Tacticity-Dependent Epitaxial Crystallization of Poly(l-lactic acid) on an Oriented Polyethylene Substrate

Abstract

The influence of tacticity on the bulk crystallization and polyethylene (PE)-induced epitaxial crystallization of poly­(l-lactic acid) (PLLA) was investigated. It was found that cold-crystallization of PLLA with different tacticities from the amorphous glassy state is easier than the melt-crystallization from the isotropic melt and the cold-crystallization of PLLA at the PE surface takes place earlier than in bulk. The time-lag between the PE-induced epitaxial crystallization and bulk cold-crystallization of PLLA increases with decreasing tacticity. For example, while the cold-crystallization of the PLLA sample with 95% tacticity in the bulk and at the PE surface starts almost simultaneously, the bulk cold-crystallization of PLLA with a tacticity of 89% starts at a time point where over 90% PLLA already crystallized epitaxially on the PE substrate. The almost simultaneous start of bulk and PE-induced cold-crystallization of high tacticity PLLA enables the creation of a cross-hatched lamellar structure, which is composed of PLLA lamellae oriented along the PE chain direction and tilted ca. 64° with respect to the PE chain direction. The big time-lag between PE-induced epitaxy and bulk cold-crystallization of low-tacticity PLLA results in the disappearance of tilted PLLA lamellae. This further confirms that the PLLA lamellae aligned in the chain direction of PE are caused by PE-induced epitaxial crystallization, whereas the tilt lamellae are caused by the homoepitaxy of PLLA based on the parallelism of the helical paths.

In Situ SAXS and WAXD Investigations of Polyamide 66/Reduced Graphene Oxide Nanocomposites During Uniaxial Deformation

“…Pashley indicated that 10–15% disregistries of lattice matching were the upper limit of epitaxy . There is also another surface-induced crystallization referred to as graphoepitaxy that is based on surface topology of the substrate as indicated by the crystallization of poly­( l -lactic acid) on polyethylene and polypropylene substrates. − …”

Structure and Mechanical Property of Melt-Drawn Oriented PLA Ultrathin Films

“…The highly oriented structure of PLAs is most frequently produced by fiber-spinning, − or epitaxial crystallization on oriented substrates. − Much research progress on the crystal structure and mechanical properties of stretched PLA films and subsequent annealing has also been achieved. − For example, it has been demonstrated that high orientation and crystallinity of stretched samples can effectively improve the strength, modulus, and toughness of PLAs. The early works focus, however, mainly on the uniaxial or biaxial stretching of the nonoriented PLA films in amorphous or crystalline states.…”

Directional Epitaxy Intensifying Intermolecular =C–H···O=C Hydrogen Bonding to Expedite Bulk Crystallization of Biobased Poly(butylene 2,5-furandicarboxylate)

“…33 Meanwhile, a molecular-level comprehension of the epitaxial mechanism remains controversial and is still far from conclusive. 15,[21][22][23][24][25][26][27][28][29]32,33 Therefore, understanding and controlling polymer crystallization via epitaxy especially in bulk materials on foreign surfaces have been and remain to be a serious challenge in polymer physics. [13][14][15][16][17]33 Furan-aromatic polyesters produced from biomass-derived furandicarboxylic acid (FDCA) are considered the renewable alternatives of petrol-based benzene-aromatic polyesters.…”

“…Moreover, the important effect of secondary nucleation upon epitaxial crystallization between polymers was also highlighted . Very recently, Tong and co-workers have approved that lattice matching is indeed required but insufficient for successful epitaxy and the critical nuclei size dominates the epitaxial growth. , Despite the extensive studies with diverse systems over the past decades, regulating polymer crystallization via epitaxy is nowadays hardly desirable. ,,− More importantly, due to the very short range of interfacial epitaxial interaction, the past works involve polymer epitaxy focused on very thin films with the main evidence of crystal morphologies and orientation relationships obtained by atomic force microscopy, transmission electron microscopy, and electron diffraction, ,− while polymer epitaxy from bulk crystallization is seldom concerned and the featured evidence is essentially missing . Meanwhile, a molecular-level comprehension of the epitaxial mechanism remains controversial and is still far from conclusive. ,− ,, Therefore, understanding and controlling polymer crystallization via epitaxy especially in bulk materials on foreign surfaces have been and remain to be a serious challenge in polymer physics. − , …”