Hydrogels have received considerable attention as an innovative material due to their widespread applications in various fields. As a soft and wet material, its mechanical behavior is best understood in terms of the viscoelastic response to the periodic deformation, which is closely related to the microscopic chemically/physically cross-linked structures. Herein, a dual-cross-linked (DC) hydrogel, where a physically cross-linked network by ionic coordination (Fe3+) is imposed on a chemically cross-linked poly(acrylamide-co-acrylic acid) network, was studied in detail by rheology and proton multiple-quantum (MQ) NMR spectroscopy. Rheology experiments revealed the diverse temperature- and strain-frequency-dependent viscoelastic behaviors for DC hydrogels induced by the dynamic Fe3+ coordination interactions, in contrast to the single chemically cross-linked (SC) hydrogels. During the shear experiment, the trivalent Fe3+ complex with moderate/weak binding strength might transform to those with strong binding strength and serve as permanent-like cross-linkages to resist the periodic deformation when a large strain frequency was applied. The viscoelastic behaviors of the DC hydrogels were strongly affected by the monomer ratio (CAAc/CAAm) and Fe3+ concentrations; however, the chemically cross-linked density did not change with CAAc/CAAm, while the physically cross-linked density was greatly enhanced with increasing Fe3+ concentrations. Besides, the DC hydrogels have less contents of network defects in comparison to the SC hydrogels. The heterogeneous structural evolution with increasing the Fe3+ concentration and monomer ratio was also quantitatively determined and elucidated by proton MQ NMR spectroscopy. In addition, the moduli (G′, G″) of DC hydrogels were almost an order magnitude higher than that of the corresponding SC hydrogels, demonstrating the significant contribution of Fe3+ coordination to the mechanical properties, in consistent with the high activation energy of viscoelasticity for the physically cross-linked network as obtained from the variable-temperature shear rheology experiments. The experimental findings obtained from the rheology and proton MQ NMR experiments can be correlated with and complementary to each other. Herein, a combination of rheology and proton solid-state NMR is well demonstrated as an effective and unique way for establishing the relationship between microscopic structures and macroscopic viscoelastic properties.

Multicompartment vesicles of ferrocene-containing triblock terpolymer containing on–off switchable pores in the vesicular membrane are prepared by seeded RAFT polymerization. In these multicompartment vesicles, the incompatible solvophobic poly(4-vinylbenzyl ferrocenecarboxylate) (PVFC) and poly(benzyl methacrylate) (PBzMA) blocks form the porous phase-segregated membrane and the solvophilic poly[2-(dimethylamino) ethyl methacrylate] block locates at the inner and outer sides of the membrane. These porous multicompartment vesicles are redox-responsive and the membrane pores can be on–off switched through redox triggering. These porous multicompartment vesicles are deemed to be new nanoassembly of ABC triblock terpolymer and are anticipated to be a smart host to load and release guests.

•Alkyl tail length determines phase behavior of side-chain-jacketed polyacetylenes.•Reentrant isotropic phase separates lamellar and columnar phases.•Transition from reentrant isotropic to columnar phase is entropy driving.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.

•Rigid, semi-rigid and mobile components are detected.•The role of each components in the reversible heat shrink process is identified.•The semi-rigid crystalline component is found to be the reversible switching phase.•A model was proposed to describe the molecular mechanism.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.

A two-stage transition upon crossing the glass transition of polystyrene with increasing temperature was precisely determined and interpreted by using solid-state nuclear magnetic resonance (SSNMR), 1H-1H dipolar couplings based double quantum-filtered (DQF) and dipolar filter (DF) experiments and 13C chemical shift anisotropy (CSA) based centerband-only detection of exchange (CODEX) experiment are used to fully characterize the time scale of molecular motions during the glass transition. While differential scanning calorimetry (DSC) and CODEX experiment predicted the first stage of glass transiton, DQF and DF experiments provided the evidence for the second stage transition during which the time scale of molecular motions changed from very slow (t > ms) to very fast (t < µs). The first stage of glass transition begins with the occurrence of remarkable slow re-orientation motions of the polymer backbone segments and ends when the degree of slow motion reaches maximum. The onset and endpoint of the conventional calorimetric glass transition of polystyrene can be quantitatively determined at the molecular level by SSNMR. In the second stage, a subsequent dramatic transition associated with the melting of the glassy components was observed. In this stage liquid-like NMR signals appeared and rapidly increased in intensity after a characteristic temperature Tf (~1.1Tg). The signals associated with the glassy components completely disappeared at another characteristic temperature Tc (~1.2Tg).

The macro-RAFT agent mediated dispersion copolymerization of two monomers, in which one is hydrophobic and the other is hydrophilic, is proposed to conveniently tune the morphology of the in situ synthesized block copolymer nano-objects. The poly(ethylene glycol) trithiocarbonate macro-RAFT agent mediated dispersion copolymerization of styrene and 4-vinylpyridine (St/4VP) in alcoholic solvent affords the in situ synthesis of the diblock copolymer nano-objects of poly(ethylene glycol)-block-poly(4-vinylpyridine-co-styrene) [PEG-b-P(4VP-co-St)]. It is found that, the morphology of the PEG-b-P(4VP-co-St) diblock copolymer nano-objects can be easily tuned either by changing the polymerization degree of the random P(4VP-co-St) block or the molar ratio of the PS/P4VP segments in the random P(4VP-co-St) block. The poly(ethylene glycol) trithiocarbonate macro-RAFT agent mediated dispersion copolymerization of St/4VP is compared with the dispersion RAFT polymerization of St, and the advantage of the dispersion RAFT copolymerization in tuning the block copolymer morphology is demonstrated. Our study is believed to be a promising extension of the polymerization induced self-assembly (PISA) under dispersion RAFT polymerization.

Chitosan-based nanoparticles (NPs) are widely used in drug and gene delivery, therapy, and medical imaging, but a molecular-level understanding of the internal morphology and nanostructure size, interface, and dynamics, which is critical for building fundamental knowledge for the precise design and efficient biological application of the NPs, remains a great challenge. Therefore, the availability of a multiscale (0.1–100 nm) and nondestructive analytical technique for examining such NPs is of great importance for nanotechnology. Herein, we present a new multiscale solid-state NMR approach to achieve this goal for the investigation of chitosan–poly(N-3-acrylamidophenylboronic acid) NPs. First, a recently developed 13C multiple cross-polarization magic-angle spinning (MAS) method enabled fast quantitative determination of the NPs’ composition and detection of conformational changes in chitosan. Then, using an improved 1H spin-diffusion method with 13C detection and theoretical simulations, the internal morphology and nanostructure size were quantitatively determined. The interfacial coordinated interaction between chitosan and phenylboronic acid was revealed by one-dimensional MAS and two-dimensional (2D) triple-quantum MAS 11B NMR. Finally, dynamic-editing 13C MAS and 2D 13C–1H wide-line separation experiments provided details regarding the componential dynamics of the NPs in the solid and swollen states. On the basis of these NMR results, a model of the unique nanostructure, interfacial interaction, and componential dynamics of the NPs was proposed.Keywords: chitosan; coordinated interaction; morphology; nanoparticles; solid-state NMR

A variety of multiscale solid-state NMR techniques were used to characterize the heterogeneous structure and dynamics of the interphase and cross-linked network in nanostructured epoxy resin/block copolymer (ER/BCP) blends, focusing on the role of ER-miscible blocks containing poly(ε-caprolactone) (PCL) or poly(ethylene oxide) (PEO) blocks having different intermolecular interactions with ER. 1H spin-diffusion experiments indicate that the interphase thickness of PEO-containing blends is obviously smaller than that of PCL-containing blends. High-resolution 1H fast magic-angle spinning (MAS) spin-exchange experiments reveal detailed interfacial mixing between ER and BCPs for the first time, and two different types of interphase structure are found. 1H fast MAS double-quantum filter experiments provide a fast and convenient detection of interphase composition, including immobilized BCPs and partially cured or local damaged ER network. The driving force for the interphase formation and miscibility in PCL-containing blends was successfully determined by high-resolution 13C CPMAS experiments, demonstrating the formation of hydrogen bonds between PCL and ER; competing hydrogen bonding interactions were also found when ER was blended with PEO-b-PCL (EOCL). A new calculation method is proposed to quantitatively determine the distribution of different blocks in the interphase and dispersed phase for PCL-containing blends in combination with 13C CPMAS and 1H spin-diffusion experiments. A 13C T1 spin–lattice relaxation experiment provides a quantitative determination of the amount of local destroyed network in the interphase. Furthermore, it is found that incorporation of BCPs induces unexpected enhanced rigidity of the cross-linked network. On the basis of NMR results, we propose a model to describe the unique structure and dynamics of the interphase and cross-linked network as well as their underlying formation mechanism in ER/BCP blends.

Polyurethane material is widely utilized in industry and daily life due to its versatile chemistry and relatively easy handling. Here, we focused on a novel thermally reversible cross-linked polyurethane with comprehensive remarkable mechanical properties as reported in our recent work (Adv. Mater. 2013, 25, 4912). The microphase-separated structure and heterogeneous segmental dynamics were well revealed by T2 relaxometry experiments, which was also first utilized to in situ monitor the reversible cross-linking associated with Diels–Alder (DA) and retro-Diels–Alder (RDA) reactions. On the basis of T2 relaxometry results, we determined the actual temperature of the (R)DA reaction as well as the corresponding activation energies of the motion of soft segments. Besides, the roles of the temperature and cross-linker contents on the microdomain structure and dynamics are discussed in detail. It is found that the microphase separation is enhanced by the increase of temperature as well as the incorporation of cross-linkers. Also, the polyurethane samples are still thermal-stable even at a high temperature beyond the disassociation of the cross-linkages. Furthermore, Baum–Pines and three-pulse multiple-quantum NMR experiments are utilized to investigate the heterogeneous structures and dynamics of the mobile and rigid segments, respectively. Both the results obtained from the T2 relaxometry and multiple-quantum NMR experiments are in good agreement with the macroscopic mechanical properties of the polyurethane. Finally, it is also well demonstrated that proton T2 relaxometry combined with multiple-quantum NMR is a powerful method to study the heterogeneous structures and dynamics of a multiphase polymer system.

Materials with asymmetric structures are attractive for wide applications in chemistry and materials science. Two-dimensional Janus disks or nanosheets are particularly appealing because of the unique shape and the distinctive self-assembled structures. A facile and versatile method for the synthesis of amphiphilic Janus Laponite disks is proposed in this paper. Positively charged PS spheres were prepared by ATRP emulsion polymerization. Upon addition of aqueous dispersion of negatively charged Laponite disks into PS emulsions, the nanosized disks were adsorbed onto the surface of PS particles via electrostatic interaction. One side of a Laponite disk touches the surface of a colloidal particle, and the other side faces the medium. After addition of positively charged polymeric micelles or quaternized poly(2-(dimethylamino)ethyl methacrylate) (q-PDMAEMA) chains into the aqueous dispersions of the colloidal particles, the micelles or polymer chains were immobilized onto the Laponite disks, and Janus disks were produced on particle templates. After centrifugation and redispersion of the colloidal particles into a good solvent, amphiphilic Janus Laponite disks with PS chains on one side and hydrophilic q-PDMAEMA or polymeric micelles on the other side were obtained. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) were used to characterize the Janus disks. Self-assembly of the Janus disks at liquid–liquid interface and in selective solvents was investigated. Similar to small molecular surfactants, the amphiphilic Janus disks can self-assemble at liquid–liquid interface, resulting in a decrease of the interfacial tension and emulsification of oil droplets in water. In a THF–methanol mixture at a volume ratio of 1:6, PS brushes on the Janus disks collapse forming two-layer face-to-face stacks. The distinctive self-assembled structures were analyzed by TEM and AFM.

In recent years, self-healing materials have attracted increasing attention due to their potentially spontaneous self-repairing ability after mechanical damage. Here, we focus on a supramolecular self-healing rubber based on fatty acids following the work of Leibler and co-workers. We study the heterogeneous network structure and hydrogen bond dynamics as well as its significant aging properties using several experimental techniques. NMR experiments reveal that the rubber is basically a two-component system, with a ∼85% fraction of material rich in hydrogen-bonded structures and associated aliphatic moieties, undergoing a glass transition just below ambient temperature, and the other one being comprised of more mobile aliphatic chains. Changes in the IR bands corresponding to the NH bending and CO stretching vibrations show that water in the rubber not only takes the role of a plasticizer, reducing the glass transition temperature of the main component, but also is involved in changes of the hydrogen-bonding network. On the basis of shear rheology experiments and proton low-field NMR, we deduce that the rubber undergoes irreversible chemical cross-linking reactions at temperatures above 110 °C, going along with a weakening of its self-healing ability.

The CODEX (center-band only detection of exchange) NMR experiment is widely used for the detection of slow motions in organic solids, especially polymers. However, the RIDER (relaxation-induced dipolar exchange with recoupling) effect may result in artificial exchange signals in the CODEX pure exchange spectrum, which greatly limits the application of CODEX method. Herein, we investigate the distance range that the RIDER effect can reach by performing CODEX experiments on two typical organic solids, hexadecyltrimethylammonium bromide (CTAB) and semi-crystalline polyamide-6 (PA6) where there are no slow molecular motions at room temperature. Our experimental results demonstrate that generally two-bond distance is far enough to ignore the RIDER effect resulted from the dipolar interactions between 13C and the fast relaxing heteronucleus 14N. From the built-up curve of RIDER signals as a function of recoupling time and mixing time, it is clearly revealed that the RIDER effect can greatly affect the signal from 13C directly bonded with 14N. However, this RIDER effect accounts less than 3% of the reference intensity for signals from 13C not directly bonded with 14N if typical recoupling (∼0.5 ms) and mixing times (∼0.5 s) are used for the investigation of slow motions. When longer recoupling and mixing time are used, there are small RIDER signals even for the 13C far away from the 14N. These signals, to a large degree, result from the spin diffusion effect and/or the special microscopic molecule arrangement. However, they are so small compared to the reference signal (∼5%) that they can be ignored. Finally, according to the simulation results, it is worth noting that the RIDER signal is still generally negligible compared to the signals due to slow motions if the chemical shift anisotropy reorientation during the mixing time is not too small(larger than 20°) under the condition of 4tr recoupling time at the magic-angle-spinning speed of 6.5 kHz.Graphical abstractHighlights► Two-bond distance is far enough to suppress RIDER effect. ► For remote 13C, RIDER signal is rather small, generally no more than 5%. ► For remote 13C, RIDER signal is negligible to jump motions larger than 20°. ► CODEX can be used to detect slow motions in 14N-containing organic solids.

The structure and dynamic behavior of mobile components play a significant role in determining properties of solid materials. Herein, we propose a novel real-time spectrum-editing method to extract signals of mobile components in organic solids on the basis of the polarization inversion spin exchange at magic angle (PISEMA) pulse sequence and the difference in 13C T1 values of rigid and mobile components. From the dipolar splitting spectrum sliced along the heteronuclear dipolar coupling dimension of the 2D spectrum, the structural and dynamic information can be obtained, such as the distances between atoms, the dipolar coupling strength, the order parameter of the polymer backbone chain, and so on. Furthermore, our proposed method can be used to achieve the separation of overlapped NMR signals of mobile and rigid phases in the PISEMA experiment. The high efficacy of this 2D NMR method is demonstrated on organic solids, including crystalline l-alanine, semicrystalline polyamide-6, and the natural abundant silk fibroin.

Glass transition behavior of hydrogen bonded polymer blends of poly(vinyl phenol) (PVPh) and poly(ethylene oxide) (PEO) is systematically investigated using normal differential scanning calorimetry (DSC) and recently developed multifrequency temperature-modulated DSC (TOPEM), in combination with Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) techniques, focusing on the effect of the PEO molecular weight on the spatial and dynamic heterogeneity. It is found, for the first time, that both the glass transition temperature (Tg) and activity energy (Ea) of the blends strongly depend on PEO molecular weight, and a common turning point, which separates the rapid and slow increasing regions, can be found. The interchain hydrogen bonding interactions, both determined by FTIR measurements and obtained from the Kwei equation, decrease with increasing PEO molecular weight, indicating a decrease of the componential miscibility. A series of parameters related to the microscopic spatial and dynamic heterogeneity, such as the activity energy, fragility, nonexponential factor and the size of cooperatively rearranging regions, are calculated from frequency dependency complex heat capacity measured using TOPEM. It is found that each of these parameters monotonically changes with increasing the PEO molecular weight during the glass transition process, demonstrating that hydrogen bonding interaction is the key factor in controlling the spatial and dynamic heterogeneity, thus the glass transition. NMR relaxation results reveal the existence of obvious phase separation large than 5 nm, implying that the cooperatively rearranging regions should be closely related to the interphase region between the two components. The above obtained origin and evolution of spatial and dynamic heterogeneity provide a new insight into the glass transition behavior of polymer blends.

This work aimed to compare two types of affinity ligands, i.e. polymeric and monomeric ligands, by investigating their adsorption affinity, capacity and selectivity to oligopeptide. The peptide NH2-VVRGCTWW-COOH (VW-8) was chosen as the target adsorbate, while histidine (His), aspartic acid (Asp), and leucine (Leu) were selected as the ligands, respectively. For each kind of ligand, both monomeric (M) and polymeric (P) forms were introduced onto the Sepharose matrix respectively to obtain the corresponding adsorbents. Both affinity tests using isothermal titration calorimetry (ITC) and adsorption capacities using static adsorption experiments indicated that the adsorbents with polymeric ligands (MX-P) exhibited better adsorption ability for VW-8 than the adsorbents with monomeric ligands (MX-M). In particular, the MX-PHis exhibited its affinity constant of 2.39 × 106 M−1 and its adsorption capacity of 77.4 mg/g for VW-8, which was approximately 8–10 times higher than that of MX-MHis. Such distinct adsorption abilities between polymeric and monomeric ligands were interpreted based on nuclear magnetic resonance (NMR) and ITC data, and the results indicated that such better characters of polymeric ligands were ascribed to their good flexibility which facilitated the cooperative effects as well as the accessibility of ligands to the peptide. Additionally, the selective adsorption experiments indicated that all the adsorbents with polymeric ligands exhibited good selectivity to the peptide VW-8.Graphical abstractThis work compared two types of affinity ligands, i.e. polymeric and monomeric ligands, by investigating their adsorption affinity, capacity and selectivity to the oligopeptide NH2-VVRGCTWW-COOH (VW-8).Research highlights► Polymeric ligands exhibited better adsorption capacity and affinity than monomeric ligands. ► Polymeric ligands were characterized by its good flexibility. ► Polymeric ligands exhibits better adsorption selectivity than monomeric ligands.

An efficient method for identifying different types of carbon groups (CH3, CH2, CH, and quaternary carbons) in organic solids is proposed by utilizing the combination of a two-dimensional (2D) 13C–1H polarization inversion spin exchange at magic angle (PISEMA) NMR experiment and numerical simulation results of simple isolated 13C–1H dipolar coupling models. Our results reveal that there is a unique line shape of the 13C–1H dipolar splitting pattern and a corresponding characteristic splitting value for each carbon group, based on which different carbon types can be distinguished unambiguously. In particular, by using this method, the discrimination and assignment of overlapped signals from different types of carbons can be achieved easily. The efficacy of this method is demonstrated on typical solid small molecules, polymers, and biomacromolecules.

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.

By adjusting the solution pH value below the isoelectric point (pI) of silk fibroin (SF) protein, the SF was in the cation state and it could interact strongly with unmodified anionic montmorillonite (MMT) surface. In this way, novel SF-MMT nanocomposites with good clay dispersion were successfully obtained, which were confirmed by X-ray diffraction and transmission electron microscopy. Further 1H CRAMPS and 13C CP/MAS NMR experimental results revealed that β-sheet content of SF was remarkably enhanced for nanocomposite prepared below the pI of SF (SF-MMTA) due to the strong interaction between MMT and SF. In SF-MMTA nanocomposite, clay layers acting as an efficient nucleator could efficiently enhance the β-sheet crystallization. On the contrary, SF preserved the native random coil conformation in SF-MMTN nanocomposites due to the weak interaction between MMT and SF. A tentative model was suggested and used to explain the mechanism of clay dispersion and conformational transition of silk protein.