Crystallite size in drawn and then annealed polypropylene

Crystallite sizes have been obtained from the breadth of equatorial x-ray reflections from polypropylene samples subjected to a draw ratio of 6 at 21°C and then annealed at 155°C, 140°C, and 120°C, respectively. For all samples it was found that the ratio of the dimension of the crystallite perpendicular to the {110} planes to that perpendicular to the (040) plane is a constant. The ratio of the lateral crystallite size to the meridional long period was also found to be constant, independent of annealing temperature. In contrast, the thickness of the crystallites in the direction parallel to the draw direction, as calculated from the meridonal long period and density data, was not proportional to the lateral crystallite size.     

Plastic deformation of polypropylene VII. Long period as function of temperature and rate of drawing

Polypropylene fibers prepared by quenching in ice-water were drawn at 25, 80, 120, and 140°C to a draw ratio between 6 and 8 at draw rates 0.05, 0.5, 5, and 50 cm/min. The long period increases almost linearly with the draw rate for drawing at 25°C and decreases for drawing at higher temperatures. The effect in the latter cases is an annealing effect. As a consequence of the shorter exposure of the drawn fibers to the high temperature at higher draw rate, the long-period growth proceeds for a shorter time and hence results in a smaller increase of long period. At 25°C, however, the long-period growth is negligible. The increase of long period with draw rate is the consequence of higher adiabatic heating as calculated from the energy input during the plastic deformation which transforms the spherulitic into the fibrous structure. One concludes that the long period established during this transformation depends on the maximum temperature reached in the micronecking zone and not on the macroscopically observed temperature of the sample in the neck.     

NMR observations of drawn polymers. V. Sorption into drawn and undrawn polyethylene

NMR measurements on undrawn polyethylene (PE) samples in contact with a solvent such as C₂Cl₄ indicate an increase in the mobility of the mobile chain segments as compared to dry samples. Highly drawn PE shows no such effect. This is because S_a, the sorption per unit mass of noncrystalline material present, decreases from 20.9 wt.-% (dry basis), found for undrawn quenched PE, to 0.63 wt.-% after drawing (S_a determined at 25°C. and 0.80 vapor activity). Drawing also reduces the segment mobility according to the NMR spectrum. It is shown that these effects are caused by considerable structural changes occurring in the noncrystalline regions of PE upon drawing. Annealing of drawn PE samples at successively higher temperatures leads to a gradual relaxation of the noncrystalline regions towards the state characteristic of undrawn PE. With increasing annealing temperature S_a as well as the mobility approach values found with undrawn PE.     

Thermodynamische Stabilität makromolekularer Kristalle

The density of free energy of a chain crystal contains two terms of opposed sign dependent on the numberN of chain elements in the straight section of the macromolecules between the surfaces of the crystal perpendicular to thec-axis. The surface energy contributes a positive term decreasing withN. The amplitude Φ of the periodic lattice field opposing the chain translation in thec-axis yields the negative term. Due to the incoherent longitudinal thermal vibration of the four first-order neighbours of any chain the fieldΦ is smeared out. Its amplitude decreases the more the higherN and hence yields an increase in free energy density with increasingN. As a consequence of the opposite sign of surface energy and lattice field changes withN the free energy density shows a minimum at finiteN corresponding to the thermodynamically stable crystal thickness. With increasing temperature and lower interaction between adjacent chainsN increases in perfect qualitative agreement with experimental data.     

NMR observations of drawn polymers. IX. Chain mobilization and water mobility in nylon 66

Wide-line NMR spectra of nylon 66 fibers have been obtained at different alignment angles between the fiber axis and the magnetic field, at varying water contents (H₂O and D₂O), and at different temperatures. At 28°C the spectrum of the dry fibers consists of a nearly structureless broad line. At water regains of 1.4% by weight (dry basis) and higher a sharp line appears which originates from highly mobile water molecules. The width of this line decreases with increasing water content, implying an increase of water mobility. Moreover, the width is a function of the alignment angle; this shows that the water is not reorienting isotropically owing to specific water-polymer interaction. The amount of mobile water is always smaller than the amount of water absorbed. At water contents close to saturation, a mobile polymer line appears with a width intermediate between the broad line (immobile polymer) and the sharp water line. This line, most clearly observed at an alignment angle of 0°, is due to a shift of the α_a process to lower temperatures in the presence of water. A similar line is observed in the dry fibers at 120°C. It is shown that the α_a process decreases the NMR second moment only slightly. The shift of the high temperature drop in second moment to lower temperatures in the presence of water is therefore interpreted as due to a shift of the α_c process, and not of the α_a process, to lower temperatures.     

Synthesis of new functionalized imidazo[2,1‐b]thiazoles and thiazolo[3,2‐a]pyrimidines

5‐Oxo‐5H‐[1,3]thiazolo[3,2‐a]pyrimidine‐6‐carboxylic acid ( 4 ), and 6‐methylimidazo[2,1‐b]thiazole‐5‐carboxylic acid ( 17 ) were reacted with amines 6a‐i by the reaction with oxalyl chloride and N, N‐di methyl‐formamide as a catalyst into primary and secondary amide derivatives 7‐14 and 19‐22. From compound 24 N,N'‐disubstituted ureas 26, 27 and perhydroimidazo[1,5‐c]thiazole 29 derivatives of imidazo[2,1‐b]thiazole were prepared. By nmr analysis of compound 29 , the existence of two stereoisomers resulting from both optical, due to centre of chirality at C7′a, and conformational isomerism, due to restricted C5? N6′ bond rotation were proved.     

7th International k0-Users’ Workshop

Journal of Radioanalytical and Nuclear Chemistry -     

Simulation of void fraction profile evolution in subcooled nucleate boiling flow in a vertical annulus using a bubble-tracking approach

A three-dimensional bubble-tracking model of subcooled nucleate boiling flow in a vertical channel at low-pressure conditions is proposed with specific application to the case of boiling in an annulus with a central heating rod. Vapour is distributed in the liquid in the form of individually tracked bubbles. The overall behaviour of the liquid–vapour system results from motion, interaction, coalescence and boiling mechanisms prescribed mostly at the level of bubbles. Bubbles are nucleated at nucleation sites randomly distributed over the heated surface. After nucleation, bubbles slide on the heated wall, detach and then migrate into the lower-temperature region away from the heated surface, where they condense. The proposed model was applied to experiments on subcooled boiling from Purdue University (USA). Experimental and calculated void fraction radial profiles at different axial locations are compared.