Length Of Grafted Chains Research Articles

Interfacial influence on mechanical properties of polypropylene/polypropylene‐grafted silica nanocomposites

ABSTRACTThree types of polypropylene‐grafted silica (PGS‐2 K, PGS‐8 K and PGS‐30 K) with different grafting chain lengths were prepared. After melt‐blending PGS with polypropylene (PP), we studied the PP/PGS interface properties and the influence of PP/PGS interfaces on mechanical properties of nanocomposites. The strong matrix/particle interface was observed in PP/PGS‐30 K nanocomposites with 5 wt % particle loading as evidenced by 2.5 °C increased glass transition temperature (Tg) compared with neat PP, whereas the weak matrix/particle interface was observed in PP/PGS‐2 K nanocomposites with decreased Tg. The variations in the matrix/particle interfacial strength lead to a transition in the yield stress of nanocomposites. Compared with the unfilled PP, the yield stress of the PP/PGS‐2 K nanocomposites is decreased by 0.7 MPa, and the yield stress of the PP/PGS‐30 K nanocomposites is enhanced by 1.4 MPa. In addition, benefiting from good dispersion, the PP/PGS‐masterbatch nanocomposites with a strong matrix/particle interface not only exhibit increased Young's modulus and yield stress, but also the strain at break remains in line with the unfilled PP, which is in contrast to the conventional wisdom that the gain in modulus and strength must be at the expense of the decreased break strain. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45887.

Polymer-Grafted Nanoparticle Membranes with Controllable Free Volume

Polymer-based membranes play a key role in several industrially important gas separation technologies, e.g., removing CO2 from natural gas, with enormous economic and environmental impact. Here, we develop a novel hybrid membrane construct comprised entirely of nanoparticles grafted with polymers. These membranes are shown to have broadly tunable separation performance through variations in graft density and chain length. Computer simulations show that the optimal NP packing forces the grafted polymer layer to distort, yielding regions of measurably lower polymer density. Multiple experimental probes confirm that these materials have the predicted increase in “polymer free volume”, which explains their improved separation performance. These polymer-grafted NP materials thus represent a new template for rationally designing membranes with desirable separation abilities coupled with improved aging characteristics in the glassy state and enhanced mechanical behavior.

Enhancing crystallization rate and melt strength of PLLA with four‐arm PLLA grafted silica: The effect of molecular weight of the grafting PLLA chains

ABSTRACTTo improve the crystallization rate and melt strength of polylactide (PLLA), nano‐size amino silica grafted by four‐arm PLLA (4A‐PLLA) with different molecular weight was synthesized. 1H nuclear magnetic resonance proved that 4A‐PLLA had been grafted onto the surface of SiO2 successfully, and the grafting ratios and the degradation behaviors of the grafted SiO2 nanoparticles (g‐SiO2) were studied. When the grafted silica was introduced into PLLA matrix, the crystallization rate and melt strength of composites were found to be improved and the length of grafted chain played an important role. The extension rheology indicated that long grafted 4A‐PLLA on the surface of SiO2 was more efficient in enhancing the elongational viscosity of PLLA, owing to the stronger interactions between the grafted chains and the matrix. The crystallization behavior of ungrafted silica filled composite was similar to that of neat PLA, while g‐SiO2 played a role of nucleating agent. The crystallinities and the crystallization rates of the composites depended on the content of g‐SiO2 and the grafted chain length of 4A‐PLLA, especially the latter. Longer grafted chain acted as nucleation site in the matrix and significantly improved the crystallization behaviors of PLLA. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45675.

Precisely controlled two-step synthesis of cellulose-graft-poly(l-lactide) copolymers: Effects of graft chain length on thermal behavior

This study presented a method to polymerize to cellulose-graft-poly (l-lactide) (cellulose-g-PLLA) copolymers as chemical modification process to overcome disadvantage of melt processing of pristine cellulose due to the strong intermolecular hydrogen bonds. In order to maximize the chain length at the end of cellulose, we designed a precisely controlled polymerization based on a two-step synthesis; 1) short-chain copolymerization of cellulose in an ionic liquid, 2) further graft polymerization in N,N-dimethylformamide and the bulk phase. Accordingly, we could synthesize a copolymer in high yield, with the number of substitutions per glucose unit (DSPLLA) being close to the theoretical maximum value of 3 and the number of lactyl repeating units introduced per glucose unit (MSPLLA) of 212, at a high yield. While the glass transition temperature (Tg) of the copolymer was greatly decreased owing to dissociation of the hydrogen bonds of cellulose and plasticizing effects at a low PLLA content (MSPLLA = 11.2), the Tg gradually increased to attain a value close to that of pure PLLA as the content increased.The cellulose-g-PLLA with low molecular weight was amorphous phase, but the crystallinity started to show from the sample with MSPLLA of 42.0 and WPLLA = 95.0%. As the molecular weight contents of PLLA were increased, the cellulose-g-PLLA of the melting point and the heat of fusion was increased. The cellulose-g-PLLA of the strong inter- and intramolecular hydrogen bonds between the hydroxyl groups was found to affect PLLA crystallinity.

Semisoft Colloidal Crystals in Ionic Liquids.

Here, we introduce a technique for the direct formation of semisoft colloidal crystals of hybrid particles in nonvolatile ionic liquid (IL) solvents. The hybrid particles are comprised of a silica core and a densely grafted polymer brush shell, which were synthesized by surface-initiated living radical polymerization. A phase transition of the suspensions from a disordered fluid to a crystallized system was observed within a narrow concentration range. Confocal laser scanning microscopy observation and ultraviolet-visible light (UV-vis) spectrometry confirmed the highly ordered structure of the hybrid particles in ionic liquids. The effect of the hybrid particle structure on the photonic band gap of the colloidal crystals was investigated, and the band gaps varied by changing graft chain lengths. In addition, the colloidal crystal suspensions were successfully immobilized in ILs.

Temperature-Triggered Colloidal Gelation through Well-Defined Grafted Polymeric Surfaces.

Sufficiently strong interparticle attractions can lead to aggregation of a colloidal suspension and, at high enough volume fractions, form a mechanically rigid percolating network known as a colloidal gel. We synthesize a model thermo-responsive colloidal system for systematically studying the effect of surface properties, grafting density and chain length, on the particle dynamics within colloidal gels. After inducing an attraction between particles by heating, aggregates undergo thermal fluctuation which we observe and analyze microscopically; the magnitude of the variance in bond angle is larger for lower grafting densities. Macroscopically, a clear increase of the linear mechanical behavior of the gels on both the grafting density and chain length arises, as measured by rheology, which is inversely proportional to the magnitude of local bond angle fluctuations. This colloidal system will allow for further elucidation of the microscopic origins to the complex macroscopic mechanical behavior of colloidal gels including bending modes within the network.

PEGylation on mixed monolayer gold nanoparticles: Effect of grafting density, chain length, and surface curvature

PEGylation on nanoparticles (NPs) is widely used to prevent aggregation and to mask NPs from the fast clearance system in the body. Understanding the molecular details of the PEG layer could facilitate rational design of PEGylated NPs that maximize their solubility and stealth ability without significantly compromising the targeting efficiency and cellular uptake. Here, we use molecular dynamics (MD) simulation to understand the structural and dynamic the PEG coating of mixed monolayer gold NPs. Specifically, we modeled gold NPs with PEG grafting densities ranging from 0–2.76chain/nm2, chain length with 0–10 PEG monomers, NP core diameter from 5nm to 500nm. It is found that the area accessed by individual PEG chains gradually transits from a “mushroom” to a “brush” conformation as NP surface curvature become flatter, whereas such a transition is not evident on small NPs when grafting density increases. It is shown that moderate grafting density (∼1.0chain/nm2) and short chain length are sufficient enough to prevent NPs from aggregating in an aqueous medium. The effect of grafting density on solubility is also validated by dynamic light scattering measurements of PEGylated 5nm gold NPs. With respect to the shielding ability, simulations predict that increase either grafting density, chain length, or NP diameter will reduce the accessibility of the protected content to a certain size molecule. Interestingly, reducing NP surface curvature is estimated to be most effective in promoting shielding ability. For shielding against small molecules, increasing PEG grafting density is more effective than increasing chain length. A simple model that includes these three investigated parameters is developed based on the simulations to roughly estimate the shielding ability of the PEG layer with respect to molecules of different sizes. The findings can help expand our current understanding of the PEG layer and guide rational design of PEGylated gold NPs for a particular application by tuning the PEG grafting density, chain length, and particle size.

Evaluation of structural effects on the flocculation performance of a co-graft starch-based flocculant

The molecular structure of a material substantially determines its final application performance. In this work, a series of starch-based flocculants with different charge densities and average graft chain lengths were prepared by the co-graft polymerization of acrylamide and [(2-methacryloyloxyethyl) trimethyl ammonium chloride] (St-g-PAM-co-PDMC). The flocculation performance of St-g-PAM-co-PDMC was studied systematically at neutral pH using kaolin suspension and sodium humate (NaHA) aqueous solution as synthetic wastewaters. The effects of the two structural factors on the flocculation efficiency of the starch-based flocculants have been investigated. The experimental results showed that the charge density and average graft chain length contributed distinctly to flocculation performance during the removal of both kaolin particles and NaHA under insufficient and excessive flocculant dose conditions. The flocculation mechanisms of this starch-based flocculant were discussed in detail on the basis of the structure-activity relationship, which are significant to optimize the flocculation conditions and guide the development of novel high-performance flocculants.

Mizoroki–Heck Cross-Coupling Reaction of Haloarenes Mediated by a Well-Controlled Modified Polyacrylamide Brush Grafted Silica/Pd Nanoparticle System

Abstract Modified polyacrylamide brushes including phosphinite functionality grafted onto silica supported Pd nanoparticles were synthesized via RAFT polymerization technique in a controlled manner with elucidated graft density and chain length. Proper activity of the catalyst was indicated in the Mizoroki–Heck coupling reaction of various haloarenes with olefins. Different aryl iodides with electron rich and electron deficient substituent and also ortho and heterocyclic substrate showed good reactivity to generate the corresponding coupled products in good to excellent yields using low Pd loading. The turnover number (TON) for this catalyst can be reduced up to 9.5 × 103. Simple filtration, appropriate reusability and negligible palladium leaching of this catalyst are among other advantages.

Insight into the Dispersion Mechanism of Polymer-Grafted Nanorods in Polymer Nanocomposites: A Molecular Dynamics Simulation Study

Coarse-grained molecular dynamics simulations are performed to investigate the dispersion behavior and the underlying dispersion mechanism of polymer-grafted nanorods (NRs) in a polymer matrix. The influences of grafting density, grafted chain length, and the miscibility between grafted chains and matrix chains are systematically analyzed. The simulation results indicate that the dispersion state of grafted NRs is determined primarily by the excluded volume effect of grafted NRs and the interface between grafted chains and matrix chains. It is found that increasing grafting density and/or grafted chain length induces the conformational transition of grafted chains from mushroom to brush, enlarges the excluded volume of grafted NRs, and enhances the brush/matrix interface in the brush regime, resulting in the improvement of the NR dispersion state. By tuning the interaction strength between grafted chains and matrix chains in a wide range, three general categories of NR spatial organization are found: macroscopic phase separation of the NRs and polymer matrix, homogeneous dispersion of the NRs, and “tele-bridging” of the NRs via the matrix chains. The transition from a “wet” to “dry” brush is observed at the strong brush/matrix interaction. In addition, for the polymer brush grafted on the surface of NRs, the dependence of the brush thickness Tb on either grafting density Σ or grafted chain length Lg is always weaker than that of polymer brush grafted on flat surface or spherical surface, mainly due to the broader range of moving and the less stretching of polymer chains grafted to NRs. The Tb scales with Σ and Lg as Tb ∼ ΣαLgβ, where the ratio β/α is a constant equal to 3, independent of the grafted surface. In general, this work offers a deep insight into the dispersion mechanism of grafted NRs and thus is believed to provide some guidance on the design and preparation of high-performance polymer nanocomposites with tailored dispersion of NRs.

Computer simulation study on the self-assembly of unimodal and bimodal polymer-grafted nanoparticles in a polymer melt.

The controllable distribution of nanoparticles (NPs) in polymer nanocomposites (PNCs) is a challenge in materials science. An important method is grafting chains that are chemically identical to the polymer matrix on NPs. By performing comprehensive molecular dynamics simulations, the self-assembly behavior of polymer-grafted NPs in a polymer matrix is investigated in this study. The relationship between the grafted chain length N, grafting density σ and the NPs' self-assembly morphologies is studied. Phase diagrams of the NP self-assembly structures for both unimodal and bimodal grafted NP systems are constructed on a parameter space, where P is the matrix polymer chain length. NP self-assembly structures of strings, connected/sheet and small clusters are identified in different regions. In order to quantitatively characterize the NP self-assembly morphology, we define a morphological measurement parameter which characterizes the distribution of the Voronoi cell volume of the NPs. Using this parameter, we discuss the influences of both long and short grafted chains on the dispersion of bimodal polymer-grafted NPs in a polymer melt. We find that the short grafted chains can not only shield the NP surface from the polymer matrix but also elongate the long grafted chains into the polymer matrix, therefore favoring a better dispersion of NPs. Our results also indicate that the bimodal grafted NPs will not be fully dispersed until the short grafted chains are dense enough to elongate the long grafted chains, hence forming a wetting NP/matrix interface.

Progress in multi-scaled structure and related properties of elastomer nanocomposites explored by molecular dynamics simulation

In this talk I will systematically present some important simulated results of elastomer nanocomposites (ENCs) via molecular dynamics simulation. First, we studied the dispersion and aggregation behavior of bare nanoparticles (NPs) with different geometries such as spherical, sheet-like and rod-like on the molecular scale. To model small ligands used in experiments to realize better dispersion, we investigated the dispersion of NPs end-grafted with polymer chains by varying the grafted chain length and grafting density. In addition, the effect of the middle- and end-functionalization on the dispersion of NPs is also covered. Second, we probed the translational and relaxation dynamics at the chain and segmental length scales of the interfacial regions, hoping to elucidate whether “glassy polymer layers” exist around NPs. Meanwhile, the formation mechanism of bound rubber is as well included. Third, we simulated the enhancement of the stress-strain and fracture toughness induced by NPs, providing a molecular reinforcing mechanism. Fourth, the famous “Payne effect”, namely the decrease of the storage modulus as a function of the strain amplitude was examined, uncovering the underlying reason responsible for this non-linear behavior, and meanwhile how the introduced carbon nano-springs can effectively reduce the dynamic hysteresis of ENCs is illustrated. Fifth, through simulation synthesis approach, we put forward a new and achievable approach to design and prepare a nanoparticle chemical network, with the NPs acting as “giant cross-linkers” or netpoints to chemically connect the dual end-groups of each polymer chain to form a network. We find this new network structure possesses excellent static and dynamic mechanical properties, highlighting a ultralow dynamic hysteresis loss tailored for green automobile tires. In general, computer simulation is shown to have the capability to obtain some fundamental understanding of ENCs, in hopes of providing some design principles for synthesizing and fabricating multi-functional and high performance ENCs.

Construction of antifouling lumen surface on a poly(vinylidene fluoride) hollow fiber membrane via a zwitterionic graft copolymerization strategy

Abstract Construction of PVDF hollow fiber membranes with antifouling lumen surfaces is of great interest and significance for many industrial applications because the inside-out filtration mode allows good control over module hydrodynamics. For the first time, a zwitterionic polymer was immobilized on the lumen surface of a PVDF hollow fiber membrane via thermal-induced graft copolymerization with an objective to render the PVDF membrane antifouling. The graft copolymerization consists of two steps: ozone-assisted surface pre-activation and grafting of sulfobetaine methacrylate (SBMA). The grafting density and chain length were systematically optimized by manipulating the operational parameters, e.g. ozone treatment and grafting duration. The resultant membranes were studied with respect to their structure, morphology, ultrafiltration (UF) performance, while their fouling behaviors were evaluated by flux recovery ratio and the resistance-in-series model. In comparison to the pristine one, the composite membranes exhibited superior fouling resistance for a 2-h filtration of a 2.0 wt% bovine serum albumin (BSA) solution. Especially, the irreversible fouling-induced resistance was prominently reduced from 53% to 15% due to the outstanding hydration capacity of the polySBMA polymer. Consequently, the flux recovery ratio ( FRR ) increased significantly from 47% to 85%, suggesting the promising prospect of the zwitterion-grafted PVDF hollow fiber membranes for their antifouling properties. Most importantly, the improvement in antifouling was achieved without sacrificing flux or rejection efficiency. This work may provide useful insights about lumen surface nano-structuring of the inner-selective hollow fiber membranes.

Crazing of nanocomposites with polymer-tethered nanoparticles

The crazing behavior of polymer nanocomposites formed by blending polymer grafted nanoparticles with an entangled polymer melt is studied by molecular dynamics simulations. We focus on the three key differences in the crazing behavior of a composite relative to the pure homopolymer matrix, namely, a lower yield stress, a smaller extension ratio, and a grafted chain length dependent failure stress. The yield behavior is found to be mostly controlled by the local nanoparticle-grafted polymer interfacial energy, with the grafted polymer-polymer matrix interfacial structure being of little to no relevance. Increasing the attraction between nanoparticle core and the grafted polymer inhibits void nucleation and leads to a higher yield stress. In the craze growth regime, the presence of "grafted chain" sections of ≈100 monomers alters the mechanical response of composite samples, giving rise to smaller extension ratios and higher drawing stresses than for the homopolymer matrix. The dominant failure mechanism of composite samples depends strongly on the length of the grafted chains, with disentanglement being the dominant mechanism for short chains, while bond breaking is the failure mode for chain lengths >10Ne, where Ne is the entanglement length.

Tuning the Mechanical Properties of Polymer Nanocomposites Filled with Grafted Nanoparticles by Varying the Grafted Chain Length and Flexibility.

By employing coarse-grained molecular dynamics simulation, we simulate the spatial organization of the polymer-grafted nanoparticles (NPs) in homopolymer matrix and the resulting mechanical performance, by particularly regulating the grafted chain length and flexibility. The morphologies ranging from the agglomerate, cylinder, sheet, and string to full dispersion are observed, by gradually increasing the grafted chain length. The radial distribution function and the total interaction energy between NPs are calculated. Meanwhile, the stress–strain behavior of each morphology and the morphological evolution during the uniaxial tension are simulated. In particular, the sheet structure exhibits the best mechanical reinforcement compared to other morphologies. In addition, the change of the grafted chain flexibility to semi-flexibility leads to the variation of the morphology. We also find that at long grafted chain length, the stress–strain behavior of the system with the semi-flexible grafted chain begins to exceed that of the system with the flexible grafted chain, attributed to the physical inter-locking interaction between the matrix and grafted polymer chains. A similar transition trend is as well found in the presence of the interfacial chemical couplings between grafted and matrix polymer chains. In general, this work is expected to help to design and fabricate high performance polymer nanocomposites filled with grafted NPs with excellent and controllable mechanical properties.

The Conformation of Thermoresponsive Polymer Brushes Probed by Optical Reflectivity.

We describe a microscope-based optical setup that allows us to perform space- and time-resolved measurements of the spectral reflectance of transparent substrates coated with ultrathin films. This technique is applied to investigate the behavior in water of thermosensitive polymer brushes made of poly(N-isopropylacrylamide) grafted on glass. We show that spectral reflectance measurements yield quantitative information about the conformation and axial structure of the brushes as a function of temperature. We study how parameters such as grafting density and chain length affect the hydration state of a brush, and provide one of the few experimental evidences for the occurrence of vertical phase separation in the vicinity of the lower critical solution temperature of the polymer. The origin of the hysteretic behavior of poly(N-isopropylacrylamide) brushes upon cycling the temperature is also clarified. We thus demonstrate that our optical technique allows for in-depth characterization of stimuli-responsive polymer layers, which is crucial for the rational design of smart polymer coatings in actuation, gating, or sensing applications.

Grafting glycidyl methacrylate to Sepharose gel for fabricating high-capacity protein anion exchangers

To develop ion exchangers of high protein adsorption capacity, we have herein introduced atom transfer radical polymerization (ATRP) method to graft glycidyl methacrylate (GMA) onto Sepharose FF gel. GMA-grafted Sepharose FF resins of four grafting densities and different grafting chain lengths were obtained by adjusting reaction conditions. The epoxy groups on the grafted chains were functionalized by modification with diethylamine (DEA), leading to the fabrication of Sepharose-based anion exchangers of 14 different grafting densities and/or grafting chain lengths. The resins were first characterized for the effects of grafting density, chain length and ionic strength on pore sizes by inverse size exclusion chromatography. Then, the resins were evaluated by adsorption equilibria of bovine serum albumin (BSA) as a function of ionic capacity (IC) (chain length) at individual grafting densities. It was observed that at each grafting density there was a specific IC value (chain length) that offered the maximum equilibrium capacity. Of the resins with maximum values at individual grafting densities, the resin of the second grafting density with an IC value of 330mmol/L (denoted as FF-Br2-pG-D330) showed the highest capacity, 264mg/mL, about two times higher than that of the traditional ungrafted resin Q Sepharose FF (137mg/mL). This resin also showed the most favorable uptake kinetics among the resins of similar IC values but different grafting densities, or of the same grafting density but different IC values. Effects of ionic strength showed that the capacities of FF-Br2-pG-D330 were much higher than Q Sepharose FF at a wide range of NaCl concentrations (0–200mmol/L), and the uptake rates of the two resins were similar in the ionic strength range. Therefore, the dynamic binding capacity values of BSA on FF-Br2-pG-D330 were much higher than Q Sepharose FF as demonstrated at different residence times and ionic strengths. Taken together, the research has proved the success in the fabrication of high-capacity protein anion exchangers by grafting GMA onto Sepharose gel.

Tuning the wetting–dewetting and dispersion–aggregation transitions in polymer nanocomposites using composition of graft and matrix polymers

Recent simulation and experimental work on polymer nanocomposites composed of polymer grafted particles and free matrix polymers, where the graft and matrix homopolymers are chemically dissimilar and exhibit lower critical solution temperature behavior with temperature, has shown that wetting to dewetting is a gradual and distinct transition from the sharp particle dispersion–aggregation transition. In this study, using coarse-grained molecular simulations, we demonstrate that the extent of wetting of the grafted polymer layer and the particle dispersion–aggregation transition are tuned using the composition of graft and matrix polymers. Specifically, we study composites where the graft and matrix chains are random copolymers composed of attractive and athermal monomers. We maintain a dense grafting density on the spherical particles of diameter five times the monomer diameter and study matrix lengths five times that of the graft chain length or equal graft and matrix chain lengths. We vary the fraction of attractive monomers in the graft and matrix chains, graft–matrix chain composition ratio and the graft–matrix interaction strength, as characterized by the Flory–Huggins interaction parameter between graft and matrix attractive monomers: When is negative, decreasing and/or decreases the extent of grafted layer wetting by matrix chains because the enthalpic driving force for wetting is reduced. As the increases and becomes positive, the extent of wetting decreases gradually till it reaches the wetting of analogous athermal composites. That value of where the extent of wetting is the same as that of an analogous athermal polymer nanocomposite marks the onset of dispersion–aggregation transition. For symmetric graft and matrix chain compositions , the magnitude of and tunes the overall extent of wetting of the grafted particles in the dispersed state but not the dispersion–aggregation transition. Varying the asymmetry of the graft and matrix chain composition (i.e. fG/fM) tunes both the extent of wetting of the grafted layer and the dispersion–aggregation transition.

Adsorption from binary solutions on chemically bonded phases

We use density functional theory to investigate adsorption of liquid mixtures on solid surfaces modified with end-grafted chains. The chains are modelled as freely joined spheres. The fluid molecules are spherical. All spherical species interact via the Lennard-Jones (12-6) potential. The Lennard-Jones (9-3) potential describes interactions of solvent molecules with the substrate. We study the relative excess adsorption isotherms, the structure of surface layer and its composition. The impact of the following parameters on adsorption is discussed: the grafting density, the grafted chain length, interactions of solvent molecules with grafted chains and with the substrate, and the presence of active groups in grafted chains. The theoretical results are consistent with experimental observations.

Well-defined polyurethane-graft-poly(N,N-dimethylacrylamide) copolymer with a controlled graft density and grafted chain length: synthesis and its application as a Pickering emulsion

Well-defined PU-g-PDMA graft copolymers with controlled graft densities and grafted chain lengths could be facilely synthesized by combining the polyaddition reaction with the RAFT polymerization.