Thermal Properties of Polyacrylates at Metal Interfaces

Summary  The thermal properties of polyacrylates films sandwiched with metal were studied by differential scanning calorimetry. There were two glass transition relaxations, and the relaxation at the lower temperature, which was almost the same as the glass transition temperature of a bulk sample, diminished with thinning the polymer film. The glass transition relaxations at the higher temperature can be explained by the segmental motion of polymer chains interacting with metal surfaces at the interface. The results for the dielectric measurements for the sandwiched films also showed the similar relaxation modes.     

热处理对PPS结晶度的影响分析

利用差示扫描量热仪研究了不同温度热处理对聚苯硫醚(PPS)树脂的结晶度影响。结果表明,在玻璃化转变温度与熔融温度之间,PPS树脂的结晶度随热处理温度的升高呈先增大、后稳定、再减小的趋势,相对结晶度最大值为65.36%,最小值为46.14%。这是由于PPS分子活性、分子链的热氧交联反应随热处理温度的不同具有不同的表现形式,这些因素会直接影响PPS聚合物的晶核形成和晶体生长,进而影响PPS的结晶度。     

Effects of Heating,Tensile, and High Pressure Treatments on Aggregate Structure of a Low Density Polyethylene

The effects of heating, tensile, and high pressure treatments on the aggregate structure of a low density polyethylene (LDPE) are studied via modulated DSC (MDSC) and dynamic mechanical technique (DMA). The results indicate that the nonreversing heat flow can reflect the relative thermodynamic stability for crystalline structure of the polymer, and this makes nonreversing heat flow be a novel useful tool to qualitatively characterize the perfect degree of crystalline structures. During thermal treatment, it is found that there exists a low temperature melting endotherm at about 35°C. It presents an endothermal peak in nonreversing heat flow and a simultaneous glasslike transition in reversing heat flow, which behaves a feature of physical aging far above the glass transition temperature of the polymer. This suggests the existence of so-called “rigid amorphous fraction” (RAF). The structural changes of polyethylene induced by different treating methods are also investigated by DMA, and the relaxation characteristics of macromolecular chains are analyzed.     

Hydroxylation and Dehydroxylation Behavior of Silica Glass Fracture Surfaces

The hydroxylation and dehydroxylation behavior of amorphous silica fracture surfaces was studied using temperature-programmed static SIMS. The results show that vacuum heat treatments result in more extensive condensation of silanol groups on the silica glass fracture surface as compared to fumed silica (Cabosil). This is attributed to differences in the distribution of silanol groups on the two silica surfaces. The rehydration kinetics of the dehydroxylated silica fracture surfaces showed two distinct reaction rates—an initial rapid increase in the silanol concentration, followed by a slower rehydration for longer dosing times. The slower rehydration reaction was shown to follow first-order reaction kinetics with the reaction rate constant, suggesting hydrolysis of strained siloxane bonds on three-membered silicate ring structures. The much faster initial rehydration is attributed to the hydrolysis of extremely strained siloxane bonds in two-membered, edge-shared tetrahedral rings. The effect of the dehydration time and temperature (i.e., thermal history of the surface) on the rehydration kinetics is also discussed.     

Molecular design and synthesis of crosslinked polyimides using radical isomerization of vinylcyclopropane with thiols

Reactive polyimides bearing a vinylcyclopropane (VCP) moiety in the main chain were successfully synthesized from the corresponding diamine with the VCP moiety. Their radical crosslinking using a dithiol proceeded with the radical ring-opening reaction (RROR) of the VCP moiety to afford the corresponding crosslinked polyimides with the CC bonds in crosslinking moieties. Thermal properties of those crosslinked polyimides were evaluated by thermal gravimetry and differential scanning calorimetry. As a result, the increase of the crosslinking degree in the crosslinked polymer exhibited great residual weight at 600°C. In contrast, the tendency of the glass transition temperature was inverse because the increase in the amount of dithiol unit as a crosslinker would enhance the mitigation of the polymer packing structure and activates the mobility of polymer chains.     

Direct synthesis of polymer-grafted inorganic hybrids via reversible chain transfer catalyzed polymerization

In this work, a new simple and robust method for preparation of polymer-grafted inorganic hybrids through “grafting to” reaction is presented. Polymer chains were synthesized by reversible chain transfer catalyzed polymerization (RTCP) are capped with iodine according to the RTCP mechanism. The obtained iodine-capped polymer chains can react irreversibly with the hydroxyl groups available on the surface of inorganic materials through a nucleophilic substitution (SN) reaction. In this method, there is no need to modify the surface of inorganic materials or to functionalize polymer chains prior to the “grafting to” reaction. RTCP produced polystyrenes with different molecular weights, e.g., 4,000, 6,000, and 8,000 g/mol, and silica nanoparticles were employed as the polymer and inorganic materials, respectively. The resulting hybrids were characterized by Fourier transform infrared spectroscopy, thermal gravimetric analysis, and transmission electron microscopy techniques. According to the results, graft density decreased by increasing the polystyrene molecular weight. Additionally, the rheological studies of prepared polystyrene nanocomposites containing 2 wt % of the produced hybrids confirmed the better dispersion of the modified hybrids in the polystyrene matrix. The glass transition temperature (T _g) of the polystyrene nanocomposites was driven by differential scanning calorimeter technique. Analysis of nanocomposites’ T _g results revealed that increment of the grafted polymer molecular weight of hybrids increased the glass transition temperature of the prepared nanocomposites due to improvement of the dispersion level.     

Effect of water absorption on physical properties of high nitrile barrier polymers

Water absorption studies were made on a series of high nitrile barrier polymers, as a function of polymer composition, rubber content, temperature, orientation and relative humidity. Results indicate that absorbed water has a great influence on both mechanical and thermal properties of the polymer. Absorbed water in high nitrile polymers causes a large decrease in the glass transition temperature and yield stress. These effects are found to depend on polymer composition, amount of absorbed water, and degree of orientation. The significance of these results is discussed in relation to end-use properties of the polymer.     

The influence of the compatibilizer on the morphology and thermal properties of polypropylene‐layered double hydroxide composites

Polypropylene (PP)/layered double hydroxide (LDH) composites were prepared via melt‐compounding using both a carbonate‐LDH and an organo‐LDH (dodecyl benzene sulfonate DBS‐LDH) in different concentrations. Transmission electron microscopy and X‐ray diffraction analysis were used to investigate the morphology. The results showed that only by using DBS‐LDH the intercalation of polymer chains and a partial delamination were obtained. However, the introduction of maleic anhydride‐grafted polypropylene (PP‐g‐MAH), as coupling agent, favored the aggregation of the particles generating localized domains of aggregates. The thermo‐gravimetric analysis showed that PP/DBS‐LDH composites have a higher thermal stability than the pure matrix. Differential scanning calorimetry evidenced that both LDH and DBS‐LDH particles acted as nucleating agents increasing the crystallization temperature, even if, in the case of LDH the effect was observed only with the addition of the compatibilizer. The results collected by dynamic mechanical thermal analysis, beyond showing a significant increase of the matrix stiffness by incorporation of DBS‐LDH, evidenced an increase of the PP glass transition temperature (T_g) indicating a restriction of PP chain segment mobility due to the strong polymer‐particle interactions. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Disruption of self-assembly and altered mechanical behavior in polyurethane/zinc oxide nanocomposites

The addition of less than 1 vol%, 33 nm zinc oxide nanoparticles into a polyurethane matrix resulted in approximately 40% decrease in the Young's modulus, 80% decrease in strain at fracture, and 50% decrease in the storage modulus, but at the same time resulted in an ∼11 °C increase in the glass transition temperature of the polymer. These results appear to contradict the general principle observed for many polymeric systems, where higher glass transition temperature generally means higher elastic modulus. Detailed experiments with FTIR, DMTA, FE-SEM, and AFM indicated that the addition of ZnO nanoparticles disrupts the phase separation in the polymer, resulting in weaker mechanical properties. The special interaction between the particles and polymer possibly constrains the mobility of polymer chains, which increases the glass transition temperature. The most likely reason for the disruption and the nature of the interaction is the reaction between the surface hydroxyl groups of the zinc oxide nanoparticles and the isocyanate groups of the polyurethane pre-polymer.     

Properties of bulk-polymerized thermoplastic polyurethane nanocomposites

The thermal, rheological, and mechanical properties of bulk-polymerized thermoplastic polyurethane nanocomposites of reactive and non-reactive layered silicate clay were characterized as a function of the state of dispersion of particles. True exfoliated nanocomposites were produced by mixing reactive clay particles with polymer chains carrying residual isocyanate groups. On the other hand, non-reactive clay particles yielded only intercalated composites. Most significant improvement in mechanical properties were obtained when clay particles were fully exfoliated, e.g. 110% increase in tensile modulus, 170% increase in tensile strength, 110% increase in tear strength, 120% increase in fracture toughness, and 40% increase in abrasion resistance over pristine polyurethane with 5 wt% clay. In addition, the terminal dynamic rheological data showed strong dependence on the clay content, indicating substantial hindrance to chain relaxation by tethering clay particles. The peak location and the area under the peak of hydrogen-bonded carbonyl showed two distinct zones of temperature dependence, which indicate additional hydrogen bonding between polymer chains and organic modifier of reactive clays.     

Flexibility Improvement of Epoxy Resin by Using γ-Butyrolactone

The addition reaction of γ-butyrolactone with epoxy resin was carried out in CCl₄ solution, with BF₃:MEA added as a catalyst. The trioxaspiro-ortho-estering structure of prepolymer was obtained first. The ring structure was opened to form a crosslinked structure of polymer at higher temperature. The structures of prepolymer and polymer have been indentified by the infrared spectra. The thermal stability of polymer was evaluated by thermo-gravimetric analysis. It was found that the glassy temperature of the polymer increased, and that the thermal stability of the polymer in the early stage decreased with an increase in the amount of BF₃: MEA catalyst added. Moreover, the mechanical properties of a laminate made of γ-butyrolactone-modified epoxy resin with glass fiber cloth have been measured. The results indicated that the break energy decreased and the flexibility increased with an increase in the content of the modified resin in the laminate.     

Peroxide crosslinking of a styrene‐free unsaturated polyester

Thermoset unsaturated polyesters are usually obtained by the crosslinking of unsaturated polyester chains dissolved in an unsaturated, reactive, monomeric diluent, which is usually styrene. This article describes a new approach in which styrene‐free unsaturated polyester chains are intrinsically cured into a crosslinked matrix. The gel time, gel content, swelling degree, glass‐transition temperature, dynamic mechanical properties, tensile properties, and molecular weight between crosslinks (calculated according to both the Flory–Rehner equation and the theory of rubber elasticity) of the crosslinked polymer are studied as a function of the peroxide concentration. All properties change considerably upon the addition of small amounts of peroxide (between 1 and 2 wt %) and change to a lesser extent with higher peroxide concentrations (up to 6 wt %). The thermal properties of the isolated gel fraction are studied as a function of the peroxide concentration. The sol fraction demonstrates a plasticizing effect on the crosslinked network, affecting the glass‐transition temperature and stress–strain behavior of the crosslinked polymer. In light of the crosslink densities derived from swelling experiments, a molecular structure and crosslinking mechanism are suggested for the gel fractions of 1 and 6 wt % peroxide crosslinked unsaturated polyester chains. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Dielectric studies of hyperbranched aromatic polyamide and polyamide‐6,6 blends

The objective of this work was to study the interactions between polyamide‐6,6 (PA‐6,6) and hyperbranched (HB) polyamide with different functional end groups. The investigation focused on the thermal, dielectric, and viscoelastic properties of two kinds of HB polyamides, with amine and alkyl end groups, prepared by a one‐pot process, in a polyamide‐6,6 matrix. Thermal analysis (by TGA and DSC) allowed us to observe decomposition and glass‐transition temperatures of these polymers. The melting point, crystallization temperature, and crystallinity ratio remained practically independent of HB content. Dielectric relaxation spectroscopy (DRS) showed two secondary relaxation (γ and β) and one primary (α) relaxation in the HB polymers and in the blends similar to those observed in polyamide‐6,6 with comparable activation energies and distribution parameters. An increase of the glass‐transition temperature was observed, showing a reinforcement of the polymer matrix and a decrease of the molecular mobility of the polyamide chains when the percentage of amine‐terminated HB polyamide increased in the matrix. DRS results found on the alkyl‐terminated HB polymer blend were indistinct from those of the polyamide‐6,6 matrix. Viscoelastic experiments confirmed the results observed in DRS. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1522–1537, 2005     

Molecular modeling of particle reinforcement in elastomers: Effect of particle radius and volume fraction

Monte-Carlo simulations of the configurations of polymer chains in reinforced elastomers reveal extensive wrapping of the chains around particle clusters. The resulting increase in chain entanglement density leads to a 3.5 power law dependence of modulus on particle loading, in perfect agreement with experimental observation. Our model also gives a consistent molecular explanation of the Payne effect and the associated energy dissipation in strained reinforced elastomers.     

Necking mechanism and its elimination in uniaxially drawn films of poly(ethylene naphthalate) (PEN)/polyetherimide (PEI) blends

The influence of blend composition on the deformation behavior of cast amorphous PEN/PEI blends were investigated above their respected glass transition temperatures. PEN inherently shows a sharp necking phenomenon when stretched at temperatures as high as 20°C above its glass transition temperature. This was attributed to highly localized rapid alignment of naphthalene planes parallel to the surface of the films. The addition of PEI was observed to reduce this necking behavior. The neck formation completely disappears when the PEI fraction exceeds 10% in the blend. X-ray studies indicate that the increase of PEI hinders the rapid alignment of naphthalene planes parallel to the surface of the films. The presence of PEI chains in the blend was found to increase the overall friction between the polymer chains in the system and this was found to prevent the formation of highly localized necks. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2059–2074, 1997     

Torrefied biomass‐polypropylene composites

Torrefied almond shells and wood chips were incorporated into polypropylene as fillers to produce torrefied biomass‐polymer composites. The composites were prepared by extrusion and injection molding. Response surface methodology was used to examine the effects of filler concentration, filler size, and lignin factor (relative lignin to cellulose concentration) on the material properties of the composites. The heat distortion temperatures, thermal properties, and tensile properties of the composites were characterized by thermomechanical analysis, differential scanning calorimetry, and tensile tests, respectively. The torrefied biomass composites had heat distortion temperatures of 8–24°C higher than that of neat polypropylene. This was due to the torrefied biomass restricting mobility of polypropylene chains, leading to higher temperatures for deformation. The incorporation of torrefied biomass generally resulted in an increase in glass transition temperature, but did not affect melting temperature. Also, the composites had lower tensile strength and elongation at break values than those of neat polypropylene, indicating weak adhesion between torrefied biomass and polypropylene. However, scanning electron microscopy results did indicate some adhesion between torrefied biomass and polypropylene. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41582.     

Molecular Origin of the Influence of the Temperature on the Loss Factor of a Solid Propellant

This study focuses on the viscoelastic behavior of an industrial hydroxyl‐terminated polybutadiene (HTPB) based solid propellant. The analysis of the loss factor as function of temperature enables the investigation of the molecular mechanisms participating in the nonlinear viscoelastic behavior. A design of experiments determines the influences of the filler fraction, of the NCO/OH ratio, of the plasticizer content, and of the presence or absence of filler‐binder bonding agents. For all the tested materials, the loss factor as function of temperature exhibits two distinct peaks when measured by Dynamic Mechanical Analysis. Exponentially modified Gaussian distributions are applied on each peak to characterize the behavior. While the first peak is commonly associated with the rubber‐glass transition of the material, the second peak has not been clearly associated with a molecular mechanism. This study shows that the second peak of the loss factor in HTPB‐based solid propellants originates from the flow of free polymer chains in the polymer network with a reptation mechanism. The sol polymer fraction controls the area of the second peak, whereas its temperature at the maximum corresponds to an activation temperature determined by the molar masses of the sol polymer. Finally, when the propellant is stretched, a decrease in area and an increase in the temperature of the peak show that the reptation of the sol polymer chains is constrained by the network.     

The influences of plasticizer B2 mass fraction on the performances of CAB/B2 polymer composite materials: Combining experiments and simulations

Cellulose acetate butyrate (CAB)/B2 (50 wt% bis(2,2-dinitropropyl)-acetal and 50 wt% bis(2,2-dinitropropyl)-formal) polymer composite material is a commonly used binder system to prepare polymer bonded explosives (PBXs). However, few literature report the influence of plasticizer B2 mass fraction on the performance of CAB/B2 polymer composite binder systems. In this study, experiment measurements and computer simulations are utilized to explore the influences of B2 mass fraction on the glass transition temperature, mechanical property, and thermal decomposition of CAB/B2 polymer composite materials. The results show that, with an increase in B2 content, the glass transition temperature and fragility of CAB/B2 are decreased, and the plasticity and ductility of CAB/B2 are enhanced. Moreover, the thermal decomposition mechanism, decomposition products, and decomposition path of CAB/B2 are unchanged with the increase in B2 content. Besides, the researchers also demonstrate that the addition of B2 is positive to the formability of CAB/B2 systems and is negative to the thermal stability of CAB/B2 systems, whereas the content of B2 has little influences on the thermal stability of CAB/B2 systems. The results obtained from this work can provide some guidance for the designs of high-energy density PBXs.     

On the chain cross-sectional area and motion of macromolecular chains

This paper attempts to relate the chain cross-sectional area to the glass transition temperature of a polymer and to discuss the effect of the chain cross-sectional area in view of the motion of molecular chains. It has been found that the definite relationship between glass transition temperature and cross-sectionalarea can be obtained only when taking account of intermolecular interaction of polymer chains. It is considered that the chain cross-sectional area will characterize the chain flexibility of a polymer since the glass transition temperature is related both to intermolecular interaction and chain flexibility of a polymer. The concept of the structural parameter cross-sectional area per chain, first introduced by Vincent¹ and used by Boyer and Miller,^2–5 is useful in empirically correlating properties and structures of polymers. The glass transition temperature is a basic parameter of bulk polymers and is characteristic of their intermolecular interactions and chain flexibility. This paper attempts to relate chain cross-sectional areas to glass transition temperatures of polymers and to discuss the effect of the chain cross-sectional area in view of the motion of molecular chains.     

Wettability of glass substrates by molten nylon-6

A study of the adhesion of molten nylon-6 to glass, vitreous silica and dehydrated silica is performed by the sessile drop method at 243°C. The influence of polymerization conditions, nature of substrate, thermal pretreatment of the polymer and lithium chloride addition on wettability is examined. The most relevant results show that thermal pretreatment of the polymer is a determining step in wettability and that LiCl addition causes an increase of contact angle, as a consequence of the decrease in the substrate-molten polymer interactions. The results are interpreted by means of an interfacial model based on additive polar and dispersion forces.