A method was developed to measure the active centers in a titanium tetrachloridealkylaluminum catalyst system for the polymerization of ethylene. The method depends on the assertion that the active carbon atom of the growing chain is relatively basic and will react with alcohol‐O‐t. The tritium becomes incorporated in the polymer. The relationship used is N = AG/a, where N is the corrected active center concentration, A is the radioactivity of the polyethylene in counts/min./g., G is the polymer concentration at the time of sampling and a is the specific activity of the quench methanol‐O‐t in counts/min./mole. The data were corrected for the kH/kT isotope effect of 3.7 which was measured for the alcohol‐O‐t reaction. This isotope effect was unchanged over a wide range of molecular weights and reaction conditions. The basicity of the carbon on the growing polymer chain was proved by quenching the reaction with C14‐labelled alcohol. The resulting polymer was not radioactive. The values of N could be correlated with polymerization rates. Values of the active center concentration and polymer chain lifetime were calculated as a function of molecular weight using fractionated polymer. The centers are very heterogeneous and the lifetimes of polymer chains range from 1 to 30 min. (with an average of 4 min. for the particular conditions used). Calculated values of viscosity‐ (M̄v) and number‐average molecular weights as a function of time of reaction agreed well with the experimental data. Because of the long lifetime of some chains, a steady state in M̄v is not reached, and M̄v increases throughout the course of the reaction. Catalyst efficiency was calculated from a knowledge of N. The efficiency, which can vary from 0.3 to 15% based on the titanium, is a function of how the catalyst components are brought together. The value of N in these systems is about 10−3 to 10−4 moles/liter compared to 10−7 to 10−9 for the concentration of active species in free radical systems. The value of the propagation constant, kp, is comparable in both systems. It is concluded that the very rapid polymerization rate in these heterogeneous systems is due solely to high values of N. These high values of N argue against bimolecular termination of centers. The stability of the centers when refluxed under argon support this idea and cast doubt on chain termination via hydride ion transfer or transfer to solvent. The value for the activation energy of kp is 11.8 over the range of 40 to 68°C. The value of the activation energy for initiation of centers is deduced to be 11.8 kcal./mole, also.
This paper addresses the uniaxial compressive failure of unidirectional, fiber-reinforced polymer matrix composites. The material that was used in this combined experimental/numerical effort was APC-2/AS4. Failure was found to result in kink bands of distinct widths oriented at an angle to the line of loading. Kink bands were preserved by conducting a series of experiments where failure was confined. Fiber misalignments in the prepreg and from the lamination process were identified as the imperfections that, in conjunction with the nonlinear shear response of the matrix material, essentially govern the formation of zones of localized bending. This, in turn, causes fibers to break and into distinct kink bands. The process was modeled by considering a micro-section of material with wavy fibers under compression. The formulation accounted for material and geometric nonlinearities and various distributions of waviness. It was illustrated that failure is associated with a limit load instability, followed by localized fiber bending which leads to kinking.
SynopsisOne-to-one block polymers of styrene and acrylonitrile were prepared by a two step process: ( 1 ) styrene was polymerized in the presence of a dialkylphosphine and ( 2 ) the polystyrene was used to initiate the polymerization of acrylonitrile. The blocks contained 14-76% acrylonitrile and were 60-100% pure as judged by extraction with toluene a t 110°C. Phase and electron microscopy showed that the block polymers consisted of a continuous polystyrene phase when the acrylonitrile content was less than 70y0 by weight and of a continuous acrylonitrile phase when the acrylonitrile content was above 70% by weight. The polyacrylonitrile dispersed phase was regular and almost spherical. Its measured size equaled the calculated size, assuming that the spheres had a diameter of twice the length of the fully extended polyacrylonitrile chains. Polyblends of homopolyniers were similar to the block polymers in structure, except that the dispersed phase was very irregular in size and shape. The physical properties of blocks and polyblends were similar and were inferior to those of polystyrene homopolymers and styreneacrylonitrile random copolymer except for modulus and for maintenance of modulus over a temperature range. The random copolymer had the highest strength properties, which fact was attributed to the absence of a two phase system and the presence of psuedc+crosslinks. The relatively poor properties of the blocks and polyblends were ascribed to the inability of bulk polyacrylonitrile to absorb energy and to the buildup of stress concentrations a t the phase boundaries.The properties of polymers can be controlled by synthesizing molecules in which the monomer units are arranged in a specific manner. For copolymers, one type of arrangement may involve the number of A and B units which are present in the sequences and, also, the distribution of these sequences. The random placements of A and B follow from the wellknown copolymer equation. The nonrandom placements have been the subject of much recent work, most of which is concerned with establishing the existence of block polymers and the differences in solution properties between blocks and mixtures of homopolymers. 2-5 Only scanty information has been published comparing the mechanical properties of blocks with those of polyblends or iandom copolymers. For alkyd-type blocks, improvements over random polymers in toughness and properties a t high temperatures were indicated.69' For blocks of acrylonitrile and methyl methacrylate, a higher glass transition temperature than for the random
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