Studies on the interactions of CO2 with biodegradable poly(dl-lactic acid) and poly(lactic acid-co-glycolic acid) copolymers using high pressure ATR-IR and high pressure rheology

Amorphous poly(dl-lactic acid) (P_dlLA) and poly(lactic acid-co-glycolic acid) (PLGA) polymers have been used to fabricate porous scaffolds for tissue engineering applications via a supercritical foaming technique. The chemical composition of the polymers and the morphology (pore size, porosity and interconnectivity) of the scaffolds are crucial because they influence cell filtration, migration, nutrient exchange, degradation and drug release rate. To control the morphology of supercritical foamed scaffolds, it is essential to study the interactions of polymers with CO₂ and the consequent solubility of CO₂ in the polymers, as well as the viscosity of the plasticized polymers. In this paper, we are showing for the first time that well known and useful biodegradable polymers can be plasticized easily using high pressure CO₂ and that we can monitor this process easily via a high pressure attenuated total reflection Fourier transform infrared (ATR-IR) and rheology. High pressure ATR-IR has been developed to investigate the interactions of CO₂ with P_dlLA and PLGA polymers with the glycolic acid (GA) content in the copolymers as 15, 25, 35 and 50% respectively. Shifts and intensity changes of IR absorption bands of the polymers in the carbonyl region (∼1750 cm⁻¹) are indicative of the interaction on a qualitative level. A high pressure parallel plate rheometer has also been developed for the shear viscosity measurements of the CO₂-plastisized polymers at a temperature below their glass transition temperatures. The results demonstrate that the viscosities of the CO₂-plasticized polymers at 35 °C and 100 bar were comparable to the values for the polymer melts at 140 °C, demonstrating a significant process advantage through use of scCO₂. The data from the high pressure rheology and high pressure ATR-IR, combined with the sorption and swelling studies reported previously, demonstrate that the interaction and the solubility of CO₂ in PLGA copolymers is related to the glycolic acid content. As the glycolic acid ratio increases the interaction and consequent solubility of CO₂ decreases. The potential applications of this study are very broad, from tissue engineering and drug delivery to much broader applications with other polymers in areas that may range from composites and polymer synthesis through to injection moulding.     

超临界CO2辅助聚合物加工

近年来,以超临界CO₂替代聚合物加工过程中大量使用的有机溶剂实现超临界CO₂辅助聚合物加工过程已引起人们越来越多的关注。CO₂在聚合物中的溶解扩散可导致其结构和形态的变化,能够溶胀增塑聚合物并且将溶解于其中的小分子物质携带输运到聚合物基体中,进而影响聚合物的结晶及晶型转变行为,聚合物/CO₂体系界面张力以及聚合物/CO₂体系流变行为等基本物性的变化。利用聚合物基本物性的变化可实现CO₂辅助聚合物接枝反应,CO₂辅助聚合物渗透小分子物质以及CO₂辅助聚合物发泡等超临界CO₂辅助聚合物加工过程的应用。结合本研究室的实例,探讨了CO₂作用下等规聚丙烯和间规聚丙烯的结晶行为以及一种多晶型聚合物--等规聚丁烯-1的晶型转变行为;探讨了利用CO₂对等规聚丙烯、聚乳酸和聚酯三种典型的低熔体强度结晶聚合物具有的不同诱导结晶作用,调控聚合物的结晶行为,使其具备发泡所需的熔体强度,制备了具有不同结构特征的发泡聚合物材料。     

Gas Diffusion and Solubility in Poly(organophosphazenes): Results of Molecular Simulation Studies

A combination of quantum chemistry, molecular dynamics, and Monte Carlo methods have been used to investigate gas diffusion and solubility in three isomeric poly[di(butoxyphosphazenes)] and in amorphous and crystalline states of poly[bis(2,2,2-trifluoroethoxyphosphazene)] (PTFEP). In this review of recently published studies reported from our laboratory, conclusions are reached in regards to the relationship between polymer structure and gas diffusion and sorption in poly(organophosphazenes). These conclusions also serve to validate our current understanding of the nature of gas transport in other polymers. Specifically, gas diffusivity has been shown to increase with increasing side-chain and main-chain mobility as determined from vectorial autocorrelation function analysis; however, high diffusivity is accompanied by a loss in diffusive selectivity resulting in decreasing permselectivity with increasing permeability. Simulation of crystalline supercells of PTFEP indicate that gas diffusion is unrestricted in the crystalline state as has been reported only for a few other polymers, principally poly(4-methyl-1-pentene). Gas solubility in poly(organophosphazenes) correlates well with gas condensability as measured by the Lennard–Jones potential well depth parameter, ɛ/k. Exceptions are cases where specific interactions can occur between gas molecules and the polymer chain such as is the case of CO₂ and PTFEP. High-level ab initio calculations of the interaction of CO₂ with low-molecular-weight fluoroalkanes indicate the presence of a weak quadrupole–dipole interaction. Association of CO₂ with the trifluoromethyl groups of the trifluoroethoxy side chain of PTFEP has been confirmed by radial distribution function (RDF) analysis of MD trajectories. Comparison between solubility coefficients obtained from Grand Canonical Monte Carlo (GCMC) simulations of amorphous cells with experimental values of microcrystalline PTFEP indicates that gas solubility in polyphosphazenes such as PTFEP that exhibit a mesophase/crystalline state is greatly reduced. This paper is dedicated to Prof. Harry Allcock for his scientific contributions to inorganic and organometallic polymers.     

Extension of a compressible lattice model to CO2 + cosolvent + polymer systems

We have recently proposed a compressible lattice model for CO₂ + polymer systems in which CO₂ forms complexes with one or more functional groups in the polymer. Furthermore, we have shown that this model is able to simultaneously correlate phase equilibria, sorption behavior, and glass transition temperatures in such systems. In the present work, we extend the model to ternary CO₂ + cosolvent + polymer systems and demonstrate that cloud point behavior in CO₂ + dimethyl ether + poly (?-caprolactone), CO₂ + dimethyl ether + poly (isopropyl acrylate), and CO₂ + dimethyl ether + poly (isodecyl acrylate) systems can be predicted using parameters obtained from binary data. Our results also suggest that dimethyl ether may form weak complexes with poly (?-caprolactone), poly (isopropyl acrylate), and poly (isodecyl acrylate).     

Polymer melt rheology with high-pressure CO2 using a novel magnetically levitated sphere rheometer

A magnetically levitated sphere rheometer (MLSR) designed to measure viscosity of fluids exposed to high-pressure carbon dioxide has been developed. This device consists of a magnetic sphere submerged inside a test fluid within a high-pressure housing and levitated at a fixed point. The housing is constructed from an optically transparent sapphire tube. The cylindrical tube can be moved vertically to generate a shear flow around the levitated sphere. The difference in magnetic force required to levitate the sphere at rest and under fluid motion can be directly related to fluid viscosity. Rheological properties, specifically zero shear viscosities, of transparent high-pressure materials can be measured to a precision of about 5% and over a wide range of viscosities. In addition, operation at constant pressure, in concentration regimes from a pure polymer to an equilibrated polymer/supercritical fluid solution, and at shear rates over several orders of magnitude is possible, eliminating many of the disadvantages associated with other high-pressure rheometers. Experiments performed at different temperatures with a poly(dimethylsiloxane) melt at atmospheric pressure are compared with data from a commercial Couette rheometer to demonstrate device sensitivity and viability. Measurements of a PDMS melt plasticized by high-pressure CO₂ are performed to illustrate the utility of the new rheometer under high-pressure conditions. Experimental data are obtained at 30 °C, for pressures up to 20.7 MPa and CO₂ concentrations reaching 30 wt%. Viscosity reductions of nearly two orders of magnitude compared with the pure polymer viscosity at atmospheric pressure are observed. Additionally, the effects of pressure on a polymer/CO₂ system are directly investigated taking advantage of the constant pressure operation mode of the MLSR. This allows us, for the first time in experiments of polymers with supercritical fluids, to decouple the effects of CO₂ concentration and pressure in a single device.     

Evaluation of CO2-philicity and thickening capability of multichain poly(ether-carbonate) with assistance of molecular simulations

Phenyl-centered tri-chain poly(ether-carbonate) (TMA-PEC), phenyl-centered double-chain poly(ether-carbonate) (TPA-PEC), and phenyl-centered four-chain poly(ether-carbonate) (TFA-PEC) were synthesized to act as CO₂ thickener. Their solubility in CO₂ was measured by cloud point pressure. In order to explore the material characteristics that affect the solubility, dynamic simulations were used to analyze intermolecular polymer interactions, and the interaction between polymers and CO₂. It was found that TPA-PEC and TMA-PEC has better solubility than TFA-PEC in CO₂ among the three polymers while the thickening effect is poor, TFA-PEC possess the best viscosity thickening effect while the solubility in CO₂ is unfavorable. The silicone unit 1,1,1,3,5,5,5-heptamethyl-3-(3-[oxiran-2-ylmethoxy] propyl)trisiloxane modified TFA-PEC (TFA-PEC-SAGE) combine good solubility and good thickening ability together. The molecular simulations show that TPA-PEC and TMA-PEC have weaker intermolecular interactions and TPA-PEC and TMA-PEC have stronger interaction with CO₂ which are beneficial to the solubility.     

Effects of polymers as direct CO2 thickeners on the mutual interactions between a light crude oil and CO2

In this paper, two commercial polymers with relatively low molecular weights, poly vinyl ethyl ether (PVEE) and poly 1-decene (P-1-D), are tested as direct thickeners for CO₂ and their detailed effects on the mutual interactions between a light crude oil and polymer-thickened CO₂ are experimentally studied under different reservoir pressures. More specifically, the polymer cloud-point pressures are measured at different known polymer solubilities in supercritical CO₂. The equilibrium interfacial tensions (IFTs) and onset pressures of quick polymer dissolution into CO₂ are measured for the polymer?pure CO₂ systems. The polymer-swelling effect due to pure CO₂ dissolution is observed and examined. To study the mutual interactions between the light crude oil and polymer-thickened CO₂, their equilibrium IFTs are measured and their so-called minimum miscibility pressures (MMPs) and onset pressures of the initial quick light-hydrocarbons extraction by CO₂ are determined. The oil-swelling effect due to polymer-thickened CO₂ dissolution is also visualized and analyzed. All the experimental data for polymer-thickened CO₂ are compared with those for pure CO₂.     

CO2 sorption and transport behavior of ODPA-based polyetherimide polymer films

Plasticization phenomena can significantly reduce the performance of polymeric membranes in high-pressure applications. Polyetherimides (PEIs) are a promising group of membrane materials that combine relatively high CO₂/CH₄ selectivities with high chemical and thermal stability. In this work sorption, swelling, and mixed gas separation performance of 3,3′,4,4′-oxydiphthalic dianhydride (ODPA)-based PEI polymers, with 1, 2 or 3 para-aryloxy substitutions in the diamine moeiety, is investigated under conditions where commercial membranes suffer from plasticization. Particular focus is on the influence of the amount of para-aryloxy substitutions and the film thickness. Results are compared with those of commercially available polymeric membrane materials (sulphonated PEEK, a segmented block-co-polymer PEBAX and the polyimide Matrimid).The glassy polymers display increasing CO₂ sorption with increasing T_g. The larger extent of sorption results from a larger non-equilibrium excess free volume. Swelling of the polymers is induced by sorption of CO₂ molecules in the non-equilibrium free volume as well as from molecules dissolved in the matrix. Dilation of the polymer is similar for each molecule sorbed. Correspondingly, the partial molar volume of CO₂ is similar for molecules present in both regions.Mixed gas separation experiments with a 50/50% CO₂/CH₄ feed gas mixture showed high CO₂/CH₄ selectivities for the ODPA PEI films at elevated pressure. This shows that these materials could potentially be interesting for high-pressure gas separation applications, although additional gas permeation experiments using different feed gas compositions and thin films are required.     

Synthesis and gas permeability of ester substituted poly(p-phenylene)s

Polymerizations of various ester substituted 2,5-dichlorobenzoates [substituent: linear alkyl groups (1a-f), branched alkyl groups (1g-l), cyclohexyl groups (1m-o), phenyl groups (1p-r), and oxyethylene units (1s-v)] were investigated with Ni-catalyzed/Zn-mediated system in 1-methyl-2-pyrrolidone (NMP) at 80 °C. Most of monomers bearing linear and branched alkyl groups successfully polymerized to give relatively high-molecular-weight polymers (M_n = 10,000-20,800). However, the molecular weight of the polymer having eicocyl groups was low because of steric hindrance of long alkyl chain. The polymerizations of cyclohexyl 2,5-dichlorobenzoate and phenyl 2,5-dichlorobenzoate produced low-molecular-weight polymers, while the polymerizations of monomers with alkyl cyclohexyl and alkyl phenyl groups proceeded to afford polymers with relatively high-molecular-weights. The polymers possessing oxyethylene units were obtained, but the molecular weights were low when the oxyethylene chains were long. The gas permeability of membranes of poly(p-phenylene)s with alkyl chains increased as increasing the length of alkyl chain. The membranes of poly(p-phenylene)s with phenyl groups and oxyethylene units exhibited high densities and relatively low gas permeability. However, the CO₂/N₂ separation factor of membrane of poly(p-phenylene) having oxyethylene units was as large as 73.6.     

Molecular dynamics study on the dissolution behaviors of poly(vinyl acetate)-polyether block copolymers in supercritical CO2

Poly(vinyl acetate) (PVAc) could be dissolved in CO₂ at a high pressure, which limits its application. In this work, the PVAc-polyether block copolymers were constructed by introducing the CO₂-philic blocks poly(propylene oxide) (PPO) and poly(ethylene oxide) (PEO) into the PVAc molecules to increase the solubility of these polymers in supercritical CO₂ (scCO₂). The dissolution behaviors of PVAc-polyether copolymers with different structures in scCO₂ were investigated by the molecular dynamic simulation methods. First, the cohesive energy and solubility parameters of PVAc, PVAc-PEO, and PVAc-PPO copolymers were analyzed. Moreover, the mechanism of PVAc-polyether copolymers dissolving in CO₂ was investigated. The results show that PVAc-PPO molecules have a higher solubility in CO₂ because they have lower polymer–polymer and higher polymer-CO₂ interactions than PVAc and PVAc-PEO. Among the three structures of PVAc-PPO molecules, PVAc-PPO-PVAc (VPV) has a highest solubility in CO₂. Therefore, the molecular composition and structure have greatly influences on the interactions of polymer and CO₂.     

New vinyl ester copolymers as stabilisers for dispersion polymerisation in scCO2

In this paper we demonstrate new poly vinyl alkylate homopolymers and copolymers with excellent solubility in scCO₂ that can be used as stabilisers for dispersion polymerisation in scCO₂. Poly (vinyl acetate) (PVAc) was combined in various ratios with poly (vinyl butyrate) (PVBu) and poly (vinyl octanoate) (PVOc) to both tune the scCO₂-solubility and provide adequate steric stabilisation. The polymer cloud points observed were found to be dependent on the ratio of the different blocks and the molecular weights and polydispersities (PDI) of the polymers. The effectiveness of these new polymeric stabilisers for dispersion polymerisation of N-vinyl pyrrolidone (N-VP) in scCO₂ is presented.     

Poly(?-caprolactone)-block-polystyrene metallopolymers via sequential ROP and ATRP condition with in situ generated ruthenium catalyst

Poly(?-caprolactone)-b-polystyrene with vacant bipyridine coordinating sites and metallated AB type diblock with well-defined metal loci in the polymer chain were synthesized and characterized. Solution atom transfer radical polymerization (ATRP) of styrene where a poly(?-caprolactone) macromonomer acted as initiator and derivative complex of [Ru(p-cymene)Cl₂]₂ as catalyst is reported. ATRP reaction conditions with respect to polymer molecular weights and polydispersity indices (PDI) of the target bifunctional polymers were examined. Electronic absorption and emission spectra of the resultant functional polymers provided evidence of the ruthenium metal chromophores in the diblock copolymer. The thermal properties of all polymers were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), and indicated that they possess a high thermal stability and are miscible in the molten state. The semicrystalline nature of the PCL macroligand and the morphology of thin films of the metal free diblocks were also elucidated by combination of atomic force microscopy (AFM), scanning electron microscopy (SEM) and wide angle X-ray diffraction (WAXD) studies.     

Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle

This paper presents experimental results of a 20 kW vertical combustor equipped with a single pf-burner on pulverised coal combustion in air and O₂/CO₂ mixtures with NO_x recycle. Experimental results on combustion performance and NO_x emissions of seven international bituminous coals in air and in O₂/CO₂ mixtures confirm the previous findings of the authors that the O₂ concentration in the O₂/CO₂ mixture has to be 30% or higher to produce matching temperature profiles to those of coal-air combustion while coal combustion in 30% O₂/70% CO₂ leads to better coal burnout and less NO_x emissions than coal combustion in air. Experimental results with NO_x recycle reveal that the reduction of the recycled NO depends on the combustion media, combustion mode (staging or non-staging) and recycling location. Generally, more NO is reduced with coal combustion in 30% O₂/70% CO₂ than with coal combustion in air. Up to 88 and 92% reductions of the recycled NO can be achieved with coal combustion in air and in 30% O₂/70% CO₂ respectively. More NO is reduced with oxidant staging than without oxidant staging when NO is recycled through the burner. Much more NO is reduced when NO recycled through the burner (from 65 to 92%) than when NO is recycled through the staging tertiary oxidant ports (from 33 to 54%). The concentration of the recycled NO has little influence on the reduction efficiency of the recycled NO with both combustion media—air and 30% O₂/70% CO₂.     

Hydrotalcites for adsorption of CO2 at high temperature

Adsorption of carbon dioxide by hydrotalcites was investigated by using a gravimetric method at 450 ‡C. Hydrotalcites possessed higher adsorption capacity of CO₂ than other basic materials such as MgO and Al₂O₃. Two different preparation methods of hydrotalcite with varying Mg/Al ratio were employed to determine their effects on the adsorption capacity of CO₂. In addition, varying amounts of K₂CO₃ were impregnated on the hydrotalcite to further increase its adsorption capacity of CO₂. The hydrotalcite prepared by the high supersaturation method with Mg/Al=2 showed the most favorable adsorption-desorption pattern with high adsorption capacity of CO₂. K₂CO₃ impregnation on the hydrotalcite increased the adsorption capacity of CO₂ because it changed both the chemical and the physical properties of the hydrotalcite. The optimum amount of K₂CO₃ impregnation was 20 wt%. The hydrotalcite prepared by the high supersaturation method with Mg/Al=2 and 20 wt% K₂CO₃ impregnation has the highest adsorption capacity of CO₂ with 0.77 mmol CO₂/g at 450 ‡C and 800 mmHg.     

Control of main chain structure and possibility of molecular weight control of polymer on polymerization of vinyl chloride with tert-butyllithium

The controlled polymerization of vinyl chloride (VC) with tert-butyllithium (tert-BuLi) was investigated. The polymerization of VC with tert-BuLi at −30 °C proceeded to give a high molecular weight polymer in good yield. In the polymerization of VC −30 to 0 °C under nearly bulk, the relationship between the M_n of polymers and polymer yields gave a straight line passed through the origin, but the M_w/M_n of PVC was not narrow. When CH₂Cl₂ was used as polymerization solvent, the M_n of PVC increased with the polymer yield, and the M_w/M_n of 1.25 was obtained. Structure analysis of the resulting polymers indicates that the main chain structure could be regulated in the polymerization of VC with tert-BuLi. Accordingly, a control of molecular weight of polymer and main chain structure is possible in the polymerization of VC with tert-BuLi.     

In situ study of ionic conductivity for polyether-LiCF3SO3 electrolytes with subcritical and supercritical CO2

In situ measurements of the ionic conductivity were performed on polyethers, poly(ethylene oxide) (PEO) and poly(oligo oxyethylene methacrylate) (PMEO), with lithium triflate (LiCF₃SO₃) as crystalline and amorphous electrolytes, and at CO₂ pressures up to 20 MPa. Both PEO and PMEO systems in subcritical and supercritical CO₂ increased more than five fold in ionic conductivity at 40 °C composed to atmospheric pressure. The pressure dependence of the ionic conductivity for PEO electrolytes was positive under CO₂, and increased by two orders of magnitude under pressurization from 0 to 20 MPa, whereas it decreases with increasing pressure of N₂. The enhancement is caused by the plasticizing effect of CO₂ molecules that penetrate into the electrolytes.     

A density functional theory study of hydrogen recombination and hydrogen-deuterium exchange on Ga/H-ZSM-5

Density functional theory calculations have been carried out to determine the thermodynamic stability of various Ga species in gallium-exchanged ZSM-5, the thermodynamics of H₂ adsorption, and the most favorable pathway for H₂/D₂ exchange. The portion of the zeolite associated with Ga was represented by a cluster containing 7, 21, or 33 atoms. The B3LYP hybrid method was used to account for the effects of electron exchange and correlation. The most likely form of Ga expected in freshly exchanged and calcined ZSM-5 is ZGa(OH)₂. H₂ reduction of this species is projected to produce ZGa(H)(OH) and ZGa(H)₂. While the thermodynamics of H₂ desorption from ZGa(H)₂ are favorable, the process is projected to be slow because of a high activation barrier. The most favorable pathway for H₂/D₂ exchange over ZGa(H)₂ proceeds via Z(D)(Ga(H)₂(D)) as an intermediate. Similar calculations have been carried out for H₂/D₂ exchange over H-ZSM-5. This revised version was published online in June 2006 with corrections to the Cover Date.     

Formation of microcapsules of medicines by the rapid expansion of a supercritical solution with a nonsolvent

The rapid expansion from a supercritical solution with a nonsolvent (RESS‐N) was applied to the formation of polymeric microcapsules containing medicines such as p‐acetamidophenol, acetylsalicylic acid, 1,3‐dimethylxanthine, flavone, and 3‐hydroxyflavone. A suspension of medicine in carbon dioxide (CO₂) containing a cosolvent and dissolved polymer was sprayed through a nozzle to atmospheric pressure. The pre‐expansion pressure was 10–25 MPa, and the temperature was 308–333 K. The polymers were poly(L ‐lactic acid) (molecular weight = 5000), poly(ethylene glycol) (PEG; PEG4000, molecular weight = 3000; PEG6000, molecular weight = 7500; and PEG20000, molecular weight = 20,000), poly(methyl methacrylate) (molecular weight = 15,000), ethyl cellulose (molecular weight = 5000), and PEG–poly(propylene glycol)–PEG triblock copolymer (molecular weight = 13,000). The solubilities of the polymers as coating materials and these medicines as core substance were very low in CO₂. However, the solubilities of these polymers in CO₂ significantly increased with the addition of low molecular weight alcohols as cosolvents. After RESS‐N, polymeric microcapsules were formed according to the precipitation of the polymer caused by a decrease in the solvent power of CO₂. This method offered three advantages: (1) enough of the coating polymers, which were insoluble in pure CO₂, dissolved; (2) the microparticles of the medicine were encapsulated without adhesion between the particles because a nonsolvent was used as a cosolvent and the cosolvent remaining in the mixture was removed by the gasification of CO₂; and (3) the polymer‐coating thickness was controlled with changes in the feed composition of the polymer for drug delivery. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 742–752, 2003