Periodically Grafted Amphiphilic Copolymers: Effects of Steric

May 18, 2015 - Citation data is made available by participants in Crossref's Cited-by Linking service. ... Criss-cross amphiphilic polymers and analog...
2 downloads 0 Views 5MB Size
Download PDF
Article pubs.acs.org/Langmuir

Periodically Grafted Amphiphilic Copolymers: Effects of Steric Crowding and Reversal of Amphiphilicity Joydeb Mandal and S. Ramakrishnan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: Two series of periodically clickable polyesters were prepared; one of them carries alkylene segments along its backbone, whereas the other carries poly(ethylene glycol) (PEG) segments. These polyesters were clicked with either MPEG-350 azide or docosyl (C22) azide to yield periodically grafted amphiphilic copolymers (PGACs) carrying either flexible hydrophilic or crystallizable hydrophobic backbone segments. The immiscibility between hydrocarbon and PEG segments causes both of these systems to fold in either a zigzag or hairpin-like conformation; the hairpin-like conformation appears to be preferred when flexible PEG segments are present in the backbone. The folded chains further reorganize in the solid state to develop a lamellar morphology that permits the collocation of the PEG and hydrocarbon (HC) segments within alternate domains; evidence for the self-segregation was gained from DSC, SAXS, and AFM studies. SAXS studies revealed the formation of an extended lamellar structure, whereas AFM images showed uniform layered morphology with layer heights that matched reasonably well with the interlamellar spacing obtained from the SAXS study. Labeling one representative PGAC, carrying crystallizable long alkylene segments in the backbone and pendant PEG-350 side chains, with a small mole fraction of pyrene fluorophore permitted the examination of the conformational transition that occurs upon going from a good to a poor solvent; this single-chain folded conformation, we postulate, is the intermediate that organizes into the lamellar morphology.

■

intervals.12 These PGACs also adopted a folded zigzag conformation because of immiscibility-driven self-segregation of the hydrophilic pendant oligoethylene glycol segments and the hydrophobic alkylene backbone segments. Unlike in previously studied systems, it was observed that the alkylene segments crystallize and provide extra stability to the folded zigzag form. More remarkably, instead of PEG segments, if pendant perfluoroalkyl segments are incorporated, we recently showed that the immiscibility-driven folding of chains occurs in a similar manner so as to facilitate independent crystallization of both the perfluoroalkyl and backbone alkylene segments.13 To further elaborate the scope of such immiscibility-driven conformational control, we first describe here a straightforward synthesis of a series of periodically grafted amphiphilic copolymers (PGACs) carrying either one or two MPEG-350 [poly(ethylene glycol) 350 monomethyl ether] units at periodic intervals. The polymers were prepared by reacting the parent periodically clickable polyesters with MPEG-350 azide using the Cu(I)-catalyzed azide-yne click reaction, as depicted in Scheme 1. Because the PGACs carry either one or two pendant MPEG-350 chains on each repeat unit, it permitted us to examine the effect of steric crowding on the folding and crystallization of the central alkylene segment. On the basis of

INTRODUCTION Amphiphilic polymers of different types, such as block copolymers, graft copolymers, core−shell dendrimers, and hyperbranched polymers, etc., have been shown to undergo self-assembly to form a variety of interesting aggregate morphologies.1,2 Many of the previous studies of such amphiphilic systems have focused on generating aggregated structures in solution; one of the primary objectives of such studies has been to explore the possibility of using the watersoluble aggregates as drug-delivery vehicles.3−7 Another class of interesting amphiphilic polymers that has witnessed a resurgence of interest is ionenes;8 these are polymers containing charged units at periodic intervals along the backbone and may be viewed as surfactants linked in a headto-tail fashion to form a linear chain. Early studies by Kunitake et al.9,10 suggested that symmetric ionenes carrying long alkylene segments (C > 12) adopt an interesting accordiontype zigzag conformation in aqueous solutions. Some years ago, in an effort to further reinforce the folding of ionene-type polymers, we designed a novel ionene bearing donor and acceptor units in alternate alkylene segments, and we showed that this DA-ionene forms a very stable zigzag folded conformation in aqueous solution that is reinforced by charge-transfer interactions.11 More recently, we developed a nonionic analogue of ionene based on a periodically grafted amphiphilic copolymer (PGAC) containing long alkylene segments and pendant oligoethylene glycol chains at periodic © 2015 American Chemical Society

Received: April 7, 2015 Revised: May 15, 2015 Published: May 18, 2015 6035

DOI: 10.1021/acs.langmuir.5b01227 Langmuir 2015, 31, 6035−6044

Article

Langmuir

Scheme 1. Structures of the Two Types of Clickable Polyesters and Their Modification to Generate Two Series of PGACs with Reversed Amphiphilicity

Figure 1. 1H NMR spectra of the parent polyester, PE-MP, and the three clicked polyesters, namely, PE-MP-PEG 350, PE-DP-PEG 350, and PE-APPEG 350. The highlighted regions show the complete disappearance of the propargyl proton and the appearance of the triazole ring proton after the azide-yne click reaction.

the observations from DSC, SAXS, AFM, and fluorescence spectroscopy, we show that steric crowding at the graft junctions has a dramatic effect on the crystallization of the backbone alkylene segment and hence on the self-segregationinduced folding of the polymer backbone. Furthermore, to explore the effect of the reversal of amphiphilicity, we designed a second series of clickable polyesters carrying backbone PEG segments of varying length, namely, PEG 300, PEG 600, and PEG 1000; these polyesters were then clicked with docosyl (C22) azide to generate the desired PGACs with crystallizable alkyl pendant units (Scheme 1). Hydrophobically modified water-soluble polymers bearing randomly grafted long-chain alkyl segments have been investigated extensively, primarily to understand their solution behavior and self-assembly properties.14−19 However, analogous polymers with periodically located hydrophobic graft segments have received far less

attention; early reports of such systems bearing well-defined PEG segments along the backbone with pendant hydrophobic alkyl chains were explored as potential steric stabilizers for liposomes, wherein the looped PEG segments served to prevent protein absorption and enhance circulation times.20,21 Most of the earlier reports of such amphiphilic comb polymer systems have focused on their solution behavior,22 whereas few have examined their solid-state morphology; exceptions are the reports by Wright and co-workers,23−26 who utilized the periodically located hydrophobic crystallizable alkyl (C16 or C18) segments to generate lamellar morphologies and exploited the PEG domains for the dissolution of Li salts to attain good ionic conductivities. In the present study, the length of the PEG segments along the backbone was varied whereas the pendant docosyl (C22) segment was left unchanged; the objective was to examine the morphology of such systems and 6036

DOI: 10.1021/acs.langmuir.5b01227 Langmuir 2015, 31, 6035−6044

Article

Langmuir

Figure 2. 1H NMR spectra of the parent polyester, PE-PEG 300, and the three clicked polyesters, namely, DOCO-PEG 300, DOCO-PEG 600, and DOCO-PEG 1000. The highlighted regions show the complete disappearance of the propargyl proton and the appearance of the triazole ring proton after the azide-yne click reaction.

also examine the influence of the PEG chain length on the selfsegregation and crystallization of the pendant alkyl group. On the basis of the observations from DSC, SAXS, and AFM, it is proposed that the PEG backbone of these polymers adopts a hairpin-like conformation, which in turn permits the collocation and crystallization of the pendant alkyl segments.

On the other hand, in order to synthesize PGACs carrying crystallizable long hydrophobic alkyl pendant chains and a flexible hydrophilic PEG backbone, we prepared a second series of clickable hydrophilic polyesters carrying PEG segments of different lengths using the standard trans-esterification procedure; here, 2-propargyl dihexylmalonate was melt condensed with PEG of different molecular weights, namely, PEG300, PEG600, and PEG1000, in the presence of titanium isopropoxide as a transesterification catalyst at 150 °C. We chose 2-propargyl dihexylmalonate instead of 2-propargyl diethylmalonate as the monomer in order to reduce the stoichiometric imbalance that could be created by the evaporative loss of the diethyl malonate monomer during the course of polymerization; the longer hexyl chains raised the boiling points considerably while at the same time 1-hexanol was readily removed from the polymerization mixture by application of reduced pressure to drive the equilibrium to polymer formation. 2-Propargyldihexylmalonate was in turn prepared by the trans-esterification of 2-propargyl diethylmalonate in the presence of excess hexanol. Unlike the first series of polyesters, in this case, because we are using commercially available PEGs, an inevitable distribution of chain lengths would lead to a statistical distribution of segment lengths between grafting junctions; this clearly lowers the precision of the periodicity to some extent. The hydrophilic polyesters were labeled as PE-PEG 300, PE-PEG 600, and PEPEG 1000 reflecting the molecular weight of the PEG segments. The 1H NMR spectra of all three polymers, expectedly, look similar; the only difference observed in the spectra is the increase in relative intensity of the peak at ∼3.6 ppm corresponding to the PEG backbone reflecting the higher PEG content on going from PE-PEG 300 to PE-PEG 1000. The nearly complete disappearance of peaks corresponding to possible end groups upon polymerization suggests the formation of a moderately high molecular weight polymer (Figure S1, Supporting Information). All three polyesters were clicked with docosyl (C22) azide; the 1H NMR spectra of the

■

RESULTS AND DISCUSSION Because our primary objective was to study the effect of crystallization of either the backbone or pendant side chains on the polymer chain conformation, we designed two different series of periodically grafted amphiphilic copolymers (PGACs) carrying either a crystallizable backbone or pendant side chains. The PGACs carrying a crystallizable backbone were prepared by reacting the periodically clickable polyesters developed by us earlier13 with MPEG-350 azide using an azide-yne click reaction (Scheme 1). The clicked polymers were isolated after purification by three reprecipitations using THF/diethyl ether. The clicked polymers are abbreviated as PE-MP-PEG 350, PE-DP-PEG 350, and PE-AP-PEG 350; here MP stands for monopropargyl, DP stands for dipropargyl, and AP stands for allyl-propargyl, whereas PEG-350 indicates that all three polymers carry MPEG-350 pendant groups. The third polyester, namely, the one bearing one allyl and one propargyl group, was prepared to explore the possibility of orthogonally clicking two different segments using an organic azide and an organic thiol. However, our preliminary efforts to click the allyl group after clicking with the MPEG-350 azide were unsuccessful; we believe that this could be due to the inaccessibility of the buried allyl group once MPEG-350 segments have been grafted. The NMR spectra of the clicked polymers confirmed the quantitative reaction, as evident from Figure 1; in all cases, the spectra show the complete disappearance of the peak associated with the pendant propargyl group (∼2.0 ppm) and the appearance of a peak associated with the triazole ring proton (∼7.5 ppm). 6037

DOI: 10.1021/acs.langmuir.5b01227 Langmuir 2015, 31, 6035−6044

Article

Langmuir

Figure 3. DSC thermograms of different PGACs. The left panel depicts the heating/cooling thermograms of hydrophobic PGACs with pendant MPEG units, and the right panel shows the thermograms of the hydrophilic PGACs with pendant C22 chains. All scans were performed at a heating rate of 10°/min; the indicated enthalpies associated with the transitions (during heating scans, except in the case of DOCO PEG 1000) were normalized with respect to the weight fraction of the alkylene segment in the polymer.

values thus obtained are listed in the figure. It is evident that as the backbone PEG segment length increases, the melting temperature and the normalized enthalpy decrease, suggesting poorer organization of the pendant chains. In the case of hydrophobic PGACs with pendant MPEG chains, all three samples exhibited single melting and crystallization peaks; the sample with a single MPEG segment exhibited the highest melting temperature (54 °C) and normalized enthalpy (w.r.t. alkyl segment weight-fraction) and the one with two MPEG segments exhibited the lowest (11 °C) values, whereas the one with one MPEG and one allyl group exhibited an intermediate value (37 °C). The subambient melting peak (11 °C) in the polymer with two MPEG units was intriguing; in order to gain a clearer understanding, we carried out variable-temperature IR spectroscopic measurements. It is well known that symmetric and asymmetric C−H stretching vibrations of n-alkanes in a crystalline paraffinic lattice shift to higher wavenumbers upon melting.27,28 The relevant region of IR spectra of PE-DP-PEG 350, recorded at various temperatures, was plotted along with the variation of νsym and νasym stretching frequencies versus temperature (Figure S2A); both peak positions remain invariant up to 11 °C and begin to shift to higher wavenumber at 14 °C and finally level off at 23 °C. This confirmed that the subambient melting peak (11 °C) exhibited by PE-DP-PEG 350 was indeed due to the melting of the backbone alkylene segment. Similar variable-temperature IR spectroscopic measurements were also carried out for the other two samples, permitting us to draw similar conclusions about the origin of the melting peaks (Figure S2).

clicked polymers clearly confirmed a quantitative reaction. The spectra of all three clicked polyesters, along with that of one representative parent polyester, namely, PE-PEG 300, are shown in Figure 2; the complete disappearance of the peaks due to the propargyl group (∼2.0 ppm) and the appearance of a peak associated with the triazole ring proton (∼7.4 ppm) confirmed the quantitative reaction. The clicked polymers were labeled as DOCO-PEG 300, DOCO-PEG 600, and DOCOPEG 1000. The molecular weights of the clicked polymers were estimated by GPC using polystyrene standards; the molecular weight (Mw) varied from 5600 to 21 400 g mol−1 (Figures S9− 10). The molecular weights of the polyesters bearing PEG segments in the backbone were significantly lower, possibly reflecting the lower reactivity of the alkylated malonate monomer toward trans-esterification.

■

THERMAL ANALYSIS The choice of a long alkylene backbone or pendant docosyl segments was to drive intrachain intersegment immiscibility while utilizing their strong crystallization tendency as the first evidence of self-segregation. The DSC thermograms of all of the clicked polyesters are shown in Figure 3. First, we shall examine the hydrophilic PGACs that carry pendant docosyl segments; both DOCO-PEG 300 and DOCO-PEG 600 exhibit single sharp melting and crystallization transitions, whereas DOCO-PEG 1000 exhibited a single melting peak during heating and two separate transitions during the cooling scan. The sharp melting peaks in the first two cases were ascribed to docosyl segments; the melting enthalpies were normalized with respect to the weight fraction of the alkyl segment, and the 6038

DOI: 10.1021/acs.langmuir.5b01227 Langmuir 2015, 31, 6035−6044

Article

Langmuir Scheme 2. Possible Folding Pathways for Self-Segregation of the Immiscible Backbone and Pendant Side Chains

600) do not exhibit any melting transition, which suggests that a hairpin-like conformation is adopted; this would clearly impede effective crystallization of the PEG segments. Hence, lowering both the melting points and the normalized enthalpies with increases in the PEG segment length could be taken as evidence of the hairpin-like conformation of the PEG backbone. It is known that PEG chains adopt a helical conformation29 in the crystalline phase, which may require a certain minimum number of units to attain stability; this threshold apparently is not met in DOCOPEG 600. On the other hand, in the case of DOCOPEG 1000 two crystallization exotherms are seen during the cooling scan, one of which we ascribed to the crystallization of the PEG backbone and the other we ascribed to the pendant hydrocarbon chain. A closer look at the two different kinds of systems reveal that, although the relative volume fractions of the crystallizable alkylene segment are similar in both PE-MP-PEG 350 and DOCOPEG 300, both the melting transition and normalized enthalpy are higher in the case of PE-MP-PEG 350. This suggests that self-segregation between backbone and pendant side chains is most effective when the backbone adopts a folded zigzag conformation assisted by the strong crystallization tendency of the backbone.

It can be envisaged that the self-segregation of the PEG and alkyl segments could occur in two different ways as depicted in Scheme 2. In one case, the backbone adopts a folded zigzag conformation (path A), allowing the collocation of the pendant segments on either side. In the other case, the backbone segments adopt a hairpin-like conformation (path B) forcing all of the pendant segments to collocate on one side. It is evident that the crystallization of the backbone would occur readily only if the polymer backbone adopts a folded zigzag conformation (path A), whereas the crystallization of pendant side chains is possible in either case. The PGACs carrying crystallizable alkylene backbone are more likely to undergo selfsegregation via path A (because the 20-carbon backbone alkylene segment is not long enough to permit crystallization via path B). Interestingly, we observed that both the melting temperature and the normalized enthalpy in the case of PGACs carrying crystallizable long alkylene chains in the backbone are highest for PE-MP-PEG 350 (54 °C, 197 J g m−1) and lowest for PE-DP-PEG 350 (11 °C, 133 J g m−1). It is therefore evident that on going from PE-MP-PEG 350 to PE-DP-PEG 350 there is a substantial increase in steric crowding at junctions. The result is that the large volumes of the two pendant PEG segments would leave a lot more volume than can be filled by the close packing of the hydrocarbon backbone. Consequently, the crystallization of the hydrocarbon backbone will occur less effectively, which in turn lowers both transition temperatures and the enthalpies associated with these transitions. In the sample bearing an additional allyl group, namely, PE-AP-PEG 350, both the melting point and the normalized enthalpy lie in between the two extremes (37 °C, 168 J g m−1); this suggests that the allyl group is less disruptive than one additional MPEG-350 segment toward the crystallization of the central backbone alkylene segments. Although PEG segments are also known to crystallize, in all three samples the PEG length is too small to induce independent crystallization. It is important to recognize here that if the backbone adopts a folded zigzag conformation (path A) then the increase in the backbone-segment length would exert little influence on the crystallization of pendant side chains but if the polymer backbone adopts a hairpin-like conformation (path B) then the increase in the relative volume of the backbone segment will leave a lot more volume than can be filled by the crystallization of the pendant side chains. Interestingly, we observed that both the melting temperature and the normalized enthalpy in the case of PGACs carrying crystallizable long alkylene pendant side chains are highest for DOCOPEG 300 (56 °C, 114.3 J g−1) and lowest for DOCOPEG 1000 (31 °C, 69 J g−1). It is important to mention here that though pure PEG 600 is crystalline, the PEG segments in the polyester (DOCOPEG

■

SMALL-ANGLE X-RAY SCATTERING (SAXS) STUDY To understand the ordering on longer length scales that could result from the organization of individual folded chains, smallangle X-ray scattering (SAXS) measurements were carried out for both kinds of PGACs; both samples exhibit multiple scattering peaks with peak positions in the ratio of 1:2 (Figure 4), suggesting the formation of an ordered lamellar structure. The lamellar ordering clearly results from the alternate layering of crystalline hydrocarbon domains and amorphous PEG domains. It is important to mention here that we did not observe any discernible scattering profile from DOCOPEG 1000, which was also further reconfirmed by the absence of any regular pattern in the AFM images. The d spacing obtained from the SAXS study for PE-MPPEG 350 was 5.8 nm, which reflects the dimension (thickness) of the zigzag folded polymer chain, as depicted in Scheme 3. Polymers PE-AP-PEG 350 and PE-DP-PEG 350 also exhibit similar kinds of morphology, but the scattering peaks were weak and diffuse with d spacings of 5.4 and 4.7 nm, respectively; peaks only up to second order were seen, suggesting that the extent of long-range order is lower. Interestingly, it was observed that an increase in steric crowding on going from PE-MP-PEG 350 to PE-DP-PEG 350 reduces the d spacing substantially from 5.8 to 4.7 nm; this might be due to the fact that the increase in steric crowding forces the 6039

DOI: 10.1021/acs.langmuir.5b01227 Langmuir 2015, 31, 6035−6044

Article

Langmuir

DOCOPEG 600 reflects the increase in PEG segment length, which forms every alternate lamellar domain. It is needless to add that the d spacings obtained for both samples were larger than that expected for a single monolayer; it may therefore be concluded that bilayers are formed through the interdigitation of the pendant alkyl side chains while two PEG domains of adjacent layers form the alternating domains. It is important to recognize that the alternate arrangement (path A) may also yield similar values for the interlamellar spacing; therefore, it is difficult to distinguish between these two possibilities from the SAXS studies alone.30

■

ATOMIC FORCE MICROSCOPIC STUDY

To visualize the assembly of the collapsed macromolecules, we carried out atomic force microscopy (AFM) analysis of both systems. Samples for AFM study were prepared by spin coating a dilute solution (0.25 mg mL−1 in THF) of the polymer on freshly cleaved mica surfaces. A typical AFM height image of polymer PE-MP-PEG 350 is shown in Figure 5. The line scans revealed the formation of flat 2-D interconnected patterns with a remarkably uniform height of 5.8 nm, which exactly matches the d spacing obtained from SAXS measurement. In many regions, the line scans also reveal the formation of doublelayered structures with nearly double the thickness. Similarly, we carried out an AFM study of PE-AP-PEG 350, but interestingly, the height (3.7 nm) obtained (Figure S3) for this system is significantly lower than the d spacing measured by SAXS (5.4 nm). This lower value, however, was uniform throughout the sample. However, upon thermal annealing at 60 °C for 4 h, the height increases to 4.5 nm (Figure S3 B); this appears to reflect a refinement in the self-segregation process during thermal annealing. Sample PE-DP-PEG 350, on the other hand, did not exhibit any regular structure; this is due to the fact that the alkyl segment in PE-DP-PEG 350 is not in a crystalline state at room temperature (Figure 3). PGACs belonging to the other series, namely, DOCOPEG 600, also exhibited a similar kind of 2-D interconnected network pattern; the line scans reveal that the network is built up with very uniform flat structures of height 7.3 nm. Here again, in certain regions multiple layers are clearly evident; remarkably, the thicknesses of the stacked layers are nearly identical, confirming the lamellar morphology as seen in the SAXS profile. However, the height of the layers estimated by AFM (7.3 nm) is considerably larger than that estimated by SAXS (6.3 nm); this may reflect some specific aspects related to thin film formation, which are unclear at this time. The

Figure 4. SAXS profiles of different PGACs, showing lamellar morphology.

alkylene (C18) backbone to adopt considerable numbers of gauche conformations (ineffective crystallization), which was also evident from both the lower melting temperature and the normalized enthalpy in these samples. On the other hand, a comparison of the SAXS profiles of DOCOPEG 300 and DOCOPEG 600 clearly shows that DOCOPEG 300 has developed a more well-defined lamellar structure, which is consistent with the DSC observations. The d spacings estimated from SAXS profiles were 5.2 and 6.3 nm for DOCOPEG 300 and DOCOPEG 600, respectively; the increase in d spacing on going from DOCOPEG 300 to

Scheme 3. Proposed Mechanism for Enhanced Excimer Formation in Pyrene-Labeled PE-MP-PEG 350, during the Collapse of the Polymer Chain with Increasing Amounts of a Poor Solvent

6040

DOI: 10.1021/acs.langmuir.5b01227 Langmuir 2015, 31, 6035−6044

Article

Langmuir

Figure 5. AFM height image of PE-MP-PEG 350 showing the formation of both a monolayer and bilayer.

Figure 6. AFM images of DOCOPEG-600 showing the formation of a multilayered assembly. (Top) Height image along with a line scan. (Bottom) Amplitude image revealing the formation of a multilayered assembly.

the height observed for the disklike assembly. DOCOPEG 1000, on the other hand, did not exhibit any regular pattern; in this sample, the large mismatch in the relative volumes of the PEG and alkyl segments appears to hinder the formation of flat lamellar morphology.

amplitude image reveals the multilayer structures even more clearly than the height images, as seen in Figure 6. Similarly, AFM analysis of sample DOCOPEG 300 was also carried out; in this case, isolated circular domains were seen (Figure S4); the height of these domains confirms flat platelike structures with a uniform height of 6.6 nm. Here again, the values are larger than those estimated by SAXS (5.2 nm). Interestingly, in some places flat structures with an average height of 2.5 nm (Figure S5) are also seen; the height is half of

■

CONFORMATIONAL TRANSITION IN SOLUTION Fluorescence spectroscopy has been extensively used for decades to study macromolecular conformation in solu6041

DOI: 10.1021/acs.langmuir.5b01227 Langmuir 2015, 31, 6035−6044

Article

Langmuir

Figure 7. Variation of fluorescence spectra of pyrene-labeled PE-MP-PEG 350 in THF (left) as a function of added water; all spectra are normalized with respect to the invariant peak at 376 nm. Variation of normalized excimer peak intensity (a.u.) with solvent composition (vol % of water in THF), depicted on the right.

tion.31−33 To examine the evolution of the conformation of the periodically grafted amphiphilic copolymers, we incorporated a fluorophore, namely, pyrene, onto the backbone of parent polymer PE-MP; this was done by clicking 95 mol % of the propargyl groups with PEG 350 azide and the remaining 5% with pyrene azide. The exact amount of pyrene incorporated was estimated by 1H NMR spectroscopy (Figure S6) and was found to be ca. 5%. To examine if the incorporation of pyrene has impacted the self-segregation, we carried out DSC studies of the pyrene-functionalized polymer; the thermogram was only marginally different from that of the parent PGAC, confirming that low levels of pyrene incorporation did not significantly alter the conformational preference of the parent polymer (Figure S7). Considering the hydrophilic nature of the grafted PEG segments, it can be anticipated that a polar solvent would drive the chain to collapse and adopt a folded zigzag conformation as depicted in Scheme 3. To examine this conformational transition, it is important to ensure that the studies are carried out at adequately dilute concentrations, wherein interchain interactions are minimal. To ensure this, the polymer solution (0.05 mg mL−1) was taken in a fluorescence cuvette and the fluorescence spectra were recorded as a function of dilution; the spectral variation is shown in Figure S8. Normalization of these spectra with respect to the 376 nm peak revealed a complete overlap of all spectra, confirming the absence of any new feature due to interchain interactions (Figure S8). The weak excimer band is clearly due to an intrachain excimer, which expectedly remained unaffected by dilution. Pyrene has been used extensively as a probe to examine polymer chain conformation for over three decades; in most instances, the formation of an excimer has been used to reflect the compactness of the conformation.34 To examine the collapse of our PGACs, the pyrene-labeled polymer was taken in THF, and increasing amounts of water were added to it. The variation of the fluorescence spectra as a fraction of water added is shown in Figure 7. It is evident from the normalized spectra that the relative intensity of the excimer band increases dramatically with the addition of water; a plot of the variation of the excimer intensity with solvent composition reveals a steady increase up to a point, after which a sudden drop is seen. The drop has been ascribed to the macroscopic precipitation of the polymer, which essentially removes it from the sampling volume. In summary, these studies revealed the collapse of the polymer chain with increasing mole fraction of a poor solvent; we anticipated that this collapse would occur in a manner so as to minimize the

exposure of the hydrophobic alkylene segments to the polar medium, which most likely would lead to the formation of a zigzag folded conformation as depicted in Scheme 3.

■

CONCLUSIONS Periodically clickable polyesters carrying either long alkyl chains or PEG along the backbone provided an excellent opportunity to access two different types of periodically grafted amphiphilic copolymers (PGACs) through the azide-yne click reaction with PEG 350 azide or docosyl (C22) azide. It was seen that PGACs carrying crystallizable long alkylene segments in the backbone adopt a folded zigzag conformation whereas the PGACs carrying flexible PEG segments in the backbone appear to prefer a hairpin-like conformation. Immiscibility in both cases drives the chains to fold and subsequently organize laterally to form 2-D sheetlike architectures as revealed by AFM images. DSC and SAXS studies of the PGACs carrying long alkylene segments in the backbone showed that steric crowding at grafting junctions dramatically affects the crystallization of the backbone; this was evident from both the lowering of the transition temperatures and the normalized enthalpies corresponding to the melting of alkylene (HC) domains. On the other hand, DSC and SAXS studies of the PGACs carrying amorphous PEG segments in the backbone showed that the length of the PEG segment plays a crucial role in the selfsegregation of the PEG backbone and pendant docosyl sidechains and consequently the crystallization of the pendant docosyl side chains. Here again, both transition temperatures and the normalized enthalpies corresponding to the melting of the alkylene (HC) domain decreased significantly on going from DOCOPEG 300 to DOCOPEG 1000; in this case, it is hypothesized that the relative volume fraction of the amorphous backbone PEG segments controls the final morphology, in a similar manner as seen in the case of block copolymers. Solvent-dependent fluorescence studies of a pyrene-labeled PGAC bearing the alkylene backbone showed that there is a dramatic increase in the excimer emission as the chain collapses; this we attribute to the folding of the chain that substantially enhances the probability of excimer formation. In summary, periodically grafted amphiphilic copolymers present a very interesting class of systems wherein the dimensions of the immiscible domains can be very precisely controlled; folding and crystallization of the backbone segments provide much greater control over the morphology than when the crystallizable segments are present as pendant units. 6042

DOI: 10.1021/acs.langmuir.5b01227 Langmuir 2015, 31, 6035−6044

Article

Langmuir

■

(14) Payne, G. F.; Raghavan, S. R. Chitosan: a soft interconnect for hierarchical assembly of nano-scale components. Soft Matter 2007, 3, 521−527. (15) Lee, J. H.; Oh, H.; Baxa, U.; Raghavan, S. R.; Blumenthal, R. Biopolymer-Connected Liposome Networks as Injectable Biomaterials Capable of Sustained Local Drug Delivery. Biomacromolecules 2012, 13, 3388−3394. (16) Desbrieres, J.; Martinez, C.; Rinaudo, M. Hydrophobic derivatives of chitosan: characterization and rheological behaviour. Int. J. Biol. Macromol. 1996, 19, 21−28. (17) Chen, Y.; Javvaji, V.; MacIntire, I. C.; Raghavan, S. R. Gelation of Vesicles and Nanoparticles Using Water-Soluble Hydrophobically Modified Chitosan. Langmuir 2013, 29, 15302−15308. (18) Wang, W.; Qu, X. Z.; Gray, A. I.; Tetley, L.; Uchegbu, I. F. SelfAssembly of Cetyl Linear Polyethylenimine To Give Micelles, Vesicles, and Dense Nanoparticles. Macromolecules 2004, 37, 9114−9122. (19) Pramod, P. S.; Takamura, K.; Chaphekar, S.; Balasubramanian, N.; Jayakannan, M. Dextran Vesicular Carriers for Dual Encapsulation of Hydrophilic and Hydrophobic Molecules and Delivery into Cells. Biomacromolecules 2012, 13, 3627−3640. (20) Heitz, C.; Prud’homme, R. K.; Kohn, J. A New Strictly Alternating Comblike Amphiphilic Polymer Based on PEG. 2. Associative Behavior of a High Molecular Weight Sample and Interaction with SDS. Macromolecules 1999, 32, 6658−6667. (21) Auguste, D. T.; Kirkwood, J.; Kohn, J.; Fuller, G. G.; Prud’home, R. K. Surface Rheology of Hydrophobically Modified PEG Polymers Associating with a Phospholipid Monolayer at the Air-Water Interface. Langmuir 2008, 24, 4056−4064. (22) Kumar, R.; Chen, M. H.; Parmar, V. S.; Samuelson, L. A.; Kumar, J.; Nicolosi, R.; Yoganathan, S.; Watterson, A. C. Supramolecular Assemblies Based on Copolymers of PEG600 and Functionalized Aromatic Diesters for Drug Delivery Applications. J. Am. Chem. Soc. 2004, 126, 10640−10644. (23) Zheng, Y.; Chia, F.; Ungar, G.; Wright, P. V. Self-tracking in solvent-free, low-dimensional polymer electrolyte blends with lithium salts giving high ambient DC conductivity. Chem. Commun. 2000, 1459−1460. (24) Zheng, Y.; Chia, F.; Ungar, G.; Wright, P. V. Self-tracking, solvent-free low-dimensional polymer electrolyte blends with lithium salts. J. Power Sources 2001, 97, 641−643. (25) Chia, F.; Zheng, Y.; Liu, J.; Reeves, N.; Ungar, G.; Wright, P. V. Thick lamellar textures and high ambient conductivity in de-blended mixtures of low-dimensional systems of two polymers and Li salts. Electrochim. Acta 2003, 48, 1939−1951. (26) Zheng, Y.; Liu, J.; Liao, Y. P.; Ungar, G.; Wright, P. V. Structure and inter-phase stability in solvent-free low-dimensional polymer electrolytes with high lithium conductivity. Dalton Trans. 2004, 3053− 3060. (27) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. Carbon-hydrogen stretching modes and the structure of n-alkyl chains. 1. Long, disordered chains. J. Phys. Chem. 1982, 86, 5145−5150. (28) MacPhail, R. A.; Straus, H. L.; Snyder, R. G.; Elliger, C. A. C-H Stretching Modes and the Structure of n-Alkyl Chains. 2. Long, AllTrans Chains. J. Phys. Chem. 1984, 88, 334−341. (29) Takahashi, Y.; Tadokoro, H. Structural Studies of Polyethers, (-(CH2)m-O-)n. X. Crystal Structure of Poly(ethylene oxide). Macromolecules 1973, 6, 672−675. (30) Schenning, A. P. H. J.; Roman, C. E.; Weener, J. W.; Baars, M. W. P. L.; Gaast, S. J. V.; Meijer, E. W. Amphiphilic Dendrimers as Building Blocks in Supramolecular Assemblies. J. Am. Chem. Soc. 1998, 120, 8199−8208. (31) Winnik, F. M. Phase transition of aqueous poly-(Nisopropylacrylamide) solutions: a study by non-radiative energy transfer. Polymer 1990, 31, 2125−2134. (32) Weiss, S. Fluorescence Spectroscopy of Single Biomolecules. Science 1999, 283, 1676−1683. (33) Tampe, R.; Clark, B. R.; McConnell, H. M. Energy transfer between two peptides bound to one MHC class II molecule. Science 1991, 254, 87−89.

ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental methods, additional NMR spectra, GPC data, DSC thermograms fluorescence spectra, and AFM images. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01227.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

■

REFERENCES

We thank the Department of Science and Technology, New Delhi, for the J. C. Bose Fellowship (2011−2016) to S.R. and the Spectroscopy and Analytical Testing Facilities (SATF) of IISc for the DSC data. J.M. acknowledges CSIR, New Delhi, for a fellowship. We also thank Professor V. A. Raghunathan and Mr. Santosh Gupta (RRI, Bangalore) and Dr. Bhoje Gowd and Mr. P. Shaiju (NIIST, Trivandrum) for their valuable assistance in performing X-ray scattering studies and Professor M. Jayakannan and Ms. Sonashree Saxena (IISER, Pune) for providing us with the GPC data.

(1) Alexandridis, P.; Lindman, B. Amphiphilic Block Copolymers: SelfAssembly and Applications; Elsevier, 2000. (2) Rosen, B. M.; Wilson, C. J.; Wilson, D. A.; Peterca, A.; Imam, M. R.; Percec, V. Dendron-Mediated Self-Assembly, Disassembly, and Self-Organization of Complex Systems. Chem. Rev. 2009, 109, 6275− 6540. (3) Adams, M. L.; Lavasanifar, A.; Kwon, G. S. Amphiphilic block copolymers for drug delivery. J. Pharm. Sci. 2003, 92, 1343−1355. (4) Allen, C.; Maysinger, D.; Eisenberg, A. Nano-engineering block copolymer aggregates for drug delivery. Colloids Surf., B 1999, 16, 3− 27. (5) Kataoka, K.; Harada, A.; Nagasaki, Y. Block copolymer micelles for drug delivery: design, characterization and biological significance. Adv. Drug Delivery Rev. 2001, 47, 113−131. (6) Rosler, A.; Vandermeulen, G. W. M.; Klok, H. A. Advanced drug delivery devices via self-assembly of amphiphilic block copolymers. Adv. Drug Delivery Rev. 2001, 53, 95−108. (7) Discher, D. E.; Ahmed, F. Polymersomes. Annu. Rev. Biomed. Eng. 2006, 8, 323−341. (8) Williams, S. R.; Long, T. E. Recent advances in the synthesis and structure-property relationships of ammonium ionenes. Prog. Polym. Sci. 2009, 34, 762−782. (9) Kunitake, T.; Nakashima, N.; Takarabe, K.; Nagai, M.; Tsuge, A.; Yanagi, H. Vesicles of polymeric bilayer and monolayer membranes. J. Am. Chem. Soc. 1981, 103, 5945−5947. (10) Kunitake, T.; Tsuge, A.; Takarabe, K. Formation of Molecular Membranes in Water from Ionene Oligomers and Related Amphiphiles. Polym. J. (Tokyo, Jpn.) 1985, 17, 633−640. (11) De, S.; Ramakrishnan, S. Charge-Transfer Reinforced Folding of Novel Ionenes. Macromolecules 2009, 42, 8599−8603. (12) Roy, R. K.; Gowd, E. B.; Ramakrishnan. Periodically Grafted Amphiphilic Copolymers: Nonionic Analogues of Ionenes. S. Macromolecules 2012, 45, 3063−3069. (13) Mandal, J.; Prasad, S. K.; Shankar Rao, D. S.; Ramakrishnan, S. Periodically Clickable Polyesters: Study of Intrachain Self-Segregation Induced Folding, Crystallization, and Mesophase Formation. J. Am. Chem. Soc. 2014, 136, 2538−2545. 6043

DOI: 10.1021/acs.langmuir.5b01227 Langmuir 2015, 31, 6035−6044

Article

Langmuir (34) Char, K.; Frank, C. W.; Gast, A. P.; Tang, W. T. Hydrophobic attraction of pyrene-end-labelled poly(ethylene glycol) in water and water-methanol mixtures. Macromolecules 1987, 20, 1833−1838.

6044

DOI: 10.1021/acs.langmuir.5b01227 Langmuir 2015, 31, 6035−6044