Effect of Chemical Modification on Molecular Ordering in Polydiketopyrrolopyrrole Copolymers: From Liquid Crystalline to Crystalline.

The chemical architecture of conjugated polymers is often designed by contemplating and understanding the consequences of structural changes on electronic properties at the molecular level. However, even minor changes to the chemical structure of a polymer can significantly influence the packing arrangement, which also influences the electronic properties of the bulk material. Here, we investigate the molecular arrangement in the ordered state at room temperature of a series of three different polydiketopyrrolopyrroles (PDPPs) in bulk and oriented thin films in detail by wide-angle X-ray scattering and by atomic force microscopy. The changes in the chemical structure of the investigated PDPPs, namely, an additional side chain or a different flanking unit, lead to an increase in long-range order and thereby to a change in the phase state from sanidic ordered via sanidic rectangular or oblique to crystalline.

Detailed structural characterization of five water-insoluble α-glucans produced by glucansucrases from Streptococcus spp.

Water-insoluble α-glucans synthesized from sucrose by glucansucrases from Streptococcus spp. are essential in dental plaque and caries formation. Because limited information is available on the fine structure of these biopolymers, we analyzed the structures of unmodified glucans produced by five recombinant Streptococcus (S.) mutans DSM 20523 and S. salivarius DSM 20560 glucansucrases in detail. A combination of methylation analysis, endo-dextranase and endo-mutanase hydrolyses, and HPSEC-RI was used. Furthermore, crystal-like regions were analyzed by using XRD and 13C MAS NMR spectroscopy. Our results showed that the glucan structures were highly diverse: Two glucans with 1,3- and 1,6-linkages were characterized in detail besides an almost exclusively 1,3-linked and a linear 1,6-linked glucan. Furthermore, one glucan contained 1,3-, 1,4-, and 1,6-linkages and thus had an unusual, not yet described structure. It was demonstrated that the glucans had a varying structural architecture by using partial enzymatic hydrolyses. Furthermore, crystal-like regions formed by 1,3-glucopyranose units were observed for the two 1,3- and 1,6-linked glucans and the linear 1,3-linked glucan. 1,6-linked regions were mobile and not involved in the crystal-like areas. Altogether, our results broaden the knowledge of the structure of water-insoluble α-glucans from Streptococcus spp.

How entanglements determine the morphology of semicrystalline polymers.

Crystallization of polymers from entangled melts generally leads to the formation of semicrystalline materials with a nanoscopic morphology consisting of stacks of alternating crystalline and amorphous layers. The factors controlling the thickness of the crystalline layers are well studied; however, there is no quantitative understanding of the thickness of the amorphous layers. We elucidate the effect of entanglements on the semicrystalline morphology by the use of a series of model blends of high-molecular-weight polymers with unentangled oligomers leading to a reduced entanglement density in the melt as characterized by rheological measurements. Small-angle X-ray scattering experiments after isothermal crystallization reveal a reduced thickness of the amorphous layers, while the crystal thickness remains largely unaffected. We introduce a simple, yet quantitative model without adjustable parameters, according to which the measured thickness of the amorphous layers adjusts itself in such a way that the entanglement concentration reaches a specific maximum value. Furthermore, our model suggests an explanation for the large supercooling that is typically required for crystallization of polymers if entanglements cannot be dissolved during crystallization.

Young's modulus of the different crystalline phases of poly (l-lactic acid).

Young's modulus of α'- and α-crystals of poly (l-lactic acid) (PLLA), more precisely, of aggregates of isotropically arranged lamellae, has been estimated based on dynamic-mechanical analysis of sets of isotropic film samples containing largely different though well-defined amounts of crystals. Evaluation of the modulus of elasticity of these film samples yielded the dependence of Young's modulus as a function of the enthalpy-based crystallinity, increasing with the crystal fraction in the assessed range, from zero to about 75% crystallinity. Extrapolation towards 100% crystallinity suggests values of Young's modulus of around 3.7 and 4.6 GPa for isotropic aggregates of α'- and α-crystals, respectively, being only slightly higher than the modulus of the unaged glassy amorphous phase of 3.0 GPa. Noting the inherent anisotropy of the crystal modulus, suggested in the literature, the average modulus determined in this work seems to be controlled by weaker interchain secondary bonding but not the modulus in chain direction. Great effort has been undertaken to minimize errors by keeping the lamellar thickness in samples of different crystallinity constant, and by providing evidence for independence of the moduli on the spherulitic superstructure.

Bulk Enthalpy of Melting of Poly (l-lactic acid) (PLLA) Determined by Fast Scanning Chip Calorimetry.

The bulk enthalpy of melting of α-crystals of poly (L-lactic acid) (PLLA) is evaluated by fast scanning calorimetry (FSC), by correlating the melting enthalpy of samples of different crystallinity with the corresponding heat capacity at 90 °C, that is at a temperature higher than the glass transition temperature of the bulk amorphous phase and lower than the melting temperature. Extrapolation of this relationship for crystals formed at 140 °C towards the heat capacity of fully solid PLLA yields a value of 104.5±6 J g-1 when melting occurs at 180-200 °C. The analysis relies on a two-phase structure, that is, absence of a vitrified rigid amorphous fraction (RAF) at the temperature of analysis the solid fraction (90 °C). Formation and vitrification of an RAF are suppressed by avoiding continuation of primary crystallization and secondary crystallization during cooling the system from the crystallization temperature of 140 °C to 90 °C, making use of the high cooling capacity of FSC. Small-angle X-ray scattering (SAXS) confirmed thickening of initially grown lamellae which only is possible if these lamellae are not surrounded by a glassy RAF. Linear crystallinity values obtained by SAXS and calorimetrically determined enthalpy-based crystallinities agree close to each other.

Competition between crystal growth and intracrystalline chain diffusion determines the lamellar thickness in semicrystalline polymers.

The non-equilibrium thickness of lamellar crystals in semicrystalline polymers varies significantly between different polymer systems and depends on the crystallization temperature Tc. There is currently no consensus on the mechanism of thickness selection. Previous work has highlighted the decisive role of intracrystalline chain diffusion (ICD) in special cases, but a systematic dependence of lamellar thickness on relevant timescales such as that of ICD and stem attachment has not yet been established. Studying the morphology by small-angle X-ray scattering and the two timescales by NMR methods and polarization microscopy respectively, we here present data on poly(oxymethylene), a case with relatively slow ICD. It fills the gap between previously studied cases of absent and fast ICD, enabling us to establish a quantitative dependence of lamellar thickness on the competition between the noted timescales.

Dynamics and healing behavior of metallosupramolecular polymers.

Self-healing or healable polymers can recuperate their function after physical damage. This process involves diffusion of macromolecules across severed interfaces until the structure of the interphase matches that of the pristine material. However, monitoring this nanoscale process and relating it to the mechanical recovery remain elusive. We report that studying diffusion across healed interfaces and a correlation of contact time, diffusion depth, and mechanical properties is possible when two metallosupramolecular polymers assembled with different lanthanoid salts are mended. The materials used display similar properties, while the metal ions can be tracked with high spatial resolution by energy-dispersive x-ray spectrum imaging. We find that healing actual defects requires an interphase thickness in excess of 100 nm, 10 times more than previously established for self-adhesion of smooth films of glassy polymers.

The Key Role of Side Chain Linkage in Structure Formation and Mixed Conduction of Ethylene Glycol Substituted Polythiophenes.

Functionalizing conjugated polymers with polar ethylene glycol side chains enables enhanced swelling and facilitates ion transport in addition to electronic transport in such systems. Here, we investigate three polythiophene homopolymers (P3MEET, P3MEEMT, and P3MEEET) having differently linked (without spacer and with methyl and ethyl spacer, respectively) diethylene glycol side chains. All the polymers were tested in organic electrochemical transistors (OECTs). They show drastic differences in the device performance. The highest µOECT C* product of 11.5 F/cm·V·s was obtained for ethyl-spaced P3MEEET. How the injection and transport of ions is influenced by the side-chain linkage was studied with electrochemical impedance spectroscopy, which shows a dramatic increase in volumetric capacitance from 80 ± 9 up to 242 ± 17 F/cm3 on going from P3MEET to P3MEEET. Thus, ethyl-spaced P3MEEET exhibits one of the highest reported volumetric capacitance values among p-type polymers. Moreover, P3MEEET exhibits in dry thin films an organic field-effect transistor (OFET) hole mobility of 0.005 cm2/V·s, highest among the three, which is one order of magnitude higher than that for P3MEEMT. The extracted hole mobility from OECT (oxidized swollen state) and the hole mobility in solid-state thin films (OFET) show contradictory trends for P3MEEMT and P3MEEET. In order to understand exactly the properties in the hydrated and dry states, the crystal structure of the polymers was investigated with wide-angle X-ray scattering (WAXS) and grazing incidence WAXS, and the water uptake under applied potential was monitored using electrochemical quartz crystal microbalance with dissipation monitoring (E-QCMD). These measurements reveal an amorphous state for P3MEET and a semicrystalline state for P3MEEMT and P3MEEEET. On the other hand, E-QCMD confirms that P3MEEET swells 10 times more than P3MEEMT in the oxidized state. Thus, the importance of the ethyl spacer toward crystallinity and mixed-conduction properties was clearly demonstrated, emphasizing the impact of side chain linkage of diethylene glycol. This detailed study offers a better understanding of how to design high-performance organic mixed conductors.

Phenomenological Theory of First-Order Prefreezing.

Prefreezing is the prewetting of the crystalline phase at the interface of a melt to a solid substrate via a first-order phase transition. We present a phenomenological theory of prefreezing and analyze thermodynamic properties of the prefrozen crystalline layer. The theory enables a clear thermodynamic explanation of the abrupt formation of a mesoscopically thick crystalline layer during cooling and defines the corresponding transition temperature as a function of the interfacial free energies. It is shown that the interfacial energy difference γsm - ( γsc + γcm) acts as a driving force for prefreezing. The analytical results are congruent with recent experimental outcomes for poly(ε-caprolactone) crystallized on graphite via prefreezing. The calculated interfacial free energies take reasonable values being close to the experimental estimates.

Correction to Semicrystalline Block Copolymers in Rigid Confining Nanopores.

[This corrects the article DOI: 10.1021/acs.macromol.7b01567.].

Temperature-dependent IR-transition moment orientational analysis applied to thin supported films of poly-ε-caprolactone.

A novel experimental setup is described which enables one to carry out infrared transition moment orientational analysis (IR-TMOA) depending on temperature. By this, three dimensional molecular order parameter tensors of IR-active transition dipole moments with respect to the sample coordinate system can be determined in their thermal evolution (35 °C < T < 59 °C). As an example crystallinity and macroscopic order of poly-ε-caprolcatone are monitored. Both remain largely unaltered up to T ∼ 50 °C, above which they decrease. These reductions are explained as the melting of flat-on crystalline lamellae that make up about 34% of the crystalline material. The remaining crystallites are arranged into bulk-like, confined spherulitic structures and do not melt by more than (3 ± 3)%. Therefore, flat-on oriented lamellae are supposed to be kinetically favored by confinement during melt crystallization but are thermodynamically less stable than two-dimensionally confined bulk-like spherulites.

Opposing Phase-Segregation and Hydrogen-Bonding Forces in Supramolecular Polymers.

Phase segregation between different macromolecules and specific weak interactions are the basis of molecular organization in many biological systems, which are held together by attractive hydrogen bonds (H-bonds) and dissociated by phase segregation. We report significant changes in the association behavior of covalent H-bonds by the phase of attached polymer chains. Depending on the aggregation state, we observed either intact H-bonds despite segregation of the phases, or macrophase separation with a larger amount of H-bonding dissociation.

Manipulating Semicrystalline Polymers in Confinement.

Because final properties of nanoscale polymeric structures are largely determined by the solid-state microstructure of the confined polymer, it is imperative not only to understand how the microstructure of polymers develops under nanoscale confinement but also to establish means to manipulate it. Here we present a series of processing strategies, adapted from methods used in bulk polymer processing, that allow us to control the solidification of polymer nanostructures. First, we show that supramolecular nucleating agents can be readily used to modify the crystallization kinetics of confined poly(vinylidene fluoride) (PVDF). In addition, we demonstrate that microstructural features that are not traditionally affected by nucleating agents, such as the orientation of crystals, can be tuned with the crystallization temperature applied. Interestingly, we also show that high crystallization temperatures and long annealing periods induce the formation of the γ modification of PVDF, hence enabling the simple production of ferro/piezoelectric nanostructures. We anticipate that the approaches presented here can open up a plethora of new possibilities for the processing of polymer-based nanostructures with tailored properties and functionalities.

Semicrystalline Block Copolymers in Rigid Confining Nanopores.

We have investigated PLLA crystallization in lamellae-forming PS-b-PLLA confined to straight cylindrical nanopores under weak confinement (nanopore diameter D/equilibrium PS-b-PLLA period L0 ≥ 4.8). Molten PS-b-PLLA predominantly forms concentric lamellae along the nanopores, but intertwined helices occur even for D/L0 ≈ 7.3. Quenching PS-b-PLLA melts below TG(PS) results in PLLA cold crystallization strictly confined by the vitrified PS domains. Above TG(PS), PLLA crystallization is templated by the PS-b-PLLA melt domain structure in the nanopore centers, while adsorption on the nanopore walls stabilizes the outermost cylindrical PS-b-PLLA shell. In between, the nanoscopic PS-b-PLLA melt domain structure apparently ripens to reduce frustrations transmitted from the outermost immobilized PS-b-PLLA layer. The onset of PLLA crystallization catalyzes the ripening while transient ripening states are arrested by advancing PLLA crystallization. Certain helical structure motifs persist PLLA crystallization even if PS is soft. The direction of fastest PLLA crystal growth is preferentially aligned with the nanopore axes to the same degree as for PLLA homopolymer, independent of whether PS is vitreous or soft.

Unveiling the molecular mechanism of self-healing in a telechelic, supramolecular polymer network.

Reversible polymeric networks can show self-healing properties due to their ability to reassemble after application of stress and fracture, but typically the relation between equilibrium molecular dynamics and self-healing kinetics has been difficult to disentangle. Here we present a well-characterized, self-assembled bulk network based on supramolecular assemblies, that allows a clear distinction between chain dynamics and network relaxation. Small angle x-ray scattering and rheological measurements provide evidence for a structurally well-defined, dense network of interconnected aggregates giving mechanical strength to the material. Different from a covalent network, the dynamic character of the supramolecular bonds enables macroscopic flow on a longer time scale and the establishment of an equilibrium structure. A combination of linear and nonlinear rheological measurements clearly identifies the terminal relaxation process as being responsible for the process of self-healing.

Basic principles of static proton low-resolution spin diffusion NMR in nanophase-separated materials with mobility contrast.

We review basic principles of low-resolution proton NMR spin diffusion experiments, relying on mobility differences in nm-sized phases of inhomogeneous organic materials such as block-co- or semicrystalline polymers. They are of use for estimates of domain sizes and insights into nanometric dynamic inhomogeneities. Experimental procedures and limitations of mobility-based signal decomposition/filtering prior to spin diffusion are addressed on the example of as yet unpublished data on semicrystalline poly(ϵ-caprolactone), PCL. Specifically, we discuss technical aspects of the quantitative, dead-time free detection of rigid-domain signals by aid of the magic-sandwich echo (MSE), and magic-and-polarization-echo (MAPE) and double-quantum (DQ) magnetization filters to select rigid and mobile components, respectively. Such filters are of general use in reliable fitting approaches for phase composition determinations. Spin diffusion studies at low field using benchtop instruments are challenged by rather short (1)H T1 relaxation times, which calls for simulation-based analyses. Applying these, in combination with domain sizes as determined by small-angle X-ray scattering, we have determined spin diffusion coefficients D for PCL (0.34, 0.19 and 0.032nm(2)/ms for crystalline, interphase and amorphous parts, respectively). We further address thermal-history effects related to secondary crystallization. Finally, the state of knowledge concerning the connection between D values determined locally at the atomic level, using (13)C detection and CP- or REDOR-based "(1)H hole burning" procedures, and those obtained by calibration experiments, is summarized. Specifically, the non-trivial dependence of D on the magic-angle spinning (MAS) frequency, with a minimum under static and a local maximum under moderate-MAS conditions, is highlighted.

Morphology, Crystal Structure and Charge Transport in Donor-Acceptor Block Copolymer Thin Films.

We studied structure and charge transport properties of thin films of donor-acceptor block copolymers, poly(3-hexylthiophene-block-perylene bisimide acrylate), using a combination of X-ray scattering, AFM and vertical charge transport measurements in diode devices. Block copolymer self-assembly and crystallization of the individual components are interrelated and different structural states of the films could be prepared by varying preparation conditions and thermal history. Generally the well-defined microphase structures found previously in bulk could also be prepared in thin films, in addition alignment induced by interfacial interactions was observed. Microphase separated block copolymers sustain ambipolar charge transport, but the exact values of electron and hole mobilities depend strongly on orientation and connectivity of the microdomains as well as the molecular order within the domains.

Direct observation of prefreezing at the interface melt-solid in polymer crystallization.

Crystallization is almost always initiated at an interface to a solid. This observation is classically explained by the assumption of a reduced barrier for crystal nucleation at the interface. However, an interface can also induce crystallization by prefreezing (i.e., the formation of a crystalline layer that is already stable above the bulk melting temperature). We present an atomic force microscopy (AFM)-based in situ observation of a prefreezing process at the interface of a polymeric model system and a crystalline solid. Explicitly, we show an interfacial ordered layer that forms well above the bulk melting temperature with thickness that increases on approaching melt-solid coexistence. Below the melting temperature, the ordered layer initiates crystal growth into the bulk, leading to an oriented, homogeneous semicrystalline structure.

Studying Twin Samples Provides Evidence for a Unique Structure-Determining Parameter in Simplifed Industrial Nanocomposites.

The structure of styrene-butadiene (SB) nanocomposites filled with industrial silica has been analyzed using electron microscopy and small-angle X-ray scattering. The grafting density per unit silica surface ρD3 was varied by adding graftable SB molecules. By comparing the filler structures at fixed ρD3 (so-called "twins"), a surprising match of the microstructures was evidenced. Mechanical measurements show that ρD3 also sets the modulus: it is then possible to tune the terminal relaxation time of nanocomposites via the chain length while leaving the modulus and structure unchanged.

Twin polymerization at spherical hard templates: an approach to size-adjustable carbon hollow spheres with micro- or mesoporous shells.

Kitset hollow spheres: The combination of twin polymerization with hard templates makes hollow carbon spheres (HCSs) with tailored properties easily accessible. The thickness and pore texture of the HCS shells and also the diameter of the spherical cavity can be varied. The application potential of synthesized HCS is substantiated by an excellent cycling stability of lithium-sulfur batteries.