Cell Morphology and Improved Heat Resistance of Microcellular Poly(l

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Cell Morphology and Improved Heat Resistance of Microcellular Poly(Llactide) Foam via Introducing Stereocomplex Crystallites of PLA Pin Jia, Jie Hu, Wentao Zhai, Yongxin Duan, Jianming Zhang, and Chang-Yu Han Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504345y • Publication Date (Web): 06 Feb 2015 Downloaded from http://pubs.acs.org on February 19, 2015

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Industrial & Engineering Chemistry Research

Cell Morphology and Improved Heat Resistance of Microcellular Poly(L-lactide) Foam via Introducing Stereocomplex Crystallites of PLA Pin Jia1,2, Jie Hu1, Wentao Zhai2*, Yongxin Duan1*, Jianming Zhang1, and Changyu Han3

1. Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-plastics, Qingdao University of Science & Technology, Qingdao 266042, Shandong, China

2. Ningbo Key Lab of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang Province 315201, China

3. Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

*

To whom all correspondence should be addressed.

Fax: +86 0574 8668 5186 E-mail: [email protected]; [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT: The preparation of poly(L-lactic acid) (PLLA) foam with well defined cell structure and high heat resistance is critical to broaden its applications. In this study, the stereocomplex crystallites (SC) with higher melting point and heat stability was introduced into PLLA foam by melt blending the PLLA with different amount of poly(D-lactic acid) (PDLA). The crystal structure of pure PLLA and PLLA/PDLA blends formed during blending and molding process was compared. It was found that no obvious crystallization was detected in pure PLLA, while SC formed in PLLA/PDLA blends. Crystal structure and morphology evolution of PLLA and PLLA/PDLA during CO2 saturation and foaming process were investigated by combination of DSC, WAXD, FTIR and SEM techniques. The results suggested SC had higher melting peak, higher thermal stability, and smaller crystal domain size in relative to the homocrystal of PLLA, and it did not further develop with the CO2 saturation and the foaming processes, while the CO2 saturation induced the formation of the mesomorphic structure in PLLA and PLLA/PDLA blends. The mesomorphic structure transformed into more stable α form in the following foaming process. The resultant PLLA/PDLA foam exhibited significant and concurrent increase in cell density and cell structure uniformity relative to PLLA foam. The heat resistance measurement presented that thus prepared PLLA/PDLA foams had better heat resistance than PLLA foam, which was attributed to the higher melting point, higher heat stability and the higher cell nucleation ability of SC in PLLA/PDLA foams.

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1. INTRODUCTION In the past decades, polylactide (PLA), a kind of biodegradable aliphatic polyesters, has been attracting significant attention, because it is synthesized based on the annually renewable resources, such as potato and corn, and also because it is cost effective compare to other bio-based plastics.1–3 Unfortunately, PLA exhibits a low service temperature, due to its low glass transition temperature (Tg) and low crystallization rate. Therefore, it is critical to increase its heat resistance for broadening its applications. Many studies have been reported to enhance the crystallization rate and crystallinity of PLA,4–12 and a comprehensive review paper has been published recently.13 Microcellular foaming technology using supercritical or compressed CO2 as the physical blowing agent has been thought as a novel approach to increase the crystallinity of PLA.14–21 Besides the contribution to the crystallinity development, foaming introduces cellular structure and devotes to reduce the sample’s density and improve its toughness.22-24 Marubayashi et al. revealed that PLLA reached a crystallinity of 45% after being saturated at 7-15 MPa and 0-80 °C.25 Our previous study demonstrated that the equilibrium crystallinity of PLLA foamed at 6.89 MPa and 100 °C was about 38.3%,17 At a low CO2 pressure of 5 MPa, the prepared PLLA foam presented a crystallinity of about 31.0%.24 The main reason for the crystallinity development of PLLA foams is that the significant plasticization effect of CO2 endows polymer chains with fast relaxation ability, resulting in the increased crystallization rate and crystallinity.24–27 Apart from the saturated process, the foam 3



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expansion process also affects the crystallization behavior of PLA.16,17,24 It is well accepted that cell growth is a biaxial extension to the cell wall in nature.28 This action usually accompanied with a significant crystallinity increase in polymer resin.16 On the other hand, the crystallization characteristic especially for crystal domain size seems to obviously affect the polymer foaming process and thus determines the cell morphology, expansion ratio, and crystallinity of the resultant foams.17,24,29–32 A well accepted reason is that the crystal domain can act as cross-linking points to improve the polymer’s melt strength, which would potentially affect the cell growth.33-35 At the same time, the crystalline/amorphous interfaces supply the heterogeneous nucleation sites for cell nucleation, and the enhanced cell nucleation process tends to increase the cell density and decrease the average cell size.32,36 Once the induced crystallinity is too high or the crystal domain sizes are too large, however, the crystallites could induce the formation of nonuniform cell distribution and evenly the presence of unfoamed regions.17,30 Recently, Park’s group developed a modified heterogeneous nucleation theory based on the in-situ observation of cell nucleation and computer simulation.37,38 They found that the melt flow induced by the growing bubble could create a dramatic local stress variation around the solid filler. The crystal domain is usually considered as inert filler because gas cannot dissolve into it.39 Moreover, the crystal domains are linked by the surrounded amorphous chains. Once cell nucleation occurs at the crystalline/amorphous interfaces, the nucleated bubbles tend to induce 4


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high stress around the crystal domains. The increased stress variation reduces the energy barrier for cell nucleation, resulting in the enhanced cell nucleation. Meanwhile, the formed extensional and/or shear flow facilitates crystallinity development in semi-crystalline polymers,40 resulting in the increased matrix modulus, which suppresses cell growth and foam expansion. Therefore, the competition of cell growth and crystallinity development during semi-crystalline polymer foaming determines the cell morphology of foams. PLA is a chiral polymer, and has two enantiomers, poly(L-lactide) (PLLA) and poly(D-lactide) (PDLA).41 PLLA can form several kinds of crystalline modifications due to specific crystallization conditions: the α,41,42 β,42 and γ forms.43 The α crystal form, which is the most common and thermodynamically stable one, has a 103 helical chain conformation, and its characteristic diffraction peaks appear at 14.8, 16.7 19.1 and 22.4°.42,44 Many researchers have reported that the stereocomplex crystallites (SC) could be generated by the melt blending or solution mixing of PLLA and PDLA.45–49 The SC of PLA possesses a high melting temperature of about 220 °C, which is 50 °C higher than that of neat PLLA or PDLA, i.e., of about 170 °C.45 Furthermore, PLA sample with SC exhibits a much better heat resistance properties than the normal PLLA.50 These improvements are mainly due to the strong interaction between L-lactyl unit chains and D-lactyl unit chains. PLA SC crystallizes in a βc form and has mainly the 31 helical chain conformation.51 Based on the FT-IR, Zhang et al. revealed that the CH···O=C hydrogen bonding is the driving force for forming the racemic nucleation of the 5



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PLA stereocomplex crystallites.52 It is interesting to figure out the possibility for producing PLLA foams with the well defined cell structure and the improved heat resistance by introducing the SC. In this work, PLLA/PDLA blends with 0-20 wt% PDLA loadings was prepared by the melt blending and then the compression molding processes. The SC was found in the as-prepared PLLA/PDLA blends. After the CO2 saturation, the mesomorphic structure of PLLA was also observed in all samples. The influences of gas pressure on the evolution of SC and mesomorphic structure were investigated. A temperature rising process was applied to foam the gas-saturated PLLA and PLLA/PDLA blends. The foaming behaviors were investigated based on the foam expansion ratio and cell morphology, and the influences of SC on the foaming behavior of PLLA/PDLA blends were discussed. Moreover, the effect of foaming process on the crystallinity development of samples was investigated. In the last part of this study, the heat resistance of PLLA and PLLA/PDLA foams was addressed, and the influence of SC on the heat resistance of PLLA/PDLA foams was emphasized.

2. EXPERIMENTAL SECTION 2.1. Materials. PLLA (4032D), a commercial product, was supplied by Nature Works LLC. It exhibited a weight-average molecular weight (Mw) of 207000, polydispersity of 1.73 (gel permeation chromatography (GPC) analysis). The D-isomer content of PLLA is approximately 2.0%. The synthesis and purification of PDLA samples used 6


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in this study were performed according to the procedures reported previously.49 It exhibited a weight-average molecular weight (Mw) of 110000, polydispersity of 1.92 (GPC analysis). CO2 with a purity of 99.9% obtained from Ningbo Wanli Gas Corporation was used as the physical blowing agent in all experiments. 2.2. Preparation of PLLA/PDLA Blends. The PLLA and PDLA pellets were vacuum-dried at 80 °C for at least 8 h before use. The PLLA/PDLA blends were prepared using a Brabender Lab-Station. The melt compounding was performed at 180 °C for 5 min, and the rotor speed was 50 rpm. For comparison, pure PLLA was also treated with the same procedure. The contents of PDLA were 5 wt %, 10 wt %, and 20 wt % based on total blends weight. The samples are abbreviated as L/D-0, L/D-5, L/D-10, L/D-20, respectively. Specimens with a thickness of 0.15 and 0.5 mm were prepared by compression molding under 10 MPa at 190 °C and then quenched to room temperature by cold water. The sheets were cut into specimens of 15 mm × 15 mm for foaming experiments. 2.3. CO2 Saturation and Batch Foaming. The procedure of CO2 saturation and batch foaming are described as follows: The sheets of PLLA and PLLA/PDLA blends were enclosed into a high-pressure vessel. The vessel was placed in an ice water bath and the temperature was fixed at 0 °C. The vessel was flushed with low pressure CO2 for about 1 min, followed by increasing the pressure and then pressurized to the desired value. The samples were treated under this condition for 12 h to ensure equilibrium adsorption of CO2. At the 7



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end of the saturation, the vessel was released with a depressurization rate of 10 MPa/s. The saturated specimens were transferred within 1 min to the water or glycerol oil bath for 20 s with the fixed temperature to obtain microcellular foams, and the foamed structure was fixed by quenching the foams in cold water. 2.4. Characterization Differential Scanning Calorimetry (DSC). The melting behaviors of PLA samples were determined using an apparatus (Mettler Toledo DSC/TGA) calibrated with indium. For all samples, only data obtained from the first heating for assessing the possible effect of CO2 saturation and the foaming process on the crystallization behavior of the PLLA and PLLA/PDLA blends. For all CO2 saturated samples, a long time degassing (more than 1 month) at room temperature was carried out to remove the possible effect of gas plasticization on the sample’s crystallinity. All measurements were carried out with a heating rate of 10 °C/min over a temperature range from 25 to 250 °C in a dry nitrogen environment. ∆Hm1 and ∆Hm2 are the experimental melting enthalpies of homocrystal and SC, and ∆Hc is the exothermic enthalpy observed during the DSC run. Wide-angle X-ray diffraction (WAXD). WAXD measurements were carried out on Bruker D8 Advance X-ray diffractometer equipped with Cu Kα radiation at room temperature. The scattering angle ranged from 2θ=10° to 30° and running at a speed of 2 °/min. The Cu Kα radiation (λ=0.15418 nm) source was operated at 40 kV and 200 mA. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were 8


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measured by a Bruker Tensor 27 spectrometer equipped with a MCT detector, using the 0.15 mm thick samples. The normal transmission mode was employed for IR measurement. All the spectra were recorded by co-adding 32 scans at a 2 cm-1 resolution. Mass Density. The mass densities of the samples before (ρ) and after (ρf) foaming were measured via water displacement method according to ISO 1183-1987. The uptake of water by the samples can be neglected during this measurement due to a smooth skin and closed cells of these foamed samples. Scanning Electron Microscope (SEM). The cell structures were investigated by using a Hitachi S-4800 field emission SEM at 4kV. The samples were freeze-fractured in liquid nitrogen and sputter-coated with gold. The cell size and cell density were obtained through the SEM photographs. The cell density (N0), the number of cells per cubic centimeter of solid polymer, was determined using eq 1 as follows: 3/2

 nM 2  N0 =   φ  A 

(1)

Where n is the number of cells in the SEM micrograph, M is the magnification factor, A is the area of the micrograph (in cm2), and φ is the volume expansion ratio of the polymer foam, which can be calculated using eq 2 as follows:

φ=

ρ ρf

(2)

where ρ and ρf are the mass densities of PLA resin and PLA foam. Heat Resistance. The heat resistance of the PLLA and PLLA/PDLA foams is 9



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defined here as their ability to retain their expansion ratio through exposure to temperatures above Tg. In order to evaluate this stability, the foams were placed in the hot water (that is, 80, 90, and 100 °C) for 1 h and then assess the percentage of shrinkage. As the temperature is above Tg, it can induce relaxation of stretched amorphous chains and subsequently shrinkage.53 The samples were then cooled at room temperature, before mass density measurements were carried out as previously described. The volumetric shrinkage (Sv) was calculated using eq 3 as follows:54 Sv = 1 −

φf φi

(3)

where φi and φf are the initial and final expansion ratio, respectively.

3. RESULT AND DISCUSSION 3.1 The formation of SC in PLLA/PDLA blends The PLLA/PDLA blends were melt mixed at 180 °C and then were molded under compression at 190 °C. The formation of SC in PLLA/PDLA blends was confirmed by DSC, WAXD and FTIR. DSC traces obtained on the PLLA and PLLA/PDLA blends at a heating rate of 10 °C /min are given in Figure 1a. As denoted in the DSC curves, a so-called aging peak near the Tg associated with structural relaxation for all the samples appears around 64.1 °C.55 For L/D-0, the cold crystallization (Tc) and the melting of homocrystal (Tm1) are presented around 100 °C and 167 °C, respectively. For PLLA/PDLA blends, except for the Tc and the Tm1, a new melting peak, i.e., Tm2, at 218 °C is observed. This is a clear evidence of SC formation in PLLA/PDLA blends.56 Furthermore, it is seen that the intensity of 10


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Tm2 increases with increasing the PDLA loading due to the increased SC amount. WAXD can provide the quantitative information of crystalline structure, such as the lamellar thickness, long period, and lattice spacing.57 Figure 1b displays the WAXD patterns of PLLA and PLLA/PDLA blends. L/D-0 exhibits a broad diffraction peak, suggesting that the as-prepared L/D-0 sample was amorphous. For PLLA/PDLA blends, however, the newly formed characteristic peaks are observed at 2θ values of 12°, 21°, and 24°, which corresponded to the unit cell dimensions of SC.45 The increased intensities of characteristic peaks in PLLA/PDLA blends with high PDLA loadings were observed, indicated that the increased amount of SC. The formation of SC in PLLA/PDLA blends was further confirmed by FTIR as indicated in Figure 1c. It is seen that a new band was present at 908 cm-1 for PLLA/PDLA blends, and the intensity of the band increased gradually with the increase of PDLA loading. This newly formed band at 908 cm-1 was associated with the SC.58 The as-prepared L/D-0 was amorphous, and the characteristic band of homocrystal at 920 cm-1 was not observed.59 According to the DSC, WAXD, and FTIR data, we confirmed that the SC was generated in the as prepared PLLA/PDLA blends Table 1 summarizes the information of cold crystallization and crystal melting of PLLA and PLLA/PDLA blends. L/D-0 sample was amorphous, and the enthalpy of cold crystallization (∆Hc) was almost equal to that of the melting of homocrystal (∆Hm1). With the addition of PDLA, a high melting peak of Tm2 was present at 218 °C. At the same time, it is seen that the values of ∆Hc and ∆Hm1 decrease while that of ∆Hm2 increases. What’s more, the cold crystallization peak of PLLA/PDLA 11



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blends is slightly lower than that of pure PLLA sample, that is, 99 °C Vs 102 °C, which is due to the nucleation effect of stereocomplex.60,61

3.2 Evolution of SC and Homocrystal in PLLA and PLLA/PDLA blends with the CO2 saturation. The strong plasticization effect of compressed CO2 increases the mobility of polymer chains, which might facilitate the crystallinity development of PLLA and PLLA/PDLA blends. Figure 2-4 show the DSC, WAXD and FTIR data of PLLA and PLLA/PDLA blends saturated at 2.5, 3.0, and 3.5 MPa, respectively, and Table 2 summarizes the crystallization and melting behaviors of all samples under CO2 saturation. The influences of CO2 saturation on the evolution of SC and homocrystal were discussed in the following parts. The SC was observed in the CO2 saturated PLLA/PDLA blends, which was verified by the presence of Tm2 at DSC curves (Figure 2a, 3a, 4a), the presence of WAXD characteristic peaks at 12°, 21°, and 24° (Figure 2b, 3b, 4b), and the presence of FTIR characteristic peak at 908 cm-1 (Figure 2c, 3c, 4c). Furthermore, as indicated in Table 2, the ∆Hm2 of SC after CO2 saturation under 2.5-3.5 MPa was about 6.0 J/g for L/D-5, about 13.4 J/g for L/D-10, and about 34.5 J/g for L/D-20, respectively, which values were very similar to those of the as-prepared PLLA/PDLA blends as indicated in Table 1. These results suggested that the amount of SC did not increase in PLLA/PDLA blends after CO2 saturation at 2.5-3.5 MPa and 0 °C for 12 h. Two possible reasons led to this phenomenon. One possible reason was that the formed SC during the sample blending and molding 12


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process possessed high perfection. According to the crystallization kinetics data reported by many researchers,47,62 the crystallization rate of SC at 140-190 °C was rapidly, and only 0.5 min was needed to induce the formation of SC. In our study, the melt mixing time of PLLA/PDLA blends at 180 °C was 5 min and the compression molding time at 190 °C was 5 min. A long sample preparation time of 10 min might be enough to ensure the fully development of SC. The other possible reason was that the crystallization conditions for the newly SC was not being satisfied. Based on previous study,62 the crystallization temperature of SC was higher than 140 °C. While in this study, the CO2 saturation conditions of PLLA/PDLA blends was 2.5-3.5 MPa and 0 °C, the low treatment temperature might not be possible for triggering the formation of newly SC. An important feature of the CO2 saturated PLLA and PLLA/PDLA blends was the formation of mesomorphic structure. As indicated in Figure 2b, the CO2 saturation changed the WAXD diffraction curve of PLLA from the broad diffraction peak to a weak crystalline characteristic at 2φ of 16°. Furthermore, this formed structure was thought to be the mesomorphic structure, i.e., an intermediate ordering state between the crystalline state and the amorphous state.63–65 The FTIR spectra were used to further explain the presence of mesophase in the CO2 saturated PLLA. As indicated in Figure 2c, a new characteristic band is observed at 918 cm-1 for PLLA after CO2 saturation. Recently, Zhang et al.65 investigated the mesophase by FTIR and assigned the 918 cm-1 band to the mesomorphic structure of PLLA. They concluded that the PLLA chains in the mesophase were more ordered than in the 13



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amorphous state, and their ordering was also totally different with those in the normal crystalline state (α form) of PLLA. DSC curve was also used to investigate the formation of mesophase. As shown in Figure 2a, a small endothermic peak pointed by the red arrow was presented in PLLA and PLLA/PDLA DSC curves in a wide temperature range of 65-80 oC. A similar melting peak at temperature slightly above Tg was also observed at DSC curve of the stretched PLA fiber63 and the CO2 saturated PLLA.24,26 This feature was attributed to the formation of mesophase. This kind of imperfect structure could melt at the heating temperature a little bit higher than the Tg.65 The crystallization perfection of mesomorphic structure tended to increase with the CO2 pressure due to the increased mobility of polymer chains. As indicated in Figure 3a, the cold crystallization enthalpies of PLLA and PLLA/PDLA blends saturated at 3.0 MPa decreased compared to that of 2.5 MPa saturated samples. Moreover, the WAXD characteristic diffraction peak at 16° and the FTIR diffraction band at 918 cm-1 of PLLA saturated at 3.0 MPa became more distinct compared to those obtained at 2.5 MPa. These results suggested that the perfection of the formed mesomorphic structure at 3.0 MPa was higher than the obtained one at 2.5 MPa. At higher CO2 pressure of 3.5 MPa, the cold crystallinzation of PLLA was restrained totally. Moreover, the WAXD diffraction curve of the CO2 saturated PLLA had three characteristic peaks at 16°, 18.5° and 22.5°, and its FTIR spectra at 918 cm-1 possessed the increased intensity. These results indicated that the perfection of mesomorphic structure further increased with the CO2 pressure 14


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increasing from 3.0 to 3.5 MPa. In summary, the blending and molding process in this study enabled only the SC being formed in the as-prepared PLLA/PDLA blends. The CO2 saturation at 2.5-3.5 MPa and 0 °C increased the mobility of polymer chains, induced the occurrence of mesomorphic structure, while the SC was not further developed with the CO2 saturation. Based on the DSC, WAXD and FTIR analysis, the formed mesomorphic structure displayed the increased perfection with the increase of CO2 pressure.

3.3 Foaming behaviors of PLLA and PLLA/PDLA blends A temperature rising process was carried out to foam the CO2 saturated PLLA and PLLA/PDLA blends. Figure 5-7 show the cell morphology of PLLA and PLLA/PDLA foams, where the samples were saturated at 2.5-3.5 MPa and then foamed at 100 °C for 20 s. Figure 8 summarizes the expansion ratio, average cell size, and cell density of the obtained foams. The saturated L/D-0 at 2.5 MPa exhibited good foamability, because the obtained PLLA foam presented uniform cell structure, high foam expansion ratio of 28.3, large cell size of 25.5 µm, high cell density of 1.1×109 cells/cm3. Similar to L/D-0 foam, the prepared L/D-5 foam presents polygon cell structure and uniform cell size distribution, but it presents the reduced expansion ratio of 14.7, the reduced cell size of 17.9 µm, and the increased cell density of 3.1×109 cells/cm3. At higher PDLA content, the cell structure of L/D-10 and L/D-20 become elliptical. Moreover, their foam expansion ratio significantly decreases to 5.2 for L/D-10 foam and 1.7 for L/D-20 foam, 15



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respectively, and their average cell size decreases to 5.1 µm for L/D-10 foam and 1.3 µm for L/D-20 foam, respectively. For the cell density data, the increase of PDLA loadings increases the cell density of L/D-10 foam to 6.5×1010 cells/cm3 and L/D-20 foam to 5.3×1011 cells/cm3, respectively. These results suggested that the introduction of PDLA enhanced cell nucleation and suppressed cell growth and foam expansion. Figure 6 shows the cell morphology of PLLA and PLLA/PDLA foams obtained at 3.0 MPa. It is seen that L/D-0 and L/D-5 foams have uniform cell structure and similar cell morphology. The foam expansion ratio of them is 19.0 and 10.1, respectively, and the average cell size is 13.4 µm and 10.2 µm, respectively. A further increase of PDLA content seemed to affect the cell morphology of PLLA/PDLA foams significantly. For L/D-10 foam, its cell size decreases, and its cell structure distribution seems non-uniform. For L/D-20 foam, only the tiny cell structure with uniform cell size distribution is observed. At higher SEM magnification of ×10000 times, the cell structure is circular, and the ductile fracture occurred during sample preparation. The expansion ratio of L/D-10 and L/D-20 is 2.9 and 1.4, respectively, and the average cell size is 2.5 µm and 0.6 µm, respectively. In the case of cell density, in relative of L/D-0 foam, the cell density increased from 4.8 ×109 cells/cm3 to 8.1 ×109 cells/cm3 for L/D-5 foam, 2.3 ×1011 cells/cm3 for L/D-10 foam, 2.8 ×1012 cells/cm3 for L/D-20 foam, respectively. These results further verified that the introduction of PDLA facilitated the decrease in cell size and the increase in cell density. 16


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At higher CO2 pressure of 3.5 MPa, we did not observed any obvious sample expansion for PLLA and PLLA/PDLA blends. The cell morphology shown in Figure 7 explained that there were no cell morphology generated in foams except for L/D-0 and L/D-5 foams where only a few bubbles were found. As previously described in Figure 4 and Table 2, there is no obvious cold crystallization (∆Hc) detected in the DSC heating curves of all the 3.5 MPa CO2 saturated samples. And that the total melting enthalpies of homocrystal and SC (∆Hm1 + ∆Hm2), are about 35.4-40.8 J/g. This indicated that the saturated samples have totally crystallized before foaming. The sufficient crystallization induced high matrix strength, which might suppress cell growth and spoil the foamability of PLLA and PLLA/PDLA blends. The evolution of cell morphology of PLLA/PDLA foams with the addition of PDLA was attributed to the presence of SC. One widely accepted explanation is that the crystal domains cannot absorb gas and it works as the inert fillers to supply new nucleation sites for cell nucleation, and thus enhances cell nucleation and increases cell density.31,33 The other reason might be that the SC improved PLLA’s melt strength. Generally, PLLA foam possesses poor cell structure and apparent cell coalescence due to the low matrix modulus and melt strength.3,66 Crystallization of PLLA can increase its low melt strength by the network of crystals.16,67,68 It also reported that the addition of PDLA gives a strong strain hardening effect to PLLA melt.69 This could consequently resist the force of cell growth and then reduce the cell collapse. 17



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It is known that the crystallinity is a general concept that contains the information of crystal domain size, the perfection of crystallites, the crystal density, and so on. Our previous study suggested that the tiny crystal domain size and well dispersed nanosilica aggregates could improve the cell morphology of PLLA foam while keeping the gas saturated PLLA with a high induced crystallinity.71 In general, the SC possesses smaller crystal size and higher crystal density than the homocrystal of PLLA. 48,62 A direct characterization of the size of SC is shown in Figure 9a and 9b. It is observed that a few embedded spherical particles with size of 0.2-0.3 µm were located at the cell structure of L/D-5 and L/D-20 foams as pointed by red arrows. Moreover, the size of spherical particles did not changed obviously with the PDLA content and the presence of mesomorphic structure. Considering there were no any solid fillers being blended in L/D-5 and L/D-20, we thought that the particles should be the SC. The presence of spherulite structure had been observed in other semi-crystalline polymer foaming system such as PLLA foam,17 PC foam,30 PP foam,71 and PE foam.72 What’s more, as shown in Figure 9c and 9d, we only found the smooth cell wall in L/D-0 foams which were prepared by foaming the amorphous and the crystallized L/D-0. As mentioned above, the generated crystalline structure during CO2 saturation was the imperfect mesomorphic structure. As suggested by our previous study,73 the strong extension flow might induce the orientation of solid fillers. In the case of mesomorphic structure, the strong stretching and orientation action could deform it and reduce its thickness. As a consequence, we could not 18


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observe the imperfect structure in PLLA foam obtained at 2.5 and 3.0 MPa. The high perfection ensured the SC strong enough to restrain the occurrence of spherulites deformation under stretching. Therefore, we could find them in the cellular structure of PLLA/PDLA foams.

3.4 Evolution of SC and homocrystal with the foaming process During the solid state foaming, the gas saturated PLLA and PLLA/PDLA blends undergo a rapid temperature rising process and a volume expansion process. Both processes will potentially affect the evolution of SC and homocrystal of foams. Table 3 summarizes the crystallization and melting behaviors of PLLA and PLLA/PDLA foams. The ∆Hm2 was associated with the melting of SC, which was 6.0-6.5 J/g for L/D-5 foams, 13.5-13.8 J/g for L/D-10 foams, and 34.7-35.1 J/g for L/D-20 foams obtained at different conditions, which were similar to those of the as-prepared and the gas saturated PLLA/PDLA blends. These results demonstrated that the CO2 saturation and the following foaming processes did not increase the amount of SC. Figure 10 shows the WAXD diffraction curves of PLLA and PLLA/PDLA foams obtained at the saturation pressures of 2.5 and 3.5 MPa, respectively. As indicated in Figure 5 and 7, PLLA and PLLA/PDLA blends could foam at 2.5 MPa, while they could not foam at 3.5 MPa. It is seen the foamed structure at the two conditions were α form with the typical diffraction peak at 16.7°, 19.1°, 22.5° which was very different from the mesomorphic structure of the saturated PLLA and PLLA/PDLA at 2.5 and 3.5 MPa. These results indicated that the foaming 19



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Page 20 of 50

process further increased the perfection of crystal structure.

3.5 Heat resistance of PLLA and PLLA/PDLA foams The foamed materials are often used as packaging. At this situation, the material usually endures the stretching or compression actions. The heat resistance of foams under loading was investigated in this study, where the foams were stretched by force under high temperature. Figure 11 indicates the digital photographs of L/D-0 and L/D-20 foams with same length and width under same 200g loading. It is well known that the expansion ratio could influence the heat resistance properties of foams, in order to illustrate this effect, L/D-0 foams with expansion ratios of 28.3 and 2.5, and L/D-20 foam with expansion ratio of 1.7 were prepared, respectively. The air temperature was 110 °C, and the thermal treatment time was 15 min. The original length of L/D-0 and L/D-20 foams between two clamps was 2.5 cm. After the thermal treatment, the length increased up to 3.4 cm for L/D-0(a) foam, 3.2 cm for L/D-0(b) foam and 2.7 cm for L/D-20 foam. The increased length of L/D-0 foam resulted from the polymer chain relaxation, which suggested that the chain structure was not fastened by the crystalline domains completely. For L/D-20 foam, however, its crystalline domains have higher thermal stability, and the polymer chain mobility is lower. The prepared foams were thin films with the thickness of about 2 mm, the sample shrinkage during the thermal exposure at 80-100 °C for 1 h was used to further evaluate the heat resistance of foams, and the results are shown in Figure 12 and Table 4. At 80 °C, the sample shrinkage is 4.75% for L/D-0 foam, 2.75% for 20


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L/D-5 foam, 2.45% for L/D-10 foam, and 1.65% for L/D-20 foam, respectively. At 90 °C, the sample shrinkage is 10.06% for L/D-0 foam, 7.85% for L/D-5 foam, 3.10% for L/D-10 foam, and 2.05% for L/D-20 foam, respectively. At higher exposure temperature of 100 °C, the sample shrinkage is 19.25% for L/D-0 foam, 13.25% for L/D-5 foam, 7.75% for L/D-10 foam, and 5.55% for L/D-20 foam, respectively. These results demonstrated that the addition of PDLA did improve the heat resistance of PLLA foams. The influence of thermal exposure on the cellular structure of foams is shown in Figure 13. It is seen that L/D-0 foam exhibited polygonous closed cell structure and thin cell wall. After the thermal exposure, however, the cell structure collapsed and the cell wall deformed into the random shape. Moreover, for the thermal treated L/D-0 foam, a lot of tiny bubbles were observed on cell wall. These phenomena further demonstrated that PLLA foam was not thermal stable at the thermal exposure temperature of 100 °C. For L/D-20 foam, we did not observe any obvious change in cellular structure before and after thermal treatment, which should benefit from the improved thermal resistance.

4. CONCULSION In this study, the PLLA was melt blended with PDLA, and the stereocomplex crystallites (SC) with high melting peak of 218 °C and high thermal stability was prepared in PLLA/PDLA blends. PLLA and PLLA/PDLA blends were saturated by the compressed CO2. DSC, WAXD, FTIR analysis suggested that the CO2 saturation 21



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Page 22 of 50

induced the formation of the mesomorphic structure in PLLA and PLLA/PDLA blends, while it did not affect the SC. After a temperature rising process, the saturated samples expanded and the cellular structure were generated. The resultant PLLA/PDLA foam exhibited significant and concurrent increase in cell density and cell structure uniformity relative to PLLA foam. The heat resistance of foams was measured by the density reduction and the change of sample shape under loading after the thermal exposure at 80-100 °C for 1 h and 110 °C for 15 min, respectively. It was found that L/D-0 foam with only homocrystal experienced the shrinkage of 19.25% at 100 °C, cell structure collapse, and shape deformation under loading at 110 °C. For L/D-20 foams with SC and homocrystal, however, it only presented a slight sample shrinkage and deformation after the thermal exposure the same conditions.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grants 51003115). The financial support from

Taishan

Mountain

Scholar

Constructive

Engineering

Foundation

(tshw20110510) are greatly appreciated.

22


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Table 1. The thermal behaviors of the as prepared PLLA and PLLA/PDLA samples Code

Tc C

o

△Hc Tm1 △Hm1 Tm2 △Hm2 o J/g oC J/g C J/g

L/D-0

102 29.7 167

30.1

-

-

L/D-5

100 27.8 167

29.7

218

6.0

L/D-10 100 24.3 166 L/D-20 99 15.4 164

26.7 15.0

218 218

13.1 34.2

33



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Page 34 of 50

Table 2. The thermal behaviors of the saturated PLLA and PLLA/PDLA samples Code L/D-0

L/D-5

L/D-10

L/D-20

P/MPa △Hc(J/g) △Hm1(J/g) △Hm2(J/g) 2.5

32.9

34.4

-

3.0

6.2

37.4

-

3.5

-

35.4

-

2.5

30.7

32.8

6.0

3.0

4.8

29.1

6.0

3.5

-

30.6

6.1

2.5

26.5

28.3

13.4

3.0

3.5

25.5

13.5

3.5

-

26.1

13.5

2.5

16.0

16.0

34.4

3.0

2.6

17.6

34.5

3.5

-

16.1

34.7

34


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Table 3. The thermal behaviors of PLLA and PLLA/PDLA foams Code

L/D-0

L/D-5

L/D-10

L/D-20

L/D-20

P

T

△Hc

△Hm1

△Hm2

MPa

°C

J/g

J/g

J/g

2.5

100

-

41

-

3.0

100

-

40.2

-

3.5

100

-

40.3

-

2.5

100

-

34.8

6.3

3.0

100

-

34.5

6.0

3.5

100

-

33.6

6.5

2.5

100

-

29.6

13.5

3.0

100

-

29.2

13.8

3.5

100

-

29.9

13.7

2.5

100

-

16.6

34.8

3.0

100

-

17.1

35.1

3.5

100

-

16.5

34.7

2.5

100

-

16.6

34.8

2.5

110

-

17.4

33.9

2.5

120

-

16.2

34.9

2.5

130

-

17.6

33.8

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 50

Table 4. The Volumetric shrinkage of the PLLA and PLLA/PDLA foams. Code L/D-0

L/D-5

L/D-10

L/D-20

Thermal exposure conditions

Volume shrinkage (%)

80 °C

4.75

90 °C

10.06

100 °C

19.25

80 °C

2.75

90 °C

7.85

100 °C

13.25

80 °C

2.45

90 °C

3.10

100 °C

7.75

80 °C

1.65

90 °C

2.05

100 °C

5.55

36


Page 37 of 50

SC

(a)

L/D-20

(b) SC

Intensity /a.u.

Exo

L/D-10

L/D-5 L/D-0

SC

L/D-20 L/D-10 L/D-5 L/D-0

50

100

150

200

250

10

15

20

25

30

ο

Temperature /°C

2θ /

908

(c)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


L/D-20 L/D-10 L/D-5 L/D-0

960

920

880

840

−1

Wavenumber /cm

Figure 1. Characterization of crystallization behavior and crystalline structures of the as prepared PLLA and PLLA/PDLA samples: (a) DSC heating traces; (b) WAXD profiles; (c) IR spectra.

37



(a)

L/D-20

(b)

SC

SC

SC mesophase

L/D-10

Intensity /a.u.

L/D-20

Exo

L/D-5 L/D-0 Untreated

L/D-10 L/D-5

L/D-0

Untreated

50

100

150

200

250

10

15

ο

Temperature / C

20

25

o

2θ /

908

918

(c)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 50

L/D-20 L/D-10 L/D-5 L/D-0 Untreated

960

920

880

840

-1

Wavenumber /cm

Figure 2. Characterization of crystallization behavior and crystalline structures of the sautrated PLLA and PLLA/PDLA samples at 2.5 MPa: (a) DSC heating traces; (b) WAXD profiles; (c) IR spectra.

38


Page 39 of 50

L/D-20

SC

(a)

(b) SC

L/D-10 SC

Intensity /a.u.

mesophase

Exo

L/D-5 L/D-0

L/D-20

L/D-10 L/D-5 L/D-0

50

100

150

200

250

10

15

o

Temperature / C

20

25

30

o

2θ /

908

918

(c)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


L/D-20 L/D-10 L/D-5 L/D-0

960

920

880

840

-1

Wavenumber /cm


39



(a)

L/D-20

(b)

SC mesophase

SC

L/D-10 SC

Intensity /a.u.

Exo

L/D-5

L/D-0

L/D-20 L/D-10 L/D-5 L/D-0

50

100

150

200

250

10

15

20

25

o

o

2θ /

908

Temperature / C

(c)

918

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 50

L/D-20

L/D-10 L/D-5 L/D-0

960

920

880

840

-1

Wavenumber /cm


40


Page 41 of 50

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a

b

c

d

Figure 5. SEM micrographs of PLLA and PLLA/PDLA foams saturated at 2.5 MPa and foamed at 100 °C. (a) L/D-0 foam; (b) L/D-5 foam; (c) L/D-10 foam; (d) L/D-20 foam.

41



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

b

c

d

e

f

Page 42 of 50

Figure 6. SEM micrographs of PLLA and PLLA/PDLA foams saturated at 3.0 MPa and foamed at 100 °C. (a) L/D-0 foam; (b) L/D-5 foam; (c,e) L/D-10 foam; (d,f) L/D-20 foam.

42


Page 43 of 50

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a

b

c

d

Figure 7. SEM micrographs of PLLA and PLLA/PDLA foams saturated at 3.5 MPa and foamed at 100 °C. (a) L/D-0 foam; (b) L/D-5 foam; (c) L/D-10 foam; (d) L/D-20 foam.

43



28

30 2.5MPa 3.0MPa 3.5MPa

20 15 10 5 0

2.5 MPa 3.0 MPa

24

Average cell size /µm

Expansion ratio

25

20 16 12 8 4 0

L/D-0

L/D-5

L/D-10

L/D-20

L/D-0

L/D-5

L/D-10

L/D-20

13

10

12

10

3

Cell density (cells/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 50

11

10

10

10

9

10

2.5MPa 3.0MPa

8

10

L/D-0

L/D-5

L/D-10

L/D-20

Figure 8. The expansion ratio, cell density and average cell size of PLLA and PLLA/PDLA foams obtained at a foaming temperature of 100 °C

44


Page 45 of 50

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a

b

c

d

Figure 9. SEM of the L/D-5 (a), L/D-20 (b), and L/D-0 (c, d), foams. L/D-5 and L/D-20 foams were obtained by saturated at 2.5 MPa and foamed at 100 °C, respectively. L/D-0 foams were prepared by foaming the amorphous (c) and crystalline (d) saturated PLLA samples.

45



α

α

(a)

SC

α SC L/D-0 L/D-5

(b) α

SC

Intensity /a.u.

α

SC

Intensity /a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 50

SC

α

SC L/D-0 L/D-5 L/D-10

L/D-10

L/D-20 L/D-20

10

15

20

25

30

10

15

20

25

30

o

2θ /

o

2θ /

Figure 10. WAXD profiles of PLLA and PLLA/PDLA foams. The samples were saturated at 2.5 (a) and 3.5 (b) MPa and foamed at 100 oC

46


Page 47 of 50

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Figure 11. Digital photographs of L/D-0 and L/D-20 foams with hanging a weight of 200 g at 110 °C. The expansion ratio of L/D-0(a), L/D-0(b) and L/D-20 foams is 28.3, 2.5 and 1.7, respectively.

47



24 o

100 C o 90 C o 80 C

20

Shrinkage /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 50

16 12 8 4 0

L/D-0

L/D-5

L/D-10

L/D-20

Figure 12. Volumetric shrinkage of the PLLA and PLLA/PDLA foams. The foams were prepared by saturated at 2.5 MPa and foamed at 100 °C for 20 s. The foams were thermally treated within the hot water with temperature of 80-100 °C.

48


Page 49 of 50

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Before thermal treatment

After thermal treatment

L/D-0

L/D-0

L/D-20

L/D-20

Figure 13. The change of cellular structure of L/D-0 and L/D-20 foams before and after thermal exposure at 100 °C for 1 h.

49



The TOC graphic:

24 o

100 C o 90 C o 80 C

20

Shrinkage /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 50 of 50

16 12 8 4 0

L/D-0

L/D-5

L/D-10

L/D-20

50


Cell Morphology and Improved Heat Resistance of Microcellular Poly(l

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