Article pubs.acs.org/Macromolecules
Programming of Temperature-Memory Onsets in a Semicrystalline Polyurethane Elastomer Nikolaus Fritzsche and Thorsten Pretsch* BAM Federal Institute for Materials Research and Testing, Division 6.5, Polymers in Life Science and Nanotechnology, Unter den Eichen 87, 12205 Berlin, Germany S Supporting Information *
ABSTRACT: We demonstrate that phase-segregated poly(ester urethane) (PEU) with crystallizable switching segments of poly(1,4-butylene adipate) (PBA) excels as high-performance temperature-memory polymer. Temperature-memory effects (TMEs) with regard to strain and stress recovering could be programmed by polymer elongation at temperatures below or within the PBA melting transition, followed by cooling under constant stress below the PBA crystallization transition and unloading. Beyond that conventional approach, a novel TME programming route was designed, mostly consisting in specimen elongation and unloading at the same temperature. As a result, an enhanced control over the onsets of strain and stress recovering could be achieved. With these findings, the TME could be exploited to switch quick response (QR) codes in recently developed information carriers from unreadable to readable. We conjecture that such behavior can be programmed into virtually all semicrystalline elastomers and anticipate applicability as label technology to monitor temperature abuse of food and pharmaceuticals.
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INTRODUCTION Shape-memory polymers (SMPs) belong to the emerging class of smart materials. SMPs can be stabilized in a temporary shape and recover deformations of several hundred to even thousand percent in strain in response to heating.1−4 Because of the ability of SMPs to perform complex movements on demand, many groups worldwide, industrial and academic, are pursuing high-tech applications including aerospace,5−8 textile,9−11 packaging,12−14 and biomedical ones.15−22 Among others, research also focused on active disassembly,23−27 actuators,28−32 adhesive systems,33−35 mandrels,36 energy storage,37−39 microfluidic devices,40,41 temperature sensing devices,42,43 and switchable information carriers.44−47 The functional performance of SMPs like thermoplastic polyurethane elastomers, which are structurally composed of netpoints interconnected by switching segments, can be tailored by varying synthesis and manufacturing strategies. Some of the tunable key parameters are the polymer architecture including the molecular weight of the switching segment,48−50 microphase separation,51,52 and cross-link density.53 In particular, postpolymerization cross-linking of structural motifs, which are part of the polymer repeat units, by curing54 or electron beam irradiation55 is an appropriate tool to set the thermal and mechanical properties of polyurethane SMP systems. Once fabricated, triggering of the shape memory effect (SME) requires a preceding thermomechanical treatment, socalled programming or functionalization. This usually necessitates complete devitrification or melting of the switching © 2014 American Chemical Society
segment prior to deformation, whereupon polymer chains are aligned and the conformational entropy of the system is reduced,56,57 and cooling under constraint conditions to fix the temporary shape by crystallizing or vitrifying the switching segment. When heated again above the phase transition temperature, drastic growth in entropy drives shape recovering, culminating in the almost complete restoration of the permanent shape. By contrast, when applying a deformation at a temperature Td within the glass or melting transition of the switching segment and cooling the polymer below the corresponding thermodynamic phase transition, temperaturememory effects (TMEs) can be verified in an ensuing heating step.16,58−60 The TME is defined as the ability of a polymer to generate a substantial response in strain (under stress-free conditions), particularly a peak in strain recovery rate,61 or a maximum recovery stress59 (under constant strain conditions) close to Td. For this reason, temperature-memory behavior opens the door for gaining precisely control over the switching behavior of polymers without modifying the synthesis route or manufacturing process. Temperature-memory properties can be explained by contemplating the phase transition behavior of the switching segment, which in case of a glass transition can be understood as an infinite number of single sharp transitions.62−64 When applying a deformation within a polymer’s Received: June 5, 2014 Revised: July 4, 2014 Published: July 15, 2014 5952
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Scheme 1. Molecular Structure of the Repeat Units in PEU
the presence of crystalline PBA units in the PEU structure at 23 °C, is supplied in the Supporting Information (Figure S2). Characterization Methods. Differential scanning calorimetry (DSC) scans were run with an EXSTAR DSC7020 from Seiko Instruments Inc. The sample weight was approximately 5 mg. The sample was heated from −90 to 90 °C with a rate of 10 °C min−1. Dynamic mechanical analysis (DMA) was conducted with a Netzsch DMA 242, operating in single cantilever bending mode. Here, PEU samples with dimensions of 5 mm × 2 mm × 2 mm were used. The sample was heated from −150 to 100 °C with a rate of 1 °C min−1. In parallel, the storage modulus E′ and the loss modulus E″ were determined. Since DMA results were found to be highly sensitive to the thermal history of PEU (e.g., storage conditions), samples were annealed for 10 min at 60 °C and stored at 23 °C (50% air humidity) for at least 1 week, before their viscoelastic properties were studied. In situ wide-angle X-ray scattering (WAXS) was performed at the synchrotron microfocus beamline “μSpot” at Bessy II (Helmholtz Centre Berlin for Materials and Energy). Details regarding the specimen preparation, experimental setup, and measurement conditions are given in the Supporting Information. The degree of PEU crystallinity χc was estimated from the obtained WAXS patterns by dividing the integrated intensity due to scattering from crystallites Icryst through the totally scattered intensity Itotal:
glass transition temperature (Tg) range, some subunits undergo the transition and are soft (amorphous), while others remain hard (glassy). Hence, the soft subunits can more easily be deformed and the deformation be fixed in the course of cooling by vitrifying the switching segment. Upon heating, the mobility of subunits increases step by step again and the gain in entropy drives shape recovering. Since a similar mechanism seems to apply to semicrystalline polymers around their melting/ crystallization transitions, the temperature-memory behavior appears to be dictated by those fractions of amorphous “memory units”, which solidify in the course of programming. According to the underlying phase transitions, polymers for which TMEs have been verified can at least be divided into two groups. The first one includes candidates characterized by a fairly broad glass transition like vulcanized rubber,58 acrylatebased polymers,16 poly(ether urethane) and radio opaque composites thereof,65 perfluorosulfonic acid ionomer (Nafion),61,66 poly(methyl methacrylate)/poly(ethylene glycol) (PEG) semi-interpenetrating polymer networks,67 and nanocomposites based on carbon nanotubes embedded in poly(vinyl alcohol).59 The second group comprises polymers with a broad crystallite melting transition such as poly[ethylene-ran-(vinyl acetate)] copolymers containing crystallizable polyethylene segments,60 poly(ester urethanes) (PEUs) with crystallizable poly(ε-caprolactone) (PCL) segments,68 and photo-crosslinked star PCL−PEG networks.69 Against this background, the overarching goal of this contribution was to find out if there is an enhanced control over temperature-memory behavior of semicrystalline elastomers through modification of the most prominent programming strategy.60,68,69 To find out the answer, PEU with crystallizable poly(1,4-butylene adipate) (PBA) segments and pronounced viscoelastic properties was examined.70,71 In essence, the influence of unloading temperature (Tu) upon the strain and stress recovery characteristics was studied. Thereupon, TMEs, defined by controllable onset recovery temperatures, could efficiently be realized. Beyond a detailed thermomechanical investigation, mechanistic dissimilarities toward the hitherto employed standard programming route60,68,69 are discussed. In a last step, we followed an application oriented approach by switching the quick response (QR) codes of recently developed information carriers44 at predefined temperatures from machine unreadable to readable. The detected behavior could qualify them as temperature monitoring labels in cold chains.
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χc =
Icryst Itotal
(1)
Thermomechanical measurements (except those on QR code carriers) were conducted with an electromechanical testing system (Zwick/Roell Z005), which was equipped with a thermochamber (Zwick/Roell) and a temperature controller (Eurotherm 2261e). Test procedures were designed with the software testXpert II (V 3.31). Some of the experimental details are given in the Supporting Information. In general, the stress σ was calculated by dividing the force through the initial cross section of the specimen and the strain ε was determined from crosshead displacement. For temperature-memory programming, a PEU specimen was elongated with a rate of 30 mm min−1 up to a maximum strain εm of 100% at deformation temperature Td between −20 and 40 °C. The Young’s modulus E was calculated from the initial slope of the stress− strain curve as ratio of stress to strain. After 5 min, the specimen was cooled to the unloading temperature Tu = −20 °C and unloaded (route 1, Td > Tu) or directly unloaded at Td, before it was cooled to −20 °C (route 2, Td = Tu). In any case, an unloading rate of 1 mm min−1 was chosen. The fixed strain εu, which is the strain after programming a temporary shape, was determined at −20 °C and the thermoresponsiveness investigated during heating from −20 to 80 °C both under stress-free and constant strain recovery conditions. To quantify strain recoverability, the strain εp after recovering a permanent shape was recorded at 80 °C. All measurements were conducted with uniform heating and cooling rates of 3 °C min−1. In further experiments, programming via route 2 was applied with maximum strains up to 400%. Again, the thermoresponsiveness was investigated during heating. The strain fixity ratio Rf and the total strain recovery ratio Rr,tot are important figures of merit for the material’s strain fixing ability and strain recoverability. They are defined by eqs 2 and 3
EXPERIMENTAL SECTION
Materials. The herein investigated PEU was Desmopan DP 2795A SMP from Bayer MaterialScience AG. Samples of the polymer were received as 2 mm thick injection-molded plaques. Detailed information regarding the two-step synthesis process is given in a patent72 and a previous publication.50 In the physically cross-linked structure, the soft segment was built up by PBA (Mw = 3500 g mol−1) and the hard segment was composed of 4,4′-methylenediphenyl diisocyanate (MDI) and a 1,4-butanediol (BD) chain extender (see Scheme 1). An ATR-FT-IR spectrum of pristine PEU including a signal assignment is given in the Supporting Information (Figure S1). A wide-angle X-ray scattering (WAXS) diffraction pattern, which proves
Rf =
εu εm
R r,tot = 5953
(2)
εm − εp εm
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and have been determined accordingly. In the literature, the temperature corresponding to the maximum strain recovery rate is sometimes referred to as switching temperature Tsw.73 Since in our case clear maxima in strain recovery rate were not always detectable, Tsw was defined as that temperature, at which half of the strain release εu − εp was accomplished. A detailed description regarding the determination of Tsw, the maximum recovery stress σmax, and the stress recovery temperature Tσ,max is given in the Supporting Information. The thermomechanical studies were completed by tensile tests at Td = −30, −40, −50, and −60 °C, which were run with the abovementioned deformation rate. Preparation and Thermomechanical Investigation of QR Code Carriers. For sample preparation, PEU was surface-dyed and subjected to laser treatment according to ref 44. For temperaturememory programming, a specific QR code carrier dimensioning was developed, which was suitable for tensile deformation-determinated functionalizations (Figure 1).
later thermomechanical tests were performed at appropriate temperatures. The calorimetric properties of PEU were characterized by a PBA melting transition between 33 and 54 °C, a PBA crystallization transition spreading from 14 to −20 °C, and a PBA glass transition at −45 °C (Figure 2a). As characteristic for physically cross-linked PEUs,49,50,74 the storage modulus exhibited a two-step decrease in the DMA when passing the glass and the melting transition (Figure 2b). The tan(δ) peak, which often is used to determine the glass transition temperature Tg in polymers, was located at −35 °C. In comparison with the DSC results, Tg of urethane-based SMPs is commonly a few degrees higher in the DMA due to differences in testing procedures and chain dynamics under the respective conditions.75 To elucidate the influence of programming upon the temperature-memory behavior of PEU, attention was devoted to two scenarios. In the first one, a tension was applied at a deformation temperature Td, before the specimen was cooled under constraint conditions to −20 °C, corresponding to the DSC offset PBA crystallization temperature, and unloaded. Thus, a rather conventional approach was followed by selecting Td above the unloading temperature Tu (Td > Tu, route 1).60,68,69 In the other case, a novel route was explored, consisting of uniaxial stretching and unloading at the same temperature (Td = Tu, route 2) and completed with specimen cooling to −20 °C. Independent of the programming route, a maximum tensile strain εm of 100% was applied at Td (Figure 3). The stress−strain curves, which were essentially identical for both routes, unambitiously show the interference of Young’s moduli and yield stresses through stepwise solidification of the PBA switching segment on cooling due to a growing degree of PBA crystallinity (crystals were serving as fillers) and an associated gain in physical cross-link density. Further deformations at −30, −40, −50, and −60 °C gave continuously growing Young’s moduli and yield stresses; the corresponding stress−strain curves are also included in Figure 3. Below the PBA glass transition, vitrification and the accompanying specimen embrittlement only allowed for tensile deformation at very small strains. Subsequent to programming via the two routes, specimens were heated from −20 to 80 °C under stress-free or constant strain recovery conditions (Figures 4 and 5).
Figure 1. Sketch of an information carrier, containing an inversed QR code with the encoded information “http://www.bam.de/en/index. htm”. The dimensions given within the sketch are in millimeters (clamping distance a0 = 13.5 mm). During thermomechanical investigations, changes in elongation were followed. Experimental details related to the shape programming and recovering of information carriers and a description of QR code investigations in view of legibility are given in the Supporting Information.
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RESULTS AND DISCUSSION Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to study the soft segmental phase transition behavior of PEU (Figure 2) and to ensure that
Figure 2. Thermal and mechanical properties of PEU as determined by DSC (a, first heating and cooling) and DMA (b, temperature dependence of E′ and tan δ). 5954
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Figure 3. Engineering stress−strain curves of PEU at different temperatures (strain rate = 300% min−1). The corresponding Young’s moduli were 1341 MPa (−60 °C), 1145 MPa (−50 °C), 746 MPa (−40 °C), 483 MPa (−30 °C), 282 MPa (−20 °C), 217 MPa (−10 °C), 156 MPa (0 °C), 144 MPa (10 °C), 116 MPa (20 °C), 103 MPa (30 °C), and 43 MPa (40 °C).
Figure 5. Temperature-memory properties of PEU (programmed via route 2) under stress-free and constant strain recovery conditions (a), including the evolution of Tsw,on, Tsw, Tσ,on, and Tσ,max with Td (b).
According to the thermomechanical protocols of route 1, strain and stress recovering started in the early phase of heating (Figure 4a). This lack of recovery control is a deficiency that runs like a thread through the thermomechanical studies on semicrystalline temperature-memory polymers.60,68,69 Despite this disadvantage, pronounced strain recovery behavior was detected (Figure 4a, upper part), which is in agreement with the current literature. Moreover, it is obvious that the switching temperature Tsw grew with the increment of deformation temperature. In turn, under constant strain recovery conditions, maximum recovery stresses σmaxs were generated at continuously growing temperatures Tσ,maxs (Figure 4a, lower part). Over the whole temperature range investigated, Tsw and Tσ,max tended to increase almost linearly with Td (Figure 4b). In quantitative terms, Tsw deviated in average from Td by 3 °C and Tσ,max from Td by 9 °C. Insofar, PEU complied with the key characteristics of a temperature-memory polymer (Tsw ≈ Tσ,max ≈ Td). Heating of specimens, which were programmed via route 2, gave a different but also characteristic picture of strain and stress recovering (Figure 5). Again, temperature-memory
Figure 4. Temperature-memory properties of PEU (programmed via route 1) under stress-free and constant strain recovery conditions (a), including the evolution of Tsw,on, Tsw, Tσ,on, and Tσ,max with Td (b).
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Table 1. Temperature-Memory Properties of PEU Programmed via Routes 1 and 2a stress-free recovery conditions
a
route
Td (°C)
1 1 1 1 1 1 2 2 2 2 2 2 2
−10 0 10 20 30 40 −20 −10 0 10 20 30 40
Rf (%) 79 88 90 90 91 94 56 50 47 46 45 36 18
± ± ± ± ± ± ± ± ± ± ± ± ±
2 2 2 2 2 2 2 2 2 2 2 2 2
εp (%) 12 13 11 13 14 14 12 10 11 13 12 12 13
± ± ± ± ± ± ± ± ± ± ± ± ±
3 3 3 3 3 3 3 3 3 3 3 3 3
Rr,tot (%) 88 87 89 87 86 86 88 90 89 87 88 88 87
± ± ± ± ± ± ± ± ± ± ± ± ±
3 3 3 3 3 3 3 3 3 3 3 3 3
Tsw,on (°C) −15 −12 −9 −13 −14 −7 −13 −5 6 15 25 35 43
± ± ± ± ± ± ± ± ± ± ± ± ±
1 1 1 1 1 1 1 1 1 1 1 1 1
constant strain recovery conditions Tsw (°C) −2 4 12 21 30 36 2 9 17 25 33 38 49
± ± ± ± ± ± ± ± ± ± ± ± ±
3 3 3 3 3 3 3 3 3 3 3 3 3
Tsw,off (°C) 50 50 50 48 48 51 49 49 48 49 50 49 50
± ± ± ± ± ± ± ± ± ± ± ± ±
3 3 3 3 3 3 3 3 3 3 3 3 3
Tσ,on (°C) −20 −20 −20 −20 −20 −20 −20 −9 −1 9 20 31 42
± ± ± ± ± ± ± ± ± ± ± ± ±
2 2 2 2 2 2 2 2 2 2 2 2 2
Tσ,max (°C) 6 11 19 28 37 45 19 27 32 36 43 47 52
± ± ± ± ± ± ± ± ± ± ± ± ±
2 2 2 2 2 2 2 2 2 2 2 2 2
σmax (MPa) 4.7 5.1 4.6 3.9 3.0 2.1 3.1 2.8 2.5 2.0 1.5 1.1 0.2
± ± ± ± ± ± ± ± ± ± ± ± ±
0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2
Tσ,off (°C) 53 52 51 51 50 54 54 54 54 54 54 55 56
± ± ± ± ± ± ± ± ± ± ± ± ±
2 2 2 2 2 2 2 2 2 2 2 2 2
The maximum strain applied was consistently 100%. The errors of the individual parameters were estimated from repeated measurements.
Figure 6. Diffraction patterns of an in situ WAXS measurement on a PEU specimen, which was programmed via route 2 (Td = 40 °C) and heated from 9 to 60 °C with 3 °C min−1 (a) and the corresponding evolution in overall crystallinity χc (b).
growing Td (Figure 4a, upper part). This behavior is attributed to an increasing fraction of crystallizable polymer chains supplied by the PBA phase. When considering the specimen strains after unloading in route 2 (Figure S4, Supporting Information), a decreasing elasticity toward lower Tds is evident. Essentially, polymer chains that were in the rubbery state determined the amount of strain upon stress release at Td. In turn, a growing fraction of crystalline constituents progressively blocked entropy elastic recovering at lower deformation temperatures, whereupon the fixed strain εu and thus Rf remarkably increased (Figure 5a, upper part). Hence, a different tendency could be verified compared with the results of programming route 1. One further facet of thermoresponsiveness is the width of the temperature-memory transition. In route 1, it was continuously in between 60 and 75 °C, thus covering large parts of the investigated temperature range. In the strain recovery curves of other temperature-memory polymers, which were programmed in a similar way, the same limitation is apparent.68,69 Moreover, it is clearly visible in most measurement protocols exhibited by Kratz et al.60 In contrast, the width of the temperature-memory transition in route 2 steadily decreased with rising Tds from approximately 70 to 10 °C (Figure S5, Supporting Information). The thermoresponsiveness in route 1 was dominated by the temperature stability of PBA crystals, which were established under constrained conditions during cooling. By contrast in route 2, the melting of PBA crystals, which were expected to form during cooling in the absence of an external
behavior could be verified (Figure 5a), characterized by almost linear rises of Tsw and Tσ,max with Td (Figure 5b). In contrast to specimens which were programmed via route 1, the most noticeable aspect was that the onset temperatures Tsw,on and Tσ,on, which marked the beginning of strain decrease and stress increase, were strongly depending on Td. The two onset temperatures differed from Td by an average of 5 and 1 °C, respectively. This impressively illustrates that a temperaturememory onset programming could be achieved over a broad temperature range. For a better overview, all relevant thermomechanical parameters are supplied in Table 1. In routes 1 and 2, the thermal (predeformation) history and the deformation rate were essentially identical. This ensured that PEU specimens had about the same PBA morphology after elongation. As a main difference, stress release was not allowed in route 1, before PBA crystallization was mostly completed. In the course of cooling, tension declined most likely due to PBA crystallization (Figure S3, Supporting Information). Maintaining the elongation during cooling inevitably gave good strain fixities, exemplified by the Rf parameter, and comparatively higher maximum recovery stresses in the ensuing heating run (Table 1). Again, this finding is in agreement with current literature.60,68,69 The reason was that the attempt of elastic recovering during programming was also blocked by those PBA units, which crystallized on cooling. As such, lower states of entropy could be conserved and a higher gain in entropy achieved on heating. In the route 1 series, Rf increased with 5956
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Figure 7. Stress−strain behavior of PEU during programming via route 2 (a) and strain−temperature behavior during subsequent heating (b). In the course of loading, maximum strains εm of 100−400% were applied with a rate of 30 mm min−1 at Td = 10 and 40 °C.
87 to 95%. Similarly, Rf increased at Td = 40 °C from 18% (εm = 100%) to 37% (εm = 400%) and Rr,tot from 87 to 94%. The improvement of thermomechanical parameters was attributed to strain-induced PBA crystallization, which gained in significance at higher strains (Figure 7a). For instance, PEU with completely amorphous switching segments even starts crystallizing upon deformation at εm > 200%, when selecting a slow loading rate of 0.1 mm min−1.71 The freshly formed PBA crystals served as physical cross-links, which could be molten when activating the temperature-memory effect. These findings unequivocally demonstrate that a careful selection of maximum strains in route 2 allows for a fine-tuning of the most relevant thermomechanical parameters. In a last step, we addressed the challenge of switching QR code carriers, which were recently introduced by our group44 and, based on the above studied PEU, at predefined temperatures. Therefore, TME programming was carried out according route 2, and the thermoresponsiveness adjacently followed (Figure 8). From the results of our thermomechanical experiments, decreasing shape fixities were assumed when applying the same elongation at growing Tds. Therefore, slightly growing elongations were selected and applied at higher Tds when programming information carriers. This way, decreasing shape fixities, which nominally were 60% (Td = 0 °C), 57% (Td = 10 °C), and 54% (Td = 20 °C), could be compensated, and comparable elongations and distortion patterns could be obtained at −20 °C (Figure 8 and Figure S7, Supporting Information). In a subsequent heating, QR codes could be switched on demand from unreadable to readable at 0 °C (Td = 0 °C), 10 °C (Td = 10 °C), and 20 °C (Td = 20 °C). Thus, the switching performance was no longer restricted by the PBA melting transition region. Most strikingly, the programmed distortion of the QR code could be deskewed by the reading software of the employed smartphone at similar elongations of around 85%. It is noteworthy that the individual forms of the QR code carriers were stable at any state exhibited in Figure 8. Beyond that, preliminary results on stability investigations showed that TME-programmed QR code carriers, which were stored at 0 °C for 4 weeks, were able to maintain the TME as outlined above. Based on our new insights of TME programming, a facile tuning of information release temperatures could be realized. This could be exploited to monitor cold chain temperature abuse. Therefore, we anticipate application potential as
load, virtually did not affect the recovery process. Instead, specimen recovering was preferentially driven by the melting of PBA crystallites, which were thermally stable at T ≥ Td and thus defined by the PBA predeformation morphology. For proof, a DSC measurement was conducted on a sample of a specimen, which was programmed via route 2 (Td = 40 °C, Figure S6, Supporting Information). In sharp contrast to the first DSC heating scan on pristine PEU (Figure 2a), further signals appeared at 26 and 35 °C and were also assigned to PBA melting. This demonstrates that an extension of the PBA melting transition toward lower temperatures could be achieved by programming. In a further attempt to elucidate the temperature stability of PBA crystals, in situ WAXS was carried out on a PEU sample, which was programmed via route 2 (Td = 40 °C, Figure 6). At 9 °C, the diffraction pattern exhibited dominating reflexes at 2θ = 21.7°, 22.5°, and 24.2°, which were assigned to the crystalline PBA phase and superimposed by a broad amorphous halo.76 In a subsequent heating, the reflexes gradually disappeared until a specimen temperature of 50 °C was reached, but the amorphous halo persisted (Figure 6a). Same as for the DSC measurement (Figure S6, Supporting Information), it could be shown that PBA melting was not restricted to small temperature areas. Most interestingly, a small step in the decrease of overall crystallinity χc could be detected close to Td (Figure 6b) and PBA melting was completed exactly at Tsw,off (Table 1). We conclude that strain release must have been initiated by the melting of those PBA crystals, which had the highest thermal stability and were already existing in the predeformation state. Apart from that, it is noticeable that the programming route did not impair the total strain recovery ratio Rr,tot, which consistently was in between 86 and 90% (Table 1). To confirm our concept for higher strains, route 2 programming was carried out again (Figure 7). This time, deformation temperatures of 10 and 40 °C and maximum strains in between 100 and 400% were exemplarily applied (Figure 7a); programmed specimens were then heated under stress-free recovery conditions from −20 to 80 °C (Figure 7b). As demonstrated by the measurement protocols, temperature-memory behavior could be verified. In accordance with the above-mentioned results, the onsets of specimen recovering deviated in average from Td by 5 °C (Td = 10 °C) and 3 °C (Td = 40 °C). Advantageously, Rf could be increased at Td = 10 °C from 46% (εm = 100%) to 63% (εm = 400%) same as Rr,tot from 5957
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Figure 8. Thermoresponsiveness of three QR code carriers, which were subjected to temperature-memory programming via route 2 including tensile deformations am/a0 of 150%, 155%, and 165% at Td = 0, 10, and 20 °C, respectively, unloading and cooling to −20 °C. The information release temperatures are marked with asterisks. As a measure for the elongation of QR code carriers at the different temperatures, the vertical distance between the two clamps of the tensile testing machine is provided in the white fields of the single images.
behavior (route 2), width of temperature-memory transitions (route 1 and 2), DSC thermogram of PEU programmed via route 2, superimposed bicolor images of programmed QR code carriers. This material is available free of charge via the Internet at http://pubs.acs.org.
temperature monitoring labels for food packaging and pharmaceuticals.
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CONCLUSIONS Thermally stimulated temperature-memory effects could be implemented in PEU by means of two programming routes. When following traditional route 1 (Td > Tu), programming supplied higher fixed strains and recovery stresses, but the recovery transitions almost covered the whole temperature range investigated. This is in sharp contrast to the results of newly introduced route 2 (Td = Tu), which gave precise control over the thermoresponsiveness of a temperature-memory polymer. This included an accurate setting of the onset temperatures of both strain and stress recovering and thus the transition width. Dissimilarities in specimen behavior were ascribed to unequal thermal stabilities of physical cross-links (PBA crystallites), which were established under constrained conditions during cooling (route 1) or already existing in the predeformation states (route 2). We conjecture that other semicrystalline elastomers provide similar behavior, which could render route 2 a better selection compared with route 1. Although the concept is still in its infancy, the introduced programming route could be exploited to tune the information release temperatures of QR code carriers. As a result, unprecedented switching performances could be realized which no longer necessitates extensive synthesis efforts to enhance control over the phase transition behavior of the switching segment. This advance may pave the way for the monitoring of temperature abuses of food or pharmaceuticals and thereby generate a novel practical application for temperature-memory polymers.
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AUTHOR INFORMATION
Corresponding Author
*Ph +49 30 8104 3804; fax +49 30 8104 1617; e-mail thorsten.
[email protected] (T.P.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge financial support from the German Federal Ministry of Education and Research (BMBF, project funding reference number 16 V0043) and thank Dietmar Neubert (BAM) for conducting the DSC measurements, Petra Fengler for performing the DMA measurement, Josefine Buschke for carrying out thermomechanical measurements, and Franziska Emmerling and Ralf Bienert for their support regarding the WAXS measurements. The authors thank Jürgen Hättig (Bayer MaterialScience AG) for fruitful discussions and Bayer MaterialScience AG for kindly providing the PEU.
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REFERENCES
(1) Lendlein, A.; Kelch, S. Angew. Chem., Int. Ed 2002, 41 (12), 2034−2057. (2) Liu, C.; Qin, H.; Mather, P. T. J. Mater. Chem. 2007, 17 (16), 1543−1558. (3) Voit, W.; Ware, T.; Dasari, R. R.; Smith, P.; Danz, L.; Simon, D.; Barlow, S.; Marder, S. R.; Gall, K. Adv. Funct. Mater. 2010, 20, 162− 171. (4) Heuwers, B.; Quitmann, D.; Katzenberg, F.; Tiller, J. C. Macromol. Rapid Commun. 2012, 33 (18), 1517−1522. (5) Lan, X.; Liu, Y.; Lv, H.; Wang, X.; Leng, J.; Du, S. Smart Mater. Struct. 2009, 18 (2), No. 024002.
ASSOCIATED CONTENT
S Supporting Information *
Experimental details, ATR-FT-IR and WAXS data; stress evolution during cooling (route 1), loading and unloading 5958
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dx.doi.org/10.1021/ma501171p | Macromolecules 2014, 47, 5952−5959
