W. J. MacKnight, G. E. Leroi and A. V. T O ~ O I S ~ Y
Princeton University Princeton, New Jersey
Physical Chemistry of Crosslinked Polysulfide Elastomers
The basic idea of polymer science is the conception of the lmear polymer chain, a few examples of which are shown: -CH2CHnCHzCH2CH1CH~CHtCHHCH1CHHCHpolymethylene (high density polyethylene)
-CH,CHCH,~HCH~CHCH,CHCH~~H AHJ
I
CeHs
I
CsHj
atactic polystyrene
Atactic polystyrene, which is the product of commerce, differs from isotactic polystyrene in that the phenyl groups are not stereochemically regular. In atactic polystyrene the phenyl groups are randomly "up" and "down." Ethylene propylene copolymer contains random sequences of ethylene and propylene units as well as random "up" and "down" placement of methyl groups. In the case of small molecules, crystallization in the
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solid state is a very nearly universal tendency. However, this is by no means the case with macromolecules, many of which may be in an amorphous condition in the solid state due to structural irregularities. Thus, polymethylene and isotactic polystyrene are semicrystalline (partly crystalline, partly amorphous) up to their melting points of 137°C and 250°C respectively. Above their melting points these polymers become wholly amorphous. By contrast, atactic polystyrene and ethylene-propylene copolymer (mole ratio ethylene propylene in the range of 1 :1 to about 2: 1) are completely amorphous at all temperatures because of their structural irregularities. All amorphous polymers possess a glass transition temperature T , which is conveniently determined as the temperature at which a plot of specific volume versus temperature shows a change of slope. Below To an amorphous polymer is in a hard glassy state wherein the individual chain segments are fixed on the sites of an irregular quasi-lattice. They can execute vibratory motions around these bed positions, hut do not exhibit significant diffusional and translational motion from one site to another. Above To, the polymer segments begin to exhibit longer range diffusional motions which increase in velocity and amplitude with increasing temperature. The effect of molecular structure on T , is discussed in reference (1). Lightly or moderately crosslmked amorphous polymers are in a true rubbery state at temperatures about 30 degrees above T,. Crosslinking prevents molecular flow and insures reversible elastic behavior in the
absence of chemical cleavage of the resulting molecular network. Rubber articles are shaped and molded while in a flowable h e a r molecular form, and crossliked in the final stage of fabrication. Ethylene-propylene copolymer (2: 1) of high molecular weight has a T, of -60°C. This polymer can be crosslinked by heating it in the absence of oxygen with small quantities of organic peroxides. This causes carbon-carbon linkages to form between chains by removal of hydrogen atoms. In fact, copolymers of this type are expected to be important synthetic rubbers in the near future. By contrast atactic pdystyrene, with a T,of 100°C, is an important plastic. Its useful range is up to 100°C, and it is obviously used in its glassy state. Polysulflde Polymers
We shall consider here an elastomer based on the ethylene tetrasulfide chain: -CH,CH2SSSSCH2CH3SSSSCH1CH1SSSSCHRCHHSSSSpolyethylene tetrasul6de
This linear polymer has a T , of -24'C. Crosslinking may be introduced in this polymer during the polymerization process. In contrast with other crosslinked polymers, it can be molded because of interchange reactions which occur at moderate temperatures between its tetrasulfide linkages. The family of polysulfide elastomers is of practical value in specialty uses as oil resistant rubbers for gaskets and sealants, and for solid propellants. Polysulfide polynlers are prepared by an interfacial polymerization process (2). For polyethylene tetrasulfide polymer, a suspension of ethylene dichloride is prepared in aqueous sodium tetrasulfide using small amounts of a weak wetting agent to stabilize the suspension. Upon warming the reaction proceeds as follows: polyethylene tetrasulfide
,
!
'
The polymerization can be controlled to prepare polymers of very high molecular weight. By using a mixture of ethylene dichloride and 1,2,3trichloropropane in t,he polysulfide reaction, infinite networks are formed. If the halides are con~pletely reacted, each molecule of original trichloropropane is a network junction (or crosslink) with three network .chains emanating from it. The number of network a / 2 times the number of network junctures smce each network chain is shared by two junctures. The polysulfide polymers have a distribution of polysulfide linkages (monosulfide, disulfide, trisulfide, tetrasulfide, pentasulfide, and perhaps higher) inas~nuchas the original tetrasulfide solution very probably has S; SZ=, S3=, S1-, SS= ions. However, there is undoubtedly a predominance of S h ions and a predominance of tetrasulfide linkages in the polymer. For this reason we shall simplify the terminology and refer to polysulfide linkages as tetrasulfide linkages. The average number mof tetrasulfide linkages per network chain depends on the mole fraction X of trihalide in the dihalide-trihalide mixture. It can he seen that:
Rubber Elasticity and Crosslink Efficiency
For a rectangular strip of rubber stretched from an initial length Lo to a stretched length L, the stress required to maintain that length is given by rubber elasticity theory (3, 4):
In equation ( Z ) , F is the force per unit area of unstretched rubber that is required to maintain the length I,. It is expressed in units of dynes/cme. R is the gas constant (in cgs units), T is the absolute temperat,ure, n is the moles of network chains per cc, and 4 is a correction factor, which is unity for an ideal rubber and is fairly close to unity for most actual rubbers. A network chain is defined as the portion of the mocular network between adjacent crosslinks. I n the polymers prepared for the experiment in this paper, three network chains emanate from each crosslink and each network chain is shared by two crosslinks. The quantity n can be calculated for these polymers by the following equation (5): 3 =
i pe Mnz
X
+ M R ( I - X)
In equation (3), MA and Ma are, respectively, the molecular weights of the trifunctional and difunctional chain units: (-SSCH2CHCH2SS--) I and (-SSCHsCHsSS-)
The quantity X in equation (3) is the mole fraction of trifunctional units, p is the density and e is the efficiency of the crosslinking reaction. The mole fraction X of trifunctional units is obtained from the weights of ethylene dichloride and 1,2,3-trichloropropane used in the polymerization recipes. The density of the polymer formed is 1.6 gm/cma. The efficiency e would be unity if the reaction were conlplete, if ethylene chloride and trichloropropane were equally reactive to Na& and if no cyclization were to occur. Equation (2) can therefore be rewritten as:
I t is clear that by an experimental determination of F for a given value of L/Lo, the quantity e+ can be calculated. This quantity is expected to be smaller than unity but larger than 0.1. Measurement of Rate of Bond Interchange
From the equation of state for rubber elasticity it is clear that for a crosslinked rubber sample maintained at ('011313111 extwsion, rile utrrssjil; a t time 1 is related to the i~~irial stress! 0, 2s lollo\\-i:
where N(t) is the number of moles of network chains per cubic centimeter supporting the stress at time t, and N(0) is the number of moles of network chains supporting the stress a t time zero. Volume 42, Number I, January 1965
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Suppose that the network chains are undergoing an interchange reaxtion. Only those chains N(t) which have not yet undergone interchange a t time t are still supporting the stress a t time t. The rate law for the disappearance of N(t) is
very short network chains and promote the formation of a network of relatively uniform chain length. I n any case, m of equations (5)-(12) is to be identified with ft of equation (1). The Experimenl
In equation (6), m is the number of bonds per network chain which are susceptible to interchange and kl is the specfic rate constant for the interchange reaction. In this case m is the number of tetrasulfide linkages Der network chain. Although for the crosslinked polysulfide rubbers the stress decay is certainly due to an interchange reaction, the rate controlling steo of the interchancre mav be the initial cleavage of the p~lysulfidelinkage. Integration of equation (6) gives
-
A
-
N ( t ) = N(0) e l / v c b 7 ~ h=
l/klm
The complete experiment consists of the following parts: preparation of sample, measurement of equilibrium stress, measurement of stress decay a t various temperatures. One or more of these sections may be omitted a t the discretion of the instructor. Preparation of Sample
A convenient apparatus is shown in Figure 1. The following sample preparation is designed to yield
(7)
(8)
From equation (5) The experimental stress decay curves adhere rather closely to equation (9), which is the law for Maxwellian decay of stress. The value of T., can be obtained from an experimental stress-relaxation curve as the time at which f(t)/f(O) = l/e. From the experimental value of T C ~and the known values of m for each of the crosslinked samples, the value of the rate constant k, can he obtained (6) by the useof equation (8). The rate constant k, for each of the rubber samples may he interpreted by the equation obtained from absolute reaction rate theory:
where AH$ and AS$ are the enthalpy and entropy of activation, respectively. Inasmuch as the concentration of tetrasulfide linkages available for interchange remains constant at a value of mN(0) moles of linkages per cubic centimeter, the moles of interchanges q(t) occurring per cuhic centimeter per unit time is governed by the followingequation dt
=
k,mN(O)
(11)
(It must be recognized that not all these interchanges are effective in causing stress decay, because a network chain that has undergone a single interchange is no longer effective in supporting stress. Hence, any subsequent interchanges do not decrease N(t).) Integrating equation (11) we obtain:
Equations (5) through (12) are based on the assump tion that all of the network chains contain the same number m of tetrasulfide linkages. In fact the number of tetrasulfide linkages is distributed about an average value m given by equation (1). However the assumption that m is constant appears to he a good approxima tion, because the experimental stress decay curves obey equation (9) quite accurately. I t is probable that geometric and steric effects prevent the formation of 6
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Journal of Chemical Education
SO0 ML RESIN FLASK
Figure 1.
A p p a r o h for preparation of polysulflde polymers.
approximately one mole of 10% crosslinked polyethylene tetrasulfide. (The procedure may be modified according to the amount of product desired, remembering that the aqueous polysulfide solution should he present in 10% excess.)
commercially as Nekal BX),50% aqueous NaOH, lead acetate test paper, and 50% aqueous HBO,.
Procedure: Add 1.1 moles of N&a.la to the resin flask. Stir and heat whiie adding 5 rnl of Nekal BX solution (5% by weight aqueous solution) and 4 ml of 50% NaOH from the addition funnel. Then add 30 ml of 25% MgCl~6Hz0by drops. This provides a fine suspension of Mg(OH)%,which serves as a nucleating agent for the polymerization reaction. When the temperature of the reaction mixture reaches 90°C, remove the heat and slowly add 1 mole of mixed halide (90 mole % 1,2-dichloroethane and 10 mole % 1,2,3-trichloropropane) from the addition funnel. The mixed halide should he added over a period of about one hour. Heating is not necessary during this stage. After addition of halide is complete, heat for two hours a t 90°C to
insure complete reaction. At this time the product is present in the form of a latex which settles readily to the bottom of the reaction vessel. The latex is then washed repeatedly with warm water until the supernatant liquid fails to discolor lead acetate test paper. After washmg, the latex is coagulated to form a rubbery mass by acidification with 50% aqueous HzSOa to a pH of 4-5. The rubber crumb should be washed 2&30 times to remove inorganic salts. I t is then dried in a vacuum oven at 50°C for 24 hours. I n order to prepare the elastomer for stress relaxation studics, it is necessary to mold it into a thin sheet. This may be accomplished on a hydraulic press. Sample dimensions may vary within rather wide limits, but a sample size of 6 X 2 X 0.2 cm has been found to be suitable. A metal mold should be constructed and the sample molded on the press a t 40,000 psi for 2 min at 150°C. If the sample is not homogeneous, the molding should be repeated. If desired, this section of the experiment may be omitted and samples obtained commercially from the Thiokol Chemical Corporation. Measurement of Equilibrium F
A stress relaxation balance with associated constant temperature oven is depicted in Figure 2. Such a balance may be readily constructed or, alternatively, a connnercial apparatus is available1. I n this part of the experiment the constant temperature oven is not necessary, as measurements are to be carried out at room temperature. The procedure is simply to evaluate experimentally the quantities involved in equation (4). Lo is the separation of the upper and lower jaws under no stress. The positions of the jaws are located with a cathetometer. Elongationsof about 10% (L) are appropriate, accomplished by using the micrometer, and then adding weights to the balance pan and holding the equilibrium for at least 10 minutcs. The force, F i n equation (4), is calculated from
where m in the mass in grams applied to the sample and A is the cross-sectional area of the sample in cm2. Known values of F, Lo, and L allow the computation of e+. Measurement of Stress Decoy at Vorious Temperatures
The procedure necessary to obtain f(t)/f(O) as a function of time is quite similar to that detailed above. The constant temperature oven shown in Figure2 contains a blower in the rear of the oven to circulate air. The speed of the blower is regulated by a rheostat. Heating is achieved by two electrical heaters. One heater is connected to an Aminco thermoregulator and supersensitive relay; the other heater is controlled by a rheostat. Temperature control to *O.l°C can be achieved if the heater connected to the rheostat is 1 "Stress Relaxation Modulus Balance Mk 1,'' Bulletin issued by the Hunter-Bristol Division, Thiokol Chemical Cop., Bristol, Pa., Dec. 1, 1960. This describes a relaxation balance and oven at unit prices of $842.00 and $962.42 respectively. These instruments me available from the Bristol Division of the Thiokol
Chemical Corp., P. 0. Box 27, Bristol, Pa.
Figwe 2.
Strerr relaxation bolance ond conrtont temperature oven
adjusted so that the other heater operates approximately 50y0 of the time. After the sample is clamped and the apparatus zeroed, the oven is heated to the desired temperature (e.g. 70°C). Since relative stress only is to be measured, it is not necessary to obtain Lo for this part of the experiment. The sample should be annealed at the temperature of measurement for approximately one hour. Then, with the sample under zero tension, sufficient weight should he placed on the pan so that the balance will be zeroed at approximately 10% sample elongation. The micrometer is used to stretch the sample until equilibrium is attained, at which point the timer is started. The initial weight is taken to he f(0) and weights are then removed from the pan in 1-g increments until f(t)/f(O) is about 0.2. The time a t which equilibrium is established a t each stress is recorded. Aplot of f(t)/f(O) as a function of log time is made from, which r,b and kl may be obtained. The process is repeated a t 70a, 80°, 90°, and 100°C. The activation energy for scission of the polysulfide bond is obtained from an Arrhenius plot and AS$ and AHt. are evaluated using equation (10). Liferature Cited
TOBOLSKY, A. V., "Structure and Properties of Polymers," John Wiley & Sons, Inc., New York, 1960, chap. 2. (2) BERENBAUM, M. B., AND PANEK, J. R., "Polyethers, Part 111: Palydkylene Sullides and Other Polythioethers," N. G. GAYLORD, Editor, Interscience, New York, 1962. (3) TOBOLSKY, A. V., CLFLLSON, D. W., AND INDICTOR, N., J.
(1)
Polymer Sei., 54, 175 (1961).
(4) (5)
CIPERRI, A,, J. Polymer Sei., 54, 149 (1961). TOBOLSKY, A. V., BEEYERS, R. B., AND OWEN, G. D. T., J.
(6)
TOBOLSKY, A. V., BEEVERS, R. B., AND OWEN,G. D. T.,J.
Colloid Sei., 18, 353 (1963).
Colbid Sei., 18, 359 (1963).
Volume 42, Number I , January 1965
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