Materials by Solid-State NMR - American Chemical Society

Article pubs.acs.org/Macromolecules

Insights into Shape-Memory Poly(ε-caprolactone) Materials by SolidState NMR Silvia Borsacchi,*,†,∥,⊥ Katia Paderni,‡,∥ Massimo Messori,‡,∥ Maurizio Toselli,§,∥ Francesco Pilati,‡,∥ and Marco Geppi†,∥ †

Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, 56126 Pisa, Italy Dipartimento di Ingegneria “Enzo Ferrari”, Università di Modena e Reggio Emilia, via Vignolese 905/A, 41125 Modena, Italy § Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, viale Risorgimento 4, 40136 Bologna, Italy ∥ INSTM, via G. Giusti 9, 50121 Firenze, Italy ‡

S Supporting Information *

ABSTRACT: Polymeric materials showing shape-memory behavior are attracting large interest especially in the field of biomaterials. Despite the large number of studies aimed at charaterizing macroscopic features, detailed investigations of properties at the nanometric scale and molecular level are still very few. In this work we present a multinuclear and multitechnique solid-state NMR investigation on recently developed hybrid materials formed by sol− gel cross-linked alkoxysilane terminated poly(ε-caprolactone), showing very interesting shape-memory behavior. By investigating several spectral and relaxation properties of 29Si, 13C, and 1H nuclei present in the hybrid material, we could characterize and compare the structural, dynamic, and phase properties of a sample fixed in a stretched temporary shape with those of a sample in the permanent shape. Interesting differences could be observed: in particular, the sample fixed in the temporary shape showed a larger amount (40% instead of 35%) of crystalline phase and amorphous domains in which polymeric chains experienced a more restricted molecular mobility, as inferred from proton T2 values. Moreover, from 13C spectra recorded by varying the orientation of the sample with respect to the direction of the magnetic field, it was possible to clearly detect that about 90% of PCL chains, mainly in the crystalline domains, but also in the amorphous ones, were strongly aligned along the stretching direction. If subjected to heating, so to remove the temporary shape, the sample showed a melting temperature a few degrees higher, a sligthly more rigid melt phase, and a complete loss of molecular alignment.

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INTRODUCTION

Solid-state NMR is known to be very effective in providing detailed information on the phase, structural and dynamic properties of amorphous and semicrystalline polymers, both in their pristine state and in blends or composites,7 and its application to shape-memory polymeric materials can shed light on the phenomena occurring at nanometric and molecular level related to the shape-memory behavior. In spite of the relatively large interest in shape-memory polymers, studies of their properties at a nanometric and molecular scale are relatively few, and, in particular, solid-state NMR has been exploited only in very few cases.8−10 The studies reported so far on two different shape-memory polymer materials, mainly based on the analysis of proton homonuclear dipolar couplings (through the measurement of spin−spin relaxation time T28 and double-quantum coherences9), showed that the deformation in a stretched temporary shape corresponds to an increase of the dipolar couplings,

Shape-memory materials are a class of the so-called smart materials that show the capability to be formed in a permanent shape and to be programmed for one or more temporary shapes, while spontaneously recovering their original permanent shapes from temporary deformations upon exposure to an external stimulus. Various types of stimuli (electrical and magnetic fields, UV and infrared irradiation, moisture, solvent, and pH change) can be used to trigger the shape recovery process, and among them, temperature is the most frequently employed. One of the most attracting applications of shape-memory materials is in the field of biomaterials, for instance as drug delivery systems and devices for minimally invasive surgery, such as degradable sutures, stents, orthodontic appliances, etc.1,2 Several reviews have been published in the last years focusing the attention on the mechanisms of shape-memory programming and recovery and on actual and potential applications of shape-memory polymers and composites.3−6 © XXXX American Chemical Society

Received: March 19, 2014 Revised: May 16, 2014

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dx.doi.org/10.1021/ma5005716 | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Cross-Linking of α,ω-Triethoxysilane-Terminated Poly(ε-caprolactone) (PCL) through the Sol−Gel Process

5 mg were analyzed under a thermal program consisting in a heating run at 20 °C·min−1 from −30 to +100 °C, followed by a cooling run at 10 °C·min−1 to −30 °C by using a TA Instrument DSC 2010 purged with nitrogen. Tm and Tc were derived from heating and cooling scans, respectively. A further thermal cycle at 20 °C·min−1 from −120 to +40 °C was performed in order to evaluate the glass transition temperature. The degree of crystallinity of PCL was calculated by considering a melting enthalpy of 134.9 J·g−1 for the 100% crystalline PCL,15 referring the measured melting enthalpy to the actual PCL weight fraction (that is taking into account the presence of silica). Solid State NMR Analysis. The solid-state NMR experiments were carried out on two specimens of the same PCL_Si sample: one in the permanent shape (PCL_Siperm), the other one fixed in the temporary shape (PCL_Sitemp). For MAS spectra and relaxation time measurements samples were prepared cutting small pieces of the specimens and packing them into rotors and 5 mm NMR tubes, respectively. Differently, for the 13C line shape analysis experiments, disks with a diameter of about 10 mm of PCL_Siperm and PCL_Sitemp were inserted into two different 10 mm diameter NMR tubes. In the case of PCL_Sitemp, the disks were packed so to align as better as possible their stretching directions (Figure 1). All the NMR samples were prepared operating several degrees below the melting temperature of the material.

ascribable to a partial elongation and alignment in the stretching direction of the polymer chains; interestingly it was also shown that the macroscopic recovery of the permanent shape corresponded to the recovery of the previous dipolar couplings and thus of the starting molecular arrangement. In this work, we present a solid-state NMR investigation on semicrystalline networks based on α,ω-triethoxysilane terminated poly(ε-caprolactone) (PCL) cross-linked by sol−gel chemistry (Scheme 1). These materials, already investigated in previous works, showed a satisfying one-way shape memory behavior11,12 and a significant two-way shape memory response.13,14 Moreover, PCL is known to meet some basic requirements for biomedical applications, such as low toxicity, biodegradability, and potential thermally stimulated recovery at the human body temperature. With the aim of getting insights into the changes occurring at molecular level and at the nanometric scale as a consequence of the shape-memory phenomenon, we tried to obtain a detailed picture of these materials in both permanent and temporary, uniaxially stretched, shapes, exploiting several nuclear properties observable by means of solid-state NMR. In particular, 13C selective high-resolution spectra, 1H spin−lattice (T1), and free induction decay (FID) analysis allowed us to obtain information on the phase and dynamic properties of PCL in both unstretched and stretched state. Moreover, the analysis of 13 C line shapes in static spectra of the stretched sample recorded at different values of the angle between the stretching direction and the magnetic field direction provided a clear and quantitative picture of the orientation of PCL chains in the temporary shape.

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EXPERIMENTAL SECTION

Materials. The materials were prepared starting from α,ωhydroxyl-terminated PCL (molecular weight: 3400 g·mol−1) triethoxysilane end-capped through bulk reaction with 3-(triethoxysilyl)propyl isocyanate. The subsequent cross-linking reaction of triethoxysilane terminated PCL was carried out by means of a sol−gel approach, through hydrolysis and condensation reactions of alkoxysilane terminal groups to generate silica-like domains as cross-linking points. More details concerning the materials preparation are reported in the Supporting Information. The sample was obtained as a thin circular sheet (average thickness: 250 μm; approximate disc diameter: 140 mm). Uniaxially stretched specimens were prepared cutting 60 × 15 mm2 strips from the circular sheet, then stretched at 50 °C with a draw rate of 100 mm/min up to an average deformation of 50%. Immediately after stretching, specimens were put in a refrigerator at −10 °C under fixed strain conditions and left overnight to fix the temporary stretched shape. The grips were then removed and the materials obtained were coded as PCL_Sitemp (temp: temporary stretched shape) to distinguish them from PCL_Siperm (perm: permanent unstretched shape). During storage attention was paid to keep the temperature below the melting temperature of the material to prevent some recovery of the permanent shape. Thermal Characterization. Thermal properties were investigated by using differential scanning calorimetry (DSC). Specimens of about

Figure 1. Preparation of stretched PCL_Sitemp sample for solid-state NMR 13C line shape analysis. Disks of stretched PCL_Sitemp sample were packed so to align as better as possible their stretching directions, which were also perpendicular to the long axis of the cylinder.

29 Si and 13C solid-state NMR spectra were recorded on a dualchannel Varian InfinityPlus 400 spectrometer, working at a Larmor frequency of 400.02, 100.59, and 79.47 MHz for hydrogen-1, carbon13, and silicon-29, respectively, equipped with a MAS probehead for rotors with an outer diameter of 3.2 mm, with a 1H 90° pulse duration of 2 μs. 29Si CP-MAS spectra were recorded under high-power decoupling from protons, using a recycle delay of 3 s, a contact time of 3 ms, a MAS frequency of 3 kHz and accumulating 10000 transients.13C CP-MAS spectra were recorded under high-power decoupling from protons, using a recycle delay of 3 s, a contact time of 2 ms, a MAS frequency of 3.5 kHz and accumulating 600 transients. The same conditions were used to record 13C Delayed CP-MAS16 spectra, for which a delay of 100 μs was inserted between the 90° 1H pulse and the CP irradiation, in order to let the magnetization of protons with short T2 to completely decay.

B


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13 C CP spectra for anisotropic line shape analysis were recorded without MAS (static), under high-power decoupling from protons, using a goniometric probehead and standard NMR glass tubes with a diameter of 10 mm. The 1H 90° pulse duration was 8 μs and 1200 transients were accumulated for each spectrum. The NMR tube stays with its long axis perpendicular to the magnetic field direction: spectra were recorded rotating by different angles the tube about its axis (Figure 1). All the 13C spectra were recorded at a temperature of 5 ± 0.5 °C, using air as cooling gas, precooled in an ice bath. Simulations of the 13C anisotropic line shapes were performed by means of a software purposely written using Mathematica.17 29 Si and 13C chemical shift scales were set using as secondary references 3-trimethylsilyl-1-propanesulfonic acid sodium salt, and hexamethylbenzene, respectively, and TMS as primary reference. Proton FIDs and T1 were measured using a Varian XL-100 spectrometer coupled with Stelar lock and PC-NMR acquisition systems, at a Larmor frequency of 24.1 MHz, with a 1H pulse duration of 3.9 μs. FIDs were recorded using a solid-echo pulse sequence, with a dwell time of 0.25 μs, a recycle delay of 1.5 s and accumulating 200 transients. T1’s were measured with the inversion recovery pulse sequence followed by solid-echo, using 23 delays, variable between 1 ms and 1.5 s, a recycle delay of 1.5 s and accumulating 32 transients for each delay value. Temperature was controlled within ±0.1 °C. At each temperature, the sample was let to equilibrate for 10 min.

to the sensitivity of 29Si isotropic chemical shift, observable in 29 Si high-resolution solid-state NMR spectra, to the chemical environment of silicon nuclei. In Figure 2, the 29Si CP-MAS

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RESULTS AND DISCUSSION Materials and Thermal Properties. PCL with shape memory properties was prepared exploiting sol−gel chemistry to achieve cross-linking of alkoxysilane terminated PCL precursor. The mild sol−gel curing allowed the obtainment of a covalently cross-linked network with Si−O−Si moieties behaving as net-points.11,13 More details concerning materials preparation and properties are reported in the Supporting Information. The melting temperature, Tm, crystallinity content, χc, and crystallization temperature on cooling, Tc, of PCL_Sitemp and PCL_Siperm were measured by DSC. The results are summarized in Table 1 while the thermograms are reported

Figure 2. 29Si CP-MAS spectrum of PCL_Siperm, recorded at a spinning frequency of 3 kHz and a temperature T = 20 °C.

spectrum of the PCL_Si sample in its permanent shape (PCL_Siperm) is shown. The use of cross-polarization (CP), which allows a sensitivity increase but makes the signal integrals not quantitative, was substantially imposed by the very small amount of silicon nuclei present in the sample. Two intense signals can be observed at about −68 and −60 ppm, ascribable to fully condensed T3 (Si(R)(OSi)3) and partially condensed T2 (Si(R)(OH/OR)(OSi)2) silicon nuclei, respectively. While a very weak signal, arising from T1 (Si(R)(OH/OR)2(OSi)) silicon nuclei could be present at about −52 ppm, the presence of a signal due to T0 (Si(R)(OH/OR)3) nuclei, due to uncondensed triethoxysilane groups, predicted at about −43 ppm, can be ruled out. These results clearly indicate that condensation successfully occurred among the different triethoxysilane groups giving rise to relatively highly condensed silica domains, which act as cross-links among different PCL chains. 13 C MAS Spectra. Figure 3a reports the 13C CP-MAS spectra of PCL_Siperm and PCL_Sitemp. The absence of signals due to 13C nuclei of ethoxy groups (expected at 18 and 60 ppm) indicates that uncondensed SiOR groups were nonetheless successfully hydrolyzed. On the basis of literature data on PCL,18 the signals assignment is quite straightforward and it is reported in Figure 3a. The comparison between the two spectra shows that they are substantially identical, not only in the position of the peaks but also in their shape, line width and relative intensities, indicating the absence of any significant difference in the very local environment of PCL carbon nuclei in the permanent or temporary shape of the sample. In Figure 3b, the region of the 13C CP-MAS spectrum containing the signals of the aliphatic carbons of PCL_Siperm is reported in comparison with the same region of the 13C Delayed CP-MAS spectrum, which selectively contains only signals from carbon nuclei belonging to domains having a significant degree of molecular mobility.

Table 1. Results of the Thermal Characterization of PCL_Sitemp and PCL_Siperm (Tm, Melting Temperature; ΔHm, Enthalpy of Melting; χc, Degree of Crystallinity; Tc, Crystallization Temperature on Cooling) sample code

Tm (°C)

ΔHm (J/g)

χc (%)

Tc (°C)

PCL_Sitemp PCL_Siperm

32.8 29.0

51.3 39.0

39 30

−7.0 −7.6

in the Supporting Information (Figure S1). The glass transition temperature of PCL_Siperm was evaluated (Tg = −55.5 °C) from the DSC thermogram reported in the Supporting Information (Figure S2). The melting temperature and the crystallinity content of PCL_Sitemp are slightly higher with respect to the values of PCL_Siperm. This could be ascribed to an enhanced crystallization in PCL_Sitemp cooled under stretching conditions. On the other hand, as expected, PCL_Sitemp and PCL_Siperm show similar crystallization temperatures as the different thermo-mechanical histories to which they were subjected were reset by heating them above the melting temperature. 29 Si MAS Spectra. Before investigating the shape-memory behavior of PCL_Si materials, we used 29Si NMR to verify the result of the hydrolysis and condensation reactions that occurred during sol−gel reactions. This was possible thanks C


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Figure 3. (a) 13C CP-MAS spectra of (bottom) PCL_Siperm and (top) PCL_Sitemp, recorded at a spinning frequency of 3.5 kHz and a temperature T = 5 °C. Asterisks indicate spinning sidebands. The signals assignment is reported in the figure. (b) Comparison between the 0−80 ppm region of the13C CP-MAS and Delayed CP-MAS spectra (the Delayed CP-MAS spectrum can be recognized by its higher noise level) of PCL_Siperm.

Figure 4. (a) T2 and (b) weight percentages of the different functions (Exp = exponential, Weib = Weibullian, Gau = Gaussian) used for reproducing 1 H FIDs at variable temperature. In part a, T2 values of Exp/Weib and Gau can be read on the left and right y axes, respectively. Full blue and open red symbols are used for PCL_Siperm and PCL_Sitemp, respectively. Squares refer to the Gaussian functions, representing the proton nuclei in rigid domains (crystalline PCL); circles are used for the exponential/Weibullian functions representing the proton nuclei in mobile domains (amorphous PCL); triangles refer to the Weibullian function that reproduces the FIDs of the samples cooled back after melting. Uncertainties were ±0.2 μs, ±2 μs, and ±0.5 for T2 of the Gaussian function, T2 of the exponential/Weibullian function, and weight percentages w, respectively. 1

H FID Analysis. For better investigating the phase properties of PCL in these materials and following in particular their evolution in the temperature range relevant to the shape memory phenomenon, spin−spin relaxation times (T2) of 1H nuclei were measured at low magnetic field, through the analysis of the on-resonance FIDs. The proton FID (signal in the time domain) is directly analyzed without resorting to Fourier transformation; in the presence of a weak static magnetic field indeed it is possible to record the on-resonance proton FID of a sample, that is the FID free from oscillations due to chemical shift effects. The FID (F(t)) can be then reproduced, through a fitting procedure, with a linear combination of analytical functions, usually chosen among Pake, Abragamian, Gaussian, Weibullian, and exponential:20−23

It is possible to observe that, apart from the esteric carbon, whose signal is not observed in the Delayed CP-MAS spectrum essentially due to the absence of close hydrogen-1 nuclei, all PCL carbon nuclei give rise to signals, even if very weak, in the Delayed CP/MAS spectrum, indicating the presence in the material of mobile domains, that must be identified with the PCL amorphous fraction (well above Tg that is −55 °C). It can be observed that in addition to C2 and C6,18 also carbon C5 resonates at slightly but detectably different chemical shift in amorphous and crystalline domains. These effects can be ascribed to the different conformational features of PCL in the two phases; in particular the chemical shift values recorded for signals arising from amorphous domains are in very good agreement with those reported for PCL in solution.19 The sample in the temporary shape substantially showed the same spectral behavior.

F (t ) =

∑ wi × fi (t ) i

D


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shape-memory polymers fixed in a temporary shape.8,9 It is worth to notice that even if the differences between corresponding values of the two samples are small, they are larger than the uncertainty on the values themselves (±0.2 μs for T2 of the Gaussian function, ±2 ms for T2 of the exponential/Weibullian function, ±0.5 for the weight percentages). Moreover, the variable temperature data show that the melting process of the crystalline phase in PCL_Sitemp is slower than in PCL_Siperm (at T = 30 °C about 20% of crystalline phase is still present), and it occurs at a slightly higher temperature. Above Tm, the fact that the FIDs can be reproduced with a single Weibullian function indicates that the melt and the amorphous phases at these temperatures present similar dynamic properties, even if it is worth to notice that the Weibullian function itself describes a situation in which a distribution of relaxation occurs. Moreover, looking at T2 values, it is possible to observe that PCL_Sitemp seems to remain, even above Tm, slightly but systematically more rigid than PCL_Siperm (T2 of PCL_Siperm is larger than T2 of PCL_Sitemp). When the samples are cooled from 60 °C back to 20 °C the bicomponent FIDs are not recovered and the single Weibullian functions, with a decreasing T2, still well describe the FIDs. This indicates that crystallization does not occur at these temperatures in the lapse of time of the measurements, in agreement with the undercooling observed by means of DSC analysis. 1 H Spin−Lattice Relaxation Times. In order to obtain information on the fast polymer motions, that is those occurring with a characteristic frequency of the order of the Larmor frequency, we measured proton spin−lattice relaxation times (T1) at a Larmor frequency of 24.1 MHz for PCL_Siperm and PCL_Sitemp. In Figure 5 1H T1 measured by increasing the temperature from 16 to 60 °C and then cooling back to 20 °C are reported. Proton T1’s in the solid state are heavily affected by the spindiffusion process, which tends to average different intrinsic T1’s present in a sample to a single value. The completeness of the

Each ith function represents a motionally distinct protons fraction and it is characterized by a T2i and a weight percentage wi, corresponding to the percentage of the represented 1H nuclei of the sample (F(t) is normalized so that F(0) = 100). The best linear combination of functions is chosen on the basis of the Occam’s Razor principle and of the minimization of the χ2 of the fitting, while T2i and wi are obtained as fitting parameters. Out of the rigid lattice regime (where motions, if present, have characteristic frequencies smaller than the dipolar static interactions and they do not affect T2), T2 monotonically increases with increasing mobility.24 For both PCL_Siperm and PCL_Sitemp the proton FID was recorded and analyzed heating from about 15 °C (that is below Tm) up to 60 °C, and then cooling back to 15 °C. For temperatures below Tm the proton FIDs of both PCL_Siperm and PCL_Sitemp resulted to be well reproduced by a linear combination of a Gaussian function (f(t) = exp(−(t/ T2)2)) with a T2 of about 22−23 μs and an exponential function (f(t) = exp(−(t/T2))) with a T2 of about 200 μs. At the lowest temperatures the weight percentage w of the Gaussian function was about 40% for PCL_Sitemp and 35% for PCL_Siperm. In Figure 4 the values of w and T2 of the two functions are plotted as a function of temperature. At a temperature of about 30 °C, very close to the melting temperature of the crystalline phase, Tm, as determined by DSC, the weight percentage of the Gaussian function strongly decreases and then it becomes zero at the higher temperatures. When a temperature of 40 °C is reached, only one Weibullian function (f(t) = exp(−(t/T2)1.5)) with a T2 of about 500 μs, progressively increasing with increasing temperature, was sufficient for well reproducing the experimental FIDs of both samples. After having reached a temperature of 60 °C, the samples were gradually cooled down to 16 °C: in spite of the cooling, both samples FIDs were still reproducible with a Weibullian function at each temperature, with a decreasing but still long T2. The physical interpretation of these results is quite straightforward. At temperatures below Tm the short T2 Gaussian and the long T2 exponential functions can be mainly ascribed to the PCL rigid crystalline and mobile amorphous fractions, respectively. At the lowest temperatures the weight percentage w of the Gaussian function is about 40% for PCL_Sitemp and 35% for PCL_Siperm, in very good agreement with the crystallinity degree determined from DSC (Table 1) (the slightly higher values obtained by FID analysis are probably due to the capability of this technique to reveal even nanosized crystalline domains). Between 30 and 40 °C the disappearance of the Gaussian component is in very good agreement with the melting of the crystalline phase as determined by DSC. For both PCL_Siperm and PCL_Sitemp the T2 of the Gaussian function below Tm remains constant with temperature, while that of the exponential function increases with temperature, indicating that, approaching Tm, the mobility of the amorphous phase gradually increases, while that of the crystalline phase remains substantially unchanged. By comparing the data of PCL_Siperm and PCL_Sitemp it is possible to notice that the sample in the temporary shape is characterized by a more rigid amorphous phase (shorter T2 of the exponential function) and a slightly higher degree of crystallinity (larger weight of the Gaussian function), in agreement with the stiffening already observed for different

Figure 5. Proton T1 measured at variable temperature. Blue and red symbols are used for PCL_Siperm and PCL_Sitemp, respectively. Circles are used for the heating and triangles for the cooling scan, respectively. Lines are drawn only as a guide for the eyes. The uncertainty on T1 values is about ±0.2 ms. E


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magnetic field direction occur with the same probability, every chemically inequivalent carbon-13 nucleus gives rise to a broad signal, whose shape depends on the chemical shift tensor. Differently, when a preferred molecular orientation with respect to the magnetic field is induced, as it should happen in our case thanks to the uniaxial stretching of the specimen, the 13C line shape characteristically changes with changing the orientation of the sample with respect to the magnetic field direction. The spectrum is determined by δzzLAB, which can be conveniently expressed as28

averaging depends on sample heterogeneity and spin-diffusion coefficient. In this case we measured a single T1 for both PCL_Siperm and PCL_Sitemp. The spin-diffusion coefficient D can be expressed as D = ⟨r2⟩/(nτ), where r2 is the main square diffusive path length, τ is the time for a step in a random walk, and n = 2, 4, 6 for diffusion in 1, 2, 3 dimensions, respectively. If values of 10−16 m2 s−1 and 6, that are known to be good for polymers, are taken for D and n respectively, and τ is taken equal to typical T1 values (of the order of 100 ms), an r value of the order of 10 nm is obtained.24,7 From this background it is usually considered that, if a single T1 of the order of 100 ms is measured, as in this case, the sample is homogeneous on an approximately 10 nm scale. It is known from the literature18,25,26 that T1 relaxation for pristine PCL below Tm is mainly determined, as usually happens for semicrystalline polymers, by the segmental motions of the polymer chains in the amorphous domains above Tg.27 In this case it is possible to observe that both samples show a minimum of T1 at about 20 °C, which means that at this temperature the frequency of the chain motions of amorphous PCL approximately matches the Larmor frequency (24.1 MHz). PCL_Sitemp shows, in proximity of the minimum, values of T1 systematically higher than those of PCL_Siperm, and this is in agreement with the lower amount of amorphous phase in PCL_Sitemp. At temperatures higher than 20 °C, polymer chains motions become faster than the Larmor frequency determining an increase of T1 that becomes equal for the two samples once the melting has occurred. Decreasing the temperature from 60 °C down to 30 °C, the T1’s of the two samples remain equal and similar to the values measured during the heating scan for PCL_Siperm. At 20 °C both samples show T1 values clearly smaller than those measured at the same temperatures during the heating scan. This is in agreement with the presence of a single supercooled homogeneous amorphous phase, as already highlighted by proton FID analysis. 13 C Variable-Angle Static Spectra. The few solid-state NMR results so far published on polymeric shape-memory materials, indicating a stiffening of the polymers in the temporary shape with respect to the permanent shape, have been interpreted in terms of a partial alignment of the polymeric chains along the stretching direction,8,9 but so far an evidence of this has been provided only for PCL/polyurethane films from wide angle X-ray diffraction measurements, concerning the crystalline domains only.8 With the aim of going in depth into this issue, we have directly investigated, by means of 13C experiments, the degree of molecular order that PCL assumes in these hybrid PCL_Si materials as a consequence of the macroscopic deformation. Following the approach proposed by Asakura et al.,28,29 we performed an analysis of the 13C anisotropic line shape on a PCL_Sitemp sample purposely prepared for the NMR investigation, by varying the value of the angle between the magnetic field direction and the sample stretching direction (see Experimental Section and Figure 1). Using a suitable goniometric probe, 13C static spectra were recorded at different values of the angle between the sample stretching and the magnetic field directions, at a temperature of 5 °C, that is below Tm. When magic angle spinning is not used, 13C nuclei in solid samples give rise to broad signals that, under high power decoupling from 1H nuclei, reflect the anisotropy of the chemical shift interaction, mathematically represented by a tensor (δ). In a so-called “powder sample”, that is a sample in which all the molecular orientations with respect to the

δzzLAB = RT(αL , βL , γL)RT(αF , βF , γF )δ PAFR(αF , βF , γF )R (αL , βL , γL)

where LAB, PAF and F indicate, respectively, a laboratory frame with the z axis along the magnetic field direction, the Principal Axes Frame of the chemical shift interaction, that, for a given nucleus in a molecule, has a precise orientation and makes the tensor diagonal, and a frame fixed on the film and having the z axis coincident with the stretching direction. R(αF, βF, γF) and R(αL, βL, γL) are the rotation matrices transforming the PAF to the F frame and the F to the LAB frame, respectively, and α, β, and γ are the respective Euler angles.34 Making the reasonable assumption of a cylindrical symmetry of the sample about the stretching direction, γF and γL can be taken equal to zero. βL is the angle between the stretching and the magnetic field directions, αL ranges from 0 to 360°, while αF and βF describe the orientation of the PAF with respect to the film and are not known a priori. As previously done by Asakura on PCL,29 we decided to focus on the esteric COO signal, whose relatively isolated position in the spectrum makes easier the analysis of its anisotropic line shape. In Figure 6, the static 13C CP spectrum of PCL_Siperm is shown; by

Figure 6. 13C static spectrum of PCL_Siperm. It can be noticed how the pattern arising from the esteric carbon is in the range 100−270 ppm, and it is not superimposed to the patterns arising from the other carbons, all of which are limited to the region below 100 ppm.

recording the spectrum at different values of βL we could verify that, as expected, it was completely invariant. From the discontinuities of the line shape it was possible to determine the principal values of the chemical shift tensor (δPAF) of COO (δ11 = 259 ppm, δ22 = 141 ppm, δ33 = 115 ppm), finding a good agreement with literature data.29,18 These values were used in F


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the simulations of the line shapes observed for the PCL_Sitemp sample at different values of βL, taking x = 1, y = 2, and z = 3. In Figure 7, the experimental COO line shapes recorded as a

Figure 8. Orientation of the PAF and F frames relative to the COO and the film plane, respectively. ZF coincides with the stretching direction.

percentage of the crystalline fraction (about 35%, as determined from both DSC and 1H FID analysis): this is in part certainly due to the use of cross-polarization, which intrinsically favors signals of carbon nuclei in rigid domains, but a partial alignment of the amorphous fraction induced by sample stretching is also likely. Considering the crystal structure reported in the literature for neat PCL,31 in which the CO direction is approximately perpendicular to the chain long axis and the results obtained from these measurements, it is possible to sketch a molecular picture of the stretched film as reported in Figure 9, in which

Figure 7. 13C experimental (left) and simulated (right) anistropic line shapes of COO carbon in PCL_Sitemp at the reported nominal values of the angle βL between the film stretching and the magnetic field directions (the effective values of βL used in the simulations differed by −15° with respect to the nominal ones, as explained in the text). Best simulated line shapes were obtained with the Euler angles between PAF and F frame αF = 5° and βF = 90° and assuming a 10% contribution of the powder pattern arising from non oriented molecules.

function of βL are shown. Simulations of the line shapes were performed, searching the values of αF and βF that could give the best agreement with the experimental profiles. In the simulations the contribution of a nonoriented fraction of molecules was also considered, and its weight percentage was optimized. Moreover, a Gaussian distribution of βL values has been used, which should reflect the not perfect alignment of the drawing directions of the different disks, with an optimized standard deviation value of 20°. In Figure 7, the best-simulated spectra are shown, which were obtained with αF = 5° and βF = 90° and a 10% contribution of the powder pattern arising from non oriented molecules. We have to mention that in order to obtain a good agreement with the experimental spectra, it was necessary to use values of βL shifted by −15° with respect to the nominal experimental values; indeed this shift is also in agreement with the substantial identity between the pairs of line shapes recorded at nominal βL values of 0° and 30°, and of 90° and 120°, which otherwise would be difficult to be explained. This discrepancy between nominal and actual values of βL could be due to an initial misalignment of −15° between the sample stretching direction and the magnetic field direction caused by the experimental difficulty in perfectly controlling the sample position during its insertion in the probe. Since the orientation of the COO PAF on the molecular fragment is known from the literature18,30 (Figure 8), the values found for αF = 5° and βF = 90° unambiguously indicate that in the stretched film, at the experimental temperature of 5 °C, the normal to the COO plane is perpendicular to the stretching direction. It is worth noticing that the weight percentage of the oriented fraction (about 90%) is much higher than the

Figure 9. Sketch of the PCL chains orientation in stretched PCL_Si films at T = 5 °C.

the long molecular PCL axis is approximately aligned along the film stretching direction, and the CO bond lies on the plane perpendicular to the stretching direction, where, however, it does not assume a single orientation. It must be mentioned that, by means of infrared dicroism and X-ray diffraction techniques, a similar alignment of PCL chains was already observed as the most likely occurring in PCL (Mw = 65000) films, both pure and in blends,32,33 either stretched or crystallized under strain. The same orientation has been also observed in the case of composites with CaCO3.29 This is the first time in which a similar evidence is obtained for novel shape-memory sol−gel materials in which silica domains act as cross-links between PCL oligomers. These data prove that in G


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these materials, during the fixing of the temporary shape by cooling under stretching, PCL crystallizes aligning to a very large extent its chains with the stretching direction, even in the presence of silica cross-links. We also recorded 13C spectra after heating PCl_Sitemp sample above Tm; as expected, considering the loss of the temporary shape, the spectra did not depend on the value of βL and were substantially equal to those of PCL_Siperm.

Present Address ⊥

Istituto di Chimica dei Composti OrganoMetallici, Consiglio Nazionale delle Ricerche, CNR, Via G. Moruzzi 1, 56124 Pisa, Italy

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

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Notes

CONCLUSIONS The application of different solid-state NMR techniques, for the investigation of several nuclear properties of different nuclei, to a shape-memory hybrid PCL film both in its permanent shape and fixed in a stretched temporary shape allowed us to characterize in detail the molecular and phase features of this material and their variation with shape memory behavior. We could verify that the sol−gel reaction among the terminal alkoxysilane groups, with which PCL was functionalized, produced cross-links made of highly condensed silica domains, necessary for shape memory behavior. The combined investigation of 1H and 13C relaxation and spectral properties allowed us to find that, when the hybrid polymeric film is first heated above the melting temperature of crystalline PCL, Tm, then stretched and fixed in the temporary shape by lowering the temperature below Tm, the amount of polymer crystalline phase formed is larger than in the starting material and the polymer chains align to a very large extent with the stretching direction. Moreover PCL chains in the amorphous phase has a lower degree of molecular mobility than in the amorphous domains of the starting permanent shape and appear partially aligned along the stretching direction. When the sample in the temporary shape is again heated, the melting transition is observed to occur at slightly higher temperature than for the sample in the permanent shape. Above the transition, with the loss of the temporary shape, the chains preferential alignment disappears, but the previously stretched sample still maintains a slightly lower molecular mobility. This work is one of the few solid-state NMR extensive investigations of an hybrid polymeric system exhibiting shape memory behavior so far reported. We believe that the results obtained provide new insights, of possible general value, on the variation of the structural, dynamic, and phase properties of a shape-memory polymeric material between its permanent and temporary shape. We believe that this can add a piece of knowledge on the attractive family of shape-memory materials, and especially on their features at the molecular scale, that have been much less investigated in comparison with macroscopic performances.

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The authors declare no competing financial interest.

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ASSOCIATED CONTENT

S Supporting Information *

Details on materials preparation and DSC thermograms. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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