Nucleation of Polymer Crystals: The “δ Mystery”

Nucleation of Polymer Crystals: The “δ Mystery

Extensive Development of Precursory Helical Pairs Prior to Formation of Stereocomplex Crystals in Racemic Polylactide Melt Mixture

Melt crystallization of racemic polylactide (equimolar PLLA/PDLA) blend upon slow cooling (1 °C/min from 270 °C) was studied via a combination of wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FTIR). Results indicated extensive development of racemic (3₂/3₁) helical pairs below 220 °C, followed by emergence of a broad mesomorphic peak in the WAXS profile below 190 °C; the intensity of this mesophase peak started to decrease at 150 °C, with concomitant emergence of WAXS- or DSC-discernible formation of stereocomplex (β_c) crystals. Isothermal measurements at 200 vs 170 °C revealed the presence of low vs high populations of helical pairs; β_c crystals were observed to develop only at 170 °C but not at 200 °C, indicating the need for adequate population of racemic helical pairs for formation of their mesomorphic clusters in the melt matrix as precursors of β_c nuclei. The clear change in the melt structure <i>well before</i> the formation of incipient β_c crystals reflects strong driving force under large supercooling toward transformation, but the transformation process is kinetically suppressed: only after extensive development of racemic helices and emergence of mesomorphic clusters in the melt matrix may nucleation occur. These observations suggest that the nucleation process proceeds in elementary units of preformed helical pairs in the melt matrix, with an intermediate stage of clustered helical pairs before incipience of β_c crystals

Well-Packed Chains and Aggregates in the Emission Mechanism of Conjugated Polymers

We synthesized dialkoxy-substituted poly[phenylene vinylene]s (dROPPV-1/1, 0.2/1, and 0/1) consisting of two repeating units with different side-chain lengths (methoxy and 3,7-dimethyloctyloxy). These polymers can serve as a model system to clarify roles of aggregates (the sites with ground-state interchain interactions) and the independent chain segments in the well-packed chains (the chain segments that are compactly packed without interaction) in the emission mechanism of conjugated polymers. Due to the packing of polymer chains, films of all of these polymers are accessible to interchain excitations, after which excitons can re-form to result in delayed luminescence. Besides, some chains form aggregates so that the delayed luminescence is no more the ordinary single-chain emission but red-shifted and less structured. Not only the re-formation of these indirect excitons but also the aggregation of chains are facilitated in the polymers with short methoxy side groups, revealing that both packing and aggregation of chain segments require a short spacing between polymer chains. However, the incorporation of other side chains such as the 3,7-dimethyloctyloxy group to dROPPVs is necessary for the formation of aggregates because these long branched side chains can reduce the intrachain order imposed by the short methoxy groups, which accounts for the absence of aggregate emission in the well-studied poly[2,5-dimethoxy-1,4-phenylene vinylene]. This study reveals that the well-packed chains do not necessarily form aggregates. We also show that the photophysical properties and the film morphology of conjugated polymers can be deliberately controlled by fine-tuning of the copolymer compositions, without altering the optical properties of single polymer chains (e.g., as in dilute solutions)

Fine Tuning the Purity of Blue Emission from Polydioctylfluorene by End-Capping with Electron-Deficient Moieties

We propose a simple way to achieve pure blue emission and improved device efficiency via capping poly(9,9-dioctylfluorene) (PFO) with electron-deficient moieties (EDMs, such as oxadiazole (OXD) and triazole (TAZ)), which can induce a minor amount of long conjugating length species (regarded as β phase) to control extents of energy transfer from amorphous matrix to the β phase. The device efficiency of PFO end-capped with TAZ is higher than that with para-tert-butyl phenyl (TBP) by a factor of 2 (with CsF/Al as cathode), and its electroluminescent spectrum remains stable and with pure blue emission during cyclic operations (C.I.E. color coordinates x = 0.165, y = 0.076, independent of operating voltage and within the limit for pure blue emission x + y < 0.30). The improvement of device efficiency is dependent on the structure of EDM, such as size and planarity. The deep blue emission is originated from the incomplete energy transfer from amorphous matrix to the β phase induced by the end-cappers

Nanoscale Ordered Structure Distribution in Thin Solid Film of Conjugated Polymers: Its Significance in Charge Transport Across the Film and in Performance of Electroluminescent Device

We use ultrahigh-vacuum conducting atomic force microscopy to probe the local current distributions in spin-cast thin films from the solutions of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and poly(9,9-di-n-octyl-2,7-fluorene) (PFO). We found that spatially homogeneous distribution of the ordered structures (well-packed chains and/or aggregates) in MEH-PPV can be controlled by the selection of solvent or mixed solvent, by which effects of spatial charge transport distribution in MEH-PPV thin films on the performance of polymer light-emitting diodes (PLEDs) are unambiguously clarified. For PFO thin film, after the treatment by immersing in the mixed nonsolvent composed of a solvent and nonsolvent, the ordered structures (β-phase) are generated; its excess content can result in highly conducting regions. However, the device efficiency can be promoted significantly by optimizing the content of β-phase

Sequential Epitaxial Organization of Poly(9,9-di‑<i>n</i>‑octyl-2,7-fluorene) in an Eutectic System

The phase diagram of a binary eutectic system of poly­(9,9-di-<i>n</i>-octyl-2,7-fluorene) (PFO) and hexamethylbenzene (HMB) has been established via onset melting temperatures. Upon homogeneous mixing, the growth of PFO nematic phase in binary mixtures exhibits both lyotropic and thermotropic features. During cooling, a smectic phase of PFO developed epitaxially on the surface of prior-developed HMB crystals, establishing layer stacking via the packing of side chains. Upon the annealing at 120 °C, further epitaxial organization of PFO smectic packing on HMB crystalline substrate results in the growth of β crystalline. Both smectic and β phases of PFO yield characteristic ultraviolet (UV) absorption at 434 nm wavelength and photoluminescence at the wavelength of 441 nm. While HMB crystals were sublimated prior to the annealing treatment, α crystalline form developed instead through the transformation of smectic packing. The UV absorption and fluorescence of α form are relatively shorter wavelengths. As attributing the rise of energy bandgaps to the effect of backbone conformation, organization of less coplanar backbones within α crystalline form is proposed as a result of the lack of epitaxial effect. After the growth of smectic phase via the monotropic behavior of PFO in eutectic mixture, the following routes of phase transformation upon annealing hence unveiled the growth mechanisms of both α and β crystalline forms of PFO

Equilibrium Melting Temperature Depression in Syndiotactic Poly(styrene-<i>stat</i>-3-methylstyrene) and Poly(styrene-<i>stat</i>-4-methylstyrene)

After randomly incorporating 2–21 mol % of 3-methylstyrene (3MS) or 4-methylstyrene (4MS) into the syndiotactic polystyrene (sPS) backbone, the effects of the comonomer units on the equilibrium melting temperature (Tm°) depression were examined within the Sanchez–Eby theoretical framework. In situ small-/wide-angle X-ray scattering (SAXS/WAXS) was used to investigate the morphological evolution upon heating. The WAXS profiles showed that the α- or β-dominated crystals in syndiotactic poly­(styrene-stat-3-methylstyrene) (sPS-3MS) and poly­(styrene-stat-4-methylstyrene) (sPS-4MS) all consistently reach complete melting with no signs of interphase transformation. By analyzing the corresponding SAXS heating profiles with a polyradius cylinder form factor, the correlations between the crystal thickness (lc) and temperature were identified; the corresponding Tm° value was then determined via constructing the Gibbs–Thomson melting line and extrapolating it to the infinite crystal thickness. Generally, the Tm° of α and β crystals decreases with increasing comonomer content, and the Tm° for the β phase was always higher than that of the α counterpart. The level of Tm° depression was interpreted in terms of the Sanchez–Eby theory, showing that the introduction of methyl groups into the β structure has higher penalty energy than that into the α structure in both sPS-3MS and sPS-4MS cases, suggesting that incorporating the methyl groups more significantly destabilizes the β crystals and leads to a further preference for α crystal formation. Comparing the β crystals between sPS-3MS and sPS-4MS systems, the penalty energy for incorporating 4MS units into the β lattice is greater than that in the 3MS case; for the α crystals, the penalty energy for incorporating 3MS units is greater than that in the 4MS case

Crystallization of α versus β Phases in Syndiotactic Poly(styrene-<i>stat</i>-3-methylstyrene) and Poly(styrene-<i>stat</i>-4-methylstyrene)

By incorporating different levels of methyl groups at meta- or para-positions (3- vs 4-methylstyrene) as comonomer units into syndiotactic polystyrene (sPS), we examined the effects of comonomer structure/content on the polymorphic behavior. Upon melt-crystallization, wide-angle X-ray scattering (WAXS) profiles revealed that syndiotactic poly­(styrene-stat-3-methylstyrene) (sPS-3MS) formed predominantly β-crystals for 3MS content up to 11 mol %, followed by dominance of α-crystals at a higher 3MS level (21 mol %); in the case of syndiotactic poly­(styrene-stat-4-methylstyrene) (sPS-4MS), predominant formation of β-crystals occurs only at 2 mol % 4MS content, followed by dominance of α-crystals at higher comonomer levels (5 and 10 mol %). Upon cold-crystallization, both sPS-3MS and sPS-4MS consistently gave only α-crystals. It appears that the formation of α-phase is consistently preferred in these random copolymers. WAXS profiles showed that the characteristic reflections generally shift to lower q-positions with increasing comonomer content, i.e., crystals in copolymers have greater unit cell parameters, except for the β-structure of sPS-4MS. Hence, comonomer units are believed to be incorporated into α-crystals in both sPS-3MS and sPS-4MS or β-structure in sPS-3MS but fully excluded from the β-structure of sPS-4MS. Results of molecular mechanics computation showed that the packing energy increases upon incorporation of comonomer units, the effect being particularly strong for β-crystals of sPS-4MS. The significantly suppressed formation of β-crystals in sPS-4MS may thus be simply attributed to the strongly increased interchain packing energy due to the protruding 4-methyl groups; this steric repulsion is weaker for β-crystals in the sPS-3MS series, consistent with the delayed preference of α-crystals to higher 3MS contents

Thickening-Induced Faceting Habit Change in Solution-Grown Poly(l-lactic acid) Crystals

Morphological evolution of poly(l-lactic acid) (PLLA) crystals obtained via crystallization at Tc = 85 to 97 °C from 0.1% solutions in mixed xylene was closely followed via transmission electron microscopy. Upon stirring-enhanced nucleation after the adopted incubation scheme (ti = 10 h at Ti = 90 °C), there emerged “a axis” lenticular crystals of the imperfectly packed α form, as indicated by the presence of the presumably forbidden (010) spot in the selected-area electron diffraction along the [001] zone. Subsequently observed during further growth was a gradual change from lenticular to truncated lozenge with {100} and {110} facets, driven by the outward propagation of the thickened interior (truncated lozenge in shape) within the lenticular crystal. The width of the {100} facets clearly decreased with decreasing Tc, resulting in rhombic lozenges with {110} edges at 87 °C. This change in faceting habit is interpreted in terms of the competition between primary growth (controlled by surface-nucleation) and interior thickening (controlled by the reeling-in of chains from the primary thin rim via slip diffusion on the {110} surfaces). Eliminating the stirring- enhanced nucleation procedure, we have observed direct transition from lenticular to rhombic lozenge crystals at Tc = 85 °C without interior thickening, that is, bypassing entirely the truncated lozenge stage. It is hence proposed that the lamellar thickening is intrinsically a nucleation-and-growth process, initiated by the (presumably thick) nucleus from stirring, followed by the reeling-in process of stems in the rim. The supercooling dependence of the reeling-in rate is anisotropic and decreases most significantly along [100] directions with increasing Tc. Therefore, the {100} facets of truncated lozenge develop wider at higher Tc

Competition between Fullerene Aggregation and Poly(3-hexylthiophene) Crystallization upon Annealing of Bulk Heterojunction Solar Cells

Concomitant development of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) aggregation and poly(3-hexylthiophene) (P3HT) crystallization in bulk heterojunction (BHJ) thin-film (ca. 85 nm) solar cells has been revealed using simultaneous grazing-incidence small-/wide-angle X-ray scattering (GISAXS/GIWAXS). With enhanced time and spatial resolutions (5 s/frame; minimum q ≈ 0.004 Å–1), synchrotron GISAXS has captured in detail the fast growth in size of PCBM aggregates from 7 to 18 nm within 100 s of annealing at 150 °C. Simultaneously observed is the enhanced crystallization of P3HT into lamellae oriented mainly perpendicular but also parallel to the substrate. An Avrami analysis of the observed structural evolution indicates that the faster PCBM aggregation follows a diffusion-controlled growth process (confined by P3HT segmental motion), whereas the slower development of crystalline P3HT nanograins is characterized by constant nucleation rate (determined by the degree of supercooling and PCBM demixing). These two competing kinetics result in local phase separation with space-filling PCBM and P3HT nanodomains less than 20 nm in size when annealing temperature is kept below 180 °C. Accompanying the morphological development is the synchronized increase in electron and hole mobilities of the BHJ thin-film solar cells, revealing the sensitivity of the carrier transport of the device on the structural features of PCBM and P3HT nanodomains. Optimized structural parameters, including the aggregate size and mean spacing of the PCBM aggregates, are quantitatively correlated to the device performance; a comprehensive network structure of the optimized BHJ thin film is presented