K. I?. KWEIAR’R T. K. KTVEI
2 146
Vol. 66
THE SORPTION OF ORGANIC VAPORS BY POLYOLEFINS BY El. P. KIWEI Newark College of Engineering, Newark, New Jersey ASD
T. E(.KV~EI
interchemical Corporation, Central Research Laboratory, New York, AT. Y , Received April 8, 1968
The equilibrium sorption of aliphatic and aromatic hydrocarbons by isotactio and by completely amorphous polypropylene was studied. The contribution of the enthalpy change of network deformation is analyzed. I n addition, the average molecular weight of polypropylene chain between crystalline regions is calculated.
Introduction The sorption of ore;$^ vapors by polyethylene acloompanied by a sigat temperaturn nificant change in Thus the isosteric enthalpy of sorption,edefined by eq. 1
(An11m r p RT2
(1)
can he estimated from the data of Rogers and coworkers1 to be in the neighborhood of 1 kcal./mole far the system hexane- olyetbylane in the temperature range 0 to 30”. he enthalp of sorption is composed of enthalpies of-mixing, %~J,,,, and of network deformation, (AM1)eI. It is eustomary to neglect the latter quantity,2 with the assumption that the elastic properties of the network stmcture do not deviate, to any significant extent, from ideal rubber elasticity. The enthalpy of vapor sorption then oan be equated to the enthalpy of mixing, which conventionally is expressed in terms of the net interaction energies of solvent-polymer pair contacts.? It i8 difficult to erstand, however, that the pair contact inter energy of two chemiodly alike substances such as hexane and polyethylene. can reach the magnitude of 1 kcal./mole, The sorptim d toluene vapor by polystyrene,& and the sarption of various Cs to Cs aliphatic hydrocarbon vapors by polyisobutylene4 show no temperature coefiici hence (AH2)sorp for the above systems, mu nwgligihle. The heat of mixing n-heptane with polyisobatglem (PIB) &SKI has been ayixwpw$ imetricdly; by W a t t m and oo-wo~ker8~;(AHL)mix can be calculated from their data to be slightly negative, i.e., about -60 eal./mole. Recently, Flory, Ciferri, and Cbiang6 c0,efficientof the swelling measured: the tempera d PE by n-octacosane at 140-180” (above the melting point of PE crystallites) and concluded that the heat Qf mixing was al-
F
(1) C. E. Rogers, V. Stannett, and M. Sswarc, J . Polymer Scz., 45, 61 (1960). ( 2 ) P. J. Flory, ”Principles of Polymer Chemistry,” Cornell University Press, Ithaoa, X. Y., 1963, chapters 11-13, J . Am. Chem. Soc., 7 8 , 5222 (1956); P. J. Flory, C. A. J. Hoeve, and A. Ciferri, J . Polymer Scz., 34, 337 (1959).
(3) C. E. H. B a a n , R. F. J . Freeman, and A. R Kammaliddin, Trans. Faraday Soc., 46, 677 (19.50). (4) R. Prager, E. B a d e y , a n d F A. Long, J . Am. Chem. Soc., 76, 2742 (1953). ( 5 ) C Watteis, H. Daust, and M. Rinfret, Can. J . Chem., 38, 1087 (1960). (6) P. J. Ylory A. Ciferri, and R Chiang J . Am. Chem. Soc., 83, 1023 (1961).
most negligible. The vapor sorption experiments Rogers and co-workers Were Carried out near room temperature, at which the amorphous regions of the PE film were under severe strain as aresult of the restriction imposed by the neighboring crystallites. It is possible that the deformation of such a network produces changes in chain conformation with a concomitant change in the potential energy af the n e t ~ o r k . ~ The above situation is unlikelv to occur when amorphous polymers are used as absorbents. There is reason to suspe_ct,therefore, that the difference between the (AHl)sorp values for the sorption of aliphatic hydrocarbons by P E and- by PIB originates, not from the difference in (AHI),~,,but rather from the contribution of the (AH1),1 term. The present investigation \?as undertaken to elucidate the contribution of (AHl),ltp the sorption of organic vapors by polycrystalline materials. Isotactic polypropylene (PP) film was used because no change in the crystallinity of PP with temperature has been reported between 25 and 60”; the ambiguity ari&ng from the phenomenon of crystallite “melt-out,”8 such as in the case of low density PE, thus is minimized. The significance of energy and entropy changes in the swelling of a non-ideal network is discussed briefly. In addition, the length of the repeating units in the amorphous region of PF is estimated. For the purpose of COMparison, experiments also were conducted with eompleteIy amorphous PP. Theoretical Considerations Enthalpy Change in Network Swelling.-According to the current viewpoint, the energy change in the deformation of a network structure may arise from the change in chain conformation from the low energy to the high energy state. It is assumed that Me1=
gw
(2)
where g is the number of low energy-to-high energy conversions, each of which is characterized by an energy change w. In the swelling of a polymer by solvent molecules, the polymer chain segments, originally occupying a volume V , are redistributed in a volume V AV, where AV is the volume of the solvent imbibed by the polymer at equilibrium. Many segments of the polymer chain are displaced by the solvent molecules from their original posi-
+
( 7 ) A. Cifeni, C. A. J. Hoeve, and P. J. Flory. zbzd., 83, 1015 (1961). (8) 8. S.Michaels and H. J. Bixler, J . Polymer Sm., 5 0 , 393 (1961).
THESORPTION OF ORGANICVAPORSBY POLYOLEFINS
Nov., 1962
2 147
tions in the lattice to new positions. I n the course of this process, some of the displaced segments or their neighbonr will be forced to assume conformationally high energy states. As a first approximation, it is assumed that g is proportional to the number of such displacements, or. the number of solvent-polymer contacts, p12
differ from that of completely amorphous polymer chains. The parameter x' of our interest is not The quantities dx'/dT necessarily the same as XI. and dxl/dT, however, are identical because both represent pair contact interaction energies. An alternate form of eq. 8 is
(3) provided that the tendency for the solvent molecules to form clustersQis small. The factor q is the probability that each solvent-polymer contact is accompanied by a change in the chain conformation of the polymer. According t o the lattice theory of polymer solutionsj2
In
g = 4P12
1212
=
x w 2
(4)
where nl is the inumber of solvent molecules, v2 is the volume fraction of the polymer, and x is the lattice coordination number. Upon substitution in eq. 2, there is obtained
AHel
Q , ~ ~ V Z
(5)
and
a1 =
In
01
+ uz + !3
((a)02vzl/a
-
s)
(9)
u2
M C
where X represents the difference between x' for polycrystalline material and x1 for the completely amorphous polymer. The evaluation of the various parameters €h, 1, and M , from the vapor sorption data can be carried out as follows; the crystallites are considered as cross-link junctions with a functionality f equal to twice the number of chains in the cross-section of a The term (21/2)/f can be neglected because .f is much greater than 2v2. The molecular weight of cross-linked chain M , is the average molecular weight between crystallites. Upon simplification and rearrangement,9eq. 9 becomes
qxw (6) Differentiation of AH,1 with respect to n1 gives Eh
P l V (CY)OZ
u2-6/8
(10)
MC
A plot of the left hand side us. p 7 1 7 ~ 2 - ~ / 'will yield a straight line with ( C Y ) ~ ~as / Mthe , slope and (xl Eh, which is characteristic of the network, but is X EIJRT) as the intercept with the ordinate. independent of the nature of t;he swelling solvent. If a suitable solvent can be found so that the net Eq. 7 also demands that ( A H I ) , ~is directly pro- pair contact energy of the polymer and the solvent portional to 2122. is zero, the quantity dx'/dT or dxr/dT is zero. Swelling Equilibrium.-The equation for swell- A plot of (xl X Eh/RT) us. 1/11 then will result ing equilikiim now can be rewritten to include in a straight line with e h / R as its slope. The interthe energy contribution of network deformation. cept of this straight line with its ordinate is (xl The elastic free energy AFel involved in the expan- A). The value of Eh obtained by this procedure must sion of a network structure is2 agree with eq. 7, if the above analysis is self consistent. AtFei = AH,, - TAX,, In order to establish whether the net pair contact energy of a given solvent with the amorphous and fraction of the isotactic polypropylene film is zero, the sorption of organic vapors by a completely amorphous polypropylene sample also was studied. (Many aliphatic hydrocarbons fulfill this requirement.) The parameter XI can be obtained readily from the relationship4
It readily can be seen that (Afll)Q1is a function of
+
+
+ +
+
At swelling equilibrium, the equation of state may be written as In ul = In
VI
+ + 02
X'VZ~
+
Equation 8 is of the same form as .the conventional eh/&5")v22 replaces x11/2. one except that (x' The state of randomness of the chains of the amorphous fraction of a polycrystalline polymer may
+
(9) B. H. Zimm and J. L. Lundberg, J . P h y s . Chem., 60,425 (1956).
In a1
= In u1
+ + xlv22 v1
(11)
Experimental Materials.-Completely amorphous polypropylene (sample A) from Hercules Powder Go. has limited solubility in xylene at room temperature. Film was cast from xylene solution over mercury at 70°, followed by subsequent evacuation in vucuo to remove any residual solvent. The density of the resultin$ material is 0.854 g./cm.3 at 25" and 0.827 g./crn.3 at 50 . Isotactic olyprogylene film (sample 3) from Avisun Co. wasused. $he density of the film is 0.908 g./cm.3 at 16' and (10) C . E. Rogers, V. Stannett, and M. Bzwarc, ibid., 63, 1406 (1959).
I 2’ > 3 and undissociated acid and to solvation of the protonated amines by the benzene x-electrons. Primary and secondary aromatic amines showed a weakening of base strength in benzene relative to the aliphatic and tertiary aromatic amines. Heats of reaction were estimated for 1:2 and 1:3 molar amine: acid associations.
.
Introduction The dissociation constants of amines in water are used commonly as a measure of base strength. It is well known that discrepancies occur in comparing base strength values (dissociation constants) with molecular structure of amines. Solvation of an amine moleculle or its conjugate acid with water molecules limits direct comparison of dissociation constants of primary, secondary, and tertiary amine~.~A J non-solvating solvent would appear the best choice for obtaining these base strength values. Forman and Humee have correlated Taft Zcr* values with heats of neutralization (AH,) of aliphatic primary and straight chain secondary amines in acetonitrile. They found AH, for (1) R. P. Bell and J. W. Bayles, J . Chem. Soc., 1518 (1952).
(2) A. F. Trotman-Diokenson, ibid., 1293 (1949). (3) H. K. Hall, Jr., J. Phys. Chem., 60, 63 (1956). (4) H. K. Hall, Jr., J . A m . Chem. Soc., 79, 5441 (1957). (5) M. M. Davis and H. B. Hetaer, J. Res. Natl. Bur. Std., 60, 569 (1958), RP 2871. (6) E. J. Forman and D. N. Hume, J . Phus. Chem., 63, 1949 (1959).
rneta- and para-substituted aromatic amines correlated with pKb(H20) values, but “no obvious correlation” of AHn with pKb(HZO) values for non-aromatic amines was obtained. Deviation of branched chain secondary and tertiary aliphatic amines was explained by the B strain hypothesis. Limited solvation of amines and ammonium ions in acetonitrile explained these results. Preliminary thermometric titrations of amines in benzene with trichloroacetic acid by the author indicated a greater spread of AHn values than those obtained by Forman and Hume in acetonitrile. Solvation of amine molecules and cations should be less in benzene. This study concerns the heats of neutralization of amines in benzene with a reference acid, and the correlation of these reaction heats with base strength values in various solvents. Experimental Materials.-Reagent grade benzene and trichloroacetic acid from Baker and Adamson Go. were used throughout.
