Analysis of multiple melting behavior of spherulites comprising ring‐band shell/ringless core in polymorphic poly(butylene adipate)

Abstract

In‐depth interpretation of ring‐banded spherulitic morphology, crystals, polymorphism, and complex melting behavior in poly(1,4‐butylene adipate) (PBA) were analyzed via a procedure of designing composite core‐shell spherulites, in which two lamellar patterns (ring‐band vs. ringless) were packed by subjecting to crystallization at two‐step temperature schemes with specific temperatures and times. By heating to 52 °C and holding at that temperature for 30 min annealing, the core can be stripped off by melting, and analysis specifically on the ring‐shell portion (with the ringless core stripped by controlled melting) proves that the highest melting peak (P4 at 55–57 °C) is likely associated with melting of the ring‐band lamellae. Furthermore, the unusually complex multiple melting in PBA can be attributed to all three widely proposed mechanisms: (1) multiple types of lamellae preexisting in crystallized PBA, (2) scan/heating induced remelting/reorganization, and (3) polymorphism of dual crystal cells. In addition, this study evidently shows that the extinction rings within the ring‐banded shell, regardless of alternate edge‐on and flat‐on mechanism or alternative origins, can be of all singly α‐crystal form, either initially or upon postheating temperature‐induced transformation. Thus, the type of crystal forms (α or β) in polymorphic PBA is mainly associated with temperature of crystallization (Tc = 28 or 35 °C), and not likely with lamellar orientation (flat‐on or edge‐on). © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 892–899, 2008

Coexisting Straight, Radial, and Banded Lamellae on the Six Corners of Hexagon‐Shaped Spherulites in Poly(l‐Lactide)

“…Some semicrystalline polymers or polymer blends have been reported to form ring‐banded spherulites at specific crystallization conditions . However, the formation of ring‐banded spherulite morphology remains one of the most challenging and complex issues in the field of crystalline polymer morphology.…”

“…Ring‐banded spherulites can be found in many polymers of various structures. Polyesters are among the most studied polymers that show ring‐banded spherulites upon melt crystallization; in addition, poly(vinylidene fluoride) (PVDF) (at T c = 168 °C), blends of liquid‐crystalline poly(aryl ether ketone) (LC‐PAEK), and poly(ether ether ketone) (PEEK) (by dynamically cooling from 380 °C to ambient termperature), or block copolymers such as poly(ε‐caprolactone‐ block ‐ethylene oxide) have also been shown to exhibit ring‐banded spherulites. Different from the conventional ring‐banded spherulites whose formation mechanism has been approached from a lamellar twisting model, the periodical growth/precipitation‐induced concentric banded spherulites, usually non‐birefringent in alternating rings, are derived from a diffusion process along the radial direction.…”

Atomic Force Microscopy Characterization and Interpretation of Thin‐Film Poly(butylene adipate) Spherulites with Ring Bands

“…To utilize the free‐surface nature of AFM in dealing with the difficulty and complexity in the subject of ring‐banded crystals in polymers, even more innovative procedures are needed. Earlier studies have demonstrated the feasibility of compacting two‐ or three‐alternating layered spherulites in semicrystalline poly(1,4‐butylene adipate) (PBA)14 by subjecting it to stepwise crystallization. The sequence and number of layers in thin‐film confined spherulites can be designed by alternating steps of isothermal crystallization temperature with proper holding time.…”

“…Alternating ring‐banded/ring‐less layers can be packed into a single spherulite by crystallization under stepwise temperature schemes. PBA is quite unusual in that it is known to possess polymorphic crystal cells and is capable of developing ring‐band or regular Maltese‐cross spherulites in two proximate temperature regimes,14 while at yet higher temperatures, a morphology of straight hexagonal lamellae replaces the round‐shaped spherulites 15. The spherulite that consists of two patterns in a single spherulite could then be used for convenient in situ AFM characterization for close comparison and analyses of lamellae in ring‐banded versus ring‐less crystals.…”

Effect of hydrogen bonding strength on the morphology and polymorphism of poly(butylene adipate)

“…A series of methods are adopted to control the polymorphic behavior of PBA . First, modulation of crystallization temperature provides an effective way to control the crystal form of PBA . The α‐form PBA is generally formed during isothermal crystallization at high temperatures (eg, above 33°C).…”