Rheological investigation of the influence of molecular structure on natural and accelerated UV degradation of linear low density polyethylene

Metallocene and Ziegler-Natta (ZN) linear low density polyethylenes (LLDPEs) of different branch types and contents as well as linear high density polyethylene (HDPE) were exposed to natural and accelerated weather conditions. The degree of UV degradation of exposed samples was measured by rheological techniques and results were compared with unexposed polymers. Dynamic shear measurements were performed in an ARES rheometer in the linear viscoelastic range. The degree of enhancement or reduction in viscosity and elasticity was used as a measure of the degree of cross-linking or chain scission, respectively. The degradation results of LLDPE suggest that both cross-linking and chain scission are taking place. Chain scission dominated the degradation at high levels of short chain branching (SCB) and long exposure times. The degradation mechanism of m-LLDPE and ZN-LLDPE is similar; however, m-LLDPE showed a higher degradation rate than ZN-LLDPE of similar M_w and average SCB. ZN-LLDPE was found to be more stable than a similar m-LLDPE. Comonomer type had little influence on degradation. Dynamic shear rheology was very useful in revealing the influence of different molecular parameters and it exposed the degradation mechanism.     

微波无极紫外光助双氧水处理活性艳红X-3B染料废水研究

活性艳红X-3B(C.I.ReactionRed 2)是一种单偶氮染料,其应用范围广泛,可用于棉、粘纤、涤/棉、锦纶、蚕丝、羊毛、锦/粘等织物的染色和丝绸的印花,因此其废水量大且色度深。目前染料废水的生物处理方法脱色效果差、占地面积大、处理周期长,传统的物理法和化学法也存在着成本高、不能彻底矿化染料分子等缺点。近年来发展起来的光化学处理染料废水的方法及各种光化学组合技术日益受到人们关注,但其中使用的普通紫外灯存在使用寿命短、启动慢、发光不稳定、电极材料易损等缺点[1],致使光化学方法处理废水的成本很高。本文利用微波产生的高频…     

On the effectiveness of different additives and concentrations on the re-building of the molecular structure of degraded polyethylene

Mechanical recycling is an easy and economic way to re-use plastic waste as secondary materials, but, in general, their properties are worse with respect to the reclaimed materials and the virgin polymer. The aim of this work was to study the effect of concentration and reaction kinetics of two additives, an ethylene-co-glycidyl methacrylate (Lotader) and a hydroxylamine derivative (CGX), in the re-building of a degraded polyethylene. CGX is a nitroxyl radical generator able to form branching in polyolefins while the epoxy groups of Lotader can react with the functional groups present in the recycled polyethylene. The results indicate that the CGX has a higher reaction rate than Lotader, probably due to its lower molecular weight and the different reaction path. As for the effect of concentration, as expected, a higher amount of additive accelerates and increases re-building, especially when CGX is used. The melt strength increases with the additive content and the mechanical properties show a significant reduction at the highest concentrations of both additives due to excessive cross-linking.     

反应挤出接枝共聚反应表观链增长常数的测量

以预辐照的LLDPE反应挤出接枝MAA作为研究对象，通过在双螺杆挤出机中的不同部位取样，用FFIR分析接枝物及均聚物的产量，并用ESR测试链增长自由基浓度，设计了测定接枝和均聚反应初始阶段的表观链增长常数的方法,结果表明，在本实验测试范围内表观接枝链增长常数不同于均聚链增长常数；所得的测量值符合理论预测值，证明测试方法适合高温熔融体系的研究。     

Kinetics of the thermal degradation of poly(lactic acid) obtained by reactive extrusion: Influence of the addition of montmorillonite nanoparticles

A reactive extrusion-calendering process was used in order to manufacture sheets with a nominal thickness of 1 mm of poly(lactic acid) and its nanocomposite with 2.5% of an organo-modified montmorillonite. During processing, the properties of the melt were stabilized and enhanced by the addition of 0.5% of a styrene-acrylic multi-functional-epoxide oligomeric reactive agent. The general analytical equation has been used in order to evaluate the kinetic parameters of the thermal degradation of poly(lactic acid) obtained by reactive extrusion and its nanocomposite. Various empirical and theoretical solid-state mechanisms have been tested to elucidate the best kinetic model. In order to reach this goal, master plots have been constructed by means of standardized conversion functions. Given that it is not always easy to visualize the best accordance between experimental and theoretical values of standardized conversion functions, a new index has been developed to quantitatively discern the best mechanism. By doing that, it has been possible to determine the right activation energy of the thermal degradation. It has been demonstrated that the best theoretical mechanism was the random scission of macromolecular chains within the polymer matrix. This was also in accordance with an empirical kinetic model based on an autocatalytic kinetic model. The presence of montmorillonite nanoparticles has been beneficial and has enhanced the thermal stability of poly(lactic acid).     

Influences of molecular weight and crystalline structure on fracture behavior of controlled-rheology-polypropylene prepared by reactive extrusion

The molecular weight of ethylene-block-co-polypropylene (co-PP) was adjusted by reactive extrusion with the incorporation of dicumyl peroxide (DCP), and the effect of molecular weight on the crystallization behavior, crystal morphology, and fracture behavior was investigated. It was found that, with increasing DCP content, the molecular weight (MW) decreased and the polydispersity (M_w/M_n) slightly decreased. After modification, the number of spherulites with obscure boundaries increased, and the size of the spherulites was more even due to increasing amount of grafting and micro-cross-linking structures, generated in co-PP degradation, which were acting as nucleating agents. Evaluated by essential work of fracture method, the specific essential work of fracture, w_e, was found to be strongly dependent on the molecular weight, especially, on the number average molecular weight (M_n) linearly, while the specific non-essential work of fracture, βw_p, was enhanced with decreasing z-average molecular weight (M_z), probably owing to the reduction of ultra-high molecular weight component in degraded co-PP.     

New approach on the development of plasticized polylactide (PLA): Grafting of poly(ethylene glycol) (PEG) via reactive extrusion

In this work, new ways of plasticizing polylactide (PLA) with low molecular poly(ethylene glycol) (PEG) were developed to improve the ductility of PLA while maintaining the plasticizer content at maximum 20 wt.% PLA. To this end, a reactive blending of anhydride-grafted PLA (MAG-PLA) copolymer with PEG, with chains terminated with hydroxyl groups, was performed. During the melt-processing, a fraction of PEG was grafted into the anhydride-functionalized PLA chains. The role of the grafted fraction was to improve the compatibility between PLA and PEG. Reactive extrusion and melt-blending of neat and modified PLA with PEG did not induce any dramatic drop of PLA molecular weight. The in situ reactive grafting of PEG into the modified PLA in PLA/PEG blends showed a clear effect on the thermal properties of PLA. It was demonstrated by DSC that the mobility gained by PLA chains in the plasticized blends yielded crystallization. The grafting of a fraction of PEG into PLA did not affect this process. However, DSC results obtained after the second heating showed an interesting effect on the T_g when 20 wt.% PEG were melt blended with neat PLA or 10 wt.% MAG-PLA. In the latter case, the T_g displayed by the reactive blend was shifted to even lower temperatures at around 14 °C, while the T_g of neat PLA and PLA blended with 20 wt.% PEG was around 60 and 23 °C, respectively. Regarding viscoelastic and viscoplastic properties, the presence of MAG-PLA does not significantly influence the behavior of plasticized PLA. Indeed, with or without MAG-PLA, elastic modulus and yield stress decrease, while ultimate strain increases with the addition of PEG into PLA.     

The use of SSA fractionation to detect changes in the molecular structure of model ethylene-butene copolymers modified by peroxide crosslinking

Four model ethylene-butene copolymers of different molecular weights modified with various concentrations of peroxide were analyzed by a DSC based successive self annealing method. The original copolymers had the same intra and intermolecular homogeneous branching distribution along the linear chains with approximately 2.4% mol of ethyl branches. The copolymers with average molecular weights of 29,000, 45,000, 81,000 and 125,000 g/mol were modified with different amounts of 2,5-dimethyl-2,5-di(tert-butyl peroxy)-hexane (DBPH) as a crosslinking initiator. The molecular changes induced by the reaction with the peroxide affect the semicrystalline structure of the material. Variations in the crystal thickness distributions of the material as a consequence of the modification are related to the peroxide induced free radical reactions.     

过氧化苯甲酰的热分解研究

The thermal decomposition process of benzoyl peroxide was investigated by Accelerating Rate Calorimeter. The curves of thermal decomposition temperature and pressure versus time for the systems were obtained. The curves of temperature rising-rate versus thermal decomposition temperature were also obtained. After the data revision disposal and analysis processing, thermal decomposition parameters and kinetic data of benzoyl peroxide were calculated, respectively.     

Reactive functionalization of poly(lactic acid), PLA: Effects of the reactive modifier,initiator and processing conditions on the final grafted maleic anhydride content and molecular weight of PLA

A response surface methodology (RSM) design was used to analyze the effects of maleic anhydride (MA) and 2,5-bis(tert-butylperoxy)-2,5-dimethyl hexane (Luperox or L101) content, and TSE screw speed on the degree of grafted MA (MA_g) and number average molecular weight (M_n) of maleated PLA (PLA-g-MA), which can be used as a reactive compatibilizer in production of PLA blends with various components. PLA-g-MA's FTIR peaks indicated that MA was grafted onto the PLA backbone and oligomeric MA was also present. A maximum of 0.52 wt% MA_g determined by titration was achieved at the expense of a 50% reduction of M_n and an increase in the polydispersity index to around 2.0. Generally, increasing L101 increased the degree of grafting and decreased M_n. L101 and MA_g had a large effect on the stability of PLA-g-MA's M_n during storage. Nominally, amounts of MA equal to 4.5 wt%, L101 between 0.45 and 0.65 wt%, and screw speed of 20 rpm provided the optimal conditions for grafting MA onto PLA.     

The effect of UV light on the thermooxidative stability of linear low density polyethylene films crosslinked by ionizing radiation

The effect of ultraviolet (UV) light on the thermooxidative stability of Linear Low Density Polyethylene(LLDPE) films was studied. LLDPE was stabilized with phenolic type antioxidant known as Irganox 1010, hindered amine light stabilizer known as Chimmasorb 944 and phenolic type gamma stabilizer. The influence of these additives on the thermooxidative stability of gamma and UV irradiated LLDPE were investigated by isothermal Differential Scanning Calorimeter (DSC). The oxidation induction time (OIT) experiments indicate that antirad free LLDPE films which contains antioxidant and UV stabilizer are more sensitive to gamma and UV radiation. On the other hand, films which contain antirad and irradiated to different doses of γ-radiation demonstrated improved thermooxidative stability.     

Wide-and small-angle x-ray scattering studies on the real structure of fractions of (ethylene-α-olefine)-copolymers

Fractions according to the degree of branching of (Ethylene/1-Buten)-Copolymer (LLDPE) were investigated by wide-and small-angle scattering. The thickness of the crystallites is nearly independent of the degree of branching and their volumes reach a limit according to the smallest stable crystal. The mean lattice disfortions also do not change remarkably with the degree of branching but the components of theg _ik tensors do so. The possibility of partial incorporation of side groups into the crystal lattice is discussed. Comparison of wide-and small-angle scattering shows that only the part of the crystallites with smaller thickness builds lamellar clusters while thicker ones are individual crystals.Dedicated to Prof. Dr. W.Pechhold on the occasion of his 60th birthday     

Effect of monomer molecular flow on electrode surface on the structure of poly(α-tetrathiophene) obtained by anodic polymerization

Different structures have been found for poly(α-tetrathiophene) [poly(α-4TF)] electrosynthesized on Pt by anodic oxidation of 1.0 mM monomer solutions in media such as 45:35:20 (v/v/v) acetonitrile/THF/DMF, 45:35:20 (v/v/v) acetonitrile/ethanol/DMF and 72:28 (v/v) acetonitrile/DMF containing 0.1 M LiClO₄; as well as 72:28 (v/v) acetonitrile/DMF with 0.1 M NaClO₃, under dynamic and static conditions at 25 °C. In all cases the polymer was generated by chronoamperometry at 1.000 V vs. Ag∣AgCl, corresponding to the first oxidation peak detected by cyclic voltammetry. Uniform, adherent, insoluble and black polymer films were obtained under these conditions. The resulting structures have been elucidated by combining the information of their IR spectrum, n_ox-value and doping level of the counterion. The degree of crosslinking of every polymer has been quantified and related to the molecular flow of monomer on the Pt electrode. A monomer concentration flow between 4 × 10⁻⁶ and 5 × 10⁻⁶ mmol cm⁻² s⁻¹ was determined as the limiting value below which the polymer grows with crosslinking. This value corresponds to the electropolymerization rate of α-4TF by Pt area unit at 25 °C.     

Study by Raman spectromicroscopy of the effect of tensile deformation on the molecular structure of Bombyx mori silk

The effect of mechanical deformation on the polarized Raman spectra of a silk monofilament of the silkworm Bombyx mori has been studied in order to detect tensile stress-induced changes of the fibroin conformation and reorientation up to the breaking point. For this study, a motorized mechanical stretcher has been specifically designed to fit under the Raman microscope, thus allowing the simultaneous recording of the stress, strain and polarized Raman spectra. Due to its twin drive screw design, this stretcher allows to stretch the silk fiber symmetrically and thus to probe the same area of the fiber during the entire elongation procedure. The results obtained have allowed to clarify the assignment of some vibrational bands of silk. They also show that the intensity of some Raman bands is affected by tensile deformation. In particular, the amide III/amide I intensity ratio decreases as the strain increases and the variation is proportional to the stress applied on the fiber. These variations in intensity suggest that the alignment of the protein chains decreases with strain which might be due to the reorganization of the amorphous phase. The Raman spectra also show that several bands shift toward low wavenumbers as the strain applied increases indicating that the force constant and/or dihedral angles of the peptide bonds, in particular those involved in the β-sheets, are affected by drawing. Therefore, a local stress is directly applied on the β-sheets and the wavenumber shift observed is proportional to the stress applied on the fiber.