Article pubs.acs.org/Macromolecules
Molecular, Crystalline, and Lamellar Length-Scale Changes in the Poly(L‑lactide) (PLLA) during Cyclopentanone (CPO) Desorption in PLLA/CPO Cocrystals P. Shaiju,†,‡ N. Sanjeeva Murthy,§ and E. Bhoje Gowd*,†,‡ †
Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum 695 019, Kerala, India ‡ Academy of Scientific and Innovative Research (AcSIR), New Delhi 110 001, India § New Jersey Center for Biomaterials, Rutgers University, The State University of New Jersey, Piscataway, New Jersey 08854, United States S Supporting Information *
ABSTRACT: Polymer−solvent complexes, poly(L-lactide) (PLLA) with cyclopentanone (CPO), were studied at multiple length scales using differential scanning calorimetry, small-angle neutron scattering, Fourier transform infrared spectroscopy, and temperature-dependent wide- and small-angle X-ray scattering. PLLA crystallizes in the ε form when organic solvents such as CPO are incorporated into the crystal lattice at subambient temperatures. The transformation of this structure into the α form during solvent desorption and the accompanying changes in the lamellar structure were followed by various measurements on PLLA/CPO cocrystals. SANS data suggest that CPO is present stoichiometrically in the crystal lattice and as clusters in the interlamellar amorphous regions in the nominally dried samples. DSC thermogram showed a sharp endotherm during this ε to α transition. Xray fiber diagrams showed that the ε form transforms to the α form over a temperature range (40−55 °C) as the solvent molecules are expelled from the crystalline lattice, while maintaining chain orientation. Infrared spectra showed the splitting of the CH3 symmetric deformation band at 1383 cm−1 into a doublet (1382 and 1386 cm−1) at ε to α transition, indicating the desorption of CPO molecules from the crystal lattice. Changes in the invariant in SAXS data are interpreted as due to the migration of the solvent from the crystalline phase to the amorphous phase during the ε to α transition followed by the evaporation of the solvent from the entire polymer. During this transition, lamellae that are tilted in the presence of CPO in the crystal lattice become perpendicular to the chain axis. In addition, there are changes in long period, lamellar thickness, and amorphous thickness. Continuing the desorption to dryness by further heating results in the removal of the solvent molecules in the amorphous phase of the α form. This is accompanied by increased crystallinity. These studies show that the solvent desorption results in a precise sequence of quantifiable structural changes at multiple length scales.
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and α′, and that is above 120 °C is pure α. The β form can be obtained by the mechanical stretching of the α form at relatively high draw ratio (draw ratio ∼14) at high temperatures (close to the melting point ∼170 °C).14−16 Puiggali et al. suggested a frustrated structure for the β form in which three chains (31 helical conformation) in random up and down directions are arranged in a trigonal cell.14 The β form is thermally less stable than the α form.4 Crystallization of PLLA under 3−15 MPa CO2 at 0−20 °C and under 7−15 MPa CO2 at 30 °C will lead to the formation of α″ form, another disordered structure.12,13 The γ form can be obtained by means of epitaxial crystallization on a substrate like hexamethylbenzene. In the γ form, the unit cell is orthorhombic, where two
INTRODUCTION Poly(L-lactide) (PLLA), the most common stereoisomer of poly(lactic acid), is a widely used bio-based polymer in degradable plastics. It is also used as degradable biomaterial in many medical devices and tissue engineering.1−3 PLLA exhibits very complex polymorphic behavior and has different polymorphic forms such as α,4−6 α′(δ),7−11 α″,12,13 β,14−16 γ,17 and ε18,19 depending on the experimental conditions. Among these, the α form is the most stable one. The α form can be prepared by either solution or melt crystallization methods. It has a 103 helical chain conformation, and the helices are arranged in an orthorhombic unit cell.4,5,20 The α′ form can be prepared by isothermal crystallization at temperatures
