Some rheological properties of highly crosslinked polysiloxanes

Seven crosslinked polydimethylsiloxanes were prepared from carboxyl-terminated siloxane oligomers of different average lengths and a trifunctional imine. The 10-sec torsion modulus was measured as a function of temperature, and compression-deflection data were obtained on swollen specimens. Polymer-solvent interaction parameters were calculated from equilibrium swelling ratios and modulus data. The plots of modulus versus temperature show two distinct transitions for all the crosslinked polymers. The low-temperature transition corresponds to the siloxane chains, whereas the high-temperature transition should be attributed to the portion of the network chain derived from the crosslinking agent and the organic part of the oligosiloxane. On shortening the siloxane blocks, both transitions shift to successively higher temperatures and higher modulus levels. However, the siloxane transition temperature shows only a limited rise even when the average block length becomes quite small. The low siloxane polymers seem to exhibit two swelling maxima, each corresponding to one of the constituents. The studied materials from nonrandom copolymeric structures typical of block polymers. Such microphase systems can probably be expected in crosslinked polymers whenever the crosslinking agent participates significantly in the network chains and its compatibility with the flexible chains species is poor.     

Relaxations in the transition region of crosslinked polyesters. II. The glass transition

The glass-transition region of crosslinked polymers prepared from poly(1,2-propylene phthalate fumarate) and styrene was studied using a torsion pendulum. The glass-transition temperature and the modulus in the rubbery region of these polymers were analyzed in terms of the crosslink density. The styrene concentration at which the maximum crosslink density occurs, as estimated from the viscoelastic data, is found to be in agreement with estimates made by other workers using chemical methods.     

Glass transition temperature of glucose, sucrose, and trehalose: an experimental and in silico study

Isothermal-isobaric molecular dynamics simulations are used to calculate the specific volume of models of different amorphous carbohydrates (glucose, sucrose, and trehalose) as a function of temperature. Plots of specific volume vs temperature exhibit a characteristic change in slope when the amorphous systems change from the glassy to the rubbery state. The intersection of the regression lines of data below (glassy state) and above (rubbery state) the change in slope provides the glass transition temperature (T(g)). These predicted glass transition temperatures are compared to experimental T(g) values as obtained from differential scanning calorimetry measurements. As expected, the predicted values are systematically higher than the experimental ones (about 12-34 K) as the cooling rates of the modeling methods are about a factor of 10(12) faster. Nevertheless, the calculated trend of T(g) values agrees exactly with the experimental trend: T(g)(glucose) < T(g)(sucrose) < T(g)(trehalose). Furthermore, the relative differences between the glass transition temperatures were also computed precisely, implying that atomistic molecular dynamics simulations can reproduce trends of T(g) values in amorphous carbohydrates with high quality.     

Polymerization and crosslinking characteristics of a 3-methoxybutyl acrylate

Polymers of 3-methoxybutyl acrylate (3-MBA) were prepared by máss, solution, and emulsion polymerization techniques. The 3-MBA polymers could be converted from soft, rubbery, soluble, thermoplastic films to hard, glossy, flexible, crosslinked films when exposed to air and/or transition metal catalysts at elevated temperatures. The crosslinked polymers are resistant to common organic solvents and to mineral acids. Strong alkalis degraded the crosslinked polymers. The second-order transition temperature of poly-3-MBA is ?56°C. as determined by volume dilatometry. A comparison of the crosslinking properties of poly-3-MBA and other alkyl and alkoxyalkyl polymers is discussed. An autoxidative mechanism is proposed for the crosslinking of 3-MBA polymers.     

Nonlinear Viscoelasticity Raised at Low Temperatures by Intermolecular Cooperation of Bulk Amorphous Polymers

We performed dynamic Monte Carlo simulations of stress relaxation in parallel-aligned and uniaxially stretched bulk amorphous polymers at low temperatures.We observed an extra-slowing down in the early stage of stress relaxation,which causes nonlinear viscoelasticity as deviated from Debye relaxation and Arrhenius-fluid behaviors observed previously at high temperatures.Meanwhile,fluctuation analysis of stress relaxation revealed a substantial increase in the stretch fractions of polymers at the transient periods of high-temperature Debye relaxation.Structural analysis of free volume further revealed the scenario that,at low temperatures,the modulus of polymer entropy elasticity decreases with temperature and eventually loses its competition to the imposed modulus (Deborah number becomes larger than one),and hence upon stress relaxation under constant strains,monomers are firstly accumulated nearby two stretching ends of polymers,resulting in tentative global jamming like physical cross-linking there,and thus retarding the coming transient state of stress relaxation.We concluded that intermolecular cooperation raises physical crosslinking for nonlinear viscoelasticity of polymer stress relaxation as well as the rubbery states unique to bulk amorphous polymers.The new microscopic mechanism of the fluid-rubbery transition of polymers may bring insights into the intermolecular cooperation mechanism of glass transition of small molecules,if the fluid-rubbery transition is regarded as an extrapolation of glass transition from low to high molecular weights.     

Fixed crosslink formation and viscoelasticity of polystyrene networks

The crosslinking formation in the 8/92 mol % random copolymer of ethynyl- styrene and styrene is clarified quantitatively from the IR spectrum analysis on the basis of the thermally homogeneous reaction of ethynylstyrenes between the chains. The viscoelastic properties of the crosslinked polystyrene, CL-P(ESt/St), are investigated based on the temperature and frequency dependencies of the dynamic Young modulus. The relaxation spectrum of CL-P(ESt/St) drops deeply like the Wedge type with a slope of −½ in the glass transition region, and became a plateau without a terminal zone in the rubbery elasticity region. The effects of the ethynyl crosslinked junctions for the micro-Brownian motions of the segments in the strand are discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3319–3327, 1999     

The linear rheological responses of wedge‐type polymers

The linear rheological responses of a series of specially designed wedge‐type polymers synthesized by the polymerization of large molecular weight monomers have been measured. These wedge polymers contained large side groups which contained three flexible branch chains per polymer chain unit. The master curves for these polymers were obtained by time temperature superposition of dynamic data at different temperatures from the terminal flow regime to well below the glass transition temperature, T_g. While these polymers maintained a behavior similar to that of linear polymers, the influence of the large side group structure lead to low entanglement densities and extremely low rubbery plateau modulus values, being near to 13 kPa. The viscosity molecular weight dependence was also somewhat higher than that normally observed for linear polymers, tending toward a power law near to 4.2 rather than the typical 3.4 found in entangled linear chains. The glassy modulus of these branched polymers is also found to be extremely low, being less than 100 MPa at T_g ?60 °C. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 899–906     

Comparison of the transient stress–strain response of rubber to its linear dynamic behavior

Master curves of the small strain and dynamic shear modulus are compared with the transient mechanical response of rubbers stretched at ambient temperature over a seven‐decade range of strain rates (10^?4 to 10³ s^?1). The experiments were carried out on 1,4‐ and 1,2‐polybutadienes and a styrene–butadiene copolymer. These rubbers have respective glass transition temperatures, T_g, equal to ?93.0, 0.5, and 4.1 °C, so that the room temperature measurements probed the rubbery plateau, the glass transition zone, and the onset of the glassy state. For the 1,4‐polybutadiene, in accord with previous results, strain and strain rate effects were decoupled (additive). For the other two materials, encroachment of the segmental dynamics precluded separation of the effects of strain and rate. These results show that for rubbery polymers near T_g the use of linear dynamic data to predict stresses, strain energies, and other mechanical properties at higher strain rates entails large error. For example, the strain rate associated with an upturn in the modulus due to onset of the glass transition was three orders of magnitude higher for large tensile strains than for linear oscillatory shear strains. © 2011 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys, 2011     

Effect of crosslink on the characteristic length of glass transition of network polymers

The characteristic length of the glass transition temperature was evaluated for crosslinked bulk polystyrenes and poly(methyl methacrylate)s by differential scanning calorimetry. The characteristic length, which corresponds to a length scale of the cooperative rearrangement due to polymer segmental relaxation, was revealed to decrease with increase in the degree of crosslink. The relative values of configurational entropy for crosslinked polymers were evaluated based on a simple polymer network model on a cubic lattice. As a result, the configurational entropy of the smallest cooperatively rearranging region was revealed to decrease with increase in the degree of crosslink, which is responsible for the above behavior of the characteristic length. The effects of crosslink on the characteristic length and the glass transition temperature were found to be stronger for crosslinked poly(methyl methacrylate)s than for crosslinked polystyrenes, which reflects the difference in the structure of crosslink segment. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1958–1966, 2006     

Preparation of poly(vinyl chloride) at low temperature by a photochemical method

A convenient method has been developed for the polymerization of vinyl chloride at low temperatures (to at least ?75°C) by using a tungsten–iodine rather than an ultraviolet lamp, and uranyl nitrate as sensitizer. The use of predominantly visible light minimizes degradation reactions sometimes encountered with ultraviolet light. Measurements of the fraction of racemic (or syndiotactic) diads and per cent crystallinity confirm that the products resemble polymers prepared by other techniques, such as polymerization initiated by a boron alkyl. It is concluded, therefore, that the method can provide a useful alternate to the more common, organometallic system. Measurements of torsional modulus as a function of temperature were also made. As the temperature of polymerization is lowered, the fraction of racemic diads and the per cent crystallinity are increased. The higher the per cent crystallinity, the higher the glass temperature, the broader the glass transition, and the higher the modulus in the rubbery state. Thus, the increased stereoregularity permits the development of a higher level of crystallinity which, in turn, restricts the mobility of the polymer chains.     

Well-defined crosslinked materials via ring opening metathesis polymerisation

The work reported here is part of our ongoing programme of work directed. towards the synthesis and characterisation of polymeric materials via ROMP-RIM and ROMP-RTM. It describes the synthesis and characterisation of well-defined linear and crosslinked polymeric materials via ROMP-RTM. The process involves in-mould polymerisation of monofunctional imidonorbornene monomers, with different alkyl side chain lengths, to give a range of linear polymers. The process also involves in-mould copolymerisation of monofunctional imidonorbornene monomers, with different alkyl side chain lengths and difunctional monomers with different alkylene spacer lengths, to produce well-defined crosslinked polymers. The glass transition temperature (T_g) of the linear polymers was found to depend on the length of the alkyl side chain. For the crosslinked materials the results show that as the percentage of the difunctional, crosslinking unit, is increased (1, 5 and 10 molar percentage of the difunctional monomer) the glass transition shifts to a higher temperature, the height of the tanδ peak decreases and the plateau shear modulus above T_g increases. These results are as expected for an increase in the crosslink density of a polymer.     

高Tg交联型二阶非线性光学聚合物的合成

合成了双羟基双氨基偶氮苯化合物，将其分别与六亚甲基二异氰酸酯的缩二脲和２，４－甲苯二异氰酸酯进行交联反应合成了两种交联型二阶非线性光学聚合物，利用ＦＴＩＲ光谱和ＤＳＣ对交反应过程和交联聚合物的玻璃化转变温度进行研究。采用ＵＶ－Ｖｉｓ吸收光谱对极化交联反应前后膜的取向及取向稳定性进行了研究。     

Dynamic mechanical analysis

Using dynamic mechanical analysis (DMA) we have studied thermal degradation for a system containing a diglycidyl ether of bisphenol A (DGEBA) and 1,3-bisaminomethylcylohexane (1,3-BAC). The changes of dynamic mechanical properties during thermal degradation indicated a shift of the glass transition temperature (T _g) to higher temperatures and a decrease in the peak value of the dynamic loss factor (tan δ) with an increasing of aging time. The value of dynamic storage modulus (E′) at the rubbery state showed an increase with aging time, whiteE′ at the glassy state only underwent a moderate change with increased thermal degradation. From these results it can be argued that thermal degradation during the stage prior to the onset of the severe degradation involves structural changes in the epoxy system, as further crosslinking and loss of dangling chains in the crosslinked network.     

Polyurethane/polystyrene one-shot interpenetrating polymer networks with good damping ability: Transition broadening through crosslinking,internetwork grafting and compatibilization

Good damping materials should exhibit a high loss factor value over a broad temperature range. Polyurethane and polystyrene are highly immiscible polymers with glass transition regions far apart. The interpenetrating polymer network topology can restrict phase separation and result in materials with a broad transition region. Simultaneous polyurethane/polystyrene interpenetrating polymer networks were synthesized by the one-shot route. Different methods of improving the miscibility of the two polymers were investigated. These included the vanation of the crosslink level in both polymer networks, the controlled introduction of internetwork grafting and the incorporation of compatibilizers into the polystyrene network. Dynamic mechanical thermal analysis indicated that the latter two were successful in achieving a compatibilization of the polymer components. With some materials, a high, broad transition region exhibiting a loss factor > 0.3 over more than 135°C was obtained. The morphology observed via transmission electron microscopy ranged from macrophase separated materials in the lightly crosslinked IPNs to a fine, microheterogeneous morphology in the grafted ones. Modulated differential scanning calorimetry measurements confirmed the trend of the glass transition locations observed with dynamic mechanical thermal analysis.     

Formation,structure, and elasticity of loosely crosslinked epoxy-amine networks. II. Mechanical and optical properties

The mechanical and optical behavior in the dry and swollen states of loosely crosslinked epoxy networks prepared from the diglycidyl ether of bisphenol A, phenylglycidyl ether, and 4,4′-diaminodiphenylmethane was investigated, and the weight fraction of sol in the networks was determined. The crosslinking density was controlled by an excess of diamine and by the fraction of monoepoxide. The reaction proceeded to almost full conversion of epoxy groups. With increasing content of monoepoxide or with increasing excess of diamine, the main transition region is shifted to lower temperatures. The dependence of the viscoelastic modulus on temperature and the optical behavior indicate that the networks are homogeneous. In all cases, the sol fraction is adequately described by the theory of branching processes (cf. Part I). The equilibrium modulus related to the dry state is the same irrespective of whether it is obtained by measurements in the dry or swollen state. The mechanical behavior in the rubbery state can be described by the theory of phantom networks with fully suppressed fluctuations of crosslinking (front factor A = 1) or by the theory of phantom networks with fully released fluctuations of crosslinks (front factor) A = f_e?2/f_e] and contribution of trapped entanglements of the Langley-Graessley type (cf. Part I). In the analysis of the equilibrium behavior, it is advantegeous to use the plot of superimposed dependences of G_e on the gel fraction, which considerably reduces the effect of experimental inaccuracy in determination of composition and degree of conversion.     

PVT properties of dodecane/polystyrene systems

The PVT properties of crosslinked polystyrene samples containing various amounts of dodecane were measured. The Tait equation was used to describe the PVT behavior of each system in both the glassy and rubbery regions. The glass transition temperature was determined from the abrupt change of the thermal expansion coefficient. Increase in the dodecane content in the samples resulted in a significant decrease of the difference between the expansion coefficients in the glassy and rubbery regions. Addition of dodecane lowered the glass transition temperature linearly. However, the dependence of the glass transition temperature on pressure was not affected by the presence of dodecane in the polymer samples. Above the glass transition temperature, the volume of the swollen polymer, V_m, could be determined by simple addition of the volumes of the pure components at the appropriate temperature and pressure; the volume change of mixing, δV_m, was independent of temperature and pressure. Below the glass transition temperature, volume additivity of the two components was also applicable after appropriate adjustment of the glass transition temperature of the polymer to that of the dodecane/polymer samples. © 1994 John Wiley & Sons, Inc.     

Side chain crosslinking of aromatic polyethers for high temperature polymer electrolyte membrane fuel cell applications

Novel aromatic polymers bearing polar pyridine units in the main chain and side chain crosslinkable hydroxyl and propargyl groups have been successfully synthesized. The polymers have been investigated in terms of their critical properties related to their application in high temperature polymer electrolyte membrane fuel cells, such as doping ability, mechanical properties, and thermal stability. Crosslinked membranes were prepared by direct crosslinking of hydroxyl side chain groups with decafluorobiphenyl used for the first time as a crosslinking agent. However, further functionalization of hydroxyl groups to the propargyl derivative has also led to crosslinked polymers after thermal curing. Both types of crosslinked membranes exhibited higher glass transition temperatures as well as lower doping levels when doped in phosphoric acid compared with the non crosslinked analogs, confirming the formation of a successfully crosslinked network. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Dynamic mechanical analysis on modified bismaleimide resins

Polyaminobismaleimides (PAMBI) with diphenyl methane, diphenyl ether and hexamethylene segments were synthesized according to Michael type addition reaction. The modification of PAMBIs with 2-glycidyl-phenyl-ether (GPE) was performed in order to ameliorate the toughness of the products. Dynamic mechanical analysis allowed the study of the processes that concur in the material through curing by tracking the storage modulus (E′) and loss factor (tanδ) changes. The small drop of E′ with increasing temperature in the glass transition region argues for crosslinked structures. The viscoelastic behavior revealed complex processes, i.e., overlapping of glass transition temperature range with intra-and inter-crosslinking.     

交联对聚己二酸乙二醇酯聚氨酯/甲基丙烯酸甲酯交联聚合物的力学阻尼、相容性及形态的影响

 用紫外光聚合方法制备出一系列端乙烯基聚己二酸乙二醇酯聚氨酯(PEAPU)和甲基丙烯酸甲酯(MMA)AB交联聚合物(ABCP)。用粘弹谱仪、透射电子显微镜(TEM)和平衡溶胀法研究了AB交联聚合物的动态力学性能、玻璃化转变温度、形态和交联密度、观察到相应于聚氨酯和PMMA相两个玻璃化转变温度,TEM照片中的微相分离是更明显的,ABCP中的两个T_g内移表明,两种成分的化学交联增加了相互的可混性、与ABCP具有相同组成的IPN有比ABCP大得多的相区。氢键能够影响ABCP的相容性、形态和动态力学性能。     

交联对聚己二酸乙二醇酯聚氨酯/甲基丙烯酸甲酯交联聚合物的力学阻尼、相容性及形态的影响

用紫外光聚合方法制备出一系列端乙烯基聚己二酸乙二醇酯聚氨酯(PEAPU)和甲基丙烯酸甲酯(MMA)AB交联聚合物(ABCP)。用粘弹谱仪、透射电子显微镜(TEM)和平衡溶胀法研究了AB交联聚合物的动态力学性能、玻璃化转变温度、形态和交联密度、观察到相应于聚氨酯和PMMA相两个玻璃化转变温度,TEM照片中的微相分离是更明显的,ABCP中的两个T_g内移表明,两种成分的化学交联增加了相互的可混性、与ABCP具有相同组成的IPN有比ABCP大得多的相区。氢键能够影响ABCP的相容性、形态和动态力学性能。