Nanoscale Order and Crystallization in POSSâPCL Shape Memory

Article pubs.acs.org/Macromolecules

Nanoscale Order and Crystallization in POSS−PCL Shape Memory Molecular Networks Bonifacio Alvarado-Tenorio,†,‡,§ Angel Romo-Uribe,*,‡ and Patrick T. Mather*,§ †

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 2, 2015 | http://pubs.acs.org Publication Date (Web): August 13, 2015 | doi: 10.1021/acs.macromol.5b01409

Departamento de Ciencias Químico-Biológicas, Instituto de Ciencias Biomédicas, Universidad Autónoma de Ciudad Juárez, Chihuahua C.P. 32310, Mexico ‡ Laboratorio de Nanopolímeros y Coloides. Universidad Nacional Autónoma de México, Cuernavaca, Mor. 62210, Mexico § Syracuse Biomaterials Institute and Department of Biomedical and Chemical Engineering, Syracuse University, Syracuse, New York 13244, United States ABSTRACT: The angstrom- and nanometer-scale crystallization and long-range ordering behavior of polyhedral oligosilsesquioxane−poly(ε-caprolactone) (POSS−PCL) shape memory nanocomposites under thermomechanical shape memory cycles and uniaxial stretching were studied by simultaneous wide-angle and small-angle X-ray scattering (WAXS/SAXS). POSS/PCL cross-linked molecular networks featuring a single POSS moiety centered between two PCL network chains, previously reported [Alvarado-Tenorio et al. Macromolecules 2011, 44, 5682−5692], with molecular weight of 2600 g/mol and exhibiting shape memory behavior, were synthesized with variation of cross-linking molar ratio (POSS−PCL diacrylate/tetrathiol cross-linker, 2:1, 2:1.5, and 2:2). The photocured networks exhibited a morphology consisting of POSS crystals embedded in an amorphous PCL matrix, and the POSS crystals were ordered in a cubic nanostructure. However, under tensile stress afforded by a shape memory cycle, the networks yielded a double-induced orientation (90° and 180°) of the POSS crystals, as indicated by the 101 reflection. Moreover, we detected stretch-induced crystallization of the otherwise amorphous PCL chains. Investigation of nanometer-scale structure by SAXS revealed long periods along the meridional and equatorial axes corresponding to a lamellar nanostructure of PCL chains coexisting with the cubic POSS superstructure. We conclude that the shape memory cycles promoted crystallization and highly ordered nanostructures.

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INTRODUCTION Shape memory polymers (SMPs) are “bottom-up” designed polymeric materials with the ability to adopt a programmed, nonequilibrium temporary shape and return to their permanent shape by means of an external stimulus, either by changes in temperature, pH, electric field, and so on.1 Copolymers containing a “switchable” soft phase of poly-ε-caprolactone (PCL) with shape memory activated by heat and designed with network architecture chemically interconnected have been positioned with significant advantages in the area of minimally invasive surgery (e.g., smart degradable sutures).1 In particular, heat-activated shape memory in such smart sutures allows for mechanically complex contractions automatically.1 Lendlein and co-workers2 have reported the synthesis and thermomechanical analysis of a copolymer with triple shape memory behavior wherein one shape change (from temporary shape A to second temporary shape B) is followed by a second shape change from shape be to the permanent shape, shape C. Whereas shapes A and B are programmed distinct physical cross-linking, shape C is defined by the covalent cross-links of a permanent network. Alteheld et al.3 have reported a series of amorphous biodegradable shape memory polymer networks from co-oligoester segments oligo[rac-lactide-co-glycolide]. These amorphous copolyester−urethane networks were © 2015 American Chemical Society

observed to be transparent and to undergo bulk degradation, and the authors suggest it a good candidate for minimally invasive surgery in areas like ophthalmology. More recently, Neffe and co-workers4 have synthesized and studied a novel biodegradable multifunctional shape memory copolymer on the basis of precursors of oligo[(ε-caprolactone)-co-glycolide] dimethacrylates. This approach combines in one material shape memory, biodegradability, and controlled release of drugs. The incorporation of the PCL blocks into covalently interconnected shape memory networks has some advantages over other biodegradable shape memory polymers.3,5−8 For instance, when using PCL as the programmable phase of the SMP, the activation temperature to recover the permanent shape can be tuned to include human physiologic temperature (37−40 °C) by selecting an appropriate molecular weight for the PCL network chains.1,4 The design of shape memory polymers adopting a network architecture including chemical cross-links or physical cross-links has been reported by other research groups with acceptable shape fixity and shape recovery, Received: June 27, 2015 Revised: August 6, 2015 Published: August 13, 2015 5770

DOI: 10.1021/acs.macromol.5b01409 Macromolecules 2015, 48, 5770−5779

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Scheme 1. Idealized Structure of POSS−PCL Chemical/Physical Double Networks (Drawing Not to Scale)

of aggregation or crystallization of POSS moieties plays a prominent role in determining such physical properties as thermal stability and mechanical modulus. Furthermore, the connectivity (or not) of pendant POSS moieties to the polymeric host influences the microstructure and physical properties in a profound manner. X-ray scattering (WAXS) and transmission electron microscopy (TEM) have shown that POSS arranges into a rhombohedral (or the equivalent hexagonal) structure and that POSS monomers decorated with alkyl units (or a norbornyl group in place of one alkyl unit) can be treated as spheres packing in the ABCA sequence. That is, spheres in one layer lie above the interstitial spaces in adjacent layers. The corner units prevent close packing of the spheres, and the nature of the corner units defines the lattice parameters.27,28 Although the application of shape memory polymers to different fields (i.e., biomedical devices) has been well studied, there is still a need to investigate the role of the short- and longrange order on the macroscopic properties, especially as the materials are being subjected to stress fields and temperature cycles. X-ray scattering, wide-angle (WAXS), and small-angle (SAXS) are well-suited techniques for this purpose as the material preparation is relatively simple, most materials are transparent to X-ray radiation, 1D and 2D patterns can be recorded, and in situ, time-resolved, and post-mortem studies are enabled due to new developments in instrumentation and high flux facilities.32−39 WAXS enables one to determine the presence and type of crystalline structure as well as overall degree of crystallinity among other properties. SAXS allows the characterization of any long-range order, which is the addition of crystalline lamellar length plus the length of the amorphous

taking advantage of segregation, crystallization, or hydrogen bonding.9−18 Although some of these investigations have been focused on optimizing shape memory performance (shape fixity, Rf, and strain recovery, Rr), a following step will be focused to phase separation where sharp thermal transitions and sharp activation temperatures are shown as well as increasing rigidity and mechanical stability of the temporal shape. This paper concerns the microstructure of SMPs bearing the polyhedral oligomeric silsesquioxane (POSS) moiety, incorporated for its potential benefit on shape memory performance. Recently, new strategies for the incorporation of the hybrid POSS moiety into biodegradable shape memory polymers have been introduced by our groups.19−25 Hybrid POSS molecules feature a well-defined and novel “bottom-up” approach toward achieving nanostructured materials and associated enhancement in thermal and mechanical properties, relying on selfassembly rather than dispersion to achieve nanostructure. In terms of biomedical applications, POSS has the advantage of being bioinert.21,26 The general molecular structure of an individual unit of POSS consists of R8Si8O12, arranged in a cage-like structure. A variety of substituents can be attached at each corner positions around the nanocage. Typically seven of these corners are occupied by identical R groups, which control the degree of compatibility of the POSS with the host polymer, while the remaining position is occupied by an organic reactive group, which provides the site for incorporation into a polymer backbone.27−29 The characteristics of POSS for the synthesis of new POSS− polymer nanocomposites have been summarized in recent reviews.30,31 Wu and Mather30 have pointed out that the state 5771


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Table 1. .Physical−Chemical Properties of POSS−PCL (P-CL) Molecular Networks sample

P-CL-A:cross-linker ratio

gel fraction (%)

Tm,POSS (°C)

ΔHm,POSS (J/g)

Tc,POSS (°C)

ΔHc,POSS (J/g)

ve (mol/cm3)

E′ (MPa) 70 °C

P-CL 2.6-net-2:1 P-CL 2.6-net-2:1.5 P-CL 2.6-net-2:2

2:1.0 2:1.5 2:2.0

87 75 66

30 32 36

3.3 3.0 2.5

16 18 21

2.2 2.2 2.2

10.5 × 10−5 5.84 × 10−5 4.55 × 10−5

0.902 0.508 0.388

The POSS−PCL cross-linked networks of this investigation are denoted as P-CL 2.6-net-x:y, where 2.6 corresponds to the target molecular weight 2600 g/mol, net stands for network, and x:y denotes the diacrylate:tetrathiol molar ratio. In this investigation the molar ratio was varied to include 2:1 (stoichiometric), 2:1.5, and 2:2. The photocured samples and their properties are summarized in Table 1. Simultaneous WAXS/SAXS Studies. Two-dimensional WAXS and SAXS patterns were obtained using a three-pinhole collimation system, S-Max3000 (Rigaku Inc.) This equipment employs Cu Kα (λ = 1.5405 Å) as the radiation source and was operated at 45 kV and 0.88 mA. WAXS patterns were recorded using a flat-plate camera and Fuji image plates; a sample-to-detector distance of 6 cm was used. The patterns were analyzed using the software POLAR v2.6 (Stonybrook Technology Inc., Stonybrook, NY). SAXS patterns were recorded using an area detector, sample-to-detector distance of 1.1 m, and 60 min exposure time. The data were recorded in the range 0.0054 < q < 0.16 nm−1, where q = (4π/λ) sin θ, λ is the radiation wavelength, and 2θ is the scattering angle. The samples were dried at 50 °C under vacuum for 12 h prior to X-ray analysis. Thermomechanical Shape Memory Characterization. Shape memory cycles as well as linear viscoelastic properties of POSS/PCL nanocomposites were determined using the Q800 dynamic mechanical analyzer (TA Instruments, New Castle DE). Typically, a POSS/PCL strip with dimensions 3.5 mm (width) × 0.3 mm (thickness) × 12 mm (length) was clamped and characterized in tension mode using a fourstep program: (1) deformation, (2) fixing, (3) unloading, and (4) recovery. The thermal transitions previously reported for POSS/PCL nanocomposites showed that the activation (i.e., recovery Tm) temperatures range from 30 to 36 °C, whereas the fixing (i.e., crystallization Tc) temperatures range from 16 to 21 °C.22 Therefore, the shape memory cycles consisted of: (1) Deformation: the sample is elongated at T = 70 °C applying a ramp force from 0.005 to 0.14 N at a rate of 0.02 N/min. (2) Fixing: under constant load, the sample is cooled at 2 °C/min to 0 °C, a temperature below its Tc. (3) Unloading: at the low temperature, the load is removed and the length of the samples is measured. (4) Recovery: shape recovery is initiated by heating the sample to 70 °C, a temperature above each sample’s Tm, at a rate of 2 °C/min. Based on this thermomechanical cycle, the shape fixing, Rf, and shape recovery, Rr, parameters are defined as11

phase contained in between lamellae for the case of semicrystalline polymers. Recently, we have reported on the micro- and nanostructure in POSS−PCL hybrid nanocomposites and their macromeric precursors, wherein two PCL chains were tethered to a single POSS moiety, thus resembling an asymmetric block copolymer with crystallizable blocks.22 The nanocomposites exhibited shape memory behavior, as previously reported by Lee et al.,19 with the molecular architecture enabling dynamic coupling between crystallization and microphase separation. Thus, such a system offered the possibility to fine-tune the microstructure on the nanometer scale with significant implications on physical properties. The results, correlated with their thermal properties, surprisingly showed a highly ordered nanoscale superstructure not only in the precursors but also in the covalently crosslinked networks, leading to the present study. Here, we have focused on the influence of stress applied during shape memory cycles on the microstructure and nanostructure of our previously reported series of POSS− PCL networks where two PCL oligomers of equal molecular weight are tethered to a single POSS moiety, with varying degree of cross-linking. Simultaneous WAXS/SAXS experiments were carried out, where the angstrom-scale order was investigated by WAXS, whereas the nanometer-scale order was investigated by SAXS. The results show not only stress-induced crystallization of POSS and PCL, but a strikingly highly ordered nanostructure featuring POSS clusters coexisting with lamellar PCL nanodomains.

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EXPERIMENTAL SECTION

Materials. The synthesis of POSS−PCL macromolecular networks was described in detail in our previous publication, and it is shown schematically in Scheme 1.19,22 Briefly, 2-ethyl-2-[3-[[(heptaisobutylpentacyclo[9.5.1.140,41.142,43.144,45] octasiloxanyl)oxy]dimethylsilyl]propoxy]methyl]-1,3-propanediol (or “TMP diol isobutyl-POSS”), hereafter referred to as POSS, was purchased from Hybrid Plastics as a pure (>99%) crystalline solid and used as received. ε-Caprolactone monomer was vacuum-distilled and stored under nitrogen prior to use. Purified ε-caprolactone, dried POSS diol, and as-received catalyst tin(II) 2-ethylhexanoate were added to a 100 mL flask under a nitrogen atmosphere, and the mixture was stirred at 140 °C for 24 h. Unreacted material was removed by the following procedure:19 the resulting telechelic diol liquid was allowed to cool to room temperature, dissolved in tetrahydrofuran (THF), and then precipitated into n-hexane, filtered, and dried. The yield for each of these reactions was typically over 90%. Then, POSS−PCL-diacrylate sample was synthesized according to the procedure described previously.19,22 For this, the POSS−PCL-diol with target molecular weight Mn = 2600 g/mol was end-capped with acrylate groups. The yield from the collected POSS−PCL-diacrylate was 90%. The synthesized POSS−PCL-diacrylate was characterized by 1H NMR, as described by Lee et al.,19 and the average molecular weight was determined to be 2650 g/mol. The weight percent of POSS present in the as-synthesized POSS−PCL-diacrylate was 39 wt %. Then, POSS− PCL networks were photocured using tetrathiol cross-linker pentaerythritol tetrakis(3-mercaptopropionate) (hereafter, “tetrathiol”) to end-link POSS−PCL diacrylate oligomers under photoinitiated thiol−acrylate addition and minor free radical polymerization.

Rf =

Lu − L i × 100% Lt − L i

(1)

Rr =

L u − Lf × 100% Lt − L i

(2)

where Li, Lt, Lu, and Lf are the sample’s initial, temporal, after stress removal, and final (recovered) length, respectively.

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RESULTS AND DISCUSSION Crystallization and Long-Range Ordering during Shape Memory Cycling. The cross-link density of the molecular networks can be reduced by incorporation of tetrathiol at a concentration exceeding the stoichiometric level, with excess cross-linking groups leading to net points with effective functionality lower than 4. As the density of covalent cross-links determines the rubber-like behavior of the samples above the melting transition of POSS, dynamic mechanical analysis was carried out within the linear viscoelastic regime to determine the rubbery modulus, Ge, of each sample and 5772


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therefore their average cross-link density, νe,46 according to the relation Ge = νe + RT. The results, listed in Table 1, indeed confirmed a reduction of cross-link density as the concentration of cross-linker agent was increased to exceed the stoichiometric level. Indeed, doubling the ratio of tetrafunctional cross-linker to difunctional macromer decreased the cross-linking density approximately 2-fold. (Note: one could adopt the approach of using substoichiometric cross-linker concentrations to reduce the cross-linking density, but this was not selected because it would yield undesirable dangling chains.) The shape memory behavior of the stoichiometric network P-CL 2.6-net-2:1 has been described by Lee et al.19 Their results showed a shape recovery and shape fixing of 95% and 99%, respectively. Here, such testing was extended to include the several cross-link densities prepared. Figure 1a shows shape

smaller POSS concentrations denoting the important effect of the hard segment content (POSS plus urethane).47 The PCL molecular weights utilized by those authors was considerable greater than this work, and the increase in fixity and recovery may well be ascribed to entanglement decoupling, resulting in more homogeneous material properties after each shape memory cycle. After the third shape memory cycle the sample was fixed and removed from the dynamic mechanical analyzer and the WAXS pattern was obtained. The shape fixing at room temperature and after the third cycle being about 70%, the patterns before and after thermomechanical cycles are shown in Figure 2a−d

Figure 2. 2D and 3D wide-angle X-ray scattering patterns of the nanocomposite P-CL 2.6-net-2:2 at room temperature. (a, b) Patterns before thermomechanical shape memory cycles. (c, d) Patterns after three shape memory cycles and being fixed at ∼70% strain at room temperature. Arrow indicates the stretching direction. Cu Kα radiation.

(as indicated, the strain direction is vertical). Consistent with our prior report,22 P-CL 2.6-net-2:2 exhibits POSS crystalline reflections indexed as 101 and 110 according to a rhombohedral crystal lattice.48 The outer amorphous halo in Figure 2a corresponds to amorphous PCL chains. The uniform intensity around the azimuth (clearly appreciated from the 3D WAXS pattern, Figure 2b) indicates that the sample has no preferred molecular orientation prior to thermomechanical cycling. However, Figure 2c,d shows that after the thermomechanical history experienced during shape memory cycling the molecular network exhibits biaxial orientation double orientation of POSS crystals and PCL preferred molecular orientation and crystallization. It is unusual to observe double orientation of POSS crystals. Reports on stretched POSS-urethanes49 and sheared POSSolefins50 have shown either no orientation of POSS crystals or only uniaxial orientation of POSS crystals, respectively, but never a biaxial double orientation of the crystalline phase. We postulate here that the double-oriented crystalline phase of POSS is due to the constrained deformation attained within the stretched covalent network. Furthermore, the stress-induced orientation and crystallization in PCL creates additional constraints that may direct the crystal orientation of POSS crystals. Further studies are being carried out to better understand the nature of POSS double crystal orientation and will be reported elsewhere.

Figure 1. (a) One shape memory cycles of the POSS−PCL nanocomposite with cross-link molar ratio 2:2 (P-CL 2.6-net-2:2). (b) Shape recovery and shape fixing parameters as a function of shape memory cycles.

memory cycles of the P-CL 2.6-net-2:2 sample (lowest crosslink density) in a typical 3D plot representation, i.e., strain as a function of temperature and applied stress. The results show an increase in strain at constant stress after each thermomechanical cycle and increase in shape recovery with each cycle. The shape recovery reached 90% after the third cycle, a relatively modest value compared with P-CL 2.6-net- 2:1 which exhibited shape recovery of 98%, although with much smaller shape fixing. This observation of greater shape fixing and relatively smaller shape recovery attained for P-CL 2.6-net-2:2 is associated with the smaller cross-link density, though the relationship may be indirect and relate also to network imperfections, as suggested by the lowering of gel fraction with increasing tetrathiol concentration (Table 1). The increase of shape fixing and recovery after each shape memory cycle has also been reported for star-shaped POSS−PCL molecular networks, where the efficiency for shape memory decreased at 5773


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reflection in the meridional trace (Figure 3b) indicates that POSS crystals are predominantly oriented orthogonal to the deformation axis. On the other hand, the greater intensity of the 110 (and 200) PCL reflection in the equatorial axis indicates that PCL crystals become oriented predominantly parallel to the deformation axis. These aspects were considered in more detail through examination of azimuthal intensity profiles. The quality of orientation of the molecular network is easily appreciated from the azimuthal intensity traces shown in Figure 4. Note that angles ϕ = 90° and ϕ = 270° correspond to the


The reflections corresponding to PCL in Figure 2c are indexed 110 and 200 according to an orthorhombic crystal lattice.51−54 Moreover, the concentration of crystalline intensity on the equatorial axis indicates that the orientation of the PCL network chains is predominantly parallel to the deformation direction, although the azimuthal spread of intensity indicates that the molecular orientation is relatively modest. The crystalline reflections from both POSS and PCL phases and anisotropy of the WAXS pattern in Figure 2c are easily appreciated in the corresponding 3D plot shown in Figure 2d. The WAXS patterns in Figure 2 were further examined by extracting the intensity traces along the equatorial and meridional (deformation) axes, and these are shown in Figure 3. The intensity trace for the undeformed PCL network is

Figure 4. Intensity traces around the azimuth for the reflections of POSS (101, 110) and PCL (110, 200) in the nanocomposite P-CL 2.6-net-2:2 after the third shape memory cycle and fixed at ∼70% strain at room temperature.

Figure 3. Intensity traces of P-CL 2.6-net-2:2 (a) before shape memory cycles; after the third shape memory cycle and fixed at ∼70% strain at room temperature: (b) meridional and (c) equatorial intensity traces. Cu Kα radiation.

deformation (meridional) axis. The azimuthal intensity trace around the 101 reflection of POSS (Figure 4a) exhibits four maxima indicative of a double (biaxial) oriented crystal. The azimuthal angular spread is a measure of the quality of orientation, and the well-defined four intensity maxima (i.e., no overlapping of the maxima) suggests considerable degree of orientation of POSS crystals.55 On the other hand, PCL exhibits only uniaxial orientation (i.e., only two azimuthal maxima), and the location of the azimuthal intensity maxima for the 110 (and 200) reflection are at 0° and 180°, thus denoting orientation parallel to the deformation axis. This is not surprising, given that architecturally, the PCL phase constitutes the load-bearing network chains of the SMP. Following the WAXS analysis above-described, the nanoscale structure of P-CL 2.6-net-2:2 before and after the third shape memory cycle was investigated from the simultaneously obtained SAXS patterns, and these are shown in Figure 5. Because of the electron density difference between crystalline and amorphous phases,36 the SAXS patterns P-CL macromolecular networks exhibited good signal-to-noise ratio, the intensity increasing considerably in the low q range (i.e., close to the beam stop). Therefore, the patterns shown in Figure 5 were logarithmically rescaled to better appreciate larger q features rendering a “grainy” appearance. Figure 5a shows that the sample exhibits nanoscale structure before any thermomechanical treatment. Moreover, the symmetry in the scattered intensity indicates that the nanostructure was unoriented. Shape memory cycle testing, however, led to anisotropic nanostructure. Figure 5b shows that the SAXS pattern after testing was anisotropic and elongated along the deformation axis. That is the nanostructure (as well as the microstructure, Figures 2−4) is deformed after strain fixation at ∼70%. Further analysis was carried out by extracting the intensity traces of

shown in Figure 3a. This shows two crystalline reflections at 2θ = 7.9° and 10.6°, pertaining to POSS 101 and 110 crystalline reflections, according to a rhombohedral unit cell.48 In addition, there is an amorphous halo centered at 2θ = 18.6°, indicating that as-prepared samples feature a microstructure with POSS crystals embedded within an amorphous PCL matrix. On the other hand, the intensity traces for the stretched sample along the meridional (Figure 3b) and equatorial (Figure 3c) axes show three crystalline reflections in place of the amorphous halo. The reflections at 2θ = 21.4° (4.15 Å) and 23.6° (3.76 Å) correspond to PCL crystalline orthorhombic phase and are indexed as 110 and 200 according to an orthorhombic phase.54 The reflections at 2θ = 7.9° (11.2 Å) and 10.6° (8.3 Å) correspond to POSS crystalline phase and are indexed as 101 and 110 according to a rhombohedral lattice or equivalently hexagonal crystal unit cell.48 Therefore, the WAXS data indicate that the thermomechanical treatment of shape memory cycle testing leads to higher ordering of PCL phase, with POSS-based rhombohedral crystalline entities now coexisting with PCL-based orthorhombic crystals. These crystalline phases are separated spatially and necessarily repeating in space (giving rise to diffraction) without macroscopic separation prohibited by covalent attachment. Note that the coexistence of crystalline entities had been observed for the PCL un-cross-linked diols22 where no constraints for crystallization were present. Furthermore, the uneven intensity of reflections between meridional (Figure 3b) and equatorial (Figure 3c) intensity traces indicates that there is preferred molecular orientation in the sample. The pronounced intensity of the 101 POSS 5774


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between chain backbone and lamellae normal56). Thus, the smallest ordered structure would comprise at least two PCL chains and two POSS layers giving 212 Å. Therefore, the first maximum at 633 Å indicates that clusters of 6 PCL/POSS crystalline regions predominate through the sample. On the other hand, the remaining maxima at 56 and 47 Å are attributed to POSS aggregates, as discussed below (see Figure 9 and ref 22). Taken together, the WAXS and SAXS analyses show that the deformed molecular network P-CL 2.6-net-2:2 exhibits coexisting POSS and PCL crystalline phases and POSS and PCL/POSS nanostructures. We postulate that this unusual molecular order is due to both the highly controlled and asymmetrical conformation of the POSS-based macromer (two PCL chains of equal length tethered to a single POSS moiety) and the covalent bonding. Further studies where we have synthesized POSS−PCL molecular networks with the same stoichiometry, but wherein POSS was incorporated in a spatially random manner, exhibited only long-range, lamellar order for PCL. The results of this investigation will be reported elsewhere. Stress-Induced Crystallization and Long-Range Ordering. The crystallization and long-range ordering was investigated for all synthesized POSS−PCL molecular networks in an effort to understand the influence of the degree of crosslinking. For this, samples were uniaxially stretched in a water bath at 80 °C using a custom-built device, and then the samples were rapidly transferred to an ice water bath to quench the structure. The samples were stretched to the maximum strain attainable (indicated in each patterns of Figure 7) before producing fracture. No attempt was made at this point of correlating this maximum strain to the degree of cross-linking in the molecular networks. For that holding constant the strain would be required. Simultaneous WAXS/SAXS patterns were acquired ex situ at room temperature. Figure 7a−f shows 2D and 3D WAXS patterns of the stretched nanocomposites. The patterns are anisotropic, indicating stress-induced molecular orientation. Moreover, we observe that all WAXS patterns exhibited 110 and 200 reflections for PCL crystalline phase and 101 and 110 crystalline reflections corresponding to POSS phase. These results show that (a) in all molecular networks the POSS crystals are biaxially oriented and (b) even under the more restricted mobility afforded by the high degree of cross-linking of P-CL 2.6-net-2:1, the PCL chains are still able to orient and crystallize. The quality of molecular orientation can be assessed from the azimuthal concentration of intensity. Figure 8a−c shows azimuthal intensity traces around each crystalline reflection in the WAXS patterns of Figure 7. The azimuthal concentration of intensity on the equatorial axis (ϕ = 0° and 180°) shows that there is higher PCL orientation along the stretching direction for the networks with lower cross-link density (P-CL 2.6-net2:1.5, and PCL 2.6-net-2:2), as anticipated. High degree of orientation is indicated by small fwhm values. Although the exact nature of the double oriented POSS crystals remains elusive, it is interesting to note that the quality of orientation of POSS crystals increases with increasing deformation, which is achieved for the network with smaller degree of cross-linking (programming stress, not strain, was held fixed across samples), PCL 2.6-net-2:2 (see Figures 7e and 8c).

Figure 5. SAXS patterns at room temperature of the P-CL 2.6-net-2:2 nanocomposite (a) before and (b) after the third shape memory cycle. Arrow indicates the stretching direction. Cu Kα radiation.

each pattern, and the Lorentz-corrected traces are shown in Figure 6.

Figure 6. SAXS intensity traces of P-CL 2.6-net-2:2 nanocomposite: (a) before shape memory cycles, (b) equatorial, and (c) meridional traces from sample after three shape memory cycles and fixed at ∼70% strain at room temperature.

Figure 6a shows that the undeformed molecular network exhibits an intensity maxima denoting long-range order with characteristic dimensions of 558 and 45 Å. Because of the amorphous nature of the PCL phase in the undeformed molecular network (see Figures 2 and 3), this structure may be ascribed to POSS crystalline clusters. Considering that samples were dried at 50 °C prior to all X-ray studies, including SAXS, we suggest that the POSS-based nanostructure observed developed as a result of the thermal history experienced. Shape memory cycle testing led to changes in the SAXS intensity traces, shown in Figures 6b and 6c for the equatorial and meridional axes, respectively. Multiple intensity maxima were observed, denoting a structure with unusual long-range order. The equatorial trace (Figure 6b) shows maxima at 633 and 211 Å, corresponding to a first and third order of a lamellar structure. On the other hand, the meridional trace (Figure 6c) shows intensity maxima at 633, 211, and 70 Å, corresponding to first, third, and ninth order of a lamellar structure. It should be stressed that in the molecular network POSS and PCL are covalently joined, and therefore the stretch-induced long-range ordering would involve both species. PCL of 2650 g/mol crystallizes as folding once and the lamellae are tilted 35° (angle 5775


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Figure 7. WAXS patterns at room temperature of uniaxial stretched PCL 2.6 networks with diacrylate:tetrathiol molar ratio of (a, b) 2:1, (c, d) 2:1.5, and (e, f) 2:2. Arrow indicates the stretching direction. Cu Kα radiation.

The nanoscale structure of the stretched macromolecular networks was also investigated by collecting the SAXS patterns; these are shown in Figure 9. The patterns of P-CL 2.6-net-2:1.5 and P-CL 2.6-net-2:2 are clearly anisotropic, thus denoting preferred orientation of the nanostructure. Detailed investigation of each SAXS patterns was carried out by analyzing the intensity traces along the equatorial and meridional axes, the results being shown in Figure 10. As in the case of the network experiencing multiple shape memory cycles (PCL 2.6-net-2:2, Figure 5), the stretched networks exhibit strikingly ordered nanostructures. Inspection of the intensity traces revealed that the intensity maxima correspond to PCL/POSS lamellar morphology coexisting with POSS cubic nanostructure. In particular, the network PCL 2.6-net-2:1 (Figure 10a, trace i) exhibited intensity maxima at 636, 91, and 53 Å, corresponding to 1st, 7th, and 12th order of a lamellar morphology ascribed to PCL/POSS. Moreover, on the same intensity trace there are intensity maxima at 75, 62, and 46 Å, corresponding to the simple cubic unit cell of POSS crystalline aggregates previously reported,22 where the interplanar distances are given by d111 = a/√3, d200 = a/2, and d220 = a/(2√2), with a = 130 Å. The SAXS pattern of P-CL 2.6-net-2:1.5 molecular network is quite intriguing as it appears more anisotropic than the SAXS patterns of the other networks despite having experienced relatively small strain, about 33%. The meridional trace (Figure 10a, trace ii) features a strong intensity maxima at 230 Å not seen for the other networks. Moreover, the intensity maxima at

Figure 8. Intensity traces around the azimuth of POSS (101, 110) and PCL (110, 200) crystalline reflections in stretched P-CL 2.6-networks: (a) P-CL 2.6-net-2:1, (b) P-CL 2.6-net-2:1.5, and (c) P-CL 2.6-net2:2.

75 and 46 Å would correspond to third and fifth orders. Therefore, for this particular network the crystalline POSS aggregates appear to be arranged in a lamellar morphology. Moreover, the intensity maxima at 230 Å appears to overwhelm the intensity maxima at 636 Å, although maxima at 91 and 56 Å are still clearly visible. We deduce therefore that the nanostructure of P-CL 2.6-net-2:1.5 molecular network consists of POSS lamellae coexisting with PCL lamellae. Indeed, such a morphology has been reported for high molecular weight POSS-polyethylene copolymers.49 A schematic representation suggested for the crystalline arrangement of POSS and PCL after straining the molecular networks is shown in Scheme 2. 5776


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Scheme 2. Stylized Sketch of Phase Behavior of POSS Crystals and Amorphous PCL Network in a POSS−PCL Cross-Linked Network

Figure 9. SAXS patterns at room temperature of stretched P-CL 2.6 molecular networks: (a) 2:1, (b) 2:1.5, and (c) 2:2. Arrow indicates the stretching direction. Cu Kα radiation.

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CONCLUSIONS In this study we have investigated the influence of uniaxial stress on microstructure of asymmetric POSS−PCL shape memory molecular networks, where a single, pendant POSS moiety is centered between two PCL chains. The results revealed stress-induced crystallization and nanometer-scale crystalline ordering of an otherwise amorphous PCL phase. The PCL crystal ordered in an unoriented lamellar morphology. On the other hand, POSS crystals adopted a biaxial orientation under the applied stress field. Strikingly, the stress field induced the ordering of POSS crystals into cubic or lamellar nanostructures, depending of the degree of crosslinking of the network. Whereas further investigation needs to be carried out to understand the origin of this molecular and nanoscale ordering, the results of our investigation showed that control of molecular conformation can produce the coexistence of crystalline structures and highly ordered morphologies.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (A.R.-U.). *E-mail [email protected] (P.T.M.).

Figure 10. (a) Meridional and (b) equatorial traces from SAXS patterns of the stretched P-CL networks (i) P-CL 2.6-net-2:1, (ii) PCL 2.6-net-2:1.5, and (iii) P-CL 2.6-net-2:2. The inset numbers correspond to long-range periodicities.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B. Alvarado-Tenorio was supported by a postgraduate fellowship from the Mexican Council for Science and Technology (CONACyT) at Syracuse University. A.R.-U. acknowledges the financial support of PASPA-UNAM through a Sabbatical

The scheme depicts the crystallization of initially amorphous PCL chains upon the applied stress field and the biaxial reorientation of POSS rhombohedral crystals. 5777


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fellowship. This research was partially supported by CONACyT’s Ciencia Basica program CB-2011 (grant 168095) and the NSF’s Materials World Network program (DMR0758631).


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Nanoscale Order and Crystallization in POSSâPCL Shape Memory

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