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). 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.     

Radiation-induced solid-state polymerization of derivatives of methacrylic acid. II. Postirradiation polymerization of octadecyl methacrylate

Octadecyl methacrylate (mpc ≈ 12°C.) polymerized readily in the solid state in the temperature range ?30 to +12°C. after gamma irradiation at ?196°C. The initial rate of polymerization and the “limiting” conversion increased with radiation dose and temperature. The temperature dependence of the rate corresponded to an “apparent” activation energy of 20 kcal./mole. Difficulties were experienced with polymerization during separation of the polymer from residual monomer, but these were minimized by using low radiation doses and a hot, selective solvent. The maximum conversion achieved was 70%. The polymer was crosslinked, even at low conversions.     

Radiation-induced polymerization of cyclohexene sulfide in the solid state

Radiation-induced solid-state polymerization of cyclohexene sulfide has been investigated. Differential thermal analysis shows that this compound has a phase transition point at ?74°C and behaves as a plastic crystal in the temperature range from ?74 to ?20°C (melting point). By rapid cooling, this plastic crystal was easily supercooled, and below ?166°C a glassy crystal, i.e., a supercooled nonequilibrium state of plastic crystal, was obtained. In-source polymerization proceeded in the plastic crystalline state. Postpolymerization of glassy crystalline monomer irradiated at ?196°C occurred above ?166°C (glass transition point) during subsequent heating.     

ESR study of radiation-induced polymerization of methyl methacrylate on solid surfaces

ESR spectra of methyl methacrylate (MMA) adsorbed on solid surfaces such as silica, alumina, and CPG-10 show a seven-line absorption due to monomer radicals when γ-irradiated at ?196°C. These spectra turn into conventional nine-line spectra by annealing at ?86°C as a result of polymerization. The intensity ratio of the four-line component to the five-line component changes with further polymerization. The ratio depends on the kind of solid and the smallest value is obtained in the spectra of alumina-MMA. The temperature of irradiation strongly affects the value in alumina-MMA. These results suggest that MMA is strongly adsorbed and deformed the conformation on alumina surface.     

Crystallization during polymerization of poly-p-xylylene

Crystallization during polymerization of p-xylylene from the gas phase has been studied between 200 and ?196°C. From room temperature to ?17°C the polymer crystal morphology changes in that the crystallinity decreases. In this range the process is thought to be of the successive polymerization and crystallization type. The morphology is in agreement with this mechanism, of the folded-chain β-polymorph type with proper epitactic orientation of the chains with respect to the support surface. At ?78°C an intermediate, poorly crystallized polymer results. At 196°C the reaction is most likely of the simultaneous polymerization and crystallization type. The morphology is, in agreement with the changed mechanism, of a metastable, irregularly folded β-polymorph type with no orientation of the chains relative to the support surface. No significant changes in molecular weight were observed in the polymers produced between 26 and ?196°C.     

Equilibrium anionic polymerization of n-valeraldehyde

This paper describes an anionic polymerization of n-valeraldehyde (VA) in tetrahydrofuran (THF) initiated by benzophenone-monolithium complex in a high vacuum system. In spite of the deposition of the resulting polymer an equilibrium state between the monomer and the polymer was observed at a temperature range of ?90 to ?68°C. From the linear relationship between the equilibrium monomer concentration and the polymerization temperature, values of ?5.3 ± 0.3 kcal/mole and ?25.7 ± 1.4 cal/mole-deg, respectively, were evaluated for the enthalpy change and the entropy change in the present system. The effect of polar substituents on the polymerizability of aldehydes is discussed from the comparison of these values with those in the case of β-methoxypropionaldehyde.     

ESR spectra of poly(methacrylic acid) and poly(methyl methacrylate): Reinterpretation of the 9-line spectrum

The temperature dependence of the ESR spectra of poly(methacrylic acid) and poly-(methyl methacrylate) γ-irradiated at room temperature was studied between ?196°C and +25°C. The conventional 9-line spectrum was observed throughout this range with no significant spectral change, in contrast to the propagating radical ··· CH₂? °C(CH₃)COOR found in methacrylic acid monomer or barium methacrylate dihydrate irradiated at ?196°C. In addition, the irradiation of methacrylic acid monomer with a low dose at 0°C gave the same 13-line spectrum as that of the propagating radical obtained by the irradiation at ?196°C, while prolonged irradiation at 0°C gave the same conventional 9-line spectrum as that of poly(methacrylic acid) or poly(methyl methacrylate). The conventional 9-line spectrum has a much weaker 4-line component than that of the propagating radical. The difference comes from the surrounding matrix, and the conventional 9-line spectrum is well interpreted by introducing the concept of the distribution of the conformational angle in the irregular polymer matrix. From simulation of the ESR spectrum, it was found that the intensity of the 4-line component is very sensitive to the distribution, and that the observed 9-line spectrum is well reproduced assuming a Gaussian distribution (half-height width of 5–6°) around the most probable conformation which is nearly the same as that of the propagating radical, where the conformational angles of the two C? H_β bonds to the half-filled p-orbital are 55° and 65°.     

Crystallization during polymerization of poly-p-xylylene. II. Polymerization at low temperature

The polymerization of p-xylylene was followed with a newly designed differential thermal analysis system at temperatures between ?196°C and ?20°C. It was found that at the lower temperatures the monomer condenses first to the crystalline monomer before simultaneous polymerization and crystallization. At the higher temperatures, polymerization and crystallization are successive. The data are in agreement with the morphology and crystal structure data derived in Part I of this series of papers on crystallization during polymerization of poly-p-xylylene.     

Kinetics of the γ-radiation-induced polymerization of methyl methacrylate in the solid state

Studies have been made of the γ-radiation-induced polymerization of methyl methacrylate in bulk, in the solid state at a temperature of ?65°C. and a radiation intensity of 346,000 rad/hr. The reaction was found to have an extremely long induction period (～50 hr.) when pure monomer was used, and to be first-order with respect to polymer concentration. This first-order dependency was confirmed by a series of irradiations in which 0.6% poly(methyl methacrylate) was dissolved in the monomer before irradiation. These irradiations showed no induction period. Nuclear magnetic resonance spectroscopy indicated a much more heterotactic polymer than that obtained in the liquid state at ?49°C.     

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.     

Thermal polymerization of N-tert-butylacrylamide in 1,4-dioxane

The kinetics of the thermal polymerization of N-tert-butylacrylamide were investigated in 1,4-dioxane as solvent, in the 65–80°C temperature range. It was found that the overall rate of polymerization which was determined by a gravimetric method is proportional to the 1.9 power of monomer concentration at 70°C. The rate of initiation was determined by ESR spectroscopy using DPPH as an inhibitor, and it was found that the order of initiation rate is 1.8 with respect to monomer concentration at 70°C. The overall activation energy for the thermal polymerization of N-tert-butylacrylamide was found to be 64 ± 9 kJ mol^?1 in the 65–80°C temperature range. The activation energy for the rate of initiation was also determined and it was found to be 90 ± 23 kJ mol^-1.     

Polymerization of coordinated monomers. I. Stereoregulation in the free-radical polymerization of methyl methacrylate-zinc chloride or methyl methacrylate–stannic chloride complexes

Stereoregulation in free-radical polymerization was studied for the polymerization of the 2:1 or 1:1 complex of methyl methacrylate with ZnCl₂ or SnCl₄. The complexes were polymerized with the use of a free-radical initiator or γ-ray irradiation either in the liquid or solid state at various temperatures ranging from ?196 to 110°C, and the tacticities of the resulting polymers were determined by NMR spectroscopy. The polymers had different and characteristic values of tacticities depending upon the complex species, i.e., the kind of metal chloride and the stoichiometry. The tacticities were found to be independent of the polymerization temperature in both the liquid and solid states, in contrast with the fact that tacticities of the polymer from pure monomer changed markedly with the temperature. A temperature dependence appeared in the polymerization system, which contained more monomer than that corresponding to the 2:1 complex. The effect of the viscosity or the solid phase on the stereoregulation was examined in comparison with the polymerization of a mixture of methyl methacrylate and liquid paraffin. Two possible explanations regarding the stereoregulation mechanism are offered in relation to the structures of the complexes.     

Radiation-induced polymerization of 1,2-cyclohexene oxide in the plastic crystalline state

Radiation-induced solid state polymerization of 1,2-cyclohexene oxide has been investigated. By the differential thermal analysis and x-ray diffraction analysis, it was found that this compound has a phase transition point at ?81°C and behaves as a plastic crystal in the temperature range from ?81°C to ?36°C (melting point). The in-source polymerization proceeded not only in the plastic crystalline state but also in the ordinary crystalline state at temperatures below the phase transition point. The overall rate of polymerization and the rate of chain transfer to monomer in the plastic crystalline state were larger than those in the ordinary crystalline state by a factor of about forty, but the kinetic mechanisms were identical, i.e., the termination mechanisms were unimolecular in both solid states. In contrast, the kinetic mechanisms in the plastic crystalline state and in the liquid state were different. From these observations, the effects of molecular motion and molecular arrangement on the polymerizations of 1,2-cyclohexene oxide in the three phases were discussed.     

Four-center type photopolymerization in the crystalline state. VI. Kinetic and structural study of the polymerization of p-phenylenediacrylic acid diethyl ester

The effect of temperature on the four-center type photopolymerization has been investigated for p-phenylenediacrylic acid diethyl ester over a wide temperature range including crystal transition point (56°C) and melting point (96°C) of monomer. With the elevation of temperature between ?50 and 15°C, the polymerization rate in the initial stage increased and the degree of polymerization decreased monotonously, while the rate in the later stage decreased above ?25°C. With irradiation at above 25°C, the monomer crystals became sticky, and the polymerization was suppressed at the stage of oligomerization with low conversion. This tendency was enhanced above the crystal transition point, giving mainly dimer in low yield. Above the melting point, only radical polymerization occurred with the aid of oxygen. The steric configuration of the products in the crystalline state was 1,3-trans with respect to the cyclobutane ring. Peaks in NMR spectra of all products were assigned to the protons involved in four compounds up to tetramer. Various results obtained have been interpreted in terms of the change, as a function of temperature, from a topochemical polymerization which proceeds under a control of the monomer lattice to a photoinitiated vinyl-type polymerization in the disordered state. It is concluded that a rigid crystal lattice is indispensable for the four-center type photopolymerization to proceed smoothly.     

Structure and optical characteristics of poly(p-phenylenevinylene) prepared by vapor deposition polymerization

Films based on poly(p-phenylenevinylene) are prepared by pyrolitic polymerization of α,α′-dichloro-p-xylene. During monomer precipitation, the temperature on a substrate is 25, 50, or ?196°C. Subsequent annealing of the precursor at 250°C yields the final product: the copolymer of p-phenylenevinylene and p-xylylene with an approximate composition of 4: 1. The surface morphology, structure, and optical characteristics of the polymer are studied. The mean-square surface roughness of the precursor is 5 nm. Thermal treatment increase the samples’ roughness up to 10 nm. When the precursor is transformed into poly(p-phenylenevinylene), the roughness coefficient decreases from 0.85 ± 0.05 to 0.74 ± 0.05 owing to the formation of a rougher surface. Characterization of the optical characteristics of the synthesized poly(p-phenylenevinylene) shows that the maximum effective conjugation chain length achieves 12 units in the copolymer prepared when the temperature on the substrate is ?196°C. As the temperature on the substrate increases, the conjugation length decreases to 8 units upon precipitation. Luminescence analysis reveals the effective excitation-energy transfer from short chain fragments of poly(p-phenylenevinylene) to long chain fragments. Electron parameters of the material are estimated: i.e., the band gap, the Huang-Rhys factor, the Stokes shift, and the oscillation energy of molecules.     

Anionic bulk polymerization of α-methylstyrene. Stopping of polymerization due to vitrification and equilibrium in bulk

Anionic living polymerization of α-methylstyrene containing a small amount of THF (less than 10%) was studied at temperatures between ?30°C and 50°C. At any temperature studied, a certain quantity of monomer remained without further polymerization. The effect of temperature and THF content on the final state was completely different in low and high temperature regions; at temperatures lower than ca. 20°C, the final monomer concentration decreased with increasing polymerization temperature and THF content. This is explained by the concept of “stopping of polymerization due to vitrification” of the polymerizing mixture. In fact, the final reaction mixture is really glassy in most cases and the red color of living polymer buried in the glass is discolored only very slowly when exposed to air. Detailed analysis of the results showed that the vitrification stopping holds only approximately. At temperatures higher than ca. 30°C, a normal equilibrium between propagation and depropagation holds, and the final monomer concentration increased with temperature. It is, however, far less than the equilibrium monomer concentration obtained in solution polymerization at the same temperature, and it increased appreciably with the increase in THF content. It is shown that the behavior of the equilibrium for the whole concentration range can be explained satisfactorily by a thermodynamic theory of ternary mixture.     

Anionic polymerization of acrylates. III. Polymerization of 2-ethylhexyl acrylate initiated by lithium-tert-butoxide

The polymerization of 2-ethylhexyl acrylate (EtHA) initiated with lithium-tert-butoxide (t-BuOLi) in tetrahydrofuran (THF) and in the temperature range between ?60 and 20°C was investigated. The reaction rate is distinctly temperature-dependent and at ?60°C is already very low, similarly to the polymerization of methacrylates. Molecular weights of the polymers thus formed, particularly at higher temperatures, are inversely proportional to conversion of the monomer due to the slow initiation reaction. This is documented by the low consumption of alkoxide even at long reaction times, which also depends on the reaction temperature. At higher temperatures the polymerization stops spontaneously, due to the greater extent of autotermination reactions. The weak initiating efficiency of the alkoxide decreases still more with decreasing concentration of the monomer during the polymerization, as confirmed by the concentration dependence of the reaction rate in toluene at ?20°C. The results suggest a negligible initiating effect of alkoxides in complex bases, particularly at lower polymerization temperatures. © 1992 John Wiley & Sons, Inc.     

Solid-State Polymerization in Binary Component Systems at Low Temperature

Abstract

Radiation-induced polymerization in binary component systems of acrylonitrile-methacrylonitrile and acrylonitrile-vinyl acetate was studied at ?196°C. A mixture of two-component homopolymers was obtained from the acrylonitrile-methacrylonitrile system, which forms a eutectic mixture. When the mixture of acrylonitrile with vinyl acetate is cooled quickly from room temperature, a glassy state can be obtained. It was found that the copolymerization is possible in the glassy state at ?196°C, and the monomer reactivity ratios were determined as r ₁ = 6.0 and r ₂ = 0.2 (M ₁ = acrylonitrile), which coincides with the reported values on the radical copolymerization at room temperature.     

Polymerization of vinyl chloride by alkyllithium compounds

The polymerization of vinyl chloride initiated by alkyllithium compounds was investigated. The effect of temperature, initiator concentration, and monomer concentration on the conversion and the properties of the resulting polymers were studied. The optimum temperature in the investigated range (between ?20°C and +20°C) was +5°C. The conversion is directly proportional to the concentration of both the initiator and the monomer. The molecular weight is inversely proportional to the initiator concentration and directly proportional to the monomer concentration. Under optimum conditions the molecular weight of the polymers is as high as 140,000. These results differ by an order from hitherto published data on the nonradical polymerization of vinyl chloride. The proportion of isotactic and syndiotactic structures resulting from the presence of tert-butyllithium does not differ from that obtained by radical polymerization, but the occurrence of anomalous structures is reduced to a minimum. The stability of the macromolecules is higher. A mechanism of the polymerization is suggested.