Infrared dichroism of glassy polycarbonate at small strains

The dichroism of the 889, 1364, and 3063 cm^?1 infrared absorption bands of glassy, amorphous polycarbonate has been measured as a function of the strain in the range 0 to 2% at 23°C. The data obtained for these three bands superpose rather well over this strain range. Negligible dichroism is observed up to about 0.6% strain; above this level, the dichroic ratio increases in an approximately linear manner. Independent mechanical data, obtained under comparable conditions of time-scale and temperature, are cited which show that a transition from approximately linear to marked non-linear viscoelastic behavior occurs with glassy polycarbonate in the range 0.7 to 1.0% strain. The coincidence on the strain axis of a relatively abrupt increase in optical absorption anisotropy with a distinct change in Young's modulus is discussed in terms of a recent molecular theory of deformation of glassy polymers. It is suggested that the data are consistent with the view that the transition from linear to nonlinear viscoelastic behavior in glassy polycarbonate is marked by the onset of significant rotation around backbone bonds.     

Some optical and mechanical properties of poly(vinyl chloride)

The optical and mechanical properties of poly(vinyl chloride) film were examined by observing both the stress and birefringence during stretching at constant rate, during relaxation at constant length and during a dynamic birefringence experiment. Experiments were also done by varying the temperature at constant length. The changes in birefringence are interpreted in terms of changes in negative distortional birefringence, changes in positive orientation birefringence, and possible reversible changes in birefringence with temperature arising from conformational changes in the polymer chain and changes in the contribution of birefringent crystals.     

Effect of morphology on the mechanical and rheo-optical properties of a styrene–butadiene–styrene block copolymer

The mechanical and rheo-optical properties of a styrene–butadiene–styrene block copolymer of a given chemical composition are dependent upon the morphology of the polymer as affected by the solvent system from which a polymer film is cast. Films cast from methyl ethyl ketone and from toluene are compared. Properties found to differ are the stress–strain curve, the birefringence–strain curve, stress relaxation birefringence relaxation, and the dynamic mechanical spectra.     

Phenomenological analysis of elongational flow birefringence in semidilute solutions of hydroxypropylcellulose

The relaxation time of a polymer chain in an elongational flow field was investigated for hydroxypropylcellulose (HPC) semidilute solution systems by two methods: phenomenological analysis of elongational flow-induced birefringence, and dynamic light scattering (DLS) and rheological measurements. To understand the relaxation time of an entangled semiflexible polymer solution in an elongational flow field, scaling analysis of the elongational flow-induced birefringence curve was performed. The results of both temperature and concentration scaling analyses showed that birefringence curves at different temperatures and at several HPC concentrations were described well by a universal birefringence–strain rate curve. This scaling behavior was compared with the "fuzzy cylinder" model. The critical strain rate corresponded to the correlation time of the slow relaxation mode determined by DLS measurement and the relaxation spectrum obtained by dynamic viscoelasticity measurement. The elongational flow-induced birefringence observed in an HPC semidilute solution was concluded to be attributed to the orientation of the HPC segment in the entangled molecular system, because the dominant relaxation mode is found to be the concentration fluctuation of an entangled molecular cluster in a quiescent state.     

Comparison of mechanical and molecular measures of mobility during constant strain rate deformation of a PMMA glass

We performed constant strain rate deformation and stress relaxation on a poly(methyl methacrylate) glass at T_g – 19 K, utilizing three strain rates and initiating the stress relaxation over a large range of strain values. Following previous workers, we interpret the initial rate of decay of the stress during the relaxation experiment as a purely mechanical measure of mobility for the system. In our experiments, the mechanical mobility obtained in this manner changes by less than a factor of 3 prior to yield. During these mechanical experiments, we also performed an optical measurement of segmental mobility based on the reorientation of a molecular probe; we observe that the probe mobility increases up to a factor of 100 prior to yield. In the post‐yield regime, in contrast, the mobilities determined mechanically and by probe reorientation are quite similar and show a similar dependence on the strain rate. Dynamic heterogeneity is found to initially decrease during constant strain rate deformation and then remain constant in the post‐yield regime. These combined observations of mechanical mobility, probe mobility, and dynamic heterogeneity present a challenge for theoretical modeling of polymer glass deformation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1957–1967     

Structure and dynamics of a bisurea-based supramolecular polymer in n-dodecane

The structure and dynamics of a supramolecular polymer formed by a bisurea-type compound, 2,4-bis(2-ethylhexylureido)toluene (EHUT), in an apolar solvent, n-dodecane (C12), were examined in detail. The EHUT/C12 organo-gel system forms long, dynamic chain-like supramolecular polymers, which lead to an entangled network showing remarkable viscoelastic behavior with two major relaxation modes. A slow relaxation mode with an approximately constant relaxation time, tauS, was observed in a flow region and the other, fast, relaxation mode with a time tauF1 (相似文献   

Athermal simulation of plastic deformation in amorphous solids at constant pressure

An algorithm is introduced for the molecular simulation of constant-pressure plastic deformation in amorphous solids at zero temperature. This allows to directly study the volume changes associated with plastic deformation (dilatancy) in glassy solids. In particular, the dilatancy of polymer glasses is an important aspect of their mechanical behavior. The new method is closely related to Berendsen's barostat, which is widely used for molecular dynamics simulations at constant pressure. The new algorithm is applied to plane strain compression of a binary Lennard-Jones glass. Conditions of constant volume lead to an increase of pressure with strain, and to a concommitant increase in shear stress. At constant (zero) pressure, by contrast, the shear stress remains constant up to the largest strains investigated (ε = 1), while the system density decreases linearly with strain. The linearity of this decrease suggests that each elementary shear relaxation event brings about an increase in volume which is proportional to the amount of shear. In contrast to the stress–strain behavior, the strain-induced structural relaxation, as measured by the self-part of the intermediate structure factor, was found to be the same in both cases. This suggests that the energy barriers that must be overcome for their nucleation continually grow in the case of constant-volume deformation, but remain the same if the deformation is carried out at constant pressure. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2057–2065, 2004     

Dynamic strain-birefringence apparatus for polymer films

A new apparatus for investigating dynamic strain birefringence in polymer films at frequencies up to 10 Hz and at various degrees of internal sample orientation is described. Samples are elongated at constant low rates while simultaneously being strained sinusoidally. Fast changes in sample retardance are recorded while slow changes are automatically compensated with a servo-controlled Soleil-Babinet compensator. The signal-to-noise ratio of the system is greatly enhanced by incorporating a highly monochromatic laser light source, a synchronous amplifier, and a light beam modulator based on a rotating polarizer. Data obtained from this apparatus can be used to elucidate polymer relaxation mechanisms at various frequencies over a wide range of static strains.     

Rheo-optical studies of polyurethane block polymers

Stress and birefringence relaxation have been measured for two polyurethane block polymers at several temperatures to 140°C. Superposition of the stress curves is possible, but the existence of multiple relaxation mechanisms makes such a process of questionable validity. Simple superposition of the birefringence relaxation was not possible. The time and temperature dependence of the stress-optical coefficient implies different mechanisms for mechanical and optical relaxation process. Analysis of the birefringence data requires consideration of nonorientational sources of birefringence.     

Rheooptical investigation of the crystallization of poly(ethylene terephthalate) under tensile strain

The crystallization of poly(ethylene terephthalate) under uniaxial tensile strain at different extension rates was investigated with optical polarimetry in a temperature range between the glass-transition temperature and the quiescent crystallization temperature. The evolution of the optical properties of the polymer, including the turbidity, birefringence, and dichroism, were monitored simultaneously with the mechanical parameters. To complete the semicrystalline microstructure characterization of the polymer under strain, an online wide-angle X-ray diffraction (WAXD) technique was used in separate experiments, which were performed under the same thermomechanical conditions. For real-time measurements, a high-energy synchrotron radiation source was used. The optical properties provided information about both the crystalline and amorphous phases, whereas the WAXD patterns essentially gave information about the crystalline phase. The two experimental techniques were then used in a complementary way to characterize the semicrystalline microstructure. Significant deviations from the stress-optical rule were found. This was attributed to both transient effects and the appearance of crystallites, which consisted of highly oriented molecular segments that could contribute to the optical anisotropy but not necessarily to the stress. The behavior of the optical dichroism was found to be qualitatively different from that of the birefringence. The latter monotonically increased with the strain, whereas the former first increased with the strain, passed through a maximum, and then decreased to a steady-state value. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1915–1927, 2004     

Stress birefringence in amorphous polymers under nonisothermal conditions

The relationship of birefringence to stress in an amorphous polymer was studied, with emphasis on conditions of high stress and rapid cooling. The latter (nonisothermal) conditions are important in connection with studies of polymer processing operations. Polystyrene was pulled at a constant elongation rate (0.075 to 2 sec^?1 in the present and related work) under both isothermal conditions (in the range 120 to 157°C) and nonisothermal conditions (with cooling rates in the range 0.6 to 1.7°C/sec). Generally we conclude that stress in proportional to birefringence under a wide range of conditions, except that a nonlinear regime appears at stresses higher than about 10⁷ dyn/cm². In this regime, stress increases more rapidly with deformation than does birefringence.     

Stress-optical properties of bimodal polymer networks

The optical properties of bimodal poly(dimethylsiloxane) (PDMS) networks were studied with special emphasis on the non-linear stress optical properties exhibited by these materials. In particular the effect of chain length, and junction functionality on the strain induced birefringence was investigated. It is shown that for the non-linear properties to clearly manifest themselves a critical concentration of short chains is essential and that the junctions are tetra-functional. However, all bimodal compositions studied were found to exhibit a non-linear variation of birefringence with strain irrespective of the junction functionality. The optical properties of the unimodal networks were found to vary linearly with stress and strain as expected. The transition from non-linear to linear optical behavior on increasing the molecular weight of the short chains is also established.     

Molecular orientation behavior of poly(vinyl chloride) as influenced by the nanoparticles and plasticizer during uniaxial film stretching in the rubbery stage

The structural evolution during uniaxial stretching of poly(vinyl chloride) films was studied using our real time spectral birefringence stretching machine. The effect of clay loading and the amount of plasticizer as well as the rate effects on the birefringence development and true mechanical response are presented with a final model summarizing the molecular phenomena during stretching. Mechano‐optical studies revealed that birefringence correlated with mechanical response (stress, strain, work) nonlinearly. This was primarily attributed to the preexisting strong network of largely amorphous chains connected via small crystallites that act as physical crosslinking points. These crystallites are not easily destroyed during the high‐speed stretching process as evidenced from the birefringence–true strain curves along with the X‐ray crystallinity measurements. At high speeds, the amorphous chains do not have enough time to relax and hence attain higher orientation levels. The crystallites, however, orient more efficiently when stretched at slow speeds. Apparently, some relaxation of the surrounding amorphous chains helps rotate the crystallites in the stretching direction. Overall birefringence is higher at high stretching speeds for a given true strain value. When the nanoparticles are incorporated, the orientation levels are increased significantly for both the crystalline and amorphous phases. Nanoplatelets increase the continuity of the network because they have strong interaction with the amorphous chains and/or crystallites. This in turn helps transfer the local stresses to the attached chains and increase the orientation levels of the chains. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 724–742, 2005     

Orientation of cylindrical microdomains of triblock copolymers by in situ stress–strain‐ birefringence measurements

The orientation behavior of polystyrene‐block‐hydrogenated polyisoprene‐block‐polystyrene (SEPS) with cylindrical microdomains of polystyrene (PS) dispersed in the rubbery segments was investigated by simultaneous measurements of stress and birefringence during uniaxial stretching. The stress increased sharply with strain below the yield strain and then it gradually increased. In contrast, the birefringence changed little below the yield strain, increased sharply with strain above the yield strain up to a strain of 0.5, and then gradually increased. The characteristic birefringence behavior is attributed to the form birefringence induced by the orientation and the parallel arrangement of the cylindrical microdomains associated with the orientation of the rubbery segments. The orientation function of the cylindrical microdomains f evaluated by analyzing the form birefringence agrees well with that obtained from the SAXS result. The f was almost zero below the yield strain and it increased sharply with strain up to a strain of 0.5 and then was constant above a strain of 0.5. These results suggest that the orientation of the cylindrical microdomains occur above the yield strain up to a strain of 0.5 and that the orientation does not increase above a strain of 0.5 in spite of the continuous orientation of the rubbery ethylene–propylene segments. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 715–723, 2009     

Thermal characterization of a series of poly(vinylidenefluoride–chlorotrifluoroethylene–trifluoroethylene) terpolymer films

A number of solution-casted poly(vinylidenefluoride–chlorotrifluoroethylene–trifluoroethylene) [P(VDF–CTFE–TrFE)] terpolymer films with different CTFE content have been characterized by a series of thermal analysis techniques, including thermogravimetric analysis (TG), differential scanning calorimetry, dynamic mechanical analysis (DMA) and thermal mechanical analysis (TMA). The work intends to provide more comprehensive information about thermal behavior of these ferroelectric polymers. TG results suggest that the introduction of the CTFE units slightly decreases the thermal stability of the polymer due to the instability of C–Cl bond during heating. DMA detected a relatively weak α_a relaxation and a broad α_c relaxation in the samples of low CTFE content. These two relaxation processes completely mixed together in the sample with high CTFE content, revealing the crystalline structures in the polymer, become a more imperfect and diffuse state as CTFE units increasing. The polymer with less CTFE units possesses an enhanced stiffness due to its higher degree of crystallinity. A contraction process after a slight amount of thermal expansion upon heating is detected by TMA, due to the release of internal tensile strain/stress generated during solidification of the films. The higher crystallinity of the polymer film generated the greater strain/stress, leading to the larger degree of shrinkage. Also, the higher melting point of the polymer with less CTFE units allows the film soften at a higher temperature.     

Apparatus for infrared measurement of sorption/desorption in strained polymeric films

A new apparatus has been developed for optical measurement of sorption/desorption in transparent polymer films at a given strain or stress. The technique utilizes a chosen infrared absorption frequency of the diffusing vapor in a spectral region where the film has negligible absorption. From the time dependence of the IR absorption at this frequency the sorption/desorption behavior of the film may be determined at any strain or stress. The simultaneous measurement of mechanical relaxation as a function of the amount of sorbed vapor is also possible. The results presented here show the applicability of the apparatus for determining the transport and mechanical properties of a low-density polyethylene film in ethyl acetate vapor at 30°C.     

The influence of sub-Tg annealing on environmental stress cracking resistance of polycarbonate

To explore the effect of physical aging on environmental stress cracking (ESC) behavior of polycarbonate (PC), sub-T_g annealing was utilized as a method for accelerated aging. Injection molded samples were annealed at 130 °C for different time varying up to 96 h. A three point bending apparatus was used to evaluate critical stress for crazing and to record the variation of stress with immersion time at constant strain. The ESC results indicated that the critical stress for crazing initiation of PC in ethanol is increased by sub-T_g annealing. However, the resistance of annealed PC to ESC with immersion time during the stress relaxation test depends on the level of initial stress. When a relatively low initial stress was used, a short time (24 h) of sub-T_g annealing reduced the stress relaxation rate and decreased the number of cracks on the surface of PC. However, under higher initial stress, the stress relaxation rate of PC had a slight change only when the annealing time was prolonged about threefold (72 h). This can be explained by the formation of cohesional entanglement sites during the sub-T_g annealing process, which was demonstrated by the thermal and dynamic mechanical tests.     

Thermodynamic theory of viscoelasticity

The relaxation spectra in polymers arise from the existence of many possible modes for dissipating the strain energy raised by the imposed force. These modes are made up by coupling the simplest and fastest mode of relaxation involving the rotation of a conformer, typically represented by the picosecond rotation of the carbon to carbon bond. This fast relaxation process cannot take place easily in the condensed state crowded by the densely packed conformers, necessitating cooperativity among them. The domain of cooperativity grows at lower temperatures, toward the infinite size at the Kauzman zero entropy temperature. From the temperature dependence of the domain size, the well-known Vogel equation is derived, which is numerically equivalent to the empirical WLF and free volume equations. The molar volume is a crucial factor in determining the molar free volume and, therefore, in determining theT _g of a material. The molar ΔC _P is proportional to the logarithmic molar volume, and is greater for a polymer with a higherT _g, but ΔC _P per gram for it is smaller, as it is proportional to (logM) divided byM, whereM is the molecular weight of the conformer. From this theory, it is possible to predict the dependence of the characteristic relaxation time on temperature if eitherT _g or the conformer size is known, since one can be derived from the other. From the Vogel equation with all parameters thus derived, it is possible to obtain a master relaxation curve and the spectrum from one set of dynamic mechanical data taken at one frequency over a range of temperatures. Whereas the linear viscoelastic principle is limited to small strains only, a real polymer is often deformed well beyond such a limit. Above a certain limit of strain energy level, linear viscoelastic deformation is no longer possible and the plastic deformation takes over. However, because a polymer typically manifests a spectrum of relaxation times, its behavior is a combination of viscoelastic and plastic behaviors. The ratio between the two behaviors depend on the rate of deformation, and can be precisely predicted from the linear viscoelastic relaxation spectrum. The combined behavior is termed viscoplasticity, and it applies to a wide range of practically important mechanical behaviors from the flow of the melt to the yield and fracture of glassy and crystalline solids.     

Unusual contributions of molecular architecture to rheology and flow birefringence in hyperbranched polystyrene melts

With the increase in sophisticated synthesis methods, it appears that polymer architecture may be a tunable property. Therefore, the role of architecture in rheological and processing properties has received renewed attention, mainly because of dendrimer synthesis and metallocene‐catalyst technology. Linear polymers and hyperbranched polymers represent two ends of branching complexity. Some previous studies have suggested that hyperbranched polymers may behave like unentangled polymers, whereas others have proposed that they exhibit the properties of soft colloids. In an effort to compare the responses of linear and hyperbranched polymers, we synthesized starlike hyperbranched polystyrenes (HBPSs) of various branch lengths and numbers of branches. The HBPSs used in this study were unentangled or weakly entangled, allowing us to study the effect of branch density more readily. Two linear polystyrene (L‐PS) melts and two HBPSs were studied. Using a custom‐built rheooptical apparatus, we characterized the rheology and flow birefringence of these materials. To our knowledge, these are the first flow birefringence measurements on highly branched polymer melts. Our results suggest that the flow behavior of HBPS is significantly different from that of L‐PS: (1) HBPS shows nonterminal behavior in the low‐frequency rheological response; (2) when the stress‐optical rule (SOR) holds, the stress‐optical coefficient of HBPS is much lower than those of analogous linear polymers; and (3) when the branch density is high and the branch length is sufficiently low, the SOR fails for these homopolymer melts. A significant increase in the birefringence for a given amount of stress in the low‐frequency region suggests that there may be a soft core in these materials due to the strong preferential radial orientation of chain segments near the center of a molecule versus those near the periphery. The predominantly elastic response of the soft structures may be responsible for the enhanced form birefringence. Our preliminary results indicate that these materials may exhibit both polymeric and soft‐colloid natures. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2562–2571, 2001     

High-temperature mechanical behavior of unplasticized poly(vinyl chloride)

We studied the high-temperature mechanical behavior of unplasticized poly(vinyl chloride) by stress relaxation experiments covering a temperature range of 100–180°C and a time interval of 0.01–3600 sec at an elongation of 40%. The polymer was observed to respond elastomerically within these set temperature limits. The molecular basis for this behavior is linked to the partial crystalline nature of this material. The crystalline phase is postulated to be the seat of the relaxation process taking place in the rubbery response region, and evidence is presented supporting this point of view. A comparison is made between free volume variations arising from thermal expansion and those originating instead from uniaxial straining. It is also pointed out that, following linear viscoelastic theory, the viscosity is a time-dependent quantity, and it is shown that for this polymer its variation with temperature at short times is ninefold less than the corresponding variations in its limiting value. Finally it is noted that the measured high-temperature tensile strength corresponds to a chain bond strength approximately 1/300 of the theoretical value.