(7) ( l / n ) - 1 = -a;
n = (1 -- a ) - l
Substituting equation 8 in equation 10 results in the final relationship
(8)
B = ( 1 - a ) (1__a>cl-ai-'
Solving for K in equation 7 I\: = (Fn)"
(0)
K and B may be related by substit,uf,ingequatJion 6 in the above expression B =
(Fn)"/jl
(11)
By comparing the exponents of equations 2 and 4 X:=n-l
(12
Substituting equation 8 in equation 12
(10)
k = a(1
- a)-1
(I3
SPECIFIC HEAT OF SYNTHETIC HIGH POLYRIRRS: I\r. POLYCAPROLA4CTARI BY PAULMARX,C. 117. SMITH,A. E. WORTHINGTON AND MALCOLM DOLE Contribution from the Cheinical Laboratory of Northwestern University, Evanston, Illinois Received March 11, 1965
Polycaprolactam, or 6 Nylon, has been studied with respect to its specific heat using a new calorimetric system over the temperature range -20 t o 280" in flake, drawn, undrawn filament and annealed forms. The results are compared with similar measurementsoon 6-6 Nylon. Apparently, depolymerization in the melt on annealing lowers the glass transition temperature about 15 . The undrawn filaments exhibit irreversible recrystallization effects from 90 to 160". The crystallinity of 6 Nylon is slightly less than that of 6-6 Xylon, but exhibits a maximum in the case of the drawn and undcawn fibers a t 190". At room temperature the crystallinity of the drawn fibers is less than that of the undrawn.
I. Introduction Polycaprolactam or G Nylon is chemically identical with po1:vhesamethylene diamine, G-G Xylon, escept for 1ie:id-to-tail arrangement of its imine and cnrlionyl groups as compared to the head to head arrangement of the imine or carbonyl groups i i i G-G Nylon. For this reason it appeared to be of iiiterest to measure tlie specific heat of G Nylon for comparison with previous measurements on 6-6 Nylon.' Such nieasiirements include the mmsurement of heats of fusion, heats of transition, heats of crystallizatioii and heats associated with second order or glass? trmisitmions. -4s far as we ara aware there have beeii n o previously published data for the specific lieat of G Njrloii. Brill3 made an X-ray study of G ant1 G-G Nylon as a function of temperature. He demonstrated that the 020 mid 220 lattice spacings increased more rapidly with temperature in the case of the G-G hr.vlon than in tlie case of G Nylon and equalled the 200 spacing a t IG5" giviiig to the G-G Yylon a pseudo hexagonal st,racture a t that temperat'iire. This hesagoiial structure esists in cross-secl,ion only. In the case of G-G Nylon the breaking: of hydrogeu boiids followed by the sliiftiiig of adjacent chains and :t rotational oscillat'ion of aljoiit GO" of the chain segments permits the formation of new hydrogen bonds aucl the estahlishnieiit of tjhe pseudo hexagoiial st,ructure. In G Nylon the segmental rotation does not result in coiitlitions fai.or,zble for hydrogen bond formation, heiice G Nylon does not make tlie trniisition to pseudo hesagolid symmet,ry. Mikhailov and Klesmaii4 strudied tlie effect of ex(1) R. C. Willloit and Rf. Dole, THISJ O U R N A L57, , 1 4 (1953). (2) P. J. Flor,y, "Principles of Polymer Chemistry," Cornell Univcrsity Press, Itliaca, N. Y., 1953, p. 5G; a n d W. Raustnonn, Chem. Reus., 43, 219 (1948), hove reviewed t h e subject of second order and class transitions. ( 3 ) R . Brill, J . pralil. C h e m . , 161, 44 (1042). ( 4 ) N . 1'. llikliailox. and 1'. 0. Klesnmn, DoHadu A k o d . Nnirb.. TJ.S.S.R.. 91,$19 (l$l.X3): C . A . , 48, 20 (l:l54); 48, I3RB!l ( l f l , i 4 ) .
tent of crystallization on the thermal properties of G Nylon. Making a thermographic and X-ray analysis of samples prepared from an e\.aporation of a formic acid solution or by slow cooling and samples prepared hy rapid cooling, they concluded that 6 Nylon could exist, in two forms, an imperfect, unstable crystalline form, and a glassy amorphoiis form. The sloivly cooled form exhihit,ed a single endothermic effect a t 200-216", while the amorphons form showed t,wo endothermic effects, at, 120-158' and a t 216-222'. The 120-150" range is also referred to as n range of temperat>ureswhere \.itrificntion occurs. They speak of a ''1ie:Lt of fusion" of t'he crystalline form as 12.4 cal./g., and of t8heglassy modification as 9.4 cal./g.
11. Experimental Details The following samples of 6 Nylo115 were studied: 1. Estracted 6 Nylon flake containing npprosimately 1.7.% water estractables having a number average inolecular weight, on an extimacted basis of about 20,000. 2. Undrawn G Nylon yarn (1050 denier, 34 filaments) containing about 4% water estractables and having a number average molenul:u weight on :in extracted Imsis of about 18,500. 3. Di,an-n 6 Nylon yarn (210 denier, 34 filaments) containing about 4% water ext,ractnbles, and having a number average molecular weight on an estmcted basis of about, 18,500. The above samples were used as received from the du Pont Company except that the samples were dried t o constant weight by evacurtt,ion before each specific heat measurement. "Melt annealed" samples were prepared by slowly cooling the G Nylon while still in t,he calorimeter after the 6 Nylon had been heated above the melting point. It was assumed that due t,o depolymerization in the melt the melt annealed samples contained llyo of monoinri, (caprolactam) wlien heated5 to 282", and 7y0 non no me^^ i f heated6 t o 235'. Other annealed samples ~vet'e p r c ~ ~ i : i ~ ~ t ~ I by slow cooling from a temperature somewhat I)clow the melting point,. These were called "210 or 190' a n i i ~ : d ~ ~ I " Nylon. (5) Generously forwarded t o 11s hy Dr. ,I. Ziiiiiiierninii of t l i p Nylon Research Division, E. I. ( l i i Pont de N e i i ~ o i i r sand Clo. (G) Information received froni Dr. R . E. Wilfong, Nylon Rrsewrli Dii.isioii, E. I. dii Pont de N e i i i o i i r s and Co.
P. MARX,C. W. SMITH,A. E. WORTHINGTON A N D M. DOLE
1016
'\
+-
I
I
SPECIFIC HEAT OF UNDRAWN S-NYLON 0
Vol. 58
0 SERIES 1 SERIES
0.40
I
0
20
40
GO
80
0
20
40
GO
80
T , "C.
T,"C.
Fig. 1.-Comparison of the specific heat of undiawn G Nylon (solid line) with that of undrawn G-G Nylon (dotted line) over the temperature range -20 to 80'.
Fig. 3.-Compnrison of the specific heat of melt annealed G Nylon (solid line) with that of 6-6 Nylon (dotted line)
A new improved calorimeter and bimetallic adiabatic
jacket' were used for the specific heat measurements on 6 Nylon. The watt-hour meter for the measurement of the electrical energy input previously describeds was calibrated7 by measuring the energy required to melt a known weight of ice in the calorimeter. The heat capacity of the empty calorimeter was found by calibration? using synthetic sapphire, A1203, whose specific heat has been accurately determined by Ginnings and F u r ~ k a w a . ~The technique of applying corrections for temperature unbalance between the Calorimeter and adiabatic jacket previously described'O was considerably simpler in this research as compared to that on 6-6 Nylon,' because of the improved design of the calorimeter and the m e of heat shields to close the top and bottom ends of the adiabatic jacket.? Only one correction factor was applied instead of two or more. In one calibration experiment using A1203 sixteen heat capacity results for the total system of calorimeter plus contents deviated
the data obtained in two independent series of measurements fluctuated on the average +0.4y0 from a smooth curve; over the temperature range 40 to SOo, the fluctuations decreased to an average of &0.33%, but over the range 80 to 160' where probably some recrystallization took place in the polymer, the average fluctuations rose to f0.93%. In the liquid range, 240 to 270°, where strains and irregularities similar to those of the solid state would not be expected to exist, the average fluctuations fell to f0.25% for the data of three series despite the higher temperature and the greater difficulty of carrying out specific heat measurements a t such temperatures. The uncertainty in the absolute values a t the high temperatures may be as high as f 0 . 5 % .
111. Experimental Results and Discussion A. Second Order or Glass Transition Effects.Both 6 and 6-6 Nylon exhibit changes of specific heat with temperature suggestive of second order or glass transitions. By a second order or glass transition we mean a discontinuity in the specific 0.45 heat-temperature curve, the specific heat suddenly SPECIfIC HEAT OF increasing with the slope of the specific heatDRAWN 6 - N Y L O N SERIES temperature curve usually being greater a t temperatures above the second order transition than below. The transitions are not sharp, nor are they transitions between two phases in thermodynamic equilibrium.ll Because they are probably kinetic effects with the magnitude of the effect depeiidiiig on the time of observation, these transit,ions should more properly be called glass transit,ions.? The glass transitions occur probably only I 0.30 b' - 1 - - -L.- -. 1 -1 --A in the amorphous regions of the polymer, inasmuch O 20 40 (i0 80 as they seem to be far more pronounced and defiT , "C. nit,e in polyvinyl chlorideI2and polyethylene tereFig. 2.-Comparison of the specific heat of drawn 6 Nylon phthalatjel3which are much more amorphous than (solid line) with that f: drawn G-6 Nylon, ovei the teniperathe polyamides. Another indicat,ion of the importure range -20 to 80 . tance of the amorphous regions for the glass transifrom a smooth curve to an average extent of 0.07%. The tions is the fact that the glass transition temperaspecific heat data for the high polymers are less accurate, however. The uncertainty depends partly on the experi- tures for G and 6-G Nylon are the same (at least for mental methods and technique, but also on the reproduci- the flake form) despite the fact that their masimum bility of the behavior of the polymers themselves, as pre- melting points (determined by the crystalline reviously noted.' As an example, let us consider the undrawn gions) differ by 40". fibers which represent the most rapidly quenched and the Examples of glass transitions in this research are least reproducible samples. Over the temperature range from -20 to +40°, which covers the glass transition range, shown in Fig. 1, undrawn 6-Nylon a t 30-40°; and Fig. 3, melt-annealed 6-Nylon at 15-20". Granular (7) A. E. Worthington. Paul Marx and M. Dole, paper submitted G Nylon also shows a glass transition at 40-50". for publication. (8) M . Dole, N. R. Larson, J. A. Wethington, Jr., and R. C. WilThe lowering of the glass transition temperatlure hoit, Rev. Sci. Instru., 22, 818 (1951). on melt-annealing the 6 Nylon is probably the re-
'
D
0
-
(9) D. C. Ginnings a n d G. T. Frirukawa. J . A m .
CArn. S o c . , 75, 5 2 2
(1953). (10) M. Dole, W. P. Hettinger. .Jr,, N. Larson, J. A. Wethington, Jr., and A . W o i t l i i n q t o i i . Aru. S r t . J u ~ ~ J 22, I ~ , 81? , (lrlnl).
e.
(11) J. E. Mayer and S.B. Streeter. J . Chern. P h ~ s .?, , 1010 (1!139). ( 1 2 ) Unpublished d a t a of S. Alford and M. Dole. (13) 1.JnliublifiIiPd data of C. W. Siiiitli a n d 31, r h i h
THESPECIFIC HEATO F P O L Y C A P R O L A C T A M
Oct., 1955
70
00
110
130
150
170
T , "C. Fig. 4.--Comparison of the specific he:tt of undrawn 6Nylon (solid line) with that of 6-G Nyloii (dotted liiic) over the temperature range 70 to 170".
sult of the depolymerization of the material that occurs a t high temperatures, the monomer and dimer content rising t o l l from 4 weight per cent.6 Table I contailis data that illustrate the relationship between monomer content and the glass transition temperature. The glass transition temperature given is the temperature a t which the slope of the specific heat temperature curve is a masimum.
80
100
1017
120 T , "C.
140
lG0
180
Fig. 5.-Comparison of the specific heat of drawn 6 Nylon (solid line) with that of 6-6 Nylon (dotted line) over the temperature range 80 to 180".
not reproduced by the annealed samples, they undoubtedly represent irreversible transitions of segments of the chains from metastable states to states of lower energy. The transitions might also represent small crystallizations taking place. Three fairly well defined minima seem to exist over this range. Drawing the undrawn filaments eliminated these fluctuations. In the case of the melt annealed TABLE I sample, Fig. 6, the specific heat rose smoothly with temperature oi~erthe temperature interval, 80 to G L A S S TRANVTION TEMPERATURE AND MONOMER CONTENT Approx. % 180". Neither the pre-melt annealed nor the Transition of cyclic granular 6 Nylon exhibited any uiiusual specific temp., monomer Form OC. present heat effects in this temperature range. Flake 45-48 1.7 It should be noted that the specific lieat data are Undrawn filaments 37 4 sensitive indications of transformattions within the Pre-melt annealed 38 4 solid. Thus, the area between a stmight line conMelt annealed 20 11 necting the peaks of the specific heat temperllture The monomer possibly acts as a plasti~izerl~ i n curve for the undrawn sample at 125 and 144" and the amorphous regions of the polymer, thus per- the curve itself is equal to 0.5 cal./g. The evolution mitting the onset of the molecular vibr at'ions re- of this quantity of heat could be produced by the sponsible for the appearance of the glass transition crystallization of only :about lY0 of the polymer. In the case of 6-G Nyloii, the annealed polymer a t 20 instead of 40". Conversely drawing the fibers produces a peculiar peak in the specific heat exhibited a well defined mssimum in the specific curve a t 115" in the case of 6 Nylon, Fig. 5 , and a heat cur\.e a8ta temperxture of 165" where the crysless pronounced peak a t 135" in the case of 6-6 t8alstructure of G-6 Nylon changes from triclinic to Nylon. There is a definite slight "hump" in the a pseudo liesagonal type l:itJtice as tliscoIrered by specific heat curve of drawn 6 Nylon a t 42". When Brill.3 There was no indication from the X-ray int,he drawn fibers are annealed just below their vestjigation of a cryst,al st,ructure change a t 1G5" melting point, the pre-melt annealing, tlie effect atJ i n tlie case of 6 Nylon, nor was tliere :tnythitig pe115" disappears and the glass transition reappears culiar in the specific heat in this temperature range. a t 40". We interpret these observations on the Brill first considered the possil)ilit8ythat the inasupposition that drawing the polymer produces I)ilit8y of G Nylon to make the cryst8al structure mechanical strains in the amorphous regions which tr:uisition resn1t)ed from :I, hindering of rotation or raise the activation energy required for the onset of rotationid vilirations in G Nylon as compared to G-G the molecular motion responsible for the glass tra.n- Nylon, but he then rejected this explanation in fasition ; annealing eliminates the strains and permits vor of one in which he postulated that in the case of the molecular motions t o commence a t their nur- 6-G Nylon a rotation of the chains through ail angle of about 60' as well as a shift of neighboring chains mal temperature of 40". of molecules relative to each other in the direction B. Recrystallization in the Solid State.-Much more pronounced than the glass transition effects of the long asis made possible further hydrogen are the unusual fluctuations in speci.6~heat of the bonding between the chains. I n G Nylon a similar undrawn 6 Nylon over the temperature range 90 to rotation is not favorable for the formation of hydro160" as illustrated in Fig. 4. As these effects are gen bonds. The specific heat data favor this second interpretation of Brill's rather than the first, (14) Tli. Gasi:, Kunslstofe, 43, 16 (1053),measured the specific heat inasmuch as if the first were true, one might expect of polyvinyl chloride as a function of plasticizer content a n d demona higher specific heat a t 165' for G-6 Nylon than Rtrsted t h a t t l i i ! glass transition range broadens orit t o lower temfor 6 Nylon, but the opposit,eis true. peratures a8 the content of plasticiner increases.
80
GO
-
e-
SPECIFIC H E 4 1 ff 4NNEAED 6 NYLON
.%0.52 c (
'
0 ,
I-- --/
0,
0.47
-
I
D
X
SERIES
-
r
f / ' ~
11 I
80
120
/'
LI? F,
-
100
Vol. 59
/' ~~,-~ MLT
I
2
.3
,
P. A I i i w , C. W .SRIITII, .A. E. WORTHINGTON AND M. DOLE
1018
-~
140
1GO
-
I ~
.-sh Y
~
3
,
8
5 40 k
180
T , "C. Fig. O.-Coinparison of the specific heat of melt annealed G Nylon (solid line) with that of G-G Nylon (dotted line) over the temperature range 80 t.o 180 .
C. The Percentage Crystallinity of 6 Nylon.In Fig. 7 the percentage crystallinity of undrawn, drawn and melt annealed 6 Nylon is given. These curves were calculated from the following assumptions: (1) the enthalpy of the amorphous 6 Nylon down to 160" is given by the equation for the enthalpy, H L , of liquid 6 Nylon as deduced from measurements of the specific heat in the liquid range, or
20
0
100
200
300
T , "C. Fig. 7.-Comparison of the percentage of crgat,allinitg of different forms of G Nylon: 0, melt annealed; 0 , rindrnwn filaments; f, drawn filaments.
ues, and that in contrast to 6-G Nylon the crystnlliiiity of the drawn filaments is slightly less than that) C,(liquid) = 0.577 + 3.36 X 10-'2' of the undrawn a t room temperature. This latter HL = H L , 280' 0.577(T - 280) f observation was also noted in the case of drawn and 1.68 X l o W 4T( 2 - (280)?) undrawn p01yethylene.l~ The decreased cryst,allinity of annealed G Nylon with respect t,o nnwhere T is the temperature in degrees centigrade. nealed 6-6 Nylon may he the result of the presence (2) The eritJha,lpy of the amorphous G Nylon of monomer and diiner produced by the annealing. from zero to 160" is given by adding the heat of fuBetween 190 and 225" the actual heat of fusion sion to the estimated enthalpy of perfect)ly crystal- would be approximately 0.67 X 44 or 29 cal./g. line G Nylon, H,, or This is considerably more than the estimate of 12.4 cal./g. due to Mikhailov and T
