Living radical polymerization (LRP) techniques and their ability to improve the morphology of crosslinked polymer networks by controlling polymer chain growth are reviewed. Recent successes in the creation of improved molecularly imprinted polymer networks are also discussed. LRP offers the ability to control molecular weight, polydispersity, and tacticity while reducing microgel formation in polymers created via free‐radical polymerization (FRP). The improved network architecture of polymers created via LRP has great potential, especially when considering imprinted networks which have traditionally been plagued by heterogeneity in network morphology and binding affinities. Using LRP can considerably improve template recognition and further delay template transport in imprinted polymers.
Fully exfoliated PS/clay nanocomposites were prepared via FRP in dispersion. Na‐MMT clay was pre‐modified using MPTMS before being used in a dispersion polymerization process. The objective of this study was to determine the impact of the clay concentrations on the monomer conversion, the polymer molecular weight, and the morphology and thermal stability of the nanocomposites prepared via dispersion polymerization. DLS and SEM revealed that the particle size decreased and became more uniformly distributed with increasing clay loading. XRD and TEM revealed that nanocomposites at low clay loading yielded exfoliated structures, while intercalated structures were obtained at higher clay loading.
The thermal conductivity of a rubber compound is studied as a function of its state of curing. The device is presented and the calculations in order to obtain samples with controlled and homogeneous vulcanization rates are performed. The hot disk technique is used to measure the thermal conductivity of the rubber. This transient, plane‐source and non‐destructive method allows rapid and accurate measurement of the thermal conductivity based on the measurement of the electrical resistance of a plane sensor placed between two identical samples. The obtained results show that the thermal conductivity may vary significantly as a function of vulcanization rate. The effect of this variation on the prediction of the reaction progress is discussed.
The copolymerization behavior of mixtures of an acidic and a basic monomer were studied as a function of the monomer feed composition. The four systems investigated in this way comprised APSA/1‐VIm, APSA/4‐VP, APSA/DAMA, and SSA/1‐VIm. The corresponding polyelectrolytes were obtained by free radical polymerization of the monomer mixtures in solution, while the isolated monomeric salts of equivalent amounts of both monomers were prone to spontaneous polymerization. For APSA/1‐VIm, APSA/4‐VP, and APSA/DAMA, a donor‐acceptor relationship was found, whereas for SSA/1‐VIm the SSA was favorably incorporated over all feed ratios. High Tg and complex viscosity data indicated a considerable degree of ionic, thermally reversible crosslinking within the polymers that were thermally stable up to at least 230 °C.
Blends of nanosized pigment particles and polymers are widely believed to offer the potential for the design of novel or at least improved materials. This review critically evaluates the recent literature with regard to the following issues: (a) why and how does the size of the particles matter, (b) what are the requirements to create compatibility between amorphous polymers and nanoparticles, (c) carbon allotropes as nanosized pigments, (d) bulk polymerization of monomer/pigment mixtures, (e) interaction of growing chains with the particles in the polymerization, (f) depletion flocculation as a mechanism to counteract homogeneous distribution of the particles in the polymer matrix and ways to suppress the undesirable flocculation, and (g) optical properties of the blends as well as methods of optical characterization.
This paper reports the properties of highly oriented nanocomposite tapes based on isotactic PP and needle‐like sepiolite nanoclay, obtained by a solid state drawing process. The intrinsic 1D character of sepiolite allows its exploitation in 1D objects, such as oriented polymer fibres and tapes, where it can be uniaxially oriented upon drawing. A synergistic increase in mechanical properties is presented for highly drawn tapes (λ ≤ 20) and low filler loadings (≤2.5 wt.‐%), which can not be simply explained by micromechanical composite models. Instead, mechanical properties are intimately related to the dispersion state of the nanoclays in PP, the rheological properties of the nanocomposites and the polymer morphology.
Summary: An organic‐inorganic hybrid material consisting of a 3‐(methacryloxy)propyl functionalized SiO2/MgO framework was synthesized. This hybrid was successfully reacted with styrene, butyl acrylate and butyl methacrylate via a free radical emulsion polymerization to form polymer composites. The polymer composites were investigated by means of FT‐IR spectroscopy, TGA, DSC and rheometry. It is shown that the polymer is linked covalently to the organic/inorganic hybrid. Although the polymer content is rather low, the composites exhibit a polymer‐like character and enhanced mechanical properties compared to the corresponding homopolymers.
Summary: In this work, epoxy resin/CaCO3 nanocomposites were prepared by in situ and inclusion polymerization. Nanoparticles with a size of 30–40 nm were dispersed efficiently in bisphenol‐A alkaline solution before polymerization and the dispersion could be kept in the resultant composite by a reaction that took place at the nanoparticle surface and among the nanoparticles when epichlorohydrin was added. Furthermore, the slightly conglomerated nanoparticles could even be separated by epoxy resin growing among them. This method showed a better dispersion of nanoparticles compared with solution‐blending as observed with TEM. Owing to better combination of epoxy resin and nanoparticles, the resultant nanocomposite showed a 12 °C increase in Tg compared to the nanocomposite prepared by solution‐blending. Tensile test revealed that the tensile strain of nanocomposites rises as the nano‐CaCO3 content increases.
The dispersion mechanism of nanoparticles through in situ and inclusion polymerization. 相似文献
Spherical silica particles with pseudo‐inverse opal structure are synthesized by using pomegranate‐like polymer microparticles as templates. A micro‐dispersion polymerization occurring in the suspended monomer droplets in the presence of a silica precursor leads to the formation of nearly monodisperse polymer sub‐particles of about 1 µm size, randomly‐packed within a 30–100 µm polymer particle. The polymerization is followed by an acid‐catalyzed reaction that induces formation of silica in the interstices between the sub‐particles within a polymer particle. Spherical PIOS particles are eventually produced by selectively removing the polymer template by pyrolysis. The PIOS particles show large specific surface areas with unique pore geometry and pore size distribution.
Copolymers of 3,4‐ethylenedioxythiophene and 3‐methylthiophene have been prepared by recurrent potential pulses using monomer mixtures with various concentration ratios, their properties being compared with those of the corresponding homopolymers. In addition, different technological applications have been tested for the generated copolymers. Results indicate that the properties of the copolymers are closer to those of poly(3,4‐ethylenedioxythiophene) than to those poly(3‐methylthiophene). Furthermore, the ability of the copolymers to store charge and to interact with plasmid DNA suggest that they are very promising materials.
The crosslinking of thin liquid PDMS layers by three different technically relevant processes, H2 radio‐frequency plasma treatment, Xe2* excimer VUV irradiation, and low‐energy electron beam processing is investigated. The modifications to the layers due to the processing are monitored by means of RAIRS. Plasma processing of liquid PDMS leads to a direct conversion to a SiOx‐like material of the topmost layers, whereas a gradual transition from PDMS to the same product is observed upon VUV irradiation. Electron exposure does not induce oxidation. The initiating steps of the conversion induced by the interaction with VUV photons, low‐energy electrons, or their combined effect with ions and H atoms in the plasma are discussed. The latter creates a high density of damage sites.
The effect of hydrophilic and hydrophobic nanosilica on the morphological, mechanical and thermal properties of polyamide 6 (PA) and poly(propylene) (PP) blends is investigated by extrusion compounding. Depending on the difference between the polymer/nanoparticle interfacial tensions, different morphologies are obtained as highlighted by TEM and SEM. Hydrophobic nanosilica migrates mainly at the PA/PP interface, which leads to a clear refinement of PP droplet size. The macroscopic properties of the hybrid blends are discussed and interpreted in relation with the blend morphology and melt‐mixing procedure. The control over coalescence allows a morphology refinement of the blends and improves mechanical properties.
An effective approach is presented for designing and tailoring diffusion‐controlled systems for targeted release rate profiles. The diffusion‐controlled system consists of PBA networks with precisely controlled crosslink densities via RAFT copolymerization of BA with ethylene glycol dimethacrylate, which gives a desired diffusivity. One‐dimensional releases in the crosslinked PBA matrix (CPM) slabs are evaluated with a hydrophobic dye. Fick's second law is used to model the transient mass transfers in the CPMs with a mixed Newton‐Tikhonov regularization method for determination of their diffusion coefficients. A two‐layer CPM (CPM‐2) with controlled crosslink densities is constructed. The dye release results from the CPM‐2 agreed well with theoretical predictions.
Summary: A semi‐batch process using nitroxide mediated polymerization, was explored for the design of low molecular weight solvent‐borne coatings, typical of those used in the automotive industry. While living radical polymerization (LRP) offers many advantages in the control of polymer chain microstructure that may confer important physical and chemical property benefits to coatings, adapting LRP to a semi‐batch process poses significant challenges in the design and operation of the process. Using styrene monomer, various two‐component initiating systems (free radical initiator, 4‐hydroxy‐TEMPO) were studied to understand the effects of different initiators on the course of polymerization. In addition, an alkoxyamine was synthesized and used as the initiating source. The initiators Luperox 7M75 and Luperox 231 give higher polymerization rates and reasonable control over polymerization, while benzoyl peroxide (BPO), Vazo 67, and the alkoxyamine are less effective. The number of polymer chains in the final product is always less than the theoretical value, reflecting poor initiation efficiency, probably resulting from undesirable termination reactions that become important due to the nature of the semi‐batch process. Adding camphorsulfonic acid (CSA) or charging initiator concurrently with monomer during semi‐batch feed, can increase the polymerization rate while maintaining the living character of the polymerization. The copolymerization of styrene and butyl acrylate is also shown to exhibit living character.
Schematic representation of the exchange reaction to produce N‐TEMPO capped polymer chains. 相似文献
Summary: Five new crosslinkers for use in dental composites were synthesized. Four are based on TBHMA: 1 via reaction of TBBr and Bisphenol A; 2 by hydrolysis of t‐butyl groups of the first monomer to give a diacid derivative; 3 by conversion of the first monomer to an amide derivative using benzyl amine; 4 by conversion of the first monomer to amide derivative using APTES. The AHM‐based monomer 5 was synthesized from the Michael addition of APTES to AHM. The photopolymerization behaviors of the synthesized monomers with Bis‐GMA, TEGDMA and HEMA were investigated using photodifferential scanning calorimetry at 40 °C using DMPA as photoinitiator. The polymerization rates and degrees of conversion for mixtures of any of the monomers 1 – 4 with Bis‐GMA:TEGDMA were found to be similar to Bis‐GMA:TEGDMA, higher than Bis‐GMA:HEMA, and also higher than mixtures with Bis‐GMA:HEMA. The incorporation of TBHMA‐based monomers into the conventional resin mixture (Bis‐GMA and TEGDMA) reduced the polymerization shrinkages. Monomer 5 and its mixtures polymerized much faster and to higher degrees of conversion than the other investigated systems, however, this system exhibited the largest volume shrinkage.
Structures of some of the new crosslinkers synthesized. 相似文献
The impacts of nanoclays on the miniemulsion polymerization kinetics of styrene were studied. It was found that both RP and the fractional conversion decreased upon increasing the organoclay content in the miniemulsion system. In the presence of nanoclay the molecular weight of polystyrene nanoclay composite is lower and the particle size polydispersity of the final composite latex is greater than that of pure styrene miniemulsion polymerization. The effect of the nanoclays is mainly caused by the destabilization of the miniemulsion by the organoclay particles. The increase in the monomer viscosity and the decrease in the diffusion rate of the monomer and the living polymer inside the monomer droplet also accounts for the reduction in the polymerization rate.
This paper investigates the effect of both the clay loading and the monomer feed rate on the morphology and properties of poly(styrene‐co‐butyl acrylate)‐clay nanocomposites prepared in emulsion polymerization. Analysis by X‐ray diffraction (XRD) and transmission electron microscopy (TEM) of the nanocomposites prepared by batch polymerization showed that the polymer clay nanocomposites (PCNs) with 1–3 wt.‐% clay loading resulted in intercalated structures, while exfoliated structures were obtained at 10 wt.‐% clay loading. The polymerization was also carried out with semi‐batch polymerization. The morphology, thermal stability, and mechanical properties of nanocomposites obtained were found to be more strongly dependent on the clay/polymer ratio than the monomer feed rate.
The influence of electron beam (EB) irradiation on the mechanical properties of biopolymers from modified linseed oil is studied. The thermoset is prepared by copolymerizing norbornenyl‐functionalized linseed oil and dicyclopentadiene (DCPD) by ring‐opening metathesis polymerization (ROMP). EB irradiation of the bulk polymer results in a substantial increase in the crosslinking density. The residual carbon‐carbon double bonds remaining after ROMP are expected to act as further crosslinking sites upon exposure to the high‐energy electrons. The increase in the crosslinking density is studied by DMA and sol/gel fraction measurements from Soxhlet extraction. Tensile testing reveals that Young's modulus and tensile strength are enhanced after EB irradiation.
Polymerization rate and copolymerization parameters of the free‐radical copolymerization of AMPS with 1‐VIm was studied as a function of the monomer feed and the pH value in ethanol. It was found that neutral and basic monomer mixtures containing the sodium salt of AMPS polymerized faster and led to polymers with a higher proportion of NaAMPS incorporated than those monomer mixtures containing the free acid. Additionally, based on the experimental data, copolymerization parameters of rAMPS = 0.3 and r1‐VIm = 0.13 were calculated for polymerization in acidic solution and rAMPS = 4.1 and r1‐VIm = 0.1 for polymerization in basic and neutral solutions. Finally, the thermal stability, rheological behavior, and intrinsic viscosity were determined for the polymers.
Lipase‐catalyzed polycondensation of two biobased diacids, 1,12‐dodecanedioic acid and 1,14‐tetradecanedioic acid, with 1,8‐octanediol was achieved using immobilized Lipase B from Candida antarctica. The procedure resulted in partially renewable prepolymers, while poly(octylene adipate) from petroleum‐based adipic acid was also synthesized for comparison reasons, revealing a dependence of the enzymatic polymerization degree on monomer composition. The prepolymers were further submitted to bulk postpolymerization at temperatures in the vicinity of their melting point under flowing nitrogen. The intrinsic viscosity increase was found up to 12%, with no significant impact on the polyesters thermal properties.
