Photo-mediated copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) “click” reactions for forming polymer networks as shape memory materials

The formation of polymer networks polymerized with the Copper (I) – catalyzed azide – alkyne cycloaddition (CuAAC) click reaction is described along with their accompanying utilization as shape memory polymers. Due to the click nature of the reaction and the synthetic accessibility of azide and alkyne functional-monomers, the polymer architecture was readily controlled through monomer design to manipulate crosslink density, ability for further functionalization, and the glass transition temperature (55–114 °C). Free strain recovery is used to quantify the shape memory properties of a model CuAAC network resulting in excellent shape fixity and recovery of 99%. The step growth nature of this polymerization results in homogenous network formation with narrow glass transitions ranges having half widths of the transition close to 15 °C for these materials resulting in shape recovery sharpness of 3.9%/°C in a model system comparable to similarly crosslinked chain growth polymers. Utilization of the CuAAC reaction to form shape memory materials opens a range of possibilities and behaviors that are not readily achieved in other shape memory materials such as (meth) acrylates, thiol-ene, thiol-Michael, and poly(caprolactone) based shape memory materials.     

On reducing anisotropy in 3D printed polymers via ionizing radiation

The mechanical properties of materials printed using fused filament fabrication (FFF) 3D printers typically rely only on adhesion among melt processed thermoplastic polymer strands. This dramatically limits the utility of FFF systems today for a host of manufacturing and consumer products and severely limits the toughness in 3D printed shape memory polymers. To improve the interlayer adhesion in 3D printed parts, we introduce crosslinks among the polymer chains by exposing 3D printed copolymer blends to ionizing radiation to strengthen the parts and reduce anisotropy. A series polymers blended with specific radiation sensitizers, such as trimethylolpropane triacrylate (TMPTA) and triallyisocyanurate (TAIC), were prepared and irradiated by gamma rays. Differential scanning calorimetry (DSC), tensile testing, dynamic mechanical analysis (DMA) and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) were employed to characterize the thermomechanical properties and the chemical structure of the various polymers. TAIC was shown to be a very effective radiation sensitizer for 3D printed sensitized polylactic acid (PLA). The results further revealed that crosslinks induced by radiation temperatures near T_g of shape memory systems have prominently enhanced the thermomechanical properties of the 3D printed polymers, as well as the solvent resistance. This enables us to deliver a new generation of inexpensive 3D printable, crosslinked parts with robust thermomechanical properties.     

Hyperbranched‐modified epoxy thermosets: Enhancement of thermomechanical and shape‐memory performances

In this study a complete characterization of the thermomechanical and shape‐memory properties of epoxy shape‐memory polymers modified with hyperbranched polymer and aliphatic diamine was performed. Focusing on the mechanical properties that are highly desirable for shape‐memory polymers, tensile behavior until break was analyzed at different temperatures and microhardness and impact strength were determined at room temperature. As regards shape memory performance, the materials were fully characterized at different programming temperatures to study how this influenced the recovery ratio, fixity ratio, shape‐recovery velocity, and switching temperature. Tensile testing revealed a peak in deformability and in the stored energy density at the onset of the glass transition temperature, demonstrating that this is the best programming temperature for obtaining the best shape‐memory performances. The Young's moduli revealed more rigid structures in formulations with higher hyperbranched polymer content, while microhardness showed higher values with increasing hyperbranched polymer content due to the increased crosslinking density. Impact strength was greatly improved as the aliphatic diamine content increases due to the energy dissipation capability of its flexible structure. As regards the shape‐memory properties, increasing the programming temperature has a minor effect on formulations with a lower hyperbranched polymer content and worsens these properties when the hyperbranched polymer content is increased. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44623.     

Synthesis and Thermomechanical Behavior of (Qua)ternary Thiol-ene(/acrylate) Copolymers

The objective of this work is to characterize and understand the structure-to-thermomechanical property relationship in thiol-ene and thiol-ene/acrylate copolymers in order to complement the existing studies on the kinetics of this polymerization reaction. Forty-one distinct three- and four-part mixtures were created with systematically varied functionality, chemical structure, type and concentration of crosslinker. The resulting polymers were subjected to dynamic mechanical analysis and tensile testing at their respective glass transition temperature, T_g, to quantify and understand their thermomechanical properties. The copolymer systems exhibited a broad range of T_g, rubbery modulus – E_r and failure strain. The addition of a difunctional high-T_g acrylate to several three-part systems increased the resultant T_g and E_r. Higher crosslink densities generally resulted in higher stress and lower strain at failure. The tunability of the thermomechanical properties of these copolymer systems is discussed in terms of inherent advantages and limitations in light of pure acrylate systems.     

Facile tailoring of thermal transition temperatures of epoxy shape memory polymers

A critical parameter for a shape memory polymer (SMP) lies in its shape memory transition temperature. For an amorphous SMP polymer, it is highly desirable to develop methods to tailor its T_g, which corresponds to its shape memory transition temperature. Starting with an amine cured aromatic epoxy system, epoxy polymers were synthesized by either reducing the crosslink density or introducing flexible aliphatic epoxy chains. The thermal and thermomechanical properties of these epoxy polymers were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). All the crosslinked epoxy polymers with T_g's above room temperature were found to possess shape memory properties. Overall, our approach represents a facile method to precisely tune the T_g of epoxy SMP polymers ranging from room temperature to 89 °C.     

乙烯基对苯二甲酸类甲壳型液晶高分子的形状记忆性能

通过自由基聚合合成了四种基于乙烯基对苯二甲酸类甲壳型液晶高分子（PBPCS,PMPCS,PDCHVT,PbiPCS）。TGA和DMA表征结果表明,这四种甲壳型液晶高分子均具有较好的热稳定性,其侧基末端基团刚性越大,相对应的片材在30～80℃温度区间刚性越大。形状记忆弯角回复测试表明：四种聚合物热压成型的片材都具有很好的形状记忆固定率,均接近100%。形状记忆回复效果与聚合物侧基的末端基团有关,侧基末端基团分别为对丁氧基苯基和环己基（对应聚合物分别为PBPCS和PDCHVT）时,片材显示出较好的形状记忆性能,形状记忆回复率分别为87%和100%。将PDCHVT通过熔融纺丝制成纤维,采用形状记忆循环DMA测试表征其形状记忆性能,结果表明PDCHVT纤维具有稳定的优异的形状记忆性能。     

Finite strain 3D thermoviscoelastic constitutive model for shape memory polymers

A 3D thermoviscoelastic model is proposed to represent the thermomechanical behavior of shape memory polymers. The model is based on a physical understanding of the material behavior and a mechanical interpretation of the stress–strain–temperature changes observed during thermomechanical loading. The model is thermodynamically motivated and is formulated in a finite strain framework in order to account for large strain deformations. Model predictions capture critical features of shape memory polymer deformation and, in some cases, provide very favorable comparisons with experimental results. POLYM. ENG. SCI. 46:486–492, 2006. © 2006 Society of Plastics Engineers.     

Shape‐memory behavior of thermally stimulated polyurethane for medical applications

Shape memory polymers (SMPs) have been of great interest because of their ability to be thermally actuated to recover a predetermined shape. Medical applications in clot extracting devices and stents are especially promising. We investigated the thermomechanical properties of a series of Mitsubishi SMPs for potential application as medical devices. Glass transition temperatures and moduli were measured by differential scanning calorimetry and dynamic mechanical analysis. Tensile tests were performed with 20 and 100% maximum strains, at 37 and 80°C, which are respectively, body temperature and actuation temperature. Glass transitions are in a favorable range for use in the body (35–75°C), with high glassy and rubbery shear moduli in the range of 800 and 2 MPa respectively. Constrained stress–strain recovery cycles showed very low hysteresis after three cycles, which is important to know for preconditioning of the material to ensure identical properties during applications. Isothermal free recovery tests showed shape recoveries above 94% for MP5510 thermoset SMP cured at different temperatures. One material exhibited a shape fixity of 99% and a shape recovery of 85% at 80°C over one thermomechanical cycle. These polyurethanes appear particularly well suited for medical applications in deployment devices such as stents or clot extractors. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3882–3892, 2007     

Nanoindentation of shape memory polymer networks

This work examines the small-scale deformation and thermally induced recovery behavior of shape memory polymer networks as a function of crosslinking structure. Copolymer shape memory materials based on diethylene glycol dimethacrylate and polyethylene glycol dimethacrylate with a molecular weight of 550 crosslinkers and a tert-butyl acrylate linear chain monomer were synthesized with varying weight percentages of crosslinker from 0 to 100%. Dynamic mechanical analysis is used to acquire the bulk thermomechanical properties of the polymers, including the glass transition temperature and the elastic modulus over a wide temperature range. Instrumented nanoindentation is used to examine ambient temperature deformation of the polymer networks below their glass transition temperature. The glassy modulus of the networks measured using nanoindentation is relatively constant as a function of crosslinking density, and consistent with values extracted from monotonic tensile tests. The ambient temperature hardness of the networks increases with increasing crosslinking density, while the dissipated energy during indentation decreases with increasing crosslinking density. The changes in hardness correlated with the changes in glass transition but not changes in the rubbery modulus, both of which can scale with a change in crosslink density. Temperature induced shape recovery of the indentations is studied using atomic force microscopy. For impressions placed at ambient temperature, the indent shape recovery profile shifts to higher temperatures as crosslink density and glass transition temperature increase.     

Sterilization of Thiol‐ene/Acrylate Based Shape Memory Polymers for Biomedical Applications

A fundamental study on the sterilization of thiol‐ene/acrylate polymers for biomedical applications is presented. These polymer networks belong to the emerging field of shape memory polymers and have the capability to undergo softening after insertion into the body. The impact of various sterilization methods, such as radiation, steam, and ethylene oxide on the thermomechanical properties of these stimuli responsive materials is investigated. Time and temperature dependent thermomechanical properties of sterilized and nonsterilized samples are determined by means of dynamic mechanical analysis in an aqueous environment to allow testing of polymers in phosphate buffered saline. The findings show that ethylene oxide sterilization is appropriate for thiol‐ene and thiol‐ene/acrylate based shape memory polymers. This method does not adversely affect thermomechanical and self‐softening properties and after sterilization, endotoxin levels remain below the thresholds recommended in the FDA Guidance.

     

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     

The role of branching architecture in shape memory semi-IPNs: Shape memory effect and tube model analysis

The impact of branching architecture of one continuous uncrosslinked phase on properties of classic shape memory semi-interpenetrating polymer networks (semi-IPNs) was explored. Crosslinked poly (methyl methacrylate) (PMMA)/star-shaped polyethylene glycol (PEG) (PMMA/SPEG) semi-IPNs and PMMA/linear PEG (PMMA/LPEG) semi-IPNs were synthesized with the same PEG content. Mechanical properties, phase structure, thermal properties, dynamic mechanical properties, and shape memory properties of these two semi-IPNs systems were compared. Due to the better compatibility of SPEG in the PMMA network, which was derived from little crystallization compared with PMMA/LPEG semi-IPNs, PMMA/SPEG semi-IPNs exhibited a combination of large tensile strength and high elongation at break. PMMA/SPEG semi-IPNs, which had little crystallization exhibited superior shape recovery versus PMMA/LPEG semi-IPNs, which had more crystallization. Moreover, the higher the crystallinity in PMMA/PEG semi-IPNs was the worse long-term temporary shape retention. Based on tube model theory, the high shape recovery capacity of PMMA/SPEG semi-IPNs is mainly ascribed to the retraction of free PEG arms, which is entropically favorable and thermally activated due to the fluctuations of the path length. This result is supported by stress relaxation analysis and the influence of long shape fixity time on shape fixity ratio for these two systems.     

Modeling thermomechanical behaviors of shape memory polymer

The thermomechanical constitutive equations are critical for shape memory polymers (SMPs) in analyzing their shape, memory, and recovery responses under different constraints. In this study, a new physical‐based, temperature and time‐dependent constitutive model was proposed. The deformation mechanisms of this class of functional materials were explained, and the theoretical predicting values by different models were compared with available experimental results. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Dependency of the shape memory properties of a polyurethane upon thermomechanical cyclic conditions

The relationship between the shape memory properties and thermomechanical cyclic conditions was investigated with a type of shape memory polyurethane (SMPU). The thermal and dynamic mechanical properties of the polyurethane were examined by using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). It was found that the SMPU exhibited good shape memory effects (SMEs) at deformation temperatures ranging from T_g to T_g + 25 °C. The strain recovery ratios increased with the increase of deformation speed and with the decrease in maximum strain. The recovery ratios also increased with increasing fixing speed. Therefore, in practical applications, in order to obtain better SMEs, the SMPU should be cooled to its frozen state as soon as possible after being deformed at a high temperature. The ‘fixity’ dramatically increased with the decrease in fixing temperature. To obtain optimal SMEs, the polymer has to be reheated up to the temperature at which the polymer deformed. In addition, the recovery ratios of the SMPU could increase slightly with the increase of recovery time. Copyright © 2004 Society of Chemical Industry     

Matrix physical structure effect on the electro-optic characteristics of thiol-ene based H-PDLC films

The physical and mechanical properties of several thiol-ene based polymers and their mixtures with the liquid crystal, E7, were characterized to probe their relationship with the liquid crystal film electro-optic performance properties. Kinetic data suggests that high conversion is achieved for each thiol-ene combination. Pre-polymerization phase diagrams indicate that each thiol-ene/E7 mixture phase separates well below room temperature, and thus prior to polymerization at room temperature all are in a single phase. Holographic polymer dispersed liquid crystals (HPDLC) were fabricated for several thiol-ene and E7 mixtures, and electro-optical parameters characterized to probe the relationship between the thiol-ene network properties and the electro-optic performance of the HPDLCs. The photocured matrices exhibited glass transitions and tan δ peak maxima that ranged from temperatures below 0 °C to well above room temperature. There is a clear correlation between the physical nature of the matrix and the electro-optic switching parameters with H-PDLC films fabricated from trithiol-pentaerythritol triallylether, all of which exhibit glass transition temperatures below 0 °C, having the fastest switching times and lowest switching voltages at room temperature. Also, in each case higher liquid crystalline concentration resulted in lower switching voltages.     

Thiol-ene Reaction as Reversible Covalent Bond for the Design of Shape-Memory Polymers

Besides a stable phase, shape-memory polymers require an additional switchable moiety. In addition to thermal transitions and supramolecular interactions, these units can also be based on covalent bonds. Herein, the use of the reversible thiol-ene reaction as reversible cross-linker for the design of shape-memory polymers is demonstrated. A facile route to polymer networks with a thiol-ene acceptor and a comonomer (butyl methacrylate or 2-ethylhexyl methacrylate) cross-linked by dithiols is introduced. The thermal and mechanical properties of the resulting polymers are characterized in detail. Hereby, the polymers feature excellent shape-memory behavior with fixity and recovery rates above 90%. This study shows that the thiol-ene cross-linker can function as both, the stable and the switchable structural moiety rendering the usage of a covalent cross-linker unnecessary. This partial reversibility can also be proven by temperature-depending Raman spectroscopy.     

Enhanced shape memory effect of poly(L-lactide-co-ε-caprolactone) biodegradable copolymer reinforced with functionalized MWCNTs

Data from comprehensive thermomechanical tests of poly(L-lactide-co-ε-caprolactone) biodegradable shape memory polymer (SMP) reinforced with pristine and functionalized multiwalled carbon nanotubes (MWCNTs) are reported. The SMP specimens tested up to 500% strain and between 25 °C and 70 °C temperatures. The incorporation of functionalized MWCNTs leads to the best overall reinforcing effect in tensile modulus, yield stress, tensile strength and elongation at failure. Thermo mechanical experiments resulted that the functionalized MWCNTs increased the glass transition range of composites and changed the melting point of composites slightly. The crystallinity of composites was increased with increment of MWCNTs in composites. The shape fixity and shape recovery of composites increased slightly, while the recovery stress increased 46%. It is found that the functionalized MWCNTs prepare an effective physical cross linking and switching segments in polymer composites.     

Effect of including a hydrophobic comonomer on the rheology of an acrylamide-acrylic acid based copolymer

Synthesis and design of polymer systems based on acrylamide for enhanced oil recovery (EOR) is essential for reservoirs with high salinity and high temperature conditions. The use of associative monomers or the modification of the polymers with hydrophobic functional groups represents a promising alternative that extends the use of chemical EOR. In this study, terpolymers based on acrylamide, acrylic acid and butyl methacrylate were synthesized and the rheological properties of aqueous solutions of the obtained polymers at different pH values, and salt concentrations were evaluated. The results show that at alkaline conditions the viscosity of aqueous solutions of a polymer synthesized with 68.6 wt% of acrylamide, 22.9 wt% of acrylic acid and 8.6 wt% of butyl methacrylate increases by a factor of more than 1,000 at a 3 wt% concentration. Also, all polymers with the hydrophobic modification showed higher viscosity in saline solutions compared to their acrylamide-acrylic acid analogue.     

A constitutive model of thermoviscoelastic semicrystalline shape memory polymer considering viscous dissipation in the amorphous phase

Mathematical modeling has increasingly recognized as a powerful tool that could aid the understanding of shape memory behavior in semicrystalline shape memory polymer (SMP). Up to now, studies have not fully taken into account the viscous effect of the amorphous phase in the whole shape memory cycle, which causes a more realistic prediction of the SMP behavior. In this work, a constitutive thermoviscoelastic model was developed to predict the thermomechanical behavior of semicrystalline SMP. The simulated results of the proposed model for a typical uniaxial deformation were compared with the case having no dissipation effect, also with experimental data. The accuracy improvements in the results of the stress–strain trends together with fixing ratio and recovery ratio obtained from the modified model were significant. The results were in good agreement with the experimental data. The modified model revealed a real and more accurate trend by considering viscous dissipation.     

Thermomechanical properties and deformation behavior of a unidirectional carbon-fiber-reinforced shape memory polymer composite laminate

A shape memory polymer (SMP) demonstrates large reversible deformation functionality upon exposure to heating stimuli. In this study, the thermomechanical properties and deformation behavior of a unidirectional carbon-fiber-reinforced SMP composite (SMPC) laminate were studied. The findings can be used as a basis to design angle-ply laminated plates, woven laminated plates, or special laminated structures used for space deployment. The fundamental static and dynamic mechanical properties of SMP and SMPC were characterized. The fiber-reinforced SMPC exhibited local postmicrobuckling behavior and obtained a high-reversible macroscale strain of 9.6%, which enabled the high-reversible deformation to be used for foldable structures in space. The state of critical failure of bending deformation was determined through microscale morphology observations and provided the upper limit in the design of SMPC structures. The evolution of the key shape memory properties (e.g., recovery speed and recovery ratio) during deformation cycles was characterized, and it offered the general recovery performance of a space deployable structure. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48532.