Differences in the Raman spectrum of the α‐ and β‐polymorph of poly(propylene) (PP) have been identified in the form of band shifts distinct for the specific polymorph. A new method has been developed, which can ascertain the presence of the α‐polymorph. These band shifts have been used to profile both the polymorphs and variations in the crystallinity across the whole spherulite have been ascertained by carrying out a spatially resolved profiling of the degree of crystallinity. The crystalline domains in PP have been qualitatively assessed using the band shifts in the Raman spectra. This approach has then been applied to study local variations in the morphology of welds between PP plates, wherein the complex interplay between the heating and cooling patterns as well as the mechanical forces due to shearing gives rise to the development of a different morphological structure in the weld. Using Raman microscopy, a reduction in the crystallinity (≈18%) in the region of the weld seams and sporadic formation of β‐spherulites in the weld core is observed. The methodology developed has been applied to investigate a weld between PP‐H and PP‐R (PP with random ethylene copolymer), to ascertain the variations occurring upon welding in such materials.
The design, synthesis, and investigation of two series of side‐on dendrimers displaying nematic liquid crystal behavior at room temperature are described. The dendritic structures are derived from poly(amidoamine) (PAMAM‐(NH2)n) or poly(propylene imine) dendrimers (PPI‐(NH2)n) (with n = 4, 8, 16, 32, and 64) containing lateral attached mesogenic units in the periphery. The materials are characterized by 1H, 13C nuclear magnetic resonance (NMR) spectroscopy, matrix‐assisted laser desorption/ionization‐mass (MALDI‐TOF MS) spectrometry, differential scanning calorimetry (DSC), polarized optical microscopy (POM), and x‐ray diffraction spectroscopy. All dendrimers exhibit nematic mesophase behavior at room temperature. Conoscopic studies reveal that the nematic phase is uniaxial. The relationship between the structure of the two dendrimer series and the mesomorphic behavior is discussed.
The conformation of a helical type polymer chain in the crystalline phase prefers the state with lower energy. The molecular simulation of the melting behaviors of α and β isotactic poly(propylene) (iPP) crystals with ultrafast heating shows that the ratio of right‐handed and left‐handed helix stays at 50% in α‐iPP and changes from 100% to 50% in β‐iPP before and after melting. Till date, the formation of β form crystal in iPP has not been understood well together with the mechanism of the crystallization process. One of the approaches to produce β‐iPP is to shear the melt. The influence of shear flow on the crystallization behavior of iPP was investigated via ex situ and in situ X‐ray measurements. It was found that preceding or simultaneously initiated formation of β‐iPP together with α‐iPP occurs under proper shear conditions. The β‐rich iPP sample could no longer form β crystal at 135 °C after heating to the β‐melting temperature with the melting of β‐iPP and the retaining of oriented α‐iPP, which is contrary to the accepted formation process of β‐iPP. A mechanism was proposed to understand the shear‐induced β crystal in iPP, considering the mesophase and stereocomplex type helical conformation of the polymer chains.
Vis‐breaking or rheology control is an important technical process to improve the processability of impact poly(propylene) copolymers (IPCs). In the vis‐breaking process the molar mass and its dispersity are reduced and the crystallinity changes and polar carbonyl functionalities are introduced due to the reaction of the polymer with peroxide. Although the fundamental principles of vis‐breaking are well understood, not much research has been devoted to the investigation of the molecular changes brought about by the vis‐breaking process. In the present study, the effect of vis‐breaking on the molecular parameters of IPCs is elucidated using advanced fractionation methods. After the analysis of the bulk sample properties, the samples are fractionated using preparative temperature rising elution fractionation and the fractions are analyzed by Fourier transform infrared spectroscopy, differential scanning calorimetry, crystallization analysis fractionation, and high temperature high performance liquid chromatography techniques. For the first time, an effective multidimensional analytical approach is established to study compositional heterogeneities in vis‐broken IPC materials.
Nanocomposites of isotactic polypropylene (iPP) containing modified multi‐walled carbon nanotubes (MWCNTs) are prepared by melt mixing. MWCNTs with alkyl groups (alkyl‐MWCNTs) and with methyl‐PEG (mPEG‐MWCNTs) are used, as well as unmodified MWCNTs, to prepare the three series of nanocomposites. The surface treatment and the content of MWCNTs affect the mechanical properties of the iPP. In all cases, theoretical models are used to estimate the effect of surface treatment. The crystallization rates of nanocomposites in both isothermal and non‐isothermal crystallization conditions are measured. The nanocomposites containing unmodified MWCNTs crystallize faster than those with modified MWCNTs.
An investigation of the structure–property relationships of poly(dimethyl siloxane) (PDMS) polyoxamide segmented copolymers reveals the impact of oxamide spacing and PDMS molecular weight on physical properties. Varying the length of the methylene spacing between the oxamide hydrogen bonding groups in the hard segment (HS), as well as the PDMS soft segment molecular weight, provides insight into the influence on thermal and mechanical properties. Bulk polycondensation of ethyl‐oxalate‐terminated PMDS oligomers with diamines yields optically clear, thermoplastic elastomers with excellent mechanical properties. The structure–property investigation reveals optimum mechanical properties of PDMS polyoxamide segmented copolymers occur with the smallest spacing between oxamide groups in the HS.
Poly(methyl methacrylate)‐block‐poly(4‐vinylpyridine), polystyrene‐block‐poly(4‐vinyl pyridine), and poly(ethylene glycol)‐block‐poly(4‐vinylpyridine) block copolymers are synthesized by successive atom transfer radical polymerization (ATRP), single‐electron‐transfer nitroxide‐radical‐coupling (SET‐NRC) and nitroxide‐mediated polymerization (NMP). This paper demonstrates that this new approach offers an efficient method for the preparation of 4‐vinylpyridine‐containing copolymers.
Herein, a feasible protocol of nanohybrid shish kebab (NHSK) in isotactic poly(propylene) (iPP)/low density polyethylene (LDPE) blend is shown, where iPP nanofibrils in situ generate during melt compounding followed by hot stretching, thus serving as shish to induce the kebabs of LDPE under extensional flow, therewith leading to greatly improved mechanical properties. Although scanning electron microscope and small angle X‐ray scattering results reveal classic characteristic of NHSK, wide angle X‐ray diffraction demonstrates that some LDPE chains within the kebabs of LDPE are actually tilted off the flow direction. Through the comparison of LDPE lamellae epitaxially growing on iPP nanofibrils substrate under quiescent condition, it is concluded that the extensional flow affects the formation of NHSK notably, mainly by enhancing nuclei density and further influencing the twist of LDPE lamellae. Moreover, of particular significance is the methodology based on practical processing of extrusion in this work, making it promising to industrially achieve NHSK in polymer blend with advanced performance.
The variation of the drift mobility of positive and negative charge carriers in films of anthracene‐containing poly(p‐phenylene‐ethynylene)‐alt‐poly(p‐phenylene‐vinylene)s ( AnE‐PVs ), differently substituted, is investigated as a function of the applied electric field. Branched 2‐ethylhexyl and linear alkoxy side chains of different lengths are considered, as well as well‐defined and random distributions of lateral substituents. The same conditions are used both for the deposition of the polymer films and for their characterization, which allows for the establishment of a clear relationship between the chemical structure and the charge carrier mobility.
Using poly(vinyl alcohol) as a drug model, the swelling behaviors of thermosensitive poly(N‐isopropylacrylamide) gel with various concentrations of PVA are investigated. The swelling ratio in PVA9000 is larger than that of PVA89000 at the same composition because the fully hydrolyzed PVA acts as a steric hindrance. A lattice‐based molecular thermodynamic model is used to obtain interaction parameters and then directly applied to describe the swelling behaviors of the gel. Using only one adjustable parameter, the PVA selectivity can be theoretically predicted and the ternary phase diagrams of PNIPA/water/PVA system can be generated. Lastly, the calculated results are compared with the swelling data for PNIPA/PVA copolymer gel, and are found to be in good agreement with the proposed model.
2‐Hydroxyethyl methacrylate (HEMA) is incorporated to N,N‐dimethylaminoethyl methacrylate (DMAEMA) fabricating a series of stimuli‐responsive copolymer hydrogels by free radical copolymerization. The increasing comonomer content via adding more HEMA results in a higher network density and, therefore, a lower equilibrium swelling ratio. The uniaxial compressive testing results showed that increasing HEMA in the feed improved the mechanical strength of P(DMAEMA‐co‐HEMA) hydrogels. As HEMA increased, the elastic modulus as well as the effective crosslinked chain density increased. The work strives to provide method to tune mechanical and physical properties for weakly basic copolymer hydrogels based on (meth)acrylate polycations, which is hopefully to guide the design of hydrogels with desirable properties.
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.
Highly monodisperse poly(2‐hydroxyethyl acrylate)‐grafted poly(methyl methacrylate) (PMMA) microspheres are prepared by a facile photoinitiated dispersion polymerization in ethanol/water mixtures at room temperature. Poly(2‐hydroxyethyl acrylate) homopolymers with different molecular weights are synthesized via reversible addition‐fragmentation chain transfer (RAFT) solution polymerization and employed as steric stabilizers in photoinitiated dispersion polymerization. The effect of macro‐RAFT agent (both concentration and molecular weight) on particle diameter is studied in detail. Kinetic study indicates that the polymerization proceeded rapidly with 96% yield being reached with 1 h of UV irradiation. Finally, the obtained hydroxyl‐rich PMMA microspheres are used as building blocks for the preparation of covalently cross‐linked colloidosomes.
Size matters: the interface of nanodroplets in an inverse miniemulsion is used to produce polyphosphate nanocapsules by interfacial polycondensation. Phosphorus acts as a probe for 31P NMR spectroscopy to compare the polycondensation at the nanoscopic droplet with macroscopic interfaces proving less unwanted hydrolysis in miniemulsion by 1H–31P 2D‐heteronuclear multiple bond correlation spectra.
Poly(2‐(3‐butenyl)‐2‐oxazoline)s (PBOX) with glucose‐S‐butyl (Glc) and perfluoroalkyl‐S‐butyl (F) side chains (three samples: Glc/F = 100/0, 0/100, and 88/12) are synthesized by ring‐opening polymerization of 2‐butenyl‐2‐oxazoline and thiol‐ene click photochemistry, and their thermal properties are analyzed by direct‐pyrolysis mass spectrometry. Significant changes in the thermal stability and thermal‐degradation products are observed depending on the structure of the side chain. The thermal degradation of PBOX‐Glc and PBOX‐F homopolymers starts with loss of side chains at relatively low temperatures and successive cleavage along the side and main chains follows. The stability of the glucose units of the PBOX‐Glc/F copolymer is significantly increased by the presence of perfluoroalkyl groups, attributable to OH···F hydrogen bonding interactions during pyrolysis.
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.
Biocompatible polymer nanoparticles are of high interest in chemical, biological, and medical research as potential drug delivery systems. These nanoparticles must be nontoxic and metabolizable. Poly(lactic acid) (PLA) meets these requirements and therefore is widely used for biomedical applications. The miniemulsion/solvent evaporation procedure is a very efficient approach to produce PLA nanoparticles with controlled size and morphology using preformed polymer. In this paper, the synthesis of PLA‐based nanoparticles functionalized with either carboxyl groups or fluorescent molecules is presented. Both functionalities are covalently attached to the particle, thus any physical desorption and leakage of the dye upon storage or experiments performed under physiological conditions, for example during uptake by living cells, can be avoided. The fluorescence properties and stability of the obtained nanoparticles in aqueous solutions with different ionic strength are studied by fluorescence correlation spectroscopy and dynamic light scattering.
An N‐vinylimidazole‐based monomer carrying an adamantyl group, (1‐vinyl‐2‐yl)methyladamantan‐1‐ylcarbamate ( 3 ), is synthesized. The terpolymerization with N‐vinylimidazole ( 4 ) and 1‐vinyl‐2‐(hydroxymethyl)imidazole ( 2 ) yields terpolymers ( 5a–b ) with high upper critical solution temperature (UCST)‐type transitions in water. The solubility can be influenced by the addition of various amounts of randomly methylated β‐cyclodextrin (RAMEB‐CD). It is found that supramolecular host/guest interaction clearly controls the solution behavior.
Early stages of polymeric nanoparticle formation are monitored by in situ synchrotron small‐angle X‐ray scattering (SAXS) experiments. A new strategy enables the preparation of stable polymeric nanoparticles from homopolymers (not copolymers) of one type only and without any assembly‐triggering additives. Poly(ethylacrylic acid) (PEA) in an aqueous solution is tuned to the pH, where it exhibits thermosensitivity and the formation of nanoparticles is temperature‐induced. The kinetics of this formation are clearly monitored by SAXS. Experiments indicate the slightly elongated, not exactly spherical shape of the resulting nanoparticles, which is also confirmed by cryo‐transmission electron microscopy (cryo‐TEM). Quasielastic neutron scattering (QENS) shows that the PEA nanoparticles are relatively loose with a lot of solvent inside, in agreement with results from previous light scattering work.
Commercially available hydrophobic porphyrins are investigated as an environmentally friendly catalytic system for iron‐mediated atom transfer radical polymerization (ATRP) of poly(ethylene glycol) monomethyl ether methacrylate (PEGMA). Polymerizations in organic solvent are optimized using activators generated by electron transfer (AGET ATRP), based on iron(III)–porphyrin complexes, and tin(II) 2‐ethyl hexanoate or ascorbic acid as a reducing agent. Copper‐free PPEGMA macromolecules are obtained with high conversion, controlled molecular weight, and polydispersity index compared with standard copper‐based ATRP. The facile preparation and availability of the catalyst, together with its expected low toxicity, represent clear advantages for the synthesis of PPEGMA‐based materials for biomedical use.
下载免费PDF全文