Full control of polymer properties during the two‐stage on‐demand free‐radical photopolymerization of fiber‐reinforced polymers

The effect of a two‐step free‐radical photopolymerization of an acrylate resin on the polymer properties in the presence of glass fibers is studied. It is found that a first irradiation leading to a partial conversion is effective for the fabrication of a preimpregnated glass‐fiber composite, which can be further processed and fully polymerized through a second irradiation. DMA analysis evidences the formation of a first relatively soft polymer embedding unreacted double bonds during the preirradiation. Further process allows the completion of the photopolymerization together with a reinforcement of the polymer network. This obviously affects the final mechanical properties of the photocomposite. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1440–1447     

Spectra analysis for macromolecular systems: Chain‐length effects in polymer particles

A complete set of frequencies for the density of states spectrum g(ω) is obtained for 6000‐atom polymer particles involving all 18,000 degrees of freedom. We efficiently generate the particle using molecular dynamics and molecular mechanics, and systematically analyze the spectra and thermal properties as a function of chain lengths: from one to 100 monomers. The density of states spectrum g(ω) shows a higher number of low‐frequency modes for the polymer particle with a smaller chain length, which results in a higher heat capacity at low temperature. Gibbs free energy was also calculated for the formation of a particle from a crystal as a function of temperature to evaluate thermal stability of those systems. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1812–1823, 2000     

Gas‐free initiators for high‐temperature free‐radical polymerization

Quaternary ammonium persulfates as free‐radical initiators for high‐temperature polymerization are synthesized and their shelf‐life stability investigated. These initiators do not have gaseous byproducts and are therefore ideal for frontal polymerization, a process in which polymeric materials are produced via a thermal front that propagates through the unreacted monomer/initiator solution. Quaternary ammonium persulfate initiators offer additional qualities such as high solubility in organic media and low volatility, which are desirable for frontal polymerization. The initiators are synthesized using two procedures, and the initiating efficacy of the respective products is compared to a peroxide initiator in the frontal polymerization of 1,6‐hexanediol diacrylate. Of all the quartenary ammonium persulfates synthesized, tricaprylmethylammonium (Aliquat) persulfate (APSO) is the best initiator for frontal polymerization because it is soluble in organic media, is very reactive, and does not produce volatile byproducts under decomposition. A study of the decomposition kinetics of APSO is performed, and frontal polymerization is proposed as a quicker analytical technique to assay the purity. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3984–3990, 2000     

Phase separation in dilute polymer solutions at high‐rate extension

In this article the demixing instability and phase segregation in unentangled polymer solutions of semiflexible chains at high‐rate uniaxial extension above the coil to stretched coil transition was studied. Orientation of the stretched chains was described in terms of an effective potential field. Based on the free energy analysis it was shown that the flow‐induced orientation of polymer segments could drastically reduce the energy of their steric repulsion. As a result attraction between the chains gain more importance, and this effect lead to the demixing process and eventual segregation of polymer from the solvent if the strain rate exceeds some critical value. A mean‐field theory was developed to study this flow‐induced phase separation effect. The phase diagrams of the system showing the spinodal and binodal transitions at different extension rates were calculated and discussed. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1066–1073     

Tensile creep of thermoplastics: Time‐strain superposition of non‐iso free‐volume data

The dimensional stability of thermoplastics is characterized by their tensile compliance D(t,σ,T) as a function of time t, stress σ, and temperature T. Creep retardation times are controlled by the free volume available for underlying molecular (segmental) motions. Tensile deformation of polymeric materials, whose Poisson ratio is smaller than 0.5, is accompanied by volume dilatation that can be identified with an increase in available free volume. Consequently, a steady increase in strain with time during tensile creep experiments accounts for shortening of the retardation times. The superposition of as‐received tensile compliance curves is difficult because any point of a curve requires a shift factor along the time axis that differs from those of other points. In this article, tensile creep at a constant stress and temperature is viewed as a non‐iso free‐volume process. A procedure is proposed to transform as‐received data to a pseudo‐iso free‐volume state that eliminates this deficiency and permits construction of a generalized compliance curve for the pseudo‐iso free‐volume state. This curve can be used for calculation of real‐time‐dependent compliance for any selected stress in the range of reversible deformations. As the superposed curve can be generated with several short‐term creep tests (e.g., 100 min) for a series of stresses, the proposed procedure saves experimental time. The effects of physical aging on tensile compliance (observed previously by other researchers) are interpreted in terms of the proposed approach in appendix A . © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 736–748, 2003     

Virtual‐system‐coupled adaptive umbrella sampling to compute free‐energy landscape for flexible molecular docking

A novel enhanced conformational sampling method, virtual‐system‐coupled adaptive umbrella sampling (V‐AUS), was proposed to compute 300‐K free‐energy landscape for flexible molecular docking, where a virtual degrees of freedom was introduced to control the sampling. This degree of freedom interacts with the biomolecular system. V‐AUS was applied to complex formation of two disordered amyloid‐β (Aβ_30–35) peptides in a periodic box filled by an explicit solvent. An interpeptide distance was defined as the reaction coordinate, along which sampling was enhanced. A uniform conformational distribution was obtained covering a wide interpeptide distance ranging from the bound to unbound states. The 300‐K free‐energy landscape was characterized by thermodynamically stable basins of antiparallel and parallel β‐sheet complexes and some other complex forms. Helices were frequently observed, when the two peptides contacted loosely or fluctuated freely without interpeptide contacts. We observed that V‐AUS converged to uniform distribution more effectively than conventional AUS sampling did. © 2015 Wiley Periodicals, Inc.     

Impact of average free‐volume element size on transport in stereoisomers of polynorbornene. I. Properties at 35 °C

This article demonstrates that transport of gases through glassy polymers is significantly influenced not only by the absolute amount but also by the distribution of free volume. Two stereoisomers of polynorbornene with nearly equivalent total free volume, but markedly different average free‐volume sizes, were evaluated. The free‐volume element size was probed with positron annihilation lifetime spectroscopy, wide‐angle X‐ray scattering, gas sorption, and molecular modeling. The permeation, sorption, and diffusion of light gases were measured in each stereoisomer at 35 °C. All analytical techniques indicated that one isomer (labeled as Architecture II) had a larger average free‐volume element size but fewer elements. This isomer also had a very slightly higher bulk density (1.000 vs 0.992 g/cm³ for the other stereoisomer). Architecture II also had gas sorption and diffusion coefficients that were two to three times those of the less dense counterpart. These differences have been attributed to differences in the free‐volume element size available within the polymer matrix. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2185–2199, 2003     

A solvation‐free‐energy functional: A reference‐modified density functional formulation

The three‐dimensional reference interaction site model (3D‐RISM) theory, which is one of the most applicable integral equation theories for molecular liquids, overestimates the absolute values of solvation‐free‐energy (SFE) for large solute molecules in water. To improve the free‐energy density functional for the SFE of solute molecules, we propose a reference‐modified density functional theory (RMDFT) that is a general theoretical approach to construct the free‐energy density functional systematically. In the RMDFT formulation, hard‐sphere (HS) fluids are introduced as the reference system instead of an ideal polyatomic molecular gas, which has been regarded as the appropriate reference system of the interaction‐site‐model density functional theory for polyatomic molecular fluids. We show that using RMDFT with a reference HS system can significantly improve the absolute values of the SFE for a set of neutral amino acid side‐chain analogues as well as for 504 small organic molecules. © 2015 Wiley Periodicals, Inc.     

Carrier ampholyte‐free free‐flow isoelectric focusing for separation of protein

Free‐flow isoelectric focusing (FFIEF) has the merits of mild separation conditions, high recovery and resolution, but suffers from the issues of ampholytes interference and high cost due to expensive carrier ampholytes. In this paper, a home‐made carrier ampholyte‐free FFIEF system was constructed via orientated migration of H⁺ and OH^? provided by electrode solutions. When applying an electric field, a linear pH gradient from pH 4 to 9 (R² = 0.994) was automatically formed by the electromigration of protons and hydroxyl ions in the separation chamber. The carrier ampholyte‐free FFIEF system not only avoids interference of ampholyte to detection but also guarantees high separation resolution by establishing stable pH gradient. The separation selectivity was conveniently adjusted by controlling operating voltage and optimizing the composition, concentration and flow rate of the carrier buffer. The constructed system was applied to separation of proteins in egg white, followed by MADLI‐TOF‐MS identification. Three major proteins, ovomucoid, ovalbumin and ovotransferrin, were successfully separated according to their pI values with 15 mmol/L Tris‐acetic acid (pH = 6.5) as carrier buffer at a flow rate of 12.9 mL/min.     

Atomistic detailed free‐energy landscape of intrinsically disordered protein studied by multi‐scale divide‐and‐conquer molecular dynamics simulation

Calcineurin (CaN) is a eukaryotic serine/threonine protein phosphatase activated by both Ca²⁺ and calmodulin (CaM), including intrinsically disordered region (IDR). The region undergoes folding into an α‐helix form in the presence Ca²⁺‐loaded CaM. To sample the ordered structure of the IDR by conventional all atom model (AAM) molecular dynamics (MD) simulation, the IDR and Ca²⁺‐loaded CaM must be simultaneously treated. However, it is time‐consuming task because the coupled folding and binding should include repeated binding and dissociation. Then, in this study, we propose novel multi‐scale divide‐and‐conquer MD (MSDC‐MD), which combines AAM‐MD and coarse‐grained model MD (CGM‐MD). To speed up the conformation sampling, MSDC‐MD simulation first treats the IDR by CGM to sample conformations from wide conformation space; then, multiple AAM‐MD in a limited area is initiated using the resultant CGM conformation, which is reconstructed by homology modeling method. To investigate performance, we sampled the ordered conformation of the IDR using MSDC‐MD; the root‐mean‐square distance (RMSD) with respect to the experimental structure was 2.23 Å.     

Apparent relaxation‐time spectrum cutoff in dilute polymer solutions: An effect of solvent dynamics

The apparent short time cutoff of the relaxation‐time spectrum at surprisingly long times for polymers in solution is a well known but not yet understood observation. To elucidate its origins we revisit viscoelastic and oscillatory flow birefringence data for solutions and melts of two linear polymers (polystyrene and polyisoprene) and present new measurements of oscillatory flow birefringence of the latter. Previous measurements have suggested that the “flexibility” of both polymers in solution is smaller than in the melt on the basis of the breadth of the relaxation‐time spectrum of the solution as compared with that of the melt. Our new measurements have explored a higher effective frequency range than was previously possible. This has allowed us to observe the effect of the rotational relaxation time of the solvent on the dynamics of the solution at high frequencies. To obtain the polymer global motion contribution, one now needs to subtract from the solution properties a frequency‐dependent complex solvating environment contribution. We show that the decrease in apparent “flexibility” for solutions arises from the presence of a solvent that exhibits a rotational relaxation time and thus simple viscoelastic behavior somewhat near the frequency window of the experiment. Although recent predictions of a model for a chain in a solvent with a single relaxation time are in qualitative agreement with our results, our data suggest that the solution results may reflect the influence of solvent on the development of the “entropic spring” forces at short times. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2860–2873, 2001     

Short‐time stretch relaxation of entangled polymer solutions investigated using full Rouse model predictions

We conduct a systematical investigation into the short‐time stretch relaxation behavior (i.e., shorter than the Rouse time but sufficiently longer than the glassy time) of entangled polymer liquid in single‐step strain flows, on the basis of theory/data comparisons for a broad series of type‐A entangled polymer solutions. First, within existing normal‐mode formulations, the Rouse model predictions on a full‐chain stretch relaxation in single‐step strain flows are derived for a popular 1‐D model proposed within the Doi–Edwards tube model, as well as for the original 3‐D model for nonentangled systems. In addition, an existing formula for the aforementioned 1‐D model that, however, rested upon a consistent‐averaging or the so‐called uniform‐chain‐stretch approximation is simultaneously examined. Subsequently, the previously derived formulas on chain stretch relaxation are directly incorporated into a reliable mean‐field tube model that utilizes the linear relaxation spectrum and the Rouse time constant consistently determined from linear viscoelastic data. It is found that the predictions of the 1‐D model differ substantially from that of the original 3‐D model at short times. Theory/data comparisons further indicate that the 1‐D model without approximations seems able to describe fairly well the nonlinear relaxation data under investigation. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1199–1211, 2006     

End‐group fidelity in nitroxide‐mediated living free‐radical polymerizations

In this study, new nitroxides based on the 2,2,5‐trimethyl‐4‐phenyl‐3‐azahexane‐3‐oxy skeleton were used to examine chain‐end control during the preparation of polystyrene and poly(t‐butyl acrylate) under living free‐radical conditions. Alkoxyamine‐based initiators with a chromophore attached to either the initiating fragment or the mediating nitroxide fragment were prepared, and the extent of the incorporation of the chromophores at either the initiating end or the propagating chain end was determined. In contrast to 2,2,6,6‐tetramethyl piperidinoxy (TEMPO), the incorporation of the initiating and terminating fragment into the polymer chain was extremely high. For both poly(t‐butyl acrylate) and polystyrene with molecular weights less than or equal to 70,000, incorporations at the initiating end of greater than 97% were observed. At the terminating chain end, incorporations of greater than 95% were obtained for molecular weights less than or equal to 50,000. The level of incorporation tended to decrease slightly at higher molecular weights because of the loss of the alkoxyamine propagating unit, which had important consequences for block copolymer formation. These results clearly show that these new α‐H nitroxides could control the polymerization of vinyl monomers such as styrene and t‐butyl acrylate to an extremely high degree, comparable to anionic and atom transfer radical polymerization procedures. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 4749–4763, 2000     

Self‐diffusion of small molecules into rubbery polymers: A lattice free‐volume theory

In the framework of the free‐volume (FV) theory, a new equation was derived for the evaluation of self‐diffusion coefficients of small molecules in polymers above the mixture glass transition temperature. The derivation of the equation turned out to be straightforward once the equivalence between the free volume and the unoccupied volume given by thermodynamic lattice theories is assumed. A parameter evaluation scheme is proposed, which is substantially simpler compared with the conventional Vrentas–Duda approach, even without losing generality. The key assumption is discussed, and its consistency is verified from a numerical viewpoint. A comparison with experimental solvent self‐diffusion coefficients for several solvent/polymer binary systems confirmed that the proposed theory presents good correlative ability over wide temperature and composition ranges. Moreover, the introduced thermodynamic foundation allows one to easily include the pressure effect too. In the frame of the proposed lattice free volume theory, the sizes of the polymer jumping units decrease with temperature and increase with pressure. Such behavior converges with theoretical expectations and opens the way for a predictive FV theory. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 529–540, 2010     

Improvements of network approach for analysis of the folding free‐energy surface of peptides and proteins

Folding network is an effective approach to investigate the high‐dimensional free‐energy surface of peptide and protein folding, and it can avoid the limitations of the projected free‐energy surface based on two‐order parameters. In this article, we present improvements of the effectiveness and accuracy of the folding network analysis based on Markov cluster (MCL) algorithm. We used this approach to investigate the folding free‐energy surface of the beta‐hairpin peptide trpzip2 and found the folding network is able to determine the basins and folding paths of trpzip2 more clearly and accurately than the two‐dimensional free‐energy surface. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Pressure–volume–temperature of molten and glassy polymers

The pressure–volume–temperature (PVT) dependencies of several amorphous polymers (PS, PC, PPE, and PPE/PS 1:1 blend) in the glassy and molten state were studied. The Simha–Somcynsky (S–S) lattice‐hole equation of state (EOS) was used. Fitting the PVT data in the molten state to the EOS yielded the free volume quantity, h = h(T, P), and the characteristic reducing parameters, P*, V*, and T*. The data within the glassy region were interpreted assuming that the latter parameters are valid in the molten and vitreous state, than calculating h = h(T, P) from the experimental values of V = V(T, P). Next, the frozen free volume fraction in the glass was computed as FF = FF(P). The FF values of polystyrene (PS) resins at ambient pressure showed little scattering (FF_P=1 = 0.691 ± 0.008), while their P‐dependencies varied, reflecting the thermodynamic history of the glass formation as well as the PVT measurements protocol. The pressure gradient of T_g was compared with the Ehrenfest relation for the second‐order transition; here also agreement depended on the method of vitrification. The experimental values of FF at ambient pressure decreased with increasing values of the characteristic temperature reducing parameter, T*. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 270–285, 2007.     

Phase diagram for microphase separation transition in poor solvent polymer solutions

In the present paper, we consider the possibility of microphase separation transition in poor solvent polymer solutions. It is shown that this phenomenon can take place if the following two conditions are fulfilled: i) there is a large entropic contribution to the entropy of polymer/solvent mixing, i.e., solvent acts like a plastisizer; ii) this entropic contribution is nonlocal. Both conditions are met below the glass transition temperature for the pure polymer near the so-called Berghmans point when the glass transition curve intersects the liquid-liquid phase separation curve for polymer solutions. The phase diagram for the microphase separation transition is calculated within the framework of weak segregation approximation first proposed by Leibler for block-copolymer systems. The regions of stability of different microdomain structures (lamellar, triangular, body-centered-cubic) are obtained. It is shown that under certain conditions the phase diagram can have two critical points related to the macro- and microphase separation respectively.This paper is dedicated to Prof. E. W. Fischer on the occasion of his 65th Birthday.This work was done in the course of the Humboldt Research Award stay of A.R. Khokhlov at the Max-Planck-Institute for Polymer Research in Mainz. During this stay A.R.K. greatly benefited from numerous discussions with Professor E.W. Fischer who introduced him to the fascinating field of glass transition in polymer systems and formulated several new directions for future research.     

Heterochain model for simultaneous long‐chain branching and crosslinking. II. Application to free‐radical polymerization

The matrix formula developed in the context of hetrochain theory, M?_w = M?_wp + WF ( I ? M )^?1 S , was applied to describe the molecular weight development during free‐radical homopolymerization. All of the required probabilistic parameters are expressed in terms of the kinetic‐rate constants and various concentrations. In free‐radical polymerization, the primary chains are formed consecutively, and the number of heterochain types, N, is extrapolated to infinity. Practically, such extrapolation can be conducted on the basis of the calculated values for only three different N values with sufficient accuracy. This matrix formula is valid regardless of the chemical and reactor systems used, as long as the primary chain‐connection statistics is considered Markovian. The gel point can be determined simply by solving an equation det( I ? M ) = 0. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2791–2800, 2004     

Theoretical simulation of thermally induced phase separation in a main‐chain liquid‐crystalline polymer solution

Phase diagrams of main‐chain liquid‐crystalline polymer (MCLCP) solutions have been calculated self‐consistently on the basis of a simple addition of the Flory–Huggins free energy for isotropic mixing, the Maier–Saupe free energy for nematic ordering, and the Flory free energy for chain rigidity of the MCLCP backbone. The calculated phase diagram is an upper critical solution type overlapping with the nematic–isotropic transition. The phase diagram consists of liquid–liquid, liquid–nematic, and pure nematic regions. Subsequently, the dynamics of thermally induced phase separation and morphology development have been investigated by the incorporation of the combined free energy density into the coupled time‐dependent Ginzburg–Landau (model C) equations, which involve conserved compositional and nonconserved orientational order parameters. The numerical calculations reveal a variety of the morphological patterns arising from the competition between liquid–liquid phase separation and nematic ordering of the liquid‐crystalline polymer. Of particular interest is the observation of an inflection in the growth dynamic curve, which may be attributed to the nematic ordering of the MCLCP component, which leads to the breakdown of the interconnected domains. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 913–926, 2003