Control of molecular aggregation in symmetrically substituted π-conjugated bulky poly(p -phenylenevinylene)s and their copolymers

Interfacial Modulation of Graphene by Polythiophene with Controlled Molecular Weight to Enhance Thermal Conductivity

With a trend of continuing improvement in the development of electronic devices, a problem of serious heat accumulation has emerged which has created the need for more efficient thermal management. Graphene sheets (GNS) have drawn much attention with regard to heat transfer because of their excellent in-plane thermal conductivity; however, the ultrahigh interfacial thermal resistance between graphene lamellae has seriously restricted its practical applications. Herein, we describe heat transfer membranes composed of graphene which have been modified by intrinsic thermally conductive polymers with different molecular weights. The presence of macromolecular surface modifiers not only constructed the graphene heat transfer interface by π-π interactions, but also significantly enhanced the membranes' in-plane thermal conductivity by utilizing their intrinsic heat transfer properties. Such results indicated that the in-plane thermal conductivity of the fabricated membrane exhibits a high in-plane thermal conductivity of 4.17 W m-1 K-1, which, containing the GNS modified with 6000 g/mol (Mn) of poly(3-hexylthiophene) (P3HT), was 26 times higher that of poly (vinylidene fluoride) (PVDF). The P3HT molecular chain with specific molecular weight can form more matching structure π-π interactions, which promotes thermal conductivity. The investigation of different molecular weights has provided a new pathway for designing effective interfacial structures to relieve interface thermal resistance in thermally conductive membranes.

Secondary Structure in Nonpeptidic Supramolecular Block Copolymers

Proteins contain a level of complexity-secondary and tertiary structures-that polymer chemists aim to imitate. The bottom-up synthesis of protein-mimicking polymers mastering sequence variability and dispersity remains challenging. Incorporating polymers with predefined secondary structures, such as helices and π-π stacking sheets, into block copolymers circumvents the issue of designing and predicting one facet of their 3D architecture. Block copolymers with well-defined secondary-structure elements formed by covalent chain extension or supramolecular self-assembly may be considered for localized tertiary structures.In this Account, we describe a strategy toward block copolymers composed of units bearing well-defined secondary structures mixed in a "plug-and-play" manner that approaches a modicum of the versatility seen in nature. Our early efforts focused on the concept of single-chain collapse to achieve folded secondary structures through either hydrogen bonding or quadrupole attractive forces. These cases, however, required high dilution. Therefore, we turned to the ring-opening metathesis polymerization (ROMP) of [2.2]paracyclophane-1,9-dienes (pCpd), which forms conjugated, fluorescent poly(p-phenylenevinylene)s (PPVs) evocative of β-sheets. Helical building blocks arise from polymers such as poly(isocyanide)s (PICs) or poly(methacrylamide)s (PMAcs) containing bulky, chiral side groups while the coil motif can be represented by any flexible chain; we frequently chose poly(styrene) (PS) or poly(norbornene) (PNB). We installed moieties for supramolecular assembly at the chain ends of our "sheets" to combine them with complementary helical or coil-shaped polymeric building blocks.Assembling hierarchical materials tantamount to the complexity of proteins requires directional interactions with high specificity, covalent chain extension, or a combination of both chemistries. Our design is based on functionalized reversible addition-fragmentation chain-transfer (RAFT) agents that allowed for the introduction of recognition motifs at the terminus of building blocks and chain-terminating agents (CTAs) that enabled the macroinitiation of helical polymers from the chain end of ROMP-generated sheets and/or coils. To achieve triblock copolymers with a heterotelechelic helix, we relied on supramolecular assembly with helix and coil-shaped building blocks. Our most diverse structures to date comprised a middle block of PPV sheets, parallel or antiparallel, and supramolecularly or covalently linked, respectively, end-functionalized with molecular recognition units (MRUs) for orthogonal supramolecular assembly. We explored PPV sheets with multiple folds achieved by chain extension using alternating pCpd and phenyl-pentafluorophenyl β-hairpin turns. Using single-molecule polarization spectroscopy, we showed that folding occurs preferentially in multistranded over double-stranded PPV sheets. Our strategy toward protein-mimicking and foldable polymers demonstrates an efficient route toward higher ordered, well-characterized materials by taking advantage of polymers that naturally manifest secondary structures. Our studies demonstrate the retention of distinct architectures after complex assembly, a paradigm that we believe may extend to other polymeric folding systems.

High fluorescence intensity poly(aryl ether ketone)s containing tetraphenylethylene moieties: preparation, characterization and fluorescent properties

PAEK with tetraphenylethylene groups and relationships between fluorescence intensity and temperature.

Linearly polarized emission from self-assembled microstructures of mesogenic polythiophenes

Polarized electroluminescence from ordered mesogenic polythiophenes.

Amphiphilic π-conjugated poly(m-phenylene) photosensitizer for the Eu3+ ion: the role of macromolecular chain aggregation on the color tunability of lanthanides

Here, we report new carboxylic functionalized poly(phenylene)s and their oligomers as selective and efficient photosensitizers for Eu(3+) ions. Palladium-catalyzed Suzuki polycondensation was developed for the synthesis of carboxylic functionalized π-conjugated materials. The chemical structures of the polymer skeleton were varied using two anchoring groups consisting of ethylhexyloxy and methoxy substitution in the chain backbone. The molecular weights of the polymer samples were obtained in the range of 4000-8000 containing 20 aromatic units in the chain. Photoexcitation of the oligomer-Eu(3+) complexes resulted in magenta color emission as a result of the combination of partial blue self-emission from the chromophores along with the red color from the metal center. The ethylhexyl substituted polymer-Eu(3+) complex showed complete excitation energy transfer from the macromolecular chains to the metal center and produced bright and sharp red emission. The polymer-containing methoxy unit was found to show largely self-emission rather than photoexcitation to the metal center. Singlet and triplet energy levels of the complexes and chromophores revealed that both oligomers and polymers have almost identical energy levels for photosensitizing Eu(3+) ions. The polymers possessed typical amphiphilic structures via a rigid aromatic hydrophobic core and hydrophilic anionic periphery for self-organization in water. Both dynamic light scattering and atomic force microscope analysis confirmed the existence of the spherical shape nanometer size aggregates of the polymer chains in water. The branched ethylhexyl polymer showed the formation of loosely packed 500 nm aggregates whereas tightly packed 200 nm particles are produced by the methoxy substituted rigid polymer. These molecular aggregates behaved as templates for complexation as well as photosensitizing of the Eu(3+) ions. The loosely packed nanoaggregates (ethylhexyl polymer) contain Eu(3+) ions in the entire scaffold and showed efficient and complete energy transfer from the conjugated chain to metal ions. The tightly packed rigid-chains in methoxy polymer restricted the complete energy transfer to metal center. The molecular self-organization of the polymers played a crucial role on the efficient energy transfer from the polymer chain to metal center, more specifically Eu(3+) ion-based red emission.