Zai-Zai Tong

Hierarchical structures of olefinic blocky copolymer/montmorillonite nanocomposites with collapsed and intercalated clay layers

The hierarchical structures of olefinic blocky copolymer (OBC)/organically modified montmorillonite (OMMT) nanocomposites with different dispersion states of clay, including collapsed and intercalated OMMT (c-OMMT and i-OMMT), were characterized by various techniques. It is observed that, there are more densely stacked clay layers in single clay particles with a smaller size but larger distance between the adjacent clay particles in OBC/c-OMMT. c-OMMT exhibits stronger nucleation ability on crystallization of OBC than i-OMMT. This leads to a smaller size of the macroscopic crystals, thicker lamellar crystals, and more amorphous phase included in the inter-lamellae but less amorphous phase among different macroscopic crystals in OBC/c-OMMT. These differences in the hierarchical structure are responsible for the more obvious strain-hardening behavior but smaller strain at break of the OBC/c-OMMT nanocomposites.

Regulation of the self-assembly morphology of azobenzene-bearing double hydrophobic block copolymers in aqueous solution by shifting the dynamic host–guest complexation

The double hydrophobic azobenzene-containing diblock copolymers, namely, poly(L-lactide)-b-poly[6-(4-(4-methoxyphenylazo)phenoxy)hexyl methacrylate] (PLLA-b-PMMAZO), were successfully prepared by sequential atom transfer radical polymerization (ATRP) and ring opening polymerization. These block copolymers (BCPs) can self-assemble into various morphologies in aqueous solution via host–guest interaction between the azobenzene (azo) groups and methyl-β-cyclodextrin (β-CD). The complexation equilibrium between azo and β-CD can be shifted by changing the length of the hydrophobic PLLA block, the micelle concentration or the β-CD/azo molar ratio; thus the micellar morphology and the size of the PLLA-b-PMMAZO/β-CD complexes are altered. A vesicle-to-sphere-to-entrapped vesicle transition was observed at β-CD/azo = 1 as the PLLA block length increases. In addition, a large β-CD/azo ratio or higher micelle concentration leads to a vesicle-like morphology, while entrapped vesicles tend to be formed at a smaller β-CD/azo ratio or a lower micelle concentration. It is found that prolongation of the UV irradiation time can also induce a vesicle-to-sphere-to-entrapped vesicle transition of the micellar morphology. Such a transition is reversible upon irradiation of visible light. In summary, the self-assembly behavior of PLLA-b-PMMAZO/β-CD complexes can be readily regulated by different methods. Moreover, the complexation between the azo groups and β-CD is inhomogeneous, which is responsible for the formation of entrapped vesicles and the microphase separation in the core of the spherical micelles and entrapped vesicles.

Hierarchical self-assembly, photo-responsive phase behavior and variable tensile property of azobenzene-containing ABA triblock copolymers

A series of triblock copolymers with liquid crystalline (LC) poly{6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate} (PMMAZO) as the end blocks and rubbery poly(n-butyl acrylate) (PnBA) as the midblock were synthesized. The effect of the interplay between the LC ordering and microphase separation on the hierarchical assembly of the triblock copolymers was studied. It is found that microphase separation at a larger scale can affect the LC ordering at a smaller scale, such as the stacking of the LC moieties, LC temperature and the domain size of the LC phase. On the other hand, alteration of the LC ordering, such as isotropization and smetic-to-nematic transition, may also lead to an order–order transition (OOT) or change in the long period of the microphase-separated structure. UV light can trigger the isomerization of the azobenzene LC moieties, which can be further amplified and exerts an effect on the microphase separation behavior, including the regularity of the microphase-separated structure and the OOT. The triblock copolymers also exhibit light-variable tensile properties. The results reveal that the phase behavior and mechanical properties of this type of triblock copolymer can be readily regulated by light, thus it may be used as smart and functional thermoplastic elastomer.

Chain Structure, Aggregation State Structure, and Tensile Behavior of Segmented Ethylene–Propylene Copolymers Produced by an Oscillating Unbridged Metallocene Catalyst

Segmented ethylene-propylene copolymers (SEPs) with different propylene contents were prepared by an unbridged metallocene bis(2,4,6-trimethylindenyl)zirconium dichloride [(2,4,6-Me3Ind)2ZrCl2] catalyst. Due to oscillation of the unbridged ligands in the catalyst, the SEPs are composed of segments with low propylene contents, alternated by the segments with high propylene contents. Such a chain structure was verified by (13)C NMR and successive self-nucleation and annealing (SSA). As the propylene/ethylene feed ratio during copolymerization increases, the comonomer contents in both segments are increased, leading to noncrystallizability of the high propylene segments and smaller crystallinity of the low propylene segments. Consequently, SEPs may be used as thermoplastic elastomers (TPEs). The aggregation state structures at nano- and micro-scales were characterized with small angle X-ray scattering, transmission electron microscopy and polarized optical microscopy, and compared with those of ethylene-octene multiblocky copolymers (OBCs) with similar crystallinity. It is found that SEPs form thinner lamellar crystals with a lower melting temperature due to shorter length and higher comonomer content of the low propylene segments. Moreover, the short length of the high propylene segments in SEPs results in an evidently thinner amorphous layer among the lamellar crystals, thus lots of amorphous phases are excluded out of the interlamellae. Accordingly, ill-developed spherulites or even bundle crystals are formed in SEPs, as compared with the well-developed spherulites in OBCs. SEPs exhibit the tensile property of typical TPEs with diffused yielding and large strain at break.

Isothermal crystallization kinetics of multi-walled carbon nanotubes-graft-poly(ε-caprolactone) with high grafting degrees

The isothermal crystallization kinetics of poly(e-caprolactone) (PCL) grafted on multi-walled carbon nanotubes (MWNTs-g-PCL) with high grafting degrees were studied and compared with those of neat PCL and a PCL/MWNTs blend. The Avrami exponent n altered from 3.0 for the neat PCL to nearly 2.0 for the grafting polymers. The grafting polymers exhibit a significantly faster crystallization rate but lower crystallinity than the neat PCL and PCL/MWNTs blend. In the grafting polymers, MWNTs exert both stronger nucleation and confinement effects on the crystallization of PCL than those for the PCL/MWNTs blend. The grafting density affects crystallization of PCL more severely than the molecular weight of the grafting PCL. The crystallization activation energy of the grafting polymers is smaller than that of the PCL/MWNTs blend but larger than that of the neat PCL. The grafted PCL chains are over-crowded and highly stretched, and the PCL crystals lie on the surface of MWNTs with the b-axis basically perpendicular to the surface.

Composition heterogeneity of tetrafluoroethylene-hexafluoropropylene random copolymers characterized by successive self-nucleation and annealing

In the present work, successive self-nucleation and annealing (SSA) was applied to a series of tetrafluoroethylene-hexafluoropropylene random copolymers (FEPs). Multiple melting peaks were observed for all FEP samples after SSA thermal treatment. The lamellar crystal thicknesses were calculated from the melting temperatures, and the mass percentages of the crystals of specific thickness were obtained from the areas of the melting peaks. As a result, distributions of the lamellar thickness, which can be correlated to the composition distribution, were determined. It was found that the composition distribution of the FEP samples tended to become more heterogeneous as the content of hexafluoropropylene (HFP) comonomer increases. Samples with the same HFP content might also have different composition distributions.