Nanocellular and needle-like structures in poly(L-lactic acid) using spherulite templates and supercritical carbon dioxide

Abstract

Poly(L-lactic acid) (PLLA) foams with unique nanocellular and needle-like morphology were successfully prepared by combining spherulite templating and supercritical carbon dioxide (CO₂) foaming. The corresponding crystalline morphology formed under supercritical CO₂ in PLLA was illustrated to investigate the foaming behavior of spherulites in detail. It was found that not only the degree of crystallinity but also the crystalline morphology played a vital role in cell nucleation and growth, thus the foam morphology. Nanocellular structure was primarily generated for the PLLA foamed at 100 °C and 12–24 MPa. Moreover, the morphological transition from approximate circular cells to needle-like cells occurred around 16 MPa at 100 °C because of the constraint of lamellae, and the two different structures coexisted at 100 °C and at pressures ranging from 16 to 24 MPa. The results indicated that the expansion ratio of spherulite was bigger than that of PLLA foam.

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Introduction

Nanocellular materials have aroused great interest in both industrial and academic fields due to their tremendous potential for applications such as molecular filtration, tissue engineering and catalysis.¹ Many researchers have been engaged in the development of a promising approach to prepare new materials with nanostructure. Nowadays supercritical CO₂ foaming technology is an increasingly popular method to introduce nano- and micro-cell structure into polymers because of the features of the environmental benignancy, easy volatilization and considerable solubility in CO₂-philic polymers.^1–6 By utilizing the significant solubility difference between fluorinated blocks and the non-CO₂-philic blocks, Yokoyama et al. extensively investigated the tunable nanocellular structure in block polymers using supercritical CO₂.^7,8 Nevertheless, without the help of non-CO₂-philic blocks in block copolymers, the fabrication of nano-scaled structure would be confronted with numerous difficulties. Our paper wants to display a particular method to prepare nanostructure in semicrystalline polymers by using spherulites as templates.

Dissolved CO₂ can induce crystallization, depress the melting temperature, facilitate the formation of unique crystalline morphology and shift the curve of crystallization rate vs. temperature to lower temperature in semicrystalline polymers by means of significantly swelling molecular chains.^9–13 These physical changes will inevitably influence the following cell nucleation and growth in semicrystalline polymers. For instance, by inducing a crystallinity gradient accompanied by a matrix stiffness gradient in PLLA using subcritical CO₂, Liao et al. reported the skin-core structure foam with elongated microcellular structure in the core and submicro- to micro-sized pores within the skin.¹⁴ Similar to the above research, however, a large amount of work has only been focusing on the cell nucleation and growth in the amorphous regions and/or at the interface of the crystalline and amorphous regions in the semicrystalline polymer/CO₂ systems. Little work has been published on the detailed description of cell nucleation and growth in the internal parts of crystalline domains such as spherulites. Besides, some researchers considered that the rigid amorphous layers with a thickness of about several nanometers located in between lamellae could hardly participate to cell nucleation and growth due to the tight restriction of lamellae in the spherulites.¹⁵

Recently, our research group has studied the microcellular skin-core structure of dispersed polystyrene (PS) droplets which were embed in the continuous PLLA matrix phase.¹⁶ The unique foam morphologies generated in dispersed domains further aroused the interest to explore the foaming behavior of spherulites in multi-phase semicrystalline polymers. To our best knowledge, our work is the first report on the formation of nano-scaled needle-like cells in the interlamellar regions of spherulite in PLLA, and clearly shows the transition from the approximately circular cells to the radial needle-like cells. In order to uncover the origin of the spherical foam morphology consisting of random nanocells and radial needle-like cells in PLLA, the spherulite structure formed under supercritical CO₂ is also illustrated.

Experimental section

Sample preparation

PLLA pellets (Nature Works 2002D) were compression-molded into sheets with a thickness of 600 μm at 200 °C for 5 min under 5 MPa after preheating for 3 min, followed by quenching in ice-water. The amorphous PLLA sheets were placed into a high pressure vessel and exposed to supercritical CO₂ under the experimental temperatures and pressures for 4 h. The sorption equilibrium of CO₂ in PLLA can be reached within 4 h under the experimental foaming conditions.^17,18 Following the saturation step, the high pressure vessel was depressurized rapidly (about 5–7 s) to ambient pressure and cooled to room temperature within about 2–3.5 min.

To prepare the unfoamed PLLA sheets, after the saturation process the vessel was cooled to room temperature within 8 minutes. Then the vessel was depressurized slowly to atmospheric pressure. In order to investigate the cell nucleation and growth in spherulites, the crystalline morphology of PLLA treated under supercritical CO₂ was needed to be inspected. Therefore, the upper surface of the unfoamed sheet was etched by a water–methanol (1 : 2 by volume) mixture solution including 0.025 mol L⁻¹ of sodium hydroxide for 12 h at 12 °C, and subsequently the etched surface was cleaned by distilled water and ultrasonication.¹⁹

Characterization

DSC measurements were carried out by Q20TA instruments in the temperature range from 10 to 190 °C at a heating rate of 10 °C min⁻¹. A scanning electron microscopy (FEI inspect F50) was used to observe the foam structure on the cross section and the etched crystalline morphology. The average cell diameter (D) and the average spherulite diameter were estimated by image analysis software (Image-Pro Plus). The density (ρ) of the foamed PLLA was determined by the water displacement method.²⁰ Details regarding the statistical treatments of the degree of crystallinity (χ_c), the average cell diameter, the average spherulite diameter, the cell density (N₀), the spherulite density and the volume expansion ratios of foams and spherulites are provided in the ESI.†

Results and discussion

Fig. 1 illustrates the cell morphology of PLLA foamed at 100 °C and 12 to 24 MPa. For the PLLA foams prepared at 12 MPa, not only numerous nano- and submicro-sized cells but also very high cell density (summarized in Table 1) appears. The extremely high cell density is directly attributed to the great gas nucleating efficiency in PLLA/CO₂ system. Fig. 2 shows the corresponding crystalline morphology where the intercrystalline amorphous regions are removed by etching. From Fig. 2a₁ and a₂, the intercrystalline amorphous regions are clearly distributed over the tight networks which are made up of lamellae. During the crystallization process, the growing crystallites helped to exclude CO₂ from the crystal growth front into the intercrystalline amorphous regions because CO₂ cannot diffuse into the crystalline regions but the amorphous regions.^21–23 The interfaces of lamellae and intercrystalline amorphous regions are the preferred nucleation sites due to the lower nucleation energy barrier. Furthermore, the highly dispersed amorphous regions increase the interface area, which creates more heterogeneous nucleation sites and makes nucleation easier. Once the critical cells were formed stably, they would grow. However, the expansion of cells was restricted, since the tight lamellae network decreased the mobility of polymer chains and increased the matrix stiffness. As a result, large numbers of nanocells emerged. From literatures, nanocellular structure also appeared in the amorphous phase between crystalline lamellae in isotactic polypropylene.^5,6 From Fig. 3a, all the cells generated are below 280 nm and the fraction of cells below 100 nm (nanocells) is as high as 91%.

Fig. 1 SEM micrographs of PLLA foams obtained at 100 °C and different pressures: (a₁) and (a₂) 12 MPa; (b₁) and (b₂) 16 MPa; (c₁) and (c₂) 20 MPa; (d₁) and (d₂) 24 MPa.

Table 1 The structural information of PLLA foamed and unfoamed at 100 °C and different pressures

Saturation pressure	P (MPa)	12	16	20	24
a Df was defined as the average spherulites diameter which was measured from the SEM images of PLLA foams, whereas Du referred to that estimated from the unfoamed PLLA after etching treatment.b Nu was the spherulites density which was estimated from the SEM images of CO2-treated PLLA after etching.
Foam density	ρ (g cm^−3)	1.13	1.10	1.05	1.03
Cell diameter	D (nm)	60.1	73.2	85.0	83.5
Cell density	N0 (10^14 cm^−3)	1.05	1.65	1.69	1.71
Crystallinity	χc (%)	36.4	40.8	40.2	40.0
Spherulite diameter	Df (μm)^a	—	4.0 ± 0.4	4.6 ± 0.5	4.8 ± 0.3
Spherulite diameter	Du (μm)^a	2.6 ± 0.5	2.8 ± 0.3	3.2 ± 0.5	4.2 ± 0.8
Spherulite density	Nu (10^10 cm^−3)^b	3.36	2.99	3.36	1.37

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Fig. 2 SEM images of etched PLLA sheets after the CO₂ treatment at 100 °C under different pressures: (a₁) and (a₂) 12 MPa; (b₁) and (b₂) 16 MPa; (c₁) and (c₂) 20 MPa; (d₁) and (d₂) 24 MPa.

Fig. 3 Cell size distribution of PLLA foams prepared at the indicated conditions.

By increasing the pressure to 16 MPa, needle-like cells, which are radially distributed from a common center, occur with the nano- and submicro-sized cells. This texture reveals the architecture of the corresponding spherulites which consist of closely packed lamellae and interlamellar rigid amorphous layers, as shown in Fig. 2b₂. The viscosity of the interlamellar material would be lower at higher pressures because of the significant plasticizing effect of CO₂. This viscosity reduction would consequently promote the cell growth. That is, cells developed in the interlamellar regions should have grown larger due to both the high CO₂ concentration and the decreased viscosity. However, those cells actually grew along the radial direction of spherulites because of the constraint of neighboring lamellae, resulting in the formation of needle-like cells with long axis ranging from being on the order of submicro-meter to micro-meter and short axis from nano-meter to submicro-meter scale. Additionally, more cells would coalesce at higher pressure because of lower viscosity. Therefore, cell coalescence could also make a contribution to the formation of needle-like cells in the interlamellar amorphous regions. The increase of pressure to 16 MPa produced an increase of both D and N₀, which is suggested to be ascribed to the reduction in the viscosity of the interlamellar materials and the increase in crystallinity, respectively.

With further increasing pressure to 20 and 24 MPa, needle-like cells still coexist with nano- and submicro-sized cells. The distribution of cells in the foamed spherulites is in accord with that of rigid amorphous regions in the unfoamed spherulites, showing that these rigid amorphous regions greatly influence the foam structure including its size and shape. That is, the amorphous regions with bigger size benefit the cell growth, and needle-like amorphous regions located in between lamellae can induce the radial cell growth. It is found that, N₀ increases gradually at pressures ranging from 16 to 24 MPa while the degree of crystallinity varies little. According to the classical nucleation theory, the decreased nucleating energy barrier at higher pressure is responsible for the increase of N₀. From Fig. 3b and c, by increasing pressure from 16 to 20 MPa, the fraction of nanocells decreases from about 81% to 75% and the average cell diameter increases from 73.2 to 85.0 nm. The fraction of nanocells and the average cell diameter vary little with further increasing pressure to 24 MPa. In addition, the cell size distribution at 12–20 MPa shifts broader with pressure and the distribution peak moves to bigger size by further increasing the pressure to 24 MPa. This is because that the viscosity reduction and cell coalescence at higher pressure resulted in bigger difference in cell size.

A physical interaction evidently took place between spherulites and cells during the process of cell growth. The spherulites could restrict the cell growth due to the special crystalline structure, and in the meantime the cells could broaden the lamella spacing and expand the spherulites in order to reach thermodynamic equilibrium. The expansion ratios of spherulites at 16–24 MPa are summarized in Fig. 4. From Fig. 4, the expansion ratios of foams are small and increase gradually with saturation pressure compared with those of spherulites. In view of the higher foamability of the amorphous regions outside spherulites, the bigger expansion ratios of spherulites indicate that not all the spherulites take part in cell nucleation and growth. Cells may hardly nucleate and grow in the spherulites located in the skin due to large amounts of gas quickly diffusing out of the skin after depressurization.

Fig. 4 Expansion ratio of PLLA (Φ_f) and spherulites (Φ_s) foamed at 100 °C and different pressures.

Fig. 5 illustrates the schematic diagram of cell growth in PLLA at different pressures. Nanocells appear in the interlamellar layers due to both the high cell density and the stiff matrix at 12 MPa CO₂. By increasing saturation pressures to 16 MPa and above, the decrease of viscosity in the interlamellar amorphous regions promotes cell growth and coalescence. During the process of cell growth, needle-like cells gradually develop in the needle-like amorphous regions located in the spherulites because of the restriction of lamellae.

Fig. 5 Schematics of cell growth in the interlamellar layers of spherulites.

Conclusions

Nanocellular and needle-like structure was successfully obtained in PLLA by using spherulite templates and supercritical CO₂. Dissolved CO₂ could induce the formation of lamellar network and spherulites in PLLA. The spherulites consisted of the closely packed lamellae and the interlamellar amorphous regions in the CO₂-treated PLLA. Moreover, the interlamellar amorphous regions were characterized by the approximately circular and needle-like shape. The occurrence of the needle-like cells in the spherulites of PLLA foams was attributed to the lamellar constraint and the cell coalescence during the cell growth. The results clearly indicated that not only the crystallinity but also the crystalline morphology had a great effect on the foaming behavior. The distinctive morphologies of PLLA foams were tunable by regulating the physical properties of matrix treated under CO₂. The resultant nanocellular PLLA foams are probably attractive for catalysis materials and low thermal conductivity materials.

Acknowledgements

This work is supported by National Natural Science Foundation of China (no. 51373103 and 51421061), Science and Technology Department of Sichuan Province (2015HH0026 and 2013GZ0152) and the opening project of the Key Laboratory of Polymer Processing Engineering, Ministry of Education, China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03682h

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