Session L22: Focus Session: Directed Assembly of Hybrid Materials I - Crystallization and Multicomponent Systems

Manipulating the morphologies and lamellar orientations of substrate-supported polyester films using end-grafted poly(methacrylate) brushes

Crystallization of polymeric materials on solid substrates has technological and scientific importance in applications such as coatings, electronic devices and solar cells. Crystalline morphologies and orientations of polymer near the polymer/substrate interface can be greatly altered by tuning the specific interactions between polymer and substrate. In this talk, we will show the effect of end-grafted poly(methacrylate) brushes in controlling the spherulitic morphologies and lamellar assembly patterns of thin polyesters films on glass substrate. Poly(methyl methacrylate) (PMMA) and poly(benzyl methacrylate) (PBzMA) brushes were grafted on glass surface using surface-initiated atom transfer radical polymerization method. The crystalline morphologies and lamellar orientations of polyesters on the brush-grafted substrate were then investigated using polarized optical microscopy (POM) and atomic force microscopy (AFM). The results clearly showed that the spherulitic morphologies of polyesters are strongly depending on interaction strength between polyesters and brushes.   

Crystallization in sequence-defined peptoid diblock copolymers induced by microphase separation

Atomic level synthetic control over a polymer's chemical structure is desired for tuning the microphase separation and other properties of crystalline block copolymers. In order to explore the impact of side chain structure on crystallization behavior, we designed a series of chemically-defined, highly monodisperse peptoid diblock copolymers poly-N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine-block-poly-N-decylglycine (pNte-b-pNdc) with volume fraction of pNte (?Nte) values ranging from 0.28 to 0.71 and a polydisperisty indices ? 1.0003. Both monomers have nearly identical molecular volumes but the pNte block is amorphous while the pNdc block is crystalline. We demonstrate that all the block copolypeptoids self-assemble into lamellar microphases driven by crystallization of the pNdc block. To our knowledge, there are no previous reports of crystallization of a polymer chain induced by microphase separation. These investigations show that polypeptoids provide a unique platform for examining the intertwined roles of side chain organization on the thermodynamic properties of diblock copolymers.   

Polymer Crystallization at Liquid-Liquid Interface

Curved space is incommensurate with typical ordered structures with three-dimensional translational symmetry. However, upon assembly, soft matter, including colloids, amphiphiles, and block copolymers, often form structures depicting curved surface/interface. On the other hand, twisted and curved crystals are often observed in crystalline polymers. Various mechanisms have been proposed for these non-flat crystalline morphologies. In this presentation, we will discuss the recent development of crystallization at flat and curved liquid/liquid (L/L) interface. We show that structure, morphology and chain folding behaviors are strongly affected by L/L interfacial energy and polymer chain ends. Polymer crystallization behavior at L/L interface will be compared with solution and bulk crystallization.   

Kinetics of nucleation and crystallization of poly(epsilon-caprolactone) - multiwalled carbon nanotube composites

The nucleation efficiency of multi-wall carbon nano-tubes (MWCNT) in poly($\varepsilon $-caprolactone) (PCL), as an example, was tested for a wide range of temperatures and cooling rates and compared to the efficiency of homogeneously formed nuclei. The temperature range below the maximum of crystallization rate is generally not accessible for non-isothermal cooling experiments because the sample becomes amorphous at the needed cooling rates. Isothermal experiments after fast quenches extend the temperature range down to and below the glass transition. The employed differential fast scanning calorimeter (DFSC) allows cooling at rates up to 100,000 K/s and precise adjustment and control of isothermal conditions in the time range from 10$^{-4}$ to 10$^{4}$ s and longer. As shown in previous work, heterogeneous crystal nucleation dominates at low supercooling, revealing a significant dependence of crystallization rate on MWCNT concentration. Nevertheless, no saturation of the nucleation activity at a MWCNT loading of 0.2 to 0.5 wt{\%} as seen in slow DSC experiments was observed at the much higher cooling rates employed here. At high supercooling, where homogeneous nucleation is prevalent, the addition of MWCNT does not enhance neither reduce the crystallization rate. At the temperature of maximum homogeneous nucleation rate, formation of homogeneous nuclei always dominates crystallization. \\[4pt] E. Zhuravlev, J.W.P. Schmelzer, B. Wunderlich, C. Schick, Kinetics of nucleation and crystallization in poly(epsilon caprolactone) (PCL) Polymer, 52 (2011) 1983-1997.   

Two-dimensional folded chain crystals composed of a single isotactic poly(methyl methacrylate) chain observed by atomic force microscopy

We successfully visualized crystallization behavior of a single isolated polymer chain at a molecular level by atomic force microscopy (AFM). Previously, we found that isotactic poly(methyl methacrylate) (it-PMMA) formed two-dimensional folded chain crystals upon compression of its Langmuir monolayer on a water surface, and the molecular images of the crystals deposited on mica were clearly visualized by AFM (Kumaki, et al. JACS 2005, 127, 5788; J. Phys. Chem. B 2013, 117, 5594). In the present study, a high-molecular-weight it-PMMA was diluted in a monolayer of an it-PMMA oligomer which cannot crystallize due to the low molecular weight. At a low surface pressure, isolated amorphous chains of the high-molecular-weight it-PMMA solubilized in the oligomer monolayer were observed. On compression, the isolated chains converted to crystals composed of a single chain. Detailed AFM observations of the crystals indicated that the crystalline nuclei preferably formed at the ends of the chains, and the size of the nuclei was almost independent on the molecular weight of the it-PMMA in a wide range.   

Structure Formation of Spinning Polymer Fibers Studied by Monte Carlo Simulations

We performed dynamic Monte Carlo simulations of lattice polymer solutions to investigate the solidification process in a fluid filament after extruded from a spinneret into a coagulation bath. We observed skin-core structure formation under the interplay of phase separation and polymer crystallization. We found that a radial temperature gradient dominates the formation of a highly oriented solid skin, while a radial influx of non-solvent dominates the formation of a concentrated core. The processing parameters can adjust and even eliminate the skin-core structure in the fibers. Our molecular-level observations facilitate a better understanding of the fiber processing for the industry to make high quality fibers.   

Precise Tetrahedral Giant Molecules Based on Polyhedral Oligosilsesquioxane (POSS) Nano-atoms

The assembly of building blocks with specific shape and symmetry in 3D space is a long-lasting topic in scientific research. If ``nano-atoms'' are placed on the apexes of a rigid polyhedron linker to form a larger faceted giant molecule, such molecules would amplify the symmetry of the linkers and result in giant polyhedra molecules. When four POSS cages are linked to the apex of a tetrahedron, we obtain a giant tetrahedron. Depending on the linkers, it can be a semi-rigid or a rigid giant polyhedron. An interesting approach is to utilize the sp3-carbon or adamantane core to introduce the Td symmetry, and utilize ``click reaction'' to connect four hydrophobic isobutyl-POSS (BPOSS) at four corners. Our preliminary results show that the giant tetrahedron Tetra-4BPOSS forms an interdigitated diamondoid structure. In these giant polyhedra, we can use different ``nano-atoms'' with different functional groups, which may also act as an additional factor to affect the final ordered structures. The progresses of our research lead to three hydrophobic and one hydrophilic HPOSS (HPOSS represents seven hydroxyl group functionalized POSS), and two hydrophobic BPOSS and two hydrophilic HPOSS.   

Novel Polymer Ferroelectric Behavior via Crystal Isomorphism and Nanoconfinement Effect

Despite comprehensive understanding of novel ferroelectric [i.e., relaxor ferroelectric (RFE) and antiferroelectric (AFE)] behaviors for ceramics, RFE and double hysteresis loop (DHL) behaviors have just emerged for ferroelectric crystalline polymers since the past 15 years. A number of applications such as electrostriction, electric energy storage, and electrocaloric cooling have been realized by utilizing these novel ferroelectric properties. However, the fundamental understanding is still lacking. In this invited talk, we intend to unravel the basic physics behind these novel ferroelectric behaviors via systematic studies of poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)]-based terpolymers and e-beam irradiated copolymers. It is found that both crystal internal structure and crystal-amorphous interaction are important for achieving the RFE and DHL behaviors. For the crystal internal structure effect, friction-free dipole switching and nanodomain formation by pinning the polymer chains are essential, and they can be achieved via the mechanism of crystal repeating unit isomorphism. Physical pinning [e.g., in P(VDF-TrFE)-based terpolymers] induces a reversible RFE$\leftrightarrow $FE phase transition and thus the DHL behavior, whereas chemical pinning [e.g., in e-beam irradiated P(VDF-TrFE)] results in the RFE behavior. Finally, the crystal-amorphous interaction (or the nanoconfinement effect) results in a competition between the polarization and depolarization local fields. When the depolarization field becomes stronger than the polarization field, a DHL behavior can also be observed. Obviously, the physics is different from ceramics and can be largely attributed to the long chain nature of semicrystalline ferroelectric polymers. This understanding will help us design new ferroelectric polymers with improved electroactive properties and/or better applications.   

When Do Semi-crystalline Polymer Fold during Crystallization?

Semi-crystalline polymers are crystallized as folded chains in thin lamellae of ca. 5-20 nm from random coils in the melt and solution states. Lauritzen--Hoffman theory implied the crystallization process is dominated by sequential stem deposition on the growth front. Conversely, Allegra proposed a bundle model in which aggregates of 10-20 stems are produced by folding in the pre-stage of crystallization. The pre-folded chains are kinetically deposited on the growth front and thus determine the morphology at different crystallization temperatures. The folded chains preserve their own chain-folding directions, numbers, and fractions as a function of concentrations and supercooling, which would provide detailed chain-folding mechanism. We recently developed a new strategy using $^{\mathrm{13}}$C-$^{\mathrm{13}}$C double-quantum NMR to investigate chain-trajectory of $^{\mathrm{13}}$C selectively labeled polymer in bulk crystals. Here, we report how re-entrance sites, fraction, and number of folded chains of \textit{isotactic} poly(1-butene) in form III single crystals depends on supercooling conditions and solvent effects. On the basis of molecular level structures, we will discuss about chain-folding process of $i$PB1 in dilute solutions.   

Chain-Folding Structures of a Semi-crystalline Polymer in Bulk and Single Crystals Elucidated by 13C--13C Double Quantum NMR

Semi-crystalline polymers are crystallized as folded chains in thin lamellae of ca. 5-20 nm from random coils in the melt and solution states. However, understanding of detailed chain-folding structure and crystallization mechanism are still challenging issue due to various experimental limitations. We recently developed a new strategy using $^{13}$C--$^{13}$C double-quantum (DQ) NMR with selectively $^{13}$C isotope labeled \textit{isotactic} poly(1-butene) form I to investigate chain-trajectory in solution and melt grown crystals at various \textit{Tc}s. This new method can determine the re-entrance sites, the successive folding number ($n)$, and the fractions ($F)$ of chain-folding in a wide Tc range. In melt grown crystals at \textit{Tc} $=$ 95 $^{\circ}$C, a comparison of experimental and simulated DQ efficiency determined that the polymer chains alternatively change chain-folding directions and the stems tightly pack via intramolecular interactions, and the fraction (F) of adjacent re-entry structure ranges from 70{\%} at n $=$ 4 to 100{\%} at mixed structures of n $=$ 1 and 2. Furthermore, DQ efficiency is independent of Tc in bulk crystals. This means chain-folding do not change in a wide Tcs.   

Torsional Tapping atomic force microscopy for molecular resolution imaging of semicrystalline polymers

Torsional tapping atomic force microscopy (TTAFM) provides a considerable improvement in signal-to-noise when compared with conventional AFM imaging approaches. This enables the routine use of ultra-sharp whisker tips and leads to true molecular resolution imaging in the crystalline and crystal-amorphous interface zones in semi-crystalline samples. Peak-to-peak resolution below 0.4 nm is obtainable even on topographically rough samples. Here we will present the result of recent studies showing the molecule by molecule chain structure of various polymer samples including polyethylene and polypropylene, showing how chain conformation within the crystal and at the crystal-amorphous and crystal-air interface is influenced by processing conditions. Of particular interest are observations of the roughness of the crystal fold surface at the nanometer level even on samples that have been annealed for long times. It is also clear that the crystal surface that is presented is not always dominated by the chain like nature of the molecules, but in some cases can have a more complex character that might strongly influence how the process of crystallization should be modelled. Data on the chain level internal structure of bulk samples as revealed by cryo-microtoming, will also be discussed.