An official website of the United States government
Polymer crystallization is a process which connects the initial amorphous state with the final semicrystalline state. It is important to elucidate the amorphous structure which determines the crystallization pathway. In this work, we report quantitative analysis of the spatial proximity of poly(Lactic acid) (PLA) racemate before and after stereocomplex (SC) crystallization by using 13C selective isotope labeling and two-dimensional solid-state (ss) NMR techniques. It is found that i) the PLA racemate forms SC structure prior to crystallization (chiral recognition), ii) fraction of the chiral recognition segments (f) is extremely high, 94% in a low molecular weight (M) racemate while a high M one possesses only f = 10%, and iii) the f value for the former and latter is surprisingly in accordance with the f value after SC crystallization and SC crystallinity, respectively. From the observed analogies between the initial glassy and final crystalline structures, it is concluded that pre-existing chiral recognition fraction governs the formations of SCC.
more »
« less
This content will become publicly available on October 21, 2026
Analogies of the Spatial Proximity of Polymer Racemate Glass and Crystal as Revealed by NMR Crystallography: “Freezing in” Crystallization
The crystallization pathway of long and flexible polymer chains is debatable because of the lack of an initial melt/glass structure. To identify the crystallization pathway, we focus on two binary blends of poly(lactic acid) racemates that form stereocomplex crystals (SCCs). NMR crystallography is used to identify the stereocomplex (SC) structure and SC fraction with or without long-range order. There are significant structural analogies between glass and crystals for both high-molecular-weight (M) and low-M racemates. The observed analogies and kinetics of crystallization indicate that polymer crystallization proceeds via chain segments moving the least possible distance (“freezing in” mechanism) and that topological constraints govern nucleation barriers.
more »
« less
- PAR ID:
- 10645105
- Editor(s):
- Korley, LaShanda
- Publisher / Repository:
- ACS
- Date Published:
- Journal Name:
- ACS Macro Letters
- Volume:
- 14
- Issue:
- 10
- ISSN:
- 2161-1653
- Page Range / eLocation ID:
- 1543 to 1548
- Subject(s) / Keyword(s):
- Polymer crystallization mechanism, polymer racemate, solid state NMR spectroscopy
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Chain entanglements play a crucial role in polymer crystallization, yet their effects on crystallization remain not fully understood. Freeze-drying is one way to potentially preserve disentangled states of long polymer chains. In fact, it is known that freeze-drying (FD) significantly accelerates the crystallization kinetics of semicrystalline polymers. However, the chain-level structure of the FD polymer chains without a long-range order (glass) has been a debatable matter. In this study, we investigate the effect of freeze-drying on single chain-level structures of 13CH3 enriched poly(L-lactic Acid) and 13CH enriched poly(D-lactic acid) racemate by using 1H-1H spin diffusion via 13C detection solid-state NMR spectroscopy. Spatial distributions of PLLA and PDLA glassy chains in the range of a few Å – 30 nm are evaluated via 1H-1H spin diffusion. This analysis provides core-shell morphology of single chains where the outer shell layers include both PDLA and PLLA mixture and the inner core possess a single component.more » « less
-
Abstract An improved design is described for ferroelectric crystals and implemented with the “methoxyphenyl series” of acetophenone azines, (MeO−Ph, Y)‐azines with Y = F (1), Cl (2), Br (3), or I (4). The crystal structures of these azines exhibit polar stacking of parallel beloamphiphile monolayers (PBAMs). Azines 1, 3, and 4 form true racemates whereas chloroazine 2 crystallizes as a kryptoracemate. Azines 1–4 are helical because of the N−N bond conformation. In true racemates the molecules of opposite helicity (M and P) are enantiomers A(M) and A*(P) while in kryptoracemates they are diastereomers A(M) and B*(P). The stacking mode of PBAMs is influenced by halogen bonding, with 2–4 showcasing a kink due to directional interlayer halogen bonding, whereas fluoroazine 1demonstrates ideal polar stacking by avoiding it. Notably, (MeO−Ph, Y)‐azines display a stronger bias for dipole parallel alignment, attributed to the linearity of the biphenyl moiety as compared to the phenoxy series of (PhO, Y)‐azines with their non‐linear Ph−O−Ph moiety. The crystals of 1–4 all feature planar biphenyls and this synthon facilitates their crystallization through potent triple T‐contacts and enhances their nonlinear optical (NLO) performance by increasing conjugation length and affecting favorable chromophore conformations in the solids.more » « less
-
The recently discovered Cu46Zr33.5Hf13.5Al7 (at.%) bulk metallic glass (BMG) presents the highest glass-forming ability (GFA) among all known copper-based alloys, with a record-breaking critical casting thickness (or diameter) of 28.5 mm. At present, much remains to be explored about this new BMG that holds exceptional promise for engineering applications. Here, we report our study on the crystallization behavior of this new BMG, using isochronal and isothermal differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM). With the calorimetric data, we determine the apparent activation energy of crystallization, the Avrami exponent, and the lower branch of the isothermal time–temperature–transformation (TTT) diagram. With XRD and TEM, we identify primary and secondary crystal phases utilizing samples crystallized to different degrees within the calorimeter. We also estimate the number density, nucleation rate, and growth rate of the primary crystals through TEM image analysis. Our results reveal that the crystallization in this BMG has a high activation energy of ≈360 kJ/mole and that the primary crystallization of this BMG produces a high number density (≈1021 m−3 at 475 °C) of slowly growing (growth rate < 0.5 nm/s at 475 °C) Cu10(Zr,Hf)7 nanocrystals dispersed in the glassy matrix, while the second crystallization event further produces a new phase, Cu(Zr,Hf)2. The results help us to understand the GFA and thermal stability of this new BMG and provide important guidance for its future engineering applications, including its usage as a precursor to glass–crystal composite or bulk nanocrystalline structures.more » « less
-
Vitrimers, dynamic polymer networks with topology conserving exchange reactions, have emerged as a promising platform for sustainable and reprocessable materials. While prior work has documented how dynamic bonds impact stress relaxation and viscosity, their role on crystallization has not been systematically explored. Precise ethylene vitrimers with 8, 10, or 12 methylene units between boronic ester junctions were investigated to understand the impact of bond exchange on crystallization kinetics and morphology. Compared to linear polyethylene which has been heavily investigated for decades, a long induction period for crystallization is seen in the vitrimers ultimately taking weeks in the densest networks. An increase in melting temperatures ( T m ) of 25–30 K is observed with isothermal crystallization over 30 days. Both C 10 and C 12 networks initially form hexagonal crystals, while the C 10 network transforms to orthorhombic over the 30 day window as observed with wide angle X-ray scattering (WAXS) and optical microscopy (OM). After 150 days of isothermal crystallization, the three linker lengths led to double diamond (C 8 ), orthorhombic (C 10 ), and hexagonal (C 12 ) crystals indicating the importance of precision on final morphology. Control experiments on a precise, permanent network implicate dynamic bonds as the cause of long-time rearrangements of the crystals, which is critical to understand for applications of semi-crystalline vitrimers. The dynamic bonds also allow the networks to dissolve in water and alcohol-based solvents to monomers, followed by repolymerization while preserving the mechanical properties and melting temperatures.more » « less
