Impact of Entanglement on Folding of Semicrystalline Polymer during Crystallization.

Upon cooling, semicrystalline polymers experience crystallization and form alternatively stacked layers consisting of thin crystal lamellae and amorphous ones. The unique morphology, crystallinity, and crystallization kinetics highly depend on the molecular weight. Therefore, it is deduced that entanglement impacts crystallization kinetics, as well as hierarchically crystalline structures. However, the impact of entanglement on folded crystalline chains has not been well understood due to experimental difficulties. In this work, chain-folding structures for seven 13C CH3 labeled poly(l-lactic acid)s with various molecular weights (Mws) were investigated by 13C-13C double quantum NMR spectroscopy. As a result, chain-folding events were categorized into three different Mw regimes: (i) The lowest Mw sample (2K g/mol) adopts an extended chain conformation (folding number, n = 0) (regime I); (ii) Intermediate Mw ones possess mixtures of non- and once-folded structures, and the once-folded fraction suddenly increases above the entanglement length (Me), up to Mw = 45K g/mol (regime II); (iii) The high Mw ones (Mw > 45K g/mol) adopt the highest chance for an adjacent re-entry structure with n = 1.0 in the well-developed entangled network (regime III). It was suggested that entanglement induces folding of the semicrystalline polymer.

Stochastic Bullvalene Architecture Modulates Structural Rigidity in π-Rich Macromolecules.

The synthesis and processing of π-rich polymers found in novel electronics and textiles is difficult because chain stiffness leads to low solubility and high thermal transitions. The incorporation of "shape-shifting" molecular cages into π-rich backbone provides an ensemble of structural kinks to modulate chain architecture via a self-contained library of valence isomers. In this work, we report the synthesis and characterization of (bullvalene-co-phenylene)s that feature smaller persistence lengths than a prototypical rigid rod polymer, poly(p-phenylene). By varying the amount of bullvalene incorporation within a poly(p-phenylene) chain (0-50 %), we can tune thermal properties and solution-state conformation. These features are caused by stochastic bullvalene isomers within the polymer backbone that result in kinked architectures. Synthetically, bullvalene incorporation offers a facile method to decrease structural rigidity within π-rich materials without concomitant crystallization. VT NMR experiments confirm that these materials remain dynamic in solution, offering the opportunity for future stimuli-responsive applications.

Meso- and Macroporous Polymer Gels for Efficient Adsorption of Block Copolymer Surfactants.

An understanding of surfactant adsorption at solid-liquid interfaces is important for solving many technological problems. This work evaluates surfactant adsorption abilities of high surface area (200-600 m2/g), high porosity (>90%), hierarchically structured open pore polymer gels. Specifically, the interactions of a nonionic block copolymer surfactant, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO), with three polymer gels, namely, syndiotactic polystyrene (sPS), polyimide (PI), and polyurea (PUA) offering different surface energy values, are evaluated at surfactant concentrations below and well above the critical micelle concentration (CMC). Two distinct surfactant adsorption behaviors are identified from the surface tension and nuclear magnetic resonance data. At concentrations below CMC, the surfactant molecules adsorb as a monolayer on polymer strands, inferred from the Langmuir-type adsorption isotherm, with the adsorbed amount increasing with the specific surface area of the polymer gel. The study reports for the first time that the gels show a strong surfactant adsorption above CMC, with the effective surfactant concentration in the gel reaching several folds of the CMC values. The effective surfactant concentration in the gel is analyzed using surfactant micelle size, polymer surface energy, and pore size of the gel. The findings of this study may have strong implications in liquid-liquid separation problems and in the removal of small dye molecules, heavy metal ions, and living organisms from aqueous streams.

Asymmetric Molecular Dynamics and Anisotropic Phase Separation in the Cocrystal of the Crystalline/Crystalline Polymer Blend.

Semicrystalline polymers are categorized as either mobile or fixed crystals, depending on chain mobility in the crystalline region. In this work, we investigate molecular dynamics and phase structure in the cocrystal consisting of fixed and mobile polymer crystals by solid-state (ss) nuclear magnetic resonance (NMR) spectroscopy. It is demonstrated that (i) the mobile component begins large amplitude motions associated with crystal-crystal transition, while fixed ones keep their rigidity in the cocrystal, and (ii) asymmetric molecular dynamics leads to nanosegregations into mobile- and fixed-rich domains in the cocrystal below the melting temperature (Tm). The observed phase separation induced by asymmetric molecular dynamics is similar to the phase separation of the miscible amorphous polymer blend; however, it is limited to two dimensions due to the parallel packing of the stems inside the cocrystal, as well as chain connectivity at the crystalline-amorphous boundary.

Polymer Chains Fold Prior to Crystallization.

There are long-standing debates in crystallization mechanism of polymer chains at the molecular levels: Which comes first, chain folding or lamellae formation during crystallization? In this study, we report the local chain trajectory of 13C-labeled semicrystalline polymer in an extreme case of rapidly quenched glassy state as well as thermodynamically stable crystals formed via different pathways from glass and melt. Magnetically dipole interactions do not require a long-range order of molecular objects and thus enable us to trace the local chain trajectory of polymer chains even in a glassy state. To accurately characterize the local chain trajectory of polymer glass, the natural abundance effect on 13C-13C double-quantum (DQ) nuclear magnetic resonance (NMR) signal is re-examined using extended chain conformation. As results, it is found that glassy chains adopt the same adjacent re-entry structure (adjacent re-entry number, n = 1) with the melt- and cold-grown crystals. From these results, it is concluded that (i) folding occurs prior to crystallization and (ii) melt and cold crystallization do not induce additional folding but proceed with rearrangements of polymer chains in the existing templates.

Non-thermal plasma-assisted rapid hydrogenolysis of polystyrene to high yield ethylene.

The evergrowing plastic production and the caused concerns of plastic waste accumulation have stimulated the need for waste plastic chemical recycling/valorization. Current methods suffer from harsh reaction conditions and long reaction time. Herein we demonstrate a non-thermal plasma-assisted method for rapid hydrogenolysis of polystyrene (PS) at ambient temperature and atmospheric pressure, generating high yield (>40 wt%) of C1-C3 hydrocarbons and ethylene being the dominant gas product (Selectivity of ethylene, SC2H4 > 70%) within ~10 min. The fast reaction kinetics is attributed to highly active hydrogen plasma, which can effectively break bonds in polymer and initiate hydrogenolysis under mild condition. Efficient hydrogenolysis of post-consumer PS materials using this method is also demonstrated, suggesting a promising approach for fast retrieval of small molecular hydrocarbon modules from plastic materials as well as a good capability to process waste plastics in complicated conditions.

Isolation and Characterization of Allomelanin from Pathogenic Black Knot Fungus-a Sustainable Source of Melanin.

Melanin, a widespread pigment found in many taxa, is widely recognized for its high refractive index, ultraviolet (UV) protection, radical quenching ability, metal binding, and many other unique properties. The aforementioned characteristic traits make melanin a potential candidate for biomedical, separation, structural coloration, and space applications. However, the commercially available natural (sepia) and synthetic melanin are very expensive, limiting their use in various applications. Additionally, eumelanin has been the primary focus in most of these studies. In the present study, we demonstrate that melanin can be extracted from the pathogenic black knot fungus Apiosporina morbosa with a yield of ∼10% using the acid-base extraction method. The extracted melanin shows irregular morphology. Chemical characterization using X-ray photoelectron spectroscopy, infrared spectroscopy, and solid-state nuclear magnetic resonance spectroscopy reveals that the melanin derived from black knots is the less explored nitrogen-free allomelanin. Additionally, the extracted melanin shows broadband UV absorption typical of other types of melanin. Because of the wide availability and low cost of black knots and the invasive nature of the fungus, black knots can serve as an alternative green source for obtaining allomelanin at a low cost, which could stimulate its use as an UV light absorber and antioxidant in cosmetics and packaging industries.

Elucidating the Molecular Interactions of Encapsulated Doxorubicin within a Nonionic, Thermoresponsive Polyester Coacervate.

Thermoresponsive polymers that display a lower critical solution temperature (LCST) are attractive drug delivery systems (DDSs) due to their potential to encapsulate and release therapeutics in a sustained manner as a function of temperature input. To attain the full potential of such DDSs, methods that illustrate the details of drug-polymer interactions are necessary. Here, we synthesized a nonionic, coacervate-forming, thermoresponsive polyester to encapsulate doxorubicin (Dox) and used solution state NMR spectroscopy and fluorescence microscopy techniques to probe the interactions between the polymer and Dox at the molecular level. The incomplete dehydration provides a matrix for encapsulation of sensitive therapeutics and preserving their activity, while the low hysteresis property of the polyester provides rapid transition from soluble to coacervate phase. Saturation transfer difference (STD) NMR revealed the Dox-polymer interactions within the coacervates. 1H-1H nuclear Overhauser effect spectroscopy (NOESY) cross-peak differences of Dox confirmed the Dox-polymer interactions. Diffusion-ordered spectroscopy (DOSY) revealed the slower diffusion rate of Dox in the presence of polyester coacervates. These studies illustrate how the state of the polyester (below and above LCST) affects the polyester-Dox interactions and offers details of the specific functional groups involved in these interactions. Our results provide a framework for future investigations aimed at characterizing fundamental interactions in polymer-based DDSs.

Mechanism of UVA Degradation of Synthetic Eumelanin.

Eumelanin is a ubiquitous natural pigment that has a broad absorption across ultraviolet (UV, 100-400 nm) and visible wavelengths (400-700 nm) and can protect against radiation. Synthetic eumelanin with properties similar to natural eumelanin has been made using dopamine or dihydroxyindole. Here, we use solid-state nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy to elucidate the chemical structure of synthetic eumelanins (made from dopamine and l-3,4-dihydroxyphenylalanine precursors) and investigate how their structures change after intensive UVA (315-400 nm) exposure. We first confirm that polydopamine has indole units. Upon UV exposure, the pyrrole ring in this indole unit remains intact, and a fraction of the six-membered benzyl ring is broken and the indole potentially converted to furo[3,4-b]pyrrole. This change in the chemical structure is accompanied by a release of carbon dioxide. In addition, the sepia (natural) eumelanin used for comparison is more stable than the synthetic eumelanin. Understanding the UVA degradation mechanism of eumelanin will help reveal the role of eumelanin in skin cancer and in the design of more efficient UV stabilizers.

Tuning the Intercage Distance in Charge-Regulated Blackberry-Type Assemblies through Host-Guest Chemistry.

Charged or neutral adamantane guests can be encapsulated into the cavity of cationic metal-organic M6 L4 (bpy-cage, M=PdII (2,2'-bipyridine), L=2,4,6-tri(4-pyridyl)-1,3,5-triazine) cages through hydrophobic interaction. These encapsulations can provide an approach to control the net charge on the resulting cage-guest complexes and regulate their charge-dominated assembly into hollow spherical blackberry-type assemblies in dilute solutions: encapsulation of neutral guests will hardly influence their self-assembly process, including the blackberry structure size, which is directly related to the intercage distance in the assembly; whereas encapsulating negatively (positively) charged guests resulted in a shorter (longer) intercage distance with larger (smaller) assemblies formed. Therefore, the host-guest chemistry approach can be used to tune the intercage distance accurately.

Elucidation of the hierarchical structure of natural eumelanins.

Eumelanin is one of the most ubiquitous pigments in living organisms and plays an important role in coloration and UV protection. Because eumelanin is highly cross-linked and insoluble in solvents, the chemical structure is still not completely known. In this study, we used atomic force microscopy, X-ray photoelectron spectroscopy and solid-state nuclear magnetic resonance (NMR) to compare intact eumelanosomes (pigment granules mostly made of eumelanin) from four phylogentically distant species: cuttlefish (Sepia officinalis) inks, black fish crow (Corvus ossifragus) feathers, iridescent wild turkey (Melleagris gallopavo) feathers and black human hair. We found that eumelanosomes from all four species are composed of subunit nanoparticles with a length of 10-60 nm, consistent with earlier observations in eumelanosomes from the sepia ink and human hair. The solid-state NMR results indicate the presence of quinone methide tautomers in all four eumelanins. We also found clear differences in the UV absorbance, the ratio of 5,6-dihydroxyindole-2-carboxylic acid/5,6-dihydroxyindole and protonated aryl carbon ratios in sepia eumelanin relative to the other three. This comparison of natural eumelanin across a phylogenetically broad group of organisms provides insights into the change in the eumelanin structure over the evolutionary history and enables the production of synthetic eumelanin with properties that are similar to natural eumelanin.

Stoichiometry and Packing Structure of Poly(lactic acid) Stereocomplex as Revealed by Solid-State NMR and 13C Isotope Labeling.

Poly(l-lactic acid) (L)/poly(d-lactic acid) (D) blends form a stereocomplex (SC) at a mixing ratio of 7/3-3/7. The stoichiometry and packing structure of L/D in the SC are controversial topics because the SC is semicrystalline and because the enantiomeric pair has the same chemical structure. In this study, both the stoichiometry and packing structure of 33% 13C CH3-labeled (l) L/nonlabeled D blends at mixing ratios of 7/3-3/7 were investigated by using solid-state (SS) NMR. The 13C CO signals in natural abundance provided the fractions of the SC (ΦSC), α, and amorphous regions of l-L/D blends. Moreover, the 33% 13CH3-labeled signals could determine the fraction of only l-L in the SC (ΦL) and amorphous region. These two data sets allowed us to determine the stoichiometry of l-L/D in the SC (ΦL-SC/ΦD-SC) to be 1/1. 13C-13C double-quantum (DQ) buildup curves of l-L in the SC followed one universal curve even at different mixing ratios. Comparison of the experimental and simulated DQ curves led to the conclusion that all SC crystals adopt a regular packing structure at varied mixing ratios.

Chain Trajectory, Chain Packing, and Molecular Dynamics of Semicrystalline Polymers as Studied by Solid-State NMR.

Chain-level structure of semicrystalline polymers in melt- and solution-grown crystals has been debated over the past half century. Recently, 13C⁻13C double quantum (DQ) Nuclear Magnetic Resonance (NMR) spectroscopy has been successfully applied to investigate chain-folding (CF) structure and packing structure of 13C enriched polymers after solution and melt crystallization. We review recent NMR studies for (i) packing structure, (ii) chain trajectory, (iii) conformation of the folded chains, (iv) nucleation mechanisms, (v) deformation mechanism, and (vi) molecular dynamics of semicrystalline polymers.

Hierarchical Self-Organization of AB n Dendron-like Molecules into a Supramolecular Lattice Sequence.

To understand the hierarchical self-organization behaviors of soft materials as well as their dependence on molecular geometry, a series of AB n dendron-like molecules based on polyhedral oligomeric silsesquioxane (POSS) nanoparticles were designed and synthesized. The apex of these molecules is a hydrophilic POSS cage with 14 hydroxyl groups (denoted DPOSS). At its periphery, there are different numbers (n = 1-8) of hydrophobic POSS cages with seven isobutyl groups (denoted BPOSS), connected to the apical DPOSS via flexible dendron type linker(s). By varying the BPOSS number from one to seven, a supramolecular lattice formation sequence ranging from lamella (DPOSS-BPOSS), double gyroid (space group of Ia3̅d, DPOSS-BPOSS2), hexagonal cylinder (plane group of P6mm, DPOSS-BPOSS3), Frank-Kasper A15 (space group of Pm3̅n, DPOSS-BPOSS4, DPOSS-BPOSS5, and DPOSS-BPOSS6), to Frank-Kasper sigma (space group of P42/mnm, DPOSS-BPOSS7) phases can be observed. The nanostructure formations in this series of AB n dendron-like molecules are mainly directed by the molecular geometric shapes. Furthermore, within each spherical motif, the spherical core consists hydrophilic DPOSS cages with flexible linkages, while the hydrophobic BPOSS cages form the relative rigid shell, and contact with neighbors to provide decreased interfaces among the spherical motifs for constructing final polyhedral motifs in these Frank-Kasper lattices. This study provides the design principle of molecules with specific geometric shapes and functional groups to achieve anticipated structures and macroscopic properties.

Controlling the enthalpy-entropy competition in supramolecular fullerene liquid crystals by tuning the flexible chain length.

We present here that in two-dimensional (2D) fullerene supramolecular liquid crystals (SLCs), the phase diagram and lamella thickness of SLCs and 2D crystals can be tuned by the flexible alkyl tail and spacer length, due to their different effects on enthalpy and entropy changes during SLC formation.

Isoexergonic Conformations of Surface-Bound Citrate Regulated Bioinspired Apatite Nanocrystal Growth.

The superior biomechanical properties of bone and dentin are dictated, in part, by the unique plate-like morphology of hydroxyapatite (HAP) nanocrysals within a hierarchically assembled collagen matrix. Understanding the mechanism of crystal growth and thus morphology is important to the rational design of bioinspired apatite nanocrystals for orthopedic and dental applications. Citrate has long been proposed to modulate apatite crystal growth, but major questions exist regarding the HAP-bound citrate conformations and the identities of the interacting functional groups and HAP surface sites. Here, we conducted a comprehensive investigation of the mechanism from the angstrom to submicrometer scale by detailed correlation of the results of high-level metadynamics simulations, employing force-fields benchmarked to experiment and density functional theory calculations with the results of high resolution transmission electron microscopy, nuclear magnetic resonance spectroscopy, solution analysis, and thermogravimetric analysis. Crystal morphology changed from needle- to plate-like with increasing citrate concentration. Citrate adsorbed more strongly on the HAP (100) face than on the (001) face, thus resulting in preferential growth in the [001] direction and the plate-like morphology. Two very different bound conformations were obtained, involving interactions of either one or both terminal carboxyl groups with three or five surface calcium ions, respectively, and a hydrogen bond between the citrate hydroxyl and the HAP surface. Remarkably, despite fewer interaction sites in the single bound carboxyl conformation, the structures were isoexergonic, so both exist at equilibrium. Identification of the former conformation is significant because it allows a greater adsorption density than is traditionally assumed and can help explain concentration-dependence of citrate in modulating crystal morphology. These unique results were enabled first by the application of advanced metadynamics, a technique necessary for the accurate simulation of ionic materials but which is rarely employed in the biomaterials and biomineralization fields and second by the detailed correlation of computational, spectroscopic, and analytical results.

Chain Trajectory of Semicrystalline Polymers As Revealed by Solid-State NMR Spectroscopy.

Over the last half century, a chain-folding structure of semicrystalline polymers has been debated in polymer science. Recently, 13C-13C double quantum (DQ) NMR spectroscopy combined with 13C selective isotope labeling has been developed to investigate re-entrance sites of the folded chains, mean values of adjacent re-entry number ⟨n⟩ and fraction ⟨F⟩ of semicrystalline polymers. This viewpoint highlights the versatile approaches of using solid-state (ss) NMR and isotope labeling for revealing (i) chain trajectory in melt- and solution-grown crystals, (ii) conformation of the folded chains in single crystals, (iii) self-folding in the early stage of crystallization, and (iv) unfolding of the folded chains under stretching.

Unfolding of Isotactic Polypropylene under Uniaxial Stretching.

Despite numerous investigations on polymer processing, understanding the deformation mechanisms of semicrystalline polymer under uniaxial stretching is still challenging. In this work, 13C-13C Double Quantum (DQ) NMR was applied to trace the structural evolution of 13C-labeled isotactic polypropylene (iPP) chains inside the crystallites stretched to an engineering strain (e) of 21 at 100 °C. DQ NMR based on spatial proximity of 13C labeled nuclei proved conformational changes from the folded chains to the locally extended chains induced by stretching. By combining experimental findings with literature results on molecular dynamics, it was concluded that transportation of the crystalline chains plays a critical role to achieve large deformability of iPP.

Three-Dimensional Conformation of Folded Polymers in Single Crystals.

The chain-folding mechanism and structure of semicrystalline polymers have long been controversial. Solid-state NMR was applied to determine the chain trajectory of (13)C CH3-labeled isotactic poly(1-butene) (iPB1) in form III chiral single crystals blended with nonlabeled iPB1 crystallized in dilute solutions under low supercooling. An advanced (13)C-(13)C double-quantum NMR technique probing the spatial proximity pattern of labeled (13)C nuclei revealed that the chains adopt a three-dimensional (3D) conformation in single crystals. The determined results indicate a two-step crystallization process of (i) cluster formation via self-folding in the precrystallization stage and (ii) deposition of the nanoclusters as a building block at the growth front in single crystals.

Composition and Function of Spider Glues Maintained During the Evolution of Cobwebs.

Capture silks are an interesting class of biological glues that help spiders subdue their prey. Viscid capture silk produced by the orb web spiders is a combination of hygroscopic salts that aid in water uptake and interact with adhesive glycoproteins to make them soft and sticky. The orb was a stepping stone to the evolution of new web types, but little is known about the adhesives in these webs. For instance, cobweb spiders evolved from orb-weaving ancestors and utilize glue in specialized sticky gumfoot threads rather than an elastic spiral. Early investigation suggests that gumfoot adhesives are quite different viscid glues because they lack a visible glycoprotein core, act as viscoelastic fluids rather than solids, and are largely invariant to humidity. Here, we use spectroscopic and staining methods to show that the gumfoot silk produced by Latrodectus hesperus (western black widow) is composed of hygroscopic organic salts and water insoluble glycoproteins, similar to viscid silk, in addition to a low concentration of spider coating peptides reported before. Our adhesion studies reveal that the organic salts play an important role in adhesion, similar to that seen in orb web spiders, but modulating function at much lower humidity. Our work shows more similarities in the viscid silk produced by orb web and cobweb spiders than previously anticipated and provide guidelines for developing synthetic adhesives that can work in dry to humid environments.