Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Confinement Effects on the Crystallization of Poly(3hydroxybutyrate) Xiying Dai,† Huihui Li,† Zhongjie Ren,† Thomas P. Russell,†,§ Shouke Yan,*,† and Xiaoli Sun*,† †
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State Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China § Department of Polymer Science and Engineering, University of Massachusetts Amherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *
ABSTRACT: Poly(3-hydroxybutyrate) (PHB) is taken as an example to explore (i) whether the confined crystallization occurring in anodized aluminum oxide (AAO) nanopores is the same as that in ultrathin films, and (ii) whether the interfacial effect of curve surface is the same as flat surface. The crystallization behavior of PHB in AAO and thin films (sandwiched between two plates) has been compared. A curvature-dependent crystallization behavior of PHB is identified. Stable intermediate structures of PHB confined in narrow pores (diameter 100 nm), crystallization occurs both in the center and interfacial regions in contact with AAO inner wall. This implies that the strength of interfacial layer weakens with decreased curvature and has been further proved by the crystallization behavior of its sandwiched ultrathin film. It is found that the highly restricted interface layer incapable of crystallization is about 30 nm for film, which is much thinner than that for nanorods (approximately 100 nm). We have also identified two different relaxations of PHB nanorods corresponding to the interfacial effect and spatial confinement, respectively. While the relaxation correlated to the interfacial effect with a slower relaxation time strengthens, the relaxation corresponding to spatial confinement with a faster relaxation time (bulklike α-relaxation) weakens with decreased pore size. This is completely different from that of sandwiched thin film, where only a thickness-independent α-relaxation same as bulk is observed 1 (Macromolecules 2006, 39, 5967). Therefore, the reduced crystallization kinetics of thin film is attributed to the reduction of long-range chain mobility. By contrast, the inhibited crystallization of PHB nanorods in AAO is attributed to the reduced segmental dynamics and the reduced chain mobility of PHB layer at interface.
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crystallization kinetics with film thickness. The glass transition temperature, Tg, remains constant for PHB for different thickness.1 The lamellar orientation and stability of crystals are dependent on the interfacial boundary and the film thickness.17−22 For example, a crystal structure of PHB with larger (020) crystal plane distance of α-form exists near the free surface of the film which transforms into the highly ordered αform during heating process.17 PHB is used in the field of regenerative medicine and tissue engineering, such as nerve guides and artificial esophagus.18 Also, it is frequently used as thin film (2D) or is found in nature, that is, in the cell, as a nanoparticle (0D). Consequently, understanding the kinetics of crystallization and the dynamics of the polymer chain under these different confinement conditions can have significant impact.
INTRODUCTION Poly(3-hydroxybutyrate) (PHB), one of the most abundant poly(3-hydroxyalkanoates) (PHAs) polyesters, can be produced by many bacteria and stored as energy and carbon in cells.2−6 Native PHB storage granules in cells are amorphous7 and, once isolated from the cell, are highly crystalline.8 The inability of PHB in the granules to crystallize has been attributed to confinement effects.9,10 Although the crystallization of polymers in a confined geometry is of fundamental importance, it is also of importance to applications11−13 where the crystalline structure, degree of crystallinity, or spatial distribution of the crystals can be used to manipulate the mechanical,14 biodegradable,15 and biocompatible properties.16 Recently, studies of PHB crystallization in confined geometries focused on thin films, where there is confinement only along one direction, that is, normal to the surface. Napolitano et al. use broadband dielectric spectroscopy (BDS) and show that the reduced mobility of the polymer at substrate interface played a fundamental role in the reduction in © XXXX American Chemical Society
Received: May 23, 2018 Revised: July 11, 2018
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DOI: 10.1021/acs.macromol.8b01083 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
spectroscopy (IR). Polymer dynamics were assessed by BDS. Using these combined methodologies, we investigated the influence of curvature on the segmental dynamics and crystallization as a function of the pore diameter. We find an interface-induced layer, characterized by a highly constrained relaxation, at the walls of the membrane. As the pore size is decreased, the relaxation associated with interfacial effects having a slower relaxation time strengthens, while the relaxation associated with spatial confinement having a faster relaxation time weakens. There is crossover from a dominance of confinement to interfacial effects at a pore diameter of ∼100 nm. The highly restricted interface layer with ultraslow segmental dynamics closely correlates with the crystallization behavior and freezes the PHB into intermediate structures in narrow pores (diameter
