Preparation of Porous Poly(vinyl alcohol)‐Silk Fibroin (PVA/SF) Blend Membranes

The relation of PVA/SF blending ratio and freezing temperature with the morphology, fine structure and properties of porous PVA/SF blend membranes prepared by means of freeze drying was investigated. It was indicated that the pore diameter of the blend membranes remarkably decreased and the pore density obviously increased with increasing proportion of PVA or decreasing freezing temperature. With increasing proportion of PVA, the crystallinity of the blend membrane increased. When the blend ratio of PVA/SF was larger than 25/75 or 50/50, the strength, the elongation and the initial tensile modulus of the blend membrane increased somewhat and the compressibility decreased a little with increasing proportion of PVA or decreasing freezing temperature. Therefore, by increasing the proportion of PVA or decreasing the freezing temperature, porous SF/PVA blend membranes could be prepared which had smaller pore diameter, larger pore density, higher crystallinity, strength and elongation.     

Microstructure and mechanical properties of poly(L‐lactide) scaffolds fabricated by gelatin particle leaching method

Biodegradable poly(L ‐lactide) (PLLA) scaffolds with well‐controlled interconnected irregular pores were fabricated by a porogen leaching technique using gelatin particles as the porogen. The gelatin particles (280–450 μm) were bonded together through a treatment in a saturated water vapor condition at 70°C to form a 3‐dimensional assembly in a mold. PLLA was dissolved in dioxane and was cast onto the gelatin assembly. The mixtures were then freeze‐dried or dried at room temperature, followed by removal of the gelatin particles to yield the porous scaffolds. The microstructure of the scaffolds was characterized by scanning electron microscopy with respect to the pore shape, interpore connectivity, and pore wall morphology. Compression measurements revealed that scaffolds fabricated by freeze‐drying exhibited better mechanical performance than those by room temperature dying. Along with the increase of the polymer concentration, the porosity of the scaffolds decreased whereas the compressive modulus increased. When the scaffolds were in a hydrated state, the compressive modulus decreased dramatically. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1373–1379, 2005     

Effect of [6,6]‐phenyl‐C61‐butyric acid methyl ester on the morphology of poly(3‐hexylthiophene) film

The change of morphology of poly(3‐hexylthiophene) (P3HT) film as a result of blending with [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) was studied using a freeze‐dry method. A porous structure was observed as the P3HT/PCBM solution was freeze‐dried. The pore size decreased as the proportion of PCBM increased in the P3HT/PCBM blended film. Additionally, the freeze‐dried P3HT/PCBM film was more resistant to the formation of PCBM crystals than that prepared by a spin‐coating method during the thermal annealing process. Homogeneous distribution of PCBM in the freeze‐dried P3HT/PCBM film was the main reason for the reduction of large PCBM crystal formation. Copyright © 2011 Society of Chemical Industry     

High‐tensile‐strength polyvinyl alcohol films prepared from freeze/thaw cycled gels

The mechanical properties and molecular structure of a poly(vinyl alcohol) (PVA) film, which was obtained by eliminating water from a PVA hydrogel using repeated freeze/thaw cycles, were investigated by tensile tests, thermal analysis, and X‐ray diffraction measurements. The mechanical properties of PVA with 99.9% saponification were measured as a function of the number of freeze/thaw cycles performed. The tensile strength and Young's modulus increased and the elongation at break decreased with increasing freeze/thaw cycles. The tensile strength and Young's modulus of PVA films obtained after seven freeze/thaw cycles were as high as 255 MPa and 13.5 GPa after annealing at 130°C. Thermal analysis and X‐ray diffraction measurements revealed that this is because of a high crystallinity and a large crystallite size. A good relationship between the tensile strength and the glass transition temperature was obtained, regardless of the degree of saponification and annealing conditions. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40578.     

The effect of freeze‐drying parameters on the cure characteristics of freeze‐dried BSA‐loaded silicone elastomer

To formulate therapeutic proteins into polymeric devices the protein is typically in the solid state, which can be achieved by the process of freeze‐drying. However, freeze‐drying not only risks denaturing the protein but it can adversely affect the cure characteristics of protein‐loaded silicone elastomers. This study demonstrates that a variation in the parameters of the freeze‐dryer can significantly affect the residual moisture content of freeze‐dried BSA, which in turn has an effect on the bulk density and flow properties of the BSA. The bulk density and flow properties of the BSA subsequently affect the cure characteristics of BSA‐loaded silicone elastomers. An increase in the residual moisture content results in the freeze‐dried BSA having a decreased bulk density and poor flow properties which can have a detrimental effect on the cure characteristics of a freeze‐dried BSA‐loaded silicone elastomer. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Bioactive monetite‐containing whisker‐like fibers reinforced chitosan scaffolds

The aim of this work was to develop bioactive chitosan scaffolds reinforced with monetite‐containing whisker‐like fibers. The fibers synthesized by homogeneous precipitation were characterized as monetite/hydroxyapatite short fibers (MAFs), using XRD, FTIR and SEM. The pure chitosan and MAFs/chitosan composite scaffolds were produced by freeze‐drying, and characterized with respect to porosity, pore size, swelling behavior, compressive strength and modulus, and in vitro bioactivity. The incorporation of MAFs in chitosan matrices led to increase the pore size, according to the evaluation by FE‐SEM, and decrease the porosity of composite scaffolds. The swelling ratio decreased as MAFs content of scaffolds increased. The compressive strength and modulus of scaffolds were improved by an increase in MAFs content. The noncross‐linked scaffolds with a chitosan: MAFs weight ratio of 1:1 (CW3) showed a porosity of 75.5%, and the strength and modulus of 259 kPa and 2.8 MPa in dry state, respectively. The crosslinking by glutaraldehyde resulted in improved mechanical properties. The strength and modulus of cross‐linked CW3 scaffolds in wet state reached to 345 kPa and 1.8 MPa, respectively. The in vitro bioactivity of the reinforced scaffolds, evaluated by FE‐SEM/EDS, XRD, and ATR‐FTIR, was confirmed by the formation of a carbonated apatite layer on their surfaces when they soaked in simulated body fluid (SBF). The results of this initial study indicate that the monetite‐containing whisker‐like fibers may be an appropriate reinforcement of chitosan scaffolds.     

The effect of desiccation on the porosity and pore structure of freeze dried hardened portland cement and slag-blended pastes

Mercury porosimetry studies of hardened cement pastes of Portland cement and blast furnace slag blended cements have been conducted after freeze drying cured samples. The samples have been aged after freeze drying in a desiccating atmosphere so as to evaluate the effect of aging on the pore structure of freeze dried hardened Portland cement paste prepared with and without admixtures. Results indicate that after 24 hours of freeze drying the hardened paste shows negligible change in porosity and pore structure with aging in a desiccating atmosphere.     

Polyvinyl alcohol‐polyethylene oxide‐carboxymethyl cellulose membranes for drug delivery

The purpose of this research was to develop blends of poly(vinyl alcohol) (PVA)‐poly(ethylene oxide) (PEO) and carboxymethyl cellulose (CMC) by two approaches: solvent casting and freeze‐drying to develop membranes for various biomedical applications. The PVA/PEO/CMC blends in different compositions of 90/10/20, 80/20/20, 70/30/20, 60/40/20, and 50/50/20 were prepared and were coated on polyester (PET) nonwoven fabric and were subsequently freeze‐dried (FD). The influence of PEO concentration on the blend membranes was investigated and characterized by X‐ray diffraction (XRD), differential scanning calorimetry, and attenuated total reflectance‐fourier transform infra‐red (ATR–FTIR) techniques. The water vapor transmission rate (WVTR), swelling behavior, and surface morphology of the FD membranes was also investigated. It was observed that an increase of PEO concentration in blends makes the membranes more fragile. However, the coating of this blend on PET fabric helps in developing the stable membrane. Swelling of the membranes decreased with the increase in the PEO concentration. XRD showed decrease in crystallinity with increase in concentration of PEO. Morphological studies showed a highly porous structure with interconnected pores. The total porosity of the membranes was found to be in the range 89–92%. The FD membranes were found to have WVTR in the range 2000–3000 g/m²/day. A model drug, ciprofloxacin hydrochloride was also incorporated in the matrix and drug release was studied. The antimicrobial nature of the membranes was monitored against E. coli by zone of inhibition method. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

A biaxial stretched β‐isotactic polypropylene microporous membrane for lithium‐ion batteries

In this article, microporous polypropylene (PP) membranes were produced with TMB‐5 as β‐crystal nucleating agent by biaxial stretching. Influences of different concentration of TMB‐5 were studied using differential scanning calorimetry and X‐ray diffraction. It was found that the highest crystallinity was reached when the nucleating agent content was 0.5 wt %. The effect of stretching temperature and stretching ratio on pore size distribution and porosity of the membranes were investigated by scanning electron microscopy and mercury porosimeter, respectively. And physical properties, such as tensile strength, permeability, and puncture resistance of the microporous membrane prepared at the optimized conditions, were evaluated. Compared with commercial PP separator membrane, the as‐prepared microporous membrane shows similar uniform pore size distribution and exhibits slightly higher porosity and ionic conductivities. When used as lithium‐ion separator, the experimental film shows more stable cycling performance than the commercial one. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45825.     

Effect of freeze‐thaw pretreatment on thermal drying process and physicochemical properties of chitosan

The effect of freeze‐thaw pretreatment on the thermal drying process and physicochemical properties of chitosan was investigated in this study. Results showed that the freeze‐thaw treatment changed the form of chitosan paste and reduced 75.6–77.7% of the water content. The freeze‐thaw treatment decreased the drying time of chitosan from 16–19 h to 2.75–4 h and the dried product was loosely packed powder. After freeze‐thaw treatment, the molecular weight of chitosan was unchanged during the thermal drying. The heat‐induced browning effect of chitosan during drying was greatly alleviated by the pretreatment. Furthermore, the pretreatment increased the 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) radical‐scavenging activity of dried product by 40.4–59.8%. The molecular weight, color, and DPPH radical‐scavenging activity of the pretreated dried chitosan product were close to those of freeze‐dried product. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41017.     

Preparation,microstructure, and thermal properties of phenolic cryogel composite plates

Phenolic hydrogel plates were successfully fabricated by a microemulsion‐templated sol gel polymerization method. Then, the phenolic cryogel plates were obtained from the hydrogel plates freeze‐dried by the lyophilizer. Ultimately, the phenolic cryogel plates (PCPs) and fiber glass cloth layers were compounded to be the phenolic cryogel composite plates (PCCPs). The pore morphology, mechanical, and thermal properties of the composites were investigated. The experimental results showed that the aerogels made by freeze‐drying method and disposed by acetic acid and tert butyl alcohol had the best microstructure. In addition, it was discovered that the tensile strength of the plates had equally increased for approximately eight times after compounding with the glass fiber cloth layers and the mechanical properties of the 20 wt% and 25% solid content PCPs achieved the optimal value compared with PCPs of other contents. Moreover, it was found that the cryogels had excellent thermostability, and their thermal conductivity decreased with the reducing of the solid contents, what's more, the joining of cloth layers had the increased the composites’ thermal conductivity to some degree. Finally, considering the requirement of low density, good mechanical, and thermal performance, the 20 wt% solid content PCCP had the best comprehensive performance compared with others. POLYM. COMPOS., 38:2294–2300, 2017. © 2015 Society of Plastics Engineers     

One‐ or two‐dimensional channel structures and properties of piezoelectric composites via freeze‐casting

By employing carefully tailored tert‐butyl alcohol (TBA)‐based freeze‐casting parameters, a large amount of porosity (>70 vol%) and one or two‐dimensional pore channels created were produced into alkali niobate‐based (NKN) ceramics. The relationship between processing factors and microstructures has here been studied, in terms of (i) porosities controlled by adjusting the solid loading in the initial slurry and (ii) strategically attempted freezing direction to make varied pore channels, in which two freezing directions from the bottom or side of mold can produce unidirectional elongated and radially centrosymmetric microstructures, respectively. In addition to that, NKN/epoxy composites with 3‐1 or 3‐2 type polymer channels in the NKN matrix have been fabricated by infiltration of the polymer into the porous NKN hosts. The effect of the channel directions on the mechanical and piezoelectric properties of the composites was investigated for varied volume fractions of the active ceramic phase, mechanical loading, and the poling direction, leading to very high‐piezoelectric g₃₃ coefficients at >60 mV·m/N in the composites with unique channel structures.     

High‐strength regenerated cellulose fibers spun from 1‐butyl‐3‐methylimidazolium chloride solutions

High‐performance regenerated cellulose fibers were prepared from cellulose/1‐butyl‐3‐methylimidazolium chloride (BMIMCl) solutions via dry‐jet wet spinning. The spinnability of the solution was initially evaluated using the maximum winding speed of the solution spinning line under various ambient temperatures and relative humidities in the air gap. The subsequent spinning trials were conducted under various air gap conditions in a water coagulation bath. It was found that low temperature and low relative humidity in the air gap were important to obtain fibers with high tensile strength at a high draw ratio. From a 10 wt % cellulose/BMIMCl solution, regenerated fibers with tensile strength up to 886 MPa were prepared below 22 °C and relative humidity of 50%. High strengthening was also strongly linked with the fixation effect on fibers during washing and drying processes. Furthermore, an effective attempt to prepare higher performance fibers was conducted from a higher polymer concentration solution using a high molecular weight dissolving pulp. Eventually, fibers with a tensile strength of ～1 GPa and Young's modulus over 35 GPa were prepared. These tensile properties were ranked at the highest level for regenerated cellulose fibers prepared by an ionic liquid–based process. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45551.     

Highly porous Y2SiO5 ceramic with extremely low thermal conductivity prepared by foam‐gelcasting‐freeze drying method

Foam‐gelcasting‐freeze drying method is developed to fabricate porous Y₂SiO₅ ceramic with ultrahigh porosity of 92.2%‐95.8% and isotropous multiple pore structures. As prepared porous samples have quite low shrinkages of 0.8%‐1.9% during demolding and drying processes, lightweights of 0.19‐0.35 g/cm³, and extremely low thermal conductivities of 0.054‐0.089 W·(m·K)^?1. Our approach combines the merits of foam‐gelcasting method and freeze drying method. It is a simple and effective method to fabricate porous ceramics with very high porosity and extremely low thermal conductivity through low shrinkage of green body and near net complex shape forming.     

Physicochemical characterization of hydrogels based on polyvinyl alcohol–vinyl acetate blends

Hydrogels made of polyvinyl alcohol–vinyl acetate and its blends with water soluble polymer were studied in terms of swelling behavior, microstructure, and dynamic mechanical properties. Hydrogels prepared by blending polyvinyl alcohol–vinyl acetate with either polyacrylic acid or poly(4‐vinyl pyridine) exhibited a strong pH dependency. When poly(vinyl pyrrolidone) was used for blending, an unusual pH dependency was observed. An increase in the equilibrium water content in all systems resulted in an increase in the freezable water as determined by DSC. Critical point drying led to a striated surface on polyacrylic acid–polyvinyl alcohol–vinyl acetate hydrogels, whereas a porous structure was observed on the freeze‐dried poly(vinyl pyrrolidone)–polyvinyl alcohol–vinyl acetate gels. Hydrogels with elevated storage modulus were obtained when either polyvinyl alcohol–vinyl acetate alone or polyacrylic acid–polyvinyl alcohol–vinyl acetate blends were thermally treated at high temperatures (i.e., 150°C). Low storage modulus was observed for both poly(vinyl pyrrolidone) and poly(4‐vinyl pyridine)‐containing hydrogels. Temperature dependency of storage modulus from 20 to 60°C was observed only for poly(4‐vinyl pyridine)–polyvinyl alcohol–vinyl acetate hydrogels. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3578–3590, 2001     

The influence of solvent properties on the performance of polysulfone/β‐cyclodextrin polyurethane mixed‐matrix membranes

This study investigates the effect of solvent properties on the structural morphology and permeation properties of polysulfone/β‐cyclodextrin polyurethane (PSf/β‐CDPU) mixed‐matrix membranes (MMMs). The membranes were prepared by a modified phase‐inversion route using four different casting solvents [dimethyl formamide (DMF), dimethyl sulfoxide (DMSO), dimethyl acetamide (DMA), and N‐methyl‐2‐pyrrolidone (NMP)]. While DMSO‐based membranes demonstrated particularly high permeability (ca 147 L/m²h.bar), their crystallinity was low compared to MMMs prepared using DMA, DMF and NMP due to the formation of thin active layers on their surfaces. Cross‐sectional morphology revealed that the MMMs have a dense top skin with finger‐like inner pore structures. Membranes prepared using NMP displayed the highest hydrophilicity, porosity, and crystallinity due to the low volatility of NMP; DMF membranes exhibited superior mechanical and thermal stability due to its (DMF) high hydrogen bonding (δH) values. Thus, the morphological parameters, bulk porosity, and flux performance of MMMs have a significant inter‐relationship with the solubility properties of each solvent (i.e., δH, density, volatility, solubility parameter). © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2005–2014, 2013     

Effect of PEG additives on properties and morphologies of membranes prepared from poly(2,6‐dimethyl‐1,4‐phenylene oxide) by benzyl bromination and in situ amination

This study investigated the effect of PEG additive on the structure formation and permeation properties of membranes. The membranes were prepared from a bromomethylated poly(2,6‐dimethyl‐1,4‐phenylene oxide)/chlorobenzene/ethanol system using the phase inversion method with PEG as an additive. As expected, PEG with a fixed molecular weight (e.g., PEG 600) acted as a pore‐forming agent, and membrane porosity increased as the PEG content of the casting solution increased. However, when the PEG content was fixed, the effect of PEG on the membrane properties and morphology was largely dependent on its molecular weight. It was found that when the molecular weight of PEG was less than 800, it acted as a pore former, but when the molecular weight of PEG was more than 1000, the pore size and porosity of the resulting membrane decreased. These results can be explained by the membrane‐forming system's thermodynamic and kinetic properties, which can be assessed by coagulation value and viscosity. Furthermore, the membranes were characterized for pure water flux and rejection of solute and by SEM observation. The filtration results agreed well with the SEM observations. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2414–2421, 2005     

Novel preparative method for porous hydrogels using overrun process

Porous poly(vinyl alcohol) (PVA) hydrogels were prepared using the overrun process which is usually used in manufacturing ice cream. The pores in the hydrogel formed exhibit dual‐pore structure due to the injection of air bubbles and ice recrystallization. Morphological investigation revealed that overrun‐processed hydrogels had closed pore structures and that their pore size and size distribution had been influenced by the impeller rate and concentration of polymer solution. The closed‐pore structure was reformed into interconnected open‐pore structure at lower concentrations of the solution. The freeze–thawing process, which takes place in PVA cross‐linking, has no effect on the bubble structure, but removes the small pores formed during ice recrystallization. Besides the swelling ratio of overrun‐processed PVA hydrogels is increased tenfold in comparison with non‐porous hydrogels. Overrun‐processed hydrogels showed more rapid swelling kinetics than freeze‐dried and film‐like hydrogels due to their larger surface area. In the future, the overrun process can be applied to prepare porous scaffolds containing proteins, such as growth factors and other cytokines, without denaturation, because it operates at a low temperature. Copyright © 2004 Society of Chemical Industry     

Freeze‐drying of aqueous solution frozen with prebuilt pores

To save drying time and increase productivity, a novel idea was proposed for freeze‐drying of liquid materials by creating an initially unsaturated frozen structure. An experimental investigation was carried out aiming at verifying the idea using a multifunctional freeze‐drying apparatus. Mannitol was selected as the primary solute in aqueous solution. Liquid nitrogen ice‐cream making method was used to prepare the frozen materials with different initial porosities. Results show that freeze‐drying can be significantly enhanced with the initially unsaturated frozen material, and substantial drying time can be saved compared with conventional freeze‐drying of the initially saturated one. Drying time was found to decrease with the decrease in the initial saturation. The drying time for the initially unsaturated frozen sample (S₀ = 0.28 or 0.69 of initial porosity) can be at best 32% shorter than that required for the saturated one (S₀ = 1.00 or zero porosity). This unique technique is easy to implement and improves the freeze‐drying performance of liquid materials. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2048–2057, 2015     

Influence of Processing Methods on Mechanical and Structural Characteristics of Vacuum Microwave Dried Biopolymer Foams

Control of physical and mechanical properties of biopolymer (derived from food hydrocolloid) porous solids in terms of stress strain relationship during compression, porosity and pore size would enable their use for a wider range of purposes. Different types of dried cellular biopolymer foams were produced using different food hydrocolloids such as locust bean and alginate gums, gelatin, low and high methoxy pectin, methyl cellulose and starches (corn and tapioca) at various proportions. First different types of wet hydrogels were prepared by varying gel processing methods. Then they dried using microwave energy under vacuum called vacuum microwave drying. Before performing the drying process the initial Young's modulus of the hydrogels was measured. Pore size analysis and distribution percentage were done using mercury pore size analyser after drying. Relationship between the pore size distribution after drying and the initial Young's modulus was developed. Compressive test was performed for dried porous solids and true stress strain relationship curves were developed to classify nature of dried foams obtained from various gel types. Scanning Electron Microscopic study of individual samples was performed to view the internal structure of dried porous biopolymers.