Temperature-Induced Morphological Transitions of Poly

Sep 15, 2017 - Synthesis of diblock copolymer nano-assemblies: Comparison between PISA and micellization. Habib Khan , Mengjiao Cao , Wenfeng Duan ...
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Temperature-Induced Morphological Transitions of Poly(dimethylacrylamide)−Poly(diacetone acrylamide) Block Copolymer Lamellae Synthesized via Aqueous PolymerizationInduced Self-Assembly Xiao Wang, Jiamin Zhou, Xiaoqing Lv, Baohua Zhang, and Zesheng An* Institute of Nanochemistry and Nanobiology, College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China S Supporting Information *

ABSTRACT: Aqueous dispersion polymerization of diacetone acrylamide (DAAM) by chain extension from a hydrophilic poly(N,N-dimethylacrylamide) (PDMA30) macromolecular chain transfer agent (macro-CTA) to produce PDMA30−PDAAM x block copolymer nano-objects was investigated in detail by systematically varying solids content and degree of polymerization of the core-forming PDAAM, leading to the formation of pure lamellae, mixed lamellae/ vesicles, and pure vesicles as revealed by dynamic light scattering (DLS), transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). PDMA30−PDAAMx lamellae were found to span an unprecedented wide space in the morphology phase diagram. Moreover, in situ cross-linking of lamellae via statistical copolymerization of DAAM with an asymmetric cross-linker allyl acrylamide and the effect of cross-linking density on the colloidal and morphological stabilities were studied, representing the first report on in situ cross-linking of lamellae during polymerization-induced self-assembly (PISA). Finally, reversible, temperature-induced morphological transitions from lamellae to worms/spheres on cooling were investigated by DLS, TEM, 1H NMR spectroscopy, and rheology. The kinetics of the temperature-dependent morphological transitions and the rheological properties could be tuned by the cross-linking density.

1. INTRODUCTION

scope of PISA for the controllable synthesis of polymeric nano-objects that have now found increasing applications.21−29 Although a few polymerization techniques have been used,30−36 reversible addition−fragmentation chain transfer (RAFT)37 remains the predominantly exploited controlled radical polymerization in PISA synthesis because of its excellent monomer tolerance and mild conditions. Both RAFT-mediated emulsion and dispersion polymerizations have been successfully conducted with a variety of combinations of stabilizing and core-forming blocks having been reported. Emulsion PISA formulations are based on water-immiscible monomers which produce insoluble core-forming polymers.38−47 In contrast, dispersion PISA formulations involve using soluble monomers to produce core-forming polymers that are plasticized by the solvent to various degrees depending on the polymer−solvent solubility parameters.48 This qualitative difference explains, at least in part, why thermodynamically stable higher-order morphologies are more easily accessible under suitable dispersion polymerization conditions (e.g., relatively short

1−3

Polymerization-induced self-assembly (PISA) is an efficient and robust method for the synthesis of block copolymer nanoobjects. This is an alternative to the well-known traditional selfassembly of block copolymers, which typically produces lowconcentration nanoparticles via time-consuming processing steps.4−6 PISA allows simultaneous chain extension from a stabilizer block and in situ self-assembly of the growing block copolymers to produce a wide range of block copolymer morphologies without postpolymerization processing steps. Importantly, these morphologies are usually accessed at high polymer concentrations, an attribute that is highly desirable for industrial scale-up.7,8 Reliably targeting a specific morphology can be assisted by first constructing a phase diagram by systematically tuning the block ratio and solids content.9,10 More recently, it has been shown that other parameters including the degree of solvophilicity of the core-forming block,11−13 the incompatibility between the blocks,14 topology,15 and architecture16 as well as the ratio of homopolymer/diblock copolymer17−19 or triblock/diblock copolymer20 mixtures can all be harnessed to modulate nano-object morphologies, thus greatly expanding the versatility and © XXXX American Chemical Society

Received: July 31, 2017 Revised: September 5, 2017

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is highly soluble in water, so aqueous PISA can be conducted over an exceptionally large solids range.8 In addition to having a nicely balanced hydrophilic/hydrophobic structure, PDAAM is capable of forming multiple intra/intermolecular hydrogen bonds because it has both hydrogen bond donating and accepting moieties, which not only reinforces the insolubility of PDAAM but also offers exciting opportunities for new morphologies to be discovered and new thermoresponsive copolymers to be synthesized.86 Therefore, PDAAM-based PISA formulations have attracted increasing attention from several research groups. Cai et al. reported visible-lightmediated PISA of DAAM at room temperature, which revealed interesting film/silk-to-ribbon-to-vesicle morphological transitions.8 Our group also reported photocontrolled PISA of DAAM using a supramolecular photocatalyst.87 Sumerlin and co-workers demonstrated that significant modulation of worm length could be achieved through copolymerization of DAAM with a hydrophilic comonomer N,N-dimethylacrylamide (DMA) as a means to tune the hydrophilicity/hydrophobicity balance of the core-forming block.13 In a more relevant study by Armes and co-workers, constructing a phase diagram by systematically varying the degree of polymerization of both the acid-functionalized poly(N,N-dimethylacrylamide) (PDMA) stabilizing block and the PDAAM core-forming block enabled pure spheres, worms, and vesicles to be reliably produced.85 The pure worm phase was found to occupy an extremely narrow window with only one copolymer composition, specifically PDMA40−PDAAM99, being identified to form pure worms at 20% solids. While in most cases the PDMA− PDAAM nano-objects were insensitive to changes in either solution pH or temperature, the PDMA40−PDAAM99 worms exhibited weak pH and temperature responses. Partial morphological transitions were observed from pure worms to worms/spheres on increasing the solution pH from pH 2−3 to pH 9 at 20 °C or from pure worms to worms/vesicles on heating to 50 °C. Given that multiple morphological transitions from bilayer vesicles to worms/spheres have been reported for PHPMA-based PISA formulations,88,89 further studies on temperature-induced morphological transitions involving bilayer structures for PDAAM-based PISA formulations are necessary. In this article, we report the formation of PDMA30− PDAAMx lamellae over an unprecedented wide space via aqueous PISA of DAAM using PDMA30 as a stabilizer block. Identification of such a wide range of lamellae was enabled by the construction of a phase diagram and the use of complementary imaging techniques including transmission electron microscopy (TEM), atomic force microscopy (AFM), and scanning electron microscopy (SEM), which suggested that many of the bilayer structures that might otherwise be mistaken as vesicles were actually lamellae. We further report our studies on the in situ cross-linking of lamellae via statistical copolymerization of DAAM with an asymmetric cross-linker allyl acrylamide (ALAM) and examine the effect of cross-linking density on the colloidal and morphological stabilities. To the best of our knowledge, in situ cross-linking of PISA-generated lamellae has not been reported up to date. Finally, temperature-induced morphological transitions involving lamellae were investigated by TEM, 1H NMR spectroscopy, and rheology.

stabilizing block and relatively high polymer concentration) than in emulsion PISA, with kinetically frozen spheres more frequently being observed in the latter.49 It should be emphasized that besides solvent plasticization monomer partition into the core of nano-objects further contributes to the flexibility and mobility of the core-forming block, thus facilitating morphological transitions in dispersion PISA.12,50 For these reasons, considerably more studies have focused on dispersion PISA.23,28,29,51−62 Dispersion PISA has been conducted in various solvents including both polar and nonpolar solvents.63,64 The use of water for dispersion PISA is particularly attractive not only because water is cheap and environmentally benign but also high polymerization rates can be achieved and the obtained materials can be used for biomedical applications after simple purification via dialysis.65 Although aqueous PISA formulations are still limited due to the special requirement for water-soluble monomer/insoluble polymer, the number has been steadily growing over the past decade. Initially, aqueous dispersion PISA formulations comprised thermoresponsive polymers as the core-forming blocks such as poly(N-isopropylacrylamide) and poly(oligo(ethylene glycol) methyl ether (meth)acrylate)66−68 because many water-soluble polymers are known to have thermal transitions with either a lower critical solution temperature (LCST) or an upper critical solution temperature (UCST); above LCST or below UCST the polymers become (at least partially) dehydrated and thus insoluble in water.69,70 This temperature-dependent solubility makes thermoresponsive polymers suitable for aqueous dispersion polymerization and in the presence of a suitable cross-linker for the synthesis of thermoresponsive core−shell nanogels.71−77 Now aqueous PISA has further evolved to incorporate water-insoluble core-forming polymers which typically have a subtly balanced hydrophilic/hydrophobic structure and hence are weakly or moderately hydrophobic.14,78 One such notable example is aqueous PISA formulations based on poly(2-hydroxypropyl methacrylate) (PHPMA) as the coreforming polymer, which was originally developed by Armes et al. and now has become arguably the most studied aqueous dispersion PISA system.79 Interestingly, temperature-induced morphological transitions have been reported for PHPMAbased nano-objects; for example, cooling a poly(glycerol monomethacrylate)-b-PHPMA (PGMA54−PHPMA140) freestanding worm gel at 21 °C produced a freely flowing sphere dispersion at 4 °C.21 More recent studies on dispersion PISA in both polar (e.g., ethanol) and nonpolar (e.g., dodecane) organic solvents have suggested that such temperature-induced morphological transitions may be general for dispersion PISA formulations, albeit in organic solvents thermally induced morphological transitions usually occur on heating.80,81 This phenomenon has been explained by enhanced surface plasticization on cooling for aqueous dispersions or on heating for nonaqueous dispersions, which increases the effective volume of the stabilizer block and hence reduces the packing parameter.82 Poly(diacetone acrylamide) (PDAAM) is another waterinsoluble polymer that has been only recently reported by our group and others for aqueous dispersion PISA with several remarkable features having been noted.83,84 PDAAM bears a pendent ketone group in its repeat unit which can be conveniently exploited for efficient functionalization or crosslinking of the produced nano-objects using oxime or hydrazone chemistry.13,83,85 The diacetone acrylamide (DAAM) monomer B

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Scheme 1. Schematic Representation of (a) Synthesis of Linear and Cross-Linked Diblock Copolymer Lamellae via RAFT Aqueous Dispersion Polymerization of DAAM Using PDMA30 Macro-CTA and (b) Temperature-Induced Morphological Transitions

conditions. The mean thickness of lamellae was determined from around 50 lamellae for each sample. Scanning electron microscopy (SEM) was performed on a Hitach S-4800 at an operating voltage of 10 kV. The dispersions were diluted to 0.05% or 0.1% w/v and applied to aluminum foils which were then attached to steel stubs using carbon adhesive. After drying at room temperature for 2 h, the samples were made conductive by sputtering a thin layer of platinum. Rheology tests were performed on an Anton Paar MCR302 rheometer in the temperature range 0−50 °C, at a temperature ramping rate of 2 °C/ min with a temperature resolution of 0.01 °C, controlled by a Peltier plate and active hood. The frequency was maintained at 1 Hz, and the strain was maintained at 0.1% during the variable-temperature tests. DMF Resistance Tests. Dispersions were diluted to 1% w/v with DMF and then used for DLS characterization. To redisperse the nanoobjects from DMF back to water, the DMF dispersions were dialyzed against water in dialysis tubes (MWCO 1 kg/mol) for 24 h. The redispersed nano-objects were used for DLS and TEM characterization. Study of Temperature-Induced Transitions. The PDMA30− PDAAM70 dispersions (12% w/v) were kept at desired temperatures to induce morphological transitions. At predetermined time intervals, aliquots were taken and diluted to 0.1% w/v for DLS and TEM characterization. Synthesis of PDMA30 Macro-CTA via Solution Polymerization. PDMA30 macro-CTA was synthesized as previously reported.90 In brief, 2-ethylsulfanylthiocarbonylsulfanylpropionic acid methyl ester (0.4330 g, 1.929 mmol) and DMA (7.6117 g, 0.0767 mol) were dissolved in 16 mL of DMF. After the solution was degassed with nitrogen for 30 min in an ice/water bath, it was heated to 70 °C and the polymerization was initiated by injection of a degassed AIBN (6.3 mg, 0.038 mmol) solution via a microsyringe. After 2 h, the monomer conversion was determined to be 75% via 1H NMR spectroscopy analysis. The solution was precipitated three times in excess diethyl ether to give the PDMA30 macro-CTA. Mn,theory = 3.2 kg/mol, Mn,GPC = 4.2 kg/mol, and Đ = 1.09. Synthesis of PDMA30−PDAAMx Block Copolymer NanoObjects via Aqueous Dispersion Polymerization. A PDMA30 macro-CTA was used as the stabilizer block for RAFT dispersion polymerization of DAAM to synthesize linear PDMA30−PDAAMx

2. EXPERIMENTAL SECTION Materials. Diacetone acrylamide (DAAM, 99%) from Aladdin, 2,2′-azobis(2-methylpropionamidine) dihydrochloride (V-50, 97%) from J&K Scientific, and N,N-dimethylformamide (DMF, AR) from Sinopharm Chemical Reagent were used without further purification. N,N-Dimethylacrylamide (DMA, 99%) from J&K Scientific was passed through a column of Al2O3 to remove the inhibitor. 2,2′-Azobis(2methylpropionitrile) (AIBN, CP) from Sinopharm Chemical Reagent was recrystallized from methanol twice. PDMA30 macro-CTA and allyl acrylamide (ALAM) were synthesized as previously reported.90 Characterization and Methods. 1H NMR spectroscopy was acquired on a Bruker AV 500 MHz spectrometer in CDCl3 or DMSO-d6, and chemical shifts were reported using solvent residue as the reference. Variable-temperature 1H NMR spectroscopy of PDMA30− PDAAM70 block copolymer (prepared at 12% w/v) was recorded on a Varian Mercury plus (500 MHz) spectrometer in D2O at an increment of 2 °C. Gel permeation chromatography (GPC) was performed on a Waters Alliance e2695 GPC system. Columns consisting of a styragel guard column and Waters Styragel HR3-HR5 (molecular weight range 5.0 × 102−4.0 × 106 g mol−1) were used for separation of linear block copolymers, while columns consisting of an Org guard column and Org D2500 and D5000 were used for separation of branched/crosslinked block copolymers. A 2414 refractive index (RI) detector was used using DMF (HPLC grade, containing 1 mg mL−1 LiBr) as the eluent at a flow rate of 0.8 mL min−1. Molecular weights and dispersities were reported against PMMA standards (molecular weight range 2.4 × 102−1.0 × 106 g mol−1) using Empower 2 software. Dynamic light scattering (DLS) analysis of the nano-object dispersions diluted to 0.1% w/v was conducted on a Malvern ZS90 at a detection angle of 90°. Z-average diameter (Dh) and polydispersity (PDI) were analyzed using the Malvern software. Transmission electron microscopy (TEM) was performed on a Jeol 200CX microscope operating at 200 kV. The nano-object dispersions were diluted to 0.1% or 0.2% w/v for the preparation of TEM samples on carbon-coated copper grids. After brief drying at room temperature the TEM samples were further dried under vacuum at 40 °C for 12 h. Atomic force microscopy (AFM) was performed on a Shimadzu SPM-9600 in the phase mode. The nanoobject dispersions were diluted to 0.01% w/v, which were spread on freshly cleaved mica pieces and subjected to drying under ambient C

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Figure 1. (a) Kinetic plots for the RAFT aqueous dispersion polymerization of DAAM ([PDMA30]/[DAAM]/[V-50] = 1/100/0.05, solids 12% w/ v, 70 °C). Molar ratio of [ALAM]/[DAAM] = 0 (black) or 2% (red). (b) Mn and Đ evolution during the synthesis of linear PDMA30−PDAAM100. GPC evolution for the synthesis of linear PDMA30−PDAAM100 (c) and cross-linked PDMA30−P(DAAM100-ALAM2) (d). (Note the separation of linear and cross-linked samples was performed using different columns.)

Figure 2. (a) Morphology phase diagram of linear PDMA30−PDAAMx block copolymer dispersions. Phase regions consist of lamellae (L), mixed phase of lamellae and vesicles (L+V), and vesicles (V). Representative GPC traces (b) and TEM images (c) for (i) PDMA30−PDAAM130 at 12% w/ v, (ii) PDMA30−PDAAM110 at 25% w/v, and (iii) PDMA30−PDAAM200 at 12% w/v. block copolymer nano-objects in the absence of the asymmetric crosslinker ALAM or cross-linked PDMA30−P(DAAMx-ALAMy) block copolymer nano-objects in the presence of ALAM. PDMA30− P(DAAM100-ALAM2) block copolymer nano-objects prepared at 20% w/v was taken as an exemplary synthesis and was described as follows: PDMA30 (0.0381 g, 0.012 mmol), DAAM (0.2019 g, 1.2 mmol), and ALAM (2.6521 mg, 0.024 mmol) were dissolved in deionized water (2 mL). The reaction solution was degassed with nitrogen for 0.5 h at 0 °C. After thermal stabilization at 70 °C, 100 μL of aqueous solution of V-50 (containing V-50 0.1615 mg, 0.6 μmol) was injected via a microsyringe to start the polymerization. After 12 h, the monomer conversion was determined to be >99% as revealed by 1 H NMR spectroscopic analysis (500 MHz, DMSO-d6).

3. RESULTS AND DISCUSSION A PDMA30 macro-CTA was prepared by RAFT solution polymerization in DMF and used as the stabilizer block for RAFT aqueous dispersion polymerization of DAAM to synthesize PDMA−PDAAM diblock copolymer nano-objects. The number-average degree of polymerization (DP) of purified PDMA was determined via 1H NMR spectroscopic analysis (Figure S1) by comparing the integrals of the dimethyl groups (e) of DMA units with those of the methyl ester protons (h) of the CTA end-group. The polymer was denoted as PDMA30. Mn,theory = 3.2 kg/mol, Mn,GPC = 4.2 kg/mol, and Đ = 1.09 (Figure S2). As shown in Scheme 1, linear PDMA30−PDAAMx and in situ cross-linked PDMA30−P(DAAMx-ALAMy) block copolymer D

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Figure 3. TEM, SEM, and AFM images for (a−c) linear PDMA30−PDAAMx dispersions at various PDAAM DPs and solids (w/v), and (d) crosslinked PDMA30−P(DAAM100-ALAM2) dispersion.

phase diagram (Figure 2a). The morphology evolves from pure lamellae to a mixture of lamellae and vesicles to pure vesicles as the DP increases. It seems the region with mixed morphologies is very narrow for solids ≤20% w/v while the pure phases of lamellae and vesicles are bounded by a relatively broad region of mixed phases for higher solids. Nevertheless, it is emphasized that pure lamellae cover a remarkably large space in the phase diagram, which is not very common in other PISA formulations.87 Some representative TEM images for different phases are depicted in Figure 2c. At 12% w/v, PDMA30− PDAAM130 formed a pure lamellae phase while PDMA30− PDAAM200 formed a pure vesicle phase. At a much higher solids content (25% w/v), PDMA30−PDAAM110 formed a mixed phase of lamellae and vesicles. GPC analysis indicates these block copolymers have monomodal molecular weight distributions with low dispersities. In addition, SEM and AFM were applied as complementary characterization methods for the PDMA30−PDAAMx lamellae in cases where lamellae might be mistaken as vesicles. As shown in Figure 3, several PDMA30−PDAAMx dispersions at various PDAAM DPs and solids were analyzed by TEM, SEM, and AFM. While some of the linear PDMA30−PDAAM100 lamellae (12% w/v) show curling rims with slightly higher thickness than that in the middle, the linear PDMA30−PDAAM130 lamellae (12% w/v) are more flat with a more uniform thickness across the lamellae. Some of the PDMA30−PDAAM150 lamellae may perhaps resemble vesicles at first glance. However, genuine vesicles formed by the same diblock copolymer typically exhibit wrinkles with a darker periphery in their TEM images.90 Also, SEM and AFM analysis of this sample suggests lamellae

nano-objects were synthesized employing the PDMA30 macroCTA via RAFT aqueous dispersion polymerization of DAAM (or DAAM plus ALAM) at 70 °C. Kinetic studies were performed by periodic sampling of the reaction dispersion. The kinetic plots (Figure 1a) show typical two-stage polymerization kinetics corresponding to an initial solution polymerization and a subsequent accelerated polymerization in monomer-swollen particles after nucleation.50 Compared with the synthesis of linear PDMA30−PDAAM100, a reduction in polymerization rate is observed when targeting cross-linked PDMA30−P(DAAM100ALAM2) with 2 mol % of ALAM (relative to DAAM), which is consistent with our previous results90 and can be explained by increased viscosity due to the formation of branched structures.91 GPC analysis (Figure 1b,c) for linear PDMA30− PDAAMx shows a linear evolution of Mn with DAAM conversion and a relatively low dispersity of the final diblock copolymer (Đ = 1.24), typical of a well-controlled RAFT polymerization. In contrast, the GPC traces (Figure 1d) recorded for the synthesis cross-linked PDMA 30 −P(DAAM100-ALAM2) become increasingly broad with increasing monomer conversion, and the dispersity significantly increases from 1.27 (25 min, 20% conversion) to 1.45 (35 min, 39% conversion) to 1.66 (45 min, 56% conversion) and to 2.49 (55 min, 72% conversion), suggesting formation of progressively more branched structures as the conversion increases. Next, the synthesis of PDMA30−PDAAMx via RAFT aqueous dispersion polymerization was systematically investigated by varying solids content and DP of PDAAM, and the morphologies were revealed by DLS, TEM, SEM, and AFM analysis, which were used for construction of a morphology E

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Figure 4. TEM images of linear PDMA30−PDAAM100 and cross-linked PDMA30−P(DAAM100-ALAMy) lamellae at 12% w/v solids with different amounts of ALAM (mol %, relative to DAAM) before DMF resistance tests (a−c, g−i) and their corresponding dispersions after DMF resistance tests (d−f, j−l).

Figure 5. TEM images of linear PDMA30−PDAAM70 and cross-linked PDMA30−P(DAAM70-ALAMy) dispersions synthesized at 12% w/v solids with different amounts of ALAM (mol %, relative to DAAM) at different temperatures.

In situ cross-linking is a straightforward strategy to afford nano-objects with structural integrity; however, to the best of our knowledge, in situ cross-linking of lamellae during PISA has yet to be demonstrated. Therefore, in situ cross-linking of lamellae was conducted via statistical copolymerization of DAAM with an asymmetric cross-linker ALAM, and the effect of cross-linking density on the colloidal and morphological stabilities was also inspected. Colloidally stable cross-linked lamellae dispersions were obtained when targeting DP of 100 at 12% w/v solids with 0.5−2.5 mol % of ALAM (relative to DAAM), but macroscopic precipitate was observed with ≥3 mol % of ALAM. A comparison of the TEM images of the linear and cross-linked lamellae can be found in Figure 4a−c,g− i. With increasing cross-linking density, the lamellar morphology is retained up to 2 mol % of ALAM while a large portion of vesicles appeares for 2.5 mol % of ALAM. AFM analysis reveals

rather than vesicles. Height analysis of the AFM images indicates the lamellae become thicker from ∼3 nm (Figure 3a, PDMA30−PDAAM100 at 12% solids) to ∼5 nm (Figure 3b, PDMA30−PDAAM130 at 12% solids) and to ∼6 nm (Figure 3c, PDMA30−PDAAM150 at 20% solids) due to increased DP of the core-forming block. For comparison, the SEM and AFM images of vesicles appeared in the cross-linked PDMA30− P(DAAM100-ALAM2) dispersion (12% solids) are shown in Figure 3d. It is evident that the vesicles are collapsed with uneven surfaces, whereas the lamellae have a more uniform thickness. It should be emphasized that some of the lamellae have remarkably rounded edges (the reviewers of the manuscript pointed out that lamellae typically have irregular shapes92), which is not understood at the present but is believed the shape of lamellae may be related to the nature of block copolymers and synthetic conditions. F

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Figure 6. Temperature-dependent rheology sweeps of linear PDMA30−PDAAM70 (a) and cross-linked PDMA30−P(DAAM70-ALAMy) dispersions with 1 mol % (b) and 2 mol % (c) of ALAM (relative to DAAM) at 12% w/v solids. The tests were performed at a fixed frequency of 1.0 Hz and 0.1% strain. The cooling/heating rate used for the rheology sweeps was 2 °C/min.

investigated the effect of temperature on the kinetics of morphological transitions (Figure S3), which indicated morphological transitions from lamellae to worms/spheres were enhanced at lower temperatures, as expected. In order to elucidate the mechanism for the temperature-induced morphological transitions, variable-temperature 1H NMR spectra were taken in the temperature range of 9−51 °C (Figure S4), which, however, revealed essentially no obvious change in the degree of hydration of the core-forming PDAAM block, confirming that PDAAM is only insoluble in water rather than being thermoresponsive. The morphological transitions can be explained by the interfacial plasticization mechanism as proposed by Armes and co-workers.82 Thus, lowering temperature only increases the hydration of several PDAAM units at the PDMA/PDAAM interface, which accounts for a minor portion of the PDAAM block. This explains why no obvious hydration changes were observed by variable-temperature 1H NMR spectroscopy. However, the hydration change of this minor portion of the PDAAM units at the interface is enough to induce morphological transitions from lamellae to worms/ spheres. The reversible temperature-induced gelation−degelation of linear and cross-linked nano-objects was further characterized by rheology (Figure 6). At the onset of the cooling process, the free-flowing dispersions have low values of storage modulus G′ and loss modulus G″, with G″ being higher than G′. The linear sample has very close values of G′ and G″ on cooling from 50 °C to the crossover point at 35 °C, suggesting a viscoelastic dispersion and a slow morphological transition from lamellae to worms. Below the critical gelation temperature (CGT) of 35 °C, G′ becomes obviously larger than G″, suggesting formation of a worm gel. Further cooling leads to progressively larger moduli and thus a stronger gel due to accumulated formation of worms. During the heating process, both G′ and G″ drop due to the worms aggregating and transforming back to lamellae. The G′ and G″ curves cross over again at 22 °C on heating, which is lower than that on cooling due to slower kinetics for worms to re-form lamellae. It is somewhat surprising that the two cross-linked samples show similar rheological behavior to the linear sample with only some subtle differences being observed. As previously discussed, it took a relatively long period of time for the PDMA30−P(DAAM70-ALAM0.7) lamellae (1 mol % of ALAM) to dissociate into worms/spheres at 10 °C while the PDMA30−P(DAAM70-ALAM1.4) lamellae (2 mol % of ALAM) showed no changes in morphology over the same period of time at 10 °C. However, rheology tests suggest that both the cross-linked samples experience morphological transitions from lamellae to worms upon cooling and such transitions are reversible with some degrees of hysteresis. We

the thickness of lamellae notably increases from 3 nm (Figure 3a, PDMA30−PDAAM100 at 12% solids) to 5 nm (Figure 3d, PDMA30−P(DAAM100-ALAM2) at 12% solids) upon crosslinking, which can be explained by the enhanced rigidity of the block copolymers due to cross-linking. The ability of these linear and cross-linked lamellae to resist solvent dissolution during solvent switching was evaluated by DMF resistance tests. The dispersions were diluted in DMF and then dialyzed against water for 24 h to remove DMF. The TEM images of linear and cross-linked samples after DMF resistance tests are shown in Figure 4d−f,j−l. Linear PDMA30− PDAAM100 and cross-linked lamellae with less than 1 mol % of ALAM were disrupted by DMF (Figure 4d−f), which was consistent with the DLS data (in DMF, Dh < 30 nm) shown in Table S2. On the contrary, cross-linked lamellae with 1.5−2.5 mol % of ALAM (Figure 4j−l) remained as stable lamellae after solvent switching. These results suggest that block copolymer lamellae can be effectively in situ cross-linked during PISA under suitable conditions to maintain the morphology that is resistant to dissolution by a good solvent as DMF used in this study. Interestingly, linear PDMA30−PDAAMx nano-objects with PDAAM DPs from 60 to 90 underwent reversible temperatureinduced morphological transitions. A 12% w/v aqueous dispersion of linear PDMA30−PDAAM70 lamellae formed a free-flowing dispersion at the polymerization temperature of 70 °C, which became a free-standing gel on cooling to room temperature, and turned back to a free-flowing dispersion when reheated to 70 °C for 2 h. The TEM images in Figure 5a−c show that the morphology of linear PDMA30−PDAAM70 nanoobjects transformed from lamellae at 70 °C to a mixture of worms (major population) and spheres (minor population) at 10 °C for 6 h and back to lamellae when reheated to 70 °C for 2 h. Cross-linked PDMA30−P(DAAM70-ALAMy) lamellae at 12% w/v solids with 1−2.5 mol % of ALAM were used to explore the effect of cross-linking density on the temperatureinduced morphological transitions. Compared with linear PDMA30−PDAAM70, the TEM images in Figure 5d−f indicate that cross-linked lamellae with 1 mol % of ALAM essentially maintained the lamellar morphology at 10 °C for 8 h but gradually turned to worms/spheres at 10 °C after a long period of time (100 h), which suggests the temperature-induced morphological transition is delayed by the cross-links due to reduced mobility of the hydrophobic block. No obvious morphological transitions for cross-linked nano-objects with 2 and 2.5 mol % of ALAM were observed at 10 °C for 100 h (Figure 5g−l), suggesting the morphologies were completely immobilized at these two cross-linking densities, which is also consistent with the results from DMF dissolution tests. We also G

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attribute this discrepancy in morphological transitions of these cross-linked samples to the different measurement conditions in TEM and rheology studies. In TEM studies the samples were kept still at 10 °C, but in rheology tests the samples were subjected to gradual temperature changes and perhaps more importantly the samples were subjected to shear forces, which can enhance the dissociation of lamellae. It is noticeable that as the degree of cross-linking increases from 0 to 2 mol %, the CGT changes from 35 to 32 to 28 °C on cooling and from 22 to 25 to 29 °C on heating. In addition, the 2 mol % cross-linked copolymer seems to be more elastic than the other two samples as evidenced by the close resemblance of its profiles during the cooling and heating cycles.

4. CONCLUSION A detailed morphology phase diagram has been constructed for RAFT aqueous dispersion polymerization of DAAM using PDMA30 as a macro-CTA. Lamellae were found to exist over an unprecedented wide phase space. In situ cross-linking of lamellae during PISA synthesis was realized for the first time using an asymmetric cross-linker. Lamellae synthesized with cross-linker higher than 1 mol % (relative to DAAM) were able to resist DMF dissolution. Linear PDMA30−PDAAMx lamellae with x in the range 60−90 were thermoresponsive. Reversible temperature-induced morphological transitions from lamellae to worms/spheres were observed on cooling. Rheology experiments confirmed formation of worm gels on cooling of the lamellae dispersions. Cross-linking density was demonstrated to tune the kinetics of morphological transitions as well as the rheology properties of the dispersions during temperature-induced morphological transitions.

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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.7b01644. Characterization data of PDMA30, tabulated data for dispersion polymerization and DMF resistance tests, TEM images of temperature-dependent morphologies, and variable-temperature 1H NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.A.). ORCID

Zesheng An: 0000-0002-2064-4132 Author Contributions

X.W. and J.Z. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank financial support by National Natural Science Foundation of China (21674059) and assistance of Instrumental Analysis and Research Center, Shanghai University.

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REFERENCES

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