Poly(l-lactic acid): Uniplanar Orientation in Cocrystalline Films and Structure of the Cocrystalline Form with Cyclopentanone

Abstract

Guest molecules, like cyclopentanone (CPO), N,Ndimethylformamide (DMF) tetrahydrofuran (THF) and 1,3−dioxolane (DOL), which are able to cocrystallize with poly(L-lactic acid) (PLLA), can also generate high degree of crystalline phase orientation, by simple sorption in amorphous unoriented PLLA films. This crystalline phase orientation is defined as a ∥ c ∥ (a-parallel, c-parallel) because the ac plane of most crystallites is preferentially parallel to the film plane. This crystalline phase orientation, which is achieved without stretching even for high thickness films, is unprecedented for PLLA and can be maintained after suitable guest exchange procedures. The reported results confirm the recent hypothesis that cocrystallization with lowmolecular-mass guest molecules could be a common route for getting uniplanar orientations of polymer crystalline phases, even in the absence of stretching. Wide angle X-ray diffraction analyses of unoriented and oriented PLLA samples have allowed proposing the structure of the cocrystalline form of PLLA with CPO. In particular, four PLLA chains exhibiting the 10/7 helical conformation and 16 CPO guest molecules are packed according the space group P2 1 2 1 2 1 in an orthorhombic lattice with a = 1.61 nm, b = 1.26 nm, and c = 2.90 nm. The presence in PLLA cocrystalline structures of dense bc layers with close-packed polymer helices allows an easy rationalization of the observed uniplanar orientation. ■ INTRODUCTIONPolymeric cocrystalline forms, i.e. structures were a polymeric host and a low-molecular-mass guest are cocrystallized, although early recognized in literature, have been ignored for many decades. 1−15 Only in the last 2 decades have a few polymeric cocrystalline forms received attention in material science, mainly due to their ability to produce nanoporouscrystalline forms, 16−21 as a consequence of guest removal. 16,22,23 In the past decade, it has been also found that cocrystallization with suitable guest molecules of two commercial polymers: syndiotactic polystyrene (s-PS) and poly(2,6-dimethyl-1,4-phenylene ether) (PPO), can lead without any stretching procedure to high thickness f ilms (up to 200− 300 μm) with high degrees of different kinds of uniplanar orientations of the crystalline phases, i.e., with high degrees of parallelism of well-defined crystal planes with respect to the film plane. 24−34 In particular, for s-PS films, three different kinds of uniplanar orientations can be achieved by simple procedures involving cocrystallization in the presence of suitable guest molecules (solution crystallization, 24−26 solventinduced crystallization in amorphous samples, 27−30 or solventinduced recrystallizations of suitable crystalline films 31 ). The three observed uniplanar orientations correspond to the three simplest orientations of a high planar-density layer (formed by close-packed alternated enantiomorphous s-PS helices) with respect to the film plane. 32,33 As for PPO, two different kinds of uniplanar orientations have been recently obtained by cocrystallizat...

Fabrication of Polymer Structure among Fibrous Structure of Poly(lactic acid) Gel and Improvement of Physical Properties

“…This indicates that once the fibrous structure of the ε‐crystals is formed, the gels are stable even after the ε‐crystals changed to another crystals. The transition from ε‐crystals to α‐crystals by increasing the temperature, and coexistence of these two types of crystals were also reported by Shaiju et al and Rizzo et al The transition temperature of ε‐crystals to α‐crystals in cyclopentanone at 40—55 °C is comparable with that in DMF, which was detected in our previous work …”

c-Perpendicular Orientation of Poly(ʟ-lactide) Films

“…It is well known that PLLA is able to form host–guest co-crystalline forms with suitable guest molecules like cyclopentanone, tetrahydrofuran, 1,3-dioxolane, γ-butyrolactone and N , N -dimethylformamide (DMF), [ 18 , 22 , 23 , 24 , 25 ] and some crystalline structures have also been proposed [ 18 , 22 ].…”

“…The degree of orientation of the crystalline phase, f c , was calculated by using Herman’s orientation function [ 9 , 18 ]: where cos 2 γ was calculated by the azimuthal distribution of the main reflection (110/200, at 2θ = 16.7°), for the EDGE patterns. Based on these assumptions, when f c is equal to 0, a random crystallite orientation occurs, while when f c is equal to –0.5 the c axes of all crystallites are perfectly perpendicular to the film plane.…”

“…The degree of orientation of the crystalline phase, f c , was calculated by using Herman's orientation function [9,18]:…”

“…Furthermore, for PLLA, a relevant commercial biodegradable polymer, crystallization as induced by sorption of several guest molecules (cyclopentanone, tetrahydrofuran, 1,3-dioxolane and N , N -dimethylformamide) in amorphous films can lead to co-crystalline phases with a high degree of orientation. In particular, a uniplanar orientation with ac-planes of crystallites being preferentially parallel to the film plane (defined as a-parallel, c-parallel, shortly a || c || ) has been recently observed [ 18 ].…”