The 50 most-cited articles in Macromolecules, 2016

Recently, polymerization-induced self-assembly (PISA) has become widely recognized as a robust and efficient route to produce block copolymer nanoparticles of controlled size, morphology, and surface chemistry. Several reviews of this field have been published since 2012, but a substantial number of new papers have been published in the last three years. In this Perspective, we provide a critical appraisal of the various advantages offered by this approach, while also pointing out some of its current drawbacks. Promising future research directions as well as remaining technical challenges and unresolved problems are briefly highlighted.

Abstract Block polymers have undergone extraordinary evolution since their inception more than 60 years ago, maturing from simple surfactants to an expansive class of macromolecules encoded with exquisite attributes. Contemporary synthetic accessibility coupled with facile characterization and rigorous theoretical advances have conspired to continuously generate fundamental insights and enabling concepts that target applications spanning chemistry, biology, physics, and engineering. Here, we parse the vast literature to examine the forefront of the field and identify exciting themes and challenging opportunities that portend a bracing future trajectory. This Perspective celebrates the visionary role played by Macromolecules in advancing our understanding of this remarkable class of materials.

In this Perspective we summarize the most widely used definitions of free volume and illustrate the differences between them, including the important distinction between total free volume and excess free volume. We discuss the implications when alternative estimates for free volume are inserted into relationships that connect experimentally measured properties (e.g., the viscosity) to free volume, such as those proposed by Doolittle, Fox and Flory, Simha and Boyer, Cohen and Turnbull, and Williams, Landel, and Ferry. Turning to the results of our own locally correlated lattice (LCL) model, we demonstrate, by analyzing data for a set of over 50 polymers, that our calculations for total percent free volume not only lead to a predictive relationship with experimental glass transition temperatures but also allow us to place the different definitions of free volume within a physical picture of what the proposed contributions represent. We find that melts go glassy upon reaching a “boundary” of minimum (total) percent free volume that depends roughly linearly on temperature. We interpret…

Covalent polymeric networks composed of imine cross-linkages have been prepared by condensation polymerization of poly(ethylene glycol) bis(3-aminopropyl) with 1,3,5-triformylbenzene with an equal molar ratio of amine and aldehyde functionalities in organic solvents with varying polarity and in neat condition. The polymer networks exhibit malleability and self-healing characteristics. Rheological measurements revealed that longer reaction time is required to reach the gel point (i.e., crossover of G ′ and G ″) in polar solvents than in nonpolar solvents. The malleability of the solvent-swelled polymer network is also strongly dependent on the solvent polarity. Polymer gels in polar solvents are more malleable than those in nonpolar solvents, as supported by the dynamic mechanical analysis. These results are consistent with faster dynamic imine bond exchange in the polar solvents relative to the nonpolar solvents, thus requiring higher functionality conversion to form an elastic network in the polar solvent relative to the nonpolar solvent. The imine–imine bond exchange kinetic was…

Anion exchange membranes are an important component in alkaline electrochemical energy conversion and storage devices, and their alkaline stability plays a crucial role for the long-term use of these devices. Herein, a systematic study was conducted for the analysis of polymer backbone chemical stability in alkaline media. Nine representative polymer structures including poly(arylene ether)s, poly(biphenyl alkylene)s, and polystyrene block copolymers were investigated for their alkaline stability. Polymers with aryl ether bonds in their repeating unit showed poor chemical stability when treated with KOH and NaOCH 3 solutions, whereas polymers without aryl ether bonds [e.g., poly(biphenyl alkylene)s and polystyrene block copolymers] remained stable. Additional NMR studies and density functional theory (DFT) calculations of small molecule model compounds that mimic the chemical structures of poly(arylene ether)s confirmed that electron-withdrawing groups near to the aryl ether bonds in the repeating unit accelerate chemical degradation. Results from this study suggest that the use of…

It is challenging to develop hydrogels with a combination of excellent mechanical properties, versatile functions, and good processability. Here we report a physical hydrogel of poly(acrylamide- co -acrylic acid) (P(AAm- co -AAc)) cross-linked by carboxyl–Fe 3+ coordination complexes that possesses high stiffness and toughness, fatigue resistance, and stimulation-triggered healing along with shape memory and processing abilities. The copolymers have randomly dispersed AAm and AAc repeat units, making the physical cross-links with different strength. The strong coordination bonds and their associations serve as permanent cross-links, imparting the elasticity, whereas the weak ones reversibly rupture and re-form, dissipating the energy. Furthermore, a stick–slip instability is observed during the tearing test, which should be associated with the specific nature of metal-coordination bonds that are strong yet fragile. Because of the dynamic nature of coordination bonds, both tensile and tearing mechanical properties are rate dependent. By tuning the bond strength via pH, the gels show…

Abstract The effective and safe capture and storage of radioactive iodine (129I or 131I) is of significant importance during nuclear waste storage and nuclear energy generation. Here we present detailed evidence of highly efficient and reversible iodine capture in hexaphenylbenzene-based conjugated microporous polymers (HCMPs), synthesized via Buchwald–Hartwig (BH) cross-coupling of a hexakis(4-bromophenyl)benzene (HBB) core and aryl diamine linkers. The HCMPs present moderate surface areas up to 430 m2 g–1, with narrow pore size distribution and uniform ultramicropore sizes of less than 1 nm. Porous properties are controlled by the strut lengths and rigidities of linkers, while porosity and uptake properties can be tuned by changing the oxidation state of the HCMPs. The presence of a high number of amine functional groups combined with microporosity provides the HCMPs with extremely high iodine affinity with uptake capacities up to 336 wt %, which is to the best of our knowledge the highest reported to date. Two ways to release the adsorbed iodine were explored: either slow release…

Anion exchange membrane fuel cells (AEMFCs) have been developed as promising energy conversion devices for stationary and mobile applications due to their potentially low cost. To realize high-performance AEMFCs, new polymeric membranes are needed that are highly conductive and chemically stable. Here we report a systematic study of anion exchange membranes (AEMs) with multiple cations per side chain site to demonstrate how this motif can boost both the conductivity and stability of poly(2,6-dimethyl-1,4-phenylene oxide)-based AEMs. The highest conductivity, up to 99 mS/cm at room temperature, was observed for a triple-cation side chain AEM with 5 or 6 methylene groups between cations. This conductivity was considerably higher than AEM samples based on benzyltrimethylammonium or benzyldimethylhexylammonium groups with only one cation per side chain site. In addition to high conductivity, the multication side chain AEMs showed good alkaline and dimensional stabilities. High retention of ion exchange capacity (IEC) (93% retention) and ionic conductivity (90% retention) were observed…

Reinforcing rubbers and expanding their application galleries are two important issues in material science and engineering. In this work, we demonstrate a bioinspired design of high-performance and macroscopically responsive diene-rubber by engineering sacrificial metal–ligand motifs into a chemically cross-linked architecture network. The metal–ligand bonds are formed through the coordination reaction between the pyridine groups in butadiene–styrene–vinylpyridine rubber (VPR) and metal ions. Under external load, the metal–ligand bonds serve as sacrificial bonds that preferentially rupture prior to the covalent network, which dissipates energy and facilitates rubber chain orientation. Based on the function mechanisms, the modulus, tensile strength, and toughness of the samples are simultaneously improved without sacrificing the extensibility, and these properties can be conveniently tuned by varying the structure parameters of the covalently cross-linked network and metal–ligand bonds. Moreover, the dissociation/re-formation of metal–ligand bonds upon heating/cooling can endow VPR…

Abstract Aliphatic polyester block polymers constitute a highly useful and amazingly versatile class of self-assembled materials. Analogous to styrenic block polymers in both design and function, the property profiles of these degradable materials can be precisely tailored by altering the chemical structure of the components. Driven by this ideal, we have examined the impact of n-alkyl substituents on the polymerization thermodynamics and kinetics of substituted δ-valerolactone monomers and developed guiding design principles based on critical structure–property relationships in the resulting aliphatic polyesters. Under bulk room temperature conditions the polymerization rate depends strongly on substituent position and exhibits a more modest dependence on alkyl length (from −CH3 to −(CH2)8CH3). The enthalpy and entropy of polymerization are significantly influenced by substituent position, but both are largely insensitive to n-alkyl length. However, the physical properties of the resulting aliphatic polyesters depend much more on substituent length than on substituent position.…

Abstract In the Brookhart type α-diimine palladium catalyst system, it is highly challenging to tune the polymer branching densities through ligand modifications or polymerization conditions. In this contribution, we describe the synthesis and characterization of a series of α-diimine ligands and the corresponding palladium catalysts bearing both the dibenzhydryl moiety and with systematically varied ligand sterics. In ethylene polymerization, it is possible to tune the catalytic activities ((0.77–8.85) × 105 g/(mol Pd·h)), polymer molecular weights (Mn: (0.2–164.7) × 104), branching densities (25–116/1000C), and polymer melting temperatures (amorphous to 98 °C) over a very wide range. In ethylene–methyl acrylate (E–MA) copolymerization, it is possible to tune the catalytic activities ((0.3–8.8) × 103 g/(mol Pd·h)), copolymer molecular weights (1.1 × 103–79.8 × 103), branching densities (30–119/1000C), and MA incorporation ratio (0.4–13.8%) over a very wide range. The molecular weights and branching densities could also be tuned in α-olefin polymerization. The tuning in polymer…

Block copolymers are known for their intricate morphology. We review the state of the art of block copolymer membranes and discuss perspectives in this field. The main focus is on pore morphology tuning with a short introduction on nonporous membranes. The two main strategies for pore formation in block copolymer membranes are (i) film casting and selective block sacrifice and (ii) self-assembly and nonsolvent-induced phase separation (SNIPS). Different fundamental aspects involved in the manufacture of block copolymer membranes are considered, including factors affecting the equilibrium morphology in solid films, self-assembly of copolymer in solutions, and macrophase separation by solvent–nonsolvent exchange. Different mechanisms are proposed for different depths of the SNIPS membrane. Block copolymer membranes can be prepared with much narrower pore size distribution than homopolymer membranes. Open questions and indications of what we consider the next development steps are finally discussed. They include the synthesis and application of new copolymers and specific…

We use time-domain thermoreflectance (TDTR), and the generation and detection of longitudinal and surface acoustic waves, to study the thermal conductivity, heat capacity, and elastic properties of thin films of poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), polyacrylamide (PAM), poly(vinylpyrrolidone) (PVP), methyl cellulose (MC), poly(4-styrenesulfonic acid) (PSS), poly( N -acryloylpiperidine) (PAP), poly(methyl methacrylate) (PMMA), and a polymer blend of PVA/PAA. The thermal conductivity of six water-soluble polymers in the dry state varies by a factor of ≈2, from 0.21 to 0.38 W m –1 K –1, where the largest values appear among polymers with a high concentration of hydrogen bonding (PAA, PAM, PSS). The longitudinal elastic constants range from 7.4 to 24.5 GPa and scale linearly with the shear elastic constants, suggesting a narrow distribution of Possion’s ratio 0.35 < ν < 0.40. The thermal conductivity increases with the average sound velocity, as expected based on the model of the minimum thermal conductivity. The thermal conductivity of polymer blends of PVA (0.31 W m –1…

Recently, polyampolytes have been discovered to form hydrogels that possess high toughness, full resilience, and self-healing between two cut surfaces. The self-healing of this class of hydrogels is based on the re-forming of the multiple ionic bonds at the fractured surfaces, in which the mobility of the polymer segments and strength of the ionic bonds play an important role. In this work, we study the effects of healing temperature and chemistry of the polyampholyte hydrogels (chemical cross-linker density and chemical structure of the monomers) on the healing kinetics and healing efficiency. The high healing temperature substantially accelerates the self-healing kinetics. Chemical cross-linking reduces the self-healing efficiency. Monomers with more hydrophobic feature give a low self-healing efficiency. For polyampholyte physical hydrogels with a softening temperature below the room temperature, excellent healing efficiency (∼84% on average and maximum 99%) was observed without any external stimuli. We found a correlation between the self-healing efficiency and the fraction of…

The applications of hydrogels are severely limited by their weak mechanical properties. Despite recent significant progress in fabricating tough hydrogels, it is still a challenge to realize high stretchability, toughness, and recoverability at the same time in a hydrogel. Herein, we develop a novel class of dual physically cross-linked (DPC) hydrogels, which are triggered by clay nanosheets and iron ions (Fe 3+ ) as cross-linkers. First, clay nanosheets induce the formation of the first cross-linking points through the interaction of hydrogen bonds with poly(acrylamide- co -acrylic acid) (PAm- co -Ac) chains. Then the secondary cross-linking points are introduced by ionic coordinates between Fe 3+ and −COO– groups of PAm- co -Ac polymer chains. The mechanical properties of DPC hydrogels can be tuned readily by varying preparation parameters such as clay concentration, Fe 3+ concentration, and molar ratio of Ac/Am. More importantly, the optimal DPC hydrogels possess high tensile strength (ca. 3.5 MPa), large elongation (ca. 21 times), remarkable toughness (ca. 49 MJ m –3 ), and good…

The production of a range of acrylate and acrylamide polymers in completely open reaction vessels has been achieved utilizing the PET-RAFT polymerization technique with zinc tetraphenylporphyrin (ZnTPP) as photoredox catalyst. Polymerization was conducted under extremely mild reaction conditions; low-intensity yellow light, ambient temperatures, and dimethyl sulfoxide (DMSO) as solvent were used. The resulting polymers display characteristics typical of RAFT polymerization, with narrow molecular weight distributions (typically, Đ < 1.10) and controlled molecular weights. One of the advantages of performing PET-RAFT using ZnTPP is the possibility to polymerize monomer in open vessels (i.e., in the presence of oxygen). Oxygen tolerance in DMSO was investigated and attributed to energy transfer from ZnTPP to oxygen to generate singlet oxygen. The effect of changing catalyst concentration and light intensity in these systems has been investigated. Extension of this polymerization technique to a flow system has demonstrated the robustness and effortless scalability of these systems.

Fully bio- and CO 2 -sourced non-isocyanate polyurethanes (NIPUs) were synthesized by reaction of carbonated soybean oil (CSBO) either with biobased short diamines or amino-telechelic oligoamides derived from fatty acids to achieve respectively thermoset or thermoplastic NIPUs. Biobased carbonated vegetable oils were first obtained by metal-free coupling reactions of CO 2 with epoxidized soybean oils under supercritical conditions (120 °C, 100 bar) before complete characterization by FTIR, 1 H NMR, and electrospray ionization mass spectroscopy (ESI-MS). In a second step, biobased NIPUs were produced by melt-blending of the so-produced cyclocarbonated oil with the biobased aminated derivatives. The thermal and mechanical properties of resulting polymers were found to be depending on the cyclocarbonated vegetable oil/amine ratio. More precisely, short diamines and CSBO led to the formation of cross-linked NIPUs, and the resulting tensile and thermal properties were poor. In contrast, elastomeric NIPUs derived from oligoamides and CSBO exhibited a better rigidity, an improved…

Reinforced silica–epoxy vitrimer nanocomposites have been made in a solvent-free, easily processable, and economical way with filler contents up to 40 wt %. Increasing silica content leads to higher modulus materials in both glassy and rubbery regions. These nanocomposites are insoluble like permanent cross-linked networks but can completely relax stresses by thermoactivated exchange reactions that rearrange the network topology. Furthermore, the surface functionalization of particles improves the dispersion of fillers and accelerates the relaxation process. Similar results were obtained with industrial precipitated silica, which would allow vitrimer nanocomposites to be produced on an industrial scale.

Flow-induced crystallization (FIC) of polymers is a long-standing, industrial relevant, nonequilibrium thermodynamic challenge. Thanks to the development of in situ time and spatial resolved techniques like rheology and synchrotron radiation X-ray scattering, substantial progress on the understanding of FIC has been achieved in past 20 years. In this Perspective, we first discuss some recent modifications and improvements on early coarse-grained approaches like the coil–stretch transition model for shish formation and the entropic reduction model for the enhancement of nucleation rate. Then breaking out the two-phase model of classical nucleation theory, flow-induced coil–helix transition, density fluctuation or phase separation, and isotropic–nematic transition are considered as intermediate orders in FIC. Establishing flow morphology and phase diagrams is essential for revealing the nonequilibrium nature of FIC, which will serve as structural roadmap for the processing of high performance polymer products.

Abstract We propose a theoretical framework for dealing with a transient polymer network undergoing small deformations, based on the rate of breaking and reforming of network cross-links and the evolving elastic reference state. In this framework, the characteristics of the deformed transient network at microscopic and macroscopic scales are naturally unified. Microscopically, the breakage rate of the cross-links is affected by the local force acting on the chain. Macroscopically, we use the classical continuum model for rubber elasticity to describe the structure of the deformation energy, whose reference state is defined dynamically according to when cross-links are broken and formed. With this, the constitutive relation can be obtained. We study three applications of the theory in uniaxial stretching geometry: for the stress relaxation after an instantaneous step strain is imposed, for the stress overshoot and subsequent decay in the plastic regime when a strain ramp is applied, and for the cycle of stretching and release. We compare the model predictions with experimental data…

Covalent adaptable networks (CANs; also known as dynamic covalent networks or vitrimers) are appealing for developing simple and efficient techniques for recycling thermosetting polymers. In this paper, ethylene glycol (EG) is used as a solvent to enable pressure-free surface welding, surface repair, and recycling of a malleable epoxy where the transesterification-type bond exchange reaction (BER) imparts a dynamic nature to the covalent network. At a high temperature, the EG molecules participate in the BER, leading to dissolution of the epoxy network. If the EG is allowed to evaporate, the dissolved epoxy can re-form into a solid. The effects of EG content, temperature, and catalyst concentration on EG-assisted BERs are investigated. It is found that the amount of EG can be adjusted to tune the solution/solid transformation: An excessive amount of EG is required to dissolve the epoxy; on the other hand, a shortage of EG can shift the reaction back to re-form the polymer. Furthermore, the catalyst concentration defines the point at which dissolution initiates, while the degradation…

The relationship between glass transition ( T g ) and ionic conductivity (σ) of an amorphous crosslinked polymer electrolyte membrane (PEM) was examined based on ion–dipole complexation between dissociated lithium cations and ether oxygen of poly(ethylene glycol diacrylate) and plasticization by succinonitrile (SCN). In a binary PEM consisting of a lithium salt/polymer network, T g increased due to a strong ion–dipole interaction, whereas σ declined due to lower ion mobility coupled to reduced chain mobility. Above the threshold salt concentration of 7 mol %, dual loss tangent peaks were observed in dynamic mechanical studies, which may be ascribed to segmental relaxations of ion–dipole complexed networks and that of polymer chains surrounding the undissociated lithium salt acting like “fillers”. Upon SCN plasticization, these two peaks merged into one that was further suppressed below T g of the pure network, whereas σ improved to the superionic conductor level. The role of plasticization on the ionic conductivity enhancement is discussed.

Abstract Self-consistent field theory (SCFT) is a powerful tool for the design and interpretation of experiments on block polymer materials. In this Perspective, we lower the barrier to entry to the use of SCFT by experimental groups by two means. First, we present a pedagogical introduction to an improved version of the open-source Polymer Self-Consistent Field (PSCF) software package and of the underlying theory. Second, we discuss methods for generating robust initial guesses for the fields that are computed in SCFT. To demonstrate our approach, we present two case studies in which a typical desktop computer has been used to simulate the structure of (i) body-centered cubic, face-centered cubic, A15, and Frank–Kasper σ sphere-forming phases of a diblock copolymer melt and (ii) two core–shell morphologies of ABAC tetrablock terpolymers. A companion Web site provides all of the relevant software and detailed instructions for reproducing all results contained herein.

We investigate the nonlinear shear and uniaxial extensional rheology of entangled polystyrene (PS) melts and solutions having the same number Z of entanglements, hence identical linear viscoelasticity. While experiments in extensional flows confirm that PS melts and solutions with the same Z behave differently, respective transient and steady data in simple shear over the largest possible range of rheometric shear rates (corresponding to Rouse−Weissenberg numbers from 0.01 to 40) demonstrate that melts and solutions exhibit identical behavior. Whereas the differences between melts and solutions in elongational flows are due to alignment-induced friction reduction (more effective in melts than in solutions), in shear flows they disappear since the rotational component reduces monomeric alignment substantially. Recent molecular dynamics simulations of entangled polymers show that rotation induces molecular tumbling at high shear rates, and here a tube-based model involving tumbling effects is proposed in order to describe the response in shear. The main outcome is that tumbling can…

The development of advanced elastomers with a combination of high strength, large extensibility, and excellent flex-cracking resistance is a huge challenge. In this contribution, we proposed a novel strategy to engineer a multinetwork by incorporating weaker sacrificial hydrogen bonds and stronger Zn-based units into a chemically cross-linked cis -1,4-polyisoprene network. The dynamic nature allows the sacrificial bonds to be ruptured and re-formed, resulting in high stretchability. During external loading, the sacrificial bonds rupture prior to fracture of the covalent network, thus dissipating energy efficiently and facilitating chain orientation to produce improved tensile modulus and fracture toughness as well as significant enhancement of flex-cracking resistance. We propose that the enhanced cracking resistance may originate from the energy dissipation and re-forming of sacrificial bonds, a new mechanism alternative to strain-induced crystallization. Overall, this concept provides unique inspiration for the design of advanced elastomers with excellent mechanical properties…

Quaternized chitosan (QCh) was homogeneously synthesized by reacting chitosan with 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHPTAC) in alkali/urea aqueous solution for the first time. The structure and solution properties of QCh were characterized by using element analysis, FT-IR, 13 C NMR, SEC-LLS, rheology, viscometer, and ξ-potential measurements. Subsequently, polyelectrolyte complex (PEC) hydrogels were constructed by in situ polymerization of acrylic acid (AA) monomers in the concentrated QCh solution. The structure and mechanical behavior of the prepared hydrogels were systematic studied. Because of the high charge density and solubility of QCh, strong electrostatic interactions were formed in the hydrogels and endowed them tough with self-recovery properties. The mechanical behavior of the hydrogels was accurately tuned from stiff and viscoelastic to soft and elastic by changing the poly(acrylic acid) (PAA) content. The regulation mechanism relied on the remarkable difference in the chain segmental mobility between QCh and PAA. Moreover, the QCh/PAA PEC hydrogels…

To get intrinsic adhesion or healing properties in a cross-linked rubber, two levers are possible: the inherent relaxation in permanently but lightly cross-linked elastomers or the relaxation due to exchange reactions as recently reported in the case of vitrimers. The former is associated with dangling chain motion and cannot be controlled. Lightly cross-linked rubbers may show interesting adhesion and healing properties, but at the expense of mechanical properties: ultimate properties are limited, and materials are subjected to creep even at low temperature. Conversely, exchange reactions may be triggered by temperature, providing the healable materials with strong elastomeric properties in a wide temperature range. Here a model system based on epoxidized natural rubber is presented which enables to differentiate and quantify the part of each process. A comparative study including stress relaxation, swelling experiments, and adhesion measurements highlights the advantages of the vitrimer chemistry on inherent relaxation as a proof of concept.

A scaling model of self-similar conformations and dynamics of nonconcatenated entangled ring polymers is developed. Topological constraints force these ring polymers into compact conformations with fractal dimension d f = 3 that we call fractal loopy globules (FLGs). This result is based on the conjecture that the overlap parameter of subsections of rings on all length scales is the same and equal to the Kavassalis–Noolandi number O KN ≈ 10–20. The dynamics of entangled rings is self-similar and proceeds as loops of increasing sizes are rearranged progressively at their respective diffusion times. The topological constraints associated with smaller rearranged loops affect the dynamics of larger loops through increasing the effective friction coefficient but have no influence on the entanglement tubes confining larger loops. As a result, the tube diameter defined as the average spacing between relevant topological constraints increases with time t, leading to “tube dilation”. Analysis of the primitive paths in molecular dynamics simulations suggests a complete tube dilation with the…

A carboxylic acid functional trimer made from the reaction of isosorbide with maleic anhydride was used to cross-link epoxidized sucrose soyate (ESS), resulting in bio-based, degradable thermosets having a good combination of hardness and flexibility. This work addresses some critical needs for thermosets such as improving the sustainability of raw materials, enabling recycling, and achieving both good material hardness and flexibility/ductility simultaneously. In this paper, a dicarboxylic acid, MI, was synthesized from isosorbide and maleic anhydride and characterized in detail by FTIR and 1 H NMR. It was utilized to cross-link ESS without using extra catalyst or toxic compounds except for ethanol and water. For comparison, a dicarboxylic acid from 1,3-propanediol and maleic anhydride (MP) was also synthesized and used to cross-link ESS. Because of the carbon–carbon double bond in conjugation with the carboxylic acid group, both MI and MP showed high reactivity toward ESS and could cross-link ESS with the assistance of water and ethanol. The MI cross-linked thermosets exhibited…

The thiol–Michael adduct is used as a thermoresponsive dynamic cross-linker in polymeric materials. Recently, the thiol–Michael reaction between thiols and conjugated alkenes has been used as a ligation reaction for polymer synthesis and functionalization. Here, the thiol–Michael reaction is demonstrated to be thermally responsive and dynamic. Small molecule model experiments demonstrate the potential for the thiol–Michael adducts to be used in dynamic covalent chemistry. Thiol–acrylate adducts are incorporated into a cross-linker to form a soft polymeric material. These thiol–Michael cross-linked materials display healing after being cut and malleability characteristics at 90 °C. Additionally, the data suggest that there is limited creep and stress relaxation at room temperature with complete recovery of creep once the strain is removed. These thiol–Michael cross-linked polymers show dynamic properties upon thermal stimulus, with long-term stability against mechanical deformation in the absence of this stimulus, opening the way for them to be used in various applications.

Double network (DN) gels, consisting of a brittle first and flexible second network, have been known to be extremely tough and functional hydrogels. In a DN gel subjected to force, the brittle first network breaks prior to the fracture of the flexible network. This process, referred to as internal fracture, dissipates energy and increases the energy required to completely fracture DN gels. Such internal fracture macroscopically appears as a yielding-like phenomenon. The aim of this paper is to investigate the relationship between the yield point and the first network molecular structure of DN gels to more deeply understand the internal fracture mechanism of DN gels. To achieve this goal, we synthesized DN gels having a tetra-PEG first network, which is known to be a nearly ideal and well-controlled network gel. We have found that yielding of the DN gels occurs when the first network strands reach their extension limit (finite extensibility), regardless of their deformation mode. This conclusion not only helps by further understanding the toughening mechanism of DN gels but also…

Polymerization-induced self-assembly (PISA) was achieved by conducting an initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP) at low ppm of copper catalyst concentration. A poly(oligo(ethylene oxide) methyl ether methacrylate) 50 (POEOMA 50 ) macroinitiator and stabilizer was synthesized by an aqueous ICAR ATRP using Cu II Cl 2 /tris(pyridin-2-ylmethyl)amine (TPMA) complex. Subsequently, the dispersion polymerization of benzyl methacrylate (BnMA) in ethanol was realized with a Cu II Br 2 /TPMA complex either at room temperature or at 65 °C using V-70 or AIBN as radical initiators, respectively. The effect of catalyst concentration, radical initiators, targeted degree of polymerization (DP) of PBnMA, solids content, and temperature on the molecular characteristics and self-assembly behavior of block copolymers POEOMA–PBnMA was evaluated by gel permeation chromatography (GPC), transmission electron microscopy (TEM), and dynamic light scattering (DLS). Block copolymers assembled into spheres, wormlike aggregates, and vesicles with diameters…

Here we report the design and synthesis of a new conjugated microporous polymer based on a BODIPY dye (CMPBDP) which has shown excellent luminescence properties and an efficient heterogeneous photocatalytic activity. CMPBDP shows high thermal stability and high surface area with microporous size and efficiently catalyzes the selective oxidation of thioanisole into the corresponding sulfoxide via singlet oxygen under visible light (up to 500 nm). The photocatalytic rate of the heterogeneous materials is 4-fold faster that the corresponding soluble (used as reference) under the same reaction conditions and can be reused several times.

Self-assembly of amphiphilic molecules in water is a cornerstone to build compartmentalized materials toward unique functions, whereas it is yet challenging to create uniform, discrete, and size-controlled nanocompartments. This paper is to report that precision random copolymers, amphiphilic with hydrophilic poly(ethylene glycol) (PEG) and hydrophobic dodecyl pendants, induce precision self-assembly and self-recognition in water to form uniform, tunable, and self-sorting nanoparticles with inner-core hydrophobic compartments covered by PEG chains; the copolymers have been obtained via living or free radical copolymerization. The nanoparticles allow the on-target and predictable control of size, molecular weight, and aggregation number by tuning the primary structure of the copolymers; even mixtures of the copolymers with different composition underwent self-sorting to provide size-controlled discrete compartments.

The unique properties of ionic liquids (ILs) have made them promising candidates for electrochemical applications. Polymerization of the corresponding ILs results in a new class of materials called polymerized ionic liquids (PolyILs). Though PolyILs offer the possibility to combine the high conductivity of ILs and the high mechanical strength of polymers, their conductivities are typically much lower than that of the corresponding small molecule ILs. In the present work, seven PolyILs were synthesized having degrees of polymerization ranging from 1 to 333, corresponding to molecular weights (MW) from 482 to 160 400 g/mol. Depolarized dynamic light scattering, broadband dielectric spectroscopy, rheology, and differential scanning calorimetry were employed to systematically study the influence of MW on the mechanism of ionic transport and segmental dynamics in these materials. The modified Walden plot analysis reveals that the ion conductivity transforms from being closely coupled with structural relaxation to being strongly decoupled from it as MW increases.

We explore the relationship between the morphology and ionic conductivity of block copolymer electrolytes over a wide range of salt concentrations for the system polystyrene- block -poly(ethylene oxide) (PS- b -PEO, SEO) mixed with lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI). Two SEO polymers were studied, SEO(16–16) and SEO(4.9–5.5), over the salt concentration range r = 0.03–0.55. The numbers x and y in SEO( x – y ) are the molecular weights of the blocks in kg mol –1, and the r value is the molar ratio of salt to ethylene oxide moieties. Small-angle X-ray scattering was used to characterize morphology and grain size at 120 °C, differential scanning calorimetry was used to study the crystallinity and the glass transition temperature of the PEO-rich microphase, and ac impedance spectroscopy was used to measure ionic conductivity as a function of temperature. The most surprising observation of our study is that ionic conductivity in the concentration regime 0.11 ≤ r ≤ 0.21 increases in SEO electrolytes but decreases in PEO electrolytes. The maximum in ionic conductivity…

A novel strategy to obtain sulfur-rich polybenzoxazine copolymers by reacting allyl functional benzoxazine (BA-ala) and elemental sulfur was described. Simultaneous inverse vulcanization and ring-opening reactions of benzoxazine generated soluble copolymers in specific feed ratios. Parameters such as monomer structure and feed ratios on the polymerization were studied. The thermal stability of the copolymers was investigated and compared to that of polybenzoxazines derived from neat BA-ala by using thermogravimetric analysis and differential scanning calorimetry. The surface properties of the materials as examined by scanning electron microscopy confirmed that elemental sulfur and benzoxazine copolymers can be produced without a phase separation at the micrometer level. Moreover, a sponge-like insoluble macroporous polybenzoxazine networks was obtained at the 20 wt % feed ratio of sulfur.

Connecting continuum-scale ion transport properties such as conductivity and cation transference number to microscopic transport properties such as ion dissociation and ion self-diffusivities is an unresolved challenge in characterizing polymer electrolytes. Better understanding of the relationship between microscopic and continuum scale transport properties would enable the rational design of improved electrolytes for applications such as lithium batteries. We present measurements of continuum and microscopic ion transport properties of nonflammable liquid electrolytes consisting of binary mixtures of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and perfluoropolyethers (PFPE) with different end groups: diol, dimethyl carbonate, ethoxy–diol, and ethoxy–dimethyl carbonate. The continuum properties, conductivity and cation transference number, were measured by ac impedance spectroscopy and potentiostatic polarization, respectively. The ion self-diffusivities were measured by pulsed field gradient nuclear magnetic resonance spectroscopy (PFG-NMR), and a microscopic cation…

Tunable mechanical response under dynamic and static loading is desirable for many technological applications. Traditionally, mechanical performance of polymeric materials is controlled by modulating structural (i.e., molecular weight, chain packing, or cross-link density) or temporal parameters (such as kinetics of the exchange of dynamic cross-linkers). Metal–ligand interactions are uniquely suited to control both structural and temporal parameters as the thermodynamics and kinetics of mechanically active cross-linkers can be varied by careful selection of metal without significant synthetic modification of the polymer backbone. Here, we have demonstrated that it is possible to engineer desired mechanical properties in a metallopolymer with a high degree of tunability by simply changing the type and amount of added metal. Specifically, we cross-linked an imidazole-containing brush copolymer system with the divalent cations of zinc, copper, and cobalt. Using rheology and tensile experiments, we have correlated the emergent mechanical properties to the stoichiometric ratio of ligand…

Kinetics of II to I polymorphic transformation in isotactic polybutene-1 (PB-1) and its annealing temperature and time dependencies have been investigated by means of differential scanning calorimetry and in situ wide-angle X-ray diffraction techniques. The PB-1 samples were isothermally crystallized into metastable form II crystalline modification followed by annealing at a lower temperature ( T l ) and at a higher temperature ( T h ) subsequently or at a single temperature ( T s ) to promote polymorphic transition from form II to I. This solid-to-solid phase transition was shown to be a two-step process including nucleation and growth suggested by the result that more form I was obtained after being annealed at T l and T h than annealed at T s for the same period. Annealing at T l benefits nucleation due to internal stress induced by unbalanced shrinkage of amorphous and crystalline phases because of their different thermal expansion coefficients, while annealing at T h is beneficial to growth owing to rapid segmental diffusion at that temperature. At a given annealing time at T l…

Here, taking a polythiophene derivative (PBDD4T) as a starting polymer, we tried to increase the rotation barrier and hence stabilize its backbone conformation by introducing fluorine into the β- and β′-position of the α-linked bithiophene segments and then synthesized a new polymer named as PBDD4T-2F. Our results demonstrate that the rotation barrier between the α-linked bithiophene significantly increases after the fluorination, so PBDD4T-2F has a more stable backbone conformation than PBDD4T. Compared to PBDD4T, PBDD4T-2F shows stronger aggregation effect in solution state and more compact π–π stacking in solid thin film and also possesses deeper HOMO level. These properties make PBDD4T-2F being an ideal donor material in PSCs. When blended with PC 71 BM, a fullerene acceptor, the PBDD4T-2F-based device showed a power conversion efficiency (PCE) of 9.04%, which is 38% higher than that of the PBDD4T-based device; when blended with ITIC, a non-fullerene acceptor, the PBDD4T-2F-based device showed a PCE of 8.69%, which is almost 20 times higher than that of the PBDD4T-based device.…

Renewable, biodegradable polymers, such as aliphatic polyesters, based on sustainable sources have attracted considerable interest as alternatives to petroleum-based polymers. One limiting factor in the development of aliphatic polyesters as replacements for these materials has been their relatively low glass transition temperatures ( T g ). For example, commercially available poly(lactic acid) has a T g of approximately 60 °C. Epoxide/anhydride copolymerizations offer an alternative to the ring-opening polymerization of lactones for the synthesis of aliphatic polyesters and allow for tuning of polymer properties through two distinct monomer sets. We synthesized six partially or fully renewable tricyclic anhydrides and copolymerized them with propylene oxide (PO) and cyclohexene oxide (CHO). By varying both the epoxide and the anhydride, we were able to tune the T g of the resulting polymers over a nearly 120 °C range from 66 °C to an exceptionally high 184 °C. Polymers produced with PO had a lower range of T g values (66–108 °C) and higher molecular weights up to 32.2 kDa, while…

Materials that display thermally activated delayed fluorescence (TADF) have recently been identified as the third generation emitters for organic light-emitting diodes (OLEDs). However, there are only a few reported examples of polymeric TADF materials. This study reports a series of polymers with an insulating backbone and varying ratios of 2-(10 H -phenothiazin-10-yl)dibenzothiophene- S, S -dioxide as a pendant TADF unit. Steady-state and time-resolved fluorescence spectroscopic data confirm the efficient TADF properties of the polymers. Styrene, as a comonomer, is shown to be a good dispersing unit for the TADF groups, by greatly suppressing the internal conversion and triplet–triplet annihilation. Increasing the styrene content within the copolymers results in relatively high triplet energy, small energy splitting between the singlet and triplet states (Δ E ST ), and a strong contribution from delayed fluorescence to the overall emission. Green emitting OLED devices employing these polymers as spin-coated emitting layers give high performance, which is dramatically enhanced in…

Softwood (guaiacylic) lignin-based methacrylate polymers (LBMPs) that exhibit excellent glass transition temperatures ( T g ’s), desirable thermal stabilities (greater than 100 °C above T g ), and intermediate shear-flow resistances, in comparison to polystyrene and poly(methyl methacrylate), are reported herein. Different R-groups ( p -position hydrogen, methyl, ethyl, and formyl groups) in otherwise homologous LBMPs impart distinct characteristics to the flow behavior and thermal properties of these bio-based polymers, which permit the investigation of unique structure–property relationships. More specifically, the zero-shear viscosities (η 0 ’s) for the LBMPs span nearly 2 orders of magnitude as the R-group is varied, while the characteristic degradation temperatures differ more modestly (by ≈50 °C over the same series of polymers), and the T g ’s exhibit minimal, yet application relevant, variations between ≈110 and ≈130 °C. These property differences were probed independent of tacticity, molecular weight, and dispersity effects due to the nature of the well-controlled…

Thermoplastic elastomers are attractive materials because of their ability to be melt-processed, reused, and recycled, unlike chemically cross-linked elastomers such as rubber. We report the synthesis and mechanical properties of polyolefin-based thermoplastic elastomer block copolymers. A simple one-pot procedure is employed, using a living arylnaphthyl-α-diimine Ni(II) “sandwich” complex to generate high crystallinity hard blocks from 1-decene and low crystallinity soft blocks from ethylene. Various block structures are accessed, ranging from a diblock up to a heptablock copolymer. Statistical copolymers of 1-decene and ethylene are also synthesized for comparison. All resulting polymers behave as elastomers, with properties that modulate with hard and soft block composition, block architecture, and polymerization solvent. Triblock copolymers demonstrate strain at break values up to 750%, with elastic strain recoveries up to 85%. Interestingly, statistical copolymers demonstrate strain at break values upward of 1120% and elastic strain recoveries up to 77%. Creep experiments were…

The thermal stability of several crystal polymorphs in polyamide 11 (PA11) and polyamide 6 (PA6) has been investigated by means of in situ X-ray experiments. In the case of PA11, δ′ and α′ phases display a Brill transition far below the melting point. Both phases transform into a (pseudo)-hexagonal HT δ phase above 100 °C. The latter turns back into the most stable α′ phase upon cooling. PA6 exhibits similar thermally induced crystal transitions. In situ X-ray investigations refute the occurrence of a β → α transition upon heating, whereas the existence of a pseudohexagonal HT phase is suggested as in PA11. For both polymers, the present study underlines the major role of crystal perfection of the most stable α phase on the existence of a Brill transition. The combination of in situ structural information with thermal analysis allows to propose a thermodynamic scheme to describe the Brill transition in both polyamides.

The emergence of light regulated controlled/living radical polymerization adds a new layer of control over polymerization. Light mediated polymerizations afford simple and facile route to modulate polymerization rate through manipulation of light intensity and by switching on/off the light source. Such mediation has resulted in the synthesis of 3D surfaces with both spatial and temporal control. However, these techniques present a major limitation in terms of oxygen tolerance and require the use of organic solvent. In this contribution, we report an efficient aqueous polymerization system capable of being activated under visible light in the presence of oxygen. We perform aqueous photopolymerization in the presence of water-soluble zinc porphyrin photocatalyst (Zn(II) meso-tetra(4-sulfonatophenyl)porphyrin, ZnTPPS 4– ) with ascorbic acid as singlet oxygen quencher in both open and closed vessels. Polymers could be prepared without prior deoxygenation with good control over the molecular weight and polydispersity. In addition, polymerization in the presence of air could be achieved…

Rheology studies were performed on tough PVP- in situ -PAAm hydrogels physically cross-linked by cooperative hydrogen bonding to understand their viscoelastic response and, hence, the interactions and microstructure. The viscoelasticity of the PVP- in situ -PAAm hydrogels was strongly affected by the monomer ratio ( C AAm / C VP ). Hydrogels prepared with a high monomer ratio exhibited weak time, temperature and frequency dependence of the viscoelastic properties, similar to those of chemically cross-linked hydrogels. The storage modulus ( G ′) of the gels was much greater than the loss moduli ( G ″) and low loss factor (tan δ < ∼ 0.1), which indicated that they were solid-like, and mostly elastic. These supramolecular gels exhibited a strain- and C AAm / C VP -dependent reversible gel (solid) to viscoelastic liquid transition due to the dynamic nature of the cooperative hydrogen bonds. That transition also coincided with the onset of nonlinear viscoelastic behavior. The addition of a low molecular weight compound, urea, that competes for hydrogen bonding sites weakens the gel by…

Shape memory thermoplastic polyurethane (SMTPU) containing isophorone diisocyanate (IPDI) in hard segment has excellent shape recoverability even after large strain deformation. However, the underlying mechanism of shape recovery remains unclear. In this study, 1,4-phenylene diisocyanate (PPDI) in the polyurethane is gradually substituted by IPDI, and multiscale effects are examined by normal and dichroic Fourier transform infrared spectroscopy (FTIR), small-angle X-ray scattering (SAXS), single-molecule force spectroscopy (SMFS), and mechanical test. Contradictory to the traditional conclusion, the degree of microphase separation decreases as the content of IPDI increases, while the macroscopic shape recoverability is largely improved. With dichroic FTIR and SAXS, we find that the morphology of hard phases changed from lamellar-like to fibrillar-like, which is more stable under stretching. SMFS experiments discover that IPDI could increase the elasticity of polymer chain and could endow the hard phases with “elastic” under stress. With these two factors, we are able to explain the…

We report a photoinduced electron/energy transfer–reversible addition-fragmentation chain transfer (PET-RAFT) dispersion polymerization mediated by 5,10,15,20-tetraphenyl-21 H,23 H -porphine zinc (ZnTPP) under low energy red (λ max = 635 nm) or yellow light (λ max = 560 nm). By varying the degrees of polymerization for the hydrophobic block, nanoparticles of different morphologies (spheres, wormlike micelles, and vesicles) were formed at high monomer conversion (>98%) under visible light irradiation. Interestingly, encapsulation of the ZnTPP catalyst into the nanoparticle core was achieved by direct dialysis against water with no significant change in nanoparticle morphology. These aqueous ZnTPP-loaded nanoparticles were demonstrated to have potential applications in photodynamic therapy owing to their ability to generate singlet oxygen under visible light irradiation. Finally, the ability of ZnTPP to generate singlet oxygen was exploited to perform polymerization without traditional deoxygenation procedures by addition of a singlet oxygen quencher (ascorbic acid).