Morphologies and Crystallization Behaviors in Melt-Miscible Crystalline/Crystalline Blends with Close Melting Temperatures but Different Crystallization Kinetics

Abstract

Poly(L-lactic acid) (PLLA) and poly(oxymethylene) (POM), with very close melting temperatures (T m ), can crystallize simultaneously or separately in their blends depending on composition and crystallization temperature (T c ), resulting in various types of morphology. It is mainly attributable to the greatly different crystallization kinetics of PLLA and POM. At a content of POM (φ POM ), 3 wt % < φ POM < 20 wt %, PLLA crystallization kinetics are comparable to POM, and therefore two type spherulites exhibit "side-byside" simultaneous growth with the penetration of PLLA spherulites into POM crystals. Although crystal growth rate (v c ) of POM is still a bit faster than that of PLLA, for φ POM = 3 wt %, the nucleation of POM is restrained and POM spherulites can only develop on the propagating PLLA growth fronts with the generation of novel "core−shell" blended spherulites. For 20 wt % ≤ φ POM < 80 wt %, interspherulitic growth of PLLA inside the pre-existing matrix of POM spherulites causes the formation of interpenetrated blended spherulites, owing to the large discrepancy in kinetics. At φ POM ≥ 80 wt %, PLLA molecular chains are redistributed into the interlamellar level regimes within the POM spherulites and can only crystallize into tiny crystals (owing to strong confinement). PLLA/POM blends provide a perfect example and new insights for understanding the crystallization of miscible crystalline/crystalline polymer blends (with very similar T m 's), in which kinetic factors could play a significant role in crystallization behaviors and morphology.

Crystalline and Spherulitic Morphology of Polymers Crystallized in Confined Systems

“…Researchers have found that the binary miscible crystalline/crystalline polymers show the confined and fractional crystallization behavior because of the phase separation and segregation in different length scales during crystallization process. These crystalline/crystalline polymer blends included poly(ethylene terephthalate) (PET)/poly(butylene terephthalate) (PBT) [75], poly(3-hydroxybutyrate) (PHB)/poly(ethylene oxide) (PEO) [76,77], poly(vinylidene fluoride) (PVDF)/poly(butylene adipate) (PBA) [78][79][80], PVDF/poly(butylene succinate-co-butylene adipate) (PBSA) [81], PVDF/PHB [82], PVDF/poly(butylene succinate) (PBS) [83][84][85], PBS/PEO [86][87][88][89][90][91][92], PBS/PBA [93,94], poly(butylene adipate-co-butylene succinate) (PBAS)/PEO [95], poly(ethylene succinate) (PES)/PEO [96], PLLA/poly(oxymethylene) [97,98], and so on. Expect for the miscible blends with one or two crystalline homopolymers, the miscible copolymer/copolymer blends having crystalline components or blocks could also show the confined crystallization behavior [99,100].…”

Crystallization behavior in miscible blends of poly(ε‐caprolactone) and poly(hexylene adipate) with similar thermal properties studied by time‐resolved Fourier transform infrared spectroscopy

“…There has been a growing interest over the last decade to investigate crystalline/crystalline binary polymer blend systems because of the different morphological structures that can be obtained due to the crystallization of both polymers depending on the polymers' thermal properties, crystallization condition, and blend composition . This makes it a challenge and interesting to understand the crystallization behavior of such systems when both components crystallize.…”

“…Most miscible crystalline/crystalline blends reported so far in the literature can be categorized as those in which the individual component have; (1) large ΔT m or (2) small ΔT m. In type (1) blends, sequential crystallization has been observed where the high T m component crystallizes first even when its content was low in the blend. The low T m component either remained amorphous in the interspherulitic, interlamellar, and interfibrillar of the crystallized component or nucleated and crystallized in these confinements at large supercooling.…”

“…The mechanism of fractional crystallization is well‐documented in the literature . Type (ii) blends exhibit simultaneous crystallization of both the components, sometimes resulting in co‐crystallization and the formation of interpenetrating spherulites . Co‐crystallization in miscible crystalline/crystalline blends is rare and limited to polymer pairs with similar chemical structures.…”

Synergistic effect of thermoplastic phenolic resin and multiwalled carbon nanotubes on the crystallization of polyoxymethylene

“…The POM bars should also be extruded very slowly without pulling and depending only on the pressure of the head part of the extruder, and must be shaped in an oil bath of 125-135 C. Most importantly, due to the high crystallinity with coarse crystalline grains, the POM articles are very brittle. [3,8] Therefore, the crystallization behavior, including crystallization kinetics [2,[10][11][12][13][14][15][16][17][18][19][20][21][22] and nucleation, [23][24][25][26] as well as the toughness [3,[27][28][29][30][31][32][33][34][35][36][37][38] of POM, have always been its main research fields.…”