Thermal Conductance of Poly(3-methylthiophene) Brushes.

A wide variety of recent work has demonstrated that the thermal conductivity of polymers can be improved dramatically through the alignment of polymer chains in the direction of heat transfer. Most of the polymeric samples exhibit high conductivity in either the axial direction of a fiber or in the in-plane direction of a thin film, while the most useful direction for thermal management is often the cross-plane direction of a film. Here we show poly(3-methylthiophene) brushes grafted from phosphonic acid monolayers using surface initiated polymerization can exhibit through-plane thermal conductivity greater than 2 W/(m K), a 6-fold increase compared to spin-coated poly(3-hexylthiophene) samples. The thickness of these films (10-40 nm) is somewhat less than that required in most applications, but the method demonstrates a route toward higher thermal conductivity in covalently grafted, aligned polymer films.

Three-Dimensional Conformation of Folded Polymers in Single Crystals.

The chain-folding mechanism and structure of semicrystalline polymers have long been controversial. Solid-state NMR was applied to determine the chain trajectory of (13)C CH3-labeled isotactic poly(1-butene) (iPB1) in form III chiral single crystals blended with nonlabeled iPB1 crystallized in dilute solutions under low supercooling. An advanced (13)C-(13)C double-quantum NMR technique probing the spatial proximity pattern of labeled (13)C nuclei revealed that the chains adopt a three-dimensional (3D) conformation in single crystals. The determined results indicate a two-step crystallization process of (i) cluster formation via self-folding in the precrystallization stage and (ii) deposition of the nanoclusters as a building block at the growth front in single crystals.

Folding of Polymer Chains in the Early Stage of Crystallization.

Understanding the structure formation of an ordered domain in the early stage of crystallization is one of the long-standing issues in polymer science. In this study, we investigate the chain trajectory of isotactic polypropylene (iPP) formed via rapid and deep quenching, using solid-state NMR spectroscopy. Comparisons of experimental and simulated 13C-13C double quantum (DQ) buildup curves demonstrated that individual iPP chains adopt adjacent re-entry sequences with an average folding number ⟨n⟩ = 3-4 in the mesomorphic form, assuming an adjacent re-entry fraction ⟨F⟩ of 100%. Therefore, long flexible polymer chains naturally fold in the early stage of crystallization, and folding-initiated nucleation results in formation of mesomorphic nanodomains.

Molecular Structural Basis for Stereocomplex Formation of Polylactide Enantiomers in Dilute Solution.

Poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) alternatively pack with each other and form stereocomplex crystals (SCs). The crystal habits of SCs formed in the dilute solution highly depend on the molecular weight (⟨Mw⟩). In this study, we investigated chain-folding (CF) structure for 13C labeled PLLA (l-PLLA) chains in SCs with PDLAs that have either high or low ⟨Mw⟩s by employing an advanced Double Quantum (DQ) NMR. It was found that the ensemble average of the successive adjacent re-entry number ⟨n⟩ for the l-PLLA chains drastically change depending on ⟨Mw⟩s of the counter PDLA chains in the SCs. It was concluded that the CF structures of l-PLLA depending on ⟨Mw⟩s of PDLA determine the crystal habits of SCs.