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PubMed2001

Crystallization of paracetamol from solution in the presence and absence of impurity.

The bulk crystallization of paracetamol has been examined under controlled conditions in the presence and absence of the additive p-acetoxyacetanilide (PAA), as a function of both supersaturation and additive levels. The induction time to nucleation was found to increase with increase in PAA concentration in solution. The product micro-crystals were characterized for shape and strain/defect content using electron and optical microscopy and X-ray Laue diffraction techniques, respectively. A change in crystal habit of the pure crystals from columnar (dominant [110]) to plate-like (dominant [001]) was observed to occur with an increase in supersaturation level, whilst the addition of PAA invariably led to the development of columnar crystals with an aspect ratio that varied with impurity level and supersaturation. HPLC showed the PAA to be incorporated into the crystals with an average segregation coefficient of 14-18% depending on the supersaturation. The ready incorporation of PAA is attributed to the molecular similarity of this molecule to that of the host material. The incorporation is shown to cause a significant increase in the mosaic spread, implying the development of a significant strain/defect content in the crystals. The influence of the impurity on the time to nucleation is probably due to its effect in blocking the development of the critical nucleus. The potential implications of such variations in morphology and strain content in the design of the physical and chemical properties of the resulting particulates are discussed.

Acetaminophen↗
PubMed2002

Crystal growth and structure of a new hormonal derived compound.

The 17alpha-acetoxy-6-hydroxymethyl-3,20-dioxo-19-nor-pregna-4,6-diene (C(23)H(30)O(5)), a new hormonal derived compound presenting a potential therapeutic interest for a pharmaceutical formulation, raises a problem as it precipitates spontaneously when in contact with an aqueous solution. The solubilization of this potential pharmaceutical drug was investigated to control and/or prevent crystal formation in the presence of water. The solubility limit of this potential drug, either in Cremophor(R) EL or RH40, was determined to be 2.6% (w/w). In addition, crystal growth of this compound solubilized in Cremophor(R) and in contact with aqueous environment was undertaken to improve crystal quality and size and subsequently to determine its three dimensional structure. We used a system that allows slow single crystal growth at room temperature. The crystallization system that we set-up comprised an interface between two solutions: one containing the compound solubilized either in Cremophor(R) RH40 or EL and the other containing water and glycine. The crystal structure was established by single crystal X-ray diffraction analysis. The structure refinement study revealed the presence of one water molecule with strong intermolecular hydrogen bonding leading to an atomic arrangement corresponding to an infinite chain in the [001] direction.

Crystallization↗
PubMed2002

Hydroxyapatite crystals as a local delivery system for cisplatin: adsorption and release of cisplatin in vitro.

This study describes the characteristics of the in vitro binding and release of the anti-tumor drug cisplatin by slurries of synthetic hydroxyapatite crystals carried out in aqueous media. The adsorption of cisplatin by slurries of hydroxyapatite and its release were found to depend significantly on the ionic composition of the aqueous media used. At a constant pH of 7.4, significantly more cisplatin is adsorbed by the hydroxyapatite crystals in the slurry from a chloride-free phosphate buffered solution or a Tris buffered solution than from a buffered phosphate solution containing chloride ions. The amount of hydroxyapatite-bound cisplatin desorbed into solution was also progressively increased as a function of the increasing concentration of chloride in the equilibrating solution. Very little hydroxyapatite-bound cisplatin was released from the crystals in either a Tris or phosphate buffer. These results suggest that it is the hydrated derivatives of cisplatin which are involved in the adsorption of cisplatin by hydroxyapatite crystals. The adsorption data can be expressed as a Freundlich isotherm from which the association constant can be calculated. The rate of release of cisplatin bound to crystals of hydroxyapatite is relatively slow even at the maximum concentration of chloride ions in the phosphate buffer. Approximately 33% of the total cisplatin bound to the crystals of hydroxyapatite was released after 4.25 days. An additional 15% of the remaining cisplatin bound to the hydroxyapatite cyrstals was released after an additional equilibration with fresh buffer for two weeks (58% of the total cisplatin originally bound). These findings suggest that cisplatin bound to slurries of hydroxyapatite crystals may be useful in the local treatment of malignant tumors.

Adsorption↗
PubMed2000

Piezoelectric quartz crystal based label-free analysis for allergy disease.

This work presents a piezoelectric (Pz) quartz crystal based label-free quantification of total IgE and allergen-specific IgE in human sera for allergy testing. An evaluation of the different brands of crystals was first initiated with respect to variability in mass sensitivity, frequency measurement reliability and stability, and surface roughness. Thereafter, for total IgE quantification. a direct assay format was adopted. By means of thioctic acid (TA) and coupling reagents, anti-human IgE antibodies were immobilized on AT-cut Pz crystals (10 MHz). The modified crystals could detect serum IgE directly corresponding to a downward frequency shift. The results showed that silver-coated crystals as compared with their gold-coated counterparts provided approximately 1.5 times higher mass detection sensitivity for total IgE in the range of 5-300 IU/ml with a linear regression line, y = 1.8957 x + 1.5603, R2 = 0.995. For the detection of allergen-specific IgE, a sandwiched assay format was used. As the allergen-modified sensor surface captured various classes of associated antibodies (IgE, IgG, etc) and interfering serum proteins as well, the initial frequency shift downwards caused by sera sample incubation would not be proportional to specific IgE levels. Thus, following sample incubation, a second incubation step with secondary anti-human IgE was added to recognize IgE from other bound substances. The frequency shift after secondary antibody binding reflected the amount of allergen-specific IgE proportionally. Compared with 10 MHz crystals, the 20 MHz counterparts provided approximately four times higher mass detection sensitivity for allergen specific IgE in the range of 0.15-17.5 IU/ml with a linear regression line, y = 50.525 x + 107.777, R2 = 0.954. Total IgE and allergen specific IgE assay results of real patients' sera using the Pz sensors agreed well with those obtained by commercially available test kits with correlation coefficient 0.96-0.98. The possibility of regenerating the quartz crystals for further re-use was also dealt with.

Allergens↗
PubMed2002

Importance of detergent and phospholipid in the crystallization of the human erythrocyte anion-exchanger membrane domain.

Three-dimensional crystals were obtained for the membrane domain of the human erythrocyte anion exchanger (AE1, Band 3). Protein homogeneity and stability and the delicate balance between the detergent used and the amount of phospholipids copurifying are critical to the formation of three-dimensional crystals of the AE1 membrane domain. While deglycosylation improved the protein homogeneity, its stability was significantly increased by inhibitor binding. Size-exclusion chromatography showed that the protein was monodisperse in detergents with acyl chains of 10-12 carbons over a pH range of 5.5-10.0. This pH range and the detergents that retained the protein's monodispersity were used for crystallization screening. Crystals were obtained with the protein purified in C(12)E(8), dodecylmaltoside, decylthiomaltoside, and cyclohexyl-hexylmaltoside. Five to 13 lipid molecules per protein were required for the protein crystal formation. Those crystals grown in dodecylmaltoside diffracted X-rays to 14 A. With these factors taken into consideration, ways to further improve the crystal quality are suggested.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed2003

Crystals of P2 myelin protein in lipid-bound form.

The P2 protein of peripheral nervous system myelin induces experimental allergic neuritis in rats, a model of Guillain-Barré syndrome in humans. Previous purification procedures have used acid extraction to obtain the protein in lipid-free form (LF-P2). Here, we have purified the P2 protein in lipid-bound form (LB-P2) by extracting myelin with the detergent CHAPS, followed by Cu(2+)-affinity column chromatography. All myelin lipids were present in the preparation as shown by high-performance thin-layer chromatography and mass spectrometry. The LB-P2 preparation, which differs from LF-P2 in solubility and in the secondary-structure composition, was dialyzed to remove unbound lipids and excess detergent and crystallized using the hanging-drop vapor diffusion technique. Crystals of lipid-bound P2 appeared usually very reproducibly within 2 weeks at pH 5.7 in polyethylene glycol 6000 (PEG6000) at concentrations of 20-30% (w/v), and larger crystals were obtained by additional sitting-drop crystallization. X-ray diffraction showed reflections up to 2.7A. The crystallization conditions (25-30% PEG6000, pH 5.0) and the unit cell dimensions (a = 94.5A, b = 94.5A, c=74.2A, alpha = beta = 90 degrees, gamma = 120 degrees ) of LB-P2 were different from those earlier described for LF-P2 (10% PEG4000, pH 3, and unit cell dimensions a = 91.8A, b = 99.5A, c = 56.5A, alpha = beta = gamma = 90.0 degrees ). It is important that P2 has been crystallized with specifically bound lipids; therefore, solving this new crystal structure will reveal details of this protein's behavior and role in the myelin sheath.

Animals↗
PubMed2003

New techniques in macromolecular cryocrystallography: macromolecular crystal annealing and cryogenic helium.

Cryocrystallography is used today for almost all X-ray diffraction data collection at synchrotron beam lines, with rotating-anode generators, and micro X-ray sources. Despite the widespread use of flash-cooling to place macromolecular crystals in the cryogenic state, its use can ruin crystals, trips to the synchrotron, and sometimes even an entire project. Annealing of macromolecular crystals takes little time, requires no specialized equipment, and can save crystallographic projects that might otherwise end in failure. Annealing should be tried whenever initial flash-cooling causes an unacceptable increase in mosaicity, results in ice rings, fails to provide adequate diffraction quality, or causes a crystal to be positioned awkwardly. Overall, annealing improves the quality of data and overall success rate at synchrotron beam lines. Its use should be considered whenever problems arise with a flash-cooled crystal. Helium is a more efficient cryogen than nitrogen and will deliver lower temperatures. Experiments suggest that when crystals are cooled with He rather than N2, crystals maintain order and high-resolution data are less affected by increased radiation load. Individually or in combination, these two techniques can enhance the success of crystallographic data collection, and their use should be considered essential for high-throughput programs.

Cold Temperature↗
PubMed2003

Protein crystallization by capillary counterdiffusion for applied crystallographic structure determination.

Counterdiffusion crystallization in capillary is a very simple, cost-effective, and practical procedure for obtaining protein crystals suitable for X-ray data analysis. Its principles have been derived using well-known concepts coupling the ideas of precipitation and diffusion mass transport in a restricted geometry. The counterdiffusion process has been used to simultaneously screen for optimal conditions for protein crystal growth, incorporate strong anomalous scattering atoms, and mix in cryogenic solutions in a single capillary tube. The crystals obtained in the capillary have been used in situ for X-ray analysis. The implementation of this technique linked to the advancement of current crystallography software leads to a powerful structure determination method consolidating crystal growth, X-ray data collection, and ab initio phase determination into one without crystal manipulation. We review the historical progress of counterdiffusion crystallization, its application to X-ray crystallography, and ongoing tool development for high-throughput protein structure determination.

Crystallization↗
PubMed1996

Monoclonal antibody recognition of cholesterol monohydrate crystal faces.

BACKGROUND: The immune system can elicit antibodies against a wide variety of antigens. We have proposed that crystal surfaces may also operate as antigens, binding specific antibodies. Here we exploit the crystal surfaces of cholesterol monohydrate to investigate antibody-surface recognition at the molecular level. RESULTS: Four monoclonal antibodies were selected. Two specifically interact with cholesterol monohydrate crystals, and one with 1,4-dinitrobenzene crystals. The fourth interacts nonselectively with various solid substrates. The relative reactivities of the four antibodies to the different surfaces of cholesterol monohydrate and to other surfaces were compared. The nonspecific antibody adsorbs mainly at imperfections. Of the two specific antibodies, one shows a clear preference for one set of faces, relative to others, the second adsorbs selectively at one face of cholesterol monohydrate crystals. CONCLUSIONS: Monoclonal antibodies can be selected that specifically bind to the crystal surfaces of cholesterol monohydrate. The binding sites of such antibodies appear to recognize a number of molecular moieties, exposed at the surface in a specific structural organization. Different antibodies recognize different structural organizations with varying degrees of selectivity. Antibody-crystal surface interactions may serve as convenient models for studies aimed at an understanding of the molecular bases of antibody recognition.

Animals↗
PubMed1992

Comparison of hydrogen exchange rates for bovine pancreatic trypsin inhibitor in crystals and in solution.

Hydrogen exchange rate constants for the 17 slowest exchanging amide NH groups in bovine pancreatic trypsin inhibitor (BPTI) were measured in solution and in form II and form III crystals. All 17 amide hydrogens are buried and intramolecularly hydrogen bonded in the crystal structure, except Lys 41 which is buried and hydrogen bonded to a buried water. Large-scale crystallization procedures were developed for these experiments, and rate constants for both crystal and solution exchange were measured by 1H NMR spectroscopy of exchange-quenched samples in solution. Two conditions of pH and temperature, pH 9.8 and 35 degrees C, and pH 9.4 and 25 degrees C, bring two groups of hydrogens into the experimental time window (minutes to weeks). One consists of the 10 slowest exchanging hydrogens, all of which are associated with the central beta-sheet of BPTI. The second group consists of seven more rapidly exchanging hydrogens, which are distributed throughout the molecule, primarily in a loop or turn. In both groups, most hydrogens exchange more slowly in crystals, but there is considerable variation in the degree to which the exchange is depressed in crystals. Many differences observed for the more rapidly exchanging hydrogens can be attributed to local surface effects arising from intermolecular contacts in the crystal lattice. Within the slower group, however, a very large effect on exchange of Ile 18 and Tyr 35 appears to be selectively transmitted through the matrix of the molecule.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1993

Structure and thermodynamics of nonalternating C.G base pairs in Z-DNA: the 1.3-A crystal structure of the asymmetric hexanucleotide d(m5CGGGm5CG).d(m5CGCCm5CG).

We have solved the single-crystal X-ray structure of the complementary hexanucleotides d(m5-CGGGm5CG) and d(m5CGCCm5CG). The hexamer duplex was crystallized as Z-DNA, but contains a single C.G base pair that does not follow the alternating pyrimidine/purine rule for Z-DNA formation. This is the first crystal structure which serves to illustrate the structural consequences of placing a cytosine in the sterically disfavored syn conformation. In addition, since these sequences are not self-complementary, the individual strands of this asymmetric hexamer are unique in sequence and therefore distinguishable in the crystal lattice. Nevertheless, the crystal of this duplex is isomorphous with other Z-DNA hexamer structures. The asymmetry of this hexamer sequence required that the structure be solved using two unique models, which are distinguished by the orientation of hexanucleotides in the crystal lattice. In one model (the GG model) the cytosine in the syn conformation is packed against the terminal guanine base of a symmetry-related hexamer, while in the alternative model (the CC model) this cytosine sits exposed in a solvent channel of the lattice. We find that neither model alone can completely account for the observed electron densities. The two models ultimately were refined together. A composite structure consisting of 65% GG model and 35% CC model refined to an R-factor of 19.3%, which was significantly lower than refinements using either model alone. A detailed analysis of these two structures shows that, in spite of the out-of-alternation C.G base pair, the features characteristic of Z-DNA have been maintained. Both models, however, show significant local structural adjustments to accommodate the single cytosine base which is forced to adopt the syn conformation in each hexamer. In general, it appears that in order to relieve the energetically unfavorable steric contacts between the cytosine base in the syn conformation and the deoxyribose sugar, the base is forced into a highly buckled conformation, and that this large buckle in turn alters the conformation of neighboring residues. This unusual conformation also significantly weakens base-stacking interactions between the cytosine in syn and the adjacent residues in the helix and affects the exposure of the bases to solvent. We conclude that this crystal structure provides a molecular rationale for why nonalternating bases are energetically disfavored in Z-DNA.

Base Composition↗
PubMed1987

Sequence dependence of the B to Z transition in crystals and aqueous NaCl solutions for deoxyoligonucleotides containing all four canonical DNA bases.

A laser Raman study has been made on the conformation of a series of self-complementary octameric deoxynucleotides that contain all four canonical deoxynucleotide bases [guanine (G), cytosine (C), adenine (A), and thymine (T)] in order to determine which sequences will crystallize in the Z form and which sequences will go into the Z form in aqueous solution at high salt concentrations (4-6 M NaCl). All four octadeoxynucleotides, d(TGCGCGCA) (I), d(CACGCGTG) (II), d(CGTGCACG) (III), and d(CGCATGCG) (IV), have been crystallized from low-salt solutions. The Raman spectra of microcrystals show that I, II, and IV crystallize in a rigorous Z form while III crystallizes in the B form. Sequences I and II go into a Z form in 4-6 M NaCl solution at 0 degrees C while sequences III and IV remain in the B form in 6 M salt. There are substantial differences in the Raman spectra of oligonucleotides in the Z form found in the crystal and in high-salt solutions. The Raman spectra of the Z forms in 6 M NaCl solution at 0 degrees C are not linear combinations of the Raman spectra of the complete Z form in the crystal and the complete B form in low-salt solutions. The terminal residues of these oligomers do not appear to be in a strict Z form. A detailed analysis of the ring puckers and syn/anti conformation for all of the residues both in solution and in the crystal has been made.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Composition↗
PubMed2006

Rhodopsin photointermediates in two-dimensional crystals at physiological temperatures.

Bovine rhodopsin photointermediates formed in two-dimensional (2D) rhodopsin crystal suspensions were studied by measuring the time-dependent absorbance changes produced after excitation with 7 ns laser pulses at 15, 25, and 35 degrees C. The crystalline environment favored the Meta I(480) photointermediate, with its formation from Lumi beginning faster than it does in rhodopsin membrane suspensions at 35 degrees C and its decay to a 380 nm absorbing species being less complete than it is in the native membrane at all temperatures. Measurements performed at pH 5.5 in 2D crystals showed that the 380 nm absorbing product of Meta I(480) decay did not display the anomalous pH dependence characteristic of classical Meta II in the native disk membrane. Crystal suspensions bleached at 35 degrees C and quenched to 19 degrees C showed that a rapid equilibrium existed on the approximately 1 s time scale, which suggests that the unprotonated predecessor of Meta II in the native membrane environment (sometimes called MII(a)) forms in 2D rhodopsin crystals but that the non-Schiff base proton uptake completing classical Meta II formation is blocked there. Thus, the 380 nm absorbance arises from an on-pathway intermediate in GPCR activation and does not result from early Schiff base hydrolysis. Kinetic modeling of the time-resolved absorbance data of the 2D crystals was generally consistent with such a mechanism, but details of kinetic spectral changes and the fact that the residuals of exponential fits were not as good as are obtained for rhodopsin in the native membrane suggested the photoexcited samples were heterogeneous. Variable fractional bleach due to the random orientation of linearly dichroic crystals relative to the linearly polarized laser was explored as a cause of heterogeneity but was found unlikely to fully account for it. The fact that the 380 nm product of photoexcitation of rhodopsin 2D crystals is on the physiological pathway of receptor activation suggests that determination of its structure would be of interest.

Animals↗
PubMedDate not supplied

Crystallization, stability, and enzymatic degradation of poly(L-lactide) thin film.

Poly(L-lactide) (PLLA) thin film with 100 nm thickness was crystallized at 160 degreesC for 20 min from the melt obtained at 220 degreesC. Hexagonal crystals with three types of growth (derivative growth lamellae, overgrowth multistacked lamellae, and undergrowth multistacked lamellae) were simultaneously observed by atomic force microscopy (AFM). These phenomena are due to the differences of the formative points of secondary crystal nuclei against the basal lamella. Enzymatic degradation of PLLA thin film revealed two types of amorphous regions. These regions were identified as the free amorphous region around the crystals and the restricted amorphous region between the crystal and glass substrate. In situ observation of thermal behavior of lamellar crystals was performed to understand the correlation between the chain folding and stability of the crystal by using temperature-controlled AFM. The morphology of the sectors with [100] growth plane had changed to a comblike morphology despite the fact that the [110] growth plane remained unchanged, suggesting that the stability of the chain folding and the chain-packing state affected the thermal behavior.

Crystallization↗
PubMedDate not supplied

Melting and crystallization behaviors of biodegradable polymers enzymatically coalesced from their cyclodextrin inclusion complexes.

Inclusion complexed (IC) and coalesced biodegradable poly(epsilon-caprolactone) (PCL), poly(L-lactic acid) (PLLA), and their diblock copolymer (PCL-b-PLLA) were achieved by forming ICs between host alpha-cyclodextrin(alpha-CD) and guest PCL, PLLA, and PCL-b-PLLA, followed by removing the alpha-CD host with an amylase enzyme. FTIR spectra of the coalesced polymers reveal that the host alpha-CD can be completely removed, without polymer degradation, by treatment with an amylase enzyme. The melting and crystallization behavior of these CD-IC treated polymers, which are crystallizable, biodegradable, and bioabsorbable, are investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. Results show that coalescence increased the crystallinities of the homopolymers but decreased that of the diblock copolymer. The Avrami exponent (n), derived from both isothermal and nonisothermal crystallization models for homo-PCL and -PLLA and the PCL and PLLA blocks in the diblock copolymer samples coalesced from their ICs, is close to 4, indicating homogeneous crystallization, whereas crystallization of the blocks in the as-synthesized diblock copolymer yields an Avrami exponent around 3, indicating heterogeneous crystallization. All of these results demonstrate that the PCL and PLLA homopolymers and blocks in the IC-coalesced samples are more readily and homogeneously crystallized than those in the as-synthesized samples or their physical blend, even though the level of crystallinity in the IC-coalesced diblock copolymer is significantly lower. Moreover, unlike the as-synthesized diblock copolymer, the crystallization of PCL and PLLA blocks in the IC-coalesced diblock copolymer are not influenced by their covalent connection.

Biodegradation, Environmental↗
PubMedDate not supplied

Eggshell matrix protein mimics: designer peptides to induce the nucleation of calcite crystal aggregates in solution.

Ansocalcin is a novel goose eggshell matrix protein with 132 amino acid residues, which induces the formation of polycrystalline calcite aggregates in in vitro crystallization experiments. The central region of ansocalcin is characterized by the presence of multiplets of charged amino acids. To investigate the specific role of charged amino acid multiplets in the crystal nucleation, three short peptides REWD-16, REWDP-17 (containing charged doublets), and RADA-16 (alternating charged residues) were synthesized and characterized. The aggregation of these peptides in solution was investigated using circular dichroism, intrinsic tryptophan fluorescence, and dynamic light scattering experiments. The peptides REWD-16 and REWDP-17 induced the polycrystalline calcite crystal aggregates, whereas RADA-16 did not induce significant changes in calcite crystal morphology or aggregate formation in in vitro crystallization experiments. The lattice and morphology of the calcite crystals were characterized using X-ray diffraction and scanning electron microscope. The results discussed in this paper reveal the importance of multiplets of charged amino acid residues toward the nucleation of polycrystalline calcite crystal aggregates in solution.

Amino Acid Sequence↗
PubMedDate not supplied

Preparation and crystallization kinetics of new structurally well-defined star-shaped biodegradable poly(L-lactide)s initiated with diverse natural sugar alcohols.

This study presents syntheses, structural characterization, and crystallization kinetic investigation of new structurally well-defined star-shaped poly(l-lactide)s (PLLAs). First, a series of new 3- to 6-arm star-shaped PLLAs were synthesized through SnOct(2) catalyzed ring-opening polymerization of (l)-lactide with natural sugar alcohols of glycerol, erythritol, xylitol, and sorbitol as the favorable initiators. Subsequently, their chemical structures were characterized by means of GPC, NMR, and viscometer with respect to the star-shaped structures, demonstrating the well-defined arm structures as evidenced on the g(1/2)/g' values, where g and g' denote the ratios of mean-square radius of gyration and intrinsic viscosity of a star-shaped polymer to those of a linear structural reference with similar absolute molecular weight. Furthermore, spherulite morphologies and growth rates were studied by a polarized microscopy (POM) for the synthesized star-shaped PLLAs with different molecular weights, and it was found that the more arms of a star-shaped PLLA finally resulted in a lower spherulite growth rate. With regard to the crystallization kinetics of these star-shaped PLLAs, isothermal and nonisothermal crystallization were examined by differential scanning calorimeter (DSC). It was found that Avrami exponent n values of isothermal crystallization were almost independent of the isothermal crystallization temperature T(c) for different series of star-shaped PLLAs. In contrast, the values of Avrami exponent n were observed to strongly depend on the star-shaped structures with different arms, implying their distinct nucleation mechanisms, and the more arms of a star-shaped PLLA led to a slower isothermal crystallization rate. On the basis of a modified Avrami equation, new light was shed on the nonisothermal crystallization kinetics for the star-shaped PLLAs, and the activation energies were found to vary from 146.86 kJ/mol for the linear PLLA EG-3 to 221.23 kJ/mol of the star-shaped S-3, demonstrating much decreased crystallizabilities of star-shaped PLLAs with more arms.

Biodegradation, Environmental↗
PubMed2006

Formation of a unique crystal morphology for the poly(ethylene glycol)-poly(epsilon-caprolactone) diblock copolymer.

The crystallization behaviors of the poly(ethylene glycol)-poly(epsilon-caprolactone) diblock copolymer with the PEG weight fraction of 0.50 (PEG(50)-PCL(50)) was studied by DSC, WAXD, SAXS, and FTIR. A superposed melting point at 58.5 degrees C and a superposed crystallization temperature at 35.4 degrees C were obtained from the DSC profiles running at 10 degrees C/min, whereas the temperature-dependent FTIR measurements during cooling from the melt at 0.2 degrees C/min showed that the PCL crystals formed starting at 48 degrees C while the PEG crystals started at 45 degrees C. The PEG and PCL blocks of the copolymer crystallized separately and formed alternating lamella regions according to the WAXD and SAXS results. The crystal growth of the diblock copolymer was observed by polarized optical microscope (POM). An interesting morphology of the concentric spherulites developed through a unique crystallization behavior. The concentric spherulites were analyzed by in situ microbeam FTIR, and it was determined that the morphologies of the inner and outer portions were mainly determined by the PCL and PEG spherulites, respectively. However, the compositions of the inner and outer portions were equal in the analysis by microbeam FTIR.

Calorimetry, Differential Scanning↗