Preparation and glass transition temperature of hyperbranched poly[allyl methyl maleate‐co‐N‐propyl maleimide]

The kinetics and molecular weight averages of the hyperbranched polymers formed by the alternating copolymerization of equimolar allyl methyl maleate (AMM) and N‐n‐propyl maleimide (PMI) were investigated. The yields, molecular weight averages, and polydispersity indices as well as the branching degrees of the produced copolymers increased with increasing initiator concentrations and prolonged polymerization time. The trends of the experimental molecular weights as determined by size exclusion chromatography were in good agreement with the theoretical predictions. The molecular weight distribution indices fit the curve given by M_w/M_n = 1/(1‐x_D), and the molecular weights fit the curve given by M_w = 4076/(1‐x_D)², where x_D was the conversion of vinyl groups. DSC studies demonstrated a nonlinear relation of T_g values to the reciprocal of molecular weight (M), and T_g values decreased with the increase of molecular weight. For the T_g values of highly branched polymers in high molecular weight range, a relation of T_g = T + k/M was obtained, where T was obtained by extrapolating to infinite molecular weight and k was a constant. T was 136°C, and k = 2.9 for this work. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1941–1947, 2005     

Stereocomplex formation between poly(L‐lactic acid) and poly(D‐lactic acid) with disproportionately low and high molecular weights from the melt

Poly(L ‐lactic acid) (PLLA) and poly(D ‐lactic acid) (PDLA) with very different weight‐average molecular weights (M_w) of 4.0 × 10³ and 7.0 × 10⁵ g mol^?1 (M_w(PDLA)/M_w(PLLA) = 175) were blended at different PDLA weight ratios (X_D = PDLA weight/blend weight) and their crystallization from the melt was investigated. The presence of low molecular weight PLLA facilitated the stereocomplexation and thereby lowered the cold crystallization temperature (T_cc) for non‐isothermal crystallization during heating and elevated the radial growth rate of spherulites (G) for isothermal crystallization, irrespective of X_D. The orientation of lamellae in the spherulites was higher for the neat PLLA, PDLA and an equimolar blend than for the non‐equimolar blends. It was found that the orientation of lamellae in the blends was maintained by the stereocomplex (SC) crystallites. Although the G values are expected to decrease with an increase in X_D or the content of high‐molecular‐weight PDLA with lower chain mobility compared with that of low‐molecular‐weight PLLA, G was highest at X_D = 0.5 where the maximum amount of SC crystallites was formed and the G values were very similar for X_D = 0.4 and X_D = 0.6 with the same enantiomeric excess. This means that the effect of SC crystallites overwhelmed that of chain mobility. The nucleating mechanisms of SC crystallites were identical for X_D = 0.1–0.5 in the T_c range 130–180 °C. Copyright © 2011 Society of Chemical Industry     

Temperature and concentration dependence of diffusion coefficient in dilute solutions

Temperature and molecular weight dependence of k_D in D(C) = D(O) [1 + Ck_D], where D(C) is the diffusion coefficient for the density fluctuations in a dilute polymer solution, is investigated by first expressing D(C) as a function of the static structure factor S(q,C) within the framework of the Kirkwood-Riseman theory. The continuous transition of k_D from negative values under theta conditions to positive values in good solvents is calculated using various models for the intermolecular interaction potential and the results are presented graphically as function of a reduced variables $\text{S}\text{?}\text{R}{}_{\mathrm{H}}$ that combines both molecular weight and temperature effects. It is shown that the negative value of k_D at the theta temperature can be explained at least partially, in terms of an increase in the chain dimensions of two overlapping molecules. The concentration dependence of the self-diffusion coefficient is also discussed.     

Pulsed electron beam irradiation of dilute aqueous poly(vinyl methyl ether) solutions

A dilute aqueous solution of the temperature-sensitive polymer, poly(vinyl methyl ether) (PVME), was irradiated by a pulsed electron beam in a closed-loop system. At temperatures, below the lower critical solution temperature (LCST), intramolecular crosslinked macromolecules, nanogels, were formed. With increasing radiation dose D the molecular weights M_w increase, whereas the dimensions (radius of gyration R_g, hydrodynamic radius R_h) of the formed nanogels decrease. The structure of the PVME nanogels was analyzed by field emission scanning electron microscopy (FESEM) and globular structures with d=(10-30) nm were observed. The phase-transition temperature of the nanogels, as determined by cloud point measurements, decreases from T_cr=36 °C (non-irradiated polymer) to T_cr=29 °C (c_p=12.5 mM, D=15 kGy), because of the formation of additional crosslinks and an increase in molecular weights. The same behavior was observed for a pre-irradiated PVME (γ-irradiation) with higher molecular weight due to intermolecular crosslinks. After pulsed electron beam irradiation the molecular weight again slightly increases whereas the dimension decreases. Above D=1 kGy the calculated ρ-parameter (ρ=R_g/R_h) is in the range of ρ=0.5-0.6 that corresponds to freely draining globular structures.     

Lindemann-like size-independent glass-transition criterion for polymers

It is well known that the intrinsic melting mechanism is independent of crystal size according to Lindemann's melting criterion. In order to probe whether the glass transition mechanism is also size-independent, segment dynamics of free-standing polystyrene (PS) films is determined by considering the temperature- and thickness-dependent number of styrene segments N_α(T,D) in the cooperative rearranging region (CRR). Under the help of Adams-Gibbs glass transition theory and molecular dynamics simulation, N_α(T,D) function is established and it decreases as D decreases or T increases. However, N_α[T_g(D),D] at the glass transition temperature T_g(D) is size-independent, which is consistent with the simulation results obtained by Donth's method. Meanwhile, its relative temperature function N_α{[T − T_g(D)]/T_g(D)} is also size-independent. Therefore, N_α[T_g(D),D] function as a criterion for glass transition, which describes the physical nature of the glass transition, is similar to the vibrational amplitude in Lindemann's melting criterion.     

Synthesis and Properties in Solution of Gaussian Homo Asymmetric Polysiloxanes with a Bulky Side Group

Ring opening polymerization (ROP) of 1,3,5-tri-n-hexyl,1,3,5-trimethylcyclotrisiloxane (D ₃ ^Hexa ) and 1,3,5-tri-n-heptyl,1,3,5-trimethylcyclotrisiloxane (D ₃ ^Hepta ) was promoted by acid-treated synthetic silica–alumina to obtain Gaussian homo asymmetric polysiloxanes. M_w was above 70?kg/mol, meaning that homo asymmetric bulky side-group polysiloxane chains with high molecular weight were obtained. The material was treated in an acidic medium to improve the contents of acid sites and successfully tested as an inorganic acidic catalyst for ROP of D ₃ ^Hexa and D ₃ ^Hepta cyclosiloxanes. The samples of poly(methylhexylsiloxane) (PMHS) and poly(methylheptylsiloxane) (PMHepS) obtained were structurally characterized mainly by ²⁹Si NMR. All the experimental values including the refractive index increment (dn/dc), the second virial coefficient (A₂), the square root of the mean square radius of gyration ( $ \langle {{\text{RMS}}_{\text{radius}}}^{ 2} \rangle^{ 1/ 2} $ ), the average molecular weight (M_w), the average molecular numeral (M_n), and the weight polydispersity (M_w/M_n) were obtained using a gel permeation chromatography/light scattering (GPC/LS) coupled system. The A₂ experimental value for the two polymers (between 4 and 6.5?×?10^?4?mol/mL?g²) indicated that toluene was a good solvent. In addition, PMHS and PMHepS $ \langle {{\text{RMS}}_{\text{radius}}}^{ 2} \rangle^{ 1/ 2} $ were greater than 30?nm, indicating that larger chains of high molecular weight were obtained.     

Enhanced Mass Selectivity in Thermal Field-Flow Fractionation Due to the Temperature Dependence of the Transport Coefficients

Abstract

Mass selectivity in thermal field-flow fractionation (ThFFF) is governed by the dependence of molecular weight M on the ratio of the diffusion coefficient D to the thermal diffusion coefficient D _T. Although studies indicate that D _T is independent of molecular weight in homopolymers (meaning that mass selectivity depends on differences in D alone), the greatest possible dependence of D on M predicted by polymer theory is not enough to account for the selectivity obtained in certain polymer-solvent systems. However, the increased selectivity can be explained by the temperature dependence of D/D _T. Thus, lower molecular weight polymers, which are less compressed against the cold wall, experience a higher mean temperature. Since values of D/D_T increase with temperature, the decrease in retention with M is greater than that predicted without considering the temperature dependence. We quantify the increased selectivity for polystyrene in ethylbenzene using a temperature-dependent value of D/D _T in the concentration profile. When the temperature dependence of D/D _T is removed, the selectivity decreases from 0.64₆ to 0.57₄. Although it does not constitute proof, the lower value is consistent with D _T being independent of M.     

Power law and scaling for molecular weight dependence of crystal growth rate in polymeric materials

The molecular weight (M) dependence of the linear crystal growth rate (G) and the influence of the super-cooling on the relationship between M and G were studied. The molecular weight dependence of G has been expressed generally as G∝M^α at a given super-cooling. The temperature dependence of G shows a bell shape with the maximum growth rate (G_max). The value of α was −0.5 at the temperature (T_cmax) of G_max. However, the small super-cooling and the small molecular weight gave a large negative value of α. In other words, the value of α was dependent not only on the degree of super-cooling (ΔT) but also on the molecular weight. The effect on α by these two factors (ΔT and M) goes off to zero at T_cmax and α yields to −0.5. G_max can be defined as a characteristic intrinsic value to the crystal growth behavior. The molecular weight dependence of G_max was scaled and expressed as a −0.5 power to molecular weight for all crystalline polymers.     

Transport parameters and solubility coefficients of polymers at their glass transition temperatures

Many parameters of polymers exhibit breaks when temperature passes through glass transition. It is also often assumed that fractional free volume (FFV) at the glass transition temperature (T_g) has a standard value (the isofree volume concept). As gas diffusion (D) and permeability (P) coefficients depend on FFV, and mechanism of sorption and permeation is different above and below T_g, a question can be asked if D and P parameters of various gases in polymers have standard values at corresponding T_g, and, if not, how the values of D(T_g) and P(T_g) vary with T_g in different polymers. To examine this problem, two approaches were used: (1) extrapolation to T_g of numerous P and D values measured at ambient temperatures; (2) an analysis of direct data obtained in different polymers at their T_g. In both cases, qualitatively similar results were obtained: the D(T_g) and P(T_g) values increase with growing T_g independently of the nature of gas. Permselectivity P_i(T_g)/P_j(T_g) and selectivity of diffusion D_i(T_g)/D_j(T_g) are reduced when T_g increases. The dependence of the solubility coefficients S(T_g) = D(T_g)/P(T_g) is much weaker than those of D(T_g) and P(T_g). This conclusion was confirmed by the results of direct measurements of S in a wide range of temperature including T_g for several gas/polymer systems. An analysis of the results of positron annihilation studies of free volume in polymers led to the conclusion that the observed increases in the D(T_g) and P(T_g) values with T_g are caused mainly by thermal activation of diffusion processes at T_g. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1691–1705, 2000     

Dualism of the hydration water visualized in drying processes of foods

The dualism of the hydration water in seven foods was visualized, focusing on both water diffusivity (De, m²s^?1) and apparent molecular mobility (T₂, relaxation time, s). On the pre-exponential factor (δDe₀) of De, the compensation effects between ln(δDe₀) and the activation energy (E_D) of De demonstrated the difference in the water diffusion mechanisms among the foods. On the values of T₂ at the self-organized temperature (1/T) of hydration water, it yielded a common compensation effect between ln(T₂) and the activation energy (E_SO) of self-organization, demonstrating a common self-organization mechanism.     

Tensile strength elevation of brittle polymers by entanglements

Plots of tensile strength (T) versus reciprocal number average molecular weight (M^?1) have been made using previously reported data for linear polymers tested in the glassy state. Over a wide range of molecular weights there is conformity to Flory's empirical equation T = A - BM^?1, in which A and B are constants. Values of M obtained by extrapolation to T = 0 correlate with critical values of molecular weight which are diagnostic of incipient formation of an entangled network. The entanglement thesis is developed further by reference to a model in which brittle strength is attributed to the breaking of covalent backbone bonds. Theoretical values are calculated which exceed experimental values by a factor of only three. Such close agreement is attributed to the insensitivity of glassy linear polymers to flaws.     

Dynamics of macromolecular chains. 13C spin relaxation study of short-chain polystyrenes in deutero-chloroform solution

The results of T₁, T₂ and nuclear Overhauser effect (NOE) measurements on molecular weights 510–110 000 at weight fractions 0.05–0.60 in the title system suggest that previously neglected entanglement effects are highly significant for the reorientational processes in polymer chains and that previous T₂ estimates from bandwidths may be too low. T₂ is usually equal to T₁, and deviates only at high concentrations in these systems. The data can be represented by a double exponential reorientational correlation function of the form

$\text{G(τ)= A}\text{exp}\text{(}\text{? τ}\text{τ}{}_{\mathrm{A}}\text{) + B(}\text{? τ}\text{τ}{}_{\mathrm{B}}\text{)}$

where A + B = 1. The correlation times τ_A and τ_B are typically 1.5 × 10^?10 and 1.5 × 10^?9s, respectively. B increases with increasing concentration and molecular weight. The terminal phenyl group rotation is quite free in contrast to the backbone phenyls, and a spinning ratio above 15 has been estimated in a 60% solution for this group. Signal assignments and relaxation times (T₁) are given for 15 different ¹³C signals of these polymers at high concentrations.     

Influence of two‐stage drawing conditions on ultradrawing behavior of gel films of ultrahigh‐molecular‐weight polyethylene and low‐molecular‐weight polyethylene blends

The influence of two‐stage drawing conditions on the ultradrawing behavior of the gel films of ultrahigh‐molecular‐weight polyethylene/low‐molecular‐weight polyethylene blends is reported in this article. The critical draw ratios (λ_c) of the gel films prepared near their critical concentrations were found to depend significantly on the draw ratio attained in the first drawing stage (D_1r) and on the temperature utilized in the second drawing stage (T_sec). After drawing the gel films to a fixed draw ratio in the first drawing stage, each two‐stage drawn gel film was made to exhibit a maximum λ_c (λ_cmax) by drawing the drawn gel film at its corresponding optimum T_sec. In addition, the optimum T_sec was found to increase significantly with the D_1r value of the drawn gel films. It is worth noting, on the other hand, that the λ_cmax of two‐stage drawn gel films increased consistently with an increasing D_1r until its value reached an optimum value of 160. These results clearly suggest that, as T_sec and D_1r are increased to their optimum values, the λ_cmax of the two‐stage drawn gel films can be improved further so as to be higher than those of the corresponding one‐stage drawn gel films. These interesting phenomena were investigated in terms of reduced viscosities of the solutions and by an analysis of the thermal, birefringence, and tensile properties of the drawn gel films. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1890–1901, 2001     

Limitation and evaluation of the two-broad standard (TBS) method of calibration of aqueous SEC

Few years ago, the two broad standard method of molecular weight (MW) calibration was proposed¹ and the presence of negative σ² (peak dispersion coefficient) was found to be somewhat disturbing. The limitation of the method was not specified. Since large negative values of σ² cannot be tolerated, herein is reported another type of instrumental spreading function for which the method may seem to apply, and an evaluation of this method. In its evaluation, plots of log_e (intercept of a linear molecular weight calibration curve), that is, log_e(D₁) vs. the corresponding slope of the molecular weight calibration curve, D₂, which were found to be linear, were used. The systems employed were Dextran/Corning controlled porous glass (CPG-10) packing in well-chosen mobile phase.     

Diluent effects on carbonate mobility in bisphenol-A polycarbonate in the solid state

The effect of miscible low molecular weight additives on the mobility of the carbonate group in bisphenol-A polycarbonate (BPAPC) has been studied using n.m.r. and dielectric relaxation experiments in the solid state. Proton-enhanced dipolar-decoupled carbon-13 n.m.r. spectra of BPAPC, isotopically enriched at the carbonate position, are obtained without magic-angle sample spinning. The resolved chemical shift anisotropy allows study of nuclear spin relaxation for the carbonate groups in the polymer that have different orientations relative to the static magnetic field in the laboratory frame. The spin-lattice relaxation time in the rotating frame (T_1?) is measured at a motional-probe frequency of 50 kHz for the undiluted polymer and for BPAPC-diluent blends containing either dibutylphthalate or dinitrobiphenyl. The T_1? exhibits some dependence on orientation in all systems studied. In the blend containing dibutylphthalate (DBP), T_1? is decreased by a factor of two for all orientations of the carbonate group. This implies that DBP substantially increases the spectral density of 50 kHz motions in the carbonate region of the polymer at ambient temperature. In contrast, dinitrobiphenyl does not significantly alter the Fourier component of thermal fluctuations at 50 kHz. Dielectric relaxation measurements at 10 kHz reveal that the primary (T_g) and secondary (β) motional processes in BPAPC are affected by low molecular weight additives. An intermediate relaxation process appears in the temperature interval between the glass transition temperature (T_g) and the sub-T_g β-relaxation (T_β) in the polymer-diluent blends. The n.m.r. spin-lattice relaxation rate in the rotating frame, T^?1_1?, correlates well with the relative magnitude of the dielectric dissipation factor (tan δ_ε) between T_g and T_β.     

The effect of molecular weight and casting solvent on the miscibility of polystyrene-poly(α-methyl styrene) blends

The glass transition temperatures (T_g) have been measured for blends of polystyrene and poly(α-methyl styrene) in the molecular weight ranges: polystyrene, 2030 < M < 250 000, and poly(α-methyl styrene), 17 000 < M < 250 000. The presence of either one T_g or two has been used as a criterion to determine the miscibility of each blend over a range of compositions, and the T_gs were obtained from measurements of the temperature dependence of the heat capacities of the blends. A sinlge T_g was observed over the entire composition range for blends in which the component molecular weights were both less than 70 000 g mol^?1, when these were cast from toluene solutions. When propylene oxide solutions were used to prepare the blends, this limit dropped to M = 50 000 g mol^?1. By using the appearance of two T_gs as an indication that phase separation had taken place, it was possible to establish regions of miscibility and immiscibility as a function of casting solvent and molecular weight for both components. The change in heat capacity at the glass transition was measured and it was found that for miscible blends the heat capacity changes are similar to the calculated values, but for immiscible systems the measured change is smaller than expected.     

Synthesis and thermal characterization of novel adamantane-based polysiloxane

Novel polysiloxane derivative having adamantyl moiety in the main chain (P1) was synthesized and characterized by differential scanning calorimetry (DSC), thermogravimetry (TG), and X-ray diffraction analysis. P1 was obtained by bulk polycondensation without catalysts as well as solution polycondensation of novel disilanol monomer, i.e., 1,3-bis[4-(dimethylhydroxysilyl)phenyl]adamantane (M1), which was prepared by the Grignard reaction using chlorodimethylsilane and 1,3-bis(4-bromophenyl)adamantane, followed by the hydrolysis catalyzed by 5% palladium on charcoal. The molecular weight of P1 was dependent on the concentration of M1 in solution polycondensation, and the high concentration of M1 would result in the high average molecular weight of P1. P1 exhibited the good solubility in common organic solvents, such as tetrahydrofuran (THF), chloroform, dichloromethane, and toluene. The glass transition temperature (T_g) of P1 determined from DSC would be dependent on the average molecular weight of P1. The highest T_g was 115 °C and much higher than that of poly(tetramethyl-1,4-silphenylenesiloxane) (−20 °C). The melting temperature (T_m) of P1 seemed to be independent of the average molecular weight of P1 and was in the range of 153-157 °C, which was comparable to the T_m of poly(tetramethyl-1,4-silphenylenesiloxane). The temperature at 5% weight loss (T_d5) of P1 determined by TG was also comparable to that of poly(tetramethyl-1,4-silphenylenesiloxane), indicating that P1 is a new polysiloxane derivative with the high T_g as well as good thermostability.     

Anomalous galvanomagnetic properties of graphite in the quantum limit

Recent experiments on the galvanomagnetic effects of graphite in the quantum limit revealed that (1), σ_xyH is not a constant but depends on the field strength; (2), the resistivity ? ? σ_xx^?1 at low temperatures has a field dependence of? = ∣(A + BHⁿ), n ? 1; and (3), the ? vs T curve has a maximum at about T = 25K. These results cannot be explained by a simple theory. However, in consideration of the appearance of the bound states D⁰ and A⁰ in the quantum limit we can provide a qualitative explanation of the above observed results, where D correspond to donor and A represents acceptor. In the quantum limit the possibility of the transitions D⁺ + (?e) → D⁰ and A^? + (+e) → A⁰ is pointed out. Co-existence of the ionized impurity scattering and the neutral impurity scattering explains the qualitative features of the (H,T)-dependence of the resistivity at low temperatures. At high temperatures it is necessary to consider the phonon scattering and the carrier-carrier scattering. Without the carrier-carrier scattering the (H,T)-dependence of ? at T > 25K cannot be explained.     

Temperature annealing of radiation defects in xCu2Se-(1 − x)As2Se3 glasses: Dependence on composition

The temperature annealing of radiation defects (RDs), which are broken bonds of network atoms, has been studied for a number of glasses of quasi-binary system xCu₂Se-(1 ? x)As₂Se₃ γ-irradiated at 77 K. The density distributions of localized states caused by defects have been determined based on the temperature dependences of the relative intensity I(T)/I(77) of the EPR spectrum of RDs and according to the energy of the thermal release of D(E); and it was found that their shape is close to Gaussian. There is a significant shift in the curves of I(T)/I(77) to lower temperatures with increasing Cu content in the network. In glasses with copper, the width of D(E), and the density of localized states increase with increasing x. It is assumed that the dependence of E ^m (x) (E ^m —energy, which corresponds to the maximum of the distribution of D(E)), correlates with the change in the negative effective interelectron correlation energy U _eff on x. In As₂Se₃ glasses, in the temperature range of 77 to ～175 K, values of I(T)/I(77) > 1 are observed, which was explained by the appearance of thermally induced defects.