Calculations of diffusion coefficients of cyclic poly(dimethylsiloxanes)

The Kirkwood equation and a Monte-Carlo technique which employs Metropolis sampling have been used to calculate diffusion coefficients (D) for low molar mass cyclic poly(dimethylsiloxanes). Correlations between calculated and experimental values of D show that the effective segment size is larger for cyclic than for linear PDMS. Thus, free-draining flow is reduced for the cyclic species consistent with its greater segment density. The cyclic molecules also show enhanced diffusion coefficients at very low molar masses which may be associated with the rigid toroidal shapes of these molecules.     

Preparation and characterization of graft copolymers of methyl methacrylate and poly(n-hexyl isocyanate) macromonomers

Living poly(n-hexyl isocyanate) (PHIC) was deactivated with methacryloyl chloride to produce methacryl-terminated poly(n-hexyl isocyanate) (PHIC-MA) rodlike macromonomers. Radical copolymerization of methyl methacrylate (MMA) with PHIC-MA was performed using 2,2′-azobis(isobutyronitrile) as an initiator in benzene at 60 °C to prepare poly(methyl methacrylate)-graft-poly(n-hexyl isocyanate) (PMMA-graft-PHIC) graft copolymers. The monomer reactivity ratios of MMA (M₁) and PHIC-MA (M₂) were evaluated as r₁=11.5 and r₂=∼0, exhibiting remarkably lower reactivity of PHIC-MA macromonomer than that of common macromonomers. The resultant graft copolymers were characterized using gel permeation chromatography equipped with low-angle laser light-scattering to determine the molecular weights, and equipped with a refractive index detector and an ultraviolet light detector to estimate a PHIC weight fraction of PMMA-graft-PHIC at the ith elution volume of the GPC chromatogram. There are 2-3 PHIC grafts per PMMA molecule, and the PHIC rodlike chains might be difficult to introduce into the PMMA main chains having higher molecular weights. A specific dimension of PMMA-graft-PHIC in solution was discussed in detail.     

Diffusion of poly(vinylpyrrolidone) through cellulose ester membranes

Diffusion coefficients of poly(vinylpyrrolidone) in aqueous solutions were ascertained by the diaphragm method as a function of molecular weight and temperature using mixed cellulose ester (Millipore) membranes. The results agree satisfactorily with diffusion coefficients obtained by other methods. Separation of polymer according to molecular size takes place on diffusion as a function of membrane pore diameter. Also, interaction between the polymer and membrane has been observed resulting in a distribution coefficient between solution and membrane larger than 1. The energy and entropy of activation, repectively, increase with polymer chain length, and their magnitudes are in agreement with the view that segment mobility is operative for the diffusion process.     

Electrochemical behaviors of poly(ferrocenylsilane) solutions

The electrochemical behaviors of two kinds of poly(ferrocenylsilanes) (PFS) with different substituent groups in CH₂Cl₂ solutions were investigated by means of cyclic voltammetry (CV) and electrochemical quartz crystal microbalance. The results demonstrated that the CV processes of the PFS on the glass carbon electrode surface in CH₂Cl₂ solutions are the diffusion‐controlled reversible processes. The oxidative state of PFS forms an adsorption layer on the electrode surface during the oxidation process. The interaction of active ferrocene centers along the main chain induces the stepwise redox process, and makes the CV waves of the PFS solutions present two couples of peaks. The diffusion coefficients of PFS in CH₂Cl₂ solution are much larger than those of PFS in films. The different electrochemical behaviors of the two PFS solutions indicated that the molecular structure of polymer has influence on the electrochemical properties of the PFS. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 789–794, 2007     

Studies on the characterization of partly hydrolysed derivatives of poly(vinyl acetate) and their red iodine complex

Three samples of poly(vinyl acetate) 36% hydrolysed, respectively, by saponification, transesterification and acid-equilibrium have been characterized through the refractive indices and light-scattering properties of their solutions in 10% aqueous methanol at 25°C. The original poly(vinyl acetate) sample and its iodine/iodide complex have been similarly characterized, respectively, in methanol at 5°C and in 9.1% methanolic water over the range 5–15°C. The apparent weight-average molecular weights of the vinyl polymers show that the three hydrolytic processes degrade the 130 000 average molecular weight of the original poly(vinyl acetate) macromolecules hardly at all. It is suggested that values of the apparent second virial coefficient might be used as criteria for the characterization of hydrolysed poly(vinyl acetate) specimens which have otherwise identical degrees of hydrolysis and blockiness of their residual acetate groups, but contrasting polydisperse characteristics and surface-active properties. The apparent average root-mean-square radius of gyration of the poly(vinyl acetate) macromolecules is reduced to ≈33% or less of its value on formation of the complex. The characteristics and structure of such complexes are discussed.     

Studies on Permeation,Rejection, and Transport of Aqueous Poly(ethylene Glycol) Solutions using Ultrafiltration Membranes

Abstract

The permeate flux and retention of aqueous solutions of poly(ethylene glycols) (PEG) with different molecular weights ranging from 4000 to 35,000 Da have been investigated using various compositions such as 100/0, 90/10, 80/20, and 70/30 wt% of cellulose acetate (CA)/sulfonated poly(etheretherketone) (SPEEK) ultrafiltration blend membranes. The factors affecting the rejection rate and permeate flux such as molecular weight of PEGs, concentration of the solute, composition of the membranes, and transmembrane pressures have been studied. It is seen that the increase in the concentration of PEG results in the decreased permeate flux and increased rejection for increasing CA content in the membranes. A similar observation in the flux and rejection was made for increasing the molecular weight of PEGs. Further, the mass transfer, diffusion, and true retention coefficients of the solute have been studied with different operating variables like molecular weight and concentration of PEGs. An increase in the molecular weight of PEGs results in the decrease of mass transfer and diffusion coefficients and increase of the true retention coefficient. A reverse trend is observed with increasing concentrations of PEG.     

Preparation and characterization of poly(vinyl alcohol) nanofiber mats crosslinked with blocked isocyanate prepolymer

Poly(vinyl alcohol) (PVA) nanofibers crosslinked with blocked isocyanate prepolymer (BIP) were successfully prepared using the electrospinning process and subsequent thermal treatment. Fourier transform infrared spectroscopy and solid‐state ¹³C NMR spectroscopy demonstrated that chemical crosslinks between the hydroxyl group of PVA and the isocyanate group of BIP were formed. Thermogravimetric analysis and differential scanning calorimetry results indicated that when the BIP content was increased, the thermal stability of PVA/BIP nanofibers increased, and the crystallinity of PVA decreased. Field emission scanning electron microscopy was used to measure the average diameter (200–300 nm) of the electrospun PVA/BIP nanofibers. The water contact angles were 10.2° and 113° for the pristine PVA nanofibers and PVA nanofibers crosslinked with 8 wt% BIP, respectively. The tensile strength of the crosslinked PVA nanofibers was 53.7 MPa, which was seven times higher than that of pristine PVA. The improved tensile strength and water resistance of the crosslinked PVA/BIP nanofibers were due to a combination of increased crosslinking density and decrease in the number of hydroxyl groups on the surface of the PVA/BIP nanofibers. Copyright © 2010 Society of Chemical Industry     

Effect of crosslinking on mechanical and viscoelastic properties of semiinterpenetrating polymer networks composed of poly(vinyl chloride) and isocyanate crosslinked networks

Semiinterpenetrating polymer networks (SIPNs) of PVC/isocyanate/poly‐triol were prepared by premixing small (150 μm dia.) porous (30% voids) unplasticized PVC particles, 10% by weight of isocyanate, and a triol at different OH/NCO mol ratios. Three types of isocyanates (methylene bis‐phenyl diisocyanate (MDI), oligomeric MDI isocyanates (PAPI), and toluene diisocyanate (TDI) prepolymer/polytriol) were used. Two‐roll milling was followed by hot‐press curing. The tensile, flexural, and impact strengths increased when small amounts of crosslinked isocyanate networks were created in PVC. The isocyanate/polyol hydroxyl stoichiometry was varied, and the effects of crosslinking on the tensile, impact, and flexural strengths of PVC/isocyanate/triol SIPNs were examined. The strength increments were greater when the OH/NCO mole ratio went from 0 to 0.25, than when it went from 0.25 to 1.0. In many cases, increasing OH/NCO mol ratio from 0.5 to 1.0 decreased tensile, impact, and flexural strengths. Both PAPI and MDI (30% NCO content) gave bigger improvements in the these mechanical strengths than the TDI (only 9.7% NCO). These SIPN blends exhibited lower tan δ peak temperatures and a single distinct loss modulus, E″, peak values at lower temperatures than those of PVC that had been exposed to the same processing temperatures. Substantial amounts of isocyanate networks exist in SIPN phases according to DMTA studies. The OH/NCO ratio did not generally correlate with the decreases in the glass transition temperatures in these three sets of blends. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1402–1411, 2000     

Unique waterborne systems based on TMXDI (meta) aliphatic isocyanate and TMI (meta) unsaturated isocyanate

TMXDI (meta) aliphatic isocyanate and TMI (meta) unsaturated isocyanate have been formulated into unique waterborne polymer systems. Poly(urethane) dispersions based on TMXDI (meta) aliphatic isocyanate are characterized by very low volatile organic component (VOC) levels as no solvents are required, excellent toughness and flexibility, high solids capability and low viscosity. Acrylic latexes modified with TMI (meta) unsaturated isocyanate are characterized by higher hardness levels with little loss of flexibility, and improved chemical and solvent resistance when compared to non-modified systems. Application areas for these coatings include automotive isocyanatoethylmetacrylate (OEM) and refinish, as well as trade sale paints.     

Effect of tacticity of poly(methyl methacrylate) on the miscibility with poly(vinyl pyrrolidone)

Isotactic, atactic, and syndiotactic poly(methyl methacrylates) (PMMA) (designated iPMMA, aPMMA, and sPMMA) with approximately the same molecular weight were mixed separately with poly(vinyl pyrrolidone) (PVP) primarily in chloroform to make three polymer blend systems. Differential scanning calorimetry (DSC) was used to study the miscibility of these blends. The results showed that the tacticity of PMMA has a definite impact on its miscibility with PVP. The aPMMA/PVP and sPMMA/PVP blends were found to be miscible because all the prepared films showed composition-dependent glass-transition temperatures (T_g). The glass-transition temperatures of the aPMMA/PVP blends are equal to or lower than weight average and can be qualitatively described by the Gordon–Taylor equation. The glass-transition temperatures of the other miscible blends (i.e., sPMMA/PVP blends) are mostly higher than weight average and can be approximately fitted by the simplified Kwei equation. The iPMMA/PVP blends were found to be immiscible or partially miscible based on the observation of two glass-transition temperatures. The immiscibility is probably attributable to a stronger interaction among isotactic MMA segments because its ordination and molecular packing contribute to form a rigid domain. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 3190–3197, 2001     

Microwave dielectric relaxation study of poly(propylene glycol) in dilute solution

Dielectric relaxation studies of poly(propylene glycol), average molecular weight 2000 g mol⁻¹, in dilute solution of cyclohexane, decaline, benzene and carbon tetrachloride have been carried out at 10.10 GHz and 35 °C. Average relaxation time τ₀, and relaxation times corresponding to segmental motion τ₁, group rotations τ₂ and dipole moment µ have been determined. It is found that τ₀, τ₁ and µ are influenced by the solvent environment while the τ₂ value is solvent‐independent. A comparison has been made with the dielectric behaviour of poly(ethylene glycol), average molecular weight 1500 g mol⁻¹, in dilute solutions of benzene and carbon tetrachloride because both systems have an equal number of monomer units. The effect of methyl side‐groups on dielectric relaxation in poly(propylene glycol) molecules is discussed. The Kirkwood correlation factor is also evaluated in dilute solutions with concentration variation and it is found that these molecules exist in cluster form due to intermolecular hydrogen bonding. © 2000 Society of Chemical Industry     

Self diffusion of small molecules in aqueous solution of poly(vinylpyrrolidone)

The self-diffusion coefficients of small penetrants were measured in aqueous solutions at varying concentrations of poly(vinylpyrrolidone). Measurements have been performed using the n.m.r. pulsed-gradient spin-echo (PGSE) technique and the classical gradient diffusion (CGD) method, modified for ternary systems. A good agreement was found between the two, confirming the validity of the latter. The results have been quantitatively analysed by a free-volume approach adapted for diffusion of a solute in moderately concentrated polymer solutions. From this model a linear relationship is predicted between ln $\text{D}\text{D}{}_{\mathrm{<mtext>o</mtext>}}$ and ?^?1, the reciprocal volume fraction of solvent, which was also found experimentally for all diffusants studied. An enhanced concentration dependence of penetrant diffusion with increasing size of the diffusing molecules was observed. These findings are in agreement with predictions from the free-volume theory.     

Morphology and thermal analysis of poly(ethylene-b-vinylacetate) copolymers

Summary Studies of crystallization kinetics and morphological analysis on three samples of poly(ethylene-b-vinylacetate) multiblock copolymers have been performed. The copolymers contain 28% by weight of vinylacetate and their macromolecules are builted up by different numbers of blocks, whose lengths are constant in all cases. An original method of staining puts into evidence the segregation of the polyethylenic blocks in crystalline domains, whose dimensions are less than 50 Å, in good agreement with the molecular characteristics. A structural model, involving fringed micellae for the rigid segments, is suggested and supported by values of the Avrami index, less than 2. The crystallization rates are also discussed as functions of the molecular weights of the samples.     

Chemorheology of poly(urethane/isocyanurate) formation

Poly(urethane/isocyanurate) is a major resin used in the structural reaction injection molding (SRIM) process. In this study, the kinetics and viscosity changes of a poly(urethane/isocyanurate) resin were investigated by the adiabatic temperature/viscosity rise method and differential scanning calorimetry. The effect of the catalyst concentration and the volume ratio of isocyanate to polyol was investigated. It is found that isocyanate trimerization depends on the diffusion effect resulted from the urethane formation. A kinetic model including the diffusion effect is developed, which is able to predict the experimental results. The reaction induced viscosity rise depends on the concentration and the molecular weight of the formed polymer. The measured adiabatic viscosity rise correlates well with the calculated molecular weight growth using the recursive method.     

An inelastic electron tunnelling spectroscopy (IETS) study of poly(vinylacetate) poly(methyl methacrylate) and poly(vinylalcohol) adsorbed on aluminium oxide

IETS is used to investigate the adsorption of poly(vinylacetate) (PVA), poly(methylmethacrylate) (PMMA), and poly(vinylalcohol) (PVOH) on aluminium oxide. These polymers are of interest in the field of adhesion science, and until now synthetic macromolecules have not been studied in this way. Both commercially available polymers and those synthesized in our laboratory have been used. On the basis of IET spectra presented here, and existing i.r. spectra it is believed that PMMA and PVA undergo ester cleavage at the oxide surface leading to their subsequent adsorption. For PMMA this is thought to be via carboxylate anions generated on the polymer side groups, while PVA is expected to be adsorbed as PVOH. Bonding of PVOH to the oxide is not fully understood, but may occur by the formation of an AlOC bridge. Another possibility for the above polymers, that of intermolecular hydrogen bonding between polar polymer side groups, and adsorbed hydroxyl species present on the oxide surface, cannot be ruled out.     

Solvolytic liquefaction of coal in the presence of poly (vinylcyclohexane)

Coal liquefaction in the presence of poly(vinylcyclohexane) (PVCH) was studied at 400 °C under an atmosphere of nitrogen. PVCH promoted coal conversion better than tetralin (27.8% for tetralin, 47.2% for preheated PVCH). The effects of the initial molecular weight of PVCH, of free radicals formed during a decrease within molecular weight, and of changes in molecular structure with decomposition were examined. Conversion increased with increased initial molecular weight between 112 amu (ethylcyclohexane) and 700 amu (low molecular weight PVCH); no effect was found for molecular weights greater than 700. The best conversion was obtained when preheated PVCH was used, resulting from formation of allylic hydrogens. Aromatic structures were not found in the preheated PVCH. It is concluded that PVCH with molecular weights of ≈ 700 amu with allylic hydrogens present promoted the liquefaction of coal better than tetralin.     

Solution properties of poly(diallyldimethylammonium chloride) (PDDA)

Size exclusion chromatography (SEC) with dual detection, i.e. employing a refractive index (RI), concentration sensitive, detector together with a multiangle light scattering (MALS) detector which is sensitive to molecular size, has been applied to study the solution properties of poly(diallyldimethylammonium chloride) (PDDA) in water containing different electrolytes, namely: NaCl, NaBr and LiCl, at 25 °C. The analysis of a single highly polydisperse sample is enough for obtaining calibration curves for molecular weight and radius of gyration and the scaling law coefficients. The effect of the ionic strength on the conformational properties of the polymer can also be analyzed and unperturbed dimensions can be obtained by extrapolation of the values measured in a good solvent. The values of the characteristic ratio of the unperturbed dimensions thus obtained were: 17, 11 and 17, respectively, for NaCl, NaBr and LiCl solutions. Viscosity and conductivity measurements support the results obtained by SEC. Moreover, the experimental results are in good agreement with the theoretical calculations performed by combining molecular dynamics and Monte Carlo sampling procedures.     

Hydrolytic degradation of poly(ethylene terephthalate)

Molecular weight is an important factor in the processing of polymer materials, and it should be well controlled to obtain desired physical properties in final products for end‐use applications. Degradation processes of all kinds, including hydrolytic, thermal, and oxidative degradations, cause chain scission in macromolecules and a reduction in molecular weight. The main purpose of this research is to illustrate the importance of degradation in the drying of poly(ethylene terephthalate) (PET) before processing and the loss of weight and mechanical properties in textile materials during washing. Several techniques were used to investigate the hydrolytic degradation of PET and its effect on changes in molecular weight. Hydrolytic conditions were used to expose fiber‐grade PET chips in water at 85°C for different periods of time. Solution viscometry and end‐group analysis were used as the main methods for determining the extent of degradation. The experimental results show that PET is susceptible to hydrolysis. Also, we that as the time of retention in hydrolytic condition increased, the molecular weight decreases, but the rate of chain cleavage decreased to some extent, at which point there was no more sensible degradation. The obtained moisture content data confirmed the end‐group analysis and viscometry results. Predictive analytical relationships for the estimation of the extent of degradation based on solution viscosity and end‐group analysis are presented. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 2304–2309, 2007     

异端基遥爪聚乙二醇的合成

采用3种新式引发剂,即2-(苄氧基）乙醇钾、2-(四氢-2H-吡喃-2-氧基)乙醇钾、单丙烯基乙二醇钾引发环氧乙烷阴离子开环聚合,反应条件为25 ℃、48 h、醇与萘钾摩尔比例1∶1,得到3种异端基遥爪聚乙二醇。以2-(苄氧基）乙醇钾引发聚合所得产物为起始物,经一系列反应,得到两种两端均为活性基团的异端基遥爪聚乙二醇,这种方法具有普适性。通过1HNMR及GPC手段,表征了产物的结构、分子量及分子量分布。结果表明可以得到高产率、分子量可控且分布窄的异端基遥爪聚乙二醇。     

Nafcillin release from poly(acrylic acid–co–methyl methacrylate) hydrogels

Summary Copolymeric poly(acrylic acid-co-methyl methacrylate) hydrogels for three different compositions: (90/10), (80/20) and (60/40), have been studied. Drug release has been examined as a function of the hydrogel composition by HPLC (High Pressure Liquid Cromatography). The release experiments were carried out at 37 °C. The fraction of available drug release was linear in t^1/2. The values of the diffusional coefficient (0.50<n<1.0) indicate that the nafcillin release mechanism from the hydrogels in study is non-Fickian. The diffusion coefficients for this drug release have been calculated. The molecular diffusion of nafcillin through hydrogels is controlled by the swelling.