Structural recovery mechanism after shear induced orientation of multiwalled carbon nanotube in polypropylene matrix

In this work the flow induced orientation and the governing mechanism of structural recovery of multi-walled carbon nanotube (MWCNT) filled polypropylene nanocomposites were investigated. A series of linear and nonlinear melt rheological measurements including stress growth and time sweep experiments were performed at different temperatures to study the structural breakdown, nanoparticles orientation, subsequent structural recovery and MWCNT loadings. The results showed that the structural recovery occurred in two stages. The first stage, initial agglomeration, showed a quick recovery which was independent of temperature, can be interpreted in terms of inter-particle van der Waals interactions. This structural recovery stage had major contribution in the storage modulus increment. The second stage of the recovery, secondary agglomeration, was slower and dependent on temperature, can be attributed to rotary diffusion of nanoparticles. This stage had minor contribution to the storage modulus increase. Storage modulus increment in both of these agglomeration was attributed to the increase of nanotube-nanotube contacts. Both of these stages were confirmed by transmission electron micrographs. These result were in a good agreement with those calculated using van der Waals and diffusion concepts.     

Equibiaxial extension of a thermotropic liquid-crystalline polymer Flow induced orientation, relaxation of orientation, and strain recovery

The effects of equibiaxial extensional flow on the structural state of a thermotropic liquid crystal aromatic copolyester are studied through wide-angle X-ray scattering (WAXS), strain recovery above the melting point, and differential scanning calorimetry (D.S.C.). For that purpose, samples were melted, stretched and cooled to room temperature. WAXS patterns and strain recovery show that equibiaxial extension (achieved through the lubricated squeezing experiment) results in a planarly-oriented state, i.e. preferred molecular orientation perpendicular to the compression direction accompanied by random molecular orientation within the plane of extension. This flow-induced oriented state may relax partially if not prevented by stretching in the super-cooled liquid state (10-30 K below the melting point). Quenching immediately after deformation at a high temperature can also result in the retention of a high degree of orientation, but it lowers the degree of crystallinity. D.S.C. data show that low cooling rates and large extensions lead to the highest crystallinity. Strain recovery upon reheating is remarkably high for samples with high orientation. This large recovery is attributed to the elasticity of the macrostructure which is formed by the LCP molecules (structural elasticity) and not to the elasticity of the molecules.     

Real-time X-ray scattering study during heating of oriented injection-molded polyethylene

Highly oriented linear polyethylene was prepared by elongational flow injection molding. The changes in crystal orientation were investigated as a function of temperature by real-time wide-angle X-ray diffraction. Additionally, the influence of molecular weight upon the microstructure and the changes in orientation, during heating near the melting point, and after cooling have been examined. A shish-kebab structure is inferred for the high molecular weight samples (M_w≥10⁵) from SAXS observations, while for samples with M_w<10⁵ only an oriented lamellar structure is found. Consequently, a higher thermal stability is shown by the higher molecular weight samples. Furthermore, a recovery of crystal orientation on rapid cooling of the samples from the melt is only observed for samples with M_w≥10⁵. The results are discussed in terms of a preferential recrystallization of chain-folded lamellae, on cooling, onto the shish fibrils which survive at high temperature.     

RECOVERY OF AMORPHOUS POLY(ETHYLENE TEREPHTHALATE)FILM UNIAXIALLY DRAWN JUST BELOW THE GLASS TRANSITION TEMPERATURE*

Isothermal recovery in the macroscopic length of homogeneously deformed specimens of amorphous poly(ethyleneterephthalate) (PET) film sample uniaxially drawn at 69℃ to the draw ratios λ_0=1.26～2.20 were studied at temperaturesaround the glass transition temperature (T_g = 73℃). Experimental results indicate that the length recovery took place in twodistinct steps: a fast first step (fast relaxation) followed by a slow second step (slow relaxation). The relaxation processeswere accompanied by the reversion of trans-conformers (1340 cm~(-1)) to gauche, and the dichroic function of the 1340 cm~(-1)band characterizing the segmental orientation along the chain direction decreased to a very low value at the end of the fastrelaxation. This fact led us to assign the fast relaxation as the segmental orientation while the slow relaxation as relaxation ofthe global chain orientation. It was found that the slow relaxation follows a single exponential function, with relaxation timesstrongly dependent on the temperature resembling the glass transition process. The fast relaxation does not follow a singleexponential decay, presumably a distribution of relaxation times is involved.     

Shape memory effects of poly(ethylene terephthalate-co-ethylene succinate) random copolymers

Random copolymers based on terephthalate acid, succinic acid and ethylene glycol, with thermally induced shape memory, were synthesized via melt polycondensation. The chemical structures of these poly(ethylene terephthalate-co-ethylene succinate) copolymers (PET-co-ES) were ascertained by ¹H NMR spectroscopy. The thermal and viscoelastic characteristics of these copolymers were studied in terms of the succinic acid content using differential scanning calorimetry and dynamic mechanical analysis. The shape memory effects of the copolymers were examined using the strain test. The experimental results suggested that all copolymers exhibited shape memory above the glass transition temperature and that the highest shape recovery rate was 90%. The shape recovery rates of all test samples declined with the number of cycles. This decrease in the shape recovery rate may result from the change in the degree of polymer orientation and/or crystallinity during repeated testing.     

分子链大尺度高度取向小尺度接近无规取向态的无规聚苯乙烯

在Ｔｇ以上３０℃对无规聚苯乙烯（ａ ＰＳ）薄膜进行了热拉伸，然后用各种实验方法测定的结果表明，该试样在红外二向色性，光学双折射及广角Ｘ 光衍射（ＷＡＸＤ）方面表现出各向同性；但是在热膨胀、溶剂溶胀以及应力 应变行为方面表现出显著的各向异性．以上结果表明，该试样小区域基本上无取向，而各向异性的性质是和试样中存在的大区域取向相联系的．     

Low‐Temperature FT‐NIR Spectroscopy of Strain‐Induced Orientation and Crystallization in a Poly(dimethylsiloxane) Network

Summary: The mechanical properties, morphology, and orientation of a poly(dimethylsiloxane) (PDMS) network have been studied during cyclic elongation and recovery by simultaneous Fourier‐transform near‐infrared polarization spectroscopy at temperatures ranging from room temperature to −40 °C. Completely different orientation/recovery mechanisms and changes in the state of order of PDMS were detected as a consequence of cyclic loading/unloading with decreasing temperature. The differences observed at −20 °C compared to room temperature are explained in terms of conformationally regular chain segments, whereas the cooling to −40 °C leads to the formation of lamellar crystals.

Stress/strain diagrams of the elongation/recovery‐cycles of the PDMS films.     

Drawing by solid-state coextrusion of blends of atactic and isotactic polystyrene with poly(2,6-dimethyl-1,4-phenylene oxide)

Atactic polystyrene (aPS)/poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and isotactic polystyrene (iPS)/PPO compatible blends of varied composition were subjected to solid-state coextrusion. The efficiency of drawing, orientation, and crystallinity development were studied as a function of composition and draw ratio. The efficiency of drawing, as measured by elastic recovery, is high for coextrusion at temperatures ?40°C above the glass transition temperature of the particular blend. The maximum attainable draw ratio for the blends decreased with increasing PPO concentration; the highest blend draw ratio attained was 6.5 for 25 wt % PPO. The orientation on drawing, as measured by birefringence, increased with draw but decreased with increasing PPO component at the same draw ratio. When PPO was <50% in iPS/PPO blends, iPS crystallized on draw. The morphology of drawn blends was studied by electron microscopy and wide-angle x-ray scattering.     

拉伸取向尼龙1010样品的抗张回复

研究了拉伸取向尼龙 10 10的抗张回复现象 .残存应变ε′表征了由链滑移而产生的永久形变 ,回复应变(ε -ε′)表征了键角变化和链段取向产生的形变 .当拉伸温度T=18℃ (T 相似文献   

Probing the electric field alignment of a thermotropic liquid crystalline polymer by synchrotron radiation

The orientation of a cyclic side-chain thermotropic liquid crystalline material in an AC field was monitored in real-time using synchrotron radiation. Monitoring the realignment processes in the millisecond-to-minute time-scale was made possible by the high X-ray flux. Orientation parameters and response times were calculated as a function of temperature and frequency. Response times decreased exponentially with temperature due to a decrease in the viscosity. Very little dependence of the response time on frequency was observed, except at low temperatures, where a switch from homeotropic to planar alignment of the molecules was detected. This reorientation of the director was studied in real-time and the resulting complex diffraction patterns were due to equal but opposite director rotations from an alignment parallel to the applied electric field to an alignment perpendicular to the applied electric field. The orientation parameters were highest in the central portion of the mesophase temperature range. At temperatures near clearing, the net degree of orientation diminished. Cooling through the mesophase with an applied electric field resulted in much larger orientation parameters than could be obtained by aligning at a fixed temperature in the mesophase.     

Determination of transition moment directions by means of dichroic spectra in stretched polymer films. Mechanism of solute orientation

The orientation of planar molecules in stretched polymer films has been studied by means of absorption spectroscopy with linearly polarized light. Solute orientation was considered as a function of the solute structure, polymer structure, polymer orientation and of the temperature. We conclude that orientation of the solute cannot derive primarily from attractive interactions between solute and polymer matrix.     

Predicting the location of weld line in microinjection‐molded polyethylene via molecular orientation distribution

The microstructure and molecular orientation distribution over both the length and the thickness of microinjection‐molded linear low‐density polyethylene with a weld line were characterized as a function of processing parameters using small‐angle X‐ray scattering and wide‐angle X‐ray diffraction techniques. The weld line was introduced via recombination of two separated melt streams with an angle of 180° to each other in injection molding. The lamellar structure was found to be related to the mold temperature strongly but the injection velocity and the melt temperature slightly. Furthermore, the distributions of molecular orientation at different molding conditions and different positions in the cross section of molded samples were derived from Hermans equation. The degree of orientation of polymeric chains and the thickness of oriented layers decrease considerably with an increase of both mold temperature and melt temperature, which could be explained by the stress relaxation of sheared chains and the reduced melt viscosity, respectively. The level of molecular orientation was found to be lowest in the weld line when varying injection velocity, mold temperature, and melt temperature, thus providing an effective means to identify the position of weld line induced by flow obstacles during injection‐molding process. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1705–1715     

侧链型偶氮聚合物液晶在不同温度条件下的光致取向

在高于和低于玻璃化转变温度 (Tg)的温度范围内 ,研究了聚甲基丙烯酸 ( 6 [4 ( 4 氰基偶氮苯 )苯氧基 ]己酯 ) (Poly( 6 [4 ( 4 cyanophenylazo)phenoxy]hexylmethacrylate) (PM6ABCN) )聚合物膜的光致取向行为 .实验结果表明 ,在温度低于Tg 时 ,实验温度对取向速率没有影响 ;高于Tg 温度时 ,取向的速率随温度升高而变大 ,取向过程的表观活化能为 3 2 76kJ mol.通过对Tg 温度以下的光致取向进行分析 ,对光致取向过程提出了一个新的微相模型 ,微相内的温度能够达到Tg 以上 ,而实验中控制的样品宏观温度不变 ;高于Tg 温度时的光致取向过程受到宏观温度的控制 ,其表观活化能远远小于热致的各向同性相I(Isotropicphase,I)到向列相N(Nematicphase,N)相转变表观活化能 ( 3 40 2kJ mol) ,表明光致取向过程对温度的依赖性较小     

分子链大尺度取向对非晶态聚对苯二甲酸乙二酯结晶行为的影响

用ＤＳＣ和溶剂诱导结晶（ＳＩＮＣ）的方法对比研究了（ＧＯＬＲ）态和未取向聚对苯二甲酸乙二酯（ＰＥＴ）纤维样品的结晶行为．实验结果表明，样品的大尺度取向可有效地降低样品的冷结晶温度（Ｔｃｃ），证明大尺度取向对样品的结晶行为可起到促进作用．     

Wide angle X‐ray diffraction and Fourier transform infrared dichroism studies of stretched‐recovery‐restretched process of polyurethane/clay nanocomposite

An intercalated polyurethane (PU) /clay nanocomposite was prepared by in situ intercalative polymerization. The PU/clay nanocomposite pellet or film samples were stretched‐recovery‐restretched, using selfmade microstretching tools. The changes of the basal spacings of clay and the orientation of polymer chain segments during the stretched‐recovery‐restretched process were studied by wide angle X‐ray diffraction (WAXD) and Fourier transform infrared (FTIR) dichoism techniques. The WAXD results show that the basal spacing of clay did not change obviously, indicating that no macromolecular chains entered or moved out of the interlayer space, and the orientations of both hard and soft segments inside the interlayer space did not change obviously, either. The FTIR dichroism tests suggest that outside the interlayer space, the orientation of the hard chain segment increased, decreased, and then increased again during the stretched‐recovery‐restretched process. However, no obvious changes of the degree of orientation of the soft segment were observed during the processes, the slightly orientation might be released during the relaxation process before the measurements. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 654–660, 2007     

Survival and recovery of yeast cells after simultaneous treatment of UV light radiation and heat

Cell survival, synergistic interaction, liquid-holding recovery (LHR) kinetics and inactivation forms after the simultaneous treatment with UV light (254 nm) and various high temperatures were studied in diploid yeast cells Saccharomyces cerevisiae. The synergistic interaction was observed within a certain temperature range in which there was a temperature that maximizes the synergistic effect. The LHR study revealed that both the extent and the rate of recovery greatly decreased with the increase in exposure temperature. A quantitative approach describing the LHR process as a decrease in the effective radiation dose was used to estimate the probability of recovery per unit time and the irreversible component of damage. Using the experimental data obtained and the mathematical model described, it was shown that the irreversible component, i.e. the fraction of cells incapable of recovery, increased with the exposure temperature, whereas the recovery constant, i.e. the probability of recovery per unit time, was independent of the exposure temperature. The increase in the irreversible component was accompanied by an increase in cell death without postirradiation division. It is concluded based on this that the synergistic interaction of UV light radiation and hyperthermia in yeast cells is not related to the impairment of the recovery process itself and that it may be attributed to an increased yield of the irreversible damage.     

The influence of flow-induced crystallization on the impact toughness of high-density polyethylene

The relation between the impact toughness and flow-induced crystalline orientation of high-density polyethylene (HDPE) was investigated. Flow-induced crystalline orientation was created in the samples via injection moulding and the amount of orientation was controlled through variation of processing conditions (injection temperature) and sample thickness. The impact toughness behaviour was found to be strongly dependent on the amount of crystalline orientation, whereas the loading direction also had a strong influence, e.g. giving highest impact properties in flow direction. Subsequently, injection moulded samples of HDPE modified with calcium carbonate filler particles were tested. In this case a similar relation between crystalline orientation and loading direction was found, whereas the total amount of flow-induced crystallization was observed to be strongly influenced by the presence of the filler particles.     

Orientation analysis of uniaxially drawn polycarbonate films,doped with fluorescent molecules,by UV-dichroism,fluorescence polarization and birefringence

Bis-phenol-A-polycarbonate (PC) is dyed in solution with fluorescent molecules of different shapes and cast to films. Rectangular samples, cut from these films, are drawn varying draw ratio and drawing temperature. The orientation of the fluorescent probes is investigated by dichroism and fluorescence polarization, that of the polymer segments by birefringence. Hermans' orientation factors obtained by dichroism and fluorescence polarization are in good agreement. Long rodlike fluorescent molecules exhibit greater orientation factors than smaller ones. The correlation between segmental and probe orientation is discussed. Keeping the birefringence constant, the probe orientation factors depend on drawing temperature.     

Temperature gradient-cooled smectic A phase orientation and its effects on Cl orientation in ferroelectric liquid crystals

The temperature gradient cooling of ferroelectric liquid crystals (FLCs) confined to a closed long narrow rectilinear space causes the molecules constituting the smectic layers to acquire significant movement. The ordinary layer structure of the SmA phase consequently undergoes deformation which in turn determines the mode of the SmA SmC* phase transition. SmA stripe-shaped texture was found to result from molecular movement only when the direction of the temperature gradient is the same as the rubbing direction. For FLCs whose SmA temperature range exceeds 20 C, the SmA phase undergoes virtually defect-free C1 orientation without change to C2 orientation, when the direction of the temperature gradient is opposite that of rubbing. Defect-free C2 orientation is possible irrespective of the SmA temperature range in temperature gradient cooling. C1 and C2 orientations may combine with no zigzag defects through the use of such FLCs.     

一种反铁电液晶薄膜相变过程的变温红外光谱研究

通过变温红外光谱对反铁电液晶MHOCPOOB薄膜相变过程中的分子构象、排布及相互作用的变化进行了研究.结果表明,室温时,薄膜中的分子烷基链同时含有zigzag和gauche两种构象.随着温度的升高,有序的zigzag构象转化为无序的gauche构象,链的扭曲程度增加.但S^*IA到S^*CA的转变并不引起烷基链构象和取向发生明显变化.刚性核中的羰基与相邻的苯环形成共轭体系,苯环之间相互倾斜排列,在相变过程中羰基与苯环的共平面作用逐渐被打破,且在相变点苯环间的二面角明显增大.