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Effect of low molecular weight materials on physical and mechanical properties of gelatin films
1288
ZH. F. MOTElVEVA et al.
stress relaxation were plotted for the latter under conditions of uniaxial compression with different initial strains. This is exemplified b y the set of relaxation curves shown in Fig. 2 for polymer III. These data were replotted in a-e coordinates to determine the critical stress (act) values, i.e. the maximum stresses retained in the samples during time t (Fig. 3). The maximum on the curves marks the value of ac~ for a given period of relaxation. As m a y be seen from Fig. 3, the a¢~ values for the poly-l,3,7-triehloroheptadiene are fairly high, and exceed the corresponding values for PMMA, being close to those found for polycarbonares [8]. Translated by R. J. A. H~.~-DRY REFERENCES 1. A. P. SUPR1~, T. A. SOBOLEYA, I. I. V 0 ~ T S E V A and T. T. YASIL'EVA, U.S.S.R. Pat. 333255, 1972; Byull. izob., No. 13, 1972 2. A. P. SYPRUN, V. M. ZHULIN, T. A. SOBOLEVA, I. I. VOINTSEVA and T. T. VASIL'EVA, U.S.S.R. Pat. 325235, 1972; Byull. izob., No. 3, 1972 3. H. PRIM.AS, R. ERNST and R. ARNDT, Paper presented at the International Meeting of Molecular Spectroscopy, Bologna, 1959 4. A. P. SYPRUN and T. A. SOBOLEVA, Vysokomol. soyed. 6: 1128, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 6, 1242, 1964) 5. A. A. ASKADSKII and G. L. SLONIMSKII, Vysokomol. soyed. B12: 1917, 1971 (Not translated in Polymer Sei. U.S.S.R.) 6. I. I. VOINTSEVA, A. P. SUPRUN and T. A. SOBOLEVA, Vysokomol. soyed. A16: 998, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 5, 1153, 1974) 7. L. A. VUD, Uspekhi khimii 9: 1340, 1940 8. G. L. SLONIMSKII, A. I. MZHEL'SKII and A. A. ASKADSI(II~ Vysokomol. soyed. A12: 1161, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 1314, 1970)
EFFECT OF LOW MOLECULAR WEIGHT MATERIALS ON PHYSICAL AND MECHANICAL PROPERTIES OF GELATIN FILMS* ZH. F. !~OTEI~EVA, G. I. BURDYGINA, I. M. FRIDI~AN and P. V. KOZLOV All-Union Scientific Research Institute of Cinematography (Received 4 October 1972)
A study was made of low molecular weight substances on the heat stability, thermal contraction and mechanical properties of gelatin films. I t was shown t h a t the heat stability of gelatin decreases considerably on adding compounds containing more t h a n two polar groups and which have no marked effect on the helical structure of * Vysokomol. soyed. AI6: No. 5, 1113-1124, 1974.
Physical and mechanical properties of gelatin films
1289
gelatin. For materials, which hinder spiralling in gelatin macromolecules during film formation it is not the temperature but super-contraction which decreases. I t was found that the effect of low molecular weight substances on thermal contraction of gelatin films (at 20 to 120°), is correlated with their effect on the temperature variation of super-contraction of gelatin. Materials which reduce the thermal contraction of gelatin films normally reduce heat stability. Mechanical properties of gelatin films depend on the structural ordering and moisture content of gelatin. Materials which reduce the spiralling of gelatin macromolecules, or lower the moisture content of the gelatin system increase the brittleness of gelatin films. The elastic properties of films increase if materials introduced enable the moisture content of films to he increased considerably without altering the structure of gelatin. A LAROE n u m b e r o f p a p e r s h a v e b e e n c o n c e r n e d w i t h t h e s t u d y o f t h e effect o f low m o l e c u l a r w e i g h t s u b s t a n c e s on t h e p r o p e r t i e s a n d s t r u c t u r e o f fibrillar a l b u m e n s , p a r t i c u l a r l y collagen a n d g e l a t i n [1-3]. H o w e v e r , if t h e effect o f t h e s e m a t e r i a l s o n t h e p r o p e r t i e s o f solutions a n d g e l a t i n gels has b e e n s t u d i e d suffic i e n t l y [2, 4], t h e p r o p e r t i e s o f g e l a t i n in t h e solid s t a t e a f f e c t e d b y low m o l e c u l a r w e i g h t s u b s t a n c e s h a v e s c a r c e l y b e e n e x a m i n e d u p t o t h e p r e s e n t tim(:,. T h e use o f low m o l e c u l a r w e i g h t s u b s t a n c e s for t h e m o d i f i c a t i o n o f t h e b e h a v i o u r o f g e l a t i n in v a r i o u s d i r e c t i o n s is o f c o n s i d e r a b l e i n t e r e s t f o r d e v e l o p i n g m a t e r i a l s w i t h t h e r e q u i s i t e c o m p l e x o f p r o p e r t i e s . I t was s h o w n [5] t h a t low m o l e c u l a r w e i g h t m a t e r i a l s a d d e d t o g e l a t i n films m a y g r e a t l y r e d u c e h e a t s t a b i l i t y o f gelatin. A c c o r d i n g l y , t h i s p a p e r seeks t o e x a m i n e t h e effect o f f u n c t i o n a l g r o u p s o f low molectflar w e i g h t s u b s t a n c e s o f different c h e m i c a l s t r u c t u r e a n d c o n t e n t o n t h e p h y s i c a l a n d m e c h a n i c a l p r o p e r t i e s o f g e l a t i n films a t different t e m p e r a t u r e s a n d h u m i d i t i e s a n d establish t h e n a t u r e o f p l a s t i c i z a t i o n o f gelatin.
OBJECTS AND METHODS OF INVESTIGATION Highly viscous photographic gelatin was used. Films were obtained from 5% aqueous gelatin solutions on a polyethylene or polyethylene terephthalate substrate by gel-formation at room temperature and from 15-19°/0 solutions on an aeetyl cellulose substrato by special spraying machines with cooling at 0 ° and drying the film at room temperature. I t is known that these conditions of film formation promote the restoration of collagen-like spiral conformation of gelatin macromolecules [2, 6]. Materials of various classes of compounds and of different types and numbers of functional groups were used. Thus, for example, polyhydric alcohols, urea and its derivatives, amino-acids, hydroxy-benzenes, amino-alcohols, amides, inorganic salts and some other compounds were used. The amount of substances added to the fihns varied between 2.5 and 30 to 60 ~ of the weight of the air dry gelatin. The upper limit was determined in most cases by compatibility with gelatin in films. The maximum content of substances in the films corresponded to a proportion which, when exceeded, caused turbidity during formation. The effect of low molecular weight substances on heat stability (super-contraction) and contraction of gelatin films under the action of heat was examined by linear dilatometry at temperatures of 18 to 230 °. The tests were made in a U P P device [7] using a method previously described [8]. The variation of linear dimensions of specimens was determined with an accuracy of u p to 2/~m. The structure of gelatin in films with the added materials mentioned was studied by X-ray micrography in a URS-55 device using monochromatic copper radiation and a nickel filter. The effect of low molecular weight materials on the
1290
Zrr. F. M o ~ - z v A e$ a/.
mechanical properties of gelatin films was studied in relation to the variation of the impact strength of films determined with a pendulum impact apparatus. Tests were carried out at 20 and 70°. Results concerning the impact strength of films are average values of 20 to 30 parallel tests. Prior to dilatometric tests the films were conditioned in a relative atmospheric humidity ~ 6 5 % (over saturated Na~O, solution) and before impact tests at : 65 and 0% (over P~Os) for 6 to 7 days up to practically equilibrium moisture content. RESULTS
The effect of low molecular weight substances on the properties of gelatin films has been e x a m i n e d in two directions. The first is related to t e m p e r a t u r e transitions in gelatin, p a r t i c u l a r l y the t e m p e r a t u r e of s u p e r - c o n t r a c t i o n T, upeon, which is the u p p e r limit of heat stability of gelatin a n d to the film c o n t r a c t i o n u n d e r the a c t i o n of hear, according to the t y p e a n d a m o u n t s o f low molecular w e i g h t materials added.
5#
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Fie. 1. Relation between the variation of linear dimensions (A) of gelatin films with triethanolamine (a), glycinc (b) and films obtained from a mixture of water and formamide (v} and temperature: a: •--0; 2--5; 3--10; 4--20; 5--30; 6--40; 7--50 wt. % triethanolamine; b: 1--0; 2--2.5; 3--10; 4--20; 5--30 wt. % glycine; c; •--0; 2--0.5; 3--5; 4--10; 5--20 vol. % formamide in the mixture.
Physical and mechanical properties of gelatin films
1291
The second direction is related to the effect of these additives on the mechanical properties of gelatin films, particularly on the resistance of the latter to mechanical impact. These two directions essentially cover the basic complex of physical and mechanical properties of gelatin film materials, which is not only of scientific interest, but also has practical application. As a result of the investigation it was found that the effect of low molecular weight materials on heat resistance, thermal contraction and mechanical properties of gelatin films is characterized by specific features. Heat resistance of gelatin. Figure la and b show the temperature relations which govern the variation of linear dimensions of gelatin films with varying ~riethanolamine and glycine contents. Similar cl~rves were plotted for all materials examined. From dilatometric test results curves were plotted to show the relation between Tsup¢on of gelatin and the number of moles of materials added per 1 kg gelatin (Fig. 2). Curves indicate that for most materials, with an increase in concentration, Tsupconof gelatin decreases. The reduction of Tsupcon of gelatin depends to a large extent on the chemical structure of compounds introduced, the number and position of functional groups contained and therefore, the type of interaction with albumen macromolecules. Thus for example, the reduction of Tsupcon of gelatin influenced mostly by materials containing at least three functional groups (triethanolamine, glycerine, glutamic acid, urea, thiourea, etc.). Furthermore, the reactivity of functional groups and their three-dimensional arrangement in the structure of the material is significant. Of the compounds containing hydroxyl with the same number of OH groups triethanolamine in particnlar reduces Tsupcon of gelatin to a greater extent than glycerine. For example, with triethanolamine and glycerine concentrations of 2 mole/kg the former reduces Tsupcon of gelatin by 100 and the latter, by 70 ° (Fig. 2a). With the same concentration (2 mole/kg) also the value of Tsupconof gelatin is also reduced by 70 ° by thiourea, urea nitrate (Fig. 2b) and glutamic acid (Fig. 2c), i.e. materials containing carboxyls, amino-, nitro-, thio- and carbonyl groups and not hydroxyls in the molecule. In other words, in spite of the varying interaction of these compounds with albumen macromolecules [3, 4], the same effect is essentially observed in the variation of T~upcon of gelatin. However, results concerning the variation of Tsupcon of gelatin under the effect of urea and derivatives such as thiourea and urea nitrate prove that not only the number of reactive groups (which accounts for the strong effect on Tsupconof gelatin of all the three materials) but also the type determining interaction with gelatin is significant. Thus, for example, urea normally reacts in the amide form of H2N--CO--NH~ [9], while thiourea, in contrast to urea mainly interacts in the enol form H 2 N - - C ( - - S H ) = = N H [9], in which the SH group becomes the donor and the = N H group is the acceptor during the formation of the H bond [4], thus intensifying the denaturing action, which is also confirmed by a greater reduction of Tsupeo=of gelatin.
1292
ZH. F. Mo~zvEVA et al.
In a certain concentration range urea nitrate has an even more significant effect (Fig. 2b). Low molecular weight materials containing two or one functional groups normally slightly reduce T~up~on of gelatin or do not change it at all (ethylene Tsupcon , °O
22O
,~
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11
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4
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I
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Fro. 2. Relation between Tsupeon of gelatin films and the concentration of low molecular weight substances added: a: 1--triethanolamine; 2--glycerine; 3--rosorcine; 4--thiodiglycol; g--Na salt of toluenesulpho acid; 6--1,4-butylene glycol; 7--ethylene glycol; 8 ethylene chlorohydrin; b: 9-- urea nitrate, 10-- thiourea; 11-- urea; 12-- dimethyl sulphoxide; 13 -- aeetamide; 14--formamide; c: lg--glutamic acid; 16--aspartic acid; 17--valine; 18-glycine; d: 19--potassium thioeyanate; 20--potassium iodide; 21--lithium bromide. glycol, ethylene chlorohydrin, dimethylsulphoxide, some amino-acids, etc.). Investigations carried out with ethylene glycol produced somewhat unexpected results. The addition to the films of even 8 mole/kg ethylene glycol reduces the value of Tsupcon of gelatin slightly (~ 15°), whilst 5 mole/kg glycerine reduces the value of Tsupcon b y 115 ° (Fig. 2a). Both glycerine and ethylene glycol are, under certain conditions solvents of gelatin [4], both substances belonging to the same class of compounds containing only one type of functional group --hydroxyl. According to results in the literature, both materials increase the strength and melting point of gelatin gels [4, 10-12]. Nevertheless, a marked difference is observed in their effect on the behaviour of gelatin during heating in the solid state. This fact m a y be due to at least two possible causes. First, the presence in the glycerine molecule of three hydroxyl groups, which increases the likelihood
Physical and mechanical properties of gelatin films
1293
of blocking the functional groups of albumen. As mentioned, all materials with three polar groups markedly reduce Tsup¢on. Secondly, it m a y be due to a steric factor, which is apparently of significance for rigid-chain polymers in the solid state, when molecular interaction reaches the upper limit. Indeed, if ethylene glycol is capable of undergoing intramolecular combination via a hydrogen bl)nd [4], which increases molecular density even more, glycerine which lacks this property and has larger molecular dimensions, may cause a marked disintegration in gelatin structure in the solid state, thus reducing molecular interaction and increasing the state of stress of undecomposed bonds in the polymer. This assumption is confirmed by a fairly sudden reduction in the value of Tsupcoa of gelatin under the effect of resorein (similar to glycerine) which, like ethylene glycol, has two OH groups, but is an aromatic compound, with molecules orientated perpendicular to the direction of the macromolecular axis of albumen [3]; this increases the distance between them and thus increases the state of stress of adjacent bonds. Furthermore, the Na salt of toluene sulphonic acid containing only one polar group, but belonging also to aromatic mononuclear compounds capable of orientation during adsorption of albumen by macromolecules [3], like resorcin and glycerine (Fig. 2a), considerably reduces Tsupcon. For glycerine heat breaks down the weakened bonds and c~uses conformation transition at lower temperatures than for ethylene glycol. An increase ill the distance between OFI groups in 1,4-butylene glycol compared with ethylene glycol causes a more noticeable reduction of Tsupcon. The value of Tsupcondecreases even more markedly with thiodiglycol, which contains a thio-group, as well as two hydroxyls (Fig. 2a). Interesting results were obtained when studying the.effect of amino-acids oll the behaviour of gelatin during heating. Curves indicate (Fig. 2c) that aminoacids of the glycine and valine types slightly affect the value of Tsupconof gelatin. Asparagine [5] has an even smaller effect. Aspartic and glutamic acids in concentrations of up to 1.5 mole/kg also greatly influence Tsupoo. as, for example, do urea and its derivatives. Consequently, the presence of even one carboxyl group in molecules of these amino-acids and the more remote arrangement of this group from other polar groups increase their activity in gelatin which is shown by a more sudden reduction of Tsupcon. It should be noted that the concentration relationship of the variation of Tsupoon on adding glycine is extremal; with a glycine concentration higher than 1.3 mole/kg Ts~poonincreases (Fig. 2c, curve 18). It has been noted in the literature that some amino-acids have an inhibiting action during denaturing of albumens, the effectiveness of this inhibiting action of glycine being of primary importance [13]. Glycine, apparently, has a stabilizing action even in the case of gelatin above a certain critical concentration, thus preventing the super-contraction of gelatin. The mechanism of this effect, evidently, involves structural crosslinking of gelatin by glycine molecules to form fairly strong crosslinks. However, this assumption cannot account for reduction in T~upconby glycine below the critical concentration, which requires further study.
1294
ZH. F. MO~E~rEVAet
al.
Curves in Fig. 2 indicate t h a t some of the organic compounds, e.g. e t h y l e n e c h l o r o h y d r i n (Fig. 2a), dimethylsulphoxide, f o r m a m i d e and a c c t a m i d e (Fig. 2b) do n o t over the entire range of c o n c e n t r a t i o n alter Tsupcon. All these materials are, a t t h e same time, solvents of gelatin [4]. F u r t h e r , it is well k n o w n t h a t considerable a m o u n t s of ethylene c h l o r o h y d r i n a n d d i m e t h y l s u l p h o x i d e (over 25 vol. %) result in decomposition of the helical s t r u c t u r e of gelatin in aqueous solutions [14, 15]. Despiralization of gelatin macromolecules in aqueous solut i o n (Table 1) is also observed on adding a fair a m o u n t of formamide, which a m o n g organic solvents has the highest dissolving power for gelatin. TABLE
1. E F F E C T OF L O W M O L E C U L A R ~VEIGHT M A T E R I A L S ON T H E S T R U C T U R E OF G E L A T I N IN FILMS
Material
-
-
Glycerine Ethylene glycol Diethylene glycol Triethanolamine Glycino Urea Thiourea Resorcin Potassium thiocyanate Formamide ,,
Coneentration of material, mole/kg gelatin
Main interferences, in order of magnitude, calculated from the centre 1 2 3 inteninteninten/~ sity A sity A sity
-5.4 8-0 5-9 2-7 2.6 5-0 2.6
11.60 10"79 12.45 12.08 11.95 10.07 11.63 11.25
Strong Very weak Strong Weak ,, Strong Average Very weak
4.15 Strong 4.15 ,, 4.14 i ,, 4.17 " e 4"30 Avera~ 4.02 Strong 4.08 ,, 4.03 ,,
2-0
10.32
Average
3.91
2.0
11.30
Weak
3.80
Avera~ e 2.88
11.5
,,
4.15
Strong
4.20
,,
10, wt.% 20, wt.
%
-
-
,,
2.79 Average 2.80 ,, 2.79 ,, 2-85 ,, 2.89 ,, 2.65 ,, 2.77 Weak 2.78 Very weak 2.66 Weak
2-80 -
Very weak ,, -
Figure lc, indicates t h a t as the c o n t e n t o f f o r m a m i d e increases in gelatin films, it is s u p e r - c o n t r a c t i o n and n o t t e m p e r a t u r e which decreases. W i t h a formamide c o n c e n t r a t i o n of 20 v o l . % , gelatin in films o b t a i n e d f r o m this m i x t u r e becomes fully a m o r p h o u s (interferences at 2.8 and 11.4 A are a b s e n t f r o m the X - r a y p h o t o g r a p h of this film) which accordingly reduces t h e super-contract i o n of gelatin (Fig. lc, curve 5). Tsurcon in this case remains u n c h a n g e d a n d is e q u a l t o T~upco= of gelatin o b t a i n e d f r o m an aqueous solution. I n a d d i t i o n to the organic c o m p o u n d s examined, we studied t h e effect o f some inorganic substances, p a r t i c u l a r l y neutral salts, on the properties of gelatin in t h e solid state. The d e n a t u r i n g a c t i o n of salts such as KSCN, L i B r a n d K I
Physical and mechanical properties of gelatin films
1295
on albumens has been known for a long time [1]. The fact previously established t h a t anions and cations have an independent effect on albumens, which is normally in accordance with the Hofmeister series [1, 16], also holds good for the behaviour of gelatin in the solid state. The highest adsorption is found with salts which contain SCN- and the I - is adsorbed to a lesser extent. In the adsorption of gelatin LiBr Li + cation is mainly significant, its adsorption properties being lower than those of SCN- and I - in salts of potassium, although in the case of LiBr B r - is also adsorbed by albumen. Figure 2d shows that the salts indicated have a variable effect on Tsupcon, this effect also depending on the adsorption properties of ions of these salts by macromolecules of albumen. The reduction of Tsupcon is most marked in the case of KCNS, a strong denaturing agent of collagen [1], gelatin in solutions [17] and other albumens [18]. As expected, LiBr has the least effect and the curve of K I occupies an intermediate position. Consequently, the type of action of the neutral salts indicated on Tsupconshows a direct relation with adsorption properties of ions and agTees with corresponding results for other fibrillar albumens. A study of the behaviour of gelatin from the point of view of the effect of low molecular weight materials on albumen structure indicates that the heat resistance of gelatin is considerably reduced on using materials which do not essentially hinder the spiralization of gelatin macromolecules during film formation via the gel-like state (e.g. triethanolamine, glycerine, etc., Table 1). However,~ the collagen-like spiral structures obtained are thermally unstable as a conse-. quence of partial disintegration or weakening by these materials of bonds both directly participating in spiralization of gelatin and of secondary bonds, stabilizing in some way the spiral structural formations of albumen. In this case the action of these materials and temperature supplement each other. Materials, reacting with cis-amide bonds of gelatin [4], i.e. preventing the spiralization of macromolecules (e.g. dimethylsulphoxide, formamide, etc.) do not, in practice, reduce Tsupcon. With considerable concentrations of these materials films behave like those in "hot" gelatin [19], i.e. the extent of supercontraction and not the value of Tsupcon alters. This fact proves that the degree of spiralization of gela-. tin macromolecules decreases in the presence of these additives without altering the type of bond formed, which confirms the earlier conclusions [12] concerning the equivalence of action of increased temperatures during film formation and similar materials. Consequently, the absence of any variation in the heat stability of gelatin during heating is the result of irreversible structural conversion of gelatin macromolecules in solutions during film formation. However, it will be shown that as a result of structural changes the mechanical strength of gelatin films deteriorates. Finally, compounds capable of crosslinking gelatin macromoleeules, or substances able to form an intramolecular hydrogen bond (ethylene glycol) do not practically affect the heat stability of gelatin, which results in only one func-
1296
Z m F. MOTENEVA e$ al.
tional group with increased proton activity [4] remaining in the material; this is clearly insufficient for a significant reduction of Tsupcon. Thermal contraction of gelatin. Temperature relationships governing the variation of linear dimensions of gelatin films provide important information concerning the effect of active low molecular weight substances on the type a n d extent of deformation (contraction) b y heat. It has been shown previously [19] that gelatin films 'during heating at temperatures of 20 to 120 ° undergo contraction, dependent on water deaorption from gelatin. Gelatin films contract to the extent of ~ 4%, i.e. contraction is high enough to be the source of residual stress. A study of the effect of low molecular weight active additives on thermal contraction of gelatin films is therefore of undoubted interest. t TABLE
2. C O M P A R A T I V E
DATA
OF
THE
MOLECULAR
ABSORPTION WEIGHT
PROPERTIES
A m o u n t o f w a t e r , ~o
Material
adsorbed at ~=65°//o
desorbed at (o=0~/o
18.5 70.2 38.2 43-9
18.3 69.5 -43.6
31.1
31.2
OF
GE~LATIN
AND
LOW
SURSTA:NCES $
A m o u n t o f w a t e r , ~o
Material
adsorbed at ~=65~o
desorbod at ~=0~o
42.0 0 0 0
42.6 0 0 0
i
Gelatin [ KCNS I Ethylene glycol Glycerine Triethanolamine
Formamide Glycine Resorcin Thiourea
* Data were obtained by keeping the materials under these conditions for 7 days.
From curves showing the temperature relationship which governs the variation of linear dimensions of gelatin films with different contents of low molecular weight materials (e.g. Fig. la, b) film contraction to 120 ° was calculated and curves plotted to show the relation between contraction and the molar concentration of materials added to the films per kg gelatin (Fig. 3). These curves show that the materials studied have a variable effect on the contraction of gelatin films. There is a clear correlation between concentration relations of the variation of Tsupeon (Fig. 2) and the contraction (to 120 °) of gelatin. A comparison of Figs. 2 and 3 indicates that materials which cause marked reduction of Tsup~on cause a significant reduction and contraction of films and vice versa, materials which have no effect on Tsupcon (except glycine, which will be mentioned below) do not alter in practice the extent of film contraction over the temperature range indicated. A reduction in the contraction of gelatin films at temperatures lower than 120 °, according to the content of low molecular weight materials m a y be due to at least two factors: l) reduction in the initial moisture content of films, dependent on the blocking of polar groups of gelatin b y active substances introduced and 2) relaxation processes facilitated in the disordered regions of gelatin
Physical and mechanical properties of gelatin films
1297
structure formed during film formation b y active substances present in the solution, which reduce molecular interaction in the polymer. We made the first assumption studying the effect of some materials on the absorption properties of gelatin films b y a gravimetric method with a relative atmospheric humidity of 65%. Prior to placing the films under the humidity conditions indicated they were kept at a relative humidity of 0% to achieve constant moisture content. The amount of moisture absorbed b y the films was calculated in terms of dry gelatin without considering the weight of low molecular weight substances contained. Curves in Fig. 4 indicate that with small amounts (up to ~ 1 mole/kg) of various low molecular weight substances a general tendency is observed to reduce absorption properties of gelatin, which may be due to the blockage of polar groups. An increase in the amount of materials introduced in gelatin either further reduces the absorption properties of gelatin (e.g. thiourea, resorcin and glyeine), or produces an extremal relation of the absorption properties of gelatin with the presence on the curves of a minimum at certain concentrations of materials (e.g. for glycerine, triethanolamine, potassium thioeyanate). It should be noted that a reduction in the absorption properties of gelatin over the entire concentration range of additives is observed on adding materials, of which the absorption properties under these conditions are almost absent (Table 2). The absorption properties of gelatin are therefore determined in this case only b y the number of polar groups remaining free. At the same time an increase is observed i n the absorption properties of gelatin after achieving a certain minimum with concentrations determined individually, for those materials of which the absorption properties under the relative atmospheric humidity conditions indicated exceed several times the values for gelatin itself (Table 2). Consequently, after a maximum possible blockage of functional groups of gelatin the active substances begin to absorb moisture themselves. A comparison of concentration relationships which govern. the variation of contraction and the absorption properties of gelatin films enables us to regard the reduction of thermal contraction of films containing small amounts of all substances investigated (except for ethylene glycol) as the result of reduced initial moisture content of gelatin. On increasing the concentration of additives a reduction in the thermal contraction of gelatin films in some cases takes place as a result of relaxation processes during film formation (glycerine, triethanolamine, etc.), while in others, as a result of a further reduction in the moisture content of gelatin (resorcine, thiourea, etc.), or as a consequence of a combination of the factors mentioned. Of all the materials examined it was only in one case, glycine, when there was no correlation between the variation of Tsupcon of gelatin and its thermal contraction. Up to a glycine concentration of 1.3 mole/kg both Tsupoon (Fig. 2c) and the contraction of gelatin (Fig. 3c) decrease. Above this critical concentration Tsupcon begins to increase, approximating to initial gelatin while film contraction continues to decrease rapidly. This fact is, apparently, due to the cross-
1298
ZH. F. MOTENEVA et o~.
linking action of glycine, which not only stabilizes T~upcon, but also creates a rigid frame of crosslinks to prevent the compression of gelatin during desorption of water. It could be assumed that with a content in the films in excess of a certain TABLE
3. ~-]FlVECT OF L O W M O L E C U L A R W E I G H T S U B S T A N C E S ON T H E I M P A C T S T R E N G T H OF GELATII~ F I L M S AT I N C R E A S E D T E M P E R A T U R E
c, mole/kg gelatin
Material
-Glycerine Ethylene glycol Triethanolamine Glycine
Impact strength at 70 °, kg.cm/ /cma
Material
~=6~% ~=0%
-5.4 8.0 1"3 1.3
25 30 20 17 13
15 17 20 20 8
I m p a c t strength at 70% kg.cm/ /cma
c, mole/kg gelatin
Thiourea Resoricine KCNS KI
~=65% ~=o%
2-6 2.0 2.0 1.3
7 7 0 4
18 12 0 9
concentration glycine prevents water desorption from gelatin, or films containing glyeine have a much lower initial moisture content. Both should result in lower film contraction during heating. Investigations of the absorption properties of gelatin films with glycine showed that the moisture content of these films with a
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O
I
2
I
[
4
d
I
I
2
1
I
C, mole/hff
FIG. 3. Relation b e t w e e n t h e contraction of gelatin films in the temperature range of 2 0 t o 1 2 0 ° a n d t h e concentration of low molecular weight substances introduced. See Fig. 2 for explanations.
Physical and mechanical properties o£ gelatin ~lm~
1299
relative atmospheric humidity of 65 % is indeed lower than the moisture conten~ of films without glycine (Fig. 4, eur~,e 5). However, even with m a x i m u m glycine concentration moisture content in films is 14%, i.e. high enough to cause film contraction during desorption. I n addition, the temperature relations which
2 40
0
-
4O
80
I
l
120 T,°O i
~-5-
~-lO-
0
I
I
I 3
I fs I
8
-18-
6'
0, mo/e/h~7 FiG. 4
FIo. 5
Fxo. 4. Relation between moisture absorption by gelatin films under conditions of relative atmospheric humidity o£ 65% and the concentrations of triethanolamino (1), glycerine (2), ethylene glycol (3), glycine (4), potassium thiooyanate (5), thiourea (6) and resorcin (7). FIe. 5. Temperature relation governing the weight variation of a gelatin film with 2.5 molo/kg glyeino (2) and without glycine (1). govern the weight variation of gelatin films with or without glycine (tests were carried out heating the specimens at a rate of 2 deg/min in a thermal balance produced by the French firm DAM) arc almost identical (Fig. 5) which proves t h a t water is desorbed without hindrance from gelatin films containing glycine. All this is contrary to the assumption made. The only apparent cause remains the crosslinking action of glycine which, similar to conventional tanning, reduces the thermal contraction of gelatin films and stabilizes Tsupcon which, naturally, requires experimental verification. Mechanical properties of gelatin. The effect of low molecular weight substances on the mechanical properties of gelatin films was evaluated from the variation of impact strength (Fig. 6a, Table 3). One material was selected for investigation from each class of compounds. With an increase in the content of most low molecular weight substances studied the impact strength of gelatin films decreases both with normal (at q----65%) and reduced (at q : 0 % ) moisture content (Fig. 6). A similar pattern is also observed when testing gelatin films at increased temperature (Table 3). Gelatin films with ethylene glycol, glycerine [20] and tricthanolamine additives are exceptions which under conditions of normal relative atmo-
1300
ZH. F. MOTEN~VAet a/.
spheric humidity with a certain fairly high concentration increase the elasticity of gelatin films, which is characterized by a two to three-fold increase in impact strength (Fig. 6a). However, a comparison of the concentration relation of the impact strength of gelatin films (Fig. 6a) and the absorption properties of these films (Fig. 4) and the absorption properties of materials added to the films (Table
~150
zook I
/
I
/
I /
A
/2
zo
,.%
0
I
2
3
4
58
0
Gmole/kg
FTG. 6. Relation between the impact strength of gelatin films and the concentration of low molecular weight substances under conditions of relative atmospheric humidity of 65 (a) and 0% (b): 1--triethanolamine; 2--glycerine; 3--ethylene glycol; 4--glycine; 5--potassium thiocyanate; 6--thiourea; 7--resorcin; 8--potassium iodide. 2) indicates that the mechanical properties of gelatin films are in direct relation to the moisture content of the system formed by gelatin and the material. When as a result of the high absorption properties of low molecular weight substances the moisture content of the entire system at some concentrations increases to a value exceeding the moisture content of gelatin without additives, the impact strength of these films increases as well. At the same time a reduction in the moisture content of films (Fig. 4), which is particularly marked in the case of materials incapable of absorbing moisture being added to the films (e.g. thiourea, resorcine, glycine (Table 2)), increases the brittleness of gelatin under conditions of normal relative atmospheric humidity. The brittleness of gelatin films also increases on adding materials which influence the formation of gelatin structure during film formation, independent of the absorption properties themselves. This is clearly seen be the example of potassium thiocyanate (Table l, Fig. 6a) and formamide (Table 4). Results of investigating the structure and mechanical properties of gelatin films with formamide, interacting with gelatin and preventing spiral formation of its macromolecules showed that with an increase in formamide content in the films, the degree of macromolecular ordering of gelatin decreases which markedly reduces impact strength.
Physical and mechanical properties of gelatin films
1301
Consequently, the mechanical properties of gelatin films depend both on moisture content and on the structure of gelatin. Therefore, 'any additives which lower the absorption properties of gelatin or reduce macromolecular ordering, TABLE 4.
EFFECT
O F F O R M A M I D E ON T H E M E C H A N I C A L
PROPERTIES
OF
Amount of formamide in the solution, vol. ~o 0 10 20
GELATIN
FILI~IS
Impact strength, kg-em/ cm
~=65%
~=0%
70 3O 0
40 20 0
i.e. influence the restoration of collagen-like helical structures during film formation, will increase brittleness. The problem concerning the effect of moisture content of gelatin on mechanical properties is of particular scientific and practical interest and should be examined specially. This is particularly important as the role of water in the structure of collagen is now fairly well known [21-23]. I t has been shown that, on reducing the moisture content of this biopolymer, the degree of structural ordering decreases until macromolecular spirals disappear completely when drying the specimens. This structural variation in itself should cause a loss in a certain complex of mechanical properties. The water which does not take part in helical structure formation of gelatin apparently functions as a plasticizer of collagen and gelatin. This is confirmed by data [24] concerning the mechanical properties of gelatin films with different moisture contents and the impact strength of films with glycerine, ethylene glycol and triethanolamine contents (Fig. 6a) which greatly increase the moisture content of the entire system as a result of additional moisture adsorption by these materials. Translated by E. SEMERE REFERENCES
1. K. N. GUSTAVSOIq, The Chemistry and Reactivity of Collagen, New York, 1956 2. A. WlE.I*, Makromolekulyarnaya khimiya zhelatina (Macromolecular Chemistry of Gelatin). Pishchevaya prom., 1971 3. A. N. MEKHAILOV, Kollagen kozhnogo pokrova i osnovy ego pererabotki (Collagen of the Integument and Principles of Treatment). Legkaya promyshlennost', 1971 4. J. Q. UMRERGER, Photogr. Sci. Engng. 11: 385, 1967 5. G. I. BURDYGINA, O. A. ALEKSINA, I. M. FRIDMAN and P. V. KOZLOV, Trudy NIKFI, 58, 1970 6. V. P. MERZLO~r, Dissertation, 1965 7. I. F. KAIMIN', Plast. massy, No. 9, 62, 1966 8. G. I. BURDYGINA, I. M. FRIDMAN and P. V. KOZLO~r, Trudy NIK_FI, 58, 1970 9. A. Ye. CHICHIBABIM, Osnovnye nachala organicheskoi khimii (Fundamentals of Organic Chemistry). vol. 1, Goskhimizdat, 1953 10. D. N. OSOKII~A,Kolloidn. zh. 19: 713, 1957
1302
S.I. KUCHANOV
11. J. R. NIXON, P. P. GEORGAKOPOULUS and J. E. (~ARLESS, J. Pharm. Pharmac. 18: 283, 1966 12. L. Z. ROGOVINA, G. L. SLONIMSKII and L. L. AKSENOVA, Vysokomol. soyed. AI3: 1451, 1971 (Translated in Polymer Sei. U.S.S.R. 13: 7, 1631, 1971) 13. M. ZHOLI, Fizieheskaya khimiya denaturatsii belkov (Physical Chemistry of Denaturing Albumens). Izd. "Mir", 1968 14. P. V. KOZLOV, A. I. UNDZENAS, V. P. IYIERZLOV and S. G. ROZENBERG, I)okl. AN SSSR 185: 118, 1969 15. A. I. UNDZENAS, Dissertation, 1968 16. A. N. MIKHAILOV, Fiziko-khimicheskie osnovytekhnologiikozhi (Physical and Chemical Principles of Leather Technology). Gizlegprom, 1949 17. E. O. KRAEIYIER, J. Phys. Chem. 45: 660, 1941 18. L. MANDEI,KERN, W. F. MEYER and A. F. DIORIO, J. Phys. Chem. 66: 375, 1962 19. G. I. BURDYGINA, I. M. FRIDMA_N, P. V. KOZLOV and V. A. KARGIN, Vysokomol. soyed. A l l : 118, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 1, 132, 1969) 20. Zh. F. MOTENEVA, G. I. BURDYGINA, I. M. FRIDMAN and P. V. KOZLOV, Mezhdunarodnyi kongress po fotograficheskoi nauke (International Congress on Photography). Moscow, 1970 21. N. G. YESIPOVA, N. S. ANDREYEVA and G. V. GATOVSKAYA, Biofizika 3: 529, 1958 22. H. J. C. BERENDSEN, J. Chem. Phys. 36: 3297, 1962 23. L. P. KAYUSHIN (Ed.), Sb. Sostoyanie i rol' vody v biologicheskikh ob'ektakh (The State and Role of Water in Biological Objects). Izd. "l~auka", 1967 24. Yu. K. GODOVSKII, I. I. MAL'TSEVA and G. L. SLONIMSKII, Vysokomol. soyed. A13: 2768, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 12, 1971)
THEORETICAL STUDY OF THE MICRO-HETEROGENEITY OF POLYCONDENSATION COPOLYMERS* S. I. K u c ~ A ~ o v Institute of Electrochemistry, U.S.S.R. Academy of Sciences (Received 5 October 1972) The relation between the type of distribution of units in polycondensation copolymers and the method of relying the initial monomers and their activities was examined theoretically. A comparison of the micro-heterogeneity coefficient calculated with experimental values in the literature showed satisfactory quantitative agreement in most eases. Distribution functions were calculated according to the lengths of various monomer units in oopolymer maeromolecules. THE p r o b l e m of micro-heterogeneity, i.e. the n a t u r e o f the distribution of units in linear macromoleeules of p o l y c o n d e n s a t i o n eopolymers, irrespective of m o i s t u r e content, has n o t been sufficiently studied. Owing t o the d e v e l o p m e n t of t h e high resolution N I ~ R m e t h o d it has r e c e n t l y been possible to examine c o p o l y m e r * Vysokomol. soyed. A16: No. 5, 1125-1132, 1974.
ZH. F. MOTElVEVA et al.
stress relaxation were plotted for the latter under conditions of uniaxial compression with different initial strains. This is exemplified b y the set of relaxation curves shown in Fig. 2 for polymer III. These data were replotted in a-e coordinates to determine the critical stress (act) values, i.e. the maximum stresses retained in the samples during time t (Fig. 3). The maximum on the curves marks the value of ac~ for a given period of relaxation. As m a y be seen from Fig. 3, the a¢~ values for the poly-l,3,7-triehloroheptadiene are fairly high, and exceed the corresponding values for PMMA, being close to those found for polycarbonares [8]. Translated by R. J. A. H~.~-DRY REFERENCES 1. A. P. SUPR1~, T. A. SOBOLEYA, I. I. V 0 ~ T S E V A and T. T. YASIL'EVA, U.S.S.R. Pat. 333255, 1972; Byull. izob., No. 13, 1972 2. A. P. SYPRUN, V. M. ZHULIN, T. A. SOBOLEVA, I. I. VOINTSEVA and T. T. VASIL'EVA, U.S.S.R. Pat. 325235, 1972; Byull. izob., No. 3, 1972 3. H. PRIM.AS, R. ERNST and R. ARNDT, Paper presented at the International Meeting of Molecular Spectroscopy, Bologna, 1959 4. A. P. SYPRUN and T. A. SOBOLEVA, Vysokomol. soyed. 6: 1128, 1964 (Translated in Polymer Sci. U.S.S.R. 6: 6, 1242, 1964) 5. A. A. ASKADSKII and G. L. SLONIMSKII, Vysokomol. soyed. B12: 1917, 1971 (Not translated in Polymer Sei. U.S.S.R.) 6. I. I. VOINTSEVA, A. P. SUPRUN and T. A. SOBOLEVA, Vysokomol. soyed. A16: 998, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 5, 1153, 1974) 7. L. A. VUD, Uspekhi khimii 9: 1340, 1940 8. G. L. SLONIMSKII, A. I. MZHEL'SKII and A. A. ASKADSI(II~ Vysokomol. soyed. A12: 1161, 1970 (Translated in Polymer Sci. U.S.S.R. 12: 1314, 1970)
EFFECT OF LOW MOLECULAR WEIGHT MATERIALS ON PHYSICAL AND MECHANICAL PROPERTIES OF GELATIN FILMS* ZH. F. !~OTEI~EVA, G. I. BURDYGINA, I. M. FRIDI~AN and P. V. KOZLOV All-Union Scientific Research Institute of Cinematography (Received 4 October 1972)
A study was made of low molecular weight substances on the heat stability, thermal contraction and mechanical properties of gelatin films. I t was shown t h a t the heat stability of gelatin decreases considerably on adding compounds containing more t h a n two polar groups and which have no marked effect on the helical structure of * Vysokomol. soyed. AI6: No. 5, 1113-1124, 1974.
Physical and mechanical properties of gelatin films
1289
gelatin. For materials, which hinder spiralling in gelatin macromolecules during film formation it is not the temperature but super-contraction which decreases. I t was found that the effect of low molecular weight substances on thermal contraction of gelatin films (at 20 to 120°), is correlated with their effect on the temperature variation of super-contraction of gelatin. Materials which reduce the thermal contraction of gelatin films normally reduce heat stability. Mechanical properties of gelatin films depend on the structural ordering and moisture content of gelatin. Materials which reduce the spiralling of gelatin macromolecules, or lower the moisture content of the gelatin system increase the brittleness of gelatin films. The elastic properties of films increase if materials introduced enable the moisture content of films to he increased considerably without altering the structure of gelatin. A LAROE n u m b e r o f p a p e r s h a v e b e e n c o n c e r n e d w i t h t h e s t u d y o f t h e effect o f low m o l e c u l a r w e i g h t s u b s t a n c e s on t h e p r o p e r t i e s a n d s t r u c t u r e o f fibrillar a l b u m e n s , p a r t i c u l a r l y collagen a n d g e l a t i n [1-3]. H o w e v e r , if t h e effect o f t h e s e m a t e r i a l s o n t h e p r o p e r t i e s o f solutions a n d g e l a t i n gels has b e e n s t u d i e d suffic i e n t l y [2, 4], t h e p r o p e r t i e s o f g e l a t i n in t h e solid s t a t e a f f e c t e d b y low m o l e c u l a r w e i g h t s u b s t a n c e s h a v e s c a r c e l y b e e n e x a m i n e d u p t o t h e p r e s e n t tim(:,. T h e use o f low m o l e c u l a r w e i g h t s u b s t a n c e s for t h e m o d i f i c a t i o n o f t h e b e h a v i o u r o f g e l a t i n in v a r i o u s d i r e c t i o n s is o f c o n s i d e r a b l e i n t e r e s t f o r d e v e l o p i n g m a t e r i a l s w i t h t h e r e q u i s i t e c o m p l e x o f p r o p e r t i e s . I t was s h o w n [5] t h a t low m o l e c u l a r w e i g h t m a t e r i a l s a d d e d t o g e l a t i n films m a y g r e a t l y r e d u c e h e a t s t a b i l i t y o f gelatin. A c c o r d i n g l y , t h i s p a p e r seeks t o e x a m i n e t h e effect o f f u n c t i o n a l g r o u p s o f low molectflar w e i g h t s u b s t a n c e s o f different c h e m i c a l s t r u c t u r e a n d c o n t e n t o n t h e p h y s i c a l a n d m e c h a n i c a l p r o p e r t i e s o f g e l a t i n films a t different t e m p e r a t u r e s a n d h u m i d i t i e s a n d establish t h e n a t u r e o f p l a s t i c i z a t i o n o f gelatin.
OBJECTS AND METHODS OF INVESTIGATION Highly viscous photographic gelatin was used. Films were obtained from 5% aqueous gelatin solutions on a polyethylene or polyethylene terephthalate substrate by gel-formation at room temperature and from 15-19°/0 solutions on an aeetyl cellulose substrato by special spraying machines with cooling at 0 ° and drying the film at room temperature. I t is known that these conditions of film formation promote the restoration of collagen-like spiral conformation of gelatin macromolecules [2, 6]. Materials of various classes of compounds and of different types and numbers of functional groups were used. Thus, for example, polyhydric alcohols, urea and its derivatives, amino-acids, hydroxy-benzenes, amino-alcohols, amides, inorganic salts and some other compounds were used. The amount of substances added to the fihns varied between 2.5 and 30 to 60 ~ of the weight of the air dry gelatin. The upper limit was determined in most cases by compatibility with gelatin in films. The maximum content of substances in the films corresponded to a proportion which, when exceeded, caused turbidity during formation. The effect of low molecular weight substances on heat stability (super-contraction) and contraction of gelatin films under the action of heat was examined by linear dilatometry at temperatures of 18 to 230 °. The tests were made in a U P P device [7] using a method previously described [8]. The variation of linear dimensions of specimens was determined with an accuracy of u p to 2/~m. The structure of gelatin in films with the added materials mentioned was studied by X-ray micrography in a URS-55 device using monochromatic copper radiation and a nickel filter. The effect of low molecular weight materials on the
1290
Zrr. F. M o ~ - z v A e$ a/.
mechanical properties of gelatin films was studied in relation to the variation of the impact strength of films determined with a pendulum impact apparatus. Tests were carried out at 20 and 70°. Results concerning the impact strength of films are average values of 20 to 30 parallel tests. Prior to dilatometric tests the films were conditioned in a relative atmospheric humidity ~ 6 5 % (over saturated Na~O, solution) and before impact tests at : 65 and 0% (over P~Os) for 6 to 7 days up to practically equilibrium moisture content. RESULTS
The effect of low molecular weight substances on the properties of gelatin films has been e x a m i n e d in two directions. The first is related to t e m p e r a t u r e transitions in gelatin, p a r t i c u l a r l y the t e m p e r a t u r e of s u p e r - c o n t r a c t i o n T, upeon, which is the u p p e r limit of heat stability of gelatin a n d to the film c o n t r a c t i o n u n d e r the a c t i o n of hear, according to the t y p e a n d a m o u n t s o f low molecular w e i g h t materials added.
5#
-N
/5#
'
5#
~/
/50
50
/50
~ °C
IH2
I f [l," L_
Fie. 1. Relation between the variation of linear dimensions (A) of gelatin films with triethanolamine (a), glycinc (b) and films obtained from a mixture of water and formamide (v} and temperature: a: •--0; 2--5; 3--10; 4--20; 5--30; 6--40; 7--50 wt. % triethanolamine; b: 1--0; 2--2.5; 3--10; 4--20; 5--30 wt. % glycine; c; •--0; 2--0.5; 3--5; 4--10; 5--20 vol. % formamide in the mixture.
Physical and mechanical properties of gelatin films
1291
The second direction is related to the effect of these additives on the mechanical properties of gelatin films, particularly on the resistance of the latter to mechanical impact. These two directions essentially cover the basic complex of physical and mechanical properties of gelatin film materials, which is not only of scientific interest, but also has practical application. As a result of the investigation it was found that the effect of low molecular weight materials on heat resistance, thermal contraction and mechanical properties of gelatin films is characterized by specific features. Heat resistance of gelatin. Figure la and b show the temperature relations which govern the variation of linear dimensions of gelatin films with varying ~riethanolamine and glycine contents. Similar cl~rves were plotted for all materials examined. From dilatometric test results curves were plotted to show the relation between Tsup¢on of gelatin and the number of moles of materials added per 1 kg gelatin (Fig. 2). Curves indicate that for most materials, with an increase in concentration, Tsupconof gelatin decreases. The reduction of Tsupcon of gelatin depends to a large extent on the chemical structure of compounds introduced, the number and position of functional groups contained and therefore, the type of interaction with albumen macromolecules. Thus for example, the reduction of Tsupcon of gelatin influenced mostly by materials containing at least three functional groups (triethanolamine, glycerine, glutamic acid, urea, thiourea, etc.). Furthermore, the reactivity of functional groups and their three-dimensional arrangement in the structure of the material is significant. Of the compounds containing hydroxyl with the same number of OH groups triethanolamine in particnlar reduces Tsupcon of gelatin to a greater extent than glycerine. For example, with triethanolamine and glycerine concentrations of 2 mole/kg the former reduces Tsupcon of gelatin by 100 and the latter, by 70 ° (Fig. 2a). With the same concentration (2 mole/kg) also the value of Tsupconof gelatin is also reduced by 70 ° by thiourea, urea nitrate (Fig. 2b) and glutamic acid (Fig. 2c), i.e. materials containing carboxyls, amino-, nitro-, thio- and carbonyl groups and not hydroxyls in the molecule. In other words, in spite of the varying interaction of these compounds with albumen macromolecules [3, 4], the same effect is essentially observed in the variation of T~upcon of gelatin. However, results concerning the variation of Tsupcon of gelatin under the effect of urea and derivatives such as thiourea and urea nitrate prove that not only the number of reactive groups (which accounts for the strong effect on Tsupconof gelatin of all the three materials) but also the type determining interaction with gelatin is significant. Thus, for example, urea normally reacts in the amide form of H2N--CO--NH~ [9], while thiourea, in contrast to urea mainly interacts in the enol form H 2 N - - C ( - - S H ) = = N H [9], in which the SH group becomes the donor and the = N H group is the acceptor during the formation of the H bond [4], thus intensifying the denaturing action, which is also confirmed by a greater reduction of Tsupeo=of gelatin.
1292
ZH. F. Mo~zvEVA et al.
In a certain concentration range urea nitrate has an even more significant effect (Fig. 2b). Low molecular weight materials containing two or one functional groups normally slightly reduce T~up~on of gelatin or do not change it at all (ethylene Tsupcon , °O
22O
,~
I4
/dO
11
/00 I
220 -
I
I
I
I
3
IgO
I
II
I
/2
5
l
c
/7
3
5
~'
0, mole/kg
~
d
140 I
0
I
I
Z
4
I
]
6'
I
0
I
2
I
L
4
I
I
I
6'
C, mole/kd
Fro. 2. Relation between Tsupeon of gelatin films and the concentration of low molecular weight substances added: a: 1--triethanolamine; 2--glycerine; 3--rosorcine; 4--thiodiglycol; g--Na salt of toluenesulpho acid; 6--1,4-butylene glycol; 7--ethylene glycol; 8 ethylene chlorohydrin; b: 9-- urea nitrate, 10-- thiourea; 11-- urea; 12-- dimethyl sulphoxide; 13 -- aeetamide; 14--formamide; c: lg--glutamic acid; 16--aspartic acid; 17--valine; 18-glycine; d: 19--potassium thioeyanate; 20--potassium iodide; 21--lithium bromide. glycol, ethylene chlorohydrin, dimethylsulphoxide, some amino-acids, etc.). Investigations carried out with ethylene glycol produced somewhat unexpected results. The addition to the films of even 8 mole/kg ethylene glycol reduces the value of Tsupcon of gelatin slightly (~ 15°), whilst 5 mole/kg glycerine reduces the value of Tsupcon b y 115 ° (Fig. 2a). Both glycerine and ethylene glycol are, under certain conditions solvents of gelatin [4], both substances belonging to the same class of compounds containing only one type of functional group --hydroxyl. According to results in the literature, both materials increase the strength and melting point of gelatin gels [4, 10-12]. Nevertheless, a marked difference is observed in their effect on the behaviour of gelatin during heating in the solid state. This fact m a y be due to at least two possible causes. First, the presence in the glycerine molecule of three hydroxyl groups, which increases the likelihood
Physical and mechanical properties of gelatin films
1293
of blocking the functional groups of albumen. As mentioned, all materials with three polar groups markedly reduce Tsup¢on. Secondly, it m a y be due to a steric factor, which is apparently of significance for rigid-chain polymers in the solid state, when molecular interaction reaches the upper limit. Indeed, if ethylene glycol is capable of undergoing intramolecular combination via a hydrogen bl)nd [4], which increases molecular density even more, glycerine which lacks this property and has larger molecular dimensions, may cause a marked disintegration in gelatin structure in the solid state, thus reducing molecular interaction and increasing the state of stress of undecomposed bonds in the polymer. This assumption is confirmed by a fairly sudden reduction in the value of Tsupcoa of gelatin under the effect of resorein (similar to glycerine) which, like ethylene glycol, has two OH groups, but is an aromatic compound, with molecules orientated perpendicular to the direction of the macromolecular axis of albumen [3]; this increases the distance between them and thus increases the state of stress of adjacent bonds. Furthermore, the Na salt of toluene sulphonic acid containing only one polar group, but belonging also to aromatic mononuclear compounds capable of orientation during adsorption of albumen by macromolecules [3], like resorcin and glycerine (Fig. 2a), considerably reduces Tsupcon. For glycerine heat breaks down the weakened bonds and c~uses conformation transition at lower temperatures than for ethylene glycol. An increase ill the distance between OFI groups in 1,4-butylene glycol compared with ethylene glycol causes a more noticeable reduction of Tsupcon. The value of Tsupcondecreases even more markedly with thiodiglycol, which contains a thio-group, as well as two hydroxyls (Fig. 2a). Interesting results were obtained when studying the.effect of amino-acids oll the behaviour of gelatin during heating. Curves indicate (Fig. 2c) that aminoacids of the glycine and valine types slightly affect the value of Tsupconof gelatin. Asparagine [5] has an even smaller effect. Aspartic and glutamic acids in concentrations of up to 1.5 mole/kg also greatly influence Tsupoo. as, for example, do urea and its derivatives. Consequently, the presence of even one carboxyl group in molecules of these amino-acids and the more remote arrangement of this group from other polar groups increase their activity in gelatin which is shown by a more sudden reduction of Tsupcon. It should be noted that the concentration relationship of the variation of Tsupoon on adding glycine is extremal; with a glycine concentration higher than 1.3 mole/kg Ts~poonincreases (Fig. 2c, curve 18). It has been noted in the literature that some amino-acids have an inhibiting action during denaturing of albumens, the effectiveness of this inhibiting action of glycine being of primary importance [13]. Glycine, apparently, has a stabilizing action even in the case of gelatin above a certain critical concentration, thus preventing the super-contraction of gelatin. The mechanism of this effect, evidently, involves structural crosslinking of gelatin by glycine molecules to form fairly strong crosslinks. However, this assumption cannot account for reduction in T~upconby glycine below the critical concentration, which requires further study.
1294
ZH. F. MO~E~rEVAet
al.
Curves in Fig. 2 indicate t h a t some of the organic compounds, e.g. e t h y l e n e c h l o r o h y d r i n (Fig. 2a), dimethylsulphoxide, f o r m a m i d e and a c c t a m i d e (Fig. 2b) do n o t over the entire range of c o n c e n t r a t i o n alter Tsupcon. All these materials are, a t t h e same time, solvents of gelatin [4]. F u r t h e r , it is well k n o w n t h a t considerable a m o u n t s of ethylene c h l o r o h y d r i n a n d d i m e t h y l s u l p h o x i d e (over 25 vol. %) result in decomposition of the helical s t r u c t u r e of gelatin in aqueous solutions [14, 15]. Despiralization of gelatin macromolecules in aqueous solut i o n (Table 1) is also observed on adding a fair a m o u n t of formamide, which a m o n g organic solvents has the highest dissolving power for gelatin. TABLE
1. E F F E C T OF L O W M O L E C U L A R ~VEIGHT M A T E R I A L S ON T H E S T R U C T U R E OF G E L A T I N IN FILMS
Material
-
-
Glycerine Ethylene glycol Diethylene glycol Triethanolamine Glycino Urea Thiourea Resorcin Potassium thiocyanate Formamide ,,
Coneentration of material, mole/kg gelatin
Main interferences, in order of magnitude, calculated from the centre 1 2 3 inteninteninten/~ sity A sity A sity
-5.4 8-0 5-9 2-7 2.6 5-0 2.6
11.60 10"79 12.45 12.08 11.95 10.07 11.63 11.25
Strong Very weak Strong Weak ,, Strong Average Very weak
4.15 Strong 4.15 ,, 4.14 i ,, 4.17 " e 4"30 Avera~ 4.02 Strong 4.08 ,, 4.03 ,,
2-0
10.32
Average
3.91
2.0
11.30
Weak
3.80
Avera~ e 2.88
11.5
,,
4.15
Strong
4.20
,,
10, wt.% 20, wt.
%
-
-
,,
2.79 Average 2.80 ,, 2.79 ,, 2-85 ,, 2.89 ,, 2.65 ,, 2.77 Weak 2.78 Very weak 2.66 Weak
2-80 -
Very weak ,, -
Figure lc, indicates t h a t as the c o n t e n t o f f o r m a m i d e increases in gelatin films, it is s u p e r - c o n t r a c t i o n and n o t t e m p e r a t u r e which decreases. W i t h a formamide c o n c e n t r a t i o n of 20 v o l . % , gelatin in films o b t a i n e d f r o m this m i x t u r e becomes fully a m o r p h o u s (interferences at 2.8 and 11.4 A are a b s e n t f r o m the X - r a y p h o t o g r a p h of this film) which accordingly reduces t h e super-contract i o n of gelatin (Fig. lc, curve 5). Tsurcon in this case remains u n c h a n g e d a n d is e q u a l t o T~upco= of gelatin o b t a i n e d f r o m an aqueous solution. I n a d d i t i o n to the organic c o m p o u n d s examined, we studied t h e effect o f some inorganic substances, p a r t i c u l a r l y neutral salts, on the properties of gelatin in t h e solid state. The d e n a t u r i n g a c t i o n of salts such as KSCN, L i B r a n d K I
Physical and mechanical properties of gelatin films
1295
on albumens has been known for a long time [1]. The fact previously established t h a t anions and cations have an independent effect on albumens, which is normally in accordance with the Hofmeister series [1, 16], also holds good for the behaviour of gelatin in the solid state. The highest adsorption is found with salts which contain SCN- and the I - is adsorbed to a lesser extent. In the adsorption of gelatin LiBr Li + cation is mainly significant, its adsorption properties being lower than those of SCN- and I - in salts of potassium, although in the case of LiBr B r - is also adsorbed by albumen. Figure 2d shows that the salts indicated have a variable effect on Tsupcon, this effect also depending on the adsorption properties of ions of these salts by macromolecules of albumen. The reduction of Tsupcon is most marked in the case of KCNS, a strong denaturing agent of collagen [1], gelatin in solutions [17] and other albumens [18]. As expected, LiBr has the least effect and the curve of K I occupies an intermediate position. Consequently, the type of action of the neutral salts indicated on Tsupconshows a direct relation with adsorption properties of ions and agTees with corresponding results for other fibrillar albumens. A study of the behaviour of gelatin from the point of view of the effect of low molecular weight materials on albumen structure indicates that the heat resistance of gelatin is considerably reduced on using materials which do not essentially hinder the spiralization of gelatin macromolecules during film formation via the gel-like state (e.g. triethanolamine, glycerine, etc., Table 1). However,~ the collagen-like spiral structures obtained are thermally unstable as a conse-. quence of partial disintegration or weakening by these materials of bonds both directly participating in spiralization of gelatin and of secondary bonds, stabilizing in some way the spiral structural formations of albumen. In this case the action of these materials and temperature supplement each other. Materials, reacting with cis-amide bonds of gelatin [4], i.e. preventing the spiralization of macromolecules (e.g. dimethylsulphoxide, formamide, etc.) do not, in practice, reduce Tsupcon. With considerable concentrations of these materials films behave like those in "hot" gelatin [19], i.e. the extent of supercontraction and not the value of Tsupcon alters. This fact proves that the degree of spiralization of gela-. tin macromolecules decreases in the presence of these additives without altering the type of bond formed, which confirms the earlier conclusions [12] concerning the equivalence of action of increased temperatures during film formation and similar materials. Consequently, the absence of any variation in the heat stability of gelatin during heating is the result of irreversible structural conversion of gelatin macromolecules in solutions during film formation. However, it will be shown that as a result of structural changes the mechanical strength of gelatin films deteriorates. Finally, compounds capable of crosslinking gelatin macromoleeules, or substances able to form an intramolecular hydrogen bond (ethylene glycol) do not practically affect the heat stability of gelatin, which results in only one func-
1296
Z m F. MOTENEVA e$ al.
tional group with increased proton activity [4] remaining in the material; this is clearly insufficient for a significant reduction of Tsupcon. Thermal contraction of gelatin. Temperature relationships governing the variation of linear dimensions of gelatin films provide important information concerning the effect of active low molecular weight substances on the type a n d extent of deformation (contraction) b y heat. It has been shown previously [19] that gelatin films 'during heating at temperatures of 20 to 120 ° undergo contraction, dependent on water deaorption from gelatin. Gelatin films contract to the extent of ~ 4%, i.e. contraction is high enough to be the source of residual stress. A study of the effect of low molecular weight active additives on thermal contraction of gelatin films is therefore of undoubted interest. t TABLE
2. C O M P A R A T I V E
DATA
OF
THE
MOLECULAR
ABSORPTION WEIGHT
PROPERTIES
A m o u n t o f w a t e r , ~o
Material
adsorbed at ~=65°//o
desorbed at (o=0~/o
18.5 70.2 38.2 43-9
18.3 69.5 -43.6
31.1
31.2
OF
GE~LATIN
AND
LOW
SURSTA:NCES $
A m o u n t o f w a t e r , ~o
Material
adsorbed at ~=65~o
desorbod at ~=0~o
42.0 0 0 0
42.6 0 0 0
i
Gelatin [ KCNS I Ethylene glycol Glycerine Triethanolamine
Formamide Glycine Resorcin Thiourea
* Data were obtained by keeping the materials under these conditions for 7 days.
From curves showing the temperature relationship which governs the variation of linear dimensions of gelatin films with different contents of low molecular weight materials (e.g. Fig. la, b) film contraction to 120 ° was calculated and curves plotted to show the relation between contraction and the molar concentration of materials added to the films per kg gelatin (Fig. 3). These curves show that the materials studied have a variable effect on the contraction of gelatin films. There is a clear correlation between concentration relations of the variation of Tsupeon (Fig. 2) and the contraction (to 120 °) of gelatin. A comparison of Figs. 2 and 3 indicates that materials which cause marked reduction of Tsup~on cause a significant reduction and contraction of films and vice versa, materials which have no effect on Tsupcon (except glycine, which will be mentioned below) do not alter in practice the extent of film contraction over the temperature range indicated. A reduction in the contraction of gelatin films at temperatures lower than 120 °, according to the content of low molecular weight materials m a y be due to at least two factors: l) reduction in the initial moisture content of films, dependent on the blocking of polar groups of gelatin b y active substances introduced and 2) relaxation processes facilitated in the disordered regions of gelatin
Physical and mechanical properties of gelatin films
1297
structure formed during film formation b y active substances present in the solution, which reduce molecular interaction in the polymer. We made the first assumption studying the effect of some materials on the absorption properties of gelatin films b y a gravimetric method with a relative atmospheric humidity of 65%. Prior to placing the films under the humidity conditions indicated they were kept at a relative humidity of 0% to achieve constant moisture content. The amount of moisture absorbed b y the films was calculated in terms of dry gelatin without considering the weight of low molecular weight substances contained. Curves in Fig. 4 indicate that with small amounts (up to ~ 1 mole/kg) of various low molecular weight substances a general tendency is observed to reduce absorption properties of gelatin, which may be due to the blockage of polar groups. An increase in the amount of materials introduced in gelatin either further reduces the absorption properties of gelatin (e.g. thiourea, resorcin and glyeine), or produces an extremal relation of the absorption properties of gelatin with the presence on the curves of a minimum at certain concentrations of materials (e.g. for glycerine, triethanolamine, potassium thioeyanate). It should be noted that a reduction in the absorption properties of gelatin over the entire concentration range of additives is observed on adding materials, of which the absorption properties under these conditions are almost absent (Table 2). The absorption properties of gelatin are therefore determined in this case only b y the number of polar groups remaining free. At the same time an increase is observed i n the absorption properties of gelatin after achieving a certain minimum with concentrations determined individually, for those materials of which the absorption properties under the relative atmospheric humidity conditions indicated exceed several times the values for gelatin itself (Table 2). Consequently, after a maximum possible blockage of functional groups of gelatin the active substances begin to absorb moisture themselves. A comparison of concentration relationships which govern. the variation of contraction and the absorption properties of gelatin films enables us to regard the reduction of thermal contraction of films containing small amounts of all substances investigated (except for ethylene glycol) as the result of reduced initial moisture content of gelatin. On increasing the concentration of additives a reduction in the thermal contraction of gelatin films in some cases takes place as a result of relaxation processes during film formation (glycerine, triethanolamine, etc.), while in others, as a result of a further reduction in the moisture content of gelatin (resorcine, thiourea, etc.), or as a consequence of a combination of the factors mentioned. Of all the materials examined it was only in one case, glycine, when there was no correlation between the variation of Tsupcon of gelatin and its thermal contraction. Up to a glycine concentration of 1.3 mole/kg both Tsupoon (Fig. 2c) and the contraction of gelatin (Fig. 3c) decrease. Above this critical concentration Tsupcon begins to increase, approximating to initial gelatin while film contraction continues to decrease rapidly. This fact is, apparently, due to the cross-
1298
ZH. F. MOTENEVA et o~.
linking action of glycine, which not only stabilizes T~upcon, but also creates a rigid frame of crosslinks to prevent the compression of gelatin during desorption of water. It could be assumed that with a content in the films in excess of a certain TABLE
3. ~-]FlVECT OF L O W M O L E C U L A R W E I G H T S U B S T A N C E S ON T H E I M P A C T S T R E N G T H OF GELATII~ F I L M S AT I N C R E A S E D T E M P E R A T U R E
c, mole/kg gelatin
Material
-Glycerine Ethylene glycol Triethanolamine Glycine
Impact strength at 70 °, kg.cm/ /cma
Material
~=6~% ~=0%
-5.4 8.0 1"3 1.3
25 30 20 17 13
15 17 20 20 8
I m p a c t strength at 70% kg.cm/ /cma
c, mole/kg gelatin
Thiourea Resoricine KCNS KI
~=65% ~=o%
2-6 2.0 2.0 1.3
7 7 0 4
18 12 0 9
concentration glycine prevents water desorption from gelatin, or films containing glyeine have a much lower initial moisture content. Both should result in lower film contraction during heating. Investigations of the absorption properties of gelatin films with glycine showed that the moisture content of these films with a
4~
× ~
f~
/4" is
-×
2
~
a
g
C
I
O
I
2
I
[
4
d
I
I
2
1
I
C, mole/hff
FIG. 3. Relation b e t w e e n t h e contraction of gelatin films in the temperature range of 2 0 t o 1 2 0 ° a n d t h e concentration of low molecular weight substances introduced. See Fig. 2 for explanations.
Physical and mechanical properties o£ gelatin ~lm~
1299
relative atmospheric humidity of 65 % is indeed lower than the moisture conten~ of films without glycine (Fig. 4, eur~,e 5). However, even with m a x i m u m glycine concentration moisture content in films is 14%, i.e. high enough to cause film contraction during desorption. I n addition, the temperature relations which
2 40
0
-
4O
80
I
l
120 T,°O i
~-5-
~-lO-
0
I
I
I 3
I fs I
8
-18-
6'
0, mo/e/h~7 FiG. 4
FIo. 5
Fxo. 4. Relation between moisture absorption by gelatin films under conditions of relative atmospheric humidity o£ 65% and the concentrations of triethanolamino (1), glycerine (2), ethylene glycol (3), glycine (4), potassium thiooyanate (5), thiourea (6) and resorcin (7). FIe. 5. Temperature relation governing the weight variation of a gelatin film with 2.5 molo/kg glyeino (2) and without glycine (1). govern the weight variation of gelatin films with or without glycine (tests were carried out heating the specimens at a rate of 2 deg/min in a thermal balance produced by the French firm DAM) arc almost identical (Fig. 5) which proves t h a t water is desorbed without hindrance from gelatin films containing glycine. All this is contrary to the assumption made. The only apparent cause remains the crosslinking action of glycine which, similar to conventional tanning, reduces the thermal contraction of gelatin films and stabilizes Tsupcon which, naturally, requires experimental verification. Mechanical properties of gelatin. The effect of low molecular weight substances on the mechanical properties of gelatin films was evaluated from the variation of impact strength (Fig. 6a, Table 3). One material was selected for investigation from each class of compounds. With an increase in the content of most low molecular weight substances studied the impact strength of gelatin films decreases both with normal (at q----65%) and reduced (at q : 0 % ) moisture content (Fig. 6). A similar pattern is also observed when testing gelatin films at increased temperature (Table 3). Gelatin films with ethylene glycol, glycerine [20] and tricthanolamine additives are exceptions which under conditions of normal relative atmo-
1300
ZH. F. MOTEN~VAet a/.
spheric humidity with a certain fairly high concentration increase the elasticity of gelatin films, which is characterized by a two to three-fold increase in impact strength (Fig. 6a). However, a comparison of the concentration relation of the impact strength of gelatin films (Fig. 6a) and the absorption properties of these films (Fig. 4) and the absorption properties of materials added to the films (Table
~150
zook I
/
I
/
I /
A
/2
zo
,.%
0
I
2
3
4
58
0
Gmole/kg
FTG. 6. Relation between the impact strength of gelatin films and the concentration of low molecular weight substances under conditions of relative atmospheric humidity of 65 (a) and 0% (b): 1--triethanolamine; 2--glycerine; 3--ethylene glycol; 4--glycine; 5--potassium thiocyanate; 6--thiourea; 7--resorcin; 8--potassium iodide. 2) indicates that the mechanical properties of gelatin films are in direct relation to the moisture content of the system formed by gelatin and the material. When as a result of the high absorption properties of low molecular weight substances the moisture content of the entire system at some concentrations increases to a value exceeding the moisture content of gelatin without additives, the impact strength of these films increases as well. At the same time a reduction in the moisture content of films (Fig. 4), which is particularly marked in the case of materials incapable of absorbing moisture being added to the films (e.g. thiourea, resorcine, glycine (Table 2)), increases the brittleness of gelatin under conditions of normal relative atmospheric humidity. The brittleness of gelatin films also increases on adding materials which influence the formation of gelatin structure during film formation, independent of the absorption properties themselves. This is clearly seen be the example of potassium thiocyanate (Table l, Fig. 6a) and formamide (Table 4). Results of investigating the structure and mechanical properties of gelatin films with formamide, interacting with gelatin and preventing spiral formation of its macromolecules showed that with an increase in formamide content in the films, the degree of macromolecular ordering of gelatin decreases which markedly reduces impact strength.
Physical and mechanical properties of gelatin films
1301
Consequently, the mechanical properties of gelatin films depend both on moisture content and on the structure of gelatin. Therefore, 'any additives which lower the absorption properties of gelatin or reduce macromolecular ordering, TABLE 4.
EFFECT
O F F O R M A M I D E ON T H E M E C H A N I C A L
PROPERTIES
OF
Amount of formamide in the solution, vol. ~o 0 10 20
GELATIN
FILI~IS
Impact strength, kg-em/ cm
~=65%
~=0%
70 3O 0
40 20 0
i.e. influence the restoration of collagen-like helical structures during film formation, will increase brittleness. The problem concerning the effect of moisture content of gelatin on mechanical properties is of particular scientific and practical interest and should be examined specially. This is particularly important as the role of water in the structure of collagen is now fairly well known [21-23]. I t has been shown that, on reducing the moisture content of this biopolymer, the degree of structural ordering decreases until macromolecular spirals disappear completely when drying the specimens. This structural variation in itself should cause a loss in a certain complex of mechanical properties. The water which does not take part in helical structure formation of gelatin apparently functions as a plasticizer of collagen and gelatin. This is confirmed by data [24] concerning the mechanical properties of gelatin films with different moisture contents and the impact strength of films with glycerine, ethylene glycol and triethanolamine contents (Fig. 6a) which greatly increase the moisture content of the entire system as a result of additional moisture adsorption by these materials. Translated by E. SEMERE REFERENCES
1. K. N. GUSTAVSOIq, The Chemistry and Reactivity of Collagen, New York, 1956 2. A. WlE.I*, Makromolekulyarnaya khimiya zhelatina (Macromolecular Chemistry of Gelatin). Pishchevaya prom., 1971 3. A. N. MEKHAILOV, Kollagen kozhnogo pokrova i osnovy ego pererabotki (Collagen of the Integument and Principles of Treatment). Legkaya promyshlennost', 1971 4. J. Q. UMRERGER, Photogr. Sci. Engng. 11: 385, 1967 5. G. I. BURDYGINA, O. A. ALEKSINA, I. M. FRIDMAN and P. V. KOZLOV, Trudy NIKFI, 58, 1970 6. V. P. MERZLO~r, Dissertation, 1965 7. I. F. KAIMIN', Plast. massy, No. 9, 62, 1966 8. G. I. BURDYGINA, I. M. FRIDMAN and P. V. KOZLO~r, Trudy NIK_FI, 58, 1970 9. A. Ye. CHICHIBABIM, Osnovnye nachala organicheskoi khimii (Fundamentals of Organic Chemistry). vol. 1, Goskhimizdat, 1953 10. D. N. OSOKII~A,Kolloidn. zh. 19: 713, 1957
1302
S.I. KUCHANOV
11. J. R. NIXON, P. P. GEORGAKOPOULUS and J. E. (~ARLESS, J. Pharm. Pharmac. 18: 283, 1966 12. L. Z. ROGOVINA, G. L. SLONIMSKII and L. L. AKSENOVA, Vysokomol. soyed. AI3: 1451, 1971 (Translated in Polymer Sei. U.S.S.R. 13: 7, 1631, 1971) 13. M. ZHOLI, Fizieheskaya khimiya denaturatsii belkov (Physical Chemistry of Denaturing Albumens). Izd. "Mir", 1968 14. P. V. KOZLOV, A. I. UNDZENAS, V. P. IYIERZLOV and S. G. ROZENBERG, I)okl. AN SSSR 185: 118, 1969 15. A. I. UNDZENAS, Dissertation, 1968 16. A. N. MIKHAILOV, Fiziko-khimicheskie osnovytekhnologiikozhi (Physical and Chemical Principles of Leather Technology). Gizlegprom, 1949 17. E. O. KRAEIYIER, J. Phys. Chem. 45: 660, 1941 18. L. MANDEI,KERN, W. F. MEYER and A. F. DIORIO, J. Phys. Chem. 66: 375, 1962 19. G. I. BURDYGINA, I. M. FRIDMA_N, P. V. KOZLOV and V. A. KARGIN, Vysokomol. soyed. A l l : 118, 1969 (Translated in Polymer Sci. U.S.S.R. 11: 1, 132, 1969) 20. Zh. F. MOTENEVA, G. I. BURDYGINA, I. M. FRIDMAN and P. V. KOZLOV, Mezhdunarodnyi kongress po fotograficheskoi nauke (International Congress on Photography). Moscow, 1970 21. N. G. YESIPOVA, N. S. ANDREYEVA and G. V. GATOVSKAYA, Biofizika 3: 529, 1958 22. H. J. C. BERENDSEN, J. Chem. Phys. 36: 3297, 1962 23. L. P. KAYUSHIN (Ed.), Sb. Sostoyanie i rol' vody v biologicheskikh ob'ektakh (The State and Role of Water in Biological Objects). Izd. "l~auka", 1967 24. Yu. K. GODOVSKII, I. I. MAL'TSEVA and G. L. SLONIMSKII, Vysokomol. soyed. A13: 2768, 1971 (Translated in Polymer Sci. U.S.S.R. 13: 12, 1971)
THEORETICAL STUDY OF THE MICRO-HETEROGENEITY OF POLYCONDENSATION COPOLYMERS* S. I. K u c ~ A ~ o v Institute of Electrochemistry, U.S.S.R. Academy of Sciences (Received 5 October 1972) The relation between the type of distribution of units in polycondensation copolymers and the method of relying the initial monomers and their activities was examined theoretically. A comparison of the micro-heterogeneity coefficient calculated with experimental values in the literature showed satisfactory quantitative agreement in most eases. Distribution functions were calculated according to the lengths of various monomer units in oopolymer maeromolecules. THE p r o b l e m of micro-heterogeneity, i.e. the n a t u r e o f the distribution of units in linear macromoleeules of p o l y c o n d e n s a t i o n eopolymers, irrespective of m o i s t u r e content, has n o t been sufficiently studied. Owing t o the d e v e l o p m e n t of t h e high resolution N I ~ R m e t h o d it has r e c e n t l y been possible to examine c o p o l y m e r * Vysokomol. soyed. A16: No. 5, 1125-1132, 1974.