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Three-dimensional shape control is an enabler of dexterous motion in nature. Herein, we report on the thermally initiated out-of-plane (torsional) responses observed in a series of glassy, liquid crystalline polymer networks prepared with a range of cross-link densities. The three-dimensional shape of these materials is strongly dictated by both cross-link density as well as the preparation conditions (polymerization temperature). All of the materials examined herein undergo torsional inversion of the handedness with increasing temperature. The temperature at which the material flattens (crossover between handedness inversion) can be positioned by the polymerization temperature. Monoliths prepared with either multimaterial composition or multidirector orientations are shown to exhibit spatial variation in shape adaptivity.

Azobenzene-functionalized polyimide materials can directly transduce light into mechanical force. Here, we examine the impact of polymer backbone rigidity on the photomechanical response in a series of linear, azobenzene-functionalized polymers. The rigidity of the backbone was varied by the polymerization of five dianhydride monomers with a newly synthesized diamine (azoBPA-diamine). The azobenzene-functionalized linear polymers exhibit glass transition temperatures (Tg) ranging from 276 to 307 °C and maintain excellent thermal stability. The photomechanical response of these materials was characterized by photoinduced cantilever bending as well as direct measurement of photogenerated stress upon exposure to linearly polarized, 445 nm light. Increasing the rigidity of the polymer backbone increases the magnitude of stress that is generated but decreases the angle of cantilever deflection.

Photoresponsive polymers convert a light stimulus input into a mechanical output (work). Photoinduced conformational changes, such as within azobenzene, dictate molecular-level distortions that sum...

The potential for wireless transduction of input light energy into mechanical outputs has led to a reinvigorated pursuit of photomechanical effects in polymeric materials and composites. We report here on factors influencing the photochemical mechanism (and thus the mechanical output) in monodomain azobenzene-functionalized liquid crystal polymer networks. Through systematic examination of a representative material with both mechanics and spectroscopic characterization the prevalence of the trans–cis and trans–cis–trans mechanisms is elucidated. Furthermore, the role of light intensity in generating heat (photothermal effects) is also reported.

To understand the formation mechanism of nonflat polymer single crystals, two types of triblock copolymers with a middle crystalline block and two amorphous, immiscible end-blocks were designed and synthesized. Specifically, polystyrene-block-poly(ethylene oxide)-block-poly(1-butene oxide) and polystyrene-block-poly(ethylene oxide)-block-polydimethylsiloxane were examined. When the end-blocks possess different volumes and are microphase separated onto the opposite sides of the single crystal lamella formed by the middle crystalline block, unbalanced surface stress can be generated. As a result, large scrolled single crystals (∼80 μm) were grown from dilute solution using the self-seeding procedure at low supercoolings. The scrolling direction was identified to be along the (120) planes based on transmission electron microscopy (TEM) observations of the sedimented scrolled single crystals, which is in line with the fact that the scrolling occurs along the planes with the highest coefficient of thermal expa...

We report the synthesis of amorphous, heat-resistant (Tg ranging from 220 to 246 °C) polyimides cross-linked with a novel tris(azobenzeneamine) cross-linker and examine the photodirected bending of cantilevers composed of these materials to exposure to linearly polarized, 442 nm light. Increasing the cross-linker concentration from 5 to 20 mol % in the network not only serves to increase the Tg and modulus but also results in a considerable increase in photomechanical response observed as an increase in bending angle from 5° to 20°. Adjustment of the orientation of the electric field of the light polarization to the cantilever axis is shown to generate forward and backward (bidirectional) bending. Upon removal of the incident light, the cantilevers exhibit photoelastic behavior by restoring to the original vertical position.

We report on the influence of cross-link density of azobenzene-containing liquid crystal polymer networks (azo-LCNs) on the thermomechanical properties and the laser-directed bending of cantilevers consisting of these materials. Cross-link density of azo-LCN was increased by adjusting the length of photocuring from 1 to 120 min. The storage modulus (E′), loss modulus (E′′), and glass transition temperature (Tg) of the azo-LCNs increase with cross-link density. Increasing the cross-link density of the polydomain azo-LCN reduces the magnitude of the bending angle of the cantilevers. The relationship between the thermomechanical and photomechanical properties of the polydomain azo-LCN reported here is further elucidated in the examination of laser-directed bending over a wide range of temperature. The temperature dependence of the equilibrium photodriven bending angle is shown to be strongly related to the temperature dependence of the storage modulus for a given azo-LCN sample. Normalizing the temperature d...

Biodegradable PLGA oligomers were synthesized to feature a hybrid organic−inorganic moiety, POSS, incorporated at the center of each chain by using it as the ring-opening initiator. After end-capping with vinyl groups, the macromers were photocured into networks with POSS content ranging from 10 to 41 wt %. Increasing POSS inclusion increased the crystallinity of the network while decreasing the degradation rate due to its hydrophobic and nonhydrolyzable properties. It was found that co-curing PLGA oligomers with and without POSS in the backbone could be utilized to create networks with a reduced POSS loading level, at a given cross-link density, allowing tuned crystallinity. Hydrolytic degradation of the co-cured networks was slower than for the PLGA homonetwork but still led to complete degradation after 14 weeks in PBS buffer at 37 °C. The networks exhibited versatile shape memory properties, exploiting the combination of covalent cross-linking and the glass transition, POSS-phase melting temperature, ...

Vinyl sulfone-terminated polyethylene glycol-poly(l-lactide) (VS−PEG−PLLA) diblock copolymer was designed as an amphiphilic structure containing a VS group with high reactivity toward thiols. Prior to the copolymer synthesis, heterobifunctional PEG with a VS group at one terminus was prepared and then PLLA was introduced to the other end of PEG. The chemical structure and molecular weight of the resulting products was assigned by 1H NMR and GPC. As micellar properties of VS−PEG−PLLA under aqueous environment, the critical micelle concentration and mean size of VS−PEG−PLLA were found to be approximately 59.8 mg/L and 34 nm, respectively. Kinetic studies of the VS-functionalized PEG−PLLA micelle showed that the reaction rates were affected by various factors but most of cysteine-contained peptides reacted rapidly within 1 h via Michael-type addition. Therefore, this thiol-reactive polymeric micelle is expected to be very useful for a functional nanocarrier system through the effective conjugation of cystein...

Biodegradable shape memory polymers are exciting materials, based on a combination of controlled degradation and triggered actuation that enables the design of unique medical devices. However, their synthesis requires precise control over thermomechanical properties often not possible with common building blocks for biodegradable polymers. In the present work, we report the preparation of “double networks” that feature a superposition of a covalent network with a percolative physical network, the latter being derived from a well-defined crystalline phase of strongly hydrophobic polyhedral oligosilsesquioxane (POSS) moieties. Each covalent network chain features polycaprolactone (PCL) tethers on a single POSS moiety by virtue of its use as a difunctional initiator for PCL ring-opening polymerization. Such network chains were synthesized to feature molecular weight values that ranged from 2000 to 4000 g/mol (polydispersity index

Lamellar single crystals grown in dilute solutions can be used as templates for tethered chain analysis. Two series of diblock copolymers, poly(ethylene oxide)-block-polystyrene (PEO-b-PS) and poly...

The dispersion characteristics of organoclay in nanocomposites based on end-functionalized polystyrene (PS) and end-functionalized polystyrene-block-polyisoprene (SI diblock) copolymer were investi...

The effects of triblock copolymer architecture (ABA-type vs ABC-type) and the degree of functionalization on the organoclay dispersion in nanocomposites were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), and linear dynamic viscoelasticity. For the study, a lamella-forming polystyrene-block-polyisoprene-block-polystyrene (SIS triblock) copolymer and a homogeneous polyisoprene-block-polystyrene-block-polybutadiene (ISB triblock) copolymer were synthesized via sequential anionic polymerization. Subsequently, the polyisoprene block of the SIS triblock copolymer was hydroxylated yielding an SIOHS triblock copolymer, and the polybutadiene block of the ISB triblock copolymer was hydroxylated yielding an ISBOH triblock copolymer. The degree of hydroxylation in both SIOHS and ISBOH triblock copolymers was varied. Both the unfunctionalized (SIS and ISB) and functionalized (SIOHS and ISBOH) triblock copolymers were used to prepare nanocomposites with two different types of organ...

The effect of the microstructure of polydiene blocks on order-disorder transition (ODT) in diblock copolymers containing polystyrene (PS) or poly(vinylcyclohexane) (PVCH) blocks was investigated. For the study, a series of polystyrene-block-polybutadiene (SB diblock) copolymers with varying microstructures in the polybutadiene (PB) block and polystyrene-block-polyisoprene (SI diblock) copolymers with varying microstructures in the polyisoprene (PI) block were synthesized. Subsequently, either only the PB block in an SB diblock copolymer was selectively saturated to obtain PS-block-poly(ethylene-co-1-butene) (PEB) copolymer or both blocks were saturated to obtain PVCH-block-PEB copolymer. Also, either only the PI block in an SI diblock copolymer was selectively saturated to obtain PS-block-poly-(ethylene-alt-propylene) (PEP) copolymer or both blocks were saturated to obtain PVCH-block-PEP copolymer. The ODT temperature (T ODT ) of each block copolymer was determined from oscillatory shear rheometry. It has been found that (i) the TOOT of an SB diblock copolymer with predominantly 1,2-addition in the PB block is much higher than that of an SB diblock copolymer with predominantly 1,4-addition in the PB block, (ii) the TOOT of an SI diblock copolymer with high vinyl content (predominantly 3,4- and 1,2-additions) in the PI block is much higher than that of an SI diblock copolymer with predominantly 1,4-addition in the PI block, (iii) the T ODT s of PS-block-PEB and PS-block-PEP copolymers are considerably higher (>100 °C) than those of the unsaturated precursors regardless of the microstructures of polydiene block, and (iv) the T ODT s of PVCH-block-PEB and PVCH-block-PEP copolymers are higher or lower than those of the unsaturated precursors depending upon the microstructure of polydiene block. By curve fitting the experimental phase diagrams, which were obtained from cloud point measurement, to the theoretical phase diagrams based on the Flory-Huggins theory, temperature-dependent segment-segment interaction parameters (χ) were determined for PS/PB, PS/PEB, PVCH/PEB, PS/PI, PS/PEP, and PVCH/ PEB pairs with varying microstructures of polydienes. It is found that while values of χ for the polymer pairs thus determined explain satisfactorily the experimentally determined T ODT s of all the block copolymers investigated, the conformational asymmetry parameter (e) explains the experimentally determined T ODT s of the completely saturated PVCH-block-PEB and PVCH-block-PEP copolymers, but not of the partially saturated PS-block-PEB and PS-block-PEP copolymers. Further, it is found that values of e do not explain the effect of microstructures of PB block on the T ODT of SB diblock copolymer with varying microstructures. Thus, we conclude that a correlation between e and χ seems to exist for completely saturated block copolymers, but not for partially saturated block copolymers.

The effect of flexible spacer length on the rheology of side-chain liquid-crystalline polymers (SCLCPs) was investigated. For the study, a liquid-crystalline monomer, [n-[(4-cyanobiphenyl)oxy]alkyl]carboxylic acid (nCN-COOH) with varying methylene groups (n = 5, 7, and 11), was grafted onto a nearly monodisperse hydroxylated polyisoprene, yielding SCLCPs, poly{[n-[(4-cyanobiphenyl)oxy]alkyl]carbonyl}oxyisoprenes (PI-nCN). Differential scanning calorimetry has shown that the clearing temperature (Tcl) of PI-nCN increases with increasing n. Polarizing optical microscopy has shown that PI-5CN has a nematic mesophase, while both PI-7CN and PI-11CN have a smectic mesophase. The linear dynamic viscoelasticity, steady shear flow, and transient/intermittent shear flows of PI-nCN were investigated. It was found that flexible spacer length has a profound influence on the rheological responses of PI-nCN. The following observations have been made. (i) At temperatures below Tcl the complex viscosity (|η*|) of PI-5CN e...

The linear dynamic viscoelasticity, and transient and intermittent shear flows of organoclay nanocomposites, which were prepared with poly(ethylene-ran-vinyl acetate) (EVA), poly(ethylene-ran-vinyl alcohol) having 53 mol % vinyl alcohol (EVOH-53), or poly(ethylene-ran-vinyl acetate-ran-vinyl alcohol) having 5 mol % vinyl alcohol (EVAOH-5) were investigated. For the study, two organoclays (Southern Clay Products) were employed: (i) Cloisite 30B (having the gallery distance of 1.85 nm) treated with a surfactant (MT2EtOH) containing tallow, quaternary ammonium chloride, and hydroxyl groups and (ii) Cloisite 15A (having the gallery distance of 3.15 nm) treated with a surfactant (2M2HT) containing hydrogenated tallow, quaternary ammonium chloride but no hydroxyl group. Thus, a total of six nanocomposites were prepared by melt blending using a Brabender mixer. Before taking rheological measurements, the nanocomposites were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). ...

Phase transitions in sphere-forming polystyrene-block-polyisoprene-block-polystyrene (SIS triblock) copolymer and its blends with polystyrene (PS) were investigated using oscillatory shear rheometry, transmission electron microscopy, and small-angle X-ray scattering (SAXS). For the investigation, a commercial grade of high-molecular-weight SIS triblock copolymer (Vector 4113, Dexco Polymers) and a low-molecular-weight SIS triblock copolymer (SIS-100) synthesized in our laboratory were used. It is found from oscillatory shear rheometry and SAXS that both Vector 4113 and SIS-100 undergo order−disorder transition (ODT) having a characteristic feature of lattice disordering/ordering transition (LDOT) of spheres, giving rise to a phase consisting of micelles with short-range liquidlike order at thermal equilibrium above ODT and demicellization/micellization transition (DMT), as a pseudo phase transition, giving rise to a micelle-free phase with only thermal composition fluctuations. The LDOT temperature (TLDOT...

New hybrid polyoxazolines (CubePOZO) were synthesized by ring-opening polymerization of 2-methyl-2-oxazoline initiated by functionalized polyhedral oligomeric silsesquioxane (POSS) with various feed ratios. The hybrid micelles derived from CubePOZO with both seven hydrophobic cyclopentyl groups of POSS and polyoxazoline (POZO) as a hydrophilic segment were prepared in an aqueous phase. Compared to POZO initiated by cyclohexyl iodide or methyl p-toluenesulfonate, increased hydrophobicity caused by the structure of POSS in CubePOZO played a major role in the micelle formation. This depended on the amount of POSS incorporated into the POZO. The CubePOZO100 and CubePOZO200 formed micelles in an aqueous phase with critical micelle concentrations (cmc) of 55 and 100 mg/L, respectively. Transparent polymer hybrids, homogeneous dispersion of hydrophobic POSS bonded covalently to POZO in the silica gel matrix at the molecular level, were obtained through hydrogen-bonding interaction between CubePOZO and silica gel...

Linear dynamic viscoelastic properties of functionalized block copolymer/organoclay nanocomposites were investigated. The organoclay employed (Cloisite 30B, Southern Clay Products) is natural clay ...

The effect of saturation of polydienes with varying microstructures on the phase behavior of poly(vinylcyclohexane) (PVCH)/poly(ethylene-co-1-butene) (PEB) and PVCH/poly(ethylene-alt-propylene) (PE...

A nematic side-chain liquid-crystalline polymer (SCLCP) was synthesized by grafting a liquid-crystalline monomer, 6-[(4-cyano-4‘-biphenyl)oxy]hexanoic acid (5CN-COOH), onto a nearly monodisperse hydroxylated polyisoprene. The linear dynamic viscoelasticity, steady shear flow, transient and intermittent shear flows, and stress relaxation of the SCLCP in the nematic state were investigated. For comparison, the rheological behavior of a main-chain liquid-crystalline polymer (MCLCP) was also investigated. Similarities and dissimilarities in the rheological behavior between the SCLCP and MCLCP are presented. Some of the important similarities observed are as follows. Upon start-up of shear flow, both SCLCP and MCLCP exhibited a very large overshoot peak of first normal stress difference (N1+ ) and shear stress (σ+ ). The values of first normal stress difference in steady shear flow were positive in both SCLCP and MCLCP over the entire range of shear rates tested. Some of the important dissimilarities observed ...

Microphase separation was induced from homogeneous polystyrene-block-polyisoprene (SI diblock) copolymer by grafting of a liquid-crystalline (LC) monomer onto the polyisoprene (PI) block. The origin of the formation of microdomain structure in the side-chain LC block copolymer from a homogeneous SI diblock copolymer is explained in terms of a dramatic increase in repulsive segmental interactions between the constituent blocks. For the investigation, three homogeneous SI diblock copolymers with varying block length ratios (SI-5/6, SI-10/6, and SI-14/3) were synthesized via anionic polymerization in tetrahydrofuran as solvent. This resulted in PI blocks having the following microstructures: 34% 1,2-addition, 59% 3,4-addition, and 7% 1,4-addition. Subsequently, the PI blocks in each of the SI diblock copolymers were first hydroxylated via hydroboration/oxidation reactions to yield hydroxylated SI diblock copolymers (SI-5/6-OH, SI-10/6-OH, and SI-14/3-OH), and then a liquid-crystalline (LC) monomer, 6-[(4-cyano-4'-biphenyl)oxy]hexanoic acid (5CN-COOH), synthesized in our laboratory, was grafted onto the hydroxylated PI blocks (PI-OH), yielding side-chain LC diblock copolymers (SI-5/6-5CN, SI-10/6-5CN, and SI-14/3-5CN). The disappearance of the -COOH and -OH groups after coupling reaction was confirmed using 1 H nuclear magnetic resonance spectroscopy and infrared spectroscopy. Transmission electron microscopy revealed that at room temperature SI-5/6-5CN has spherical microdomains, SI-10/ 6-5CN has hexagonally packed cylindrical microdomains, and SI-14/3-5CN has lamellar microdomains. The LC phase of the side-chain block copolymers was identified using polarizing optical microscopy. The clearing temperature (T cl ), determined by differential scanning calorimetry, of the side-chain LC block copolymers is found to be ca. 84 °C, whereas the T cl of LC monomer 5CN-COOH is 170 °C. Oscillatory shear rheometry indicates that the sphere-forming block copolymer SI-5/6-5CN undergoes a lattice disordering/ordering transition at 140 °C and a demicellization/micellization transition at 155 °C, while both the cylinder-forming block copolymer SI-10/6-5CN and lamella-forming block copolymer SI-14/3-5CN undergo microphase separation transition at temperatures much higher than 240 °C, the highest experimental temperature employed. Binodal curves exhibiting upper critical solution temperature were constructed, via cloud point measurement, for four pairs of polystyrene (PS) and PI grafted with 5CN-COOH (PI-5CN). An expression for the temperature dependence of the Flory-Huggins interaction parameter X for the PS/(PI-5CN) pair was determined and compared with the X expression for the PS/PI pair. The origin of the formation of microdomain structure in the side-chain LC block copolymers SI-5CN from a homogeneous SI diblock copolymer is explained in terms of molecular weight, block composition, and X for the PS/(PI-5CN) pair.

Three homogeneous polystyrene-block-polyisoprene (SI diblock) copolymers with varying block length ratios (SI-10/6, SI-5/6, and SI-14/3) were synthesized via anionic polymerization in tetrahydrofuran as solvent, giving rise to the microstructures of polyisoprene (PI) block of 34% 1,2-addition, 59% 3,4-addition, and 7% 1,4-addition. Subsequently, the SI diblock copolymers were subjected to hydroboration/oxidation reactions to yield hydroxylated SI diblock (SI-OH) copolymers (SI-10/6-OH, SI-5/6-OH, and SI-14/3-OH). After the hydroxylation, the molecular weight of SI-OH is increased by 3−9%, depending on the block length ratio of the corresponding SI diblock copolymer. Infrared (IR) spectroscopy has shown that the SI-OH diblock copolymers form hydrogen bonding at room temperature. Transmission electron microscopy has revealed that at room temperature SI-10/6-OH and SI-5/6-OH have lamellar microdomains while SI-14/3-OH has hexagonally packed cylindrical microdomains. Oscillatory shear rheometry has indicated ...

Organic−inorganic polymer hybrids were prepared by an in-situ method. In the presence of ruthenium salt, the cross-linking reaction of bipyridyl-containing polymer was carried out together with the sol−gel reaction of tetramethoxysilane. This methodology includes the simultaneous formation of silica gel and organic gel. Using polystyrene or poly(ethylene oxide) having bipyridyl groups, organic−inorganic polymer hybrids could be prepared. The obtained polymer hybrids having IPN (interpenetrating polymer network) structure showed excellent solvent-resistant property.