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Polyurethane elastomers have been widely used in the industry and daily life due to their versatile physical and chemical properties. Therefore, a fundamental understanding of the structures and dynamics at a molecular level will provide piercing insights into the precise design and application of new polyurethane materials. In this study, we mainly focused on investigating the strain-induced structural and dynamic changes in a typical polyurethane elastomer composed of poly(e-caprolactone) (PCL) and 4,4′-diphenylmethylene diisocyanate (MDI) as the soft and hard segments, respectively. Obvious strain-hardening phenomenon was observed during the mechanical tensile test, and a systematic comparison was performed on the fractured and pristine samples. DSC results revealed that the crystallization of PCL chains was still going on after the sample was fractured, and the crystallite structures became stable after physical aging at 25 °C for two days. 1H solid-state nuclear magnetic resonance (NMR) experiment was further employed to determine the fraction of mobile PCL chains that were converted to crystallites during stretching. Besides, the microphase separation was also significantly enhanced in the fractured sample. The mobility of amorphous PCL chains was largely reduced due to the strain-induced crystallization of nearby PCL segments, as revealed by the 1H magic-sandwich echo (MSE) NMR experiments. 1H multiple quantum (MQ) NMR experiments also quantitatively revealed the strain-induced orientation of amorphous PCL chains in the fractured sample, indicating that the PCL crystallites were acting as the physical cross-linkages to prevent the contraction of the elongated PCL chains when the sample is fractured.

Inspired by the nature material, a crosslinked network structure of polyurethane/epoxy resin composite material was built through Diels-Alder (DA) reversible covalent bond reaction between the dienes and dienophile group on the side chain of epoxy resin and polyurethane respectively. The synthesized composite material combined the advantages of polyurethane and epoxy resin. The hard segments of polyurethane and epoxy resin were closely combined through the DA reaction and high density hydrogen bonding. The strength, hardness and toughness of the composite material were greatly enhanced. DA covalent bonds can be reversible with varying the temperature, which led to the heat reprocessing ability of the synthesized composite material. The well-established structure-property relationship shown in this paper could further provide guidance for fabrication of high performance recyclable material with precisely controllable microstructures and behaviors.

Polyurethane materials (PUs) have been widely used in industry and daily life due to the versatile chemistry. However, despite the rapid advance in synthetic chemistry, it still remains a significant challenge for the facile fabrication of PUs with a single cross-linked network embedded with excellent mechanical properties and recyclability. Herein, in this study, we proposed a simple strategy to fabricate a high-performance recyclable cross-linked PU and revealed the relationship between microscopic structure and macroscopic properties. The UPy (2-ureido-4-[1H]-pyrimidione) motifs were incorporated into the backbone chains of PUs, where the quadruple hydrogen bonding interactions between UPy dimers can significantly enhance the mechanical strength and toughness. Furthermore, a single small molecular Diels-Alder adduct was utilized as the chemical crosslinker, rendering the final cross-linked PUs healable and recyclable. The thermal reversibility of the Diels-Alder reaction was well verified by DSC and solid-state NMR spectroscopy. Notably, it was found that the incorporation of UPy motifs could enhance the strain-induced crystallization (SIC), leading to a large stress at break. The structural and dynamic changes induced by SIC were quantitatively addressed by proton multiple-quantum NMR spectroscopy and SAXS experiments, where SIC further imposed restrictions on the mobility of surrounding polymer chains in the soft domain and led to the change of microphase separated nanostructures. Overall, a simple strategy is proposed here for the facile fabrication of high performance recyclable PUs, and the detailed investigation here on the structure-property relationship may further provide insights into developing high performance polymeric materials.

Understanding the shape memory properties of heat-shrink polymers (HSPs) at the molecular level is crucial for the design and synthesis of advanced HSP materials. Herein, we employed a variety of in situ variable-temperature (VT) solid-state nuclear magnetic resonance (NMR) techniques, in combination with other methods, to investigate the evolution of the individual components of a poly(ethylene-co-vinyl acetate)-based HSP with mobility contrast and segmental orientation during the heat-shrink process. In situ VT 1H T2 relaxometry experiments clearly revealed the presence and evolution of rigid, semi-rigid and mobile components associated with stable crystallites and crosslinkage, less-stable crystallites and the amorphous phase in HSPs with increasing temperature, respectively. In particular, the reversible switching phase should be predominately attributed to the semi-rigid crystalline components, which dramatically decreased after the onset temperature and completely disappeared at the end temperature used in the heat-shrink process. The fixed phase associated with the rigid crosslinkage was observed at high temperatures. Furthermore, the activation energy (Ea) of the mobile components decreased after the heat-shrink process, indicating the chain relaxation of deformed segments in the expanded sample. This was confirmed by Baum−Pines 1H double-quantum experiments, which also revealed an inflection point of the chain mobility at the onset temperature (∼330 K) of the heat-shrink process, at which the restricted mobile chains in the expanded sample are nearly completely relaxed. This imbues HSPs with the ability to shape change. In addition, two-dimensional wide-angle X-ray diffraction (WAXD) indicated that the weak orientation of crystalline domains in HSP disappears after the heat-shrink process. Based on the NMR and WAXD experimental results, a model was proposed to describe the molecular mechanism underlying HSPs' shape memory properties. Finally, proton T2 relaxometry combined with multiple-quantum NMR was confirmed to be a powerful method to study HSPs shape memory properties.

We have investigated phase behaviors of a series of polyphenylacetylene derivatives, poly[di(n-alkyl) ethynylterephthalates] (Pn, n is the number of carbon atoms of n-alkyl group, from 2 to 14), which are largely influenced by the length of n-alkyl tails on 2,5-position of the phenyl group. With short alkyl groups (n ≤ 6), Pns form columnar liquid crystalline (Col) phases due to the “jacketing effect” of side-chains. As the alkyl tails become longer (n ≥ 8), an isotropic phase, which can be considered as a reentrant one (Ire), is identified between lamellar phase (Lam) at low temperatures and Col phase at high temperatures. This unusual phase behavior is determined by the motional state of the side-chains. Thanks to in-situ variable temperature solid-state NMR experiments, the motion of main- and side-chains in different phases were distinguished, providing strong evidence for the entropy effect of side-chains which drives Ire and then Col phase in Pns (n ≥ 8) upon heating.

The adsorbed species on nanofillers is one of the most important factors that may influence the performance of polymer nanocomposites. Herein, we performed a systematic investigation of the effects of the residual surfactant introduced by a synthetic method on the glass transition temperature ( T g ) of polystyrene/gold nanoparticle nanocomposites. The surfactant was found to cause an obvious decrease in the T g of polystyrene confined on gold. Furthermore, we used dipolar-filtered 1 H solid-state nuclear magnetic resonance to characterize the mobility of the molecules confined on gold. It was impossible to completely remove the surfactant once it had been adsorbed on the gold nanoparticles; thus it is important to choose suitable methods of synthesis, and special attention should be paid to the adsorbed species on nanofillers.

Hydrogenation has been proved to be an efficient way to remove the toxicity of phthalate plasticizer. However, other influences of this hydrogenation are still unknown. Here we chose di-2-ethylhexyl phthalate (DOP) and di(2-ethylhexyl) cyclohexane-1,2-dicarboxylate (DEHHP) to study the influence on interaction with poly(vinyl chloride) (PVC). By combining experiment and calculation, we found the interaction was stronger in PVC/DEHHP than in PVC/DOP. Low-Field 1H NMR results showed that PVC chains could restrict much more DEHHP molecules than DOP. FTIR results showed that the interaction exists in form of hydrogen bonding complex, and it was stronger in PVC/DEHHP than in PVC/DOP system. Combined with FTIR results, theoretical calculation results revealed the three-center hydrogen bonded structure of the complex. Both the proportion and the binding energy of pre-complex in DEHHP are much larger than in DOP. Here, the hydrogenation-induced change of interaction was elucidated systematically and could be generalized to other phthalate plasticizers.

1H spin-diffusion solid-state NMR, in combination with other techniques, was utilized to investigate the effect of molecular architecture and temperature on the interphase thickness and domain size in poly(styrene)-block-poly(butadiene) and poly(styrene)-block-poly(butadiene)-block-poly(styrene) copolymers (SB and SBS) over the temperature range from 25 to 80 °C. These two block copolymers contain equal PS weight fraction of 32 wt%, and especially, polystyrene (PS) and polybutadiene (PB) blocks are in glass and melt state, respectively, within the experimental temperature range. It was found that the domain sizes of the dispersed phase and interphase thicknesses in these two block copolymers increased with increasing temperature. Surprisingly we found that the interphase thicknesses in these two block copolymers were obviously different, which was inconsistent with the theoretical predictions about the evolution of interphase in block copolymer melts by self-consistent mean-field theory (SCFT). This implies that the interphase thickness not only depends strongly on the binary thermodynamic interaction (χ) between the PS and PB blocks, but also is influenced by their molecular architectures in the experimental temperature range.

First, the NMR method proposed in our previous work was improved to provide more accurate measurement of interphase thickness in multiphase polymers. Then the improved method, in combination with other techniques, was applied to elucidate the phase behavior, miscibility, heterogeneous dynamics and microdomain structure in thermoset blends of unsaturated polyester resin (UPR) and amphiphilic poly(ethylene oxide)- block -poly(propylene oxide)- block -poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymer. The experimental results were compared with those of epoxy resin (ER)/PEO–PPO–PEO blends to systematically elucidate the influence of binary polymer–polymer interaction on the phase behavior, domain size and especially the interphase thickness in thermoset blends of UPR and ER, respectively, with the same PEO–PPO–PEO triblock copolymer. It was found that UPR/PEO–PPO–PEO exhibits strong phase separation with considerably small interphase, and only a small fraction of PEO is mixed with UPR. Whereas ER/PEO–PPO–PEO exhibits weak phase separation with thick interphase, and a large amount of PEO is intimately mixed with ER. It was suggested that the thermodynamic interaction between the block copolymer and cross-linked thermoset resin is one of the key factors in controlling the phase behavior, domain size and interphase thickness in these blends. These NMR results are qualitatively in good agreement with the previous theoretical prediction of interphase properties between two immiscible polymers. Our NMR works on different thermoset blend systems with weak and strong microphase separations clearly demonstrate that the improved NMR method is a general and useful method for measuring the interphase thickness and elucidating the phase behavior and subtle microdomain structure in multiphase polymers with detectable heterogeneous dynamics.

Disorderly exfoliated telechelic liquid polybutadiene/organo-clay nanocomposite gel was, for the first time, successfully prepared at room temperature of 26 °C by simply compounding of carboxyl-terminated polybutadiene (CTPB) with organo-clay containing surfactant modifier with two alkyl tails. The dispersion behavior of organo-clay in liquid polybutadiene/organo-clay nanocomposite gels were systematically investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), rheology and compatibility experiments, focusing on the influence of the functional end-groups of the liquid rubber and the alkyl tails number of the organic modifier on the clay exfoliation. A unique liquid-like reversible rheological behavior was observed for CTPB/exfoliated clay nanocomposite gels with organic modifier having two alkyl tails, which is totally different with that of other samples (HTPB/C18-clay, HTPB/D18-clay and CTPB/C18-clay). And it is noteworthy that CTPB exhibits a higher level of dispersion with an organo-clay with two alkyl tailed surfactant than that with one alkyl tail.

A novel cytocompatible graft copolymer of chitosan and l -lactic acid (CL) was prepared by grafting l -lactic acid onto the amino groups in chitosan without a catalyst. The structures of the CL graft copolymers were characterized by FTIR, 13C-NMR and X-ray measurements. Degree of substitution and side-chain length were evaluated from salicylaldehyde and elemental analysis. The tensile strength and water uptake of the CL copolymers films were investigated as a function of feed ratio of LA/CS. The influence of pH on the swelling behavior of the copolymer films was determined and interpreted. Fibroblast culture was performed to evaluate cell proliferation on the copolymers films. The results showed that the cell growth rate on the copolymers films is faster than chitosan obviously.