Polymerization of para-xylylene derivatives (parylene polymerization). I. Deposition kinetics for parylene N and parylene C

Kinetic aspects of parylene N [unsubstituted poly(para-xylylene)] and Parylene C [monochlorosubstituted poly(para-xylylene)] were studied. The conversion of starting material (dimer of either p-xylylene or chloro-para-xylylene) to polymer is quantitative (ca. 100%). Consequently, the total polymer formed in a closed system is directly proportional to the amount of dimer charged. However, the percentage of the total amount of polymer formed which deposits on substrate surfaces, placed in the deposition chamber, as well as the polymer film growth rate are dependent on operational factors such as the temperature of the substrate, sublimation of dimer temperature, flow pattern of the reactive species, etc. Parylene C, being a heavier and more polar molecule, has the tendency to deposit easily in the deposition chamber compared to the deposition of Parylene N. Parylene C also has a higher ceiling temperature for deposition than Parylene N. This situation has been investigated from the viewpoint of excess thermal energy which hinders polymer formation (deposition) due to the exceedingly high entropy change necessary for polymer deposition to occur. The addition of a cool (i.e., room temperature) inert gas was shown to increase the deposition of Parylene N on substrate surfaces placed in the deposition chamber. The deposition increase and acceleration of deposition (film growth) rate were found to be related to the size and molecular weight of the inert gas pressure maintained in the system. The accelerating effect is explained by the increase in third-body collisions to dissipate the excess thermal energy of the reactive species.     

Polymerization of para-xylylene derivatives (parylene polymerization). III. Heat effects during deposition of parylene N at different temperatures

Thermal effects accompanying the vacuum deposition of poly-para-xylene (Parylene N) at different temperatures have been studied by following the changes in the temperature of the substrate. Similarly to the case of polychloro-para-xylylene (Parylene C), two distinct exothermic effects were observed; one discrete, resulting in sharp exothermic spikes and the other continuous, resulting in the shift of the baseline. The spike effect, attributed to the solid-state polymerization of para-xylylene, is observed at the low-temperature range, the upper limit of which moves higher for higher deposition rates. The shift of a baseline as a function of deposition temperature exhibits two maxima, one independent of deposition rate and the second moving toward higher temperatures (that is, toward the first maximum) for higher deposition rates. First maximum falls at about ? 72°C and is attributed to the melting point of para-xylylene crystals. X-ray diffraction studies of polymer samples have shown that the existence of the second maximum is always followed by the appearance of an additional crystalline phase in the respective range of deposition temperatures. When the deposition rate is high enough, the second maximum merges with the first one, or virtually disappears. Under such conditions the new crystalline phase is no more detectable. It is postulated that the evolution of the additional amount of heat resulting in the appearance of the second maximum is due to the cyclization reaction and the formation of cyclic oligomers. This reaction very likely requires a particular spatial arrangement of monomer molecules and, therefore, it is suggested to take place in certain domains of the crystalline lattice.     

Polymerization of para-xylylene derivatives (parylene polymerization). II. Heat effects during deposition of parylene C at different temperatures

Thermal effects accompanying vacuum deposition of poly(chloro-para-xylylene) in the temperature range between ?196 and 0°C have been studied using two separate methods. One is based on the recording of the rate of evaporation of liquid nitrogen and it is used for the deposition at ?196°C, and the second involves the recording of changes in the substrate temperature and is used for the deposition in the range of ?162 to 0°C. These methods enable us to observe two distinct effects: fast (discrete), resulting in the appearance of sharp, exothermic spikes; and slow (continuous), resulting in the shift of the baseline. The shift of the baseline exhibits a well-defined maximum at about ?65°C and this temperature is attributed to the melting point of the monomer. The fast process always occurs below this temperature and is explained as a solid state, chain addition polymerization. The quantification of the heat effect at ?196°C strongly suggests that the quinonoid form of the monomer participates in the propagation step of this chain reaction. The fast (solid state) and the continuous modes of polymerization may occur simultaneously in the range of about ?140 and ?65°C. The frequency of the initiation which is the formation of dimer radical seems to control the occurrence of these two modes of polymerization.     

Polymerization of para-xylylene derivatives (parylene polymerization). IV. Effects of the sublimation rate of di-p-xylylene on the morphology and crystallinity of parylene N deposited at different temperatures

The effect of the sublimation rate of di-p-xylylene on the crystallinity and morphology of Parylene N deposited on stainless steel was studied as a function of substrate temperature. For a given rate of dimer sublimation, the deposition rate increases with decreasing substrate temperature. Increasing the sublimation rate of the dimer increases the deposition rate 10-fold, decreases the crystallinity, and shifts the appearance of the hexagonal β structure towards higher substrate temperature for samples synthesized from room temperature (RT) to ?60°C. Solution annealing resulting from solvent extraction, and isothermal annealing, increase the crystallinity of the polymers and result in structures containing both α and β polymorphs. The surface topology, as revealed by scanning electron microscopy (SEM), for polymers synthesized from RT to ?40°C shows a globular structure, whereas low temperature samples exhibit a rod-type morphology. For higher sublimation rates of the dimer, SEM micrographs show that oligomeric species start appearing on the polymer films after a period of 4–5 days. Solvent extraction removes the oligomeric crystals, and GPC analysis of the resulting extract indicates that most of the oligomers range in molecular weight from 100 to 900. The cross-sectional morphology for fractured low temperature samples, however, reveals different morphologies as polymerization proceeds. It is postulated that in the temperature range ?50 to ?78°C, both surface condensation and surface adsorption of monomer occurs, leading to different morphologies and lower crystallinity. The polymer synthesized at liquid nitrogen temperature shows the presence of voids along with different morphologies. X-ray diffractograms of polymers synthesized at liquid nitrogen reveal a considerable amount of amorphous phase in the films. Hence, it is inferred that, although the liquid nitrogen polymerization is a solid state polymerization of the crystalline monomer, it does not lead to 100% crystalline material, and the reasons for this are discussed.     

Polymerization of para-xylylene derivatives. V. Effects of the sublimation rate of di-p-xylylene on the crystallinity of parylene C deposited at different temperatures

Poly(chloro-p-xylylene) was synthesized in a manner similar to poly(p-xylylene) using Gorham's method at various cryogenic temperatures. The effect of the sublimation rate of dimer on the kinetics of deposition, crystallinity, and crystalline structure was studied. Increasing the sublimation rate of the dimer increases the deposition rate similar to that of poly(p-xylylene). However, an increase in crystallinity, in contrast to Parylene N, is observed, although, in general, Parylene C has lower crystallinity relative to Parylene N. No polymorphism is observed either by decreasing the deposition temperature or by increasing the sublimation rate of the dimer. Solution annealing and isothermal annealing both bring about crystallization without any structural transformation. Solution annealing removes the oligomers and dimers, but no crystalline oligomers are ever detected under the scanning electron microscope (SEM). The surface topology of films synthesized from ambient temperature to ?40°C is very similar to Parylene N. At lower temperatures, in the region ?50 to ?60°C, a rod-type morphology is observed similar to Parylene N. The surface topology of samples synthesized at ?196°C is totally different from that of Parylene N. All low temperature synthesized samples are amorphous.     

Experimental and theoretical study of AC electrical conduction mechanisms of semicrystalline parylene C thin films

The electrical conduction mechanisms of semicrystalline thermoplastic parylene C (-H(2)C-C(6)H(3)Cl-CH(2)-)(n) thin films were studied in large temperature and frequency regions. The alternative current (AC) electrical conduction in parylene C is governed by two processes which can be ascribed to a hopping transport mechanism: correlated barrier hopping (CBH) model at low [77-155 K] and high [473-533 K] temperature and the small polaron tunneling mechanism (SPTM) from 193 to 413 K within the framework of the universal law of dielectric response. The conduction mechanism is explained with the help of Elliot's theory, and the Elliot's parameters are determined. From frequency- and temperature-conductivity characteristics, the activation energy is found to be 1.27 eV for direct current (DC) conduction interpreted in terms of ionic conduction mechanism. The power law dependence of AC conductivity is interpreted in terms of electron hopping with a density N(E(F)) (~10(18) eV cm(-3)) over a 0.023-0.03 eV high barrier across a distance of 1.46-1.54 ?.     

The morphology of gel-spun polyethylene fibers,investigated by solid-state 13C NMR

Gel-spun polyethylene fibers were analyzed at room temperature with ¹³C NMR, using both, CP-MAS and BILEV (Bloch-decay with two-level decoupling). The analysis shows the existence of three different components in the fiber sample—a crystal component, an amorphous component and a third component, named the oriented, mobile component. This latter component has a ¹³ C chemical shift that is similar to the crystalline chemical shift, but with a mobility, expressed by T₁, that is closer to the amorphous component. The chemical shift and T₁ are as follows: 34.06 ppm and 28.1 s for the crystalline part; 31.70 ppm and 0.3 s for the amorphous part; and 34.06 ppm and 1.8 s for the oriented mobile component. The percentages are 63.2% crystalline; 34.0% oriented mobile, 2.8% amorphous component. Using proton spin-diffusion measurements, it was possible to estimate the domain size of the crystalline and oriented mobile components to be 62.8 and 13.2 nm, respectively, in agreement with the results of a full-pattern x-ray study on the same sample. After melting of the fiber at 450 K and recrystallinzation on cooling, the oriented, mobile component is dramatically reduced. © 1994 John Wiley & Sons, Inc.     

Polymerization photoinitiated by carbonyl compounds. VI. Mechanism of benzil photoinitiation

The mechanism of the photoinitiation of the vinyl polymerization sensitized by benzil and 4,4′-dimethoxybenzil was studied. The monomers considered were methacrylic acid esters and styrene derivatives. All these monomers are efficient quenchers of the excited triplet benzil. However, the initiation efficiency of the benzil is important only when styrene derivatives are employed as monomers. The main polymerization process follows a simple free radical mechanism. The initiation step is a consequence of the interaction (triplet benzil–monomer double bond) through a charge transfer complex.     

Diheterocyclic compounds from dithiocarbamates and derivatives thereof. VI. unsymmetrical N1,N4-Bis(2-benzazolyl)sulphanilamides

The title compounds 9 are readily synthesized from dithiocarbamic acid derivative 5 by stepwise formation of the heterocyclic rings. Differences in reactivity of dithiocarbamates and dithiocarbonimidates make group protection unnecessary.     

Aminobutadienes. VI. Polymerization and Copolymerization of 2-Phthalimido-1,3-butadiene

2-Phthalimido-1,3-butadiene (2-PB) was polymerized either radically or thermally in bulk and in solution. While the polymer obtained by solution polymerization was soluble in some solvents such as halogenated hydrocarbons, dioxane, and dimethylformamide and had a softening point in the range of 160–170°C., that obtained by polymerization in bulk was insoluble in any solvent and only swollen on being immersed in such solvents as above. The reduced viscosity of the soluble polymer obtained by solution polymerization was approximately 1.0, and this value remained almost unchanged with varying polymerization time. Likewise the cationic polymerization in acetylene tetrachloride or in chloroform at 20°C. with the use of cationic catalysts such as boron trifluoride and stannic chloride was attempted, but no formation of polymer was observed. This monomer preferentially reacted with acrylonitrile, methyl methacrylate, styrene, and N-vinylphthalimide to form the respective copolymers; it reacted somewhat less readily with vinyl acetate. The monomer reactivity ratios in the copolymerization with styrene were calculated by the Fineman and Ross method and found to be r₁ (2-PB) = 5.2 and r₂ (styrene) = 0.11, respectively, from which the Q, e parameters were successively evaluated to be Q = 5.0 and e = ?0.05. The fact that e value is close to zero, easily explains why this monomer can copolymerize well both with acrylonitrile, which has a highly positive value of e (1.2) and with styrene, for which e is considerably negative (-0.8).     

The morphology of zinc dimethacrylate reinforced elastomers investigated by SEM and TEM

The morphologies of various ZDMA-reinforced elastomers, including styrene butadiene rubber (SBR), ethylene-propylene-diene monomer (EPDM), nitrile-butadiene rubber (NBR), ethylene-propylene monomer (EPM), poly(α-octylene-co-ethylene) elastomer (POE) and hydrogenated nitrile-butadiene rubber (HNBR), were studied by using SEM and TEM. The observation on the compounds showed that during the compounding process, the dimension of ZDMA particles reduced, and could even form dispersion structures with nanometer size (<100 nm). It is shear stress of compounds during mixing rather than polarity of matrix rubber that plays the most important role to determine dispersion state of ZDMA in compounds. High shear stress facilitates dispersion of ZDMA. Only in elastomers having the lower shear stress such as POE and EPM, original dimension features of ZDMA particles make considerable effects on dispersion level of ZDMA in compounds. The observation on cured composites displayed that there are two kinds of micro-dispersed structures: micron dispersion—residual ZDMA particles and nano-dispersion—the aggregate of poly-ZDMA. The higher saturation and polarity of rubbers and the better dispersion level of ZDMA in compounds benefit in situ polymerization of ZDMA, resulting in the lower amount of residual ZDMA particles (micron dispersion). In the elastomer with higher saturation such as POE, EPM, EPDM and HNBR, the dimensions of nano-dispersions are slightly larger. For the ZDMA/POE, formula effect on morphology of the composite was also discussed. It was found that the loading of ZDMA and peroxide impact remarkably on the amount and dimension of nano-particles in the composite.     

Polymerization of phenylacetylenes. VI. Chain-transfer reaction in the polymerizations of phenylacetylene and styrene by WCl6

Chain-transfer reactions to alkylbenzenes were investigated in the polymerizations of phenylacetylene and styrene by WCl₆ in benzene at 30°C. In the polymerization of phenylacetylene, alkylbenzenes did not work as chain-transfer agents, and further ethyl iodide was not a terminating agent. These findings suggest that the polymerization of phenylacetylene by WCl₆ differs from the conventional cationic or anionic mechanisms. On the other hand, the ability of alkylbenzenes as chain-transfer agents in the polymerization of styrene by WCl₆ increased in the following order: toluene < p-xylene < m-xylene < o-xylene. This order is similar to that in the polymerization by SnCl₄. These results indicate that the polymerization of styrene by WCl₆ proceeds by a conventional cationic mechanism.     

Polymerization of iso-butyl vinyl ether by oxychlorides of group V and VI elements

Chromyl chloride (CrO₂Cl₂) causes facile polymerization of iso-butyl vinyl ether (IBVE). In contradiction to the existing report, CrO₂Cl₂ alone or in combination with conventional modifiers fails to produce stereoregular poly(IBVE). Because of the fast polymerization at ambient temperature (30°), the kinetics of the polymerization of IBVE by CrO₂Cl₂ were studied at 0° and 15°. The polymerization is first order in CrO₂Cl₂ and second order in IBVE upto a threshold monomer concentration thereafter becoming independent of [IBVE]. The polymerization is inhibited by pyridine. Addition of HCl to CrO₂Cl₂ increases both rate of polymerization (R_P) and [η]. Added water does not affect R_P upto [H₂O]/[CrO₂Cl₂] ≈ 1.33 but thereafter enhances it sharply upto [H₂O]/[CrO₂Cl₂] ≈ 5.32. With increasing [H₂O], [η] falls to a limiting value. [η] is independent of [CrO₂Cl₂] but increases with [IBVE] upto 1.0mole/l. Both R_P and [η] show a maximum at 47° in the Arrhenius plot over the range 10–72°. The features have been explained on the basis of a conventional cationic mechanism.     

Complex morphology of melt-spun nylon-6 fibres investigated by 1H double-quantum-filtered NMR spin-diffusion experiments.

The complex morphology of high-speed melt-spun nylon-6 fibres hydrated with D2O was investigated using 1H double-quantum-filtered spin-diffusion NMR experiments. The magnetisation exchange from selected crystalline domains along the fibrils and interfibrils was simulated with the help of a three-dimensional solution of a spin-diffusion equation approximated by a product of one-dimensional analytical NMR signals, which correspond to a lamellar morphology. This allows to measure the sizes of crystalline and less-mobile amorphous domains along the fibrils, as well as the diameter of the fibrils and interfibril distances. A series of nylon-6 fibres with extreme values of winding speed and draw ratio was investigated. The changes detected in the domain size along the fibrils and interfibrils show the same trend in the data obtained from wide-angle X-ray diffraction and small-angle X-ray scattering.     

Sorption of U(VI) on magnetic sepiolite investigated by batch and XANES techniques

Additional experimental cross sections were deduced for the long half-life activation products (¹⁷²Hf and ¹⁷³Lu) from the alpha particle induced reactions on ytterbium up to 38 MeV from late, long measurements and for ¹⁷⁵Yb, ¹⁶⁷Tm from a re-evaluation of earlier measured spectra. The cross-sections are compared with the earlier experimental datasets and with the data based on the TALYS theoretical nuclear reaction model (available in the TENDL-2014 and 2015 libraries) and the ALICE-IPPE code.

     

Block copolymerization. VI. Polymerization of pivalolactone with macromolecular initiators from polystyrene and polytetrahydrofuran

Polymerization of pivalolactone with polystyryl sodium or polystyryl ethoxysodium in tetrahydrofuran resulted in homopolymer mixtures. Block copolymers of pivalolactone and styrene were obtained by the polymerization of pivalolactone with sodium polystyrene carboxylate in tetrahydrofuran containing dimethyl sulfoxide. Block copolymers of pivalolactone and tetrahydrofuran were obtained by the polymerization of pivalolactone with polytetrahydrofuran containing carboxylate endgroups. The mechanism of the initiation reaction and various factors affecting block efficiency are discussed.     

Polymerization of acetylenic derivatives. XXVII. Synthesis and properties of isomeric poly-N-ethynylcarbazole

Poly-N-ethynylcarbazole (PEC) was prepared thermally, by using Ziegler-Natta catalysts, and also by using TiCl₄ and phosphine complexes for initiation. The spectral data (infrared, NMR, x-ray diffraction, and ultraviolet) were best interpreted based on a cis-transoidal/cis-cisoidal stereoblock structure for (both soluble and insoluble) polymers prepared by Ziegler-Natta catalysts and on a trans-cisoidal structure for the polymers prepared with the other catalysts. The cis-transoidal/cis-cisoidal stereoblock structure isomerized thermally into a trans-cisoidal structure, at temperatures which were dependent on the type of initiation.     

Pyrimidine derivatives. VI. Synthesis of mono-, di-, tri-, and tetrabromo-substituted pyrimidine derivatives

The following bromo-2,4(1H,3H)-pyrimidinediones possessing a bromo substituent at the 5-position and side chains at the 1- and 6-positions were prepared. The three types of mono-bromo derivatives are: 1-(bromoalkyl)-3,6-dimethyl- 3a-d , 5-bromo-3,6-dimethyl-1-(hydroxyalkyl)- 4a-d , and 1-(acetoxyalkyl)-5-bromo-3,6-dimethyl-2,4(1H,3H)-pyrimidinediones 11a-d . The three types of dibromo derivatives are: 5-bromo-1-(bromoalkyl)-3,6-dimethyl- Sa-d , 1-(acetoxyalkyl)-5-bromo-6-bromomethyl- 8a, 8c , and 8d , and 5-bromo-6-bromomethyl 1-(hydroxyalkyl)-2,4(1H,3H)-pyrimidinediones 9a, 9c , and 9d . Likewise one group of tribromo and one group of tetrabromo derivatives are: 5-bromo-1-(bromoalkyl)-6-bromomethyl -7a-d and 5-bromo-1-(bromoalkyl)-6-dibromomethyl-2,4(1H,3H)-pyrimidinediones 6a-d .     

Polymeric catechol derivatives. IV. Polymerization behavior of 4-vinylcatechols and some properties of their polymeric derivatives

The polymerization and copolymerization of 4-vinylcatechols, such as 2-(0-methyl)-4-vinylcatechol (I), 3,4-dimethoxystyrene (II), and 3,4-methylenedioxystyrene (III), were investigated in cyclohexanone at 30°C, using tri-n-butylborane as an initiator. The reactions yielded vinyl polymers and copolymers. The copolymerization parameters of I–III were determined; their Q and e values were found to be similar to those of styrene and vinylhydroquinone. The copolymerization of I–III gave copolymers of a highly alternating character. The thermal stability of the polymers and copolymers so obtained was also studied. The redox potentials of hydroloyzed poly(I) were examined; the reverse “polymer effect” was observed.