Negative thermal expansion of poly(vinylidene fluoride) and polyethylene tie molecules: A molecular dynamics study

The mechanism of thermal actuation for poly(vinylidene fluoride) (PVDF) and polyethylene (PE) tie molecules has been investigated using molecular dynamics simulations. Tie molecules are found in semicrystalline polymers and are polymer chains that link two (or more) crystalline lamellae, allowing for the transfer of force between these regions. A novel simulation technique has been developed to enable measurement of changes in the tie molecule length upon heating. We investigate the dependence of the percentage actuation observed upon heating, on the external applied force that stretches the tie molecules, the temperature range used for heating as well as the length and the number of tie molecules. Two molecular level mechanisms for actuation are identified. An entropically driven mechanism occurs at low applied forces and is applicable to all flexible polymers. A second mechanism due to conformational changes is observed for PVDF but not for PE at intermediate applied forces. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2223–2232     

Tensile yield of isotactic polypropylene in terms of a lamellar‐cluster model

In this study, the structural factors controlling the yield in isotactic polypropylene materials were theoretically investigated. To describe the yielding behavior of spherulitic polypropylenes, we introduced a new structural unit, lamellar clusters, which are several stacked lamellae bound by tie molecules. It was shown that tie molecules between adjacent lamellar clusters produce a concentrated load acting on the cluster surface, leading to the bending deformation of the lamellar clusters. The yielding behavior can be explained if one assumes that the disintegration of the lamellar clusters occurs when the elastic‐strain energy stored by the bending deformation reaches a critical value. By applying the fracture theory of composites to a system consisting of lamellar clusters and tie molecules, we found the yield stress σ_y to be proportional to , in which E_Y is the Young's modulus and U_y is the yield energy. The proportional coefficient between σ_y and depends only on the cluster size and tie‐molecule density, so this proportionality is expected to be true for other spherulitic semicrystalline polymers such as polyethylenes, being independent of temperature and tensile rate. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1037–1044, 2000     

Tear anisotropy in films blown from polyethylenes of different macromolecular architectures

Blown films of different types of polyethylenes, such as branched low‐density polyethylene (LDPE) and linear high‐density polyethylene (HDPE), are well known to tear easily along particular directions: along the film bubble's transverse direction for LDPE and along the machine direction (MD) for HDPE. Depending on the resin characteristics and processing conditions, different structures can form within the film; it is therefore difficult to separate the effects of the crystal structure and orientation on the film tear behavior from the effects of the macromolecular architecture, such as the molecular weight distribution and long‐chain branching. Here we examine LDPE, HDPE, and linear low‐density polyethylene (LLDPE) blown films with similar crystal orientations, as verified by through‐film X‐ray scattering measurements. With these common orientations, LDPE and HDPE films still follow the usual preferred tear directions, whereas LLDPE tears isotropically despite an oriented crystal structure. These differences are attributed to the number densities of the tie molecules, especially along MD, which are considerably greater for linear‐architecture polymers with a substantial fraction of long chains, capable of significant extension in flow. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 413–420, 2005     

Non-iterative and exact method for constraining particles in a linear geometry

We present a practical numerical method for evaluating the Lagrange multipliers necessary for maintaining a constrained linear geometry of particles in dynamical simulations. The method involves no iterations and is limited in accuracy only by the numerical methods for solving small systems of linear equations. As a result of the non-iterative and exact (within numerical accuracy) nature of the procedure, there is no drift in the constrained geometry, and the method is therefore readily applied to molecular dynamics simulations of, for example, rigid linear molecules or materials of non-spherical grains. We illustrate the approach through implementation in the commonly used second-order velocity-explicit Verlet method. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 911-916, 2005     

Environmental stress cracking resistance of polyethylene: The use of CRYSTAF and SEC to establish structure–property relationships

Nineteen commercial high‐density polyethylene resins made with different polymerization processes and catalyst types were analyzed by high‐temperature size exclusion chromatography and crystallization analysis fractionation. The information obtained with these characterization techniques on the polymer chain structure was correlated to environmental stress cracking resistance. Environmental stress cracking resistance increases when the molecular weight and concentration of polymer chains that crystallize in trichlorobenzene between 75 and 85 °C increase. Polymer chains present in this crystallization range are assumed to act as tie molecules between crystal lamellae. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1267–1275, 2000     

Synthesis,characterization, and application of melamine‐based dendrimers supported on silica gel

Dendrimers based on melamine have been covalently attached to silica gel with convergent and divergent approaches. These approaches yield different products. Each step of the synthesis can be monitored with Fourier transform infrared spectroscopy, thermal gravimetric analysis, and X‐ray photoelectron spectroscopy. The organic molecules can be isolated by chemical etching the organosilica material with HF and then analyzed by mass spectrometry. The convergent approach, which relies on the solution‐phase synthesis of the dendrimer before its attachment onto silica gel, produces materials that are ～20% w/w organic regardless of the generation of the dendrimer used in the preparation of these composites and without apparent defects in the dendrimer structure. Materials prepared with the divergent approach show increasing percentages of organic material with each generation, but the dendrimers show structural defects including incomplete branching. In a survey of sequestration potential, materials obtained with the divergent approach removed more atrazine from solution than the more homogeneous materials obtained from the convergent approach. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 168–177, 2005     

An analytical technique for measuring relative tie-molecule concentration in polyethylene

While consideration of the crystalline domains have long dominated research in understanding the properties of semicrystalline polymers, a satisfactory understanding of crack growth in these materials can only be realized by developing corresponding analytical tools to characterize the amorphous region. Since slow stable cracks in these materials preferentially form between crystalline lamellae, the role of tie molecules—the amorphous chains that bridge crystalline lamellae—are particularly important in this regard. Unfortunately, there is no method readily available for quantitative assessment of tie molecules. Through deformation and subsequent chlorination of polyethylene films, it is demonstrated that infrared dichroism can be used to determine relative tie-molecule concentration. Using this technique, one can a priori predict which resin in a series having comparable densities but widely varying molecular weights or comonomer distributions exhibits better crack resistance.     

Role of tie molecules in the yielding deformation of isotactic polypropylene

We examined the effects of the tie‐molecule fraction on the yielding behavior of two isotactic polypropylenes, one having little ethylene content and the other as the homopolymer with no ethylene. The tie‐molecule fraction of the samples used in this study was controlled by blending ethylene‐α‐olefin of an α‐olefin content above 50 mol % in the blend of which the copolymers were incorporated into the amorphous regions of polypropylene (PP). An excellent linear relationship was observed between the measured yield stress and the tie‐molecule fraction estimated from the Huang–Brown model, suggesting that the tie‐molecule fraction and lamellar stiffness determine whether the lamellar fragmentation is easily activated or not, depending on the PP composition. Furthermore, an extended Huang–Brown model predicts a lamellar cluster connecting about five lamellae, which has a potential to account for morphological transformation of the spherulitic structure into a fibrillar one. Comparing the immiscible blends showing a phase‐separated morphology with the partially miscible blends mentioned above, the yield stress was lowered by the presence of rubber phase, apparently in a similar manner; but the yielding processes were clearly discriminated between both cases when the yield stress was plotted against the tie‐molecule fraction. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 357–368, 1999     

Preparation and characterization of the chromatographic properties of ethylene glycol dimethacrylate/divinylbenzene polymeric microspheres

The preparation of porous ethylene glycol dimethacrylate/divinylbenzene polymeric microspheres is presented. These materials were investigated as column packings for high‐performance liquid chromatography. Their porous structures in both the dry and wet states were also studied. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3049–3058, 2005     

Synthesis of carbohydrate‐segmented polydimethylsiloxanes by hydrosilylation

Carbohydrate‐modified polysiloxanes have been presented several times within the last decade. In this work, a new route to carbohydrate‐segmented polysiloxanes is presented. A series of allyl‐group‐containing bifunctional carbohydrate derivatives was synthesized and reacted with hydrodimethylsilyl‐terminated polysiloxane in hydrosilylation reactions with Speier's catalyst. The carbohydrate monomers and the resulting materials were fully characterized with ¹H and ¹³C NMR spectroscopy. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3814–3822, 2005     

Hybrid metal–polymer composites from functional block copolymers

The combination of metals and polymers in hybrid materials is a research area of great current interest. A number of methods for controlling the positioning of metallic species within polymer matrices on the nanometer scale have been developed. This highlight focuses on the use of functional block copolymers for the localization of metal species, especially nanoparticles, on the nanometer scale through block copolymer phase segregation. Research from the author's group on the use of alkyne‐functional block copolymers for the preparation of cobalt‐containing materials is discussed in this context. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4323–4336, 2005     

Structure and properties of the δ‐form and mesophase of syndiotactic polystyrene membranes prepared from different organic solvents

Syndiotactic polystyrene (sPS) membranes were prepared with different organic solvent systems and compared to get the information about the δ‐form complexing behavior of sPS. Further, the guest molecules included in the clathrate δ form of sPS are removed by stepwise extraction method. The conformational changes during the TTGG helical formation of sPS/organic solvent systems have been identified by FTIR spectroscopy, and it was concluded that the TTGG helices were constructed in regular sequences, which depends on the nature of the respective solvents. The TTGG content in the mesophase is found to be increased by removing the guest molecules. The structural changes of sPS/organic solvent systems have been characterized by WAXD analysis. Moreover, the different clathrate structures were found and showed the different crystalline reflections in the WAXD profiles, which are significantly changed with the kind of guest solvent included in sPS. The content of solvents in the clathrated sPS and the desorption temperatures were determined by thermal analysis. The resulted mesophase of sPS membrane contains the nanoporous molecular cavities that depend on the size of the guest molecule. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1873–1880, 2005     

Effect of orientation on the free volume and oxygen transport of a polypropylene copolymer

The effect of uniaxial orientation on the free‐volume and oxygen‐transport properties of a propylene copolymer with 4.5 wt % ethylene was examined. The free‐volume hole size and hole density were measured with positron annihilation lifetime spectroscopy. Subsequently, the free‐volume characteristics were correlated with the oxygen‐transport properties. Orientation had only a small effect on the total amount of free volume: a small increase in the hole density was offset by a small decrease in the hole size. As a result, the oxygen solubility and amorphous‐phase density were unchanged by orientation. However, a pronounced decrease in the oxygen diffusivity when the draw ratio exceeded 6 indicated a change in the dynamic free volume. This was attributed to an increasing number of taut tie chains, which retarded oxygen diffusion. The reduced amorphous chain mobility was also manifest in the increased glass‐transition temperature, decreased bulk thermal expansivity, and decreased expansivity of free‐volume holes. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1230–1243, 2005     

The synthesis of novel aliphatic bis(acenaphthelene) reactive monomers

Novel aliphatic‐bridged bis(acenaphthalene)s are synthesized and evaluated as potential precursors to high‐temperature reactive monomers. It was found that as the length of the aliphatic bridging unit increased, the thermal stability of bis(acenaphthalene) increased and melting point decreased. The use of an alternative route for the introduction of the reactive vinyl group resulted in a dramatic increase in monomer purity. The increase in monomer purity resulted in the creation of a processing window that enabled samples to be melt cast prior to polymerization, greatly improving their suitability as high‐temperature polymers. Polymerized samples were found to have high thermal stability, indicating they have potential as high temperature materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43:5072–5082, 2005     

A mesoscopic Archimedean tiling having a new complexity in an ABC star polymer

The Archimedean tiling (3².4.3.4) is a regular but complex polygonal assembly of equilateral triangles and squares. This tiling pattern with mesoscopic repeating distance has been found for an ABC star‐branched three‐component polymer composed of polyisoprene, polystyrene, and poly(2‐vinylpyridine). In this structure the environment of a molecule splits into multiple sites and two microdomains with different sizes and shapes are formed for one component. This complexity is the first observation in complex polymer systems and can lead to a new type of mesoscale self‐organization. The tiling pattern has been observed for the other materials on much shorter length‐scale; therefore, the experimental fact observed in the present study is demonstrating that the complexity is universal over different hierarchies. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2427–2432, 2005     

Synthesis and self-organization of rod–dendron and dendron–rod–dendron molecules

We synthesized molecules containing one or two dendritic segments and a rigid-rod-like segment with their structures in the solid state. The molecules with rod–dendron or dendron–rod–dendron architecture had biphenyl ester rigid segments and 3,4,5 tris(n-dodecyloxy)benzoate of first or second generation as their dendritic segments. The variables investigated included the rod segment length as well as dendron generation, and all materials obtained were characterized by optical microscopy, differential scanning calorimetry, and X-ray scattering. Depending on the size of the rod segment and generation number of the dendritic segment, molecules organized into smectic, columnar, or cubic phases, and the symmetries observed were dominated by the anisotropic rod–rod interactions. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3501–3518, 2003     

High resolution electron microscopy of ordered polymers and organic molecular crystals: Recent developments and future possibilities

High Resolution Electron Microscopy (HREM) has made it possible to directly image the detailed organization of a variety of polymers and organic molecular crystals. For organic materials it is imperative to use low dose techniques that minimize the structural reorganizations that inevitably occur during electron beam irradiation. This article reviews recent developments in low dose HREM from our own laboratory and elsewhere. The developments in closely related microstructural characterization techniques are also reviewed. In the future, the ability to correct the spherical aberration of the objective lens, the use of low voltages to increase contrast, and the use of time‐resolved techniques are expected to open new avenues for the ultrastructural investigations of organic materials. New sample preparation techniques, such as the ability to make thin samples by focused ion beam (FIBs), to cut samples with an oscillating diamond knife, and to more conveniently prepare cryogenically solidified specimens, are also expected to be of increasing importance. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1749–1778, 2005     

Molecular structure of agarose chains in thermoreversible hydrogels revealed by means of a fluorescent probe technique

Thermoreversible hydrogels of agarose with both high and low molecular weights were studied by means of a fluorescence probe method using 1‐anilino‐8‐naphthalene sulfonic acid (ANS). The fluorescence spectra of ANS in agarose/water changed markedly with agarose concentration. Analysis of the spectra shows that this change is not due to a shift of the whole spectrum, but that there are two fluorescent components of ANS in addition to the fluorescence peak at 542 nm due to free ANS molecules in water. The fluorescence component at 461 nm is assumed to be from ANS intercalated within either a single 3₁ helix‐form agarose chain or nonpolar region produced by chemical defect, and the fluorescence component between 507 and 522 nm is assumed to be from ANS in aggregated regions of agarose chains having a loose helical structure. There is no doubt that a certain number of water molecules are included in the aggregated region. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 680–688, 2005     

Rheological properties of crystallizing polylactide: Detection of induction time and modeling the evolving structure and properties

Linear viscoelastic properties of polymer melts are highly sensitive to any structural changes, including molecular weight changes and the formation and growth of crystallites. Here, we make use of this sensitivity to study the homogeneous crystallization of polylactide. As this polymer is rather quickly susceptible to thermal degradation even at moderate temperatures, it is essentially impossible to study homogeneous crystallization in the absence of degradation. Thus, the evolution of complex viscosity of a polylactide due to thermal degradation in the absence of crystallization was studied. A simple empirical model is used to characterize the variation of complex viscosity due to thermal degradation and to determine the induction time of homogeneous crystallization at wide range of degrees of supercooling. Next, the evolution of complex viscosity due to crystallization was measured at several temperatures. Based on the results, a phenomenological model describing the viscosity evolution during homogeneous crystallization is proposed and validated. Finally, the linear viscoselastic data in the early stages of crystallization are shown to be consistent with gelation due to the formation of a network of tie molecules between spherulites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 812–822, 2010     

Synthesis and characterization of poly(DTC‐b‐PEG‐b‐DTC) triblock and poly(TMC‐b‐DTC‐b‐PEG‐b‐DTC‐b‐TMC) pentablock copolymers and kinetics of the polymerization

Dihydroxyl capped biodegradable poly(DTC‐b‐PEG‐b‐DTC) (BCB) triblock copolymer and poly(TMC‐b‐DTC‐b‐PEG‐b‐DTC‐b‐TMC) (ABCBA) pentablock copolymer have been synthesized by PEG and BCB copolymer as macroinitiator in the presence of yttrium tris(2,6‐di‐tert‐butyl‐4‐methylphenolate). The copolymers without random segments have been thoroughly characterized by ¹H, ¹³C‐NMR, SEC, and DSC. Molecular weights of the obtained copolymers are dependent on the amount of PEGs and coincide with the theoretical values. The exchange reaction of yttrium alkoxide and hydroxyl end group is essential for controlling the products' molecular weight. Their thermal behaviors are relevant to the chain lengths of PEG and PDTC segments. The Monte Carlo method has been developed to estimate the chain propagation constant and exchange reaction constant. In average, one exchange reaction will occur after approximately six monomer molecules insert into the growing chain. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1787–1796, 2005