A 3D finite deformation constitutive model for amorphous shape memory polymers: A multi-branch modeling approach for nonequilibrium relaxation processes

Shape memory polymers (SMPs) are materials that can recover a large pre-deformed shape in response to environmental stimuli. For a thermally activated amorphous SMP, the pre-deformation and recovery of the shape require the SMP to traverse its glass transition temperature (T_g) to complete the shape memory (SM) cycle. As a result, the recovery behavior of SMPs shows strong dependency on both the pre-deforming temperature and recovery temperature. Generally, to capture the multitude of relaxation processes, multi-branch models (similar to the 1D generalized viscoelastic model or Prony series) are used to model the time-dependent behaviors of polymers. This approach often requires an arbitrary (usually numerous) number of branches to capture the material behavior, which results in a substantial number of material parameters. In this paper, a multi-branch model is developed to capture the SM effect by considering the complex thermomechanical properties of amorphous SMPs as the temperature crosses T_g. The model utilizes two sets of nonequilibrium branches for fundamentally different modes of relaxation: the glassy mode and Rouse modes. This leads to a significant reduction in the number of material parameters. Model simulation comparisons with a range of thermomechanical experiments conducted on a tert-butyl acrylate-based SMP show very good agreement. The model is further utilized to explore the intrinsic recovery behavior of an SMP and the size effects on the free recovery characteristics of a magneto-sensitive SMP composite.     

Shape memory property of microcrystalline cellulose–poly(ε-caprolactone) polymer network with broad transition temperature

Preparation of shape memory polymers (SMPs) with broad transition temperature was an effective method to realize multishape memory effect. In this study, a novel SMP with a broad glass transition temperature (T _g) based on microcrystalline cellulose was prepared. The structure of the SMP was analyzed by Fourier transform infrared spectroscopy and proton nuclear magnetic resonance, which can prove the successful synthesis of the material. The thermal properties were investigated with differential scanning calorimetry and dynamic mechanical analysis (DMA). The dual- and multishape memory effects were also quantificationally analyzed by DMA. Further, the influence of programming temperature within T _g on dual-shape memory effect was investigated, and a 1D model was built to explain their relationship.     

Functional Fatigue of Shape Memory Polymers

In the present study the functional fatigue in the commercial SMP Veriflex® which is associated with repeating up to 20 programming/1‐WE cycles has been examined. The material is characterized by a glass temperature T_g of 67 °C, above which it looses all its strength. During tensile testing at 21 °C (T < T_g), stresses steadily increase to 26 MPa as strains approach the rupture strain of 7.6 %. At 80 °C (T > T_g) the material can be strained up to 225 % before rupture occurs while stresses slowly increase to values as low as 0.4 MPa. Thermomechanical cycles including programming, cooling and unloading and heating to trigger the one way effect result in an increase of irreversible strain associated with a corresponding decrease of recovery ratio at the beginning of thermomechanical cycling. In parallel a wavy surface structure evolves during cycling.     

Chemical and thermomechanical tailoring of the shape memory effect in poly(ε-caprolactone)-based systems

The thermally activated shape memory response of polymeric materials results from a combination of the material molecular architecture with the thermal/deformational history, or ‘programming’. In this work, we investigate the shape memory response of systems based on poly(ε-caprolactone) (PCL) so as to explore the adoption of proper chemical and thermomechanical tailoring routes. Cross-linked semicrystalline PCL-based materials are prepared by different molecular architectures starting from linear, three- and four-arms star PCL functionalized with methacrylate end groups, allowing to tune the melting temperature, T _m, ranging between 36 and 55 °C. The materials’ ability to display the shape memory is investigated by the application of proper thermomechanical cycles on specimens deformed at two different temperatures (23 and 65 °C, i.e. below and above the T _m, respectively). The shape memory response is studied under dynamic thermal conditions in thermally activated recovery tests, to identify the typical transformation temperatures, and under isothermal conditions at given recovery temperatures, to monitor shape recovery as a function of time. All the specimens are capable of full recovery on specific thermal ranges influenced by both melting and deformation temperatures. Specimens deformed above T _m are able to recover the whole deformation in a very narrow temperature region close to T _m, while those deformed at room temperature display broader recovery processes, those onset at about 30 °C. Isothermal tests reveal that when the deformed material is subjected to a constant recovery temperature, the amount of recovered strain and the time required strongly depend on the particular combination of melting temperature, deformation temperature and recovery temperature.     

Self-assembled multi-layered carbon nanofiber nanopaper for significantly improving electrical actuation of shape memory polymer nanocomposite

This study presents an effective approach to significantly improve the electrical properties of shape memory polymer (SMP) nanocomposites that show Joule heating triggered shape recovery. Carbon nanofibers (CNFs) were self-assembled to form multi-layered nanopaper to enhance the bonding and shape recovery behavior of SMP, respectively. It was found that both glass transition temperature (Tg) and electrical properties of the SMP nanocomposites have been improved by incorporating multi-layers of self-assembled nanopapers. The electrically actuated shape recovery behavior and the temperature profile during the actuation were monitored and characterized at a voltage of 30 V.     

Thermo-Mechanical Properties of Glass Fiber Reinforced Shape Memory Polyurethane for Orthodontic Application

Objectives: Glass fiber reinforced shape memory polyurethane (GFRSMPU) has great potential to be an alternative kind of material for orthodontic archwires for overcoming the disadvantages of metal wires in terms of esthetic and allergy and deficiency of pure shape memory polyurethane (SMPU) wires in mechanical properties. The objective of this study was to investigate the thermo-mechanical properties and shape recovery functions of GFRSMPU and evaluate the feasibility of using this composite for orthodontic archwires. Material and methods: GFRSMPU were made from short cut glass fibers and SMPU by mixing extrusion. Scanning electron microscope (SEM) and differential scanning calorimetry (DSC) were performed to investigate the distribution of glass fibers in the mixture and glass transition temperature (T_g). Then the thermo-mechanical properties, including tensile modulus, flexural modulus and stress relaxation effects, were measured. Furthermore, shape recovery functions of GFRSMPU characterized by the shape recovery ratio and force were investigated through shape recovery tests, typodont models and finite element analysis (FEA). Results: SEM images indicated that an excellent dispersity of glass fibers was obtained after double-extrusion. DSC experiments showed T_g was not enormously affected with the existence of glass fibers, but the mechanical properties of GFRSMPU were greatly improved. Shape recovery tests showed reduction of shape recovery ratio of the GFRSMPU material with the addition of glass fibers, but dentition aligning time was reduced by 50% in the simulation performed on identical typodont models with GFRSMPU archwires filled with 30?wt.% glass fibers. The FEA results illustrated that the reacting forces of GFRSMPU archwires with 30?wt.% glass fiber was increased by 96.36% compared with pure SMPU archwires. Conclusions: The mechanical properties of GFRSMPU can be considerably improved by adding glass fibers, and the shape memory function would be well preserved too. Enhanced SMPU owns a good application prospect in orthodontics for dentation aligning on the preliminary stage, as well as other medical fields.     

Effects of thermal rates on the thermomechanical behaviors of amorphous shape memory polymers

Shape memory polymers (SMPs) are polymers that can recover a large pre-deformed shape in response to environmental stimuli, such as temperature, light, etc. For a thermally triggered (or activated) amorphous SMP, the pre-deformation and recovery of the shape require the temperature of the material to traverse the glass transition temperature T _g under constrained or free conditions. In this paper, effects of thermal rates on the thermomechanical behaviors of amorphous SMPs are investigated. Under uniaxial compression, during a temperature cycle (cooling followed by heating), the stress decreases to zero as the temperature decreases to below the glass transition temperature, and increases to a value larger than the initial stress (termed stress overshoot) as the temperature is raised above the glass transition temperature. These observations are examined by a thermoviscoelasticity model that couples the nonequilibrium structural relaxation and temperature dependent viscoelastic behavior of the material. In addition, using this model, stress-temperature behaviors during temperature cycles with various thermal rate conditions and tensile loading conditions are studied.     

Dynamic mechanical analysis of carbon/epoxy composites for structural pipeline repair

The viscoelastic behavior of a carbon fiber/epoxy matrix composite material system used for pipeline repair has been evaluated though dynamic mechanical analysis. The effects of the heating rate, frequency, and measurement method on the glass transition temperature (T_g) were studied. The increase in T_g with frequency was related to the activation energy of the glass transition relaxation. The activation energy can be used for prediction of long term performance. The measured tan delta peak T_g’s of room temperature cured and post-cured composite specimens ranged from 60 to 129 °C. Analysis of T_g data at various cure states was used to determine use temperature limits for the composite repair system.     

Physico-Chemical Characterization and Stability Study of Glassy Simvastatin

The objective of the present study was to investigate the glass forming capability of a model drug simvastatin. The glassy material produced by melt quench technique was subjected to physico-chemical characterization and subsequent stability and enthalpy relaxation study. The chemical stability of drug during preparation of glass was tested by High Performance Liquid Chromatography (HPLC) and Infrared (IR) spectroscopy. The presence of amorphous form was confirmed by DSC and XRPD. Surprisingly, glassy simvastatin was almost stable throughout the period of stability, inspite of its T_g being relatively low. The stability and very low enthalpy recovery of glassy simvastatin perhaps could be attributed to strong inter-molecular hydrogen bonding.     

Influence of the epoxidation degree of a polystyrene–polybutadiene–polystyrene (SBS) triblock copolymer on the compatibilization with an organomodified nanoclay

In this work, the addition of organophilic-modified montmorillonite into polystyrene–polybutadiene–polystyrene (SBS) triblock copolymers was investigated with and without the use of epoxidized SBS as a compatibilization agent. The nanocomposites were prepared by melting mixture at 60 rpm and 130 °C. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), tensile tests, and dynamic-mechanical analysis (DMA). XRD and TEM showed the formation of an intercalated dispersion of clay platelets oriented on the SBS surface. The AFM showed the typical lamellar microstructure of the styrene and butadiene phases of SBS. The estimation of the average distance between the styrene lamellae by AFM analysis showed that the platelets are anchored between the phases, and this structural feature caused an increase in elastic modulus. DMA analysis showed that the T _g of butadiene decreased in the nanocomposites. The decrease of the T _g and the increase in the elastic modulus are correlated to mechanisms at the micro- and the macro-scales, respectively. The decrease in the T _g indicates flexibilization at the interface, whereas the intercalation of the platelets restricted the interphase macroscale deformation.     

Synergistic effect of siloxane modified aluminum nanopowders and carbon fiber on electrothermal efficiency of polymeric shape memory nanocomposite

The present study reports an effective approach of significantly enhancing electrothermal efficiency and shape recovery performance of shape memory polymer (SMP) nanocomposite, of which shape recovery was induced by electrically resistive heating. Metallic aluminum (Al) nanopowders synthesized from Al³⁺ solution were chemically grafted onto carbon fiber. Siloxane groups were grafted onto surfaces of the Al nanopowders to enhance the interfacial bonding between the carbon fiber and SMP matrix via van der Waals force and covalent bond, respectively. The siloxane modified Al surfaces could improve both the electrically induced shape recovery performance and electrothermal efficiency through facilitating the electrically resistive heating from carbon fiber into the SMP matrix. Effectiveness of the synergistic effect between siloxane modified Al surface and carbon fiber was demonstrated to achieve the electrical actuation for SMP nanocomposites at a low electrical voltage below 4.0 V.     

On the cyclic material stability of shape memory polymer

This paper presents the experimental study of a Thermoplastic Polyurethane (TPU) based Shape Memory Polymer (SMP) produced from granulates of commercially available Estane^TM ETE75DT3 NAT022 (Oevel Westerlo, Belgium). This polymer is characterized by Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Thermal Analysis (DMTA). The experimental procedure was designed to study the functional properties (shape fixity and shape recovery) of this shape memory polymer in multiple programming/shape‐recovery cycles at three different programming temperatures. The results are displayed in the temperature‐stress‐strain space with focus on parameters which characterize the functional fatigue and material stability of the tested polymer during consecutive cycles. These results give a better understanding of this material class that has a potential for actuator applications in engineering.     

Effect of high temperature annealing on the electrical performance of titanium/platinum thin films

In this study, the influence of post deposition annealing steps (PDA) on the electrical resistivity of evaporated titanium/platinum thin films on thermally oxidised silicon is investigated. Varying parameters are the impact of thermal loading with maximum temperatures up to T_PDA = 700 °C and the platinum top layer thickness ranging from 24 nm to 105 nm. The titanium based adhesive film thickness is fixed to 10 nm. Up to post deposition annealing temperatures of T_PDA = 450 °C, the film resistivity is linearly correlated with the reciprocal value of the platinum film thickness according to the size effect. Modifications in the intrinsic film stress strongly influence the electrical material parameter in this temperature regime. At T_PDA > 600 °C, diffusion of titanium into the platinum top layer and its plastic deformation dominate the electrical behaviour, both causing an increase in film resistivity above average.     

Service life assessment and moisture influence on bio-based thermosetting resins

In this study, three different types of bio-based resins are compared to a conventional oil-based epoxy in terms of moisture uptake, long-term properties and its influence of moisture and glass transition temperature, T _g. Moisture uptake is determined by means of gravimetric method, time temperature superposition (TTSP), and T _g data obtained in dynamic mechanical thermal analysis (DMTA). Moisture uptake show Fickian diffuison behavour for all resins, saturation level and diffusion coefficient however differ. The long-term properties is characterised by creep compliance master curves created by means of TTSP. The examined bio-based resins are compatible to the reference epoxy in term of stability up to 3–10 years. Comparison between master curves for virgin, wet, and dried material show that moisture present in the specimen increases creep rate, and that some of this increase remains after drying of samples. T _g measurements show that moisture inside the specimen decreases T _g; this is anticipated because of the plasticizing effect of water. The overall conclusions are that the bio-based resins of polyester, and epoxy type are comparable in performance with oil-based epoxy, LY556 and they can be used to develop high-performance composites.     

Tin fluorophosphate nonwovens by melt state centrifugal Forcespinning

This report describes the direct melt processing of inorganic tin fluorophosphate (TFP) glass fibers with average diameters ranging from 2 to 4 µm via centrifugal Forcespinning. This was accomplished by using a TFP glass with low glass transition temperature (T _g) and the melt processing capability of Forcespinning. The thermal behavior of TFP glass fibers was investigated by differential scanning calorimetry and thermogravimetric analysis, while the compositional evolution of the fibers was studied using energy-dispersive spectrometry and Fourier-transform infrared spectroscopy. These fibers exhibited excellent thermal stability after thermal post-treatment at 300 °C. The T _g of the thermally treated fibers increased by 100 °C compared to the bulk material. The fibers were found to undergo dehydration and loss of fluorine during thermal treatment, resulting in a rigid and crosslinked glass network with enhanced thermal stability and increased T _g. The enhanced thermal stability demonstrated the potential of TFP fibers for high temperature catalysis and chemical filtration applications.     

Isothermal cure characterization of fumed silica/epoxy nanocomposites: The glass transition temperature and conversion

The glass transition temperature (T_g) and conversion (α) were measured for a cycloaliphatic epoxy/anhydride system incorporated with different contents of hydrophobic fumed silica. Samples isothermally cured at varying cure temperatures and times were analyzed by differential scanning calorimetry (DSC). T_g vs. ln(time) data at an arbitrary reference temperature were superposed by time–temperature shifts to form a master curve for the kinetically controlled reaction, and the shift factors were used to calculate an Arrhenius activation energy. Influence of the hydrophobic fumed silica was investigated from T_g and α data. For low conversions, the incorporated fumed silica may reduce segmental mobility, giving rise to an increase in T_g. For high conversions, there was a significant depression of T_g which may be associated with the presence of residual hydroxyl groups on the surface of hydrophobic fumed silica. The positive and negative effects of the fumed silica on T_g and α are discussed.     

Thermodynamic Temperatures of the Triple Points of Mercury and Gallium and in the Interval 217 K to 303 K

We measured the acoustic resonance frequencies of an argon-filled spherical cavity and the microwave resonance frequencies of the same cavity when evacuated. The microwave data were used to deduce the thermal expansion of the cavity and the acoustic data were fitted to a temperature-pressure surface to deduce zero-pressure speed-of-sound ratios. The ratios determine (T–T₉₀), the difference between the Kelvin thermodynamic temperature T and the temperature on the International Temperature Scale of 1990 (ITS-90). The acoustic data fall on six isotherms: 217.0950 K, 234.3156 K, 253.1500 K, 273.1600 K, 293.1300 K, and 302.9166 K and the standard uncertainties of (T−T₉₀) average 0.6 mK, depending mostly upon the model fitted to the acoustic data. Without reference to ITS-90, the data redetermine the triple point of gallium T_g and the mercury point T_m with the results: T_g/T_w = (1.108 951 6 ± 0.000 002 6) and T_m/T_w= (0.857 785 5 ± 0.000 002 0), where T_w = 273.16 K exactly. (All uncertainties are expressed as standard uncertainties.) The resonator was the same one that had been used to redetermine both the universal gas constant R, and T_g. However, the present value of T_g is (4.3 ± 0.8) mK larger than that reported earlier. We suggest that the earlier redetermination of T_g was erroneous because a virtual leak within the resonator contaminated the argon used at T_g in that work. This suggestion is supported by new acoustic data taken when the resonator was filled with xenon. Fortunately, the virtual leak did not affect the redetermination of R. The present work results in many suggestions for improving primary acoustic thermometry to achieve sub-millikelvin uncertainties over a wide temperature range.     

Composite coatings of CoCrAlY plus platinum

The addition of noble metals has been reported to improve the corrosion resistance of aluminide and overlay coatings for gas turbine airfoils. The production and structure of a CoCrAlY + Pt coating are described. This coating is superior in oxidation and hot corrosion resistance to a coating made of the same CoCrAlY material without platinum. The 0.6% ductility temperature T_d for these two coating is similar. However, mechanical damage greatly raises the T_d value of CoCrAlY + Pt. This is attributed to a favorable residual stress system locally destroyed by the damage.     

Failure analysis of a GFRP pipe for oil transport

The study was conducted on a pipe sample of GFRP. There was widespread damage in the internal structure of the tube. The flow area was blocked due to landslides helical fibers. The matrix was an epoxy–anhydride system (Diglycidyl Ether of Bisphenol A). There was an advanced state of diffusive processes that reduced the glass transition temperature (T_g) of the material to critical levels. The material showed a major irreversible hydrolytic attack. The mechanical behavior would indicate a considerable degree of structural compromise. The sample showed an advanced state of degradation, with clear signals of processes activated at temperatures close to T_g and catalyzed by the diffusion of low molecular weight organic species. The irreversible chemical degradation process especially attacked the fiber–matrix interface reducing the mechanical performance.     

Preparation of sol–gel-based nanostructured hybrid coatings; part 1: morphological and mechanical studies

Attempts have been made using sol–gel-based precursors to produce hybrid organic–inorganic clearcoats. To this end, a typical automotive acrylic/melamine clearcoat with tetramethyl ortosilicate (TEOS) and methacryoloxy propyltrimethoxysilane (MEMO) were used to obtain nanostructured silica clusters produced in situ embedded in the polymeric matrix. Microscopic techniques including scanning electron microscope (SEM), atomic force microscope (AFM), and transmission electron microscope (TEM) were utilized to investigate the morphology of coatings. The effect of each precursor on coating mechanical properties was also studied using dynamic mechanical thermal analysis (DMTA) as well as micro and nanoindentation techniques. It was found that using TEOS and MEMO (in non-hydrolyzed state), the mechanical properties of the resulting films were negatively influenced. The decreased hardness, lower T _g and cross-linking density, and reduced elastic modulus were observed with non-hydrolyzed precursors. In addition, the phase separation of organic and inorganic domains occurred in the presence of pristine sol–gel precursors. However, using hydrolyzed precursors (HTEOS and HMEMO), the mechanical properties were notably improved. While HTEOS resulted in an increase in coating T _g, and cross-linking density as well as improved elastic modulus and hardness, HMEMO caused an increase in coating hardness but lowered coating T _g and cross-linking density.