Stephen Z. D. Cheng

The role of metastability in polymer phase transitions

Abstract Polymer phases can be described in the same way as phases in other condensed matter using a number density operator and its correlation functions. This description requires the understanding of both symmetry operations and order at different atomic and molecular levels. Statistical mechanics provides a link between the microscopic description of structure and motion and the macroscopic thermodynamic properties. Within the limits of the laws of thermodynamics, polymers exhibit a rich variety of phase transition behaviours. By definition, a first-order phase transition in a temperature–pressure ensemble describes a transformation which involves a discontinuous change of all the thermodynamic functions but the Gibbs free energy at the transition temperature. Higher-ordered phase transitions are classified as critical phenomena. Of special interest is the role of metastability in phase and phase transition behaviours. A classical metastable state possesses a local free energy minimum, but it is not at the global stable equilibrium. Further, the existence of circumstantial metastability need to be invoked based on the constraints of size, dimensionality, order and symmetry; examples include polymorphism, mesophase concepts, crystal size, and thin film effects. Metastable behaviour is also observed in phase transformations that are impeded by kinetic limitations along the pathway to thermodynamic equilibrium. This is illustrated in structural and morphological investigations of crystallization and mesophase transitions, liquid–liquid phase separation, vitrification and gel formation, as well as combinations of all such transformation processes. In these cases, the metastable state often becomes the dominant state for the entire system, and is observed over a range of time and size scales. This review will describe the general principles of metastability in polymer phases and phase transitions and will provide illustrations from current experimental works in selected areas together with raising so far unaddressed conceptual issues of wider applicability to phase transformations in general.

Dianhydride architectural effects on the relaxation behaviors and thermal and optical properties of organo-soluble aromatic polyimide films

Abstract Dianhydrides of specific molecular architecture was designed and synthesized based on 2,2′-disubstituted 4,4′,5,5′-biphenyltetracarboxylic dianhydrides (2,2′-disubstituted BPDAs). Eight dianhydrides were polymerized with two 4,4′-diamino-2,2′-disubstituted biphenyl diamines (trifluoromethyl disubstituted groups, or PFMB, and methyl disubstituted groups, or DMB) to obtain two series of PFMB- and DMB-based aromatic polyimides. As the backbone structures of these two series of polyimides are unchanged throughout each series, the effects of the 2,2′-disubstituted groups of both the dianhydride and diamine constituents on the solubility and thermal and optical properties as well as the relaxation behavior of these polyimides can be identified. It was found that the PFMB-based polyimides with 2,2′-disubstituted BPDAs show excellent solubility while the DMB-based polyimides with the same dianhydrides are less soluble. The same trends can be found for both thermal and thermo-oxidative stability and optical transparency in the ultraviolet and visible light regions. These two series of polyimides exhibit glass transition temperatures ( T g ) which show a competition between chain rigidity and linearity with regards to molecular packing. When the size of the 2,2′-disubstituted groups is small and their shape is close to spherical, the T g initially increases with the size of these 2,2′-disubstituted groups. This is because of the fact that the steric hindrance of these groups prevents the appearance of a cis -conformation of BPDA. However, once these groups possess large size and exhibit anisotropic shapes, their effect on the molecular packing becomes dominant and the T g starts to decrease. Further, this is the first time that three relaxation processes (the β 1 , β 2 , and α processes) were observed above room temperature in these aromatic polyimides. We have identified that the β 1 process is attributed to the local motion of the diamine constituents while the β 2 process is caused by the local motion of the dianhydride constituents. The α process is associated with the glass transition. The cooperativity of the molecular motion associated with the β 1 and β 2 processes are also discussed.

Diamine architecture effects on glass transitions, relaxation processes and other material properties in organo-soluble aromatic polyimide films

Abstract A series of twelve aromatic diamines, 4,4′-diamino-2,2′-disubstitutedbiphenyls, has been designed and synthesized. These diamines were reacted with 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) to form polyimides via a one-step polycondensation method. All of the resulting polyimides could be dissolved in common organic solvents and exhibited excellent film forming ability. At the same time, their inherent high thermal and thermo-oxidative stability of these polyimides was retained in the films. Because of the incorporation of disubstituted groups at the 2- and 2′-positions of these biphenyl diamines, their crystallinity was suppressed to the level that they were in complete amorphous state. Further, the conjugation of the phenylene and imide groups in these polyimide films was interrupted, leading to clear blue shifts during light transmission. As this series of polyimides possessed the same backbone, the chain rigidity and linearity changed very little throughout the series. However, the molecular packing was affected by the introduction of different disubstituted pendant groups. Each polyimide film exhibited an α relaxation process related to the glass transition. This relaxation changed significantly with the size and the shape of the disubstituted pendant groups. In addition to this process, each of these polyimide films displayed a sub-glass transition, the β relaxation process, which was initiated by motion of the 4,4′-diamino-2,2′-disubstituted biphenyls. This study provided an opportunity to investigate how disubstituted pendant groups affected the α and β relaxation behaviors of these polyimides. With an increase of the sizes and the shape anisotropy of the disubstituted pendant groups at the 2- and 2′-position, the nature of the motion regarding to the β relaxation was found to evolve from a non-cooperative process to a cooperative one, while the glass transition temperature (the α relaxation temperature) correspondingly decreased.

Simultaneously strong and tough ultrafine continuous nanofibers.

Strength of structural materials and fibers is usually increased at the expense of strain at failure and toughness. Recent experimental studies have demonstrated improvements in modulus and strength of electrospun polymer nanofibers with reduction of their diameter. Nanofiber toughness has not been analyzed; however, from the classical materials property trade-off, one can expect it to decrease. Here, on the basis of a comprehensive analysis of long (5-10 mm) individual polyacrylonitrile nanofibers, we show that nanofiber toughness also dramatically improves. Reduction of fiber diameter from 2.8 μm to ∼100 nm resulted in simultaneous increases in elastic modulus from 0.36 to 48 GPa, true strength from 15 to 1750 MPa, and toughness from 0.25 to 605 MPa with the largest increases recorded for the ultrafine nanofibers smaller than 250 nm. The observed size effects showed no sign of saturation. Structural investigations and comparisons with mechanical behavior of annealed nanofibers allowed us to attribute ultrahigh ductility (average failure strain stayed over 50%) and toughness to low nanofiber crystallinity resulting from rapid solidification of ultrafine electrospun jets. Demonstrated superior mechanical performance coupled with the unique macro-nano nature of continuous nanofibers makes them readily available for macroscopic materials and composites that can be used in safety-critical applications. The proposed mechanism of simultaneously high strength, modulus, and toughness challenges the prevailing 50 year old paradigm of high-performance polymer fiber development calling for high polymer crystallinity and may have broad implications in fiber science and technology.

Hard and soft confinement effects on polymer crystallization in microphase separated cylinder-forming PEO-b-PS/PS blends

Abstract A lamellae-forming poly(ethylene oxide)-b-polystyrene (EOS) has been blended with a polystyrene homopolymer (PS) and a PS oligomer (PSO), respectively, to obtain miscible polymer blends (denoted as EOS/PS-32 and EOS/PSO-32, respectively). Both blends exhibit cylindrical microphase morphologies, with the PEO volume fractions being 0.32. The order–disorder transition temperatures (TODT) of both blends are 175 and 84°C, respectively, as determined by temperature-dependent small angle X-ray scattering experiments. The glass transition temperature of the PS matrix (TgPS) for the EOS/PS-32 blend is 64°C as determined by differential scanning calorimetry (DSC), while that for the EOS/PSO-32 blend is only 16°C. Thus, by controlling the crystallization temperatures (TcPEO), two kinds of nano-confined PEO crystallizations have been achieved in these blends: when TODT≫TgPS>TcPEO in the EOS/PS-32 blend, the PEO-block crystallization is confined under a hard PS confinement, while for TODT>TcPEO≳TgPS in the EOS/PSO-32 blend, the PEO-block crystallization is confined under a soft PS confinement. DSC and wide-angle X-ray experiments show that the crystallizations of the PEO blocks in these two confinement environments behave differently. The PEO-block crystallization kinetics in the hard confinement is much slower than that in the soft confinement. The DSC kinetics studies show that for Tc

Isotacticity effect on crystallization and melting in polypropylene fractions: 1. Crystalline structures and thermodynamic property changes

Abstract A set of polypropylene (PP) fractions with similar molecular masses and distributions but different isotacticities have been studied through wide-angle X-ray diffraction, small-angle X-ray scattering and differential scanning calorimetry measurements. The crystal unit-cell parameters, crystallinity, apparent crystal size and lamellar crystal thickness are found to be dependent on crystallization temperature and isotacticity. The equilibrium thermodynamic properties (melting temperature and heat of fusion) for these PP fractions were determined following two extrapolation methods. These fractions can be thought of as stereo-copolymers, with configurational defects along the chains. A uniform inclusion model proposed by Sanchez and Eby can be applied to describe the crystals formed in these fractions. Both equilibrium and non-equilibrium data are discussed.

A high-performance aromatic polyimide fibre: 1. Structure, properties and mechanical-history dependence☆

Abstract A new segmented rigid-rod polyimide has been synthesized from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB). This polyimide is soluble in hot m-cresol, allowing fibres to be spun from an isotropic solution using a dry-jet wet spinning method. The as-spun fibres have low tenacities and low moduli, but they can be drawn at high temperatures (> 380°C) under tension to large draw ratios (up to 10 times), which produces a remarkable increase in strength and modulus. Drawn fibres display a tensile strength of about 25 g den−1 (3.2 GPa) and an initial modulus higher than 1000 g den−1 (130 GPa). BPDA-PFMB fibres show excellent thermal stability and retain relatively high strength and modulus at elevated temperatures. Annealed BPDA-PFMB fibres display distinct wide-angle X-ray patterns, from which a monoclinic unit cell has been determined. Furthermore, changes in crystallographic c-axis, apparent crystal sizes, degree of crystallinity, crystal orientation and thermomechanical properties have been observed with different draw ratios.

Giant surfactants provide a versatile platform for sub-10-nm nanostructure engineering

The engineering of structures across different length scales is central to the design of novel materials with controlled macroscopic properties. Herein, we introduce a unique class of self-assembling materials, which are built upon shape- and volume-persistent molecular nanoparticles and other structural motifs, such as polymers, and can be viewed as a size-amplified version of the corresponding small-molecule counterparts. Among them, “giant surfactants” with precise molecular structures have been synthesized by “clicking” compact and polar molecular nanoparticles to flexible polymer tails of various composition and architecture at specific sites. Capturing the structural features of small-molecule surfactants but possessing much larger sizes, giant surfactants bridge the gap between small-molecule surfactants and block copolymers and demonstrate a duality of both materials in terms of their self-assembly behaviors. The controlled structural variations of these giant surfactants through precision synthesis further reveal that their self-assemblies are remarkably sensitive to primary chemical structures, leading to highly diverse, thermodynamically stable nanostructures with feature sizes around 10 nm or smaller in the bulk, thin-film, and solution states, as dictated by the collective physical interactions and geometric constraints. The results suggest that this class of materials provides a versatile platform for engineering nanostructures with sub-10-nm feature sizes. These findings are not only scientifically intriguing in understanding the chemical and physical principles of the self-assembly, but also technologically relevant, such as in nanopatterning technology and microelectronics.

Selective assemblies of giant tetrahedra via precisely controlled positional interactions

Creating unusual nanostructures Self-assembly often occurs when dissimilar molecular fragments are forced together by covalent bonding. Surfactants or block copolymers are two common examples. Huang et al. grafted four different nanoparticles, based on polyhedral oligomeric silsesquioxanes with slightly different compositions, onto a single tetrahedal core (see the Perspective by Yang). Depending on the type of nanoparticle, they assembled into a range of defined, ordered supramolecular lattices similar to a range of metal alloys. These include phases that have higher coordination numbers than usually found in the packing of spherical objects. Science, this issue p. 424; see also p. 396 Tetrahedrally connected nanoparticles self-assemble into complex ordered phases. [Also see Perspective by Yang] Self-assembly of rigid building blocks with explicit shape and symmetry is substantially influenced by the geometric factors and remains largely unexplored. We report the selective assembly behaviors of a class of precisely defined, nanosized giant tetrahedra constructed by placing different polyhedral oligomeric silsesquioxane (POSS) molecular nanoparticles at the vertices of a rigid tetrahedral framework. Designed symmetry breaking of these giant tetrahedra introduces precise positional interactions and results in diverse selectively assembled, highly ordered supramolecular lattices including a Frank-Kasper A15 phase, which resembles the essential structural features of certain metal alloys but at a larger length scale. These results demonstrate the power of persistent molecular geometry with balanced enthalpy and entropy in creating thermodynamically stable supramolecular lattices with properties distinct from those of other self-assembling soft materials.

Isotacticity effect on crystallization and melting in polypropylene fractions: 3. Overall crystallization and melting behaviour

Abstract A set of polypropylene (PP) fractions with similar molecular masses and molecular mass distributions but different isotacticities have been investigated and their overall crystallization and crystal melting behaviours determined by differential scanning calorimetry and time resolved small-angle X-ray scattering experiments. The observations indicate that when the supercooling is high ( ΔT > 48 K), only poor and imperfect crystals can form, which are continuously annealed to more perfect crystals upon heating after isothermal crystallization. On the other hand, below ΔT = 48 K, two different crystal morphologies are recognized with their own crystallization kinetics and thermal stabilities. The Avrami treatment for the overall crystallization kinetics of these PP fractions generally reveals a low Avrami exponent ( n ) compared with the literature data. The overall crystallization and crystal melting behaviour can be correlated to the crystal morphology change with supercooling, in particular to the ‘cross-hatching’ lamellar phenomenon uniquely observed in the case of PP. The isotacticity effect on the overall crystallization processes and melting behaviour are also focused upon.