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.     

In-plane deformation of shape memory polymer sheets programmed using only scissors

This paper describes an unconventional, yet simple method to program sheets of shape memory polymer into a variety of two dimensional (2D) structures. The final shape is “encoded” by physically cutting an initial design out of a pre-strained film. The orientation of the initial cut-out relative to the direction of strain and the subsequent relaxation of strain via heating defines the final shape. The appeal of the approach described here is that an easy, low-cost cutting method can achieve a similar shape memory effect attained by more complex processing techniques. Unlike conventional methods, where the final shape of a shape memory polymer must be defined a priori, the direction of cutting of the polymer defines its final shape without any complex pre-programmed strain profiles. A geometric model relating the resolved 2D polymer shape to the initial shape and strain orientation reveals linear correlation between the model-predicted and experimentally-observed shapes. In addition to demonstrating the principle with simple rectangular shapes, we suggest geometries related to encryption and high aspect ratio fibers.     

Improvement in shape memory in magnesium niobate modified PZST

A modified lead zirconate stannate titanate system (PZST) has been studied for shape memory effect. Addition of magnesium niobate (MN) slows down the dipole relaxation process, leading to increased (∼3-fold improvement) remnant strain in the PZST system (0.99PZST–0.01PMN). Room temperature X-ray diffraction patterns before poling (antiferroelectric (AFE) tetragonal) and after poling (ferroelectric (FE) rhombohedral), clearly demonstrate that the transition to the ferroelectric phase is stable even in the absence of any electric field. A small applied electric field (∼1.7 kV/cm) in the opposite direction was required to bring the sample back to its original shape. Field-induced strain butterfly loops taken at 50 Hz show that the material response time is quite small.     

A novel type of shape memory polymer blend and the shape memory mechanism

A novel styrene-butadiene-styrene tri-block copolymer (SBS) and poly(?-caprolactone) (PCL) blend were introduced for its shape memory properties. Compared to the reported shape memory polymers (SMPs), this novel elastomer and switch polymer blend not only simplified the fabrication process but also offer a controllable approach for the study of mechanisms and the optimization of shape memory performances. Microstructures of this blend were characterized by differential scanning calorimetry (DSC), AFM microscope observation and tensile test. DSC results demonstrated the immiscibility between SBS and PCL. AFM images and stress-strain plot further confirmed the two-phase morphology within the blend. It was found that the SBS and PCL continuous phases contributed to the shape recovery and shape fixing performances, respectively. A detailed shape memory mechanism for this type of SMP system was then concluded and an optimized SMP system with both good recovery and fixing performances was designed from this mechanism.     

A novel approach to predict the recovery time of shape memory polymers

In this paper a novel approach is presented for prediction of the recovery time for a shape memory polymer. The Transient Stress Dip Tests of Fotheringham and Cherry are used to determine the two parameters of a Kelvin-Voigt element. The characteristic retardation time of this element can then be calculated to predict the recovery time. It is shown that this approach is successful in predicting the recovery times for a shape memory polymer drawn and recovered under a range of temperatures. Furthermore it is shown that the ratio of the recovery stress to the draw stress is independent of the drawing conditions to a very good approximation.     

Shape memory properties of main chain bile acids polymers

Main chain bile acid polymers obtained via entropy-driven ring-opening metathesis are biodegradable polymers with excellent shape memory properties. X-ray scattering was used to study the conformational changes implicated in the shape memory effect of these polymers. Strain induced orientation and ordering was observed upon the drawing of the polymer films. A correlation between ordering and the shape memory performances is observed.     

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.     

A novel supramolecular shape memory material based on partial α-CD-PEG inclusion complex

A novel supramolecular shape memory material was prepared based on partial α-CD-PEG inclusion complex, which contains α-CD-PEG inclusion crystallites as a fixing phase and naked PEG crystallites as a reversible phase. The recovery ratio of these materials could reach 97%. The characteristics of the material were investigated and a mechanism for the shape memory behavior was proposed.     

Nanoclay-tethered shape memory polyurethane nanocomposites

The study investigated shape memory properties of nanoclay-tethered polyurethane nanocomposites. Polyurethanes based on polycaprolactone (PCL) diol, methylene diisocyanate, and butane diol and their nanocomposites of reactive nanoclay were prepared by bulk polymerization in an internal mixer and the values of shape fixity and shape recovery stress were determined as function of clay content. The melting point of the crystalline soft segment was used as the transition temperature to actuate the shape memory actions. It was seen that clay particles exfoliated well in the polymer, decreased the crystallinity of the soft segment phase, and promoted phase mixing between the hard and soft segment phases. Nevertheless, the soft segment crystallinity was enough and in some cases increased due to stretching to exhibit excellent shape fixity and shape recovery ratio. A 20% increase in the magnitude of shape recovery stress was obtained with the addition of 1 wt% nanoclay. The room temperature tensile properties were seen to depend on the competing influence of reduced soft segment crystallinity and the clay content. However, the tensile modulus measured at temperatures above the melting point of the soft segment crystals showed continued increases with clay content.     

Impact of shape-memory programming on mechanically-driven recovery in polymers

Shape-memory polymers (SMPs) are a class of mechanically functional “smart” materials defined by their ability to change shape upon exposure to an environmental stimulus. The shape-memory effect has traditionally been activated by thermal mechanisms via heating the polymer above a transition temperature to increase chain mobility and initiate shape recovery. This study proposes a unique approach to mechanically drive recovery in SMP networks using external forces to facilitate shape change in a material with stored strain. SMP networks were synthesized from tert-butyl acrylate and poly(ethylene glycol) dimethacrylate in three network compositions. Networks were tailored to maintain a constant glass transition temperature (∼52 °C) with increasing crosslinking density, shown by rubbery modulus values of 1.2, 3.1, and 8.2 MPa. Hollow SMP cylinders were axially elongated (programmed) to stored strain levels of approximately 25%. A second set of samples was machined to match the programmed dimensions of the SMP sample set. Compression testing revealed that the compressive strength and energy required for deformation for the programmed SMP samples were on average 62% and 52% of the as-machined samples’ values, respectively. The ratios between programmed and as-machined samples’ compressive properties were independent of both crosslinking density and temperature up to the onset of glass transition. Lastly, an interference-fit test model was used to demonstrate that mechanically-driven SMPs could immediately create and maintain a stronger fixation force compared to as-machined samples and thermally-driven SMP samples. This study introduces an approach to drive shape change that mitigates the time-temperature dependence and discusses the potential of this mechanism for biomedical devices.     

Synthesis and characterization of high temperature cyanate-based shape memory polymers with functional polybutadiene/acrylonitrile

New thermosetting shape memory cyanate polymers (SMCPs) modified with polybutadiene/acrylonitrile (PBAN) were synthesized and compared with polyethylene glycol (PEG)-modified SMCPs for integration into the family of high temperature shape memory polymers with controllable glass transition temperatures (T_g) used in the aerospace industry. The materials were characterized in terms of microstructure, thermal properties, mechanical properties and shape memory properties by Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis, and tensile tests. Differing from the SMCP with PEG, the new cyanate-based shape memory polymer with PBAN (T_g ∼255.1.0 °C) had better shape memory properties and higher thermal stability (relatively high initial degradation temperature and high char residue value at 800 °C). Both of the SMCPs with PBAN and PEG displayed exceedingly high glass transition temperatures over 241.3 °C and higher toughness than unmodified polycyanurate. These qualities render them desirable candidates as matrices in polymer composites, particularly for space applications.     

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.     

Semi-crystalline two-way shape memory elastomer

Most shape memory polymers are rigid and exhibit only one way (irreversible) shape memory effect. This work explored the two-way (reversible) shape memory effect for semi-crystalline elastomeric networks. These semi-crystalline networks with various gel fractions and crosslinking densities were synthesized via radical initiated crosslinking of poly(ethylene-co-vinyl acetate). The thermal, mechanical, and the two-way shape memory properties of the resulting materials were investigated. The critical impact of the gel fraction on the two-way shape memory behaviors was revealed. The results from this study establish the basis for potential applications involving soft reversible actuation.     

Modeling shape memory effect in uncrosslinked amorphous biodegradable polymer

Shape memory effect (SME) is critical for minimally invasive surgical procedures in medicine. In this paper, the shape memory behavior of amorphous biodegradable polymer, poly(d,l-lactide-co-glycolide), is experimentally investigated. Based on the experimental observations and the understanding of the underlying mechanism of SME, a one-dimensional constitutive model is derived to describe the shape memory behavior in the context of (1) the stress-strain behavior in deformation, (2) the isothermal recovery and (3) the recovery at constant heating rate, by using a set of model constants. By fine tuning the model constants, a good agreement between the experimental results and computer predictions was achievable.     

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.     

High transition temperature shape memory polymers (SMPs) by telechelic oligomer approach

This work describes the synthesis and comparative shape memory properties of cross-linked networks derived from epoxy and cyanate ester monomers containing polyether oligomers as reactive shape memory segments. The hydroxy telechelic oligomers viz. polyethyleneglycol (PEG), polypropyleneglycol (PPG), and polytetramethyleneglycol (PTG) are reacted with epoxy–cyanate ester matrix resulting in shape memory polymers with high transition temperatures. The soft oligomer segments act as flexible linker unit which interconnect oxazolidone, isocyanurate and triazine ring structures in the cross-linked polymer. The resultant cyclomatrix SMPs exhibit high transition temperatures 132, 178 and 161 °C respectively for PEG, PPG and PTG integrated SMPs. The E_g/E_r ratios are increased in the order PEG < PTG < PPG. The PTG and PPG based SMPs show shape retention of 99% and shape recovery of >98% with recovery time <100 s. All the SMPs display good thermal stabilities (both inert and oxidative) above 275 °C.     

Radiation crosslinked shape-memory polymers

Shape-memory polymers (SMPs) are active smart materials with tunable stiffness changes at specific, tailored temperatures. The use of thermoset SMPs has been limited in commodity applications because a variety of common low-cost plastics processing techniques are not possible with network polymers. In this study of thermoset SMPs, beyond adjusting the glass transition temperature (T_g) between 25 and 75 °C and tuning the recoverable force between 0.5 and 13 MPa, a novel manufacturing process, Mnemosynation, is described. The customizable mechanical properties of traditional SMPs are coupled with traditional plastic processing techniques to enable a new generation of mass producible plastic products with thermosetting shape-memory properties: low residual strains, tunable recoverable force and adjustable T_g. The results of this study are intended to enable future advanced applications where mass manufacturing, the ability to accurately and independently position T_g and the ability to tune recoverable force in SMPs are required.     

Viscoelastic characteristics of shape memory polymers

Shape memory polymers (SMPs) can keep a temporary state and subsequently recover to the original shape through a prescribed thermomechanical process. Although different theoretical models have been presented, the viscous effects were seldom considered. This article aims to provide an insight into the viscoelastic property of SMPs and its effect on the functional realization. Systematic thermomechanical experiments were performed. Special considerations were focused on the viscoelastic response of SMPs in the vicinity of the glass transition temperature T_g. The relations between shape switching transition temperature T_tran and T_g were also discussed. The results confirm that T_tran departs from T_g due to the viscoelastic effect and does not keep a constant value during heating and cooling processes. The viscoelastic effect reaches to maximum value at T_g, then decreases slowly at cooling and quickly at heating. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Novel moisture-sensitive shape memory polyurethanes containing pyridine moieties

In this communication, novel moisture-sensitive shape memory polyurethane (SMPU) was synthesized from 1,6-hexamethylene diisocyanate (HDI) and N,N-bis(2-hydroxylethyl) isonicotinamide (BINA) and 1,4-butanediol (BDO). Results show that the BINA based SMPUs have excellent moisture absorption properties which are not only influenced by the temperature, but also by the relative humidity (RH). As a result, high strain recovery with fast recovery speed is obtained after immersion in the moisture condition for a short time. FT-IR spectra provide a proof to the mechanism of moisture-sensitive SME which is based on the dissociation or disrupt of hydrogen bonding in the pyridine ring induced by moisture absorption. Thus, the strain recovery is achieved in the moisture-sensitive SMPUs by decreasing the glass transition temperature (T_g) below ambient temperature without external heating.     

Co-extruded multilayer shape memory materials: Nano-scale phenomena

Shape memory polymers (SMPs), which demonstrate the ability to possess multiple shapes, are traditionally produced from copolymers and recently from blends. These materials often have phase separated morphologies that possess domain sizes on either the nano- or micro-scale. The observed properties, specifically the shape memory behavior, can be significantly altered by a change in the domain size; however, doing this often requires modification to the materials or material production process. Forced assembly multilayer co-extrusion was used to produce shape memory materials with a continuous layered structure that can be easily tailored to cover layer thicknesses ranging from the nano- to the micro-scale. Upon decreasing the layer thickness of polyurethane/polycaprolactone (PU/PCL) layered films, improvement in the shape fixity and recovery ratios tracked with layer thickness. The improvement in properties was attributed to a change in the PCL crystal orientation from randomly oriented in microlayers to in-plane lamella orientation in nanolayers.