A Monte Carlo simulation study of the mechanical and conformational properties of networks of helical polymers. Part II. The effect of temperature

In a recent article (Carri GA, Batman R, Varshney V, Dirama TE. Polymer 2005;46:3809 [17]) we presented a model for networks of helical polymers. In this article we generalize our results to include the effect of temperature and focus on the mechanical, conformational and thermo-elastic properties of the network. We find that the non-monotonic stress-strain behavior observed at constant temperature also appears in the stress-temperature behavior at constant strain. The origin of this behavior is traced to the induction and melting of helical beads by the application of large strains or reduction in temperature. Other conformational properties of the polymer strands are also discussed. We also study the network entropy and heat capacity, and find a non-monotonic dependence on temperature and strain. Our study shows that the entropy is controlled by the helical content whenever the latter is significant. Otherwise, the entropy corresponds to the one of a network made of random coils. In addition, the study of the heat capacity shows that strain shifts the helix-coil transition temperature significantly. Other results are also discussed.     

Latex interpenetrating polymer networks based on acrylic polymers. III. Synthesis variations

Two latex interpenetrating polymer networks, one from a supposedly compatible pair and the other from an incompatible pair of polymers, were prepared by two-stage emulsion polymerization. The synthesis conditions in each case were varied by altering the ratio of the glassy polymer to the rubbery polymer and also by reversing the order of synthesis. Fabricated samples of these latex interpenetrating polymer networks were subjected to hardness, stress-strain, and dynamic mechanical measurements. Hardness and stress-strain measurements showed that, although the second formed polymer dominated the final mechanical properties, the compatibilities of the polymer pairs played an important role. Dynamic mechanical analysis for the inverse systems showed evidence of enhanced mixing.     

Thiol-ene Reaction as Reversible Covalent Bond for the Design of Shape-Memory Polymers

Besides a stable phase, shape-memory polymers require an additional switchable moiety. In addition to thermal transitions and supramolecular interactions, these units can also be based on covalent bonds. Herein, the use of the reversible thiol-ene reaction as reversible cross-linker for the design of shape-memory polymers is demonstrated. A facile route to polymer networks with a thiol-ene acceptor and a comonomer (butyl methacrylate or 2-ethylhexyl methacrylate) cross-linked by dithiols is introduced. The thermal and mechanical properties of the resulting polymers are characterized in detail. Hereby, the polymers feature excellent shape-memory behavior with fixity and recovery rates above 90%. This study shows that the thiol-ene cross-linker can function as both, the stable and the switchable structural moiety rendering the usage of a covalent cross-linker unnecessary. This partial reversibility can also be proven by temperature-depending Raman spectroscopy.     

Supramolecular approach for synthesis and functionalization of cyclic macromolecules

This report highlights our recent findings concerning the synthesis of cyclic polymers based on supramolecular chemistry as well as the stereochemical recognition of helices by a cyclic macromolecule consisting of helical peptides and porphyrins. The first part will focus on an electrostatic self-assembly and covalent fixation strategy for the efficient synthesis of cyclic polymers. It has been shown that a unimeric polymer assembly is formed exclusively from the linear polymer precursor having cyclic ammonium salt end groups carrying dicarboxylate counter ions. An effective synthesis of cyclic polymers has been achieved by the subsequent covalent transformation of the ammonium salt groups. The process has been extended to the synthesis of cyclic macromonomers, which produced a unique polymer network having both covalent and mechanical linkages. The second part will focus on the stereochemical recognition of helices by a cyclic host. α-Aminoisobutyric acid (Aib) peptide-based cyclic hosts having metalloporphyrin units have been synthesized for guest binding and chiroptical sensing. By using these cavities, biologically important “peptide bundling” has been realized through complexation of helical peptide guests, where the three helical chains, two of which are from the host and one from the guest, are harmonized stereochemically in a confined cavity, leading to intense chiroptical outputs in the absorption bands of the metalloporphyrin units. The selective peptide bundling events based on helical senses of the host/guest molecules has also been achieved with a chiral conformational matching.     

Improvements in the stress-strain behavior of urethane rubbers by bimodal network formation

Urethane rubbers find increasing application as binders for composites. Typical examples are solid propellants. The rubber networks are formed by end linking hydroxy-terminated prepolymers with tri- or higher functional isocyanates. A recent trend in solid propellant technology is the replacement of the traditional low-energy binders with “energetic” binders containing nitro, nitrato, or azido groups. Since these energetic polymers create relatively short interchain lengths between the cross-link points, the binders give notoriously poor mechanical properties. Our study demonstrates that significant improvements in the stress-strain behavior are attained with bimodal modifications of the energetic binders, that is, by blending these energetic short chains with very long chains prior to curing into rubbers. Molecular aspects of the improvement have been examined in terms of polymer types, crosslinkers, viscoelastic factors, and solid filler content. Results indicate that the improvement is primarily due to the extent of nonaffine deformation of the bimodal rubber network.     

Photopolymerized Thiol-Ene Systems as Shape Memory Polymers

In this study we introduce the use of thiol-ene photopolymers as shape memory polymer systems. The thiol-ene polymer networks are compared to a commonly utilized acrylic shape memory polymer and shown to have significantly improved properties for two different thiol-ene based polymer formulations. Using thermomechanical and mechanical analysis, we demonstrate that thiol-ene based shape memory polymer systems have comparable thermomechanical properties while also exhibiting a number of advantageous properties due to the thiol-ene polymerization mechanism which results in the formation of a homogeneous polymer network with low shrinkage stress and negligible oxygen inhibition. The resulting thiol-ene shape memory polymer systems are tough and flexible as compared to the acrylic counterparts. The polymers evaluated in this study were engineered to have a glass transition temperature between 30 and 40 °C, exhibited free strain recovery of greater than 96% and constrained stress recovery of 100%. The thiol-ene polymers exhibited excellent shape fixity and a rapid and distinct shape memory actuation response.     

Structure and dynamics of polymeric systems

Two problems that we encounter in the structure formation of polymeric systems are reviewed. One is the dynamics of phase separation of polymer blends and the other is the intramolecular structure formation of associating polymers. In the case of phase separation of polymer blends, we review the model and the simulation method that is suitable for large-scale computer simulations of phase separation of binary fluid mixtures. We also show that simulation results are in quantitative agreement with experimental results of polymer blends. In the case of associating polymers, we treat the intramolecular structure formation in single associating polymers. In order to study the structure formation in polymers with strong attractive interactions, we employ the multicanonical simulation method. We show that a two-step intramolecular conformational transition occurs in periodic associating polymers where associative groups are periodically placed along the chain. With decreasing the temperature, a transition from random-coil conformations to micelles occurs and multiple flower-type micelles are formed via the transition. The number of the associative groups forming a micelle core is limited by the excluded volume effect of loop chains around micelle cores. By this effect, two intramolecular micelles are formed for long polymer chains with 60 bonds via the coil-to-micelle transition. By further decreasing the temperature, we find that another transition, i.e., a micelle-to-micelle transition takes place. At this transition point, the two intramolecular micelles merge into one micelle.     

Structural and mechanical properties of advanced polymer gels with rigid side-chains using coarse-grained molecular dynamics

Computational modeling was utilized to design complex polymer networks and gels which display enhanced and tunable mechanical properties. Our approach focuses on overcoming traditional design limitations often encountered in the formulation of simple, single polymer networks. Here, we use a coarse-grained model to study an end-linked flexible polymer network diluted with branched polymer solvent chains, where the latter chains are composed of rigid side-chains or “spikes” attached to a flexible backbone. In order to reduce the entropy penalty of the flexible polymer chains these rigid “spikes” will aggregate into clusters, but the extent of aggregation was shown to depend on the size and distribution of the rigid side-chains. When the “spikes” are short, we observe a lower degree of aggregation, while long “spikes” will aggregate to form an additional secondary network. As a result, the tensile relaxation modulus of the latter system is considerably greater than the modulus of conventional gels and is approximately constant, forming an equilibrium zone for a broad range of time. In this system, the attached long “spikes” create a continuous phase that contributes to a simultaneous increase in tensile stress, relaxation modulus and fracture resistance. Elastic properties and deformation mechanisms of these branched polymers were also studied under tensile deformation at various strain rates. Through this study we show that the architecture of this branched polymer can be optimized and thus the elastic properties of these advanced polymer networks can be tuned for specific applications.     

Latex interpenetrating polymer networks based on acrylic polymers. II. The influence of the degree of network compatibility on morphology

Two latex interpenetrating polymer networks, one prepared from a pair of supposedly compatible polymers and the other from an incompatible pair, were investigated using transmission electron microscopy and dynamic mechanical analysis. From the results, it was proposed that both interpenetrating polymer networks consisted of latex particles with essentially coreshell morphologies. Evidence for a core-shell structure was more marked for the materials synthesised from the incompatible polymers. The other polymer pair showed indications of a significant amount of mixing.     

Formulation of new synthetic binders: Thermomechanical properties of resin/recycled polymer blends

Synthetic binders are special paving materials manufactured by mixing polymers, resins, and oils. These materials may have improved mechanical properties as compared with the traditional modified bitumen. This work is part of a comprehensive study on the design of synthetic binders with selected mechanical properties. In this sense, upgraded mechanical properties of the final synthetic binder can be attained by understanding and correlating the mechanical properties of its individual constituents as a function of composition and temperature. With this aim, this work deals with the thermomechanical properties of recycled polymer/resin blends, over a wide range of temperature and composition. Recycled polymer/resin blends are thermorheologically complex materials, due to the development of multiphase domains depending on polymer concentration. Thus, these blends show a predominantly gel‐like behavior at high polymer concentrations and a predominantly viscous behavior, with high thermal susceptibility, for low polymer concentrations. The dynamic viscosities of the blends, as a function of polymer concentration and temperature, can be predicted using a logarithmic mixing rule. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Semiinterpenetrating polymer networks based on polyurethane and polyvinylpyrrolidone. I. Thermodynamic state and dynamic mechanical analysis

Semiinterpenetrating polymer networks (semi‐IPNs) based on polyurethane (PU) and polyvinylpyrrolidone (PVP) have been synthesized, and their thermodynamic characteristics, thermal properties, and dynamical mechanical properties have been studied to have an insight in their structure as a function of their composition. First, the free energies of mixing of the two polymers in semi‐IPNs based on crosslinked PU and PVP have been determined by the vapor sorption method. It was established that these constituent polymers are not miscible in the semi‐IPNs. The differential scanning calorimetry results evidence the T_g of polyurethane and two T_g for PVP. The dynamic mechanical behavior of the semi‐IPNs has been investigated and is in accordance with their thermal behavior. It was shown that the semi‐IPNs present three distinct relaxations. If the temperature position of PU maximum tan δ is invariable, on the contrary, the situation for the two maxima observed for PVP is more complex. Only the maximum of the highest temperature relaxation is shifted to lower temperature with changing of the semi‐IPNs composition. It was concluded that investigated semi‐IPNs are two‐phase systems with incomplete phase separation. The phase composition was calculated using viscoelastic properties. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 852–862, 2001     

固化动力学对PU/VER同步互穿网络聚合物性能的影响

互穿网络聚合物(IPNs)是一种特殊的交联聚合物合金.本研究以聚氨酯(PU)和乙烯基酯(VER)预聚物为原料,在室温下合成了互穿网络聚合物.采用傅立叶红外光谱法跟踪了网络形成的动力学过程并进行半定量分析,研究了固化体系对互穿纲络聚合物的摩擦学性能、力学性能及光学性能等的影响.结果显示,两种预聚物在固化过程中虽遵循不同的聚合机理,却相互影响制约.通过改变引发剂和催化剂的配比得到的样品显示出不同特性.当VER引发剂的用量为0.75%,PU催化剂的用量为0.6%时,两网络可基本实现同步互穿;同步互穿网络聚合物显示出良好的耐磨性,在实验条件下涂层寿命可达28.81 min,且力学性能优异,在可见光波长范围内具有良好的透光性, 450 nm处的透光率可达85%.     

Single- and double-stranded helical polymers: synthesis, structures, and functions

Biological macromolecules, such as DNA and proteins, possess a unique and specific ordered structure, such as a right-handed double helix or a single alpha-helix. Those structures direct the sophisticated functions of these molecules in living systems. Inspired by biological helices, chemists have worked to synthesize polymers with controlled helicity, not only to mimic the biological helices but also to realize their functions. Although numerous synthetic polymers that fold into a single-handed helix have been reported, double-stranded helical polymers are almost unavailable except for a few oligomers. In addition, the exact structures of most helical polymers remain obscure. Therefore, the development of a conceptually new method for constructing double-stranded helical polymers and a reliable method for unambiguously determining the helical structures are important and urgent challenges in this area. In this Account, we describe the recent advances in the synthesis, structures, and functions of single- and double-stranded helical polymers from our group and others and provide a brief historical overview of synthetic helical polymers. We found unique macromolecules that fold into a preferred-handed helix through noncovalent bonding interactions with specific chiral guests. During the noncovalent helicity induction process, these guest molecules significantly amplified chirality in a dynamic helical polymer. During the intensive exploration of the helicity induction mechanism, we observed an unusual macromolecular helical memory in dynamic helical polymers. Furthermore, we found that rigid-rod helical poly(phenylacetylene)s and poly(phenyl isocyanide)s showing a cholesteric or smectic liquid crystal self-assemble to form two-dimensional crystals with a controlled helical conformation on solid substrates upon exposure to solvent vapors. We visualized their helical structures including the helical pitch and handedness by atomic force microscopy (AFM). We propose a modular strategy to construct complementary double helices by employing chiral amidinium-carboxylate salt bridges with m-terphenyl backbones. The double-stranded helical structures were characterized by circular dichroism in solution and X-ray diffraction of the crystals or the direct AFM observations. Serendipitously, we found that oligoresorcinols self-assemble into well-defined double helices resulting from interstrand aromatic stacking in water. These oligoresorcinols bound cyclic and linear oligosaccharides in water to form rotaxanes and hetero-double helices, respectively. The examples presented in this Account demonstrate the notable progress in the synthesis and structural determination of helical polymers including single- and double-stranded helices. Not only do we better understand the principle underlying the generation of helical conformations, but we have also used the knowledge of these unique helical structures to develop novel helical polymers with specific functions.     

The effect of water‐soluble polymers on rheology of microfibrillar cellulose suspension and dynamic mechanical properties of paper sheet

Rheological properties of fiber/polymer suspensions and dynamic mechanical analysis (DMA) of paper sheets containing the same polymers were measured. Correlations between viscoelastic properties of suspensions and strength of paper sheet are presented. Rheological properties of suspensions of microfibrillar cellulose (MFC) and a set of water soluble polymers were measured. Rheological properties of these complex fluids vary considerably depending on the added polymer. A suspension of fiber and carboxymethyl cellulose (CMC) exhibits a viscosity higher than the sum of the viscosity of the individual components in the suspension. In contrast, when cationic starch (CS) is used together with the fiber, the yielding behavior rather than the viscosity is characteristic of the suspension. Dynamic mechanical properties of paper sheets containing CMC or CS as additives were studied at different humidity levels. Different yielding behavior observed in oscillatory rheology can be correlated with straining behavior in dynamic mechanical properties. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Latex interpenetrating polymer networks based on acrylic polymers. IV. The influence on mechanical properties of the time of swelling the seed particles with the second monomer

Two latex interpenetrating polymer networks, one based on a partially compatible pair and the other based on an incompatible pair of polymers, were prepared by a two-stage emulsion polymerization. To investigate the effect of swelling the first formed polymer particles with the second monomer, the second stage of the synthesis was conducted after allowing the second monomer to be in contact with the seed latex for specified periods of time. Fabricated samples of these interpenetrating polymer networks were subjected to hardness, stress–strain, and dynamic mechanical measurements. The results showed an enhancement in mixing of the two networks in the case of the partially compatible pair and a detectable increase in the level of mixing for the incompatible pair.     

Gas-filled polymers. III. Mechanical behavior of polycarbonate and poly(vinyl chloride)

Gasification behavior and its effects on mechanical properties were determined for amorphous polycarbonate (PC) and poly(vinyl chloride) (PVC). Nitrogen-gasified PC and PVC exhibit interior regions containing gas bubbles surrounded by surface layers of void-free polymer, while in the helium-gasified polymers no gas bubbles could be observed. Scanning electron microscope (SEM) observations of the bubbles in nitrogengasified PC indicate that the bubble walls are smooth and featureless (in contrast to the diffuse walls with fibrils of polymer extending into the bubbles observed previously in gasified polyethylene). For both PC and PVC, neither the yield stress nor the elongation to fracture showed any appreciable variation between gasified and ungasified material. The lack of a significant effect of gas bubbles on the drawing behavior in these glassy polymers stands in contrast with the pronounced effect noted with semicrystalline polyethylene. The origin of this difference in behavior and its relation to the crystallization process in polyethylene are discussed.     

Blends of triazine‐based hyperbranched polyether with LDPE and plasticized PVC

Triazine‐based hyperbranched polyether was obtained by earlier reported method and blended with low density polyethylene (LDPE) and plasticized poly(vinyl chloride) (PVC) separately to improve some desirable properties of those linear polymers. The properties like processability, mechanical properties, flammability, etc. of those linear polymers were studied by blending with 1–7.5 phr of hyperbranched polyether. The mechanical properties were also measured after thermal aging and leaching in different chemical media. SEM study indicates that both polymers exhibit homogenous morphology at all dose levels. The mechanical properties like tensile strength, elongation at break, hardness, etc. of LDPE and PVC increase with the increase of dose level of hyperbranched polyether. The flame retardant behavior as measured by limiting oxygen index (LOI) for all blends indicates an enhanced LOI value compared to the polymer without hyperbranched polyether. The processing behavior of both types of blends as measured by solution viscosity and melt flow rate value indicates that hyperbranched polyether acts as a process aid for those base polymers. The effect of leaching and heat aging of these linear polymers on the mechanical properties showed that hyperbranched polyether is a superior antidegradant compared to the commercially used N‐isopropyl‐N‐phenyl p‐phenylene diamine. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 648–654, 2007     

Environmental degradation of some polymer blends. Blends of polystyrene with acrylonitrile butadiene styrene,poly(vinyl chloride), and polybutadiene and blends of polybutadiene with poly(vinyl chloride)

One class of polymer/additive which has become increasingly important is polymer blends. In this study the ultimate tensile strength, elongation at break, and the modulus of acrylonitrile–butadiene–styrene, poly(vinyl chloride), polybutadiene and polystyrene and their blends have been studied over an entire binary composition range. We have correlated these mechanical properties to their degradation behavior under natural and accelerated weathering by measurement of various indices during thermal and natural weathering. It was found that during natural weathering the presence of polystyrene in acrylonitrile–butadiene–styrene (ABS) improved the weatherability of ABS; the converse was true when the blends were heated in an air oven at 100°C. It was also found that the weatherability of PB was improved in the presence of polystyrene and large improvement in the rigidity was observed. Similarly, from a measurement of carbonyl index, it was found that PVC has a stabilizing effect on PB. In many cases, the 50:50 composition of the polymers gave the best compromise of good mechanical properties, heat stability, and outdoor weathering. The mechanisms of possible interactions between the degrading polymers are discussed.     

Electroconductive Adhesives: Comparison of Three Different Polymer Matrices. Epoxy,Polyimide and Silicone

Electrically conductive organic adhesives are used in the microelectronics manufacturing industry for the attachment of silicon dies. These adhesives are composite materials which owe their conductivity to the incorporation of silver flakes. Several polymers have been formulated into electrically-conductive adhesives to meet different applications in the microelectronics industry; these are an epoxy resin, a polyimide and a silicone polymer. The purpose of this paper is to examine properties of these die-bonding adhesives in order to determine the advantages or disadvantages of these materials. This study offers a comparison of hardening chemistry, chemical purity, processing, electrical, thermal, and mechanical properties of three conductive adhesives based on an epoxy, a polyimide and a silicone polymer. We discuss correlation of composite properties with the structure of each matrix. The results indicate that the choice of the matrix is dictated by the application for which the electronic grade conductive adhesive is to be used and the desired properties for best reliability and performance.     

Insight into how molecular structures of thiophene-based conjugated polymers affect crystallization behaviors

Combining the structural characterization of solution crystals fabricated from thiophene-based conjugated polymers with different molecular structures and a theoretical investigation of the polymer conformational transformability leads to an interesting discovery of the relationship between the molecular structures and their crystallization behaviors. The chain folding or nonfolding behavior of thiophene-based conjugated polymers in crystallization, an important factor to shape polymer crystals, is determined by their molecular structures, and can be estimated by the inter-ring rotation energy barriers of the polymer backbones. A quantitative theoretical calculation is proposed to evaluate the inter-ring rotation energy barriers, and the values are correlated with the experimentally observed chain folding or nonfolding behavior. The higher percentage of type I inter-ring σ bond (CH₃ and H are at 3 and 3′ position of adjacent aromatic rings, respectively) or the lower average rotation barrier in polymer backbones creates higher capability of polymer conformational transformation and higher tendency of chain folding. Our study provides a valid prediction of the crystallization behavior of thiophene-based conjugated polymers through a theoretical evaluation of conjugated polymer molecular structures, and offers an essential understand of the structure-property relationship of conjugated polymers.