Fracture Behavior of Polyrotaxane-Toughened Poly(Methyl Methacrylate).

The fracture behavior of polyrotaxane (PR)-modified poly(methyl methacrylate) (PMMA) was investigated. PR is a supramolecule with rings threaded onto a linear backbone chain, which is capped by bulky end groups to prevent the rings from de-threading. The ring structure is α-cyclodextrin (CD), and it can be functionalized to enhance its affinity with the hosting polymer matrix. Adding only 1 wt % of PR containing methacrylate functional groups (mPR) at the terminal of some of the polycaprolactone-grafted chains on CD promotes massive crazing, resulting in a significant improvement in fracture toughness while maintaining the modulus and transparency of the PMMA matrix. Dynamic mechanical analysis and atomic force microscopy studies reveal that mPR strongly interact with PMMA, leading to higher molecular mobility and enhanced molecular cooperativity during deformation. This molecular cooperativity may be responsible for the formation of massive crazing in a PMMA matrix, which leads to greatly improved fracture toughness.

Polymer Brush Formation Assisted by the Hierarchical Self-Assembly of Topological Supramolecules.

The development of methods for the polymer brush layer formation on material surfaces to improve the surface properties has been researched for decades. Here, we report a novel approach for the formation of a polymer brush layer on materials and the alteration of the surface properties using a pseudo-polyrotaxane nanosheet (PPRNS). In the PPRNS, ß-cyclodextrin (CD) selectively covered the central poly(propylene oxide)29 segment of the carboxyl-terminated poly(ethylene oxide)75-b-poly(propylene oxide)29-b-poly(ethylene oxide)75 (COOH-EO75PO29EO75) triblock copolymer to form columnar crystals. The EO chains of COOH-EO75PO29EO75 then adopt polymer brush conformations and exhibit an oil-repellent property on the material surfaces. Based on the flexibility derived from the nanosheet structure, the PPRNS showed high adhesion to the Blu-ray disk substrate (1D bending), polystyrene spherical beads (2D bending), and random rough surface of pork skin. The PPRNS is expected to become a new method for obtaining polymer brush layers and improving the surface properties irrespective of the material type.

Molecular Recognition of Fluorescent Probe Molecules with a Pseudopolyrotaxane Nanosheet.

Pseudopolyrotaxane nanosheets (PPRNS) are ultrathin two-dimensional (2D) materials fabricated via supramolecular self-assembly of ß-cyclodextrin (ß-CD) and poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymers. In this study, the molecular loading of various fluorescent probe molecules onto PPRNS was systematically investigated. 1H NMR study for R6G absorption to PPRNS indicated that the small hydrophobic groups, such as the methyl group, of R6G were absorbed by PPRNS. Consistently, the fluorescent probes without methyl groups were not absorbed. These results indicate that PPRNS has a molecular recognition absorption property based on the host-guest interaction of the functional groups on probe molecules and molecular-sized spaces of PPRNS surfaces, which may be vacant ß-CDs and voids between ß-CD columns. The absorbed amount of the molecular probes onto PPRNS was investigated by UV-vis spectra, and the absorption behavior could be described well by the Langmuir absorption isotherm. This is consistent with the suggested model that the probes are absorbed onto the PPRNS surfaces. This study demonstrates that PPRNSs can be applied as adsorbents for toxic compounds, drug delivery systems, and 2D sensors.

Precise control of cyclodextrin-based pseudo-polyrotaxane lamellar structure via axis polymer composition.

Self-assembly of cyclodextrin (CD) with guest polymers has attracted much attention owing to its biocompatibility and accessibility. In this study, we investigate the composition effect of poly(ethylene oxide)m-b-poly(propylene oxide)n-b-poly(ethylene oxide)m (EOmPOnEOm) triblock copolymers on lamellar or plate structures formed by complexation with ß-CD. EO5PO29EO5, EO14PO29EO14, and EO75PO29EO75 show periodic lamellar morphology consisting of single-crystalline pseudo-polyrotaxane (PPR) nanosheets with a thickness equal to the central PO length. This is because ß-CDs selectively cover the PO component and cause the microphase separation between ß-CD and EO layers. The thickness of the EO layers increases linearly with increasing number of EO units, which suggests that the EO chains are constrained into virtual cylinders with the diameter of the ß-CD. This means that we can precisely control the thickness of both the crystal (ß-CD and PO) and the amorphous (EO) layers in the lamellar structure. In contrast, EO2PO29EO2 forms a thin plate structure, where not only PO but also EO chains are covered with ß-CD. Furthermore, the length of the central PO component is necessary to form the lamellar structure with the phase separation between the ß-CD and EO layers. These findings provide a more fundamental understanding to enhance the variety and applicability of CD-based self-assembled materials.

Autonomously isolated pseudo-polyrotaxane nanosheets fabricated via hierarchically ordered supramolecular self-assembly.

We succeeded in obtaining autonomously isolated nanosheets consisting of pseudo-polyrotaxane (PPR) fabricated via hierarchically ordered supramolecular self-assembly of ß-cyclodextrin and a poloxamer by introducing charged groups to the axis ends of the poloxamer. The isolated PPR nanosheets exhibited a tunable structural coloration and were aligned using a strong magnetic field.

Drastic Change of Mechanical Properties of Polyrotaxane Bulk: ABA-BAB Sequence Change Depending on Ring Position.

Polyrotaxane (PR), consisting of many ring molecules and an axis polymer, is a typical supramolecular structure with unique topological characteristics. In this study, we demonstrated the drastic change of the macroscopic mechanical properties depending on the ring position of PR in bulk. Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer was employed as an axis polymer to control the position of ß-cyclodextrin (ß-CD). To transfer the ß-CD positions, hydroxypropyl groups (HPPR) and hydrophobic trimethyl silyl groups (TMS-HPPR), which have hydrophilic and hydrophobic ß-CD, respectively, were synthesized. ß-CDs in HPPR were localized on a central PPO segment and formed crystal domains. The axis polymer of HPPR could not bridge ß-CD crystal domains, resulting in a melt state at high temperature. On the other hand, ß-CDs in TMS-HPPR were transferred to both PEO segments and formed crystal domains. The axis polymer in TMS-HPPR could bridge the ß-CD crystal domains, resulting in an elastic state even at high temperature. We succeeded in demonstrating the potential ability of PR: the macroscopic mechanical properties of PR can be changed from a melt state to an elastic one by manipulating the ring positions.