04-2

240 K. Kaji

agrees with the prediction of Eq. (7) but, surprisingly, it remains almostconstant at higher temperatures. This strange phenomenon might be re-lated to a memory effect of the glass. On the basis of these observations,the great difference between melt and glass crystallization in number den-sity of spherulites, shown by van Krevelen [5], might be explained by theassumption that crystal nuclei are formed within A\ for the former case,A may become infinite as the temperature approaches T8. Furthermore, Ais of the order of tens of nanometers in the case of crystallization fromthe glass below 1150C. Hence, there might exist a critical crystallizationtemperature between 115 and 12O0C. Again, the cause for this is unknown,but it might be related to the range of molecular interaction of Eq. (7).

4. Cause of the spinodal decomposition process

The SD is a phase separation process, usually occurring in systems con-sisting of more than two components, such as solutions and blends. In thepresent case, however, the pure PET system employed is composed of onecomponent. Then, what triggers such an SD-type phase separation? Doi etal [14,26] proposed a dynamic theory of the isotropic-nematic phase tran-sition for liquid crystalline polymers and showed that the orientation ofrod-like molecules causes an SD-type phase separation. Our experimentalfinding of SD may be due to such orientational fluctuations. In 1956, Flory[48] proposed a concept of two-step crystallization — parallel orientationof the rigid segments and their closest packing. We believe our finding isdirect evidence of his concept, though Flory did not predict the SD.

Here, we first elucidate the Doi theory in some detail and then show ourexperimental results to clarify the cause of SD, based on the assumptionthat the partial extension of polymer chains triggers the parallel orientationof the stiff segments during the induction period of crystallization.

4.1. Doi's theory

Figure 12 is a schematic diagram, explaining Doi's theory [14,26]. It is wellknown that polymer molecules assume a random coil (Gaussian) confor-mation and are entangled with one another. When the sample is quencheddown to a temperature at which the polymer can crystallize, the polymerchains tend to assume a crystalline conformation which is energetically themost stable. However, this is a difficult process because of two resistantfactors, z.e., chain entanglements and the increase of excluded volumes ofcrystalline sequences. The entanglements greatly delay the process and theextension of the crystalline sequences makes the system unstable becauseof the rapid increase in their excluded volumes. The latter relation is givenby Doi et al [14]

VexCi = 2dL2\ sin Θ \ (S)

Handbook of Thermoplastic Polymers: Homopolymers, Copolymers, Blends, and CompositesEdited by Stoyko Fakirov

Copyright © 2002 WILEY-VCH Verlag GmbH, WeinheimISBN: 3-527-30113-5

Structure Formation in PET During the Induction Period of Crystallization 241

1.

2.

4.

5.

Structural change in the induction period ofpolymer crystallization

Change in chain conformation

PE: gauche —>· trans

OIncrease in the length of stiff segments

(increase in the persistence length of polymer chain)

3. Increase in the excluded volume of

stiff segments

—>> the system becomes unstable.

Excluded volume:

where L and b are length and diam-

eter of the stiff segments; b ~ d.

AS Θ -> O, Vexcl -> O.

Parallel orientation of stiff segments

—>· stabilization of the system.Critical concentration:

i/* = 4.19/dL2

Microphase separation due to ori-

entational fluctuations (spinodal

decomposition type): kinetics of

isotropic-nematic phase transition

of liquid crystals (Doi's theory)

DD

26L2| sin θ\

OD

-DD

OD

Figure 12. Schematic diagram explaining the kinetic theory by Doi et al on the

spinodal decomposition due to orient at ional fluctuations. DD: disordered domain;OD: ordered domain

242 K. Kaji

where L and d are the length and diameter of rod-like or crystalline seg-ments, respectively, and Θ is the angle between neighboring rods.

When the average length of crystalline sequences exceeds a criticalvalue, they start to orient parallel to one another, to reduce the excludedvolume or the free energy of the system because, as easily seen from Eq. (8);the completely parallel orientaion makes the excluded volumes zero whilethe completely perpendicular orientation results in their maximum value.This critical density of stiff segments in the bulk system, above which thesegmental orientation is induced, can also be calculated [14]

ι/* = 4.19/dL2. (9)

According to Doi's theory, such parallel orientation does not occur homo-geneously in the system, but it involves an SD-type microphase separationinto the oriented and unoriented domains. When this theory is applied toflexible polymers, such as PET, one has to assume the hypothetical freelyjointed chain model [49] where the polymer chain consists of connectedstiff segments, with a fixed length, L, being equal to the persistence length,frp, of the real chain. This theoretical prediction actually agrees with ourobservations, as shown in the next subsection.

4.2. Change in chain conformation during the induction period

As described above, Doi's theory leads to a prediction that SD is triggeredby the extension of unoriented crystalline sequences in the induction pe-riod. In order to confirm this prediction, the transition from the amorphousto the crystalline chain conformation should be followed as a function ofannealing time. We have investigated this transition, using SANS and FT-IR techniques [22,42]. The persistence length of PET during the inductionperiod was measured by a time-resolved SANS technique with deuteriumlabeling. In the FT-IR measurements of PET, we used the band of transconformation of the ethylene glycol segments at 973Cm"1 [50] and an in-ternal standard band at 794Cm"1 [51]. Figure 13 shows the annealing timedependences of the persistence length and the relative amount of trans con-formation of PET when the quenched glassy sample was annealed at 8O0C.In these experiments, however, the induction period was about 100 min,which is slightly shorter than that for the SAXS experiments (120min). Itis seen in the figure that the increase in the amount of trans conformationcorresponds well to that in persistence length. The persistence length ofthe PET chain in the quenched glass is 1.22nm, but within an annealingtime of 1-2 min it increases to 1.32nm and then levels off, keeping almostconstant until about 50 min. After a slow rise, at about 100 min it rapidlycontinues to increase.

Now, let us examine the criterion for the stiff segment orientation usingEq. (9). The density v of the stiff segments in the melt-quenched glassis calculated to be 3.9segments/nm3 from the macroscopic density of the


1.5

s

t1'4

1CD

I 1.3

£

1.2l

- o bp ·* ;• FT-IR * -

: , , J:J τ ^λ·ί :

P , , , , , , , , ι , , , , , , , , ι ι . , , "

0.50

- 0.45

-0.40

- 0.35

l 10

Annealing time (min)100

Figure 13. Annealing time dependences of the persistence length and the relativeamount of trans conformation for PET, isothermally crystallized at 8O0C fromthe glass [42]

sample, and the molecular weight of the stiff segment is almost equal tothe monomer mass of PET. The critical density if* calculated from Eq. (9)is 4.5segments/nm3, which is larger than z/, and hence the random coil isstable in the glassy or molten state. The initial increase of the persistencelength to 1.32nm after 1-2 min decreases v* to 3.6segments/nm3, whichis smaller than ι/, so that it induces SD due to the segment orientationfluctuations, to reduce the increase in excluded volume.

A similar discussion has also been reported for the cases of iPS andsPS [23,24]. It has clearly been confirmed by FT-IR spectroscopy that theiPS polymer chain starts to assume the crystalline conformation (3/1 he-lix) during the induction period. This supports the concept that the stiffsegments correspond to those with crystalline conformation.

4.3. Orientational fluctuations during the induction period

According to Doi's theory [14,26], the SD-type microphase separation iscaused by parallel orientation of rigid segments. If so, a possibility exists ofexperimental detection of the orientational fluctuations during the induc-tion period by a DPLS technique. Using Stem's method [52,53], we havesucceeded in observing such orientational fluctuations during the glass crys-tallization of PET [25], as well as for that of iPS [23] and sPS [24]. Figure 14shows the time evolution of DPLS curves and their integrated intensitiesin the case of PET. During the induction period of ca. 100 min, the DPLS

244 K. Kaji

100

.·§

ίο

0.1

) Annealing time O 133 min

+100 minooooc-oooooo^o co« oc^c-oo ΟΟΟΟΦΦΟΦΟΛ

V 50 min

O 25 min

• 3 min

o o°o

·Μ> ·ϊ A

» 9

Ι ι ι ι t I ! t ι i l

41 »

Ί i ι ι* Id I I 1

4.5

•S'S

s? o.i -

0.01100 200 300 400

Annealing time (min)500

Figure 14. Time dependence of the depolarized light scattering curves (a) andtheir integrated intensities (b) for PET, crystallized at 8O0C from the glass [25]

curves are independent of the scattering vector Q, i.e., the sizes of the ori-ented domains are sufficiently small, as compared to the wavelength of thelight used. After crystallization, spherulites or crystalline entities may haveformed because the scattering curves considerably depended on Q, thoughthey are not reproduced here. The integrated intensities in the very earlystage increase exponentially with time, in agreement with the kinetics ofSD due to orientational fluctuations, according to Doi's theory.

Kimura et al found for PEN [54] and iPS [55] that a magnetic orienta-


tion of the polymer melt occurs in the magnetic field during the inductionperiod of crystallization near the melting point. This may be evidence offormation of oriented clusters formed prior to crystallization.

5. Secondary microphase separation: SAXS before WAXS

As mentioned in the previous section, an SD-type microphase separationdue to orientational fluctuations occurs prior to crystallization. The densedomains caused by this microphase separation are assumed to have a ne-matic structure. The next problem is whether the crystal nuclei are formeddirectly from the nematic structure. In this connection, we recall a re-cent experimental finding that an SAXS peak, the so-called long periodpeak, emerges before the crystalline WAXS peaks. This was first discov-ered for the crystallization from oriented melts, i.e., the crystallization oflow density polyethylene (PE) during the melt spinning process and fromthe stretched state of a crosslinked PE melt, by Katayama et al. [56,57].

Subsequently, it was reconfirmed in the crystallization from the orientedglassy state by Strobl et al. [58,59]. Cakmak et al. [60] and Terrill et al. [61]confirmed it from the real time SAXS and WAXS studies on melt spinningand extrusion, respectively, using synchrotron radiation. Furthermore, Ter-rill et al. [61] showed that such a phenomenon occurs even in the case ofcrystallization from quiescent melts. More recently, Wang et al. [62] alsofound such a phenomenon using fractionated iPP. These findings suggestthat the secondary microphase separation (probably SD-like) actually oc-curs by inducing the phase transition from nematic to smectic in the denseregions. In conclusion, it can be assumed that the crystal nucleation beginsin the smectic domains after the secondary microphase separation.

6. Model of the crystallization mechanism in polymers

Recently, Olmsted et al. [18] proposed a generic temperature-density dia-gram for a polymer melt, indicating that when a polymer melt is quenchedto the crystallization temperature, binodal or spinodal decomposition oc-curs depending on the degree of supercooling. The key concept of thistheory is to take into account the free energy of the change in chain con-formation, coil to helix (crystalline); the helical stiff segments induce orien-tation fluctuations when they exceed a critical length, resulting in binodalor spinodal decomposition. The coil-to-helix transformation correspondsto the increase in the length of rod-like molecules in Doi's theory. Further-more, the phase diagram by Olmsted indicates that there is a gap betweenthe melting temperature Tm and the binodal temperature T&, and, in thistemperature range of Tm — T&, the usual homogeneous crystal nucleationis expected [63].

246 K. Kaji

TS<TX< Tb

Small spherulites

Figure 15. Structural formation model for the initial stage of polymer crystalliza-tion. BD - nucleation and growth of oriented domains, SD - spinodal decompo-sition into oriented and unoriented domains; T&, T5, and Tx - binodal , spinodaland crystallization temperatures, respectively; I — isotropic, N — nematic, S —smectic, and C — crystalline

Based on the above concept, we present a schematic diagram of poly-mer crystallization (Figure 15). At higher temperatures between T& (bin-odal temperature) and T5, (spinodal temperature), nucleation and growthof oriented (nematic) domains of rigid segments occur, while at lower tem-peratures below T5, a spinodal decomposition type microphase separationinto oriented and unoriented domains occurs. After such a microphase sep-aration, crystallization occurs in the oriented (nematic) domains. Beforecrystal nucleation, however, microphase separation occurs again in the ne-matic domain, involving a nematic-to-smectic phase transition. This sec-ondary microphase separation corresponds to the so-called SAXS beforeWAXS phenomenon.

This mechanism leads to the important conclusion that the higher ordertexture of crystalline polymers greatly changes depending on whether thecrystallization temperature is above or below the spinodal temperature.We have recently succeeded in taking light micrographs, illustrating thisdrastic change.


7. Conclusions

In this chapter, we reviewed the structure formation processes during theinduction period prior to crystallization for the cases of glass and meltcrystallization of PET. It was found that the spinodal decomposition typemicrophase separation actually occurs in the induction period, owing to ori-ent ational fluctuations of the stiff segments. The characteristic wavelengthof this SD was a few tens of nm for glass crystallization at a temperaturevery slightly above T9 and several μτη for melt crystallization.

The most impressive result is that we have succeeded in observing di-rectly in the light microscope the spinodal pattern of PET that emergesduring the induction period of melt crystallization and disappears shortlyafter crystallization.

Time-resolved FT-IR measurements revealed that polymer chains startto assume crystalline conformation (generally helical conformations) dur-ing the induction period, and depolarized light scattering measurementssuggested that the orientation fluctuations actually occur prior to crystal-lization. Crystal nucleation may begin in such oriented domains.

Finally, a model of the crystallization mechanism was proposed, basedon Doi and Olmsted theories. Thus, two different types of microphase sepa-ration in the oriented (nematic) dense and unoriented (isotropic) less densedomains occur depending on the crystallization temperature: nucleation-and-growth and SD-type at higher and lower crystallization temperatures,respectively. In turn, the second microphase separation occurs with a tran-sition from the nematic phase to the smectic phase in the nematic domains.

Acknowledgements

The author wishes to thank his coworkers, especially Dr. Imai, Dr. Nishida,Dr. Kanaya, and Dr. Matsuba, for their infinite efforts in the realizationof this work. This work was supported by NEDO International Joint Re-search Grant for the project "Fundamental Studies on Crystallization ofPolymers", by a Grant-in-Aid for Scientific Research on Priority Area"Cooperative Phenomena in Complex Liquids", a Grant-in-Aid for Scien-tific Research (A) on "Structure Formation of Polymers during the Induc-tion Period of Crystallization" and a Grant-in-Aid for Scientific Researchon Specified Area (B) "Mechanism of Polymer Crystallization" from theJapanese Ministry of Education, Science, Culture, and Sports.

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