Monomer Feed Ratio Research Articles

In-chain multi-functionalized polystyrene by living anionic copolymerization with 1,1-bis(4-dimethylaminophenyl)ethylene: Synthesis and effect on the dispersity of carbon black in polymer-based composites

In-chain multi-functionalized polystyrene possessing definite geminal dimethylamino groups along the polymer backbone, poly(styrene-co-1,1-bis(4-dimethylaminophenyl)ethylene) (poly(styrene-co-BDADPE)), has been designed and synthesized via living anionic copolymerization of styrene with 1,1-bis(4-dimethylanimophenyl)ethylene in benzene at 50 °C, using sec-butyllithium as initiator. The amine functionality was readily controlled by the monomer feed ratio and molecular weight. Compositions of styrene/BDADPE were 9.0–57.9 when ratios of styrene/BDADPE = 0.45–7.79 and Mn = 4.7–15.5 kg/mol. The incorporation of 1,1-bis(4-dimethylanimophenyl)ethylene unit resulted in an increase in glass transition temperature of copolymer and was hindered in the presence of Lewis base. The monomer reactivity ratio for styrene, r1 = 53.4, was obtained. Such in-chain multi-functionalized polystyrene significantly improved the dispersity of carbon black in the corresponding composites, as verified by SEM observation. This in-chain multi-functionalization of matrix polymer via anionic copolymerization employing 1,1-bis(4-dimethylaminophenyl)ethylene as comonomer provides a facile and effective method to prepare carbon black-based polymer composites with good dispersity.

Electrochemical synthesis of poly(indole‐co‐thiophene) on low‐nickel stainless steel and its anticorrosive performance in 0.5 mol L−1H2SO4

AbstractThe electrochemical synthesis of poly(indole‐co‐thiophene) copolymer coatings was achieved on low‐nickel stainless steel (LN SS) with the cyclic voltammetry technique using indole and thiophene monomers in acetonitrile medium containing lithium perchlorate. The optimization of synthesis parameters such as monomer feed ratio and various scan rates was studied and also their influence on the morphology of the copolymer coatings on LN SS. For the first time, a possible radical cationic electro‐copolymerization mechanism is also proposed in order to understand the electrochemical synthesis. The bonding and structure of the as‐synthesized coatings were characterized using Fourier transform infrared and 1H NMR spectroscopies. The surface morphology and composition of the coatings were also assessed using scanning electron microscopy and energy‐dispersive X‐ray analysis. It was observed that changes in morphology occurred which had a marked and significant effect on the electrochemical behaviour of the coated LN SS confirmed using electrochemical techniques of potentiodynamic polarization and electrochemical impedance spectroscopy in aqueous 0.5 mol L−1 H2SO4 solution. The obtained results revealed that the copolymer coatings on LN SS provided significant corrosion protection in the acid medium. It was also found that a 1:1 ratio of indole to thiophene yielded the most stable and corrosion‐protective copolymer coating. © 2013 Society of Chemical Industry

Structures and properties of thermoregulated acrylonitrile–methyl acrylate sheet containing microphase change materials

AbstractA series of melt processable poly(acrylonitrile–methyl acrylate) copolymers with a feeding ratio of 85/15 mol% (abbreviated as 85/15 AN/MA below) containing 0–25 wt% of microencapsulated phase change materials (micro‐PCMs) were synthesized by aqueous precipitation polymerization. The number average sequence lengths (NASLs) and tacticity of 85/15 AN/MA were determined using 13C‐NMR. The results showed that the NASL of contiguous AN sequences is 10.00 and the tacticity is atactic with approximately 51% m and 49% r diads (the relative orientation of two adjacent cyanogroup). 85/15 AN/MA containing various contents of micro‐PCMs were processed into sheets in melt condition at 210°C and 20 MPa. The structures and properties of the copolymers and sheets containing micro‐PCMs were studied by using Fourier transform infrared spectroscopy, nuclear magnetic resonance (1H‐NMR), scanning electron microscopy, differential scanning calorimetry, gel permeation chromatography, X‐ray diffraction, and dynamic mechanical analysis. The composition of AN/MA copolymer agreed well with its monomer feed ratio. The melting enthalpy of the sheets containing 25 wt% of micro‐PCMs was approximately 25 J/g, which increased steadily as the micro‐PCMs content increased. The modulus of the sheets decreased with the micro‐PCMs content increased. The glass transition temperature (Tg) of the sheets was in the range of 79.7–81.7°C, which also increased with the micro‐PCMs content increasing. POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers

Synthesis of indolo[3,2-b]carbazole-based random copolymers for polymer solar cell applications

In addition to preparing two indolocarbazole-based random copolymers (named as r-PICTBT1 and r-PICTBT2), this work investigated their feasibility for bulk heterojunction polymer solar cells (PSCs). These copolymers consisted of commercially available 3,9-dibromo-5,11-dioctyl-5,11-dihydroindole[3,2-b]carbazole, 2,5-bis(trimethylstannyl) thiophene and dibromobenzo[c][1,2,5]thiadiazole by varying the feed in ratios via Stille cross-coupling reactions. The photophysical and electrochemical properties of the resulting copolymers could be fine-modulated easily by adjusting the feed ratios of monomers. Both copolymers in the thin film state exhibited two obvious peaks and a vibronic shoulder in the absorption spectra. Electrochemical experiments indicated that the highest occupied molecular orbital energy levels were −4.95, −5.00eV; meanwhile, the lowest unoccupied molecular orbital energy levels were −3.38, −3.54eV for r-PICTBT1 and r-PICTBT2, respectively. Bulk heterojunction PSCs composed of an electron-donor copolymer blended with an electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) at a weight ratio of 1:1 or 1:3 were investigated. Moreover, the r-PICTBT2/PC71BM-based (w/w=1:1) PSC performed the best with an open-circuit voltage of 0.54V, short-circuit current of 6.83mA/cm2, fill factor of 0.44, and power conversion efficiency of 1.63%.

Enzymatic Synthesis of Poly(butylene-co-sebacate-co-glycolate) Copolyesters and Evaluation of the Copolymer Nanoparticles as Biodegradable Carriers for Doxorubicin Delivery

Aliphatic copolyesters consisting of diester, diol, and glycolate repeat units were enzymatically synthesized for the first time via lipase-catalyzed polycondensation reactions. Copolymerization of ethyl glycolate (EGA) with diethyl sebacate (DES) and 1,4-butanediol (BD) in the presence of Candida antarctica lipase B (CALB) resulted in the formation of poly(butylene-co-sebacate-co-glycolate) (PBSG) copolyesters with molecular weight (Mw) up to 28000 and typical polydispersity between 1.2 and 1.8. The synthesized copolymers contained 10–40 mol % glycolate (GA) units depending on the monomer feed ratio employed. DSC analyses show that the copolyesters with 12–38% GA content are semicrystalline materials that melt between 43 and 59 °C. Free standing nanoparticles with an average size ranging from 250 to 400 nm were successfully fabricated from these PBSG copolymers using a single emulsification-solvent evaporation process. PBSG copolyesters were found to be hydrolytically degradable and doxorubicin- (DOX-) encapsulated PBSG nanoparticles exhibited slow and sustained release of the drug in PBS solution at 37 °C over an extended period of time (60 days). Cellular uptake studies indicate that the drug-loaded PBSG particles are absorbed by a large percentage (up to 95%) of Hela cancer cells within 4 h incubation time. In vitro cytotoxicity investigations reveal that at a same DOX concentration (0.125–2.0 μM), DOX-encapsulated PBSG nanoparticles possess either higher or comparable cytotoxicity toward Hela cells than the free drug DOX·HCl. These results suggest that the PBSG nanoparticles are promising carriers for controlled release delivery of DOX to treat cancers.

Polycationic nanoparticles synthesized using ARGET ATRP for drug delivery

This work provides a systemic comparison for ARGET ATRP and UV-initiated polycationic nanoparticles for drug delivery and a guide to deciding which type of polycationic nanoparticles have the best properties for specific applications. Polycationic nanoparticles were synthesized using a previously developed UV-initiated photoemulsion polymerization or a newly developed ARGET ATRP synthesis technique. The effect of the ratio of hydrophobic monomer in the feed was evaluated. Increasing the feed ratio of hydrophobic monomer was necessary to maintain biocompatibility and pH-responsive membrane disruptive characteristics when switching from the UV-initiated polymerization to ARGET ATRP. The resulting polycationic nanoparticles have utility as drug delivery carriers for hydrophobic drugs and/or nucleic acids.

Liquid Crystalline Behaviors of Polycholesterylmethacrylate and Poly(Cholesterylmethacrylate Co-1-hexene), Their Synthesis and Characterization

The synthesis of polycholesterylmethacrylate and poly(cholesterylmethacrylate-co-1-hexene) at different monomer feed ratios with their liquid crystalline behaviours have been reported in this article. Liquid crystalline phases of the polymer and copolymers have been detected under the hot stage microscope and also by differential scanning calorimetric and X-ray diffraction technique. The polymers are characterized by IR spectroscopy, proton NMR spectroscopy and GPC system. The reactivity ratios of cholesterylmethacrylate and 1-hexene are found to be r1 = 2 and r2 = 1.6, respectively. The number average molecular weight of the polymer is 1.01 × 104 Daltons and that of the copolymers are much higher than the polymer.

Molecular weight and architectural dependence of well-defined star-shaped poly(lysine) as a gene delivery vector.

A series of well-defined star-shaped polypeptides were successfully synthesised by the ring opening polymerisation (ROP) of the N-carboxyanhydride (NCA) of ε-carbobenzyloxy-l-lysine (ZLL) using a range of generations of polypropylene imine (PPI) dendrimers as multifunctional initiators. The monomer feed ratio and dendrimer generation were varied to afford a series of polypeptide dendrimer hybrids with superior structural versatility and functionality. Subsequent protecting group removal yielded star-shaped poly(lysine) of controlled variation in polypeptide chain length and arm multiplicity. Star-shaped PLL polymers were used to prepare pDNA and siRNA to form "polyplexes" to determine their ability to complex different nucleic acid cargoes and were compared with linear PLL polyplex controls. Significant differences in size and surface charge were seen between star-shaped PLL polyplexes and linear PLL polyplexes for both cargoes. The star-shaped polypeptides were capable of more effective complexation of both nucleic acids at low N/P ratios compared to linear PLL as evidenced by zeta potential and electrophoretic data. This was particularly evident in siRNA polyplexes as linear PLL failed to completely complex siRNA into nanocomplexes of appropriate size for cell transfection i.e. <200 nm in size, while star poly(lysine) formed siRNA polyplexes <100 nm at certain N/P ratios, albeit strongly dependent on the particular molecular weight and architecture, as analysed by dynamic light scattering (DLS). Atomic force microscopy (AFM) identified discrete spherically shaped polyplexes for all star-shaped polypeptide-based polyplexes while linear PLL formed elongated irregular shaped complexes. This difference in morphology may go some way towards explaining the 300-fold increase in luciferase expression seen for star-shaped PLL polyplexes G5(64)-PLL40 compared to linear PLL pGLuc polyplexes in epithelial cells. Each of the PPI-PLL polymers appeared to be capable of protecting the nucleic acid cargoes from degradation by the relevant nuclease enzyme as effectively as the positive control polyethyleneimine (PEI) polyplexes. Overall the promising nucleic acid complexation, sizing, morphology and protection capacity of two different genetic "cargoes" highlight the potential of polypeptide dendrimer hybrids as gene delivery vectors.

Regulation of fibrinolytic protein adsorption on polyurethane surfaces by modification with lysine-containing copolymers

A new strategy to regulate the attachment of plasminogen and its activator, t-PA, to a polyurethane surface is reported. The method is based on the one-step graft copolymerization of a lysine-containing methacrylic monomer and hydroxyethyl methacrylate (HEMA). The copolymer-modified PU surfaces showed high capacity for plasminogen and t-PA binding. The release of pre-adsorbed t-PA from the lysine-containing PU surfaces in contact with plasma was also investigated. It was shown that the interactions of plasminogen and t-PA on this type of surface can be controlled simply by changing the monomer feed ratio, and thus the relative quantities of HEMA and lysine in the grafts. This approach may be useful for the development of fibrinolytic, blood-contacting materials and, more generally, for controlling protein-surface interactions for biocompatibility.

Renewable carvone-based polyols for use in polyurethane thermosets

A series of carvone-derived statistical copolymers, CMXD (CM = carvomenthide; X = %D; D = dihydrocarvide), were prepared by ring-opening polymerization. The olefinic side chains were subsequently modified by the thiol–ene reaction using mercaptoethanol to provide CMXD′ (D′ = hydroxy-functionalized dihydrocarvide unit). The resulting samples, having hydroxy functionality dictated by the original monomer feed ratio, were exploited as renewable polyols in the synthesis of polyurethanes. The polyurethane films were characterized by tensile testing, differential scanning calorimetry and dynamic mechanical analysis. Polyols with 4–5 hydroxyls per chain produced mechanically robust thermoplastic polyurethanes.

UV initiated thiol–ene chemistry: a facile and modular synthetic methodology for the construction of functional 3D networks with tunable properties

A facile methodology for the fabrication of functional crosslinked three dimensional (3D) networks has herein been explored via the benign and UV initiated thiol–ene coupling (TEC) chemistry. The careful selection of monomers or polymers and their feed ratio resulted in straightforward design of organic, inorganic and hydrogel networks with readily available alkenes or thiol functional groups. All crosslinked networks were fabricated within 1 second of UV exposure at wavelengths of 320–390 nm and generally exhibited excellent gel fractions around 90%. By introducing off-stoichiometric thiol and ene (OSTE) monomer feed ratios the window of mechanical properties could be manipulated. For the organic triazine system, the Young's modulus was altered from 780 MPa at an equimolar monomer ratio to soft 106 kPa for 2.5 equiv. with excess of thiol compared to enes. Postfunctionalizations with hydrophilic polyethylene glycols or acrylic acid and hydrophobic heneicosafluorododecyl acrylate were explored for the manipulation of functional networks. In this case, the rigid networks with excess of thiols were used as model substrates of which the initial contact angle (CA) of 60° was decreased to 43° by the introduction of acrylic acid and increased to 140° by successful attachment of fluorinated molecules. Finally, amalgamating micropatterning strategy with simple postfunctionalizations of hydrophobic groups resulted in superhydrophobic rigid surfaces with a CA of 173°.

Effects of the acceptor conjugation length and composition on the electrical memory characteristics of random copolyimides

AbstractResistive‐switching memories based on copolyimides (coPIs), PI‐NTCDIX and PI‐BTCDIX, with different compositions of 4,4′‐diamino‐4″‐methyltriphenylamine (AMTPA), 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride, and N,N′‐bis‐(4‐aminophenyl)‐1,8:4,5‐naphthalenetetracarboxydiimide (NTCDI) or N,N′‐bis‐(4‐aminophenyl)‐1,2:4,5‐benzenetetracarboxydiimide (BTCDI) have been developed. By varying the feed ratio of monomers, PI‐NTCDIX and PI‐BTCDIX showed tunable optical and electronic properties through the charge transfer (CT) between AMTPA and NTCDI or BTCDI. The memory devices based on PI‐NTCDIX exhibited the tunable electrical bistability from the volatile dynamic random access memory to nonvolatile write once read many memory characteristics as the NTCDI composition increased. The OFF/ON electrical switching transition was mainly attributed to the CT mechanism for the charge separated high conductance, based on the analysis of model compounds and density functional theory calculation. Also, the volatility of the memory device depended on the stability of CT complex. The long conjugation and high electron affinity of the NTCDI moiety stabilized the radical anion generated in the CT complex and prevented the recombination of segregated radical species even through applying the high positive or negative voltage. On the other hand, the memory devices based on PI‐BTCDIX showed a rather unique behavior compared with those based on PI‐NTCDIX. At the low BTCDI composition, the device exhibited volatile memory property. However, no switching behavior was observed at the high BTCDI composition due to the low highest occupied molecular orbital energy level of BTCDI. Combining these results and our previous study on perylenebisimide (PBI), we concluded that memory characteristics could be tailored by changing the conjugation length (PBI &gt; NTCDI &gt; BTCDI) and the acceptor composition in random coPIs. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Solubility Data of Comonomer Pairs Relevant to Aqueous‐Phase Study in the Emulsion Copolymerization

In emulsion copolymerizations, the two monomers might not be equally incorporated in the copolymers. Besides copolymerization in the particles, also copolymerization in the aqueous phase can take place which affects the formation of oligomeric radicals responsible for the entry of radicals into the particles. In describing copolymerization in emulsion polymerization, one can use the normal reactivity ratios in combination with the monomer feed ratios in the phase at hand (either water phase or particle phase). Most of the research works done were focused on the feed ratio of the monomers in the particle phase. This is because of the interest in the final product. But in case of water‐soluble monomers, where significant amount of polymerization already occurs in the water phase, feed ratios of monomers in the water phase have to be known. Even for less water‐soluble monomers, the feed ratios in the aqueous phase are important to know, related to entry of radicals. Therefore, to calculate feed ratios in water phase, solubilities of mixtures of monomers in this phase have been measured by using a UV‐Vis technique. It is shown that the solubility of one monomer is influenced by the presence of another monomer.

Tailor‐made starch‐based gels bearing highly acidic groups

AbstractA facile synthesis route was employed to prepare starch‐based hydrogels that contained highly acidic groups. The gels were obtained through the free radical cross‐linking reaction between an unsaturated starch‐based macromonomer and acrylic acid (AA)/sodium vinyl sulfonate or AA/itaconic acid monomer pair, and denoted as SAS or SAI. The structure of the gels was verified with FTIR, XRD, and TGA measurements. It was found that the swelling ratios of SAI and SAS depended on the feeding ratio of monomers and could be 6.6 and 12.1, respectively. Rhodamine B was bound into the gels via electrostatic interaction and able to be released again. In particular, the hydrolysis of starch was enhanced with either hydrogel. Both the encapsulation capacity for the cationic electrolyte and catalysis effect of the gels were in the order of SAS&gt;SAI. The results indicated that the network structure and performances of the starch‐based gels could be regulated by simply varying the components and ratios of monomers.

Synthesis of new bio-based polycarbonates derived from terpene

In this work, a bio-based polycarbonate has been synthesized successfully from terpene diphenol and diphenyl carbonate by melt polymerization without using any catalysts. The polymerization process involves no usage of toxic phosgene. The reaction parameters such as monomer feed ratio, polymerization temperature and time have been systematically examined. A little excess of diphenyl carbonate for terpene diphenol affords the highest molecular weight. The chemical structure of the product is identified by 1H NMR and FT-IR. The DSC analysis shows that the glass transition temperature of the present bio-based polycarbonate is much higher than that of a conventional bisphenol A-based polycarbonate due to the rigid molecular structure of terpene diphenol. In addition, a series of copolycarbonates with adjustable glass transition temperature are prepared from terpene diphenol and bisphenol A. The present polycarbonate and copolycarbonates with high thermal stability synthesized via an environmental benign process have large potential for bio-based engineering plastics in various industrial fields.

Novel hybrid fluoro-carboxylated copolymers deposited by initiated chemical vapor deposition as protonic membranes

Copolymers of 1H, 1H, 2H, 2H,-perfluorodecyl acrylate and methacrylic acid are synthesized by initiated chemical vapor deposition (iCVD) to serve as proton exchange membranes for potential application in miniaturized fuel cells. The copolymerization of hydrophobic and hydrophilic monomers is difficult by conventional liquid-phase technique. We demonstrate that the use of iCVD is very promising for the synthesis of these membranes. Changing the monomer feed ratio we have a systematical control over the copolymer composition. The calculation of the copolymer ratios shows a moderate alternate composition of the copolymer which is advantageous to obtain highly conductive (acid functionalities) and stable (hydrophobic functionalities) membrane in the operating conditions of the fuel cells. The copolymers show a high degree of chain mobility when passing from dry to hydrated state as demonstrated by XPS analysis and by the high water contact angle hysteresis measured (>30°). An ionic conductivity of 70 mS/cm is obtained in conditions of highest percentage of hydrophobic functionalities. This conductivity value is comparable with the ionic conductivity of commercial Nafion®.

Kinetic Analysis of A2 + AB2 Polymerization Approach

AbstractThe kinetic model of the co‐polycondensation with A2 and AB2 type monomers is developed and the analytical expressions of the various molecular parameters of the products are derived rigorously. The monomer feed ratio (α) of A2 to AB2 significantly affects the molecular parameters and the critical condition of gelation. Gelation can be avoided if α is &gt;$1 + \sqrt {3} /2$. At the critical state, the degree of branching decreases firstly and reaches its minimum value at about α≈0.22. Then, it increases with increasing α‐value. In comparison with experimental results, non‐equal reactivity of the active groups should be considered. magnified image

One‐Step Preparation and pH‐Tunable Self‐Aggregation of Amphoteric Aliphatic Polycarbonates Bearing Plenty of Amine and Carboxyl Groups

This paper reports a novel amphoteric aliphatic polycarbonate bearing both amine and carboxyl groups. In the absence of protection-deprotection chemistry, the multi-functionalized copolymer is synthesized by one-step enzymatic copolymerization. The influences of the reaction conditions including monomer feed ratio and polymerization time are explored. The simultaneous incorporation of amine and carboxyl functionalities provides the copolymer with a pH-tunable self-aggregation feature, leading to various aggregation states including precipitated agglomerate, well-dispersed positively or negatively charged nanoparticles in a controlled manner. The copolymer displays minimal cytotoxicity to 293T and HeLa cells.

Synthesis of poly[3‐(methacryloylamino) propyl trimethylammonium chloride‐co‐methacrylic acid] copolymer hydrogels for controlled indomethacin delivery

AbstractStimulus responsive hydrogels are being considered as one of the most exciting biomaterials of current generation. A novel hydrogel based on poly 3‐[(methacryoylamino) propyl trimethylammonium chloride‐co‐methacrylic acid] (PMAPTACMAAc) copolymer was synthesized by free radical aqueous copolymerization. The water uptake of the hydrogels was investigated as a function of temperature and pH. The in vitro release properties of the PMAPTACMAAc hydrogels were analyzed under simulated body fluid (pH 7.4) by loading indomethacin (IND) as a model drug. The XRD study of the hydrogel revealed the amorphous nature of the copolymer and provided evidence that crystalline IND was entrapped in the hydrogel matrix. The DSC study showed the presence of single glass transition temperature in the thermogram which indicated the formation of random copolymers. The prepared hydrogels showed typical polyampholyte behavior and swelling was affected by the monomer feed ratios in the hydrogels. The morphological study on the hydrogels showed the three dimensional (3D) porous nature of the PMAPTACMAAc‐5 gel with pore size ranging from 10 to 40 μm. The kinetics of the cumulative IND release shows that the gel follows a non‐Fickian release mechanism. In vitro cytotoxicity of the hydrogel (PMAPTACMAAc‐5 gel) on RAW 264.7 murine macrophages showed that the hydrogel was biocompatible and not toxic as the viability was maintained. The hydrogel with a 90 : 10 feed ratio of MAAc and MAPTAC was better than the other developed formulations. The results suggest that the hydrogels could be employed as a sustained release formulation for targeted delivery of IND, for example to the colon. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Synthesis, characterization and enzymatic degradation of novel biodegradable copolymers of 5-allyloxy-1,3-dioxan-2-one with ε-caprolactone

Ring-opening copolymerization of 5-allyloxy-1,3-dioxan-2-one (ATMC) with ε-caprolactone (CL) was successfully performed for the first time in bulk at 140 °C using Sn(Oct)2 as the catalyst. Novel biodegradable copolymers with different compositions were characterized by 1H NMR, GPC, DSC, X-ray diffraction and static contact angle method. The copolymer composition was relevant to the monomer feed ratio, while the ATMC incorporation in the copolymers was less than that in the feed. With the increase of ATMC molar feed ratio from 25% to 75%, the Mn of the resulting copolymers decreased from 30,500 to 14,300 g/mol, while the static contact angle of the copolymers increased from 92.7° to 98.2°. The relationship between the copolymer glass transition temperature and composition was in agreement with the Fox equation. In vitro cytotoxicity studies using MTT assay demonstrated that the copolymer had low cytotoxicity compared to poly(ε-caprolactone) (PCL). Moreover, solution cast films were allowed to degrade in a pH 7.4 phosphate buffer solution containing Pseudomonas lipase. Weight loss data showed that the copolymers with ATMC content higher than 46% were not degradable and the enzymatic degradation rate increased with CL content. The areas of the diffraction peaks decreased gradually with increasing degradation time according to XRD analysis. Little changes of molecular weight can be detected during enzymatic degradation, in agreement with a surface erosion mechanism shown by SEM.