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1964 Volume 6 Issue 5
1964, 6(5): 337-341
Abstract:
1964, 6(5): 342-349
Abstract:
Polydimethylsiloxane containing hydroxyl end groups can be vulcanized at room tern-perature by using ethyl orthosilicate as crosslinking agent and dibutyltin dilaurate as catalyst．The generally accepted mechanism of the vulcanization is given in Eq．(1)．However，in actual practice，the relative quantity of ethyl orthosilicate to hydroxysiloxane used is always many times larger than the stoichiometric ratio．The excess of ethyl orthosilicate should render the chain terminal unreactive toward the crosslinking reaction as shown in Eq．(2)，thus questioning the validity of the proposed mechanism．In our experiments carried out in moist air(Table 1)，the rate of vulcanization in-creased with the increase of the amount of ethyl orthosilicate，and the reverse was true when vulcanization took place in dry sealed tubes(Table 2)．This indicates that under dry condition，the excess ethyl orthosilicate retardS the crosslinking action as expected，while，in the presence of water，ethyl orthosilicate is hydrolysed to form polyethylsilicate having increased opportunity for the crosslinking．Consequently，the vulcanization should proceed according to Eqs．(3) and (4) where polyethylsilicate，instead of ethyl ortho-silicate，is the real crosslinking agent，as is generally proposed．When heated to 250℃ in air，the tensile strength of the room temperature vul-canized silicone rubber dropped to almost zero within an hour，and then gradually re-covered on prolonged heating (Fig．1b)．The recovery was not observed in N2 atmosphere (Fig．1c)．Similar results were obtained when polydimethylsiloxane alone was heated with dibutyltin dilaurate．In air，the viscosity at first dropped and then rose again (Fig．3a)，whereas，in N2，there was no such increase in viscosity after the initial drop (Fig．3b)．A small amount (0.1％) of dibutyltin dilaurate caused a significant decrease of molecular weight of polydimethylsiloxane on heating，while further increase of the catalyst produced only a small effect (Fig．4)．However，when water was added，great decrease in molecular weight was noted (Fig．5)．The effect of increasing amount of laurie acid on the molecular weight is shown in Fig．6．All these seem to indicate that the initial drop of the tensile strength of the room temperature vulcanized silicone rubber is due t0 the breaking of the Si—O—Si bond of polydimethylsiloxane by a trace of water in the system under the catalytic action of laurie acid or of dibutyhin dilaurate (Eq．5)．The recovery of the tensile strength when heated jn air for a long time iS caused bv the oxidation of the methyl group of the dimethylsiloxane in the presence of dibutyltin dilaurate to form new crosslinking units．
Polydimethylsiloxane containing hydroxyl end groups can be vulcanized at room tern-perature by using ethyl orthosilicate as crosslinking agent and dibutyltin dilaurate as catalyst．The generally accepted mechanism of the vulcanization is given in Eq．(1)．However，in actual practice，the relative quantity of ethyl orthosilicate to hydroxysiloxane used is always many times larger than the stoichiometric ratio．The excess of ethyl orthosilicate should render the chain terminal unreactive toward the crosslinking reaction as shown in Eq．(2)，thus questioning the validity of the proposed mechanism．In our experiments carried out in moist air(Table 1)，the rate of vulcanization in-creased with the increase of the amount of ethyl orthosilicate，and the reverse was true when vulcanization took place in dry sealed tubes(Table 2)．This indicates that under dry condition，the excess ethyl orthosilicate retardS the crosslinking action as expected，while，in the presence of water，ethyl orthosilicate is hydrolysed to form polyethylsilicate having increased opportunity for the crosslinking．Consequently，the vulcanization should proceed according to Eqs．(3) and (4) where polyethylsilicate，instead of ethyl ortho-silicate，is the real crosslinking agent，as is generally proposed．When heated to 250℃ in air，the tensile strength of the room temperature vul-canized silicone rubber dropped to almost zero within an hour，and then gradually re-covered on prolonged heating (Fig．1b)．The recovery was not observed in N2 atmosphere (Fig．1c)．Similar results were obtained when polydimethylsiloxane alone was heated with dibutyltin dilaurate．In air，the viscosity at first dropped and then rose again (Fig．3a)，whereas，in N2，there was no such increase in viscosity after the initial drop (Fig．3b)．A small amount (0.1％) of dibutyltin dilaurate caused a significant decrease of molecular weight of polydimethylsiloxane on heating，while further increase of the catalyst produced only a small effect (Fig．4)．However，when water was added，great decrease in molecular weight was noted (Fig．5)．The effect of increasing amount of laurie acid on the molecular weight is shown in Fig．6．All these seem to indicate that the initial drop of the tensile strength of the room temperature vulcanized silicone rubber is due t0 the breaking of the Si—O—Si bond of polydimethylsiloxane by a trace of water in the system under the catalytic action of laurie acid or of dibutyhin dilaurate (Eq．5)．The recovery of the tensile strength when heated jn air for a long time iS caused bv the oxidation of the methyl group of the dimethylsiloxane in the presence of dibutyltin dilaurate to form new crosslinking units．
1964, 6(5): 350-355
Abstract:
The effect of oxygen in the suspension polymerization of tetrafluoroethylene initia-ted by ammonium persulfate has been investigated．As usual，oxygen acts as an inhibi-tor in this radical initiated polymerization and the induction period increases with the amount of oxygen present in the system．In the presence of oxygen，pH of the aqueous medium drops considerably after polymerization，with simultaneous production of fluoride ions, the concentration of which increases with botll the amount of oxygen and poly-merization conversion．The product obtained from the polymerization of tetrafluoroethylene in the presence of oxygen contains considerable amount of carboxylic and carboxylate groups as in-dicated by its infrared spectra．According to the measurements of fluoride ion and oxalic acid concentrations of the resulting aqueous medium，it seems that the polymeric peroxide，七(CF2一CF2)n一O—O+，formed in the initial stage of polymerization in the presence of oxygen，does not decompose instantaneously and completely during the course of polymerization．The value of n is generally greater than one．The heat stability of the polymers thus formed is poor--low in thermal decomposi-tion temperature and fast in thermal decomposition rate．The initial thermogravimetric loss is probably due to the decomposition of carboxylate groups attached in the polymer．
The effect of oxygen in the suspension polymerization of tetrafluoroethylene initia-ted by ammonium persulfate has been investigated．As usual，oxygen acts as an inhibi-tor in this radical initiated polymerization and the induction period increases with the amount of oxygen present in the system．In the presence of oxygen，pH of the aqueous medium drops considerably after polymerization，with simultaneous production of fluoride ions, the concentration of which increases with botll the amount of oxygen and poly-merization conversion．The product obtained from the polymerization of tetrafluoroethylene in the presence of oxygen contains considerable amount of carboxylic and carboxylate groups as in-dicated by its infrared spectra．According to the measurements of fluoride ion and oxalic acid concentrations of the resulting aqueous medium，it seems that the polymeric peroxide，七(CF2一CF2)n一O—O+，formed in the initial stage of polymerization in the presence of oxygen，does not decompose instantaneously and completely during the course of polymerization．The value of n is generally greater than one．The heat stability of the polymers thus formed is poor--low in thermal decomposi-tion temperature and fast in thermal decomposition rate．The initial thermogravimetric loss is probably due to the decomposition of carboxylate groups attached in the polymer．
1964, 6(5): 356-362
Abstract:
The intrinsic viscosities of five polystyrene (PS) fractions in mixed solvents of methylethyl ketone (MEK) and hexane were determined．When the intrinsic viscosities are plotted against the solvent compositions，there appears a maximum at the volume fraction of hexane γ=0.13． The light scattaring measurements of a PS fraction in seven mixed solvents of varied compositions show that the values of the second virial coefficient A，also pass through a maximum at γ=0.13． The above results are inter-preted by means of solubility parameters，that of PS lies between those of MEK and hexane．From our experimental results．a solubility parameter of 8.8 for PS is suggested．The θ-composition of PS-MEK-hexane at 30.0℃ was determined both from light scattering and viscosicy measurements；the results obtained are in agreement with each other and γθ=0.45．
The intrinsic viscosities of five polystyrene (PS) fractions in mixed solvents of methylethyl ketone (MEK) and hexane were determined．When the intrinsic viscosities are plotted against the solvent compositions，there appears a maximum at the volume fraction of hexane γ=0.13． The light scattaring measurements of a PS fraction in seven mixed solvents of varied compositions show that the values of the second virial coefficient A，also pass through a maximum at γ=0.13． The above results are inter-preted by means of solubility parameters，that of PS lies between those of MEK and hexane．From our experimental results．a solubility parameter of 8.8 for PS is suggested．The θ-composition of PS-MEK-hexane at 30.0℃ was determined both from light scattering and viscosicy measurements；the results obtained are in agreement with each other and γθ=0.45．
1964, 6(5): 363-369
Abstract:
A new initiator system，di-tert-butyl peroxide in the presence of MgO or BaO，has been proposed for the telomerization of tetrafluoroethylene with lower aliphatic alcohols．Comparative studies showed that the presence of BaO enhanced the rate Of telomeriza-tion and increased the total yield of telomers，as well as lowered the average molecular weight of telomers．This effect i decreased in the following order：methanol>ethanol>iso-propyl alcohol．The telomerization was studied over a wide range of C2F4 alcohol molar ratios．The yield of 1：1 adduct could be satisfactorily controlled by means of the molar ratio． In the case of methanol，the percentage yield of 1：1 adduct was increased from 25．8 to 57％ by increasing the molar ratio from 0.10 to 0.28．The chain transfer constant was found to follow qualitatively the sequence：methanol
A new initiator system，di-tert-butyl peroxide in the presence of MgO or BaO，has been proposed for the telomerization of tetrafluoroethylene with lower aliphatic alcohols．Comparative studies showed that the presence of BaO enhanced the rate Of telomeriza-tion and increased the total yield of telomers，as well as lowered the average molecular weight of telomers．This effect i decreased in the following order：methanol>ethanol>iso-propyl alcohol．The telomerization was studied over a wide range of C2F4 alcohol molar ratios．The yield of 1：1 adduct could be satisfactorily controlled by means of the molar ratio． In the case of methanol，the percentage yield of 1：1 adduct was increased from 25．8 to 57％ by increasing the molar ratio from 0.10 to 0.28．The chain transfer constant was found to follow qualitatively the sequence：methanol
1964, 6(5): 370-372
Abstract:
Nitrile rubber-resin mixtures were found to be good sealants for precision instrument cabins as well as large engine cases owing to their excellent adhesion to metals，good water-tight and air-tight properties，and appropriate resistance to heat and ageing．During preparation of such mixtures，the author noticed that in the presence of metallic oxides，sulphur and T.M.T.D.，the solution mixture gelatinized gradually at room temperature while the free sulphur content gradually decreased．The vulcanizing effects were further proved by the solvent extraction test of the gelatinized mass，although the crosslinking was found to be far much less than those caused by vulcanizing at higher temperatures．
Nitrile rubber-resin mixtures were found to be good sealants for precision instrument cabins as well as large engine cases owing to their excellent adhesion to metals，good water-tight and air-tight properties，and appropriate resistance to heat and ageing．During preparation of such mixtures，the author noticed that in the presence of metallic oxides，sulphur and T.M.T.D.，the solution mixture gelatinized gradually at room temperature while the free sulphur content gradually decreased．The vulcanizing effects were further proved by the solvent extraction test of the gelatinized mass，although the crosslinking was found to be far much less than those caused by vulcanizing at higher temperatures．
1964, 6(5): 373-376
Abstract:
1964, 6(5): 377-381
Abstract:
Triisobutyl aluminium．titanium tetrachloride can induce cis-1,4-polymerization of butadiene when a certain amount of iodine is added．The effects of molar ratio of triisobutyl aluminium to titanium tetrachloride,molar ratio of iodine to titanium tetra-chloride，concentration of titanium tetrachloride and polymerization temperature on con-version，molecular weight，and microstructure of the polymer have been studied． It is shown that iodine reacts with triisobutyl aluminium to form diisobutyl aluminium iodide which is one of the real components of the catalyst．If diethylaluminium iodide is used instead of iodine，cis-1，4-polymerization can likewise be induced．Wjthout triisobutyl aluminium．polymerization does not occur．Experiments indicate that triisobutyl alumi-nium functions as a reducing agent which reduces titanium from tetravalent state to divalent state．If tetrabutyl titanate instead of titanium tetrachloride is used in corn-bination with iodine and triisobutyl aluminium, cis-1，4-polymerization of butadiene also occurs.
Triisobutyl aluminium．titanium tetrachloride can induce cis-1,4-polymerization of butadiene when a certain amount of iodine is added．The effects of molar ratio of triisobutyl aluminium to titanium tetrachloride,molar ratio of iodine to titanium tetra-chloride，concentration of titanium tetrachloride and polymerization temperature on con-version，molecular weight，and microstructure of the polymer have been studied． It is shown that iodine reacts with triisobutyl aluminium to form diisobutyl aluminium iodide which is one of the real components of the catalyst．If diethylaluminium iodide is used instead of iodine，cis-1，4-polymerization can likewise be induced．Wjthout triisobutyl aluminium．polymerization does not occur．Experiments indicate that triisobutyl alumi-nium functions as a reducing agent which reduces titanium from tetravalent state to divalent state．If tetrabutyl titanate instead of titanium tetrachloride is used in corn-bination with iodine and triisobutyl aluminium, cis-1，4-polymerization of butadiene also occurs.
STUDIES ON POLYORGANOSILOXANES——Ⅳ. THE EFFECT OF ALKALI ON THE CROSSLINKING OF POLYDIMETHYLSILOXANES
1964, 6(5): 382-384
Abstract:
Low molecular weight polydimethylsiloxanes containing high concentration of alkali was prepared by heating potassium hydroxide with octamethylcyclotetrasiloxane, KOH+n(CH3)2SiO]4一KO[(CH3)2Si十OH．When this mixture was heated，either in N2 or in O2，the amount of insoluble gel formed was the same．Methane was the sole gas evolved in this reaction，indicating that Si—CH3 is attacked by KOH，forming CH4 and Si-OK，which，through hydydo-lysis and condensation．forms Si—O—Si linkage．The rate of methane evolution at 150°using 2.3 to 4.8％ of KOH in polydimethylsiloxane is shown in Figs．1 and 2，Fig．3 gives the rate at various temperatures using 3％ KOH．
Low molecular weight polydimethylsiloxanes containing high concentration of alkali was prepared by heating potassium hydroxide with octamethylcyclotetrasiloxane, KOH+n(CH3)2SiO]4一KO[(CH3)2Si十OH．When this mixture was heated，either in N2 or in O2，the amount of insoluble gel formed was the same．Methane was the sole gas evolved in this reaction，indicating that Si—CH3 is attacked by KOH，forming CH4 and Si-OK，which，through hydydo-lysis and condensation．forms Si—O—Si linkage．The rate of methane evolution at 150°using 2.3 to 4.8％ of KOH in polydimethylsiloxane is shown in Figs．1 and 2，Fig．3 gives the rate at various temperatures using 3％ KOH．
1964, 6(5): 385-388
Abstract:
Maron’s theory of non-electrolyte solutions has been quite successful for the treat-ment of thermodynamic properties of polymer solutions．However，it has so far been applied to the solutions of polymer in good solvents，where the correct evaluation of eoo is of no importance．It is hereby shown that in order to make the θ-condition deduced from Maron’s cheory to be consistent with the condition A2=O，q should vanish for a polymer in θ-solvent．Consequently a new approximation for the value of eoo is proposed to meet the condition qθ=O，that is，eoo =1-2[n]θ,v instead of eoo=4 used bv Maron and his coworkers．This modification is applied to the treatment of osmotic pressure data of a polydimethylsiloxane fraction in toluene and in butanone at 25℃． If unmodified．the solution in butanone would give a negative value for the second virial coefficient which is evidently unreasonable．
Maron’s theory of non-electrolyte solutions has been quite successful for the treat-ment of thermodynamic properties of polymer solutions．However，it has so far been applied to the solutions of polymer in good solvents，where the correct evaluation of eoo is of no importance．It is hereby shown that in order to make the θ-condition deduced from Maron’s cheory to be consistent with the condition A2=O，q should vanish for a polymer in θ-solvent．Consequently a new approximation for the value of eoo is proposed to meet the condition qθ=O，that is，eoo =1-2[n]θ,v instead of eoo=4 used bv Maron and his coworkers．This modification is applied to the treatment of osmotic pressure data of a polydimethylsiloxane fraction in toluene and in butanone at 25℃． If unmodified．the solution in butanone would give a negative value for the second virial coefficient which is evidently unreasonable．
1964, 6(5): 389-393
Abstract:
Desalting the sodium chloride from water solution of 9-aminopelargonic acid by elec-trodialysis．using “Permaplex”C-20 and A-20 ion-exehange membranes has been studied．Experiments indicate the following operating conditions to be the optimum for a bulk solution containing 3.5 gm of 9-aminopelargonic acid and 1.5 gm of sodium chloride in 100 ml of the solution：1． pH value of the solution in central conpartment=7.5--7.8；2．Maximum current density=about 8 mA／cm2；applied voltage=65 V；3．Concentration of anolyte一sodium hydroxide=0.00135±0.0002 N，and concen-tration of catholyte-hydrochloride acid=0.0040±0.0002 N：4．Electrodialysis is stopped when the specific conductivity of the solution reaches to 5×10-3Q-1cm-1．After desalting the sodium chloride by electrodialysis，9-aminopelargonic acid was crystalized twice from water to give the pure product，mp．186—189℃，ash content 0.1％，chloride ion trace．The yield is about 93％．
Desalting the sodium chloride from water solution of 9-aminopelargonic acid by elec-trodialysis．using “Permaplex”C-20 and A-20 ion-exehange membranes has been studied．Experiments indicate the following operating conditions to be the optimum for a bulk solution containing 3.5 gm of 9-aminopelargonic acid and 1.5 gm of sodium chloride in 100 ml of the solution：1． pH value of the solution in central conpartment=7.5--7.8；2．Maximum current density=about 8 mA／cm2；applied voltage=65 V；3．Concentration of anolyte一sodium hydroxide=0.00135±0.0002 N，and concen-tration of catholyte-hydrochloride acid=0.0040±0.0002 N：4．Electrodialysis is stopped when the specific conductivity of the solution reaches to 5×10-3Q-1cm-1．After desalting the sodium chloride by electrodialysis，9-aminopelargonic acid was crystalized twice from water to give the pure product，mp．186—189℃，ash content 0.1％，chloride ion trace．The yield is about 93％．
