Controlling the Polymorphic Behaviors of Semicrystalline Polymers

Controlling the Polymorphic Behaviors of Semicrystalline Polymers with Cyclodextrins Cristian C. Rusa,| Min Wei,§ Todd A. Bullions,⊥ Mariana Rusa,| Marian A. Gomez,‡ Francis E. Porbeni,| Xingu Wang,| I. Daniel Shin,† Charles M. Balik,# Jeffery L. White,& and Alan E. Tonelli*,|

CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 6 1431-1441

Fiber & Polymer Science Program, North Carolina State University, Raleigh, North Carolina 27695-8301, Emory University, School of Medicine, Atlanta, Georgia, Virginia Polytechnic Institute, 132 Norris Hall, Blacksburg, Virginia 24061-0219, Instituto de Ciencia y Tecnologia de Polimeros, Calle Juan de la Cierva, 3, 28006 Madrid, Spain, Department of Pharmaceutical Sciences, Campbell University, P. O. Box 1090, Buies Creek, North Carolina 27506, Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina 27695-7907, and Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204 Received June 3, 2004; Revised Manuscript Received July 14, 2004

ABSTRACT: We present a review of our initial studies concerning the control of polymorphism in semicrystalline polymers with cyclodextrins (CDs). CDs are cyclic starch oligomers with six (R-CD), seven (β-CD), and eight (γ-CD) R-1,4-linked glucose units possessing bracelet structures with hydrophobic and hydrophilic interiors and exteriors, respectively. They are able to act as hosts to form noncovalent inclusion compounds (ICs) with a large variety of guest molecules, including a wide range of high molecular weight guest polymers. In polymer-CD-ICs, the CD host crystalline lattice consists of hexagonally packed CD stacks with guest polymers occupying the narrow channels (∼0.5-1.0 nm) extending down the interiors of the stacked CDs. As a consequence, the included guest polymers must adopt highly extended conformations and are segregated from neighboring guest polymer chains. When the host CDs are appropriately removed from polymer-CD-ICs, the included guest polymers are forced to coalesce into a pure polymer solid, which has been observed to affect their conformations, morphologies, and, for crystallizable polymers, even their polymorphism. Here we present a comparison of the polymorphism observed in nylon-6, PET, polycarbonate, and the polyolefins, isotactic-polypropylene and -poly-1-butene, coalesced from their CD-ICs with that observed when they are crystallized from their melts and/or solutions. Generally, a higher level of crystallinity and higher melting and melt recrystallization temperatures are observed for the CD-IC coalesced samples, and they also often crystallize into different polymorphs. Introduction Cyclodextrins (CDs), cyclic starch oligomers, may be represented as shallow truncated cones consisting of six, seven, or eight glucose units, and are named alpha (R), beta (β-), or gamma (γ-) CDs, respectively (see Figure 1). Although the depth of the cavities for the three CDs is the same (∼7.9 Å), their cavity diameters are ∼5, 6, and 8 Å, respectively.1 CD molecules can complex and host both polar and nonpolar molecules, including polymers, as guests inside their cavities due to their unique structure.2-7 Our research group has recently reported that CDs may act as hosts in the formation of inclusion compounds (ICs) with various high molecular weight polymers. Polymer-CD-ICs are crystalline compounds obtained by threading of the doughnut-shaped CD molecules onto the guest polymer chains. Once guest polymer chains are included inside the CD cavities, they are * To whom correspondence should be addressed. E-mail: [email protected]; phone: 919-515-6588; fax: 919-515-6532. | Fiber & Polymer Science Program, North Carolina State University. § Emory University. ⊥ Virginia Polytechnic Institute. ‡ Instituto de Ciencia y Tecnologia de Polimeros. † Department of Pharmaceutical Sciences, Campbell University. # Materials Science & Engineering, North Carolina State University. & Department of Chemistry, North Carolina State University.

Figure 1. Structure and dimensions of cyclodextrins (CDs).

segregated from neighboring polymers chains by the walls of the CD crystalline lattice and are forced to adopt highly extended conformations by the narrow host CD channels (see Figure 2). We have shown that coalescence of guest polymers from their CD-IC crystals can result in a significant improvement of their physical properties

10.1021/cg049821w CCC: $27.50 © 2004 American Chemical Society Published on Web 09/08/2004

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Figure 2. Schematic representation of the reorganization of polymers through formation of and coalescence from their CD-ICs.

caused by modification of the structures, morphologies, and even conformations that are observed for their coalesced bulk samples.8-10 In addition, recently we have also developed a novel approach for forming intimately mixed polymer blends by coalescing polymer pairs (removal of CDs) from their common CD-ICs.11-16 Here we summarize our observations concerning the structures, morphologies, conformations, and properties of semicrystalline polymers that are solidified by coalescence from their CD-ICs (see Figure 2), with particular reference to the polymorphism exhibited by their crystalline regions, which can be altered from those normally produced by crystallization of their completely disordered randomly coiling polymer chains from solutions and melts. CD-IC coalesced samples of PET,8 nylon 6,9 bisphenol A polycarbonate (PC),8 and the isotactic polyolefins [polypropylene (i-PP) and poly-1-butene (iPB)]10 are considered and compared to their as-received and solution-cast samples. Experimental Procedures Details concerning the polymer samples used, the formation of and coalescence from their CD-ICs, and the characterization of both the coalesced and control, as-received and solutioncast samples can be found in the original references.8-10 Here we additionally offer the following comments: (i) coalescence was achieved by washing the polymer-CD-ICs with a solvent good for CDs, but one that does not dissolve the guest polymer, or by treatment with an amylase enzyme that degrades only the host CDs, and (ii) coalescence of the guest polymers from their CD-ICs was always conducted at temperatures below their Tg values. These procedures were used in an attempt to preserve, as much as possible, the extended, unentangled nature of the included guest polymers (Figure 2) in their coalesced, solid samples.

Results and Discussion PET. Figure 3 presents the WAXS diffractograms of as-received (asr)-, solution-cast (sc)-, and coalesced (coa)PET samples. Diffraction peaks at 2θ ) 16.5, 23.2, and

Figure 3. X-ray diffraction of as-received, solution-cast, and IC-coalesced PET, from bottom to top, respectively.

26.0°, which have been assigned17 to the (010), (110), and (100) lattice planes, are evident in the sc- and coaPET samples. The (100) peak is commonly enhanced in oriented PET samples,18 such as uniaxially drawn fibers and films, and is clearly more pronounced in the coalesced sample. The close similarity in the WAXS diffraction peaks and, although not presented here, melting temperatures observed8 for the sc- and coa-PET

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Figure 4. The all-trans and kink conformers of PET.

Figure 5. Crystallization enthalpies, Hc, observed at different cooling rates for as-received (lower) and γ-CD-IC-coalesced (upper) PETs.

samples suggests that they have both solidified into the same all-trans conformation crystal structure (Figure 4), but with a significantly higher degree of crystalline orientation in the coalesced sample. This is likely a consequence of the highly ordered environment from which the coa-PET was crystallized, as compared to the randomly coiling environment from which the sc-PET was obtained (See Figure 2). Coalescence of PET from its γ-CD-IC has not altered its crystalline polymorph, but instead has produced a semicrystalline sample with a high degree of crystallinity and with some orientation of its crystalline regions. Molecular modeling19,20 of the conformations permitted to PET chains confined to cylinders with diameters comparable to γ-CD (Figure 1) indicate that the ethylene glycol fragment (-O-CH2-CH2-O-) must adopt g( t gkink conformations (Figure 4). Upon coalescence, facile counter rotations about the -O-CH2- and -CH2-Obonds from g( and g- to the trans (t) conformation, which require only a very modest amount of swept-out volume,21 permit a rapid and extensive crystallization of the coalesced PET chains.8,22 On the other hand, entangled PET chains in solution and the melt adopt randomly coiling conformations dominated by -CH2CH2- bonds in the g( conformations,23,24 and so they must undergo a conformational transition requiring a much larger swept-out volume to crystallize, which explains the normally slow crystallization of PET from its melt. The results produced by this contrasting conformational behavior can be seen in Figure 5, where

Figure 6. Polarized light micrographs of melt-crystallized asreceived (top) and coalesced (bottom) PETs.

the heats of crystallization observed by DSC for asr- and coa-PETs upon cooling from their melts at various rates are presented.22 While coa-PET crystallizes even during the fastest cooling rates, asr-PET is quenched to a totally amorphous sample once the cooling rate exceeds ∼100 °C/min. In fact, at the highest observed cooling rate, coalesced PET crystallizes at a temperature below the generally accepted Tg of PET (∼70 °C). The very different crystallization abilities of coa- and asr-PETs are also reflected in the morphologies of their

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Figure 8. Dependence of 1H rotating-frame relaxation time T1F(1H) on temperature for as-received (asr), precipitated (ppt), and coalesced (coa) PETs.

Figure 7. FTIR spectra of asr-, sc-, and coa-PET samples, from bottom to top, respectively.

melt-crystallized samples. In Figure 6 coa-PET shows much more uniform and extensive crystallization of smaller spherulites than the asr-PET, although each were crystallized at 25 °C after cooling from their melts on the hot-stage.22 Coa-PET is also able to repeatedly crystallize rapidly from its melt even after being held at T > Tm for extensive periods. We believe this unusual behavior is a consequence of the g( t g- kink conformations of included PET chains, which are largely retained after coalescence from PET-γ-CD-IC by the PET chains in the melt and, after crystallization, in the noncrystalline sample regions. These extended, noncrystalline conformations are consistent with the mesomorphic form of PET observed by X-ray diffraction25,26 on samples drawn below Tg. In addition, solid-state FTIR and 13C NMR observations of coa-PET8,22 indicate that the noncrystalline chains are indeed adopting the g( t g- kink conformations. For example, in Figure 7 the FTIR spectra of asr-, sc-, and coa-PETs are compared. Notice the much improved resolution of the coa-PET spectrum, where nearly every vibrational absorption is resolved to the baseline. The generally broad IR bands observed in asr- and sc-PETs, and most other polymer samples, are likely due to the large variety of polymer conformations and packing environments surrounding each vibrating molecular bond or group. In contrast, the improved conformational order, and resultant chain packing, in the noncrystalline regions of coa-PET result in a highly resolved spectrum. coa-PET, as a consequence of its formation by coalescence from its γ-CD-IC, has a high level of crystallinity and noncrystalline regions that are conformationally ordered. As pointed out, this results in repeated rapid crystallization from its melt, likely due to retention

there of the extended g( t g- kink conformations. In fact, coa-PET does not exhibit a glass-transition in the DSC, even after rapid quenching from its melt.8,22 This is supported by the 13C NMR-observed 1H rotatingframe relaxation times, T1F(1H), measured for asr-, ppt-, and coa-PET samples shown in Figure 8. [Note that precipitated (ppt)-PET is obtained by a precipitation process, without the use of γ-CD, which results in samples very similar to coa-PET in both their structural and thermal behaviors.] The abrupt change in the T1F(1H)s of asr-PET, which reflect kHz segmental motions, as the temperature is increased above Tg is in marked contrast to the coa- and ppt-PET samples, which show no such abrupt change in motion throughout the entire investigated range of temperatures. Thus, the noncrystalline regions in coa-PET are unable to experience a glass transition, and so are not truly amorphous in the dynamic, as well as the conformational, sense. PC. The WAXS diffractograms observed for asr-, sc-, and coa-PCs are presented in Figure 9. The sc- and coaPC samples show similar diffractograms, with the main diffraction peak at 2θ ) 18°, which has been observed in noncrystalline PC samples and attributed to amorphous PC chains with trans carbonate groups.27 A weak peak at 2θ ∼ 25° in the coa-PC sample may result from an increase in the ordering along the PC chain axis retained from its highly extended conformation in the γ-CD-IC channels, which must necessarily contain alltrans carbonate groups. This peak moves to 2θ ∼ 2223° in the sc-PC sample, implying a crystalline form less dense than coa-PC. Consequently, we suggest coa-PC may contain two crystalline polymorphs, whereas scPC contains only a single polymorph. DSC observations presented in Figure 10 show a double melting endotherm for coa-PC with the major peak at 246 °C and a smaller shoulder at 233 °C, while sc-PC has a single melting endotherm at 233 °C. These observations are consistent with the presence of two and single crystalline polymorphs in coa-PC and sc-PC, respectively. In addition, in the second DSC heating scans following cooling from their melts, coa-PC exhibits

Review


Figure 11. FTIR spectra of (a) as-received and (b) coalesced nylon 6.

Figure 9. X-ray diffraction of (a) as-received, (b) solutioncast, and (c) IC-coalesced PC.

Figure 10. DSC heating scans of coa-PC runs I (a) and II (b); sc-PC runs I (c) and II (d); and (e) asr-PC run I.

a Tg that is 6 °C higher than observed for sc-PC, and neither sample shows a melting peak. Unlike coa-PET, it appears that melting largely destroys the reorganization of PC that was produced by coalescence from its γ-CD-IC. Nylon 6. Nylon 6 exhibits polymorphic structures that contain two types of stable crystal forms: a monoclinic R-form and a pseudohexagonal (or monoclinic) γ-form, whose populations are sensitive to the method of sample preparation. The γ-form crystal of nylon 6 can be transformed into the R-form by annealing or by drawing.28,29 Conversely, the R-form of nylon 6 can be transformed to the γ-form by treatment with iodine.30 The change in polymorphic structures of nylon 6 results

from the spatial rearrangement of the hydrogen bonding between the carbonyl oxygens of one polyamide molecular chain and the amide hydrogens in the neighboring polyamide molecular chain. In the γ-form nylon 6 crystals, nonplanar polyamide molecules adopt a parallel-chain arrangement of hydrogen bonding, whereas the polyamide molecules in the monoclinic R-form crystal are fully extended, all-trans, and planar and are packed in the more stable antiparallel chain arrangement of hydrogen bonds. The R-form is more stable than the γ-form presumably because of shorter, stronger hydrogen bonds31 and the lower energy of the fully extended, all-trans conformation of its chains. Because the R-form nylon 6 has a more extended chain conformation (larger unit cell C-axis value, which is along the chain direction) than the γ-form, we attempted to induce nylon 6 to crystallize exclusively in the R-form polymorph by forming and disassociating the nylon 6-R-CD-IC and recovering the coalesced nylon 6. The overall crystallinity, hydrogen-bonding density, and possibly chain orientation were expected to increase as a result of this processing method, since parallel, nonplanar (γ) or antiparallel, planar, and extended (R) polymer chains are necessary to achieve interchain hydrogen-bonding and nylon 6 crystallization. After removing the host R-CD from the nylon 6 inclusion complex, we first examined the coalesced and as-received samples by ATR-FTIR. The two infrared spectra look broadly similar, and no vibrational bands characteristic of cyclodextrin (1026, 1079, and 1158 cm-1) remained, indicating the guest nylon 6 polymer chains had been coalesced from the host R-CD channels with complete removal of R-CD. However, it was also evident that the bands in the region between 1300 and 800 cm-1 showed clear differences between the two nylon 6 samples (Figure 11). FTIR band assignments for both R and γ crystal forms of nylon 6 have been reported in the literature.32-34 The bands at 928, 959, and 1200 cm-1 were attributed to the R-crystalline phase, whereas the band at 973 cm-1 was attributed to the γ-crystalline phase. In Figure 11, for the as-received nylon 6 chips, there is a strong band at 973 cm-1, indicating that these samples contain a considerable amount of the γ-crystal form, with only a small amount of the R-crystal form indicated by the weak bands at 928, 959, and 1200 cm-1. In contrast,

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Figure 12. Wide-angle X-ray diffraction patterns of (a) asreceived and (b) coalesced nylon 6. Table 1. Comparison of asr- and coa-Nylon 6 Hydrogen-Bonding Related FTIR Bands bandwidth at half-maximum (cm-1) sample

3297 cm-1

1637 cm-1

as-received nylon 6 nylon 6 coalesced from its R-CD-IC

76 36

33 24

the FTIR spectrum of nylon 6 coalesced from its R-CDIC does not show the band at 973 cm-1, but strong bands at 930, 959, and 1200 cm-1, indicating a much higher fraction of R-form crystals. Another interesting result is suggested by the absorption peak at 1030 cm-1. According to FTIR studies of nylon 6 yarns, it is clear that the intensity of this peak increases with increasing yarn draw ratio, and presumably reflects the increased orientation of the constituent nylon 6 chains.35 In the FTIR spectrum of the sample coalesced from its inclusion complex, there is a very strong peak at 1030 cm-1, in contrast to the as-received nylon 6. This may demonstrate that the extended, planar conformation adopted by nylon 6 in the R-CDIC is substantially retained after coalescence, leading to an improved orientation of the extended nylon 6 chains. The amide units provide hydrogen bonding between polymer chains, giving nylon 6 hydrogen-bonding dependent crystalline behavior. The crystalline N-H‚‚‚‚ OdC hydrogen bond density in nylon 6 can be estimated by measuring the widths of N-H and OdC stretching peaks at 3297 and 1637 cm-1, respectively.36 The sharper the peaks, the more crystalline are the N-H‚ ‚‚‚OdC hydrogen bonds, because of the greater homogeneity of their structural environment in comparison to the N-H‚‚‚‚OdC hydrogen bonds in the heterogeneous environments characteristic of the amorphous portions of the nylon 6 sample. The normalized peak widths at half-maximum observed for these two characteristic bands in the nylon 6 samples are tabulated in Table 1. Compared to the as-received sample, the coalesced nylon 6 has much narrower hydrogen bond related peaks than the as-received nylon sample, thereby indicating a higher overall crystallinity. Wide-angle X-ray diffraction is often used to determine the total crystallinity and the ratio of R- and

Figure 13. CP-MAS 13C NMR spectra of as-received (top) and coalesced (bottom) nylon 6.

γ-crystalline phases for nylon 6.37,38 In the X-ray diffraction patterns of nylon 6, the diffraction peak at 2θ ) 21.8° is contributed by the γ-form crystal of nylon and a pair of peaks at 2θ ) 20° and 24° are distinctive features of the R-form crystal of nylon 6. The single diffraction peak of γ-form nylon 6 is contributed by the (200) plane, and the two well-separated diffraction peaks for the R-form come from (200) and (002) planes, respectively. Figure 12 shows the WAXD patterns for as-received and coalesced nylon 6. In the pattern of asreceived nylon 6, it is apparent that there is a substantial amount of γ-form crystal, although the peak at 21.8° is overlapped. However, the 21.8° diffraction peak almost disappears in the X-ray pattern of the R-cyclodextrin inclusion complex treated nylon. For the coalesced nylon 6, two strong diffraction peaks characteristic for the R-form crystal, with much less scattering from the unstable γ-form crystals and amorphous material, can be observed. Solid-state NMR is one of the most powerful and versatile tools to study polymer structure, morphology, and dynamics. The 13C resonances of CH2 groups in nylon 6 occur in the range of 15-50 ppm vs TMS and overlap strongly. This frequency range includes resonances from the amorphous, γ- and R-crystalline phases. Resonances of the amorphous phase, which are generally much broader than the crystalline resonances, are not separately visible, while the narrower line widths of resonances from the crystalline fraction cause them to dominate the CP/MAS spectrum. The lines assigned here are therefore essentially due to the crystalline fraction. Some methylene carbons have a chemical shift that is sensitive to the crystalline modification, and therefore, a distinction between the different crystalline phases is possible.39,40 The peak at 43.9 ppm is assigned to C1 in the R-polymorph and the peak at 41.3 ppm to C1 in the γ-form. The CP/MAS 13C NMR spectra of asreceived nylon 6 and nylon 6 coalesced from its R-CDIC are given in Figure 13, where it can be seen that

Review


Figure 15. (a) TGA profiles and (b) thermal degradation rates for as-received (lower decomposition temperature) and coalesced (higher decomposition temperature) nylon 6 samples.

Figure 14. (A) DSC heating scans of coalesced (top) and asreceived (bottom) nylon 6 samples. (B) DSC cooling scans of as-received (top) and coalesced (bottom) nylon 6 samples. Table 2. Thermal Properties Obtained from DSC for As-Received and Coalesced Nylon 6 thermal properties

as-received nylon 6

Tm (°C) ∆Hf (J/g) crystallinity (%)a Tcc (°C) ∆Hcc (J/g) temp range of crystallization (°C)

215.6 55.8 29.4 179.9 60.2 190.7-156.8

a

The ∆Hf of 100% crystalline nylon 6 is

coalesced nylon 6 219.3 100.4 52.8 180.0 66.2 188.6-161.8

taken41

as 190 J/g.

R-form crystals dominate the crystalline regions of nylon 6 coalesced from its R-CD-IC. DSC measurements were conducted to obtain the melting and crystallization behaviors of nylon 6 asreceived and coalesced from its R-CD-IC, although this is not an unambiguous way to distinguish between the two crystal forms of nylon 6. Figure 14 shows the DSC heating and cooling scans and Table 2 lists the melting temperatures (Tm), the crystallization temperatures (Tcc) observed upon cooling from the melt, and the crystallinity of both nylon 6 samples. Compared to the asreceived sample, elevated Tm and Tcc and an almost doubled crystallinity were found for the sample coalesced from its R-CD-IC. It is known that R-form nylon 6 crystals have a higher Tm than γ-form.42 Therefore, the DSC results, again, reveal that R-form crystals are the dominant component in the coalesced material. Finally Figure 15 shows the TGA profiles for asreceived and coalesced nylon 6 samples. The coalesced nylon 6 is observed to have an ∼30 °C higher thermal degradation temperature than the as-received nylon 6. Once again, this likely reflects a different organization of nylon 6 chains in the coalesced sample compared to the normal morphology found in the as-received bulk polymer. Isotactic Polyolefins (i-PP and i-PB). Depending on preparation procedures, i-PP can be crystallized in four different crystal forms, R-, β-, γ-, and smectic,

Figure 16. X-ray diffractograms of as-received (a), precipitated (b), and coalesced (c) i-PP.

distinguished by different arrangements of the 31-helical chains in their unit cells.43-53 The R-form is the thermodynamically stable crystalline modification, which is usually obtained under common processing conditions. i-PB can crystallize in five different crystal phases, depending on the preparation method, which differ in their helical conformations and chain packing.54-64 One of these phases, named Form III (orthorhombic unit cell and a 41 helical conformation), is rarely observed and can only be obtained by crystallizing from selected solvents. Of the remaining crystal phases, Form II is unstable, converting over time to the most prevalent phase, Form I. Form II (tetragonal unit cell and an 113 helical conformation), is commonly obtained by melt crystallization and transforms slowly and irreversibly to Form I (hexagonal unit cell and 31 helical conformation) at room temperature without constraints. Form I′ and Form II′ are formed upon crystallization from the melt under high pressure. Moreover, Form I′ and Form III crystals have been found to form on solution crystallization depending on the solvent, concentration, and crystallization temperature. Wide-angle X-ray diffractograms of the as-received, precipitated, and coalesced i-PPs in Figure 16 clearly indicate that only the R-form is present in the asreceived and coalesced samples, and a very poor R-form or almost a smectic form is obtained in the precipitated i-PP.65 The R-form is characterized by strong diffraction peaks at 2θ ) 13.9 (110), 16.9 (040), 18.6 (130), and 21.8° (041). No peaks are evident at 2θ ) 16.0 or 20.1° corresponding to the β- or γ-phases, respectively.66

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Figure 17. MDSC cooling scans of as-received (a), precipitated (b), and coalesced (c) i-PP.

Figure 18. Second heating scans of as-received (a), precipitated (b), and coalesced (c) i-PP. (a′), (b′), and (c′) represent the nonreverse heat flows of the above corresponding total signals.

Careful analysis of all diffractograms reveals that the γ-CD inclusion/coalescence process does not modify the crystalline form of the included i-PP, but yields a higher crystallinity for the coalesced i-PP. γ-CD-IC formation isolates the included i-PP chains and the narrow host γ-CD channels forces them to adopt a highly extended conformation.67 Since the coalescence is a rapid process, the polymer chains do not have enough time to reorganize and readopt their more entangled and randomly coiling conformations. Consequently, the MDSC data

show an increase of about 27% in the crystallinity of coalesced i-PP in comparison with as-received i-PP. MDSC thermograms were run for as-received, precipitated and coalesced i-PP at a heating rate of 5 °C/ min. First heating scans of all three samples exhibit a single melting endotherm peak near 157 °C. However, on cooling from their melts at a rate of 5 °C/min, crystallization peaks occurring at different temperatures can be noticed in Figure 17. A significant increase in the crystallization rate was obtained for coalesced i-PP,

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Figure 19. WAXD patterns of the as-received (a), precipitated (b), and coalesced (c) i-PB.

which crystallizes at 130 °C compared with crystallization temperatures of 117 °C for the as-received i-PP and 121 °C for the precipitated i-PP. As mentioned above, this behavior has been observed in other semicrystalline coalesced polymers, such as PET, and can be explained by considering that the included polymer chains retain a certain degree of their extended and unentangled natures even after coalescence, thereby facilitating their rapid crystallization.8,22 All i-PP samples were kept in the melt at 200 °C for 3 min. However, the melt-phase memory effect in i-PP is very well-known and leads to crystallization rates highly conditioned by thermal history.66,68 In a previous study, heating in the melt at 210 °C for 10 min was determined to be sufficient for investigating the nonisothermal crystallization of i-PP in the absence of melt-phase memory effects.69 Second heating thermograms in Figure 18 reveal double melting endotherms for all three i-PP samples. According to the literature, the double melting peaks


are related to recrystallization and reorganization of imperfect monoclinic crystals. The peak at the lower temperature represents the melting of the imperfect crystals formed during the cooling process, whereas the higher temperature peak is due to the melting of more stable lamellae derived from recrystallization or reorganization of the imperfect crystals.65 Zhu et al. noticed that a decrease in the cooling rate leads to a larger fraction of the sample that crystallized perfectly with a diminishing fraction obtained upon recrystallization.65 This results in a decrease of the endotherm area at the higher temperature and an increase of the endotherm area at lower temperature, because of the decreased reorganization and recrystallization. Finally, only one endotherm appears at a 5 °C/min heating rate. All i-PP samples in our study were cooled to 50 °C, at 5 °C/min, from the melt. Two endothermic peaks with almost the same area (54/46 relative ratio area) were revealed for the as-received i-PP in the second heating scan. However, the precipitated and coalesced i-PP present a larger fraction of perfect crystals that melt at lower temperature against a smaller fraction of the recrystallized, further perfected ones. The relative areas calculated after deconvolution of the double endothermic peak are 95/5 and 96/4 for precipitated and coalesced i-PP, respectively. The total signal of the MDSC heat flow may be separated into reversing and nonreversing heat flow components, as a result of the temperature modulation employed. Thermal transitions in the reversing signal arise from thermodynamic phenomena, such as Tg and melting, while the nonreversing signal reflects kinetic phenomena, including evaporation and recrystallization. For the first time, the recrystallization of imperfect lamellae is supported by our MDSC study, which shows in the nonreversing heat flow an exothermic recrystallization occurring at a temperature be-

Figure 20. DSC scans of as-received (a), precipitated (b), and coalesced (c) i-PB. a, b, c are the first heating scans; a′, b′, c′ are the second heating scans, and a′′, b′′, c′′ are the intervening cooling scans.

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tween the two endothermic peaks in the total heat flow signal. As presented in Figure 18, the areas of these exothermic peaks are in good correlation with the relative areas observed in the second heating scans mentioned above. X-ray diffractograms of as-received, precipitated, and coalesced i-PBs are presented in Figure 19. The diffractograms of both as-received and precipitated i-PB clearly show the Form I polymorph with the (110) reflection at 2θ ) 9.9°, the (300) reflection at 2θ ) 17.3°, and the (220) reflection at 2θ ) 20.2°. In contrast, the coalesced i-PB predominantly adopts Form II, along with some form III, which are normally obtained from melt or solution casting, respectively. The characteristic diffraction peaks of Form II appear at 2θ ) 11.8 (200), 16.8 (220), and 18.1° (213). Form III has a strong reflection (110) at 2θ ) 11.8° and three weak (200), (111), and (120) reflections at 2θ ) 13.8, 16.8, and 20.7°, respectively. Although very noisy, the peak at 13.8° indicates the presence of Form III. The diffraction peak of Form III at 2θ ) 20.7° overlaps with a diffraction peak belonging to Form I′. However, the weak peak at 2θ ) 10.0° reveals a (110) reflection from Form I or Form I′. Forms I and I′ cannot be distinguished by X-ray but exhibit different Tm values in the DSC.61 Interestingly, after quiescent storage for 6 months at room temperature, coalesced i-PB, with a high molecular weight of 380 000, shows exactly the same diffraction pattern. It is very well-known in the literature that Form II (tetragonal) is metastable and transforms to Form I (hexagonal) at atmospheric pressure and room temperature with a half-life in the range of 250 to 1600 min.61 The same crystalline polymorph stability has not been observed for low molecular weight i-PB following coalescence from its γ-CD-IC. As was previously mentioned, CD-IC formation forces the included polymer chains to adopt highly extended conformation(s) in the host γ-CD narrow channels, which in the case of i-PB favors the formation of Form I crystals, with a more extended conformation, from among its polymorphs. However, an investigation of the phase transformations in i-PB upon drawing has demonstrated the formation of Form II upon tensile drawing and a strong dependence of the deformation process on the crystal form of the initial starting sample.64 To discern among different crystalline forms observed in the X-ray patterns, DSC scans were run for the asreceived, precipitated and coalesced i-PBs (see Figure 20). The endothermic peak at 125 °C in the first scan of as-received i-PB corresponds to Form I (Figure 20a). In the second heating scan, a melting peak at 110 °C is evident for the tetragonal Form II, which is always obtained by recrystallization from the melt. The multiple endothermic peaks in the first DSC run of coalesced i-PB (Figure 20c) indicate an interesting polymorphism for this sample. The first endothermic peak at around 90 °C and a second endotherm with a maximum at 99 °C may be assigned to Form III and Form I′, which have been reported to have a Tm between 90 and 100 °C.53 The third peak at 115 °C corresponds to the melting of Form II. The DSC results are in very good correlation with X-ray data of i-PB. Consequently, we may conclude that the coalesced i-PB mainly adopts Form II, together with small frac-

Review

tions of Form III and I′. After melting, the coalesced i-PB behaved the same as as-received i-PB, and entirely recrystallized into Form II (endothermic peak at 110 °C in the second scan). As shown in Figure 20b, the precipitated i-PB control sample presents a main endothermic peak at 124 °C (Form I melting) and a significant, but smaller endotherm, which may correspond to the melting of Form III. Since no diffraction peaks belonging to Form III were identified in the X-ray pattern of precipitated i-PB, one can conclude that this sample is dominated by Form I crystals. Like the asreceived i-PB, this sample completely transforms to Form II upon recrystallization from the melt (see second DSC scan). It is also noteworthy to mention the observation of a similar, yet even more significant, increase in the recrystallization kinetics from the melt of coalesced i-PB (crystallizes at 71 °C upon cooling), compared with the as-received and precipitated i-PBs (both crystallize at 42 °C upon cooling). Like coalesced i-PP, i-PB apparently retains some memory of its coalesced structure even after melting. Acknowledgment. We are indebted to the National Textile Center, US Department of Commerce, and NC State University for financially supporting the research. References (1) Harada, A.; Li, J.; Kamachi, M. Nature 1994, 370, 126. (2) Huang, L.; Allen, E.; Tonelli, A. E. Polymer 1999, 40, 3211. (3) Lu, J.; Shin, I. D.; Nojima, S.; Tonelli, A. E. Polymer 2000, 41, 5871. (4) Shuai, X. T.; Porbeni, F. E.; Wei, M.; Shin, I. D.; Tonelli, A. E. Macromolecules 2001, 34, 7355. (5) Harada, A.; Nishiyama, T.; Kawaguchi, Y.; Okada, M.; Kamachi, M. Macromolecules 1997, 30, 7115. (6) Do Nascimento, G. M.; Da Silva, J. E. P.; De Torresi, S. I. C.; Santos, P. S.; Temperini, M. L. A. Mol. Cryst. Liq. Cryst. 2002, 374, 53. (7) Li, J. Y.; Yan, D. Y.; Jiang, X. L.; Chen, Q. Polymer 2002, 43, 2625-2629. (8) Bullions, T. A.; Wei, M.; Porbeni, F. E.; Gerber, M. J.; Peet, J.; Balik, M.; White, J. L.; Tonelli, A. E. J. Polym. Sci., Part B 2002, 40, 992. (9) Wei, M.; Davis, W.; Urban, B.; Song, Y.; Porbeni, F. E.; Wang, X.; White, J. L.; Balik, C. M.; Rusa, C. C.; Fox, J.; Tonelli, A. E. Macromolecules 2002, 35, 8039. (10) Rusa, C. C.; Rusa, M.; Gomez, M.; Shin, I. D.; Fox, J. D.; Tonelli, A. E. Macromolecules 2004, 37, in press. (11) Rusa, C. C.; Tonelli, A. E. Macromolecules 2000, 33, 5321. (12) Wei, M.; Tonelli, A. E. Macromolecules 2001, 34, 4061. (13) Bullions, T. A.; Edeki, E. M.; Porbeni, F. E.; Wei, M.; Shuai, X.; Rusa, C. C.; Tonelli, A. E. J. Polym. Sci., Part B 2003, 41, 139. (14) Uyar, T.; Rusa, C. C.; Tonelli, A. E., submitted for publication. (15) Rusa, C. C.; Uyar, T.; Rusa, M.; Wang, X.; Hunt, M. A.; Tonelli, A. E. J. Polym. Phys., Part B 2004, in press. (16) Rusa, C. C.; Wei, M.; Shuai, X.; Bullions, T. A.; Wang, X.; Rusa, M.; Uyar, T.; Tonelli, A. E. J. Polym. Phys., Part B 2004, in press. (17) Lindner, W. L. Polymer 1973, 14, 9. (18) Goschel, U. Polymer 1996, 37, 4049. (19) Tonelli, A. E. Comput. Theor. Polym. Sci. 1992, 2, 80. (20) Auriemma, F.; Corradini, P.; Guerra, G.; Petraccone, V.; Vacatello, M. Macromol. Chem. Theor. Simul. 1995, 4, 165. (21) Tonelli, A. E. Polymer 2002, 32, 637. (22) Wei, M.; Bullions, T. A.; Rusa, C. C.; Wang, X.; Tonelli, A. E. J. Polym. Sci., Part B. 2004, 42, 386. (23) Williams, A. D.; Flory, P. J. J. Polym. Sci. Part A-2 1967, 5, 417. (24) Schmidt-Rohr, K.; Hi, W.; Zumbulyadis, N. Science 1998, 280, 714.

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Controlling the Polymorphic Behaviors of Semicrystalline Polymers

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