Relative reactivities and graft distributions of polystyrene Macromers® in vinyl chloride copolymerization

Polystyrene macromonomers terminated with methacrylate, vinyl ether, or maleic half ester functionalities were evaluated in free radical initiated copolymerizations with vinyl chloride in aqueous suspension polymerization. Macromers® (M₁) terminated with methacrylate disappeared very rapidly in copolymerization with vinyl chloride (M₂). The relative reactivity ratio, r₂, was determined to be 0.05 in good agreement with literature values of about 0.04. Vinyl ether-terminated Macromers® had unexpectedly uniform reactivity with vinyl chloride in early conversion samples, but macromonomer conversion was incomplete. Macromers® having maleic half ester functionality were incorporated rapidly in vinyl chloride copolymerization at pH 2.5 (r₂ = 0.13). However, at pH 10 these Macromers® had reduced reactivity (r₂ = 0.34), which improved graft polymer uniformity. These Macromer® copolymerization relative reactivities are shown to be useful in predicting and controlling graft densities and graft polymer heterogeneity which influence morphology, processing, and mechanical properties.     

The effects of the synthesis parameters on the xerogels structures and on the swelling parameters of the poly(methacrylic acid) hydrogels

Poly(methacrylic acid) hydrogels were synthesized. The effects of the synthesis parameters: the neutralization degree of methacrylic acid and the concentrations of monomer, crosslinker and initiator on the xerogels structural properties: the xerogel density (ρ_xg), the number average molar mass between the network crosslinks (M_c), the crosslink degree (ρ_c), the number of elastically effective chains totally induced in a perfect network per unit volume (V_e), the distance between the macromolecular chains (ξ) and the equilibrium swelling degree (SD_eq) and the swelling kinetics was investigated. As the concentrations of crosslinker, monomer and initiator increase, the value of ρ_xg, ρ_c and V_e increases and decreases the value of M_c, ξ and SD_eq. With the increase in the neutralization degree of methacrylic acid, the values of ρ_xg, M_c, ξ, SD_eq increase, while the ρ_c and V_e decrease. The xerogels structural properties, SD_eq and swelling kinetic parameters are mainly in power law form functional relationships with the synthesis parameters as well as with the xerogels crosslinking degree.     

Synthesis, characterization and biodegradation of poly(ester amide)s based hydrogels

A series of new biodegradable hybrid hydrogels were designed and fabricated from a new family of amino acid-based functional poly(ester amide) (PEA-AG) and commercial poly(ethylene glycol) diacrylate (PEG-DA) or Pluronic diacrylate (Pluronic-DA) by UV photocrosslinking. These biodegradable hybrid hydrogels were characterized in terms of equilibrium swelling ratio (Q_eq), compression modulus by dynamic mechanical analysis (DMA), and interior morphology by scanning electron microscope (SEM). Both the precursors’ chemical structures and feed ratio had significant effect on the properties of the hybrid hydrogels. All these hybrid hydrogels had a three-dimensional porous network structure. The hydrophobicity, crosslinking density and mechanical strength of the hybrid hydrogels increased with an increase in allylglycine (AG) content in the PEA-AG, but the swelling and pore size of the hybrid hydrogels decreased. The biodegradation rate of these hybrid hydrogels in an enzyme (α-chymotrypsin) solution was faster than in a pure PBS buffer control, and the biodegradation rate increased with an increase in α-chymotrypsin concentration and allylglycine content.     

Preparation,characterization, and water‐sorption study of polyvinyl alcohol based hydrogels with grafted hydrophilic and hydrophobic segments

The interpenetrating polymer network hydrogels based on poly(vinyl alcohol) were obtained by graft copolymerization of acrylamide and styrene onto polyvinyl alcohol in the presence of N,N′‐methylene bisacrylamide as a crosslinking agent. The hydrogels were characterized by optical microscopy, scanning electron microscopy, infrared spectral analysis, differential scanning calorimeter, and thermogravimetric analysis. The hydrogels showed enormous swelling in aqueous medium and displayed swelling characteristics, which were highly dependent on the chemical composition of the hydrogels and pH of the swelling medium. The kinetics of water uptake and the mechanisms of water transport were studied as a function of composition of the hydrogel and pH of the swelling medium. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1129–1142, 2005     

Effect of different preparation methods on structure and properties of chitosan/poly-lactic acid blend porous membrane

Two kinds of blend solutions were used to prepare chitosan (CS)/poly-l-lactic acid (PLLA) blend membranes by the immersion precipitation phase inversion method. CS/PLLA blend membrane was fabricated by mixing CS-acetic acid solution and PLLA-dioxane solution, chitosan powder (CSP)/PLLA blend membrane was fabricated by mixing CSP and PLLA-dioxane solution. The membrane structure and properties were characterized by Scanning Electron Microscope, Wide Angle X-ray Diffraction, porosity, water vapor transmission rate and swelling property. The results showed that CS content in CSP/PLLA blend membrane was nearly 70 times higher than in CS/PLLA blend membrane. Two types of blend membranes took on an unsymmetrical structure with a skin layer, a microvoid sub-layer and a porous bottom surface. But CSP/PLLA blend membrane exhibited a porous skin layer, while the skin layer of CS/PLLA blend membrane was impact. The WAXD analysis revealed that PLLA and CS were amorphous in CS/PLLA blend membrane, while there were PLLA and CS crystalline in CSP/PLLA membrane, which resulted in the higher initial modulus of CSP/PLLA blend membrane. The porosity, WVTR and equilibrium swelling (Q _eq ) of CSP/PLLA blend membrane were higher than that of CS/PLLA blend membrane, especially Q _eq . The swelling kinetics results showed that in the initial swelling, water molecules diffusion followed Non-Fickian diffusion for CS/PLLA blend membrane swelling, and the swelling of CSP/PLLA blend membrane fitted with the relaxation diffusion model. The swelling kinetics studies for whole swelling revealed that the swelling degree and the initial swelling rate of CSP/PLLA membrane were significantly higher than that of CS/PLLA membrane, although the total swelling rate of CSP/PLLA blend membrane decreased lightly.     

Separation of the effects of pH and polymer concentration on the swelling pressure and elastic modulus of a pH-responsive hydrogel

The elastic shear modulus G and swelling pressure ω are studied for a basic, pH-responsive hydrogel synthesized by crosslinking copolymerization of co-monomers hydroxypropyl methacrylate and N,N-dimethylaminoethyl methacrylate with crosslinker tetraethylene glycol dimethacrylate. Under normal conditions of use as a “smart” material, hydrogel swelling ratio Q and pH vary simultaneously, but here G and ω values are presented as a function of pH with Q held constant and vice versa. At fixed pH, G decreases with increase in Q in a power law dependence, as predicted by the Flory-Rehner model. However, at fixed Q, G increases with decrease in pH (i.e., increase in degree of ionization). The pH effect is more pronounced than the volume effect, thus the hydrogel stiffens as it swells in response to pH change. At high pH, ω values of the uncharged hydrogel obey the Flory-Rehner model, whereas explicit ionic contributions can be identified at lower pH values.     

Synthesis and super-swelling behavior of a novel protein-based superabsorbent hydrogel

Summary In this work, we synthesize a novel protein-based superabsorbent hydrogel and study its swelling behavior. The crosslinking graft copolymerization of acrylic acid (AA) onto the hydrolyzed collagen as a protein backbone was carried out in a homogeneous medium. Potassium persulfate (KPS) as an initiator and N,N^′-methylene bisacrylamide (MBA) as a crosslinker were used. The product’s structure was established using FTIR spectroscopy. We were systematically optimized the certain variables of the graft copolymerization (i.e. the monomer, the initiator, and the crosslinker concentration) to achieve a hydrogel with maximum swelling capacity. Under this condition, maximum capacity of swelling in distilled water was found to be 920 g/g. Morphology of the optimized sample was examined by scanning electron microscopy (SEM). The swelling ratio in various salt solutions was also determined. Additionally, the swelling of superabsorbing hydrogels was measured in solutions with pH ranged from 1 to 13. The synthesized hydrogel exhibited a pH-responsiveness character so that a swelling-collapsing pulsatile behavior was recorded at pH 2 and 8.     

Synthesis and characterization of alternating copolymers of thiophene-containing N-phenyl maleimide and styrene by photoinduced radical polymerization and their use in electropolymerization

A novel N-(4-(3-thienyl methylene)-oxycarbonylphenyl) maleimide (MBThi) monomer was synthesized by the esterification reaction of maleimidobenzoic acid (MBA) with 3-thiophene methanol. Photoinduced radical polymerization was employed to prepare the alternating copolymers of MBThi with styrene (St) at room temperature using ω,ω-dimethoxy-ω-phenylacetophenone (DMPA) as photoinitiator. Different copolymerization conditions were examined to estimate the influence of the used solvents and comonomers' total molar concentration on the conversion, the number-average molecular weight (M_n), and polydispersity index (M_w/M_n) of the resulting polymers. Thermal behavior of the alternating copolymers (PSt-alt-MBThi) was also investigated by thermogravimetrical analysis (TGA) and differential scanning calorimetry. Moreover, the obtained alternating copolymers were employed in electropolymerization experiments and random conjugated graft copolymers with thiophene or pyrrole were synthesized through their pendant thienyl groups. These polymers were characterized by cyclic voltammetry (CV), FTIR and scanning electron microscopy (SEM). Conductivity measurements were carried out by the four-probe technique.     

Preparation conditions and swelling equilibria of dextran hydrogels prepared by some crosslinking agents

Crosslinking reactions of Dextran (Dx) (M_n of 2.0×10⁶ g mol⁻¹) with some selective Cl-, P- and N-containing functional monomers such as epichlorohydrin (ECH), phosphorus oxychloride (POC1₃) and N,N′-methylenebisacrylamide (MBAM) were carried out in the basic aqueous solutions (2.8 N NaOH) at 25-50 °C. The optimum conditions of the effective swelling and crosslinking for the each system studied were found in copper (CuCl₂·2H₂O) solution. The percent swelling, equilibrium swelling, initial rate of swelling, swelling rate of constant, equilibrium water content, and diffusion type and constant values were evaluated for Dx/crosslinker (CL) systems at 1 mg/100 ml copper (CuCl₂·2H₂O) solution. A substantial difference of these parameters observed for the various Dx/CL systems was explained by the effect of nature of crosslinking agents on the mechanism of crosslinking and swelling processes. It was shown that S_eq and M_c values increase depending on the nature of CLs in the following order: ECH>MBAM>POCl₃. General scheme and proposed mechanism of crosslinking reactions in the Dx/CL systems were also described.     

Development of a superporous hydroxyethyl cellulose‐based hydrogel by anionic surfactant micelle templating with fast swelling and superabsorbent properties

The self‐assembling anionic surfactant, sodium n‐dodecyl sulfonate (SDS) micelles were used as pore‐forming templating for fabricating novel superporous hydroxyethyl cellulose‐grafting‐poly(sodium acrylate)/attapulgite (HEC‐g‐PNaA/APT) hydrogels. The network characteristics, morphologies of the hydrogels and removing of SDS micelles from the final product by washing with ethanol/water (v/v, 7 : 3) procedure were determined by Fourier transform infrared spectroscopy and scanning electron microscopy, as well as by determination of swelling ratio, swelling rate, and stimuli response to salts and pHs. The results showed that the added‐SDS concentration significantly affected the morphologies and pore structure of the hydrogel, and 2 mM SDS facilitates to form a homogeneous and well‐defined pore structure in the gel network to extremely improve the swelling ratio and swelling rate. The 2 mM SDS‐added superporous HEC‐based hydrogel not only had highest equilibrium swelling ratio (Q_eq, 1118, 102 g g^?1 in distilled water and 0.9 wt % NaCl solution), rapid swelling rate (k_is, 5.2840 g g s^?1), also showed multistimulus responses to salts and pHs, which may allow its applications in several areas such as adsorption, separation and biomedical materials. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42027.     

Synthesis of PAA-g-PNIPAM well-defined graft polymer by sequential RAFT and SET-LRP and its application in preparing size-controlled super-paramagnetic Fe3O4 nanoparticles as a stabilizer

A series of well-defined double hydrophilic graft copolymers, poly(acrylic acid)-g-poly(N-isopropylacrylamide) (PAA-g-PNIPAM), was employed as a novel water-soluble coating for constructing superparamagnetic iron oxide nanoparticles. The copolymer was synthesized via a three-step procedure: firstly, a well-defined hydrophobic PtBA-based backbone, poly(tert-butyl 2-((2-chloropropanoyloxy)-methyl)acrylate)-co-poly(tert-butyl acrylate), (PtBCPMA₁₉-co-PtBA₁₈), was prepared through RAFT copolymerization of a new trifunctional acrylic monomer, tert-butyl 2-((2-chloropropanoyloxy)methyl)acrylate and tert-butyl acrylate; secondly, taking this backbone as a macroinitiator to initiate SET-LRP of N-isopropylacrylamide resulted in well-defined (poly(tert-butyl 2-((2-chloropropanoyloxy)methyl)-acrylate)-co-poly(tert-butyl acrylate))-g-poly(N-isopropylacrylamide) ((PtBCPMA-co-PtBA)-g-PNIPAM) amphiphilic graft copolymers with relatively narrow polydispersities (M_w/M_n ≤ 1.31); thirdly, handling (PtBCPMA-co-PtBA)-g-PNIPAM in acidic conditions afforded PAA-g-PNIPAM graft copolymers. The resulting PAA-g-PNIPAM copolymers were directly utilized as a polymeric stabilizer in the preparation of superparamagnetic Fe₃O₄ nanoparticles. The particle size can be readily tuned in the range of 12.1–23.2 nm by varying the amount of PAA-g-PNIPAM copolymer or the length of PNIPAM side chain. Besides, the structure and properties of prepared Fe₃O₄/polymer nanocomposites were characterized by XRD, FT-IR, TGA, TEM, and magnetic measurement in detail.     

Time dependence of the aggregation of star-shaped poly(2-isopropyl-2-oxazolines) in aqueous solutions

The paper concerns the analysis of time t _eq required to equilibrium state achievement in aqueous solutions of star-shaped poly(2-isopropyl-2-oxazolines) (PiPrOx) after changing temperature. The discussed data were obtained for PiPrOx differing in arm number and length. For all samples, high t _eq values, half an hour at least, were obtained because of rather high intramolecular density. The dependence t _eq on temperature displayed maximum near the phase separation beginning due to the aggregate growth and redistribution of scattering particles. The maximum times t _eq increased symbatically with arm number and length. The higher energy of the hydrogen bond formed by deuterium isotope leads to the growth of the t _eq values as compared to solutions in H₂O.     

Synthesis and characterization of butyl acrylate graft sodium salt of partially carboxymethylated starch

Graft copolymers were synthesized by graft copolymerization of butyl acrylate (BA) onto sodium salt of partially carboxymethylated starch (Na‐PCMS). Ceric ammonium nitrate (CAN), a redox initiator, was used for initiation of graft copolymerization reaction. All the experiments were run with Na‐PCMS having degree of substitution, DS = 0.35. The grafting reaction was characterized by parameters such as % total conversion (%Ct), % grafting (%G), % grafting efficiency (%GE), and % add‐on. Graft copolymers were characterized by infrared spectral analysis and scanning electron microscopy. Variables affecting graft copolymerization reaction such as nitric acid concentration, reaction time, reaction temperature, and ceric ion concentration were investigated. The results revealed that 0.3M CAN as initiator, 0.3M HNO₃, with reaction time 4–4.5 h at 25–30°C were found as suitable parameters for maximum yield of graft copolymerization reaction. © 2006 Wiley Periodicals, Inc. JAppl Polym Sci 102: 3334–3340, 2006     

Effects of different pH medium on swelling properties of 1,2,4,5-benzenetetracarboxylic-chitosan-filled chitosan bio-composites

Investigating the swelling properties of chitosan (Cs) film was deemed meaningful, as it plays an important role in predicting the life span of the film. Due to the limits in stability properties, the insertion of reinforcement agent is expected to increase the properties of Cs film. To this purpose, 1,2,4,5-benzenetetracarboxylic-chitosan (BTC) filler was inserted into the Cs matrix. The effect on the degree of swelling (Q _t ) and the rate of swelling (Q _r ) of the composite film at varying compositions of BTC filler (0, 2, 4, 6, 8, 10 and 12 wt/v%) was investigated. The Q _r and Q _t both decrease with an increasing BTC content, which may be attributed to the looser packaging structure, and the improvement of the hydrophobic character of the composites film. Thus, the addition of BTC filler, up to 10 wt/v%, makes the Cs film more stable with a prolonged swelling time. Meanwhile, electrostatic interaction and hydrogen bonding between the swelling medium and neutral groups, of the polymeric chains of the composites, contributed to the obtained values of Q _t and Q _r . The FTIR results support the argument for the Q _t and Q _r values of different compositions of BTC filler in the Cs matrix, in the different swelling medium (pH 2–14).     

Dynamic swelling behavior of pH‐sensitive anionic hydrogels used for protein delivery

There have been many attempts to use anionic hydrogels as oral protein delivery carriers because of their pH‐responsive swelling behavior. The dynamic swelling behavior of poly(methacrylic acid‐co‐methacryloxyethyl glucoside) and poly(methacrylic acid‐g‐ethylene glycol) hydrogels was investigated to determine the mechanism of water transport through these anionic hydrogels. The exponential relation M_t/M_∞ = ktⁿ (where M_t is the mass of water absorbed at time t and M_∞ is the mass of water absorbed at equilibrium) was used to calculate the exponent (n) describing the Fickian or non‐Fickian behavior of swelling polymer networks. The mechanism of water transport through these gels was significantly affected by the pH of the swelling medium. The mechanism of water transport became more relaxation‐controlled in a swelling medium of pH 7.0, which was higher than pK_a of the gels. The experimental results of the time‐dependent swelling behaviors of the gels were analyzed with several mathematical models. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1606–1613, 2003     

Synthesis and characterization of novel pH‐sensitive chitosan‐poly(acrylamide‐co‐itaconic acid) hydrogels

Novel pH‐sensitive chitosan‐poly(acrylamide‐co‐itaconic acid) hydrogels were prepared by free radical copolymerization of acrylamide and itaconic acid (IA) in chitosan solution. The hydrogels were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry and the swelling ratios of the hydrogels in water (pH 6.8) and pH 1.2. The influence of composition on the thermal properties of the hydrogels was assessed. The glass transition temperatures of the samples increased with IA content, ranging from 110 to 136 °C. Swelling of the hydrogels was found to obey second‐order kinetics with respect to the remnant swelling, indicating that diffusion is controlled by the relaxation of chains. The equilibrium swelling degree was strongly dependent on pH and composition. At both pH values the highest water uptake was obtained for the IA‐free sample M1. From the equilibrium swelling results the average molar mass between crosslinks, M_c, and the crosslink density of the chitosan‐poly(acrylamide‐co‐itaconic acid) samples were calculated. The results evidenced the reinforcing effect of IA on the hydrogel structure. It is concluded that these highly swellable pH‐sensitive hydrogels can be useful for applications in biomedicine and pharmacy. © 2013 Society of Chemical Industry     

pH-dependent swelling of hydrogels containing highly branched polyamine macromonomers

We examine the pH-dependent swelling of end-linked hydrogels containing high concentrations of amine-functional macromonomers. Gels are formed by end-linking of epoxide-terminated, linear poly(ethylene glycol) (PEG) to either amine-terminated poly(amidoamine) (PAMAM) dendrimers or highly branched poly(ethyleneimine) (PEI). After extraction in neutral water, the hydrogels are swollen in aqueous solutions of HCl or NH₄OH to vary the external pH. Equilibrium volume swelling ratios (Q_s) pass through a maximum value (Q_max) at an external pH denoted as pH^∗ which is approximately 4-5 for the gels studied. The swelling behavior is modeled using Donnan equilibrium theory to describe the ion swelling pressure, with the Flory-Rehner phantom network expression representing the elastic and mixing contributions to the free energy. The model accurately predicts the maximum in swelling near pH = 4-5, but overestimates Q_max for several of the gels due to neglecting the finite extensibility of the short linear PEG chains.     

A Comparative Study on Curing Characteristics,Mechanical Properties,Swelling Behavior,Thermal Stability,and Morphology of Feldspar and Silica in SMR L Vulcanizates

Abstract

Comparison studies on effects of feldspar and silica (Vulcasil C) as a filler in (SMR L grade natural rubber) vulcanizates on curing characteristics, mechanical properties, swelling behavior, thermal analysis, and morphology were examined. The incorporation of both fillers increases the scorch time, t ₂, and cure time, t ₉₀, of SMR L vulcanizates. At a similar filler loading, feldspar exhibited longer t ₂ and t ₉₀ but lower values of maximum torque, M_HR, and torque difference, M_HR–M_L than did silica-filled SMR L vulcanizates. For mechanical properties, both fillers were found to be effective in enhancing the tensile strength (up to 10 phr), tensile modulus, and hardness of the vulcanizates. However, feldspar-filled SMR L vulcanizates showed lower values of mechanical properties than did silica-filled SMR L vulcanizates. Swelling measurement indicates that swelling percentages of both fillers-filled SMR L vulcanizates decrease with increasing filler loading whereas silica shows a lower swelling percentage than feldspar-filled SMR L vulcanizates. Scanning electron microscopy (SEM) on fracture surface of tensile samples showed poor filler–matrix adhesion for both fillers with increasing filler loading in the vulcanizates. However, feldspar-filled SMR L vulcanizates showed poorer filler–matrix adhesion than did silica-filled SMR L vulcanizates. Thermogravimetric analysis (TGA) results indicate that the feldspar-filled SMR L vulcanizates have higher thermal stability than do silica-filled SMR L vulcanizates.     

Synthesis and properties of novel polyvinyl alcohol–lactic acid gels

This article describes a process for esterifying polyvinyl alcohol (PVA) with L‐lactide (LLA) and D,L‐lactic acid (LA), using ethyl acetate and N,N′‐dimethyl formamide at temperatures varying from 120 to 150°C. The grafting process was carried out under nitrogen to avoid possible oxidation or other degradation of the process ingredients and product. Lower T_g values were obtained for the PVA/LLA graft copolymers of higher LLA content suggesting some compatibility in the amorphous phase. Higher T_g values were observed for PVA/LA graft copolymers that yielded tough polymer films. The structure of the copolymers was studied by solid‐state ¹³C‐NMR, infrared spectroscopy, and differential scanning calorimetry (DSC). The PVA/LA films exhibited melt processability and good mechanical properties such as yield strength, tensile energy at break, modulus, and elongation at break. The polymer films produced through compression molding at 100°C showed good swelling properties. The transport coefficient (n) values determined from the plot of log(M_t/M_∞) vs. log t indicate Fickian behavior, and they are consistent with the reported literature values for other PVA systems. The nature of water in gels [bound water (W_b), freezing (W_f), and freezing bound (W_fb), and water content (W_t)] was evaluated from DSC data. The results demonstrate that PVA/LA hydrogels with good combination of thermal, physicomechanical, and swelling properties can be prepared via the lactic acid esterification of PVA polymer process described. Besides being melt processable, the PVA/LA gels exhibit a melting point, which indicates possibly use of higher temperatures. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Characterization of commercial linear low‐density polyethylene by TREF‐DSC and TREF‐SEC cross‐fractionation

A new technique for characterization of linear low‐density polyethylene (LLDPE) is presented in this report. The molecular structure of two commercial LLDPEs, produced by copolymerization of ethylene with 1‐butene over a Ziegler‐Natta and a metallocene catalyst, was investigated. The LLDPE resins were fractionated by temperature rising elution fractionation (TREF), and the TREF fractions were further analyzed by size exclusion chromatography and differential scanning calorimetry (DSC) coupled with successive nucleation/annealing (SNA). The cross‐fractionation techniques provided detailed information about the molecular structure of different types of LLDPEs; of particular interest is the TREF‐SNA‐DSC cross‐fractionation which allowed a direct observation of methylene sequence distribution and thus short chain branch (SCB) distribution. TREF‐size exclusion chromatography cross‐fractionation showed that the molar mass of the Ziegler‐Natta LLDPE increased monotonically with decreasing SCB, whereas the plot of M_w vs SCB for the metallocene LLDPE showed a maximum. TREF‐SNA‐DSC cross‐fractionation clearly showed that the metallocene LLDPE only had intramolecular heterogeneity in SCB distribution, whereas the Ziegler‐Natta LLDPEs exhibited both intermolecular and intramolecular heterogeneity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 960–967, 2000