A Vesicle‐to‐Worm Transition Provides a New High‐Temperature Oil Thickening Mechanism

Abstract Diblock copolymer vesicles are prepared via RAFT dispersion polymerization directly in mineral oil. Such vesicles undergo a vesicle‐to‐worm transition on heating to 150 °C, as judged by TEM and SAXS. Variable‐temperature 1H NMR spectroscopy indicates that this transition is the result of surface plasticization of the membrane‐forming block by hot solvent, effectively increasing the volume fraction of the stabilizer block and so reducing the packing parameter for the copolymer chains. The rheological behavior of a 10 % w/w copolymer dispersion in mineral oil is strongly temperature‐dependent: the storage modulus increases by five orders of magnitude on heating above the critical gelation temperature of 135 °C, as the non‐interacting vesicles are converted into weakly interacting worms. SAXS studies indicate that, on average, three worms are formed per vesicle. Such vesicle‐to‐worm transitions offer an interesting new mechanism for the high‐temperature thickening of oils.

Small-molecule amphiphiles such as surfactants are wellknown to self-assemble in aqueous solution to form spherical micelles, [1] lamellae or vesicles (liposomes). [2] In 1976, Israelachvili and co-workers introduced the concept of ageometric packing parameter for surfactants, [3] thus allowing their morphology to be predicted based on the relative dimensions of the hydrophilic and hydrophobic components.I n1 995, Eisenberg and co-workers [4] reported the first examples of block copolymer vesicles using highly asymmetric polystyrene-poly(acrylic acid) diblock copolymers.S elf-assembly occurred on addition of water, anon-solvent for polystyrene, to adilute copolymer solution in DMF.Bates and co-workers reported the formation of well-defined block copolymer worms in aqueous solution using poly(ethylene oxide)polybutadiene diblock copolymers. [5] Antonietti and Fçrster subsequently extended the packing parameter concept to include block copolymer spheres,worms and vesicles. [6] More recently,p olymerization-induced self-assembly (PISA) has provided aversatile and convenient route for the synthesis of block copolymer spheres,worms or vesicles at relatively high copolymer concentrations in either aqueous, [7] alcoholic [8] or non-polar [9] media. Ty pically,r eversible addition-fragmentation chain transfer (RAFT) dispersion polymerization is utilized to chain-extend as oluble macromolecular chain transfer agent (macro-CTA) with amiscible monomer to form an insoluble polymer block, which drives in situ self-assembly. Tr ansmission electron microscopy (TEM) studies have shed new light on the nature of the worm-to-vesicle transition that can occur under these conditions,which proceeds via a"jellyfish" intermediate. [10] PISA formulations have enabled the rational synthesis of low-polydispersity vesicles using abinary mixture of two macro-CTAs [11] and the precise mechanism of vesicle growth during such syntheses has been recently elucidated. [10a, 12] PISA enables the convenient preparation of thermoresponsive diblock copolymer nano-objects.F or example, aw orm-to-sphere transition occurs on cooling poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) (PGMA-PHPMA) diblock copolymer nanoparticles,because the core-forming PHPMA block becomes more hydrated at lower temperatures. [13] In contrast, worm-to-sphere transitions are observed on heating certain alcoholic and non-polar PISA formulations. [13,14] In both cases,t he ingress of solvent into the cores increases the effective volume fraction of the stabilizer block and hence triggers the morphological transition. Such worm-to-sphere transitions lead to degelation, because the multiple inter-particle contacts formed by the anisotropic worms cannot be maintained by the isotropic spherical nanoparticles.
In the present study,w er eport at hermally triggered vesicle-to-worm transition for aP ISA formulation in an onpolar solvent (mineral oil). Small-angle X-ray scattering (SAXS), rheology and 1 HNMR studies confirm that this order-order transition leads to much higher viscosity at elevated temperature,which suggests aunique oil-thickening mechanism.
Al ow-polydispersity poly(stearyl methacrylate) (PSMA; mean degree of polymerization = 13) macro-CTAwas chainextended via RAFT dispersion polymerization of benzyl methacrylate (BzMA) monomer in mineral oil to generate well-defined PSMA 13 -PBzMA 96 diblock copolymer vesicles at 10 %w/w solids (Scheme 1). ABzMA conversion of 96 %was obtained for this PISA formulation within 5h at 90 8 8C, as judged by 1 HNMR spectroscopy.T HF GPC analysis confirmed that the resulting PSMA 13 -PBzMA 96 diblock copolymer chains exhibited ar elatively narrow molecular weight distribution (M w /M n = 1.17). Moreover,t he clear shift in the molecular weight distribution curve relative to that of the PSMA 13 macro-CTAindicated ahigh blocking efficiency (see Figure S1 in the Supporting Information). TEM images indicate aw ell-defined vesicular morphology,s ee Figure 1a. SAXS studies conducted on the 5.0 %w /w dispersion of PSMA 13 -PBzMA 96 nanoparticles at 20 8 8C(red data, Figure 2) indicated ag radient of approximately À2a tl ow q,a s expected for vesicles,w ith characteristic local minima corresponding to the outer vesicle dimensions (q % 0.05 nm À1 )and the vesicle membrane thickness, T m (q % 0.5 nm À1 ).
Representative SAXS patterns for 5.0 %w /w PSMA 13 -PBzMA 96 nanoparticles in mineral oil ( Figure 2) recorded at 20 8 8C(red data), 90 8 8C(blue data) and 130 8 8C(green data) are very similar, indicating that the vesicular morphology is retained throughout this temperature range.N otably,t he local minimum at high q ( % 0.5 nm À1 )g radually shifts to higher q,s uggesting that the vesicle membrane thickness decreases from 8.8 nm to 7.5 nm over this temperature range. These observations are consistent with greater solvation of the BzMA residues near the block junction on heating, which reduces the effective volume fraction of this membraneforming block (hence reducing the mean membrane thickness). On heating to 135 8 8C, as hallower gradient is observed at low q (orange data) and the local minimum at high q is shifted to lower q.T his indicates the onset of the vesicle-toworm transition. SAXS patterns continued to evolve on further heating,w ith ap ure worm phase eventually being obtained at 145 8 8C( pink data) and 150 8 8C( black data). This morphological assignment is based on ag radient of approximately À1a tl ow q,a sw ell as the loss of the feature at q % 0.05 nm À1 which represents the overall vesicle diameter.
Fitting SAXS patterns acquired at 20 8 8Cand 150 8 8Cusing well-known vesicle [15] and worm-like micelle [16] models enabled the mean overall vesicle diameter, D out ,v esicle membrane thickness, T m ,w orm thickness, T w ,a nd worm contour length, L w ,t obe determined (see Figure 3). At 150 8 8C, the local minimum at q % 0.5 nm À1 indicates that T w = 14.5 nm, which is somewhat larger than the T m observed at 20 8 8C (8.8 nm). Similar differences were also observed by Rank et al., [17] who reported that poly(2-vinyl pyridine) 66 -poly(ethylene oxide) 46 (P2VP 66 -PEO 46 )d iblock copolymer vesicles formed worms on cooling from 25 8 8Ct o48 8C. These P2VP 66 -PEO 46 vesicles exhibited a T m of 12 nm at 25 8 8Ccompared to a T w of 16 nm for worms formed from the same diblock copolymer at 4 8 8C. This was attributed to the interdigitated core-forming blocks producing am ore densely packed core within the vesicle membrane-with little or no interdigitation occurring for the corresponding worms. [17] Geometric calculations based on the PSMA 13 -PBzMA 96 vesicle/worm dimensions determined from SAXS analysis indicate that, on average,e ach vesicle dissociates to form three worms on heating from 20 8 8Ct o1 50 8 8C( see Supporting Information). This observation is in rather good agreement with the ratio of Scheme 1. Synthesis of poly(stearyl methacrylate) 13 -poly(benzyl methacrylate) 96 (PSMA 13 -PBzMA 96 ) vesicles via RAFT dispersion polymerization of benzyl methacrylate at 10 %w/w solids in mineral oil at 90 8 8C.   Thea pparent degree of solvation of the PBzMA coreforming block within the PSMA 13 -PBzMA 96 nano-objects was monitored by variable temperature 1 HNMR spectroscopy. First, the initial PSMA 13 -PBzMA 96 vesicles were transferred from mineral oil into d 26 -dodecane via three centrifugationredispersion cycles (see Supporting Information). TEM studies of the final diluted copolymer dispersion in ndodecane confirmed that the vesicles survived this solvent exchange (see Figure S2). 1 HNMR studies of 5.0 %w /w PSMA 13 -PBzMA 96 nano-objects in d 26 -dodecane were conducted from 25 to 150 8 8C(see Figure 4). This aliphatic solvent was selected because it is very similar to the mean chemical composition of mineral oil, which is not commercially available in deuterated form. TheP BzMA benzylic proton signal "b" at 4.9 ppm became progressively more intense relative to the oxymethylene proton signal "a" of the PSMA block at 4.0 ppm, which confirms greater solvation of the PBzMA block. This indicates increasing solvent ingress into the PBzMA membranes at elevated temperatures,c ausing achange in the preferred diblock copolymer morphology via surface plasticization. [14b] Allowing for the known subtle differences in solvation between mineral oil and n-dodecane, [14c] these NMR data are both physically reasonable and also consistent with the SAXS observations shown in Figure 2.
It is well-known that dispersions of diblock copolymer worms form free-standing gels at sufficiently high copolymer concentrations. [5,18] Moreover,athermally-triggered wormto-sphere transition results in rapid in situ degelation. [13,14] A 10 %w/w dispersion of PSMA 13 -PBzMA 96 vesicles in mineral oil was studied to assess the effect of the vesicle-to-worm transition on its rheological behavior (see Figure 5). On heating from 20 8 8Ct o1 30 8 8C, the storage (G')a nd loss (G'') moduli are reduced and G' always remains lower than G'', which indicates that the dispersion becomes less viscous.Such an inverse relationship between solution viscosity and temperature is well-known for most fluids. [19] However,onfurther heating from 130 to 135 8 8C, G' increases by more than five orders of magnitude up to around 1Pa. Thus the latter temperature corresponds to the critical gelation temperature (CGT), above which the dispersion behaves as av iscoelastic gel (since G' now exceeds G''). These observations are supported by the SAXS data shown in Figure 2, where PSMA 13 -PBzMA 96 vesicles are observed at temperatures   between 20 8 8Ca nd 130 8 8C, and the onset of the vesicle-toworm transition occurred at around 135 8 8C. Thef irst appearance of anisotropic worm-like particles as judged by SAXS is in close agreement with the CGT determined by rheology, which strongly suggests that multiple inter-worm contacts are responsible for the observed gelation. Moreover,varying the target mean degree of polymerization of the membraneforming PBzMA block from 85 to 100 enables the thermoresponsive behavior of the precursor vesicles to be tuned:a n upturn in complex viscosity is observed over a2 08 8Cr ange (see Figure S3).
There are rather few literature examples of significantly higher viscosities being achieved on increasing the solution temperature,w ith most of these studies involving aqueous formulations. [13,20] However,t hese thermal transitions typically occur at relatively low temperatures (4 8 8Cto308 8C) and usually increase the aqueous solution viscosity by less than an order of magnitude.Asfar as we are aware,there is just one literature example of an aqueous formulation that is directly analogous to the thermal transition described herein:certain ionic surfactant vesicles can form ah ighly viscous dispersion of anisotropic micelles on heating to 60 8 8C. [21] Of perhaps more relevance to the present study,o il thickening has been widely reported for low molecular weight gelators. [22] However,s uch gelators are usually heated to obtain ah omogeneous solution, with gelation then occurring on cooling. Thus our observations suggest that diblock copolymer vesicles can provide anew high-temperature oil-thickening mechanism to rival that recently achieved using thermosensitive graft copolymers. [23] In summary,PSMA 13 -PBzMA 96 vesicles prepared directly in mineral oil via PISA undergo an order-order morphological transition on heating from 20 8 8Ct o1 50 8 8C. This system has been characterized using TEM, SAXS,rheology and variable temperature 1 HNMR spectroscopy.T he latter technique confirms that greater solvation of the PBzMA core-forming block occurs at elevated temperatures.S uch surface plasticization triggers the observed morphological transition above 135 8 8C, as indicated by SAXS,TEM and rheology studies.This vesicle-to-worm transition provides au nique high-temperature oil-thickening mechanism that may offer some commercial utility.