Linear-dendritic block copolymers: The state of the art and exciting perspectives

Concurrent with the rapid development of both dendrimers and hyperbranched polymers, a novel class of block copolymer architectures has emerged from the combination of these dendritic architectures with linear chains, the “linear-dendritic block copolymers” (LDBCs). This review gives a comprehensive summary of the state of the art in this rapidly developing field from pioneering early work to promising recent approaches.The different strategies leading to these hybrid architectures with either perfect dendrimer/dendron building blocks or imperfect, yet more conveniently accessible hyperbranched segments, are reviewed and compared. The consequences of the unusual polymer topology for supramolecular structures both in solution and in the solid state are summarized, and important differences in comparison with classical linear block copolymer structures are highlighted. Current challenges in the area of block copolymers, nanotechnology and potential applications of linear-dendritic block copolymers are also considered.     

Hydrogenated linear block copolymers of butadiene and isoprene: effects of variation of composition and sequence architecture on properties

The effect of variations in molecular architecture and composition on bulk properties is reported for a series of well characterized hydrogenated block copolymers of butadiene (HB) and isoprene (HI), each with a total molecular weight of ～ 200 000 and a narrow distribution ($\text{Mw}\text{Mn}\text{<1.17}$). The polymers were synthesized by sequential anionic polymerization followed by hydrogenation, using p-toluenesulphonylhydrazide. The material properties of the homopolymeric HI and HB were also investigated. As expected, HI is rubbery at room temperature and HB is a tough semicrystalline plastic with properties similar to those of a low density polyethylene, LDPE. The crystallinity, density and ΔH_t for all of the block copolymers were found to be linearly dependent on HB content indicating that little mixing exists between the HB and HI blocks in the solid state. Although the solution cast films of the block copolymers were spherulitic, the quenched films displayed no distinct structure on the supermolecular level indicating that the aggregation of the crystallites was more random in these films. The stress-strain properties of triblock copolymers with different block sequence, HBIB and HIBI, and a di-block copolymer, HBI, were similar in bulk behaviour to each other in the high and the intermediate butadience content (50–90%). This was related to the fact that the mechanical properties were determined predominantly by the behaviour of the more continuous HB phase. For the lower butadiene compositions (7–29%), there was a major difference in the behaviour of polymers with different block architecture. HBIB polymers were thermoplastic elastomers, whereas HIBI polymers behaved like an uncured particulate filled rubber. This difference was related to the presence of permanent ‘entanglements’ in HBIB polymers. The permanent entanglements which act as a physical crosslink are a consequence of the anchorage of the HB end blocks in the semicrystalline domains. No such arrangement is possible for either the HIBI or HBI polymers. The hysteresis behaviour of HBIB polymers were strongly dependent on butadiene content, decreasing with lowering of the concentration of the semicrystalline HB. This dependence was related to the continuity of the crystalline microdomains. All the members of HIBI series (and the HBI we considered) showed large hysteresis behaviour. This large energy loss during cyclic deforn ation in these polymers was related to the absence of the permanent anchor points arising from end block crystallization.     

Self-assembly of polystyrene-b-poly(4-vinylpyridine) in deoxycholic acid melt

It is well known that amphiphilic block copolymers in selective solvents self-assemble into micellar structures, where solvophilic blocks tend to contact with solvents while solvophobic blocks are shielded from the solvents. Different from the conventional micellization in liquid systems, we report that the block copolymer, polystyrene-b-(4-vinylpyridine) (PS-b-P4VP), can self-assemble in melted deoxycholic acid (DCA) at high temperatures and the structures are retained in “solid state” after being cooled down to room temperature. Probing by transmission electron microscopy (TEM), we found that a series of self-assembled structures, including spherical micelles, wormlike micelles and vesicles can be obtained by varying the length of the block copolymers and the morphologies are dependent on the annealing temperature and time. We also demonstrate how to extract the structures that are trapped in solid state by removing DCA using appropriate solvents. The extracted vesicles, which are loaded with solid molecules, are potential for applications in nanocapsules and controlled release.     

Degradation and geometric isomerization of poly(N-propargylamides)

The stability of several poly(N-propargylamides) was investigated in solution and in solid state on the basis of molecular weight change with time, and further their thermal stability was investigated by TGA. When the stability of poly(N-propargylamides) with varying pendent groups was compared, polymers with pendent groups of moderate size showed the highest stability in solution. Too short and too bulky pendent groups were not favorable for the stability of polymers. When poly(N-propargylheptanamide) (poly(6)) was stored in THF as solution at −20 °C in the absence of oxygen in dark, its degradation rate was the lowest. The degradation rate of poly(6) depended on the solvents used, which may be related to different solubility of oxygen in these solvents. Polymers with high cis contents degraded faster than polymers with low cis contents did. Addition of TEMPO and DPPH into the poly(6)/THF solution more or less depressed the degradation of poly(6). The degradation of polymer main chain in solution was always accompanied by the decrease of cis content, i.e. geometric isomerization from cis- to trans-structure. When the polymers were stored in the solid state at −20 °C, the polymers having alkyl pendent groups with moderate length were more stable than those with bulky pendent groups. Geometric isomerization occurred along with degradation in the solid state as well.     

Organic/inorganic hybrid composites prepared by polymerization compounding and controlled free radical polymerization

A new method to produce highly filled and well dispersed polymer solid composites using controlled free radical polymerization has been developed. Grafting of polymers onto ultrafine silica was done in bulk polymerization at 120 °C in presence of N-tert-butyl-1-diethylphosphono2,2-dimethyl propyl nitroxide (DEPN) as a nitroxide stable free radical. Optimum conditions for tert-butyl hydroperoxide grafting onto fumed silica were first determined. The percentage of grafting, the architecture of grafted polymers, the length of chains, and the polydispersity index can be controlled at will using this approach. The effect of the number of grafted polymer chains combined with its molecular weight on the processing of these materials was investigated. The syntheses performed in this work gave grafting percentages of polymers and copolymers ranging from 12 to 88 wt%. All ‘synthesized’ composites gave stable suspensions in toluene and tetrahydrofuran.     

Synthesis and luminescent properties of block copolymers based on polyfluorene and polytriphenylamine

For the preparation of block copolymers containing polyfluorene (PF) and hole transporting segment, PF homopolymers with diphenylamine terminals were synthesized by Suzuki coupling polymerization. The terminals of PF were converted to polytriphenylamine (PTPA) block by C–N coupling polymerization to give PTPA-block-PF-block-PTPA (PF-PTPA) triblock copolymers with different PTPA chain lengths. These polymers were soluble in common organic solvents and readily formed thin films by solution processing. All of the polymers exhibited similar UV absorption and PL emission properties both in chloroform solution and in film state. PF-PTPA block copolymers showed relatively high HOMO compared with that of PF by cyclic voltammetry. Compared with corresponding PF homopolymers, the EL devices based on PF-PTPA block copolymers showed higher luminance and current efficiency than those of PF homopolymers because of the improvement of hole injection by the introduction of PTPA segment.     

A new cationic surfactant N,N′-dimethyl-N-acryloyloxyundecyl piperazinium bromide and its pH-sensitive gels by microemulsion polymerisation

A piperazine-based cationic surfactant, N,N′-dimethyl-N-acryloyloxyundecyl piperazinium bromide (DAOUPB) was synthesised by a two-step procedure. The monomer was polymerised in two new microemulsion systems: (i) DAOUPB/water/methyl methacrylate (MMA):hydroxyethylmethacrylate (HEMA) and (ii) DAOUPB/water/acrylonitrile with ethyleneglycol dimethacrylate (EGDMA) as the crosslinking agent. Transparent solid polymeric materials were obtained by photo-initiated polymerisation of some of these microemulsion compositions. Most of the bicontinuous microemulsions investigated gelled within 10 min resulting in transparent solid polymers. The electron micrographs of the polymers obtained from microemulsion compositions containing MMA:HEMA did not show any micropores, while those obtained using acrylonitrile revealed the existence of open-cell type micropores and also of the bicontinuous nature of the system. The width of the bicontinuous structure (micropores) was about 33 nm in the dry state, with long and winding channels of random distribution. The swelling of the gels was highly sensitive to pH.     

New linear π-conjugated polymers via Suzuki coupling of (1Z, 3Z)-1,4-dibromo-1,4-diaryl-buta-1,3-diene with aromatic diborates: Synthesis and photophysical properties

A series of new linear π-conjugated polymers have been synthesized via Suzuki coupling of (1Z, 3Z)-1,4-dibromo-1,4-diaryl-buta-1,3-diene with aromatic diborates. The structures were characterized by NMR spectroscopy, IR spectroscopy and GPC. All of them exhibit good thermal stability with high decomposition temperature over 340 °C. It was found that the absorption and emission of the polymers can be adjusted through changing the side aromatic group, and the partial twisted structure of two aromatic groups at the end carbon of buta-1,3-diene unit can hinder the interchain interaction of conjugated main chain and improve the photophysical properties of the polymers in the solid state. The electrochemical and electroluminescent properties of the polymers were primarily studied.     

Organized structures in amorphous styrene/cis-1,4-isoprene block copolymers: low angle X-ray scattering and electron microscopy

It is shown that amorphous styrene/cis-1,4-isoprene block copolymers form organized structures in concentrated solutions as well as in the solid state. The effects of the nature and the concentration of the solvent, the composition of the copolymer and the temperature on the parameters determined by low angle X-ray scattering and electron microscopy are discussed. It is observed that, independent of the nature or concentration of the solvent, a given block copolymer exhibits only one ordered structure which may be lamellar or hexagonal.     

Preparation of pH-sensitive amphiphilic block star polymers,their self-assembling characteristics and release behavior on encapsulated molecules

Poly(ethylene glycol) (PEG), a polymer with excellent biocompatibility, was widely used to form nanoparticles for drug delivery applications. In this paper, based on PEG, a series of pH-sensitive amphiphilic block star polymers of poly(ethylene glycol)-block-poly(ethoxy ethyl glycidyl ether) (PEG-b-PEEGE) with different hydrophobic length were synthesized by living anionic ring-opening polymerization method. The products were characterized using ¹H NMR and gel permeation chromatography. These copolymers could self-assemble in aqueous solution to form micellar structure with controlled morphologies. Transmission electron microscopy showed that the nanoparticles are spherical or rodlike with different hydrophilic mass fractions. The pH response of polymeric aggregates from PEG-b-PEEGE was detected by fluorescence probe technique at different pH. A pH-dependent release behavior was observed and pH-responsiveness of PEG-b-PEEGE was affected by the hydrophobic block length. These results demonstrated that star-shaped polymers (PEG-b-PEEGE) are attractive candidates as anticancer drug delivery carriers.     

Core-shell polyacrylate and polystyrene-block-polyacrylate stars

The polymerization of p-(iodomethyl)styrene (PIMS) yields well-defined branched polymers with reactive iodomethyl groups. The branched poly[p-(iodomethyl)styrene] was used as the transfer agent in the iodine mediated radical polymerization of vinyl monomers. The polymerization proceeds in a controlled way and yields polystyrene and poly(t-butyl acrylate) star polymers with reactive groups at the end of their arms. Polymers so obtained were also used to prepare stars with block copolymer arms: polystyrene-block-poly(t-butyl acrylate). The characterization of star structures was performed by NMR and gel permeation chromatography with absolute molar mass detection (MALLS). Preliminary characterization of the thermal properties of these novel materials is reported.     

Easy obtainment of three-dimensional coordination polymer of zinc 1,2,4,5-benzenetetracarboxylate with para-amino-pyridinium cations

Within a wide study on coordination polymers coming from 1,2,4,5-benzenetetracarboxylate combined with aromatic amines and first row transition metal ions, we isolated and characterized a peculiar tridimensional coordination polymer made by an anionic mesoporous scaffold of Zn(II)-benzeneteracarboxylate whose charge is balanced by included p-aminopyridinium cations stacked up along two quasi-perpendicular crystallographic directions. The particular features of the solid state structure, easily obtained in standard conditions, prompts a wider research of possible related derivatives.     

The effects of the selectivity of the toluene/ethanol mixture on the micellar and the ordered structures of an asymmetric poly(styrene-b-4-vinylpyridine)

The effects of the solvent selectivity of toluene/ethanol mixtures on the micellar and ordered structures of an asymmetric diblock copolymer of PS(19.6 K)-b-P4VP(5.1 K) in the dilute (1 wt%) and semi-dilute (8 wt%) solutions, as well as in the gel and solid films, were studied using small angle X-ray scattering (SAXS), generalized indirect Fourier transform (GIFT), and transmission electron microscopy (TEM) methods. The solvent selectivity was controlled by ? (weight percentage of ethanol in toluene/ethanol mixture). Individual micelles, space-filled micellar structure (without three-dimensional order), and three-dimensionally ordered gel and solid structures were observed from the 1 and 8 wt% solutions, the gel, and the solid film, respectively. In the 1 wt% solution, the individual micellar structures were strongly dependent on ?; the spherical micelles with P4VP core at ? = 0, the unimer state at 10 ≤ ? ≤ 50, the spherical micelles with PS core at ? = 60, the cylindrical micelles with PS core at ? = 70 and 80, and precipitation at ? = 90 and 100 were observed. The 8 wt% solution was close to overlap concentration with the unimer state in the regions of 20 ≤ ? ≤ 40. In the gel, the ordered structure was observed in the sequence of bcc, hexagonal, gyroid, lamellar, reverse hexagonal and random as ? increased, and could be explained by the change of the relative volume fraction of each block as ? changed, similar to the phase sequence in the phase diagram of the diblock copolymer. The solid films showed the various kinetically frozen ordered microstructures such as randomly packed sphere, hexagonal, gyroid, hexagonally perforated lamella, reversed hexagonal, and randomly packed cylinder, which were controlled by the solvent quality in the gel before solidification. We believe that these results can be applied to photonic crystals, self-assembled nano-patterning, and functional nanoparticles in which the structural control is most important.     

Effect of Molecular Weight on the Strength of Nylon 6 and PET Fibers

Abstract

The mechanical properties of polymers depend on their morphological characteristics. With many systems and especially with semicrystalline polymers, it is possible to achieve significant changes in structure and properties by varying their thermal and stress histories. Considering the broad range of polymer applications it would, therefore, be desirable to have a quantitative understanding of the effects of structure on the properties of polymers in the solid state. The morphology of polymers is, however, often so complex that the interpretation of the mechanical responses must be restricted to qualitative considerations. In particular, the analysis of strength of un-oriented semicrystalline polymers is quite complex because of large plastic deformations which usually precede the rupture. However, by orienting the polymers in the form of fibers, it is possible to achieve structures which can be ruptured in tension with minimum plastic deformation. Under these conditions, the analysis of mechanical properties is greatly facilitated for fibers. Nevertheless, we still lack a quantitative understanding of the mechanical responses of polymeric fibers mainly because their structure has not yet been described with a precision allowing rigorous treatments.     

RAFT cryopolymerizations of acrylamides and acrylates in dioxane at −5 °C

Reversible addition-fragmentation chain transfer (RAFT) cryopolymerizations of acrylamides and acrylates were successfully carried out at −5 °C with cumene hydroperoxide/ascorbic acid as redox initiation couple and 2-dodecylsulfanyl- thiocarbonylsulfanyl-2-methylpropinoic acid as chain transfer agent. The cryopolymerization features of N,N-dimethylacrylamide (DMA) and tert-butyl acrylate (tBA) were investigated in view of kinetics, molecular weight and its distribution by proton nuclear magnetic resonance analysis and gel permeation chromatography. Furthermore, sequential block cryopolymerizations of N-isopropylacrylamide were performed with the obtained trithiocarbonate- functionalized PDMA or PtBA as macro-CTA and the corresponding block polymers were obtained. All the results demonstrated that these cryopolymerizations bear all the characteristics of controlled/living radical polymerizations.     

Formation of phase structure and crystallization behavior from disordered melt for ethylene-isoprene block copolymers and their blends

The structure formation and crystallization kinetics in crystallization from a disordered melt were investigated for a polyethylene-polyisoprene block copolymer (LEI) having M_n = 3.2 × 10⁴ and 53 wt% of polyethylene content and for its blends with the corresponding homopolymers, polyethylene (PE) and polyisoprene (PIp), using synchrotron small-angle X-ray scattering techniques (SAXS) and differential scanning calorimetry (DSC). For LEI copolymer and the blends, no microphase separation structure was observed in the molten state. In the crystalline state of the neat LEI, the first and higher order scattering peaks were clearly observed, in which the intensity of the higher order peaks was considerably strong. This unusual behavior of the higher order peaks was explained by the lamellar insertion model of Hama and Tashiro. From the analyses based on this model and one-dimensional electron density correlation function with a three phase model, the phase structure in the crystalline state of the neat LEI was concluded to be a regular lamellar structure consisting of crystalline lamella of PE block and amorphous layers of PE and PIp blocks. This phase structure was quite different from that reported previously for a polyethylene-polyisoprene block copolymer (HEI) with a higher molecular weight in which HEI crystallized with keeping the microphase separation structure in the melt. For the blends of LEI with PIp homopolymer, the phase structure is affected by the blend composition, while for the blends with PE homopolymer, the phase structure depended on the crystallization temperature as well as the molecular weight and composition of the added PE. The Avrami index was 2-3 for neat LEI, all blends and PE homopolymers.     

New luminescent 1,2,4-triazole/thiophene alternating copolymers: Synthesis, characterization, and optical properties

New five-membered ring heteroaromatic copolymers composed of 1-alkyl-1H-1,2,4-triazole and thiophene or bithiophene units were prepared by palladium-catalyzed polycondensation. The copolymers were soluble in organic solvents, and showed number-average molecular weights of 6200-23,700 in the GPC analysis. NMR spectroscopy revealed that the copolymers had a regio-random molecular structure. The optical properties, as well as the electrochemical properties, of the copolymers in solutions and films were determined. The polymers exhibited blue photoluminescence (PL) with an emission peak at about 420 nm and quantum yields of 36-43% in solutions, and the PL peak shifted to 470-480 nm in films. X-ray diffraction data suggested that the polymers formed a π-stacked structure in solid state.     

Syntheses, characterization, and energy transfer properties of benzothiadiazole-based hyperbranched polyfluorenes

A series of benzothiadiazole-based (BT) hyperbranched polyfluorene copolymers with various branching degrees (5-40%) were designed and synthesized. TGA and film annealing tests showed the substantial thermal stability of these highly branched polymers. The optical performance of the polymers in solutions and as films, and their electrochemical properties were characterized. The energy transfer (ET) processes in these hyperbranched conjugated polymers, both in solutions and in the solid state, were also investigated. With the change of the solution concentration and the branching degree, the energy transfer efficiency of the polymers varied in solutions and the main photoluminescence (PL) peaks changed from blue to green region. As films, only green light emitted from BT units. In addition, the PL efficiency of the films decreased dramatically with the increase of branching degrees. All these features demonstrated that highly branched structure would effectively impede the intra- and interchain energy migration, especially in solutions, and remarkably influence the ET process in the solid state, which resulted in low PL efficiency.     

Chloromodified ethylene propylene diene polymers

The heterogeneous gas/solid chlorination of various ethylene propylene diene polymers has been studied at temperatures between 0 and 75°C at conditions which are unfavourable for free radical reactions. The structure of the chloromodified polymers has been cleared by n.m.r. and from these data a chlorination mechanism has been proposed. An equation for the diffusion controlled chlorination rate has also been given as well as some cure characteristics and mechanical properties of allylic chlorine containing chloromodified ethylene propylene dicyclopentadiene polymers.     

Modelling the structural and physicomechanical properties of substituted poly(p-phenylene)s using molecular mechanical and molecular orbital methods

Conducting polymers are important technological materials that are finding increasing use in batteries and display devices. The conformation and packing of these polymers in the amorphous glassy state are poorly understood, despite the fact that they dictate their most important physical and mechanical properties. The processing of currently known conducting polymers is difficult and there is a strong incentive to increase their processability through functionalization. Developing an ability to predict the structure and structure-property relations of conducting polymers in the bulk will help with the design of new structures that combine processability with favourable electronic properties and facilitate their use in future high-technology applications. In this work, we concentrate on substituted poly(p-phenylene)s. Detailed atomistic molecular models have been developed with the help of molecular mechanics and semi-empirical quantum mechanical calculations using Cerius and MOPAC V6.0 program packages and structural, volumetric, and mechanical properties, e.g. geometrical values, densities, have been calculated by simulations on these models. The results from both methods have been compared with simulated and experimental data and conclusions have been drawn on the methodology and the approximations used. This study was used to compare with results obtained on unsubstituted poly(p-phenylene)s carried out earlier and to continue to develop our methodology for calculating structure, physical and mechanical properties that will be generally applicable to conductive polymers.