The preparation of novel nanoparticles is described, which consist of a poly(lactic acid) (PLA) core obtained by the nanoprecipitation method with a shell made of oppositely charged β‐cyclodextrin polymer (poly‐β‐CD) assembled using the layer‐by‐layer technique. Characterization of these nanoparticles is accomplished through dynamic light scattering, ζ‐potential measurements, X‐ray photoelectron spectroscopy and electron microscopy. The release of a loaded lipophilic molecule, benzophenone (BP), is mainly controlled by diffusion of BP and by its host–guest interaction with the β‐CD cavities of the adsorbed poly‐β‐CD. Increasing the number of poly‐β‐CD layers results in a better control of the release through the partition equilibrium between free BP and BP included in the cavities inside the multilayers.
A novel spacer‐free liquid crystalline polyrotaxane (LCPR) is synthesized by directly grafting 4‐phenylazobenzoyl chloride onto the hydroxyl of threaded α‐cyclodextrin (α‐CD) as the side‐chain. It is found that LCPR can form nematic LC phase above 180 °C or under the concentration of 5–0.2 wt% in dimethyl sulfoxide (DMSO) solution. The mechanisms of formation of thermotropic and lyotropic LC phase are proposed, respectively, which show the mobility of both threaded α‐CDs and substituted azo‐mesogens is responsible for the formation of LC phase. This mechanism is further demonstrated by the formation of crystalline phase under UV radiation, which is characterized by polarizing optical microphotograph and UV measurements.
Unsaturated polyesters are synthesized via ring‐opening copolymerization of α‐methylene‐δ‐valerolactone and δ‐valerolactone. These polyesters 4a–c are mixed with ethyl methacrylate (EMA), 2‐hydroxyethyl methacrylate (HEMA), and α‐methylene‐δ‐valerolactone (α‐MVL), respectively. Then, crosslinking is carried out by free radical polymerization initiated by an azo‐initiator. A second glass transition is found with incorporation of HEMA and α‐MVL. These findings indicate the formation of phase‐separated polyester blocks crosslinked with the poly(meth)‐acrylic‐segments, respectively poly(α‐methylene‐δ‐valerolactone) segments.
In the present paper, new photoinitiators based on the meta‐terphenyl scaffold in combination with an iodonium salt and 9H‐carbazole‐9‐ethanol (CARET) are proposed for the free radical promoted cationic polymerization of epoxides upon visible light exposure using light emitting diodes at 405, 455, and 470 nm. Remarkably, a new high performance additive (i.e., CARET) is proposed here for cationic polymerizations. CARET shows some important advantages compared to other reported additives such as N‐vinylcarbazole or benzyl alcohol used as references. Excellent polymerization initiating abilities are found and a full picture of the involved chemical mechanisms is provided.
Novel branched polyoxymethylene copolymers are synthesized by cationic copolymerization of 1,3,5‐trioxane (TOX) with 3‐(alkoxymethyl)‐3‐ethyloxetane (ROX) using BF3·Et2O as an initiator. Four oxetane derivatives with different side‐chain lengths (from 1 to 6 carbons) are tested for copolymerization. The copolymer composition is controlled by the feed ratio of ROX, and influenced by the chain length of alkyl group on ROX. The incorporation ratio and side‐chain length of the ROX unit have great influence on the thermomechanical properties and crystallinity of the copolymers.
Copolymerization of ethylene with 4‐penten‐1‐ol (4P1O) is conducted by using a half‐titanocene catalyst. The incorporation of 4P1O is high as approximately 10 mol%. The assignments of the 13C NMR spectrum and the analysis of the sequence distribution show the existence of not only the alternative structure but also the repeated 4P1O units. The product of the reactive ratios is calculated as ca. 2.
The homo‐ and copolymerizations of 1,3‐butadiene and isoprene are examined by using neodymium isopropoxide [Nd(Oi‐Pr)3] as a catalyst, in combination with a methylaluminoxane (MAO) cocatalyst. In the homopolymerization of 1,3‐butadiene, the binary Nd(Oi‐Pr)3/MAO catalyst works quite effectively, to afford polymers with high molecular weight ( ≈ 105 g mol‐1), narrow molecular‐weight distribution (MWD) (/ = 1.4–1.6), and cis‐1,4‐rich structure (87–96%). Ternary catalysts that further contain chlorine sources enhance both catalytic activity and cis‐1,4 selectivity. In the copolymerization of 1,3‐butadiene and isoprene, the copolymers feature high , unimodal gel‐permeation chromatography (GPC) profiles, and narrow MWD. Most importantly, the ternary Nd(Oi‐Pr)3/MAO/t‐BuCl catalyst affords a copolymer with high cis‐1,4 content in both monomer units (>95%).
Polystyrenes (PS) end‐functionalized with perfluorotridecyl (PFTD), perfluorodecyl (PFD), and perfluoroheptyl (PFH) groups are synthesized using the corresponding perfluoroalkyl initiators by atom transfer radical polymerization. Polymer thin films deposited on silicon wafers are studied by dynamic nanoindentation (NI). NI measurements of the 15k PFTD‐PS sample showed increases of about 80 and 300% in the storage and loss moduli, respectively, compared to the 15k PS homopolymer. Measurements made on different regions of the PFTD‐functionalized PS show a fivefold greater standard deviation compared to the 15k PS homopolymer. The effects for the PFD and PFH groups are smaller in all respects consistent with expectations.
An oxacyclophane framework is modified to include π‐conjugated segments positioned so as to allow an optimized face‐to‐face π‐stacking interaction between them. This unique architecture is first employed for the preparation of model compounds with defined interaction between two small molecular organic chromophores. The interaction allows facile through‐space energy transfer between the chromophores when they are held in the appropriate geometry. The oxacyclophane is thus employed in a polymer with consecutive short stretches of phenylene ethynylene such that polymer chain conjugation requires through‐space interaction between segments rather than the through‐bond π‐conjugation that typifies traditional conjugated polymers. The extent of the through‐space interactions along the polymer backbone is explored through photophysical measurements, calculations, X‐ray diffraction, and comparison with a variety of model and control materials.
Size exclusion chromatography coupled with light scattering (SEC/MALS), dynamic light scattering (DLS), steady‐state and time‐resolved fluorescence, as well as molecular dynamics (MD) simulations are used to study the behavior of several poly(ethylene glycol) (PEG)/α‐cyclodextrins (αCDs) polyrotaxanes (PRs) in solution. The number of CD units in any of the PRs studied is always smaller than that required to saturate the PEG chains. These PRs seem to aggregate in dimethyl sulfoxide (DMSO) solution. The presence of hairpins in the non‐saturated PRs contributes to diminishing their expected large dimensions. Intra‐ and intermolecular interactions and forces responsible for hairpins and aggregation are investigated.
Functional nanowires with photochromic spiropyran (SP) species are prepared by reversible addition–fragmentation transfer (RAFT) dispersion polymerization of styrene using poly(4‐vinylpyridine‐co‐spiropyranyl methacrylate) as a macro‐RAFT agent, and then electrospinning technology is used to fabricate fibers from the functional nanowires. The photoisomerization of SP in the nanowires and fibers is studied, and reversible photochromism for both nanowires and fibers is observed. Since the merocyanine (ME form) of the SP emits fluorescence and the SP form is non‐fluorescent, the fluorescence of the nanowires and the fibers can be switched on and off upon alternating UV and visible‐light irradiation.
The Passerini three‐component reaction is applied to synthesize, in a one‐step procedure, diverse asymmetric α,ω‐dienes containing an acrylate and a terminal olefin. Such monomers are well known to undergo head‐to‐tail acyclic diene metathesis (ADMET) polymerization due to the high cross‐metathesis selectivity between acrylates and terminal olefins. Additionally, amphiphilic block copolymers are synthesized using a monofunctional PEG480 monoacrylate, which acts as a selective chain‐transfer agent during the polymerization process. Thus, control over the molecular weight of the amphiphilic ADMET polymers is shown by using different ratios of monomer and chain‐transfer agent. All the polymers are thoroughly characterized, and their ability to form nanoparticles in aqueous solution is studied.
Polystyrene (PS) film containing 1,4‐bisbenzil is efficiently crosslinked in two steps. In the first step, visible light (λ > 400 nm) causes molecular oxygen to insert between two carbonyl groups of one of the 1,2‐dicarbonyl groups. The peroxide is subsequently decomposed by absorption of another photon, forming acyloxy radicals, which add to the aromatic ring of the PS chain. The remaining 1,2‐dicarbonyl group is then photoperoxidized to form PS with pendant benzoyl peroxide moieties. In the second step, pendant benzoyl peroxide groups are decomposed thermally to form acyloxy macroradicals responsible for the crosslinking. Crosslinking proceeds simultaneously with degradation. Finally, the gel content in the film may exceed 80 wt%.
Here, the formation of nanoparticles based on a microemulsion approach and the use of polymer surfactants are described. Therefore, two amphiphilic poly(2‐oxazoline) block copolymers P1 and P2 with alkyne groups in their hydrophobic block have been synthesized by ring‐opening, cationic polymerization. The polymers P1 and P2 are employed in a microemulsion process to stabilize the particle core by core cross‐linking of 1,6‐hexanediol diacrylate (HDDA) using either AIBN as azo‐initiator or 2‐propanethiol as a photo‐initiator for the polymerization reaction. The results show that particle size can be controlled by sonication time, the hydrophilic–hydrophobic balance of the polymer surfactant, and the ratio of polymer surfactant versus HDDA giving access to water‐soluble nanoparticles in a size range of 10–70 nm.
A narrow‐bandgap conjugated polymer, PFDTBTzQ‐2OC1, is prepared by alternating [1,2,3]triazolo[4,5‐g]quinoxaline and 9,9‐didodecyl‐fluorene. With a bandgap of 1.63 eV, this polymer has wide absorption ranging from 300–760 nm in film. Bulk heterojunction solar cells fabricated by blending PFDTBTzQ‐2OC1 with [6,6]‐phenyl‐C71‐butyric acid methyl ester exhibit a maximum power conversion efficiency of 1.31%, with a short‐circuit current density of 1.98 mA cm–2, an open‐circuit voltage of 0.74 V, and a fill factor of 0.47.
Recently, graphene and its derivative nanosheets have been considered as potential candidates for gas barrier membranes. Here, the effect on the water‐vapor barrier properties of incorporating graphene nanoplatelets into a UV‐curable perfluoropolyether methacrylic oligomer is investigated. By combining the hydrophobicity of the highly fluorinated polymer with the tortuous path induced by the graphene sheets, the barrier properties of the crosslinked films are further enhanced, acting both on the thermodynamic (low solubility) and kinetic (longer diffusion path) contribution of the water‐vapor diffusion through the polymeric films. Good performance in increasing water‐vapor barrier properties at a very low filler loading is obtained: the permeability is halved in the presence of only 0.3 wt% graphene.
Dynamic crystallization (DC) is a new characterization technique for measuring the chemical composition distribution (CCD) of semicrystalline copolymers. This technique fractionates polymers based on chain crystallizabilities under a constant cooling rate; a solvent is also fed through the column at a constant flow rate during the crystallization to enhance the physical separation of the polymer fractions. In this work, a DC model for ethylene/1‐olefin copolymers on the basis of population balance, crystallization kinetics, and axial dispersion is proposed. This model is found to describe the experimental DC profiles of an ethylene/1‐octene copolymer at various operation conditions very well.
The self‐organization of colloids into defined structures offers the possibility to develop novel materials with exciting properties. However, this requires the understanding and application of the forces governing interparticle association. A novel approach for a size‐dependent anisotropic assembly between nanoparticles is achieved. Janus‐like iron oxide/polystyrene hybrid nanocolloids are prepared by heterophase polymerization and selectively coated with silica on the iron oxide face to gradually form a cavity. Hence, a shallow surface around the hydrophobic polymer face is created, enabling smaller particles of the same nature to be locked by shape complementarity and colloidal steric stabilization. Further coating with silica fixes these assemblies and allows quantitative analysis of the interaction on a nanoscale.
Dithiobenzoates are among the most popular agents for reversible addition‐fragmentation chain transfer (RAFT) polymerization. This is attributed to the better control over molecular weight and end‐group fidelity found in RAFT polymerization of methacrylates and methacrylamides. However, in polymerization of styrenes, acrylates, and acrylamides, their use has diminished, mainly in favour of trithiocarbonates, because of issues with retardation, as well as hydrolytic and thermal instability. This paper critically assesses developments in understanding the mechanism and kinetics of dithiobenzoate‐mediated RAFT polymerization from 2006 to 2013, with specific reference to the choice of reagents, polymerization conditions, side reactions, and factors leading to retardation.
In this study, in situ small‐angle X‐ray scattering (SAXS), in situ Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy (TEM) are used to monitor the formation of ordered mesophases in cured mixtures of phenolic resin and the diblock copolymer poly(ethylene oxide‐block‐ε‐caprolactone) (PEO‐b‐PCL). SAXS and TEM analyses reveal that the mesophase of the phenolic/PEO‐b‐PCL mixture transforms sequentially from disordered to short‐range‐ordered to hexagonal‐cylindrical to gyroidal during the curing process when using hexamethylenetetramine (HMTA) as a cross‐linking agent, indicating that a mechanism involving reaction‐induced microphase separation controls the self‐assembly of the phenolic resin. In situ SAXS is also used to observe the fabrication of mesoporous phenolic resins during subsequent calcination processes.
下载免费PDF全文