Effect of molecular parameters on thermomechanical behavior of side-on nematic liquid crystal elastomers

We investigated the structure–property relationship of liquid crystal elastomers (LCEs) obtained from a series of nematic side-on monomers. A new synthetic strategy was developed to obtain the acrylate monomers (n-ADBB), which gave us the opportunity to easily modify the spacer lengths of the monomers. Through magnetic field alignment, well-defined LCE micropillars were fabricated from the monomers by a method combining soft lithography and photopolymerization/photocrosslinking of the monomers and a crosslinker. The influence of structural parameters on the thermomechanical deformation of the microstructures was studied through microscopic observations. The study quantitatively revealed the correlation of thermomechanical behavior of the microstructured LCEs with the crosslinking density and length of the flexible spacer linking the mesogenic core to the backbone. With a proper control of the structural parameters, optimized performances such as large reversible contraction, good elasticity and mechanical robustness were demonstrated for this type of LCEs.     

Dielectric relaxation of binary mixtures of alcohols with acrylic esters

A dielectric relaxation study on alcohol–acrylic ester binary mixtures has been carried out at different concentrations using time domain reflectometry (TDR). The least‐squares fit method has been used to obtain the dielectric parameters (the static dielectric constant ?₀ and the relaxation time τ). By using these parameters, the Bruggeman factor, the Kirkwood correlation factor, and excess inverse relaxation time were determined and discussed to yield information on the molecular structure and dynamics of the mixture. The ?₀ and τ values decreased with an increase in the percent of acrylic ester in alcohol for all the systems. The value of t increased with an increase in chain length of both the alcohol and acrylic ester, whereas the reverse trend is observed for ?₀. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A new semi-phenomenological approach to predict the stress relaxation behavior of thermoplastic elastomers

In this paper we report on a new semi-phenomenological approach to predict the stress relaxation behavior of thermoplastic elastomers at long times. This approach relies on the method of Gurtovenko and Gotlib [J Chem Phys 115 (2001) 6785], which has originally been conceived to describe the relaxation dynamics of inhomogeneously crosslinked polymers forming agglomerations of crosslinks. In this work we demonstrate that the method can be extended to predict the stretched exponential stress decay of homogeneously crosslinked thermoplastic elastomers, which are subjected to an extensional strain pertaining to the nonlinear regime of mechanical properties. In our approach thermal fluctuations induce fluctuations in size of domains of crosslinks via a chain-pullout mechanism. We compare our theoretical predictions to the experimental measurements of Hotta et al. [Macromolecules 35 (2002) 271] performed on poly(styrene-isoprene-styrene) triblock copolymers, which are composed of hard domains of polystyrene embedded in a rubbery polyisoprene matrix. Our study confirm the importance of the chain-pullout mechanism in the stress relaxation process and demonstrates the involvement of multiple time- and structural-length-scales.     

影响丙烯酸酯树脂相对分子质量的因素研究

用溶液聚合法制备丙烯酸酯合成树脂。采用凝胶渗透色谱法测定制备树脂的数均分子量。研究了溶液聚合过程影响丙烯酸酯树脂分子量的因素。研究结果表明：随着单体浓度的增加,丙烯酸酯树脂的分子量增大；丙烯酸酯分子曼随着聚合反应温度、引发剂浓度、甲基丙烯酸甲酯含量的增加而降低；在苯和醋酸丁酯混合溶剂中,制备出的树脂分子量最大,而在异丙苯和醋酸丁酯混合溶剂中,制备出的树脂分子量最小；在反应体系中加入少量的2-巯基乙醇,可以显著降低树脂的分子量。     

Effect of the stress relaxation property of acrylic pressure‐sensitive adhesive on light‐leakage phenomenon of polarizer in liquid crystal display

The acrylic pressure‐sensitive adhesive systems with the different stress relaxation abilities are prepared by varying the curing agent from 0.3% to 3.0% and the effect of the stress relaxation ability of pressure‐sensitive adhesive system on the durability of LCD polarizer was investigated. The stress relaxation ability of the crosslinked pressure‐sensitive adhesive systems was measured by transient tensile test and dynamic viscoelastic test. And the durability was evaluated by the light‐leakage test of polarizer coated with the pressure‐sensitive adhesive after it was aged under the condition of 60°C and the relative humidity of 95% for 72 h. It was observed that the stress relaxation ability of pressure‐sensitive system is decreased with the crosslinking density, as intended. And the pressure‐sensitive adhesive with the lower stress relaxation ability exhibited the lower light‐leakage and consequently the higher durability than the other PSA systems. This observation is obviously contradictory to the previous design strategy of PSA system and the reason for this observation was discussed briefly. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Effects of the molecular structure of impact modifier and compatibilizer on the toughening of PBT/SBS/PS‐GMA blends

This work aims at studying the toughening process of poly(butylene terephthalate) (PBT) through its blends with styrene‐butadiene‐styrene block copolymers (SBS), in the presence of poly(styrene‐ran‐glicydil methacrylate) (PS‐GMA) as reactive compatibilizer. High values of impact strength were attained for PBT/SBS blends without the compatibilizer; however, this improvement is achieved for blends with SBS having similar viscosity compared to PBT, at high SBS content (40 wt %) and for blends prepared under specific processing conditions. The efficiency of the in situ compatibilization of PBT/SBS blends by PS‐GMA was found to be strongly dependent on the SBS and PS‐GMA molecular characteristics. Better compatibilizing results were observed through fine phase morphologies and lower ductile to brittle transition temperatures (DBTT) as the interfacial interaction and stability of the in situ formed compatibilizer are maximized, that is, when the miscibility between SBS and PS‐GMA and reaction degree between PBT and PS‐GMA are maximized. For the PBT/SBS/PS‐GMA blends under study, this was found when it is used the SBS with higher polystyrene content (38 wt %) and with longer PS blocks (M_w = 20,000 g mol^?1) and also the PS‐GMA with moderate GMA contents (4 wt %) and with molecular weight similar to the critical one for PS entanglements (M_c = 35,000 g mol^?1). © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5795–5807, 2006     

Irradiation effects on the relaxation behaviour of EPDM elastomers

γ‐irradiation of ethylene–propylene diene monomer (EPDM) elastomers under oxidant atmosphere was carried out in order to change their mechanical and dielectric behaviour. Three different formulations of EPDM (70 wt% ethylene, 28 wt% propylene; diene monomer: 2 wt% norbornene) were studied: a non‐crosslinked EPDM terpolymer, a crosslinked EPDM and a crosslinked EPDM stabilized with an antioxidant. Dielectric and mechanical relaxation show a β‐sub‐glass relaxation at about ?120 °C (1 Hz) and an α‐relaxation at ?15 °C (1 Hz) associated with the glass transition but influenced by the effects of irradiation. The local mobility associated with the β‐relaxation is only weakly influenced by γ‐irradiation up to 450 kGy. The α‐process is shifted to higher temperatures as a result of crosslinking and changes in the semicrystalline structure. The amplitude of the dielectric α‐process increases as a result of the formation of oxidized species during irradiation under oxygen. In contrast, the mechanical α‐relaxation amplitude decreases as a result of physical and chemical cross‐linking. It was shown that the main factors that determine the crosslinking/chain scission balance are (1) the presence of oxygen together with the irradiation dose, (2) the dose rate and (3) the initial crosslink density of the EPDM material. As a result, the individual contribution of crosslinking and crystallization, and therefore the understanding and prediction of the properties after γ‐irradiation can only be deduced after comparison of the polymer behaviour below and above its melting temperature. Copyright © 2004 Society of Chemical Industry     

Effect of the molecular structure of polyurethane elastomers on the thermomechanical properties of PUE/PC blends

Polyurethane elastomers (PUEs) based on 4,4′‐diphenylmethane diisocyanate (MDI), 1,4‐butanediol (BDO) and two kinds of aliphatic polycaprolactone (PCL) diols with molecular weight of 1000 Da and 2000 Da have been synthesized and melt‐blended with polycarbonate (PC). The compatibility of PC and PUEs was investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM). The results indicated that the glass transition temperature (T_g) of PC decreased by 0–40°C when 0–10 wt % of PUEs incorporated into the PC matrix. Phase separation in the blends was not detected by means of DSC characterization, but measurements of DMA and SEM indicated that phase separation existed in the blends of PC and PUEs synthesized with 1000 Da PCL‐diol. As for PUEs/PC blend in which 2000 Da PCL‐diol as PUEs' soft segments, it turned from completely compatible to partially when the NCO/OH ratio for the PUEs prepolymer was increased from 2 : 1 to 4 : 1. The compatibilities of PC and PUEs were greatly influenced by the molecular weight of polyols and the ratio of NCO/OH in the PUE prepolymer, higher molecular weight of polyols and lower NCO/OH ratio resulted in better compatibility. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of the sol fraction and hydrostatic deformation on the viscoelastic behavior of prestrained highly filled elastomers

This study focuses on the relations between the microstructure and the viscoelastic behavior of an industrial solid propellant belonging to the class of highly filled elastomers. Precisely, the study aims at determining the impact on the viscoelastic behavior of the presence of the sol fraction inside the polymer network. The sol fraction is the part of the binder that a good solvent can extract. The solid propellant is swollen to various extents by solutions of plasticizer and polymer molecules. This swelling leads to a hydrostatic deformation of the polymer network, corresponding to an extension or contraction loading for each specimen. Prestrained dynamic mechanical analysis tests, superimposing a small oscillating strain on a prestrain, characterize the viscoelastic behavior. The degree of swelling of the network and the effective filler fraction drive the viscoelastic response. In addition, the mechanical behavior does not depend on the chemical nature of the introduced sol fraction. Moreover, a nonlinear behavior, i.e., an increase in both storage and loss moduli with increasing prestrain, is initiated at low prestrain. This nonlinearity depends on the contraction or extension of the network and could result from particles aligning with prestrain, which is expected in such highly filled materials. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Thermally stimulated current studies on molecular relaxation and blow moulding of poly(ethylene terephthalate)

The thermally stimulated current (TSC) technique has been used to investigate molecular relaxation in poly(ethylene terephthalate) (PET) films unstretched and biaxially stretched at 90 and 95 °C. Unstretched PET films show two peaks at 77 and 90 °C corresponding to α and ρ relaxation processes, respectively. The α relaxation is associated with the main glass transition of the material. The ρ peak with lower intensity is attributable to permanent dipoles. Both biaxially stretched samples show one TSC peak at 95 °C, supposed to correspond to ρ relaxation. The disappearance of the α peak, accompanied by the displacement of the ρ peak to higher temperature, is the result of the higher thermal stability of the permanent dipoles, which is strongly influenced by the stiffening of amorphous parts and the crystallization by stretching. In both stretched samples, the continuous distribution of pre-exponential factors over activation energies observed might correspond to a single relaxation mode. The kinematics of stretching PET has been discussed in terms of activation energy and temperature dependence of relaxation time. © 1999 Society of Chemical Industry     

Effect of acrylic core–shell rubber particles on the particulate flow and toughening of PVC

Different types of acrylic core–shell rubber particles with a poly(butyl acrylate) (PBA) core and a grafted poly(methyl methacrylate) (PMMA) shell were synthesized. The average size of acrylic core–shell latex particles ranged from 100 to 170 nm in diameter, having the core gel content in the range of 35–80%. The melt blending behavior of the poly(vinyl chloride) (PVC) and the acrylic core–shell rubber materials having different average particle sizes and gel contents was investigated in a batch mixing process. Although the torque curves showed that the particulate flow of the PVC in the blends was dominant, some differences were observed when the size and gel content of the particles varied. This behavior can be attributed to differences in the plasticizing effect and dispersion state of various types of core–shell rubber particles, which can vary the gelatin process of the PVC in the mixing tool. On the other hand, the highest toughening efficiency was obtained using core–shell rubber particles with the smallest particle size (i.e., 100 nm). The results showed that increasing the gel content of the core–shell impact modifiers with the same particle size improved the particle dispersion state in the PVC matrix. The toughening efficiency decreased for the blends containing 100 and 170 nm rubber particles as the gel content increased. Nevertheless, unexpected behavior was observed for the blends containing 140 nm rubber particles. It was found that a high level of toughness could be achieved if the acrylic core–shell rubber particles as small as 100 nm had a lower gel content. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

阳离子杂质对腈纶染色性能的影响

讨论了影响腈纶丝束染色性能的因素 ,分析了钙等阳离子对丝束上色率的影响机理及影响程度。通过线性回归的方法对生产中的实测数据进行处理 ,找出系统中钙离子浓度和丝束上色率偏差的线性关系 ,并提出了预防和处理钙离子浓度过高的几点措施     

Effect of the elastomer viscosity on the morphology and impact behavior of injection molded foams based on blends of polypropylene and polyolefin elastomers

The impact resistance of injection-molded polypropylene (PP) parts is severely reduced when they are foamed. It is necessary to implement strategies, such as elastomer toughening, to increase the impact behavior of foamed parts. However, the knowledge on the effect of elastomer addition on the morphology, cellular structure, and impact of injection-molded cellular parts is very limited. In this work, foamed parts based on blends of PP and polyolefin elastomers have been produced and characterized. A high and a low viscosity octene-ethylene copolymer (EOC) and a high viscosity butene-ethylene copolymer (EBC) were employed. The blends have been thermally and rheological characterized. Solids materials and foams (relative density 0.76) were injection-molded. The solid phase and cellular structure morphologies were studied using scanning electron microscopy. The results showed that elastomer toughening has been successful to obtain an improvement of the impact behavior in solid and cellular polymers. In this case, EOC materials provide an appropriate interfacial adhesion and optimized cellular structure which results in high impact resistance. The optimum elastomer to improve the properties is the EOC with a higher viscosity which provides impact resistance with n values below 3 due to the toughening of polymer matrix, thick skin thickness, and low cell size.     

超高相对分子质量聚乙烯接枝丙烯酸的制备

以丙烯酸为单体,采用γ射线辐照引发技术制备了超高相对分子质量聚乙烯接枝丙烯酸(UHMWPE- g-AA);利用傅里叶变换红外光谱和差示扫描量热法表征了接枝物的结构和热性能;用化学滴定法测定了接枝物的接枝率;研究了UHMWPE-g-AA对聚酰胺(PA)1010/UHMWPE-g-AA/UHMWPE共混物力学性能的影响。实验表明：接枝物在1716cm~(-1)处有明显的羰基特征吸收峰,说明AA分子确实被接枝到UHMWPE分子链上;接枝率随单体浓度、辐照剂量及辐照时间的增加而增加;加入UHMWPE-g-AA后,UHMWPE与PA1010的相容性得到了改善,PA1010/UHMWPE-g-AA/UHMWPE共混物的缺口冲击强度是PA1010/UHMWPE共混物的1.5倍,达到72J/m。     

Use of Functional (Meth)acrylic Cross‐Linked Polymer Microparticles as Toughening Agents for Epoxy/Diamine Thermosets

(Meth)acrylic cross‐linked polymer microparticles (CPM, also named microgels) were used as toughening agent for an epoxy/amine network. CPM were mainly based on butyl acrylate and consequently they were rubbery at ambient temperature. Various types of reactive groups were introduced onto the CPM: epoxy, carboxy (meth)acrylic double bonds, and epoxy + acrylic double bonds, carboxyl + methacrylic double bonds. Non functional microparticles were also used. Before any reaction, most of the CPM were soluble in the thermoset precursors. Nevertheless, the CPM functionality strongly influenced their initial miscibility in the epoxy‐amine monomers and their final dispersion in the cross‐linked matrix, as well as the mechanical properties of the network. Non‐functional CPM did not lead to a high increase of fracture toughness because of the low adhesion between microparticles and epoxy matrix. However, fracture toughness was increased with reactive CPM because of better adhesion between the microparticles and the matrix. The best toughness was obtained with microparticles containing two types of reactive groups, allowing at the same time cross‐linking reactions between CPM and chemical bonding between CPM and the epoxy matrix. In this case, fracture toughness can be greatly improved, up to 3‐times if the chemical composition of the microparticles was wisely chosen, without significantly reducing the thermal properties.

Viscoelastic properties of toughened DGEBA/MCDEA networks.     

高分子聚合物松弛行为及模型的研究进展

高分子聚合物材料作为一种使用频率高且应用范围广的材料,其松弛行为对于其加工制品的精度具有重要的影响,且高分子化学合成的结果会最终影响高分子聚合物的松弛行为。由此总结了高分子聚合物松弛行为的特点,整理了其力学松弛及模型、松弛时间的研究进展。并由此提出现阶段高分子聚合物的松弛行为研究的局限性,并指出未来的研究方向。     

Effect of matrix chain entanglement on toughening mechanism evolution of acrylic impact modifier toughened methyl methacrylate-N-phenylmaleimide copolymers

The toughening mechanism evolution of acrylic impact modifier (AIM) toughened methyl methacrylate-N-phenylmaleimide copolymers (PMMA-PMI) was investigated in terms of matrix chain entanglement density (ν_e), especially within the range of matrix ν_e?≈?0.1 mmol/cm³. After respectively blended the PMMA-PMI having designed ν_e with the same content preformed AIM, it was showed that the notched impact strength of the blends increase with PMMA-PMI ν_e enhancement. The morphological analyses revealed that the extent of shear yielding gradually enlarged when matrix ν_e is higher than 0.1061 mmol/cm³. Whereas, in the AIM/PMMA-PMI blend with lower matrix ν_e, the toughening mechanism transformed into matrix crazing triggered by PBA cavitation. The results suggest that the response of the matrix with different ν_e to the rubber particle cavitation dominate toughening mechanism and mechanical performance of AIM/PMMA-PMI blends, and it is believed that matrix ν_e is an inherent factor to toughening mechanism evolution.     

Effect of a homogeneous magnetic field on the viscoelastic behavior of magnetic elastomers

Viscoelastic behavior of highly elastic magnetic elastomers has been studied by three different experimental techniques: elongation, static and dynamic shears. It has been shown that the elastic modulus of the materials increases considerably in an external homogeneous magnetic field of up to 0.3 T (100-fold increase of the tangential modulus has been observed at small 1-4% deformations). The appearance of the new effect of pseudo-plasticity induced by the magnetic field has been observed leading to a considerable (up to 100-fold) increase in the shear loss modulus of the composites.     

抗起球干法腈纶的研究与开发

分析了干法腈纶纤维抗起球机理,研究了纺丝过程以后的工艺条件,通过降低纤维的钩接强度和钩接伸长,使织物表面的绒毛在摩擦力的作用下自行脱落,不再缠结成球或成球后迅速脱落,以达到提高产品抗起球性能的目的.通过该方法生产的丝束或毛条加工制成织物,抗起球等级可达到3级以上.