E.s.r. and saturation transfer e.s.r. spectroscopy of spin-probed poly(vinyl acetate): A new spectral transition correlating with glass transition temperature

From the temperature dependence of the rotational frequency of a nitroxide spin probe dispersed in poly(vinyl acetate) the size of the polymer segment involved in the glass to rubber relaxation can be estimated. The calculation is based on a free volume model which relates f, the ratio of the volumes of the spin probe and the polymer segment, to the rotational frequency of the probe. An increase in f with increasing size of the probe is demonstrated. Saturation transfer electron spin resonance spectra of two nitroxide probes are shown to undergo a rapid change with temperature at a temperature T_R which is characteristic of both the probe and its environment. T_R corresponds to a pseudo-isofrequency point and is analogous to, though at a lower characteristic frequency than, the widely-used parameter T_50G. For the systems examined, T_R correlates with the glass to rubber relaxation.     

Glass transition temperature prediction of polymers through the mass-per-flexible-bond principle

A semi-empirical method based on the mass-per-flexible-bond (M/f) principle was used to quantitatively explain the large range of glass transition temperatures (T_g) observed in a library of 132 l-tyrosine derived homo, co- and terpolymers containing different functional groups. Polymer class specific behavior was observed in T_g vs. M/f plots, and explained in terms of different densities, steric hindrances and intermolecular interactions of chemically distinct polymers. The method was found to be useful in the prediction of polymer T_g. The predictive accuracy was found to range from 6.4 to 3.7 K, depending on polymer class. This level of accuracy compares favorably with (more complicated) methods used in the literature. The proposed method can also be used for structure prediction of polymers to match a target T_g value, by keeping the thermal behavior of a terpolymer constant while independently choosing its chemistry. Both applications of the method are likely to have broad applications in polymer and (bio)material science.     

Conformational effects on glass transition temperature and relaxation phenomena of polymers

The glass transition temperature, T_g, of a polymer has been defined as the temperature at which the segmental motions of molecular chains in the quasiequilibrium glassy state overcome the intermolecular attractions. As the thermodynamic criterion of T_g, both of the conditions ΔF = 0 and ?f_r = f^v_r should be accepted, where f_r and f^v_r are the conformational free energy and the intermolecular cohesive free energy per structural unit, respectively, and ΔF the free energy difference per molar chain between the frozen solid part and the flow part, still unfrozen at a given temperature in the vicinity of T_g. The equation for T_g has been derived by use of the criterion for T_g in the present work. The conformational effect on relaxation phenomena (the WLF equation) in polymers has been thermodynamically examined by use of the partition function taking into account both the conformational character of the polymer and the free volume for the polymer liquid. The constant C′₂ in the WLF equation, the ratio of $\text{ø}{}_{\mathrm{g}}\text{α}{}_{\mathrm{f}}$ (with α_f and ø_g being the difference in the volume expansion coefficients and the volume fraction ø at T_g, respectively), derived from the partition function, are in good agreement with the experimental values.     

Facile tailoring of thermal transition temperatures of epoxy shape memory polymers

A critical parameter for a shape memory polymer (SMP) lies in its shape memory transition temperature. For an amorphous SMP polymer, it is highly desirable to develop methods to tailor its T_g, which corresponds to its shape memory transition temperature. Starting with an amine cured aromatic epoxy system, epoxy polymers were synthesized by either reducing the crosslink density or introducing flexible aliphatic epoxy chains. The thermal and thermomechanical properties of these epoxy polymers were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). All the crosslinked epoxy polymers with T_g's above room temperature were found to possess shape memory properties. Overall, our approach represents a facile method to precisely tune the T_g of epoxy SMP polymers ranging from room temperature to 89 °C.     

Relationships between Tg and cohesive energy: 2. Prediction of Tg of homopolymers

An empirical equation expressing T_g of polymers as a function of cohesive energy and steric factor is suggested so that the parameter V_S in this equation may be considered to be an additive quantity. From the data on 44 polymers including 6 homologous series the increments of V_S were calculated for individual structural units. The agreement of the calculated with the measured values of T_g found for homopolymers and copolymers is discussed.     

Exceptional chiroptical behavior near ceiling temperature in free radical polymerization of menthyl 2-acetamidoacrylates

Effect of polymerization conditions on chiroptical properties of polymer has been studied in the polymerization of (−)- and (+)-menthyl 2-acetamidoacrylates using radical initiators under the conditions with various temperatures, monomer concentrations, and reaction times. Specific rotation and circular dichroism of the resulting polymers indicated that a ceiling temperature (T_c) affected the chiroptical properties of the polymers and the polymerizations would give preferentially a helical polymer through a radical vinyl polymerization near T_c. In addition, the helical structure of the polymer was maintained intact even heating at 120 °C in anisole.     

Effects of temperature on elastic behavior of short compact polymers

In this paper, we study further to explore the effects of temperature on the elastic behavior of short compact polymers. Average conformations and thermodynamics statistical properties at various temperatures T are calculated here. Different chain lengths N and elongation ratio λ are also considered simultaneously. From the plots of f and f_U vs. elongation ratio at low temperature, we can know that compact polymers are more close to the native states. With temperature decreasing deeply, polymer chains have the tendency to form globular structures. The results are concluded from: the ratio of 〈L₁²〉/〈L₂²〉 increase abruptly with temperature decreasing at low temperature, and both characteristic ratio 〈R²〉/Nb² and average energy per bond 〈U〉 decreases abruptly with temperature decreasing at low temperature, here L₁², and L₂² are the eigenvalues of the radius of gyration tensor S (L₁²≤L₂²). We also analyze the relationship between the heat capacity C_V and temperature T for different chain lengths in the process of tensile elongation. The coil-to-globule transition temperature T_c can be estimated from the location of the peak on the heat capacity plot as a function of temperature. The plots of 〈R²〉 as a function of chain length N at different temperatures are also shown, and the correlation 〈R²〉∼N^α is obtained at T>T_c or T<T_c, while at T=T_c, the plots are irregular, here α depends on temperature and elongation ratio simultaneously. Elastic force (f), energy contribution to elastic force (f_U), and the ratio f_U/f are also discussed at various temperatures. These investigations may provide some insights into elastic behaviors of compact polymers at different temperatures, especially at low temperature.     

A metallurgical approach to the pre-yield and yield behavior of glassy polymers

This paper discusses some new mechanical and small angle neutron scattering (SANS) data on glassy polymers, both thermoplastics and thermoset resins, from the point of view of dislocation-like defects introduced in the molecular chain arrangement by deformation. In the pre-yield stage, a new parameter, the work-hardening rate K is introduced and its measurement is defined. Experiments are reported which show that K can be used as a very sensitive probe for microstructural changes during physical aging or curing. In one hand, the theory of yielding is revisited to make clear how dislocations and their propagation in polymers depend on specific features like entanglements and chain stiffness. On this basis, experimental internal stresses and activation volumes at yield (i.e., the temperature slope of yield stress) are accounted for. On the other hand, SANS data provide us with experimental evidence at the scale of 10 to 20 Å of the dislocation nature of the molecular “shear defects” introduced in the polymer by deformation. Finally, temperature is known to have a pronounced influence on yield processes. It is shown that two distinct deformation modes exist below and above a critical temperature T_c. Above T_c, a dislocation climb, which probably involves β-processes, gives rise to a “diffusional” deformation mode where chains within a (diffuse) shear band are no longer oriented. A tentative formalization of this behavior, and its relation to the small strain creep of polymers, are then presented.     

Thermo-optic coefficients of polymers for optical waveguide applications

Polymers with diverse chemical structure were evaluated to establish an empirical relationship between their thermo-optic coefficients (dn/dT) and coefficients of thermal expansion (α) according to the Lorentz-Lorenz equation. The results have showed that, regardless their chemical structure and T_gs, all the evaluated polymers follow a single linear relationship between their dn/dT values and α values with the slope (δn/δT)_ρ equal to 0.56 and the interception (ρδn/δρ)_T equal to −3.7×10⁻⁶. The relationship can be used to estimate the dn/dT value of a given polymer using its α value, which is widely available and can be easily measured using TMA and other conventional equipment, and the estimated dn/dT value is useful for developing polymers and evaluating device performance in optical waveguide applications.     

Effect of molecular oxygen on the n.m.r. relaxation times of some aromatic polymers

Proton spin-lattice (T₁) and spin-spin (T₂) relaxation times are reported for poly(2-vinylpyridine), poly(1-vinylanthracene) and polycarbonate in air, oxygen and in vacuo. The results substantiate earlier findings that oxygen complexes with the aromatic rings giving rise to T₁ minima which are not intrinsic to the polymers. This effect appears to be fairly general for aromatic containing polymers. For those polymers containing low temperature relaxations intrinsic to the polymer, the oxygen paramagnetic effect can complicate the relaxation behaviour and, in some cases, totally mask the intrinsic processes. The transitions in T₁ and T₂, due to the torsional oscillation of the anthracene side group in poly(1-vinylanthracene), is much better defined than the corresponding transitions for previously reported aromatic vinyl polymers. The activation energy for the motion of the side group is comparable to that of polystyrene and poly(N-vinylcarbazole).     

Correlation between glass transition temperature and molecular mass in non-polymeric and polymer glass formers

It is shown that the glass transition temperature T_g of molecular (non-polymeric) glass formers correlates with molecular mass M as T_g(M) ∝ M^α, α = 0.51 ± 0.02. The subclasses of molecular glasses with homologous chemical structure but different M exhibit a similar universal correlation with significantly lower scatter. A possible explanation of T_g vs M correlation in molecular glasses is suggested. Comparing molecular glasses with polymers we found that in polymers T_g(M) dependence at small M (short chains) is similar to that in molecular glasses. At further increasing of the chain length the T_g(M) dependence in polymers begin to deviate from the universal T_g(M) correlation of molecular glasses and eventually saturates at some polymer specific T_g∞ value. We conclude that at least a substantial part of T_g(M) dependence of low-M polymers is common with molecular glasses mechanism that does not require chain-like structure. In particular, the model of T_g(M) dependence in polymers based on additional free volume on chain ends is not fully adequate at small M. Our picture provides an alternative explanation that in polymers a mechanism is in action which leads to a saturation of the normal T_g(M) dependence common with molecular glasses.     

From the glassy state to ordered polymer structures: A microhardness study

The review covers the understanding of the nanostructure development in glassy and semicrystalline polymers as revealed by indentation hardness methods. The microhardness of polymer glasses is discussed emphasizing the influence of thermal history and physical ageing. The correlation between hardness and glass transition temperature is brought in. Furthermore, the role played by the lamellar morphology in the case of amorphous blends of a block copolymer and a glassy homopolymer is highlighted. A discussion on the influence of filler structure on the microhardness of polymer glasses is introduced. Indentation hardness is presented as a valuable tool to study the kinetics of crystallization from the glassy state. As an example, distinct results on polymer systems under different confinement conditions are shown. The nanostructure-microindentation hardness correlation in the case of semicrystalline polymers and the influence of degree of crystallinity and crystal thickness for various flexible and semirigid polymer systems are recalled. A comprehensive discussion of the creep properties of polymer materials is offered. Concerning deformation mechanisms, experimental results show that for polymers with low degree of crystallinity and T_g below room temperature, a large deviation from the microhardness additivity law is always found. This is due to a different deformation mechanism with respect to that envisaged for polymer materials with T_g above room temperature. The assumption that microhardness approaches zero for amorphous materials above T_g is experimentally confirmed. In the case of an oriented material, it is shown that indentation hardness is capable to detect the gradual appearance of phases of intermediate order. In addition, the study of the creep properties also yields valuable information on the internal degree of order of the oriented system. Finally, an overview of the future perspectives of the application of depth-sensing indentation to the study of polymer materials is offered.     

Optimization of poly(1,4-cyclohexylidene cyclohexane-1,4-dicarboxylate) (PCCD) preparation for increased crystallinity

A key factor, which affects the crystallization temperature on cooling (T_c) of PCCD is the cis/trans isomer ratio of the cyclohexyl diester in the polymer. Isomerization of pure dimethyl-trans-1,4-cyclohexanedicarboxylate can occur during the polymerization, and the T_c on cooling decreases along with the trans content. The isomerization reaction is enhanced with temperature, time and catalyst amount, and these variables should be minimized to prepare PCCD polymers with high T_c. However, these same variables also control the molecular weight growth of the polymer, and so a compromise between the best conditions for high T_c and those for high M_w must be made. A set of optimized conditions were obtained leading to PCCD polymers with M_w of 75,000-80,000 and T_c on cooling of 164-167 °C. Solid state polymerization was used to prepare high molecular weight PCCD with a high level of crystallinity (T_c on cooling ∼193 °C). It was also shown that adding small amounts of supplementary diols facilitates PCCD preparation by ensuring that high molecular weight PCCD polymers will be obtained even when the stoichiometry of monomer feed is off by >3%, i.e. conditions which would otherwise lead to low M_w. Finally, less crystalline PCCD's have been prepared via either incorporation of diethylene glycol or increasing the cis-diester amount in the polymer.     

Structural relaxation in chalcogenide glasses

The structural relaxation of chalcogenide glasses is discussed within Tool–Narayanaswamy–Moynihan (TNM) formalism. The TNM parameters for more than 70 different glassy compositions are compared on the basis of the relaxation rate defined as R_f(ΔT) = −(dT_f/dlogt)_i at the inflection point of the isothermal relaxation curve plotted on a logarithmic timescale. The R_f(ΔT) depends on the TNM parameter ß and the parameter σ, combining the nonlinearity parameter x, the effective activation energy h^* or the fragility m. It is shown that R_f(10) estimated at 10 K below T_g is useful for the prediction of structural relaxation kinetics in different amorphous materials. The chalcogenide glasses are, for example, compared with oxide glasses and organic polymers. For all these materials, the R_f(10) versus σ plot shows a well-defined pattern that is thoroughly discussed.     

Orientation and relaxation in uniaxially-stretched poly(2,6-dimethyl 1,4-phenylene oxide) — atactic polystyrene blends

Infra-red measurements of the dichroic ratio of atactic polystyrene and poly(2,6-dimethyl 1,4-phenylene oxide) absorption bands provide a valuable method for the determination of orientation as well as relaxation of chains of both polymers during stretching of their compatible blends. Influence of strain rate, temperature of stretching, and molecular weight of the polymers on orientation of both polymer chains in blends containing up to 35% PPO has been studied. Orientation relaxation for both polymers has been analysed using Lodge's constitutive equation. Master curves have been obtained for PPO and PS in the blends at a reference temperature T₀ = T_g + 10°C. Results are interpreted in terms of an hindrance of relaxation of PS chains induced by interaction with a highly-oriented PPO network which slowly relaxes.     

Flow and dynamic behavior of dilute polymer solutions in hydrodynamic chromatography

Recently, hydrodynamic chromatography (HDC) has become an important probe for determining the molecular size or molecular shape in the sub-micron range. Although a lot of studies on HDC were performed, the clear understanding on the transport behavior of polymer solutions in porous media has not been achieved yet. In this study, the flow and dynamic behavior of polymer molecules in a packed HDC column is fully analyzed by extending the molecular kinetic approach of dilute polymer solution in aconfined geometry to elucidate the effects of relativeparticle sizes as well as theflow strength on theretention factor (R _f ). R _f equation of each simple polymer model was developed- and the numerical simulation was worked out to illustrate the R _f for rigid rod(RR) polymers. Theoretical predictions were in remarkable agreement with our experimental results of xanthan gum and other published data despite of several approximations. Significant size effects were observed, and for RR model the R _f decreased with increasing the flow strength within a particular range. This feature emphasized a transition behavior from weak to strong flow due to theorientalional effect of xanthan molecules. It should be noted that our major concern is restricted solely to the hydrodynamic force.     

Probe dependency of polymer‐plasticizer and polymer‐polymer interaction parameters in inverse gas chromatography

Inverse gas chromatography has been widely used to determine the Flory–Huggins parameter, χ, between a plasticizer and a polymer, or between two polymers. Many studies showed that interaction parameters may be probe dependent. In a recent study it was proposed that, when a specific interaction occurred between two polymers, the probes had less interaction with the polymers, leading to a lower solubility parameter for polymer blends than the volume average of the components. An equation was derived to relate the probe dependency to the deviation of solubility parameter of polymer mixtures. Here this approach is applied to plasticized poly(vinyl chloride) (PVC) and a copolymer, and to poly(vinylidene fluoride)–poly(ethyl methacrylate) blends. For a PVC and epoxidized oil system the relative deviation of specific retention volume showed two trends, with saturated hydrocarbons as one group, and polar and aromatic probes as another group. For the poly(vinylidene fluoride)/poly(ethyl methacrylate) system the plot of retention volume deviation versus solubility parameter of probes also showed separate trends for n‐alkanes, esters, and alcohols. But the plot of ?₂?₃RT(χ₂₃/V₂) versus solubility parameter had better linearity for the systems studied. The slope of this plot was used as an indicator for miscibility. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Dilute solution properties of hydrophobically associating polyacrylamide: fitted by different equations

In this paper six different equations were used to fit the data of dilute solution of both nonionic (PBAM3) and ionic (PBAMS) hydrophobically associating polyacrylamide with and without the addition of NaCl. The results showed that Fedors equation was the most accurate equation to describe the dilute solution properties of these kinds of polymers. The lower value of the polymer concentration parameter (C_m) in Fedors equation was corresponded to the higher value of the constant k in Schulz-Blaschke equation for PBAM3 system. Two types of polymers had anti-polyelectrolyte effect. In dilute solution the PBAMS polymer chains had a more extended conformation than those of PBAM3 due to the existence of ionic groups.     

Elevated temperature strength and thermal shock behavior of hot-pressed carbon fiber reinforced TiC composites

In order to improve the elevated strength and thermal shock resistance of TiC materials, 20vol% short carbon fiber-reinforced TiC composite (C_f/TiC) was produced by hot pressing. With carbon fiber addition, the strength and fracture toughness of TiC is increased remarkably, and the elastic modulus and thermal expansion coefficient are decreased. The strength value of C_f/TiC composite is 593 MPa at room temperature and 439 MPa at 1400°C, and the fracture toughness value at room temperature is 6.87 MPa m^1/2. The thermal stress fracture resistance parameter, R, thermal stress damage resistance parameter, R^IV, and thermal stress crack stability parameter, R_st, are all increased. The residual strength decreases significantly when the thermal shock temperature difference, ΔT, is higher than 900°C, and the residual strength is 252 MPa when ΔT is 1400°C. Carbon fiber reinforced-TiC composite exhibits superior resistance to thermal shock damage compared with monolithic TiC. The catastrophic failure induced by severe thermal stresses can be prevented in C_f/TiC composite.     

pH-Controlled association of PEG-containing terpolymers of N-isopropylacrylamide and 1-vinylimidazole

Temperature- and pH-controlled association of terpolymers of N-isopropylacrylamide (NIPA) with 1-vinylimidazole (VI) and polyethylene glycol (PEG) has been investigated by light scattering and atomic force microscopy (AFM) in situ. The polymers contained 0-15 mol% VI and 0-2 mol% PEG. The phase transition temperatures (T_p) have been in the range of 32-45 °C and exhibited significant dependence on the pH of solution in the pH range between 5 and 8. The T_p of the polymers increased with increasing VI content and with decreasing pH, confirming major effect of VI ionization status on T_p. The presence of PEG grafts in the polymer structure had augmenting effect on the magnitude of pH-responsiveness and on the pH-independent colloidal stability of the polymer particles formed above T_p. Incorporation of VI into the polymer structure had similar, but pH-dependent effect on colloidal stabilization of the polymer particles. The size of the particles formed after the phase transition is driven by the association of the collapsed NIPA segments in the globule conformation and it decreased with decreasing pH. The phase transition temperature of the polymers could be adjusted to increase from temperatures below, to temperatures above body temperature upon decreasing pH from 7 to 6, suggesting that such polymers could provide a material platform for a variety of biomedical applications. AFM analysis in situ showed a fully reversible formation of particles in the solutions of the polymers above their T_p.