The phase transition of materials in extreme environments is an important research direction in condensed matter physics. With the rapid development of high-pressure physics in the fields of materials, the condensed matter evolution of polymer materials composed of flexible long-chain molecules in high-pressure environments has become an important research issue.
To investigate this issue, we have invented an in-situ high-pressure multi-optical observation instrument [1,2]. This instrument integrates three observation methods, including microscopy optics, polarization optics, and small angle laser scattering. It can achieve accurate control of sample temperature and pressure within the wide range of -56~400 ℃ and 0.1~40 MPa. It can also in-situ observe the condensed structure and evolution behavior of polymer/high-pressure gas systems, such as melting, phase separation, foaming, and crystallization. The evolution behavior of homochiral Poly(lactic acid) (PLA) in high-pressure Carbon Dioxide (CO2) was studied by using this instrument. It was found that homochiral PLA formed two types of vortex-shaped dendrites with opposite spiral directions in high-pressure CO2, namely two different morphology chirality of crystals (https://doi.org/10.1038/s41467-024-51292-y).
In the classical phase transition theory, the ordered phase transition is accompanied by symmetry breaking, but our study found that homochiral PLLA also forms an achiral snowflake-shaped dendrite in high-pressure CO2, as shown in Figure 1. This means that ordered phase transition has undergone chiral symmetry restoration. To understand this anomalous phenomenon, we conducted multi-level characterization studies on the material structures involved in this phenomenon using various instruments.
Fig. 1 Crystal morphology of PLA enantiomer observed by in-situ high-pressure visualization system. A PDLA dendrites with right-hand spiral (Crystallization temperature Tc = 80℃, crystallization pressure, i.e., CO2 pressure Pc = 500 Psi). B PDLA snowflake crystals without spiral (Tc = 60℃, Pc = 1400 Psi). C PDLA dendrites with left-hand spiral (Tc = 60℃, Pc = 1250 Psi). D PLLA dendrites with left-hand spiral (Tc = 80℃, Pc = 500 Psi). E PLLA snowflake crystals without spiral (Tc = 60℃, Pc = 1400 Psi). F PLLA dendrites with right-hand spiral (Tc = 60℃, Pc = 1250 Psi).
From the perspective of monomers, we detected the chiral characteristics of two types of PLAs in the experiment by using Electronic Circular Dichroism (ECD), as shown in Figure 2A. This chirality depends on the chirality of the repeating units that make up PLA. The four covalent bonds of chiral carbon atom in lactic acid monomer are respectively connected to a hydrogen atom, an oxygen atom in a hydroxyl group, a carbon atom in a methyl group, and a carbon atom in a carboxyl group. These four completely different functional groups form mirror symmetric left-handed and right-handed lactic acid monomers, which respectively construct Poly(L-lactic acid) (PLLA) and Poly(D-lactic acid) (PDLA). This study performed the high-pressure crystallization treatments on the high-purity samples of these two types of homochiral PLAs.
Fig. 2 Chirality information at different levels, A ECD spectra of PDLA and PLLA in Acetonitrile (AcCN) solution, PDLAcd means the CD signal of PDLA solution, the unit mdeg means millidegree which equals to 0.001 degree. B and C VCD spectra of crystalline PLLA solid sample, the sample number represents the crystallization conditions, for example, 50-1000 means a crystallization temperature of 50 ℃ and a CO2 pressure of 1000 Psi. The Y-axis of B and C means the difference in absorption of left-handed and right-handed polarized light by samples. D AFM height image of left-handed spiral crystals (Tc = 50℃, Pc = 1250 Psi). E AFM height image of right-handed spiral crystal (Tc = 70℃, Pc = 1000 Psi). F AFM height image of non-spiral crystal (Tc = 60℃, Pc = 1500 Psi). G One-dimensional (1D) GIWAXS data. H One-heating Differential Scanning Calorimetry (DSC) data of PLLA samples crystallized in high-pressure CO2. I Simultaneously changing the temperature from 50 to 70°C and adjusting the CO2 pressure from 1300 to 1000 Psi induces a change in the spiral chirality of PDLA crystals. J Changing the CO2 pressure from 1250 to 750 Psi at 60°C induces a reversal in the spiral chirality of PDLA crystals.
 From the perspective of molecules, we characterized the helical chirality of molecules in the crystal by using Vibrational Circular Dichroism (VCD) spectroscopy. The results showed that the helical chirality of PLA molecular chains in the crystal was not affected by crystallization temperature and CO2 pressure, as shown in Figures 2B and 2C. Based on the coupled oscillator model and the signatures of the split-type Cotton effect in the VCD spectra [3], the Cotton effect in the three crystalline PLLA samples shown in Figures 2B and 2C is identified as negative chirality. Namely, the helical conformations of the crystalline PLLA molecular chains are left-handed. It is also easy to speculate that the helical conformation of the PDLA molecular chains in the crystal are right-handed. This indicates that the high-pressure environment does not alter the helical chirality of PLA molecular chains, but changes the morphological chirality of PLA crystal superstructures.
From a crystal perspective, using Atomic Force Microscopy (AFM), we found that the symmetrical crystal planes of the single crystal at the growth front of PLA dendrites exhibit asymmetric growth directions, resulting in two different growth modes, i.e. clockwise bending and counterclockwise bending, as shown in Figures 2D and 2E, and ultimately forming two types of PLA dendrites with contrary spiral chirality. The Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) technique helped us discover the differences between these two types of helical chiral crystals, as shown in Figure 2G. The crystal form of spiral crystals whose spiral chirality are consistent with the chirality of PLA molecules (left spiral dendrites of PLLA or right spiral dendrites of PDLA) is α crystal, while spiral crystals whose spiral chirality are opposite to the chirality of PLA molecules (right spiral dendrites of PLLA or left spiral dendrites of PDLA) contain β crystal. The non-helical snowflake crystals formed by homochiral PLA contain both α and β crystals. We explained the formation mechanism of two types of dendritic crystals with different spiral chirality generated by homochiral PLA based on the chain tilt characteristics of α crystals and the asymmetric crystal plane characteristics of β crystals [4].
Our research findings have potential application value. Firstly, high-pressure CO2 can flexibly change the growth direction of PLA dendrites, thereby achieving programmed control of crystal morphology, as shown in Figures 2I and 2J. This provides a new method for controlling the crystal superstructure morphology in polymer films. Secondly, β crystals can enhance the tensile strength and heat resistance of PLA. This study proposes a method for preparing PLA β crystals in a high-pressure CO2 environment. Compared with the pressure forming method with mechanical pressure exceeding 100 MPa [5,6], the entropy effect of CO2 can be used to prepare PLA β crystals at temperatures of 50-60 ℃ and pressures of 5-10 MPa, which has significant advantages in reducing the energy consumption and production cost of PLA β crystal preparation.
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Reference
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