Triple-Shape Memory Materials via Thermoresponsive Behavior of Nanocrystalline Non-Isocyanate Polyhydroxyurethanes

Crystallization of long <i>n</i>-alkyl side chains within the confined environment of nonisocyanate polyhydroxyurethane (PHU) networks renders PHUs thermoresponsive, enabling thermomechanical programming of temperature-induced shape changes. Key intermediates of shape memory PHUs are highly branched, semicrystalline polyamidoamine curing agents tailored by amidation of a polyamine-terminated hyperbranched polyethylenimine with semicrystalline long chain behenic acid. Both cure temperature and content of <i>n</i>-alkyl side chains, varied independently, govern crystallization behavior, phase separation and mechanical properties of semicrystalline PHU networks obtained by curing pentaerythritol-based polyfunctional cyclic carbonates with hyperbranched, semicrystalline polyamidoamines. As compared to conventional PHUs, the incorporation of hydrophobic, crystalline <i>n</i>-alkyl side chains significantly lowers hydrophilicity. Typically, the <i>n</i>-alkyl side chains of behenic amides in PHU networks melt at temperatures varying between 40 and 75 °C. According to analyses by means of atomic force microscopy (AFM) and differential scanning calorimetry (DSC) crystallization of the behenic amide side chains accounts for nanophase separation producing nanocrystalline PHUs with programmable shapes. Hence, controlled PHU crystallization and PHU nanostructure formation afford thermomechanical programming of PHU triple-shape memory materials memorizing two different shapes in addition to the original shape within a single shape memory cycle. Opposite to conventional polyurethanes, triple-shape memory PHUs require neither the use of isocyanates nor phosgene

Reversible Nucleation, Growth, and Dissolution of Poly(γ-benzyl l‑glutamate) Hexagonal Columnar Liquid Crystals by Addition and Removal of a Nonsolvent

We have investigated the process of nucleation and growth and its reversal (i.e., dissolution) of ordered poly­(γ-benzyl l-glutamate) (PBLG) objects in thin film solutions containing a few percent of α-helical PBLG dissolved in chloroform. Nucleation, growth, and dissolution rate were controlled by adding and removing, respectively, defined amounts of a nonsolvent (methanol), introduced through the vapor phase by regulating its flow rate and vapor pressure. Adding methanol to the isotropic polymer solution allowed for the induction of nucleation and growth, even with polymer solutions of very low concentrations, which were significantly below the solubility limit (equilibrium volume fraction). The variation of the number density of nuclei with the supersaturation ratio was found to fit well the predictions of the classical nucleation theory, for all equilibrium concentrations. For a given supersaturation ratio, fewer objects were nucleated for lower equilibrium concentrations

Semicrystalline Long-Chain Polyphosphoesters from Polyesterification

Semicrystalline aliphatic polyphosphoesters can be obtained in a one-step approach by polyesterification of readily available bio-based long-chain diols with dichloro­phosphorus compounds. Nonadecane-1,19-diol and tricosane-1,23-diol with respectively methyl­phosphonic dichloride or phenyl dichlorophosphate yield polymers (<b>PPE19Me</b>, <b>PPE23Me</b>, <b>PPE19­(OPh)</b>, and <b>PPE23­(OPh)</b>) with molecular weights <i>M</i>_n up to 3 × 10⁴ g mol^–1. DSC analysis of these polymers and a C₁₂ analogue reveal significantly enhanced crystallinities and melting points (up to <i>T</i>_m = 87 °C) with increasing methylene sequence length. DMA on injection-molded samples shows glass transitions at −19 °C (<b>PPE19Me</b>) and −12 °C (<b>PPE23Me</b>). Single crystals of <b>PPE19Me</b> accommodate a single C₁₉ repeat unit, as concluded from a lamella thickness of only 3 nm thick determined by AFM. Hydrolytic degradation of solid polymer samples under ambient conditions occurred only to a minimal extent over three months by hydrolysis of very small amounts of in-chain anhydride defects

Self-Interference of Exciton Emission in Organic Single Crystals Visualized by Energy-Momentum Spectroscopy

We employ energy-momentum spectroscopy on isolated organic single crystals with micrometer-sized dimensions. The single crystals are grown from a thiophene-based oligomer and are excellent low-loss active waveguides that support multiple guided modes. Excitation of the crystals with a diffraction-limited laser spot results in emission into guided modes as well as into quasi-discrete radiation modes. These radiation modes are mapped in energy-momentum space and give rise to dispersive interference patterns. On the basis of the known geometry of the crystals, especially the height, the characteristics of the interference maxima allow one to determine the energy dependence of two components of the anisotropic complex refractive index. Moreover, the method is suited to identify the orientation of molecules within (and around) a crystalline structure

Reversibly Slowing Dewetting of Conjugated Polymers by Light

Dewetting, i.e., the retraction of a fluid from a surface it “dislikes”, is a macroscopic phenomenon controlled through parameters like viscosity and surface tension on length-scales much larger than the size of the molecules. So far, dewetting was known to proceed in the same manner, independent of the dewetting film being illuminated by light or not, e.g., through an optical microscope. Here, we demonstrate that the velocity of dewetting of conjugated polymers can be reversibly tuned through appropriate exposure to light. We relate this observation to the absorption of photons of suitable energy resulting in the generation of excitons which may partially delocalize along and across polymer chains and so induce changes in polymer chain conformation. Such changes, in turn, may cause stiffening or overlap of polymer chains and thus lead to macroscopically detectable differences in behavior of an ensemble of conjugated molecules expressed via material properties like viscosity

High-Temperature Stability of Dewetting-Induced Thin Polyethylene Filaments

We investigated the stability and crystallization behavior of a polygon network of thin filaments of ultrahigh molecular weight polyethylene, generated by dewetting of a spin-coated film of hot <i>p</i>-xylene solution on a solid substrate. Because of the shear forces occurring in the course of dewetting, the PE chains in the filaments were expected to be highly stretched, corroborated by their partial decoration with edge-on lamellar crystals resulting in so-called shish-kebab structures. Above an annealing temperature of 134 °C, the filaments exhibited a liquid-like behavior and were thus prone to decay to droplets according to the Plateau–Rayleigh instability. Intriguingly, even after prolonged annealing at temperatures up to almost 100 °C above the melting temperature of polyethylene for times significantly longer than the corresponding bulk reptation time, semicylindrical PE filaments as small as 50 nm did <i>not</i> decay into droplets. Furthermore, after such prolonged annealing and subsequent quenching to room temperature, these filaments were still capable of inducing shish-kebab structures. We tentatively conclude that polymer chains confined in nanoscopic filaments kept some memory of the dewetting-induced stretching and can undergo only slower conformational changes and rearrangement processes than observed in an equilibrated melt

Tuning Morphologies of Langmuir Polymer Films Through Controlled Relaxations of Non-Equilibrium States

Langmuir polymers films (LPFs) frequently form nonequilibrium states which are manifested in a decay of the surface pressure with time when the system is allowed to relax. Monitoring and manipulating the temporal evolution of these relaxations experimentally helps to shed light on the associated molecular reorganization processes. We present a systematic study based on different compression protocols and show how these reorganization processes impact the morphology of LPFs of poly­(γ-benzyl-l-glutamate)­(PBLG), visualized by means of atomic force microscopy. Upon continuous compression, a fibrillar morphology was formed with a surface decorated by squeezed-out islands. By contrast, stepwise compression promoted the formation of a fibrillar network with a bimodal distribution of fibril diameters, caused by merging of fibrils. Finally, isobaric compression induced in-plane compaction of the monolayer. We correlate these morphological observations with the kinetics of the corresponding relaxations, described best by a sum of two exponential functions with different time scales representing two molecular processes. We discuss the observed kinetics and the resulting morphologies in the context of nucleation and growth, characteristic for first-order phase transitions. Our results demonstrate that the preparation conditions of LPFs have tremendous impact on ordering of the molecules and hence various macroscopic properties of such films

Systematic Control of Self-Seeding Crystallization Patterns of Poly(ethylene oxide) in Thin Films

Using optical microscopy and atomic force microscopy, we studied systematically crystallization patterns in thin films of a low molecular weight poly­(ethylene oxide) (PEO) resulting from a kinetically controlled self-seeding approach. In particular, the influence of seeding temperature (<i>T</i>_s) and heating rate (<i>V</i>_h) on the various resulting crystallization patterns was investigated. Crystallization at 49 °C resulted in dendritic PEO crystals consisting of almost exclusively twice-folded chains. Upon heating these crystals, we observed crystal thickening due to a reduction in the average number of chain folds. On the basis of the detected morphology, we deduced that the density of seeded PEO crystals decreased when increasing <i>T</i>_s from 54 to 57 °C. At the highest <i>V</i>_h (i.e., 100 °C/min), only a few well-separated faceted single crystals of PEO were grown from individual seeds. In contrast to such random distribution of crystals, because of a faster reduction of chain folds at the edges of PEO lamellae, an almost continuous sequence of seeded crystals was formed at the periphery of the original crystals at significantly lower <i>V</i>_h (i.e., 10 °C/min). Interestingly, reflecting the different metastable states within the initial crystal resulting from seeding at <i>T</i>_s = 54 °C, the seeding probability for crystals at the diagonals was higher than for the major side branches. In addition, we estimated activation energies (213–376 kJ/mol) for thickening of PEO lamellar crystal from an Arrhenius-type behavior of the lateral spreading rates as a function of <i>V</i>_h. Our findings suggest that the interplay between thickening and melting of metastable states within the initial crystals is considered as responsible for the resulting nucleation density and crystal morphology induced by self-seeding

Morphological Changes of Isotactic Polypropylene Crystals Grown in Thin Films

Morphological variations of lamellae of isotactic polypropylene (iPP) grown in thin films have been examined experimentally by optical microscopy (OM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). A flower-shaped morphology of iPP crystals, composed of several petal-like lamellae radiating from a nucleus, was typically found. At crystallization temperatures (<i>T</i>_c) below 135 °C, initially petal-like lamellae with a flat α-iPP backbone and many regular branches were formed, which were able to induce epitaxial nucleation of γ-iPP, resulting in features similar to a dendrite growing in the plane of the slow growth direction (i.e., <i>b</i>-axis of α-iPP). With increasing <i>T</i>_c, these dendritic structures disappeared gradually, and the lamellae exhibited a faceted lath-like shape for <i>T</i>_c > 150 °C. Interestingly, periodic lateral splitting (the crystal splayed into a pair of branches) at the fast growth plane was observed at a critical width (<i>W</i>_max) which increased with <i>T</i>_c. In particular, the measured temperature dependence of the products of <i>W</i>_max²<i>G</i> (<i>G</i> represents the growth rate along the <i>a</i>*-axis) was found to be constant. We discuss the role of the diffusion field at the growth front and epitaxial crystallization with respect to morphological changes of iPP lamellae in thin films

Anisotropic Photophysical Properties of Highly Aligned Crystalline Structures of a Bulky Substituted Poly(thiophene)

The photophysical properties of a phenyl-substituted poly­(thiophene), poly­(3-(2,5-dioctylphenyl)­thiophene) (PDOPT), were studied as a function of polarization and degree of orientation of the crystalline structure. Under well-chosen controlled conditions, large-sized spherulitic crystals of PDOPT were successfully prepared from the melt. From polarized optical microscopy and X-ray diffraction, the molecular orientation of PDOPT within the spherulite was determined, indicating that the fastest growth direction of the spherulite was the <i>a</i>-axis. This implied that crystallization of PDOPT was directed by the packing of the side chains rather than the backbones, which are significantly separated. As the crystalline lamellae were all radially oriented, the local absorbance strongly depended on the polarization of the incoming light. Compared to randomly oriented crystals in a quenched and thus rapidly crystallized sample, PDOPT spherulites displayed red-shifted absorption and emission spectra, combined with a reduced photoluminescence quantum yield. Even for these markedly separated polymer backbones (1.47 nm), the reduced photoluminescence suggests an enhancement of interchain interactions of highly ordered bulky substituted polythiophene induced by crystallization