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Chapter 8

Isothermal Thickening and Thinning Processes in Low Molecular Weight Poly(ethylene oxide) Fractions Crystallized from the Melt Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: July 30, 1999 | doi: 10.1021/bk-2000-0739.ch008

Effects of Molecular Configurational Defects on Crystallization, Melting, and Annealing 1

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Er-Qiang Chen , Song-Wook Lee , Anqiu Zhang , Bon-Suk Moon , Ian Mann , Frank W. Harris , Stephen Z. D. Cheng , Benjamin S. Hsiao , Fengji Yen , and Ernst D. von Meerwall 1

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Maurice Morton Institute and Department of Polymer Science, The University of Akron, Akron, OH 44325-3909 Department of Chemistry, The State University of New York at Stony Brook, Stony Brook, NY 11794-3400 Department of Physics, The University of Akron, Akron, OH 44325

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Three two-arm poly(ethylene oxide) fractions (PEOs) with molecular weights of 2220 g/mol for each arm have been prepared by a coupling reaction using 1,4-, 1,3-, and 1,2-benzene dicarbonyl dichloride. The two arms at the coupling agents thus form angles of 180°, 120°, and 60°, respectively. Self-diffusion coefficients of these two-arm PEOs in the melt are surprisingly different. Wide angle X-ray diffraction patterns reveal that these PEOs possess the identical crystal structure to that of linear PEO. Observations of time-resolved synchrotron small angle X-ray scattering (SAXS) indicate that the samples crystallized below 38°C forming crystals with non-integrally folded (NIF) overall molecular conformations (OMCs). Crystals with integrally folded (IF) OMCs form when the crystallization tempearture (T ) is above 44°C,. Two different crystal populations with extended and once-folded OMCs are observed in both the 1,4- and 1,3-two-arm PEOs. Only one crystal population with mixed IF OMCs is found for the 1,2-two-arm PEO. The dependence of long period as a function of T for these twoarm PEOs is remarkably similar to the melting temperature response to T . The annealing effect is examined for samples crystallized at 32°C, subsequently heated to 50°C, and isothermally annealed for various periods of time. A partial melting upon heating and recrystallization during annealing can be identified. c

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Corresponding author.

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© 2000 American Chemical Society

Cebe et al.; Scattering from Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

119 In the past three decades, low-molecular-weight (LMW) poly(ethylene oxide) fractions (PEOs) have played an important role in understanding polymer crystallization (1-8), In our efforts, we have reported that an initial transient state during the LMW PEO crystallization can be recognized as non-integral folding chain (NIF) crystals which form prior to the final state of integral folding chain (IF) crystals (9-15). In order to investigate the molecular architecture effects on LMW PEO crystallization behavior, several experiments have been designed: molecular weight dependence (15), end group effects (16), and defects at the center of chains (17). Recently, we have focused on the crystallization and melting behavior of three different two-arm PEOs. These PEOs possess an identical MW of 2220 g/mol for each arm (M = 2220) and the coupling agents used are 1,4-, 1,3-, and 1,2-benzene dicarbonyl dichloride. The two arms at the coupling agents thus form angles of 180°, 120°, and 60°, respectively (18). It has been found that configurational defects at the center of each of the two-arm PEO chains substantially affect their overall molecular conformation (OMC) in the crystalline state. Wide angle X-ray diffraction (WAXD) expriments indicate that these PEOs exhibit the same crystal structure as that of pure PEO. Upon crystallization at low undercooling (AT), such as at a crystallization temperature (T ) of 48°C, differential scanning calorimetry (DSC) results reveal two crystal populations. Small angle X-ray scattering (SAXS) experiments also identifies two different long periods. It is speculated that one of the crystal populations possesses an extended OMC in these two-arm PEOs, and thus, one layer of defects is present in between two neighboring lamellae and the long period is smaller. The second crystal population consists of a once-folded OMC. Two layer defects are thus included in between the neighboring lamellae. The crystals with once-folded OMC represent the more stable form compared to those containing the extended OMC. In varying the linkage from the 1,4- to 1,2-positions at the coupling agents, the oncefolded OMC population increases under the same crystallization conditions (18). In this publication, we attempt to understand the crystallization, melting, and annealing behaviors of these three two-arm PEOs using the combined experimental techniques of time-resolved simultaneous synchrotron WAXD, SAXS, and DSC.

Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: July 30, 1999 | doi: 10.1021/bk-2000-0739.ch008

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Experimental Section Materials synthesis and characterization. The synthesis of two-arm PEOs has been described in our previous publications (17,18). In brief, 1,4-, 1,3-, and 1,2benzene dicarbonyl dichlorides were used as coupling agents in conjunction with a LMW PEO fraction (2220 g/mol) [a,œ-methoxy-hydroxy-poly(ethylene oxide), HO(CH CH -0) -CH ]. Further fractionation was also performed. Gel permeation chromatography (GPC) experiments using tetrahydrofuran at 30°C were carried out to measure the number average MW (M ) and polydispersity. The GPC was calibrated using standard linear PEOs over a MW range of 500 to 600,000 g/mol. Fourier transform infrared spectroscopy (FTIR, Mattson Galaxy 5020) measurements were carried out between 500 and 4000 cm" in order to identify the end groups and 2

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120 the coupling agents of the PEOs. A Knauer vapor pressure osmometer (VPO) was used to determine the M in toluene at 40°C. Sucrose octaacetate was used in the same solvent and concentration to calibrate the VPO. Finally, light scattering (LS, Wyatt Dawn F) experiments were conducted to insure the accuracy of the polydispersities by measuring the weight average MWs. Self-diffusion coefficients of these two-arm PEOs in the melt were measured via a pulsed-gradient spin-echo (PGSE) nuclear magnetic resonance (NMR) method (19-21). The principle echo onresonance without Fourier transform was measured using radio frequency phasesensitive detection. Its attenuation in the presence of a pair of applied magnetic field gradient pulses was detected. The samples were measured at 60.5, 80.5, and 100.5°C. The PEO critical entanglement MW in a monodisperse melt is 4400 g/mol (22). Equipment and experiments. DSC (TA2000 system) experiments were carried out to study crystallization, melting, and annealing behaviors of these PEOs. The DSC was calibrated with standard materials. Isothermal crystallization was conducted by quenching the samples from the melt to a preset T and held for various crystallization times (t ). In the low arrange, a self-seeding technique was employed for the isothermal crystallization as described previously (4). The crystallized samples were then heated above the melting temperature (T ) at a rate of 5°C/min. Annealing experiments were conducted after quenching the PEO samples from the melt and held for 30 min at 32°C for isothermal crystallization. The samples were then heated to 50°C and isothermally annealed at that temperature for different periods of times (t ). The melting traces of the annealed crystals were recorded by DSC. Time-resolved synchrotron WAXD and SAXS experiments were carried out on the synchrotron X-ray beam line X27C of the National Synchrotron Light Source at Brookhaven National Laboratories. The wavelength of the X-ray beam was 0.1307 nm. Isothermal crystallizations were carried out on a customized two-chamber hot stage. Temperature control precision was ±0.5°C. Annealing experiments were conducted using the identical thermal history to that of the DSC experiments. Position sensitive proportional counters were used to record the diffraction and scattering data. The diffraction counter was calibrated using silicon crystals of known size. The scattering counter was calibrated with duck tendon scattering peaks at q values of 0.109 nm" , 0.22 nm , 0.33 nm" , etc. (q= 4nsin®X, where λ is the wavelength of X ray radiation). The Lorentz correction was performed by multiplying the intensity / (counts per second) by q . The relative invariant g'was calculated based on f (IIb)q dq which covers a q range between 0.08 and 2 nm* (where l is the intensity of PEO liquid scattering obtained using the Porod's law extrapolation) (23).


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Results and Discussion Molecular analyses and characterizations. Following the coupling reactions, there remains a mixture of the linear "parent" and two-arm PEOs in the samples. It is necessary to carefullyfractionatethe mixture. Table 1 lists the analytical results for the MW and MW distribution as measured by GPC, VPO, and LS upon fractionation.


121 The samples possess narrow MW distributions. FTIR results indicate that these twoarm PEOs do not possess an absorption band at 3500 cm" which originates from hydrogen bonding and OH stretching vibration. The vibration band of the ketone group present in these PEOs is also observed at 1720 cm" (17,18). WAXD patterns of the samples crystallized at different T s show that the two-arm and linear PEO crystals possess identical structures. Therefore, the defects do not appear to change the crystal structure of the PEOs (17,18). 1

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Table 1. Molecular characteristics of linear and two-arm PEO fractions a

M

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My/My?

Mb n

Average length (nm) 14.0 29.0 29.0 29.0

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e

Linear PEO 1,4-two-arm 1,3-two-arm 1,2-two-arm

2200 4490 4500 4500

1.02 1.05 1.05 1.05

2220 4550 4550 4550

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—

2220 2220 2220

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Number average molecular weight from GPC. Number average molecular weight from VPO. Polydispersities from GPC, and independently checked via LS by measuring the weight average MWs. d Number average molecular weight of each arm. The average chain lengths were calculated from the equation / = M /d, d = 158.2 (4) for the linear fractions. For the two-arm PEO fractions, the average chain lengths were calculated by doubling the linear fraction length and adding the size of the coupling agent. c

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Self-diffusion coefficients (Ds) measured at three different temperatures using PGSE-NMR for the linear and two-arm PEOs are shown in Figure 1. The Ds of an a,co-methoxy-poly(ethylene oxide) (MPEO) with a MW of 4250 g/mol are larger (i.e. faster) than those of the two-arm PEO(M = 2220)s at the same temperature despite both fractions having almost identical overall molecular lengths. Furthermore, the D of the 1,4-two-arm PEO is approximately 15% higher than those of the 1,3- and 1,2two-arm PEOs. This indicates that the molecular dynamics in the melt may be different due to the locations of the two PEO arms at the coupling agents. The activation energies of the linear and two-arm PEOs can be calculated using the Arrhenius equation. A value of 30 ± 2 kJ/mol was determined for these fractions which is in good agreement with the data reported in our earlier work (20). Isothermal crystallization behavior of the two-arm PEOs. Isothermal crystallization processes of these three two-arm PEOs were monitored by simultaneous measurements of synchrotron WAXD and SAXS as shown in Figures 2a and 2b respectively, for the 1,4-two-arm PEO crystallized at 32°C. The WAXD patterns in Figure 2a demonstrate the development of crystallinity as t increases. The overall crystallization is complete within approximately 4 min, after which the crystallinity reaches a maximum. The SAXS patterns shown in Figure 2b have taken into account the Lorentz correction. An apparent thinning process can be observed during the onset of crystallization. A broad scattering peak is initially observed, with a maximum corresponding to a long period of 18.6 nm at t = 1 min. The peak intensity gradually increases with increasing t and the peak width at the half-maximum a

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Figure 1. Relationship between the self-diffusion coefficients and temperature linear and two-arm PEOs.



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Figure 2. Set of WAXD (a) and SAXS (b) time resolved synchrotron data for 1,4 arm PEO crystallized at 32 X.


124 narrows, indicating that the long period becomes more uniform and the layer correlation improves. During the progression of crystallization, the first-order SAXS maximum shifts to a larger q value and the long period reaches 15.3 nm at t = 4 min (the apparent thinning process). Within 3 min, the total decrease of the long period is 3.3 nm. A similar phenomenon can be observed for the 1,3-two-arm PEO crystallized at 32°C. Its long period decreases from 18 to 16.7 nm after t = 4 min. Therefore, the long period change of the 1,3-two-arm PEO is 1.3 nm, which is less than that of the 1,4-two-arm PEO. In addition, the initial scattering peak in the crystallization is asymmetric with a broad shoulder on the low q side. Both sets of WAXD and SAXS patterns of the 1,2-two-arm PEO crystallized at 32°C are shown in Figures 3a and 3b, respectively. Once again, the maximum crystallinity is reached within 4 min (Figure 3a). However, unlike the 1,4- and 1,3two-arm PEOs, the long period of the 1,2-two-arm PEO does not appear to thin. The asymmetric SAXS peak with a long period of 16.4 nm gradually increases in scattering intensity without changing the peak shape (Figure 3b). This peak is broader than those of the 1,4- and 1,3-two-arm PEOs, indicating that the lamellar layer correlation length is lower than the other two-arm PEOs. Upon increasing T to 40°C, the overall crystallization rates of these two-arm PEOs decrease. Based on the WAXD patterns in Figures 4a and 5a for the 1,4- and 1,2-two-arm PEOs, respectively, the time needed for completing the crystallization at 40°C is approximately 10 min. In Figure 4b, the SAXS patterns manifest the apparent thinning process of the 1,4-two-arm PEO. The initial long period is 20.6 nm and it decreases to 16.0 nm after t = 14 min. For the 1,3-two-arm PEO crystallized at 40°C, the crystallization also starts with a broad SAXS peak. The apparent thinning process can be identified by the long period changing from 19.5 to 18.1 nm within 14 min. Figure 5b shows that the SAXS peak of the 1,2-two-arm PEO is rather broad having a maximum at 18.1 nm. This value remains practically unchanged. In all three two-arm PEOs, the SAXS peak widths at the half-heights at T = 40°C are narrower than those corresponding to T = 32°C. Upon further increase in T , a self-seeding process must be utilized in order to accelerate the crystallization. Figures 6 and 7 are two sets of WAXD and SAXS patterns of 1,4- and 1,2- two-arm PEOs, respectively, crystallized at 48 °C after selfseeding. In all three two-arm PEOs, the crystallinity plateaus at t > 30 min (Figures 6a and 7a). During the development of crystallinity, these two-arm PEOs also exhibit an apparent thinning behavior which ceases when the crystallinity reaches its maximum. From Figure 6b, the initial long period of the 1,4-two-arm PEO is found to be 24.0 nm. The broad SAXS peak increases in intensity with increasing t , whereas the peak position shifts to higher q values. More importantly, this peak gradually evolves into two separate scattering peaks. Although their peak positions are in close proximity, these two peaks can be identified at t = 8 min. The corresponding long periods of these two peaks decrease slightly with prolonged t , reaching 19.8 and 17.3 nm after t = 30 min. A similar observation in the 1,3-two-arm PEO is also observed. The long period at initial crystallization is approximately 26.0 nm. After t = 10 min, a separate shoulder on the low q side of the scattering peak becomes c


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1,4-two-arm PEO Τ = 40 °C

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44°C, the 1,4- and 1,3-two-arm PEOs possess two long periods, e.g., nearly at 20.0 nm and 17.5 nm respectively at 48°C, indicating the coexistence of two separate crystal populations (Figures 8a and 8b). As described previously (18), the two-arm PEO crystals with once-folded OMC possess a thicker long period than that of the extended OMC. The long period of 20.0 nm is therefore be associated with the crystals having the once-folded OMC, and the long period of 17.5 nm is attributed to the crystals with the extended OMC. Furthermore, compared to the long period of 14.8 nm for the extended chain crystals of the linear "parent" PEO, one may estimate the thickness of the configurational defect layer to be approximately 2.5 nm. On the other hand, only one scattering maximum can be recongnized for the 1,2-two-arm PEO crystallized at T > 44 °C (Figure 8c). The corresponding long period is almost identical to that of crystals with once-folded OMC of the 1,3-two-arm PEO. This implies that the 1,2-two-arm PEO possesses only one population of crystals which have predominantly the once-folded OMC when crystallized at low AT. At T < 38 °C, only one long period appears, ranging between 15.5 and 17.0 nm. Note that this long period is usually thinner than that of the extended or once-folded OMCs. It can be speculated that these crystals contain irregular NIF OMCs. Furthermore, the changes of long periods with respect to T s for the 1,2- and 1,3-two-arm PEOs are similar, even though the 1,2-two-arm PEO has a single long period at T > 44°C. In summary, the crystallization behavior and lamellar morphology of two-arm PEOs are affected by the types of defect linkages at the center of the two PEO arms. Melting behavior of the linear and two-arm PEOs. Detailed melting behavior of the 1,4-two-arm PEOs has been discussed in reference 18. Figure 9 shows a set of DSC heating diagrams for the 1,3-two-arm PEO following crystallization at different T s. It is evident that at relatively low T s (< 38°C), the peak temperatures of melting endotherm are almost constant and the lowest compared to that of crystals grown at T s above 38 °C. Between 38 and 44°C, the T increases about 2°C, and for T > 44°C, two endothermie peaks can be found. Upon further increases in T , the T s increase slightly. Figures 10a - 10c represent the summary of the melting behavior for these two-arm PEOs at different isothermal T s. Common features can be noted in the other two PEOs. The crystals formed at T < 38°C are speculated to contain NIF c


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Figure 8. Relationships between the long periods at thefinalstage of crystalliza and T s for three two-arm PEOs: (a) 1,4-two arm, (b) 1,3-two-arm, and (c) 1 arm PEOs. c


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1,3-two-arm PEO Τ =32 °C


34 °C 36 °C 38 °C 40 °C 42 °C 44 °C 46 °C 48 °C

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Temperature (°C)

Figure 9. Set of DSC heating diagrams for 1,3-two-arm PEO at a heating rate o 5 °C/min at different T s. c


133 OMC. These constant T s observed may be due to annealing during heating (10,11). With further increases in T , the T starts to increase. Above T = 44°C, two separate melting processes can be observed for the 1,4- and 1,3-two-arm PEOs; the lower T most likely represents the crystals with the extended OMC, and the higher one reprents the once-folded OMC (Figures 10a and 10b). This identification has been confirmed by the results obtained from time-resolved SAXS data. It is interesting to note that Figures 8a and 8b are remarkably similar to Figures 10a and 10b with respect to the long periods and T s changing with T . The only apparent difference is in the 1,2-two-arm PEO (Figure 10c), wherein, two melting endotherms appear above T > 44 °C from DSC, but in Figure 8c, only one long period can be observed. However, the lower temperature endotherm of the 1,2-twoarm PEO possesses less than 25% of the overall heat of fusion. Several possibilities may explain this apparent difference (Figures 8c and 10c). The most likely possibility is that the crystal having the lower T consists of a mixture of both OMCs, although it is uncertain whether they represent a eutectic system or a solid solution. As long as the crystals possess mixed OMCs on a nanoscopic scale, only a relatively broad scattering peak can be found in the 1,2-two-arm PEO. Further studies are necessary to understand this observed difference. Annealing behavior in the two-arm PEOs. It has been found that for the twoarm PEO crystals formed at T < 38°C, even for a prolonged t (such as days), cannot change either their long periods (in SAXS) or apparent T s (in DSC heating at 5°C/min). The question is whether the crystals formed at high ATs (such as at 32°C) can be annealed at high temperatures (such as at 50°C) to form two separate crystals with different OMCs, similar to observations of isothermal crystallization directly from the melt. The annealing experiments show that after the 1,4-two-arm PEO is completely crystallized at 32°C, heated to 50°C, and annealed for different t s, the T increases from 54.6°C to 55.2°C at t = 120 min. When t is further increased to 900 min, the T increases to 55.4°C. Similar behavior of the annealing process can also be observed in the 1,3- and 1,2-two-arm PEOs. Figure 11 describes the T changes during annealing at different t s. It is evident that the initial increase of T is rather quick, followed by a slow development. In the 1,4-two-arm PEO, the increment of T for the crystals formed at 32°C before versus after annealing at 900 min at 50°C is 0.8°C (55.4°C versus 54.6°C). For the 1,3- and 1,2-two-arm PEOs, the increments are 1.3°C (56.4°C versus 55.1°C) and 1.9°C (56.8 C versus 54.9°C), respectively. This indicates that annealing at higher temperatures can improve the thermodynamic stability of the crystals (increasing T ). The annealing effect is more predominant in the 1,2-two-arm PEO compared to the 1,4-two-arm PEO. Interestingly, the annealing experiments lead to only one melting endotherm at 55.4°C for the 1,4-two-arm PEO. When the crystals are grown at 50°C directly from the melt, two T s of 54.8°C (the T of extended OMC crystals) and 55.8°C (the T of once-folded OMC crystals) are observed. Therefore, the T of 55.4°C associated with the annealed crystal is between the T s of the crystals with the extended and once-folded OMCs. Similar observations can be found in the 1,3-two-arm PEO; 56.4°C for the annealed crystals compared with 55.8°C and 57.3°C for the extended m

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30 32 34 36 38 40 42 44 46 48 50 52 T (°C) C

Figure 10. Relationships between T s and T s of three two-arm PEOs: (a) 1,4-twoarm, (b) 1,3-two-arm, and (c) 1,2-two-arm PEOs. Thefilledsymbols are results of peak separation. m

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Figure 11. Relationship between T s and t s for three two-arm PEOs after crys zation at 32 °C and heated to 50 V for annealing at t s. m

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136 and once-folded OMC crystals respectively. In the 1,2-two-arm PEO, it is 56.8°C for the annealed crystals compared with 56.6°C and 57.6°C for the crystals grown isothermally at 50°C. The results indicate that the annealed crystals may not possess the purely extended or once-folded OMC on a nanoscopic scale. On the contrary, during annealing the most probable transformation of the NIF crystals formed at T = 32°C is to form a mixture containing extended and once-folded OMCs. The annealing behavior of these two-arm PEOs crystallized at low T s is also investigated by time-resolved synchrotron SAXS and WAXD. For example, Figures 12a - 12c describe the long period (Figure 12a), crystallinity (Figure 12b), and relative invariant g'(Figure 12c) changes for the 1,4-two-arm PEO during heating and annealing. The temperature profile of the experiment is also included in Figure 12c. Figure 12a demonstrates an initial long period of approximately 14.5 nm. The long period starts to increase at a 44°C, reaching 16.4 nm at 50°C. Annealing isothermally at 50°C leads to a further increase of the long period to 17.8 nm after t = 4 min. Little increase in the long period can be found for prolonged t s. It is noted that 44°C is about 10°C below the peak temperature of the melting endotherm, and is even 4 °C lower than the starting T (48.0°C). Similar behavior can be observed in the 1,3- and 1,2-two-arm PEOs. The heating event clearly involves a thickening process of the original thin NIF long period formed at 32°C, which may be achieved by motion of the chain segments. During the thickening, the defects must diffuse to and concentrate on the crystal surfaces. Either the extended or once-folded OMC may thus form in the crystals (depending on the local free energy barriers of motion). The crystals therefore contain a mixture of both IF OMCs showing a single long period with an intermediate spacing and a relatively broad scattering peak. For instance, the annealed crystals of the 1,4-two-arm PEO result in a long period of 17.8 nm at 50°C, while the long periods are 17.3 nm and 19.8 nm for the extended and once-folded OMC crystals isothermally crystallized at 48°C directly from the melt, respectively. Since the long periods after annealing are closer to that of the extended OMC crystal rather than the once-folded OMC crystal, the annealed crystals most likely possess a predominantly extended OMC. Furthermore, since the thickening starts at 44°C, the T $ of the crystals grown at T < 38°C observed in DSC should be representative of the already thickened crystals during heating. The crystallinity (Figure 12b) initially remains constant at 0,79 before it starts to decrease (when the long period starts to increase) dropping to 0.68 at 50°C, which is followed by a slow increase. The development of the relative invariant g'with time can be recognized as a two-step process (Figure 12c); an initial β'increases during heating, reaching a maximum at 50°C, followed by a continuous but gentle decrease. In order to explain the Q change with time in this annealing process, it should be noted that Q' χ Κφ (1 - φ ), where Κ is a constant, is the mean square of the electron density difference between the crystalline and amorphous polymer, and φα is the volume crystallinity. Therefore, dQ'/dt χΚ(1 -2φ )άφ /άΙ. Based on the WAXD results, the weight crystallinity (and therefore, φ ) of each the two-arm PEOs is always higher than 0.5 during heating and annealing. This gives rise to a c

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Figure 12. Real-time long period (a), crystallinity (b), and relative invariant Q changes for 1,4-two-arm PEO crystallized at 32 °C and annealed at 50

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