Sol-Gel Transition and Equilibrium Shear Modulus of Poly(vinyl chloride) and Plasticizers

The elasticity of poly(vinyl chloride) gels with molecular weight distribution (M_w/M_n), of 2.16 have been studied in the region beyond their gel points. Dynamic storage modulus G′, and equilibrium gel shear modulus of elasticity G_e, at low frequencies (ω) have specific developments as a function of polymer concentrations c, and plasticizers. The scaling elasticity from G_e = kε^z equation holds at different PVC plasticizer gels. The scaling exponent z, and constant k. ε is defined as the relative distance, ε = (|c − c_g|)/c_g, the calculated z = 2.45 ± 0.15. Furthermore, this analysis provides constant k with certain informations about the dependency of gel elasticity on the kind of plasticizer. Near the sol-gel transition temperature T, G_e decreases rapidly with increasing temperature. The normalized moduli G_eM/cRT, of the gels at different temperature, and/or c were dependent on the relative distance from the gelation point ε, and PVC and plasticizers concentration respectively. These results suggested mesh size of gel network near the gelation point for PVC with bis(2-ethylhexyl) phthalate (DOP) or di-n-butyl sebacate (DBS) plasticizers that has been newly reported.     

Influences of nonsolvent and temperature on critical viscoelastic behaviors of ternary polyacrylonitrile solutions around the sol-gel threshold

Viscoelastic experiments were performed to study the influence of nonsolvent and temperature on critical viscoelastic behaviors of ternary polyacrylonitrile (PAN) solutions around the sol-gel threshold. The dynamic critical parameters around the sol-gel threshold were determined using dynamic rheometer. The sol-gel transition takes place at a critical gel temperature at which the scaling law of G′(ω) ∼ G″(ω) ∝ ωⁿ holds, allowing an accurate determination of the critical gel temperature by means of the frequency independence of the loss tangent. Although the gel points of PAN solutions increase with increasing H₂O content, the results show that the scaling exponent n at the gel point is found to be universal for all ternary PAN solutions, which is independent of temperature and H₂O content, indicating the similarity of the fractal structure in the critical PAN gels. The gelation of ternary PAN solutions induced by adding a nonsolvent and by decreasing the temperature is demonstrated to be a thermoreversible process, which implies that the PAN gels are physical gels. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2637–2643, 2008     

Molecular weight effects on gelation and rheological properties of konjac glucomannan–xanthan mixtures

Fractions of konjac glucomannan (KGM) with various viscosity‐average molecular weights (M_v) ranging from 4.00 × 10⁵ to 2.50 × 10⁶ were prepared by hydrolysis degradation in hydrochloride acid/ethanol. Effect of M_v of KGM on the critical gelation temperature (T_gel) determined by Winter–Chambon criterion and the elasticity of KGM/xanthan mixed gels, a kind of binary gel formed by synergistic gelation, were investigated by dynamic viscoelastic measurements. It was shown that the value of T_gel of the gel was shifted to a higher temperature and the gel strength was enhanced when M_v of KGM was increased. The critical M_v (1.91 × 10⁶) was observed, above which the T_gel and elasticity of the mixed gels showed no or slight increase. It was suggested that T_gel and elasticity of KGM/xanthan mixed gels mainly depend on the structure of junction zones driven by the strong interaction between KGM and xanthan, which was gradually improved with increasing M_v of KGM. It was found that the critical strain and yield stress of the mixed gels increased monotonically with the increasing M_v of KGM. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 313–321, 2010     

Small‐angle x‐ray scattering study of thermoreversible poly(vinyl chloride) gels

Poly(vinyl chloride) (PVC)/bis(2‐ethylhexyl)phthalate (DOP) gels were prepared at room temperature from tetrahydrofuran solutions of PVC and DOP. PVC/DOP gels of different molecular weights at various PVC concentrations (c) were investigated with small‐angle X‐ray scattering (SAXS). The mean distance between two neighboring inhomogeneities (D) and two characteristic lengths, the intrainhomogeneity distance (d₁) and interinhomogeneity distance (d₂), were evaluated from Bragg's law and the distance distribution function, respectively. Both D and d₂ can be expressed by a power‐law relation (e.g., D and d₂ ∝ c^?0.5). After a period of rapid cooling to 25 °C from the sol state, the structural evolution was examined with time‐resolved SAXS measurements. An Avrami analysis with the SAXS invariant data revealed that the growth kinetics of PVC/DOP gels was one‐dimensional growth from predetermined nuclei, regardless of c. These results suggest that the PVC/DOP gels are constructed from a fibrillar structure that forms gel structures at high concentrations or low temperatures. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2340–2350, 2001     

Gelation Studies: Comparison of the Critical Exponents Obtained by Dynamic Light Scattering and Rheology, 2

Summary: The sol–gel transition of two thermoreversible gelling mixtures made of xanthan gum and locust‐bean gum has been studied by using in situ, time‐resolved dynamic light scattering (DLS) and in situ rheology. A critical dynamical behavior was observed near the sol–gel transition, which was characterized by the presence of power‐law spectra over three and four decades in the time‐intensity correlation function g₂(t) − 1 ∼ t^−μ and over four and three decades in the oscillatory shear experiment G′(ω) ∼ G″(ω) ∼ ωⁿ. A comparison of the critical exponents obtained (μ₁ ≈ 0.36, μ₂ ≈ 0.32 and n₁ ≈ 0.62, n₂ ≈ 0.67) was made as a function of the dependence of the two mixing ratios according to the theory by Doi and Onuki. New experiments were also performed to compare the critical exponents on such a thermoreversible system.

Double‐logarithmic plot of the time‐intensity correlation functions g₂(t) − 1 versus the delay time, t, at a 90° scattering angle and at several temperatures of the mixture 1.     

Polysiloxane Gel with Low Crosslinking Density for Make-up Cosmetics

Silicic anhydride has conventionally and routinely been used as a gelling agent for silicones. In the present study, the mechanism wherein a crosslinked silicone having a specific crosslinking density is swollen in the presence of a silicone oil having a low viscosity to form a stable gel was applied to the manufacture of foundations, and optimum conditions that provide favorable functions and satisfactory organoleptic sensation were studied. Polymethylhydrogen siloxanes with a low crosslinking density were prepared by subjecting α,ω-divinyl polydimethyl siloxane to addition polymerization. The viscoelasticities of the gels formed by the resultant products and silicone oil were investigated. based on these results, it is concluded that the most suitable product is a low crosslinking density material obtained by adding α,ω-divinyl polydimethyl siloxane (M¯_w: 1.0 kDa) to polymethyl hydrogen siloxane having an average molecular weight of 2.5 kDa under the conditions wherein the concentration of dimethylpolysiloxane (viscosity: 6 × 10⁶ m² S^-1) is 60%. This product provides an enhanced viscosity at a reduced solids content. © 1997 John Wiley & Sons, Ltd.     

Thermoreversible gelation of helical polypeptide/single‐walled carbon nanotubes and their solid‐state structures

Single‐walled carbon nanotubes (SWCNTs) have been functionalized with poly(γ‐benzyl‐L ‐glutamate)s (PBLGs) having well‐defined polymer molecular weight (M_n = 7.5–21.1 kg·mol^?1) and molecular weight distribution (PDI = 1.05–1.20) by a graft‐to method. Toluene solutions containing 5 wt % free PBLG and variable amounts of PBLG‐functionalized SWCNTs (PBLG‐SWCNTs) form gels at room temperature. Differential scanning calorimetry (DSC) analysis reveals that the gelation occurs thermoreversibly, in accord with previous studies on the pristine PBLG/toluene gels. The heat of gel melting (ΔH_m) is slightly elevated for the composite gels compared with the pristine gel, which suggests enhanced interactions between PBLGs in the former. But the gelation temperatures of the composites are unaffected by the presence of PBLG‐SWCNTs. Small‐angle X‐ray scattering (SAXS) analysis of the composite and pristine gels at different temperatures by the Guinier method suggests that PBLG‐SWCNTs promote interactions between PBLG rods, as indicated by the larger PBLG bundle size with increasing PBLG‐SWCNT content in the gel and the melt state. W/SAXS analysis of the dry gels reveals that PBLG‐SWCNTs induce significant changes in the PBLG packing order, resulting in a nematic phase, in contrast to a weakly ordered smectic C phase containing tilted PBLG rods that is observed in the pristine gel. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Cyclodextrin gels which have a temperature responsiveness

β-Cyclodextrin (CD) gels crosslinked by epichlorohydrin (CD-gel) were modified with poly(N-isopropylacrylamide) (PIPA; M_n 4900) chains. The CD residue in the gel associated with 8-anilino-1-naphthalenesulfonic acid (ANS) more strongly than the free PIPA-carrying CD. The van't Hoff plot for complexation of ANS with the gel drifted largely from linear relationship above the coil-globule transition temperature of individual PIPA chains. Furthermore, a relatively more significant temperature effect on the stereoselective adsorption of phenylalanine to the CD gel was observed by the modification with PIPA chains. These results imply that the ability of the CD-gel–PIPA conjugate to form inclusion complex could be skillfully controlled by temperature. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1535–1541, 1997     

Thermoreversible gels of polymers in recycled motor oil

A study of the dynamic viscoelastic properties of gels of Ethylene Vinyl Acetate (EVA) and Styrene‐Butadiene‐Styrene (SBS) copolymers in recycled motor oil is presented. Both systems form gels with enhanced elastic moduli, with respect to SBS/aromatic oil gels which have been used to develop synthetic binders. Although the procedure described by Winter is conveniently applied to obtain gel‐sol transition of EVA/oil gels, it is not suitable for SBS/oil gels which do not give rise to a homogeneous liquid when they melt. For EVA/oil gels the relaxation exponent at the gel point is Δ=0.5, which according to Muthukumar's model corresponds to a fractal dimension d_f=2. The variation of the elastic modulus with polymer concentration follows the scaling law G_e‐cⁿ, with n=2.8 for EVA/oil and n=1.3 for SBS/oil. In the case of EVA/oil gels the validity of theoretical models relating fractal dimension to n exponent is discussed.     

Synthesis of poly[arylene ether sulfone-b-vinylidene fluoride] block copolymers

A series of α,ω-dihydroxy polyarylene sulfones (PAES) were synthesized comprising bisphenol A (PAES1, M_n=1800, 4900, and 9500 daltons), 4,4^′-biphenol (PAES2, M_n=4100 daltons), and hexafluorobisphenol A (PAES3, M_n=3300 daltons). These were reacted with α,ω-dibromo poly(vinylidene fluoride) (PVDF, M_n=1200 daltons) prepared by telomerization, to yield block copolymers possessing rigid and flexible segments. Block copolymers were characterized by FTIR, NMR, GPC, DSC, TGA and TEM. In several cases the block copolymers exhibited distinct thermal transitions, i.e. T_m and T_g for PVDF and PAES segments, respectively. Where observable, T_g of PAES domains in the block copolymers occurred at a temperature lower than the corresponding PAES homopolymer due to the flexible nature of the surrounding PVDF domains. Block copolymers exhibited a similar thermal stability to the corresponding PAES homopolymers but higher stability than the PVDF homopolymer, and much higher still than α,ω-dibromo PVDF. TEM analyses indicate that phase separation of PAES and PVDF domains occurs on the nanometer scale.     

The effects of solvent type and aging on structural development in poly(vinyl chloride) thermoreversible gels

We investigated the effects of solvent type (mono‐ester vs. di‐ester solvent) and aging on the structural development in the poly(vinyl chloride)/butyl benzoate (PVC/BB) and PVC/dibutyl phthalate(DBP) gels, as well as on their viscoelastic and mechanical behaviors. It was found that aged PVC/DBP gels held at RT for 7 days exhibit an improvement of about 100% in storage modulus (G′) compared to fresh gels, with a sudden drop in G′ around 50 °C, whereas the storage moduli of the PVC/BB gels decrease monotonically with temperature, irrespective of the postaging time. These different behaviors of the PVC/BB and PVC/DBP gels arise mainly because of the difference in the network structure produced by the formation of the polymer‐solvent complex between the C?O groups of the solvent and the polarized hydrogen moieties of PVC, as was confirmed with small angle X‐ray scattering and uniaxial tensile experiments. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 263–271, 2008     

Osmotic deswelling of weakly charged poly(acrylic acid) solutions and gels

The osmotic pressure of weakly charged aqueous poly(acrylic acid) (PAA) solutions and the swelling pressure PAA gels were studied by osmotic deswelling at different degrees of ionization (α). In solution, the osmotic pressure was found to scale linearly with concentration, whereas the scaling power of the swelling pressure of gels was higher (1.66). The effect of the ionization degree on the osmotic coefficient in PAA solutions was in agreement with the theory of Borue and Erukhimovich [Macromolecules, 21 , 3240 (1988)]. Ionization increases the swelling capacity of the PAA gels until a plateau is reached at about 35% neutralization. The concentration at equilibrium swelling scales as C_e ～ α^?0.6. The contribution of the network to the gel swelling pressure is evaluated by subtracting the osmotic pressure of the polymer solution at the same concentration and degree of ionization. In swollen gels the extended network opposes swelling. As the gel is osmotically deswelled, a state of zero network pressure exists at a certain concentration, below which the network elasticity favors swelling. The crossover concentration shifts to lower values as the degrees of ionization increases. © 1995 John Wiley & Sons, Inc.     

Chromogenic WPA/TiO 2 Hybrid Gels and Films

A novel low-temperature electrochromic and photochromic material in the form of gels and thin solid films composed of H₃PW₁₂O₄₀·nH₂O (WPA) incorporated in titanium oxide gel issued of Ti(OPr ⁱ )₄ with addition of acetic acid were made via the sol-gel route by the dip-coating technique. Gels in bulk and thin film forms have been studied with the help of FT-IR transmission, reflection absorption and Raman spectroscopy. Similarity of the characteristic band frequencies with pure WPA confirmed that entrapped Keggin salt is well preserved inside the gel. Photochromism of WPA/TiO₂ gels which appears after short exposure to UV (λ=366 nm) radiation is related to the excitation effect of branched alcohol leading to formation of carbonyl modes due to the alcohol transformation to aldehyde. Electrochromic properties of WPA/TiO₂ gel films were tested in aqueous 0.001M HClO₄ and 0.1M LiClO₄ electrolytes as well as in the sol-gel electrochromic device (ECD) composed of LiClO₄ doped ormolyte and Sb:Mo:SnO₂ counter electrode film with optically passive response. Slight improvement of the WPA/TiO₂ intercalation/deintercalation kinetics was obtained with the films exhibiting one order of magnitude higher electronic conductivity which were made with addition of in situ polymerized pyrrole with Fe(NO₃)₃.     

SURFACE RHEOLOGY OF POLYMERIC SOLIDS

Surface molecular motions of amorphous polymeric solids have been directly measured on the basis of scanningviscoelasticity microscopic (SVM) and lateral force microscopic (LFM) measurements. SVM and LFM measurements werecarried out for films of conventional monodisperse polystyrene (PS) with sec-butyl and proton-terminated end groups atroom temperature. In the case of the number-average molecular weight, M_n, less than ca. 4.0×10~4, the surface was in a glass-rubber transition state even though the bulk glass transition temperature, T_g was far above room temperature, meaning thatthe surface molecular motion was fairly active compared with that in the bulk. LFM measurements of the, monodisperse PSfilms at various scanning rates and temperatures revealed that the time-temperature superposition was applicable to thesurface mechanical relaxation behavior and also that the surface glass transition temperature, T_g~σ, was depressed incomparison with the bulk one even though the magnitude of M_n was fairly high at 1.40×10~5. The surface molecular motionof monodisperse PS with various chain end groups was investigated on the basis of temperature-dependent scanningviscoelasticity microscopy (TDSVM). The T_g~σs for the PS films with M_n of 4.9×10~6 to 1.45×10~6 measured by TDSVMwere smaller than those for the bulk one, with corresponding M_ns, and the T_g~σs for M_ns smaller than ca. 4.0×10~4 were lowerthan room temperature (293 K). The active thermal molecular motion at the polymeric solid surface can be interpreted interms of an excess free volume near the surface region induced by the surface localization of chain end groups. In the case ofM_n=ca. 5.0×10~4, the T_g~σs for the α, ω-diamino-terminated PS (α,ω-PS(NH_2)_2) and α, ω-dicarboxy-terminated PS (α, ω-PS(COOH)_2) films were higher than that of the PS film. The change of T_g~σ for the PS film with various chain end groups canbe explained in terms of the depth distribution of chain end groups at the surface region depending on the relativehydrophobicity.     

Influence of initial polymer concentration in solution and weight-average molecular weight on the drawing behavior of polyethylenes

The influence of initial polymer concentration in solution (c), weight-average molecular weight (M_ω), and drawing temperature on the solid-state drawing behavior of linear polyethylenes was investigated. Optimum conditions, with respect to maximum attainable draw ratio, are observed in isothermal drawing experiments. Moreover, it is shown that high maximum attainable draw ratios can also be obtained upon multistage drawing of UHMW-PE (ultrahigh-molecular-weight polyethylene, M_ω > 10⁶ g/mol) gel films cast from concentrated solutions. The high maximum attainable draw ratio in combination with the high molecular weight (M_ω > 10⁶ g/mol) and polymer concentration (c = 10% w/v) is of particular interest because it results in tapes or fibers with a high Young's modulus (100 GPa) and tensile strength (2.5–3.5 GPa). It is also shown that the maximum attainable draw ratio of polyethylenes scales with the Bueche parameter (c · M_ω) to the ?0.5 power. This experimental observation indicates that intermolecular interactions not only dominate the rheological properties of polyethylene melts and concentrated solutions, but also strongly influence the solid-state drawing behavior of linear polyethylenes.     

Thermal analyses of poly(N-isopropylacrylamide) in aqueous solutions and in nanocomposite gels

The coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPA) prepared by free-radical redox polymerization in aqueous solutions and its nanocomposite (NC) gels were investigated by differential scanning calorimetery. The lower critical solution temperatures (LCST) of aqueous solutions of PNIPA of different molecular weights were not significantly affected by molecular weight (M _w: 0.19?×?10⁶?4.29?×?10⁶?g?×?mol^?1) or polymer concentration (1?10?wt%), although the enthalpy of transition increased with molecular weight, at M _w (<1.2?×?10⁶ g?×?mol^?1). The glass-transition temperature of PNIPA in the dried state also remained constant (138?°C), regardless of molecular weight. On the other hand, the enthalpy of the coil-to-globule transition of PNIPA in NC gels consisting of a PNIPA/clay network decreased with increasing clay concentration (C _clay), while the onset temperature (≡LCST) was almost constant, regardless of C _clay. The PNIPA chains in NC gels could be classified into the following three types: P-1, which exhibits a normal LCST transition, similar to that of linear PNIPA; P-2, exhibiting restricted transition at higher temperatures as a result of interactions with the clay; and P-3, which does not undergo that transition because of stronger restrictions. It was found that the proportion of P-3 increases with increasing C _clay. However, some P-1 and P-2 was still observed, even in NC gels with high C _clay. That the transition to the hydrophobic globular state was restricted by interactions with the clay was confirmed by measurements on PNIPA after removal of the clay from NC gels.     

Polymerization of vinyl chloride at reduced monomer accessibility. II. Polymerization at subsaturation pressure with emulsion PVC as seed

Vinyl chloride was polymerized at 59–92% of saturation pressure in a water-suspended system at 45–65°C with an emulsion poly(vinyl chloride) (PVC) latex as a seed. A water-soluble initiator was used in various concentrations. The monomer was continuously charged as vapor from a storage vessel kept at lower temperature. Characterization included determination of molecular-weight distribution and degree of long-chain branching by gel permeation chromatography (GPC) and viscometry, thermal dehydrochlorination, and microscopy. The polymerization rate decreases with decreasing pressure but is reasonable even at the lowest pressure. The molecular weight decreases with decreasing pressure and increasing initiator concentration and also with increasing polymerization temperature, if the initiator concentrations are chosen to give a constant initiator radical concentration. The degree of long-chain branching increases with increasing initiator concentration and decreasing monomer pressure but is unaffected by the polymerization temperature, if the initiator radical concentration is kept constant. The thermal stability decreases with decreasing M _n, while the degree of long-chain branching has only a minor influence. The most important factor in the system influencing the molecular parameter is the monomer accessibility.     

Induction and Acceleration of Bonelike Apatite Formation on Tantalum Oxide Gel in Simulated Body Fluid

Untreated tantalum metal forms bonelike apatite layer on its surface in a simulated body fluid (SBF) after a long period. The apatite formation on the tantalum metal is significantly accelerated, when the metal was previously subjected to NaOH and heat treatments to form an amorphous sodium tantalate on its surface. The fast formation of the apatite on the NaOH- and heat-treated tantalum metal was explained as follows. The sodium tantalate on the surface of the metal releases the Na⁺ ion via exchange with H₃O⁺ ion in SBF to form a lot of Ta-OH groups on its surface. Thus formed Ta-OH groups induce the apatite nucleation and the released Na⁺ ion accelerates the apatite nucleation by increasing ionic activity product of the apatite in SBF due to increase in OH^– ion concentration. In the present study, in order to confirm this explanation, apatite formations on sodium tantalate gels with different Na/Ta atomic ratios, which were prepared by a sol-gel method were investigated. It was found that even Na₂O-free tantalum oxide gel forms the apatite on its surface in SBF. This proves that the Ta-OH groups abundant on the gel can induce the apatite nucleation. The apatite-forming ability of the gels increased with increasing Na/Ta atomic ratios of the gels. The sodium-containing tantalum oxide gels released the Na⁺ ion, the amount of which increased with increasing Na/Ta atomic ratios of the gels. The released Na⁺ ion gave an increase in pH of SBF. These results prove that the apatite nucleation induced by the Ta-OH groups is accelerated with the released Na⁺ ion by increasing ionic activity product of the apatite in SBF.     

Spinel-Mullite Composites with Optical Properties

The aim of this paper was to study the synthesis and characterization of spinel-containing mullite based materials, using sol-gel techniques. Several gels were prepared, with nominal compositions 3(Al_2−2xM_x Ti_xO₃)·2SiO₂ and 3(Al_2−xM_xO₃)·2SiO₂, with M=Ni⁺² or Co⁺² and 0.0≤x≤0.2, by hydrolysis and condensation of mixtures of aluminum, silicon and titanium alkoxides and nickel chloride. Dried gels were homogeneous and displayed a glass transition at around 750°C, which indicated that the system could be described as an amorphous silicoaluminate network. Crystallization pathway of gels were followed using differential thermal analysis and X-ray diffraction patterns of samples thermal treated at temperatures in the range between 800 and 1400°C. A two-phase aluminate spinel-mullite arrangement was detected at temperatures around 1200°C. The microstructure of the final product was interesting, because the minor secondary phase was homogeneously dispersed in the mullite matrix. Chemical and thermal resistance of diphasic materials were tested and the results indicate that these materials can be used as high temperature ceramic pigments.     

Composition-controlled metal-insulator transitions and minimum metallic conductivity in the oxide systems LaNi1−xMxO3 (M = Cr,Mn, Fe,or Co)

The composition-controlled metal-insulator transition in the perovskite systems LaNi_1?xM_xO₃ (M = Cr, Mn, Fe, and Co) has been investigated by transport measurements over the temperature range 12–300 K. These systems, which have critical electron densities (n_c) in the range (1–2) × 10²⁰ electrons cm^?3, exhibit sharp metal-insulator transitions at the base temperature. The corresponding minimum metallic conductivity (σ_min), separating the localized and itinerant electronic regimes, is of the order of 10² ohm^?1 cm^?1. Particular attention is paid to the idea of σ_min scaling with n_c, and our present results are compared with earlier studies of the metal-insulator transition in low (e.g., Ge:Sb) and high (e.g., metal-ammonia, supercritical Hg) electron-density systems. A link is established between the transport and magnetic properties of the title systems at the metal-insulator transition.