Polymer Spherulites I. Birefringence and Morphology J. M. Marentette and G. R. ~ r o w n '
McGill University, 801 Sherbrooke St. W., Montreal, PO, Canada H3A2K6 Awide variety of substances, ranging from naturally occurring minerals to stereochemically regular synthetic polymers crystallize in the form of spherulites. Differences in chemical composition among these materials lead to significant differences in crystal packing, and, therefore, spherulite birefringence, morphology, and growth rate. The growth of polymer spherulites from the melt, when viewed with a polarized light microscope, is a phenomenon that is truly striking in its beauty and apparent simplicity. Spherulites, such as that shown in Figure 1,are regular birefringent structures with spherical symmetry in three dimensions and circular symmetry in thin section. They are composed of crystallite fibrils that grow radially outward from the nucleus as shown schematically in Figure 2a. Low-angle branching occurs along the fibrils due to the presence of defects in fibril structure, or material, usually residual amorphous polymer or foreign particles, that have not been incorporated into the spherulite. Branching is responsible for the space-filling geometry of spherulites. Within a fibril, polymer chain segments or %terns'' are laid down on the growth surface in an analogous manner to bricklaying (see Fig. 2b). It is important to note that the stems are oriented perpendicular to the spherulite radius. Stem length, that is fibril thickness, is typically -0.1 wm and is a characteristic specific to a given polymer and set of crystallization conditions. In general, macromolecules possess a n end-to-end distance in the crystalline conformation that is greater than the stem length and consequently must fold to accommodate the fibril thickness. Depending on the relative lengths of the stems and the chains, a given chain may fold numerous times before crystallization is complete. Re-entry into the fibril after folding may be ad-
' Author to whom correspondenceshould be addressed. stem Figure2 (a)Diagram of polymer sphe~lite with radially growing crystallite fibrils;(b) enlarged oblique view of tip of spherulite fibril, growth direction G; the crystalline chain segments or "stems"(i.e.,the rectangular blocks) are laid down perpendicular to the fibril direction.
Figure 1. Optical micrograph (taken using a polarized light microscope)of spherulites of poly(ethyleneoxide) (PEO) T,40.0 'C exhibiting fine texture.
jaceut or nonadjacent. The noncrystalline chain folds and loose chain ends constitute part of the amorphous material. A straightforward, detailed examination of spherulite structure is given in the text by Sharples ( I ) . The presentation of fundamental concepts of polymer crystallization in lectures, and in many texts, can be made very effective through the use of photomicrographs. In addition, simple laboratory experiments or demonstrations can be developed easily, and a t a relatively low cost, using polarized light microscopy, a tool that is available in most chemistry and chemical engineering departments. Previously crystallized samples can be used to illustrate spherulite birefringence and morphology, as described in Volume 70 Number 6 June 1993
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this paper. Crystallization kinetics can be studied with the help of a microscope hot stage, - as discussed in a future paper. Crystallization of Polymers from the Melt The apparently simple process of crystallization of polymers from the melt is, in fact, a complex pmcess that results in the ordering of long, randomly coiled polymer chains in the melt into s~ecific.three-dimensional chainfolded arrangements in k e c r y s h i n e state. The ordering Drocess involves a decrease in entronv. the maenitude of which depends largely on the deg&'of orden'ng in the melt: hence. accordine to the definition of Gibbs' free en-
this entropy decrease must be compensated by a negative change in enthalpy in order to cause a favorable change in free energy. As the temperature of a polymer sample is decreased to its melting temperature (T,),the volume available to the macromolecules decreases, restricting their rotational and translational freedom. This restriction of movement, which is favored by the presence of substituent groups that promote intramolecular attraction andor rigid close-packing of chain segments, allows neighboring segments to align in an ordered fashion in close-packed arrays known as crystallites. Variations in the density of the melt created bv chain ordering (homogeneous nucleation) or an impurity~heterogeneous~nuclea~on) can a d as a nucleus from which a spherulite can form. Spherulites and the Polarized Llght Microscope Spherulites are birefringent structures that can range in size from a~proximatelv1 urn to 1mm in diameter, depending onihe nucleatibn density. They are not visible in ordinam light but can be examined in thin section between crossedpol&s using a polarized light microscope. As such, they provide a simple illustration of birefringence. When the topic of polymer crystallization is presented, a knowledge of birefringence is often assumed, but undergraduate students rarely have experience with the concept beyond exposure to equations given in elementary physics texts. A polymer melt is amorphous and therefore optically isotmpic, and no light is viewed through the analyzer when the polan are crossed. On the other hand, polarized light of amplitude A that is incident upon optically anisotropic, doubly refracting regions such as spherulites in the melt is resolved into a fast component and a slow component vibrating in the directio;~ of the mutually principal refractive indices of the crystalline structure, n, and n,(nee Fia. 3a). In soherulites. the orincioal refractive indices are r a z a l and tangential. ~ u ethe iifferencebetween n. and n*. the two comoonents of lieht travel through the sample at different'velocities a n i therefore exit with a path difference or retardation. If nl> nz, then the amplitude of the slow component is given by (A cos 0), and the amplitude of the fast component is given by (A sin 8) (see Fig. 3a); if nl< nz, then the opposite is true. When the refractive index parallel to the radial direction of the spherulite is greater than that perpendicular to the radial direction. the s~heruliteis said to be "positive". When the conveneis true, the spheruliteis "negativen.Most polymer spherulites are negatively birefringent. In the analyzer, which is set perpendicular to the polarizer, the two components are recombined to yield a single component vibrating in the direction of the analyzer, with amolitude (A sin 0 cos 0 + A cos 0 sin 0) (see Fie. 3b). The paih difference causes constructive and destrugive interference of soecific waveleneths of white lieht. The resultant wavele&ths yield theinterference calk of the sam-
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Figure 3. (a) Resolution of plane polarized light of amplitude A into hvo components along the principal refactve indices of a sample, n, and n2;(b) resolution of the light transmitted by the sample by an analyzer that is positioned at right angles to the polarizer in (a).(After Saville, 6. P. In Applied Polymer Micmscopy; Hernsiey, D. A,, Ed.; Elsevier: New York, 1989;p 89.)
d e . which varv in accordance with Newton's series. The ~ a l t e s cross e &tinction pattern characteristic of regularly birefrineent soherulites (that is. the black cross clearlv visible on k e spierulite in k g . 1)is observed due to thLcoincidence of the mutually perpendicular optic axes of the spherulite with the respective orientations of the ~olarizer and the analyzer. The r&ions of the sample that hsve optic axes oarallel to the vibration direction of the nolarizer wiU tran'mit the light from the polarizer, but thisalightwill not be observed through the analyzer when the latter is perpendicular to the polarizer; the regions of the sample with optic axes perpendicular to the vibration direction of the polarizer will not transmit any light, so that in this case also, no light will be transmitted through the analyzer. The regions ofthe sample that have optic &es that a k not per~endicularto either the ~olarizeror the analvzer transmit iight through the analyzer and are seen as thk bright areas of the spherulite in Figure 1. The s i m and magnitude of the birefringence of a spherulite can provide &formation about the relative o&ntation of crvstalline substructures within the s~hemlite.Dmviding tge unit cell of the polymer is known. ~ e n e k y , polarized light microscopes are equipped with accessories that can be inserted between the obiective and the analyzer to obtain fixed or variable reta;dation, thereby permittinedetermination of the retardation ofa wide raneeof samples. The sign of the birefringence of a sample exhibiting first-order birefringence colors can be determined using a first-order red (or sensitive tint) plate or a quarterwave plate. The direction of vibration of the slower compo-
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two principal refractive indices, that is. the birefringence. This relation permits a very accurate determination of birefringence, providing the measurement of the sample thickness is accurate. sample-thickness can be measured with a micrometer screw gauge or the fine focusing micrometer on most microscopes. In the latter method, the microscope is focused on the top surface of the coverslip and then the slide surface. The sample thickness is the difference between these two readings less the wverslio thickness. I t is imoortant to take the avcrage of a ;umber ofmenfiurements ofa eiven sitmple to allow for variations in sample and glass thicknesses. The details of the procedure for the accurate determination of interference colors of different classes of samples, using a quartz wedge plate and t h e Michel-Levy interference color chart, which provides a graphical relation among the three parameters in eq 2, are given in many polarized light microscope instruction manuals and numerous references (see, for example, 24).
I.
1
Fig-re 4 C ose-.o of ieGal ,e, o ~ f . i g e rslm e r * te of PEO a v. :ro..l any com3eisa1ors: 0, ~ t n sensl ve In1 pale, c N i n q.aner-nav? p ale Tre nraton a recl ons ot ine. ow avc fa?. components Variations in Spherullte Birefringence and Morphology of ~ n inr mc compensators are no calea a1 me .pper r q n l
,
nent usually is marked by the manufacturer on these plates. When the slow component of light transmitted by the sample is parallel to that of the sensitive tint plate, tbat is, the sample is positively birefringent, then retardation is added to the sample, which changes the interference color to a shade of blue. If the slow component is perpendicular to tbat of the sample, tbat is, the sample is negatively birefringent, then retardation is subtracted from the sample, which changes the interference wlor to a shade of yellow. In the case of a quarter-wave plate, when the slow components of the plate and the sample are parallel, the sample bright white or yellow; when the - appears -. components are perpendicul&, the sample appears dark gray or black (2).Figure 4 is a series of photomicrographs of a typical spherulite of poly(ethy1ene oxide) (PEO), viewed through crossed polars, without a compensator (Fig. 4a), and with sensitive tint and quarter-wave plates (Figs. 4b and 42,respectively). Comparison of the vibration direction of the slower comwnent of liebt of the comoensators with the colors obseked using t l k e plates indicates that the given spherulite is negatively birefringent; a positively birefringent spherulite would exhibit similar colors, but the oattem would be rotated bv 90'. ~nowiedgeofthe interference coiors and the thickness of a sample permit calculation of the birefringence from the relation (3) r = t(nz-nl)
Although the basic structure of most spherulites resembles the schematic diagram presented in Figure 2, obervation of the growth of spherulites of different polymers a t a variety of crystallization temperatures reveals that spherulite birefringence and morphology vary with chemical identity, unit cell, and crystallization temperature. Certain polymers, such a s linear polyethylene (7)and poly-3-hydroxybutyrate (PHB)(8)exhibit regularly spaced & c e n t & extinction rings or bands in addition to radial extinction. In wneral. concentric rines are attributed to cooperative $sting df radial fibrils aiong the axes of growing crystallite fibrils. These patterns range from rough, herringbone bands to regular rings (Fig. 5 ) depend-
(2)
where r is the retardation or interference color (nm), t, the thickness (nm), and (nz - nl), the difference between the
Flgure 5. Spherulites of PHB 1, 50.0 'C showing relatively regular rings. Volume 70
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different morphologies at a given T,also have different radial growth rates. Sample Experiment A simple two-part experiment illustrates some basic features of spherulite birefringence and morphology. The first part consists of the preparation and analysis of a sample consisting of distincily negatively hirefringent sphemliies of PEO that exhibit a well-defined Maltese cross and reeular structure. This sample can then act as a referencelfor iPP spherulites of varying birefringence and morphology grown in the second part of the experiment.
Ire 6. Spherulites of iPP: (a) Types I (dark) and Ill (bright), T, .O ' C ; (b) mixed type, T, 127.0 ' C .
ing on the polymer and the crystallization conditions. A detailed discussion of the relationship among the above five parameters is beyond the scope of undergraduate and many graduate courses; however, the tremendous variety in the appearance of sphemlites crystallized under different conditions can be illustrated by isotactic polypropylene 1iPP) ~ -.. ,. Isotactic polypropylene (high molecular weight, T, approximately 172 'C, available from Aldrich, for example) is particularly interesting because it forms five different types of sphemlites, classified according to crystal stmcture, birefringence, banding and crystallization temperature, over a 20-degree temperature range (9).The polymer crystallizes in a monoclinic structure in Types I and 11, and a "mixed" type, and in a hexagonal structure in Types I11 and IV. Types I and I1 resemble the sphemlite of PEO shown in Figure 1(also a monoclinic unit cell) and are visually indistinguishable; but Type I (the dark spherulites in Fig. 6a) is positively birefringent and crystallizes below -134 'C; whereas, Type I1 is negatively birefringent and crystallizes above -134 'C. The mixed type (Fig. 6b) is positively and negatively birefringent and crystallizes below -140 'C. Types 111 (T, < 127 'C) (the bright sphemlite in Fig. 6a) and N (T, 127-132 OC) appear much brighter than the monoclinic types, and they are both negatively birefringent. They also have longer induction times, higher radial growth rates, and lower melting temperatures than the monoclinic types. Jagged concentric banding distinguishes Type N from Type 111. It is important to note that, as has been observed in the case of iPP, spherulites of 438
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Part 1. Poly(ethye1ene oxide) Poly(ethylene oxide), a highly crystalline polymer that crystallizes mainly in the monoclinic unit cell, is readily available (from Aldrich, for example). A molecular weight of -1 x lo5 is recommended for this study. To prepare a thin section of reference s~hemlites.a small amount (2-5 me) of PEO is placed on ;standard glass microscope slide and melted at 100 'C (approximately30'above theequilibrium melting temperature of the polymer). Spacer shims ( - 1 6 20 um in thickness,. which can be cut from thin aluminum foii, are positioned on either side of the molten sample before placement of a standard glass coverslip. The sample is melted for a total of 10 min to erase its thermal historv and to reduce the nucleation densitv at T. so that individual sphemlites can be obsemed. A prknelt: inc time of 10 min at 100 'C is adequate in many cases, although the time required is sampl&dependent.~ollow~ ing premelting, the sample is cooled to 40 'C, where it is allowed to attain thermal equilibrium and crystallize completely. - so thermal crystallization can be observed under the polarized light microscope using a hot stage such as a Mettler FP-52 (see ref. 4 for alternatives). Most hot stages are designed to permit flow of inert gas (such as nitrogen) over the sample to minimize degradation at elevated temperatures. There is an induction time of seconds to minutes prior to sphemlite nucleation and growth for PEO at this T,. The induction time ia generally shortened by the presence of im~uritiesin the melt that Dromote he&roeeneous nucleation. The fullv crvstallized PEO s~hemlitesare then nhotographed under the polarized li&t minoscope, with kossed ~olars.Color DhotQeraDhsshould be obtained without anv ;ompensator', and-thin with the sensitive tint and qu&ter-wave plates. Details of spherulite structure, birefringence sign, pattern, and colors should be noted for all of the ~hotoeraohs.If a uuartz wedge is available. the numerical kalueif ;he birefrhgence can be measured. The reader is referred to other sources for the details of this more involved procedure (2-6).
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Part 2. lsotactic Polypropylene A series of samples of iPP sphemlites grown at various temperatures between 120 and 140 'C are prepared as described above, but a premelting temperature of 210 'C is used instead of 100 'C. In addition, the nucleation density of iPP is simificantlv hieher than that of PEO: therefore. observationof indivi&a'isphemlites is more &cult and requires higher magnification than in the case of PEO sphemlites. These samples also should be photographed using color film, with and without compensators. The results should be analyzed as above and compared with those obtained with PEO. This analysis can be used to identify the various sphemlite types. The preceding description of variations in iPP sphemlite birefringence and morphology and Figure 6 are intended to aid in the analysis.
Conclusion
Examination of polymer spherulites provides students with a vivid, nearly molecular-level illustration of the relevant polymer theory as well as the concept of birefringence. Samples of semicrystalline polymers are commereially available and require minimal preparation prior to spherulite growth. Isotadic polypropylene provides an excellent examole of the uossible variations in birefrineence and morphology that arise due to variations in crystallization temperature and unit cell. Comparison of iPP with other polymers, such as PEO, demonstrates that birefringence and morphology also vary with the chemical nature of the polymer. An additional aspect of polymer crystalliza-
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tion and crystallization kinetics is discussed in a future paper. Literature Cited 1. Sharpies, A. Intmdudion to Polymer Cry~tollirofion, Edward Arnold: New York, l%fi
2. Hartshame, N. H.; h a r t , A CrystnLr andth~PderiaingMirrosmp,4th d.; Elmier: N m Ymk, 1970. 3. Delly, J. G. Indust. IlDS 1813.ID.4440. 4. Maaon, C. W Handbook of Ckemiml Micmsew, 4th d.; Wiley: New Yo&, 1983:pp 189-199. 5. Saville,B.P. l"qopllodP0lyrnDlLiphf M i e i e p y ; Hemeley,D.A.,
Ed.;Elsener:New
York. 198% pp 73-109. 6. Rachaw, T 0.;Rochaw, E. 0 . h htmdudion fo M i w m p y by M e w oflight, E k Imm,X-Rays, or U I f f fund: F'lenum: NewYork. 1978. 7. Chiu, G.:Alamo,R. G.; Mandelkem. L J Polym. Sci., P d y m Phys. Edn. 1890,28, ,207-1Pll .. .. -. . . 8. Barham,P.J.:Kelle~A.;Otun,E.L.;Hahes,P.A. J.Makc Sci. 1984,19,2781-2794. 9. Padden, F J ,Jr.;Keith, H.D. J Appl Phya. 1869,30,141%1484.
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