The Microstructure of Photographic Gelatin Binders
`
`o
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`J. E. Jolley
`
`C. Uliricli and G. Nawn
`-
`The Interactions of Polyanions with Gelatins
`jectronic Spectra of Cyanine Dyes at Low Temperature. Part II. Luminescence Spectra
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`I W. Cooper, Sandra P. Lovell, and W. West
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`Effect of Benzotriazole on Color Development Kinetics
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`0 M. R. V. Sayhun
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`Charge Transfer in Electrotax° Development by Reversal Liquid Toning
`E. C. Hatter and E. C. Giaimo
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`A Study of the Relation of Electron Affinity to the Photoconductivity of Doped
`Poly-N-Vinylcarbazole Films: A New Method of Analyzing the Photoresponse
`0
`William J. Wagner and Earl l.. Gasner
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`Apparatus and Method for Measurement of Charge Density and Capacitance in
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`0 Robert B. Comizzoli
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`

`PHOTOGRAPHIC SCIENCE AND ENGINEERING
`Volume 14, Number 3, May-June I970
`
`The Microstructure of Photographic Gelatin Binders
`
`J. E. JOLLEY, E. I. du Pont de Nemours & C0,, Inc., Photo Products Department,
`Wilmington, Delaware
`19898
`
`Review of measurements made on gelatin solutions and dried layers indicates a structural be-
`havior similar to other semicrystalline polymeric materials.
`In aqueous solution,
`the gelatin
`molecules exist as single chains in random configurations, sheathed by water molecules.
`This
`random structure persists under conditions of "hot" drying, e.g., at 50°C, but partial crystalliza-
`tion {i.e., gelation] occurs on "cold" drying, e.g., at 10°C. A new method of estimating the degree
`of crystallinity from measurement of the heat of melting of an anhydrous gelatin layer is pro-
`posed.
`This gives an estimate of 20% for the maximum crystollinity.
`This concept of gelatin
`structure is related to published reports on properties important for a binder of silver halide
`grains in a photographic emulsion.
`
`Gelatin has been used in photographic emulsions
`for about 100 years and plays a variety of important
`roles.
`Its chief functions are as a protective colloid,
`ripening agent, sensitizer, halogen acceptor, and
`binder, and much research has been directed at
`understanding how and why it functions in each of
`these roles."‘ Gelatin’s binder properties have
`probably received least attention, and this paper
`will concentrate on this aspect. The author will
`show that when viewed from the point of view of
`polymer physics, the characteristics of gelatin film
`structure and gelation fit conventional theories.
`It
`is the purpose of this paper to present a unified
`picture of the microstructural features and prop-
`erties of gelatin films that hopefully will provide a
`useful framework for photographic scientists in all
`fields, although some aspects will necessarily be
`Simplified because of the limitations of time. Much
`of the information presented is from the literature,
`but some original data is included.
`The general properties that are desired in a photo-
`graphic binder are that it forms transparent films,
`strong enough to be handled in both the wet and
`dry state, which are insoluble, yet highly permeable,
`to both water and ionic solutions. These are the
`kind of physical properties that typically depend
`upon the microstructure of a material, that is to
`333% the way in which the molecules are arranged in
`the material. Most solid materials such as plastics
`...,.___
`
`l-utorial paper presented at the Annual Conference of the
`I‘i_"’llvE(l
`“fl-3’ of Photographic Scientists and Engineers, Los Angeles, Cali-
`F“'”3- May 14, 1969. Received July 16,1969; revised January 19 and
`fbwary 24. mo.
`2- H- W. Woocl,J. P.’:otogr.Sci., 9: 151i19E-1} (78 references).
`- H. lrie. -J‘. Soc. Sci. Piwtogr. Japan, 25: 59(l9l32} (35 references).
`- C- E. K. Mees and T. H. James. The Theory of the Photographic
`-Process, 3rd ed., Macmillan, New York, 1965, Chapter 3.
`4‘ H" B‘-"ail-ion. J. Wintogr. Sci, 15: 20‘?U9I:'7J {I05 references}.
`
`and metals have microstructures that are greatly
`influenced by thermal treatments, e. g., in quenching
`and annealing operations. Although it is not gen-
`erally thought of as such, gelatin is a thermoplastic
`polymer, and it would be surprising if its micro-
`structure did not also depend upon its thermal
`history.
`
`The Molecular Structure and Origin of Gelatin
`
`The gelatin molecule is a long chain polymer
`molecule made up of amino acids joined together
`by CO—NH linkages called peptide bonds.
`Its
`empirical formula is__cleceptively simple:
`
`i
`i"t‘CHH
`ll
`0 e
`
`In a typical photographic gelatin there may be
`500-1000 amino acid units, or residues, in the chain.
`There are 18 different amino acids in gelatin, and
`although their precise sequence in the chain has not
`been elucidated, the general arrangement is clear.
`Three amino acids make up about 55% of the gela-
`tin molecule. They are Glycine (G), Praline (P),
`and Hydroxy proline (Hi, and each gives a non-
`polar residue. They occur frequently in the ter-
`polymer sequence +G —P—I-1+... Glycine appears
`to occur at every third site along the whole chain
`with amino acids filling in. About 20 % of the amino
`acids are ionizable, some being acidic and some
`basic, giving rise to an amphoteric character to the
`169
`
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`
`

`

`JOLLEY
`
`P S & E, Vol. 14, 1970
`
`(the neutral, J{—G—P—H+,,, sequences) and seg.
`ments that do not crystallize (the polar sequences}.
`In the manufacture of most photographic gelatin
`bone Collagen fosseinl
`is broken down by alkaline;
`hydrolysis.
`If done carefully,
`this yields pre_
`dominantly single-stranded gelatin molecules, but
`too vigorous hydrolysis leads to products in which
`the gelatin backbone is broken in preference to the
`interchain crosslinks,
`leading to highly branched
`structures. For simplicity I will refer to single
`chains throughout
`the discussion, but
`the same
`principles apply to the more highly branched gela.
`tins. For more detailed information on the molec-
`ular structures of gelatin and collagen, and the man.
`ufacture of gelatin, those interested are referred to
`three recent books.7—‘3
`
`The Microstrucfurol Features of Gelatin Films
`
`In aqueous solution gelatin molecules exist as
`single chains, completely sheathed by water mole-
`cules. The segments of the chain are in a continual
`state of rapid motion, rotating about the backbone
`bonds to various degrees depending on the chemical
`nature of the bonds, the pH, and the temperature.
`This conformation is known as the random coii
`configuration. On drying down a thin layer of a
`solution of gelatin, 3. clear film is obtained, and it is
`well known that the film has different microstruc-
`ture and properties depending upon the tempera-
`ture of drying. 19-“
`In the subsequent discussion I
`will consider two extreme examples, “hot dried"
`film, dried at 50°C, and “cold dried” film, dried at
`10°C.
`On x-ray examination, the hot dried film gives
`only a broad diffuse diffraction pattern typical of
`liquids or glassy materials, indicating no ordering of
`the gelatin molecules. The gelatin molecules are
`randomly arranged, essentially like their configura-
`tion in solution except that the motion is frozen out.
`Hot dried films thus have a relatively simple ran-
`dom coil, or amorphous, structure.
`In contrast, cold dried gelatin films give an x-ray
`pattern with a series of diffracted arcs and spots
`indicating the presence of a definite crystalline
`species with a high degree of order. The spacings
`are identical to those found in collagen but. have
`different orientations, indicating the same crystal
`structure, but a different alignment of the crystal-
`lites.
`If the film is stretched during the cold drying
`step, the diffraction pattern obtained is essentiall_}'
`identical to that of a natural collagen fiber. Thi§ 13
`illustrated in Fig. 3, where the x-ray diifractlfm
`pattern of a gelatin film stretched to 3 times. 1135
`length during drying is compared to that of ‘:1 P1999
`of natural collagen fiber
`(kangaroo tail
`tend011s
`
`7. A. Vein, The Macronioiecufnr Chemistry of Gefarin, Academic P1999’
`New York, 1964.
`.
`8. G. N. Ramachandran, E1-1., Treatise on £‘ol!t‘agen. Vol. 1. Academic
`Press. New York, 1967.
`9. R. J. Croome and F. G. Clegg, P.’ao:ograp!i ic Geiotfn, The Fot‘-9' P'“’3s'
`London. 1365.
`‘
`10. E. M. Bradbury and C. Martin, Proc. Roy. Soc-., A214: 18311952!-
`11. D. W. Jopling, J. Appi. Chain. (London), 6: 'a'9(l956).
`
`Figure l. Gelulin moiecule.
`G — Glycine
`P
`Proiine
`H
`Hydroxyproline
`R
`Other amino acid residues
`
`over-all molecule. Figure 1 illustrates a portion of
`a gelatin molecule schematically, showing a non-
`polar (G—-P——I-1),. sequence and polar (G—R—-R},.
`sequences.
`The gelatin molecule is the basic building block of
`collagen, the most abundant protein in the animal
`kingdom, where it occurs in the skin, bone, and
`connective tissue of mammals. Collagen is an
`insoluble fibrous protein, with a high degree of
`order as indicated by its x-ray diffraction pattern.
`From the x-ray diff'raction pattern, it has been de-
`duced“-° that the highly ordered crystalline regions
`of collagen contain basic crystalline units that con-
`sist of three gelatin molecules wound around each
`other in a ropelike fashion to form what is known as
`a triple-stranded helix, illustrated in Fig. 2. Not
`all of the gelatin molecule can participate in such a
`highly organized crystalline structure, however, but
`the nonpolar +G—-P-—H—)~... sequences can, and fit
`perfectly to give a structure that accounts exactly
`for the observed x-ray pattern. The three chains
`are held together in the helical conformation by
`lateral interchain hydrogen bonds, although agree-
`ment has not been reached over whether there is
`one H bond per +G-~—P—H-)- unit,-5 or twofi
`In
`the one bonded structure, the H bond is between
`the amide hydrogen (NH) of the glycine residue on
`one chain, and the carbonyl oxygen { C0) of the pro-
`line residue on the neighboring chain. The highly
`ordered regions of collagen fibers contain bundles of
`such triple-stranded helical units, which in polymer
`terminology are called crystallites.
`In the non-
`crystalline regions of collagen, the unordered single-
`stranded gelatin chains are joined at various points
`via covalent crosslinks formed between their polar
`groups, giving collagen its toughness and insolubil-
`ity.
`The gelatin molecule then contains segments that
`can crystallize
`into triple-stranded crystallites
`
`5. A. Rich and F. H. C. Crick,J. Moi. Ha'o£.,3: 483N961).
`G. G. N. Rnmaclitlndrsln and G. Kartlia. Nature. 175! -'393(1955}-
`
`Mylan v. Qualicaps, IPR2017-00203
`QUALICAPS EX. 2012 - 4/11
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`QUALICAPS EX. 2012 — 5/11
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`

`172
`
`JOLLEY
`
`P 5 a. E, Vol. 14, 1970
`
`points and heats of fusion in a differential scanning
`calorimeter. 15 All
`the samples
`showed similar
`behavior~a melting point at about 230°C, which
`occurred with heats of melting in the range of 3 to 4
`cal fig. Subsequent x-ray examination showed that
`the samples are no longer crystalline, confirming
`that the melting corresponds to the melting of the
`triple-stranded crystallites to the random coil
`arrangement of the single gelatin molecules. The
`correspondence of the melting points of the gelatin
`film and the collagen reinforces the conclusion
`drawn from the x-ray dilfraction data that the crys-
`talline species is the same in both. Further, and
`more importantly, the similarity of their heats of
`fusion indicates that the total amount of crystal-
`linity is about the same in the cold dried gelatin
`film as in the collagen samples.
`If the heat of fusion of completely crystalline
`gelatin were known, the absolute amount of crys-
`tallinity could be deduced from the experimentally
`measured heat of fusion. The heat of fusion of
`crystalline polymers can be obtained from the de-
`pression of their melting point by a diluent. 15-"
`We have measured the depression of the melting
`point of cold dried gelatin by water and calculate
`from our preliminary data” that the heat of fusion
`of completely crystalline gelatin is approximately
`17 cal Hg. As a first approximation it can therefore
`be concluded that in the cold dried gelatin film and
`collagen samples studied, approximately 2.0% of
`their molecules are in the triple-stranded helical
`structure,
`the remainder being single-stranded
`gelatin molecules. The principal differences be-
`tween the crystallites of collagen and those of a cold
`dried gelatin are in their morphology, i.e., in their
`size, number, and perfection.
`In natural collagen
`the crystallites are larger, more perfect, and there-
`fore fewer in number than in a crystalline gelatin
`film. Experimental support for this is found in
`electron microscopy of collagen, in the sharpness of
`the melting transition” and in the sharpness of the
`diffracted x-ray beams in collagen. 15
`On a molar basis, the heat of fusion of completely
`crystalline +G—P—-H+ sequences works out to 4.5
`kcal,-ltermonomer unit. The theoretical under-
`standing of the melting process in polymers has not
`advanced sufficiently to allow us to relate this heat
`of fusion precisely to the number of hydrogen bonds
`per unit, but it is nevertheless of a magnitude that
`is consistent with the breaking of one or two hydro-
`gen bonds per +G—P—H+ unit. A comparison
`with the melting points and heats of fusion of a
`number of typical hydrogen-bonded linear con-
`densation polymers (Table I} shows gelatin to be
`quite comparable to these synthetic materials.
`Since we have concluded that even the crystalline
`gelatin films contain only about 20% crystallinity,
`
`15. Details will be given i.n a forthcoming publication.
`16. P. J. Flory, Principles of Polymer Chemistry, Cornell University
`Press, Ithaca, New York, 1953, p. 563 et seq.
`‘I7. L. Mnnclelkern, Crystallization of Polymers, Mr.-G raw-H ill. New York,
`1964, Chapter 3.
`. A. Vols, The Mncmmofecuiar Chcmistfiv of Gelatin, AI:a(lemic Press,
`New York, 1964. p. 385.
`
`the major component of both hot and cold dried
`gelatin films and natural collagen is therefore
`amorphous gelatin,
`i.e., individual single-stranded
`gelatin molecules. We saw that in hot dried gelatin
`films they are randomly arranged in the film, and
`this is further confirmed by the absence of any
`shrinkage in such films, even when heated as high
`as 260°C where they are soft and beginning to
`decompose. A collagen fiber or a square of cold
`dried crystalline film, on the other hand, changes di.
`mensions suddenly at the melting point,
`i.e., at
`230°C when dry, or at lower temperatures when
`plasticized with water. The change in dimensions
`is such that the fiber shrinks to Li of its original
`length, and the square shrinks rapidly to perhaps
`1/; of its original area, at the same time expanding
`to 4 times its original thickness. A large part of this
`shrinkage is due to the melting of the crystallites
`from elongated helices to random coils, but there
`would also be a contribution from the relaxation of
`any elongated or stretched single gelatin molecules
`(if they were present) upon melting of the crystalline
`tie points. Unfortunately, no one has calculated
`the contribution of the melting to shrinkage, so it is
`not possible to decide at the moment whether it is
`necessary to assume any orientation of the amor-
`phous molecules.
`the structural features of hot
`To summarize,
`dried amorphous films and cold dried crystalline
`gelatin films just described are shown schematically
`in Figure 4. The hot dried films are structureless
`and consist of randomly arranged single gelatin
`chains. The cold dried films consist essentially of
`single-chain gelatin molecules tied together by
`triple-stranded crystallites
`that
`are randomly
`aligned in the plane of the film. There may be
`some elongation and planar orientation of the amor-
`phous molecules.
`One remaining major structural feature we have
`not yet considered is covalent crosslinking or hard-
`ening.3 The crystallites we have discussed so far
`are often referred to as crosslinks. They do act as
`crosslinks, but it must be remembered that they are
`reversible crosslinks and can be easily removed DI.‘
`changed by appropriate manipulation of physit_381
`conditions, e.g., heat or water. The crystalhte
`crosslinks of a gelatin film are therefore mobile and
`the film microstructure cannot be considered 1_Je1'-
`manent until it is locked in by chemical means via a
`crosslinking
`reaction. Crosslinking agents,
`01’
`hardeners, are compounds that are capable Of
`forming strong primary chemical bonds to two 01'
`more gelatin molecules. Formaldehyde is pr_0b'
`ably the best known hardener for gelatin and 15 3
`very effective crosslinker even at very low C011‘
`centrations.
`It is generally considered to ‘react
`with the lysine component of gelatin {_a primary
`amine), i.e., in the polar amorphous regions of the
`polymer, and only a relatively small number 0
`crosslinks effectively convert the gelatin into 0119
`huge molecule. The resultant restrictions to the
`relative movement of the individual gelatin chains
`prevents further changes in microstructure.
`
`Mylan v. Qualicaps, IPR2017-00203
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`

`

`P53. E,vo:.14,I970
`
`MICROSTRUCTURE OF PHOTOGRAPHiC GELATIN BINDERS
`
`TABLE I. Thermodynamic Properties of Selected
`Crystalline Polymers
`
`Polymer
`
`1u-10 Nylon"
`5 Nylon“
`Polyethylene
`terephthalate"
`Gelatin
`
`M el ting
`Point (°C)
`
`.11-I
`(cal,-"gl
`
`216
`225
`
`270
`230
`
`25
`45
`
`28
`17
`
`AH
`(kcal
`mole}
`
`8.3
`5. 1
`
`5 . 5
`4 . 5
`
`.-TL_ Mmdgjkern, (:’:-_w,-rrxiiizcxrion of Polymers. McGraw-Hill, New York,
`1964.l3lJ-119"120-
`
`We have done a simple experiment to illustrate
`that formaldehyde crosslinking takes place prefer-
`entially in the amorphous regions of cold dried
`gelatin. We exposed a cold dried (i.e., crystalline)
`gelatin film to formaldehyde vapors so that it was
`rendered insoluble to boiling water and its swelling
`capacity considerably reduced. We then measured
`its crystalline melting point and heat of fusion and
`found them to be essentially unchanged from the
`unhardened control. Thus, while the amorphous
`regions of the gelatin were crosslinked, the crystal-
`lites were not attacked by formaldehyde. This is
`also consistent with the view that the crystalline
`regions are formed largely by the nonpolar +G—P
`_—H+ ter-polymer, while the reactive polar groups
`in the amorphous segments of the molecule form
`interchain crosslinks.
`
`Gelution and the Formation of Crystollinity
`
`The formation of an amorphous film structure on
`drying a thin layer of gelatin solution at 50°C is
`relatively easy to understand. As water evaporates
`away, the layer remains liquid and the random
`nature of the gelatin molecules in solution persists
`until eventually the viscosity becomes so high that
`no ordering is possible and the random configuration
`becomes frozen into the solid film.
`Gelatin solutions that are cold dried undergo two
`successive steps. Before the actual drying begins,
`the solution undergoes the phenomenon of gelation.
`I will first review this phenomenon, which occurs
`when a gelatin solution is cooled down and typically
`begins at about 30°C. On gelation a three-di-
`mensional network is formed throughout the gelatin
`solution, and it has been recognized for some years
`that the tie points that hold this gel network to-
`gether are crystalline in nature.”-'3-'“" That is,
`the tie points are tiny crystallites in which the gela-
`tln molecules are probably in their triple-stranded
`Configuration. They are formed randomly through-
`out the solution by a crystallization process wher-
`9‘{9I' three individual gelatin chains come together
`Wlth the appropriate neutral amino acid sequence,
`e1}9T'€)',.
`and geometry. The network is a three-
`-_..._
`dlrnensional one because a single gelatin molecule
`
`19- C. Robinson. in Nature and Structure of Cotrlageri, ed. by J. T. Randall,
`20 Butterworths. London U953), 12. 96.
`- H- Boedtkemnd P. Doty, J. Phys. cr.._=m., 5s: sssoas-1).
`
`Fig. 4.
`
`[cl Hot-dried amorphous film and {bl cold-dried crystalline film.
`
`may participate in more than one crystallite tie
`point. The process is not really very different from
`that which occurs when a solution of a crystalline
`organic or inorganic salt is cooled, except for the
`efiect of the long polymer chain. Thus, when a salt
`solution is cooled,
`it reaches a saturation point
`below which minute crystals are formed randomly
`throughout the solution. Unlike a polymer solu-
`tion, however, these are not connected to each other
`and are free to settle to the bottom of the solution.
`In view of our estimate of “-20% for the amount
`of crystallinity in the dried film, this represents an
`upper limit to the crystallinity of the gel. The
`structure of a gelatin gel is summarized schemati-
`cally in Fig. 5 where a multitude of small randomly
`
`Fig. 5. Gel structure.
`
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`

`JOLLEY
`
`P S 8. E, Vol. 14, 1970
`
`NUCLEfl\T|ON
`
`MOLECULAR
`MOTION
`I
`
`I
`/
`
`/
`K’fi'*CRYSTALL|ZATlON
`
`!
`GLASS
`TRANSITION
`
`+
`MELTING
`POINT
`
`TEMPERFATURE —I--
`
`Fig. 6. Crysialliz ufi on kinetics.
`
`arranged crystallites are shown connecting the bulk
`of the randomly coiled single gelatin molecules.
`This structure has been shown to account well for
`the major features of a gel, i.e., its discrete melting
`point, optical activity,
`rigidity, and rubberlike
`elasticity.“ *9‘
`Further support for this structure and insight into
`its details are obtained by considering the corre-
`spondence between gelation and the crystallization
`of polymers. The theory of crystallization of poly-
`mers has been developed and refined during the
`past 10 years and is well summarized in Mandel-
`kern’s book. ” The essential features are as follows.
`
`Experimentally it is found that on cooling a poly-
`mer melt, crystallization begins at the melting point
`but is very slow at that temperature.
`If the tem-
`perature is lowered below the melting point, the
`rate of crystallization at first increases, achieves a
`maximum, then decreases to zero at the polymer’s
`glass transition temperature. The explanation of
`this behavior is that two processes are necessary for
`crystallization to take place—nucleation and growth
`-—and these have opposite dependences on temper-
`ature. Thus, nucleation,
`the formation of the
`initial crystalline sites, increases as the temperature
`is lowered, since there is then a higher probability of
`finding the chain segments in their lower energy
`crystalline configurations. Crystalline growth, on
`the other hand, requires motion of chain segments
`at the growing nuclei, and is favored by higher
`temperatures. The net result is a temperature at
`which there is a maximum rate of crystallization.
`These relationships are shown schematically in
`Figure 6.
`
`2].. R. J. Croome and F. G. C-legg. Plaotograpfiaic Gofarin, The Focal
`Press, London, 1965, p. 41.
`22. G. Stainsby, Ed., Recent Achxmces In Gelatin and Glue Research,
`Pergnmmon Press, New York, 1958.
`23. A. G. Ward. J. Pfiotogr. Sch, 9: 550.961).
`24- A- Vcis. The Macmmolecufar Chennai:-y of Geir.-tin, Academic Press,
`New York, 1964, Chapter 5.
`
`It follows further that the morphology of the
`crystallinity, depends upon the temperature of
`crystallization. Thus, crystallization at tempera.
`tures only a little below the melting point occurs
`with a preponderance of growth over nucleation, so
`that materials crystallized under these conditions
`contain fewer, but larger and more perfect cg-3,3-
`tallites than materials crystallized at lower tam-
`peratures. The lower
`the temperature of cryg-
`tallization,
`the smaller and less perfect are the
`crystallites.
`An example of how well gelation conforms to
`these concepts is illustrated by the data of Flory
`and Weaver” on the optical rotation developed in
`gelatin gels when gelled at different temperatures.
`They found that the development of helical struc.
`ture, as measured by optical rotation,
`increased
`both in rate of formation and total amount as the
`temperature was lowered below the melting point.
`Their data is shown in Fig. 7, and it can be seen
`that it conforms well to the kinetics of crystalliza-
`tion just described.
`The dependence of the properties of gelatin gels
`on their gelation temperature is also consistent with
`crystallization theory. For example,
`it
`is well
`known that the melting point of gels formed close to
`the gelation point are higher than those formed at
`lower temperatures2'3—-a result we can attribute to
`the greater stability of the more perfect crystal-
`lites. Also, the greater rigidity of gels formed at
`lower temperatures is due to their greater number
`of tie points throughout the gel.
`An elegant piece of work published recently by
`Beier and Engel“ further illustrates the correspon-
`dence of gelation and crystallization. They cooled
`dilute solutions of an acid-soluble calfskin gelatin to
`various temperatures and studied the nature of the
`gels formed by a variety of physical
`techniques.
`They found that at temperatures just a few degrees
`below the melting point, structure formation was
`slow, but the material formed was rod—like collagen
`units practically identical
`to the original native
`calfskin collagen, i.e., large perfect crystals in our
`terminology. At lower temperatures, they found
`less of this material and a progressively greater
`amount of network structure in which short, triple‘
`stranded, helically wound segments were randonlll’
`connected by nonhelical chain sections.
`.
`In summary, it seems highly likely that. gelation
`occurs in gelatin solutions via the formation of tri-
`ple-stranded crystallites, whose size, number, and
`perfection depend upon the thermal history of the
`gel. The cooler
`the gelation temperature,
`the
`more crystalline it is, but the smaller and less Per’
`feet are its crystallites.
`'
`_
`When the gelled film is dried, there is a C0l'lSld9r'
`able reduction in volume due to water loss, amount“
`ing to approximately a 13-fold reduction f0!‘ 3
`gelatin solution of 10% by weight. When the lay?’
`is coated on a base all of this volume reduction 13
`forced to take place in the thickness direct1011-
`
`25. J. E. Eldridge and J. D. Fen-y.J. Phys. C.F:em., 58: 992 {I954}-
`2fi. G. Beier and J. Engel, Biochemistry, 5: 27-’l4(l5J65l.
`
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`

`958-
`
`5, Vol. 14, 1970
`
`MICROSTRUCTURE OF PHOTOGRAPHIC GELATIN BINDERS
`
`the isotropic gel network structure initially
`nsiderably distorted and deformed by
`-
`I
`t'he.f(;rLe'{j[‘he crystallites are squashed into the
`and: (if the film and so arrive in the planar orienta-
`. gfouna in the final dried film. The orientation
`tlgthe C;-ystallites will obviously be greatest in films
`gamed from dilute solutions. Stretching of the
`and,”-n1y oriented single molecules,
`if it