Synthesis,Characterization and Properties of Nitropolybutadiene as Energetic Plasticizer for NHTPB Binder

The nitration of low molecular weight polybutadiene (PB) by a convenient and inexpensive procedure was investigated. To retain the unique physico‐chemical properties of the plasticizer, it was nitrated to an extent of 10 % double bonds. The product nitropolybutadiene (NPB) was characterized by FT‐IR and ¹H NMR spectroscopy as well as GPC, DSC, and TGA methods. The kinetic parameters for the decomposition of NPB from room temperature to 400 °C were obtained from non‐isothermal DSC. The changes in glass transition temperature (T _g) and inert uncured binder systems were used for determination of its efficiency as plasticizer. NPB was used in cured and unfilled nitro‐hydroxyl terminated polybutadiene (NHTPB) binder. Isothermal thermogravimetric analysis (Iso‐TGA) was employed to determine the migration rate in cured and unfilled HTPB binder systems compared to the dioctyladiphate (DOA) plasticizer. It was found that the exudation of the NPB plasticizer is slower than that of the DOA plasticizer. Thus, the NHTPB/NPB binder system (binder/plasticizer) presents more convenient mechanical properties than HTPB/DOA and is a promising new energetic binder system for polymer bonded explosives.     

Glass Transition Temperature and the Nature of the Amorphous Phase in Semicrystalline Polymers: Effects of Drawing,Annealing and Hydration in Polyamide 6

Abstract

The influence of the organization of the amorphous chains segments on the glass transition temperature (T_g) in semicrystalline polymers is analysed by studying the effects of drawing, annealing and hydration in polyamide 6 fibers. We consider the role of three of the features of the amorphous phase: orientation (configurational entropy), density (free volume) and confinement (segmental mobility). Three classes of amorphous phases are identified; two of these are constrained in the intercrystaline regions, at the fold surfaces (between the lamellae within the lamellar stack) and at the stem surface (growth surface of the lamellae or between the fibrils). The third species is the bulk amorphous phase outside the lamellar stacks, and constitutes a large fraction of the amorphous phase especially at low crystallinities. Because the small fraction of the amorphous chain segments in the intercrystalline regions, and because they are in confined spaces, we suggest that these interlamellar and the interfibrillar components do not contribute significantly to the observed major glass transition peak. Rather, it is the amorphous region outside the lamellar stack that determines the T_g. T_g increases upon drawing and decreases upon annealing (heat setting). Our data suggest that orientation has a direct influence on T_g and can easily be measured whereas the influence of crystallinity is more complex. The influence of orientation of T_g can be understood in terms of a two T_g model in which the oriented amorphous component has a higher T_g than the unoriented component.     

On the Correlation between Miscibility and Solubility Properties of Energetic Plasticizers/Polymer Blends: Modeling and Simulation Studies

In this paper, systems for which miscibility (hydroxyl‐terminated polybutadiene–dioctyl adipate or HTPB–DOA) or immiscibility (HTPB–diethylene glycol dinitrate or HTPB–DEGDN) have been firmly established were used to test the usefulness of an atomistic molecular mechanics model. Two specific aspects were discussed: miscibility assessment of a plasticizer/polymer blend, and predictions of the enthalpy of vaporization. Simulations were carried out using Amorphous Cell and Discover packages of the Material Studio software using Compass force field for all calculations. A good agreement has been found for miscibility observations of blends, and for solubility parameter, density, and derived enthalpy of vaporization for pure substances. Therefore, the approach proposed in this work is a useful tool to provide insights on miscibility and properties of a given polymer/plasticizer blend. In addition, it is a promising technique to help in screening among several plastic bonded explosive (PBX) formulations prior to real experimental tests.     

Influence of alternating sequential fraction on the melting and glass transition temperatures of ethylene-tetrafluoroethylene copolymer

The melting (T_m) and glass transition (T_g) temperatures of a series of ethylene (E)-tetrafluoroethylene (TFE) copolymer (ETFE) have been found to show unique dependence on the TFE content with the minimal and maximal points. These behaviors have been interpreted successfully on the basis of the degree of alternation of E and TFE monomeric units along the skeletal chain. The melting point of a perfectly alternating copolymer is estimated to be 295 °C on the basis of the dependence of T_m using a modified Flory’s equation. The corresponding T_g was estimated as 145 °C by applying a modified Gibbs-Damnation’s equation.     

Energetic analysis of the two PMMA chain tacticities and PMA through molecular dynamics simulations

Simulated dilatometry techniques have been applied to compute the glass transition temperatures, T_gs, of the two poly(methyl methacrylate) (PMMA) chain tacticities, and poly(methyl acrylate) (PMA). Since the difference in T_gs between the two configurations was accurately simulated, further analysis could be carried out. This article more particularly deals with energetic and structural analysis of the difference. Thus this analysis showed that the non-bond energy and the bending angle energy associated with the intradiad backbone angle, principally contribute to the energetic difference between the two PMMA configurations. Following the free volume theory, these two energetic variations allow an increase in T_g in comparison to PMA, and an enlargement of the difference in the T_gs between the two PMMA configurations. Actually, these two energetic contributions stem from the substitution of the hydrogen atom attached to the chiral carbon atom in the PMA repeat unit by a methyl group. The same behavior is encountered with the two poly(ethyl methacrylate) (PEMA) chain tacticities.     

PVC modification with new functional groups. Influence of hydrogen bonds on reactivity, stiffness and specific volume

The chemical modification of PVC with new bifunctional thiol compounds is reported. Aliphatic as well as aromatic reactives were tested and the influence of protic and non-protic functionalities on reactivity was studied. The chemical structure of the polymers was analyzed using ¹H NMR spectroscopy.The structural changes in the modified samples were monitored by means of density and T_g determinations, which are a measure of interchain spacing and chain stiffness. While protic functionalities lead to polymers with strongly enhanced T_g values indicating a considerable stiffening of the system due to physical interaction by hydrogen bonds, the softening-point temperature of PVC modified with non-protic substituents does not change very much.The interchain spacing of the polymer is not significantly altered by the presence of mobile hydrogen in the polymer.     

Variations in the glass transition temperature of polyester with special architectures confined in thin films

Variations of the polymer dynamic of different systematically varied polyester architectures in the confinement of thin films were studied by temperature dependent spectroscopic vis-ellipsometry. The architectures were tailored in order to evaluate (a) the impact of different polymer backbones (hyperbranched, branched or linear and aromatic, aromatic-aliphatic or aliphatic), (b) the influence of functional groups (hydroxyl, benzoyl, tert-butyldimethylsilyl) and (c) the role of interfacial interactions (attractive, repulsive) with the silicon substrate. Possible reasons for the deviation of the glass transition temperature T_g in thin polymer films (10-800 nm) from the bulk value are described and compared to the literature. It was found that the functional groups of the applied polymers have the largest effect on T_g. Beside interfacial interactions, chemical and physical reactions in the polymer film are playing a significant role.     

Experimental Investigation on the Damping Behaviors of Epoxies with Different Epoxy Value

In this article, five different epoxies including a new kind of flexible epoxy having low glass transition temperatures (11 ～ 28°C) were prepared using polyamine as curing agent. Damping mechanical tests show that compared with other common available epoxies, the flexible epoxy has high loss factor over broad frequency and temperature range. Activation energy corresponding to glass transition process of different epoxies has been calculated from the temperature corresponding to tan δ_max values, obtained at different measurement frequencies. The maximum value of loss factor is 0.71 and the Tg varies from 11 to 28°C, indicating the flexible epoxy can be used as damping polymer materials in common temperature or frequency range.     

Effect of Maltodextrin Addition during Spray Drying of Tomato Pulp in Dehumidified Air: I. Drying Kinetics and Product Recovery

This work investigates the effect of maltodextrin addition on the drying kinetics and the stickiness during spray drying of tomato pulp in dehumidified air. A pilot-scale spray dryer was employed for the spray-drying process. The modification made to the original design consisted in connecting the spray dryer inlet air intake to an absorption air dryer. Twenty-seven different experiments were conducted varying the dextrose equivalent (DE) of the maltodextrin, the ratio (tomato pulp solids)/(maltodextrin solids), and the inlet air temperature. Data for the residue remaining on the walls were gathered. Furthermore, the effect of maltodextrin addition on the drying kinetics and the stickiness of the product was investigated using a numerical simulation of the spray-drying process modeled with the computational fluid dynamics (CFD) code Fluent. The code was used to determine the droplet moisture content and temperature profiles during the spray-drying experiments conducted in this work. The stickiness was determined by comparing the droplet temperature with its surface layer glass transition temperature (T_g ). The T_g was determined using a weighted mean rule based on the moisture content profiles calculated by the CFD code and the experimental data of T_g , which were obtained for the different tomato pulp and maltodextrin samples and fitted to the Gordon and Taylor model.     

Mechanical behavior of adhesive anchors under high temperature exposure: Experimental investigation

The improvement in mechanical and adhesion properties of polymer resins have allowed to progressively substitute cast-in place rebars by polymer-based anchors in some applications, by providing equivalent or even higher mechanical properties at ambient temperature. However, a temperature increase has the effect of weakening the bond and leads to a significant decay in the bearing capacity of the adhesive anchors.This paper presents a study of the phenomena that occur at high temperature in an adhesive anchor when exposed to high temperatures by means of two pull-out test procedures and by thermomechanical characterization of the polymer resin. Results showed that the resin glass transition is the responsible for the decay in the fire resistance of adhesive anchors. The paper highlights the non-conservative aspect of the current design method used to calculate the fire resistance of chemical anchors and proposes to consider the pull-out curves as input data.     

Formation of porosity and change in binder pitch properties during thermal treatment of green carbon materials

The formation of porosity during heat treatment of an extruded mixture of coke particles and a binder pitch was investigated using image analysis of polished cross-sections of shaped samples. The porosity due to pitch devolatilisation takes place during a rather narrow interval of temperature when the sample is heated at 12 K/hour. Only a small fraction of pores is formed when the shaped sample is submitted to an intermediate isothermal step which allows the departure of volatile compounds from the binder. Physico-chemical changes occurring in the pitch during pyrolysis were followed by measuring glass transition temperature in relationship with weight loss. The results indicate that in sufficiently slow heating conditions, low molecular compounds essentially diffuse through the fluid pitch and evaporate at the external surface of the sample during pyrolysis. As a result, no marked porosity is formed during baking. In contrast, a high heating rate allows low molecular mass compounds to accumulate inside the sample and the formation of a significant porosity is due to the evaporation inside the shaped sample of accumulated low molecular mass compounds.     

聚合物在超临界CO2中玻璃化转变研究进展

简要介绍了聚合物的玻璃化转变以及超临界CO2在高分子科学中广阔的应用前景,着重对超临界CO2环境下聚合物玻璃化转变的实验研究方法和理论研究的进展进行了综述。     

Quantitative structure-property relationships for composites: prediction of glass transition temperatures for epoxy resins

The glass transition temperatures of nine stoichiometric resin systems of tetraglycidyl-4,4′-diamino-diphenylmethane (TGDDM), triglycidyl p-amino phenol and diglycidyl ether of bisphenol A with 4,4′-diaminodiphenylsulphone (DDS), diethyl-toluenediamine and dimethylthiotoluenediamine were calculated using group interaction modelling (GIM) and atomic additivity (AA) methods. The input parameters were generated from kinetics simulation, which outputs the structure information for the cured systems. The modelling parameters were also applied to four non-stoichiometric systems of TGDDM and DDS. The predicted values from GIM were in good quantitative agreement with measured results from temperature modulated differential scanning calorimetry for all systems studied. Compared to GIM, the AA method gave inferior predictions for the highly crosslinked systems, especially for those, where epoxy was in excess.     

Single droplet drying: Transition from the effective diffusion model to a modified receding interface model

Drying experiments on single droplets of aqueous amorphous polymer solution show morphological changes towards the end of drying that result in an under-prediction of the drying rate using an effective diffusion based model. Alternately, other researchers argue that the receding interface model more accurately reflects the physics of drying by predicting a fixed droplet radius once a specified surface condition is reached, usually the saturation concentration. However, this surface condition is not adequate for many skin forming materials. The conditions at which droplet radial contraction ceases will be determined by the balance between internal moisture loss causing a collapsing pressure and the mechanical strength of the surface skin. Because measurements and prediction of surface stress are difficult, it is proposed that they are related to the state of the polymer solution at the surface which is defined by the proximity of the surface temperature to its glass transition temperature, (T − T_g). In this work, an effective diffusion model is used to predict ideal shrinkage until a critical temperature difference or (T − T_g)_crit is reached where the surface of the droplet becomes fixed and the skin grows towards the droplet centre, that is, as a receding interface. For maltodextrin DE5, a (T − T_g)_crit of 20 °C was found to provide an accurate prediction of the drying rate. While these results show (T − T_g)_crit is indicative of mechanical stress development, it points to a need for further understanding of mechanical stress development in skin forming polymers during drying.     

Investigation of the glass transition temperature and damping of an acrylic/epoxy bonding tape using the peak damping method

The dynamic properties of a structural tape used for adhesive bonding applications have been measured at different temperatures to determine its glass transition temperature and damping properties. For this purpose, free layer beams consisting of a base layer steel and the tape layer were vibrated using a resonant beam technique with free-free end conditions. To measure the dynamic values (elastic modulus and loss factor) of the tape, the necessary equations were derived and the frequency dependence of the beams was investigated from –55°C to +60°C. Three beams with different layers were tested. Results have shown that as the temperature increases, the elastic modulus of the tape decreases, while the loss factor of the tape increases up to 20°C and then decreases to a constant level. The results from the three beams are in agreement, showing that the glass transition temperature of the tape is about 20°C, which implies viscoelastic properties at room temperature.     

Powder coatings and differential scanning calorimetry: the perfect fit

A thermosetting powder coating is the ideal field of application for differential scanning calorimetry (DSC) analysis. A review of the existing techniques is done and a new product characterization approach is discussed in comparison with some traditional methods. Time savings, effectiveness and compliance with the objectives, can be advantageously obtained by using DSC analysis versus alternative methods (gel time, solvent cure test, storage stability, mechanical test, etc.). An attempt is made to provide a quick ‘one shot' analytical test to predict storage stability, curing behaviour, processing conditions and in some way the end-use performance of powder coatings.     

On the Influence of Glass Transition on Shrinkage in Convective Drying of Fruits: A Case Study of Banana Drying

The ideal shrinkage model assumes that the extent of shrinkage is equal to the volume of liquid water removed from the dried medium. Generally if a material undergoes glass transition during the drying process, shrinkage will no longer be ideal. The aim of this study was to observe how the glass transition temperature influences the shrinkage kinetics. Cylindrical banana samples were dried. Shrinkage extent was significant for all drying conditions (temperature: 30–50°C, relative humidity: 0–80%). Deviation from linearity was found to be affected not only by drying air temperature but also by its relative humidity.