College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, Sichuan, People’s Republic of China
Commercially available PLA used in this study was an injection-grade granular material purchased from Nature Works (trade name 4032D). The M n and M w were 6.24×10 4 g·mol -1 and 9.32×10 4 g·mol -1 , respectively.
The self-made crystallization apparatus which can load a high-pressure stress is schematically shown in Figure S1. The diameter of the plunger is 20 mm, and the length of the cylindrical channel is 88 mm. Hydrostatic pressure supplied from a hydraulic jack can be up to 1 GPa, while the magnitude of pressure inside the channel can be measured via the modulated pressure meter with an a) Author to whom correspondence should be addressed. Electronic mail: [email protected] 1
accuracy of ±0.5 MPa, and the process temperature is controlled via a thermocouple mounted 15 mm away from the channel, which ensured a temperature accuracy of 1 °C inside the cell. Then the sample inside the channel was heated by an electrical heating sheath, which was controlled by a temperature controller. Before the experiment, temperature of the inside channel was carefully calibrated with the in-situ temperature measured by the thermocouple.
Schematic of the high-pressure cell (1. Plunger; 2. Mould Core; 3. Sample; 4. Mould; 5. Heating ring; 6. Thermocouple)
Prior to the high-pressure treatment, the original PLA material was dried in a vacuum oven for 12 h at 55 ºC to evaporate excess moisture. Afterwards, 4 g of PLA was first put inside the channel of the mould, heated to 185 ºC and kept the temperature for 30 min. Then the melted resin was cooled to the desired crystallization temperature at a rate of 2.5 ºC /min and held at the crystallization temperature for 10 min. The crystallization temperatures were set to 120, 130, 150, 2
175 and 185 ºC, respectively. After that a predetermined pressure value of 300 MPa was applied to the sample during 2 min. Both the temperature and pressure for the system were kept for 3 h. Finally the material was cooled to room temperature at a rate of 2.5 ºC /min and released the pressure (as shown in Figure S2).
Temperature protocol and pressure conditions used during the experiments In addition, an experiment was conducted to confirm the state of PLA before high pressure exerted. Figure S3 shows the DSC curves of PLA samples cooling from 185 ºC at a rate of 2.5 ºC /min and holding at the crystallization temperatures of 120, 130, 150 and 175 ºC for 10 min. It is evident that all specimens kept the melting state before high pressure applied, so we can make a conclusion that the crystallization of PLA belongs to high-pressure crystallization from the melt instead of annealing under high pressure. 3
The DSC curves of PLA samples cooling from 185 ºC and holding at crystallization temperatures of 120, 130, 150 and 175 ºC for 10 min
The melting behaviors of PLA samples crystallized at high pressures were studied employing TA Q1000 V7.3 differential scanning calorimeter. The calibration was performed with indium and all tests were carried out in ultra pure nitrogen as purge gas. Samples (about 7mg) enclosed in aluminum pans were heated from 40 to 190 ºC at a scanning rate of 10 ºC /min. Melting point temperatures were determined from the melting curves as peak temperatures. The fractional crystallinity (
X c %
) was calculated by using the equation as follows.
0 . (1) where
is the cold crystalline enthalpy;
is the melting enthalpy;
is the melting enthalpy of 100 % crystalline PLA, which is set as 93 J/g in this study.
The 2D-WAXD measurements were conducted on the synchrotron light source under room temperature in the National Synchrotron Radiation Laboratory (NSRL), Hefei. The distance from sample to detector was 250 mm, the wavelength of the monochromated X-ray was 0.15418 nm, and the beam size is ca. 400µm. Then the 2D-WAXD images were collected with an X-ray CCD detector (Model Mar345). An aluminum oxide (Al 2 O 3 ) standard was used to calibrate the scattering angle for WAXD, and air scattering was subtracted. Linear WAXD profiles were obtained from circularly integrated intensities of 2D-WAXD image patterns acquired, Gaussian functions were applied to fit the amorphous background, and an iterative peak-fit procedure was used to fit the crystalline reflections of WAXS profiles.
Structure of the samples was examined by a field emission scanning electron microscopy (Inspect-F, FEI, Finland), operating in high vacuum and with an accelerating voltage of 20 kV. To expose the interior of PLA the specimens were frozen in liquid nitrogen for about 40 min, then quickly impact fractured. In addition, a procedure was devised for removal of the amorphous part. We prepared reagents by dissolving 0.1 wt % sodium hydroxide in a mixture of two parts by volume concentrated distilled water ( 33 %) and one part methyl alcohol (67 %). The etching was conducted at room temperature for 40 h. Then, the samples were carefully washed with a procedure described elsewhere.
2,3 Prior to SEM examination the freshly broken surfaces were sputtered with gold. Origination of the double melting peaks is demonstrated as shown in Figure S4. 5
Heating curves of samples crystallized at 185 ºC, 300MPa for 3 hours with different scanning rate of (a)10, (b)20, (c)30 ºC /min respectively
1 Y. T. Shieh, and G. L. Liu, J. Polym. Sci. Polym. Phys. 45, 466 (2007). 2 R. H. Olley, and D. C. Basset, Polymer 23 (12), 1707 (1982). 3 P. Yu, H. Y. Bai, J. G. Zhao, C. Q. Jin, and W. H. Wang, Appl. Phys. Lett. 90, 051906 (2007). 6