Junsong Lia,
Guangjian Heb,
Xia Liao*a,
Hao Xua,
Qi Yanga and
Guangxian Li*a
aCollege of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan 610065, China. E-mail: xliao@scu.edu.cn; guangxianli@scu.edu.cn; Tel: +86-28-8540-8361
bKey Laboratory of Polymer Processing Engineering, Ministry of Education, South China University of Technology, Guangzhou 510641, China
First published on 14th April 2015
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.
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.
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
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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. |
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 (1014 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 (1010 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. |
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. 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.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03682h |
This journal is © The Royal Society of Chemistry 2015 |