Thermal study of synthesis of cryptohalite

The present work represents a thermal study of synthesis of cryptohalite (Ammonium silicon hexafluoride) by sintering of quartz with ammonium fluoride using a derivatograph. The reaction products were identified microscopically and by using a Siemens Crystalloflex diffractometer. The DTA curves indicate that the intensive formation of cryptohalite takes place at 125–155C by an endothermic reaction. Cryptohalite is unstable and dissociates at 320–335C as represented by the sharp and large endothermic peaks at these temperatures.The resulted cryptohalite is colorless in thin sections and crystallizes in cubic system, in the form of octahedral crystals with perfect (111) cleavage. The dimorph bararite is not detected in all runs.

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Zusammenfassung Mittels eines Derivatographen wurde die Synthese von Kryptohalit (Ammoniumsiliziumhexafluorid) durch Sintern von Quarz mit Ammoniumfluorid thermisch untersucht. Die Reaktionsprodukte wurden mikroskopisch und mit Hilfe eines Siemens-Crystalloflex Diffraktometers identifiziert. Die DTA-Kurven zeigen, da\ die intensive Bildung von Kryptohalit in einer endothermen Reaktion bei 125–155C abläuft. Wie durch die scharfen und intensiven endothermen Peaks bei 320–335C gezeigt wird, ist Kryptohalit bei dieser Temperatur instabil und dissoziiert.Das erhaltene Kryptohalit ist in dünnen Schnitten farblos und kristallisiert im kubischen System in der Form von oktaedrischen Kristallen mit perfekter (111) Spaltbarkeit. Das dimorphe Bararit konnte in keinem der Versuche beobachtet werden.

     

Extended Hückel MO calculations of the conformation of several amino acids

The conformation of the amino acid residues, glycine, alanine, proline, and phenylalanine have been predicted from molecular orbital calculations of appropriate model compounds. Using the current main chain rotation convention (, ) the principle conformations were found to be glycine (0, 0), alanine (240, 240), proline (120, 330) and phenylalanine (30, 330). Several secondary conformations were also found for glycine. A comparison of the predicted conformations is in good agreement with experimental data on comparable residues or model compounds.Supported by National Institutes of Health Grants No. FR 5409-07 and GM 16312-01.Recipient of a Public Health Service Research Career Development Award AM 1159-01.     

Measurement of specific heat and energetic characterization of materials up to high temperatures

A new DSC system has been developed which not only allows quantitative results in the temperature of –160C to 700C, but also allows the quantitative determination of a variety of material properties up to 1500C. For example, the specific heat of materials can be measured to at least 1400C, while enthalpies, etc. can be measured to 1500C.

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Zusammenfassung Ein neuartiges DSC Messystem (Netzsch DSC 404) wurde entwickelt, das sich durch hohe Reproduzierbarkeit der Basislinie, grosse Empfindlichkeit und breiten Temperaturanwendungsbereich (–160C bis 700C resp. bis 1500C) auszeichnet. Die Messanordnung ermöglicht die Verwendung von unterschiedlichen GasatmosphÄren als auch Messungen im Vakuum. Es werden Beispiele der Bestimmung von SchmelzvorgÄngen, der Glasumwandlungstemperatur, der KristallinitÄt und der spezifischen WÄrme, sowohl für Polymere als auch für anorganische Materialien dargestellt und diskutiert.

     

Surface tension as a function of temperature for some molten metals

Results are given for Sn, In, and Ge from the melting points up to 1700 C, for Pb up to 1400 C, for Tl up to 1100 C, for Bi up to 1300 C, and for Cd up to 600 C. In every case the surface tension is a linear function of temperature. Estimates are made of the critical temperatures of Cd and Ge. It is shown that deductions on the structure of the melts can be drawn from surface-tension data.     

Relaxation during internal rotation ethane and hydrogen peroxyde

An ab initio SCF-LCAO-MO study of the relaxation process during internal rotation has been performed for ethane and hydrogen peroxyde. A large gaussian basis set has been used, with polarization functions. The total energy has been optimized with respect to the bond lengths and bond angles. The computed barrier for the ethane molecule is 3.07 kcal/mole with the optimized geometry (experimental 2.93 kcal/mole). For hydrogen peroxyde, this yields a cis-barrier of 10.9 kcal/mole (experimental 7.0 kcal/mole) and a trans barrier of 0.6 kcal/mole (experimental 1.1 kcal/mole), with a dihedral angle equal to 123 (experimental 111–120). The eclipsed or cis conformations are found to have more open structures than the staggered or equilibrium conformations.

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Zusammenfassung Der Relaxationsproze\ wÄhrend der inneren Rotation wurde für Äthan und Wasserstoffperoxid mit Hilfe einer ab initio SCF-LCAO-MO-Rechnung untersucht. Dabei wurde eine gro\e Basis von Gau\funktionen mit Polarisationsfunktionen benutzt. Die Gesamtenergie wurde unter Variation der BindungslÄnge und Bindungswinkel optimiert. Die berechnete Rotationsbarriere mit der optimalen Geometrie betrÄgt 3,07 kcal/Mol für Äthan (experimentell 2,93 kcal/Mol). Für Wasserstoffperoxid ergibt sich eine cis-Barriere von 10,9 kcal/Mol (experimentell 7,0 kcal/Mol) und eine trans-Barriere von 0,6 kcal/Mol (experimentell 1,1 kcal/Mol) sowie ein Verdrillungswinkel von 123 (experimentell 111–120). Die verdeckten oder cis-Konformationen besitzen mehr offene Strukturen als die gestaffelten oder die Gleichgewichtskonformationen.

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Résumé Une étude des phénomènes de relaxation liés à la rotation interne a été effectuée pour l'ethane et l'eau oxygénée par la méthode ab initio SCF-LCAO-MO. Avec une base étendue de fonctions gaussiennes comprenant des fonctions de polarisation, on minimise l'énergie par rapport aux longueurs et aux angles des liaisons. La barrière calculée pour l'ethane est de 3,07 kcal/mole (valeur expérimentale 2,93 kcal/mole). Pour l'eau oxygénée, on trouve pour les barrières cis 10,9 kcal/mole, trans 0,6 kcal/mole et pour l'angle dièdre 123 (values expérimentales 7,0 et 1,1 kcal/mole et 111–120). Les conformations éclipsée ou cis possèdent des structures plus «ouvertes» que les conformations en étoile ou trans.

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Quantum Mechanical Calculations on Barriers to Internal Rotation. Part VI. Preceding paper, Ref. [35].     

Dynamic mechanical properties of irradiated polyvinyl chloride

Summary Effects of gamma radiation on the dynamic mechanical properties of polyvinyl chloride (PVC) have been studied at audio frequencies from 80 K to 450 K for doses up to 1400 megarads. Two damping peaks were observed between 80 K and 400 K. The onset of main relaxation near 350 K shifted to lower temperatures at low doses, and then shifted to higher temperatures at a dose of 1400 megarads. Only slight changes are discernible in the damping peak near 250 K upon irradiation, but definite variations are noted in the modulus curves. Upon irradiation to 270 megarads an additional damping peak appeared near 150 K accompanied by an increase in modulus at lower temperatures. The height of this peak increased with increasing radiation dose.Crosslinking was confirmed from rubber elastic behavior, swelling, and solvent extraction studies. Some tentative explanations for the changes occurring in the dynamic mechanical properties of irradiated PVC are given in terms of crosslinking, loss of crystallinity, and dehydrochlorination followed by the formation of conjugated series of double bonds.

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Zusammenfassung Es wurde die Wirkung von-Strahlung auf die dynamisch-mechanischen Eigenschaften von Polyvinylchlorid (PVC) bei Hörfrequenzen von 80 K bis 450 K für Dosen bis zu 1400 Megarad untersucht. Zwei Dämpfungspiks zeigen sich zwischen 80 und 400 K. Das Einsetzen der Hauptrelaxation nahe 350 K verlagert sich zu tieferen Temperaturen bei kleineren Dosen, um dann zu höheren Temperaturen bei einer Dosis von 1400 Megarad hinaufzugehen. Nur leichte änderungen sind im Dämpfungspik nahe 250 K durch die Strahlung zu beobachten; aber in der Modulkurve werden definierte Variationen erkennbar. Nach Bestrahlung mit 270 Megarad erscheint ein zusätzlicher Dämpfungspik nahe 150 K, begleitet durch ein Anwachsen des Moduls zu tieferen Temperaturen. Die Höhe dieses Piks wächst mit wachsender Strahlungsdosis.Die Vernetzung wurde aus dem gummielastischen Verhalten, der Quellung und aus Lösungsmittel-Extraktionen bestimmt. Die im dynamisch-mechanischen Verhalten durch die Strahlung des PVC auftretenden änderungen werden auf Grund der Vernetzung, der Abnahme an Kristallinität, der Dehydrochlorierung — gefolgt durch die Bildung von Serien konjugierter Doppelbildungen — erklärt.

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This work was supported in part by the National Science Foundation and the National Aeronautics and Space Administration.     

Thermal behaviour of polyetherimide peek blends

PEEK is characterised by high impact and fatigue resistance andT _g of 145C. Blends of PEEK and PEI have been made and scanning electron micro-graphs of the broken specimen show that the two polymers are completely miscible in all proportions. The study also shows that PEEKPEI 5050 blend, can be used as matrix for composite applications with appreciable enhancement ofT _g to 177C.     

Analytische Dichtegradienten-Zentrifugation bei Temperaturen zwischen +4?C und +40?C

Zusammenfassung Es wird gezeigt, da\ die bei der analytischen Dichtegradienten-Zentrifugation benötigten Parameter, die von Hearst u. Mitarb. tabelliert wurden, auch für andere Temperaturen als 25C gültig sind. Das ist von gro\er Wichtigkeit, da hochempfindliche Enzyme nur bei Temperaturen um + 4C lÄnger als 24 h nativ bleiben. Man ist deshalb auf eine Zentrifugation bei dieser Temperatur angewiesen. Weil man oft nur sehr wenig Substanz isolieren kann, bleibt nur die Dichtegradienten-Zentrifugation als Methode der Wahl. Ein Vergleich der gemessenen Molekulargewichte bei 6C und 25C zeigt keine Abweichungen, die über die Me\genauigkeit hinausgehen.

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Analytical density gradient centrifugation at temperatures between +4C and +40 C

It is shown that all the parameters needed for analytical density gradient centrifugation tabulated by Hearst et al. for 25C are valid for other temperatures, too. This will be important in the investigation of highly sensitive enzymes which are only stable at temperatures near +4C over a period of 24 h. Comparison of molecular weights determined at 6C und 25C shows no difference greater than 2%, which will be within the experimental error.

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Das Bundesministerium für Bildung und Wissenschaft hat diese Arbeiten gro\zügig unterstützt.     

Oxidation of [Co(en)2(SOCH2CO2)]+ by S2O82− and by IO4− in binary aqueous solvent mixtures at pressures up to 1.25 kbar

Second order rate constants and activation parameters H, S, and V have been measured for the oxidation of [Co(en)2(SOCH2CO2)]+ by S2O82– and by IO4– in highly aqueous H2O – t-BuOH mixtures. The changes in solvation on going from the initial to the transition state are discussed on the basis of the transfer functions Gto, Hto and Sto. Whereas Gt changes smoothly as the proportion of t-BuOH increases, the plots of Ht and TSt exhibit mirror behaviour and pass through extrema located around x2(t- BuOH)=0.038. Information on the role of solvation is complemented by the determination of activation volumes. These are discussed in terms of intrinsic and solvational contributions. It is proposed that changes in hydrophobic hydration are of principal importance in determining the response of H, S, and V to changes in solvent composition in H2O – t-BuOH mixtures.     

Effect of the network structure on the flammability ofcis-1,4-polyisoprene vulcanizates

The combustibilities ofcis-1,4-polyisoprene and its peroxide and sulphur vulcanizates have been investigated by means of the oxygen index method. The temperature distribution in the sample and the molecular weight of the liquid products formed as a result of elastomer destruction in the combustion zone have been measured. It has been found that the combustibility ofcis- 1,4-polyisoprene diminishes as its cross-link density increases. This effect is much stronger for sulphur than for peroxide vulcanizates. The investigations lead to the conclusion that a mechanism of network degradation and volatile sulphur compounds penetrating to the flame are responsible for this difference.

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Zusammenfassung Die Brennbarkeit des unvernetztencis-1,4-Polyisoprens und seiner Peroxid- und Schwefelvulkanisate wurde mit der Sauerstoffindexmethode untersucht. Messungen der Temperaturverteilung im Prüfling wurden ebenfalls ausgeführt und das Molekulargewicht der infolge der Zersetzung der Elastomere in der Verbrennungszone entstehenden flüssigen Produkte mit der Methode der dynamischen Osmometrie bestimmt. Es wurde festgestellt, daß die Brennbarkeit des cis-1,4-Polyisoprens mit zunehmender Vernetzungsdichte abnimmt, und zwar viel ausgeprägter im Falle der Schwefel- als im Falle der Peroxidvulkanisate. Es wurde gefolgert, daß flüchtige, in die Flamme gelangende Schwefelverbindungen als Verbrennungsinhibitoren wirken.

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-1,4- . , . , -1,4- . . , .

     

Diffusion of linear aliphatic esters in polyethylene as a function of chain length of the esters,pressure and temperature

Diffusion of linear aliphatic mono- and diesters (C _N ) havingN main chain atoms (N=13–68) in bulk medium-density polyethylene (MDPE) has been studied under hydrostatic pressures up to 2500 bar at temperatures between 60°C and 125°C. Three triglycerides, phenyl stearate, and p-aminoazobenzene (pAAB, 80°C) as the diffusants and low-density (LDPE) and high-density (HDPE) polyethylenes as polyethylene substrate were used for comparison. Diffusion coefficientD was determined from concentration distribution of the diffusants through stacked PE sheets as substrate. Regarding the linear esters at 90°C, the relationshipD N holds at constant pressures. Under the atmospheric pressure, became –2.10 in accordance with de Gennes's proposal (1971)D N ^–2 as well as with the experimental results reported by Klein and Briscoe (1979) forN larger than 30.D's for the glycerides deviate from the relationshipD N ^–2 toward the smaller values by comparison at the sameN. The exponent is pressure-dependent. It decreases with increasing pressure according to =–2.10–0.000942P, whereP is measured by the unit of bar. Plots of lnD vsP for all the diffusants show linear relationships with negative slopes, from which activation volume for the diffusion V was calculated. At 90°C, V increases slowly with increasingN and increasingV _Ki, the intrinsic molecular volume of the diffusant, from 39.3 cm³/mol for ethyl caprate (C ₁₃,V _Ki=136 cm³/mol) to 76.8 cm³/mol for behenyl behenate (C₄₅,V _Ki=466 cm³/mol). Observed V s are explainable on the basis of the reptation mode of the chain molecule diffusion. V s for C₂₅ and C₄₅ are found to increase with increasing degree of crystallinity where MDPE, heat-treated MDPE, LDPE, and HDPE were used. The results obtained by varying temperature are as follows. V for C₄₅ was always found to be larger than C₂₅. Both decreased linearly with increasing temperature, giving two linear lines with different slopes whose extensions intersected at 132°C, the melting point of the MDPE, where the difference in V disappeared. The apparent activation energiesE _Ds for the diffusion of C₂₅ and C₄₅ increased linearly with increasing pressure, whose slopes are explainable according toE _D=E ₀+PV [1-(dln V /dlnT) _P ].     

Isokinetic relationships (IKRs) in organometallic and coordination chemistry: Reactions of Ru5C(CO)14{P(OPh)3} and other complexes

The linear compensation plot of H versus S for associative substitution reactions of Ru5C(CO)14 {P(OPh3)} with seven P-donor nucleophiles suggests that an isokinetic temperature, Tiso, of 253±10K exists. A detailed statistical analysis by the Linert-Exner method shows that the data are consistent with there being a genuine isokinetic temperature at 245K at which reactions with six out of the seven nucleophiles proceed at the same rate. This can be shown more easily, graphically more vividly, and with quantitatively the same or better results, by a simplified version of an earlier method due to Krug, Greiger et al. in which H values are found to depend linearly on corresponding values of G calculated at a suitably chosen temperature. This isokinetic behaviour is closely related to that shown by the linear free energy analysis of the rates in terms of the electronic and steric properties of the nucleophiles. The temperature dependence of the sensitivity of the rates to these electronic and steric properties suggests that the major factors involved are entropic rather than enthalpic, reactions with larger nucleophiles actually being favoured by enthalpic factors. Steric profiles obtained at different temperatures all pass through a common point with an isokinetic cone angle of 153°. A few examples of other reactions of organometallic or coordination compounds that show linear compensation plots of H versus S are also analysed by the Linert- Exner and Krug-Greiger methods. Some do show unambiguous isokinetic behaviour but others do not, even though the compensation plots appear to be linear.     

Thermal research of montmorillonite synthetized from the oxides SiO2-Al2O3-MgO-CaO

The thermal behaviour of synthetic montmorillonite of relatively simple chemical composition has been studied and interpreted. Montmorillonite was prepared by hydrothermal synthesis at 300° and 8.8 MPa during 165 hours. With the methods of DTA, DTG, TG, X-ray diffraction analysis, infrared spectroscopy and electron microscopy it was proved that the synthetic montmorillonite is a monomineral sample and no differences from natural montmorillonite were observed.

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Zusammenfassung Das thermische Verhalten synthetischer Montmorillonite verhältnismässig einfacher chemischer Zusammensetzung wurde untersucht und gedeutet. Montmorillonit wurde durch hydrothermische Synthese bei 300°C und 8.8 MPa in 165 Stunden hergestellt. Durch die Methoden der DTA, DTG, TG, Röntgendiffraktionsanalyse, Infrarotspektroskopie und Elektronenmikroskopie wurde erwiesen, daß synthetische Montmorillonit eine monominerale Probe ist und keine Abweichungen von natürlichem Montmorillonit beobachtet werden konnten.

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. 165 300° 8.8 . , , , , , .

     

Study of phenolic resin/EVA blends by thermal analysis

The properties of polymeric blends originate from the synergistic association of their components. In this investigation, phenolic resins obtained by the reaction of cashew-nut shell liquid (CNSL) and aldehyde are used in several applications. Mixtures of CNSL with industrial reject ethylene-co-vinyl acetate (EVA reject) were prepared with an EVA reject content up to 70%. The thermal compatibility and stability were evaluated by means of thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC). For blends containing a high percentage of EVA reject, the TG curves clearly show two decomposition stages, one at 350C and the other at 450C (onset 467C). The DIG curves of the blend containing 70% CNSL exhibit decomposition at 240C. The DSC curves show that the samples containing a high percentage of EVA reject are incompatible, withT _g values around –30C.The authors would like to thank PETROBRAS/CENPES/DIQUIM for the NMR facilities and thermal measurements.     

Molecular structure of aniline in the gaseous phase: A concerted study by electron diffraction and ab initio molecular orbital calculations

The molecular structure of free aniline has been investigated by gas-phase electron diffraction and ab initio MO calculations at the HF and MP2 levels of theory, using the 6-31G^*(6D) basis set. Least-squares refinement of a model withC _s symmetry, with constraints from MP2 calculations, has led to an accurate determination of the C-C-C angle at theipso position of the benzene ring, =119.0±0.2 (where the uncertainty represents total error). This parameter provides information on the extent of the interaction between the nitrogen lone pair and the system of the benzene ring, and could not be determined accurately by microwave spectroscopy. The angles at theortho, meta, andpara positions of the ring are 120.3±0.1, 120.7±0.1, and 119.0±0.3, respectively. Important bond distances are r _g(C-C)=1.398±0.003 å andr _g(C-N) =1.407±0.003 å. The effective dihedral angle between the H-N-H plane and the ring plane, averaged over the large-amplitude inversion motion of the amino group, is ¦¦=44±4. The equilibrium dihedral angle is calculated to be 41.8 at the HF level and 43.6 at the MP2 level, in agreement with far-infrared spectroscopic information. The MO calculations predict that the differencer(C_ortho-C_meta) -r(C_ipso-C_ortho) is 0.008–0.009 å. They also indicate that the nitrogen atom is displaced from the ring plane, on the side opposite to the amino hydrogens. The displacement is 0.049 å at the HF level and 0.072 å at the MP2 level. The two calculations, however, yield very different patterns for the minute deviations from planarity of the ring carbons.     

Swelling equilibrium and preferential sorption of crosslinked poly(methyl methacrylate) in the mixture acetonitrile + n-butanol

Measurements of preferential sorption,( ₃), determined by refractometry, and swelling equilibrium, ₃ ^–1, of PMMA networks have been carried out in the cosolvent mixture MeCN+BuOH at 25 and 49 C. With an intermediate mixture composition, ₃ ^–1 passes through a maximum at both temperatures. At 25 C MeCN is preferentially adsorbed by the network over most of the composition range, but a small inversion is detected. At 49 C MeCN is preferentially adsorbed over all the composition range.The behavior of the system crosslinked PMMA/MeCN + BuOH is compared with the results obtained for solutions of linear PMMA in mixtures formed by the same two solvents, MeCN and BuOH.     

Messungen der dielektrischen Anisotropie im GHz-bereich

Zusammenfassung Es wird eine Methode beschrieben, die Anisotropie der DK an festen Proben im Mikrowellengebiet zu erfassen. Die zu untersuchende Probe wird auf eine 3/4-Quarzglasunterlage in ein Spannungsmaximum des elektrischen Feldes einesH ₁₁-Resonators gebracht. Durch die spezielle Wahl desTE ₁₁-Wellentyps kann die DK unter jedem Winkel zum elektrischen Feldvektor bestimmt werden. Die Reproduzierbarkeit ist ±1.10^–3, Wesentlich bei dieser neuentwickelten Meßmethode ist. daß das zu messende Material als dünne Platte, z. B. als verstreckte Folie vorliegen kann. Daher läßt sich DK-Anisotropie und optische Anisotropie auch an ein und derselben Probe messen.Messungen an Polystyrol zeigten erwartungsgemäß den Zusammenhang =2nn. Für das polare PMMA liegt eine Größenordnung höher. Die Anisotropie hängt bei derart verstreckten Polymeren von den Verstreckungsbedingungen in gleicher Weise wie Wärmeleitung und Doppelbrechung ab.Mit 11 AbbildungenVortrag, gehalten auf der Sitzung des Fachausschusses Physik der Polymeren auf der Regional-Tagung der Physikalischen Gesellschaft in Freudenstadt am 4. April 1967.Zugleich Auszug aus der Diplomarbeit vonH. W. Schulze.     

The kinetics and mechanism of aquation and base hydrolysis of the cis-[Co(cyclen)Cl2]+ cation (cyclen=1,4,7,10-tetraazacyclododecane)

The aquation of cis-[Co(cyclen)Cl2]+ (cyclen=1,4,7,10-tetraazacyclododecane) has been studied over a range of temperatures with 0.1 mol dm–3 HNO3 as solvent. At 25°C, kaq=4.5×10–3 s–1 with H=78 kJ mol–1 and S298=–21 J K–1 mol–1. Base hydrolysis of cis-[Co(cyclen)Cl2]+ is extremely rapid with kOH=2.1×107 dm3 mol–1 s–1 at 25°C and I=0.1 mol dm–3. This is the largest rate constant so far reported for the base hydrolysis of a cis-dichloro-complex of a saturated macrocycle. The activation parameters, H=53 kJ mol–1 and S298=73 J K–1 mol–1 are consistent with a mechanism in which deprotonation of the substrate is rate-determining. This conclusion is confirmed by the observation of general base catalysis by formate ion. The Brønsted value for the reaction is ca. 0.72 and SN1(CB) and E2 mechanisms are considered to account for the kinetic results. Base hydrolysis of cis-[Co(cyclen)(OH)Cl]+ has also been studied in the pH range 6.5 to 8.7. The value of kOH=3.8×102 dm3 mol–1 s–1 at 25°C and I=0.1 mol dm–3 with H=110 kJ mol–1 and S298=171 J K–1 mol–1 are consistent with an SN1(CB) mechanism.     

Growth transition and morphology change in polypropylene

The growth rate of isotactic polypropylene is deduced from microscopic observations during isothermal crystallizations. A change in the growth regime is observed at 138 C and interpreted as a Regime III Regime II transition, according to Hoffman's kinetic theory of polymer crystallization. A Regime II Regime I transition is also theoretically predicted at 155 C, i. e. at a temperature outside the investigated temperature range. The Regime III Regime II transition is related to the positive to negative change in the spherulite birefringence, which is generally attributed to a change in the organization of crystalline lamellae: quadritic arrays of intercrossing lamellae atT _c < 138 C (Regime III) and preferentially radiating lamellae atT _c > 138 C (Regime II). It is suggested that such a morphological change could be interpreted using the concept of non-adjacent re-entry introduced in Hoffman's kinetic theory. This interpretation could also explain the interspherulitic ruptures observed in negative spherulites.     

Thermal properties of polysaccharides at low moisture

Differential scanning calorimetry (DSC) has been used to probe ordered structures and glassing behaviour for a range of agars containing < 25% w/w water. Most commercial agars are supplied in an ordered (double-helical) state, show an endothermic helix-to-coil transition above 100C at low-moisture, and require 90–100C for solubilisation in excess water. Agars dried from the coil (single-chain) state show no corresponding endothermic transitions and only require a minimum of 45C for aqueous dissolution. Evidence from helix-to-coil transition enthalpies, equilibrium water content as a function of relative humidity, and solid-state¹³C NMR spectroscopy suggests that water molecules are associated enthalpically with double-helical agar. Single-chain agar is apparently not obtained in a glassy state by direct drying from solution, but in common with double-helical forms, exhibits rubber/glass transition behaviour following heating (in a DSC pan) to 180C.For part 1, see Ref. [19].We thank Liam Horgan and Ger Cooney (Quest International, Cork, Ireland) for supplying roller-dried and spray-dried agar samples, Arthur Darke for NMR measurements and Karen Smith for text processing.