摘要:

AbstractRules for equilibrium phase behaviour, laid down long ago on the basis of the two laws of thermodynamics, appear to be valid also for many polymer systems in which aggregates of various topology occur. The phase rule offers a useful tool for the construction and understanding of phase diagrams for systems in which order of some kind is prevalent in certain ranges in the diagram. ‘Double sickle’ type phase diagrams, found for surfactant solutions, microemulsions as well as block‐copolymer solutions can be traced back to the considerable loss of translational entropy which accompanies aggreg

ISSN:0258-0322    

DOI:10.1002/masy.19900390103

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractConsequences for classification, structure and properties are assessed in the simplest case of an uncrystallizable chemically unspecific polymer, atactic polystyrene (a‐PS), for these studies, in closely monodisperse form. The mechanism first proposed by Berghmans and collaborators, namely liquid‐liquid phase separation as intercepted by glass transition was reaffirmed, followed up by the establishment of the character of the connectivity required for the still disperse phase‐segregated and vitrified system to become a gel. These connectivity types, leading to three distinctly different gel species were identified according to whether connectivity is through (i) individual solvated molecular chains, (ii) continuous glassy phase, (iii) adhesive contact of the glassy phase. (i) corresponds to the familiar gel consistency of swollen rubber, (ii) to porous glass or foam (closed or open), or to glassy network, (iii) to the colloidal gel of loosely packed spheres forming ramifying aggregates. The above generic definition widens the convential concept of a gel creating a unifying umbrella with predictive potential for material design.The existence of the above three connectivity types was confirmed morphologically. The origin of their existence and their delineation could be attributed to the combined concentration and molecular weight dependence of the solution which was to undergo phase separation. In particular, for the higher molecular weights (M), type (i) arises in the most dilute solutions, with overlap conditions following the c* criterion, but for a somewhat larger radius of gyration than usual for c*(c* = critical concentration).For the highest M‐s, the limiting solution concentration increasingly departs from the above: the corresponding gel type now becomes as for type (iii).For the highest M‐s the change‐over from gel type (i) to (ii) occurs at highly asymmetric phase ratios, corresponding to very small amounts of concentrated phase giving rise to interpenetrating networks (IPN). It was shown by combination of light scattering and morphological tests that this corresponds to passing from a nucleated to spinodal phase segregation mechanism, revealing (by morphological means) a very sharply defined boundary between the two.The different gel types had widely differing consistencies: (i) rubbery, (ii) glassy, fragile for IPN, robust for foams and porous glass, (iii) paste like. (i) and (ii) were characterised in terms of G'(storeage) modulus extrapolated to zero strains and frequency, confirming the consistency tests, with a dramatic transition between the two.Beyond the issue of gelation itself that of treating two‐fluid separation, as interrupted by glass transition, in terms of equilibrium thermodynamics has been raised. For this a conceptual framework has been laid down together with some preliminary experiments on delayed, incomplete phase separation in the glass. We believe the above to be of wider relevance to the glassy state and phase studi

ISSN:0258-0322    

DOI:10.1002/masy.19900390104

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractCollegial dispersion forces originate from a coordination of the temporary electronic dipoles of a series of molecules. To insure this coordination the nuclei on a row have to replace their individual vibrations by coupled ones. Such row is then rigid. For polymers collegial forces can already be temporarily active in given parts of a solution but are regularly annihilated by intrusion of the solvent. However the regular passage through the rigid state modifies the distribution of the dipole moments during the interval that solvent or ligand molecules are in contact with the segment. In a rigid domain the correlation parameter of Kirkwood g is zero. The existence of such rigidity in solution is demonstrated by the low experimental value of gmAfor the dipoles of the complexes between poly(ethyl methacrylate) and phenols. Due to the large number of segments that separate the complexes the classical explanation of stiffness of the chain due to individual interactions between neighbours does not hold.

ISSN:0258-0322    

DOI:10.1002/masy.19900390105

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThe thermoreversible gelation of solutions of isotactic poly(methyl methacrylate) is investigated. Amorphous gels can be prepared in l‐butanol by a combination of a liquid‐liquid demixing with an upper critical demixing temperature and a glass transition. Annealing of the demixed solutions above their glass transition temperature TG, results in the formation of a crystalline gel. In oxylene, crystalline gels are formed by a liquid‐liquid demixing with an lower critical demixing temperature and an annealing at room temperature. Very fast gelation is observed to occur far below room temperature in solvents like 2‐b

ISSN:0258-0322    

DOI:10.1002/masy.19900390106

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractThree groups of problems concerning ordered structures in solutions of stereoregular poly(methyl meth‐acrylates) (PMMA) were studied by spectroscopic methods: (i) Self‐aggregation of syndiotactic (s) PMMA in solution was followed by means of NMR and IR spectroscopy; it has been established that double helices are formed during the initial stage of aggregation, and activation and equilibrium thermodynamic parameters (ΔH, ΔS) which characterize the transition coil–double helix in s‐PMMA were determined. (ii) NMR spectroscopy has demonstrated that association in solutions of atactic PMMA proceeds via interactions of stereocomplex type (interaction between m‐diads and r‐tetrads). (iii) It has been found that13C CP/MAS NMR spectra of crystalline forms of PMMA (prepared from aggregated solutions by solvent evaporation) conform to the double‐helix structure o

ISSN:0258-0322    

DOI:10.1002/masy.19900390107

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractFrom studies on dilute and semi‐dilute solutions it will be shown that gelation in atactic polystyrene proceeds via the creation of a nematic‐like order (formation of a polymer‐solvent intercalate with the highly syndiotactic sequences). A similar mechanism holds for isotactic polystyrene (intercalate wherein the chain adopts a near‐31helical form). It will be emphasized that the gelation phenomenon depends upon both the solvent size and the volume of the cavities created by the phenyl groups along the chain. Finally, diffraction experiments on well‐organized structures (giant spherolites of 150–200/μm diameter) grown from isotactic polystyrene solutions and possessing the same melting point as the gel show the absence of 121helices. Therefore, gel formation is not due to the growth of this

ISSN:0258-0322    

DOI:10.1002/masy.19900390108

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractWith the aid of the Boltzmann superposition principle, the infinitesimal flow behaviour of a substance can be described in terms of a memory integral. The memory function in this integral can be expanded in a set of relaxation times and relaxation strengths: the relaxation spectrum. On basis of a specific form of the fluctuation‐dissipation theorem, a physical meaning is proposed for the relaxation spectrum. The theory of complex functions enables one to show that the relaxation spectrum, determined from dynamic mechanical measurements, has a unique meaning. The infinitesimal flow behaviour, expressed in terms of the relaxation spectrum, is used to develop a general description of the phenomenon of thermoreversible gelation. This description method was applied to a styrene‐isoprene‐styrene triblock copolymer sol

ISSN:0258-0322    

DOI:10.1002/masy.19900390109

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractA gel was obtained from reacting a α, ω–hydroxy‐terminated polyester oligomer and a diisocyanate in styrene. Two types of morphologies were possible which are dependent on the quantity of catalyst. The rheological experiments led to the discovery that the “gel point” was not well defined and that the gel formation was a gradual process. A certain chain length was necessary for the appearance of the gel properties. The physical cross‐links could be destroyed an

ISSN:0258-0322    

DOI:10.1002/masy.19900390110

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractWe describe phase separations seen in the poly (γ‐benzyl α, L‐glutamate) (PBLG)/benzyl alcohol (BA) system. At temperatures below about 60°C, this system gels. Possible explanations of the system's rigidity include both crystallisation and spinodal decomposition. Recent work on atactic polystyrene (a‐PS) gels has suggested these are the result of a phase separation arrested by vitrification. An analogy between a‐PS and PBLG gels is suggested whereby crystallisation (in the latter) plays the rǒle of vitrification in

ISSN:0258-0322    

DOI:10.1002/masy.19900390111

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY

摘要:

AbstractEvidence that ligand‐induced de‐gelation of Cr(+3)/polyacrylamide (PAAm) gels involves simple extraction of Cr(+3) from the gel matrix has been obtained from rheological studies. Kinetic experiments provide evidence that de‐gelation involves dissociative ligand exchange at Cr(+3). On the basis of de‐gelation results acetate is suggested to be a poor thermodynamic model for the bonding of PAAm to Cr(+3) and it is hypothesized that Cr(+3)/PAAm complexation may involve bonding to both a polymer carboxylate and a neighboring amid

ISSN:0258-0322    

DOI:10.1002/masy.19900390112

出版商:Hüthig&Wepf Verlag    

年代:1990

数据来源: WILEY