The last decade has witnessed drastic improvements of the electronic properties, environmental and operational stability, and processibility of organic semiconductors (OSCs). [1,2] Designing new materials with high carrier mobilities, μ, remains one of the main research objectives to enable faster operation and lower power consumption of circuits and addressing of advanced liquid crystal and organic lightemitting diode displays. [1,3] Yet despite exploring a wide range of material systems, charge carrier mobilities in excess of 10 cm 2 V −1 s −1 have only been achieved in very few molecular semiconductors and highly aligned polymers. [4][5][6] At present, despite significant general advances in the comprehension of transport physics, a Molecular vibrations play a critical role in the charge transport properties of weakly van der Waals bonded organic semiconductors. To understand which specific phonon modes contribute most strongly to the electron-phonon coupling and ensuing thermal energetic disorder in some of the most widely studied high-mobility molecular semiconductors, state-of-the-art quantum mechanical simulations of the vibrational modes and the ensuing electronphonon coupling constants are combined with experimental measurements of the low-frequency vibrations using inelastic neutron scattering and terahertz time-domain spectroscopy. In this way, the long-axis sliding motion is identified as a "killer" phonon mode, which in some molecules contributes more than 80% to the total thermal disorder. Based on this insight, a way to rationalize mobility trends between different materials and derive important molecular design guidelines for new high-mobility molecular semiconductors is suggested.

The crystallization of poly­(ε-caprolactone) (PCL) and isotactic polypropylene (iPP) infiltrated in nanoporous anodic alumina oxide (AAO) templates was reexamined to demonstrate the importance of obtaining polymer-free, clean AAO surfaces on the nucleation, size dependence of crystallization temperature (T c), and texture. The AAO pore diameters cover a broad range from 400 to 20 nm. When the AAO templates were completely free of any residual polymer on their surfaces, differential scanning calorimetry (DSC) experiments exhibited a single crystallization peak for all the samples with different AAO pore sizes. A drastic decrease in T c with density of domains indicated a transition from heterogeneous to homogeneous/surface nucleation. A regular decrease of T c with pore size was observed in the low T c regime, as a result of the volume dependence of nucleation events. The chain alignment of the two polymers infiltrated in AAO was studied by two-dimensional wide-angle X-ray scattering (WAXS). By comparing the experimental and simulated WAXS patterns, the orientation modes of the polymers were identified and compared with previous studies.

The determination of intrinsic chain stiffness of conjugated polymers is challenging, in particular, for scattering techniques because of their strong light absorption and structural instability due to the complicated intra-/intermolecular interactions. In this work, the chain conformation and aggregation formation of a high charge mobility donor−acceptor polymer (DPPDTT) are systematically investigated by using small-angle neutron scattering (SANS) and static/dynamic light scattering (SLS/DLS). On the one hand, chloroform was chosen as a good solvent, in which SANS reveals a rod-like geometry with a radius of ∼15 Å. Once the absorption effect is properly accounted for, SLS shows a power law of 1 between the radius of gyration (R g ) and molecular weight (M w ) and a negative second virial coefficient (A 2 ). On the other hand, 1,2-dichlorobenzene was chosen as a poor solvent, in which SANS, SLS/DLS, and atomic force microscopy (AFM) reveal a strong temperature-/concentration-dependent assembling behavior. The results provide a general picture of the multiscale assembly process of conjugated polymers.

In this work, a complete study of the effect of different organocatalysts on the step-growth polyaddition of a 5-membered dicyclic carbonate, namely, diglycerol dicarbonate with a poly(ethylene glycol)-based diamine in bulk at 120 °C was first carried out. The reaction was found to be dramatically catalyst dependent, higher rates being observed in presence of strong bases, such as phosphazenes (t-Bu-P4 or P4) and 5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). Unexpectedly, the as-formed urethane linkages entirely vanished with time, as evidenced by FTIR and 13 C NMR spectroscopies, while signals due to urea bond formation progressively appeared. Advantage of the chemical transformation occurring from urethane to urea linkages was further taken, by optimizing the polymerization conditions to access a range of poly(hydroxyurea-urethane)s (PHUUs) with precise urethane to urea ratio in a one pot process. Characterization of corresponding polymers by rheological measurements showed that the storage modulus reached a plateau at high temperatures and at high urea contents. The application temperature range of poly(hydroxyurea-urethane)s could thus be increased from 30 °C to 140 °C, as for regular polyurethanes. Furthermore, SAXS and phase-contrast microscopy images demonstrated that increasing the urea content improved the phase separation between soft and hard segments of these PHUUs. Altogether, this novel, straightforward, efficient and environmentally-friendly strategy enables the access to non-isocyanate poly(urea-urethane)s with tunable urethane to urea ratio from 5-membered dicyclic carbonates following an organocatalytic pathway.

Triple crystalline triblock terpolymers are materials with remarkable semicrystalline superstructures. In this work, we report for first time the alternating triple lamellar morphology that self-assembles inside spherulites of a triblock terpolymer composed of poly(ethylene oxide) (PEO), poly(ε-caprolactone) (PCL), and poly(L-lactide) (PLLA). The morphology of the PEO-b-PCL-b-PLLA triblock terpolymer is compared to an analogous PCL-b-PLLA diblock copolymer. Both diblock and triblock form a single phase in the melt. Two crystallization protocols were employed to create particular crystalline morphologies. In both cases, the isothermal crystallization of the PLA block is induced first (at 81 °C, a temperature above the melting points of both PCL and PEO blocks) and PLLA spherulites form a template, whereupon cooling the other two blocks can crystallize within the PLLA interlamellar spaces. WAXS analysis demonstrated the double crystalline and triple crystalline nature of the materials. The lamellar structure was evaluated by AFM observations and SAXS measurements. Moreover, theoretical SAXS curves of one-dimensional structural models were calculated. AFM micrographs of the triblock terpolymer evidenced the three different lamellae of PLLA, PCL and PEO that coexist together within the same spherulite. Three different lamellar thickness were determined, and their dimensions suggested that all blocks crystallized in chain-folded conformations. The evolution of the triple lamellar morphology during heating of tricrystalline samples was followed by in situ synchrotron SAXS measurements. The theoretical analysis of the SAXS curves of the triblock terpolymer allowed us to propose a stacking morphological model, in which a particular trilayer structure exists, where one lamella of PCL or one lamella of PEO is inserted randomly between two adjacent PLLA lamellae.

We have characterized a series of aliphatic polycarbonates synthesized by organocatalysis containing a variable number of methylene groups (n CH2 ) in their repeating units ranging from n CH2 = 6 to 12. The melting and crystallization behavior, and crystalline structures were studied by differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FT-IR), and wide-angle X-ray scattering (WAXS). We found a clear even–odd effect in terms of thermal properties and crystalline structure, for n CH2 = 6 to 9, and a saturation of the even–odd effect, for n CH2 = 10 to 12. These results were independent of the crystallization conditions employed: nonisothermal, isothermal, and successive self-nucleation and annealing. The even–odd region showed that the even samples had higher melting temperatures than the odd ones and a monoclinic unit cell. On the other hand, the odd samples showed an orthorhombic unit cell. Both even and odd samples exhibited a trans-conformation, with a dilution of the impact of the carbonyl group as evidenced by the weakening of the crystalline memory effect as n CH2 increases, independent of the even or odd nature of the samples. In the saturation region, methylene, instead of the carbonyl groups, dominated the behavior, resulting in thermal properties that changed almost linearly with n CH2 . The unit cells were all orthorhombic, and the strength of the memory effect was similar, as n CH2 increased. Accordingly, the samples showed a shift of the FT-IR bands toward a PE-like dominated conformation.

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Metrics & MoreArticle RecommendationsCONSPECTUS: Crystallization of polymeric materials under nanoscopic confinement is highly relevant for nanotechnology applications. When a polymer is confined within rigid nanoporous anodic aluminum oxide (AAO) templates, the crystallization behavior experiences dramatic changes as the pore size is reduced, including nucleation mechanism, crystal orientation, crystallization kinetics, and polymorphic transition, etc. As an experimental prerequisite, exhaustive cleaning procedures after infiltrations of polymers in AAO pores must be performed to ensure producing an ensemble of isolated polymer-filled nanopores. Layers of residual polymers on the AAO surface percolate nanopores and lead to the so-called "fractionated crystallization", i.e., multiple crystallization peaks during cooling.Because the density of isolated nanopores in a typical AAO template exceeds the density of heterogeneities in bulk polymers, the majority of nanopores will be heterogeneity-free. This means that the nucleation will proceed by surface or homogeneous nucleation. As a consequence, a very large supercooling is necessary for crystallization, and its kinetics is reduced to a first-order process that is dominated by nucleation. Self-nucleation is a powerful method to exponentially increase nucleation density. However, when the diameter of the nanopores is lower than a critical value, confinement prevents the possibility to self-nucleate the material.Because of the anisotropic nature of AAO pores, polymer crystals inside AAO also exhibit anisotropy, which is determined by thermodynamic stability and kinetic selection rules. For low molecular weight poly(ethylene oxide) (PEO) with extended chain crystals, the orientation of polymer crystals changes from the "chain perpendicular to" to the "chain parallel to" the AAO pore axis, when the diameter of AAO decreases to the contour length of the PEO, indicating the effect of thermodynamic stability. When the thermodynamic requirement is satisfied, the orientation is determined by kinetics including crystal growth direction, nucleation, and crystal growth rate. An orientation diagram has been established for the PEO/AAO system, considering the cooling condition and pore size.The interfacial polymer layer has different physical properties as compared to the bulk. In poly(L-lactic acid), the relationship between the segmental mobility of the interfacial layer and crystallization rate is established. For the investigation of polymorphic transition of poly(butane-1), the results indicate that a 12 nm interfacial layer hinders the transition of Form II to Form I. Block and random copolymers have also been infiltrated into AAO nanopores, and their crystallization behavior is analogously affected as pore size is reduced.