Surface Hydrophilicities and Enzymatic Hydrolyzability of Biodegradable Polyesters, 2

Control of the surface hydrophilicities and enzymatic hydrolyzability of hydrophobic aliphatic polyesters such as poly(ε‐caprolactone) (PCL) and poly(L ‐lactide) [i.e. poly(L ‐lactic acid) (PLLA)] was attempted by coating with hydrophilic poly(vinyl alcohol) (PVA). The PVA coating was carried out by immersion of the PCL and PLLA films in PVA solutions. The effects of PVA coating on the hydrophilicities were monitored by dynamic contact angle measurements, while the enzymatic hydrolyzability of the PVA‐coated PCL and PLLA films was evaluated by the weight losses after Rhizopus arrhizus lipase‐ and proteinase K‐catalyzed hydrolysis, respectively. It was found that the PVA coating successfully enhanced the hydrophilicities of the aliphatic polyester films and significantly suppressed enzymatic hydrolyzability of the aliphatic polyester films, excluding the PCL film coated at a very low concentration such as 0.01 g · dL^?1 and the crystallized PLLA film coated at 1 g · dL^?1, for which slight enhancement and no significant enhancement, respectively, were observed in the enzymatic hydrolyzability. Moreover, the hydrophilicities and enzymatic hydrolyzability of the aliphatic polyester films were controllable to some extent by varying the PVA solution concentration and the film crystallinity.

Advancing contact angle (θ_a) of PCL, PLLA‐C, and PLLA‐A films before and after the PVA coating by immersion in 1 g · dL^?1 solution.     

Solvent-induced morphological diversification in poly(l-lactide-b-?-caprolactone) block copolymer thin films

Biodegradable block copolymer of poly(l-lactide-b-?-caprolactone) (P(LA-b-CL)) was dissolved in various solvents with different solubility as well as volatility, and spin-cast on a highly oriented pyrolytic graphite (HOPG) to prepare thin films. The surface morphologies were observed by using atomic force microscopy (AFM) in a dynamic force (tapping) mode. Particle like morphology was found in the thin films prepared form the dichloromethane and acetone. Higher volatility of dichloromethane and acetone resulted in the reflection of the particle like objects in the solution to HOPG substrate. In contrast, the P(LA-b-CL)s in toluene and 1,4-dioxane exhibited different morphologies compared to those in dichloromethane and acetone. Lower volatility of toluene and 1,4-dioxane assisted the epitaxial crystallization of PCL component along the HOPG lattice, that was revealed by enzymatic degradation of PLLA component by proteinase K. Thus, adjusting the solubility and solvent volatility for the film formation provided morphological divergence of the P(LA-b-CL) block copolymer, and this technique would be applicable for the surface patterning of biodegradable polymers.     

Enzymatic degradation of block copolymers obtained by sequential ring opening polymerization of l-lactide and ?-caprolactone

Copolymers of ?-caprolactone and l-lactide with different molar ratios were prepared via sequential ring opening polymerization (ROP) of both monomers. The resulting PCL-PLLA-PCL triblock copolymers were characterized by using NMR, SEC, DSC and XRD. One melting peak corresponding to the PCL block was detected, but the presence of PLLA decreased the crystallinity of PCL. Enzyme-catalyzed biodegradation of solution cast films was investigated at 37 °C in the presence of Pseudomonas lipase. It was observed that the PLLA component retarded the degradation of the block copolymer as compared to the PCL homopolymer. Therefore, the enzymatic degradation rate can be adjusted by varying the composition of the copolymers. ¹H NMR and SEC data showed no significant chemical composition or molecular weight changes during degradation, indicating that the degradation proceeded according to a surface erosion mechanism. ESEM confirmed surface erosion with appearance of a rugged morphology.     

Effects of molecular weight and small amounts of d-lactide units on hydrolytic degradation of poly(l-lactic acid)s

Films of poly(l-lactic acid) (PLLA) with different number-average molecular weights (M_n) and d-lactide unit contents (X_d) were made amorphous and the effects of molecular weight and small amounts of d-lactide units on the hydrolytic degradation behavior in phosphate-buffered solution at 37 °C of PLLA were investigated. The degraded films were investigated using gravimetry, gel permeation chromatography, polarimetry, differential scanning calorimetry, X-ray diffractometry, and tensile testing. To exclude the effects of crystallinity on the hydrolytic degradation, the films were made amorphous by melt-quenching. The incorporation of small amounts of d-lactide units drastically enhanced the hydrolytic degradation of PLLA. In the period of 0-32 weeks, the hydrolytic degradation rate constant (k) of PLLA films increased with increasing X_d, while the k values did not depend on M_n. This means that the effects of X_d on the hydrolytic degradation rate of the films are higher than those of M_n. In contrast, in the period of 32-60 weeks neither X_d nor M_n was a crucial parameter to determine k values, probably because in addition to these parameters the differences in the amount of catalytic oligomers accumulated in films and crystallinity affect the hydrolytic degradation behavior of the films. The initially amorphous PLLA films remained amorphous even after the hydrolytic degradation for 60 weeks.     

Porous Biodegradable Polyesters, 3. Preparation of Porous Poly(ε‐caprolactone) Films from Blends by Selective Enzymatic Removal of Poly(L‐lactide)

Porous poly(ε‐caprolactone) (PCL) films were prepared by the removal of poly(L ‐lactide) (PLLA) from phase‐separated PLLA/PCL blend films using the selective Proteinase K™‐catalyzed hydrolysis of PLLA and subsequent elution of its water‐soluble oligomers and monomer into the surrounding hydrolysis media. Polarimetry, gravimetry, and differential scanning calorimetry (DSC) confirmed the complete removal of PLLA molecules from the blend films within 5 d of the Proteinase K‐catalyzed hydrolysis and therefore the formation of porous PCL films when the initial PLLA content [X_PLLA(0)(w/w) = PLLA/(PCL + PLLA)] of the blend films was in the range 0.3–0.5. The fragmentation of the blend film with X_PLLA(0) = 0.7 occurred when the Proteinase K‐catalyzed hydrolysis was continued for longer than 5 d. These findings exhibited that both the PLLA‐rich and PCL‐rich phases were continuous in the blend films for X_PLLA(0) ranges of 0.3–0.7 and of 0.3–0.5, respectively, and that the PCL‐rich phase became dispersed when X_PLLA(0) was increased to 0.7. The dependence of enzymatic hydrolysis rate on X_PLLA(0) strongly suggests that the Proteinase K‐catalyzed hydrolysis of the blend films occurs at the interfaces of PLLA‐rich and PCL‐rich phases as well as at the film surfaces.     

Synthesis and characterization of photocrosslinkable poly(l-lactide)s with a pendent cinnamate group

In this report, we describe the synthesis and characterization of photosensitive poly(l-lactide) with a pendent cinnamate group. α,ω-Dihydoxy terminated poly(l-lactide) (PLLA-diol) [molecular weights (MW); 2000, 4000 and 9000 g/mol ] was chain-extended with a diacyl chloride of 5-cinnamoyloxyisophthalic acid (ICA) to obtain high-molecular-weight photocrosslinkable poly(l-lactide)s (ICA/PLLA). The resulting polyesters were characterized by proton nuclear magnetic resonance spectroscopy, gel permeation chromatography, ultraviolet–visible spectroscopy, differential scanning calorimetry, thermomechanical analysis and thermogravimetry. The glass transition temperature (T_g), the melting temperature (T_m) and the degree of relative crystallinity (X_c) of ICA/PLLAs increased with the increasing MW of the PLLA-diols. The photosensitive ICA/PLLAs were irradiated with a 400 W high-pressure mercury lamp (λ > 280 nm) for various times to produce the PLLA gel films without a photoinitiator. The crosslinking rate monitored by a UV–vis spectrum decreased with the increasing MW of the PLLA-diols. The crosslinking of the ICA/PLLA ?4000 film enhanced the T_g slightly and the tensile strength and Young’s modulus significantly, while reduced the T_m and X_c. The enzymatic degradation was measured by the weight loss of the films in a phosphate buffer solution with proteinaze-k. The crosslinking of the films decreased markedly the degradation rate.     

Bimodal swelling responses in microgel thin films

A series of studies on microgel thin films is described, wherein quartz crystal microgravimetry (QCM), surface plasmon resonance (SPR), and atomic force microscopy (AFM) have been used to probe the properties of microstructured polymer thin films as a function of film architecture and solution pH. Thin films composed of pNIPAm-co-AAc microgels were constructed by using spin-coating layer-by-layer (scLbL) assembly with poly(allylamine hydrochloride) (PAH) as a polycationic "glue". Our findings suggest that the interaction between the negatively charged microgels and the positively charged PAH has a significant impact on the pH responsivity of the film. These effects are observable in both the optical and mechanical behaviors of the films. The most significant changes in behavior are observed when the motional resistance of a quartz oscillator is monitored via QCM experiments. Slight changes to the film architecture and alternating the pH of the environment significantly changes the QCM and SPR responses, suggesting a pH-dependent swelling that is dependent on both particle swelling and polyelectrolyte de-complexation. Together, these studies allow for a deeper understanding of the morphological changes that take place in environmentally responsive microgel-based thin films.     

Degradation behaviors of polyester monolayers at the air/water interface: Alkaline and enzymatic degradations

The degradation kinetics of Langmuir monolayer films of a series of biodegradable polyesters has been studied to investigate the effect of degradation medium, alkalinity and enzymes. The degradation behavior of polyester monolayers strongly depended on both degradation medium and surface pressure. As the surface pressure was increased, the degradation rates of poly(l-lactide) (PLLA) and poly[(R)-3-hydroxybutyrate] (P(3HB)) increased in both degradation media. When monolayers were exposed to an alkaline subphase, the degradation of PLLA monolayers occurred at relatively low surface pressures; the PLLA monolayers were hydrolyzed at pH 10.5 regardless of surface pressure, while the alkaline degradation of P(3HB) monolayer occurred over a constant surface pressure of 7 mN/m at pH 11.8. These results have been explained by the difference in hydrophilic/hydrophobic balance of the polymers; PLLA is more hydrophilic than P(3HB). In contrast, the enzymatic degradations of both polymer monolayers occurred at higher constant surface pressures than those of the alkaline treatment; 7 mN/m for PLLA and 10 mN/m for P(3HB). This behavior was attributed to the enzymes being much larger than the alkaline ions: the enzymes need a larger contact area with the submerged monolayers to be activated.     

Thermodynamic theory of thin liquid films including line tension effects

This paper presents the contemporary state-of-art of the phenomenological thermodynamic theory of the thin foam and emulsion films, both symmetrical and asymmetrical ones. The roots of this theory are in the Gibbs' theory of capillarity. Two basic approaches — with two Gibbs dividing surfaces and with three surfaces of tension, are described. The generalization of the theory for systems with more complex geometry is commented. The ways of determining of the thermodynamic thickness of the film are described. The basic thermodynamic quantities of the thin film: disjoining pressure, tension of the film and surface tension of the film, are defined. The tangential mechanical equilibrium conditions with two types of contact angles, θ_h, and θ₀, are discussed. The effect of line tension of the three-phase contact-line perimeter on the film contact angles is elucidated.     

Organic vapor sensing properties and characterization of α-naphthylmethacrylate LB thin films

Determination of organic vapor sensing properties of α-Naphthylmethacrylate (α-NMA) monomer based Langmuir-Blodgett (LB) thin films was aimed in this study. LB thin film fabrication was performed on quartz glass and quartz crystal substrates in order to investigate the characterization and organic vapor properties of α-NMA materials by using UV-Visible, Atomic Force Microscopy (AFM) and Quartz Crystal Microbalance (QCM) techniques. π-A isotherm graph was taken and a suitable surface pressure value were primarily determined as 13?mN m^?1 for successful α-NMA LB thin film fabrication. Transfer ratio value was found to be ≥ 0.93 for quartz glass and quartz crystal substrates. The typical frequency shift per layer was obtained as 16.93?Hz/layer and the deposited mass onto a quartz crystal was calculated as 271.30?ng/layer (1.02?ng mm^?2). The sensing responses of α-NMA LB films against dichloromethane, chloroform, toluene and m-xylene were measured by QCM system. Dichloromethane created the maximum shift in the resonance frequency than other organic vapors used in this study. Results exhibited that α-NMA LB thin films were potential candidates for organic vapor sensing applications, especially high sensitive detection of dichloromethane at room temperature.     

Photodegradation of biodegradable polyesters: A comprehensive study on poly(l-lactide) and poly(?-caprolactone)

The photodegradation of melt-crystallized and amorphous-made poly(l-lactide) (PLLA-C and PLLA-A, respectively) and cast-crystallized poly(?-caprolactone) (PCL) was investigated comprehensively for the periods up to 200 h using gel permeation chromatography, differential scanning calorimetry, tensile testing, and polarization optical microscopy. The photodegradation of PLLA and PCL films proceeds via a bulk erosion mechanism, indicating that UV penetrates the specimens with no significant reduction in its intensity, irrespective of the chemical structure and the crystallinity of biodegradable polyesters. The photodegradability of PCL chains was higher than that of PLLA chains. This strongly suggests that the chemical structure of the two sequential groups adjacent to the ester oxygen rather than the density of ester group is crucial to determine the photodegradability of biodegradable polyesters. Although PLLA chains are photodegradable even in the crystalline regions, their photodegradability is lower than that in the amorphous regions. The significant increase in weight-average molecular weight (M_w)/number-average molecular weight (M_n) was observed for PLLA-A and PCL films, even when the decrease in M_n by UV irradiation was small. Most of the tensile properties of PLLA and PCL films remained unchanged during UV irradiation, while solely the elongation at break of PCL film significantly decreased. This result reflects that among the tensile properties the elongation at break was most sensitive to the change in molecular characteristics of biodegradable polyesters by UV irradiation. The contrast between bright and dark parts of Maltese crosses remained unchanged for the spherulites in PLLA-C and PCL films even after UV irradiation for 200 h. This result exhibits that the cleaved fraction of the tie chains was too low to cause the traceable disorientation of lamellae.     

Structural and optical properties of TiO2 thin films prepared by spin coating

Transparent semiconducting thin films of titanium oxide (TiO₂) were deposited on glass substrates by the sol–gel method and spin-coating technique. The physical properties of the prepared films were studied as a function of the number of spun-cast layers. The microstructure and surface morphology of the TiO₂ films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), with respect to the film thickness. The XRD analysis reveals that the films are polycrystalline with an anatase crystal structure and a preferred grain orientation in the (101) direction. The morphological properties were investigated by AFM, which shows a porous morphology structure for the films. The optical properties of the films were characterized by UV–Visible spectrophotometry, which shows that the films are highly transparent in the visible region and their transparency is slightly influenced by the film thickness, with an average value above 80 %. The dependence of the refractive index (n), extinction coefficient (k), and absorption coefficient (α) of the films on the wavelength was investigated. A shift in the optical band gap energy of the films from 3.75 to 3.54 eV, as a function of the film thickness, has been observed.     

Preparation and Characterization of Polyhydroxyalkanoates Macroporous Scaffold Through Enzyme-Mediated Modifications

Polyhydroxyalkanoates (PHAs) are hydrophobic biodegradable thermoplastics that have received considerable attention in biomedical applications due to their biocompatibility, mechanical properties, and biodegradability. In this study, the degradation rate was regulated by optimizing the interaction of parameters that influence the enzymatic degradation of P(3HB) film using response surface methodology (RSM). The RSM model was experimentally validated yielding a maximum 21 % weight loss, which represents onefold increment in percentage weight loss in comparison with the conventional method. By using the optimized condition, the enzymatic degradation by an extracellular PHA depolymerase from Acidovorax sp. DP5 was studied at 37 °C and pH 9.0 on different types of PHA films with various monomer compositions. Surface modification of scaffold was employed using enzymatic technique to create highly porous scaffold with a large surface to volume ratio, which makes them attractive as potential tissue scaffold in biomedical field. Scanning electron microscopy revealed that the surface of salt-leached films was more porous compared with the solvent-cast films, and hence, increased the degradation rate of salt-leached films. Apparently, enzymatic degradation behaviors of PHA films were determined by several factors such as monomer composition, crystallinity, molecular weight, porosity, and roughness of the surface. The hydrophilicity and water uptake of degraded salt-leached film of P(3HB-co-70%4HB) were enhanced by incorporating chitosan or alginate. Salt-leached technique followed by partial enzymatic degradation would enhance the cell attachment and suitable for biomedical as a scaffold.     

Properties and morphology of poly(L-lactide). II. Hydrolysis in alkaline solution

Hydrolysis of poly(L -lactide) (PLLA) films in 0.01N NaOH at 37°C was investigated by gel permeation chromatography, differential scanning calorimetry, scanning electron microscopy, and polarizing optical microscopy. The change in molecular weight distribution and surface morphology of PLLA films during hydrolysis revealed that PLLA film hydrolysis in dilute alkaline solution proceeded mainly via the surface erosion mechanism. An insignificant dependence of the rate of weight loss per unit surface area on the PLLA film thickness also supported this conclusion. Etching of the outside of PLLA spherulites resulted in preferred hydrolysis of PLLA chains in the amorphous region. The disorientation of lamella and inhomogeneous erosion in the spherulites implied that hydrolysis of PLLA chains occurred predominantly in the amorphous region between the crystalline regions in the spherulites. The rate of weight loss per unit surface area decreased linearly with the increase in the initial crystallinity of PLLA film, while the radius of spherulites had practically no significant effect on the hydrolysis of PLLA film. The specific low molecular weight of PLLA chains produced by hydrolysis increased with the rise in annealing temperature of the PLLA film, suggesting that the PLLA chains released were the component of one fold in the crystalline region. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 59–66, 1998     

Microstructure and optical properties of Ag-doped ZrO2 thin films prepared by sol gel method

Ag doped ZrO₂ thin films were deposited on quartz substrates by sol–gel dip coating technique. The effect of Ag doping on tetragonal to monoclinic phase transformation of ZrO₂ at a lower temperature (500 °C) was investigated by X-ray diffraction. It is found that the Ag doping promotes the phase transformation. The phase transformation can be attributed to the increase in the tetragonal grain size and concentration of oxygen vacancies in the presence of the Ag dopant. Accumulation of the Ag atoms at the film surface and surface morphology changes in the films were observed by AFM as a function of varying Ag concentration. X-ray photoelectron spectroscopy gave Ag 3d and O 1s spectra on Ag doped thin film. The chemical states of Ag have been identified as the monovalent state of Ag⁺ ions in ZrO₂. The Ag doped ZrO₂ thin films demonstrated the tailoring of band gap values. It is also found that the intensity of room temperature photoluminescence spectra is suppressed with Ag doping.     

Miscibility and hydrolytic degradation in alkaline solution of poly(l-lactide) and poly(p-vinyl phenol) blends

Poly(l-lactide) (PLLA) was melt-blended with poly(p-vinyl phenol) (PVPh) using a two-roll mill, and the miscibility between PLLA and PVPh and degradation of the blend films were investigated. It was found that PLLA/PVPh blend has miscibility in the amorphous state because only single T_g was observed in the DSC and DMA measurements. The T_g of the PLLA/PVPh blend could be controlled in the temperature range from 55 °C to 117 °C by changing the PVPh weight fraction. In alkaline solution, degradation rate of PLLA/PVPh blends was faster than that of neat PLLA because PVPh could dissolve in alkaline solution. The surface morphology of degraded PLLA and PLLA/PVPh blend were observed by SEM. The surface morphology of degraded PLLA/PVPh blend was finer than that of PLLA. Young's modulus of PLLA/PVPh blend increased with increasing PVPh content. Yield stress of PLLA/PVPh blends whose PVPh content was less than 30 wt% kept the level of about 55 MPa and that of PLLA/PVPh blend whose PVPh content was 40 wt% is much lower than that of neat PLLA.     

Degradation behaviour of PCL/PEO/PCL and PCL/PEO block copolymers under controlled hydrolytic,enzymatic and composting conditions

Short-term hydrolytic and enzymatic degradation of poly(ε-caprolactone) (PCL), one series of triblock (PCL/PEO/PCL) and the other of diblock (PCL/PEO) copolymers, with a low content of hydrophilic PEO segments is presented. The effect of the introduction of PEO as the central or lateral segment in the PCL chain on copolymer hydrolysis and biodegradation properties was investigated. FTIR results revealed higher hydrolytic degradation susceptibility of diblock copolymers due to a higher hydrophilicity compared to PCL and triblock copolymers. Enzymatic degradation was tested using cell-free extracts of Pseudomonas aeruginosa PAO1, for two weeks by following the weight loss, changes in surface roughness, and changes in carbonyl and crystallinity index. The results confirmed that all samples underwent enzymatic degradation through surface erosion which was accompanied with a decrease in molecular weights. Diblock copolymers showed significantly higher weight loss and decrease in molecular weight in comparison to PCL itself and triblock copolymers. AFM analysis confirmed significant surface erosion and increase in RMS values. In addition, biodegradation of polymer films was tested in compost model system at 37 °C, where an effective degradation of block copolymers was observed.     

Preparation and Characterization of Nanocrystalline Fe-Doped TiO2 Film on Different Substrates and Its Application in Degrading Dyeing Water

TiO₂ and Fe-doped TiO₂ thin films were prepared on the substrates (glass plates and glass microballs) after dipped in colloid and calcined at 500°C. The films have been characterized by UV-absorption, x-ray diffraction and atomic force microscopy (AFM). The investigated Fe-doped TiO₂ (A nominal 10 wt% Fe₂O₃ was impregnated) thin films were obtained by sol-gel method. The characterization results suggest that the calcined thin films primarily consist of TiO₂ anatase. And the light absorption curve of the TiO₂ films modified with Fe showed that red shift had happened by the dope of Fe. TiO₂/beads as photocatalyst were used to degrade the simulated dyeing water which contains reactive deep-blue dye (K-R). Through the degradation experiment, we found that the dope of Fe promoted the photocatalytic activities. The results showed that the dyeing water can be decomposed more effectively by the photocatalytic oxidation of Fe/TiO₂ film on glass micro-balls than glass plates.     

Slope analysis and scaling analysis of electrodeposited thin films

We introduce slope analysis as a straightforward complement to scaling analysis for characterizing the morphology of electrodeposited thin films. The surface slope θ, defined as the angle between the local surface normal and the film normal, is calculated as a function of position for electrodeposited Cu films of increasing thickness t. We show that the mean value of θ increases with t and demonstrate the close relationship between this observation and the increase in local roughness found in previous studies from scaling analysis.     

Poly(L‐lactide)/C60 nanocomposites: Effects of C60 on crystallization of poly(L‐lactide)

The effects of solvent type and concentration of C₆₀ on the crystallization of poly(L ‐lactide) (PLLA) during solvent evaporation, heating from room temperature, and cooling from the melt were investigated by polarized optical microscopy and differential scanning calorimetry. The addition of C₆₀ enhanced the PLLA crystallization during solvent evaporation, during heating of the melt‐quenched films, and during cooling from the melt of As‐cast films, except for heating and cooling of the PLLA film with 1 wt % of C₆₀ prepared with dichloromethane. In the case of solvent evaporation, the difference in crystallinity between the PLLA films with and without C₆₀ became higher for the solvent with a lower boiling point. In the case of heating of melt‐quenched films, the addition of C₆₀ had a small effect on the crystallinity of PLLA, whereas significantly lowered the peak top and ending temperatures of cold crystallization, except for melt‐quenched PLLA film with 1 wt % of C₆₀ prepared with dichloromethane. The crystallinity of PLLA was determined by the solvent type, rather than by the C₆₀ concentration. In the case of cooling from the melt of As‐cast films, the addition of C₆₀ elevated the crystallinity and cold crystallization temperature values of PLLA films, except for PLLA films prepared with dichloromethane. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2167–2176, 2007