Nanoconfinement crystallization of frustrated alkyl groups: crossover of mesophase to crystalline structurew

Nanoconfined crystallization and phase transition are currently attracting much interest as aspects of polymer crystallization, which provide an approach for understanding the formation mechanism of a hierarchy of ordered structure and the transient mesophase in the confined conditions, typically in the length scale of B10 nm. Characteristic of nanoscopic confined crystallization is the reorganization of the frustrated crystallizable components into the conformational ordered packing structure upon reaching the critical crystal nucleus size. Longer CH2 sequences in alkyl nanodomains (B5 nm), separated by the polymeric backbones, exhibit crystalline structure obviously different to that of the typical main chain polymers, and the phase transformations of frustrated CH2 groups show size-dependent behavior. The combination of the length scale of side groups and nanoconfinement could be a good approach for in-depth understanding of the confinement crystallization behavior and the mesophase evolution of polymer crystallization. In this communication, by using the N-docosylated polyethyleneimine (PEI22C) as an example, we intend to describe the nanoconfined crystallization of the C22 side groups along the flexible polymer backbones, which leads to the new phase formation and different mesophase evolution in alkyl nanodomains. The frustrated mobility and chain packing contribute to the coexistence of orthorhombic (bO), monoclinic (bM) and hexagonal (aH) in PEI22C, and these are three cases of dependence on the length scale of side groups controlling the nano-sized crystals. This is the first example of the polymorphic phase transformations in one-component systems, in comparison with the typical side-chain crystallization polymers showing only the metastable hexagonal phase in alkyl nanodomains. The grafting procedure of N-alkylated polyethyleneimine (PEI(n)Cs) has been published elsewhere. The molecular structure and composition of PEI22C were examined with H NMR and FT-IR (ESI, Fig. S1). Due to the steric hindrance of longer C22 side groups, only 40% substitution degree was achieved. The crystalline structures and phase transitional behaviors of PEI22C were investigated by differential scanning calorimetry (DSC) and one-dimensional (1D) wide/small-angle X-ray scattering. High-resolution FT-IR spectroscopy was used to analyze the fine packing mode and microstructural variation of CH2 groups in nanoscopic confinement. The experimental and characterization details are provided in ESI. Confined C22 side groups (PEI22C) exhibit three endothermic peaks at 26.4, 34.4 and 49.5/58.5 1C, respectively (ESI, Fig. S2), while the free ones (n-docosane) show two transition processes, arising from the solid-to-solid phase transition and the mesophase-to-isotropic state at 33.8 and 45 1C (ESI, Fig. S3), which is assigned to triclinic and hexagonal phase, respectively. Comparing the enthalpy changes in the confined and free nanoscale domains, only part of the CH2 sequences in the confined C22 side groups stay in the crystalline state (68.7% of C22 side chains enter into the crystal lattice), due to the frustrated mobility and molecular chain packing of alkyl groups. Fig. 1 represents the 1D WAXD and SAXS patterns of PEI22C recorded as a function of temperature from 20 to 75 1C in the phase transition process of confined C22 alkyl nanodomains. The WAXD scattering (Fig. 1A) atB21.51 and B23.91 (d spacings of B4.12 and B3.7 Å) is relatively sharp, ranging from 0 to 20 1C, which is characteristic of the orthorhombic phase (bO phase). Upon heating to 30 1C, the intensities of the diffraction peak atB23.91 gradually decrease and one new peak at B23.31 (d spacing of B3.8 Å is characteristic of bM phase indicated by the green line) appears accompanied by a bO shoulder (the blue line), showing that bO phase gradually is transformed into bM phase because of the distortion of CH2 chains to the end group packing planes. After 40 1C, the single strong peak atB21.61 a State Key Laboratory of Hollow Fiber Membrane Materials and Processes, Institute of Functional Fiber, Tianjin Polytechnic University, Tianjin 300160, China. E-mail: [email protected] b Institute of Textile & Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China c Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E-mail: [email protected] w Electronic supplementary information (ESI) available: Experimental details and results of DSC and FT-IR. See DOI: 10.1039/c0cc05245k ChemComm Dynamic Article Links