摘要:

AbstractThe advent of the homogeneous, termination‐free anionic polymerization systems has proven to be a valuable synthesis tool for the polymer scientist. The absence of a spontaneous termination reaction in these systems makes it feasible to form a variety of interesting polymeric materials, i.e., linear, block polymers, star‐shaped polymers, and well‐defined graft polymers, as well as chains garnished at the end (or ends) with reactive groups. Recently linear block polymers of styrene and butadiene have achieved commercial importance. This review covers the synthesis methods of these block polymers and touches briefly on the formation of other block polymers by diverse synthesis r

ISSN:0449-2994    

DOI:10.1002/polc.5070260103

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractAn important new class of polymers, the thermoplastic elastomers, was announced by the Shell Chemical Company (U.S.A.) in 1965. These new products may be formed into useful articles by modern rapid thermoplastics processing techniques, such as injection molding, and without any chemical vulcanization step provide most of the useful physical properties of vulcanized rubber. High resilience, high tensile strength, highly reversible elongation, and abrasion resistance are obtained. The thermoplastic elastomers consist of ordered, block copolymers of the general structure A‐B‐A, where A is a thermoplastic block polymer and B is an elastomeric block polymer. Choice of monomers, block length, and the weight fractions of A and B are crucial in achieving elastomeric performance. An example is the polystyrene–polybutadiene–polystyrene block copolymer (S‐B‐S) where the molecular weights of S and B and the weight fraction of S are restricted. A two‐phase system is formed, with the middle‐block phase constituting a continuous three‐dimensional elastomeric network and the dispersed end‐block phase serving as multijunction points for the ends of the middle blocks. These systems, without vulcanization, have rubber‐like properties similar to those of conventional rubber vulcanizates but flow as thermoplastics at temperatures above the glass transition of the end block. The behavior is fully temperature reversible. Melt viscosity behavior, measured as a function of shear and temperature, is similar to that of conventional thermoplastics. Activation energies obtained at constant shear stress vary with temperature: at high temperatures they are between those of the pure homopolymers and at low temperatures they approach that of the thermoplastic part of the molecule. Melt viscosities, however, are very much higher than those of either homopolymer of the same total molecular weight. An additional energy term is indicated in the flow process which arises from the transfer of the end block from one aggregate to another, in the process being forced to pass through the elastomeric matrix with which it is thermodynamically incompatible. The classical kinetic theory of rubber elasticity can be applied to these polymers, treating the end‐block phase as hard discrete particles which do not contribute to the elastic network For example, equilibrium modulus or swelling measurements are used to calculate the concentration of elastically effective chains or effective crosslink density. The resulting effective elastic chain length (Mc) is thus identified, not with the middle‐block segment length, but with the normal entanglement length. Hence, normal entanglement junctions in the elastomeric matrix behave as strong effective crosslinks because the ends of the middle block are securely anchored in the end‐block aggregates. High tensile strengths, in the absence of reinforcing fillers or crystallization, may be attributed to a highly perfected network (Case theory) or to the inertial masses of the discrete end‐block aggregates (Bueche theory), or to both. The physical properties of the thermoplastic elastomers over a range

ISSN:0449-2994    

DOI:10.1002/polc.5070260104

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

ISSN:0449-2994    

DOI:10.1002/polc.5070260105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractMicrophase separation occurs in many block copolymers to give domain structures. In this first paper in a series dealing with domain formation and the consequences thereof, a theory is presented for the formation of spherical domains in A‐B block copolymers. The theory establishes criteria for the formation of domains and their size in terms of molecular and thermodynamic variables. It is shown that the considerable loss in configurational entropy due to the constraints on the spacial placement of chains in a domain structure requires that the critical block molecular weights required for domain formation are many‐fold greater than required for phase separation of a simple mixture of the component blocks. The relation between domain radius R and molecular dimensions is obtained from the requirement that space in the domain must be filled with a constant density of segments. Segment densities are evaluated from solutions of the diffusion equation, treating the constraints on chain placement as boundary value problems. This gives the relationship R = 4/31/2, where1/2is the root‐mean‐square end‐to‐end chain length. Because of chain perturbations in a domain system,1/2is larger than the unperturbed value01/2normally expected for bulk polymers. A means to evaluate the perturbations is shown. The agreement between the predictions of the present theory and the limited published experimental information appears quite s

ISSN:0449-2994    

DOI:10.1002/polc.5070260106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractA study has been carried out on block polymers of the A‐B‐A type, the thermoplastic elastomers, where A represents polystyrene and B polybutadiene or polyisoprene. The objective was to relate the mechanical properties of these elastomers to their molecular architecture. For this purpose a series of styrene‐butadiene and styrene‐isoprene block polymers were synthesized by means of organolithium initiators, in which the block lengths of the polystyrene and polydiene were varied. It was found that the stress‐strain properties of styrene‐butadiene polymers are mainly dependent on the polystyrene content, regardless of the block sizes, and that the center, elastic block does not appear to behave as the “molecular weight between crosslinks” of these networks. However, the monodispersity of the chain length of these elastic blocks does appear to contribute substantially to higher tensile strengths. The polystyrene appears to be aggregated in small domains, of the order of several hundred angstrom units, and these undergo an irreversible deformation under stress, which is, however, completely recoverable above the Tgof the polystyrene. At high polystyrene contents (∼40%) these elastomers exhibit an irreversible yield point at very low elongations, and this is ascribed to the presence of a continuous phase of connected polystyrene domains, which can be re‐formed by raising the temperature above the Tgof the polystyrene. Although the stress‐strain curves are not affected by the thermal history of the sample, the tensile strength, especially at high styrene contents, is strongly dependent on “annealing” of any frozen stresses in the polystyrene phase. The useful range of block molecular weights is about 10,000–20,000 for polystyrene and 40,000–80,000, respectively, for polybutadiene. The lower limit is probably governed by the minimum polystyrene chain length required to insure the formation of a heterogeneous phase; while the upper limit is set by the high viscosity of both blocks, which might seri

ISSN:0449-2994    

DOI:10.1002/polc.5070260107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractThe morphology of solution cast films of butadiene and styrene block polymers in both stretched and unstretched state has been studied by electron microscopy, and has been related to the mechanical behavior of these materials. These micrographs confirm the indications from dynamic properties that this material consists of a two‐phase system in which the relationship between the phases is sensitive to the solvent system used in casting. Although the basic morphological unit is a polystyrene sphere of 100 Å diameter dispersed in a matrix of polybutadiene, the interaction between spheres is complex. These spheres are more deformable than would be predicted from their glass transition temperature. The dynamic mechanical properties, stress‐strain properties, and stress relaxation properties of block polymers of styrene and dienes can be explained in terms of the morphology and changes of morphology on stretching in the electron micros

ISSN:0449-2994    

DOI:10.1002/polc.5070260108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractUntil recently it has been believed that either stress‐crystallization or the presence of reinforcing filler is necessary for the development of high strength in elastomers. Now it has been observed that gum vulcanizates of isoprene/acrylonitrile copolymers have tensile strengths in the order of 3000–4000 psi in the absence of fillers. In this case crystallization does not occur and the strength is associated with the ability of the polymer molecules to become highly oriented on stretching. Similar observations have been made with ABA block copolymers polystyrene‐polybutadiene‐polystyrene which are highly elastomeric in the absence of chemical crosslinks. The elastomeric properties and high strength of the block copolymers are attributed to the mutual incompatibility of the A and B blocks which leads to phase separation on a microscopic scale. The effective network structure can be temporarily destroyed by heating the polymer above the softening temperature of the resinous blocks which makes the material readily processable. The initial decrease in retractive force with increasing temperature up to 60–70°C is attributed to an increase of mobility of the resinous domains in the rubbery matrix. At higher temperatures the crosslinking effectiveness decreases markedly due to softening of the resinous regions. The process of yielding, hysteresis effects, and delayed elastic creep arise because of viscous forces which oppose the motions of the colloidal polystyrene domains. The strength and effectiveness of physical crosslinks in block copolymers shows a similar temperature dependence as has been observed for other filled and unfilled vulcanizates. The process of self‐reinforcement in isoprene‐acrylonitrile copolymers and polystyrene‐polybutadiene‐polystyrene block copolymers is attributed to viscous effects which make possible the alignment of a large number of polymer chains in the st

ISSN:0449-2994    

DOI:10.1002/polc.5070260109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractThe stress‐strain‐optical properties of three elastomeric styrene‐butadiene block copolymers containing 31, 40, and 49 wt‐% styrene were studied as a function of temperature. The mechanical and the optical properties indicate that these materials are two‐phase systems in which the polybutadiene chains form an elastomeric phase and the polystyrene a glassy phase with the latter providing physical crosslinks. Birefringence measurements indicate that the decrease in modulus and strength of these materials is associated with a decrease in the concentration of elastically effective network chains. The independence of the stressoptical coefficient on temperature suggests that the decrease in concentration of elastically effective chains is not due to the onset of rubber‐like behavior of flow within the polystyrene regions themselves, at least for temperatures below about 70°C. Rather, the decrease seems to be associated with the increased mobility of the polybutadiene chains at higher temperatures which also leads to an increase in the rate of stress relaxation. Birefringence measured during extension and retraction showed that the stress‐strain hysteresis is due to restricted mobility of the polybutadiene chain segments rather than to permanent viscous flow or to a change in the effective network structure of the block copolymers. The ultimate properties of these rubbers correlated well with the effective network structure in unde

ISSN:0449-2994    

DOI:10.1002/polc.5070260110

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

ISSN:0449-2994    

DOI:10.1002/polc.5070260111

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY

摘要:

AbstractA study was made of the stress–strain and ultimate properties in simple tension of an elastomeric styrene‐butadiene‐styrene block copolymer (Kraton 101) and also of a similar material (Thermolastic 226) that contains about 35% plasticizer as well as inorganic pigments. Stress–strain data were obtained at crosshead speeds from 0.02 to 20 in./min at temperatures from ‐40 to 60°C. The relaxation rate, derived from the data at constant extension rates, was about 8% per decade of time for both materials at temperatures from −40 to about 40°C and at extensions from about 20% up to 400%. Above −30°C, the shift factor log aTwas found to vary linearly with temperature. These findings indicate that the time and temperature dependence of the mechanical properties results primarily from the plastic (or viscoelastic) characteristics of the styrene domains. The tensile strength for Kraton 101 below 40°C is somewhat greater than 4000 psi, sensibly independent of extension rate and temperature. For the highly plasticized Thermolastic 226, the tensile strength at an extension rate of 1.0 min−1increases from 2200 psi at 0°C to 3600 psi at −40°C. Above 40°C for Kraton 101 and above 0°C for Thermolastic 226, the tensile strengths are quite dependent on extension rate and temperature owing to the increased ductility of the styrene domains. The high strength of these materials results from the uniformly dispersed styrene domains of colloidal dimensions. To obtain a crack of sufficient size to satisfy an energetic criterion for self‐sustained high‐speed propagation, domains must be disrupted. The plastic characteristics of the domains have a controlling effect on crack growth and thus on the ultimate properties of the materials. The strength and extensibility of other elastomers are considered in relation to

ISSN:0449-2994    

DOI:10.1002/polc.5070260112

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1969

数据来源: WILEY