摘要:

With a view to a study of fold configurations, the crystal and molecular structure of a large cyclic paraffin (CH2)34was determined by x‐ray diffraction. Three‐dimensional data were collected and the structure refined using a structure‐factor least‐squares computer program. The standard deviations in the final carbon atom coordinates were 0.02 Å. The configuration of the molecule was that of a ``collapsed'' ring, with two parallel linear chains linked by two terminal carbon atoms at either end. The chains packed together to give a triclinic subcell with dimensionsa=4.246  Å&agr;=60∘30′b=5.470  Å&bgr;=74∘08′c=2.564  Å&ggr;=96∘47′.Restraints imposed by the fold configuration gave rise to a departure from optimum packing of the chains at some distance from the fold. A rough estimate of the surface free energy of polymer faces containing similar folds was made.

ISSN:0021-8979    

DOI:10.1063/1.1709092

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

An electron diffraction study of thin (100–200 Å) vapor‐deposited films of poly‐p‐xylylene which had been annealed to the beta modification reveals that the majority of crystallites are very specifically oriented with (10·0) prism faces in the plane of the film. Since the polymer chains are aligned in thec‐axis direction [00·1], it follows that the molecules within oriented crystallites are lying in the film plane. A smaller number of crystallites are randomly oriented in these films. These conclusions are qualitatively extended on the basis of x‐ray diffraction data to include films as thick as 4 mil. This study corroborates the hexagonal assignment to the beta cell, witha=20.52 andc=6.581 Å. The level of orientation is essentially independent of the nature of the crystalline and amorphous substrates upon which the films were deposited, but is inversely proportional to the partial pressure of thep‐xylylene monomer in the vapor phase above the substrate.

ISSN:0021-8979    

DOI:10.1063/1.1709093

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The rheo‐optical properties of PVC homopolymer films, prepared by solvent casting, were studied by measuring length and birefringence changes during creep experiments at constant load. Both temperature and stress level were varied in these experiments; however, the effects of stress level were particularly emphasized. Temperature was varied in the range of 30°–140°C. An unusual phenomenon of stepwise extension was sometimes observed, particularly in the case of unannealed films. There seems to be a unique relation between birefringence and strain over a wide range of experimental conditions, and this relation is significantly different from that corresponding to a rubbery network. Cold drawing is observed at high stress levels. A pattern of curves very similar to that obtained for different temperatures at a constant load, is produced by varying the stress level at a constant temperature. However, the curves at different stress levels and different temperatures do not superpose when shifted vs log time. Optical‐creep compliance (birefringence/stress ratio) shows two stages of increase in creep experiments at different stress levels at a single temperature, indicating two birefringence mechanisms. The second mechanism appears to be associated with whitening of the sample. This second‐stage process is less clearly visible in the mechanical compliance curves. Annealing conditions seem to have very important effects on the film properties observed. A three‐dimensional isothermal surface can be used to describe the relation of strain to stress and log time conveniently. The linear relation between yield stress and temperature, which extrapolates to zero stress near the glass transition temperature, was investigated in more detail by use of these creep experiments. A stress‐temperature equivalence of 6.79 kg/cm2(or 6.66×106dyn/cm2) tensile stress per degree centigrade was obtained. From the results, it appears that the cold‐drawing phenomenon can be described as a stress‐induced glass transition. The glass transition must, therefore, be regarded not simply as a temperature, but as a phenomenon which is a function of temperature, stress level, and time.

ISSN:0021-8979    

DOI:10.1063/1.1709094

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Certain properties of crystalline polymers can be associated with the presence of local defects, e.g., deviations of short segments of the chain from the predominant conformation allied with the crystal structure. General methods are developed by which such chain defects may be simulated by computer techniques. The application of these methods to the polyethylene system gives detailed information concerning: stable defect structures, low‐energy conversion paths between stable structures (molecular motions), interactions in defect assemblies, and intermolecular interactions between chains and crystalline environments.Such an investigation of all fold structures containing seven (or less) dihedral angles indicates the existence of a small number of stable but immobile structures. The intramolecular energy of these structures is somewhat high and they do not pack efficiently. Expansion of fold structures to include eight and nine dihedral angles and slight relaxation of orientation constraints provides lower‐energy structures which are mobile, more efficient packing, and a low‐energy path for conversion between diagonal andb‐axis folds.In addition, stable structures and motions of other local defects (i.e., kinks and jogs) are identified, rigid segment rotation is found to be questionable, and certain mechanical properties are interpreted in terms of the over‐all results.

ISSN:0021-8979    

DOI:10.1063/1.1709095

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A mechanism is proposed for transverse slip on {100} ⟨010⟩ in polyethylene, in terms of the motion and multiplication of dislocations caused by chain ends. Some of these defects, the jogged dislocations, are able to move at stresses somewhat below the yield stress. However, the extent of transverse slip and of the resulting transverse textures is limited by chain folds and the requirement of chain continuity. Slip displacements parallel to the chain direction are then favored.The mismatch requirements for dislocations with Burgers vectors having components in the chain direction are determined. The Burgers vectors are ⟨0,b, nc⟩ on {100} and ⟨a, b, nc⟩ on {110}; the latter can dissociate into two types of partials with Burgers vectors ⟨a/2,b/2,nc⟩ or ⟨a/2,b/2, (n−½)c⟩, wheren=1, 2&cellip;. Such defects can act in pairs [u, v, w] and [u¯, v¯, w] producing displacements parallel to the chain direction.

ISSN:0021-8979    

DOI:10.1063/1.1709096

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Observations of the propagation of plastic band fronts in nonoriented Nylon 6–10 monofilaments have been made as a function of load and temperature, and analyzed in terms of dislocation dynamics. From the shapes and the velocities of the band fronts, dislocation velocities are estimated and found to have a stress dependence similar to that found for nonpolymeric crystalline materials. The measurements yield the following quantities: characteristic drag parameter (3×109dyn/cm2), dislocation density (105/cm2), dislocation multiplication coefficient (20/cm), length of propagating front (10−2cm), and activation energy for the dislocation motion (0.3 eV). The maximum strain required to initiate flow in Nylon is about 40%, independent of the load and strain rate, and in agreement with previous experiments of Milkowitz, and of Williams and Bender. Measurements by Williams and Bender for Nylon 66 have also been analyzed. From the stress dependence of creep incubation times, reasonable values for the mean dislocation velocity in Nylon 66, and the associated activation energy, have been obtained. It is tentatively concluded that the rate of breaking of hydrogen bonds limits dislocation mobility in Nylons 6–10 and 66. The glide system appears to be (100) [001] within the crystallites.

ISSN:0021-8979    

DOI:10.1063/1.1709097

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A striking change in coherence is obtained when single‐crystal platelets of linear polyethylene are recrystallized by annealing at temperatures between 120°C and the melting point, 137°C. Instead of showing micaceous cleavage, the sintered crystal mat is ductile and malleable in shear, yet still will not flow under compression.Annealing provides enough toughness to the crystal mat to allow a deformation which elongates it several‐fold. Hot calendering allows a much greater elongation than cold rolling and produces a tough, crackly, clear film. The sintered aggregate can be easily hand drawn hot to extremely high draw ratios, often above ×40. The product has very good tensile properties when tested as a fiber. A two‐way hot calendering operation produces a film with outstanding tear strength and poor puncture and impact strengths.The coherence caused by the annealing of the platelets is analogous to the coherence developed in powder metallurgy or in sintering of ceramic bodies. In polymer crystals, annealing has been shown to cause the molecules to refold into longer fold periods, leaving holes. These holes provide the growing room for the refolding of chains from adjacent platelets; a cooperative intermeshing of chains occurs. This creates one large crystal aggregate which has a homogeneous distribution of sites of weakness. It is proposed that many annealing treatments of bulk crystalline polymers cause this intermeshing to occur, and a new interpretation is at hand for the well‐known relationship of strength vs annealing temperature.

ISSN:0021-8979    

DOI:10.1063/1.1709098

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

During the first stage of plastic deformation before the neck formation the crystal lamellae are rotated into the position of maximum compliance to applied stress that causes the stress‐strain curve to drop to the plateau characteristic for deformation with neck propagation. The plastic‐flow pattern in the neck results in an asymmetry of strain momentum imposed on tie molecules which during annealing lets the lamellae on the sample surface rotate about an axis parallel to the surface and perpendicular to the draw direction. The temperature at which such a rotation starts increases with the draw ratio. Concurrently thecaxis moves away from the draw direction. The rotation of lamellae in the surface layer is nearly prevented by keeping the sample at constant length during annealing and in the interior of the sample by the surrounding lamellae even if the sample is free to shrink. The difference in long period which depends on the temperature of drawing in bulk samples and on the thickness of original crystals in extremely thin films is most probably a consequence of different geometry of heat dissipation in both cases.

ISSN:0021-8979    

DOI:10.1063/1.1709099

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Experiments have been carried out to characterize the geometry of the neck which forms during the cold drawing of poly (methyl methacrylate). Sequence photography was used in conjunction with tensile stress‐strain measurements. A series of tests were carried out at various temperatures, strain rates, and amounts of molecular preorientation to determine the effect of each of these variables on neck geometry. It was found that: (1) The neck geometry does not stabilize at the point where the load first reaches its steady‐state value, but at an elongation much beyond this point. (2) As the applied strain rate is increased, the steepness of the neck shoulder passes through a maximum. Further increase in the strain rate causes the neck to become more diffuse. (3) A similar maximum in neck sharpness is observed vs temperature. (4) Preorientation drastically reduces the tendency for yielding to become localized in a well‐defined neck. By combining neck‐profile photographs and load‐extension curves during drawing, it was possible to construct a series of ``true'' stress‐strain curves for poly (methyl methacrylate) at various temperatures, strain rates, and degrees of molecular preorientation, which should represent very closely the homogeneous stress‐strain characteristics of the material itself.

ISSN:0021-8979    

DOI:10.1063/1.1709100

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The fine‐scale structure that exists in polytetrafluoroethylene fibers produced by different methods was investigated by electron microscopy and electron diffraction. These fibers were formed by mechanically abrading the surface and by fracture of compacted granular grade PTFE, and by stripping the fibers from the surface of Teflon‐coated aluminum foil. Bright‐field electron microscope observations indicate that the fibers are composed of microfibers or fibrils with a nearly uniform diameter of 450 Å. These fibrils are composed of platelet‐like units 60 Å thick, oriented normal to the fiber axis, and are organized into blocks on the order of 475 Å in length. Electron diffraction from unshadowed individual fibers indicates that the crystal size must be fairly large, with few defects. These observations are well supported by the dark‐field micrographs of the fibers. The dark‐field photographs show crystalline regions along these fibrisl, which appear to be multiples of the fundamental 450‐Å fibrill diameter, with a range in lnegth of 160–7000 Å. Additional observations of the fibrils indicate that these molecular packets can slide past one another, either individually or in groups. Examples of slip or shear, bowing, rotation, and kinking of the platelet‐like structures have been observed.

ISSN:0021-8979    

DOI:10.1063/1.1709101

出版商:AIP    

年代:1967

数据来源: AIP