Polymorphism of a semi-crystalline diketopyrrolopyrrole-terthiophene polymer

Few semiconducting polymers are known that possess more than one semi-crystalline structure. Guidelines for rationalizing or creating polymorphism in these materials do not exist. Two different semi-crystalline polymorphs, β₁ and β₂, and an amorphous α phase have recently been identified for alternating diketopyrrolopyrrole-quaterthiophene copolymers (PDPP4T). The polymorphs differ structurally by the π–π stacking distance, and electronically by the optical bandgap and charge carrier mobility. Here we investigate the corresponding terthiophene (PDPP3T) derivatives, to study the effect of the relative orientation of adjacent DPP units on the polymorphism. In PDPP3T, the relative orientation of DPP units alternates along the chain, while in PDPP4T it is constant. We show that the two polymorphs, β₁ and β₂, can also be generated for a PDPP3T polymer in solution and thin film. Interestingly, compared to PDPP4T, more solvents can induce the two distinct semi-crystalline polymorphs for PDPP3T via a β₁ → α → β₂ polymorphic transition.     

Anisotropic plasticity and chain orientation in polymer glasses

The anisotropic mechanical response of oriented polymer glasses is studied through simulations with a coarse-grained model. Systems are first oriented by uniaxial compression or tension along an axis. Then the mechanical response to subsequent deformation along the same axis or along a perpendicular axis is measured. As in experiments, the flow stress and strain hardening modulus are both larger when deformation increases the degree of molecular orientation produced by prestrain, and smaller when deformation reduces the degree of orientation. All stress curves for parallel prestrains collapse when plotted against either the total integrated strain or the degree of molecular orientation. Stress curves for perpendicular prestrains can also be collapsed. The stress depends on the degree of strain or molecular orientation along the final deformation axis and is independent of the degree of orientation in the perpendicular plane. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1473–1482, 2010     

Conformational Entropy as a Means to Control the Behavior of Poly(diketoenamine) Vitrimers In and Out of Equilibrium

Control of equilibrium and non‐equilibrium thermomechanical behavior of poly(diketoenamine) vitrimers is shown by incorporating linear polymer segments varying in molecular weight (MW) and conformational degrees of freedom into the dynamic covalent network. While increasing MW of linear segments yields a lower storage modulus at the rubbery plateau after softening above the glass transition (T_g), both T_g and the characteristic time of stress relaxation are independently governed by the conformational entropy of the embodied linear segments. Activation energies for bond exchange in the solid state are lower for networks incorporating flexible chains; the network topology freezing temperature decreases with increasing MW of flexible linear segments but increases with increasing MW of stiff segments. Vitrimer reconfigurability is therefore influenced not only by the energetics of bond exchange for a given network density, but also the entropy of polymer chains within the network.     

Computational study of the transamination reaction in vinylogous acyls: Paving the way to design vitrimers with controlled exchange kinetics

One of the key points in the design of vitrimers is controlling the associative exchange kinetics. One common chemistry used in vitrimers is based on the dynamic amine exchange reaction of vinylogous acyl compounds in presence of free amine. Understanding the reaction mechanism is essential to assist the optimization of the reaction conditions as well as the molecular structure of the reactant compounds in the pursuit of new materials. In this work, a computational study has been performed to explore different reaction mechanisms in neutral, acidic and in basic conditions or in the presence of Lewis acids, as well as the effect of chemical modifications in the exchange reaction. The results reveal that the formation of hydrogen bonds are a key feature and that the vinylogous urea improves the transamination compared to vinylogous urethane. The esteric hindrance of the amino group in the vinylogous compound also plays an important role. Finally, the nature of the free amine can improve the reactivity by equilibrating two contrary effects: the basicity favors the nucleophilic attack and the conjugated acidity favors the protonation. The findings of this theoretical work shed light in the design of new vitrimers with controlled exchange kinetics by chemical modifications.     

Optimization of clamping force for low-viscosity polymer injection molding

Due to its low cost and high efficiency, injection molding is used for the mass production of many plastic products nowadays. However, when processing low-viscosity plastic materials, i.e., materials with an excellent fluidity, an inappropriate setting of the clamping force often results in a poor appearance and dimensional accuracy of the final product. Thus, operators usually take the upper limit of the clamping force as a default in setting up the machine in an attempt to improve the quality of the molded parts. However, such an approach shortens the machine and mold life, increases the energy consumption, and leads to poor air venting. Consequently, more scientific methods for determining the clamping force setting are required. To meet this demand, the present study proposes a clamping force search methodology for determining the optimal clamping force setting of a hydraulic cylinder clamping injection molding machine in the processing of low-viscosity plastics such as thermoplastic polyurethane (TPU) and polypropylene (PP). Based on the characteristic extracted from the sensing tie-bar elongation profile under different clamping force settings, a regression analysis on these data points is implemented to seek for an optimal clamping force. The experimental results show that for an injection molding machine with a hydraulic cylinder clamping mechanism, the effect of the mold temperature on the clamping force is sufficiently small to be ignored, which has an impact on the toggle type clamping unit. Furthermore, compared to traditional methods, the optimal clamping force obtained using the method proposed in the present study results in a significant improvement in the yield rate. Overall, the results confirm that for low-viscosity polymer resins, the optimal clamping force determined using the proposed method results in a higher and more consistent quality of the molded parts than that achieved using the proper clamping force setting for ordinary-viscosity resins.     

Cyclic nonlinear behavior of a glassy polymer using a contact method

The effects of repeated large strain shear cycles on the dynamics of a glassy acrylate polymer are investigated using an original contact method. It is based on the measurement of the shear properties of thin (about 50 μm) polymer films geometrically confined within contacts between elastic substrates. Under small amplitude (300 nm–10 μm) oscillating lateral displacements, friction at the contact interface can be neglected and the measurement of the contact lateral response thus provides information about the rheology of the sheared polymer film. Using this approach, the complex shear modulus of the polymer film can be measured both in the linear (viscoelastic) and in the nonlinear regimes. The investigations are focused on the changes in mechanical properties induced in a large strain regime where the polymer glass is cyclically sheared up to the yield point. During the application of large strain cycles, the mechanical response of the polymer glass slowly evolves toward a quasi stabilized state which is described from the measurement of an apparent–strain dependent–complex shear modulus. When the applied strain is increased by a tenfold factor, this apparent shear modulus decreases by about one decade. These underlying changes are investigated from a consideration of the time dependent linear viscoelastic properties after the mechanical stimulus. Both mechanical rejuvenation and recovery (ageing) effects are evidenced. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

New trends in industrial polymer research

In the past decades a shift in paradigm took place in industrial polymer research. Only a few new polymers were developed based on new monomeric building blocks. The main focus is now on tailoring “new polymers” with well-defined structure and properties based on a set of low cost “old” monomers using controlled polymerization mechanisms.     

Reactive compatibilization of polyamide‐6 (PA 6)/polybutylene terephthalate (PBT) blends by a multifunctional epoxy resin

A multifunctional epoxy resin has been demonstrated to be an efficient reactive compatibilizer for the incompatible and immiscible blends of polyamide‐6 (PA 6) and polybutylene terephthalate (PBT). The torque measurements give indirect evidence that the reaction between PA and PBT with epoxy has an opportunity to produce an in situ formed copolymer, which can be as an effective compatibilizer to reduce and suppress the size of the disperse phase, and to greatly enhance mechanical properties of PA/PBT blends. The mechanical property improvement is more pronounced in the PA‐rich blends than that in the PBT‐rich blends. The fracture behavior of the blend with less than 0.3 phr compatibilizer is governed by a particle pullout mechanism, whereas shear yielding is dominant in the fracture behavior of the blend with more than 0.3 phr compatibilizer. As the melt and crystallization temperatures of the base polymers are so close, either PA or PBT can be regarded as a mutual nucleating agent to enhance the crystallization on the other component. The presence of compatibilizer and in situ formed copolymer in the compatibilized blends tends to interfere with the crystallization of the base polymers in various blends. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 23–33, 2000     

Oscillatory shear-accelerated exfoliation of graphite in polypropylene melt during injection molding

In this study, good dispersion status of graphite in a nonpolar, intractable polymer, i.e. polypropylene (PP), was realized in melt processing by using a specific dynamic packing injection molding (DPIM) technique. The exfoliation extent of graphite increased remarkably from the skin zone to the core zone of the molded part, as confirmed by combination of WAXD, SEM and TEM analyses, indicating an accelerated exfoliation occurred during the DPIM processing. This phenomenon is due to decreased melt flow channel and increased melt viscosity as the solidification takes place from the wall into the center, which leads to greatly increased shear force. The good dispersion of graphite results in obvious reinforcements of both tensile strength and impact strength by adding moderate amount of graphite. The present study proposes a promising route for realizing the large-scale fabrication of structural parts of polymer/exfoliated-graphite nanocomposites with excellent mechanical properties.     

Filler networks in elastomers

Elastomers are soft materials that can be reinforced by dispersing into them nanosized solid particles. Common examples of the latter are silica or carbon black aggregates. However, the mechanism of reinforcement is still not yet fully understood. Our work consists in investigating by small-angle X-ray scattering (SAXS) the structure of the aggregate network spreading throughout the matrix in the initial sample and its modification during and after straining (elongation). The goal is to relate the macroscopic mechanical behaviour with the structure of the aggregate network. The present paper is a qualitative overview of recent results obtained on well defined composites.     

Nanocomposites of polyethylene with polyhedral oligomeric silsesquioxane: from thermoplastics to vitrimers through silyl ether metathesis

In this contribution, we reported the transformation of linear polyethylene-polyhedral oligomeric silsesquioxane (PE-POSS) copolymers into PE vitrimers through silyl ether metathesis. First, the PE-POSS copolymers were synthesized via the ring-opening metathesis polymerisation (ROMP) of cyclooctene with heptaisobutyl POSS-norbornene followed by subsequent hydrogenation reaction. The PE-POSS copolymers were microphase -separated, and the POSS cages were aggregated into the microdomains with the size of 10–20 nm. The thermoplastic PE-POSS nanocomposites were transformed into the PE vitrimers via direct silyl ether metathesis, which was catalyzed with zinc trifluoromethyl sulfonate [Zn(OTf)₂]. It was found that the cross-linking with the dynamic covalent bonds occurred within the POSS microdomains. The cross-linking significantly influenced the morphologies and thermal and mechanical properties of the materials. More importantly, the cross-linked PE products displayed the self-adaptivity at elevated temperature, that is, the PE-POSS thermoplastics were transformed into PE-polysilsesquioxane vitrimers. Because of the formation of cross-linked networks, the PE vitrimers were endowed with shape memory properties. In the meantime, the permanent shapes of the shape memory PEs can readily be reprogrammed by taking advantage of the self-adaptivity of the vitrimers.     

Transfer of load from matrix to fiber in self-reinforced polymer composites

The tensile properties of self-reinforced polypropylene composites, obtained by rapid extension of an isotactic polypropylene/atactic polypropylene melt, have been measured and correlated to morphological parameters derived from x-ray experiments. The longitudinal morphology of the core-fibrils is found to be independent of sample composition, while the lateral thickness of the fibers varies between 220 Å for iPP and 110 Å for the blend containing 50 wt.% aPP. Critical fiber lengths, as a function of sample composition and the elastic modulus and the yield stress of the fibers, could be determined. While the number of corefibrils increases with mass fraction of atactic polypropylene, the length of the lateral interface between fiber and matrix and the interface volume decreases with increasing aPP mass fraction. It is shown that this interface is responsible for the mechanical behavior of the composite by effecting the transfer of load from matrix to fiber.     

Measurement of size‐dependent glass transition temperature in electrospun polymer fibers using AFM nanomechanical testing

The glass transition temperature (T_g) of individual electrospun polymer polyvinyl alcohol fibers of varying diameter was measured using atomic force microscopy (AFM) based nanomechanical thermal analysis. Indentation and bending of individual electrospun fibers using AFM allowed the calculation of the elastic modulus of the polyvinyl alcohol (PVA) fibers across a range of different temperatures. The elastic modulus of electrospun PVA fibers was observed to decrease significantly when passing through T_g, which allowed accurate determination of T_g. The T_g of electrospun PVA fibers was shown to decrease for smaller fiber diameters especially for fiber diameters below 250 nm. This size‐dependent glass transition behavior of electrospun PVA fibers is indicated as being due to polymer chain confinement. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Investigation of structural changes in semi-crystalline polymers during deformation by synchrotron X-ray scattering

The mechanical behavior of polymer materials is strongly dependent on polymer structure and morphology of the material. The latter is determined mainly by processing and thermal history. Temperature-dependent on-line X-ray scattering during deformation enables the investigation of deformation processes, fatigue and failure of polymers. As an example, investigations on polypropylene are presented. By on-line X-ray scattering with synchrotron radiation, a time resolution in the order of seconds and a spatial resolution in the order of microns can be achieved. The characterization of the crystalline and amorphous phases as well as the study of cavitation processes were performed by simultaneous SAXS and WAXS. The results of scattering experiments are complemented by DSC measurements and SEM investigations. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1574–1586, 2010     

Preparation and properties of organic–inorganic hybrid gel films based on polyvinylpolysilsesquioxane synthesized from trimethoxy(vinyl)silane

Polyvinylpolysilsesquioxane (PVPS) organic–inorganic hybrid gel films comprising polyethylene and siloxane backbone linkages were prepared through two routes: trimethoxy(vinyl)silane (VTS) was first subjected to radical polymerization of the vinyl groups, followed by acid‐catalyzed hydrolytic polycondensation of the trimethoxysilyl groups (route A) to afford PVPS; in the second route PVPS was prepared in reverse order (route B). PVPS gel films were transparent and homogeneous. We find that the mechanical and heat‐resisting properties correlate both to the degree of polymerization and the degree of cross‐linking. Copyright © 2003 John Wiley & Sons, Ltd.     

Study of the segmental dynamics in semi-crystalline poly(lactic acid) using mechanical spectroscopies

The glass transition of poly(L-lactic acid) (PLLA) occurs not far above typical service temperatures (room or body temperatures) which has consequences on the material properties during its use, such as damping or the occurrence of structural relaxation. This work aims at characterising the glass transition dynamics of a semi-crystalline PLLA using both dynamic mechanical analysis (DMA) and thermally stimulated recovery (TSR). The main viscoelastic parameters have been characterised at 1 Hz using DMA and the master curve obtained after isothermal experiments at different temperatures provided a full picture of the solid-state rheological behaviour throughout a wide frequency range. The activation energies calculated from the shift factors agree with the TSR ones, exhibiting a maximum near the T(g). Above the T(g), the results can be described with the WLF model. In the glassy state, the activation energy decreases with decreasing temperatures being always higher than the prediction of the Adam and Gibbs theory, at least down to temperatures 30 degrees C below the T(g). This suggests the existence of non-arrested degrees of freedom in the glassy state, being consistent with the existence of a significant degree of cooperativity in the TSR results.     

Making polymer concrete and polymer mortar using synthesized unsaturated polyester resins from poly(ethylene terephthalate) waste

Depolymerization of poly(ethylene terephthalate), PET, waste was studied in the presence of manganese acetate catalyst and propylene glycol (PG) at different weight ratios. The glycolyzed products were analyzed for hydroxyl value and the amount of free glycol. A series of unsaturated polyesters based on the glycolyzed products, maleic anhydride and styrene, were prepared. Molecular weights and curing behavior of these polyesters were determined. Polymer concrete (PC) and polymer mortars (PM) made with these resins were investigated for their compressive strength. It has been found that the properties of PC and PM obtained from resins based on recycled PET are comparable to PC and PM made from virgin materials.     

An overview of boron nitride based polymer nanocomposites

Recently, boron nitride (BN) based materials have received significant attention in both academic and industrial sectors due to its interesting properties like large energy band gap, good resistance to oxidation, excellent thermal conductivity, thermal stability, chemical inertness, significant mechanical property and widespread applications. This review article deals with the preparation and properties of boron nitride and its nanocomposites with various polymers. Diverse polymers have been explored for the preparation of boron nitride filled polymer nanocomposites by adopting different mixing methods. Properties of the resulting polymer nanocomposites mainly depend up on filler size and dispersion, mixing conditions and type of interaction between polymer matrix and the filler. Herein, the structure, preparation and properties of various boron nitride based polymer nanocomposites are reviewed in detail along with a brief overview of different classes of BN nanomaterials.     

Flow injection fluorimetric determination of beta-estradiol using a molecularly imprinted polymer

The steroidal compound 17beta-estradiol (E2) is a major estrogen now widely studied because of its potential endocrine disruption effects. The present paper describes a simple, rapid, flow injection (FI) method for the fluorimetric determination of beta-estradiol involving a molecularly imprinted polymer (MIP) packed into a microcolumn. Microwave-assisted extraction, a relatively new technique, was used to help remove the template from the MIP; this improved extraction efficiency. The optimization of the FI parameters and the intrinsic fluorescence of beta-estradiol allowed its direct determination without a derivatization step. When the possible interference of other estrogens was examined, the system displayed excellent specificity for beta-estradiol; no response to the natural estrogens estrone and estriol and the xenoestrogen bisphenol A was recorded. Under optimum experimental conditions, linear calibration graphs were obtained from 4 to 80 microg L(-1) with a detection limit of 1.12 microg L(-1).     

Effect of short glass fiber on structure and viscoelastic behavior of olefinic polymers synthesized with metallocene catalyst

Several composites were prepared on the basis of an ethylene homopolymer and different copolymers of ethylene and 1‐hexene, synthesized with a metallocene catalyst, as matrices and a content of a 5 wt % of short glass fiber. The effect of the fiber incorporation on the structure and mechanical and viscoelastic behaviors was analyzed for the different samples. The glass fibers induced a slightly higher crystallinity, and the crystallite morphology significantly changed (long spacings and crystal orientation). The incorporation of fibers did not reinforce the different matrices under study at this low content; consequently, the mechanical parameters, such as Young's modulus, yielding stress, and microhardness, were lower in the composites as compared with those values found in the neat polyolefins. The location and apparent activation energies of distinct relaxation processes are also discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1244–1255, 2003