Chirality transfer from molecule to crystal morphologyFigure 1 illustrates the crystal morphology of homochiral PDLA and PLLA in high-pressure CO2. Each homochiral PLA exhibits three distinct morphologies of dendritic crystals: (1) The dendritic crystals generate a vortex-like pattern with a spiral chirality matching the molecule chirality (Fig. 1a, b); (2) snowflake-like dendrites without spiral chirality (Fig. 1c, d); (3) The dendritic crystals generate a vortex-like pattern with a spiral chirality contrary to the molecular chirality (Fig. 1e, f). Supplementary Discussion gives the definition rules of morphology chirality of these PLA crystals. Supplementary Figs. 4–10 provide in situ images of the growth process of crystals with mentioned morphologies. The morphologies of these three types of dendrites mean the occurrence of chiral amplification, chiral disappearance, and chiral reversal in the chirality transfer from molecules to macroscopic crystals, respectively. Among them, the chirality disappearance means that chirality symmetry restoration is realized in the crystallization process of homochiral polymer.Fig. 1: Crystal morphology and spiral curvature statistic of PLA enantiomer observed by in situ high-pressure visualization system.a PLLA dendrites with left-hand spiral (Crystallization temperature Tc = 80 °C, crystallization pressure, i.e., CO2 pressure Pc = 500 Psi). b PDLA dendrites with right-hand spiral (Tc = 80 °C, pc = 500 Psi). c PLLA snowflake crystals without spiral (Tc = 60 °C, Pc = 1400 Psi). d PDLA snowflake crystals without spiral (Tc = 60 °C, Pc = 1400 Psi). e PLLA dendrites with right-hand spiral (Tc = 60 °C, Pc = 1250 Psi). b, f PDLA dendrites with left-hand spiral (Tc = 60 °C, Pc = 1250 Psi). The scale bars in (a–f) represent 100 μm, and the spiral directions of dendrites are marked in the bottom left corner of the images. g Spiral chirality and curvature of PLA dendritic crystal after treatment under different crystallization temperatures and different CO2 pressures. The curvature of bent branch crystals in Supplementary Figs. 11 and 12 is measured by the Kappa plugin of ImageJ software, with 12 randomly measured branches for each image. IQR stands for inter-quartile range. Here we define left-handed spiral (curving clockwise) as positive curvature, while right-handed spiral (curving counterclockwise) as negative curvature.Generation conditions are the key to understand the hierarchical transfer of chirality. From Fig. 1g and Supplementary Fig. 12, PLLA crystals exhibit a vortex shape with a left-handed spiral under a temperature of 70–80°C or a CO2 pressure of 250–750 Psi. Conversely, under a temperature of 50–60°C and a CO2 pressure of 1000 Psi, they exhibit a vortex shape with a right-handed spiral. Supplementary Fig. 13 shows that PLA mainly generates spherulites without spiral features under atmospheric pressure. So, high-pressure CO2 is the necessary condition to reveal the morphology chirality of PLA dendritic crystal. From Supplementary Fig. 14, with the increase of CO2 pressure at 60 °C, spiral chirality of PLA crystal undergoes chiral amplification, chiral disappearance, chiral reversal, and chiral disappearance again. The non-spiral snowflake crystals can be found between two opposite chiral morphologies or under high-pressure conditions (e.g., Pc = 1600 Psi). Thus, the effect of CO2 on the chiral characteristics of PLA dendrites should be primarily investigated.The phase field simulation of Gránásy et al.18 indicated that impurity particles can change the growth direction of achiral polymer dendrites, but it leads to dizzy dendrites with random directions. Prud’homme et al.19 found that unequal mixing of PLLA and PDLA also can generate vortex-like dendrites. Although there is a correlation between molecular chirality and crystal chirality, molecular chirality is not a decisive factor of crystal chirality. Figure 2 gives the chirality information of PLA monomer and conformational chirality of PLLA molecular chains in crystals with different spiral chirality formed under high-pressure CO2.Fig. 2: Chirality information at different levels.a Right-handed spiral crystals of PLLA (Tc = 50 °C, Pc = 1000 Psi). b Left-handed spiral crystals of PLLA (Tc = 70 °C, Pc = 750 Psi). c Achiral dendritic crystal of PLLA (Tc = 60 °C, Pc = 1000 Psi). The scale bar lengths of (a–c) are 250 μm. d, e Electronic circular dichroism (ECD) and ultraviolet-visible (UV–vis) absorption spectra of PDLA and PLLA in Acetonitrile (AcCN) solution, PDLAcd means the CD signal of PDLA solution, PDLAabs means the UV-vis absorption signal of PDLA solution. The unit mdeg means millidegree which equals to 0.001 degree. f–i Vibrational circular dichroism (VCD) and corresponding Fourier transform infrared (FT–IR) absorption spectra of crystalline PLLA solid sample, the sample number represents the crystallization conditions, for example, 50–1000 means a crystallization temperature of 50 °C and a CO2 pressure of 1000 Psi. The Y-axis of (f) and (h) means the difference in absorption of left-handed and right-handed polarized light by samples.From Fig. 2d, PLLA and PDLA solutions show a positive and a negative Cotton effect at 210 nm, respectively. According to Ho’s research20,21, the ECD signals result from the interaction of chiral entities in chiral polymers. In this way, the absolute configurations of chiral monomers in PLAs can be identified. Here, PLLA was selected to be treated under high-pressure CO2. From Fig. 2f, g, the three PLLA crystalline samples show a similar split-type Cotton effect with an inflection point at 1760 cm−1 corresponds to the characteristic absorption of the C=O stretching motion of the ester group in PLLA, whose electric transition moment perpendicular to the helical axis of the chiral PLLA chain. Based on the coupled oscillator model and the signatures of the split-type Cotton effect in the VCD spectra20, the Cotton effect in the three crystalline PLLAs is identified as negative chirality. Namely, the helical conformations of the three crystalline PLLAs are left-handed. The induced VCD signals of the absorption bands of the C–O–C vibration ranged from 1000 to 1250 cm−1 (Fig. 2h, i) also show the same trend and point to a left-handed helical conformation of PLLA chains21. These results mean that the three PLLA samples shown in Fig. 2a–c with different morphology chirality have the same helical conformation chirality of molecular chain in crystal. So, the reversal and disappearance of macroscopic spiral chirality of PLLA crystals are not decided by the helical chirality of crystalline chains.Hierarchical chirality transfer mechanismAt present, four reasons are mainly considered as the causes of polymer growth deviating from the crystallographic direction22: rhythmic growth23, self-induced compositional field24, successive screw dislocations25, and unbalanced stresses on fold surface26,27. In Fig. 3a, b, bamboo-like dendrites appear in both the left-handed and right-handed spiral crystals of PDLA. The bamboo-like structure is formed by rhythmic growth28, so rhythmic growth is not a factor determining the spiral chirality of PDLA. In addition, in Fig. 3c–e, no matter which kind of spiral chirality, similar melt fields are self-induced by the crystal growth at the front of the dendrites. These self-induced melt fields are all shown as melt enrichment states, providing a high concentration of melt supply for crystal growth. This means that the self-induced field is also not the determining factor for the spiral growth of dendritic crystals.Fig. 3: Microscopic morphology and structural characteristics of PLA crystal.a, b Bamboo-like characteristics formed by the rhythmic growth of left-handed and right-handed spiral PDLA crystals. c–e Melt enrichment zones at the edges of three crystal morphologies imaged by the white light interferometer (WLI). f–i Atomic force microscopy (AFM) height images of Bi-layered lamellae originated from self-induced nucleation and spiral dislocations, as well as their height curves at the red dashed line. j, k Height images and peak force error images of multi-layer lamellae. The rotation angle between the top lamella and the bottom lamella is marked in (k).The PLA dendrites are composed of multilayer stacked lamellae (Fig. 3f–i), Some of them (Fig. 3f) originated from self-induced nucleation29, and others (Fig. 3g) from screw dislocation30,31. In Fig. 3g, there are two screw dislocations with opposite helical directions in the same dendrite. Therefore, there is no one-to-one correspondence between the helical direction of screw dislocations and the dendritic spiral direction. When the layer number of stacked lamellae increases, the multi-layered structure will be rotated32 and the azimuth angle between the top and bottom layer will be deflected by 1°–3° (Fig. 3j, k). This cumulative deviation will only be reflected in the upper layer of the multi-layer structure; however, the growth direction of dendrites is determined by the basal layer lamellae. From Fig. 4a, the bending of multi-layer layers originated from screw dislocations or self-induced nucleation lags the basal layer lamellae. The growth direction of the upper layer crystals and the entire dendrite is dominated by the basal layer crystal. The research of Shen et al.33 also supports this viewpoint.Fig. 4: Crystal structure of PDLA spiral dendrites.a AFM height image of left-handed spiral crystals (Tc = 50 °C, Pc = 1250 Psi). b AFM height image of right-handed spiral crystal (Tc = 70 °C, Pc = 1000 Psi). c, d AFM height image of non-spiral crystal (Tc = 60 °C, Pc = 1500 Psi). e–h Height curves at the lines in (a–d), separately. Two-dimensional (2D) Grazing incidence wide-angle X-ray scattering (GIWAXS) patterns of PDLA crystal generated at (i) Tc = 60 °C, Pc = 0 Psi, (j) Tc = 50 °C, Pc = 1250 Psi, (k) Tc = 60 °C, Pc = 1500 Psi, and (l) Tc = 70 °C, Pc = 1500 Psi, respectively. m One-dimensional (1D) GIWAXS data. n Transmission electron microscopy (TEM) images of PDLA crystal generated at Tc = 60 °C, Pc = 1250 Psi. o One-heating differential scanning calorimetry (DSC) data of PLLA samples (Number average molecular weight Mn = 6452 g mol−1, Polydispersity Index PDI = 1.58) crystallized in high-pressure CO2, the sample number represents the crystallization conditions, for example, 50–500 means a crystallization temperature of 50 °C and a CO2 pressure of 500 Psi.After excluding rhythmic growth, self-induced field, and screw dislocation mentioned above, the unbalanced stress on the fold surface becomes the possible factor inducing the spiral growth of the basal layer of PDLA spiral dendrites. The unbalanced stress is caused by the differential congestion on the fold surface. Based on Fritzsching’s research34, amorphous chain segments can cause density anomalies on the lamellae surface and further induce chain tilt. From Fig. 4b, f, it is obvious that the two lateral sides of the lamellar crystal of the right-handed spiral crystal exhibit different tilt angles, and Supplementary Fig. 16b, d shows another similar example. This indicates that the molecular chains arranged along the thickness direction of lamellar crystal are tilted. On the surface of polymer lamellar crystal, cilia (chain ends), tie molecules, and folding chains constitute the amorphous layer. These disordered chain segments need more space than the ordered crystalline chain which will cause density anomalies of the crystal surface, resulting in surface stress. Surface stress induces chain tilt33,35, which in turn leads to an asymmetric growth on the lateral side of the lamellar crystal, finally resulting in the formation of bent dendrites with spiral morphology.However, there is no obvious difference between the two lateral sides of lamellar crystal in the left-hand spiral dendrites formed by PDLA, and the molecular chains are not significantly tilted (Fig. 4a, e and Supplementary Fig. 16a, c). So, the chiral reversal may no longer be attributed to surface stress or chain tilt. Figure 4i–l shows the 2D GIWAXS images of PDLA crystals. It shows that crystals of all the samples have a preferred orientation in the in-plane direction, and the lamellar crystals are mainly flat on the substrate. This is consistent with the AFM results shown in Figs. 3 and 4 and Supplementary Fig. 15. Figure 4m shows 1D GIWAXS data of all samples. We found that β crystals appeared in the PDLA crystal samples in addition to the common α and α’ (δ) crystals. For samples crystallized at 60 °C under atmospheric normal pressure, the characteristic peaks mainly appear at 16.4° and 18.9°, corresponding to the (200)/(110) crystal face and (203) crystal face of α’ (δ) crystal type, respectively36. For the right-handed spiral dendrite formed at 70 °C under 1500 Psi pressure, the characteristic peaks mainly appear at 16.8° and 19.1°, corresponding to the (200)/(110) crystal face and (203) crystal face of α crystal respectively36. For the left-handed spiral dendrite samples crystallized at 50 °C at 1250 Psi, the characteristic peaks mainly appear at 17.1° and 19.6°, corresponding to the (200) crystal face and (203) crystal face of β crystal, respectively37,38.Figure 4n shows the TEM diffraction pattern of the left-handed spiral dendrite of PDLA. Two features can be observed from the pattern: (1) The diffraction spots are hexagonally symmetric, indicating that the unit cell is trigonal; (2) Two landmark reflections are indexed 120 and 210 and are located between the 300 and 030 reflections (Supplementary Fig. 17). These two features indicate that the observed PLA crystals are β crystals39,40,41.In addition, Fig. 4o presents the DSC curves of PLLA crystalline samples during the first heating process. From the curves, the main melting peaks of the 4 curves occur around 155 °C. However, in the 50–1500 and 60–1500 curves, the second melting peak appears around 147.8 °C. Based on the crystal transition mechanism discussed by Ru et al.38, the relatively low temperature melting peak can be attributed to β crystals. As the temperature increases, the β crystals melt and recrystallize to form α crystals, followed by the melting peak of α crystals at a higher temperature. The phenomenon of dual melting peaks again proves the presence of β crystal.The above findings suggest that the shift in spiral chirality within vortex-like dendrites coincides with a transformation in crystal structure. Specifically, the right-hand spiral PDLA dendrites correspond to the α crystal form, while the left-hand spiral PDLA dendrites correspond to the β crystal form. PDLA snowflake dendrites lacking spirals contain a mixture of both α and β crystals (or α’, α, and β). Consequently, the direct growth of achiral snowflake crystals arises from the neutralizing of opposing spiral directions corresponding to the two distinct crystal forms.To further validate the correlation between the spiral chirality of PLA dendrites and the β crystal form, Fig. 5 illustrates the crystallization outcomes of PLLA with varying molecular weights under conditions of 50 °C and 1000 Psi. As the molecular weight increases, the proportion of β crystals also increases. In the PLLA1 sample (Mn = 577 g mol−1), predominantly α crystals are formed with only a small fraction of β crystals exhibiting weak bending and almost no discernible spiral morphology. Conversely, in PLLA2 and PLLA3 with higher molecular weights, the presence of β crystal peaks becomes pronounced. Correspondingly, the crystal morphology displays distinct counterclockwise bent dendrites, representing the right-handed spiral morphology. This evidence further substantiates the correlation between chiral reversal and the β crystal structure.Fig. 5: The effect of molecular weight on the crystal structure and spiral morphology of PLLA crystalized in 50 °C and 1000 Psi CO2.a GPC data. b 1D GIWAXS data. c–e 2D GIWAXS of PLLA1, PLLA2, and PLLA3. Crystal microscopy images of PLLA1, PLLA2, and PLLA3, and the lengths of scale bars of three microscopy images (f–h) are 250 μm.Furthermore, upon comparing Fig. 5d, e, it is observed that the PLLA2 sample predominantly displays in-plane diffraction spots, with dendrites primarily composed of flat-on lamellar crystals. In contrast, the PLLA3 sample exhibits diffraction rings, indicative of a relatively disordered arrangement of lamellar crystals within the sample. This arrangement includes not only flat-on lamellae but also edge-on or inclined lamellar crystals. Interestingly, despite the disordered arrangement of crystallites, there is no discernible impact on the curvature of PLLA dendrites. This implies that while the disorderly arranged lamellar crystals may cover and conceal the underlying lamellar crystals, they do not influence the bending direction of the underlying lamellar crystals.Entropy effect of high-pressure CO2
High-pressure crystallization experiments have demonstrated that PLA can generate β crystal under mechanical pressure exceeding 100 MPa and the temperature near the melting point (160–180 °C)38,42. A question arises: how can CO2, with a pressure of about 10 MPa, induce PLA β crystal at low temperatures (50–60 °C)? The Flory-Huggins binary interaction parameter χ of PLA and CO2 indicates that CO2 is a poor solvent for PLA43. In addition, considering that the molecular weights of PLA in this study are lower than or close to the critical entanglement molecular weight44 of PLA, the molten PLA molecule chain can be assumed as a rod-shaped molecule with high orientation entropy. According to Onsager theory45 and entropy-induced phase transition theory46, the thermal motion of poor solvents will provide entropy increase to compensate for the ordered arrangement (orientation entropy decrease) of rod-shaped molecules. In the crystallization process of the PLA/CO2 system, the energy change induced by CO2 mainly includes the adsorption energy between CO2 and PLA, and the translational energy of CO2 molecules desorbed from PLA crystal.For PLLA/CO2 systems, there are two specific interactions between CO2 and electron-donating groups (carbonyl and ether) or some polymers with Lewis base properties47. The interaction energy between CO2 and carbonyl and ether (\({E}_{{{{\rm{CO}}}}_{2}-{{\rm{carbonyl}}}}\), \({E}_{{{{\rm{CO}}}}_{2}-{{\rm{ether}}}}\)) is approximately 2.2 kcal mol−1 obtained by ab initio calculation47. Here, we assume that the binding probabilities of CO2 molecule with carbonyl and ether of PLA molecular chain are equal, and then the CO2 adsorption energy change \(\triangle {G}_{{\mbox{a}}}\) that needs to be overcome for 1 mol PLA monomer to form crystals is approximately as:$$\triangle {G}_{{\mbox{a}}}=\frac{1}{2}{n}_{1}\left({E}_{{{\mbox{CO}}}_{2}-{\mbox{carbonyl}}}+{E}_{{{\mbox{CO}}}_{2}-{\mbox{carbonyl}}}\right)$$
(1)
where \({n}_{1}\) is the molar number of CO2, which can be calculated by \({n}_{1}=\left(72{w}_{1}\right)/\left[44\left(1-{w}_{1}\right)\right]\). \({w}_{1}\) is the mass fraction, which can be calculated by Sanchez–Lacombe (S-L) equation of state48,49, and the calculation process is given in detail in the Supplementary Discussion. The translational energy \({\triangle G}_{{{{\rm{t}}}}}\) of CO2 desorbed from PLA crystal can be calculated by \({\triangle G}_{{{{\rm{t}}}}}={n}_{1}({G}_{{{{\rm{m}}}},{{{\rm{t}}}}}-{G}_{{{{\rm{m}}}},{{{\rm{t}}}}}^{{{{\rm{\theta }}}}})\), where \({G}_{{{{\rm{m}}}},{{{\rm{t}}}}}\) means the molar translational energy of CO2, \({G}_{{{{\rm{m}}}},{{{\rm{t}}}}}^{{{{\rm{\theta }}}}}\) means the molar translational energy of CO2 at ideal state (\(P\) = 101,325 Pa and \(T\) = 273.15 K). Based on Sackur–Tetrode formula50,\({G}_{{{{\rm{m}}}},{{{\rm{t}}}}}\) of CO2 can be calculated as:$${G}_{{{{\rm{m}}}},{{{\rm{t}}}}}= {H}_{{{{\rm{m}}}},{{{\rm{t}}}}}-T{S}_{{{{\rm{m}}}},{{{\rm{t}}}}}=\frac{5}{2}{RT}-{RT}\left({{\mathrm{ln}}}\frac{{q}_{{{{\rm{t}}}}}}{{N}_{{{{\rm{a}}}}}}+\frac{5}{2}\right) \\= -{RT}\left\{{{\mathrm{ln}}}\left[\frac{{\left(2\pi {mkT}\right)}^{\frac{3}{2}}}{{{N}_{{{{\rm{a}}}}}h}^{3}}{V}_{{{{\rm{m}}}}}\right]\right\}$$
(2)
where \({H}_{{{{\rm{m}}}},{{{\rm{t}}}}}\) is the molar translational enthalpy, \({S}_{{{{\rm{m}}}},{{{\rm{t}}}}}\) is the molar translational entropy, \(R\) is the gas constant, \({N}_{{{{\rm{a}}}}}\) is the Avogadro constant, \({q}_{{{{\rm{t}}}}}\) is the translational partition function, \(h\) is the Planck constant, \(k\) is the Boltzmann constant, and the molar volume of CO2 can be estimated as \({V}_{{{{\rm{m}}}}}=\frac{{RT}}{P}\). The mass \(m\) of CO2 molecules can be determined by \(m=M/{N}_{{{{\rm{a}}}}}\), where \(M\) is the molar mass of CO2. The values of parameters in the above model and references are listed in Supplementary Table 1.According to density functional calculations, the energy of monomer in the unit cell of PLA β crystal exceeds that of PLA α crystal by 0.7 kcal mol−1 51. Figure 6a, b shows the calculation results of the above equations. Under low temperature and high-pressure conditions, the difference between the translational energy and the adsorption energy (\({\triangle G}_{{{{\rm{t}}}}}-{\triangle G}_{{{{\rm{a}}}}}\)) caused by CO2 is larger than the energy gap required to generate PLA β crystal, theoretically (Fig. 6a). This result proves possibility that high-pressure CO2 induces PLA β crystal at low temperatures. Furthermore, the energy contribution of CO2 translational entropy \({T\triangle S}_{{{{\rm{t}}}}}\) is more significant than the adsorption energy change \({\triangle G}_{{{{\rm{a}}}}}\) and the translational enthalpy \({\triangle H}_{{{{\rm{t}}}}}\) of CO2 (Fig. 6b). This result means that translational entropy increase of CO2 plays a key role in the formation of PLA β crystal.Fig. 6: The energy calculation results and control of chirality transfer of PLA in high-pressure CO2.a Calculation results of energy contribution of CO2 to PLA crystallization. b Adsorption energy change \({\triangle G}_{{{{\rm{a}}}}}\), translational enthalpy change \({\triangle H}_{{{{\rm{t}}}}}\) and energy contribution of translational entropy \({T\triangle S}_{{{{\rm{t}}}}}\) caused by CO2 at a crystallization temperature of 50 °C, where \({\triangle H}_{{{{\rm{t}}}}}\) and \({\triangle S}_{{{{\rm{t}}}}}\) were calculated by \({\triangle H}_{{{{\rm{t}}}}}={n}_{1}({H}_{{{{\rm{m}}}},{{{\rm{t}}}}}-{H}_{{{{\rm{m}}}},{{{\rm{t}}}}}^{{{{\rm{\theta }}}}})\) and \({\triangle S}_{{{{\rm{t}}}}}={n}_{1}({S}_{{{{\rm{m}}}},{{{\rm{t}}}}}-{S}_{{{{\rm{m}}}},{{{\rm{t}}}}}^{{{{\rm{\theta }}}}})\), respectively. The superscript \({{{\rm{\theta }}}}\) means the ideal state whose \(P\) = 0.1 MPa and \(T\) = 273.15 K. c Changing the CO2 pressure from 1250 to 750 Psi at 60 °C induces a reversal in the spiral chirality of PDLA crystals. d 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.Given that high-pressure CO2 serves as the primary factor inducing chiral reversal in PLA dendrites, we can exert control over the chirality and morphology of PLA dendrites through manipulation of CO2 pressure. As depicted in Fig. 6c, d, the chirality of PDLA dendrites undergoes immediate reversal upon alterations in CO2 pressure or crystallization temperature, resulting in the S-shaped or Z-shaped dendrites with opposite chirality. Additionally, Supplementary Figs. 18–20 provide further insights into the real-time changes in dendrite chirality and crystal morphology. Thus, the crystal morphology can be flexibly programmed via high-pressure CO2, giving a straightforward and cost-effective approach to regulating the pattern of crystal superstructure (spiral structure with the growth in three dimensions) and crystal chirality of PLA films.
Chirality hierarchical transfer in homochiral polymer crystallization under high-pressure CO2
