Article pubs.acs.org/Macromolecules
Designing Multiple-Shape Memory Polymers with Miscible Polymer Blends: Evidence and Origins of a Triple-Shape Memory Effect for Miscible PLLA/PMMA Blends Cédric Samuel,*,† Sophie Barrau,‡ Jean-Marc Lefebvre,‡ Jean-Marie Raquez,† and Philippe Dubois† †
Laboratory of Polymeric and Composite Materials (LPCM), Center of Innovation and Research in Materials and Polymers (CIRMAP), University of Mons - UMONS, Place du Parc 23, B-7000 Mons, Belgium ‡ Unité Matériaux et Transformations (UMET), CNRS UMR 8207, Université Lille 1, Sciences et Technologies, Bât. C6-107, F-59655 Villeneuve d’Ascq, France S Supporting Information *
ABSTRACT: Shape memory properties of polymers represent one of the most expanding fields in polymer science related to numerous smart applications. Recently, multiple-shape memory polymers (multiple-SMPs) have attracted significant attention and can be achieved with complex polymer architectures. Here, miscible PLLA/ PMMA blends with broad glass transitions are investigated as an alternative platform to design multiple-SMPs. Dual-shape memory experiments were first conducted at different stretching temperatures to identify the so-called “temperature memory effect”. The switch temperature of the symmetric 50% PLLA/50% PMMA blend smoothly shifted from 70 to 90 °C for stretching temperatures increasing from 65 to 94 °C, attesting for a significant “temperature memory effect”. Asymmetric formulations with 30% and 80% PMMA also present a “temperature memory effect”, but the symmetric blend clearly appeared as the most efficient formulation for multiple-shape memory applications. A programming step designed with two successive stretchings within the broad glass transition consequently afforded high triple-shape memory performances with tunable intermediate shapes, demonstrating that the symmetric blend could represent an interesting candidate for future developments. Advanced shape recovery processes are consistent with a selective activation of specific “soft domains” or nanodomains arising from the broad glass transition and the large distribution of relaxation time observed by DSC and dielectric spectroscopy. Polarized IR measurements pointed out that the composition of activated/oriented “soft domains” could vary with stretching temperature, giving rise to the “temperature-memory effect”. Consequently, from a polymer physics standpoint, nanoscale compositional heterogeneities within the symmetric blend could be suspected and discussed on the basis of available models for miscible blends and for multiple-SMPs.
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INTRODUCTION
great importance in the aerospace and electronics industry, for instance. Most SMPs are dual-shape memory polymers (dual-SMPs) that are able to store two different shapes: temporary and permanent shapes. The shape memory effect is based on a single transition (a glass transition or a melting transition) to fix the temporary shape after a programming step, classically including a deformation at temperature higher than the corresponding transition followed by a fast cooling. For an efficient dual-shape memory effect, the memorization of the initial shape requires the presence of chemical and/or physical cross-links (crystallites for semicrystalline polymers and entanglements for amorphous polymers).8,9 Recently, new
Shape memory polymers (SMPs) currently represent an expanding field of high technological interests for the development of innovative materials and smart applications. These mechanically active polymers are able to change ondemand their shape and to generate mechanical stresses and strains upon the application of various stimuli (temperature, UV irradiation, electric and magnetic field, water contact, etc.).1−4 Most of applications concern thermally actuated SMPs for advanced medical applications with the elaboration of smart stents, sutures, and other invasive devices, which could change their shape near body temperature.5 SMP were also introduced in numerous and ongoing engineering applications such as microfluidic devices, self-healing materials, heat-shrinkable foils and tubes, self-deployable structures, structures for active disassembly, smart actuators, and sensors,1−7 all of them of © XXXX American Chemical Society
Received: April 23, 2014 Revised: September 7, 2014
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dx.doi.org/10.1021/ma500846x | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
could represent an attractive pathway to multiple-SMPs, especially as far as easy processing is concerned. In our previous investigations on miscible polymer blends based on poly(L-lactide) (PLLA), we evidenced the miscibility between PLLA and poly(methyl methacrylate) (PMMA) after meltblending at 210 °C using twin-screw extrusion technologies.23,24 As a consequence of the miscibility, the glass transition temperature and the α-relaxation temperature are finely tuned with the amount of PMMA incorporated into PLLA. In addition, these transitions broadened significantly for miscible blends, and the symmetric 50% PLLA/50% PMMA presented the broadest glass transition with a range of temperatures between approximately 60 and 100 °C. Therefore, the use of miscible polymer blends could make a radical change in the design of multiple-SMP systems, and to the best of our knowledge, the use of such melt-blended miscible polymer pairs was only documented for dual-SMP purposes,26,27 but multipleSMPs have never been investigated. From our past experiences on the thermomechanical properties of miscible PLLA/PMMA blends, the development of multiple-SMPs based on such a blend with a broad glass transition has been investigated. Although other miscible blends could be found in the literature, the PLLA/PMMA binary system has been used as a model miscible blend to evaluate the multiple-shape memory performances and limitations as a function of composition and, in particular, the broadness of the glass transition. Thermal and thermomechanical properties were first assessed with DSC and DMA experiments in film tension mode, followed by a systematic investigation of the “temperature memory effect” as a function of the blend composition. A triple-shape memory system is presented on a selected formulation with a focus on the tuning of the intermediate shapes. A final part is dedicated to underlying physical phenomena with correlations between triple-shape memory effects, the distribution of the relaxation time, and the resultant macromolecular orientation after the stretching step. The existence of heterogeneities in the symmetric blend is finally discussed.
functionalities for future applications have been reported for SMPs with the successful memorization of three, four, and even five different shapes using advanced macromolecular compositions and architectures/networks. Multiple-shape memory polymers (multiple-SMPs) could be basically prepared by introducing several distinct and well-defined glassy and/or melting domains. For example, poly(ε-caprolactone)/poly(cyclohexyl methacrylate) networks10 and poly(ε-caprolactone) networks with grafted poly(ethylene glycol) side chains11 could memorize and recover an additional shape upon an appropriate thermomechanical programming and recovery steps. In this case, the triple-shape memory effect can be merely tuned upon the weight composition of different domains existing within the SMPs. Interestingly, several authors have successfully synthesized advanced macromolecular architectures with a broad glass or melting transition and demonstrated the high efficiency of these multiple-SMPs.12−19 Among them, cross-linked poly[ethylene-ran-(vinyl acetate)],12 semi-interpenetrated networks of poly(ethylene oxide) and poly(methyl methacrylate),14 Vshaped poly[stryrene-grad-(methyl acrylate)],16 and norbornene-based copolymers17 represent the most efficient systems ever developed for multiple-SMP purposes. These systems are able to store up to five shapes related to broad glass transitions and/or broad melting transitions as high as 100 °C. Here, the intermediate shapes can be finely tuned upon the prior thermomechanical programming step without the need of changing polymer composition, representing a real advantage in terms of robustness, recycling and reuse of multiple-SMPs. The thermomechanical programming step and especially stretching temperatures play a key role for multiple-SMPs. For instance, Nafion, a cross-linked fluoropolymer with ionic chemical functions, was one of the first multiple-SMPs to be intensively characterized about the influence of the thermomechanical programming step on the multiple-shape memory response.15,18 The authors were able to develop a quasi-perfect triple-shape memory system based on two successive stretchings at 100 and 60 °C within the broad glass transition from 55 to 130 °C. The intermediate shapes could be finely tuned by varying the amount of strain level imposed at each stretching temperature, and more interestingly the recovery of the first and the second shape was observed in vicinity of the corresponding initial stretching temperature. This effect was also mentioned by several authors as the so-called “temperature memory effect”, meaning that the material could efficiently store the temperature at which the deformation was applied.12,13,15,18,20 The memory of the stretching temperature during the thermomechanical programming step clearly represents a key behavior for multiple-SMPs as the recoveries of multiple shapes could be theoretically performed at various predefined temperatures. The importance of the “temperature memory effect” appears of such an interest that authors have tried to identify the physical phenomenon responsible for it. Small-angle neutron scattering experiments revealed that the orientation of ionic clusters in Nafion is highly dependent on the stretching temperature, thus providing a physical interpretation for the multiple-shape memory effects.18 However, these multiple-SMPs still require the synthesis of complex macromolecular architectures, and other simple systems are believed to fulfill the requirements for multipleSMP response in terms of the broadness at the level of the glass and melting transitions. In this respect, broad glass transitions are classically encountered for miscible polymer blends,21−25 and this area
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EXPERIMENTAL SECTION
Materials. Poly(L-lactide), hereafter called PLLA, was kindly supplied by NatureWorks LLC (grade 4032D, D-isomer
