3D printing of shape memory polymers

Shape memory polymers (SMPs) are polymers which ''remember'' their original shape and can return to it after deformation, if an external stimulus—often an increased temperature – is applied. Some SMPs can be 3D printed, typically by fused deposition modeling (FDM). The most well-known SMP is poly(lactic acid), which belongs to the most often used materials in FDM 3D printing. There are; however, many more SMPs which can be 3D printed to combine the possibilities to prepare new, sophisticated shapes with the opportunity to restore these shapes after undesirable or intentional deformation. This review gives an overview of several 3D printable SMPs, their mechanical characteristics and their possible applications.     

New directions in the chemistry of shape memory polymers

The rapidly expanding field of shape memory polymers (SMPs) is driven by a growing number of potential applications, such as biomaterials, optics, and electronics. The basic concept involves polymers that can be trapped in a thermodynamically-unfavorable shape, then triggered by an external stimulus to return to their original shape, doing useful work in the process. Part of the attraction of using SMPs is that the energy released during actuation is stored in the polymer itself, rather than requiring an external force to change shape. This approach is beneficial for applications where external actuation is impossible or inconvenient. Polymers are also advantageous over shape memory metal alloys or ceramics in that there are endless combinations of functional groups and material properties to suit a variety of purposes, based on the monomers and polymerization conditions chosen. This advantage of SMPs is of particular interest in the development of materials with additional, desirable physicochemical attributes that are not necessarily coupled to the shape memory (SM) behavior itself. The SM behavior is quantitatively measured to facilitate comparison of various polymer systems, and researchers have used a number of defining parameters to guide the development and characterization of materials with extremely precise and reliable SM responses. In this review, recent trends in the structural or chemical characteristics of SMPs are explored, with an emphasis on how the molecular structure and functionality of each polymer affects its mechanical response.     

热敏形状记忆高分子材料及其应用

介绍了热敏形状记忆高分子材料及其应用。热敏形状记忆高分子材料是通过温度的变化实现形状的记忆与改变,利用其形状记忆功能,热敏形状记忆弹性体可应用在热缩管、油田封隔器、医用外科、保险杠等领域。     

Linear Shape Memory Polyester with Programmable Splitting of Crystals

Shape memory polymers (SMPs)are widely used owing to their ability to change shapes under external stimuli. Conventional covalently crosslinked SMPs have limitations in biomedical applications. This article presents a linear shape memory biodegradable polyester without chemical crosslinks or multiblock structures. A new programming protocol is developed to split the crystals into two parts with different melting transitions through partial melting/recrystallization. The split crystals play different roles in fixation and recovery process to complete a shape memory cycle. The ratio between the partitioned crystals affects the fixed rate and recovery rate. The shape memory performance can be optimized by controlling the partial melting temperature and pre-stretching of the sample. Examples of complicated shape changes demonstrate the effectiveness of the proposed technique. The method is applicable to crystallizable linear polymers and has potential applications in implantation devices.     

Recent advances in polymer shape memory

Traditional shape memory polymers (SMPs) are those capable of memorizing a temporary shape and recovering to the permanent shape upon heating. Although such a basic concept has been known for half a century, recent progresses have challenged the conventional understanding of the polymer shape memory effect and significantly expanded the practical potential of SMPs. In this article, notable recent advances in the field of SMPs are highlighted. Particular emphasis is placed on how the new developments have changed the conventional view of SMPs, what they mean for practical applications, and where the future opportunities are.     

Shape-Memory Polymers Hallmarks and Their Biomedical Applications in the Form of Nanofibers

Shape-Memory Polymers (SMPs) are considered a kind of smart material able to modify size, shape, stiffness and strain in response to different external (heat, electric and magnetic field, water or light) stimuli including the physiologic ones such as pH, body temperature and ions concentration. The ability of SMPs is to memorize their original shape before triggered exposure and after deformation, in the absence of the stimulus, and to recover their original shape without any help. SMPs nanofibers (SMPNs) have been increasingly investigated for biomedical applications due to nanofiber’s favorable properties such as high surface area per volume unit, high porosity, small diameter, low density, desirable fiber orientation and nanoarchitecture mimicking native Extra Cellular Matrix (ECM). This review focuses on the main properties of SMPs, their classification and shape-memory effects. Moreover, advantages in the use of SMPNs and different biomedical application fields are reported and discussed.     

Porous inorganic-organic shape memory polymers

Thermoresponsive shape memory polymers (SMPs) are a type of stimuli-sensitive materials that switch from a temporary shape back to their permanent shape upon exposure to heat. While the majority of SMPs have been fabricated in the solid form, porous SMP foams exhibit distinct properties and are better suited for certain applications, including some in the biomedical field. Like solid SMPs, SMP foams have been restricted to a limited group of organic polymer systems. In this study, we prepared inorganic-organic SMP foams based on the photochemical cure of a macromer comprised of inorganic polydimethylsiloxane (PDMS) segments and organic poly(ε-caprolactone) (PCL) segments, diacrylated PCL(40)-block-PDMS(37)-block-PCL(40). To achieve tunable pore size with high interconnectivity, the SMP foams were prepared via a refined solvent-casting/particulate-leaching (SCPL) method. By varying design parameters such as degree of salt fusion, macromer concentration in the solvent and salt particle size, the SMP foams with excellent shape memory behavior and tunable pore size, pore morphology, and modulus were obtained.     

Soybean oil‐based shape‐memory polyurethanes: Synthesis and characterization

Shape‐memory polymers (SMPs) have wide range of applications due to their ability to sense environmental stimuli and reshape from a temporary shape to a permanent shape. Plant oil‐based polymeric materials are highly concerned in recent years in consideration of petroleum depletion and environmental pollution. However, plant oil‐based polymers are rarely investigated regarding their shape‐memory characteristics though bio‐based SMPs are highly desired nowadays. In this study, a series of soybean oil‐based shape‐memory polyurethanes (SSMPUs) are prepared through a mild chemo‐enzymatic synthetic route, and their properties are fully characterized with tensile testing, DSC, dynamic mechanical analysis (DMA), and shape‐memory testing. Results show that SSMPUs are soft rubbers with tensile strength in the range of 1.9–2.2 MPa and glass transition temperature in the range of 2–5°C, and possess good shape recoveries at RT when stretching ratio is 10, 20, and 30%, respectively. This work would promote the development of high‐value‐added plant oil‐based shape‐memory polyurethanes. Practical applications: Using annual renewable plant oil as feedstock, the synthesized SSMPUs show good shape recovery properties, which will make them applicable as potential alternatives to petroleum‐based shape‐memory materials. The simple and mild preparation process also contributes to the further exploration of plant oil to value‐added functional materials.     

Photothermal-Responsive Crosslinked Liquid Crystal Polymers

Crosslinked liquid crystal polymers (CLCPs) are a promising type of smart polymer material with excellent two-way shape memory behavior under the stimulation of various external conditions. Among the various external stimuli, light with the advantages of instantaneity, high accuracy, remote controllability, pollution-free, etc., has been widely used in the control of CLCPs. Traditional photoresponsive CLCP systems based on photoisomerization are limited by the penetration depth of ultraviolet light, the number of useful photochemical reactions, and long-term stability. Recently, a strategy for designing photoresponsive CLCPs based on the photothermal effect has attracted scientific attention. Photothermal materials in the CLCP matrix can convert light into heat through plasma resonance or energy transition under the light irradiation of a certain wavelength, and locally heat the CLCP matrix to beyond the liquid crystal-to-isotropic phase transition temperature, thus achieving reversible macroscopic deformation. Combining photothermal materials and CLCPs can decrease the dependence on specific photosensitive groups such as azobenzene mesogens and endow photoresponsive CLCPs with design flexibility. Herein, the photothermal-responsive CLCPs under the stimulations of different light wavelengths and their application fields are reviewed. In addition, the current challenges in the field of photothermal-responsive CLCPs are summarized, and a brief outlook on future development is proposed.     

光热转换材料在相变储能领域的研究进展

相变储能是热储能的一种,即利用相变材料的储热特性来储存或释放热量,达到调控温度的效果。但相变材料往往不具备光吸收能力,不能及时收集太阳光,导致其光热转换效率较低。将相变材料与光热转换材料复合可以在增强吸光能力的同时将获得的能量存储在相变材料中,赋予复合相变材料高光热转换能力。该文对光热转换材料进行了分类,介绍了其光热转换机理、对紫外光-可见光-近红外光的吸收能力以及在相变领域的应用。此外,还阐述了光热复合相变材料 的复合策略,包括浸渍法、溶胶-凝胶法、涂层法和改性微胶囊法,分析表明,不同复合策略下制备的光热复合相变材料的光吸收能力、导热系数、光热转换效率几乎都得到了提高。因此,将光热转换材料拓展到相变储能领域将进一步优化太阳能资源。     

Sustainable shape memory polymers based on epoxidized natural rubber cured by zinc ferulate via oxa-Michael reaction

Although various shape memory polymers (SMPs) or diverse applications have been widely reported, the SMPs based on rubbers have been rarely realized due to the low triggering temperature of rubbers. In another aspect, the SMPs based on sustainable substances are highly desired for the growing shortage in fossil resources. In the present study, we accordingly developed the sustainable SMPs with tunable triggering temperature, based on natural rubber (NR) and ferulic acid (FA) as the raw materials. Specifically, the SMPs are based on a crosslinked network of epoxidized natural rubber (ENR) crosslinked by in situ formed zinc ferulate (ZDF) via oxa-Michael reaction. The excellent shape memory effect (SME) is found in these SMPs, as evidenced by the high fixity/recovery ratio and the tunable triggering temperature. With the incorporation of natural halloysite nanotubes (HNTs), the stress and recovery rate of the SMPs are found to be tunable, which widens the application of this kind of SMPs. The combination of adoption of sustainable raw materials, and the excellent and tunable SME makes these SMPs potentially useful in many applications, such as various actuators and heat-shrinkable package materials.     

Recent advance on constitutive models of thermal‐sensitive shape memory polymers

Shape memory polymers (SMPs) are a novel class of shape memory materials which can store a deformed (temporary) shape and recover an original (permanent) shape under a shape memory thermomechanical loading–unloading cycle. The deformation mechanisms of SMPs are very complicated, but the SMPs also have a lot of advantages and the widespread application value and prospect. So developing proper constitutive models that describe thermomechanical properties of SMPs and the shape memory effect is very challenging and of great theoretical and application value. Based on the deformation mechanisms and considerable experimental investigations of SMPs, researchers have developed many constitutive models. This article investigates the deformation mechanism and introduces the recent research advance of the constitutive models of thermal‐sensitive SMPs. Special emphases are given on the micromechanical constitutive relations in which the deformation is considered being based on the microstructure of the SMPs. Finally, the lack of research and prospects for further research are discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Shape memory polymers based on uniform aliphatic urethane networks

Aliphatic urethane polymers have been synthesized and characterized, using monomers with high molecular symmetry, to form amorphous networks with very uniform supermolecular structures, which can be used as photo‐thermally actuable shape memory polymers (SMPs). The monomers used include hexamethylene diisocyanate (HDI), trimethylhexamethylenediamine (TMHDI), N,N,N′,N′‐tetrakis(hydroxypropyl)ethylenediamine (HPED), triethanolamine (TEA), and 1,3‐butanediol (BD). The new polymers were characterized by solvent extraction, NMR, XPS, UV/VIS, DSC, DMTA, and tensile testing. The resulting polymers were found to be single phase amorphous networks with very high gel fraction, excellent optical clarity, and extremely sharp single glass transitions in the range of 34–153°C. Thermomechanical testing of these materials confirms their excellent shape memory behavior, high recovery force, and low mechanical hysteresis (especially on multiple cycles), effectively behaving as ideal elastomers above T_g. We believe these materials represent a new and potentially important class of SMPs, and should be especially useful in applications such as biomedical microdevices. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Photosensitive Polymers

Abstract

The present summary covers photophysical and photochemical processes in polymers and their applications; photopolymerization is not included. The main research interests in the field of photosensitive polymers has now shifted from fundamental studies on photophysical and photochemical processes to the area of applications such as photoresponsive polymers, image forming and optical memory materials.     

基于氟硼二吡咯的共轭聚合物研究进展

氟硼二吡咯（BODIPY）类功能染料因具有良好的光子及电子性质已经得到广泛的应用,对其共轭聚合物的研究则在近年受到广泛关注。与BODIPY单体相比,BODIPY的共轭聚合物具有更窄的带隙、吸收光谱的红移及更强的电子传导能力。BODIPY共轭聚合物在有机半导体材料及有机太阳能电池等方面具有广泛的应用前景。总结了BODIPY共轭聚合物的制备方法、结构与性质的关系以及应用等方面,提出了BODIPY共轭聚合物设计合成策略及未来的发展趋势。     

Noble metals on the nanoscale: optical and photothermal properties and some applications in imaging, sensing, biology, and medicine

Noble metal nanostructures attract much interest because of their unique properties, including large optical field enhancements resulting in the strong scattering and absorption of light. The enhancement in the optical and photothermal properties of noble metal nanoparticles arises from resonant oscillation of their free electrons in the presence of light, also known as localized surface plasmon resonance (LSPR). The plasmon resonance can either radiate light (Mie scattering), a process that finds great utility in optical and imaging fields, or be rapidly converted to heat (absorption); the latter mechanism of dissipation has opened up applications in several new areas. The ability to integrate metal nanoparticles into biological systems has had greatest impact in biology and biomedicine. In this Account, we discuss the plasmonic properties of gold and silver nanostructures and present examples of how they are being utilized for biodiagnostics, biophysical studies, and medical therapy. For instance, taking advantage of the strong LSPR scattering of gold nanoparticles conjugated with specific targeting molecules allows the molecule-specific imaging and diagnosis of diseases such as cancer. We emphasize in particular how the unique tunability of the plasmon resonance properties of metal nanoparticles through variation of their size, shape, composition, and medium allows chemists to design nanostructures geared for specific bio-applications. We discuss some interesting nanostructure geometries, including nanorods, nanoshells, and nanoparticle pairs, that exhibit dramatically enhanced and tunable plasmon resonances, making them highly suitable for bio-applications. Tuning the nanostructure shape (e.g., nanoprisms, nanorods, or nanoshells) is another means of enhancing the sensitivity of the LSPR to the nanoparticle environment and, thereby, designing effective biosensing agents. Metal nanoparticle pairs or assemblies display distance-dependent plasmon resonances as a result of field coupling. A universal scaling model, relating the plasmon resonance frequency to the interparticle distance in terms of the particle size, becomes potentially useful for measuring nanoscale distances (and their changes) in biological systems. The strong plasmon absorption and photothermal conversion of gold nanoparticles has been exploited in cancer therapy through the selective localized photothermal heating of cancer cells. For nanorods or nanoshells, the LSPR can be tuned to the near-infrared region, making it possible to perform in vivo imaging and therapy. The examples of the applications of noble metal nanostructures provided herein can be readily generalized to other areas of biology and medicine because plasmonic nanomaterials exhibit great range, versatility, and systematic tunability of their optical attributes.     

Electrical actuation and shape recovery control of shape-memory polymer nanocomposites

Shape-memory polymers (SMPs) are one of the most popular smart materials due to their light weight and high elastic deformation capability. The synergistic effect of carbon nanofiber (CNF) and carbon nanofiber paper (CNFP) on the electro-actuation of SMP nanocomposites was studied. The electrical conductivity of SMPs was significantly improved by incorporating CNF and CNFP into them. The dynamic mechanical analysis result reveals good thermal stability of SMP nanocomposites even after they were mixed with CNFs. A vision-based control system is designed to precisely control the shape recovery of SMP composites. Any quasi-state shape between the permanent shape and a temporary shape can be achieved by adjusting the electrical energy input. Experimental results demonstrated that (1) compared with the baseline material, the full recovery time of the SMP nanocomposites was decreased by 1000% to less than 80 s; (2) a good repeatability was shown in the developed vision system in 10 experimental trials and the accuracy of the controlled deflection angle of SMPs was within a 5% error bound.     

Electrospun Nanofibers: Materials,Synthesis Parameters,and Their Role in Sensing Applications

Since the last decade, electrospinning is garnering more attention in the scientific research community, industries, applications like sensing (glucose, H₂O₂, dopamine, ascorbic acid, uric acid, neurotransmitter, etc.), biomedical applications (wound dressing, wound healing, skin, nerve, bone tissue engineering, and drug delivery systems), water treatment, energy harvesting, and storage applications. This review paper provides a brief overview of the electrospinning method, history of the electrospinning, factors affecting the electrospun nanofibers, and their morphology with different materials and composites (metals, metal oxides, 2D material, polymers and copolymers, carbon-based materials, etc.) used in the electrospinning technique with optical spinning parameters. Moreover, this paper deliberates the application of electrospun nanofibers and fibrous mats for sensing (electrochemical, optical, fluorescence, colorimetric, mechanical, photoelectric, mass sensitive change, resistive, ultrasensitive, etc.) in most illustrative representations. In the end, the challenges, opportunities of the electrospun nanofibers, and new direction for future progress are also discussed.     

Co-extruded multilayer shape memory materials: Comparing layered and blend architectures

Shape memory polymers (SMPs) are a class of materials that exhibit the ability to form multiple temporary shapes, with shape change most often occurring upon exposure to heat. Applications of SMPs can be found in many areas such as sensors, packaging, smart fabrics, and most commonly medicine. Often, thermoplastic SMPs are based on block copolymer or blend morphologies that create two distinct phases, which are on the nano- or micro-scale respectively, to facilitate shape fixing and shape recovery. Forced assembly multilayer co-extrusion of commercially available polyurethane (PU) and polycaprolactone (PCL) polymers was used to create a continuous periodic alternating layer architecture that exhibits shape memory behavior. Similar shape memory properties were observed between PU/PCL layers and blends at 50/50 volume composition; however, offset compositions showed significantly different behavior. The layered structure was maintained across all compositions, as compared with blends that exhibit a composition dependent morphology. The difference in morphology was directly attributed to the difference in shape memory behavior observed between layered and blend films with domain sizes on the micro-scale.     

Reconfiguration and shape memory triggered by heat and light of carbon nanotube–polyurethane vitrimer composites

Thermoset shape‐memory polymers (SMPs) are widely applied because of their superiority in maintaining permanent shapes. However, the inferiority of this material is also conspicuous, namely the loss of reprocessing ability owing to the chemically crosslinked structure. Fortunately, a new class of SMPs, known as “vitrimers,” was discovered, which can be reshaped or reprocessed via topological rearrangement due to the existence of dynamic covalent bonds. Thus, this new thermoset SMP could become a novel solution. In this paper, carbon nanotube–polyurethane vitrimer (CNT‐PUV) composites have been prepared, which possess the capability of thermally induced shape memory based on entropy changes and thermal reconfiguration based on transcarbamoylation reactions of carbamate bonds. In addition, the introduction of CNTs endows them with properties of near‐infrared (NIR) triggered shape memory and reconfiguration due to the photothermal conversion effect of CNTs. Besides, due to the character of the NIR laser, step‐by‐step shape recovery of CNT‐PUVs is realized from predefined temporary shapes to a permanent shape. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45784.