Mechanically Robust, Self-Healable Polymers Usable under High Humidity: Humidity-Tolerant Noncovalent Cross-Linking Strategy

Although mechanically robust polymer materials had not been thought to self-heal, we recently found that poly(ether thiourea) PTUEG₃, which is a glassy polymer with high mechanical strength, self-heals even at ambient temperatures. This finding updated the above preconception. Nevertheless, it should also be noted that PTUEG₃, under high humidity, absorbs water and is plasticized to lose its mechanical strength. Humidity-induced plasticization is a general problem for polymers with polar groups. Herein, we report that PTUEG₃, if designed by copolymerization to contain only 10 mol % of a dicyclohexylmethane (Cy₂M) thiourea unit (TUCy₂M), serves as a humidity-tolerant, mechanically robust polymer material that can self-heal at ambient temperatures. This copolymer contained, in its ether thiourea (TUEG₃)-rich domain, a humidity-tolerant, noncovalently cross-linked 3D network with mechanical robustness formed by stacking of the Cy₂M group. The present work provides a promising design strategy for mechanically robust, self-healable polymers usable under high humidity.

Variable temperature solid-state NMR spectral and relaxation analyses of the impregnation of polyethylene glycol (PEG) into coniferous wood

To investigate the behaviours of polyethylene glycol (PEG) and its interaction with biomass constituents in coniferous wood (Japanese cypress), variable temperature solid-state NMR spectra and relaxation times were measured from 20-80 °C. Signal intensities in the 1H and 13C PST-MAS NMR spectra changed depending on both the measurement temperature and the melting point of the impregnated PEG. In the 13C CP-MAS NMR spectra with increasing temperature, although the signal intensities of biomass constituents slightly decreased, signal intensities of PEG molecules in the cypress maximized at 80 °C. PEG impregnation into cypress decreased the T 1H values at 80 °C for short to medium chain PEG in the liquid phase while it decreased T 1H values at ambient temperature for long chain PEG in the solid phase because the interactions of PEG molecules and the biomass constituents of coniferous wood were different for different chain lengths of the PEG. These variable temperature measurements of both solid-state NMR spectra and relaxation time indicated that impregnation of longer chain PEG molecules produced higher hydrophobicity because of the increased steric hinderance of PEG attached to carbohydrates. The variable temperature measurements also showed that long chain PEG molecules were restricted to the lumen while short to medium chain length PEG molecules infiltrated into the intercellular region of the cell wall in addition to the lumen. These results obtained from the variable temperature NMR measurements were also supported by ATR-IR spectroscopy analyses.

Multi-scale instrumental analyses of plasticized polyhydroxyalkanoates (PHA) blended with polycaprolactone (PCL) and the effects of crosslinkers and graft copolymers

Details of the mechanism underlying the tensile properties of plasticized polyhydroxyalkanoates (PHA) including poly(butylene succinate) (PBS) were investigated by blending with poly(ε-caprolactone) (PCL) as well as the addition of compatibilizers. Multi-scale instrumental analyses employed micro-focus X-ray CT to provide micro-scale morphology information on the order of ten microns while solid-state NMR spectral and relaxation time analyses contributed knowledge of the environment and molecular mobility of each constituent at the molecular to nano-scale. The blend of plasticized PHA with 50% PCL adopted a sea-island morphology to improve elongation at break in a quasi-static tensile test, which was dominated by the tensile properties of the added PCL. However, impact tensile properties were less improved by PCL addition, because its molecular mobility was suppressed by blending. Meanwhile, peroxy crosslinkers changed the sea-island morphology to homogenous in X-ray CT observations. Although the homogenous morphology sharply lowered the elongation at break in a quasi-static tensile test, the homogenous morphology improved impact tensile properties. Furthermore, graft polymers having acrylonitrile–styrene side-chains did not change the sea-island morphology but increased the molecular mobility of PBS in the plasticized PHA. This weak interaction between the plasticized PHA and PCL improved tensile properties in both quasi-static and impact tensile tests.

Multi-scale instrumental analyses showed that the crosslinker changed the morphology to homogenous while the graft polymer increased molecular mobility.

Solid-State Nuclear Magnetic Resonance (NMR) and Nuclear Magnetic Relaxation Time Analyses of Molecular Mobility and Compatibility of Plasticized Polyhydroxyalkanoates (PHA) Copolymers

The molecular mobility and compatibility of plasticized polyhydroxyalkanoates (PHA) were investigated, focusing on changes due to copolymerization using either flexible poly (butylene succinate) (PBS) or rigid poly(lactic acid) (PLA) units. For the case of a poly(3-hydroxybutyrate) (PHB) unit in plasticized PHA, copolymerization of either PBS or PLA decreased ¹H and 13C spin-lattice relaxation times in the laboratory frame (T₁H and T₁C) in the same manner, while PBS produced a lower ¹H spin-lattice relaxation time in the rotating frame (T₁ρH) than PLA. Both the signals of ¹H MAS (magic-angle spinning) and 13C PST (pulse saturation transfer) MAS nuclear magnetic resonance (NMR) spectra were sharpened and increased by copolymerization with PBS. A variable temperature relaxation time analysis showed that the decrease of T₁H values was dominated by the ¹H spin diffusion via the interface between PHB and the added polyester because of the good compatibility. Meanwhile, the decrease of T₁C values was dominated by increasingly rapid molecular motions of PHB because of the lowered crystallinity due to the plasticization. Slow molecular motions (kHz order) were enhanced more by the addition of PBS than PLA, although rapid molecular motions (MHz order) were enhanced by either polyester. Several NMR parameters were beneficial for analyzing the manufacturing process as the indexes of polymer compatibility and molecular motions.

Nucleating and Plasticization Effects in Drawn Poly(Lactic Acid) Fiber during Accelerated Weathering Degradation

Changes in the polymer properties of poly(lactic acid) (PLA) fibers during drawing and degradation processes were analyzed using solid-state NMR, with the goal of elucidating morphological changes that influence fiber tensile properties. Combination of X-ray diffraction (XRD) and differential scanning calorimeter (DSC) indicated that the drawn PLA fibers consisted of different proportions of α crystalline and amorphous forms. ¹³C CP-MAS NMR spectra showed amorphous-like broad singlet signals, of which the full width at half maximum (FWHM) decreased with increasing crystallinity and crystal orientation. The T₁H value decreased by interaction with additives and increased with increasing crystal orientation. The interaction with additives also reduced T₁C values, which increased with increasing crystallinity. Use of organic clay enhanced the crystallization of high draw-ratio PLA fibers due to nucleation, which increased tensile strength; this effect gradually decreased with time during accelerated weathering. In contrast, the plasticization due to the addition of flexible polymers increased fiber elongation, which rapidly dropped during the degradation. Changes of FWHM, T₁H, and T₁C values indicated that the degradation occurred at sites within the amorphous portions of the PLA fibers containing organic clay, while the flexible polymers were preferentially degraded if they were present in the PLA fibers.