A predictive parameter for the shape memory behavior of thermoplastic polymers

We present an experimental and modeling study of the effect of programming conditions on the shape-memory behaviors of amorphous thermoplastic polymers. Experimentally we measure the influence of deformation temperature, strain rate and relaxation time on the thermomechanical properties and shape-memory response of poly(para-phenylene), which is a stiff and strong aromatic thermoplastic. To understand the underlying mechanism, we develop a viscoelastic model, which contains multiple discrete relaxation processes with broad distribution of relaxation time. The model parameters of the relaxation spectrum are obtained from the master curve of small strain–stress relaxation tests using time-temperature superposition. The model predictions show good agreement with experimental observations, including the stress response and shape-memory response under various conditions. We applied the model to study the effect of the programming conditions on the shape recovery performance. The results show that the relaxation modulus at the end of the programming process was a predictor of the recovery speed and recoverable strain ratio. This provides a design metric to optimize the shape programming process for shape recovery. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1405–1414     

Pecularities of the thermomechanical behaviour of ultra-high molecular weight linear polyethylene and its blends with linear polyethylene of normal molecular weight

Ultra-high molecular weight polyethylene UHMWPE (M _w=4 · 10⁶,I _s=O g/ 10 min), high density polyethylene of normal molecular weight NMWPE (I _s= 4.8 g/10 min) and their blends have been investigated by means of thermomechanical loading in constant and impulse regime. It has been established that after melting, NMWPE passes to a viscous-liquid state. After melting at 138 °C UHMWPE passes to a high-elastic state. The transition of UHMWPE to a viscous-liquid state takes place at temperatures higher than 180 °C and is accompanied by a high-elastic reversible deformation. The blends of UHMWPE with 10 and 20 mass % of NMWPE show a plateau on the thermomechanical curves, corresponding to a high-elastic state, in a shorter temperature range where the deformation is greater. The blends containing the higher percent of NMWPE show thermomechanical curves lacking such a plateau. All blends are characterized by a singular thermomechanically defined temperature of melting, which increases with increase of UHMWPE content. The existence of the high-elastic state in the curves of UHMWPE and its blends containing NMWPE less than 30 mass % above their melting temperatures is explained by the high degree of physical crosslinking of UHMWPE.     

Positron annihilation and differential scanning calorimetry studies of polyacrylamide and poly(dimethylacrylamide)/poly(ethylene glycol) blends

Positron annihilation lifetime spectroscopy and differential scanning calorimetry (DSC) measurements were performed for blends of polyacrylamide (PAM) and poly(ethylene glycol) (PEG) and blends of poly(dimethylacrylamide) (PDMAM) and PEG. The samples were prepared by codissolution in a concentration range of 0–100 wt % PEG. The thermal behavior, characterized by DSC measurements, showed similar variations of the glass‐transition temperatures (T_g's) with the PEG concentration for the two systems. Pure PAM and PDMAM presented T_g's of 188 and 111 °C, respectively. A relatively small and nearly linearly decreasing T_g was observed for the two systems in the range of 20–80 wt % PEG. PEG crystals were present in all blend compositions, and no melting point depression was observed. The thermal results pointed to the partial miscibility of the blends. The degree of crystallinity of PEG increased with increasing PEG concentration for the PDMAM/PEG systems. The ortho‐positronium lifetime (τ₃) increased with increasing PEG concentration for both blends. However, the parameter of the ortho‐positronium formation probability (I₃) decreased with the PEG concentration. The product τI₃, which was proportional to the total free volume fraction, was approximately constant with the PEG concentration for PDMAM blends and increased with the PEG concentration for PAM systems. This result may be interpreted as a consequence of a more heterogeneous structure in PAM blends. Scanning electron microscopy micrographs of blends with 40 and 80 wt % PEG provided evidence of the regions associated with PEG crystallites. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1493–1500, 2003     

New approach on the development of plasticized polylactide (PLA): Grafting of poly(ethylene glycol) (PEG) via reactive extrusion

In this work, new ways of plasticizing polylactide (PLA) with low molecular poly(ethylene glycol) (PEG) were developed to improve the ductility of PLA while maintaining the plasticizer content at maximum 20 wt.% PLA. To this end, a reactive blending of anhydride-grafted PLA (MAG-PLA) copolymer with PEG, with chains terminated with hydroxyl groups, was performed. During the melt-processing, a fraction of PEG was grafted into the anhydride-functionalized PLA chains. The role of the grafted fraction was to improve the compatibility between PLA and PEG. Reactive extrusion and melt-blending of neat and modified PLA with PEG did not induce any dramatic drop of PLA molecular weight. The in situ reactive grafting of PEG into the modified PLA in PLA/PEG blends showed a clear effect on the thermal properties of PLA. It was demonstrated by DSC that the mobility gained by PLA chains in the plasticized blends yielded crystallization. The grafting of a fraction of PEG into PLA did not affect this process. However, DSC results obtained after the second heating showed an interesting effect on the T_g when 20 wt.% PEG were melt blended with neat PLA or 10 wt.% MAG-PLA. In the latter case, the T_g displayed by the reactive blend was shifted to even lower temperatures at around 14 °C, while the T_g of neat PLA and PLA blended with 20 wt.% PEG was around 60 and 23 °C, respectively. Regarding viscoelastic and viscoplastic properties, the presence of MAG-PLA does not significantly influence the behavior of plasticized PLA. Indeed, with or without MAG-PLA, elastic modulus and yield stress decrease, while ultimate strain increases with the addition of PEG into PLA.     

Photocurable biodegradable poly(ɛ-caprolactone)/poly(ethylene glycol) multiblock copolymers showing shape-memory properties

Biodegradable multiblock copolymers were synthesized by a polycondensation of poly(ɛ-caprolactone) (PCL) diols of molecular weight (MW)=3,000 and poly(ethylene glycol)s (PEG) of MW=3,000 with 4,4′-(adipoyldioxy)dicinnamic acid (CAC) dichloride as a chain extender in diphenyl ether at 180 °C for 2 h, and were characterized by GPC, ¹H-NMR, FTIR, UV, DSC, and WAXS. These photosensitive copolymers were irradiated by a 400-W high-pressure mercury lamp (λ>280 nm) from 5–60 min to form a network structure. The gel contents increased with irradiation time, and attained ca. 90% after 60 min for all copolymers. The degree of swelling in a distilled water at ambient temperature, and the rate of degradation in a phosphate buffer solution (pH 7.2) at 37 °C increased with increasing PEG components. The shape-memory tests were performed by a cyclic thermomechanical experiments for the photocured CAC/PCL/PEG (75/25) films. The film with a gel content of 57% showed the best shape-memory property with strain fixity rate of 100% and strain recovery rate of 88%.     

Effect of polyethylene glycol on the crystallization and impact properties of polylactide‐based blends

Polylactide (PLA) was plasticized by polyethylene glycols (PEGs) with five different molecular weights (M_w = 200–20,000 g/mol). The effects of content and molecular weight of PEG on the crystallization and impact properties of PLA were studied by wide‐angle X‐ray diffraction, differential scanning calorimetry, scanning electron microscopy, transmission electron microscopy, and V‐notched impact tests, respectively. The results revealed that PEG‐10,000 could significantly improve the crystallization capacity and impact toughness of PLA. When the PEG‐10,000 content ranged from 0 to 20 wt%, the increases in both V‐notched Izod and Charpy impact strengths of PLA/PEG‐10,000 blends were 206.10% and 137.25%, respectively. Meanwhile, the crystallinity of PLA/PEG‐10,000 blends increased from 3.95% to 43.42%. For 10 wt% PEG content, the crystallization and impact properties of PLA/PEG blends mainly depended upon PEG molecular weight. With increasing the M_w of PEG, the crystallinity and impact strength of PLA/PEG blends first decreased and then increased. The introduction of PEG reduced the intermolecular force and enhanced the mobility of PLA chains, thus improving the crystallization capacity and flexibility of PLA. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of polyethylene glycol plasticizer on long‐term antibacterial activity and the release profile of bacteriocin nisin from polylactide blends

This work describes the synergetic effect of polyethylene glycol (PEG) in polylactide (PLA) blends, wherein the polyether acts as both the plasticizer and functional additive, ensuring the long‐term antimicrobial activity of bacteriocin nisin. Two types of PEG with the molecular weights of 1000 and 6000 g.mol⁻¹ (20 wt.%) were used to plasticize the PLA blends. The aforementioned bacteriocin nisin, at concentrations ranging between 0.02 and 0.15 wt.% (8000‐60 000 IU.g⁻¹), was incorporated into the samples by the solvent cast technique. The effect of various PEG on the structural, mechanical, and thermal properties of the PLA‐based blends were investigated by scanning electron microscopy, Fourier transform infrared spectroscopy, stress‐strain analysis, differential scanning calorimetry, and dynamic mechanical analysis, respectively. The antibacterial activity of the samples was detected by the agar diffusion technique against Micrococcus luteus. Furthermore, the antibacterial properties of the samples were tested according the ISO 22196 standard against Gram‐negative (Escherichia coli) and Gram‐positive (Staphylococcus aureus, Listeria monocytogenes) bacterial strains. The nisin was detected by high performance liquid chromatography, the device having been equipped with a UV/vis detector. The results show that the PEG, besides its plasticizing effect, significantly enhances the release profile and sustains long‐term antibacterial activity of nisin in a PLA matrix.     

Synthesis of a random copolymer and its compatibility for thermotropic liquid crystalline polymer/poly(ether ether ketone) blends

A random copolymer (RCP) containing poly(ether ether ketone) (PEEK) and thermotropic liquid crystalline polymer (TLCP) segments was synthesized. Its chemical structure and liquid crystalline properties were characterized by FT‐IR, differential scanning calorimetry (DSC) and polar light microscopy (PLM) respectively. A single glass transition temperature (T_g) at 134.0°C, a melting temperature (T_m) at 282.0°C and a temperature of ignition (T_i) at 331.3°C can be observed. Blends of PEEK and TLCP with and without RCP as compatibilizer were prepared by extrusion and the effect of RCP on the thermal properties, dynamic mechanical properties, morphology and static tensile mechanical properties of blends was investigated by means of DSC, dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), etc. Dynamic mechanical measurements indicated that there appeared to be only a single tan δ peak resulting from the glass transition of the PEEK‐rich phase and the T_g value shifted towards higher temperature due to the presence of compatibilizer, as suggested partial compatibility. Morphological investigations showed that the addition of RCP to binary blends reduced the dispersed phase size and improved the interfacial adhesion between the two phases. The ternary compatibilized blends showed enhanced tensile modulus compared to their binary blends without RCP. The strain at break decreased for the ternary blends due to embrittlement of the matrix by the incorporation of some RCP to the matrix phase. Copyright © 2007 John Wiley & Sons, Ltd.     

Mechanical, thermal and degradation properties of poly(d,l-lactide)/poly(hydroxybutyrate-co-hydroxyvalerate)/poly(ethylene glycol) blend

The mechanical, thermal and biodegradable properties of poly(d,l-lactide) (PDLLA), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and poly(ethylene glycol) (PEG) blends were studied. The influence of PEG on the tensile and impact strengths of the blends was investigated. The results showed that the toughness and elongation at break of the PDLLA/PHBV (70/30) blends were greatly improved by the addition of PEG, and the notched Izod impact strength increased about 400% and the elongation at break increased from 2.1% to 237.0%. The thermal and degradation properties of the blends were investigated by differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA), it was found that the thermal stability of PHBV in the presence of PDLLA was improved. The degradation test showed that the addition of PEG could notably accelerate the biodegradation of the blends in the soil at room temperature, and the mass loss is about 20% after 30 days of the storage.     

Synthesis and characterization of novel biodegradable unsaturated poly(ester amide)/poly(ethylene glycol) diacrylate hydrogels

A series of novel biodegradable hydrogels were designed and synthesized from four types of unsaturated poly(ester amide) (UPEA) and poly(ethylene glycol) diacrylate (PEG‐DA) precursors by UV photocrosslinking. These newly synthesized biodegradable UPEA/PEG‐DA hydrogels were characterized by their gel fraction (G_f), equilibrium swelling ratio (Q_eq), compressive modulus, and interior morphology. The effect of the precursor feed ratio (UPEAs to PEG‐DA) on the properties of the hydrogels was also studied. The incorporation of UPEA polymers into the PEG‐DA hydrogels increased their hydrophobicity, crosslinking density (denser network), and mechanical strength (higher compressive modulus) but reduced Q_eq. When different types of UPEA precursors were coupled with PEG‐DA at the same feed ratio (20 wt %), the resulting hydrogels had similar Q_eq values and porous three‐dimensional interior morphologies but different G_f and compressive modulus values. These differences in the hydrogel properties were correlated to the chemical structures of the UPEA precursors; that is, the different locations of the >C?C< double bonds in individual UPEA segments resulted in their different reactivities toward PEG‐DA to form hydrogels. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3932–3944, 2005     

Thermal properties of anhydride-cured bio-based epoxy blends

Epoxidized palm oil (EPO) (0–12 wt%) was added into petrochemical-based epoxy blends (diglycidyl ether of bisphenol-A (DGEBA)/cycloaliphatic epoxide resin/epoxy novolac resin) to develop a thermal curable bio-based epoxy system. The thermal behaviors of the EPO, epoxy blends (EB), and bio-based epoxy blends (EB/EPO) were characterized using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMT) and thermo-mechanical analysis (TM). The glass transition temperature (T _g) and storage modulus (E′) of the EB/EPO system was reduced with the increasing of the EPO loading. This is attributed to the plasticizing effect of the EPO. It was found that epoxy blends with higher loading of EPO possessed higher coefficient of thermal expansion (CTE) and tanδ value. This is due to the increase of the free volume and chain flexibility in the three-dimensional network of the epoxy blends. The internal thermal stresses of the EB/EPO were decreased as the increasing loading of EPO, owing to the reduction of crosslink density, modulus of elasticity, and T _g in the epoxy blends.     

Synthesis,thermal characterization,and tensile properties of alipharomatic polyesters derived from 1,3‐propanediol and terephthalic,isophthalic, and 2,6‐naphthalenedicarboxylic acid

In this work, three alipharomatic polyesters—poly(propylene terephthalate) (PPT), poly(propylene isophthalate) (PPI), and poly(propylene naphthalate) (PPN)—were prepared and studied with the aliphatic diol 1,3‐propanediol and the corresponding aromatic diacids. Their synthesis was performed by the two‐stage melt polycondensation method in a glass batch reactor. The thermal characterization of these polyesters was carried out with different thermal techniques such as simultaneous thermogravimetry/differential thermal analysis, thermomechanical analysis (TMA), and dynamic thermomechanical analysis. From the recorded values for the glass‐transition temperature (T_g) and melting temperature with all the aforementioned techniques, it could be said that they were in good agreement. According to the thermogravimetric results, PPT and PPI showed about the same thermal stability, whereas PPN seemed to be somewhere more thermostable. Remarkably, a transition existed immediately after T_g that was realized by the first derivative of TMA, and it was characterized as a midrange transition. For all polyesters, the average coefficient of linear thermal expansion was calculated with TMA. The secondary relaxations T_β and T_γ, recorded with dynamic mechanical thermal analysis, were mainly affected by the kinds of monomers. Concerning the mechanical properties, PPN had the highest tensile strength at break, whereas PPT had the highest elongation at break. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3998–4011, 2005     

Body temperature triggered shape-memory polymers with high elastic energy storage capacity

Shape-memory polymers (SMPs) that respond near body temperature are attracting broad interest, especially in the biomedical fields. In this study, the triggering temperature of poly(caprolactone) SMP networks is precisely adjusted by inclusion of non-crystallizable molecular linkers and by variation of prepolymer molecular weight. Longer, non-crystalline linkers and lower molecular weight prepolymers interfere with crystallization, lowering the transition temperature. Networks are prepared with crystallization temperatures that are beneath the human body temperature and yet are above room temperature. Upon cooling such amorphous networks to room temperature, crystallization is sluggish. There, elastomers can be easily strained by several hundred-percent to induce crystallization, thereby fixing strained states. If subsequently heated, programmed SMPs can release significant amounts of stored strain energy (∼3 MJ/m³). SMPs that combine elastic energy storage and exhibit triggering temperatures near the human body temperature could benefit emerging applications in the biomedical space. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1397–1404     

Shape memory effect and recovery stress property of carbon nanotube/waterborne epoxy nanocomposites investigated via TMA

Shape memory polymers (SMPs) have received great attention and scientific interest in widespread technological development during last few decades. Besides the development of novel SMPs, various techniques have been practiced for characterization of shape memory effect (SME) of SMPs. In this study, the shape memory effect and recovery stress property of the carbon nanotube (CNT)/waterborne epoxy (WEP) nanocomposites below and above the glass transition temperature (T_g) of the nanocomposites and under isostrain and isostress were systematically investigated via thermal mechanical analysis (TMA), respectively. The experimental results showed that the nanocomposites exhibit excellent shape memory effect. The shape memory fixity and recovery ratios were approximately 100% even below glass transition temperature (T_g). A remarkable point is that the strain of the nanocomposites suddenly increased with the temperature decreasing in a certain period of the heating-cooling cycles under isostress condition and the strain increment increased with temperature in general. Especially at low temperature, the recovery stress was very sensitive to temperature under isostrain condition of ±0.25 °C temperature with differential of 25.5 °C developed pressure difference of 0.20 MPa. Moreover, TMA is a practical method for quantifying the SME and recovery stress properties of SMPs and their composites.     

Complex phase behavior and state of miscibility in Poly(ethylene glycol)/Poly(l‐lactide‐co‐ε‐caprolactone) Blends

A miscibility and phase behavior study was conducted on poly(ethylene glycol) (PEG)/poly(l ‐lactide‐ε‐caprolactone) (PLA‐co‐CL) blends. A single glass transition evolution was determined by differential scanning calorimetry initially suggesting a miscible system; however, the unusual T_g bias and subsequent morphological study conducted by polarized light optical microscopy (PLOM) and atomic force microscopy (AFM) evidenced a phase separated system for the whole range of blend compositions. PEG spherulites were found in all blends except for the PEG/PLA‐co‐CL 20/80 composition, with no interference of the comonomer in the melting point of PEG (T_m = 64 °C) and only a small one in crystallinity fraction (X_c = 80% vs. 70%). However, a clear continuous decrease in PEG spherulites growth rate (G) with increasing PLA‐co‐CL content was determined in the blends isothermally crystallized at 37 °C, G being 37 µm/min for the neat PEG and 12 µm/min for the 20 wt % PLA‐co‐CL blend. The kinetics interference in crystal growth rate of PEG suggests a diluting effect of the PLA‐co‐CL in the blends; further, PLOM and AFM provided unequivocal evidence of the interfering effect of PLA‐co‐CL on PEG crystal morphology, demonstrating imperfect crystallization in blends with interfibrillar location of the diluting amorphous component. Significantly, AFM images provided also evidence of amorphous phase separation between PEG and PLA‐co‐CL. A true T_g vs. composition diagram is proposed on the basis of the AFM analysis for phase separated PEG/PLA‐co‐CL blends revealing the existence of a second PLA‐co‐CL rich phase. According to the partial miscibility established by AFM analysis, PEG and PLA‐co‐CL rich phases, depending on blend composition, contain respectively an amount of the minority component leading to a system presenting, for every composition, two T_g's that are different of those of pure components. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014 , 52, 111–121     

Sodium Alginate and Poly(ethylene glycol) Blends: Thermal and Morphological Behaviors

Sodium alginate (SA) was blended with varying amounts of poly(ethylene glycol) (PEG) viz., 10, 20, 30, 40 and 50 wt % by using water as a solvent. The obtained SA/PEG blends have been characterized for thermal behavior by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) and surface morphology by scanning electron microscopic (SEM) methods. DSC analysis indicates the increase in glass transition temperature (T_g) of the blends with an increase in PEG content in the blend, which is due to chain entanglement. TGA results reveal the enhancement of thermal stability of SA/PEG blends in terms of the onset of degradation and percentage of weight loss. SEM photomicrographs shows the two phase morphology. This result indicates the immiscible nature of the SA/PEG blends.     

A unified modeling approach for amorphous shape memory polymers and shape memory polymer based syntactic foam

Shape memory polymers (SMPs) and shape memory polymer composites have drawn considerable attention in recent years for their shape memory effects. A unified modeling approach is proposed to describe thermomechanical behaviors and shape memory effects of thermally activated amorphous SMPs and SMP‐based syntactic foam by using the generalized finite deformation multiple relaxation viscoelastic theory coupled with time–temperature superposition property. In this paper, the thermoviscoelastic parameters are determined from a single dynamic mechanical analysis temperature sweep at a constant frequency. The relaxation time strongly depends on the temperature and the variation follows the time–temperature superposition principle. The horizontal shift factor can be obtained by the Williams–Landel–Ferry equation at temperatures above or close to the reference temperature (T_r), and by the Arrhenius equation at temperatures below T_r. As the Arruda–Boyce eight‐chain model captures the hyperelastic behavior of the material up to large deformation, it is used here to describe partial material behaviors. The thermal expansion coefficient of the material is regarded as temperature dependent. Comparisons between the model results and the thermomechanical experiments presented in the literature show an acceptable agreement. Copyright © 2016 John Wiley & Sons, Ltd.     

Dynamic mechanical and thermal properties of physically compatibilized natural rubber/poly(methyl methacrylate) blends by the addition of natural rubber‐graft‐ poly(methyl methacrylate)

The dynamic mechanical and thermal properties of natural rubber/poly (methyl methacrylate) blends (NR/PMMA) with and without the addition of graft copolymer (NR‐g‐PMMA) have been investigated. Dynamic mechanical spectroscopy is used to examine the effect of compatibilizer loading on storage modulus (E′), loss modulus (E″) and loss tangent (tan δ) at different temperatures and at different frequencies. The morphology of the blends indicates that the size of the dispersed phase decreased by the addition of a few percent of the graft copolymer followed by a leveling off at higher concentrations. This is an indication of interfacial saturation. Attempts have been made to correlate morphology with dynamic mechanical properties. Various models have been used to fit the experimental viscoelastic results. Differential scanning calorimetry has been used to analyze the glass‐transition temperatures of the blends. The thermal stability of the blends has been analyzed by thermogravimetry. Compatibilized blends are found to be more thermally stable than uncompatibilized blends. Finally the miscibility and mechanical properties of the blends annealed above T_g are evaluated. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 525–536, 2000     

Synthesis and characterization of block poly(ester‐ether‐urethane)s from bacterial poly(3‐hydroxybutyrate) oligomers

Telechelic hydroxylated poly(3‐hydroxybutyrate) (PHB‐diol) oligomers have been successfully synthesized in 90–95% yield from high molar mass PHB by tin‐catalyzed alcoholysis with different diols (mainly 1,4‐butanediol) in diglyme. The PHB‐diol oligomers structure was studied by nuclear magnetic resonance, Fourier transformed infrared spectroscopy MALDI‐ToF MS, and size exclusion chromatography, whereas their crystalline structures, thermal properties and thermal stability were analyzed by wide angle X‐ray scattering, DSC, and thermogravimetric analyses. The kinetic of the alcoholysis was studied and the influence of (i) the catalyst amount, (ii) the diol amount, (iii) the reaction temperature, and (iv) the diol chain length on the molar mass was discussed. The influence of the PHB‐diol molar mass on the thermal stability, the thermal properties and optical properties was investigated. Then, tin‐catalyzed poly(ester‐ether‐urethane)s (PEEU) of M_n = 15,000–20,000 g/mol were synthesized in 1,2‐dichloroethane from PHB‐diol oligomers (P_ester) with modified 4,4'‐MDI and different polyether‐diols (P_ether) (PEG‐2000, PEG‐4000, and PPG‐PEG‐PPG). The influence of the PHB‐diol chain length, the P_ether/P_ester ratio, the polyether segment nature and the PEG chain length on the thermal properties and crystalline structures of PEEUs was particularly discussed. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1949–1961     

Characterization of Shape-Memory Trifunctionally Cross-Linked Polyurethanes,with Varying Hard and Soft Segments

A family of potential shape-memory trifunctionally cross-linked polyurethanes (SMPs) was achieved with varying the hard segments. The molecular weight of the soft segment poly(oxytetramethylene) (PTMO) was held constant. The polymers were employed with and without chain extender. They were characterized by means of structural studies and creep tests over a range of temperatures, and hence their shape-memory performance was determined. More crystallization was observed in the materials with hard domains based on the flexible diisocyanate 4,4′-dibenzyl diisocyanate (DBDI) extended with 1,4-butanediol (BDO). A series of key parameters characterizing shape-memory performance was determined: the temperature position of the peak (T_peak), the half-width of the peak (δT), and the minimum value of the 30 s isochronal creep modulus E(30 s) E_min. While the peak temperature and modulus increased with increasing cross-link density, the width of the peak was observed to decrease with increasing cross-link density. The materials were found to vary systematically with details of the network structure.