Search for: All records

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. 

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. 

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. 

In the earlier theoretical research, impact of entanglement on folding during crystallization was minimized. The combination of 13C isotope labeling and NMR spectroscopy allows us to quantitatively determine stem to stem distance as well as chain folding distance, hence, we are able to probe chain-level structure. Our recent work indicated that polymer chains are possible to fold prior to crystallization. In this poster, we would like to investigate the folding structure of a semi-crystalline polymer in melt-grown crystals (mgc) by using solid-state NMR spectroscopy and SAXS measurement. First, various 13C enriched poly(L-lactic acid) (PLLA) samples with different molecular weights (Mw = 2.5k – 300k g/mol) across critical entanglement length (Mc = 16k g/mol) were prepared in order to observe the molecular weight dependence of folding structure of PLLA. We revealed that entanglements influence the folding number during crystallization. Second, we attempt to observe the entanglement effect through diluting entanglement density, i.e., blending the PLLA above and below the Mc with different ratio and molecular weight. Based on the experimental results, we would like to highlight the impact of entanglements on folding of semicrystalline polymer in the melt-grown crystal.