSSION 3.1. Fourier Transform Infrared (FTIR) Analysis. FTIR spectroscopy was used to detect the miscibility of PLA with POM. Figure 1 compares FTIR spectra between neat PLA, neat POM, and the PLA nucleated by POM. The two strong absorption bands at 1089 and 1184 cm−1 were attributed to the C(O)−O−C stretching vibrations of PLA and also were noted in the PLA/POM blends.23 For neat POM, the major absorption bands at 903, 933, 1092, and 1238 cm−1 are due to the symmetric and asymmetric stretching vibrations of the C−O−C ether groups.24,25 The hump at ∼980 cm−1 for neat POM was further deconvoluted, revealing a characteristic peak at 960 cm−1 attributed to the C−O stretching vibration. The 16755



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shows the spherulitic morphology of all compositions after crystallization at 130 °C for 30 min. As expected, typical Maltese-cross spherulites were observed for neat PLA (Figure 2a). Compared with PLA, neat POM clearly showed larger spherulites. The introduction of POM moderately increased the number of spherulites and decreased the spherulitic dimension. When the POM content reached 5 wt %, well-defined banded spherulites were observed with alternative positive and negative birefringence along the radial and circumferential directions (Figure 2e). In contrast, talc appeared to be more effective on nucleation for the crystallization of PLA than POM as it resulted in more and smaller spherulites at the same addition levels (see Figure S1 in the Supporting Information). The similar nucleating effect by talc was reported by others; for example, the peak cold crystallization temperature shifted to a lower temperature and the crystallinity increased from 10.0% to 17.9% with 2 wt % talc.26 Figure 3 shows the morphological evolution of spherulites with time for neat PLA and POM5 at 130 °C. For neat PLA, no clear birefringence was detected within the first 9 min. With a further increase in time, the number of crystal nuclei and spherulitic dimension gradually increased, and some of the spherulites were about to impinge on each other. Compared with neat PLA, POM5 showed much faster spherulitic growth as the birefringence of all crystal nuclei appeared within the first minute. The bright and dark rings alternated to form the banded spherulites in POM5. These optical images suggest that (i) the nucleation was slow for neat PLA but fast for POM5, and (ii) three-dimensional (3D) crystal growth occurred to both. The crystal growth rate, G (μm/min), which is the slope of the line of the spherulite radius versus time, was computed to be 2.6 μm/min for POM 5 but 0.86 μm/min for neat PLA. It is clear that the addition of 5 wt % POM significantly increased the crystallization and also changed the crystallization morphology from typical spherulites to banded ones with alternative negative and positive birefringence along the radial direction.

Figure 1. FTIR spectra for neat POM, neat PLA, POM1, POM2, and POM5.

spectra of POM1 and POM2 were nearly identical to that of neat PLA, and did not exhibit the characteristic peaks of POM at 903, 933, and 1238 cm−1. However, the spectrum of POM5 showed two new absorption bands at 935 and 1001 cm−1, compared to that of neat PLA; the band at 935 cm−1 was originated from the C−O−C stretching vibration of POM at 933 cm−1 but shifted downward. The absorption band at 1001 cm−1 was a new peak that was originated from neither PLA nor POM. Based on the chemical structure of PLA and POM, the band at 1001 cm−1 might be ascribed to the formation of C− O−C between the carbonyl group of PLA and C−H of POM. This result suggests the existence of certain chemical interaction between PLA and POM. Thus, PLA and POM were approximated to be partially miscible, which was verified by DMA test in the following section. 3.2. Crystalline Morphology of the Nucleated PLA. The effects of POM and talc on the crystal morphology of PLA were studied using polarized optical microscopy. Figure 2

Figure 2. Polarized optical photomicrographs for (a) neat PLA, (b) neat POM, (c) POM1, (d) POM2, and (e) POM5, which crystallized isothermally at 130 °C for 30 min. The scale bar represents 20 μm. 16756



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Figure 3. Polarized optical photomicrographs showing crystal growth with time under isothermal crystallization at 130 °C: (a) neat PLA and (b) POM5. The scale bar represents 20 μm.

bands in spherulites. The imbalanced stress at lamellar fold surfaces is usually regarded as the driving force for the lamellar twisting to form the helical band-like morphology shown in Scheme 1.28 As a result, we hypothesize that the chiral effect of PLA is necessary for the lamellar twisting along the radial growth direction, which results in the banded spherulites. The chiral effect is enhanced by the introduction of the POM facilitating the formation of the banded spherulites. The band spacing was found to decrease with the increase of POM content. In addition, the helical twisting power (i.e., the inverse value of the helical pitch length of twisting bands) increased with the content of the POM. Thus, the formation of banded spherulites is enhanced by the POM acting as a diluent to cause more-imbalanced surface stresses;29 the surface stresses may be facilitated further by the specific interaction between PLA and

To further investigate the mechanism of banded spherulites, the crystal morphologies of all compositions comprising POM were measured at 130 °C and shown in Figure 4. The welldefined banded spherulites with characteristic Maltese-cross extinction patterns were observed, regardless of the POM content. The Maltese cross is known to emerge from a specific array of birefringent spherulites. Negative birefringence results from spherulites in which radial lamellae are dominant, while positive birefringence is due to spherulites with predominantly tangential lamellae. Negatively and positively birefringent spherulites are alternatively arranged to form the Maltese cross.27 It is generally believed that the formation of the banded spherulites is attributed to the lamellar twisting along the radial growth direction. The overall lamellar twisting may be induced by the formation of screw dislocations to generate extinction 16757



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Figure 4. Polarized optical photomicrographs after isothermal crystallization at 130 °C for 30 min for the PLA/POM blends with various POM contents: (a) 5%, (b) 10%, (c) 25%, and (d) 50%. The scale bar represents 20 μm.

result of the high cooling rate and interfacial reactions between PLA and POM during the molding process. Also, POM is known to show the greatest growth rate of spherulites at ∼80− 85 °C.31 Figure 5b shows the thermograms from the first cooling scan for the same compositions in Figure 5a. During the cooling process, the melt crystallization peaks were only observed for POM1, POM2, and POM5. The melt crystallization temperature shifted upward, from 96.7 °C to 110.7 °C, as the POM content increased from 1% to 5%, and the normalized enthalpy of the melt crystallization and hence the crystallinity increased as well. These may be attributed to the incorporation of POM as a polymeric nucleating agent. 3.4. Isothermal Crystallization Kinetics of the POM Nucleated PLA. Figure 6 shows the DSC thermograms as a function of time under isothermal crystallization at 108 °C. The crystallization peak time of neat PLA was 11.2 min. POM1 showed a broader peak at 14 min, presumably implying that the addition of 1% POM somehow hindered the crystallization of PLA. This was because POM at this concentration was completely dissolved in PLA, as discussed below. With further increases in POM content, the crystallization peak time shortened and the peak became narrower, indicating that the crystallization rate of PLA significantly increased. In terms of increasing the crystallization rate of PLA, POM is still inferior to some inorganic NAs. As shown in Figure S2 in the Supporting Information, the talc-nucleated PLA exhibited shorter peak crystallization times and sharper exothermic peaks, which was in agreement with that reported in the literature.25 Isothermal crystallization kinetics for neat PLA and the PLA nucleated by POM and talc were theoretically analyzed via the

Scheme 1. The Lamellar Geometry of Banded Spherulites

POM (recall Figure 1), which would induce different chain folding within the blends.30 3.3. Effects of POM on Crystallization Behaviors of PLA. Figure 5a shows the first heating curves of DSC thermograms for the PLA nucleated by POM, and the results are summarized in Table 1. Neat PLA exhibited a Tg of ∼55 °C and a cold crystallization peak at 110 °C. With the incorporation of POM in the compositions, the Tg of PLA was reduced to ∼53 °C, respectively. With the incorporation of POM, the cold crystallization peak of PLA also narrowed and shifted to lower temperatures. In Table 1, the difference between the melting enthalpy and cold crystallization enthalpy (ΔHm − ΔHc) for POM1, POM2, and POM5 was 1.99, 2.84, and 6.7 J/g, respectively, much greater than that of neat PLA (0.58 J/g), which further implies that the incorporation of POM improved the crystallization of PLA. Interestingly, two cold crystallization peaks were noted for the compositions containing POM. The major peak at ∼100 °C was assigned to the cold crystallization of PLA, while the minor peak at ∼80 °C might be attributed to the cold crystallization of POM, because of the incomplete and retarded crystallization of POM as a 16758



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Figure 6. Comparison of crystallization isotherms obtained at 108 °C as a function of time between neat PLA, POM1, POM2, and POM5. The curves were vertically shifted for legibility.

By plotting log{−ln[1 − X(t)]} vs log(t), n and K values may be directly obtained from the slope and the intercept, respectively, from the early linear segment. The half-time of crystallization, t1/2, is calculated from the following equation:

t1/2 =

Table 1. Parameters of Melting and Crystallization Derived from the DSC Curves for Neat PLA, POM, POM1, POM2, and POM5 First Cooling Scan

composition

Tg (°C)

Tcc,1 (°C)

Tcc,2 (°C)

*ΔHm − ΔHc (J/g)c

Tmc (°C)

*ΔHm,c (J/g)c

PLA POM1 POM2 POM5

56.5 53.4 53.3 52.7

n.a. 81.5 81.7 73.0

110.0 105.2 104.8 102.3

0.58 0.92 0.94 1.80

n.a. 96.7 107.5 110.7

n.a. 1.58 1.66 2.40

a

b

a

Tcc,1 is the cold crystallization temperature for POM. bTcc,2 is the cold crystallization temperature for PLA. cNormalized heat enthalpy (ΔH) is the total enthalpy excluded the portion of POM by weight fraction.

Avrami model,32−34 which is described by the following equation: 1 − X(t ) = exp( −Kt n)

(2)

where X(t) is the relative crystallinity, n is the Avrami exponent (which is dependent on the crystal geometry, the nucleation mode (simultaneous or sporadic), and rate determination mode (diffusion or contact)), and K is the isothermal crystallization rate constant. By taking a logarithmic transformation of eq 2, a new equation is obtained: log{−ln[1 − X(t )]} = n log(t ) + log(K )

(4)

Table 2 summarizes n and t1/2 values at various crystallization temperatures. The n value of neat PLA was ∼2.7−3.6, which was comparable to those which has been reported by other researchers. Kolstad et al. reported that the n value for pure PLLA was 2.5−3.3 at Tc = 90−130 °C,35 and Iannace and Nicolais reported an n value of 2.8−3.2 at Tc = 90−130 °C.36 Neat PLA showed a crystallization mechanism of spherical growth via diffusion from thermal (sporadic) nucleation. The PLA nucleated by POM had an n value of 3.1−3.9, implying a crystallization mechanism of spherical growth upon contact from thermal (sporadic) nucleation.37−39 As listed in Table S1 in the Supporting Information, the n values for talc1 and talc2 ranged from 2.9 to 4, also indicating the same crystallization mechanism. In addition, the n value of talc5 was ∼2.7, probably suggesting a thermal nucleation process, followed by mixed two-dimensional (2D) and three-dimensional (3D) crystal growth. Because of the coexistence of homogeneous and heterogeneous nucleation in the PLA matrix, the n value between neat PLA and PLA nucleated by POM and talc are partially overlapped. Generally, t1/2 decreased with the introduction of a nucleating agent and increased with the temperature. For PLA nucleated by POM, t1/2 first increased (POM1) and then significantly decreased (POM2 and POM5). Based on these, we suppose that POM may be totally dissolved in PLA under certain content. To verify it, dynamic mechanical analysis was conducted for POM1, POM2, and POM5. As shown in Figure 7, the Tg of PLA was shifted toward a lower temperature with the POM content. According to the Fox equation,40 1.4% POM was dissolved in PLA in the case of POM5. Therefore, for PLA with 1% POM, POM was likely to be completely dissolved in PLA. In other words, 1% POM was a threshold for a nucleating effect, and no nucleating effect was observed at a POM addition of ≤1%. 3.5. Optical Transparency of Neat PLA and the POM Nucleated PLA. Figure 8 shows the qualitative comparison of

Figure 5. Comparison of DSC thermograms of (a) the first heating scan and (b) the first cooling scan between neat PLA, POM1, POM2, and POM5.

First Heating Scan

⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ K ⎠

(3) 16759



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Table 2. Isothermal Crystallization Kinetics Parameters of PLA and Its Composites at 106−111 °C 106 °C PLA POM1 POM2 POM5

107 °C

108 °C

109 °C

111 °C

n

t1/2 (min)

n

t1/2 (min)

n

t1/2 (min)

n

t1/2 (min)

n

t1/2 (min)

2.66 3.17 3.08 3.30

8.28 9.66 2.40 1.30

3.15 3.13 3.56 3.44

9.36 10.86 2.80 1.51

3.37 3.36 3.57 3.75

10.63 11.87 4.07 1.81

3.51 3.00 3.77 3.72

12.09 16.35 4.63 1.93

3.57 3.88 3.71 3.75

14.49 17.07 5.59 2.31

Figure 9. UV-vis transmittance spectra of neat PLA, neat POM, POM1, POM2, POM5, talc1, talc2, and talc 5.

Figure 7. Comparison of loss tangent (i.e., tan δ) of dynamic mechanical analysis as a function of temperature between neat PLA, neat POM, and PLA nucleated by POM.

at a wavelength of 550 nm, while neat POM showed only 23% light transmittance. At the same wavelength, the light transmittance of PLA nucleated by 1, 2, and 5 wt % POM was improved to 92%, 94% and 96%, respectively, partly implying that the incorporation of a low content of POM led to a smaller size of spherulites. In contrast, the transmittance of the PLA nucleated by talc was drastically reduced as the talc content increased, which was due to the strong light scattering by relatively large talc particles. Thus, the addition of POM into PLA would be more suitable for applications requiring optical transparency. The refractive index of PLA (1.482−1.492)41 is very similar to that of POM (1.476−1.492),42 which most likely accounts for the high transparency of the PLA/POM compositions. Since the two polymers have almost identical refractive indices, the loss of light transmission from scattering at the interface is minimized. In addition to the similar refractive index, the reduced size of spherulites of PLA might enhance the level of transparency slightly, but its role is believed to be secondary. The transparency of a polymer blend may be indicative of a certain level of miscibility between the components, namely, PLA and POM may be partially miscible. The lowered transmittance of the PLA nucleated by talc is believed to result from two aspects: enhanced light scattering, as a result of the incorporation of (large and agglomerated) talc particles, and increased crystallinity of PLA by talc.43,44 3.6. X-ray Diffraction (XRD) Analysis. Figure 10a shows the X-ray diffraction (XRD) patterns of neat PLA, neat POM, POM1, POM2, and POM5 that were annealed at 120 °C for 45 min prior to the testing. The neat PLA displayed two main diffraction peaks: a strong diffraction peak at 2θ = 16.6°, which corresponded to the (200, 110) reflection of the α-form orthorhombic crystal lattice, and a weak peak at 2θ = 18.9°, which corresponded to the (203) reflection.45−47 The neat POM showed a strong diffraction peak at ∼23° which was

Figure 8. Optical photomicrograph showing comparison of transparency between neat PLA, neat POM, POM1, POM2, POM5, talc1, talc 2, and talc 5 on a background paper.

transparency through an optical image between neat PLA, neat POM, and the PLA nucleated by POM and talc at various concentrations. The specimens were all standard tensile bars prepared by injection molding. Neat PLA appeared translucent because of its low crystallinity but large spherulites, while neat POM was completely opaque, because of its high crystallinity. POM1, POM2, and POM5 all showed enhanced transparency relative to the neat PLA, partly indicating that the addition of the POM significantly decreased the size of spherulites. In contrast, these PLA nucleated by talc became opaque and showed the least transparency in all these compositions. The obvious differences in transparency were further revealed by their transmittances of the UV-vis spectra shown in Figure 9. Neat PLA exhibited a moderate visible transmittance of ∼83% 16760



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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J. Zhang). *E-mail: [email protected] (J. J. Huang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial supports from the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service (Grant No. 200735504-17818).

Figure 10. XRD patterns for neat PLA, neat POM, POM1, POM2, and POM5 prepared by annealing at 120 °C for 45 min.

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ascribed to the (100) reflection of its hexagonal crystal lattice.48,49 Both POM2 and POM5 showed prominent diffraction peaks at 16.6°, 18.9°, and 23°, which was characteristic of neat PLA and POM crystals, respectively. Hence, the introduction of POM did not altered the crystal structure of PLA, since the characteristic diffraction peaks of PLA and POM were maintained without any new peaks forming. However, the peak intensities increased with increase in the POM content, indicating that the introduction of POM could increase the crystallinity of PLA. A broad shoulder appeared in POM1, implying that 1% POM may not be enough for effective crystallization of the PLA.

REFERENCES

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4. CONCLUSIONS The incorporation of polyoxymethylene (POM) as a polymeric nucleating agent in poly(lactic acid) (PLA) significantly accelerated the crystallization of PLA, as seen in the reduced half time of crystallization. Application of the Avrami model to the isothermal crystallization kinetics of PLA suggested that the addition of POM resulted in thermal (sporadic) nucleation and three-dimensional (3D) crystal growth for PLA. However, the addition of 1% POM slightly decelerated the crystallization rate of PLA. It was further noted that ∼1.4 wt % POM was dissolved in the PLA phase, based on the estimation using Fox equation. Therefore, when the loading level of POM was low, e.g., 1% POM or less, no nucleating effect was noted, because POM would be completely dissolved in PLA. A new vibration peak was noted in the Fourier transform infrared (FTIR) spectra of the POM-nucleated PLA, suggesting that there was probably a favorable interaction between POM and PLA at the molecular level that accounted for the alteration of PLA crystallization. Interestingly, the well-defined banded spherulites, derived from the lamellar twisting along the radial growth direction of lamellae, emerged in PLA when the POM content exceeded 5 wt %. The addition of POM also slightly reduced the spherulitic size of PLA. Furthermore, the incorporation of POM as a nucleating agent enhanced the level of transparency of PLA (i.e., greater visible transmittance), which is contrary to the cases of most inorganic nucleating agents, such as talc. Such an improvement in transparency resulted mainly from the almost same refractive indexes of PLA and POM and smaller spherulites. 16761



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