Anomalous rheological properties of polyethylene molecular composites

The effect of molecular weight on the rheological properties in the molten state has been studied for binary blends of high‐density polyethylene obtained by the Zieglar–Natta catalyst and low‐density polyethylene produced in an autoclave process. The blends composed of high‐density polyethylene with a high molecular weight and low‐density polyethylene show a higher drawdown force than the individual pure components, whereas the blends of high‐density polyethylene with a low molecular weight and low‐density polyethylene do not exhibit anomalous behavior. The pronounced drawdown force for the former blend system is attributed to the viscous enhancement in the linear viscoelastic region as well as the nonlinear strain‐hardening behavior in the elongational viscosity. POLYM. ENG. SCI. 46:1284–1291, 2006. © 2006 Society of Plastics Engineers     

Miscibility in molecular composites of polyamide-imide/polyetherimide

Summary Compositional dependence of miscibility in molecular composites of polyamide-imide/polyetherimide prepared by coagulation of the blend of the two polymers from dimethyl acetamide solutions has been established by calorimetric and dynamic mechanical thermal studies. This study reports on the thermodynamics of miscibility in the molecular composites.     

Thermal analysis studies on thermoplastic rigid rod molecular composites

Rigid rod molecular composites involve molecular level mixing of two polymers having greatly disparate chain conformations (one rodlike and one flexible coil like). Morphology studies have shown that in such materials the bundle diameter of these rigid rod molecules is less than 5 nm. The rigid rod molecule used is poly(para-phenylene benzobisthiazole) (PPBT). The thermoplastic flexible polymer matrix is Nylon-66, DuPont's Zytel 42. Thermal analysis studies on these materials indicate that the expected T_g shifts, as observed for blends of two flexible polymers, and depression of T_m with increased rod molecule composition does not occur.     

聚合物／聚合物分子复合材料的研究动态

综合讨论了聚合物／聚合物分子复合材料的定义、特征和优势,重点介绍了分子复合材料相容的有效途径及分子复合材料的常用制备方法。并简述了热固性分子复合材料的研究情况及分子复合材料研究前景。     

蜂窝状块体硅胶/分子筛复合物吸附性能

采用静态法测定了蜂窝状块体硅胶/分子筛复合物的吸附性能,并采用吸附床结构模拟转轮除湿过程,测定了复合物动态吸附性能;探讨了复合物在吸附过程中温、湿度分布;研究了处理风参数（风速,进口温度、湿度）对块体吸附剂吸附性能的影响。结果表明：块体硅胶/分子筛复合物的除湿性能明显高于硅胶,尤其在较高温度和较低湿度条件下。在处理风温度较低、风速较小及处理风湿度较大的前提下,块体复合物除湿性能较好。     

Tailoring molecular structure via nanoparticles for solvent-free processing of ultra-high molecular weight polyethylene composites

Composites of conventional ultra-high molecular weight polyethylene (UHMWPE) and nanoparticles, such as carbon nanotubes or ceramics, require special processing techniques due to the high melt viscosity of the polymer matrix. Recently, we have shown that polymerization with a single-site catalytic system in suitable reaction conditions produces “disentangled” UHMWPE that can be processed in the solid state. In this study, nanoparticles have been used as carriers for the single-site catalytic system in the polymerization of UHMWPE. The high-surface area of the nanoparticles, coupled with controlled reaction conditions, favors the growth of polyethylene chains with a reduced number of entanglements. This novel synthetic route offers several advantages: 1) the catalytic system is more stable and less fouling occurs during the polymerization reaction; 2) nanoparticles are directly embedded in an otherwise intractable polymer matrix; 3) the low amount of entanglements in the UHMWPE matrix allows the resulting composites to be processed in the solid state well below the equilibrium melting temperature in a broad temperature window, to give high strength/high modulus tapes.     

Piezoelectric properties of polypeptide-PMMA molecular composites fabricated by contact charging

The most common piezoelectric materials (PM) in use today are ceramic crystals which are heavy and brittle despite high piezoelectricity. Polymer-based PM is an alternative to ceramic crystals but they have lower piezoelectric coupling constants which deteriorates quickly at high temperature. In an effort to develop non-brittle and light PM with stable piezoelectric properties, we explored fabrication of composite materials comprising piezoelectric α-helical poly(α-amino acids), poly(γ-benzyl α,l-glutamate) (PBLG) and matrix polymer, poly(methylmethacrylate) (PMMA). Thick composite disks were created by contact charging of PBLG-MMA solution mixture followed by curing the MMA matrix in a designed mold. Compared to our prior method of corona discharge, this new method allowed the application of predefined electrical fields to the PMMA solution with little MMA evaporation. This communication presents the fabrication and characterization of a series of PBLG-PMMA composite disks with various PBLG compositions prepared under different poling conditions. The results show for the first time that all PBLGs can be poled in the direction normal to the disk surface and that the poled PBLGs within the PMMA matrix are directly responsible for the piezoelectricity of the composite materials. The two-polymer composite system allows independent modulation of film’s mechanical properties and piezoelectricity at a molecular level.     

Bisbenzocyclobutene: A thermoset matrix host for rigid-rod molecular composites

An approach to realize the reinforcement of a thermoset system at the molecular level by rigid-rod polymers was investigated. A mixture of bisbenzocyclobutene (BCB)-terminated imide oligomers constitutes the thermosetting component, and the rigid-rod polymer utilized in the present study was poly(p-phenylene benzothiazole) (PBT). The cure chemistry of the thermoset materials is based on the ability of benzocyclobutene functions to homopolymerize under the influence of heat. Thermal properties as well as film processing and mechanical properties of PBT/BCB thermoset composites are presented.     

Synthesis of hyperbranched polybenzoxazoles and their molecular composites with epoxy resins

Hyperbranched aromatic polymers have attracted great attention recently because they combined the processability of hyperbranched polymers and the high‐level performance of aromatic polymers. Here, a one‐pot strategy for the synthesis of hyperbranched Polybenzoxazoles (HBPBOs) by polycondensation of 2,2‐Bis (3‐amino‐4‐hydroxyphenyl) hexafluoropropane and 1,3,5‐benzenetricarboxylic acid in Polyphosphoric acid was reported. The HBPBOs exhibited good solubility in organic solvents because of the branched structure and the flexible hexafluoropropane groups in main chains. The structure and terminal functional groups could be tailored by adjusting the molar ratio of two monomers. FT‐IR, NMR and XRD measurements confirmed the structure of HBPBOs, while thermogravimetric analysis (TGA), UV‐vis, and photoluminescence spectra, combined with the comparison with linear PBOs demonstrated the intriguing optical properties and good thermal stabilities of HBPBOs. The good solubility of HBPBOs also permitted their usage as molecular reinforcement for polymer composites as demonstrated in this study of HBPBOs/epoxy composites. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41942.     

Functionalizing and molecular bonding nanoscale silicate-polymer composites of epoxies and Polyacrylates

The nanometer-thin silicate platelets (NSP) were prepared by exfoliating sodium silicate clays (Na⁺-MMT). The silanol groups on the edge of NSP were organically functionalized via covalent bonding with 3-aminopropyltriethoxysilane (APTES) through sol-gel reaction to generate the amine-functionalized NSP (NSP-amines). Moreover, the NSP-amines were further allowed to react with glycidyl methacrylate (GMA) through epoxy-ring opening reaction to afford the NSP with covalently bonded acrylates (NSP-acrylates). The blending of NSP-acrylate in an UV-curable resin afforded the coating films of improved hardness from 4H to 6H by loading only 0.5 wt% and to 8H by loading 1.0 wt% of NSP-acrylate. By comparison, blending NSP-amines instead of NSP-acrylates in thermal-curable epoxy resins improved the films’ hardness from HB to H, 2H, 3H, and 4H when loading 0.5 wt%, 1 wt%, 2 wt%, and 3 wt%, respectively. The reversed utilization of using NSP-acrylates in epoxy and NSP-amines in acrylate resin demonstrated the nanocomposites of less effect than the analogous examples. The NSP-amines and NSP-acrylates were characterized by ²⁹Si solid-state NMR for the evidence of Si-O-R bonding and the organic fraction variation was studied by thermo-gravimetric analysis (TGA).

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Graphical abstract The conventional method of fabricating nanocomposites by the dispersion technique was further advanced by the molecular bonding with nanoclays, which was exemplified by nanometer-thin silicate platelets (NSP) in tethering amine and acrylate monomers. The covalently bonded NSP-monomers participated the resin polymerization into the network matrices in molecular-level connection, and subsequently enhanced their physical properties such as hardness, transparency and scratch-free in film performances in epoxies and acrylate resins.

     

Comparative studies of ultra high molecular weight polyethylene fiber reinforced composites

Ultra high molecular weight polyethylene fiber is a very promising material for making light-weight high strength and high impact resistant composites, especially for ballistic protective shields. Three commercially available materials designed specifically for ballistic applications are Spectra® woven cloth, Dyneema Fraglight® nonwoven felt, and Spectra Shield® Plus PCR prepreg were chosen for parallel comparisons. The high-temperature high-pressure sintering process was applied to all three materials. The physical, thermomechanical, and microstructural properties of the consolidated products were studied and compared, including their crystallinity, molecular orientation, impact resistance, interlaminar adhesion, flexural properties, and thermoformability. The differences in these materials and their structures are reflected in the different properties of the final products. The influence of different processing conditions on the properties also differs for each material. It is concluded that matrix free Spectra cloth composite has dominant advantages over the other two materials. POLYM. ENG. SCI., 47:1544–1553, 2007. © 2007 Society of Plastics Engineers     

The morphology of rigid-rod polymers and molecular composites resulting from coagulation processing

The effects of processing conditions on the morphology of molecular composite films are examined by optical and electron microscopy. During coagulation processing from solutions in methanesulfonic acid (MSA), rigid-rod polymer, such as poly(p-phenylenebenzobisoxazole) (PBO), phase separates into undesirable aggregates. The coagulant and the method of its introduction have been found to exert a strong impact on the final film morphology. A quench of a PBO solution in MSA into a water bath results in a three dimensional interconnected network of PBO, while a slower introduction of water results in a more amorphous material. A computer simulation program of the coagulation process has been developed to better understand the different structures emerging from coagulation processing of molecular composites. The simulation results are in qualitative agreement with the experimental observations.     

Miscibility in poly-p-phenylene terephthalamide/nylon 6 and nylon 66 molecular composites

Miscibility and crystallization of poly-p-phenylene terephthalamide (PPTA)/nylon 6 and nylon 66 composites prepared by coagulation of isotropic ternary sulfuric acid solutions were studied. The apparent crystallinity of nylon 6 and nylon 66 in molecular composites was investigated by differential scanning calorimetry and Fourier transform infrared spectroscopy. The solvation of nylon 6 crystals in PPTA matrix was observed when the PPTA content exceeds 70 wt%. Cross-hydrogen bonding seems to be responsible for the virtual disappearance of nylon 6 crystals. Specific interaction between PPTA and nylon 6 macromolecules and phase separation during thermal treatment has been discussed.     

Miscibility and mechanical properties of aryl-aliphatic polyamides/nylon 6 molecular composites

Summary A series of aryl-aliphatic polyamides, copoly(4,4-diaminobenzanilide-adipamide/2,6-naphthalamide)s, with feed mole ratios of adipic acid/2,6-naphthalene dicarboxylic acid of 1/9, 3/7, and 5/5 were synthesized. A new family of molecular composites based on the synthesized aryl-aliphatic polyamides and nylon 6 has been discovered. The molecular composites were found to have at least partial miscibility between aryl-aliphatic polyamides and nylon 6. Well-defined aryl-aliphatic polyamide microfibrils a few nanometers in diameter were observed in the molecular composites. 10 wt% aryl-aliphatic polyamide clearly promoted the toughness of nylon 6.     

Thermal decomposition of poly(ethylene terephthalate)/mesoporous molecular sieve composites

The nonisothermal and isothermal degradation processes of poly(ethylene terephthalate)/mesoporous molecular sieve (PET/MMS) composites synthesized by insitu polymerization were studied by using thermogravimetric analysis in nitrogen. The nonisothermal degradation of the composite is found to be the first-order reaction. An isoconversional procedure developed by Ozawa is used to calculate the apparent activation energy (E), which is an average value of about 260 kJ/mol with the weight conversion from 0% to 30%, and is higher than that of neat PET. Isothermal degradation results are confirmed with the nonisothermal process, in which PET/MMS showed higher thermal stability than neat PET. The polymer in mesoporous channels has more stability due to the protection of the inorganic pore-wall. These results indicate that mesoporous MMS in PET/MMS composites improve the stability of the polymer. Translated from Polymer Materials Science and Engineering, 2006, 22(1): 64–67 [译自: 高分子材料与工程]     

Electrostatic phenomena and molecular motion at interfaces of glass/polymer composites

A boundary zone of filler-affected polymeric network is a possible element in the microstructure of composites. The intensity of adsorption potentials for polymeric material on the filler and therefore its surface energy should be an important variable in boundary zone properties. This work has involved the modification of surface energy in a glass filler material through the introduction of externally induced electrostatic charge that is through the creation of a ‘synthetic’ zeta potential on filler particles. The interactions of charged and uncharged glass surfaces with a polymeric matrix were studied by thermally stimulated discharge (TSD), and also by contact angle measurements. The experimental system studied was an aluminosilicate glass as the filler material, and poly(tetraethylene glycol dimethacrylate) as the organic matrix. Contact angle measurements revealed enhanced wettability for the matrix-forming monomer on glass surfaces charged negatively by electric fields. TSD analysis was also carried out on pure polymer, and composite materials containing either uncharged or bipolar filler particles (particles exposed to an external electrical potential). TSD spectra suggest a suppressed level of molecular motion in composites with fillers of high electrostatic surface energy. Also, activation energies calculated from TSD data are higher in bipolar filler composites. The higher activation energies are consistent with the possibility that a less mobile interfacial zone polymerizes around higher surface energy filler particles.     

Rheological behavior of low molecular weight polystyrene composites containing monodisperse crosslinked polystyrene beads

Steady shear viscosities and dynamic moduli of polymer composites, consisting of crosslinked polystyrene beads and low molecular weight polystyrene matrix, were measured in a cone-and-plate rheometer at different temperatures. Viscosities and dynamic moduli were found to be very sensitive to filler loading and measurement temperature. Steady shear viscosities of 30% and 40% loaded low molecular polystyrene composites showed a power-law behavior over the entire range of shear rates. Storage and loss moduli were initially linear with frequency on double logarithmic plots, with limiting slopes of 0.3 and 0.1. At high concentration of filler particles, they showed a flat plateau at low frequencies, indicating that these systems exhibit a yield behavior. A 20% PS composite loaded with beads of high crosslink densities resulted in poor dispersion of beads as a result of poor dispersion of particles. PS beads 1.16 μm in diameter showed a higher viscosity. It is due to the apparent increase in loading resulting from broken particles. At low measurement temperature, filler effects were suppressed by high viscosity matrix and showed a similar rheological behavior to high molecular weight by PS matrix. We suggest that rheological behavior reflects the state of dispersion of beads in the matrix.     

Molecular composites for molecular reinforcement: A promising concept between success and failure

The basic principles of molecular reinforcement and especially the specific approaches to obtain homogeneous composites with molecularly dispersed rigid rods are focused on and discussed. Brief overviews and successful examples of the available data covering the main characteristics are summarized.