Showing papers by "3M published in 1962"

Polymer NMR spectroscopy. VI. Methyl methacrylate–styrene and methyl methacrylate–α‐methylstyrene copolymers

Abstract: The NMR spectra of methyl methacrylate–styrene and methyl methacrylate–α-methylstyrene copolymers exhibit, among other features of interest, an unexpected multiplicity in the 6.4–7.8 τ region, characteristic of methoxyl protons. This multiplicity is believed due to magnetic shielding by styrene units. By comparing the results of a statistical analysis of monomer sequence probabilities with the relative areas of the methoxyl peaks, it is shown that the degree of shielding of the methoxyl protons is dependent not only upon the presence of styrene units as nearest neighbors along the chain but also upon their stereochemical configuration with respect to the methyl methacrylate units. By means of reasonable assumptions it is possible to explain the copolymer spectra and to give a partial description of the stereochemical configuration of the methyl methacrylate–styrene copolymers. The NMR method should be valuable in the analysis of other copolymers, particularly block and graft copolymers.

Reproduction of television signals from photographic disc recordings

Abstract: 6. In combination in a system for reproducing .Iadd.video .Iaddend.information from a rotatable medium where the medium has characteristics at progressive positions along a spiral path for modifying energy directed at the medium in representation of .Iadd.video .Iaddend.information previously recorded on the medium where the information recorded on the medium at each successive position along the spiral path represents substantially less than a complete visual image, means operatively coupled to the medium for producing a continuous rotation of the medium, pickup means disposed relative to the medium including, means for directing .[.energy.]. .Iadd.a beam of energy having a radius not greater than approximately one micron to enable the reproduction of the video information .Iaddend.at the medium at each instant to obtain a modification of the energy passing from the medium at that instant in accordance with the variations in the characteristics of the medium at the progressive positions along the spiral path, and means responsive to the modified energy at each instant for producing signals having characteristics representative at that instant of the information recorded at the progressive positions along the spiral path on the medium, .[.and.]. means operatively coupled to the pickup means for varying in synchronism at each instant the position of the means for directing energy and the means responsive to the modified energy in a direction having a radial component to obtain a .Iadd.coarse .Iaddend.tracking at each instant of the characteristics at the progressive positions along the spiral path on the medium.Iadd., and means for varying in synchronism at each instant the position of the signal producing means relative to the energy directing means to obtain a fine tracking along the spiral path on the medium. .Iaddend.

The chemistry of xylylenes. XIII. The problem of crystallinity and crosslinking in poly(p‐xylylene)

Abstract: The problem of crystallinity and crosslinking in poly(p-xylylene) was re-investigated with the use of polymer that was prepared by polymerization of p-xylylene in solution. It was found that amorphous polymer is converted irreversibly to α-crystalline polymer at 50–140°C. The α-form in turn is converted irreversibly to the β-form at 220–260°C. The rate of change and the ultimate proportion of change is dependent upon the temperature. The polymer appears to soften and reharden at 220–250°C. and subsequently to melt at 410–420°C. This is followed by rapid thermal degradation. It was shown that the properties associated with a three-dimensional network are imparted to poly(p-xylylene) by reactions that occur when the mixture of insoluble polymer and residual monomer (<0.01M) is warmed from −78°C. to room temperature. This result appears to be independent of reactions that commonly afford covalent crosslinks. On the other hand, there is a close correlation with the experimental conditions that afford low molecular weight cyclic poly(p-xylylene). It is spectulated that some cycllization might occur in such a way that two or more chains of poly(p-xylylene) from an interlaced structure which might be responsible for the properties of poly(p-xylyene) heretofore attributed to covalent crosslinks.

The chemistry of xylylenes. XIV. The moldability and thermal stability of poly(p‐xylylene) and related polymers

Abstract: Freshly prepared α-poly(p-xylylene) softens at 220–260°C. and then rehardens as it forms β-poly(p-xylylene). Consequently, homogeneous objects can be fabricated at this temperature range by simultaneous application of heat and pressure. This “moldable” poly(p-xylylene), however, becomes nonmoldable within a week if allowed to age at room temperature. It is speculated that this change in moldability with aging at room temperature may be due to formation of tiny areas of crystalline α-polymer which serve as seeds for rapid conversion from α- to β-polymer before melt-flow can occur in the critical temperature range of 220–260°C. of the heating cycle. The shelf life of the moldable polymer is increased considerably, however, by polymerization of the monomer in the presence of a molar equivalent of trialkylphosphines of trialkylphosphites. In vacuum, poly(p-xylylene) has excellent thermal stability below 400°C., but in air, oxidative degradation is serious even at about 225°C. The moldability of p-xylylene polymers is improved by copolymerization with a variety of olefinic and inorganic molecules, but the thermal stability, thermal oxidative stability, and sometimes the electrical properties of the resultant product are inferior to those of unmodified poly(p-xylylene). These polymers appear to be resistant to degradation by γ-irradiation.

Trifluoronitrosomethane/tetrafluoroethylene copolymer: Degradation by heat and by radiation

Abstract: Thermal degradation of trifluoronitrosomethane/tetrafluoroethylene copolymer yields COF2 and CF3NCF2 as gaseous decomposition products. The copolymer weight loss appears first order in copolymer. The activation energy is 58 kcal./mole, and the first-order rate constant is approximately k1 (min.−1) = 3.1 ′ 1021 exp {-29,047/T} determined in the 250–265°C. range. A rather large weight fraction of low molecular weight species exists in some nonfractionated copolymer samples leading to rapid initial weight losses upon heating. Thermogravimetric and differential thermal analyses were used to observe a thermal runaway decomposition at temperatures above 270°C. Gamma radiations from spent atomic fuel elements produce random scissions in the copolymer chains followed by COF2 and CF3NCF2 evolution. At 23°C. the total energy absorption per primary chain scission is Ed = 44 e.v./scission. For each chain scission an average of approximately 5.3CF2N(CF3)OCF2 units decompose to COF2 and CF3NCF2. In systems containing these product gases at appreciable pressure an adduct, (CF3)2NCOF, is produced. Ultraviolet light of 2537 A. wavelength photolyzes the copolymer producing random chain scissions and the gases COF2 and CF3NCF2. The quantum yield for primary random chain scission at 23°C. is ϕd = 0.91 × 10−3 scissions/absorbed photon; the equivalent total energy absorption per event is Ed = 5400 e.v./scission. An average of approximately 3.6 CF2N(CF3)OCF2 units decompose to COF2 and CF3NCF2 for each chain scission.

Curable plastic composition

Abstract: 10. A stable unitary liquid polymer composition protected from moisture and capable of being shipped and stored in a single container and later being deposited in place without agitation or intermixing with other material for complete curing by contact of its surface with surroundings containing essentially only moisture, said composition comprising essentially (a) a liquid polyalkylene polysulfide polymer having thoroughly dispersed therein, (b) a dormant curing agent in an amount sufficient to cure said polymer which is activated by the presence of moisture, and (c) an alkaline desiccating deliquescent accelerating agent adapted and sufficient to maintain said polymer in dry condition during shipment and storage and to attract and absorb moisture from its surroundings after deposition in place to hasten the curing of said polymer by said curing agent, wherein (b) and (c) are different substances and said accelerating agent is watersoluble and composed of at least one number of the group consisting of the alkaline earth metal oxides and hydroxides.

Foveal Vision: Dependence of Threshold Energy on the Visual Angle of a Circular Target*

Abstract: It is shown that circular nonmoving targets, detected at threshold, which appear as a “point source” for low values of the subtended visual angle (target diameter), pass smoothly into a subjective annular shape for larger diameters. The annulus is the locale of the luminance gradient and therefore provides the significant visual information. The annulus width is 0.9 min of arc when the adapting luminance B is 102 ft-L and rises to 1.3 min as B falls to 10−3 ft-L. Because of this continuous transition from a point source to an annulus, the “critical visual angle” is at best only an approximation.Over a range of B from 10−3 to 102 ft-L and a duration of stimulus from 10−2 to 1 sec the threshold energy for a point source rises 140 fold. The corresponding rise in the threshold energy per sq min of annulus area is about 1600 fold.The trends in threshold energy and in threshold contrast with increasing target size are believed to measure some of the effects of retinal (or neural) interaction and inhibition in the human fovea.

Recent studies of treeing breakdown

Abstract: Trees can propagate in epoxy resins at stresses below 100 vpm. Resistance to treeing probably can not be correlated with other commonly measured breakdown properties. Continuing study is necessary to determine (a) the service conditions in which treeing is an important consideration, and (b) the range of test conditions which reliably rate this characteristic of materials.