Micellar Morphologies of Poly(ε-caprolactone)-<i>b</i>-poly(ethylene oxide) Block Copolymers in Water with a Crystalline Core

The self-assembly of poly(ε-caprolactone)-b-poly(ethylene oxide) block copolymers (PCLnPEO44 and PCLnPEO113) with narrow polydispersity in aqueous medium was studied using transmission electron microscopy. In this system, the formed micelles are composed of a crystalline PCL core and a soluble PEO corona. We demonstrated that the PCL-b-PEO block copolymers can form micelles with abundant morphologies, depending on the lengths of the blocks and composition. It is observed that for PCLnPEO44 the micellar morphology changes from spherical, rodlike, wormlike, to lamellar, as the length of the PCL block increases. In contrast, most of PCLnPEO113 (n = 21−147) block copolymers form spherical micelles, and only PCL232PEO113 exhibits mixed spherical and lamellar micellar morphologies. The effect of microstructure on micellar morphology was semiquantitatively interpreted in terms of reduced tethering density (σ). It is found that lamellar micelles are formed when σ is smaller than a critical value of between 3.0 and 4.8. A larger σ indicates crowding of the tethered chain, and spherical micelles tend to be formed

Micellar Morphologies of Poly(ε-caprolactone)-<i>b</i>-poly(ethylene oxide) Block Copolymers in Water with a Crystalline Core

The self-assembly of poly(ε-caprolactone)-b-poly(ethylene oxide) block copolymers (PCLnPEO44 and PCLnPEO113) with narrow polydispersity in aqueous medium was studied using transmission electron microscopy. In this system, the formed micelles are composed of a crystalline PCL core and a soluble PEO corona. We demonstrated that the PCL-b-PEO block copolymers can form micelles with abundant morphologies, depending on the lengths of the blocks and composition. It is observed that for PCLnPEO44 the micellar morphology changes from spherical, rodlike, wormlike, to lamellar, as the length of the PCL block increases. In contrast, most of PCLnPEO113 (n = 21−147) block copolymers form spherical micelles, and only PCL232PEO113 exhibits mixed spherical and lamellar micellar morphologies. The effect of microstructure on micellar morphology was semiquantitatively interpreted in terms of reduced tethering density (σ). It is found that lamellar micelles are formed when σ is smaller than a critical value of between 3.0 and 4.8. A larger σ indicates crowding of the tethered chain, and spherical micelles tend to be formed

Cooperative Effect of Electrospinning and Nanoclay on Formation of Polar Crystalline Phases in Poly(vinylidene fluoride)

Poly(vinylidene difluoride)/organically modified montmorillonite (PVDF/OMMT) composite nanofibers were prepared by electrospinning the solution of PVDF/OMMT precursor in DMF. Wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) show that in the bulk of the PVDF/OMMT precursor OMMT platelets are homogeneously dispersed in PVDF and can be both intercalated and exfoliated. It is found that the diameter of the PVDF/OMMT composite nanofibers is smaller than that of the neat PVDF fibers because the lower viscosity of PVDF/OMMT solution, which is attributed to the possible adsorption of PVDF chains on OMMT layers and thus reduction in number of entanglement. The crystal structure of the composite nanofibers was investigated using WAXD and Fourier transform infrared (FT-IR) and compared with that of thin film samples. The results show that the nonpolar α phase is completely absent in the electrospun PVDF/OMMT composite nanofibers, whereas it is still present in the neat PVDF electrospun fibers and in the thin films of PVDF/OMMT nanocomposites. The cooperative effect between electrospinning and nanoclay on formation of polar β and γ crystalline phases in PVDF is discussed. The IR result reveals that electrospinning induces formation of long trans conformation, whereas OMMT platelets can retard relaxation of PVDF chains and stabilize such conformation due to the possible interaction between the PVDF chains and OMMT layers. This cooperative effect leads to extinction of nonpolar α phase and enhances the polar β and γ phases in the electrospun PVDF/OMMT composite nanofibers

Microphase Separation with Sub‑3 nm Microdomains in Comb-Like Poly(<i>n</i>‑alkyl acrylate) Homopolymers Facilitated by Charged Junction Groups between the Main Chains and Side Chains

The phase structure with a small domain size in polymers is expected to provide a template for lithography to fabricate electronic devices, while the uniformity and thermal stability of the phase structure are vital in lithography. In this work, we report an accurately microphase-separated system of comb-like poly(ionic liquid) (PIL)-based homopolymers containing imidazolium cation junctions between the main chain parts and the long alkyl side chains, poly(1-((2-acryloyloxy)ethyl)-3-alkylimidazolium bromide) (P(AOEAmI-Br)). The ordered hexagonally packed cylinder (HEX) and lamellar (LAM) structures with small domain sizes (sub-3 nm) were successfully achieved. Since the microphase separation was induced by the incompatibility between the main chain parts and the hydrophobic alkyl chains, the microdomain spacing of the ordered structure was independent of the molecular weight and molecular weight distribution of P(AOEAmI-Br) homopolymers and could be precisely regulated by changing the length of the alkyl side chains. Importantly, the microphase separation was promoted by the charged junction groups; thus, the phase structure and domain size of P(AOEAmI-Br) exhibited excellent thermal stability

Straightening Single-Walled Carbon Nanotubes by Adsorbed Rigid Poly(3-hexylthiophene) Chains via π–π Interaction

The straightened morphology of single-walled carbon nanotubes (SWCNTs) driven by rigid and conjugated regioregular poly­(3-hexyl­thiophene) (rr-P3HT) and the related mechanism were studied with transmission electron microscopy (TEM), atomic force microscopy (AFM), and spectroscopic methods. It was found that bent SWCNTs could be straightened significantly in solutions with low SWCNT concentration, which was quantitatively confirmed by the increased persistence length. UV–vis and photoluminescence spectroscopic studies revealed that there exists strong π–π interaction between P3HT chains and SWCNTs. The P3HT chains also became more rigid after interaction with SWCNTs. We proposed that SWCNTs were straightened by the coaxially adsorbed P3HT chains instead of epitaxial P3HT crystals. Multiwalled carbon nanotubes (MWCNTs) could not be straightened by P3HT under the same conditions, showing that the straightening force was limited. This result may be useful in preparation of aligned arrays of SWCNTs/conjugated polymer supramolecular structures

Influence of Ionic Species on the Microphase Separation Behavior of PCL‑<i>b</i>‑PEO/Salt Hybrids

The microphase separation behavior of the hybrids of poly­(ε-caprolactone)-b-poly­(ethylene oxide) (PCL-b-PEO) with different inorganic salts at various doping ratios (r) was studied by temperature-variable SAXS. It was observed that the salts could induce microphase separation to form ordered structure in the originally miscible melt of PCL-b-PEO. The effects of the metal ion and anion were correlated with the competitive interactions of PEO/salt and PCL/salt, which were characterized by FT-IR and DSC, respectively. It was found that at lower doping ratios the salts preferentially interacted with PEO. The larger association number of the metal ion and stronger association between PEO and salt led to a lower onset doping ratio for formation of ordered structure (r0). At higher doping ratios the salt interacted with PCL as well. When the metal ion exhibited a highly selective interaction toward PEO, a more ordered structure with a higher order–order transition temperature (TODT) tended to be formed. The anion in the salt also affected the interactions of PEO/salt and PCL/salt. Weaker Lewis basicity of the anion would result in a stronger interaction of PEO/salt and thus a lower r0. The results showed that the microphase separation behavior of the PCL-b-PEO/salt hybrids was sensitive to the competitive interactions of the salt with the PCL and PEO blocks

Fabrication of High χ‑Low <i>N</i> Block Copolymers with Thermally Stable Sub‑5 nm Microdomains Using Polyzwitterion as a Constituent Block

In this work, we used zwitterionic poly­(4-vinylpyridine) propane-1-sulfonate (PVPS) as a constituent block to construct high χ-low N block copolymers (BCPs) with different neutral polymers as the other block, including polystyrene (PS), poly­(ethylene oxide) (PEO), and poly­(l-lactide) (PLLA). Lamellar structures with sub-5 nm microdomains were observed in all three types of BCPs. Due to the tendency of self-aggregation induced by electrostatic interaction in polyzwitterion, the Flory–Huggins parameters (χ) between PVPS and most neutral polymers are relatively high, which provides a facile and efficient way to fabricate high χ-low N BCPs. In addition, the dimension of the sub-5 nm structures formed in PVPS-containing BCPs showed high thermal stability with a small fluctuation (±0.1 nm) of domain spacings upon heating

Competition of Crystalline and Liquid Crystalline Moieties in Self-Assembly of Poly(oxyethylene) Cholesterol Ethers

Self-assembly of a series of poly(oxyethylene) (POE) cholesterol ethers (ChEOn, n = 5, 10, 15, 20, 24, 30, and 45) bearing both liquid crystalline (LC) and crystalline moieties was studied by differential scanning calorimetry, wide-angle X-ray diffraction, Raman spectrometry, and small-angle X-ray scattering. In ChEO5 where POE is amorphous, the LC moiety was found to be dominant in determining morphology, and the repeating lamellar structure of ChEO5 is composed of double layers of cholesterol and a single layer of amorphous POE. In ChEO10 and ChEO15, LC and crystalline phases coexist and polymorphism is observed. The repeating lamellar structures of ChEO10 and ChEO15 are similar to that of ChEO5, except for the crystalline helical conformation of POE. With further increase in the chain length of POE, the crystalline POE becomes dominant in determining morphology, and the LC phase is not detected. The crystalline conformation of POE induces LC moieties to pack more closely, and the two LC layers gradually merge into a single LC layer in the repeating lamellar structure. Nonisothermal and isothermal crystallization experiments show that the preexisting LC phase can nucleate and accelerate POE crystallization, whereas the dimension of crystal growth of POE is reduced

Effect of Substrate and Molecular Weight on the Stability of Thin Films of Semicrystalline Block Copolymers

The thermal stability of the thin film morphology of two symmetric oxyethylene/oxybutylene block copolymers (E76B38 and E114B56) on mica and silicon was investigated via atomic force microscopy (AFM). It is found that morphological transition of EmBn thin films during melting is strongly dependent on the molecular weight of the diblock copolymers and their interaction with the substrate. For E76B38 on mica, a single-layered structure transforms into a double-layered structure upon melting, but the same polymer on silicon retains a single-layered structure after melting and spreads quickly to wet-out the silicon surface. Conversely a longer polymer, E114B56, has a thin film on mica that does not change much after melting of the crystalline E block. A mechanism was proposed to explain the relative stability of E76B38 and E114B56 thin films upon melting. Internal stress is produced during melting and can be released along two directions. The release along the vertical direction is restricted by the energy barrier related to the segregation strength, and the release along the horizontal direction is dependent on the mobility of block copolymer related to the interaction between the block copolymer and the substrate. Domain size affects the release rate of the internal stress along the horizontal direction and thus the thermal stability of EmBn thin films. Switching between horizontal and vertical releases can be realized by controlling the domain size of the thin films

Straight and Rod-like Core–Sheath Crystals of Solution-Crystallized Poly(ε-caprolactone)/Multiwalled Carbon Nanotube Nanocomposites

The crystal morphology of poly­(ε-caprolactone)/multiwalled carbon nanotube (PCL/MWCNT) blends and MWCNT-<i>g</i>-PCL grafting polymers crystallized in <i>n</i>-hexanol was investigated. Two typical morphologies are observed: a straight and rod-like core–sheath structure with embedded MWCNTs as the core and PCL polycrystals of high crystallinity as the sheath, and a bent double-layer structure with MWCNTs covered by a PCL layer of low crystallinity. It is found that thinner (outer diameter <15 nm) and shorter (length <2 μm) MWCNTs are easier to be straightened by PCL crystals, and the grafted PCL chains have weaker crystallizability due to structural confinement and thus a weaker ability of straightening MWCNTs. Electron diffraction and high-resolution transmission electron microscopy reveal that the PCL crystals are randomly orientated with the <i>b</i>-axis perpendicular to the MWCNT surface. The growth direction of the PCL crystals is not perpendicular to the axis of MWCNT, possibly due to the nucleation effect of the preadsorbed PCL chains in the solution, which helically wrap MWCNTs. This leads to wrapping and straightening of MWCNTs by rigid PCL crystals