The Role of Polymer Crystallizability on the Formation of Polymer

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The role of polymer crystallizability on the formation of polymer-urea-inclusion compounds Jialong Shen, Shanshan Li, Yavuz Caydamli, Ganesh Narayanan, Nanshan Zhang, Owen Harrison, Shiaoching Tse, and Alan E Tonelli Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00240 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on April 1, 2018

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Crystal Growth & Design

The role of polymer crystallizability on the formation of polymer-ureainclusion compounds Jialong Shen1, Shanshan Li1, Yavuz Caydamli1, Ganesh Narayanan1, Nanshan Zhang1, Owen Harrison2, Shiaoching Tse3, and Alan E. Tonelli1* 1

Fiber & Polymer Science Program, North Carolina State University, Raleigh, NC 27695-8301, United States 2 Polymer & Color Chemistry, Department of Textile Engineering Chemistry & Science, North Carolina State University, Raleigh, NC 27695-8301, United States 3 North Carolina School of Science and Mathematics, 1219 Broad Street, Durham, North Carolina 27705, United States *

To whom correspondence should be addressed ([email protected]).

Abstract Polymer-Urea Inclusion compounds (P-U-ICs) were formed using a series of linear aliphatic polyesters with varying crystallizabilities; from highly crystalline to wholly amorphous. The traditional hexagonal P-U-ICs were obtained irrespective of the crystallinities of the neat guest polyesters. Two distinct co-crystallization mechanisms were evident based on the observation of the change in thermal stabilities of the ICs using DSC and the crystal morphologies by SEM; one involves polymer chain folding back and forth in a lamella-like crystal structure and the other grows much like short chain molecule U-IC absent of chain reentering different channels. For polymers with sufficient chain length, their inherent flexibility is the key factor determining the co-crystallization mechanism while their crystallizability affects the kinetics, whose consequence is more pronounced during recrystallization from melt. The amorphicity induced by random ester group placement is an interchain property, which does not play a role in affecting IC thermal stability. Rather, increasing the average ester group content can be understood as introducing more defects in the IC crystal and therefore reduces its thermal stability. The understanding gleaned in this study provides a new avenue for designing P-U-ICs to be used both in theoretical modeling and engineering of high-performance materials.

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introductions Many polymers1–16 have been reported to form inclusion compounds (ICs) with small molecule hosts, such as urea, which forms narrow hydrogen bonded channels hosting extended and isolated polymer chains. The rearrangement of polymer chains in this unique solid-state environment has attracted both theoretical and practical interest. Theoretically, it provides a model system where the observation of polymer behaviors, i.e., conformations, dynamics etc., in a confined environment can be compared to that of the ordered bulk samples and leads to a means of isolating intrinsic intrachain contribution from that of the complex cooperative interchain interactions.17–22 For practical uses, the extended chain conformations of polymers in ICs were harvested in the coalescence23–26 process using a solvent and non-solvent for urea and polymer, respectively. Although both of the above mentioned usages assume the physical picture of guest polymer chains residing in host urea channels like those of urea inclusion compounds formed with short chain guest molecules with a well-elucidated structure27–38, the detailed cocrystallization mechanism between polymer and urea is largely unclear. Specifically, whether the inherent polymer crystallizability plays any significant role in the IC formation processes and how polymer’s crystallization behavior is changed or modified in the IC formation? Experimental observations made on the ability of amorphous polymers to form urea inclusion compounds are scarce. The only reference that touched upon amorphous polymer urea inclusion compound was a comparison between the use of cyclodextrin (CD) and urea inclusion compounds to blend immiscible amorphous PMMA and PVAc pairs.39 Though the similar blending effect between using CD and urea suggests the formation of common urea inclusion compounds by the two polymers, the crystal structure of the IC was not specified. Part of the dilemma is that the very structural factors that defeat polymer crystallinities are usually irregular


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structures, such as bulky side groups with random tacticity, which would inevitably alter the urea channel structure if it were to form inclusion compound at all. Hence, linear polymers that, in their extended chain conformations, would physically fit the narrow channel yet possess different crystallizabilities will provide insight into the co-crystallization mechanism of polymer-urea-ICs. Recently, we have synthesized40 a series of linear aliphatic tetra-polyesters with varying crystallizability, and some of them were completely amorphous in the bulk. In the current work, we have made polymer urea inclusion compounds (P-U-ICs) with these tetra-polyesters using a solution cocrystallization method. The inclusion of polyester chains in the narrow urea hexagonal channels was confirmed by various methods. Factors determining the cocrystallization mechanisms and their consequences are discussed. Experimentals Materials Urea (Sigma Cat. # U5378, ≥98%), methanol (BDH, ACS, 99.8%+), acetone (BDH, ACS, 99.5%+), ethylene glycol (Sigma-Aldrich, anhydrous 99.8%), 1,3-propanediol (Aldrich, 98%), 1,6-hexanediol (Aldrich, 99%), succinic acid (Alfa Aesar, 99+%), and adipic acid (Sigma, 99%) were used without further purification. Synthesis of copolyesters The linear aliphatic tetra-polyesters were synthesized by step-growth polymerization using varying amounts of each of the two diols (1,3-propanediol and 1,6-hexanediol) and the two diacids (succinic acid and adipic acid) as monomers. The detailed experimental descriptions were reported elsewhere.40 The tetra-polyester series were named according to nomenclature of the copolymerization of the two homopolyesters; i.e., poly(trimethylene succinate) (P34) and poly(hexamethylene adipate) (P66). For example, P3466-20/80 represents poly(20% trimethylene



succinate-co-80% hexamethylene adipate). In addition, the other two possible homopolyesters, poly(trimethylene adipate) and poly(hexamethylene succinate), were named consistently as P36 and P64, respectively. Sample codes of the polyesters and their corresponding compositions are summarized in Table 1. Formation of inclusion compounds A solution co-crystallization method was used to form the polymer-urea-ICs, and an example procedure is described as follows. In an Erlenmeyer flask, 1 gram of polyester sample was dissolved in 25 ml of acetone heated at 50 °C with magnetic stirring. In a separate flask, 7 grams of urea was dissolved in 35 ml of methanol at 50 °C. After forming both solutions, the polymer acetone solution was added dropwise to the urea methanol solution with continuous stirring and heating. Heating was ended when the addition was completed, while the stirring was continued until room temperature was reached. The flask was then set aside for re-crystallization overnight. The rate of IC formation was visible as a fine white powder that was seen to vary with the crystallization abilities of the polyester samples. The most crystallizable polyester P66 formed a white powder IC with urea during the addition process, while the inferior crystallizer P34 formed IC slowly overnight. To test this hypothesis, another amorphous co-polyester formed between P34 and P24 with a 60%:40% composition was used to make an IC with urea using the same procedure. Replacing the better crystallizer P66 with an inferior crystallizer P24, their copolyester is considered to be less crystallizable than all the other samples in the P3466 series. The resultant solution failed to produce solid precipitate after two days at room temperature. Solid precipitate could only be obtained after the solution was kept overnight in a refrigerator at 4 °C. All solid precipitated samples were first filtered out and air dried and then were kept in a vacuum oven set at 60 °C for at least 24 hours before characterization.


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FTIR analysis Fourier Transform Infrared Spectroscopy (FTIR) analysis is generally used to confirm the co-existence of both the host and guest molecules in the intended IC sample and to infer the formation of inclusion compounds from the peak position shifts and shape change due to the interaction between the host and guest in the IC crystal lattice. However, other characterization methods need to be considered at the same time for the definite verification of the IC formation. For each IC sample, 64 scans with a resolution of 4 cm-1 were collected in the range of 700-4000 cm-1 on a Nicolet OMNI Germanium Crystal ATR sampling head accompanying a Nicolet Nexus 470 FTIR Spectrometer. DSC thermal analysis Differential scanning calorimetry (DSC) is used to observe the melting behaviors of ICs and their free constituent molecules. On the first heating scan, the IC crystal usually melts at a temperature different from that of the pure urea or pure polymer, which, on the other hand, do not manifest their individual thermal signatures (melting or Tg) when included in the IC crystal lattice. On the second heating scan, diminishing observation of or disappearance of the melting peak of the IC and the appearance of the melting peak or Tg of the free urea or polymer confirms that the IC crystal indeed included the two pure components. A Perkin Elmer Diamond DSC-7 instrument that was calibrated using a three-point method with undecane, indium, and zinc and was used in this study. About 3-5 mg of each sample were encapsulated in DSC aluminum pans and heated and cooled at a rate of 10 °C/min between -65 and 163 °C with nitrogen as the purge gas. Glass transition temperatures (Tgs) and melting temperatures (Tms) and its peak areas were calculated with Pyris software.



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XRD Wide angle X-ray diffraction was used to confirm the formation of P-U-ICs with hexagonal crystal structures. A Philips XLF, ATPS X-ray diffractometer with an OMNI Instruments customized auto-mount and copper tube, which produces X-rays with a wavelength of 1.54 Å was used. The diffraction intensities were measured every 0.1° over the 2θ range of 540°. SEM The FEI Verios 460L field-emission scanning electron microscope (FESEM) was used to observe IC crystal morphology. Fine IC powders were first sprinkled onto the carbon tape on the sample holder and then blown with a nitrogen gun to remove any loose powder. No conductive coating was applied to the samples.

Table 1 Thermal properties of polyesters before inclusion in and after liberated from U-IC Sample Feed ratio (mol%)* Tm (℃) code PD HD SA AA Before

Tm (℃) After

∆Tm (℃)

Tg(℃) Before

Tg(℃) After

∆Tg (℃)

P34

50

-

50

-

-

-

-

-34.9

-29.2

5.7

P36

50

-

-

50

34.7

26.5

-8.2

-60.8

-50.3

10.5

P346680/20 P346670/30 P346660/40 P346650/50 P346640/60 P346630/70 P346620/80 P64

40

10

40

10

-

-

-

-43.7

-39.1

4.6

35

15

35

15

-

-

-

-49.3

-43.7

5.6

30

20

30

20

-

-

-

-52.9

-47.0

5.9

25

25

25

25

11.7

5.8

-5.9

-55.3

-50.1

5.2

20

30

20

30

17.7

15.1

-2.6

-57.4

-46.7

10.7

15

35

15

35

27.9

27.0

-0.9

-

10

40

10

40

37.8

36.5

-1.3

-

-

50

50

-

54.3

50

-4.3

-


-

-

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-

P66

50

-

50

54.1

53.2

*PD=1,3-Propanediol HD=1,6-Hexanediol SA=Succinic Acid

-0.9

-

-

-

AA=Adipic Acid

Results and Discussions Polymer crystallizability and P-U-IC formation The FTIR spectrum of P66-U-IC is compared to that of the previously confirmed PCL-UIC, pure PCL, and pure tetragonal urea.(See Figure 1) It is evident that both P66 and urea are present in the IC sample and that the urea N-H stretching in the range of 3200-3500 cm-1 changes shape from that of neat U. The carbonyl stretching is located at the same position as that of the PCL-U-IC and is slightly shifted to larger wavenumber from the pure PCL value of 1723 cm-1. This was previously determined to be a shift due to PCL crystallinity.41,42 It is resonable to expect that carbonyl groups have very different interactions depending on whether they are within or outside of the PCL crystal. The carbonyl peaks for both P66-U-IC and PCL-U-IC shifted to a value corresponding to the amorphous or melted PCL. This indicates that polyester chains are separately included in different channels and are devoid of polymer to polymer interactions which are present in pure polymer crystals. The observations described above for P66-U-IC and PCL-U-IC are universal for all polyester-U-IC investigated in this study. (See Figure 2) DSC 1st heating scans (See Figure 3) confirmed that all polyesters are included in the IC crystals based on the disappearance of melting and/or the Tg of corresponding neat polyesters. For some of the samples, free urea peaks at around 134 °C are present due to the excess amount of urea used in the P-U-IC formation procedure. For all but the P34-U-IC, the larger melting peaks located between 137 to 147 °C are assigned to the IC melting peaks. P34-U-IC has a peculiar broad peak at 115 °C, which is too low for it to be convincingly assigned to free urea. Additional work was done to study the origin of this melting peak and will be discussed in detail



in the next section. The melting peaks of ICs were observed to shift to lower temperatures with higher ester group content irrespective of the crystallinities of the neat guest polyesters. The amorphicity of the neat polyesters with medium-range feed ratios is caused by the mismatch of the distribution of the ester groups between adjacent chains and therefore is an inter-chain property, which does not exist for the isolated chains. Increasing the average ester group content can be understood as introducing more defects in the IC crystal and therefore reduces its thermal stability. On the 2nd heating scans, Tgs and melting peaks for polyesters emerge along with free tetragonal urea melting peaks. (See Figure 4) This observation confirms that the assigned IC peaks indeed correspond to the IC crystals containing both polyesters and urea. It is also noticeable that some of the samples manifest IC peaks that can be attributed to the partially recrystallized IC during the cooling scan, while others, namely those ICs made from amorphous polyesters, do not. The correlation between the crystallizability of the guest polymer and their ability to form urea inclusion compounds from the melt is evident, though an answer for its cause is not readily apparent. It is however reasonable to expect that distinct co-crystallization mechanisms and their resulting structures cause the disparity. For those more crystallizable samples, a dominant role is played by polymer in crystallizing in its own fashion, i.e., by rapid chain folding into lamella, and urea simply finds the stabilizing entity (extended polymer chain) to co-crystallize with into a stable hexagonal inclusion compound. This polymer crystallization driven IC adopts lamella-like crystals that are radially stacked, as will soon be discussed in the SEM results. Consequently, the proposed switch board-like structures weave the whole IC structure together, which may well-explain the increased thermal stability of the ICs beyond that of pure urea. This is not the case for short aliphatic chains whose ICs melt at higher temperature


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with higher molecular weight, but do not exceed that of the pure tetragonal urea melting peak.27,28 As the polyester crystallizability decreases, largely caused by the decrease in the number of flexible methylene units, the kinetics of the co-crystallization process with U is reduced. Although it is possible to rely on urea co-crystallization as the driving force to form IC in the solution where chain rearrangements are energetically much easier and faster, these IC crystals, once melted, are difficult to recrystallize back together again from the melt upon cooling at a regular cooling rate due to the large energy barrier associated with chain rearrangement in the viscous polymer melt. XRD diffractograms definitively confirm the formation of hexagonal urea crystal structure. In Figure 5, Homopolyester-Urea-ICs are compared with the regular tetragonal urea crystal structures. Peaks at 2θ of 12.5, 14.9, 20.5, 21.7, 23.2, 25.1, 26.4, and 27.4 are identical to that of the hexagonal PE-U-IC43 whose crystal structure was confirmed by single crystal diffraction experiment3 to be identical to that of the traditional Urea-Paraffin-IC hexagonal structure determined by Smith44. On the other hand, tetragonal urea has very different characteristic peaks at 22.4, 24.7, 29.4, 31.7, and 35.6. It is evident that the major peak in the neat urea diffractogram shifts leftward to a smaller 2θ corresponding to a larger distance between parallel planes in real space in the IC. This larger spacing between planes of hexagonal urea crystal structure accommodates the inclusion of polymer chains within its crystal lattice. All IC samples are compared in Figure 6, where the shift of the major peak toward smaller 2θ with the increase of 34 content was not significant. In other words, increasing the ester group content does not affect the dimension of the IC significantly. SEM was used to observe crystal morphological differences among four homopolyesterUrea-ICs that are closely similar at the atomic scale as evidenced by their nearly identical XRD



diffractograms. (See Figure 7) It is evident that U-ICs formed with the more crystallizable homopolyesters (P64 and P66) manifest distinct morphologies from those formed with the less crystallizable ones (P34 and P36). This explains the nuances in the XRD diffractograms of the two sets: ICs made with those polyesters having inferior crystallizability have a sharper triplet between 2θ of 25-27°. (See Figure 6) The change in the co-crystallization kinetics with polymer crystallizability is corroborated, i.e., polymer-crystallization-driven vs. urea-crystallizationdriven. The readily crystallizable polyesters possess radially stacked lamella-like crystals that resemble polymer spherulites, while those slower crystallizing polyesters form IC crystals with urea characteristics. The average thickness of the lamella-like crystal sheets were estimated from SEM image to be 311 and 108 nm, for P64-U-IC and P66-U-IC, respectively. P36-U-IC formed regular hexagonal prisms with stacks of crystal platelets clearly shown from the side view. This type of platelet structure has been observed in PE, PTHF, PEO, and several aliphatic polyesters.3–6,8 Though taken with a grain of salt, the average distance between the apparent ridges (or valleys) of two platelets was also estimated from SEM image to be about 2.8 micrometers, which is about a hundred times thicker than typical polymer lamellae. It is also interesting to note the topology with material protruding out along the channel axis supporting a mechanism of crystal growth proposed by Hollingsworth et al.34for small molecule urea inclusion compounds. Polymers possess polydispersity and therefore naturally generate dangling chains around which urea wraps. Such ledges then serve as binding sites for polymer to sit and become wrapped around and propagate laterally. In the case of P34-U-IC, the crystals appear to be irregular and smooth. Chenite et al.5 made the observation on PEO that low molecular weight PEO formed ICs with smooth sides, while those having higher molecular weight formed stacks of platelets. It is thus reasonable to


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conclude that the formation of the platelets is related to polymer chain folding. Judging by its Tg, which is sensitive to polymer molecular weight, the liberated P34 should have molecular weight typical of step-growth polymers and similar to all other polyesters in this study. A difference lies, however, in its chain folding ability. With the highest ester group content in the series, P34 and urea formed IC without much of the P34 folding back and forth to stabilize the crystal and form platelets. Instead, it grows like short chain molecule U-ICs as reflected by its low thermal stability. To summarize, despite their vastly different co-crystallization kinetics and superficial morphologies, the only structural feature that separates P34 from the others is its lack of widely distributed lamella-like structure. This is therefore implying a different co-crystallization mechanism, which would lead to its peculiar thermal properties discussed in the next section. The thermal stability of IC crystals P34-U-IC has a melting peak at 115 °C, which is much lower than both neat tetragonal urea and hexagonal urea ICs. Similar behavior was observed for P2434(40%60%)-U-IC that has a well-defined hexagonal urea crystal structure compared to the high-melting PCL-U-IC (See Figure 8) yet possesses a low melting peak between 115 and 122 °C (See Figure 9(a)). To elucidate the nature of this low melting peak, DSC was subsequently run only up to 130 °C in the first heating scan to melt the first peak. (See Figure 9(b)) It is found on the 2nd heating scan that this low temperature peak is irreversible and there only remains one large peak where the 2nd high temperature peak was initially. At the same time, the 2nd heating scan shows the Tg of the polyester which was not observable in the first heating due to the separation of polymer chains in different urea channels. It is this lower melting peak whose corresponding crystal structure contains polymer chains. The question remains as to whether the higher temperature peak



corresponds to hexagonal structure or tetragonal structure. From the room temperature XRD of the IC (See Figure 10 middle panel), there appears very little of the neat U tetragonal crystal structure. But after being annealed at 130 °C (See also Figure 10 top panel), the hexagonal P-UIC crystal structure transformed into the neat tetragonal U crystal structure indicating that the high temperature peak belongs to the melting of tetragonal U which does not exist in a large quantity at the beginning of the heating. As a result, there must be a transition from hexagonal to tetragonal crystal structure between the two peaks. The heat of transformation could be small due to the compensating effect of melting and recrystallization. This type of transformation was previously observed in paraffin urea ICs and monosubstituted aliphatic compound urea ICs.27,28 There it was concluded that the increase in the carbon chain length will increase the thermal stability of the IC, but it was never more stable than the neat U tetragonal structure. Instead of regarding it to be caused by an increase in molecular weight, we can also view it as diluting chain end or functional group contents. While continuous long hydrocarbon chains are “perfect” for IC stability, chain ends or functional groups may act like defects in the crystal structure. The same can be said of our polyester series; larger ester group content equates to less perfect structure and lower thermal stability. This explains well the very low melting peak for those P-UICs whose formation do not proceed through a polymer chain folding mechanism. P3466-80/20 is the closest sample to P34 in terms of ester group content. According to estimated diad probabilities40 (64% P34; 4% P66; 16% P36; 16% P64), there is still 36% of all other three repeating units. This significant percentage will likely provide enough flexible structural units for it to form IC with chain folding and thus yields a melting temperature higher than that of the pure tetragonal urea. Conclusions


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A series of linear aliphatic polyesters (P) with varying crystallizabilities successfully formed P-U-ICs in their traditional hexagonal crystal structures as confirmed by XRD. The crystallinity of the neat guest polyesters is not a requirement for its formation process, because the amorphicity caused by the mismatch of the placement of ester groups between adjacent chains has an interchain origin and has no consequence for isolated chains. Rather, the average ester group content seems to be negatively affecting the IC thermal stability and can be understood as defects that alter the perfect IC crystal structure. Two distinct co-crystallization mechanisms were evident based on the observation of the change in thermal stabilities of the ICs using DSC and the crystal morphologies determined by SEM. The first one involves polymer chain folding back and forth in a lamella-like crystal structure and the other grows much like short chain molecule U-ICs with an absence of chains folding and reentering different channels. For polymers with sufficient chain length, their inherent flexibility is the key factor determining the co-crystallization mechanism. A majority of flexible linear polymers are predicted to proceed through a chain folding P-U-IC mechanism. With a decrease in inherent flexibility, there is an increasing tendency for the polymer to persist in the direction of the channel axis. This type of chain rearrangement is atypical of polymer crystalllization from the melt, and can only be accommodated in solution where mass transport is more rapid. This explains well the disparities between IC recrystallization from the melts of different polyesters in the series. Finally, we expect that the preparation of highly extended polymer chains residing inside and then coalesced out of their U-IC channels, using the principles described in this study will find wide applications in both the theoretical understanding and engineering of their high performance materials. Acknowledgement



We acknowledge graduate assistantships provided by the Department of Textile Engineering Chemistry & Science at North Carolina State University. The authors also acknowledge the use of the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation.


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Figure 1 FTIR spectra of Urea, an example of polyester poly(ε-caprolactone) (PCL), PCL-U-IC and P66-U-IC.


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Figure 2 FTIR spectra of all samples showing the existence of both constituents in each sample.



Figure 3 DSC 1st heating of polyesters urea inclusion compound showing in the range from 75 to 165 °C. No thermal event was detected at temperature lower than this range.


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Figure 4 DSC 2nd heating of polyesters urea inclusion compounds. Tg and/or Tm show up on the 2nd heating but not in the 1st heating confirming inclusion compound formation.



Figure 5 XRD diffractograms of homopolyester urea inclusion compounds and urea.


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Figure 6 XRD diffractograms of all P-U-ICs.



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P34-U-IC

P36-U-IC

P64-U-IC

P66-U-IC

Figure 7 SEM images of homopolyester-urea-inclusion compounds.


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Figure 8 XRD diffractogram of P2434-U-IC vs. PCL-U-IC



(a)

(b)

Figure 9 DSC thermograms of (a) P2434-U-IC with regular temperature range, and (b) P2434-U-IC, where the first heating scan run did not exceed 130° C.


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Figure 10 XRD diffractograms of neat U (bottom), original P2434-U-IC (middle), and after it has been annealed at 130° C (top).



For Table of Contents Use Only The role of polymer crystallizability on the formation of polymer-ureainclusion compounds Jialong Shen1, Shanshan Li1, Yavuz Caydamli1, Ganesh Narayanan1, Nanshan Zhang1, Owen Harrison2, Shiaoching Tse3, and Alan E. Tonelli1* 1

Fiber & Polymer Science Program, North Carolina State University, Raleigh, NC 27695-8301, United States 2 Polymer & Color Chemistry, Department of Textile Engineering Chemistry & Science, North Carolina State University, Raleigh, NC 27695-8301, United States 3 North Carolina School of Science and Mathematics, 1219 Broad Street, Durham, North Carolina 27705, United States *

To whom correspondence should be addressed ([email protected]).

Urea Channel Polymer Chain

Polymer Chain Flexibility

Synopsis The traditional hexagonal polymer-urea inclusion compounds (ICs) were obtained using a series of linear aliphatic polyesters with varying crystallizabilities. The tendency of polymer chain folding back and forth and therefore interweaving the IC crystals during the co-crystallization with urea was found to increase with increasing inherent polymer chain flexibilities.


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The Role of Polymer Crystallizability on the Formation of Polymer

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