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Chemical Analysis of Synthetic Fibers.

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Chemical Analysis of Synthetic Fibers[**]
By Dietmar Nissen, Volker Rossbach, and Helmut Zahnl'l
Dedicated to Professor Paul Schlack on the occasion of his 75th birthday
The analytical characterization of synthetic fibers has not kept pace with their development
and production. Whereas the "second-generation" fibers have already conquered the market,
the chemical investigation methods used until now for the three classical fiber polymers
of the polyester, polyamide, and polyacrylonitrile types are still frequently unsatisfactory.
Though they satisfy the requirements of production supervision and quality control, they
do not supply enough information to enable us to understand and to explain the degradation
of and damage to the polymers under hydrolytic, thermal, or thermooxidative influences.
This report presents methods of chemical analysis that open up new possibilities for investigations of this nature on synthetic fibers. The results reported for commercial products must
not be taken as an assessment of quality. They are simply intended to show the diversity
to be expected in the chemical properties and to emphasize the practical basis of the
chemical analysis of synthetic fibers.
1. Introduction
"One of the most frustrating problems in the early production of synthetic fibers was the inability to define polymer
quality other than in terms of drawability and spinnability."
This sentence from Schfack"' shows how far it was necessary to come in order to reach the present position in
the analytical characterization of synthetic fiber
polymers'2,'1. However, though a number of physical
methods are now available, particularly for routine investigations, it is essential for synthetic fiber research to possess
substantially complete and absolute numerical values for
the chemical properties of the fibers, i. e. not simply comparison values such as are obtained in many test methods.
It appears from the literature''] that fundamental new
developments of fiber polymers are probably to be expected
in the immediate future only to a limited extent and for
special applications. In any case, the chemical modification
of the three classical fiber polymers (Fig. 1) polyester,
polyamide, and polyacrylonitrile is becoming increasingly
important, and considerable problems are at the same
time being raised for analytical chemistry. Methods from
the analysis of natural macromolecules can help to solve
these problems, as will be shown in the following discussion.
Thus end group determination methods for polyesters and
polyamides can be based on the well-known methods for
the sequencing of proteins, such as the hydrazinolysis
method of Akabori161and the dinitrophenylation technique
of Sanger"]. A method for the determination of acidic
groups in cellulose by staining with methylene blue[8]can
also be applied to the investigation of polyacrylonitrile,
p~lyamide'~],
and polyester[g,9a, l4].
The polymers spun into filaments are not molecularly
uniform substances, but contain a certain percentage of
['I
Dr. D. Nissen, Dr. V. Rosshach, and Prof. Dr.-Ing. H. Zahn
Deutsches Wollforschungsinstitut an der Technischen Hochschule
51 Aachen, Veltmanplatz 8 (Germany)
[**] Expanded version of a lecture to the 4th Textile Technical Forum,
Schloss Elmau, 1972; 7th report in the series: Synthetic Fibers in the
Wool Industry. 6th report: W L. Lindner, Polymer 14, 9 (1973).
602
oligomers (polymeranalogs having low molecular weights),
which have proved valuable as model substances for the
polymers, but have an adverse effect in the technical processing of the fibers. Analysis of the oligomers is therefore
of great practical importance.
Polyester, PES (polyethylene terephthalate, PET [Terylene type])
H-[NH-(CH2)s-CO-InOH
Polyamide 6, PA 6 (polycaprolactam [Perlon])
H-[NH-(CH2)6-NH-CO-(CH2)4-CO-]nOH
Polyamide 6.6 (polyhexamethylene adipamide [Nylon type])
Polyacrylonitrile (modified), PAC
R1,RZ=various neutral, acidic, or basic groups; Ri,R2=alkyI or
hydrogen
Fig. 1. Structuial formulas of synthetic fiber polymers of the polyester,
nylon, and polyacrylonitrile types [ 5 ] .
The chemical characterization of commercial synthetic
fibers also includes the investigation of any extraneous
substances present, since these help to determine the processability of the fibers and their properties in use. However,
the heterogeneity of these classes of substances, such as
spinning oils, delustering agents, optical brighteners, light
stabilizers, heat stabilizers, antistatics, and catalyst residues, makes both qualitative and quantitative determinations difficult, so that practically no generally valid, standardized analytical methods have been developed so far.
2. Determination of the Structural Units
of Synthetic Fiber Polymers
Since the discovery of paper chromatography and later
of thin layer chromatography, these methods have become
Angew. Chem. internat. Edit. 1 Vol. 12 (1973)
/ No. 8
essential aids in the analysis of synthetic fibers because
of their ease of use. Once the polymer has been broken
down into its monomeric units or their derivatives, these
methods rapidly yield qualitative information on the composition of the macromolecules. Gas chromatography is
also suitable for such routine determinations['0], and is
also used for determination of the undesirable diethylene
glycol structure [middle part of ( I ) ] in polyethylene terephthalate" 'I.
methylene acetates (CH3CO-[OCHz-lnOCOCH3; n = 1
to 20), though the molecular homogeneity of these substances was difficult to check by the means available at
that time. This opened up a field of research whose importance was underlined by Carotherdt71in 1930 in his suggestion that low molecular weight homologs be isolated from
synthetic polymers for chemical and physical structural
investigations aimed at obtaining an understanding of the
high polymers. Nine years later, Schlack and Kunz['81
L
Besides paper chromatography and thin layer chromatography, which have been in use for some time for the
determination of the structural units of commercial
nylon[' 21,
and polyester['31, paper electrophoresis, which has been recommended for the analysis
of hydrolysis products of nylons['3], has also proved useful
in the qualitative determination of the monomer units
of copolyesters['4] (Fig. 2).
I
Bt
I
10
Q
15
8
-
20
4
L
25
30
Fig. 2. Electropherogram of HCI hydrolysis products of modified polyester
fibers (reference substance for the Rr values: Neucoccin [C. 1. 162551).
A) Unitika copolyether ester, B) DuPont Dacron 65, C ) reference substances (a: p-($hydroxyethoxy)benzoic acid, b: terephthalic acid, c: 5-sulfoisophthalic acid).
By thin layer chromatography of hydrolysis products of
the silk-like Qiana fibers, it is possible to show with the
aid of reference substances that this product is the polyamide of bis(4-aminocyclohexyI)methane and n-decanedicarboxylic acid (2)["].
On the other hand, the structural units of copolymers
of acrylonitrile cannot be determined by this simple
method, since the paraffinic polymer chain (Fig. 1) cannot
be hydrolyzed.
3. Importance of the Oligomers to the
Analysis of Synthetic Fibers
As early as 1925, Staudinger and Liithy[l6I described the
isolation of methyloligooxymethylene methyl ethers
(CH3-[OCH2-].0CH3;
n = 1 to 14) and acetyloligooxyAngew. C h e m . internat. Edit. / Vol. 12 (1973) / N o . 8
JY
extracted the cyclic dimer ( 3 ) and trimer ( 4 ) as well
as monomeric lactam from caprolactam polymers.
Attempts made between then and 1967 to synthesize
polymers with definite chain lengths or ring sizes from
the monomers to allow the study of changes in the physical
and chemical properties with increasing degree of polymerization have been reviewed by Heidernand").
To avoid the difficulties associated with the direct synthesis
of oligomers, repeated efforts were made to obtain the
pure oligomers by fractionation of the extract from the
polymers1201,
since the formation of oligomers is a phenomenon that accompanies melt condensation in particular,
and oligomers can therefore be easily isolated from such
as "natural", though usually undesirable, components. The most important of the fiber-forming polymers
produced by melt condensation are the nylons and
polyesters.
The specificsynthesis of oligomers with degrees of polymerization greater than 8 is difficult if not impossible by
conventional methods, not only because of solubility factors. As a result of incomplete reaction, costly purification
of the resulting mixture is necessary in every case[21.221.
This is also the case when the Merrifield
is used for the preparation of higher linear polyamide
and ethylene terephthalate oligomers, i. e. the purity of
the products is usually unsatisfactory[24- 261,which agrees
with experience in peptide chemistry. In the preparation
of acrylic acid oligomers, the use of molecularly uniform
phenol-formaldehyde condensates as a template should
help to provide definite p r o d u c t ~ [ ~ ~ J .
Following the remarkable successes of Rothe et
K ~ s c h [ ~Repinr281,
~],
and P e ' n i s s ~ n [in
~ ~the
] synthesis of
higher linear and cyclic oligomers of the nylon 6, nylon
6,6, and ethylene terephthalate series, it can nowadays
be said that for most purposes, oligomers with degrees
of polymerization of less than 10 are quite satisfactory
as model substances for the polymers. Thus from the heptamer upward, the dimethyl esters of polyethylene terephthalate do not differ from the polymer in their solubility,
melting point, X-ray powder pattern, or IR
In the nylon 6,6 series, X-ray studies show the same super603
structure for the tetramer and pentamer as for the
polymer["J.
An important objective of oligomer research has thus been
achieved, namely the provision of substances for model
reactions in which the molecular uniformity of the compounds ensures a simpler situation than in the case of
an unfractionated polymer. Oligomers with degrees of polymerization of less than 10 are adequate for this purpose,
and have already been used in many cases (Table 1).
With the continuous development of new homopolymer
and copolymer synthetic fibers, the need to prepare suitable
oligomers for their analytical investigation will continue
in the future. Table 2 shows the results of attempts to
isolate oligomers from Qiana poly[(4,4'-methylenedicyclohexy1)dodecanamidel (2)" 'I.
4. Determination of the Functional Groups
of Polyesters, Nylons, and Polyacrylonitrile
Table I . Scalar properties of polymers, investigated with oligomers as
model substances [19, 311.
Melting point
Viscosity
UV and IR absorption
Crystallinity and conformation IX-ray studies, IR and NMR spectroscopy,
density measurements)
Influence of chain length and structure on a covalently bonded chromophore (2,4-dinitrophenyl, 4-nitro- I -naphthyl residue) [32, 331
Dyeing mechanism (crystalline ponceau 6 R rC.1.441, 2.4-dini~ro-1-fluorobenzene, disperse dyes)
Absorption of moisture
Rate of hydrolysis
Photostability [34]
Thermostability [34. 351
The functional groups (-OH,
-COOH,
-SO,H,
-OS03H, -NH,, -CN) of the three fiber polymers
polyester, nylon, and polyacrylonitrile and of the modified
types derived from them are of great importance to their
processability and to their properties in use, and play
a crucial part in cases of damage. The specific determination
of these groups is therefore important for the evaluation
of a polymer and the detection of changes due to external
influences such as hydrolysis, thermolysis, thermal oxidation, and irradiation.
4.1. Determination of Acidic Groups
Interest is now being focused mainly on the manner in
which the oligomers are formed and on their behavior
in the production and processing of fiber-forming polymers.
Thus the difficulties encountered in the processing (spinning, texturing, dyeing) of polyethylene terephthalate,
which are caused by the escape mainly of the cyclic trimer
( 5 ) from the fiber, have led even recently to investigations
having practical a ~ p e c t s [ ~ ~ - These
~ ' 1 . investigations are
greatly facilitated by the model substances. Attempts have
recently been made to decrease the content of oligomer
by extraction of the granulated polymer before spinning
in order to obtain polyester filaments with better properties
for HT (high-temperature)
Table 2. Analytical characterization of Qiana oligomers [IS].
I
Linear pentamer
Mol Wt.
Elemental
anal)si>
H
C
calc. 73.60
exp. 73.40
N
exp.
R , \slue
H
11.18
0
8 63
8.30
N
6 92
6 92
0
096[a]
0 0 [b]
220
c
substance insoluble
C
74.25
74. I8
10.90
6.87
7 13
cyclic monomer
I
2038
exp. 2050
Cdk.
CdlC
I
I
I
10 89
I1 I1
79
71
0 83 [a]
094[b]
> 350 C (subllmdble)
[d] Eiuent. 2-butanol/formic acid/acetic acid/water [22]
[h] Fluent: tetrahydrofuran/cyclohexane/water [39].
604
4.1. I . Carboxyl End Groups of Polyesters
The reason why every polyethylene terephthalate (PET,
polyesters of the Terylene type) contains carboxyl end
groups as well as the hydroxyl groups resulting from the
synthesis method is the thermal cleavage of the polymer
chains due to the relatively high polycondensation ternperatures and the residence times required to obtain high
molecular w e i g h t ~ [ ~ ' - The
~ ~ ] resulting
.
ratio of hydroxyl
to carboxyl groups provides a measure for the assessment
of the chemical quality of the polyester[43! Thus hydroxyl
groups are particularly important to the thermal stability
of p o l y e ~ t e r s [ ~since,
~ . ~ ~as] ,long as there are free hydroxyl
end groups present in the molten polymer, the vinyl ester
groups that are formed together with carboxyl groups
on chain scission are transesterified with formation of
a new polyester linkage[441.A carboxyl group is thus formed
at the expense of a hydroxyl group, but the average degree
of polymerization has not decreased. On heat treatment
of fibers below the melting point between 180 and 220 C,
on the other hand, an increase in molecular weight occurs
with consumption of both types of end groups[33,46-471.
The content of carboxyl groups is an important factor
in connection with the susceptibility to hydrolysis[481,since
these end groups have a catalytic effect in this p r o c e ~ s [ ~ ~ l .
Their number can be increased by hydr~lytic~"'~
or photoI y t i ~ [ ~ damage
~ . ~ ' ] and by thermal o x i d a t i ~ n [ ~ ~ ~ ~ ~ ] .
The determination of carboxyl end groups in PET is thus
not only important for the determination of the number
average molecular weight, but also serves for the elucidation of damage to the polymer by hydrolysis, heat, and
light. In view of this fact, it seems peculiar that acidimetric
titration is still the most commonly used method for the
determination of cdrboxyl groups in PET, with addition
of an indicator for visual end point determination'541or
with photornetri~['~',p o t e n t i o r n e t r i ~ ' ~or~ ~conductomet,
end point determination, since titration methods
are not specific for end groups and can give reasonably
reliable results only up to an average molecular weight
of 20000. Above this limit, the ratio of the end groups
(which are present in very small concentrations) to frequently unidentifiable impurities that interfere with the
titration becomes increasingly unfavorable. This disadvantage cannot be overcome by the use of larger polymer
samples and dilution of the titrant[s81.
A-25 ion exchanger, which is then washed with water.
The adsorbed terephthalomonohydrazide is desorbed with
a measured elution volume of 0.1 N hydrochloric acid
and determined photometrically at the absorption maximum of 240nm. The content of carboxyl end groups is
found from a calibration curve plotted for synthesized
terephthalomonohydrazide.
Chemical methods have also been described for the determination of carboxyl end groups in aliphatic polyesters,
but these have not been used for PET. The conversion
of the cdrboxyl groups into methyl ester groups by reaction
with diazomethane should allow the determination of carboxyl groups by a methoxyl group
Alternatively, these groups may be allowed to react with phenyl
isocyanate in the presence of pyridine. The resulting anilide
end groups are determined by hydrolysis and photometric
determination of the aniline formedlhOl.
4.1 . I . 1. Principles of the Determination of Carboxyl End
Groups by Hydrazinolysis
A new possibility for the chemical determination of carboxyl groups in PET is offered by the application of the
principle developed by Akabo.ri ut a/.['], according to which
C-terminal amino acids in proteins are determined by
h y d r a z i n o l y ~ i s ~The
~ ~ hydrazinolytic
].
degradation of PET,
which proceeds readily, has already been used for a wide
range of purposes, p.9. for the separation of PET from
mixtures with wool["1, for the investigation of the harmful
action oforganic bases on PET
for the determination of the degree of crosslinking of cured polyester
and for the determination of methyl ester groups
and diethylene glycol groupings in PET[' 1.43.'J1. However,
no analytical use has so far been made of the fact that
the reaction of hydrazine with PET leads to quantitative
cleavage of ester linkages in the polymer, with liberation
of terephthalomonohydrazide ( 7 ) as the hydrazinium salt
from C-terminal[*] terephthalic acid units, together with
large quantities of terephthalodihydrazide (6) as the main
product. The monohydrazide can be separated from the
reaction mixture by ion exchange (Fig. 3) and determined
photometrically.
I
02
-
06
01
RF
I
I
08
10
Fig. 3. Thin layer chromatogram of the hydrazinolysis products of a
polyethylene terephthalate. A = before ion exchange: B = after ion
exchange: C =reference substances ( a - terephthalomonohydrazide ( 7).
b: terephthalodihydrazide (6)) Silica gel H F 254 (Merck, Darmstadt.
Germany): dioxane!dinmonia ( 2 5 %,)/water(65 : X : 27).
Under these analytical conditions, the. reaction of PET
with hydrazine proceeds uniformly and does not lead to
the formation of secondary hydrazides, which could be
formed at higher temperatures if the excess of reagent
is too ~rnall[~~1.
4.1.1.2. Application of Hydrazinolysis
The contents ofcarboxyl end groups found for polyethylene
terephthalate fibers are between 20 and 59 meq/kg of
fiber, depending on the manufacturer and the type of fiber
(Table 3): these values agree with recent literature
data[4S.661
Table 3. Content of carboxyl end groups in commercial polyethylene
terephthalate fibers.
Producer,
trade name.
degree of delustering [a]
IC'I Terylene W I I , gl
ICI Terylene W 16, hm
Enka Diolen, gl
Enka Diolen FL. gl
Hoechst Trevira 220. gl
Hoechst Trevira 220, mt
Horchst Trevira SFO, mt [d]
I-loechst Trevira 560, mt [d]
Rhodiaceta Tergal, hm
Rhodiaceta Tergal. mt
COOH [meq kg]
Hydrdzino1)sis
Colorimetric
method [b]
indicator
method [c]
32
37
40
59
20
?7
--
17
21
25
26
37
64
56
20
34
I.?
16
-.
3.5
gl= bright. hm = semi-dull. mt =dull.
[b] Coefficient of variation =9.5 %: confidcncc belt = 2.0mcqjb.g (for
95 % statistical certainty)
[c] Coefficient of variation = 7 5 Y , . confidcncc belt = k2.0mcqikg (for
95 %, statistical certainty).
[d] Chemically modified polyester high-shrinkage fibers.
[d]
The polyester material is allowed to react for five hours
at 70 C with 250 times as much of a hydrazine/dioxane/nbutanol mixture ( I 5 : 70: 15). The hydrazinolysis mixture
is evaporated and then extracted with water. The extraction
solution is discontinuously adsorbed on QAE Sephadex
[*] In this connection the term "C-terminal" means cdrboxyl terminal.
The results are compared with values obtained by a colorimetric indicator method. The principle of this method
was used by If m ~ / r y [ ~ 'for
1 the determination of traces
605
of acid in dimethyl terephthalate. In analogy with this
method, the color reaction between the indicator bromophenol blue in its anionic form and the carboxyl groups
is used for the quantitative photometric determination
of the latter in PET. The decrease in the blue color is
proportional to the number of equivalents of acid present
in the solution.
The inadequacy of this non-specific method can be seen
on comparison of the results (Table 3) with those obtained
by hydrazinolysis. The differences which are sometimes
pronounced are due to the fact that ionic fiber additives
do not interfere with the end-groupspecific formation of
terephthalomonohydrazide (7) and its detection, whereas
they can influence the color change of the indicator. Similar
differences are also found in the determination of amino
end groups in nylon fibers by titration on the one hand
and by dinitrophenyfation on the other (Table 7).
The reaction in the homogeneous phase in the system
calcium chloride/methanol/acetic anhydride, based on a
method given by Blackburn and Phillips[771for the methyl
esterification of wool, also leads to only 58% reaction
of the carboxyl gr~ups[~*I.
4.1.3. Acidic Groups of Polyacrylonitrile
The acidic groups in polyacrylonitrile fibers may be
strongly acidic sulfo or sulfate groups and weakly acidic
carboxyl groups. The sulfo groups constitute about 80%
of the total content of strongly acidic
They
are introduced into the polymer either by the catalyst
as end groups[791(reactions (a)-(d)) or with comonomers
(Fig. 4) that are polymerized into the polyacrylonitrile[80]
or grafted onto the preformed
Carboxyl
groups can also be formed by hydrolysis of nitrile and
other groups during p o l y m e r i z a t i ~ n [ ~ or
~ . ~process~I
ingra51.
4.1.2. Carboxyl End Groups of Nylons
Radical generation
The carboxyl groups of nylons are partly responsible for
KO-SO~-O-O--SO~-OK
2 'O-SO2-0K
(a)
KO-S0~-O-O-S0~-OK
+ 0 2 + 2'S02-OK
the net charge of these polymers, and hence, according
to Zollinger[68],influence their dyeability by acidic dyes.
KO-S02-O-O-S02-OK
+ Fe2+ +
(b)
They are mainly determined by titration in benzyl alcohol
KO-SO2-O+ KO-S02-0 + Fe3+
according to Waltz and Taylor[69]in many variants[70?
Initiation
The end point is determined conductometrically, potentioKO-SO; + H,C=CH-CN
+ KO-SO2-CHz-CH-CN
(c)
metrically, or visually with color indicators. Because of
KO-S02-0'
+ H2C=CH-CN
KO-SO2-0-CH2-CH-CN
the high dissolution temperature of nylons in benzyl alcohol (approx. 160°C), considerable quantities of benzoic
acid are formed during the analysis. Attempts have been
7H3
made to eliminate this methodical error by very precise neutral: H C-CH
H C-C
HzC=CH
-1
-1
I
standardization of the analytical conditions. Other solvents
COOCH,
COOCH,
OOC-CH,
described for the determination of carboxyl groups, such
methyl acrylate
vinyl acetate
methyl methacrylate
as 2,2,2-trifluoroethan01"'~~~~
and P-phenylethan~l'~~',
have only occasionally been used.
COOH
--t
+
In Putzold's method[7z1,the nylons dissolved in trifluoroethanol are titrated conductometrically with aqueous
sodium hydroxide solution. As in the acidimetric titration
in benzyl alcohol[69],the graphical determination of the
equivalence point in this method leads to unsatisfactory
results. On the other hand, definite reproducible values
are obtained by mathematical evaluation, in which the
regression lines for both branches of the titration curve
are determined, the transition section being disregarded,
and the intersection is determined mathematically.
In this way, carboxyl group contents of 50 to 68 meq/kg,
depending on the manufacturer and the intended use, are
found for a number of commercial nylon fibers (Table
7).
The titrimetric determination of the carboxyl end groups
is more problematic than that of the amino end groups,
owing to the lack of suitable solvents for nylons. Nevertheless, only one method for the specific chemical determination of carboxyl end groups has been reported in the
literature. Stuudinger and S~hnelZ['~]
esterifed the carboxyl
groups in the heterogeneous phase with a solution of diazomethane in ether and determined the methoxyl content
by the Zeisel method. This method has also been used
more recently[751.As was shown by Smith[761,however,
25 YOof the carboxyl groups remain unesterified even after
reaction for seven days.
606
acidic:
H,C=CH
6OOH
acrylic acid
basic:
H C-CH
-1
CSH4N
vinylpyridine
I
lizc=y
CH,-COOH
itaconic acid
H C-CH
-1
CONH,
acrylamide
1%C-CH
-1
CBH~-SO~H
styrenesulfonic acid
H2C-CH2
N
''
H
ethyleneimine
Fig. 4. Typical comonomers for the preparation of polyacrylonitrile fibers
[SO, 821.
Beckmann and Glenz[861were the first to carry out quantitative determinations of acidic groups in acrylonitrile
polymers by titration. Kirby and B a l d ~ i n [later
~ ~ ]showed
that the values obtained are too low. These authors developed a method in which the strongly acidic groups and
the weakly acidic groups can be determined separately
by potentiometric titration. However, since many of the
acidic groups are present in the polymer as salts, an ion
exchange must be carried out before the titration.
The sulfonate and sulfate groups can also be determined
by IR ~ p e c t r o s c o p y ~however,
~ ~ ] ; this method requires a
difficult calibration with radioactive sodium.
Hilden[8s-901describes a simpledetermination of the acidic
groups of polyacrylonitrile, in which the polymer is dyed
Angew. Chem. internat. Edit.
1 Vol. 12 ( 1 9 7 3 ) J
No. 8
with mcthylenc blue [C'. I. 52015l. :I basic dye that was
used eai-lier for the inbestigation of libel damage in polj;lei-) lonitrilelL)ll.The fibers are dyed in the heterogeneous
phase at viii-iotis pli v;ilties to achicvc selective bonding
o f the dye to tlie strongl) acidic gi-otips in the acidic
range and t o the strongly acidic and weakly acidic gi-oups
in the neutral I-angc: the content of ueakl) acidic groups
can then be ealculatcd b j difference. l.ahle 4 shous the
results obtained in this wa) ior ;I sei-ies o f comniercial
Pc'lyncrylonitrilc fihers.
only a feu chemical determination niethods are to be
found in the literature.
n
Apart from the fact tliat the Iistci-ogencous reaction makes
quantitati\c d j e satui-ation of the functional groups problematic"", since the accessibilit) of these groups depends
o n rhc ten1pcr;tttlre. the fine htrlictllrc of the filxr18('.''.Tl.
and the iondisti-ib~ition~"'~.thcdctcrmination
ofthe ~ e i i l \ l j
acidic groups is ;11so difficult for otlici i-exons. M'hcrcas
the bonding of the dye to the stronpl! acidic gi-oups i:,
piii-tl) covalent in charxtcrl'"1. the d ~ molecules
c
hondcd
h) t h c ~ c a l \ l acidic
)
gi-oups pal-tlj dissociate again during
tlic extraction for the i-emwal of unhotlnd d j c . The \ulucs
for the strongl) acidic giotips thus agree n i t h other literuturc ciat;i'"-". M Iici-e;is those for the weahlj acidic gi-otips
iirc considcrabl) IOMW.
A qualitative method is based on the ninhydrin react i o n ' 1i'41.
f < k . / i r v i . c'r ul.1 'lI5l developed a quantitative
m e t h od, w h ich shou Id be part ic ti I a r l y su i t a ble for the determination o f lo^ contents of primal-y amino end groups,
and which involves reaction with succindialdehyde t o form
p j rrolyl end groups, which are then determined colorimeti-icall) by the Ehrlich reaction. 2.4-Dinitro-l -fluor-oben~ e n e( D N F B ) is the reagent most commonly used for the
chemical determination of amino end grotips.
4.2.1. Principles of the Dinitrophenylation of Nylons
4.2. 1)ctermination of the Basic End Groups of Nylons
In the an:tlqsis of n! Ions. the derei-mination of the basic
group^. 11 hich ~ J - Cgcncrall! r<fcrrcd to siitipl) 21s amino
. iiio1-ci i i i p o r t a n ~11iari that o f the carboxjl ~ U O L I ~ S .
since ~ h scontent o f h a i c gt-wips cietcimiines the abilitj
oi' thc f i I x 1 - S t o hs d!
ith I-caciivc. acidic. and 2: 1
metal compleu d)cs'"-'.
1. and is also importanl for ihe
ucLvgnit ion 01' fihci. d a m a g e due t o ac~dsl"~I,
heat'"''.
OI. liglitl""l. Thc h:ihic. snci grotips 'ire mainly detei-mined
in pi-acl icc b , ~photometric 01 conductomctric t i t r a t i o n
oi- h! titration
itli ths addition of color indicators~l""].
Wliat \\:is s a i d about tlii, titrinictric determination of carbox! I gi-otips i n polycstcrs is ;ilso applicable to these analytical methods: t l i q i11-cnoii-specific. and so d o not allow
;I differentiation betfieen basic end gro~ipsand other hasic
gi-oiips (total hasicity).
Despite the gi-ou.ing inipori;incc of the analysis of basic
end grotips. partlciiiarl) for newl) developed special endgrotip modified o r copolymerized nyloiis {Fig. 5)'
lo.''.
As early as 1953, one of us and Rtrtliyrhrrl"' dinitrophenylated nylon 6 (Fig. 6) and nylon 6.6 by a procedure
taken fi-om wool analysisr'"('l.
I'ig 6. Rcaction of n y l o n 6 w i t h 7.4-diniiro-l-fluorobc1~~cnc
(DNI-13).
However, structure-dependent errors have been detected
in this method~lO'.
l o S l . Thus since the diffusion of the
rcagcnt is rate-determining i n the heterogeneous phase.
fine-structure parameters such as the degree of orientation
ol the fihers inflticnce the accessibility and reaction of
the amino groups (Fig. 7).
607
100
1
Table 5. Activation parameters, determined according to the Eyring
theory of the actwated complex, for the dinitrophenylation of E-aminocaproic acid and nylon 6 at 30'C.
e-Aminocaproic acid
Nylon 6
"Y
20
60
LO
["I
+
Fig. 7. Proportion A of amino groups that can be dinitrophenylated
with 2,4-dinitro-l-fluorobenzene
in nylon 6 fibers with different degrees
of orientation. Abscissa: azimuthal half widths of the 020 reflection
as a measure of the degree of orientation of the fibers.
Garrnon and Gibson['"l therefore carried out the dinitrophenylation in the homogeneous phase with a very large
excess of reagent, with the result that the photometric
determination of the dinitrophenylated end groups
becomes inaccurate. These authors also failed to consider
that the total basicity and the content of amino groups
may be different in commercial nylon fibers. Basic fiber
additives may react with DNFB, a fact that must be disregarded in this method, which also allows no differentiation
between different dinitrophenylatable groups such as primary and secondary amino groups and amidine groups.
Dinitrophenylation in the homogeneous phase in the system 2,2,2-trifluoroethanoI/sodium hydrogen carbonate/
water132*lo' eliminates these difficulties and also allows
a differentiation between the basic end groups mentioned.
The nylon is dissolved in a 1.7% solution of DNFB in
2,2,2-trifluoroethanol (nylon concentration : 1%), and 5%
aqueous sodium hydrogen carbonate solution is added
(6%). After 14 hours the DNP-nylon (DNP=2,4-dinitrophenyl) is precipitated with 10 times as much water at
pH 3, freed from adhering reagent by extraction with
ethyl acetate, and dried. The DNP-nylon is then dissolved
in trifluoroethanol and determined photometrically at the
350nm absorption maximum. The content of amino end
groups is determined from a calibration curve plotted
for DNP-oligomers from the nylon 6 and nylon 6,6 series,
for which the chromophore has been found to be independent of the structure.
A DNFB-catalyzed condensation of the amino and carboxyl groups such as was observed by Heikens et ~ l . [ ' ' ' ~
for E-aminocaproic acid in an ethanol-water mixture containing hydrogen carbonate takes place only to a very
small extent in this model substance under the above
analytical conditions (content of DNP-E-aminocaproyl-Eaminocaproic acid <5mol-%). This is probably due to
the large excess of reagent.
Since the activation parameters of the dinitrophenylation
for E-aminocaproic acid and for the polymer have been
found to agree (Table 5), so that the "principle of equal
reactivity"[1I 2 , i 131 IS
. satisfied, a condensation can also
be ruled out for the polymer. Thus in the dinitrophenylation
of nylon, no amino groups are lost by side reactions.
608
AH+
[kcal/mol]
AG*
[kcal/mol]
AS
[cal deg-
15.0
16.7
22.6
23.1
- 23.9
- 20.2
I
mol-
'1
No molecular weight fractionation occurs in the isolation
of the DNP-nylon, as is shown by the correlation of the
values obtained with those found potentiometri~ally[~'1
in the molecular weight range between 6000 and 18000
(Table 6). If the analytical conditions are adhered to, the
dinitrophenylation proceeds quantitatively, as difference
analyses by the method of Richter et
have shown.
Table 6. Determination of the basic groups of nylon 6 (without added
regulators) [a] by potentiometry 1691 and by dinitrophenylation.
a.
17500
14000
12200
9000
6 300
Basic groups [meq/kg]
Potentiometry
DNP method
58
72
82
54
67
77
110
101
160
147
[a] These products were kindly made and characterized for us by BASF,
Ludwigshafen.
4.2.2. Use of Dinitrophenylation
4.2.2.1. Determination of Primary Amino Groups
Amino group contents of between 26 and 65 meq/kg of
fiber have been found for commercial fibers of the nylon 6
and nylon 6,6 types, depending on the manufacturer
and the intended use (Table 7).
End group equivalence of the free and acylated amino
groups to the carboxyl groups has been found for
unbranched polycaprolactam[' '1. This is confirmed by
the complete end group analysis of nylon 6 fibers (Table
7), but only if the total amino group content is calculated
from the dinitrophenylation values and compared with
the content of carboxyl groups found conductometrically.
Alkalimetric titration usually gives excessively high contents of amino groups because of its non-specificity. This
also explains why the number average molecular weights
calculated from the dinitrophenylation values are higher
than the values given in the literature[1161or at the upper
limit of the range" '1, since the latter were all determined
by titrimetric methods.
For a complete end group analysis, the acylated amino
groups must also be determined. The acyl residues are
derived from monofunctional carboxylic acids (acetic acid,
propionic acid, benzoic acid) added to the nylon 6 and
nylon 6,6 polymerization mixtures as molecular weight
stabilizers. Because of these regulators, equivalence of the
free end groups cannot be expected in technical nylon.
A method is described in the literature for the determination of the acetyl residues[118.1191,in which the acetic
Angew. C h r m . internat. Edit. / Vol. 12 (1973)
/ No. 8
Table 7. End group content and molecular weight (Rn)
of commercial nylon fibers.
[rll [ I 141
[dkl
Producer,
trade name
Basic groups [meq, kg]
DNP method
Potentiometry
~ 9 1
R-CO-NH
[meq/kgl
13.
COOH
[meqikgl
Conductometry [72]
R"
Enka Perlon
105
49
70
6
53
18500
Bayer Perlon
I .07
25
23
26
50
Phrix Perlon
Emser-Werke Grilon
0 92
1.09
I 07
40
36
51
18
53
19800
18000
42
17
57
18200
45
46
26
54
16000
65 (prim.)
3 3 (sec.)
not
titratable
II
68
135M)
Rhodiaceta Nylon
Monsanto Nylon
Reproducibility of the methods used
Confidence belt (for 95 Yo statistical certainty)
[meqikgl
Variation coefficient [%I
Method
DNP method
2.7
i0.9
Acetyl determination
15.4
k4.8
4.0
k2.7
Conductometry
acid liberated by phosphoric acid is distilled azeotropically
with xylene and then titrated. The following procedure[32J
is simpler. The nylon is digested with 50% sulfuric acid,
and the organic acids are steam distilled and then titrated
potentiometrically. The pK value gives an indication of
the acid present.
Though this method has a fairly high coeflicient of variation
(Table 7), the statement by Dinse and Praeger["sJ that
microanalytical or semimicroanalytical acetyl determinations on nylons are impossible does not appear to be
justified.
4.2.2.2. Determination of Secondary Amino Groups
and Detection of Amidine Groups
Mclntire et U I . [ ' ~ ~ J in
, 1953, described the possibility of
differentiating between primary and secondary amines by
dinitrophenylation, since the absorption maximum of the
primary DNP-amine at 350nm is displaced to 390nm
for secondary DNP-amine (Fig. 8).
The spectra of a large number of dinitrophenylated primary
and secondary amines show that when the alkyl residue
H-[CH,-],
has a chain length of n > 1, the chromophore
absorbs independently of the structure of the rest of the
molecule.
The presence of secondary amino groups can therefore
bededuced from theextinction ratio E350/E390o f a dinitrophenylated polyamide. The spectral behavior is evaluated
for the quantitative determination of primary and secondary amino groups in polymers by a two-component analysis
developed on the basis of measurements on DNP-monoalkylamines and DNP-dialkylamines.
c,
=
(7.49 x E350 - 3.62 x E3'O) x lo-' rnol/l
c2 = (7.21 x E390 - 3.41 x € ' 5 0 ) x
mol!
c,,c 2 =concentration of primary and secondary amino groups
P 5 0 . E390=extinctions at 350 and 390 nm
In addition to many other products that are formed by
the action of heat on aliphatic polyamides, particularly
in the presence of oxygen, a number of primary and secondary amines, amino acids, and other compounds containing
nitrogen, which should react with DNFB because of their
structure, have been described['z1- 1231. However, since
it has been necessary until now to liberate and isolate
these substances by hydrolysis in order to detect them,
there are only a few known groupings that are formed
in the polymer during thermal degradation and can be
dinitrophenylated there. Kumerbeek et cil.[' 241 showed that
in nylon 6 and nylon 6,6 at high temperatures, two amino
end groups react with elimination of ammonia, so that
a secondary amino group is formed with chain doubling.
270
310
350
390
h"n+
430
L7D
Fig. 8. Spectra of dinitrophenyl-n-propylamine(-) and dinitrophenyl
mol/l).
di-n-propylamine (....) in 2,2,2-trifluor.oethanoI (c=5.1 x
Angew. Chem. internat. Edit. / Vol. 12 (1973) / No. 8
+
NHa
2 R-NH2 + R-NH-R
R = HO-[CO-(CH2)s-NH-],CO-(CH2)s or
H 0-[CO-(CH
,),-CO-NH-(CH
2)6-NH-]nCO-(CH2),-CO-NH-(CH
609
2),,
In NMR studies on the gelation of nylon 6 and nylon
6,6, secondary and tertiary amino groups were recently
detected in the polymer itselF' l S l .
are present. a change in thc extinction ratio E350;E3y0
is found in relation to tinti-catcd nylon 6 containing only
primary amino groups (Fig.9. curve a).
This may be taken as the first direct evidence that the
There is also a possibility that nylon 6 contains the semicyclic amidines of the type ( 8 ) postulated by S ~ h k i r c . k [ ' ~ ~ ! non-deaminatable basic groups formed in nylon 6 by the
action of heat are amidine groupings. The spectra of nylon
though these have not yet been directly detected.
6.6 samples treated in the same way indicate that amidines
are also formed here bq thermolysis. Attempts by Schkuck
and Ric4irr.1i31Jto prove the existence of these groups
in peramidinated polycaprolactam by removal and isolation of the amidine heterocycle were successful only for
low molecular weight model substances. On the other
hand. they have been detected by thin layer chromatograin
phy['"] and determined indirectly by titration""'
For nylon 6,6, a reaction between the cyclopentanone
cationically
polymerized
nylon
6.
Schiff bases mentioned by CZ'ik0f/7~'~~1and DNFB must
be considered, and dinitrophenylation of the pyridincs
detected by Goodriiun~' is also possible. All these basic
groups are included in the results of titrimetric determinations, and are generally referred to simply as amino groups,
despite their different pl< values.
2-Benzylamino-I -aza-I -cycloheptene ( 9 ) and 2-butylamino-I-aza-I-cycloheptene (fU) can be used as model substances for the investigation of semicyclic amidine groups
in thermally damaged nylon 61'")1.
U
Dinitrophenylation experiments on tl- :se compounds[ 1
show that both the exocyclic and the c idocyclic amidinc
nitrogen react with DNFB, but that he reaction does
not proceed uniformly['29d1.
A comparison of the spectrum of peran dinated aiid dinitrophenylated nylon 6 with that ofa thi rmolyzed product
(30h at 185-195 C in the absence of oxygen) that had
subsequently been deaminated" 301and 1 ien dinitrophenylated shows good agreement (Fig. 9, cur es b and c respectively).As when diiiitrophenylated secon lary amino groups
4.3. Determination of the &drox!l
Pol! esters
End Groups of
For a chemical determination of the hydroxyl cnd groups,
these groups must first be allowed to react with a reactive
compound. Because of the low content of these groups
in fiber-forming polycstersl"l, calculation by measurement
of the excess reagent after the manner of a back-titration.
as suggested by a number of
'('1 .seems probIematicllJ'l. Dii-cct methods are based on the determination of the groups introduced into the polymer in polymeranalogous reactions (carbo~yll'~'',
haloacy1"4"-'J81. N plienylcarbamoyll-'". ' "). ' .")I . benzoyll I ' I 1 .
sulfosalicyl
groupsli5"). Ilowcver, analjses of this nature require an
additional determination of the original carboxyl group
content (this also applies to the repeatedly dcscribcd determinations by exchange of active hydrogen atoms with
isotopes' I 5-'1) or a mtdtiple quantitative reaction of the
polymer.
The reaction with phenyl isocyanate was first used bj,
Gric,h/ and R.'c,~rc, for the determination of hydroxyl groups
in PETl1"]]. These authors. and also Bcrtzct. and Mtrrigold1 5J! and Gtr~./ordand Roserihotrriil Is51.who allowed
PET to react with hexamethylene diisocyanate, determined
the liydroxyl group content from a nitrogen analysis of
the polycster urethane obtained: because of the very low
nitrogen contents, however, this cannot yield satisfactory
results. The accuracq of the phenyl isocyanate method
was improved by the work of Kct-ri[""1 and of Hcw/ri.d'('~.
but the analytical procedure required is complicated. The
polyester urethane formed is hydi-olyzed l o remove aniline,
which is diazotized, coupled uith 1 -naplitl~yletli~leuediamine 01-2-naphthol. and determined colorimetrically.
The use of a colored isocyanate, on the other hand, decisively simplifies the determination of the hydroxyl end
&roups?
4.3.1. Principles of the Determination of Hydroxyl
End Groups with 4-Nitro-I-naphthyl Isocyanate
The reaction of PET with 4-nitro-I-naphthyl isocyanate
("1)
yields a yellow R.'-(4-nitro-I-naplithyI) urethane of
the polymer. whose chromophoi-c absorbs independently
of the structure of the rest of the molecule. as has bcen
610
shown for N-(4-nitro-I-naphthyl) urethanes of ethylene
terephthalate oligomers (Fig. 10).
the isocyanate takes place to a negligible extent, if at
all, under the conditions
The polyester material is dissolved in nitrobenzene, and
sufficient NNI in nitrobenzene is added to make the solution 1 % with respect to polyester and 1.8'% with respect
to NNI. After five hours at 130 C , the W(4-nitro-l-naphthyl) urethane of the polyester is precipitated in ten times
as much ether, reprecipitated twice from nitrobenzene with
ether, and finally extracted with ether. The polyester urethane is dissolved in phenollo-dichlorobenzene (3 :2) and
determined photometrically at 375 nm. The content of hydroxyl groups is calculated with the aid of a calibration
curve plotted for W(4-nitro- I-naphthyl) urethanes of the
Table 8 shows a comparison of the results of the NNI
method with those of the method described by Coni.d'J21,
which requires a double determination of the carboxyl
group content before and after the reaction of the hydroxyl
groups with succinic anhydride and has a large confidence
belt" 39J.
Table 8. Determination of the hydroxyl end groups of commercial PET
fibers by reaction with succinic anhydride [ 1421 and with 4-nitro-I-naphthy1 isocyanate ("1).
Producer,
trade name.
degree of delustering [a]
Hocchst Trevira 220. mt
Enka Diolcn FL, gl
ICI Tcrylcne W16, hm
Rhodiaceta Tergdl. mt
OH [meq;kgl
with succinic
with NNI
anhydride [b]
[c]
55
54
66
60
19
73
51
57
gl =bright, hm = semi-dull, mt =dull.
[b] Initial content of carboxyl groups- sec Table 3.
[c] Coefficient of variation =2.4 "h:confidence belt = k 2 . 2 mcq;kg (for
95 % statistical certainty).
[d]
4.3.2. Application of the Hydroxyl End Group
Determination with 4-Nitro-I-naphthyl Isocyanate
The hydroxyl group contents found by the NNI method
for PET homopolyesters are between 35 and 79 meq/kg,
depending on the manufacturer and the intended use
(Table 9).
These values together with the results obtained by the
hydrazinolysis method give remarkable differences in the
ratios of hydroxyl to carboxyl groups in the fibers investigated; this ratio can play an important part in the thermal
behavior of the p0lyester~'~.~~1.
The molecular weights
(number average) were calculated from the results given
by the determination methods developed for both end
groups, the methyl ester end groups that may be present
in very small concentrations in PET" 571 being disregarded.
As was shown by Zitnrtrurnitrnr7 and Bwkrr['"], however,
a complete end group analysis should include testing for
methyl ester groups, since these can interfere with the
polycondensation, and their content in the PET should
yield information on the course of the production process.
a,,
Fig 10. Calibration line for the determination of thc hydroxyl end group
concentration from the molar extinction (at 375nm) of PET treated
with 4-nitro-I-naphthyl isocyanate(NN1):Thecalibration line was plotted
for the N-(4-nitro-I-iiaphthyl) urethanes i i l j - 114, of cthqlene terephthalate oligomers
ethylene terephthalate series. The hydroxyl end groups
are thus determined by a single chemical reaction of the
polymer. This leads to only small quantities of by-products
of the reagent, which can be removed without difficulty
by reprecipitation. As investigations on oligomers have
shown, reaction of carboxyl groups of the polyester with
The molecular weights obtained for the PET homopolyester fibers of the normal type are between 23000 and
28000. and correspond to data in recent publicatiOnS[40.15.hh.1 5 8 J. L ow molecular weights (approx. 17000)
were found for the two pilling-resistant PET fiber types;
the low molecular weights are probably responsible for
this property of the
611
Table 9. End group content and molecular weight (Mn)of commercial PET fibers.
Producer,
trade name,
degree of delustering [a]
En] [ I 561
[dllgl
COOH [meqlkgl
H ydrazinolysis
OH [meqlkgl
NNl method
[OH]
[COOH]
R
Hoechst Trevira 220, mt
Hoechst Trevira 220, gl
Enka Diolen. gI
Enka Diolen FL, gl
ICI Terylene W1 I . gl
ICl Terylene W 16, hm
Rhodiaceta Tergal. mt
Rhodiaceta Tergal, hrn
0.47
0.44
0.47
22
20
40
59
32
37
26
25
54
52
2.5
2.6
0.9
I .o
26 300
27 800
26 700
16800
26 300
17200
24 100
22 700
0.38
0.44
0.37
0.47
0.45
35
60
44
1.4
2.1
2.2
2.5
79
57
63
[a] gl= bright, hm =semi-dull, mt =dull
5. Determination of Comonorners in Polyesters
and Polyacrylonitrile
5.1. Investigation of Chemically Modified Polyesters
The difficulty of dyeing of polyester fibers and their suscep
tibility to pilling can be problematic in practice. In efforts
to overcome these difficulties, the homopolyester of terephthalic acid and ethylene glycol has been subjected to
numerous chemical modifications by copolymerization of
a third ~ o m p o n e n t [ ~ ~In. ~this
~ 1way,
.
the dyeability with
dispersed dyes can be improved (Table 10) or dyeing with
basic dyes can become possible (Fig. 11). On the other
hand, no technically and economically satisfactory solution
has so far been found to the problem of the modification
of PET for dyeing with anionic dyes, owing to the thermal
sensitivity of the required modifying components containing basic nitrogen.
There have been only a few publications dealing with
the quantitative determination of comonomers in chemically modified polyesters. These are concerned with the
I ) incorporation of co-components during polymer production
HoocuCmH
X-CH,-H,C"CH~~H,-X
I
R
R
= -SO,Na,
-R'-S03Na. -SOzNa.
X = -OH, -COOR
- S 0 2 0 - C ~H,
-PO(ONa),
-OK, -COOK
HOOC 0 ; O
C O O
ONa
H
::
HOCH,-P-C
OK
I
H,OH
2) incorporation of anionically active substances prior to or during the
spinning process
3) after-treatment of filaments and fibers with SOJ,SO2CI2,and CIS03H
Fig. 11. Modification of PET to allow dyeing with cationic dyes [66].
examination of the total hydrolysis product of the polymer
and the use of spectroscopic detection methods (IR,
NMR)[ Ol.
Table 10. Modifying components for PET to improve the affinity for disperse dyes [66]
Dicarboxylic
acids
Hydroxy
carboxylic acids
HOOC-(CHz),-COOH
n = 3,
4, 6, 7. 8 , 10
HO-(CHz),-COOH
Diols and monofunctional
polyalkylene oxides
HC+(CHz),-OH
n = 3.4. 5
HoocUcooN
n=O,1
, & O ~ c o o H
HOOC
H O O C ~ ( C H , ) ~ - ~ O , - ( C H , ~ ~ ~ C O O H
n .-. 10...150
RO-(CHz-CHz-O). H
iisl0
... 150
[a] Substitution products are also suitable
612
Angew. Chem. internat. Edit.
/ Vol. 12
( 1 9 7 3 ) 1 No. 8
' 1.;"..,.
5.1.1. Determination of Aromatic Comonomers
with Acidic Substituents
The hydrazinolysis method is also suitable for the investigation of chemically modified polyesters, and in addition
to the determination of carboxyl end groups it also allows
the quantitative determination of aromatic comonomers
with acidic substituent~[~~I.
Thus on ion exchange of the
hydrazinolysis products of basically dyeable polyester
fibers copolymerized with sodium 5-sulfoisophthalate, the
hydrazides of the comonomer are isolated because of their
unchanged sulfo group, as well as terephthalomonohydrazide.
The ion-exchange eluate may be regarded as a two-component system if no differentiation is made between the
sulfoisophthalic acid derivatives (dihydrazide and monohydrazide) that are isolated together with terephthalomonohydrazide. A quantitative simultaneous determination of the comonomer content and of the carboxyl end
groups, which are equivalent to the C-terminal terephthalic
acid, thus becomes possible. Terephthalomonohydrazide
( I 5) and 5-sulfoisophthalodihydrazide (16 ) are used as
calibration substances for the development of a two-component analysis based on the difference in the spectral
behavior of the compounds (Fig. 12).
I
320
l"nFig 12. UV spectra (in 0.1 N HCI) of terephthaiomonohydrazide ( l S j ,
p-(p-hydroxyethoxy)henzoic acid (17) ( c = 4 x
mol/l), and 5-sulfoisophthalodihydrazide (16) (c=2 x lo-' mol/l).
5.1.2. Determination of Aromatic Hydroxy Carboxylic
Acids
The ester linkages in homo- and copolyether ester fibers
with p-(P-hydroxyethoxy)benzoic acid (I 7) as the
monomer unit or comonomer unit are also quantitatively
broken by hydrazine, though only after a longer reaction
time.
H,NHN-OC
H,NHN-OC~COOH
nCO-NHNHz
''
0
COOH (17)
HO-CH,-CH,-O
cl
=
(6.21 x E240 - 1.04 x E212)x
cz
=
(2.62 x E2" - 0.51 x
E2"O)
+ HOCH,-CH,-O
=
\
COOQ H ~0N N H ,
molfl
x lo-' molfl
c I rc2 = concentration of terephthalomonohydrazide and 5-sulfoisophthalodihydrazide respectively
EZ4O,€ ' I 2
1
0
extinctions at 240 and 212 nm
For three basically dyeable PET copolyester fibers,
comonomer contents of 1.6 to 1.8 mol % of sodium
5-sulfoisophthalate were found, based on the monomer
unit [OCHzCH200C-CsH4-C0]
(Table 11). These
results are lower than the values given in the literature1160. 1611.
In the case of poly-Cp-(2-ethylenoxy)]benzoate (A-Tell)
(18), the hydrazinolysis method results in the isolation
of p-(J3-hydroxyethoxy)benzoicacid ( 1 7), which is equivalent to the C-terminal carboxyl groups of the polymer,
and which is quantitatively determined with the corresponding pure compound as the calibration substance
(Table 11).
The hydrazinolysis products of the copolyether ester, on
the other hand, yield terephthalomonohydrazide (1 5) and
p-(P-hydroxyethoxy)benzoicacid (I 7), which correspond
to different C-terminal monomer units. The ratio of the
numbers of monomer units carrying carboxyl groups
Table 11. Contents of carboxyl end groups and of comonomers in copolyester and homopolyether ester fibers.
Producer,
trade name
C- terminal
terephthalic
acid
Immol/kgl
C- terminal
p-(P-hydroxyethoxy)benzoic acid
[mmol/kgl
5-Sulfoisophthalic acid
fmmol/kgl
DuPont Dacron 64c
DuPont Dacron 65
DuPont Dacron 89
Unitika Copolyether ester
Unitika A-Tell
(homopolyether ester)
21
35
38
29
-
-
86
94
82
8
32
-
-
Angew. Chem. internat. Edit. / Vol. 12 (1973) f No. 8
61 3
can be determined by a photometric two-component
analysis developed with the pure compounds. This may
be regarded a s the first step toward a chemical sequence
analysis of this polymer. which has so far been attempted
only with the aid of high-resolution N M R spectroscopy“ 601.
5.2. Investigation of Copolymerized Pol! acrylonitrile
The comonomers present in commercial polyacrylonitrile
fibers (Fig. 4) can be determined directly, according to
H i l d m , by hydrolysis of the nitrile groups[’*I, with quantitative formation of carboxyl groups and ammonia; the
ammonia can be easily determined by titration, and the
content of nitrile groups can be calculated from the result.
I-CIIz<
€I-CHz< €1-It,
I
CzN
1%
I
-
+2il,O
[-CtIz<
I I-C HZ-CI 1-1”
I
COOH
R
+ NII,
However, no differentiation is possible in this method
between hydrolyzable nitrile groups and amide groups.
The content of incorporated comonomer is deduced from
the difference between the nitrile content expected for pure
acrylonitrile and that found for the polymer in question.
The comonomer contents obtained in this way vary widely
from one fiber to another (6 to 149’0. Table 12).
6. Determination of Extraneous Substances
Dyeability and strength. as the most important textile
properties, are determined by the chemical structure and
fine structure of the polymers. The extraneous substances.
however, must also be taken into accoiint in the analysis
of synthetic fibers. These include preparing agents. delustering agents. optical brighteners, light stabilizers. heat
stabilizers, antistatics, efc., which are added to the fiber
polymers to modify their application properties.
Since the substances mentioned belong to very different
classes, which may be present P . ~ J . even in a single
spinning oil, it seems impossible to find any generally
applicable methods. Since the treatment of synthetic fibers
with these additives is undergoing constant devcloprnent
and modification, it is extremely difficult to analyze them
qualitatively and quantitatively1 I ”~‘I.
However, the importance of these investigations is shown
4.9. by the influence of surface-active compounds,
1’.
g.
in spinning oils, on the viscosity
on titrimetric end group determinations[32.”I, and on the dyefastness of the fibers[’6’’. Methods for the analysis of
wool[ib’] appear to be of only limited value for the determination of extractable ionic substancest32.3 3 1 .
A spectrophvtometric determination of phosphorus, which
is important in connection with stabilizers containing phosp h o r ~ ~ [has
~ ’ ~been
, described by Telep and Ehr/ich[’hyl.
Dinse and E i ~ e i t “ ~have
~ ] developed a polarographic
method for the determination of titanium dioxide in PET
and nylon 6 materials. Polarography””’, atomic absorption spectroscopy” 721, X-ray fluorescence spectroscopy,
and neutron activation a n a l y ~ i s 13~3 .~“*I. are suitable for
Table 12. Contents of acrylonitrile and of comonomers i n commercial polyacrylonitrile fibers [8X]
Producer.
trade name
~~~~~
Experimental
acrylonitrile
content
Calculated comonomer content [%]
from nitrile
from nitrogen
group contcnt
content
90 5
89 9
904
86 0
95
10.1
96
140
91 2
94 I
91 0
91 5
xx
[“%I
~~
Bayer Dralon
Bayer Dralon bifilar
DuPont Orlon 42
DuPont Orlon 74
DuPont Orlon 75
Dul’ont Orlon 7 5 B
i’hrix Rcdon F
Hoechst DoIan 50
59
90
85
As a compnrison, the comonomer content was also determined from the content of nitrile groups, which can be
calculated from the result ofa Kjeldahl nitrogen determination. In the case of Orlon 74, a considerably higher
comonomer content is found by hydrolysis than by elemental analysis: this means that part of the nitrogen contained
in this fiber is present in an unhydrolyzable form. This
fact, the characteristic IR spectrum, and the dyeability
with acid dyes18xl support the assumption that this fiber
contains a heterocyclic amine (e.y. vinylpyridine), a conclusion that cannot be drawn from the nitrogen analysis
alone.
614
96
99
95
96
89
9x
$9
71
the determination of titanium and for the analysis of traces
of heavy metals, which may come from impurities, catalyst
residues, or labeling additives. The determination of the
heavy metal content shows very clearly the importance
of the analysis of extraneous substances, since heavy metals
can influence the stability of the fibers to light, heat, and
thermal oxidation.
7. Conclusion
In the foregoing discussions, we have described new specific
chemical methods for the dctermination of functional
groups in polymers, which allow more far-reaching conclusions to be drawn regarding the chemical structure and
the behavior of synthetic fibers than v.9. from most of
the titration methods used. Methods were shown for the
rccognition of fiber damage and the qiiantitative chemical
determination ofcomonomers. I t is clear that the chemistry
of natural macromolecular products, which has already
provided the stimulus for the synthesis of fiber-forming
polymers. is also a fruitful basis for their analysis.
C f i ~t h r ~ r i k D ~ / J / . - ~ . / Z V
1.J 1<rrrmrr,
II.
D i p l . - C / ~ ? iR
. . A /tt/oi:f;
D ipl .- C'f i P I i I . D. M ii Ilcv-Sch I I Irr fiw p i w i iss ion t o I ISO
5 0 1 1 1 ~ ' of thcir rc~sl~trrl~h
r1~Slrlr.s. ct+ arc' also yrtrtclfi'l io rhc
c ~ o r i i p r r r i i c ~ rlirrt
s
.supplicd the fiber niairritrl. 12+ //itrn/i the
Ltrrit/iwrrii/ fiir For.sc~/iirrighi~irn , L f i i i i s t i ~fiir 1~2fs.srr1schr$
ri~itlForsc./i~~~
d ic y~ s Lrriitlcs Nortlrhciii- 1t't..stfLr/rri trntl th1.
I'c~rhti~ir/
t l c ~c ~ / i c ~ r ~ i i s c ~ /1iid~i.stric
zcri
,fbr thcir slrppori 01' rhis
work.
tiiit/
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oii
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,
Received: October 31. 1972 [ A 953 II:]
G e r m a n version: Angcw. ('hem <S5. 691 (1973)
Translated by Lxprcss Translation Service. I.ondon
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Chemistry and Applications of Liquid Crystals
By Ralf Steinstrasser and Ludwig Pohl[*]
Approximately 5 % of all organic compounds are transformed at their melting point into
liquid crystals- thermodynamically stable, anisotropic liquids which in contrast to isotropic
melts appear turbid and are also known as mesophases. Such melts are classed as smectic,
nematic, and cholesteric liquid crystalline phases, depending upon the arrangement of the
constituent molecules. The discovery of numerous potential applications during the past
ten years has awakened the study of liquid crystals from its former slumber as a physical
curiosity and placed it in the limelight of the scientific stage. Uses in display systems
for measured values and for computer and process data, as well as for remote controlled
timetables, for windows of variable light-transmission, etc., appear particularly promising.
Not only black-and-white contrasts are now possible but also color production.
1. Introduction
2.1. Smectic Liquid-Crystalline Phases
Directionally dependent (anisotropic) properties are
observed independently of the state of aggregation only
for substances having a regular arrangement of constituent
molecules. If the order is three-dimensional in nature then
the substance is a crystalline solid, while two- or one-dimensional order is characteristic of crystalline liquids or liquid
crystals[’ - 51.
Smectic phases have a two-dimensional structure. As seen
in Figure 1 their molecules are arranged in layers. Since
hardly any interaction occurs between the ends of the
molecules the layers can readily slip over one another.
The high viscosity and surface tension of smectic phases
are a consequence of the high degree of order.
The main emphasis of the present progress report will
be placed on recent applications of liquid crystals and
on the connection between chemical structure and the
principal physical effects.
2. Structure and Properties of
Liquid-Crystalline Phases
Smectic, nematic, and cholesteric liquid-crystalline phases
can readily be distinguished on the basis of their optical,
rheological, and thermodynamic properties. The structures
characteristic of smectic phases were first observed with
soaps (Greek crpqypcl) under the polarizing microscope.
The nematic phases owe their name to their threadlike
(Greek vbpclros) appearance under the polarizing microscope. And finally the designation cholesteric derives from
cholesterol whose derivatives were the first compounds seen
to exhibit such phases.
[*] Dr. R. Steinstrasser and Dr. L. Pohl
Zentrallahoratorium fur Industriechemikalien
und Analytisches Zentrallaboratorium der E. Merck
61 Darmstadt, Frankfurter Strasse 250 (Germany)
Angew. Chem. internat. Edit.
Vol. 12 (1973)
1 No. 8
Fig. 1. Structural model of a smectic phase.
Depending on the molecular arrangement it is possible
to distinguish at least five different smectic states which
are classified by the symbols A to E according to H .
Sackmann and Demud21. Three additional smectic phases
designated F, GI6], and HI7] have also been proposed.
Pure compounds too, on warming or cooling, can pass
through several smectic phases bounded by first-order
617
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