Stereocomplex formation in ABA triblock copolymers of poly(lactide) (A) and poly(ethylene glycol) (B)

Two series of triblock copolymers of poly(ethylene glycol) (PEG, number-average molecular weight [bar M ]n = 6000) and poly(L-lactide) (PLLA) or poly(D-lactide) (PDLA) were prepared by ring-opening polymerization of lactide initiated by PEG end groups using stannous octoate as a catalyst, either in refluxing toluene or in the melt at 175°C. The weight percentage of PLA in the polymers varied between 15 and 75 wt.-%. Blends of polymers containing blocks of opposite chirality were prepared by co-precipitation from homogeneous solutions. The melting temperatures of the crystalline PEG and PLA phases strongly depended on the composition of the polymers. The melting temperature of the PLA phase in the blends was approximately 40°C higher than that of the single block copolymers. Stereocomplex formation between blocks of enantiomeric poly(lactides) in PEG/PLA block copolymers was established for the first time. Water uptake of polymeric films prepared by solution casting was solely determined by the PEG content of the film

Block copolymers of poly(L-lactide) and poly(ε-caprolactone) or poly(ethylene glycol) prepared by reactive extrusion

Blends of poly(L-lactide) (PLLA) and poly(-caprolactone) (PCL) were prepared in a co-rotating twin screw miniextruder (40 rpm, 200°). It was attempted to prepare multiblock copolymers by allowing a controlled number of transesterification reactions. Various cat-alysts (n-Bu3SnOMe, Sn(Oct)2, Ti(OBu)4, Y(Oct)3, para-toluene sulphonic acid) were introduced to promote these transesterification reactions. However, PLLA degradation by ring-closing depolymerization was the dominant reaction in every case. Alternatively, after showing that L-lactide can be conveniently polymerized in the extruder, L-lactide and hy-droxyl functionalized prepolymers of PCL or poly(ethylene glycol) (PEG) were fed to the extruder in the presence of stannous octoate. Monomer conversions of over 90% and effective transformation of all hydroxyl end groups present were generally reached. Di-and triblock copolymers could be prepared in this way with characteristics very similar to polymers prepared in a batch-type process, but with considerably reduced reaction times in a fashion, which is, in principle, scaleable to a continuous process for the production of such block copolymers

Determination and Verification of PVB Interlayer Modulus Properties

Interlayer modulus properties are important for the calculation of the load resistance of laminated glass configurations. The determination of interlayer properties can be made on the interlayer material directly, or indirectly through measurements on laminated glass test specimens. One challenge relates to the four-decade range of modulus values that interlayer materials go through with variations of temperatures and duration that are relevant to building elements. Furthermore, in determination of properties directly on the interlayer, e.g. by dynamic mechanical analysis, methodological choices and data treatment play an important role, apart from experimental and material variation. To some extent, this also holds true for modulus determination on laminated glass specimens, with the added challenge that the properties of a relatively soft material are derived from the response of a rigid assembly. In this contribution, three material models are provided for of all major PVB interlayer formulations (conventional, structural, acoustic trilayer), as determined by dynamic mechanical analysis and compared to modulus values obtained in four-point bending experiments. The validated material models can provide modulus data relevant for glass design for a wide range of load scenarios in a concise format

Polymerization of ethylene oxide using yttrium isopropoxide

Well defined poly(ethylene oxide)s were prepared using yttrium isopropoxide as an initiator. End group analysis using 1H- and 13C NMR spectroscopy revealed that only polymers with isopropyl ether and hydroxyl end groups were produced. The molecular weight is controlled by the initial amount of initiator added and low polydispersity polymer (Mw/Mn ≈ 1.1) was isolated. Sequential polymerization indicated the suitability of this initiator for macromolecular engineering

Is there a "native" band gap in ion conducting glasses?

It is suggested that the spectrum of ion site energies in glasses exhibits a band gap, establishing an analogy between ion conducting glasses and intrinsic semiconductors. This implies that ion conduction in glasses takes place via vacancies and interstitial ions (as in crystals).Comment: 3 page

Legumes traditionnels du Cameroun, une etude agro-botanique

Cameroon has a remarkable abundance of vegetable species. A great variety of local and introduced vegetable crops are grown and together with a significant number of wild and semi-wild plants, occasionally cultivated, form a valuable complementary food in the daily diet.Primarily the aim of this study is to present an inventory of traditional vegetable species in Cameroon, indicating their importance in local agriculture, their significance as food and their nutritional value. The second objective is a taxonomical study of these vegetable crops, in order to expose, and where feasible eliminate, the existing widespread confusion in their scientific nomenclature. Botanical descriptions and practical differential keys, based on those parts of the plants that are actually used, are provided. This study deals with 67 traditional vegetables, 20 species of which are discussed in greater detail. The materials and data were gathered mainly in Cameroon during the period 1975 - 1979, and subsequently elaborated upon at the Department of Plant Taxonomy of the Agricultural University, Wageningen, The Netherlands.Chapter I contains a general introduction. The working plan (chapter 2) clarifies the selection of the 20 primary species (paragraph 2.1). Further detailed information is supplied regarding the materials used and the applied methods of investigation (paragraph 2.2).Chapter 3 presents the five agro-ecological regions into which Cameroon can be divided (paragraph 3. 1). A review is given of the physical and biological environment (paragraph 3.2) and of the agriculture and animal husbandry (paragraph 3.3). Various aspects pertaining to vegetables are discussed, such as the different types of vegetable crops (paragraph 3.4. 1), local and introduced vegetables (paragraph 3.4.2), the inventory of the vegetables in the different agro- ecological regions (paragraph 3.4.3) and a classification of wild and/or cultivated species (paragraph 3.4.4). Finally the various types of vegetable production are dealt with (paragraph 3.5).Chapter 4 is based on nutritional research and surveys which have been carried out in Cameroon since 1953. The first part is devoted to a survey of food and nutrition in the different agro-ecological regions (paragraph 4. 1). In the second part an account is given of the preparation and utilisation of vegetables including their nutritional value (paragraph 4.2).Chapter 5 deals with the botanical study of the vegetable species. Differential keys to groups of species (paragraph 5. 1. 1) and to individual species (paragraph 5.1.2), based on characteristics of the edible product, are supplied. In the next paragraph (5.2) all 67 species are arranged in alphabetical order, primarily in family order and subsequently according to the genera and species. This paragraph also provides concise information concerning the 47 secondary species.The 20 primary species are considered in a final paragraph (5.3), also in alphabetical order.The format of the text in the paragraphs 5.2 and 5.3 is essentially similar for each species. In paragraph 5.2 the author and the original publication of the species are cited, along with the current synonyms, pertinent literature, vernacular names, the occurrence in each of the five agro-ecological regions, a concise botanical description, a few relevant notes, representative herbarium specimens from Cameroon and finally the applicability as a vegetable. Furthermore paragraph 5.3 provides an etymological explanation of the scientific name and the typification, while synonymy, literature, geographic distribution and botanical description are discussed at length. Notes in this paragraph elaborate the typification. Any other relevant questions are subsequently dealt with, such as the recurring confusion with other species of the same genus, infraspecific variation and the distinction of cultivar-groups or cultivars, some cytogenetic and evolutionary aspects, and the use as a vegetable. Finally all the herbarium material examined from Cameroon is listed.Each species is accompanied by a botanical drawing of the edible parts of the plant in paragraph 5.2. Species dealt with in paragraph 5.3 are fully illustrated. In some cases a photograph is added. In conclusion a general bibliography and indices of scientific and vernacular plant names are provided