Article pubs.acs.org/Macromolecules
Control of the Crystalline Properties of 2‑Isopropyl-2-oxazoline Copolymers in Condensed State and in Solution Depending on the Composition Natalia Oleszko-Torbus,* Wojciech Wałach, Alicja Utrata-Wesołek, and Andrzej Dworak Centre of Polymer and Carbon Materials, Polish Academy of Sciences, ul. M. Curie - Skłodowskiej 34, 41-819 Zabrze, Poland S Supporting Information *
ABSTRACT: Copolymers of 2-isopropyl- (iPrOx) and 2-npropyl-2-oxazoline (nPrOx) were obtained, and attempts to control their crystallization both in condensed state and in solution were made. The homopolymer of nPrOx showed a weaker crystallization tendency than PiPrOx; nevertheless, the frequently encountered assumption that it is completely amorphous and is not able to crystallize was found to be unjustified. By increasing the amount of nPrOx in copolymers, their crystallization ability decreased both in the condensed state and in solution. The highest degree of crystallization was achieved for copolymer iPrOx/nPrOx of the composition 85:15 mol %, and χc values of ∼60% in condensed state and ∼45% in water were obtained. On the other hand, for the copolymer with 50 mol % of nPrOx no crystalline fraction was observed, even when it was subjected to mild thermal treatment, both in the condensed state and in solution. However, when copolymers were subjected to more rigorous external conditions, such as exposure to high, predefined temperature for a significantly extended time, the crystallization of seemingly amorphous copolymer could be forced.
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INTRODUCTION Poly(2-oxazoline)s (POxs) are known as thermoresponsive synthetic polymers with good biocompatibility that are obtained via cationic ring-opening polymerization of five-membered cyclic imino ethers.1,2 The interesting properties of POxs have caused a significant revival of interest in this class of polymers. They show side-chain-dependent properties due to the presence of a strong amide dipole together with a nonpolar alkyl side chain in their structure. The nature of the side chain determines the properties such as solubility, thermosensitivity, the glass and melting transitions, and hence the crystallinity. These structure− property relationships can be simply adjusted in a controlled manner, opening up a route to POxs macromolecular engineering and thus the potential applications of these materials.3−6 POxs containing less than four carbon atoms in the side chain are soluble in water. Poly(2-methyl-2-oxazoline) (PMOx) is soluble in water regardless of the temperature. Homopolymers of 2-ethyl- (PEOx), 2-isopropyl- (PiPrOx), 2-cyclopropyl(PcPrOx), and 2-n-propyl-2-oxazoline (PnPrOx) have limited temperature-dependent solubility and exhibit LCST-type liquid−liquid phase separation.7−10 POxs containing four or more carbon atoms in the side chain are not soluble in water at room temperature, but some of them exhibit UCST-type phase transition.11 The glass transition is observed for POxs with less than four carbon atoms in the side chain, and generally, the transition temperature decreases linearly with an increasing number of C atoms.12,13 The crystallization ability increases with an increasing length of the side chain of POxs, starting from poly(2-butyl-2-oxazoline). While these POxs show an endother© XXXX American Chemical Society
mic melting peak, surprisingly, the length of the side chain does not significantly influence the value of the melting temperature, which is approximately 150 °C.12 Poly(2-isopropyl-2-oxazoline), which shows melting at a temperature of approximately 200 °C, is an exception to this rule. Despite its relatively short side chain, PiPrOx is known for its high crystallization tendency. The first systematic studies on the crystallization of POxs were reported by Litt et al.14 This study was focused mainly on POxs in the condensed state. An approximate packing of the polymer segments in the unit cell and densities of monomer units per unit cell were calculated. Over time, it became known that an unusual heat-induced crystallization behavior of PiPrOx in condensed state occurs in the 120−170 °C temperature range. However, the mechanism involving conformational changes of PiPrOx chains and other POxs during crystallization in the condensed state was still poorly understood. In later years, Sun and Wu15 studied this issue in detail by using IR and Raman spectroscopy combined with molecular dynamics simulations. They simulated different conformations of PiPrOx chains and proposed a model of a crystalline chain with alternate isopropyl groups and a slightly distorted backbone. Additionally, the crystallization of POxs in solutions, similar to that for proteins, has been widely described. Initially, Schlaad et al. found that poly(2-isopropyl-2-oxazoline) is able to crystallize upon prolonged annealing of its aqueous solution at temperReceived: July 31, 2017 Revised: September 4, 2017
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DOI: 10.1021/acs.macromol.7b01639 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules atures above LCST.16−18 Since then, crystallization of different thermoresponsive POxs has been investigated, also in different solvents.19−23 It was found that in the case of the aqueous solutions of thermoresponsive POxs the polymer-rich phase formed at temperatures above LCST (or below UCST) facilitated the organization into a unit cell. Additionally, crystallization occurred for POx dissolved in organic solvents (lack of thermosensitivity), confirming that thermoresponsive behavior is not a precondition for the crystallization of POx but rather that an appropriate saturation of the solution with the polymer is required.21 Theoretical and experimental studies on the mechanism of POxs crystallization in solutions using the example of PiPrOx were conducted mainly by Winnik,24 Wu,25,26 and Atilgan et al.27 It was established that during nucleation cleavage of hydrogen bonds occurs, and water molecules are expelled following chain arrangement due to the amide dipolar orientation. At this stage, the polymer adopts a conformation in which the trans and gauche conformers coexist. Then, a further chain ordering process occurs, and the polymer chains adopt mostly all-trans conformation. This conformation is stabilized and remains predominant upon the cooling of the solution. This conformation also promotes a partial organization of the chains in a dense liquid phase, leading to the crystal growth stage. Although PiPrOx is nontoxic, displays an LCST-type phase separation near the physiological temperature range, and has found some application even for its semicrystalline forms, e.g., as microspheres for carbohydrate−protein recognition22 or layers for human dermal fibroblasts culture and detachment,28 it is known that the precipitated, crystalline phase is not soluble, even after the cooling of the solution. This disqualifies such systems from applications where a thermoresponsive behavior is necessary. Thus, it is highly important to develop a method for the control of the crystallization of PiPrOx and other POxs systems without significantly influencing the LCST. In this work, attempts to control the crystallization of 2isopropyl-2-oxazoline copolymers were made, both in solution and in the condensed state. Copolymers of 2-isopropyl- with 2-npropyl-2-oxazoline were obtained. We expected that the crystallization ability of the copolymer decreases with increasing amounts of nPrOx in the polymer chain. Here, we also examined whether it is possible to completely eliminate the crystallization of 2-isopropyl-2-oxazoline copolymers.
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copolymers (DP) at full conversion, calculated as [M]0/[I]0, was 100. In the case of copolymers, three compositions of comonomers were assumed with the contents of nPrOx equal to 15, 30, and 50 units. For all syntheses, acetonitrile and methyl 4-nitrobenzenesulfonate were used as the solvent and initiator, respectively. Polymerizations were carried out at 75 °C to full conversion of monomers (checked by GC). Then, npropylamine was added, and the mixture was kept for 10 min at room temperature under stirring. Then, acetonitrile was evaporated, water was added, and the obtained polymer was dried by lyophilization. Measurements. The compositions of the copolymers were calculated from the 1H NMR spectra. The spectra were recorded using a Bruker Ultrashield spectrometer operating at 600 MHz in CDCl3 as the solvent. Molar mass and molar mass dispersity of homo- and copolymers were determined in DMF using a GPC MALLS system equipped with a multiangle laser light scattering detector (DAWN EOS, Wyatt Technologies, λ = 658 nm) and a refractive index detector (Δn-1000 RI WGE DR Bures, λ = 620 nm). The results were evaluated using the ASTRA software (Wyatt Technologies). The refractive index increment for copolymers were calculated from the weight ratios and the dn/dc values of the homopolymers. Thermal properties of homo- and copolymers (Tm, crystallization, enthalpy of transitions) were determined by differential scanning calorimetry. DSC measurements were carried out using a TA-DSC Q2000 apparatus (TA Instruments, New Castle, DE) under a nitrogen atmosphere with a flow rate of 50 mL/min. The measurements were taken in the range from 0 to 220 °C. The heating rate for the standard measurement was 10 °C/min and was equal to 2.5 °C/min for the socalled slow measurement. The enthalpy of melting or crystallization (ΔH) was calculated as the area under the peak, limited by the baseline. The data were collected and then analyzed using a Universal Analysis 2000 with Universal V4.5a software. The sample crystallinity was measured by a wide-angle X-ray diffractometer (WAXS) TUR-M62 equipped with a HZG-3 goniometer, using Cu Kα radiation. Calculations of the intensities and positions of peaks were carried out using the WAXSFIT software. The degree of sample crystallinity χc was determined by integrating the intensities of the crystalline reflections and amorphous halos and was calculated as the ratio of the area under the diffraction peaks to the total area of the diffraction curve. Samples from the solutions were prepared by heating the aqueous solutions of copolymers (1 g/L) for 24 h or 12 days at 70 °C. Then, the solutions were frozen in liquid nitrogen and lyophilized. Samples from the melt were prepared by annealing for 1 or 12 h at the copolymer crystallization temperature (previously determined by DSC) and then cooled to room temperature at a rate of 10 °C/min. The morphology of the copolymer structures was analyzed by scanning electron microscopy (SEM). SEM analysis was performed using a ESEM Quanta 250 FEG (FEI Co.) microscope with EverhartThornley detector (ETD) under a high vacuum mode. The preparation of the samples for SEM was as follows: 10 μL aliquots of water suspensions of copolymers (1 g/L) preheated for 24 h at 70 °C or for 12 days at 70 °C were placed on a mica surface, frozen in liquid nitrogen, and lyophilized. Thermosensitivity of the copolymers was analyzed by a Jasco V-530 UV−vis spectrophotometer equipped with a programmable Medson MTC-P1 thermocontroller. The transmittance of the aqueous copolymer solutions (5 g/L) was monitored at the wavelength λ = 550 nm as a function of temperature. The cloud point temperature value (TCP) was defined as the temperature at which the transmittance of the polymer solutions reached 50% of its initial value.
EXPERIMENTAL SECTION
Materials. Butyronitrile (>99%, Aldrich), isobutyronitrile (99.6%, Aldrich), 2-aminoethanol (99%, Aldrich), cadmium acetate (>98%, Fluka), methyl 4-nitrobenzenesulfonate (99%, Aldrich), and n-propylamine (>99%, Aldrich) were used as received. 2-n-Propyl- (nPrOx) and 2-isopropyl-2-oxazoline (iPrOx) were synthesized according to the procedure of Witte and Seeliger.29 Briefly, eqiumolar amounts of butyronitrile or isobutyronitrile were added to 2-aminoethanol, and the mixture was heated under reflux in the presence of cadmium acetate at 100 °C. After the full conversion of reactants (checked by gas chromatography (GC)), the monomer was distilled. Raw nPrOx and iPrOx were dried over KOH, distilled under reduced pressure, then dried over CaH2, and distilled again. Monomers with the purity of 99.8% were used for polymerizations. Acetonitrile (for HPLC, POCH) was dried over CaH2 and distilled. N,N-Dimethylformamide (DMF) for gel permeation chromatography was distilled under reduced pressure, and then 5 mmol/L of LiBr was added. Synthesis of Homo- and Copolymers. Poly(2-isopropyl-2oxazoline) (PiPrOx), poly(2-n-propyl-2-oxazoline) (PnPrOx), and copolymers of iPrOx and nPrOx were obtained via cationic ringopening polymerizations according to a procedure described previously.30 The theoretical degree of polymerization of homo- and
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RESULTS AND DISCUSSION Synthesis and Characterization of Homo- and Copolymers. Poly(2-isopropyl-2-oxazoline) (PiPrOx), poly(2-n-propyl-2-oxazoline) (PnPrOx), and copolymers of iPrOx and nPrOx were obtained via cationic ring-opening polymerization initiated by methyl 4-nitrobenzenesulfonate. The theoretical degree of polymerization was 100 for all polymers and was obtained as [M]0/[I]0. Linear, thermoresponsive polymers with narrow B
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Macromolecules Table 1. Characterization Data of the Obtained Homo- and Copolymers 1
H NMR
symbol
A
PiPrOx P(iPrOx-nPrOx)1 P(iPrOx-nPrOx)2 P(iPrOx-nPrOx)3 PnPrOx
iPrOx iPrOx iPrOx iPrOx
B nPrOx nPrOx nPrOx nPrOx
mol % A 100 85 70 50
GPC MALLS
mol % B
Mn (g/mol)
Mw/Mn
TCP (°C) (UV−vis)
15 30 50 100
15000 13100 17200 11500 8700
(1.01) (1.03) (1.21) (1.09) (1.03)
37 36 31 29 25
molar mass dispersity were obtained, and good agreement of molar mass with the theoretical values was achieved (Table 1). Copolymerization of iPrOx with nPrOx led to copolymers of gradient distribution of comonomers. Precise kinetics of this reaction has been shown elsewhere.13 Briefly, the reactivity ratios for the studied copolymerizations were calculated based on the monomer consumption rates and were equal to rnPrOx = 2.75 and riPrOx = 0.40. The r values were transformed into addition probabilities of the comonomers. A simple Monte Carlo simulation yields visual data regarding the copolymers’ structure. The probable distribution of units along the chain for obtained copolymers of different compositions is presented in Figure 1. A schematic representation of the calculated gradient structure of the obtained copolymers is presented in Figure S1 of the Supporting Information. Figure 2. DSC traces of PnPrOx at heating rates of 10 and 2.5 °C/min and PiPrOx at heating rate of 10 °C/min.
as PiPrOx of the highest obtained degree of crystallinity χc (the highest achieved χc for PiPrOx in condensed state was 68%21). Therefore, PnPrOx was isothermally crystallized according to ref 21. The polymer was annealed for 1 h at the highest temperature recorded by DSC before melting (90 °C) and then cooled to room temperature at a rate of 10 °C/min. This sample was named PnPrOxbulk1. Additionally, PnPrOx was annealed for an extended time (12 h), cooled to room temperature at the same rate, and named PnPrOxbulk2. DSC traces recorded with the heating rate of 10 °C/min, and WAXS curves for both samples are presented in Figure 3. An endothermic, not symmetric peak in the ∼100−140 °C range can be seen for PnPrOx annealed for 1 and 12 h. The presence of two maxima of the peak is possibly due to the imperfect morphologies of the ordered structures. The enthalpy of melting is similar for both samples regardless of the time of annealing and is much higher when compared to nonannealed PnPrOx (Figure 2). On the other hand, ΔH is significantly lower compared to PiPrOx (ΔH = 38 J/g21). The degree of crystallinity is the same for both PnPrOx samples regardless of the annealing time. This may indicate that after 1 h of annealing an equilibrium of PnPrOx crystallization was already achieved. The χc of approximately 55% is the highest obtained degree of crystallinity for PnPrOx in the condensed state. This is substantially lower than for PiPrOx.21 Nonetheless, the common assumption in the literature that PnPrOx is completely amorphous and does not exhibit a tendency to crystallization appears to be unjustified. Crystallization of Copolymers of iPrOx and nPrOx in Condensed State. The crystallization of thermoresponsive PiPrOx-based materials may disqualify such systems from applications where thermoresponsive behavior is necessary. In this work, we aimed to investigate how the crystallization ability of PiPrOx is changed when nPrOx units were incorporated
Figure 1. Probable distribution of units along the chain for P(iPrOxnPrOx)1, P(iPrOx-nPrOx)2, and P(iPrOx-nPrOx)3.
Crystallization of PnPrOx in Condensed State. It was reported that poly(2-n-propyl-2-oxazoline) is not able to crystallize, does not melt, and shows only a glass transition in the vicinity of 40 °C.12 However, for the DSC curves obtained in this work, we observed an endothermic peak in the 120−140 °C range that was assigned to the melting of this polymer. The melting peak is significantly broader, less sharp, and of lower energy (ΔH) than in the case of PiPrOx (Figure 2). The endothermic peaks were observed both for standard DSC measurement (with the heating rate of 10 °C/min) and in the case of heating at a slow rate (2.5 °C/min). Moreover, in contrast to PiPrOx, PnPrOx does not exhibit cold crystallization (reorganization of the polymer chain above Tg leading to crystallization during the DSC measurement) at any applied heating run. This confirms our expectations that PnPrOx has weaker crystallization ability than PiPrOx. We wanted to compare the amount of crystalline fraction of PnPrOx crystallized in the condensed state during the same time C
DOI: 10.1021/acs.macromol.7b01639 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. DSC traces at a heating rate of 10 °C/min (top) and X-ray diffraction curves (bottom) of PnPrOxbulk1 and PnPrOxbulk2.
Figure 4. DSC traces of copolymers of iPrOx with nPrOx at heating rate of 10 and 2.5 °C/min (samples without previous thermal treatment).
into the chain. It seems that the separation of the iPrOx segments in the chain by the units with lower crystallization ability (nPrOx) will probably weaken the strong interactions responsible for organization of the chains into a unit cell. This should disrupt the ordering of the macromolecules and reduce the tendency of the resulting polymer to crystallization without significantly influencing the solution behavior (thermosensitivity). For that purpose, three gradient copolymers of iPrOx with nPrOx were obtained with molar ratios of the second comonomer of 15% (P(iPrOx-nPrOx)1), 30% (P(iPrOxnPrOx)2), and 50% (P(iPrOx-nPrOx)3). The thermal properties of the copolymers were studied by DSC, and the obtained results are shown in Figure 4.
An endothermic peak is observed during the so-called standard DSC measurement (with a heating rate of 10 °C/min) for the copolymer with 15% of nPrOx, indicating the melting of the crystalline phase. The peak is wide (from 170 to 210 °C) and not as sharp and symmetric as in the case of PiPrOx. During this measurement, no cold crystallization is observed, already indicating the lowered tendency to organization of P(iPrOxnPrOx)1 compared to PiPrOx. However, in the case of heating at a slow rate (of 2.5 °C/min), both exothermic (at temperatures ranging from 110 to 170 °C) and subsequent endothermic peaks (from 170 to 190 °C) are observed. These are attributed to cold crystallization and melting of the crystalline phase, respectively. An increase of nPrOx content in the copolymer to 30% resulted in a lack of melting (for the heating rate of 10 °C/min). Ordering D
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Figure 5. DSC traces at a heating rate of 10 °C/min (top) and X-ray diffraction curves (bottom) of isothermally crystallized copolymers of iPrOx with nPrOx (1 h of annealing).
and crystallization of this copolymer at the 130−150 °C temperature range followed by melting at 150−180 °C are observed only during the DSC measurement at the low heating rate. However, these effects are of significantly lower energy (ΔH) than in the case of P(iPrOx-nPrOx)1. It is important to note that for both P(iPrOx-nPrOx)1 and P(iPrOx-nPrOx)2 copolymers the temperatures of crystallization and melting of crystalline fraction are slightly shifted toward lower temperatures compared to PiPrOx. In the case of P(iPrOx-nPrOx)3 (with 50% of nPrOx), neither crystallization nor melting is observed during the standard or slow DSC measurement, indicating the low tendency to crystallize of this copolymer. Thus, it can be concluded that the crystallization ability of PiPrOx may be significantly reduced by increasing the nPrOx content in the copolymer, until 50 mol % of nPrOx is incorporated. The copolymers were subjected to isothermal crystallization. Therefore, the copolymers were annealed for 1 h at chosen temperatures and then cooled to room temperature at a rate of 10 °C/min. The temperature of annealing for copolymers was chosen as the highest temperature value after which an exothermic peak was recorded during the slow DSC measurement. The temperatures were 170 and 150 °C for P(iPrOxnPrOx)1 and P(iPrOx-nPrOx)2, respectively. As is shown in Figure 4, the effect of cold crystallization and subsequent melting of crystalline fraction shifts toward lower temperatures with increasing nPrOx content in the copolymer. Thus, although cold crystallization was not observed for the P(iPrOx-nPrOx)3 at any applied heating run during the DSC measurement, the polymer was annealed at 120 °C. This temperature was named its apparent crystallization temperature. Isothermally crystallized
samples were named P(iPrOx-nPrOx)1bulk1, P(iPrOx-nPrOx)2bulk1, and P(iPrOx-nPrOx)3bulk1. The properties of these copolymers revealed by DSC and WAXS are shown in Figure 5. For P(iPrOx-nPrOx)1bulk1, a broad and sharp endothermic peak with a significantly higher enthalpy compared to the nonannealed P(iPrOx-nPrOx)1 is observed in the DSC trace with the heating rate of 10 °C/min. For P(iPrOx-nPrOx)2bulk1 and P(iPrOx-nPrOx)3bulk1, a small endothermic peak appeared, even though these copolymers did not melt in the case of nonannealed samples (assuming the same DSC heating rate). Additionally, the melting of the crystalline fraction is slightly shifted toward lower temperatures for the obtained copolymers with increasing nPrOx content in the copolymer. This means that annealing of copolymers for 1 h at an appropriate temperature gave rise to the ordering of polymer chains and the formation of a crystalline fraction. However, while for P(iPrOx-nPrOx)1bulk1 the degree of crystallinity is significant (χc ∼ 60%), for the latter copolymers the amorphous phase is predominant (see WAXS curves in Figure 5). This indicates that the crystallization ability decreases with the nPrOx content in the copolymer chain. In the WAXS curve of P(iPrOx-nPrOx)1bulk1, diffraction peaks at 2θ = 7.74° (11.4 Å) and 17.32° (5.1 Å) and two smaller peaks at 21.40° (4.2 Å) and 23.90° (3.7 Å) can be seen. Similar peaks are also present in the PiPrOx diffraction curve,16,21 indicating that these crystalline products have similar unit cells. Crystallization of Copolymers of iPrOx and nPrOx in Water. Except of ability to crystallization in condensed state, some thermoresponsive poly(2-alkyl-2-oxazoline)s tend to crystallize in the solutions. This occurs when the polymer concentration in some regions of the solution reaches a certain E
DOI: 10.1021/acs.macromol.7b01639 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. SEM micrographs (top) and X-ray diffraction curves (bottom) for homo- and copolymers of iPrOx incubated in water (1 g/L, 24 h of annealing at 70 °C).
Figure 7. SEM micrographs. Comparison of sizes (A) and the amounts (B) of the structures of PiPrOxwater1 and P(iPrOx-nPrOx)1water1 after 24 h at 70 °C (1 g/L).
value and so-called polymer-rich phase predominates (solution is supersaturated). Under such conditions, the polymer chains undergo conformational changes that facilitate organization into
unit cell and crystallization. As a result, insoluble, crystalline material is precipitated. In the case of PiPrOx, the crystalline precipitate consists mostly of uniform, spherical particles of F
DOI: 10.1021/acs.macromol.7b01639 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules about 2 μm in diameter, composed of a porous fiber mesh.18 The precipitated, crystalline phase is not soluble, even after the cooling of the solution. Such behavior disqualifies these systems from applications where a thermoresponsive behavior is necessary. To examine whether the nPrOx units in the nPrOx/iPrOx copolymers influence their crystallization ability also in the solution, the copolymers were dissolved in water and incubated at 70 °C for 24 h. Under these conditions, since the temperature is above the phase transition of the copolymers (Table 1), the polymer-rich phase predominates, and this may facilitate the crystallization. The samples of copolymer solutions after incubation at the elevated temperature were named P(iPrOxnPrOx)1water1, P(iPrOx-nPrOx)2water1, and P(iPrOx-nPrOx)3water1. For P(iPrOx-nPrOx)1water1, similarly to incubated PiPrOx, a milky solution was obtained. The precipitate did not dissolve after cooling the solution to room temperature, indicating the crystallization of the copolymer. Conversely, for P(iPrOxnPrOx)2water1 and P(iPrOx-nPrOx)3water1 a milky solution after changing the temperature to room value became transparent, and a precipitate was no longer observed. Thus, it seems that no crystallization occurred. The morphology and crystallinity of the structures formed under the prolonged incubation of copolymers (24 h at 70 °C) were compared to the structures of the incubated PiPrOx (Figure 6). Structures obtained as a result of prolonged annealing of the aqueous solution of P(iPrOx-nPrOx)1 are slightly different from those obtained from the incubation of the PiPrOx solution. In contrast to the incubated PiPrOx structures, no individual spherical particles are present for P(iPrOx-nPrOx)1water1 because these merge with each other into clusters. Additionally, they have smooth surfaces without fibrils, unlike PiPrOx “cotton balls” that are composed of a porous fiber mesh.18 They are also slightly smaller by approximately 0.5 μm (Figure 7A), and their number is significantly lower than in the case of PiPrOx (Figure 7B). While examination of WAXS curve confirms that an amorphous fraction predominates in the P(iPrOx-nPrOx)1water1 sample, a small but pronounced diffraction peak at 2θ = 7.74° indicates the presence of a very low amount of the crystalline fraction (the high content of amorphous fraction does not permit the calculation of χc). This means that the P(iPrOx-nPrOx)1water1 structures, formed under prolonged incubation in the polymerrich phase state, are considerably less crystalline than those obtained for the heated PiPrOx solution. This may indicate that the nPrOx units disrupt the ability of PiPrOx to crystallize in water. The crystallization ability decreases with the nPrOx content in the investigated copolymers. In the case of P(iPrOxnPrOx)2water1 and P(iPrOx-nPrOx)3water1, although the solutions were incubated in the polymer-rich phase state, no precipitation was observed when the temperature was lowered to the room temperature. This indicates a lack of crystallization in the solution, confirming the conclusion drawn from the WAXS curves where no diffraction peaks are observed. SEM micrographs of these copolymers show smooth structures of several nanometers resembling grains that stick with each other for P(iPrOx-nPrOx)2water1, while in the case of P(iPrOx-nPrOx)3water1 no structures of defined morphology can be seen at all. To summarize briefly, it appears that the copolymerization of iPrOx with nPrOx in the amount of 15 mol % led to copolymers that have a tendency to crystallize both in the condensed state and in water, similar to the homopolymer of PiPrOx. The copolymer with 30 mol % of nPrOx is able to crystallize to a small
degree in the condensed state but does not exhibit crystalline properties when the solution is heated in the polymer-rich phase state for 24 h in water. Incorporation of 50 mol % of nPrOx results in a copolymer that is not able to crystallize both in the condensed state and in water (after 24 h of heating). Is It Possible To Completely Eliminate the Crystallization of iPrOx−nPrOx Copolymers? It is known that many factors influence the crystallization of proteins (to which POxs are often compared) and some polymers.31 Some of these are highly crystalline at room temperature without any thermal treatment, while others require prolonged incubation at elevated temperature or nucleating agents for crystallization. We sought to determine whether the crystallization could be forced for a seemingly amorphous (co)poly(2-oxazoline)s (e.g., P(iPrOxnPrOx)3) or intensified for POx with a low tendency to crystallize (e.g., P(iPrOx-nPrOx)2) both in the condensed state and in water. Therefore, in this study, we proposed more rigorous crystallization conditions and the copolymers were exposed to a high, predefined temperature for a significantly prolonged time. In the case of copolymers in the condensed state, they were annealed for 12 h (instead of 1 h) at their temperature of crystallization (150 °C for P(iPrOx-nPrOx)2) or at apparent temperature of crystallization (120 °C for P(iPrOx-nPrOx)3) and then cooled to room temperature at a rate of 10 °C/min. Copolymers prepared in this way were named P(iPrOxnPrOx)2bulk2 and P(iPrOx-nPrOx)3bulk2, respectively. P(iPrOx-nPrOx)3 (50 mol % of nPrOx) did not exhibit crystalline properties both in condensed state and in water (see Figures 4 and 6). DSC traces presented in Figure 8 show how the time of annealing of a seemingly amorphous copolymer changes its thermal properties.
Figure 8. DSC traces at a heating rate of 10 °C/min of P(iPrOxnPrOx)3 (without previous thermal treatment), P(iPrOx-nPrOx)3bulk1 (annealed for 1 h), and P(iPrOx-nPrOx)3bulk2 (annealed for 12 h).
The P(iPrOx-nPrOx)3 directly after synthesis and without any thermal history does not show melting. The annealing of this copolymer at its apparent crystallization temperature (120 °C) for 1 h (P(iPrOx-nPrOx)3bulk1) or 12 h (P(iPrOx-nPrOx)3bulk2) caused the melting peaks to appear in the DSC traces (Figure 8). Thus, we can conclude that the crystalline fraction in the copolymer is formed during prolonged annealing and subsequent slowly cooling to room temperature (conditions of isothermal crystallization), even if this copolymer did not previously show any crystalline fraction. As the time of annealing is extended, the more pronounced and sharp endothermic peak G
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Figure 9. DSC traces at a heating rate of 10 °C/min (top) and X-ray diffraction curves (bottom) of P(iPrOx-nPrOx)2bulk2 and P(iPrOx-nPrOx)3bulk2 (12 h of annealing).
nPrOx content, as seen in the DSC trace (Figure 9). Additionally, for the higher degree of crystallinity, the higher value of ΔH is observed according to the known relationship described in ref 21. Simultaneously with studies of the copolymers’ crystallization in the condensed state, the copolymers were incubated in water at the polymer-rich phase state (at 70 °C) for a significantly extended time12 days (instead of 24 h). The copolymer solutions after this incubation time were named P(iPrOxnPrOx)2water2 and P(iPrOx-nPrOx)3water2. Slightly milky solutions with the precipitate were observed when the solutions were cooled to room temperature after incubation. The morphology and crystallinity of structures formed under prolonged incubation of both copolymer samples were studied by SEM and WAXS (Figure 10). The morphology of the structures obtained after annealing the aqueous solutions of copolymers for 12 days is significantly different from those annealed for 24 h (P(iPrOx-nPrOx)2water1 and P(iPrOx-nPrOx)3water1, Figure 6). Here, a network-like structure with separate objects possessing a fibril-like morphology can be distinguished. The length of the fibrils ranged from a few to several micrometers, with widths of several nanometers. No spherical, compact particles of regular diameter and shape that are composed of fibers are observed as in the case of the crystalline material obtained during the prolonged incubation of PiPrOx in water.18 On the other hand, a similar network-like structure of the precipitate was observed for the PiPrOx crystallized in nonaqueous media.21
can be seen in the DSC trace, with the significantly higher value of ΔH. This suggests that the crystallization of apparently amorphous POx can be “forced” in this way. DSC and WAXS curves of P(iPrOx-nPrOx)2bulk2 and P(iPrOx-nPrOx)3bulk2 are compared in Figure 9. For P(iPrOx-nPrOx)2bulk1 and P(iPrOx-nPrOx)3bulk1 annealed for 1 h, where the amorphous phase was predominant (Figure 5), increasing the time of annealing to 12 h led to an increase in the degree of crystallinity to 50% and 40%, respectively. This means that for the copolymers with the predominant amorphous fraction the amount of the crystalline phase could be significantly raised by prolonged annealing at elevated temperature. Although we did not investigate the crystallization equilibrium, on the basis of literature data, we assume that the reached crystallinity is probably close to equilibrium.21 For comparison, P(iPrOx-nPrOx)1, which is already highly crystalline after 1 h of annealing at its crystallization temperature (60% of crystalline fraction, see Figure 5) after prolonged heating (170 °C for 12 h, then cooled to room temperature at the rate of 10 °C/min; P(iPrOx-nPrOx)1bulk2), exhibited the same degree of crystallization as that annealed for 1 h (see Figure S2). Thus, it may be concluded that 60% is the highest possible content of the crystalline fraction in the iPrOx/nPrOx copolymers (comparatively, the highest degree of crystallinity achieved for iPrOx homopolymer in the condensed state was χc ∼ 68%21). Similarly, as in the case of copolymers annealed for 1 h, the melting peak of the copolymers annealed for 12 h is slightly shifted toward lower temperatures with the increasing amount of H
DOI: 10.1021/acs.macromol.7b01639 Macromolecules XXXX, XXX, XXX−XXX
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Figure 10. SEM micrographs (top) and X-ray diffraction curves (bottom) of P(iPrOx-nPrOx)2water2 and P(iPrOx-nPrOx)3water2 (1 g/L, 12 days of annealing at 70 °C).
obtained during PiPrOx crystallization in water were obtained. Incorporation of 30% and 50% of nPrOx led to copolymers with low crystallization ability in condensed state. However, much prolonged annealing of these copolymers led to a significant increase of their crystallinity to χc ∼ 50% and χc ∼ 40%, respectively. Additionally, copolymers of 30% and 50% of nPrOx did not crystallize in water in the polymer-rich phase state under standard conditions. Only much prolonged annealing led to the precipitate of fibril-like morphology (similar to PiPrOx crystallites in nonaqueous media) with a very small amount of the crystalline fraction. Incorporation of a certain amount of nPrOx allows the control of the crystallization process and enables us to obtain copolymers with desired χc. It is not possible to completely eliminate the crystallization of nPrOx/iPrOx copolymers because in every case the chain ordering and crystallization may be forced by very prolonged annealing at an elevated temperature, both in condensed state and in solutions. This should be taken into account when planning possible applications of nPrOx/iPrOx copolymers.
In the WAXS curves, a very small characteristic diffraction peak at 2θ = 7.74° can be seen for both copolymers and at 2θ = 17.32° only for P(iPrOx-nPrOx)2water2, indicating a very small amount of the crystalline fraction in the precipitate (the predominant amorphous fraction does not permit the calculation of χc). Both WAXS curves and morphology studies indicate that structures of P(iPrOx-nPrOx)2water2, and P(iPrOx-nPrOx)3water2 formed under significantly prolonged incubation of polymer solutions for 12 days in the polymer-rich phase state, despite a predominated amorphous fraction, are more crystalline than those obtained from incubation of these copolymers for 24 h. It can be concluded that the crystallization of apparently amorphous 2-oxazoline copolymers can be “forced” by prolonged annealing of copolymers both in solution and in condensed state at an elevated temperature. While crystallization in water takes much longer and the final obtained material is predominantly amorphous, in the condensed state crystallization occurs faster and to a much greater degree.
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SUMMARY AND CONCLUSIONS The control of the crystallization of nPrOx/iPrOx copolymers was attempted both in condensed state and in solution. The homopolymer of nPrOx had a lower tendency to crystallize when compared to PiPrOx, and χc ∼ 55% was its highest achieved degree of crystallinity in condensed state. Copolymerization of nPrOx with iPrOx led to gradient copolymers of lowered crystallization ability compared to PiPrOx. Incorporation of 15% of nPrOx led to copolymers crystallized in condensed state with crystallinity of up to χc ∼ 60%. This copolymer also crystallized in aqueous solution in the polymer-rich phase state up to χc ∼ 45%, and spherical particles that are somewhat similar to those
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01639. Schematic representation of the calculated gradient structures of copolymers, DSC trace and X-ray diffraction curve of P(iPrOx-nPrOx)1bulk2 sample (PDF) I
DOI: 10.1021/acs.macromol.7b01639 Macromolecules XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Author
*(N.O.-T.) E-mail:
[email protected]. ORCID
Natalia Oleszko-Torbus: 0000-0002-2315-9343 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Science Centre, project 2016/21/D/ST5/01951. The authors are indebted to Anna Hercog (Centre of Polymer and Carbon Materials, Polish Academy of Sciences) for SEM measurements, to Dr. Henryk Janeczek (Centre of Polymer and Carbon Materials, Polish Academy of Sciences) for DSC measurements, and to Dr. Marian Domański (Centre of Polymer and Carbon Materials, Polish Academy of Sciences) for WAXS measurements.
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