Search

The molecular weight dependence of the lower critical solution temperature of poly(N,N-diethylacrylamide) was studied with 11 samples of the polymer with a number-average molecular weight ([M.sub.n]) ranging from 9.6 x [10.sup.3] to 1.3 x [10.sup.6] g [mol.sup.-1] and relatively narrow polydispersity indices from 1.19 and 2.60. These samples were obtained by fractional precipitation of the polymer. LCST values of polymers were measured by turbidimetry and microcalorimetry. An inverse dependence of LCST on the molar mass was found and the LCST of the samples remained more or less a constant above a critical molecular weight of ca. 2 x [10.sup.5] g [mol.sup.-1]. The enthalpy and the entropy changes as well as the LCST of the polymer depend strongly on the molar mass of the polymer, especially in low molecular weight range. Key words: poly(N,N-diethylacrylamide), LEST, thermosensitive, phase diagram, effect of molecular weight.

Bile acids are gaining increasing importance as building blocks in the development of novel polymeric materials. This is evidenced by the growing number of publications advocating the advantages of their incorporation in the design and construction of materials. Composed of a rigid steroid backbone, functional groups with potential towards diverse reactions, and a biocompatible framework, there are various ways in which these molecules can be utilized to afford biomaterials via distinct architectures. Soft materials utilize the intrinsic capacity of bile acids to self-assemble and have seen a range of applications, most notably in the field of drug delivery. On the other hand, there is also the possibility of including bile acids in the polymer backbone, which has been used in the preparation of elastomers. This review discusses a selection of materials that can be prepared using bile acids and the advantages afforded by these molecules. Focus will be on the development of soft and hard materials, where soft materials are described as being held by weak intermolecular interactions, whereas hard materials are mechanically stronger with bile acids covalently incorporated in the polymer network. [ABSTRACT FROM AUTHOR]

Abstract: Various polymer microspheres have demonstrated their ability to form crystalline colloidal arrays in relatively concentrated suspensions. The behaviors of such structures are closely related to the characteristics and properties of the microspheres, which could be easily tuned with the choice of monomers and the polymerization procedure. Three commonly used kinds of microspheres are reviewed here: charged hard microspheres, microgels and core–shell particles with a hydrogel shell. The light diffraction displayed by the resulting crystalline colloidal arrays has triggered an increasing interest for potential applications such as optical switches and biosensors. [Copyright &y& Elsevier]

Abstract: Bile acid-based polymers are promising new materials for biomedical applications. The determination of their molar mass, as for other novel polymers, has been difficult, due to the lack of suitable standards for size exclusion chromatography (SEC). In order to solve this problem, a family of main-chain bile acid-based oligo-esters has been synthesized by acyclic diene metathesis to be used as analogues in such analysis. These oligomers have been characterized by SEC, MALDI-TOF mass spectrometry and NMR spectroscopy. The results show that SEC with polystyrene standards tends to overestimate the molar mass of these materials and that a correction factor between 0.50 and 0.60 should be used for more accuracy. [Copyright &y& Elsevier]

Polymer microspheres can be employed to deliver medication in a rate-controlled and sometimes targeted manner. Medication is released from a microsphere by drug leaching from the polymer or by degradation of the polymer matrix. Since the rate of drug release is controlled by these two factors, it is important to understand the physical and chemical properties of the releasing medium. This review presents the methods used in the preparation of microspheres from monomers or from linear polymers and discusses the physio-chemical properties that affect the formation, structure, and morphology of the spheres. Topics including the effects of molecular weight, blended spheres, crystallinity, drug distribution, porosity, and sphere size are discussed in relation to the characteristics of the release process. Added control over release profiles can be obtained by the employment of core-shell systems and pH-sensitive spheres; the enhancements presented by such systems are discussed through literature examples. [Copyright &y& Elsevier]

Poly(β-cyclodextrin) (PCD) resins were prepared by a crosslinking reaction of β-cyclodextrin with different amounts of epichlorhydrin. Some hydroxyl groups of these polymers were functionalized with alkyl quaternary ammonium groups. The polymers were tested for their ability to bind several bile salts (including the sodium salts of cholic acid, glycocholic acid, and chenodeoxycholic acid), individually and competitively, from phosphate buffer solutions. In all cases, the aminated PCD resin had a higher binding capacity for bile salts. The binding of chenodeoxycholate by the resins was much more effective than that of cholate and its conjugate, which indicates the importance of the host cavity size relative to that of the guest molecules. The degree of hydrophobicity of bile acids also plays a role in their binding by PCD resins. The variable temperature studies indicate that the electrostatic interactions become weaker at higher temperatures while the hydrophobic interactions are not as much affected. [ABSTRACT FROM AUTHOR]

Abstract: A cholic acid-imprinted polymer has been prepared by a hybrid imprinting method involving the use of a template-containing monomer, 3α-methacryloyl cholic acid methyl ester. The removal of the template can afford the polymer with cavities and carboxylic acid groups well positioned in these cavities for the specific binding of cholic acid via hydrogen bonding. The maximum binding capabilities for the imprinted and non-imprinted polymers were determined to be 344.8 and 43.6μmol/g, respectively. Scatchard analysis indicates the existence of two kinds of binding sites on the imprinted polymer. The imprinted polymers exhibit high affinity and good recognition selectivity for cholic acid in comparison to other compounds with similar molecular structures. The size-specific binding and hydrogen bonding may be both important for the binding of cholic acid by the imprinted polymers. [Copyright &y& Elsevier]

We report sequential self-assembly of low molecular weight asymmetric star-like polymers from nanometric spherical micelles (core diameter 8–12 nm) to colloidal spherical superstructures ( d > 500–1000 nm) in aqueous media, as directed by quadruple hydrogen bonding moieties activated by solvent exchanges. The molecules consist of a rigid asymmetric steroidal cholic acid (CA) core, whose hydroxyl groups are first grafted with four oligomeric allyl glycidyl ether (AGE) chains involving six repeat units. The allylic double bonds are thiol-clicked with cysteamine (HS-(CH 2 ) 2 -NH 2 ), and in the final step their amino end groups are conjugated with 2-ureido-4[ 1H ]-pyrimidinone (UPy) via isocyanate chemistry using UPy-(CH2)6-NCO to obtain asymmetric star-like facially amphiphilic molecules denoted as CA[(AGE-NH 2 ) 6 ] 4 -(UPy) n . They self-assemble into spherical micelles in dimethyl sulfoxide (DMSO), which suppresses the dimerization of UPys. Solvent exchange from DMSO to water activates the inter-micellar hydrogen bonds via the peripheral UPy units, which triggers the formation of spherical submicron to micron scale superstructures. Based on three-dimensional (3D) reconstruction using transmission electron tomography (ET), we show that such structures are composed of densely packed micellar network. Our work shows that surfactant-like molecules, characteristically leading to micelles, can undergo complex and hierarchical self-assembly to microscale by equipping the solubilising chains with hydrogen bonding units that can be switched ON and OFF by sequential solvent exchanges. [ABSTRACT FROM AUTHOR]

Abstract: Main chain bile acid polymers obtained via entropy-driven ring-opening metathesis are biodegradable polymers with excellent shape memory properties. X-ray scattering was used to study the conformational changes implicated in the shape memory effect of these polymers. Strain induced orientation and ordering was observed upon the drawing of the polymer films. A correlation between ordering and the shape memory performances is observed. [Copyright &y& Elsevier]

Abstract: Four methacrylate derivatives of steroid molecules, namely cholic acid methyl ester, lithocholic acid methyl ester, cholesterol and dihydrocholesterol, were synthesized. A 10-carbon spacer separates the rigid core and the polymerizable methacrylate group. These monomers and their corresponding polymers were characterized for their liquid crystalline properties by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. It was found that only the compounds bearing the planar cholesterol-based moieties, and not those bearing the esterified bile acid groups, possess liquid crystalline phases. A detailed X-ray study showed that the dihydrocholesterol polymer possesses the same two mesophases as the cholesterol polymer, but with reduced thermal stability. [Copyright &y& Elsevier]