Structured growth from sheaf-like nuclei to highly asymmetric morphology in poly(nonamethylene terephthalate)

Abstract

Thorough microscopy analyses are done on dissecting 3D interiors of poly(nonamethylene terephthalate) (PNT) banded spherulites. Growth of a specific type (Type-I) of banded spherulites is initiated from highly elongated sheaf-like nuclei, with asymmetrically grown lamellae packed as a multi-shell structure, leading to resemblance of a dual-face spheroid. Top-surface bands are characterized with discrete/ unconnected dots of crystals aligned as zig-zag circles, which correspond to interior layered lamellae ends surfacing to top. Thus, the banded spherulites display not only inter-band discontinuity, but also discontinuity among the dotted crystals aligned on the top-surface ridges. 3D analysis exhibits that the banded spherulites possess a Janus-face morphology of compounded shell-and-kink lamellae assembly with dual discontinuities.Organic or inorganic compounds, most notably calcium carbonate (CaCO 3 ), can react in a suitable regulating medium to form products of crystals displaying specic or diversied periodic assemblies on surfaces that are termed as surface-relief patterns that are intensively studied as model bio-mimetic systems.1-9 Additionally, small-molecule organic compounds such as phthalic acids have been reported to form periodic surface-relief patterns (concentric bands) upon solution evaporation induced crystallization.10-12 Many organic polymers have also been known to display periodic ring patterns in meltcrystallized spherulites since the 1950's; however, albeit possessing similar or same periodic bands as those surface-relief patterns in small-molecule compounds, 1-9 the mechanisms and formation of the ring bands have been interesting and intriguing subjects in numerous studies for past many decades; yet, the subjects in polymer periodic patterns have remained to be argumentative for half a century, partially owing to an extra structural difference of existence of chain folding in the longchain backbones of polymers. Periodic ringed structures in spherulites of polymers, copolymers, or their blends have been discovered, studied and debated for a long time.13-22 Numerous re-visits or revisions of the model of continuous lamellar twisting for periodic banding have been attempted recently by investigators.23,24 Lamellar helix-twisting 360 -angle continuously like DNA molecules, had been one of the most oen suggested mechanisms in explaining how crystal lamellae grow into periodic bands, as viewed in polarized microscopy. [25][26][27][28][29] In past decades, many investigators attributed the model of DNAlike continuous helix twisting of lamellae to surface stresses caused by chain-tilting in repeated folding; some other investigators even claimed that the twisting of polymer lamellae may be driven by polymer molecular chirality, which can be considered as a special form of chain tilting. 30-35The same phenomenon is also found in poly(trimethylene terephthalate) (PTT) that banding is attributed by some investigators to be due to chain tiling in repeated folding in the lamellae by Ivanov et al. ...

Crystallization in arylate polyesters to periodically ringed assembly

“…An even more advanced study has revealed the shell-like structure in Type-1 ringed PNT spherulites. [35] Figure 12 shows This result is a sufficient and novel evidence to prove that the corrugate-board (flute-liner) structure of the interior ringed PNT spherulites is responsible for the periodic optical banding. Once again, the interior lamellae assembly proves agreement between the crystal periodicity with the optical banding is valid not just for the three types of PTT bands discussed earlier but also for the PNT ringed spherulites (Type-1).…”

“… However, if a fixed T max is used, variation of T c can lead to similar effects of varying the relative fractions of Type‐1 versus Type‐2 ringed PNT spherulites. Figure shows POM micrographs for two ringed types of PNT (Type‐1 vs Type‐2 bands) as function of T c (at 60°C, 65°C, 70°C, or 85°C), respectively, by quenching from T max = 120°C . Very small‐size Type‐1 ringed PNT spherulites are found at lower T c ; on the other hand, larger‐size Type‐1 spherulites are at higher T c 's.…”

“…Optically ringed bands in crystallized polymers have been conventionally regarded as “concentric” multiple rings by many investigators for simplicity, or perhaps owing to insufficient‐resolution of the optical microscopy; yet in recent studies, clearer evidence (by high‐resolution atomic‐force or scanning microscopies) has been disclosed that many of the banding patterns, if not all, are actually Archimedean spirals initiating from a nuclei center, as proved in many polymer systems . The nuclei region might be straightly sheaf‐like, yet, bending and rotation take place immediately at the ends of straight‐sheaf nuclei crystals during subsequent growth.…”

“…atomic-force or scanning microscopies) has been disclosed that many of the banding patterns, if not all, are actually Archimedean spirals initiating from a nuclei center, as proved in many polymer systems. [35][36][37] The nuclei region might be straightly sheaf-like, yet, bending and rotation take place immediately at the ends of straight-sheaf nuclei crystals during subsequent growth. In addition, mechanisms are yet to be expounded for the common phenomena that the ring bands in polymer spherulites can start as "Archimedean spirals" initiating from the Film sample thickness generally affects the inter-ring spacing, and often also influences the formation of ring patterns in aryl polyesters, if the film thickness is kept in a range, say one to several tens of micrometers.…”

“…The result reveals that both types of ringed spherulites (termed Type-1 and -2) co-exist at this specific T c = 85 C, but with various relative fractions depending on T max and T c , and the volume and number percentages of Type-1 increase over that of Type-2 with increasing T max [27,35]. However, if a fixed T max is used, variation of T c can lead to similar effects of varying the relative fractions of Type-1 versus Type-2 ringed PNT spherulites.Figure 2shows POM micrographs for two ringed types of PNT (Type-1 vs Type-2 bands) as function of T c (at 60 C, 65 C, 70 C, or 85 C), respectively, by quenching from T max = 120 C [27,35]. Very small-size Type-1 ringed PNT spherulites are found at lower T c ; on the other hand, larger-size Type-1 spherulites are at higher T c 's.…”