Crystallization of Supramolecular Pseudoblock Copolymers

Because of the presence of supramolecular bonds, the crystallization process of supramolecular pseudoblock copolymers (SPBCP) is more complex in comparison to conventional covalently bonded block copolymers (BCP). Thus, supramolecular binding motives included on the polymer chain-ends display additional dynamic effects as well as possible nuclei for the crystallization. In this article we systematically study nonisothermal crystallization processes in SPBCP’s consisting of a crystallizable poly­(ε-caprolactone) (PCL) connected via triple hydrogen bonds to either a short alkyl-modified 2,4-diaminotriazine, or bound to a large block of amorphous poly­(isobutylene) (PIB). The crystallization of the PCL is studied with both groups acting as supramolecular barriers for the crystallization process, either during nucleation or during crystal growth. A strong influence of the short alkyl-modified 2,4-diaminotriazine barrier on the crystallization temperature of the PCL compared to the control sample devoid of this compound is observed. In contrast, the large polymer block (PIB) acting as a barrier causes a strong decrease of the crystallization temperature and fractionated crystallization of SPBCP consisting of smaller PCL-chains is observed

Molecular Order in Cold Drawn, Strain-Recrystallized Poly(ε-caprolactone)

Biaxial order in free-standing films of poly­(ε-caprolactone) (PCL), induced by plastic deformation and fibrillation, is studied by infrared transition moment orientational analysis (IR-TMOA) and X-ray diffraction (pole figures). This enables one to determine the order parameter tensor for the different molecular moieties with respect to the sample coordinate system. The main chains of the polymers are aligned with the stretching direction (<u><i>x</i></u>), leading to a strong order of the crystallites (Hermans orientation function, <i>S</i>_<i>xx</i> = 0.9 ± 0.1), and less ordered, amorphous regions (<i>S</i>_<i>xx</i> = 0.34 ± 0.1). The microscopic biaxiality of the system, |<i>S</i>_<i>yy</i> – <i>S</i>_<i>zz</i>| ≈ 0.1, is caused by the macroscopically asymmetric deformation perpendicular to the stretching direction. Cold drawing leads to a reduction in crystallinity and distorted crystallites in the fibrils, indicating the absence of “melting and recrystallization” or “fine slip” processes

Influence of Fullerene Grafting Density on Structure, Dynamics, and Charge Transport in P3HT‑<i>b</i>‑PPC₆₁BM Block Copolymers

A series of tailor-made poly­(3-hexylthiophene)-<i>block</i>-PPCBM (P3HT-<i>b</i>-PPCBM) block copolymers incorporating P3HT as donor and a polystyrene with pendant fullerenes (PC₆₁BM) as acceptor block (PPCBM) is presented. The grafting density of PC₆₁BM was varied between 26 and 60 wt %. This has high impact on structure formation, molecular dynamics, and charge transport. It causes considerable increase in glass transition temperature (<i>T</i>_g from 150 to 200 °C). The <i>T</i>_g of the amorphous PPCBM block restricts the dynamics of structure evolution of the block copolymer resulting in an incomplete microphase separation, although structural studies revealed a donor–acceptor nanostructure of 30–40 nm in bulk and thin films. All block copolymers exhibit ambipolar charge transport in organic field-effect transistors. Further, the most densely grafted system showed 2 orders of magnitude higher electron mobility. Thus, the fullerene grafting density turned out as a key parameter in designing P3HT-<i>b</i>-PPCBM systems for tuning phase separation and charge transport

Determination of the Crystallinity of Semicrystalline Poly(3-hexylthiophene) by Means of Wide-Angle X‑ray Scattering

Temperature-dependent small-angle and wide-angle X-ray scattering (SAXS/WAXS) measurements on a series of chemically well-defined and highly regioregular poly­(3-hexylthiophenes) were analyzed to determine absolute values of the crystallinities. The analysis is based on the evaluation of the scattered intensity from the amorphous regions providing an easy and fast method for the determination of the crystallinity in the class of side chain substituted polymers. The resulting values are in the range of 68–80% at room temperature depending on the molecular weight. Based on these values, an extrapolated reference melting enthalpy of a 100% crystalline material was determined (Δ<i>H</i>_m^∞ = 33 ± 3 J/g) for use in DSC measurements. For higher molecular weights a decrease of the crystallinity was observed which can be explained by the onset of chain folding as deduced from the analysis of the SAXS patterns. An in-depth analysis based on Ruland’s method showed that the crystalline regions of P3HT exhibit a large amount of internal disorder

Nanostructure and Rheology of Hydrogen-Bonding Telechelic Polymers in the Melt: From Micellar Liquids and Solids to Supramolecular Gels

Polymers with hydrogen-bonding groups in the melt state often combine the ability to form specific supramolecular bonds with a tendency for unspecific aggregation and microphase separation. Using a combination of small-angle X-ray scattering and shear spectroscopy, we present a study of structure formation and rheological properties of such a case, an exemplary series of telechelic polyisobutylenes, functionalized with hydrogen-bonding end groups. Unspecific interaction between hydrogen-bonding groups leads to the formation of micelles. For monofunctional samples, we observe ordering at lower temperatures, induced by a temperature dependent concentration of the micelles. The rheological properties of these systems can be mapped to the behavior of a concentrated colloidal fluid or solid. For bifunctional polymers with complementary hydrogen-bonding groups, interaction between micellar aggregates leads to network formation and solidlike properties at lower temperatures induced by gelation without ordering. Only in this case the supramolecular bonds directly determine the rheological properties

What Controls the Structure and the Linear and Nonlinear Rheological Properties of Dense, Dynamic Supramolecular Polymer Networks?

We investigated a series of telechelic polyisobutylenes, previously shown to exhibit self-healing, by means of small-angle X-ray scattering and rheology. All samples form a dense, dynamic network of interconnected micelles resulting from aggregation of the functional groups and leading to viscoelastic behavior. The dynamic character of this network manifests itself in the appearance of terminal flow at long time scales. While the elastic properties are distinctly molecular weight dependent, the terminal relaxation time is controlled by the functional end groups. The yielding properties under large deformation during startup shear experiments can be understood by a model of stress activation of the dynamic bonds. Stress relaxation experiments help to separate the nonlinear response into two contributions: a fast collapse of the network and a slow relaxation, happening on the time scale of the terminal relaxation. The latter is also known to control self-healing of the collapsed structure

Influence of Chain Topology on Polymer Dynamics and Crystallization. Investigation of Linear and Cyclic Poly(ε-caprolactone)s by ¹H Solid-State NMR Methods

We report on the investigation of cyclic and comparable linear poly(ε-caprolactone)s (PεCL) with molecular weight between 50 and 80 kg/mol with regard to chain mobility in the melt and crystallinity using low-field solid-state ¹H NMR. Our results from NMR Hahn echo and more advanced multiquantum measurements demonstrate a higher segmental mobility of cyclics in the melt as compared to their linear counterparts. Rheological experiments indicate that the cyclics are less viscous than the linear analogues by about a factor of 2, confirming the NMR results. FID component analysis shows higher crystallinities of the cyclic samples by some percent under the condition of isothermal crystallization at 48 °C, suggesting that due to their enhanced overall mobility in the melt, the cyclics reach a more perfect morphology leading to higher crystallinity. We compare this finding with results from DSC measurements obtained under identical conditions and critically evaluate the applicability of polymer crystallinity determination from nonisothermal crystallization investigations by DSC. We further highlight the use of nucleating agents to investigate the particular effect of crystal growth on (nonisothermal) crystallization, separated from the influence of nucleation. These experiments indicate a faster crystal growth for cyclic samples

Thermotropic Behavior, Packing, and Thin Film Structure of an Electron Accepting Side-Chain Polymer

We report on the phase behavior and the structure of poly­(perylene bisimide acrylate), an electron accepting semiconductor polymer with disclike side-chain units, in comparison to the corresponding low molecular weight perylene bisimide. By combination of DSC, optical microscopy, and temperature-dependent small-angle and wide-angle X-ray scattering, we show that both compounds display a lamello-columnar packing. While the perylene bisimide model compound crystallizes, the polymeric architecture of poly­(perylene bisimide acrylate) suppresses order, leading to a 2D lamello-columnar liquid crystalline phase. The structure of the side-chain polymer in thin films with different thermal treatments as observed by GIWAXS correlates well with previously observed largely different electron mobilities. Such a polymeric, liquid crystalline compound combines the advantages of molecular order and easy processability, together with the film forming properties of polymeric materials

Phase Separation in the Melt and Confined Crystallization as the Key to Well-Ordered Microphase Separated Donor–Acceptor Block Copolymers

Microphase-separated donor–acceptor block copolymers have been discussed as ideal systems for morphology control in organic photovoltaics. Typical microphases as known from coil–coil systems were not observed in such systems due to crystallization dominating over microphase separation. We show how this problem can be overcome by the synthesis of high molecular weight block copolymers leading to a high enough χ<i>N</i> parameter and microphase separation in the melt. A combination of copper-catalyzed azide-alkyne click reaction and nitroxide mediated radical polymerization (NMRP) was used for the synthesis of donor–acceptor poly­(3-hexylthiophene)-<i>block</i>-poly perylene bisimide acrylate (P3HT-<i>b</i>-PPerAcr) block copolymers. With this synthetic strategy, high molecular weights are possible and no triblock copolymer byproducts are formed, as observed with former methods. Two different block copolymers with a high molecular weight P3HT block of 19.7 kg/mol and a PPerAcr content of 47 and 64 wt % were obtained. X-ray scattering measurements show that the diblock copolymers exhibit microphase separation in the melt state. Furthermore, upon cooling confined crystallization occurs inside the microphase separated domains without destroying the microphase order. The observed microstructures fit well to the respective volume fractions and the crystalline packing within the individual blocks is analogous to those in the respective homopolymers. For the first time, typical lamellar or cylindrical phase separated structures as known for amorphous coil–coil systems are realized for a crystalline–liquid crystalline, donor–acceptor block copolymer. A similar block copolymer synthesized with an earlier method exhibits a crystallization-induced microphase separation

Regioregular Polymer Analogous Thionation of Naphthalene Diimide–Bithiophene Copolymers

Polymer analogous thionation of the n-type conjugated polymer PNDIT2 is investigated using Lawesson’s reagent (LR). Detailed high-temperature NMR spectroscopic investigations show that due to the copolymer structure, two out of the four available carbonyl groups present in the naphthalene diimide (NDI) comonomer are sterically less hindered and react preferentially. This leads to regioselective thionation in the <i>trans</i>-configuration even for a large excess of LR. For high degrees of O/S conversion, signals of minor intensity show up in addition pointing to undesired side reactions. These signals could not be eliminated despite further optimized reaction conditions including different aromatic solvents and reaction temperatures. Compared to PNDIT2, the resulting 2S-<i>trans</i>-PNDIT2 features strong aggregation, lower solubility, an 80 nm bathochromic shift of the charge-transfer band, a by 0.22 eV lower LUMO energy level, a lower thermal stability, and higher melting temperatures (<i>T</i>_m). As the combination of the lower thermal stability and higher melting points renders the characterization of thermal transitions challenging, fast scanning calorimetry (flash-DSC) is successfully used to determine <i>T</i>_m. With increasing O/S conversion, <i>T</i>_m first increases but then decreases, which is ascribed to a combined effect of stronger main chain interactions and increasing chemical defects. Microstructural order and field-effect electron mobilities decrease with increasing O/S conversion compared to PNDIT2