Crystals in Situ Induced by Supercritical CO2 as Bubble Nucleation

Article pubs.acs.org/IECR

Crystals in Situ Induced by Supercritical CO2 as Bubble Nucleation Sites on Spherulitic PLLA Foam Structure Controlling Junsong Li, Xia Liao,* Qi Yang, and Guangxian Li College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065 Sichuan, China S Supporting Information *

ABSTRACT: The CO2-induced crystals were successfully utilized to regulate the structure of the foamed poly(L-lactic acid) (PLLA) spherulites with large numbers of cells ranging from nanoscale to microscale in three temperature regions. Region I (60−80 °C) was characterized by the double melting peaks and the nanocells with the dramatically improved cell density. By increasing the temperature to Region II (90−110 °C), more highly perfect crystals and the unique spherulitic foam morphology were developed. In Region III (115−120 °C) double melting endotherms emerged again, and microcells were formed. The double melting endotherms in Regions I and III originated from the imperfect crystals created during the saturation and cooling stage, respectively. A bimodal structure with microcells surrounded by nanocells occurred at 115 °C and 20 MPa. It was shown that cells could nucleate and grow in between the neighboring lamellar stacks and that the cells interacted with the spherulites.

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INTRODUCTION PLLA is considered as a promising candidate for the disposable petroleum-based materials and as a preferred biomedical material used for implanting, cell culture, and tissue engineering.1,2 However, the main bottleneck limiting the mass commercial applications of PLLA is the brittleness. Supercritical carbon dioxide foaming technology can effectively increase the toughness by introducing microcells and/or nanocells into the polymer matrix and simultaneously shows the lowered processing temperature resulting in the less high-temperature molecule decomposition due to the CO2 plasticization effect. The toughening effect works efficiently only when a sufficient number of cells smaller than critical crack size are created in polymers. In order to maximize the positive impact of the incorporated cells on the mechanical properties, large numbers of research groups have attempted to develop nanofoams with cells on the order of 100 nm or less ever since the invention of microcellular foams.3−5 The common strategy is to include a second material possessing higher strength. The stiffer material could decrease the cell growth rate by strengthening the matrix. On the other hand, according to the heterogeneous nucleation theory, good quantities of phase interfaces are formed to boost cell nucleation density upon the incorporation of the second phase. The more gas the cell nucleation consumes, the less gas the cell growth gains. Consequently, the cell growth rate decreases further. The added material can be either a polymer or a nanofiller. In fact, adding nanofiller is the most popular way to regulate cell size and density.6 It is flexible to control the cell nucleation and growth by varying the size, content, and surface © XXXX American Chemical Society

chemistry of nanofillers. However, the inferior compatibility between the dispersed fillers and the matrix is adverse to the mechanical properties of foam. Moreover, the added fillers probably destroy the biodegradability of PLLA and sacrifice its certain application like biomedicine. If the crystalline phase is created as the control agent for foam structure in PLLA, not only can the ticklish problems on the loose interface bonding and the tight nanofiller aggregation be perfectly avoided but also the heat resistance and mechanical properties are enhanced. A semicrystalline polymer like PLLA is generally regarded as a multiphase system where crystalline domains disperse in the amorphous matrix. The crystallization rate of PLLA is very slow under atmosphere. However, the crystallization process can be accelerated under compressed CO2, because the dissolved CO2 induces the swelling of polymers and decreases the energy barrier for the molecular retraction and fold.7 Furthermore, the intense interaction between the electron-donating carbonyl oxygen of PLLA and the electron-accepting carbon of CO2 greatly promotes the CO2 dissolution and then reinforces the plasticization effect.2,8 PLLA can even be sufficiently crystallized at 0 °C under 3 MPa CO2 for 2 h due to the strong plasticization effect.9 The crystalline domains are considered to be impenetrable for CO2 molecules because of the compact Received: Revised: Accepted: Published: A

June 7, 2017 September 5, 2017 September 7, 2017 September 7, 2017 DOI: 10.1021/acs.iecr.7b02348 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

the real melting temperature under CO2. Based on the previous research, it is reasonable to speculate that foaming also takes place in the interlamellar amorphous domains of the CO2induced spherulites. So both an ironclad proof and a detailed investigation about the formation and evolution of cells in the CO2-induced spherulites are imperative. In the present work, the special structure and morphology of the crystals in situ developed under CO2, which are varied by the CO2 treatment conditions, are employed to tailor the cell nucleation and growth in PLLA. The unique transitions from nanocells to microcells through needlelike cells and bimodal cells in the spherulites formed under CO2 are specifically investigated in detail. As the foam structure is closely related to the crystalline structure, the thermal behavior of the corresponding foams is first studied to uncover the crystallization behavior of PLLA under CO2. The crystalline morphology of PLLA saturated under CO2 is also revealed by etching to better clarify the morphological conversion of foam.

chain packing and expel CO2 from the advancing crystalline− amorphous phase interface during the crystallization. Several research groups have studied the crystallization kinetics of PLLA under CO2 in detail by high-pressure differential scanning calorimeter (DSC),7,10−14 high-pressure Fourier transform infrared spectroscopy,15 and ultrasonic device.16 On the whole, the increased chain mobility could enhance the crystallization rate by increasing the crystal growth rate in the crystal-growth controlled region and depress the crystallization rate by preventing the formation of stable crystal nuclei in the nucleation controlled region. As a result, the crystallization rate vs temperature curve shifts to lower temperature, and simultaneously the maximum crystallization rate increases with increasing pressure. CO2 also exerts a significant influence on the crystalline morphology9,17−19 and crystal form.9,20 With the consideration of both the plasticization effect and the excusion effect of CO2, Saito et al. reported that the crystalline morphology of iPP varied from spherulites to distorted domian crystals and then to the needle crystals by melt crystallization in CO2.17,18 Zhang et al. obtained the unique unclassical ringbanded spherulites in six-arm star-shaped poly(ε-caprolactone) via CO2 treatment.19 Marubayashi et al. showed that the crystal modification changed from α″ to α forms in the presence of CO2 in PLLA, and that the rodlike crystalline superstructure on a nanometer scale was formed at 0−10 °C under 7−15 MPa and 0−20 °C under 3 MPa.9,20 With a deeper insight into the properties of polymer under CO2, the crystalline morphology and structure could be better tailored through the processing parameters to design the desired foam structure. The effect of crystallization on the foaming behavior is extensively investigated in many semicrystalline polymers, such as polyethylene,21,22 polypropylene (PP),21,23−25 poly(ethylene terephthalate),21,26 poly(vinylidene fluoride),27 poly(ester amide),28 polyphenylene sulfide,29 poly(ε-caprolactone),30,31 and PLLA.2,7,32−40 The high energy interface between the crystalline and amorphous domains leads to less Gibbs free energy necessary for cell nucleation and therefore acts as heterogeneous nucleating sites. Plenty of papers focus on the impact of crystalline content on the cell nucleation and growth. However, relatively little work has been published on the foam morphology control via crystalline structure on the scale of a single spherulite. Zhao et al. reported that submicrosized cells appeared in the interlamellar amorphous regions of spherulites in isotactic PP (iPP).23 They further obtained nanocells in the amorphous domains confined by shish kebab crystalline domains.25 However, those crystalline structures in their research had been formed before CO2 saturation, and CO2induced crystals were not used to regulate the foam morphology. The crystallization process of PLLA takes much more time under atmosphere compared with that of iPP. Therefore, Taki et al. employed the CO2-induced spherulites as bubble nucleation agents to in situ observe the foaming process in PLLA and vividly revealed that cells preferred to nucleate around the growing spherulites.37 Nevertheless, they did not further demonstrate whether or not cells nucleated in the CO2induced spherulites. More recently, Nofar et al. reported the significant effect of the crystals generated during the CO2 saturation and the foaming and cooling on the structure of polylactide bead foams with double melting peaks.41−43 However, neither did they compare the morphology and structure of the crystals formed under CO2 with that of the foams, nor did they study the double melting behavior of polylactide foams prepared at lower temperatures far away from

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EXPERIMENTAL SECTION Materials. PLLA pellets (Nature Works 2002D, Mw = 1.38 × 105 g/mol, Mw/Mn = 1.56) with a D-content 4.3% were supplied by Unic Technology (Suzhou) Ltd., China. Commercial purity CO2 (99.9%) was used as a physical blowing agent. Sample Preparation. PLLA sheets about 600 μm thickness were prepared by compression molding at 200 °C under 5 MPa for 5 min using hot press, followed by quenching in ice−water. The amorphous PLLA sheets were placed into a high pressure vessel and exposed to supercritical CO2 under the desired pressures and temperatures for 4 h so as to reach the solubility equilibrium of CO2 in PLLA.44,45 The temperature was controlled at an accuracy of ±0.5 °C. Following the saturation step, the high pressure vessel was depressurized quickly (over 5 to 7 s) to ambient pressure and simultaneously cooled to room temperature within 1−3.5 min. The foamed specimens were aged under dry conditions at room temperature for at least 1 week prior to further study. The preparation of unfoamed PLLA spherulites has been described elsewhere.36 Characterization. Thermal analysis was carried out by DSC Q20 (TA Instruments, USA). A specimen of 5−6 mg in weight was sealed in an aluminum pan and heated from 25 to 190 °C at a heating rate of 10 °C/min under a nitrogen gas atmosphere. The degree of crystallinity (χc) for each specimen was calculated from the following equation χc (%) =

ΔHm − ΔHc ΔHm0

× 100 (1)

where ΔH0m is the heat of fusion for perfectly crystalline PLLA (93 J/g), ΔHm is the heat of fusion for PLLA during a DSC heating run, and ΔHc is the heat of crystallization for PLLA during a DSC heating run. The temperature modulated DSC (TMDSC) measurements were performed on a Netzsch DSC 204 F1 instrument with 5− 6 mg samples at a heating rate of 3 °C/min having an oscillation period of 60 s and amplitude of 0.5 °C. XRD measurements were conducted on a DX-1000 system with Cu Kα radiation (λ = 0.154 nm) operating at 40 kV and 25 mA in the diffraction angle range from 5 to 40° with a scanning rate of 0.06°/s. Time sweep experiments were carried out on a rheometer (MCR-102, Anton Paar) with parallel-plate geometry to learn B

DOI: 10.1021/acs.iecr.7b02348 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. DSC curves of PLLA foamed at (a) 20 MPa and the indicated temperatures and (b) 100 °C and the indicated pressures.

the variation of viscosity during isothermal crystallization. The angular frequency and strain were 1 Hz and 5%, respectively. Disk-shaped PLLA was heated to 180 °C and held for 5 min to erase the thermal history. Then the sample was cooled rapidly to the desired temperature. Once the desired temperature was reached, the variation of viscosity with time was recorded. The morphology of the foamed and unfoamed specimens was characterized by scanning electron microscopy (SEM, JEOL SJM-5900VL) and image analyses. The method of determining the average cell diameter (D) was clarified in detail elsewhere.36 The cell density (N0), that is, the number of cells per cubic centimeter of nonporous sample before conditioning with CO2, was determined using the following equations Nf N0 = , 1 − Vf

⎛ nM2 ⎞3/2 Nf = ⎜ ⎟ , ⎝ A ⎠

Vf =

π 3 D × Nf 6 (2)

where Nf is the number of cells per cubic centimeter of the foamed sample, Vf is the void volume fraction, n is the number of cells on the SEM image, M is the magnification factor, and A is the area of the micrograph (in cm2).

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RESULTS AND DISCUSSION Thermal Behavior of scCO2-Foamed PLLA. DSC curves of the PLLA foamed at 20 MPa and 60−120 °C are shown in Figure 1a. Foams prepared at 60−80 °C and 110−120 °C display double melting endotherms, while each of those obtained at 90−100 °C exhibits a single melting endothermic peak. In the following discussion, these melting peaks are denoted as L, L′, and H. The double melting behavior, which is a common characteristic for many semicrystalline polymers, has been explained by the melting−recrystallization process which occurred during the heating run.46−49 That is, the melting of PLLA proceeded through the fusion of a certain amount of original crystals, recrystallization, and the final melting of more perfect crystals which were partly grown during primary crystallization and partly formed through the reorganization process.50 For the PLLA foamed at 110−120 °C, besides the

Figure 2. Peak temperatures of PLLA foams obtained at (a) 20 MPa and different temperatures and (b) 100 °C and different pressures.

C


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Figure 3. TMDSC curves for PLLA samples foamed at 20 MPa and different temperatures: (a) 70, (b) 100, (c) 110, and (d) 120 °C. The insets in parts a and b refer to the fitted peaks of the reversible and total double melting endotherms, respectively.

temperature, while that of the recrystallized crystals through the heating run remains almost constant. To disclose the origin of the double melting behavior during the heating run, the TMDSC results are displayed in Figure 3. TMDSC has special advantages over conventional DSC for investigating the complicated thermal process by dividing the total heat flow signal into a reversible part relative to heat capacity and a nonreversible one arising from kinetic effects.51,52 Chosen as examples, the reversible, total, and nonreversible heat flow curves of TMDSC for PLLA foams prepared at 20 MPa and 70, 100, 110, and 120 °C are presented. It is found that the total heat flow curves are similar to the corresponding conventional DSC curves conducted at a low heating rate. From Figure 3a, double melting peaks in the reversible heat flow curve can be observed in the range from 120 to 160 °C, corresponding to L and H in the total heat flow curve, respectively. Moreover, the low reversible melting peak is much bigger than L, whereas the high one is smaller than H. In the nonreversible heat flow curve, a big exothermic peak ascribed to the recrystallization appears to the accompaniment of the low reversible endothermic peak. Obviously, the low reversible melting endotherm is partially offset by the nonreversible exotherm to result in L having small intensity in the total heat flow curve. However, both the high reversible and nonreversible melting peaks positively contribute to H

double melting peaks, a small and broad exothermic peak exists prior to the melting endothermic peaks in each DSC curve, implying that the crystallization during the CO2 saturation period was insufficient. In contrast, the exothermic peak cannot be observed for the foams obtained at 60−100 °C, showing that the PLLA was completely crystallized. From Figure 1a, the exothermic peaks of the DSC curves at 110, 115, and 120 °C appear at about 89.7, 91.0, and 114.2 °C, respectively. According to the perfection of crystals developed in PLLA foam, the saturation temperature can be divided into three temperature regions, Regions I (60−80 °C), II (90−110 °C), and III (115−120 °C). The characteristics of the melting and crystallization behavior in three temperature regions are as follows. Region I (60−80 °C): L is related to the crystals formed by the primary crystallization during the saturation process, while H corresponds to those recrystallized after the (partial) fusion of initial crystals. From Figure 1a, it is clear that the relative intensity of L to H increases with the saturation temperature, suggesting the formation of more initial crystals with increasing perfection during the saturation process and the decrease in the extent of recrystallization during the heating run. From Figure 2a, Tm(L) increases gradually with the saturation temperature, whereas Tm(H) is almost constant. This result indicates that the thickness of initial lamellae increases with the saturation D


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Figure 4. TMDSC curves for PLLA samples foamed at 100 °C and different pressures: (a) 12, (b) 16, (c) 24, and (d) 28 MPa.

with the low melting peak denoted by L′ moving to lower temperature and the high one shifting to higher temperature, as shown in Figure 1a. From Figure 3c, the double melting endothermic peaks are closely connected in the reversible, total, and nonreversible heat flow curves. Because of the lack of the exothermic peak during melting in the nonreversible heat flow curve, it can be inferred that the structural reorganization did not happen. Therefore, PLLA foamed at 110 °C is suggested to belong to Region II. Based on this result and the XRD curves (shown in Figure 5), it follows that the double melting peaks originate from dual lamellae populations instead of meltrecrystallization and dual crystal structures. The enthalpies of the cold-crystallization peak and the separated L′ in the total heat flow curve are 3.12 J/g and 15.52 J/g, respectively. The result shows that the appearance of L′ cannot be entirely ascribed to cold-crystallization. The crystals corresponding to L′ are considered to be formed in the cooling process following the rapid depressurization. On the one hand, PLLA treated at 110 °C under 20 MPa CO2 was insufficiently crystallized. On the other hand, the molecular mobility was still high during cooling due to the CO2 plasticization effect. Therefore, the crystals developed in the saturation period could further facilitate the crystallization during the cooling process. Although the crystals corresponding to peak L′ has a low

having big intensity. It can be concluded that the melting of initial crystals and the recrystallization process occur almost simultaneously. The enthalpy of the nonreversible recrystallization peak is 10.85 J/g. The reversible melting enthalpy of the recrystallized crystals is 8.68 J/g by fitting the reversible double melting peaks using the Gaussian model. Therefore, the nonreversible melting enthalpy of the recrystallized crystals is 2.17 J/g. The results indicate that the melting of the recrystallized crystals contains not only a reversible signal but also a nonreversible part. In addition, the melting enthalpy in the nonreversible heat flow curve is 5.82 J/g from Figure 3a. So the nonreversible melting enthalpy of the initial crystals is 3.65 J/g. It follows that the melting of the initial crystals also contains reversible and nonreversible parts. Region II (90−110 °C): With an increase in the saturation temperature to 90 and 100 °C, H merges with L and fades away at the high temperature side. The disappearance of H demonstrates that the recrystallization process no longer exists and that highly perfect crystals have been formed during the saturation process, as revealed by the results of TMDSC. From Figure 3b, it can be evidently seen that the melting of initial crystals contains both reversible and nonreversible parts, which agrees with the conclusion above. By further increasing the saturation temperature to 110 °C, however, foamed PLLA displays double melting peaks again E


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Figure 7. Cell morphology of PLLA foams prepared at 20 MPa and different temperatures: (a) 60 °C; (b) 70 °C; (c) 80 °C. The magnification factor is held constant.

ambient pressure to about 120 °C at 20 MPa. The depression of Tm of PLLA is consequently estimated to be around 2 °C/ MPa, which is almost consistent with the measurement results (dTm/dp = 2.1 °C/MPa, in the pressure range of 0.1−15 MPa) by high pressure DSC.14 The incomplete crystallization of PLLA leads to the appearance of peak L′ and the cold crystallization peak during the heating run. After the rapid depressurization, the cooling process is considered as the nonisothermal crystallization. The starting temperature of the nonisothermal crystallization depends on the saturation temperature. That is, the higher the saturation temperature, the higher the starting temperature. The increasing starting temperature is responsible for the gradual increase in peak temperature and peak height of L′ for PLLA foams prepared at 110−120 °C. For the PLLA foamed at 115 °C, the melting behavior is the most complex of all, just like its complicated cell structure (Figures 9a1-a2)). The crystals corresponding to the high melting endotherm possibly consist of the contributions of the primary crystallization, cold crystallization, and reorganization process. To emphasize the recrystallization process, the high melting endothermic peak is denoted by H. In our previous communication, the foam morphology and structure of PLLA at 100 °C and different pressures have been studied in detail.36 In the present paper, the thermal behavior of PLLA foam prepared at different pressures is illustrated in Figures 1b, 2, and 4. For the sample obtained at 12 MPa, three endotherms appear in the range from 130 to 160 °C in the DSC curve. These peaks are assigned as E, L, and H from low to high temperatures, respectively. The TMDSC results confirm that the occurrence of peaks L and H originates from the melt-recrystallization process, as shown in Figure 4a. The endotherm corresponding to E is clearly present in the nonreversible heat flow curve, while no endothermic peak is displayed in the same temperature range in the reversible heat flow curve. Accordingly, E is attributed to the nonreversible enthalpy relaxation of the rigid amorphous fraction in PLLA instead of the melting of crystals, similar to annealing peak.53−55 From Figure 4a the nonreversible enthalpy relaxation occurs at the beginning of melting. However, as

Figure 5. XRD patterns of the PLLA foams (20 MPa, 60−120 °C) and the amorphous PLLA film.

Figure 6. Time sweep rheological curves of PLLA at different temperatures under atmosphere pressure.

melting temperature and lamellar thickness, they possess relatively high perfection. Region III (115−120 °C): At 115−120 °C, peak H emerges again and coexists with L′. From Figure 2a, Tm(L′) increases with the saturation temperature, whereas Tm(H) decreases abruptly. The steep drop of the crystallinity shows that the crystallization rate of PLLA under CO2 falls sharply. The crystallization rate is too low to obtain stable crystals at 120 °C under 20 MPa CO2. As a result, the value of Tm(H) at 120 °C is even lower than the value of Tm(L) at 110 °C. Therefore, it is suggested that the Tm of PLLA is reduced from about 160 °C at F


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Industrial & Engineering Chemistry Research Table 1. Structural Information of PLLA Foam Prepared at 20 MPa and Different Temperatures T (°C) χc (%) D (μm) N0 (1014 cm−3) a

60

70

80

90

100

110

115

34.5

34.7 0.062 0.51

37.3 0.054 1.69

39.4 0.075 1.55

40.2 0.085 1.69

36.1 0.110 0.89

28.7 0.053 (6.2)a 1.76 (2.04 × 10−5)a

The numbers shown in the parentheses refer to the average cell diameter and cell density of microcells at 115 °C.

temperature of PLLA under CO2 and the saturation temperature, resulting in the decreased crystallization rate (crystal nucleation rate) and the incomplete crystallization during the saturation period. The crystals related to L and L′ are formed before and after the depressurization process, respectively. It follows that the physical properties (especially the crystallization rate) of PLLA treated with 28 MPa CO2 at 100 °C resembles that at 20 MPa and 110 °C. Additionally, the physical resemblance is directly reflected in the similar thermal behavior such as the cold crystallization temperature, the heat of cold crystallization, the heat of fusion, and the peak temperatures of double melting endotherm. Therefore, the corresponding relationship between the saturation temperature and pressure is estimated to be 1.25 °C/MPa, which is lower than that between the melting temperature and pressure (about 2 °C/ MPa). From the literature, the pressure dependence of Tg, crystallization temperature (Tc), and Tm differs from each other.10,56−58 Therefore, it could be inferred that the relationship between temperature and pressure strongly relies on the selected physical state of the polymer/CO2 system. X-ray Diffraction Patterns of scCO2-Foamed PLLA. XRD was used in this study to characterize the crystal polymorphism of PLLA foams. Figure 5 shows the raw XRD patterns of PLLA foams obtained with 20 MPa CO2 at 60−120 °C compared with that of the amorphous film. According to the discussion above, during the cooling stage the crystalline content at 120 °C is bigger than that at 115 °C. However, a strong diffraction peak appears around 2θ = 16.7° for the foams prepared at 115 °C, whereas an extremely weak diffraction peak is observed at 120 °C. The results indicate that a large number of crystals were formed during the saturation process rather than after that at 115 °C, compared to that at 120 °C. Therefore, Tm(H) at 115 °C is the highest in Figure 2a. Indexing of the diffraction peaks is based on the reported crystal structure for the α and α′ forms of PLLA.47,59−66 The reflection of the (211) plane, which appears only in the α form,48,61−66 can be seen at 2θ = 22.4° for foams obtained at 60−115 °C. In addition, the characteristic diffraction of the α′ form of PLLA, attributed to (116) plane at 24.5°,48,61−66 is absent for all foamed samples. Therefore, the α crystal form is induced in PLLA foams prepared at 20 MPa and 60−115 °C. From Figure S2, the crystal form seems unaffected by varying the pressure from 12 to 28 MPa. Rheological Analysis of PLLA. Figure 6 shows the variation of complex viscosity (η*) of PLLA in air at 110− 150 °C. The selected temperature range is close to the melting temperature of PLLA in air. Although the data are obtained at atmosphere pressure, they still could reflect the trend of viscosity variation for PLLA saturated with CO2 at temperatures near the melting region.24 For the PLLA at 110−130 °C, the complex viscosity increases slightly at the first several minutes due to the unstable sample temperature, then remains constant, and at last further increases with time because of crystallization. As expected, the complex viscosity decreases a

Figure 8. SEM photographs of PLLA foams prepared at 20 MPa and different temperatures: (a1) and (a2) 90 °C; (b1) and (b2) 100 °C; (c1) and (c2) 110 °C. The magnification factor is held constant for a1-c1 and a2-c2, respectively.

mentioned above, the recrystallization process and the melting of initial crystals almost start contemporaneously. Therefore, it can be concluded that the enthalpy relaxation peak must have partially covered the recrystallization peak in the nonreversible heat flow curve. For the PLLA foamed at 16 MPa, E shifts to higher temperature, and L disappears in Figure 1b. From Figure 4b it is found that the intensity of the melting endotherm keeps almost constant at 151−155 °C in the reversible heat flow curve, indicating that crystals with higher thermal stability are formed during the heating run, so the recrystallization process actually exists during the melting. The disappearance of L in the total curve is possibly due to the severe overlap between the enthalpy relaxation and the recrystallization process in the nonreversible heat flow curve. For foams obtained at 20 and 24 MPa, highly perfect crystals are developed during the CO2 saturation period. With further increasing the saturation pressure to 28 MPa, the sample demonstrates double melting peaks again with the low endotherm moving to lower temperature. This is because an increase in pressure causes a further decrease of the difference between the melting G


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Figure 9. Cell morphology of PLLA foams obtained at 20 MPa and different temperatures: (a1)-(a3) 115 °C and (b1) and (b2) 120 °C. (a3) and (b2) are the cell walls of the corresponding foam.

Moreover, the restricted cell growth induced by the high viscoelasticity leads to a low CO2 consumption rate and favors the cell nucleation.67 Therefore, the nucleation rate is greatly high. Figure 8 displays the SEM photographs of PLLA foams in Region II. The novel foamed spherulite morphology, which consists of nano- and submicrosized cells and radially distributed needlelike cells, appears in an exquisite manner. It is well-known that CO2 can diffuse into the amorphous regions rather than the crystalline parts and that foaming only takes place where CO2 dissolves. Therefore, from the morphological texture it can be concluded that spherulites are made up of the lamellae and the interlamellar amorphous layers. This conclusion is experimentally verified by etching the unfoamed PLLA treated under the same saturation conditions, as shown in Figures 10d-f. In our last paper, a similar foam morphology consisting of needlelike cells and nanocells was also developed with increasing pressure.36 The result once again shows that the similar physical state of PLLA under CO2 can be reached by adjusting either the saturation temperature or pressure. An increase in the saturation temperature from 80 to 90 °C and above gradually decreases the viscosity of interlamellar layers, increases the CO2 diffusion rate and the cell growth rate, and induces the nanocell to grow further. However, because of the tight constraint of the neighboring lamellae, nanocells actually grow along the long axis of the lamellae, resulting in high aspect ratio cells with long axes ranging from the submicrometer to micrometer scale and short axes on the nanometer and submicrometer scale. Besides, cell coalescence also makes a contribution to the formation of needlelike cells. With increasing the saturation temperature to 110 °C, open cells are present in the interlamellar regions due to cell rupture induced by the further reduction in viscosity, as shown in Figures 8c1-c2. It is found that the material between two neighboring needlelike cells at 110 °C is thinner than those at 90−100 °C. If the material located in between neighboring needlelike cells only stands for a single lamella, the thickness of lamella formed at 110 °C is smaller than that at lower temperatures. This result contradicts the conclusions drawn from the DSC curves.

great deal with increasing the temperature. The onset time of the abrupt growth in complex viscosity increases with increasing temperature, indicating the depressed crystallization rate at high temperature. The slope of complex viscosity vs time decreases with increasing temperature, showing that the crystallization rate decreases with temperature. At 140−150 °C, the crystallization rate is too low, and the complex viscosity decreases because of molecular degradation. At 110−130 °C, the crystals formed during the isothermal process could restrict the mobility of neighboring molecules, act as physical network junctions, and progressively increase the viscosity and modulus. When the crystals become sufficiently interconnected, i.e., the storage modulus equals to the loss modulus displayed in the inset, PLLA transfers from the liquid-state to the solid-state. Subsequently, the complex viscosity increases faster. As the crystallization is finished, there is little variation for the complex viscosity and a platform emerges in the curve. Transitions of Cell Structure in the Foamed Spherulites. Figure 7 shows the PLLA foam morphology in Region I characterized by the nanoscaled cells. At 60 °C, merely sporadic nanocells are locally generated. However, by increasing the saturation temperature to 70 and 80 °C, a substantial number of nano- and submicrosized cells appear. The corresponding crystalline morphology of PLLA treated under scCO2 is illustrated in Figures 10a-c. It is found that each spherulite is closely connected to the neighboring spherulites and crystallites. As a result, a tight physical network composed of lamellae is evolved. The physical lamellae network could cause a sharp rise in the viscosity of PLLA, as revealed in the rheological results. At 60 °C PLLA is too stiff to participate in cell nucleation and growth. An increase in temperature to 70− 80 °C induces the foaming by decreasing the matrix viscosity. However, because the viscosity is still greatly high, the growth rate of cell nuclei remains very low, resulting in the formation of nano- and submicrosized cells. From Table 1, the cell density listed is extremely high and up to 13−14 orders of magnitude, indicating excellent gas nucleation efficiency. From Figures 10bc, the nano- and submicrosized amorphous regions are highly dispersed over the lamellar network, resulting in the formation of large numbers of heterogeneous nucleation interface. H


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Accordingly, piled-lamellae behave as a single block to restrict the cell growth instead of each individual lamella during foaming. Although the neighboring lamellae could impose a restriction on the cell expansion, the growth of needlelike cells and other cells in the interlamellar regions in turn leads to the split of adjoining piled-lamellae from each other and hence the expansion of spherulites. The expansion ratios of PLLA and spherulites are discussed elsewhere.36 From Figures 8a2-b2, it is obviously seen that numerous nanocells are generated in the blocks (piled-lamellae) located in between neighboring needlelike cells. CO2 treatment could probably induce the loose packing of lamellae, leading to the cell nucleation and growth in the less ordered regions of the blocks. The less ordered regions might be attributed to the CO2 exclusion effect.9 The nanoscaled amorphous regions in the unfoamed spherulites, as shown in Figures 10d2-e2, probably illustrate the loose packing of lamellae. It is interesting to note that nanocells and needlelike cells also turn up at the centers of foamed spherulites in Figures 8a2-b2, corresponding to the nanoscaled and needlelike amorphous regions at the centers of unfoamed spherulites in Figures 10d2-e2. It follows that the crystal nuclei of spherulites can participate in cell nucleation and growth. In addition, several needlelike cells are embedded in the boundaries of the foamed spherulites in Figure 8b2, which could be suggested to originate from the impingement of spherulites revealed in Figure 10e2. The cell morphology of PLLA in Region III is illustrated in Figure 9. Micrometric cells appear considerably under these foaming conditions. Unlike the PLLA foamed at lower temperatures, the foamed spherulite structure is not recognizable any more. The foam prepared at 115 °C is characterized by the bimodal cell morphology in which the average cell diameters of microcells and nanocells are 6.2 μm and 53 nm, respectively. The bimodal cell structure is closely connected to the crystalline morphology. According to the analyses above, the nano- and submicrosized cells located in the microcellular wall are formed in the interlamellar amorphous layers of spherulites. From Figure 10g2, there are large amorphous regions in the spherulites besides the lamellae and interlamellar amorphous layers. In addition, large amorphous regions also exist between neighboring spherulits. It can be inferred that microcells are developed at the large amorphous regions located both inside and outside the spherulites because of little constraint on the cell growth there. That is, a faster growth rate can be obtained at the large amorphous regions compared to that at the interlamellar regions. During the cell growth stage, microcells deplete large amounts of CO2, which further depresses the growth rate of cells in the cell wall. As a result, needlelike cells disappear. By further increasing the temperature to 120 °C, it is difficult for PLLA to crystallize. The viscosity of PLLA is the lowest due to the highest saturation temperature and the minimum crystalline content. Therefore, cell nuclei could grow fastest and finally evolve into microcells. Nanocells and needlelike cells could not be seen any more, as shown in Figure 9b2. The density of PLLA foam at 120 °C and 20 MPa is as low as 0.25 g/cm3, while that at 100 °C and 20 MPa is 1.05 g/cm3. The result shows that the expansion ratio of nanofoam is lower in spite of the higher cell density compared with that of microcellular foam. The inner surface of microcell at 120 °C is smooth, whereas the inner surface at 115 °C is quite rough from Figure 9a2. More specifically, there are a number of nano- and submicrosized cells and/or sporadic microcells on the inner

Figure 10. Crystalline morphology of PLLA treated at 20 MPa and different temperatures: (a1) and (a2) 60 °C; (b1) and (b2) 70 °C; (c1) and (c2) 80 °C; (d1) and (d2) 90 °C; (e1) and (e2) 100 °C; (f1) and (f2) 110 °C; (g1) and (g2) 115 °C. The magnification factor is held constant for a1-g1 and a2-g2, respectively. The yellow ellipses in part g2 refer to the large amorphous regions inside the spherulite. I


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Figure 11. Cell size distribution of PLLA foamed at 20 MPa and the indicated temperatures.

microcells at 115 °C could be ascribed to the cell coalescence. The cell density of microcells at 115 °C is over 3 times higher than that at 120 °C. In addition to the heterogeneous nucleation effect, the CO2 exclusion effect probably accelerates cell nucleation. The crystallization still proceeds for PLLA at 115 °C before depressurization. Therefore, the CO2 exclusion effect induces a local increase of gas concentration in the interface between the crystalline domains and the large amorphous regions during the crystallization.22,31,37 The higher CO2 concentration could decrease the nucleation energy barrier and promote the microcell nucleation. The origin of peak L′ in the DSC curves at 110−120 °C has been clarified above. As expected, Tm(L′) and the peak

surface at 115 °C. The nanocells and needlelike cells on the surface may be formed in the interlamellar layer and/or the interface between the crystalline domains and the large amorphous regions. The sporadic microcells on the surface may be developed in the large amorphous regions and/or the interface between the crystalline domains and the large amorphous regions. During the cell growth stage, the cell wall between two neighboring microcells becomes thinner as the cell growth imposes an extensional stress on the surrounding polymer melt.67 At a given moment, microcells encounter the cells on the inner surface. Subsequently, cell coalescence occurs with further cell growth because of the lower viscosity. Therefore, the rough surface inside the J


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Figure 12. Cell morphology (a)-(b), crystalline morphology (c)-(d), and cell size distribution (e) of PLLA obtained at 100 °C and 28 MPa. The DSC curves of foams at the indicated conditions are shown in part f. The N0, D, and χc of PLLA foam prepared at 100 °C and 28 MPa are 1.21 × 1014 cm−3, 0.091 μm, and 37.2%, respectively. The average spherulite diameter of the etched PLLA is 3.3 ± 0.4 μm.

Figure 13. Schematics of the cell structure transition mechanism in PLLA spherulites at 70−115 °C under 20 MPa CO2.

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resulting in the needlelike structure. Cell coalescence in the interlamellar amorphous regions also makes important contributions to the formation of needlelike cells. Consequently, spherulitic foam morphology consisting of nanocells and needlelike cells is developed. The decreased viscosity also promotes the mobility of chain, and then perfect crystals are obtained in Region II. The results show that the nanocellular growth has little influence on the crystalline structure. With further increasing the temperature to 115 °C, some parts of spherulites become molten, while the rest remains highly crystallized. Larger amorphous regions with a lower viscosity greatly promote the cell growth, while the crystalline regions still impose a tight restriction on the cell growth. As a result, a bimodal structure with the microcells surrounded by nanocells is obtained. The microcellular growth causes the chain orientation in the nearby polymer matrix, which promotes the crystal nucleation. A number of the nucleated crystals probably hinder the mobility of neighboring molecular chains. Consequently, less perfect crystals are developed during the cooling process in Region III. In addition, the difference in the growth rate and sites among microcells perhaps induces the irregular expansion of spherulites.

intensity increase with the saturation temperature. However, the crystals related to L′ become imperfect at 115−120 °C. The imperfectness of crystal at high temperatures is considered to be induced by microcell. Cell growth causes the extensional strain in the nearby polymer matrix, which is similar to biaxial stretching.39,68 As a result, the crystal nucleation energy barrier is decreased, and the crystal nucleation rate is enhanced in the following cooling stage. However, the appearance of large numbers of crystal nuclei may hinder the motion of polymer chains, leading to the formation of defective crystals. It can be inferred that the molecular orientation effect induced by microcell is much stronger than that by nanocell. Therefore, melt-recrystallization is observed for the foams at 115−120 °C and absent at 110 °C during the heating run. The cell size distribution of PLLA foams becomes slightly narrow and then gradually broad as the temperature is increased from 70 to 80 °C and from 90 to 110 °C, respectively, as shown in Figure 11. The fraction of cells below 100 nm (nanocells) increases from about 91% to 95% by increasing the temperature from 70 to 80 °C, which agrees with the variation trends of D and N0. In Region II, the increase of temperature from 90 to 110 °C causes a gradual decrease in the fraction of nanocells from 80% to 65%. The broadening of cell size distribution and the reduction in the fraction of nanocell can be explained as follows. PLLA treated at higher temperature has lower viscosity, which could accelerate the cell growth rate, enhance cell coalescence, and result in bigger cell diameter. Then the fraction of nanocells decreases, and the difference in cell diameter enlarges. The bimodal cell distribution of PLLA foamed at 115 °C is clearly illustrated in Figures 11f-g. The size distribution of cells in the cell wall is narrowest, and the fraction of nanocells is up to 95%. For the foams prepared at 110 °C and 20 MPa and at 100 °C and 28 MPa, the physical state of PLLA under CO2 has proved to be similar from the point of view of the crystallization behavior. The similar physical state also generated the similar cell morphology, as shown in Figures 8c1-c2 and 12a-b. All the cells obtained at 100 °C and 28 MPa are below 0.75 μm, and the fraction of cell below 0.75 μm at 110 °C and 20 MPa is about 99.4%. Although the cell morphology prepared at 110 °C and 20 MPa resembles the one at 100 °C and 28 MPa, their structural characteristics are slightly different. Both the decrease of temperature from 110 to 100 °C and the increase of pressure from 20 to 28 MPa increase the solubility of CO2 in PLLA, which promotes the cell nucleation. With the increase of N0 from 0.89 × 1014 cm−3 to 1.21 × 1014 cm−3, D decreases from 110 to 91 nm. Meantime, the fraction of nanocells increases from 65% to 72%. It can be concluded that the CO2 solubility also exerts a significant influence on the cell structure. Schematics of Cell Morphological Transitions in Foamed Spherulites. Figure 13 is a schematic illustration representing the cell nucleation and growth at 70−115 °C. At 70−80 °C, cells nucleate at the interface of lamellae and amorphous layers. Nanofoams are obtained due to both the heterogeneous nucleation and the too stiff matrix in the presence of crystalline network. The too high viscosity also depresses the molecular mobility and probably induces the formation of less perfect crystals in Region I. By increasing the saturation temperature to 90−110 °C, the matrix viscosity decreases, and the lamellar thickness increases. The former helps the cell growth, whereas the latter facilitates the lamellae to strongly constrain the cell growth. Therefore, cells only grow along the radial direction of spherulites,

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CONCLUSIONS The thermal behavior of PLLA foam prepared by scCO2 and its morphological transitions from nanocells to needlelike cells, then to microcells in the foamed spherulites, was investigated in detail. Both the multiple melting and the foam structure of PLLA were closely related to the crystalline behavior under CO2. The cell structure could be well regulated by tailoring the crystalline structure and morphology of PLLA. It was found that nanosized and needlelike amorphous regions formed in the CO2-induced spherulites offered the space necessary for cell growth. The presented work described a clear relationship between the crystalline structure and the foam structure, which provided a distinctive way to fabricate nanofoams.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b02348. DSC curves at different heating rates for PLLA foams; XRD patterns of the PLLA foams; and the average spherulite diameter of PLLA (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86 28 8540 8361. E-mail: [email protected]. ORCID

Xia Liao: 0000-0002-4093-0507 Qi Yang: 0000-0002-3831-1002 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51373103 and 51421061) and the Science and Technology Department of Sichuan Province, China (2015HH0026 and 2013GZ0152). L


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Crystals in Situ Induced by Supercritical CO2 as Bubble Nucleation

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