Triple‐shape‐memory polymer films created by forced‐assembly multilayer coextrusion

Triple‐shape‐memory polymers are capable of memorizing two temporary shapes and sequentially recovering from the first temporary shape to the second temporary shape and eventually to the permanent shape upon exposure to a stimulus. In this study, unique three‐component, multilayered films with an ATBTA configuration [where A is polyurethane (PU), B is ethylene vinyl acetate (EVA), and T is poly(vinyl acetate) (PVAc)] were produced as a triple‐shape‐memory material via a forced‐assembly multilayer film coextrusion process from PU, EVA, and PVAc. The two well‐separated thermal transitions of the PU–EVA–PVAc film, the melting temperature of EVA and the glass‐transition temperature of PVAc, allow for the fixing of the two temporary shapes. The cyclic thermomechanical testing results confirm that the 257‐layered PU–EVA–PVAc films possessed outstanding triple‐shape‐memory performance in terms of the shape fixity and shape‐recovery ratios. This approach allowed greater design flexibility and simultaneous adjustment of the mechanical and shape‐memory properties. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44405.     

Monolayer film behavior of polymers and their mixtures

The monomolecular film behavior of polyvinyl acetate (PVAc), polyethyl acrylate (PEA) and mixtures of these ranging in concentration ratio of PEA to PVAc from 0.008 to 11.301 has been studied using a Langmuir-type filmbalance over an aqueous 0.01N HCl substrate. Hysteresis was observed in the compression–expansion cycle for PVAc. A discussion of polymer–polymer interaction in a mixed monomolecular film at an interface is presented. The extent of this interaction for films comprising PVAc and PEA has been determined in terms of deviations of the film area from the ideal behavior. The data on the film areas of PVAc and PEA are discussed in terms of the molecular orientation of these polymers on the surface. A simple equilibrium thermodynamic treatment is applied to the data on mixed monolayer films. The implications resulting from this study are discussed.     

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.     

Fabrication of Shape‐Memory Aerogel Based on Chitosan/Poly(ethylene glycol) Diacrylate Semi‐Interpenetrating Networks via a Facile and Eco‐Friendly Strategy

While the field of shape memory polymers (SMPs) has developed rapidly, it is still highly challenging to obtain SMPs in the form of aerogels (SMPAs) due to the unique technique used for the fabrication of the aerogels and their high porosity. Herein, a thermally induced SMPA based on chitosan/poly(ethylene glycol) diacrylate (CS/PEGDA) semi‐interpenetrating networks is reported that are produced using an eco‐friendly strategy. The main network is responsible for the shape memory effect (SME) and can be easily tuned by varying the feed ratio of the two PEGDA precursors, which have different molecular weights. The crystalline segment in poly(ethylene glycol) diacrylate (PEGDA) with higher molecular weight acts as the molecular switch, and the PEGDA with lower molecular weight endows the network with an efficient degree of crosslinking. Meanwhile, the chitosan (CS) is interpenetrated into the main network to enhance the aerogel. The SME is realized both at the macroscale and the microscale, as is further demonstrated for three different models with various shapes.     

Synthesis of poly(cyclooctene) by ring‐opening metathesis polymerization: Characterization and shape memory properties

Cis‐cyclooctene was polymerized via ring‐opening metathesis polymerization using a well‐defined ruthenium catalyst (Grubbs' type) under varying reaction conditions. Control over molecular weight was achieved by the inclusion of a chain transfer agent and its influence on the behavior of the obtained polymers was evaluated. The resulting polymers were characterized by means of differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical thermal analysis. Taking into account their thermal behavior, samples of appropriate molecular weight were subjected to a suitable treatment by chemical crosslinking to obtain a material showing thermally induced shape memory effect. The material recovers its original shape after several cycles of deformation into different shapes. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Improvement of film properties of vinyl acetate based emulsion polymers by using different types of maleic acid diesters

Two types of maleic acid diesters, dibutyl maleate (DBM) and dioctyl maleate (DOM) were used as comonomers in semicontinuous emulsion copolymerization of vinyl acetate (VAc) in order to improve the film properties of poly(vinyl acetate), PVAc emulsion polymer. The effects of the comonomer type and comonomer ratio on minimum film forming temperature (MFFT), glass transition temperature (T_g), polymer structure, molecular weights, water contact angle and water resistance of PVAc latex films were examined. It was found that MFFT and T_g of the PVAc emulsion polymer decreased by the presence of the maleic acid disters in copolymer composition. This decrease was more affected by the increasing content and alkyl chain length of the comonomers. The molecular weights of the emulsion polymers were also affected by the comonomers and their ratios. Moreover, hydrophobicity and water resistance of the PVAc latex films were increased by using DBM and DOM as comonomer.     

Characterization of nanocellulose‐ reinforced shape memory polyurethanes

BACKGROUND: Shape memory polymers are capable of fixing a transient shape and of recovering their original dimensions by the application of an external stimulus. Their major drawback is their low stiffness compared to smart materials based on metals and ceramics. To overcome this disadvantage, nanocellulose was utilized as reinforcement. RESULTS: Composites were prepared by casting stable nanocellulose/segmented polyurethane suspensions. The heat of melting of the polyurethane soft segment phase increased on cellulose addition. Composites showed higher tensile modulus and strength than unfilled films (53% modulus increase at 1 wt% nanocellulose), with higher elongation at break. Creep deformation decreased as cellulose concentration increased (36% decrease in 60‐minute creep by addition of 1 wt% nanocellulose). The nanocomposites displayed shape memory properties equivalent to those of the neat polyurethane, with recoveries of the order of 95% (referred to second and further cycles). CONCLUSIONS: It is possible to markedly improve the rigidity of shape memory polymers by adding small amounts of well‐dispersed nanocellulose. However, this improvement did not have substantial effects on the material shape fixity or recovery. Shape memory behavior seems to continue to be controlled by the polymer properties. Copyright © 2007 Society of Chemical Industry     

Effect of molecular weight on shape memory behavior in polyurethane films

Understanding the relationship between the number‐average molecular weight (M_n) and the shape memory behavior of polymers is crucial for a complete picture of their thermomechanical properties, and hence for the development of smart materials, and, in particular, in textile technology. We report here on the study of shape memory properties as a function of M_n of polymers. Shape memory polyurethanes (SMPUs) of different M_n were synthesized, with various catalyst contents or molar ratio(r = NCO/OH) in the composition. In particular, two types of SMPU, namely T_m and T_g types according to their switch temperature type, were synthesized to compare the influence of M_n on their shape memory behavior. X‐ray diffraction, differential scanning calorimetry, dynamic mechanical analysis, and shape memory behavior results for the SMPUs are presented. The results indicate that the melting temperature (T_m), the glass transition temperature (T_g), the crystallinity, and the crystallizability of the soft segment in SMPUs are influenced significantly by M_n, before reaching a critical limit around 200 000 g mol^?1. Characterization of the shape memory effect in PU films suggests that the T_m‐type films generally show higher shape fixities than the T_g‐type films. In addition, this shape fixity decreases with increasing M_n in the T_g‐type SMPU, but the shape recovery increases with M_n in both types of SMPU. The shape recovery temperature, in contrast, decreases with M_n as suggested by the result of their thermal strain recovery. It is concluded that a higher molecular weight (M_n > 200 000 g mol^?1) is a prerequisite for SMPUs to exhibit higher shape recovery at a particular temperature. Copyright © 2007 Society of Chemical Industry     

Shape‐memory behavior of high‐strength amorphous thermoplastic poly(para‐phenylene)

The purpose of this study was to investigate the shape‐memory behavior of poly(para‐phenylene) (PPP) under varying programming temperatures, relaxation times, and recovery conditions. PPP is an inherently stiff and strong aromatic thermoplastic, not previously investigated for use as a shape‐memory material. Initial characterization of PPP focused on the storage and relaxation moduli for PPP at various frequencies and temperatures, which were used to develop continuous master curves for PPP using time–temperature superposition (TTS). Shape‐memory testing involved programming PPP samples to 50% tensile strain at temperatures ranging from 155°C to 205°C, with varying relaxation holds times before cooling and storage. Shape‐recovery behavior ranged from nearly complete deformation recovery to poor recovery, depending heavily on the thermal and temporal conditions during programming. Straining for extended relaxation times and elevated temperatures significantly decreased the recoverable deformation in PPP during shape‐memory recovery. However, PPP was shown to have nearly identical full recovery profiles when programmed with decreased and equivalent relaxation times, illustrating the application of TTS in programming of the shape‐memory effect in PPP. The decreased shape recovery at extended relaxation times was attributed to time‐dependent visco‐plastic effects in the polymer becoming significant at longer time‐scales associated with the melt/flow regime of the master curve. Under constrained‐recovery, recoverable deformation in PPP was observed to have an exponentially decreasing relationship to the bias stress. This study demonstrated the effective use of PPP as a shape‐memory polymer (SMP) both in mechanical behavior as well as in application. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42903.     

Compostability of cellulose acetate films

Composting is an accelerated biological decay process viewed by many to be a potential solution to the solid-waste management crisis existing in many parts of the world. As part of a program to develop environmentally nonpersistent polymers that are compatible with a composting environment, we have developed a bench-scale compost methodology that emulates a high efficiency municipal windrow composting operation. A series of cellulose acetate films, differing in degree of substitution, were evaluated in this bench-scale system. In addition, commercially available biodegradable polymers such as poly(hydroxybutyrate-co-valerate) (PHBV) and polycaprolactone (PCL) were included as points of reference. Based on film disintegration and on film weight loss, cellulose acetates, having degrees of substitution less than approximately 2.20, compost at rates comparable to that of PHBV. NMR and GPC analyses of composted films indicate that low molecular weight fractions are removed preferentially from the more highly substituted and slower degrading cellulose acetates. © 1994 John Wiley & Sons, Inc.     

Rheological,crystallization and foaming behaviors of high melt strength polypropylene in the presence of polyvinyl acetate

Polyvinyl acetate (PVAc) is a kind of CO₂-philic materials with high solubility of CO₂. For improving the supercritical carbon dioxide (Sc-CO₂) foaming behavior of isotactic polypropylene (iPP), a high melt strength polypropylene (HMSPP) was prepared using styrene (St) as grafting monomer. The effect of PVAc on the preparation, rheological, crystallization and foaming behaviors of HMSPP was investigated. The high temperature gel permeation chromatography (HT-GPC) results showed that the PVAc had a promotive effect on melt grafting reaction. With the addition of PVAc, the weight-average molecular weight (M_w) of HMSPP increased from 217,158 to 240,733 g/mol. Thus, the HMSPP presented higher complex viscosity and storage modular, and lower loss angle, which indicated that the melt viscosity and melt strength of HMSPP was increased by adding PVAc. The crystallization behavior of HMSPP was investigated using differential scanning calorimetry (DSC). Double crystallization peaks were observed on the DSC cooling curves of HMSPP in the presence of PVAc, which was ascribed to incomplete molten of iPP with long chain branching (LCB) structure at low end melting temperature. Moreover, the prepared HMSPP exhibited better foaming behavior in the presence of PVAc. With the addition of PVAc, the average cell diameter of HMSPP decreased from 93 to 59 μm, and the cell density increased from 2.83?×?10⁷ to 9.79?×?10⁷ cell/cm³.     

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.     

The morphology,properties, and shape memory behavior of polylactic acid/thermoplastic polyurethane blends

Biodegradable polylactic acid (PLA) was compounded with thermoplastic polyurethane (TPU) by twin‐screw extrusion at weight ratios of 90/10, 80/20, 70/30, and 60/40. The blends were investigated based on their phase morphology, thermal and mechanical properties, and shape memory properties. The tensile results showed that PLA was successfully toughened by TPU. When the TPU content was 40%, the elongation‐at‐break increased to 400%. The SEM morphology showed that TPU was dispersed uniformly in the PLA matrix; DMA and DSC results indicated that the two polymers were immiscible. Most interestingly, it was found that the blends exhibited a shape memory behavior and, unlike most of the existing shape memory polymers (SMPs), the PLA/TPU blends could be deformed at room temperature without an extra heating and cooling step. During the deformation process, TPU acted as a toughening agent that prevented the PLA/TPU blends from breaking; thus, the temporary shape could be kept and internal stress was stored in the blends. Upon heating to above the glass transition temperature of PLA (about 60°C), the deformed parts regained their original shapes quickly along with the release of the stress. POLYM. ENG. SCI., 55:70–80, 2015. © 2014 Society of Plastics Engineers     

Blends of poly(vinyl acetate) and poly(ethyl methacrylate): Some mechanical properties

Poly(vinyl acetate) (PVAc) and poly(ethyl methacrylate) (PEMA) were solution blended in chloroform and cast into films on mercury surface. Some mechanical properties of the films were studied with the Instron Testing Machine. Tensile strength (TS), initial modulus (IM) and elongation at break of the films were found to depend highly on blend composition, and increased above the values for the pure polymers, each showing a peak at about 20% PEMA. The peak values of TS, IM, and elongation at break depended on an important factor, (M _v)_rc the ratio of the molecular weights of (PEMA) to (PVAc). Improvements to the mechanical properties of the polymers due to blending were considered to be as a result of the presence of favorable and strong (PVAc-PEMA) intermolecular interactions which reveal miscibility and compatibility of the polymers. A lower critical value (M _v)_rc of 4.9 was found, above which no phase separation would be expected on blending these two polymers to the extent of 20% by weight of PEMA.     

Partially constrained recovery of (meth)acrylate shape‐memory polymer networks

The purpose of this study was to characterize the partial strain recovery of a thermoset shape‐memory polymer under a constraining stress. Three polymer networks were synthesized from tert‐butyl acrylate and poly(ethylene glycol) dimethacrylate (PEGDMA) solutions. The molecular weight and the weight fraction of the PEGDMA crosslinking monomer was altered systematically to maintain a constant glass transition temperature (T_g = 54°C) but tailorable rubbery moduli, which varied by almost an order of magnitude for the three polymer networks (E = 1.8–11.3°MPa). The shape‐recovery behavior of the polymers under a constraining stress was characterized for programming temperature below (20°C) and above (70°C) the T_g. The experiments revealed a peak in the recovered strain for samples programmed at 20°C. Recovered strain scaled linearly with the constraining stress by the rubbery modulus. The work performed by the shape‐memory polymer networks was observed to be primarily a function of constraining stress and crosslinking density, while programming temperature had a relatively mild influence; however, the efficiency of the shape‐memory effect was shown to be a function of constraining stress and programming temperature, but was independent of crosslinking density. Maximum work efficiencies (up to 45%) were observed for programming temperature of 70°C. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Stress - Optical behavior of Poly(m-xylylenediamine adipamide) (Nylon MXD6): Influence of molecular weight

The effect of molecular weight and rubbery state uniaxial stretching conditions on the mechano-optical behavior of Poly(m-xylylenediamine adipamide) (Nylon MXD6) films was investigated using a real time spectral birefringence stretching machine. The stress optical behavior exhibits a multi stage behavior that depends on process conditions as well as molecular weight. At low stretching temperatures and high rates the stress optical behavior was found to start with an initial glassy photo elastic behavior. Decreasing stretching rate or increasing processing temperature was found to eliminate this glassy photoelastic regime leading to the observation of a linear initial stress optical behavior past a temperature of 95 °C (T_ll). The stress optical constant (SOC) was about the same for both M.W. materials stretched at temperatures past T_ll, at 2.7 GPa⁻¹ for HPA6 and 2.61 GPa⁻¹ for LPA6. Following this initial regime, the behavior is controlled by the competition between orientation and relaxation during deformation. If the chain orientation relaxation is not suppressed by increasing the stretching rate and/or the molecular weight or by decreasing temperature, the material strain crystallizes.     

Solvent effects on electrochemical behavior of poly(ferrocenylsilane) films

The electrochemical behaviors of poly(ferrocenylsilane) (PFS) films in organic solutions were investigated by means of cyclic voltammetry (CV) and electrochemical quartz crystals microbalance (EQCM). The influences of solvent on the electrochemical behavior of the films were discussed. In “good” solvents, the supporting electrolytes dissociated completely, the films were solvent-swollen moderately which provided a favorable condition for the electrolyte ions penetrating through, and the CV behavior of the films exhibited reversible or nearly reversible features. With increasing the carbon chain length of solvent molecule, however, the polarity of solvent reduced, which conduced to decrease the dissociation of electrolyte and the swelling of the polymer film. The efficiency of electrochemical reaction in the film was depressed, and the CV behavior of the film exhibited low reversibility. The solvent effects on the oxidation process of films exhibited more noticeable than the reduction process. The results supported the viewpoint that penetration of the electrolyte anions played an important role on the charge balance and transfer in the films during the CV process. The different electrochemical behaviors of the two PFS films in various organic solutions indicated that molecular structure of polymer had important influence on the electrochemical properties of the PFS.     

Plastic deformation and tearing in high density polyethylene blown films

Polymer films produced by tubular film blowing have a unique morphology that results from the large elongational flow in melt draw down and biaxial orientation due to bubble blow-up. Three high density polyethylene (HDPE) blown films were produced under similar processing conditions from resins which varied principally in molecular weight (MW) and molecular weight distribution (MWD). Scanning electron microscopy (SEM) showed that the lower MW and narrower MWD resin produced film which had a uniaxial orientation of stacked lamellar crystals. The higher MW (HMW) and broad MWD resins produced films consisting of a network of nearly orthotropically oriented lamellar stacks. Greater high molecular weight fraction (MW > 10⁶) in the resin resulted in more random orientation. The influence of these different structures on properties was studied by examining the plastic zone formation at crack tips and uniaxial tensile deformation with the SEM and comparing them to the macroscopic stress-strain behavior. A continuous deformation of the network structure was observed in the HMW films. Lamellar deformation occurred primarily in regions of stacks oriented parallel to the tensile axis. Macroscopic yield occurred at 6 to 10 percent strain via a shearing and opening the lamellar crystals. Irreversible deformation occurred from ?50 to 400 percent strain by transformation of the oriented lamellae to microfibrils. Eventually all the lamellar stacks in the network become aligned with the tensile axis. This process was found to improve the tear resistance in the crack propagation experiments. The lamellar stacks in the network orient perpendicular to the crack independent of crack propagation direction, insuring a more uniform transmission of stress and preventing local yielding. The tensile modulus, yield stress, and ultimate strength were highest in the film containing more high molecular weight polymer.     

Further experimental evidences of the compatibility of poly(vinyl acetate) with poly(ethyl methacrylate) and its dependence on the ratio of the molecular weight of the two homopolymers

Using various fractions of poly(ethyl methacrylate) (PEMA) and poly(vinyl acetate) (PVAc) of very narrow molecular weight distribution, a very wide range of 2–120 of molecular weight ratio M¯_ν PEMA/M¯_ν PVAc, (M¯_ν)_r was obtained. Studies of some tensile mechanical properties of films of the blends produced from solution on mercury confirmed strong dependence of the improvements of PVAc properties for blend with 18% PEMA on (M¯_ν)_r, especially in the range 5 ≤ (M¯_ν)_r < 100. The density of the films of the blend was much higher than those of the individual polymers and increased monotonically with (M¯_ν)_r. Optical micrographs of the films of the blends showed interactions between the two polymers with 18% PEMA composition, which appears to be more intimate as (M¯_ν)_r increases, as further evidence of compatibility and miscibility of the two polymers.     

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.