isocyanate imparts extremely good light stability and weather resistance to urethane coatings (Gruber, 1965). To evaluate these topcoats, they were compared with the presently specified aircraft acrylic (MIL-L-81352), epoxy-polyamide (MIL-C-22750C), and aliphatic polyurethane (MIL-C-81773A/AS) coatings, These coatings were subjected to more than 15 tests to evaluate their durability as compared to the above coatings, and proved superior in all aspects. Several high gloss formulations in various colors have been weathered in Washington, D.C. for more than a year with excellent retention of gloss. In addition to the tests specified for aliphatic polyurethanes, a series of corrosion test specimens was prepared by coupling 7075-76 aluminum panels to 2024-ST aluminum blocks with a pattern of eight cadmium-plated steel fasteners. No sealant was used during assembly, thus enabling corrosion to proceed more rapidly and to compare the protection of the coating systems by themselves. These specimens were stressed and exposed to 500 hr of 5% salt fog and an additional 1000 hr to acidulated salt fog (ASTM: B 287-62). These tests compared the specified polyurethane coating, MIL-C-81773, with the NRL-developed fluorinated polyurethane over various primer systems. The primers included the specified epoxy-polyamide (MIL-P-23377C) and two NRL-formulated phenoxy systems, both over the MIL-(2-5541 conversion coating and on bare aluminum. The results showed: (1) that there is inadequate adhesion of the primer without the conversion coating, (2) in all cases the fluorinated urethane gives better corrosion and blister resistance than the specified polyurethane, and (3) over the conversion coating, phenoxy primer without isocyanate curing agent shows comparable or superior primer performance to the specified epoxy-polyamide primer. For primary service evaluation the surface of two engine hatches and an area of the topside and leading edge of a wing adjacent to an engine from one of NRL's (2-121 Constellations was painted with the fluorinated urethane coating systems. These test areas were observed as to degree of soiling and ease of cleaning after 6 months of flight operations. After washing the test and surrounding areas with MIL-C-22543 cleaner, it was found that both the clear and pigmented NRL coatings were more readily
cleaned than adjacent urethane paint and resisted permanent "burnt-in" soil from hot exhaust gases and engine oil. As previously mentioned, these polymers have surface energies which are low enough to "wet" Teflon. It was found that small-particle-size TFE (pigment) could be combined with the fluorinated urethane up to 70 vol '-70. However, loadings of 25-40% by volume or less give films which not only are very hydro- and organo-phobic but have low coefficients of friction about the same as Teflon (0.10-0.19 static and 0.04-0.08 kinetic) and low ice adhesion. These properties suggest many other uses for these coatings. Acknowledgment This research was conducted a t the Baval Research Laboratory under the sponsorship of the Naval Air Systems Command, United States Department of the Navy. 'Literature Cited Farah, B. S., Gilbert, E. E.,Sibilia, J. P., J . Org. Chem., 30, 998 (1965). Gruber, H . . J . O i l C d o r ChemMs Assoc., 48, 1069 (1965) Griffith, J. R . . Preprint 29. 157th National Meeting of the American Chemical Society, Division of Organic Coatings and Plastics Chemistry, No. 1, pp 253-256, 1969. Griffith, J. R . , Field. D. E., "Fluorinated Network Polymers," Naval Research Reviews, pp 15-26, Dec 1973. Griffith, J. R.. Field, D . E., O'Rear, J . G., Preprint 34, 167th National Meeting of the American Chemical Society, Division of Organic Coatings and PlasticsChemistry, No 1, pp 709-714, 1974. Griffith, J. R . , O'Rear, J. G . , Reines. S. A,. Preprint 31, 161st National Meeting of the American Chemical Society, Division of Organic Coatings and Plastics Chemistry, No. 1, pp 546-551, 1971 Griffith, J R . , Quick, J. E.. Advan. Chem. Ser.. No. 92. 8 (1970) McBee, E. T., Marzluff, W F . , Pierce, 0 R , J . Amer. Chem. Soc.. 74, 444 ( 1952) O'Rear, J. G . , Griffith, J. R., Preprint 31, 162nd National Meeting of the American Chemical Society. Division of Organic Coatings and Plastics Chemistry. No. 2 , p 634, 1971 O'Rear. J. G., Griffith, J. R . , Reines. S. A,, J . Paint Techno/.. 43, No. 552, 113 (1971). Reines, S. A,, Griffith, J. R., O'Rear, J. G . , Preprint 30, 160th National Meeting of the American Chemical Society, Division of Organic Coatings and Plastics Chemistry, No. 2. pp 263-68, 1970. Reines, S A,, Griffith, J. R . , O'Rear, J. G . , J Org. Chem., 35, 2722 (1970) Reines, S. A,, Griffith. J. R . , O'Rear, J. G . , J. Org. Chem.. 36, 1209 (1971).
Received f o r reuieu September 16, 1974 Accepted D e c e m b e r 3, 1974
I nerting Characteristics of Halogenated Hydrocarbons (Halons) Satya N. Bajpai* and John
P. Wagner
Factory Mutual Research Corporation, Norwood, Massachusetts 02062
Flammability diagrams for fuel-air-Halon systems at ambient temperature and pressure are presented. Six gaseous fuels (hydrogen, methane, ethylene, propane, butane, and isobutane) and two Halons (1301 and 1211) were investigated in this study. A n empirical relationship i s postulated between the flame temperature (referred to stoichiometric fuel-oxidant composition) and the Halon inerting concentration (measured at the "peak" of the flammability envelope). On this basis, it is shown that the higher the flame temperature of a fuel-oxidant system the higher the Halon inerting concentration requirement for that system.
Introduction It is generally well known in the field of combustion and, in particular, the fire protection industry that Halons 54
Ind. Eng. Chern., Prod. Res. Dev., Vol. 14, No. 1, 1975
(halogenated hydrocarbons) 1301 (CF3Br) and 1211 (CFZClBr) are effective fire suppressants for a large variety of fuels. Published studies dealing with various as-
pects of combustion suppression are too numerous to cite in detail in this report. For example, the well-known study of Friedman and Levy (1957) contains 345 references. Following this comprehensive report additional surveys have appeared (Fristrom, 1967; McHale, 1969). Based on these studies, it is generally agreed that Halons act chemically to suppress combustion mainly by two processes: (1) decreasing the flame propagation velocity and (2) narrowing the limits of flammability. The studies of Friedman and Levy, Fristrom, and McHale are of a fundamental nature, emphasizing chemical kinetics and structure of flames containing inhibitors. It is recognized that design of fire-explosion protection systems employing Halons requires information on the critical concentration of the inhibitor needed to “inert” a flammable fuel-air mixture. Presently, the specifications contained in the National Fire Protection Association Standards (1972a,b) are based on “peak” (i e., critical) Halon concentrations obtained by the following techniques: the Bureau of Mines 2-in. and 4-in. inerting apparatus (Coward and Jones, 1952), the IC1 (Lewis, 1972), modified version of the Bureau of Mines set-up, and du Pont’s Mason J a r Technique (Floria, 1972), substantiated in a few cases by large-scale test results. However, the data reported in the NFPA Standards are not consistent in that they were obtained by a number of investigators using different techniques over a considerable period of time. Therefore, employing certain features of both Bureau of Mines and IC1 techniques, apparatus and procedures have been developed for collecting a consistent and meaningful set of data on the inerting characteristics of Halons 1301 and 1211 on a variety of gaseous fuels. Factors Influencing Selection of Inerting Tube a n d Operating Procedure Since the principal objective of this study is to determine “peak” inerting concentrations which are to be used with some degree of confidence in large-scale Halon system design, the selection of an inerting tube oriented vertically with upward flame propagation over downward propagation readily follows. That is, the literature data show that the flammability range for upward propagation is wider (lean limit is lower while the rich limit is higher) than for downward propagation. For horizontal propagation, the limits lie between these extremes. Furthermore, this is substantiated clearly by Coward and Jones (1952), who show that, for prevention of industrial hazards, the limits should be taken for upward propagation. Briefly stated, a proper experimental procedure requires that homogeneous mixtures of fuel, Halon, and air (in known concentrations) be subjected to a source of energy (usually high-energy spark) sufficient to produce a flame. Data are then acquired to define the flammability envelopes for various mixtures and are generally expressed as plots of per cent fuel L I ~ per cent Halon. The factor of greatest practical significance is the percentage of Halon required to prevent flame propagation in all possible mixtures. This is known as peak inerting concentration. It is generally used to assess the effectiveness of the inerting agent based on the criterion that the lower the peak inerting concentration the better the agent. The question of whether a flammable mixture does indeed exist is based on the distance a flame propagates (generally determined by visual observation) in a tube (usually of 2-in. and 4-in. diameter and 4-ft to 6-ft length). Serious disagreement exists among various authors regarding the definition of the term “inerting.” For example, the Bureau of Mines (Coward and Jones, 1952) defines “inert” as the inability of a flame to propagate be-
226m
,
I
‘I“’”
1
11
1 ,
t
TCZgt ‘2 305”
9
-ab
Ssqurnfial Timer -*I Induction coil
@ Figure 1. Inerting apparatus setup: 1, Matheson rotameters; 2, packed bed mixer: 3 , rubber insulation; 4, O-ring seal: 5, spark gap, -1 cm; 6, needle point electrodes; ’7, electrode point to packed bed separation distance, -1 cm; 8, glass bead ( b - i n . diameter) packed bed (2.0 cm thick); 9, SS screen: 10, aluminum block to support the inerting tube; 11. l14-in. diameter hole bored through the supporting block for the inlet of ternary mixture: 12, thermocouple rig; 13. Marinite lid.
yond 50% of the tube length. Freon Products Laboratory of du Pont (Floria, 1972) defines “inert” as allowing no flame extension when a match head ignition source is used and no flame when spark ignition sources are used. It is interesting to note that the latter definition is clearly more rigid than the former one. Often it is not explicitly stated and yet generally assumed that these definitions and flammability criteria are adequate for the construction of the flammability envelopes and hence the peak inerting concentrations. However, recently it has been suggested by Lovachev, et al. (1973), that the extent of flame propagation in a tube cannot be used as a criterion to define the flammability limits. Xevertheless, in the absence of any firm recommendations, it seems that a criterion based on the extent of flame propagation in a tube is still an appropriate one, if only in a practical sense. There is one additional point that requires some clarification concerning the possibility of contamination of the walls of the flammability tube. Frequently, inerting data are judged to be inaccurate and, therefore, discarded upon noting the presence of surface contamination on the tube. It must be emphasized that discarding data on such a basis contradicts the results of Lewis and von Elbe (19431, who showed that widely different surfaces (clean Pyrex and Pyrex coated with sodium tungstate or potassium chloride or silver) did not influence the flammability limits. It was further pointed out that these surfaces covered a wide range of efficiencies for chain termination. The important factors as outlined form the basis for development of the inerting tube and experimental procedure which follows. Inerting Apparatus a n d Procedure The Inerting Apparatus. A diagrammatic sketch of the inerting tube and ignition assembly is given in Figure 1. The important parts of this setup are the inerting tube containing a thermocouple rig and set of electrodes and auxiliary flow metering and recording apparatus. Ignition takes place at the base of the tube and flame propagation is vertically upward. The upward propagation along the length of the tube (visually and by means of thermocouples) and the velocity of propagation (thermocouple reInd. Eng. Chern., Prod. Res.
Dev., Vol. 14, No.
1, 1975
55
2
l 1
I
4
J
/
i
0
i
i
i
i
i Halon i i IXn,vd-pct h i b
i
i
i
l
3
i
b
J
0
0 25
0 50
0 75 T ~ m r , seconds
IC
15
I25
Figure 4 . Limits of flammability of methane-air-Halon mixtures at 25°C and 760 mm Hg.
1301
Figure 2. Typical thermocouple response to the arrival of flame front propagating upward in the inerting tube (mixture composition; 5 % propane, 5% Halon 1301, 90% air), ,
100
90+
80
----,
,
,
,
,
%air=lOO%-% hydrogen-%Halon 1301
,
,
4
~
Halon Ix)I,wI-pct
30
40
Figure 5 . Limits of flammability of ethylene-air-Halon mixtures at 25°C and 760 mm Hg.
1301
Halon 130l,vol-pcl
Figure 3. Limits of flammability of hydrogen-air-Halon 1301 mixtures at 25°C and 760 mm Hg.
sponse over a fixed distance) were measured. The inerting tube is a 5.8-cm Pyrex glass tube having a n overall length of 168 cm. The effective length, z e , the distance from the electrodes up to the Marinite cap containing the upper thermocouple, is approximately 127 cm. This is close to the standard length range recommended by the Bureau of Mines (Coward and Jones, 1952). The tube is supported vertically a t its base on a n aluminum block and enclosed inside a protective Plexiglas frame. A 2-cm thick bed of glass beads is provided in the cavity of the aluminum support block to ensure uniform distribution of the Halon-fuel-air mixture. A set of needle point electrodes was mounted 1 cm above the bead surface in a horizontal position in the aluminum block. The electrodes were electrically isolated from the block by rubber insulation in order to prevent charge leakage. The voltage input to the electrodes was supplied from a n induction coil to produce a fixed spark gap of 9-10 mm. This is consistent with the recommendations of Coward and Jones, who state that a n effective ignition source can usually be obtained by a n electric spark from a n induction coil with a spark gap several millimeters long. The thermocouple setup employs four rapid-response chromel-alumel thermocouples, each having a diameter of 0.004 in. enclosed within a stainless steel sheath of 0.02in. diameter. All four thermocouples were enclosed inside a single ?$-in. stainless tube. Four ?hG-in. diameter holes were drilled along the tube length and thermocouple beads were located just outside these holes a t 30.5, 44.7, 30.5 cm apart (see Figure 1). The thermocouple rig was supported by a Marinite cap such that it remained in contact with the inner wall of the inerting tube while 56
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 1, 1975
t
f
+ " \
+ * P
I+
0'
i
i i
4 - 5
Q 4
6 i Halon Ix)l,vol-pcl
ib
1'1
ii ii
(4
Figure 6. Limits of flammability of propane-air-Halon mixtures at 25°C and 760 mm Hg.
1301
thermocouple beads were facing the central axis of the tube. Possible disturbances of the flowfield due to the presence of the rig inside the inerting tube are considered to be of little or no consequence, since the limiting fuel-lean and fuel-rich values for all fuels tested agree closely with published values (Zabetakis, 1965). The thermocouple outputs were followed on a highspeed oscillographic recorder-Honeywell Model 906A. Each thermocouple recorded the arrival of the flame kernel a t its respective position. An Eagle Sequential Timer, Model 5OA6, was used to accomplish two functions: (1) control the duration of electrode firing for 2 sec and ( 2 ) trigger the oscillographic photosensitive chart paper a t the instant firing was started. Procedure. Proper mixing of the components of the ter-
,
2o-----r--
10,
1
%air = 100%-% is0butane - % H a h I30I
,
,
I
,
, I '
%air~IOO%-%metham - % H a h 1211
18t
.
l6! I
7-
2'/C 0 I-
o
2
" ----A -l_-i__L
I
2
3
4
Figure 7 . Limits of flammability of isobutane-air-Halon mixtures at 25'C and 760 mm Hg. 10
1
5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 Halon I3Ol,vol-pct
1301
3 4 Halon 1211,vol-pct
5
6
7
Figure 10. Limits of flammability of methane-air-Halon mixtures at 25°C and 760 mm Hg
_ - - ~
~
2
%air = 100% - % ethylene -
36 -
1211
Halon I2 I I
32t
-
28 -
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-6
.
2 4-
*
*
:%
%
f
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3-
a-
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o---
2
3
5 6 7 8 9 Halon l3Ol,vol-pct
4
1011
Figure 8. Limits of flammability of butane-air-Halon mixtures at 25°C and 760 mm Hg.
,
,
1
l 2 3 4 5 6 1 8 9 l O l 1 1 2 l 3 l 4 Halon 121 I , "01-pct
121314
1301
Figure 11. Limits of flammability of ethylene-air-Halon mixtures at 25°C and 760 mm Hg.
1211
%air = 100%-% propane -%Halon I2I I
; i l I;
: !?T? i
O'
'
5
'
IO
'
I5
' 20 ' 25 30 Halon 1211,vol-pct
'
35
'
40
'
45
Figure 9. Limits of flammability of hydrogen-air-Halon mixtures at 25°C and 760 mm Hg.
41 3c
1211
nary mixture (Halon-fuel-air) was accomplished using the setup shown in Figure 1. Precalibrated rotameters with needle valve control were used to meter the individual species flows. Each component was first premixed in a packed bed of glass beads in a 1-ft long x 1-in. diameter stainless steel pipe and subsequently passed through the second packed bed contained in the aluminum support block. The mixture composition was verified for selected fuels and the two Halons by removing samples a t the top of the inerting tube with a n airtight syringe followed by gas chromatographic analysis. Excellent agreement was obtained between the expected mixture composition and the chromatographic results. Consequently, it was considered unnecessary to repeat this confirmation for each test. Purging of the inerting tube for 10 min a t a flow rate of 1500 cclmin (i e., four tube volumes) prior to the start of each ignition was found sufficient to expel air and gaseous species generated or accumulated during the preceding experiment. Routine cleansing of the inner walls of the in-
O
0
i
z
i
i
*\
1
3
4
i
b
7
e
6
1
o
Halon 12ll,vol-pct
Figure 12. Limits of flammability of propane-air-Halon mixtures at 25°C and 760 mm Hg.
1211
erting tube with methyl ethyl ketone (MEK) followed with acetone removed condensable liquids. It is probable that trace amount of species remained on the tube wall for those tests in which the tube was not cleaned; however, repeated comparisons between tubes cleaned after each test and those cleaned intermittently did not produce noticeable changes in the limits of flammability. This is consistent with the findings of Lewis and von Elbe (1943). After completion of purging, the on-off valve at the outlet side of the packed bed mixer was closed and the electrodes were fired for 2 sec in order to attempt ignition of the quiescent ternary mixture. A typical thermocouple response to the arrival of flame L
.. -
Ind
Eng Chem., Prod Res. Dev , Vol. 14, No 1, 1975
57
I
O
,
,
.
,
I
%air=100%-%isobutane-%Halon 1211
i
9,
'\ \
*
\'
:/%
+ IF O
i
Z
~
~ ~ ~ ~ Halon 12ll,vol-pet
~
S
I
O
I
'
I
I
~
I
~
I
~
Figure 13. Limits of flammability of isobutane-air-Halon 1211 mixture at 25°C and 760 mm Hg. 4
2
%air =100%-%butane-%Halon 1211
7-
1
Fuel
Halon
Halon inerting concn, vol %
Hydrogen Methane Ethylene Propane n-Butane Isobutane Hydrogen Methane Ethylene Propane *Butane Isobut ane
1301 1301 1301 1301 1301 1301 1211 1211 1211 1211 1211 1211
28 6.8 10.5 8 .O 7.2 7.2 32.5 4.3 12 .o 7 .O 6.3 6.3
1%
I OL.LI-8 I
2
3
4
,
5
,
6
1
1
,
,
7 8 9 1 0 1 Halon l2ll,vol-pct
1
,
I
1
2
'
1
3
,
1
1
4
Figure 14. Limits of flammability of butane-air-Halon 1211 mixtures at 25°C and 760 mm Hg. kernel is shown in Figure 2. Visual observations were also made to record the height to which flame propagated. Our criterion for inerting limits is based on propagation less than 25% of the tube height, i . e . , no propagation beyond the locations of TC1 in Figure 1. In other words, if TC1 responds, the mixture is considered flammable. Results and Discussion Flammability Envelopes a n d P e a k Inerting Values. Flammability envelopes for six gaseous fuels with two Halons (each fuel and Halon paired separately) were constructed by following the experimental technique described previously. The flammability envelopes for these combinations are given in Figures 3 to 14. Lower and upper flammability limits were also determined for all fuels except for hydrogen, in order to determine how well our results compared with the classic work of Zabetakis (1965) of the Bureau of Mines on flammability limits of hydrocarbons. It may be noted that our values for upper and lower flammability limit are in excellent agreement with the results of Zabetakis. Table I presents the concentrations of Halons 1301 and 1211 for various fuels a t the peak of the flammability envelopes. The significance of these values is that they provide the minimum concentration of Halon required to render all proportions of a fuel-oxidant system nonflammable. Flame Temperature a n d P e a k Inerting Values. Figure 15 shows the functional relationship of Halon inerting concentration a t the peak of the flammability envelope and the fuel-oxidant flame temperature (as referred to stoichiometric composition). It is noted that the inerting concentration requirement increases with increasing flame temperatures. 'It is also evident that Halon 1211 is more effective than Halon 1301 for fuel-oxidant systems having flame temperatures less than 1940°C. However, for fuel58
Ind. Eng. Chern., Prod. Res. Dev., Vol. 14, No. 1, 1975
oxidant systems exceeding a flame temperature of 1940°C a reversal in this effectiveness trend is noted. The chemistry of flame inhibition by Halons is very complex and, a t best, incompletely understood. At this time, no firm explanation can be provided to account for these observed trends except to say that stoichiometric flame temperature of a fuel-oxidant system may serve to predict the critical Halon concentration requirement for a given system. Conclusions The apparatus and procedure described herein can be used to accurately construct the flammability envelopes of any flammable ternary mixture a t ambient temperature and pressure. The flammability envelopes and, in particular, the concentration of Halons 1301 and 1211 a t the "peak" of the flammability envelope, may be useful in evaluating agent flame suppression effectiveness. The stoichiometric flame temperature of the fuel-oxidant system shows promise for predicting the inerting concentration of Halons 1301 and 1211; however, both additional data and subsequent theoretical analyses are needed to substantiate this correlation unequivocally. Literature Cited Coward H F Jones G W , U S Bur MinesBuli 503 (1952) F l o r t a , J A , 'du Pont FE 1301 Fire Extlngbishant Fuel Inerting Concen-
tration-Summary Report." du Pont de Nemours & Co., Inc., Wilmington, Del., Mar 1972. Friedman, R., Levy, J . B.. "Survey of Fundamental Knowledge of Mechanisms of Action of Flame-Extinguishing Agents," WADC Technical Reoort 56-568. Jan 1957. Frl'strom, R . M., Fire Res. Abstr.Rev.. 9, 125 (1967). Lewis, B., von Elbe, G . , J . Chem. Phys.. 11, 75 (1943) Lewis, D. J., "Method Used for Peak Flammability Determination," Imperial Chemical Industries, Mond Division, Cheshire, U . K . , 1972. Lovachev. L. A , , e t a / . , Combust. Flame, 20, 259 (1973) McHale, E. T.. Fire Abslr. Rev., 90 (19691. National Fire Protection Association, "Halogenated Fire Extinguishing
Agent Systems-Halon 1301," NFPA Standard 12A. 1972a. National Fire Protection Association, "Haolgenated Fire Extinguishing Agent Systems-Halon 121 1," NFPA Standard 128, 1 9 7 % Zabetakis, M . G., U . S . Bur. Mines Buil. 627 (1965).
Received for revieu September 19, 1974 Accepted D e c e m b e r 18, 1974 The f i n a n c i a l s u p p o r t of E. I. du Pont de N e m o u r s & Co., Inc., W i l m i n g t o n , Del., i s g r a t e f u l l y acknowledged.
Morphology of Amorphous Polymers P. H. Geil Department ot Macromolecular Science, Case Western Reserve University, Cleveiand, Ohfo 44 709
It has been conventional to rely upon concepts and measurements of free volume in characterizing the properties of amorphous polymers. As Petrie (1975) has stated, free volume is the primary physical parameter affecting the physical properties of polymers in the glassy state and is also of concern, along with the normally assumed entanglements of the molecules, in characterizing various properties a t temperatures above T g . These features are recognized and accepted by nearly all polymer scientists. In this presentation, however, we consider a more controversial subject, one which is just beginning to receive substantial attention. Ever since the beginnings of polymer science it has been generally assumed and accepted that amorphous polymers, both in the glassy state and above TL',consist of randomly coiled, entangled chains with no local order (Flory, 1953). This model should have been rejected on the basis of density considerations alone. As pointed out by Robertson (1965), a collection of randomly coiled molecules would have a considerably lower density than observed for any amorphous polymer (typically 85% of the perfect crystal density). This has, however, been the basis for nearly all discussion of physical properties of glassy and molten polymers. I t is proposed here (and some of the initial evidence reviewed) that this assumption is incorrect-that amorphous polymers consist of small (ca. 30100 .A) domains in which there is a local ordering or alignment of neighboring segments. An amorphous polymer, we suggest, can most simply be looked a t as being composed of numerous, small, nematic-liquid-crystal-like domains, with the majority, but not all, of the molecules running from one domain to another. In the glassy state this structure will be frozen whereas above TI(there will be a continual redistribution of segments among the domains, individual domains forming and disappearing. In a crystalline polymer it is now accepted that per cent crystallinity is a first-order parameter characterizing their properties, with lamellar structure (thickness, orientation, interconnection, defect content and type, surface structure, etc.) being second-order parameters (see chapter by Clark (1975)). Two samples of a given polymer, with the same crystallinity as determined by X-ray or density, can have significantly different properties if, for example, their thermal histories and, therefore, their morphology
differ. Likewise, it is proposed that free volume should be considered a first-order parameter characterizing the properties of glassy amorphous polymers, but that its distribution, as determined by the size, interconnection, internal order, etc., of the domains, i.e , the morphology of the amorphous polymer, should be considered as a second-order parameter We do not propose a complete review of the research to date on the morphology of amorphous polymers; such a review was recently published by Yeh (1972a). Rather, we consider only some of the salient features, pointing out as well some of the remaining problems. In particular we consider electron microscope evidence for domains of local order in several amorphous polymers (crystallizable and noncrystallizable), the effect of deformation and annealing on this order, means and need of characterizing the degree of order and domain size other than electron microscopy, and initial studies of the effect of the domains on physical properties. Because of the recent publication of Yeh's review article, we will emphasize results obtained in our own laboratories. Electron Microscope Observations Figure 1 is one of the better micrographs showing the domain structure or nodules (as they frequently have been called) in an amorphous polymer. In contrast to many published micrographs for which thin film samples were used, this micrograph is from a platinum-carbon (Pt-C) replica of a thick, quenched film of polyethylene terephthalate (PET). Films thin enough to directly insert in the microscope (i e., < 500 .A thick, termed "thin" films in the following), with or without shadowing, usually yield both better and more reproducible micrographs; this feature has been ascribed to obscuring of the surface of thick samples by exuded low molecular weight polymer, additives, or other impurities. It is noted here, and discussed further below, that this is one of many major problems in this research; not only is there considerable variation in apparent morphology in thin film samples presumably prepared in an identical manner, but it has been nearly impossible to directly examine. by electron microscopy, the morphology of samples thick enough for physical property studies. The nodules in Figure 1 have an average diameter of ea Ind. Eng. Chem., Prod. Res. Dev., Vol. 1 4 , No. 1, 1975
59
