High‐pressure viscosity of polystyrene solutions in toluene + carbon dioxide binary mixtures

High‐pressure viscosity of polystyrene (M_w = 50,000, M_w/M_n ≤ 1.06) solutions in toluene and in mixtures of toluene + carbon dioxide was measured using a falling cylinder‐type viscometer at 320, 340, and 360 K, and up to 35 MPa. Solutions with polystyrene concentrations of 3, 5, and 7 wt % were investigated. Carbon dioxide levels in the range from 0 to 14.7 wt % were evaluated. Viscosity was observed to vary linearly with pressure at the temperatures and polymer concentrations investigated. Viscosity of the polymer solutions decreased as the concentration of carbon dioxide in the mixture was increased. The largest viscosity reduction was observed at the lowest temperature and at the highest concentration of polymer. The viscosity of the solutions was correlated with the solution density for different compositions. It was found that solutions of the same density have different viscosities, depending upon the carbon dioxide concentration in the mixture. The solutions with the higher carbon dioxide content display the lower viscosities at a given density. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 306–315, 2000     

High-pressure phase behavior in polyethylene/n-butane binary and polyethylene/n-butane/CO2 ternary systems

Solubility of polyethylene molecular weight standards (M_w = 2150, 16,400, 108,000, and 420,000 and M_w/M_n = 1.14, 1.16, 1.32, and 2.66, respectively) has been studied in near- and supercritical n-butane and n-butane/CO₂ mixtures at pressures up to 70 MPa. For each polyethylene/solvent system at selected compositions, demixing pressures have been determined using a high-pressure variable-volume view-cell at temperatures up to 200°C. Solutions in pure n-butane are found to display LCST (lower critical solution temperature)-type behavior. The behavior of the solutions in n-butane/CO₂ mixtures are observed to change from the LCST to the UCST (upper critical solution temperature) with increasing CO₂ content in the binary solvent. Sanchez–Lacombe theory has been used to model these systems. The predictions correctly describe the nature of the phase diagrams for both binary and ternary systems and the calculations are in reasonable agreement with experimental data. © 1994 John Wiley & Sons, Inc.     

Phase behavior and density of polysulfone in binary fluid mixtures of tetrahydrofuran and carbon dioxide under high pressure: Miscibility windows

Mixtures of tetrahydrofuran (THF) and carbon dioxide (CO₂) were identified as new solvent systems for polysulfone. The miscibility and density of polysulfone in binary fluid mixtures of THF and CO₂ were investigated from 300 to 425 K at pressures up to 70 MPa. The influence of the CO₂ and polysulfone concentrations was studied, with the concentrations of the other two components kept constant. At a 4.5 wt % polymer concentration, the demixing pressures in a 10 wt % CO₂ and 90 wt % THF mixture increased with temperature (310–425 K) from 15 to 40 MPa. With increasing CO₂ concentration (from ca. 10 to 14 wt %), a significant increase (from 15 to 70 MPa at 310 K) was observed in the demixing pressures. Furthermore, with an increasing amount of CO₂, the nature of the phase boundary shifted from lower critical solution temperature behavior to upper critical solution temperature behavior. The influence of the polymer concentration was studied in the 0–5 wt % range at two CO₂ levels, with solvent compositions of 10 wt % CO₂ and 90 wt % THF and 13 wt % CO₂ and 87 wt % THF. The system with a higher level of CO₂ (13 wt %) showed highly unusual phase behavior: on pressure–composition and temperature–composition diagrams, the system displayed two distinct regions of miscibility. In the system with 10 wt % CO₂, the distinct regions of miscibility that were observed in the system with 13 wt % CO₂ partially overlapped and led to a W‐shape phase boundary. The densities of the polymer solutions were measured from the one‐phase region through the demixing point into the two‐phase region at a constant temperature. No significant change in density was found around the phase boundary; this indicated that the coexisting phases had similar densities, as is often the case with liquid–liquid phase separation in polymer solutions under high pressure. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2357–2362, 2002     

Preparation of porous poly(L‐lactic acid)‐co‐(trimethylene‐carbonate) structures using supercritical CO2 as antisolvent and as foaming agent

In this work, porous structures of poly(l ‐lactic acid)‐co‐(tri‐methylene‐carbonate) (PLLA‐co‐TMC) were successfully fabricated using two experimental methods, that is, using supercritical CO₂ as antisolvent and as foaming agent through the pressure induced phase separation technique. Considering the phase inversion method, the effect of the initial polymer concentration of the solution, pressure, and temperature on the morphology of the final porous structure (pore size, porosity, and cell density) was investigated. The L–L demixing process was suggested as the dominant mechanism for the phase separation and pore production. The temperature window, for which PLLA‐co‐TMC porous structures are successfully produced using the pressure induced phase separation technique, was determined at 150 and 210 bar. The effect of temperature on the final porous structure was investigated. POLYM. ENG. SCI., 57:1005–1015, 2017. © 2016 Society of Plastics Engineers     

Comparison of Sanchez–Lacombe and SAFT model in predicting solubility of polyethylene in high-pressure fluids

Sanchez–Lacombe and SAFT (statistical associating fluid theory) models are used to describe phase behavior of polyethylene solutions (M_w = 2,150, 16,400, 108,000, and 420,000, and M_wM_n = 1.14, 1.16, 1.32, and 2.66, respectively) in n-pentane and in n-butane at high pressures. In order to test the predictive capability of the two models, all the predictions were conducted without any adjustment of the binary interaction parameter. Even though both models correctly predict the general trends of the phase envelopes and the LCST (lower critical solution temperature) nature of the systems, SAFT gives predictions that are much closer to the experimental data than the Sanchez–Lacombe model. © 1995 John Wiley & Sons, Inc.     

Synthesis of poly(propylene carbonate) from highly active,inexpensive achiral (Salph)Co(III)X as initiator and bis(triphenyl phosphine) iminium as co‐initiator

Copolymerization of carbon dioxide with racemic propylene oxide has been investigated in the presence of an inexpensive achiral (Salph)Co(III)X [Salph is N,N′‐bis(3,5‐di‐tert‐butylsalicylidene) phenylenediimine and X is pentaflorobenzoate] as initiator and [PPN]⁺Cl^? ([PPN] is bis(triphenylphosphine) iminium) as co‐initiator. Effects of monomer‐to‐initiator ratio, initiator/co‐initiator ratio, and reaction conditions like stirring rate, temperature, and pressure of CO₂ on the molecular weight, yield, and selectivity of poly(propylene carbonate) over propylene carbonate have been studied. The initiator used in the study has been found to be highly active at milder conditions of pressure and temperature, giving a product with maximum M_w of 14.8 × 10³ g/mol at 25 bar and 50°C. The conversion increases with an increase in stirring rate and then becomes almost constant at 1100 rpm and above, indicating that the reaction is no longer limited by mass transfer. The molecular weight M_w of the polymer has been found to increase with increasing monomer‐to‐initiator ratio up to 3000:1, but it starts decreasing with a further increase in monomer‐to‐initiator ratio, giving a polymer of lower M_w. The activity of the initiator is considerably affected by pressure, temperature, time, and amount of co‐initiator. The polymeric product has low polydispersity (near unity) with negligible formation of polypropylene oxide. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43099.     

Controlled antibody release from a matrix of poly(ethylene-co-vinyl acetate) fractionated with a supercritical fluid

A new method is presented for controlling the rate of antibody (Ab) release from an inert matrix composed of poly(ethylene-co-vinyl acetate) (EVAc), a biocompatible polymer that is frequently used to achieve controlled release. Using supercritical propane, a parent EVAc sample (M_n = 70 kDa, M_w/M_n = 2.4) was separated into narrow fractions with a range of molecular weights (8.7 < M_n < 165 kDa, 1.4 < M_w/M_n < 1.7). Solid particles of Ab were dispersed in matrices composed of different polymer fractions and the rate of Ab release into buffered saline was measured. The rate of Ab release from the EVAc matrix depended on molecular weight: > 90% of the incorporated Ab was released from low molecular weight fractions (M_n < 40 kDa) during the first 5 days of release, while < 10% was released from the high molecular weight fraction (M_n > 160 kDa) during 14 days of release. No significant differences in polymer composition, glass-transition temperature, or crystallinity were identified in the different molecular weight fractions of EVAc. Mechanical properties of the polymer did depend on the molecular weight distribution, and correlated directly with Ab release rates. Because it permits rapid and reproducible fractionation of polymers, supercritical fluid extraction can be used to modify the performance of polymeric biomaterials. © 1993 John Wiley & Sons, Inc.     

Hydrophobically modified poly(sodium 2‐acrylamido‐2‐methylpropanesulfonate): synthesis and viscosity of aqueous solution

BACKGROUND: Hydrophobically modified polyelectrolytes are widely used polymers due to their good water solubility, stretched configuration in water and strong hydrophobic association. The study reported here aimed at researching the double action of hydrophobic association and electrostatic effect of novel hydrophobically modified polyelectrolytes in solution. RESULTS: A series of novel hydrophobically modified polyelectrolytes were synthesized by micellar copolymerization with various feed ratios of sodium 2‐acrylamido‐2‐methylpropanesulfonate, N‐n‐dodecylamine and sodium dodecylsulfonate. Their structure was characterized using Fourier transform infrared spectroscopy, nuclear magnetic resonance and gel permeation chromatography, and the viscosities of their aqueous and salt solutions were studied. CONCLUSION: The results show that the addition of the hydrophobic comonomer results in a decrease in molecular weight (M_w). The smaller the initial number of hydrophobes in one micelle, the higher is M_w of the resulting copolymer. The viscosity of PAD‐1.73 polyelectrolyte is less sensitive to salt than those of the others. According to the zero shear viscosity and corresponding concentration, the critical cluster‐forming concentration, critical overlap concentration and critical entanglement concentration of these polymer solutions were determined. Moreover, in the dilute regime the viscosity decreases with increasing salinity, while in the semi‐dilute regime the viscosity decreases first and then increases. It is suggested that in dilute and semi‐dilute regimes, hydrophobic intramolecular association and intermolecular association dominate, respectively. Copyright © 2009 Society of Chemical Industry     

Solid–fluid phase behaviour of linear polyethylene solutions in propane, ethane and ethylene at high pressures

The solid–fluid phase transitions for a series of linear polyethylene fractions (M_w: 800, 7000, 23 625, 52 000, 59 300 g/mol) in propane, ethane and ethylene were measured in the temperature range from 360 to 423 K and at pressures up to 2000 bar. Conditions of precipitation of the solid polyethylene from supercritical solutions seem to have minor influence on dissolution of the solid polymer in supercritical solvents. In general, an increase of pressure results in a shift of the solid–fluid phase transition to higher temperatures, i.e. solubility of polyethylene decreases. The solid–fluid phase boundaries for systems composed of low-molecular weight polyethylene and propane show temperature minima. The effect is not observed in the high-molecular weight polyethylene + propane systems and systems with ethane or ethylene as solvents. It was observed that the temperature of the solid–fluid phase transition measured at a constant pressure and a constant composition is not a monotonic function of molecular weight of the polymer. The order of the dissolution temperatures depends on pressure.     

An amphoteric water‐soluble copolymer. II. Effect of its molecular weight on the dispersion of barium titanate in water

An amphoteric water‐soluble copolymer, that is, polyacrylamide/(α‐N,N‐dimethyl‐N‐acryloyloxyethyl)ammonium ethanate (PAAM/DAAE) was synthesized and it showed the ability to disperse BaTiO₃ (BT) particles in aqueous solutions. In this work, the effect of molecular weight of this polymer on the dispersing properties was further examined. The results indicate that the effectiveness of three polymer samples with different molecular weights in the dispersion of BT particles is P2 (M_w = 1.1 × 10⁵) > P1 (M_w = 1.2 × 10⁴) > P3 (M_w = 3.0 × 10⁵). Apparently, P2 is most effective in dispersing the particles, reducing the viscosity of the suspensions, and obtaining highest green and sintered densities. This is attributed to the highest adsorption of this polymer onto BT powder, and causes strongest electrostatic and steric repulsions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 886–891, 2006     

Schiff base‐substituted polyphenol: synthesis,characterisation and non‐isothermal degradation kinetics

BACKGROUND: Polymers of phenols and aromatic amines have emerged as new materials in fields such as superconductors, coatings, laminates, photoresists and high‐temperature environments. The stability, kinetics and associated pollution of the thermal decomposition of oligophenols are of interest for the aforementioned fields. RESULTS: A new Schiff base polymer, derived from N,N′‐bis(2‐hydroxy‐3‐methoxyphenylmethylidene)‐2,6‐pyridinediamine, was prepared by oxidative polycondensation. Characterisations using Fourier transform infrared, UV‐visible, ¹H NMR and ¹³C NMR spectroscopy, thermogravimetric/differential thermal analysis, gel permeation chromatography, cyclic voltammetry and conductivity measurements were performed. The number‐average (M_n) and weight‐average molecular weight (M_w) and dispersity (D = M_w/M_n) of the polymer were found to be 61 000 and 94 200 g mol^?1 and 1.54, respectively. Apparent activation energies of the thermal decomposition of the polymer were determined using the Tang, Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose and Coats–Redfern methods. The most likely decomposition process was a D_n deceleration type in terms of the Coats–Redfern and master plot results. CONCLUSION: The mechanism of the degradation process can be understood through the use of kinetic parameters obtained from various non‐isothermal methods. Copyright © 2009 Society of Chemical Industry     

Precipitation polymerization of PLGA with Sn(Oct)2/BuOH in supercritical carbon dioxide

In order to increase molecular weight of PLGA, BuOH was introduced as a co‐initiator to Sn(Oct)₂ in ring‐opening precipitation polymerization of L ‐lactide and glycolide in supercritical carbon dioxide (ScCO₂). The polymerization mechanism of the reaction processes on Sn(Oct)₂ with BuOH was explored. The positive effect of BuOH on polymer molecular weight was studied. PLGA with higher M_w (of 19,500 g/mol) and narrower PDI (of 1.12) was successfully prepared by ring‐opening precipitation polymerization in ScCO₂ using Sn(Oct)₂/BuOH as an efficient catalytic system. Influence of reaction conditions, such as monomer feed concentration, reaction temperature, pressure, and time, were also investigated in detail. POLYM. ENG. SCI., 54:704–710, 2014. © 2013 Society of Plastics Engineers     

Preparation and morphology characterization of microcellular styrene‐co‐acrylonitrile (SAN) foam processed in supercritical CO2

Microcellular polymeric foam structures have been generated using a pressure‐induced phase separation in concentrated mixtures of supercritical CO₂ and styrene‐co‐acrylonitrile (SAN). The process typically generates a microcellular core structure encased by a non‐porous skin. Pore growth occurs through two mechanisms: diffusion of CO₂ from polymer‐rich regions into the pores and also through CO₂ gas expansion. The effects of saturation pressure, temperature and swelling time on the cell size, cell density and bulk density of the porous materials have been studied. Higher CO₂ pressures (hence, higher fluid density) provided more CO₂ molecules for foaming, generated lower interfacial tension and viscosity in the polymer matrix, and thus produced lower cell size but higher cell densities. This trend was similar to what was observed in swelling time series. While the average cell size increased with increasing temperature, the cell density decreased. The trend of bulk density was similar to that of cell size. © 2000 Society of Chemical Industry     

Solvent effect on the lower critical solution temperature of biodegradable thermosensitive poly(organophosphazenes)

Summary The solvent effect on the lower critical solution temperature (LCST) of poly(organophosphazenes) with methoxy-poly(ethylene glycol) (MPEG) and amino acid esters as side groups was examined in terms of the structure of polyphosphazenes in aqueous solutions containing one of the organic solvents selected from monoalcohols, ethylene glycol derivatives, alkylamines, and other common solvents. When such a solvent was added to the aqueous solutions of the polymers, their LCST was found to be mainly dependent on the hydrophobic and hydrophilic properties of the solvents. Most of the alcohols and amines with shorter alkyl chains increased the LCST of the polymers but those with longer chains decreased the LCST. Trifluoroethanol (TFE) showed a strong LCST decreasing effect in spite of its short chain, which seems to be due to its strong hydrophobicity. Temperature-induced molecular weight fractionation of the polymer bearing MPEG350 (M _w= 350) and L-aspartic acid ethyl ester as a side group was carried out by using the LCST decreasing effect of TFE, and the fractionated samples were characterized by gel permeation chromatography (GPC) and ¹H- and ³¹P NMR spectroscopies. Thus it has been shown that a polymer may be fractionated to the higher and lower molecular weight fractions with smaller polydispersity indices (PDI): the polymer with the weight-average molecular weight (M _w) of 73,500 with PDI of 5.56 was fractionated to those of 106,000 with PDI of 4.37 and 11,000 with PDI of 1.86. Received: 8 September 2000/Revised version: 6 November 2000/Accepted: 9 November 2000     

Nitroxide‐mediated synthesis of styrenic‐based segmented and tapered block copolymers using poly(lactide)‐functionalized TEMPO macromediators

Poly(styrene)‐poly(lactide) (PS‐PLA), poly (tert‐butyl styrene)‐poly(lactide) (PtBuS‐PLA) diblocks, and poly(tert‐butyl styrene)‐poly(styrene)‐poly(lactide) (PtBuS‐PS‐PLA) segmented and tapered triblocks of controlled segment lengths were synthesized using nitroxide‐mediated controlled radical polymerization. Well‐defined PLA‐functionalized macromediators derived from hydroxyl terminated TEMPO (PLA_T) of various molecular weights mediated polymerizations of the styrenic monomers in bulk and in dimethylformamide (DMF) solution at 120–130°C. PS‐PLA and PtBuS‐PLA diblocks were characterized by narrow molecular weight distributions (polydispersity index (M_w/M_n) < 1.3) when using the PLA_T mediator with the lowest number average molecular weight M_n= 6.1 kg/mol while broader molecular weight distributions were exhibited (M_w/M_n = 1.47‐1.65) when using higher molecular weight mediators (M_n = 7.4 kg/mol and 11.3 kg/mol). Segmented PtBuS‐PS‐PLA triblocks were initiated cleanly from PtBuS‐PLA diblocks although polymerizations were very rapid with PS segments ～ 5–10 kg/mol added within 3–10 min of polymerization at 130°C in 50 wt % DMF solution. Tapering from the PtBuS to the PS segment in semibatch mode at a lower temperature of 120°C and in 50 wt % DMF solution was effective in incorporating a short random segment of PtBuS‐ran‐PS while maintaining a relatively narrow monomodal molecular weight distribution (M_w/M_n ≈ 1.5). © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Synthesis of star‐shaped poly(ϵ‐caprolactone) by samarium‐based tetrafunctional initiator and its dilute‐solution properties

An in situ–generated tetrafunctional samarium enolate from the reduction of 1,1,1,1‐tetra(2‐bromoisobutyryloxymethyl)methane with divalent samarium complexes [Sm(PPh₂)₂ and SmI₂] in tetrahydrofuran has proven to initiate the ring‐opening polymerization of ?‐caprolactone (CL) giving star‐shaped aliphatic polyesters. The polymerization proceeded with quantitative conversions at room temperature in 2 h and exhibited good controllability of the molecular weight of polymer. The resulting four‐armed poly(?‐caprolactone) (PCL) was fractionated, and the dilute‐solution properties of the fractions were studied in tetrahydrofuran and toluene at 30°C. The Mark–Houwink relations for these solvents were [η] = 2.73 × 10^?2M_w^0.74 and [η] = 1.97 × 10^?2M_w^0.75, respectively. In addition, the unperturbed dimensions of the star‐shaped PCL systems were also evaluated, and a significant solvent effect was observed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 175–182, 2006     

Measurement and prediction of LDPE/CO2 solution viscosity

When CO₂ is dissolved into a polymer, the viscosity of the polymer is drastically reduced. In this paper, the melt viscosities of low‐density polyethylene (LDPE)/supercritical CO₂ solutions were measured with a capillary rheometer equipped at a foaming extruder, where CO₂ was injected into a middle of its barrel and dissolved into the molten LDPE. The viscosity measurements were performed by varying the content of CO₂ in the range of 0 to 5.0 wt% and temperature in the range of 150°C to 175°C, while monitoring the dissolved CO₂ concentration on‐line by Near Infrared spectroscopy. Pressures in the capillary tube were maintained higher than an equilibrium saturation pressure so as to prevent foaming in the tube and to realize single‐phase polymer/CO₂ solutions. By measuring the pressure drop and flow rate of polymer running through the tube, the melt viscosities were calculated. The experimental results indicated that the viscosity of LDPE/CO₂ solution was reduced to 30% of the neat polymer by dissolving CO₂ up to 5.0 wt% at temperature 150°C. A mathematical model was proposed to predict viscosity reduction owing to CO₂ dissolution. The model was developed by combining the Cross‐Carreau model with Doolittle's equation in terms of the free volume concept. With the Sanchez‐Lacombe equation of state and the solubility data measured by a magnetic suspension balance, the free volume fractions of LDPE/CO₂ solutions were calculated to accommodate the effects of temperature, pressure and CO₂ content. The developed model can successfully predict the viscosity of LDPE/CO₂ solutions from PVT data of the neat polymer and CO₂ solubility data.     

Polymerization of olefins through heterogeneous catalysis. XIV. The influence of temperature in the solution copolymerization of ethylene

The influence of temperature variation on the kinetics and the polymer properties in the homo- and copolymerization of ethylene in a solution reactor is discussed. The Polymerization is conducted in a semibatch mode at 320 Psig total reactor pressure for 10 min polymerization time. Temperature variations in the range 145–200°C in both home-and copolymerization of ethylene with 1-octene shows that the highest catalyst yield was obtained at temperature of 165–175°C. At the optimal temperature, a high initial maximum in the rate of ethylene consumption is attained in a few seconds followed by a relatively slow decay when compared with polymerization conducted at higher temperatures. Polymerization at temperatures ≥ 185°C resulted in a lower peak in the consumption rate of ethylene accompanied by a rapid decay with time. In the case of ethylene/1-Octene copolymerization, a rather low comonomer incorporation level is obtained at the conditions employed; the 1-octene incorporated was only 0.2–0.7 mol %. Higher M_w values, of about 350,000 at 145°C, are obtained in homopolymerization in comparison to M_w values obtained in copolymerization, of about 195,000 at the same temperature. Over the temperature range of 145–200°C, both M_w and M_n values vary by about 40%. © 1993 John Wiley & Sons, Inc.     

A new technique for foaming submicron size poly(methyl methacrylate) particles

About 0.7–2 μm diameter poly (methyl methacrylate) (PMMA) foamed particles were prepared via thermally induced phase separation (TIPS) from a PMMA/ethanol mixture and vacuum dried. It was found that ethanol, known to be a poor solvent to PMMA, could dissolve PMMA when the temperature was over 60°C. The solubility of PMMA (M_w = 15,000 and M_w = 120,000) in ethanol was measured and was found to increase as the temperature increased. PMMA particles on the scale of submicron and single micron diameter could be precipitated from the PMMA/ethanol solution by temperature quenching. Then, since the precipitated particles contained a certain amount of ethanol, the precipitated particles could be foamed using the ethanol as a foaming agent in a vacuum drying process. Vacuum drying at temperatures slightly below the glass transition temperature of the polymer could make the particles foam. The effects of foaming temperature and the molecular weight of the polymer on the size of foamed particles were investigated. The experimental results showed that the vapor pressure and the molecular weight of the polymer are key factors determining the expandability of the micro particles. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

In vitro release of complexed pDNA from biodegradable polymer films

The controlled delivery of low‐molecular weight drugs and proteins from biodegradable polymers has received considerable attention. However, controlled release studies of pDNA from such polymers have not been reported to date. In this study, a plasmid DNA was complexed with the cationic polymer called polyethylenimine (PEI). This gene vector has been shown to be very effective in transfecting cells. The complexed DNA were then incorporated into different types of poly‐lactic‐co‐glycolic acid (PLGA) film; PLGA 53/47 (M_w 90 kDa), 50/50 (M_w 11 kDa, end group is lauryl ester) and 75/25 (M_w 120 kDa). Their release profiles from a buffer solution were studied. An initial (small) burst release of PEI‐DNA from film was observed in PLGA 53/47 and 50/50, followed by a plateau phase and finally a rapid erosion‐controlled release. For PLGA 50/50, the rapid release started after 14 days; erosion‐controlled release for PLGA 53/47 started after 9 days; for PLGA 75/25, the release rate was governed by an initial burst release (10%) followed by a slow release controlled by diffusion. No obvious erosion‐controlled release rate was observed for this polymer up to 27 days. Thus, the controlled release of complexed DNA follows the general features exhibited by lower‐ M_w drugs. This is of significance in designing gene vector matrices that offer the promise of more lasting gene therapy compared with particulate formulations. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008