Influences of phase composition and thermomechanical conditions on shape memory properties of segmented polyurethanes with amorphous reversible phase

We synthesized series of shape memory polyurethanes with amorphous reversible phase (T_g‐SMPUs) and systematically studied their microphase structure and shape memory properties. The T_g‐SMPUs having no or less hard phase showed lower shape recovery. When the volume fraction of hard phase was in the range of 20–30%, the T_g‐SMPUs exhibited the highest shape recovery. As the fraction of hard phase increased further the shape recovery decreased, because more polymer components with higher glass transition temperatures (T_gs) would participate in strain storage. For the T_g‐SMPUs having similar T_gs, those polymers having higher volume fraction of hard phase exhibited higher shape fixity, broader shape recovery region, and larger recovery stress. Increasing deformation strain could raise shape fixity and recovery stress but broaden shape recovery region. The highest recovery stress of a material could be achieved when the deformation occurred at its glass transition temperature below which decreasing deformation temperature could not increase recovery stress further. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Physical and Mechanical Behavior of Polymer Glasses. VI. the Role of Free Volume

Abstract

For various polymer glasses, the temperature-induced recovery of residual deformation was studied. The ratio between the low-temperature and high-temperature recovery components is controlled by the difference between deformation temperature and glass transition temperature T _g of polymer samples independently of their chemical structure. This ratio correlates with polymer macroscopic mechanical characteristics such as elastic modulus and yield stress. Experimental results were treated in terms of the dynamics of segmental mobility within different structural sublevels with different packing densities. To correlate this mechanical response with the structural state of glassy polymers, positron annihilation lifetime spectroscopy (PALS) was used. For different polymer glasses, the microscopic segmental mobility and resultant macroscopic mechanical properties were shown to be controlled only by the development of the adequate free volume content which depends on the difference between testing temperature and T _g . These results allowed us to propose the general correlation between microstructure, microscopic molecular mobility, and Macroscopic mechanical behavior of polymer glasses.     

Shape memory behavior of poly(methyl methacrylate)-graft-poly(ethylene glycol) copolymers

This paper describes a shape memory behavior of graft copolymers poly(methyl methacrylate)-graft-poly(ethylene glycol) (PMMA-g-PEG). In shape memory test, the sample was deformed from its original shape to a temporary shape above glass transition temperature (T_g), cooled below T_g to fix the temporary shape, and subsequently heated above T_g for spontaneous recovery to the original shape. By grafting PEG onto PMMA backbone, shape memory ability was drastically enhanced than PMMA homopolymer. The shape recovery ratio was decreased with the increase in the shape deformation temperature. With considering a good miscibility of backbone and side chain in PMMA-g-PEG, this shape memory ability may be related to a physically cross-linked network structure by chain entanglement of the comb-like graft copolymer. Stress relaxation measurements were investigated in order to confirm the effect of the graft chains on the shape memory behavior.     

Molecular dynamics predictions of thermal and mechanical properties of thermoset polymer EPON862/DETDA

We use molecular dynamics (MD) to perform an extensive characterization of the thermo-mechanical response of a thermoset polymer composed of epoxy EPON862 and curing agent DETDA. Our simulations, with no adjustable parameters, show that atomistic simulation can capture non-trivial behavior of amorphous thermosets including the role of polymerization degree, thermal history, strain rate and temperature on the glass transition temperature (T_g) and mechanical response (including ultimate properties) and lead to predictions in quantitative agreement with experiments. We find a significant increase in T_g, Young’s modulus and yield stress with degree of polymerization while yield strain is significantly less sensitive to it. For structures cured beyond the gel point (percolation of a 3D network) conversion degree and temperature affect yield stress in a similar way with yield stress linearly dependent on T−T_g; however, we find non-linear and non-universal relationship below the gel point. Our results show that a relative small variation in polymerization degree (∼5%) can explain the spread in experimental measurements of T_g and elastic constants available in the literature.     

Correlation between glass transition temperature and molecular mass in non-polymeric and polymer glass formers

It is shown that the glass transition temperature T_g of molecular (non-polymeric) glass formers correlates with molecular mass M as T_g(M) ∝ M^α, α = 0.51 ± 0.02. The subclasses of molecular glasses with homologous chemical structure but different M exhibit a similar universal correlation with significantly lower scatter. A possible explanation of T_g vs M correlation in molecular glasses is suggested. Comparing molecular glasses with polymers we found that in polymers T_g(M) dependence at small M (short chains) is similar to that in molecular glasses. At further increasing of the chain length the T_g(M) dependence in polymers begin to deviate from the universal T_g(M) correlation of molecular glasses and eventually saturates at some polymer specific T_g∞ value. We conclude that at least a substantial part of T_g(M) dependence of low-M polymers is common with molecular glasses mechanism that does not require chain-like structure. In particular, the model of T_g(M) dependence in polymers based on additional free volume on chain ends is not fully adequate at small M. Our picture provides an alternative explanation that in polymers a mechanism is in action which leads to a saturation of the normal T_g(M) dependence common with molecular glasses.     

Thermally stimulated recovery of plastic strain in crosslinked and uncrosslinked epoxy/amine systems

Effects of crosslinks on plastic strain recovery in epoxy glass was studied by means of thermally stimulated strain recovery (TSSR) and differential scanning calorimetry (DSC) techniques. Two types of samples were prepared from bisphenol-A type epoxide monomer: one had a crosslinked structure cured by 4,4′-diaminodiphenylmethane and the other had a linear uncrosslinked structure polymerized by aniline. Both specimens were compressed first in the rubbery state and subsequently compressed further in the glassy state. After compressed in the glassy state, the specimens were subjected to the TSSR and the DSC measurements. The TSSR results indicated that the amount of plastic strain recovering at temperatures below the glass transition temperature T_g was larger for the crosslinked sample than for the linear sample. On the other hand, the DSC results indicated that the amount of exothermic heat flow at temperatures below T_g was less for the crosslinked sample than for the linear sample. These results presumably indicate that, for the crosslinked epoxy glass deformed in the glassy state, glass-like strain recovers quite cooperatively with rubber-like strain at temperatures below T_g, in contrast to independent recovery of these strains in the linear epoxy glass.     

Thermomechanical characterization of a shape memory polymer based self-repairing syntactic foam

While the current self-healing approaches such as micro-capsules, hollow fibers, thermally reversible covalent bonds, ionomers, incorporation of thermoplastic particles, etc., are very effective in self-healing micro-length scale damage, self-healing of structural scale or macro-length scale damage remains one of the grand challenges facing the self-healing community. We believe that self-healing of structural damage may need multiple steps, at least two steps: close then heal (CTH), similar to the biological healing of wounds in the skin. In a previous study [1], it has been proven that the confined shape recovery functionality of a shape memory polymer (SMP) based syntactic foam can be utilized to repair structural damage such as impact damage repeatedly, efficiently, and almost autonomously. The purpose of this study is to investigate the effect of various design parameters on the closing efficiencies of both the pure SMP and the SMP based syntactic foam. A systematic test program is implemented, including glass transition temperature (T_g) determination by dynamic mechanical analysis (DMA), isothermal compressive constitutive behavior at various temperatures, and stress-controlled uniaxial compression programming and shape recovery. During thermomechanical cycle testing, two stress levels are utilized for programming and three confinement conditions (fully confined, partially confined, and free) are investigated for shape recovery. It is found that the programming stress is restored under confined recovery conditions, which helps in fully closing the crack; the foam shifts the T_g higher and increases the stiffness at temperatures above the T_g; higher programming stresses lead to slightly higher shape fixity but lower shape recovery in free recovery cases; a higher programming stress also results in a higher peak stress for confined recovery conditions; while the peak stress recovered is controlled by thermal stress, the final stress recovered is controlled by the programming stress, which is stored and recovered using an entropic mechanism. This study lays a solid foundation for using shape memory polymer based composites to self-repair macro-length scale damage.     

Structure-property relations of polyethertriamine-cured bisphenol-A-diglycidyl ether epoxies

The relations between the chemical and physical network structure, the deformation and failure processes and the tensile mechanical properties of polyethertriamine-cured bisphenol-A-diglycidyl ether epoxies are reported for a series of epoxy glasses prepared from a range of polyethertriamine concentrations. Near-infra-red spectroscopy indicates that these glasses form exclusively from epoxide-amine addition reactions. Their T_g exhibits a maximum and swell ratio a minimum at the highest crosslink density. Stress-birefringence studies reveal that these highly crosslinked glasses are ductile and undergo necking and plastic deformation. The plastic deformation initially occurs homogeneously but ultimately becomes inhomogeneous and shear bands develop. Tensile failure occurs in the high strain shear band region. The ultimate tensile strain of these epoxies attains a maximum of 15% for the highest crosslinked glass. Off stoichiometric networks fail at lower strains because such networks inherently contain more defects in the form of unreacted ends. The density, yield stress, tensile strength, and modulus of these glasses all decrease with increasing polyethertriamine concentration as a result of increasing free volume because of the poor packing ability of the amine molecule. A slight minimum is superimposed on this downtrend in density and modulus with increasing amine content at the highest crosslink density because of geometric constraints imposed on segmental packing by the network crosslinks. The ability of these crosslinked glasses to undergo deformation is discussed in terms of the free volume and the crosslinked network topography. Network failure is considered in terms of stress-induced chain scission which is determined by the concentration ad extensibility of the least extensible network segments.     

Effect of physical aging on the shape-memory behavior of amorphous networks

This paper presents an experimental and modeling study of the effects of physical aging on the shape-memory performance of (meth)acrylate-based networks composed of tert-butyl acrylate (tBA) crosslinked by various concentrations of poly(ethylene glycol dimethacrylate) (PEGDMA). The experiments measured the unconstrained recovery response of samples stored at 20 °C (T_g ? 36 °C) for zero to 180 days and evaluated the effects of storage on the strain fixity, activation temperature, and initial recovery rate. A thermoviscoelastic model recently developed for amorphous networks near the T_g was applied to study the influence of structural and viscoelastic relaxation and the aging time and temperature on the recovery response. Results showed that the activation temperature and the initial recovery rate increased with the aging time, producing a sharper initial recovery response. The thermoviscoelastic model predicted that the magnitude of these effects depended on the aging temperature. There was an optimum aging temperature that maximized the initial recovery rate. These results suggest that physical aging can be manipulated to accelerate the recovery performance of shape-memory polymer devices.     

Influence of copolymerisation on fracture behaviour of aliphatic polyketones

High strain rate tensile impact properties of aliphatic polyketone terpolymers were investigated and related to the polymer chain structure. Aliphatic polyketones were used as a model system, by changing the termonomer content and type. Aliphatic polyketone is a perfectly alternating copolymer and the structure was changed with the addition of a few mol% of termonomer: propylene, hexylene and dodecene. Studied were the thermal properties with DSC and DMTA, tensile behaviour, notched tensile impact behaviour, notched Izod properties and the temperature development during deformation. The perfectly alternating copolymer had a melting point of 257 °C, a T_g at 15 °C, a high crystallinity (48%), a high yield stress (77 MPa) and yield strain (31%) but a relatively low fracture strain (85%) and an impact strength (notched Izod) of 13 kJ/m². Increasing the propylene content to 6%, lowered the melting temperature to 224 °C, without changing the T_g. The modulus and yield stress were lowered but the impact strength improved. Increasing the length of the termonomer while keeping the T_m at 224 °C lowered the T_g, the modulus, the yield stress but strongly improved the impact resistance. The longer termonomers, with a lower yield stress, reduced the necking behaviour. The temperature increase in front of the notch was about 85 °C. By adding termonomers to aliphatic ketones, the notched impact behaviour improved significantly at the cost of modulus and yield stress.     

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.     

Time–temperature dependence of thermomechanical recovery of cold-curing structural adhesives

The thermomechanical recovery behavior of a commercial cold-curing structural epoxy adhesive was investigated. Exposing the adhesive to temperatures above the glass transition temperature T_g significantly increased the latter although the curing degree increased only marginally. Cooling the adhesive from temperatures above T_g to temperatures below T_g led to full recovery of the mechanical properties (tensile stiffness and strength). The secondary bonds between the polymer chains fully reformed after cooling. Temporarily exceeding the T_g did not therefore result in any degradation of the material—on the contrary, the mechanical properties significantly improved due to post-curing. Simulating temperature fluctuations by eight consecutive cycles across glass transition also improved the mechanical properties and did not lead to any degradation. An existing model for predicting temperature-dependent mechanical properties was extended to also describe the recovery behavior. The simulations agreed well with the experimental results.     

Rheological analysis of polyvinyl butyral near the glass transition temperature

In many polymer manufacturing operations, the material is processed near the glass transition temperature (T_g). Examples are thermoforming, blow molding, film blowing, hot embossing, forging, plastic welding, and de‐airing in safety glass lamination. In these processes, solid‐like behaviors such as strain hardening and yielding play important roles. These material properties cause the material to flow (or deform) in a way that substantially differs from a polymer melt. In order to understand the flow behavior near the T_g, polyvinyl butyral (PVB), a rubbery polymer used in safety glass lamination, was studied in this work. The flow properties of the polymer above the T_g were characterized by using both shear and elongational rheometers, and a tensile tester. The measured flow properties were described by a viscoelastic constitutive model.     

Mechanical properties of plastic‐bonded explosive binder materials as a function of strain‐rate and temperature

Compression measurements were conducted on three explosive formulation binders, extruded Estane, plasticized Estane, and plasticized hydroxyl‐terminated polybutadiene, as a function of temperature and strain rate. The mechanical response of the Estane was found to exhibit the strongest dependency on strain rate and temperature and higher flow strength for similar test conditions of the three materials tested. Plasticized Estane was less sensitively dependent on strain rate and temperature, followed by the plasticized HTPB. The visco‐elastic recovery of all three binders is seen to dominate the mechanical behavior at temperatures above the glass transition temperature (T_g). There is a pronounced shift in the apparent T_g to higher temperatures as the strain rate is increased. Two distinct behaviors are observed in the binders below the T_g. At low strain rates, the binders exhibit a yield behavior, followed by a drop in the flow stress, which may or may not recover. At high strain rates, the load drop does not occur and the flow stresses either gradually increase, as in plasticized HTPB, or it levels out as seen in the Estane‐based binders. POLYM. ENG. SCI., 46:812–819, 2006. © 2006 Society of Plastics Engineers     

From the glassy state to ordered polymer structures: A microhardness study

The review covers the understanding of the nanostructure development in glassy and semicrystalline polymers as revealed by indentation hardness methods. The microhardness of polymer glasses is discussed emphasizing the influence of thermal history and physical ageing. The correlation between hardness and glass transition temperature is brought in. Furthermore, the role played by the lamellar morphology in the case of amorphous blends of a block copolymer and a glassy homopolymer is highlighted. A discussion on the influence of filler structure on the microhardness of polymer glasses is introduced. Indentation hardness is presented as a valuable tool to study the kinetics of crystallization from the glassy state. As an example, distinct results on polymer systems under different confinement conditions are shown. The nanostructure-microindentation hardness correlation in the case of semicrystalline polymers and the influence of degree of crystallinity and crystal thickness for various flexible and semirigid polymer systems are recalled. A comprehensive discussion of the creep properties of polymer materials is offered. Concerning deformation mechanisms, experimental results show that for polymers with low degree of crystallinity and T_g below room temperature, a large deviation from the microhardness additivity law is always found. This is due to a different deformation mechanism with respect to that envisaged for polymer materials with T_g above room temperature. The assumption that microhardness approaches zero for amorphous materials above T_g is experimentally confirmed. In the case of an oriented material, it is shown that indentation hardness is capable to detect the gradual appearance of phases of intermediate order. In addition, the study of the creep properties also yields valuable information on the internal degree of order of the oriented system. Finally, an overview of the future perspectives of the application of depth-sensing indentation to the study of polymer materials is offered.     

Rheological behavior of bitumen modified by reclaimed polyethylene and polypropylene from different recycling sources

This work studied the rheological, thermal, and mechanical properties of bitumen modified by reclaimed polyethylene (PE), and polypropylene (PP) from different recycling sources. The rutting resistance under high temperature of polymer modified bitumen (PMB) was investigated by rheologically temperature ramp test and multiple stress creep recovery (MSCR) test. It is found that for some modified bitumen, a plateau of complex modulus G^* could be formed with temperature increment, revealing rheological stability. Furthermore, these samples with rheological stability revealed a higher creep recovery and lower creep compliance measured by the MSCR test. Glass transition temperature (T_g) measured by dynamic mechanical analysis was used to evaluate the crack resistance under a low temperature of PMB. The influence of recycled PE on the T_g of modified bitumen was different from that of recycled PP modified bitumen, as compared with corresponding virgin polymer modified bitumen. A possible reason for the various effect of recycling sources on the service property of modified bitumen was explored by crystallization and melting behaviors of polymer in bitumen since that polymer with higher crystallinity degree could endow the modified bitumen stiffness, which was closely, related to their service property especially the rutting resistance.     

Nucleation as a new concept for morphology adjustment of crystalline thermosetting epoxy polymers

Partially crystalline thermosetting epoxy polymers of defined heterogeneous morphology are herein presented. Crystalline poly(ε-caprolactone) (PCL) domains are covalently integrated into the cationically polymerized epoxy network. Although crystallinity is common in thermoplastic polymers, these new materials are among the first examples of partially crystalline thermosetting polymers. Nucleation is crucial and leads to a defined heterogeneous morphology. Homogeneous and heterogeneous polymers with identical composition can be prepared by initiation below or above the melting point of the PCL, respectively. While the homogeneous polymer showed the expected decrease in crosslinking density of the epoxy network and low glass transition temperature (T_g), the epoxy phase is not substantially influenced in the case of the heterogeneous morphology, showing two separated modulus changes at the melting point of PCL and the T_g of the epoxy matrix polymer.     

Effects of molecular entanglement on molecular dynamics and phase-separation kinetics of poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) blends

Poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) (PMMA/SMA) blends with various compositions were prepared through solution casting and melt blending. Two preparation routes, solution casting and melt blending, were used to achieve different degrees of molecular entanglement in the samples with solution casting giving rise to a lower degree of entanglement. Therefore, the effect of molecular entanglement on molecular dynamics and phase-separation kinetics of PMMA/SMA blends was investigated by using broadband dielectric spectroscopy and small-angle laser light scattering (SALLS). Molecular entanglement is found to have a pronounced effect on the α-relaxation process. The glass transition temperature (T_g) is related to the degree of entanglement and a higher degree of entanglement can result in a higher T_g which shifts to a higher temperature after annealing. The relaxation time (τ) of the α-relaxation process is lower for lower degrees of entanglement. Neither the dynamics nor the distribution width of the β-relaxation process is affected by degree of entanglement, regardless of the blend composition. The kinetics of phase-separation by spinodal decomposition (SD) in PMMA/SMA blends are however significantly influenced by the degrees of entanglement with decomposition rate being higher at lower degrees of entanglement.     

The effects of silver nitrate on the structure and properties of polyurethanes containing pyridyl units

In this study, 2,6-pyridinedimethanol was used as a chain extender to synthesize a new polyurethane, PDM-PU. Further, various amounts of silver nitrate were incorporated to produce PDM-PU/AgNO₃ complexes. FT-IR and UV–Vis analyses confirm the formation of complex in the PDM-PU/AgNO₃. DSC and DMA results show that the glass transition temperature (T _g), dynamic T _g and storage modulus at 25 °C of the PDM-PU/AgNO₃ complexes increase with increasing AgNO₃ content. This is due to the formation of complex structure that can restrict the segmental motion of polymer chains. The TGA and stress–strain test results show that the thermal decomposition temperature, tensile strength and elongation at break increase with the AgNO₃ content initially. Then, they decrease inversely. This indicates that the formation of complex structure raises these properties when the AgNO₃ content is below certain value. But as more coordinate bonds were formed, the specimens become brittle. In addition, the crosslink effect caused by coordinate bonds inhibits the dissolution of polymer chains and thereby reduces the swelling degree of the complexes in solvent. Furthermore, AgNO₃ imparts antibacterial activity against S. aureus and K. pneumoniae to the complexes     

Understanding the effect of chain entanglement on the glass transition of a hydrophilic polymer

In this article, we report an interesting phenomenon of the glass transition temperature (T_g) deviation of a hydrophilic polymer. Polyacrylamide (PAL) samples with different extents of chain entanglement were prepared by spray drying and solution casting. We found that the glass transition temperature increases as the extent of chain entanglement decreases upon the sub‐T_g annealing. The water content in the PAL matrix is found with no direct correlation to T_g. However, the observation of a faster diffusion process of water in the disentangled PAL matrix offers an evidence of a faster relaxation process of disentangled PAL molecules. The T_g increase of the disentangled PAL samples is believed to be associated with the increased molecular interaction during the chain relaxation process upon the sub‐T_g annealing. A macroscopic evidence is the fact that the density of the hot‐laminated samples increases as the extent of chain entanglement decreases. A thermodynamic analysis suggests that enthalpy more than entropy favors an elevated T_g of a disentangled hydrophilic polymer upon the sub‐T_g annealing. We believe that this research provides new understanding of T_g of the hydrophilic polymers, which are being extensively used in bio‐related studies. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011